Microchannel Apparatus

0001The present invention relates to a microchannel apparatus within which are metal surfaces coated with a buffer layer and interfacial layer and optionally a catalytically active layer.

0002Hydrogen and hydrocarbon conversion reactions including such as steam reforming, water-gas shift reactions, methanol synthesis and catalytic combustion are well known. These reactions are usually carried out at temperatures between 150 and 1000°C. Currently these reactions are industrially run using catalyst pellets which consist of an active catalytic metal or metal oxide deposited on high surface area ceramic pellets.

0003Foam or monolith catalysts are known that have three layers (1) porous support, (2) interfacial layer, and (3) catalyst metal as described in [1]. In making these catalysts, the interfacial layer has been deposited by various methods including solution impregnation techniques. The catalyst layer may be deposited by solution impregnation techniques. The interfacial layer has greater surface area than the porous support whereas the porous support has greater mechanical strength than the interfacial layer.

0004The porous-support may be a metal or ceramic foam. Metal foams are highly thermally conductive and easy to machine. The sponge-like mechanical properties allow convenient sealing in a reaction chamber via mechanical contact. The closely matched thermal expansion between the metal foam and the housing reaction chamber minimizes cracking of the porous support and minimizes gas channeling around the porous support at higher reaction temperatures. Pestryakov <i>et al</i> prepared metal foam supported transition metal oxide catalysts with [1] and without [2] an intermediate gamma-alumina layer for the oxidation of n-butane. Kosak [3] examined several approaches to disperse precious metals on various metal foams where the surface was pre-etched with HCl solution, and reported that electroless deposition provides the best adhesion of precious metals to the foam supports. Podyacheva et al. [4] also synthesized foam metal supported LaCoO₃ perovskile catalyst with a porous alumina intermediate for methane oxidation. Despite all of the potential advantages with metal foam supported catalysts, metal foam has low corrosion resistance and its nonporous and smooth web surfaces have provided poor adhesion to ceramic materials, and these materials are prone to spalling of interfacial layers after thermal cycling because of the mismatch in thermal expansion.

0005In order to increase corrosion resistance, methods such as diffusion alloying with Al, Cr, and Si have been used to fabricate ferritic steels, which are typically used for the manufacturing of high temperature furnace elements (about 1200°C) [5]. When the aluminum containing ferritic steels are appropriately heat-treated, aluminum migrates to the alloy surface and forms a strongly adhering oxide film which is resistant to oxygen diffusion. Such ferritic steel foils have been used to fabricate metal monoliths with z10 ppi (pores per inch) open cells [6]. However, the search for the similar alloy foams with pores suitable for catalytic applications (&lt;20ppi, 80ppi preferred) has been fruitless. This has been attributed to both the immature methods for making the finer Al-ferritic steel foams and the lack of the alloy precursors for making the foams.

References

0006<ol id="ol0001" compact="compact"><li>1. <nplcit id="ncit0001" npl-type="s"><text>A.N. Pestryakov, A.A.Fyodorov, V.A.Shurov, M.S.Gaisinovich, and I.V.Fyodorova, React.Kinet.Catal.Lett., 53 [2] 347-352 (1994</text></nplcit>).</li><li>2. <nplcit id="ncit0002" npl-type="s"><text>A.N. Pestryakov, A.A.Fyodorov, M.S.Gaisinovich, V.P.Shurov, I.V.Fyodorova and T.A.Gubaykulina, React.Kinet.Catal.Lett., 54 [1] 167-172 (1995</text></nplcit>).</li><li>3. <nplcit id="ncit0003" npl-type="b"><text>J.R. Kosak. A Novel Fixed Bed Catalyst for the Direct Combination of H2 and O2 to H2O2, M.G.Scaros and M.L.Prunier, Eds., Catalysis of Organic Reactions, Marcel Dekker, Inc. (1995), p1 15-124</text></nplcit>.</li><li>4. <nplcit id="ncit0004" npl-type="s"><text>O.Y.Podyacheva, A.A.Ketov, Z.R.Ismagilov, V.A.Ushakov, A.Bos and H.J.Veringa, React.Kinet.Catal.Lett., 60 [2] 243-250 (1997</text></nplcit>).</li><li>5.<nplcit id="ncit0005" npl-type="s"><text> A.N.Leonov, O.L.Smorygo, and V.K.Sheleg, React. Kinet.Catal. Lett., 60 [2] 259-267 (1997</text></nplcit>).</li><li>6. <nplcit id="ncit0006" npl-type="b"><text>M.V.Twigg and D.E.Webster. Metal and Coated-Metal Catalysts, A Cybulski and J.A.Moulijn, Eds., Structured Catalysts and Reactors, Marcel Dekker, Inc. (1998), p59-90</text></nplcit>.</li></ol>

