Microwave-assisted depolymerization of various types of waste lignins over two-dimensional CuO/BCN catalysts

Abstract

Valorization of lignin to valuable chemicals and biofuels increases the economic viability of sustainable biorefineries. This work aimed at elucidating how the lignin structures recovered from various agricultural and industrial residues governed the downstream catalytic conversion. Three types of lignins, namely bio-enzymatic lignin (BL), organosolv lignin (OL), and Kraft lignin (KL), were fully characterized by HSQC-NMR, TGA, FTIR, and SEM to obtain a detailed description of their structures. In consideration of redox-active CuO and highly active carbon-modified boron nitride (BCN) in oxidative dehydrogenation, a two-dimensional CuO/BCN catalyst was prepared and explored in microwave-assisted lignin conversion to improve the yields of aromatic monomers. BL achieved the highest yield of monomers (10 wt%) over the CuO/BCN catalyst after the 3rd cycle in 30 min under mild conditions (200 °C). The yields of bio-oils reached 70 wt% in 10 min when BL and OL were used as the substrate. High efficiency of the microwave-assisted reaction was illustrated by comparing with that of the hydrothermal reaction. This work demonstrated strong dependence of the conversion efficiency on the interunit linkages and functional groups of lignin structures. The strong metal-support interaction between CuO and BCN not only facilitated lignin depolymerization via the promoted electron transfer, but also enhanced the stability of Cu catalysts under hydrothermal conditions. In addition, elucidation of the catalyst redox evolution shed light on the role of the CuO/BCN catalyst in lignin depolymerization in recycle runs.