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.     

Co-based zeolitic imidazolate framework ZIF-9 membranes prepared on α-Al2O3 tubes through covalent modification for hydrogen separation

Hydrogen has been regarded as the most promising clean and renewable energy. Beside the production of the hydrogen, the separation of hydrogen is also an import issue before it can be used in fuel cells. Membrane-based separation technologies have gained considerable attentions due to its high efficiency and low energy consumption. Zeolite imidazolate framework (ZIF) membranes have drawn intense interest due to their zeolite-like properties such as permanent porosity, uniform pore size and exceptional thermal and chemical stability. It is rather challenged to prepare well-intergrown Co-based zeolitic imidazolate frameworks (ZIFs) membranes on porous α-Al₂O₃ tubes since Co-based ZIFs prefer to form crystals in the synthesis solution rather than grow as membrane layer on the support surface. In this work, we report the preparation of high-quality ZIF-9 membrane with high H₂/CO₂ selectivity and excellent thermal stability by using 3-aminopropyltriethoxysilane (APTES) as a covalent linker to modify the α-Al₂O₃ tube. Due to the formation of covalent bonds between APTES and ZIF-9, ZIF-9 nutrients are bound to the support surface, thus promoting the growth of dense and phase-pure ZIF-9 membrane with a thin thickness of about 4.0 μm. The gas separation performances of the ZIF-9 membrane were evaluated by single gas permeation and mixture gas separation of H₂/CO₂, H₂/N₂ and H₂/CH₄, respectively. The mixture separation factors of H₂/CO₂, H₂/CH₄, and H₂/N₂ of the ZIF-9 membrane are 21.5, 8.2 and 14.7, respectively, which by far exceeds corresponding Knudsen coefficients. Moreover, the as-prepared ZIF-9 membrane exhibits excellent stability at a relatively broad range of operating temperature, which is beneficial for the industrial application of hydrogen separation or further membrane reactor.     

Gas sorption in H2-selective mixed matrix membranes: Experimental and neural network modeling

Robust artificial neural network (ANN) was developed to forecast sorption of gases in membranes comprised of porous nanoparticles dispersed homogenously within polymer matrix. The main purpose of this study was to predict sorption of light gases (H₂, CH₄, CO₂) within mixed matrix membranes (MMMs) as function of critical temperature, nanoparticles loading and upstream pressure. Collected data were distributed into three portions of training (70%), validation (19%), and testing (11%). The optimum network structure was determined by trial-error method (4:6:2:1) and was applied for modeling the gas sorption. The prediction results were remarkably agreed with the experimental data with MSE of 0.00005 and correlation coefficient of 0.9994.     

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.     

Advances in materials process and separation mechanism of the membrane towards hydrogen separation

The increased demand for a reliable and sustainable renewable energy source encourages the hydrogen-based economy. For the same, membrane separation approaches were reviewed as an advantageous process over contemporary techniques due to the environmentally friendly nature, economically viable pathway, and easily adaptable technology. A comprehensive assessment for the advancements in the type of membranes namely, polymeric and mixed matrix membranes (MMMs) has been delineated in the present article with the fabrication methodologies and associated mechanism for hydrogen separation. In hydrogen separation mechanism of the membrane, depends on the morphology of the membrane (dense or porous). The existence of pores in membranes offers various gas transport mechanisms such as Knudsen diffusion, surface diffusion, capillary condensation, molecular sieving mechanisms were observed, depending on the pore size of membranes and in dense membrane gas transport through the solution-diffusion mechanism. In polymer membrane, hydrogen separation occurs mainly due to solubility and diffusivity of gases. The hydrogen separation mechanism in MMMs is very complex due to the combining effect of polymer and inorganic fillers. So, the gas separation performance of MMMs was evaluated using the modified Maxwell model. Moreover, adequate polymeric material and inorganic fillers have been summarised for MMMs synthesis and highlighting the mechanism for gas transport phenomena in the process. Several types of materials implemented with polymeric matrix examined in the literature, amongst these functionally aligned CNTs with Pd-nanoparticles dispersed in polymer matrix were observed to reveal the best outcome for the hydrogen separation membrane due to the uniform distribution of inorganic material in the matrix. Henceforth, the agglomeration gets reduced promoting hydrogen separation.     

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.     

