Niobium-doped vanadium/phosphorus mixed oxide catalyst

The invention relates to a process for the preparation of vanadium-phosphorus mixed oxide catalysts containing niobium (Nb) as an activator, which is used as catalysts for the production of maleic anhydride by the selective oxidation of n-butane, a catalyst obtainable by said method, and a method for the preparation of anhydride maleic acid using said catalyst.

[0002] Maleic anhydride is a well known and versatile intermediate for the production of unsaturated polyester resins, chemical intermediates such as butanediol and tetrahydrofuran, pharmaceuticals and agrochemicals. It is produced by partial oxidation of aromatic (e.g. benzene) or non-aromatic (e.g. n-butane) hydrocarbons. Oxidation is carried out in the gas phase, in the presence of a heterogeneous catalyst in a fixed, fluidized bed or riser reactor.

[0003] The main component of the catalyst for the oxidation of non-aromatic hydrocarbons, such as n-butane, to maleic anhydride is vanadyl pyrophosphate, (VO) 2P2O7, which is obtained by thermal treatment of vanadyl orthophosphate hemihydrate with formula (VO) HPO4 * 0.5H2O, acting as a catalyst precursor.

[0004] Conventional methods for producing the precursor require a reduction of the pentavalent vanadium compound under conditions that favor the formation of tetravalent vanadium (average oxidation state +4) and the reaction of tetravalent vanadium with phosphoric acid.

[0005] The prior art describes various manufacturing methods that generally require the use of vanadium pentoxide (V2O5) as a source of vanadium (see e.g. US 5,137,860 and [0006] EP 0 804 963 A1). One of the factors reducing V⁵⁺ to V⁴⁺ is an aqueous solution of hydrogen chloride. Organic reducing compounds of the primary and secondary aliphatic alcohols or aromatic alcohols such as isobutyl alcohol and benzyl alcohol are also used. Isobutyl alcohol is the most commonly used organic reducing agent because it combines optimal characteristics both as a solvent and in terms of redox properties, favoring a complete redox reaction to form a tetravalent vanadium, which reacts with phosphoric acid to form a precursor, i.e. vanadyl orthophosphate hemihydrate formula (VO) HPO4 * 0.5H2O.

[0007] Both vanadyl pyrophosphate and vanadyl orthophosphate hemihydrate may be modified by the addition of an activating element selected from the groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, VB, VIA, VIB and VIIIA of the periodic table, or by adding a mixture of these types of elements.

[0008] The patent literature claims that the catalytic performance of vanadyl pyrophosphate can be significantly improved by adding said elements. A comprehensive review of activators and their role has been described by GJ Hutchings in Appl. Catal., 1991, 72, 1-32, and in Stud. Surf. Sci. Catal. Preparation of Catalysts VI, (G. Poncelet et al., Eds.), Vol. 91, Elsevier Science, Amsterdam, 1995, p. 1.

[0009] The prior art mentions niobium among activators that improve the catalytic efficiency of vanadyl pyrophosphate, but the results obtained are not entirely satisfactory.

1. I. Mastuura, et al. (Catal. Today, 1996, 28, 133-138) co-precipitated V and Nb in aqueous solution and treated the residue with benzyl alcohol by heating the whole under reflux. The resulting solid product was activated in the presence of a reaction mixture containing air and n-butane. Nb-modified catalysts show higher activity, and the best results are obtained for high activator concentrations (atomic ratio V / Nb = 4).

2. PG Pries de Oliveira, et al. (Catal. Today, 2000, 57, 177-186) prepared the VPO precursor in isobutyl alcohol and introduced NbPO4 before nucleation of vanadyl orthophosphate hemihydrate. The catalyst precursor was activated in the reactor under a butane / air atmosphere. The addition of Nb reduces the time required to achieve a constant catalyst yield from 120 hours to 40 hours. Higher activity was noted for the catalyst with the addition of an activator compared to the catalyst without the addition of an activator. The best results were noted for high activator concentrations (atomic ratio V / Nb = 6.4).

