Fractionation Of Lignin From Steam-exploded Corn Stalk And Lignin-based Materials Preparation

As one of the three major components of lignocellulose, lignin is the most abundant source of natural aromatic polymers. Currently, most industries using lignocellulose as raw material discharge lignin as waste. The main reasons for the above phenomenon are the low purity and activity, and high polydispersity of isolated lignin. In this thesis, a steam explosion-derived refining was firstly employed to enhance lignin purity and activity. Moreover, novel lignin fractionation methods were established to reduce lignin polydispersity. Finally, the applications of isolated lignin as feedstock for adhesive and phenolic foam were investigated.The main research results obtained are as follows:(1) The structural changes of corn stalk lignin (CSL) during steam explosion were firstly investigated by gel permeation chromatography (GPC), FTIR, C/H two dimensional NMR and quantitative NMR analysis. The results showed the main reactions of CSL during the high-temperature and short duration cooking process caused by steam explosion were the depolymerization of CSL. CSL depolymerization resulted in the decrease of molecular weight and the obvious enhancement of active groups, such as phenolic hydroxyl and carbonyl, which facilitated the subsequent isolation and applications of lignin.(2) The effects of steam explosion intensity on the purity of lignin were systematically investigated to optimize the treatment conditions. Under the optimized steam explosion conditions, the effects of steam explosion on the kinetics of lignin extraction were also studied. The results showed that hemicellulose were partly degraded during steam explosion, which reduced the carbohydrates content in the lignin extracted liquor and led to the improvement of lignin purity. Under the optimized steam explosion conditions, the lignin accounted for 70.52% of total solid content in the extracted liquor, which increased 95.40% compared with the lignin content (36.09%) in the extracted liquor from untreated corn stalk. The extraction kinetics of lignin from untreated and steam exploded corn stalk showed that steam explosion significantly improved the lignin extraction rate and the activation energy of lignin extraction after steam explosion decreased from 19.47 kJ/mol to 12.11 kJ/mol. The main reason for the enhanced kinetics of lignin extraction was the depolymerization during steam explosion, which destroyed lignin net structure and increased its phenolic hydroxyl content, resulting in the easier dissolution.(3) Novel lignin fractionation methods by gradient acid precipitation and sequential dissolution in ethanol solutions were established to obtain lignin fractions with lower polydispersity and more homogenous structure. Lignin fractionation by gradient acid precipitation was easier to operate and mainly applied in lignin recovery from alkaline solutions, which could eliminate alkali and soluble carbohydrates in the alkaline solutions. Sequential dissolution in ethanol solutions was always employed for solid lignin fractionation. As the solvents used were ethanol and water, the facile fractionation process was non-toxic and low-cost compared with traditional multi-solvents lignin fractionation. The characterization of fractionated lignin indicated that low molecular weight lignin contained more phenolic acids and non-conjugated carbonyl structures which were generated by lignin depolymerization during steam explosion while high molecular weight lignin possessed more guaiacyl units and showed relatively higher thermal stability.(4) Due to the high lignin content, the alkaline extraction liquor from steam exploded corn stalk was concentrated and directly hydroxymethylated without further purification process for the substitution of phenolic resin as plywood adhesive. Under the optimized methylolation conditions, the modified lignin liquor was found to successfully substitute 50% of phenolic resin as wood adhesive with lower free formaldehyde and phenol content. And the bonding strength was 1.16±0.16MPa, which satisfied the standards of class I plywood. The effect of lignin polydispersity on the properties of lignin-based adhesive was also investigated. The results showed that the effect of molecular weight on bonding strength of lignin-based adhesive was related to the substitution rate. The reduction of molecular weight facilitated increase of bonding strength with lower substitution rate. However, when the substitution rate was higher, the lignin activity differences caused by molecular weight change showed little effect on the bonding strength since part of the lignin acted as filler.(5) The phenolic foam preparation by lignin from steam exploded corn stalk was systematically studied. The feasibility of using the alkaline extraction liquor from steam exploded corn stalk as raw materials to prepare phenolic foam was firstly analyzed; lignin depolymerization by phenolation was then adopted to further improve the properties of lignin-based phenolic foam; finally a novel process named two-step acid foaming process, was established to reduce the resin viscosity and prepare high lignin content phenolic foam. The results showed lignin-based phenolic foam has uniform closed cell structure and the addition of lignin resulted in the rise of foam density and compression strength. With the decrease of lignin molecular weight, the overall properties of foam were improved. Thus, the depolymerized lignin obtained from phenolation was used to prepare lignin-based foam with superior properties. The depolymerized lignin was found to successfully replace 40% phenol and further increase of substitution rate led to the sharp rise of resin viscosity which resulted in the failure of foaming. Therefore, two-step acid foaming process was established. In this process, the synthesized high lignin content resin was firstly neutralized by p-toluenesulfonic acid. The surface activity of p-toluene sulfonate formed during the neutralization process prevented the coagulation of lignin-based phenolic resin and reduced the viscosity effectively. Meanwhile, hydrochloric acid with higher acidity was then employed to cure the resin and improve the mechanical properties of the foam. Through the two-step acid foaming process, lignin could substitute up to 70% phenol to prepare phenolic foam. The prepared form showed uniform structure and the compression strength was 0.272 MPa, which could meet the requirement of phenolic foam application as insulation materials.