EP1984114A2

EP1984114A2 - Preparing a compound comprising a combination of two crystal phases

Info

Publication number: EP1984114A2
Application number: EP07730972A
Authority: EP
Inventors: Jean-Marc Millet; Philippe Lacorre; Quyen Huynh
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite du Maine
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite du Maine
Priority date: 2006-02-14
Filing date: 2007-02-14
Publication date: 2008-10-29
Also published as: JP2009526730A; US20090171118A1; FR2897350A1; WO2007093702A2; JP5225865B2; US8207080B2; FR2897350B1; WO2007093702A3

Abstract

The invention concerns a compound comprising a combination of two crystal phases. The first crystal phase corresponds to the formula: AaEbVcModPeOfHg wherein A is an alkali-metal; E is Te, Sb or Bi; and 0 ≤ a ≤ 3, 0 < b ≤ 3, 0 ≤ c ≤ 3, 0 < d ≤ 13, 0 < e ≤ 2, 0 ≤ g ≤ 3. The second crystal phase corresponds to the formula ZgMohXiOj wherein: Z is selected among trivalent rare earths; X is selected among the elements V, Ga, Fe, Bi, Ce, Ti, Sb, Mn, Zn, Te; and 0 < g ≤ 3, 0 ≤ h ≤ 3, 0 ≤ i ≤ 1. The indices f and j represent the number of oxygen atoms required for satisfying the relative valency and atomic proportions of the elements present. The invention also concerns the method for preparing said compound, and its use in particular as catalyst for oxidizing alkanes.

Description

Preparation of a compound comprising the combination of two crystalline phases

The present invention relates to a compound comprising a combination of two crystalline phases, a crystalline phase being of the phosphomolybdic type, and a process for its manufacture and several uses of this compound.

The compound according to the invention is more particularly intended to catalyze certain reactions of oxidation of alkanes, and in particular the oxidation of isobutane to methacrylic acid and methacrolein, which leads to the formation of methyl methacrylate.

In order to obtain methacrylic acid and methacrolein by oxidation of isobutane, two publications mentioned below are known, catalysts comprising two phases, in which one of the two phases is of the phosphomolybdic type.

In the publication of Q. Deng et al., J. Mol. Catal.A, 229, (2005) 165, the compound used as a catalyst consists of a phospho (arsenio) molybdic acid phase and a phase based on ferrous iron orthophosphate. In both phases, some of the phosphorus must be substituted with arsenic. This catalyst therefore contains a large amount of arsenic, the toxicity of which limits the industrial use of the catalyst.

The publication of T. Ushikobo, Catalysis Today, 78, (2003) 79-84, discloses a catalyst comprising a first phase formed of a heteropoly acid of formula (PMon VO ₄₀ ) and a second phase consisting of a tantalum oxide Ta ₂ O ₅ previously treated with sulfuric acid. This catalyst has a relatively low productivity, of the order of 30 g.Kg ^-1 _Ca t _a -h ^"1 . In addition, the stability over time of the sulphated oxides is unsatisfactory, and the selectivity of the catalyst to methacrylic acid relative to methacrolein is low.

The object of the invention is to provide a compound which can be used as a catalyst which does not have the abovementioned disadvantages, that is to say a compound which does not contain toxic elements and exhibits stable catalytic properties over time.

According to a first aspect, the subject of the invention is a compound comprising the combination of two crystalline phases. The first crystal phase is phosphomolybdic type and has the formula (1): A _a E _b V _c P _d Mo _e O _f M _g (1) wherein:

- A is an alkali metal; E is selected from the elements Te, Sb or Bi, preferably Te or Bi;

the indices a, b, c, d, e, g are such that: 0 ≤ a <3, 0 <b <3, 0 <c <3, 0 <d <13, 0 <e ≤2, 0 ≤ g <3, and f represents the number of oxygen atoms needed to satisfy the valence and relative atomic proportions of the elements present.

In a preferred embodiment, the alkali metal A is cesium.

Element E is preferably tellurium.

