Towards the rational design of efficient organic structure-directing agents for zeolite synthesis

An Aluminosilicate Zeolite Containing Rings of Tetrahedral Atoms with All Odd Numbers from Five to Eleven

Herein we report the synthesis, structure solution, and catalytic properties of PST-31, which has an unprecedented framework topology. This high-silica (Si/Al=16) zeolite was synthesized using a pyrazolium-based dication with a tetramethylene linker as an organic structure-directing agent (OSDA) in hydroxide media. The PST-31 structure is built from new building layers containing four-, five-, six-, and seven-membered rings, which are connected by single four-membered rings in the interlayer region to form a two-dimensional pore system. Its channels consist of [4.5⁶ .6.9.11] and [5.6.7.9.10.11] cavities and are thus delimited by nine-, ten-, and eleven-membered rings. The OSDA cations in as-synthesized PST-31 were determined to reside without disorder in the large [4² .5¹⁴ .6⁴ .7² .9⁴ ] cavities composed of smaller [4.5⁶ .6.9.11] and [5.6.7.9.10.11] ones, leading to a symmetry coincidence between the OSDA and the surrounding zeolite cavity. The proton form of PST-31 was found to be selective for the cracking of n-hexane to light olefins.

A Mechanistic Investigation of Sustainable Solvent-Free, Seed-Directed Synthesis of ZSM-5 Zeolites in the Absence of an Organic Structure-Directing Agent

The solvent-free, seed-directed synthesis using natural precursors has emerged as a sustainable route for the synthesis of zeolite. Albeit the significant progress in the synthesis techniques, the crystallization behaviors of zeolites are somewhat elusive. Herein, we performed a detailed investigation of the crystallization behaviors of ZSM-5 zeolites synthesized through the solvent-free, seed-directed route using rice husk silica as starting materials. The crystallization at 180 °C is completed rapidly in 10 h, with an ultrahigh zeolite yield of at least 95%. Moreover, we evaluated the crystallization kinetics at different temperatures using the nonlinear Avrami equation and found instantaneous nucleation with three-dimensional growth in the studied temperature range, with activation energies for nucleation, transition, and crystal growth of 137, 51, and 51 kJ mol-1, respectively, indicating that nucleation is the rate-determining step. Further investigation of the structural and morphological evolution revealed a preference for secondary nucleation over the seed-growth mechanism. Crystallization proceeds via structural rearrangement within the solid system. We anticipate that our work will provide extensive insights that increase the understanding of zeolite crystallization and expand the highly sustainable production of zeolites.

Local Distortions in a Prototypical Zeolite Framework Containing Double Four-Ring Cages: The Role of Framework Composition and Organic Guests*

Cube-like double four-ring (d4r) cages are among the most frequent building units of zeolites and zeotypes. In materials synthesised in fluoride-containing media, the fluoride anions are preferentially incorporated in these cages. In order to study the impact of framework composition and organic structure-directing agents (OSDAs) on the possible occurrence of local distortions of fluoride-containing d4r cages, density functional theory (DFT) calculations and DFT-based molecular dynamics simulations were performed for AST-type zeotypes, considering four different compositions (SiO2 , GeO2 , AlPO4 , GaPO4 ) and two different OSDA cations (tetramethylammonium [TMA] and quinuclidinium [QNU]). All systems except SiO2 -AST show significant deformations, with a pyritohedron-like distortion of the d4r cages occurring in GeO2 - and GaPO4 -AST, and a displacement of the fluoride anions towards one of the corners of the cage in AlPO4 - and GaPO4 -AST. While the distortions occur at random in TMA-containing zeotypes, they exhibit a preferential orientation in systems that incorporate QNU cations. In addition to providing detailed understanding of the local structure of a complex host-guest system on the picosecond timescale, this work indicates the possibility to stabilise ordered distortions through a judicious choice of the OSDA, which might enable a tuning of the material's properties.

