FR2992236A1 - Preparing catalyst support used in Fischer-Tropsch reaction, comprises providing beta-silicon carbide support, preparing titanium dioxide precursor solution, impregnating support in solution, and drying and calcining impregnated support - Google Patents

Abstract

Preparing a catalyst support based on silicon carbide (SiC) coated with at least partially titanium dioxide, comprises providing beta -silicon carbide support having high porosity, preparing at least one titanium dioxide precursor solution, impregnating the support in the solution, drying the impregnated support, and calcining the impregnated support to convert the titanium dioxide precursor to titanium dioxide. Independent claims are included for: (1) the catalyst support obtained by the process; and (2) the catalyst obtained by depositing an active phase on the support, where the active phase is cobalt metal and/or iron metal.

Description

TECHNICAL FIELD OF THE INVENTION The invention relates to heterogeneous catalysis, and more particularly supports for catalysts and catalysts capable of being used in heterogeneous catalysis. . In particular, it relates to a novel catalyst support and a novel catalyst for the Fischer-Tropsch reaction. This new catalyst support belongs to porous supports based on silicon carbide (SiC), in particular based on 13-SiC, modified by a surface deposition of TiO 2. State of the art The Fischer-Tropsch reaction converts a mixture of CO and hydrogen into hydrocarbons. There are two types of catalysts for the Fischer-Tropsch reaction: iron catalysts, which work optimally at a temperature of about 350 ° C (so-called "high temperature FT catalyst"), and catalysts based on cobalt, which work at a lower temperature, generally below 250 ° C. They are mainly composed of an active phase and an oxide support. A summary of the state of the art on catalysts for the cobalt-based FT reaction is given in the article "On the selectivity of cobalt-based Fischer-Tropsch catalysts: Evidence for a common precursor for methane and long-chain hydrocarbons "by S. Logdberg et al., published in 2010 in J. Catalysis (doi: 10.1016 / jcat.2010.06.007). The oxide supports are mainly made of alumina, silica and optionally titanium dioxide. The oxide supports have excellent properties making it possible to make active catalysts for the FT reaction, but they also suffer from disadvantages such as their very low thermal conductivity, a low hydrothermal resistance, the presence of acidic sites on the surface (alumina), a low mechanical strength especially for extrudates (silica and titanium dioxide) and low attrition resistance for microbeads used in bubbling beds (in particular for silica and TiO 2).

In order to increase the mechanical and hydrothermal stability of the oxide supports, it has been sought to modify these supports. There may be mentioned alumina promoted with lanthanum oxide La203 (US Pat. Nos. 5,537,945 and US Pat. No. 6,255,358 (Energy International Corp.), US Pat. No. 7,163,963 (Conoco Phillips Co.)), Si (US patent application 2005/0124490 (Chevron Texaco Corp.), US Pat. No. 7,365,040 (Sasol Technology)) and Ti or Zr (US Patent 6,975,7209 (Sasol Technology)). Spinel supports have also been proposed (patent applications VVO 2006/067285 (French Petroleum Institute) and US 2007/0161714), as well as supports in amorphous silica (US patent application 2010/0311570) to increase the resistance to attrition. The alumina used is generally predominantly alumina-y, but alumina supports comprising a majority of α-alumina (US Pat. No. 7,351,679 (Statoil ASA)) have also been used. It has been reported in the literature that TiO2 supports can be used to manufacture extremely selective (05+) cobalt-based catalysts for FT (see publications in Catalysis Today, Vol 100 (2005), p. 343-347 and in Journal of Catalysis, 236 (2005), 139, and Applied Catalysis A: General, 210 (2001) 137-150). However, the authors mention that their mechanical resistance is too weak. TiO2 is generally used to increase interactions with particles of the active phase. It has been reported in the literature that a silica support coated with a TiO 2 layer makes it possible to increase the activity and the selectivity of a silica support. The results obtained on this type of TiO2-promoted silica-based support (see the publication "Influence of Support Preparation Methods on Structure and Catalytic Activity of Co / TiO 2 -5 02 for Fischer-Tropsch Synthesis" of S. Mu et al. , published in 2009 in Catal. Lett. 133, pp. 341-345, and the publication of S. Hinchiranan et al., published in the journal Fuel Proc., Technol., 89, pp. 455-459). a significant improvement in the catalytic activity in FT synthesis. The introduction of TiO 2 into the support also modifies the selectivity to liquid hydrocarbons. Nevertheless, depending on the deposition mode and configuration of the FT test, fixed bed or bubbling bed, the influence is different, eg an improvement in the case of a FT test in fixed bed mode and a slight fall in the case a shake-bed test. In all cases, the improvement of the catalytic activity in FT is attributed to a better dispersion of the cobalt particles with a smaller size in the presence of TiO 2. Finally, in fixed bed tubular reactors, the use of catalysts supported on p-sic makes it possible to temper the thermal variations in the catalytic bed by virtue of the high thermal conductivity of the p-sic material. All of these advantages make it possible to carry out the FT synthesis under more severe conditions in order to improve the productivity of the process. Nevertheless, a catalyst supported on a p-sic material is less active than its counterparts supported on oxides.

