First principles investigation into the metal catalysed 1,2 carbon shift reaction for the epimerization of sugars

Deciphering the Structure of Tungstate and Molybdate Complexes with Glucose, Mannose, and Erythrose

Combining nuclear magnetic resonance (NMR), X-ray absorption spectroscopy near-edge structure (XANES), and density functional theory (DFT), we elucidate the structures of tungstate and molybdate with sugars of interest in the conversion of biomass to platform chemicals (glucose, mannose, and erythrose). We highlight a number of complexes, including one nearly isostructural structure that is formed with each metal-sugar combination. We also emphasize the singular reactivity of erythrose that undergoes retro-aldolization at room temperature.

Elucidation of Metal-Sugar Complexes: When Tungstate Combines with d-Mannose

The control of metal-sugar complexes speciation in solution is crucial in an energy transition context. Herein, the formation of tungstate-mannose complexes is unraveled in aqueous solution using a multitechnique experimental and theoretical approach. ¹³C nuclear magnetic resonance (NMR), as well as ¹³C-¹H and ¹H-¹H correlation spectra, analyzed in the light of coordination-induced shift method and conformation analysis, were employed to characterize the structure of the sugar involved in the complexes. X-ray absorption near edge structure spectroscopy was performed to provide relevant information about the metal electronic and coordination environment. The calculation of ¹³C NMR chemical shifts for a series of tungstate-mannose complexes using density functional theory (DFT) is a key to identify the appropriate structure among several candidates. Furthermore, a parametric study based on several relevant parameters, namely, pH and tungstate concentration, was carried out to look over the change of the nature and concentrations of the complexes. Two series of complexes were detected, in which the metallic core is either in a ditungstate or a monotungstate form. With respect to previous proposals, we identify two new species. Dinuclear complexes involve both α- and β-furanose forms chelating the metallic center in a tetradentate fashion. A hydrate form chelating a ditungstate core is also revealed. One monotungstate complex appears at high pH, in which a tetrahedral tungstate center is bound to α-mannofuranose through a monodentate site at the second deprotonated hydroxyl group. This unequalled level of knowledge opens the door to structure-reactivity relationships.

H3PMo12O40 Immobilized on Amine Functionalized SBA-15 as a Catalyst for Aldose Epimerization

In this work various amount of phosphomolybdic acid (PMo) were immobilized on amine functionalized SBA-15 and used as heterogeneous catalysts in the epimerization of glucose in aqueous solution. 13.3PMo/NH₂-SBA-15 exhibited the best catalytic performance with a glucose conversion of 34.8% and mannose selectivity of 85.6% within two hours at 120 °C. The activation energy of 80.1 ± 0.1 kJ·mol^-1 was lower than that of 96 kJ·mol^-1 over the homogeneous H₃PMo₁₂O₄₀ catalyst. The catalytic activities of 13.3PMo/NH₂-SBA-15 for the transformation of some other aldoses including mannose, arabinose and xylose were also investigated.

Catalytic Isomerization of Biomass-Derived Aldoses: A Review

Selected aldohexoses (D-glucose, D-mannose, and D-galactose) and aldopentoses (D-xylose, L-arabinose, and D-ribose) are readily available components of biopolymers. Isomerization reactions of these substances are very attractive as carbon-efficient processes to broaden the portfolio of abundant monosaccharides. This review focuses on the chemocatalytic isomerization of aldoses into the corresponding ketoses as well as epimerization of aldoses at C2. Recent advances in the fields of catalysis by bases and Lewis acids are considered. The emphasis is laid on newly uncovered catalytic systems and mechanisms of carbohydrate transformations.