Preparation of thermosensitive cellophane-graft-N-isopropylacrylamide copolymer membranes and permeation of solutes through the membranes

Thermosensitive membranes with high mechanical strength were prepared by heterogeneous graft copolymerization of N-isopropylacrylamide (NIPAAm) onto cellophane in a nitric acid solution using cerium ammonium nitrate as an initiator, and the permeation behavior of solutes such as lithium chloride and poly(ethylene glycol)s (PEGs) through the membranes at various temperatures was investigated. The degree of graft copolymerization of NIPAAm on cellophane depended on temperature, time, initiator concentration, and so on. The copolymer membranes having a high content of the NIPAAm moiety could be obtained at 25°C for 24 h. The permeation of Li⁺ through the membranes was affected by temperature, i.e., the permeation rate of Li⁺ increased with increasing temperature up to 32°C and then decreased rapidly above 35°C. The permeation rate of Li⁺ through the copolymer membranes at 40°C decreased considerably, but that at 20°C decreased slightly with an increasing amount of the NIPAAm moiety in the membranes. The permeation rate of PEGs with a molecular weight more than 1000 through the cellophane-g-NIPPAm copolymer membranes was considerably suppressed and only the permeation rate of PEG300 increased with increasing temperature up to 35°C and then decreased at 40°C. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 209–216, 1997     

Partial pore blockage and polymer chain rigidification phenomena in PEO/ZIF-8 mixed matrix membranes synthesized by in situ polymerization

Nanostructured zeolitic imidazolate frameworks(ZIF-8) was incorporated into the mixture of poly(ethylene glycol) methyl ether acrylate(PEGMEA) and pentaerythritol triacrylate(PETA) to synthesize mixed matrix membranes(MMMs) by in situ polymerization for CO_2/CH_4 separation. The solvent-free polymerization between PEGMEA and PETA was induced by UV light with 1-hydroxylcyclohexyl phenyl ketone as initiator. The chemical structural characterization was performed by Fourier transform infrared spectroscopy. The morphology was characterized by scanning electron microscope. The average chain-to-chain distance of the polymer chains in MMMs was investigated by X-ray diffraction. The thermal property was evaluated by differential scanning calorimetry. The CH_4 and CO_2 gas transport properties of MMMs are reported. The relationship between gas permeation–separation performances or physical properties and ZIF-8 loading is also discussed. However, the permeation–separation performance was not improved in Robeson upper bound plot compared with original polymer membrane as predicted. The significant partial pore blockage and polymer rigidification effect around the ZIFs confirmed by the increase in glass temperature and the decrease in the d-spacing, were mainly responsible for the failure in performance improvement, which offset the high diffusion induced by porous ZIF-8.     

Gas permeation and separation properties of sulfonated polyimide membranes

Permeability and selectivity of pure gas H₂, CO₂, O₂, N₂ and CH₄ as well as a mixture of CO₂/N₂ for sulfonated homopolyimides prepared from 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA) and 2,2-bis[4-(4-aminophenoxy)phenyl] hexafluoro propane disulfonic acid (BAPHFDS) were measured and compared to those of the non-sulfonated homopolyimide having the same polymer backbone. The polyimide in a proton form (NTDA-BAPHFDS(H)) displayed higher selectivity of H₂ over CH₄ without loss of H₂ permeability. Strong intermolecular interaction induced by sulfonic acid groups decreased diffusivity of the larger molecules. The CO₂/N₂ (19/81) mixed gas permeation was investigated as a function of humidity. With increasing relative humidity from 0% RH to 90% RH, the CO₂ permeability for NTDA-BAPHFDS(H) polyimide increased by more than one order of magnitude, and the selectivity of CO₂/N₂ also increased twice or more. On the other hand, the gas permeability for the non-sulfonated polyimide slightly decreased with increasing humidity. NTDA-BAPHFDS(H) polyimide displayed a CO₂ permeability of 290×10⁻¹⁰ cm³ (STP) cm/(cm² s cmHg) and a separation factor of CO₂/N₂ of 51 at 96% RH, 50 °C and total pressure of 1 atm.     

