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), 29Si and 27Al magic-angle spinning (MAS) nuclear magnetic resonance spectroscopy (NMR) and 13C 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 sites 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.