FR2993795A1 - Preparing supported catalyst based on metal particles, comprises providing aqueous solution of metal precursor, heating porous support, contacting aqueous solution with porous support by dry impregnation, and drying the product - Google Patents

Abstract

Preparing a supported catalyst based on metal particles, comprises (a) providing an aqueous solution of at least one metal precursor at a first temperature, (b) heating a porous support at a second temperature that is greater than the first temperature, (c) contacting the aqueous solution with the porous support by dry impregnation, (d) drying the product, (e) optionally calcining the product, and (f) reducing the product. Independent claims are included for: (1) process for selective hydrogenation of carbon monoxide to 1-6C-alcohol, methanol or dimethyl-ether, comprising contacting a hydrocarbon filler comprising at least one polyunsaturated compound in the presence of hydrogen with the supported catalyst, where the hydrocarbon filler consists of catalytic cracking fraction, steam cracking C2 fraction, steam cracking C3 fraction, steam cracking C4 fraction, steam cracking C5 fraction and steam cracking gasolines; and (2) isomerizing, hydroisomerizing or hydrogenolysis of hydrocarbon compound comprising contacting the hydrocarbon filler with the supported catalyst in the presence of hydrogen.

Description

The invention relates to a method for preparing supported catalysts based on metal particles for controlling the metallic dispersion of said particles on the support. It also relates to the use of the catalysts obtained by the process in transformation reactions of unsaturated organic compounds, and in particular in selective hydrogenation. Monounsaturated organic compounds such as, for example, ethylene, propylene, are the source of the manufacture of polymers, plastics and other value-added chemicals. These compounds are obtained from natural gas, naphtha or gas oil which have been treated by steam cracking or catalytic cracking processes. These processes are operated at high temperature and produce, in addition to the desired monounsaturated compounds, polyunsaturated organic compounds such as acetylene, propadiene and methylacetylene (or propyne), 1-2-butadiene and 1-3 -butadiene, vinylacetylene and ethylacetylene, and other polyunsaturated compounds whose boiling point corresponds to the C5 + gasoline fraction (gasolines containing hydrocarbon compounds having 5 or more carbon atoms). These polyunsaturated compounds are very reactive and lead to spurious reactions in the polymerization units. It is therefore necessary to eliminate them before upgrading the sections containing mono-unsaturated organic compounds. Selective hydrogenation is the main treatment developed to specifically remove undesired polyunsaturated compounds from pyrolysis or catalytic cracking species containing mainly mono-olefins. It allows the conversion of the polyunsaturated compounds to the corresponding alkenes or aromatics, avoiding their total saturation and thus the formation of the corresponding alkanes. Selective hydrogenation catalysts are generally based on Group VIII metals of the periodic table of elements, preferably palladium or nickel. The active phase of the catalysts is in the form of small metal particles deposited on a support which may be a refractory oxide in the form of beads, extrudates, trilobes or in forms having other geometries. The metal content but also the dispersion of the metal particles are part of the criteria which have an importance on the activity and the selectivity of these catalysts.

The macroscopic distribution of the metal particles in the support is also an important criterion, mainly in the context of rapid and consecutive reactions such as selective hydrogenations. It is generally advantageous that these elements are located in a crust at the periphery of the support in order to avoid problems of intragranular material transfer that can lead to defects in activity and loss of selectivity. It is known to one skilled in the art that for the hydrogenation reactions of polyunsaturated molecules such as diolefins or acetylenics, the reaction rate depends on the size of the metal particles, this result is generally described under the term "Sensitivity to structure". An optimum is generally observed for a size of the order of 3 to 4 nm, this value being able to vary as a function, in particular, of the molecular mass of the reagents (described in the documents M. Boudart, WC Cheng, J. Catal., 106, 1987 , 134, and S. Hub, L. Hilaire, R. Touroude, Appl., Catal., 36, 1992, 307). Many catalyst synthesis methods for controlling the metal dispersion have been presented in the literature by Che et al. (Advances in Catalysis, 36, pp. 55-172, 1989). On the other hand, according to the metal precursor used, it is known to prepare catalysts having modulated metal dispersions (J. P. Boitiaux et al., Applied Catalysis, 6, pp. 41-51, 1983). It is also known from the literature (Y. Fort et al., Journal of Molecular Catalysis A-Chemical, 112 (2), pp. 311-316, 1996) that the solvent as well as the temperature used during the Pt-based catalyst preparation step on alumina has an impact on catalytic performance. It is known from EP0879641 to dry-impregnate a solution of palladium precursor on a spray support in order to preferentially locate the metal phase at the periphery of the support. This preparation step is preferably carried out at ambient temperature, a high temperature being able to induce the undesirable formation of a metal film on the surface of the support. Similarly, it is known from US 2005/0154236 to prepare a metal catalyst by mixing a carrier and a vaporizable solid precursor in temperature to perform a chemical vapor deposition (or CVD for "Chemical Vapor deposition "according to the English terminology) of the metal phase.