0007<patcit id="pcit0001" dnum="US3907708A"><text>US-A-3,907,708</text></patcit> discloses multi-layer supported catalysts comprising a metal support, a layer of metal aluminide, a layer of α-alumina and a catalytic surface layer.

0008Although not claimed in this application, the disclosure includes a catalyst that has at least three layers, (1) porous support, (2) buffer layer, (3) interfacial layer, and optionally (4) a catalytically active layer. In some embodiments, the buffer layer, which is disposed between the porous support and the interfacial layer, contains at least two compositionally different sublayers. The buffer layer typically provides a transition of thermal expansion coefficient from the porous support to the interfacial layer thereby reducing thermal expansion stress as the catalyst is heated to and cooled from high operating temperatures. The buffer layer also reduces corrosion and oxidation of the porous support, and minimizes side reactions catalyzed by the surface of the porous support.

0009A method for making the multi-layer catalyst (at least three layers) has the steps of (1) selecting a porous support, (2) depositing a buffer layer on the porous support, (3) depositing an interfacial layer thereon, and optionally (4) depositing a catalytically active layer onto or integral with the interfacial layer; wherein the buffer layer is disposed between the porous support and the interfacial layer. Better results can be obtained where the buffer layer is vapor deposited. The catalytically active layer can be deposited after or during the deposition of the interfacial layer.

0010Advantages of the porous support with a buffer layer and an interfacial layer, may include: better match of thermal expansion coefficients and better stability to temperature changes, reduction of side reactions such as coking, desired metal-oxide interactions, strong bonding to a high-surface-area interfacial layer, and enhanced protection of the underlying porous support.

0011The present invention relates to the coating of metal surfaces within microchannel apparatus with the aforementioned buffer and interfacial layers, rather than the use of a porous support.

0012In accordance with the present invention there is provided a microchannel apparatus in accordance with claim 1 hereinafter.

0013The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.

BRIEF DESCRIPTION OF THE DRAWINGS

0014<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is an enlarged cross section of a catalyst.</li><li><figref idref="f0001">FIG. 2a</figref> is a graph of weight gain (via oxidation) versus time for a stainless steel foam (top line) and a stainless steel foam coated with titania (bottom line) at 580°C (dotted line).</li><li><figref idref="f0002">FIG. 2b</figref> is a graph of weight gain (via oxidation) versus time for a nickel foam (top line) and a nickel foam coated with titania (bottom line) at 500°C.</li><li><figref idref="f0003">FIG. 3</figref> is a pair of photomicrographs comparing the effect of thermal cycling on a stainless steel foam that has a titania buffer layer and an alumina wash coat (left) and a stainless steel foam that has an alumina wash coat (no buffer layer, right).</li></ul>

0015A catalyst is depicted in <figref idref="f0001">FIG. 1</figref> having a porous support 100, a buffer layer 102, an interfacial layer 104, and, optionally, a catalyst layer 106. Any layer may be continuous or discontinuous as in the form of spots or dots, or in the form of a layer with gaps or holes.

0016The porous support 100 may be a porous ceramic or a metal foam. This catalyst is not strictly in accordance with the present invention which requires a microchannel apparatus having metal surfaces within (rather than a porous support) to which the buffer and interfacial layers are applied.

