FR3137848A1 - METHOD FOR PREPARING A CATALYST COMPRISING A GROUP VIII METAL AND A SUPPORT IN THE FORM OF A MONOLITH OR A FOAM COATED WITH ALPHA ALUMINA - Google Patents

Abstract

Process for preparing a selective hydrogenation catalyst comprising an active phase based on at least one group VIII metal, deposited on an alpha alumina, and a support in the form of a monolith or a foam ceramic or metallic, which process comprises at least the following steps: - supply a gamma alumina powder comprising a specific surface area of between 100 and 500 m²/g; - bringing the gamma alumina powder into contact by coating with a support in the form of a monolith or a ceramic or metallic foam; - calcination at a temperature between 900°C and 1300°C; - bringing the calcined catalyst precursor into contact with a solution comprising a precursor of the active phase comprising a Group VIII metal; - drying then calcination of the material.

Description

METHOD FOR PREPARING A CATALYST COMPRISING A GROUP VIII METAL AND A SUPPORT IN THE FORM OF A MONOLITH OR A FOAM COATED WITH ALPHA ALUMINA

The subject of the invention is a process for preparing a selective hydrogenation catalyst comprising an active phase based on a Group VIII metal deposited on an alpha alumina and a support in the form of a monolith or a ceramic foam. The catalyst can be used in a process for the selective hydrogenation of polyunsaturated compounds in a hydrocarbon feed, in particular in C2-C5 steam cracking cuts and steam cracking gasolines.

State of the art

Many catalysts comprising a support in the form of a monolith or foam have been developed and manufactured in recent years with different manufacturing techniques. The monolith supports can be made of ceramic materials such as alumina or silicon carbide or zirconium or cordierite. Supports in the form of monoliths also exist with metallic materials, for example steel, stainless steel and many other types of metals. The foam supports can be made of ceramic materials, such as alumina or silicon carbide or zirconium. Foam supports also exist with metallic materials, for example nickel, aluminum, nickel-chrome, nickel-chrome-aluminum, and many other types of metals.

Metal or ceramic monoliths can be used in various catalytic applications, including in the treatment of exhaust gases (US1969/3441381, US1971/35971653) or as a NO _x reduction catalyst (Tomasic, V. 2007), or still in selective hydrogenation of hydrocarbon feeds comprising polyunsaturated compounds.

The review article “ Selective hydrogenation of 1,3-butadiene in the presence of 1-butene under liquid phase conditions using structured catalysts ” by FJ Méndeza et al. published in Catalysis Today 289 (2017) 151–161 focuses on the use of a catalyst support in the form of metal foams or monoliths for the selective hydrogenation of 1,3-butadiene. This document discloses that the use of a foam or monolith support coated with active phase of NiPd/(CeO ₂ -Al ₂ O ₃ ) gives good results in terms of conversion and in terms of selectivity in selective hydrogenation of 1,3-butadiene. Catalysts whose support is in the form of a monolith have a layer thickness of the active phase of 18 µm or 20 µm.

The article “ Catalyst deactivation in liquid- and gas-phase hydrogenation acetylene using a monolithic catalyst reactor” by Asplud et al. published in Catalysis Today, vol. 24 (181-187) 1995, focuses on the use of a catalyst support in the form of a ceramic α-alumina monolith for the selective hydrogenation of acetylene. This document discloses the use of a monolith support impregnated directly with PdCl ₂ on the walls of the monolith, the active phase obtained based on palladium having a thickness of 200 µm.

The Applicant has developed a new process for preparing a catalyst comprising an active phase based on a group VIII metal deposited on an alpha alumina, and a support in the form of a monolith or a ceramic or metallic foam, said method comprising a step of bringing a gamma alumina powder into contact with a support in the form of a monolith or a ceramic or metallic foam, followed by a calcination step at high temperature in order to transform the gamma alumina into an alpha alumina, and finally a step of impregnation of the precursor of the active phase on the calcined catalyst precursor comprising an alpha alumina. This particular preparation process allows good adhesion of the active phase on the walls of the support after coating with gamma alumina, making it possible to treat the monolith or foam at very high temperatures without the adhesion deteriorating, and this even for final catalysts comprising a very low specific surface area.

Objects of the invention

The subject of the present invention is a process for preparing a selective hydrogenation catalyst comprising an active phase based on at least one metal from group VIII, deposited on an alpha alumina, and a support in the form of a monolith or a ceramic or metallic foam, which process comprises at least the following steps:

a) a gamma alumina powder is supplied comprising a specific surface area of between 100 and 500 m²/g;

b) said gamma alumina powder from step a) is brought into contact by coating with a support in the form of a monolith or a ceramic or metallic foam to obtain a catalyst precursor comprising a gamma alumina ;

c) the catalyst precursor obtained at the end of step b) is calcined at a temperature between 900°C and 1300°C to obtain a calcined catalyst precursor comprising an alpha alumina;

d) the calcined catalyst precursor obtained at the end of step c) is brought into contact with a solution comprising at least one precursor of the active phase comprising at least one metal from group VIII to obtain a material;

e) the material obtained at the end of step d) is dried at a temperature below 250°C and then the dried material is calcined at a temperature between 250°C and 550°C.

According to one or more embodiments, said Group VIII metal is palladium.

According to one or more embodiments, the palladium content is between 0.005 and 1% by weight of palladium element relative to the total weight of the catalyst.

In one embodiment according to the invention, when the support is in the form of a ceramic or metallic foam, said step b) comprises the following sub-steps:

b1) the gamma alumina powder is dispersed in a solution based on nitric acid and water, then the dispersed catalyst precursor is ground so as to obtain a suspension;

b2) the ceramic or metallic foam is coated with said suspension obtained at the end of step b1) by soaking-withdrawal;

b3) we carry out:

- either a centrifugation step of the coated ceramic or metal foam obtained at the end of step b2);

- either a step of blowing the coated ceramic or metallic foam obtained at the end of step b2);

b4) optionally, the coated ceramic or metallic foam obtained at the end of step b3) is dried at a temperature between 80°C and 220°C;

b5) the coated ceramic or metallic foam obtained at the end of step b3), optionally at the end of step b4), is calcined at a temperature between 250°C and 550°C.

