Catalytic oxidation. II. Silica supported noble metals for the oxidation of ethylene and propylene

The catalytic oxidation of C₂H₄ and C₃H₆ was studied over silica-supported Pt, Pd, Ir, Ru, and Rh. The specific activities fell in the order listed and correlated with the percentage d-character of the metal and fairly well with its atomic radius. The partial oxidation products were characterized. Acetaldehyde and acetic acid were produced from C₂H₄ in all cases, but only with Pd and Ir were the quantities significant. Products from C₃H₆ were more varied. C₃ products in substantial amounts were formed over Rh, Ru, and Pd, the selectivity decreasing in the order given. Somewhat smaller amounts of C₂ products were also formed over these catalysts. The principal product over Ir was acetic acid; the most active catalyst, Pt, was relatively unselective. Three different types of pressure dependencies were found: the rate of total oxidation decreased with olefin pressure and increased with oxygen pressure over Pt and Pd; the rate increased with olefin pressure and was inhibited by oxygen over Rh and Ir; and the rate increased with oxygen pressure, but was nearly independent of olefin pressure over Ru. Activation energies varied from 16 to 29 kcal/mole and tended to correlate inversely with the catalytic activity. The order of activity depended upon the ability of the metal to activate O₂.