Preparation of fischer-tropsch catalysts via hydrotalcites

Abstract

The present invention concerns a method of preparing a catalyst by preparing a hydrotalcite having the formula: Md 2+ aMt 3+ b(OH)(2a+2b)(CO3)b / 2; wherein Md 2+ is a divalent metal cation; Mt 3+ is a trivalent metal cation; and the ratio of a and b is in the range of from about 2.0 : 1.0 to about 4.5 : 1.0; and subsequently reducing the hydrotalcite by heating in a reducing atmosphere to a temperature of at least 600 °C to produce the catalyst, wherein the catalyst has the formula of any one of formulas: Md 0 aMt 3+ bO3b / 2; Md 0 aMt 2+ bOb; or Md 0 aMt 0 b. The invention also concerns a catalyst made by the method; and a process for producing a Fischer-Tropsch hydrocarbon product comprising the step of contacting the catalyst with synthesis gas.

Description

IMPROVEMENTS IN AND RELATING TO PREPARATION OF CATALYSTS FIELD OF THE INVENTION

[0001] The present invention concerns a method of preparing a catalyst. The invention also concerns a catalyst made by the method; and a process for producing a Fischer-Tropsch hydrocarbon product comprising the step of contacting the catalyst with synthesis gas. BACKGROUND OF THE INVENTION

[0002] Methane is available in large quantities in many areas of the world. Some methane is generated from refinery applications while large amounts of methane, as the principal constituent of natural gas, are found in deposits in various areas.

[0003] Methane can be used directly as a gas for heating purposes and the like when the source of methane is relatively close to the end user. However, if the methane must be transported over long distances, the methane is preferably transported as a liquid.

[0004] Methane is also used as a starting material to produce hydrocarbons. The conversion of methane is normally carried out in a two-step procedure involving reforming the methane to produce hydrogen and carbon monoxide i.e., synthesis gas (syngas), and then converting the syngas to higher hydrocarbons in a Fischer-Tropsch type reaction. Both steps of the process are well-known and can be readily illustrated: the first step by U.S. Pat. Nos. 1,711,036, 1,960,912 and 3,138,438; the second step by U.S. Pat. Nos. 4,477,595, 4,542,122 and 4,088,671.

[0005] In addition, gasification of renewable feedstocks of biological origin is becoming a more common route to making syngas without using natural gas as a starting component.

[0006] Many attempts at providing effective catalysts for selectively converting syngas to hydrocarbons have previously been disclosed in the art.

[0007] Chester et al., U.S. Pat. No. 4,523,047 disclose employing a catalyst system comprising zeolite ZSM-45 in combination with tungsten, vanadium, molybdenum, rhodium, nickel, cobalt, chromium, manganese, platinum or lead to produce liquid hydrocarbons from syngas in a Fischer-Tropsch synthesis slurry reactor system.

[0008] Cobalt-containing catalysts are also well-known in the art for use in Fischer- Tropsch synthesis. Payne et al., U.S. Pat. No. 4,542,122 disclose a catalyst composition comprising cobalt or thoria-promoted cobalt on a titania support for converting syngas to C10+ linear paraffins and olefins. Further, several prior disclosures describe employing various iron- cobalt spinels as catalysts in Fischer-Tropsch synthesis. See, for example, Soled et al., U.S.Pat. No.4,518,707 (high surface area iron-cobalt spinels which are fully reduced / carburized to selectively convert syngas to alpha-olefins); Fiato et al., U.S. Pat. No. 4,537,867 (promoted iron-cobalt spinels containing low levels of cobalt to selectively convert syngas to C2to C6olefins with low CH4production); Soled et al., U.S. Pat. No.4,544,671 (high surface area iron- cobalt spinels which are fully reduced / carburized to selectively convert syngas to alpha- olefins); Fiato et al., U.S. Pat. No. 4,544,672 (reduced and carbided unsupported iron-cobalt single phase spinels containing low levels of cobalt to selectively produce low molecular weight olefins); Fiato et al., U.S. Pat. No.4,544,674 (alkali promoted iron-cobalt single phase spinels containing low levels of cobalt to selectively produce low molecular weight olefins); Soled et al., U.S. Pat. No.4,584,323 (copper promoted iron-cobalt spinels to convert syngas to alpha olefins); and Soled et al., U.S. Pat. No. 4,607,020 (copper promoted iron-cobalt spinels carbided in-situ in the reactor to selectively convert syngas to alpha-olefins).

[0009] Also known in the art for use in Fischer-Tropsch hydrocarbon synthesis are ruthenium based catalysts. For example, Madon, U.S. Pat. No.4,477,595, describes employing ruthenium catalysts supported on titanium oxide, niobium oxide, vanadium oxide or tantalum oxide to produce C5 to C40 hydrocarbons in a Fischer-Tropsch hydrocarbon synthesis process; and Wachs et al., U.S. Pat. No.4,861,747 describe a catalyst comprising ruthenium supported on a non-crystalline surface-modified oxide containing titania support for producing substantially alcohol free hydrocarbon products having high concentrations of internal olefins in a Fischer-Tropsch hydrocarbon synthesis process. Further, cobalt-ruthenium catalysts for Fischer-Tropsch hydrocarbon synthesis have been described in, e.g., Iglesia et al., U.S. Pat. No.4,738,949 and Iglesia et al., U.S. Pat. No.4,822,824 (cobalt and ruthenium deposited on a titania support).

[0010] In addition, copper promoted iron-manganese catalysts are described in Fiato et al., U.S. Pat. No. 4,618,597 for the conversion of CO / H2into alpha-olefins. The iron-manganese spinels are prepared by utilizing an alpha-hydroxy aliphatic carboxylic acid which acts as a solubilizing agent for the iron and manganese salts in aqueous solution. Representative examples of such acids are given as glycolic, malic, glyceric, mandelic, tartaric, lactic acids and mixtures thereof.