According to one or more embodiments, said support is in the form of a metal foam chosen from nickel, aluminum, iron, copper, nickel-chromium, nickel-chromium-aluminum, nickel-iron-chromium-aluminum foams , iron-chrome-aluminum, nickel-aluminum, stainless steel.

According to one or more embodiments, said support is in the form of a ceramic foam chosen from alumina foams (Al ₂ O ₃ ), silica-alumina, silicon carbide (SiC), phosphorus-alumina, magnesia ( MgO), zinc oxide, zirconium oxide (ZrO ₂ ), cordierite (Al ₃ Mg ₂ AlSi ₅ O ₁₈ ).

According to one or more embodiments, said catalyst comprises a geometric surface area of between 1000 and 7000 m²/m ³ .

According to one or more embodiments, said catalyst comprises a pore diameter of between 0.2 and 1.5 mm.

In one embodiment according to the invention, when the support is in the form of a ceramic or metallic monolith, said step b) comprises the following substeps:

b1') the gamma alumina powder is dispersed in a solution based on nitric acid and water, then the dispersed catalyst precursor is ground so as to obtain a suspension;

b2') the support in the form of a ceramic or metallic monolith is coated with said suspension obtained at the end of step b1') by soaking-withdrawal;

b3') a step is carried out of blowing the coated ceramic or metallic monolith obtained at the end of step b2');

b4') optionally, the coated ceramic or metallic monolith obtained at the end of step b3') is dried at a temperature between 80°C and 220°C;

b5') the coated ceramic or metallic monolith obtained at the end of step b3'), optionally at the end of step b4'), is calcined at a temperature between 250°C and 550°C.

According to one or more embodiments, said support is in the form of a metal monolith chosen from steel, stainless steel, nickel, aluminum, iron, copper, nickel-chromium, nickel-chromium-aluminum, iron-chromium monoliths. -aluminum.

According to one or more embodiments, said support is in the form of a ceramic monolith chosen from alumina (Al ₂ O ₃ ), silica-alumina, silicon carbide (SiC), phosphorus-alumina, magnesia ( MgO), zinc oxide, zirconium oxide (ZrO ₂ ), cordierite (Al ₃ Mg ₂ AlSi ₅ O ₁₈ ).

According to one or more embodiments, said support comprises a number of channels per unit of length (CPSI) between 300 and 1200.

According to one or more embodiments, the geometric surface of said catalyst is between 1500 m²/m ³ and 5000 m²/m ³ .

According to one or more embodiments, said support in the form of a monolith or a ceramic or metallic foam supplied in step d) is previously calcined in air at a temperature between 300°C and 800° C for a period of between 2 and 8 hours.

detailed description Definitions

In the following, the groups of chemical elements are given according to the CAS classification (CRC Handbook of Chemistry and Physics, publisher CRC press, editor-in-chief DR Lide, ^81st edition, 2000-2001). For example, group VIII (or VIIIB) according to the CAS classification corresponds to the metals of columns 8, 9 and 10 according to the new IUPAC classification.

The total pore volume and the specific surface area (S _BET ) of the alumina are measured by mercury porosimetry according to standard ASTM D4284-92 with a wetting angle of 140° using an Autopore® IV model device from the brand Micromeritics®.

Monolithic or foam supports (ceramic or metallic) are generally characterized by the density and size of the channels, more specifically by the number of channels per unit of length which is called CPSI (“ Channels per square inch ” according to English terminology). -Saxon). As its abbreviation indicates, it corresponds to the number of channels intercepted by a section of 1 x 1 inch (“ inch ” according to Anglo-Saxon terminology) or 2.54 x 2.54 cm. In the present application, the monolithic or foam supports (ceramic or metallic) are characterized by the number of channels per unit length (CPSI). It should be noted that the CPSI value of a catalyst comprising such a monolith or foam support does not change, whatever the thickness of the layer of the active phase of the catalyst.

The porosity of the foam or monolith can be calculated by the following formula:

with :

ε: porosity or void ratio of the foam or monolith;

ρ _m : density of the foam or monolith;

ρ _mat : density of the foam or monolith material.

The group VIII metal content is measured by X-ray fluorescence.

Preparation process

The process for preparing a selective hydrogenation catalyst comprising an active phase based on at least one group VIII metal, deposited on an alpha alumina, and a support in the form of a monolith or a ceramic or metallic foam, which process comprises at least the following steps:

a) a gamma alumina powder is supplied comprising a specific surface area of between 100 and 500 m²/g;

b) said gamma alumina powder from step a) is brought into contact by coating with a support in the form of a monolith or a ceramic or metallic foam to obtain a catalyst precursor comprising a gamma alumina ;

c) the catalyst precursor obtained at the end of step b) is calcined at a temperature between 900°C and 1300°C to obtain a calcined catalyst precursor comprising an alpha alumina;

d) the calcined catalyst precursor obtained at the end of step c) is brought into contact with a solution comprising at least one precursor of the active phase comprising at least one metal from group VIII to obtain a material;

e) the material obtained at the end of step d) is dried at a temperature below 250°C then the dried material is calcined at a temperature between 250°C and 550°C;

f) optionally, a step of reduction of the catalyst obtained at the end of step e) is carried out in the presence of a reducing gas at a temperature between 80°C and 450°C.

The steps of said preparation process are described in detail below.

