Cellulose pretreatment with inorganic salt hydrate: Dissolution, regeneration, structure and morphology

The special molecular structure and properties of cellulose restrict its resource utilization. In this work, two kinds of common inorganic salt hydrates (ZnBr2·RH2O and FeCl3·RH2O) were used to pretreat microcrystalline cellulose (MCC) by magnetic stirring method at temperatures below 80 °C. It was found that while high temperatures could shorten the dissolution time of MCC, while inorganic salt hydrates with either low water (ZnBr2·3.5H2O and FeCl3·5H2O) or high water (ZnBr2·5H2O and FeCl3·8H2O) content can prolong the dissolution time of MCC. The reactivity of regenerated cellulose (RC) after dissolution was evaluated by enzyme hydrolysis reaction. Moreover, ZnBr2·4H2O and FeCl3·6H2O have better pretreatment effect. The structure and surface morphology of MCC before and after pretreatment were investigated by XRD, FT-IR, SEM, AFM and TGA. The MCC was converted from cellulose Ⅰ to cellulose II through the pretreatment. With the extension of dissolution time, the strength of hydrogen bond and thermal stability of RC were both slightly weakened, Moreover, the crystallinity and degree of polymerization (DP) of RC decreased, and the surface morphology of RC was more rougher, more uneven and dispersed compared with the smooth and compact MCC. The adsorption rate curves showed that RC-ZnBr2·4H2O was more conducive to the adsorption of cellulase, thereby accelerating the rate of RC hydrolysis. Furthermore, ZnBr2·4H2O is a well reusable pretreatment solvent that can maintain a good pretreatment effect on cellulose after five cycles.