Keiji Itabashi

A Working Hypothesis for Broadening Framework Types of Zeolites in Seed-Assisted Synthesis without Organic Structure-Directing Agent

Recent research has demonstrated a new synthesis route to useful zeolites such as beta, RUB-13, and ZSM-12 via seed-assisted, organic structure-directing agent (OSDA)-free synthesis, although it had been believed that these zeolites could be essentially synthesized with OSDAs. These zeolites are obtained by adding seeds to the gels that otherwise yield other zeolites; however, the underlying crystallization mechanism has not been fully understood yet. Without any strategy, it is unavoidable to employ a trial-and-error procedure for broadening zeolite types by using this synthesis method. In this study, the effect of zeolite seeds with different framework structures is investigated to understand the crystallization mechanism of zeolites obtained by the seed-assisted, OSDA-free synthesis method. It has been found that the key factor in the successful synthesis of zeolites in the absence of OSDA is the common composite building unit contained both in the seeds and in the zeolite obtained from the gel after heating without seeds. A new working hypothesis for broadening zeolite types by the seed-assisted synthesis without OSDA is proposed on the basis of the findings of the common composite building units in zeolites. This hypothesis enables us to design the synthesis condition of target zeolites. The validity of the hypothesis is experimentally tested and verified by synthesizing several zeolites including ECR-18 in K-aluminosilicate system.

Formation of Hierarchically Organized Zeolites by Sequential Intergrowth

Hierarchically organized porous materials can provide multidimensional spatial networks on different length scales with improved characteristics relevant to molecular diffusion. Zeolites that are microporous crystalline materials having pores and channels at molecular dimensions are of great importance for industrial applications. However, the presence of only micropores in zeolite frameworks often limits the molecular diffusion and therefore, restricts the transport of bulky molecules. This problem can be resolved by shortening the effective diffusion path lengths, which has been achieved by miniaturizing zeolite crystals, delaminating or exfoliating layered zeolites, synthesizing zeolite nanosheets, and introducing mesopores into zeolite particles. Among these solutions, the fabrication of hierarchical zeolites with microand mesoporosity is of interest because it combines intrinsic micropores with bypass-interconnected mesopores, and therefore, enhances both the micropore accessibility and molecular traffic within zeolite particles. Hierarchical zeolites have been produced using several techniques, including top-down desilication by alkali postsynthetic treatment and bottom-up directed assembly by hard or soft templates. The hard-template method requires multistep procedures and is therefore unfavorable for large-scale production. Alternatively, the direct introduction of organic mesopore-generating agents (mesoporogens) during zeolite synthesis can create uniform mesopores. The use of such mesoporogens is currently one of the most promising methods for the single-step construction of hierarchical zeolites. Progress has been made using well-designed mesoporogens composed of long hydrophobic chains and hydrophilic zeolitic structure-directing groups to generate zeolites with tunable mesoporosity and to direct the hierarchical assembly of zeolite nanosheets, yielding mesoporous zeolites with house-of-cards-like structures. These hierarchical nanosheets showed excellent catalytic performance in several important reactions because the presence of thin layers with specific crystalline faces facilitates catalysis at the exteriors or pore mouths. Such mesoporogens are probably necessary for the direct, singlestep synthesis of hierarchical zeolites. Herein we report an alternative, mesoporogen-free approach for the construction of hierarchically organized MFI zeolites by sequential intergrowth using a simple organic structure-directing agent (OSDA). The selection of an appropriate OSDA and optimized synthesis conditions that can form plate-like zeolites with enhanced 908 rotational intergrowths seems to be a key to achieving a hierarchical structure with three classes of porosity in one structure: the intrinsic microporosity of the zeolite framework together with mesoporosity existing within the zeolite plates and macroporosity stemming from the complex intergrown structure. Epitaxial and rotational intergrowths are commonly observed in many zeolites. We have hypothesized that by engineering the zeolite intergrowths, hierarchically organized zeolites can be constructed without the need for mesoporogens. In particular, we have focused on the MFI zeolite because it is an excellent catalyst in many industrial processes and a promising material for membrane separation. MFI zeolite that contains sinusoidal channels along the a axis interconnected with straight channels along the b axis is often formed with 908 rotational intergrowths, in which substantial (h00) faces are epitaxially overgrown on the (0k0)

Synthesis and characteristic properties of siliceous mordenite

Abstract In the present paper we report a synthesis of siliceous mordenite without using any organic bases or seeds for crystallization, and correlation between SAR, X-ray diffraction patterns and characteristic adsorption properties.

Crystal structure of synthetic mordenites

Abstract Crystal structures of synthetic sodium mordenites with different Si Al ratios have been studied by X-ray diffraction methods. Framework geometries of the synthetic mordenites are very similar to those of natural mordenites. Atomic positions and occupancies of extraframework cation and water molecules of synthetic mordenites with different Si Al ratios have been determined.

Seed‐Assisted, One‐Pot Synthesis of Hollow Zeolite Beta without Using Organic Structure‐Directing Agents

Hollow aluminosilicate zeolite beta was successfully synthesized by adding CIT-6, that is, zincosilicate zeolite, which has the same topology as beta, as seeds to the Na-aluminosilicate gel without the need for organic structure-directing agents. One important factor in the successful organic structure-directing agent (OSDA)-free synthesis of hollow beta crystals is the solubility of the seed crystals in alkaline media. CIT-6 was less stable than aluminosilicate zeolite beta in alkaline media and the solubility changed depending on whether the crystals were calcined or not. The hollow beta could be obtained by using the uncalcined CIT-6 seed crystals. The volumes of intra-crystalline voids were tuned by changing the reaction time and the initial gel compositions, such as the SiO2/Al2O3 and Na2O/SiO2 ratios. We estimated that the intra-crystalline voids were formed through the dissolution of the seed crystals, just after the crystal growth of new beta on the outer surface of the seeds. In addition, new crystal growth toward inside of the void was also observed by TEM. On the basis of the characterization data, such as chemical analysis, N2-adsorption/desorption measurements, and TEM observation, a formation mechanism of the intra-crystalline voids is proposed and discussed.