Step a)

According to step a), a gamma alumina is supplied comprising a specific surface area of between 100 and 500 m²/g, preferably between 120 and 450 m²/g, and even more preferably between 150 and 300 m²/g, said alumina gamma in the form of a powder.

Alumina may include impurities such as metal oxides of groups IIA, IIIB, IVB, IIB, IIIA, IVA according to the CAS classification, preferably silica, titanium dioxide, zirconium dioxide, oxide zinc, magnesium oxide and calcium oxide, or alkali metals, preferably lithium, sodium or potassium, and/or alkaline earth metals, preferably magnesium, calcium, strontium or barium or even sulfur.

According to step a), the alumina is in the form of a powder comprising a median diameter less than or equal to 250 µm, preferably between 10 and 250 µm, and even more preferably between 50 and 150 µm. The term "median diameter" here designates the diameter of an equivalent sphere such that 50% of the particles, by volume, have a greater diameter and 50% a smaller diameter. The alumina can be formed into powder by any technique well known to those skilled in the art, for example by grinding and sieving.

Advantageously, the total porous volume of the alumina is between 0.1 and 1.5 cm ³ /g, preferably between 0.35 and 1.2 cm ³ /g, and even more preferably between 0.4 and 1.0 cm ³ /g, and even more preferably between 0.45 and 0.9 cm ³ /g.

Step b)

According to step b), the gamma alumina powder supplied in step a) is brought into contact with a support in the form of a monolith or a ceramic or metallic foam.

Advantageously, the support in the form of a monolith or a ceramic or metallic foam used in step b) is previously calcined in air at a temperature between 300°C and 800°C for a period advantageously included between 2 and 8 hours in order to promote adhesion of the alumina powder to said support.

Bringing the gamma alumina powder into contact with the support in the form of a monolith or a ceramic or metallic foam is carried out by coating (“ washcoat ” according to Anglo-Saxon terminology).

In one embodiment according to the invention, a step b) is carried out of bringing the gamma alumina powder from step a) into contact with a support in the form of a ceramic or metallic foam. In this particular embodiment, step b) advantageously comprises the following sub-steps:

b1) the gamma alumina powder is dispersed in a solution based on nitric acid and water, then the dispersed catalyst precursor is ground so as to obtain a suspension (“slurry” according to Anglo-Saxon terminology) advantageously by means of a ball mill, for a period preferably between 15 and 30 hours, more preferably between 18 and 28 hours, so as to obtain a suspension advantageously comprising particles of calcined catalyst precursor comprising an average size less than 1 µm;

b2) the ceramic or metallic foam is coated with said suspension obtained at the end of step b1) by soaking-withdrawal;

b3) we carry out:

- either a centrifugation step (“spin-coating” according to Anglo-Saxon terminology) of the coated ceramic or metallic foam obtained at the end of step b2) in order to remove the excess slurry, preferably at a speed between 4000 and 8500 rpm, preferably between 4500 and 8000 rpm;

- either a step of blowing the coated ceramic or metallic foam obtained at the end of step b2), advantageously using an inert gas or under air. The blowing technique is well known to those skilled in the art.

Preferably, a centrifugation step is carried out on the coated ceramic or metallic foam obtained at the end of step b2) in order to remove the excess slurry, preferably at a speed of between 4000 and 8500 rpm, preferably between 4500 and 8000 rpm;

b4) optionally, the coated ceramic or metallic foam obtained at the end of step b3) is dried at a temperature between 80°C and 220°C, preferably between 90°C and 150°C, for a period advantageously between 6 and 30 hours, preferably between 8 and 24 hours;

b5) the coated ceramic or metallic foam obtained at the end of step b3), optionally at the end of step b4), is calcined at a temperature between 250°C and 550°C, preferably between 320°C and 450°C, for a period advantageously between 1 and 6 hours, preferably between 2 and 4 hours.

When the catalyst support is in the form of a ceramic foam, said foam is preferably chosen from alumina (Al ₂ O ₃ ), silica-alumina, silicon carbide (SiC), phosphorus-alumina, magnesia foams. (MgO), zinc oxide, zirconium oxide (ZrO ₂ ), cordierite (Al ₃ Mg ₂ AlSi ₅ O ₁₈ ). Preferably, said ceramic foam is made of alumina (Al ₂ O ₃ ), silica-alumina, phosphorus-alumina, or silicon carbide (SiC).

When the catalyst support is in the form of a metal foam, said foam is preferably chosen from nickel, aluminum, iron, copper, nickel-chromium, nickel-chromium-aluminum, nickel-iron-chromium-foams. aluminum, iron-chrome-aluminum, nickel-aluminum, stainless steel (316L, 310SS). Preferably, said metal foam is chosen from aluminum, nickel, nickel-chrome, nickel-chrome-aluminum foams. These foams can include additives such as molybdenum, manganese or even phosphorus.

In another embodiment according to the invention, a step b) is carried out of bringing the gamma alumina powder from step a) into contact with a support in the form of a ceramic or metallic monolith. In this particular embodiment, step b) advantageously comprises the following sub-steps:

b1') the gamma alumina powder is dispersed in a solution based on nitric acid and water, then the dispersed catalyst precursor is ground so as to obtain a suspension (“slurry” according to Anglo-Saxon terminology) advantageously by means of a ball mill, for a period preferably between 15 and 30 hours, more preferably between 18 and 28 hours, so as to obtain a suspension advantageously comprising particles of calcined catalyst precursor comprising a lower average size at 1 µm;

b2') the ceramic or metallic monolith is coated with said suspension obtained at the end of step b1') by soaking-shrinkage;

b3') a step of blowing the coated ceramic or metallic monolith obtained at the end of step b2') is carried out advantageously using an inert gas. The blowing technique is well known to those skilled in the art;

b4') optionally, the coated ceramic or metallic monolith obtained at the end of step b3') is dried at a temperature between 80°C and 220°C, preferably between 90°C and 150°C, for a duration advantageously between 6 and 30 hours, preferably between 8 and 24 hours;

b5') the coated ceramic or metallic foam obtained at the end of step b3'), optionally at the end of step b4'), is calcined at a temperature between 250°C and 550°C, preferably between 320°C and 450°C, for a period advantageously between 1 and 6 hours, preferably between 2 and 4 hours.