[0011] Kim et al., U.S. Pat. No. 4,624,968 disclose a two stage Fischer-Tropsch hydrocarbon synthesis process wherein an iron-based catalyst, e.g., iron / cesium / zinc / potassium, iron / manganese / potassium, iron / cobalt / potassium, is employed in the first stage to selectively produce olefins; and a ruthenium-based catalyst, e.g.,ruthenium / titanium oxide, ruthenium / aluminium oxide, ruthenium / niobium oxide, ruthenium / silicon oxide, is employed in the second stage to selectively produce paraffins.

[0012] Cobalt-oxide spinel catalysts have also been described in the literature. Fornisari et al., "Cobalt Mixed Spinels as Catalysts for the Synthesis of Hydrocarbons," Ind. Eng. Chem. Res., Vol.26, No.8, pp.1500-1505 (1987), reports that catalysts consisting of cobalt, copper, zinc, and chromium mixed oxides have improved selectivity to hydrocarbons where the catalyst contains comparable amounts of cobalt and copper. Selyama et al., "Characterization and Activity of Some Mixed Metal Oxide Catalysts," Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, pp. 19-27 (1987), reports that spinel type oxides, e.g. CuCo2O4 and CoNiO4, show activity and selectivity for biacetyl formation. Van der Riet et al., "Selective Formation of C3Hydrocarbons from CO+H2 using Cobalt-Manganese Oxide Catalysts," J. Chem. Soc., Chem. Commun., pp. 798-99 (1986) reports that selective formation of C3hydrocarbons is obtained when cobalt-manganese oxide catalysts are employed in hydrocarbon synthesis processes.

[0013] Also, cobalt with a Re promoter on a titania support is described in Mauldin, US 4,670,475 and shown to be a very effective catalyst for converting syngas to higher molecular weight paraffins and superior to cobalt alone without Re.

[0014] However, despite the prior art disclosures there still exists a need in the art for improved catalysts which under Fischer-Tropsch conditions selectively produce hydrocarbons, especially high molecular weight paraffins, with a high conversion rate of carbon monoxide and with low methane production. There is a need to enhance catalyst activity, for example, by increasing the number of active surface sites per gram of catalyst.

[0015] Clay minerals are composed of layers of metal or non-metal oxides and hydroxides stacked one on top of the other. In the case of the widely found cationic clays, interlayer cations (Na+, Ca2+etc.) neutralize the negatively charged oxide / hydroxide sheets. The far less common anionic clays have positively charged metal oxide / hydroxide layers with anions located interstitially. Many of these are based on double hydroxides of such main group metals as Mg and Al and transition metals such as Ni, Co, Cr, Zn, Fe often together with Al. These clays have a structure similar to brucite [Mg(OH)2] in which the divalent ions are octahedrally surrounded by hydroxyl groups with the resulting octahedra sharing edges to form infinite sheets. In these anionic clays some of the divalent ion is isomorphously replaced by a trivalent ion, such as Al3+. The Mg2+, Al3+, OH- layers are then positively charged necessitating charge balancing by insertion of anions between the layers. One such clay is hydrotalcite in which the carbonate ion is the interstitial anion. Rhombohedral hydrotalcite has the idealized unit cellformula [Mg6Al2(OH)16]CO3.4H2O. The ratio of Mg / Al in hydrotalcite can vary between 1.7 and 4 and various other divalent and trivalent ions may be substituted for the magnesium and aluminium. In addition, the anion, which is carbonate in hydrotalcite, can vary in both the naturally occurring and synthetic varieties being replaced by a variety of simple anions such as NO3-, Cl-, OH-, SO4-2etc. in naturally occurring varieties and by more complicated pillaring organic, inorganic, and organic-inorganic ion combinations in synthetic varieties. Hydrotalcites containing the large pillaring anions are generally made by substituting a hydrotalcite containing a simple anion by the larger pillaring anion. Substitution techniques which have been used are ion exchange and acid treatment in the presence of the desired replacing anion. Through changes in the size of the pillar used to separate the sheets in the clay structure, the pore size of the clay may be tailored to a particular use.

[0016] In U.S. Patent Nos.4,458,026 and 4,476,324 and “Catalytic reactions by thermally activated, synthetic, anionic clay minerals”, J. Cat., Vol.94, No.2, pp.547-557 (1985), Reichle describes catalytic reactions including aldol condensations using synthetic hydrotalcites containing smaller anions and also large organic anions such as long chain, aliphatic, alpha- omega dicarboxylates. Mg+2, Co+2, Ni+2, Cu+2are among other divalent cations that can be part of the hydrotalcite structure.

[0017] Preston et al., US 5,250,279 describes the preparation of Mg-Al hydrotalcite using Mg hydroxide, alumina trihydrate and alkali metal aluminate or carbonate or sodium bicarbonate solutions heated hydrothermally at 100-160 °C.

[0018] Fujii et al., EP 1472262 A1 describes synthetic hydrotalcites of the general formula [M2+1-xM3+ x(OH)2]x+[Anx / n.mH2O]xwhere M2+is a divalent cation, M3+is a trivalent cation and An-is an organic anion selected from straight chain carboxylates of C16-C18 acids, carboxylates of aromatic acids, carboxylates of acrylic acid, unsaturated carboxylates of methacrylic acid, unsaturated carboxylates of vinylacetic acid and C2and higher organic acids containing heteroatoms such as nitrogen, phosphorous, sulfur and halogens are disclosed, along with methods of synthesis and uses.

[0019] Bhattacharyya et al., EP 0536879 A1 describes locating metal oxo species in the anion layer in a hydrotalcite, with such anions as B(OH)4-, [B3O3(OH)4]-, [B3O3(OH)5]2-, [B4O5(OH)4]2-, V2O74-V4O124-, V10O286-, as well as Nb6O198-, HNb6O197-, H2Nb6O196-, NbO43-, Ta6O198-, HTa6O197-,TaO43-, Mo7O246-, HW6O215-, and Keggin-type ions such as PW11O397-and SiW11O397-which are stable at a pH above about 6.