The object of the present invention is to prepare a new type of silicon carbide (SiC) based support for the Fischer-Tropsch synthesis reaction (SFT) which has a better stability and a better yield as well as a better selectivity.

OBJECT OF THE INVENTION According to the invention, the problem is solved by depositing the active phase on a very finely divided titanium oxide (TiO 2) layer, consisting of nanoparticles, which covers at least part of the macroporous and mesoporous surface. porous support porous SiC.

The silicon carbide is preferably 13-SiC having at least one macroporosity and one mesoporosity. It may be, for example, granules, beads or a 13-SiC foam. TiO2 is preferably anatase. The inventors have observed that when the cobalt is deposited on a support according to the invention based on silicon carbide (and in particular on a cellular foam made of 13-SiC) covered with TiO 2, this makes it possible to increase significantly the FT synthesis activity, while maintaining a relatively high liquid hydrocarbon selectivity greater than 90%. The method for depositing the TiO2 surface layer is easy to implement for all types and forms of support. This mixed support has the interesting properties coming from both the beta polytype of silicon carbide ([3-SiC) and titanium dioxide, in particular in its anatase form. Among these properties, there may be mentioned for 3-SiC its excellent thermal conductivity, mesoporous macroporous bimodal porosity, excellent chemical and hydrothermal resistance as well as high mechanical strength. Titanium dioxide in its anatase form allows fine dispersion of active phase particles.

The synthesis method according to the invention makes it possible to control the morphology and the size of the supports as a function of the configuration of the reactors used. Catalysts prepared from these TiO2-coated carriers exhibit high SFT activity compared to that measured on a homologous catalyst deposited on 3-SiC without a TiO 2 layer. In addition, the catalytic performances of these catalysts supported on 3-SiC coated TiO2 are very stable over time. According to an essential aspect of the invention, the deposition of TiO 2 is carried out by a sol-gel type process involving a liquid phase of a TiO 2 precursor. This gives better results compared to a deposit of TiO 2 crystals, for example from a dispersion of TiO 2 powder (known process leading to a deposit called "wash-coat"). More particularly, this process according to the invention involves the following steps: (a) supplying a high-porosity 13-SiC support, (b) preparing a solution of at least one TiO 2 precursor, (c) impregnating said supported by said solution, (d) drying said impregnated support, (e) calcining said impregnated support to convert said TiO 2 precursor to TiO 2. This method, which forms the first subject of the invention, leads to the formation of TiO 2 crystallites on the mesoporous and macroporous surface. This avoids the formation of a crust on the macroporous surface which prevents the access of the reaction gases to the mesopores and micropores. This is reflected by the fact that the porous distribution of the support is not significantly modified by the deposition of TiO2 crystallites on the mesoporous and macroporous surface of the support.

In this process, said 13-SiC support can be in particular in the form of extrudates, granules, beads, microbeads, or cellular foam. The specific surface of the support resulting from the process according to the invention is at least 15 m 2 / g, and preferably at least 20 m 2 / g. The microporous contribution to the specific surface of said support is advantageously less than 5 m 2 / g. g, and even more preferably less than 3.5 m 2 / g. Said calcination (step (e)) is preferably carried out at a temperature between 400 ° C and 1000 ° C, and preferably between 500 ° C and 900 ° C. The temperature rise is advantageously with a slope of between 1.5 ° C./min and 3.5 ° C./min, and preferably between 1.5 ° C./min and 2.5 ° C./min.

A second object of the invention is a catalyst support that can be obtained by the method according to the invention, described above. Advantageously, its mass content (y%) of TiO 2 relative to that (100-y) of SiC is between 3% and 30%, and preferably between 5% and 20%. A third object is a catalyst obtained by the deposition of an active phase on the catalyst support according to the invention, characterized in that said active phase is metallic cobalt, metallic iron or a mixture of both, finely divided.

Advantageously, it comprises between 5% and 40% by weight and preferably between 10% and 25% by weight (expressed relative to the weight of the support) of cobalt on a support whose TiO 2 / SiC mass is between 5% and 20%. . Advantageously, the content of TiO 2 is less than 0.02 g per m 2 of surface area of the SiC support, and preferably less than 0.013 g per m 2 of specific surface area of the SiC support. A fourth object of the invention is the use of a catalyst support according to the invention or a catalyst according to the invention for the Fischer-Tropsch reaction. A final subject of the invention is a catalytic process for converting CO and hydrogen into hydrocarbons, characterized in that it involves a catalyst according to the invention. Figures 1 to 10 illustrate embodiments of the invention.

