WO2009106474A2

WO2009106474A2 - Phase-enriched movtenb mixed oxide catalyst and methods for the preparation thereof

Info

Publication number: WO2009106474A2
Application number: PCT/EP2009/051961
Authority: WO
Inventors: Olaf Timpe; Ayyamperumal Sakthivel; Annette Trunschke; Robert SCHLÖGL
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-02-25
Filing date: 2009-02-19
Publication date: 2009-09-03
Anticipated expiration: 2010-08-25
Also published as: WO2009106474A3; DE112009000404T5

Abstract

The present invention provides a method for preparing a metal oxide catalyst material comprising oxides of molybdenum (Mo), vanadium (V), tellurium (Te) and niobium (Nb), said process comprising the steps of (i) subjecting a catalyst precursor mixture comprising molybdenum (Mo), vanadium (V), Tellurium (Te) and niobium (Nb) to a calcination treatment in an oxygen-containing atmosphere at a temperature of 150 to 400°C to prepare a calcined catalyst material and (ii) subjecting the calcined catalyst material to a treatment at a pressure of at least 10MPa and a temperature of at least 400°C in the presence of an inert fluid phase to prepare a catalyst material comprising oxides of molybdenum (Mo), vanadium (V), Tellurium (Te) and niobium (Nb). This method results in catalyst materials having very high contents of crystallographic Ml phase and excellent propane conversion, rates and selectivities for acrylic acid in the oxidation of propane.

Description

Phase-enriched MoVTeNb mixed oxide catalyst and methods for the preparation thereof

FIELD OF THE INVENTION

The present: invention concerns a metal oxide catalyst (material) and methods for the preparation thereof, as well as its use in the oxidation of hydrocarbons . More specifically, the present invention relates to a catalyst (material) comprising oxides of Mo, V, Te, and Nb, said catalyst being strongly enriched in ¥.1 phase, and its use in the oxidation of hydrocarbons to e.g. unsaturated carboxylic acids such as acrylic acid and methacrylic acid.

BACKGROUND ART

Multi-metal oxides based on molybdenum, vanadium, tellurium and niobium with a nominal composition of approximately M^ol^v0.3^Te0.23^Nb0.125⁰X ^are known from US 5,380,933 and have been reported to achieve outstanding performance as catalyst in the oxidation of alkanes, in particular in the conversion of propane to acrylic acid. Despite their considerable chemical complexity, such materials normally essentially consist of two orthorhombic phases known as Ml and M2 [T. Ushikubo, K. Oshinxa, A. Kayou and M. Hatano, Studies in Surface Science and Catalysis 112 (1997) 473], Minority phases like (MOQ .93V0.07) 5^14 ' M0C3 and TeMθ5θ^g [M. Baca, A. Pigamo, J. ~L . Dubois and J. M. M. Millet, Topics in Catalysis 23 (2003) 39, J. M. Lopez Nieto, P. Botella, B. Solsona and J.M, Oliver, Catalysis Today 81 (2003) 87] may also form depending on the preparation method and the activation procedure applied. In the Ml phase (ICSD 55097 [Inorganic Crystal Struczure Database, Fachinformationszentrum (FIZ) Karlsruhe, Germany]), corner-linked MOg (M=Mo, V} octahedrons are assembled forming 6- and 7-membered rings, partially accommodating TeO units [P. DeSanto, D.J. Buttrey, R. K. Grasselli, CG. Lugmair, A. F. Volpe, B. H. Toby and T, Vogt _r Zeitschrift fur Kristallographie 219 (2004) 152, J. M. M. Millet, H. Roussel , A. Pigamo, J. L. Dubois and J. C. Jumas, Applied Catalysis A: General 232 (2002) 77, H. Tsuji, K. Oshima and Y. Koyasu, Chemistry of Materials 15 (2003) 2112, H. Murayama, D. Vi try, W. Ueda , G. Fuchs , M. Anne and J. L. Dubois, Applied Catalysis A: General 318 (2007) 137]. Niobium has been postulated to be exclusively located in the centre of a MO7 pentagonal bi-pyramidal unit sharing edges with the surrounding octahedrons [P. DeSanto, et al. ibid.; H. Murayama, et al., ibid.]. The (001) planes are congruently stacked along the [001] direction, forming a bronze-like structure similar to the structure of Csø _ 7 (Nb2 _, 7W2 3)Ol4 (ICSD 67974 [Inorganic Crystal Structure Database, Fachinformat ionszentrum (FIZ) Karlsruhe, Germany; M. Lundberg and M. Sundberg, Ultramicroscopy 52 (1993) 429]). The M2 phase differs from Ml by the absence of the pentagonal bi- pyramidal unit, and the 7-membered ring [P. DeSanto, et al., ibid.]. The formulae of the refined unit cells have been determined to be M07 _ g^vl .2^NbTe0.937°28.9 ^{for M1 and} M°4.31^V1.36^^el .81^N^0.33^19.81 f°^r M2, respectively [P. DeSanto, et al., ibid.].

The selective oxidation of propane has been assigned to the presence of Ml phase. M2 phase on its own does not activate propane but selectively oxidizes the propylene intermediate. Although correlations between phase composition and catalytic properties are still actively debated, it has been generally accepted that the high selectivity towards acrylic acid is associated with the presence of Ml phase [M. Barca et al . , Topics in Catalysis, and W. Ueda, D. Vitry and T. Katou, Catalysis Today 99 (2005) 43]. To investigate the mechanism underlying this activity, synthesis of phase-pure Ml catalysts in large batches is a basic prerequisite. Generally, two different preparation methods known as "slurry method" or "hydrothermal treatment" have been used to synthesize MoVTeNb mixed oxide catalysts.

According to the slurry method as described for instance in WO 2006/008177, ammonium heptamolybdate, ammonium metavanadate and telluric acid are dissolved in water, followed by the addition of ammonium niobium oxalate to precipitate a slurry, which is then spray-dried. After a calcination step in air at about 275°C, the final activation of the catalyst is conducted substantially in the absence of oxygen at much higher temperatures of typically 600 to 65O⁰C. This preparation method usually results in catalysts composed of several phases including a mixture of Ml and M2 phase [P. Beato, A, Blume , F. Girgsdi.es, R. E. Jentoft, R. Schlόgl , O. Timpe, A. Trunschke, G. Weinberg, Q. Basher, F. A, Hamid, S. B. A. Hamid, E, Omar and L. Mohd Salim, Applied Catalysis A: General 301 (2006) 131 and US 5,422,328] . The purification by post-treatment with nitric acid [H. Hibst, F. Rosowski and G. Cox, Catalysis Today 111 (2006) 234] or H₂O₂ [M. Baca and J. M. M. Millet, Applied Catalysis A: General 219 (2005) 61] gives access to phase-enriched Ml catalysts. These post- treatments are, however, considered cumbersome and do not always lead to phase pure material or alter the chemical composition of the Ml phase.

The alternative hydrothermal synthesis route to MoVTeNb oxide catalysts has for instance been described by W. Ueda , D. Vitry, T. Kato, N. Watanabe and Y. Endo, Research on Chemical Intermediates 32 (2006) 217. According to Ueda, MoVTeNb oxide is hydrothermally synthesized by dissolving (NH4)gMθ7θ24 and telluric acid in water and adding solutions of VOSO4 and niobium oxalate thereto. The resulting slurry is stirred at 8O⁰C before being introduced into an autoclave which is heated at 175⁰C for 48 hours. After drying the dark blue powder obtained, the catalysts were calcined under nitrogen flow at 500 or 600⁰C for 2 hours prior to catalytic tests. Ueda specifically evaluates the catalytic properties of orthorhombic Mo^Vg _ 25^Te0. ll^Nb0.12⁰X ^and found low conversion and medium selectivity values of 33% C3H8 conversion and 62% acrylic acid selectivity, respectively.

In a second publication, Ueda [W. Ueda, D. Vi try, Y. Morikawa and J. L. Dubois, Applied Catalysis A: General 251 (2003) 411] prepared orthorhombic MoVTeNb mixed oxides also applying a synthesis temperature of 448K (175⁰C) and a synthesis time of 48 hours. Various other authors conducted the hydrothermal synthesis under similar conditions in a teflon-lined stainless-steel autoclave using different metal-containing compounds and various metal stoichiometries [P. Botella, B. Solsona, A. Martinez-Arias , J. M. Lopez Nieto, Catalysis Letters 14 (2001) 149; P. Botella, J. M. Lopez Nieto, B. Solsona, A. Mifsud and F. Marquez, Journal of Catalysis 209 (2002) 445/ P. Botella, E. Garcia-Gonzάlez , J. Lopez Nieto and Y. Moreno, Catalysis Togay 99 (2005) 51; F. Ivars, P. Botella, A, Dejoz, J. M. Lopez Nieto, P. Concepciόn and M.I. Vazquez, Topics in Catalysis 38 (2006) 59]. After a heat treatment over 2 hours at 600⁰C in nitrogen of the hydrothermally prepared precursor they obtained generally M1/M2 phase mixtures. It would appear from F. Ivars et al , , ibid, that Ml had been obtained as one major phase with a nominal metal stoichiometry of Mo/V/Te/Nb = 1/0.2/0.17/0.17 and oxalate /molybdenum molar ratios in the range between 0.2 and 0.6. The as-synthesized metal stoichiometry of materials which seemed to contain higher amounts of Ml phase corresponded to Mo/V/Te/Nb = 1/0.20-0.27/0.19-0.22/0.19-0.21.

