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In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/catalysis Page 1 of 13Catalysis Science & Catalysis Science & Technology Dynamic Article Links ► Technology Cite this: DOI: 10.1039/c0xx00000x ARTICLE TYPE www.rsc.org/xxxxxx Reactivity of Ethanol over Hydroxyapatite-Based Ca-enriched Catalysts with Various Carbonate Contents † Lishil Silvester a,b , Jean-François Lamonier a,b , Jérémy Faye a,b , Mickaël Capron a,b , Rose-Noëlle Vannier a,b,c , Carole Lamonier a,b , Jean-Luc Dubois d, Jean-Luc Couturier d, Christophe Calais d, Franck Dumeignil a,b,e,* Manuscript 5 Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x 45 derived, can also be produced directly from biomass, but through The Guerbet reaction of ethanol to heavier products was routes which are still not economically feasible at the moment 5,6 performed over a series of extensively characterized (biotech and syngas conversion). Then, finding a chemical carbonate-containing hydroxyapatites (HAPs) with different alternative route to transform ethanol to heavier alcohols such as 10 Ca/P ratios, and thus different densities, strengths and butanol could represent an attractive option with respect to the natures of acid and basic sites. These properties were 50 biorefinery concept of producing a diversity of chemicals and 7 correlated with the reactivity of the solids and an optimal fuels. Such a transformation is possible through the Guerbet Accepted ratio between the amount of acid and basic sites was reaction. evidenced ( ca. 5). The best performances were accordingly Both homogeneous and heterogeneous catalysts have been reported for the production of heavier alcohols through the 15 obtained over a Hap-CO 3 catalyst, which gave a yield of 30% 55 Guerbet reaction. The Guerbet reaction using homogeneous in heavier alcohols at a 40% ethanol conversion. catalysts was first examined using transition metal complexes such as Rh, Ru, Pt, and Ir under very mild conditions of 110- Introduction 8-11 140 °C at atmospheric pressure. Catalytic activity of Rh The Guerbet reaction, of which the name originates from the complex was found to be very high compared with that of Ru and Marcel Guerbet’s discovery, is an organic reaction in which a 60 Ir complexes. Pd and Au complexes are inactive in the Guerbet 20 primary aliphatic alcohol is converted to its β-alkylated dimer reaction because it is difficult to generate stable hydride metal with the loss of one molecule of water. Marcel Guerbet initially complexes. 8,9 Carlini et al. disclosed the Guerbet reaction using found that 1-octanol, thus a primary alcohol, is transformed to homogeneous Ni and Pd-based catalysts in the presence of basic Technology C16 and C24 alcohols when heated at 200 °C in the presence of alkoxides, but using a higher reaction temperature of 200 °C. 9,12- 14 65 Catalytic systems based on Cu, Ni, Rh, Ru and Pd metals in its sodium derivative or with sodium derivatives of other & 1 25 alcohols. Further, Marcel Guerbet reported that this reaction is combination with soluble alkoxides as basic components were also applicable to secondary alcohols by demonstrating the also employed in the Guerbet reaction to synthesize iso butanol. 13- production of higher chain alcohols, namely 4-methylpentanol 16 The main disadvantage of using excess alkoxide is linked to and 2,4 -dimethylheptanol, from iso propanol, with, e.g. , 3- metal leaching, and also to the production of alkali hydroxides 11-16 methylbutyric acid, acetic acid and formic acid formed as by- 70 like sodium hydroxide, which is not environmental friendly. 2 30 products. Weizmann et al . reported that the presence of an α- So, in order to make the Guerbet reaction economically feasible methylene group in the immediate vicinity of the hydroxyl group for industrial applications, there is a need for developing is a requisite for the aldol condensation step and hence for the heterogeneous catalysts, which can also be environmental Guerbet reaction. They have also confirmed that the Guerbet friendly in terms of catalyst separation and reusability. Science reaction between primary and secondary alcohols is 75 As we will comment in the next page, the Guerbet reaction 35 predetermined by the fact that an aldehyde condenses with a needs multifunctional catalysts, especially behaving both acid and ketone, most of time giving an alkylidene-ketone even though base sites. Ueda et al . reported the condensation of various there are some exceptions. 