WO2003028881A1

WO2003028881A1 - Processes for selective oxidative dehydrogenation of alkanes

Info

Publication number: WO2003028881A1
Application number: PCT/US2002/030554
Authority: WO
Inventors: Billy B. Bardin; Robin R. Hammond
Original assignee: Union Carbide Chemicals & Plastics Technology Corporation
Priority date: 2001-10-04
Filing date: 2002-09-25
Publication date: 2003-04-10

Abstract

Selective oxidative dehydrogenation of alkanes to produce olefins is accomplished using certain polyoxometalate catalysts. The polyoxometalate catalysts can provide a selectivity to the olefin of at least 50 percent.

Description

PROCESSES FOR SELECTIVE OXIDATINE DEHYDROGEΝATIOΝ OF ALKAΝES

The present invention relates to processes for the selective oxidative dehydrogenation of alkanes to produce olefins using certain polyoxometalate catalysts.

Polyoxometalates and heteropolyacids and their preparation are described by Pope et al., "Heteropoly and Isopoly Oxometalates," Springer-Nerlag, New York (1983).

Polyoxometalates consist of a polyhedral cage structure, either in an isopoly or a heteropoly arrangement ([M_mO_y]^p" or [X_xM_mO_y]^p", respectively) bearing a negative charge which is balanced by cations that are external to the cage. If the cations are protons, then the compound is an isopolyacid (IP A) or in the case of a compound with a heteroatom (for example, H₃[PW₁₂O₄₀]), a heteropolyacid (HP A). If the cations are not all hydrogen, but either metals such as an alkali metal, such as potassium, sodium, or lithium, as in K₃P ₁₂ O₄o, or ammonium, as in (NH₄)₃PW₁₂O o, then it is referred to as a heteropolyacid salt or a polyoxometalate (POM) salt. The term "polyoxoanion" is also used in the art to describe the cage-like metal oxide structure constituting the primary building block of these compounds. For purposes of the present invention, these compounds are referred to as polyoxometalates and the term polyoxoanion ("POA") is reserved for describing the anionic cage-like portion of the compound (for example, [PW₁₂O₄o]^"3).

As described by Pope et al., supra, heteropolyacids and heteropolyoxometalates are cage-like structures with a primary, generally centrally located atom(s) surrounded by a cage framework, that contains a plurality of metal atoms, bonded to oxygen atoms. The central element of heteropolyacids and polyoxometalates is different from metal atoms of the framework and is sometimes referred to as the "hetero" element or atom; the condensed coordination elements are referred to as the "framework" or addenda atoms. The framework metal atoms are ordinarily transition metals. As described by Pope et al, supra, the majority of heteropolyacids and polyoxometalates have a centrally located heteroatom ("X") usually bonded in a tetrahedral fashion through four oxygen atoms to the "framework" metals ("M "). The framework metals, in turn, (i) are usually bonded to the central atom in an octahedral fashion through oxygens ("O"), and (ii) are bonded to four other framework metals through oxygen atoms and (iii) have a sixth non-bridging oxygen atom known as the "terminal oxygen" atom.

