NO323825B1 - Process for synthesizing hydrocarbons in the presence of a catalyst comprising a Group VIII metal applied to silica alumina - Google Patents

Description

The present invention relates to a method for synthesizing hydrocarbons from a mixture comprising CO-(CO2)-H2 (ie a mixture comprising carbon monoxide and hydrogen and possibly carbon dioxide, known as synthesis gas). This method comprises using a catalyst comprising at least one group VIII metal, preferably cobalt, placed on a silica-alumina, prepared by co-precipitation and calcination at a temperature in the range from about 700°C to about 1200°C for at least 6 hours, so that it has an effective surface of less than 260 m<2>/g.

A person skilled in the art will be aware that synthesis gas can be converted to hydrocarbons in the presence of a catalyst containing transition metals. Such a conversion, carried out at high pressure and under pressure, is known in the technical literature as the Fischer-Tropsch synthesis. Metals from group VIII in the periodic table,

such as iron, ruthenium, cobalt and nickel, catalyze the conversion of the CO-(CO2)-H2 mixtures (ie a mixture of carbon monoxide, hydrogen and possibly carbon dioxide, known as synthesis gas) into liquid and/or gaseous hydrocarbons.

Different methods have been described and developed in the prior art with the aim of improving the production of Fischer-Tropsch catalysts based on cobalt placed on different supports. The most commonly used carriers are alumina, silica and titanium dioxide, sometimes modified by means of additional elements.

International patent WO-A-99/39825 describes the use of a carrier comprising a titanium dioxide base, in which a binder consisting of silica and alumina is incorporated. The solidity properties of the provided catalyst are improved, in particular for use in a three-phase reactor, commonly known as a suspension reactor. Such a reactor is usually of the suspension bubble column type.

WO-A-99/42214 describes the addition of a stabilizing element to a carrier which is partially soluble in an aqueous acidic or neutral medium, selected from alumina, titanium oxide and magnesia, used to prepare a catalyst active in Fischer-Tropsch- the synthesis. Alumina is the preferred carrier.

The stabilizing agent can be selected from the group consisting of: Si, Zr, Cu, Zn, Mn, Ba, Co, Ni and/or La. It can significantly reduce the solubility of the carrier in acidic or neutral water solutions. The preferred stabilizer is silica, added in the form of one organic silicon compound by deposition on the pre-prepared alumina carrier; at least 0.06 and at most 2.8 silicon atoms are added per square meter bearing.

Cobalt-based Fischer-Tropsch catalysts, as described in the above-mentioned invention and used in a three-phase reactor, can lead to significant losses of catalyst in the produced paraffin waxes, through the formation of sub-microscopic sieve fractions (fines). The catalyst losses, expressed in relation to the cobalt, can amount to 50 mg of cobalt per kilogram of wax.

United States Patents US-A-5,169,821 and US-A-5,397,806 describe the incorporation of silicon, zirconium or tantalum into a cobalt-based catalyst supported on TiO 2 in the form of anatase, to make it stable during high-temperature regeneration.

WO-A-96/19289 describes the use of a catalyst for converting synthesis gas to hydrocarbons based on cobalt, ruthenium or iron placed on a . mesoporous, crystalline aluminum silicate with a special pore structure.

US-A-4 497 903 describes the incorporation of cobalt into the crystalline layers of

an aluminum silicate. The provided catalyst is active in the conversion of synthesis gas to liquid hydrocarbons, consisting mainly of branched hydrocarbons with a high octane number.

The above-mentioned patents deal with the stabilization of carriers used to produce catalysts for converting synthesis gas to hydrocarbons. Other patents in the previous subject area must also be mentioned.

US-A-4 013 590 describes the stabilization of oxide carriers, in particular alumina, by addition of organosilicon compounds, drying and thermal decomposition. The amounts of added silicon in relation to alumina correspond to 1 to 6 silicon atoms per square meters. The carriers provided have improved mechanical properties and are used in the automobile's afterburner.

European patent application EP-A-0 0 184 506 describes the production of catalyst supports consisting of alumina agglomerates, stabilized by the addition of silica in a water solution, drying and thermal decomposition, with improved mechanical properties, in particular with regard to the formation of fine particles during wear. Such carriers are used in hydrogen treatment, hydrocracking, hydrogenation and dehydration of hydrocarbons or other organic compounds.

US-A-5 045 519 describes a process for the production of silica-alumina which leads to a product of high purity and which is heat resistant.

It is produced by hydrolysis of an aluminum alkoxide and the simultaneous or subsequent addition of orthosilicic acid, which has been previously purified using ion exchange. The silica aluminas provided are used as a carrier for a desulphurisation catalyst, in DeNOx catalysis, for oxidation, in hydrocracking, in weak hydrocracking, for the car's exhaust gas catalysis, and in isomerisation.

