Naixin Wang

Coordination-Driven In Situ Self-Assembly Strategy for the Preparation of Metal–Organic Framework Hybrid Membranes†

Metal-organic frameworks (MOFs) have emerged as porous solids of a superior type for the fabrication of membranes. However, it is still challenging to prepare a uniformly dispersed robust MOF hybrid membrane. Herein, we propose a simple and powerful strategy, namely, coordination-driven in situ self-assembly, for the fabrication of MOF hybrid membranes. On the basis of the coordination interactions between metal ions and ligands and/or the functional groups of the organic polymer, this method was confirmed to be feasible for the production of a stable membrane with greatly improved MOF-particle dispersion in and compatibility with the polymer, thus providing outstanding separation ability. As an experimental proof of concept, a high-quality ZIF-8/PSS membrane was fabricated that showed excellent performance in the nanofiltration and separation of dyes from water.

Nanodisperse ZIF-8/PDMS hybrid membranes for biobutanol permselective pervaporation

Alcohol-permselective membranes may play an increasingly important role in bioalcohol production. Developments for this membrane mostly involve hybrid membranes. Obtaining high compatibility and nanodispersion of inorganic nanoparticles in the polymer matrix is the key to fabricating hybrid membranes with high pervaporation performance. In this study, a homogeneous, nanodisperse ZIF-8/PDMS membrane was prepared by repeated immersion of a polysulfone supporting membrane in a dilute ZIF-8/PDMS suspension and subsequent removal of defects using a concentrated PDMS solution. To improve the nanoscale dispersion of ZIF-8, the nascent ZIF-8 suspension was directly dispersed in a PDMS solution without drying. This procedure avoids aggregation and redispersion of ZIF-8 nanoparticles after forming a powder. Analyses confirmed that the ZIF-8/PDMS dispersion effectively diminished aggregation between nanoparticles and led to the formation of a well-dispersed ZIF-8/PDMS membrane. A homogeneous and thin ZIF-8/PDMS permselective layer was obtained by adjusting the preparation conditions. The prepared ZIF-8/PDMS membrane exhibited a high separation factor (52.81) and high flux (2800.5 g m−2 h−1) in the separation of 5.0 wt% n-butanol–water solution at 80 °C. By comparing the powder-dispersed ZIF-8/PDMS hybrid membrane with the suspension-dispersed ZIF-8/PDMS membrane, we found that the latter showed much higher performance in butanol separation. Therefore, the nanodisperse ZIF-8/PDMS membrane has great potential applications for in situ recovery of biobutanol.

Covalent crosslinked polyelectrolyte complex membrane with high negative charges towards anti-natural organic matter fouling nanofiltration

Removal of natural organic matter (NOM) from drinking water by membrane technology is attracting increasing attention. However, the fouling of the membrane by NOM is one of the biggest obstacles restricting its widespread application. Therefore an anti-NOM fouling polyelectrolyte complex (PEC) membrane was obtained by creating a negatively charged multilayer on a polyacrylonitrile (PAN) supporting membrane using a layer-by-layer assembly method. To improve the stability of the PEC membrane, the electrostatically assembled (poly(ethyleneimine)/poly(sodium 4-styrenesulfonate))n/PAN membranes were crosslinked by glutaraldehyde. It was found that the zeta potential of the membrane surface decreased after chemical crosslinking, which further improved the electrostatic repulsion to NOM and thus improved the anti-NOM fouling property. Results of a 30 day nanofiltration operation showed the crosslinked membrane had good stability and gave a higher rejection of NOM; the permeance of the crosslinked membrane was double that of the uncrosslinked membrane.

Nanoconfined Zeolitic Imidazolate Framework Membranes with Composite Layers of Nearly Zero Thickness

The key to preparing dense composite membranes is reducing the thickness of the composite layer with stable separation performance. Herein, we report a nanoconfined composite membrane prepared by in situ growth of Zeolitic Imidazolate Framework (ZIF) nanocrystals in the nanoporous layer of the substrate via a fine-tuning contra-diffusion method. The thickness of the composite layer on the membrane surface was nearly zero. The formed ZIF nanoconfined composite membranes showed state-of-art flux and high stability in removing dyes from water. This new strategy is expected to offer great opportunities for the potential practical application of polymer-supported metal-organic framework (MOF) composite membranes.

