Surface reactions of oxygen ions. 5. Oxidation of alkanes and alkenes by O2- on magnesium oxide

Abstract

The superoxide ion on the magnesium oxide surface was produced by reacting oxygen with magnesium oxide containing trapped electrons. Four different superoxide species on different sites were identified by NMR spectroscopy and differential thermal analysis. Stoichiometric reactions of the superoxide ion with C/sub 1/-C/sub 4/ alkanes and alkenes at 175/sup 0/C were slower than the corresponding reactions with O(-1) and O/sub 3/(-1) on magnesium oxide. Methane and ethylene reacted only at temperatures above 300/sup 0/C and yielded mainly carbon dioxide. Propylene gave no gaseous products at 175/sup 0/C, but IR spectroscopy revealed that it adsorbed as formate and acetate ions which were converted to carbonates at higher temperatures and desorbed as acetaldehyde and methanol. The 1-butene formed 2-butanol, methanol, acetaldehyde, and acrolein above 300/sup 0/C. The analyses of the oxygen-containing and hydrocarbon products suggested that the initial step of the surface reaction is hydrogen abstraction and that the resulting radicals react with lattice oxygen to form carboxylate ions or with HO/sub 2/(-1) to form alkoxy or epoxide intermediates.