[0020] Di Fronzo et al., “Co-based hydrotalcites as new catalysts for the Fischer-Tropschsynthesis process”, Fuel, Vol.119, pp.62-69 (2104) discloses the preparation of some Co-Zn- Al hydrotalcites and tested them for Fischer-Tropsch synthetis but only after “The calcined catalysts were initially reduced in situ by flowing hydrogen for 4 h at 90.0 Nml min-1, 350 °C and 0.8 MPa. This is below the full reduction temperature of the cobalt.” Bianchi et al. “Co- Zn-Al Based Hydrotalcites as Catalysts for Fischer-Tropsch Process” at Innovative Applications in Petrochemistry and Refining conference held in Dresden, Germany between 4- 6 October 2011 discloses the preparation of Co-Zn-Al based hydrotalcites being activated at 350 °C as catalysts for Fischer-Tropsch synthesis. Jung et al., “Effect of cobalt supported on meso-macro porous hydrotalcite in Fischer-Tropsch synthesis” RSC Adv., Vol.6, pp.104280 – 104293 (2016) discloses a Mg-Al hydrotalcite based cobalt catalyst and its use for Fischer- Tropsch synthesis.

[0021] The present invention is primarily concerned with the Fischer-Tropsch reaction. To this end, the present invention provides an improved catalyst for selectively converting syngas to hydrocarbons. Despite the attempts to enhance the efficiency of Fischer-Tropsch synthesis using state-of-the-art catalysts, a need exists for the development of catalysts which under Fischer-Tropsch conditions selectively produce hydrocarbons, especially high molecular weight paraffins, with a high conversion rate of carbon monoxide and with low methane production. The present invention seeks to mitigate the above-mentioned need in the art. SUMMARY OF THE INVENTION

[0022] The present invention provides, according to a first aspect, a method of preparing a catalyst comprising the steps of: a) contacting a source of a divalent metal cation Md2+and a source of a trivalent metal cation Mt3+in an aqueous medium in the presence of carbonate anion CO32-at a pH of at least 8.0 to form a mixture; b) maintaining the mixture under conditions suitable to crystallize a hydrotalcite; c) recovering the hydrotalcite; and d) reducing the recovered hydrotalcite by heating in a reducing atmosphere to produce the catalyst.

[0023] It has been found that using the synthesis method described above in which a catalyst is produced by reduction of an intermediate hydrotalcite provides a catalyst with high metal loading and a high metal dispersion as compared to catalysts prepared by known methods, such as impregnation of a metal salt onto a support.

[0024] According to the second aspect of the invention, there is provided a catalyst madeby the method of the first aspect. According to the third aspect of the invention, there is provided a process of producing a Fischer-Tropsch hydrocarbon product comprising the step of contacting a catalyst of the second aspect with synthesis gas comprising hydrogen and carbon monoxide under conditions suitable for Fischer-Tropsch synthesis to generate the Fischer-Tropsch hydrocarbon product.

[0025] It will be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the methods of the invention may incorporate any of the features described with reference to the products of the invention and vice versa. DESCRIPTION OF THE DRAWINGS

[0026] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

[0027] Figure 1 shows the reduction profile of a reference catalyst comprising 11.15 weight percent cobalt and 0.9 weight percent rhenium on a titania support;

[0028] Figure 2 shows the reduction profile of the sample 1 Co / Al hydrotalcite is shown on a thermogravimetry plot with weight loss (TG) and the derivative of the weight loss (DTG) plotted against temperature;

[0029] Figure 3 shows a curve representing the weight gain of the reduced catalyst having the formula Co3AlO1.5upon exposure to a 1% N2O / 99% Ar gas stream at 30 °C;

[0030] Figure 4 shows a transmission electron microscopic (TEM) micrograph of the reduced catalyst having the formula Co3AlO1.5showing large number of cobalt crystallites around 10 nm in size;

[0031] Figure 5 shows a TEM micrograph of the reference catalyst; and

[0032] Figure 6 shows the surface-volume-averaged diameter (Dsv) particles size analysis of the reference catalyst; wherein Dsv=(∑NiDi3) / ∑NiDi2). DETAILED DESCRIPTION OF THE INVENTION

[0033] It has been found that a catalyst having a high metal content and a small particle size thereby providing a high active surface area may result when the catalyst is prepared according to the method described in the present disclosure. The utilization of such catalysts can offer economic benefits in the large-scale production of Fischer-Tropsch hydrocarbon products.

[0034] The divalent metal cation Md2+may be any divalent metal which has catalytic activity in the zero oxidation state, that is, when reduced to the metallic state. Optionally, the divalent metal cation Md2+is selected from the group consisting of Co2+, Ni2+, Fe2+, Cu2+, Zn2+and mixtures thereof . Optionally, the divalent metal cation Md2+is selected from the group consisting of Co2+, Ni2+, Cu2+, and mixtures thereof. Optionally, the divalent metal cation Md2+is selected from the group consisting of Co2+, Ni2+, Fe2+, Cu2+, Zn2+, Mg2+, Ca2+, and mixtures thereof. The trivalent metal cation Mt3+is optionally selected from the group consisting of Al3+, Mn3+, Fe3+and mixtures thereof. The trivalent metal cation Mt3+is optionally selected from the group consisting of Al3+, Mn3+and mixtures thereof. Optionally, the trivalent metal cation Mt3+is selected from the group consisting of Al3+, Mn3+, Fe3+, Co3+, and mixtures thereof. Preferably, the trivalent metal cation Mt3+is Al3+. In a particular embodiment, source of a divalent metal cation Md2+is a source of Co2+and the source of a trivalent metal cation Mt3+is a source of Al3+. The source of aluminium cation is optionally selected from a group consisting of aluminium hydroxide, aluminium trihydroxide, (boehmites and pseudoboehmites), basic aluminium carbonate, aluminium hydroxide-alkali carbonate complex, aluminium aminoacid salt, aluminium alcoholate, water-soluble aluminium salts, and water-soluble aluminate. The source of divalent metal cation Md2+is optionally selected from a group consisting of oxides, hydroxides, carbonates and water soluble salts, e.g., cobalt cation is optionally selected from a group consisting of cobalt oxide, cobalt hydroxide, cobalt carbonate and water soluble cobalt salts.