The literature published on hydrothermal synthesis relates to the preparation of rather small amounts of catalytic material . Attempts of the present inventors to scale up hydrothermal synthesis to catalyst amounts of more than 5g seem to indicate that this encounters difficulties especially regarding the reproducibility of the results. Further structurally related MoVNbTe oxide catalysts, and their use in the preparation of acrylic acid from propane, are disclosed in EP 0 608 838 A2 , US 6,432,870 Bl, US 6,734,136 B2, US 6,610,639 B2, EP 1 346 766, US 6,060,419, US 6,426,433 Bl and US 6,541,664 Bl.

It is one object of the present invention to provide a method for the preparation of a MoVTeNb mixed oxide catalyst that is strongly enriched in Ml phase.

It Is one further object of the present invention to provide a method for the preparation of MoVTeNb mixed oxide catalysts showing a greater conversion, and/or a greater selectivity in the oxidation of hydrocarbons, in particular in the oxidation of propane to acrylic acid.

It is one further object of the present invention to provide a method for the preparation of MoVTeNb mixed oxide catalysts that allows the preparation of greater catalyst amounts and preferably is suitable for the production of industrially relevant amounts .

It is one further object of the present invention to provide a stable MoVTeNb mixed oxide catalyst which shows a greater conversion, and/or greater selectivity in the oxidation of hydrocarbons, in particular in the oxidation of propane to acrylic acid.

It Is one further object of the present invention to provide a method for the oxidation of hydrocarbons using such catalysts, In particular the oxidation of alkanes such as propane to unsaturated carboxylic acids or the ammoxidation of propane or propylene to acrylonitrile . SUMMARY OF THE INVENTION

The present invention relates to:

(I) A method for preparing a metal oxide catalyst material comprising a mixed oxide comprising molybdenum (Mo) , vanadium (V) , Tellurium (Te) and niobium (Nb) , said process comprising the steps of

(i) subjecting a catalyst precursor mixture comprising molybdenum (Mo) , vanadium (V) , tellurium (Te) and niobium (Nb) to a calcination treatment in an oxygen-containing atmosphere at a temperature of 150 to 400°C to prepare a calcined catalyst material,

(ii) subjecting the calcined catalyst material to a treatment at a pressure of at least lOMPa and a temperature of at least 400⁰C in the presence of an inert fluid phase to prepare a catalyst material comprising a mixed oxide comprising molybdenum (Mo) , vanadium (V) , tellurium (Te) and niobium (Nb) ;

(II) .1) a metal oxide catalyst material comprising a mixed metal oxide comprising molybdenum (Mo) , vanadium (V) ,

Tellurium (Te) and niobium (Nb) wherein said metal oxide catalyst material has a crystallinity degree of at least 80wt.-% and a content of Ml phase in an amount of at least 85wt.-% based on the entire amount of crystalline metal oxide phase and is obtainable by the above method, and preferably has an average composition which lies within the ranges defined by general formula (I) :

MoV_aTe_bNb_cZ_dO_{x {I}}

wherein a = 0.15-0.50, b = 0.02-0.45, in particular 0.05-0.40, c = 0.05-0.45_r d < 0.05 and x is the molar number of oxygen binding to the metal atoms present in this mixed metal oxide which follows from the relative amount and valence of the metals elements, and 2 is at least one element selected from Ru, Mn, Sc, Ti, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Rh, Pd, In, Sb, Ce, Pr, Nd, Te, Sm, Tb, Ta, W, Re, Ir, Pt, Au, Pb, and Bi, as well as

2) a metal oxide catalyst material comprising a mixed oxide comprising molybdenum (Mo) , vanadium (V) , Tellurium (Te) and niobium (Nb) wherein said metal oxide catalyst material has a crystallinity degree of at least 80wt.~% and comprises Ml phase in an amount of at least 85wt.-% based on the entire amount of crystalline metal oxide catalyst phase and an average composition within the ranges defined by the following formula (II) :

MoV_aTe_bNb_cZ_dO_{x (II)}

wherein a = 0.28-0.40, preferably 0.29-0.38, in particular 0.30-0.36, b = 0.03-0.30, in particular 0.05-0.25, c = 0.05-0.20, and d and x are as defined above;

III. a method for producing an oxidized hydrocarbon from a hydrocarbon, which comprises subjecting the hydrocarbon to a vapour phase catalytic oxidation reaction in the presence of oxygen and a catalyst comprising a metal oxide catalyst material as defined above, preferably a method wherein the hydrocarbon is an alkane and the oxidized hydrocarbon is an unsaturated carboxylic acid; and

IV. a method for increasing the content of Ml phase in a calcined metal oxide catalyst comprising a mixed oxide comprising molybdenum (Mo) , vanadium (V) , Tellurium (Te) and niobium (Nb), which comprises the step of subjecting the calcined metal oxide catalyst to a treatment at a pressure of at least lOMPa and a temperature of at least 400⁰C in the presence of an inert fluid phase,, which can be advantageously used in the regeneration of these calcined catalysts.

DESCRIPTION OF THE FIGURES

Fig. 1 shows a SEM image of a catalyst material sample (MoVTeNbO_x) which was treated according to Example 1 at 500⁰C (773K) in superheated water prior to a final activation treatment .

Fig. 2 shows a SEM image of the catalyst material sample according to Fig. 1 after the final activation treatment.

Fig. 3 shows XRD patterns [(a) to (c) ] of samples taken at various stages of the preparation of MoVTeNbO_x according to example 1 and one Ml reference material [ (d) ] .

DETAILED DESCRIPTION OF THE INVENTION

In the following the use of "comprising" also includes more restricting embodiments where "comprising" is replaced by "essentially consisting of" or "consisting". Further, hereinafter the explicit disclosure of a broader quantitative range of values (e.g. a~d) together with an included (preferred) narrower range (e.g. b-c) also discloses the two possible part-ranges lying within the overall range on either side of the narrower range. Hence, within this broader quantitative range of values (e.g. a-d) further embodiments are characterized by all ranges (e.g. a-b, a-c, b-d or c-d) which can be formed by combining any of the upper limit or the lower limit of this explicitly defined broader quantitative range with the lower limit or the upper limit of any explicitly defined and included (preferred) narrower range .

I. Catalyst preparation method

According to the first step (i) of the claimed preparation method, a catalyst precursor mixture comprising at least the four elements molybdenum (Mo) , vanadium (V) , tellurium (Te) and niobium (Nb) and optionally other catalyst metal elements such as Z defined below (in the following also abbreviated as "catalyst metals") is subjected to a calcination treatment in an oxygen-containing atmosphere (air or a synthetic oxygen- containing atmosphere) at a temperature of 150 to 400⁰C to prepare a calcined catalyst precursor.

The catalyst precursor mixture to be calcined in the first step of the claimed method can be obtained according to any commonly known process.

As "starting material" for the catalyst precursor mixture we understand chemical compounds that comprise catalyst metals and can be converted to metal oxides in the calcination treatment or already contain such oxides (the expression "can be converted" does not exclude that this conversion already takes place at least in part at an earlier point in time, for instance during drying at higher temperature) . The starting material is preferably selected from salts of the catalyst metals, metal-containing acids, in particular oxyacids, metalorganic compounds, organic metal complexes and metal oxides. More preferably it is selected from organic salts and (oxy) acids. It is possible to prepare the catalyst precursor mixture by combining as many metal-containing starting materials as metals are to be included in the final catalyst or by adding at least one starting material that contains more than one of these metals.

Preferably, the catalyst precursor mixture is prepared in a wet mixing method which preferably comprises the steps of

(a) preparing a solution or dispersion of starting materials

{e.g. salts and/or acids) comprising molybdenum (Mo), vanadium (V), tellurium (Te) and niobium (Nb), and optionally at least one further metal to be added to the catalyst such as element Z defined below, in a solvent,

(b) optionally precipitating dissolved starting materials of these metals,

(c) removing the solvent: from the solution, dispersion or optionally the precipitate to form a solid mixture, and

(d) drying said solid mixture to form a catalyst precursor mixture .

Steps (b) , (c) and (d) can be performed subsequently or simultaneously, as in one preferred embodiment of the present invention (spray-drying) .

To prepare solutions or dispersions that contain the catalyst metals, suitable starting materials are typically dissolved or dispersed in predetermined ratios in an appropriate solvent, if required under heating, and combined.

