Zeolite Synthesis from a Charge Density Perspective: The Charge Density Mismatch Synthesis of UZM-5 and UZM-9

Abstract

A charge density model of aluminosilicate zeolite synthesis is presented. This model has been applied to the charge density mismatch (CDM) synthesis of UZM-5 and UZM-9 zeolites at 150 and 100 degrees C, respectively, using the same synthesis mixture that includes tetraethylammonium (TEA(+)), tetramethylammonium (TMA(+)), and Na+ ions as structure-directing agents (SDAs). It allows a seamless description of the contributions of both the hydroxide and SDA components of the CDM barrier to zeolite synthesis. The syntheses are described as temperature-driven confrontations with the CDM barrier, resulting in disproportionation to solution and solid products with diverging charge densities. The presence of the CDM barrier and this tunable disproportionation in charge density, along with the suitable choice of SDA concentrations, allows a flexible and cooperative participation of SDAs, as the synthesis medium initially forms aluminosilicate networks that maximize Coulombic stabilization under the conditions at hand. The UZM-5 synthesis at 150 degrees C is characterized by much higher fractional Si and Al yields (0.85 Si and 0.94 Al vs 0.30 Si and 0.70 Al) and a higher Si/Al ratio (ca. 7 vs 3) compared to UZM-9 synthesis at 100 degrees C. Unlike the latter case, TEA(+) plays an important role in the nucleation of UZM-5. However, TMA(+) was found to be essential for the nucleation of both zeolites. While Na+ is required to crystallize UZM-9, the nucleation rate of UZM-5 is about twice as fast in the absence of Na+. On the other hand, the crystal growth rate of this small-pore zeolite is over 10 times faster with Na+ present, giving a considerably larger crystallite size.