Pd encapsulated and nanopore hollow fiber membranes: Synthesis and permeation studies

New types of supported Pd membranes were developed for high temperature H₂ separation. Sequential combinations of boehmite sol slip casting and film coating, and electroless plating (ELP) steps were designed to synthesize “Pd encapsulated” and “Pd nanopore” membranes supported on -Al₂O₃ hollow fibers. The permeation characteristics (flux, permselectivity) of a series of unaged and aged encapsulated and nanopore membranes with different Pd loadings were compared to those of a conventional 1 μm Pd/4 μm γ-Al₂O₃/-Al₂O₃ hollow fiber membrane. The unaged encapsulated membrane exhibited good performance with ideal H₂/N₂ separation factors of 3000–8000 and H₂ flux 0.4 mol/m² s at 370 °C and a transmembrane pressure gradient of 4 × 10⁵ Pa. The unaged Pd nanopore membranes had a lower initial flux and permselectivity, but exhibited superior performance with extended use (200 h). At the same conditions the unaged 2.6 μm Pd nanopore membrane had a H₂ flux of 0.16 mol/m² s and separation factor of 500 and the unaged 0.6 μm Pd nanopore membrane had a H₂ flux of 0.25 mol/m² s and separation factor of 50. Both nanopore membranes stabilized after 40 h of operation, in contrast to a continued deterioration of the permselectivity for the other membranes. An analysis of the permeation data reveals a combination of Knudsen and convective transport through membrane defects. A phenomenological, qualitative model of the synthesis and resulting structure of the encapsulated and nanopore membranes is presented to explain the permeation results.     

Characterization of microporous membranes for use in membrane contactors

Methods of selecting applicable membranes for use in membrane contactors for flue gas desulfurization are proposed in this paper. The mass transfer mechanism for SO₂ diffusion through gas filled pores is explored by simple measurements in order to identify suitable membrane structures for use in contactors for flue gas cleaning. It is attempted to correlate the experimentally determined membrane mass transfer coefficient to intrinsic physical properties of the membrane by applying theoretical and empirical correlations for the porosity-tortuosity relationship of the porous structure. Thereby limiting fluxes can be predicted with good accuracy from data quoted in the manufactures catalogue.     

Gas permeation properties of asymmetric carbon hollow fiber membranes prepared from asymmetric polyimide hollow fiber

Asymmetric carbon hollow fiber membranes were prepared by pyrolysis of an asymmetric polyimide hollow fiber membrane, and their mechanical and permeation properties were investigated. The carbon membrane had higher elastic modulus and lower breaking elongation than the polyimide membrane. Permeation experiments were performed for single gases such as H₂, CO₂, and CH₄, and for mixed gases such as H₂/CH₄ at high feed pressure ranging from 1 to 5 MPa with or without toluene vapor. The permeation properties of the carbon membranes and the polyimide membrane were compared. There was little change in the properties of the carbon membranes with a passage of time. The properties were hardly affected by the feed pressure, whether the feed was accompanied with the toluene vapor or not, because the carbon membranes were not affected by compaction and plasticization.     

Gas permeation properties of ethylene vinyl acetate–silica nanocomposite membranes

The effect of silica nanoparticles on the gas separation properties of ethylene vinyl acetate (EVA) copolymer containing 28% vinyl acetate has been investigated. The EVA and hybrid EVA–silica membranes were prepared via thermal phase inversion method. Silica nanoparticles prepared by hydrolysis of tetraethylorthosilicate (TEOS), through the sol–gel mechanism. The prepared membranes were characterized using FT-IR, SEM, DSC and XRD methods. FT-IR and SEM results indicated the nanoscale dispersion of silica particles in polymer matrix. As confirmed by XRD and DSC analyses, increasing the silica content enhances the amorphous regions significantly. Gas permeation of EVA–silica nanocomposite membranes with silica contents of 5, 6 and 10 wt.% was studied for N₂, O₂, CO₂ and CH₄ single gases at pressures of 4, 6 and 8 bar. The obtained results suggest a significant increase in permeability of all gases and an increase in CO₂/N₂ and CO₂/CH₄ gases selectivities upon increasing the silica content. The possible reasons for such behavior were stated and discussed. The pressure dependence of the gas permeabilities of the membranes was also investigated.     

O/O and H/D separation factors for liquid/vapor permeation of water through an hydrophobic membrane

Liquid/vapor permeation of water through commercial grade hydrophobic PTFE membranes at 323<P(Torr)<160, shows enhancements in logarithmic separation factors, ln[(i'/i)] = In [(χ'/χ) _downstream/ (χ'/χ) _upstream], which differ markedly for isotope separation of oxygen and hydrogen. The prime refers to the lighter isotope and χ is the mole fraction. Thus ln[(16/18)] is found to be as large as 6 times the ratio of the vapor pressure isotope effect, ln(_o = ln (P^o'/P^o), but ln[(H/D)]/ln[_o(H/D)] is only as large as 1.3. The difference in enhancement factors indicates that membrane transport mechanisms must be different for the two separations. The unusually large separation factor for ¹⁶O/¹⁸O may be of practical interest.     

