The Module Exploration On Gluconic Acid-based Pretreatment And Enzymatic Hydrolyzability Of Agricultural Straws

This thesis focuses on the efficient conversion and utilization of xylan and cellulose in straw raw materials,with functional oligosaccharides and enzymatic hydrolysis of glucose coproduction as the target,and self-supplied green and mild gluconic acid as the catalyst,based on experimental model research method,the reaction mechanism and optimal process conditions of acid pretreatment chemical process and cellulase hydrolysis biological process of three typical raw materials: sorghum straw,corn straw and wheat straw.This paper provides a theoretical basis and technical support for the research on the efficient utilization of lignocellulosic raw materials,including straw,for resource extraction and industrialization.Based on the characterization and analysis of the main chemical structure parameters of the raw materials,the paper focuses on the mathematical calculation of the relationship between the hemicellulose removal rate,cellulose crystallinity index,cellulase adsorption rate(accessibility)and cellulase hydrolysis yield of the three straw raw materials under gluconic acid pretreatment conditions(intensity),and the comparative study of the classical dilute sulfuric acid pretreatment technology.A new applicable model was proposed.The main results of the study are as follows.(1)Kinetic basis of the reaction of xylan acid pretreatment process.The kinetic model of gluconic acid-catalyzed directed hydrolysis of xylan for the preparation of oligosaccharides was studied and established using Box-Behnken experimental design method with straw extracted xylan as the substrate,and the influence of the main process parameters on the hydrolysis of xylan,oligosaccharides and the generation of degradation products was analyzed.The highest yield of 57.73% oligosaccharide was obtained by pretreating xylan with 4.6 wt% gluconic acid at 167 °C for 28 min at a solid-liquid ratio of 1:10.Among them,the order of intensity of process conditions affecting the directed hydrolysis of straw xylan to make oligosaccharides was: acid concentration < holding time < reaction temperature.Under the optimum process conditions,44.9g/L of oligosaccharide was obtained at a solid-liquid ratio of 1:7.5,which was higher than 1:10and 1:5.(2)Gluconic acid pretreatment of sorghum straw and its cellulase hydrolysis.Based on the results of gluconic acid-catalyzed hydrolysis of straw xylan,a Box-Behnken experimental design approach was used to analyze the intensity of each major process condition affecting the preparation of oligosaccharides from sorghum straw.Under the optimum pretreatment conditions,50.3% oligosaccharide yield was obtained by pretreating sorghum straw at 168 °C for 35 min using 7.5 wt% gluconic acid and a solid-liquid ratio of 1:10.Using gluconic acid pretreatment of sorghum straw solid residue as a substrate,a cellulase hydrolysis yield of 90.8% was obtained using an enzyme addition of 20 FPU/g cellulose for 108 h at 50 °C.A positive correlation was found between the pretreatment removal rate of xylan-like hemicelluloses from sorghum straw and the enzymatic hydrolysis yield directly affecting the residual cellulose,but the relationship between the two was not a simple linear one.(3)Technical applicability of gluconic acid pretreatment to enzymatic hydrolysis of straw feedstock.The technical applicability of gluconic acid catalytic pretreatment and enzymatic hydrolysis model theory for corn straw and wheat straw feedstocks was further investigated based on the results of the study guided by sorghum straw feedstock.As the degree of hemicellulose fraction removal from straw feedstock during gluconic acid catalyzed pretreatment increased,the crystallinity index of cellulose and enzyme accessibility showed a corresponding increase.Based on the regression analysis of several new models,we found that the 5PL model was suitable for fitting the cellulase hydrolysis rate to the xylan pretreatment removal rate and cellulose crystallinity index,the Deming model was suitable for fitting the cellulase hydrolysis efficiency and enzyme accessibility,while the 4PL model is suitable for fitting the relationship between xylan removal rate,cellulose crystallinity index and cellulase accessibility.(4)Key influencing factors for the hydrolysis of straw polysaccharide fraction by acid pretreatment.Based on the above results for gluconic acid-catalyzed sorghum straw,the common mechanisms and their technical applicability of the acid method for pretreatment of different types of feedstocks,such as corn straw,wheat straw,sorghum straw and bagasse,were further investigated and validated using the classical dilute sulfuric acid method as a control.The results showed that the pretreatment methods based on protonic acid catalysis had consistent patterns of influence on the xylan acid-catalyzed hydrolysis and the enzymatic hydrolysis performance of cellulose for straw-based feedstocks.The regression analysis of mathematical models for the main factors affecting the cellulase hydrolysis efficiency of acid pretreatment straw revealed that the 5PL model was suitable for fitting the enzyme hydrolysis efficiency with xylan removal rate and cellulose crystallinity index,while the Allosteric sigmoidal model was suitable for fitting the cellulase accessibility with enzyme hydrolysis efficiency.