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 been studied by Si-29 CP/MAS NMR and Al-27 NMR spectroscopy. After deammoniation 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 shifts of Si(Al) signals of maximum 0.8 ppm and of about 2.7 ppm and 4.0 ppm, respectively, were observed. This is explained by a protonation-induced increase in the mean Si-O-T bond angles of SiOHAl groups. Simultaneously, an increase in the aluminium quadrupole coupling constant from about 5.0 MHz to about 16.0 MHz was determined indicating a strong protonation-induced shear strain of framework AlO4 tetrahedra. On the other hand, the proton transfer from Si-O-Al bridges to basic probe-molecules causes a decrease in the aluminium quadrupole coupling constant which corresponds to a higher AlO4 symmetry. The high symmetry of AlO4 tetrahedra in zeolite HZSM-5 loaded with one methanol molecule per SiOHAl group, the mobility of the hydroxyl protons existing in this sample 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 exchange of hydroxyl protons between bridging OH groups, hydrogen-bonded methanol molecules and methoxonium ions. At the temperature of T = 85 K methanol molecules are fixed at the zeolite framework by hydrogen bonds.