The principal framework metal, M, is effectively limited to only a handful of metals including molybdenum, tungsten, vanadium, niobium and tantalum or mixtures thereof. According to Pope et al., supra, this is due to the necessary condition that suitable metals have an appropriate cation radius and ionic charge, and have the ability to facilitate good dπ- pπ interactions. Among the successful candidates, molybdenum and tungsten share a common feature; namely, the expansion of valences of their metal cations from four to six. The coincidence of these characteristics allows these metals to form stable heteropolyacids and polyoxometalates. In recent years, other elements have been successfully substituted into polyoxoanion structures primarily formed from the previously listed addenda atoms. Okuara et al., Adv. Catal.. 41, 113, (1996), report that titanium, zirconium, chromium, manganese, rhenium, iron, cobalt, rhodium, nickel, copper, zinc, technetium, gallium, indium, thallium, and lead have been substituted into polyoxoanions. Conventional heteropolyacids (and polyoxoanions thereof) can be described by the general formula H_e(X M_nO_y)^"e. In this formula, X, the heteroatom, is frequently phosphorus. However, other suitable heteroatoms can be found in every family of the periodic table except, at present, the noble gases. In addition to phosphorus, elements such as antimony, silicon, boron, germanium, and arsenic are most common. The subscript k is preferably 1, but can be as high as 5. The symbol M is molybdenum, tungsten, or vanadium and n will vary from 5-20. The subscript y is usually about 40, but can be as low as 18 or as high as 62. The notation e is the negative charge on the (X_kM_nO_y) polyoxoanion and will vary from case to case, but e is always the number of protons needed to balance the formula. In a typical such heteropolyacid, k=l, n=12 and y=40 as in H₃PMo₁₂O₄o and the polyoxometalate salt l^ PWnNO^.

As described in Pope et al., supra, heteropolyacids are known to exist in a variety of structures including the Keggin, Dawson and Anderson structures. The different structures correspond to the specific geometry of particular heteropolyacid compositions and vary according to the coordination chemistry and atomic radii of the metals present. Ellis et al., in U.S. 4,898,989, issued Feb. 6, 1990, disclosed framework-substituted heteropolyacids and polyoxometalates having improved activity for the conversion of alkanes to alcohols. The improvement in catalyst activity was achieved by replacing certain framework atoms M (and the oxygen atoms doubly bonded to them) with zinc or other transition metals or combinations thereof. This twelve-cornered polyhedron structure is the metal atom cage-like configuration of a typical Keggin ion heteropolyacid. Between any two metal atoms of the framework of the cage is an oxygen atom, and from each metal atom is also a doubly bonded oxygen. Each of the metal atoms is bonded through oxygen to the central metal atom. The structure of polyoxometalates of other kinds (for example, Dawson ions, Anderson ions) can have different polyhedral structures.

The M atoms that are replaced according to U.S. 4,898,989, supra, are the three metal atoms in a single triangular face, not just any metal atoms as would happen in a random replacement. Another way of characterizing the regioselective, triangular insertion of the substituted metal atoms ("M"'), is that the M¹ atoms are each joined to each other (through oxygen atoms).

A typical heteropolyacid has the formula H₃PMo₁₂O_4u. When three Mo=O units are replaced with, for example, iron (Fe), the resulting framework substituted heteropolyacid has the formula H₆PMo₉Fe₃O₃ . Thus, the general formula of the regioselectively substituted heteropolyacids described above becomes:

H_e(X_kM_π M'_mO_y)-^e where k is 1-5, n is 5-19, m is 1-3 and y is 18-61. In this formula, M' comprises zinc or any of the transition metals, namely the Group IIIA-NIII metals of the periodic table. Preferably the transition metal is from Group NIII or the first row of Group INA-NII, that is, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, platinum (Group NIII) or titanium, vanadium, chromium, managanese (INA-NII, first row). Among the more preferred M' metals are iron, manganese, vanadium and combinations of nickel and iron or other transition metal (s). The three M¹ atoms do not have to be the same. However, the three M' must be different than the three M atoms replaced.

Heteropolyacids can be fundamentally prepared by dissolving the desired metal oxides in water, adjusting the pH to approximately 1-2 with acid (for example, HC1) to provide the necessary H⁺ cations, and then evaporating water until the heteropolyacid precipitates. If polyoxometalate salt is desired, a salt such as KC1 is added. The polyoxometalate ordinarily precipitates without need for an evaporation step. The desired proportion of the metal oxides may vary somewhat from the theoretical amount required for the desired product. Polyoxometalates also may be prepared by mixing oxide precursors under acidic or basic conditions in boiling aqueous or organic solutions. Examples of suitable oxide precursors include metal containing organic or inorganic compounds such as ammonium metatungstate, ammonium heptamolybdate, molybdenum acetate dimer, and metal nitrates, chlorides or bromides. The polyoxometalates are recovered by precipitation or extraction. The final product is recovered, washed, and dried. Acid forms of these polyoxometalates can be prepared by redissolving the solid product and using ion exchange. The existence of the heteropolyacid structure is confirmed by their characteristic NMR and/or IR spectra, which, as explained in Pope et al., supra, are now known for various heteropolyacids. In addition, UN-vis spectroscopy can be used to verify formation of polyoxometalate structures.