[0035] The source of the divalent cation Md2+and the source of the trivalent metal cation Mt3+may both be soluble in water such that the mixture formed in step a) is an aqueous solution. Alternatively, one or both of the source of the divalent cation Md2+and the source of the trivalent metal cation Mt3+may be insoluble in water such that the mixture formed in step a) is a solid-liquid mixture, for example, a slurry. Examples of water-soluble sources of aluminium may include aluminium sulphate, aluminium chloride, aluminium nitrate, aluminium acetate, and alkali aluminates, such as sodium aluminate. Examples of water insoluble sources of aluminium are gamma, theta or alpha alumina, alumina oxyhydroxide including boehmite and pseudoboehmite or aluminium trihydroxides, including bayerite and gibbsite. It is possible to form aluminium hydroxide, aluminium hydroxide-alkali carbonate complexes, basic aluminium carbonate or aluminium aminoacids, salts in situ, preceding the described catalyst synthesis. Examples of water-soluble sources of divalent metal cation Md2+may include mineral acid salts, such as chlorides, nitrates and sulphates, e.g., mineral acid salts of cobalt,such as cobalt chloride, cobalt nitrate and cobalt sulphate. Examples of water-insoluble divalent metal cation Md2+components may include oxides, hydroxides, and carbonates, e.g., cobalt oxide, cobalt hydroxide, and cobalt carbonate.

[0036] The source of a divalent metal cation Md2+and the source of a trivalent metal cation Mt3+are contacted in an aqueous medium in the presence of carbonate anion CO32-at a pH of at least 8.0, optionally at least 9.5, for example in the range of from 8.0 to 13.0, for example a pH in the range of from 9.5 to 11.0, to form a mixture capable of producing a hydrotalcite when heated in step b).

[0037] In an embodiment, the carbonate anion may originate as a counterion present in the source of a divalent metal cation Md2+and / or the source of a trivalent metal cation Mt3+. In another embodiment, a separate source of carbonate anion may be added to the aqueous medium in step a). Such procedure may advantageously be omitted when the carbonate salt of the divalent metal cation Md2+and / or the trivalent metal cation Mt3+is used, since in those cases the reaction system may contain sufficient carbonate anion.

[0038] In order to maintain the pH of the entire system at the pH of at least 8.0 during the reaction, alkaline substances such as an alkali metal hydroxide, for example, sodium hydroxide, and / or an alkali metal carbonate such as sodium carbonate may be added to the aqueous medium. Also, when a water-soluble trivalent metal cation Mt3+salt (e.g., aluminium salt) and / or a water-soluble divalent metal cation Md2+salt (e.g., cobalt, nickel, or copper salt) is used, it is convenient to use the combination of alkali carbonate and alkali hydroxide as the source of carbonate, so that control of pH of the reaction system and the carbonate ion supply can be achieved by single operation. The addition of an alkali metal hydroxide and / or an alkali metal carbonate may advantageously be omitted when the carbonate salt of the divalent metal cation Md2+and / or the trivalent metal cation Mt3+is used in a sufficient amount to maintain a pH of at least 8.0 during the reaction. This has the advantage of not requiring the washing and filtering steps of removing the alkali.

[0039] The ratio of divalent metal cation Md2+to trivalent metal cation Mt3+is in the range of from about 2.0 : 1.0 to about 4.5 : 1.0, optionally in the range from 2.0 : 1.0 to 4.1 : 1.0, for example in the range from 2.5 : 1.0 to 3.5 : 1.0. Such ranges have been found to give good yield of hydrotalcite crystals with a relatively high content of the divalent metal which results in a catalyst of relatively high metal loading. Preferably, the divalent metal cation Md2+and trivalent metal cation Mt3+are used in stochiometric ratio.

[0040] If the source of the divalent metal cation Md2+and / or the source of the trivalentmetal cation Mt3+are only partially water-soluble such as aluminium hydroxide, aluminium oxide hydroxides or aluminium oxyhydroxides, basic aluminium carbonate, aluminium aminoacid salt and aluminium hydroxide-alkali carbonate complex, or are water-insoluble, such as divalent metal cation Md2+oxides, hydroxides and carbonates, e.g., cobalt oxide, cobalt hydroxide and cobalt carbonate, they may be added to the aqueous medium in solid form, or as a liquid suspension, such as an aqueous suspension. Alternatively, when the source of a divalent metal cation Md2+and a source of a trivalent metal cation Mt3+are soluble in water, it is preferred to add those to the aqueous medium in the form of aqueous solution. The mixture formed in step a) may therefore be an aqueous solution if the source of a divalent metal cation Md2+and the source of a trivalent metal cation Mt3+are both soluble in water or may be an aqueous suspension or slurry if one or both of those is insoluble in water.

[0041] In step b), the conditions suitable for crystallizing a hydrotalcite will depend on the particular source of a divalent metal cation M2+and a source of a trivalent metal c 3+ d ation Mt employed. The temperature is optionally in the range of from about 0 °C to about 300 °C, optionally in the range of from 0 °C to about 200 °C, optionally 100 °C to 200 °C. The duration of step b) will depend on the temperature and on the particular source of a divalent metal cation Md2+and source of a trivalent metal cation Mt3+used, but is optionally in the range of from 1 hour to 48 hours. Where the source of divalent metal cation Md2+and the source of a trivalent metal cation Mt3+are insoluble in water such as cobalt carbonate or nickel carbonate or copper carbonate with aluminium oxide hydroxides or aluminium oxyhydroxides, step b) can be carried out under hydrothermal conditions, for example at temperatures up to about 200 °C.

[0042] The resulting hydrotalcite crystallized in step b) optionally has a structure according to formula (I): M 2+ 3+ d aMt b(OH)(2a+2b)(CO3)b / 2 (I) wherein Md2+is a divalent metal cation; Mt3+is a trivalent metal cation; and the ratio of a and b is in the range of from about 2.0 : 1.0 to about 4.5 : 1.0, optionally in the range from 2.0 : 1.0 to 4.1 : 1.0, for example in the range from 2.5 : 1.0 to 3.5 : 1.0. Md2+is optionally selected from the group consisting of Co2+, Ni2+, Fe2+, Cu2+, Zn2+and mixtures thereof. Mt3+is optionally selected from the group consisting of Al3+, Mn3+, Fe3+and mixtures thereof. In a preferred embodiment, the hydrotalcite has the formula Md2+3Mt3+1(OH)8(CO3)0.5, e.g., Co3Al1(OH)8(CO3)0.5. Optionally, in such preferred embodiment, a water-soluble aluminium salt and a water-soluble cobalt salt are contacted to form a homogeneous aqueous solution, and an alkali metal carbonate solution is added to maintain a pH of at least 8.0, thereby alsoproviding hydroxide and carbonate anions. Optionally, in step c) the resultant precipitate is recovered, for example by filtering, and is then washed, typically with water, and then dried to provide the hydrotalcite.

