Synergistic enhancement of catalytic hydrodeoxygenation performance by oxygen vacancies and frustrated Lewis pairs

Abstract

The catalytic hydrodeoxygenation (CHDO) of lignin into saturated cycloalkanes not only enhances the efficient utilization of lignin but also reduces reliance on high-density liquid fuels (HDLFs), given the importance of saturated cycloalkanes as key constituents of HDLFs. The main challenge in lignin CHDO towards HDLFs is efficiently removing oxygen-atom while maintaining high selectivity for the desired saturated cycloalkanes. Herein, we report for the first time the fabrication of a Ni/N_0.8-CeO₂-500 composite with abundant oxygen vacancies (OVs) and adjacent frustrated Lewis pairs (FLPs), achieved through the capping effect of ionic liquids. The FLPs of N/Ce³⁺ within Ni/N_0.8-CeO₂-500, mediated by OVs, not only enhance the dispersion of Ni species but also effectively facilitate the conversion of H₂ into active hydrogen (H*) through relay catalysis involving the Ni species. This integration, combining the oxygen atom-specific recognition of OVs with the adjacent FLPs of N/Ce³⁺ for the tandem generation of H*, significantly promotes the adsorption/cleavage of C_ar/alk−O−C_alk bonds, oxygen-atom removal, and hydrogenation of aromatic rings, ultimately catalyzing the one-step production of saturated cycloalkanes. The synergistic catalytic interplay results in a remarkable yield of saturated cycloalkanes (up to 88.2 wt%) during the CHDO of Kraft lignin, representing the highest reported value under similar conditions to date. A combination of diverse characterizations, experimental analyses, and kinetic studies collectively reinforces the notion that in-situ N-doping of CeO₂ increases the electron cloud density around Ce species, forming N-Ce^δ+ entities that, at high temperatures, create OVs and adjacent FLPs of N-Ce³⁺, collectively enhancing lignin CHDO to yield saturated cycloalkanes.