Heterogeneous Monometallic and Bimetallic Catalysts for Deoxydehydration and Catalytic Transfer Hydrogenation: Valorization of Sugar Alcohols and Acids

Lignocellulosic biomass provides a renewable resource for sustainable routes to biofuels and biochemicals through catalytic conversion. Oxorhenium catalysts effectively transform biomass-derived polyols into olefins via deoxydehydration with a secondary alcohol reductant. In attempts to develop a reusable solid catalyst and to investigate a possible mechanism for deoxydehydration by a heterogeneous catalyst, unsupported rhenium oxide nanoparticles (ReOx NPs) were prepared. The nanoparticle catalyst with 3-octanol as a reductant catalyzed deoxydehydration of various polyols to corresponding alkenes with high efficiency. In-situ and ex-situ X-ray spectroscopy analysis showed the changes in oxidation states and structures of ReOx NPs during heterogeneous deoxydehydration reaction. This, combined with kinetic studies and kinetic isotope effect, proposed a possible mechanism involving a ReVII/ReV redox pair for deoxydehydration driven by heterogeneous ReOx catalyst and alcohol reductants.Multi-functional catalysis including deoxydehydration with monometallic and bimetallic heterogeneous catalysts was studied for the valorization of sugar acids. First, a combination of deoxydehydration and catalytic transfer hydrogenation by a bifunctional Pt-ReOx/C catalyst achieved a one-step conversion of mucic acid, a C6 sugar acid, to adipates in high yield (85%). Isopropanol was employed as a hydrogen donor for both reactions. The catalyst was reusable and regenerated multiple times. Spectroscopic studies of Pt-ReOx/C demonstrated ReVII and Pt0 as the relevant species of deoxydehyration and catalytic transfer hydrogenation, respectively. Based on all observations, a bifunctional mechanism was proposed. This bifunctional catalysis was also achieved with Ir-ReOx/C, allowing significantly reduced metal contents in the catalyst. Similarly, deoxydehydration and transfer hydrogenation of tartaric acid, a C4 sugar acid, converted to succinates with monometallic ReOx NPs. Further dehydration/dealcoholization and transfer hydrogenation over ReOx NPs produced γ-butyrolactone and tetrahydrofuran, realizing a one-step conversion of sugar acids to C4 commodity products.