Influence of cocations on the location and coordination geometry of Cu2+ in M+ A zeolites (M+= Na, K, Tl, Rb, Cs, NH4 and CH3NH3). An electron spin resonance and electron spin–echo modulation spectroscopic study

Abstract

The changes in the locations and coordination geometries of Cu²⁺ in various cationic forms of A zeolite has been examined by electron spin resonance and electron spin–echo modulation techniques. It is observed that Cu²⁺ interacts with six deuterons (three water molecules) in NaA in the α-cages, while it interacts with only two deuterons (one water molecule) in the α-cage in the other monovalent ion-exchanged A zeolites. The changes in the coordination of Cu²⁺ as a function of hydration conditions have been monitored and an anomalous irreversible change in the immediate environment of Cu²⁺ in KA and TIA is observed. After dehydration at T > 353 K in KA and TIA rehydration does not restore the spectra seen in fresh samples. Instead the spectra in such rehydrated samples are identical to those in NaA. It is also found that on ammonia-exchange and partial dehydration Cu²⁺ exhibits markedly different behaviour in NaA when compared with exchange in NaA or in NH₄A zeolites. The difference in the initial Cu²⁺ coordination in NaA as against the other A zeolites is explained in terms of α-cage crowding by the monovalent cations.