Figure 1 shows porous distribution curves by nitrogen sorptometry. Figure 1A shows these curves for three samples: 13-SiC (diamonds); 10% TiO2 / 13-SiC according to Example 2 (squares); 21% TiO2 / 79% 3-SiC / according to Example 4 (triangles). FIG. 1B shows, on the extended abscissa, these curves for 13-SiC (diamonds) and 10% TiO2 / 13SiC according to Example 2 (squares), these two samples being the same as in FIG. 1A, and in addition the curve corresponding to 10% Co / 10% TiO2 / 13-SiC (cross) according to Example 7. Figure 2 shows X-ray diffraction patterns of 13-SiC and 10% TiO2 / 13-supported cobalt catalysts -SiC after calcination under air at 350 ° C for 2 hours and a reduction under H2 at 300 ° C. For comparison the diagrams of the support alone, pure and covered with TiO2, are also presented in the same figure. Insert: Magnification on the diffraction peak of metallic cobalt showing a widening of the width at half height in the presence of TiO 2. Samples: (a) 0-SiC; (b) 10% TiO2 / O-sic (according to Example 2); (c) 10% Co / O-sic (according to Example 5); (d) 10% Co / 10% TiO2 / O-sic (according to Example 7).

Figure 3 shows micrographs obtained by scanning electron microscopy (SEM) at different magnifications of two samples: (A) and (B): 10% Co / 0-sic (according to Example 5). (C) and (D): 10% Co / 10% TiO2 / O-sic (according to Example 7). The length of the bars is indicated at the bottom right of each micrograph: 1 pm (A) and (C), 200 nm (B) and (D). FIG. 4 shows a micrograph obtained by transmission electron microscopy (TEM) of an area of the 10% TiO 2/13-SiC support obtained according to example 2. FIG. 4 (a) shows the TEM image, FIG. 4 (b) the chemical mapping obtained in energy filtering mode to identify the zones containing Ti (in white) and those containing Si (in dark). . It is observed that the TiO 2 layer does indeed cover the SiC particles. The length of the bar corresponds to 50 nm. FIG. 5 shows two views obtained by transmission electron microscopy in energy filtering mode (EFTEM) and in three-dimensional representation of a zone of the catalyst 10% Co / 21% TiO 2 -79% O-sic according to Example 9 (comparative) in chemical mapping mode (both images correspond to the front and back of the same area). We see the distribution of TiO2 (in white) and SiC (in gray) which are distributed in surface of the solid. Two types of cobalt particles (in black) are observed: large particles deposited on the SiC zones and a few, much smaller, particles deposited on the TiO 2 zones. FIG. 6 shows two other micrographs obtained by transverse section electron microscopy of two particles (FIGS. 6 (a) and (b)) of 10% Co / 21% TiO 2 -79% O-sic catalyst according to Example 9 (comparative). The images were taken in spectroscopic mode of electron energy loss (EELS). Colors: white = Co; black = Ti; Gray = Si. Figure 7 relates to a 10% Co / 10% TiO 2 / SiC catalyst according to the invention. It shows a micrograph obtained by transmission electron microscopy (TEM) (FIG. 7 (a)) and energy filtering mode (EFTEM) (FIGS. 7 (b), 7 (c), 7 (d) respectively for FIG. mapping of Si, Ti and Co)) of a 10% Co / 10% TiO 2/13-SiC catalyst zone according to Example 7. Figure 7 (d) estimates an average particle size of cobalt at about 20 nm, the length of the bar being 100 nm. Figures 7 (e) and 7 (f) show the same images as Figures 7 (a) and 7 (c); they are slightly enlarged. The areas marked (1) and (2) in Figs. 7 (e) and 7 (f) are enlarged in Figs. 7 (g) and (h) for area (1) and Figs. 7 (i) and (j) for zone (2), knowing that FIGS. 7 (g) and (i) are TEM micrographs and FIGS. 7 (h) and (j) are EFTEM micrographs of titanium; the length of the bar is 50 nm.

FIG. 8 shows the activity (ASM) and the selectivity of liquid hydrocarbons (SC5 +) obtained with the 10% Co / 10% TiO2 / 13-SiC catalyst according to Example 7 as a function of the duration of the test. The H21 CO ratio was 2, the reaction temperature was 215 ° C, the total pressure 40 atmospheres, the hourly space velocity (GHSV) 2850 h-1.

