WO2018177709A1

WO2018177709A1 - Multilayer selective hydrogenation catalyst, preparation and use of same

Info

Publication number: WO2018177709A1
Application number: PCT/EP2018/055714
Authority: WO
Inventors: Malika Boualleg; Anne-Claire Dubreuil; Anne-Agathe Quoineaud
Original assignee: IFP Energies Nouvelles
Priority date: 2017-03-29
Filing date: 2018-03-08
Publication date: 2018-10-04
Also published as: FR3064500A1

Abstract

A multilayer catalyst comprising at least one refractory oxide selected from the group consisting of silica, alumina and silica-alumina, and an active phase comprising between 0.01 and 2% by weight of a Group VIII metal relative to the weight of the catalyst, said catalyst comprising: a porous support comprising at least said refractory oxide; a first layer, at least partially covering said porous support, comprising at least said Group VIII metal; a second layer, at least partially covering said first layer, comprising at least said refractory oxide; characterised in that: at least 80% by weight of the Group VIII metal relative to the total weight of the Group VIII metal is distributed in said first layer, the thickness of said first layer being between 10 and 600 µm; at least 1% by weight of said refractory oxide relative to the total weight of the catalyst is distributed in said second layer, the thickness of said second layer being between 1 and 300 µm. The methods for preparing said catalyst and the use of same in the selective hydrogenation of a polyunsaturated hydrocarbon feedstock are also disclosed.

Description

MULTILAYER CATALYST FOR SELECTIVE HYROGENATION, PREPARATION AND SAME

USE

Technical area

The selective hydrogenation process makes it possible to convert the polyunsaturated compounds of the petroleum fractions by conversion of the most unsaturated compounds to the corresponding alkenes, avoiding total saturation and thus the formation of the corresponding alkanes.

The object of the invention is to provide an improved performance catalyst and a process for the selective hydrogenation of polyunsaturated hydrocarbon compounds present in hydrocarbon cuts, C2-C5 steam cracking cups and steam cracking gasolines.

State of the art

The catalysts for selective hydrogenation of polyunsaturated compounds are generally based on Group VIII metals of the periodic table of elements such as nickel or palladium. Typically, the metal is in the form of nanometric metal particles deposited on a support which may be a refractory oxide. The metal content of the group VIII, the possible presence of a second metal element, the size of the metal particles and the distribution of the active phase as well as the nature and the porous distribution of the support are parameters which may have an influence on the performance of the catalysts.

Indeed, the macroscopic distribution of the metal particles in the support is an important criterion, mainly in the context of rapid and consecutive reactions such as selective hydrogenations. It is generally necessary for these elements to be 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. For example, the document US2006 / 025302 describes a catalyst for the selective hydrogenation of acetylene and diolefins, comprising palladium distributed in such a way that 90% of the palladium is introduced into the catalyst in a lower crust located at the periphery of the support. and thickness at 250 μηι.

However, such catalysts lead to the formation of fines, generated by attrition, during the various phases of loading and / or unloading of the catalysts and / or during the various catalyst transport operations and / or during its use. However, most of the active phase of the catalyst being located at the periphery of the support, in the form of a very thin crust, a significant amount of active phase can be lost. As a result, the catalyst loses activity. The service life of the catalyst is therefore considerably reduced.

To overcome this problem, US Pat. No. 3,925,253 discloses a process for preparing a catalyst that is more resistant to the attrition phenomenon, said catalyst comprising at the periphery a layer of silicon oxide (silica) or of lanthanum oxide or of copper. The catalysts prepared by this method exhibit improved attrition resistance, but with reduced accessibility to the active phase. However, the selective hydrogenation reactions are extremely rapid and the polyunsaturated compounds to be hydrogenated will not have access to the active phase (at most 500 μηι).

An object of the present invention is to meet the above drawbacks by providing a catalyst which is resistant to attrition and which retains good activity in selective hydrogenation of polyunsaturated compounds to monounsaturated compounds. The Applicant has surprisingly discovered that a multilayer catalyst comprising a porous support containing a refractory oxide, a first layer containing the active phase, covering at least partly the porous support, and a second layer containing at least one refractory oxide covering at least less in part said first layer, retains good activity in selective hydrogenation even after use or a loading / unloading operation of the catalyst with respect to catalysts not comprising a refractory oxide layer covering at least partly the active phase. Such a catalyst can be obtained by a preparation process comprising at least one step of contacting the porous support with at least one organic compound comprising a carboxylic acid function, said step being able to be carried out either before, after or simultaneously with step of contacting the active phase with the porous support.

Objects of the invention

A first subject of the invention relates to a multilayer catalyst comprising at least one refractory oxide selected from the group consisting of silica, alumina and silica-alumina, and an active phase comprising between 0.01 and 2% by weight of a group VIII metal with respect to the weight of the catalyst, which catalyst comprises:

a porous support comprising said refractory oxide;

a first layer, at least partially covering said porous support, comprising at least said group VIII metal; a second layer, at least partially covering said first layer, comprising at least said refractory oxide;

characterized in that

at least 80% by weight of said Group VIII metal relative to the total weight of group VIII metal is distributed in said first layer, the thickness of said first layer being between 10 and 600 μηι;

- At least 1% by weight of said refractory oxide relative to the total weight of the catalyst is distributed in said second layer, the thickness of said second layer being between 1 and 300 μηι.

Preferably, said second layer completely covers said first layer.

Advantageously, the Group VIII metal is palladium or nickel.

More preferably, the Group VIII metal is palladium.

Advantageously, the porous support is alumina.

Preferably, the porous support is in the form of extrudates or balls.

Another object according to the invention relates to a process for preparing a catalyst according to the invention, comprising the following steps:

a) said porous support is brought into contact with at least one solution containing at least one organic compound comprising at least one carboxylic acid function;

b) said porous support is brought into contact with at least one solution containing at least one group VIII metal precursor; steps a) and b) being performed separately, in any order, or simultaneously

c) the impregnated support is dried at a temperature between 15 ° C and less than or equal to 250 ° C;

d) the dried support resulting from step c) is calcined at a temperature greater than 250 ° C. but less than 900 ° C.

Preferably, the molar ratio between said organic compound is said group VIII metal is between 0.1 and 4.0.

In one embodiment of the invention, steps a) and b) are performed simultaneously.

Preferably, said organic compound comprises between 2 and 7 carbon atoms.

More preferably, the organic compound is chosen from hydroxyacetic acid (glycolic acid), 2-hydroxypropanoic acid (lactic acid), 2-hydroxypropanedioic acid (tartronic acid) and 2-hydroxybutanedioic acid (malic acid). , 2-hydroxypropane-1,2,3-tricarboxylic acid (citric acid), 2,3-dihydroxybutanedioic acid (tartaric acid), 2,2'-oxydiacetic acid (diglycolic acid), acid 2-oxopropanoic acid (pyruvic acid), 4-oxopentanoic acid (levulinic acid). Even more preferentially, the organic compound is citric acid.

