High Hydrogen Permselectivity Research Articles

Vacuum‐Assisted Confined Growth of MOF@COF Composite Membranes with Enhanced Hydrogen Permselectivity

With ordered and periodic network structures, adjustable pore sizes and high chemical stability, covalent organic frameworks (COFs) have drawn much attention for the fabrication of superior separation membranes. However, it is challenging to prepare COF membranes with a molecular sieving property for gas separation due to their relatively large pore size. In this work, we develop the MOF‐in‐COF concept for vacuum‐assisted synthesis of metal‐organic framework (MOF) ZIF‐8 inside the pores of TB‐COF formed from trihydroxy‐benzene‐tricarbaldehyde (T) and diamino‐biphenyl‐disulfonic acid (B), thus constructing a novel ZIF‐8@TB‐COF membrane. Attributing to the formation of a well‐defined one‐dimensional (1D) nanoscale transport channel, the ZIF‐8@TB‐COF membrane displays a high hydrogen permselectivity. At 100 °C and 200 kPa, the mixture separation factors of H2/CO2, H2/CH4 and H2/C3H8 are 21.9, 63.1 and 134.4, respectively, which are much higher than those of the pristine TB‐COF membrane due to the precise size sieving channels brought about by the incorporated MOF. The synthesis of ZIF‐67@TB‐COF membrane demonstrates the versatility of the synthesis strategy.

Development of high hydrogen permselective membranes by using a vapor treatment method

Abstract In recent years, the depletion of energy resources and global warming have led to the promotion of new energy resources. Hydrogen is attracting attention as one of the energy sources for the carbon-neutral society by 2050. Membrane separation is a method for separating and purifying hydrogen. In this study, we focused on developing high hydrogen permselectivity of silica membranes. We investigated the conditions necessary for membrane production to obtain a thin, dense separation layer. The results exhibited that the hydrogen permeation selectivity was enhanced in the silica membrane through the utilization of a 2-step deposition technique, which involves the combination of alternative feed and counter-diffusion chemical vapor deposition (CVD). The H2 permeance achieved by the 2-step method is 1.4×10−6 [mol m−2 s−1 Pa−1], representing 1.4 times higher than that of counter-diffusion CVD. In addition, the permeance ratio, which represents hydrogen selectivity, was significantly enhanced with the 2-step method, with H2/C3H8 = 260, twice as high as with CVD. Elemental mapping analysis of the cross-sectional structure revealed a concentration gradient in the deposited silica within the intermediate layer. This indicates that the silica separation layer may become an asymmetric structure. The 2-step method is expected to yield a thinner dense layer, which could contribute to improved permeance of silica separation membranes by reducing permeation resistance.

Comprehensive Review on High Hydrogen Permselectivity of Palladium Based Membranes: Part II

This article completes the presentation of various techniques reducing concentration polarisation in palladium based membranes for supplying ultra-high purity hydrogen to a polymer electrolyte fuel cell (PEFC), such as the implementation of baffles and the use of microchannel configuration. The present paper also reviews and reports the current methods for estimating hydrogen permeation flux under concentration polarisation influence, which will be a useful guide for academics and industrial practitioners.

Comprehensive Review on High Hydrogen Permselectivity of Palladium Based Membranes: Part I

Palladium based membranes are widely used for supplying ultra-high purity hydrogen to a polymer electrolyte fuel cell (PEFC) installed on small vehicles and various electronic devices. Compared to pressure swing adsorption (PSA), the use of palladium based membrane is more economical for small size (small capacity) applications. The transportation of hydrogen through a palladium based membrane is governed by Sieverts’ Law and quantified with Fick’s First Law. Since the 20th century, the fabrication of high-performance palladium based membrane for enhanced hydrogen recovery performance has become practical. However, along with the improvement in hydrogen recovery performance, concentration polarisation becomes unavoidable because hydrogen permeation flux starts to affect hydrogen concentration at the membrane surface. Various parametric studies have investigated the effects of membrane thickness, hydrogen molar fraction and total upstream and downstream pressures on concentration polarisation level. The influence of membrane temperature, permeability, type and number of species in the hydrogen mixture, diffusivity of the hydrogen mixture, system configurations and flow patterns are also reported and comprehensively reviewed in this paper. Part II will complete the presentation.

