Unexpected Low Temperature Activity with Low N2 O Emission of Stabilized Al-rich Zeolite Beta for Selective Catalytic Reduction of NOx.

The low temperature activity of Fe-loaded zeolites as selective catalytic reduction of NOx by NH3 (NH3 -SCR) catalysts is a critical drawback for practical application. Here, we found unexpected improvement of low temperature activity by our proposed post-synthetic treatment. An Al-rich zeolite beta (Si/Al=5) is employed as the catalyst support, and the parent sample is dealuminated for higher hydrothermal stability, followed by the liquid-mediated stabilization treatment and impregnation. It is found that stabilized samples feature excellent low temperature activity and high N2 selectivity even for a long-term operation, along with the ability to maintain high NOx conversion after aging. The improved SCR activity should be attributed to abundant acid sites in Al-rich framework and better stabilization of monomeric iron species after the stabilization treatment. Furthermore, the low yield of side product N2 O is probably due to the absence of the generation of NH4 NO3 during NH3 -SCR catalyzed by Fe-loaded zeolites.

Revealing the evolution of local structures in the formation process of alkaline earth metal cation-containing zeolites from glasses.

Alkaline earth metal cations are ubiquitously present in natural zeolites but less exploited in synthetic zeolites due to their low solubility in water, and hence it remains elusive how they contribute to zeolite formation. Herein, harmotome, a PHI-type zeolite with Ba2+, is readily synthesized from a Ba-containing aluminosilicate glass. This glass-to-zeolite transformation process, in particular the structure-regulating role of Ba2+, is investigated by anomalous X-ray scattering and high-energy X-ray total scattering techniques. The results demonstrate that the steady Ba2+-aluminosilicate interactions not only help prevent the precipitation of barium species under alkaline synthetic conditions, but also dictate the local structures with distinct interatomic distances between the Ba2+ and the surrounding aluminosilicate species throughout the transformation process, which lead to the successful formation of harmotome without detectable impurities. This study highlights the usefulness of the comprehensive X-ray scattering techniques in revealing the formation scheme of the zeolites containing specific metal species. In addition, a promising alternative approach to design and synthesize zeolites with unique compositions and topologies by using well-crafted glasses with suitable metal cation dopants is demonstrated.

Tracking Sub-Nano-Scale Structural Evolution in Zeolite Synthesis by In Situ High-Energy X-ray Total Scattering Measurement with Pair Distribution Function Analysis.

The crystallization mechanism of zeolites remains unclarified to date because of lack of effective techniques in characterizing the local structures of amorphous precursors under synthetic conditions. Herein, in situ high-energy X-ray total scattering measurement with pair distribution function analysis is performed throughout the hydrothermal synthesis of SSZ-13 zeolite to investigate the amorphous-to-crystalline transformation at the sub-nano level in real time. Ordered four-membered rings (4Rs) are dominantly formed during the induction period, prior to the significant increase in the number of symmetric six- and eight-membered rings (6Rs and 8Rs) in the crystal growth stage. These preformed ordered 4Rs contribute to the formation of d6r and cha composite building units containing 6Rs and 8Rs with the assistance of the organic structure-directing agent, leading to the construction of embryonic zeolite crystallites, which facilitate the crystal growth through a particle attachment pathway. This work enriches the toolbox for better understanding the crystallization pathway of zeolites.

Revealing scenarios of interzeolite conversion from FAU to AEI through the variation of starting materials.

Interzeolite conversion, which refers to the synthesis of zeolites using a pre-made zeolite as the starting material, has enabled promising outcomes that could not be easily achieved by the conventional synthesis from a mixture of amorphous aluminum and silicon sources. Understanding the mechanism of interzeolite conversion is of particular interest to exploit this synthesis route for the preparation of tailor-made zeolites as well as the discovery of new structures. It has been assumed that the structural similarity between the starting zeolite and the target one is crucial to a successful interzeolite conversion. Nevertheless, an image as to how one type of zeolite evolves into another one remains unclear. In this work, a series of dealuminated FAU zeolites were created through acid leaching and employed as the starting zeolites in the synthesis of AEI zeolite under various conditions. This experimental design allowed us to create a comprehensive diagram of the interzeolite conversion from FAU to AEI as well as to figure out the key factors that enable this kinetically favourable crystallization pathway. Our results revealed different scenarios of the interzeolite conversion from FAU to AEI and pinpointed the importance of the structure of the starting FAU in determining the synthesis outcomes. A prior dealumination was proven effective to modify the structure of the initial FAU zeolite and consequently facilitate its conversion to the AEI zeolite. In addition, this strategy allowed us to directly transfer the knowledge obtained from the interzeolite conversion to a successful synthesis of the AEI zeolite from dealuminated amorphous aluminosilicate precursors. These results offer new insights to the design and fabrication of zeolites via the interzeolite conversion as well as to the understandings of the crystallization mechanisms.