The present invention relates to a method for synthesizing hydrocarbons from a mixture comprising carbon monoxide and hydrogen in the presence of a catalyst comprising at least one group VIII metal, preferably cobalt, placed on a special silica-alumina, where the silica-alumina in the method is produced by precipitation of defects and calcination at a temperature in the range from about 700°C to about 1200°C for at least 6 hours, so that said silica-alumina has an effective surface of less than 260 m<2>/g. The catalyst is preferably used in suspension in a liquid phase in a completely stirred autoclave of the type three-phase reactor or suspension bubble column. It is also suitable for use in a stationary layer.

The applicant has discovered that the use of a silica-alumina support, prepared by co-precipitating at least one soluble aluminum compound and at least one silicon compound, washing, drying and calcining at a high temperature for a time sufficient to promote interactions between the alumina and silica (Al-O-Si bonds), after impregnation with at least one group VIII metal, preferably cobalt, can provide a catalyst that is particularly active in a process for synthesizing hydrocarbons from a mixture comprising carbon monoxide and hydrogen. Furthermore, said catalyst has improved mechanical properties, in particular when, as is preferred, it is used in suspension in a liquid phase in a three-phase reactor, and it has better resistance to wear and tear.

The term "joint precipitation", as used by the Applicant, involves a method in which at least one aluminum compound, which is soluble in a neutral or acidic medium, as will be described below, and at least one silicon compound, as will be described below , are brought into contact, simultaneously or one after the other, in the presence of at least one precipitating and/or co-precipitating compound, so that a mixed phase is provided which essentially consists of hydrated alumina-silica, which may be homogenized by intense stirring, cutting , colloid milling or by a combination of these individual operations.

As will be described below, the applicant identifies the methods of absolute co-precipitation from solutions or through the reaction of an absolute solution and a submicron colloidal suspension, for example of silicic acid, in the presence of at least one inorganic precipitant, from sequential processes of co-precipitation, in which a first aluminum compound, which is soluble in an acidic medium, is precipitated in the form of a hydroxide, oxohydroxide or a hydroxycarbonate from a water-soluble salt or from an aluminum alkoxide or alcoholate, then mixed with a second silicon compound and, simultaneously or immediately afterwards, is added to at least one precipitant, as will be described below. The silicon compound itself is selected from the group consisting of silicic acid, Ludox in its ammonia-containing form, and quaternary ammonium silicates, as previously described in the European patent application, used pure or as a mixture. The two above-mentioned embodiments constitute the most well-known methods for co-precipitation.

A third method consists of preparing, by precipitation, at least one compound of aluminium, hydroxide, oxohydroxide or hydroxycarbonate, then mixing it with at least one silicon compound, such as a silicic acid in water solution or in the form of a submicron colloidal suspension or a hydrosol. The provided mixture is then homogenized to the micronic scale and preferably to the nanometric scale, by means of intense stirring, shearing, colloid grinding or by a combination of these individual operations, which is known to a person skilled in the art. This embodiment is adapted to sequential joint precipitation.

