Effects of cation siting and spin–spin interactions on the electron paramagnetic resonance (EPR) of Cu exchanged X Faujasite zeolite

Copper(II) exchanged Na X Faujasite zeolite was cation exchanged at levels from one Cu(II) in 30 unit cells (0.033 Cu(II)/UC) to 38 Cu(II) per unit cell (38 Cu/UC) and was examined by continuous wave and two-pulse and three-pulse electron paramagnetic resonance (EPR) at temperatures from 10 K to 300 K. In this work exchange of Cu²⁺ into X Faujasite zeolite is shown by EPR spectral and pulsed EPR relaxation measurements to begin into site I′, where it lies coordinated to a hexagonal prism face with Si:Al ratios of predominantly 4:2 and 5:1. Spin–spin interactions influence EPR g-value averaging, spin–spin relaxation, and spin spectral diffusion in a manner highly dependent on Cu exchange. Spin–lattice relaxation is relatively independent of exchange. The marked increase observed in spin–spin relaxation and g-value averaging at 8 Cu/UC and an effective Cu–Cu distance of 1.2 nm can be understood in terms of filling sodalite cages with an average of 1 Cu²⁺ each.