Study On Extraction Of Lignin From Rice Straw And Its Directional Depolymerization To Prepare Monophenolic Compounds

Rice and other crop straw resources are abundant in China,and the advancement of its lignin and other components of high-efficiency utilization technology is of great significance to the sustainable and healthy development of the related bio-refining industry.At present,the utilization technology of lignocellulose is mainly focused on utilization of cellulose and hemicellulose components,but lignin component is discharged or burned as cheap by-products,affecting the economic,social and environmental benefits of lignocellulose processing industry.The status of lignin component extraction,directional depolymerization and transformation technology has become technical bottlenecks restricting its industrial development.This paper proposes a solution scheme on the extraction of rice straw lignin based on the p-toluenesulfonic acid aqueous solution(p-TsOH),and the preparation technology of monophenolic compounds by directional depolymerization of p-toluenesulfonic acid rice straw lignin.Firstly,the extraction of lignin from rice straw based on p-TsOH aqueous solution were studied.The effects of p-TsOH solution concentration,temperature and time on the separation in cellulose,hemicellulose(or degradation products)and lignin were studied,and the separation products of cellulose,xylose solution and p-TsOH lignin were obtained,respectively.Based on the extraction rate of lignin as the objective,a multi-factor optimization model was established by using the central combinatorial design method.The optimum conditions of the extraction of rice straw lignin based on the p-TsOH aqueous solution were determined as followed: 70.7 wt% p-TsOH,80.5℃ and for 33 min.The experimental extraction rate of lignin was 78.5%,and the relative error with prediction result was 0.51%.It was shown the model was reliable.The chemical structure,molecular weight and distribution of p-TsOH lignin were analyzed by some physicochemical characterization methods.It was found that the p-TsOH lignin obtained from rice straw with p-TsOH treatment belonged to GSH-type lignin.Lignin was depolymerized and condensed to a certain extent,and the active groups such as phenolic hydroxyl groups and carbonyl group were increased.The relative molecular weight of p-TsOH lignin was significantly reduced.During the thermal decomposition process,the maximum weight loss temperature and final residual solids of p-TsOH lignin were 344℃ and 25.0%,respectively.The experimental study on preparation of monophenolic compounds by directional depolymerization from rice straw lignin was carried out.In Pt/C catalyst system,the effects of various lignin extraction methods on lignin depolymerization products were investigated.Meanwhile,some acid catalysts such as HZSM-5 or p-TsOH were introduced into the Pt/C catalyst system.At 275℃ and 60 min,the yields of monophenolic compounds obtained by the Pt/C,the combination of Pt/C and HZSM-5,the combination of Pt/C and p-TsOH were 4.44%,5.46% and 9.41%,respectively.The experimental results showed that the bifunctional catalyst system composed of Pt/C and p-TsOH had the best effect.Finally,the optimal conditions for preparation of monophenolic compounds by directed depolymerization of rice straw p-TsOH lignin were determined experimentally,which were the reaction temperature of 275℃,the time of 60 min,and the amount of composite catalyst of 0.2 wt% Pt/C and 0.3 wt% p-TsOH.Under the above process conditions,the yield of monophenolic compounds was reached to 10.05%,accounting for 13.75% of the total amount of liquid-phase depolymerization products,and among which the selectivity of 4-ethylphenol was the highest(4.56%).