Designing superhydrophobic surfaces with SAM modification on hierarchical ZIF-8/polymer hybrid membranes for efficient bioalcohol pervaporation

Inspired by the complementary roles of surface energy and roughness on natural nonwetting surfaces, a superhydrophobic surface has been successfully designed and prepared by self-assembled monolayer modification on a hierarchical ZIF-8/polymer hybrid membrane. The as-prepared membrane exhibited the best overall performance for n-butanol pervaporation.

Nanostructure array assisted aggregation-based growth of a Co-MOF-74 membrane on a Ni-foam substrate for gas separation

Metal-organic frameworks (MOFs) are promising candidates for membrane-based gas separation due to their regular crystalline lattices with relatively well-defined pore structures. To date, the preparation of integrated MOF membranes grown on porous substrates is still a major challenge. Herein, a facile aggregation-based growth method is developed to prepare a Co-MOF-74 (referred to as Co2(dobdc), H4dobdc = 2,5-dihydroxyterephthalic acid) membrane on a nanostructure array modified nickel (Ni) foam substrate. Ni3S2 nanorod arrays directly grown from the Ni-foam substrate can aggregate and effectively regulate the growth of Co-MOF-74 crystals along a given direction. The effect of reaction time on membrane growth is discussed. The resulting Co-MOF-74 membrane presents good H2 permeance and H2/CO2 selectivity.

Self-modified fabrication of inner skin ZIF-8 tubular membranes by a counter diffusion assisted secondary growth method

The ability to fabricate high-quality MOF-based membranes is critical and highly desired due to their outstanding performance in molecule separations and sensors. In this work, a water stable MOF, ZIF-8 is chosen as the typical example to validate a new self-modified counter diffusion assisted secondary growth method for the fabrication of inner skin tubular MOF membranes. The preparation route involves two main steps: counter diffusion for modifying the rough surface of industrial ceramic substrate (plugging pores in the ceramic tube) with homogenous ZIF-8 nanocrystals, and the secondary solvothermal growth for fabricating continuous ZIF-8 membranes on the inner surface of the tubular substrate. The ZIF-8 membranes prepared by this method are characterized in detail. The pre-modification process and the use of a low-reactivity growth solution containing zinc acetate favor the growth of dense ZIF-8 membrane in the solvothermal process.

Tuning molecular sieving channels of layered double hydroxides membrane with direct intercalation of amino acids

Two-dimensional (2D) materials have been widely used to construct efficient molecular sieving and transport channels in membrane separation. However, controlling the architecture and properties of the separation channel is still a great challenge. In this study, we propose a facile and feasible approach to prepare layered double hydroxide (LDH) composite membranes on tubular ceramic substrates by using amino acids (glycine, serine and alanine) as intercalation molecules. The interlayer spacing formed in the stacked LDH nanosheets provides transport channels for water molecules, and is modified through in situ intercalation of amino acid molecules with different molecular size and hydrophilic properties. The amino acid-intercalated LDH membranes exhibited high permeance without sacrificing rejection compared with pristine LDH membranes. Particularly, the glycine-intercalated LDH composite membrane with expanded hydrophilic transport channels shows a permeance of 566 L m−2 h−1 MPa−1 and a rejection of about 98.5% towards Eriochrome Black T (EBT) molecules. The permeance is 2.2-fold higher than that of the pristine LDH membrane. Moreover, molecular simulation was also used to interpret the intercalation and separation mechanism of the LDH membranes. We anticipate that this study may extend the materials and methods to regulate and control the transport channels in membranes.

A vertically channeled lamellar membrane for molecular sieving of water from organic solvents

Molecular sieve-based membranes are considered to be promising for achieving both high selectivity and permeability. Layered double hydroxides (LDHs) are typical two-dimensional crystalline compounds with highly uniform interlayer galleries that can be used to construct efficient molecular transport pathways. Herein, vertically channeled laminates are constructed on a tubular alumina substrate using CoAl–LDH as building blocks. Good-quality laminates are crystallized by direct nucleation and growth onto the alumina, in which alumina substrates act as both reactants and supports. Vertically aligned interlayer galleries are formed as sieving and transport channels for water molecules with the oriented nucleation effect of NH4F. Compared with the zigzag pathways formed by two dimensional materials parallel to the substrate, the straight vertical channels have higher molecular transfer efficiency. As a result, the robust CoAl–LDH membranes show much higher and more stable organic solvent dehydration performance than most membranes under a wide range of feed conditions. This work thus demonstrates that LDH-based composite membranes are highly promising as the next generation of membranes for molecular sieving.