Synergy of Vicinal Oxygenated Groups of Catalysts for Hydrolysis of Cellulosic Molecules

Carbon materials bearing carboxylic acids and phenolic groups efficiently catalyze the hydrolysis of cellulose. In this work, we demonstrate that salicylic acid and phthalic acid show higher activity than other substituted benzoic acids as models of catalytic sites on carbons in the hydrolysis of cellobiose and cellulose. Notably, their turnover frequencies are larger than those of o-chlorobenzoic acid and o-trifluoromethylbenzoic acid, despite their lower acid strength. The high catalytic performance of salicylic acid and phthalic acid is not attributed to a reduction of activation energy but to an increase in the frequency factor. Nuclear magnetic resonance and density functional theory studies indicate that one oxygenated group forms a hydrogen bond with a hydroxyl group in cellobiose, which boosts the probability of attack of the neighboring carboxylic acid on the glycosidic bond. The computation also predicts a hydrolysis mechanism including an S_N1 reaction with anomeric inversion, which reasonably accounts for the experimental results in the conversion of cellobiose.