Catalytic Transfer Hydrogenolysis of Switchgrass Lignin with Ethanol Using Spinel-Type Mixed-Metal Oxide Catalysts Affords Control of the Oxidation State of Isolated Aromatic Products

Chemical reductions of lignin are useful to remove oxygen and create product slates that can function as renewable platform molecules for new fuels and chemicals. Catalytic transfer hydrogenolysis (CTH) is an underexplored method to reduce lignin that obviates the use of dangerous and nonrenewable hydrogen gas. While noble metals are used extensively as catalysts for transfer hydrogenation, sustainability remains a major challenge to their deployment. In this work, we synthesized mixed-metal oxides of earth-abundant Co and Ni and characterized the catalysts using powder X-ray diffraction (XRD). Catalyst reactivity for the CTH of acetophenone was also assessed. Among the catalysts tested, spinel NiCo2O4 demonstrated the highest conversion of acetophenone (75%) and the highest selectivity for ethylbenzene (90%); thus, we applied it to valorize switchgrass lignin, extracted under mild operating conditions by cosolvent enhanced lignocellulosic fractionation (CELF). The catalytically depolymerized lignin showed an increase in the number of selectively deoxygenated monomeric compounds. As demonstrated using 2D-NMR spectroscopy, the lignin displayed highly reduced aliphatic carbons, resulting from the catalyst-mediated reduction reaction at the C alpha sites. These results are critical to the further development of the lignin-first biorefinery as they demonstrate the use of sustainable catalyst materials and mild transformation conditions to generate and refine a suite of new bioproducts.