Separation and permeation characteristics of a DD3R zeolite membrane

Permeation of various gases (carbon dioxide, nitrous oxide, methane, nitrogen, oxygen, argon, krypton, neon) and their equimolar mixtures through DD3R membranes have been investigated over a temperature range of 220–373 K and a feed pressure of 101–400 kPa. Helium was used as sweep gas at atmospheric pressure. Adsorption isotherms were determined in the temperature range 195–298 K, and modelled by a single and dual site Langmuir model. The permeation flux is determined by the size of the molecule relative to the window opening of DD3R, and its adsorption behaviour. As a function of temperature, bulky molecules (methane) show activated permeation, weakly adsorbing molecules decreasing permeation behaviour and strongly adsorbing molecules pass through a maximum. Counter diffusion of the sweep gas (helium) ranged from almost zero up to the order of the feed gas permeation and was strongly influenced by the adsorption of the feed gas.

DD3R membranes have excellent separation performance for carbon dioxide/methane mixtures (selectivity 100–3000), exhibit good selectivity for nitrogen/methane (20–45), carbon dioxide and nitrous oxide/air (20–400), and air/krypton (5–10) and only a modest selectivity for oxygen/nitrogen (2) separation. The selectivity of mixtures of a strongly and a weakly adsorbing component decreased with increasing temperature and pressure. The selectivity of mixtures of weakly adsorbing components was independent of pressure.

The permeation and separation characteristics of light gases through DD3R membranes can be explained by taking into account: (1) steric effects introduced by the window opening of DD3R leading to molecular sieving and activated transport, (2) competitive adsorption effects, as observed for mixtures involving strongly adsorbing gases, and (3) interaction between diffusing molecules in the cages of the zeolite.     

Anti-trade-off in dehydration of ethanol by novel PVA/APTEOS hybrid membranes

Novel organic–inorganic hybrid membranes were prepared through sol–gel reaction of poly(vinyl alcohol) (PVA) with γ-aminopropyl-triethoxysilane (APTEOS) for pervaporation (PV) separation of ethanol/water mixtures. The membranes were characterized by FTIR, EDX, WXRD and PALS. The amorphous region of the hybrid membranes increased with increasing APTEOS content, and both the free volume and the hydrophilicity of the hybrid membranes increased when APTEOS content was less than 5 wt%. The swelling degree of the hybrid membranes has been restrained in an aqueous solution owing to the formation of hydrogen and covalent bonds in the membrane matrix. Permeation flux increased remarkably with APTEOS content increasing, and water permselectivity increased at the same time, the trade-off between the permeation flux and water permselectivity of the hybrid membranes was broken. The sorption selectivity increased with increasing temperature, and decreased with increasing water content. In addition, the diffusion selectivity and diffusion coefficient of the permeants through the hybrid membranes were investigated. The hybrid membrane containing 5 wt% APTEOS has highest separation factor of 536.7 at 50 °C and permeation flux of 0.0355 kg m⁻² h⁻¹ in PV separation of 5 wt% water in the feed.     

Permeation and separation of styrene/ethylbenzene mixtures through cross-linked poly(hexamethylene sebacate) membranes

Poly(hexamethylene sebacate) (PHS) which has strong affinity for styrene was selected as membrane material, and the characteristics of permeation and separation for the styrene/ethylbenzene mixtures through these PHS cross-linked with N,N,N′,N′-tetraglycidyl m-xylenediamine(TETRA-DX) membrane by pervaporation were investigated. The cross-linked PHS membranes exhibited a styrene permselectivity for the styrene/ethylbenzene mixtures and the permeation rate increased with increasing styrene in the feed solution. The permselectivity of their membranes was strongly governed by the sorption separation process depending on the difference of the solubility between styrene and ethylbenzene. The molecular weight of PHS had also influence to the separation factor and permeation rate in pervaporation.     