FIG. 9 shows the activity (ASM) and the selectivity of liquid hydrocarbons (SC5 +) obtained on the 10% Co / 10% TiO2 / 13-SiC catalyst according to Example 7 as a function of the reaction temperature. The H2 / CO ratio was 2, the total pressure 40 atmospheres, the hourly space velocity (GHSV) = 3800 h-1. Figure 10 shows a second area of the same sample as Figure 7. Figures 10 (a), (c) and (e) are TEM micrographs, Figures 10 (b), (d) and (f) are micrographs EFTEM titanium. Figures 10 (c) and (d) correspond to area (1), Figures 10 (e) and (f) to area (2). Detailed Description 1. Definitions In the context of the present invention, the term "specific surface" means the specific surface area determined according to the method of Brunauer, Emmet and Teller (BET method), well known to those skilled in the art and described in particular in the NF X 11-621 standard. The porosity of a material is usually defined by reference to three categories of pores distinguished by their size: microporosity (diameter less than 2 nm), mesoporosity (diameter between 2 nm and 50 nm) and macroporosity (diameter greater than about 50 nm). In some embodiments of the present invention, a p-sic foam is used which is porous open-cell foam. Here we mean by "foam foam" a foam that has both a very low density and a large pore volume. The size of the cell aperture is variable and is typically between about 800 and 6000 μm. Such a foam can be prepared using known techniques. It has a very low microporosity. Mesoporosity is essentially related to the bridges that form the alveoli. The open macroporosity of such a foam may vary from 30 to 95%, especially from 50 to 90%, and its density may be between 0.05 g / cm3 and 0.5 g / cm3. In general, for its application as a catalyst or catalyst support, below a density of 0.05 g / cm 3, there are problems with the mechanical strength of the foam, whereas above 0, 5 g / cm3, the alveolar porous volume will be reduced and the pressure drop will increase without providing a functional advantage. Advantageously, the density is between 0.1 and 0.4 g / cm 3. In other embodiments of the present invention, 13-SiC is used in the form of extrudates, granules, microbeads or grains. This material can be prepared using known techniques. 2. Mode of synthesis of the support According to one embodiment, a 3-SiC support of high porosity is impregnated with an organic solution of a TiO 2 precursor. Said TiO2 precursor may be an organic precursor, in particular an alcoholate. Ti (i -OCl3-17) 4 (abbreviated TIPP) is preferred. The solvent may be an alcohol, for example ethanol or i-propanol. Advantageously, a solution of ethanol (preferably anhydride to prevent the hydrolysis of the TIPP) containing Ti (i -OC31-17) 4 (distributed for example by the company Acros) is used. In an advantageous embodiment, the Ti / Si molar ratio is between 2.5% and 10% (ie a TiO 2 filler between 5% and 20% by mass relative to the total SiC mass). After impregnation, the solid is dried, for example in an oven at 110 ° C. for 8 hours. Advantageously, the solid is left after impregnation at ambient temperature (for example for 4 hours) before drying. The conversion of the precursor to TiO 2 is carried out by calcination (advantageously in air), preferably at a temperature of between about 400 ° C. and about 1000 ° C.). The temperature rise slope is advantageously between 1.5 ° C / min and 3.5 ° C / min, and preferably about 2 ° C / min. For example, the calcination temperature may be 600 ° C, and the treatment time at this temperature may be 5 hours.

By way of comparison, a hybrid material containing similar TiO 2 and SiC contents can also be prepared, but in which the two phases are distributed throughout the solid mass, as opposed to the materials preferred here in which a SiC core is covered by a layer of TiO2; such a hybrid medium is not within the scope of the present invention.