The second crystalline phase corresponds to formula (2):

ZgMOhXiO _j (2) in which:

Z is chosen from trivalent rare earths;

X is selected from the elements V, Ga, Fe, Bi, Ce, Ti, Sb, Mn, Zn, Te, preferably V, Ga, Fe, Bi, Ce, Ti, Mn, Zn, Te;

the indices g, h, and i are such that: 0 <g <3, 0 <h <3, 0 <i <1 and j represents the number of oxygen atoms necessary to satisfy the valence and the atomic proportions relating to the elements present.

Advantageously, the first and second crystalline phases are free of Sb atom which has a high toxicity, thus allowing an easier use of the compound on an industrial scale. In a preferred embodiment, the element Z represents lanthanum.

Also according to a preferred embodiment, the element X is vanadium.

According to an advantageous embodiment, the proportion of the second crystalline phase is less than or equal to 50% by weight relative to the total weight of the compound.

The first crystalline phase can meet the formula Cs ₂ Teo, 3Vo, -ιHo, ₄ PMo-i ₂ θ ₄ o. The second crystalline phase can meet the formula La ₂ Mo ₂ Og or the formula

₂ MB-1, ₁ 9Vo IO _8, 95, the vanadium has been partially substituted for molybdenum in relation to the above formula.

As examples of compounds according to the invention, mention may be made of those formed, respectively, of the combination of a first crystalline phase of formula Cs ₂ Teo, 3Vo, 1Ho, ₄ PMO ₂ C ₄ O ₄ and a second crystalline phase of formula ₂ Mo ₂ O ₉ , and the combination of a first crystalline phase of formula Cs ₂ Teo _{, 3} Vo _, 1Ho, ₄ PMO ₂ 0 ₄ o and a second crystalline phase of formula La ₂ Mo ₁₁₉ Vo _1I O ₈₁₉₅ . In general, the process for preparing a compound according to the invention comprises the following steps:

- synthesis of the first crystalline phase in the form of a first powder;

- synthesis of the second crystalline phase in the form of a second powder; mixing said first and second powders by grinding.

This method has the advantage not only of allowing great flexibility in the experimental conditions used, since each phase is prepared separately, its formula can thus be easily adapted to the application in question, but also to be simple to implement. the final mixture of the two phases is carried out by simple mechanical mixing of the powders.

The synthesis of the first crystalline phase can comprise the following successive steps:

preparation of an aqueous solution containing phosphomolybdic acid and a compound of element E; mixing said aqueous solution with an aqueous solution containing a salt of element A;

precipitation, drying and calcination of said mixture so as to obtain a solid phase;

mixing said solid with a solution of toluene containing a vanadium compound;

- filtration and drying at room temperature.

Among the compounds of element E, mention may be made of the acids of element E, such as telluric acid, chlorides or alkoxides of element E. Among the salts of element A, mention may be made of carbonates or nitrates. Cesium carbonate is particularly preferred.

Among the vanadium compounds, mention may be made of vanadium oxide or vanadium acetylacetonate.

The synthesis of the second crystalline phase may comprise the following successive steps: - grinding mixture of molybdenum oxide MoO ₃ in the solid state and an oxide of element Z in the solid state;

heating said mixture to a temperature of the order of 500 ° C .; - Performing successive anneals of said mixture, at a temperature above 500 ⁰ C, until the final product in powder form. When element Z represents La, the annealing temperature is between about 850 ^° C. and 960 ^° C. The oxide used is then La ₂ O ₃ . When it is desired to obtain a second crystalline phase of formula (2) containing an element X, that is to say in which the index i is different from 0, the molybdenum oxide is partially replaced by a compound of the element X. If X represents vanadium, the compound used may be vanadium oxide or ammonium vanadate.

A compound according to the invention is advantageously used as a catalyst for the oxidation of alkanes, in particular isobutane, propane and pentane. It can also be advantageously used as a catalyst for the oxidation of isobutene and methacrolein to methacrylic acid.