The subsequent isolation of a dry catalyst precursor mixture can be effected in an usual manner by e.g. filtration, evaporation to dryness (e.g. rotary evaporation), spray drying, freeze drying, drying at ambient air and/or vacuum drying .

Solvents that can be used in the preparation of the catalyst precursor mixture are not specifically limited, and preferred solvents include water, alcohols, preferably methanol, ethanol, propanol and butanol, diois, such as ethylene glycol or propylene glycol, and other polar solvents. Water is more preferred. Further, any suitable mixture of the above solvents can be used.

Suitable starting materials (metal sources} for Mo, V, Te and ϊSIb oxides are for instance those described in US 5,380,933 (col. 3, line 27 to 57} and/or US 6,710,207 (col. 8, lines 12 to 30) _r and Include salts and acids (normally oxyacids} of the desired metal elements. The salts are selected in a manner that after calcining only metals elements and oxygen remain in the calcined catalyst precursor since all other constituents are volatile or rendered volatile by decomposition or oxidation. For this reason, the use of ammonium salts of the metal element (or the corresponding oxyacid) , organic salts such as oxalates, alkoxides or acetylacetonates , organic metal complexes or metalorganic compounds is preferred. Moreover, the selected salts and acids are preferably soluble or at least dispersible in the selected solvent such as water. According to one embodiment, the water-solubility of the starting salt or acid is at least 0.1 mol metal per 11 of water at 373K (100⁰C) . Suitable starting salts and acids include for instance ammonium para- or heptamolybdate, molybdenum oxalate, ammonium metavanadate_r vanadium oxalate, telluric acid and ammonium niobium oxalate.

According to one embodiment, a solution of the V source (e.g. an aqueous ammonium metavanadate solution) and a solution of the Te source (e.g. an aqueous solution of telluric acid) are added to a solution of the Mo source (e.g. an aqueous solution of ammonium heptamolybdate) , preferably after heating the Mo solution, followed by the addition of the solution of a Kb source (e.g. an aqueous solution of ammonium niobium oxalate) .

According to one further and preferred embodiment, the dry catalyst precursor mixture is prepared by spray-drying a clear aqueous solution prepared from molybdenum oxalate, telluric acid, vanadium oxalate and ammonium niobium oxalate. If spray drying is used, drying temperatures in the range of more than 100 to 250⁰C, for instance 150 to 220⁰C are preferred, especially if the catalyst precursor mixture was prepared from aqueous solutions or dispersions.

It is preferred that the drying process does not eliminate any remaining moisture in the material to be calcined. Typically, the drying process (e.g. spray-drying) is terminated if the particles to be calcined do no longer agglomerate. Excessive drying is to be avoided in order to preserve residual moisture, which is believed to be beneficial in transport phenomena. Excessive drying occurs if the dried particles start to dust.

According to one further embodiment, the catalyst precursor mixture to be calcined is prepared under so-called "hydrothermal" conditions, that is in the presence of water under elevated temperature and pressure. The starting material for the hydrothermal preparation is preferably selected from the aforementioned catalyst metal-containing compounds, more preferably from inorganic and organic salts and (oxy) acids. Suitable starting salts and acids include for instance ammonium para- or heptamolybdate, molybdenum oxalate, ammonium metavanadate, vanadium oxalate, vanadylsulfate (VOSO4) and hydrates thereof, telluric acid and ammonium niobium oxalate. The hydrothermal preparation comprises the steps of

(a) dissolving or dispersing starting materials containing the catalyst metals, if required under heating,

(b) combining, optionally under heating, the resulting solution (s) and/or dispersion (s) to an aqueous mixture containing the catalyst metals in the desired ratio for the final catalyst material,

(c) heating the aqueous mixture at elevated pressure whereby a solid material is formed, (d) collecting and drying rhe solid material formed in step (C) .

Step (c) is preferably conducted at a temperature of above 100⁰C up to 25O⁰C, for instance 120 to 200⁰C, preferably 13O⁰C to ISO⁰C at a pressure of more than 0.2 MPa (2 bar) . There is no specific upper pressure limit (it is for instance conceivable to work with pressure up to 9MPa) . For practical reasons, normally pressures of 0.3 to 2MPa, such as 0.4 to 1 MPa are used. Typically, the reaction is conducted in a closed reactor, such as an autoclave to make use of the autogeneous pressure generated at higher temperatures by the existing water. Furthermore, the reaction time (at the target temperature) can be varied considerably and may for instance range from 24 to 144 hours, 80 hours to 130 hours being preferred. It is not considered necessary to work under oxygen-free conditions but it is preferred to replace air existing in the reactor by an inert gas. The reaction is preferably conducted m autoclaves made from a suitable inert material (e.g. Hastelloy C276) or having a suitable inner lining (e.g. Teflon} .

Regarding further embodiments and conditions, reference can be made to the previous description, as long as the same do not contradict the hydrothermal preparation method.

The calcination and activation of a catalyst precursor mixture that was hydrothermally prepared under properly selected conditions may lead to Ml rich catalyst materials as shown in comparative example 1. However, the resulting catalyst materials show an insufficient performance, in particular in terms of propane conversion to acrylic acid. A high performance can only be achieved by an intermediate p/T treatment, i.e. step (ii) of the claimed preparation method as explained in further detail below. The molar ratio of starting materials in terms of metal components is chosen in accordance with the desired final composition of the catalyst under consideration of possible changes as discussed below. The final composition is a mixed metal oxide which comprises the metal oxides of Mo, V, Te and Nb, and optionally oxides of other metal elements, as long as these do not adversely affect the function of the resulting material as a catalyst in the oxidation reactions referred to herein. Preferably, the final average catalyst composition lies within the ranges defined by general formula {I) :

MoV_aTe_bNb_cZ_dO_{x (I)}

wherein a = 0.15-0.50, b - 0.02-0.45, in particular 0.05- 0.40, c = 0.05-0.45, d < 0.05 and x is the molar number of oxygen binding to the metal atoms present in this mixed metal oxide which follows from the relative amount and valence of the metal elements, and Z is at least one element selected from Ru, Mn, Sc, Ti, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Rh, Pd, In, Sb, Ce, Pr, Nd, Te, Sm, Tb, Ta, W, Re, Ir, Pt, Au, Pb, and Bi.

"Average" composition means the composition as can be determined with techniques such as XRF particularly suitable for analyzing the bulk elemental composition. The EDX measurements conducted in the experimental section for determining average bulk compositions gave also reliable results .

According to one embodiment of the present invention d is 0 in formula (I) .

If in the compound of the formula (I) the at least one optional element Z is present (i.e. d > 0), it is preferably at least one element selected from Ru, Mn, Cr, Fe, Co, Ni, Zr, Rh, Pd, In, Sb, Ce, Ta, W, Pt and Bi. More preferred are compounds of formula (I) , wherein Z, if present, is at least one element selected from Cr and Ni. Another preferred embodiment relates to the use of Ru, Cu, Rh, Re and/or Mn as Z element, Ru, Mn and Cu, in particular Ru and Mn being particularly preferred- If element Z is present, the lower limit of d is preferably 0.0005, in particular 0.001.

According to one more preferred embodiment, the final average catalyst composition lies within the ranges defined by general formula (II) :

MoV_aTe_bNb_cZ_dO_{x (II}}

wherein a = 0.28-0.40, preferably 0.29-0.38, in particular 0.30-0.36, b = 0.03-0.30, in particular 0.05-0.25, c = 0.05- 0.20, and d and x are as defined above.

When selecting the ratio of starting materials it should be considered that during the calcination step and the subsequent treatment at high pressure and high temperature (in the following referred to as "p/T treatment"} the molar ratio of catalyst metals may change to a certain extent.

Since the V content seems to be enriched in the final composition, the V ratio in the starting material can be lower than specified in formula (I) above, for instance by 2- 20%, 5-15% or 8-13% lower. Accordingly, in the starting material mixture, variable "a" characterizing the V content may be in the order from 0.15 to 0.30. Since the volatility of Te often leads to a Te loss in the final catalyst composition over the starting material mixture, the Te ratio can be higher in the starting material mixture than specified in formula (I) above although this is not preferred. Preferably, the Te content in the starting material composition is as low as possible to effectively generate Mi phase and the process conditions are optimized to minimize Te loss . In one embodiment the catalyst precursor mixture to be calcined, and thus also the final catalyst material, can include a solid, diluent. As diluent, any inert material can be used that can withstand the conditions of the calcination, the p/T treatment and optionally the activation treatment, does not interact with the metal oxide catalyst material such that the catalytic activity thereof would be impaired, and does not react with the starting materials, intermediates or final products of the oxidation reaction.

The presence of a solid diluent may be beneficial for various reasons. First, preferred diluents are characterized by a higher thermal conductivity than the catalytically active metal oxide material. This ensures a better heat transport management and prevents the formation of hot spots during the use of the catalyst, which could lead to undesired side reactions or lowering the catalyst's life. Second, the diluent functions as a separating agent for the catalytically active material and counteracts any sintering processes, which may occur between the grains of catalyst material. Further, the diluent may also improve the surface properties of the catalyst.

