Article pubs.acs.org/IECR
Insights into the Primary Decomposition Mechanism of Cellobiose under Hydrothermal Conditions Zainun Mohd Shafie, Yun Yu,* and Hongwei Wu* School of Chemical and Petroleum Engineering, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia ABSTRACT: This paper reports a systematic investigation on the primary decomposition mechanism and kinetics of cellobiose under hydrothermal conditions at 200−275 °C and a wide initial concentration range of 10−10,000 mg L−1. Isomerization of cellobiose to cellobiulose (glucosyl-fructose) and glucosyl-mannose dominates the primary reactions of cellobiose decomposition, contributing to 71−93% of cellobiose decomposition depending on reaction conditions. In contrast, cellobiose hydrolysis to glucose makes only limited contributions (6−27% depending on reaction conditions) to the primary decomposition of cellobiose. This indicates that hydroxyl ions have a more significant effect to catalyze the isomerization reactions to produce cellobiulose and glucosyl-mannose. The catalytic effect of hydronium ions is weak probably because of the high affinity of hydronium ions for water molecules, which reduces the availability of hydronium ions for catalyzing the hydrolysis reaction. At increased temperatures, the affinity of hydronium ions for water molecules decreases because of the weakened hydrogen bonds in water, leading to an increase in the selectivity of the acid-catalyzed hydrolysis reaction. A higher initial cellobiose concentration also promotes hydrolysis reaction due to the formation of acidic products at the early stage of cellobiose decomposition. As a result of the reduced molar ratio of ion product to cellobiose, the activation energies of both isomerization and hydrolysis reactions increase with an increase in initial concentration, leading to an increase in the apparent activation energy of cellobiose hydrothermal conversion. Previous studies25−27 by this group indicated that the primary hydrothermal decomposition reactions of cellulose produce a series of sugar products including glucose and its oligomers with various degrees of deploymerization. The studies on further conversion of glucose oligomers into monomer or other decomposed products under hydrothermal conditions are scarce, apart from several limited investigations28−30 on hydrothermal decomposition of cellobiose (the simplest glucose oligomer linked by the β-1,4 glycosidic bond) at high temperatures (>300 °C). However, the cellobiose hydrothermal decomposition mechanism at low temperatures (e.g.,