The compound according to the invention is particularly effective as a catalyst for the oxidation of isobutane to methacrylic acid and methacrolein, as the examples hereinafter describe. The preparation of methacrylic acid and methacrolein from isobutane comprises passing a gaseous mixture containing isobutane and water, and optionally an inert gas and / or molecular oxygen, on a compound according to the invention. The tests carried out have shown that a synergistic effect between the two phases of the compound according to the invention took place, the observed activity does not correspond to the simple sum of the activities of the pure phases. This synergistic effect between the two phases is due to the fact that the second crystalline phase not only plays a supporting role, but also a co-catalyst role by selectively converting isobutane to isobutene, the isobutene formed being then selectively converted. methacrylic acid on the phosphomolybdic compound forming the first phase. The present invention is illustrated hereinafter by concrete examples of embodiment, to which it is however not limited. Examples 1 and 2 describe the preparation and characterization of compounds according to the invention, as well as the study of their catalytic properties in the oxidation reaction of isobutane to methacrylic acid and methacrolein. Various methods have been used to characterize these compounds. These characterization methods are described below. Chemical analysis

For the chemical analysis of these compounds, the cesium was determined by atomic emission in air-acetylene flame (on a spectrometer marketed by Perkin-Elmer) and the other elements were determined by atomic emission in a plasma ICP (Inductively Coupled Plasma), on a spectrometer marketed by Spectro. The wavelengths used for the analysis are shown in Table 1.

Table 1

Measurement of the specific surface area

Specific surfaces were measured by the BET method. The amount of nitrogen adsorbed at -196 ° C was measured by volumetry. Before each measurement, the samples were desorbed for 2 hours under a secondary vacuum at 250 ^° C.

Infrared spectrometry

The infra-red spectra were recorded in transmission between 4000 and 400 cm ^-1 on a Fourier transform apparatus, marketed by BRUCKER under the reference VECTOR 22. The samples were prepared in the form of pellets after a dilution of about 1 mg of solid in 300 mg of KBr.

Catalytic Tests The catalytic tests for the oxidation reaction of isobutane were carried out on an apparatus such as that shown diagrammatically in FIG.

The gases used (hydrogen, oxygen, isobutane, nitrogen) are distributed respectively by the valves 1a, 1b, 1c and 1d, by means of mass flowmeters 2a, 2b, 2c and 2d of BROOKFIELD type allowing precise regulation of the respective flow rates. The hot box 4 is maintained at 169 ° C by a temperature control system 3 associated with an oven 5, to avoid condensation in the pipes. The water is synthesized in the hot box 4 from oxygen and hydrogen on a platinum catalyst supported on Al ₂ O ₃ . The box 4 is equipped with a four-way valve 9. A fixed-bed reactor Pyrex 6 fixed with a condensation system 7 is used. The catalyst is placed on a frit and a thermowell (not shown) allows the measurement of the temperature directly in the catalytic bed. It is this temperature that is reported in all the tables of results. Typically, the masses of the tested catalysts are 2.0 g.

The condensation system 7 is installed at the outlet of the reactor 6 in order to trap the condensable organic compounds. The trap containing an aqueous hydroquinone solution is kept at 0 ° C. in ice. The non-condensable gases (CO, CO ₂ , C ₄ H ₁₀ , C ₄ H ₈ , N ₂ and O ₂ ) are analyzed online by chromatography after the trapping system.

The analysis system consists of two gas chromatographs and a liquid chromatograph. The first two, mounted in line, allow gas analysis. The first 10 chromatograph equipped with a molecular sieve 11 (CP-I MOLS ^® 5Â EVE), a packed column 13 (Porapak Q ^®) and a six-way valve 12, permits the separation and quantitation of CO , CO ₂ , N ₂ and O ₂ . The device is equipped with a detector 14 which is a katharometer, and the carrier gas is helium. This system allows the detection of isobutane and isobutene, but not their separation. A second chromatograph (not shown for reasons of simplification) equipped with a filled column (Silica-PIot ^® ) is used to effect this separation. Four analyzes are performed for each temperature, which represents a condensation time of about 120 minutes.

