Gas transport behavior of mixed-matrix membranes composed of silica nanoparticles in a polymer of intrinsic microporosity (PIM-1)

Recently, high-free volume, glassy ladder-type polymers, referred to as polymers of intrinsic microporosity (PIM), have been developed and their reported gas transport performance exceeded the Robeson upper bound trade-off for O₂/N₂ and CO₂/CH₄. The present work reports the gas transport behavior of PIM-1/silica nanocomposite membranes. The changes in free volume, as well as the presence and volume of the void cavities, were investigated by analyzing the density, thermal stability, and nano-structural morphology. The enhancement in gas permeability (e.g., He, H₂, O₂, N₂, and CO₂) with increasing filler content shows that the trend is related to the true silica volume and void volume fraction.     

Pervaporation of alcohols through highly permeable PIM-1 polymer films

Polycondensation material PIM-1 has attracted the attention of researchers owing to its high transport parameters in gas separation and a high free volume. The pervaporation characteristics of PIM-1 have been systematically studied. Lower aliphatic alcohols (CH₃OH, C₂H₅OH, and n-C₄H₉OH) and water were selected as objects of research. The rates of mass transfer for individual components at various temperatures have been estimated, and for the ethanol-water binary mixture, the process of separation has been examined. The films based on the polymer under study exhibit the properties of organophilic membranes and are characterized by high permeability with respect to alcohols. The apparent activation energy of permeability is low. This behavior is common for pervaporation membranes based on glassy polymers with a high free volume. The parameters of separation for the water-alcohol mixture surpass corresponding values for typical organophilic membranes based on PDMS. These parameters are commensurable with the values observed for membranes based on poly(trimethylsilylpropyne). At the same time, PIM-1 does not demonstrate a rapid decrease in permeability in the course of time.     

Crosslinking and stabilization of nanoparticle filled PMP nanocomposite membranes for gas separations

Poly(4-methyl-2-pentyne) (PMP) has been crosslinked using 4,4′-(hexafluoroisopropylidene) diphenyl azide (HFBAA) to improve its chemical and physical stability over time. Crosslinking PMP renders it insoluble in good solvents for the uncrosslinked polymer. Gas permeability and fractional free volume (FFV) decreased as crosslinker content increased, while gas sorption was unaffected by crosslinking. Therefore, the reduction in permeability upon crosslinking PMP was due to decrease in diffusion coefficient. Compared to the pure PMP membrane, the permeability of the crosslinked membrane is initially reduced for all gases tested due to the crosslinking. By adding nanoparticles (FS, TiO₂), the permeability is again increased; permeability reductions due to crosslinking could be offset by adding nanoparticles to the membranes. Increased selectivity is documented for the gas pairs O₂/N₂, H₂/N₂, CO₂/N₂, CO₂/CH₄ and H₂/CH₄ using crosslinking and addition of nanoparticles. Crosslinking is successful in maintaining the permeability and selectivity of PMP membranes and PMP/filler nanocomposites over time.     

Azide‐based Cross‐Linking of Polymers of Intrinsic Microporosity (PIMs) for Condensable Gas Separation

Cross‐linked polymers of intrinsic microporosity (PIM)s for gas separation membranes, were prepared by a nitrene reaction from a representative PIM in the presence of two different diazide cross‐linkers. The reaction temperature was optimized using TGA. The homogenous membranes were cast from THF solutions of different ratios of PIM to azides. The resulting cross‐linked structures of the PIMs membranes were formed at 175 °C after 7.5 h and confirmed by TGA, XPS, FT‐IR spectroscopy and gel content analysis. These resulting cross‐linked polymeric membranes showed excellent gas separation performance and can be used for O₂/N₂ and CO₂/N₂ gas pairs, as well as for condensable gases, such as CO₂/CH₄, propylene/propane separation. Most importantly, and differently from typical gas separation membranes derived from glassy polymers, the crosslinked PIMs showed no obvious CO₂ plasticization up to 20 atm pressure of pure CO₂ and CO₂/CH₄ mixtures.

