The Effect And Mechanism Study Of Ultrasonic-assisted Free Nitrous Acid Pretreatment On Cellulose Biomass Enzymatic Hydrolysis Process

The increasing global energy demand and the continuous consumption of fossil fuels have driven the development and utilization of renewable energy.Cellulosic biomass,due to its wide availability and low cost,has been used for the production of second-generation biofuels,offering a promising solution to the energy problem.However,the complex and tightly-packed structure of cellulose biomass severely limits its subsequent bioconversion.Despite various pretreatment methods being employed to improve the enzymatic conversion of cellulose,the lack of understanding of the structural changes in cellulose under pretreatment conditions hinders the development of efficient utilization technologies.Based on this,this study investigates the impact of pretreatment techniques on the enzymatic saccharification performance of cellulose biomass and explores the underlying structural changes mechanisms.The research focuses on the following three aspects:(1)Investigating the influence and mechanism of traditional ultrasonication pretreatment on the enzymatic saccharification efficiency of cellulose.(2)Exploring the effects and mechanisms of pretreatment using free nitrous acid(FNA)combined with ultrasonication on cellulose.(3)Using sugarcane bagasse as representative cellulose biomass,studying the impact and mechanism of combined pretreatment on its enzymatic saccharification,and analyzing the feasibility of pretreatment in practical production processes.The main research findings of this study are as follows:1)Using ultrasonication pretreatment as a representative,the enzymatic saccharification efficiency and structural changes of cellulose under pretreatment conditions are analyzed.The study demonstrates that ultrasonication pretreatment can improve the enzymatic saccharification efficiency of cellulose by 119.74%.The increase in hydrolysis efficiency is attributed to the changes in cellulose properties rather than an increase in solubility.In addition,isothermal titration calorimetry analysis shows that cellulose hydrolysis is an entropy-driven reaction dominated by hydrophobic forces,and ultrasonication treatment alters the properties and thermodynamic parameters of cellulose,thereby enhancing the enzymatic reaction activity.Cellulose treated with ultrasonication exhibits a loose,rough,and disordered morphology,accompanied by a decrease in crystalline structure integrity.Under the influence of ultrasonication,the average cross-sectional area of cellulose increases from2231.25 to 4269.73(?)~2,the crystallinity decreases from 68.38%to 61.95%,cellulose transforms from type I to type II,and hydrophilicity and enzyme accessibility increase.Furthermore,Fourier-transform infrared spectroscopy combined with two-dimensional correlation spectroscopy(2D-CoS)confirms that the sequential displacement of hydroxyl groups and intermolecular/intramolecular hydrogen bonds,which affect cellulose crystalline structure and stability,is the reason for the ultrasonication-induced transformation of cellulose crystalline structure.2)This study focuses on cellulose as the research subject and investigates the effectiveness and mechanisms of Free Nitrous Acid(FNA)treatment and the combination of ultrasonication and FNA treatment.The results show that compared to the untreated group,the cellulose enzymatic saccharification efficiency is improved by94.61%and 137.04%with FNA treatment and the combination of ultrasonication and FNA treatment,respectively.After FNA pretreatment,cellulose exhibits a more fragmented morphology,and its molecular weight decreases from 3552 to 2250.FNA treatment increases the proportion of cellulose Iαfrom 0.01%to 16.82%.This increase may be attributed to the presence of numerous active nitrogen intermediates,active nitrogen radicals,and nitrogen-containing groups in the FNA solution,which can modify cellulose and increase the proportion of cellulose Iα.The combination of ultrasonication and FNA pretreatment not only significantly improves the enzymatic saccharification efficiency but also greatly increases the proportion of cellulose II.Furthermore,after FNA modification,further ultrasonication treatment leads to a reduction in the content of intermolecular and intramolecular hydrogen bonds in cellulose,but the relative proportion of intermolecular hydrogen bonds increases,resulting in a silk-like appearance of cellulose molecules.3)In the study using sugarcane bagasse as the actual biomass waste,ultrasonication pretreatment,FNA pretreatment,and the combination of ultrasonication and FNA pretreatment were conducted to explore the effectiveness and mechanisms of these three pretreatment methods.The results show that the three pretreatment methods increase the glucose yield of sugarcane bagasse by 24.86%,20.67%,and 25.49%,respectively.Ultrasonication causes physical fragmentation of sugarcane bagasse while creating deeper and denser pores on the surface of the bagasse.FNA pretreatment opens up the entangled lignin and hemicellulose on the surface of sugarcane bagasse,exposing cellulose and stretching the cellulose fibers,increasing the intermolecular hydrogen bonds in cellulose,and making it longer and more orderly.The combination of ultrasonication and FNA pretreatment can destroy the structure of sugarcane bagasse while forming dense pores on the surface,significantly increasing the enzyme-accessible surface area and greatly enhancing the enzymatic saccharification rate.The pretreatment increases the crystallinity of sugarcane bagasse and promotes the transformation of cellulose from type I to type II,which is one of the reasons for the improvement in enzymatic saccharification efficiency.The ultrasonication and FNA combination pretreatment effectively breaks down lignin and hemicellulose,targeting functional groups including ether and ester bonds,resulting in the disintegration and modification of sugarcane bagasse,greatly increasing the efficiency of enzymatic saccharification.In summary,this thesis explores the influence and mechanisms of traditional ultrasonication pretreatment on the morphological structure and bioavailability of cellulose biomass,and further improves the bioconversion efficiency of cellulose by using ultrasonication in combination with FNA treatment.The effectiveness of the pretreatment methods is verified using sugarcane bagasse as the target biomass.The research results presented above elucidate the mechanisms by which pretreatment affects the bioconversion of cellulose and provide technical support and theoretical basis for the development of novel pretreatment methods for efficient utilization of cellulose biomass.