Adsorbate interactions on copper-exchanged X-type zeolite catalysts studied by e.p.r. spectroscopy

Abstract

E.p.r. spectra of a dehydrated copper-exchanged X-type zeolite have been obtained which suggest that the copper (II) ions are distributed in at least two magnetically non-equivalent environments. The copper (II) ions in one of these environments exhibit a preferential reactivity towards a variety of simple molecules. It is postulated that these cations are located in sites II' in the sodalite cage, whereas the less reactive cations are in the relatively inaccessible sites I'. The reaction of hydrogen with the zeolite results in a reversible reduction of copper (II) to copper (I). Carbon monoxide, but-1-ene, cyclopropane and 3,3-dimethylbut-1-ene have a similar effect and observed changes in the e.p.r. spectra are interpreted in terms of the formation of a copper complex. An analogous complex was readily formed with ammonia, but interaction of the larger pyridine molecule with the copper (II) ions was much less facile. The observed enhancement in reactivity of pyridine subsequent to reaction of 3,3-dimethylbut-1-ene with the zeolite indicated that migration of the copper (II) ions had occurred. Further evidence of cation migration to more accessible locations was provided by preferential oxygen pressure broadening of the e.p.r. spectrum attributed to the copper complex species. The implications of these observations to catalytic reactions on copper-exchanged zeolites is discussed.