Process for upgrading ketones

Abstract

A process for the upgrading of ketones wherby a ketone is reacted with hydrogen in the presence of a catalyst system comprising i) a condensation catalyst and ii) a seperate hydrogenation catalyst, th

Description

Field of the Invention The present invention relates to a catalytic process for the upgrading of ketones, especially the upgrading of acetone to C9 and C12 products. Declaration of funding The project leading to this application has received funding from the European Union’s Horizon 2020 research and innovation programme “PYROCO2” under grant agreement number 101037009. Background The condensation of acetone to oligomeric products has been well studied. The reaction begins with the formation of diacetone alcohol “DAA” which can be dehydrated and further oligomerised to give a variety of C6, C9 and C12 products. Key products and intermediates in these reactions are: isopropyl alcohol “IPA” (C3), diacetone alcohol “DAA” (C6), mesityl oxide “MO” (C6), methyl isobutyl ketone “MIBK” (C6), methyl isobutyl carbinol “MIBC” (C6), phorone “PHO” (C9), diisobutyl ketone “DIBK” (C9), 3,3,5-trimethylcyclohexanone “DHIPHO” (C9) and 2,6,8-trimethyl-4-nonanone “TMN” (C12). C9 and C12 compounds are commercially important because they correspond to the predominant carbon fractions in jet fuel. One class of condensation catalyst which has been studied in this reaction is hydrotalcites. Hydrotalcites are a family of layered magnesium- and aluminium-containing carbonate materials which can be described by the general formula Mg6Ah(OH)i6CO3.nH2O. The use of Mg-AI hydrotalcites as catalysts for the conversion of acetone to MIBK is known and is described in the following articles: “One-step synthesis of methyl isobutyl ketone from acetone with calcined Mg / AI hydrotalcite-supported palladium or nickel catalysts" Applied Catalysis A: General 169 (1998) 207-214; “Environmentally-benign liquid-phase acetone condensation process using novel heterogeneous catalyst’ Chapter 16 194-205 Nikolopoulous et al. Green Chemical Syntheses and Processes ACS Symposium Series: American Chemical Society, Washington, DC, 2000; “Influence of the metal function in the “one-pot" synthesis of 4-methyl-2-pentanone (methyl isobutyl ketone) from acetone over palladium supported on Mg(AI)O mixed oxide catalysts" Catalysis Letters Vol. 71, No. 3-4, 2001; “Single-stage liquid-phase synthesis of methyl isobutyl ketone under mild conditions" Journal of Molecular Catalysis A: Chemical 219 (2004) 273-281; “Acetone condensation and selective hydrogenation to MIBK on Pd and Pt hydrotalcite-derived Mg-AI mixed oxide catalysts” Applied Catalysis A: General 296 (2005) 128-136; “A hydrotalcite-based catalyst system for the single-stage liquid-phase synthesis of MIBK’ Applied Catalysis A: General 307 (2006) 231-238; “Ni-containing mixed oxides as catalysts for the one-step synthesis of methyl-isobutyl-ketone (MIBK) from acetone” C. R. Chimie 12 (2009) 12961304. Whilst C9 and C12 species may be formed as a by-product in these reactions, the major species is MIBK. To the present inventors’ knowledge there is only one previous report in which C9 and C12 species are made from acetone to any appreciable extent. The article “Synthesis of Acetone-Derived Cs, Cg, and C12 Carbon Scaffolds for Chemical and Fuel Applications’’ ChemSusChem 2016, 9, 3382-3386 describes a catalyst system of Amberlyst™ 15 and Ni / SiCb-AhOs. In an example acetone was treated with the catalyst system under at atmosphere of Ar / H2 (94:6; 1.7 MPa) in cyclohexane at 120 °C to produce a mixture of ketones and aromatic products; the C6 yield was 33% and the C9+C12 yield was 51%. Amberlyst™ is a styrene-divinylbenzene copolymer having sulfonic acid functional groups. A disadvantage of the catalyst system used in this reference is that ionexchange resins typically have low thermal stability which limits the temperature under which the reaction can be performed. A further disadvantage of this catalyst system is that the Ni / SiCb-AhOshas a high Ni loading of 65 wt.