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 molecularhydrogen-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 CO 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.
Biomass conversion to fuels and chemicals provides sustainability, but the highly oxygenated nature of a large fraction of biomass-derived molecules requires removal of the excess oxygen and partial hydrogenation in the upgrade, typically met by hydrodeoxygenation processes. Catalytic transfer hydrogenation is a general approach in accomplishing this with renewable organic hydrogen donors, but mechanistic understanding is currently lacking. Here, we elucidate the molecular level reaction pathway of converting hemicellulose-derived furfural to 2-methylfuran on a bifunctional Ru/RuO x /C catalyst using isopropyl alcohol as the hydrogen donor via a combination of isotopic labeling and kinetic studies. Hydrogenation of the carbonyl group of furfural to furfuryl alcohol proceeds through a Lewis acid-mediated intermolecular hydride transfer and hydrogenolysis of furfuryl alcohol occurs mainly via ring-activation involving both metal and Lewis acid sites. Our results show that the bifunctional nature of the catalyst is critical in the efficient hydrodeoxygenation of furanics and provides insights toward the rational design of such catalysts.
Biomass-derived furans offer sustainable routes to adipic acid (AA), a key chemical in Nylon-6,6 synthesis. In this work, we show that tetrahydrofuran-2,5-dicarboxylic acid (THFDCA) is a viable precursor for AA production, achieving up to 89% yield in a metal-free system containing HI and molecular H2 in a propionic acid solvent at 160 °C. Reactivity studies demonstrate that the interplay between HI, H2, and the solvent is essential for effective THFDCA ring opening. By measuring the reaction orders of HI and molecular H2 and calculating an acid–base equilibrium constant in a nonaqueous solvent, we show that HI plays a multifaceted role in the reaction by acting both as a proton source and an iodide source to selectively cleave C–O bonds without overhydrogenation of carboxylic acid groups. Using reactivity studies, kinetic measurements, and first-principles computational insights, we demonstrate that metal-free activation of molecular H2 plays a key role in the reaction, following HI-mediated cleavage of the etheric C–O bond in THFDCA.
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