Structural Changes of Lignin during OrganoCat Processing

The isolation of high-quality lignin from lignocellulose plays a pivotal role in the holistic use of biomass. Hence, modern approaches aim to avoid recondensation reactions of lignin during extraction and aim to maintain most of the native structure. This increases its solubility and the proportion of cleavable beta aryl linkages, which can be of advantage in the subsequent process chain. As the lignin structure varies greatly between different plant groups, understanding the effect of lignin structural features on processing and vice versa is a prerequisite for industrial application. In this study, a two-step process using concentrated phosphoric acid in a swelling step and then a biphasic fractionation process utilizing diluted phosphoric acid under mild conditions was used as a benchmark to extract lignin from 10 structurally and compositionally different plant materials. Pyrolysis-gas chromatography-mass spectrometry and 1H-13C-heteronuclear single quantum correlation-nuclear magnetic resonance were used to identify the monomer unit composition and the linkage proportion of lignin before and after extraction and combined with size exclusion chromatography, wet chemical analysis of the biomass, and fractionation yields to identify correlations before and after processing. After processing, the relative beta aryl ether linkage content decreased greatly from 40-90 to 20-40 linkages per 100 monomer units. The molecular weight (M w) of extracted lignin was measured to be between 2000 and 6800 Da, indicating that the fractionation process produces relatively uniform lignin in these aspects from different plant sources. Nevertheless, lignin structure properties such as beta aryl and resinol linkages showed an influence on delignification and lignin size.