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The micropore volumes and effective pore sizes of two types of silicalite-1 membranes were compared with those of a typical silicalite-1 powder. The silicalite-1 membrane with fewer grain boundaries in the membrane layer showed similar micropore volume and effective pores size to those of the silicalite-1 powder. In contrast, when the silicalite-1 membrane contained many grain boundaries, relatively small micropore volume and effective pore size were observed, suggesting that narrowing and obstruction of the micropore would occur along grain boundaries due to the disconnection of the zeolite pore. The silicalite-1 membrane with fewer grain boundaries exhibited relatively high permeation properties for C6-C8 hydrocarbons. There was an over 50-fold difference in benzene permeance between these two types of membranes. We concluded that it is important to reduce grain boundaries and improve pore-connectivity to develop an effective preparation method for obtaining a highly permeable membrane.

This study investigated the permeation behaviors of n-hexane and 2-methylpentane through two-types of silicalite-1 membranes that have different pore-connectivity. The permeation mechanisms of these hydrocarbons were able to be explained by the adsorption–diffusion model. In addition, the fluxes through silicalite-1 membranes could be expressed by the modified Fick’s first law. The hydrocarbon fluxes through S-1S with better pore-connectivity were ca. 3–20 times larger than those through S-1M with poor pore-connectivity. For these membranes with different pore-connectivity, the activation energy of diffusion of n-hexane was 17.5 kJ mol−1 for the membrane with better pore-connectivity and 18.0 kJ mol−1 for the membrane with poorer pore-connectivity, whereas for 2-methylpentane it was 17.9 and 33.0 kJ mol−1, respectively. We concluded that the pore-connectivity in silicalite-1 membrane significantly influences the molecular diffusivities.

Silicalite-1 membrane was prepared on an outer surface of a tubular α-alumina support by a secondary growth method. Changes of defect amount and crystallinity during secondary growth were carefully observed. The defect-less well-crystallized silicalite-1 membrane was obtained after 7-days crystallization at 373 K. The silicalite-1 membrane became (h0l)-orientation with increasing membrane thickness, possibly because the orientation was attributable to “evolutionally selection”. The (h0l)-oriented silicalite-1 membrane showed high p-xylene separation performance for a xylene isomer mixture (o-/m-/p-xylene = 0.4/0.4/0.4 kPa). The p-xylene permeance through the membrane was 6.52 × 10−8 mol m−2 s−1 Pa−1 with separation factors αp/o, αp/m of more than 100 at 373 K. As a result of microscopic analysis, it was suggested that a very thin part in the vicinity of surface played as effective separation layer and could contribute to high permeation performance.

We demonstrated high-resolution imaging of atomic columns in zeolite frameworks with spherical aberration-corrected transmission electron microscopy (AC-HRTEM). An MFI-type zeolite was observed by AC-HRTEM with optimized optical setup. Compared with the conventional imaging mode based on a positive spherical aberration, the negative spherical aberration imaging (NCSI) mode sharpened image contrasts at atomic column positions. The projected atomic columns of zeolite are so complex that sharp image contrast can help to distinguish each atomic column position.

The catalytic behavior of various noble metal nanoparticles (NPs) supported directly on multiwalled carbon nanotubes (MWCNTs) was observed using environmental transmission electron microscopy (E-TEM). Gasification of the MWCNTs via catalytic hydrogenation or oxidation progressed at ∼450°C in conjunction with certain noble metal NP catalysts at the interface between MWCNTs and the NPs. During such catalytic reactions, the NPs were observed to move rapidly over the MWCNT surfaces. The mobility and wettability of the NPs varied depending on the particular metal NPs employed and the ambient atmosphere. While rhodium NPs exhibited high wettability under both hydrogen and oxygen atmospheres, the wettability of platinum, palladium and iridium NPs on MWCNTs varied with the atmosphere. The metal NPs seemed to have high degrees of crystallinity while their morphologies fluctuated throughout the catalytic reactions. [ABSTRACT FROM AUTHOR]