U.S. 4,803,187, issued Feb. 7, 1989, disclosed how to prepare heteropolyacids and polyoxometalates with random substitution of framework metals, such as H (PMo₈N₄O₄o); K_ό (SiMoi iMnOsg) and K₅ (P i ιNO₄o). The latter, for example, may be prepared by dissolving 45.0 g of 12-tungstophosphoric acid, H₃(PW₁₂O₄o), in 105 ml of water. With stirring, the pH is adjusted to about 5.2 with potassium bicarbonate. The mixture is then heated to 70°C and 6.0 g of vanadyl sulfate (NOSO₄) in 15 ml water is added. The solution is cooled and KC1 is added to precipitate the K₅(PWπNO₄o) product. ). See also U.S. 5,510,308, especially for Cu and N substitution.

Heteropolyacids and polyoxometalates have been proposed for use in a variety of applications. In the area of catalysis, they have been used in connection with the oxidation of propylene and isobutylene to acrylic and methacrylic acids, oxidation of aromatic hydrocarbons; olefin polymerization; olefin epoxidation; and hydrodesulfurization processes.

The use of heteropolyacids and polyoxometalates for the catalytic air oxidation of alkanes to alcohols, such as butane to butanol, is also known. See, for example, M. Ai, "Partial Oxidation of n-Butane with Heteropoly Compound-based Catalysts," "Proceedings of the 18th International Congress on Catalysis, Berlin," 1984, Nerlag Chemie, Nol. 5, page 475. In addition, the use of heteropolyacids and polyoxometalates under mild reaction conditions for the liquid phase oxidation of alkanes has been disclosed. See, Lyons et al., U.S. 4,803,187, supra.

Heteropolyacids and polyoxometalates, methods of preparation, and methods of use have also been disclosed for oxidation of alkanes to alcohols. See, Lyons et al., U.S. 4,859,798, issued Aug. 22, 1989; Ellis et al., U.S. 4,898,989, supra; Lyons et al., U.S. 4,916,101, issued Apr. 10, 1990; Ellis et al., U.S. 5,091,354, issued Feb. 25, 1992; and Shaikh et al., U.S. 5,334,780, issued Aug. 2, 1994.

It has also been disclosed that substitution of Group NIII and other transition metals as framework elements in a heteropolyacid or polyoxometalate catalyst enhances catalytic oxidation activity for the oxidation of alkanes to alcohols. See, Ellis et al., U.S. 4,898,989, supra; and Ellis et al., U.S. 5,091,354, supra.

T. Jinbo et al., "Method for the Manufacture of Acroleic Acid or Acrylic Acid, and Catalysts Used Therein," Japanese Patent Application Public Disclosure No. H6-218286, Aug. 9, 1994, disclosed the conversion of propane to acrolein and/or acrylic acid catalyzed by extra-framework metal substituted heteropolyacids; that is, cation-exchanged heteropolyacids. A single framework mono-substituted heteropolyacid, H₄PMθπNO₄o, showed moderate selectivity for acrylic acid and a poor conversion rate.

G. Centi et al., "Selective Oxidation of Light Alkanes: Comparison between Vanadyl Pyrophosphate and N-Molybdophosphoric Acid," Catal.Sci.Technol., Proc. Tokyo Conf., 1st Meeting, 1990, 225-30, 227, disclosed that the randomly framework- substituted H₅PMo₁oN2θ₄o has been found to be more active than (NO)₂P₂O₇ for catalyzing oxidation of propane to acrylic acid.

