Upgrading of biomass-derived 2-hexanol to liquid transportation fuels on Cu-Mg-Al mixed oxides. Effect of Cu content

Abstract

The gas-phase synthesis of high molecular weight compounds of application as liquid transportation fuels from 2-hexanol was studied on Cu-Mg-Al mixed oxides with different copper content (0.3-61.2%) and a Mg/Al =1.5 molar ratio. Catalysts were prepared by coprecipitation and characterized by several techniques such as BET surface area, XRD, TPD of CO2, TPR and N2O decomposition. Yields of up to 87% were obtained for compounds in the C9-C24 range, 80% of which were suitable as jet fuels and the rest as diesel substitutes. This product pool was a hydrophobic mixture of ketones, alcohols and hydrocarbons with 160-200 g/mol average molecular weight and an O/C atomic ratio as low as 0.04. Because low copper content catalysts are hard to reduce, on these materials the reaction occurs via a base-catalyzed mechanism involving consecutive dehydrogenation, C-C bond formation, dehydration and hydrogenation steps, that forms mainly even carbon atom number products. Partially reduced Cun+ atoms contribute to promote a distinct pathway toward odd products. In contrast, on high copper content oxides the reaction yields similar amounts of even and odd products and proceeds by a bifunctional Cu0-base mechanism in which the C-C coupling is rate-limiting.