In this study, a facile and novel facilitated transport mixed matrix membrane (FT-MMM) was fabricated by incorporating molecular amine functionalized polydopamine (PDA) into Pebax MH 1657. Polydopamine submicroparticles were chemically modified by tetraethylenepentamine (TEPA). The catechol groups on PDA can react with amino groups through the Michael addition reaction and Schiff base reaction. Grafting molecular amines onto PDA submicroparticles can not only improve interfacial compatibility between the fillers and a polymeric matrix but also enhance the facilitated transport of CO2 in the membrane because of the abundant CO2-philic groups. The effects of modified PDA submicroparticle content on the membrane permselectivity were investigated, and it was found that FT-MMM with a loading of 5% PDA submicroparticles exhibited the optimum CO2/CH4 separation property under humid conditions. The selectivity of Pebax-PDA/TEPA(5) membrane was 27.5, about 1.5-fold higher than that of the pristine Pebax membrane under identical operating conditions, while the membrane permeability remained comparable. Furthermore, the effects of operating pressure and temperature on the membrane separation performance were evaluated as well.

A kind of calcium phosphate-mineralized chitosan beads (chitosan–CaP) was prepared via a one-pot method by adding droplets of Ca2+-containing chitosan aqueous solution into phosphate-containing sodium tripolyphosphate aqueous solution. The chitosan beads formed immediately coupled with in situ precipitation of calcium phosphate on the surface. The antiswelling properties of hybrid beads were greatly improved with the swelling degree as low as 5%. The morphology of the resultant chitosan–CaP hybrid beads was observed by scanning electron microscopy (SEM). Yeast alcohol dehydrogenase (YADH) was encapsulated in the hybrid beads with an about 40% lower enzyme leakage compared with that in the pure chitosan beads. The optimal temperature and pH value for enzymatic conversion catalyzed by YADH immobilized in the chitosan–CaP beads were 30 °C and 7.0, respectively, which were identical to those for free YADH. The immobilized YADH displayed obviously higher thermal stability, pH stability, recycling stability, and storage stability than the free YADH counterpart.

•Polymer brushes functionalized double-shelled organic submicrocapsules are prepared.•Organic submicrocapsules are used to fabricate mixed matrix membrane.•CO2 transport passageways are widened due to the presence of polymer brushes.•Permeability and selectivity of membranes are tested for CO2/CH4(N2) systems.•Gas separation performance of MMMs surpasses Robeson upper bound revised in 2008.Appropriate facilitated transport carriers and water environment are two principal factors for efficient CO2 membranes separation. In this study, the facile distillation-precipitation polymerization (DPP) and atom transfer radical polymerization (ATRP) methods are consecutively used to prepare polymer brushes functionalized double-shelled organic submicrocapsules, and the resultant organic submicrocapsules are incorporated into sulfonated poly(ether ether ketone) (SPEEK) to fabricate mixed matrix membranes (MMMs). The doubled-shelled structure provides abundant facilitated carriers by the outer shell and high water retention property by the inner shell simultaneously. The organic submicrocapsules (b-IM@PMMA) are homogeneously embedded in the SPEEK matrix, and abundant amine groups on polymer brushes of outer shell increase the content of CO2-facilitated transport sites in MMMs. The inner shell renders the organic submicrocapsules of higher water content, yielding the MMMs with enhanced water retention properties. Moreover, the incorporation of organic submicrocapsules increases the water uptake and water retention capacity of MMMs. The MMMs doped with organic submicrocapsules display better CO2 separation performance than the MMMs doped with double-shelled organic submicrocapsules without brushes. The polymer brushes functionalized double-shelled organic submicrocapsules act as water reservoirs to not only offer more water for the dissolution of CO2 gas molecules, but also construct and widen interconnected continuous CO2-facilitated transport passageways. The highest CO2/CH4(N2) selectivity is 73.8(76.3) for the SPEEK/b-IM@PMAA MMMs (at a CO2 permeability of 2236 Barrer) in pure gas, surpassing the 2008 Robeson upper bound.

Simultaneous manipulation of vehicle-type and Grotthuss-type proton conduction within proton exchange membranes (PEMs) to induce satisfactory proton conductivity is crucial and challenging for promising environmentally friendly devices such as PEM fuel cells. In this study, a facile one-pot biomimetic mineralization approach is proposed for the construction of binary SiO2–TiO2 nanoparticles with a tunable ratio of silica to titania. Then the nanoparticles are functionalized by acid–base pairs and introduced into a Nafion matrix to fabricate novel hybrid membranes. The interaction between functional groups on SiO2–TiO2 binary nanoparticles and the polymer endows the hybrid membrane with good interfacial compatibility and enhanced dimensional stability. The incorporation of acid–base pairs reduces the activation energy for proton transfer; as a result, the hybrid membrane exhibits the highest proton conductivity of 1.37 × 10−2 S cm−1 at 26.1% RH and 80 °C, which is two orders of magnitude higher than that of recast Nafion. Compared with recast Nafion, a 51.3% increase in maximum power density is achieved for the Nafion/Si1–Ti2-160 hybrid membrane at 60 °C.

