Sorption properties of hydrogen-selective PDMS/zeolite 4A mixed matrix membrane

The present study explores the fundamental science of estimating sorption of gases in membranes comprised of inorganic porous fillers within a polymer matrix with a novel semi-empirical correlation. The sorption properties of H₂, C₃H₈, CO₂ and CH₄ were determined in polydimethylsiloxane (PDMS)/zeolite 4A mixed matrix membranes (MMMs) to assess the viability of these membranes for hydrogen purification and natural gas sweetening. Zeolite filling in MMMs results an increase in solubility over neat PDMS membrane. In addition, incorporation of zeolite 4A to PDMS membrane improved H₂ permeation and H₂/CH₄ selectivity. The results confirmed that zeolite 4A can significantly improve the separation properties of poorly H₂-selective PDMS membrane from 0.7 up to 11 and this overcomes the Robeson upper-bound limitation. This improvement was explained referring the Flory–Huggins interaction parameter within MMMs.     

Experimental investigation of natural polysaccharide-based mixed matrix membrane modified with graphene oxide and Pd-nanoparticles for enhanced gas separation performance

In this work, we proposed a mixed matrix membrane prepared by using a glycerol modified guar gum (GGP) polymer matrix incorporated with graphene oxide (GO). The influence of varying GO concentration on the gas separation performance was investigated and 2 wt% was found to be the optimum concentration for high performance. The 2 wt% GO mixed matrix membranes were further modified with Pd nanoparticles. When GO, and Pd nanoparticles were mixed, CO₂ permeability increased by 49.94%, while the permeability of H₂ gas molecules decreased by 98.11%, respectively, compared to the pristine GGP membrane. The selectivity of CO₂/H₂ was obtained as 18.27. The glass transition temperature of the membrane increased from 85 to 95.2 °C, tensile strength and elongation of the break were significantly improved by 29.09% and 84.37% through the addition of Pd and GO into the membrane. The scanning electron microscopy revealed a dense top surface after GO nanosheets incorporation. Further, the thermogravimetric analysis proposes that the modified membrane is thermally stable than GGP. Henceforth, the study suggests GO incorporation and Pd nanoparticles modification of guar gum membrane is a promising gas separation membrane with potentially high selectivity for CO₂ gas.     

Hydrogen separation and purification with poly (4-methyl-1-pentyne)/MIL 53 mixed matrix membrane based on reverse selectivity

The effect of MIL 53 (Al) metal organic framework on gas transport properties of poly (4-methyl-1-pentyne) (PMP) was determined based on reverse selectivity. Mixed matrix membranes (MMMs) were fabricated considering various weight percent of MIL 53 particles. The reverse MMMs permselectivities were evaluated through measurement of pure CO₂ and H₂ permeation together with calculation of CO₂/H₂ selectivity. The PMP/MIL 53 (Al) MMMs exhibited privileged CO₂/H₂ permselectivity in comparison with the neat PMP. In addition, CO₂ solubility coefficient was significantly increased with increasing the MIL 53 loading, while the H₂ solubility coefficient was almost remained unchanged. Moreover with increasing the feed pressure the permeability of CO₂ and CO₂/H₂ selectivity were dramatically enhanced, especially at higher filler loadings. Therefore, it was observed that the reverse selectivity of MMMs was enhanced so that the Robeson upper bound was overcome. The best yielding membranes (PMP/30 wt.% MIL 53) represented the CO₂ permeability and CO₂/H₂ selectivity of 377.24 barrer and 24.91 for pure gas experiments respectively.     