Recent progress and perspectives on the structural design on metal-organic zeolite (MOZ) frameworks

As a typical group of coordination polymers, metal-organic zeolite (MOZs) frameworks inherit the topological and structural advantages of inorganic zeolites and display great application potential in many areas, including gas adsorption/separation, catalysis, luminescence and chemical sensing. In this review, we outline the recent progress in the synthesis, functionalization and application of metal-organic zeolite frameworks, mainly focusing on the basic structural design principle and synthesis strategy on 4-connect inorganic nodes and 2-connect organic linkers. Employing different valent metals, small inorganic TO₄^2- units and high-nuclear clusters as 4-connect nodes, we derived multi-types of MOZs with a modified framework charge, improved stability and enhanced photo-/eletrocatalytic activity. Besides, the selection, functionalization and defect-engineering on the 2-connect ligands generated different topological and functional MOZs. Finally, the future trends and some perspectives in this area are outlined.

Effect of mono- and divalent extra-framework cations on the structure and accessibility of porosity in chabazite zeolites

The differences in valence and size between extra-framework cations exert a significant effect on the nitrogen sorption ability in the synthesised chabazite zeolites (K-CHA, Cs-CHA, Ca-CHA, Ba-CHA, Sr-CHA and Zn-CHA).

A dramatic conformational effect of multifunctional zwitterions on zeolite crystallization

Carnitine functions as a mesoporogen in LTA zeolite synthesis whereas its structural analogue acetylcarnitine acts as a crystal growth modifier. An array of experimental and theoretical studies reveal a remarkable effect of molecular conformation on the actual roles of organic functional groups during zeolite crystallization.

Synthesis and Characterization of a New Aluminosilicate Molecular Sieve from Aluminosilica Perhydrate Hydrogel

A novel structure aluminosilicate molecular sieve, named BUCT-3, was prepared by dynamic hydrothermal synthesis, and the critical factor to obtain the new structure is using an active silicon and aluminum source, aluminosilica perhydrate hydrogel. Meanwhile, only high content of O-O bonds can ensure the pure phase of BUCT-3. Through the characterization of x-ray powder diffraction (XRD), Fourier transform infrared spectra (FTIR), scanning electron microscopy (SEM), and so on, some structure and morphology information of BUCT-3 molecular sieves as well as the special silicon and aluminum source was obtained. It’s worth noticing that the O-O bonds of reactants can be reserved in the products, and thus, help us to get a new structure with cell parameters a = 8.9645 Å, b = 15.2727 Å, c = 11.3907 Å, α = 90°, β = 93.858°, γ = 90°. The crystal system is monoclinic. Though the thermostability of BUCT-3 is not satisfactory, its potential application derived from O-O bonds cannot be neglected.

Rational Manipulation of Stacking Arrangements in Three-Dimensional Zeolites Built from Two-Dimensional Zeolitic Nanosheets

Unit-cell-thin zeolitic nanosheets have emerged as fascinating materials for catalysis and separation. The controllability of nanosheet stacking is extremely challenging in the chemistry of two-dimensional zeolitic materials. To date, the organization of zeolitic nanosheets in hydrothermal synthesis has been limited by the lack of tunable control over the guest-host interactions between organic structure-directing agents (OSDAs) and zeolitic nanosheets. A direct synthetic methodology is reported that enables systematic manipulation of the aluminosilicate MWW-type nanosheet stacking. Variable control of guest-host interactions is rationally achieved by synergistically altering the charge density of OSDAs and synthetic silica-to-alumina composition. These finely controlled interactions allow successful preparation of a series of three-dimensional (3D) zeolites, with MWW-layer stacking in wide ranges from variably disorder to fully ordered, leading to tunable catalytic activity in the cracking reaction. These results highlight unprecedented opportunities to modulate zeolitic nanosheets arrangement in 3D zeolites whose structure can be tailored for catalysis and separation.

Local Structures of Two-Dimensional Zeolites-Mordenite and ZSM-5-Probed by Multinuclear NMR

Mesostructured pillared zeolite materials in the form of lamellar phases with a crystal structure of mordenite (MOR) and ZSM-5 (MFI) were grown using CTAB as an agent that creates mesopores, in a one-pot synthesis; then into the CTAB layers separating the 2D zeolite plates were introduced by diffusion the TEOS molecules which were further hydrolyzed, and finally the material was annealed to remove the organic phase, leaving the 2D zeolite plates separated by pillars of silicon dioxide. To monitor the successive structural changes and the state of the atoms of the zeolite framework and organic compounds at all the steps of the synthesis of pillared MOR and MFI zeolites, the nuclear magnetic resonance method (NMR) with magic angle spinning (MAS) was applied. The 27Al and 29Si MAS NMR spectra confirm the regularity of the zeolite frameworks of the as synthetized materials. Analysis of the 1H and 13C MAS NMR spectra and an experiment with variable contact time evidence a strong interaction between the charged "heads" -[N(CH3)3]+ of CTAB and the zeolite framework at the place of [AlO4]- location. According to 27Al and 29Si MAS NMR the evacuation of organic cations leads to a partial but not critical collapse of the local zeolite structure.