3. AM Duarte de Farias et al. (J. Catal. 2002, 208, 238-246) dissolved niobium ethoxide in isobutyl alcohol and used it as a reducing agent to prepare the Nb-modified catalyst precursor. The precursor was activated under reaction conditions. The catalyst with the addition of an Nb activator (atomic ratio V / Nb = 100) has a higher activity compared to a VPO catalyst without the addition of an activator, however, the authors found that the selectivity for maleic anhydride is not improved after Nb doping.

4. R. Higgins, GJ Hutchings (US Patent 4,147,661 (1979), property of ICI Ltd.) prepared a catalyst with the addition of Nb activator in isobutyl alcohol using hydrogen chloride gas as a reducing agent. The patent uses large amounts of activator (atomic ratio V / Nb = 14) and activation is carried out in a test tube in the presence of an air / n-butane reaction mixture.

In summary, in the prior art, a positive Nb effect is obtained by using large amounts of activator (low atomic ratio V / Nb: references 1,2 and 4) and / or in the case of thermal processing of the precursor in the reactor to convert it to vanadyl pyrophosphate, using a n-butane / air mixture (references 1,2,3 and 4). This in turn requires a activation period catalyst during which the conversion of n-butane and the yield for maleic anhydride are far from optimal values, which is unfavorable from the point of view of commercial applications. Furthermore, according to the prior art, the positive Nb doping effect results in a more active catalyst, however, when small amounts of Nb are used (reference 3), the selectivity for maleic anhydride is not improved.

[0011] We have now found that a positive effect on both the catalyst activity and its selectivity towards maleic anhydride can be obtained by activating the VPO catalyst with very small amounts of Nb. A positive effect is obtained by combining a specific precursor production method with a specific heat treatment of the precursor to convert it into an active vanadyl pyrophosphate catalyst. Atomic ratio P / V in

The prepared mixture has an optimal value which is a function of the amount of Nb added. The precursor is prepared in an organic composition with the right composition, avoiding the use of hazardous or corrosive reducing agents such as HCl, which requires special construction materials. Another advantage of the present invention is that the thermal treatment of the precursor to convert it to an active catalyst vanadyl pyrophosphate is carried out outside the reactor, so the catalyst, after loading into the reactor, gives optimal catalytic efficiency from the very beginning.

[0012] According to the present invention, the precursor may conveniently be prepared according to the method described in patent application WO 00/72963 (Lonza SpA).

[0013] WO 00/72963 describes a method for producing a vanadium-phosphorus mixed oxide precursor in which the vanadium reducing agent, in the presence of a phosphorus source, is an organic medium that contains (a) isobutyl alcohol or a mixture of isobutyl alcohol and benzyl alcohol, and (b ) polyhydric alcohol in a weight ratio (a) to (b) of 99: 1 to 5:95. The most preferred polyhydric alcohols are C2-C4 alkanediols, 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol and 1,4-butanediol. A preferred alcohol mixture contains 5 to 30 mole% polyhydric alcohol relative to isobutyl alcohol.

[0014] WO 00/72963 discloses that the catalyst precursor, even after drying, contains a certain amount of organic compounds from the organic medium in which the reaction took place, which are difficult to remove. Those amounts of organic compounds that remain trapped in the precursor are a fundamental parameter that can positively affect the performance of the active catalyst obtained after heat treatment. WO 00/72963 describes a method of controlling the carbon content of a precursor of mixed vanadium-phosphorus oxide catalyst to provide a high quality catalyst precursor that when activated gives excellent conversion results of aromatic hydrocarbons to maleic anhydride.

[0015] We have now found that it is possible to further improve the efficiency of the catalyst by adding small amounts of Nb compounds or its salts to the mixture for the preparation of the catalyst precursor, the mixture containing a source of vanadium, a phosphorus source, an organic medium that is both a solvent and a reducing agent, and an additive selected from the group consisting of benzyl alcohol and polyhydric alcohols, which enables the heat treatment of the precursor in the presence of steam according to a method similar to that described in EP 0804963 A1 (property of Lonz).

[0016] EP 0804963 A1 describes calcination and activation following the steps:

a) pre-heating the catalyst precursor from room temperature to a temperature not exceeding 250 ⁰C

b. further heating at atmospheric pressure greater than about 200 ⁰C up to a temperature of at least 380 ⁰C to 600 ⁰C

c) maintaining the temperature reached in step b) above atmospheric pressure, and

d) cooling the activated catalyst.

