Zeolite
Synthesis from a Charge Density Perspective:
The Charge Density Mismatch Synthesis of UZM‑5 and UZM‑9
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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 °C,
respectively, using the same synthesis mixture that includes tetraethylammonium
(TEA<sup>+</sup>), tetramethylammonium (TMA<sup>+</sup>), and Na<sup>+</sup> 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 °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 °C. Unlike the latter case, TEA<sup>+</sup> plays an important
role in the nucleation of UZM-5. However, TMA<sup>+</sup> was found
to be essential for the nucleation of both zeolites. While Na<sup>+</sup> is required to crystallize UZM-9, the nucleation rate of
UZM-5 is about twice as fast in the absence of Na<sup>+</sup>. On
the other hand, the crystal growth rate of this small-pore zeolite
is over 10 times faster with Na<sup>+</sup> present, giving a considerably
larger crystallite size