•Hybrid polyamide NF membranes incorporated with PEG-POSS were fabricated.•Membrane surface hydrophilicity, smoothness and electronegativity were increased.•Hybrid membranes exhibited enhanced water permeation and antifouling property.In order to fabricate nanofiltration (NF) membranes with high flux and anti-fouling properties, polyethylene glycol-functionalized polyhedral oligomeric silsesquioxane (PEG-POSS) nanoparticles were incorporated into the polyamide membrane during the interfacial polymerization of piperazine (PIP) and trimesoyl chloride (TMC) on polyethersulfone ultrafiltration substrate. The hydrophilicity, surface smoothness and electronegativity of the hybrid membrane were increased compared with the pristine polyamide membrane. The optimal preparation condition determined according to desalination performance was as follows, PEG-POSS content of 0.5% (w/v), PIP and TMC concentration of 0.1% (w/v) and reaction time of 2 min. The PEG-POSS nanoparticles constructed water channels networks and increased free volume of polyamide layer, contributing to the enhanced water permeation of hybrid membrane. The highest pure water flux of 38.7 L m−2 h−1 at 0.2 MPa which was nearly twice that of pristine polyamide membrane was achieved without reducing the salt rejection of Na2SO4 (87.1–91.6%). Meanwhile, the as-fabricated membranes exhibited exceptional anti-fouling performance. The flux recovery ratio (FRR) of the hybrid membrane reached up to 97.0%, 98.4% and 94.5% using bovine serum albumin, humid acid and sodium alginate as model foulants.Download high-res image (179KB)Download full-size image

•The TFC membranes modified by attapulgite(ATP) nanorods were prepared.•The tubular structure of ATP provided the accessorial channels for water transfer.•The PA-ATP(5)/PES membrane exhibited highest water flux of 229.5 L m-2 h-1 MPa-1.•The TFC membranes exhibited high rejections to Na2SO4 (92%) and dyes (87-100%).•The antifouling performance of ATP-modified NF membrane was enhanced.A hybrid thin film composite membrane with high flux and enhanced antifouling performance was prepared by incorporating attapulgite (ATP) nanorods using the interfacial polymerization reaction of piperazine (PIP) and trimesoyl chloride (TMC). Fourier transform infrared spectroscopy (FTIR) confirmed the presence of ATP nanorods in the polyamide (PA) layer. The surface morphology and hydrophilicity of the thin film composite membranes were probed by scanning electron microscopy (SEM) and water contact angle goniometer. The existence of attapulgite nanorods in the polyamide layer increased the membrane hydrophilicity and the water flux up to 229.5 L m−2 h−1 MPa−1, which was 1.3-fold higher than PA membrane. Furthermore, the ATP-modified nanofiltration membrane kept a high salt rejection of 92% for Na2SO4 and dye rejection of 94.7% for Orange GII. Furthermore, the attapulgite nanorods also endowed the polyamide nanofiltration membrane with enhanced antifouling performance, a remarkably higher flux recovery ratio than the pristine polyamide membrane for various kinds of foulants including bovine serum albumin, humic acid and sodium alginate.Download high-res image (280KB)Download full-size image

•Pegylated POSS was employed to fabricate nanocomposite membranes.•Nanocomposite membranes possessed very thin active layers of 180–200 nm.•A large loading of pegylated POSS up to 50 wt% with a well dispersion was achieved.•Composite membranes showed enhanced permeability and selectivity for ethanol dehydration.Fabrication of nanocomposite membranes with high nanoparticle loading is a significant challenge because of the serious agglomeration of nanoparticles at high concentration. In this study, poly(ethylene glycol)-functionalized polyoctahedral oligomeric silsesquioxanes (PEG@POSS) nanoparticles were incorporated into a sodium alginate (SA) matrix to fabricate nanocomposite membranes. A large loading of PEG@POSS up to 50 wt% with a well dispersion in the membrane was achieved due to the good compatibility between PEG@POSS and alginate chains. Benefiting from the nanostructured size of PEG@POSS (1–3 nm), the obtained nanocomposite membranes possessed very thin active layers of 180–200 nm. A large quantity of hydrophilic PEG side chains enhanced the membrane surface hydrophilicity and offered massive water binding sites in the membranes. Homogeneously dispersed high content PEG@POSS nanoparticles increased the free volume of the nanocomposite membranes while maintaining a suitable free volume cavity size for water permeation. The optimum separation performance with a permeation flux of 2500 g m−2 h−1 and a separation factor of 1077 for dehydration of 90/10 wt% ethanol/water feed was achieved. Additionally, the effects of operation temperature and feed concentration on separation performance were investigated systematically.Download high-res image (218KB)Download full-size image

