EPR Study of Copper-Exchanged Zeolites:  Effects of Correlated g- and A-Strain, Si/Al Ratio, and Parent Zeolite

A series of copper-exchanged zeolites with varying Si/Al ratios were investigated using EPR spectroscopy. EPR spectra were recorded for hydrated and dehydrated Cu-ZSM5 (Si/Al = 16, 28, 44, and 144), CuY (Si/Al = 2, 48), Cu-mordenite (Si/Al = 5, 15, 46), and Cu-Beta (Si/Al = 18). EPR parameters were obtained by a previously described linear least-squares fitting procedure that was modified to incorporate coaxial, rhombic g- and A-tensors and to include correlated g- and A- strain. The EPR spectra for all of the hydrated copper-exchanged zeolites exhibited similar spectral features and similar EPR parameters suggesting that the zeolite surroundings have little effect on the electronic environment of the hydrated copper complexes in the different zeolites. The EPR spectra for the dehydrated copper-exchanged zeolites, such as Cu-ZSM5, CuY, and Cu-mordenite, contained overlapping spectral features that were more difficult to analyze. In particular, the EPR spectra of dehydrated Cu-mordenite and CuY suggested the presence of two copper sites with very different EPR parameters. Differences in g- and A-strain for the hydrated and dehydrated copper-exchanged zeolites were discussed in terms of site heterogeneity.