Resource Utilization And Electrocatalytic Hydrogen Evolution Performance Of Sugarcane Bagasse Using Ultrasonic And Microwave-assisted Deep Eutectic Solvents

Biomass is one of the most abundant renewable resources on earth,which can be converted into chemicals and energy by biological or chemical methods.Sugarcane bagasse is the main by-product of the sugar industry,accounting for 24%～28%of the total weight of sugarcane.It has the advantages of large yield,low price,renewable,and degradable.Therefore,it is of great significance for the high-quality development of the sugar industry to carry out chemical conversion and energy utilization.However,sugarcane bagasse is mainly composed of cellulose,hemicellulose,and lignin,and the complex connection among the three endows its natural resistant structure,thus,hindering its efficient conversion and utilization.Therefore,it is necessary to take effective pretreatment measures to destroy the resistant structure and improve the utilization rate.Deep eutectic solvent（DES）is a green solvent with good solubility and stability,which has been gradually applied in biomass pretreatment in recent years.However,it is difficult to systematically and deeply study the pretreatment mechanism of biomass by DES and find a more efficient pretreatment method for the high-value biomass utilization.In this dissertation,using sugarcane bagasse as raw material,an ultrasonic and microwave-assisted binary DES and ternary DES pretreatment systems were constructed to depolymerize sugarcane bagasse and selectively dissolve lignin and hemicellulose.Besides,the effect of pretreatment on cellulose conversion,nanocellulose preparation,and lignin degradation was studied,revealing the mechanism of action of DES on sugarcane bagasse pretreatment to depolymerize its resistant structure.In addition,the electrocatalytic hydrogen evolution materials were prepared based on bagasse-derived carbon,and the effects of sugarcane bagasse’s carbon and heteroatom doping on the electrocatalyst activity were systematically studied.The primary investigations and conclusions are as follows:（1）To explore the depolymerization effect of binary DES on sugarcane bagasse,the sugarcane bagasse was pretreated by sweeping-frequency ultrasound and fixed-frequency ultrasound combined with binary DES.The pretreatment mechanism of ultrasound combined with DES on sugarcane bagasse was revealed based on composition,enzymatic hydrolysate,surface morphology,crystallinity and chemical structure.The results showed that when sweeping-frequency ultrasound was combined with choline chloride-lactic acid to pretreat sugarcane bagasse at 120°C for3 h,the cellulose content in the regenerated bagasse was 67.11%,the lignin removal rate was 85.62%.The liquid fraction’s glucose,xylose,and arabinose recoveries were3.97%,8.47%,and 16.07%,respectively.After enzymatic hydrolysis of the regenerated bagasse pretreated with the optimal conditions,the glucose,xylose,and cellobiose yields were 86.76%,39.68%,and 20.76%,respectively.The studies showed that ultrasound combined with DES pretreatment could effectively destroy hydrogen bonds,ether bonds,and carbon-carbon bonds in sugarcane bagasse.Hydrogen bonds were formed between the halogen anions in DES and lignin,which promoted the cleavage of ether bonds or ester bonds between lignin and polysaccharide.（2）To further explore the depolymerization effect of ternary DES on sugarcane bagasse,the bagasse was pretreated by ultrasound combined with ternary DES.The pretreatment mechanism of ultrasound combined with ternary DES on sugarcane bagasse was revealed from the composition,enzymatic hydrolysate,surface morphology,crystallinity and chemical structure.The results showed that after pretreating sugarcane bagasse with choline chloride-glycerol-Fe Cl₃·6H₂O at 120°C for 3 h,the cellulose content in the regenerated bagasse was 47.67%,and the lignin removal rate was 82.71%.Under the same reaction conditions,the cellulose content and lignin removal rate in the regenerated bagasse increased to 66.17%and 86.39%,respectively,after the ultrasound was combined with choline chloride-glycerol-Fe Cl₃·6H₂O pretreatment.In addition,the enzymatic hydrolysis product yield of regenerated bagasse pretreated by ultrasound combined with ternary DES pretreatment was significantly higher than that of ternary DES.A mechanistic study showed that there was more Cl^-in ternary DES to form hydrogen bonds with the hydroxyl group of lignin,which accelerated the cracking of the ether bond and ester bond of lignin,thus facilitating the removal of lignin.