Critical Assessment of Electron Spin Resonance Studies on Cu(I)−NO Complexes in Cu−ZSM-5 Zeolites Prepared by Solid- and Liquid-State Ion Exchange

Abstract

Cu(I)-NO adsorption complexes were formed over Cu-ZSM-5 zeolites prepared by (i) solid-state ion exchange of NH(4)-ZSM-5 with CuCl and (ii) liquid-state ion exchange of ZSM-5 with Cu(CH(3)COO)(2). Electron spin resonance spectroscopy revealed the formation of two different Cu(I)-NO species A and B in both systems, whose spin Hamiltonian parameters are comparable with those already reported for the Cu(I)-NO species formed over 66% Cu(II) liquid-state ion-exchanged Cu-ZSM-5 materials. The population of the species A and B differs for the two systems studied. Formation of species B is more favored in the solid-state ion-exchanged Cu-ZSM-5 when compared to the liquid-state exchanged zeolite. The X-, Q- and W-band electron spin resonance spectra recorded at 6 and 77 K reveal the presence of a rigid geometry of the adsorption complexes at 6 K and a dynamic complex structure at higher temperatures such as 77 K. This is indicated by the change in the spin Hamiltonian parameters of the formed Cu(I)-NO species in both the liquid- and solid-state ion-exchanged Cu-ZSM-5 zeolites from 6 to 77 K. Possible models for the motional effects found at elevated temperatures are discussed. The temperature dependence of the electron spin phase memory time measured by two-pulse electron spin-echo experiments indicates, likewise, the onset of a motional process of the adsorbed NO molecules at temperatures above 10 K. The studies support previous assignments where the NO complexes are formed at two different Cu(I) cationic sites in the ZSM-5 framework and highlight that multifrequency electron spin resonance experiments at low temperatures are essential for reliable determination of the spin Hamiltonian parameters of the formed adsorption complexes for further comparison with Cu(I)-NO complex structures predicted by quantum chemical calculations.