%. CN102190568A describes a method for synthesizing MIBK which is carried out in the presence of a Pd-Ca / AhCh catalyst. The Pd-Ca / AhCh catalyst is prepared by depositing palladium chloride onto a Ca / AhCh support prepared by impregnating an alumina carrier with a calcium salt. The selectivity for MIBK in the examples is in excess of 93%, with isopropranol (C3) and mesityl oxide (C6) also being formed as by-products. There is a need for alternative catalyst systems having a greater selectivity towards C9 to C12 products, ideally which can tolerate high reaction temperatures and ideally not requiring high loadings of transition metals. Summary of Invention The inventors have found that calcium-containing condensation catalysts which are a mixed oxide material comprising aluminium, calcium and optionally magnesium oxides, show a surprisingly high selectivity for ketone upgrading, especially for C9 and C12 products when acetone is used as the substrate. The condensation catalyst is used in conjunction with a hydrogenation catalyst; the hydrogenation catalyst may be a separate catalyst or may be provided by supporting active metals on the condensation catalyst. The term “upgrading” as used herein refers to the coupling together of two or more molecules of a ketone substrate. The steps typically include condensation and hydrogenation. In a first aspect the invention relates to a process for the upgrading of ketones, comprising the step of reacting a ketone and hydrogen together in the presence of a catalyst system comprising: (i) a condensation catalyst; and (ii) a separate hydrogenation catalyst; wherein the condensation catalyst is a mixed oxide material comprising aluminium, calcium and optionally magnesium; wherein the condensation catalyst has a Ca:(Mg+AI) weight ratio of 0.25 <Ca:(Mg+AI) <2.5; and wherein the condensation catalyst has a Mg:AI weight ratio of 0 <Mg:AI <3.0. In a second aspect the invention relates to a process for the upgrading of ketones, comprising the step of reacting a ketone and hydrogen together in the presence of a catalyst system comprising: a condensation catalyst; wherein the condensation catalyst is a mixed oxide material comprising aluminium, calcium and optionally magnesium; wherein the condensation catalyst has a Ca:(Mg+AI) weight ratio of 0.25 <Ca:(Mg+AI) <2.5; wherein the condensation catalyst has a Mg:AI weight ratio of 0 <Mg:AI <3.0; and wherein a transition metal is supported on the condensation catalyst and there is no separate hydrogenation catalyst. Whilst the catalyst system is expected to have applicability in the upgrading of a variety of ketones, it is particularly preferred that the ketone is a C3 (acetone), C4 (butanone) or C5 (2-pentanone or 3-pentanone) ketone. Acetone is particularly preferred. Detailed Description Any sub-headings are for convenience only and are not intended to limit the disclosure. In the following various weight ratios of metals are referred to. The weight of each element in the condensation catalyst can be determined by inductively coupled plasma mass spectrometry (ICP-MS). Condensation catalyst The condensation catalyst is a mixed metal oxide comprising aluminium, calcium and magnesium. The metals are present in the oxidic form. In embodiments where a separate hydrogenation catalyst is present, it is preferred that the condensation catalyst is free of metals other than aluminium, calcium and magnesium, i.e. the condensation catalyst has content of any metal other than Al, Ca and Mg which is <0.5 wt.% when measured by ICP-MS, preferably less than 0.2 wt.%, more preferably less than 0.1 wt.%. It will be appreciated that metals besides Al, Ca and Mg may be present at the impurity level because of impurities present in the raw materials used in manufacture of the condensation catalyst. In some embodiments, described further below, a transition metal is supported on the condensation catalyst, and there is no separate hydrogenation catalyst. In these embodiments the catalyst serves a dual role as a condensation catalyst and a hydrogenation catalyst. The transition metal should be selected from those which are active for the hydrogenation of C=C and / or C=O bonds under the reaction conditions, more preferably a metal having higher selectivity towards C=C hydrogenation compared to C=O hydrogenation, so as to avoid direct hydrogenation of the ketone reagent. In these embodiments it is preferred that the content of any metal other than Al, Ca, Mg and the transition metal, is <0.5 wt.% when measured by ICP-MS, preferably less than 0.2 wt.%, more preferably less than 0.1 wt.%. Palladium is a preferred transition metal and in these embodiments it is preferred that the content of any metal other than Al, Ca, Mg and Pd, is < 0.5 wt.% when measured by ICP-MS, preferably less than 0.2 wt.%, more preferably less than 0.1 wt.