     

Stability of polyynes in air and their degradation by ozonolysis

Polyyne solutions in n-hexane or other aliphatic hydrocarbons having C₆H₂, C₈H₂ and C₁₀H₂ as main components were found to be unstable at relatively high concentration (≈10⁻² M) and to separate in few hours a brown precipitate. This phenomenon does not occur in more dilute solutions. The precipitate recovered from the mentioned solutions was analysed by FT-IR spectroscopy and thermogravimetry and found to be constituted by polyyne chains crosslinked and oxidized by oxygen resembling in some way the product obtained by photolysing acetylene in water solution and in presence of air. Polyynes react very quickly with ozone producing as main product polymeric ozonides which are insoluble in hydrocarbons and which have been studied with FT-IR spectroscopy. The oxidized nature of the chains highly crosslinked by ozonides, peroxide and oxygen bridges has been confirmed. The oxidized polyyne fraction remaining in solution after ozonolysis was studied by electronic absorption spectroscopy and by liquid chromatography. Extensive chain breaking was found as expected although some degree of unsaturation was preserved even in the final oxidized products.     

Preparation and characterization of a composite PDMS membrane on CA support

Polydimethylsiloxane (PDMS) is the most commonly used membrane material for the separation of condensable vapors from lighter gases. In this study, a composite PDMS membrane was prepared and its gas permeation properties were investigated at various upstream pressures. A microporous cellulose acetate (CA) support was initially prepared and characterized. Then, PDMS solution, containing crosslinker and catalyst, was cast over the support. Sorption and permeation of C₃H₈, CO₂, CH₄, and H₂ in the prepared composite membrane were measured. Using sorption and permeation data of gases, diffusion coefficients were calculated based on solution‐diffusion mechanism. Similar to other rubbery membranes, the prepared PDMS membrane advantageously exhibited less resistance to permeation of heavier gases, such as C₃H₈, compared to the lighter ones, such as CO₂, CH₄, and H₂. This result was attributed to the very high solubility of larger gas molecules in the hydrocarbon‐based PDMS membrane in spite of their lower diffusion coefficients relative to smaller molecules. Increasing feed pressure increased permeability, solubility, and diffusion coefficients of the heavier gases while decreased those of the lighter ones. At constant temperature (25°C), empirical linear relations were proposed for permeability, solubility, and diffusion coefficients as a function of transmembrane pressure. C₃H₈/gas solubility, diffusivity, and overall selectivities were found to increase with increasing feed pressure. Ideal selectivity values of 9, 30, and 82 for C₃H₈ over CO₂, CH₄, and H₂, respectively, at an upstream pressure of 8 atm, confirmed the outstanding separation performance of the prepared membrane. Copyright © 2009 John Wiley & Sons, Ltd.     

Engineering plasticization resistant gas separation membranes using metal–organic nanocapsules

Membrane technologies hold great potential for industrial gas separation. Nevertheless, plasticization, a common phenomenon that is responsible for the loss of gas pair selectivity and the decrease of membrane lifespan, is one of the top challenges withholding the deployment of advanced membrane materials in realistic applications. Here, we report a highly generalizable approach, that utilizes PgC₅Cu, a copper metal–organic nanocapsule (MONC) containing 24 open metal sites (OMSs) as a multi-dentate node to coordinatively crosslink polymers. By adding merely 1–3 wt% of PgC₅Cu, a wide range of carbonyl group-containing polymers can be effectively crosslinked. Through rigorous dissolution tests, molecular dynamic simulations, and in situ FT-IR spectroscopy, we qualitatively and quantitatively unveiled the coordinative binding nature at the polymer–MONC interface. As a result, we produced a series of composite membranes showing near complete plasticization resistance to CO₂, C₂H₄, and C₂H₆ under high pressure with no loss of mechanical and gas transport properties.

