Thermal dehydration of D-glucose monohydrate in solid and liquid states

The physico-geometrical reaction pathway and kinetics of the thermal dehydration of D-glucose monohydrate (DG-MH) dramatically alter by the melting of the reactant midway through the reaction. By controlling the reaction conditions, the thermal dehydration of DG-MH was systematically traced by thermoanalytical techniques in three different reaction modes: (1) solid-state reaction, (2) switching from a solid- to liquid-state reaction, and (3) liquid-state reaction. Solid-state thermal dehydration occurred under isothermal conditions and linear nonisothermal conditions at a small heating rate (β ≤ 1 K min^-1) in a stream of dry N₂. The kinetic behavior comprised the presence of an induction period and a sigmoidal mass loss process characterized by a derivative mass loss curve with a symmetrical shape under isothermal conditions, resembling the autocatalytic reaction in homogeneous kinetic processes. When DG-MH was heated at a larger β (≥2 K min^-1), the melting of DG-MH occurred midway through the thermal dehydration process, by which a core-shell structure of molten DG-MH and surface product layer of crystalline anhydride was produced. Subsequently, thermal dehydration proceeded as a complex multistep process. Furthermore, the thermal dehydration initiated at approximately the melting point of DG-MH upon the application of a certain water vapor pressure to the reaction atmosphere, and proceeded in the liquid-state, exhibiting a smooth mass loss process to form crystalline anhydride. The reaction pathway and kinetics of the thermal dehydration of DG-MH and the corresponding changes with the sample and reaction conditions are discussed on the basis of the detailed kinetic analysis.