The silica-alumina used in the process according to the present invention is preferably a silica-alumina which is homogeneous according to the micrometric measurement unit, and in which the amount of anionic impurities (for example SO4<2>', Cl ) and cationic impurities (for example Na<*> ) is preferably less than 0.1% by weight, more preferably less than 0.05% by weight. The silica-alumina used in the method according to the invention is produced by co-precipitation, as stated above. As an example, the silica-alumina carrier, which is used in the method according to the invention, can be produced by absolute co-precipitation under controlled, stationary operating conditions (average pH, concentration, temperature, average residence time) by reacting a basic, silicon-containing solution, for example in the form of sodium silicate, optionally aluminium, for example in the form of sodium aluminate, with an acidic solution containing at least one aluminum salt, for example aluminum sulphate. At least one carbonate, or CO2, can optionally be added to the reaction agent. ;After co-precipitation, the carrier is provided by filtration and washing, optionally washing with an ammonia-containing solution to extract the residual sodium by means of ion exchange, drying and shaping, for example by spray drying, then calcination, preferably in air in a rotary kiln and at a high temperature , usually in the range of about 700°C to about 1200°C, for a time sufficient to induce the formation of interactions between the alumina and the silica, usually at least 6 hours. These mutual influences lead to a better mechanical strength in the carrier and thus in the catalyst used in the method according to the invention. A further method for producing the silica-alumina according to the invention consists in producing, from a water-soluble alkaline silicate, a solution of silicic acid, hereinafter referred to as orthosilicic acid (H2SiO4, H2O), de-cationized by means of ion exchange and then simultaneously added in a cationic aluminum salt in solution, for example the nitrate, and ammonia under controlled operating conditions; or to add the orthosilicic acid solution to the cationic aluminum salt in solution and co-precipitate the provided solution with ammonia under controlled operating conditions, resulting in a homogeneous product. C02 can optionally be added to the reaction agent. After filtering and washing, drying with shaping and calcination between about 700°C and about 1200°C for at least 6 hours, a silica-alumina carrier is provided which can be used in the method according to the invention. ;An additional method of preparing the silica-alumina according to the invention consists in precipitating the aluminum hydrate, as described above, washing it and then mixing it with dilute orthosilicic acid to provide a suspension, which is then intensively homogenized by intense stirring and shearing . An Ultraturrax turbine or a Staro turbine can be used, or a colloid mill, for example an IKA colloid mill. The homogeneous suspension is then spray dried, as described above, and calcined between about 700°C and about 1200°C for at least 6 hours. A silica-alumina which can be used in the method according to the invention is then provided. A preferred method, described in US-A-5,045,519, consists in preparing a decationized orthosilicic acid, as described above, and then simultaneously adding it to at least one C2 to C20 alkoxide, or in an aluminum trihexanoate, and in desalted water for to cause hydrolysis; or the solution of decationized orthosilicic acid can be supplied to the hydrolysis product from an aluminum oxide, such as an aluminum trihexanoate. After intense homogenization of the suspension by vigorous stirring or colloidal thickening, as described above, a possible adjustment of the solids content by filtration and then renewed homogenization, the product is dried and shaped, then calcined between about 700°C and about 1200°C for at least 6 hours . In all the described methods of manufacture, during any manufacturing step, it may be desirable to add a small proportion of a stabilizing element selected from the group consisting of lanthanum, praseodymium and neodymium. The stabilizing element is preferably added in the form of a soluble salt, for example a nitrate. Preferably, a soluble salt from at least one stabilizing element is added to the aqueous, cationic aluminum salt or, as described in US-A-5,045,519, simultaneously with or immediately after the aluminum compound is brought into contact with the orthosilicic acid in an aqueous medium, this the orthosilicic acid itself is added simultaneously with or immediately after the hydrolysis of at least one aluminum alkoxide. The alumina precursors used in the present invention thus differ from those mentioned in the previous field through the following properties: They are completely soluble in an aqueous medium: cationic aluminum salts, for example the nitrate, or in an organic medium, for example : aluminum hexanoate in a hexanol medium. ;They are soluble in an acidic medium, and consist of at least one alumina hydrogel and/or alcohol gel, such as aluminum hydroxides or oxohydroxides, for example aluminum hydrates, such as microcrystalline or amorphous trihydroxide, "pseudoboehmite", "boehmite", bayerite, hydrargillite, diaspore ; kneading of at least one aluminum hydroxycarbonate. The term "soluble in an acidic medium", as used by the Applicant, implies that bringing them into contact before the addition of any silicon, immediately after co-precipitation and washing, with an acid solution, for example sulfuric or nitric acid, result in complete dissolution or, if substantially crystalline, the formation of a submicron hydrosol of "boehmite" in which most of the alumina is clearly dissolved. This dissolving ability is a required characteristic property according to the invention, and is applicable to alumina hydrogels or alcohol gels before