When the catalyst support is in the form of a metal monolith, said monolith is preferably chosen from monoliths made of steel, stainless steel (316L, 310SS), nickel, aluminum, iron, copper, nickel-chromium, nickel-chromium -aluminum, iron-chrome-aluminum.

When the catalyst support is in the form of a ceramic monolith, said monolith is preferably chosen from alumina (Al ₂ O ₃ ), silica-alumina, silicon carbide (SiC), phosphorus-alumina, magnesia monoliths. (MgO), zinc oxide, zirconium oxide (ZrO ₂ ), cordierite (Al ₃ Mg ₂ AlSi ₅ O ₁₈ ). Preferably, said ceramic monolith is made of alumina (Al ₂ O ₃ ), silica-alumina, phosphorus-alumina, or silicon carbide (SiC).

At the end of step b), the support in the form of a monolith or a ceramic or metallic foam is coated with a layer of gamma alumina. The thickness of the layer depends on the number of coating steps carried out. Step b) can be repeated several times depending on the thickness of the desired gamma alumina layer. Advantageously, this preparation route makes it possible to obtain a layer of gamma alumina on the walls of the support of between 10 µm and 150 µm, preferably between 20 µm and 100 µm and even more preferably between 30 µm and 90 µm.

Step c)

According to step c), the catalyst precursor comprising a gamma alumina obtained at the end of step b) is calcined, preferably under combustion air, and more preferably methane combustion air, comprising between 40 and 80 grams of water per kg of air, an oxygen level between 5% and 15% volume and a CO ₂ level between 4% and 10% volume. The calcination temperature is between 900°C and 1300°C, preferably between 1000°C and 1250°C, and even more preferably between 1000°C and 1150°C. The calcination of the support is carried out at high temperature in order to transform the gamma alumina into an alpha alumina.

The calcination time is generally between 0.25 hours and 10 hours. Longer durations are not excluded, but do not necessarily bring improvement.

Step d)

According to step d), the calcined catalyst precursor comprising an alpha alumina obtained at the end of step c) is brought into contact with a solution comprising at least one precursor of the active phase comprising at least one metal from the group VIII to obtain a catalyst precursor. The deposition of the precursor of the active phase comprising at least one group VIII metal can be carried out by any method well known to those skilled in the art. In particular, said step d) 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 d) is preferably carried out by impregnation consisting for example of bringing the calcined catalyst precursor obtained at the end of step c) into contact with at least one solution, aqueous or organic (for example methanol or ethanol or phenol or acetone or toluene or dimethyl sulfoxide (DMSO)) or consisting of a mixture of water and at least one organic solvent, containing at least one precursor of the active phase comprising at least one group VIII metal at least partially in the dissolved state, or else by bringing the calcined catalyst precursor obtained at the end of step c) into contact with at least one colloidal solution of at least one precursor of the active phase comprising at least one metal from group VIII, in oxidized form or in reduced form. Preferably, the solution is aqueous. The pH of this solution can be modified by the possible addition of an acid or a base.

Preferably, said step d) is carried out by dry impregnation, which consists of bringing the calcined catalyst precursor obtained at the end of step c) into contact with a solution, containing at least one active phase precursor comprising at least one metal from Group VIII, the volume of the solution of which is between 0.3 and 1.5 times the pore volume of the support to be impregnated.

The Group VIII metal of the active phase is preferably chosen from nickel, platinum and palladium. Preferably, the Group VIII metal is palladium or platinum and more preferably palladium.

When the metal of group VIII is palladium, the concentration of palladium in solution is adjusted according to the pore volume of the support still available so as to obtain on the final catalyst a palladium content generally between 0.005 and 1% by weight of said element relative to to the total weight of the catalyst, preferably between 0.01 and 0.7% by weight, and even more preferably between 0.02 and 0.5% by weight, even more preferably between 0.025 and 0.3% by weight.

When the precursor of the active phase based on at least one metal from group VIII, preferably palladium, is introduced into an aqueous solution, a precursor is advantageously used in the form of nitrate, carbonate, chloride, sulfate, etc. hydroxide, hydroxycarbonate, formate, acetate, oxalate, complexes formed with acetylacetonates, or even tetrammine or hexammine complexes, or any other inorganic derivative soluble in aqueous solution, which is brought into contact with said support. Preferably, the precursor is selected from sodium chloropalladate and palladium nitrate.

When the precursor of the active phase based on at least one metal from Group VIII, preferably palladium, is introduced in the form of a colloidal suspension, a colloidal suspension of metal oxide from Group VIII or d is prepared. Group VIII metal hydroxide, preferably palladium oxide or palladium hydroxide, in aqueous phase by mixing an aqueous solution (I) comprising at least one hydroxide selected from the group consisting of hydroxides of group VIII alkalis and alkaline earth hydroxides and an aqueous solution (II) comprising at least one active phase precursor of palladium. Preferably, said colloidal suspension is generally obtained by hydrolysis of the metal cation, preferably palladium, in an aqueous medium, which leads to the formation of particles of metal oxide or hydroxide, preferably palladium, in suspension. The aqueous solution of alkali hydroxide or alkaline earth hydroxide is generally selected from the group consisting of aqueous solutions of sodium hydroxide, aqueous solutions of magnesium hydroxide. Preferably, preferably the aqueous solution is an aqueous solution of sodium hydroxide.

