Fusheng Pan

Recent advances in the fabrication of advanced composite membranes

Composite membranes comprising a continuous polymer phase and a dispersed filler phase have revealed appealing potential in selective transport of molecules and ions. The multiphase characteristics of composite membranes provide more degrees of freedom to manipulate multiple interactions, tailor multiscale structures, and integrate multiple functionalities, compared to pristine polymer membranes. In this feature article, we have reviewed the various methods for the fabrication of composite membranes. In particular, we have thoroughly discussed two typical methods: the physical blending method and the sol–gel method. For each method, the major advances and challenges have been summarized. We have also tentatively delineated the new generation of composite membranes.

Fabrication of Ultrathin Membrane via Layer-by-Layer Self-assembly Driven by Hydrophobic Interaction Towards High Separation Performance

A novel and facile layer-by-layer (LbL) self-assembly process driven by hydrophobic interaction and then reinforced by hydrogen bond was developed to prepare ultrathin membranes. Gelatin (GE) and tannic acid (TA) were alternately deposited on polyacrylonitrile (PAN) ultrafiltration membranes to obtain GE/TA membranes. The required number of deposition cycles for acceptable permselectivity of membrane was greatly reduced compared with that of the traditional LbL self-assembly process and could be ascribed to the rapid growth of membrane thickness and the integrity of the innermost gelatin layer. Higher surface hydrophilicity and more appropriate free volume characteristics were obtained for GE/TA multilayer membranes compared with pristine gelatin membrane. Moreover, the GE/TA multilayer membrane exhibited improved stability even at high water content of 30 wt %. The membrane separation experiments with pervaporation dehydration of ethanol aqueous solution as a model system demonstrated the GE/TA multilayer membrane achieved higher water permselectivity than the pristine gelatin membrane. High operation stability was acquired in the long-term membrane separation test.

Efficient CO2 capture by humidified polymer electrolyte membranes with tunable water state

Polymer electrolyte membranes containing alkali or alkaline-earth metal salts were designed and utilized for CO2 capture. These membranes showed higher CO2 permeability than the un-doped control membrane due to the increase of water content, and CO2/gas selectivity was simultaneously enhanced due to the “salting-out” effect, which was strongly dependent on the content of bound water. More specifically, water content, water state and separation performance of polymer electrolyte membranes were strongly dependent on the salt type: (1) membranes containing alkaline-earth metal salts displayed a higher amount of bound water than those containing alkali cations, because the hydration energy of the alkaline-earth cation is relatively larger than that of the alkali cation; (2) the salts (KCl and CaCl2) that can efficiently interrupt chain packing by metal–polymer complexation facilitated the diffusion of water molecules into the polymer matrix and thus increased the total amount of absorbed water. As a consequence, CaCl2-doped membranes showed the highest CO2 permeability (2030 Barrer) and a high separation factor (108 for CO2/N2 and 31 for CO2/CH4) at 2 bar (gage pressure) and 298 K for fully humidified gas streams. The effects of annealing conditions and feed pressure were also explored to elucidate the relevant separation mechanism of the polymer electrolyte membrane.

Incorporating Zwitterionic Graphene Oxides into Sodium Alginate Membrane for Efficient Water/Alcohol Separation

For the selective water-permeation across dense membrane, constructing continuous pathways with high-density ionic groups are of critical significance for the preferential sorption and diffusion of water molecules. In this study, zwitterionic graphene oxides (PSBMA@GO) nanosheets were prepared and incorporated into sodium alginate (SA) membrane for efficient water permeation and water/alcohol separation. The two-dimensional GO provides continuous pathway, while the high-density zwitterionic groups on GO confer electrostatic interaction sites with water molecules, leading to high water affinity and ethanol repellency. The simultaneous optimization of the physical and chemical structures of water transport pathway on zwitterionic GO surface endows the membrane with high-efficiency water permeation. Using dehydration of water/alcohol mixture as the model system, the nanohybrid membranes incorporating PSBMA@GO exhibit much higher separation performance than the SA membrane and the nanohybrid membrane utilizing unmodified GO as filler (with the optimal permeation flux of 2140 g m(-2) h(-1), and separation factor of 1370). The study indicates the great application potential of zwitterionic graphene materials in dense water-permeation membranes and provides a facile approach to constructing efficient water transport pathway in membrane.

Embedding dopamine nanoaggregates into a poly(dimethylsiloxane) membrane to confer controlled interactions and free volume for enhanced separation performance

In this study, a series of hybrid membranes with high separation performance and superior swelling-resistance were fabricated by incorporating metal ion-chelated dopamine nanoaggregates into a poly(dimethylsiloxane) (PDMS) bulk matrix membrane. The concomitant hydrogen bond, metal-organic coordination and π-complexation interactions render the synergy of a favorable free volume property, reinforced chain rigidity and facilitated transport function within the membranes. The membranes displayed simultaneously enhanced permeation flux and enrichment factors when utilized for model gasoline separation. Especially, when the weight fraction of dopamine/Cu reached 5.0 wt%, the membrane displayed an optimum separation performance with a permeation flux of 7.42 kg m−2 h−1 (2.7 times as much as that of the PDMS control membrane) and an enrichment factor of 4.81 (11% more than that of the PDMS control membrane). Thanks to the elevated cohesive energy and the chain extension effect, the swelling degree of the PDMS-dopamine/Cu membranes decreased remarkably with the dopamine/Cu content. This study may provide a novel route to the design and fabrication of robust, high-performance hybrid membranes to meet diverse energy and environment-related application requirements.

