Transformations of Simple Oxides and Reactions of Isopropanol during Their Interactions under Supercritical Conditions

The transformations of a series of simple oxides (Al2O3, SiO2, La2O3, CeO2, PrO2) during their treatment with isopropanol (i-C3H7OH) in an autoclave under supercritical (SC) conditions and with subcritical water vapor were studied. In the presence of water vapor at 350°C, the specific surface areas of oxides decrease most significantly, and some of them (aluminum, lanthanum, and praseodymium oxides) are converted into hydroxides. Praseodymium oxide PrO2 is almost completely converted within 6 h under the action of i-C3H7OH at 350°C, which is accompanied by its reduction and partial conversion into hydroxide. In the case of CeO2, there were no transformations (including hydration and reduction) in i-C3H7OH or water vapor, and the specific surface area did not change. The differences in the behavior of CeO2 and PrO2 are explained by the difference in the values of the fourth ionization potential of the Ce and Pr atoms and, as a consequence, in the binding energies of oxygen in higher oxides. The products of conversion of i-C3H7OH were detected, whose composition depends on the type of the oxide being treated. The transformation of i-C3H7OH is dominant in the presence of Al2O3 and mostly gives dehydration products (propylene, diisopropyl ether). In the presence of La, Ce, and Pr oxides, dehydrogenation is dominant, forming acetone and hydrogen (mostly in the presence of La2O3). The catalytic nature of isopropanol decomposition in the presence of the given simple oxides was established. Silica SiO2, which has no pronounced acid-base and redox properties, showed the least activity in the decomposition of i-C3H7OH. It was assumed that when treated with SC isopropanol, the oxides are structured under the action of water vapor, which is the product of the transformation of i-C3H7OH. The catalytic activity of the autoclave material (stainless steel 12Kh18N10T) in the decomposition of i-C3H7OH along both routes—dehydration and dehydrogenation—was noted.