Cooperative Reformable Channel System with Unique Recognition of Gas Molecules in a Zeolitic Imidazolate Framework with Multilevel Flexible Ligands

We report a cooperative reformable channel system in a ZIF-L crystal arising from coexistence of three types of local flexible ligands. The reformable channel is able to regulate permeation of a nonspherical guest molecule, such as N₂ or CO₂, based on its longer molecular dimension, which is in a striking contrast to conventional molecular sieves that regulate the shorter cross-sectional dimension of the guest molecules. Our density functional theory (DFT) calculations reveal that the guest molecule induces dynamic motion of the flexible ligands, leading to the channel reformation, and then the guest molecule reorientates itself to fit in the reformed channel. Such a unique “induced fit-in” mechanism causes the gas molecule to pass through six-membered-ring windows in the <i>c</i>- crystal direction of ZIF-L with its longer axis parallel to the window plane. Our experimental permeance of N₂ through the ZIF-L membranes is about three times greater than that of CO₂, supporting the DFT simulation predictions

Bifunctional Polymer Hydrogel Layers As Forward Osmosis Draw Agents for Continuous Production of Fresh Water Using Solar Energy

The feasibility of bilayer polymer hydrogels as draw agent in forward osmosis process has been investigated. The dual-functionality hydrogels consist of a water-absorptive layer (particles of a copolymer of sodium acrylate and <i>N</i>-isopropylacrylamide) to provide osmotic pressure, and a dewatering layer (particles of <i>N</i>-isopropylacrylamide) to allow the ready release of the water absorbed during the FO drawing process at lower critical solution temperature (32 °C). The use of solar concentrated energy as the source of heat resulted in a significant increase in the dewatering rate as the temperature of dewatering layer increased to its LSCT more rapidly. Dewatering flux rose from 10 to 25 LMH when the solar concentrator increased the input energy from 0.5 to 2 kW/m². Thermodynamic analysis was also performed to find out the minimum energy requirement of such a bilayer hydrogel-driven FO process. This study represents a significant step forward toward the commercial implementation of hydrogel-driven FO system for continuous production of fresh water from saline water or wastewaters

Hydrophilic Nanowire Modified Polymer Ultrafiltration Membranes with High Water Flux

Germanate nanowires/nanorods with different lengths were synthesized and used as additives for the fabrication of polymer composite membranes for high-flux water filtration. We for the first time demonstrated that at a small nanowire/nanorod loading (e.g., <0.5 wt % on the basis of poly­(ether sulfone)), the length of germinate nanowires was a key parameter in determining their migration and diffusion in the polymer solution, and thus affecting polymer precipitation in the membrane formation process. In particular, short Ca₂Ge₇O₁₆ nanowires with an average length of 138.7 nm and an average diameter of 12.7 nm, and Zn₂GeO₄ nanorods with an average length of 400 nm and an average diameter of 18.7 nm quickly diffused out of the membrane, leading to a higher pore density on the active layer in comparison with the pristine membranes. The addition of short Ca₂Ge₇O₁₆ nanowires resulted in greater pore sizes than the addition of Zn₂GeO₄ nanorods because the out-diffusion of the former was faster than that of the latter. In contrast, the addition of long Ca₂Ge₇O₁₆ nanowires with lengths of several tens to hundreds of micrometers and an average of 27.3 nm was not effective in promoting the pore formation because of partial embedment of nanowires. Poly­(ether sulfone) composite membranes prepared by adding a small amount of Zn₂GeO₄ nanorods exhibited dramatically enhanced water permeation without losing rejection property. For example, the poly­(ether sulfone) (PES) composite membrane prepared with 0.3 wt % Zn₂GeO₄ nanorods exhibited the highest flux, 1294.5 LMH, which was 3.5 times of the pristine (PES) membrane (384.2 LMH). Our work provides a new strategy for developing high-performance ultrafiltration membranes for practical industrial filtration applications

