International Patent Application WO 98/02244 discloses a process for the metathesis of alkanes in which one or more alkanes is/are reacted over a solid compound comprising a metal hydride grafted to a solid support. The preparation of the solid compound comprises first grafting an organometallic compound to a support formed from a solid oxide and then hydrogenolysis of the grafted compound, carried out by treatment of the latter with hydrogen or another reducing agent at a temperature of 150°C, so as to form a metal hydride grafted to the support. This metal hydride is subsequently employed as catalyst in reactions for the metathesis of alkanes.

It has been found that by preparing this solid compound under different conditions, in particular by hydrogenolysis carried out in a specific and high temperature range, a supported metal compound is obtained which is different from that described above and which exists in the form of a composition of two metal entities.

Furthermore, it has been observed that this novel supported metal compound surprisingly exhibits a greatly increased activity with respect to the solid compounds known to date, in particular in reactions for the metathesis of alkanes.

The present invention first of all relates to a supported metal compound comprising a solid support to which are grafted at least two types of metal atom, Me,

one in a form (A) of a metal compound where the metal atom is bonded to at least one hydrogen atom and/or to at least one hydrocarbon radical, and the other in a form (B) of a metal compound where the metal atom is bonded solely to the support and optionally to at least one other element which is neither a hydrogen atom nor a hydrocarbon radical.

The term"metal atom, Me, grafted to a support"is understood to mean generally a metal atom, Me, which is fixed by at least one single or multiple bond to the support and which is in particular bonded directly to at least one atom constituting the solid support.

The compound according to the invention is characterized in particular in that at least two different types of metal atom, Me, are grafted to the same solid support: on the one hand, a metal atom, Me, bonded to at least one hydrogen atom and/or to at least one hydrocarbon radical, and, on the other hand, a metal atom, Me, alone, that is to say without a bond other than with the support and optionally with at least one component which is neither a hydrogen atom nor a hydrocarbon radical. Thus, the supported metal compound generally comprises, on the same solid support, at least two types of metal atom, Me, bonded to the said support, one in the form (A) of a metal hydride and/or of an organometallic group in which the metal atom, Me, is bonded to at least one hydrocarbon radical, R, and the other in the form (B) of a metal atom, Me, bonded only with the support and optionally with at least one component which is neither a hydrogen atom nor a hydrocarbon radical.

The Periodic Table of the Elements cited subsequently is that provided by the IUPAC in 1991 and which is found, for example, in"CRC Handbook of Chemistry and Physics", 76th Edition (1995-1996), by David R. Lide, published by CRC Press, Inc.

(USA).

The metal atoms, Me, can correspond to identical or different metals for each of the forms (A) or (B). Furthermore, the metal atoms, Me, present in the form (A) can be identical to or different from those present in the form (B).

More particularly, the metal atom, Me, can be at least one metal chosen from the lanthanides, the actinides and the transition metals from Groups 2 to 12 of the Periodic Table of the Elements. It is preferable to choose at least one metal from the lanthanides, the actinides and the transition metals from Groups 3 to 6, in particular from Groups 4

to 6 and in particular from Groups 5 and 6 of the Periodic Table of the Elements. The metal atom, Me, can be in particular at least one metal chosen from scandium, yttrium, lanthanum, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cerium and neodymium. Preferably, it can be chosen from titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten and more particularly from tantalum, chromium, vanadium, niobium, molybdenum and tungsten.

The metal atom, Me, of the compound according to the invention can be present in the forms (A) and (B) at any degree of oxidation, in particular at a degree of oxidation ranging from one to its maximum value. The degree of oxidation of the metal atoms, Me, present in the form (A) can be identical to or different from that of the metal atoms, Me, present in the form (B).

The solid support to which the metal atoms, Me, are fixed in the forms (A) and (B) can be any solid support, in particular a solid support essentially comprising atoms M and X which are different from one another and which are generally bonded to one another via single or multiple bonds, so as to form in particular the molecular structure of the solid support. The term"support essentially comprising atoms M and X"is understood to mean generally a support which comprises, as predominant constituents, the atoms M and X and which can additionally comprise one or more other atoms capable of modifying the structure of the support. The atom M of the support can be at least one of the elements chosen from the lanthanides, the actinides and the elements from Groups 2 to 15 of the Periodic Table of the Elements. The atom M of the support can be identical to or different from the metal atom, Me. The atom M can be at least one of the elements chosen in particular from magnesium, titanium, zirconium, cerium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, boron, aluminium, gallium, silicon, germanium, phosphorus and bismuth. The atom M of the support is preferably at least one of the elements chosen from the lanthanides, the actinides and the elements from Groups 2 to 6 and from Groups 13 to 15 of the Periodic Table of the Elements, in particular from silicon, aluminium and phosphorus.

The atom X of the support, which is different from the atom M, can be chosen from at least one of the elements from Groups 15 and 16 of the Periodic Table of the Elements, it being possible for the element to be alone or to be itself optionally bonded

to another atom or to a group of atoms. When the atom X of the support is chosen in particular from at least one of the elements from Group 15, it can be optionally bonded to another atom or to a group of atoms chosen, for example, from a hydrogen atom, a halogen atom, in particular a fluorine, chlorine or bromine atom, a saturated or unsaturated hydrocarbon radical, a hydroxyl group of formula (-OH), a hydrogensulphide group of formula (-SH), alkoxide groups, thiolate groups, silylated (or silane) groups or organosilylated (or organosilane) groups. Preferably, the atom X is at least one of the elements chosen from oxygen, sulphur and nitrogen and more particularly from oxygen and sulphur.

The atoms M and X, which generally represent the essential constituents of the solid support, can be in particular bonded to one another via single or double bonds. In one of the preferred alternative forms, the solid support can be chosen from oxides, sulphides and azides in particular of M, and mixtures of two or three of the oxides, sulphides and/or azides. More particularly, the support can be an oxide of M and can be chosen from simple or mixed oxides of M or from mixtures of oxides of M. The support can, for example, be chosen from metal oxides, refractory oxides and molecular sieves, in particular from silica, alumina, aluminosilicates or aluminium silicates which are simple or modified by other metals, zeolites, clays, titanium oxide, cerium oxide, magnesium oxide, niobium oxide, tantalum oxide and zirconium oxide. The support can also be a metal oxide or a refractory oxide, optionally modified by an acid, and can optionally comprise in particular an atom M bonded to at least two atoms X different from one another, for example the oxygen atom and the sulphur atom. Thus, the solid support can be chosen from sulphated metal oxides or refractory oxides, for example a sulphated alumina or a sulphated zirconia. The support can also be chosen from metal sulphides or refractory sulphides and sulphided metal oxides or sulphided refractory oxides, for example a molybdenum sulphide, a tungsten sulphide or a sulphided alumina. The support can also be chosen from azides, in particular boron azide.

The solid support preferably comprises, as essential constituents, the atoms M and X described above and generally has the advantage of exhibiting in particular, at its surface, atoms X capable of forming part of the coordination sphere of the metal atoms, Me, present in the forms (A) and (B). Thus, the atom X of the support, which is bonded on the one hand to at least one metal atom, Me, can advantageously be bonded, on the

other hand, to at least one atom M of the support, this being the case whatever the form (A) or (B). The bonds between X and M, on the one hand, and between X and Me, on the other hand, can be single or double bonds. Furthermore, the number of bonds between X and M, on the one hand, and between X and Me, on the other hand, depends both on the degrees of oxidation of X, of M and of Me and on the nature of the single or double bonds between X and M, on the one hand, and between X and Me, on the other hand.

