Method for preparing catalyst supports

Abstract

A method for preparing a catalyst support comprising the steps of: (i) preparing an aqueous slurry of titania precursors, (ii) spray-granulating the aqueous slurry in a fluidised bed spray granulator

Description

Field This invention relates to a method for preparing catalyst supports and in particular to titania microsphere catalysts supports, useful for preparing cobalt-containing Fischer-Tropsch catalyst precursors and catalysts. Background Fischer-Tropsch catalysts having a nominal diameter less than about 1 mm offer the possibility of high reaction rates and improved selectivity to C5+ hydrocarbons. Such catalysts are usually associated with use in a suspended state, such as in a slurry bubble column reactor, because their use in a fixed bed at a commercial scale would normally be rendered impractical by an unacceptably high pressure drop. WO2012 / 146903 A1 discloses a Fischer-Tropsch process using a fixed bed of small particle catalyst wherein the catalyst is disposed within a plurality of catalyst containers placed within externally-cooled reaction tubes. The catalyst particles may have a diameter of from about 100 pm to about 1 mm. Titania microspheres provide an ideal support for fixed-bed cobalt Fischer-Tropsch catalysts because they may be regular in shape, essentially free of poisons and can have a large median pore diameter, facilitating growth of long hydrocarbon chains on small well dispersed cobalt crystallites. However, preparation of titania microspheres can introduce undesirable catalyst poisons or produce particles with insufficient strength or porosity. For example, titania microspheres prepared from titania particles created by flame hydrolysis of titanium tetrachloride provides microspheres with a high chloride content that creates a risk of stress corrosion cracking of austenitic stainless steels typically used in catalyst manufacturing equipment. Furthermore, uniform particle size is highly desirable to ensure adequate flow of reactants and products from the catalyst when disposed as a catalyst bed. Fluidised bed spray granulation is useful for preparing small particles. Fluidized bed spray granulation is a technique of particle agglomeration brought about by the spray addition of a coating liquid or suspension onto a bed of powder particles that is transformed to a fluid-like state by the passage of air or another gas through it. Fluidized bed spray granulation is described, for example, in “Fluidised bed spray granulation: Investigation of the coating process on a single sphere", in Chemical Engineering and Processing: Process Intensification, vol. 36, issue 6, p 443-457, Dec 1997. An essential difference between spray drying and fluidised bed spray granulation lies in the way the particles are formed: in spray drying, a droplet containing fine particles loses water by evaporation and shrinks to form an agglomerate, whereas in spray granulation growing particles are sequentially coated with layers of a slurry that are dried to form a layered structure. WO2017042611 (A1) discloses a method of forming a plurality of porous ceramic particles including forming the plurality of porous ceramic particles using a fluidised bed spray granulation process conducted in a batch mode. The method described is distinct from continuous spray fluidization processes, which generally include continuous separation and recycle of undersized and / or oversized material. WO2018169753 (A1) discloses a batch method of forming a catalyst carrier by steps of: forming a porous ceramic particle using a spray granulation process comprising a first batch spray fluidization forming cycle; and sintering the porous ceramic particle at a temperature of at least about 350°C not greater than about 1400°C, wherein the first batch spray fluidization forming cycle comprises repeatedly dispensing finely dispersed droplets of a first coating fluid onto air borne porous ceramic particles, wherein the ceramic particles comprise a core region composition, wherein the first coating fluid comprises a first coating material composition; and wherein the first coating material composition is different from the core region composition. Continuous spray granulation processes are also known. In these methods, a solution, suspension or melt is continuously applied to core particles in the fluidized bed so that the granulate grows layer by layer like an onion. This process is often used in a continuous fluidized bed system with air-classifying discharge. Summary of Invention A problem with continuous fluidised bed spray granulation methods is that they can produce fine particulate or undersized material that is required to be separated from the product and discarded or reprocessed. This has been found to be a particular problem in preparing titania microspheres, where the recycle of fines is hindered by the properties of the recovered particles. Therefore, reducing the quantity of fines and so increasing the efficiency of the spray granulation method for titania microspheres is highly desirable. The Applicants have found that using different particle size titania precursors in a fluidised bed spray granulation method to prepare titania microspheres, generates high purity microspheres of suitable size with desirable textural properties in high yield with reduced waste. The method produces titania microspheres suitable for use as catalyst supports. The present specification provides a method for preparing a catalyst support comprising the steps of: (i) preparing an aqueous slurry of titania precursors, (ii) spray-granulating the aqueous slurry in a fluidised bed spray granulator to form titania microspheres and fine particles, (iii) separating the fine particles from the titania microspheres and recycling them to the fluidised bed spray granulator, and (iv) recovering and calcining the titania microspheres, wherein the titania precursors comprise a first titania precursor and a second titania precursor and the volume-median diameter of the first titania precursor is less than the volume-median diameter of the second titania precursor. The method has been surprisingly found to produce lower amounts ofundersized or oversized particles compared to methods using a single titania precursor. Accordingly, the overall efficiency of the process may be improved without harming the quality of the titania microspheres. The invention further provides a titania microsphere catalyst support obtainable by the method. The invention further provides a method for making a catalyst precursor and a catalyst using a titania microsphere catalyst support obtained by the method. The catalyst support, catalyst precursor and catalyst are all particularly suitable for use in a Fischer-Tropsch process to synthesise hydrocarbons from a synthesis gas comprising hydrogen and carbon monoxide. Brief description of the drawings The invention will further be described by reference to the accompanying drawings, in which: Figure 1 is a flowsheet depicting an embodiment of a spray granulation method according to the invention; and Figure 2 is an optical micrograph image of a titania microsphere catalyst support prepared by the method depicted in Figure 1. Detailed Description As indicated in the summary section, the present specification provides a method for preparing a catalyst support comprising the steps of: (i) preparing an aqueous slurry of titania precursors, (ii) spray-granulating the aqueous slurry in a fluidised bed spray granulator to form titania microspheres and fine particles, (iii) separating the fine particles from the titania microspheres and recycling them to the fluidised bed spray granulator, and (iv) recovering and calcining the titania microspheres, wherein the titania precursors comprise a first titania precursor and a second titania precursor and the volume-median diameter of the first titania precursor is less than the volume-median diameter of the second titania precursor. The method has been surprisingly found to produce lower amounts of undersized or oversized particles compared to methods using a single titania precursor. Accordingly, the overall efficiency of the process may be improved without harming the quality of the titania microspheres. By “titania” we mean titanium dioxide, TiO2. The first and second titania precursors are both formed of titania but have different volume-median