In the past decade, a huge development in rational design, synthesis, and application of molecular sieve membranes, which typically included zeolites, metal–organic frameworks (MOFs), and graphene oxides, has been witnessed. Owing to high flexibility in both pore apertures and functionality, MOFs in the form of membranes have offered unprecedented opportunities for energy-efficient gas separations. Reports on the fabrication of well-intergrown MOF membranes first appeared in 2009. Since then there has been tremendous growth in this area along with an exponential increase of MOF-membrane-related publications. In order to compete with other separation and purification technologies, like cryogenic distillation, pressure swing adsorption, and chemical absorption, separation performance (including permeability, selectivity, and long-term stability) of molecular sieve membranes must be further improved in an attempt to reach an economically attractive region. Therefore, microstructural engineering and architectural design of MOF membranes at mesoscopic and microscopic levels become indispensable. This review summarizes some intriguing research that may potentially contribute to large-scale applications of MOF membranes in the future.

AbstractMetal–organic framework (MOF) nanosheets could serve as ideal building blocks of molecular sieve membranes owing to their structural diversity and minimized mass-transfer barrier. To date, discovery of appropriate MOF nanosheets and facile fabrication of high performance MOF nanosheet-based membranes remain as great challenges. A modified soft-physical exfoliation method was used to disintegrate a lamellar amphiprotic MOF into nanosheets with a high aspect ratio. Consequently sub-10 nm-thick ultrathin membranes were successfully prepared, and these demonstrated a remarkable H2/CO2 separation performance, with a separation factor of up to 166 and H2 permeance of up to 8×10−7 mol m−2 s−1 Pa−1 at elevated testing temperatures owing to a well-defined size-exclusion effect. This nanosheet-based membrane holds great promise as the next generation of ultrapermeable gas separation membrane.

The effort on electrochemical reduction of CO2 to useful chemicals using the renewable energy to drive the process is growing fast recently. In this review, we introduce the recent progresses on the electrochemical reduction of CO2 in solid oxide electrolysis cells (SOECs). At high temperature, only CO is produced with high current densities and Faradic efficiency while the reactor is complicated and a better sealing technique is urgently needed. The typical electrolytes such as zirconia-based oxides, ceria-based oxides and lanthanum gallates-based oxides, anodes and cathodes are introduced in this review, and the cathode materials, such as conventional metal–ceramics (cermets), mixed ionic and electronic conductors (MIECs) are discussed in detail. In the future, to gain more value-added products, the electrolyte, cathode and anode materials should be developed to allow SOECs to be operated at temperature range of 573–873 K. At those temperatures, SOECs may combine the advantages of the low temperature system and the high temperature system to produce various products with high current densities.The typical electrolytes, anodes and cathodes used in solid oxide electrolysis cells for the electrochemical reduction at high temperature are discussed in this review. Download high-res image (149KB)Download full-size image

A new method for hydrogen separation to acquire high-purity hydrogen using an oxygen-permeable ceramic membrane is proposed and verified in this work. A high hydrogen separation rate of up to 16.3 mL cm−2 min−1 was achieved on an asymmetric dual-phase membrane at 900 °C. No performance degradation was observed in a long-term operation with the feed gas containing 200 ppm H2S.

•Carbon membranes were prepared from pyrolysis of OPBI polymer membranes.•High temperature promote the generation of compact and ordered graphite structures.•The membranes carbonized at 750 °C revealed optimal CO2/CH4 separation performances.•A structure-performance relationship has been studied based on characterizations.A poly [2, 2′-(p-oxydiphenylene)-5, 5′-bibenzimidazole] (OPBI) was employed to pyrolyze under an inert Ar atmosphere to produce alumina-supported carbon molecular sieve membranes (CMSMs) for CO2/CH4 gas separation. Pyrolysis temperature was also varied from 550 °C to 750 °C to tailor the carbon microstructure and then optimize the separation performances of CMSMs. A structure-performance relationship was established that high pyrolysis temperature can promote the generation of a compact and ordered graphite carbon structure and further give rise to a remarkable combinations of separation factor and gas permeability of CMSMs. Much effort has been made to reduce the thickness of CMSMs via lowering the concentration of polymer precursors or increasing the dip-coating withdraw speed. The optimal thinner CMSMs separately prepared from diluted OPBI polymer solution and increased dip-coating withdraw speed revealed a remarkable improvement of CO2 permeance, transcending the Robeson's upper bound of polymer membranes and of great potential in natural gas purifications.

AbstractMetal–organic framework (MOF) nanosheets could serve as ideal building blocks of molecular sieve membranes owing to their structural diversity and minimized mass-transfer barrier. To date, discovery of appropriate MOF nanosheets and facile fabrication of high performance MOF nanosheet-based membranes remain as great challenges. A modified soft-physical exfoliation method was used to disintegrate a lamellar amphiprotic MOF into nanosheets with a high aspect ratio. Consequently sub-10 nm-thick ultrathin membranes were successfully prepared, and these demonstrated a remarkable H2/CO2 separation performance, with a separation factor of up to 166 and H2 permeance of up to 8×10−7 mol m−2 s−1 Pa−1 at elevated testing temperatures owing to a well-defined size-exclusion effect. This nanosheet-based membrane holds great promise as the next generation of ultrapermeable gas separation membrane.

•Dual-ligand ZIFs with diverse topologies are prepared via ZIF-108 ligand exchange.•ZIF-108 provides active coordination sites and secondary building blocks.•Ligand exchange is a heterogeneous nucleation process with ZIF-108 as seeds.•Ligand exchange has much lower activation energy than solvothermal synthesis.•ZIF-108 performs as seeds to develop oriented dual-ligand ZIF-78 films.Post-synthetic ligand exchange of metal-organic frameworks (MOFs) is a feasible target-oriented method of preparing dual-ligand MOF crystals with fine-tuneable compositions and functions. Herein, a series of dual-ligand zeolitic imidazolate framework (ZIF) crystals with SOD, GME and GIS topologies were constructed by ligand exchange of the mono-ligand parent materials ZIF-108, Zn(2-nitroimidazolate)2. In-depth investigations on the formation processes of the daughter ZIFs indicated that ligand exchange is a heterogeneous nucleation process using ZIF-108 as seeds. The metastability of ZIF-108 makes the ligand substitution of ZIF-108 require lower activation energy than homogeneous nucleation. This ligand exchange process paves the way for facile synthesis of dual-ligand and dual-metal-dual-ligand ZIF crystals with diverse porous networks. Based on these results, c-out-of-plane oriented dual-ligand ZIF-78 films were synthesized through the evolutionary selection in a van der Drift's type growth originated from randomly oriented ZIF-108 seed layer.

Metal pyrophosphates have attracted considerable interests due to their high proton conductivity and potentially wide applications in the temperature range of 100–400 °C. However, great difference in conductivity was reported by different groups on the same pyrophosphates. The reason for the huge difference is still in debate up to now, and there is no coherent standpoint in literatures on the proton conduction mechanism. In this study, we chose Fe0.4Nb0.5P2O7, which was reported showing high proton conductivity recently, as an example to disclose the reason inducing the divergence in proton conductivity and conduction mechanism. We found that the as-prepared pyrophosphate grains have three layers, i.e. crystalline pyrophosphate core, amorphous phosphate shell in the middle and gel-type shell composed of amorphous phosphorus species as the outermost layer. The content of amorphous phosphorus species decreases with the increase of the calcination temperature of pyrophosphates, and the calcination temperature-dependent residual soluble phosphorus curve extremely coincides with the conductivity curve. Thus, the proton conduction of pyrophosphates is realized via a gel-type shell formed by residual amorphous phosphorus species on surfaces of pyrophosphate grains. We suggested that the phosphorus content is the key factor to explain the great difference in conductivity of pyrophosphates prepared by different groups.

