Study On Bond-breaking Mechanism Of Bamboo Primary Components In Exogenous Organic Acid-GVL/H₂O System

As the excessive consumption of non-renewable fossil fuels and the resulting increasing deterioration of global environmental problems,the production of high value-added chemicals and fuels using woody biomass to replace fossil-based products in the future has become the researching focus in the past two decades.Because it is with the advantages of high efficiency on component separation,green,and environmentally friend,theγ-valerolactone（GVL）/water-acid system has become one of the vital solvent systems in recent years.However,the degradation mechanism of carbohydrates and lignin is still not clarified in the organic weak acid-GVL/water system,creating a serious obstacle on high efficient preparation of cellulose products with high polymerization degree.For this purpose,this paper conducted studies on the ionization behavior of weak organic acids in the GVL/water system,the bond-breaking kinetics of glycoside bonds and lignin dimers.On this basis,the preparation technology of high polymerization degree cellulose was lucubrated adopting appropriate exogenous organic acids and GVL/water-solvent system,aiming to provide theoretical guidance for the preparation of high value-added products by GVL/water system.Firstly,this paper studied on the influence of GVL/H₂O solvent composition on the hydrogen ion ionization characteristics of organic weak acids（formic acid and acetic acid）and inorganic strong acids（sulfuric acid and hydrochloric acid）using conductometry.The results show that the hydrogen ion ionization capacity of the above-mentioned typical acids gradually decreases with the increase of the GVL ratio in solvent,since the solvation effect of GVL molecules on hydronium ions increase the chemical potential of hydronium ions.Moreover,the hydrogen ion ionization capacity of sulfuric acid and hydrochloric acid is higher than formic acid and acetic acid by 5 orders of magnitude.In order to minimize the degradation of cellulose in the biomass pretreatment process,formic acid and acetic acid should be used as the acid source of GVL/H₂O.Furthermore,the paper researched on relationships of the ionization equilibrium constants of formic acid and acetic acid with the GVL ratio in solvent,and established the mathematical model to decribe this relationship.Considering the fact that hydrogen ion ionization capacity of formic acid is higher than that of acetic acid by one order of magnitude,this paper will use formic acid as the exogenous acid to prepare bamboo regenerated cellulose-grade fiber,in order to achieve high-efficiency component separation under the premise of ensuring no significant degradation on cellulose.Then,this paper studied on the degradation behaviors of carbohydrate and lignin model compounds,ie.,cellobiose,xylobiose,guaiacylglycerol-β-guaiacyl ether,in the typical solvent ratio（GVL mass fraction 0%-80%）.The results show that the hydrolysis degree of cellobiose decreases with the increase of the GVL ratio,indicating that the increase of GVL content is beneficial to inhibit the hydrolysis of cellobiose.With the GVL content from 0%to 60%,the degradation degree of xylobiose does not be significantly varied.However,using an 80%of GVL proportion,the degradation degree of xylobiose is significantly reduced.In order to remove the hemicellulose of bamboo as much as possible in the subsequent pretreatment process,the proportion of GVL in the solvent should not be less than 60%.For theβ-O-4 model compound,increasing the GVL ratio in solvent at low temperature is beneficial to elevate its degradation rate.As increasing the GVL ratio in solvent at high temperature,its degradation rate will be reduced.Moreover,due to the overall increasing of reaction rate at high temperature,the lignin model compound still has a high degradation degree at high temperature with high GVL ratio.Afterward,this paper established the degradation kinetic models of cellobiose and guaiacylglycerol-β-guaiacyl ether.It is found that cellobiose follows the pseudo first-order reaction kinetics in the range of0-80%of the solvent GVL ratio.The degradation of ligninβ-O-4 model compound follows the pseudo-first-order reaction kinetics in the solvents with GVL ratio from 0%to 20%,but it follows the pseudo-secondary reaction kinetics in the solvents with GVL ratio from 40%to 80%.By researching on the ratio variation of degradation rate coefficients of cellulose dimer and ligninβ-O-4 dimer(K _fiber/K_GBG),it is found that this ratio increases as reaction temperature.Using the reaction temperature of not higher than 160℃,the K _fiber/K_GBG value shows a rapid decline firstly and then slow decrease trend as increasing of GVL ratio.As the reaction temperature is not lower than 170℃,it shows the trend of firstly increase and then decrease.In view of the fact that the K_fiber/K_GBGvalue has dropped to a low level and the variation is not significant with GVL content in solvent from 40 to 80%,the paper will select this solvent ratio as the solvent condition for bamboo pretreatment.Finally,using green bamboo as the raw material,the thesis investigated the effects of GVL ratio in solvent,formic acid concentration,and reaction time at the reaction temperature of 180℃on the recieved pulp’s cellulose,hemicellulose,lignin,cellulose polymerization degree and the hydrolysate composition.Considering the various factors such as kappa number,viscosity,polymerization degree,and cellulose and hemicellulose content in solid samples,the pulping process conditions are selected in the paper as a reaction temperature of 180°C,a solvent GVL ratio of 60%,and a formic acid concentration of 0.5 mol/L,a holding time of 180 min.Using these conditions,the pulp kappa number,cellulose polymerization degree,hemicellulose content,and pulp yield is 27.4,1193,4.54%,49.0%,respectively.The regenerated cellulose grade fiber with high polymerization degree can be prepared after bleaching treatment.