At the end of step d), the catalyst precursor obtained is generally matured in the wet state for 5 minutes to 12 hours, preferably for 5 minutes to 6 hours, preferably at a temperature between 15° C and 45°C. Longer durations are not excluded, but do not necessarily bring improvement.

Step e)

The material obtained at the end of step d) is dried at a temperature below 250°C, more preferably between 30°C and 220°C, more preferably between 90°C and 180°C. The drying time is advantageously between 0.5 hour and 20 hours. Longer durations are not excluded, but do not necessarily bring improvement.

Drying is generally carried out under combustion air of a hydrocarbon, preferably methane, or under heated air comprising between 0 and 80 grams of water per kilogram of combustion air, an oxygen level of between 5% and 25% volume and a carbon dioxide level of between 0% and 10% volume.

After drying, the dried material is calcined, preferably under combustion air, and more preferably methane combustion air, comprising between 40 and 80 grams of water per kg of air, an oxygen level of between 5% and 15% volume and a CO ₂ rate of between 4% and 10% volume. The calcination temperature is between 250°C and 550°C, preferably between 300°C and 480°C, and even more preferably between 320°C and 450°C. The calcination time is generally between 0.25 hours and 10 hours. Longer durations are not excluded, but do not necessarily bring improvement.

Step f) (optional step)

The catalyst obtained at the end of step e) generally undergoes a reduction step. 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 transformation is carried out, or ex-situ. Preferably, this step is carried out at a temperature 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 preferably comprising between 25 vol% and 100 vol% of hydrogen, for example 100 vol% of hydrogen. The hydrogen is optionally supplemented by an inert gas for reduction, preferably argon, nitrogen or methane.

The reduction generally includes a temperature rise phase then a plateau.

The duration of the reduction stage is generally between 1 hour and 40 hours, preferably between 2 hours 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.

Catalyst

The catalyst obtained by the preparation process according to the invention comprises an active phase based on at least one metal from group VIII and a support in the form of a monolith or a ceramic or metallic foam coated with an alpha alumina.

The Group VIII metal of the active phase is preferably chosen from nickel, platinum and palladium. Preferably, the Group VIII metal is palladium or platinum, and more preferably platinum.

When the Group VIII metal is palladium, the palladium content is generally between 0.005 and 1% by weight of said element relative to the total weight of the catalyst, preferably between 0.01 and 0.7% by weight, and even more preferably between 0.02 and 0.5% by weight, even more preferably between 0.025 and 0.3% by weight.

The catalyst may further comprise, as active phase, a group IB element, preferably chosen from silver and copper. Preferably, the element of group IB is money. The group IB element content is preferably between 0.01 and 0.3% by weight relative to the total weight of the catalyst, more preferably between 0.015 and 0.2% by weight.

Support

The support comes in the form of a monolith or ceramic or metallic foam.

i) Ceramic or metallic monolith

When the support is in the form of a ceramic or metallic monolith, said support comprises a number of channels per unit length (CPSI) between 300 and 1200.

Preferably, the number of channels per unit length (CPSI) of said support is between 300 and 1200, preferably between 350 and 1000, more preferably between 400 and 700, and even more preferably between 450 and 750.

Preferably, the geometric surface of said catalyst is between 1500 m²/m ³ and 5000 m²/m ³ , preferably between 1500 m²/m ³ and 4000 m²/m ³ , and even more preferably between 2000 m²/m ³ and 4000 m²/ ^m3 .

Preferably, the thickness of the catalyst wall is between 0.08 mm and 0.5 mm, more preferably between 0.1 mm and 0.4 mm.

Preferably, the porosity rate of said catalyst is between 15% and 90%, preferably between 20% and 90%, and more preferably between 20% and 70%.

Preferably, the active phase is in the form of a layer on the walls of said support, the thickness of said active phase layer being between 30 µm and 150 µm, preferably between 60 µm and 100 µm, and again more preferably between 60 µm and 90 µm.

When the catalyst support is in the form of a metal monolith, said monolith is preferably chosen from monoliths made of steel, stainless steel (316L, 310SS), nickel, aluminum, iron, copper, nickel-chromium, nickel-chromium -aluminum, iron-chrome-aluminum.

When the catalyst support is in the form of a ceramic monolith, said monolith is preferably chosen from alumina (Al ₂ O ₃ ), silica-alumina, silicon carbide (SiC), phosphorus-alumina, magnesia monoliths. (MgO), zinc oxide, zirconium oxide (ZrO ₂ ), cordierite (Al ₃ Mg ₂ AlSi ₅ O ₁₈ ). Preferably, said ceramic monolith is made of alumina (Al ₂ O ₃ ), silica-alumina, phosphorus-alumina, or silicon carbide (SiC).

ii) Metallic or ceramic foam

When the support is in the form of a metal or ceramic foam, said catalyst comprises a geometric surface area of between 1000 and 7000 m²/m ³ and a pore diameter of between 0.2 and 1.5 mm.

Preferably, the geometric surface of said catalyst is between 2000 and 5000 m²/m ³ , and even more preferably between 2000 and 4000 m²/m ³ .

Preferably, the diameter of the pores of said catalyst is between 0.3 and 1.5 mm, and even more preferably between 0.5 and 1.5 mm.

Preferably, the porosity rate of said catalyst is between 75 and 90% when the support is a ceramic foam.

Preferably, the porosity rate of said catalyst is between 75 and 95% when the support is a metal foam.

When the catalyst support is in the form of a metal foam, said foam is preferably chosen from nickel, aluminum, iron, copper, nickel-chromium, nickel-chromium-aluminum, nickel-iron-chromium-foams. aluminum, iron-chrome-aluminum, nickel-aluminum, stainless steel (316L, 310SS). Preferably, said metal foam is chosen from aluminum, nickel, nickel-chrome, nickel-chrome-aluminum foams. These foams can include additives such as molybdenum, manganese or even phosphorus.

