Preparation And Structural Characteristics Of Lignin Based On Steam Explosion From Corncob And Cassava Residue

Lignin is the second most abundant renewable natural biopolymers. It is also the exclusive renewable aromatic polymers in nature. Lignin is composed by p-hydroxyphenyl, guaiacyl and syringyl via C-C and C-O-C linkages. Lignin is treated as a kind of waste and used for heat generation. Lignin with high purity and low molecular weight is an idea alternative for preparation of aromatic chemicals and functional bio-based materials. Original lignin usually shows high molecular weight up to thousands and thousands. Lignin prepared by different processes shows varied molecular weight arranging from several thousands to more. This process usually carried out in hard conditions.Here we provided a method combined process which could reap pure, high-yield and low molecular weight lignin. Corncob was experienced a steam explosion (0.5 wt% H2SO4、1.5 MPa、5 min) combined with liquid hot pressure alcohol (80% alcohol,160℃,120 min). During the process, cellulose and hemicellulose were easy digested into value-added products (xylose and glucose). The results showed that:(1) the lignin rich fraction was pure and low molecular weight (Mw=683 g·mol-1, Mn=504 g·mol-1). Many C-C and C-O-C linkages were cleaved in different extend according to HSQC; (2) hemicellulose was utterly removed and transformed into xylose; (3) the cellulose rich fraction was easy to digestion enzymatically and the glucose conversion was 95.7% of the theoretical value. After delignification, enzymatic hydrolysis of cellulose rich fraction increased from 23% to 95%.In comparison with diluted acid steam explosion combined with liquid hot pressure alcohol, alkaline steam explosion was studied to extract lignin from corncob. In this study, corncob was impregnated in different NaOH solution (3 wt%,6 wt%,9 wt%) and experienced steam explosion under varied severities (3.05,3.45,3.64). The lignin carbohydrate complex from alkali (NaOH) steam explosion lignin was determined by FTIR, GPC and 2D NMR to compare their structural variation. It indicated that lignin repolymerization occurred and carbohydrate-lignin linkages varied under different conditions during the combined pretreatment. The cellulose from delignified corncob showed the formation from cellulose I to cellulose II. The synergistic effect of the combined pretreatment led to the structural change of cellulose for the high enzymatic hydrolysis increasing the glucose conversion yield higher than 90%.Another work in this thesis was the preparation of lignin from cassava residue. Diluted acid steam explosion and alkali extraction (0.5 wt%NaOH, 70℃,4h) to extract lignin. The highest purity of lignin (93.2%) was obtained under the condition of 0.9 wt% H2SO4,0.9 MPa,11 min. The highest yield of lignin (73.8%) came from the 0.6 wt% H2SO4,1.2 MPa,11 min. This rough lignin was purified by alcohol. It showed good capability of antioxidant showed that it had potentials to be a food-originated antioxidant in the future.