In situ transmission electron microscopy (TEM) was performed to observe the hydrogenation of Mg-Ni films in a hydrogen atmosphere of 80-100 Pa. An aberration-corrected environmental TEM with a differential pumping system allows us to reveal the Angstrom-scale structure of the films in the initial stage of hydrogenation: first, nucleation and growth of Mg2NiH4 crystals with a lattice spacing of 0.22 nm in an Mg-rich amorphous matrix of the film occurs within 20 s after the start of the high-resolution observation, then crystallization of MgH2 with a smaller spacing of 0.15 nm happens after approximately 1 min. Our in situ TEM method is also applicable to the analysis of other hydrogen-related materials. [ABSTRACT FROM AUTHOR]

Zeolites have potential application as ion-exchangers, catalysts and molecular sieves. Zeolites are once again drawing attention in Japan as stable adsorbents and solidification materials of fission products, such as 137Cs+ from damaged nuclear-power plants. Although there is a long history of scientific studies on the crystal structures and ion-exchange properties of zeolites for practical application, there are still open questions, at the atomic-level, on the physical and chemical origins of selective ion-exchange abilities of different cations and detailed atomic structures of exchanged cations inside the nanoscale cavities of zeolites. Here, the precise locations of Cs+ ions captured within A-type zeolite were analyzed using high-resolution electron microscopy. Together with theoretical calculations, the stable positions of absorbed Cs+ ions in the nanocavities are identified, and the bonding environment within the zeolitic framework is revealed to be a key factor that influences the locations of absorbed cations. [ABSTRACT FROM AUTHOR]

Abstract  Oriented ZSM-5 seed crystals on an α-Al2O3 porous substrate were hydrothermally treated in a raw sol. The ZSM-5 membranes were fabricated via secondary growth of the seed crystals. For some samples, the seed-applied substrate was fired at 300 or 600 °C before the secondary growth in order to enhance adhesion between the substrate and the seed crystals. The influence of the firing on the subsequent secondary growth of the seed crystals was examined by XRD, SEM, and TEM. The TEM images of the sample fired at 300 °C showed that the resulting membrane was continuous, and in the membrane, large ZSM-5 particles were distributed in a porous matrix. The ZSM-5 particles were slightly smaller than the used seed crystals. HR-TEM observations showed that the porous matrix is comprised of ZSM-5 micro-crystals, and the part adjacent to the large ZSM-5 crystals has the same crystallographic orientation as the large crystals. The TEM images of the sample fired at 600 °C showed that the resulting layer is comprised of particles with a core-shell structure. The core consisted of ZSM-5 micro-crystals, whereas the shell was composed of large ZSM-5 rod-like crystals. It is inferred that the formation of these interesting microstructures is related to the degradation of the template agent, NPr4OH, in the seed crystals by firing at 300 and 600 °C. The part including no template is dissolved by a hydrothermal treatment, and the dissolved species is re-crystallized via reaction with a template agent in the used raw sol, resulting in the formation of interesting microstructures. [ABSTRACT FROM AUTHOR]

The article looks at a study which demonstrate a postsynthetic approach to tune the composition of small-pore zeolites using a previously unexplored strategy named pore-opening migration process (POMP). It mentions acid treatment assisted by stabilization of zeolite framework by organic cations in pores is proven to be successful for removal of Al species from zeolite via POMP. It also mentions small-pore zeolites are gaining increasing attention owing to their superior catalytic performance.

The article discusses a study on hydrogen separation from methane and light hydrocarbons for clean energy applications. Results showed that the graphene-wrapped-membrane fouling index (G-MFI) membranes can efficiently separate hydrogen with high selectivity but with 7-day stability and readiness for industrial applications. Also noted is the methodology's possible extension to the design of novel separation membranes for any target gas which can lead toward green technologies.