0017The buffer layer 102 has different composition and/or density than both the support and the interfacial layers, and preferably has a coefficient of thermal expansion that is intermediate the thermal expansion coefficients of the porous support and the interfacial layer. The buffer layer is a metal oxide. Applicants discovered that vapor-deposited layers are superior because they exhibit better adhesion and resist flaking even after several thermal cycles. More preferably, the buffer layer is Al₂O₃, TiO₂, SiO₂, and ZrO₂ or combinations thereof. More specifically, the Al₂O₃ is α-Al₂O₃, γ-Al₂O₃ and combinations thereof. α-Al₂O₃ is more preferred because of its excellent resistance to oxygen diffusion. Therefore, it is expected that resistance against high temperature oxidation can be improved with alumina coated on the porous support <b>100.</b> The buffer layer may also be formed of two or more compositionally different sublayers. When the porous support <b>100</b> is metal, for example a stainless steel foam, a preferred embodiment has a buffer layer <b>102</b> formed of two compositionally different sub-layers (not shown). The first sublayer (in contact with the porous support <b>100</b>) is preferably TiO₂ because it exhibits good adhesion to the porous metal support <b>100.</b> The second sublayer is preferably α-Al₂O₃ which is placed upon the TiO₂. In a preferred embodiment, the α-Al₂O₃ sublayer is a dense layer that provides excellent protection of the underlying metal surface. A less dense, high surface area alumina interfacial layer may then be deposited as support for a catalytically active layer.

0018Typically the porous support <b>100</b> has a thermal coefficient of expansion different from that of the interfacial layer <b>104.</b> Accordingly, for high temperature catalysis (T > 150 °C) a buffer layer <b>102</b> is needed to transition between the two coefficients of thermal expansion. The thermal expansion coefficient of the buffer layer can be tailored by controlling the composition to obtain an expansion coefficient that is compatible with the expansion coefficients of the porous support and Interfacial layers. Another advantage of the buffer layer <b>102</b> is that it provides resistance against side reactions such as coking or cracking caused by a bare metal foam surface. For chemical reactions which do not require large surface area supports such as catalytic combustion, the buffer layer <b>102</b> stabilizes the catalyst metal due to strong metal to metal-oxide interaction. In chemical reactions which require large surface area supports, the buffer layer <b>102</b> provides stronger bonding to the high surface area interfacial layer <b>104.</b> The buffer layer is free of openings and pin holes - this provides superior protection of the underlying support. Thus, the buffer layer is nonporous. The buffer layer is between 0.05 and 10 µm thick, more preferably, less than 5 µm thick. The buffer layer should exhibit thermal and chemical stability at elevated temperatures.

0019The interfacial layer 104 is comprised of metal oxide. The interfacial layer provides high surface area and/or provides a desirable catalyst-support interaction for supported catalysts. Examples of metal oxides include, but are not limited, to γ-Al₂O₃, SiO₂, ZrO_z, TiO₂, tungsten oxide, magnesium oxide, vanadium oxide, chromium oxide, manganese oxide, iron oxide, nickel oxide, cobalt oxide, copper oxide, zinc oxide, molybdenum oxide, tin oxide, calcium oxide, aluminum oxide, lanthanum series oxide(s), zeolite(s) and combinations thereof. The interfacial layer <b>104</b> may serve as a catalytically active layer without any further catalytically active material deposited thereon. Usually, however, the interfacial layer <b>104</b> is used in combination with catalytically active layer <b>106.</b> The interfacial layer may also be formed of two or more compositionally different sublayers. Preferably, the interfacial layer thickness ranges from about 0.5 to about 100 µm, more preferably from about 1 to about 50 µm. The interfacial layer can be either crystalline or amorphous and has a BET surface area of at least 1 m²/g.

0020The catalytically active material <b>106</b> (when present) can be deposited on the interfacial layer <b>104.</b> Alternatively, a catalytically active material can be simultaneously deposited with the interfacial layer. The catalytically active layer (when present) is typically intimately dispersed on the interfacial layer. That the catalytically active layer is "disposed on" or "deposited or" the interfacial layer includes the conventional understanding that microscopic catalytically active particles are dispersed: on the support layer (i.e., interfacial layer) surface, in crevices in the support layer, and in open pores in the support layer. The catalytically active layer may Include: catalyst metals, Including but not limited to noble metal, transition metal and combinations thereof; metal oxides, including but not limited to, oxides of alkali elements, alkaline earth elements, boron, gallium, germanium, arsenic, selenium, tellurium, thallium, lead, bismuth, polonium, magnesium, titanium, vanadium, chromium, manganese, iron, nickel, cobalt, copper, zinc, zirconium, molybdenum, tin, calcium, aluminum, silicon, lanthanum series element(s), and combinations thereof; composites; zeolite(s); nitrides; carbides; sulfides: halides; phosphates; and combinations of any of the above.

