Dissolution Processing of Cellulosics
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The dissolution of cellulosic biomass is a critical step for the efficient utilization of this renewable resource for the preparation of high value-added functional polymers and chemicals and also biofuels. The crystalline structure and extended noncovalent interactions render the cellulosic biomass recalcitrant to processing and constrain its efficient utilization. Despite research efforts, important aspects of cellulose dissolution remain not well-understood. We have developed a phenomenological model that captures the phenomena governing the dissolution of semicrystalline polymers, e.g., solvent diffusion, transformation from crystalline to amorphous domains, specimen swelling, and polymer chain disentanglement. [Ghasemi, M.; Singapati, A. Y.; Tsianou, M.; Alexandridis, P., Dissolution of semicrystalline polymer fibers: Numerical modeling and parametric analysis. AIChE Journal 2017, 63 (4), 1368-1383. DOI: 10.1002/aic.15615]. This model fits well experimental data for swelling and dissolution of cotton fibers in the ionic liquid 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), and allows the quantification of the solvent effectiveness in cellulose decrystallization and chain disentanglement. [Ghasemi, M.; Alexandridis, P.; Tsianou, M., Cellulose dissolution: Insights on the contributions of solvent-induced decrystallization and chain disentanglement. Cellulose 2017, 24 (2), 571-590. DOI: 10.1007/s10570-016-1145-1] The model provides microscopic/internal information (e.g., concentration profiles of solvent and cellulose) on the cellulose fibers during dissolution. The model also provides macroscopic/integrated information such as the time evolution of fiber diameter, degree of crystallinity, and fraction of un-dissolved fiber as well as the total dissolution time and the fiber decrystallization time. The model has been employed to assess the controlling mechanism of cellulose dissolution over a broad range of solvent abilities in terms of decrystallization and disentanglement. [Ghasemi, M.; Tsianou, M.; Alexandridis, P., Assessment of solvents for cellulose dissolution. Bioresource Technology 2017, 228, 330-338. DOI: 10.1016/j.biortec.2016.12.049]. The developed model has also employed to addresses the impact of crystallinity and size on the kinetics of cellulose swelling and dissolution. We further address the kinetics of swelling and dissolution of cellulose particles in conditions emulating large-scale processing where the particles exhibit a distribution of size. To this end, we have developed a dissolution ensemble model in which the behavior of a population of polymer particles is obtained from an ensemble of individual particle dissolution models. To the best of our knowledge, this is the first study that (i) provides a microscopic description for cellulose dissolution that captures macroscopic experimental observations; (ii) distinguishes and quantifies the effectiveness of solvent toward cellulose decrystallization and chain disentanglement and allows to assess relative contributions of these two factors in dissolution kinetics of cellulose; (iii) identifies conditions where cellulose dissolution is constrained by the lack of solvent abilities in decrystallization or in chain disentanglement; (iv) quantifies the impact of the degree of crystallinity and of the fiber diameter on the kinetics of crystalline cellulosic fiber swelling and dissolution; and (v) applies the ensemble modeling approach for dissolution of polydisperse cellulosic biomass particles. The obtained results provide guidelines for the selection of solvent systems and experimental conditions that can improve the extent and speed of cellulose dissolution. The findings also provide recommendations for an efficient pretreatment strategy (e.g., treatment to reduce crystallinity or milling to decrease particle size) to facilitate an enhanced conversion of cellulosic biomass. The obtained insights offer useful suggestions for effectively processing of cellulosic biomass (grinding and milling coarse particles and controlling the polydispersity of the mixture) in order to enhance utilization of this renewable resource.