The condensable products (methacrylic and acetic acids, methacrolein, acetone and acrylic acid) are analyzed on a chromatograph CHROMPACK 9001 equipped with a FID detector comprising a filled column 16 (Silica-PIOT ^® ) and a column 17 (CP-Sil ^® ), and a CPWAX58 / FF ^® column. The measurement system is shown schematically under the reference 18, and the reference system is shown schematically under reference 19. The carrier gas used is nitrogen. The injection is carried out using a CHROMPACK automatic sample changer type CP9005. The volume injected is 0.5 mL and 5 injections per sample are made. Uncertainties on measurements are less than 2%. The chromatogram obtained shows the characteristic peaks of methacrylic acid, acrylic acid, acetic acid and methacrolein. The activities and selectivities were calculated by considering the following reaction products: methacrylic acid (AMA), methacrolein (MA), acetic acid (AAc), acrylic acid (Acr), acetone ( Ace), CO and CO ₂ .

In conversion and selectivity calculations, an F factor was introduced for internal calibration to eliminate errors due to dilution effects. This factor is determined by the ratio of the number of moles of initial nitrogen to the number of moles of nitrogen measured at the outlet of the reactor. The conversion of the reagent R (isobutane or oxygen) is thus defined by the equation:

N (R) _O -N (R) F ^{C (R) =} N (R) ₀ wherein N (R) ₀ is the number of moles of reactants R in the initial reaction mixture, and N (R) is the number of moles of reagent R at time t during the test.

The selectivity (S) of the various products is defined by the following equation: _{c (r)} - N (P) ^* n (P) ΣN (P) * n (P) wherein N (P) is the number of moles of product P at time t during the test, nc (P) is the carbon (or oxygen) number in the product molecule P. The product yield P (R (P)) is defined by the equation:

R (P) = C (RfS (P)

The carbon and oxygen balances were calculated for each analysis by measuring the amounts of products produced and conversions to reagents. These balances are made taking into account the following equations:

C ₄ H ₁₀ + 2 O ₂ -> C ₄ H ₆ O ₂ + 2 H ₂ O

C ₄ H ₁₀ + 3/2 O ₂ - * C ₄ H ₆ O + 2 H ₂ O

C ₄ H ₁₀ + 5/2 O ₂ 2 2 C ₂ H ₄ O ₂ + H ₂ O

3C ₄ H ₁₀ + 7/2 O ₂ ^ 4 C ₃ H ₆ O + 3 H ₂ O 3C ₄ H ₁₀ + 15/2 O ₂ - * 4 C ₃ H ₄ O ₂ + 7 H ₂ O

C ₄ H ₁₀ + 13/2 O ₂ ^ 4 CO ₂ + 5 H ₂ OC ₄ H ₁₀ + 9/2 O ₂ ^ 4 CO + 5 H ₂ O The experimental conditions of the catalytic tests are as follows, unless otherwise stated: two grams of catalyst are charged to the reactor. The reaction temperature varies between 310 and 370 ^° C. The total flow rate varies between 14 and 24 ml.min ^{-1, the} measured density of the catalysts being 1, 2, the contact time is typically of the order of 4, 8 s for one gram of catalyst, ie an hourly space velocity of 2600 h ^-1 . The catalysts were tested under standard feed conditions defined by the following ratios: C ₄ H 0/0 ₂ / H ₂ θ / N ₂ = 27 / 13.5 / 10 / 49.5.

In order to ensure the absence of conversion of isobutane without catalyst, the reactor was tested in a vacuum at 35 ^° C. At this temperature, no conversion of isobutane was observed. The linear evolution of the conversion to isobutane as a function of the isobutane flow rate was also monitored.

Example 1

Compound of formula Cs ₂ Te ₀ , 3 Vo, iHo, ₄ PM- _{I 2} O ₄₀ / La ₂ Mo ₂ O ₉ Two solutions are prepared independently of one another. The first solution is prepared by solubilizing 8.16 g of phosphomolybdic acid (marketed by Fluka under the reference 79560) and 0.18 g of telluric acid (marketed by Interchim under the reference 014197) in 140 ml of water. The second solution is prepared by solubilizing 1, 3 g of cesium carbonate

(marketed by Interchim, under the reference 012887) in 0.4 ml of water.