Preparation of CoB/ZIF-8 supported catalyst by single step reduction and its activity in hydrogen production

CoB/ZIF-8 supported catalysts were successfully prepared using Co/Zn-ZIF-8 as the precursor by single-step reduction, which was applied in hydrogen release from the hydrolysis of NaBH₄. Reducible Co ions of Co/Zn-ZIF-8 can be partially in-situ transformed into CoB by direct reduction, whereas ZIF-8 framework structure can be well preserved due to the resistance of Zn to reducing ambiences. Accordingly, CoB active components can be highly loaded onto ZIF-8 support to produce CoB/ZIF-8 catalysts. The texture evolution of Co/Zn-ZIF-8 during reduction was investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscope and nitrogen adsorption–desorption isotherms. Compared with the reduction of Co-ZIF-67, the framework structure of Co/Zn-ZIF-8 can be effectively preserved although Co ions of Co/Zn-ZIF-8 were partially reduced into cobalt-based alloy. In the hydrogen release from hydrolysis of NaBH₄, CoB/ZIF-8 supported catalyst exhibits excellent catalytic activity. The effect of NaOH concentration, NaBH₄ concentration and reaction temperature on hydrolysis reaction of NaBH₄ was deeply studied based on this catalyst. Compared with other published catalysts, this catalyst exhibits relatively low activation energy of about 57.72 kJ mol^?1.     

The evaluation of methane mixed reforming reaction in an industrial membrane reformer for hydrogen production

In this work, the performance of an industrial dense PdAg membrane reformer for hydrogen production with methane mixed reforming reaction was evaluated. The rate parameters of mixed reforming reaction on a Ni based catalyst optimized by using the experimental results. One-dimensional models have been considered to model the steam reforming industrial membrane reformer (SRIMR) and mixed reforming industrial membrane reformer (MRIMR). The models are validated by experimental data.The proficiency of MRIMR and SRIMR at similar conditions used as a basis of comparison in terms of temperature, methane conversion, hydrogen yield, syngas production rate and CO₂ flow rate. Results revealed that the methane conversion, hydrogen yield and syngas production rate in MRIMR is considerably higher than SRIMR. Furthermore, the operation temperature of MRIMR could be 195 °C lower than that for SRIMR. This would contribute to a major decrease in process costs as well as a reduction in catalyst sintering. On the other hand, although MRIMR consumes CO₂, the exited CO₂ flow rate at the SRIMR is three times more than that of at the MRIMR, which is a main advantage of MRIMR from the environmental issues point of view.     

High performance ZIF-8/PBI nano-composite membranes for high temperature hydrogen separation consisting of carbon monoxide and water vapor

Two types of advanced nano-composite materials have been formed by incorporating as-synthesized wet-state zeolitic imidazolate frameworks-8 (ZIF-8) nano-particles into a polybenzimidazole (PBI) polymer. The loadings of ZIF-8 particles in the two membranes (i.e., 30/70 (w/w) ZIF-8/PBI and 60/40 (w/w) ZIF-8/PBI) are 38.2 vol % and 63.6 vol %, respectively. Due to different ZIF-8 loadings, variations in particle dispersion, membrane morphology and gas separation properties are observed. Gas permeation results suggest that intercalation occurs when the ZIF-8 loading reaches 63.6 vol %. The incorporation of ZIF-8 particles significantly enhances both solubility and diffusion coefficients but the enhancement in diffusion coefficient is much greater. Mixed gas tests for H₂/CO₂ separation were conducted from 35 to 230 °C, and both membranes exhibit remarkably high H₂ permeability and H₂/CO₂ selectivity. The 30/70 (w/w) ZIF-8/PBI membrane has an H₂/CO₂ selectivity of 26.3 with an H₂ permeability of 470.5 Barrer, while the 60/40 (w/w) ZIF-8/PBI membrane has an H₂/CO₂ selectivity of 12.3 with an H₂ permeability of 2014.8 Barrer. Mixed gas data show that the presence of CO or water vapor impurity in the feed gas stream does not significantly influence the membrane performance at 230 °C. Thus, the newly developed H₂-selective membranes may have bright prospects for hydrogen purification and CO₂ capture in realistic industrial applications such as syngas processing, integrated gasification combined cycle (IGCC) power plant and hydrogen recovery.     

