Structured oxidation catalysts

0001The present invention relates to the preparation and use of structured catalysts for catalytically converting alkanes, alkenes and mixtures thereof to their corresponding oxygenates, including unsaturated carboxylic acids and esters thereof, by vapor phase oxidation. In particular, the invention is directed to preparation of such catalysts in three-dimensional forms that provide improved thermal stability, improved thermal integration and improved mass transfer coupled with reduced pressure drop during specific catalytic conversions of alkanes, alkenes and mixtures thereof. In addition, the invention includes one or more chemical and/or physical modifications of structured catalysts, which in turns improves their efficiency and selectivity for converting alkanes, alkenes and mixtures thereof to their corresponding oxygenates. The invention is further directed to methods for preparing structured catalysts and to vapor phase catalytic processes using the structured catalysts, including catalytic oxidations of alkanes, alkenes and mixtures thereof.

0002The selective partial oxidation of alkenes to unsaturated carboxylic acids and their corresponding esters is an important commercial process. However, the selective and efficient partial oxidation/dehydrogenation of alkanes to products including olefins, unsaturated carboxylic acids and esters of unsaturated carboxylic acids using conventional fixed bed reactors have significant disadvantages, including high pressure drop in the catalyst bed, inadequate mass transfer during catalysis, and thermal instability of the catalyst; all which contribute to the resulting non-uniform access of reactants to the catalyst, non-uniformity of the catalytic surface and non-optimal local process conditions. An unsatisfactory product distribution results as a consequence.

0003International Patent Publication No. WO 99/55459 discloses a exhaust gas catalytic converter comprising a monolithic catalyst in the form of a honeycomb having a plurality of parallel channels defined by the honeycomb walls. The catalyst has different zones along the length of the channels, each zone defined by their coating or lack of a coating, and the zone extend for a length of the channel in which there is the same coating and architecture. Moreover, soluble components in the coating compositions are fixed in their respective zones. Unfortunately, there are a number of limitations associated with such layered catalyst composites. One inherent limitation of the catalysts is that they are designed for three way conversions (TWC), namely three different types of catalytic conversions, the reduction of nitrogen oxides to nitrogen, the oxidation of carbon monoxide to carbon dioxide and the oxidation of hydrocarbons. In addition to the inherent non-optimal distributions of different products, there are other limitations including pressure drop in the catalyst bed and inadequate mass transfer during catalysis. The latter issue is particularly of concern when the catalyst comprises mixed metal oxides and when the catalytic conversion include multiple conversions, such as the conversion of alkanes to their corresponding oxygenates. One impediment to the provision of a commercially viable process for such catalytic oxidations is the identification of an optimal structured catalyst, structured catalysts or structured catalyst system, processed and/or prepared in three-dimensional forms that provide improved thermal stability, improved thermal integration and improved mass transfer coupled with reduced pressure drop during specific catalytic conversions of alkanes, alkenes and mixtures thereof. Such structured catalysts would provide improved alkane/alkene conversions and product selectivities, which in turn provides increased yields of corresponding unsaturated products.

0004The inventors have discovered structured catalysts useful for converting alkanes, alkenes and mixtures thereof to their corresponding oxygenates. The structured catalysts, prepared from both mixed metal oxide catalysts and modified mixed metal oxide catalysts are processed in to three-dimensional forms that provide improved thermal stability, improved thermal integration and improved mass transfer coupled with reduced pressure drop during specific catalytic conversions of alkanes, alkenes and mixtures thereof. Inventors have further discovered that one or more chemical, physical and combinations of chemical and physical modifications to the structured catalysts results in unexpected improvements in alkane/alkene conversions, product selectivities and yields of oxygenates as compared to corresponding unmodified structured catalysts.

0005Accordingly, there is provided a structured catalyst comprising: one or more mixed metal oxide catalysts, each catalyst sequentially deposited as essential elements, in random order, the relative amounts of elements satisfying the expression         MoV_aNb_bX_cZ_dO_n wherein X is at least one element selected from the group consisting of Te and Sb, Z is at least one element selected from the group consisting of W, Cr, Ta, Ti, Zr, Hf, Mn, Re, Fe, Ru, Co, Rh, Ni, Pd, Pt, Ag, Zn, B, Al, Ga, In, Ge, Sn, Si, Pb, P, Bi, Y, Ce, rare earth elements and alkaline earth elements, 0.1 ≤ a ≤ 1.0, 0.01 ≤ b ≤ 1.0, 0.01 ≤ c ≤ 1.0, 0 ≤ d ≤ 1.0 and n is determined by the oxidation states of the other elements; wherein the one or more metal oxide catalysts are self-supported and further comprise a three dimensional structure having openings that facilitates movement of gas phase reactants and products. The structured catalyst is useful for oxidizing alkanes, alkenes and combinations of alkanes and alkenes.

0006There is also provided a structured catalyst useful for oxidizing alkanes, alkenes and combinations of alkanes and alkenes comprising: one or more mixed metal oxide catalysts in contact with a three dimensional form of continuous unitary structures having openings that facilitates movement of gas phase reactants and products.

0007According to one embodiment, the structured catalysts are in the form of three-dimensional structures selected from the group consisting of ceramic foams and ceramic monoliths comprising ceramics including cordierite, alumina, zirconia, silica, aluminosilicate zeolites, phosphosilicate zeolites PSZ, and other zeolites, wherein the catalyst or catalysts are present on or inside walls of the openings. The ceramic acts as a support and the catalyst or catalysts are deposited on the surface by coating or are deposited inside the support by methods including impregnation, wash coating, slurry dip-coating, chemical vapor deposition (CVD), physical vapor deposition (PVD) and precipitation. Ceramic foams are prepared by chemical vapor deposition (CVD) and physical vapor deposition (PVD). The foam structures comprise specific numbers of pores per inch. The monoliths comprise specific numbers of cells per inch. The structured catalysts permit high space velocities of reactants and products with a corresponding minimized pressure drop. According to a separate embodiments, the structured catalysts are in the form of three-dimensional structures selected from the group consisting of metallic foams and metallic monoliths, extruded catalysts, membrane catalysts having permeable walls between openings, catalysts arranged in arrays, catalysts having openings including grooves, channels and other passages created by techniques including corrugation, stacking staggering and superimposing, fibrous catalysts, woven catalysts, mesh catalysts, non-woven catalysts, multi-layered catalysts joined together by a thermally conductive connection and coated with an oxidation barrier, perforated ceramic and metallic disks, microreactor catalysts wherein the catalysts is fabricated as a microreactor, structured composite catalysts containing combinations of three-dimensional structures, and modified structured catalysts wherein the structured catalysts undergoes one or more chemical, physical or combinations of chemical and physical treatments modifying the structured catalyst as compared to the unmodified structured catalysts.

0008The present invention also provides a process for improving one or more performance characteristics of one or more mixed metal oxide catalysts comprising the step of: preparing one or more metal oxide catalysts as a self supporting structured catalyst that facilitates movement of gas phase reactants and products or depositing one or more mixed metal oxide catalysts on a three dimensional form of continuous unitary structures having openings that facilitates movement of gas phase reactants and products; wherein catalyst performance characteristics of the structured catalyst is improved as compared to corresponding performance characteristics of the one or more unstructured (unmodified) mixed metal oxide catalysts. According to one embodiment, the structured catalyst unexpectedly provides improved selectivities and yields of oxygenates including unsaturated carboxylic acids from their corresponding alkanes, alkenes and combinations of corresponding alkanes and alkenes at constant alkane/alkene conversion as compared to corresponding unstructured mixed metal oxide catalysts.

0009The present invention also provides a process for improving one or more performance characteristics of one or more mixed metal oxide catalysts, comprising the steps of: <ul id="ul0001" list-style="none" compact="compact"><li>a) preparing one or more metal oxide catalysts into a structured catalyst that facilitates movement of gas phase reactants and products or depositing one or more mixed metal oxide catalyst on a three dimensional form of continuous unitary structures having openings that facilitates movement of gas phase reactants and products; and</li><li>b) treating the structured catalyst with one or more chemical treatments, one or more physical treatments and one or more combinations of chemical and physical treatments;</li></ul> wherein catalyst performance characteristics of the modified structured catalysts is improved as compared to corresponding performance characteristics of the one or more unmodified structured catalyst. According to one embodiment, the modified structured catalyst unexpectedly provides improved selectivities and yields of oxygenates including unsaturated carboxylic acids from their corresponding alkanes, alkenes and corresponding combinations of alkanes and alkenes at constant alkane/alkene conversion as compared to the corresponding unmodified structured catalyst.

0010The invention also provides a process for preparing a structured catalyst comprising the step of preparing a three dimensional form of continuous unitary structures having openings that is self supporting and facilitates movement of gas phase reactants and products by sequentially depositing a catalyst composition comprising one or more mixed metal oxides, in random order, as essential elements. According to one embodiment, the structured catalysts are in the form of three-dimensional structures selected from the group consisting of ceramic foams and ceramic monoliths comprising ceramics including cordierite, alumina, zirconia, silica, aluminosilicate zeolites, phosphosilicate zeolites PSZ, and other zeolites, wherein the catalyst or catalysts are present on or inside walls of the openings. The ceramic acts as a support and the catalyst or catalysts are deposited on the surface by coating or are deposited inside the support by methods including impregnation, wash coating, slurry dip-coating, chemical vapor deposition (CVD), physical vapor deposition (PVD) and precipitation. Ceramic foams are prepared by chemical vapor deposition (CVD) and physical vapor deposition (PVD). The foam structures comprise specific numbers of pores per inch. The monoliths comprise specific numbers of cells per inch. The structured catalysts permit high space velocities of reactants and products with a corresponding minimized pressure drop. According to a separate embodiments, the structured catalysts are in the form of three-dimensional structures selected from the group consisting of metallic foams and metallic monoliths, extruded catalysts, membrane catalysts having permeable walls between openings, catalysts arranged in arrays, catalysts having openings including grooves, channels and other passages created by techniques' including corrugation, stacking staggering and superimposing, fibrous catalysts, woven catalysts, mesh catalysts, non-woven catalysts, multi-layered catalysts joined together by a thermally conductive connection and coated with an oxidation barrier, perforated ceramic and metallic disks, microreactor catalysts wherein the catalysts is fabricated as a microreactor, structured composite catalysts containing combinations of three-dimensional structures, and modified structured catalysts wherein the structured catalysts undergoes one or more chemical, physical or combinations of chemical and physical treatments modifying the structured catalyst as compared to the unmodified structured catalysts.

0011The invention also provides a process for preparing a structured catalyst comprising the step of preparing a three dimensional form of continuous unitary structures having openings that is self supporting and facilitates movement of gas phase reactants and products by sequentially depositing a catalyst composition comprising, in random order, as essential elements, the relative amounts of the elements satisfying the expression         MoV_aNb_bX_cZ_dO_n wherein X is at least one element selected from the group consisting of Te and Sb, Z is at least one element selected from the group consisting of W, Cr, Ta, Ti, Zr, Hf, Mn, Re, Fe, Ru, Co, Rh, Ni, Pd, Pt, Ag, Zn, B, Al, Ga, In, Ge, Sn, Si Pb, P, Bi, Y, Ce, rare earth elements and alkaline earth elements, 0.1 ≤ a ≤ 1.0, 0.01 ≤ b ≤1.0, 0.01 ≤ c ≤ 1.0, 0 ≤ d ≤ 1.0 and n is determined by the oxidation states of the other elements.

0012The invention also provides a process for preparing a structured catalyst comprising the step of: preparing a three dimensional form of continuous unitary structures having openings that is self supporting and facilitates movement of gas phase reactants and products by sequentially depositing both a catalyst support comprising, in random order, of at least one element selected from the group consisting of O, Al, B, Si, P, Zr, Nb, Ta, W and a catalyst composition comprising, in random order, as essential elements, the relative amounts of the elements satisfying the expression         MoV_aNb_bX_cZ_dO_n wherein X is at least one element selected from the group consisting of Te and Sb, Z is at least one element selected from the group consisting of W, Cr, Ta, Ti, Zr, Hf, Mn, Re, Fe, Ru, Co, Rh, Ni, Pd, Pt, Ag, Zn, B, Al, Ga, In, Ge, Sn, Si Pb, P, Bi, Y, Ce, rare earth elements and alkaline earth elements, 0.1 ≤ a ≤ 1.0, 0.01 ≤ b ≤ 1.0, 0.01 ≤ c ≤ 1.0, 0 ≤ d ≤ 1.0 and n is determined by the oxidation states of the other elements.

0013The invention also provides a process for preparing one or more structured catalysts, comprising the steps of : <ul id="ul0002" list-style="none" compact="compact"><li>a) providing a catalyst support;</li><li>b) sequentially depositing on the catalyst support a catalyst composition comprising, in random order, as essential elements, at least one layer comprising Mo, at least one layer comprising V, at least one layer comprising Te, and at least one layer comprising X, wherein X is at least one element selected from the group consisting of Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pd, Pt, Sb, Bi, B, In and Ce, to form a loaded support, said sequential vapor deposition providing relative amounts of said elements such that, after a calcination of said loaded support, the relative amounts of the elements satisfy the expression         Mo_aV_bTe_cX_d wherein a, b, c and d are the relative atomic amounts of the essential elements Mo, V, Te and X, respectively, and, when a = 1, b = 0.01 to 1.0, c = 0.01 to 1.0 and d = 0.01 to 1.0; and</li><li>c) calcining said loaded support.</li></ul>

0014The present invention also provides a process for producing C2-C8 oxygenates, including C2-C8 unsaturated carboxylic acids, which comprises the step of contacting a corresponding alkane, alkene or a mixture of corresponding alkane and alkene to a vapor phase catalytic oxidation reaction with one or more structured catalysts comprising one or more mixed metal oxides; wherein yield and selectivity of the oxygenates, including unsaturated carboxylic acids, is improved using the one or more structured catalysts as compared to the one or more corresponding unmodified mixed metal oxide catalysts.

0015The present invention also provides a process for producing C2-C8 oxygenates, including C2-C8 unsaturated carboxylic acids, which comprises the steps of: <ul id="ul0003" list-style="none" compact="compact"><li>a) modifying one or more structured catalysts comprising one or more mixed metal oxides using one or more chemical treatments, one or more physical treatments and one or more combinations of chemical and physical treatments; and</li><li>b) contacting a corresponding alkane, alkene or a mixture of corresponding alkane and alkene to a vapor phase catalytic oxidation reaction with one or more the modified structured catalysts comprising one or more mixed metal oxides; wherein yield and selectivity of the oxygenates, including unsaturated carboxylic acids, is improved using the one or more modified structured catalysts as compared to the one or more corresponding unmodified structured catalysts.</li></ul>

0016The invention also provides a process for preparing unsaturated carboxylic acids from corresponding alkanes, alkenes, or corresponding alkanes and alkenes, the process comprising the step of: <ul id="ul0004" list-style="none" compact="compact"><li>passing a gaseous alkane, alkene or alkane and alkene, and molecular oxygen to a reactor, the reactor including one or more structured catalysts, including modified structured catalysts, cumulatively effective at converting the gaseous alkane, alkene, or alkane and alkene to its corresponding gaseous unsaturated carboxylic acid; wherein the reactor is operated at a temperature of from 100°C to 600°C. According to one embodiment, one or more structured catalysts comprising one or more mixed metal oxides, including modified mixed metal oxides, are used and a conventional reactor is used with the alkane or alkane and alkene having a reactor residence time of greater than 100 milliseconds. According to a separate embodiment, a short contact time reactor is used with the alkane or alkane and alkene having a reactor residence time of no greater than 100 milliseconds.</li></ul>

0017The structured catalyst comprises one or more modified mixed metal oxide catalysts having the empirical formula:         MoV_aNb_bX_cZ_dO_n wherein X is at least one element selected from the group consisting of Te and Sb, Z is at least one element selected from the group consisting of W, Cr, Ta, Ti, Zr, Hf, Mn, Re, Fe, Ru, Co, Rh, Ni, Pd, Pt, Ag, Zn, B, Al, Ga, In, Ge, Sn, Pb, P, Bi, Y, rare earth elements and alkaline earth elements, 0.1 ≤ a ≤ 1.0, 0.01 ≤ b ≤ 1.0, 0.01 ≤ c ≤ 1.0, 0 ≤ d ≤ 1.0; n is determined by the oxidation states of the other elements According to one embodiment, the structured catalyst comprises one or more modified mixed metal oxide catalysts having the empirical formula:         MeMoV_aNb_bX_cZ_dO_n wherein M_e is at least one or more chemical modifying agents, X is at least one element selected from the group consisting of Te and Sb, Z is at least one element selected from the group consisting of W, Cr, Ta, Ti, Zr, Hf, Mn, Re, Fe, Ru, Co, Rh, Ni, Pd, Pt, Ag, Zn, B, Al, Ga, In, Ge, Sn, Pb, P, Bi, Y, rare earth elements and alkaline earth elements, 0.1 ≤ a ≤ 1.0, 0,01 ≤ b ≤ 1.0, 0.01 ≤ c ≤ 1.0, 0 ≤ d ≤ 1.0 and n, e are determined by the oxidation states of the other elements.

0018As used herein, the term "structured catalyst" refers to any catalyst, including mixed metal oxide catalysts, prepared and or fabricated in to in a three dimensional form of continuous unitary structures having openings that facilitate movement of gas phase reactants and products. The term "modified structured catalyst" which is equivalent to "treated structured catalysts" which is also equivalent to "post-treated structured catalysts" refers to any chemical, physical and combinations of chemical and physical modification or modifications of one or more structured catalysts as compared to corresponding structured catalysts having undergone no such modification or modifications (also referred to as unmodified structured catalysts, equivalently referred to as untreated structured catalysts). Modifications to structured catalysts include, but are not limited to, any differences in the modified structured catalysts as compared to corresponding unmodified structured catalysts. Suitable modifications to structured catalysts include, for example, structural changes, spectral changes (including position and intensity of characteristic X-ray diffraction lines, peaks and patterns), spectroscopic changes, chemical changes, physical changes, compositional changes, changes in physical properties, changes in catalytic properties, changes in performance characteristics in conversions of organic molecules, changes in yields of organic products from corresponding reactants, changes in catalyst activity, changes in catalyst selectivity and combinations thereof. This includes one or more chemical modifying agents, one or more physical processes and combinations of one or more chemical modifying agents and one or more physical processes.