When the catalyst support is in the form of a ceramic foam, said foam is preferably chosen from alumina (Al ₂ O ₃ ), silica-alumina, silicon carbide (SiC), phosphorus-alumina, magnesia foams. (MgO), zinc oxide, zirconium oxide (ZrO ₂ ), cordierite (Al ₃ Mg ₂ AlSi ₅ O ₁₈ ). Preferably, said ceramic foam is made of alumina (Al ₂ O ₃ ), silica-alumina, phosphorus-alumina, or silicon carbide (SiC).

Preferably, the active phase is in the form of a layer on the walls of said support, the thickness of said layer of active phase being between 10 µm and 150 µm, preferably between 20 µm and 100 µm and even more preferably between 30 µm and 90 µm.

Use of the catalyst

The catalyst obtained by the process according to the invention can be used in a process for the selective hydrogenation of polyunsaturated compounds containing at least 2 carbon atoms per molecule, such as diolefins and/or acetylenes and/or alkenylaromatics, contained in a hydrocarbon feed having a final boiling point less than or equal to 300°C, which process being carried out at a temperature between 0 and 300°C, at a pressure between 0.1 and 10 MPa, at a ratio molar hydrogen/(polyunsaturated compounds to be hydrogenated) of between 0.1 and 10 and at an hourly volume rate (VVH) of between 0.1 and 200 h ^-1 when the process is carried out in the liquid phase, or at a molar hydrogen ratio /(polyunsaturated compounds to be hydrogenated) between 0.5 and 1000 and at an hourly volume rate between 100 and 40,000 h ^-1 when the process is carried out in the gas phase.

Monounsaturated organic compounds, such as ethylene and propylene, are the source of the manufacturing of polymers, plastics and other value-added chemicals. These compounds are obtained from natural gas, naphtha or diesel which have been treated by steam cracking or catalytic cracking processes. These processes are operated at high temperature and produce, in addition to the sought-after 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 parasitic reactions in the polymerization units. It is therefore necessary to eliminate them before valorizing these cuts.

Selective hydrogenation is the primary treatment developed to specifically remove unwanted polyunsaturated compounds from these hydrocarbon feedstocks. It allows the conversion of polyunsaturated compounds to the corresponding alkenes or aromatics while avoiding their total saturation and therefore the formation of the corresponding alkanes or naphthenes. In the case of steam cracking gasolines used as feedstock, selective hydrogenation also makes it possible to selectively hydrogenate the alkenyl aromatics into aromatics while avoiding the hydrogenation of the aromatic nuclei.

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 includes 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 alkenyl aromatic function.

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

The C2 steam cracking cut, advantageously used for implementing 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 cracked C2 cuts, between 0.1 and 1% by weight of C3 compounds may also be present.

The C3 steam cracking cut, advantageously used for implementing 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 C2 compounds and C4 compounds may also be present.

A C2 - C3 cut can also be advantageously used for implementing 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 of ethylene, of the order of 5% by weight of propylene, the remainder being essentially methane, ethane and propane. This filler can also contain between 0.1 and 2% by weight of C4 compounds.

The C4 steam cracking cut, advantageously used for implementing 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% by weight of 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 implementing the selective hydrogenation process according to the invention, has for example the following composition: 21% by weight of pentanes, 45% by weight of pentenes, 34% by weight of pentadienes.

The steam cracking gasoline or pyrolysis gasoline, advantageously used for the implementation of the selective hydrogenation process according to the invention, corresponds to a hydrocarbon cut whose boiling temperature is generally between 0 and 300 ° C, of 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, etc.). ). Steam cracked gasoline generally includes 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 from 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 alkenyl aromatic compounds, all of 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 in accordance with the selective hydrogenation process according to the invention is a C2 steam cracking cut, or a C2-C3 steam cracking cut, or a steam cracking gasoline.

The selective hydrogenation process according to the invention aims to eliminate said polyunsaturated hydrocarbons present in said feed to be hydrogenated without hydrogenating the monounsaturated hydrocarbons. For example, when said feedstock 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, we aim to eliminate butadiene, vinylacetylene (VAC) and butyne, in the case of a C5 cut, we aim to eliminate pentadienes. When said feedstock is a steam cracking gasoline, the selective hydrogenation process aims to selectively hydrogenate said polyunsaturated hydrocarbons present in said feedstock to be treated so that the diolefinic compounds are partially hydrogenated into mono-olefins and that the styrenic and indenic compounds are partially hydrogenated into corresponding aromatic compounds while avoiding the hydrogenation of the aromatic nuclei.

The technological implementation of the selective hydrogenation process is for example carried out by injection, in ascending or descending current, of the polyunsaturated hydrocarbon feed and hydrogen into at least one fixed bed reactor. Said reactor can be of the isothermal type or of the adiabatic type. An adiabatic reactor is preferred. The polyunsaturated hydrocarbon feed can advantageously be diluted by one or more re-injection(s) of the effluent, coming from said reactor where the selective hydrogenation reaction occurs, at various points of the reactor, located between the inlet and the reactor outlet in order to limit the temperature gradient in the reactor. The technological implementation of the selective hydrogenation process according to the invention can also be advantageously carried out by the implantation of at least said supported catalyst in a reactive distillation column or in reactors – exchangers or in a slurry type reactor . The hydrogen flow can be introduced at the same time as the feed to be hydrogenated and/or at one or more different points in the reactor.

The selective hydrogenation of the C2, C2-C3, C3, C4, C5 and C5+ steam cracking cuts can be carried out in the gas phase or in the liquid phase, preferably in the liquid phase for the C3, C4, C5 and C5+ cuts and in the gaseous for cuts C2 and C2-C3. A reaction in the liquid phase makes it possible to lower the energy cost and increase the cycle time of the catalyst.