Surface and interface engineering in CO2-philic based UiO-66-NH2-PEI mixed matrix membranes via covalently bridging PVP for effective hydrogen purification

We report on the fabrication of the defect-free mixed-matrix membrane (MMM) based on the polyethylenimine (PEI) matrix with uniformly dispersed metal-organic framework (MOF) filler UiO-66-NH₂, covalently bonded by polyvinylpyrrolidone (PVP). The key feature of the molecular level-controlled filler deposition in prepared UiO-66-NH₂-PVP-PEI membranes was bridging the MOF particles to the PEI polymer matrix via PVP polymer chains. Such an approach improved the polymer-filler interface interactions and boosted the MOF dispersion into the polymer matrix for higher MOF loadings up to 23 wt %. The overall membrane structure and properties were characterized using FTIR, XRD, TG, DSC, SEM and 3D optical profiler techniques. Obtained results revealed the uniform dispersion of UiO-66-NH₂, the strong polymer-filler interface interactions and entanglement of PEI with UiO-66-NH₂-PVP. Furthermore, the outstanding CO₂/H₂ separation performance was determined for the UiO-66-NH₂-PVP-PEI membrane with 18 wt % of MOF loading; the average CO₂ permeability of 394 Barrer and the separation factor of 12 for circa 100 h of the membrane testing overcome the 2008 Robeson reverse upper bound limit. Such improved CO₂/H₂ separation performance was achieved due to the combination of the diffusion-solution mechanism with the preferential adsorption of the CO₂ via the reversible bicarbonate reaction with amino groups of the UiO-66-NH₂ and PEI which acts as fixed CO₂ carrier sites in MMM structure.     

Preparation, testing and modelling of a hydrogen selective Pd/YSZ/SS composite membrane

A palladium selective tubular membrane has been prepared to separate and purify hydrogen. The membrane consists of a composite material, formed by different layers: a stainless steel support (thickness of 1.9 mm), an yttria-stabilized zirconia interphase (thickness of 50 μm) prepared by Atmospheric Plasma Spraying and a palladium layer (thickness of 27.7 μm) prepared by Electroless Plating. The permeation properties of the membrane have been tested at different operating conditions: retentate pressure (1-5 bar), temperature (350-450 °C) and hydrogen molar fraction of feed gas (0.7-1). At 400 °C, a permeability of 1.1 × 10⁻⁸ mol/(s m Pa^0.5) and a complete selectivity to hydrogen were obtained. The complete retention of nitrogen was maintained for all tested experiment conditions, with both single and mixtures of gases, ensuring 100% purity in the hydrogen permeate flux.A rigorous model considering all the resistances involved in the hydrogen transport has been applied for evaluating the relative importance of the different resistances, concluding that the transport through the palladium layer is the controlling one. In the same way, a model considering the axial variations of hydrogen concentration because of the cylindrical geometry of the experimental device has been applied to the fitting of the experimental data. The best fitting results have been obtained considering Sieverts’-law dependences of the permeation on the hydrogen partial pressure.     

Fabrication,characterization, and application of palladium composite membrane on porous stainless steel substrate with NaY zeolite as an intermediate layer for hydrogen purification

Palladium composite membrane with excellent stability was successfully prepared using the electroless plating (ELP) route on a porous stainless steel (PSS) support for hydrogen separation. In order to modify the average pore size of PSS support and to prevent inter-metallic diffusion, the NaY zeolite layer was coated on the PSS support with the seeding and secondary growth method. A high-temperature membrane module was designed by Solid work software and fabricated from 316 L stainless steel with a knife-edge seal. The microstructures and morphologies of the samples were analyzed using XRD, BET, AFM, FESEM and EDX techniques. Permeation experiments were carried out with binary mixtures of H₂/N₂ with various ratios (90/10, 75/25 and 50/50) and pure H₂ and N₂ at different temperatures (350, 400 and 450 °C) and feed pressures (200–400 kPa). Hydrogen permeation tests showed that the membrane with a thickness of about 7 μm had a hydrogen permeance of 6.2 × 10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5 with an ideal H₂/N₂ selectivity of 736, at 450 °C. In addition, the results of stability tests revealed that the membrane could remain stable during a long-term operation by varying temperature and feed gases.     

