Topochemistry And Structural Characterization Of Lignin During Diluted Acid Pretreatment Of Miscanthu×Giganteus

The complexity of cell wall micro-structure and heterogeneous distribution of components in the gramineous lignocelluloses are believed to be the natural biomass recalcitrance.Prefreatments are required to overcome the recalcitrance of lignocellulosic plant cell walls prior to efficient separation and conversion of cell wall components. In the present work, microscopic and microspectroscopic techniques were used to investigate the complexity of Miscanthus* giganteus (Mxg) cell wall ultrastructure and lignin dissolving mechanisms in the diluted acid (DA) pretreatment process. FT-IR, NMR and other techniques were also preformed to study the various structures of MWLs isolated from the diluted acid pretreatment and characterize the structure of lignin obtained from an integrated process based on diluted acid pretreatment and alkaline treatment. The main conclusions obtained were as follows:1. Anatomical observations indicated that the Mxg contained a variety of cell types, including epidermis, bast fibers, vascular bundles, protoxylem vessels, metaxylem vessels, sclerenchyma fibers and parenchymas. TEM images exhibited that the fiber wall was differentiated into secondary wall (SW), which has multilayers in partly cells, cell corner middle lamella (CCML) and compound middle lamella (CML), while AFM images revealed that the microfibril angles were 70°-80° and 25°-30°. As determined by CRM and FM, the heterogeneity in lignin distribution was demonstrated that the ligninfication of sclerenchyma fibers and parenchymas is lower than vesels in vascular bundles, meanwhile, both content of lignin and hydroxycinnamic acids (HCA) in sclerenchyma fibers cell wall were CCML>CML>SW.2. The results of CRM and HPAEC revealed that DA pretreatment released hemicellluloseds and partially removed lignin and HCA.The removal rate of lignin and HCA followed the decreasing order: SW>CML>CCML, meanwhile, the semiquantitative results indicated the preferential removal of lignin in SW and CML and no preference in CCML. Milled wood lignin (MWL) were isolated from untreatment and pretreatment raw materials and characterized by GPC, FT-IR and 2D-HSQC NMR. It was confirmed that diluted acid pretreatment may decrease the carbonhydrate content in MWLs, cleave β-O-4 linkage, decrease molecular weight and increase the S/G ratio from 0.69 to 0.78.3. An integrated process based on diluted acid preatment and alkaline treatment were performed for the production of lignin from Mxg. The yield of lignin was risen from 31.55% to 60.58% adding the diluted acid preatment. The high purity and lowver molecular weight were obtained from the process and characterized by 2D-HSQC NMR, FT-IR and DTA. It was found that the lignin was HGS type with free ferulic acid and p-coumaric acid. The major interunits were β-O-4, β-β, β-5, β-1 and α,β-diary ethers, et al. Moreover, the results of DTA revealed that diluted acid pretreatment may offer a higher thethermostability of Iignin obtained from the integrated process.