the addition of silicon. ;They are only formed after being brought into contact with at least one silica compound and possibly other metals, as will be described below. ;They differ from activated aluminas, such as gamma, delta, theta, eta, pure alpha aluminas or mixtures thereof, which are at best partially soluble in a neutral aqueous medium or preferably an acidic medium, as described within the above previously mentioned subject area, and which are produced before the addition of silicon, at which point they are supplemented with cobalt and used for synthesizing hydrocarbons from synthesis gas. Also, in this case, which does not form part of the present invention, the required property is minimal dissolution of the carrier. The carriers of silica-alumina, which are used in the present invention, preferably contain between 0.5% by weight and 30% by weight of silica, more preferably between 1% by weight and 20% by weight, and most preferably between 1.4% by weight and 15% by weight % silica, relative to the anhydrous product. They can also contain from 0.1% by weight to 5% by weight, preferably from 0.5% by weight to 2% by weight, of at least one NfeOa oxide with at least one M metal selected from the group consisting of lanthanum, praseodymium and neodymium. ;The carrier is preferably processed into a finely calibrated powder with a grain size of 800 microns (nm) or less, preferably in the range from 10 to 500 µm, more preferably in the range from 10 to 300 nm, most preferably in the range from 20 to 150 um, for optimal application in the presence of a liquid phase. The single drying-plus-shaping step is preferably performed by spray drying to provide substantially spherical microdroplets less than about 800 µm in size. ;After drying, the product is calcined, for example in air and in a convection oven at a temperature in the range from about 700°C to about 1200°C, preferably in the range from 800°C to 1100°C, and over a time which is sufficient to provide an effective surface of less than 260 m<2>/g, preferably less than 220 m<2>/g, more preferably to an effective surface in the range from 130 to 200 m<2>/g, and most preferably in the range from 130 to 190 m<2>/g. Said calcination step usually lasts at least 6 hours, preferably at least 10 hours, more preferably at least 15 hours. As an example, said silica-alumina can be calcined for 12 hours at 1050°C. ;It is also possible to initiate calcination at a low temperature, i.e. at a temperature in the range from 350°C to 550°C for at least 1 hour, preferably at least 3 hours, and then raise the temperature to a temperature in the range from about 700° C to approximately 1200°C. In a further embodiment, the support is first calcined at 550°C for three hours, and then treated in an air/H 2 O mixture at 800°C for 24 hours to provide the required effective surface. In a further variant, the support is in the form of spheres or extrudates with an equivalent diameter in the range from 2 to 10 mm, for use in a stationary bed. ;The catalyst support for use in the method according to the present invention has microhomogeneity (i.e. according to the micrometer scale), determined by means of microanalysis using a Castaing microprobe, so that the atomic ratio Si/Al, measured decentralized (locally) in several sections ) of the carrier particles, do not fluctuate by more than 20% around the mean value. The carriers which emerge from the catalysts used in the method of the invention preferably have a "nanometric" homogeneity, i.e. according to the nanometer scale. One method that can be used to characterize the supports, and in particular to determine their homogeneity, is the transmission electron microscope (TEM). With this in mind, an electron microscope (JEOL 2010 or Philips Tecnai20F, with possible scanning) equipped with an energy dispersive spectrometer (EDS) is used for X-ray analysis (for example Tracor or Edax). The EDS detector must enable light element detection. The combination of these two devices, TEM and EDS, combines image production and decentralized (local) chemical analysis with good spatial resolution. ;For this type of analysis, the samples are finely dry-crushed in a mortar; the powder is then incorporated into a resin to provide ultra-thin sections approximately 70 nanometers (NM) in thickness. These sections are captured on copper filters (-screens) coated with an amorphous, perforated carbon film and serve as a carrier. They are then brought into the microscope for observation and analysis under high vacuum. The image allows the sample areas to be easily distinguished from the resin areas. A specific number of analyses, a minimum of 10, preferably in the range from 15 to 30, are then carried out on different areas of the incorporated sample. The diameter of the electron beam to analyze the areas (approximately determine the size of the analyzed areas) is at most 50 mm, preferably 20 nm, more preferably 10, 5, 2 or 1 nm in diameter. In scan mode, the analyzed area will be a function of the size of the scanned area and not the size of the beam, which is usually reduced. Semi-quantitative processing (semi-quantitative analysis) of the X-ray spectrographs (-spectra) obtained using an EDS spectrometer can provide the relative concentration of Al and Si (in atomic %) and the Si/Al ratio for each of the analyzed areas. The mean ratio, Si/Alm, can be calculated, together with the standard deviation o for this target set. The method according to the present invention uses catalysts in which the relative standard deviation a (with regard to the value Si/Alm) is less than 30%, preferably 20%, for an overall Si/Al ratio that is preferably in the range from 0.1 to 10. This overall ratio can be measured using other techniques that are routinely used for this type of analysis (for example, X-ray fluorescence). ;The catalyst used in the method according to the invention consists of at least one element from group VIII (element