High-Performance Ionic Diode Membrane for Salinity Gradient Power Generation

Salinity difference between seawater and river water is a sustainable energy resource that catches eyes of the public and the investors in the background of energy crisis. To capture this energy, interdisciplinary efforts from chemistry, materials science, environmental science, and nanotechnology have been made to create efficient and economically viable energy conversion methods and materials. Beyond conventional membrane-based processes, technological breakthroughs in harvesting salinity gradient power from natural waters are expected to emerge from the novel fluidic transport phenomena on the nanoscale. A major challenge toward real-world applications is to extrapolate existing single-channel devices to macroscopic materials. Here, we report a membrane-scale nanofluidic device with asymmetric structure, chemical composition, and surface charge polarity, termed ionic diode membrane (IDM), for harvesting electric power from salinity gradient. The IDM comprises heterojunctions between mesoporous carbon (pore size ∼7 nm, negatively charged) and macroporous alumina (pore size ∼80 nm, positively charged). The meso-/macroporous membrane rectifies the ionic current with distinctly high ratio of ca. 450 and keeps on rectifying in high-concentration electrolytes, even in saturated solution. The selective and rectified ion transport furthermore sheds light on salinity-gradient power generation. By mixing artificial seawater and river water through the IDM, substantially high power density of up to 3.46 W/m² is discovered, which largely outperforms some commercial ion-exchange membranes. A theoretical model based on coupled Poisson and Nernst–Planck equations is established to quantitatively explain the experimental observations and get insights into the underlying mechanism. The macroscopic and asymmetric nanofluidic structure anticipates wide potentials for sustainable power generation, water purification, and desalination

Supramolecular Self-Assembly Induced Adjustable Multiple Gating States of Nanofluidic Diodes

Artificial nanochannels, inheriting smart gating functions of biological ion channels, promote the development of artificial functional nanofluidic devices for high-performance biosensing and electricity generation. However, gating states of the artificial nanochannels have been mainly realized through chemical modification of the channels with responsive molecules, and their gating states cannot be further regulated once the nanochannel is modified. In this work, we employed a new supramolecular layer-by-layer (LbL) self-assembly method to achieve reversible and adjustable multiple gating features in nanofluidic diodes. Initially, a self-assembly precursor was modified into a single conical nanochannel, then host molecule-cucurbit[8]­uril (CB[8]) and guest molecule, a naphthalene derivative, were self-assembled onto the precursor through an LbL method driven by host-enhanced π–π interaction, forming supramolecular monolayer or multilayers on the inner surface of the channel. These self-assemblies with different layer numbers possessed remarkable charge effects and steric effects, exhibiting a capability to regulate the surface charge density and polarity, the effective diameter, and the geometric asymmetry of the single nanochannel, realizing reversible gating of the single nanochannel among multiple rectification and ion-conduction states. As an example of self-assembly of supramolecular networks in nanoconfinements, this work provides a new approach for enhancing functionalities of artificial nanochannels by LbL supramolecular self-assemblies. Meanwhile, since the host molecule, CB[8], used in this work can interact with different kinds of biomolecules and stimuli-responsive chemical species, this work can be further extended to build a novel stable multiple-state research platform for a variety of uses such as sensing and controllable release

Aqueous Phase Synthesis of ZIF‑8 Membrane with Controllable Location on an Asymmetrically Porous Polymer Substrate

In this study, we have demonstrated a simple, scalable, and environmentally friendly route for controllable fabrication of continuous, well-intergrown ZIF-8 on a flexible polymer substrate via contra-diffusion method in conjunction with chemical vapor modification of the polymer surface. The combined chemical vapor modification and contra-diffusion method resulted in controlled formation of a thin, defect-free, and robust ZIF-8 layer on one side of the support in aqueous solution at room temperature. The ZIF-8 membrane exhibited propylene permeance of 1.50 × 10^–8 mol m^–2 s^–1 Pa⁻¹ and excellent selective permeation properties; after post heat-treatment, the membrane showed ideal selectivities of C₃H₆/C₃H₈ and H₂/C₃H₈ as high as 27.8 and 2259, respectively. The new synthesis approach holds promise for further development of the fabrication of high-quality polymer-supported ZIF membranes for practical separation applications