0021Catalysts hereinbefore described can also be characterized by the properties they exhibit. Factors that can be controlled to affect these properties include: selection of the porous support, buffer, interfacial, and catalytically active layers; gradation of thermal expansion coefficients, crystallinity, metal-support interactions, deposition techniques and other factors as are apparent in view of the descriptions herein. Use of a buffer layer combined with routine experimentation utilizing these factors allows the production of catalysts for catalyzing a variety of chemical reactions. Such catalysts exhibit one or more of the following properties: (1) adhesion - after 3 thermal cycles in air, the catalyst exhibits less than 2% (by area) of flaking as viewed by SEM (scanning electron microscope) analysis; (2) oxidation resistance. After heating at 580°C in air for 2500 minutes, the catalyst increases in weight by less than 5%, more preferably less than 3%; still more preferably, after heating at 750°C in air for 1500 minutes, the catalyst increases in weight by less than 0.5%. Weight gain is measured by thermal gravity analysis (TGA). Each thermal cycle consists of heating from room temperature to 600°C in air at a heating rate of 10°C/min, maintaining the temperature at 600°C for 3000 minutes, and cooling at a rate of 10°C/min. The catalyst preferably has a surface area, as measured by BET, of greater than about 0.5 m²/g, more preferably greater than about 2.0 m²/g.

0022The disclosure further provides a catalytic process comprising passage of at least one reactant into a reaction chamber comprising the catalyst, conversion of said at least one reactant into at least one product, and passage of the product out of the reaction chamber. In a preferred embodiment, the catalytic process is conducted in an apparatus having microchannels. Examples of suitable microchannel apparatus and various process related factors are described in <patcit id="pcit0002" dnum="US5611214A"><text>U.S. Patents Nos. 5,611,214</text></patcit>, <patcit id="pcit0003" dnum="US5811062A"><text>5,811,062</text></patcit>, <patcit id="pcit0004" dnum="US5534328A"><text>5,534,328</text></patcit>, <patcit id="pcit0005" dnum="US6200536B"><text>6,200,536</text></patcit>, <patcit id="pcit0006" dnum="US6129973A"><text>6,129,973</text></patcit>, <patcit id="pcit0007" dnum="US6451864B"><text>6,451,864</text></patcit>, <patcit id="pcit0008" dnum="US6540975B"><text>6,540,975</text></patcit>, <patcit id="pcit0009" dnum="US6616909B"><text>6,616,909</text></patcit>, <patcit id="pcit0010" dnum="US6488838B"><text>6,488,838 (filed Aug. 17, 1999</text></patcit>) and <patcit id="pcit0011" dnum="US6192596B"><text>6,192,596 (filed Mar. 8, 1999</text></patcit>).

0023In embodiments of the present invention the metal surfaces within microchannel apparatus can be coated with both the buffer and the interfacial layers. This is done by vapor deposition. Preferred coating materials include titania and and 5-10% SiO₂/Al₂O₃. The interior surfaces of the reaction chamber, heat exchanger and other surfaces of microchannel apparatus may be coated. In some embodiments, the walls of a reaction chamber can be coated with an optional buffer layer, an interfacial layer, and a catalytically active material - typically the catalytically active material and the interfacial layer combine to form a supported catalyst. Coatings can also be applied to metal walls in tubes and pipes that form connections to or within microchannel apparatus.

0024Catalytic processes which may be conducted include: acetylation, addition reactions, alkyiation, dealkylation, hydrodealkylation, reductive alkylation, amination, aromatization, arylation, autothermal reforming, carbonylation, decarbonylation, reductive carbonylation, carboxylation, reductive carboxylation, reductive coupling, condensation, cracking, hydrocracking, cyclization, cyclooligome zation, dehalogenation, dimerization, epoxidation, estehfication, exchange, Fischer-Tropsch, halogenation, hydrohalogenation, homologation, hydration, dehydration, hydrogenation, dehydrogenation, hydrocarboxylation, hydroformylation, hydrogenolysis, hydrometallation, hydrosilation, hydrolysis, hydrotreating, hydrodesulferization/hydrodenitrogenation (HDS/HDN), isomerization, methanol synthesis, methylation, demethylation, metathesis, nitration, oxidation, partial oxidation, polymerization, reduction, steam and carbon dioxide reforming, sulfonation, telomerization, transesterification, trimerization, water gas shift (WGS), and reverse water gas shift (RWGS).

