| Dissolving pulp is a kind of green biomass raw and processed materials made from renewable plantation woods,widely used in the fields of textile,new materials,and national defense and others.At present,dissolving pulp has great market demand and long-term development prospects because of its green environmentally friendly characteristics.The related problems to the dissolving pulp developmentneed to be improved: lower pulping yield,poor reactivity,shortage of raw material supply as well as the low value conversion of dissolved lignin and hemicellulose.Based on the concept of biomass refining,this paper carried out the study on the preparation of dissolving pulp,for the first attempt,by applying green environmental friendly acid hydrotrope(H2O/p-Ts OH)and deep eutectic solvent(DES)together with ultrasonic treatment to explore a new process scheme for preparing dissolving pulp by green solvent method,and to study the mechanism of lignin dissolution,to find out the possibility of utilization of high-value added lignin.Accordingly,the specific research route and contents of this paper are as follows:Firstly,to expand the selectivity of the raw materials of softwood dissolving pulp,this work has tried to use the fast-growing Radiata pine as raw materials and to study the practicality of utilizing recently widely used prehydrolysis sulphate pulping technology combined with(OO)D0Eop D1D2 of two stages O2-delignification and four stages ECF bleaching process for production of high performance dissolving pulp.The work further investigated the selectivity of the softwood raw material for preparing the dissolving pulp with such technology through comprehensive evaluation of chemical analysis of raw materials;analysis of components of prehydrolate;and analysis of the finished product quality of the dissolving pulp prepared by this method.The results have shown that Radiata pine meets the conditions of making dissolving pulp by chemical analysis of cellulose,hemicellulose,and lignin and extractives contents.The optimized parameters for P-facor and H-factor were tried and found to be 480 and 1500,respectively by overall balance of removal rate of hemicellulose and lignin and also the loss rate of cellulose.Under such conditions,the brightness reached 91%ISO,degree of polymerization reached 910,?-cellulose content was up to 95%,and pentosan content was smaller than 2%.Through comprehensive comparedthe quality of commercial high-end coniferous dissolving pulps,it is shown that the dissolving pulp of Radiata pine,prepared by prehydrolysis kraft technology followed by(OO)D0Eop D1D2 process fullfil the requirements of high-performance dissolving pulp.Therefore,this study provides a technical idea for the preparation of high-end softwood dissolving pulp and widens the selectivity of raw materials.Secondly,this paper has explored the technology and mechanism to produce the dissolving pulp with acid hydrotrope(H2O/p-Ts OH).The study probed the feasibility of the preparation process of dissolving pulp by 1).Choosed three different fibers as raw materials,i.e.softwood Radiata pine,hardwood poplar,and annual grass fiber reed;2).Observed the reactions and effect of the green environmentally-friendly H2O/p-Ts OH with different fibers;3).Investigated the difference of dissolution mechanism of lignin and hemicellulose and analyzed the cellulose retention value.The results showed that under the mild conditions,i.e.temperature 80? and time 30 minutes,lignin removal efficiency is 78.16% for reed,51.37 % for poplar,and 28.13% for Radiata pine;while hemicellulose dissolution rate is 85.56% for reed,79.56% for poplar,and 65.42% for radiata pine.By applying FT-IR?TGA?GPC?31P NMR and 2D HSQC NMR and other means,the lignin in raw materials,lignin extracted from prehydrolysates,and residual lignin in fibers are characterized.According to the analysis resultes,it was found that the S-lignin and H-lignin are easier to be removed compared with G-lignin,indicating that the lignin of nonwood and hardwood materials is more easily to be removed,but the lignin of softwood is more difficult to be dissolved and removed due to its higher content of G-lignin of chemically stable structures.Meanwhile,it was found that the extracted lignin from prephydrolysate preserved relatively complete chemical structrue,lower polydispersity index,and lower molecular weight,making extracted lignin has potential higher value of utilization.This study found that the mild conditions are suitable for dissolving pulp made from poplar materials,i.e.temperature 90? and time 60 minutes,thereby reaching removal rate up to 90% for both hemicellulose and lignin.However,the poplar raw material are still not fully dispersed into pulp state,which needs further improvement so as to realize the dispersion of fibers into pulp and meet the requirement of pulping.Afterwards,with conduct the ultrasonic treatment,the fibers could be fully dispersed to form pulp.Then,two stage oxygen delignification and four stage ECF bleaching process were used,resulting the dissolving pulp of brightness 