Crystal Transformation in Zeolitic-Imidazolate Framework

The phase transformation of a zinc-2-methylimidazole-based zeolitic-imidazolate framework (ZIF), from a recently discovered ZIF-L to ZIF-8, was reported. ZIF-L is made up of the same building blocks as ZIF-8, having two-dimensional crystal lattices stacked layer-by-layer. Results indicated that the phase transformation occurs in the solid phase via the geometric contraction model (R2), a kinetic model new to ZIF. The phase transformation was monitored by means of ex situ powder X-ray diffraction, nitrogen sorption, Fourier transform infrared spectroscopy, selected-area electron diffraction, scanning electron microscopy, and in situ nuclear magnetic resonance spectroscopy. This work also demonstrates the first topotactic phase transformation in porous ZIFs, from a 2D layered structure to a 3D structure, and provides a new insight into metal–organic framework crystallization mechanisms

Carbon Nanotube Networks as Nanoscaffolds for Fabricating Ultrathin Carbon Molecular Sieve Membranes

Carbon molecular sieve (CMS) membranes have shown great potential for gas separation owing to their low cost, good chemical stability, and high selectivity. However, most of the conventional CMS membranes exhibit low gas permeance due to their thick active layer, which limits their practical applications. Herein, we report a new strategy for fabricating CMS membranes with a 100 nm-thick ultrathin active layer using poly­(furfuryl alcohol) (PFA) as a carbon precursor and carbon nanotubes (CNTs) as nanoscaffolds. CNT networks are deposited on a porous substrate as nanoscaffolds, which guide PFA solution to effectively spread over the substrate and form a continuous layer, minimizing the penetration of PFA into the pores of the substrate. After pyrolysis process, the CMS membranes with 100–1000 nm-thick active layer can be obtained by adjusting the CNT loading. The 322 nm-thick CMS membrane exhibits the best trade-off between the gas permeance and selectivity, a H₂ permeance of 4.55 × 10^–8 mol m^–2 s^–1 Pa^–1, an O₂ permeance of 2.1 × 10^–9 mol m^–2 s^–1 Pa^–1, and an O₂/N₂ ideal selectivity of 10.5, which indicates the high quality of the membrane produced by this method. This work provides a simple, efficient strategy for fabricating ultrathin CMS membranes with high selectivity and improved gas flux

Rapid Construction of ZnO@ZIF‑8 Heterostructures with Size-Selective Photocatalysis Properties

To selectively remove heavy metal from dye solution, inspired by the unique pore structure of ZIF-8, we developed a synthetic strategy for rapid construction of ZnO@ZIF-8 heterostructure photocatalyst for selective reduction of Cr­(VI) between Cr­(VI) and methylene blue (MB). In particular, ZnO@ZIF-8 core–shell heterostructures were prepared by in situ ZIF-8 crystal growth using ZnO colloidal spheres as template and zinc source within 8–60 min. The shell of the resulting ZnO@ZIF-8 core–shell heterostructure with a uniform thickness of around 30 nm is composed of ZIF-8 crystal polyhedrons. The concentration of organic ligand 2-methylimidazole (Hmim) was found to be crucial for the formation of ZnO@ZIF-8 core–shell heterostructures. Different structures, ZnO@ZIF-8 core–shell spheres and separate ZIF-8 polyhedrons could be formed by altering Hmim concentration, which significantly influences the balance between rate of Zn²⁺ release from ZnO and coordinate rate. Importantly, such ZnO@ZIF-8 core–shell heterostructures exhibit size-selective photocatalysis properties due to selective adsorption and permeation effect of ZIF-8 shell. The as-synthesized ZnO@ZIF-8 heterostructures exhibited enhanced selective reduction of Cr­(VI) between Cr­(VI) and MB, which may find application in the dye industry. This work not only provides a general route for rapid fabrication of such core–shell heterostructures but also illustrates a strategy for selectively enhanced photocatalysis performance by utilizing adsorption and size selectivity of ZIF-8 shell