FIELD OF THE INVENTION

The invention relates to a catalyst containing an active cobalt phase, deposited on a support comprising alumina, silica or silica-alumina, said support containing a mixed oxide phase containing cobalt and/or nickel, said catalyst has been prepared by introducing at least one organic compound of the family of dilactones. The invention also relates to the method for preparing same and to the use thereof in the field of Fischer-Tropsch synthesis processes.

PRIOR ART

The Fischer-Tropsch synthesis processes make it possible to obtain a wide range of hydrocarbon cuts from the CO+H2mixture, commonly referred to as synthesis gas or syngas.

The simplified stoichiometric equation (limited in the equation below to the formation of alkanes) of the Fischer-Tropsch synthesis is written:
nCO+(2n+1)H2→CnH2n+2+nH2O

The catalysts used in Fischer-Tropsch synthesis are usually supported catalysts based on alumina, silica or silica-alumina or combinations of these supports, the active phase mainly consisting of iron (Fe) or cobalt (Co) optionally doped with a noble metal such as platinum (Pt), rhodium (Rh) or ruthenium (Ru).

The addition of an organic compound to Fischer-Tropsch catalysts to improve their activity was recommended by a person skilled in the art.

Many documents describe the use of various ranges of organic compounds as additives, such as nitrogen-containing organic compounds and/or oxygen-containing organic compounds.

In particular, U.S. Pat. Nos. 5,856,260 and 5,856,261 respectively teach the introduction, during the preparation of the catalyst, of polyols of general formula CnH2n+2Oxwith n being an integer between 2 and around 6, and x being an integer between 2 and 11 or sugars of monosaccharide or disaccharide type, sucrose being particularly preferred.

Patent application US 2005/0026776 teaches the use of chelating compounds of the following types: nitrilotriacetic acid (NTA), trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CyDTA) or ethylenediaminetetraacetic acid (EDTA), or else glycine, aspartic acid or citric acid for obtaining a catalyst with a reduced size of Co3O4crystallites. Other documents teach the use of polyethers (WO 2014/092278 and WO 2015/183061), glyoxylic acid (WO 2015/183059), unsaturated dicarboxylic acids (US 2011/0028575) or else of multifunctional carboxylic acids of formula HOOC—(CRR1)n—COOH with n≥1 in the preparation of Fischer-Tropsch catalysts (WO 98/47618).

Patent application FR 3050659 describes the use of lactones, of linear mono- and diesters and of cyclic carbonates in order to increase the activity and the selectivity of catalysts for Fischer-Tropsch synthesis.

Document WO 2012/013866 discloses the use of a cyclic oligosaccharide, in particular cyclodextrin, as additive of a Fischer-Tropsch catalyst. This document also describes the use of a support based on silica-alumina optionally containing a spinel.

Irrespective of the compounds selected, the induced modifications do not always make it possible to increase the catalyst performance enough to make the process profitable. Furthermore, it is often very complicated to carry out the industrial deployment thereof as the methods are complex to implement.

Consequently, it appears essential, for catalyst manufacturers, to find new catalysts for Fischer-Tropsch synthesis with improved performance.

AIMS AND SUMMARY OF THE INVENTION

According to a first aspect, the invention relates to a catalyst containing an active cobalt phase, deposited on a support comprising alumina, silica or silica-alumina, said support containing a mixed oxide phase containing cobalt and/or nickel, said catalyst being prepared by a process comprising at least:a) a step of bringing a support comprising alumina, silica or silica-alumina into contact with at least one solution containing at least one precursor of cobalt and/or of nickel, then drying at a temperature below 200° C. and calcining at a temperature of between 700° C. and 1200° C., so as to obtain a mixed oxide phase containing cobalt and/or nickel in the support,
then carrying outb) a step of bringing said support containing said mixed oxide phase into contact with at least one solution containing at least one precursor of cobalt,c) a step of bringing said support containing said mixed oxide phase into contact with at least one organic compound of the family of dilactones,
steps b) and c) being able to be performed separately, in any order, or at the same time,d) then carrying out a step of drying at a temperature below 200° C.

Not one of the documents mentioned relating to the additives describes a cobalt-based catalyst, deposited on a support containing a mixed oxide phase containing cobalt and/or nickel prepared by means of an organic compound of the family of dilactones.

The applicant has indeed observed that the use of an organic compound of the family of dilactones as an organic additive during the preparation of a catalyst containing an active cobalt phase, deposited on a support comprising alumina, silica or silica-alumina, said support also containing a mixed oxide phase containing cobalt and/or nickel made it possible to obtain a catalyst for Fischer-Tropsch synthesis displaying improved catalytic performance.