Reaction Kinetics Regulated Formation of Short-Range Order in an Amorphous Matrix during Zeolite Crystallization.

The crystallization of zeolites, a disorder-to-order transformation of aluminosilicates, has not been thoroughly understood because the nucleation events in the amorphous matrix are difficult to recognize from the diverse structural changes, especially for the dense hydrogel systems. Therefore, relationships between the synthesis conditions, the generated amorphous species, and the crystallization behavior of zeolites remain unclear. Herein, by comparatively investigating the structural evolution of the aluminosilicate matrix in a dense hydrogel system when different Si reactants (fumed silica and silicate solution) are employed, we demonstrate that the reactivity of the reactants and the kinetics of the condensation reaction is critical to the formation of short-range order in an amorphous matrix, which greatly influences the nucleation frequency of zeolites. It was revealed that an amorphous solid containing plentiful Al-rich four-membered rings and Si-rich six-membered rings could be produced when fumed silica gradually reacted with sodium aluminate solution at 80 °C. It is considered that the interaction between these rings promotes the construction of the essential building units of zeolite X (FAU). In contrast, a complex aluminosilicate matrix was formed immediately when sodium silicate solution was mixed with sodium aluminate solution due to the intense condensation reaction. Furthermore, this complex matrix became more stable when the reactant mixture was hydrothermally treated at 80 °C, which significantly impedes the crystallization process. Aging the reactant mixture at ambient temperature before heating, instead, facilitated the formation of short-range order in the amorphous matrix, which increases the nucleation frequency of zeolites.

Extremely Stable Zeolites Developed via Designed Liquid-Mediated Treatment.

Improving the stability of porous materials for practical applications is highly challenging. Aluminosilicate zeolites are utilized for adsorptive and catalytic applications, wherein they are sometimes exposed to high-temperature steaming conditions (∼1000 °C). As the degradation of high-silica zeolites originates from the defect sites in their frameworks, feasible defect-healing methods are highly demanded. Herein, we propose a method for healing defects to create extremely stable high-silica zeolites. High-silica (SiO2/Al2O3 > 240) zeolites with *BEA-, MFI-, and MOR-type topologies could be stabilized by significantly reducing the number of defect sites via a liquid-mediated treatment without using additional silylating agents. Upon exposure to extremely high temperature (900-1150 °C) steam, the stabilized zeolites retain their crystallinity and micropore volume, whereas the parent commercial zeolites degrade completely. The proposed self-defect-healing method provides new insights into the migration of species through porous bodies and significantly advances the practical applicability of zeolites in severe environments.

Toward Efficient Synthesis of Chiral Zeolites: A Rational Strategy for Fluoride-Free Synthesis of STW-Type Zeolite.

The STW-type zeolite is attractive for developing novel enantioselective syntheses/separation of chiral compounds because it is the only chiral zeolitic microporous material whose enantioenriched synthesis has been achieved. In addition to the conventional industries in which zeolites are used, STW should have diverse industrial applications in the pharmaceutical and food industries. However, the toxic and caustic fluoride required for synthesizing STW severely hinders its commercialization by mass production. Herein, we report the first example of fluoride-free STW synthesis, in which the two roles of fluoride-formation of a zeolitic framework rich in tetravalent T-atoms and promotion of double 4-membered ring unit formation-were substituted by dry gel conversion and Ge addition, respectively. The STW obtained was highly crystalline, with a similar micropore volume and thermal stability as those of original fluoride-based STW. Our approach is promising not only for the fluoride-free synthesis of enantiomeric STW but also for general fluoride-free syntheses.

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.

Ultrafast Encapsulation of Metal Nanoclusters into MFI Zeolite in the Course of Its Crystallization: Catalytic Application for Propane Dehydrogenation.

Encapsulating metal nanoclusters into zeolites combines the superior catalytic activity of the nanoclusters with high stability and unique shape selectivity of the crystalline microporous materials. The preparation of such bifunctional catalysts, however, is often restricted by the mismatching in time scale between the fast formation of nanoclusters and the slow crystallization of zeolites. We herein demonstrate a novel strategy to overcome the mismatching issue, in which the crystallization of zeolites is expedited so as to synchronize it with the rapid formation of nanoclusters. The concept was demonstrated by confining Pt and Sn nanoclusters into a ZSM-5 (MFI) zeolite in the course of its crystallization, leading to an ultrafast, in situ encapsulation within just 5 min. The Pt/Sn-ZSM-5 exhibited exceptional activity and selectivity with stability in the dehydrogenation of propane to propene. This method of ultrafast encapsulation opens up a new avenue for designing and synthesizing composite zeolitic materials with structural and compositional complexity.