In order to avoid any confusion in the notations, it is appropriate to note in the following "x% TiO 2 / SiC" the solids formed by a surface layer of TiO 2 (or at least by individual particles of TiO 2) deposited on an SiC support at a concentration of x% based on the weight of SiC, and "y% TiO 2 -SiC" mixed solids (here also called: "hybrids") containing y% of TiO 2 and (100-y)% of SiC distributed in the mass of the material, knowing that x and y express mass percentages. The method according to the invention ensures a good dispersion of the TiO2 fine particles over the entire macro-porous and mesoporous surface, external and internal, of the support. It avoids the formation of thick layers or crusts. The deposition of crystallites of TiO2 from a liquid dispersion according to the state of the art (forming a layer which the skilled person calls "wash coat") does not make it possible to obtain such a fine dispersion in depth of the support 13-SiC. The 13-SiC support can have any suitable geometric shape. It may be in particular extruded granules. It can also be 13-SiC foam. These supports are known per se and may be prepared by one of the known methods, namely: (i) impregnation of a polyurethane foam with a suspension of a silicon powder in an organic resin (Prin process see EP 0 624 560 Bi, EP 0 836 882 B1 and EP 1 007 207 A1); (ii) the reaction between SiO vapors with reactive carbon at a temperature of between 1100 ° C. and 1400 ° C. (Ledoux process, see EP 0 313 480 B1); or (iii) crosslinking, carbonizing and carburizing a mixture of a liquid or pasty prepolymer and a silicon powder (Dubots process, see EP 0 440 569 B1 and EP 0 952 889 B1). 3. Method of Preparation of the Catalyst on the Support On the TiO 2 / SiC support according to the invention, an active phase consisting of one or more transition metals is then deposited. Iron, cobalt or a mixture of both is preferred. This deposition is advantageously carried out by the method of impregnating the pore volume with a solution of a precursor of the active phase, which is known to those skilled in the art. Said precursor may be a solution of at least one organometallic compound of the metal which will constitute the active phase, or an organic salt thereof. After the impregnation, the solid is dried (preferably at a temperature of between 100 ° C. and 140 ° C.) and then calcined (preferably in air at a temperature of between 250 ° C. and 450 ° C., and preferably with a slope of heating between 0.6 ° C / min and 1.6 ° C / min) to obtain an oxide of said metal. The active phase is obtained by reducing the oxide precursor, preferably at a temperature between 200 ° C and 380 ° C (and preferably with a heating slope of between 2 ° C / min and 4 ° C / min). The average size (d) of the active phase particles (i.e. their average diameter) is advantageously between 15 nm and 40 nm. It can be estimated either from the size of the precursor oxide, which is chemically stable and therefore easier to handle for its characterization (for iron and cobalt, the particle size is approximately 0.75 times that of the particles of oxide), or from the broadening of the diffraction peak according to the well-known Scherrer formula: d = kA / (T cos 0) where A represents the wavelength of the incident radiation, T the width halfway up the diffraction peak, k is a constant and 0 is the wave deviation half.

In an advantageous embodiment of this step, after impregnation, the solid is dried in ambient air for 4 hours and then in an oven at 110 ° C. for 8 hours. The solid thus dried is calcined in air at 350 ° C. (slope of 1 ° C./min) for 2 hours in order to obtain the oxide precursor of the catalyst, Co304 / xTiO 2 / SiC. The catalyst is obtained by reducing the oxide precursor under hydrogen flow at 300 ° C. (slope of 3 ° C./min) for 6 hours. The catalyst is then denoted yCo / xTiO 2 / SiC with y which represents the load (in percent) of cobalt on the catalyst ([Co] / [TiO 2 + SiC] and x which represents the load (in percent) of the TiO 2 on the support SiC ([TiO 2] / [SiC]) The average particle size of Co is about 20 nm 4. Use of the catalyst The catalyst according to the invention is especially suitable for the Fischer-Tropsch reaction. any suitable geometric shape, and may be in particular in the form of granules, beads, microbeads, extrusions, or in the form of foam plates or cylinders 5. Advantages The additional advantages in the use of such a catalytic system compared to those reported at present in the literature are the following: (i) an easy shaping of the support depending on the nature of the reactor involved, (ii) a perfect control of the meso- and macro-porous support allowing a better ac transferability of reagents and better evacuation of reaction intermediates; (iii) higher thermal conductivity of the support, relative to silica or alumina, to reduce the formation of hot spots on the catalyst surface, knowing that the hot spots can induce a degradation of the selectivity and also favor the sintering of the particles of the active phase. (iv) higher mechanical strength compared to macroscopic TiO2 supports (extruded, foam, rings, beads, etc.) and better resistance to attrition, because TiO 2 is found in the porosity of the support and not on its surface . This attrition resistance is a particularly important property if the catalyst is used in the form of microbeads in a "slurry" type reactor. EXAMPLES Examples 1 to 3 (according to the invention): Deposition of TiO 2 on SiC support Porous SiC grains having a specific surface area of 40 m 2 / g and a porous distribution free of micropores were supplied. The pore volume of the grains was impregnated with an ethanol solution containing Ti (i -OC31-17) 4 in an amount necessary to deposit a Ti load corresponding to 5% TiO 2, 10% TiO 2 and 15% TiO 2. relative to the weight of SiC, respectively for the materials of Examples Nos. 1, 2 and 3. After impregnation, the solids were left at ambient temperature for 4 hours and then dried in an oven at 110 ° C. for 8 hours. The conversion of the precursor salt to TiO 2 was then carried out by calcination in air at 600 ° C. for 5 hours with a temperature rise slope of 2 ° C./min. The materials thus obtained have specific surface areas respectively of 38 m 2 / g, 41 m 2 / g and 41 m 2 / g for Examples 1, 2 and 3. The porous distribution of the starting SiC and the 10% TiO 2 / SiC support are reported. in Figure 1A. Figure 4 (b) shows an Energy Filtered Transmission Electron Microscopy (EFTEM) image obtained on the 10% TiO 2 / SiC support. Image # 4 (a) on the left is a cut-off TEM image for an overview of the sample. The image of FIG. 4 (b) clearly shows the presence of a thin layer of TiO 2 covering the porous surface of the SiC grain. Example No. 4 (Comparative) Preparation of TiO2-SiC Mixed Support Containing 21% TiO2 and 79% SiC A TiO2-SiC mixed material was prepared in the following manner: 1620 g 1520 g of novolac solid phenolic resin, 600 g of TiO 2 powder (Degussa-Evonik P25, BET surface area of about 50 m 2 / g, average particle size of about 20 nm), 78 g of Hexamethylene Tetramine (HMT), 30 g of Zusoplast PS1 plasticizer, 200 g of a 35% solution of polyvinyl alcohol and 1195 g of water.