0019As used herein, the term "modified catalyst" which is equivalent to "treated catalysts" which is also equivalent to "post-treated catalysts" refers to any chemical, physical and combinations of chemical and physical modification or modifications of one or more prepared catalysts as compared to corresponding catalysts having undergone no such modification or modifications (also referred to as unmodified catalysts, equivalently referred to as untreated catalysts). The term is described and defined in co-pending U. S. Provisional Application Ser. No. 60/523,297. This includes one or more chemical modifying agents (e.g. a reducing agent such as an amine), one or more physical processes (e.g. mechanical grinding at cryogenic temperatures also referred to as "cryo-grinding") and combinations of one or more chemical modifying agents and one or more physical processes. The term "cryo" in front of any treatment term refers to any treatment that occurs with cooling, under freezing temperatures, at cryogenic temperatures and any use of cryogenic fluids. Suitable cryogenic fluids include, but are not limited to for example, any conventional cryogens and other coolants such as chilled water, ice, compressible organic solvents such as freons, liquid carbon dioxide, liquid nitrogen, liquid helium and combinations thereof. Suitable chemical and physical modification of prepared (untreated) catalysts results in unexpected improvements in treated catalyst efficiency and selectivity in alkane, alkene or alkane and alkene oxidations as compared to corresponding untreated catalysts and improved yields of oxygenated products using modified catalysts using modified catalysts as compared to unmodified catalysts. The term prepared catalysts refers to unmodified catalysts. The prepared catalysts are obtained from commercial sources or are prepared by conventional preparative methods, including methods described herein. The term "treated catalysts" and "modified catalysts" does not refer to or include regenerated, reconditioned and recycled catalysts. The term conditioning refers to conventional heating of prepared metal oxide catalysts with gases including hydrogen, nitrogen, oxygen and selected combinations thereof.

0020As used herein, the term "cumulatively converting" refers producing a desired product stream from one or more specific reactants using one or more modified catalysts and modified catalyst systems of the invention under specific reaction conditions. As an illustrative example, cumulatively converting an alkane to its corresponding unsaturated carboxylic acid means that the modified catalyst(s) utilized will produce a product stream comprising the unsaturated carboxylic acid corresponding to the added alkane when acting on a feed stream(s) comprising the alkane and molecular oxygen under the designated reaction conditions.

0021As used herein, mixed metal oxide catalyst refers to a catalyst comprising more than one metal oxide. The term "catalytic system" refers to two or more catalysts. For example, platinum metal and indium oxide impregnated on an alumina support defines both a catalytic system and a mixed metal oxide catalyst. Yet another example of both is palladium metal, vanadium oxide and magnesium oxide impregnated on silica.

0022Accordingly, structured catalysts of the present invention are utilized for oxidizing alkanes, alkenes and combinations of alkanes and alkenes. Any structured catalyst of this type that is prepared and or fabricated into a three dimensional form of continuous unitary structures having openings that facilitate movement of gas phase reactants and products is useful in accordance with the invention. The openings are physical features that facilitates movement of gas phase reactants and products during catalytic conversions of carbon containing molecules. One advantage of structured catalysts is that they permit high space velocities of reactants and products with a corresponding minimized pressure drop as compared to unmodified catalysts and catalysts having random orientations of catalysts particles.

0023Suitable structured catalysts include, but are not limited to for example, monolithic catalysts comprising continuous unitary structures including many openings selected from pores, cells, channels, and other narrow passages, membrane catalysts having wall structures that are permeable in addition to openings; arranged catalysts comprising unitary structures ordered in arrays and other patterns; and composite structures prepared and or fabricated from combinations thereof. In monolithic catalysts, the openings are uniform and are oriented in a regular patterns, including straight, angular, zig-zag patterns and irregular patterns. The cross-section of the monolithic catalysts openings reveals ordered patterns, including but not limited to honeycomb structures. Catalysts are dispersed uniformly over the global repeating structure of the monolith, are deposited within the layer of materials that comprise the walls of the openings structure. Monoliths comprise both ceramics, including both metal oxide and non-metal oxide ceramics, and metals. In membrane catalysts the wall structures comprises uniform and non-uniform arrangements of features providing wall permeability. Catalysts are dispersed uniformly on the wall structures or in the wall structures. Arranged catalysts are oriented in directions of the flow of reactants and products, including perpendicular to the direction of reactant/product flow.

0024Suitable monolithic catalysts of the invention comprise structured catalysts that are prepared to have three dimensional structures and are self-supported and structured catalysts including one or more catalysts and a catalytic support. Suitable examples of the former monolithic catalysts include, but are not limited to for example, ceramic foams prepared from CVD or PVD of metal oxides resulting in a self-supported structured catalyst comprising one or more mixed metal oxides and one or more mixed metal oxides foams or monoliths fabricated in the form of a microreactor or arrays of microreactors having mechanically produced openings (referred to as cells) selected from pores, cells, channels, and other narrow passages. The foams structures include from 30 to 150 openings per inch. The monolithic structures include from 200 to 800 cells per inch. Suitable examples of the latter monolithic catalysts include, but are not limited to for example, ceramic foams and ceramic monoliths comprising ceramic supports including, but not limited to for example, cordierite, alumina, zirconia, silica, aluminosilicate zeolites, phosphosilicate zeolites PSZ, and other zeolites, wherein the catalyst or catalysts are present on or inside walls of the openings. The catalyst or catalysts are deposited on the surface of the support or are deposited inside the support. The catalyst or catalysts are deposited on the surface by coating or are deposited inside the support by methods including, , but not limited to for example, impregnation, wash coating, slurry dip-coating, chemical vapor deposition (CVD), physical vapor deposition (PVD) and precipitation. Other suitable examples of the latter monolithic catalysts include, but are not limited to for example, metallic foams and metallic monoliths, extruded catalysts wherein the extrudate has a regular structure, catalysts having openings including grooves, channels and other passages created by techniques including corrugation, stacking staggering and superimposing, perforated ceramic and metallic disks, microreactor catalysts wherein the catalysts is fabricated as a microreactor, structured composite catalysts containing combinations of three-dimensional structures, and modified structured catalysts wherein the structured catalysts undergoes one or more chemical, physical or combinations of chemical and physical treatments modifying the structured catalyst as compared to the unmodified structured catalysts.

0025Suitable membrane catalysts of the invention comprise permeable wall structures in addition to openings formed from the wall structures. Radial mass transport occurs by diffusion through the openings of the wall structures. As a consequence, mass fluxes through the wall structures are often small. Suitable examples of membrane catalysts include, but are not limited to for example, organic membrane having the catalyst or catalysts deposited on or incorporating within the catalyst, inorganic membrane catalysts, including dense membrane catalysts incorporating metals, non-metals and metal oxides, and porous membrane catalysts, both types having the catalyst or catalysts deposited on or incorporating within the catalyst.

0026Suitable arranged catalysts of the invention comprise unitary structures arranged in arrays and oriented with respect to the direction of reactant/product flow. The arranged structured catalysts provide relatively fast mass transport over the reaction zone, typically oriented perpendicular to the direction of reactant/product flow. Suitable examples of arranged catalysts include, but are not limited to for example, particulate catalysts arranged in arrays, extruded catalysts arranged in arrays, fibrous catalysts, woven catalysts, mesh catalysts, non-woven catalysts, and multi-layered catalysts joined together by a thermally conductive connection and coated with an oxidation barrier.

0027The structured catalysts of the invention are prepared by conventional methods well known in the art. Methods for making structured catalysts are described by X. Xu and J. A. Moulijn in Chapter 21, pp. 599-615 in "Structured Catalysts and Reactors", edited by A. Cybulski and J. A. Moulijn, Marcel Dekker, New York (1998). Methods for making catalytic foams are described in U. S. Pat. Nos. 6,103,149; 6,040,266; 5,780,157; 5,283,109; and 5,154,970. Suitable structured catalysts fabricated as a microreactor and structured catalysts having mechanically produced microfeatures are described in International Publication No. WO 03/106386.

0028Modified structured catalysts are obtained by treating one or more metal oxides with one or more chemical, physical and combinations of chemical and physical treatments or are obtained by preparing structured catalysts from one or more modified mixed metal oxide catalysts.

0029Chemical treatments, resulting in treated/modified catalysts, include one or more chemical modifying agents. Suitable chemical modifying agents include, but are not limited to for example, oxidizing agents selected from hydrogen peroxide, nitrogen, nitric acid, nitric oxide, nitrogen dioxide, nitrogen trioxide, persulfate; reducing agents selected from amines, pyridine, hydrazine, quinoline, metal hydrides, sodium borohydride, C1-C4 alcohols, methanol, ethanol, sulfites, thiosulfites, aminothiols; combinations of oxidizing agents and reducing agents; acids selected from HCl, HNO3, H2SO4; organic acids, organic diacids, acetic acid, oxalic acid, combinations of C1-C4 alcohols and C1-C4 organic acids, oxalic acid and methanol; inorganic bases selected from NH3, NH4OH, H2NNH2, HONH2, NaOH, Ca(OH)2, CaO, Na2CO3, NaHCO3, organic bases selected from ethanol amine, diethanolamine, triethanolamine; pH adjustments; peroxides selected from inorganic peroxides, H2O2, organic peroxides, tBu2O2; chelating agents, ethylenediamine, ethylenediaminetetraacetic acid (EDTA); electrolysis including electrolytic reduction; treatment with high energy radiation including ultraviolet and X-ray radiation; and combinations thereof.

0030Physical treatments, resulting in treated/modified catalysts, include one or more physical processes. Suitable physical processes include, but are not limited to for example, cooling, cryogenic cooling, pressure cooling, compacting under pressure, high pressure die pressing, thermolyzing (also referred to as polymer burn off), mechanical grinding at cryogenic temperatures, high shear grinding at cryogenic temperatures, cryo-milling, cryo-densifying, cryo-stressing, cryo-fracturing, cryo-pelletizing, deforming, wash coating, molding, forming, shaping, casting, machining, laminating, drawing, extruding, lobalizing, impregnating, forming spheres (spherolizing or jetting), slurrying, cryo-slurrying, preparing shelled catalysts (shelling), multi-coating, electrolyzing, electrodepositing, compositing, foaming, cryo-fluidizing, cryo-spraying, thermal spraying, plasma spraying, vapor depositing, adsorbing, ablating, vitrifying, sintering, cryo-sintering, fusing, fuming, crystallizing, any altering of catalyst crystal structure, polycrystallizing, recrystallizing, any surface treating of the catalyst, any altering of catalyst surface structure, any altering of catalysts porosity, any altering of catalyst surface area, any altering of catalyst density, any altering of bulk catalysts structure, reducing the particle size of the primary catalyst particles in combination with cooling or thermolyzing the catalyst, and any combinations of chemical and physical treatments, including but not limited to solvent extraction, Soxhlet extraction, batch solvent extraction, continuous flow solvent extraction, extraction in supercritical solvents, contacting the catalyst with one or more leaching agents including solvents, altering catalyst pH, any chemical treatments used in modifying catalyst surface structure, mechanical grinding in supercritical solvents, chemisorbing one or more chemical agents, ultrasonification using one or more solvents selected from organic solvents such as alcohols and amines ultrasonification, and any physical treatments employing solvents under supercritical conditions. According to a separate embodiment, modified catalysts include one or more further chemical and/or physical treatments of already modified catalysts.

0031Any one or more metal oxide catalysts are usefully modified and used to prepare both structured catalysts and modified structured catalysts utilized in catalytic conversions of molecules containing carbon in accordance with the invention. According to one embodiment, the structured catalysts comprise both unmodified and modified mixed metal oxide catalysts useful for catalytically converting alkanes, alkenes and combinations of alkanes and alkenes to their corresponding oxygenates. The modified structured catalysts are prepared using one or more chemical, physical and combined chemical and physical treatments to provide modified structured catalysts.

0032According to one embodiment of the invention, suitable prepared catalysts used and modified in accordance with the invention are one or more mixed metal oxide catalysts having a catalyst having the empirical formula         MoV_aNb_bX_cZ_dO_n wherein X is at least one element selected from the group consisting of Te and Sb, Z is at least one element selected from the group consisting of W, Cr, Ta, Ti, Zr, Hf, Mn, Re, Fe, Ru, Co, Rh, Ni, Pd, Pt, Ag, Zn, B, Al, Ga, In, Ge, Sn, Pb, P, Bi, Y, rare earth elements and alkaline earth elements, 0.1 ≤ a ≤ 1.0, 0.01 ≤ b ≤ 1.0, 0.01 ≤ c ≤ 1.0, 0 ≤ d ≤ 1.0 and n is determined by the oxidation states of the other elements. Preparation of the mixed metal oxide (MMO) catalysts is described in U. S. Patent Nos. 6,383,978; 6,641,996; 6,518,216; 6,403,525; 6,407,031; 6,407,280; and 6,589,907; U. S. Publication Application No. 20030004379; U. S. Provisional Application Ser. Nos. 60/235,977; 60/235,979; 60/235,981; 60/235,984; 60/235,983; 60/236,000; 60/236,073; 60/236,129; 60/236,143; 60/236,605; 60/236,250; 60/236,260; 60/236,262; 60/236,263; 60/283,245; and 60/286,218; and EP Patent Nos. EP 1 080 784; EP 1 192 982; EP 1 192 983; EP 1 192 984; EP 1 192 986; EP 1 192 987; EP 1 192 988; EP 1 192 982; EP 1 249 274; and EP 1 270 068. The synthesis of such MMO (mixed metal oxide) catalysts is accomplished by several methods well known by those having skill in the art. A precursor slurry of mixed metal salts is first prepared by conventional methods and methods described above that include, but are not limited to for example, rotary evaporation, drying under reduced pressure, hydrothermal methods, co-precipitation, solid-state synthesis, impregnation, incipient wetness, sol gel processing and combinations thereof. After the precursor slurry is prepared it is dried according to conventional drying methods including, but not limited to for example, drying in ovens, spray drying and freeze drying. The dried precursor is then calcined to obtain prepared MMO catalysts using well known techniques and techniques described above to those having skill in the art including, but not limited to for example, flow calcinations, static calcinations, rotary calcinations and fluid-bed calcinations. In some cases the prepared MMO catalysts are further milled to improve their catalytic activity.

0033It is noted that promoted mixed metal oxides having the empirical formulae Mo_jV_mTe_nNb_yZ_zO_o or W_jV_mTe_nNb_yZ_zO_o, wherein Z, j, m, n, y, z and o are as previously defined, are particularly suitable for use in connection with the present invention. Additional suitable embodiments are either of the aforesaid empirical formulae, wherein Z is Pd. Suitable solvents for the precursor solution include water; alcohols including , but not limited to, methanol, ethanol, propanol, and diols, etc.; as well as other polar solvents known in the art. Generally, water is preferred. The water is any water suitable for use in chemical syntheses including, without limitation, distilled water and de-ionized water. The amount of water present is preferably an amount sufficient to keep the elements substantially in solution long enough to avoid or minimize compositional and/or phase segregation during the preparation steps. Accordingly, the amount of water will vary according to the amounts and solubilities of the materials combined. Preferably, though lower concentrations of water are possible for forming a slurry, as stated above, the amount of water is sufficient to ensure an aqueous solution is formed, at the time of mixing.

0034According to a separate embodiment of the invention, suitable prepared mixed metal oxide catalysts used and modified in accordance with the invention are one or more promoted mixed metal oxide catalysts having the empirical formula         J_jM_mN_nY_yZ_zO_o wherein J is at least one element selected from the group consisting of Mo and W, M is at least one element selected from the group consisting of V and Ce, N is at least one element selected from the group consisting of Te, Sb and Se, Y is at least one element selected from the group consisting of Nb, Ta, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Sb, Bi, B, In, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Hf, Pb, P, Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, and Z is selected from the group consisting of Ni, Pd, Cu, Ag and Au; and wherein, when j = 1, m = 0.01 to 1.0, n = 0.01 to 1.0, y = 0.01 to 1.0, z = 0.001 to 0.1 and o is dependent on the oxidation state of the other elements. Preparation of the mixed metal catalysts is described in U. S. Patent Nos. 6,383,978; 6,641,996; 6,518,216; 6,403,525; 6,407,031; 6,407,280; and 6,589,907; U. S. Provisional Application Ser. Nos. 60/235,977; 60/235,979; 60/235,981; 60/235,984; 60/235,983; 60/236,000; 60/236,073; 60/236,129; 60/236,143; 60/236,605; 60/236,250; 60/236,260; 60/236,262; 60/236,263; 60/283,245; and 60/286,218; and EP Patent Nos. EP 1 080 784; EP 1 192 982; EP 1 192 983; EP 1 192 984; EP 1 192 986; EP 1 192 987; EP 1 192 988; EP 1 192 982; and EP 1 249 274.

0035According to a separate embodiment of the invention, suitable prepared catalysts modified and used in accordance with the invention are one or more mixed metal oxide catalysts having the empirical formula         A_aD_bE_cX_dO_e wherein A is at least one element selected from the group consisting of Mo and W, D is at least one element selected from the group consisting of V and Ce, E is at least one element selected from the group consisting of Te, Sb and Se, and X is at least one element selected from the group consisting of Nb, Ta, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Sb, Bi, B, In, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Hf, Pb, P, Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu; and a = 1, b = 0.01 to 1.0, c = 0.01 to 1.0, d = 0.01 to 1.0, and e is dependent on the oxidation state of the other elements. The catalyst composition is treated to exhibit peaks at X-ray diffraction angles (29) of 22.1°, 27.1°, 28.2°, 36.2°, 45.2°, and 50.0°, with a relative increase in a diffraction peak at the diffraction angle (29) of 27.1 degrees when compared with an untreated catalyst of like empirical formula.