Bioinspired Approach to Silica Nanoparticle Synthesis Using Amine-Containing Block Copoly(vinyl ethers): Realizing Controlled Anisotropy.

Core-shell polymer-silica hybrid nanoparticles smaller than 50 nm in diameter were formed in the presence of micelles of poly(2-aminoethyl vinyl ether-block-isobutyl vinyl ether) (poly(AEVEm-b-IBVEn)) through the hydrolysis and polycondensation of alkoxysilane in aqueous solution at a mild pH and temperature. The size of the nanoparticles as well as the number and size of the core parts were effectively controlled by varying the molecular weight of the copolymers. The polymers could be removed by calcination to give hollow silica nanoparticles with Brunauer-Emmett-Teller surface areas of more than 500 m2 g-1. Among these, silica nanoparticles formed with poly(AEVE115-b-IBVE40) displayed an anisotropy of single openings in the shell. The use of an alternative copolymer, poly(AEVE-b-2-naphthoxyethyl vinyl ether) (poly(AEVE113-b-ßNpOVE40)), yielded core-shell nanoparticles with less pronounced anisotropy. These results showed that the degree of anisotropy could be controlled by the rigidity of micelles; the micelle of poly(AEVE115-b-IBVE40) was more deformable during silica deposition than that of poly(AEVE113-b-ßNpOVE40) in which aromatic interactions were possible. This bioinspired, environmentally friendly approach will enable large-scale production of anisotropic silica nanomaterials, opening up applications in the field of nanomedicine, optical materials, and self-assembly.

Insights into the ion-exchange properties of Zn(ii)-incorporated MOR zeolites for the capture of multivalent cations.

Understanding the properties of zeolites for cation exchange is important because the ion-exchange performance largely determines their suitability in applications such as catalysis and adsorptive separation. We synthesized a Zn(ii)-incorporated mordenite-framework aluminosilicate zeolite (Zn,Al-MOR), in which both Zn and Al are substituted in the framework, and studied its ion-exchange behavior for multivalent cations. For comparison, the original aluminosilicate mordenite (Al-MOR) was also synthesized with a composition adjusted to ensure that its charge density was similar to that of Zn,Al-MOR. While the incorporation of Zn(ii) led to a slower kinetic process, the selectivities and the exchange capacities toward multivalent cations (especially divalent cations) were significantly improved. Herein, we discussed the mechanism responsible for improving the ion-exchange performance in the presence of Zn(ii) and found that the incorporation of Zn(ii) led to a significant improvement in the ion-exchange temperature dependence of the MOR, which led to the ability to enhance ion-exchange capacity through temperature control during actual application. It was also revealed that the presence of Zn(ii) significantly improves selectivity and spontaneity toward the exchange of multivalent cations, Ni2+. Moreover, XRD and nitrogen-adsorption/desorption analyses revealed that the structures of the materials are maintained during the ion exchange, which is indicative of superior structural stability and tolerance to ion exchange.

Continuous flow synthesis of ZSM-5 zeolite on the order of seconds.

The hydrothermal synthesis of zeolites carried out in batch reactors takes a time so long (typically, on the order of days) that the crystallization of zeolites has long been believed to be very slow in nature. We herein present a synthetic process for ZSM-5, an industrially important zeolite, on the order of seconds in a continuous flow reactor using pressurized hot water as a heating medium. Direct mixing of a well-tuned precursor (90 °C) with the pressurized water preheated to extremely high temperature (370 °C) in the millimeter-sized continuous flow reactor resulted in immediate heating to high temperatures (240-300 °C); consequently, the crystallization of ZSM-5 in a seed-free system proceeded to completion within tens of or even several seconds. These results indicate that the crystallization of zeolites can complete in a period on the order of seconds. The subtle design combining a continuous flow reactor with pressurized hot water can greatly facilitate the mass production of zeolites in the future.

Bridging the Gap between Structurally Distinct 2D Lamellar Zeolitic Precursors through a 3D Germanosilicate Intermediate.

