3 research outputs found
Zeolite Synthesis from a Charge Density Perspective: The Charge Density Mismatch Synthesis of UZM‑5 and UZM‑9
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
Crystallization Mechanism of Zeolite UZM‑5
A reliable formation pathway for
UZM-5 zeolite crystals in the
presence of tetraethylammonium, tetramethylammonium, and Na<sup>+</sup> ions at 150 °C has been proposed based on the <sup>13</sup>C MAS NMR and IR spectra of a series of solid products recovered
as a function of time during the crystallization process, as well
as on the crystal structure of as-made UZM-5 determined using synchrotron
powder X-ray diffraction and Rietveld analyses. The nucleation of
this cage-based small-pore zeolite begins with the construction of
the largest 26-hedral <i>lta</i>-cages among its four different
structural units. The next step is the attachment of 14-hedral <i>wbc</i>-cages to the preorganized <i>lta</i>-cage
at shared 6-rings in an appropriate orientation that will allow the
growth of two <i>wbc</i>-cage layers linked by 8-hedral <i>rth</i>-cage formation along both <i>a</i> and <i>b</i> axes. The resulting interlayer space is readily converted
to a layer of <i>lta</i>-cages by interconnecting two opposing <i>wbc</i>-cages, with the concomitant formation of interlayer <i>d4r</i>-cages and 8-rings. Over the outer surface of the resulting
UZM-5 nuclei, which resembles one-half of an <i>lta</i>-cage,
the crystal growth may take place in a self-assembled manner as described
above
Formation Pathway for LTA Zeolite Crystals Synthesized via a Charge Density Mismatch Approach
A solid understanding of the molecular-level mechanisms
responsible
for zeolite crystallization remains one of the most challenging issues
in modern zeolite science. Here we investigated the formation pathway
for high-silica LTA zeolite crystals in the simultaneous presence
of tetraethylammonium (TEA<sup>+</sup>), tetramethylammonium (TMA<sup>+</sup>), and Na<sup>+</sup> ions as structure-directing agents (SDAs)
with the goal of better understanding the charge density mismatch
synthesis approach, which was designed to foster cooperation between
two or more different SDAs. Nucleation was found to begin with the
formation of <i>lta</i>-cages rather than the notably smaller <i>sod</i> and <i>d4r</i>-cages, with concomitant incorporation
of TMA<sup>+</sup> and Na<sup>+</sup> into a very small amount of
the solid phase with a low Si/Al ratio (ca. 2.5). The overall characterization
results of our work demonstrate that <i>sod</i>-cages are
first built around the preorganized <i>lta</i>-cages and
that <i>d4r</i>-cages are in turn constructed by the progressive
addition of low-molecular-weight (alumino)Âsilicate species, which
promotes the formation and growth of embryonic LTA zeolite crystals.
We also show that the crystal growth may take place by a similar process
in which TEA<sup>+</sup> is also incorporated, forming a single LTA
zeolite phase with a higher Si/Al ratio (ca. 3.3)