Suitable diluents may be selected from alumina, zirconium oxide (zirconia) , cerium oxide (CeC^), SiC and silica. According to one embodiment, the diluent is treated with a solution containing at least one metal defined in formula (I) prior to its admixture to the catalyst precursor material or a starting material thereof.

Preferably the optionally pretreated and dried diluent is subjected to the same calcination procedure, as described for the catalyst precursor material, before it is combined with the catalyst starting material. Thus, the optionally pretreated diluent preferably undergoes this calcination twice, once prior to mixing with catalyst starting material and a second time together with this catalyst precursor material .

Preferably the weight ratio of the diluent to the metal oxide catalyst material is not more than 1.5:1 and especially not more than 1:1, e.g. not more than 0.5:1.

The diluent can be added at any time prior to the calcination procedure, i.e. it can be mixed with the metal-comprising catalyst starting materials in a dry or a wet state or, if the catalyst precursor mixture is prepared using a solvent, it can be added to the solvent to precipitate the catalyst starting materials on the diluent in the process of preparing the catalyst precursor mixture .

Irrespective of which preparation procedure is chosen and whether a diluent is present in the resulting solid material (catalyst precursor mixture) and/or this solid material has been coated onto a carrier (please see catalyst description in item II), the catalyst precursor mixture is then subjected to a calcination in an oxygen-containing atmosphere (step i) at a temperature of 150-400°C, preferably 200-350⁰C, more preferably 250-300⁰C. This procedure is intended to remove volatile constituents and convert catalyst metal elements to their oxides .

For convenience, this calcination reaction is usually conducted at atmospheric pressure. In principle it is, however, also possible to conduct this step under slightly elevated or slightly reduced pressure, for instance within the range of atmospheric pressure + 50% or ± 20%. As oxygen- containing atmosphere, air or a synthetic oxygen-containing atmosphere can be used. Depending on the other process conditions, oxygen is normally not employed in contents of more than 50 vol.-%. Suitable oxygen volume ratios are for instance 1 to 40 vol.%, 5 to 35 vol.-% or 10 to 30 vol.-%. The remainder is nitrogen as in air or any other inert gas such as Ar or He.

In principle, any suitable reactor can be used for conducting the calcination. Especially if organic metal salts are used as starting material, it is preferred to calcine in a rotating oven which seems to enhance the complete removal of organic constituents.

The catalyst precursor mixture is gradually heated from the starting temperature (usually room temperature, that is 2O⁰C) to the final calcination temperature, preferably at a rate of 1 to 20 K/min, in particular 5 to 15 K/min. Calcination is then conducted at the final temperature of 150 to 400⁰C, preferably 200-350⁰C, more preferably 250-300⁰C, preferably over a time period of 20 min to 20 hrs, for instance 30 min to 10 hrs, 1 hr to 5 hrs or 1 hr to 3 hrs.

During calcination the oxygen-containing atmosphere is preferably moved continuously relative to the catalyst precursor mixture. The expression "moved" relates to embodiments according to which fresh oxygen-containing atmosphere is supplied to replenish possibly consumed and therefore oxygen-depleted atmosphere. Suitable supply rates are for instance in the order of 10 to 100 ml/min, in particular 30 to 70 ml/min, based on a weight of catalyst precursor mixture of 1-50 g.

According to the second step (ii) of the catalyst preparation method, the calcined catalyst material is subjected to a high pressure and high temperature treatment (in the following also referred to as "p/T treatment") at a pressure of at least 10 mPa and a temperature of at least 400⁰C in the presence of an inert fluid phase. The inventors have surprisingly found that this p/T treatment greatly enhances the formation of Ml phase in the final catalyst material, either during the p/T treatment itself or in a subsequent activation treatment.

There is no specific upper limit regarding the pressure and temperature to be used in this treatment. Accordingly, it is for instance possible to conduct the treatment at pressures of up to 50 MPa or more and temperatures of up to 700⁰C or more. However, very high temperatures and pressures expose the material of the reactor to higher loads. Therefore, it is preferred from an industrial perspective to use the lowest possible temperature and pressure values that are sufficiently effective in forming Ml phase or increasing its content. Further embodiments of the temperature range to be used include 420 to 6G0°C, 440 to 560⁰C and 460 to 540⁰C. According to further embodiments, the pressure can be adjusted within the range of 12 to 40 MPa, 14 to 35 MPa, 15 to 30 MPa and 16 to 25 MPa.

The p/T treatment of the present invention is conducted in the presence of an inert fluid phase. This treatment is believed to favor transport reactions thereby triggering dissolution, phase reorganization and recrystallization processes, which enhance the formation of Ml phase. The inert fluid phase may be a vapor, liquid, critical or supercritical phase under the conditions of the p/T treatment. The term "liquid" is intended to denote all conditions of pressure and temperature where the phase is not in a vapor state and a critical or supercritical state has not yet been reached.

The compound forming the inert fluid phase (in the following referred to as "phase-forming compound") should not decompose under the conditions of the p/T treatment. Without wishing to be bound by these mechanistic considerations it would appear that the phase-forming compound might be capable of at least partially dissolving calcined catalyst material thereby enhancing transport and recrystallization processes. Moreover, it seems preferred if the following requirements are fulfilled:

* The phase-forming compound is a small molecule having preferably a molecular weight of less than 150, more preferably less than 100, in particular less than 80.

* The phase-forming compound possesses at least one polar bond between two different atoms such as C-O, C=O, O-H, S=O, while compounds (molecules) solely consisting of the same atom type and molecules constituted solely by nonpoiar C-C bonds and C-H bonds are less preferred.

* According to one embodiment, the phase-forming compound consists of at least two different chemical elements selected from the group consisting of C, S, 0 and H. Compounds containing nitrogen atoms are less preferred.

Three examples of suitable phase-forming compounds are water, CO2 and SO₂. Their critical data are Tc[K]=647.35 and Pc[mPa]=22.11 for water, Tc[Kj=304.15 and Pc [mPa] =7.37 for CO2 and Tc [K] -430.65 and Pc[mPa]-7.88 for S02. It can be seen that within the required conditions for the p/T treatment, CO2 and SO2 are always used at supercritical conditions, whereas water can be equally used as vapor or liquid (that is in a subcritical state) , at the critical point or under supercritical conditions.

The p/T treatment of the present invention can be conducted in any apparatus withstanding the required high temperature and pressure conditions. Typically, it is conducted in a tightly sealed autoclave or reactor. To ensure that the pressure loss over the treatment time is as small as possible, specific seal materials, which do not loose their sealing properties under the required pressure and temperature conditions, are preferably used. Copper and silver seals constitute for instance such materials . The p/T treatment of the present invention is preferably conducted over 0.5 to 30 hrs, for instance 1 to 25 hrs, or 2 to 20 hrs.

This p/T treatment is normally conducted after air existing in the reactor (e.g. autoclave} has been replaced by an inert gas such as nitrogen, argon or helium followed by the addition of the phase-forming compound. However, it does not appear to be necessary to work under essentially oxygen-free conditions and it is believed that a minor content of oxygen, for instance less than 1 vol.-%, or less than 0.5 vol.-% based on the total amount of fluid media does not adversely affect the p/T treatment.

The metal oxide catalyst material obtained after the p/T treatment may already show a high Ml phase content, such as at least 85wt.~%, at least 90wt.-% or at least 95wt.-% as well as other characteristics explained later in connection with the claimed metal oxide catalyst. Then, it is not necessary to further enhance the Ml phase content by subjecting the catalyst to the following "activation treatment" .

According to one preferred embodiment, normally after an intermediate cooling step, this second thermal treatment (also referred to as "activation treatment") follows the p/T treatment to further enhance the crystallinity (and thereby also the Ml content), if necessary. The activation treatment proceeds in an inert atmosphere, preferably in nitrogen gas or argon gas, at a temperature of 350-700°C, preferably 550- 68O⁰C, even more preferably 58Q-670°C, in particular 590 to 67O⁰C. During this activation treatment it is particularly important to work in the absence of oxygen since the mixed metal oxide (such as Ml phase itself) obtained after the p/T treatment is in a not fully oxidized state (as it contains at least in part molybdenum (Mo) in an oxidation state of less than +VI and at least in part vanadium (V) in an oxidation state less than +V) and complete oxidation to M0O3 or V₂O₅, respectively, is to be avoided.

For the sake of convenience, this activation treatment is usually conducted at atmospheric pressure. In principle it is, however, also possible to conduct the treatment under slightly elevated or slightly reduced pressure, for instance within the range of atmospheric pressure + 50% or ± 20%. The treatment time in this step is also not specifically limited, and is preferably 0.5-30 h, more preferably 1-20 h and in particular 1-10 h.

