NATURAL-ABUNDANCE 2-DIMENSIONAL SI-29 MAS NMR INVESTIGATION OF THE 3-DIMENSIONAL BONDING CONNECTIVITIES IN THE ZEOLITE CATALYST ZSM-5

Two-Dimensional 29Si solid-state NMR experiments have been used to investigate the three-dimensional bonding connectivities in zeolite ZSM-5 and also the changes that are induced in the structure by increasing the temperature and by the presence of sorbed p-xylene. 2D INADEQUATE experiments gave substantially better results than 2D COSY experiments on the same samples. In the case of the orthorhombic form (12 T-sites) which is formed by increasing the temperature or at a loading of 2 sorbed molecules of p-xylene per unit cell, almost all of the expected connectivities were observed. For the monoclinic form, 38 of the total of 48 connectivities were clearly observed, allowing the assignment of the spectrum. When the number of sorbed p-xylene molecules is increased above 5 molecules per unit cell, a phase transition from the orthorhombic form with 12 T-sites to another orthorhombic form with 24 T-sites occurs. Due to symmetry, for each of these structures, two possible assignments exist which are compatible with the connectivity pattern. In each case the 2D INADEQUATE experiments yielded sufficient information to determine these two possible assignments. However, without an unambiguous starting point the decision in favor of one assignment against the other cannot be made on the basis of the NMR data alone. Correlating the 29Si chemical shifts with geometric parameters from X-ray diffraction experiments, however, makes it possible to discriminate in favor of one set of assignments for the orthorhombic (12 T-sites) and yields unambiguous solutions for the monoclinic and the orthorhombic (24 T-sites) forms. © 1990, American Chemical Society. All rights reserved.