[0017] In the event that after the precalcination step from 200 to 330 ⁰C, the calcination and activation procedure will be carried out, in particular in points b) to d), the final catalyst is characterized

EP 1 633 482 increased activity with respect to the same catalyst prepared without Nb. When a different type of thermal treatment is used than that described in EP 0804963 A1, no positive effect on the catalytic performance is observed, or a negative effect is observed.

[0018] According to the present invention, it is possible to make the most of the positive effect of Nb on the catalytic efficiency of vanadyl pyrophosphate, when the calcination and activation of the precursor is carried out under specific conditions outside the reactor, where the activated catalyst after loading into the reactor shows optimal catalytic performance from the very beginning . This is a significant convenience compared to the prior art, due to the omission of the calcination period and activation inside the reactor (at a loss of efficiency) and due to the optimal catalytic efficiency (conversion of n-butane to form maleic anhydride) from the very beginning.

[0019] As a source of vanadium, tetravalent or pentavalent vanadium compounds are used. Typical examples of these, but not limited to, include vanadium tetrachloride (VCl4), vanadium oxybromide (VOBr3), vanadium pentoxide (V2O5), vanadyl phosphate (VOPO4 * nH2O), and vanadium tetroxide. Vanadium pentoxide is the preferred source of vanadium.

[0020] The source of niobium can be all available salts and compounds such as NbCl5, oxohydrate

Nb or niobium ammonium oxalate complex.

[0021] In addition to Nb, which is the activator selected according to the present application, the precursor may be accompanied by elements acting as an activator and selected from groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, VB, VIA, VIB and VIIIA periodic table, or a mixture of these elements.

[0022] Preferred additional activating elements are selected from the group consisting of zirconium, bismuth, lithium, molybdenum, boron, zinc, titanium, iron and nickel.

[0023] Orthophosphoric acid (H3PO4) is a preferred source of phosphorus.

[0024] A preferred organic medium that simultaneously acts as a solvent and reducing agent as described in patent application WO 00/72963 includes (a) isobutyl alcohol or a mixture of isobutyl alcohol and benzyl alcohol, and (b) polyhydric alcohol by weight (a ) to (b) from 99: 1 to 5:95. The most preferred polyhydric alcohols are C2-4 alkanediols, 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol and 1,4-butanediol. A preferred alcohol mixture contains 5 to 30 mole% polyhydric alcohol based on isobutyl alcohol.

[0025] In a preferred embodiment, the vanadium source, together with the phosphorus source, is suspended in an organic medium and the whole is mixed at a temperature of 90 to 200 ⁰C, more preferably from 100 to 150 ⁰C for a period of 1 hour to 24 hours.

[0026] The ratio of the niobium source to the vanadium source is such that the V / Nb atomic ratio is in the range from 250: 1 to 60: 1.

[0027] The ratio of vanadium source to phosphorus source in the prepared mixture is preferably such that the P / V atomic ratio ranges from 1: 1 to 1.8: 1, more preferably 1.1: 1 to 1.6: 1. Using a V / Nb atomic ratio lower than 100, better results are obtained when the ratio of vanadium source to phosphorus source is such that the P / V atomic ratio in the prepared mixture is in the range from 1.3: 1 to 1.6: 1.

EP 1 633 482 [0028] After precipitation, the vanadyl orthophosphate precursor is filtered off, washed and then dried, preferably at a temperature of 120 to 200 ⁰C, and precalcines at temperatures from 200 to 330 ⁰C.

[0029] In a preferred embodiment, the vanadyl orthophosphate precursor may be described by the formula (VO) HPO4 * aH2O * MmPpOy, where M is Nb and optionally one element acting as an activator selected from the groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, VB, VIA, VIB and VIIIA of the periodic table, or a mixture of these elements, and is a number from 0.3 to 0.7, m is a number from 0.004 to 0.017, p is a number from 0.004 to 0.017 and y corresponds to the amount of oxygen needed to ensure the requirements arising from the valence of all elements present in the compound.