Synthesis of nano-sheets seeds for secondary growth of highly hydrogen permselective ZIF-95 membranes

Recently, more and more attentions have been focused on the preparation of metal-organic framework (MOF) membranes for gas or liquid separations. However, it is rather difficult to prepare a dense MOF membrane by a direct solvothermal synthesis route because of poor heterogeneous nucleation of MOF on the native ceramic support surface. Chemical modifications or seeds coating are usually carried out to promote nucleation and growth of MOF membranes. In this work, we report the synthesis of ZIF-95 nano-sheets as seeds for secondary growth of highly hydrogen permselective ZIF-95 membranes. Under the action of the vacuum force, the ZIF-95 seeds in the colloidal suspensions are attracted and transported onto the whole area of the support surface continuously and uniformly, thus forming a continuous and uniform ZIF-95 seeding layer, which is helpful to promote the nucleation and growth of ZIF-95 membranes. After solvothermal synthesis at 120 °C for 48 h, a well-intergrown ZIF-95 membrane was formed on the seeding support surface. Attributing to a preferentially affinity to CO2 and a narrow pore size of ~0.37 nm, the developed ZIF-95 membrane shows high hydrogen permselectivity for the separation H2 from other large molecules. At 200 °C and 1 bar, the mixture separation factors of H2/CO2, H2/N2, and H2/CH4 are 41.6, 36.8, and 40.3, respectively, with H2 permeances over 1.7 × 10−7 mol m−2 s−1·Pa−1.

Synthesis and characterization of a silica-alumina composite membrane and its application in a membrane reactor

Abstract Hydrothermally stable silica-alumina composite membranes were synthesized through chemical vapor deposition (CVD) of tetraethylorthosilicate (TEOS) and aluminium tri-sec-butoxide precursor at 923 K on porous alumina supports. The membranes showed high hydrogen permselectivity (order of 10−7 mol m−2 s−1 Pa−1) comparable to that of pure silica membranes but with superior hydrothermal stability, and were used in a membrane reactor. The permeation of small gas species (H2, He, Ne) was well explained by a solid-state diffusion mechanism, involving jumps of the permeating species between solubility sites. The permeation mechanism of large gas molecules (CH4, CO2, N2) was explained by the gas translation mechanism involving large pore defects. Steam methane reforming (SMR) on a Ni/MgO-SiO2 catalyst was carried out at 923 K in the membrane reactor and in a conventional packed-bed reactor. The membrane contributed to an increase in the hydrogen production rate by the selective extraction of hydrogen from the reaction zone.

Confined and in-situ zeolite synthesis: A novel strategy for defect reparation over dense Pd membranes for hydrogen separation

Abstract The manufacturing of ultrathin Pd based hydrogen separation membranes with high permeation performance has great potential in industrial applications. However, there usually exists a trade-off between reduced thickness and high hydrogen permselectivity which are regarded as a pair of contradictory requirements. In this study, a novel technique of confined and in-situ zeolite synthesis for defects reparation is proposed to improve the hydrogen permselectivity of preformed Pd membranes without further increasing their thickness. Two technology routes are developed for different defect dimensions. Zeolites can only grow inside large defects in one route, which is suitable for repairing broken membranes. In the other route, a thin zeolite layer is formed on Pd membrane surface, covering small defects for further enhancing selectivity. The purity of permeate hydrogen is enhanced from 99.884 to 99.994% after zeolite reparation. High purity hydrogen (99.986%) can be separated from 75% H2/N2 mixture with a hydrogen recovery of 91% by the repaired membrane at 500 °C and 0.4 MPa.

Highly hydrogen-permselective zeolitic imidazolate framework ZIF-8 membranes prepared on coarse and macroporous tubes through repeated synthesis

Highly hydrogen-permselective zeolitic imidazolate framework ZIF-8 membranes are prepared on coarse and macroporous Al2O3 tubes through a repeated synthesis strategy for the separation of H2/CO2, H2/CH4, and H2/C3H8 mixtures. Since the amino-groups of 3-aminopropyltriethoxysilane (APTES) can coordinate to the free Zn2+ centers and thus bind the growing nano-crystals directly, the nucleation and growth of well intergrown ZIF-8 membranes could be promoted through a repeated synthesis of the ZIF-8 layer on APTES-modified macroporous Al2O3 tubes. The SEM and XRD characterizations indicate that a dense and phase-pure ZIF-8 membrane with a thickness of about 25.0μm can be formed on the alumina tubes by microwave synthesis at 105°C for 3h, and no cracks, pinholes or other defects were observed in the membrane layer. The ZIF-8 membranes were evaluated in single gas permeation and mixed gas separation. Attributed to the preferential adsorption affinity and capacity as well as a highly porous structure with nano-sized channels, the ZIF-8 membrane displays a high hydrogen permselectivity. For the separation of equimolar H2/CO2, H2/CH4, H2/C3H8 mixtures at 100°C and 2bar, the mixture separation factors of H2/CO2, H2/CH4, and H2/C3H8 are 5.6, 44.0, 328.6, with a H2 permeance of about 1.1×10−7molm−2s−1Pa−1, which is promising for potential applications in hydrogen purification and separation.

