Theoretical studies of Cu(i) sites in faujasite and their interaction with carbon monoxide

Abstract

Sitting, coordination, and properties of Cu(I) cations in zeolite faujasite are investigated using a combined quantum mechanics-interatomic potential function method. The coordination of Cu(I) ions depends on their location within the zeolite lattice. Cu(I) located inside the hexagonal prisms (site I′) and in the plane of six-membered aluminosilicate rings on the walls of sodalite units (site II) is threefold coordinated, whereas Cu(I) located in the supercages (site III) is twofold coordinated. In agreement with available experimental data Cu(I) appears to be more strongly bound in sites I′ and II than in site III. The binding energy of site II Cu(I) ions increases with the number of Al atoms, but only closest Al atoms have a substantial influence. The CO molecule binds more strongly onto sites with weaker bound cations and lower coordination. We assign the two CO stretching IR bands observed for Cu(I)–Y zeolites to sites II with one Al (2157–2161 cm⁻¹) and two Al atoms (2140–2148 cm⁻¹) in the six-membered aluminosilicate ring. For Cu(I)–X we tentatively assign the high frequency band to site III (2156–2168 cm⁻¹) and the low-frequency band to site II with three Al atoms in the six-membered ring (2136–2138 cm⁻¹).