Catalytic valorization of the acetate fraction of biomass to aromatics and its integration into the carboxylate platform

Abstract

In many plant species, the acetate fraction is the fourth most prominent fraction by weight after cellulose, hemicellulose and lignin, and can be easily extracted as a single stable molecule, acetic acid, at high yields. Despite this, upgrading the acetate fraction of biomass has received very limited attention. Here, we demonstrate a valorization route for the acetate fraction as well as mixtures of acetic acid and other volatile fatty acids produced from the polysaccharide fraction. Aqueous solutions of acetic acid, including solutions produced during steam explosion pretreatment and subsequently purified can be upgraded at high selectivity to a valuable mixture of aromatics, substituted cycloalkenes and gas olefins in a single step using Cu/ZrO₂. The catalyst displays remarkable stability despite the presence of acids, water and other biomass-derived impurities. We also show that acetic acid can be further valorized over the same catalyst by converting it in the presence of butanoic acid that was produced in a consolidated bioprocess from the same pretreated wood that was the source of the acetic acid. In this case, the acetic acid rapidly ketonizes with the butanoic acid and the resulting beta-ketones further condense to form aromatics and cycloalkenes with a higher average carbon number than those produced solely from acetic acid. Overall, our process yields a biomass-derived organic oil consisting of aromatics and cycloalkenes that spontaneously separates from water, can be tuned by varying the incoming mixture of carboxylic acids and has suitable properties for being used as a direct blend with aviation fuel.