Study of the acidic character of modified metal oxide surfaces using the test of isopropanol decomposition

Abstract

The evolution of the acid/base properties of a series of oxide supports (alumina, magnesia and silica) modified by increasing loadings of additive ions (Li⁺, Ni²⁺, and SO₄ ^2-) from 1 to 50% of the support surface coverage is reported using the catalytic test of isopropanol decomposition, studied as a function of the reaction temperature. The calculated kinetic parameters E_a, A, and ΔS^≠ permit interpretation of the reaction mechanism with relation to the acidity/basicity of the modified surfaces. The series of alumina oxides, due to the amphoteric properties of the surfaces, decomposed isopropanol through an E₂ mechanism leading to propene and di-isopropyl ether formation. The selectivity to the two products was dependent on the strength of the basicity (addition of lithium and nickel) or on the acidity (addition of sulfate) of the surfaces. Magnesia series oxides dehydrated isopropanol through an E_1b mechanism due to the presence of very strong basic surfaces possessing some weak acid sites. The very weak amphoteric character of silica was strengthened by the loading of the three additives; the modified silica surfaces displayed enhanced decomposition activity with respect to pure silica.