Partially exchanged Cs-salts of heteropolyacids have been found to be more active than pure heteropolyacids for catalyzing oxidation of lower alkanes. Ν. Mizuno et al., "Catalytic Performance of Cs₂.₅Feo.os H₁.₂₆PNMo₁₁O₄o for Direct Oxidation of Lower Alkanes," J. Mol. CaτaL, A. 114, 309-317 (1996).

When a combination of the unsubstituted heteropolyacid, H₃PMo₁₂O₄o, and N₂O₅ - P₂O₅ is used to catalyze oxidation of propane to acrylic acid, this unsubstituted heteropolyacid is disclosed as enhancing the formation of acetic acid byproduct. M. Ai, "Oxidation of Propane to Acrylic Acid," Catalysis Today. 13 (4), 679-684 (Eng.) (1992). N. Mizuno et al, Applied Catalysis A: General 128, L165-L170 (1995), reported that Fe⁺³ or Ni⁺² exchange for H⁺ ; and N^1"5 mono-substitution for Mo⁺⁶ in Cs₂.₅ Ho.₅PMθι₂ O₄o enhanced the catalytic activity for direct oxidation of propane to acrylic acid. Of the catalysts tested, Cs₂.₅ Feo.os Ho.₅PMoπNO₄₀ gave the highest yield of acrylic acid. Ueda et al., Chemistry Letters. 541, 2 (1995), reported that propane was catalytically oxidized to acrylic acid and acetic acid with molecular oxygen over unsubstituted heteropolymolybdophosphoric acids which were treated with pyridine.

While the processes disclosed in the references noted above may be effective for the oxidation of alkanes to produce unsaturated oxygenated compounds, for example, acrylic acid, using polyoxometalate catalysts, such processes are not disclosed to be effective for the production of olefins, such as propylene.

Oxidative dehydrogenation of propane to yield propylene over fully protonated copper and vanadium substituted heteropolyacid catalysts was presented by Bardin et al., Applied Catalysis, 185, 283, (1995). Conversions of only approximately 1 percent were observed in this work. The catalysts did not exhibit high activities due to thermal instability after calcination above 400 °C. Mizuno et al, Applied Catalysis. 146, L249, 1996, and Mizuno et al, J. Catalysis, 178, 391-4, (1998), reported low temperature oxidative dehydrogenation of propane (380°C) over cesium and copper salts of heteropolyacids. The reported results show higher selectivity to carbon monoxide than to propylene.

Accordingly, processes are desired for the oxidative dehydrogenation of alkanes to selectively produce olefins, particularly relative to unsaturated oxygenated compounds and carbon oxides, using polyoxometalate catalysts.

In accordance with the present invention, processes and catalysts are provided for the selective oxidative dehydrogenation of alkanes to produce olefins relative to unsaturated oxygenated compounds, such as acrylic acid. The processes comprise contacting the alkane with an oxidizing agent, such as molecular oxygen, and a polyoxometalate catalyst having the formula:

C_aH_(e-az)(X_kM_m-xM'_xM"_nO_yr where cation C is selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, vanadium, chromium, lanthanum, manganese, iron, cobalt, ruthenium, copper, actinide metal, lanthanide metal, metal oxy ion, ammonium, tetraalkylammonium, pyridinium, quinolinium, protonated aromatic amines and protonated aliphatic amines, or combinations thereof; X is a Group IA, IIA, IIIA, IN A, NA, VIA, VILA or transition metal; M is molybdenum or tungsten or combinations thereof; M' is vanadium; M" is independently zinc or a transition metal different from M and M', or combination thereof; z is the charge on said cation C; a is the number of cations C; e is the charge of anion (X_kM_m-xM'_xM"_nO_y); k is 1 to 2, m is 5 to 18, n is 0 to 3; y is 18 to 62; and x is 0 to 10:

Quite surprisingly, it has been found in accordance with the present invention that by providing the catalyst with a molar ratio of M/M', for example, molybdenum/vanadium, of at least 12:1 that the selectivity of the reaction to the olefin can be at least about 50 percent. Figure 1 illustrates the electronic spin resonance of a catalyst suitable for use in accordance with the present invention.

