Mechanism of glucose conversion in supercritical water by DFT study

Using density function theory (DFT) simulations with B3LYP/AGU-cc-pVDZ level of theory, six chemical reaction pathways of glucose decomposition in supercritical water were proposed to explore the formation mechanism of some main outcomes (levoglucosan, 5-hydroxymethylfurfural, hydroxylacetaldehyde, erythrose, glyceraldehyde and fructose). In addition, seven potential dehydration ways in glucose were investigated with and without the assistance of water molecule. All dehydration reactions are accelerated when water molecules take part in the reaction, because it can effectively lower the energy barrier of dehydration reaction. In supercritical water surrounding with assistant of water molecule, pathways 3 and 4 are preferred routes of glucose conversion to hydroxylacetaldehyde and erythrose with the lowest energy barrier of 127 kJ/mol. While a higher energy barrier (163 kJ/mol) is required to form glyceraldehyde and fructose in pathways 5 and 6. Levoglucosan and 5-hydroxymethylfurfural are hard to generate during this chemical processes because of their higher energy barrier without water participating in. It seems like that water molecule acts as a magic catalyst that can transfer hydrogen atom in dehydrations, keto-enol tautomerization and structure rearrangement, which reducing the distance of hydrogen atom moving, result in reducing these reactions energy barriers dramatically. The computational results open a window to produce hydroxylacetaldehyde and erythrose in theory. Moreover, it sheds some light on the various proportions of different products in conversion of glucose in supercritical water, as well as suggesting concrete reaction pathways to form these main products, contributing to the elaboration of the mechanism of glucose conversion and cellulose decomposition from a molecule level.