<i>In Situ</i> Synthesis of Thin Zeolitic–Imidazolate Framework ZIF‑8 Membranes Exhibiting Exceptionally High Propylene/Propane Separation

Metal–organic frameworks (MOFs) are a class of hybrid porous crystalline materials comprising of metal centers coordinated to organic linkers. Owing to their well-defined pores and cavities in the scale of molecules combined with abundant surface chemistry, MOFs offer unprecedented opportunities for a wide range of applications including membrane-based gas separations. It is not straightforward (often requiring multiple steps) to prepare membranes of MOFs due to the fact that the heterogeneous nucleation and growth of MOF crystals on porous supports are not generally favored. Furthermore, the performance of polycrystalline MOF membranes strongly depends on the membrane microstructure, in particular, the grain boundary structure. Here we report a simple one step <i>in situ</i> method based on a counter-diffusion concept to prepare well-intergrown ZIF-8 membranes with significantly enhanced microstructure, resulting in exceptionally high separation performance toward propylene over propane

Hot Electrons Generated from Doped Quantum Dots via Upconversion of Excitons to Hot Charge Carriers for Enhanced Photocatalysis

We show that hot electrons exhibiting the enhanced photocatalytic activity in H₂ production reaction can be efficiently generated in Mn-doped quantum dots via the “upconversion” of the energy of two excitons into the hot charge carriers. The sequential two-photon-induced process with the long-lived Mn excited state serving as the intermediate state is considered as the pathway generating hot electrons. H₂ production rate from doped quantum dots is significantly higher than that from undoped quantum dots and also exhibited the quadratic increase with the light intensity, demonstrating the effectiveness of the hot electrons produced in doped quantum dots in photocatalytic reaction. Due to the very long lifetime of Mn excited state (∼6 ms) in the doped quantum dots, the sequential two-photon excitation requires relatively low excitation rates readily achievable with a moderately concentrated solar radiation, demonstrating their potential as an efficient source of hot electrons operating at low excitation intensities

Heteroepitaxially Grown Zeolitic Imidazolate Framework Membranes with Unprecedented Propylene/Propane Separation Performances

Propylene/propane separation is one of the most challenging separations, currently achieved by energy-intensive cryogenic distillation. Despite the great potential for energy-efficient membrane-based separations, no commercial membranes are currently available due to the limitations of current polymeric materials. Zeolitic imidazolate framework, ZIF-8, with the effective aperture size of ∼4.0 Å, has been shown to be very promising for propylene/propane separation. Despite the extensive research on ZIF-8 membranes, only a few reported ZIF-8 membranes have displayed good propylene/propane separation performances presumably due to the challenges of controlling the microstructures of polycrystalline membranes. Here we report the first well-intergrown membranes of ZIF-67 (Co-substituted ZIF-8) by heteroepitaxially growing ZIF-67 on ZIF-8 seed layers. The ZIF-67 membranes exhibited impressively high propylene/propane separation capabilities. Furthermore, when a tertiary growth of ZIF-8 layers was applied to heteroepitaxially grown ZIF-67 membranes, the membranes exhibited unprecedentedly high propylene/propane separation factors of ∼200 possibly due to enhanced grain boundary structure

Time-Dependent Ni²⁺-Ion Exchange in Zeolites Y (FAU, Si/Al = 1.56) and Their Single-Crystal Structures

Seven single crystals of fully dehydrated Ni²⁺-exchanged zeolite Y (Si/Al = 1.56) were prepared via cation exchange of Na₇₅–Y (|Na₇₅|[Si₁₁₇Al₇₅O₃₈₄]-FAU) by flowing 0.05 M aqueous solutions of Ni­(NO₃)₂·6H₂O (pH 4.9 and 294 K) with various ion-exchange times. Their crystal structures were completely determined by single-crystal synchrotron X-ray diffraction techniques in cubic space group <i>Fd</i>3̅<i>m</i> at 100(1) K. In all seven structures, Ni²⁺ ions occupy sites I, I′, and II, and sometimes site II′ or a second site II, or both, preferring site I; residual Na⁺ ions in crystals 1 and 2 are found at site III and/or a second site III. The level of Ni²⁺ exchange monotonically increased from 75.2 to 89.6% [from 28.2(2) to 33.6(8) Ni²⁺ ions per unit cell] with increasing exchange time until 18 h. The dealumination of the zeolite frameworks was observed in the center of sodalite cavities after 24 h, suggesting that this process occurs during ion exchange, or subsequent dehydration. H⁺ ions are present in all seven crystals for a charge balance. Both the unit cell constants and Na⁺ contents decreased with increasing levels of Ni²⁺ exchange and ion-exchange time

Selective Removal of Radioactive Cesium from Nuclear Waste by Zeolites: On the Origin of Cesium Selectivity Revealed by Systematic Crystallographic Studies

Selective ion-exchange with zeolites has been considered as one of the most promising means to remove a radioactive isotope of cesium, ¹³⁷Cs, present in low concentration in seawater. However, there has been no report on the fundamental structure–property relation of zeolite-based Cs ion-exchangers. In this study, we investigate the origin of the selectivity of the radioactive cesium isotope in zeolite frameworks using zeolite A (LTA) as a model system. We prepared seven single crystals of fully dehydrated and partially cesium exchanged Zeolite A (LTA) with different Cs⁺/Na⁺ ratios. Their single-crystal synchrotron X-ray diffraction experiments revealed the significant differences in the degree of exchange and the site selectivity of Cs⁺ ions depending on the initial Cs⁺ concentrations in given ion exchange solutions. The degree of Cs⁺-ion exchange increases from 15.8 to 44.2% as the initial Cs⁺ concentration increases and the Na⁺ content decreases. In addition, it was found that Cs⁺ ions are energetically preferred and occluded in the center of eight-oxygen rings. With this finding, we tested the Cs adsorption capacity of pure zeolite Rho which has much more eight-oxygen rings than zeolite A along with commercial faujasite-type zeolite and titanosilicate from deionized water and seawater. Zeolite Rho showed significantly better performance on the Cs removal in the presence of high salt contents (i.e., seawater) than faujasite-type zeolite and titanosilicate