Ultra-small metal–organic nanocapsules (MONCs) with open metal sites (OMSs) are used as multi-dentate nodes to form coordinative crosslinking networks with polymers.     

Synthesis and characterization of NiO nanoclusters via thermal decomposition

Thermal decomposition process has been developed to synthesize nickel oxide (NiO) nanoclusters via the reaction between a new precursor, nickel oxalate [Ni(O₄C₂)(H₂O)₄] and oleylamine (C₁₈H₃₇N). The combination of triphenylphosphine (C₁₈H₁₅P) and C₁₈H₃₇N were added as surfactants to control the particle size. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA) and ultraviolet–visible (UV–Vis) spectroscopy. The synthesized NiO nanoclusters have a cubic structure with average size 2–10 nm.     

Gas permeation through hydrogels : II. Poly(vinylalcohol-co-itaconic acid) membranes

Permeability and diffusion coefficients of O₂, He, CO₂ and C₄H₆ were measured in water,swollen poly(vinylalcohol-co-itaconic acid) membranes having various water contents from 0.48 to 0.83. The permeability coefficients of CO₂ and C₄H₆ were found to depend on the upstream pressure, while the permeability coefficients of O₂ and He were independent of the pressure. With decreasing pressure the permeability coefficients of CO₂ and C₄H₆ increased, and the pressure dependence became larger with decreasing water content of the membranes. A parallel permeation model based on the two states of water in the water-swollen membranes could be applied successfully to CO₂ and C₄H₆.     

Structure and gas transport characteristics of triethylene oxide-grafted polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene

Polymeric membrane-based gas separation technology has significant advantages compared with traditional amine-based CO₂ separation method. In this work, SEBS block copolymer is used as a polymer matrix to incorporate triethylene oxide (TEO) functionality. The short ethylene oxide segment is chosen to avoid crystallization, which is confirmed by differential scanning calorimetry and wide-angle X-ray scattering characterizations. The gas permeability results reveal that CO₂/N₂ selectivity increased with increasing content of TEO functional group. The highest CO₂ permeability (281 Barrer) and CO₂/N₂ selectivity (31) were obtained for the membrane with the highest TEO incorporation (57 mol%). Increasing the TEO content in these copolymers results in an increase in CO₂ solubility and a decrease in C₂H₆ solubility. For example, as the grafted TEO content increased from 0 to 57 mol%, the CO₂ solubility and CO₂/C₂H₆ solubility selectivity increased from 0.72 to 1.3 cm³(STP)/cm³ atm and 0.47 to 1.3 at 35°C, respectively. The polar ether linkage in TEO-grafted SEBS copolymers exhibits favorable interaction with CO₂ and unfavorable interaction with nonpolar C₂H₆, thus enhancing CO₂/C₂H₆ solubility selectivity.     

Synthesis and properties of clay-based superabsorbent composite

A novel superabsorbent composites based on acrylic acid, acrylamide, and inorganic clay mineral-attapulgite were synthesized through a solution polymerization to improve water and saline absorbencies. The superabsorbent composite was characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The effects of saline solutions, amount of initiator, crosslinker and attapulgite on the water absorbency of superabsorbent composites were investigated. The water retention test of superabsorbent composites were also carried out. The superabsorbent composite exhibited improved water and saline absorbencies compared with that of crosslinked poly(acrylic acid-co-acrylamide) superabsorbent polymer. The water absorbency of the superabsorbent composite synthesized under optimal synthesis conditions with an attapulgite content of 10% reaches more than 1400 g H₂O/g and 110 g H₂O/g in distilled water and 0.9% NaCl solution, respectively.     