The high-energy nature of grain boundaries makes them a common source of undesirable phase transformations in polycrystalline materials. In both metals and ceramics, such grain-boundary-induced phase transformation can be a frequent cause of performance degradation. Here, we identify a new stabilization mechanism that involves inhibiting phase transformations of perovskite materials by deliberately introducing nanoparticles at the grain boundaries. The nanoparticles act as “roadblocks” that limit the diffusion of metal ions along the grain boundaries and inhibit heterogeneous nucleation and new phase formation. Ba0.5Sr0.5Co0.8Fe0.2O3−δ, a high-performance oxygen permeation and fuel cell cathode material whose commercial application has so far been impeded by phase instability, is used as an example to illustrate the inhibition action of nanoparticles toward the phase transformation. We obtain stable oxygen permeation flux at 600 °C with an unprecedented 10–1000 times increase in performance compared to previous investigations. This grain boundary stabilization method could potentially be extended to other systems that suffer from performance degradation due to a grain-boundary-initiated heterogeneous nucleation phase transformations.

Anti-corrosive coatings based on layered double hydroxides (LDHs) have been considered as promising alternatives to conventional chromate-containing conversion coatings. Among various LDHs, carbonate-intercalated LDH coatings with a c-axis preferred orientation should be the optimum structure for protecting metals against corrosion. Herein we successfully prepared NiAl–CO3 LDH coatings on aluminium plates in one step. Particularly it was found that CO2 dissolved in the precursor solution exerted great influence on the microstructure and anti-corrosion capacity of prepared LDH coatings. Trace amounts of CO2 in the precursor solution led to the formation of ab-oriented 7 μm-thick LDH coatings, while preferentially c-oriented LDH coatings with an average thickness of 12 μm were formed from CO2-saturated precursor solutions. A DC polarization test demonstrated that preferentially c-oriented LDH coatings exhibited much higher anti-corrosion performance than ab-oriented LDH coatings possibly due to the decreased density of mesoscopic defects. Simultaneously, CO2, the green gas, was also positively utilized.

A transition layer between Ce0.8Sm0.2O2−δ (SDC) electrolyte and Sr0.8Co0.8Fe0.2O3−δ (SCF0.8) nano cathode was introduced to improve electrochemical performances of solid oxide fuel cells (SOFCs). A calcining – stripping method was used to introduce the transition layer on the SDC electrolyte. Then a nano porous cathode was coated on the transition layer. The microstructure of the nano cathode was optimized by adjusting its calcining temperature. It was found that the transition layer played a vital role in improving the electrochemical performances of the nanostructured cathode by enhancing the interface connection between cathode and electrolyte. Good electrochemical performance was obtained as the nano cathode calcined at 700 °C after introducing the transition layer. The polarization resistance of the anode-supported SOFC at 700 °C was significantly reduced from 0.20 Ω cm2 to 0.05 Ω cm2, while the power density of the single cell was increased from 116 to 444 mW cm−2.

•Principle of hydrogen production from water splitting in MIEC membrane reactors.•Factors affecting hydrogen production rate and membrane stability.•Coupling water splitting reaction and catalytic oxidation reactions in membrane reactors.Hydrogen as a clean energy carrier for power generation through fuel cells has attracted much attention. Water is a suitable source for hydrogen production by the splitting reaction. Significant amount of hydrogen can be produced at moderate temperature if a mixed ionic-electronic conducting (MIEC) membrane is used to remove the produced oxygen from water splitting reaction, although the equilibrium constant for water splitting is small. In this review, the principle of hydrogen production via water splitting in MIEC membrane reactor is illustrated, and the factors affecting the hydrogen production rate (such as membrane materials, thickness, modification of membrane surface) and the stabilities of membranes under high oxygen chemical potential gradient are discussed. Furthermore, following the concept of process intensification, the researches on water splitting coupling with other catalytic reactions in membrane reactors are summarized.

•A novel precipitation-aging method to produce single-crystal Mn3−xCoxO4 octahedra.•The products respectively show {111} and {011} facets of cubic and tetragonal phases.•Well-defined octahedra controlled by rates of spinel nucleation and octahedra growth.•Single crystal x = 1.0&0.5 octahedra respectively show high 4e− and 2e− ORR selectivity.•High ORR activity and selectivity depend on octahedral shape, facet and Mn4+/B ratio.Single-crystal (Mn,Co)3O4 octahedra were synthesized through a novel precipitation-aging method. Well-defined single-crystal octahedra can be formed by carefully controlling the precipitation-dissolution equilibrium, oxygen concentration, and solution temperature during synthesis to match the rates of spinel nucleation and octahedra growth. The single-crystal (Mn,Co)3O4 octahedra expose {111} and {011} facets of cubic and tetragonal phases, respectively, depending on the Mn/Co ratio. However, only single-crystal octahedra of Mn2CoO4 and Mn2.5Co0.5O4 with exposed {011} facets show the highest selectivity towards 4e− and 2e− oxygen reduction reactions (ORR) in alkaline solution, respectively. Furthermore, the single-crystal Mn2CoO4 octahedra with exposed {011} facets shows ∼30 times higher area specific activity for ORR than that of nanoparticles with random/mixed facets. The high electrocatalytic activity and selectivity towards ORR are correlated with the octahedral shape, the exposed facet, and the ratio of cations on the exposed facets (especially the octahedrally coordinated Mn4+ cations). This facet dependent catalytic performance provides a new route for obtaining highly selective and active ORR electrocatalysts.Single crystal (Mn,Co)3O4 octahedra synthesized by a novel precipitation-aging method show high electrocatalytic activity and facet-tuned selectivity towards either the 4e− or 2e− process of oxygen reduction reaction.

•Three new ZIF nanocrystals were prepared.•Additive-free hydrothermal and solvothermal methods were employed.•The nanocrystals possess permanent microporous structure and good thermal stability.•Good candidates for being used as adsorbent or membrane materials.For the first time, three kinds of ZIF (zeolitic imidazolate framework) nanocrystals were synthesized with narrow size distribution under additive-free hydrothermal and solvothermal conditions. The crystal size was ca. 60, 75 and 220 nm for ZIF-71-EIM, ZIF-93 and ZIF-8-MCIM, respectively. The N2 adsorption isotherms and TG analysis indicate that the nanocrystals possess permanent microporous structure and good thermal stability. These new ZIF nanocrystals hold promising potentials for the applications in the fields of nanotechnology devices and nanocomposite membranes.

The metal–organic framework ZIF-8, which undergoes hydrolysis under hydrothermal conditions, is endowed with high water-resistance after a shell-ligand-exchange-reaction. The stabilized ZIF-8 retains its structural characteristics with improved application performances in adsorption and membrane separation.

The spinel-type oxides of (Mn, Co, Cu)3O4 prepared via a citric–EDTA acid process were investigated as candidate cathodes of intermediate temperature solid oxide fuel cells (IT-SOFCs). (Mn, Co)3O4 spinel oxide shows a phase transition from tetragonal to cubic when the doping amount of cobalt element increases. Their electric conductivities increase with the cobalt content and are enough high for them used as cathodes of IT-SOFCs. A fuel cell with (Mn, Co)3O4 spinel cathode was successfully evaluated based on YSZ electrolyte. (Mn, Co)3O4 spinel cathodes show good electrochemical activities, demonstrating the feasibility of the spinel oxide being a cathode of IT-SOFC. As copper doped into (Mn, Co)3O4 spinel, the Ppeak for Cu0.5MnCo1.5O4 cathode rise to 343, 474 and 506 mW cm−2 at 700, 750 and 800 °C, respectively. The results reveal that the spinel-type oxides are promising cathodes for IT-SOFCs, especially for Cu0.5MnCo1.5O4.Highlights► Spinel-type oxides are feasible to be candidate cathodes for IT-SOFC. ► The fuel cell with (Mn, Co)3O4 spinel cathode shows good electrochemical performance. ► As Cu doped into (Mn, Co)3O4, an enhanced performance is obtained for Cu0.5MnCo1.5O4.

