Components Dissociation Of Gramineous Feedstock And Construction And Application Of Lignin-based Gel Materials

With the continuous development of industrialization,increasing consumption of fossil resources,and excessive emission of greenhouse gases,the production of clean energy and materials with renewable lignocellulosic biomass resources has become a research hotspot in today’s society.However,the three components of biomass are complex in structure,and tightly bound together by chemical and physical bonds to form a complex three-dimensional spatial network structure,which constitute the natural anti-degradation barrier of the plant cell walls,making the lignocellulosic biomass in a single component utilization mode with low conversion rate for a long time.To realize full components utilization of lignocellulosic biomass materials,pretreatment is essential to be performed to achieve efficient separation of cellulose,hemicelluloses,and lignin.In this work,different pretreatment techniques were selected to deconstruct two gramineous materials（hybrid Pennisetum and tobacco stalk）,respectively,to improve the enzymatic saccharification rate of cellulose and isolate hemicelluloses and lignin which inhibit enzymatic hydrolysis of cellulose.Subsequently,the isolated hemicelluloses and lignin were structurally analyzed to reveal their solubilization behavior,dissociation mechanisms,and structural changes during the pretreatments,providing a theoretical basis for the subsequent high-value utilization.Based on the structural characteristics of lignin,two self-catalytic systems with redox activity were designed using sodium lignosulfonate and isolated alkali lignin,respectively.Benefiting from the catalytic systems,multifunctional organohydrogels were fabricated rapidly at room temperature.The excellent electrical conductivity and stretchability of the gels make them ideal candidates for flexible wearable and self-powered energy storage electronics.（1）A two-step integrated treatment combining microwave-assisted hydrothermal pretreatment（MHTP）with dilute alkali treatment was proposed to deconstruct hybrid Pennisetum.During the MHTP,part of hemicelluloses in raw material were rapidly depolymerized into xylo-oligosaccharide（XOS）and water-soluble hemicelluloses（SHs）with low molecular weight,thus facilitating the dissociation of lignin in the subsequent alkali treatment process.When the hydrothermal conditions were 190℃and 20 min,82.3%of lignin was obtained by the MHTP-dilute alkali integrated treatment.Lignin underwent both depolymerization and condensation reactions during the MHTP,in which the depolymerization reaction was dominate.The depolymerization reaction resulted in the release of lignin with smaller molecular weights（from 4980 g/mol to 2910?4170 g/mol）and more phenolic hydroxyl groups（from 1.58 mmol/g to 1.97?2.99 mmol/g）,which can be utilized as antioxidants or reducing agents.Meanwhile,the remaining hemicelluloses in the hydrothermal residue were further separated in dilute alkali post-treatment process,obtaining alkali-soluble hemicelluloses（AHs）.Structural characterization of the two types of hemicellulosic fractions showed that the SHs depolymerized during the hydrothermal process at high temperatures were mixed polysaccharides,mainly composed ofβ-（1→4）-xylan andβ-glucan.In comparison,the AHs released by the two-step integrated treatment were mainlyβ-（1→4）-xylan.The effective removal of hemicelluloses and lignin significantly improved the enzymatic hydrolysis rate of the feedstock（92.4%）,making it an ideal raw material for bioethanol production.（2）In view of the fact that the above dilute alkali-based integrated treatment is effective in deconstructing gramineous feedstock,five different pretreatment technologies（freeze-drying,alkali pre-swelling,ultrasound-assisted alkali extraction,ball-milling,and hydrothermal pretreatment）were combined with dilute alkali cooking,respectively,to treat tobacco stalk for isolating its hemicelluloses and lignin and improving the enzymatic saccharification rate of cellulose.Taking into account the yields of hemicelluloses（76.9%）and lignin（25.8%）and glucose conversion rate（93.5%）of cellulose,mechanical ball-milling was determined to be superior to the other four pretreatment methods.The hemicelluloses released from tobacco stalk by the integrated treatment based on ball-milling and dilute alkali cooking had a main structural model of glucuronoarabinoxylans.To investigate the natural structure of tobacco stalk lignin,a method of mechanical ball-milling coupled with enzymatic hydrolysis was developed to isolate enzymatic residue lignin（DEL）with relatively complete structure and ultrahigh yield of 99.8%.Compared with ALs,DEL contained moreβ-O-4,β-βandβ-1 linkages,but its purity and thermal stability were relatively poor.Significantly,both DEL and ALs contained no p-coumaric acid and ferulic acid units in their structures.For ALs,except for hydrothermal and ball-milling pretreatments,the other three pretreatment methods had no obvious effect on the structure.This study provides a theoretical basis for the separation and high-value utilization of the three biostreams of tobacco stalk.（3）To solve the problem of time and energy consumption in the preparation process of conventional hydrogels,a novel lignin-based macromolecule self-catalytic system was designed,namely sodium lignosulfonate-copper（II）ions(Ls-Cu²⁺).At room temperature without removal of oxygen,the catalytic system can not only generate semiquinone free radicals but also induce the efficient activation of persulfate initiator to generate hydroxyl free radicals,thus triggering the rapid polymerization of polymer monomers（N-hydroxyethyl acrylamide）in water-glycerol binary solvent to prepare conductive organohydrogels.In particular,the introduction of glycerol further accelated the gelation,so that it could be completed rapidly within 30 s.Due to the hydrogen-bond interactions among water,glycerol,and PHEAA chains,the organohydrogels exhibited extreme freezing and drying resistance（-50–60℃）and extensive adhesion（?8.4 k Pa）to various substrates（e.g.,wood,aluminum,steel,polytetrafluoroethylene,glass）.In addition,the presence of lignin endowed the organohydrogels fascinating UV-blocking ability（?100）.These excellent multifunctional properties make the organohydrogel（water/glycerol=2/1）an ideal candidate for epidermal sensor to monitor human motion,including joint flexion with large deformations and physiological pronunciation with small deformations,over a wide temperature range of-20–60°C.The construction of lignin-based self-catalysis system provides a new method for large-scale production of multifunctional organohydrogels.（4）To expand the utilization of lignin,the polyphenol hydroxy alkali lignin（AL）isolated above was used to construct self-catalytic system of AL-Cu²⁺in alkaline water-ethylene glycol（EG）binary solvent.Benefiting from the catalytic system,multifunctional polyacrylamide organohydrogels were prepared rapidly at room temperature（1 min）.In particular,the introduction of EG not only conferred extreme environmental applicability（-40–60°C）to the gels but also improved their stretchability（?800%elongation）and self-adhesive property（?31.4 k Pa）.The organohydrogel（water/EG=2/3）was used as flexible electrode to assemble a single-electrode friction nanogenerator（O-TENG）that can efficiently collect mechanical energy and convert it into electrical energy to power low-power commercial electronic devices.The maximum output voltage,current,and charge of the O-TENG with a gel area of 2×3 cm²were 220 V,4.5 p A and 0.07 p C,respectively.Even if the gel was stored at-40°C or 60°C,the assembled O-TENG still enjoyed a stable electrical output capacity（>130 V）.In addition,the O-TENG has a self-powered sensing function,which could be used as a tactile sensor to monitor human motion.The multifunctional organohydrogels constructed in this study promote the development of noval flexible wearable energy devices.