91.08%ISO,degree of polymerization 897,alpha-cellulose content 93.34% and pentason 1.9%.Through the comprehensive assessment of the quality of the hardwood dissolving pulps on the market and test results of reactivity performance,it is shown that the quality of poplar dissolving pulp which was prepared by the H2O/p-Ts OH plus untrasonic ttreatment followed by(OO)D0EOPD1D2 process is close to that of market high-performance dissolving pulp of hardwood.Meanwhile,the p-toluenesulfonic acid(p-Ts OH)in the water-soluble system has advantage to be recycled by industrial recrystallization technology.The dissolved lignin has the characteristics of relatively complete chemical structure,small molecular weight and low PDI,resulting to possibility of high-value added utilization.Thus,this new technology provides a route for the preparation of high-performance dissolving pulp from hardwood based on the biomass refining and comprehensive utilization.However,the lignin removal ability of coniferous wood is still not enough in this system therefore it is necessary to find other new solvents to expand the suitability of softwood raw materials.Lastly,the application of prehydrolysis combined with deep eutectic solvent(DES)followed by ultrasonic treatment was established with poplar and radiata pine as raw materials.The hemicellulose is partially dissolved by prehydrolysis but,most of the lignin and remaining hemicellulose were removed by DES treatment;unbleached pulp was obtained by ultrasonic treatment;the final products of dissolving pulp was formed with O2-delignification and ECF bleaching process.Because there is only water,choline chloride and lactic acid were used in the process;the technology belongs to the environmental friendly green solvents and system.Through the analysis of the components of the wood chips after the above treatment,the best prehydrolysis conditions were determined: P-factor of poplar and Radiata pine are 400 and 480,respectively;under such conditions,removal rates of hemicellulose of poplar and Radiata pine are 61.45% and 51.68%;after DES combined reactions,removal rate of lignin reached 90.45% and 89.68%,and hemicellulose removal rate is 90% above for both poplar and radiata pine.But at this point,the raw materials are still not fully dispersed into pulp yet,and then the pulp is further dispersed by ultrasonic treatment;finally O2-delignification and ECF bleaching process was used to prepare the bleached dissolving pulp.The quality of dissolving pulp made from poplar is brightness 90.87%ISO,degree of polymerization 757,alpha-cellulose 94.31%,pentason 2.01% and correspondingly,those from radiata pine is brightness 90.11%,degree of polymerization 857,alpha-cellulose content 94.01%,and pentosan 1.91%.Through the comprehensive evaluation of the quality of dissolving pulp on the market and the test of reactivity performance,it is shown that the dissolving pulp of radiata pine and poplar,prepared by the prehydrolysis-deep eutectic solvent(DES)assisted by ultrasonic technology,is close to the quality of high-end performance dissolving pulp on the market.At the same time,the dissolved lignin was analyzed by using GPC,2D HSQC and NMR techniques and found the dissolved lignin has the characteristics of relatively complete chemical structure,small molecular weight and low PDI.Therefore,the development and application of prehydrolysis/DES assisted by ultrasonic treatment provides a set of ideas of pulping technology based on the comprehensive utilization of biomass refining for the preparation of high-performance dissolving pulp from wood fiber materials.Meanwhile,it solves the problem that H2O/p-Ts OH alone is not suitable for making coniferous wood dissolving pulp and overcomes the limitation of softwood raw materials selection.Compared with the traditional acid sulfite and PHK process,the acid hydrotrope (H2O/p-Ts OH)and deep eutectic solvent(Ch Cl/LA)system have the characteristics of low reaction temperature and short time.In particular,the temperature of p-Ts OH system is only 90?,which is much lower than the traditional 140-170?.Easy separation of dissolved lignin and degraded hemicellulose from hydrolysate,and the solvent can be recycled & reused is the advantage also which benefits to zero emission of pollution.At the same time,the molecular structure of the dissolved lignin is complete,and the molecular weight and polydispersity index are low(the dissolved lignin from DES is lower than the one from H2O/p-Ts OH).In the dissolution process of lignin and hemicellulose,the degradation of cellulose is low,and the yield & degree of polymerization of dissolving pulp are high.The DES system is suitable for both hardwood and softwood is not suitable for softwood raw materials,and disaggregates the limitation that H2O/p-Ts OH is not suitable for softwood.This study provides a set of new technical routes and ideas for use of lignocellulosic materials to prepare high-performance dissolving pulp based on biomass refining and comprehensive utilization. |
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