selected from elements from groups 8,9 and 10 in the new periodic table) supported on a silica-alumina with the above-described properties, and produced by common precipitation. ;The element from group VIII in the periodic table is preferably chosen from iron, cobalt and ruthenium. More preferably, the Group VIII metal is cobalt. One technique for producing the catalyst, which is particularly applicable for carrying out the method according to the invention, is the impregnation of an aqueous solution of a precursor to the metal from group VIII in the periodic table, preferably cobalt, for example an aqueous solution of salts such as cobalt nitrates. The weight content of the Group VIII metal, based on the total catalyst weight, is usually in the range from 0.1% to 50%, preferably in the range from 1% to 30%. ;The catalyst can also contain at least one additional element, for example an activator, for example at least one element selected from the group consisting of ruthenium, molybdenum and tantalum, or reducing agents (reducibility promoters), such as platinum, palladium or ruthenium. The weight amount of additional raw material, based on the total catalyst weight, is usually in the range of 0.01% to 5%. These additional elements are preferably added at the same time as the group VIII metal or, in a preferred variant, in at least one subsequent step. In a particular embodiment of the invention, the catalyst contains both cobalt and ruthenium. In a further particular embodiment of the invention, the catalyst contains cobalt and tantalum. ;The mechanical strength of the catalyst according to the invention is improved in the case of a catalyst comprising a carrier consisting exclusively of alumina or silica, or titanium dioxide. The mechanical strength of the catalyst according to the invention is determined by measuring the change in particle size at the end of a specific test period using a three-phase reactor. The thus produced catalysts result in particularly stable performances in Fischer-Tropsch synthesis and in a conversion of synthesis gas into a mixture of linear and saturated hydrocarbons containing at least 50% by weight C5+ hydrocarbons and less than 20% methane based on the total of produced hydrocarbons. The following conditions are usually used for said catalysts when synthesizing hydrocarbons: The catalyst comprising at least one group VIII metal impregnated on the silica-alumina support, as described above, is dried and then calcined. The catalyst is then pre-reduced using at least one reducing compound, for example selected from the group consisting of hydrogen, carbon monoxide and formic acid, optionally mixed with an inert gas, for example nitrogen, in a reducing compound/(reducing compound + inert gas) mof ratio in the range from 0.001:1 to 1:1. The reduction can be carried out in the gas phase at a temperature in the range from 100°C to 600°C, preferably in the range from 150°C to 400°C, at a pressure in the range from 0.1 to 10 MPa and at a space velocity per hour in the range from 100 to 40,000 mixing volumes per catalyst volume per hour. This reduction can also be carried out in the liquid phase, under the same operating conditions as in the gas phase, the catalyst being then suspended in an inert liquid phase (also known as the solvent), for example a paraffin fraction comprising at least one hydrocarbon containing at least 5, preferably at least 10, carbon atoms per molecule. When the catalyst is used in a three-phase reactor, it may be advantageous to use, which is preferable, the same inert solvent used during the reaction. Highly preferred, a paraffin fraction from the Fischer-Tropsch process is used, for example a fraction of kerosene or gas oil. This reduction is preferably carried out in situ (on site), i.e. in the reactor which is then used to carry out the Fischer-Tropsch synthesis. The catalyst used in the method according to the invention can also be reduced ex situ or at an external facility, i.e. not in a Fischer-Tropsch synthesis reactor, or the process can even be carried out outside the industrial area. The reduction can then possibly be carried out by a company that is used to carrying out processing at external facilities. ;In such a case, the catalyst is reduced under the operating conditions described above. After reducing and cooling the reduced catalyst to at least 100°C, the catalyst is preferably mixed, in an amount of from 10% to 80% by weight at ambient air temperature, with solid paraffin waxes which are diluted to liquefy the waxes. Paraffin wax from a Fischer-Tropsch process is preferably used. After mixing, the resulting suspension is drop coagulated by throwing it against a belt collector, followed by cooling. The product provided is in corriform with an equivalent diameter (the diameter of the sphere with an equivalent volume) ranging from about 5 to about 20 mm in diameter. These catalyst grains can be loaded directly into the Fischer-Tropsch reactor. The conversion of synthesis gas to hydrocarbons is then carried out under a total pressure which is usually in the range from 0.1 to 15 MPa, preferably in the range from 1 to 10 MPa; the temperature is generally in the range from 150°C to 350°C, preferably in the range from 170°C to 300°C. The space velocity per hour is normally in the range from 100 to 20,000 synthesis gas volumes per catalyst volume per hour (f<1>), preferably in the range from 200 to 10,000 f<1>, more preferably in the range from 400 to 5,000 f<1>, and the H2/CO ratio in the synthesis gas is normally in the range from 1:2 tii 5 :1, preferably in the range from 1.2:1 to 2.5:1. The catalyst can be used in the form of a finely calibrated powder with a grain size of less than 800 microns (µm), preferably in the range from 10 to 500 µm, more preferably in the range from 10 to 300 µm, highly preferred in the range from 20 to 150 µm, and even more preferably in the range from 20 to 120 µm, where it is used in