Zehai Xu

Transformation of metal-organic frameworks for molecular sieving membranes

The development of simple, versatile strategies for the synthesis of metal-organic framework (MOF)-derived membranes are of increasing scientific interest, but challenges exist in understanding suitable fabrication mechanisms. Here we report a route for the complete transformation of a series of MOF membranes and particles, based on multivalent cation substitution. Through our approach, the effective pore size can be reduced through the immobilization of metal salt residues in the cavities, and appropriate MOF crystal facets can be exposed, to achieve competitive molecular sieving capabilities. The method can also be used more generally for the synthesis of a variety of MOF membranes and particles. Importantly, we design and synthesize promising MOF membranes candidates that are hard to achieve through conventional methods. For example, our CuBTC/MIL-100 membrane exhibits 89, 171, 241 and 336 times higher H2 permeance than that of CO2, O2, N2 and CH4, respectively.

Assembly of MOF Microcapsules with Size‐Selective Permeability on Cell Walls

The assembly of metal-organic frameworks (MOFs) into microcapsules has attracted great interest because of their unique properties. However, it remains a challenge to obtain MOF microcapsules with size selectivity at the molecular scale. In this report, we used cell walls from natural biomaterials as non-toxic, stable, and inexpensive support materials to assemble MOF/cell wall (CW) microcapsules with size-selective permeability. By making use of the hollow structure, small pores, and high density of heterogeneous nucleation sites of the cell walls, uniform and continuous MOF layers could be easily obtained by inside/outside interfacial crystallization. The prepared MOF/CW microcapsules have excellent stability and enable the steady, slow, and size-selective release of small molecules. Moreover, the size selectivity of the microcapsules can be adjusted by changing the type of deposited MOF.

Ultrathin metal–organic framework membrane production by gel–vapour deposition

Ultrathin, molecular sieving membranes composed of microporous materials offer great potential to realize high permeances and selectivities in separation applications, but strategies for their production have remained a challenge. Here we show a route for the scalable production of nanometre-thick metal–organic framework (MOF) molecular sieving membranes, specifically via gel–vapour deposition, which combines sol–gel coating with vapour deposition for solvent-/modification-free and precursor-/time-saving synthesis. The uniform MOF membranes thus prepared have controllable thicknesses, down to ~17 nm, and show one to three orders of magnitude higher gas permeances than those of conventional membranes, up to 215.4 × 10−7 mol m−2 s−1 Pa−1 for H2, and H2/C3H8, CO2/C3H8 and C3H6/C3H8 selectivities of as high as 3,400, 1,030 and 70, respectively. We further demonstrate the in situ scale-up processing of a MOF membrane module (30 polymeric hollow fibres with membrane area of 340 cm2) without deterioration in selectivity.MOF-based membranes have shown great promise in separation applications, but producing thin membranes that allow for high fluxes remains challenging. Here, the authors use a gel–vapour deposition strategy to fabricate composite membranes with less than 20 nm thicknesses and high gas permeances and selectivities.

Metal–organic framework channelled graphene composite membranes for H2/CO2 separation

Graphene-based materials offer great potential in gas separation. However, it remains a challenge to control the interlayer structure to obtain membranes with high permeability. We describe a route for the fabrication of metal–organic framework (MOF) channelled graphene composite membranes with molecular sieving properties by in situ crystallization. Based on polar oxygen groups, extended interlayer spacing and electronegativity, MOF nanosheets can easily be impregnated into interlayers of reduced graphene oxide (rGO), which strongly anchor and bolster up rGO by coordination bonds to form a porous architecture with uniform nanochannels. The H2/CO2 selectivity of ZIF-8/rGO membranes reached about 26.4. Moreover, many other MOF materials were also inserted between rGO sheets to obtain membranes with both high selectivity (larger than 10.3) and superior permeability (in the range of 73–21 112 barrer).