A Uniformly Oriented MFI Membrane for Improved CO2 Separation

Membrane separation of CO₂ from natural gas, biogas, synthesis gas, and flu gas is a simple and energy‐efficient alternative to other separation techniques. But results for CO₂‐selective permeance have always been achieved by randomly oriented and thick zeolite membranes. Thin, oriented membranes have great potential to realize high‐flux and high‐selectivity separation of mixtures at low energy cost. We now report a facile method for preparing silica MFI membranes in fluoride media on a graded alumina support. In the resulting membrane straight channels are uniformly vertically aligned and the membrane has a thickness of 0.5 μm. The membrane showed a separation selectivity of 109 for CO₂/H₂ mixtures and a CO₂ permeance of 51×10^?7 mol m^?2 s^?1 Pa^?1 at ?35 °C, making it promising for practical CO₂ separation from mixtures.     

Vapour permeation and sorption in fluoropolymer gel membrane based on ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulphonyl)imide

The emissions of hydrocarbons from fossil fuels into atmosphere entail both an economic loss and an environmental pollution. Membrane separations can be used for vapour recovery and/or vapour removal from the permanent gas stream, given that the appropriate membrane is identified. A neat poly(vinylidene fluoride-co-hexafluoropropylene) membrane is impermeable to both the representatives of aliphatic hydrocarbons and branched hydrocarbons, namely hexane and isooctane, whereas the permeation flux is enhanced by the presence of 80 mass % of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulphonyl)imide in the membrane, as detailed in this work. The permeabilities of hydrocarbon vapours were determined from the binary mixture containing hydrocarbon and nitrogen to simulate the real input of an air stream containing a condensable hydrocarbon. The diffusion coefficient determined from sorption measurements was higher for hexane, as would be expected for a smaller molecule, whereas both the sorption isotherms and permeabilities of the hydrocarbons studied were found to be almost identical. It is possible that the sorption effect predominates in the transport mechanism for VOCs/N₂ separations.     

Entropy effects during sorption of alkanes in zeolites

Recent developments in Configurational-Bias Monte Carlo (CBMC) techniques allow the accurate calculation of the sorption isotherms for alkanes, and their mixtures, in various zeolites. The CBMC simulations give new insights into subtle entropy effects affecting mixture adsorption. Three types of entropy effects can be distinguished. (1) Size entropy effects favour the component with the smaller number of C atoms because the smaller molecule finds it easier to fill in the 'gaps' within the zeolite matrix at high molecular loadings. (2) Configurational entropy effects come into play for mixtures of alkanes that differ in the degree of branching. For a mixture of linear and branched alkanes with the same number of C atoms, configurational entropy effects favour the linear isomer because such molecules 'pack' more efficiently within, say, the intersecting channel topology of MFI zeolite. (3) Length entropy effects comes into force for sorption of linear and branched alkanes within the cylindrical channels of say AFI and MOR zeolites; here the double branched alkane has the shortest length and can be packed more efficiently within the channels. We demonstrate that CBMC simulations allow the efficient screening of zeolite structures for a given separation duty and aid the development of novel separation processes exploiting entropy effects.     

Effect of structure on the temperature dependence of gas transport and sorption in a series of polycarbonates

The permeabilities and solubilities of five gases are reported for bisphenol-A polycarbonate (PC), tetramethyl polycarbonate (TMPC), and tetramethyl hexafluoro polycarbonate (TMHFPC) at temperatures up to 200°C. The temperature dependence of permselectivity is discussed in terms of solubility and diffusivity selectivity changes with temperature for CO₂/CH₄ and He/N₂ gas separations. The activation energies for permeation and diffusion and the heats of sorption are also reported for each gas in the three polycarbonates. Analysis of these values provides a better fundamental understanding of the effect of polymer-penetrant interactions and polymer backbone structure on the temperature dependence of the transport and sorption properties of gases in membrane separation processes. Important factors affecting the solubility and diffusivity selectivity losses or gains with increased temperature are also identified through correlation of these data with physical properties of the gases and polymers. These conclusions provide a framework for choosing the most promising membrane materials for particular gas separations at elevated temperatures. © 1994 John Wiley & Sons, Inc.     