The catalyst precursor mixture is gradually heated from the starting temperature {usually room temperature, that is 20⁰C) to the final activation temperature, preferably at a rate of 0.5 to 5 K/min, in particular 1 to 3 K/min.

II. Metal oxide catalyst and material

The present invention also provides a new and superior MoVTeNb mixed oxide catalyst material which shows excellent conversion rates and/or selectivities in the oxidation of hydrocarbons, for instance to unsaturated carboxylic acids. The catalyst material is preferably essentially crystalline which is to be understood as a preferred crystallinity degree of at least 80wt.-%, more preferably at least 85wt.-%, even more preferably at least 90wt.-%, even more preferably at least 95wt.-% based on the total amount of metal oxide catalyst phase (note: of course, neither optionally present diluent nor carrier are considered part of this metal oxide catalyst phase) . The crystallinity can be determined by XRD with an internal standard as explained in the experimental section . The claimed mixed oxide catalyst material is also characterized by a very high content of Ml phase of at least 85wt.-%, preferably at least 90wt.-%, more preferably at least 95wt.-% Ml phase based on the total amount of crystalline metal oxide catalyst phase. The amount of Ml phase in wt.-% can be determined as described in the experimental section.

In line with the present invention, the expression ,,Ml phase" is to be understood as orthorhorabic phase characterised by the presence of (1) corner-linked MOg (M=Mo, V) octahedrons forming 6- and 7-membered rings and (2) NbC>7 pentagonal bipyramidal units sharing edges with surrounding corner- linked MOg octahedrons. These structural elements are believed to allow an unambiguous distinction from M2 phase. Of course, other structural properties as set forth under "background art" can be used to define the Ml phase.

Moreover, Ml phase shows a characteristic needle shape with average length values of typically 100 to 400 nm, e.g. 200 to 300 nm, and average diameter values of 50 to 300 nm, e.g. 100 to 250 or 100 to 200 nm. Activation at higher temperatures generally seems to favour the formation of greater average diameters. Morphology studies and shape analysis can be performed using scanning electron microscopy (SEM) . For SEM investigations the samples were deposited on carbon tape without any pre-treatraent. From the SEM images, size distributions of the Ml needles were obtained by measuring the length and diameters of 300 arbitrarily selected Ml needles .

According to one embodiment of the claimed catalyst material {'"^■first catalyst material") , a MoVTeNb mixed oxide catalyst material comprising oxides of molybdenum (Mo) , vanadium (V) , tellurium (Te) and niobium (Nb), and optionally further catalyst metals such as element Z defined before, is provided which comprises Ml phase in an amount of at least 85wt.-%_r based on the entire amount of crystalline metal oxide catalyst phase, and is obtainable by the above explained preparation method.

The average composition of this catalyst material is preferably within the ranges defined by the above general formula (I)_f in particular the above formula (II) .

Surprisingly, it has been found that such a catalyst material shows a superior performance over comparable hydrothermaliy prepared catalyst materials with respect to conversion and/or selectivity. The reason why the preparation method, especially the p/T treatment, optionally in combination with a subsequent activation treatment, exerts this positive impact on catalyst performance has not yet been fully investigated. It is believed though that other preparation- related factors than the content of Ml phase contribute to this positive finding.

According to one further embodiment ("second catalyst material"), the present invention provides a novel catalyst material comprising oxides of molybdenum (Mo) , vanadium (V) , tellurium (Te) and niobium (Nb) and optionally further catalyst metals such as element Z defined before characterized in that it has a Ml phase content of at least 85wt.-% and an average composition that is within the ranges defined by the aforementioned formula (II) .

This embodiment involves catalyst compositions that show a superior performance irrespective of their preparation process. It is however preferred to prepare these Ml-rich catalyst material compositions in accordance with the previously described preparation method since this is believed to result in additional benefits in terms of catalyst performance. Both the first and the second catalyst material of the invention can be optionally further described by their specific surface. The specific surface area of the catalyst material as measured according to the BET method with nitrogen described in the examples is preferably greater than 5 m^/g, in particular greater than 6 m^/g, for instance 6 to 40 m²/g, e.g. 7-20 m²/g.

Specifically, it has been found that the first and second catalyst material of the invention preferably show a propane conversion of at least 40%, preferably at least 45%, and/or a selectivity for acrylic acid of at least 65%, preferably at least 70%, under the hydrocarbon oxidation conditions specified in the examples .

The catalyst material of the invention can be used as catalyst as obtained after the p/T treatment or, if applicable, the activation treatment, that is as powder. According to one embodiment, catalyst particles of a definite size are formed from the obtained powdery catalyst material (which may also contain a diluent) .

There is no specific limitation regarding the size of the catalyst particles to be used. The following structural features are however preferred.

The macroscopic size (average longest diameter) of the individual catalyst particles preferably ranges from 0,5 to 10mm. Catalyst particles of this size can be obtained by processes known in the art, for instance by pressing a dried catalyst starting material, newly crushing the pressed material and carrying out size-selecting steps such as sieving, before conducting the calcination step. Alternatively, the already calcined material is pressed, newly crushed and subjected to size-selecting steps such as sieving. Instead of pressing, an extrudate may be formed. Pressing and extrusion may be equally conducted with p/T- treated and optionally activated catalyst material.

According to one further embodiment, the final catalyst material is coated onto a carrier according to techniques known in the art. This coating of the respective catalyst (precursor) materials can be equally effected at an earlier stage, for instance prior to the calcination treatment, prior to the p/T treatment or prior to the optional activation treatment .

The carrier, which is preferably inert, can have any shape and surface structure. However, regularly shaped, mechanically stable bodies such as spheres, rings, tube sections, half-rings, saddles, spirals Gr honeycomb carrier bodies or carrier bodies provided with channels such as, for example, fibre mats or ceramic foams are preferred. The size and shape of the carrier bodies is determined, for example, by the dimensions, primarily the internal diameter of the reaction tubes if the catalyst is used in tube or tube-bundle reactors. The diameter of the carrier body should then be between 1/2 and 1/10 of the internal diameter of the reactor. In the case of fluidised bed reactors, the carrier dimensions are determined, for example, by the fluid dynamics In the reactor. Suitable materials are, for example, steatite, duranite, stoneware, porcelain, silicon dioxide, silicates, aluminium oxide, aluminates, silicon carbide or mixtures of these substances. Tube sections, rings or spheres made of ceramic, silicon carbide or carbon are preferably used.

The proportion of the layer applied to the carrier is preferably 1 to 30% by weight, particularly preferred 2 to 20% by weight, based on the total mass of the catalyst material (exclusive optionally present diluent) . The thickness of the layer is preferably 5 to 300 μm, particularly preferred 5 to 10 μm. In one preferred embodiment, the selected carrier is coated with an aqueous slurry of the starting materials for the catalyst precursor material, or a suspension thereof in an organic solvent such as for example toluene, followed by the removal of water or organic solvent and the process steps described in item I (calcination, p/T treatment, optional activation) . If the carrier is coated at a later stage the same technique can be employed under use of an optional heat treatment after the removal of water or organic solvent to fix the catalyst material on the carrier.

III. Method for oxidizing a hydrocarbon

One further aspect of the present invention relates to a method for oxidizing a hydrocarbon in the presence of oxygen and a catalyst as defined above.

According to one preferred embodiment, this method comprises subjecting a hydrocarbon, in particular an alkane to a vapour phase catalytic oxidation reaction to produce an unsaturated carboxyiic acid. This embodiment is preferably applied to Cl to C5 alkanes. For example, when propane or isobutane is used as starting alkane, acrylic acid or methacrylic acid will be obtained, respectively, in good yield.

In this embodiment the catalyst of the invention can be used under conventional conditions to convert hydrocarbons to unsaturated carboxyiic acids. The reaction is preferably conducted in fixed bed reactors, for instance tubular fixed bed reactors. For convenience, atmospheric pressure can be used whilst the reaction proceeds similarly under lower or higher pressures. Preferably, an inert gas (e.g. nitrogen) and/or steam are admixed to the hydrocarbon (e.g. propane) and oxygen. A standard feed composition is for instance alkane (e.g. propane} /oxygen/nitrogen/steam of 1/2-2.2/18- 17.8/9 (molar ratio) . Preferred reaction temperatures range from 350 to 45O⁰C. The molar amount of steam (H2O) based on the total molar amount of hydrocarbon, O2, inert gas (e.g. N2) and steam (H2O) can be varied considerably with the catalyst of the invention. Suitable results are achieved with molar amounts of preferably 5-65%, for instance 10-50%.

The present inventors have found that Ml-rich catalyst samples are surprisingly stable under conditions of use as described in example 4. After 50 hours essentially no loss of catalytic activity was observed. A phase analysis of catalyst samples used over this period of time did also not reveal any noticeable reduction of Ml phase content. This makes the catalyst of the present invention a particularly attractive choice for industrial methods for the oxidation of hydrocarbons as described hereinbefore wherein the same catalyst batch is used over several months, for instance at least 3 months, at least 6 months, or even at least one year.