Advantageously, step a) and / or b) of the process according to the invention is (are) carried out by dry impregnation. Advantageously, step a) bringing said porous support into contact with said solution containing at least one organic compound comprising at least one carboxylic acid function and / or step b) bringing said porous support into contact with said solution comprising at least one at least one group VIII metal precursor is produced by adding said solution containing at least one organic compound comprising at least one carboxylic acid function and / or said solution comprising at least one group VIII metal precursor on said support at a flow rate inclusive between 1 and 20 liter (s) per hour, said porous support being contained in a rotary impregnating device operating at a rotation speed of between 10 and 20 rpm.

Another subject of the invention relates to a process for the selective hydrogenation of a feedstock comprising polyunsaturated compounds containing at least 2 carbon atoms per molecule contained in a hydrocarbon feed having a final boiling point of less than or equal to 300 ° C by contacting said feedstock with hydrogen and at least one catalyst according to the invention or prepared according to the invention, which process is carried out at a temperature between 0 and 300 ° C, at a pressure between 0.1 and 10 MPa, at a molar ratio of hydrogen / (polyunsaturated compounds to be hydrogenated) of between 0.1 and 10 and at an hourly space velocity of between 0.1 and 200 h ^-1 when the process is carried out in the liquid phase or at a molar ratio of hydrogen / (polyunsaturated compounds to be hydrogenated) of between 0.5 and 1000 and at an hourly volume rate of between 100 and 40,000 h ^-1 when the process is carried out in the gas phase.

Description of figures

FIG. 1 is an image obtained by MRI technique, spin echo sequence, T1 contrast of a catalyst according to the prior art comprising an alumina support and comprising a palladium active phase located at the periphery of the porous support.

FIG. 2 is an image obtained by MRI technique, spin-echo sequence, T1 contrast for palladium and T2 for citric acid, of a catalyst according to the invention in the form of an extruded form in which the support porous material is coated with a first layer comprising an active phase based on palladium, said first layer being completely coated with a second layer comprising alumina. FIG. 3 is an image obtained by MRI technique, spin-echo sequence, T1 contrast for palladium and T2 for citric acid, of a catalyst according to the invention in the form of a ball in which the support porous material is coated with a first layer comprising an active phase based on palladium, said first layer being completely coated with a second layer comprising alumina.

Detailed description of the invention

1. Definitions

In the following, the groups of chemical elements are given according to the classification CAS (CRC Handbook of Chemistry and Physics, publisher CRC press, editor-in-chief D.R. Lide, 81 st edition, 2000-2001). For example, group IB according to the CAS classification corresponds to the metals of column 1 1 according to the new IUPAC classification.

Definition of the metallic dispersion of the particles (D)

Particle dispersion is a unitless number, often expressed in%. The dispersion is all the greater as the particles are small. It is defined in the publication of R. Van Hardeveld and F. Hartog, "777th Statistics of Surface Area and Surface Sites on Metal Crystals", Surface Science 15, 1969, 189-230. Definition of layer thickness (also called crust thickness)

In order to analyze the distribution of the metal phase on the support, a crustal thickness is measured by microprobe of Castaing (or microanalysis by electron microprobe). The device used is a CAMECA XS100, equipped with four monochromator crystals allowing the simultaneous analysis of four elements. The technique of analysis by microprobe of Castaing consists in the detection of X-radiation emitted by a solid after excitation of its elements by a beam of electrons of high energies. For the purposes of this characterization, the catalyst grains are embedded in epoxy resin pads. These pads are polished until the cup to the diameter of the balls or extruded and metallized by carbon deposition metal evaporator. The electronic probe is scanned along the diameter of five balls or extruded to obtain the average distribution profile of the constituent elements of the solids.

When the active element is crusted, its local concentration generally decreases progressively when measured from the edge of the catalytic grain inwards. A distance from the edge of the grain, at which the local content of the active element becomes zero, can often not be determined accurately and reproducibly. In order to measure a crustal thickness that is significant for the majority of the active element particles, crust thickness is defined as the distance to the edge of the grain containing 80% by weight of active element.

It is defined in the publication by L. Sorbier et al. To measure a crustal thickness that is significant for the majority of active element particles, crust thickness can alternatively be used to measure crustal thickness. be defined as the distance to the edge of the grain containing 80% by weight of active element From the distribution profile obtained by the Castaing microprobe (c (x)), the cumulative quantity Q (y) of active element in the grain as a function of the distance y at the edge of the grain of radius r.

For a marble:

For an extruded:

with

r: grain radius;

y: distance to the edge of the grain;

x: integration variable (position on the profile).

It is assumed that the concentration profile follows the diameter taken from x = - r to x = + r (x = 0 being the center).

Q {r) is the total amount of the element in the grain. Then we solve numerically the following equation in y:

Since c is a strictly positive function, Q is therefore a strictly increasing function and this equation has only one solution which is the crust thickness.

Characterization by MRI

The nuclear magnetic resonance imaging (MRI) technique is used to characterize the spatial location of the metal phase and the organic compound.

The apparatus used is a Bruker® 9.1 T NMR spectrometer equipped with a high-power amplifier for generating radio frequency pulses at the Larmor frequency proton, x, y, z gradient system (750 x750x750 G / cm) and a 5 mm measuring probe (micro5).

For the characterization, a catalyst ball or extrusion is introduced into a capillary and then into a 5 mm diameter MRI tube and finally introduced into the measuring probe. Characterization by MRI consists of the detection of the proton signal in the x, y, z directions by contrast in relaxation time. The relaxation phenomenon consists in the return to equilibrium of the magnetic magnetization created during the radiofrequency pulse by the "spin-lattice" relaxation, called T1 transverse relaxation, and the "spin-spin" relaxation, named longitudinal T2 relaxation. . MRI characterizations consist of a judicious choice of parameters of the pulse sequence so as to obtain a T1 or T2 contrast. The sequence "Spin Echo", denoted SE, is the pulse sequence used.

The person skilled in the art knows how to adapt the key parameters of the spin echo sequence, namely the voxel and pixel sizes as well as the inter-echo and repetition times which are chosen as a function of the spatial resolution of the relaxation times. characteristics of each of the components present: metallic phase, organic compound and support. This technique makes it possible to measure the thickness of the crust (layer thickness) of the refractory oxide layer located at the extreme periphery of the catalyst.

Attrition test

The attrition test is conducted according to the standard, ASTM D4058-96 (2015), Standard Test Method for Attrition and Abrasion of Catalysts and Catalyst Carriers.

100 grams of previously sieved catalyst are subjected to 30 minutes of mechanical stress (60 rpm) before being sieved and re-weighed to obtain the attrition rate. At the end of this test, the fines produced by the attrition of the catalysts are collected and weighed. A percentage of fines generated by this test is then obtained. The purpose of this characterization technique is to determine the wear of a catalyst or catalyst support caused by interparticle friction or wall effects. This test applies to shaped materials, mainly logs, extrusions (quadrilobes, trilobed). This method should simulate the production of fines during transport, the loading and unloading of the catalyst, as well as its use.