[0043] As mentioned above, first row transition elements other than cobalt can form similar hydrotalcite phases with aluminium. For example, copper or nickel can form aluminium containing hydrotalcites with ratios in the same range of from about 2.0 : 1.0 to about 4.5 : 1.0, optionally in the range from 2.0 : 1.0 to 4.1 : 1.0, for example in the range from 2.5 : 1.0 to 3.5 : 1.0 by similar types of reactions as described for cobalt. For example, solutions of water- soluble nickel salts or copper salts such as nickel nitrate or copper nitrate together with water- soluble aluminium compounds may be dissolved in water and precipitated with sodium hydroxide with air present. Alternatively, the water-soluble nickel salts or copper salts may be mixed with sodium hydroxide and sodium carbonate. Reactions can also be performed under hydrothermal conditions up to about 200 °C with insoluble nickel or copper carbonates and aluminium oxide hydroxides or oxyhydroxides. The resulting precipitates can be easily identified with X-ray diffraction.

[0044] In step c) the hydrotalcite crystallized in step b) may be recovered from the mixture by any suitable means, for example, by known solid-liquid separation means such as filtration or centrifugation, optionally followed by drying to provide the dry hydrotalcite. The hydrotalcite formed can be easily identified with X-ray diffraction.

[0045] In step d), the recovered hydrotalcite is reduced by heating in a reducing atmosphere to produce the catalyst. Any suitable conditions may be employed which are sufficient to reduce at least a portion, or preferably substantially all, of the divalent metal cation Md2+to an oxidation state of 0, that is, metallic Md. Optionally, during the reduction step a part or substantially all of the trivalent metal cation Mt3+may also be reduced to an oxidation state of +2 or 0, that is, to Mt2+or Mt0, respectively. Optionally, the catalyst has the formula of any one of formulas (II), (III) or (IV), or is a mixture thereof: M 0 d aM3+ t bO3b / 2 (II); M 0 M2+ d a t bOb (III); M 0 d aM0 t b (IV).

[0046] The catalyst has the formula (II) when only the divalent metal cation Md2+is fully reduced and the trivalent metal cation Mt3+is not reduced. Alternatively, the catalyst has the formula (III) when the divalent metal cation Md2+is fully reduced and the trivalent metal cation Mt3+is reduced to become divalent. Alternatively, the catalyst has the formula (IV) when boththe divalent Md2+and trivalent metal cations Mt3+are fully reduced. All the first row transition metal hydrotalcites may be reduced in a flowing stream of hydrogen gas at temperatures of at or above about 600 °C. The hydrotalcite may be heated in step d) to a temperature of at least 400 °C, optionally at least 550 °C, for example at or above 600 °C. The heating takes place in a reducing atmosphere, for example, an atmosphere comprising hydrogen. In an embodiment the reducing atmosphere is hydrogen.

[0047] In some cases an intermediate structure may be formed prior to the reduction. For example, in the case where Md2+is Ni2+and Mt3+is Al3+the hydrotalcite formed is nickel aluminium hydrotalcite, which may on heating convert to a metastable intermediate having a rock salt structure. Generally, the transformation from hydrotalcite to rock salt in the nickel or magnesium aluminium hydrotalcites occurs on heating in the range from about 400 °C to 650 °C in a non-reducing atmosphere. That rock salt intermediate may then be reduced in step d) by heating in a reducing atmosphere as described herein, to provide the catalyst of the invention. References herein to reducing the recovered hydrotalcite by heating in a reducing atmosphere should be understood to include converting the hydrotalcite to an intermediate form having the rock salt structure and then reducing such an intermediate having a rock salt structure by heating in a reducing atmosphere.

[0048] The inventors have found that the catalyst particles so formed may be smaller than those that would be formed by a calcination of the corresponding carbonates or nitrate precursors (without the presence of the trivalent cation Mt3and / or without the hydrotalcite structure) at the same temperature, followed by reduction in a reducing atmosphere, indicating that the presence of trivalent metal cation Mt3+may promote increased metal surface area in the resulting catalyst. Also, the concentration of the reduced metal derived from the divalent metal ion Md2+may be higher than is generally possible to obtain in catalysts prepared by conventional methods such as impregnation of a support material. Such high metal loadings with small metal particle sizes are desirable as catalysts for Fischer Tropsch synthesis of hydrocarbons.

[0049] In the case of a reduced version of the hydrotalcite, the catalyst made by the method of the invention may have an average particle diameter of no more than 300 Å, optionally no more than 200 Å . The catalyst optionally has an average particle diameter in the range of about 50 Å to 200 Å, optionally in the range of from 80 Å to about 120 Å , optionally in the range of from about 90 Å to about 110 Å , such as about 100 Å. The average size of the catalyst particlesmay be measured by any suitable method, for example, by the gas titration method described below. Alternatively, the average size of the catalyst particles may be a surface-volume- averaged diameter (Dsv) particles size obtained by TEM. Other methods to determine the catalyst particles size include including X-ray diffraction line broadening.

[0050] The catalyst made by the method of the invention may comprise at least 50 wt%, optionally at least 60 wt%, for example at least 70 wt% of the reduced divalent metal, Md0. Optionally, the catalyst made by the method of the invention comprises no more than 90 wt%, optionally no more than 80 wt% of the reduced divalent metal. Optionally, the catalyst comprises from 10 wt% to 50 wt%, optionally from 15 wt% to 40 wt% of an oxide of the trivalent metal Mt. In an embodiment the oxide of the trivalent metal Mtis Al2O3.