According to one further embodiment, a saturated or unsaturated hydrocarbon, such as C3 to C7 alkane or alkene is oxidized ( "ammoxidation") in the presence of oxygen, ammonia and the catalyst of the invention to the corresponding nitril. Acrylonitrile or methacrylonitrile can be produced in this manner from propane, propylene, isobutane or isobutene, respectively. This embodiment is conducted under conditions usual in the art involving for instance a reaction temperature of 350 to 500⁰C, e.g. 400 to 500⁰C and atmospheric pressure up to 3 atm. As oxygen source air is usually employed.

IV. Method for increasing the Ml content of calcined metal oxide catalyst

The concept underlying the present invention is equally applicable to increase the Ml phase content of calcined MoVTeNb oxide catalyst (material) that has a different history than defined in the claimed preparation method. In this manner it is possible to increase the performance of already calcined catalyst (material) which has for instance undesirably high contents of the amorphous phase or M2 phase.

Accordingly, one further aspect of the present invention relates to a method for increasing the content of Ml phase in a calcined metal oxide catalyst material comprising oxides of molybdenum (Mo), vanadium (V), Tellurium (Te) and niobium (Nb) , and optionally at least one further catalyst metal element such as Z defined above, which comprises the step of subjecting the calcined metal oxide catalyst to a treatment at a pressure of at least lOMPa and a temperature of at least 400⁰C in the presence of an inert fluid phase. This treatment can be conducted under the same conditions as set forth in item I for the p/T treatment conducted as second step of claimed preparation method.

According to one preferred embodiment of the present method, the p/T-treated catalyst is subjected to an activation treatment in an inert gas atmosphere at a temperature of 350 to 700⁰C, in particular 580-670⁰C. Regarding details and embodiments of this activation treatment reference is also made to the preparation method of the invention (cf . item I) .

The present method is preferably applied to catalysts having an average composition as described in items I and II, for instance catalyst materials of formula (I) or (II) and embodiments thereof.

The method can be used to increase the Ml phase content by for instance at least 10wt.-%, at least 20wt.~%, at least 30wt.-%, at least 40wt.-%, at least 50wt.-%, at least 60wt.- %, at least 70wt.-%, or at least 80wt.-%, depending on the Ml content of the calcined catalyst material to be treated and the target value. According to one embodiment, the calcined metal oxide catalyst material has a content of less than 70wt.-%, e.g. less than 60wt.-%, less than 50wt.-%, or less than 40wt.-% Ml phase prior to the treatment and at least 85wt.-% Ml phase, preferably at least 90wt.-% Ml phase, more preferably at least 95wt.-% Ml phase after the treatment each based on the total amount of crystalline metal oxide catalyst phase.

This method of increasing the Ml content can be advantageously used in the regeneration of catalysts which never showed a higher content of Ml phase or at least partially lost their original Ml phase content during use. In this preferred embodiment, the calcined catalyst material to be treated is removed from a (usually continuous) process for producing an unsaturated carboxylic acid, which comprises subjecting an alkane to a vapour phase catalytic oxidation reaction in the presence of oxygen and said calcined catalyst (as described in item III), and recycled at least in part to said process after the method of increasing the content of Ml phase has been performed.

The present invention will now be explained in more detail by the following inventive and comparative examples and their use in a representative hydrocarbon oxidation reaction.

Examples

The following analytic techniques were used for evaluating catalysts of the present invention and comparative catalysts

X-ray powder diffraction (XED)

Determination of Ml phase content (wt.-%) and cristallinity (wt.-%) The phase composition of the catalysts was determined by X- ray diffraction performed on laboratory diffractometers, either a STOE STADI P transmission dif fractoraeter equipped with a focusing primary Ge⁰ (111) monochromator and a position sensitive detector, using Cu-K(Xi radiation, or a Bruker AXS D8 Advance Bragg-Brentano diffractoraeter equipped with a secondary graphite monochromator and scintillation counter, using Cu-K_α^₄.2 radiation.

The Ml phase content in (wt.-%) was determined by fitting calculated diffraction patterns to the experimental X-ray diffraction data employing the R.ietveld method (quantitative Rietveld analysis), using the "TOPAS" software (v.3, Bruker AXS) . In its basic form, this analysis yields the mass fractions (weight-%) of those crystalline phases in a phase mixture which have a known crystal structure, the sum of which is defined as 100%. Accordingly, the content of amorphous material can only be determined as explained later.

To the knowledge of the present inventors, catalysts having a chemical composition as considered in the present invention form as main phases only Ml and M2 and occasionally one or more among five crystalline minority phases, that is (Mo, V, Nb) 5O2_.4 and VQ _ 95M00.97O5, M0O2 , M0O3 and TeMθ5θ^6-

The crystal structures of Ml and M2 published by DeSanto et al. [Z. Kristaliogr. (2004) 219, 152-165] (ICSD entries 55097 and 55098, respectively; ICSD = Inorganic Crystal Structure Database, copyright 2007 by Fachinformationszentrum (FIZ) Karlsruhe, Germany) are used to calculate the theoretical diffraction patterns of these phases. The M1:M2 ratio is defined as the ratio of the corresponding mass fractions found.

If required, the theoretical diffraction patterns of minority phases are taken into account based on the structural information for Mo₅O₁₄ (ICSD-27202) and V₀.95M00.97O5 (ICSD- 39386), MoO₂ (ICΞD-80830) , MoO₃ ( ICSD-35076) , and TeMo₅O₁₆ (ICSD-69063) . If optional catalyst elements such as Z (in formula I and II) are present (d < 0.05 based on MojJ they do not alter the diffraction pattern of the crystalline phases to an extent that this needs to be considered within the preciseness of the determination. In the very unlikely case that the crystalline metal oxide phase would contain unknown crystalline material in greater amounts, a crystallographic analysis allows considering its diffraction pattern in the determination of the Ml content.

According to one alternative description of a metal oxide catalyst material with very high Ml phase contents, the XRD pattern shows exclusively diffractions signals that can be assigned to Ml phase and is substantially free of other well- defined diffraction peaks. The term "well-defined diffraction peak" is to be understood as relating to a diffraction signal having an FWHM (full width at half maximum), i.e. the width of the peak at 50% of its height above the baseline, of at most 3° in 2 theta.

The amount of (X-ray) amorphous material can be quantified by extending above method if the sample is mixed before analysis in a known ratio with an internal diffraction standard of known crystallinity and suitable linear attenuation, e.g. NIST SRM 674b Ti02 (μ = 536 cm^"1 for Cu-Ka radiation) which is commercially available from National Institute of Standards and Technology (NIST) , USA. Now, the sum of the crystalline phases is not defined as 100% anymore, but the fit results are re-scaled according to the known mass fraction (weight-%) of the internal standard. The difference between the sum of the re-scaled crystalline phase mass fractions and 100% is then assigned to amorphous material. Finally, all mass fractions determined this way are re-scaled again to values corresponding to the original sample (i.e. before mixing with the standard) , In this manner, the degree of crystailinity of the sample (in wt.-%) can be determined. SEM and EDX

Morphology studies and shape analysis can be performed using scanning electron microscopy (SEM) . For SEM investigations the samples were deposited on carbon tape without any pre- treatment. A Hitachi S-5200 with a PGT spirit EDX system and a Hitachi S-4800 with an EDAX genesis EDX detector were used. EDX studies in the SEMs were carried out with an accelerated voltage of 10 kV while SEM images were acquired at 2 kV to optimize surface resolution. The dry sample was crushed using a mortar and pestle. The crushed powder was dispersed in dry form on a carbon coated copper grid. In order to calculate the bulk composition of a given compound via EDX, several measurements have to combined and averaged due to the area covered by EDX being relatively small. A meaningful value can be obtained by averaging the values obtained from 50 EDX measurements .

Physisorption of nitrogen

Measurement of nitrogen adsorption at -196°C was performed using the Autosorb-1 instrument (Quantachrome) . The Brunauer- Emmett-Teller (BET) method was used to determine the surface area of solid materials. The instrument measures the amount of nitrogen adsorbed onto the catalyst surface at some equilibrium vapour pressure by the static volumetric method. The surface area is calculated from the amount of adsorbed nitrogen forming a monolayer assuming that the molecular cross-sectional area for nitrogen is 16.2 A². Previous to the adsorption, the samples have to be treated in vacuum several hours at temperatures between 80~300°C to remove molecules adsorbed from the environment. The catalyst of the invention was evacuated for 4 h at 80°C. Afterwards, known quantities of N₂ are admitted into a sample cell containing the catalyst maintained at -196⁰C. The BET analysis requires a linear plot of the amount of the adsorbed nitrogen at a relative pressure p/po (p is the equilibrium vapour pressure measured, po is the equilibrium vapour pressure of liquid nitrogen) , which is restricted to a limited region of the adsorption isotherm, usually in the p/po range 0.05 to 0.3. Full adsorption/ Resorption isotherms in the p/po range 0.001 to 1 were recorded. The Quantachrome AUTOSORB software was used to determine the BET surface area of the catalyst of the invention taking into account 5 data points in the linear relative pressure range of the adsorption isotherm (p/po pressure range 0.05 to 0.3) . The amount of sample is around 0.1 g.

