Hydroxyl radicals-mediated oxidative cleavage of the glycosidic bond in cellobiose by copper catalysts and its application to low-temperature depolymerization of cellulose

Abstract

As the most abundant source of biomass in nature for sustainable production of fuels and chemicals, efficient depolymerization of cellulose under mild conditions, due to the difficulty in selective cleavage of its β-1,4-glycosidic bonds, still remains challenging. Here, we report a novel method for oxidative cleavage of the glycosidic bonds by free radicals. Probed by the cellobiose reaction, it was found that ·OH radicals, generated from the decomposition of H2O2 catalyzed by CuSO4 or CuO/SiO2, were efficient for selective conversion of cellobiose to glucose and gluconic acid at a low temperature of 333 K, and their selectivities reached 30.0% and 34.6%, respectively, at 23.4% cellobiose conversion. Other radicals, such as ·SO4−, also exhibited high efficacy in the cellobiose reaction. Mechanistic studies suggest that the oxidative cleavage of the β-1,4-glycosidic bond by the free radicals involve formation of the carbon radical intermediate via abstraction of the H atom dominantly at the C1 position. Following this oxidative mechanism, treatment of microcrystalline cellulose with ·OH by impregnation with H2O2 and CuSO4 catalyst at 343 K led to significant enhancement in its hydrolysis efficiency. These results demonstrate the effectiveness of this new method in the oxidative cleavage of glycosidic bonds, and its viability for the efficient depolymerization of cellulose at low temperatures, which can be further improved, for example, by exploring new free radicals and optimizing their reactivity and selectivity.