New polyimide-polyaniline hollow fibers: Synthesis, characterization and behavior in gas separation

New polyimide-polyaniline hollow fibers were produced by dissolution of the polymers in NMP and dry/wet spinning of the resulting solution in a non-solvent (H₂O). The morphology and thermal properties of the fibers, were examined by means of SEM and TGA, and FTIR spectroscopy was used for the study of their chemical structure. Permeability and selectivity measurements in different gases (He, H₂, CH₄, CO₂, O₂ and N₂) were performed in order to evaluate the performance of the membrane in gas separation applications. The results indicate that the novel membrane is a well structured hollow fiber, thermally stable up to 500°. The introduction of polyaniline into the polyimide matrix, results in a great enhancement in fiber permeability (60-600 times) possibly due to increase of the total free volume due to the introduction of shorter polyaniline molecules in the matrix, allowing larger quantities of gases to pass through the composite membrane. Perm-selectivity ratios for the composite membranes H₂/CH₄, He/N₂, H₂/N₂ and H₂/CO₂ were found lower by a factor of 6.4, 8.9, 7.7 and 1.47, respectively, compared to membranes produced using only polyimide. The opposite effect was observed for CH₄/CO₂ and N₂/CO₂ perm-selectivity ratios that showed an increase by a factor of 3.52 and 5.2, respectively. The ratio CH₄/CO₂ is of particular interest for natural gas purification purposes.     

Anhydrous proton conducting membranes based on crosslinked graft copolymer electrolytes

A comb-like copolymer consisting of a poly(vinylidene fluoride-co-chlorotrifluoroethylene) backbone and poly(hydroxy ethyl acrylate) side chains, i.e. P(VDF-co-CTFE)-g-PHEA, was synthesized through atom transfer radical polymerization (ATRP) using CTFE units as a macroinitiator. Successful synthesis and a microphase-separated structure of the copolymer were confirmed by proton nuclear magnetic resonance (¹H NMR), FT-IR spectroscopy, and transmission electron microscopy (TEM). This comb-like polymer was crosslinked with 4,5-imidazole dicarboxylic acid (IDA) via the esterification of the –OH groups of PHEA and the –COOH groups of IDA. Upon doping with phosphoric acid (H₃PO₄) to form imidazole–H₃PO₄ complexes, the proton conductivity of the membranes continuously increased with increasing H₃PO₄ content. A maximum proton conductivity of 0.015 S/cm was achieved at 120 °C under anhydrous conditions. In addition, these P(VDF-co-CTFE)-g-PHEA/IDA/H₃PO₄ membranes exhibited good mechanical properties (765 MPa of Young's modulus), and high thermal stability up to 250 °C, as determined by a universal testing machine (UTM) and thermal gravimetric analysis (TGA), respectively.     

Oriented Ultrathin π-complexation MOF Membrane for Ethylene/Ethane and Flue Gas Separations

Rational design and engineering of high-performance molecular sieve membranes towards C₂H₄/C₂H₆ and flue gas separations remain a grand challenge to date. In this study, through combining pore micro-environment engineering with meso-structure manipulation, highly c-oriented sub-100 nm-thick Cu@NH₂-MIL-125 membrane was successfully prepared. Coordinatively unsaturated Cu ions immobilized in the NH₂-MIL-125 framework enabled high-affinity π-complexation interactions with C₂H₄, resulting in an C₂H₄/C₂H₆ selectivity approaching 13.6, which was 9.4 times higher than that of pristine NH₂-MIL-125 membrane; moreover, benefiting from π-complexation interactions between CO₂ and Cu(I) sites, our membrane displayed superior CO₂/N₂ selectivity of 43.2 with CO₂ permeance of 696 GPU, which far surpassed the benchmark of other pure MOF membranes. The above multi-scale structure optimization strategy is anticipated to present opportunities for significantly enhancing the separation performance of diverse molecular sieve membranes.     