diameters. The term volume-median diameter D[v,0.5], sometimes given as D50 or DO.5, is the diameter where 50% of the distribution is above and 50% is below. It is defined by Dr Alan Rawle in the paper "Basic Principles of Particle Size Analysis” available from Malvern Instruments Ltd, Malvern, UK (www.malvern.co.uk), and is calculated from the particle size analysis which may conveniently be effected by laser diffraction for example using a Malvern Mastersizer™. Particle sizes are suitably determined according to ASTM D4464. The volume-median diameter, D[v,0.5], of the titania microsphere catalyst support obtainable by the method may be in the range 100 to 1000 pm (microns) but is preferably in the range 200 to 900 pm, more preferably 300 to 750 pm, most preferably 300 to 550 pm. The titania microspheres preferably have a sphericity (qi), of at least about 0.90, more preferably at least about 0.95. The spherical nature of the particles ensures that they flow readily when being manipulated and pack consistently in the catalyst bed to provide an acceptable pressure drop over the reactor in which they are placed. The volume-median diameter of the first titania precursor is less than the volume-median diameter of the second titania precursor. The difference in volume-median diameter between the titania precursors may be greater than 5 pm, preferably greater than 10 pm, more preferably greater than 15 pm, most preferably greater than 20 pm. The first titania precursor may be a powder formed by flame hydrolysis of titanium compounds, such as titanium tetrachloride (TiCk). Flame hydrolysis of titanium compounds is known and described, for example in US5451390. The volume-median diameter, D[v,0.5], of the first titania precursor may be 10 pm or less, and is preferably in the range 2 to 8 pm. The first titania precursor may comprise rutile as well as anatase and may have a rutile content in the range of 2 to 20% by weight, suitably 8 to 16% by weight. Such materials are commercially available, such as Evonik P25™. The second titania precursor may be a powder prepared by precipitation from aqueous titanium solutions, such as aqueous metatitanic acid or titanylsulphate solutions. Suitable precipitation methods are disclosed in WO2017 / 211710 A1. The second titania precursor may consist essentially of anatase. The volume-median diameter, D[v,0.5], of the second titania precursor may be 30 to 80 pm, and is preferably in the range 35 to 50 pm. Such materials are commercially available, such as Venator Hombicat FT™. The difference in particle size of the two titania precursors has been found surprisingly to improve both the efficiency and quality of the titania microsphere product, and also surprisingly provides improved catalyst performance. The method comprises preparing an aqueous slurry of the first and second titania precursors. The slurry may contain other liquids in addition to water, such as alcohols or glycols. The total titania precursor content of the slurry may be in the range of 10 to 30% by weight. Both titania precursors are typically in the form of powders that may be mixed with water using conventional mixing equipment to form the slurry. The relative amounts of the first titania precursor and second titania precursor in the slurry may be varied to achieve the desired properties. The first titania precursor may be present in an amount in the range of 50 to 99% by weight, preferably 75 to 99% of the combined first and second titania precursors. The slurry is fed to the inlet of a fluidised bed spray granulator. The process is operated continuously, i.e. with continuous feeding of the titania precursors, continuous recovery of the titania microspheres and fine particles, and simultaneous recycling of the fine particles to the spray granulator. Hence, unlike the aforesaid WO2017042611 (A1) and WO2018169753 (A1), there is no separate batch formation of cores and coated cores in the method. The fluidised bed spray granulator may be of any design suitable for continuous spray granulation. Normal operating conditions for continuous fluidised spray granulation may be used on the current process. Suitable spray granulation equipment is available from several manufacturers, including Glatt GmbH, Germany. The product mixture recovered from the spray granulator is a mixture of spherical titania particles of different sizes. The product powder therefore is subjected to a stage in which the titania microspheres of the desired particle size range are separated from the undersized or fine particles. The separation or classification of the titania microspheres may be achieved using conventional separation equipment, such as by sieving or by cyclone separation, or by other techniques that utilise the different size, shape or density the particles. A preferred method for separating the fine particles from the spray granulate product mixture is by sieving. The method provides a product cut of titania microspheres and an amount of undersized or fine particles. The method may also produce oversized particles. The product cut may have a particle size in the range of from 50 pm to 1250 pm or in the range of from 150 pm to 1150 pm, or in the range of from 200 pm to 1000 pm, or in the range of from 200 pm to 750 pm. In a preferred embodiment, the product cut has a particle size in the range of from 300 pm to 500 pm. The fines particles have a particle size below the product cut ranges and the oversize particles have a particle size above the product cut ranges. Fine particles are separated from the spray granulated product mixture and recycled to the spray granulator. The amount of material separated and recycled therefore depends on the selection of the desired titanium microsphere particle size. In a preferred method, a first portion of the fine particles is recycled to the fluidised bed spray granulator directly. Direct addition of the first portion of the fine particles to the spray granulator may be achieved using conventional spray granulating equipment and methods We have found that the first portion preferably does not include very fine particles, e.g. particles with a particle size below 50 pm or below 100 pm or below 200 pm. The proportion of the fine particles recycled directly may be in the range or 10 to 50% by weight of the recovered fine particles. In a preferred method, a second portion of the fine particles is recycled to the fluidised bed spray granulator by redispersing them as an aqueous slurry. The aqueous slurry containing the second portion of the fine particles, which may be termed the second aqueous slurry, may be recycled to the spray granulator itself or it may be combined with the aqueous slurry of the first and second titania precursors, i.e. the first aqueous slurry, fed to the spray granulator. The second portion of the fine particles preferably consists of very fine particles separated from the spray granulate product mixture, e.g. having a particle size below 50 pm or below 100 pm or below 200 pm. The second portion of the fine particles is preferably all of the recovered fine particles of this particle size. In addition to recycling the second portion of the fine particles in this way, at least a portion of the oversized particles may be recycled to the fluidised bed spray granulator by redispersing them to form an aqueous slurry. All of the oversized particles may be recycled to maximise the yield of on-size titania microspheres. The second portion of fine particles and the oversized particles may be redispersed separately or may be combined and redispersed together to form the second aqueous slurry before feeding the second aqueous slurry to the spray granulator or combining the second aqueous slurry with the first aqueous slurry of the first and second titania precursors. In a preferred embodiment, the second aqueous slurry comprising the second portion of fine particles and optionally the oversized particles is combined with the first aqueous slurry of the first and second titania precursors. The second portion of the fine particles, and the oversized particles if present, may be redispersed using known mixing equipment, e.g. using a high shear mixer. The proportion of re-dispersed recycled particles in the slurry is preferably less than or equal to 30% by weight on the total solids, more preferably less than or equal to 15% by weight on total solids. Too high an amount and