Study On Mechanism Of Selective Separation For Hemicellulose And Dissolution Of Cellulose By EmimAc/Alkali Solvent System

Lyocell is widely used in clothing,industrial filter materials,non-woven fabrics and other fields due to its good skin affinity,biodegradability,moisture permeability and water permeability.However,the development of Lyocell industry is limited by the high cost of dissolving pulp and the low stability of solvent in the preparation process.As a novel kind of green solvent,ionic liquid has attracted wide attention in the field of biomass component dissolution and separation due to its unique properties.Ionic liquid can not only selectively remove hemicellulose from chemical pulp to prepare dissolved pulp,but also efficiently and safely dissolve cellulose as a solvent to prepare Lyocell.Due to the disadvantages of high viscosity,which is not conducive to the dissolution of cellulose and hemicellulose,relatively low removal rate of hemicellulose and long dissolution time of cellulose,the application of ionic liquid in the preparation of dissolving pulp and cellulose dissolution is limited.Herein,1-ethyl-3-methylimidazole acetate/alkali water system（EmimAc/alkali-a）was introduced to study the effect and mechanism of selective dissolution of hemicellulose from chemical pulp.What’s more,the dissolution effect and mechanism of aqueous EmimAc system on cellulose were also studied in order to provide theoretical guidance and technical support for the preparation of coupling dissolved pulp and Lyocell/Tencel production using chemical pulp as raw material.The main contents are as follows.（1）The effect of sodium hydroxide on hydrogen bonding of water and NaOH aqueous（NaOH-a）on the anion and cation structure of EmimAc was tested by the attenuated total reflection spectroscopy（ATR-IR）.The results showed that the addition of NaOH decreased the intensity of absorption peak at 3500-3100 cm^-1,and the“continuous absorber”vibration peak caused by hydroxide hydrate formed at 3100-1700 cm^-1.When NaOH-a was added to EmimAc,the absorption peaks of imidazole cation（Emim⁺）and acetate anion（Ac^-）shifted blue.NaOH can hydrogen bond with both anions and cations of EmimAc,and it has greater effect on anions.On one hand,the addition of NaOH can solvate effectively imidazole cation,and reduce the effect of Emim⁺and Ac^-;on the other hand,it can reduce the action of H₂O interacting with Ac^-repairing the occupation of Ac^-active site by water.Furthermore,the higher the concentration of NaOH,the better the repair effect of Ac^-active site.（2）The effect and mechanism of EmimAc/20%NaOH-a system on the selective removal of cellulose from hardwood chemical pulp were studied.With increasing the NaOH-a concentration from 0%to 5%,the removal rate of hemicellulose increased from 63.7%to 76.5%.Further increasing NaOH-a concentration to 10%,the removal rate of hemicellulose decreased,and the loss rate of cellulose reached 19.7%.The results also indicated that the crystal structure of cellulose remains cellulose I and part of unstable Iαchanges to Iβ.Moreover,the thermal stability of cellulose was improved.When the concentration of NaOH-a increased from 0%to 5%,theβ-αvalue of EmimAc/20%NaOH-a increased from 0.09 to 0.17.Theβ-αvalue will increase the selective dissolution ability of hemicellulose in EmimAc.（3）The influence and mechanism of alkali concentration on the solubility of cellulose in EmimAc/alkali-a system.When the concentration of NaOH-a increased from 0%to 5%,the dissolution time of 2 wt%cellulose in EmimAc/10%NaOH-a decreased from 180 min to 30 min,and the dissolution concentration increased from 2.5 wt%to 4.1 wt%.When the concentration of NaOH-a increased to 10%,the dissolution time decreased from 30 min to 25 min,the solubility increased from 4.1wt%to4.3wt%.These results indicated that NaOH can promote the dissolution of cellulose in EmimAc aqueous solution.NMR showed that the absolute value of C2 chemical shift of EmimAc in EmimAc/10%NaOH-a/cellulose dissolution system was less than C10;the absolute value of C2 chemical shift of EmimAc in the EmimAc/10%H₂O/cellulose dissolution system was greater than that of C10.In EmimAc aqueous solution,the action between anions and cellulose was enhanced by adding alkali,which accelerated the dissolution rate of cellulose and improved the solubility of cellulose.（4）The influence of anions and cations on the selectivity of hemicellulose removal was studied.The influence of anions was studied by using NaOH-a and Na Cl-a as cosolvent.Theβ-αvalue of EmimAc/20%NaOH-a was higher than that of EmimAc/20%Na Cl-a.The hemicellulose removal rate of EmimAc/20%NaOH-a system was 74.8%,while that of EmimAc/20%Na Cl-a was only 49%.The influence of anions on the solvation of Emim⁺is greater than that of cations.The NaOH-a,KOH-a and LiOH-a were used as co-solvents to explore the effects of different alkali metal cations on the selective removal of hemicellulose.The results showed that the alkali metal cations play an important role in the selective dissolution of hemicellulose,EmimAc/20%NaOH-a>EmimAc/20%KOH-a>EmimAc/20%LiOH-a.The crystalline structure of cellulose remained unchanged,but the increased crystallinity and polymerization,and thermal stability increased.（5）The effect of alkali on cellulose dissolution.The dissolution time of cellulose by EmimAc/10%NaOH-a,EmimAc/10%KOH-a and EmimAc/10%LiOH-a is 30min,80min and 85min,respectively.The monoclinic crystal ratio of cellulose dissolved into EmimAc/10%LiOH-a is higher than that of the other two alkali systems,and the crystal structure of cellulose is more stable.No matter which EmimAc/10%alkali-a system was choosed,the degree of polymerization and crystallinity of regenerated cellulose would become lower,and the crystal type would changed from cellulose typeⅠto typeⅡ.Furthermore,the thermal stability of regenerated cellulose was lower than that of raw cellulose.