0036In this regard, in addition to the above noted peak at 27.1 degrees, the preferred mixed metal oxide exhibits the following five main diffraction peaks at specific diffraction angles (29) in the X-ray diffraction pattern of the treated mixed metal oxide (as measured using Cu-Kα radiation as the source): <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="3" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="52.50mm" /><colspec colnum="2" colname="col2" colwidth="52.50mm" /><colspec colnum="3" colname="col3" colwidth="52.50mm" /><thead valign="top"><row rowsep="1"><entry namest="col1" nameend="col3" align="center">X-ray lattice plane</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="center">Diffraction angle 2θ (±0.3°)</entry><entry namest="col2" nameend="col2" align="center">Spacing medium (Å)</entry><entry namest="col3" nameend="col3" align="center">Relative intensity</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="char" char=".">22.1°</entry><entry namest="col2" nameend="col2" align="char" char=".">4.02</entry><entry namest="col3" nameend="col3" align="center">100</entry></row><row><entry namest="col1" nameend="col1" align="char" char=".">28.2°</entry><entry namest="col2" nameend="col2" align="char" char=".">3.16</entry><entry namest="col3" nameend="col3" align="center">20~150</entry></row><row><entry namest="col1" nameend="col1" align="char" char=".">36.2°</entry><entry namest="col2" nameend="col2" align="char" char=".">2.48</entry><entry namest="col3" nameend="col3" align="center">5~60</entry></row><row><entry namest="col1" nameend="col1" align="char" char=".">45.2°</entry><entry namest="col2" nameend="col2" align="char" char=".">2.00</entry><entry namest="col3" nameend="col3" align="center">2~40</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="char" char=".">50.0°</entry><entry namest="col2" nameend="col2" align="char" char=".">1.82</entry><entry namest="col3" nameend="col3" align="center">2~40</entry></row></tbody></tgroup></table></tables>

0037The intensity of the X-ray diffraction peaks may vary upon the measuring of each crystal. However, the intensity, relative to the peak intensity at 22.1° being 100, is usually within the above ranges. Generally, the peak intensities at 2θ = 22.1° and 28.2° are distinctly observed. However, so long as the above five diffraction peaks are observable, the basic crystal structure is the same even if other peaks are observed in addition to the five diffraction peaks (e.g. at 27.1 degrees), and such a structure is useful for the present invention. Preparation of the mixed metal catalysts is described in U. S. Patent Application Publication No. 20020183547 and European Patent Publication No. EP 1 249 274.

0038Other suitable prepared catalysts modified using the invention include those described in U.S. Patent No. 5,380,933 discloses a method for producing an unsaturated carboxylic acid comprising subjecting an alkane to a vapor phase catalytic oxidation reaction in the presence of a catalyst containing a mixed metal oxide comprising, as essential components, Mo, V, Te, O and X, wherein X is at least one element selected from the group consisting of niobium, tantalum, tungsten, titanium, aluminum, zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, antimony, bismuth, boron, indium and cerium; and wherein the proportions of the respective essential components, based on the total amount of the essential components, exclusive of oxygen, satisfy the following relationships: 0.25 &lt; r(Mo) &lt; 0.98, 0.003 &lt; r(V) &lt; 0.5, 0.003 &lt; r(Te) &lt; 0.5 and 0.003 &lt; r(X) &lt; 0.5, wherein r(Mo), r(V), r(Te) and r(X) are the molar fractions of Mo, V, Te and X, respectively, based on the total amount of the essential components exclusive of oxygen.

0039Yet other suitable examples of prepared catalysts modified using the invention include those described in Published International Application No. WO 00/29106 discloses a catalyst for selective oxidation of propane to oxygenated products including acrylic acid, acrolein and acetic acid, said catalyst system containing a catalyst composition comprising         Mo_aV_bGa_cPd_dNb_eX_f wherein X is at least one element selected from La, Te, Ge, Zn, Si, In and W,    a is 1,    b is 0.01 to 0.9,    c is >0 to 0.2,    d is 0.0000001 to 0.2,    e is >0 to 0.2, and    f is .0 to 0.5; and wherein the numerical values of a, b, c, d, e and f represent the relative gram-atom ratios of the elements Mo, V, Ga, Pd, Nb and X, respectively, in the catalyst and the elements are present in combination with oxygen.

0040Yet other suitable examples of prepared catalysts modified using the invention include those described in Japanese Laid-Open Patent Application Publication No. 2000-037623 and European Published Patent Application No. 0 630 879 B1. Other suitable catalysts for a variety of vapor phase oxidation reactions are described fully in U.S. Patent Nos. 6,383,978, 6,403,525, 6,407,031, 6,407,280, 6,461,996, 6,472,552, 6,504,053, 6,589,907 and 6,624,111.

0041By way of an illustrative example, when a mixed metal oxide of the formula Mo_aV_bTe_cNb_dO_e (wherein the element A is Mo, the element D is V, the element E is Te and the element X is Nb) is to be prepared, an aqueous solution of niobium oxalate and a solution of aqueous nitric acid may be added to an aqueous solution or slurry of ammonium heptamolybdate, ammonium metavanadate and telluric acid, so that the atomic ratio of the respective metal elements would be in the prescribed proportions. In one specific illustration, it is further contemplated that a 5% aqueous nitric acid is mixed with niobium oxalate solution in a ratio of 1:10 to 1.25:1 parts by volume acid solution to oxalate solution, and more preferably 1:5 to 1:1 parts by volume acid solution to oxalate solution.

0042For example, when a promoted mixed metal oxide of the formula Mo_jV_mTe_nNb_yAu_zO_f wherein the element J is Mo, the element M is V, the element N is Te, the element Y is Nb, and the element Z is Au, is to be prepared, an aqueous solution of niobium oxalate may be added to an aqueous solution or slurry of ammonium heptamolybdate, ammonium metavanadate, telluric acid and ammonium tetrachloroaurate, so that the atomic ratio of the respective metal elements would be in the prescribed proportions.

0043A unmodified mixed metal oxide (promoted or not), thus obtained, exhibits excellent catalytic activities by itself. However, the unmodified mixed metal oxide is converted to a catalyst having higher activities by one or more chemical, physical and combinations of chemical and physical treatments.

0044Modified metal oxide catalysts are obtained by treating chemical, physical and combinations of chemical and physical treatments of suitable prepared metal oxide catalyst. Optionally, the modified catalysts are further modified by conventional processing techniques well known to persons having skill in this art.

0045Chemical treatments, resulting in treated/modified catalysts, include one or more chemical modifying agents. Suitable chemical modifying agents include, but are not limited to for example, oxidizing agents selected from hydrogen peroxide, nitrogen, nitric acid, nitric oxide, nitrogen dioxide, nitrogen trioxide, persulfate; reducing agents selected from amines, pyridine, hydrazine, quinoline, metal hydrides, sodium borohydride, C1-C4 alcohols, methanol, ethanol, sulfites, thiosulfites, aminothiols; combinations of oxidizing agents and reducing agents; acids selected from HCl, HNO3, H2SO4; organic acids, organic diacids, acetic acid, oxalic acid, combinations of C1-C4 alcohols and C1-C4 organic acids, oxalic acid and methanol; inorganic bases selected from NH3, NH4OH, H2NNH2, HONH2, NaOH, Ca(OH)2, CaO, Na2CO3, NaHCO3, organic bases selected from ethanol amine, diethanolamine, triethanolamine; pH adjustments; peroxides selected from inorganic peroxides, H2O2, organic peroxides, tBu2O2; chelating agents, ethylenediamine, ethylenediaminetetraacetic acid (EDTA); electrolysis including electrolytic reduction; treatment with high energy radiation including ultraviolet and X-ray radiation; and combinations thereof.

0046Physical treatments, resulting in treated/modified catalysts, include one or more physical processes. Suitable physical processes include, but are not limited to for example, cooling, cryogenic cooling, pressure cooling, compacting under pressure, high pressure die pressing, thermolyzing (also referred to as polymer burn off), mechanical grinding at cryogenic temperatures, high shear grinding at cryogenic temperatures, cryo-milling, cryo-densifying, cryo-stressing, cryo-fracturing, cryo-pelletizing, deforming, wash coating, molding, forming, shaping, casting, machining, laminating, drawing, extruding, lobalizing, impregnating, forming spheres (spherolizing or jetting), slurrying, cryo-slurrying, preparing shelled catalysts (shelling), multi-coating, electrolyzing, electrodepositing, compositing, foaming, cryo-fluidizing, cryo-spraying, thermal spraying, plasma spraying, vapor depositing, adsorbing, ablating, vitrifying, sintering, cryo-sintering, fusing, fuming, crystallizing, any altering of catalyst crystal structure, polycrystallizing, recrystallizing, any surface treating of the catalyst, any altering of catalyst surface structure, any altering of catalysts porosity, any altering of catalyst surface area, any altering of catalyst density, any altering of bulk catalysts structure, reducing the particle size of the primary catalyst particles in combination with cooling or thermolyzing the catalyst, and any combinations of chemical and physical treatments, including but not limited to solvent extraction, Soxhlet extraction, batch solvent extraction, continuous flow solvent extraction, extraction in supercritical solvents, contacting the catalyst with one or more leaching agents including solvents, altering catalyst pH, any chemical treatments used in modifying catalyst surface structure, mechanical grinding in supercritical solvents, chemisorbing one or more chemical agents, ultrasonification using one or more solvents selected from organic solvents such as alcohols and amines ultrasonification, and any physical treatments employing solvents under supercritical conditions. According to a separate embodiment, modified catalysts include one or more further chemical and/or physical treatments of already modified catalysts.

0047According to one embodiment, modified catalysts are further modified by one or more physical treatments including, but not limited to for example, heating, drying, cooling, freeze, pressure cooling, thermal die pressing, high pressure die pressing, thermal and high pressure die pressing, thermal high shear milling and grinding, thermal depolymerizing, thermolyzing (also referred to as polymer burn off), mechanical grinding at cryogenic temperatures, mechanical grinding at elevated temperatures, thermal milling, cryo-milling, thermal shearing, cryo-shearing, cryo-densifying, densification, coagulation, flocculation, sedimenting, lyophilizing, agglomerating, reducing particle size of primary particles, increasing surface area of primary particles, thermal and cryo-compacting, thermal and cryo-compressing, thermal and cryo-stressing, cryo-fracturing, shear loading, thermal and cryo-shear loading, drawing, thermal and cryo-drawing, thermal and cryo-centrifuging, thermal and cryo-granulating, thermal and cryo-spray drying, atomizing, thermal and cryo-dry pressing, cryo-pressing, heat pressing, dry compacting, cryo-compacting, heat compacting, isocompacting, thermal and cryo-isocompacting, thermal and cryo-peretizing, thermal and cryo-roll pressing, thermal and cryo-deforming, jiggering, thermal and cryo-molding, thermal and cryo-forming, thermal and cryo-shaping, thermal and cryo-casting, thermal and cryo-machining, thermal and cryo-laminating, thermal and cryo-tape casting, fiber drawing, thermal and cryo-fiber drawing, thermal and cryo-fiber extruding, thermal and cryo-extruding, thermal and cryo-lobalizing, thermal and cryo-impregnating, forming sphere forming (spherolizing or jetting), slurrying, cryo-slurrying, preparing shelled catalysts (shelling), multi-coating, electrolyzing, electrodepositing, compositing, rolling, roll forming, foaming, cementing, fluidizing, cryo-spraying, thermal spraying, plasma spraying, vapor depositing, adsorbing, ablating, firing, vitrifying, sintering, cryo-sintering, pre-shaping before extruding, thermal and cryo-pre-shaping before extruding, lobalizing, fusing, thermal fusing, fuming, coking, colloidalizing, crystallizing, thermal and cryo-crystallizing, any altering of crystal structure, polycrystallizing, recrystallizing, any surface treating, any altering of surface structure, any altering of porosity, any altering of density, any altering of bulk structure, altering catalyst pH, any chemical treatments used in modifying catalyst surface structure, mechanical grinding in supercritical solvents, chemisorbing one or more chemical agents, ultrasonification using one or more solvents selected from organic solvents, aqueous solvents and combinations of organic and aqueous solvents including, but limited to for example, acids, alcohols, chelating agents and amines ultrasonification, and any physical treatments employing solvents under supercritical conditions and any combinations thereof.

0048Other suitable treatments involve combinations of one or more chemical modifying agents and one or more physical processes, resulting in treated/modified catalysts. Suitable examples include, but are not limited to for example, solvent extraction using a Soxhlet extractor, extraction using a Parr bomb, solvent extraction using microwave radiation, batch solvent extraction, continuous flow solvent extraction, leaching, altering pH, any surface treatments, grinding in supercritical solvents, extraction in supercritical solvents, chemisorption, ultrasonification using one or more solvents selected from organic solvents such as alcohols and amines; and combinations thereof.

0049According to one embodiment of the invention, a structured mixed metal oxide catalyst is prepared by employment of a deposition technique selected from chemical vapor deposition (CVD) or physical vapor deposition (PVD). In other words, the layers of catalyst precursor materials are applied to the support by PVD or CVD, in each case under reduced pressure, i.e. at less than 10 mbar, preferably less than 1 mbar. Possible PVD methods are vapor deposition, sputtering and anodic or cathodic arc coating. Possible CVD methods are thermal- or plasma-supported gas-phase deposition. Plasma-supported methods, such as sputtering or arc coating are preferred, sputtering being particularly preferred.

0050In arc coating, the coating material is removed by means of an electric arc, which leads to a high degree of ionization of the coating material in the process gas atmosphere. The support to be coated can be provided with a bias voltage, which is generally negative and leads to intensive ion bombardment during coating.

0051In sputtering, the materials to be coated are applied in solid form as a target to the cathode of the plasma system, sputtered under reduced pressure (preferably from 5 x 10^-4 to 1 x 10^-1 mbar) in a process gas atmosphere and deposited on the support. The process gas usually comprises a noble gas, such as argon.

0052Various versions of the sputtering method, such as magnetron sputtering, DC or RF sputtering, bias sputtering or reactive sputtering and combinations thereof are suitable for the production of the presently contemplated layers. In magnetron sputtering, the target to be puttered is present in an external magnetic field which concentrates the plasma in the region of the target and hence increases the sputtering rate. In DC or RF sputtering, the sputtering plasma is excited in a conventional manner by DC or RF generators. In bias sputtering, a generally negative bias voltage which leads to intensive bombardment of the support with ions during coating is applied to the support to be coated.

0053In reactive sputtering, reactive gases, such as hydrogen, hydrocarbons, oxygen or nitrogen are mixed in the desired amount with the process gas at a suitable time. As a result, the relevant metal oxide, nitride, carbide, carbide oxide, carbide nitride, oxide nitride or carbide oxide nitride layers can be deposited directly by sputtering a metal, for example, in the presence of hydrocarbons, oxygen and/or nitrogen, in the process gas.

0054The desired layer thickness, chemical composition and microstructure may be obtained, as described below, by way of controlling the deposition parameters such as process gas pressure, process gas composition, sputtering power, sputtering mode, substrate temperature and deposition time.

0055PVD/CVD methods allow the layer thickness to be changed in a manner which is very reproducible and, as a result of the deposition parameters (e.g., deposition rate, deposition time), simple. The layer thickness can be readily chosen from a few atomic layers to about 100 mµ. For supported catalysts, catalyst layer thicknesses are preferably from 5 nm to 50 mµ, in particular from 10 nm to 20 mµ, very particularly from 10 nm to 10 mµ, and most particularly from 10 nm to 100 nm.

0056PVD/CVD technologies, in particular sputtering technology, offer very considerable freedom with regard to the chemical composition of the deposited catalyst precursor layers. The spectrum of layers which can be produced ranges from two- or three- to multi-component materials. Multi-component materials are usually prepared by introducing a suitable target into the coating unit and by subsequently sputtering the target in a noble gas plasma, preferably argon. Suitable targets are homogeneous metal targets or homogeneous alloy targets, which are prepared in a known manner by melting processes or by powder metallurgy methods, or inhomogeneous mosaic targets, which are prepared by joining together smaller pieces having different chemical compositions or by placing or sticking small, disk-like material pieces on homogeneous targets. Alternatively, metallic alloys can be prepared by simultaneously sputtering two or more targets of different compositions. The supports to be coated are arranged so that they are exposed in an advantageous manner to the flow of material produced by the sputtering of the various targets. In an advantageous arrangement, the supports to be coated are passed periodically through the simultaneously burning sputtering plasmas, a layer whose composition is periodically modulated through the layer depth being applied to the supports. The modulation period may be adjusted within wide limits by the sputtering power of the individual targets and by the speed of the periodic movement of the supports. In particular, by setting a very small modulation period, it is also possible to achieve a very thorough mixing of the individual layers and hence deposition of a homogeneous alloy.

0057The preparation of mixed oxide, nitride or carbide systems can be carried out either by sputtering of corresponding oxide, nitride or carbide targets, or by the reactive sputtering of metal targets in corresponding reactive gas plasmas. By appropriately controlling the reactive gas flow during the reactive sputtering, it is also possible to achieve partial oxidation, nitride formation or carbide formation in the alloy layer. For example, in alloys of noble and non-noble metals, selective oxidation of the non-noble metal component can be achieved by skillful adjustment of the oxygen gas flow.

0058Another commonly used PVD method is the sequential PVD deposition of different metals, such as Te, Nb, V; Mo, etc. in vacuum conditions (base pressure &lt; 1 x 10^-6 Torr). The metal sources are made by melting individual metal powders into different crucibles. The PVD system is typically equipped with multiple pockets that house multiple crucibles containing different metals. During PVD, an individual metal source is heated by electron beam, and the deposition rate is typically monitored using a quartz crystal balance that is located near the substrate.

0059With the stated deposition methods, it is also possible to produce thin gradient layers whose composition is varied in a defined manner with increasing layer depth. The variation of the composition can be controlled in a simple manner by the corresponding deposition parameters (for example, sputtering power, in the case of simultaneous sputtering, reactive gas flow, etc.). Moreover, non-periodic layer systems, e.g., layer systems comprising different metallic alloys or composite layers consisting of metallic and oxide layers, are also possible.