The alkane starting materials suitable for use in accordance with the present invention include straight and branched-chain alkane compounds suitable for conversion to olefins. Preferred among these are light alkanes comprising three to ten carbon atoms. More preferred feedstocks for the process of the present invention comprise propane and isobutane, or mixtures thereof, which may be oxidized by the process of the present invention to form propylene and isobutene, respectively.

The feedstock may comprise a combination of alkanes, preferably C₃ -do alkanes. Typically, the feedstock will comprise from 5 to 80 mole percent of the alkanes based on the total moles of the feedstock. In addition, the purity of the starting material is not critical, although it is preferable to avoid the presence of compounds that may poison the catalyst. As a result, the feedstock may, in addition to the alkane or alkanes of interest, further comprise water, methane, inert gases, such as nitrogen or helium, as well as impurities such as, for example, carbon monoxide or carbon dioxide. Suitable oxidants for use in the process of the invention include, for example, air, molecular oxygen and other oxidants, such as nitrogen oxides. Preferred among these are air and molecular oxygen.

In one embodiment, the POM used in the processes of the invention comprises 9 to 11 atoms of a first framework metal selected from the group consisting of molydenum, tungsten and vanadium, and 2 to 3 atoms of a second framework metal such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, platinum, copper or zinc, which second metal is zinc or a transition metal different from the first framework metal. The second framework metals (M') are site-specific, regioselective substitutions wherein each M¹ is bound through an oxygen atom to another M'.

The central or hetero element, X, of the POM components of the catalyst useful in the process of the present invention is selected from the elements of Group IA, IIA, IIIA, IN A, NA, VIA, or VII A of the Periodic Table or from the transition elements; it may, for example, be phosphorus, silica, aluminum, germanium or the like.

The POM component used in the process of the invention may contain second framework metals that have been substituted into the framework thereof, replacing an equivalent number of the first framework metals. Such substituting metals may, for example, be titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, platinum, copper, zinc or combinations thereof. The second framework metal (M¹) is, by definition, different from the first framework metal (M). When there are more than one M' atoms, each M' is bound through an oxygen atom to another M'. The atoms which have been replaced in such substitution may be for example molybdenum, tungsten, vanadium or combinations thereof, as disclosed in Ellis and Lyons U.S. 4,898,989, supra. The number of framework atoms replaced may be from 1 to 3 or more, and the substituting metals, which are different from the replaced metal, may each be the same metal, for example iron, or may be different from each other, for example two or three different metal atoms; for example, one iron atom may replace one tungsten atom; two iron atoms may replace two tungsten atoms; three iron atoms may replace three tungsten atoms; two atoms, different from each other, for example iron and cobalt, may replace two tungsten atoms; three atoms, different from each other, for example iron, cobalt and nickel, may replace three tungsten atoms; two atoms of iron and one atom of cobalt may replace three tungsten atoms; and so on. Replacement of three framework atoms of a POM by three atoms, different from the framework atom, two of which replacing atoms are selected from the group consisting of iron, chromium, manganese or ruthenium, and the third of which is different from the two just referred to and is a transition metal, is disclosed by Lyons et al., U.S. 5,091,354.

In accordance with the present invention, the molar ratio of M/M' is at least 12: 1, preferably at least 24: 1 and more preferably at least 48: 1. Quite surprisingly, it has been found that in accordance with the present invention that by providing a molar ratio of MM' in the catalyst of at least 12:1 it is possible to obtain selectivities to the olefin of at least 50 percent, more preferably at least 60 percent, and most preferably at least 70 percent. As used herein, the term "selectivity" means the number of moles of olefin produced in the reaction divided by the number of moles of alkane reacted expressed as a percentage.

