Catalytic Upgrading of Biomass-Derived Carboxylic Acids to Fuels and Chemicals

Our reliance on fossil fuels, which is unsustainable due to dwindling reserves, has led to various environmental issues. Lignocellulosic biomass could serve as a renewable source of carbon and energy for the production of fuels and chemicals. While several valorization routes of biomass-derived sugars (via furans or sugar alcohols) exist, the upgrading of the biological carboxylate platform is relatively unexplored, mainly due to historical focus on ethanol production via fermentation. Research on biomass-derived carboxylic acids could increase our knowledge on carboxylic acid upgrading and open up new avenues towards sustainable production of fuels and chemicals from lignocellulosic biomass.

The production of olefins from carboxylic acids via tandem hydrogenation/dehydration was carried out, with olefin selectivities > 90% at close to 99% conversion of carboxylic acids using a Cu/SiO2-Al2O3 catalyst. Interestingly, a sudden selectivity switch from olefins to predominantly alkanes was observed at full conversion. Using various intermediate products and catalyst surface studies, this selectivity switch phenomenon was ascertained to originate from the adsorption of small amounts of carboxylic acid on the catalyst surface, which prevented the binding and subsequent overhydrogenation of olefins. This finding suggests that carboxylic acids has potential to be used in influencing product distributions of other catalytic reactions, especially if selective double bond preservation is required. This phenomena could also be used to explain the difference in catalyst performance observed when using commercial model compounds versus real biomass-derived feed, given the prevalence of carboxylic acid impurities in biomass-derived streams. This route represents a single-step upgrading of carboxylic acids to olefins with no loss in carbon efficiency and without use of expensive stoichiometric reagents, which is important for economical production of bulk chemicals from lignocellulosic biomass.

In parallel, a study on carboxylic acid upgrading via single-step ketonization/cascade aldol condensation to an aviation fuel additive was performed over Cu/ZrO2. Biomass-derived acetic acid and butanoic acid was converted to an organic oil composed of aromatics and cycloalkenes with a carbon number range of C8 - C16 at mass yields of around 20 wt %. By-products include gaseous hydrocarbons (mainly propylene) and water resulting from condensation. This organic oil represents up to 96% deoxygenation of the starting carboxylic acids, and has been tested to be compatible as a 10 vol % blend with Jet A-1 fuel in terms of specific energy and distillation properties. While previous research on ketonization and aldol condensation have performed the reactions separately, this route combines both in a single step to upgrade short carboxylic acids to long chain hydrocarbons, while valorizing the often neglected acetate fraction of biomass. This study shows a combination of C-C coupling and extensive deoxygenation, both crucial towards successful upgrading of lignocellulosic biomass to fuels.