Efficient metal ion sieving in rectifying subnanochannels enabled by metal-organic frameworks.

Biological ion channels have remarkable ion selectivity, permeability and rectification properties, but it is challenging to develop artificial analogues. Here, we report a metal-organic framework-based subnanochannel (MOFSNC) with heterogeneous structure and surface chemistry to achieve these properties. The asymmetrically structured MOFSNC can rapidly conduct K+, Na+ and Li+ in the subnanometre-to-nanometre channel direction, with conductivities up to three orders of magnitude higher than those of Ca2+ and Mg2+, equivalent to a mono/divalent ion selectivity of 103. Moreover, by varying the pH from 3 to 8 the ion selectivity can be tuned further by a factor of 102 to 104. Theoretical simulations indicate that ion-carboxyl interactions substantially reduce the energy barrier for monovalent cations to pass through the MOFSNC, and thus lead to ultrahigh ion selectivity. These findings suggest ways to develop ion selective devices for efficient ion separation, energy reservation and power generation.

Incorporation of Homochirality into a Zeolitic Imidazolate Framework Membrane for Efficient Chiral Separation.

Homochiral metal-organic frameworks (MOFs) have gained much attention because of their chiral properties and disposition for chiral separation. However, the fabrication of high-quality homochiral MOF membranes remains challenging because of the difficulty in controlling growth of MOF membranes with chiral functionalities. A homochiral zeolitic imidazolate framework-8 (ZIF-8) membrane is reported for efficient chiral separation. The membrane is synthesized by incorporating a natural amino acid, l-histidine (l-His), into the framework of ZIF-8. The homochiral l-His-ZIF-8 membrane exhibits a good selectivity for the R-enantiomer of 1-phenylethanol over the S-enantiomer, showing a high enantiomeric excess value up to 76 %.

Thermoresponsive Amphoteric Metal-Organic Frameworks for Efficient and Reversible Adsorption of Multiple Salts from Water.

Regenerable, high-efficiency salt sorption materials are highly desirable for water treatment. Here, a thermoresponsive, amphoteric metal-organic framework (MOF) material is reported that can adsorb multiple salts from saline water at room temperature and effectively release the adsorbed salts into water at elevated temperature (e.g., 80 °C). The amphoteric MOF, integrated with both cation-binding carboxylic groups and anion-binding tertiary amine groups, is synthesized by introducing a polymer with tertiary amine groups into the cavities of a water-stable MOF such as MIL-121 with carboxylic groups inside its frameworks. The amphoterized MIL-121 exhibits excellent salt adsorption properties, showing stable adsorption-desorption cycling performances and high LiCl, NaCl, MgCl2 , and CaCl2 adsorption capacities of 0.56, 0.92, 0.25, and 0.39 mmol g-1 , respectively. This work provides a novel, effective strategy for synthesizing new-generation, environmental-friendly, and responsive salt adsorption materials for efficient water desalination and purification.

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.

Ultrafast selective transport of alkali metal ions in metal organic frameworks with subnanometer pores.

Porous membranes with ultrafast ion permeation and high ion selectivity are highly desirable for efficient mineral separation, water purification, and energy conversion, but it is still a huge challenge to efficiently separate monatomic ions of the same valence and similar sizes using synthetic membranes. We report metal organic framework (MOF) membranes, including ZIF-8 and UiO-66 membranes with uniform subnanometer pores consisting of angstrom-sized windows and nanometer-sized cavities for ultrafast selective transport of alkali metal ions. The angstrom-sized windows acted as ion selectivity filters for selection of alkali metal ions, whereas the nanometer-sized cavities functioned as ion conductive pores for ultrafast ion transport. The ZIF-8 and UiO-66 membranes showed a LiCl/RbCl selectivity of ~4.6 and ~1.8, respectively, which are much greater than the LiCl/RbCl selectivity of 0.6 to 0.8 measured in traditional porous membranes. Molecular dynamics simulations suggested that ultrafast and selective ion transport in ZIF-8 was associated with partial dehydration effects. This study reveals ultrafast and selective transport of monovalent ions in subnanometer MOF pores and opens up a new avenue to develop unique MOF platforms for efficient ion separations in the future.

Periodic oscillation of ion conduction of nanofluidic diodes using a chemical oscillator.

