DEUTERIUM NMR CHARACTERIZATION OF BRONSTED ACID SITES AND SILANOL SPECIES IN ZEOLITES

Solid-state deuterium NMR has been used to characterize Bronsted acid sites and nonacidic silanol species in D-Y, D-mordenite, and D-ZSM-5 zeolites. Bronsted acid deuterons may be static, with quadrupole coupling constants (QCC) near 240 kHz and asymmetries (eta) of 0. These acidic deuterons may also be motionally averaged to give QCC = 120 kHz and eta = 1, consistent with jumping between two oxygen sp(3) orbitals. These results indicate that the acid deuteron does not remain fixed in the Al-O-Si plane. Isolated silanol species display a narrow, axially symmetric powder pattern, consistent with an Si-O-D bond angle of 116 degrees. Most of the silanol species are more densely packed, and the variety of hydrogen-bonded positions possible results in a Gaussian peak about 56 kHz wide. Deuterium NMR shows promise as a technique for characterizing both acidic and nonacidic hydroxyl groups in zeolites.