Effect of Crystalline Structure on the Catalytic Hydrolysis of Cellulose in Subcritical Water

Depolymerization of cellulose, the most abundant biomass in nature, is a critical step for its catalytic conversion to fuels and chemicals. While cleavage of its glycosidic bond by acid hydrolysis is the rate-determining step to depolymerize cellulose, disrupting its robust crystalline structure is equally important. In this work, we examined the hydrolysis of cellulose of four different crystalline allomorphs, i.e., I, II, III, and IV, with respect to the conversion rate, change in the crystalline structure, and the degree of crystallinity and polymerization during the reaction. Independent of their crystalline structure, the four cellulose samples converted following the first-order reaction kinetics with no essential influence on the product selectivity. However, the rate constants were largely different and decreased in the following sequence: cellulose II > III > I > IV. The high rate of cellulose II is caused by its higher reaction probability, as reflected by its preexponential factor, which is several orders of magnitude higher than that for the other cellulose samples, which overcompensated its high apparent activation energy. It is found that cellulose I and IV undergo surface reactions at 478-508 K, whereas cellulose II and III swell at the reaction temperatures, which allows the hydrolysis reaction to occur in the whole swollen regions, leading to higher accessibility of the glycosidic bond to the H⁺ catalyst and consequently higher conversion rates. These findings provide the mechanistic basis for an alternative strategy to enhance the efficacy in depolymerization of cellulose via tuning of crystalline phases.