Lignin Isolation And Lignocellulose Gels Preparation Based On LiCl/DMSO Solution

To improve the basic knowledge of the properties and utilization of soybean straw, its chemical components, anatomical structure, and elements distribution were observed at first by the optical microscope, SEM-EDXA, fiber analyzer, and ICP-OES. Then the dissolution and regeneration performances in LiCl/DMSO solvent system of the ball-milled or enthylenediamine(EDA) pretreated soybean straw were also investigated in detail. Based on the up research, the ball-milled lignocellulose dissolution-regeneration was combined with the traditional enzyme hydrolysis to isolate virgin lignin with relatively high yield and minimal structural alternation. And the lignin products were characterized by using FT-IR, UV, 2D-HSQC-NMR and so on. After that, the lignocellulose gels and thermo-responsive gels with high surface specific area and great water absorption were fabricated through the gelation of the soybean straw/LiCl/DMSO solution system. The key results were as follows:Among three fractions of soybean straw, the pod comprised nearly half of the weight of the whole straw, followed by the stem at 44.6%, with the root comprising the smallest amount. Morphologically, the soybean straw had much shorter but wider fibers than other crops, and its average length, width, and length-width ratio of soybean straw were 0.46 mm, 242 μm, and 19, respectively. There were three main tissues---the ground tissue, the vascular tissue, and the dermal tissue systems in the stem, and the intimal layer and leathery layer in the longitudinal section of the pod. In addition, the stem and root had more lignin and carbohydrates than the pod which had more extractives, nitrogen and crude protein. The inorganic elements contents in the stem ranged as K>Ca>P>Na>Mg>Al>Fe>Zn>Mn>Cu; For the pod fraction, the sequence was Ca>K>Mg>P>Na>Fe>Al>Mn>Zn>Cu, and for the root was K>Ca>Na>P>Mg>Al>Fe>Zn>Cu>Mn. Clearly, K, Ca, P, Na, and Mg were the main inorganic components in soybean straw fractions, and all these inorganic and mental elements were distributed across the whole stem or pod in different amounts.Both the ball-milling and EDA pretreatment help to promote the dissolution of soybean straw in 8% LiCl/DMSO solvent system to form homogeneous lignocellulose solution. If using short ball-milling duration, the soybean straw formed hazy suspensions in 8% LiCl/DMSO solvent system. As the ball-milling duration was prolonged to 4 h or 6 h, it could be dissolved completely, yielding translucent homogeneous solutions. And the extractable lignin yields of the stem and pod after 4 h of ball-milling were 11.4% and 14.3%, respectively. It means that the alternation in the aromatic part of lignin was negligible after 4 h of ball-milling. However, the destruction of cellulose’s crystalline regions in the milled stem and pod was significant even at a ball-milling of 0.5 h. After dissolution, most of the ball-milled or EDA pretreated stem could be regenerated by being poured into excess water. Greater amounts of carbohydrates in the stems were retained as short ball-milling times(0.5 h or 1 h), whereas because of the cross-linked polymer, the amount of lignin retained was greater than that of carbohydrates retained at long times(2 h or 4 h). In addition, approximately 61.4% of EDA pretreated stem could be recovered as a regenerated fraction. The regeneration performance of lignin and cellulose were better than that of hemicellulose, and because the EDA pretreatment did not involve ball-milling which could reduce the polymerization degree of the carbohydrates in the treated materials, the viscosities of the stem and pod solutions were much higher than those of the ball-milled stem and pod solutions. Such homogeneous solutions should be benefit for the chemical modification of lignocellulose to allow for the preparation of the lignocellulose gels with special properties, which was also illustrated in this thesis.To improve the yield of cellulolytic enzyme lignin(CEL) and minimize the structural changes induced by isolation, the dissolution-regeneration of ball-milled soybean stem in LiCl/DMSO solvent was applied prior to enzymatic hydrolysis. The yield of dissolution-regeneration-CEL(named as DR-CEL) was much greater than that of traditional CEL. According to the results form FT-IR, UV and 1H-13C-NMR, the soybean straw lignin fractions were typical GSH type lignin and incorporated with p-coumarate and ferulate. The lignin fractions were mainly composed of β-O-4’ ether bonds, β-β’/α-O-γ’/γ-O-α’, β-5’/α-O-4’ and β-1’ linkages. Most importantly, the combination of total dissolution-regeneration and enzyme hydrolysis of ball-milled lignocellulose appeared to be a superior and promising approach for isolating virgin lignin with relatively high yield but minimal structural alternations.The lignocellulose gels with mesopore structures were prepared via dissolving EDA pretreated soybean straw in LiCl/DMSO solvent system and sequent coagulation and solvent replacement with ethanol, and lyophilization. The appropriate delignification and lignocellulose concentration help to form continuous 3D network with proper specific surface area and pore size. As the delignification went on, the thermal stability improved, and the char yield decreased because of the separation of carbohydrates and lignin as well as the crystallinity increasing of the lignocellulose. However, excessive delignification was harmful for the three-dimensional network formation, the uniformity of the pores and the swelling performance of the lignocellulose gels. In addition, it was possible to improve the properties of lignocellulose gels by properly increasing the concentration of lignocellulose materials.Semi-interpenetrating polymer network(SIPN) strategy was employed to fabricate a novel gels composed of lignocellulose without delignification and poly(N-isopropylacrylamide)(PNIPAAm) in the presence of N,N-methylenebisacrylamide(MBA) as the crosslinker and 2,2’-azobisisobutyronitrile(AIBN) as the initiator. The results from FT-IR and TG proved that the lignocellulose and NIPAAm matrix were linked to each other via chemical or physical linkages. These lignocellulose gels exhibited three-dimensional network structure with great porosity, rapid swelling, and favorable thermal responsibility. As the cross-linker dosage increased, the crosslinking density improved, but the average pore size decreased and the pore wall thickened, which lead to the swelling and deswelling abilities weaken. As a result, it was possible to control the swelling behavior of gels by varying the dosage of cross-linker. Additionally, the existence of the lignin in the lignocellulose also had impacts on the swelling behavior of the gels.