Catalytic hydrodeoxygenation and C–C coupling of lignin and its derivatives into renewable jet-fuel-range cycloalkanes

Cycloalkanes with the carbon numbers C9–C16 are ideal jet-fuel components and are mainly synthesized by the hydrogenation of petroleum-derived benzenes and the cyclization reactions of linear alkanes. The catalytic conversion of lignin and its derivatives, intrinsically embodying carbocyclic structures, to jet-fuel-range cycloalkanes has been demonstrated as a potential green and economical route, which can improve the sustainability of sustainable aviation fuels (SAFs) as well as reduce the overall greenhouse gas emissions. Direct hydrodeoxygenation (HDO) as well as C–C coupling relay hydrodeoxygenation are the two main routes for the production of cycloalkanes from lignin and its derivatives. In this review, first, the HDO of lignin C–O derivatives to monocycloalkanes over metal–acid catalysts was considered a model reaction to provide an understanding of the catalytic structure–activity relationship of the indispensable HDO process. Then, the production of lignin jet fuel via the simultaneous depolymerization and HDO of real lignin was discussed, followed by the C–C coupling relay hydrodeoxygenation route for polycycloalkanes production, including the alkylation relay hydrodeoxygenation route, aldol condensation relay hydrodeoxygenation route and one-pot conversion route. Furthermore, this paper attempts to highlight the remaining challenges and provide some perspectives for the future design of structure-specific cycloalkanes, aiming to provide insights into the viable utilization of lignin to obtain C9–C16 ideal jet-fuel-range cycloalkanes.