Towards lignin valorisation: comparing homogeneous catalysts for the aerobic oxidation and depolymerisation of organosolv lignin

Abstract

Lignin is a unique potential source of aromatic chemicals derived from renewable resources. Homogeneous oxovanadium, copper and cobalt complexes exhibit intriguing selectivity for the aerobic oxidation of complex lignin models, affording some control of C–C, C–H and C–O bond cleavage. In this work these catalysts are compared, along with a metal-free variant, for their performance in oxidation and depolymerisation of organosolv lignin, as determined by gel permeation chromatography (GPC), quantitative-heteronuclear single quantum correlation (q-HSQC) and ³¹P NMR spectroscopy of derived phosphite esters. Although most catalysts oxidized the lignin extracts, the oxovanadium(V) complexes, [VO(OⁱPr)L_n] (L_n = dipic or (HQ)₂ where dipic = dipicolinate and HQ = 8-oxyquinoline) demonstrated the highest degree of lignin depolymerisation. Further ligand development is achieved with a tethered base (pyridine) in the novel BPAMP(V^V)(O)(OⁱPr) catalyst [H₂PAMP = N,N-bis(3,5-di-tert-butyl-2-phenol)-N-(methylpyridine)amine]. Efficient oxidation and some degree of depolymerisation is achieved with this new bifunctional catalyst without the addition of exogenous base.