A novel approach to in-situ synthesize and encapsulate phosphotungstic acid into the cavity of MIL-101(Cr) using Na2WO4·2H2O and Na2HPO4 as precursors is presented to increase the acid loading content (31.4 wt.%). The phosphotungstic acid-encapsulating MIL-101(Cr) (HPW@MIL101) is introduced in sulfonated poly(ether ether ketone) (SPEEK) to prepare SPEEK/HPW@MIL101 nanohybrid membranes for PEMFC applications. Due to the introduction of HPW@MIL101, proton-conducting nanochannels are constructed both in the cavity of MIL101 and at the interface between HPW@MIL101 and SPEEK. Meanwhile, due to the hygroscopicity of phosphotungstic acid, the membrane dehydration at elevated temperatures is alleviated. The proton conductivity at low relative humidity is remarkably enhanced. The nanohybrid membrane with 9 wt.% HPW@MIL101 exhibits proton conductivity of 272 mS cm−1 at 65 °C, 100% RH and 6.51 mS cm−1 at 60 °C, 40% RH, which are 45.5% and 7.25 times higher than those of pristine SPEEK membrane (187 mS cm−1 and 0.898 mS cm−1), respectively. The single H2/O2 fuel cell with SPEEK/HPW@MIL-9 membrane acquires the power density of 383 mW cm−2 at 100% RH, which is 27.2% higher than that of pristine SPEEK membrane. The peak power density of SPEEK/HPW@MIL-9 membrane at 55% RH is 2.97 times higher than that of pristine SPEEK membrane (79 mW/cm2).

Biological membranes possess hierarchical channels and thus exhibit ultrahigh permselectivity for molecules and ions. Intrigued by the delicate structure and transport mechanisms in biochannels, polymer composite membranes with selective transport channels are successfully fabricated for diverse energy- and environment-related applications. This tutorial review aims to present the latest progress in the design and construction of selective molecule/ion transport channels within polymer composite membranes with emphasis on the regulation of the physical and chemical microenvironments of these channels. The concluding remarks are presented with a tentative perspective on the future research and development of highly efficient channel-facilitated molecule/ion transport across polymer composite membranes.

Membrane processes have evolved as a competitive approach in CO2 separations compared with absorption and adsorption processes, due to their inherent attributes such as energy-saving and continuous operation. High permeability membrane materials are crucial to efficient membrane processes. Among existing membrane materials for CO2 separations, polymer-based materials have some intrinsic advantages such as good processability, low price and a readily available variety of materials. In recent years, enormous research effort has been devoted to the use of membrane technology for CO2 separations from diverse sources such as flue gas (mainly N2), natural gas (mainly CH4) and syngas (mainly H2). Polymer-based membrane materials occupy the vast majority of all the membrane materials. For large-scale CO2 separations, polymer-based membrane materials with high CO2 permeability and good CO2/gas selectivity are required. In 2012, we published a Perspective review in Energy & Environmental Science on high permeability polymeric membrane materials for CO2 separations. Since then, more rapid progress has been made and the research focus has changed significantly. This review summarises the advances since 2012 on high permeability polymer-based membrane materials for CO2 separations. The major features of this review are reflected in the following three aspects: (1) we cover polymer-based membrane materials instead of purely polymeric membrane materials, which encompass both polymeric membranes and polymer–inorganic hybrid membranes. (2) CO2 facilitated transport membrane materials are presented. (3) Biomimetism and bioinspired membrane concepts are incorporated. A number of representative examples of recent advances in high permeability polymer-based membrane materials is highlighted with some critical analysis, followed by a brief perspective on future research and development directions.

In this study, multifunctional chitin microspheres are synthesized and utilized as a platform for multiple potential applications in enzyme immobilization, catalytic reduction and adsorption. Porous chitin microspheres with an average diameter of 111.5 μm and a porous architecture are fabricated through a thermally induced phase separation method. Then, the porous chitin microspheres are conferred with surface multifunctionality through facile coordination-enabled self-assembly of tannic acid (TA) and titanium (TiIV) bis(ammonium lactate)dihydroxide (Ti–BALDH). The multipoint hydrogen bonds between TA and chitin microspheres confer the TA–TiIV coating with high adhesion capability to adhere firmly to the surface of the chitin microspheres. In view of the biocompatibility, porosity and surface activity, the multifunctional chitin microspheres are used as carriers for enzyme immobilization. The enzyme-conjugated multifunctional porous microspheres exhibit high catalytic performance (102.8 U·mg–1 yeast alcohol dehydrogenase). Besides, the multifunctional chitin microspheres also find potential applications in the catalytic reduction (e.g., reduction of silver ions to silver nanoparticles) and efficient adsorption of heavy metal ions (e.g., Pb2+) taking advantages of their porosity, reducing capability and chelation property.Keywords: chitin; coordination-enabled self-assembly; multifunctionality; porous microspheres; TA−TiIV coating

The graphene oxide (GO) sheets are functionalized by histidine molecules and incorporated into sulfonated poly (ether ether ketone) (SPEEK) matrix to fabricate hybrid polymer electrolyte membranes for direct methanol fuel cells (DMFCs). The loading of functionalized GO is varied to investigate its influence on cross-sectional morphology, crystalline structure, polymer chain stiffness, thermal stability and fractional free volume of membrane, etc. The acidic −SO3H groups (proton donors) in SPEEK and basic imidazole groups (proton acceptors) in histidine molecules form acid-base pairs and transport protons synergistically, thus yielding efficient proton channels inside the hybrid membranes. The maximum proton conductivity (at 100% RH) of hybrid membranes is elevated by 30.2% compared with the plain SPEEK membrane at room temperature. The functionalized GO flakes also confer the hybrid membranes low methanol permeability in the range of 1.32–3.91 × 10−7 cm2 s−1. At the filler content of 4 wt%, the hybrid membrane shows a superior selectivity of 5.14 × 105 S s cm−3 and its maximum power density of single DMFC cell (43.0 mW cm−2) is 80.7% higher than that of plain SPEEK membrane.