Hydrogen solubility in PdCuAg ternary alloy films prepared by electrodeposition

PdCuAg films were prepared by pulsed co-electrodeposition on Ti substrate. The film composition, determined by energy dispersive spectroscopy ([Pd] = ∼35–65 at.%, [Cu] = ∼35–65 at.%, [Ag] = ∼0–15 at.%) was controlled by varying the electrolytic bath composition. X-ray diffraction analyses showed that all the as-deposited films present a monophased face-centered cubic (fcc) structure with an anisotropic deformation of the crystalline structure. Scanning electron microscopy (SEM) observations indicated that all the films present a cauliflower-like morphology. Their hydrogen solubility was evaluated electrochemically in NaOH media. It was shown that the H solubility in the alloy increases with the Pd content. At fixed Pd content, the variation of the H solubility with the Cu or Ag content is more complex and depends on the Pd content. Lastly, a major decrease of the H solubility was observed for films annealed at 400 °C due to the resulting fcc-to-bcc phase transition.     

Synthesis of ZIF-8 derived porous carbon/NiS hexahedral composite and its application in improving the electrochemical hydrogen storage properties of Co–P material

Thermal treatment of zinc-based MOF (ZIF-8) is conducted to prepare ZIF-8 derived porous carbon (ZIF-8-C). ZIF-8-C/NiS hexahedral composites with different C/Ni mole ratios (C@NiS-2, C@NiS-4 and C@NiS-6) are synthesized by solvothermal method. Co–P hydrogen storage material is prepared via mechanical alloying. Then, composites of Co–P coated with NiS, ZIF-8-C and C@NiS are obtained by ball-milling. Eventually, C@NiS-4 coated Co–P electrode exhibits higher discharge capacity of 624.8 mAh/g than separate NiS or ZIF-8-C modified Co–P and original Co–P electrodes. The HRD, corrosion resistance and kinetics properties of Co–P are also improved after C@NiS-4 loading. The enhanced kinetics performance and electrochemical activities of Co–P + C@NiS-4 may be due to the synergistic effect between flexible porous carbon ZIF-8-C and active NiS nanosheets, which can further accelerate the hydrogen diffusion during the charging/discharging processes.     

Hydrogen solubility in PdCuAu alloy thin films prepared by electrodeposition

PdCuAu alloy thin films were prepared by co-electrodeposition over a large composition range ([Pd] = 14–74 at.%, [Cu] = 2–82 at.%, [Au] = 0–66 at.%) and then annealed at 400 °C. The influence of the alloy composition and annealing treatment on the crystalline structure, film morphology and hydrogen solubility (at room temperature) of the various PdCuAu alloys was investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical method, respectively. All the as-deposited samples displayed a monophased face-centered-cubic (fcc) structure. After annealing, a face-centered-cubic to body-centered-cubic (bcc) phase transition was observed. Thanks to the stabilizing effect of Au on the bcc structure this phase transition occurs over a large composition range. The hydrogen solubility increased with the Pd content in the alloy and with the Au content at a constant Pd concentration. The fcc to bcc phase transformation resulted in a major decrease of the hydrogen solubility in the alloy.     

Efficient hydrogen production over MOFs (ZIF-67) and g-C3N4 boosted with MoS2 nanoparticles

MOFs (ZIF-67) and g-C₃N₄ catalyst-modified MoS₂ nanoparticles are prepared by means of doping g-C₃N₄ in the process of ZIF-67 formation and then introducing MoS₂ nanoparticles on the surface of collaborative structure between MOFs and g-C₃N₄. The MOFs (ZIF-67) and g-C₃N₄ catalyst-modified MoS₂ photocatalyst exhibits efficient hydrogen production with about 321 μmol under visible light irradiation in 4 h, which is almost about 30 times higher than that of over the pure g-C₃N₄ photocatalyst. A series of characterization studies such as SEM, XRD, TEM, EDX, XPS, UV–vis DRS, FTIR, transient fluorescence and electro-chemistry show that the novel structure of g-C₃N₄ and MOF is formed, the more active sites appears and the efficiency of photo-generated charge separation is improved. MoS₂, as a narrow band semiconductor, is grafted on the surface of g-C₃N₄/MOF, which could effectively harvest visible light and swift charge separation. The results are well mutual corroboration with each other. In addition, a eosin Y-sensitized reaction mechanism is introduced.     