[0030] The precalcined precursor, before activation treatment, can be formed into a shape suitable for final use. These types of procedures can include wet milling to a specific particle size, the introduction of abrasion resistance additives, and the formation of the right shape. The microspheres that are most convenient for use as a fluidized bed catalyst can be obtained by spray drying as described, for example, in US 4,654,425. For fixed bed reactors, the catalyst can be formed to the desired shape by pelleting or extrusion.

[0031] Further transformation of the precalcined precursor to the active catalyst is carried out by a heat treatment similar to that described in document EP 0804963 A1, where (a) heating under increased pressure in an atmosphere containing steam to a temperature of 380 to 600 ⁰C, (b) maintaining the temperature obtained in step (a) under increased pressure, and (c) cooling the activated catalyst. Such thermal treatment is preferably carried out outside the reactor used to produce maleic anhydride, in a fluidized bed or furnace.

[0032] The product obtained after activation has the structure of an active catalyst, vanadyl pyrophosphate, ready to be introduced into the reactor and used for the conversion of non-aromatic hydrocarbons, such as butane, to maleic anhydride.

[0033] Such processes are well known in the art, e.g. from US 4,594,433, US 5,137,860 or US 4,668,652.

[0034] A non-aromatic hydrocarbon, selected from C4-10 aliphatic hydrocarbons, preferably n-butane, is introduced into the reactor together with oxygen or oxygen-containing gas at a temperature from 320 to 500 ⁰C, and converts to maleic anhydride.

[0035] The conversion takes place in a fluidized bed or fixed bed reactor, preferably a fluidized bed reactor. The following examples are intended to illustrate the invention only and are not to be construed as limiting it.

[0036] In the examples below, the carbon content of the precursor was determined by burning in pure oxygen at high temperature using the apparatus and procedure described below, the resulting carbon dioxide was detected by infrared analysis.

Camera: ELTRA 900CS

Measuring range: 0.001 - 100% by weight C

Sensitivity: 0.0001% by weight C

Sample time: 90 s

Sample samples: 0.1 -0.5 g

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Furnace temperature: 400 - 1500 ⁰C

Oxygen purity: 99.5% min.

Oxygen flow rate: 4 L / min

Procedure:

The furnace was heated to 1330 ⁰C and the oxygen flow was opened 10 minutes before starting the analysis.

A detector for high carbon content was selected and calibrated using standard samples with known carbon content. The sample weight used was 150 ± 10 mg.

Example 1 (V / Nb atomic ratio 160) [0037] To a 30 L reactor with a fitted thermometer, mechanical stirrer and distillation column with a reflux condenser and a drier, 1.968 g V2O5, 65 g niobium ammonium oxalate and 2.484 g 100% H3PO4 ( atomic ratio P / V = ​​1.17), the whole suspended in 16,700 g of a mixture of isobutyl alcohol and 1,4-butanediol (80:20). The mixture was stirred and refluxed for 8 hours. The color of the mixture changed from red-brown to light green.

[0038] The mixture was cooled to room temperature, then filtered and washed with isobutyl alcohol. Then the solid residue was dried at 150⁰C and precalcined at 300 temperature ⁰C. The carbon content of the precalcined precursor was 1.7% by weight.

[0039] The solid residue was then treated in the milling and spray drying steps to obtain the typical spherical fluidized bed shape as described in Example 1 in US 4,654,425.

[0040] The solid residue was calcined in a hydrothermal fluidized bed as described in Example 4 in EP 0804963 A1.

Example 2 (atomic ratio V / Nb 200) [0041] The procedure of Example 1 was repeated, except that 52 g of niobiumammonium oxalate was used. The carbon content of the precalcined precursor was 1.8% by weight.

[0042] The solid residue was then treated in milling and spray drying steps to obtain the typical spherical fluidized bed shape as described in Example 1 in US 4,654,425.

[0043] The solid residue was calcined in a hydrothermal fluidized bed as described in Example 4 in EP 0804963 A1.

Example 3 (Atomic ratio V / Nb 120) [0044] The procedure of Example 1 was repeated, except that 87 g of niobiumammonium oxalate was used. The carbon content of the precalcined precursor was 1.7% by weight.