A novel strategy for surface modification of polyimide membranes by vapor-phase ethylenediamine (EDA) for hydrogen purification

In this study, vapor-phase ethylenediamine (EDA) is utilized to specifically modify the physicochemical properties of the outer surface of polyimide membranes without modifying the internal membrane structure for hydrogen purification. The surfaces of polyimide membranes before and after EDA-vapor modification have been characterized by Fourier transform infrared-attenuated total reflectance (FTIR-ATR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), which confirmed the modification mechanism including the conversion of imide groups into amide groups with simultaneous cross-linking between polymer chains and a physical decrement in d-space. Based on pure gas permeation tests, only a 10-min vapor-phase EDA treatment can significantly improve H₂/CO₂ selectivity (up to ∼100). This is attributed to intensive surface modification by EDA vapor, hence rendering this simple and yet novel technique more effectively for hydrogen purification than the conventional solution approach. Although the H₂/CO₂ separation performance in mixed gas tests is not as superior as that in pure gas permeation tests, mixed gas results affirmed impressive H₂/CO₂ separation performance of vapor-phase EDA modified polyimide membranes. This novel vapor modification strategy appears to be promising for large-scale processes, especially the modification of hollow fiber membranes for industrial hydrogen purification.     

Preparation of a novel Pd/layered double hydroxide composite membrane for hydrogen filtration and characterization by thermal cycling

A layered double hydroxide (LDH) layer was grown directly on a porous stainless steel (PSS) surface to reduce the pore opening of the PSS and to be a middle layer retarding Pd/Fe interdiffusion. A thin Pd film (∼7.85 μm) was plated on the modified PSS tube by an electroless plating method. A helium leak test proved that the thin Pd on the LDH-modified PSS substrate was free of defects. The membrane had a H₂ flux of 28–36 m³/(m² h) and H₂/He selectivity larger than 2000 at a pressure difference of 1 bar. Thermal cycling between room temperature and 673 K was performed and showed that the membrane exhibited good permeance and selectivity. Long-term evaluation (1500 h) of the membrane at 673 K showed static results of H₂ flux (∼30 m³/(m² h)) and H₂/He selectivity (∼2000) over the 1500 h test period.     

Molecular-level mixed matrix membranes comprising Pebax and POSS for hydrogen purification via preferential CO2 removal

The molecular-level mixed matrix membranes (MMMs) comprising Pebax^® and POSS have been developed by tuning the membrane preparation process in this work. They exhibit a simultaneous enhancement in CO₂ permeability and CO₂/H₂ selectivity by optimizing the POSS content at extremely low loadings. This is mainly attributed to the large cavity of POSS itself and its effect on the segmental-level polymeric chain packing. More interestingly, the Pebax^®/POSS MMMs reveal a much higher separation performance in the mixed gas test than that in the pure gas test. The highest CO₂/H₂ selectivity reaches 52.3 accompanied by CO₂ permeability of 136 Barrer at 8 atm and 35 °C. This is due to the CO₂-induced plasticization that improves the free volume and polymer chain mobility, hence benefiting the interaction between the polymer matrix and penetrant CO₂. These features may ensure the superiority of Pebax^®/POSS molecular-level MMMs as CO₂-selective membranes in the industrial application of hydrogen purification.     

Preparation of dense composite membrane with Ba-cerate conducting oxide and rapidly solidified Zr-based alloy

Hydrogen separation with dense ceramic membranes is non-galvanic, i.e. it does not require any electrode or an external power supply to drive the separation, and the hydrogen selectivity is almost 100% because the membrane contains no interconnected porosity. In this study, a mixed proton-electron conducting perovskite made from BaCe_0.9Y_0.1O_3-δ (BCYO) was prepared using a solid-state reaction, whereas a rapidly solidified Zr-based alloy (RSZ) was obtained via a melt-spinning process at a specified cooling rate. Finally, the BCYO/RSZ composite membrane was successfully fabricated by aerosol deposition (AD) at room temperature. The powders and composite membranes were characterized by high-temperature X-ray diffraction (HTXRD), particle size analysis (PSA), scanning electron microscopy (SEM), and X-ray elemental mapping (XRM). The hydrogen permeability of the dense BCYO/RSZ composite membrane was measured with the change of temperature. Under a pure hydrogen atmosphere at 773 K-1073 K, the BCYO/RSZ composite membrane exhibited higher permeability compared with the sole BCYO membrane over the entire investigated temperature range.     