The metal atom, Me, in the form (A) is bonded, on the one hand, to the support, in particular to at least one atom constituting the support, preferably the atom X of the support as described above, in particular via a single or double bond, and, on the other hand, to at least one hydrogen atom and/or to at least one hydrocarbon radical, R, in particular via a metal-carbon single, double or triple bond. The hydrocarbon radical, R, can in particular be saturated or unsaturated, can have in particular from 1 to 20, preferably from 1 to 10, carbon atoms and can be chosen in particular from alkyl radicals, in particular linear or branched and aliphatic or alicyclic radicals, for example from alkyl, alkylidene or alkylidyne radicals, in particular Cl to CIO radicals, aryl radicals, in particular C6 to CIO radicals, and aralkyl, aralkylidene or aralkylidyne radicals, in particular C7 to C14 radicals.

The metal atom, Me, present in the form (A) in the compound according to the invention can be bonded to the hydrocarbon radical R via one or more carbon-metal single, double or triple bonds. It may be a carbon-metal single bond, in particular of type: in this case, the hydrocarbon radical can be an alkyl radical, in particular a linear or branched radical, or an aryl radical, for example the phenyl radical, or an aralkyl radical, for example the benzyl radical or the radical of formula (C6H5-CH2-CH2-). The term"alkyl radical"is generally understood to mean a monovalent aliphatic radical originating from the removal of a hydrogen atom from the molecule of an alkane or of an alkene or of an alkyne, for example a methyl radical (CH3-), an ethyl radical (C2H5-), a propyl radical (C2H5-CH2-), a neopentyl radical ((CH3) 3C-CH2-), an allyl radical (CH2=CH-CH2-) or an ethynyl radical (CH=C-). The alkyl radical can, for example, be of formula (R'-CH2-) where R'itself represents a linear or branched alkyl radical.

It may also be a carbon-metal double bond, in particular of type a : in this case, the hydrocarbon radical can be an alkylidene radical, in particular a linear or branched

radical, or an aralkylidene radical. The term"alkylidene radical"is generally understood to mean a bivalent aliphatic radical originating from the removal of two hydrogen atoms on the same carbon from the molecule of an alkane or of an alkene or of an alkyne, for example a methylidene radical (CH2=), an ethylidene radical (CH3- CH=), a propylidene radical (C2H5-CH=), a neopentylidene radical ( (CH3) 3C-CH=) or an allylidene radical ((CH2=CH-CH=). The alkylidene radical can, for example, be of formula (R'-CH=), where R'represents a linear or branched alkyl radical. The term "aralkylidene radical"is understood to mean generally a bivalent aliphatic radical originating from the removal of two hydrogen atoms on the same carbon from an alkyl, alkenyl or alkynyl linking unit comprising an aromatic group.

It may also be a carbon-metal triple bond: in this case, the hydrocarbon radical can be an alkylidyne radical, in particular a linear or branched radical, or an aralkylidyne radical. The term"alkylidyne radical"is generally understood to mean a trivalent aliphatic radical originating from the removal of three hydrogen atoms on the same carbon from the molecule of an alkane or of an alkene or of an alkyne, for example an ethylidyne radical (CH3-C=), a propylidyne radical (C2Hs-C_), a neopentylidyne radical ((CH3) 3C-C-) or an allylidyne radical (CH2=CH-C-=). The alkylidyne radical can, for example, be of formula (R'-C_), where R'represents a linear or branched alkyl radical. The term"aralkylidyne radical"is understood generally to mean a trivalent aliphatic radical originating from the removal of three hydrogen atoms on the same carbon from an alkyl, alkenyl or alkynyl linking unit comprising an aromatic group.

More particularly, the hydrocarbon radical R can be chosen from methyl, ethyl, propyl, isobutyl, neopentyl, allyl, neopentylidene, allylidene and neopentylidyne radicals.

The metal atom, Me, present in the form (B) in the compound according to the invention, is preferably bonded solely to the support, in particular to one or more atoms constituting the essential components of the support, more particularly to one or more atoms X of the support as described above, for example via single or double bonds.

The metal atom, Me, present in the form (B) can optionally be bonded, in addition to the support, to at least one other component which is neither a hydrogen atom nor a hydrocarbon radical. The other component can, for example, be at least one

of the elements from Groups 15 to 17 of the Periodic Table of the Elements, which element can be alone or can itself optionally be bonded to at least one hydrogen atom and/or to at least one hydrocarbon radical and/or to at least one silylated (or silane) group or organosilylated (or organosilane) group. More particularly, the metal atom, Me, can optionally be bonded, in addition to the support, to at least one atom of the elements chosen from oxygen, sulphur, nitrogen and halogens, for example fluorine, chlorine or bromine. Thus, for example, the metal atom, Me, can optionally be bonded, by a single bond, to one or more halogen atoms, in particular fluorine, chlorine or bromine. It can also optionally be bonded, via a double bond, to one or more oxygen or sulphur atoms, in particular in the form of a metal oxide or metal sulphide, or, via a single bond, to at least one oxygen or sulphur atom itself bonded to a hydrogen atom or to a saturated or unsaturated hydrocarbon radical having in particular from 1 to 20, preferably from 1 to 10, carbon atoms or to a silylated or organosilylated group, for example in the form of a metal hydroxide, hydrogensulphide, alkoxide or thiolate. The metal atom, Me, can also optionally be bonded, via a single bond, to an amido (or amide) group, for example of formulae (NH2-), (NHR-) or (NRR'-) in which R and R', which are identical or different, represent saturated or unsaturated hydrocarbon radicals having in particular from 1 to 20, preferably from 1 to 10, carbon atoms or silylated or organosilylated groups ; it can also be bonded, via a double bond, to an imido (or imide) group of formula (NH=) or, via a triple bond, to a nitrido (or azide) group of formula (N=.).

The novelty of the supported metal compound according to the invention comes from the fact that the metal atom, Me, is fixed to the solid support in the two forms (A) and (B). In particular, per 100 mol of the metal, Me, fixed to the support, the supported metal compound according to the invention can comprise: (a) from 5 to 95 mol, preferably from 10 to 90 mol, in particular from 20 to 90 mol, especially from 25 to 90 mol, or more particularly from 30 to 90 mol, of the metal, Me, in the form (A), and (b) from 95 to 5 mol, preferably from 90 to 10 mol, in particular from 80 to 10 mol, especially from 75 to 10 mol, or more particularly from 70 to 10 mol, of the metal, Me, in the form (B).

It has been particularly surprising to observe that, when the supported metal

compound according to the invention is used as catalyst in alkane metathesis reactions, it exhibits, by an as yet unexplained synergistic effect, a greatly increased catalytic activity, in particular with respect to a compound which is exactly identical but which comprises only the form (A), it being known that it had been observed, furthermore, that the form (B) alone results in a supported metal compound which is inactive in the catalysis of such reactions.

The metal atom, Me, can be present in the compound according to the invention, on the one hand, in the form (A), which can correspond to the general formula: [support (M, X) M-X ;] X ;-Me-Ry (1) and, on the other hand, in the form (B), which can correspond to the general formula : [support (M, X) M-Xj] xj-Me (2) and/or optionally [support (M, X) M-Xk] xkMe-Qz (2') in which general formulae Me represents an atom of at least one metal chosen from the lanthanides, the actinides and the transition metals from Groups 2 to 12 of the Periodic Table of the Elements, R represents a hydrogen atom and/or a hydrocarbon radical, in particular a saturated or unsaturated radical, having in particular from 1 to 20, preferably from 1 to 10, carbon atoms and being chosen in particular from alkyl, alkylidene, alkylidyne, aryl, aralkyl, aralkylidene and aralkylidyne radicals, [support (M, X) ] represents the support mainly composed of M and X, which are different and represent the atoms constituting the essential elements of the support, bonded to one another, M being in particular identical to or different from Me and representing at least one atom chosen from the lanthanides, the actinides and the elements from Groups 2 to 15 of the said Table and X representing at least one atom chosen from the elements from Groups 15 and 16 of the said Table, the atom being alone or itself optionally bonded to another atom or to a group of atoms chosen in particular from a hydrogen atom, a halogen atom, for example a fluorine, chlorine or bromine atom, a saturated or unsaturated hydrocarbon radical, a hydroxyl group (-OH), a hydrogensulphide group (- SH), alkoxide groups, thiolate groups or silylated or organosilylated groups, Xi, Xj and Xk, which are identical or different, represent atoms X at the surface of the support, Q represents neither a hydrogen atom nor a hydrocarbon radical but preferably represents an atom of at least one of the elements from Groups 15 to 17 of the said Table chosen in particular from oxygen, sulphur, nitrogen and halogens, for example fluorine, chlorine