suspension in a liquid phase. It can also be used in the form of particles with an equivalent diameter in the range from approximately 2 to 10 mm, preferably in the range from 3 to 8 mm, when used in a stationary layer. The method according to the present invention can be used with said catalyst deposited in a stationary layer. In such a method, the reaction takes place in the gas phase. The mechanical strength of the catalyst described above is often sufficiently high that it can be handled and loaded into such a reactor without danger of disintegration. The method according to the invention can also, which is preferred, be carried out in a three-phase reactor, in which the catalyst is in suspension in an inert liquid phase (solvent). As an example, a fully stirred reactor can be used, such as an autoclave or a three-phase bubble column type reactor (also known as a suspension bubbler). ;The catalyst can be advantageously used in a three-phase reactor, preferably in a suspension bubble reactor, as this type of operation enables: optimal use of the catalyst's performance (activity and selectivity), by limiting intragranular diffusion phenomena; substantial limitation of heat effects in the catalyst comet, which is surrounded by a liquid phase. ;This embodiment requires that the catalyst and the reaction products be split off. Under these conditions, the catalyst used in the method according to the invention has improved mechanical properties, which leads to optimal separation of the catalyst and the products, and an improved service life. Said catalyst has an improved wear resistance, i.e. thus a substantial reduction in the quantity of the finest sieve fraction which is formed when a three-phase reactor is used. One possible explanation for this improvement is the presence of more significant and greater number of interactions between the alumina and the silica in the co-precipitated silica alumina. In summary, the invention relates to a method for synthesizing hydrocarbons from a mixture comprising carbon monoxide and hydrogen in the presence of a catalyst, which comprises at least one group VIII metal placed on a silica-alumina, which is produced by co-precipitation and calcination at a temperature in the range from about 700°C to about 1200°C for at least 6 hours, so that said silica-alumina has an effective surface of less than 260 m<2>/g. In a preferred embodiment, the silica-alumina is calcined at a temperature in the range from 700°C to 1200°C for at least 10 hours. In a further preferred embodiment, the silica-alumina is initially calcined at a temperature in the range of about 350°C to about 550°C for at least 1 hour, then at a temperature in the range of about 700°C to about 1200°C in at least 6 hours. The silica-alumina is preferably homogeneous according to micrometric units of measurement, and more preferably the amount of anionic and cationic impurities is less than 0.1% by weight. The silica-alumina preferably contains 0.5% by weight to 30% by weight of silica based on the anhydrous product, and the group VIII metal content is in the range from 0.1% by weight to 50% by weight. The Group VIII metal is preferably cobalt. The catalyst for the method according to the invention may optionally also contain at least one additional element selected from the group consisting of: ruthenium, molybdenum, tantalum, platinum and palladium. It may also contain from 0.1% by weight to 5% by weight of at least one M 2 O 3 oxide from at least one M metal selected from the group consisting of lanthanum, praseodymium and neodymium. The catalyst is preferably used in suspension in a liquid phase, in a three-phase reactor, usually in the form of a powder with a grain size of less than 800 microns. Said catalyst can, however, be used in a stationary layer in the form of particles with an equivalent diameter in the range from 2 to 10 mm. The following examples illustrate the invention. ;EXAMPLE 1 (comparative): Catalyst A ;Catalyst A, Co/SiO 2 -Al 2 O 3 ) was prepared by impregnating cobalt nitrate in a powder of silica-alumina. The silica-alumina was first produced by joint precipitation of sodium silicate, sodium aluminate, aluminum sulphate and sulfuric acid, thus providing a final composition with SiO2/Al2O3 = 5/95, and an effective surface of 220 m<2>/g after 6 hours of calcination at 600°C. The provided suspension was spray dried and the provided carrier was in the form of a powder with a particle size in the range of 20 to 150 microns. The catalyst from the impregnation step was dried and calcined at 400°C for 2 hours. The content of cobalt metal was 13% by weight. ;EXAMPLE 2 (in accordance with the invention): Catalyst B ;Catalyst B, Co/SiO 2 -AI 2 O 3, was prepared by impregnating cobalt nitrate in a silica-alumina, prepared by the precipitation of fats from a mixture of silicic acid H 2 S 1 O 4 and aluminum nitrate, in which ammonia was been added. After spray drying, the provided carrier was in the form of 40 to 120 micron microspheres. After calcination at 700°C for 6 hours, the silica-alumina composition was SiO 2 /Al 2 O 3 10/95 and the effective surface was 170 m<2>/g. The catalyst from the impregnation step was dried and calcined at 400°C for 2 hours. The content of cobalt metal was 12.5% by weight. ;EXAMPLE 3 (in accordance with the invention): Catalyst C ;Catalyst C, Co/SiO 2 -Al 2 O 3 , was prepared by impregnating cobalt nitrate in a silica-alumina of the type Siralox 5 (sold by Condea), prepared by adding silicic acid in a the hydrolysis aluminum alkoxide. The support provided was calcined at 100°C for 12 hours, its effective surface was 150 m<2>/g and its weight content of SiO2 was 5%. The infrared spectrum of the hydroxyls, provided after pelletizing the support alone and vacuum treatment at 500 °C, showed the presence of a band at 3750 cm"<1> related to SiOH, where there was added a large peak between 3750 and 3725 cm<*1> related to AI-OH, with a lower intensity.