There is broad scientific interest in lamellar zeolitic materials for a large variety of technological applications. The traditional synthetic methods towards two-dimensional (2D) zeolitic precursors have made a great impact in the construction of families of related zeolites; however, the connection between structurally distinct 2D zeolitic precursors is much less investigated in comparison, thereby resulting in a synthetic obstacle that theoretically limits the types of zeolites that can be constructed from each layer. Herein, we report a Ge-recycling strategy for the topotactic conversion between different 2D zeolitic precursors through a three-dimensional (3D) germanosilicate. Specifically, the intermediate germanosilicate can be constructed within 150 min by taking advantage of its structural similarity with the parent lamellar precursor. This process enables the conversion of one 2D zeolite structure into another distinct structure, thus overcoming the synthetic obstacle between two families of zeolitic materials.

Synthesis of New Microporous Zincosilicates with CHA Zeolite Topology as Efficient Platforms for Ion-Exchange of Divalent Cations.

There is growing interest to develop zeolite materials capable of stabilizing divalent cations such as Cu2+ , Fe2+ , and Ni2+ for catalytic applications. Herein the synthesis of a new microporous zincosilicate with CHA zeolite topology is reported for the first time, by particularly focusing on the mixing procedures of the raw materials to prevent the precipitation of zinc oxides/hydroxides and the formation of impurity phases. The obtained zincosilicate CHA products possess remarkably higher ion-exchange ability for catalytically useful, divalent cations, demonstrated here using Ni2+ as an example, compared to that of aluminosilicate and zincoaluminosilicate analogs. It is anticipated that these zincosilicate CHA materials can be an efficient platform for several important catalytic reactions. In addition, the present finding would provide a general guideline for effective substitution of other heteroatoms into the zeolite frameworks.

Crystallization of a Novel Germanosilicate ECNU-16 Provides Insights into the Space-Filling Effect on Zeolite Crystal Symmetry.

Synthesis of new zeolites involving organic molecules relies heavily on the trial-and-error approach, because it is difficult to interpret the determining effects of organics on zeolite crystal symmetry. Here, the intrinsic relationships among the space-filling of organics, included volume of channels, and zeolite crystal symmetry, are systematically demonstrated by experimental and computational means. Under controlled conditions, the "dimer" and "monomer" organics of 1-ethyl-3-methylimidazolium selectively direct different, but related, germanosilicates, the ECNU-16 with a new topology and the existing IM-16 with the UOS topology, respectively. The comprehensive computational study reveals that the zeolite phase selectivity is determined by the unique space-filling behavior of the "dimer" and "monomer" organics, which is closely correlated to their rotation freedom, as well as the included volume of host zeolite channels. The elucidation of this crucial space-filling effect from the fundamental viewpoint will provide new guidelines for the rational design and synthesis of new zeolites in future.

Directing Aluminum Atoms into Energetically Favorable Tetrahedral Sites in a Zeolite Framework by Using Organic Structure-Directing Agents.

The Al location in zeolites can have massive influences on the zeolite properties because it directly correlates with the cationic active sites. Herein, the synthesis of IFR zeolites with controlled Al distribution at different tetrahedral sites (T sites) is reported. The computational calculations suggest that organic structure-directing agents (OSDAs) used for zeolite synthesis can alter the energetically favorable T sites for Al. Zeolite products synthesized under identical conditions but with different OSDAs are found to have altered fractions of Al at different T sites in accordance with the energies derived from the zeolite-OSDA complexes. Our finding thus provides evidence for the ability of OSDAs to direct Al into more energetically favorable T sites, thereby offering rational synthetic guidelines for the selective placement of Al into specific crystallographic sites.

Organic-Free Synthesis of a Highly Siliceous Faujasite Zeolite with Spatially Biased Q4 (nAl) Si Speciation.

We report the most siliceous FAU-type zeolite, HOU-3, prepared via a one-step organic-free synthesis route. Computational studies indicate that it is thermodynamically feasible to synthesize FAU with SAR=2-7, though kinetic factors seemingly impose a more restricted upper limit for HOU-3 (SAR≈3). Our findings suggest that a slow rate of crystallization and/or low concentration of Na+ ions in HOU-3 growth mixtures facilitate Si incorporation into the framework. Interestingly, Q4 (nAl) Si speciation measured by solid-state NMR can only be modeled with a few combinations of Al positioning at tetrahedral sites in the crystal unit cell, indicating the distribution of Si(-O-Si)4-n (-O-Al)n species is spatially biased as opposed to being random. Achieving higher SAR is desirable for improved zeolite (hydro)thermal stability and enhanced catalytic performance, which we demonstrate in benchmark tests that show HOU-3 is superior to commercial zeolite Y.

