Production of jet-fuel-range molecules from biomass-derived mixed acids

Abstract

Biomass has received considerable attention as a feedstock for the replacement of crude oil for producing both energy and high-value chemicals. In this work, we use a combination of chemical and biological processing to produce long-chain linear and branched ketones with low oxygen content. A mixture of medium-chain-length carboxylic acids was obtained by methane-inhibited, open-culture anaerobic fermentation of lignocellulosic biomass, and this mixture was further oligomerized using heterogeneous chemical catalysis. The products fall in the range of C₁₀–C₂₀ molecules that can potentially be blended with existing hydrocarbon jet fuels. We used a Pd/CeZrO_x catalyst to achieve >90% yield to C₁₁₊ ketones starting from C₂–C₄ mixed acids. The acids are first recovered from the fermentation broth as ethyl esters by reactive distillation using Amberlyst-45 as a catalyst. We evaluated the activity of several bifunctional catalysts for upgrading these ethyl esters into long-chain ketones, finding that 0.25 wt% Pd/CeZrO_x was most active. Using a combination of experimental reaction kinetics measurements and gas-phase thermodynamics calculations, we postulate a reaction network that explains the production of the most abundant products via a combination of direct ester ketonization, dehydration, and hydrogenation.