Exploring the origins of crystallisation kinetics in hierarchical materials using in situ X-ray diffraction and pair distribution function analysis

The discovery of novel catalytic materials is predicated on understanding contemporary synthetic processes. With this fundamental knowledge in place it becomes possible to modify the final material with subtle changes to the synthesis process. In this vein, hierarchical materials, formed by the addition of a mesoporogen within the hydrothermal synthesis, have attracted a significant amount of attention due to their catalytic benefits over analogous microporous species. In this work we monitor the hydrothermal synthesis in situ of a hierarchical and a microporous aluminophosphate, for the first time, combining total scattering and pairwise distribution function data. In doing so we observe the local formation of the species, and the longer range crystallisation processes concurrently.

Thermodynamic rules for zeolite formation from machine learning based global optimization

While the [TO4] tetrahedron packing rule leads to millions of likely zeolite structures, there are currently only 252 types of zeolite frameworks reported after decades of synthetic efforts. The subtle synthetic conditions, e.g. the structure-directing agents, pH and the feed ratio, were often blamed for the limited zeolite types due to the complex kinetics. Here by developing machine learning global optimization techniques, we are now able to establish the global potential energy surface of a typical zeolite system, Si x Al y P z O2H y-z with 12 T atoms (T: Si, Al and P) that is the general formula shared by CHA, ATS, ATO and ATV zeolite frameworks. After analyzing more than 106 minima data, we identify thermodynamic rules on energetics and local bonding patterns for stable zeolites. These rules provide general guidelines to classify zeolite types and correlate them with synthesis conditions. The machine learning based atomistic simulation thus paves a new way towards rational design and synthesis of stable zeolite frameworks with desirable compositions.

Synthesis and Post-Synthesis Transformation of Germanosilicate Zeolites

Zeolites are one of the most important heterogeneous catalysts, with a high number of large-scale industrial applications. While the synthesis of new zeolites remain rather limited, introduction of germanium has substantially increased our ability to not only direct the synthesis of zeolites but also to convert them into new materials post-synthetically. The smaller Ge-O-Ge angles (vs. Si-O-Si) and lability of the Ge-O bonds in aqueous solutions account for this behaviour. This Minireview discusses critical aspects of germanosilicate synthesis and their post-synthesis transformations to porous materials.

3D-3D topotactic transformation in aluminophosphate molecular sieves and its implication in new zeolite structure generation

Zeolites have unique pore structures of molecular dimensions and tunable compositions, making them ideal for shape selective catalysis and separation. However, targeted synthesis of zeolites with new pore structures and compositions remains a key challenge. Here, we propose an approach based on a unique 3D-3D topotactic transformation, which takes advantage of weak bonding in zeolites. This is inspired by the structure transformation of PST-5, a new aluminophosphate molecular sieve, to PST-6 by calcination. The structure of nano-sized PST-5 crystals is determined by 3D electron diffraction. We find that the 3D-3D topotactic transformation involves two types of building units where penta- or hexa-coordinated Al is present. We apply this approach to several other zeolite systems and predict a series of new zeolite structures that would be synthetically feasible. This method provides a concept for the synthesis of targeted zeolites, especially those which may not be feasible by conventional methods.