Enhanced H2 production by using La5.5WO11.25-δ-La0.8Sr0.2FeO3-δ mixed oxygen ion-proton-electron triple-conducting membrane

Although lanthanum tungstates (Ln_nWO_12-δ) show superior CO₂-tolerance compared to the traditional perovskite-type oxides, their hydrogen permeation fluxes are not competitive. Herein, a mixed oxygen ion-proton-electron triple-conducting membrane with a nominal composition of La_5.5WO_11.25-δ-La_0.8Sr_0.2FeO_3-δ (LWO-LSF) was developed for H₂ production. The triple-conducting membrane is composed of a LWO phase with proton conductivity and a LSF phase with mixed oxygen ion-electron conductivities. In the LWO-LSF membrane, proton (H⁺) permeation and oxygen ion (O²⁻) counter-permeation property was simultaneously displayed. The improved H₂ production can be ascribed to (1) hydrogen permeated as H⁺ through LWO phase, and (2) hydrogen produced from water splitting that is enhanced by O²⁻ counter-permeation through LSF phase. A higher H₂ flux of 0.15 mL min⁻¹ cm⁻² was achieved at 900 °C using LWO-LSF triple-conducting membrane, compared with the conventional proton-electron conducting membranes LWO or La_5.5WO_11.25-δ-La_0.8Sr_0.2CrO_3-δ (LWO-LSC). Furthermore, the constant H₂ fluxes in various atmospheres indicated the good stability of LWO-LSF membrane in simulated raw hydrogen.     

Effect of hydrogen on superplastic deformation of (TiB + TiC)/Ti–6Al–4V composite

Ti–6Al–4V matrix composite reinforced with TiB plus TiC was prepared and hydrogenated. The phases were identified by X-ray diffraction (XRD). Microstructures were examined by optical microscopy (OM). Dependence of transus temperatures of the composite on hydrogen concentration was determined. The result shows hydrogen decreases transus temperatures of the composite significantly and increases the amount of β phase. Superplastic deformation of the hydrogenated composites was performed. Hydrogen decreases the optimum superplastic temperatures and increases the optimum superplastic strain rate. The flow stresses of hydrogenated composites decrease greatly compared to unhydrogenated composite. The strain rate sensitivity index m of the composite increases with increasing hydrogen concentration and reaches maximum 0.327 at 0.85 wt.% H. Meanwhile, the activation energy Q decreases with increasing hydrogen concentration and ranges in 488–382 kJ/mol. Hydrogen promotes recrystallization of the composite and refines the microstructure during superplastic deformation.     

Hydrogen permeation through dual-phase ceramic membrane derived from automatic phase-separation of SrCe0.50Fe0.50O3-δ precursor

Dense ceramic membranes with mixed protonic-electronic conductivity have been widely studied because of their 100% H₂ selectivity and directly integrated advantage with high-temperature chemical reactions. In this study, Sr-based dual-phase ceramic membrane SrCe_0.95Fe_0.05O_3-δ-SrFe_0.95Ce_0.05O_3-δ (SCF-SFC) with mixed protonic-electronic conductivity was obtained by automatic phase-separation of SrCe_0.5Fe_0.5O_3-δ (SCF55) precursor. After calcination at 1350 °C, the rationally designed SCF55 precursor auto-decomposed into two thermodynamically stable oxides: Ce-rich phase SrCe_0.95Fe_0.05O_3-δ and Fe-rich phase SrFe_0.95Ce_0.05O_3-δ that functioned as protonic and electronic conductors, respectively. The compositions and microstructures of the auto-formed phases were studied via XRD and SEM analyses. The dual-phase SCF-SFC membrane shows a high hydrogen permeation flux of 0.38 mL min⁻¹ cm⁻² at 940 °C. Stability tests indicated that the SCF-SFC membrane exhibited higher and more stable hydrogen permeation flux with less degradation under CO₂-containing atmospheres compared with the BaCe_0.15Fe_0.85O_3-δ-BaCe_0.85Fe_0.15O_3-δ (BCF-BFC) membrane. This significant improvement can be attributed to the lower CO₂ adsorption and reduced carbonate formation which is indicated by thermogravimetric analysis.     

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.