0060The microstructure (e.g., phase distribution, crystallite shape and size, crystallographic orientation) and the porosity of the layers can be controlled within wide limits by the choice of suitable deposition parameters. For example, DC magnetron sputtering of a metallic target at a pressure of from 4 x 10^-3 to 8 x 10^-3 mbar leads to very dense and hence pore-free layers, whereas a column-like morphology with increasing porosity is observed at a sputtering pressure above 1 x 10^-2 mbar. In addition to the sputtering pressure, the substrate temperature and any applied bias voltage have a considerable effect on the microstructure.

0061Examples of suitable supports are moldings of glass, quartz glass, ceramic, titanium dioxide, zirconium dioxide, alumina, aluminosilicates, borates, steatite, magnesium silicate, silica, silicates, metal, carbon (e.g., graphite), or mixtures thereof. The support may be porous or non-porous. Suitable moldings include, for example, strans, pellets, wagon wheels, stars, monolith, spheres, chips, rings or extrudates. Spheres, pellets and strands are particularly preferred.

0062In order to achieve uniform coating of the supports, it is advantageous to keep the supports in random motion during deposition or by the use of suitable mechanical apparatus having good flow mechanical properties. Suitable mechanical apparatus includes, e.g., periodically moved cages, drums, shells or channel in which the supports are caused to make random movements. The mechanical apparatus must, of course, have suitable openings to permit the passage of the deposition material or access by any plasma required.

0063In one particularly preferred aspect of the present invention, the ceramic support structure is an open or closed cell ceramic foam or monolith. More preferably, the ceramic is made from a material selected from the group consisting of cordierite, alumina, zirconia, partially stabilized zirconia (PSZ), niobium, and mixtures thereof. Of course, other like materials may also be employed. The foam structure preferably has 30 to 150 pores per inch. The monoliths may have 200 to 800 cells per inch.

0064These forms for the support permit high space velocities with a relatively minimal pressure drop. The skilled artisan will be familiar with such configurations and the manner of making the same, in view of teachings such as "Structured Catalysts and Reactors, A. Cybulski and J. A. Moulijn (Eds.), Marcel Dekker, Inc., 1998, p. 599-615 (Ch. 21): X. Xu and J. A. Moulijn, "Transformation of a Structured Carrier into Structured Catalyst".

0065Structures including a fibrous or fabric support may also be employed. For instance, ceramic oxide fabric catalyst supports, fibrous ceramic composite catalysts, or a combination, provide other attractive supported structures, which are easily formed and are readily scaled to fit commercial reactors. These types of structures, which may or may not be self-supporting, preferably will resist thermal shock under the reaction conditions of interest and will generally avoid hot-spot induced circumstances, such as a meltdown. These structures may be formed into any of a variety of three-dimensional configurations, and may employ one or more different fiber diameters, may be woven, unwoven or a mixture thereof, or even braided or otherwise aggregated into a suitable configuration, mesh or otherwise.

0066It will be appreciated as to the support structures disclosed herein that plural layers may be employed, with each layer having the same or different structure, composition, orientation, or other characteristic relative to a previous layer. For instance, a catalyst bed may contain a stack or layers of fabric disks formed from ceramic oxide fabric supported catalysts or the fibrous ceramic composite catalysts. Individual layers may or may not be self-supporting. Preferably, however, the combination embodied in the overall structure is generally self-supporting. When employed herein, ceramic oxide fibers may be comprised of alumina, silica, boria, cordierite, magnesia, zirconia, or a combination of any of these oxides.

0067It will be appreciated that the supports of the present invention, though discussed above in the context of preferred groups of materials may be selected from any of a number of different materials, such as (without limitation) a ceramic selected from the group consisting of cordierite, alumina, zirconia, partially stabilized zirconia (PSZ), niobium, silica, boria, magnesia, titania and mixtures thereof. The groups discussed herein are thus not intended as limiting.

0068In another embodiment, multi-layer structures may include a stack of a plurality of perforated plates (e.g., thin, circular perforated metal disks), preferably joined together by a thermally conductive connection. The plates may be coated with an oxidation barrier, to thereby serve as thermal shock resistant catalyst supports for active catalyst materials. By way of illustration, recognizing that the teachings are applicable to other material systems or configurations, the catalyst preparation for this aspect includes fabricating a stack of thin, circular perforated metal disks and joining them together by a thermally conductive connection. The multi-disk structure is scaled at a high temperature for sufficient time to grow an alumina layer. The multi-layer structure is impregnated with the active catalyst precursor material, dried and calcined to the result in a monolith catalyst. In one example, the multi-layer structure is scaled, or pretreated, by heating in air or oxygen at 900° C to 1200° C, for a period of time ranging from about 10 - 100 hours, to form a thin, tightly adhering oxide surface layer which protects the underlying support alloy from further oxidation during high temperature use. The surface layer also preferably functions as a diffusion barrier to the supported metal catalyst, thus preventing alloying of the catalyst metal with the alloy of the catalyst support. For example, the protective surface layer may be composed predominantly of alpha-alumina, but also contain a small amount of yttrium oxide. After pretreatment, the multi-layer support structure is coated with a catalyst metal, or catalyst precursor material.

0069The supported catalysts as described herein may be further performance tuned as desired, and may be varied in their stacking, layering, or other integration characteristics in the reactor system in such a manner to improve reaction productivity. For example, in one aspect, it may be beneficial to initially provide an oxidative dehydrogenation active catalyst (supported as described herein or unsupported) upstream in the reactor system for the conversion of an alkane to alkylene (e.g., propane to propylene) in the cases of pure, mixed and/or recycle streams. These forms might then be followed by supported or unsupported selective oxidation catalysts towards acid production.

0070The present mixed metal oxide catalyst (or combination of catalyst and support) can be prepared in a suitable manner such as that illustrated in the following discussion. Turning now in more specific detail to the first aspect of the present invention, the mixed metal oxide is prepared by introducing a metal and/or series of metals into a catalyst precursor admixture, such as by deposition. As discussed herein, the step of deposition is accomplished by employment of a deposition technique selected from chemical vapor deposition or physical deposition.

0071Generally, the metal compounds contain elements Mo, V, Te and X, as previously defined.

0072Once obtained, the catalyst precursor may be calcined into its desired supported form or into another suitable form. The calcination may be conducted in an oxygen-containing atmosphere or in the substantial absence of oxygen, e.g., in an inert atmosphere or in vacuo. The inert atmosphere may be any material which is substantially inert, i.e., does not react or interact with, the catalyst precursor. Suitable examples include, without limitation, nitrogen, argon, xenon, helium or mixtures thereof. Preferably, the inert atmosphere is argon or nitrogen. The inert atmosphere may flow over the surface of the catalyst precursor or may not flow thereover (a static environment). When the inert atmosphere does flow over the surface of the catalyst precursor, the flow rate can vary over a wide range, e.g., at a space velocity of from 1 to 500 hr^-1.

0073The calcination is usually performed at a temperature of from 350°C to 850°C, preferably from 400°C to 700°C, more preferably from 500°C to 640°C. The calcination is performed for an amount of time suitable to form the aforementioned catalyst. Typically, the calcination is performed for from 0.5 to 30 hours, preferably from 1 to 25 hours, more preferably for from 1 to 15 hours, to obtain the desired mixed metal oxide.

0074In a preferred mode of operation, the catalyst precursor is calcined in two stages. In the first stage, the catalyst precursor is calcined in an oxidizing environment (e.g. air) at a temperature of from 275°C to 400°C, preferably from 275°C to 325°C for from 15 minutes to 8 hours, preferably for from 1 to 3 hours. In the second stage, the material from the first stage is calcined in a non-oxidizing environment (e.g., an inert atmosphere) at a temperature of from 500°C to 700°C, preferably for from 550°C to 650°C, for 15 minutes to 8 hours, preferably for from 1 to 3 hours. Optionally, a reducing gas, such as, for example, ammonia or hydrogen, may be added during the second stage calcination.

0075In a particularly preferred mode of operation, the catalyst precursor in the first stage is placed in the desired oxidizing atmosphere at room temperature and then raised to the first stage calcination temperature and held there for the desired first stage calcination time. The atmosphere is then replaced with the desired non-oxidizing atmosphere for the second stage calcination, the temperature is raised to the desired second stage calcination temperature and held there for the desired second stage calcination time.

0076Although any type of heating mechanism, e.g., a furnace, may be utilized during the calcination, it is preferred to conduct the calcination under a flow of the designated gaseous environment. Therefore, it is advantageous to conduct the calcination in a bed with continuous flow of the desired gas(es) through the bed of solid catalyst precursor particles.

0077With calcination, a catalyst is formed having the formula Mo_aV_bTe_cX_dO_e wherein Mo is molybdenum; V is vanadium; Te is tellurium; X is as previously defined; O is oxygen; a, b, c and d are as previously defined; and e is the relative atomic amount of oxygen present in the catalyst and is dependent on the oxidation state of the other elements

0078The oxide obtained by the above-mentioned method may be used as a final catalyst, but it may further be subjected to heat treatment usually at a temperature of from 200° to 700°C for from 0.1 to 10 hours.

0079The resulting modified mixed metal oxide (promoted or not) may be used by itself as a solid catalyst. The modified catalysts are also combined with one or more suitable carriers, such as, without limitation, silica, alumina, titania, aluminosilicate, diatomaceous earth or zirconia, according to art-disclosed techniques. Further, it may be processed to a suitable shape or particle size using art disclosed techniques, depending upon the scale or system of the reactor.

0080Alternatively, the metal components of the modified catalysts are supported on materials such as alumina, silica, silica-alumina, zirconia, titania, etc. by conventional incipient wetness techniques. In one typical method, solutions containing the metals are contacted with the dry support such that the support is wetted; then, the resultant wetted material is dried, for example, at a temperature from room temperature to 200°C followed by calcination as described above. In another method, metal solutions are contacted with the support, typically in volume ratios of greater than 3 : 1 (metal solution : support), and the solution agitated such that the metal ions are ion-exchanged onto the support. The metal-containing support is then dried and calcined as detailed above.

0081According to a separate embodiment, modified catalysts are also prepared using one or more promoters. The starting materials for the above promoted mixed metal oxide are not limited to those described above. A wide range of materials including, for example, oxides, nitrates, halides or oxyhalides, alkoxides, acetylacetonates, and organometallic compounds may be used. For example, ammonium heptamolybdate may be utilized for the source of molybdenum in the catalyst. However, compounds such as MoO₃, MoO₂, MoCl₅, MoOCl₄, Mo(OC₂H₅)₅, molybdenum acetylacetonate, phosphomolybdic acid and silicomolybdic acid may also be utilized instead of ammonium heptamolybdate. Similarly, ammonium metavanadate may be utilized for the source of vanadium in the catalyst. However, compounds such as V₂O₅, V₂O₃, VOCl₃, VCl₄, VO(OC₂H₅)₃, vanadium acetylacetonate and vanadyl acetylacetonate may also be utilized instead of ammonium metavanadate. The tellurium source may include telluric acid, TeCl₄, Te(OC₂H₅)₅, Te(OCH(CH₃)₂)₄ and TeO₂. The niobium source may include ammonium niobium oxalate, Nb₂O₅, NbCl₅, niobic acid or Nb(OC₂H₅)₅ as well as the more conventional niobium oxalate.

0082In addition, with reference to the promoter elements for the promoted catalyst, the nickel source may include nickel(II) acetate tetrahydrate, Ni(NO₃)₂, nickel(II) oxalate, NiO, Ni(OH)₂, NiCl₂, NiBr₂, nickel(II) acetylacetonate, nickel(II) sulfate, NiS or nickel metal. The palladium source may include Pd(NO₃)₂, palladium(II) acetate, palladium oxalate, PdO, Pd(OH)₂, PdCl₂, palladium acetylacetonate or palladium metal. The copper source may be copper acetate, copper acetate monohydrate, copper acetate hydrate, copper acetylacetonate, copper bromide, copper carbonate, copper chloride, copper chloride dihydrate, copper fluoride, copper formate hydrate, copper gluconate, copper hydroxide, copper iodide, copper methoxide, copper nitrate, copper nitrate hydrate, copper oxide, copper tartrate hydrate or a solution of copper in an aqueous inorganic acid, e.g., nitric acid. The silver source may be silver acetate, silver acetylacetonate, silver benzoate, silver bromide, silver carbonate, silver chloride, silver citrate hydrate, silver fluoride, silver iodide, silver lactate, silver nitrate, silver nitrite, silver oxide, silver phosphate or a solution of silver in an aqueous inorganic acid, e.g., nitric acid. The gold source may be ammonium tetrachloroaurate, gold bromide, gold chloride, gold cyanide, gold hydroxide, gold iodide, gold oxide, gold trichloride acid and gold sulfide.

0083Modified catalysts of the invention have different chemical, physical and performance characteristics in catalytic reactions of carbon based molecules as compared to unmodified catalysts. According to one embodiment, the treated catalyst exhibits changes in X-ray lines, peak positions and intensity of such lines and peaks as compared with corresponding X-ray diffraction data for corresponding unmodified catalysts. Such difference indicate structural differences between the modified and unmodified catalysts and are born out in the catalytic activity and selectivity. For example, compared with an untreated catalyst composition, a treated catalyst composition of the present invention exhibits an X-ray diffraction pattern having a relative increase in a diffraction peak at a diffraction angle (2θ) of 27.1 degrees when compared with an untreated catalyst, which may exhibit no peak at all at 27.1 degrees.

0084The relative difference between peak intensities of treated versus untreated compositions may be greater than 5%, more preferably greater than 10%, and still more preferably greater than 20% of the intensity of the untreated catalyst composition at the diffraction angle (2θ) of 27.1 degrees. Without intending to be bound by theory, it is believed that at least two phases (A and B) are present in the resulting mixed metal oxide catalyst and the treatment of the catalyst precursor with a source of NO_x results in an increase in phase B relative to phase A in the resulting catalyst. The increase in phase B is believed to contribute to improved performance of the catalyst in terms of selectivity, reactivity and yield.

0085Modified catalysts of the invention have improved performance characteristics as compared to unmodified catalysts in catalytic processes comprising any carbon containing molecule. According to one embodiment of the invention, the modified catalysts have improved performance characteristics as compared to unmodified catalysts in processes for preparing dehydrogenated products and oxygenated products from alkanes and oxygen, alkenes and oxygen and combination of alkanes, alkenes and oxygen. The reactions are typically carried out in conventional reactors with the alkanes catalytically converted at conventional residence times (> 100 milliseconds) in conventional reactors. According to a separate embodiment the reactions are carried out at short contact times (≤ 100 milliseconds) in a short contact time reactor. Suitable alkanes include alkanes having straight or branched chains. Examples of suitable alkanes are C₂-C₂₅ alkanes, including C₂-C₈ alkanes such as propane, butane, isobutane, pentane, isopentane, hexane and heptane. Particularly preferred alkanes are propane and isobutane.

0086Modified catalysts of the invention convert alkanes, alkenes or alkanes and alkenes to their corresponding alkenes and oxygenates including saturated carboxylic acids, unsaturated carboxylic acids, esters thereof, and higher analogue unsaturated carboxylic acids and esters thereof. The modified catalyst and catalytic systems are designed to provide specific alkenes, oxygenates and combinations thereof. Alkanes are catalytically converted to one or more products in a single pass, including corresponding alkenes. Any unreacted alkane, alkene or intermediate is recycled to catalytically convert it to its corresponding oxygenate. According to one embodiment, alkenes produced from dehydrogenation of corresponding alkanes using catalyst systems of the invention are deliberately produced as in-process chemical intermediates and not isolated before selective partial oxidation to oxygenated products. For example, when catalytically converting an alkane to its corresponding ethylenically unsaturated carboxylic acid, any unreacted alkene produced is recovered or recycled to catalytically convert it to its corresponding ethylenically unsaturated carboxylic acid product stream.

0087According to a separate embodiment, alkanes, alkenes or alkanes and alkenes are also catalytically converted to its corresponding oxygenates through two or more catalytic zones. For example, an alkane is catalytically converted to its corresponding saturated carboxylic acid in a first catalytic zone or layer of a mixed catalyst bed. The saturated carboxylic acid, in the presence of an additional formaldehyde stream, to its corresponding higher analogue ethylenically unsaturated carboxylic acid in a second catalytic zone or layer of a mixed bed catalyst. In a specific example, propane is catalytically converted to propionic acid and the propionic acid in the presence of formaldehyde is catalytically converted to methacrylic acid.

0088As used herein, the term "higher analogue unsaturated carboxylic acid" and "ester of a higher analogue unsaturated carboxylic acid" refer to products having at least one additional carbon atom in the final product as compared to the alkane or alkene reactants. For example given above, propane (C₃ alkane) is converted to propionic acid (C₃ saturated carboxylic acid), which in the presence of formaldehyde is converted to its corresponding higher analogue (C₄) carboxylic acid, methacrylic acid using catalysts of the invention.

0089Suitable alkenes used in the invention include alkenes having straight or branched chains. Examples of suitable alkenes include C₂-C₂₅ alkenes, preferably C₂-C₈ alkenes such as propene (propylene), 1-butene (butylene), 2-methylpropene (isobutylene), 1-pentene and 1-hexene. Particularly preferred alkenes are propylene and isobutylene.

0090Suitable aldehydes used in the invention include for example formaldehyde, ethanal, propanal and butanal.

0091Modified catalysts and catalyst systems of the invention convert alkanes, alkenes or alkanes and alkenes to their corresponding oxygenates including saturated carboxylic acids having straight or branched chains. Examples include C₂-C₈ saturated carboxylic acids such as propionic acid, butanoic acid, isobutyric acid, pentanoic acid and hexanoic acid. According to one embodiment, saturated carboxylic acids produced from corresponding alkanes using catalyst systems of the invention are deliberately produced as in-process chemical intermediates and not isolated before selective partial oxidation to oxygenated products including unsaturated carboxylic acids, esters of unsaturated carboxylic acids, and higher esters of unsaturated carboxylic acids. According to a separate embodiment, any saturated carboxylic acid produced is converted using catalysts of the invention to its corresponding product stream including an ethylenically unsaturated carboxylic acid, esters thereof, a higher analogue unsaturated carboxylic acid or esters thereof.