the efficiency with which titania microspheres of the desired particle size range are formed is reduced. It is desirable to include one or more binders in the slurry to improve the cohesion of the particles prepared by the method prior to the calcination step. The binder may be any suitable binder used in catalyst support preparation. Suitable binders include organic binders, such as polyvinylalcohol, and inorganic binders, such as clays, alumina trihydrate and boehmite. Organic binders are preferred because they may be removed by a subsequent calcination step without leaving residues that might affect the performance of the catalysts. Binders may be included in the slurry in amounts in the range 1 to 15% by dry weight. The Applicants have found polyvinyl alcohol binders to be particularly useful in the present method at levels between 2 and 10% by weight (on a dry basis) in the slurry. Such binders are commercially available. A suitable polyvinylalcohol binder is Kuraray Poval™ 20-98 LA. If desired, one or more organic or inorganic pore structure modifiers may also be included in the slurry in addition to the titania precursors. If desired, one or more support modifiers may be applied to the titania microspheres, such as one or more compounds of metals selected from the group consisting of zirconium, lanthanum, cerium, yttrium and neodymium. The metal compounds may be metal nitrates and / or acetates. Upon calcination, the metal compounds are converted into their corresponding refractory oxides. Desirably such modification is performed by impregnation of the titania microspheres with the metal compound in aqueous solution, followed by drying and calcining. The total refractory oxide content of the modified titania microsphere catalyst support, after calcination, may be in the range of 0.1 to 15% by weight. Such modified titania supports are described in WO2022053773 A1. The recovered titania microspheres form a free-flowing powder but contains residual moisture in the pores. To produce the titania microsphere catalyst support, the recovered material is subjected to a heat treatment, which may be termed a calcination. The calcination may also convert any metal compounds of modifiers added to the precursors or microspheres upstream into their oxidic form. The heat treatment may be performed conventionally in air or by heating the recovered material under nitrogen or another suitable gas mixture or by heating under vacuum. The heat treatment may be performed in a static oven such as a tray calciner, or a moving bed oven, preferably a rotary calciner or a fluidized bed calciner. The calcination temperature may be in the temperature range 350 to 900°C, preferably 350 to 750°C, more preferably 400 to 500°C. Too low and the organic binder may not be adequately decomposed, too high and textural properties such as BET surface area and pore volume may be adversely affected. Anatase is converted to rutile at temperatures above 600°C. The catalyst support is preferably low in elements that have been found to be poisons for the Fischer-Tropsch catalyst, such as sulphur and the alkali metals. Accordingly, the sulphur content of the catalyst support is desirably less than 30 ppmw, more preferably less than 25 ppmw, and the alkali metal (e.g. Na or K) content of the modified titania catalyst support is desirably each less than 50 ppmw. Furthermore, to avoid corrosion of catalyst manufacturing and handling equipment, the chloride content of the catalyst support is preferably less than 650 ppmw, more preferably less than 600 ppmw. The pore volume of the titania microsphere support may be in the range 0.20 to 0.60 cm3 / g, preferably 0.30 to 0.50 cm3 / g, more preferably 0.35 to 0.45 cm3 / g. The average pore diameter of the titania microsphere support is preferably in the range 30 to 60 nm, more preferably 40 to 50 nm. The BET surface area of the titania microsphere support may be in the range 25 to 75 m2 / g, preferably 45 to 55 m2 / g. BET surface area and nitrogen pore volume may be measured using nitrogen physisorption techniques. The BET Surface area may be determined in this way according to ASTM Method D 3663-03. Pore volume may also be determined using mercury porosimetry. As the pore diameters are relatively large, mercury porosimetry, also known as mercury intrusion porosimetry, may be more suitably used. Mercury intrusion porosimetry is a well-known technique that involves exposing a powdered sample to mercury under pressure and measuring changes in volume. A particularly suitable method for establishing pore volume in this way is ASTM Method D4284-03. Average pore diameter may be calculated as being four times the pore volume divided by the surface area (4V / A). This relationship is derived from the geometry of a right circular cylinder with diameter D and height h, for which the surface area (A) is given by nDh and the volume (V) is given by 7tD2h / 4. The method provides titania microsphere powders suitable for use as catalyst supports. The catalyst supports may be used to prepare catalyst precursors and catalysts. Accordingly, the titania microsphere catalyst support may be treated with one or more catalytic metals to form catalyst precursors or catalysts suitable for use in catalytic processes. The one or more catalytic metals include transition metals and platinum group metals. For example, the titania microsphere maybe treated with one or more metals selected from Cu, Ni, Fe, Mn, Co, Pt. Pd, Rh, Ru, Wand Mo. The titania microsphere catalyst support has been found to be especially suitable for Fischer-Tropsch catalyst precursors and catalysts suitable for the formation of hydrocarbons. Fischer-Tropsch catalyst precursors may be prepared by impregnating the titania microsphere support with a cobalt compound. This may conveniently be performed by forming a solution of the cobalt compound and applying it to the titania microsphere support. The cobalt compound may be any suitably soluble cobalt compound, such as cobalt salts or cobalt complexes, but cobalt nitrate is particularly suitable as it introduces no catalyst poisons and is relatively straightforward to handle compared to other compounds. The cobalt compound is preferably cobalt (II) nitrate hexahydrate that may be dissolved in its water of hydration, for example by heating to above 50°C, to form a concentrated solution of molten cobalt nitrate. Alternatively, the cobalt nitrate may be dissolved in water or another solvent to form a more dilute solution. Water is the preferred solvent. The impregnated titania microsphere support is dried and calcined to form cobalt oxide crystallites within the pores of the titania microsphere support. The impregnation, drying and / or calcination may be repeated if necessary to achieve the desired metal loading. The impregnation may be performed by adding a cobalt compound solution to the titania microsphere support in a suitable mixer, such as a ploughshare mixer. The volume of the cobalt compound solution used preferably approximates to the total pore volume of the titania microsphere support. Such an impregnation, which may be termed a “dry impregnation” or “incipient wetness impregnation” advantageously improves the deposition of cobalt compounds within the pores of the titania microsphere support and minimises the amount of drying required. If desired, suitable compounds of additives and / or promoters may be added in suitable amounts to the cobalt compound solution. Alternatively, they may be combined with the catalyst precursor before or after drying and / or before a final calcination. The impregnated titania microsphere support is dried to remove the solvent, typically water, present in the cobalt compound solution and desirably leave a cobalt compound in the pores of the support. The drying step is preferably performed at a temperature in the range 50 to 150°C. The drying step may be performed at atmospheric pressure in air or an inert gas such as nitrogen, or under a vacuum. The drying time may be in the range 0.5 to 16 hours as required, although more preferably it is performed at 80 to 120°C for 1 to 5 hours. The dried impregnated titania microsphere support is a free-flowing powder containing one or more cobalt compounds, preferably partially hydrated cobalt nitrate, such as Co(N03)2.xH20 where x <6 and water is used as the solvent. To produce the catalyst precursor, the dried material is