ZIF-78 is a prototypical mixed-ligand compound among metal–organic frameworks (MOFs), with the composition of divalent zinc cations bridging by 2-nitroimidazolate (nIm) and 5-nitrobenzimidazolate (nbIm) anions. ZIF-78 belongs to hexagonal space group (P63/mmc) with a GME topology. Owing to the anisotropic pore structure and good affinity to CO2, ZIF-78 has potential applications in adsorption and separation fields. Herein, morphological control of ZIF-78 crystals was achieved via solvothermal synthesis. XRD, SEM, NMR, and adsorption characterizations were performed to study the crystalline phase, morphology, composition and adsorption behavior of obtained crystals, respectively. ZIF-78 micro-crystals with shape of the hexagonal rods were prepared with the assistance of triethylamine. The size and the aspect ratios of ZIF-78 microrods were manipulated from the “slender” to the “squat” via varying the nutrient concentration, the ligand concentration or relative molar ratio of nIm to nbIm.Graphical abstract.Highlights► Morphological control of mixed-ligand metal–organic frameworks was achieved via solvothermal synthesis. ► ZIF-78 hexagonal microrods with well defined crystal faces were prepared with the assistance of triethylamine. ► The size and the aspect ratios of the ZIF-78 microrods were modulated by varying the synthesis parameters.

•Three one-pot methods were comparatively investigated for the powders synthesis.•Membranes derived from solid state reaction method show the highest permeation flux.•Microstructure influences the formation of continuous electronic conduction network.•The membrane shows CO2-stable and high flux swept by CO2.As alternatives of single-phase mixed conducting materials, dual-phase materials have been suggested as candidates for application as oxygen separation membranes, since it is difficult to meet all the requirements in a single-phase membrane material. The influence of synthetic methods on the performance of the 75 wt.%Ce0.85Sm0.15O1.925–25 wt.%Sm0.6Sr0.4Al0.3Fe0.7O3 (SDC–SSAF) dual-phase membranes has been investigated. Three one-pot methods, i.e. the solid state reaction (SSR), EDTA-citrate complex (EC) and co-precipitation (CP) methods, were used to prepare the SDC–SSAF powder. The structure, surface morphologies, electrical conductivity, oxygen permeation, and stability in a CO2 atmosphere were investigated. It was found that the membrane derived from the SSR method shows the highest oxygen permeation flux and total conductivity. The significant differences between the performances of the dual-phase membrane derived from the different methods relates to the different microstructures developed during membrane preparation, which further influences the formation of a continuous electronic conduction network across the membranes. The stability of the dual-phase membrane was studied by treating the membrane materials under a CO2 atmosphere and by sweeping the membrane with pure CO2. The results show that the membrane is CO2-stable and is potentially integrated with the oxyfuel process for CO2 capture.

Twin growth in the synthesis of b-oriented MFI films is successfully suppressed by applying microwave irradiation on a b-oriented MFI seed layer, relying on the nucleation-related bottleneck effect. Electrochemical oxidation experiments demonstrated the importance of twin suppression in enhancing the diffusion of guest molecules in MFI films.

Ceria-based dual-phase membranes showing high oxygen permeation fluxes and stability under a CO2 environment are promising materials for CO2 capture via an oxyfuel route. The high oxygen permeation fluxes compared with other dual-phase membranes are derived from the mixed conducting properties of the perovskite oxides used in the dual-phase membranes.

The design of oxide ceramic dual-phase membranes is discussed in detail from the point of view of solid state chemistry. Dual-phase membranes with high performance have been designed by considering the permeability, stability and compatibility of the dual-phase system. It is deduced that dual-phase membranes made of Fe-based perovskite oxides (such as Sm0.6Sr0.4FeO3, mixed conductors) and Ce-based fluorite oxides (such as Ce0.85Sm0.15O1.925, ionic conductors) have both good permeability and stability. A dual-phase membrane made of an ionic conductor and pure electronic conductor was studied for comparison purposes, with a nominal composition of 75 wt.%Ce0.85Sm0.15O1.925–25 wt.%Sm0.6Sr0.4CrO3. Experimental investigation of selected membranes is reported with attention to processing, microstructures, conductivity and oxygen permeation properties. Microstructure effects of dual-phase membranes on oxygen exchange reactions and bulk oxide ionic transport were investigated by AC impedance spectroscopy, high resolution transmission electron microscopy and oxygen permeation. Oxygen permeation experiments revealed when the Fe-based mixed conducting perovskite is used in dual-phase membranes, higher oxygen permeation fluxes were achieved than that of the Cr-based pure electronic conducting perovskite was used in the dual-phase membrane. The phenomenon can be well explained by the mutual blocking of electronic and ionic transport by pure electronic conductors and ionic conductors, and the enrichments of Cr-based impurities between the ionic conducting grains. Membranes with various weight ratios of the two phases were synthesized to find the optimal composition. 75 wt.%Ce0.85Sm0.15O1.925–25 wt.%Sm0.6Sr0.4FeO3 was found to be the best membrane from the point of view of both oxygen permeability and stability. The idea of designing dual-phase membranes was verified by the comparison.Graphical abstractHighlights► Composition and microstructure of membrane were optimized. ► Impurities and pure electronic conductor block ionic transport. ► Clear grain boundaries and the mixed conductor help the ionic transport.

SrCo0.8Fe0.2O3-δ is a controversial material whether it is used as an oxygen permeable membrane or as a cathode of solid oxide fuel cells. In this paper, carefully synthesized powders of perovskite-type SrxCo0.8Fe0.2O3-δ (x = 0.80–1.20) oxides are utilized to investigate the effect of A-site nonstoichiometry on their electrochemical performance. The electrical conductivity, sintering property and stability in ambient air of SrxCo0.8Fe0.2O3-δ are critically dependent on the A-site nonstoichiometry. Sr1.00Co0.8Fe0.2O3-δ has a single-phase cubic perovskite structure, but a cobalt-iron oxide impurity appears in A-site cation deficient samples and Sr3(Co, Fe)2O7-δ appears when there is an A-site cation excess. It was found that the presence of the cobalt-iron oxide improves the electrochemical performance. However, Sr3(Co, Fe)2O7-δ has a significant negative influence on the electrochemical activity for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The peak power densities with a single-layer Sr1.00Co0.8Fe0.2O3-δ cathode are 275, 475, 749 and 962 mW cm−2 at 550, 600, 650 and 700 °C, respectively, values which are slightly lower than those for Sr0.95Co0.8Fe0.2O3-δ (e.g. 1025 mW cm−2 at 700 °C) but much higher than those for Sr1.05Co0.8Fe0.2O3-δ (e.g. only 371 mW cm−2 at 700 °C). This remarkable dependence of electrochemical performance of the SrxCo0.8Fe0.2O3-δ cathode on the A-site nonstoichiometry reveals that lower values of electrochemical activity reported in the literature may be induced by an A-site cation excess. Therefore, to obtain a high performance of SrxCo0.8Fe0.2O3-δ cathode for IT-SOFCs, an A-site cation excess must be avoided.

We have fabricated BaCe0.8Gd0.2O3−δ (BCGO) thin films with thickness in the range of 5–10 μm over substrates composed of Ce0.8Gd0.2O1.9 (CGO)-NiO using a colloidal spray deposition method. A perovskite-type BaZr0.2Co0.4Fe0.4O3−δ (BZCFO) was employed as a novel cathode material. The performances of solid oxide fuel cells were investigated from 450 °C to 600 °C. The fuel cell with the BCGO film of 9 μm in thickness showed maximum power outputs of 237, 192, 136 and 89 mW cm−2 at 600, 550, 500 and 450 °C, respectively. The impedance measurements at open circuit conditions showed the polarization resistances of the electrode were about 0.25 and 1.00 Ω cm2 at 600 and 500 °C, respectively. The low interfacial resistances indicated that the BZCFO was a promising cathode material for low-temperature proton-conducing solid oxide fuel cells.Highlights► We have fabricated BaCe0.8Gd0.2O3 films using a colloidal spray deposition method. ► A perovskite-type BaZr0.2Co0.4Fe0.4O3−δ was employed as a novel cathode material. ► The cell shows much lower interfacial polarization resistance at low temperatures.