Single- and dual-phase capillary membranes prepared through plastic extrusion method for oxygen permeation

Two capillary membranes, single-phase Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) and dual-phase 75 wt% Ce_0.85Sm_0.15O_1.925 - 25 wt% Sm_0.6Sr_0.4Cr_0.3Fe_0.7O_3-δ (SDC-SSCF), with dense cross section, were successfully prepared through the plastic extrusion method. The dual-phase capillary membrane shows higher strength compared to the BSCF counterpart, while the two capillary membranes exhibit much higher fracture strength than those of hollow fiber membranes. The oxygen permeation fluxes of both membranes increase with the increase of temperature and flow rate of sweep gas at the ambient pressure, and can be greatly improved by applying high pressures to the feed side. The oxygen permeation flux of BSCF capillary membrane is up to 19.5 mL cm^?2 min^?1 when 0.5 MPa air was applied to the feed side at 900 °C, which is one order of magnitude higher than that of SDC-SSCF capillary membrane. Thus, both capillary membranes have their own advantages and meet applications under different operation conditions.     

Enhanced water desalination performance through hierarchically-structured ceramic membranes

Developments of membrane water desalination are impeded by low water vapor flux across the membrane. We present an innovative membrane design to significantly enhance the water vapor flux. A bilayer zirconia-based membrane with a thick hierarchically-structured support and a thin functional layer is prepared using a combined freeze drying tape casting and screen printing method. The hierarchically-structured YSZ support has a porosity of 42.6%, pores of 4.5 μm or larger, and a relatively low tortuosity of 1.58 along the thickness direction. The bilayer membrane is then converted from naturally hydrophilic to hydrophobic via grafting with a fluoroalkylsilane. A water flux of 28.7 Lm⁻² h⁻¹ and a salt rejection of 99.5% are achieved by exposing the functional layer to 80 °C salt water of 2 wt.% NaCl and the support layer to 20 °C distilled water. These results are the best performing ones for ceramic membranes in direct contact membrane distillation operation.     

Preparation of poly(vinyl alcohol)-graft-N-isopropylacrylamide copolymer membranes with triphenylmethane leucocyanide and permeation of solutes through the membranes

Thermo- and photosensitive hydrogel membranes were prepared by graft copolymerization of an N-isopropylacrylamide (NIPAAm) and triphenylmethane leucocyanide (LeCN) monomer onto poly(vinyl alcohol) (PVA). The yield of the graft copolymerization as well as the content of the poly(NIPAAm) segments grafted onto PVA was depressed by copolymerization of the LeCN monomer. The change in the permeation rate of poly(ethylene glycol)s through the hydrogel membranes was induced photochemically by dissociation of LeCN moieties in the membranes. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1931–1937, 1998     

Nitric oxide and carbon monoxide permeation through glassy polymeric membranes for carbon dioxide separation

Minor components present in polymeric membrane gas separation can have a significant influence on the separation performance. Carbon monoxide and nitric oxide exist in post-combustion gas streams and can therefore influence CO₂ transport through membranes designed for that application. Here, the permeability of nitric oxide (NO) through three glassy polymeric membranes (polysulfone, Matrimid 5218 and 6FDA-TMPDA) was determined and found to be less than the CO₂ but greater than the N₂ permeability in each membrane. This study also investigated the influence of 1000 ppm CO on the mixed gas permeability of CO₂ and N₂ for two glassy polymeric membranes; polysulfone and 6FDA-TMPDA. For both membranes, CO competitive sorption resulted in a reduction in the measured permeability of CO₂ and N₂ even though present at only low concentration.     

Analysis of oxygen permeation through dense ceramic membranes with chemical reactions of finite rate

The oxygen permeation through oxygen ionic or mixed-conducting ceramic membranes under reaction conditions was examined with a model taking into account of different electrical transport mechanisms (p-type and n-type transports) and finite reaction rate. It was demonstrated that with a reaction consuming oxygen in one side of the membrane, the oxygen partial pressure in the reaction side decreases and the oxygen permeation flux increases with the increase in the reaction rate for both the p-type and the n-type transport dominated mechanism. The increase in reaction rate causes a transition of the transport mechanism from p-type to n-type. This transition leads to an increase in the permeation flux by up to 30 times. This effect offers one explanation for the large discrepancies in published permeation data for membrane reactors of partial oxidation reaction employing an oxygen permeable dense ceramic membrane. For a membrane with a specific transport mechanism, the increase in the reactant partial pressure causes an increase in the reaction rate and oxygen permeation flux. However, the increase in the inlet inert gas amount has a complicated effect on the oxygen permeation flux because it lowers both oxygen partial pressure and the reaction rate at the same time.     

