Multi‐Nuclear 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 29Si CP/MAS NMR and 27Al NMR spectroscopy. After deammoniation of zeolites Y (nSi/nAl = 2.6 and 8.0), mordenite (nSi/nAl = 8.0) and ZSM‐5 (nSi/nAl = 22.0) 29Si NMR high‐field shifts of Si(1 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 at the temperature of T = 295 K by 27Al, 2H and 1H 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.