One-Pot Synthesis of Mesoporous Anatase-TiO2(B) Mixed-Phase Nanowires Decorated with Sulfur and Fe2O3 Nanoparticles for Visible-Light Photochemical Oxidation

We present a facial one‐pot synthesis method for fabrication of mesoporous anatase‐TiO2(B) mixed‐phase nanowires (TNWs) decorated with sulfur and Fe2O3 nanoparticles (NPs). Through one‐pot calcination, control of morphology and crystal phase for TNWs was easily achieved. After calcined at 773 K, mixed‐phase heterostructured TNWs with mesoporous structure and high surface area were obtained. This can not only lead ultrafine NPs well‐disperse in porous matrix, but also greatly enhance interfacial separation and transfer of photo‐induced charges between various planes. Moreover, sulfur doping improved acid/light properties of solid and brought more orbital‐overlapping between active sites and H2O2. The prepared S‐Fe2O3/anatase‐TiO2(B) TNWs had excellent catalytic ability in visible‐light assisted Fenton oxidation at neutral pH, which was attributed to the synergistic effect of sulfur species, NPs and TNWs. The progresses are valuable in developing highly efficient and green heterogeneous catalysts.

Preparation of Cu2O nanowire-blended polysulfone ultrafiltration membrane with improved stability and antimicrobial activity

Polysulfone (PSF) membranes have been widely applied in water and wastewater treatment, food-processing and biomedical fields. In this study, we report the preparation of modified PSF membranes by blending PSF with Cu2O nanowires (NWs) to improve their stability and antifouling activity. Synthesis of novel Cu2O NWs/PSF-blended ultrafiltration membrane was achieved via phase inversion method by dispersing one-dimensional Cu2O nanowires in PSF casting solutions. Various techniques such as XRD, SEM, TEM, and EDS were applied to characterize and investigate the properties of nanowires and membranes. The introduced Cu2O nanowires can firmly be restricted into micropores of PSF membranes, and therefore, they can effectively prevent the serious leaking problem of inorganic substances in separation process. The blended PSF membranes also provided enhanced antimicrobial activity and superior permeation property compared to pure PSF membrane. The overall work can not only provide a new way for preparation of novel blended membranes with multidimensional nanomaterials, but can also be beneficial to solve the annoying problem of biofouling.Graphical Abstract

The Researches on Polymers of Intrinsic Microporosity Membranes for Separation

Polymers of intrinsic microporosity (PIMs) are novel class of porous materials with well-defined microcavities and high free volume elements. As the typical soluble PIMs, PIM-1 has received much attention in the field of gas separation, pervaporation and organic solvent nanofiltration, due to its microporosity, easy processability, high surface area and superior hydrophobicity. The PIM membrane with unique rigid and contorted macromolecular backbone structure showed excellent performance in gas and liquid separation, especially high CO2 permeance and excellent CO2/N2 and CO2/CH4 separation factors. In general, the PIM membranes with unique characteristics will be the most promising candidates for industrial separation.

Preparation of Modified Graphene Oxide Nanomaterials for Water and Wastewater Treatment

Because of the population explosion, industrialization and environmental disruption which cause big water crisis to all human beings, the development of high-efficient water treatment technologies is highly desirable. Recently, graphene oxide (GO) has drawn wide attention from the scientific community. the two-dimensional GO nanosheets possess high specific surface area and high surface activity, which make GO-based nanomaterials have great potential in various fields. In this paper, recent methods on modification of graphene oxide nanomaterials are introduced and compared. The prospects of GO-based nanomaterials applied in water treatment from three aspects of absorbent, catalysis and membrane separation are further analysed.

Advanced membrane bioreactors systems: New materials and hybrid process design

Membrane bioreactor (MBR) is deemed as one of the most powerful technologies for efficient municipal and industrial wastewater treatment around the world. However, low microbial activity of activated sludge and serious membrane fouling still remain big challenges in worldwide application of MBR technology. Nowadays, more and more progresses on the research and development of advanced MBR with new materials and hybrid process are just on the way. In this paper, an overview on the perspective of high efficient strains applied into MBR for biological activity enhancement and fouling reduction is provided first. Secondly, as emerging fouling control strategy, design and fabrication of novel anti-fouling composited membranes are comprehensively highlighted. Meanwhile, hybrid MBR systems integrated with some novel dynamic membrane modules and/or with other technologies like advanced oxidation processes (AOPs) are introduced and compared. Finally, the challenges and opportunities of advanced MBRs combined with bioenergy production in wastewater treatment are discussed.