%. The condensation catalyst has a Ca:(Mg+AI) weight ratio that is in the range of 0.25 to 2.5 i.e. 0.25 <Ca:(Mg+AI) <2.5. For the avoidance of doubt, a catalyst which contains 40 parts by weight Ca, 5 parts by weight Mg and 40 parts by weight Al, would have a Ca:(Mg+AI) weight ratio of 40 : 45 or 0.89. When the Ca:(Mg+AI) weight ratio is below 0.25 both the conversion and selectivity are impaired. A preferred Ca:(Mg+AI) weight ratio is 0.25 to 1.5, preferably 0.25 to 1.0, preferably 0.25 to 0.50, preferably 0.25 to 0.45. The condensation catalyst has a Mg:AI weight ratio that is in the range of 0 to 3.0 i.e. 0 <Mg:AI <3.0. For the avoidance of doubt, a catalyst which contains 30 parts Mg and 40 parts Al would have a Mg:AI weight ratio of 30 : 40 or 0.75. Magnesium is an optional component in the condensation catalyst, but when Mg is present the Mg:AI weight ratio is at most 3.0. It is preferred that Mg is present because better selectivities can be achieved using Ca-Mg-Al materials compared to Ca-AI materials. For optimal selectivity towards the desired condensation products a preferred Mg:AI weight ratio is 1.0 to 3.0 i.e. 1.0 <Mg:AI <3.0 , preferably 1.5 to 3.0, preferably 2.0 to 3.0, preferably 2.25 to 3.0. A Mg:AI weight ratio of 1.6 to 2.6 is also particularly preferred. Without wishing to be bound by any theory, it is currently thought that the presence of one or more of the following phases within the condensation catalyst is beneficial for achieving high selectivity towards condensation products: Ca(OH)2, CaCOs, CaO and / or Ca^AI^Oss (mayenite). Therefore, in a preferred embodiment the condensation catalyst includes one or more phases selected from the group consisting of: Ca(OH)2, CaCOs, CaO and / or Cai2Ali4O33 (mayenite). Preparation of the condensation catalyst The Ca-AI or Ca-Mg-AI oxide materials used as the condensation catalyst can be prepared by a variety of different routes known to those skilled in the art, such as hydrothermal synthesis or co-precipitation. A preferred route comprises combining magnesium oxide and alumina together as a slurry in water together with an acid. The mixing is preferably carried out above room temperature, preferably at 50-90 °C, for a duration sufficient to achieve thorough mixing. The duration of mixing will depend on the scale, but at pilot scale stirring at 80 °C for 2.5 hours was adequate. This is followed by addition of a calcium salt to the Mg oxide / alumina slurry, then spray-drying and calcining. It is particularly preferred that Ca(OH)2 is used as the calcium salt. Without wishing to be bound by theory, it is thought that the use of Ca(OH)2 in production may alter the proportion of acid / base sites in the catalyst in a way which promotes the condensation reaction. Hydrogenation catalyst The role of the hydrogenation catalyst is to hydrogenate unsaturated intermediates (in the case of acetone condensation C6, C9 and C12 intermediates) formed by the condensation catalyst in order to drive the conversion of the ketone substrate towards the condensation products. The hydrogenation catalyst includes a transition metal, which should be selected from those which are active for the hydrogenation of C=C and / or C=O bonds under the reaction conditions, more preferably a transition metal having higher selectivity towards C=C hydrogenation compared to C=O hydrogenation so as to avoid direct hydrogenation of the substrate (e.g. acetone to IPA). Preferred transition metals are platinum group metals because of their activity as hydrogenation catalysts. A preferred transition metal is palladium because of its high activity and selectivity towards C=C hydrogenation. In some embodiments the catalyst system comprises a condensation catalyst and a separate hydrogenation catalyst, i.e. there are two distinct catalysts. The condensation catalyst and hydrogenation catalyst may be located in separate sections of the catalyst bed. Alternatively, condensation catalyst and hydrogenation catalyst may be uniformly distributed throughout the catalyst bed, e.g. as a uniform mixture of particles of each catalyst. When the catalyst system includes a condensation catalyst and a separate hydrogenation catalyst, the hydrogenation catalyst is preferably a transition metal supported on a support. Preferred transition metals are platinum group metals because of their activity as hydrogenation catalysts. A preferred transition metal is palladium because of its high activity and selectivity towards C=C hydrogenation. Any suitable support may be used. Preferred supports include metal oxides and carbon. In other embodiments a transition metal is