Heterogeneous Catalytic Transfer Hydrogenation as an Effective Pathway in Biomass Upgrading

Reducing oxygen content in biomass-derived feedstocks via hydrodeoxygenation (HDO) is a key step in their upgrading to fuels and valuable chemicals. Organic molecules, e.g., alcohols and formic acid, can donate hydrogen to reduce the substrate in a process called catalytic transfer hydrogenation (CTH). Although it is practiced far less frequently than molecular hydrogen-based HDO processes, CTH has been proven to be an efficient and selective strategy in biomass upgrading in the last two decades. In this paper, we present a selective review of recent progress made in the upgrade of biomass-derived feedstocks through heterogeneous CTH, with a focus on the mechanistic interpretation. Hydrogenation and cleavage of C=O and C-O bonds, respectively, are the two main categories of reactions discussed, owing to their importance in the HDO of biomass-derived feedstocks. On acid base catalysts, Lewis acid base pair sites, rather than a single acid or base site, mediate hydrogenation of carbonyl groups with alcohols as the hydrogen donor. While acid base catalysts typically only catalyze the hydrogenation of carbonyl groups with alcohols as the hydrogen donor, metal-based catalysts are able to mediate both hydrogenation and hydrogenolysis reactions with either alcohols or formic acid. Several model reactions involving platform chemicals in biomass upgrading, e.g., 5-hydroxymethylfurfural, levulinic acid, and glycerol, are used in the discussion to illustrate general trends. Because alcohols are typically both the hydrogen donor and the solvent, the donor and solvent effects are intertwined. Therefore, solvent effects are discussed primarily in the context of sugar isomerization and reactions with formic acid as the hydrogen donor, in which the solvent and hydrogen donor are two separate species. Current challenges and opportunities of future research to develop CTH into a competitive and complementary strategy of the conventional molecular-hydrogen-based processes are also discussed.