Variable-Temperature Electron Paramagnetic Resonance Studies of Copper-Exchanged Zeolites

Abstract

Copper-exchanged zeolites, Beta and ZSM-5, were studied using variable-temperature electron paramagnetic resonance (EPR) spectroscopy to probe changes in the local environment of the Cu2+centers when samples were dehydrated and heated in flowing helium or under reagent flow. Hydrated samples of Cu-ZSM-5 and Cu-Beta exhibited EPR spectra consistent with EPR signals previously assigned to Cu2+in distorted octahedral coordination. EPR spectra of dehydrated Cu-Beta and Cu-ZSM-5 showed the presence of coordination environments that were similar to EPR signals previously assigned to Cu2+in distorted square pyramidal and distorted square planar environments. An empirical model is presented that correlatesg∥andA∥for a series of copper-exchanged zeolites and model compounds and provides additional insight into the coordination environment of Cu2+in copper-exchanged zeolites. The empirical model links a number of past EPR studies on different copper-exchanged zeolites and provides an explanation for the observed trends in EPR parameters related to the charge at the copper center. The EPR spectra for dehydrated Cu-ZSM-5 and Cu-Beta samples exhibited a temperature dependence. The EPR spectrum of Cu-ZSM-5 and Cu-Beta recorded at 673 K showed an increase ing∥and a decrease inA∥when compared with the EPR spectrum recorded at room temperature. These changes in spectral parameters are attributed to changes in the electronic environment of the Cu2+species through modification of the coordination environment.