Indeed, the catalyst according to the invention displays increased activity and increased selectivity relative to catalysts containing a mixed oxide phase containing cobalt and/or nickel in their support but prepared without additivation by an organic compound of the family of dilactones or relative to additivated catalysts with no mixed oxide phase containing cobalt and/or nickel in the support. The use of such an organic compound during the preparation of a cobalt-based catalyst comprising a support containing a mixed oxide phase containing cobalt and/or nickel seems to have a synergistic effect on the activity and selectivity in a Fischer-Tropsch process.

Without being bound to any theory, it was demonstrated that such a catalyst has a dispersion of the cobalt that is substantially greater than that exhibited by catalysts prepared in the absence of such an organic compound. This results in the presence of a greater number of active sites for the catalysts prepared in the presence of at least one organic compound of the family of dilactones, even if this dilactone compound is at least partially eliminated afterwards by a drying and optionally a calcining.

According to one variant, the content of mixed oxide phase in the support is between 0.1% and 50% by weight relative to the weight of the support.

According to one variant, the mixed oxide phase comprises an aluminate of formula CoAl2O4or NiAl2O4in the case of a support based on alumina or on silica-alumina.

According to one variant, the mixed oxide phase comprises a silicate of formula Co2SiO4or Ni2SiO4in the case of a support based on silica or on silica-alumina.

According to one variant, the silica content of said support is between 0.5% by weight and 30% by weight relative to the weight of the support before the formation of the mixed oxide phase when the support is a silica-alumina.

According to one variant, the organic compound of the family of dilactones is selected from the group of cyclic dilactones having 4, 5, 6 or 7 ring members.

According to one variant, the organic compound of the family of dilactones is selected from the group of cyclic dilactones having 4 ring members consisting of 1,2-dioxetanedione, or from the group of cyclic dilactones having 5 ring members consisting of 1,3-dioxolane-4,5-dione, 1,5-dioxolane-2,4-dione, and 2,2-dibutyl-1,5-dioxolane-2,4-dione, or from the group of cyclic dilactones having 6 ring members consisting of 1,3-dioxane-4,6-dione, 2,2-dimethyl-1,3-dioxane-4,6-dione, 2,2,5-trimethyl-1,3-dioxane-4,6-dione, 1,4-dioxane-2,5-dione, 3,6-dimethyl-1,4-dioxane-2,5-dione, 3,6-diisopropyl-1,4-dioxane-2,5-dione, and 3,3-ditoluyl-6,6-diphenyl-1,4-dioxane-2,5-dione, or from the group of cyclic dilactones having 7 ring members consisting of 1,2-dioxepane-3,7-dione, 1,4-dioxepane-5,7-dione, 1,3-dioxepane-4,7-dione and 5-hydroxy-2,2-dimethyl-1,3-dioxepane-4,7-dione.

According to one variant, the molar ratio of organic compound of the family of dilactones introduced during step c) relative to the cobalt element introduced in step b) is between 0.01 mol/mol and 2.0 mol/mol.

According to one variant, the content of cobalt element introduced during step b) as active phase is between 2% and 40% by weight expressed as cobalt metal element relative to the total weight of the catalyst.

According to one variant, the catalyst further comprises an element selected from groups VIIIB, IA, IB, IIA, IIB, IIIA, IIIB and VA.

According to one variant, the catalyst further contains an additional organic compound other than the compound of the family of dilactones, said additional organic compound containing oxygen and/or nitrogen, and preferably comprising one or more chemical functions selected from a carboxylic, alcohol, ether, aldehyde, ketone, amine, nitrile, imide, oxime, urea and amide function.

According to one variant, after the drying step d), a calcining step e) is carried out at a temperature of between 200° C. and 550° C. in an inert atmosphere or in an oxygen-containing atmosphere.

According to one variant, the catalyst obtained in the drying step d) or obtained in the calcining step e) at a temperature of between 200° C. and 500° C., is reduced.

According to a second aspect, the invention also relates to a process for preparing the catalyst according to the invention.

According to a third aspect, the invention also relates to the use of the catalyst according to the invention in a Fischer-Tropsch synthesis process for the synthesis of hydrocarbons wherein the catalyst according to the invention or prepared in accordance with the invention is brought into contact with a feedstock comprising synthesis gas under a total pressure of between 0.1 MPa and 15 MPa, under a temperature of between 150° C. and 350° C., and at an hourly space velocity of between 100 and 20 000 volumes of synthesis gas per volume of catalyst and per hour with an H2/CO molar ratio of the synthesis gas of between 0.5 and 4.