Generally speaking, the selective hydrogenation of a hydrocarbon feed containing polyunsaturated compounds containing at least 2 carbon atoms per molecule and having a final boiling point less than or equal to 300°C is carried out at a temperature between 0 and 300°C, at a pressure between 0.1 and 10 MPa, at a hydrogen/(polyunsaturated compounds to be hydrogenated) molar ratio 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 volume of the catalyst) of between 0.1 and 200 h ^-1 for a process 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 speed VVH of between 100 and 40000 h ^-1 for a process carried out in the gas phase.

In one embodiment according to the invention, when carrying out a selective hydrogenation process in which the feedstock is a steam cracking gasoline comprising 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 hydrogen/(polyunsaturated compounds to be hydrogenated) molar ratio 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.

Even more preferably, a selective hydrogenation process is carried out in which the feedstock is a steam cracking gasoline comprising polyunsaturated compounds, the hydrogen/(polyunsaturated compounds to be hydrogenated) molar ratio 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 have a sufficient quantity to theoretically hydrogenate all the polyunsaturated compounds and to maintain an excess of hydrogen at the reactor outlet.

In another embodiment according to the invention, when carrying out a selective hydrogenation process in which the feed is a C2 steam cracking cut and/or a C2-C3 steam cracking cut 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 generally between 100 and 40,000 h ^-1 , preferably between 500 and 30,000 h ^-1 and the pressure is generally between 0.1 and 6.0 MPa, preferably between 0.2 and 5, 0 MPa.

Example 1: Pd catalyst on NiCr metal foam support

In this example, the support is a NiCr metal foam (RECEMAT® supplier) pre-treated at 600°C to eliminate organic contaminants and promote adhesion of the catalyst precursor to the support.

A suspension based on a gamma alumina powder with a specific surface area of 212 m²/g (SDA 500, Sasol) is prepared by dispersing the alumina powder in a nitric acid + water solution. This suspension is placed in a ball mill for 24 hours to homogenize and reduce the size of the alumina particles (< 1 µm).

We then carry out a coating step (in one or more coating cycles – see Table 1) via a protocol comprising the following sub-steps:

- immersion of support in the slurry (soaking-withdrawal);

- blowing under air to promote a homogeneous deposit and remove excess slurry;

- drying at a temperature of 120°C for 12 hours;

- calcination at 500°C for 2 hours.

After the coating step, the catalyst precursor obtained is calcined in a muffle furnace at 1100°C for 13 hours under a static atmosphere with a temperature rise ramp of 10°C/min.

The catalyst obtained at the end of the calcination step includes the following characteristics:

- specific surface area, measured by mercury porosimetry: 6 m²/gAl ₂ O ₃

- total pore volume, measured by mercury porosimetry: 0.65 ml/g Al ₂ O ₃ (* Volume injected at P _max ≈ 400 MPa or d _pores ≈ 3.6 nm).

A colloidal solution of palladium oxide particles is prepared with a concentration of 0.23% by weight of the element palladium in the presence of sodium hydroxide.

A volume corresponding to the total pore volume of the alumina coated on the support (VPT = 0.65 ml/g) of the colloidal solution is then deposited dropwise over the entire surface of the coated support. This is followed by maturation for 30 minutes at room temperature. The material obtained is then dried in an oven at 45°C for 48 hours then calcined in a tubular oven at 450°C for 2 hours.

Adhesion test – Protocol: in order to check the good adhesion of the active phase on the support, each catalyst sample is dried, weighed, then placed in petroleum ether then sonicated for 30 minutes at room temperature. Then, the petroleum ether is evaporated at room temperature and the catalysts are dried at a temperature of 150°C to be weighed again.

The test was carried out on 6 different samples in order to check the repeatability of the preparation process. The results are presented in Table 1 below.

Sample 1 2 3 4 5 6 Number of coating cycles 2 2 2 3 3 2 Sample mass
front ultrasonic (g) 0.816 0.852 0.691 1,096 1.055 0.709 Sample mass after ultrasound 0.807 0.839 0.686 1,085 1,047 0.708 Loss (%) 1.1 1.5 0.7 1 0.8 0.2

The preparation process according to the invention makes it possible to obtain catalysts whose active palladium phase has good adhesion to the walls of the support in the form of a NiCr metal foam, because the loss in mass after testing remains lower at 2%.

Example 2: Pd catalyst on FeCrAl monolith support

In this example, the support is a FeCrAl monolith (supplier Bercy®, CPSI = 600, height 2.5 cm) pre-treated at 500°C to eliminate organic contaminants and promote adhesion of the catalyst precursor to the support .

A suspension based on a gamma alumina powder with a specific surface area of 212 m²/g (SDA 500, Sasol) is prepared by dispersing the alumina powder in a nitric acid + water solution. This suspension is placed in a ball mill for 24 hours to homogenize and reduce the size of the alumina particles (< 1 µm).

We then carry out a coating step (in one or more coating cycles – see Table 2) via a protocol comprising the following sub-steps:

- immersion of support in the slurry (soaking-withdrawal);

- blowing under air to promote a homogeneous deposit and remove excess slurry;

- drying at a temperature of 120°C for 12 hours;

- calcination at 500°C for 2 hours.

After the coating step, the catalyst precursor obtained is calcined in a muffle furnace at 1100°C for 10 hours under a static atmosphere with a temperature rise ramp of 10°C/min.

The catalyst obtained at the end of the calcination step includes the following characteristics:

- specific surface area, measured by mercury porosimetry: 6 m²/gAl ₂ O ₃

- total pore volume, measured by mercury porosimetry: 0.48 ml/g Al ₂ O ₃ (* Volume injected at P _max ≈ 400 MPa or d _pores ≈ 3.6 nm).

A colloidal solution of palladium oxide particles is prepared with a concentration of 0.20% by weight of the element palladium in the presence of sodium hydroxide.

