Purification Of Cellulose By Lewis Acid/Lewis Base-enhanced High Temperature Liquid Water

As the most abundant natural polymer on the earth,cellulose has been widely used in the production of cellulose based materials.The high-purity cellulose,as the raw materials of cellulose based materials,is difficult to obtain due to the composition characteristic and structure characteristics of lignocellulose.In this paper,bleached pulp was treated by high temperature liquid water?HTLW?for the purification of cellulose.Optimal conditions of HTLW treatment was investigate by orthogonal experiments.Based on the orthogonal experiment,the effect of factors during HTLW treatment process,including reaction temperature,reaction time and pulp concentration,was studied by single-factor experiments.Lewis base and Lewis acid were used to enhance the effect of HTLW.The high purity cellulose was obtained by the enhanced HTLW treatment.The variation of carbohydrate content,chemical structure,crystalline structure and microstructure of cellulose were investigated.Meanwhile,the possible mechanism of treatment processes involved was discussed.Results showed that during HTLW treatment processes,the most significant factor for?-cellulose content was reaction temperature,followed by reaction time and pulp concentration,respectively.The optimal conditions of HTLW treatment were reaction temperature was 180?,reaction time was 30min,and pulp concentration was 3%.Under such conditions,the?-cellulose content of treated specimens was 89.54%,S₁₀ was 11.97%,S₁₈ was 8.56%.The chemical structure of cellulose was varied scarcely after HTLW treatment.Whereas,the amount of reduce end group increased after treatment.With the incorporation of Lewis base,i.e.NaHCO₃ and Na₂HPO₄,the purity of cellulose enhanced distinctly.The highest?-cellulose content of specimens was 92.51%.After alkali treatment,the highest?-cellulose content of specimens was 93.82%.Furthermore,the degradation of cellulose was reduced distinctly by the incorporation of Lewis base.It was evident that the hydrolytic selectivity of HTLW treatment could be improved by Lewis base.The chemical structure of cellulose was not varied with the incorporation of Lewis base.Nonetheless,the amount of reduce end group,on the basis of amount of specimens treated by HTLW,was decreased by the incorporation of Lewis base.The crystalline form of cellulose was not varied after HTLW treatment with or without the incorporation of Lewis base.Besides,the incorporation of Lewis base was benefit for the increase of crystalline index?CI?during HTLW treatment process,and the retention of CI during subsequent alkali treatment processes.The purity of cellulose could also be enhanced by the incorporation of Fe³⁺,Cr³⁺or Cu²⁺,i.e.Lewis acid.Nonetheless,the average degree of polymerization was decreased dramatically.Most of obtained specimens were powders.The diameter of these specimens distributed form 1.00?m to 100?m,which could meet the requirement of MCC.Specimens had turned into rod-like structure after treatment with the incorporation of transition metal ions.Simultaneously,latitudinal fractures and fragments of cellulose fiber could be observed in the image of specimens treated by transitional metal ions enhance HTLW.The chemical structure of cellulose was not varied with the incorporation of transitional metal ions.Nonetheless,the amount of reduce end group was increased distinctly by the incorporation of transitional metal ions.Moreover,the absorption peak,due to the stretching vibration of-OH group linked by hydrogen bonds increased distinctly after treatments.The crystalline form of cellulose was not varied after HTLW treatment with or without the incorporation of transitional metal ions.The intensity of diffraction peak of?004?plane in cellulose I was enhanced observably by the incorporation of transitional metal ions.Meanwhile,the CI of specimens increased.