MULTINUCLEAR SOLID-STATE NMR-STUDY OF THE LOCAL-STRUCTURE OF SIOHAL GROUPS AND THEIR INTERACTION WITH PROBE-MOLECULES IN DEHYDRATED FAUJASITE, MORDENITE AND ZEOLITE ZSM-5

Structural variations of Si-O-Al bridges in dehydrated zeolites have b een studied by Si-29 CP/MAS NMR and Al-27 NMR spectroscopy. After deam moniation of zeolites Y (n(Si)/n(Al) = 2.6 and 8.0), mordenite (n(Si)/ n(Al) = 8.0) and ZSM-5 (n(Si)/n(Al) = 22.0) Si-29 NMR high-field shift s of Si(Al) signals of maximum 0.8 ppm and of about 2.7 ppm and 4.0 pp m, respectively, were observed. This is explained by a protonation-ind uced increase in the mean Si-O-T bond angles of SiOHAl groups. Simulta neously, an increase in the aluminium quadrupole coupling constant fro m about 5.0 MHz to about 16.0 MHz was determined indicating a strong p rotonation-induced shear strain of framework AlO4 tetrahedra. On the o ther hand, the proton transfer from Si-O-Al bridges to basic probe-mol ecules causes a decrease in the aluminium quadrupole coupling constant which corresponds to a higher AlO4 symmetry. The high symmetry of AlO 4 tetrahedra in zeolite HZSM-5 loaded with one methanol molecule per S iOHAl group, the mobility of the hydroxyl protons existing in this sam ple and their chemical shift value observed al the temperature of T = 295 K by Al-27, H-2 and H-1 NMR, respectively, indicate a rapid exchan ge of hydroxyl protons between bridging OH groups, hydrogen-bonded met hanol molecules and methoxonium ions. At the temperature of T = 85 K m ethanol molecules are fixed at the zeolite framework by hydrogen bonds .