subjected to a heat treatment, which may be termed a calcination, to convert the cobalt compound into cobalt oxide deposited as crystallites distributed within the pore structure of the support. The heat treatment may be performed conventionally in air or by heating the dried material under nitrogen or another non-reducing gas mixture or by heating under vacuum. The heat treatment may be performed in static or moving bed ovens or, preferably, in a fluidized bed reactor. A fluidized bed reactor surprisingly has been found to produce catalyst precursors with small average cobalt oxide crystallite sizes, e.g. in the range 6 to 14 nm, especially 8 to 12 nm. The calcination temperature may be in the temperature range 220 to 320°C, preferably 240 to 300°C, to minimise sintering of the cobalt oxide crystallites and maintain the cobalt surface area. The calcination time may be in the range 0.5 to 16 hours as required, although preferably it is performed at 240 to 300°C for 1 to 4 hours. Under fluid bed conditions, the calcination gas hourly space velocity (GHSV) may be 1000 to 5000 hr1, preferably 2000 to 4000 hr1 at normal temperature and pressure (NTP). The calcination step may, if desired, be followed by a polishing step in which the calcined material is subjected to heating in a dilute hydrogen stream under conditions that do not cause reduction of the cobalt oxide to elemental form. The polishing step may advantageously be used to lower the residual nitrate content of the calcined material. The hydrogen stream may, for example consist of 0.1 to 10% by volume hydrogen, preferably 1 to 5% by volume hydrogen, in an inert gas, such as nitrogen. The pressure may be 1 to 10 bar absolute, preferably 1 to 3 bar absolute. The maximum temperature for the polishing step may be in the range 100 to 250°C, preferably 140 to 220°C. The polishing step may suitably be performed for 0.5 to 16 hours as required, although preferably it is performed at 140 to 220°C for 1 to 3 hours. The gas hourly space velocity (GHSV) of the hydrogen / inert gas stream in the polishing step may be 50 to 2000hr1, but is preferably 50 to 1000hr1, more preferably 100 to 500hr1 at normal temperature and pressure (NTP). Under these conditions essentially no reduction of the cobalt oxide takes place. The polishing step reduces residual nitrate in the catalyst precursor down to levels below 0.1% by weight so that the subsequent reduction may be performed without having to take special steps to manage the ammonia that forms during reduction. This is especially useful when the catalyst precursor is reduced in-situ, i.e. in the reactor in which it is to be used for the Fischer-Tropsch process. The cobalt content of the catalyst precursor may be in the range 5 to 25 % by weight, preferably 8 to 16 % by weight, expressed as Co on a loss free basis. Despite a relatively low cobalt content, the resulting catalysts have a surprisingly high activity per gram in the Fischer Tropsch reaction. In the active catalyst, at least a portion of the cobalt oxide in the catalyst precursor is reduced to elemental form, which increases the proportion of cobalt in the active catalyst as the oxygen atoms in the cobalt oxide are removed. Particularly effective catalysts comprise 8 to 12% by weight cobalt at a degree of reduction of the cobalt oxide of >70 mol%. The catalyst precursor possesses the same particle size characteristics as the titania microsphere support. The catalyst precursor therefore provides a high activity catalyst with an acceptable pressure drop when used in a commercial-scale fixed bed, especially when used as a fixed bed within a catalyst carrier in a multi-tubular Fischer-Tropsch reaction vessel. The catalyst precursor may optionally further comprise one or more additives, which may be present as oxides, to enhance catalyst performance in the Fischer-Tropsch process. Suitable additives are selected from oxides of one or more additive metals selected from nickel (Ni), thorium (Th), magnesium (Mg), manganese (Mn) or silicon (Si). Alternatively, or in addition, the catalyst precursor may optionally include one or more promoters, which may be present in oxide or elemental form, to improve the activation or reduction of the catalyst. Suitable promoter metals include one or more of rhodium (Rh), iridium (Ir), ruthenium (Ru), rhenium (Re), platinum (Pt) and palladium (Pd). Additives and / or promoters may be incorporated into the catalyst precursor on the titania microsphere support, before or after cobalt impregnation, by use of suitable compounds such as acids, e.g. perrhenic acid, metal salts, e.g. metal nitrates and oxy-nitrates or metal acetates, or suitable metal-organic compounds, such as metal alkoxides or metal acetylacetonates. The amount of additive metal may be varied between 1 and 15% by weight in total, preferably between 1 and 10% by weight in the catalyst precursor. The amount of promoter metal may be varied between 0.01 and 1.00% by weight in total, preferably between 0.01 and 0.50% by weight in the catalyst precursor. The catalyst precursor may be installed in the reactor in which it is to be used and, for the Fischer-Tropsch reaction, activated to form the catalyst by reduction of at least part of the cobalt oxide to elemental cobalt in-situ. Such activation may be performed using any suitable reducing agent but is preferably performed using a reducing gas stream. The reducing gas stream may be selected from a hydrogen gas stream or a synthesis gas stream comprising hydrogen and carbon monoxide. The reducing gas stream may therefore consist of 1 to 100% by volume of hydrogen in an inert gas such as nitrogen. Alternatively, a synthesis gas containing hydrogen and carbon monoxide, and optionally other components may be used. The synthesis gas may suitably be a Fischer-Tropsch synthesis gas consisting essentially of hydrogen and carbon monoxide in which the hydrogen : carbon monoxide molar ratio may be in the range 1.6 to 2.2. The reducing gas may alternatively comprise a mixture of these gases. The gas hourly space velocity (GHSV) of the reducing gas through the catalyst precursor may be 4000 to 10000 hr1 at normal temperature and pressure (NTP). The gas hourly space velocity of the reducing gas may be altered during the reduction to control the exposure of the cobalt oxide and reduced cobalt to water vapour that is formed as a byproduct during reduction. The maximum temperature used for the reduction stage may be in the range 250 to 400°C but is preferably in the range 250 to 300°C to minimise sintering of the reduced cobalt crystallites. The reduction may be performed at ambient pressure or elevated pressure, i.e. the pressure of the reducing gas may be from 1 to 50 bar abs. Alternatively, the catalyst precursor may be pre-reduced ex-situ in a reduction vessel to provide the catalyst. The pre-reduction may be performed using any suitable reducing agent but is preferably performed using a hydrogen gas stream operating in a loop with removal of by-product water from the recycled reduction gas stream. The hydrogen stream may consist of 10 to 100% by volume of hydrogen in an inert gas such as nitrogen. The concentration of hydrogen in the reducing gas stream may be varied during the reduction. The gas hourly space velocity (GHSV) of the hydrogen / inert gas stream may be 4000 to 10000 hr1 at normal temperature and pressure (NTP). The gas hourly space velocity of the reducing gas stream may be altered during the reduction to control the exposure of the cobalt oxide and reduced cobalt to water vapour. The maximum temperature used for the reduction stage may be in the range 250 to 400°C but is preferably in the range 250 to 300°C to minimise sintering of the reduced cobalt crystallites. The reduction may be performed at ambient pressure or elevated pressure, i.e. the pressure of the reducing gas may be from 1 to 50, preferably 1 to 20, more preferably 1 to 10 barabs. Catalysts in the reduced state can be difficult to handle as they can react spontaneously with oxygen in air, which can lead to undesirable self-heating and loss of activity. Consequently, the reduced catalyst may be protected by encapsulation of the reduced catalyst with a suitable barrier coating. In the case of a Fischer-Tropsch catalyst, this may suitably be a hydrocarbon wax, such as a wax produced by the Fischer-Tropsch synthesis. The encapsulating wax may be removed