The second solution is added to the first with stirring. The solid precipitate is recovered in a rotary evaporator at 80 ° C, dried in an oven at 120 ° C and calcined at 360 ° C for 6 hours (up to 5 °. Min ^'1, air flow of 50 ml. Min ^{" 1} ) The solid obtained (6.5 g) is reacted with vanadium acetylacetone (V [CsO ₂ H ₇ ] _S )

(0.16 g) (marketed by Fluka under the reference 1347-99-8) dissolved in toluene.

The progress of the reaction is followed by the color change of the solution which is initially colored, and loses its color when the vanadium has reacted with the solid which is not soluble. The reaction is complete after 10 to 12 hours under argon.

Characterization of the first crystalline phase The solid has been characterized by chemical analysis. The atomic ratios Cs / Mo, P / Mo, Te / Mo and V / Mo were calculated from the results of the chemical analysis, and the specific surface area S was measured after catalytic test. These results are shown in Table 2.

Table 2

The stoichiometries calculated correspond to those desired. The solid was also characterized by infra-red spectroscopy after annealing under nitrogen at 36 ^° C. The allocation of the bands is presented in Table 3.

Table 3

The observed lines are characteristic of the Keggin structure anion of formula [PMθi2θ4o, ₄ ]. ^" The indices a, b, c and d correspond to the oxygen atoms located at different positions in this anion. a central tetrahedron (PO ₄ ), surrounded by 12 MoO _δ octahedra in four groups of 3. The trimers, in which the octahedra share edges, are connected to each other and to the central tetrahedron by vertices. are equivalent, while those of oxygen atoms are not, there are four types of oxygen atoms: 4 oxygen atoms (O ₃ ) common to the tetrahedron (PO ₄ ) and three Mo ocβ octahedra sharing edges, 12 oxygen atoms (O _b ) common to two octahedrons sharing a vertex, 12 oxygen atoms (O _c ) common to two octahedra sharing one edge, and 12 oxygen atoms (O _d ) bound to one bond double to a single metal atom. Catalytic properties of the first crystalline phase a) Evolution of the catalytic properties as a function of the reaction time

The evolution of the catalytic properties of the first crystalline phase as a function of time has been studied. FIG. 2 shows the evolution of the conversion (C ₁ %) with time (t, in hours) during the catalytic test under the standard conditions defined above, at the reaction temperature of 35 ^° C. deactivation of the sample at the beginning of the reaction, for about 5 hours, then stabilization. Measurements of the specific surface before and after catalytic test show that it has decreased from 28 to 11.8 m ² .g ^-1 .

b) Evolution of Catalytic properties as a function of temperature

The catalytic performances of the sample are indicated in Table 4 below, the tests having been carried out at three temperatures T: 330, 345 and 350 ^° C.

Table 4

c) Evolution of Catalan properties as a function of contact time

The influence of the contact time on the catalytic properties of the sample was also studied, by modifying the flow rate of the reagents under standard test conditions, at 350 ^° C. FIG. 3 shows the evolution of the selectivities (S,% ) to methacrolein (4), meth acrylic acid (A), acetic acid (•), CO (o) and CO ₂ (D), and the conversion (C,%) of isobutane (m) depending on the contact time (t, in seconds). It is observed that the increase in the contact time leads to an increase in the conversion of isobutane, selectivities in CO, CO ₂ and acetic acid, and a decrease in the selectivity to methacrolein and methacrylic acid. d) Evolution of the catalytic properties according to the molar ratio isobutane / oxygen

Since the composition of the filler plays a very important role in the oxidation reaction of isobutane to methacrylic acid, the molar ratio of isobutane / oxygen (iBu / O ₂ ) was also chosen as a study parameter. The contact time was set at 4.8 seconds, the percentage of nitrogen and water respectively at 49.5 and 10% and the reaction temperature at 34 ^° C. The results obtained are shown in the table. 5.

Table 5

e) Evolution of the catalytic properties as a function of the partial pressure of nitrogen The influence of the partial pressure of nitrogen on the catalytic performances was also studied. The contact time was set at 4.8 sec, the isobutane / oxygen ratio at 2 and the reaction temperature at 340 ° C. The catalytic results obtained are shown in Table 6.

Table 6

f) Characterization of the first phase after catalunic test

The first phase was characterized after catalytic test by X-ray diffraction and infrared spectroscopy. The diffractogram after test is comparable to that before testing, showing that no profound change has occurred. Infrared spectroscopy characterization results are shown in Table 7.