[0051] The invention also provides a particulate catalyst having an average particle diameter of less than 300 Å and comprising at least 50 wt % of Md0.

[0052] The catalyst of the invention is useful in processes of producing a Fischer-Tropsch hydrocarbon product by contacting the catalyst with synthesis gas comprising hydrogen and carbon monoxide under conditions suitable for Fischer-Tropsch synthesis.

[0053] The synthesis gas feed used in typical Fischer-Tropsch processes can comprise a mixture of H2 and CO wherein H2 : CO are present in a ratio of at least about 1.7, preferably at least about 1.75, more preferably 1.75 to 2.5, such as at least about 2.1 and / or about 2.1 or less.

[0054] Fischer-Tropsch processes can be implemented in a variety of systems such as fixed bed, slurry bed, and multiple channel designs. In various aspects, Fischer-Tropsch processes can be employed in a wide variety of reactors, such as small reactors (e.g.1+ barrel / day) or in very large reactors (e.g. 10,000-50,000 barrels / day or more). The product, typically a hydrocarbon wax, can be used as is and / or can be converted to other (e.g. liquid) components by a variety of well-known chemical processes.

[0055] Generally, the Fischer–Tropsch process can be operated in the temperature range of about 150°C to about 320 C (302°F – 626°F) and at pressures ranging from about 100 kPaa to about 10 MPaa. Modifying the reaction conditions within the Fischer-Tropsch process can provide control over the yield and / or composition of the reaction products, including at least some control of the chain length of the reaction products. Typical reaction products can include alkanes (primary reaction product), as well as one or more of oxygenates, olefins, other hydrocarbonaceous compounds similar to hydrocarbons but which may contain one or more heteroatoms different from carbon and hydrogen, and various additional reaction by-productsand / or unreacted feed components. These additional reaction products and feed components can include H2O, unreacted syngas (CO and / or H2), and CO2, among other things. These additional reaction products and unreacted feed components can form a tail gas that can be separated from the primary reaction products of the Fischer-Tropsch process in gaseous form, as opposed to non-gaseous product, such as the more typical (desired) liquids and / or hydrocarbonaceous compounds generated by the process. When the goal of the Fischer- Tropsch process is synthesis of longer chain molecules, such as compounds suitable for use as a naphtha feed, a diesel feed, or other distillate boiling range molecules, some small (C1–C4) alkanes, olefins, oxygenates, and / or other hydrocarbonaceous compounds may be incorporated into the tail gas. The primary products from Fischer-Tropsch synthesis can be used directly, and / or can undergo further processing, as desired. For example, a Fischer-Tropsch synthesis process for forming distillate boiling range molecules can generate one or more product streams that can subsequently be dewaxed and / or hydrocracked in order to generate final products, e.g. with desired chain lengths, viscosities, and cold flow properties.

[0056] Hydroprocessing of the Fischer-Tropsch wax, when desired, can typically be accomplished at elevated temperature and pressure in the presence of hydrogen to produce materials (such as at least one non-gaseous product) that can be useful products such as diesel blending stock and / or lube base stock.

[0057] Alternatively, the catalyst described herein (for example, the catalyst derived by reducing the Ni-Al hydrotalcite or the calcined rock salt version of it) is further useful in methane steam reforming reactions by contacting the catalyst with methane and steam under conditions suitable for producing a mixture of carbon monoxide and hydrogen in a reactor, such as a reverse flow reactor. EXAMPLES

[0058] The invention is now described with reference to the following examples. These examples are provided for the purposes of illustration only and the invention should in no way be construed as being limited to these examples, but rather should be construed to encompass any and all variations which become evident as a result of the teaching provided herein. Comparative Example 1 – Reference catalyst

[0059] A reference catalyst comprising 11.15 weight percent cobalt and 0.9 weight percent rhenium on titania support described in U.S. Pat. No.6,117,814 was prepared according to the procedure described in examples 18 and with Fischer-Tropsch test results shown in Table 3 of U.S. Pat. No.6,117,814. About 280 mg of sample was loaded onto a Mettler thermogravimetricanalyzer (TG) and the atmosphere was purged to 100% hydrogen gas at 101.3 kPa (1 atmosphere). As the gas was flowed, the temperature was increased at 4 °C a minute to 375 °C and held for three hours to effectuate full reduction. The result is shown in Figure 1 whereby reduction at 375 °C for 3 hours gives a fully reduced catalyst. Example 2 – Preparation of cobalt- and aluminium-containing hydrotalcites

[0060] A series of cobalt / aluminium hydrotalcites with varying Co / Al ratios and base additions were prepared, as shown in Table 1. As an example, the synthesis of Co3Al hydrotalcite (i.e. sample number 1 described below) includes the following: a 30 mL of aqueous solution containing 14.551 g Co(NO3)2·6H2O (0.050 mol), 6.252 g Al(NO3)3·9H2O (0.017 mol) was added to a 30 ml aqueous solution containing 7.066 g Na2CO3(0.068 mol) at room temperature with stirring. 8.550 g 50% NaOH solution was used to adjust pH to 10.0. The solution was aged at room temperature for 16 hours, or overnight, then heat at 150°C for 6 hours. The resulting precipitate was filtered and washed with abundant amount of water and acetone. The sample was dried in air at room temperature first, then at 100 °C for 4 hours, yielding a purple or brown powder. The recovered hydrotalcite has a chemical formula of Co3Al(OH)8(CO3)0.5. Table 1: Preparation of Co / Al hydrotalcite phases with varying Co / Al ratios and bases r6 3 14.555 g 6.252 g 7.097 g 8.550 g 50% 9.9 6.773g, NaOH purple r Examp e – e uc on pro e o samp e o / y ro a c e rom a e

