Liquid transportation fuels from biomass-derived oxygenates: Gas-phase 2-hexanol upgrading on Cu-based mixed oxides

Abstract

The gas-phase upgrading of 2-hexanol, a model molecule of the primary conversion of sugars, toward higher molecular weight compounds of application as liquid transportation fuels was investigated on Cu-MI-MII mixed oxides (MI, MII: Mg2+, Al3+, Ce4+) at 573K and 101.3 kPa. Catalysts were prepared by coprecipitation and characterized by several techniques such as BET surface area, XRD, TPD of CO2 and NH3, TPR and N2O decomposition. The bifunctional metal-base catalytic process occurs through a series of sequential steps comprising dehydrogenation, C-C coupling, dehydration and hydrogenation reactions. Nano-sized Cu0 particles promote dehydrogenation and hydrogenation steps whereas acid-base sites provided by MI(MII)-O pairs participate in the C-C coupling reaction. In general, main products were C9-C12 compounds that represented ~60% of the product pool. Branched C9-C24 compounds such as ketones, alcohols and alkanes were obtained with yields of up to 91% on a Cu-Mg-Al mixed oxide with 8wt.% Cu (catalyst 8.0CuMgAl). This catalyst presented well dispersed Cu0 particles and a high number of base sites with moderate basic properties as well as a low number of acid sites. The rate-limiting step of the bifunctional process leading to C9-C24 products on catalyst 8.0CuMgAl was the metal-promoted hydrogenation step, but the reaction can be controlled by the C-C bond formation step on less basic catalysts. By carrying out experiments under different reaction atmospheres (N2 or H2) and at different contact times, a reaction pathway leading to formation of odd carbon atom number products (C9, C15 and C21) is postulated in contrast to the conventional aldol condensation pathway toward even carbon atom number products (C12, C18 and C24). The former prevails under conditions at which the catalyst surface is deprived of hydrogen atoms.