Coordination of Cu+ and Cu2+ ions in ZSM-5 in the vicinity of two framework Al atoms
A combined quantum mechanics/interatomic potential function technique has been used to examine Cu+/Al− sites in the vicinity of H+Al− sites and Cu2+ in the vicinity of two framework Al atoms under dehydrated conditions. The symmetrical coordination of Cu2+ ions to four framework oxygen atoms is the most stable site and Cu2+ is hard to reduce at these sites. In agreement with EPR measurements a square-pyramidal coordination of Cu2+ at these sites is suggested. However, this symmetrical coordination is attained only for specific positions of the framework aluminium pair. Coordination changes upon reduction of divalent to monovalent copper ions and upon excitation of Cu+ ions in the first triplet excited state are also investigated. The coordination of the copper ions does not significantly change during reduction in the dehydrated state. Coordination changes of Cu+ upon excitation in the triplet state and calculated transition energies for Cu+/Al− sites in the vicinity of H+Al− sites are identical to those observed for isolated Cu+/Al− sites.