It is known for example documents US7169736 and GB872785, to deposit and reduce metal precursors in solution on a support, said solutions having controlled temperatures of 20 to 100 ° C.

The dispersion of the metal particles having an influence on the activity and selectivity of the catalysts, there is thus a need to provide processes for preparing supported metal catalysts for controlling and modulating this dispersion. An object of the present invention is thus to provide a process for the preparation of supported catalysts based on metal particles to control and modulate their metal dispersion. More particularly, the invention relates to a process for the preparation of a supported catalyst based on metal particles in which a solution of metal precursor is prepared, preferably at room temperature, in the aqueous phase, and then this solution is deposited on a support porous pre-heated by dry impregnation, the catalyst precursor obtained is dried, optionally calcined and reduced. In the preparation process according to the invention, the aqueous solution of metal precursor is prepared in the absence of reducing agent. Surprisingly, the deposition of the metal phase in solution on a heated support makes it possible to control the state of dispersion of the metal particles after a drying step, an optional calcination step and a reduction step. In other words, the choice of a temperature of the heated support makes it possible to predict the dispersion of the metal particles for a given catalytic system, as well as its activity. Performing the dry impregnation step on a preheated porous support preferably without heating the impregnating solution is the key step of the process according to the invention. In general, the support temperature and the temperature of the aqueous solution are thus selected so as to obtain a catalyst having a metal dispersion exhibiting maximum activity in selective hydrogenation. The control of the metal dispersion is as fine as the metal content of the catalyst is low. The metals used to work at very low grade are usually noble metals. The effect of dispersion control is then particularly pronounced for metal catalysts containing palladium, ruthenium platinum or rhodium. These metals are thus particularly preferred. The invention also has the following advantages: the preparation of catalysts supported on heated support is simplified because it makes it possible to overcome the problem of stability of the precursor solutions, the catalysts obtained according to the process of the invention have a metallic phase which is concentrated on the surface of the support; crust catalysts, particularly suitable for selective hydrogenation reactions, are thus obtained. The catalysts obtained according to the process of the invention have modulatable catalytic properties and therefore optimized performance according to the reagents used. Another advantage of the invention is the fact that the metal dispersion of the catalyst is flexible (thanks to its preparation process) without modifying the crust thickness of the metal particles.

In the following, groups of chemical elements are given according to the CAS classification (CRC Handbook of Chemistry and Physics, publisher CRC Press, editor in chief D.R. Lide, 81st edition, 2000-2001). For example, group VIII according to the CAS classification corresponds to the metals of columns 8, 9 and 10 according to the new IUPAC classification. The invention relates to a method for preparing a supported metal catalyst for controlling and modulating its metal dispersion.

More particularly, the invention relates to a method for preparing a supported catalyst based on metal particles comprising the steps of: a) providing an aqueous solution of at least one metal precursor at a first temperature, b) heating a support porous at a second temperature, said second temperature being greater than said first temperature, C) contacting said aqueous solution obtained in step (a) with said heated porous support of step (b) by dry impregnation, d) drying the product obtained in step (c), e) optionally, calcining the product obtained in step (d), f) reducing the product obtained in step (d) or step (e). Step (a): Preparation and supply of an aqueous solution of a metal precursor According to step (a), an aqueous solution of at least one metal precursor is provided at a temperature which is lower than the temperature of the support. The metal of the precursor is selected from a group VIII, IB and / or IIB metal. Preferably, the metal is selected from the group consisting of palladium, silver, gold, platinum, rhodium, ruthenium, copper and nickel. Even more preferably, the metal is selected from the group consisting of palladium, platinum, ruthenium and rhodium. Very preferably, the metal is palladium.