Manipulating the interfacial interactions of composite membranes via a mussel-inspired approach for enhanced separation selectivity

For the broadly utilized composite membranes with dense separation layers and porous support layers, the rational manipulation of interfacial interactions between these two layers is vital in optimizing the membrane structure and the associated performance. In this study, we report a facile mussel-inspired approach to enhance the separation selectivity of composite membranes by co-depositing biomimetic adhesive dopamine and poly(ethylene imine) (PEI) on a support layer and then coating with sodium alginate (SA) as the separation layer. PEI is anchored onto the support layer surface through the reaction with dopamine during the polydopamine (PDA) formation process, thus incorporating the electrostatic attraction interaction into the interface in addition to the hydrogen bond interaction between PDA and SA. Using water/alcohol separation as the model system, the separation factor of the SA/PEI–PDA/PAN membrane can reach 1807, which is 29.6 and 6.8 times higher than those of SA/PAN and SA/PDA/PAN membranes, respectively. The remarkably enhanced separation factor arises from the optimal free volume property and swelling resistance of the membrane under the optimized interfacial interactions between the separation layer and support layer. This study may present an efficient and facile approach for tailoring the membrane structure for an enhanced separation performance.

Enhanced desulfurization performance of PDMS membranes by incorporating silver decorated dopamine nanoparticles

Dopamine–silver (DAAg) nanoparticles were synthesized in aqueous solution via bioinspired adhesion and redox reaction, and they were then incorporated into poly (dimethyl siloxane) (PDMS) matrix to prepare PDMS–DAAg hybrid membranes for pervaporative desulfurization of model gasoline. The loading content of Ag(I) in DAAg nanoparticles could reach as high as 54.03 wt%. The polymer chain flexibility and membrane swelling properties were mediated by DAAg nanoparticles. The hybrid membrane with 5.0 wt% of DAAg nanoparticles displayed an optimum separation performance with permeation flux of 8.22 kg m−2 h−1 (3-fold higher than that of PDMS control membrane) and enrichment factor of 5.03 (50% higher than that of PDMS control membrane). The enhancement of separation performance was mainly due to the facilitated transport of thiophene by reversible interaction between Ag(I) and thiophene molecules, and the moderate fractional free volume tuned by DAAg nanoparticles. Moreover, effects of operation parameters such as temperature, thiophene concentration in the feed, and feed Reynolds number on the permeation flux and enrichment factor were investigated.

Pervaporation dehydration of ethanol by hyaluronic acid/sodium alginate two-active-layer composite membranes.

The composite membranes with two-active-layer (a capping layer and an inner layer) were prepared by sequential spin-coatings of hyaluronic acid (HA) and sodium alginate (NaAlg) on the polyacrylonitrile (PAN) support layer. The SEM showed a mutilayer structure and a distinct interface between the HA layer and the NaAlg layer. The coating sequence of two-active-layer had an obvious influence on the pervaporation dehydration performance of membranes. When the operation temperature was 80 °C and water concentration in feed was 10 wt.%, the permeate fluxes of HA/Alg/PAN membrane and Alg/HA/PAN membrane were similar, whereas the separation factor were 1130 and 527, respectively. It was found that the capping layer with higher hydrophilicity and water retention capacity, and the inner layer with higher permselectivity could increase the separation performance of the composite membranes. Meanwhile, effects of operation temperature and water concentration in feed on pervaporation performance as well as membrane properties were studied.

Enhancing the permeation selectivity of sodium alginate membrane by incorporating attapulgite nanorods for ethanol dehydration

Hybrid membranes for ethanol dehydration were fabricated by blending sodium alginate with natural hydrophilic attapulgite nanorods, which contained plentiful selective channels and hydrophilic –OH groups. With the incorporation of attapulgite nanorods, the crystallinity of hybrid membranes was gradually decreased and the content of non-freezable water in hybrid membranes was increased, facilitating the solution-diffusion process of water molecules by forming hydration layers along the nanorods. The water uptake of hybrid membranes was ∼10% higher than the pristine alginate membrane while the swelling degree in feed solution was only increased by ∼1%, exhibiting good structural stability in ethanol dehydration. The optimum separation performance with a permeate flux of 1356 g m−2 h−1 and a separation factor of 2030 for dehydration of a 90/10 wt% ethanol/water feed was achieved using the hybrid membrane with 2 wt% of attapulgite nanorods. Moreover, the influences of feed temperature and feed composition on separation performance were investigated.

Creation of hierarchical structures within membranes by incorporating mesoporous microcapsules for enhanced separation performance and stability

In this study, we present a novel approach for fabricating hybrid membranes with superior separation performance and physicochemical stability by incorporating multifunctional dopamine mesoporous microcapsules upon CaCO3 template. The microcapsules are synthesized via the synergy of surface segregation, metal–organic coordination and biomimetic mineralization. The micro-scale hollow lumen and the mesoporous wall decrease diffusion resistance of the membranes by endowing smaller effective membrane thickness and introducing additional shorter permeation pathways for the penetrants, which lead to a faster mass transfer within the membranes. Meanwhile, the microcapsules bridge the polymer chains mainly owing to the numerous mesopores and unique bio-adhesion, which render the optimal polymer–microcapsule interface. Due to the hierarchical structures spanning from microscale, nanoscale to molecular-scale within the membranes, the membranes display remarkably enhanced permeation flux and desired enrichment factor when utilized for model gasoline separation. In addition, due to the elevated cohesive energy and reinforced chain rigidity, the membranes display higher thermal and mechanical stability. This study can identify a facile, generic, and efficient route to design and fabricate a variety of robust, high-performance hybrid membranes for a broad range of energy and environment-related applications.