0092Modified catalysts and catalyst systems of the invention also convert alkanes to their corresponding ethylenically unsaturated carboxylic acids and higher analogues having straight or branched chains. Examples include C₂-C₈ ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, butenoic acid, pentenoic acid, hexenoic acid, maleic acid, and crotonic acid. Higher analogue ethylenically unsaturated carboxylic acids are prepared from corresponding alkanes and aldehydes. For example, methacrylic acid is prepared from propane and formaldehyde. According to a separate embodiment, the corresponding acid anhydrides are also produced when preparing ethylenically unsaturated carboxylic acids from their respective alkanes. The modified catalysts of the invention are usefully employed to convert propane to arcylic acid and its higher unsaturated carboxylic acid methacrylic acid and to convert isobutane to methacrylic acid.

0093The modified catalysts and catalyst systems of the invention are also advantageously utilized converting alkanes to their corresponding esters of unsaturated carboxylic acids and higher analogues. Specifically, these esters include, but are not limited to, butyl acrylate from butyl alcohol and propane, β-hydroxyethyl acrylate from ethylene glycol and propane, methyl methacrylate from methanol and isobutane, butyl methacrylate from butyl alcohol and isobutane, β-hydroxyethyl methacrylate from ethylene glycol and isobutane, and methyl methacrylate from propane, formaldehyde and methanol.

0094In addition to these esters, other esters are formed through this invention by varying the type of alcohol introduced into the reactor and/or the alkane, alkene or alkane and alkene introduced into the reactor.

0095Suitable alcohols include monohydric alcohols, dihydric alcohols and polyhydric alcohols. Of the monohydric alcohols reference may be made, without limitation, to C₁-C₂₀ alcohols, preferably C₁-C₆ alcohols, most preferably C₁-C₄ alcohols. The monohydric alcohols may be aromatic, aliphatic or alicyclic; straight or branched chain; saturated or unsaturated; and primary, secondary or tertiary. Particularly preferred monohydric alcohols include methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, isobutyl alcohol and tertiary butyl alcohol. Of the dihydric alcohols reference may be made, without limitation, to C₂-C₆ diols, preferably C₂-C₄ diols. The dihydric alcohols may be aliphatic or alicyclic; straight or branched chain; and primary, secondary or tertiary. Particularly preferred dihydric alcohols include ethylene glycol (1,2-ethanediol), propylene glycol (1,2-propanediol), trimethylene glycol (1,3-propanediol), 1,2-butanediol and 2,3-butanediol. Of the polyhydric alcohols reference will only be made to glycerol (1,2,3-propanetriol).

0096The unsaturated carboxylic acid corresponding to the added alkane is the α,β-unsaturated carboxylic acid having the same number of carbon atoms as the starting alkane and the same carbon chain structure as the starting alkane, e.g., acrylic acid is the unsaturated carboxylic acid corresponding to propane and methacrylic acid is the unsaturated carboxylic acid corresponding to isobutane.

0097Similarly, the unsaturated carboxylic acid corresponding to an alkene is the α,β-unsaturated carboxylic acid having the same number of carbon atoms as the alkene and the same carbon chain structure as the alkene, e.g., acrylic acid is the unsaturated carboxylic acid corresponding to propene and methacrylic acid is the unsaturated carboxylic acid corresponding to isobutene.

0098Likewise, the unsaturated carboxylic acid corresponding to an unsaturated aldehyde is the α,β-unsaturated carboxylic acid having the same number of carbon atoms as the unsaturated aldehyde and the same carbon chain structure as the unsaturated aldehyde, e.g., acrylic acid is the unsaturated carboxylic acid corresponding to acrolein and methacrylic acid is the unsaturated carboxylic acid corresponding to methacrolein.

0099The alkene corresponding to the added alkane is the alkene having the same number of carbon atoms as the starting alkane and the same carbon chain structure as the starting alkane, e.g., propene is the alkene corresponding to propane and isobutene is the alkene corresponding to isobutane. (For alkenes having four or more carbon atoms, the double bond is in the 2-position of the carbon-carbon chain of the alkene.)

0100The unsaturated aldehyde corresponding to the added alkane is the α,β-unsaturated aldehyde having the same number of carbon atoms as the starting alkane and the same carbon chain structure as the starting alkane, e.g., acrolein is the unsaturated aldehyde corresponding to propane and methacrolein is the unsaturated carboxylic acid corresponding to isobutane.

0101Similarly, the unsaturated aldehyde corresponding to an alkene is the α,β-unsaturated carboxylic acid having the same number of carbon atoms as the alkene and the same carbon chain structure as the alkene, e.g., acrolein is the unsaturated aldehyde corresponding to propene and methacrolein is the unsaturated aldehyde corresponding to isobutene.

0102The modified catalysts are processed in to three-dimensional forms or are supported on three-dimensional support structures.

0103The support structure is three-dimensional, i.e. the support has dimensions along an x, y and z orthogonal axes of a Cartesian coordinate system, and affords a relatively high surface area per unit volume. Though lower and higher amounts are possible, in one embodiment, the support structure exhibits a surface area of 0.01 to 50 m²/g, preferably 0.1 to 10 m²/g.

0104Preferably, the support structure will have a porous structure and exhibit a pore volume percent ranging from 1 to 95%, more preferably 5 to 80%, and still more preferably 10 to 50%. Thus, the support structure permits relatively high feed velocities with insubstantial pressure drop.

0105Further, the support structure is sufficiently strong so that it does not fracture under the weight of the catalyst, which can range up to almost 100% of the weight of the combination of the catalyst and the support structure. More preferably, however, the support structure is at least 60% of the weight of the combination. Still more preferably, it is 70 to 99.99% of the weight of the combination. Even still more preferably, the support structure is 90 to 99.9% of the weight of the combination.

0106The exact physical form of the support structure is not particularly important so long as it meets the above noted general criteria. Examples of suitable physical forms of modified catalysts and supported modified catalysts include foam, honeycomb, lattice, mesh, monolith, woven fiber, non-woven fiber, gauze, perforated substrates (e.g., foil), particle compacts, fibrous mat and mixtures thereof. For these supports it will be appreciated that typically one or more open cells will be included in the structure. The cell size may vary as desired, as may the cell density, cell surface area, open frontal area and other corresponding dimensions. By way of example, one such structure has an open frontal area of at least 75%. The cell shape may also vary and may include polygonal shapes, circles, ellipses, as well as others.

0107The support structure may be fabricated from a material that is inert to the reaction environment of the catalytic reaction. Suitable materials include ceramics and their isomorphs such as silica, alumina (including α-, β- and γ-isomorphs), silica-alumina, aluminosilicate, zirconia, titania, boria, mullite, lithium aluminum silicate, oxide-bonded silicon carbide, metal alloy monoliths, Fricker type metal alloys, FeCrAl alloys and mixtures thereof. (Alternatively, the catalyst may be prepared so as to define the support structure itself, e.g., by "green" compacting or another suitable technique.)

0108The modified catalysts may be applied to the support structure using any suitable art-disclosed technique. For instance, the catalyst may be vapor deposited (e.g., by sputtering, plasma deposition or some other form of vapor deposition). The catalyst may be impregnated or coated thereon (e.g., by wash coating a support with a solution, slurry, suspension or dispersion of catalyst). The support may be coated with a catalyst powder (i.e. powder coating). (Alternatively, where the support structure is the catalyst itself, a "green" body of catalyst may be compacted to yield the desired structure.)

0109Modified catalysts of the invention include promoters, modifiers and oxidants. Promoters are usefully employed to oxidatively dehydrogenate alkanes to their corresponding alkenes. According to one embodiment the modified catalysts also incorporate finely dispersed metal particles including alloys (microns to nanometers) having high surface area. Alternatively, the modified catalyst is in the form of a fine gauze, including nanometer sized wires. The catalyst is impregnated on the support using techniques selected from metal sputtering, chemical vapor deposition, chemical and/or electrochemical reduction of the metal oxide.

0110Modifiers are usefully employed to partially oxidize alkanes to their corresponding saturated carboxylic acids and unsaturated carboxylic acids. Typical modifiers are metal oxide and MMO catalysts in the form of binary, ternary, quaternary or higher order mixed metal oxides. The modifier may preferably be present in an amount of from 0.0001 to 10 wt % of the catalyst composition (promoter plus reducible metal oxide), more preferably from 0.001 to 5 wt% of the catalyst composition, and still more preferably from 0.01 to 2 wt% of the catalyst composition.

0111Oxidants are usefully employed to partially oxidize alkanes, alkenes and alkanes and alkenes to their corresponding alkenes, saturated carboxylic acids and unsaturated carboxylic acids. Typically they are also metal oxide catalysts and MMO catalysts in the form of binary, ternary, quaternary or higher order mixed metal oxides. The promoter is typically present in an amount of from 0.0001 to 10 wt % of the catalyst composition (promoter plus reducible metal oxide), more preferably from 0.001 to 5 wt% of the catalyst composition, and still more preferably from 0.01 to 2 wt% of the catalyst composition. The modified catalyst is present alone or deposited, including impregnated, on the support in the form of finely dispersed metal oxide particles (microns to nanometers) having high surface area. The catalytic system component comprises metal oxides and metal oxides used in combination with promoters in contact with a metal oxide supported.

0112The unmodified catalysts are prepared in steps. In a first step, a slurry or solution may be formed by admixing metal compounds, preferably at least one of which contains oxygen, and at least one solvent in appropriate amounts to form the slurry or solution. Preferably, a solution is formed at this stage of the catalyst preparation. Generally, the metal compounds contain the elements required for the particular catalyst, as previously defined.

0113Suitable solvents include water, alcohols including , but not limited to, methanol, ethanol, propanol, and diols, etc., as well as other polar solvents known in the art. Generally, water is preferred. The water is any water suitable for use in chemical syntheses including, without limitation, distilled water and de-ionized water. The amount of water present is preferably an amount sufficient to keep the elements substantially in solution long enough to avoid or minimize compositional and/or phase segregation during the preparation steps. Accordingly, the amount of water will vary according to the amounts and solubilities of the materials combined. However, as stated above, the amount of water is preferably sufficient to ensure an aqueous solution is formed at the time of mixing.

0114For example, when a mixed metal oxide of the formula Mo_aV_bTe_cNb_dO_e is to be prepared, an aqueous solution of telluric acid, an aqueous solution of niobium oxalate and a solution or slurry of ammonium paramolybdate may be sequentially added to an aqueous solution containing a predetermined amount of ammonium metavanadate, so that the atomic ratio of the respective metal elements would be in the prescribed proportions.

0115Once the aqueous slurry or solution (preferably a solution) is formed, the water is removed by any suitable method, known in the art, to form a catalyst precursor. Such methods include, without limitation, vacuum drying, freeze drying, spray drying, rotary evaporation and air drying. Vacuum drying is generally performed at pressures ranging from 10mmHg to 500mmHg. Freeze drying typically entails freezing the slurry or solution, using , for instance, liquid nitrogen, and drying the frozen slurry or solution under vacuum. Spray drying is generally performed under an inert atmosphere such as nitrogen or argon, with an inlet temperature ranging from 125°C to 200°C and an outlet temperature ranging from 75°C to 150°C. Rotary evaporation is generally performed at a bath temperature of from 25°C to 90°C and at a pressure of from 10mmHg to 760mmHg, preferably at a bath temperature of from 40° to 90°C and at a pressure of from 10mmHg to 350mmHg, more preferably at a bath temperature of from 40°C to 60°C and at a pressure of from 10mmHg to 40mmHg. Air drying may be effected at temperatures ranging from 25°C to 90°C. Rotary evaporation or air drying are generally employed.

0116Once obtained, the catalyst precursor is calcined. The calcination is usually conducted in an oxidizing atmosphere, but it is also possible to conduct the calcination in a non-oxidizing atmosphere, e.g., in an inert atmosphere or in vacuo. The inert atmosphere may be any material which is substantially inert, i.e., does not react or interact with, the catalyst precursor. Suitable examples include, without limitation, nitrogen, argon, xenon, helium or mixtures thereof. Preferably, the inert atmosphere is argon or nitrogen. The inert atmosphere may flow over the surface of the catalyst precursor or may not flow thereover (a static environment). When the inert atmosphere does flow over the surface of the catalyst precursor, the flow rate can vary over a wide range, e.g., at a space velocity of from 1 to 500 hr^-1.

0117The calcination is usually performed at a temperature of from 350°C to 1000°C, including from 400°C to 900°C, and including from 500°C to 800°C. The calcination is performed for an amount of time suitable to form the aforementioned catalyst. Typically, the calcination is performed for from 0.5 to 30 hours, preferably from 1 to 25 hours, more preferably for from 1 to 15 hours, to obtain the desired mixed metal oxide.

0118In one mode of operation, the catalyst precursor is calcined in two stages. In the first stage, the catalyst precursor is calcined in an oxidizing atmosphere (e.g., air) at a temperature of from 200°C to 400°C, including from 275°C to 325°C for from 15 minutes to 8 hours, including from 1 to 3 hours. In the second stage, the material from the first stage is calcined in a non-oxidizing environment (e.g., an inert atmosphere) at a temperature of from 500°C to 900°C, including from 550°C to 800°C, for from 15 minutes to 8 hours, including from 1 to 3 hours.

0119Optionally, a reducing gas, such as, for example, ammonia or hydrogen, is added during the second stage calcination.

0120In a separate mode of operation, the catalyst precursor in the first stage is placed in the desired oxidizing atmosphere at room temperature and then raised to the first stage calcination temperature and held there for the desired first stage calcination time. The atmosphere is then replaced with the desired non-oxidizing atmosphere for the second stage calcination, the temperature is raised to the desired second stage calcination temperature and held there for the desired second stage calcination time.

0121Although any type of heating mechanism, e.g., a furnace, may be utilized during the calcination, it is preferred to conduct the calcination under a flow of the designated gaseous environment. Therefore, it is advantageous to conduct the calcination in a bed with continuous flow of the desired gas(es) through the bed of solid catalyst precursor particles.

0122With calcination, a mixed metal oxide catalyst is formed having a stoichiometric or non-stoichiometric amounts of the respective elements.

0123The invention provides also process for preparing modified mixed metal oxide catalysts that convert alkanes to their corresponding alkenes and oxygenates comprising the steps of: <ul id="ul0005" list-style="none" compact="compact"><li>mixing salts of metals selected from the group consisting of Mo, Te, V, Ta and Nb at temperatures above the melting point of the highest melting salt to form a miscible molten salt; and</li><li>calcining the mixture of salts in the presence of oxygen to provide a mixed metal oxide catalyst, optionally using a metal halide salt or a metal oxyhalide salt as solvent.</li></ul>

0124The starting materials for the above mixed metal oxide are not limited to those described above. A wide range of materials including, for example, oxides, nitrates, halides or oxyhalides, alkoxides, acetylacetonates and organometallic compounds may be used. For example, ammonium heptamolybdate may be utilized for the source of molybdenum in the catalyst. However, compounds such as MoO₃, MoO₂, MoCl₅, MoOCl₄, Mo(OC₂H₅)₅, molybdenum acetylacetonate, phosphomolybdic acid and silicomolybdic acid may also be utilized instead of ammonium heptamolybdate. Similarly, ammonium metavanadate may be utilized for the source of vanadium in the catalyst. However, compounds such as V₂O₅, V₂O₃, VOCl₃, VCl₄, VO(OC₂H₅)₃, vanadium acetylacetonate and vanadyl acetylacetonate may also be utilized instead of ammonium metavanadate. The tellurium source may include telluric acid, TeCl₄, Te(OC₂H₅)₅, Te(OCH(CH₃)₂)₄ and TeO₂. The niobium source may include ammonium niobium oxalate, Nb₂O₅, NbCl₅, niobic acid or Nb(OC₂H₅)₅ as well as the more conventional niobium oxalate.

0125Use of low-melting salts opens up a new approach to preparing mixed metal oxide catalysts. The advantages over current aqueous suspension methods include higher incorporation of sparsely soluble metal salts, better control of metal ratios, and more homogeneous catalyst systems. One unique approach is to use low-melting halides of the desired MMO metals to prepare salt solutions. Variations of this approach are discussed below in more detail.

0126Halide salts of the desired metals are combined by mixing at temperatures above the melting point of the highest melting salt. The molten salts should be miscible with each other forming a stable, homogeneous solution of molten salt.

0127One advantage of the method is that it eliminates the solubility limits inherent in aqueous slurry systems. By using molten salts, we can incorporate much higher levels of such metals as niobium, vanadium, and palladium, the salts of which have relatively low solubilities in aqueous media. These salts are readily available, relatively inexpensive, and have reasonably low melting points.

0128According to one embodiment, certain metal oxyhalides are useful as solvents in preparing metal oxides using the method. Vanadium halides such as vanadium tetrachloride, VCl₄ and vanadyl trichloride (VOCl₃), which are liquids at room temperature and are ideal solvents for the chloride salts of the other metals because of their polarity and low boiling points (BP(VCl₄) = 148°C, BP(VOCl₃) = 127°C). Metal halides are dissolved in one of these solvents in the desired mole ratios, and then excess vanadium is removed via evaporation under reduced pressure and inert atmosphere. The catalyst cake is then calcined under O₂/Argon to liberate oxides of chlorine, generating the mixed metal oxide catalyst. Alternatively, the catalyst cake can be calcined under wet Argon to generate the mixed metal oxide (MMO) catalyst and HCl. In addition, mixed metal halides (MMH) are also converted to MMO, discussed in more detail below.

0129According to a separate embodiment, volatile organic solvents having a large dielectric constant are used as solvents in preparing mixed metal oxide catalysts. A suitable example includes, but is not limited to, SO₂. The solvent then provides a homogeneous system for preparing MMO.

0130According to a separate embodiment, it is advantageous to introduce oxygen earlier in the synthesis. This is achieved by mixing metal oxides into either the molten salt solution or the VCl₄/VOCl₃ solution. This method reduces the amount of chlorine that must be removed during calcination and generates mixed oxychloride precursors that already have some of the desired characteristics of the final catalyst. One preparation is to dissolve oxides of niobium, tellurium, and molybdenum in VCl₄/VOCl₃. The resulting precursor will already have high oxygen content.