or bromine, the atom being alone or itself optionally bonded to at least one hydrogen atom and/or to at least one hydrocarbon radical, in particular a saturated or unsaturated radical, having in particular from 1 to 20, preferably from 1 to 10, carbon atoms, and/or to at least one silylated or organosilylated group, xi and y are integers which are identical or different and at least equal to 1, the sum (xi + y) being less than or equal to the degree of oxidation of the metal Me in the general formula (1), xj is an integer at least equal to 1 and less than or equal to the degree of oxidation of the metal Me in the general formula (2), xk and z are integers which are identical or different and at least equal to 1, the sum (xk + z) being less than or equal to the degree of oxidation of the metal Me in the general formula (2'), it being possible for the degrees of oxidation of the metal Me in the general formulae (1), (2) and (2') to be identical or different.

The preferred definitions of the components given above in the general formulae (1), (2) and (2') are equivalent to those described above in the general description of the compound according to the invention.

By way of illustration, the forms (A) and (B) can be represented by the following general formulae in which M, X, X ;, Xj, Xk, Me, R, Q, xi, xj, xk, y and z have the same general definitions as in the preceding general formulae (1), (2) and (2') and in particular when the atoms X, Xi, Xj and Xk represent an oxygen atom, O : the form (A) is written: [support (M, O) M-O] xi-Me-Ry (3) the form (B) is written: [support (M, O) M-O] xj-Me (4) and/or optionally: [support (M, O) M-O] xk-Me-Qz (4') or, when the atoms X, Xi, Xj and Xk, on the one hand, and M, on the other hand, respectively represent oxygen atoms, O, and silicon atoms, Si: the form (A) is written: [support (Si, O) Si-O] xi-Me-Ry (5) the form (B) is written: [support (Si, O) Si-O] j-Me (6) and/or optionally: [support (Si, O) Si-0] xk-Me-Qz (6') or, when the atoms X, Xi, X ; and Xk, on the one hand, and M, on the other hand, respectively represent oxygen atoms, O, and aluminium atoms, Al: the form (A) is written: [support (A1, 0) Al-O] xi-Me-Ry (7) the form (B) is written : [support (A1, 0) Al-O] xj-Me (8) and/or optionally: [support (AI, 0) Al-O] xk-Me-Qz (8') or, when the atoms X, Xi, Xj and Xk, on the one hand, and the atoms M and Me, on the

other hand, respectively represent oxygen atoms, O, and silicon atoms, Si, and tantalum atoms, Ta: the form (A) is written: [support (Si, O) Si-0] 2-Ta-R (9) the form (B) is written: [support (Si, O) Si-0] 3-Ta (10) and/or optionally: [support (Si, O) Si-0] 3-Ta-Q (10') or, when the atoms X, Xi, Xj and Xk, on the one hand, and the atoms M, Me and Q, on the other hand, respectively represent oxygen atoms, O, and silicon atoms, Si, tantalum atoms, Ta, and oxygen atoms 0 : the form (A) is written: [support (Si, O ? Si-0] 2-Ta-R (9) the form (B) is written: [support (Si, O) Si-0] 3-Ta=O (11) or, when the atoms X, Xi, Xj and Xk, on the one hand, and the atoms M and Me and the group Q, on the other hand, respectively represent oxygen atoms, O, and silicon atoms, Si and tantalum atoms, Ta, and the hydroxyl group, OH: the form (A) is written: [support (Si, O) Si-0] 2-TaR (9) the form (B) is written: [support (Si, O) Si-0] 3-Ta- (OH) 2 (12) or, when the atoms X, Xi, Xj and Xk, on the one hand, and the atoms M and Me and the group Q, on the other hand, respectively represent oxygen atoms, O, and silicon atoms, Si, and tantalum atoms, Ta, and the alkoxyl (or alkoxide) group, OR, where R represents a saturated or unsaturated hydrocarbon radical having from 1 to 20, preferably from 1 to 10, carbon atoms: the form (A) is written: [support (Si, O) Si-O] 2-Ta-R (9) the form (B) is written: [support (SiO) Si-0] 3-Ta-(OR) 2 (13) The forms (A) and (B) present in the compound according to the invention can be identified and characterized by various conventional analytical methods, for example by infrared spectroscopy (IR), by nuclear magnetic resonance (NMR) spectroscopy, by Extended X-Ray Absorption Fine Structure (EXAFS) spectroscopy, by elemental chemical analyses or by methods combining both a conversion by chemical reaction of the metal compound with at least one of the abovementioned spectroscopic or analytical methods. These methods are described, for example, in J. Am. Chem. Soc. (1995), 117, pages 4288 to 4294, and (1996), 118, pages 4595 to 4602.

Thus, for example, the characterization of the metal compound in the form (A) as hydride of the metal Me can be carried out according to a method combining an

exchange reaction between hydrogen and deuterium in the metal compound and an IR spectroscopic analysis in which the vibrational bands v (MeH) characteristic of the Me- H bond are seen to disappear to the advantage of the vibrational bands v (MeD) characteristic of the Me-D bond.

The characterization of the metal compound in the form (A) as organometallic compound of the metal Me can be carried out by NMR spectroscopy and by hydrolysis reaction of the compound and qualitative and quantitative analysis of the resulting gas evolution.

The metal compound in the form (B) can be characterized by various conventional methods. One of the most effective methods is EXAFS spectroscopy, which makes it possible to determine the nature and the number of X atoms of the support around the metal atom Me of the compound. Another method can combine a hydrolysis reaction of the metal compound, by bringing the latter into contact with water and qualitative and quantitative analysis of a resulting hydrogen evolution, with an EXAFS spectroscopic analysis of the product resulting from the hydrolysis, showing the formation of at least one Me-OH bond, in which bond OH represents a hydroxyl group. Another method can combine an ethanolysis reaction of the metal compound, by bringing the latter into contact with ethanol and qualitative and quantitative analysis of a resulting hydrogen evolution, for example by gas chromatography, with an IR and/or NMR and/or EXAFS spectroscopic analysis of the product resulting from the ethanolysis. Another method can also combine an absorption reaction of pyridine on the metal compound with an IR spectroscopic analysis and/or an elemental chemical analysis and/or a 15N NMR spectroscopic analysis of the product resulting from the absorption. Another method can also combine a coordination reaction of trimethyl- phosphine on the metal Me of the metal compound with an IR spectroscopic analysis and/or an elemental chemical analysis and/or a P NMR spectroscopic analysis of the product resulting from the coordination.

The present invention also relates to several processes for the preparation of the supported metal compound. A first preparation process comprises the following stages: (a) grafting an organometallic precursor (P) comprising the metal, Me, as defined above, bonded to at least one hydrocarbon ligand, to the solid support, and (b) treating the solid product resulting from stage (a) with hydrogen or a reducing

agent, at a temperature greater than the temperature, T1, at which the supported metal compound is formed solely in the form (A) and lower than the temperature, T2, at which the supported metal compound is formed solely in the form (B), the forms (A) and (B) being in particular those defined above.