supported on the condensation catalyst, and there is no separate hydrogenation catalyst. Preferred transition metals are platinum group metals because of their activity as hydrogenation catalysts. A preferred transition metal is palladium because of its high activity and selectivity towards C=C hydrogenation. Process The process involves reacting the ketone and hydrogen together in the presence of the catalyst system. The catalyst system is preferably present as a catalyst bed, which either comprises a condensation catalyst and a separate hydrogenation catalyst, or a dual condensation / hydrogenation catalyst in which a transition metal is supported on the condensation catalyst. The catalyst, or catalysts as the case may be, are preferably present in the form of particles. The reaction conditions may vary over a wide range. Typical and preferred reaction conditions are a temperature of 180-260 °C a hydrogen pressure of 10-50 bar. Temperatures and pressures towards the higher ends of these ranges may promote the unwanted hydrodeoxygenation reactions instead of the desired ketone coupling reactions. Preferred conditions are a temperature of 180-240 °C and a hydrogen pressure of 10-40 bar. Examples A design of experiments (DOE) was conducted to investigate the impact of nine different parameters relating to the preparation, catalyst composition and reaction conditions, on acetone conversion and the selectivity of the catalyst towards the desired C9 and C12 products. The DOE identified the Mg:AI weight ratio, Ca:(Mg+AI) weight ratio and the type of Ca salt used during the preparation of the condensation catalyst as particularly important factors in acetone coupling to the desired C9 and C12 products. To achieve good acetone conversion and selectivity towards C9 / C12 products it was found necessary for Ca to be present, and to control the Mg:AI ratio and Ca:(Mg+AI) ratio within certain ranges. Examples 1-3 have been selected to demonstrate the importance of including Ca such that the ratio Ca:(Mg+AI) satisfies 0.25 <Ca:(Mg+AI). Preparation of catalyst El (according to the invention) The catalyst was prepared by mixing 24 parts by weight alumina (as a 10 wt.% slurry in water) and 56 parts by weight magnesium oxide (as a 15 wt.% slurry in water) and acetic acid (84 wt.%) at room temperature, followed by stirring at 80 °C for 2.5 h. Once cooled down to room temperature, 19 parts Ca(OH)2 (as a 50% solids slurry) was added slowly to the alumina / magnesium oxide slurry with stirring. The final slurry was spray dried, sieved and then calcined at approximately 450 °C. Preparation of catalyst CE2 (comparative) The catalyst was prepared by mixing 49 parts by weight alumina (as a 10 wt.% slurry in water) and 39 parts by weight magnesium oxide (as a 15 wt.% slurry in water) and acetic acid (84 wt.%) at room temperature, followed by stirring at 80 °C for 2.5 h. Once cooled down to room temperature, 2 parts vanadium oxide (as a 13% solids slurry) and 10 parts cerium oxide (as a 28% solids slurry) were added to the alumina / magnesium oxide slurry. The final slurry was spray dried, sieved and then calcined at approximately 450 °C. Preparation of catalyst CE3 (comparative) The catalyst was prepared by mixing 30 parts by weight alumina (as a 10 wt.% slurry in water) and 70 parts by weight magnesium oxide (as a 15 wt.% slurry in water) and acetic acid (84 wt.%) at room temperature, followed by stirring at 80 °C for 2.5 h. Once cooled down to room temperature, the final slurry was spray dried, sieved and then calcined at approximately 450 °C. Preparation of catalyst CE4 (comparative) The catalyst was prepared by mixing 24 parts by weight alumina (as a 10 wt.% slurry in water) and 56 parts by weight magnesium oxide (as a 15 wt.% slurry in water) and acetic acid (84 wt.%) at room temperature, followed by stirring at 80 °C for 2.5 h. Once cooled down to room temperature, the slurry was diluted to 10% solids, spray dried, sieved and then calcined at approximately 450 °C. Preparation of catalyst E5 (according to the invention) The catalyst was prepared by mixing 24 parts by weight alumina (as a 10 wt.% slurry in water) and 56 parts by weight magnesium oxide (as a 15 wt.% slurry in water) and acetic acid (84 wt.%) at room temperature, followed by stirring at 80 °C for 2.5 h. Once cooled down to room temperature, 39 parts Ca(OH)2 (as a 50% solids slurry) was added slowly to the alumina / magnesium oxide slurry with stirring. The final slurry was spray dried, sieved and then calcined at approximately 450 °C. Preparation of catalyst E6 (according to the invention) The catalyst was prepared by mixing 48 parts by weight alumina (as a 10 wt.