（3）To depolymerize sugarcane bagasse more efficiently and add value to cellulose,microwave synergistic ternary DES combined with high-intensity ultrasound was used to pretreat sugarcane bagasse and effectively cleave the strong hydrogen bonds in it to prepare bagasse cellulose nanofibers（CNFs）.The effects of microwave synergistic ternary DES on bagasse pretreatment and the formation mechanism of CNFs were investigated by composition,chemical structure,chemical properties,surface morphology and crystallinity.The results showed that the cellulose contents in sugarcane bagasse were 56.20%and 62.60%,respectively,after microwave pretreatment（100°C for 20 min）with choline chloride-oxalic acid-Al Cl₃·6H₂O and choline chloride-lactic acid-Al Cl₃·6H₂O as the solvent.The prepared CNFs contained 0.74 and 0.84 mmol·g^-1 of carboxylic acid groups,with a crystallinity of 58.05%and 60.71%,respectively.The length of CNFs was about 400～600 nm,the width was about 15～17 nm,the height was about 6～7 nm,and the aspect ratio was between 60 and 260.（4）To further elucidate the mechanism of DES on lignin and add value to lignin,alkaline lignin was separated from bagasse and pretreated with ternary DES.The structural changes of sugarcane bagasse lignin during ternary DES pretreatment were investigated by elemental composition,chemical properties,and chemical structure.The possible degradation reactions were revealed by the degradation products.The results showed that after the pretreatment of choline chloride-lactic acid-Al Cl₃·6H₂O and choline chloride-oxalic acid-Al Cl₃·6H₂O,the yield of regenerated lignin was65.40%and 55.43%,respectively.Furthermore,after pretreatment,the contents of phenolic hydroxyl groups in regenerated lignin were 2.54 and 2.84 mmol·g^-1,and the contents of aliphatic hydroxyl groups were 0.91 and 0.79 mmol·g^-1,respectively.The mechanistic study showed that when lignin dissolved in acidic DES,a positive proton was formed at C_α,which underwent deprotonation and protonation processes to form a resonance-stabilized positive charge.This subsequently hydrated and deprotonated at C_βto form the ketones and phenols group,resulting in cleavage of the ether bond.（5）To realize the high-value utilization of bagasse in electrochemistry,a series of transition metal molybdenum disulfide（MoS₂）-supported sugarcane bagasse-derived carbon（SCBC）nanocomposites were synthesized by calcination and hydrothermal reaction.The physicochemical and electrochemical characterizations of the nanocomposites were analyzed.The effect of different bagasse carbon additions on the electrocatalytic hydrogen evolution performance of MoS₂ was studied.The results showed that MoS₂ nanosheets were uniformly loaded on bagasse carbon and showed a vertical growth trend,resulting in a synergistic effect on the hydrogen evolution reaction（HER）.As a result,SCBC/MoS₂ 1:2 showed better HER activity,with an overpotential of 142 m V,a Tafel slope of 71 m V·dec^-1,and a C_dl value of 65.26m F·cm^-2 at a current density of 10 m A·cm^-2.In addition,SCBC/MoS₂ 1:2 showed good catalytic performance and stability after 2000 cyclic voltammetry（CV）scans and a long-term test of 12 h.（6）To further improve the MoS₂HER activity,cobalt（Co）doping strategy was proposed to enhance the catalytic activity of MoS₂ and MoS₂-supported sugarcane bagasse-derived carbon conjugate（MoS₂-SCBC）.The effects of Co doping and bagasse carbon addition on the electrocatalytic HER performance of nanocomposites were studied.The results showed that in the doping process of MoS₂,Co atoms could replace Mo atoms to form covalent doping of MoS₂.Co doping can effectively adjust MoS₂ electronic structure and improve HER activity.The SCBC can enhance the electronic conductivity of MoS₂ and the stability of the mixed structure and promote the dispersion of MoS₂ and the exposure of more edge-active sites.The Co-MoS₂-0.67-SCBC-0.2 nanocomposite showed a good HER performance;the overpotential was 62 m V at 10 m A·cm^-2,the Tafel slope was 53.86 m V·dec^-1,and the C_dl value was 93.36 m F·cm^-2.In addition,Co-MoS₂-0.67-SCBC-0.2 maintained good stability after 2000 CV scans.In conclusion,this study used ultrasound combined with DES and microwave synergistic DES to depolymerize bagasse to achieve efficient utilization of each component.A highly active and low-cost non-noble metal electrocatalyst with bagasse carbon as a carrier was prepared for water electrolysis to produce hydrogen,aiming to realize the green resource utilization of bagasse.