Chitin Hydrolysis Using Zeolites in Lithium Bromide Molten Salt Hydrate

Abstract

N-Acetyl-D-glucosamine (NAG), the monomer of chitin, is used as a food supplement and a key platform chemical to access a range of organonitrogen chemicals. However, efficient and environmentally benign catalytic systems to depolymerize raw chitin into monomers are still limited. In this regard, here we report hydrolysis of untreated chitin using low-cost and recyclable zeolites in LiBr molten salt hydrate (MSH). The chabazite (CHA) zeolites (silicoaluminophosphate SAPO-34 and aluminosilicate SSZ-13) exhibited superior performance compared with other zeolites (e.g., H beta, HZSM-5, and HUSY). Under optimized conditions, nanosheet-like SAPO-34 crystals (NS-0.1) and commercial microporous SAPO-34 achieved 61 and 63% NAG yield, respectively. H+ in zeolites exchanges with Li+ in LiBr MSH, which then promotes chitin hydrolysis. Thus, the formation rate of NAG was not dependent on the textural properties of zeolites (e.g., pore size, surface area, pore volume) but rather correlated to the acid site properties. In particular, a strong negative correlation between Lewis acid density and NAG yield was observed. The work establishes a pathway to hydrolyze chitin using solid heterogeneous catalysts.