[0061] In Figure 2, the reduction profile of the sample 1 Co / Al hydrotalcite is shown on a thermogravimetry plot with weight loss (TG) and the derivative of the weight loss (DTG) plotted on the temperature axis. The lowest temperature weight losses are largely from adsorbed water, hydroxyl and carbonate removal, but the second and third peak represent the Co reduction. There is a higher temperature step that requires the reduction to proceed for a few hours at 600 °C to provide the reduced Co catalyst. The calculated cobalt and cobalt oxide contents in both the starting hydrotalcite phase and in the reduced phase are shown in Table 2. The weight loss on the reduction of Co3Al(OH)8(CO3)0.5 to Co3AlO1.5 is 38.4 weight percent. Table 2: Calculated cobalt content in sample 1 Co / Al hydrotalcite and its reduced form Co3Al(OH)8(CO3)0.5as Co3AlO1.5in reducedExample 4 – Calculation of cobalt dispersion from N2O chemisorption data

[0062] Metal dispersion is a measure of the fraction of total metal atoms that are on the external surface of the material. This is a common way to benchmark the ‘quality’ of a catalystmaterial, since the number of surface metal sites on a catalyst typically correlates with the catalytic activity of the material. Dispersion is calculated according to the following formula: ^^^^^ ^^^^^^^^^^ ^^^^^^ ^^ ^^^^^^^ ^^^^^ ^^^^^ ^^ ^^^^^ ^^^^^ ^^ ^^^^^^^^^ 100

[0063] Metal dispersion values can also be used to roughly estimate the average metal particle diameter in the system by the following formula: ^^^^^^. ^^^^^^^^ ^^^^^^^^ ^^^ Å^ ^1000 ^^^^^ ^^^^^^^^^^ ^^^ ^ ^^^^^^^^^^^

[0064] Metal surface sites are quantified by titrating surface metal atoms with an appropriately chosen probe molecule and quantifying the uptake of these molecules. In the case of cobalt metal, N2O is known to quantitatively oxidize metallic surface cobalt atoms by the chemical equation shown below:

[0065] This is acount metal surfaces sites on cobalt. This provides a method to quantify the oxygen uptake from this surface oxidation by monitoring the weight gain of surface oxidation with thermogravimetric analysis (TGA). The following is an example as to how cobalt dispersion of Co3AlO1.5is quantified with this technique. Figure 3 shows a curve representing the weight gain of the reduced catalyst having the formula Co3AlO1.5upon exposure to a 1% N2O / 99% Ar gas stream at 30 °C.

[0066] The weight gain curves follow a particular pattern where there is first a region of faster weight gain, followed by an inflection point where the weight gain becomes much slower (regions delineated with dashed lines). The point where this inflection occurs is taken as the point where the metal surface is fully oxidized and is starting to oxidize somewhat into the bulk, albeit at a much slower rate. We take the mass at this inflection point to be the approximate mass of oxygen needed to completely oxidize all the cobalt surface sites. In thegraph above, we read this value as approximately 143.3 ^^ 140.5 ^^ ^ 2.8 ^^ O . Withthis value we can do a few stoichiometric conversions to determine the number of cobalt surface sites: 0.0027 ^^ & ∙1 ^^^ & 1 ^^^ )^ 1000 ^^^^ ∙ ∙ 15.9994 ^ 1 ^^^ & 1 ^^^^ 0.175 ^^^^ ^^^^^^^ )^

[0067] The total moles of cobalt can be approximated based on the cobalt loading determined from either of the two aforementioned techniques (stoichiometrically or via TGA):0.1405 ^ ^^^^^^^ ^^^^^^^^ ^^ *+, ∙77.6 ^ )^ 1 ^^^ )^ 1000 ^^^^ ∙ ∙ 100 ^ ^^^^^^^ ^^^^^^^^ 58.933 ^ )^ 1 ^^^ ^1.85 ^^^^ ^^^^^ )^

[0068] From here, dispersion and particle size are calculated from the first two equations listed in this section: ^^.^^^ ^^^^^^^^^^ ^0.175 ^^^^ ^^^^^^^ )^ 1.85 ^^^^ ^^^^^ )^^ 100 ^ 9.5% ^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^ ^^^^^^^^ ^Å^ ^1000 9.5^ 105 Å

[0069] This means that cobalt particles are just slightly larger than 10 nm. This is close to and just slightly larger than the particles in the reference catalyst according to Example 1 which showed an 11.4% dispersion and particle diameter of 87 Å.

[0070] The N2O chemisorption measurements are in agreement with the TEM micrograph images obtained for Co3AlO1.5and the reference catalyst of Example 1, as shown on Figure 4 and 5, respectively, wherein the relative sizes of the cobalt particles are shown. ADDITIONAL EMBODIMENTS

[0071] Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described in the following clauses.

[0072] Clause 1: A method of preparing a catalyst comprising the steps of: a) contacting a source of a divalent metal cation Md2+and a source of a trivalent metal cation Mt3+in an aqueous medium in the presence of carbonate anion CO32-at a pH of at least 8.0 to form a mixture; b) maintaining the mixture under conditions suitable to crystallize a hydrotalcite; c) recovering the hydrotalcite; and d) reducing the recovered hydrotalcite by heating in a reducing atmosphere to produce the catalyst.

[0073] Clause 2: The method of Clause 1, wherein Md2+is selected from the group consisting of Co2+, Ni2+, Fe2+, Cu2+, Zn2+and mixtures thereof.

[0074] Clause 3: The method of Clause 1 or Clause 2, wherein Mt3+is selected from the group consisting of Al3+, Mn3+, Fe3+and mixtures thereof.

[0075] Clause 4: The method of Clause 1, i) wherein Md2+is selected from the group consisting of Ca2+, Mg2+, Co2+, Ni2+, Fe2+, Cu2+, Zn2+and mixtures thereof; ii) wherein Mt3+is selected from the group consisting of Co3+, Al3+, Mn3+, Fe3+and mixtures thereof; or iii) a combination of i) and ii).

[0076] Clause 5: The method of any of Clauses 1 to 4 wherein the hydrotalcite crystallized in step b) has the formula (I): M 2+ 3+ d aMt b(OH)(2a+2b)(CO3)b / 2 (I); wherein: Md2+is a divalent metal cation; Mt3+is a trivalent metal cation; and the ratio of a and b is in the range of from about 2.0 : 1.0 to about 4.5 : 1.0.

[0077] Clause 6: The method of any of Clauses 1 to 5 wherein the catalyst produced in step d) has the formula of any one of formulas (II), (III) or (IV) or is a mixture thereof: M 0 d aM3+ t bO3b / 2 (II); Md0aMt2+bOb(III); M 0 aM0 d t b (IV).