Preparation and electrochemical hydrogen storage properties of Co9S8 alloy coated with cobalt/graphene composite

A facile one-pot reduction process is used to obtain the cobalt/graphene composite (CoRGO). The CoRGO materials exhibit unique reticular globular morphology. Co₉S₈ hydrogen storage alloy is fabricated via mechanical alloying method. Different amounts of CoRGO are coated on the surface of Co₉S₈ alloy by ball milling. The electrochemical characterizations of the composites are conducted in the standard tri-electrode system. Ultimately, the CoRGO coated Co₉S₈ electrode shows preferable performance than the RGO modified alloy (603.6 mAh/g) and original alloy (577.3 mAh/g). As the additive content of CoRGO is 6 wt%, a maximum discharge capacity of 637.5 mAh/g is obtained. Furthermore, the cycle stability and high-rate dischargeability of the electrode are also enhanced. The Co particles in the CoRGO participate in the reversible redox reactions and the graphene provides high conductivity. The CoRGO with distinctive structure and morphology can not only improve the electrocatalytic activity but also increase the specific surface area of Co₉S₈ alloy. The cobalt and graphene species in the CoRGO composite serve a synergistic effect in further facilitating the hydrogen diffusion, expediting the charge transfer in/on the alloy and improving the corrosion resistance, thus enhancing the electrochemical performance and reaction kinetics of Co₉S₈ alloy.     

Nitrogen-doped graphene aerogel with an open structure assisted by in-situ hydrothermal restructuring of ZIF-8 as excellent Pt catalyst support for methanol electro-oxidation

We report an innovative strategy to prepare the porous N-doped graphene aerogel with an open structure and abundant defects by hydrothermal self-assembly of zeolitic imidazolate framework (ZIF)-8 and graphene oxide. The in-situ hydrothermal restructuring of ZIF-8 on graphene sheets plays a key role in the synthesis of the open structure and the uniform N-doping. The dissolution and restructuring of ZIF-8 on graphene oxide obviously suppress the stacking and reunion of graphene sheets to obtain the continuous macroporous structure. Moreover, the introduction of N and Zn creates the abundant N-doped sites and microporous structure. Its unique structure and composition improve the accessible surface area, the mass transfer diffusion, the dispersion and electronic structure of Pt nanoparticles, further resulting in the high catalytic performance of Pt-based catalyst for methanol oxidation reaction (MOR). Its MOR activity is about 1.8 times of commercial Pt/C, and its long cycling durability is improved by about 18.7% compared with commercial Pt/C. This work renders a promising method by utilizing ZIF-8 derivatives to synthesize the excellent N-doped carbon materials for electrochemical applications.     

Preparation of palladium membrane on Pd/silicalite-1 zeolite particles modified macroporous alumina substrate for hydrogen separation

A palladium composite membrane was successfully fabricated by electroless plating on a macroporous alumina tube. Pd/silicalite-1 zeolite particles were employed to reduce the pore size of the alumina support and improve its surface roughness. Moreover, the Pd⁰ existed in the Sil-1 particle can avoid the time consuming sensitization and activation steps for palladium seeding. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS) and X-ray diffraction (XRD) analysis were conducted for analyzing the detailed microstructure of the palladium composite membrane. The hydrogen permeation performance of the resulting palladium membrane was investigated at temperatures of 623 K, 673 K, 723 K and 773 K. The hydrogen permeance of 1.95 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ with an H₂/N₂ ideal selectivity of 1165 for the palladium membrane was obtained at 773 K. Furthermore, the resulting palladium membrane was stable for a long-term operation of 15 days at 673 K.     