The powders were mixed. The polyvinyl alcohol was diluted in the amount of water. An extrudable mixture was prepared by stirring the liquid mixture onto the powders. This mixture was extruded to form granules 3 mm in diameter. After drying in ambient air and then at 150 ° C. for 4 hours, the granules were treated at 1360 ° C. under a stream of argon for one hour. The solid obtained comprises 83.7% by weight of SiC and 16.3% by weight of TiC (ie a mole fraction of 11.5% Ti relative to the sum Ti + Si). The X-ray diffraction pattern indicated that the resulting solid is a mixture of SiC and TiC ("SiC-TiC composite"). Its BET surface area was 54 m2 / g, of which 27 m2 / g of microporous surface. Then, this SiC-TiC composite was oxidized under air at 400 ° C. for 8 hours. There was then obtained a composite of 20.6% by weight TiO 2 - 79.4% by weight SiC (ie a mole fraction of 11.5% Ti relative to the sum Ti + Si) which has a mechanical strength of 59 N / mm . Its specific surface is 83 m2 / g, including 53 m2 / g of microporous surface. Figure 5 shows that the surface of this material consists of juxtaposed areas of SiC and TiO2 in equivalent amounts. This material therefore has a composition similar to that of Example 2, but its porous properties on the one hand and its surface composition on the other hand make it a very different catalytic support.

Example 5 (Comparative): Preparation of a 10% Co / SiC Catalyst A 10% cobalt catalyst containing no titanium was prepared from the crude SiC already used in Examples 1-3. aqueous solution of cobalt nitrate impregnated on SiC by the porous volume method. The concentration of cobalt nitrate is calculated to obtain the desired cobalt charge in the final catalyst. The solid was then dried in ambient air for 4 hours, then in an oven at 110 ° C. for 8 hours. It was then calcined under air at 350 ° C (1 ° C / min) for 2 hours to obtain the catalyst oxide precursor, Co304 / SiC. The 10% Co catalyst was obtained by reducing the oxide precursor under a stream of hydrogen at 300 ° C. (slope of 3 ° C./min) for 6 hours. Its specific surface area was 33 m2 / g. The average particle size of Co is estimated at 40-50 nm (see Table 1).