X-ray fluorescence measurements (XRF)

XRF measurements were carried out on a Sequenz- Rontgenfluoreszenz-Ξpektrometersystem S4 PIONEER available from Bruker AXS GmbH, Germany. To further increase the preciseness of the method, standard samples can be prepared that are adapted to the sample to be analyzed such as M0O3, V2O5, Te (OH) Q and Nb2θ5- If required, other suitable standards can be used as known in the art.

Example 1 (treatment with water)

Starting materials

Molybdenum (VI) oxide (MoO₃, 99.5 % Fiuka) , telluric acid (Te(OH)₅, 97.5-102.5 % Aldrich), vanadium(V) oxide (V₂O₅, 99.5

% Riedel-de Haen) , ammonium niobate (V) oxalate hydrate (NH₄ [NbO (C₂O₄) ₂] -XH₂O, 99.99 % H. C. Starck) , and oxalic acid dihydrate (C₂H₂O₄^H₂O, 99.5 % ROTH) were used as received.

Distilled water was also utilized for preparation of the aqueous solutions.

Mo-V-Te-Nb-O complex oxide catalyst preparation First, the molybdenum (VI) oxide (130 mrαol) and oxalic acid dihydrate (158 mmol) were dissolved in distilled water (300 mli) under vigorous stirring at 80 ⁰C for 30 minutes. Next, the Te containing solution consisted of telluric acid (30 mmol), dissolved in distilled water (30 mL) under continuous stirring at 40 ⁰C for 15 minutes, was added to the molybdenum containing solution. The V containing solution was prepared by carefully adding oxalic acid dihydrate (56 mmol) to a suspension of the vanadiιim(V) oxide (19.5 mmol) in distilled water (50 mL) under continuous stirring at 65 ⁰C for 30 minutes. Then, this solution was added to the Mo/Te one. Afterwards, the Nb containing solution consisted of ammonium niobate(V) oxalate hydrate (16 mmol) dissolved in distilled water (30 mL) under continuous stirring at 40 ⁰C for 15 minutes was added to the Mo/V/Te containing solution. Finally, the as-derived homogeneous complex solution with nominal molar ratio Mo/V/Te/Nb being 1/0.3/0.23/0.123 was stirred at 40 ⁰C for a further 30 minutes.

To obtain a powder from this complex solution, the spray- drying procedure was performed in a Btichi 191 Mini Spray Dryer operating with inlet temperature of 150 ⁰C, outlet temperature of about 110 ⁰C, aspirator and pump levels of 100 % and 10 %, respectively, and air flow rate of ~ 600 mL/min. The resulting powdered product was quickly collected and immediately subjected to a moderate-temperature heat treatment in an atmosphere of air flow (100 mL/min) . The treatment involved a 25 minutes ramping up to 275 ⁰C and holding at this level for 1 hour. The resulting powder was thoroughly ground in an agate mortar and pestle.

Mo-V-Te-Nb-O complex oxide catalyst material was prepared via a p/T-treatment of as-derived precursor in superheated water vapour with following activation in argon. Typically, 1.6 g of precursor powder and 2.65 mL of distilled water were mixed in a stainless steel vessel (volume - 48 cm³) , the vessel was then capped by a stainless steel cover (copper gasket sets between the vessel and cover) , and placed inside a stainless steel bomb. The bomb was sealed and kept at 500 ⁰C for 2 hours. Spatula-collected product of the superheated water vapour treatment was then dried at 110 ⁰C for 2 hours in air. The p/T-treated catalyst material sample displayed a sponge- like morphology as shown in Fig. 1. Its BET surface value was 18 m²/g.

The resulting powder was thoroughly ground in an agate mortar and pestle and subjected to an activation treatment under heating in an atmosphere of argon flow (100 rrsL/min) . This treatment involved a 40 minutes ramping up to 600 ⁰C and holding at this level for 2 hours. The as-synthesized catalyst material was thoroughly ground in an agate mortar and pestle and stored in a moisture-free desiccator. The recrystaliization occurring during this activation treatment led to the formation of a virtually phase-pure material containing 95wt.-% Ml, 2wt.-% M2 and 3wt.-% Vg_, 95 M00.97O5 as determined by XRD. The activated material displayed the common needle-shaped morphology of Ml phase as shown in Fig. 2.

The average bulk composition of the resulting catalyst material was determined by EDX as Mo/V/Te/Nb = 1/0.34/0.078/0.14.

If the composition of the final catalyst material is compared with the molar ratio of starting materials, it is noted that the vanadium content is slightly increased whereas the tellurium content was considerably lower. The latter is due to the volatility of tellurium under preparation conditions. It is believed that this Te depletion does not have a major influence on the properties of the final catalyst material as long as the tellurium content can be kept within certain ranges as specified in the present application. XRD patterns of samples taken at various stages of the preparation are juxtaposed in Fig. 3. These XRD pattern relate to

a} the calcined catalyst material prior to the p/T- treatment in the presence of water, and

b} after this treatment; and

c) the final catalyst material after the activation treatment in Ar at 600⁰C.

d} The uppermost XRD pattern stems from a Ml reference material .

Fig. 3 thus confirms that the initially amorphous calcined catalyst material (Fig. 3a) undergoes substantial structural rearrangements in the high pressure/temperature (ρ/T) treatment of the present invention. It would appear that under these conditions structural units arrange predominantly along the crystallographic c axis resulting in nano- crystailine Ml as revealed by unisotropic line broadening analysis of simulated Ml patterns (P. D. Santo, D.J. Buttrey, R. K. Grasseli, CG. Lugmair, A. F. Volpe, B. H. Toby, T. Vogt, Z. Cristallogr. 219 (2004} 152} . However, the material is still X-ray amorphous (see Fig. 3b) . Crystallinity (Fig. 3c) can be achieved in the final activation treatment.

Example 2 (treatment with carbon dioxide)

Example 1 was repeated except for replacing 2.65 g H2O by 6.57 g solid CO2 (dry ice) and conducting the high pressure and temperature treatment over 15 hrs . The resulting catalyst material had almost the same chemical composition as that of example 1 and very similar properties. . Comparative Example 1 (hydrothermal preparation without subsequent p/T treatment)

Ammonium heptamolybdate (NH4 ) 6M07O24.4H2O (Merck), vanadyl sulfate VoSO₄-S H₂O (Riedel-deHaen) , telluric acid Te (OH) 5

(Aldrich) and ammonium niobium oxalate

(NH₄) [NbO(C₂CM) 2 (^H2°) 2] -3H₂O (Aldrich) were used as starting materials to prepare the initial suspension containing the catalyst metals in a molar ratio of Mo/V/Te/Nb =^■ 1/0.25/0.23/0.12. For this purpose, ammonium molybdate was added to 100 ml H₂O in an amount corresponding to 52 mmol Mo at 353 K (80⁰C) followed by the addition of vanadyl sulfate

(13 mmoi V) and telluric acid (12 mmol Te) . The resulting aqueous mixture was cooled to 313 K (40⁰C) and added to a second solution prepared by dissolving ammonium nobium oxalate in an amount corresponding to 6.5 mmol Nb in 50 ml H₂O at 313 K (4O⁰C) . After replacing residual air by nitrogen, hydrothermal synthesis was carried out in an autoclave made of Hastelloy at 448 K (175⁰C) for 48 hrs thereby producing an autogenous pressure of about 0.9 MPa. The obtained suspension was centrifuged for 20 min and the precipitate washed with 100 ml of bidistilled water for 10 min and filtered under vacuum. Finally, the solid material was dried in a muffle furnace at 353 K (80⁰C) for 16 hrs in air resulting in a catalyst precursor material.

3 g of the precursor were heated in a rotating oven in synthetic air (100 ml /min) to 598 K (325°C) at a heating rate of 10 K/min and calcined at this temperature for 1 hr. The calcined catalyst precursor material was then subjected to a final activation treatment in a rotating oven under flowing inert gas (nitrogen, 100 ml /min) for 2 hrs at 923 K (65O⁰C) . The resulting catalyst material had a BET surface of 1 m²/g and a metal composition (EDX analysis) of Mo/V/Te/Nb = 1/0.22/0.186/0.29. The Ml phase content was more than 95wt.- Example 3 {Oxidation test)

The performance of the catalysts according to Example 1 and Comparative Example 1 was evaluated in the following oxidation process.