The metal of the precursor under consideration has a degree of oxidation greater than 0. The concentration of the aqueous solution of the metal precursor is adjusted according to the desired metal mass content on the catalyst. The mass content of metal relative to the support mass is between 0.01 and 20% by weight, preferably between 0.05 and 10% by weight. When the metal is selected from the group consisting of palladium, platinum, ruthenium and rhodium, the concentration of the aqueous solution of the metal precursor is adjusted according to the desired mass content on the catalyst. In this case, the mass content of metal relative to the support mass is between 0.01 and 2% by weight, preferably between 0.05 and 1% by weight. The preparation of the aqueous metal precursor solution is preferably carried out at room temperature. The preparation of the aqueous metal precursor solution is preferably carried out at ambient pressure. The metal precursor may be in ionic form or in the form of metal oxide. In the case where the metal precursor is in ionic form, it is in salt form. Preferably, the metal precursor is a soluble salt in aqueous solution which is selected from the group consisting of metal halide, oxide, hydroxide, nitrate and sulfate.

In the case of a palladium precursor, it is generally selected from the group consisting of palladium chloride, palladium nitrate and palladium sulfate. Very preferably, the precursor salt of palladium is palladium nitrate.

In the case where the metal precursor is in the form of a metal oxide, said metal precursor is obtained by neutralization of an aqueous solution of a salt of the metal precursor with a mineral base chosen from the group formed by the alkali hydroxides and alkaline earth hydroxides. More particularly, an aqueous solution of at least one metal precursor salt as described above is first prepared and then neutralized with a mineral base selected from the group consisting of alkali hydroxides and hydroxides. alkaline earth. Preferably, the mineral base is sodium hydroxide. This variant makes it possible to form a colloidal suspension of metal oxide particles, said metal oxide particles being thus contained in the aqueous solution which will be used as an impregnation solution (in step c). According to one particularly preferred variant, a colloidal palladium suspension in the form of a metal oxide is prepared as described in document FR2922784 and according to which a colloidal suspension of palladium oxide or of palladium hydroxide is prepared in an apparatus. in the aqueous phase by mixing an aqueous solution 1 comprising at least one hydroxide selected from the group consisting of alkali hydroxides and alkaline earth hydroxides and an aqueous solution 2 comprising at least one precursor salt of palladium, the solution 2 and then the solution 1 being poured into the apparatus or solutions 1 and 2 being poured simultaneously into the apparatus, the preparation temperature being preferably between 5 ° C and 40 ° C, the the pH of said colloidal suspension preferably being between 1.0 and 3.5, the residence time of the colloidal suspension in the apparatus being preferably between taken between 0 and 20 hours. The salt of the palladium precursor is preferably selected from the group consisting of palladium chloride, palladium nitrate and palladium sulfate. Step (b): Heating the support According to step b), a porous support is heated to a second temperature, said second temperature being greater than the temperature of the aqueous solution of the metal precursor.

The support is preferably heated to a temperature greater than or equal to 40 ° C, preferably between 40 ° C and 350 ° C, and even more preferably between 50 ° C and 250 ° C, and particularly preferably between 100 ° C and 150 ° C. The support may comprise at least one refractory oxide chosen from the group consisting of oxides of magnesium, aluminum, silicon, zirconium, thorium, or titanium, taken alone or as a mixture with one another, such as silica. -alumina. Preferably, the support is an oxide of aluminum (alumina) or silica. The support may also be a coal, a silico-aluminate, a clay or any other compound known to be used as a support. Preferably, the support has a BET surface of between 5 and 300 m 2 / g, even more advantageously between 100 and 200 m 2 / g. The BET surface area is measured by physisorption with nitrogen. The total pore volume of the support is generally between 0.1 and 3 cm3 / g, preferably between 0.4 and 1.5 cm3 / g. The total pore volume is measured by mercury porosimetry according to ASTM D4284-92 with a wetting angle of 140 °, for example by means of an Autopore IIITM model device of the Microméritics TM brand. The support may be shaped into balls, extrudates, trilobes, pellets, irregular and non-spherical agglomerates whose specific shape may result from a crushing or monolithic step. Advantageously, said support is in the form of balls or extrudates. Step (c): Heated Dry Impregnation In step (c), the aqueous solution obtained in step (a) is brought into contact with said heated porous support of step (b) by impregnation with dry. Step (c) is carried out by dry impregnation, which comprises bringing the heated support into contact with the aqueous solution with a volume equal to the pore volume of the support to be impregnated. The impregnation may be carried out in one or more successive impregnations, it is preferably carried out in a single step. The dry impregnation is preferably carried out dropwise, that is to say that the solution is impregnated dropwise on the support. In a particularly preferred manner, the dry impregnation is done by drop in a single step.