The grafting stage (a) can in particular be carried out by employing an organometallic precursor (P) which comprises the metal Me, as described above, bonded to at least one hydrocarbon ligand and which can correspond to the general formula: MeR"a (14) in which formula Me has the same definition as above, and R"represents one or more identical or different and saturated or unsaturated hydrocarbon ligands, in particular aliphatic or alicyclic, preferably from C1 to C20, in particular from Cl to Clo, hydro- carbon ligands, for example having the same definition as that given above for the hydrocarbon radical R of the form (A) of the compound according to the invention. The radical R"can be chosen in particular from alkyl, alkylidene, alkylidyne, aryl, aralkyl, aralkylidene and aralkylidyne radicals and a can be an integer and equal to the degree of oxidation of the metal Me.

The metal Me can be bonded to one or more carbons of the hydrocarbon ligands, R", in particular via one or more carbon-metal single, double or triple bonds, for example single bonds of type or double bonds of s type or triple bonds, such as described above for the Me-R bond in the form (A) of the compound according to the invention.

The preparation according to this process comprises a stage (a) in which the organometallic precursor (P) is grafted to a solid support as described above and which can essentially comprise M and X atoms as defined above. The support, which is preferably a metal oxide or refractory oxide, such as silica, is generally subjected to a preliminary heat treatment which is capable in particular of producing partial or complete dehydration and/or partial dehydroxylation, in particular at a temperature ranging from 200°C to the sintering temperature of the support, for example from 200 to 1 000°C, for one or more hours, for example from 2 to 48 hours, preferably from 10 to 24 hours. The maximum temperature of the treatment is preferably less than the sintering temperature of the support. Thus, for example for a silica, dehydration and/or

dehydroxylation can be carried out at a temperature ranging from 200 to 800°C, in particular from 300 to 500°C, or at a temperature ranging from 500°C to the sintering temperature of the silica, in particular so as to form, in particular, at the surface of the support, siloxane bridges of formula Si-O-Si, in which Si and O respectively represent silicon and oxygen atoms. The temperature and the duration of the treatment can be chosen so as to create and/or to allow to remain, in the support, at predetermined concentrations, functional groups capable of grafting, by reaction, the precursor (P).

Mention may be made, among the functional groups known for the supports, of groups of formula XH in which H represents a hydrogen atom and X corresponds to the same definition as above, for example represents oxygen, sulphur or nitrogen atoms, and in particular hydroxyl groups of formula OH, or, in particular for silica, of siloxane bridges. The grafting can advantageously be carried out so as to react the precursor (P) with most of the functional groups.

The stage (a) of grafting the precursor (P) to the support can be carried out by sublimation or by bringing into contact in a liquid medium or in solution. In the case of sublimation, the precursor (P), used in the solid state, can be heated under vacuum and under temperature and pressure conditions which provide for its sublimation and its migration in the vapour state onto the support. The sublimation can be carried out at a temperature ranging from 25 to 300°C, in particular from 50 to 150°C, under vacuum. It is possible, for example, to monitor the grafting of the precursor (P) to the support using analysis by IR spectroscopy.

It is also possible, instead of sublimation, to carry out a contacting operation in a liquid or solvent medium. In this case, the precursor (P) can be dissolved in an organic solvent, such as pentane or ether, so as to form a homogeneous solution, and the support can subsequently be suspended in the solution comprising the precursor (P), or by any other method which provides for contact between the support and the precursor (P). The contacting operation can be carried out at ambient temperature (20°C) or more generally at a temperature ranging from-80°C to 150°C, under an inert atmosphere, for example under nitrogen.

The precursor (P) which has not fixed to the support can be removed, in particular by washing or by reverse sublimation.

The preparation subsequently comprises a stage (b) in which in particular a

treatment is carried out during which the precursor (P), grafted to the support, is brought into contact with hydrogen or a reducing agent which is capable in particular of converting, at least in part, the atoms of the metal Me to metal hydride by hydrogenolysis of the hydrocarbonaceous ligands of the grafted precursor (P). During this treatment, the metal Me attached to the support may possibly see its degree of oxidation reduced to a value lower than its initial value.

Stage (b) of the preparation is carried out within a specific temperature range which makes it possible to successively or simultaneously create the two forms (A) and (B) of the metal compound and in particular in desired proportions. The specific temperature range can advantageously be chosen so as to carry out, concurrently and simultaneously: (i) a reaction for hydrogenolysis of the hydrocarbon ligands of the grafted precursor (P), so as to form the metal compound in the form of a metal hydride and in particular in the form (A) as defined above and in which R represents a hydrogen atom, and (ii) a reaction for conversion of the grafted precursor (P) and/or of the solid product resulting from the hydrogenolysis reaction to a metal compound in the form (B) as defined above.

Thus, the temperature of stage (b) is chosen so that it is greater than the temperature, T1, at which the metal compound is formed solely in the form (A). It can in particular be at least 10°C, preferably at least 20°C, in particular at least 30°C or even at least 50°C, greater than the temperature Tl. In addition, it is chosen so that it is lower than the temperature, T2, ar which the metal compound is formed solely in the form (B).

It can in particular be at least 10°C, preferably at least 20°C, in particular at least 30°C or even at least 50°C, lower than the temperature T2. The temperature of stage (b) can, for example, be chosen within a range from 165 to 450°C, preferably from 170 to 430°C, in particular from 180 to 390°C, in particular from 190 to 350°C or from 200 to 320°C. Stage (b) of the preparation can take place under an absolute pressure of 10-3 to 10 MPa. The duration of stage (b) can be very variable and can range, for example, from 1 to 24 hours, preferably from 10 to 20 hours.

The first preparation process leads to the production of a supported metal compound according to the invention comprising especially the forms (A) and (B)

corresponding in particular to the following general formulae: the form (A): [support (M, X) M-Xi] x;-Me-Ry (1), and the form (B): [support (M, X) M-Xj] xj-Me (2) in which general formulae M, X, Xi, Xj, Me, xi, xj and y have the same definitions as above and R represents a hydrogen atom with y equal to 1.

A second process for the preparation of the compound according to the invention comprises the following successive stages: (a) grafting an organometallic precursor (P) comprising the metal, Me, as defined above, bonded to at least one hydrocarbon ligand, to the solid support, (b) treating the solid product resulting from stage (a) with hydrogen or a reducing agent, at a temperature capable of bringing about hydrogenolysis, preferably complete hydrogenolysis, of the hydrocarbon ligands of the grafted precursor (P), so as to form a metal hydride in the form (A) in particular as described above, and (c) heat treating the solid product resulting from stage (b), preferably in the presence of hydrogen or of a reducing agent, at a temperature greater than the temperature of stage (b) and lower than the temperature, T2, at which the supported metal compound is formed solely in the form (B) in particular as defined above.

In this process, stage (a) is precisely identical to stage (a) described above in the first process.

Stage (b) is precisely identical to stage (b) described above in the first process, except that it is carried out at a temperature which makes it possible to carry out the hydrogenolysis of the hydrocarbon ligands of the grafted precursor (P). It is possible in particular to choose a temperature which makes it possible to carry out the hydrogenolysis of most, if not all, of the hydrocarbon ligands of P and in particular to form a metal hydride comprising the metal Me at more than 95 mol%, preferably at more than 99 mol%, in the form (A) as defined above. Thus, for example, in stage (b), it is possible to choose a temperature at most equal to the temperature, Tl, at which the metal compound is formed solely in the form (A) and in particular is formed solely in the form of the metal hydride in the form (A) as described above. It is possible, for example, to choose a temperature within a range from 50 to 160°C, preferably from 100

to 160°C.