Table 7

We find the same lines as those observed on the sample before the implementation of the catalytic test. Synthesis of the second crystalline phase of formula LapMogOg

This phase is prepared by reaction in the solid state between M0O ₃ (marketed by Chernpiir under the reference 005565) and La ₂ θ ₃ (marketed by Alfa Aesar under the reference 011264). For this purpose, the reagents are weighed in stoichiometric proportions and ground in an agate mortar. La ₂ O ₃ compound was preheated to 1000 ⁰ C to prevent its hydration in time and the formation of La (OH) ₃ .

The mixture is then transferred to an alumina boat. It undergoes a preheating at 500 ⁰ C and two successive annealing with a duration of 15 hours at a temperature of 96O ⁰ C, between which the mixture is ground in acetone to homogenize and ensure a good dispersion of the grains, and this until the pure product. The purity of the product, in terms of phase composition, is controlled by X-ray diffraction.

Characterization of the second crystalline phase

The second phase was characterized by X-ray diffraction. The diffractogram of the second phase, on which the intensity is indicated in counts per second (CPS) (Figure 4), corresponds to that of the cubic phase of lanthanum molybdate.

Catalytic properties of the second crystalline phase

The catalytic performances of the second crystalline phase are indicated in Table 8 below, the test having been carried out at a temperature of 36 ^° C. with the Isobutane / 0 ₂ / H ₂ O / N ₂ = 27/13 re-equilibrium mixture. 5 / 10.0 / 49.5 and a contact time of 4.8 s.

Table 8

Preparation of the final compound

The final compound is prepared by mixing the first and second phases by simple mechanical grinding.

Different tests were carried out by varying the respective proportions of the two phases. The composition and numbers assigned to the samples corresponding to these tests are summarized in Table 9. Table 9

Catalytic properties of the final compound

The catalytic properties of compounds 1a, 1b and 1c were tested under several experimental conditions.

The results obtained are grouped together in Table 10 and in FIGS. 5a and 5b, which represent the evolution of the conversion C (in%) of isobutane (m), the selectivities S (in%) of AMA (A) and MA (4) and the yield of AMA and MA (0) at 345 ° C. (FIG. 5a) and 369 ° C. (FIG. 5b) as a function of the content by weight of lanthanum molybdate (denoted LM, in%). Each compound has been tested under standard conditions and at three different temperatures.

Table 10

There is an increase in low temperature methacrylic acid selectivity with a maximum of 30% lanthanum molybdate. The observed activity therefore does not correspond to the sum of the activities of the pure phases and a synergistic effect between phases takes place. The yield of methacrylic acid and methacrolein only drops by about 1% while the mixture contains 50% by weight of lanthanum molybdate.

The conditions of the catalytic test of the mixture with 20% molybdate (compound 1a) were modified by choosing a reaction medium richer in oxygen. The temperature is 36O ⁰ C and the contact time 4.8 seconds. The results obtained are shown in Table 11.

Table 11

Conversion increases without significant loss of selectivity. This gives a conversion of 22% with a methacrylic acid and methacrolein selectivity of 57% and a productivity which reaches 78 (gAM _A 'Kg ^-1 catalyst-h ^{~ 1} ).

X-ray diffractograms of compound 1c before catalytic test (diffractogram a) and after catalytic test (diffractogram b) are shown in FIG. 6 (the intensity is given in counts per second (CPS)). The signs (T) represent the peaks corresponding to the second crystalline phase La ₂ Mθ ₂ θg. These diffractograms do not show any phase transformation.

Example 2 Preparation of a compound of the formula Cs ₂ Teo, 3Vo, Ho _1, ₄ PMo ₁₂ 04O / La ₂ _Mo1> 9Vo, ₁ 08.95

This compound consists of a first phase identical to that of Example 1, and a different second phase.

The first crystalline phase was prepared using the same experimental conditions as for the first phase of Example 1.

