Synthesis of Zeolites via Interzeolite Transformations without Organic Structure-Directing Agents

We report synthetic protocols and guiding principles inspired by mechanistic considerations for the synthesis of crystalline microporous solids via interzeolite transformations that avoid direct intervention by organic structure-directing agents. These protocols are specifically implemented to synthesize high-silica MFI (ZSM-5), CHA (chabazite), STF (SSZ-35), and MTW (ZSM-12) zeolites from FAU (faujasite) or BEA (beta) parent materials. These transformations succeed when they lead to daughter structures with higher framework densities, and their nucleation and growth become possible by the presence of seeds or of structural building units common to the parent and target structures, leading, in the latter case, to spontaneous transformations by choosing appropriate synthesis conditions. These protocols allow the synthesis of high-silica frameworks without the use of organic templates otherwise required. The NaOH/SiO₂ ratio and Al content in reagents are used to enforce synchronization between the swelling and local restructuring within parent zeolite domains with the spalling of fragments or building units from seeds of the target structure. Seed-mediated interconversions preserve the habit and volume of the parent crystals because of the incipient nucleation of the target structure at the outer regions of the parent domains. The pseudomorphic nature of these transformations requires the concurrent nucleation of mesopores within daughter zeolite crystals because their framework density is larger than that for the parent zeolites. The approach and evidence described shows, for the first time, that a broad range of zeolites rich in silica, and thus more useful as catalysts, can be made without the organic templates originally used to discover them

Encapsulation of Metal Clusters within MFI via Interzeolite Transformations and Direct Hydrothermal Syntheses and Catalytic Consequences of Their Confinement

The encapsulation of metal clusters (Pt, Ru, Rh) within MFI was achieved by exchanging cationic metal precursors into a parent zeolite (BEA, FAU), reducing them with H₂ to form metal clusters, and transforming these zeolites into daughter structures of higher framework density (MFI) under hydrothermal conditions. These transformations required MFI seeds or organic templates for FAU parent zeolites, but not for BEA, and occurred with the retention of encapsulated clusters. Clusters uniform in size (1.3–1.7 nm) and exposing clean and accessible surfaces formed in BEA and FAU zeolites; their size remained essentially unchanged upon transformation into MFI. Encapsulation selectivities, determined from the relative hydrogenation rates of small (toluene) and large (alkyl arenes) molecules and defined as the ratio of the surface areas of all the clusters in the sample to that of external clusters, were very high (8.1–40.9) for both parent and daughter zeolites. Encapsulation into MFI via direct hydrothermal syntheses was unsuccessful because metal precursors precipitated prematurely at the pH and temperatures required for MFI synthesis. Delayed introduction of metal precursors and F^– (instead of OH^–) as the mineralizing agent in hydrothermal syntheses increased encapsulation selectivities, but they remained lower than those achieved via interzeolite transformations. These interconversions provide a general and robust strategy for encapsulation of metals when precursors can be introduced via exchange into a zeolite that can be transformed into target daughter zeolites with higher framework densities, whether spontaneously or by using seeds or structure-directing agents (SDA)

Synthesis and Catalytic Properties of Metal Clusters Encapsulated within Small-Pore (SOD, GIS, ANA) Zeolites

The synthesis protocols for encapsulation of metal clusters reported here expand the diversity in catalytic chemistries made possible by the ability of microporous solids to select reactants, transition states, and products on the basis of their molecular size. We report a synthesis strategy for the encapsulation of noble metals and their oxides within SOD (Sodalite, 0.28 nm × 0.28 nm), GIS (Gismondine, 0.45 nm × 0.31 nm), and ANA (Analcime, 0.42 nm × 0.16 nm) zeolites. Encapsulation was achieved via direct hydrothermal synthesis for SOD and GIS using metal precursors stabilized by ammonia or organic amine ligands, which prevent their decomposition or precipitation as colloidal hydroxides at the conditions of hydrothermal synthesis (<380 K) and favor interactions between metal precursors and incipient aluminosilicate nuclei during self-assembly of microporous frameworks. The synthesis of ANA requires higher crystallization temperatures (∼415 K) and high pH (>12), thereby causing precipitation of even ligand-stabilized metal precursors as hydroxides. As a result, encapsulation was achieved by the recrystallization of metal clusters containing GIS into ANA, which retained these metal clusters within voids throughout the GIS–ANA transformation