Growth of a faujasite-type zeolite membrane and its application in the separation of saturated/unsaturated hydrocarbon mixtures

Faujasite type zeolite membranes were synthesized on porous ceramic alumina supports by using direct (in situ) and secondary (seeded) growth methods. In the secondary growth method a seed layer of ZSM-2 nanocrystals (prepared according to a report by Schoeman et al. J. Colloid Interface Sci. 1995, 170, 449–456) was deposited on the surface of the support before the hydrothermal growth. For both in situ and secondary growth, the mixture composition was 4.17 Na₂O:1.0 Al₂O₃:10 TEA (triethanol ammonium):1.87 SiO₂:460 H₂O. X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron microprobe analysis (EPMA), indicate well intergrown 5–30 μm thick FAU films with Si/Al ∼1–1.5. The separation of saturated/unsaturated hydrocarbon mixtures is demonstrated over a range of temperatures (40–160°C). The mixtures examined (and the corresponding equimolar mixture separation factors) are benzene/cyclohexane (160), benzene/n-hexane (144), toluene/n-heptane (45), propylene/propane (6.2), and ethylene/methane (8.4). In all cases, the membranes are unsaturated hydrocarbon permselective. With equimolar feed mixtures (5 kPa/5 kPa benzene/cyclohexane) and in the temperature range 65–160°C, the membranes exhibit separation factor of 20–160 with the benzene flux in the range 10⁻⁴–10⁻³ mol m⁻² s⁻¹. Decreasing the total feed partial pressure (0.31/0.31 kPa benzene/cyclohexane) reduces both separation factor (12) and benzene flux. Similar trend is observed when the benzene/cyclohexane ratio in the feed mixture (0.5/9.5 kPa benzene/cyclohexane) is reduced. A sorption diffusion model based on the Stefan–Maxwell formulation has also been employed to show that the benzene/cyclohexane separation can mainly be attributed to differences of their adsorption properties.     

Influence of pore dimension and sorption configuration on the heat of sorption of hexane on monodimensional siliceous zeolites

Sorption of n-hexane on monodimensional pure silica SSZ-35, CIT-5, ZSM-12, and ZSM-22 zeolites with different pore dimension and on recently synthesized ITQ-29 was studied by IR spectroscopic and computational chemistry methods. Heats of sorption of n-hexane on these zeolites was determined experimentally from the temperature dependence of the intensity of IR bands of sorbed hexane as well as from theoretical calculations. Calculations have shown the different orientations of sorbed hexane molecules inside zeolite channels, which depend on the type of zeolite and loading. At high loadings, ordering of hexane inside the channels is observed due to optimization of sorbate-sorbate and sorbate-zeolite interaction energies. Such ordering is responsible for the increase of the sorption energy. A decrease of the sorption energy upon increasing the pore dimension of zeolite was observed, in agreement with results previously published in the literature. Effects of pore diameter of zeolites and ordering of molecules inside zeolite channels on the sorption energy of hexane are discussed.     

Sorption and pervaporation properties of crosslinked membranes of poly(ethylene oxide imide) segmented copolymer to aromatic/nonaromatic hydrocarbon mixtures

Poly(ethylene oxide imide) segmented copolymer (PEO‐PI) membranes were crosslinked by the chemical reaction between ethylene glycol diglycidyl ether and benzylalcohol groups of diamine moieties in polyimide segments at high temperatures. Sorption and diffusion of penetrants took place in poly(ethylene oxide) segment microdomains. Sorption and desorption behavior of pure vapors such as benzene (Bz), cyclohexane (Cx) and n‐hexane (Hx) was classified as the Fickian diffusion. Sorption isotherms of binary liquid mixtures could be represented by the Flory–Rehner model, but the model overpredicted the sorption amounts of Cx and Hx, leading to small predictions of sorption selectivity α_S for Bz/Cx and Bz/Hx systems. UNIFAC‐FV model fairly well predicted the sorption amounts of aromatic hydrocarbons, but significantly overestimated those of nonaromatic ones, leading to too small predictions of α_S. Pervaporation (PV) behavior of PEO‐PI membranes was governed by sorption behavior followed by membrane swelling. Diffusion coefficient weakly depended on the minimum cross section of a penetrant. The diffusivity selectivity α_D hardly depended on the feed composition and was about 1.4 and 0.75 for Bz/Cx and Bz/Hx, respectively. PV selectivity α_PV was larger for Bz/Hx than for Bz/Cx because of larger α_S. PEO‐PI membranes displayed high specific permeation flux Ql and reasonably high α_PV for aromatic/nonaromatic hydrocarbons; for example, Ql = 60 Kg μm/(m² h) and α_PV = 8 for a feed mixture containing Bz, Tol, Hx, n‐Ot and i‐Ot of 20 wt % at 353 K. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1800–1811, 2000     