2. Catalyst According to the invention, the multilayer catalyst comprises at least one refractory oxide selected from the group consisting of silica, alumina and silica-alumina, and an active phase comprising between 0.01 and 2% by weight of a metal group VIII with respect to the weight of the catalyst, which catalyst comprises:

a porous support comprising at least one refractory oxide selected from the group consisting of silica, alumina and silica-alumina;

a first layer, at least partially covering said porous support, comprising at least one Group VIII metal;

a second layer, at least partially covering said first layer, comprising at least one refractory oxide selected from the group consisting of silica, alumina and silica-alumina;

at least 80% by weight of Group VIII metal relative to the total weight of Group VIII metal is distributed in said first layer, the thickness of said first layer being between 10 and 600 μηι, preferably between 10 and 100; μηι, and more preferably between 20 and 90 μηι;

- At least 1% by weight of said refractory oxide relative to the total weight of the catalyst is distributed in said second layer, the thickness of said second layer being between 1 and 300 μηι, preferably between 1 and 200 μηι. Preferably between 1 and 30% by weight, and more preferably between 1 and 10% by weight of said refractory oxide relative to the total weight of the catalyst is distributed in said second layer, the thickness of said second layer being between 1 and 300 μηι, preferably between 1 and 200 μηι. When the group VIII metal is palladium, the average size of the metal particles is between 4 to 10 nm, preferably between 3 and 6 nm. The average size of the crystallites is deduced from the measurements of metal dispersion of the particles (D), by applying the dispersion-particle size relationships known to those skilled in the art and described in "Physico-chemical analysis of industrial catalysts, Chapter I, Editions Technip, Paris 2001 ".

When the Group VIII metal is nickel, the average size of the metal particles is less than 18 nm, preferably less than 15 nm. The average size of the crystallites is deduced from the X-ray diffraction characterization. The porous support is selected from the group consisting of silica, alumina and silica-alumina. Even more preferably, the support is alumina. The alumina can be present in all possible crystallographic forms: alpha, delta, theta, chi, rho, eta, kappa, gamma, etc., taken alone or as a mixture. Preferably the support is selected from alpha alumina, delta, teta, gamma.

The specific surface of the porous support is preferably between 50 and 250 m ² / g, preferably between 70 and 230 m ² / g, and even more preferably between 70 and 220 m ² / g. The BET surface area is measured by physisorption with nitrogen. The BET surface area is measured by nitrogen physisorption according to ASTM D3663-03 as described in Rouquerol F .; Rouquerol J .; Singh K. "Adsorption by Powders & Porous Solids: Principle, methodology and applications", Academy Press, 1999.

The total pore volume of the support is between 0.1 and 1.5 cm ³ / g, preferably between 0.5 and 1.1 cm ³ / g, and even more preferably between 0.25 and 1, 3 cm ³ / g. The total pore volume is measured by mercury porosimetry according to ASTM standard D4284-92 with a wetting angle of 140 °, for example by means of an Autopore® III model apparatus of the Microméritics® brand. In one embodiment according to the invention, the support of the catalyst is purely mesoporous, ie it has a pore diameter of between 2 and 50 nm, preferably between 5 and 30 nm and even more preferably between 8 and 20 μm. nm.

In another embodiment according to the invention, the support of the catalyst is bimodal, the first mode being mesoporous, ie it has a pore diameter of between 2 and 50 nm, preferably between 5 and 30 nm, and still preferred from 8 to 20 nm, and the second macroporous, ie it has pores with a diameter greater than 50 nm. The support used for the preparation of the catalyst according to the invention advantageously has a pore volume of pores having a pore diameter of between 50 and 700 nm less than 20% of the total pore volume of the support, preferably less than 18% of the pore volume. total support and particularly preferably less than 15% of the total pore volume of the support.

The support may optionally include sulfur. The sulfur content in the support may be between 0.0050 and 0.25% by weight relative to the total weight of the catalyst, preferably between 0.0075 and 0.20% by weight. The group VIII metal content is between 0.01 and 2% by weight of a Group VIII metal relative to the weight of the catalyst, preferably between 0.05 and 1% by weight, and even more preferentially between 0 and 1% by weight. , 1 and 0.9% by weight. Preferably, the Group VIII metal is palladium or nickel.

According to the invention, the porous support is in the form of cylindrical or multilobed beads or extrusions (trilobed, quadrilobes). Preferably the porous support is in the form of extrudates.

In a particular embodiment according to the invention, the catalyst consists of extrudates of diameter generally between 0.5 and 10 mm, preferably between 0.8 and 3.2 mm and very preferably between 1.0 and 3.2 mm. and 2.5 mm. This may advantageously be in the form of cylindrical, multilobed, trilobed or quadrilobed extrudates. Preferably its shape will be trilobed or quadrilobed. The shape of the lobes can be adjusted according to all known methods of the prior art.

In another particular embodiment according to the invention, the catalyst is in the form of beads having a diameter of between 1 and 8 mm, preferably between 2 and 7 mm.

3. Preparation Process The invention also relates to a process for preparing the catalyst. More particularly, the process for preparing the catalyst according to the invention comprises the following steps:

b) said porous support is brought into contact with at least one solution containing at least one group VIII metal precursor; steps a) and b) being performed separately, in any order, or simultaneously;

c) the impregnated support is dried at a temperature of less than or equal to 250 ° C;

d) the dried support from step c) is calcined at a temperature above 250 ° C.

Without being bound by any theory, the process according to the invention, because of its step a), makes it possible to create a refractory oxide layer that protects the layer comprising the group VIII metal. Indeed, during the contacting step a), an adsorption competition in the support between the organic compound and the metal of group VIII grows Group VIII metal complexes to migrate into a deeper layer. This generates after the step of calcination of the catalyst precursor which leads to the decomposition of the organic compound, to the formation of a second layer comprising at least partly a refractory oxide, at least partially covering the layer comprising the metal of the group VIII, thus making it possible to protect said group VIII metal, and more particularly during the use of the catalyst and the catalyst loading and / or unloading operations.

In one embodiment of the invention, step a) is carried out before step b) (pre-impregnation). In another embodiment of the invention, step b) is carried out before step a) (post-impregnation).

In yet another embodiment of the invention, steps a) and b) are performed simultaneously (co-impregnation).

When steps a) and b) are carried out successively and not simultaneously, ie step a) is carried out before step b) or step b) before step a), no intermediate drying step between said steps are not performed.

The bringing into contact of the group VIII metal precursor and the organic compound can be carried out according to all the techniques known to those skilled in the art. Preferably, the Group VIII metal precursor solution and the solution containing the organic compound are deposited by dry or excess impregnation method.

The different steps are explained in detail below.

Step a) bringing the organic compound into contact with the support

The bringing into contact of said support with at least one solution containing at least one organic compound comprising at least one carboxylic acid function, according to the implementation of said step a), can be carried out by any method well known to the man of the job. In particular, said step a) can be carried out by impregnation, dry or in excess, according to methods that are well known to those skilled in the art. Preferably, said step a) is carried out by dry impregnation, which consists in bringing the support of the catalyst into contact with a volume of said solution of between 0.3 and 1.5 times the pore volume of the support to be impregnated.

Said solution containing at least one organic compound comprising at least one carboxylic acid function may be aqueous or organic (for example methanol or ethanol or phenol or acetone or toluene or dimethylsulfoxide (DMSO)) or consists of a mixture of water and at least one organic solvent. Said organic compound is previously at least partially dissolved in said solution at the desired concentration. Preferably, said solution is aqueous or contains ethanol. Even more preferably, said solution is aqueous. The pH of said solution may be modified by the possible addition of an acid or a base. In another possible embodiment, the solvent may be absent from the impregnating solution when the organic compound is in the liquid state at the temperature and pressure of contacting the support with the organic compound.

