Interpretation of the EPR Spectra of Nitrogen-Containing Compounds Adsorbed on Copper-Exchanged Zeolites

Abstract

Electron paramagnetic resonance (EPR) spectroscopy was used to investigate adsorption of a series of amines (ammonia, methylamine, triethylamine) on Cu−ZSM5 and Cu−Beta. Several changes were observed in the EPR spectra after adsorption of the amine: an increase in spectral broadening, a shift in the copper hyperfine features, and the appearance of nitrogen hyperfine splittings in the perpendicular region of the EPR spectrum for some of the samples. For samples that exhibited resolved nitrogen hyperfine couplings, the EPR spectra were easily interpreted in terms of the number of coordinated amine molecules. For samples in which resolved nitrogen hyperfine interactions were not observed, the interpretation of the EPR spectrum was more complex. A second approach for the interpretation of the EPR spectrum involved using an empirical model correlating the EPR parameters for a series of model compounds to the number of coordinated nitrogen atoms and the charge of the copper complex. This approach was very useful for determining the number of coordinated nitrogen atoms if the charge of the complex was known or vice versa. It was determined that four molecules of ammonia or methylamine bind to the Cu2+ centers in Cu−ZSM5 and Cu−Beta, but that only one molecule of triethylamine is able to bind to the Cu2+ centers in these zeolites. For cases in which the charge of the complex and the number of ligands are both unknown, there is more uncertainty in using this empirical approach to interpret the EPR spectra. This empirical approach was further assessed by application to the EPR spectra of NO adsorbed on Cu−ZSM5 and Cu−Beta.