0131According to a separate embodiment, mixed metal halides (MMH) are also converted to MMO. Three methods for converting mixed metal halides (MMH) and mixed metal oxyhalides (MMOH) to mixed metal oxides (MMO) are described: <ul id="ul0006" list-style="none" compact="compact"><li>(A) MMH precursors are calcined under wet (1%) argon at elevated temperatures (600°C). The off-gas is scrubbed with caustic to trap the product HCl.</li><li>(B) MMH precursors are calcined under argon with low O₂ concentration. The low O₂ concentration moderates the reaction. The oxychloride gases is scrubbed with caustic.</li><li>(C) MMH precursors are chemically converted to the metal alkoxides under mild conditions, followed by calcination under O₂/Argon to generate the MMO catalyst. By using the alkoxide intermediate, the crystalline structure of the final catalyst can be altered.</li></ul>

0132The MMO prepared from the molten salt method can be prepared on support materials including metal oxide supports. One advantage of using molten salt or salt solutions in VCl₄/VOCl₃ is that it is comparatively easy to impregnate support material, such as alumina, zirconia, silica, or titanium oxide, and allows the use of either the pearl technique or sequential loading. The relatively high metal concentrations in solution enables one to increase the metal loading on the support material, providing an ideal catalyst for millisecond contact time reactions.

0133Alternatively, another approach to preparing supported MMO catalyst is addition of finely-divided support material such as aluminum oxide into the salt solution (molten salt or VCl₄/VOCl₃ solution) to create a suspension/slurry. After concentration and calcination, the final catalyst prepared is a supported MMO catalyst with significantly higher surface area.

0134According to a separate embodiment, mixed metal sulfides (MMS) and chalcogenides (MMC) are also converted to MMO. MMS are prepared using a molten metal salt mixture that incorporates a source of sulfur (such as S₈ for example). The MMS precursor is converted to a MMO by hydrolyzing the MMS and calcining the hydrolyzed mixture at 600-800° C which affords the MMO.

0135A mixed metal oxide, thus obtained, exhibits excellent catalytic activities by itself. However, the mixed metal oxide can be converted to a catalyst having higher activities by grinding.

0136There is no particular restriction as to the grinding method, and conventional methods may be employed. As a dry grinding method, a method of using a gas stream grinder may, for example, be mentioned wherein coarse particles are permitted to collide with one another in a high speed gas stream for grinding. The grinding may be conducted not only mechanically but also by using a mortar or the like in the case of a small scale operation.

0137As a wet grinding method wherein grinding is conducted in a wet state by adding water or an organic solvent to the above mixed metal oxide, a conventional method of using a rotary cylinder-type medium mill or a medium-stirring type mill, may be mentioned. The rotary cylinder-type medium mill is a wet mill of the type wherein a container for the object to be ground is rotated, and it includes, for example, a ball mill and a rod mill. The medium-stirring type mill is a wet mill of the type wherein the object to be ground, contained in a container is stirred by a stirring apparatus, and it includes, for example, a rotary screw type mill, and a rotary disc type mill.

0138The conditions for grinding may suitably be set to meet the nature of the above-mentioned mixed metal oxide; the viscosity, the concentration, etc. of the solvent used in the case of wet grinding; or the optimum conditions of the grinding apparatus. However, it is preferred that grinding is conducted until the average particle size of the ground catalyst precursor would usually be at most 20µm, more preferably at most 5µm. Improvement in the catalytic performance may be brought about by such grinding.

0139Further, in some cases, it is possible to further improve the catalytic activities by further adding a solvent to the ground catalyst precursor to form a solution or slurry, followed by drying again. There is no particular restriction as to the concentration of the solution or slurry, and it is usual to adjust the solution or slurry so that the total amount of the starting material compounds for the ground catalyst precursor is from 10 to 60 wt %. Then, this solution or slurry is dried by a method such as spray drying, freeze drying, evaporation to dryness or vacuum drying. Further, similar drying may be conducted also in the case where wet grinding is conducted.

0140The oxide obtained by the above-mentioned method may be used as a final catalyst, but it may further be subjected to heat treatment usually at a temperature of from 200° to 800°C for from 0.1 to 10 hours.

0141The mixed metal oxide thus obtained is typically used by itself as a solid catalyst, but may be formed into a catalyst together with a suitable carrier such as silica, alumina, titania, aluminosilicate, diatomaceous earth or zirconia. Further, it may be molded into a suitable shape and particle size depending upon the scale or system of the reactor. Alternatively, the metal components of the modified catalysts may be supported on materials such as alumina, silica, silica-alumina, zirconia, titania, etc. by conventional incipient wetness techniques. In one typical method, solutions containing the metals are contacted with the dry support such that the support is wetted; then, the resultant wetted material is dried, for example, at a temperature from room temperature to 200°C followed by calcination as described above. In another method, metal solutions are contacted with the support, typically in volume ratios of greater than 3 : 1 (metal solution : support), and the solution agitated such that the metal ions are ion-exchanged onto the support. The metal containing support is then dried and calcined as detailed above.

0142When using a catalyst system including two or more modified catalysts, the catalyst may be in the form of a physical blend of the several catalysts. Preferably, the concentration of the catalysts may be varied so that the first catalyst component will have a tendency to be concentrated at the reactor inlet while subsequent catalysts will have a tendency to be concentrated in sequential zones extending to the reactor outlet. Most preferably, the catalysts will form a layered bed (also referred to a mixed bed catalyst), with the first catalyst component forming the layer closest to the reactor inlet and the subsequent catalysts forming sequential layers to the reactor outlet. The layers abut one another or may be separated from one another by a layer of inert material or a void space.

0143The invention provides a process for producing an unsaturated carboxylic acid, which comprises subjecting an alkane, alkene or a mixture of an alkane and an alkene ("alkane/alkene"), to a vapor phase catalytic oxidation reaction in the presence of a catalyst containing the above promoted mixed metal oxide, to produce an unsaturated carboxylic acid.

0144In the production of such an unsaturated carboxylic acid, it is preferred to employ a starting material gas that contains steam. In such a case, as a starting material gas to be supplied to the reaction system, a gas mixture comprising a steam-containing alkane, or a steam-containing mixture of alkane and alkene, and an oxygen-containing gas, is usually used. However, the steam-containing alkane, or the steam-containing mixture of alkane and alkene, and the oxygen-containing gas may be alternately supplied to the reaction system. The steam to be employed may be present in the form of steam gas in the reaction system, and the manner of its introduction is not particularly limited.

0145Further, as a diluting gas, an inert gas such as nitrogen, argon or helium may be supplied. The molar ratio (alkane or mixture of alkane and alkene) : (oxygen) : (diluting gas) : (H₂O) in the starting material gas is preferably (1) : (0.1 to 10): (0 to 20) : (0.2 to 70), more preferably (1) : (1 to 5.0) : (0 to 10) : (5 to 40).

0146When steam is supplied together with the alkane, or the mixture of alkane and alkene, as starting material gas, the selectivity for an unsaturated carboxylic acid is distinctly improved, and the unsaturated carboxylic acid can be obtained from the alkane, or mixture of alkane and alkene, in good yield simply by contacting in one stage. However, the conventional technique utilizes a diluting gas such as nitrogen, argon or helium for the purpose of diluting the starting material. As such a diluting gas, to adjust the space velocity, the oxygen partial pressure and the steam partial pressure, an inert gas such as nitrogen, argon or helium may be used together with the steam.

0147As the starting material alkane it is preferred to employ a C_2-8 alkane, particularly propane, isobutane or n-butane; more preferably, propane or isobutane; most preferably, propane. According to the present invention, from such an alkane, an unsaturated carboxylic acid such as an α,β-unsaturated carboxylic acid can be obtained in good yield. For example, when propane or isobutane is used as the starting material alkane, acrylic acid or methacrylic acid will be obtained, respectively, in good yield.

0148In the present invention, as the starting material mixture of alkane and alkene, it is preferred to employ a mixture of C_2-8 alkane and C_2-8 alkene, particularly propane and propene, isobutane and isobutene or n-butane and n-butene. As the starting material mixture of alkane and alkene, propane and propene or isobutane and isobutene are more preferred. Most preferred is a mixture of propane and propene. According to the present invention, from such a mixture of an alkane and an alkene, an unsaturated carboxylic acid such as an α,β-unsaturated carboxylic acid can be obtained in good yield. For example, when propane and propene or isobutane and isobutene are used as the starting material mixture of alkane and alkene, acrylic acid or methacrylic acid will be obtained, respectively, in good yield. Preferably, in the mixture of alkane and alkene, the alkene is present in an amount of at least 0.5% by weight, more preferably at least 1.0% by weight to 95% by weight; most preferably, 3% by weight to 90% by weight.

0149As an alternative, an alkanol, such as isobutanol, which will dehydrate under the reaction conditions to form its corresponding alkene, i.e. isobutene, may also be used as a feed to the present process or in conjunction with the previously mentioned feed streams.

0150The purity of the starting material alkane is not particularly limited, and an alkane containing a lower alkane such as methane or ethane, air or carbon dioxide, as impurities, may be used without any particular problem. Further, the starting material alkane may be a mixture of various alkanes. Similarly, the purity of the starting material mixture of alkane and alkene is not particularly limited, and a mixture of alkane and alkene containing a lower alkene such as ethene, a lower alkane such as methane or ethane, air or carbon dioxide, as impurities, may be used without any particular problem. Further, the starting material mixture of alkane and alkene may be a mixture of various alkanes and alkenes.

0151There is no limitation on the source of the alkene. It may be purchased, per se, or in admixture with an alkane and/or other impurities. Alternatively, it can be obtained as a by-product of alkane oxidation. Similarly, there is no limitation on the source of the alkane. It may be purchased, per se, or in admixture with an alkene and/or other impurities. Moreover, the alkane, regardless of source, and the alkene, regardless of source, may be blended as desired.

0152The detailed mechanism of the oxidation reaction of the present invention is not clearly understood, but the oxidation reaction is carried out by oxygen atoms present in the above mixed metal oxide or by molecular oxygen present in the feed gas. To incorporate molecular oxygen into the feed gas, such molecular oxygen may be pure oxygen gas. However, it is usually more economical to use an oxygen-containing gas such as air, since purity is not particularly required.

0153It is also possible to use only an alkane, or a mixture of alkane and alkene, substantially in the absence of molecular oxygen for the vapor phase catalytic reaction. In such a case, it is preferred to adopt a method wherein a part of the catalyst is appropriately withdrawn from the reaction zone from time to time, then sent to an oxidation regenerator, regenerated and then returned to the reaction zone for reuse. As the regeneration method of the catalyst, a method may, for example, be mentioned which comprises contacting an oxidative gas such as oxygen, air or nitrogen monoxide with the catalyst in the regenerator usually at a temperature of from 300° to 600°C.

0154This aspect present invention is described in still further detail with respect to a case where propane is used as the starting material alkane and air is used as the oxygen source. The reaction system may be preferably a fixed bed system. The proportion of air to be supplied to the reaction system is important for the selectivity for the resulting acrylic acid, and it is usually at most 25 moles, preferably from 0.2 to 18 moles per mole of propane, whereby high selectivity for acrylic acid can be obtained. This reaction can be conducted usually under atmospheric pressure, but may be conducted under a slightly elevated pressure or slightly reduced pressure. With respect to other alkanes such as isobutane, or to mixtures of alkanes and alkenes such as propane and propene, the composition of the feed gas may be selected in accordance with the conditions for propane.

0155Typical reaction conditions for the oxidation of propane or isobutane to acrylic acid or methacrylic acid may be utilized in the practice of the present invention. The process may be practiced in a single pass mode (only fresh feed is fed to the reactor) or in a recycle mode (at least a portion of the reactor effluent is returned to the reactor). General conditions for the process of the present invention are as follows: the reaction temperature can vary from 200°C to 700°C, but is usually in the range of from 200°C to 550°C, more preferably 250°C to 480°C, most preferably 300°C to 400°C; the gas space velocity, SV, in the vapor phase reaction is usually within a range of from 100 to 10,000 hr^-1, preferably 300 to 6,000 hr^-1, more preferably 300 to 2,000 hr^-1; the average contact time with the catalyst can be from 0.01 to 10 seconds or more, but is usually in the range of from 0.1 to 10 seconds, preferably from 0.2 to 6 seconds; the pressure in the reaction zone usually ranges from 0 to 75 psig, but is preferably no more than 50 psig. In a single pass mode process, it is preferred that the oxygen be supplied from an oxygen-containing gas such as air. The single pass mode process may also be practiced with oxygen addition. In the practice of the recycle mode process, oxygen gas by itself is the preferred source so as to avoid the build up of inert gases in the reaction zone. The feed of hydrocarbon in the catalytic process is dependent on the mode of operation (e.g. single pass, batch, recycle, etc.) and ranges from 2 wt. % to 50 wt. %. According to a separate embodiment, the catalytic process is a batch process. According to a separate process, the catalytic process is run continuously. The catalytic process all conventional beds including, but not limited to static fluid beds, fluidized beds, moving beds, transport beds, moving beds such as rising and ebulating beds. Any catalytic process is carried out under steady state conditions or non steady state conditions.

0156Of course, in the oxidation reaction of the present invention, it is important that the hydrocarbon and oxygen concentrations in the feed gases be maintained at the appropriate levels to minimize or avoid entering a flammable regime within the reaction zone or especially at the outlet of the reactor zone. Generally, it is preferred that the outlet oxygen levels be low to both minimize after-burning and, particularly, in the recycle mode of operation, to minimize the amount of oxygen in the recycled gaseous effluent stream. In addition, operation of the reaction at a low temperature (below 450°C) is extremely attractive because after-burning becomes less of a problem which enables the attainment of higher selectivity to the desired products. The catalyst of the present invention operates more efficiently at the lower temperature range set forth above, significantly reducing the formation of acetic acid and carbon oxides, and increasing selectivity to acrylic acid. As a diluting gas to adjust the space velocity and the oxygen partial pressure, an inert gas such as nitrogen, argon or helium may be employed.

0157When the oxidation reaction of propane, and especially the oxidation reaction of propane and propene, is conducted by the method of the present invention, carbon monoxide, carbon dioxide, acetic acid, etc. may be produced as by-products, in addition to acrylic acid. Further, in the method of the present invention, an unsaturated aldehyde may sometimes be formed depending upon the reaction conditions. For example, when propane is present in the starting material mixture, acrolein may be formed; and when isobutane is present in the starting material mixture, methacrolein may be formed. In such a case, such an unsaturated aldehyde can be converted to the desired unsaturated carboxylic acid by subjecting it again to the vapor phase catalytic oxidation with the promoted mixed metal oxide-containing catalyst of the present invention or by subjecting it to a vapor phase catalytic oxidation reaction with a conventional oxidation reaction catalyst for an unsaturated aldehyde.

0158Turning now in more specific detail to the third aspect of the present invention, the method of the present invention comprises subjecting an alkane, or a mixture of an alkane and an alkene, to a vapor phase catalytic oxidation reaction with ammonia in the presence of a catalyst containing the above mixed metal oxide, to produce an unsaturated nitrile.

0159In the production of such an unsaturated nitrile, as the starting material alkane, it is preferred to employ a C_2-8 alkane such as propane, butane, isobutane, pentane, hexane and heptane. However, in view of the industrial application of nitriles to be produced, it is preferred to employ a lower alkane having 3 or 4 carbon atoms, particularly propane and isobutane.

0160Similarly, as the starting material mixture of alkane and alkene, it is preferred to employ a mixture of C_2-8 alkane and C_2-8 alkene such as propane and propene, butane and butene, isobutane and isobutene, pentane and pentene, hexane and hexene, and heptane and heptene. However, in view of the industrial application of nitriles to be produced, it is more preferred to employ a mixture of a lower alkane having 3 or 4 carbon atoms and a lower alkene having 3 or 4 carbon atoms, particularly propane and propene or isobutane and isobutene. Preferably, in the mixture of alkane and alkene, the alkene is present in an amount of at least 0.5% by weight, more preferably at least 1.0% by weight to 95% by weight, most preferably 3% by weight to 90% by weight.

0161The purity of the starting material alkane is not particularly limited, and an alkane containing a lower alkane such as methane or ethane, air or carbon dioxide, as impurities, may be used without any particular problem. Further, the starting material alkane may be a mixture of various alkanes. Similarly, the purity of the starting material mixture of alkane and alkene is not particularly limited, and a mixture of alkane and alkene containing a lower alkene such as ethene, a lower alkane such as methane or ethane, air or carbon dioxide, as impurities, may be used without any particular problem. Further, the starting material mixture of alkane and alkene may be a mixture of various alkanes and alkenes.

0162There is no limitation on the source of the alkene. It may be purchased, per se, or in admixture with an alkane and/or other impurities. Alternatively, it can be obtained as a by-product of alkane oxidation. Similarly, there is no limitation on the source of the alkane. It may be purchased, per se, or in admixture with an alkene and/or other impurities. Moreover, the alkane, regardless of source, and the alkene, regardless of source, may be blended as desired.

0163Accoording to a separate embodiment, a short contact reactor is employed with the one or more modified catalysts of the invention. The short contact time reactor is of a type suitable for the use of a fixed catalyst bed in contact with a gaseous stream of reactants. For instance, a shell and tube type of reactor may be utilized, wherein one or more tubes are packed with catalyst(s) so as to allow a reactant gas stream to be passed in one end of the tube(s) and a product stream to be withdrawn from the other end of the tube(s). The tube(s) being disposed in a shell so that a heat transfer medium may be circulated about the tube(s).

0164In the case of the utilization of a single catalyst or catalyst system, the gas stream comprising the alkane, molecular oxygen and any additional reactant feeds including but not limited to alkenes, oxygen, air, hydrogen, carbon monoxide, carbon dioxide, formaldehyde and alcohols, steam and any diluents including nitrogen, argon may all be fed into the front end(s) of the tube(s) together. Alternatively, the alkane and the molecular oxygen-containing gas may be fed into the front end(s) of the tube(s) while the additional reactants, steam and diluents may be fed (also referred to as staging) into the tube(s) at a predetermined downstream location (typically chosen so as to have a certain minimum concentration of product alkene present in the gas stream passing through the tube(s), e.g., 3%, preferably 5%, most preferably 7%).

