Lignin-first biorefinery approach for the valorization of cotton stalks to phenolic monomers

Abstract

Conventional biochemical conversion technologies concentrate on depolymerizing carbohydrates, leaving lignin behind as a residue and underutilized waste. Reductive catalytic fractionation (RCF) is an emerging technology for depolymerizing lignin into phenolics in the form of bio-oil at first, leaving behind carbohydrates as pulp residue. As cotton stalks make the highest contribution to agricultural residue and India is the largest cotton producing country in the world. Hence, to valorize the cotton stalks, herein, for the first time, the RCF of cotton stalks was carried out over a hydrous ruthenium oxide impregnated HY zeolite (HROY) catalyst in ethanol at 190–230 °C in hydrogen. Catalytic acidity assisted in depolymerizing native lignin, converting it into phenolics and stabilizing the obtained phenolics via hydrogenation. The maximum delignification of 88% was achieved with 18.1 wt% phenolic monomer yield and a carbohydrate retention of 92%. The thioacidolysis of cotton stalks produced 20.9% theoretical yield of phenolic monomers, corresponding to 45.7% β-ether linkages. The RCF products; bio-oil, and carbohydrate pulp were thoroughly characterized using GC/MS, ¹H NMR spectroscopy, GPC and compositional analysis, TGA, FT-IR spectroscopy, PXRD respectively. In addition, for the first time, the pulp residue was then carbonized to form carbon to find its potential for charge storage applications. Furthermore, the prepared carbon properties were demonstrated by various characterizations, such as PXRD, Raman spectroscopy, SEM, and BET analyses. The charge storage capacity of carbon from the residual pulp was compared with raw cotton stalks carbon by fabricating a coin cell-type symmetric supercapacitor to evaluate the impact of RCF on carbon.