Hydrolysis of cellulose to glucose and cello-oligosaccharides (COS) is an essential step in converting waste cellulose to useful chemicals. Carbon catalysts functionalized with oxygenated weak acid sites are effective for hydrolyzing cellulose to glucose and COS after suitable pretreatment. It is known that cellulose molecules adsorb on carbon materials via CH-π and hydrophobic interaction. However, influence of adsorption on the rate of hydrolysis is not clear. In this study we investigate the rate of hydrolysis of COS with increasing chain length, which serve as model cellulose molecules. Among various heterogeneous acid catalysts only carbon showed an increase in rate of hydrolysis with chain length as shown in the figure. Cellohexaose underwent hydrolysis eleven times faster than cellobiose under the same reaction condition for AC-Air. Carbon catalyst had a microporous structure with pores just wide enough for axial entry of COS followed by adsorption. However, the total molar adsorption of COS was same for all chain lengths and increased adsorption of larger COS was not the reason for higher activity as expected. A careful examination of evolved products suggested that hydrolysis of terminal glycosidic bonds was favored over internal glycosidic bonds. Furthermore, a decrease in activation energy was observed with increase in chain length. Based on these observations we propose that adsorption of larger COS withing the narrow pores of carbon causes structural distortion of molecule leading to reduction in activation energy required for cleavage of glycosidic bonds, which contributes towards the superior activity of carbon catalysts for cellulose hydrolysis.

Influence of chain length on hydrolysis of β-1,4 glycosidic bonds over carbon catalysts

ACS Spring Meeting 2021

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