The support is preferably heated to a temperature greater than or equal to 40 ° C, preferably between 40 ° C and 350 ° C, and even more preferably between 50 ° C and 250 ° C, and particularly preferably between 100 ° C and 150 ° C.

During dry impregnation, the aqueous solution has a temperature below the temperature of the heated support, preferably below 40 ° C. In a particularly preferred manner, the aqueous solution is at ambient temperature, that is to say that it is brought into contact with the heated support without being itself heated. In a preferred manner, the temperature of the heated support is at least 10 ° C higher than that of the aqueous solution. During dry impregnation, the moving support is impregnated in an enclosure heated to a temperature of preferably between 40 ° C and 350 ° C, more preferably between 50 ° C and 250 ° C, and so particularly preferably between 100 ° C and 150 ° C. The heated chamber may be any device known to those skilled in the art such as a double-jacketed impregnating barrel or a rotary evaporator type device consisting of a heated flask in an oil bath. In a particularly preferred manner, palladium is chosen as the metal of the metal precursor and the alumina is chosen as support, and said support is heated in step b) at a temperature of between 100 ° C. and 150 ° C. . Step (d): Drying According to step (d), the product obtained in step (c) dry impregnation is then dried to remove all or part of the water introduced during the impregnation preferably at a temperature between 50 and 250 ° C, more preferably between 70 ° C and 200 ° C. The drying can be carried out under air, or under an inert atmosphere (nitrogen for example). Step (e): Calcination (optional) According to step (e), the product resulting from the drying step (d) is then optionally calcined under a gas stream, preferably in air, under nitrogen or a mixture of these. gas, preferably at a hourly volume velocity (VVH) of between 100 and 5000 h-1, the hourly space velocity being defined as the ratio of the volume flow rate of charge to the volume of catalyst per hour. The calcination temperature is generally between 150 ° C. and 900 ° C., preferably between 200 ° C. and 500 ° C. The calcination time is generally between 0.5 hours and 24 hours, preferably from 1 hour to 12 hours. The calcination step can be operated by temperature step up to the defined maximum set temperature. Step (f): Reduction According to step (f), the product obtained in drying step (d) or in the calcining step (e) is generally reduced in a gaseous flow comprising between 25 vol% and 100 vol% of a reducing gas, preferably 100 vol% of a reducing gas. The reducing gas is preferably hydrogen. Preferably, this step is carried out at a temperature of between 50 ° C. and 500 ° C., even more preferably between 80 ° C. and 350 ° C. This reduction makes it possible to activate the said catalyst and to form metal particles in the zero state. Said reduction may be carried out in situ or ex situ, that is to say after or before the catalyst is loaded into the hydrogenation reactor. It is preferably carried out in situ, that is to say in the reactor where the catalytic conversion is carried out. Metal Dispersion The catalyst preparation process makes it possible to control the metal dispersion of the catalyst particles. The measurement of the metal dispersion can be carried out by any method known to those skilled in the art, for example by adsorption of a probe molecule or else by measurement of the particle size by transmission electron microscopy following the method proposed by Van Hardeveld. and Hartog (Surface Science, 15, pp. 189, 1969). Preferably, the measurements of metal dispersions are carried out by chemisorption of carbon monoxide CO, on the previously reduced catalyst under 1 liter of hydrogen per hour and per gram of catalyst, with a temperature rise of 300 ° C./h and a bearing for one hour at 150 ° C. The catalyst is then flushed for 1 hour at 150 ° C. under helium and then cooled to 25 ° C. under helium. The chemisorption of CO is carried out at 25 ° C. in a dynamic manner according to the usual practices known to those skilled in the art, it leads to a chemisorbed CO volume, from which the person skilled in the art can calculate the number of CO molecules. chirnisorbées. A stoichiometric ratio of one CO molecule per surface metal atom is taken into account in calculating the number of surface metal atoms. The dispersion is expressed as% of surface metal atoms relative to all of the metal atoms present in the catalyst sample. The metal dispersion of the catalysts obtained according to the process of the invention is generally between 10% and 70%, preferably between 15 and 60%. Another object of the invention is the use of the catalyst obtained by the process described above in a conversion reaction of organic compounds. Thus, the catalyst obtained according to the process of the invention can be used in reactions involving cuts or carbon-carbon bond formations. Thus, the present invention relates to a selective hydrogenation process comprising contacting a hydrocarbon feedstock in the presence of hydrogen with the catalyst prepared according to the preparation method of the invention, said feed comprising aromatic functions, ketones , aldehydes, acids and / or nitro. The invention also relates to a process for the hydrogenation of carbon monoxide to C1-C6 alcohols, methanol or dimethyl ether comprising contacting a hydrocarbon feedstock in the presence of hydrogen with the catalyst prepared according to the invention. process for the preparation of the invention.

