A reinterpretation of the EPR spectra of Cu(II) in zeolites A, Y and ZK4, based on ab initio cluster model calculations
A theoretical study is presented of the EPR spectra of the dehydrated Cu(II)-A, Cu(II)-Y and Cu(II)-ZK4 zeolites. B3LYP-DFT geometry optimizations were performed on cluster models representing six-ring sites with different Al contents, as observed for the different zeolites. All calculated structures indicated a strong preference of the Cu(II) ion for coordination to oxygens bound to Al rather than Si, together with a striving for a planar four-fold oxygen coordination in the six-rings. Depending on the number and relative positions of the aluminiums in the ring two distinct four-fold coordination modes were distinguished, containing either only one or several aluminiums “competing” for a position of both their oxygens in the first Cu(II) coordination sphere. The electronic spectra and EPR g-tensors of all optimized cluster models were calculated by means of the CASPT2 method (multiconfigurational perturbation theory based on a complete-active-space reference wavefunction), with inclusion of spin–orbit coupling. These calculations pointed to the appearance of two distinct EPR-signals in connection with the two different four-fold coordination modes. Based on the close correspondence between the calculated g-factors and the experimental EPR-signals of the three zeolites under investigation, a new interpretation of the latter signals is suggested. According to this new interpretation the occurrence of two EPR signals in zeolite Y as opposed to only one signal in zeolite A is connected to the higher Si/Al ratio in the former zeolite, rather than to a different topology (as was suggested in earlier assignments of the spectra). Our new interpretation is corroborated by the experimental EPR signals obtained for Cu-ZK4: with the same topology as zeolite A, but containing a Si/Al ratio closer to zeolite Y, two rather than one Cu(II) EPR signals were indeed observed. Finally, our calculations also indicate that, in six-rings containing more than one aluminium, Cu(II) is likely to undergo a hopping process at room temperature.