In-situ synthesis of well dispersed CoP nanoparticles modified CdS nanorods composite with boosted performance for photocatalytic hydrogen evolution

Development of photocatalysts with characters of low-cost, environment friendliness, visible light response and good performance is vital for the transformation of solar energy into hydrogen fuel. Here, we constructed CoPCdS nanorods hybrid composites via a novel two-step in-situ growth method for the first time. The obtained CoPCdS composites exhibited remarkably enhanced photocatalytic performance and excellent stability in comparison with bare CdS nanorods. Notably, the optimum H₂ evolution rate of 1 wt%CoPCdS was 9.11 times higher than that of pristine CdS. The apparent quantum efficiency of the photocatalyst was calculated to be 11.6%. The superior activity of this material could be attributed to the role of well dispersed CoP nanoparticles and the intimate interface between CoP cocatalysts and CdS nanorods, which efficiently accelerated the separation and transfer of photogenerated electrons. This work provided a new in-situ growth method for the preparation of transition metal phosphides coated photocatalysts with boosted photocatalytic activity of hydrogen evolution.     

Metal-organic framework MIL-101(Cr) based mixed matrix membranes for esterification of ethanol and acetic acid in a membrane reactor

Mixed matrix membranes (MMMs) based on the polyimide Matrimid^® (PI) with metal-organic framework (MOF) MIL-101(Cr) as porous nanostructured filler were synthesized and applied as separation element in a membrane reactor to carry out the esterification of acetic acid with ethanol. The MMMs were characterized by techniques including X-ray diffraction, IR spectroscopy and scanning electron microscopy. In order to compare the performance of MIL-101(Cr)-PI MMMs in the membrane reactor, pure PI and HKUST-1-PI membranes were also used. MMMs provided a better reactor performance than the bare PI membrane because of the increase in permeability associated to the presence of MOF as filler. The PI membrane reactor barely achieved the same conversion as a fixed bed reactor, while the MIL-101(Cr)-PI membrane showed a reactor performance similar to that of the HKUST-1-PI membrane with higher stability, as confirmed by membrane characterization after the reaction experiments.     

Preparation of new self-humidifying composite membrane by incorporating graphene and phosphotungstic acid into sulfonated poly(ether ether ketone) film

In this work, a combination of constituent materials capable of improving the moisture retention and proton conductivity (PC) was incorporated into sulfonated poly (ether ether ketone) (SPEEK) membranes in order to prepare new, self-humidifying composite membranes (SHMs) for proton exchange membrane fuel cells. The property-improving components were incorporated into the cast SPEEK film in an appropriate microstructural architecture to prepare the SHMs with increased water retention and PC. SHMs were therefore prepared with the inclusion of carboxyl-functionalized graphene (G(c)) and phosphotungstic acid (PWA) with varying proportions into the SPEEK film. The structure of the SPEEK/G(c)/PWA composite membranes was characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis, thermogravimetric analysis and X-ray photoelectron spectroscopy. The physicochemical properties of the composite membranes, such as ion exchange capacity, water uptake, thermal stability and PC, were investigated. This work provides confirmation that self-humidifying properties are improved at temperature above 60 °C through a combinational inclusion of G(c) and PWA within SPEEK and the new self-humidifying membranes have potential for use in medium temperature direct methanol fuel cells.     

Fabrication and characterization of poly(phenylene oxide)/SBA-15/carbon molecule sieve multilayer mixed matrix membrane for gas separation

A novel multilayer mixed matrix membrane (MMM), consisting of poly(phenylene oxide) (PPO), large-pore mesoporous silica molecular sieve zeolite SBA-15, and a carbon molecular sieve (CMS)/Al₂O₃ substrate, was successfully fabricated using the procedure outlined in this paper. The membranes were cast by spin coating and exposed to different gases for the purpose of determining and comparing the permeability and selectivity of PPO/SBA-15 membranes to H₂, CO₂, N₂, and CH₄. PPO/SBA-15/CMS/Al₂O₃ MMMs with different loading weights of zeolite SBA-15 were also studied. This new class of PPO/SBA-15/CMS/Al₂O₃ multilayer MMMs showed higher levels of gas permeability compared to PPO/SBA-15 membranes. The permselectivity of H₂/N₂ and H₂/CH₄ combinations increased remarkably, with values at 38.9 and 50.9, respectively, at 10 wt% zeolite loading. Field emission scanning electron microscopy results showed that the interface between the polymer and the zeolite in MMMs was better at a 10 wt% loading than other loading levels. The increments of the glass transition temperature of MMMs with zeolite confirm that zeolite causes polymer chains to become rigid.