Examples 6 to 8 (according to the invention): preparation of 10% Co / TiO 2 / SiC catalysts Example 5 was reproduced by replacing the SiC support with the solids prepared according to Examples 1 to 3. Then, respectively, the catalysts according to Examples 6, 7 and 8, all containing a load of 10% of C. The specific surface area and the average diameter of the Co particles measured on these catalysts are reported in Table 1. Compared to the specific surfaces of the supports Initially, the 10% Co catalysts deposited on the substrates coated with a TiO 2 layer do not show any significant change in the specific surface area, whereas this decreases from 40 m 2 / g to 33 m 2 / g after deposition. Co on SiC. The presence of TiO2 significantly affects the average size of cobalt particles. Indeed, it thus passes from about 40-50 nm on SiC at 20 nm when the support was previously covered with a 10% by weight layer of TiO 2. The X-ray diffraction patterns of the catalysts are shown in FIG. 2. The diffraction peaks of the TiO 2 phase are clearly visible on the diffraction diagrams. The diffraction also indicates that the reduction is (relatively) complete because no diffraction peaks corresponding to the cobalt oxide phase, Co0 and / or 00304 are observed. The enlargement of the cobalt diffraction peak (insert of FIG. 2) on the diagrams shows that there is an increase in the width at mid-height of this peak indicating that the size of cobalt particles participating in the coherent diffraction is smaller in the presence of TiO 2. The SEM images of the catalysts of Example 5 and 7 (10% Co / SiC and 10% Co / 10% TiO 2 / SiC) are shown in Figure 3 and indicate a significant decrease in the size of the cobalt particles in the presence of Ti02. These results are in agreement with those obtained by the X-ray diffraction presented above: the dispersion of the cobalt particles is therefore greatly improved by the presence of the TiO 2 surface layer. The EFTEM (Energy Filtered Transmission Electron Microscopy) images obtained on the 10% Co / 10% TiO 2 / SiC catalyst are presented in FIG. 7. FIGS. 7 (c) and 7 (d) show the dispersion of the cobalt particles on the TiO2 layer. It can be seen in the map of FIG. 7 (d) that the average size of the cobalt particles is relatively small (of the order of 20 nm) and homogeneous. This result is in good agreement with those obtained from the broadening of the diffraction peaks of cobalt presented previously (Table 1). Figure 10, which refers to another area of the same sample, corroborates these findings and conclusions. Example 9 (Comparative): Preparation of a 10% Co / 21% TiO 2 -SiC Catalyst A 10% Co catalyst was prepared according to Example 5, replacing the SiC support with the mixed 21% TiO 2 -SiC solid. prepared in Example 4. FIGS. 5 and 6 show that the catalyst obtained has two active phase populations: very large Co particles located on the surface of SiC zones and some very small Co particles deposited on TiO 2 zones. . Example 10 (Comparative): Preparation and Testing of a 10% Co / SiC Catalyst A catalyst was prepared comprising an active phase of cobalt (deposited by the method described in Example 5) on a microporous SiC support. about 100 m2 / g. Example 11 Evaluation of the Catalytic Performance of the Different Catalysts According to the Invention or According to Comparative Examples The performance of the catalysts prepared according to Examples 5, 6, 7 and 9 were evaluated in the Fischer-Tropsch reaction. The results are reported in Table 2. A doubling of catalytic activity is observed for the catalyst containing a continuous TiO 2 layer on SiC with respect to the catalyst prepared on SiC alone. The catalyst prepared from a support partially coated with TiO 2 has an intermediate activity, little improved compared to that of the catalyst containing no TiO 2.

The stability of the catalytic performance of the catalyst of Example 7 (10% Co / 10% TiO 2 / SiC) was also evaluated and the results are shown in FIG. 8. It is clearly seen in this figure that the activity and the selectivity The liquid hydrocarbons of the catalyst are extremely stable as a function of the test time.

The influence of the reaction temperature on the activity of the catalyst of Example 7 (10% Co / 10% TiO 2 / SiC) in the Fischer-Tropsch test was also evaluated and the results are shown in Figure 9. It should be noted that in these tests the hourly space velocity was increased from 2850 h-1 to 3800 h-1. This is to avoid reaching too high conversions that could induce thermal runaway problems in the catalytic bed. These thermal runaways could modify the characteristics of the active phase by sintering particles of the active phase. The activity in SFT increases significantly with the reaction temperature while the selectivity of the liquid hydrocarbons remains high and stable. The specific specific activity (ASM) reaches about 0.6 gc at 5-1, with a liquid hydrocarbon selectivity of around 90% at 225 ° C. It should also be noted that the catalyst has a relatively high stability and no deactivation has been observed at each test stage.

By way of comparison, the catalyst prepared according to Example 9 by depositing Co on a 21% TiO 2 -SiC mixed material has a specific activity which is much lower than that measured on catalysts which are the subject of the invention. These results demonstrate that the catalysts prepared according to the invention have very good catalytic performance for the FT reaction, excellent stability over time. Table 1 Characteristics of supports and catalysts used for FT synthesis Surface Size of the size of the specific Co particles Co particles [m2ig] (d (Co)) (a) [nm] (d (Co) (b) [nm Support SiC 40 - - 5% TiO 2 / SiC (Example 1) 38 - - 10% TiO 2 / SiC (Example 2) 41 - - 15% TiO 2 / SiC (Example 3) 41 - - 21% TiO 2 - 79% SiC ( eg 4) (*) 83 - - Catalyst 10% Co / SiC (eg 5) (*) 33 42 ± 10 51 ± 10 10% Co / 5% TiO2 / SiC (eg 6) 37 31 ± 10 30 ± 10 10% Co / 10% TiO2 / SiC (eg 7) 40 24 ± 10 24 ± 10 10% Co / 15% TiO2 / SiC (eg 8) NM NM NM 10% Co / 21% TiO2-79% SiC (eg 9) (*) 25.2 NM NM (a) d (Co) = 0.75 xd (Co304) (b) d (Co) = kA / (r.co) (*) eg comparative15 Table 2 Catalytic Performance in FT Synthesis on SiO promoted SiC and SiC promoted Cobalt Catalysts Reaction Conditions: Cobalt concentration = 10% by weight, H2 / CO ratio = 2, reaction temperature = 215 ° C, total pressure = 40 atmospheres, hourly space velocity (GHSV) = 2850 h-1 (2750 h-1 for the catalyst of ex. No. 9). Conversion Catalyst Conversion Selectivity [c / 0] CoTY (a) ASM [c / 0] [h4] (13) CH4 CO2 C2-C4 C5 + Ex 5: 10% Co / SiC 27 3 0 2 95 8.5 0.19 Ex 6: 34 3 0 2 95 10.4 0.23 10% Co / 5% TiO 2 / SiC Ex 7: 55 4 0 3 93 17.5 0.39 10% Co / 10% TiO 2 / SiC Ex 9, comparative: NM NM 93 NM 0.17 10% Co / 21% TiO2-SiC (a) Co-Time Yield (CoTY): The yield per cobalt site represents the number of moles of CO converted per unit mass of cobalt per hour ( ie: CO [mol] / Co [g] / [h]). (13) Specific Mass Activity (MSA): The Mass Specific Activity (MSA) is the mass of hydrocarbon (> C5) formed per gram of catalyst per hour (ie, catalyst / NM, NM means "unmeasured"). .