In a preferred embodiment according to the invention, bringing said support into contact with said solution containing at least one organic compound comprising at least one carboxylic acid function is carried out by means of a rotary impregnation device, such as a impregnating barrel (also called rotating drum). The rotary impregnating device used is preferably a conventional impregnating barrel whose enclosure can be placed under reduced pressure (approximately 20 mmHg) or under gas (nitrogen) sweep.

The rotary impregnation device is equipped with a double jacket, in which circulates a heat transfer fluid via a thermoregulator. It is thus possible to regulate a wall temperature within the impregnator and a drying time. In a preferred embodiment, the impregnation temperature is between 40 and 90 ° C, preferably between 50 and 70 ° C.

According to the invention, the rotary impregnating device in which the support has been loaded operates at a rotational speed of between 4 and 20 rpm.

As a continuous process, for example, a process may be mentioned in which the solution containing at least one organic compound comprising at least one carboxylic acid function is poured into a tank which discharges continuously into a rotating drum comprising the support to be impregnated.

Preferably, said organic compound comprising at least one carboxylic acid functional group comprises between 2 and 7 carbon atoms, preferably between 2 and 6 carbon atoms.

Said organic compound may comprise at least one second functional group chosen from ethers, hydroxyls, ketones and esters.

Even more preferentially, the organic compound comprising at least one carboxylic acid function is chosen from hydroxyacetic acid (glycolic acid), 2-hydroxypropanoic acid (lactic acid), 2-hydroxypropanedioic acid (tartronic acid), 2-hydroxybutanedioic acid (malic acid), 2-hydroxypropane-1,2,3-tricarboxylic acid (citric acid), 2,3-dihydroxybutanedioic acid (tartaric acid), 2,2 'acid -oxydiacetic acid (diglycolic acid), 2-oxopropanoic acid (pyruvic acid), 4-oxopentanoic acid (levulinic acid). More preferably, said organic compound is citric acid.

The molar ratio of said organic compound comprising at least one carboxylic acid function introduced during step a) relative to the metal of group VIII introduced in step b) is between 0.1 and 4.0 mol / mol, more preferably between 0.3 and 3.5 mol / mol.

Step b) Contacting the group VIII metal precursor with the support

The deposition of the group VIII metal precursor on said support, in accordance with the implementation of said step b), can be carried out by any method well known to those skilled in the art. In particular, said step b) can be carried out by impregnation, dry or in excess, or by deposition - precipitation, according to methods well known to those skilled in the art.

Said step b) is preferably carried out by impregnation of the support consisting, for example, in bringing said support into contact with at least one aqueous or organic solution (for example methanol or ethanol or phenol or acetone or toluene or dimethylsulfoxide (DMSO)) or consists of a mixture of water and at least one organic solvent, containing at least one group VIII metal precursor at least partially in the dissolved state, or else in the setting of contacting said support with at least one colloidal solution of at least one group VIII metal precursor, in oxidized form (nanoparticles of oxide, oxy (hydroxide) or of group VIII metal hydroxide) or in reduced form (Metallic metal nanoparticles of Group VIII metal in the reduced state). Preferably, the solution is aqueous. The pH of this solution can be modified by the possible addition of an acid or a base. According to another preferred variant, the aqueous solution may contain ammonia or ammonium ions NH ₄ ⁺ . Preferably, said step b) is carried out by dry impregnation, which consists in bringing the catalyst support into contact with a solution containing at least one precursor of the group VIII metal, the volume of the solution of which is between 0.degree. , 3 and 1, 5 times the pore volume of the support to be impregnated. When the group VIII metal precursor is introduced in aqueous solution, a group VIII metal precursor is advantageously used in the form of nitrate, carbonate, chloride, sulphate, hydroxide, hydroxycarbonate, formate, acetate, oxalate, complexes formed with acetylacetonates, or complex tetrammine or hexammine, or any other soluble inorganic derivative in aqueous solution, which is brought into contact with said support.

When the group VIII metal is palladium, the palladium precursor is preferably selected from sodium chloropalladate and palladium nitrate. When the group VIII metal precursor is introduced in the form of a colloidal suspension, a colloidal suspension of Group VIII metal oxide or Group VIII metal hydroxide is prepared in the aqueous phase by mixing an aqueous solution.

(I) comprising at least one hydroxide selected from the group consisting of alkali hydroxides and alkaline earth hydroxides and an aqueous solution (II) comprising at least one group VIII metal precursor.

When the Group VIII metal is palladium, said colloidal suspension is generally obtained by hydrolysis of the palladium cation in an aqueous medium, which leads to the formation of particles of oxide or palladium hydroxide in suspension. The aqueous alkali hydroxide or alkaline earth hydroxide solution is generally selected from the group consisting of aqueous solutions of sodium hydroxide, aqueous solutions of magnesium hydroxide. Preferably, the aqueous solution is preferably an aqueous solution of sodium hydroxide.

The precursor salt of palladium 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.

Typically, the aqueous solution comprising at least one group VIII metal precursor salt (also called solution) is supplied in a suitable apparatus.

(II)] then the aqueous solution comprising at least one alkali or alkaline earth hydroxide [also referred to herein as solution (I)]. Alternatively, the solutions (I) and (II) can be poured simultaneously into the apparatus. Preferably, the aqueous solution (II) and then the aqueous solution (I) is poured into the apparatus.

The colloidal suspension generally remains in the apparatus for a residence time of between 0 and 20 hours.

The concentrations of the solution (I) and (II) are generally chosen in order to obtain a pH of the colloidal suspension of between 1.0 and 3.5. So the pH of the colloidal suspension can be modified during this residence time by adding amounts of acid or base compatible with the stability of the colloidal suspension.

In general, the preparation temperature is between 5 ° C and 40 ° C and preferably between 15 ° C and 35 ° C.

The palladium concentration is preferably between 5 and 150 millimoles per liter (mmol / L), more preferably between 8 and 80 millimoles per liter.

The quantities of the group VIII metal precursor introduced into the solution are chosen so that the total element content of the group VIII metal is between 0.01 and 2% by weight of the catalyst mass, preferably between 0 and , 05 and 1% by weight, more preferably between 0.1 and 0.9% by weight

In a preferred embodiment according to the invention, bringing said support into contact with said solution comprising at least one group VIII metal precursor is carried out by means of a rotary impregnation device, such as an impregnating barrel ( also called rotating drum). The rotary impregnating device used is preferably a conventional impregnating barrel whose enclosure can be placed under reduced pressure (approximately 20 mmHg) or under gas (nitrogen) sweep.