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.

An inverse replication method based on porous CaCO3 templates was developed to fabricate porous magnetic polymer microspheres (PMMSs) composed of biocompatible polydopamine and magnetic Fe3O4 nanoparticles. The preparation procedure involved the synthesis of Fe3O4@CaCO3 templates, infiltration and spontaneous polymerization of dopamine in template pores and finally the mild removal of templates. The particle size, the surface morphology and the pore structure (e.g., average pore size, pore volume, surface area, etc.) of porous PMMSs were facilely tailored by altering the templates. The as-prepared polydopamine microspheres PMMSs were applied to covalently immobilize YADH for catalyzing the conversion of formaldehyde to methanol. In comparison to the enzyme-conjugated PDA-coated Fe3O4 nanoparticles (PMNPs), the immobilized enzyme on porous PMMSs exhibited remarkably enhanced activity (specific activity: 162.3 U mg−1 enzyme vs. 97.6 U mg−1 enzyme; methanol yield: 95.5% vs. 57.1%; initial reaction rate: 0.15% s−1vs. 0.08% s−1), and desirable thermal/pH/recycling/storage stabilities, and particularly, easy separation from the bulk solution by an external magnetic field.

Hollow mesoporous silica microspheres with a uniform pore size and a large surface area are synthesized and functionalized by three kinds of amino acids with different acid–base pairs, including sulfonic acid–amino groups (HMS-Cys), phosphoric acid–amino groups (HMS-Phos) and carboxylic acid–amino groups (HMS-Asp). The incorporation of these microspheres into a Nafion matrix enhances the water uptake and adjusts the organic–inorganic interface and hydrophilic ionic domains inside the membranes. Microspheres with a hollow mesoporous structure endow the membranes with strong water retention ability. The proton conductivities of hybrid membranes are more than 8 times higher than that of recast Nafion at 40 °C and 20% RH after 90 min of testing. The acid–base pairs within the membranes work as proton donors and acceptors, and the retained water molecules are used as hydrogen-bonded bridges, which reduce the energy barrier for proton conduction. At the filler content of 4 wt%, the HMS-Cys embedded membrane shows the highest proton conductivity of 1.19 × 10−1 S cm−1 (30 °C, 100% RH). In addition, hybrid membranes incorporated with amino acid functionalized HMS show small reliance on humidity. The proton conductivities of all the hybrid membranes are ∼5.29–11.1 times higher than that of the recast Nafion under 26.1% RH and 80 °C.

Composite membranes are fabricated by incorporating amino acid-functionalized graphene oxide (GO-DA-Cys) nanosheets into a sulfonated poly(ether ether ketone) (SPEEK) polymer matrix. Graphene oxide (GO) nanosheets are functionalized with amino acids through a facile two-step method using dopamine (DA) and cysteine (Cys) in succession. The CO2 separation performance of the as-prepared membranes is evaluated for CO2/CH4 and CO2/N2 systems. GO nanosheets increase more tortuous paths for larger molecules, enhancing the diffusivity selectivity. Amino acids with carboxylic acid and primary amine groups simultaneously enhance the solubility selectivity and reactivity selectivity. Accordingly, CO2 molecules can transport quickly due to the enhanced selectivity. The optimum separation performance is achieved at the GO-DA-Cys content of 8 wt% with selectivities of 82 and 115 for CO2/CH4 and CO2/N2, respectively, and a CO2 permeability of 1247 Barrer, significantly surpassing the Robeson upper bound reported in 2008. Besides, the mechanical and thermal stabilities of the composite membranes are also improved compared with the pristine SPEEK membrane.

Polyethylenimine (PEI) was immobilized by MIL-101(Cr) (∼550 nm) via a facile vacuum-assisted method, and the obtained PEI@MIL-101(Cr) was then incorporated into sulfonated poly(ether ether ketone) (SPEEK) to fabricate mixed matrix membranes (MMMs). High loading and uniform dispersion of PEI in MIL-101(Cr) were achieved as demonstrated by ICP, FT-IR, XPS, and EDS-mapping. The PEI both in the pore channels and on the surface of MIL-101(Cr) improved the filler–polymer interface compatibility due to the electrostatic interaction and hydrogen bond between sulfonic acid group and PEI, and simultaneously rendered abundant amine carriers to facilitate the transport of CO2 through reversible reaction. MMMs were evaluated in terms of gas separation performance, thermal stability, and mechanical property. The as-prepared SPEEK/PEI@MIL-101(Cr) MMMs showed increased gas permeability and selectivity, and the highest ideal selectivities for CO2/CH4 and CO2/N2 were 71.8 and 80.0 (at a CO2 permeability of 2490 Barrer), respectively. Compared with the membranes doped with unfilled MIL-101(Cr), the ideal selectivities of CO2/CH4 and CO2/N2 for PEI@MIL-101(Cr)-doped membranes were increased by 128.1 and 102.4 %, respectively, at 40 wt % filler loading, surpassing the 2008 Robeson upper bound line. Moreover, the mechanical property and thermal stability of SPEEK/PEI@MIL-101(Cr) were enhanced.Keywords: facilitated CO2 transport; MIL-101(Cr); mixed matrix membranes; polyethylenimine; sulfonated poly(ether ether ketone); uniform dispersion

