Multicentre Synergistic Polyoxometalate-based Metal–Organic Framework for One-Step Selective Oxidative Cleavage of β-O-4 Lignin Models

Abstract

A novel mixed-valence polyoxovanadate-based copper organic framework, [CuI(bbi)]2{[CuI(bbi)]2VIV2VV8O26}2H2O (NENU-MV-5, bbi=1,1'-(1,4-butanediyl)bis(imidazole)), was facilely synthesized from routine reagents under mild hydrothermal condition. Using NENU-MV-5 as heterogeneous catalyst without any co-catalyst, one-step oxidative cleavage of β-O-4 lignin to phenols and aromatic acids with high catalytic activity and selectivity was realized under oxygen atmosphere. No obvious activity decrease after five recycles indicates the excellent stability and sustainability of NENU-MV-5. The perfect catalytic performance of NENU-MV-5 can be attributed to multi-sites synergistic effect of the mixed-valence VV-O-VIV sites on polyoxovanadate for oxidation of β-O-4 alcohol to β-O-4 ketone and the Cu(I) sites on the framework for rapid cleavage of Cα-Cβ bond of β-O-4 ketone. This system represented the first co-catalyst-free example for one-step selective degradation of lignin catalyzed by a well-defined crystalline catalyst with definite composition and structure in a single solvent.