The catalyst from the impregnation step was dried and calcined at 400°C for 2 hours. The content of cobalt metal was 12.5% by weight.

EXAMPLE 4 (in accordance with the invention): Catalyst D

A catalyst D was prepared in the same way as catalyst C; lanthanum nitrate was added at the same time as the silicic acid to provide a catalyst containing 12 wt% cobalt on a support consisting of 2 wt% lanthanum oxide, 5 wt% silica and 93 wt% alumina. Its effective surface area was 145 m<2>/g after calcination for 2 hours at 400°C.

EXAMPLE 5 (comparative!: Catalyst E

A catalyst E, Co/AfeOa, was prepared by impregnating cobalt nitrate in a Puralox Scca 5-170 alumina powder with an effective surface area of 180 m<2>/g. This carrier was in the form of microspheres with a particle size ranging from 20 to 150 microns. The catalyst from the impregnation step was dried and calcined at 400°C for 2 hours. The final cobalt metal content was 12.5% by weight.

EXAMPLE 6 (comparative): Catalyst F

A catalyst F was prepared using the following steps in consecutive order: 1. Impregnation of TEOS (tetraethoxysilane) in Purlaox Scca 5-170 microspheres with an effective surface of 180 m2/g, using the method described by B. BEGUIN, E. GARBOWSKI and M. PRIMET in "Journal of Catalysis", page 595, volume 127, 1991; 2. Calcination at 500°C for 2 hours; 3. Impregnation with cobalt nitrate, drying and calcination at 400°C for 2 hours. The infrared spectrum of the hydroxyls, obtained after pelleting it

modified the support alone and vacuum treatment at 500°C, showed only the presence of a hydroxyl band at 3745 cm'<1> related to SiOH;

the alumina hydroxyl bands at 3760, 3730 and 3580 cm'<1> had disappeared. Catalyst F contained 13% by weight of cobalt and 3% by weight of silica on a carrier consisting of alumina.

EXAMPLE 7 (comparative): Catalyst G

Catalyst G, C0/S1O2-Al2O3, was prepared by impregnating cobalt nitrate in a silica-alumina powder. The silica-alumina was prepared by adding silicic acid to an alumina gel obtained by hydrolyzing an aluminum alkoxide. The provided support was calcined at 500°C for 4 hours; its effective surface was 410 m<2>/g and it comprised 20 wt% silica and 80 wt% alumina. The catalyst, which was provided after impregnation with the cobalt nitrate, was dried and then calcined at 400°C for 2 hours. The cobalt content of the catalyst was 13% by weight.

EXAMPLE 7 bis (comparative): Catalyst H

A catalyst H, Co/SiO 2 -Al 2 O 3 , was prepared by impregnating cobalt nitrate in a calibrated powder of silica-alumina. The silica-alumina powder was obtained by precipitating hydrargillite in an alkaline medium, washing it in the presence of ammonia until intense dealkalization, then intensive mixing by flowing through an IKA colloid mill in the presence of CO2 with a solution of orthosilicic acid, which itself

was provided by decationization of an alkaline silicate solution, to provide a homogeneous suspension, which was then spray dried and then calcined at 1100°C for 6 hours. A silica-alumina with one effective surface of 170 m<2>/g was provided. The support was then impregnated with cobalt nitrate, dried at 120°C for 6 hours and calcined at 400°C in nitrogen for 3 hours. Catalyst H contains 12.8% by weight of cobalt and 7% by weight of silica on a carrier consisting of silica-alumina.

EXAMPLE 7 ter: Comparative dissolution tests

10.2 grams of alumina AI2O3 in the form of the hydrate AI(OH)3 with a hydrargillite structure prepared in accordance with Example 7 bis was brought into contact with 60 milliliters of an aqueous solution of sulfuric acid containing 80 grams of pure acid for 2 hours at 80 °C, then 110°C. After 2 hours, the aluminum hydrate had completely dissolved and formed a clear solution of aluminum sulfate. Separately, 10.2 grams of alumina Al 2 O 3 in the form of Puralox Scca-5-170, used in Comparative Example 6, was contacted with 60 milliliters of an aqueous solution of sulfuric acid containing 80 grams of pure acid for 2 hours at 80°C, then 110° C. After 2 hours, the excess liquid was filtered. The total amount of aluminum in the squeezed filtrate, based on the alumina, was less than 0.5 grams. It then turns out that under operating conditions, where the activated alumina used in comparative example 6 was partially soluble, the alumina hydrates or gels referred to in the present invention were completely soluble.

EXAMPLE 8: Catalytic tests in a reactor with a stationary bed

Catalysts B to F, whose preparations were described in Examples 2 to 6, were tested in a stationary bed with gas phase in a unit with continuous operation and operating with 20 cm<3> catalyst. Initially, the catalysts were reduced in situ at 350°C for 12 hours in a mixture of hydrogen and nitrogen, containing 30% hydrogen, then for 12 hours in pure hydrogen. The conditions of the catalyst tests were as follows: • T,°C = 220°C; • Pressure = 2MPa;

Room speed per hour (HSV) = 15001'<1>;

Ha/CO mole ratio = 2/1

The results in Table 1 show that the selectivity in the method according to the invention, in the presence of catalyst B or C placed on an amorphous silica-alumina, is improved with respect to heavy products in relation to a substantially equivalent conversion.

EXAMPLE 9: Catalytic tests in a three-phase reactor

Catalysts B to G, described in Examples 2 to 8 above, were tested in a fully stirred three-phase reactor with continuous operation and operating at a concentration of 10% (molar) catalyst in suspension. The conditions of the catalyst tests were as follows: • T, °C = 230°C; • Pressure = 2 MPa; • Space velocity per hour (HSV) » 10001"<1>;

• H2/CO mole ratio = 2/1

The results in Table 2 show that the catalysts according to the invention (B to D) have improved the conversion and selectivity with respect to heavy products in relation to the comparative catalysts E, F and Q.