Mn1.5Co1.5O4 spinel oxide as a cathode or one component of a composite cathode presents no visible reaction with an Y2O3-stabilized ZrO2 electrolyte. The low electrode polarization resistances and good performance compared with traditional Sr-doped LaMnO3–YSZ composite cathodes imply promising application for the next generation of intermediate-temperature solid oxide fuel cells.

An ultrathin (300 nm) homogeneous silicalite-poly(dimethylsiloxane) (PDMS) nanocomposite membrane was fabricated on capillary support by a “Packing–filling” method. Firstly, silicalite-1 nano-crystals were deposited onto a porous alumina capillary support using dip-coating technique (packing); secondly, the interspaces among the nano-crystals were filled with PDMS phase (filling). No voids between nano-crystals and PDMS phase were observed by scanning electron microscopy (SEM), suggesting good zeolite–polymer adhesion. The membrane possesses very high flux (5.0–11.2 kg m−2 h−1) and good separation factor (25.0–41.6) for the pervaporative recovery of iso-butanol from aqueous solution (0.2–3 wt.%) at 80 °C. Such properties offer great potential towards applications in fermentation–pervaporation coupled processes. The effects of feed temperature and concentration on the pervaporation performance of this nanocomposite membrane were investigated.Graphical abstractResearch highlights▶ Packing–filling method promotes homogeneous Si-PDMS composite membrane synthesis. ▶ Capillary support decreases transport resistance. ▶ Ultrathin homogenerous membrane possesses high iso-butanol flux.

The effects of sintering temperature on the properties of Ce0.85Sm0.15O3−δ–Sm0.6Sr0.4FeO3−δ (SDC–SSF) dual-phase membranes were investigated. X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray analysis (EDX), alternating current impedance and oxygen permeation techniques were employed to study the phase structure, element transport, microstructure, grain size and oxygen permeation through dual-phase membranes sintered at different temperatures. Sintering temperature has complex effects on dual-phase membranes compared with single-phase perovskite membranes. XRD and EDX analysis results reveal that the surfaces of dual-phase membranes were mainly coated by fluorite oxide with a thickness of about 45 μm when the membrane was sintered at 1500 °C. Fluorite grains were gradually transferred from the inner layer to the outer layer with the increase of sintering temperature, which leads to a decrease in electronic conductivity of the membrane surface. The grain size increased with the rising of sintering temperature, which has negatively effect on oxygen permeation when the sintering temperature is higher than 1425 °C.Graphical abstractResearch highlights▶ Over high sintering temperature leads to enrichment of fluorite phase on surface. ▶ Enrichment of fluorite phase increases the surface resistances. ▶ Growth of grains decreases the homogeneous of the two phases and permeation fluxes. ▶ The optimal sintering temperature for Ce0.85Sm0.15O3−δ–Sm0.6Sr0.4FeO3−δ is 1425 °C.

A compact and highly b-oriented MFI monolayerwas fabricated with a novel phase-segregation-induced self-assembly method. When MFI microcrystals were dispersed uniformly in an appropriate dispersant (sec-butanol) containing a trace amount of binding agent (linoleic acid), these microbuilding blocks were spontaneously self-assembled into a compact monolayer at an air−liquid interface. In particular, it was observed that the binding agent took effect only after being phase-segregated from the aqueous solution. The influence of the kind of dispersants and binding agents on the final morphology of the as-prepared MFI monolayers was discussed. On the basis of these results, a mechanism was proposed to elucidate the driving force for the compact and oriented assembly of these MFI building blocks at the air−water interface.

Unsteady-state permeation in the initial stage was investigated on ceria-based dual-phase membranes by coating La0.6Sr0.4CoO3–δ (LSC) active porous layers onto the surface of membranes. It is found that the unsteady period is greatly influenced by the active porous layer. The membrane with LSC porous layers coated on both sides reaches a steady state immediately while starting the permeation testing. However, the membranes without LSC porous layers coated on one side or both sides need several to tens of hours to achieve the steady state. The active porous layer can improve the oxygen flux and decrease the permeation activation energy, and the membrane with coating on both sides had the highest flux and lowest Ea. In addition, the active porous layer can eliminate oxygen exchange limitations on the membrane surface. The changes of surface microstructures are suggested as the cause of unsteady-state permeation.Research highlights► Unsteady-state permeation in the initial stage of dual-phase membranes were investigated. ► The unsteady period can be shortened or eliminated by coating the active porous layers on membrane surfaces. ► The active porous layer can improve the oxygen flux and decrease the permeation activation energy. ► The changes of surface microstructures are related to the unsteady-state permeation.

Here we report a facile method to fabricate highly b-oriented and submicrometer thin MFI films on substrates. Neither an anhydrous environment nor a specifically designed structure-directing agent (SDA) is required in the whole process, and by innovation of the secondary growth process, twin growth of the b-oriented seed layer is effectively suppressed with TPAOH as the SDA for the first time. Furthermore, various substrates with different surface conditions can be directly used as substrates without premodification. A Pt electrode also was successfully used as a substrate to grow this high quality MFI film, showing excellent molecular sieving ability in aqueous solution.

BaCe0.1Co0.4Fe0.5O3−δ, a new mixed ionic–electronic conducting (MIEC) perovskite oxide, was successfully synthesized, which has high oxygen permeability and excellent structural stability. Through the XRD characterization, it was shown that only if the amount of cobalt is no more than 40%, pure perovskite phase can be obtained. Oxygen permeation flux of BaCe0.1Co0.4Fe0.5O3−δ permeable membrane reaches at 1.35 ml min−1 cm−2 at 950 °C with membrane thickness of 1 mm. In oxygen permeation operation of 270 h at 800 °C and 750 °C, no degradation of oxygen permeation flux was found. An increasing process was observed in the initial oxygen permeation of BaCe0.1Co0.4Fe0.5O3−δ which took about 40 h to reach steady state. If the thickness of the membrane is thicker than 0.7 mm, bulk diffusion is the rate-determining step of oxygen permeation. And if its thickness is thinner than 0.7 mm, oxygen permeation process is mainly controlled by surface exchange mechanism.

The phase transitions that take place in Sr1 + xCo0.8Fe0.2O3 − δ (− 0.2 ≤ x ≤ 0.1) oxides are reported here. Thermogravimetric analysis (TGA) showed that the oxides with − 0.2 ≤ x ≤ 0 were prone to undergo oxygen-vacancy disorder–order phase transitions, while others with x = 0.05, 0.1 had more stable crystal structures during oxygen-desorption processes in nitrogen. These results were further confirmed by high-temperature in-situ X-ray techniques. The changes in activation energies of three typical oxides, Sr1 + xCo0.8Fe0.2O3 − δ (x = − 0.2, 0, 0.1), used as oxygen-permeable membranes were investigated. The phase transitions in Sr1 + xCo0.8Fe0.2O3 − δ (x = − 0.2, 0) have also been detected in differential scanning calorimetry (DSC) profiles.

A highly efficient catalyst, MoV0.3Te0.17Nb0.12O, used for acrylic acid (AA) production from propane, was used as an anodic catalyst in an SOFC reactor, from which AA and electric power were cogenerated at 400–450 °C.

By surface modification of MFI microcrystals with sec-butanol and pre-coating the substrate with a temporary water layer, a convenient process is developed to directly assemble anisotropic MFI microcrystals into highly b-oriented monolayers on various substrates with different surface conditions.

