CHARACTERIZATION OF ACIDITY IN ZEOLITES AND RELATED OXIDES USING TRIMETHYLPHOSPHINE AS A PROBE

The MAS-NMR spectra of trimethylphosphine (TMP) have been used to stud y 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 che mical 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, a nd if aluminum is involved, a J(P-Al) coupling of ca. 300 Hz may resul t in a multiplet of six lines. When excess TMP is present, rapid chemi cal exchange may occur between the liquid-like molecules and those tha t are complexed at acid sites. In such cases the distinction between t he different forms of TMP is obscured. Attempts to relate Bronsted aci d strength to a magnetic parameter such as the scalar coupling constan t show promise in those cases where the majority species is of interes t. In dealuminated Y zeolites, however, it appears that a small fracti on of the protons is responsible for acid-catalyzed reactions, and the value of the J(P-H) coupling does not adequately reflect the contribu tion of these centers.