Synthesis of large-pore zeolites from chiral structure-directing agents with two l-prolinol units

In this work, we perform an in-depth experimental and computational study about the structure-directing effect of two new chiral organic quaternary ammonium dications bearing two N-methyl-prolinol units linked by a xylene spacer in para or meta relative orientation, displaying four enantiopure stereogenic centers in (S) configuration. Synthesis results show that the para-xylene derivative is an efficient structure-directing agent, promoting the crystallization of ZSM-12 (in pure-silica composition), beta zeolite (as pure-silica, or in the presence of Al or Ge), and a mixture of polymorphs C, A and B of zeolite beta (in the presence of Ge). In contrast, the meta-xylene derivative showed a much poorer structure-directing activity, yielding only amorphous materials unless Ge is present in the gel, where beta and polymorph C (together with A and B) zeolites crystallized. Molecular simulations showed that the para-xylene dication displays a cylindrical shape suitable for confining in zeolite pores, while the meta-xylene derivative has an angular shape that shifts from the typical dimensions required for 12MR zeolite channels. Despite enantio-purity of the para-xylene dication with (S,S,S,S) configuration, no enrichment in polymorph A of the zeolite beta samples obtained was observed by Transmission Electron Microscopy. With the aid of molecular simulations, the failure in transferring chirality to the zeolite is explained by the loose fit of this SDA in the large-pores of zeolite beta, and a lack of close geometrical fit with the chiral element of polymorph A, as evidenced by the very similar interaction of the cation with the two enantiomorphic space groups of polymorph A. Nevertheless, the molecular-level knowledge gained in this work can provide insights for the future design of more efficient SDAs towards the synthesis of chiral zeolites.

Multi-objective de novo molecular design of organic structure-directing agents for zeolites using nature-inspired ant colony optimization

Organic structure-directing agents (OSDAs) are often employed for synthesis of zeolites with desired frameworks. A priori prediction of such OSDAs has mainly relied on the interaction energies between OSDAs and zeolite frameworks, without cost considerations. For practical purposes, the cost of OSDAs becomes a critical issue. Therefore, the development of a computational de novo prediction methodology that can speed up the trial-and-error cycle in the search for less expensive OSDAs is desired. This study utilized a nature-inspired ant colony optimization method to predict physicochemically and/or economically preferable OSDAs, while also taking molecular similarity and heuristics of zeolite synthesis into consideration. The prediction results included experimentally known OSDAs, candidates having structures closely related to known OSDAs, and novel ones, suggesting the applicability of this approach.

Inspired by the exploratory methods of ant colonies, adaptive optimization was employed to explore the chemical space for organic molecules that guide zeolite crystallization, giving both physicochemically and economically promising molecules.

A Bottom-Up Strategy for the Synthesis of Highly Siliceous Faujasite-Type Zeolite

High-silica zeolite Y is a desired catalytic material for oil refining and the petrochemical industry. However, its direct synthesis remains a symbolic challenge in the field of zeolite synthesis, with a limited improvement of the framework SiO₂ /Al₂ O₃ ratio (SAR) from ≈5 to 9 over the past 60 years. Here, the synthesis of highly siliceous zeolite Y with tunable SAR up to 15.6 through a cooperative strategy is reported, which involves the use of FAU nuclei, a bulky organic structure-directing agent (OSDA), and a gel system with low alkalinity (named NOA-co strategy). A series of quaternary alkylammonium ions is discovered as effective OSDAs based on the NOA-co strategy, and the relevant crystallization mechanism is elucidated. Moreover, the high-silica products are demonstrated to have greatly improved (hydro)thermal stability, high concentration of strong acid sites, and uniform acid distribution, which lead to superior catalytic performance in the cracking of bulky hydrocarbons. It is anticipated that this synthetic strategy will benefit the synthesis and development of zeolitic catalysts in a wide range of reaction processes.

Identification of New Templates for the Synthesis of BEA, BEC, and ISV Zeolites Using Molecular Topology and Monte Carlo Techniques