Synthesis of the Second Crystalline Phase of the Formula LagMo ^ gVn ^ Og ^

The second crystalline phase was synthesized under experimental conditions similar to those described for the second phase of Example 1, with the exception that the compound V ₂ O ₅ (marketed by Alfa Aesar under the reference 81110) was added to MOO ₃ and La ₂ Os in stoichiometric proportions and that, after preheating, the mixture has undergone 7 successive annealings of a duration of 15 hours at a temperature of 925 ° C.

Characterization of the second crystalline phase

The second phase was characterized by X-ray diffraction. The diffractogram of the second phase (FIG. 7, in which intensity I is indicated in counts per second) corresponds to that of the cubic phase of lanthanum molybdate.

Catalytic properties of the second crystalline phase

The catalytic performances of the second crystalline phase are indicated in Table 12 below, the tests having been conducted at the temperature of 360 ^° C., with a contact time of 6 seconds, and by varying the feed conditions ( defined by the ratios C ₄ H 0/0 ₂ / H ₂ 0 / N ₂ ).

Table 12

Preparation of the final compound

Different tests were carried out by varying the respective proportions of the two phases. The composition and numbers assigned to the samples corresponding to these tests are summarized in Table 13.

Table 13

Catalytic properties of the final compound The results obtained under various reaction conditions for the compounds 2a and

2b are grouped in Table 14.

Table 14

It appears, with respect to the compounds 1a, 1b and 1c, that the catalysts obtained have both a greater activity and selectivity in AMA. The maximum activity is observed for the rich mixtures in the first crystalline phase (compound 2a) while an optimum of selectivity is obtained for the compound 2b (50% of the first crystalline phase / 50% of the second crystalline phase).

Evolution of Catalan properties as a function of CO content?

The catalytic performances of the compound according to Example 2 (Cs ₂ Te 0, 3 Ve, iHo _I 4 PMθi ₂ O 4 O / La ₂ Mθ 1,9Vo ₁ iθ 8,95) are indicated in Table 15 below and summarized in FIG. 8.

FIG. 8 represents the evolution of the productivity in (AMA + MA) (φ), the selectivity in (AMA + MA) (m) and the conversion of isobutane (A) as a function of the amount of CO ₂ in the reaction mixture.

The tests were conducted for a Cs2Teo, 3Vo, 1Ho, 4PMo1204o / La2Mo ₁ , 9Vo, 108.95 compound having a weight ratio of 50/50 between the first and second crystalline phases, a catalyst mass of 1.45 g , a contact time of 4.8 s, a temperature of 345 ^° C. The volume ratio of JC ₄ H ₁ O / O 2 / H ₂ O / N ₂ / CO ₂ is 40/20/10 / 58.5 / X where X represents the percentage of CO ₂ indicated in Table 15 below. Table 15

An increase in the selectivity for (AMA + MA) given by the sum of the selectivities for AMA and MA respectively is observed when the content of CO ₂ increases in the reaction mixture. Evolution of catalytic properties as a function of total pressure

The catalytic performances of the compound

Cs2Teo, 3Vo, iHo, ₄ PMθi ₂ θ ₄ o / La ₂ Mθi, 9Vo, iθ ₈ , 95 according to Example 2, are given in Table 16 below. The total pressure corresponds to the total pressure exerted on the reaction medium.

The tests were carried out for a compound

Cs2Teo, 3Vo, iHo, ₄ PMθi2θ ₄ O / La2Mθi, 9Vo, -ι0 ₈ , 95 having a weight ratio of 50/50 between the first and the second crystalline phase.

Two feed conditions (CA) are used: a: iC ₄ H ₁₀ / O ₂ / H ₂ O / N ₂ = 27 / 13.5 / 10 / 49.5; b: iC ₄ H ₁₀ / O ₂ / H ₂ θ / N ₂ = 40/20/10/30.

Table 16

These results demonstrate that an increase in total pressure induces a marked increase in both AMA + MA selectivity and productivity.