After 500 hours of testing, the mechanical strength of the catalysts A to H was assessed by measuring the particle size of the catalysts obtained after splitting off the reaction products. Table 3 below shows the % of catalyst particles less than 20 microns in size that were formed during the testing of catalysts A through H.

The mechanical strength of the catalysts used in the process according to the invention (B to D and H) was significantly higher compared to the catalysts A, E, F and G.

Claims (20)

1. A method for synthesizing hydrocarbons from a mixture comprising carbon monoxide and hydrogen in the presence of a catalyst comprising at least one group VIII metal placed on a silica-alumina, characterized in that the silica-alumina in the method is produced by joint precipitation and calcination at a temperature in the range from about 700°C to about 1200°C for at least 6 hours, so that said silica-alumina has an effective surface of less than 260 m<2>/g. 2. Method according to claim 1, characterized in that: the carrier of silica-alumina therein is produced by bringing at least two solutions into contact with each other under controlled operating conditions and common precipitation, drying with shaping, then calcination. 3. Method according to claim 1, characterized in that the carrier of silica-alumina therein is produced through precipitation from an alumina hydrogel, in which the mixed silica-alumina hydrogel is then provided by adding a solution of silicic acid, intense homogenization by vigorous stirring, cutting, colloidal milling, drying with shaping , and then calcination. 4. Method according to claim 1, characterized in that, in a first step, a water solution of silicic acid is prepared by decationization of a water-soluble alkaline silicate, in which this solution is then brought into contact with a water solution containing at least one cationic aluminum salt, and in which the provided water solution is precipitated simultaneously with ammonia under controlled operating conditions. 5. Method according to claim 1, characterized in that, in a first step, a water solution of silicic acid is prepared by decationization of a water-soluble alkaline silicate, in which this solution is then simultaneously brought into contact with an anhydrous solution of at least one aluminum alkoxide and with water, under controlled operating conditions, in order to hydrolyzing said alkoxide by simultaneously incorporating the silica therein, wherein the aqueous suspension provided is then homogenized by intense stirring, then dried and shaped by spray drying. 6. Method according to claim 1, characterized in that, in a first step, a water solution of silicic acid is prepared by decationization of a water-soluble alkaline silicate, in which this solution is then brought into contact with the hydrolysis product from at least one aluminum alkoxide, and in which the provided aqueous suspension is homogenized by intense stirring, then dried and shaped by spray drying. 7. Method according to any one of claims 3 to 6, characterized in that the homogenization therein is carried out by ultra-grinding in a colloid mill. 8. Method according to any one of claims 1 to 7, characterized in that the silica-alumina therein is calcined at a temperature in the range from 700°C to 1200°C for at least 10 hours. 9. Method according to any one of claims 1 to 8, characterized in that the silica-alumina therein is initially calcined at a temperature in the range from 350°C to 550°C for at least 1 hour, then at a temperature in the range from approximately 700°C to approximately 1200°C for at least 6 hours. 10. Method according to any one of claims 1 to 9, characterized in that the silica-alumina therein is homogeneous according to the micrometer scale. 11. Method according to any one of claims 1 to 10, characterized in that the silica-alumina therein is homogeneous according to the nanometer scale. 12. Method according to any one of claims 1 to 11, characterized in that the amount of anionic and cationic impurities therein is less than 0.1% by weight. 13. Method according to any one of claims 1 to 12, characterized in that the silica-alumina therein contains 0.5% by weight to 30% by weight of silica based on the anhydrous product. 14. Method according to any one of claims 1 to 13, characterized in that the content of the group VIII metal therein lies in the range from 0.1% by weight to 50% by weight. 15. Method according to any one of claims 1 to 14, characterized in that the group Wild metal therein is cobalt. 16. Method according to any one of claims 1 to 15, characterized in that the catalyst therein contains at least one additional element selected from the group consisting of: ruthenium, molybdenum, tantalum, platinum and palladium. 17. Method according to any one of claims 1 to 16, characterized in that the catalyst further contains from 0.1% by weight to 5% by weight of at least one M2O3 oxide of at least one M metal selected from the group consisting of lanthanum, praseodymium and neodymium. 18. Method according to any one of claims 1 to 17, characterized in that the catalyst therein is used in suspension in a liquid phase, in a three-phase reactor. 19. Method according to claim 18, characterized in that said catalyst therein is in the form of a powder with a particle size of less than 800 microns. 20. Method according to any one of claims 1 to 17, characterized in that the catalyst therein is used in a stationary layer in the form of particles with an equivalent diameter in the range from 2 to 10 mm.