Correlation Mechanism Of Dissolution Pattern And Microstructure Of Major Components During The Hot Water Hydrolysis Of Poplar Wood

Biomass refining technology comprehensively,efficiently,and rationally utilizes renewable resources such as lignocellulosic biomass is one of the important methods and measures to deal with the current global resource consumption crisis.Hemicellulose is a type of complex polysaccharide in the cell wall of lignocellulosic biomass,which can be widely used in food,medicine,and chemical industries.However,due to its complex and diverse structure,it is not easy to separate and purify,so it has been inefficient utilization so far.Appropriate pretreatment technologies are added to the traditional pulping process to separate and extract hemicellulose and lignin to produce high value-added products.The purpose is to increase the utilization rate of lignocellulosic biomass and reduce environmental pollution.This is one of the effective ways to solve the low resource utilization rate of traditional chemical pulping and papermaking enterprises,and it is of great significance to the resource utilization of biomass.In this paper,poplar hemicellulose was separated and degraded by hot water pretreatment under relatively mild conditions.A single factor experiment was used to analyze the wood chip structure and changes in the composition of the pre-hydrolyzed solution and the lignocellulose two-phase kinetic model was used to quantitatively analyze the reaction rate constant of glycan hydrolysis.Subsequently,sulfuric acid treatment was used to separate and purify the pre-hydrolyzed liquid,and the response surface experiment design method was used to optimize the hot water pretreatment and sulfuric acid treatment conditions of poplar wood,with the purpose of providing technical support and theoretical basis for the combination of biomass refining and pulp and paper technology.The poplar materials were pretreated by hot water with different reaction temperatures and reaction times.The reaction kinetic model of xylose and glucose in the hydrolysis solution were established using the modified Saeman biphasic kinetic model.The reaction rate constants of monosaccharides generation and degradation under different conditions were determined by nonlinear curve fitting.The activation energy of lignocellulosic hydrolysis reaction was obtained by linear fitting of Arrhenius formula.The experiments showed that xylan was divided into an easy-to-hydrolyse fraction and a difficult-to-hydrolyse fraction during the pre-hydrolysis of poplar wood.The activation energy of easy-to-hydrolysis and difficult-to-hydrolyse xylan were E_f=78.35 k J/mol,E_s=88.97 k J/mol,respectively.Furthermore,the activation energy of glucan degradation(E₃=48.06 k J/mol)was significantly lower than the activation energy of glucose degradation(E₄=90.18 k J/mol).The main factors affecting the hot water pretreatment of lignocellulosic biomass include hydrolysis temperature,heat preservation time and solid-liquid ratio,etc.The biomass material can only get a higher yield efficiency of the target product under appropriate conditions.Taking the total xylose in the hot water pretreatment liquid as the target product,the effects of different conditions on the content of total xylose,acetic acid,and lignin in the poplar pre-hydrolysis liquor and the microstructure of poplar wood chips were discussed.The single factor experiment was used to explore the changing regularity of the total xylose content in the pre-hydrolysis liquor,and the total xylose production was used as an indicator,and the response surface method was used to optimize the hot water pre-hydrolysis conditions of poplar wood.The experimental results show that hot water pretreatment of poplar wood has a significant effect on the microstructure,crystallinity,and specific surface area of poplar wood.Micro-CT and SEM have observed irregular spalling and micro-spherical lignin deposition on the surface of poplar wood chips.The regression model established by the response surface method showed that the total xylose content in the pre-hydrolysis liquor reached the optimal value of 76.54 mg/g when the hydrolysis temperature was 172.51?,the heat preservation time was 88.14 min.At the same time,the content of lignin and acetic acid are 34.14 mg/g and 11.28 mg/g respectively.The separation and purification of poplar pre-hydrolysate by sulfuric acid hydrolysis has the advantages of simple operation and less dosage of chemicals.Adding sulfuric acid solution to the hydrolysate and hydrothermal acid hydrolysis can not only make all kinds of complex glycans in the hydrolysate into monosaccharides but also make the lignin coagulate and precipitate under acidic conditions.The effects of different sulfuric acid treatment conditions on xylose concentration and lignin removal rate were compared by single factor test and response surface methodology.The lignin separated from sulfuric acid hydrolysate was characterized in detail to optimize the experimental conditions of sulfuric acid treatment hydrolysate.The experimental results show that sulfuric acid hydrolysis can effectively increase the concentration of xylose in the pre-hydrolysate and separate lignin without destroying the main structure.The optimal conditions of sulfuric acid treatment were 113.647?,heat preservation time was 72.831 min and sulfuric acid dosage was 3.255wt%,respectively.FT-IR,TGA,2D-HSQC NMR characterization results showed that after sulfuric acid treatment the obtained lignin exhibited excellent structure,such as a large number of?-O-4'bonds,good thermal stability,and high molecular weight.