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 conduct reductions of lignin that obviates the use of dangerous and non-renewable hydrogen gas. While noble metals are used extensively as catalysts for transfer hydrogenation, one major challenge for their deployment is related to their sustainability. In this work, we synthesized mixed-metal oxides of earth-abundant Co and Ni. We characterized these catalysts using powder x-ray diffraction (XRD) and tested their reactivity for CTH of acetophenone. Among the catalysts we tested, we noted that the spinel NiCo2O4 demonstrated the highest conversion of acetophenone (75%) and highest selectivity for ethylbenzene (90%), so we applied it to valorization of switchgrass lignin extracted under mild operating conditions by cosolvent enhanced lignocellulosic fractionation (CELF). The catalytically depolymerized lignin showed an increase in selectively deoxygenated monomeric compounds. Using 2D-NMR spectroscopy, we demonstrated the lignin displayed highly reduced aliphatic carbons resulting from the reduction reaction at the Cα sites mediated by our catalyst material. 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.