Abstract
Understanding the depolymerization of lignocellulosic biomass with reactive oxygen species provides insight to develop new technologies for the conversion of biomass to fermentable sugars used to produce biofuels. In this study, the effects of four oxidative pretreatment techniques on sugar recovery and lignin degradation of wheat straw were investigated, including potassium superoxide (PS), sodium persulfate (SP), sodium perborate (PB), and iron sulfate-hydrogen peroxide (FE). The pretreated biomass was then characterized in terms of lignin/ash removal, polysaccharide content, sugar recovery, functional group transformations, pyrolysis products, structural modification, and thermal behaviors as compared to untreated biomass. Among the different oxidative pretreatment methods, PS treatment appeared to be the most promising method by increasing glucan recovery from 12.7% (untreated) to 68.9% (treated) and resulting in a 27% decrease of lignin content in the residual wheat straw. FTIR and Py-GC/MS in PS and PB methods confirmed degradation of lignin via oxidative side chain cleavage and syringyl ring oxidation, whereas SP and FE methods only caused oxidative side chain cleavage. Solid-state NMR showed changes in lignin and hemicellulose/cellulose resonances correlating to aliphatic, carbonyl functionality, acetyl groups, and crystalline regions. The thermal behavior of PS-treated wheat straw revealed an increase in cellulose degradation temperature and decreases in activation energy from 259 to 223 kJ/mol compared to raw material. Following PS method (68.9% sugar recovery), SP, PB, and FE methods resulted in recovered sugar of 43.8%, 30.8%, and 28.8%, respectively as compared to 12.7% in the untreated sample. Overall, the results indicate that the aromatic ring side chains and hemicellulose acetyl groups removed by radical fragmentation, polar hydrolysis, and CC bond cleavage play a more important role in sugar recovery than side-chain oxidation.
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