The rotary impregnation device is equipped with a double jacket, in which circulates a heat transfer fluid via a thermoregulator. It is thus possible to regulate a wall temperature within the impregnator and a drying time. In a preferred embodiment, the impregnation temperature is between 40 and 90 ° C, preferably between 50 and 70 ° C. According to the invention, the rotary impregnating device in which the support has been loaded operates at a rotational speed of between 4 and 20 rpm. Above 20 rpm, the layer comprising the group VIII metal obtained on the support is too weak, ie below 20 μηι, and part of the solution containing the group VIII metal precursor is not impregnated on the support. If the rotation of the drum is too small, ie less than 4 revolutions / minute, the layer comprising the metal of group VIII obtained on the support can exceed 600 μηι thick, and the dispersion of the metal of group VIII on the support can not not be satisfactory, ie less than 15%.

According to the invention, the rate of addition of said solution to the stage containing the group VIII metal precursor on the porous support is between 1 and 20 liter (s) per hour. Above 20 liters per hour, the Group VIII metal layer obtained is too thick, ie above 600 μηι and the dispersion of the Group VIII metal is unsatisfactory, ie less than 15%.

As a continuous process, for example, there may be mentioned a process in which the group VIII metal precursor solution is poured into a tank which discharges continuously into a rotating drum comprising the support to be impregnated.

After step b), the support is generally wet-matured for 0.5 to 40 hours, preferably for 1 to 30 hours. Longer durations are not excluded, but do not necessarily improve. c) drying the catalyst precursor

The precursor of the catalyst is generally dried, preferably at a temperature of between 15 ° C. and less than or equal to 250 ° C., more preferably between 30 ° C. and 220 ° C., more preferably between 70 ° C. and 180 ° C. vs. The drying time is between 0.5 h and 20 h. Longer durations are not excluded, but do not necessarily improve.

The drying is generally carried out under combustion air of a hydrocarbon, preferably methane, or in heated air comprising between 0 and 80 grams of water per kilogram of combustion air, an oxygen content of between 5% and 25% volume and a carbon dioxide content between 0% and 10% volume. d) calcination of the dried catalyst obtained in step c)

After drying, the catalyst is calcined in air, preferably in combustion, and more preferably in a combustion air of methane, comprising between 40 and 80 grams of water per kg of air, an oxygen content of between 5% and 15% volume and a C0 _{2 content} between 4% and 10% volume. The calcining temperature is generally above 250 ° C but is below 900 ° C, preferably between about 300 ° C and about 500 ° C. The calcination time is generally between 0.25 h and 10 h. e) reduction of the oxide thus supported obtained in step d), preferably using hydrogen gas (optional step)

The catalyst is generally reduced. This step is preferably carried out in the presence of a reducing gas, either in situ, that is to say in the reactor where the catalytic conversion is carried out, or ex-situ. Preferably, this step is carried out at a temperature of between 80 ° C. and 450 ° C., even more preferably between 100 ° C. and 400 ° C.

The reduction is carried out in the presence of a reducing gas comprising between 25 vol% and 100 vol% hydrogen, preferably 100% hydrogen volume. The hydrogen is optionally supplemented with an inert gas for reduction, preferably argon, nitrogen or methane.

The reduction generally comprises a temperature rise phase and then a landing. The duration of the reduction stage is generally between 1 and 40 hours, preferably between 2 and 20 hours.

The Volumetric Hourly Speed (V.V.H) is generally between 150 and 3000, preferably between 300 and 1500 liters of reducing gas per hour and per liter of catalyst. f) passivation (optional step)

Prior to its implementation in the catalytic reactor, the catalyst according to the invention may optionally undergo a passivation step (step f) with a sulfur or oxygen compound or with CO ₂ before or after the reducing treatment step e) . This passivation step may be performed ex situ or in situ. The passivation step is carried out by the implementation of methods known to those skilled in the art.

The sulfur passivation step makes it possible to improve the selectivity of the catalysts and to avoid thermal runaways when starting new catalysts ("run away" according to the English terminology). Passivation generally consists in irreversibly poisoning with the sulfur compound the most virulent active sites, for example nickel, which exist on the new catalyst and thus in attenuating the activity of the catalyst in favor of its selectivity. The passivation step is carried out by the implementation of methods known to those skilled in the art and in particular, for example by the implementation of one of the methods described in patent documents EP0466567, US5153163, FR2676184, WO2004 / 098774, EP0707890. The sulfur compound is for example chosen from the following compounds: thiophene, thiophane, alkylmonosulfides such as dimethylsulfide, diethylsulfide, dipropylsulphide and propylmethylsulphide or an organic disulfide of formula HO-RrS-SR ₂ -OH such as di-thio-di- ethanol of the formula HO- C ₂ H 4 SSC ₂ H4 OH (often called DEODS). The sulfur content is generally between 0.1 and 2% by weight of said element relative to the mass of the catalyst.

The passivation step with an oxygenated compound or with CO ₂ is generally carried out after a reducing treatment beforehand at elevated temperature, generally between 350 and 500 ° C., and makes it possible to preserve the metallic phase of the catalyst in the presence of air. . A second reducing treatment at a lower temperature, generally between 120 and 350 ° C., is then generally carried out. The oxygenated compound is generally air or any other stream containing oxygen. 4. Use of the catalyst

Monounsaturated organic compounds such as, for example, ethylene and propylene, are at 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 + cut (hydrocarbon compounds having at least 5 carbon atoms), in particular diolefinic or styrenic or indenic compounds. These polyunsaturated compounds are very reactive and lead to spurious reactions in the polymerization units. It is therefore necessary to eliminate them before valuing these cuts.

Selective hydrogenation is the main treatment developed to specifically remove undesired polyunsaturated compounds from these hydrocarbon feeds. 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 or naphthenes. In the case of steam cracking gasolines used as a filler, the selective hydrogenation also makes it possible to selectively hydrogenate alkenyl aromatics to aromatics by avoiding the hydrogenation of the aromatic rings.

The hydrocarbon feedstock treated in the selective hydrogenation process has a final boiling point less than or equal to 300 ° C and contains at least 2 carbon atoms per molecule and comprises at least one polyunsaturated compound. The term "polyunsaturated compounds" means compounds comprising at least one acetylenic function and / or at least one diene function and / or at least one alkenylaromatic function.

More particularly, the filler is selected from the group consisting of a C2 steam cracking cut, a C2-C3 steam cracking cut, a steam cracking C3 cut, a steam cracking C4 cut, a steam cracking C5 cut and a still called steam cracking gasoline. pyrolysis gasoline or C5 + cut.

The steam cracking section C2, advantageously used for carrying out the selective hydrogenation process according to the invention, has for example the following composition: between 40 and 95% by weight of ethylene, of the order of 0.1 to 5% by weight of acetylene, the remainder being essentially ethane and methane. In certain steam cracking sections, between 0.1 and 1% by weight of C 3 compounds may also be present. The C3 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 1 to 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. A C2 - C3 cut can also be advantageously used for the implementation of the selective hydrogenation process according to the invention. It has for example the following composition: of the order of 0.1 to 5% by weight of acetylene, of the order of 0.1 to 3% by weight of propadiene and methylacetylene, of the order of 30% by weight ethylene, of the order of 5% by weight of propylene, the remainder being essentially methane, ethane and propane. This filler may also contain between 0.1 and 2% by weight of C4 compounds.