The invention also relates to a process for the isomerization, hydroisomerization or hydrogenolysis of hydrocarbon compounds comprising contacting a hydrocarbon feedstock in the presence of hydrogen with the catalyst prepared according to the preparation method of the invention.

The operating conditions used for these reactions are the following: a temperature of between 0 and 500 ° C., preferably between 25 and 350 ° C., a pressure of between 0.1 and 20 MPa, preferably between 0.1 and 10 MPa. a hourly volume velocity (VVH) of between 0.1 and 50 h -1, preferably between 0.5 and 30 h -1, for a liquid charge; and between 500 and 30,000 h -1, preferably between 500 and 15,000 h -1, for a gaseous charge. When hydrogen is present, the volume ratio of hydrogen on the feedstock is between 1 and 500 liters per liter, preferably between 10 and 250 liters per liter. The implementation of the catalyst prepared according to the method of the invention and the conditions of its use are to be adapted by the user to the reaction and the technology used. In general, the implementation is carried out by injection of the hydrocarbon feedstock to be treated and hydrogen in at least one reactor containing said catalyst, the reactor being a fixed bed, moving bed or bubbling bed, preferably in a fixed bed reactor. All of said charge is preferably injected at the reactor inlet. However, it may be advantageous, in some cases, to inject a fraction or all of said feedstock between two consecutive catalytic beds placed in said reactor. This embodiment makes it possible in particular to continue to keep the reactor operational even when the inlet of said reactor is clogged by deposits of polymers, particles or gums present in said load. Selective Hydrogenation Process According to a particularly preferred variant, the catalyst prepared according to the process of the invention is used in selective hydrogenation reactions.

The present invention thus also relates to a selective hydrogenation process comprising contacting a hydrocarbon feedstock comprising at least one polyunsaturated compound in the presence of hydrogen with the catalyst prepared according to the preparation process of the invention, said feed being selected from the group consisting of catalytic cracking cuts, C2 steam cracking cups, C3 steam cracking cups, C4 steam cracking cups, steam cracking C5 cups and steam cracking gasolines. The C5 steam cracking section and the pyrolysis gasolines are hereinafter referred to as the C5 + section.

Said hydrocarbon feedstock comprises at least one polyunsaturated compound. More specifically, said polyunsaturated hydrocarbons present in said treated feed are in particular compounds comprising at least one acetylenic function (ie at least one triple bond) and / or at least one diene function (ie say at least two double bonds). In particular, said polyunsaturated hydrocarbon feedstock may comprise at least one type of compound containing both an acetylene function and a diene function per molecule. The pyrolysis gasoline feed can additionally comprise alkenylaromatics. The steam cracking section C2, advantageously used for the implementation of the selective hydrogenation process according to the invention, has, for example, the following composition: 90% by weight of ethylene, of the order of 0.3 to 2% weight of acetylene, the remainder being essentially ethane. In certain steam cracking sections, between 0.1 and 1% by weight of C 3 compounds may also be present. The C 3 steam-cracking cut, advantageously used for carrying out the selective hydrogenation process according to the invention, has, for example, the following average composition: of the order of 90% by weight of propylene, of the order of 3 at 8% by weight of propadiene and methylacetylene, the remainder being essentially propane. In some C3 cuts, between 0.1 and 2% by weight of C 2 compounds and C 4 compounds may also be present. The C4 steam-cracking cut, advantageously used for carrying out the selective hydrogenation process according to the invention, has for example the following average mass composition: 1% by weight of butane, 46.5% by weight of butene, 51% weight of butadiene, 1.3% by weight of vinylacetylene (VAC) and 0.2% by weight of butyne. In some C4 cuts, between 0.1 and 2% by weight of C3 compounds and C5 compounds may also be present. The C5 steam-cracking cut, advantageously used for carrying out the selective hydrogenation process according to the invention, has, for example, the following composition: 21% by weight of pentanes, 45% by weight of pentenes and 34% by weight of pentadienes.

