Effect of defect-healing treatment on layered silicate precursors toward well-defined crosslinked frameworks.

The synthesis of zeolites from two-dimensional layered precursors through interlayer crosslinking of the layers is a promising avenue for realizing meticulously designed synthesis routes. However, the presence of defective silanol species in the precursors hinders the achievement of desirable synthesis outcomes. This study focuses on PREFER-a layered precursor for FER-type zeolites-which was synthesized and subjected to a liquid-mediated defect-healing treatment that we recently developed. The defect-healing process involves the use of fluoride compounds for reconstruction and organic pore fillers to stabilize the framework. The effects of the treatment on the structure, composition, and iron insertion behavior of PREFER were examined. Characterization results revealed a reduction in the number of intralayer silanol defects, whereas interlayer silanols were unaffected by the defect-healing treatment. Furthermore, the subsequent alterations observed in the crosslinking behavior with iron atoms indicated that the defect-healing treatment may enhance the insertion of iron species between the layers in more homogeneous environments compared with the untreated precursor. These findings provide valuable insights into the prospects of controlled interlayer linkage in two-dimensional zeolite materials.

Synthesis and Structural Analysis of High-Silica ERI Zeolite with Spatially-Biased Al Distribution as a Promising NH3-SCR Catalyst.

Erionite (ERI) zeolite has recently attracted considerable attention for its application prospect in the selective catalytic reduction of NOx with NH3 (NH3-SCR), provided that the high-silica (Si/Al > 5.5) analog with improved hydrothermal stability can be facilely synthesized. In this work, ERI zeolites with different Si/Al ratios (4.6, 6.4, and 9.1) are synthesized through an ultrafast route, and in particular, a high-silica ERI zeolite with a Si/Al ratio of 9.1 is obtained by using faujasite (FAU) as a starting material. The solid-state 29Si MAS NMR spectroscopic study in combination with a computational simulation allows for figuring out the atomic configurations of the Al species in the three ERI zeolites. It is revealed that the ERI zeolite with the highest Si/Al ratio (ERI-9.1, where the number indicates the Si/Al ratio) exhibits a biased Al occupancy at T1 site, which is possibly due to the presence of a higher fraction of the residual potassium cations in the can cages. In contrast, the Al siting in ERI-4.6 and ERI-6.4 proves to be relatively random.

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.

Drastic Photoemission Color Alternation from a Single Molecule as a Starting Material Introduced in Acid-Treated Zeolites: From Pure Blue to White.

Since high-purity blue- and white-light emitters are an indispensable group of materials for the creation of next-generation optical devices, a number of light-emitting materials have been developed from both inorganic and organic synthetic chemistry. However, these synthetic chemical methods are far from the perspective of green chemistry due to the multistep synthetic process and the use of toxic reagents and elements. Herein, we demonstrate that the introduction of simple unsubstituted anthracenes into zeolite-like pores can create a wide variety of luminescent materials, from ultrapure blue luminescent materials (emission peak at 465 nm with a full width of half-maximum of 8.57 nm) to efficient white luminescent materials [CIE coordination at (0.31, 0.33) with a quantum efficiency of 11.0% under 350 nm excitation light]. The method for rational design of the luminescent materials consists of the following two key strategies: one is molecular orbital confinement of the anthracene molecules in the zeolite nanocavity for regulating the molecular coordination associated with photoexcitation and emission and the other is the interaction of unsubstituted anthracenes with extra-framework aluminum species to stabilize the 2-dehydride anthracene cation in the zeolite cavity.

Atom-Selective Analyses Reveal the Structure-Directing Effect of Cs Cation on the Synthesis of Zeolites.

To understand the crystallization mechanism of zeolites, it is important to clarify the detailed role of the structure-directing agent, which is essential for the crystallization of zeolite, interacting with an amorphous aluminosilicate matrix. In this study, to reveal the structure-directing effect, the evolution of the aluminosilicate precursor which causes the nucleation of zeolite is analyzed by the comprehensive approach including atom-selective methods. The results of total and atom-selective pair distribution function analyses and X-ray absorption spectroscopy indicate that a crystalline-like coordination environment gradually forms around Cs cations. This corresponds to the fact that Cs is located at the center of the d8r units in the RHO structure whose unit is unique in this zeolite, and a similar tendency is also confirmed in the ANA system. The results collectively support the conventional hypothesis that the formation of the crystalline-like structure before the apparent nucleation of the zeolite.

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.

Dealumination of small-pore zeolites through pore-opening migration process with the aid of pore-filler stabilization.

Small-pore zeolites are gaining increasing attention owing to their superior catalytic performance. Despite being critical for the catalytic activity and lifetime, postsynthetic tuning of bulk Si/Al ratios of small-pore zeolites has not been achieved with well-preserved crystallinity because of the limited mass transfer of aluminum species through narrow micropores. Here, we demonstrate a postsynthetic approach to tune the composition of small-pore zeolites using a previously unexplored strategy named pore-opening migration process (POMP). Acid treatment assisted by stabilization of the zeolite framework by organic cations in pores is proven to be successful for the removal of Al species from zeolite via POMP. Furthermore, the dealuminated AFX zeolite is treated via defect healing, which yields superior hydrothermal stability against severe steam conditions. Our findings could facilitate industrial applications of small-pore zeolites via aluminum content control and defect healing and could elucidate the structural reconstruction and arrangement processes for inorganic microporous materials.