The C4 steam-cracking cut, advantageously used for the implementation of the selective hydrogenation process according to the invention, has for example the following average mass composition: 1% weight of butane, 46.5% weight of butene, 51% by weight butadiene, 1.3% by weight of vinylacetylene 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 and 300 ° C., preferably between 10 and 250 ° C. The polyunsaturated hydrocarbons to be hydrogenated 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% by weight for each of these cuts). For example, a charge formed of pyrolysis gasoline generally has the following composition: 5 to 30% by weight of saturated compounds (paraffins and naphthenes), 40 to 80% by weight of aromatic compounds, 5 to 20% by weight of mono-olefins, 5 to 40% by weight of diolefins, 1 to 20% by weight of alkenylaromatic compounds, all the compounds forming 100%. It also contains from 0 to 1000 ppm by weight of sulfur, preferably from 0 to 500 ppm by weight of sulfur.

Preferably, the polyunsaturated hydrocarbon feedstock treated according to the selective hydrogenation process according to the invention is a C2 steam cracking cut, or a C2-C3 steam cracking cut, or a C3 cut, or a steam cracking gasoline. The selective hydrogenation process according to the invention aims at eliminating said polyunsaturated hydrocarbons present in said feedstock to be hydrogenated without hydrogenating the monounsaturated hydrocarbons. For example, when said feed is a C2 cut, the selective hydrogenation process aims to selectively hydrogenate acetylene. When said feedstock is a C3 cut, the selective hydrogenation process 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 feed is a steam cracking gasoline, the selective hydrogenation process aims to selectively hydrogenate said polyunsaturated hydrocarbons present in said feed to be treated so that the diolefinic compounds are partially hydrogenated to mono-olefins and that the styrenic and indene compounds are partially hydrogenated to corresponding aromatic compounds by avoiding the hydrogenation of aromatic rings. The technological implementation of the selective hydrogenation process 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 to limit the temperature gradient in the reactor. The technological implementation of the selective hydrogenation process according to the invention may also be advantageously carried out by implanting at least one of said supported catalyst in a reactive distillation column or in reactor-exchangers or in a slurry-type reactor. . The flow of hydrogen can be introduced at the same time as the feedstock to be hydrogenated and / or at one or more different points of the reactor.

The selective hydrogenation of the steam-cracking cuts C2, C2-C3, C3, C4, C5 and C5 + can be carried out in the gaseous phase or in the liquid phase, preferably in the liquid phase for the C3, C4, C5 and C5 + cuts and in the phase gaseous for C2 and C2-C3 cuts. A reaction in the liquid phase makes it possible to lower the energy cost and to increase the catalyst cycle time. In general, the selective hydrogenation of a hydrocarbon feed containing polyunsaturated compounds containing at least 2 carbon atoms per molecule and having a final boiling point of less than or equal to 300 ° C. is carried out at a temperature of between 0 and 300 ° C., at a pressure of between 0.1 and 10 MPa, at a molar ratio of hydrogen / (polyunsaturated compounds to be hydrogenated) ) between 0.1 and 10 and at an hourly volume velocity VVH (defined as the ratio of the volume flow rate of charge to the catalyst volume) of between 0.1 and 200 h ^-1 for a process carried out in the liquid phase, or at a hydrogen / (polyunsaturated compounds to be hydrogenated) molar ratio of between 0.5 and 1000 and at a VVH hourly volume rate of between 100 and 40,000 h ^-1 for a process carried out in the gas phase.

In one embodiment according to the invention, when a selective hydrogenation process is carried out in which the feedstock is a steam cracking gasoline containing polyunsaturated compounds, the molar ratio (hydrogen) / (polyunsaturated compounds to be hydrogenated) is generally understood. between 0.5 and 10, preferably between 0.7 and 5.0 and even more preferably between 1.0 and 2.0, the temperature is between 0 and 200 ° C, preferably between 20 and 200 ° C and even more preferably between 30 and 180 ° C, the hourly volume velocity (VVH) is generally between 0.5 and 100 h ^"1 , preferably between 1 and 50 h ^{" 1} and the pressure is generally between 0.3 and 8.0 MPa, preferably between 1.0 and 7.0 MPa and even more preferably between 1.5 and 4.0 MPa. More preferably, a selective hydrogenation process is carried out in which the feedstock is a steam cracking gasoline comprising polyunsaturated compounds, the molar ratio of hydrogen / (polyunsaturated compounds to be hydrogenated) is between 0.7 and 5.0, the temperature is between 20 and 200 ° C., the hourly volume velocity (VVH) is generally between 1 and 50 h ^-1 and the pressure is between 1.0 and 7.0 MPa. selective hydrogenation in which the feedstock is a steam cracking gasoline containing polyunsaturated compounds, the molar ratio hydrogen / (polyunsaturated compounds to be hydrogenated) is between 1.0 and 2.0, the temperature is between 30 and 180 ° C., the Hourly volume velocity (VVH) is generally between 1 and 50 h ^-1 and the pressure is between 1.5 and 4.0 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 another embodiment according to the invention, when a selective hydrogenation process is carried out in which the feedstock is a steam-cracking cross-section C2 and / or a C2-C3 steam-cracking cross-section comprising polyunsaturated compounds, the molar ratio ( hydrogen) / (polyunsaturated compounds to be hydrogenated) is generally between 0.5 and 1000, preferably between 0.7 and 800, the temperature is between 0 and 300 ° C, preferably between 15 and 280 ° C, the speed hourly volume (VVH) is included generally between 100 and 40000 h ^-1 , preferably between 500 and 30000 h ^-1 and the pressure is generally between 0.1 and 6.0 MPa, preferably between 0.2 and 5.0 MPa.

Examples

Example 1: Preparation of a catalyst C1 (non-compliant)

A colloidal suspension of Pd oxide is prepared with stirring at 25 ° C. by diluting 230 g of a solution of palladium nitrate Pd (NO ₃ ) ₂ containing 8.5% by weight of palladium Pd with approximately 3.4 liters. demineralized water, then adding about 150ml of a sodium hydroxide solution to obtain a pH of 2.4. The suspension is then diluted with demineralized water to a volume corresponding to 100% of the pore volume of the alumina support (Vp = 0.7 ml / g), the specific surface area of which is 140 m ² / g shape of balls of diameter 2-4 mm. This solution is then impregnated in an impregnation barrel at a rotation speed of 14 rpm and a solution addition rate of 5.4 L / h on about 10 kg of an alumina heated to 20 ° C. A stage of maturation of the impregnated support before drying of a duration of 20 h is carried out under air in a confined and humid environment. The solid obtained is dried for 16 hours at 90 ° C under nitrogen flow. The catalyst is then calcined under a flow of air for 2 hours at 450 ° C.

The catalyst C1 thus prepared contains 0.19% by weight of palladium relative to the total weight of catalyst.

The characterization of the catalyst C1 by microprobe of Castaing shows that 80% of the Pd is distributed over a crust thick about 75 μηι.

The apparent dispersion of the palladium of the catalyst C1 is 23%. Example 2 Preparation of a Catalyst C2 (Conforming)

A colloidal suspension of Pd oxide is prepared with stirring at 25 ° C. by diluting 230 g of a solution of palladium nitrate Pd (NO ₃ ) ₂ containing 8.5% by weight of palladium Pd with approximately 5.6 liters. demineralised water, then add about 150mL of a sodium hydroxide solution to obtain a pH of 2.4. Citric acid is added to the colloidal solution with a citric acid to palladium acid ratio of 1.

