Rhenium-based catalysts for biomass conversion

Biomass is the most abundant and renewable resource in the pursuit of bio-value added chemicals. Platinum-group metals and typical nonprecious transition metals are considered as the active metals for catalytic conversion of biomass into value added chemicals and fuels. Rhenium (Re) in particular is a versatile, oxophilic element with a broad variety of easily accessible oxidation states ideally suited for catalytic applications. Taking advantage of Re's unique properties and the green concept of biomass, the catalytic transformation of lignocellulose with Re-based catalysts has gained considerable attention, leading to the preparation of value-added chemicals including olefins through deoxydehydration from the conversion of polyols, aromatics through depolymerization of lignin, and alkane fuels through a hydrodeoxygenation reaction. This review focuses on the catalytic conversion of lignocellulosic main components (cellulose, hemicellulose, and lignin) and their derived platform chemicals into value-added chemicals and fuels over homogeneous and heterogeneous Re-based catalysts. The reaction pathways for biomass conversion over Re-based catalysts are introduced based on different feedstocks or chemical bonds. The unique role of Re species in tailoring the active sites of catalysts for these reactions is summarized and possible reaction mechanisms are discussed. Finally, an outlook is provided to underscore both the challenges and opportunities associated with this interesting and important field, offering a comprehensive overview on the current state of Re catalysis with a low carbon footprint. This review highlights the advancements in homogeneous and heterogeneous Re-based catalysts for the conversion of biomass into value-added chemicals.