High performance FAU-type zeolite membranes were synthesized by the “in situ aging-microwave synthesis” (AM) method in a clear solution with the molar composition of 70Na2O:1Al2O3:20SiO2:2000H2O. The AM method couples in situ aging at relatively low temperature and fast crystallization under microwave heating (MH). To achieve the optimization of the synthesis parameters, the in situ aging and MH temperatures, as well as the probable ranges of the corresponding times were firstly determined by means of investigating their effects on the crystal structures and morphologies of the zeolite layers. Then the Taguchi experimental design was employed to further optimize the in situ aging and MH times of the two-stage synthesis. The as-synthesized zeolite membranes showed high pervaporation performance for the dehydration of ethanol and isopropanol aqueous solutions. Only pure water was detected in the permeate before the water content decreased to ca. 9% for water/ethanol mixtures and ca. 6% for water/isopropanol mixtures, respectively.

Oxygen permeation through Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCFO) perovskite tubular membranes was investigated under vacuum and elevated pressures conditions. This paper is focused on dependence of oxygen permeation flux on oxygen recovery, temperature and feed pressure. Oxygen flux decreases with the raise of oxygen recovery, more quickly when the recovery is larger than about 60%. Elevating the feed pressure speeds up the decrease of oxygen fluxes against oxygen recovery. Oxygen permeation flux increases with the feed pressure of air, however, the increment decreases gradually. Oxygen permeation flux and recovery increase with temperature gradually at a constant airflow rate and pressure. The permeation flux reached 9.5 cm3/cm2 min at an oxygen recovery of 48% under the condition of 925 °C and vacuum pressure of ∼100 Pa for permeation side and 7 atm for feed side. Long-term operation under vacuum and elevated pressure shows that oxygen purity larger than 99.4% can be obtained when silver was used as sealant.

By microwave-assisted hydrothermal synthesis (MAHS) method, a&b-oriented zeolite T membranes were firstly prepared on the α-Al2O3 tubes from the a&b-oriented seed layers. By adjusting the pH value of the seed suspension, zeolite T seeds about 8.5 μm long were uniformly deposited on the support surface with their a&b-axes perpendicular to the support surface. After the secondary growth, the a&b-oriented seed layers grew into a&b-oriented zeolite T membranes with the thickness of about 7 μm. A formation mechanism of the membranes was proposed, which could provide general guidance for the preparation of other types of zeolite membranes. The as-synthesized membranes displayed high pervaporation (PV) performance for alcohol/water liquid mixtures. Aiming at the application in the industrial dehydration process, the hydrothermal stability and acid stability of the zeolite T membranes were also investigated.

Zeolite T membranes were firstly prepared on the α-Al2O3 tubes by microwave-assisted in situ nucleation and secondary growth. The obtained membranes were characterized by XRD, SEM, single gas permeation, and pervaporation (PV). In the PV dehydration of ethanol and 2-propanol, the as-synthesized membranes displayed high separation performance. For the 90 wt.% alcohol/water mixtures at 338 K, the water flux reached 1.23 kg m− 2 h− 1 for the dehydration of ethanol and 1.52 kg m− 2 h− 1 for the dehydration of 2-propanol; both separation factors were higher than 10, 000.

The effect of different heteropoly acids (HPA) on the catalytic properties of Pd–HPA/SiO2 in the selective oxidation of ethylene to acetic acid was investigated in this work. The results demonstrated that a good balance of Brönsted acid sites and Pd2+ ions stabilized by Brönsted acid sites was an indispensable factor in producing acetic acid.

A pure M1 phase catalyst was originally obtained by treatment of the multi-phase Mo–V–Te–Nb–O catalyst in the reaction atmosphere and was systematically studied by some physico-chemical techniques. The results showed that the reaction treatment played an important role in improving the performance of the catalyst.

Different methods were used to eliminate the negative effect of surface Te0 in Mo-V-Te-Nb-O catalysts. The characterization and catalytic results showed that the best catalytic performance was obtained on the catalyst prepared by addition of HNO3, and the excellent catalytic behavior could be attributed to the elimination of surface Te0 and the optimum synergetic effect between the M1 and M2 phases.

Thin proton-conducting electrolyte with composition BaCe0.8Gd0.2O3−δ (BCGO) was prepared over substrates composed of Ce0.8Gd0.2O1.9 (CGO)-Ni by the dry-pressing method. Solid oxide fuel cells (SOFCs) were fabricated with the structure Ni-CGO/BCGO/Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCFO)-CGO. The performance of a single cell was tested at 600 and 650 °C, with ammonia directly used as fuel. The open circuit voltages (OCVs) were 1.12 and 1.1 V at 600 and 650 °C, respectively. The higher OCV may be due to both the compaction of the BCGO electrolyte (no porosity) and complete decomposition of ammonia. The maximum power density was 147 mW cm−2 at 600 °C. Comparisons of the cell with hydrogen as fuel indicate that ammonia can be treated as a substitute liquid fuel for SOFCs based on a proton-conducting solid electrolyte.

The electrochemical properties of an Sm0.5Sr0.5CoO3−δ/Co3O4 (SSC/Co3O4) composite cathode were investigated as a function of the cathode-firing temperature, SSC/Co3O4 composition, oxygen partial pressure and CO2 treatment. The results showed that the composite cathodes had an optimal microstructure at a firing temperature of about 1100 °C, while the optimum Co3O4 content in the composite cathode was about 40 wt.%. A single cell with this optimized C40-1100 cathode exhibited a very low polarization resistance of 0.058 Ω cm2, and yielded a maximum power density of 1092 mW cm−2 with humidified hydrogen fuel and air oxidant at 600 °C. The maximum power density reached 1452 mW cm−2 when pure oxygen was used as the oxidant for a cell with a C30-1100 cathode operating at 600 °C due to the enhanced open-circuit voltage and accelerated oxygen surface-exchange rate. X-ray diffraction and thermogravimetric analyses, as well as the electrochemical properties of a CO2-treated cathode, also implied promising applications of such highly efficient SSC/Co3O4 composite cathodes in single-chamber fuel cells with direct hydrocarbon fuels operating at temperatures below 500 °C.

Thin dense Pd/Al2O3 composite membranes were prepared by a vacuum electroless plating technique. By the influence of vacuum effect on the both sides of tubular substrate, thin dense Pd composite membranes with finer and more uniform microstructure were rapidly deposited. These features would significantly improve hydrogen permeation performance and enhance stability as compared with the conventional electroless plating method. Hydrogen permeance of these membranes up to 22.4 m3/m2 h bar and the ideal permselectivity over 3000 were obtained at 500 °C. These membranes were stable over a period of 470 h and over 10 temperature cycles and gas-exchanging cycles over the temperature range of 350–480 °C under H2 or Ar atmosphere without any significant changes for hydrogen permeation performance. Moreover, the chemical stability of these membranes in the various H2–X (X = Ar, N2, steam, CO2, CH4 and CO) mixtures were investigated. These membranes exhibited good stability over a period of 2000 h and could resist temperature fluctuations under the operating conditions.

Asymmetric dual-phase composite membranes for oxygen separation were conveniently fabricated by an acid leaching technique. A thin dense layer of Ce0.85Sm0.15O1.925/Sm0.6Sr0.4FeO3−δ was left by controlling the degree of acid leaching, and a porous substrate of Ce0.85Sm0.15O1.925 with a fluorite structure was formed after dissolution of Sm0.6Sr0.4FeO3−δ with a perovskite structure in HCl. Thus, a thin dense layer and a porous substrate can be fabricated in a single step in which traditional shrinkage mismatch and chemical reaction between thin dense layers and porous substrates can be avoided. The thickness of the dense layer can be controlled by varying the acid leaching time. Hence, dual-phase composite membranes with high oxygen flux can be obtained.

