TECHNICAL FIELD

The present invention relates to a method for preparing a supported metallic catalyst, comprising nickel and copper, intended particularly for the hydrogenation of unsaturated hydrocarbons.

The present invention also relates to the use of these catalysts in reactions for the hydrogenation of unsaturated hydrocarbons, and more particularly selective hydrogenation of olefinic fractions.

PRIOR ART

Catalysts for the selective hydrogenation of polyunsaturated compounds are generally based on metals from Group VIII of the Periodic Table of the Elements, such as nickel. The metal is in the form of nanoscale metallic particles deposited on a support which may be a refractory oxide. The content of metal from Group VIII, the optional presence of a second metal element, the size of the metallic particles and the distribution of the active phase in the support and also the nature and the pore distribution of the support are parameters which may have an influence on the performance of the catalysts.

The rate of the hydrogenation reaction is governed by several criteria, such as the diffusion of the reactants toward the surface of the catalyst (external diffusional limitations), the diffusion of the reactants in the porosity of the support toward the active sites (internal diffusional limitations) and the intrinsic properties of the active phase, such as the size of the metallic particles and the distribution of the active phase within the support.

The promotion of a nickel-based catalyst has frequently been proposed in order to improve performance levels in selective hydrogenation. For example, it is known from U.S. Pat. No. 5,208,405 to use a catalyst based on nickel and silver for the selective hydrogenation of C4-C10diolefins. Furthermore, it is known to promote nickel, predominantly present, with metals of group IB, in particular gold (FR 2 949 077) or tin (FR 2 949 078). Document FR 3 011 844 discloses a catalyst for the implementation of a selective hydrogenation method comprising a support and an active metallic phase deposited on the support, the active metallic phase comprising copper and at least one nickel or cobalt metal in a Cu:(Ni and/or Co) molar ratio greater than 1.

Moreover, prior to the employment of such catalysts and the use thereof in a hydrogenation method, a step of reducing treatment in the presence of a reducing gas is carried out so as to obtain a catalyst comprising an active phase at least partially in metallic form. This treatment makes it possible to activate the catalyst and to form metallic particles. This treatment may be carried out in situ or ex situ, that is to say after or before the catalyst is charged to the hydrogenation reactor.

SUBJECTS OF THE INVENTION

Continuing its research in the field of hydrogenating catalysts, the applicant has now discovered that it is possible to prepare catalysts which are particularly active, and particularly selective, in the selective hydrogenation of polyunsaturated hydrocarbon fractions, by successively and not simultaneously placing two specific metallic precursors in contact on a porous support, chosen from nickel precursors and copper precursors, in a specific Cu:Ni ratio, and by carrying out, after these contacting steps, an in situ reduction step in the catalytic reactor in the presence of a reducing gas, at a temperature less than 200° C. Without wishing to be bound by any theory, it has been noted by the applicant that, during the preparation of the catalyst, the presence of copper greatly improves the reducibility of the nickel on the support, regardless of the order of addition of the metallic precursors (nickel and copper), which makes it possible to carry out a step of reducing the metal elements in the presence of a reducing gas at temperatures which are lower and reaction times which are shorter than those commonly used in the prior art. The use of less severe operating conditions than in the prior art makes it possible to directly carry out the reduction step within the reactor in which it is desired to carry out the selective hydrogenation of polyunsaturated fractions. Furthermore, the presence of copper in the catalyst makes it possible to maintain good activity and a longer service life of the catalyst when the latter is placed in contact with a hydrocarbon feedstock comprising sulfur, in particular steam cracking and/or catalytic cracking C3 hydrocarbon fractions. Indeed, compared to nickel, the copper present in the catalyst more easily captures the sulfur-containing compounds included in the feedstock, thereby avoiding irreversibly poisoning the most virulent active sites of the nickel which exist on the new catalyst. Finally, the preparation method according to the invention makes it possible in particular, by carrying out two distinct steps of impregnating the metallic precursors on the support, to avoid the formation of alloys based on nickel and copper, which are not desired in the context of the present invention. Indeed, a nickel-copper alloy would lead to a poorer activity and/or selectivity than those of nickel alone.

A subject of the present invention is a method for preparing a catalyst for the selective hydrogenation of polyunsaturated hydrocarbon fractions comprising a bimetallic active phase based on a first nickel metal element, in a proportion of 1 to 30% by weight of nickel element relative to the total weight of the catalyst, and a second copper metal element, in a proportion of 0.5 to 15% by weight of copper element relative to the total weight of the catalyst, the molar ratio between the copper and the nickel being less than 1, and a support comprising at least one refractory oxide chosen from silica, alumina and silica-alumina, said method comprising the following steps:a) a step of placing the support in contact with at least one solution containing at least one nickel precursor;b) a step of placing the support in contact with at least one solution containing at least one copper precursor;the steps a) and b) being carried out separately in any order;c) at least one step of drying the catalyst precursor at the end of step a) and b), or b) and a), at a temperature less than 250° C.;d) supplying the catalyst precursor obtained at the end of step c), into a reactor for the selective hydrogenation of polyunsaturated hydrocarbon fractions, and carrying out a reduction step by placing said precursor in contact with a reducing gas at a temperature less than 200° C. and for a period greater than or equal to 5 minutes and less than 2 hours.

Preferably, step b) is carried out before step a).

Advantageously, step d) is carried out at a temperature of between 130 and 190° C.

Advantageously, step d) is carried out for between 10 minutes and 110 minutes.

Advantageously, the method according to the invention also comprises a step e) of passivation with a sulfur-containing compound after the reducing treatment step d).

Advantageously, a step of drying the catalyst precursor at a temperature less than 250° C. is carried out between step a) and step b).

Preferably, the copper content is between 0.5 and 12% by weight of copper element relative to the total weight of the catalyst.

Advantageously, the copper precursor is chosen from copper acetate, copper acetylacetonate, copper nitrate, copper sulfate, copper chloride, copper bromide, copper iodide or copper fluoride.

More preferably, the copper precursor is copper nitrate.

Advantageously, the reducing gas of step d) is dihydrogen.

Preferably, the hydrogen flow rate, expressed in l/hour/gram of catalyst precursor is between 0.01 and 100 l/hour/gram of catalyst precursor.

Preferably, a step of heat treatment of the dried catalyst precursor obtained in step c) is carried out, before step d), at a temperature of between 250 and 1000° C.

Advantageously, the support is an alumina.

Another subject according to the invention relates to a method for the selective hydrogenation of polyunsaturated compounds containing at least 2 carbon atoms per molecule, such as diolefins and/or acetylenics and/or alkenylaromatics, present in a hydrocarbon feedstock having a final boiling point of less than or equal to 300° C., which method is carried out at a temperature of between 0 and 300° C., at a pressure of between 0.1 and 10 MPa, at a hydrogen/(polyunsaturated compounds to be hydrogenated) molar ratio of between 0.1 and 10 and at an hourly space velocity of between 0.1 and 200 h−1when the method is 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 an hourly space velocity of between 100 and 40 000 h−1when the method is carried out in the gas phase, in the presence of a catalyst obtained according to the preparation method according to the invention.