3,4 primary alcohols with methanol over solid basic catalysts such as Bioethanol is one of the major compounds produced in large MgO, CaO and ZnO. 17-20 The reaction took place only over MgO, quantities from biomass, but its high vapour pressure, its affinity 80 as the other metal oxides were found to be inactive or non- 40 for water and its lower energy content are hurdles for an selective. The Guerbet reaction of alcohols has also been studied extensive application in existing fuels, while this is a major using various Mg-Al mixed oxides derived from hydrotalcite type output for this compound nowadays. Biobutanol, which is another precursors. 21-25 The rate of alcohol dehydration to ethers and Catalysis primary alcohol that can be used as a fuel or a fuel additive, but olefins is higher for Al-rich Mg yAlO x solids, but dehydrogenation also as a platform molecule from which C4 chemistry can be 85 and aldol condensation of ethanol, respectively to corresponding This journal is © The Royal Society of Chemistry 2015 Catalysis Science & Technology , 2015, [vol] , 00–00 | 1 Catalysis Science & Technology Page 2 of 13 Table 1 List and basic characteristics of the catalytic solids used in the aldehyde and higher alcohols, is favoured over Mg-rich Mg yAlO x samples. 21,25 Also, introduction of copper into the hydrotalcite- present study (from data of Ref 31). Ca/P atomic Na Specific surface Type of solid Name derived Mg-Al mixed oxide enhanced the activity of catalyst ratio (ICP) (wt.%) area (m 2.g -1) 22,23 towards the Guerbet reaction. Deficient HapD 1.62 - 124 5 Hydroxyapatite was especially found to be an active catalyst Stoichiometric Hap 1.69 - 114 for the heterogeneous gas phase reaction of ethanol. 26,27 To this Carbonated Hap-CO 1.70 - 107 respect, Tsuchida et al. reported the ethanol conversion over 3 hydroxyapatites with different Ca/P ratios up to the Carbonated HapNa-CO 3 1.72 0.64 109 27 stoichiometric value of 1.67. As a result, they obtained about Carbonate-rich HapE-CO 3 1.90 - 76 10 70% of selectivity to higher alcohols, at an ethanol conversion of Carbonate-rich HapE-Na-CO 3 2.39 < 0.3 72 50%. Indeed, the drawback of using hydroxyapatites for the Guerbet reaction is that at higher ethanol conversions, the produced primary alcohols undergo further condensation to Characterisations 28 produce heavier alcohols and other heavier products. These 60 Extensive physico-chemical characterization of the solids, Manuscript 15 heavier alcohols can however be used, e.g. , as solvents, fuel namely, X-ray diffraction (XRD), Thermogravimetric analysis additives or plasticizers, provided relevant separation operations (TGA), Scanning electron microscopy (SEM), Infrared can be performed. spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Concerning the reaction mechanism, it is most largely admitted Low-energy ion scattering spectroscopy (LEIS), Hetero nuclear that the Guerbet reaction proceeds through a series of consecutive 65 magnetic resonance spectroscopy (2D NMR), Temperature 20 steps over multifunctional catalysts (acid, base and/or redox programmed desorption (TPD-NH 3 & TPD-CO 2), XPS after sites), which were especially considered to explain the reactivity adsorption of 2-phenylethylamine (PEA-XPS) and pulse liquid 27,29 of ethanol over the aforementioned HAPs. First, chromatographic adsorption technique using benzoic acid, are dehydrogenation of the alcohol takes place over redox/acid-base reported elsewhere. 31 The present paper is then focused on the sites to form aldehydes, which further undergo an aldol 70 solids reactivity in the Guerbet reaction, in relation to the Accepted 25 condensation over basic sites to produce the corresponding aldol. aforementioned characterization results. The as-obtained aldol then undergoes dehydrogenation over acid The Guerbet reaction of ethanol was carried out using 0.20 g sites to form α-β unsaturated aldehyde or ketone that further (GHSV = 15000 mL.h -1.g -1) or 0.60 g (GHSV = 5000 mL.h -1.g -1) undergoes hydrogenation over redox/acid-base sites to form of sample under the form of a powder with a particle size 30 higher alcohols. However, Scalbert et al. recently showed that 75 between 80 and 100 mesh. 20 Vol.% ethanol in He gas were 30 over 350 °C and using a commercial HAP of which the Ca/P ratio passed through a fixed bed glass reactor at a total flow rate of was not mentioned but which is supposedly stoichiometric, 50 mL/min.