Mechanisms of Caromatic-C bonds cleavage in lignin over NbOx-supported Ru catalyst

Efficient and selective cleavage of C aromatic -C linkages in lignin to enhance the productivity of monocyclic aromatic hydrocarbons is an extremely challenging and attractive task. Previously, we reported a Ru/NbOPO 4 catalyst that realized the catalytic cleavage of both interunit C-C and C-O bonds in one-pot lignin conversions [ Chem , 2019, 5, 1521–1536]. However, the roles of NbO x species in the selective C aromatic -C cleavage are not clear and need to be extensively studied. Herein, we focus on the exploration of the catalytic mechanisms of C aromatic -C bond cleavage over NbO x -supported Ru catalyst, by comparing with those of Ru/TiO 2 , Ru/ZrO 2 , and Ru/Al 2 O 3 catalysts. It shows that the Ru/Nb 2 O 5 catalyst exhibits high activity and remarkable selectivity to monocyclic hydrocarbons. A combined diffuse reflectance infrared Fourier Transform spectra (DRIFTS) and density functional theory (DFT) calculation analysis unambiguously confirmed that (i) NbO x not only can strongly adsorb phenylcyclohexane via benzene ring, but also selectively activate C aromatic -C bond between benzene ring and cyclohexane ; (ii) NbO x can promote the protonation of phenylcyclohexane with the assistance of H 3 PO 4 ; and (iii) NbO x can significantly reduce the barrier of C aromatic -C bonds cleavage of the carbocation intermediate (C 6 H 5 (H)C 6 H 11 + ). A detail underlying mechanism has been established, which would have further implications to promote the design of new, high efficient catalysts.