Lignin Catalytic Depolymerization for Phenolic Monomers: Boosting the Selective Cleavage of beta-O-4 Bonds of Lignin by Mo=O and Al(IV)-O-BO2 Interfacial Sites in B-Mo/Sepiolite

Selective breaking of beta-O-4 bonds is a challenge for lignin depolymerization. Herein, boron (B) was utilized to construct superficial Mo=O and subsurface Al(IV)-O-BO2 interfacial sites of Mo/sepiolite (SEP), which acted as Lewis acid sites to promote beta-O-4 bond cleavage of lignin. Characterizations and density functional theory (DFT) calculations showed that B could interact with the quadridentate Al(IV) atoms of SEP to generate Al(IV)-O-BO2 and did not affect the incipient Mo=O of Mo/SEP. The experimental results demonstrated that the B content altered the catalytic performance of Mo/SEP, and B-Mo/SEP achieved 99.3% lignin liquefaction and the highest selectivity of phenol (48.5%) and ethoxyphenol (27.4%) monomers. Combined with DFT simulation, the results confirmed that the antibonding orbital of Mo=O accepted electrons of alcohol oxygen, while the B bonding orbital in Al(IV)-O-B-O-2 received electrons of benzyl oxygen, promoting the formation of C+ intermediates. Simultaneously, oxygen vacancies in SEP activated the ethanol medium to facilitate enol intermediate hydrogenation for phenolic compounds.