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⁺), 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 °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⁺ 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

Crystallization Mechanism of Zeolite UZM‑5

A reliable formation pathway for UZM-5 zeolite crystals in the presence of tetraethylammonium, tetramethylammonium, and Na⁺ ions at 150 °C has been proposed based on the ¹³C 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

Synthesis and Catalytic Behavior of Ferrierite Zeolite Nanoneedles

The proton form of nanosized, needlelike ferrierite zeolite, which was synthesized using choline and Na⁺ cations as structure-directing agents, was found to be much more efficient for the skeletal isomerization of 1-butene to isobutene than the corresponding cation form of conventional, submicrometric ferrierite with a platelike shape, mainly because of the considerably lower density of strong acid sites, but as well as a result of the higher density of 10-ring pore mouths

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⁺), tetramethylammonium (TMA⁺), and Na⁺ 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⁺ and Na⁺ 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)­silicate 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⁺ is also incorporated, forming a single LTA zeolite phase with a higher Si/Al ratio (ca. 3.3)