Directional Depolymerization And Purification Of Hemicellulose In Chloride Pretreatment

Cellulose,hemicellulose and lignin are the main components of biomass cell walls.Hemicellulose has a complex structure and is difficult to extract and purify,resulting in an underutilization.In order to promote biomass refining and improve the high-efficiency separation and high-value utilization of biomass raw materials,this paper took poplar as the research object.First,different types of metal chloride salts were used to catalyze the degradation of lignocellulose at high temperature.The metal chloride salt with the best treatment effect and its action conditions were screened out,and the optimal pretreatment process was further optimized by the response surface method.Then the pretreatment liquid was purified by ion exchange resin,and the hemicellulose polysaccharide in the pretreatment solution was extracted by the freezing-ethanol precipitation method.Finally,the hemicellulose and lignin-carbohydrate complex（LCC）in the raw material were directly pretreated to analyze the changes of components and linkages before and after pretreatment.（1）Five metal chlorides with different chemical valences were selected as catalysts,and their action behaviors on lignocellulose under high temperature water treatment were analyzed.Metal chlorides had a good extraction effect on hemicellulose.With the increase of concentration and temperature,the pretreatment effect of metal chlorides was obviously enhanced.Under certain conditions,the removal rate of hemicellulose can reach to 100%,while the retention rate of cellulose was basically above 80%,showing a good performance of selectivity.Analysis of the pretreated solid found that metal chloride pretreatment can effectively improve the crystallinity of cellulose and reduce the degree of polymerization of cellulose.The crystallinity of cellulose can be increased up to 77.99%.The degree of polymerization of cellulose was reduced by at most about 70%.This can greatly improve its chemical reactivity and the accessibility of cellulase.Based on comprehensive analysis,it was preliminarily considered that 0.075 mol/L Fe Cl₃ pretreatment at 140℃was the best pretreatment condition.（2）The pretreatment conditions of Fe Cl₃ were screened and optimized,aiming to screen out the best reaction area with high yield of hemicellulose and low content of degradation products.On the basis of the conclusions of the first part,a more specific single factor condition analysis was carried out for Fe Cl₃ pretreatment,and the optimal action condition was screened out as 0.2 mol/L Fe Cl₃ incubated at 120℃for 10 min.Then,the response surface experiment was designed to further optimize the optimal pretreatment process,and the verification experiment proved that this process was reasonable and reliable and had certain practical value.The optimal pretreatment condition was 0.16 mol/L Fe Cl₃ treatment for 13 min at 138°C.At this time,the removal rate of hemicellulose in lignocellulose was 76.21%,the retention rate of cellulose was 81.83%,and the yield rate of hemicellulose was 45.72%.The content of degradation products in the pretreatment solution was only 3.05 mg/g raw material.（3）The pretreatment solution was purified by Amberlite XAD7HP resin,and then the hemicellulose was extracted by the freezing-ethanol precipitation method.The optimal adsorption conditions for resins were uniformly mixed with ion exchange resin and pre-hydrolysis solution at a ratio of 1:7 w/v（g/ml）at 45°C for 120 min.Under these conditions,the total sugar retention rate in the pretreatment solution was 81.88%,the total acid removal rate was 87.13%,the total furan removal rate was 82.29%,and the lignin removal rate was84.58%,showing a good purification effect.At the optimal freezing temperature,the xylo-oligosaccharide contained in the hemicellulose precipitated by freezing-ethanol could reach71.11%.Finally,the effective recovery of ion exchange resin,ethanol and Fe³⁺reduced the cost and pollution,realized the sustainable development of the pretreatment process,and provided a new idea and method for the purification of the pretreatment liquid and the extraction of hemicellulose.（4）The hemicellulose and LCC model substances were extracted from the raw materials as completely as possible by the organic solvent method,and then pretreated by Fe Cl₃ solution.It was found that the main structure of poplar hemicellulose was poly-O-acetyl-4-O-methyl-D-glucuronic acid-D-xylose,and the structure of lignin in LCC was mainly S-type and G-type structural unit.The 2D-HSQC pattern analysis found that Fe Cl₃pretreatment could degrade and dissolve almost all carbohydrates without causing significant degradation of lignin.The effect of metal chloride pretreatment was revealed at the molecular level,which provided a theoretical basis for the extraction of hemicellulose by metal chloride pretreatment.