A novel multi-permselective mixed matrix membrane (MP-MMM) is developed by incorporating versatile fillers functionalized with ethylene oxide (EO) groups and an amine carrier into a polymer matrix. The as-prepared MP-MMMs can separate CO2 efficiently because of the simultaneous enhancement of diffusivity selectivity, solubility selectivity, and reactivity selectivity. To be specific, MP-MMMs were fabricated by incorporating polyethylene glycol- and polyethylenimine-functionalized graphene oxide nanosheets (PEG–PEI–GO) into a commercial low-cost Pebax matrix. The PEG–PEI–GO plays multiple roles in enhancing membrane performance. First, the high-aspect ratio GO nanosheets in a polymer matrix increase the length of the tortuous path of gas diffusion and generate a rigidified interface between the polymer matrix and fillers, enhancing the diffusivity selectivity. Second, PEG consisting of EO groups has excellent affinity for CO2 to enhance the solubility selectivity. Third, PEI with abundant primary, secondary, and tertiary amine groups reacts reversibly with CO2 to enhance reactivity selectivity. Thus, the as-prepared MP-MMMs exhibit excellent CO2 permeability and CO2/gas selectivity. The MP-MMM doped with 10 wt % PEG–PEI–GO displays optimal gas separation performance with a CO2 permeability of 1330 Barrer, a CO2/CH4 selectivity of 45, and a CO2/N2 selectivity of 120, surpassing the upper bound lines of the Robeson study of 2008 (1 Barrer = 10–10 cm3 (STP) cm–2 s–1 cm–1 Hg).

•Alkaline anion exchange membranes were prepared by blending ImPEEK and SPEEK.•IEC of the alkaline anion exchange membranes with was up to 3.15 mmol g−1.•Hydroxide conductivity up to 31.59 mS cm−1 was obtained at 30 °C.•Physical, thermal and alkaline stabilities of membranes are elevated.The development of alkaline anion exchange membrane (AEM) with both high ion conductivity and stabilities is of great significance for fuel cell applications. In this study, a facile acid-base blending method is designed to improve AEM performances. Basic imidazolium-functionalized poly (ether ether ketone) with a high functionalization degree is employed as polymer matrix to pursue high ion-exchange capacity (IEC) as well as high hydroxide conductivity, meanwhile acidic sulfonated poly (ether ether ketone) (SPEEK) is employed as the cross-linking agent to enhance the stabilities of the blend membranes. Particularly, an in-situ Menshutkin/crosslinking method is exploited to prevent the flocculation in the preparation process of blend membranes. As a result, dense and defect-free blend membranes are obtained. The blend membranes exhibit high level of IEC up to 3.15 mmol g−1, and consequently possess elevated hydroxide conductivity up to 31.59 mS cm−1 at 30 °C. In addition, benefiting from the strong electrostatic interaction introduced by the acid-base blending, the stabilities and methanol resistance of blend membranes are enhanced.

•A MOF was sulfonated with –SO3H groups by a facile post-synthesis method.•MMMs were fabricated by introducing sulfonated MOF into SPEEK.•Permeability and selectivity of MMMs were elevated in CO2/CH4 and CO2/N2 systems.•Mechanical and thermal stabilities of MMMs were improved.An octahedral metal–organic framework (MIL-101(Cr)) with a uniform size of ~550 nm was synthesized via hydrothermal reaction, and then was functionalized with sulfonic acid groups by concentrated sulfuric acid and trifluoromethanesulfonic anhydride. Mixed matrix membranes (MMMs) were fabricated by incorporating the as-prepared metal organic frameworks into sulfonated poly(ether ether ketone) (SPEEK). The gas separation performance of MMMs was investigated both in dry and humidified state. The addition of the sulfonated metal–organic framework increased the selectivity of the membranes for CO2/CH4 and CO2/N2 systems by increasing the CO2 solubility, and the diffusion of gases through the porous metal organic frameworks led to the simultaneous increase in CO2 permeability. The highest ideal selectivities for CO2/CH4 and CO2/N2 were 50 and 53 (at a CO2 permeability of 2064 Barrer) in humidified state, respectively. The increased total water in MMMs led to increased CO2 permeability and the increased bound water resulted in the improved CO2/gas selectivity. The effects of operating conditions on separation performance were investigated.