Two preferred catalysts for use in accordance with the present invention are those having the formulas:

Cs_aH_(e-az)(PM i2_-xM'_xO₄o)-^e

where M is molybdenum or tungsten, M' is vanadium, and x is 0 to 1 ; and

Cs_aFenH_(e-az-nq₎(PM _12-xM'_xO₄₀)^"e

where M is molybdenum or tungsten, M' is vanadium, Fe is iron, and x is 0 to 1 and q is the charge on Fe. In an especially preferred aspect of the invention x=0, that is, there is no vanadium in the catalyst. It is further preferred in accordance with the present invention that the catalyst has an electronic spin resonance pattern showing a peak at a g-factor of from 1.85 to 2.0. Electronic spin resonance (ESR) has been used to characterize the degree of reduction in the catalyst. This technique provides information on the electronic state of the catalyst. By using ESR, it has been discovered that the preferred selective catalysts have a pattern typical of the one shown in Figure 1. Non-selective catalysts have no major peak and therefore do not appear to be as reduced as the selective catalysts. From ESR analysis, it appears that calcining the catalyst at temperatures above about 470°C is one method of obtaining this degree of reduction. The data set forth in Figure 1 was generated using a Bruker Electron Paramagnetic

Resonance Spectrometer. The following parameters were used for data collection: microwave power = 2 mW, scan range 3480 ± 750 G, modulation amplitude = 15.0 G, and gain = 1.0 x 10⁵. Data analysis was performed using WTN-EPR software. Approximately 100 mg of sample was loaded and the spectrum was collected at ambient conditions. Figure 1 shows a typical ESR signature for an active and selective catalyst. Plotted on the ordinate is the intensity of the signal. The g-factor is plotted on the abscissa.

Preferably, the processes of the present invention are effective to inhibit the production of unsaturated oxygenated compounds, such as, for example, acrylic acid, acrolein, methacrolein, acetone, acetic acid, isobutyric acid, propionic acid, and acetaldehyde. Preferably, in accordance with the present invention the selectivity to the unsaturated oxygenated compounds is less than 50 percent, preferably less than 40 percent and more preferably less than 30 percent.

The temperature used in the process of the invention is that which favors the formation of olefins over unsaturated carboxylic acids as reaction products. The process is generally carried out at a temperature in the range from 225°C to 620°C. The preferred temperatures used in the process of the present invention are in the range of 450°C to 550°C, where alkane activation may occur more readily. The determination of the most desirable temperature for a given reaction and given catalyst within the scope of the invention is within the ability of the person skilled in the art.

The addition of water vapor to the reactant feed in the range of 0 to 50 mole percent, but preferably in the range of 5 to 20 percent, and more preferably 10 percent, slightly enhances the activity and selectivity of the catalyst.

The pressure used in the process of the invention is not critical. The process may be carried out at atmospheric pressure. Other pressures may be used, and the determination of the most desirable pressure for a given reaction within the scope of the invention is within the ability of the person skilled in the art.

The process of the invention may be carried out in any suitable reactor configuration. For example, the reaction may be performed in a fixed-bed, moving bed, fluidized bed reactor, or other suitable reactor as known by the person skilled in the art. The process of the invention is preferably carried out in the vapor phase. Preferably, the feedstock is an alkane gas. The reaction may be carried in the presence or absence of steam. An inert gas, such as nitrogen, argon, helium or the like, may also be used. When an inert, diluting gas is used in the process of the invention, determination of the molar ratio of alkane, oxidant, diluting gas and water (steam), if present, in the starting reaction gas mixture is within the ability of the skilled practitioner in the art. Determination of the gas space velocity used in the process of the invention is within the ability of the skilled practitioner in the art.

EXAMPLES The following examples are provided for illustrative purposes and are not intended to limit the scope of the claims which follow. All parts and percentages are molar unless otherwise indicated.