0025The method of making the catalyst has the steps of selecting a porous support <b>100,</b> depositing a buffer layer <b>102</b> on the porous support <b>100</b> and depositing an interfacial layer <b>104</b> thereover. Optionally a catalyst layer <b>106</b> may be deposited onto the interfacial layer <b>104</b> or both the interfacial layer and the catalyst layer may be simultaneously deposited on the buffer layer <b>102.</b>

0026Because metal has web surfaces that are nonporous and smooth, deposition of the buffer layer may be impeded. One way to mitigate this problem is to rough the metal surface via chemical etching. The adhesion of high surface area gamma-alumina supported metal catalysts to metal foam is significantly Improved when metal foam is roughed via chemical etching using mineral acid solutions, for example 0.1 to 1 M HCl. Roughed web surface also shows improved resistance to the spalling of catalyst layer under thermal cyclings. In a preferred embodiment, wherein a metal foam is used as the porous support <b>100,</b> the metal foam is etched prior to vapor depositing the buffer layer <b>102.</b> Etching is preferably with an acid, for example HCl.

0027Deposition of the buffer layer <b>102</b> is by vapor deposition including but not limited to chemical vapor deposition, physical vapor deposition or combinations thereof. Surprisingly, it has been found that vapor deposition, which is typically conducted at high temperatures, results in polycrystalline or amorphous phases that provide good adhesion of the buffer layer to the surface of the porous support. The method is particularly advantageous for adhering a metal oxide buffer layer to a metal porous support.

0028The interfacial layer 104 is formed by solution deposition using precursors as are known for these techniques. Suitable precursors include organometallic compounds, halides, carbonyls, acetonates, acetates, metals, colloidal dispersions of metal oxides, nitrates, slurries, etc. For example, a porous alumina interfacial layer can be wash-coated with PQ alumina (Nyacol Products, Ashland, MA) colloidal dispersion followed by drying in a vacuum oven overnight and calcining at 500°C for 2 hours.

0029The catalytically active material can be deposited by any suitable method. For example, catalyst precursors can be deposited on colloidal metal oxide particles and slurry coated on a buffer-coated porous support, then dried and reduced.

Example 1

0030An experiment was conducted to demonstrate certain advantages of the buffer layer which is used in the microchannel apparatus of the present invention.

0031An unetched stainless steel foam (Astromet, Cincinnati OH) was coated with 1000 Angstroms TiO₂ via chemical vapor deposition. Titanium isopropxide (Strem Chemical, Newburyport, MA) was vapor deposited at a temperature ranging from 250 to 800°C at a pressure of 13 to 13,000 Pa (0.1 to 100 torr). Titania coatings with excellent adhesion to the foam were obtained at a deposition temperature of 600°C and a reactor pressure of 400 Pa (3 torr).

0032SEM (scanning electron microscope) analysis showed that the stainless steel foam supported gamma-alumina with a TiO₂ buffer layer did not show spalling after several (3) thermal cycles from room temperature to 600°C. In a control experiment with a stainless steel foam support coated with gamma-alumina without the TiO₂ buffer layer, severe flaking or spalling of the gamma alumina under the identical testing conditions was observed. Resistance to high temperature oxidation is shown in <figref idref="f0001">figs. 2a</figref> and <figref idref="f0002">2b</figref> As can be seen in the <figref idref="f0001">Fig. 2a</figref>, uncoated steel foam rapidly oxidized (as shown by the weight gain, i.e., thermal gravity, values) while the titania coated steel oxidized relatively slowly. As can be seen in the <figref idref="f0002">Fig. 2b</figref>, uncoated nickel foam oxidized, while, under the same conditions, the titania coated nickel foam showed zero (i.e., undetectable levels of) oxidation.