•Abundant amine groups were introduced onto titania nanotubes by a facile method.•MMMs were fabricated by introducing aminated titania nanotubes into SPEEK.•CO2 transport pathway was constructed by doping aminated titania nanotubes.•Aminated titania nanotubes improved interface compatibility.•MMMs had high CO2 separation performance surpassing the 2008 upper bound line.One-dimensional aminated titania nanotubes were prepared by a facile distillation-precipitation polymerization method and were incorporated into sulfonated poly(ether ether ketone) (SPEEK) to fabricate mixed matrix membranes (MMMs). The aminated titania nanotubes (TNT-IM) with abundant amine groups increased the CO2-facilitated transport sites in MMMs. Moreover, the CO2-facilitated transport pathways (CO2-FTP) were constructed from aminated titania nanotubes in polymer matrix, and acted as the fast CO2 transport channels to transfer CO2 molecules continuously and smoothly. Water uptake and water state in MMMs also played an important role in the gas separation performance. The relationship between the water content and CO2 separation performance was investigated. Nanotubes as fillers showed water retention properties during water evaporation from the MMMs at low humidity. The highest ideal selectivities of the SPEEK/TNT-IM MMMs for CO2/CH4 and CO2/N2 were 56.8 and 62.0, respectively, with a CO2 permeability of 2090 Barrer.

•Anti-tradeoff effect was achieved by adding Janus nanoparticles into membrane.•Janus nanoparticles consisted of polydopamine (PDA) sphere and silver species.•Catechol-chelated Ag+ endowed membrane with CO2 facilitated transport property.•Ag nanoparticle endowed membrane with enhanced diffusion property.•Both CO2 permeability and CO2/gas selectivity were enhanced.Silver-containing Janus nanoparticles were synthesized by adhering silver onto polydopamine nanosphere and then were embedded in Pebax® MH 1657 polymer matrix to fabricate mixed matrix membranes for CO2 capture. The Janus nanoparticle was composed of a polydopamine nanosphere (~80 nm) with plenty of catechol-chelated Ag+ ions and a catechol-reduced metallic Ag nanoparticle (~20 nm) epitaxially growing from the surface of the polydopamine nanosphere. The highly loaded Ag+ ions (58.41 wt% of total Ag content) served as CO2 facilitated transport carriers and thereby endowed the membrane with CO2 facilitated transport ability, and meanwhile the inorganic metallic Ag nanoparticle can interfere the polymer chain packing and optimize the membrane free volume characteristics therefore affecting the diffusion behavior of gas molecules. The membrane separation performance for pure gas (CO2, CH4, and N2) and binary gas mixtures (CO2/CH4 and CO2/N2) of the Janus nanoparticle-incorporated Pebax membrane was investigated and compared with that of the polydopamine nanosphere-incorporated membrane without silver modification. An anti-tradeoff effect was achieved by incorporation of the silver-containing Janus nanoparticles. The best separation performance of the Janus-incorporated Pebax membrane for pure gas was obtained with a CO2 permeability of 150 Barrer and a CO2/gas selectivity of 26.3 for CO2/CH4 and 72.5 for CO2/N2, i.e., 83%, 42% and 39% higher than those of neat Pebax membrane, respectively. The results suggested that incorporation of the Janus nanoparticles could achieve both a diffusion tuning effect and a facilitated transport effect simultaneously.

•Composite membranes comprising nanohydrogels and Matrimid® were prepared.•Nanohydrogels increase water content and water retention capacity of membranes.•Interconnected CO2 transport passageways were constructed within membranes.•Membrane performance surpassed or was close to the 2008 Robeson upper bound line.Composite membranes were fabricated by incorporating poly(N-isopropylacrylamide) nanohydrogels (NHs) into Matrimid® 5218 matrix to improve the separation performance for CO2/CH4 and CO2/N2 mixtures. The membranes were characterized by a fourier transform infrared spectrometer (FT-IR), scanning electron microscopy (SEM), tensile test, dynamic mechanical analysis (DMA), X-ray diffraction (XRD), positron annihilation lifetime spectroscopy (PALS), the static contact angle and water content measurement. The incorporation of nanohydrogels increased the fractional free volume of the composite membranes, water uptake and water retention capacity. The composite membranes displayed better performance than the pure Matrimid® membrane. The nanohydrogels homogeneously embedded in the Matrimid® matrix acted as water reservoirs to not only provide more water for dissolving CO2, but also construct interconnected CO2 transport passageways. The as-prepared Matrimid®/NHs-20 composite membrane showed CO2/CH4 and CO2/N2 selectivities of 61 and 52 with a CO2 permeability of 278 Barrer, surpassing or being close to the 2008 Robeson upper bound lines.