Example 1 A solution of H₃PMo₁ O₄o (9.34 g of solid in 100 ml of water) was titrated with a solution of CsNO₃ (2.79 g of solid in 100 ml of water) in the appropriate stoichiometric ratios to provide Cs₂.₈Ho.₂PMo₁₂O o. The solution was evaporated to dryness and the solid was collected and calcined at 500°C. The catalyst was tested under a propane feedstream consisting of 30 percent propane, 10 percent O₂ and 60 percent N₂ in a packed bed, stainless steel, down flow reactor system. Gas phase products were analyzed online by a Hewlett Packard 6890 gas chromatograph using PoraPlot U, KCl/Al₂O₃, and molecular sieve capillary columns in conjunction with a thermal conductivity detector. Liquid phase (oxygenated) products were collected at the base of the reactor tube in a condenser cooled to 5°C and analyzed off-line using a Hewlett Packard 5890 gas chromatograph equipped with an flame ionization detector and a Carbowax capillary column. Reaction temperatures ranged from 300°C to 500°C with an operating pressure of 50 psig. A temperature range of from 475°C to 495°C demonstrated the highest conversion of propane. The catalyst provided 84.5 percent selectivity to propene at 5 percent conversion and 72.7 percent selectivity to propene at 10 percent conversion. Approximately 61 percent selectivity to propene was achieved at 16 percent conversion. Approximately 5 percent selectivity to oxygenated products was observed. Example 2

A Cs₂Feo.₂7Ho.₂PMo₁₂O₄o catalyst was prepared in a similar manner to Example 1. Using the same reactor test conditions as listed in Example 1, selectivities to propylene of 82 and 66 percent were achieved at 5 and 13 percent conversion, respectively.

Although the invention has been described above with respect to specific aspects, those skilled in the art will recognize that other aspects are intended to be within the scope of the claims that follow.

Claims

WHAT IS CLAIMED IS:

1. A process for the selective oxidative dehydrogenation of alkanes to produce olefins, the process comprising contacting an alkane with an oxidizing agent and a polyoxometalate catalyst under reaction conditions, said polyoxometalate catalyst having the formula:

C_aH_(e-az)(X_kM _m-xM'_xM"_nO_y)-^e

where cation C is selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, vanadium, chromium, lanthanum, manganese, iron, cobalt, ruthenium, copper, actinide metal, lanthanide metal, metal oxy ion, ammonium, tetraalkylammonium, pyridinium, quinolinium, protonated aromatic amines and protonated aliphatic amines, or combinations thereof; X is a Group IA, IIA, IIIA, IVA, VA, VIA, VIIA, or transition metal; M is molybdenum or tungsten or combinations thereof; M' is vanadium; M" is independently zinc or a transition metal different from M and M', or combination thereof; z is the charge on said cation C; a is the number of cations C; e is the charge of anion (X_kM _m-xM'_xM"_nO_y); k is 1 to 2, m is 5 to 18, n is 0 to 3; y is 18 to 62; and x is 0 to 10:

characterized in that when x is greater than 0 the molar ratio of M/M' is at least 12:1, and effective to provide a selectivity to the olefin of at least about 50 percent.

2. The process of Claim 1 wherein the catalyst has the formula:

Cs_aH_(e-az)(PM _12-xM'_xO₄₀)^"e

where M is molybdenum or tungsten, M' is vanadium, and x is 0 to 1.

3. The process of Claim 1 wherein the catalyst has the formula:

Cs_alFe_a2H(e_-al-a2q)(PM _12-xM'_xO₄o)^"e

where M is molybdenum or tungsten, M' is vanadium, x is 0 to 1, q is the charge on Fe, and az equals al + a2q.

4. The process of Claim 1 wherein x is greater than 0 and the molar ratio of M/M' is at least 24:1.

5. The process of Claim 1 wherein x is greater than 0 and the molar ratio of M/M' is at least 48:1.

6. The process of Claim 1 wherein x = 0.

7. The process of Claim 1 wherein said cation is lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, lanthanum, ammonium, tetraalkylammonium, pyridinium, quinolinium, protonated aromatic amines or protonated aliphatic amines, or combinations thereof.

