DOI: 10.14457/tu.the.2017.1724
title: Phase behavior and shrinking particle model of cellulose dissolution and modification in sub- and supercritical carbon dioxide switchable system: applications for fabrication of cassava-based cellulose transparent film and porous carbon electrode on lithium-air battery

description: Cellulose is one of alternative sustainable raw materials for reducing petroleum-derived polymer dependency in the manufacturing of related industries. To widen the applications of cellulose, novel approaches for dissolution and modification of cellulose are considerable interest to improve processability of poorly soluble cellulose. Due to strong intra- and inter-molecular hydrogen bonding of cellulose, homogeneous cellulose form cannot be achieved by water and most conventional solvents. This research aimed to dissolve cellulose using carbon dioxide switchable system (CO2-SWS) and to investigate the phase behavior of cellulose during dissolving in this system. Cellulose derived from a cassava pulp waste was prepared and dissolved in the CO2-SWS composed of 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU), ethylene glycol (EG) and carbon dioxide (CO2) mixture under various operating pressures (p) and temperatures (T) where CO2 exhibits properties of gaseous, sub-, and supercritical states. CO2-SWS at super- and subcritical state was an effective solvent yielding transparent cellulose solution which was further used in preparing transparent film. Moreover, applying the homogeneous cellulose solution was used as preliminary study in preparing hybrid composite cellulose/carbon black electrode as a promising candidate binder solution for application in lithium-air rechargeable battery. After initial discharging the lithium-air battery, the specific energy density was about 10-fold higher than that of conventional lithium-ion battery. The cyclic performance was poor since the lithium-air battery failed after 10th cycles; however, it can potentially extend periods of cycling time of three-cycle results published on lithium-air battery. When the gaseous phase of CO2 under lower p and T was applied, formation and precipitation of cellulose carbonate salts were observed in the CO2-SWS as verified by Fourier transform infrared spectroscopy results. Based on the experiments of homogeneous cellulose dissolution, the correlation between initial CO2 concentration and dissolving conditions was derived using regression analysis; which in turn, the established correlation strongly relied on the pressure. The correlation allowed us to estimate CO2 concentration representing the reactant concentration in CO2-SWS for determining model-predicted diffusion coefficient of CO2 ( ), completed cellulose dissolution time ( ), and cellulose dissolution conversion ( ) by using the developed  diffusion-limited shrinking particle model (SPM). The results showed that the   values range from 10-20 to 10-17 m2 s-1 depending on the dissolution conditions. The R2 value of 0.984 suggests that the predicted   and   values were in good agreement with the observation. The influence of the CO2-SWS operating conditions on the predicted numerical  ,  , and   values was investigated using analysis of Pearson’s and Spearman’s rho correlations. The estimated   and  values were inversely related with p, whereas   showed a very strong positive relationship with p.