(635a) Does Recrystallization in Aqueous Environment Affect the Reactivity of Ball-Milled Cellulose for Acid Catalyzed Hydrolysis?
Cellulose is a homopolymer of glucose units connected by Î²-1,4 glycosidic bonds; polymer chains form dense hydrogen bond network with their hydroxyl groups, resulting in a highly crystalline solid resistant to dissolution and chemical conversion. Typical depolymerisation to glucose is through acid or enzyme catalysed hydrolysis. Kinetic analyses have attempted to relate cellulose hydrolysis rate to structural parameters such as crystallinity, degree of polymerization, particle size and surface area. Literature reports suggest that cellulose crystallinity is the major factor determining reactivity. Models explain this hypothesis by differential rate of depolymerisation of amorphous and crystalline regions. Thus, a large number of pretreatment processes aiming at cellulose decrystallization have been proposed to decrease time and condition severity of cellulose conversion. However, an overlooked phenomenon is cellulose recrystallization in aqueous environment and most hydrolysis processes happen in water solutions. In this work, we studied the relationship between recrystallization of ball-milled cellulose and its difficulty to depolymerize in acid catalysed hydrolysis.
We used vibratory ball-milling of cellulose for varying periods of time to generate samples with different degrees of crystallinity. Hydrolysis with hydrochloric acid confirmed that progressive ball-milling resulted in higher rate of depolymerisation to glucose, consistent with the decrease in crystallinity. According to the current model, recrystallization of cellulose should result in a decrease of hydrolysis rate. We used hydrothermal treatment of ball-milled samples to increase the crystallinity, avoiding cellulose hydrolysis at the treatment conditions. Further, we hydrolysed the recrystallized cellulose and compared their glucose yield to ball-milled only samples. However, the glucose yield showed only slight decrease, not consistent with the increase in crystallinity.
The lack of full recalcitrance recovery suggested that there is another structural feature of ball-milled cellulose that was determining its reactivity. Particle analysis revealed fast initial decrease of size followed by a stabilization during ball-milling. Thus, it could not explain increase in hydrolysis rate of progressively ball-milled samples. Further structural analysis with gel permeation chromatography and electron spin resonance revealed that ball-milling results in chain breaking and generation of new chain ends. We hypothesize that these are accessible and highly active sites for acid catalysed hydrolysis.
Our results suggest that crystallinity is not always the major parameter affecting cellulose hydrolysis rates and depolymerisation models should be updated to include recrystallization and other structural features. Furthermore, the obtained fundamental knowledge motivates new cellulose pre-treatment processes and use of hydrolysis conditions that do not result in recrystallization.