from the catalyst once installed within the Fischer-Tropsch reactor, for example by heating the catalyst under a hydrogen gas stream or a synthesis gas stream. Alternatively, the reduced catalyst may be passivated by exposure to dilute oxygen in a carrier gas such as nitrogen, such that a protective layer of cobalt oxide forms around each cobalt crystallite. Whichever route is chosen to convert the oxidic catalyst precursor into an active FT catalyst, cobalt catalysts prepared from the titania microsphere catalyst support produced by the method provide high metal surface areas per gram of reduced metal. For example, the catalyst precursors, when reduced by hydrogen at 250°C, may have a cobalt surface area of >5 m2 / g as measured by hydrogen chemisorption. The catalyst obtained from the catalyst precursor is particularly effective for the Fischer-Tropsch synthesis of hydrocarbons. The Fischer-Tropsch synthesis of hydrocarbons using cobalt catalysts is well established. The Fischer-Tropsch synthesis converts a mixture of carbon monoxide and hydrogen to hydrocarbons, preferably hydrocarbons with a carbon chain length >5. The mixture of carbon monoxide and hydrogen is typically a synthesis gas having a hydrogen: carbon monoxide ratio in the range 1.6 to 2.2:1. The reaction may be performed in a continuous or batch process, using one or more reactors such as fixed-bed reactors, slurry-phase reactors, bubble-column reactors, loop reactors or fluidised bed reactors. The process may be operated at pressures in the range 0.1 to 10MPa and temperatures in the range 150 to 350°C. The gas-hourly-space velocity (GHSV) for continuous operation may be in the range 1000 to 25000hr1. The catalysts of the present invention are particularly suitable as fixed bed catalysts, i.e. a bed of catalyst fixed within a reaction vessel through which the reactant synthesis gas is passed. The physical properties of the catalyst precursor mean it is especially suitable for use in a catalyst carrier placed in a reaction tube within a Fischer-Tropsch reaction vessel, such as a downflow multi-tubular Fischer-Tropsch reaction vessel. By “catalyst carrier” we mean a catalyst container, for example in the form of a cup or can, configured to allow a gas and / or liquid to flow into and out of the carrier and through a bed of the catalyst or catalyst precursor disposed within the carrier. The flow of the gas and / or liquid within the catalyst carrier may be radial and / or axial. Preferably the catalyst or catalyst precursor is disposed as an annular catalyst bed within a central part of the catalyst carrier. The flow through the catalyst bed within the carrier is preferably radial. The catalyst bed may be surrounded by and spaced apart from a heat transfer zone at a periphery. The catalyst bed and heat exchange zone may be in fluid communication with each other. In a preferred arrangement, the catalyst carrier may be that disclosed in WO2016 / 050520, the contents of which are incorporated herein by reference. Thus, the catalyst carrier may comprise: a container suitable for holding the catalyst precursor or catalyst in place, said container having a bottom surface closing the container, and a top surface; a carrier outer wall extending from the bottom surface of said container to the top surface; a seal extending from the container by a distance which extends beyond the carrier outer wall; said carrier outer wall having apertures located below the seal. In a preferred arrangement, the catalyst carrier may comprise: an annular container suitable for holding the catalyst precursor or catalyst in place, said container having a perforated inner container wall defining an inner channel, a perforated outer container wall, a top surface closing the annular container and a bottom surface closing the annular container; and a surface closing the bottom of said inner channel formed by the inner container wall of the annular container. The catalyst carrier will generally be sized such that it is of a smaller dimension than the internal dimension of the reactor tube into which it is to be placed in use. The seal will be sized such that it interacts with the inner wall of the reactor tube when the catalyst carrier of the present invention is in position within the reactor tube. In use in a vertical reactor with downflow, reactants flow downwardly through the reactor tube and thus first contact the upper surface of the catalyst carrier. Since the seal blocks the passage of the reactants around the side of the carrier, the top surface thereof directs them into the inner channel defined by the inner container wall. The reactants then enter the annular container through the perforated inner container wall and then pass radially through the catalyst bed towards the perforated outer container wall. During the passage from the inner container wall to the outer container wall, the reactants contact the catalyst and the Fischer-Tropsch reaction occurs. Unconverted reactants and reaction products then flow out of the container through the perforated outer container wall. The carrier outer wall then directs reactants and products upwardly between the inner surface of the carrier outer wall and the perforated outer container wall of the annular container until they reach apertures in the carrier outer wall. They are then directed through the apertures located in the carrier outer wall and flow downwardly between the outer surface of the carrier outer wall and the inner surface of the reactor tube where heat transfer takes place. In the event that the reactor is operated such that the flow is reversed, the paths of reactants and products will also be reversed. The catalyst precursor may be loaded into the catalyst carrier and the resulting combination loaded into the reaction tubes of a Fischer-Tropsch reaction vessel for reduction and activation of the catalyst in-situ as described above. Alternatively, the combination may be subjected to a reduction ex-situ in a reduction vessel and the combination encapsulated or passivated. The resulting combination may then be safely loaded into the reaction tubes of a Fischer-Tropsch reaction vessel for a quicker and simpler in-situ activation than in the case where the oxidic catalyst precursor is used. Examples Figure 1 shows an example of a flowsheet depicting an embodiment of a spray granulation method according to the invention. In Figure 1, a binder dissolution vessel 10 is fed with water via line 12 and a binder via line 14. The binder mixture is fed via line 16 to a slurry make up vessel 18, fed with powdered first titania precursor via line 20 and second titania precursor via line 22. The first and second titania precursors are mixed with the binder and water to form a slurry, which is fed via line 24 to a slurry holding vessel 26 for later use. The slurry holding vessel 26 is connected via line 28 to the inlet of a fluidised bed spray granulator 30. The slurry is fluidised in the granulator with air fed via line 32. The fluidised bed spray granulator heats the fluidised slurry and generates a granulated material with the evolution of water, which leaves the granulator as moist air via line 34 after filtration to remove entrained solids. A microsphere product mixture is recovered from the granulator 30 and fed via line to a sieve classifier 38. The sieve classifier separates the desired particle size range of microspheres from undersized and oversized materials. The desired product is fed via line 40 to a microsphere product storage vessel 42 to be fed via line 44 to a calciner 46. The calciner heats the microsphere product to high temperatures to drive off any remaining water and decompose the binder to form the final titania microsphere catalyst support material, which is recovered via line 48. Oversized material is recovered from the sieve classifier via line 50, part may be purged via line 52 and part recycled via line 54 to a high shear mixer 56, where it is mixed with water fed via line 58 to form an oversized particle slurry. The high shear mixer breaks down the oversize particles in the oversized particle slurry and the resulting sheared mixture fed to the slurry holding vessel via line 60. Undersized particles are recovered from the sieve classifier 38 via line 62. Part is fed to the high shear mixer 56 via line 64 and part is fed directly to the fluidised bed spray granulator 30 via line 66. Part of