[0045] The solid residue was then treated in milling and spray drying steps to obtain the typical spherical fluidized bed shape as described in Example 1 in US 4,654,425.

EP 1 633 482 [0046] The solid residue was calcined in a hydrothermal fluidized bed as described in example 4 in EP 0 804 963 A1.

Comparative Example 1 (without Nb) [0047] The procedure of Example 1 was repeated, except that niobiumammonium oxalate was not used. The carbon content of the precalcined precursor was 1.8% by weight.

[0048] The solid residue was then treated in the milling and spray drying steps to obtain a typical spherical fluidized bed shape as described in Example 1 in US 4,654,425.

[0049] The solid residue was calcined in a hydrothermal fluidized bed as described in Example 4 in EP 0 804 963 A1.

Comparative Example 2 (without Nb, without 1,4-butanediol) [0050] The procedure of Example 1 was repeated, except that no niobium ammonium oxalate and 1,4-butanediol were introduced into the reactor. The carbon content of the precalcined precursor was 0.5% by weight.

[0051] The solid residue was then treated in the milling and spray drying steps to obtain a typical spherical fluidized bed shape as described in Example 1 in US 4,654,425.

[0052] The solid residue was calcined in a hydrothermal fluidized bed as described in example 4 in document EP 0 804 963 A1.

Comparative Example 3 (V / Nb atomic ratio 25) [0053] The procedure of Example 1 was repeated, except that 416 g of niobium ammonium oxalate was used. The carbon content of the precalcined precursor was 2.0% by weight. [0054] The solid residue was then treated in the milling and spray drying steps to obtain the typical spherical fluidized bed shape as described in Example 1 in US 4,654,425.

[0055] The solid residue was calcined in a hydrothermal fluidized bed as described in Example 4 in EP 0 804 963 A1.

Example 4 (atomic ratio V / Nb 80, P / V = ​​1.17) [0056] The procedure of Example 1 was repeated, except that 130 g of niobium ammonium oxalate was used. The carbon content of the precalcined precursor was 1.8% by weight. [0057] The solid residue was then treated in the milling and spray drying steps to obtain a typical spherical fluidized bed shape as described in Example 1 in US 4,654,425.

[0058] The solid residue was calcined in a hydrothermal fluidized bed as described in example 4 in document EP 0 804 963 A1.

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Example 5 (atomic ratio V / Nb 80, P / V = ​​1.46) [0059] The procedure of Example 4 was repeated, except that 3.092 g H3PO4 (100%) was used. The carbon content of the precalcined precursor was 2.0% by weight.

[0060] The solid residue was then treated in the milling and spray drying steps to obtain the typical spherical fluidized bed shape as described in Example 1 in US 4,654,425.

[0061] The solid residue was calcined in a hydrothermal fluidized bed as described in Example 4 in EP 0804963 A1.

Comparative Example 4 (without 1,4-butanediol) [0062] The procedure of Example 1 was repeated, except that no 1,4-butanediol was introduced into the reactor. The carbon content of the precalcined precursor was 0.5% by weight.

[0063] The solid residue was then treated in milling and spray drying steps to obtain the typical spherical fluidized bed shape as described in Example 1 in US 4,654,425.

[0064] The solid residue was calcined in a hydrothermal fluidized bed as described in Example 4 in EP 0804963 A1.

Comparative Example 5 (without hydrothermal treatment) [0065] The procedure of Example 1 was repeated. The carbon content of the precalcined precursor was 1.8% by weight.

[0066] The solid residue was then treated in milling and spray drying steps to obtain the typical spherical fluidized bed shape as described in Example 1 in US 4,654,425.

[0067] The solid residue was calcined according to the following procedure:

· Nitrogen atmosphere · 3 K / min.

· Final t = 550 ⁰C, maintained for 6 hours.

Catalytic suspension bed tests:

[0068] The catalysts prepared in Examples 1-5 and in Comparative Examples 1-5 were tested in metal fluidized bed reactors under the following conditions:

₃ · The catalyst was introduced into the reactor 1000 cm³ · Air flow rate 556 NL / h · N-butane feed

4.3% by volume 65 g / h · Pressure 3.0 bar (2.0 barg) [0069] The catalyst yields are summarized in Table 1.

