Characterization of acidity in zeolites and related oxides using trimethylphosphine as a probe

The MAS-NMR spectra of trimethylphosphine (TMP) have been used to study acidity on zeolites, amorphous silica-alumina, sulfated ziconia and other solid materials. Acidic protons react with the strong Lewis base to form [(CH3)(3)P-H](+) adducts that are characterized by a P-31 chemical shift of ca. -3 ppm and a J(P-H) coupling constant of ca. 500 Hz. Lewis-bound TMP exhibits resonances at considerably higher fields, and if aluminum is involved, a J(P-Al) coupling of ca. 300 Hz may result in a multiplet of six lines. When excess TMP is present, rapid chemical exchange may occur between the liquid-like molecules and those that are complexed at acid sites. In such cases the distinction between the different forms of TMP is obscured. Attempts to relate Bronsted acid strength to a magnetic parameter such as the scalar coupling constant show promise in those cases where the majority species is of interest. In dealuminated Y zeolites, however, it appears that a small fraction of the protons is responsible for acid-catalyzed reactions, and the value of the J(P-H) coupling does not adequately reflect the contribution of these centers.