The steam cracking gasoline or pyrolysis gasoline, advantageously used for carrying out the selective hydrogenation process according to the invention, corresponds to a hydrocarbon fraction whose boiling point is generally between 0 ° C. and 250 ° C. preferably between 10 ° C and 220 ° C. The polyunsaturated hydrocarbons present in said steam cracking gasoline are in particular diolefinic compounds (butadiene, isoprene, cyclopentadiene, etc.), styrenic compounds (styrene, alpha-methylstyrene, etc.) and indene compounds (indene). . Steam cracking gasoline generally comprises the C5-C12 cut with traces of C3, C4, C13, C14, C15 (for example between 0.1 and 3 wt% for each of these cuts). For example, a charge formed of pyrolysis gasoline generally has a composition in% by weight as follows: 8 to 12% by weight of paraffins, 58 to 62% by weight of aromatic compounds, 8 to 10% by weight of mono-olefins, 18 to 22 % by weight of diolefins and from 20 to 300 ppm by weight of sulfur (part by million), all the compounds forming 100%. Preferably, the polyunsaturated hydrocarbon feedstock treated according to the selective hydrogenation process according to the invention is a steam cracking gasoline. The selective hydrogenation process according to the invention aims to eliminate said polyunsaturated hydrocarbons present in said feedstock to be hydrogenated by converting said polyunsaturated hydrocarbons to the corresponding alkenes by avoiding the total saturation of said hydrocarbons so as to avoid the formation of the alkanes correspondents. For example, when said feed is a C2 cut, the selective hydrogenation process according to the invention aims to selectively hydrogenate acetylene. When said feed is a C3 cut, the selective hydrogenation process according to the invention aims to selectively hydrogenate propadiene and methylacetylene. In the case of a C4 cut, it is intended to remove butadiene, vinylacetylene (VAC) and butyne, in the case of a C5 cut, it is intended to eliminate pentadienes. When said feedstock is a steam cracking gasoline, the selective hydrogenation process according to the invention aims to selectively hydrogenate said polyunsaturated hydrocarbons present in said feedstock to be treated so that the diolefinic compounds are partially hydrogenated to mono-olefins and that the styrenic and indenic compounds are partially hydrogenated to the corresponding aromatic compounds. The technological implementation of the selective hydrogenation process according to the invention is carried out, for example, by injection, in ascending or descending current, of the polyunsaturated hydrocarbon feedstock and hydrogen in at least one fixed bed reactor. Said reactor may be of the isothermal or adiabatic type. An adiabatic reactor is preferred. The polyunsaturated hydrocarbon feedstock may advantageously be diluted by one or more re-injection (s) of the effluent, from said reactor where the selective hydrogenation reaction occurs, at various points of the reactor, located between the inlet and the outlet. the reactor outlet. The technological implementation of the selective hydrogenation process according to the invention can also be advantageously carried out by implanting at least one of said supported catalyst in a reactive distillation column or in reactor-exchangers. The flow of hydrogen can be introduced at the same time as the feedstock to be hydrogenated and / or at a different point of the reactor. The selective hydrogenation of the C2, C3, C4 and C5 + cuts can be carried out in the gas phase or in the liquid phase, preferably in the liquid phase. Indeed, a reaction in the liquid phase makes it possible to lower the energy cost and to increase the cycle time of the catalyst. For a reaction in the liquid phase, the pressure is generally between 1 and 3 MPa, the temperature between 2 and 50 ° C and the molar ratio (hydrogen) / (polyunsaturated compounds to be hydrogenated) between 0.1 and 4, preferably between 1 and 2. The VVH is between 10 and 50 h-1. For a hydrogenation reaction in the gas phase, the pressure is generally between 1 and 3 MPa, the temperature between 40 and 120 ° C, the molar ratio (hydrogen) / (polyunsaturated compounds to be hydrogenated) between 0.1 and 4 preferably between 1 and 2 and the VVH (charge rate per volume of catalyst) is between 500 h -1 and 5000 h -1. Preferably, a selective hydrogenation process is carried out in which the feedstock is a steam cracking gasoline comprising polyunsaturated compounds, the molar ratio (hydrogen) / (polyunsaturated compounds to be hydrogenated) is generally between 1 and 2, the temperature is generally between 40 ° C and 200 ° C, preferably between 50 and 180 ° C, the space hourly velocity (VVH) is generally between 0.5 h-1 and 10 h-1, preferably between 1 h- 1 and 5 h-1 and the pressure is generally between 1.0 MPa and 6.5 MPa, preferably between 2.0 MPa and 3.5 MPa. The hydrogen flow rate is adjusted in order to dispose of it in sufficient quantity to theoretically hydrogenate all of the polyunsaturated compounds and to maintain an excess of hydrogen at the outlet of the reactor. In order to limit the temperature gradient in the reactor, it may be advantageous to recycle a fraction of the effluent to the inlet and / or the middle of the reactor. The invention is illustrated by the following examples without limiting its scope. These examples show that the temperature of the support before impregnation has an influence on the metal dispersion of the catalyst particles and can thus be used to control and modulate it. Similarly, Example 3 shows that the catalysts prepared according to the process of the invention are active in selective hydrogenation. EXAMPLES Example 1 Preparation of palladium catalysts on an alumina support An aqueous solution of palladium nitrate Pd (NO 3) 2 is prepared at ambient temperature by dilution of 3.92 g of an aqueous solution of palladium nitrate Pd (NO3) 2 containing 10% by weight of palladium nitrate and 10% by weight of nitric acid (Aldrich), with demineralized water to a total volume of 61.2 ml. The pH of this solution is 0.81. This solution is then impregnated dropwise over 60 g of an alumina whose specific surface area is 140 m 2 / g and the pore volume is 1.02 ml / g. This alumina is in the form of beads whose average diameter is 3 mm. During impregnation, the support is stirred in a rotating glass flask at 50 rpm. The temperature of the support during the impregnation is regulated by an oil bath in which the glass flask is quenched. The temperature of the support before impregnation of the palladium nitrate salt solution is measured by a probe in direct contact with the support and varies from 50 ° C to 240 ° C. The catalysts obtained at different temperatures of the support are then dried under air at 120 ° C. for 12 hours and then calcined for 2 hours at 450 ° C. under a flow of air with a V.V.H. 500 h-1. The catalysts contain 0.5% by weight Pd relative to the total weight of the catalyst.