Structure-permeability relationships in silicone polymers

Permeability coefficients P for He, O₂, N₂, CO₂ CH₄, C₂H₄, C₂H₆, and C₃H₈ in 12 different silicone polymer membranes were determined at 35.0°C and pressures up to 9 atm. Values of P for CO₂, CH₄, and C₃H₈ were also determined at 10.0 and 55.0°C. In addition, mean diffusion coefficients D and solubility coefficients S were obtained for CO₂, CH₄, and C₃H₈ in 6 silicone polymers at 10.0, 35.0, and 55.0°C. Substitution of increasingly bulkier functional groups in the side and backbone chains of silicone polymers results in a significant decrease in P for a given penetrant gas. This is due mainly to a decrease in D , whereas S decreases to a much lesser extent. Backbone substitutions appear to have a somewhat lesser effect in depressing P than equivalent side-chain substitutions. The selectivity of a silicone membrane for a gas A relative to a gas B, i.e., the permeability ratio P (A)/P (B), may increase or decrease as a result of such substitutions, but only if the substituted groups are sufficiently bulky. The selectivity of the more highly permeable silicone membranes is controlled by the ratio S (A)/S (B), whereas the selectivity of the less permeable membranes depends on both the ratios D (A)/D (B) and S(A)/S(B). The permeability as well as the selectivity of one silicone membrane toward CO₂ were significantly enhanced by the substitution of a fluorine-containing side group that increased the solubility of CO₂ in that polymer.     

Molecular‐Sieving Membrane by Partitioning the Channels in Ultrafiltration Membrane by In Situ Polymerization

Commercial ultrafiltration membranes have proliferated globally for water treatment. However, their pore sizes are too large to sieve gases. Conjugated microporous polymers (CMPs) feature well‐developed microporosity yet are difficult to be fabricated into membranes. Herein, we report a strategy to prepare molecular‐sieving membranes by partitioning the mesoscopic channels in water ultrafiltration membrane (PSU) into ultra‐micropores by space‐confined polymerization of multi‐functionalized rigid building units. Nine CMP@PSU membranes were obtained, and their separation performance for H₂/CO₂, H₂/N₂, and H₂/CH₄ pairs surpass the Robeson upper bound and rival against the best of those reported membranes. Furthermore, highly crosslinked skeletons inside the channels result in the structural robustness and transfer into the excellent aging resistance of the CMP@PSU. This strategy may shed light on the design and fabrication of high‐performance polymeric gas separation membranes.     

辐照气氛对异丙氧基杯[4]冠-6辐解的影响

异丙氧基杯[4]冠-6(1,3-交替-25,27-二(2-丙氧基)杯[4]芳烃-26,28-冠-6, 简称BPC6)对高放废液中的放射性Cs离子具有很好的选择萃取性能, 然而在萃取过程中BPC6 会受到强辐射场辐照, 所以有必要研究其辐射稳定性. 本文应用气相色谱(GC)、傅里叶变换显微红外(Micro-FTIR)和核磁共振(NMR)谱等手段分析了BPC6 固体分别在O₂和N₂气氛下的γ辐照效应. 结果表明, 当剂量为1 MGy时, O₂气氛下BPC6 的辐解率明显高于N₂(分别约为10.4%和2.5%), 而且气体辐解产物也有很大差异, 在O₂气氛下主要为H₂、CH₄、CO和CO₂,而在N₂辐照气氛下还有C₂H₄、C₂H₆、C₃H₆和C₃H₈等产物. 通过综合分析气体与固体辐解产物, 我们提出BPC6在不同气氛下具有不同的辐解途径, 这将为BPC6 萃取体系的辐射效应研究提供新的方法与思路, 加深对其辐解机理的认识.     

A 2D Graphitic-Polytriaminopyrimidine (g-PTAP)/Poly(ether-block-amide) Mixed Matrix Membrane for CO2 Separation

Poly(ether-block-amide)/g-PTAP mixed matrix membranes (MMMs) were developed by incorporating different wt.% (1–10%) of a novel 2D g-PTAP nanofiller and its effects on membrane structure and gas permeability were studied. The novel 2D material g-PTAP was synthesized and characterized by various analytical techniques including field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and Raman spectroscopy. The fabricated MMMs were investigated to study the interaction and compatibility between Pebax and g-PTAP. The MMMs showed an effective integration of g-PTAP nanofiller into the Pebax matrix without affecting its thermal stability. Gas permeation experiments with MMMs showed improved CO₂ permeability and selectivity (CO₂/N₂) upon incorporation of g-PTAP in the Pebax polymer matrix. The maximum CO₂ permeability enhancement from 82.3 to 154.6 Barrer with highest CO₂/N₂ selectivity from 49.5 to 83.5 were found with 2.5 wt.% of nanofiller compared to neat Pebax membranes.     