% slurry in water) and 35 parts by weight magnesium oxide (as a 15 wt.% slurry in water) and acetic acid (84 wt.%) at room temperature, followed by stirring at 80 °C for 2.5 h. Once cooled down to room temperature, 19 parts Ca(OH)2 (as a 35% solids slurry) was added slowly to the alumina / magnesium oxide slurry with stirring. The final slurry was spray dried, sieved and then calcined at approximately 450 °C. Preparation of catalyst E7 (according to the invention) The catalyst was prepared by mixing 31 parts by weight alumina (as a 10 wt.% slurry in water) and 49 parts by weight magnesium oxide (as a 15 wt.% slurry in water) and acetic acid (84 wt.%) at room temperature, followed by stirring at 80 °C for 2.5 h. Once cooled down to room temperature, 19 parts Ca(OH)2 (as a 35% solids slurry) was added slowly to the alumina / magnesium oxide slurry with stirring. The final slurry was spray dried, sieved and then calcined at approximately 450 °C. Selectivity testing in acetone upgrading In order to remove any residual moisture formed during storage the catalysts were recalcined at 500 °C for 8 h prior to use. Reactions were performed in a 10-well Baskerville reactor equipped with a pressure gauge and a magnetic stirrer. Acetone (5 mL, 3.92 g, 0.067 mol), iso-octane (0.172 mL, 0.12 g, 1.04 mmol, internal standard), condensation catalyst (1.308 g, 25 wt.% with respect to acetone) and Pd / C (8.56 mg, 0.60 mol% Pd with respect to acetone) were added to the autoclave. The autoclave was purged 10 times with H2 then pressurised with 30 bar H2 and heated to the 220 °C. Reaction mixture was stirred with constant stirring at 800 rpm for 16 h. At the end of the reaction the autoclave was cooled, depressurised and the reaction mixture was filtered with 0.45 mm PTFE syringe filters. The filtered solutions were analysed by GC-MS and the proportion of major products (IPA (C3), MIBK (C6), DIBK (C9), DHIPHO (C9) and TMN (012)) are reported in Table 1. Mg:AI (w / w) Ca:(Mg+AI) (w / w) Other metals Conversion (%) Selectivity C3 C6 C9 C9 C12 C9+C12 IPA MIBK DIBK DHIPHO TMN - E1 2.51 0.34 - 97 2 3 45 8 42 87 CE2 0.95 0.013a V, Ce 94 2 35 53 - 9 62 CE3 3.73 0.0074a - 53 - 78 18 4 - 22 CE4 2.53 0.015a - 84 - 39 51.5 5.5 4 61 E5 2.72 2.35 - 93 - 16 47 5 31 83 E6 0.88 1.32 - 96 - 19 57 0 24 81 E7 1.64 0.83 - 97 3 8 40 8.5 40.5 89 a Originates from an impurity in the magnesium oxide raw material Table 1. E1, CE4 and E5 have approximately the same Mg:AI ratio. E1 and E5 both include Ca whereas CE4 does not. A comparison of E1, CE4 and E5 shows that the presence of Ca is beneficial both for conversion and C9+C12 selectivity. The beneficial impact of Ca on conversion and C9+C12 selectivity can also be seen by comparison of CE2 and E6. CE2 and E6 also have approximately the same Mg:AI ratio; E6 includes Ca whereas CE2 does not include any appreciable amount of Ca. A comparison of E1 and CE2 / CE3, together with further trends obtained from the DOE, showed that the presence of magnesium was beneficial for selectivity, particularly when the Mg:AI ratio was 1.0 to 3.0. Selectivity testing in 2-butanone upgrading In order to remove any residual moisture formed during storage the catalysts were recalcined at 500 °C for 8 h prior to use. Reactions were performed in Premex multi batch autoclave reactors, fitted with overhead stirrers allowing gas entrainment, with all reactors having independent gas phases and temperature and stirring control. 2-butanone (12.5 mL, 10.06 g, 0.14 mol), iso-octane (0.43 mL, 0.30 g, 2.61 mmol, internal standard), condensation catalyst (3.35 g, 25 wt.% with respect to 2-butanone) and Pd / C (17.88 mg, 0.60 mol% Pd with respect to 2-butanone) were added to the autoclave. The autoclave was purged twice with N2 and 6 times with H2 then pressurised with 15 bar H2 and heated to the 200 °C. Reaction mixture was stirred with constant stirring at 800 rpm for 16 h. At the end of the reaction the autoclave was cooled, depressurised and the reaction mixture was filtered with 0.45 mm PTFE syringe filters. The 5 filtered solutions were analysed by GC-MS and the proportion of major products (2-butanol (C4) and 5-methyl-3-heptanone (C8)) are reported in Table 2. Mg:AI (w / w) Ca:(Mg+AI) (w / w) Other metals Conversion (%) Selectivity C4 C8 2-butanol 5-methyl-3-heptanone E1 2.51 0.34 - 73 36 64 CE3 3.73 0.0074a - 60 64 36 10 Table 2.