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

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 degrees C to 190 degrees C) and with the amount of acid added (0%, 0.25%, or 1% H2SO4). Pretreatment at 190 degrees C with increasing concentrations of acid catalyst led to a decrease in glucan content, whereas the glucan content remained almost constant at 160 degrees 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 degrees C and 1% H2SO4, the majority of the beta-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.