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Table 1

<td>Example No.</td><td>t [⁰C]</td><td>Conversion [%]</td><td>MA yield [% by weight]</td><td>MA molar selectivity [%]</td>

<td> 1</td><td> 404</td><td> 81</td><td> 92</td><td> 67</td>

<td> 2</td><td> 420</td><td> 82</td><td> 89</td><td> 64</td>

<td> 3</td><td> 397</td><td> 81</td><td> 91</td><td> 66</td>

<td>Porówn.1</td><td> 422</td><td> 81</td><td> 85</td><td> 62</td>

<td>Porówn.2</td><td> 424</td><td> 80</td><td> 79</td><td> 59</td>

<td>Porówn.3</td><td> 370</td><td> 85</td><td> 49</td><td> 34</td>

<td> 4</td><td> 374</td><td> 81</td><td> 76</td><td> 56</td>

<td> 5</td><td> 395</td><td> 80</td><td> 89</td><td> 66</td>

<td>Porówn.4</td><td> 412</td><td> 80</td><td> 86</td><td> 64</td>

<td>Porówn.5</td><td> 401</td><td> 29</td><td> 28</td><td> 57</td>

[0070] Examples 4 and 5 show that in the presence of Nb at an atomic ratio lower than 100, when the P / V atomic ratio in the prepared mixture is lower than optimal, the catalyst performance is worse. In this case, the catalyst efficiency can be improved by feeding the reactor, together with air and butane, small amounts of phosphorus compounds such as organic phosphites or organic phosphates (this is well-known commercial practice usually used to maintain a constant catalyst performance over time). During the catalyst test of Example 4, after several days of observing unsatisfactory performance, triethyl phosphite was added to the reaction mixture introduced into the catalyst. After a few days, a yield of 86% by weight was obtained at 395⁰C at 80% butane conversion.

Example 6 (V / Nb atomic ratio 160, fixed bed) [0071] The procedure of example 1 was repeated for the preparation of the precacinated precursor. The carbon content of the precalcined precursor was 1.7% by weight.

[0072] The precalcined powdered precursor was mixed with 4% graphite and 4.8 x 4.8 mm round tablets were formed with a hole diameter of 1.7 mm. The precalcined precursor tablets were activated under hydrothermal conditions by heating from room temperature to 430⁰C in a controlled atmosphere: oxygen content was 13% at the beginning and 5% at the final stage, the steam content was about 50%.

Comparative Example 6 (V / Nb atomic ratio 45, fixed bed) [0073] The procedure of Example 6 was repeated, except that 51.5 g of niobiumammonium oxalate was used.

[0074] The carbon content of the precalcined precursor was 1.8% by weight.

[0075] The precalcined powdered precursor was molded and activated as described in Example 6.

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Comparative Example 7 (without Nb, without 1,4-butanediol, fixed bed) [0076] The procedure of Comparative Example 2 was repeated.

[0077] The carbon content of the precalcined precursor was 0.5% by weight.

[0078] The precalcined powdered precursor was molded and activated as described in Example 6.

Catalytic fixed bed tests:

[0079] The catalysts prepared in Example 6 and Comparative Examples 7 and 8 were tested in a fixed bed tubular reactor (h = 380 cm, ID = 2.1 cm) under the following conditions:

· Amount of catalyst loaded into the reactor 750 g · Air flow rate 2650 NL / h · N-butane cartridge

1.7% vol. 118 g / h · Catalyst yields are summarized in Table 2.

Table 2

<td>Example No.</td><td>t [⁰C]</td><td>Conversion [%]</td><td>MA yield [% by weight]</td><td>MA molar selectivity [%]</td>

<td> 6</td><td> 405</td><td> 80</td><td> 95</td><td> 70</td>

<td>Comp. 6</td><td> 400</td><td> 81</td><td> 88</td><td> 64</td>

<td>Comp. 7</td><td> 424</td><td> 76</td><td> 87</td><td> 68</td>

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