Identification of Cu(ii) coordination structures in Cu-ZSM-5, based on a DFT/ab initio assignment of the EPR spectra

Abstract

The coordination structures of Cu(II) exchanged into ZSM-5 were obtained by B3LYP-DFT geometry optimizations on cluster models, representing the cation sites. The EPR g-factors of the resulting cluster models were calculated by means of the CASPT2 method (multi-configurational perturbation theory), with the inclusion of spin–orbit coupling. In order to facilitate the confrontation of theoretical and experimental results, the EPR spectra of a selection of dehydrated Cu(II)-ZSM-5 samples are presented as well. The axially symmetric signal with g_∥ = 2.30–2.33, which is present over the whole range of copper loadings, is assigned to a five-fold or distorted three-fold Cu(II) coordination in site α, a six-ring with bridging T-site, containing 2 lattice Al's. The axially symmetric species with g_∥ = 2.26–2.28, present at medium copper loadings, is assigned to a square-planar Cu(II) coordination in six-rings and a square-pyramidal Cu(II) coordination in five-rings, with both rings containing only one Al and no extra-lattice oxygen (ELO). The near absence of the g_∥ = 2.26–2.28 signal at the highest Cu/Al ratio's is explained by the coordination of ELO to Cu(II) in these sites with one Al, yielding an EPR silent species.