Coordination of Cu 2+ in synthetic mordenites

Abstract

Electron spin resonance and diffuse reflectance spectra of Cu2+ in dehydrated mordenites were studied as a function of the type of co-exchanged cation (Na+, K+, Cs+, H+, and Ca2+), of the Si/Al ratio, and of the loading. At low, Cu2+ loadings, one axially symmetric e.s.r. signal was observed with g∥ = 2.327, g⊥ = 2.062, A∥ = 15.4 mT, A⊥ = 1.49 mT, and Q = 0.26 mT. The corresponding d-d transitions form a band with maximum at 13,700 cm−1 and shoulders at 14,800 cm−1 and 12,500 cm−1. As the Cu2+ loading increases, a second species is detected with g∥ = 2.277, g⊥ = 2.057, A∥ = 16.8 mT, A⊥ = 1.19 mT, and Q = 0.28 mT. The d-d spectrum of this species is ill-defined, but comprises a high-frequency component at 17,000 cm−1 and low-frequency components down to 8000 cm−1. The relative intensities, but not the parameters of both signals, depend on the loading, on the co-exchanged cation, and on the Si/Al ratio. An angular overlap model analysis of the data set allows one to make the following assignments: The first signal is due to Cu2+ coordinated to six oxygens of the distorted eight-ring of site A in C2h symmetry. The second set of parameters is due to Cu2+ on site E, coordinated to four oxygens of the six-rings of the 12-membered ring channel in C4v symmetry.