Lignin oxidative depolymerization to phenolic compounds in a continuous-flow manufacturing platform

Abstract

Effective oxidative depolymerization of lignin to phenolic monomers and oligomers is a crucial step in the economic feasibility of biorefinery. To date, most studies on the processing technical of lignin have focused on intermittent treatment, with little attention paid to microreactor. In this research, a novel and efficient process technical of oxidative depolymerization lignin via a continuous manufacturing platform was developed, in which the lignin alkaline solution is the disperse phase and the hydrogen peroxide is the continuous phase (oxidants, 8 wt%). The results show that different products can be obtained through varying the reaction conditions of lignin depolymerization. Under the optimized condition, 93.03 wt% conversion of raw lignin and 22.37 wt% yield of monomer was obtained. Besides, comparative characterizations on the samples of raw lignin and depolymerization products by GC-MS-FID and HSQC NMR analysis illustrated that 84.26 wt% of monomers (the sum of yield of 18.85 wt%) is phenolic compounds, with syringaldehyde (yield of 4.22 wt% and selectivity of 18.86 wt%) and vanillin (yield of 3.49 wt% and selectivity of 15.60 wt%) are the main products and the high yields attributed to the efficient cleavage of C-O linkages (mainly β-O-4 linkages) of lignin. Moreover, GPC and elemental analyses show that the light oil product with an oxygen content of 41.28% was mainly composed of dimers and trimers. This work demonstrating that continuous-flow oxidative depolymerization was an effective strategy on lignin processing technical and will be an interesting field of research on lignin comprehensive utilization.