Comparing a deep eutectic solvent (DES) to a hydrotrope for their ability to enhance the fractionation and enzymatic hydrolysis of willow and corn stover†
This work compared the effectiveness of a deep eutectic solvent system, lactic acid:betaine (DES), to a hydrotrope, p-toluenesulfonic acid (p-TsOH), for their ability to fractionate lignin under mild conditions after a mechanical pretreatment process that was applied to corn stover and willow biomass. The hydrotrope and DES were assessed based on their ability to recover carbohydrates, selectively remove lignin, affect the lignin structure and enhance the subsequent enzymatic hydrolysis of the residual cellulose. The hydrotrope and DES both exhibited the ability to remove lignin. However, the DES treatment removed lignin with greater selectivity and allowed increased cellulose and hemicellulose recovery at a pretreatment temperature of 140 °C. Despite compromising hemicellulose recovery, p-TsOH could efficiently remove 51–96% lignin at relatively low temperatures (50–80 °C). The lignins isolated from both treatments were characterized for their molecular weight and the amounts and types of hydroxyl groups using gel permeation chromatography and 31P NMR spectroscopy. It was apparent that the lignin removed using the hydrotropic treatment at 50 °C had similar molecular weight and functionality to enzymatic-mild acidolysis lignin (EMAL) isolated from the corresponding untreated corn stover and willow mechanical pulp substrates.