Retention of Cs on zeolite, bentonite and their mixtures

The sorption behavior of cesium on zeolite and bentonite minerals and their mixtures was studied by means of a batch method and a tracer technique. All experiments were carried out in the presence of CsCl spiked with ¹³⁷Cs and NaCl as a supporting electrolyte in varying concentrations. The distribution coefficients (K _D) did not show significant differences at low Cs⁺ loadings while they decreased in the high loading region. Freundlich and D-R isotherms were applied to the adsorption data of zeolite and bentonite. Adsorption capacities and mean energies calculated from D-R isotherm parameters decreased by increasing ionic strength on both minerals. The identification of the specific uptake sites was attempted on the basis of the Freundlich isotherm. Experimentally observed distribution coefficients of Cs on two mineral mixtures were smaller than theoretically calculated values, except at the highest NaCl concentration.     

Preparation of highly selective zeolite ZSM-5 membranes by a post-synthetic coking treatment

Zeolite ZSM-5 membranes with high n-butane:isobutane selectivities, e.g., 322 at 185°C, are obtained by a selective deposition of coke into non-zeolitic pores. The zeolite membranes are prepared by in situ crystallization on either bare porous α-Al₂O₃ support disks or disks that are pretreated to include a diffusion barrier. The post-synthetic coking treatment is accomplished by impregnating these membranes with liquid 1,3,5-triisopropylbenzene (TIPB) for 24 h at room temperature and then calcining them in air at 500°C for 2 h. Calcination at 500°C for up to 30 h does not destroy the high n-butane:isobutane selectivity. Thermogravimetric analysis (TGA) experiments on two model pore systems ZSM-5 (5.5 Å) and Vycor glass (40–50 Å) suggest that micro-defects are selectively eliminated by the TIPB coking treatment while the intracrystalline pore space of the ZSM-5 is not affected. The elimination of non-zeolitic pores results in a large increase of n-butane:isobutane pure gas flux ratio (45 vs. 320 at 185°C) accompanied by a fourfold reduction of the n-butane flux. The permeation experiments reveal that the n-butane flux increases nonlinearly with the partial pressure in the feed while the n-butane:isobutane pure gas flux ratio remains relatively unchanged.     

Zur Dünnschicht-Chromatographie von zinnorganischen Verbindungen

With the aid of thinlayer chromatography mixtures of tetraorganotin compounds containing no polar groups can be separated and identified if they differ in the number of phenyl groups per molecule or in the nature of their aliphatic residues (alkyl or allyl). For analytical purposes mixtures of 80% hexane and 20% benzene by volume are suitable as mobile phase. For preparative purposes (requiring multiple development) pure hexane or hexane containing 10 to 20% benzene by volume are best suited as mobile phase, where the amount of hexane depends on the problem of separation at hand. The R_f values of the mobile phases vary linearly with the number of the phenyl groups x and decrease with increasing x. The straight lines connecting the R_f values cross at x=4 (tetraphenyltin). Dithizone and silver nitrate are suitable spray reagents for organotin compounds. By spraying the plates with dithizone tetra-, tri-, and di-organotin compounds can be immediately distinguished on the basis of the ensuing colourations. With silver nitrate allyltin compounds and polystannanes can be distinguished from alkyltin compounds; tetra-phenyltin is not coloured by this reagent.     