the material in line 66 may be purged from the process via line 68. In the Examples, the following measurements were performed. Particle Size: Particle sizes were determined according to ASTM D4464 by laser light scattering using a Malvern Mastersizer 3000 laser diffraction particle size analyser, using the optical properties of TiO2 (Anatase) from the Malvern MS3000 optical property database in the instrument software. (Refractive Index 2.51, Absorption Index 0.01). Wet dispersion measurements were carried out in de-ionised water using 0% and 85% ultrasonication and a stirrer speed of 50% with a Hydro MV sample dispersion unit. Sufficient sample was added to the unit until the laser beam is obscured by between 0.1 and 20%. Five measurements were made for each aliquot of sample, with a measurement duration of 5 seconds. Dry dispersion measurements were made using 0.5 Bar and 3.0 Bar of dispersive air pressure using site compressed air, at a feed rate of 20% in an Aero dry dispersion unit with a micro-volume tray and a standard venturi. One measurement was made per aliquot of sample, with the whole aliquot being measured in each measurement. The measurement duration is 15 seconds when the laser beam is obscured by between 0.1 and 10%. To calculate the results, the Malvern software converts the resulting diffraction pattern into a particle size distribution using Mie Theory. Pore Volume. Mercury intrusion I extrusion data was measured on a Micromeritics AutoPore 9520 mercury porosimeter in accordance with ASTM Method D4284-03; Test Method for Determining Pore Volume Distributions of Catalysts by Mercury Intrusion Porosimetry. Intrusion curves were measured over the pressure range of 0.5 to 60000 psia followed by extrusion down to atmospheric pressure. An equilibration time of 15 seconds was used for each data point on both the intrusion and extrusion curves, the mercury contact angle was taken to be 140° and the mercury surface tension taken as 485 dynes / cm. Samples were dried at 115°C overnight in an oven prior to analysis. Temperature and pressure effects that are manifested during the porosimetry measurement were accounted for by blank correction runs on empty penetrometer tubes which were subsequently subtracted from the experimental data. Surface areas were measured using a Micromeritics 2420 ASAP physisorption analyser by application of the BET method in accordance with ASTM Method D 3663-03; Standard Test for Surface Area. Nitrogen was used as the adsorbate and the measurements carried out at liquid nitrogen temperature (77K). The cross-sectional area of a nitrogen molecule was taken as 16.2 A. Samples were outgassed prior to analysis by purging with dry nitrogen gas for a minimum of 1 hour at 140°C. Five relative pressure / volume data pairs were obtained over the relative pressure region of 0.05 to 0.20 P / Po inclusive. The equilibration time for each point was 10 second. Surface areas are reported based on the weight of the sample post outgassing. Average Pore Diameter. The average pore diameter was calculated from the pore volume and surface area measurements. The average pore diameter is four times the corrected pore volume divided by the calculated surface area (4V / A). This relationship is derived from the geometry of a right circular cylinder with diameter D and height h, for which the surface area (A) is given by nDh and the volume (V) is given by 7tD2h / 4. Median Pore Diameter. The median pore diameter was derived from the pore volume measurement. By “median pore diameter” we mean the pore diameter representing the midpoint of the cumulative mercury intrusion curve, corrected for interparticle void filling. Cobalt Content. Determined by ICPAES (inductively coupled plasma atomic emission spectroscopy) or ICPMS (inductively coupled plasma mass spectrometry) on the calcined catalyst precursors expressed as percentage cobalt by weight on a loss free basis. Chloride and sulphur content. Chloride and sulphur contents were measured by Combustion Ion Chromatography (CIC). The test sample was heated in a furnace in the presence of flowing moisture containing air. Halides and / or sulphur compounds present in the sample were combusted and released into the air flow. The air exiting the furnace was cooled to generate a liquid condensate. The resultant liquor was diluted to a set volume and an aliquot injected onto an ion chromatography (IC) column, thereby separating the anions of interest. The effluent from the IC column was passed through a conductivity detector generating a set of peaks corresponding to the anions present in the sample. Calibration of the detector using standard anion solutions facilitated calculation of the chloride and / or sulphur content of the sample. Cobalt Oxide Crystallite Size. The cobalt oxide crystallite size was determined by XRD using a Bruker D8 Advance X-Ray Diffractometer. The powdered catalyst precursor sample was pressed into a sample holder and loaded into the instrument. Parallel beam (Gobel mirror) optic. Software; Bruker EVA for phase identification; Topas for pattern refinement. The diffractometer Conditions were as follows: X-ray Cu Ka Wavelength 1.5406 A with Lynxeye PSD detection. Starting 2 theta 10 Finish 2 theta 130 Step 0.022 Step time, sec 4 X-ray current, mA 40 X-ray voltage, kV 40 Rietveld analysis (Bruker Topas v4.2) was used to determine cobalt oxide crystallite size. Rietveld refinement of powder XRD data starts with a calculated diffraction pattern based on symmetry information and an approximate structure. Rietveld refinement then uses a least squares minimisation to compare every observed point to the calculated plot and refines the calculated structure to minimise the difference. Anatase and rutile crystal size and content. The anatase and rutile contents of the titania microsphere supports and catalyst precursors were determined by XRD using the same equipment and methods used to determine the cobalt oxide crystallite size. Cobalt Surface Area. Cobalt metal surface areas were measured on a Micro me ritics 2480 HTP 6 Station Chemisorption Analyser. The samples were reduced with 100% vol hydrogen at 250°C for 120 minutes at a hydrogen flow rate of 200 SCCM. After the reduction stage was complete, the samples were purged with helium for 15 minutes before being cooled under vacuum to the analysis temperature of 35°C. Evacuation was continued for 45 minutes after achieving a vacuum of <10 pm Hg. The sample was then dosed with 100% vol. hydrogen over a range of pressures between 100 and 760 mmHg. At each pressure, the chemisorbed hydrogen was allowed to equilibrate, and the volume of hydrogen uptake was measure and recorded automatically. Pressure I uptake pairs were plotted to give an isotherm which exhibited a well-defined plateau region. Data points were selected within this region to achieve a best-line fit which was then extrapolated back to zero pressure. The intercept value was used to calculate the cobalt surface area using a stoichiometry of 1.0 for H2 / C0 and the cobalt surface area reported based on the mass of reduced catalyst. Example 1: Preparation of titania microspheres The method depicted and described in Figure 1 was carried out using a Glatt ProCell Pilotsystem spray granulator to provide titania microspheres. First titania precursor: Evonik Aeroxide P25™ Second titania precursor: Venator Hombikat FT™ Binder: polyvinyl alcohol: Kuraray Poval™ 20-98 LA The properties of the first and second titania precursors were as follows: First Titania Precursor Second Titania Precursor BET surface area (m2 / g) 56 45 D[v, 0.5] (pm) 3 41 Hg Intrusion Volume (cm3 / g) 0.36 0.27 Hg Median Pore Diameter (nm) 32 29 XRD Anatase (% w / w) 88 100 XRD Rutile (% w / w) 12 0 A fresh aqueous slurry feed consisting of 90% w / w First Titania Precursor and 10% w / w Second Titania Precursor (dry solids) and 5.5% w / w Binder (dry basis) was prepared and fed to the Glatt™ continuous fluidised bed spray granulator. The titania content of the aqueous slurry feed was 22% w / w. The spray granulator was operated under typical conditions. The spray granulator produced a spray granulate product mixture. Undersized particles with a particle size <350 gm and oversized particles with a particle size >500 pm were separated from a desired product cut of the titania microspheres using sieves and recycled to the spray granulator. The fine particles