Catalytic hydrogenation performance of ZIF-8 carbide for electrochemical reduction of carbon dioxide

The conversion of CO₂ electrocatalytic hydrogenation into energy-rich fuel is considered to be the most effective way to carbon recycle. Nitrogen-doping carbonized ZIF-8 is proposed as carrier of the earth-rich Sn catalyst to overcome the limit of electron transfer and CO₂ adsorption capacity of Sn. Hierarchically porous structure of Sn doped carbonized ZIF-8 is controlled by hydrothermal and carbonization conditions, which induces much higher specific surface area than that of the commercial Sn nanoparticle (1003.174 vs. 7.410 m²·g^-1). The shift of nitrogen peaks in X-ray Photoelectron Spectroscopy spectra indicates interaction between ZIF-8 and Sn, which induces the shift of electron cloud from Sn to the chemical nitrogen to enhance conductivity and regulate electron transfer from catalyst to CO₂. Lower mass transfer resistance and Warburg resistance are investigated through EIS, which significantly improves the catalytic activity for CO₂ reduction reaction (CO₂RR). Onset potential of the reaction is reduced from -0.74 V to less than -0.54 V vs. RHE. The total Faraday efficiency of HCOOH and CO reaches 68.9% at -1.14 V vs. RHE, which is much higher than that of the commercial Sn (45.0%) and some other Sn-based catalyst reported in the literature.     

Zeolite filled polyimide membranes for dehydration of isopropanol through pervaporation process

Six mixed matrix membranes (MMMs) were prepared using zeolites of 4A and ZSM-5 incorporated in polyimide of Matrimid 5218. Effects of filler type on membrane morphology and pervaporation performance of MMMs were investigated using isopropanol dehydration. In addition, effects of operating temperature (30, 40, 50, and 60 °C), feed water concentration (10, 20, 30, and 40 wt.%) and permeate side pressure (0 and 15 torr) on pervaporation performance were studied. Scanning electron microscopy (SEM) analysis showed there were good adhesion between the fillers and the polymer matrix. Zeolite 4A has a better contact with the polymer phase and thereby nearly no void is formed in the MMM structure. Pervaporation were performed based on L16 array of Taguchi method for design of experiments. The results showed that the best separation condition is achieved at temperature, feed water concentration, and permeate pressure of 30 °C, 10 wt.% water and 0 torr, respectively. Selectivities of zeolites 4A and ZSM-5 filled MMMs were calculated as 8991 and 3904 compared with 1276 measured for the neat Matrimid 5218 membrane. Permeation rates of the zeolite 4A and ZSM-5 filled MMMs and the neat polymeric membrane were found to be 0.018, 0.016, and 0.013 kg/m² h, respectively.     

Enhanced oxygen permeation through perovskite hollow fibre membranes by methane activation

Hollow fibre membranes of mixed conducting perovskite La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) were prepared via the combined phase inversion and sintering technique. The fibres were tested for air separation with a home-made reactor under the oxygen partial pressure gradient generated by the air/He streams. Some fibres were in situ activated by introducing methane in the He sweeping gas at high temperatures. The activated membranes with new morphology were created by transforming the inner densified surface layer to a porous structure. Compared to the original membranes, the activated gave appreciable higher oxygen fluxes. At 800 °C, the oxygen fluxes were increased by a factor of 10 after activation was carried out at 1000 °C for 1 h.     

Effect of temperature on gas adsorption and separation in ZIF-8: A combined experimental and molecular simulation study

In this work, the effect of temperature on adsorption of CO₂, CH₄, CO, and N₂ and separation of their binary mixtures in ZIF-8 were investigated using experimental measurements combining with molecular simulations. The results show that for pure gas adsorption, the effect of temperature is large when strong adsorption occurs, mainly due to the variation of the interaction energy between adsorbate molecules with temperature; while for gas mixtures, systems with large selectivity are more sensitive to temperature. In addition, this work shows that temperature influences the working capacity of CO₂ in temperature swing adsorption (TSA) process with the interplay of pressure, which should be considered in the design of TSA process in practical applications.     