A volume corresponding to the total pore volume of the alumina coated on the support (VPT = 0.65 ml/g) of the colloidal solution is then deposited dropwise over the entire surface of the coated support. This is followed by maturation for 30 minutes at room temperature. The material obtained is then dried in an oven at 45°C for 48 hours then calcined in a tubular oven at 450°C for 2 hours.

The catalyst is tested via an adhesion test under the same operating conditions as Example 1. The test was carried out on 4 different samples in order to check the repeatability of the preparation process. The results are presented in Table 2 below.

Sample 1 2 3 4 Number of coating cycles 2 2 2 2 Sample mass
front ultrasonic (g) 0.972 1.003 1.012 1,118 Sample mass after ultrasound (g) 0.962 0.990 0.992 1,106 Loss (%) 1 1.3 2 1.1

The preparation process according to the invention makes it possible to obtain catalysts whose active palladium phase has good adhesion to the walls of the support in the form of a metallic FeCrAl monolith because the loss in mass after testing remains less than 1.5%.

Claims (14)

Process for preparing a selective hydrogenation catalyst comprising an active phase based on at least one group VIII metal, deposited on an alpha alumina, and a support in the form of a monolith or a foam ceramic or metallic, which process comprises at least the following steps:
a) a gamma alumina powder is supplied comprising a specific surface area of between 100 and 500 m²/g;
b) said gamma alumina powder from step a) is brought into contact by coating with a support in the form of a monolith or a ceramic or metallic foam to obtain a catalyst precursor comprising a gamma alumina ;
c) the catalyst precursor obtained at the end of step b) is calcined at a temperature between 900°C and 1300°C to obtain a calcined catalyst precursor comprising an alpha alumina;
d) the calcined catalyst precursor obtained at the end of step c) is brought into contact with a solution comprising at least one precursor of the active phase comprising at least one group VIII metal to obtain a material;
e) the material obtained at the end of step d) is dried at a temperature below 250°C and then the dried material is calcined at a temperature between 250°C and 550°C. A method according to claim 1, wherein said Group VIII metal is palladium. Process according to claim 2, in which the palladium content is between 0.005 and 1% by weight of palladium element relative to the total weight of the catalyst. Method according to any one of claims 1 to 3, in which when the support is in the form of a ceramic or metallic foam, said step b) comprises the following sub-steps:
b1) the gamma alumina powder is dispersed in a solution based on nitric acid and water, then the dispersed catalyst precursor is ground so as to obtain a suspension;
b2) the ceramic or metallic foam is coated with said suspension obtained at the end of step b1) by soaking-withdrawal;
b3) we carry out:
- either a centrifugation step of the coated ceramic or metal foam obtained at the end of step b2);
- either a step of blowing the coated ceramic or metallic foam obtained at the end of step b2);
b4) optionally, the coated ceramic or metallic foam obtained at the end of step b3) is dried at a temperature between 80°C and 220°C;
b5) the coated ceramic or metallic foam obtained at the end of step b3), optionally at the end of step b4), is calcined at a temperature between 250°C and 550°C. Method according to claim 4, in which said support is in the form of a metal foam chosen from nickel, aluminum, iron, copper, nickel-chromium, nickel-chromium-aluminum, nickel-iron-chromium-aluminum foams , iron-chrome-aluminum, nickel-aluminum, stainless steel. Method according to one of claims 4, in which said support is in the form of a ceramic foam chosen from alumina foams (Al ₂ O ₃ ), silica-alumina, silicon carbide (SiC), phosphorus-alumina , magnesia (MgO), zinc oxide, zirconium oxide (ZrO ₂ ), cordierite (Al ₃ Mg ₂ AlSi ₅ O ₁₈ ). Process according to any one of claims 4 to 6, in which said catalyst comprises a geometric surface area of between 1000 and 7000 m²/m ³ . A method according to any one of claims 4 to 7, wherein said catalyst comprises a pore diameter of between 0.2 and 1.5 mm. Method according to any one of claims 1 to 3, in which when the support is in the form of a ceramic or metallic monolith, said step b) comprises the following sub-steps:
b1') the gamma alumina powder is dispersed in a solution based on nitric acid and water, then the dispersed catalyst precursor is ground so as to obtain a suspension;
b2') the support in the form of a ceramic or metallic monolith is coated with said suspension obtained at the end of step b1') by soaking-shrinkage;
b3') a step is carried out of blowing the coated ceramic or metallic monolith obtained at the end of step b2');
b4') optionally, the coated ceramic or metallic monolith obtained at the end of step b3') is dried at a temperature between 80°C and 220°C;
b5') the coated ceramic or metallic monolith obtained at the end of step b3'), optionally at the end of step b4'), is calcined at a temperature between 250°C and 550°C. Method according to claim 9, in which said support is in the form of a metal monolith chosen from steel, stainless steel, nickel, aluminum, iron, copper, nickel-chromium, nickel-chromium-aluminum, iron-chromium monoliths -aluminum. Method according to one of claims 9, in which said support is in the form of a ceramic monolith chosen from alumina (Al ₂ O ₃ ), silica-alumina, silicon carbide (SiC), phosphorus-alumina monoliths , magnesia (MgO), zinc oxide, zirconium oxide (ZrO ₂ ), cordierite (Al ₃ Mg ₂ AlSi ₅ O ₁₈ ). A method according to any one of claims 9 to 11, wherein said medium comprises a number of channels per unit length (CPSI) between 300 and 1200. Process according to any one of claims 9 to 12, in which the geometric surface of said catalyst is between 1500 m²/m ³ and 5000 m²/m ³ . Method according to any one of claims 1 to 13, in which said support in the form of a monolith or a ceramic or metallic foam supplied in step d) is previously calcined in air at a temperature between 300°C and 800°C for a period of between 2 and 8 hours.