Regulation Of Dissolving Pulp Properties By Using Heteropoly Acid-based Catalytic Hydrolysis And Its Coupling Treatments

High-purity cellulose(dissolving pulp)can be produced from lignocellulosic biomass using different methods,and it can be used to prepare viscose rayon,cellulose ethers,cellulose esters and nanocellulose products,etc.Among them,viscose rayon is the dominant one,accounting for 60-65%of the total consumption of dissolving pulp.The commercial production of dissolving pulp involves two main methods:acid sulfite(AS)and prehydrolysis kraft(PHK).PHK method is generally used in the newly-built dissolving pulp production line due to the low purity of the dissolving pulp prepared by AS method.In recent years,with the increasing demand for dissolving pulp,some treatment methods have been used to upgrade the paper-grade pulp to dissolving pulp.Because of its low investment cost,flexible and efficient process,direct purification of paper-grade pulp has attracted much attention.The alkali extraction technology,especially cold caustic extraction(CCE),is one of the most effective methods for removing hemicellulose,in fact,it is a commercial process for increasing the cellulose content.The properties of the dissolving pulp are closely related to the performances of the downstream viscose fiber in terms of production processability and product quality.The key quality indicators of dissolving pulp include viscosity,purity and reactivity.Among them,the latter two are particularly important.Due to the alkaline pulping method,dissolving pulps prepared by PHK method and CCE method both have the problem of higher purity but lower reactivity.In addition,the traditional CCE method requires large alkali consumption and requires multi-stage washing,which increases the production cost of energy consumption and equipment.Therefore,it is necessary to take some measures to reduce the alkali concentration of CCE pulp and improve the reactivity of CCE and PHK dissolving pulp.Heteropolyacid(HPA)is a new type of green solid acid catalyst with strong Br(?)nsted acidity,high stability,proton mobility and good regeneration and recycling characteristics.This thesis focuses on the HPA catalytic hydrolysis of cellulose and hemicellulose,and the improvement of the purity and reactivity of the dissolving pulp by means of coupled microwave treatment and mechanical grinding.Firstly,using PHK dissolving pulp as the raw material,we optimized the HPA catalytic hydrolysis of cellulose of dissolving pulp with the following conditions:0.01 mol/L H+concentration,10%pulp concentration 90℃ and 30 min.We compared the hydrolysis performances of four different HPAs,namely,phosphotungstic acid(PTA),silicotungstic acid(STA),phosphomolybdic acid(PMA)and silicomolybdic acid(SMA),in terms of viscosity,fiber accessibility(fiber morphology and water retention value(WRV))and Fock reactivity.The results showed that PTA was the best one,while others were in the order of STA>PMA>SMA.Therefore,PTA was chosen as the catalyst for catalytic hydrolysis of cellulose fibers in subsequent research.Besides,the recovery rate of STA,PTA and PMA were all above 87%.In addition,the catalytic activity of HPA remained very high when recycled/reused.We mainly focused on using PTA catalytic hydrolysis of cellulose fiber to activate the PHK dissolving pulp.The reactivity before and after PTA treatment,as well as the related mechanisms were investigated.The results showed the accessibility of dissolving pulp,indicated by pore size(PD),pore volume(PV),specific surface area(SSA)and water retention value(WRV),improved as a result of PTA hydrolysis treatment.Finally,the viscosity of the dissolving pulp obtained by PTA treatment was reduced from 561 mL/g to 437 mL/g,and the reactivity increased from 49.1%to 74.1%.PTA also had a high chemical stability and excellent recyclability and reusability,showing a high recycling rate(87.1%),Fock reactivity(72.5%)and catalytic activity(similar viscosity decrease and reactivity increase to the first cycle)even after 6 cycles.Secondly,in order to further improve the PTA catalytic effect,a simultaneous treatment consisting of microwave radiation and PTA catalytic(MW-PTAsim)was proposed to activate the accessibility of PHK dissolving pulp