Facilitated Transport of C<Subscript>2</Subscript>H<Subscript>4</Subscript> in a SiO<Subscript>2</Subscript>-poly(N-vinylpyrrolidone)-Ag<Superscript>+</Superscript> Inorganic-Organic Hybrid Membrane

Inorganic-organic hybrid membranes containing silica as the structure matrix, poly(N-vinylpyrrolidone) (PVP) as the organic mediating agent and silver ions as olefinic carriers were prepared using sol–gel method and dip-coating process. The structure and permeances of the membranes for N₂, He, C₂H₄, C₂H₆ at different temperatures indicated that defect-free membranes were obtained and the transportation of the C₂H₄ through the membranes followed the dissolution and diffusion mechanism. Ideal separation factors of C₂H₄/C₂H₆ through the membranes were evaluated at the temperature of 298, 373 and 423 K respectively using mixture gas of 50% C₂H₄-50% C₂H₆. The results showed that the ideal separation factors of C₂H₄/C₂H₆ through the membranes were obviously greater than the ratio of PC₂H₄/PC₂H₆ obtained from the single gas measurement due to the hindering effect by the adsorbed C₂H₄. The ideal separation factors of C₂H₄/C₂H₆ increased with temperature and reached 10 at 423 K, which suggested that C₂H₄ and C₂H₆ could be separated at lower humidity compared to the reported organic polymer/silver salt membranes in which humidified gases and higher silver loading were usually used. The transport of C₂H₄ in the inorganic-organic hybrid membrane was proposed to follow the hopping mechanism, that is, olefins moved across the fixed silver sites.     

Reforming of CO<Subscript>2</Subscript>-Containing Natural Gas Using an AC Gliding Arc System: Effects of Operational Parameters and Oxygen Addition in Feed

In this research, the reforming of simulated natural gas containing a high CO₂ content under AC non-thermal gliding arc discharge with partial oxidation was conducted at ambient temperature and atmospheric pressure, with specific regards to the concept of the direct utilization of natural gas. This work aimed at investigating the effects of applied voltage and input frequency, as well as the effect of adding oxygen on the reaction performance and discharge stability in the reforming of the simulated natural gas having a CH₄:C₂H₆:C₃H₈:CO₂ molar ratio of 70:5:5:20. The results showed marked increases in both CH₄ conversion and product yield with increasing applied voltage and decreasing input frequency. The selectivities for H₂, C₂H₆, C₂H₄, C₄H₁₀, and CO were observed to be enhanced at a higher applied voltage and at a lower frequency, whereas the selectivity for C₂H₂ showed an opposite trend. The use of oxygen was found to provide a great enhancement of the plasma reforming of the simulated natural gas. For the combined plasma and partial oxidation in the reforming of CO₂-containing natural gas, air was found to be superior to pure oxygen in terms of reactant conversions, product selectivities, and specific energy consumption. The optimum conditions were found to be a hydrocarbons-to-oxygen feed molar ratio of 2/1 using air as an oxygen source, an applied voltage of 17.5 kV, and a frequency of 300 Hz, in providing the highest CH₄ conversion and synthesis gas selectivity, as well as extremely low specific energy consumption. The energy consumption was as low as 2.73 × 10⁻¹⁸ W s (17.02 eV) per molecule of converted reactant and 2.49 × 10⁻¹⁸ W s (16.60 eV) per molecule of produced hydrogen.