Upgrading of diols by gas-phase dehydrogenation and dehydration reactions on bifunctional Cu-based oxides

Abstract

Biomass-derived short-chain polyols can be transformed into valuable oxygenates used as building blocks. The gas phase conversion of a model molecule of 1,3-diols (1,3-butanediol), was studied on bifunctional Cu–Mg, Cu–Al and Cu–Mg–Al mixed oxide catalysts that exhibit surface Cu⁰ particles and acid–base properties. A series of ZCuMgAl catalysts (Z = 0.3–61.2 wt.% Cu, Mg/Al = 1.5 molar ratio) was prepared by coprecipitation and thoroughly characterized by several techniques such as BET surface area, TPR-N₂O chemisorption, XRD and TPD of CO₂. The ZCuMgAl catalysts promote the upgrading of diols by a series of dehydrogenation and/or dehydration reactions proceeding at reaction rates that depend on the copper content (Z). The overall activity increases linearly with the amount of surface Cu⁰ species thereby confirming the participation of metallic sites in the rate-limiting steps. Besides, surface Cu⁰ sites favor the reaction pathway toward 1,3-butanediol dehydrogenation. Thus, the dehydrogenation/dehydration selectivity ratio increases with Z as a result of the enhanced amount of exposed Cu⁰ particles. ZCuMgAl catalysts with Z < 8 wt.% dehydrogenate–dehydrate–hydrogenate the diol at low rates giving mainly C₄ ketones and break the intermediates forming C₁–C₃ oxygenates; catalysts with Z > 8 wt.% have higher activity and yield valuable multifunctional C₄ oxygenates such as hydroxyketones and, to a lesser extent, unsaturated alcohols and ketones. A strongly basic Cu–Mg catalyst promotes the C–C bond cleavage reaction giving short carbon chain oxygenates at low rates; an acidic Cu–Al catalyst converts the diol into C₄ saturated ketones and olefins.