0165In the case of the utilization of catalyst systems including two or more catalysts, e.g., a first catalyst component and a second catalyst component as described above, once again the gas stream comprising the alkane, the oxygen-containing gas and any additional reactant feeds including but not limited to alkenes, oxygen, air, hydrogen, carbon monoxide, carbon dioxide, formaldehyde and alcohols, steam and any diluents including nitrogen, argon are fed to the front end(s) of the tube(s) together. Alternatively, and preferably, the alkane and the molecular oxygen-containing gas are staged into the front end(s) of the tube(s) while any additional reactant feeds, steam and diluents are staged into the tube(s) at a predetermined downstream location (typically chosen so at have a certain minimum concentration of desired product present in the gas stream passing through the tube(s),as set forth above; or in the case of the utilization of layered beds of catalyst, as described above, intermediate two layered catalyst beds).

0166Typical reaction conditions for the oxidation of propane or isobutane to acrylic acid or methacrylic acid including respective esters thereof which are utilized in the practice of the present invention include: reaction temperatures which can vary from 300°C to 1000°C, but are usually in the range of flame temperatures (from 500°C to 1000°C); the average contact time with the catalyst (i.e. the reactor residence time) is not more than 100 milliseconds, including not more than 80 milliseconds, and including not more than 50 milliseconds; the pressure in the reaction zone usually ranges from 0 to 75 psig, including no more than 50 psig.

0167The invention provides a process for preparing unsaturated carboxylic acids from corresponding alkanes, the process comprising the step of: providing one or more modified catalysts cumulatively effective at converting the gaseous alkane to its corresponding gaseous unsaturated carboxylic acid;    wherein the second catalyst layer is separated at a distance downstream from the first catalyst layer and the reactor is operated at a temperature of from 100°C to 700°C, with a reactor residence time of no less than 100 milliseconds. As a separate embodiment, a short contact time reactor is operated at a temperature of from 100°C to 700°C, with a reactor residence time of than 100 or less milliseconds. 300°C to 400°C, with a second reaction zone residence time of no greater than 100 milliseconds;

0168It is preferred to pass a gaseous stream comprising propane or isobutane and molecular oxygen to the reactor. In addition, the feed may contain an additional reactant, adjuvant such as steam or a diluent such as an inert gas, e.g., nitrogen, argon or carbon dioxide.

0169In a separate embodiment, the gaseous stream of the alkane is passed through the reactor in a single pass or wherein any unreacted alkane is recycled back into the gaseous stream of alkane entering the reactor and any saturated carboxylic acid is recycled back into the second catalyst zone to increase the overall yield of unsaturated carboxylic acid.

0170The invention also provides a process comprising the steps of: (a) converting an alkane to its corresponding products selected from alkene, unsaturated carboxylic acid, and higher analogue unsaturated carboxylic acid in a short contact time reactor using the catalyst systems of the invention; and (b) adding the resulting product or products to the front end of a second fixed bed oxidation reactor with the product(s) from the first reactor acting as feed to the second reactor. For example, propane is converted to propylene using a catalyst system as described in a short contact time reactor. The propylene is then fed to second oxidation reactor that converts its to acrylic acid. According to one embodiment this includes feeding any unreacted alkane from the first reactor to the second reactor to recycle the alkane. For example, any unreacted propane is recycled to the first SCTR or added as a feed to the second oxidation reactor. The second oxidation reactor comprises any conventional industrial scale oxidation reactor used for converting alkenes to unsaturated carboxylic acids at longer residence times (seconds). Alternatively, the second oxidation reactor comprises a second SCTR operating at millisecond residence times.

0171Any source of molecular oxygen may be employed in this process, e.g., oxygen, oxygen-enriched gases or air. Air may be the most economical source of oxygen, especially in the absence of any recycle.

0172The invention also provides a process for the production of esters of unsaturated carboxylic acids, the process comprising the step of: <ul id="ul0007" list-style="none" compact="compact"><li>passing a gaseous alkane, molecular oxygen and a gaseous alcohol to a short contact time reactor, the reactor including a mixed catalyst bed comprising (a) a first catalyst layer comprising one or more modified catalysts cumulatively effective at converting the gaseous alkane to its corresponding gaseous unsaturated carboxylic acid; wherein the catalysts of the first layer are impregnated on a metal oxide support; and (b) a second catalyst layer comprising one or more unmodified or modified catalysts cumulatively effective at converting the gaseous unsaturated carboxylic acid to its corresponding gaseous ester;</li></ul>    wherein the second catalyst' layer is separated at a distance downstream from the first catalyst layer and the reactor is operated at a one or more temperatures of from 100°C to 1000°C. In a separate embodiment the modified catalysts are partitioned into one or more zones, the first reaction zone being operated at a temperature of from 100°C to 1000°C, the second reaction zone being operated at a temperature of from 300°C to 400°C. The second catalyst comprises one or more unmodified or modified superacids.

0173According to yet another embodiment, provides a process for catalytically converting alkanes to their corresponding higher unsaturated carboxylic acids and then catalytically converting them to their corresponding esters in the presence of specific alcohols.

0174The second catalyst comprises one or more modified or umodified superacid. A superacid, according to the definition of Gillespie, is an acid that is stronger than 100% sulfuric acid, i.e. it has a Hammett acidity value H₀ &lt; -12. Representative superacids include, but are not limited to: zeolite supported TiO₂/(SO₄)₂, (SO₄)₂/ZrO₂-TiO₂, (SO₄)₂/ZrO₂-Dy₂O₃, (SO₄)₂/TiO₂, (SO₄)₂/ZrO₂-NiO, SO₄/ZrO₂, SO₄/ZrO₂Al₂O_3' (SO₄)₂/Fe₂O₃, (SO₄)₂/ZrO₂, C₄F₉SO₃H-SbF₅, CF₃SO₃H-SbF₅, Pt/sulfated zirconium oxide, HSO₃F-SO₂ClF, SbF₅-HSO₃F-SO₂CIF, MF₅/AlF₃ (M=Ta, Nb, Sb), B(OSO₂CF₃)₃, B(OSO₂CF₃)₃-CF₃SO₃H, SbF₅-SO₂-Al₂O₃, SbF₅-TiO₂-SiO₂ and SbF₅-TiO₂. Preferably, solid superacids are utilized, e.g., sulfated oxides, supported Lewis acids and supported liquid superacids. Only a small number of oxides produce superacid sites on sulfation, including ZrO₂, TiO₂, HfO₂, Fe₂O₃ and SnO₂. The acid sites are generated by treating an amorphous oxyhydrate of these elements with H₂SO₄ or (NH₄)₂SO₄ and calcining the products at temperatures of 500°C - 650°C. During the calcination, the oxides are transformed into a crystalline tetragonal phase, which is covered by a small number of sulfate groups. H₂MoO₄ or H₂WO₄ may also be used to activate the oxide.

0175In a separate embodiment of the present invention, an alcohol is reacted with an unsaturated aldehyde to form an acetal. Such reaction can be carried out by contacting the aldehyde with an excess of the anhydrous alcohol in the presence of a small amount of an anhydrous acid, e.g., anhydrous HCl. Preferably, the aldehyde and the alcohol can be passed through a bed containing an acid catalyst, e.g., through a bed of a strongly acidic ion exchange resin, such as Amberlyst 15.

0176The so-formed acetal and molecular oxygen are fed to a reactor containing at least one catalyst effective for the oxidation of the acetal to its corresponding ester. Examples of such a catalyst include well known Pd and Bi on alumina or V oxides.

0177The modified catalysts of the invention are usefully employed in catalytic processes described in a pending provisional U. S. Application (Ser. No. 06/000000). The application provides a process which addresses the problem of a decreased total yield of oxidation product in multi-stage vapor phase oxidation reactions which employ staged oxygen arrangements for conversion of lower alkanes and alkenes, and mixtures thereof, to unsaturated carboxylic acids and/or unsaturated nitriles. More particularly, it has been discovered that in such processes, the removal of at least a portion of the oxidation product from each intermediate effluent stream, for example, by inter-stage partial condensation, prior to adding more oxygen and feeding the effluent stream to the next stage, unexpectedly results in overall cumulative oxidation product yields greater than either the original single-stage system or the system including only staged oxygen arrangements.

0178The present invention provides an improved process for the production of unsaturated carboxylic acids and unsaturated nitriles from their corresponding C₂-C₈ alkanes, or mixtures of C₂-C₈ alkanes and alkenes, that utilizes a multi-stage reaction system and includes the steps of separating the oxidation product from one or more intermediate effluent streams, as well as feeding additional oxygen to reaction zones subsequent to the first reaction zone.

0179The process using modified catalysts of the invention is for producing unsaturated carboxylic acids or unsaturated nitriles by vapor phase oxidation reaction of their corresponding C₂-C₈ alkanes, C₂-C₈ alkenes, and mixtures thereof. The process of the present invention uses a reaction system, having at least two reaction zones arranged in series with one another and at least one catalyst capable of catalyzing the vapor phase oxidation reaction disposed in each of the at least two reaction zones. Furthermore, at least one intermediate effluent stream exits a preceding one of the at least two reaction zones and is at least partially fed to a subsequent one of the at least two reaction zones. The process of the present invention comprises separating the at least one intermediate effluent stream into at least an intermediate product stream comprising an oxidation product selected from the group consisting of an unsaturated carboxylic acid and an unsaturated nitrile, and an intermediate feed stream comprising starting materials selected from the group consisting of an unreacted C₂-C₈ alkane, an unreacted C₂-C₈ alkene, and mixtures thereof; feeding the intermediate feed stream to the subsequent reaction zone; and feeding an oxygen-containing gas to the subsequent reaction zone. In one alternative embodiment, two or more of the reaction zones may be contained within a single reactor vessel.

0180The separating step may be performed by cooling the at least one intermediate effluent stream such that at least a portion of the oxidation products condenses out of the at least one intermediate effluent stream. Such cooling may be achieved with a condenser. The separating step may, alternatively, be performed using an absorber.

0181In a particular application of the present invention, the C₂-C₈ alkane, C₂-C₈ alkene, or mixture thereof may comprise propane, propene, or a mixture thereof, and the oxidation product may comprise acrylic acid.

0182The process also comprises feeding ammonia-containing gas to each of the at least two reaction zones. In a particular application of the process, in which ammonia-containing gas is fed to each of the at least two reaction zones, the C₂-C₈ alkane, C₂-C₈ alkene, or mixture thereof, may comprise propane, propene, or a mixture thereof, and the oxidation product may comprise acrylonitrile.

0183Separators suitable for use with the present invention include any suitable fluid separator capable of separating a gaseous product stream into multiple streams according to composition, such as separating a gaseous output stream into a first stream containing primarily the desired reaction product(s) and a second stream containing primarily unreacted materials and by-products. For example, while not intending to be limited, the separator may be a partial condenser 16, 20, such as a conventional heat exchanger, capable of cooling the gaseous output stream sufficiently to condense and separate out at least a portion of the lowest boiling point components of the gaseous output stream would be suitable for use with the process 10 of the present invention. The coolant in such a condenser may be, for example, without limitation, cooling tower water having a temperature between 85°F and 105°F (29°C to 40°C), or chilled water having a temperature between 32°F and 40°F (0°C and 5°C). In addition, for example, the separators may include gas absorbers or gas adsorbers.

0184Suitable starting materials, which are discussed hereinafter and which are readily determinable by persons having ordinary skill in the art, are fed into the first reaction zone 12. In the first reaction zone 12, the starting materials come into contact with the catalyst and react with one another to form the desired oxidation products, as well as various side products and by-products, according to the particular types of C₂ to C₅8alkanes and alkenes used.

0185Suitable starting materials for the process 10 of the present invention depend upon the desired oxidation product and typically include, but are not limited to, a C₂ to C₈ alkane, a C₂ to C₈ alkene, or a mixture thereof, and an oxygen-containing gas, as well as, optionally, steam, diluting gases and ammonia. The starting materials may be added separately and simultaneously to the first reaction zone 12, or they may be mixed and fed to the first reaction zone 12 as one or more combined streams. For example, as explained in further detail hereinafter, the initial feed stream 22, shown in Figure 1, may be a combined stream comprising an oxygen-containing gas and a C₂ to C₈ alkane, a C₂ to C₈ alkene, or a mixture thereof. The optional supplemental streams 24, 24', 24", shown in phantom in Figure 1, may be, for example, steam-containing gases or ammonia-containing gases, depending upon the particular oxidation products desired. The optional supplemental streams 24, 24', 24" may even comprise additional C₂ to C₈ alkane, C₂ to C₈ alkene, or a mixture thereof.

0186The detailed mechanism of the oxidation reaction of the present invention is not clearly understood, but the oxidation reaction is carried out by oxygen atoms present in the above mixed metal oxide or by molecular oxygen present in the feed gas. Addition of oxygen-containing gas to the starting materials provides such molecular oxygen to the reaction system. The term "oxygen-containing gas," as used herein, refers to any gas comprising from 0.01% up to 100% oxygen, including, for example, air. Thus, although the oxygen-containing gas may be pure oxygen gas, it is usually more economical to use an oxygen-containing gas such as air, since purity is not particularly required.

0187The purity of the starting material, i.e., the C₂ to C₈ alkane, the C₂ to C₈ alkene, or the mixture thereof, is not particularly limited. Thus, commercial grades of such alkanes, or mixtures of such alkanes and alkenes, may be used as starting material for the process 10 of the present invention, although higher purities are advantageous from the standpoint of minimizing competing side reactions. In addition, mixed C₂ to C₈ alkane/alkene feeds are generally more easily obtained and may include price incentives (e.g., lower separation costs) relative to pure C₂ to C₈ alkane feeds. For example, a mixture of alkane and alkene containing a lower alkene such as ethene, a lower alkane such as methane or ethane, air or carbon dioxide, as impurities, may be used without any particular problem. Further, the starting material mixture of C₂ to C₈ alkane and alkene may be a mixture of various C₂ to C₈ alkanes and alkenes. Further details concerning the starting materials will be discussed hereinafter in connection with particular embodiments of the present invention.

0188Suitable diluting gases include, but are not limited to, one or more of carbon monoxide, carbon dioxide, or mixtures thereof, an inert gas, such as nitrogen, argon, helium, or mixtures thereof. A suitable molar ratio of the starting materials for the initial feed stream 22, (C₂ to C₈ alkane, Cz to C₈ alkene, or a mixture thereof) : (oxygen) : (diluting gas) : (H₂O), would be, for example, (1) : (0.1 to 10) : (0 to 20) : (0.2 to 70), for example, including but not limited to, (1) : (1 to 5.0) : (0 to 10) : (5 to 40).

0189Where it is desired to produce unsaturated carboxylic acids, it is beneficial to include steam among the starting materials. In such a case, for example, a gaseous input stream comprising a mixture of and oxygen-containing gas and a steam-containing C₂ to C₈ alkane, or a steam-containing C₂ to C₈ alkene, or a steam-containing mixture thereof, may be used. It is noted that the steam may be added to the first reaction zone separately from the C₂ to C₈ alkane, the C₂ to C₈ alkene, or the mixture thereof, and the oxygen-containing gas, as an initial feed stream and an optional steam stream, respectively.

0190In accordance with the process, at least a portion of the one or more oxidation products is separated from the first effluent stream, for example, by using a separator, such as the condenser, to produce an intermediate product stream and an intermediate feed stream. The intermediate product stream typically contains, but is not limited to, at least a portion of the one or more oxidation products from the first effluent stream, as well as other condensables, such as organic acids, aldehydes, ketones, and water. The intermediate product stream may be fed to additional processing apparatus (not shown) to undergo further separation and purification processes. The intermediate feed stream contains, but is not limited to, at least a portion of the unreacted oxygen, unreacted C₂ to C₈ alkane or alkene, or mixture thereof, and possibly reaction by-products such as acetic acid and carbon dioxide, and, possibly, unreacted water and unreacted ammonia, depending upon the starting materials used.

0191The cumulative yield of the desired oxidation product produced by the above-described process is greater than the cumulative yield of the desired oxidation product that is produced by a process that does not include both separating at least a portion of the one or more oxidation products from the first effluent stream, as well as feeding additional oxygen-containing gas to the second reaction zone. In addition, the cumulative yield of the one or more oxidation products produced by the above-described process is greater than the cumulative yield of the one or more oxidation products that is produced by a process that includes only feeding additional oxygen-containing gas to the second reaction zone, without separating at least a portion of the one or more oxidation products from the first effluent stream. The process allows for the use of starting materials containing a higher concentration of the C₂ to C₈ alkane, the C₂ to C₈ alkene, or mixture thereof. It is also believed that a greater portion of the oxygen in each subsequent reaction remains available for reacting and converting the C₂ to C₈ alkanes and alkenes.

0192The purity of the starting material alkene is not limited, and an alkene containing a lower alkene such as ethene, air or carbon dioxide, as impurities, may be used without any particular problem. Further, the starting material alkene may be a mixture of various alkenes. Similarly, the purity of the starting material mixture of alkene and alkane is not particularly limited, and a mixture of alkene and alkane containing a lower alkene such as ethene, a lower alkane such as methane or ethane, air or carbon dioxide, as impurities, may be used without any particular problem. Further, the starting material mixture of alkene and alkane may be a mixture of various alkenes and alkanes.

0193There is no limitation on the source of the alkene. It may be purchased, per se, or in admixture with an alkane and/or other impurities. Alternatively, it can be obtained as a by-product of alkane oxidation. Similarly, there is no limitation on the source of the alkane. It may be purchased, per se, or in admixture with an alkene and/or other impurities. Moreover, the alkane, regardless of source, and the alkene, regardless of source, may be blended as desired.

0194The detailed mechanism of the oxidation reaction of this embodiment of the present invention is not clearly understood. When it is desired to incorporate molecular oxygen in the starting materials, the oxygen-containing gas may be pure oxygen gas. However, since high purity is not required, it is usually economical to use air as the oxygen-containing gas.