Table 1 shows the total temperatures of the support to which the catalyst preparations were carried out. Catalyst Converter Temperature (° C) Catalyst A 50 Catalyst B 80 Catalyst C 110 Catalyst D 135 Catalyst E 150 Catalyst F 160 Catalyst G 180 Catalyst H 240 Table 1: Temperature of the support before impregnation for the preparation of the catalysts Example 2: Measurement of metal dispersion of the catalysts by chemisorption of carbon monoxide The measurements of metal dispersions are carried out by chemisorption of carbon monoxide CO, on the previously reduced catalyst under 1 liter of hydrogen per hour and per gram of catalyst, with a rise in temperature 300 ° C / h and a one-hour stage at 150 ° C. The catalyst is then flushed for 1 hour at 150 ° C. under helium and then cooled to 25 ° C. under helium. The chemisorption of CO is performed at 25 ° C in dynamics, it leads to a chemisorbed CO volume, from which the skilled person can calculate the number of chemisorbed CO molecules. A stoichiometric ratio of one CO molecule per atom of surface Pd is taken into account to calculate the number of surface Pd atoms. The dispersion is expressed as% of surface Pd atoms relative to all the Pd atoms present in the catalyst sample.

The metal dispersions of the various catalysts are reported in Table 2. It is observed that the choice of a certain temperature of the support makes it possible to control and modulate the dispersion of the metal particles.

TABLE 2 Measurement of the Catalyst Metal Dispersion by Chemisorption of Carbon Monoxide Example 3: Catalytic Tests in Hydrogenation of a Styrene-Isoprene Mixture in the Presence of Sulfur Compounds Before the catalytic test, the catalysts A, B, C, D, E, F, G and H are treated under a stream of hydrogen with a VVH of 500 11-1 with a temperature rise of 300 ° C / h and a bearing at a final temperature of 150 ° C for 1 hour. The catalysts are then subjected to a hydrogenation test in a perfectly stirred batch reactor of the "Grignard" type. To do this, 4 ml of reduced catalyst beads are secured to the air in an annular basket located around the stirrer. The baskets used in the reactors are Robinson Mahonnay type.

The hydrogenation is carried out in the liquid phase. The composition of the filler is as follows: 8% styrene weight, 8% isoprene weight, 74% n-heptane, 32 ppm pentanthiol and 262 ppm thiophene. The test is carried out under a constant pressure of 3.5 MPa of hydrogen and at a temperature of 45 ° C.