Material Advancements in Fabrication of Mixed‐Matrix Membranes

The mixed‐matrix membrane (MMM) is a new membrane material for gas separation and plays a vital role for the advancement of current membrane‐based separation technology. Blending between inorganic fillers like carbon molecular sieves, zeolite, metal oxides, silica and silica nanoparticles, carbon nanotubes, zeolitic imidazolate framework, metal organic framework, and glassy and rubbery polymers etc. is possible. Due to mechanical, thermal, and chemical stability, these membranes achieve high permeability and selectivity as compared to pure polymeric materials. Despite of these advantages, the MMM performances are still below industrial expectations because of membrane defects and related processing problems as well as the nonuniform dispersion of fillers in MMMs. Material selection for organic and inorganic phases, preparation techniques, material advancements, and performance of MMMs are discussed. Issues and challenges faced during MMM synthesis as well as problem solutions are highlighted.     

Design and synthesis of Al-MOF/PPSU mixed matrix membrane with pollution resistance

To enhance the performance of the polyphenylene sulfone(PPSU) membrane,a novel mixed matrix membrane with hydrophilicity and antifouling properties was prepared.Using PPSU as the ba sic membrane material,polyvinyl pyrrolidone(PVP) as the porogen,N-Methyl pyrrolidone(NMP) as the solvent,and MOF-CAU-1(Al_4(OH)_2(OCH_3)_4(H_2 N-BDC)_3·xH_2 O) as the filler,PPSU/CAU-1 mixed matrix membrane(MMM) was prepared by an immersion precipitation and phase transformation technique.By changing the amount of MOF-CAU-1,the properties and performance of the MMM membrane were investigated in terms of hydrophilicity,pore morphology,surface roughness,and dye removal.The results show that the highest pure water flux of the mixed reached 47.9 L·m~(-2)·h~(-1), when the CAU-1 addition amount was 1.0 wt%, which was 23% higher than that of the pure PPSU membrane.Both the rejection rate and the antifouling performance of the MMM membrane also noticeably improved.     

用于CO2/CH4分离的cPIM-1/ZIF-8混合基质膜的制备

自聚微孔聚合物（PIM-1）虽具有良好的CO₂渗透性能,但其气体选择性普遍较差,限制其在CO₂/CH₄分离领域的应用。本文以N,N-二甲基甲酰胺（DMF）为溶剂制备ZIF-8纳米粒子,将其引入到羧基化的PIM-1基质中,制备了cPIM-1/ZIF-8混合基质膜,用于CO₂/CH₄分离。结果表明：由于合成ZIF-8的溶剂也是cPIM-1的良溶剂,使得两者之间具有良好的界面相容性,从而使ZIF-8添加量高达质量分数45%。随着ZIF-8添加量的增加,膜的CO₂渗透速率持续增加,CO₂/CH₄选择性呈现先上升后下降的趋势。当ZIF-8添加量为质量分数25%时,膜的CO₂/CH₄分离性能最好,即CO₂渗透系数为3942 Barrer,CO₂/CH₄选择性为18.7,较cPIM-1纯膜分别提高了 84%和43%,成功地超越了Robeson分离上限。     

Tailoring the morphology of self-assembled block copolymer hollow fiber membranes

Isoporous asymmetric polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) hollow fiber membranes were successfully made by a dry-jet wet spinning process. Well-defined nanometer-scale pores around 20–40 nm in diameter were tailored on the top surface of the fiber above a non-ordered macroporous layer by combining block copolymer self-assembly and non-solvent induced phase separation (SNIPS). Uniformity of the surface-assembled pores and fiber cross-section morphology was improved by adjusting the solution concentration, solvent composition as well as some important spinning parameters such as bore fluid flow rate, polymer solution flow rate and air gap distance between the spinneret and the precipitation bath. The formation of the well-organized self-assembled pores is a result of the interplay of fast relaxation of the shear-induced oriented block copolymer chains, the rapid evaporation of the solvent mixture on the outer surface and solvent extraction into the bore liquid on the lumen side, and gravity force during spinning. Structural features of the block copolymer solutions were investigated by small-angle X-ray scattering (SAXS) and rheological properties of the solutions were examined as well. The scattering patterns of the optimal solutions for membrane formation indicate a disordered phase which is very close to the disorder-order transition. The nanostructured surface and cross-section morphology of the membranes were characterized by scanning electron microscopy (SEM). The water flux of the membranes was measured and gas permeation was examined to test the pressure stability of the hollow fibers.     

