Molybdenum sulfide-2D nanosheets offering multiple metallic sites enable different sugar epimerization reactions to rare sugars in water

Abstract

In recent years, significant efforts have been made to formulate health-beneficial low-calorie sugars that can regulate the rising problems of lifestyle disorders, such as obesity and diabetes, in humans. These sugars can be produced through a chemical catalytic pathway. In this study, we report on the proficiency of as-synthesized molybdenum sulfide (MoS₂) in the preparation of a variety of rare sugars. Analytical characterization indicates that the 2D nanosheets are largely composed of the 1T-MoS₂ form, which can offer more metallic sites as Mo⁴⁺/Mo⁶⁺ and S²⁻. They exhibited varied binding affinity with sugars with respect to the structural configuration, resulting in varying end-products. For instance, during glucose epimerization, Mo could engage in a metal–glucose complex formation, thereby yielding mannose as high as 29% with 84% selectivity in water via a C1–C2 carbon shift pathway (Bílik mechanism), while sulfur, as a nucleophile, is involved in electron sharing with deficient Mo. Meanwhile in the fructose epimerization reaction, sulfur interacts and initiates an intra-molecular proton transfer (i.e., 1,4-hydride shift) when Mo undergoes a usual metal–fructose complexation, attributed to its electron source characteristics, resulting in an ∼26% L-sorbose formation with ∼50% selectivity. The recyclability study verified the stability of MoS₂ accredited to a Mo–S covalent linkage. The results endorse the versatile characteristics of MoS₂, therefore it can help in the large-scale and sustainable preparation of rare sugars (mannose, sorbose, allulose, and tagatose), whose production is currently undertaken via costly biotechnological methods, for commercial food and pharma applications.