•MMMs were fabricated by incorporating CNTs and GO into Matrimid® matrix.•The efficient CO2 transport passageways were constructed by CNTs and GO in MMMs.•CNTs and GO exerted a favorable synergistic effect on separation performance.•Compared to pure Matrimid® membrane, CO2 permeability of the MMM increased by 331%.Mixed matrix membranes (MMMs) were fabricated by incorporating carbon nanotubes (CNTs) and graphene oxide (GO) into a Matrimid® matrix to improve CO2 separation performance. The combination of CNTs and GO exerted a favorable synergistic effect on the permselectivity of membranes. The extraordinary smooth walls of CNTs acted as a highway to render high permeability, whereas the graphene oxide nanosheets acted as a selective barrier to render high selectivity through the hydroxyl and carboxyl groups on GO surface in MMMs. The MMMs doped with both CNTs and GO had better gas separation performance than those doped with only CNTs or GO. Efficient CO2 transport pathways were constructed by homogeneous dispersion of CNTs and GO within MMMs, leading to both enhanced permeability and selectivity. The membrane combining 5 wt% of CNTs and 5 wt% of GO (Matrimid®-CNTs/GO-5/5) displayed the optimum performance with a CO2 permeability of 38.07 Barrer, a CO2/CH4 selectivity of 84.60 and a CO2/N2 selectivity of 81.00. Compared to pure Matrimid® membrane, the CO2 permeability, CO2/CH4 selectivity and CO2/N2 selectivity of the Matrimid®-CNTs/GO-5/5 membrane were increased by 331%, 149%, 147%, respectively.

Inspired by the water reserving function of vacuoles in plant cells, poly(methacrylic acid)-loaded imidazole microcapsules (IMCs-PMAA) with high water retention were prepared by distillation–precipitation polymerization and incorporated into a sulfonated poly(ether ether ketone) (SPEEK) matrix to fabricate SPEEK/IMCs-PMAA composite membranes. Compared with the SPEEK control membrane, the composite membranes exhibit higher water uptake, lower swelling degree and better methanol barrier properties. The water retention capacity of the microcapsules is optimized by the carboxylic acid groups of PMAA and the strong electrostatic attraction between the imidazole groups and sulfonic acid groups. Proton transfer pathways are constructed both inside the microcapsules and at the IMCs–SPEEK interface to enhance proton conductivity. The highest proton conductivity of composite membranes is about 5.52 × 10−2 S cm−1 at room temperature and 100% relative humidity (RH), which is more than two times of the SPEEK control membrane conductivity (2.51 × 10−2 S cm−1). In particular, the SPEEK/IMCs-PMAA-20 membrane exhibits the highest proton conductivity of 1.93 × 10−2 S cm−1 at 20% RH, which is two orders of magnitude higher than that of the SPEEK control membrane. The high water retention and proton conduction properties demonstrate that the composite membranes have a great potential application for the direct methanol fuel cell (DMFC).

Adenosine triphosphate (ATP) molecules, which contain proton-conductive phosphoric acid groups, imidazole groups and amino groups are incorporated into sulfonated poly(ether ether ketone) (SPEEK) to fabricate novel composite membranes. The morphology, structure and proton conduction abilities of the prepared membranes are investigated. ATP coalesces with SPEEK through ionic interactions between amino groups and sulfonic acid groups, which are beneficial to the stable existence of ATP molecules within polymer networks and enhance the thermal stability of the composite membranes. Due to the formation of abundant acid–base pairs among ATP molecules as well as between ATP and SPEEK chains, the proton conductivities of composite membranes under different conditions are increased in comparison with the plain SPEEK membrane, and this improvement becomes more evident when humidity is decreased. Particularly, the SPEEK/ATP composite membrane displays a highest proton conductivity of 0.198 S cm−1 at 80 °C and 100% RH. The proton conductivity of the SPEEK/ATP-20 membrane at 41% RH is nearly 30 times higher than that of the plain SPEEK membrane. The results indicate a promising potential of such acid–base pairs which contain –PO3H2 groups in enhancing the proton conduction of membrane materials.

A mild and efficient method for the construction of robust organic–inorganic hybrid microcapsules was developed by merging of covalent cross-linking and biomimetic mineralization into a layer-by-layer (LBL) self-assembly process. The diatom cell-inspired microcapsule structure had a biocompatible inner organic layer which could create a suitable microenvironment for biologically active substances inside the microcapsules and an inorganic layer which could function as a supporting membrane to maintain the intact morphology of the microcapsules. When in 40% PSS solution, only 5% of the hybrid microcapsules were deformed indicating that the hybrid microcapsule had a higher mechanical stability. The combination of the advantages of both the organic layer and the inorganic layer was applied for the immobilization of catalase (CAT). After being reused 7 times, the CAT in the hybrid microcapsules retained 78% of its initial activity. A buffering effect was created by the capsule wall and the immobilized CAT had a higher pH stability than the free CAT. After storing for 45 days, the CAT in the hybrid microcapsules retained 78% of its initial activity. It is envisaged that the as-prepared hybrid microcapsules can be extended to many applications such as biocatalysis, drug/gene delivery and biosensor fields.