清液体系中T型分子筛膜的高重复性合成与渗透汽化性能

以自制微米级分子筛为晶种,在清液体系中成功合成出高性能的T型分子筛膜,考察了硅铝比、水硅比、碱度及合成温度与时间等条件对膜的生长和渗透汽化性能的影响.结果表明,在摩尔组成为1SiO₂:0.015Al₂O₃:0.41(Na₂O+K₂O):30H₂O的清液体系中,于423K晶化6h的条件下可较快地形成一层厚度为5μm的连续致密纯相T型分子筛膜,较大缩短了膜合成时间且提高了膜致密性.在优化条件下所合成的膜具有优良的分离性能和高重复性.348K时,在10wt%水-90wt%异丙醇混合物体系中膜的渗透通量和分离因子分别高达4.20kg/(m²·h)和7800.     

Separation of p-xylene from multicomponent vapor mixtures using tubular MFI zeolite mmbranes

MFI zeolite membranes have been synthesized on tubular -alumina substrates to investigate the separation of p-xylene (PX) from m-xylene (MX) and o-xylene (OX) in binary, ternary, and simulated multicomponent mixtures in wide ranges of feed pressure and operating temperature. The results demonstrate that separation of PX from MX and OX through the MFI membranes relies primarily on shape-selectivity when the xylene sorption level in the zeolite is sufficiently low. For an eight-component mixture containing hydrogen, methane, benzene, toluene, ethylbenzene, PX, MX, and OX, a PX/(MX + OX) selectivity of 7.71 with a PX flux of 6.8 × 10⁻⁶ mol/(m² s) was obtained at 250 °C and atmospheric feed pressure. The addition of a small quantity of nonane to the multicomponent mixture caused drastic decreases in the fluxes of aromatic components and the PX separation factor because of the preferential adsorption of nonane in the zeolite channels. The nanoscale intercrystalline pores also caused serious decline in the PX separation factor. A new method of online membrane modification by carbonization of 1,3,5-triisopropylbenzene in the feed stream was found to be effective for reducing the intercrystalline pores and improving the PX separation.     

Dehydration of acetic acid by pervaporation using SPEK-C/PVA blend membranes

Sulfonated cardo polyetherketone (SPEK-C) and poly(vinyl alcohol) (PVA) blend membranes were prepared by solution casting method and used in pervaporation (PV) dehydration of acetic acid. The membranes were characterized by Fourier transform infrared (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and contact angle meter. The results show that thermal crosslinking occurred to the membrane under high temperature annealing. The effective d-spacing (inter-segmental spacing) decreased with PVA content decreasing. The hydrophilicity of the blend membrane increased with SPEK-C content increasing. Swelling and sorption experiments show that the swelling degree of the blend membrane increased, however both the sorption and diffusion selectivities decreased with increasing PVA content. The diffusion selectivity is higher than the sorption selectivity. This suggests that PV dehydration of acetic acid is dominated by the diffusion process. The pervaporation separation index (PSI) of the membrane increases with increasing PVA content and arrives at a maximum when the SPEK-C/PVA ratio is 3/2, then decreases with further addition of PVA. The membrane has an encouraging separation performance with a flux of 492 g m⁻² h⁻¹ and separation factor of 59.3 at 50 °C at the feed water content 10 wt%.     

醇/水混合物在磺化聚乙烯中空纤维阴离子交换膜中的渗透汽化

通过对聚乙烯中空纤维光化学氯磺化反应,继而胺化和季胺化,制备了磺化聚乙烯中空纤维阴离子交换膜。以恒沸组成的异丙醇/水和85wt%乙醇/水为料液,测定了不同抗衡离子膜的渗透汽化性能。表明:该膜有极高的选择性,分离系数和渗透通量与抗衡离子密切相关,对卤素离子,α_W/A大小次序为I~->Br~->Cl~-;通量大小次序与之相反。三种抗衡离子膜的平衡吸收实验表明,该阴离子膜的选择渗透性不仅与醇水在膜中的溶解度有关,而且取决于平均扩散系数。     

Enhanced CO2-selective behavior of Pebax-1657: A comparative study between the influence of graphene-based fillers