Thin NaA zeolite membranes, with uniform and small crystals, were prepared on the tubular α-Al2O3 support by adding a small amount of tetramethylammonium hydroxide (TMAOH) in the clear synthesis solution. The as-synthesized NaA zeolite membranes were characterized by XRD and SEM. The permeation properties of the membranes were evaluated by pervaporation and gas permeation. The effects of TMAOH amount on membrane formation and permeation properties were investigated. By addition of suitable amount of TMAOH in the clear synthesis solution, the crystals size of NaA zeolite could be remarkably reduced from about 10 μm to 3–4 μm, and the membrane thickness correspondingly reduced from about 16 μm to 5 μm. The thinner membrane prepared by adding TMAOH in the clear synthesis solution, with uniform and small crystal, displayed higher perm-selective properties than that without adding TMAOH. For the as-synthesized NaA zeolite membrane prepared with adding suitable amount of TMAOH (x = 1), the separation factor (water/isopropanol) was 4700 and the flux was 1.67 kg/(m2 h), which were higher than that without adding TMAOH of 339 and 1.08 kg/(m2 h), respectively. The ideal separation factor of H2/N2 was 6.60, higher than that without adding TMAOH of 3.41.

FAU-type zeolite membranes were prepared by an “in situ aging-microwave synthesis” (AM) method using a clear solution with the molar composition of 70Na2O: 1Al2O3: 20SiO2: 2000H2O. After two-stage AM synthesis, a defect-free FAU zeolite layer consisted of uniform zeolite crystals was formed on the support surface. The Si/Al ratio was ca. 1.5. The single-gas permeation and single-liquid vapor permeation properties of the as-synthesized zeolite membranes were investigated. The permselectivity of H2 and N2 was as high as 4.25 for FAU-type zeolite membranes. Pervaporation measurements were carried out for the dehydration of ethanol and isopropanol aqueous solutions. FAU-type zeolite membranes displayed relatively high permeation flux and water-selectivity.

In this study, we demonstrated a novel and convenient seeding method for preparing zeolitized diatomite with hierarchical porosity. The silicalite-1 nanocrystals, grown in-situ on the surface of diatomite starting from a steam-assisted crystallization process with the aid of CTAB, induced the crystallization of diatomite under steam at 150 °C. The hierarchical pore structure of the zeolitized diatomite was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and N2 adsorption analysis. The confinement of CTAB could be controlled by using different solvents. Dissolving in water gave rise to smaller zeolite particles, whereas with ethanol, the formation of larger particles is observed, which is because CTAB tends to form smaller micelles in ethanol than in water.

Hydrogen permeation performance of three thin palladium–copper composite membranes with different thicknesses had been studied between 398 K and 753 K. Hydrogen permeance was obtained up to 2.7 × 10− 6 mol/(m2 s Pa) with an ideal selectivity over 1000 at 753 K. The hydrogen permeation exhibited two different activation energies over the temperature range: lower activation energy of about 9.8 kJ/mol above 548 K, while higher activation energy of about 26.4 kJ/mol below 548 K. After permeation tests, the alloy membranes were characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis and in situ X-ray diffraction. Palladium segregation on the surface of these palladium–copper alloys may induce changes of hydrogen permeation performance and thus influence the activation energies.

Highly efficient and chemically compatible LnxSr1−xCoO3−δ (Ln = La, Sm, Gd, …)/Co3O4 electrocatalysts for oxygen reduction reaction are presented and the very low cathode polarization resistances and excellent performances implied their promising application for developing intermediate-temperature solid oxide fuel cells (SOFCs), as well as potential application for oxygen separation membranes.

Interests are focused on preparation of hierarchical porous materials with zeolite structures by using soft or rigid templates in order to solve diffusion and mass transfer limitations resulting from the small pore sizes of zeolites. Here we develop a convenient template-free sol–gel method to synthesize hierarchical porous materials with ZSM-5 structures. This method involves hydrothermal recrystallization of the xerogel converted from uniform ZSM-5 sol by a vacuum drying process. By utilizing this method we can manipulate the size of zeolite nanocrystals as building units of porous structures based on controlling temperature of recrystallization, consequently obtain hierarchical porous materials with different intercrystalline pore sizes and ZSM-5 structures.

High-performance silicalite membranes were successfully prepared on porous tubular silica supports by in situ hydrothermal synthesis without seeding. The separation performance of the as-synthesized silicalite membranes was evaluated by separating ethanol/water mixtures. It was found that both the aging temperature and aging time had a significant influence on the performance of silicalite membranes when the silica tubes and synthesis solution were aged together. The tubular silica supports were filled with glycerol/water mixtures to reduce the penetration of synthesis solution, and it was found that this solution-filling method could effectively improve the membrane flux by about 27% with a higher separation factor. By using two-step hydrothermal synthesis, the silicalite membrane with a total flux of 0.58 kg/m2 h and ethanol/water separation factor of 95 at 333 K was obtained. The method in this work has been demonstrated a simple and effective way to skip the deposition of pre-formed nanosized seeds on tubular supports before the secondary growth of silicalite membrane.

Uniform and dense NaA zeolite membranes were synthesized on the tubular α-Al2O3 support with a vacuum-assisted method by using low vacuum. The effect of vacuum degree on the formation of the NaA zeolite membrane was investigated. With the assistance of the vacuum, the negative influence of the gravitation force could be effectively reduced. Therefore, the zeolite particles could transport to cover the entire area of the support surface homogeneously, which facilitated formation of uniform and dense zeolite membranes. With the assistance of the vacuum, most zeolite particles migrated to the support surface for membrane formation, and the amount of crystals forming in the liquid phase could be greatly reduced. In addition, zeolite particles could be transported to the support surface rapidly, increasing the rate of dense membrane formation. The pervaporation properties of the as-synthesized zeolite membranes were evaluated by dehydration of 95 wt.% isopropanol/water mixtures at 343 K. The as-synthesized NaA zeolite membrane prepared with the vacuum-assisted method (i.e., with the assistance of 10 mmHg vacuum degrees), showed good pervaporation properties. The separation factor (water/isopropanol) was 3781 and the flux was 1.49 kg/m2h, respectively. Based on the observed results, the formation mechanism of zeolite membrane with vacuum-assisted method was discussed.

Uniform and dense NaA zeolite membranes were prepared on the tubular porous α-Al2O3 supports in the electric field, where the charged zeolite particles attracted and transported to the support surface for membrane preparation homogeneously. The membranes properties were characterized by XRD, SEM and pervaporation for dehydration of 95 wt% isopropanol/water mixture at 343 K, respectively. The applied potential had great effect on membrane morphology, membrane thickness and separation performance. Under the action of the applied electric field, the negatively charged zeolite particles could migrate to the support surface homogenously and rapidly, facilitating to form uniform and dense membranes in a short time. Most zeolite particles transported to the support surface for membrane formation in the electric field, which increased the percentage of the product contained in the support surface and thus fully utilized the zeolite particles. High-quality NaA zeolite membrane, i.e., with a separation factor (water/isopropanol) of 3281 and a flux of 1.24 kg/(m2 h), could be prepared with the electrical potential of 1.0 V.