Periodic ion-conduction oscillation of biological ion channels in a living system is essential for numerous life processes. Here we report an oscillatory nanofluidic system that can self-regulate its ion-conduction states under constant conditions. The oscillatory nanofluidic system is constructed by integrating a chemical oscillator into an artificial single nanochannel system. Oscillating chemical reactions of the pH oscillator carried out inside the nanochannel are used to switch the surface properties of the channel between highly and lowly charged states, thus realizing an autonomous, continuous and periodic oscillation of the ion conductance of the channel between high and low ion-conduction states. The ion-conduction switching is characterized by the periodic ion current oscillation of the nanochannel measured under constant conditions. The oscillation period of the nanofluidic devices decreased gradually with increasing the working temperature. This study is a potential step toward the ability to directly convert chemical energy to ion-conduction oscillation in nanofluidics. On the basis of these findings, we believe that a variety of artificial oscillatory nanofluidic systems will be achieved in future by integrating artificial functional nanochannels with diverse oscillating chemical reactions.

Hydrothermal Synthesis of Metal-Polyphenol Coordination Crystals and Their Derived Metal/N-doped Carbon Composites for Oxygen Electrocatalysis.

Cobalt (or iron)-polyphenol coordination polymers with crystalline frameworks are synthesized for the first time. The crystalline framework is formed by the assembly of metal ions and polyphenol followed by oxidative self-polymerization of the organic ligands (polyphenol) during hydrothermal treatment in alkaline condition. As a result, such coordination crystals are even partly stable in strong acid (such as 2 m HCl). The metal (Co or Fe)-natural abundant polyphenol (tannin) coordination crystals are a renewable source for the fabrication of metal/carbon composites as a nonprecious-metal catalyst, which show high catalytic performance for both oxygen reduction reaction and oxygen evolution reaction. Such excellent performance makes metal-polyphenol coordination crystals an efficient precursor to fabricate low-cost catalysts for the large-scale application of fuel cells and metal-air batteries.

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.

A Versatile Iron-Tannin-Framework Ink Coating Strategy to Fabricate Biomass-Derived Iron Carbide/Fe-N-Carbon Catalysts for Efficient Oxygen Reduction.

The conversion of biomass into valuable carbon composites as efficient non-precious metal oxygen-reduction electrocatalysts is attractive for the development of commercially viable polymer electrolyte membrane fuel-cell technology. Herein, a versatile iron-tannin-framework ink coating strategy is developed to fabricate cellulose-derived Fe3 C/Fe-N-C catalysts using commercial filter paper, tissue, or cotton as a carbon source, an iron-tannin framework as an iron source, and dicyandiamide as a nitrogen source. The oxygen reduction performance of the resultant Fe3C/Fe-N-C catalysts shows a high onset potential (i.e. 0.98 V vs the reversible hydrogen electrode (RHE)), and large kinetic current density normalized to both geometric electrode area and mass of catalysts (6.4 mA cm(-2) and 32 mA mg(-1) at 0.80 V vs RHE) in alkaline condition. This method can even be used to prepare efficient catalysts using waste carbon sources, such as used polyurethane foam.

Zeolitic Imidazolate Framework/Graphene Oxide Hybrid Nanosheets as Seeds for the Growth of Ultrathin Molecular Sieving Membranes.

A defect-free zeolitic imidazolate framework-8 (ZIF-8)/graphene oxide (GO) membrane with a thickness of 100 nm was prepared using two-dimensional (2D) ZIF-8/GO hybrid nanosheets as seeds. Hybrid nanosheets with a suitable amount of ZIF-8 nanocrystals were essential for producing a uniform seeding layer that facilitates fast crystal intergrowth during membrane formation. Moreover, the seeding layer acts as a barrier between two different synthesis solutions, and self-limits crystal growth and effectively eliminates defects during the contra-diffusion process. The resulting ultrathin membranes show excellent molecular sieving gas separation properties, such as with a high CO2 /N2 selectivity of 7.0. This 2D nano-hybrid seeding strategy can be readily extended to the fabrication of other defect-free and ultrathin MOF or zeolite molecular sieving membranes for a wide range of separation applications.

Metal-organic framework membranes fabricated via reactive seeding.

A facile reactive seeding (RS) method was developed for the preparation of continuous MOF membranes on alumina porous supports, in which the porous support acted as the inorganic source reacting with the organic precursor to grow a seeding layer.