Synthesis and Characterization of Gallosilicate Molecular Sieves with High Gallium Contents: Examples of Structure Direction Exerted by Gallium

Hydrothermal syntheses of the microporous gallosilicates TNU-1 (CGS topology), TNU-2 (GIS topology), TNU-3 (a synthetic counterpart of the natural zeolite gonnardite, NAT topology), and TNU-4 (a synthetic counterpart of the natural zeolite natrolite, NAT topology) are presented. In the range of synthesis conditions studied here, the presence of Ga itself is found to be the critical factor directing the crystallization of microporous gallosilicate materials with topologies different from those of the aluminosilicate zeolites obtained when Ga is replaced by Al. This suggests that the introduction of Ga into silicate synthesis mixtures may be a viable route for the synthesis of novel low-silica zeolite structures. The Si, Ga ordering in TNU-3 and TNU-4 materials is investigated in an attempt to understand why the crystallization of the same synthesis mixture at different temperatures results in two materials with the same topology and chemical composition but with different symmetries. A high degree of Si, Ga ordering is observed in TNU-4 by 29 Si MAS NMR, and possibly as a consequence, its 71 Ga MAS NMR spectrum clearly shows a line shape affected by second- order quadrupolar interactions rarely found in these kinds of materials.