The presence of organic structure directing agents (templates) in the synthesis of zeolites allows the synthesis to be directed, in many cases, toward structures in which there is a large stabilization between the template and the zeolite micropore due to dispersion interactions. Although other factors are also important (temperature, pH, Si/Al ratio, etc.), systems with strong zeolite-template interactions are good candidates for an application of new computational algorithms, for instance those based in molecular topology (MT), that can be used in combination with large databases of organic molecules. Computational design of new templates allows the synthesis of existing and new zeolites to be expanded and refined. Three zeolites with similar 3-D large pore systems, BEA, BEC, and ISV, were selected with the aim of finding new templates for their selective syntheses. Using a training set of active and inactive templates (obtained from the literature) for the synthesis of target zeolites, it was possible to select chemical descriptors related to activity, meaning a good candidate template. With a discriminant function defined upon MT, the screening through a database of organic molecules led to a small subset (preselection) of candidate templates for the synthesis of BEA, BEC, and ISV. As far as we know, this is the first time that topological/topochemical descriptors, which do not consider 3-D information on the molecules, have been used to predict the activity of zeolite structure directing agents (SDAs). Following the prediction of SDAs using MT, an automated approach of sequential template filling of micropores based on a combination of Monte Carlo and lattice energy minimization was applied for all the candidate templates in the three zeolites. Two results can be obtained from this: an evaluation of the quality of the molecular topology QSAR models leading to the preselection of templates, and a final selection of candidate templates for the selective synthesis of BEA, BEC, and ISV. Regarding the latter, a good template will be that which maximizes the zeolite-template dispersion interactions with one, and only one, of the three zeolites. The presented methodology can be used to find alternative (maybe cheaper or perhaps more selective) templates than those already known.

An Extra-Large-Pore Pure Silica Zeolite with 16×8×8-Membered Ring Pore Channels Synthesized using an Aromatic Organic Directing Agent

Extra-large-pore zeolites for processing large molecules have long been sought after by both the academia and industry. However, the synthesis of these materials, particularly extra-large-pore pure silica zeolites, remains a big challenge. Herein we report the synthesis of a new extra-large-pore silica zeolite, designated NUD-6, by using an easily synthesized aromatic organic cation as structure-directing agent. NUD-6 possesses an intersecting 16×8×8-membered ring pore channel system constructed by four-connected (Q⁴ ) and unusual three-connected (Q³ ) silicon species. The organic cations in NUD-6 can be removed in nitric acid to yield a porous material with high surface area and pore volume. The synthesis of NUD-6 presents a feasible means to prepare extra-large pore silica zeolites by using assembled aromatic organic cations as structure-directing agents.

Zeolite Structure Direction: Identification, Strength and Involvement of Weak CH⋅⋅⋅O Hydrogen Bonds

We demonstrate that weak CH⋅⋅⋅O hydrogen bonds (HBs) are important host-guest interactions in zeolite assemblies involving structure directing organocations. This type of HB is identified between alkyl groups of the organic structure directing agent (OSDA) and the silica framework in as-synthesized silicalite-1 of complex topology (MFI) using a combination of experimental and theoretical data obtained at low and room temperatures. The 28 weak CH⋅⋅⋅O HBs, evidenced along dynamics simulation at room temperature, represent 30 % of the energy of the Coulomb electrostatic interaction between OSDA and the zeolite framework. The strongest and most stable HB found here connects the OSDA to the [4¹ 5² 6² ] cage containing F atoms and should contribute to preserve zeolite topology during crystal growth. An inspection of other as-synthesized zeolites of very different framework topology indicates that the directional CH⋅⋅⋅O HBs have to be considered when discussing zeolite structure directing phenomena.

Understanding the impact of one-dimensional pore containing 10MR and 12MR and aluminium content on MTH reaction pathways: direct synthesis of heteroatom containing UZM-55

We directly synthesized one-dimensional zeolite UZM-55 as an aluminosilicate and catalyzed MTH to understand pore structure influence on catalytic properties.

Zeolite Synthesis Using Imidazolium Cations as Organic Structure-Directing Agents

Imidazolium-derivative cations are very interesting molecules used as organic structure-directing agents (OSDAs) for zeolite synthesis, widening the possibilities of new materials and applications in this research area. In this review, the studies performed at LABPEMOL using this kind of compound are presented after a quick overview on imidazolium derivatives. The first zeolite synthesis results that started this research study were obtained with 1-butyl-3-methylimidazolium chloride. Then, the design of new OSDAs based on the imidazolium cation, such as 1,2,3-triethylimidazolium, 2-ethyl-1,3-dimethylimidazolium and 1,2,3-triethyl-4-methylimidazolium, is reported. Afterwards, the structure-direction effect caused by the introduction of heteroatoms with already-published imidazolium derivatives (for example, the Al3+ insertion into zeolite frameworks with two different OSDAs and the silicoaluminophosphate (SAPO) synthesis using 2-ethyl-1,3,4 trimethylimidazolium cations) is discussed. Finally, we also present a quick overview of some achievements of other laboratories.

