Natsumi Kamiya

Determining the structure of a benzene7.2-silicalite-1 zeolite using a single-crystal X-ray method.

An orthorhombic benzene-silicalite-1 single crystal was obtained from a monoclinic twin crystal, and the structure was determined by a single-crystal method for the first time.

Final storage of radioactive cesium by pollucite hydrothermal synthesis

The Fukushima nuclear accident has highlighted the importance of finding a better final storage method for radioactive cesium species. Cs is highly soluble in water, and can easily exchange with other alkali ions in zeolites or clays to form stable complexes. However, Cs+ is released from Cs+ complexes into water when surrounded by an excess of water. Pollucite may be the best final storage option for Cs+, but its typical synthesis requires heating to about 1200°C in air. Here, we show that the hydrothermal synthesis of pollucite can be completed at 300°C in three hours from any zeolite or clay. Furthermore, our procedure does not require ion exchange before synthesis. Radioactive Cs is usually found in complexes with clays. At that time, this method only requires calcium hydroxide, water, and three hours of hydrothermal synthesis, so the process is both inexpensive and practical for large-scale application. Pollucite is an analog of analcime zeolite, and contains a channel system 2.8 Å in diameter, which is formed by 6-oxygen rings. As the diameter of Cs+ is 3.34 Å and each Cs+ exists independently within a separate portion of the channel, Cs+ cannot exit the pollucite framework without breaking it.

Large-quantity single crystal synthesis of TPA-ZSM-5 using KOH as a mineralizer

Single crystals of TPA-ZSM-5 were synthesized using KOH as a mineralizer instead of NaOH. The size of the TPA-ZSM-5 crystals after 120 h was approximately 100 μm. When KOH was used, the quantity of TPA-ZSM-5 crystals formed was increased, and Al distribution in the crystal was uniform. The form and quality of these crystals were suitable for single crystal X-ray diffraction. Nucleation of the analcime by-product, which is formed in the presence of NaOH, is inhibited when KOH is used, because the potassium ion does not act as a template for the analcime structure. Instead, zeolite W was observed as a by-product. However, compared to analcime, the amount of zeolite W was lower, and it was easier to separate from TPA-ZSM-5. Thus, potassium ions play an important role in the production of large quantities of high-quality ZSM-5 with a uniform Al distribution.

Crystal structure of pollucite

Abstract The crystal structure of pollucite (ANA) (Cs,Na)AlSi2O6 is characterized by single-crystal X-ray diffractometry. The space group C2/c, monoclinic, is found to be the most probable space group for this mineral based on the systematic absences of reflections. Cesium ions or water molecules are disordered at the center of a channel and are slightly apart from the special position. This is the origin of symmetry breaking and the space group becomes lower from Ia-3d (analcime) to C2/c. Sodium ions or vacancies are located at the contact point of the channel. No ordering of aluminum is observed in the pollucite structure.

Location of CO2on silicalite-1 zeolite using a single-crystal X-ray method

Abstract The location of physisorbed CO2 molecules on silicalite-1 zeolite (MFI-type) is determined using a single-crystal X-ray method. There are four sorption sites for CO2 molecules in the three-dimensional channel system of the silicalite-1. Two of them are in the straight channel in disorder (STR1 and STR2), while the other sites are in the sinusoidal channel (SIN) and at the intersection (INT). Their occupancy factors indicate that a large portion of the CO2 molecules are located in the straight channel. It is also revealed that the straight channel is the most favorable for CO2 molecules as sorption sites.

Adsorption Process of CO2 on Silicalite-1 Zeolite Using Single-Crystal X-ray Method

The process of CO2 adsorption on silicalite-1 zeolite (MFI-type) is revealed using a single-crystal X-ray method. The structure of CO2-silicalite-1 with a small amount of CO2 in the pore is determined, wherein most of CO2 molecules are located in the straight channel. It indicates the straight channel is the most stable sorption site based on the van der Waals interactions between the CO2 and the framework, and the CO2 molecules initially adsorb in the straight channel in the adsorption process. This is the first report to describe the structure of MFI-type zeolites with the adsorbate molecules occupying only the straight channel.

Simple method for preparing monoclinic single crystals of zeolite ZSM-5 and analysis of their structure

Abstract A simple method for preparing monoclinic single crystals of ZSM-5 is developed. The ZSM-5 crystals are pressed with a weight of 50 g at an angle of 45° relative to the b and c crystallographic axes of twin ZSM-5 crystals while the temperature is simultaneously increased to 573 K. The temperature is then slowly reduced to 313 K, and these steps are repeated three times. The monoclinic structure of the single crystals of ZSM-5 produced using this method is then determined.

Reanalysis of CO 2 -silicalite-1 structure as monoclinic twinning

Abstract Structural analysis of CO2-silicalite-1 crystals using the single-crystal X-ray diffraction method reveals that the crystal system is twinned monoclinic. The structure of silicalite-1 (MFI-type zeolites) loaded with CO2 has previously been solved in orthorhombic system [1], but the R value is still large. Since CO2 molecules are too small to maintain the orthorhombic structure, it is considered that the CO2-silicalite-1 crystals should be in the relaxed monoclinic twin phase. The diffraction data in the previous report is recalculated in P21/n as the monoclinic twinning.

Adsorption structure of dimethyl ether on silicalite-1 zeolite determined using single-crystal X-ray diffraction

The most stable sorption site of dimethyl ether on silicalite-1 is the sinusoidal channel. The configuration of guest molecules (linear or bent) plays an important role in determining where the stable sorption site is situated.

Entrance and diffusion pathway of CO2 and dimethyl ether in silicalite‐1 zeolite channels as determined by single‐crystal XRD structural analysis

The entrance and diffusion pathway of CO2 and dimethyl ether (DME) in MFI-type zeolite channels were investigated by a selective sealing method using large silicalite-1 crystals. The MFI-type zeolite has two kinds of orthogonal channels: straight channels and sinusoidal channels. The mouths of the straight channels are on (010) crystal faces, while those of the sinusoidal channels are on (100) faces. The channel mouths are directly sealed by silicone resin on the (100) and (010) faces so as to restrict the entrance and diffusion pathways to straight and sinusoidal channel pathways, respectively. The locations and loadings of the guest CO2 and DME molecules are determined by single-crystal X-ray diffraction structural analysis. The loadings show the difference of the adsorption rates between the pathways. The straight channel pathway is 4.2 times faster than the sinusoidal channel pathway for the CO2, and the sinusoidal channel pathway is 5.1 times faster than the straight channel pathway for the DME. It reveals their dominant pathways and the anisotropy of adsorption. The dominant pathway correlates to the stability of the channel as adsorption sites.