Hierarchical porous materials with zeolite structures show great promise in catalysis due to combining the advantages of zeolites and mesoporous materials. Here a novel template-free sol–gel method is developed to synthesize hierarchical porous silica materials. This method involves solvothermal recrystallization of the xerogel converted from uniform silicalite-1 precursor sol via vacuum drying process. The zeolite sol and the solid samples were characterized by laser light scattering, XRD, N2 adsorption/desorption isotherm, FTIR, SEM, TEM and thermal analysis technologies. The results show that we obtain two novel materials with different mesoporous structures and silicalite-1 walls by using different recrystallization media, one of which has irregular arrays of mesopores, the other possesses 3D wormhole mesoporous structure. We speculate that formation of different mesoporous structures results from different nucleation and growth process of materials

Zr-substituted Ba0.5Sr0.5Co0.8Fe0.1Zr0.1O3−δ (BSCFZO) powders for oxygen membrane were synthesized by the solid-state reaction (SSR) method. Oxygen permeation fluxes (JO2JO2) across the dense Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCFO) and BSCFZO membrane disks were measured at 973–1123 K, and they increased as BSCFO (synthesized by improved EDTA–citric acid complexing method, ECC) > BSCFO (SSR) > BSCFZO. The reduced oxygen permeability of Zr-substituting BSCFZO material can be attributed primarily to the higher oxidation state of Zr cations than ones of iron cations, thus lead to the decrease in the oxygen vacancy concentration in BSCFO. Oxygen permeation measurements with different oxygen partial pressure gradients and membrane thicknesses demonstrate the bulk oxide ionic diffusion is the rate-limiting step for the BSCFZO membrane in the range of temperatures investigated (973–1123 K). The enhanced stability of BSCFZO demonstrated by the differential thermal analysis (DTA) and high-temperature X-ray diffraction (HT-XRD) characterizations, show that the structural stability can be improved when the Fe ions in the B-sites of BSCFO materials are substituted partially by Zr cations.

High-performance silicalite-1 membranes were successfully synthesized on novel porous silica tubes by two-step in-situ hydrothermal synthesis. The flux and separation factor towards ethanol/water mixture at 60°C were 0.56 kg/(m2·h) and 84, respectively. The as-synthesized silicalite-1 membranes were characterized by scanning electron microscopy (SEM). The influence of different synthesis conditions on the separation performance of the silicalite-1 membranes was investigated. It was found that the average flux of silicalite-1 membranes was improved by about 26% after filling the silica tubes with mixed solution containing glycerol and water. After calcinating at 400°C for 5 h repeatedly, membrane synthesized on silica tube still showed high pervaporation performance towards ethanol/water mixture even at a calcination rate of 4°C/min, which suggested that silica support was more suitable for preparing high-performance silicalite-1 membranes.

Dense BaCe0.15Fe0.85O3−δ (BCF1585) ceramic membranes synthesized by the solid-state reaction (SSR) method and EDTA-citric acid (EC) process were investigated by X-ray powder diffraction, total conductivity, oxygen permeation, etc. XRD results revealed the perovskite structure of the powders prepared by the EC process was easier to be developed than that of prepared by SSR method. Membranes derived from EC had higher density, pure phase structure and fewer defects comparing to those derived from SSR method. However, membranes derived from SSR method had higher oxygen permeability. Thickness experiments revealed that the oxygen permeation fluxes of the membranes synthesized by both methods are all jointly controlled by surface exchange and bulk diffusion in the range of 0.7–2.0 mm. The long-term oxygen permeation operation revealed that the membranes derived from both methods exhibit good oxygen permeation stability.

Cobalt-free BaCexFe1−xO3−δ (x = 0.15–0.85) membranes have both high oxygen permeation flux and high structure stability even in reducing atmosphere at high temperature. The oxygen flux of BaCe0.15Fe0.85O3−δ (BCF1585) membrane reached at 0.5 ml/min cm2 at 900 °C with a thickness of 1.4 mm. Oxygen permeation results showed that oxygen permeation controlling step mainly was oxygen ion diffusion in the membrane bulk if the membrane thicker than 1.0 mm, and mainly was surface oxygen exchange if the membrane thinner than 0.78 mm. Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) porous layer coated on the membrane surfaces could effectively accelerate the surface oxygen exchange rate, thus greatly improve the oxygen permeability of the membrane. In some cases, the oxygen permeation flux of the membrane coated with BSCF was even two times higher than that of the membrane without coating. Perovskite structure of BCF1585 was still maintained at 950 °C even after it was exposed to 5% H2 + Ar mixture gas for 1 h.

In order to optimize the synthesis conditions for a specific zeolite membrane with high efficiency, and generalize this method to other types of zeolite membranes, a fundamental understanding of the membrane formation mechanism is of great importance. In present paper, gravimetric analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), attenuated total reflectance/Fourier transform infrared spectroscopy (ATR/FTIR), and gas permeation were carried out to characterize the whole formation process of LTA zeolite membrane which was synthesized by “in situ aging-microwave heating” method (AM method), and the formation mechanism of LTA zeolite membrane was proposed. With this mechanism, a gel layer is first formed on the support after in situ aging, which contains plenty of pre-nuclei. During the following microwave assistant crystallization, these pre-nuclei rapidly and simultaneously develop into crystal nuclei, and then crystal growth by propagation through the amorphous primary particles (with size of ca. 50 nm) goes on, and finally, the amorphous particles transform into LTA crystal particles with the same size. This propagation growth process is followed by or parallel with the agglomeration and densification of the primary particles. In this way, compact LTA zeolite membrane consisting of sphere grains with undefined crystal facets is obtained.

Oxygen permeation fluxes across the dense Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCFO) membrane disks were measured under an air/helium oxygen partial pressure gradient at high pressures (up to 10 atm) and various temperatures (973–1123 K). The fabricated BSCFO membrane exhibited good oxygen permeability with a high oxygen permeation flux of 2.01 ml min− 1cm− 2 (thickness: 1.37 mm) at 1123 K and 10 atm. Oxygen permeation results were analyzed theoretically using the surface exchange current model. The dependences of the oxygen permeation fluxes on the oxygen partial pressure gradient, suggested that the bulk oxygen ionic diffusion was the rate-limiting step for the overall oxygen permeation process across the BSCFO membrane. The ambipolar diffusion coefficients (Da), the oxygen vacancy diffusion coefficients (Dv) and the oxygen ionic conductivities (σi) of the BSCFO material at different temperatures (973–1123 K) were calculated. It was found that BSCFO possessed high oxygen diffusion coefficients and ionic conductivities, which resulted in the good oxygen permeability of BSCFO. In addition, the BSCFO membrane exhibited good stability of oxygen permeation at 1123 K, while the deterioration of oxygen permeation stability was observed at 1098 K due to structural changes occurring at the surface of the BSCFO membrane disk as demonstrated by XRD.

The structural stability under reducing environment and oxygen permeation fluxes of perovskite-type BaCexFe1−xO3−δ (0 ≤ x ≤ 0.15) ceramic membranes were investigated. The XRD results showed that 5% of cerium doping into the perovskite B-site can make the BaFeO3−δ transform from the hexagonal structure to the cubic structure, and make the structure stable under 10% H2–Ar mixed gas at 900 °C for 1 h, but breaks down after 5 h. Lattice parameters and oxygen non-stoichiometry of the as-prepared and reduced samples were measured. Oxygen permeation fluxes of the membranes were measured between 700 and 950 °C. Oxygen permeation during the cooling and heating circles showed that 5% of cerium doping into the perovskite B-site can avoid the phase transformation. The optimum cerium-doped amount on the B-sites was found to be 10%.

Oxygen-permeable ceramic membrane materials of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCFO) and Fe-substituted partially Ba0.5Sr0.5Co0.8Fe0.1Zr0.1O3−δ (BSCFZO) were synthesized by solid state reaction method. The BSCFO material possess purely cubic perovskite structure, while minor impurity phase (Ba,Sr)ZrO3 exists in the perovskite-type BSCFZO material. Oxygen permeability of these dense membrane disks was measured at different temperatures (973–1123 K). The reduced oxygen permeability of BSCFZO was resulted from the decrease of the oxygen vacancy concentration and ionic conductivity due to the dissolution of Zr4+ in the B-sites of BSCFO. However, good stability of oxygen permeation of BSCFZO membrane was achieved at the temperature lower than 1123 K. Structural stability of the BSCFZO material was studied by the high temperature XRD technique, which demonstrated that its stability was significantly improved when the Fe ions in the B-sites of BSCFO were substituted partially by Zr4+ ions.

