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

An attempt was made to synthesize MAZ-type zeolites with enhanced Si/Al ratio by adding organic molecules to conventional aluminosilicate hydrogels for MAZ zeolites containing sodium and tetramethylammonium cations. Molecular models of candidate templates were built and optimized inside the 12-membered ring (12-MR) pore of the MAZ structure. The hexamethonium cation, 1,6-diaminohexane 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 using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive 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 spectroscopy (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-hexanediol is not. Incorporation of 1,6-diaminohexane caused a higher incorporation of silicon in the framework, especially in the T1 sires delineating the 12-MR channels. The upper limit of silicon incorporation was investigated 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 synthesis gel. (C) 2000 Elsevier Science B.V. All rights reserved.