Structural Analysis And Self-precipitation Mechanism Of Soluble Lignin In Pre-Hydrolysis Liquor Of The Dissolving Pulp

Dissolving pulp is a high-purity refined pulp which can be used as raw material to produce a range of cellulose derivatives.Pre-hydrolysis is a critical step in the kraft-based dissolving pulp,with the aim of removing hemicellulose from the lignocellulosic material as much as possible while protecting the cellulose.Dissolving pulp production process well fit with the biorefinery,the pre-hydrolysis liquid(PHL)produced in the pre-hydrolysis process contains a large amount of organics such as xylose,glucose,acetic acid,furfural and lignin.However,the structure and composition of soluble lignin in the PHL are complex and difficult to be selectively removed,which affects the further utilization of other components in the PHL and its self-utilization.In this paper,bamboo-willow was used as raw material to explore the effect of adding phosphoric acid on the main components of material and PHL in the pre-hydrolysis stage.The results showed that adding phosphoric acid in the pre-hydrolysis stage can effectively remove hemicellulose from the bamboo-willow and prevents cellulose from degrading to a certain extent.A large amount of sugars(monosaccharides and oligosaccharides)and a small amount of lignin,acetic acid,formic acid,and furfural are found in the PHL.The dissolved hemicelluloses in the PHL were mostly xylose and glucose,and most of the sugars in the PHL were in an oligomeric form.Temperature and time are the key factors affecting the pre-hydrolysis effect,and the concentration of dissolved substances in the PHL increased with the increase of P-factor.Bamboo-willow can be used as a linocellulosic material to produce a qualified viscose grade dissolving pulp through the optimized prehydrolysis process parameter,kraft cooking and bleaching.The?-cellulose content,pentosan content and degree of polymerization(DP)of the finally prepared viscose grade dissolving pulp were 92.1%,4.77%and 675,respectively,and the reactivity was 8.10 seconds.The bamboo-willow PHL was prepared by the optimal pre-hydrolysis process parameter,the soluble lignin was separated from PHL by activated carbon adsorption,and its structure was characterized and analyzed by IR,~1H NMR,2D HSQC spectroscopy and TGA.Studies have shown that there are mainly syringyl(S)and guaiacyl(G)soluble lignin structures and a small amount of p-hydroxyphenyl(H)soluble lignin in the bamboo-willow PHL.The main linkages between soluble lignin structural units are?-?,?-O-4,?-1 and?-5.In addition,cinnamyl alcohol and ferulic acid ester structurally soluble lignin were also detected in the bamboo-willow PHL.The PHL was treated by calcium chloride,aluminum sulfate,cationic polyacrylamide and a cationic polyacrylamide/aluminum sulfate combination treatment,and the effect of treatment on the soluble lignin and sugar content of the PHL was investigated to explore soluble lignin self-precipitation mechanism.The results showed that when the optimum dosage of calcium chloride was 2.0%,the selective removal of soluble lignin was as high as 94.80%,and the soluble lignin removal was 8.09%.When the optimum dosage of aluminium sulfate is 300 ppm,the removal of soluble lignin and its selectivity removal were 18.32%and 55.46%,respectively.When cationic polyacrylamide was used to treat the PHL,the loss rate of total sugar increased with the increase of CPAM dosage.The optimum dosage of cationic polyacrylamide/aluminum sulfate combination treatment was 60/300 ppm.At this time,the soluble lignin removal was 25.0%,and the soluble lignin selective removal was 56.07%.The above results indicated that calcium chloride may remove soluble lignin from PHL by electrostatic neutralization and adsorption,aluminum sulfate removed soluble lignin by electroneutralization and flocculation precipitation,and cationic polyacrylamide increased molecular weight of soluble lignin by electroneutralization and bridging with flocculation,thus precipitating soluble lignin colloid molecules.