Structural changes in switchgrass lignin and hemicelluloses during pretreatments by NMR analysis

Switchgrass is currently being developed as a sustainable bio-energy crop due to its broad adaptability, high mass yield and low agricultural input. Its current conversion to biofuels is detrimentally impacted by its native recalcitrance which is typically addressed using chemical and/or biological pretreatments. In this study, extractives free switchgrass was pretreated with steam, dilute H2SO4 and lime at 160 °C for 1 h. The degradation and impact of pretreatment was estimated semi-quantitatively by 13C-1H HSQC (heteronuclear single quantum coherence) NMR analysis of ball milled untreated and pretreated switchgrass samples in perdeuterated pyridinium chloride-DMSO-d6 solvent system. As a result of steam pretreatment the resulting switchgrass was depleted of xylan and a slight degradation of lignin were observed. This was confirmed by the relative decrease of cross peak intensity for β-aryl ether, phenylcoumaran, resinol and dibenzodioxocin units. Significant structural changes observed due to the lime pretreatment of switchgrass was deacetylation/dissolution of hemicellulose and the extent of delignification was less however, a preferential removal p-hydroxy of benzoyl ester, ferulate and coumarate type linkages were notified from the HSQC studies. Finally the most significant degradation resulted in acid pretreatment involving ∼90% loss of hemicellulose and a substantial degradation of various lignin sub-units. These results are further supported by the composition analysis of the respective switchgrass samples.