The products of the reaction are analyzed by gas chromatography. The catalytic activities are expressed in moles of H2 consumed per minute and per gram of palladium and are reported in Table 3. Metal dispersion (%) 43 42 37 36 Catalyst Catalyst A Catalyst B Catalyst C Catalyst D Catalyst E Catalyst F Catalyst G Catalyst H 32 29 23 25 * in (moles H 2) / [minx (gram of palladium)] Table 3: Measured activities in hydrogenation of a styrene-isoprene mixture in the presence of sulfur compounds The catalysts C and D impregnated on a support of alumina at 110 ° C and 135 ° C, respectively, exhibit the optimum metal dispersion for the hydrogenation reaction of a styrene-isoprene mixture in the presence of pentanethiol and thiophene to achieve the highest activities. Catalyst Catalytic activity Catalyst A 0.45 Catalyst B 0.48 Catalyst C Catalyst D Catalyst E Catalyst F Catalyst G Catalyst H 0.50 0.47 0.52 0.54 0.50 0.49

Claims (15)

CLAIMS1 Process for the preparation of a supported catalyst based on metal particles, comprising the following steps: a) an aqueous solution of at least one metal precursor is provided at a first temperature, b) a porous support is heated to a second temperature, said second temperature being greater than said first temperature, C) contacting said aqueous solution obtained in step (a) with said heated porous support of step (b) by dry impregnation, d) drying the product obtained in step (c), e) optionally, the product obtained in step (d) is calcined, f) the product obtained in step (d) or step (e) is reduced. 2. The method of claim 1, wherein in step b) the support is heated to a temperature between 40 ° C and 350 ° C. 3. Process according to claims 1 or 2, wherein the aqueous solution has a temperature below 40 ° C. 4. Process according to claims 1 to 3, wherein in step a) the metal of the metal precursor is selected from the group consisting of palladium, silver, gold, platinum, rhodium, ruthenium. , copper and nickel. 5. Method according to claims 1 to 4, wherein the support is formed of at least one refractory oxide selected from the group consisting of oxides of magnesium, aluminum, silicon, zirconium, thorium, or titanium taken alone or mixed with each other. The process according to claims 1 to 5, wherein the metal precursor is a soluble salt in aqueous solution which is selected from the group consisting of metal halide, oxide, hydroxide, nitrate and sulphate. 7. Process according to claims 1 to 5, wherein the metal precursor is in the form of metal oxide, said metal precursor is obtained by neutralization of an aqueous solution of a salt of the metal precursor with a mineral base chosen from the group formed by alkali hydroxides and alkaline earth hydroxides. 8. Process according to claims 1 to 7, wherein, the step (d) of drying is carried out at a temperature between 50 and 250 ° C, the step (e) of calcination is carried out at a temperature between 150 ° C and 900 ° C and the step (f) of reduction is carried out in a gaseous flow comprising between 25 vol% and 100 vol% of a reducing gas at a temperature between 50 ° C and 500 ° C. 9. Process according to claims 1 to 8, wherein the metal of the metal precursor is palladium, the support is alumina, and said support is heated, in step b), at a temperature between 100 ° C and 150 ° C. 10. A selective hydrogenation process comprising contacting a hydrocarbon feedstock comprising at least one polyunsaturated compound in the presence of hydrogen with the catalyst prepared according to one of claims 1 to 9, said filler being selected from the group consisting of by cuts from catalytic cracking, C2 steam cracking cups, C3 steam cracking cups, C4 steam cracking cups, C5 steam cracking cups and steam cracking gasolines. 11. The method of claim 10 wherein the feedstock is a steam cracking gasoline comprising polyunsaturated compounds, the molar ratio hydrogen / (polyunsaturated compounds to be hydrogenated) is between 1 and 2, the temperature is between 40 ° C and 200 ° C, the space hourly velocity is between 0.5 hl and 1011-1 and the pressure is between 1.0 MPa and 6.5 MPa. 12. A selective hydrogenation process comprising contacting a hydrocarbon feedstock in the presence of hydrogen with the catalyst prepared according to one of claims 1 to 9, said feed comprising aromatic functions, ketones, aldehydes, acids and / or or nitro. 13. A process for hydrogenating carbon monoxide to C1-C6 alcohols, methanol or dimethyl ether comprising contacting a hydrocarbon feedstock in the presence of hydrogen with the catalyst prepared according to one of claims 1 to 9. 14. A process for the isomerization, hydroisomerization or hydrogenolysis of hydrocarbon compounds comprising contacting a hydrocarbon feedstock in the presence of hydrogen with the catalyst prepared according to one of claims 1 to 9. 15. Process according to claims 12 to 14, wherein the temperature is between 0 ° C and 500 ° C, the pressure is between 0.1 and 20 MPa, the hourly space velocity is between 0.1 h -1 and 50 h-1 for a liquid charge, and between 500 h-1 and 30,000 h-1 for a gaseous charge.