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 k ¼ 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 a, a six-ring with bridging T-site, containing 2 lattice Al's. The axially symmetric species with g k ¼ 2.26-2.28, present at medium copper loadings, is assigned to a squareplanar 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 k ¼ 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.

Identification and Quantification of Copper Sites in Zeolites by Electron Paramagnetic Resonance Spectroscopy

“…At high loadings the amount of species with EPR signal 1 is lower. This was explained by the formation of an EPR silent Cu 2+ species by coordination of extra-framework oxygen for charge compensation [44]. Progress was also made in this work assigning EPR signal 1 and 2 to particular sites in dehydrated Cu-MFI, but due to the complicated structure it is likely that a distribution of different sites contributes to the total EPR signal.…”

“…This coordination environment should give an overall less negatively charged perturbation of the spin-bearing 2 − 2 orbital. The site with parameters in the higher left part of the line is indeed assigned to this site as outlined below [37,44]. For Cu-CHA both set of parameters fall in the middle of the trend line.…”

“…Groothaert et al made progress towards assigning the two copper species using a combination of DFT and EPR [44]. Signal 2 with ∥ 2 = 2.32 was assigned to Cu 2+ in 2Al 6mr.…”

“…The EPR signals have been assigned on the basis of DFT calculations of the g-values [44]. EPR signal 1 with the lower g || -value is assigned to a site with only one Al in the 6mr but with a second Al in the bridge across the middle of the ring.…”

An operando optical fiber UV–vis spectroscopic study of the catalytic decomposition of NO and N2O over Cu-ZSM-5

“…In a recent EPR (electron paramagnetic resonance) study of our group [46], we found that calcined Cu-ZSM-5 samples with Cu/Al > 0.2 are characterized by a fraction of EPR-undetectable copper, which grows with increasing Cu/Al ratio. This EPR silent copper fraction took our attention since the NO decomposition activity has a S-shaped dependence on the copper exchange level in ZSM-5 [15,21,23].…”