were sieved to separate them into a first portion consisting of particles with a particle size in the range of 200 to 350 pm, and a second portion consisting of particles with a particle size <200 pm. The product cut therefore consisted of particles with a size in the range 350 to 500 pm. The volume-median diameter of the product cut of titania microspheres post calcination was 393 pm. All of the second portion of fine particles together with all of the oversized particles were re-slurried, redispersed via high shear mixing and recycled to the spray granulator with the fresh slurry feed. An amount of the first portion of fine particles was returned directly to the spray granulator with the balance being re-slurried. The method produced about 78% by weight titania microspheres, about 20% by weight fine particles and about 2% by weight oversized particles. The titania component of the spray granulator feed was approximately 90% w / w fresh powders consisting of 90% w / w First Titania Precursor and 10% w / w Second Titania Precursor and 10% w / w redispersed fines and oversized particles. Re-slurrying and redispersing recycle materials was found to be beneficial to both the overall yield of on-size particles and the stability of the continuous spray granulation process. An optical microscopy image of the product cut of the titania microspheres recovered from the spray granulator product mixture is depicted in Figure 2. The titania microsphere product cut was subjected to calcination at elevated temperature to generate a porous titania microsphere Fischer-Tropsch catalyst support. Calcination was performed in a tray calciner at temperatures in the range 300 to 700°C with a 6 hour hold time. The resulting titania microsphere product properties were as follows: Hold Temp (°C) 300 400 450 500 600 700 BET SA (m2 / g) 54 52 51 49 40 20 N2 PV (cm3 / g) 0.44 0.44 0.43 0.42 0.36 0.22 Hg I nt Vol (cm3 / g) 0.49 0.48 0.47 0.46 0.38 0.25 Hg Pore Diameter (nm) 41 44 44 45 44 54 XRD Anatase (% w / w) 88 87 87 85 74 20 XRD Rutile (% w / w) 12 13 13 15 26 80 Anatase Crystal (nm) 18 17 18 18 20 24 Rutile Crystal (nm) 30 27 28 30 39 58 As calcination temperature increases BET surface area and nitrogen pore volume decrease. The loss of surface area and pore volume is gradual up to a temperature of 500°C. Above 500°C the rate of loss of surface area and pore volume increases. Corrected mercury intrusion volume follows a trend similar to that of nitrogen pore volume. The corrected mercury intrusion volume is slightly higher than the nitrogen pore volume. This is indicative of macro porosity, which is not detected by nitrogen physisorption. The anatase content of the microspheres is reasonably constant as calcination temperature increases to 500°C. Calcination at or above 600°C results in significant conversion of anatase to rutile. Calcination at or above 600°C also significantly increases anatase and rutile crystallite sizes. Below 400°C, it was found that carbon residues from the binder were present in the titania microspheres. These results demonstrate that to obtain optimal textural properties for Fischer-Tropsch catalysts, a tray calcination temperature of 400 to 500°C is preferable. Approximately 7 kg of the titania microsphere product cut was dried at 100°C for 2 hours and tray calcined (seven trays in one oven, ca. 1000 g per tray) at 450°C for 6 hours to generate “Support B”. The characteristics of this material are compared in the following table to those of a commercially available titania microsphere product (’’Support A”). Support A Support B BET surface area (m2 / g) 53 48 Nitrogen pore volume (cm3 / g) 0.36 0.43 Corrected mercury intrusion volume (cm3 / g) 0.41 0.47 Median mercury pore diameter (nm) 39 45 Air jet attrition (% w / w) 42 50 D[v, 0.5] (pm) 394 393 XRD average anatase crystallite size (nm) 19 18 XRD average rutile crystallite size (nm) 30 29 XRD anatase (% w / w) 91 86 XRD rutile (% w / w) 9 14 Chloride (ppm w / w) 802 517 Sulphur (ppm w / w) < 30 24 Example 2: Preparation of Fischer-Tropsch Catalyst precursors (a) Catalyst A. 400.0 g of Support A were placed in a preheated Z-blade mixer. 261.5 g of cobalt nitrate hexahydrate (19.80% w / w Co) were placed in a beaker and heated. 3.2 g of tetraammine platinum (II) nitrate solution (3.367% w / w Pt) and 10.5 g of demineralized water were added and the mixture heated to ca. 75°C. The hot solution was added to the Z-blade mixer at a powder temperature of about 60°C. Mixing was continued after solution addition for 1 minute. The mass of discharged product was 665.6 g. Approximately 166 g of mixed product were placed into each of four stainless steel trays. These were placed in an oven, which had been pre-heated to 110°C. The oven temperature was held at 110°C for 3 hours. The temperature was then increased to 250°C over 1 hour 10 minutes and held at 250°C for 2 hours, after which the oven was allowed to cool to ambient temperature. (b) Catalyst B. The method of Example 2(a) was repeated using Support B. (c) Catalyst C. The method of Example 2(a) was repeated, replacing the tetraammine platinum (II) nitrate with 3.7 g of ruthenium (III) nitrosyl nitrate solution (13.880% w / w Ru). (d) Catalyst D. The method of Example 2(c) was repeated using Support B. Characterization data for Catalysts A to D are presented in the following table: Catalyst A Co / Pt / TiO2 Catalyst B Co / Pt / Ti02 Catalyst C Co / Ru / TiO2 Catalyst D Co / Ru / Ti02 BET surface area (m2 / g) 47 46 47 45 Nitrogen pore volume (cm3 / g) 0.31 0.35 0.31 0.34 Cobalt surface area (m2 / g) 5.1 4.8 4.8 5.0 Corrected mercury intrusion volume (cm3 / g) 0.31 0.36 0.31 0.37 Median mercury pore diameter (nm) 41 47 42 48 XRD average cobalt oxide crystallite size (nm) 9 10 12 12 XRD average anatase crystallite size (nm) 18 18 18 18 XRD average rutile crystallite size (nm) 29 35 32 33 XRD cobalt oxide (% w / w) 16 16 16 16 XRD anatase (% w / w) 76 73 77 73 XRD rutile (% w / w) 8 11 7 11 D[v, 0.5] (pm) 395 398 392 395 Calculated cobalt content (% w / w) 11.0 11.0 11.0 11.0 Calculated platinum content (% w / w) 0.02 0.02 0.0 0.0 Calculated ruthenium content (% w / w) 0.0 0.0 0.1 0.1 Chloride (ppm w / w) 394 311 434 324 Sulphur (ppm w / w) < 30 < 30 < 30 < 30 Example 3: Catalyst Testing The tests were performed using 0.5 g of the catalyst precursors diluted with 2.5 g SiC placed in a laboratory reactor tube with an internal diameter of 4 mm. After in-situ reduction at 300°C (ramp 1°C / min) in pure hydrogen for 7 hours using a flow rate of 60 ml / min, the temperature was then reduced to 150°C and the gas was switched to syngas (H2:CO = 2:1), and the reactors pressurized to 20 barg using a flow rate of 110 ml / min. After 6 hours, the temperature was increased (10C / min) to 210°C and left overnight for about 16 hours. The flow rates were then reduced first to 50 ml / min, then to the required flow rate to achieve 50% syngas conversion and data collection continued forthe duration of the test (about 160 hours unless otherwise stated). Inlet gases were metered into the reactor using mass flow controllers. Gaseous, liquid and solid hydrocarbon products and the aqueous phase were analyzed by gas chromatography to achieve a mass balance from which CO conversion and selectivity were calculated. The performance data for the catalysts are tabulated below. FT Catalyst Composition Catalyst A Catalyst B Catalyst C Catalyst D Syngas conversion (%) 50.5 50.1 49.0 49.4 CH4 selectivity (C mol %) 7.47 7.02 7.12 6.77 C5+ selectivity (C mol %) 87.7 88.3 88.4 88.8 Reactor temperature (°C) 209 209 210 210 Reactor pressure (barg) 20.4 20.4 20.0 20.4 GHSV (hr1) 6079 6277 6110 5913 Relative activity (in comparison to Catalyst A) 1.00 1.01 0.97 0.94 It is apparent that the platinum promoted catalysts (A and B) are marginally more active than the ruthenium promoted catalysts (C and D), with marginally higher selectivity to methane. The Relative Activity of the catalysts prepared using the Support A (Catalysts A and C) is similar to that of catalysts prepared from supports derived from the mixed titania feed (Catalysts B and D). However, in both cases (Co / Pt / TiO2 and Co / Ru / TiO2) the catalyst prepared on the support derived from Support B has lower methane selectivity and consequently higher C5+ selectivity. This property of the invention is advantageous for processes intended to generate long chain hydrocarbons