Claims

claims

A compound comprising the combination of a first and a second crystalline phase, the first crystalline phase being of the phosphomolybdic type, characterized in that the first crystalline phase corresponds to the formula (1): AaEbVcMOdPeOfHg (1) in which :

- A is an alkali metal;

E is selected from the elements Te, Sb or Bi;

the indices a, b, c, d, e, g are such that: 0 ≤ a <3, 0 <b <3, 0 ≤c <3, 0 <d <13, 0 <e <2, 0 < g ≤3, and f represents the number of oxygen atoms necessary to satisfy the valence and the relative atomic proportions of the elements present; and the second crystalline phase corresponds to formula (2):

ZgMOhXiO _j (2) in which:

Z is chosen from trivalent rare earths;

X is selected from the elements V, Ga, Fe, Bi, Ce, Ti, Sb, Mn, Zn, Te;

the indices g, h and i are such that: 0 <g <3, 0 <h <3, 0 <i ≤1, and j represents the number of oxygen atoms necessary to satisfy the valence and the atomic proportions relating to the elements present.

2. Compound according to claim 1, characterized in that A represents cesium.

3. Compound according to any one of claims 1 or 2, characterized in that E is Te or Bi.

4. Compound according to any one of claims 1 to 3, characterized in that E represents tellurium.

5. Compound according to any one of claims 1 to 4, characterized in that Z represents lanthanum.

6. Compound according to any one of claims 1 to 5, characterized in that X is V, Ga, Fe, Bi, Ce, Ti, Mn, Zn, Te.

7. Compound according to any one of claims 1 to 6, characterized in that

X represents vanadium.

8. Compound according to any one of claims 1 to 7, characterized in that the proportion of the second crystalline phase is less than or equal to 50% by weight relative to the weight of the compound.

9. Compound according to any one of claims 1 to 8, characterized in that the first crystalline phase corresponds to the formula Cs2Teo, 3V ₀ , iHo, ₄ PMθi2θ ₄₀ .

10. Compound according to any one of claims 1 to 9, characterized in that the second crystalline phase corresponds to the formula La ₂ Mθ ₂ θg.

11. Compound according to any one of claims 1 to 9, characterized in that the second crystalline phase corresponds to the formula La ₂ Mo-1, 9V ₀ , - _! O ₈ , 95.

12. Compound according to any one of claims 1 to 11, characterized in that it is formed from the combination of a first crystalline phase of formula Cs ₂ Teo, ₃ Vo, - | Ho, 4 PMθi ₂ θ ₄ o and a second crystalline phase of formula La ₂ Mo ₂ Og.

13. A compound according to any one of claims 1 to 12, characterized in that it is formed of the combination of a first crystalline phase of formula Cs ₂ Te ₀ , 3Vo, iHo, 4 PMθi2θ4o and a second phase crystalline formula of ₂ Mo-1, ₉ Vo, ₈ , 95-

14. Process for the preparation of a compound according to any one of Claims 1 to 13, characterized in that it comprises the following stages:

- synthesis of the first crystalline phase in the form of a first powder;

15. The method of claim 14, characterized in that the synthesis of the first crystalline phase comprises the following successive steps:

mixing said solid with a solution of toluene containing a vanadium compound;

- filtration and drying at room temperature.

16. Process according to claim 15, characterized in that the compound of element E is an acid, a chloride or an alkoxide, in that the salt of element A is a carbonate or a nitrate, and in that the vanadium compound is vanadium oxide or acetylacetonate.

17. The method of claim 14, characterized in that the synthesis of the second crystalline phase comprises the following successive steps:

- mixing, by grinding, of molybdenum oxide MOO ₃ in the solid state and an oxide of element Z in the solid state;

heating said mixture to a temperature of the order of 500 ^° C .; - Performing successive anneals of said mixture, at a temperature greater than

500 ⁰ C, until the final product is obtained in powder form.

18. The method of claim 17, characterized in that a compound of the element X partially replaces the molybdenum oxide.

19. The method of claim 18, characterized in that the compound of the element X is a vanadium oxide or ammonium vanadate.

20. Use of a compound according to any one of claims 1 to 13 as a catalyst for the oxidation of alkanes.

21. Use of a compound according to any one of claims 1 to 13 as a catalyst for the oxidation of isobutane to methacrylic acid and methacrolein.

22. Use of a compound according to any one of claims 1 to 13 as a catalyst for the oxidation of isobutene and methacrolein to methacrylic acid.