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

Abstract

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.