Synthesis of MIL-88B(Fe)/Matrimid mixed-matrix membranes with high hydrogen permselectivity

The incorporation of MIL-88B(Fe) can enhance H2 permeability but hinder the transport of CH4 in Matrimid-based MMMs, thus resulting in enhanced separation selectivity of H2–CH4.

Development of Pd Alloy Hydrogen Separation Membranes with Dense/Porous Hybrid Structure for High Hydrogen Perm-Selectivity

For the commercial applications of hydrogen separation membranes, both high hydrogen selectivity and permeability (i.e., perm-selectivity) are required. However, it has been difficult to fabricate thin, dense Pd alloy composite membranes on porous metal support that have a pore-free surface and an open structure at the interface between the Pd alloy films and the metal support in order to obtain the required properties simultaneously. In this study, we fabricated Pd alloy hydrogen separation membranes with dense/porous hybrid structure for high hydrogen perm-selectivity. The hydrogen selectivity of this membrane increased owing to the dense and pore-free microstructure of the membrane surface. The hydrogen permeation flux also was remarkably improved by the formation of an open microstructure with numerous open voids at the interface and by an effective reduction in the membrane thickness as a result of the porous structure formed within the Pd alloy films.

Highly Size-Selective Ionically Crosslinked Multilayer Polymer Films for Light Gas Separation

Exceptionally high hydrogen permselectivity, exceeding that of any polymeric or porous inorganic systems, is achieved using an ionically crosslinked multilayer polymer thin film.

팔라듐 합금 수소 분리막의 전처리에 관한 연구

A Pd-based hydrogen membranes for hydrogen purification and separation need high hydrogen perm-selectivity. The surface roughness of the support is important to coat the pinholes free and thin-film membrane over it. Also, The pinholes drastically decreased the hydrogen perm-selectivity of the Pd-based composite membrane. In order to remove the pinholes, we introduced various surface pre-treatment such as alumina powder packing, nickel electro-plating and micro-polishing pre-treatment. Especially, the micro-polishing pretreatment was very effective in roughness leveling off the surface of the porous nickel support, and it almost completely plugged the pores. Fine Ni particles filled surface pinholes with could form open structure at the interface of Pd alloy coating and Ni support by their diffusion to the membrane and resintering. In this study, a surface pore-free Pd-Cu-Ni ternary alloy membrane on a porous nickel substrate was successfully prepared by micro-polishing, high temperature sputtering and Cu-reflow process. And permeation and leak tests showed that the Pd-Cu-Ni ternary alloy hydrogen membrane achieved both high permeability of permation flux and infinite selectivity.

Palladium membrane on TiO 2 nanotube arrays-covered titanium surface by combination of photocatalytic deposition and modified electroless plating processes and its hydrogen permeability

Palladium membrane with high hydrogen permselectivity was successfully fabricated on the TiO 2 nanotube arrays-covered titanium surface by photocatalytic deposition (PCD) under ultraviolet irradiation and subsequently improved and thickened using the modified electroless plating method. The TiO 2 nanotube arrays were produced in situ on the surface of asymmetric porous titanium foil in 0.5 wt% hydrofluoric acid by using the anodization method. The resulting palladium membrane with thickness of about 10 μm was pure, homogeneous, lack of defects, and compact, as revealed by SEM, XRD and XPS characterization. The gas permeation experiment under temperature of 593 K–773 K and pressure of 0.3 MPa–0.7 MPa demonstrated that the palladium layers possessed high hydrogen permeability, but impermeability for nitrogen. The hydrogen permeation coefficient was 2.91 × 10 −6 mol m −2 s −1 Pa −1 at 773 K at 0.7 MPa. The activation energy for hydrogen permeation and hydrogen pressure exponent were 12.63 kJ mol −1 and 1, respectively. In addition, the durability and stability of the palladium layers bound on the TiO 2 nanotube arrays-covered titanium surface were investigated by the adhesion force test, thermal shocking test and high temperature stability test. Thus, the so-fabricated material is potentially useful for separating hydrogen from the hydrogen-containing gas at high temperature.

Pure nickel coating on a mesoporous alumina membrane: Preparation by electroless plating and characterization

This work concerns the preparation and characterization of a composite inorganic membrane based on nickel supported on a tubular porous ceramic for use in high temperature hydrogen separation. The characteristics required of this material for a high hydrogen permselectivity are a defect-free coating and thermal stability. Thus, electroless plating of nickel using hydrazine as reducing agent enables a metallic nickel coating to be deposited directly on an asymmetric alumina support having a mesoporous top layer composed of γ-alumina. To optimize conditions for reduction of the metal ion, variables including the temperature, the nature of the nickel salt, the choice of stabilizer and the hydrazine concentrations in the plating bath were studied. Metallic nickel was found to be dispersed both in and on the γ-alumina. After heat treatment at high temperature, a thin and uniform pure Ni˙ layer (thickness ≈1–1.5 μm), characterized by scanning electron microscopy (SEM) and X-ray Diffraction (XRD), was formed. The separation performance of the Ni/ceramic membrane was investigated using single gases (hydrogen, nitrogen, carbon dioxide and methane) at room temperature. The results indicate that the mechanism of hydrogen transport is related to the Knudsen diffusion.