Mesoporous silica particles were prepared for efficient immobilization of the β-glucuronidase (GUS) through a biomimetic mineralization process, in which the solution containing lysozyme and GUS were added into the prehydrolyzed tetraethoxysilane (TEOS) solution. The silica particles were formed in a way of biomineralization under the catalysis of lysozyme and GUS was immobilized into the silica particles simultaneously during the precipitation process. The average diameter of the silica particles is about 200 nm with a pore size of about 4 nm. All the enzyme molecules are tightly entrapped inside the biosilica nanoparticles without any leaching even under a high ionic strength condition. The immobilized GUS exhibits significantly higher thermal and pH stability as well as the storage and recycling stability compared with GUS in free form. No loss in the enzyme activity of the immobilized GUS was found after 30-day’s storage, and the initial activity could be well retained after 12 repeated cycles.

An inverse replication method based on porous CaCO3 templates was developed to fabricate porous magnetic polymer microspheres (PMMSs) composed of biocompatible polydopamine and magnetic Fe3O4 nanoparticles. The preparation procedure involved the synthesis of Fe3O4@CaCO3 templates, infiltration and spontaneous polymerization of dopamine in template pores and finally the mild removal of templates. The particle size, the surface morphology and the pore structure (e.g., average pore size, pore volume, surface area, etc.) of porous PMMSs were facilely tailored by altering the templates. The as-prepared polydopamine microspheres PMMSs were applied to covalently immobilize YADH for catalyzing the conversion of formaldehyde to methanol. In comparison to the enzyme-conjugated PDA-coated Fe3O4 nanoparticles (PMNPs), the immobilized enzyme on porous PMMSs exhibited remarkably enhanced activity (specific activity: 162.3 U mg−1 enzyme vs. 97.6 U mg−1 enzyme; methanol yield: 95.5% vs. 57.1%; initial reaction rate: 0.15% s−1vs. 0.08% s−1), and desirable thermal/pH/recycling/storage stabilities, and particularly, easy separation from the bulk solution by an external magnetic field.

Hollow mesoporous silica microspheres with a uniform pore size and a large surface area are synthesized and functionalized by three kinds of amino acids with different acid–base pairs, including sulfonic acid–amino groups (HMS-Cys), phosphoric acid–amino groups (HMS-Phos) and carboxylic acid–amino groups (HMS-Asp). The incorporation of these microspheres into a Nafion matrix enhances the water uptake and adjusts the organic–inorganic interface and hydrophilic ionic domains inside the membranes. Microspheres with a hollow mesoporous structure endow the membranes with strong water retention ability. The proton conductivities of hybrid membranes are more than 8 times higher than that of recast Nafion at 40 °C and 20% RH after 90 min of testing. The acid–base pairs within the membranes work as proton donors and acceptors, and the retained water molecules are used as hydrogen-bonded bridges, which reduce the energy barrier for proton conduction. At the filler content of 4 wt%, the HMS-Cys embedded membrane shows the highest proton conductivity of 1.19 × 10−1 S cm−1 (30 °C, 100% RH). In addition, hybrid membranes incorporated with amino acid functionalized HMS show small reliance on humidity. The proton conductivities of all the hybrid membranes are ∼5.29–11.1 times higher than that of the recast Nafion under 26.1% RH and 80 °C.

A mild and efficient method for the construction of robust organic–inorganic hybrid microcapsules was developed by merging of covalent cross-linking and biomimetic mineralization into a layer-by-layer (LBL) self-assembly process. The diatom cell-inspired microcapsule structure had a biocompatible inner organic layer which could create a suitable microenvironment for biologically active substances inside the microcapsules and an inorganic layer which could function as a supporting membrane to maintain the intact morphology of the microcapsules. When in 40% PSS solution, only 5% of the hybrid microcapsules were deformed indicating that the hybrid microcapsule had a higher mechanical stability. The combination of the advantages of both the organic layer and the inorganic layer was applied for the immobilization of catalase (CAT). After being reused 7 times, the CAT in the hybrid microcapsules retained 78% of its initial activity. A buffering effect was created by the capsule wall and the immobilized CAT had a higher pH stability than the free CAT. After storing for 45 days, the CAT in the hybrid microcapsules retained 78% of its initial activity. It is envisaged that the as-prepared hybrid microcapsules can be extended to many applications such as biocatalysis, drug/gene delivery and biosensor fields.

Composite membranes are fabricated by incorporating amino acid-functionalized graphene oxide (GO-DA-Cys) nanosheets into a sulfonated poly(ether ether ketone) (SPEEK) polymer matrix. Graphene oxide (GO) nanosheets are functionalized with amino acids through a facile two-step method using dopamine (DA) and cysteine (Cys) in succession. The CO2 separation performance of the as-prepared membranes is evaluated for CO2/CH4 and CO2/N2 systems. GO nanosheets increase more tortuous paths for larger molecules, enhancing the diffusivity selectivity. Amino acids with carboxylic acid and primary amine groups simultaneously enhance the solubility selectivity and reactivity selectivity. Accordingly, CO2 molecules can transport quickly due to the enhanced selectivity. The optimum separation performance is achieved at the GO-DA-Cys content of 8 wt% with selectivities of 82 and 115 for CO2/CH4 and CO2/N2, respectively, and a CO2 permeability of 1247 Barrer, significantly surpassing the Robeson upper bound reported in 2008. Besides, the mechanical and thermal stabilities of the composite membranes are also improved compared with the pristine SPEEK membrane.