The suspension is then diluted with demineralized water to a volume corresponding to 100% of the pore volume of the alumina support (Vp = 0.7 ml / g), the specific surface area of which is 140 m ² / g shape of balls of diameter 2 - 4 mm. This solution is then impregnated in an impregnation barrel at a rotation speed of 14 rpm and a rate of addition of the solution of 5.4 L h on about 10 kg of an alumina heated to 20 ° C. A stage of maturation of the impregnated support before drying of a duration of 20 h is carried out under air in a confined and humid environment. The solid obtained is dried for 16 hours at 90 ° C under nitrogen flow. The catalyst is then calcined under a flow of air for 2 hours at 450 ° C.

The catalyst C2 thus prepared contains 0.19% by weight of palladium relative to the total weight of catalyst.

The characterization of the catalyst C2 by a Castaing microprobe shows that 80% of the Pd is distributed over a crust of thickness approximately 75 μηι.

The characterization of the catalyst C2 by MRI shows the presence of a layer of alumina distributed over a crust with a thickness of about 35 μηι at the extreme periphery of the catalyst.

The apparent dispersion of the palladium of catalyst C2 is 26%.

Catalyst C2 is shown in FIG.

Example 3 Preparation of a C3 Catalyst (Non-Conforming)

A colloidal suspension of Pd oxide is prepared with stirring at 25 ° C. by diluting 230 g of a solution of palladium nitrate Pd (NO ₃ ) ₂ containing 8.5% by weight of palladium Pd with approximately 4.6 liters. demineralized water, then add about 150 mL of sodium hydroxide solution to reach a pH of 2.4. The suspension is then diluted with demineralised water to a volume corresponding to 75% of the pore volume of the alumina support (Vp = 0.7 ml / g), the specific surface area of which is 180 m ² / g form of trilobal extrusions with a diameter of 2 mm. This solution is then impregnated in an impregnation barrel at a rotation speed of 14 rpm and a solution addition rate of 5.4 l / h on about 10 kg of an alumina heated to 60 ° C. A stage of maturation of the impregnated support before drying of a duration of 20 h is carried out under air in a confined and humid environment. The solid obtained is dried for 16 hours at 90 ° C under nitrogen flow. The catalyst is then calcined under a flow of air for 2 hours at 450 ° C.

Catalyst C3 thus prepared contains 0.19% by weight of palladium relative to the total weight of catalyst.

The characterization of the C3 catalyst by a Castaing microprobe shows that 80% of the Pd is distributed over a crust with a thickness of approximately 35 μηι.

The apparent dispersion of the palladium of catalyst C3 is 25%.

Catalyst C3 is shown in FIG.

Example 4 Preparation of a C4 Catalyst (Compliant)

A colloidal suspension of Pd oxide is prepared with stirring at 25 ° C. by diluting 230 g of a solution of palladium nitrate Pd (NO ₃ ) ₂ containing 8.5% by weight of palladium. Pd with about 5.6 liters of demineralized water, then add about 150 ml of a sodium hydroxide solution to obtain a pH of 2.4. Citric acid is added to the colloidal solution with a citric acid to palladium acid ratio of 1.

The suspension is then diluted with demineralised water to a volume corresponding to 100% of the pore volume of the alumina support (Vp = 0.7 ml / g), the specific surface area of which is 180 m ² / g form of trilobal extrusions with a diameter of 2 mm. This solution is then impregnated in an impregnation barrel at a rotation speed of 14 rpm and an addition rate of the solution of 5.4 l / h on about 10 kg of an alumina heated to 20 ° C. A stage of maturation of the impregnated support before drying of a duration of 20 h is carried out under air in a confined and humid environment. The solid obtained is dried for 16 hours at 90 ° C under nitrogen flow. The catalyst is then calcined under a flow of air for 2 hours at 450 ° C.

Catalyst C4 thus prepared contains 0.19% by weight of palladium relative to the total weight of catalyst.

The characterization of the catalyst C4 by a Castaing microprobe shows that 80% of the Pd is distributed over a crust with a thickness of approximately 45 μηι.

The characterization of the C4 catalyst by MRI shows the presence of a layer of alumina distributed on a crust with a thickness of about 30 μηι at the extreme periphery of the catalyst.

The apparent dispersion of the palladium of catalyst C4 is 26%. Catalyst C4 is shown in FIG.

Example 5: Preparation of a C5 catalyst (compliant)

An aqueous solution of palladium nitrate is prepared at 25 ° C. by diluting 3.5 g of a solution of palladium nitrate containing 8.5% by weight of palladium, with demineralized water at a volume which corresponds to the volume porous alumina support (Vp = 0.7 ml / g).

Citric acid is added this solution with a molar ratio of citric acid to palladium of 1.

The suspension is then diluted with demineralized water to a volume which corresponds to 100% of the pore volume of the alumina support whose specific surface is 210 m ² / g shaped in the form of trilobal extrusions with a diameter of 2 mm. This solution is then impregnated in an impregnation barrel at a rotation speed of 14 rpm and a solution addition rate of 5.4 L / h on about 10 kg of an alumina heated to 20 ° C. A stage of maturation of the impregnated support before drying of a duration of 20 h is carried out under air in a confined and humid environment. The solid obtained is dried for 16 hours at 90 ° C under nitrogen flow. The catalyst is then calcined under a flow of air for 2 hours at 450 ° C. The catalyst C5 thus prepared contains 0.19% by weight of palladium relative to the total weight of catalyst.

The characterization of the C5 catalyst by a Castaing microprobe shows that 80% of the Pd is distributed over a crust of thickness approximately 300 μηι.

The characterization of the catalyst C5 by MRI shows the presence of a layer of alumina distributed on a crust with a thickness of about 45 μηι at the extreme periphery of the catalyst.

The apparent dispersion of the palladium of catalyst C5 is 19%.

Example 6 Preparation of a C6 Catalyst (Conform)

A colloidal suspension of Pd oxide is prepared with stirring at 25 ° C. by diluting 230 g of a solution of palladium nitrate Pd (NO ₃ ) ₂ containing 8.5% by weight of palladium Pd with approximately 5.6 liters. demineralised water, then add about 150 ml of sodium hydroxide solution to obtain a pH of 2.4. The citric acid is added by pre-impregnation on the support before impregnation of the colloidal palladium solution with a citric acid to palladium acid ratio of 1.

The suspension is then diluted with demineralised water to a volume corresponding to 100% of the pore volume of the alumina support (Vp = 0.7 ml / g), the specific surface area of which is 180 m ² / g form of extrusions. This solution is then impregnated in an impregnation barrel at a rotation speed of 14 rpm and a solution addition rate of 5.4 L / h on about 10 kg of an alumina heated to 20 ° C. A stage of maturation of the impregnated support before drying of a duration of 20 h is carried out under air in a confined and humid environment. The solid obtained is dried for 16 hours at 90 ° C under nitrogen flow. The catalyst is then calcined under a flow of air for 2 hours at 450 ° C.

The catalyst C6 thus prepared contains 0.19% by weight of palladium relative to the total weight of catalyst.

