Electron paramagnetic resonance of a nitroxide adsorbed on silica, silica-alumina, alumina, and decationated zeolites

Di-tert-butyl nitroxide (DTBN) has been adsorbed on silica, silica-alumina, alumina, and decationated Y and X zeolites. The surface epr spectra exhibit well-resolved nitrogen hyperfine splittings (hfs) from DTBN adsorbed at two distinct types of sites. One spectrum arises from DTBN hydrogen bonded to surface hydroxyls. The second arises from complexation with a surface Al³⁺ and exhibits ²⁷Al hfs. In some cases, the existence of a strongly acidic (Brønsted) site is inferred. The trends in epr results observed upon comparing different surfaces are compared to trends in catalytic activity, and poisoning experiments with CO₂ give further connection with catalytic reactions. Surface nitroxides exhibit rotational and translational motion, and from observations of the manner in which they partition themselves between H bonding and complexation to Al³⁺, the enthalpy change for transfer of a nitroxide from surface hydroxyl to aluminum coordination is positive and estimated to be ∼+5 kcal/mol.