Machine Learning Applied to Zeolite Synthesis: The Missing Link for Realizing High-Throughput Discovery

Zeolites are microporous crystalline materials with well-defined cavities and pores, which can be prepared under different pore topologies and chemical compositions. Their preparation is typically defined by multiple interconnected variables (e.g., reagent sources, molar ratios, aging treatments, reaction time and temperature, among others), but unfortunately their distinctive influence, particularly on the nucleation and crystallization processes, is still far from being understood. Thus, the discovery and/or optimization of specific zeolites is closely related to the exploration of the parametric space through trial-and-error methods, generally by studying the influence of each parameter individually. In the past decade, machine learning (ML) methods have rapidly evolved to address complex problems involving highly nonlinear or massively combinatorial processes that conventional approaches cannot solve. Considering the vast and interconnected multiparametric space in zeolite synthesis, coupled with our poor understanding of the mechanisms involved in their nucleation and crystallization, the use of ML is especially timely for improving zeolite synthesis. Indeed, the complex space of zeolite synthesis requires drawing inferences from incomplete and imperfect information, for which ML methods are very well-suited to replace the intuition-based approaches traditionally used to guide experimentation. In this Account, we contend that both existing and new ML approaches can provide the "missing link" needed to complete the traditional zeolite synthesis workflow used in our quest to rationalize zeolite synthesis. Within this context, we have made important efforts on developing ML tools in different critical areas, such as (1) data-mining tools to process the large amount of data generated using high-throughput platforms; (2) novel complex algorithms to predict the formation of energetically stable hypothetical zeolites and guide the synthesis of new zeolite structures; (3) new "ab initio" organic structure directing agent predictions to direct the synthesis of hypothetical or known zeolites; (4) an automated tool for nonsupervised data extraction and classification from published research articles. ML has already revolutionized many areas in materials science by enhancing our ability to map intricate behavior to process variables, especially in the absence of well-understood mechanisms. Undoubtedly, ML is a burgeoning field with many future opportunities for further breakthroughs to advance the design of molecular sieves. For this reason, this Account includes an outlook of future research directions based on current challenges and opportunities. We envision this Account will become a hallmark reference for both well-established and new researchers in the field of zeolite synthesis.

Synthesis and Photoelectric Properties of Metal-Organic Zeolites Built from TO4 and Organotin

Herein we report a novel strategy capable of generating a new class of metal-organic zeolite (MOZ) materials. When the MoO₄ or WO₄ tetrahedra are employed to assemble with triorganotin R₃Sn fragments, four 3D networks with the zeolite BCT topology and nonzeotype 4-connected topological net (such as lon and dia) have been generated. The photocurrent study results show that these materials have good photoelectric response and high photophysical stability.

Linking synthesis and structure descriptors from a large collection of synthetic records of zeolite materials

Correlating synthesis conditions and their consequences is a significant challenge, particularly for materials formed as metastable phases via kinetically controlled pathways, such as zeolites, owing to a lack of descriptors that effectively illustrate the synthesis protocols and their corresponding results. This study analyzes the synthetic records of zeolites compiled from the literature using machine learning techniques to rationalize physicochemical, structural, and heuristic insights to their chemistry. The synthesis descriptors extracted from the machine learning models are used to identify structure descriptors with the appropriate importance. A similarity network of crystal structures based on the structure descriptors shows the formation of communities populated by synthetically similar materials, including those outside the dataset. Crossover experiments based on previously overlooked structural similarities reveal the synthesis similarity of zeolites, confirming the synthesis–structure relationship. This approach is applicable to any system to rationalize empirical knowledge, populate synthesis records, and discover novel materials.

Understanding zeolite synthesis-structure relationships remains challenging owing to the number of variables involved in their preparation. Here the authors analyze zeolite synthetic records from the literature via machine learning and find communities of synthetically related materials with previously overlooked similarities.