Direct hydrothermal synthesis and stabilization of high-silica mordenite (Si∶Al = 25) using tetraethylammonium and fluoride ions

The direct crystallization of high-silica mordenite (MOR) zeolite using both tetraethylammonium and fluoride ions was investigated by varying the primary variables of the hydrothermal synthesis such as Si∶Al and H2O∶SiO2 ratios in the starting synthesis mixture. Under well-optimized conditions, highly crystalline MOR zeolites with a Si∶Al ratio of approximately 25 were successfully prepared. From evaluation of the thermal stabilities of the MOR zeolites obtained, it was also found that there is a good relationship between the thermal stability and the bulk Si∶Al ratio of MOR zeolites and that any NaF remaining in the zeolite crystals considerably reduces the thermal stability.

Crystallization of single phase (K, Na)-clinoptilolite

Abstract A single phase (K, Na)-clinoptilolite was hydrothermally crystallized without seed crystals from a reactant mixture of (K, Na)-aluminosilicate gel slurry through homogeneous mixing at 150 °C for 144 h. Compositions of the reactant mixtures and reaction temperatures to obtain the clinoptilolite were restricted within narrow limits in the case of syntheses without seed crystals, while the compositions and temperatures were expanded into wide ranges in the case of syntheses with 1 wt.-% of seed crystals. Coexistence of Na+- and K+-ions with appropriate ratios in the reactant mixture, and homogeneous mixing of this mixture, were indispensable for the crystallization of clinoptilolite. The crystals obtained were characterized by X-ray diffraction, chemical analysis, thermal analysis (DTA/TG), electron microscopy, and electron diffraction. The crystals were assigned as clinoptilolite, not heulandite, based on their chemical compositions and thermal stability.

Clear Aqueous Nuclei Solution for Faujasite Synthesis

Pure zeolite Y could be synthesized in a short crystallization period with agitation by adding the clear aqueous nuclei solution to a reaction mixture. Effects of addition of the nuclei solution on crystallization of zeolite Y, and physical and chemical properties of that solution were investigated by X-ray diffraction, 29 si and 27 Al NMR and adsorption measurements. It was shown that zeolitic nuclei with the faujasite structure were formed from soluble aluminosilicate species in the clear nuclei solution and the nuclei grew up, agglomerated and precipitated as an amorphous solid (by XRD) with ultra-fine crystallites. The remarkable effect of the nuclei solution was attributed to the micro size of nuclei and a large amount of those compared with seed crystals. These results supported the hypothesis that zeolite crystallization was originated from liquid phase aluminosilicates.

Seed‐Assisted Synthesis of MWW‐Type Zeolite with Organic Structure‐Directing Agent‐Free Na‐Aluminosilicate Gel System

The seed-assisted synthesis of zeolites without using organic structure-directing agents (OSDAs) has enabled alternative routes to the simple, environmentally friendly and low-cost production of industrially important zeolites. In this study, the successful seed-assisted synthesis of MCM-22 (MWW-type) zeolite with an OSDA-free gel is reported for the first time. MWW-type zeolites are obtained by the addition of as-synthesized MCM-22 seeds prepared with hexamethyleneimine (HMI) into OSDA-free Na-aluminosilicate gels. Based on the results of XRD, ICP-AES, NMR, N2 physisorption and NH3 -TPD, the product exhibited different features compared to those of the seeds. The H-form product can serve as a catalyst in Friedel-Crafts alkylation reaction of anisole with 1-phenylethanol, and its catalytic activity is comparable to the seeds. Furthermore, XRD, FE-SEM, TG-DTA, CHN, FT-IR and NMR analyses of products and intermediates provide insights into the role of seeds and occluded HMI, the crystallization process, and key factors for achieving seed-assisted synthesis of MWW-type zeolites with an OSDA-free gel system. The present results provide a new perspective for the economical and environmentally friendly production of MWW-type zeolites.

Ultratrace Measurement of Acetone from Skin Using Zeolite: Toward Development of a Wearable Monitor of Fat Metabolism

Analysis of gases emitted from human skin and contained in human breath has received increasing attention in recent years for noninvasive clinical diagnoses and health checkups. Acetone emitted from human skin (skin acetone) should be a good indicator of fat metabolism, which is associated with diet and exercise. However, skin acetone is an analytically challenging target because it is emitted in very low concentrations. In the present study, zeolite was investigated for concentrating skin acetone for subsequent semiconductor-based analysis. The adsorption and desorption characteristics of five zeolites with different structures and those hydrophobicities were compared. A hydrophobic zeolite with relatively large pores (approximately 1.6 times larger than the acetone molecule diameter) was the best concentrator of skin acetone among the zeolites tested. The concentrator developed using zeolite was applied in a semiconductor-based gas sensor in a simulated mobile environment where the closed space was frequently collapsed to reflect the twisting and elastic movement of skin that would be encountered in a wearable device. These results could be used to develop a wearable analyzer for skin acetone, which would be a powerful tool for preventing and alleviating lifestyle-related diseases.