Stage (c) of the process makes it possible, by heat treatment of the solid product resulting from stage (b), to partially convert the metal hydride in the form (A) to a metal compound in the form (B). The temperature of the heat treatment is generally greater than the temperature of stage (b) and can in particular be that used in stage (b) of the first process. It is chosen in particular so that it is greater than the temperature, Tl, at which the metal compound is formed solely in the form (A). It can in particular be at least 10°C, preferably at least 20°C, in particular at least 30°C or even at least 50°C, greater than the temperature Tl. In addition, it is chosen so that it is lower than the temperature, T2, at which the metal compound is formed solely in the form (B). It can in particular be at least 10°C, preferably at least 20°C, in particular at least 30°C or even at least 50°C, lower than the temperature T2. Thus, for example, the temperature of the heat treatment can be chosen within a range from 165 to 450°C, preferably from 170 to 430°C, in particular from 180 to 390°C, especially from 190 to 350°C or from 200 to 320°C. Generally, the proportion of the metal compounds in the two forms (A) and (B) depends essentially on the temperature of the heat treatment. The duration of the heat treatment can be chosen within a broad range and can range, for example, from 1 to 24 hours, preferably from 10 to 20 hours.

The second preparation process results in the production of a supported metal compound according to the invention comprising especially the forms (A) and (B) corresponding in particular to the general formulae (1) and (2) respectively cited above and in which R represents a hydrogen atom.

A third preparation process comprises the following successive stages: (a) grafting an organometallic precursor (P) comprising the metal, Me, as defined above, bonded to at least one hydrocarbon ligand, to the solid support comprising functional groups capable of grafting the precursor (P), by bringing the precursor (P) into contact with the solid support, so as to graft the precursor (P) to the support by reaction of (P) with a portion of the functional groups of the support, preferably from 5 to 95% of the functional groups of the support, (b) heat treating the solid product resulting from stage (a), preferably in the presence of hydrogen or of a reducing agent, at a temperature equal to or greater than the temperature, T2, at which the supported metal compound is formed

solely in the form (B) in particular as defined above, (c) grafting, to the solid product resulting from stage (b), an organometallic precursor (P'), identical to or different from (P), comprising the metal, Me, as defined above, bonded to at least one hydrocarbon ligand, the metal Me and the ligand being identical to or different from those of (P), by bringing the precursor (P') into contact with the solid product resulting from stage (b), so as to graft the precursor (P') to the support by reaction of (P') with the remaining functional groups in the support, and optionally (d) treating the solid product resulting from stage (c) with hydrogen or a reducing agent, at a temperature capable of bringing about hydrogenolysis, preferably complete hydrogenolysis, of the hydrocarbon ligands of the grafted precursor (P'), so as to form a supported metal hydride in the form (A) in particular as defined above.

Stage (a) of the process is carried out under conditions identical to those described above in stage (a) of the first process, except that the grafting of the precursor (P) is carried out partially on the support, thus leaving a portion of the support ungrafted and available for a second subsequent grafting. Thus, the first grafting can be regarded as a partial grafting which can be carried out by reacting the precursor (P) with a limited proportion of the functional groups of the support, for example from 5 to 95%, preferably 10 to 90%, in particular from 10 to 80%, especially from 10 to 75% or more particularly from 10 to 70%, of the functional groups of the support. The functional groups of the support which are capable of grafting the precursor (P) can be those mentioned above, in particular groups of formula XH in which X has the same definition as above and preferably represents at least one atom chosen from oxygen, sulphur and nitrogen, and H represents a hydrogen atom. Other functional groups of the support can. exist as for silica, for example siloxane bridges.

Stage (b) of the process comprises a heat treatment of the solid product resulting from the preceding stage, preferably in the presence of hydrogen or of a reducing agent, at a temperature in particular such that most or preferably all of the precursor (P) grafted to the support is converted to metal compound in the form (B) as defined above.

The temperature of the heat treatment can in particular be equal to or greater than the temperature, T2, at which the metal compound is formed solely in the form (B) as

defined above, for example a temperature equal to or greater than 460°C, preferably equal to or greater than 480°C, in particular equal to or greater than 500°C.

Stage (c) of the process comprises grafting an organometallic precursor (P'), identical to or different from (P), to the solid product resulting from the preceding stage.

This stage is carried out by bringing the precursor (P') into contact with the solid product resulting from the preceding stage, so as to graft, by reaction, the precursor (P') with the remaining functional groups in the support. The grafting can be carried out under conditions identical to those described above in stage (a) of the first process and result in the formation of a metal compound in the form (A) as defined above, in particular in which R represents a hydrocarbon radical, and in the continued presence of the metal compound in the form (B) formed in the preceding stage.

The third preparation process results, on conclusion of stage (c), in the production of a supported metal compound according to the invention comprising in particular the forms (A) and (B) corresponding in particular to the following general formulae: the form (A) : [support (M, X) M-Xi-Me-Ry (1), and the form (B): [support (M, X) M-Xj] xi-Me (2) in which general formulae M, X, Xi, Xj, Me, xi, xj and y have the same definitions as above and R represents a hydrocarbon radical, in particular a saturated or unsaturated hydrocarbon radical, having in particular from 1 to 20, preferably from 1 to 10, carbon atoms, as described above.

Stage (d) of the process is optional. It comprises a treatment of the solid product resulting from the preceding stage with hydrogen or a reducing agent. The treatment makes it possible in particular to carry out hydrogenolysis, preferably complete hydrogenolysis, of the hydrocarbon ligands of the grafted precursor (P'), in particular to convert the atoms of the metal Me of the grafted precursor (P') to metal hydride in the form (A) as defined above and in particular in which R represents a hydrogen atom. The treatment can be carried out at a temperature which makes it possible to form a metal hydride in the form (A), in particular a temperature at most equal to the temperature, T1, at which the metal compound is formed solely in the form (A), in which form R represents a hydrogen atom. The temperature of the treatment can be chosen within a range from 50 to 160°C, preferably from 100 to 160°C.

The third preparation process results, on conclusion of stage (d), in the production of a supported metal compound according to the invention comprising in particular the forms (A) and (B) corresponding in particular to the general formulae (1) and (2) respectively mentioned above and in which R represents a hydrogen atom.

A fourth process for the preparation of the compound according to the invention comprises the following successive stages: (a) grafting an organometallic precursor (P) comprising the metal, Me, as defined above, bonded to at least one hydrocarbon ligand, to the solid support comprising functional groups capable of grafting the precursor (P), by bringing the precursor (P) into contact with the solid support, so as to graft the precursor (P) to the support by reaction of (P) with a portion of the functional groups of the support, preferably from 5 to 95% of the functional groups, (b) heat treating the solid product resulting from stage (a), preferably in the presence of hydrogen or of a reducing agent, at a temperature equal to or greater than the temperature, T2, at which the supported metal compound is formed solely in the form (B) in particular as defined above, (c) bringing the solid product resulting from stage (b) into contact with at least one compound Y capable of reacting with the metal, Me, of the form (A) and/or (B) prepared above, the contacting operation preferably being followed by removal of the unreacted compound Y and/or by a heat treatment at a temperature lower than the sintering temperature of the support, (d) grafting, to the solid product resulting from stage (c), an organometallic precursor (P'), identical to or different from (P), comprising the metal, Me, as defined above, bonded to at least one hydrocarbon ligand, the metal Me and the ligand being identical to or different from those of (P), by bringing the precursor (P') into contact with the solid product resulting from stage (c), so as to graft the precursor (P') to the support by reaction of (P') with the remaining functional groups in the support, and optionally (e) treating the solid product resulting from stage (d) with hydrogen or a reducing agent, at a temperature capable of bringing about hydrogenolysis, preferably complete hydrogenolysis, of the hydrocarbon ligands of the grafted precursor (P'), so as to form a supported metal hydride in the form (A) in particular as

defined above.

Stages (a) and (b) of the process are identical to stages (a) and (b) described in the third preparation process.