8. The process of Claim 1 wherein "a", the number of cations, is greater than or equal to 2 when z is 1.

9. The process of Claim 1 wherein the selectivity to the olefin is at least about 60 percent.

10. The process of Claim 1 wherein the catalyst has an electron spin resonance pattern showing a peak at a g-factor of from 1.85 to 2.0.

11. The process of Claim 10 wherein the catalyst has an electron spin resonance pattern according to FIG. 1.

12. The process of Claim 1 wherein the alkane has from three to ten carbon atoms per molecule.

13. The process of Claim 12 wherein the alkane is propane and the olefin is propylene.

14. The process of Claim 1 wherein x = 1 to 10.

15. The process of Claim 1 wherein the reaction conditions include a temperature of from 225°C to 600°C.

16. The process of Claim 15 wherein the reaction conditions include a temperature of from 450°C to 550°C.

17. The process of Claim 1 where the oxidizing agent is air, molecular oxygen or a nitrogen oxide.

18. The process of Claim 1 wherein the contacting is conducted in the presence of from lppm to 20 wt. percent water based on the total weight of the water and the alkane.

19. The process of Claim 1 wherein the contacting is conducted in the presence of from 0.5 to 10 wt. percent water based on the total weight of the water and the alkane.

20. A polyoxometalate catalyst having the formula:

C_aH_(e-a2)(X_kM _m-xM'_xM"_nO_y)-^e

where cation C is selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, vanadium, chromium, lanthanum, manganese, iron, cobalt, ruthenium, copper, actinide metal, lanthanide metal, metal oxy ion, ammonium, tetraalkylammonium, pyridinium, quinolinium, protonated aromatic amines and protonated aliphatic amines, or combinations thereof; X is a Group IIIA, IV A, VA, or transition metal; M is molybdenum or tungsten or combinations thereof; M' is vanadium; M" is independently zinc or a transition metal different from M and M', or combination thereof; z is the charge on said cation C; a is the number of cations C; e is the charge of anion (X_kM _m-xM'_xM"_nO_y); k is 1 to 2, m is 5 to 18, n is 0 to 3; y is 18 to 62; and x is 0 to 6: characterized in that the molar ratio of M/M' is at least 12:1.

21. The catalyst of Claim 20 wherein x is greater than 0 and the molar ratio of M/M' is at least 24:1.

22. The catalyst of Claim 20 wherein x is greater than 0 and the molar ratio of

M/M' is at least 48:1.

23. The catalyst of Claim 20 wherein x = 0.

24. A process for the selective oxidative dehydrogenation of alkanes to olefins, the process comprising contacting an alkane with an oxidizing agent and a polyoxometalate catalyst under reaction conditions sufficient to produce an olefin with a selectivity to the olefin of at least about 50 percent, said polyoxometalate catalyst having the formula:

C_aH_(e-az)(XkM _m-xM'_xM"_nO_y)^"e

where cation C is selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, vanadium, chromium, lanthanum, manganese, iron, cobalt, ruthenium, copper, actinide metal, lanthanide metal, metal oxy ion, ammonium, tetraalkylammonium, pyridinium, quinolinium, protonated aromatic amines and protonated aliphatic amines, or combinations thereof; X is a Group IA, IIA, IIIA, IVA, VA, VIA, VIIA, or transition metal; M is molybdenum or tungsten or combinations thereof; M' is vanadium; M" is independently zinc or a transition metal different from M and M', or combination thereof; z is the charge on said cation C; a is the number of cations C; e is the charge of anion (X_kM _m-xM'_xM"_nO_y); k is 1 to 2, m is 5 to 18, n is 0 to 3; y is 18 to 62; and x is 0 to 10;

characterized in that when x is greater than 0 the molar ratio of M/M' is at least 12:1.