2D NMR characterization of wheat straw residual lignin after dilute acid pretreatment with different severities

Abstract

Abstract The chemical characteristics of wheat straw lignin pretreated under dilute acid conditions were compared. After pretreatment, the lignin content of the solid residue increased as temperature increased (from 160°C to 190°C) and with the amount of acid added (0%, 0.25%, or 1% H2SO4). Pretreatment at 190°C with increasing concentrations of acid catalyst led to a decrease in glucan content, whereas the glucan content remained almost constant at 160°C pretreatment regardless of the acid concentration. The xylan content decreased in proportion with increased acid concentration and pretreatment temperature. The residual lignins were characterized by solution-state, two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopy and size-exclusion chromatography (SEC). Results showed that more ether bonds were cleaved with increased pretreatment temperature and lower pH, whereas the levels of carbon-carbon bonded structures (e.g. phenylcoumaran and resinol units) were hardly affected. With a pretreatment of 160°C and 1% H2SO4, the majority of the β-O-4 bonds were cleaved. In addition, lignin depolymerization was more evident than repolymerization at higher pretreatment temperatures and lower pH. Documenting lignin structural changes as a function of pretreatment parameters provides a tool for biorefineries to gain flexibility in processing parameters with full control over the final properties of the products.