Stage (c) of the process comprises bringing the solid product resulting from stage (b) into contact with at least one compound Y capable of reacting with the metal, Me, of the form (A) and/or (B) prepared above. In particular, the compound Y can be chosen from molecular oxygen, water, hydrogen sulphide, ammonia, an alcohol comprising in particular from 1 to 20, preferably from 1 to 10, carbon atoms, a thiol comprising in particular from 1 to 20, preferably from 1 to 10, carbon atoms, a primary or secondary amine comprising in particular from 1 to 20, preferably from 1 to 10, carbon atoms, a molecular halogen, in particular molecular fluorine, chlorine or bromine, and a hydrogen halide, for example of formula HF, HCI or HBr. Preferably, the compound Y is chosen from molecular oxygen and molecular halogens, such as molecular fluorine, chlorine or bromine. The contacting operation can be carried out under conditions where the compound Y reacts with the solid product resulting from stage (b) and can in particular form a metal compound in the form (B) corresponding in particular to the general formula (2') mentioned above. The contacting operation is preferably followed by removal of the compound Y which is not reacted with the solid product resulting from stage (b), so as in particular to isolate the solid product resulting from the reaction with the compound Y freed in particular from the excess unreacted compound Y. The contacting operation can also optionally be followed by a heat treatment carried out at a temperature lower than the sintering temperature of the support, for example at a temperature ranging from 25 to 500°C.

Stage (d) of the process is identical to stage (c) described in the third process, except that the grafting of the precursor (P') is not carried out on the solid product resulting from stage (b) but on the solid product resulting from stage (c) of the fourth process.

The fourth preparation process results, on conclusion of stage (d), in the production of a supported metal compound according to the invention comprising in particular the forms (A) and (B) corresponding respectively to the general formulae (1) and (2') mentioned above and in which R represents a saturated or unsaturated hydrocarbon radical having in particular from 1 to 20, preferably from 1 to 10, carbon

atoms, as described above.

Stage (e) of the process is optional. It is identical to stage (d) described in the third process, except that the treatment is not carried out with the solid product resulting from stage (c) but with the solid product resulting from stage (d) of the fourth process.

The fourth preparation process results, on conclusion of stage (e), in the production of a supported metal compound according to the invention comprising in particular the forms (A) and (B) corresponding in particular to the general formulae (1) and (2') respectively mentioned above and in which R represents a hydrogen atom.

The present invention also relates to the use of the supported metal compound in particular as a catalyst, for example in a process employing hydrocarbon splitting and recombination reactions. In other words, it relates to a process employing hydrocarbon splitting and recombination reactions, which process is characterized in that it is carried out in the presence of the supported metal compound according to the invention, in particular as catalyst. The invention especially relates to a process for the metathesis of starting linear or branched alkanes, in which process the starting alkane or alkanes is/are reacted over the supported metal compound according to the invention, in which compound the form (A) comprises the metal atom, Me, bonded to at least one hydrogen atom. The form (A) of the metal compound employed in this process is essentially a metal hydride. The combination of the two forms (A) and (B) in the compound according to the invention means that the latter exhibits a greatly increased catalytic activity in reactions for the metathesis of alkanes, in comparison with a supported metal compound which is identical but which would include only the form (A), it being known that it has been observed that the form (B) is inactive in this type of reaction.

The process for the metathesis of alkanes can be carried out at a temperature chosen within a range from 25 to 300°C, preferably from 100 to 200°C. The reaction for the metathesis of alkanes can be carried out by passing the alkane or alkanes, in particular in the gas phase, over the supported metal compound. In this case, it is preferable to pass the alkane or alkanes in the gas phase at an absolute pressure equal to or greater than atmospheric pressure but lower than or equal to the condensation pressure of the alkane or of the heaviest alkane, when there are several starting alkanes.

The process can be carried out under an absolute pressure chosen within the range from 1 kPa to 10 MPa.

It is also possible to carry out the process by employing the supported metal compound according to the invention in the form of a suspension in a liquid phase comprising in particular the starting alkane or alkanes involved in the metathesis reaction.

The metathesis reaction can be carried out in the presence of at least one inert gas preferably chosen from nitrogen, helium and argon. The starting alkane or alkanes can be chosen from linear C2 to C30 alkanes, branched C4 to C30 alkanes and cyclic hydrocarbons, for example aromatic rings or saturated rings or cyclic alkane hydrocarbons substituted by at least one linear or branched alkane chain. The starting alkane or alkanes can in particular be chosen from ethane, propane, n-butane, n-pentane, isobutane, isopentane, 2-methylpentane, 3-methylpentane and 2,3-dimethylbutane. It is possible in particular to react together at least two alkanes chosen from linear alkanes, branched alkanes and cyclic hydrocarbons substituted by at least one linear or branched alkane chain.

The process for the metathesis of alkanes can advantageously be carried out continuously.

The present invention also relates to the use of the supported metal compound in other processes, such as those described below.

In other words, the invention also relates to the use of the supported metal compound preferably in a process for the manufacture of alkanes comprising, as main stage, a reaction resulting from bringing methane into contact with at least one other starting alkane (I) in the presence of the supported metal compound according to the invention, in which compound the form (A) comprises the metal atom, Me, bonded to at least one hydrogen atom and/or to at least one hydrocarbon radical, so that the reaction results in the formation of at least one or two final alkanes (II) having a number of carbon atoms lower than or equal to that of the starting alkane (I) and at least equal to 2.

The form (A) of the supported metal compound involved in this process is essentially in the form of a metal hydride and/or of an organometallic compound.

In this process, the metal compound according to the invention acts in particular as a catalyst in a reaction between methane and starting alkane or alkanes (I). Thus, for example, methane can react with at least one starting Cn alkane (I), with n being an integer equal to at least 2, preferably to at least 3, so that the reaction results in the

formation of at least one or two final C2 to Cn alkanes (II). By way of example, the reaction can be written according to one or more following equations: CH4 + CnH2n+2" Cn-aH2 (n-a) +2 + Ca+lH2a+4 in which equation n is an integer at least equal to 2, preferably at least equal to 3, and a is an integer ranging from 1 to n-1. It is surprising to find that methane, which does not comprise a carbon-carbon bond, can react directly or indirectly with another alkane in the presence of the compound according to the invention, acting as catalyst in a reaction for the metathesis of alkanes by splitting and recombination of carbon-carbon bonds.

The starting alkane or alkanes (I) employed in this process can be chosen from linear or branched acyclic alkanes and from cycloalkanes substituted by at least one linear or branched alkane chain. They can correspond to the general formula: CnH2n+2 (17) in which general formula n is an integer ranging from 2 to 60, preferably from 3 to 60, in particular from 3 to 50, especially from 3 to 20. They can also be chosen from substituted cycloalkanes of general formula: CnH2n (18) in which general formula n is an integer ranging from 5 to 60, preferably from 5 to 20, in particular from 5 to 10. They can in particular be chosen from propane, n-butane, isobutane, n-pentane, isopentane, n-hexane, n-heptane, n-octane, n-nonane and n- decane. They can be chosen more particularly from linear or branched C3 to C17 alkanes or from Cis to C6o n-or isoparaffins.

The operation of contacting the methane with at least the other starting alkane (I) can be carried out at a temperature of-30°C to +400°C, under an absolute pressure of 10-3 to 30 MPa. It can advantageously be carried out in the gas phase, in particular in at least one mechanically stirred and/or fluidized bed reactor or in at least one stationary bed or circulating bed reactor, the bed being composed essentially of the supported metal compound according to the invention. It can also be carried out in the liquid phase, the liquid phase essentially comprising the starting alkane or alkanes (I) used in the liquid state and the supported metal compound according to the invention being suspended in the liquid phase. Methane and the starting alkane or alkanes (I) can be used in a molar ratio of methane to the starting alkane or alkanes ranging from 1/10 to

500/1, preferably from 1/1 to 200/1, in particular from 1/1 to 100/1. The supported metal compound according to the invention can be present in the reaction mixture comprising methane and the starting alkane or alkanes (I) in a proportion such that the molar ratio of methane to the metal Me of the said supported metal compound is chosen within a range from 10/1 to 105/1, preferably from 50/1 to 104/1, in particular from 50/1 to 103/1. The process can advantageously be carried out continuously.

