The thermodynamic properties and role of water contained in dispersed oxides in precursor-boehmite conversion, based on the example of aluminum hydroxide and oxide under hydrothermal conditions in different environments

The hydrothermal conversion of precursors (MDGA hydrargillite/γ-Al2O3) to boehmite is studied. It is found that the specific surface area of the precursors increases and the peak of the bending vibrations of molecular water is shifted from 1640 to 1690 cm−1 for hydrargillite at the early stages of hydrothermal treatment. The heats of vaporization of water from a two-phase system (original precursor/resulting boehmite) are found to be 27.5 and 6.5 kJ/mol H2O, which is less than the tabulated value. Based on the literature data, conclusions are drawn with regard to the state of water in the structure of the treated material in the form of single molecules, dimers, and trimers: the boehmite structure is not perfect, and the heat effects of the dehydration of boehmite in forming γ-Al2O3 have values of 13.5 and 21.4 kJ/mol AlOOH. It is found that further hydrothermal treatment leads to the formation of boehmite crystals with perfect structure and, regardless of the composition of the reaction medium used for the treatment, the heat effects of the dehydration of boehmite are 26.9–27.2 kJ/mol AlOOH, which is close to the tabulated value.