Electron pair acceptor properties of alkali cations in zeolite Y: an electron spin resonance study of adsorbed di-tert-butyl nitroxide

Di-tert-butyl nitroxide (DTBN) was adsorbed at several alkali metal ion exchanged Y zeolites to investigate the electron pair acceptor properties of the monovalent zeolite cations Li+, Na+, K+, and Cs+. This probe molecule forms adsorption complexes with the Lewis acidity possessing zeolite cations and thereby the spin density distribution at the nitroxide group of the DTBN molecule changes. This can directly be detected by electron spin resonance spectroscopy via the nitrogen hyperfine coupling constants of the probe molecule. A linear dependence of the nitrogen spin density at the π-orbital of the nitroxide group on the electronegativity of the zeolite cations was found. The Q values which relate the isotropic hyperfine structure coupling constant to the π-electron spin densities were determined. In comparison with these results, the nitrogen hyperfine coupling data of DTBN sorbed on the protonated form of the Y zeolite, possessing OH groups acting as Brøsnstedt acid sites, cannot be explained within this simple electronegativity model.