[0078] Clause 7: The method of any one of Clauses 1 to 6, wherein Md2+is Co2+and Mt3+is Al3+.

[0079] Clause 8: The method of any one of Clauses 1 to 7, wherein the hydrotalcite has the formula: Co3Al1(OH)8(CO3)0.5; and the catalyst has the formula: Co3Al1O1.5.

[0080] Clause 9: The method of any one of Clauses 1 to 8, wherein the source of the divalent cation Md2+and the source of the trivalent metal cation Mt3+are soluble in water such that the mixture in step a) is an aqueous solution.

[0081] Clause 10: The method of any one of Clauses 1 to 8, wherein the source of the divalent cation Md2+and the source of the trivalent metal cation Mt3+are insoluble in water such that the mixture in step a) is a slurry.

[0082] Clause 11: The method of any one of Clauses 1 to 10, further comprising adding sodium hydroxide to the aqueous medium in step a).

[0083] Clause 12: The method of any one of Clauses 1 to 11, wherein, prior to step d), the recovered hydrotalcite is heated to a temperature in the range of from about 300 °C to about 650 °C in a non-reducing atmosphere.

[0084] Clause 13: The method of any one of Clauses 1 to 12, wherein in step d) the recovered hydrotalcite is heated in a reducing atmosphere to a temperature of at least 600 °C, or step d) includes converting the recovered hydrotalcite to an intermediate form having a rock salt structure and then reducing the intermediate having a rock salt structure by heating in a reducing atmosphere to a temperature of at least 600 °C.

[0085] Clause 14: A catalyst made by the method of any one of Clauses 1 to 13.

[0086] Clause 15: A process of producing a Fischer-Tropsch hydrocarbon product comprising the step of contacting a catalyst made by a method according to any of Clauses 1 to 13 or a catalyst according to Clause 14 with synthesis gas comprising hydrogen and carbon monoxide under conditions suitable for Fischer-Tropsch synthesis to generate the Fischer- Tropsch hydrocarbon product.

[0087] Clause 16: A process according to Clause 15 wherein the conditions suitable for Fischer-Tropsch synthesis include a temperature of about 150 °C to about 320 °C and a pressure in the range of from about 100 kPaa to about 10 MPaa.

[0088] Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims

CLAIMS 1. A method of preparing a catalyst comprising the steps of: a) contacting a source of a divalent metal cation Md2+and a source of a trivalent metal cation Mt3+in an aqueous medium in the presence of carbonate anion CO32-at a pH of at least 8.0 to form a mixture; b) maintaining the mixture under conditions suitable to crystallize a hydrotalcite; c) recovering the hydrotalcite; and d) reducing the recovered hydrotalcite by heating in a reducing atmosphere to produce the catalyst.

2. The method of claim 1, wherein Md2+is selected from the group consisting of Co2+, Ni2+, Fe2+, Cu2+, Zn2+and mixtures thereof.

3. The method of claim 1 or claim 2, wherein Mt3+is selected from the group consisting of Al3+, Mn3+, Fe3+and mixtures thereof.

4. The method of claim 1, i) wherein Md2+is selected from the group consisting of Ca2+, Mg2+, Co2+, Ni2+, Fe2+, Cu2+, Zn2+and mixtures thereof; ii) wherein Mt3+is selected from the group consisting of Co3+, Al3+, Mn3+, Fe3+and mixtures thereof; or iii) a combination of i) and ii).

5. The method of any of claims 1 to 4 wherein the hydrotalcite crystallized in step b) has the formula (I): M 2+ d aM3+ t b(OH)(2a+2b)(CO3)b / 2 (I); wherein: Md2+is a divalent metal cation; Mt3+is a trivalent metal cation; and the ratio of a and b is in the range of from about 2.0 : 1.0 to about 4.5 : 1.

0.

6. The method of any of claims 1 to 5 wherein the catalyst produced in step d) has the formula of any one of formulas (II), (III) or (IV) or is a mixture thereof: M 0 d aM3+ t bO3b / 2 (II); Md0aMt2+bOb(III); Md0aMt0b(IV).

7. The method of any one of claims 1 to 6, wherein Md2+is Co2+and Mt3+is Al3+.

8. The method of any one of claims 1 to 7, wherein the hydrotalcite has the formula: Co3Al1(OH)8(CO3)0.5; and the catalyst has the formula: Co3Al1O1.5.

9. The method of any one of claims 1 to 8, wherein the source of the divalent cation Md2+and the source of the trivalent metal cation Mt3+are soluble in water such that the mixture in step a) is an aqueous solution.

10. The method of any one of claims 1 to 8, wherein the source of the divalent cation Md2+and the source of the trivalent metal cation Mt3+is insoluble in water such that the mixture in step a) is a slurry.

11. The method of any one of claims 1 to 10, further comprising adding sodium hydroxide to the aqueous medium in step a).

12. The method of any one of claims 1 to 11, wherein, prior to step d), the recovered hydrotalcite is heated to a temperature in the range of from about 300 °C to about 650 °C in a non-reducing atmosphere.

13. The method of any one of claims 1 to 12, wherein in step d) the recovered hydrotalcite is heated in a reducing atmosphere to a temperature of at least 600 °C, or step d) includes converting the recovered hydrotalcite to an intermediate form having a rock salt structure and then reducing the intermediate having a rock salt structure by heating in a reducing atmosphere to a temperature of at least 600 °C.

14. A catalyst made by the method of any one of claims 1 to 13.

15. A process of producing a Fischer-Tropsch hydrocarbon product comprising the step of contacting a catalyst made by a method according to any of claims 1 to claim 13 or a catalyst according to claim 14 with synthesis gas comprising hydrogen and carbon monoxide under conditions suitable for Fischer-Tropsch synthesis to generate the Fischer- Tropsch hydrocarbon product.

16. A process according to claim 15 wherein the conditions suitable for Fischer-Tropsch synthesis include a temperature of about 150 °C to about 320 °C and a pressure in the range of from about 100 kPaa to about 10 MPaa.

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