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/CeZrOx 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/CeZrOx 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.
