Catalysts for the upgrade of bio-ethanol to butanol

The use of biomass as energy resource has attracted a growing interest in the last years due to the increasing energy demand and to the environmental issues related to the combustion of fossil fuels. Ethanol represents the most widerspread bio-fuel although its addition to gasoline is limited by issues as water solubility and corrosivity which would require the substitution for a higher fuel-soluble alcohol such as butanol [1]. Bio-butanol can be produced by biomass fermentation with Clostridium microorganisms (Acetone Butanol Ethanol or ABE fermentation) [2]. The fraction of ethanol in the ABE mixture can be upgraded to butanol by catalytic route according to the Guerbet reaction [3]. This reaction starts with dehydrogenation of the alcohol, followed by aldol condensation, dehydration and finally hydrogenation of the unsaturated aldehyde. Addition of a metal can promote the hydrogenation/dehydrogenation steps slightly lowering the reaction temperature. Nevertheless, in the absence of metal promoters, ethanol dehydrogenation proceeds on basic metal oxides through a dissociative adsorption of the alcohol on weak Lewis acid-strong Brönsted base pairs breaking the OH bond forming a surface alkoxide intermediate. This work summarizes the research activity done within Waste2Fuels 'Sustainable production of next generation biofuels from waste streams' project funded under the European Union's research and innovation program Horizon 2020. The work started with a preliminary screening of powder catalysts using MgO,?-Al2O3 and hydroxyapatite as oxide supports and Ru and Ni as supported metals in a lab-scale isothermal plug-flow reactor to establish the key-features associated to better butanol yields [4]. Then, the best formulation, identified as ruthenium dispersed on MgO, was reproduced on a structured substrate, consisting in commercial ?-Al2O3 pellets, in order to use the catalyst in a larger scale rig avoiding high pressure drops [5]. The catalytic pellet composition preserving the high surface of ?-Al2O3 but assuring the basic surface properties of MgO, intrinsically much more active and selective towards butanol production than alumina, was determined together with the operating parameters of the preparation technique. Finally, due to the partial deactivation of this material observed under wet feed mixture, a new catalyst, based on MgO dispersed on high surface area activated carbon, was proposed. This catalyst, in addition to better performance under dry feed condition, assigned to the high dispersion of MgO on the micro-porous support, also showed the expected water tolerance that was verified also by carrying out long run tests. Results of catalytic activity were supported by a deep chemical and physical characterization of materials such as ICP-MS analysis, SEM/EDX analysis, porosimetric analysis, TPD of NH3 and CO2, H2-TPR.