Propane Dehydrogenation over a Hydrogen Permselective Membrane Reactor

The dehydrogenation of propane to propylene has been studied in an isothermal high-temperature shell-and-tube membrane reactor containing a Pd-coated γ-Al 2 O 3 membrane and a Pt/K/Sn/Al 2 O 3 packed catalyst. A tubular Pd-coated γ-Al 2 O 3 membrane was prepared by an electroless plating method. This membrane showed high hydrogen to nitrogen permselectivities (P H 2 / N 2 = 10-50) at 400 °C and 500 °C with various transmembrane pressure drops. The employment of a membrane reactor in the dehydrogenation reaction, which selectively separates hydrogen from the reaction mixture along the reaction path, can greatly increase the conversion and enable operation of the reactor at lower temperatures. High hydrogen permselectivity has been confirmed as a key factor in determining the reactor performance of conversion enhancement.

Steam reforming of methane in membrane reactors: comparison of electroless-plating and CVD membranes and catalyst packing modes

Steam reforming of methane was studied at 773 K using a membrane reactor containing two types of supported precious metal membranes: one is an electroless-plating palladium membrane, and the others are CVD palladium, platinum, and ruthenium membranes. A CVD platinum membrane having a H 2/N 2 separation factor as high as 280 gave an excellent performance comparable to the electroless-plating palladium membrane. The advantages of the CVD platinum membrane versus the electroless-plating palladium membrane are lower metal loading and lower cost as well as a lower tendency toward hydrogen embrittlement. It is also shown in this study based on a computer simulation that compared with thermodynamic equilibrium, the non-palladium CVD membranes produced a high conversion of methane similar to an electroless-plating palladium membrane only when showing high hydrogen permselectivity. The catalyst packing in front of the membrane zone enhances the membrane reactor efficiency.

A COMPARATIVE EVALUATION OF HIGH-TEMPERATURE MEMBRANE SYSTEMS FOR CATALYTIC PROCESSING

An investigation has been carried out using mathematical simulation to evaluate the relative performance of various composite membranes incorporated in high-temperature membrane reactors. Two membrane categories (porous and dense) and six types of composite membrane systems (Pd/Ag, polyimide, silica, inert porous and Ru-dispersed porous silicalite zeolites and carbon molecular sieve) have been compared. Also considered is the special case of the Pd/Ag composite system having imperfections (pinholes and cracks). The industrially important reaction of ethylbenzene dehydrogenation over promoted iron oxide catalyst using rate constant values from the literature has been studied as the model reaction. Important characteristic results have been obtained which show that while the dense systems possess higher performance levels at lower membrane thicknesses, the porous composite systems especially those containing highly dispersed active metal particles within their micropores can have advantages due to their significantly higher contact surface to volume ratio. The study has also confirmed that high hydrogen permselectivity is a key factor in determining reactor performance in terms of conversion enhancement.

An experimental evaluation of high-temperature composite membrane systems for propane dehydrogenation

Experiments have been performed using a high-temperature membrane reactor to evaluate the relative performance of various composite membranes. Two membrane categories (porous and dense) and three types of composite membrane systems (Pd/Ag, silica and Pd-dispersed porous) have been studied and their performance compared. Also considered is the special case of the PdAg composite system having imperfections. The industrially important reaction of propane dehydrogenation over noble metal catalysts to produce propylene was selected for study. Results have been obtained which indicate that the dense PdAg composite system does possess higher performance levels in the temperature range studied. However, metal-dispersed porous systems have advantages due to their significantly higher contact surface-to-volume ratio. High hydrogen permselectivity is confirmed as a key factor in determining reactor performance in terms of conversion enhancement.

A reactor-separator incorporating porous and dense membrane systems

This study employs mathematical models to compare and assess the relative performance of various composite membranes incorporated in a high-temperature membrane reactor. Two categories (porous and dense) and five types of composite membrane systems (Pd/Ag, polyimide, silica, inert porous and Ru-dispersed porous) have been considered. Also included in the study is the special case of the Pd/Ag composite system having imperfections (pinholes and cracks). Ethylbenzene dehydrogenation over promoted iron oxide catalyst has been studied as the model reaction. Overall, the dense systems show higher performance levels at lower membrane thicknesses. However, the porous composite systems, especially those in which active metal particles have been incorporated within the micropores, can be very useful due to their significantly higher contact surface to volume ratio. The study has also confirmed that high hydrogen perm-selectivity is a key factor in determining reactor performance in terms of con-version enhancement. ©1997 SCI