Noble Metal Particles Confined in Zeolites: Synthesis, Characterization, and Applications

Noble metal nanoparticles or subnanometric particles confined in zeolites, that is, metal@zeolite, represent an important type of functional materials with typical core-shell structure. This type of material is known for decades and recently became a research hotspot due to their emerging applications in various fields. Remarkable achievements are made dealing with the synthesis, characterization, and applications of noble metal particles confined in zeolites. Here, the most representative research progress in metal@zeolites is briefly reviewed, aiming to boost further research on this topic. For the synthesis of metal@zeolites, various strategies, such as direct synthesis from inorganic or ligand-assisted noble metal precursors, multistep postsynthesis encapsulation and ion-exchange followed by reduction, are introduced and compared. For the characterization of metal@zeolites, several most useful techniques, such as electron microscopy, X-ray based spectroscopy, infrared and fluorescence emission spectroscopy, are recommended to check the successful confinement of noble metal particles in zeolite matrix and their unique physiochemical properties. For the applications of metal@zeolites, catalysis and optics are involved with an emphasis on catalytic applications including the size-dependent catalytic properties, the sintering-resistance properties, the substrate shape-selective catalysis, and catalysis modulation by zeolite microenvironment.

Targeted Synthesis of a Zeolite with Pre-established Framework Topology

Given their great potential as new industrial catalysts and adsorbents, the search for new zeolite structures is of major importance in nanoporous materials chemistry. However, although innumerable theoretical frameworks have been proposed, none of them have been synthesized by a priori design yet. We generated a library of diazolium-based cations inspired from the organic structure-directing agents (OSDAs) recently reported to give two structurally related zeolites (PST-21 and PST-22) under highly concentrated, excess-fluoride conditions and compared the stabilization energies of each OSDA cation in ten pre-established hypothetical structures. A combination of the ability of the OSDA selected in this way with the excess-fluoride approach has allowed us to crystallize PST-30, the targeted aluminosilicate zeolite structure. We anticipate that our approach, which aims to rationally couple computational predictions of OSDAs with an experimental setup, will advance further development in the synthesis of zeolites with desired properties.

Combined Alkali-Organoammonium Structure Direction of High-Charge-Density Heteroatom-Containing Aluminophosphate Molecular Sieves

The charge density mismatch concept was applied to the synthesis of high-charge-density silicoaluminophosphate SAPO-69 (OFF) and SAPO-79 (ERI) and zincoaluminophosphate PST-16 (CGS), PST-17 (BPH), PST-19 (SBS), and ZnAPO-88 (MER) molecular sieves. Combined alkali-organoammonium structure direction in these systems is thus enabled. Structure direction is treated from the perspective of stabilizing an ionic framework, the relationships between reaction charge density (OH^- /H₃ PO₄ ), alkali and organoammonium content, and ionicity of tetrahedral framework atoms in successful structure direction are presented.

Rediscovery of the Importance of Inorganic Synthesis Parameters in the Search for New Zeolites

Zeolites and related crystalline microporous materials with cavities and channels of molecular dimensions are of major importance for applications ranging from ion-exchange to adsorption and to catalysis. Because their unique shape-selective properties are closely related to the size, shape, and dimensionality of the intracrystalline channels and cavities, much interest has been devoted to the discovery of novel zeolitic materials over the last several decades. As a result, a dramatic expansion in the structural domain of crystalline microporous materials, as well as in their compositional range, has been achieved. This is largely due to the development of innovative synthetic strategies, for example, organic structure-directing agent (OSDA) design, introduction of heteroatoms like Ge in OSDA-mediated zeolite synthesis, topotactic transformation of two-dimensional layered zeolite precursors, assembly-disassembly-organization-reassembly method, etc. However, although many of these methodologies are quite successful in finding unprecedented zeolite structures, the resulting materials tend to be (hydro)thermally unstable and are often commercially impractical from a manufacturing perspective because of the high cost of the OSDA and/or heteroatom employed. Therefore, we focused on inorganic synthesis parameters as the key phase selectivity factor that has received relatively little attention in the search for new industrially relevant zeolites. This Account describes our recent efforts to find previously undiscovered aluminosilicate zeolites by boosting the roles of inorganic structure-directing agents (ISDAs) in the presence of OSDAs. They include the multiple inorganic cation and excess fluoride approaches, which aim to promote a synergistic cooperation between ISDAs and/or OSDAs and thus to hold a rational design concept, although the latter is not friendly to the practical zeolite manufacturing process due to the toxicity of fluoride. Using these two approaches, we were able to synthesize not only the second generation (PST-29) and four higher generations (PST-20 (RHO-G5), PST-25 (RHO-G6), PST-26 (RHO-G7), and PST-28 (RHO-G8)) of the RHO family of embedded isoreticular zeolites but also three other novel zeolite structures (EU-12, PST-21, and PST-22). We also explored the synthesis of a number of heteroatom-containing aluminophosphate (AlPO₄) molecular sieves with different framework structures and unusually high framework charge density through the cooperative structure direction of alkali metal and small OSDA cations or under wholly inorganic conditions. Although we need to clarify the nature and extent of interactions between the inorganic cations and framework components in synthesis mixtures, we believe that our synthetic concepts, shedding new light on the importance of inorganic synthesis parameters, will open a door for achieving many other novel zeolite structures and compositions.