fibers,thereby enhancing the reactivity of dissolving pulp.The synergy of the MW-PTAsim treatment was due to the fact that the microwave action improves the penetration of PTA into fiber structure,also,the enhanced molecular activation under the influence of the microwave field,increases the catalytic hydrolysis,both of which will lead to more extensive changes in fiber microstructure and fiber surface structure,which,in turn,further facilitates the PTA penetration and the desirable catalytic hydrolysis.In addition,the localized heating and activation,further improve the PTA catalytic hydrolysis.After MW-PTAsim treatment,the reactivity of dissolving pulp increased from 49.1%of the original pulp to 85.8%,which was better than the effect of microwave with sequential PTA hydrolysis alone(MW-PTAseq).Again,PTA recycling/reuse to the process was studied,and similar catalytic activity to fresh PTA was observed,and the PTA recycling and Fock reactivity were over 87%and 83.2%,respectively,after six cycles,indicating that microwave treatment did not change the chemical stability and reusability of PTA.Furthermore,in order to decrease the alkali concentration in the CCE treatment of paper-grade pulp for the production of dissolving pulp,the PTA-assisted low-alkali concentration(4%NaOH)CCE process was developed.The results showed that PTA catalytic hydrolysis,together with 4%CCE treatment(PTA-4%CCE)decreased the hemicellulose content in the commercial kraft pulp(KP)from 13.96%to 4.03%,and the α-cellulose content increased from 85.20%to 94.92%,well-qualified for the quality of dissolving pulp.Compared with 8%CCE treated KP pulp(α-cellulose was 94.82%,Fock reactivity was 25%),PTA-4%CCE treated dissolving pulp had higher reactivity(49.5%),which is mainly due to the improved accessibility of fibers by PTA treatment.Additionally,the hemicellulose removal rate and its dissolution selectivity of the PTA-4%CCE were higher than those of 4%CCE and 8%CCE(i.e.,without PTA),further proving that the PTA-4%CCE is an effective process to remove hemicellulose from the paper-grade pulp.The recycling/reusing rate of PTA was high,but the degraded low molecular weight hemicellulose accumulated in the recovered PTA solution,which may have a small negative effect.Finally,a specialty dissolving pulp with high purity and high reactivity(96.96%of α-cellulose)was prepared based on a three-stage process.The process,namely R/PTAr-CCE-PTAf,consisted of an initial simultaneous mechanical refining(R)and PTA hydrolysis(R/PTA),followed by a low-alkali CCE extraction and a final PTA treatment.For the process,the first two steps were used for purification and the last step was for increasing its reactivity.During the first R/PTA treatment,the mechanical refining enhanced the PTA catalytic hydrolysis;also,the refining was promoted by PTA hydrolysis;in fact,the two are truly synergistic.After R/PTA treatment,the Mw of hemicellulose in KP pulp decreased significantly,thus,with a CCE treatment at 4%NaOH concentration,the hemicellulose removal from KP pulp was efficient,reaching 85.17%after R/PTA-CCE(4%NaOH),with the resultant dissolving pulp reaching 96.96%α-cellulose content.The PTA post-treatment improved the Fock reactivity of the dissolving pulp from 54.2%(R/PTA-CCE treated pulp)to 73.8%.The used PTA was recovered by ether extraction and reused in the process.After 8 reuses/recycles,the resultant dissolving pulp had similar α-cellulose content and Fock reactivity to those from using fresh PTA.In summary,the HPA catalytic hydrolysis concept can be applied to different processes.For PHK dissolving pulp,it improves the accessibility and reactivity of dissolving pulp,and controls its viscosity.The used HPA is recyclable/reusable in the process.For upgrading kraft pulp(KP)to dissolving pulp,the PTA catalytic hydrolysis will degrade hemicellulose in KP pulp fibers,thus,decreasing its Mw and facilitating the hemicellulose removal in the CCE process,consequently,the CCE can operate at a low alkali concentration.Again,the used PTA can be reused/recycled.Overall,the PTA catalytic hydrolysis provides an effective process concept for the preparation of high-quality dissolving pulp with high purity and high reactivity in a green,economic,and efficient manner.