Table 3 Catalytic Performance in F-T Synthesis on 10Co / 10TiO2-SiC Catalyst versus Reaction Temperature. Reaction conditions: Cobalt concentration = 10% by weight, H2: CO = 2, total pressure = 40 atm, hourly space velocity (GHSV) = 3800 h-1. Temperature [C] Conversion [c / 0] Selectivity (%) CoTY (a) ASM [1-1-1] (b) CH4 CO2 C2-C4 C5 + 215 41 4.5 0 2.5 93 17.2 0, 38,220 49 5.3 0 2.8 92 20.5 0.45 225 61 5.8 0.1 2.9 91 25.7 0.56 (a) Co-Time Yield (CoTY): The yield per site of cobalt represents the number of moles of CO converted per unit mass of cobalt per hour (ie: CO [ol] / Co [g] / [h]]. (b) Mass specific activity (SSA): The specific activity Mass (ASM) represents the mass of hydrocarbon (> C5) formed per gram of catalyst per hour (gcs-igoetalizer / [h]).

Claims (13)

REVENDICATIONS1. A process for preparing an SiC-based catalyst support at least partially covered with TiO 2, characterized in that said process comprises the following steps: (a) supplying a high-porosity 13-SiC support, (b) preparing a solution of at least one TiO 2 precursor, (c) impregnating said support with said solution, (d) drying said impregnated support, (e) calcining said impregnated support to convert said TiO 2 precursor to TiO 2. The method of claim 1, wherein said 13-SiC support is in the form of extrudates, granules, beads, microbeads, or foamed foam. 3. Method according to claim 1 or 2, characterized in that the specific surface area of said support is at least 15 m2 / g, and preferably at least 20 m2 / g. 4. Method according to claim 3, characterized in that the microporous contribution to the specific surface of said support is less than 5 m 2 / g. 5. Method according to any one of claims 1 to 4, characterized in that said calcination is carried out at a temperature between 400 ° C and 1000 ° C, and preferably between 500 ° C and 900 ° C. 6. Method according to any one of claims 1 to 5, characterized in that in the calcination step, the temperature rise is with a slope of between 1.5 ° C / min and 3.5 ° C / min, and preferably between 1.5 ° C / min and 2.5 ° C / min. 7. Catalyst support obtainable by the method according to any one of claims 1 to 6. 8. catalyst support according to claim 7, characterized in that the mass ratio TiO 2 / SiC is between 3% and 30%, and preferably between 5% and 20%. 9. Catalyst support according to claim 8, characterized in that the TiO2 content is less than 0.02 g per m2 of specific surface area of the SiC support, and preferably less than 0.013 g per m2 of specific surface area of the support. SiC. 10. Catalyst obtained by the deposition of an active phase on the support according to any one of claims 7 to 9, characterized in that said active phase is cobalt metal, metallic iron or a mixture of the two, finely divided. 11. Catalyst according to claim 10, characterized in that it comprises between 5% and 40% by weight, and preferably between 10% and 25% by mass (expressed relative to the mass of the support) of cobalt on a support whose TiO 2 / SiC mass ratio is between 5% and 20%. 12. Use of a catalyst support according to one of claims 7 to 9 or a catalyst according to claim 10 or 11 for the Fischer-Tropsch reaction. 13. Catalytic process for converting CO and hydrogen into hydrocarbons, characterized in that it involves a catalyst according to claim 10 or 11.