The characterization of the C6 catalyst by a Castaing microprobe shows that 80% of the Pd is distributed over a crust with a thickness of approximately 47 μηι.

The characterization of the catalyst C6 by MRI shows the presence of a layer of alumina distributed on a crust with a thickness of about 34 μηι at the extreme periphery of the catalyst.

The apparent dispersion of the palladium of catalyst C6 is 25%.

The set of technical and morphological characteristics of the catalysts C1 to C6 are shown in Table 1 below. Table 1: Technical and morphological characteristics of catalysts C1 to C6

Example 6 Use of Catalysts C1, C2, C3, C4, C5 and C6 for the Selective Hydrogenation of a Petroleum Gasoline.

Catalytic test in hydrogenation of a styrene isoprene mixture in the presence of S.

Before the catalytic test, the catalysts A, B, C and D are treated under a stream of 1 liter of hydrogen per hour and per gram of catalyst with a temperature rise of 300 ° C./h and a plateau at 150 ° C. during 2 hours.

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, 10 ppm S introduced as pentanethiol, 100 ppm S introduced as thiophene, the solvent being n-heptane.

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 H ₂ consumed per minute and per gram of palladium and are reported in Table 2 below. The results in terms of activities measured in hydrogenation of a styrene-isoprene mixture in the presence of sulfur are shown in Table 2 below.

Table 2: Measured activities in hydrogenation of a styrene-isoprene mixture in the presence of sulfur

^* in (moles H ₂ ) / [minx (gram of palladium)]

^** % / ref corresponds to the gain converted in%, obtained relative to the reference catalyst C2 whose activity is defined at 100%. Fresh C4, C5 and C6 catalysts prepared on extruded substrates, i.e. prior to any loading / unloading, are all more active than their counterparts prepared on bales, i.e. C1 and C2 catalysts.

After two loading / unloading steps, the catalyst C1 loses 1.5% of fines and thus its catalytic activity decreases slightly and increases to 91% respectively due to a loss of palladium located at the periphery of the catalyst.

Catalyst C2 in turn loses virtually no activity due to the protection of the alumina layer generated by the co-impregnation in the presence of citric acid.

The catalyst C3 (non-compliant), after unloading loading steps, loses 3.5% of fines and thus its catalytic activity decreases and goes from 130% to 93% compared to our reference which represents a loss in activity of nearly 28%, due to a loss of palladium located at the periphery of the catalyst. While the catalyst C4 according to the invention loses a quantity of fines equivalent to the catalyst C3 (3.6%), its catalytic activity is not affected because the lost fines do not contain palladium. The attrition-impacted catalyst portion is palladium-free and essentially contains alumina and / or carbon residues essentially derived from the decomposition of citric acid.

Catalysts C5 and C6 prepared on extruded with a co-impregnation or prepreg of a colloidal solution of palladium and citric acid are more active and have a higher resistance to attrition than their counterparts prepared on beads and on extruded but without the addition of citric acid.

Catalyst C5 prepared by conventional dry impregnation, i.e. not colloidally, has thicker layer thicknesses. It is therefore a little less active than their colloidal counterparts (i.e. the C6 catalyst). However, the addition of citric acid in the impregnating solution also makes it possible not to lose activity after loading / unloading steps, this being due to the presence of the alumina layer at the extreme periphery of the catalyst.

Claims

1. Multilayer catalyst comprising at least one refractory oxide selected from the group consisting of silica, alumina and silica-alumina, and an active phase comprising between 0.01 and 2% by weight of a Group VIII metal with respect to catalyst weight, which catalyst comprises:

a porous support comprising at least said refractory oxide;

a first layer, at least partially covering said porous support, comprising at least said group VIII metal;

a second layer, at least partially covering said first layer, comprising at least said refractory oxide;

characterized in that

- At least 80% by weight of Group VIII metal relative to the total weight of Group VIII metal is distributed in said first layer, the thickness of said first layer being between 10 and 600 μηι;

The catalyst of claim 1, wherein said second layer completely covers said first layer.

The catalyst of claims 1 or 2, wherein the Group VIII metal is palladium or nickel.

4. Catalyst according to claim 3, in the group VIII metal is palladium.

Catalyst according to any one of the preceding claims, wherein the porous support is alumina.

Catalyst according to any one of the preceding claims, wherein the porous support is in the form of extrudates or balls.

Process for the preparation of a catalyst according to any one of claims 1 to 6, comprising the following steps: a) said porous support is brought into contact with at least one solution containing at least one organic compound comprising at least one carboxylic acid function;

8. Process for preparing a catalyst according to claim 7, wherein the molar ratio between said organic compound and said group VIII metal is between 0.1 and 4.0.

9. Process for preparing a catalyst according to claim 7 or 8, wherein steps a) and b) are carried out simultaneously.

A process for preparing a catalyst according to any one of claims 7 to 9, wherein said organic compound comprises between 2 and 7 carbon atoms.

1 1. A preparation process according to claim 10, wherein the organic compound is selected from hydroxyacetic acid (glycolic acid), 2-hydroxypropanoic acid (lactic acid), 2-hydroxypropanedioic acid (tartronic acid), acid 2-hydroxybutanedioic acid (malic acid), 2-hydroxypropane-1,2,3-tricarboxylic acid (citric acid), 2,3-dihydroxybutanedioic acid (tartaric acid), 2,2'-oxydiacetic acid ( diglycolic acid), 2-oxopropanoic acid (pyruvic acid), 4-oxopentanoic acid (levulinic acid).

The process of claim 11, wherein the organic compound is citric acid.

13. Process for preparing a catalyst according to any one of claims 7 to 12, wherein step a) and / or b) is (are) carried out by dry impregnation.

14. Process for preparing a catalyst according to any one of claims 7 to 13, wherein the step a) of bringing said porous support into contact with said solution containing at least one organic compound comprising at least one carboxylic acid function. and / or step b) bringing said porous support into contact with said solution comprising at least one group VIII metal precursor is carried out by adding said solution containing at least one organic compound comprising at least one carboxylic acid function and / or or said solution comprising at least one group VIII metal precursor on said support at a flow rate of between 1 and 20 liters per hour, said porous support being contained in a rotary impregnation device operating at a rotational speed of between 10 and 20 rpm.

15. A process for the selective hydrogenation of a feedstock comprising polyunsaturated compounds containing at least 2 carbon atoms per molecule contained in a hydrocarbon feed having a final boiling point of less than or equal to 300 ° C. by contacting the feedstock. said feedstock with hydrogen and at least one catalyst according to any one of claims 1 to 6 or prepared according to any one of claims 7 to 14, which process is carried out at a temperature between 0 and 300 ° C, at a pressure of between 0.1 and 10 MPa, at a molar ratio of hydrogen / (polyunsaturated compounds to be hydrogenated) of between 0.1 and 10 and at an hourly space velocity of between 0.1 and 200 h ^-1 when the process is carried out in the liquid phase, or at a molar ratio of hydrogen / (polyunsaturated compounds to be hydrogenated) of between 0.5 and 1000 and at an hourly space velocity of between 100 and 40,000 h ^-1 when the process is carried out in phases gaseous.