Mixed matrix membranes (MMMs) containing graphene-based fillers have attracted considerable attention in the field of gas separation. In this study, two types of graphene derivatives (Graphene (G) and Graphene Oxide (GO)) were embedded into the poly-ether-block-amide (Pebax) based MMM to investigate and compare CO₂/N₂ separation at various filler loadings (0.3–1 wt%). The morphologies of the prepared neat Pebax and MMMs were characterized by SEM, XRD, FTIR and DSC. Compared with the neat Pebax, the permeability of all gases was increased by adding filler content in the MMMs due to the crystallinity decrement of the polyamide (PA) segment. The best separation performance of the Pebax/G MMMs occurred at 0.7 wt%, where the CO₂ permeability increased from 26.51 to 44.78 Barrer (~1.7 times). Also, for the Pebax/GO MMMs, the CO₂ permeability was increased up to 58.96 Barrer (~2.2 times) by adding 0.5 wt% filler. This further gas permeation increment for the Pebax/GO sample was attributed to the higher affinity of GO nanosheets to CO₂ sorption, which can facilitate CO₂ gas transition through the membrane matrix. Moreover, the CO₂/N₂ ideal selectivity increased from 74.26 for the neat Pebax to 111.95 (~1.5 times) and 120.72 (~1.62 times) by adding 0.7 wt% G (Pebax/G-0.7) and 1 wt% GO (Pebax/GO-1) into Pebax, respectively. As a consequence, graphene derivatives can be recognized as a promising developer of permselectivity (permeability and selectivity) of the MMMs.     

Facile preparation of compact LTA molecular sieve membranes on polyethyleneimine modified substrates

A facile preparation strategy was proposed for preparation of compact zeolite LTA membranes on polyethyleneimine(PEI) modified substrates without seeding.Through the functionalization of substrates by using PEI,compact LTA membranes can be formed on various kinds of substrates.A well-intergrown and phase-pure LTA membrane with a thickness of about 3.0 μm is successfully prepared on the a-Al_2 O_3 disk after crystallization for 24 h at 60℃.Besides LTA membrane,wellintergrown zeolite FAU membranes can also be formed on PEI-modified a-Al_2 O_3 substrates,suggesting the universality of this strategy.The zeolite LTA membranes synthesized on PEI-modified a-Al_2 O_3 tubes we re evaluated fo r the separation of alcohols/water mixture through pervaporation.The as-synthesized zeolite LTA membranes display high pervaporation performances.For the separation of 10 wt% isopropanol/water solution at 90℃,a high separation factor of44991 and a water flux of 1.73 kg m ~2 h ~1 are achieved.     

Application of NKF-6 zeolite for the removal of U(VI) from aqueous solution

To better understand the application of NKF-6 zeolite as an adsorbent for the removal of U(VI) from radionuclides and heavy metal ions polluted water, herein, NKF-6 zeolite was employed to remove U(VI) at different experimental conditions. The influence of solid/liquid ratio, contact time, pH, ionic strength, humic substances and temperature on sorption of U(VI) to NKF-6 zeolite was investigated using batch technique under ambient conditions. The experimental results demonstrated that the sorption of U(VI) on NKF-6 zeolite was strongly dependent on pH. The sorption property of U(VI) was influenced by ionic strength at pH < 7.0, whereas was independent of ionic strength at pH > 7.0. The presence of fulvic acid or humic acid promoted the sorption of U(VI) on NKF-6 zeolite at low pH values while restrained the sorption at high pH values. The thermodynamic parameters (i.e., ΔS ⁰, ΔH ⁰, and ΔG ⁰) calculated from the temperature-dependent sorption isotherms demonstrated that the sorption process of U(VI) on NKF-6 zeolite was endothermic and spontaneous. At low pH values, the sorption of U(VI) was dominated by outer-sphere surface complexation and ion exchange with Na⁺/H⁺ on NKF-6 zeolite surfaces, while inner-sphere surface complexation was the main sorption mechanism at high pH values. From the experimental results, one can conclude that NKF-6 zeolite can be used as a potential adsorbent for the preconcentration and solidification of U(VI) from large volumes of aqueous solutions.