0195The following illustrative examples are provided to further demonstrate the utility of the present invention and are not in any way construed to be limiting. Moreover, the examples provided are representative examples that broadly enable the claimed scope of the invention. In the following Examples, "propane conversion" is synonymous with "feed conversion" and was calculated in accordance with the formulas provided earlier hereinabove. Furthermore, "AA yield" means acrylic acid yield and is synonymous with "product yield" and was calculated in accordance with the formulas provided earlier hereinabove.

0196Unless otherwise specified, all percentages recited in the following Examples are by volume, based on the total volume of the feed or product gas stream.

0197The examples set forth below are for illustrative purposes only and should not be considered as limiting the scope of the invention. For purposes of this application, "% conversion" is equal to (moles of consumed alkane (or alkane/alkene)/moles of supplied alkane (or alkane/alkene)) X 100; and "%yield" is equal to (moles of formed desired unsaturated carboxylic acid or aldehyde/moles of supplied alkane (or alkane/alkene)) X (carbon number of formed desired unsaturated carboxylic acid or aldehyde/ carbon number of the supplied alkane (or alkane/alkene)) X 100.

Example 1

0198The catalyst samples were prepared as follows. The sequential physical vapor deposition (PVD) of Te, Nb, V, and Mo on a honeycomb substrate was performed in a PVD system with a base pressure of 5×10^-7 Torr. The metal sources were made by melting individual metal powders into different crucibles. The PVD system was equipped with four pockets that house four crucibles containing Te, Nb, V and Mo, respectively. (The unique advantage of having four metal sources at the same time is that the sequential deposition can be performed without opening the vacuum system to change metal sources.) During deposition an individual metal source was heated by electron beam, and the deposition rate (typically a few nanometers per minute) was monitored using a quartz crystal balance that was located near the honeycomb substrate. Two PVD samples were sequentially deposited using the following two sequences: <ul id="ul0008" list-style="none" compact="compact"><li>Sample 1 - Mo(72 nm)/V(19 nm)/Nb(10 nm)/Te(36 nm)</li><li>Sample 2 - Te(36 nm)/Nb(10 nm)/V(19 nm)/Mo(72 nm)</li></ul>

0199The two PVD samples were each calcined in a quartz tube. Each quartz tube was placed in an oven, at ambient temperature, with a 100 cc/min flow of air through the tube, the furnace was then heated from ambient temperature to 275°C at 10°C/min and held there for one hour; then, using a 100 cc/min flow of argon through the tube, the oven was heated from 275°C to 600°C at 2°C/min and held there for two hours.

0200The calcined PVD samples were each evaluated in a fixed bed reactor at a contact time of 50 milliseconds and with a feed of 7 % propane, 22% steam and the balance air with results given in Table 1. Results were consistently reproduced with mass balances within 98 - 102 %. <tables id="tabl0002" num="0002"><table frame="all"><title>Table 1</title><tgroup cols="6" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="26.25mm" /><colspec colnum="2" colname="col2" colwidth="26.25mm" /><colspec colnum="3" colname="col3" colwidth="26.25mm" /><colspec colnum="4" colname="col4" colwidth="26.25mm" /><colspec colnum="5" colname="col5" colwidth="26.25mm" /><colspec colnum="6" colname="col6" colwidth="26.25mm" /><thead valign="top"><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Catalyst</entry><entry namest="col2" nameend="col2" align="center">Sequence</entry><entry namest="col3" nameend="col3" align="center">Temp, °C</entry><entry namest="col4" nameend="col4" align="center">% C3 Conv.</entry><entry namest="col5" nameend="col5" align="center">% AA Yield</entry><entry namest="col6" nameend="col6" align="center">% C3= Yield</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Sample</entry><entry namest="col2" nameend="col2" align="center">1</entry><entry namest="col3" nameend="col3" align="center">560</entry><entry namest="col4" nameend="col4" align="center">26</entry><entry namest="col5" nameend="col5" align="center">19</entry><entry namest="col6" nameend="col6" align="center">4</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Sample</entry><entry namest="col2" nameend="col2" align="center">2</entry><entry namest="col3" nameend="col3" align="center">480</entry><entry namest="col4" nameend="col4" align="center">32</entry><entry namest="col5" nameend="col5" align="center">21</entry><entry namest="col6" nameend="col6" align="center">3</entry></row></tbody></tgroup></table></tables>

Example 2

0201The sequential PVD of Mo, V, Te and Ti on a honeycomb support is performed in a PVD system with a base pressure of 5×10^-7 Torr. The metal sources are made by melting individual metal powders into different crucibles. The PVD system is equipped with four pockets that house four crucibles containing Mo, V, Te and Ti, respectively. During deposition the individual metal source is heated by electron beam, and the deposition rate (typically a few nanometers per minute) is monitored using a quartz crystal balance that is located near the honeycomb substrate. The PVD sample is sequentially deposited using the following sequence:<maths id="math0001"><math display="block"><mrow><mtext>Mo(72nm)/V(19nm)/Te(36 nm)/Ti(10nm)</mtext></mrow></math><img file="EP1598110A1_D0001.tif" /></maths>

0202The PVD sample is calcined in a quartz tube. The quartz tube is placed in an oven, at ambient temperature, with a 100 cc/min flow of air through the tube, the furnace is heated from ambient temperature to 275°C at 10°C/min and is held there for one hour; using a 100 cc/min flow of argon through the tube, the oven is heated from 275°C to 600°C at 2°C/min and is held there for two hours.

0203The calcined PVD sample is evaluated in a fixed bed reactor at a contact time of 50 milliseconds and with a feed of 7 % propane, 22% steam and the balance air. Consistent results are reproducible with mass balances within 98 - 102 % and are essentially equivalent to those of Example 1/Sample 1.

Example 3

0204The sequential PVD of Mo, V, Te and Ta on a honeycomb support is performed in a PVD system with a base pressure of 5×10^-7 Torr. The metal sources are made by melting individual metal powders into different crucibles. The PVD system is equipped with four pockets that house four crucibles containing Mo, V, Te and Ta, respectively. During deposition the individual metal source is heated by electron beam, and the deposition rate (typically a few nanometers per minute) is monitored using a quartz crystal balance that is located near the honeycomb substrate. The PVD sample is sequentially deposited using the following sequence:<maths id="math0002"><math display="block"><mrow><mtext>Mo(72nm)/V(19nm)/Te(36 nm)/Ta(10nm)</mtext></mrow></math><img file="EP1598110A1_D0002.tif" /></maths>

0205The PVD sample is calcined in a quartz tube. The quartz tube is placed in an oven, at ambient temperature, with a 100 cc/min flow of air through the tube, the furnace is heated from ambient temperature to 275°C at 10°C/min and is held there for one hour; using a 100 cc/min flow of argon through the tube, the oven is heated from 275°C to 600°C at 2°C/min and is held there for two hours.

0206The calcined PVD sample is evaluated in a fixed bed reactor at a contact time of 50 milliseconds and with a feed of 7 % propane, 22% steam and the balance air. Consistent results are reproducible with mass balances within 98 - 102 % and are essentially equivalent to those of Example 1/Sample 1.

Example 4

0207The sequential PVD of Mo, V, Te and W on a honeycomb support is performed in a PVD system with a base pressure of 5×10^-7 Torr. The metal sources are made by melting individual metal powders into different crucibles. The PVD system is equipped with four pockets that house four crucibles containing Mo, V, Te and W, respectively. During deposition the individual metal source is heated by electron beam, and the deposition rate (typically a few nanometers per minute) is monitored using a quartz crystal balance that is located near the honeycomb substrate. The PVD sample is sequentially deposited using the following sequence:<maths id="math0003"><math display="block"><mrow><mtext>Mo(72nm)/V(19nm)/Te(36 nm)/W(10nm)</mtext></mrow></math><img file="EP1598110A1_D0003.tif" /></maths>

0208The PVD sample is calcined in a quartz tube. The quartz tube is placed in an oven, at ambient temperature, with a 100 cc/min flow of air through the tube, the furnace is heated from ambient temperature to 275°C at 10°C/min and is held there for one hour; using a 100 cc/min flow of argon through the tube, the oven is heated from 275°C to 600°C at 2°C/min and is held there for two hours.

0209The calcined PVD sample is evaluated in a fixed bed reactor at a contact time of 50 milliseconds and with a feed of 7 % propane, 22% steam and the balance air. Consistent results are reproducible with mass balances within 98 - 102 % and are essentially equivalent to those of Example 1/Sample 1.

Example 5

0210The sequential PVD of Mo, V, Te and Mn on a honeycomb support is performed in a PVD system with a base pressure of 5×10^-7 Torr. The metal sources are made by melting individual metal powders into different crucibles. The PVD system is equipped with four pockets that house four crucibles containing Mo, V, Te and Mn, respectively. During deposition the individual metal source is heated by electron beam, and the deposition rate (typically a few nanometers per minute) is monitored using a quartz crystal balance that is located near the honeycomb substrate. The PVD sample is sequentially deposited using the following sequence:<maths id="math0004"><math display="block"><mrow><mtext>Mo(72nm)/V(19nm)/Te(36 nm)/Mn(10nm)</mtext></mrow></math><img file="EP1598110A1_D0004.tif" /></maths>

0211The PVD sample is calcined in a quartz tube. The quartz tube is placed in an oven, at ambient temperature, with a 100 cc/min flow of air through the tube, the furnace is heated from ambient temperature to 275°C at 10°C/min and is held there for one hour; using a 100 cc/min flow of argon through the tube, the oven is heated from 275°C to 600°C at 2°C/min and is held there for two hours.

0212The calcined PVD sample is evaluated in a fixed bed reactor at a contact time of 50 milliseconds and with a feed of 7 % propane, 22% steam and the balance air. Consistent results are reproducible with mass balances within 98 - 102 % and are essentially equivalent to those of Example 1/Sample 1.

Example 6

0213The sequential PVD of Mo, V, Te and Ru on a honeycomb support is performed in a PVD system with a base pressure of 5×10^-7 Torr. The metal sources are made by melting individual metal powders into different crucibles. The PVD system is equipped with four pockets that house four crucibles containing Mo, V, Te and Ru, respectively. During deposition the individual metal source is heated by electron beam, and the deposition rate (typically a few nanometers per minute) is monitored using a quartz crystal balance that is located near the honeycomb substrate. The PVD sample is sequentially deposited using the following sequence:<maths id="math0005"><math display="block"><mrow><mtext>Mo(72nm)/V(19nm)/Te(36 nm)/Ru(10nm)</mtext></mrow></math><img file="EP1598110A1_D0005.tif" /></maths>

0214The PVD sample is calcined in a quartz tube. The quartz tube is placed in an oven, at ambient temperature, with a 100 cc/min flow of air through the tube, the furnace is heated from ambient temperature to 275°C at 10°C/min and is held there for one hour; using a 100 cc/min flow of argon through the tube, the oven is heated from 275°C to 600°C at 2°C/min and is held there for two hours.

0215The calcined PVD sample is evaluated in a fixed bed reactor at a contact time of 50 milliseconds and with a feed of 7 % propane, 22% steam and the balance air. Consistent results are reproducible with mass balances within 98 - 102 % and are essentially equivalent to those of Example 1/Sample 1.

Example 7

0216The sequential PVD of Mo, V, Te and Co on a honeycomb support is performed in a PVD system with a base pressure of 5×10^-7 Torr. The metal sources are made by melting individual metal powders into different crucibles. The PVD system is equipped with four pockets that house four crucibles containing Mo, V, Te and Co, respectively. During deposition the individual metal source is heated by electron beam, and the deposition rate (typically a few nanometers per minute) is monitored using a quartz crystl balance that is located near the honeycomb substrate. The PVD sample is sequentially deposited using the following sequence:<maths id="math0006"><math display="block"><mrow><mtext>Mo(72nm)/V(19nm)/Te(36 nm)/Co(10nm)</mtext></mrow></math><img file="EP1598110A1_D0006.tif" /></maths>

0217The PVD sample is calcined in a quartz tube. The quartz tube is placed in an oven, at ambient temperature, with a 100 cc/min flow of air through the tube, the furnace is heated from ambient temperature to 275°C at 10°C/min and is held there for one hour; using a 100 cc/min flow of argon through the tube, the oven is heated from 275°C to 600°C at 2°C/min and is held there for two hours.

0218The calcined PVD sample is evaluated in a fixed bed reactor at a contact time of 50 milliseconds and with a feed of 7 % propane, 22% steam and the balance air. Consistent results are reproducible with mass balances within 98 - 102 % and are essentially equivalent to those of Example 1/Sample 1.

Example 8

0219The sequential PVD of Mo, V, Te and Pd on a honeycomb support is performed in a PVD system with a base pressure of 5×10^-7 Torr. The metal sources are made by melting individual metal powders into different crucibles. The PVD system is equipped with four pockets that house four crucibles containing Mo, V, Te and Pd, respectively. During deposition the individual metal source is heated by electron beam, and the deposition rate (typically a few nanometers per minute) is monitored using a quartz crystal balance that is located near the honeycomb substrate. The PVD sample is sequentially deposited using the following sequence:<maths id="math0007"><math display="block"><mrow><mtext>Mo(72nm)/V(19nm)/Te(36 nm)/Pd(10nm)</mtext></mrow></math><img file="EP1598110A1_D0007.tif" /></maths>

0220The PVD sample is calcined in a quartz tube. The quartz tube is placed in an oven, at ambient temperature, with a 100 cc/min flow of air through the tube, the furnace is heated from ambient temperature to 275°C at 10°C/min and is held there for one hour; using a 100 cc/min flow of argon through the tube, the oven is heated from 275°C to 600°C at 2°C/min and is held there for two hours.

0221The calcined PVD sample is evaluated in a fixed bed reactor at a contact time of 50 milliseconds and with a feed of 7 % propane, 22% steam and the balance air. Consistent results are reproducible with mass balances within 98 - 102 % and are essentially equivalent to those of Example 1/Sample 1.

Example 9

0222The sequential PVD of Mo, V, Te and In on a honeycomb support is performed in a PVD system with a base pressure of 5×10^-7 Torr. The metal sources are made by melting individual metal powders into different crucibles. The PVD system is equipped with four pockets that house four crucibles containing Mo, V, Te and In, respectively. During deposition the individual metal source is heated by electron beam, and the deposition rate (typically a few nanometers per minute) is monitored using a quartz crystl balance that is located near the honeycomb substrate. The PVD sample is sequentially deposited using the following sequence:<maths id="math0008"><math display="block"><mrow><mtext>Mo(72nm)/V(19nm)/Te(36 nm)/In(10nm)</mtext></mrow></math><img file="EP1598110A1_D0008.tif" /></maths>

0223The PVD sample is calcined in a quartz tube. The quartz tube is placed in an oven, at ambient temperature, with a 100 cc/min flow of air through the tube, the furnace is heated from ambient temperature to 275°C at 10°C/min and is held there for one hour; using a 100 cc/min flow of argon through the tube, the oven is heated from 275°C to 600°C at 2°C/min and is held there for two hours.

0224The calcined PVD sample is evaluated in a fixed bed reactor at a contact time of 50 milliseconds and with a feed of 7 % propane, 22% steam and the balance air. Consistent results are reproducible with mass balances within 98 - 102 % and are essentially equivalent to those of Example 1/Sample 1.

Example 10

0225The sequential PVD of Mo, V, Te and Sb on a honeycomb support is performed in a PVD system with a base pressure of 5×10^-7 Torr. The metal sources are made by melting individual metal powders into different crucibles. The PVD system is equipped with four pockets that house four crucibles containing Mo, V, Te and Sb, respectively. During deposition the individual metal source is heated by electron beam, and the deposition rate (typically a few nanometers per minute) is monitored using a quartz crystl balance that is located near the honeycomb substrate. The PVD sample is sequentially deposited using the following sequence:<maths id="math0009"><math display="block"><mrow><mtext>Mo(72nm)/V(19nm)/Te(36 nm)/Sb(10nm)</mtext></mrow></math><img file="EP1598110A1_D0009.tif" /></maths>

0226The PVD sample is calcined in a quartz tube. The quartz tube is placed in an oven, at ambient temperature, with a 100 cc/min flow of air through the tube, the furnace is heated from ambient temperature to 275°C at 10°C/min and is held there for one hour; using a 100 cc/min flow of argon through the tube, the oven is heated from 275°C to 600°C at 2°C/min and is held there for two hours.

0227The calcined PVD sample is evaluated in a fixed bed reactor at a contact time of 50 milliseconds and with a feed of 7 % propane, 22% steam and the balance air. Consistent results are reproducible with mass balances within 98 - 102 % and are essentially equivalent to those of Example 1/Sample 1.

Example 11

0228The sequential PVD of Mo, V, Te and Al on a honeycomb support is performed in a PVD system with a base pressure of 5×10^-7 Torr. The metal sources are made by melting individual metal powders into different crucibles. The PVD system is equipped with four pockets that house four crucibles containing Mo, V, Te and Al, respectively. During deposition the individual metal source is heated by electron beam, and the deposition rate (typically a few nanometers per minute) is monitored using a quartz crystl balance that is located near the honeycomb substrate. The PVD sample is sequentially deposited using the following sequence:<maths id="math0010"><math display="block"><mrow><mtext>Mo(72nm)/V(19nm)/Te(36 nm)/Al(10nm)</mtext></mrow></math><img file="EP1598110A1_D0010.tif" /></maths>

0229The PVD sample is calcined in a quartz tube. The quartz tube is placed in an oven, at ambient temperature, with a 100 cc/min flow of air through the tube, the furnace is heated from ambient temperature to 275°C at 10°C/min and is held there for one hour; using a 100 cc/min flow of argon through the tube, the oven is heated from 275°C to 600°C at 2°C/min and is held there for two hours.

0230The calcined PVD sample is evaluated in a fixed bed reactor at a contact time of 50 milliseconds and with a feed of 7 % propane, 22% steam and the balance air. Consistent results are reproducible with mass balances within 98 - 102 % and are essentially equivalent to those of Example 1/Sample 1.

0231While the invention has been described in conjunction with the specific embodiments set forth above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alterations, modifications and variations as fall within the spirit and broad scope of the appended claims.