Heteroatom-Substituted Delaminated Zeolites as Solid Lewis Acid Catalysts

This manuscript represents a comparative study of Lewis acid catalysis using heteroatom-substituted delaminated zeolites, which are synthesized using an approach that obviates the need for surfactants and sonication during exfoliation. The comparison involves heteroatom substitution into silanol nests of delaminated zeolites consisting of DZ-1 and deboronated UCB-4. Diffuse reflectance ultraviolet (DR-UV) spectroscopy demonstrates framework heteroatom sites, and the Lewis acidity of these sites is confirmed using infrared spectroscopy of adsorbed pyridine. The enhanced catalytic accessibility of these Lewis acid sites is confirmed when performing Baeyer–Villiger oxidation of substituted 2-adamantanones with hydrogen peroxide as the oxidant. Comparison of delaminated Sn-DZ-1 with three-dimensional Sn-Beta for this reaction shows that the delaminated zeolite is more active for bulkier ketone substrates. The role of the two-dimensional crystalline framework of the delaminated zeolite on catalysis is highlighted by comparing delaminated zeolites Sn-DZ-1 with Sn-UCB-4. The former exhibits a significantly higher activity for Baeyer–Villiger oxidation, yet when comparing Ti-DZ-1 with Ti-UCB-4, it is the latter that exhibits a significantly higher activity for olefin epoxidation with organic hydrogen peroxide, whereas both delaminated zeolites are more robust and selective in epoxidation catalysis compared with amorphous Ti/SiO₂

SSZ-52, a Zeolite with an 18-Layer Aluminosilicate Framework Structure Related to That of the DeNOx Catalyst Cu-SSZ-13

A new zeolite (SSZ-52, |(C₁₄H₂₈N)₆Na₆(H₂O)₁₈|[Al₁₂Si₉₆O₂₁₆]), related to the DeNOx catalyst Cu-SSZ-13 (<b>CHA</b> framework type), has been synthesized using an unusual polycyclic quaternary ammonium cation as the structure-directing agent. By combining X-ray powder diffraction (XPD), high-resolution transmission electron microscopy (HRTEM) and molecular modeling techniques, its porous aluminosilicate framework structure (<i>R</i>3̅<i>m</i>, <i>a</i> = 13.6373(1) Å, <i>c</i> = 44.7311(4) Å), which can be viewed as an 18-layer stacking sequence of hexagonally arranged (Si,Al)₆O₆ rings (6-rings), has been elucidated. The structure has a three-dimensional 8-ring channel system and is a member of the ABC-6 family of zeolites (those that can be described in terms of 6-ring stacking sequences) like SSZ-13, but it has cavities that are twice as large. The code SFW has been assigned to this new framework type. The large cavities contain pairs of the bulky organic cations. HRTEM and XPD simulations show that stacking faults do occur, but only at the 5–10% level. SSZ-52 has considerable potential as a catalyst in the areas of gas conversion and sequestration

SSZ-52, a Zeolite with an 18-Layer Aluminosilicate Framework Structure Related to That of the DeNOx Catalyst Cu-SSZ-13

A new zeolite (SSZ-52, |(C₁₄H₂₈N)₆Na₆(H₂O)₁₈|[Al₁₂Si₉₆O₂₁₆]), related to the DeNOx catalyst Cu-SSZ-13 (<b>CHA</b> framework type), has been synthesized using an unusual polycyclic quaternary ammonium cation as the structure-directing agent. By combining X-ray powder diffraction (XPD), high-resolution transmission electron microscopy (HRTEM) and molecular modeling techniques, its porous aluminosilicate framework structure (<i>R</i>3̅<i>m</i>, <i>a</i> = 13.6373(1) Å, <i>c</i> = 44.7311(4) Å), which can be viewed as an 18-layer stacking sequence of hexagonally arranged (Si,Al)₆O₆ rings (6-rings), has been elucidated. The structure has a three-dimensional 8-ring channel system and is a member of the ABC-6 family of zeolites (those that can be described in terms of 6-ring stacking sequences) like SSZ-13, but it has cavities that are twice as large. The code SFW has been assigned to this new framework type. The large cavities contain pairs of the bulky organic cations. HRTEM and XPD simulations show that stacking faults do occur, but only at the 5–10% level. SSZ-52 has considerable potential as a catalyst in the areas of gas conversion and sequestration

SSZ-87: A Borosilicate Zeolite with Unusually Flexible 10-Ring Pore Openings

The structure of the as-synthesized borosilicate zeolite SSZ-87 has been solved by combining high-resolution X-ray powder diffraction (XPD) and rotation electron diffraction (RED) techniques. The unit cell and space group symmetry were found from the XPD data, and were essential for the initial analysis of the RED data. Although the RED data were only 15% complete, this proved to be enough for structure solution with the program <i>Focus</i>. The framework topology is the same as that of ITQ-52 (<b>IFW</b>), but for SSZ-87 the locations of the structure directing agent (SDA) and the B atoms could also be determined. SSZ-87 has large cages interconnected by 8- and 10-rings. However, results of hydroisomerization and Al insertion experiments are much more in line with those found for 12-ring zeolites. This prompted the structure analyses of SSZ-87 after calcination, and Al insertion. During calcination, the material is also partially deboronated, and the location of the resulting vacancies is consistent with those of the B atoms in the as-synthesized material. After Al insertion, SSZ-87 was found to contain almost no B and to be defect free. In its calcined and deboronated form, the pore system of SSZ-87 is more flexible than those of other 10-ring zeolites. This can be explained by the fact that the large cages in SSZ-87 are connected via single rather than double 10-ring windows and that there are vacancies in some of these 10-rings

