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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry