Spectroscopy and Redox Chemistry of Copper in Mordenite

Abstract

Copper-containing zeolites, such as mordenite (MOR), have recently gained increased attention as a consequence of their catalytic potential. While the preferred copper loadings in these catalytic studies are generally high, the literature lacks appropriate spectroscopic and structural information on such Cu-rich zeolite samples. Higher copper loadings increase the complexity of the copper identity and their location in the zeolite host, but they also provide the opportunity to create novel Cu sites, which are perhaps energetically less favorable, but possibly more reactive and more suitable for catalysis. In order to address the different role of each Cu site in catalysis, we here report a combined electron paramagnetic resonance (EPR), UV/Vis-NIR and temperature-programmed reduction (TPR) study on highly copper-loaded MOR. Highly resolved diffuse reflectance (DR) spectra of the CuMOR samples were obtained due to the increased copper loading, allowing the differentiation of two isolated mononuclear Cu(2+) sites and the unambiguous correlation with extensively reported features in the EPR spectrum. Ligand field theory is applied together with earlier suggested theoretical calculations to determine their coordination chemistry and location within the zeolite matrix, and the theoretical analysis further allowed us to define factors governing their redox behavior. In addition to monomeric species, an EPR-silent, possibly dimeric, copper site is present in accordance with its charge transfer absorption feature at 22200 cm(-1), and quantified with TPR. Its full description and true location in MOR is currently being investigated.

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

“…For Cu-MOR two types of EPR signal is found after dehydration [37,45,46] with roughly the same values as for Cu-MFI and with the same assignment. Other groups reports a more complicated distribution of species in a Cu-MOR material [18] whereas others could only discern the species with the strongest signal [38].…”

“…All agree that the Cu site corresponding to signal 2 is dominant at low concentrations of Cu and is therefore assumed to be more stable. The Cu site corresponding to signal 1 is also observed to be more easily reduced in a hydrogen temperature programmed reduction (TPR) experiment [37].…”

“…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.…”

“…[18] have different parameters for the other site and [38] only report one site. Here the values of the Cu species from [37] are given, since the spectrum has an overall simpler appearance and is not complicated by a possible third species as in [18]. The values from [18] are given for the spectrum of the hydrated Cu zeolite since [37] does not report a spectrum of hydrated Cu-MOR.…”

“…a References [18,37,38], agree on the parameters for signal 2 in the dehydrated Cu-MOR. [18] have different parameters for the other site and [38] only report one site.…”