Crystallization of MAZ-type zeolites using tetramethylammonium, sodium andn-hexane derivatives as structure- and composition-directing agents

An attempt was made to synthesize MAZ-type zeolites with enhanced Si/Al rat io by adding organic molecules to conventional aluminosilicate hydrogels fo r MAZ zeolites containing sodium and tetramethylammonium cations. Molecular models of candidate templates were built and optimized inside the 12-membe red ring (12-MR) pore of the MAZ structure. The hexamethonium cation, 1,6-d iaminohexane and 1,6-hexanediol show the most favorable calculated Lennard- Jones potential. The crystallization in presence of these organic molecules led to the formation of MAZ-type zeolite. The samples were characterized u sing X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-di spersive analysis of X-rays (EDAX), chemical analysis, nitrogen adsorption, thermogravimetric analysis/differential thermal analysis (TGA/DTA), Si-29 and Al-27 magic-angle spinning (MAS) nuclear magnetic resonance spectroscop y (NMR) and C-13 cross-polarization (CP)/MAS NMR. The hexamethonium cation and 1,6-diaminohexane are incorporated into the MAZ crystals, whereas 1,6-h exanediol is not. Incorporation of 1,6-diaminohexane caused a higher incorp oration of silicon in the framework, especially in the T1 sires delineating the 12-MR channels. The upper limit of silicon incorporation was investiga ted by systematically reducing the aluminum content of the gel. The highest Si/Al ratio achieved by this approach was 5.3. A molecular crystallization mechanism for mazzite is proposed, explaining this upper limit as well as the crystal morphology changes depending on the aluminum content of the syn thesis gel. (C) 2000 Elsevier Science B.V. All rights reserved.