Structure-Directing Roles and Interactions of Fluoride and Organocations with Siliceous Zeolite Frameworks

Interactions of fluoride anions and organocations with crystalline silicate frameworks are shown to depend subtly on the architectures of the organic species, which significantly influence the crystalline structures that result. One- and two-dimensional (2D) ¹H, ¹⁹F, and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy measurements establish distinct intermolecular interactions among F^– anions, imidazolium structure-directing agents (SDA⁺), and crystalline silicate frameworks for as-synthesized siliceous zeolites ITW and MTT. Different types and positions of hydrophobic alkyl ligands on the imidazolium SDA⁺ species under otherwise identical zeolite synthesis compositions and conditions lead to significantly different interactions between the F^– and SDA⁺ ions and the respective silicate frameworks. For as-synthesized zeolite ITW, F^– anions are established to reside in the double-four-ring (D4R) cages and interact strongly and selectively with D4R silicate framework sites, as manifested by their strong ¹⁹F–²⁹Si dipolar couplings. By comparison, for as-synthesized zeolite MTT, F^– anions reside within the 10-ring channels and interact relatively weakly with the silicate framework as ion pairs with the SDA⁺ ions. Such differences manifest the importance of interactions between the imidazolium and F^– ions, which account for their structure-directing influences on the topologies of the resulting silicate frameworks. Furthermore, 2D ²⁹Si{²⁹Si} double-quantum NMR measurements establish ²⁹Si–O–²⁹Si site connectivities within the as-synthesized zeolites ITW and MTT that, in conjunction with synchrotron X-ray diffraction analyses, establish insights on complicated order and disorder within their framework structures

Nonaqueous Fluoride/Chloride Anion-Promoted Delamination of Layered Zeolite Precursors: Synthesis and Characterization of UCB-2

The delamination of layered zeolite precursor PREFER is demonstrated under mild nonaqueous conditions using a mixture of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride in <i>N</i>,<i>N</i>-dimethylformamide (DMF) as solvent. The delamination proceeds through a swollen material intermediate which is characterized using powder X-ray diffraction (PXRD). Subsequent addition of concentrated HCl at room temperature leads to synthesis of UCB-2 via delamination of the swollen PREFER material and is characterized using PXRD, transmission electron microscopy (TEM), and argon gas physisorption, which shows lack of microporosity in UCB-2. ²⁹Si magic angle spinning (MAS) NMR spectroscopy indicates lack of amorphization during delamination, as indicated by the entire absence of Q² resonances, and ²⁷Al MAS NMR spectroscopy shows exclusively tetrahedral aluminum in the framework following delamination. The delamination process requires both chloride and fluoride anions and is sensitive to solvent, working well in DMF. Experiments aimed at synthesizing UCB-2 using aqueous conditions previously used for UCB-1 synthesis leads to partial swelling and lack of delamination upon acidification. A similar lack of delamination is observed upon attempting synthesis of UCB-1 under conditions used for UCB-2 synthesis. The delamination of PREFER is reversible between delaminated and swollen states in the following manner. Treatment of as-made UCB-2 with the same reagents as used here for the swelling of PREFER causes the delaminated UCB-2 material to revert back to swollen PREFER. This causes the delaminated UCB-2 material to revert back to swollen PREFER. Altogether, these results highlight delamination as the reverse of zeolite synthesis and demonstrate the crucial role of noncovalent self-assembly involving the zeolitic framework and cations/anions/structure-directing agent and solvent during the delamination process

Heteroatom-Tolerant Delamination of Layered Zeolite Precursor Materials

The synthesis of the first delaminated borosilicate layered zeolite precursor is described, along with its aluminosilicate analogue, which consists of Al-containing UCB-3 and B-containing UCB-4 from as-made SSZ-70. In addition, the delamination of PREFER (which is the precursor to ferrierite zeolite) under similar conditions yields delaminated layered zeolite precursors consisting of Al-containing UCB-5 and Ti-containing UCB-6. Multinuclear solid-state NMR spectroscopy (¹¹B and ²⁷Al), diffuse-reflectance UV-vis spectroscopy, and heteroatom/Si ratios measured via elemental analysis are consistent with a lack of heteroatom leaching from the framework following delamination. Such mild delamination conditions are achieved by swelling the zeolite precursor in a fluoride/chloride surfactant mixture in DMF solvent, followed by sonication. Powder X-ray diffraction, argon gas physisorption, and chemisorption of bulky base probes strongly suggest delamination, and demonstrate a 1.5-fold increase in the number density of external acid sites and surface area of calcined UCB-3, relative to calcined Al-SSZ-70. The synthesis of microporous pockets in materials UCB-3–UCB-5 suggests the possibility of interlayer porosity in SSZ-70, which is a layered zeolite precursor material whose structure remains currently unknown. The mildness of the delamination method presented here, as well as the lack of need for acidification in the synthesis procedure, enables the delamination of heteroatom-containing zeolites while preserving the framework integrity of labile heteroatoms, which could otherwise be leached under harsher conditions