Exploring Hydrothermal Synthesis of SAPO-18 under High Hydrostatic Pressure.

The effect of external hydrostatic pressure on the hydrothermal synthesis of the microporous silicoaluminophosphate SAPO-18 has been explored. The crystallization of the SAPO-18 phase is inhibited at 150 °C under high pressures (200 MPa) when using relatively diluted synthesis mixtures. On the contrary, the use of concentrated synthesis mixtures allowed SAPO-18 to be obtained in all the studied conditions. The obtained solids were characterized with XRD, SEM, ICP-AES, TG and 27Al and 31P MAS NMR spectroscopy. The results highlight the importance of the external pressure effect on the hydrothermal synthesis of molecular sieves and its influence on the interaction between the organic molecule and the silicoaluminophosphate network.

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.

No more trial and error for zeolites.

A data-intensive approach scours known organics for synthesizing targeted zeolites.

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.

Recent progress in the improvement of hydrothermal stability of zeolites.

Zeolites have been successfully employed in many catalytic reactions of industrial relevance. The severe conditions required in some processes, where high temperatures are frequently combined with the presence of steam, highlight the need of considering the evolution of the catalyst structure during the reaction. This review attempts to summarize the recently developed strategies to improve the hydrothermal framework stability of zeolites.

Synthetic and natural MOR zeolites as high-capacity adsorbents for the removal of nitrous oxide.

N2O is typically present as a trace gas in chemical processes, but its emission causes serious environmental issues. We herein demonstrate that ion-exchanged mordenite zeolites (framework code: MOR) can exhibit high capacities for N2O adsorption under ambient conditions. In particular, a natural MOR zeolite gives an adsorption capacity as high as 0.34 mmol-N2O per g-zeolite (1 atm, 25 °C), representing the best performing material among all zeolite-based adsorbents reported so far. The results contribute toward a comprehensive understanding of the structure-activity relationship and offer insights to establishing a zeolite-based adsorption system for enriching or removing N2O.

Aryl radical initiators accumulated within layered silicates realize polystyrene with directly and regioselectively bonded aryl-terminal groups.

The arbitrary design of a terminal group of polymers exploits the still-veiled functions of polymers with potential for application in fields such as drug delivery systems, photonics, and energy conversions. Here we demonstrate for the first time that polystyrenes with directly and regioselectively bonded aryl-terminal groups can be obtained via styrene radical polymerization initialized by arbitrary aryl radicals accumulated within the interlayer space of smectite clay minerals, which can be prepared by our developed 'Clay Catalysed ab intra Deamination (CCD)' method.

Tracking the crystallization behavior of high-silica FAU during AEI-type zeolite synthesis using acid treated FAU-type zeolite.

During AEI zeolite synthesis using acid treated FAU (AcT-FAU), we found the recrystallization of high-silica FAU with high crystallinity and Si/Al ratio of 6.1 using N,N-dimethyl-3,5-dimethylpiperidinium hydroxide (DMDMPOH) after 2 h, followed by the crystallization of AEI via FAU-to-AEI interzeolite conversion at a longer synthesis time. In order to understand the formation mechanism of high-silica FAU and generalize its direct synthesis, we have investigated this synthesis process. An analysis of the short-range structure of AcT-FAU revealed that it has an ordered aluminosilicate structure having a large fraction of 4-rings despite its low crystallinity. The changes in the composition of the products obtained at different synthesis times suggested that DMDMP+ plays a certain role in the stabilization of the FAU zeolite framework. Moreover, the results of thermogravimetric analysis showed that the thermal stability of DMDMP+ changed with the zeolite conversion. To the best of our knowledge, this is the first study to clarify the structure-directing effect of DMDMP+ on FAU zeolite formation.

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.

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.

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.

Superior Ion-exchange Property of Amorphous Aluminosilicates Prepared by a Co-precipitation Method.

The development of inexpensive inorganic ion-exchangers for the purification of environmental pollutants is a social demand. Amorphous aluminosilicates with a relatively high homogeneous Al environment are prepared by a feasible co-precipitation method, i. e., mixing an acidic aluminum sulfate solution and basic sodium silicate solution, which exhibit excellent ion-exchange selectivity for Cs+ and Sr2+ . The Kd value for Sr2+ was comparable with that of zeolite 4A. The local structures and ion-exchange behavior of the amorphous aluminosilicates are systematically investigated. The ion-exchange property of the amorphous aluminosilicates can be tuned by changing the interaction between the exchangeable cation and the amorphous aluminosilicates. Also, the amorphous aluminosilicates can adsorb bulky cations that zeolites hardly adsorb due to the limitation of the miropore size of zeolites.