Shulan Ji

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.

Simultaneous Spray Self‐Assembly of Highly Loaded ZIF‐8–PDMS Nanohybrid Membranes Exhibiting Exceptionally High Biobutanol‐Permselective Pervaporation

The ability to obtain a maximum loading of inorganic nanoparticles while maintaining uniform dispersion in the polymer is the key to the fabrication of mixed-matrix membranes with high pervaporation performance in bioalcohol recovery from aqueous solution. Herein, we report the simultaneous spray self-assembly of a zeolitic imidazolate framework (ZIF)-polymer suspension and a cross-linker/catalyst solution as a method for the fabrication of a well-dispersed ZIF-8-PDMS nanohybrid membrane with an extremely high loading. The ZIF-8-PDMS membrane showed excellent biobutanol-permselective pervaporation performance. When the ZIF-8 loading was increased to 40 wt%, the total flux and separation factor could reach 4846.2 g m(-2) h(-1) and 81.6, respectively, in the recovery of n-butanol from 1.0 wt% aqueous solution (80 °C). This new method is expected to have serious implications for the preparation of defect-free mixed-matrix membranes for many applications.

Surface-Modification of Poly(dimethylsiloxane) Membrane with Self-Assembled Monolayers for Alcohol Permselective Pervaporation

The use of self-assembled monolayers (SAMs) has recently been recognized as an effective way to tailor the surface properties of films used in various applications. However, application of SAMs in the preparation of separation membranes remains unexplored. In the present study, surface-modified poly(dimethylsiloxane) (PDMS) membranes were prepared using SAMs to fabricate a membrane for use in pervaporation separation of ethanol/water mixtures. A cross-linked PDMS/polysulfone (PSf) composite membrane was transformed by introducing hydroxyl functionalities on the PDMS surface through a UV/ozone conversion process. (Tridecafluoroctyl)triethoxysilane was allowed to be adsorbed on the resulting Si-OH substrate to increase the hydrophobicity of the membrane. Results from Fourier transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectrometry, atomic force microscopy, and contact angle analyses suggest that the fluoroalkylsilane monolayer was successfully formed on the modified PDMS/PSf membrane treated by 60 min UV/ozone exposure. The newly SAM-modified membrane exhibited a separation factor of 13.1 and a permeate flux of 412.9 g/(m(2) h), which are higher than those obtained from PDMS membranes.

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.

Tubular ceramic-based multilayer separation membranes using spray layer-by-layer assembly

An automatically controlled spray-layer-by-layer (LbL) assembly method was used to build multilayers onto a tubular ceramic macroporous substrate. Poly(ethyleneimine) (PEI) and poly(acrylic acid) (PAA) were alternately sprayed onto a rotating tubular ceramic substrate. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analyses of different locations on the substrate confirmed that a fine coating formed on both the top surface and in the pores of the macroporous ceramic substrate. The substrate pretreatment using water filling greatly influenced the self-assembly process, slowing the penetration of the polyelectrolyte into the substrate pores, resulting in different multilayer morphologies. The effects of the substrate pretreatment, number of bilayers, and the addition of salt into the polyelectrolyte solution on the dye rejection performance were investigated. We found that the substrate pretreatment and the addition of NaCl to the polyelectrolyte solution could change the multilayer structure, in turn affecting the separation performance of the composite membranes. The composite membrane consisting of 5 layers of PEI/PAA had a flux of ∼10.0 kg m−2 h−1 and a rejection of >99%. Moreover, spray-LbL assembly using different polymer pairs such as poly(diallyldimethylammonium chloride)/poly(styrenesulfonate) (PDDA/PSS) and PEI/TiBisLac/PAA on vertically held tubular Al2O3 substrates was also explored. Using these materials led to the formation of different types of polymeric multilayers and in situ nanohybrid multilayers. Our study demonstrates that a multilayer membrane can be successfully prepared on a 3D support by an automated spray system, and can easily be used to rapidly coat large areas.

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.

Natural organic matter fouling behaviors on superwetting nanofiltration membranes

Nanofiltration has been widely recognized as a promising technology for the removal of micro-molecular organic components from natural water. Natural organic matter (NOM), a very important precursor of disinfection by-products, is currently considered as the major cause of membrane fouling. It is necessary to develop a membrane with both high NOM rejection and anti-NOM fouling properties. In this study, both superhydrophilic and superhydrophobic nanofiltration membranes for NOM removal have been fabricated. The fouling behavior of NOM on superwetting nanofiltration membranes has been extensively investigated by using humic acid (HA) as the model foulant. The extended Derjaguin-Landau-Verwey-Overbeek approach and nanoindentor scratch tests suggested that the superhydrophilic membrane had the strongest repulsion force to HA due to the highest positive total interaction energy (ΔG(TOT)) value and the lowest critical load. Excitation emission matrix analyses of natural water also indicated that the superhydrophilic membrane showed resistance to fouling by hydrophobic substances and therefore high removal thereof. Conversely, the superhydrophobic membrane showed resistance to fouling by hydrophilic substances and therefore high removal capacity. Long-term operation suggested that the superhydrophilic membrane had high stability due to its anti-NOM fouling capacity. Based on the different anti-fouling properties of the studied superwetting membranes, a combination of superhydrophilic and superhydrophobic membranes was examined to further improve the removal of both hydrophobic and hydrophilic pollutants. With a combination of superhydrophilic and superhydrophobic membranes, the NOM rejection (RUV254) and DOC removal rates (RDOC) could be increased to 83.6% and 73.3%, respectively.

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.

Low-Temperature Synthesis of Anatase TiO2 Nanoparticles with Tunable Surface Charges for Enhancing Photocatalytic Activity

In this work, the positively or negatively charged anatase TiO2 nanoparticles were synthesized via a low temperature precipitation-peptization process (LTPPP) in the presence of poly(ethyleneimine) (PEI) and poly(sodium4- styrenesulfonate) (PSS). X-ray diffraction (XRD) pattern and high-resolution transmission electron microscope (HRTEM) confirmed the anatase crystalline phase. The charges of the prepared TiO2, PEI-TiO2 and PSS-TiO2 nanoparticles were investigated by zeta potentials. The results showed that the zeta potentials of PEI-TiO2 nanoparticles can be tuned from +39.47 mV to +95.46 mV, and that of PSS-TiO2 nanoparticles can be adjusted from −56.63 mV to −119.32 mV. In comparison with TiO2, PSS-TiO2 exhibited dramatic adsorption and degradation of dye molecules, while the PEI modified TiO2 nanoparticles showed lower photocatalytic activity. The photocatalytic performances of these charged nanoparticles were elucidated by the results of UV-vis diffuse reflectance spectra (DRS) and the photoluminescence (PL) spectra, which indicated that the PSS-TiO2 nanoparticles showed a lower recombination rate of electron-hole pairs than TiO2 and PEI-TiO2.