The present invention also relates to the use of the supported metal compound in a process for the manufacture of alkanes, the process being in particular carried out in the presence of the supported metal compound according to the invention, in which compound the metal atom, Me, in the form (A) is bonded in particular to at least one hydrocarbon radical, for example the R radical mentioned above. The process comprises, as main stage, a crossed metathesis reaction between at least one starting alkane (I) and the compound according to the invention comprising, in the form (A), at least one hydrocarbon radical bonded to the metal, Me, so that the reaction results in the formation of at least one final alkane (II), which is a higher or lower homologue of the starting alkane (I), by splitting between the hydrocarbon radical and the metal, Me, of the supported metal compound and recombination of the said radical with at least one other radical originating from a splitting of the starting alkane (I).

The process can employ at least one starting alkane (I) chosen from linear or branched alkanes or from cycloalkanes substituted by at least one linear or branched alkane chain. The starting alkane (I) can be a C2 to Cgo, preferably C2 to Cl7, or C18 to C80 alkane. The starting alkane (1) can have a carbon number different from or identical to that of the hydrocarbon radical of the supported metal compound. In the latter case, the isomeric forms of the starting alkane (I) and of the hydrocarbon radical are preferably different.

The process can be carried out at a temperature of 20 to 400°C, preferably of 100 to 300°C, under an absolute pressure which can range from 10-3 to 10 MPa.

The process can comprise, in a first alternative form, one or more additional stages carried out before or after the main stage and comprising essentially a crossed metathesis reaction different from that of the main stage, employing at least one starting alkane (I) different from that used in the main stage, or else a supported metal compound comprising at least one hydrocarbon radical different from that of the

supported metal compound used in the main stage, or else, simultaneously, at least one starting alkane (I) and a supported metal compound which are different from those used in the main stage.

According to another alternative form, the process can comprise one or more additional stages carried out before or after the main stage and comprising essentially a conventional reaction for the metathesis of alkanes, employing at least one starting alkane identical to or different from that used in the main stage which is brought into contact with a supported metal compound according to the invention in which the form (A) comprises the metal atom, Me, bonded to at least one hydrogen atom, the metal atom, Me, being identical to or different from the metal, Me, of the supported metal compound of the main stage.

In the two alternative forms, the additional stage can be carried out before the main stage, the stream of the final alkane or alkanes resulting from the additional stage being partly or completely used as starting alkanes (I) in the main stage of the process.

Conversely, the additional stage can be carried out after the main stage and, in this case, the stream of the final alkane or alkanes (II) resulting from the main stage is partially or completely used as starting alkanes in the additional stage.

According to one or other of these alternative forms, the process can advantageously be carried out continuously.

The present invention can also relate to the use of the supported metal compound in a process for modifying the carbon skeleton of an hydrocarbon polymer, in particular a process for the conversion of a polymer of one or more monomers comprising ethylenic unsaturation into alkanes or into a hydrocarbon fraction or into oligomers or into polymers with a modified macromolecular structure, in which process the polymer is brought into contact with the supported metal compound according to the invention in the presence of hydrogen, so as to carry out in particular the catalytic hydrocracking of the polymer. The supported metal compound is preferably that having the form (A) in which the metal atom, Me, is bonded to a hydrogen atom. The polymer can be a homo-or a copolymer of at least one monomer chosen from olefinic monomers comprising a single ethylenic unsaturation, such as ethylene, propylene and styrene, or a mixture of the latter, or of at least one monomer comprising several conjugated or nonconjugated ethylenic unsaturations.

The process can be carried out at a temperature of 25 to 300°C, under an absolute hydrogen pressure which can range from 1 kPa to 50 MPa. It can be carried out in a liquid medium comprising a solvent or in the absence of solvent, for example the polymer being in the solid state or in the molten state. It can be carried out during the preparation of the polymer. The process can advantageously be carried out continuously.

The present invention can also relate to the use of the supported metal compound in a process of polymerisation or oligomerization of one or more olefin or vinyl monomers, in particular in a process wherein the monomers are brought into contact with the supported metal compound according to the invention, optionally in the presence of hydrogen, in particular in a liquid phase or in a gas phase. The monomers can be selected from olefins or alpha-olefins, e. g. ethylene, propylene, n-butene, isobutene, n-pentene, n-hexene and n-octene, from diolefin (or dienes) conjugated or non-conjugated, and from vinyl compounds, in particular from aromatic vinyl compounds, e. g. styrene. The polymerisation or oligomerization process can be a bulk process, a slurry process or a gas phase process, for example a fluidised and/or a mechanically agitated bed process. The process can be carried out at a temperature in the range from 20 to 200°C, preferably from 50 to 180°C, under an absolute pressure which can range from 0.1 to 10 MPa, preferably from 0.2 to 5 MPa. The polymerisation or oligomerization process can be carried out discontinuously or preferably continuously.

The present invention can also relate to the use of the supported metal compound in a hydrocarbon isomerization process, in particular a hydrocarbon isomerization process in which at least one hydrocarbon, for example a C4 to C20 hydrocarbon, preferably a C4 to Clo hydrocarbon, is brought into contact with the supported metal compound according to the invention, in particular in which the form (A) comprises the metal atom, Me, bonded to at least one hydrogen atom. The contacting operation can preferably be carried out in the presence of hydrogen, at a temperature of 20 to 300°C, preferably of 100 to 200°C, under an absolute pressure which can range from 1 kPa to 10 MPa. The process can be carried out in the liquid phase or, preferably, in the gas phase, it being possible for the supported metal compound to be in suspension in the liquid or gas phase, for example maintained in the

fluidized state by the gas phase comprising the hydrocarbon or hydrocarbons to be isomerized. The process can advantageously be carried out continuously.

The following examples illustrate the present invention.

Comparative Example 1 A supported tantalum compound, C1, is prepared, which compound comprises solely the form (A) in which the metal atom, Ta, is bonded to a hydrogen atom.

In a first stage, 50 mg of a silica, dehydrated and dehydroxylated beforehand at 500°C, are introduced into a glass reactor, followed by tris (neopentyl) neo- pentylidenetantalum, used as precursor and corresponding to the general formula: Ta [-CH2-C (CH3) 3] 3 [=CH-C (CH3) 3] (15) which precursor is sublimated at 80°C onto the silica and which, by reaction at 25°C with the hydroxyl groups of the silica, is grafted to the silica. The excess unreacted precursor (15) is desorbed by reverse sublimation at 80°C.

In a second stage, the metal compound, thus grafted to the silica, is subsequently treated under a hydrogen atmosphere of 80 kPa, at a temperature of 150°C, for 15 hours. A tantalum compound, C1, grafted to the silica, is thus formed by hydrogenolysis of the neopentyl and neopentylidene ligands, which compound exists solely in the form (A) in which the tantalum atom is bonded with a hydrogen atom, according to the general formula: [support (Si, O) Si-0] 2-Ta-H (16) The characteristic data of the supported tantalum compound as defined by the formula (16) are described in J. Am. Chem. Soc. , (1996) 118, pages 4595 to 4602.

Examples 2 to 5 Four supported tantalum compounds according to the invention, C2, C3, C4 and C5, are prepared, each compound comprising the two forms (A) and (B) in different proportions.

In Examples 2,3, 4 and 5, the preparation is carried out exactly as in Comparative Example 1, except that, in the second stage, the treatment under a hydrogen atmosphere is carried out at a temperature of 250°C, 300°C, 350°C and 450°C respectively, instead of 150°C. Under these conditions, the supported tantalum compounds C2, C3, C4 and C5 respectively are thus obtained.

