MECHANISTIC STUDIES OF MONOSUGAR ISOMERIZATION AND EPIMERIZATION TO LOW-CALORIE SUGARS THROUGH CHEMICAL CATALYSIS

Abstract

This study sheds light on sugar isomerization and epimerization reactions using a variety of catalysts, delving deeper than typical research by incorporating temperature dependence, reaction mechanisms, and thermodynamic considerations. Firstly, the research employs magnesium bromide (MgBr 2 ) dissolved in water as a catalyst to convert glucose into fructose. Interestingly, the process leverages two distinct pathways: a magnesium-aided 1,2-hydride shift contributing to half the conversion and a bromine-induced proton transfer responsible for the remaining half. Isotopic labeling experiments were used to confirm these pathways. This selective transformation yielded a notable 32% fructose with an impressive 76% selectivity. The MgBr 2 forms the weak water shell surrounding the Mg 2+ , which exposes the inherent catalytic activity of Mg 2+ , while simultaneously enhancing the activity of bromine, although this boost in bromine activity also leads to more side reactions. The temperature-dependent characteristics of the corresponding pathways were determined by applying the principles of transition-state Eyring and Marcus theories to the elementary steps involving a proton transfer and electron transfer, respectively, kinetic analysis using the Eyring theory revealed an activation energy barrier of 70.25 kJ/mol. The semi-classical Marcus model disclosed that the hydride shift is a normal electron transfer rate based on the localization of k ET in the λ< -ΔG o >0 region. The study then explores the use of MgBr 2 to convert galactose into tagatose, again demonstrating the involvement of both magnesium and bromine through their respective 1,2-hydride shift and proton transfer mechanisms. This process resulted in a 22% yield of tagatose with a high 73% selectivity and the formation of just one co-product, talose, at 7%. Here, the researchers employed first-principles theories alongside reaction kinetics to provide support for the preferential feasibility of the hydride shift mechanism, further confirmed by DFT modeling. Moving beyond MgBr 2 , the research introduces a novel method for L- sorbose production from fructose using molybdenum oxide (MoO 3 ) in water. This method achieved a 32% yield of L-sorbose with 55% selectivity, with the reaction favoring a 1,4-hydride shift for fructose conversion. Kinetic analysis revealed an activation energy barrier of 68.2 kJ/mol. Finally, the study examines a finely tuned MoO 3 solid acid catalyst treated with nitric acid to improve Lewis acidity and porosity. This enhanced catalyst demonstrates significant efficiency in D-talose production from D-galactose. The treatment resulted in a 25% yield of D-talose with 70% selectivity iand an impressive 98% carbon balance. Isotopic labeling combined with NMR characterization confirmed a C1-C2 carbon shift mechanism, supporting the Bílik mechanism. In conclusion, this research paves the way for efficient and selective production of valuable sugars through environmentally friendly and sustainable catalytic methods. These methods hold significant promise for applications in the food additive, fuel industry and pharmaceutical industries.