Claims

1. A method for preparing a catalyst support comprising the steps of: (i) preparing an aqueous slurry of titania precursors, (ii) spray-granulating the aqueous slurry in a fluidised bed spray granulator to form titania microspheres and fine particles, (iii) separating the fine particles from the titania microspheres and recycling them to the fluidised bed spray granulator, and (iv) recovering and calcining the titania microspheres, wherein the titania precursors comprise a first titania precursor and a second titania precursor and the volume-median diameter of the first titania precursor is less than the volume-median diameter of the second titania precursor.

2. A method according to claim 1, wherein the titania microspheres have a volumemedian diameter, D[v,0.5], in the range 100 to 1000 pm, preferably in the range 200 to 900 pm, more preferably 300 to 750 pm, most preferably 300 to 550 pm.

3. A method according to claim 1 or claim 2, wherein the titania microspheres have a sphericity (ip), of at least 0.90, preferably at least 0.95.

4. A method according to any one of claims 1 to 3, wherein the difference in volumemedian diameter between the first titania precursor and the second titania precursor is greater than 5 pm, preferably greater than 10 pm, more preferably greater than 15 pm, most preferably greater than 20 pm.

5. A method according to any one of claims 1 to 4, wherein the first titania precursor is formed by flame hydrolysis.

6. A method according to any one of claims 1 to 5, wherein the first titania precursor has a volume-median diameter, D[v,0.5], that is 10 pm or less, and is preferably in the range 2 to 8 pm.

7. A method according to any one of claims 1 to 6, wherein the first titania precursor has a rutile content in the range of 2 to 20% by weight, preferably 8 to 16% by weight.

8. A method according to any one of claims 1 to 7, wherein the second titania precursor is formed by precipitation.

9. A method according to any one of claims 1 to 8, wherein the second titania precursor has a volume-median diameter, D[v,0.5], in the range of 30 to 80 pm, and is preferably in the range 35 to 50 pm.

10. A method according to any one of claims 1 to 9, wherein the first titania precursor is present in the slurry at an amount in the range of 50 to 99% by weight, preferably 75 to 99% of the combined weight of the first and second titania precursors.

11. A method according to any one of claims 1 to 10, wherein the slurry has a total titania precursor content in the range of 10 to 30 % by weight.

12. A method according to any one of claims 1 to 11, wherein the method provides a product cut of titania microspheres, fine particles and oversized particles, the product cut has a particle size in the range of from 50 pm to 1250 pm or in the range of from 150 pm to 1150 pm, or in the range of from 200 pm to 1000 pm, or in the range of from 200 pm to 750 pm, or in the range of from 300 pm to 500 pm.

13. A method according to any one of claims 1 to 12, wherein a first portion of the fine particles is recycled to the fluidised bed spray granulator directly and a second portion of the fine particles is recycled by redispersing them to form a second aqueous slurry which is fed to the fluidised bed spray granulator.

14. A method according to claim 13, wherein the first portion does not include fine particles with a particle size below 50 pm or below 100 pm or below 200 pm.

15. A method according to claim 13 or claim 14, wherein the method also produces oversized particles that are combined with the second portion of fine particles.

16. A method according to any one of claims 13 to 15, wherein the amount of recycled particles in the slurry is less than or equal to 30% by weight of the titania solids fed to the spray granulator, preferably less than or equal to 15% by weight of titania solids fed to the spray granulator.

17. A method according to any one of claims 1 to 16, wherein one or more binders are included in the slurry.

18. A method according to claim 17, wherein the binder is an organic binder preferably a polyvinyl alcohol, or an inorganic binder, preferably a clay, alumina trihydrate, boehmite or zirconium acetate.

19. A method according to any one of claims 1 to 18, wherein the calcination step is performed at a temperature in the range 350 to 900°C, preferably 350 to 750°C, more preferably 400 to 500°C.

20. A method according to any one of claims 1 to 19, wherein the calcination step is performed using a tray calciner a fluid bed calciner or a rotary calciner.

21. A titania microsphere catalyst support obtainable by the method according to any one of claims 1 to 20.

22. A method of producing a catalyst precursor or catalyst comprising the step of producing a catalyst support according to the method of any one of claims 1 to 20 and treating the catalyst support with one or more catalytic metals.

23. A method according to claim 22, wherein the one or more catalytic metals comprise transition metals and platinum group metals, preferably one or more metals selected from Cu, Ni, Fe, Mn, Co, Pt. Pd, Rh, Ru, Wand Mo.

24. A method according to claim 22 or claim 23, wherein the one or more catalytic metals comprises cobalt.

25. A cobalt Fischer-Tropsch catalyst precursor obtainable by the method of claim 24.

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