Characteristics of ammonia permeation through porous silica membranes

A sol–gel method was applied for the preparation of silica membranes with different average pore sizes. Ammonia (NH₃) permeation/separation characteristics of the silica membranes were examined in a wide temperature range (50–400°C) by measurement of both single and binary component separation. The order of gas permeance through the silica membranes, which was independent of membrane average pore size, was as follows: He > H₂ > NH₃ > N₂. These results suggest that, for permeation through silica membranes, the molecular size of NH₃ is larger than that of H₂, despite previous reports that the kinetic diameter of NH₃ is smaller than that of H₂. At high temperatures, there was no effect of NH₃ adsorption on H₂ permeation characteristics, and silica membranes were highly stable in NH₃ at 400°C (i.e., gas permeance remained unchanged). On the other hand, at 50°C NH₃ molecules adsorbed on the silica improved NH₃‐permselectivity by blocking permeation of H₂ molecules without decreasing NH₃ permeance. The maximal NH₃/H₂ permeance ratio obtained during binary component separation was ～30 with an NH₃ permeance of ～10^?7 mol m^?2 s^?1 Pa^?1 at an H₂ permeation activation energy of ～6 kJ mol^?1. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

ZnO修饰的不锈钢网支撑体上ZIF-8膜的制备

ZIF-8因具有0.34 nm的孔道直径而被认为是最具应用前景的气体分离膜材料之一。不锈钢网（SSN）作为分离膜的支撑体具有价格低廉、易于裁剪、厚度薄等优点。采用水热法在SSN表面生长ZnO缓冲层,以ZnO修饰的SSN（ZnO/SSN）为支撑体制备ZIF-8膜。采用X射线衍射（XRD）和扫描电子显微镜（SEM）对合成的ZIF-8膜进行表征,并进行了气体渗透性能测试,结果表明,在ZnO颗粒修饰后的SSN支撑体上无需活化可制备出单一物相、无缺陷的ZIF-8膜;在室温（298 K）下,ZIF-8膜的H₂/CO₂、H₂/N₂、H₂/CH₄的理想分离系数分别为7.3、9.2、12.4;在150℃,ZIF-8膜的渗透性能稳定。     

Role of adsorption and diffusion pathways on the CO2/N2 separation performance of nanocomposite (B)-MFI-alumina membranes

This work reports a detailed study of CO₂ and N₂ adsorption and diffusion pathways on MFI-alumina hollow fibres and their role on both pure and mixture CO₂/N₂ permeation. The hollow fibres have been prepared by pore-plugging hydrothermal synthesis starting from a clear solution with molar composition 1 SiO₂: 0.45 TPAOH: 27·8H₂O: xB(OH)₃ (x=0–0.02), showing a final nanocomposite architecture. A comprehensive screening has been performed to select the most suitable model for describing CO₂ and N₂ adsorption and pure permeation in the (B)-MFI-alumina hollow fibres. The classical Ideal Adsorption Solution Theory (IAST) has been implemented to different permeation models to account for binary CO₂/N₂ permeation within the fibres.     

A pillared-layer metal-organic framework for efficient separation of C3H8/C2H6/CH4 in natural gas

Metal-organic frameworks (MOFs) have great potentials as adsorbents for natural gas purification. However, the trade-off between selectivity and adsorption capacity remains a challenge. Herein, we report a pillared-layer metal-organic framework Ni(HBTC)(bipy) for efficiently separating the C₃H₈/C₂H₆/CH₄ mixture. The experimental results show that the adsorption capacity of C₃H₈ and C₂H₆ on Ni(HBTC)(bipy) are as high as 6.18 and 5.85 mmol·g^-1, while only 0.93 mmol·g^-1 for CH₄ at 298 K and 100 kPa. Especially, the adsorption capacity of C₃H₈ at 5 kPa can reach an unprecedented 4.52 mmol·g^-1 and for C₂H₆ it is 1.48 mmol·g^-1 at 10 kPa. The ideal adsorbed solution theory predicted C₃H₈/CH₄ selectivity is as high as 1857.0, superior to most of the reported materials. Breakthrough experiment results indicated that material could completely separate the C₃H₈/C₂H₆/CH₄ mixture. Therefore, Ni(HBTC)(bipy) is a promising material for separation of natural gas.