Templated fabrication of hierarchically porous metal-organic frameworks and simulation of crystal growth

Hierarchically porous metal-organic frameworks (MOFs) have recently emerged as a novel crystalline hybrid material with tunable porosity. Many efforts have been made to develop hierarchically porous MOFs, yet their low-energy fabrication remains a challenge and the underlying mechanism is still unknown. In this study, the rapid fabrication of two hierarchically porous MOFs (Cu-BTC and ZIF-8) was carried out at room temperature and ambient pressure for 10 min using a novel surfactant as the template in a (Cu, Zn) hydroxy double salt (HDS) solution, where the (Cu, Zn) HDS accelerated the nucleation of crystals and the anionic surfactants served as templates to fabricate mesopores and macropores. The growth mechanism of hierarchically porous MOFs was analyzed via mesodynamics (MesoDyn) simulation, and then the synthetic mechanism of hierarchically porous MOFs at the molecular level was obtained. The as-synthesized hierarchically porous Cu-BTC showed a high uptake capacity of 646 mg g^-1, which is about 25% higher as compared with microporous Cu-BTC (516 mg g^-1) for the capture of toluene. This study provides a theoretical basis for the large-scale fabrication of hierarchically porous MOFs and offers a reference for the understanding of their synthetic mechanism.

A simple matrine derivative for the facile syntheses of mesoporous zeolites ITQ-37 and ITQ-43

ITQ-37 and ITQ-43 possessing mesoporous structures have aroused great interest because of their promising applications involving large molecules. However, it is challenging to synthesize these zeolites. Herein, MAS-ITQ-37 and MAS-ITQ-43 with enhanced Si/Ge ratios of 1.7 and 3.0 have been synthesized by using a simple matrine-derived organic structure-directing agent.

Ultrafast synthesis of zeolites: breakthrough, progress and perspective

An ultrafast route was established to synthesize industrially important zeolites in several minutes, which represents a breakthrough in the field of zeolite synthesis.

New trends in tailoring active sites in zeolite-based catalysts

This review discusses approaches for tailoring active sites in extra-large pore, nanocrystalline, and hierarchical zeolites and their performance in emerging catalytic applications.

Two-Step to One-Step Nucleation of a Zeolite through a Metastable Gyroid Mesophase

The importance of nonclassical nucleation pathways in the formation of complex crystals has become apparent in recent years. Nonclassical pathways were unraveled for, among others, the crystallization of proteins, colloids, and clathrates. In those cases, the formation of a metastable fluid with density close to the crystal decreases the crystallization barrier. Recent simulations indicate that mesophases can facilitate the nucleation of zeolites. Here, we use molecular simulations to investigate the role of a gyroid mesophase on the crystallization of a model zeolite from liquid. The nucleation pathway is always nonclassical. At warmer temperatures, the mechanism proceeds in two well-defined steps: nucleation of a metastable gyroid followed by its crystallization into a zeolite. At colder temperatures, the second barrier becomes negligible, and the crystallization occurs in one step. This second scenario is also nonclassical, as the critical nucleus for the crystallization has the structure of the gyroid and seamlessly transforms into a zeolite as it grows past its critical size. To our knowledge, this is the first report of a nonclassical mechanism of crystallization mediated by a mesophase.