Partial oxidation of propane was carried out in a reactor for parallel catalytic testing with twelve fixed bed quartz tubular reactors (i.d., 4 mm; length, 225 mm), working at atmospheric pressure. The catalyst material was pressed by a hydraulic press (8 ton of force on a round surface of approx. 3cm diameter} , crushed and sieved manually to particle sizes between 0.24 to 0.45 mm. The feed flow rate was fixed at a gas hourly space velocity (GHSV) of 4800 h^"1 (at STP: standard temperature pressure conditions) with a catalytic bed volume of 0.5 ml and the corresponding bed height.

The feed composition was 2.8 vol-% propane, 6.3 vol~% oxygen, 50.9 vol-% nitrogen and 40 vol-% steam. The reaction was carried out at 673 K (400⁰C) . The products were analyzed by two on-line gas chromatograph systems. At the reactor outlet, the produced gases were analyzed by two GCs and the conversion of propane, the selectivity of acrylic acid and the acrylic acid yield calculated. The first GC was configured with molecular sieve and Porapak columns coupled with a thermal conductivity detector for the analysis of inorganic gases and hydrocarbons (C1-C3) . The second GC was configured with a capillary column (HP-FFAP) coupled with a flame ionization detector for the analysis of oxygenated products .

Conversion (%) = mols of consume propane/mols of supplied propane Selectivity {%)= mols of formed acrylic acid/mols of consumed propane x 100 Yield (%) = mols of formed acrylic acid/mois of supplied propane x 100 The results are shown in the following table 1

Table 1

Catalytic Performance

Claims

1. A method for preparing a metal oxide catalyst material comprising a mixed oxide comprising molybdenum (Mo) , vanadium (V), Tellurium (Te) and niobium (Nb), said process comprising the steps of

(i) subjecting a catalyst precursor mixture comprising molybdenum (Mo), vanadium (V), tellurium (Te) and niobium (Nb) to a calcination treatment in an oxygen-containing atmosphere at a temperature of 150 to 400⁰C to prepare a calcined catalyst material,

(ii) subjecting the calcined catalyst material to a treatment at a pressure of at least lOMPa and a temperature of at least 400⁰C in the presence of an inert fluid phase to prepare a catalyst material comprising a mixed oxide comprising molybdenum (Mo) , vanadium (V) , tellurium (Te) and niobium (Nb) .

2. Method according to claim 1, wherein the catalyst material obtained in step (ii) is subjected to an activation treatment in an inert gas atmosphere at a temperature of 350 to 700⁰C.

3. Method according to claim 1 or 2 wherein said catalyst precursor mixture is made from salts and acids comprising molybdenum (Mo) , vanadium (V) , tellurium (Te) and niobium (Nb) .

4. Method according to claim 1 or 2 wherein said catalyst precursor mixture is prepared in a method comprising the steps of preparing a solution or dispersion of salts and acids comprising at least molybdenum (Mo) , vanadium (V) , tellurium (Te) and niobium (Nb) in a solvent, optionally precipitating dissolved salts and acids of these metals, removing the solvent from the solution, dispersion or optionally the precipitate to form a solid mixture and drying said solid mixture to form a catalyst precursor mixture.

5. Method according to claim 1 or 2 wherein the calcination treatment is conducted at 200 to 35O⁰C.

6. Method according to claim 1 or 2 wherein the treatment according to step (ii) is conducted at a temperature of at least 46O⁰C and a pressure of at least 16MPa.

7. Method according to claim 1 or 2 wherein the fluid phase is formed by at least one compound having a molecular weight of less than 150 and consisting of at least two different elements selected from the group consisting of C, S, 0 and H.

8. Method according to claim 7 wherein said compound is selected from carbon dioxide, water and sulphur dioxide .

9. Method according to claim 2, wherein the activation treatment is conducted at 580 to 67O⁰C.

10. Method according to claim 1 or 2 wherein the average composition of the metal oxide catalyst material is within the ranges defined by the following formula (I) :

MoV_aTe_bNb_cZ_dO_{χ (I)}

wherein a = 0.15-0.50, b = 0.02-0.45, in particular 0.05-0.40, c = 0.05-0.45, d < 0.05 and x is the molar number of oxygen binding to the metal atoms present in this mixed metal oxide which follows from the relative amount and valence of the metals elements, and Z is at least one element selected from Ru, Mn, Sc, Ti, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Rh, Pd, In, Sb, Ce, Pr, Nd, Te, Sm, Tb, Ta, W, Re, Ir, Pt, Au, Pb, and Bi,

and preferably within the ranges defined by the following formula (II) :

MoV_aTe_bNb_cZ_dO_{x (II)}

wherein a = 0.28-0.40, preferably 0.29-0.38, in particular 0.30-0.36, b = 0.03-0.30, in particular 0.05-0.25, c = 0.05-0.20, and Z, d and x are as defined above .

11. Method according to claim 1 or 2 wherein the metal oxide catalyst material has a crystallinity degree of at least 80wt.-% and a content of Ml phase in an amount of at least 85wt.-%, preferably at least 90wt.-%, more preferably at least 95wt.-%, based on the entire amount of crystalline metal oxide catalyst phase.

12. A metal oxide catalyst material comprising a mixed metal oxide comprising molybdenum (Mo), vanadium (V), Tellurium (Te) and niobium (Nb) wherein said metal oxide catalyst material has a crystallinity degree of at least 80wt.~% and a content of Ml phase in an amount of at least 85wt.-% based on the entire amount of crystalline metal oxide phase and is obtainable by a method as defined in any of claims 1 to 10.

13. Metal oxide catalyst material according to claim 12 wherein the average composition of the metal oxide catalyst material is within the ranges defined by the following formula (I) :

MoV_aTe_bNb_cZ_dO_{x (I)}

and preferably within the ranges defined by the following formula (II) :

MoV_aTe_bNb_cZ_dO_{x {II)}

14. A metal oxide catalyst material comprising a mixed oxide comprising molybdenum (Mo) , vanadium (V) , Tellurium (Te) and niobium (Nb) wherein said metal oxide catalyst material has a crystallinity degree of at least 80wt.-% and comprises Ml phase in an amount of at least 85wt.-% based on the entire amount of crystalline metal oxide catalyst phase, and the average composition of said material is within the ranges defined by the following formula (II) :

MoV_aTe_bNb_cZ_dO_{x (II)} wherein a - 0.28-0.40, preferably 0.29-0.38, in particular 0.30-0.36, b = 0.03-0.30, in particular 0.05-0.25, c - 0.05-0.20, d < 0.05 and x is the molar number of oxygen binding to the metal atoms present in this mixed metal oxide which follows from the relative amount and valence of the metals elements, and Z is at least one element selected from Ru, Mn, Sc_r Ti, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Rh, Pd, In, Sb, Ce, Pr, Nd, Te, Sm, Tb, Ta, W, Re, Ir, Pt, Au, Pb, and Bi.

15. Metal oxide catalyst material according to claim 12, 13 or 14 wherein the Ml phase content is at least 90wt.-%, in particular at least 95wt.-%.

16. Metal oxide catalyst material according to claim 12, 13 or 14 wherein said catalyst, if used in the oxidation of propane to acrylic acid, displays a propane conversion rate of at least 40% and selectivity for acrylic acid of at least 65%.

17. A method for producing an oxidized hydrocarbon from a hydrocarbon, which comprises subjecting the hydrocarbon to a vapour phase catalytic oxidation reaction in the presence of oxygen and a catalyst comprising a metal oxide catalyst material as defined in any of claims 12 to 16.

18. Method according to claim 17 wherein the hydrocarbon is an aikane and the oxidized hydrocarbon is an unsaturated carboxylic acid.

19. Method according to claim 18 wherein the aikane is propane and the unsaturated carboxylic acid is acrylic acid .

20. Method according to claim 17 wherein the hydrocarbon is an aikane or alkene, the reaction is conducted in the presence of ammonia and the oxidized hydrocarbon is a nitrile .

21. A method for increasing the content of Ml phase in a calcined metal oxide catalyst comprising oxides of molybdenum (Mo), vanadium (V), Tellurium (Te) and niobium (Nb) , which comprises the step of subjecting the calcined metal oxide catalyst to a treatment at a pressure of at least lOMPa and a temperature of at least 400⁰C in the presence of an inert fluid phase.

22. Method according to claim 21, wherein the treated calcined catalyst is subjected to an activation treatment in an inert gas atmosphere at a temperature of 350 to 700⁰C, in particular 580 to 67O⁰C.

23. Method according to claim 21 or 22 wherein the calcined metal oxide catalyst has a content of less than 70wt.~% Ml phase prior to the treatment and more than 85wt.-% Ml phase after the treatment each based on the total amount of crystalline catalytic metal oxide phase.

24. Method according to claim 21, 22 or 23 wherein the calcined catalyst to be treated is

(i) removed from a continuous process for producing an unsaturated carboxylic acid, which comprises subjecting an alkane to a vapour phase catalytic oxidation reaction in the presence of oxygen and said calcined catalyst, and

(ii) recycled at least in part to said process after the method of increasing the content of Ml phase has been performed.