Energy Analysis of Aluminosilicate Zeolites with Comprehensive Ranges of Framework Topologies, Chemical Compositions, and Aluminum Distributions.

The contents and locations of Al in the zeolite frameworks are one of the key factors determining the physicochemical properties of zeolites. Systematic evaluation of the characteristics of zeolites with a wide variety of framework topologies, a wide range of Si/Al ratios, and various locations of Al is of great significance, but very challenging due to the limitation of the realizable ranges of Al contents in zeolites as well as the limited information on the Al locations obtained from the current analytical techniques. Here, we report the systematic analysis of the energetics of aluminosilicate zeolites with 209 existing framework topologies at different Si/Al ratios using molecular mechanics. More than 43 000 initial structures were generated to give comprehensive views of the energetics of zeolites. The results coincide well with the structural knowledge obtained experimentally. It was revealed that the relation between the relative framework energies versus the Al contents varies in accordance with the topologies, suggesting that the relative stability of zeolites depends not only on the topologies, but also on the substituting contents of Al. For particular topologies with the same Al contents, in addition, comparisons between random and specific distributions of Al showed that zeolite with Al at a particular T site is energetically more stable than those with random distributions, suggesting the inherent influences of the Al locations. The contents and locations of Al in zeolites likely have a certain preference that may reflect the range of chemical compositions, the Al distributions, and consequently the physicochemical properties of realizable aluminosilicate zeolites.

Photoinduced Bending of Self-Assembled Azobenzene-Siloxane Hybrid.

A novel azobenzene-siloxane hybrid material displaying photoinduced macroscopic motions has been prepared by one-step organosilane self-assembly. Two types of alkoxysilane precursors with either pendant or bridging azobenzene groups were synthesized via thiol-ene click reactions. Hybrid films with well-ordered lamellar structures were obtained by hydrolysis and polycondensation of these precursors. The film with solely pendant azobenzene groups showed reversible and rapid d-spacing variation upon UV-vis irradiation, which was induced by the trans-cis isomerization of azobenzene moieties. The flexible, free-standing film obtained by co-condensation of two types of precursors showed reversible bending-unbending motions upon UV-vis irradiation. The partial cross-linking between the siloxane layers by bridging azobenzene groups was crucial for photoinduced distortion of the film. This film possesses high elastic modulus, good thermal stability, and shows large amplitude of photoinduced bending-unbending over a wide temperature range. This is the first report on photoinduced macroscopic motions of azobenzene-containing siloxane-based materials. These materials possess great potential for applications in smart devices and energy conversion systems.

Structure-Directing Behaviors of Tetraethylammonium Cations toward Zeolite Beta Revealed by the Evolution of Aluminosilicate Species Formed during the Crystallization Process.

Organic structure-directing agents (OSDAs) have been widely used for the synthesis of zeolites. In most cases, OSDAs are occluded in zeolites as an isolated cation or molecule geometrically fitted within the zeolite cavities. This is not the case for zeolite beta synthesized by using tetraethylammonium (TEA(+)) cation as an OSDA, in which a cluster/aggregate of ca. six TEA(+) cations is occluded intact in the cavity (i.e., the channel intersection) of zeolite beta. The structure direction of TEA(+) in such a nontypical, clustered mode has remained elusive. Here, zeolite beta was hydrothermally synthesized using TEA(+) in the absence of other alkali metal cations in order to focus on the structure-directing behaviors of TEA(+) alone. The solid products formed throughout the hydrothermal synthesis were analyzed by an array of characterization techniques including argon adsorption-desorption, high-energy X-ray total scattering, Raman and solid-state NMR spectroscopy, and high-resolution transmission electron microscopy. It was revealed that the formation of amorphous TEA(+)-aluminosilicate composites and their structural, chemical, and textural evolution toward the amorphous zeolite beta-like structure during the induction period is vital for the formation of zeolite beta. A comprehensive scheme of the formation of zeolite beta is proposed paying attention to the clustered behavior of TEA(+) as follows: (i) the formation of the TEA(+)-aluminosilicate composites after heating, (ii) the reorganization of aluminosilicates together with the conformational rearrangement of TEA(+), yielding the formation of the amorphous TEA(+)-aluminosilicate composites with the zeolite beta-like structure, (iii) the formation of zeolite beta nuclei by solid-state reorganization of such zeolite beta-like, TEA(+)-aluminosilicate composites, and (iv) the subsequent crystal growth. It is anticipated that these findings can provide a basis for broadening the utilization of OSDAs in the clustered mode of structure direction in more effective ways.