A series of cobalt-free and low cost perovskite oxygen permeable membranes based on BaCexFe1−xO3−δ (BCF) oxides was successfully synthesized and the membrane showed both high oxygen permeability and high stability under reductive atmosphere, which will be most suitable for constructing a membrane reactor for selective oxidation of light hydrocarbons to syngas or high value corresponding oxygenates.

A high quality pure hydroxy-sodalite zeolite membrane was successfully synthesized on an α-Al2O3 support by a novel microwave-assisted hydrothermal synthesis (MAHS) method. Influence of synthesis conditions, such as synthesis time, synthesis procedure, etc., on the formation of hydroxy-sodalite zeolite membrane by MAHS method was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and gas permeation measurements. The synthesis of hydroxy-sodalite zeolite membrane by MAHS method only needed 45 min and synthesis was more than 8 times faster than by the conventional hydrothermal synthesis (CHS) method. A pure hydroxy-sodalite zeolite membrane was easily synthesized by MAHS method, while a zeolite membrane, which consisted of NaX zeolite, NaA zeolite and hydroxy-sodalite zeolite, was usually synthesized by CHS method. The effect of preparation procedures had a dramatic impact on the formation of hydroxy-sodalite zeolite membrane and a single-stage synthesis procedure produced a pure hydroxy-sodalite zeolite membrane. The pure hydroxy-sodalite zeolite membrane synthesized by MAHS method was found to be well inter-grown and the thickness of the membrane was 6–7 μm. Gas permeation results showed that the hydrogen/n-butane permselectivity of the hydroxy-sodalite zeolite membrane was larger than 1000.

The dual-phase membrane of La0.15Sr0.85Ga0.3Fe0.7O3−δ–Ba0.5Sr0.5Fe0.2Co0.8O3−δ (LSGF–BSCF) was prepared successfully. This membrane was characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe micro-analyzer (EPMA). This membrane has a dense dual-phase structure: LSGF being the dense body of this membrane and BSCF as another phase running along the LSGF body. This structure is favorable for the oxygen permeation through the membrane. The oxygen permeation test shows that the oxygen permeation flux of LSGF–BSCF membrane (JO2=0.45 ml/min cm2, at 915 °C) is much higher than that of LSGF membrane (JO2=0.05 ml/min cm2). Thickness dependence of oxygen permeation indicates that the oxygen permeation is controlled by the bulk diffusion. Compared to pure BSCF, the dual-phase membrane of LSGF–BSCF is stable in reducing atmosphere.

An oxygen permeable membrane based on Ba0.5Sr0.5Co0.8Fe0.2O3−δ is used to supply lattice oxide continuously for oxidative dehydrogenation of ethane to ethylene with selectivity as high as 90% at 650 °C.

A titanium-based perovskite-type oxide was synthesized by an improved method of combining EDTA acid and citric acid complexes. High structural stability, good sintering ability, and relatively high oxygen permeation flux were obtained simultaneously for disks synthesized from this ceramic oxide.

A new perovskite material, BaCe0.1Co0.4Fe0.5O3−δ used as dense oxygen permeable membrane for partial oxidation of methane (POM) reaction was investigated. In order to improve the synergetic effects between membrane and catalyst, LiLaNiO/γ-Al2O3 catalyst was directly packed onto the surface of the membrane to carry out POM. In BaCe0.1Co0.4Fe0.5O3−δ membrane reactor, high oxygen permeation flux, high CH4 conversion and CO selectivity were obtained. At 950 °C, oxygen flux of 9.5 ml cm−2 min−1, CH4 conversion of 99% and CO selectivity of 93% were achieved with a membrane thickness of 1.0 mm. There was an induction process at the initial stage of POM, which was related to the reduction of NiO to Ni0 in LiLaNiO/γ-Al2O3 catalyst. Experiments illustrated that higher reaction temperature would shorten the induction time. During continuously operating for 1000 h at 875 °C, no degradation of performance of the membrane reaction was observed. SEM characterization also demonstrated that the membrane disc maintained an integral structure without any cracks after long-term operation.

Carbon molecular sieve membranes (CMSMs) were fabricated by pyrolysis of MOF-doped polyimide mixed matrix membranes. ZIF-108 (Zn(2-nitroimidazolate)2) was used as a dopant to tailor the micropores of the as-prepared CMSMs into narrow ultramicropores, providing a remarkable combination of permeability and selectivity of membranes in CO2/CH4, O2/N2 and N2/CH4 separation.

The metal–organic framework ZIF-8, which undergoes hydrolysis under hydrothermal conditions, is endowed with high water-resistance after a shell-ligand-exchange-reaction. The stabilized ZIF-8 retains its structural characteristics with improved application performances in adsorption and membrane separation.

A highly efficient catalyst, MoV0.3Te0.17Nb0.12O, used for acrylic acid (AA) production from propane, was used as an anodic catalyst in an SOFC reactor, from which AA and electric power were cogenerated at 400–450 °C.

Mn1.5Co1.5O4 spinel oxide as a cathode or one component of a composite cathode presents no visible reaction with an Y2O3-stabilized ZrO2 electrolyte. The low electrode polarization resistances and good performance compared with traditional Sr-doped LaMnO3–YSZ composite cathodes imply promising application for the next generation of intermediate-temperature solid oxide fuel cells.

Ceria-based dual-phase membranes showing high oxygen permeation fluxes and stability under a CO2 environment are promising materials for CO2 capture via an oxyfuel route. The high oxygen permeation fluxes compared with other dual-phase membranes are derived from the mixed conducting properties of the perovskite oxides used in the dual-phase membranes.

Twin growth in the synthesis of b-oriented MFI films is successfully suppressed by applying microwave irradiation on a b-oriented MFI seed layer, relying on the nucleation-related bottleneck effect. Electrochemical oxidation experiments demonstrated the importance of twin suppression in enhancing the diffusion of guest molecules in MFI films.

SrCo0.8Fe0.2O3-δ is a controversial material whether it is used as an oxygen permeable membrane or as a cathode of solid oxide fuel cells. In this paper, carefully synthesized powders of perovskite-type SrxCo0.8Fe0.2O3-δ (x = 0.80–1.20) oxides are utilized to investigate the effect of A-site nonstoichiometry on their electrochemical performance. The electrical conductivity, sintering property and stability in ambient air of SrxCo0.8Fe0.2O3-δ are critically dependent on the A-site nonstoichiometry. Sr1.00Co0.8Fe0.2O3-δ has a single-phase cubic perovskite structure, but a cobalt-iron oxide impurity appears in A-site cation deficient samples and Sr3(Co, Fe)2O7-δ appears when there is an A-site cation excess. It was found that the presence of the cobalt-iron oxide improves the electrochemical performance. However, Sr3(Co, Fe)2O7-δ has a significant negative influence on the electrochemical activity for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The peak power densities with a single-layer Sr1.00Co0.8Fe0.2O3-δ cathode are 275, 475, 749 and 962 mW cm−2 at 550, 600, 650 and 700 °C, respectively, values which are slightly lower than those for Sr0.95Co0.8Fe0.2O3-δ (e.g. 1025 mW cm−2 at 700 °C) but much higher than those for Sr1.05Co0.8Fe0.2O3-δ (e.g. only 371 mW cm−2 at 700 °C). This remarkable dependence of electrochemical performance of the SrxCo0.8Fe0.2O3-δ cathode on the A-site nonstoichiometry reveals that lower values of electrochemical activity reported in the literature may be induced by an A-site cation excess. Therefore, to obtain a high performance of SrxCo0.8Fe0.2O3-δ cathode for IT-SOFCs, an A-site cation excess must be avoided.

Highly efficient and chemically compatible LnxSr1−xCoO3−δ (Ln = La, Sm, Gd, …)/Co3O4 electrocatalysts for oxygen reduction reaction are presented and the very low cathode polarization resistances and excellent performances implied their promising application for developing intermediate-temperature solid oxide fuel cells (SOFCs), as well as potential application for oxygen separation membranes.