The supported tantalum compound C2 comprises the two forms (A) and (B):

(a) 72% of the tantalum atoms are in the form (A) in which the tantalum atom is bonded, on the one hand, to two oxygen atoms of the silica via single bonds and, on the other hand, to a hydrogen atom, and (b) 28% of the tantalum atoms are in the form (B) in which the tantalum atom is bonded solely to three oxygen atoms of the silica via single bonds.

The forms (A) and (B) obtained above in the compound C2 correspond to the following general formulae: at 72%, the form (A): [support (Si, O) Si-O] 2-Ta-H (16) and, at 28%, the form (B) : [support (Si, O) Si-0] 3-Ta (17) The supported tantalum compound C3 comprises the two forms (A) and (B): (a) 50% of the tantalum atoms are in the form (A) according to the formula (16), and (b) 50% of the tantalum atoms are in the form (B) according to the formula (17).

The supported tantalum compound C4 comprises the two forms (A) and (B): (a) 35% of the tantalum atoms are in the form (A) according to the formula (16), and (b) 65% of the tantalum atoms are in the form (B) according to the formula (17).

The supported tantalum compound C5 comprises the two forms (A) and (B): (a) 7% of the tantalum atoms are in the form (A) according to the formula (16), and (b) 93% of the tantalum atoms are in the form (B) according to the formula (17).

Comparative example 6 A supported tantalum compound, C6, comprising tantalum solely in the form (B) is prepared. The preparation is precisely identical to that of Comparative Example 1, except that, in the second stage, the treatment under a hydrogen atmosphere is carried out at a temperature of 500°C instead of 150°C. Under these conditions, a supported tantalum compound C6 is obtained from which 100% of the tantalum atoms are in the form (B) according to the formula (17). The characteristic data of the compound according to the formula (17) are described in the doctoral thesis of G. Saggio at the University of Lyons I, No. 250-2001. In particular, the EXAFS spectroscopy shows a Ta-O distance of 1.9 A and an environment of three oxygen atoms around each tantalum atom.

Example 7

The supported tantalum compounds C2 to C5 prepared according to the invention in Examples 2 to 5 are used successively under identical conditions in a propane metathesis reaction and are compared with the supported tantalum compounds Cl and C6 prepared in Comparative Examples 1 and 6.

For each of the tests carried out, the propane metathesis reaction is carried out under the following conditions: the supported metal compound is prepared in situ in a glass reactor as described above. The reactor is subsequently placed under vacuum, is then filled with propane up to atmospheric pressure and is heated at 150°C under stationary conditions. The formation of a mixture of methane, of ethane, of n-butane, of isobutane and, in smaller proportions, of pentane, of isopentane and of C6 homologues gradually over time is observed.

After reacting for 120 hours under these conditions, the number of moles of propane converted per mole of active tantalum and the initial rate of conversion of the propane (expressed by the number of moles of propane converted per mole of active tantalum and per hour) are measured and calculated for each of the supported tantalum compounds of Cl to C6. The results of these measurements and calculations are collated in Table 1.

Table 1 Supported metal % of % of Number of moles of Initial rate of compound tantalum in tantalum in propane converted conversion of the the form (A) the form per mole of active propane (B) tantalum C1 (comparative) 100% 0% 53 1. 0 C2 72% 28% 145 2. 3 C3 50% 50% 204 3. 6 C4 35% 65% 149 2. 0 C5 7% 93% 129 1. 5 C6 (comparative) 0% 100% 0 0 Analysis of the results of the propane metathesis shows that: - the compounds C2, C3, C4 and C5 exhibit a higher catalytic activity than that of the compound C1 ;

the compound C6 exhibits no catalytic activity.

These results show that a supported metal compound according to the invention which simultaneously comprises the two metal forms (A) and (B), has a catalytic activity in a propane metathesis reaction which is much greater than that of an identical supported metal compound but comprising solely the form (A). It is considered that this increase in activity is due to a synergistic effect between the two forms (A) and (B), it being known that it has been found that the form (B) used alone in the supported metal compound is inactive in the catalysis of propane metathesis.

Example 8 A supported tantalum compound, C7, according to the invention is prepared by the following process.

In a first stage, 1 g of a silica partially dehydroxylated at 700°C, 5 ml of n- pentane and a solution of 15 ml of n-pentane comprising 64 mg (0.14 millimol of tantalum) of the tantalum precursor corresponding to the general formula (15) are introduced into a glass reactor at ambient temperature (20°C). The mixture thus obtained is stirred for 30 minutes and the n-pentane is subsequently evaporated.

In a second stage, the solid compound thus grafted is dried under vacuum and is then treated under an atmosphere of 80 kPa of hydrogen at 500°C for 15 hours. At the end of this time, the reactor is cooled to ambient temperature (20°C) and the gas phase of the reactor is discharged.

In a third stage, 0. 8 g of the solid compound is recovered from the preceding stage and is suspended in 20 ml of n-pentane. A solution of 10 ml of n-pentane comprising 100 mg (0.21 millimol of tantalum) of the tantalum precursor corresponding to the general formula (15) is added to this suspension. After 2 hours, the resulting solid compound is filtered off, then washed 5 times using 20 ml of n-pentane each time, and dried under vacuum.

In a fourth stage, 0.75 g of the solid compound is recovered from the preceding stage and is then brought into contact with an atmosphere of 80 kPa of hydrogen. The reactor is heated to 150°C and maintained at this temperature for 12 hours. At the end of this time, the reactor is cooled to ambient temperature (20°C) and the gas phase is discharged from the reactor, so as to isolate and to recover a supported tantalum compound, C7, according to the invention comprising tantalum both in the form (A)

and in the form (B), corresponding respectively to the general formulae (16) and (17).

Example 9 A supported tantalum and zirconium mixed compound, C8, according to the invention is prepared precisely as in Example 8, except that, in the first stage, instead of introducing, into the reactor, an n-pentane solution comprising 64 mg of the tantalum precursor corresponding to the general formula (15), an n-pentane solution comprising 41 mg (0.11 millimol of zirconium) of a zirconium precursor corresponding to the general formula: Zr [-CH2-C (CH3) 314- (18) is introduced.

The other stages of the preparation remain unchanged with respect to those described in Example 8. A supported tantalum and zirconium mixed compound, C8, according to the invention is thus obtained, which compound comprises tantalum in the form (A) corresponding to the general formula (16) and zirconium in the form (B) corresponding to the general formula: [support (Si, O) Si-0] 4-Zr (19) The supported tantalum and zirconium mixed compound, C8, is used as catalyst in a propane metathesis reaction carried out under the same conditions as in Example 7, except that the compound C8 is used instead of the compounds Cl to C6. It is observed that the propane metathesis reaction progresses with an activity of the reaction similar to that obtained with the compounds C2 to C5.

Example 10 A supported tantalum and tungsten mixed compound, C9, according to the invention is prepared precisely as in Example 8, except that, in the third stage, instead of adding an n-pentane solution comprising 100 mg of the tantalum precursor corresponding to the general formula (15), an n-pentane solution comprising 98 mg (0.21 millimol) of a tungsten precursor corresponding to the general formula: W [=C-C (CH3) 3] [-CH2-C (CH3) 3] (20) is added.

The final stage of the preparation remains unchanged with respect to that of Example 8. A supported tungsten and tantalum mixed compound, C9, according to the

invention is thus obtained, which compound comprises tungsten in the form (A) corresponding to the general formula: [support (Si, O) Si-0] 3-W-H (21) and tantalum in the form (B) corresponding to the general formula (17).

The supported tungsten and tantalum mixed compound, C9, is used as catalyst in a propane metathesis reaction carried out under the same conditions as in Example 7, except that the compound C9 is used instead of the compounds C1 to C6. It is observed that the propane metathesis reaction takes place with an activity of the reaction similar to that obtained with the compounds CZ to C5.