Gas permeation properties of poly(lactic acid)

The need for the development of polymeric materials based on renewable resources has led to the development of poly(lactic acid) (PLA) which is being produced from a feedstock of corn rather than petroleum. The present study examines the permeation of nitrogen, oxygen, carbon dioxide, and methane in amorphous films of PLA cast from solution. The properties of PLA are compared to other commodity plastics and it is shown that PLA permeation closely resembles that of polystyrene. At 30°C, N₂ permeation in PLA is 1.3 (10⁻¹⁰ cm³ (STP) cm/cm² s cmHg) and the activation energy is 11.2 kJ/mol. For oxygen the corresponding values are 3.3 (10⁻¹⁰ cm³(STP) cm/cm² s cm Hg) and 11.1 kJ/mol. The values for carbon dioxide permeation are 1.2 (10⁻¹⁰ cm³ (STP) cm/cm² s cmHg) and 6.1 kJ/mol. For methane values of 1.0 (10⁻¹⁰ cm³ (STP) cm/cm² s cmHg) and an activation energy of 13.0 kJ/mol are found. Studies with pure gases show that polymer chain branching and small changes in l:d stereochemical content have no effect on permeation properties. Crystallinity is found to dominate permeation properties in a biaxially oriented film. The separation factor for a CO₂/CH₄ mixed gas system is measured between 0 and 50°C and does not deviate significantly from the calculated ideal separation factor; at 0°C the separation factor is 16, a value that suggests continued studies of PLA as a separation medium are warranted.     

Gas permeation properties of copolyetherimide based on 1,4-bis(3,4-dicarboxyphenoxy) benzene dianhydride

A series of aromatic homo- and copolyetherimides was prepared from 1,4-bis(3,4-dicarboxyphenoxy) benzene dianhydride with 2,4,6-trimethyl phenylenediamine, 3,3^′-dimethyl-4,4^′-methylene dianiline, and 3,5-diaminobenzoic acid. The gas permeability coefficients of the copolyetherimides to H₂, CO₂, O₂, N₂ and CH₄ were measured under 10 atm and at 30°C. A significant change in the permeability and permselectivity resulting from the systematic variation in chemical composition was found. The experimental values of the gas permeability coefficients and permselectivity coefficients of the copolyetherimides are in satisfactory agreement with the values estimated from the gas permeability coefficients of the constituent homopolyimides and their weight fractions.     

聚合物链长及表面吸附强度对三维受限下嵌段共聚物微观相分离的影响

利用动态密度泛函(Dynamic density functional theory, DDFT)方法研究了三维受限下嵌段共聚物的微观相分离, 讨论了共聚物链长和表面吸附强度对微观相形成与取向的影响. 体系中随机分布的等径微球提供三维限制结构, 体积分数为0.6. 增加微球的半径和体积分数, 能够使其从破坏微相规整结构的纳米掺杂过渡到提供三维限制结构. 调整嵌段共聚物与微球表面的相互作用对微相形成与取向有重要影响.     

The formulation of a self-consistent constricted variational density functional theory for the description of excited states

We outline here a self-consistent approach to the calculation of transition energies within density functional theory. The method is based on constricted variational theory (CV-DFT). It constitutes in the first place an improvement over a previous scheme [T. Ziegler, M. Seth, M. Krykunov, J. Autschbach, F. Wang, Chem. Phys. 130 (2009) 154102] in that it includes terms in the variational parameters to any desired order n including n = ∞. For n = 2, CV(n)-DFT is similar to TD-DFT. Adiabatic TD-DFT becomes identical to CV(2)-DFT after the Tamm-Dancoff approximation is applied to both theories. We have termed the new scheme CV(n)-DFT. In the second place, the scheme can be implemented self-consistently, SCF-CV(n)-DFT. The procedure outlined here could also be used to formulate a SCF-CV(n) Hartree-Fock theory. The approach is further kindred to the ΔSCF-DFT procedures predating TD-DFT and we describe how adiabatic TD-DFT and ΔSCF-DFT are related through different approximations to SCF-CV(n)-DFT.     

Metal doped silica membrane reactor: Operational effects of reaction and permeation for the water gas shift reaction

In this work, we investigate the performance of metal (Cobalt) doped silica membranes in a membrane reactor (MR) configuration for the low temperature water gas shift (WGS) reaction. The membranes were hydrostable and showed activated transport even after 2 weeks exposure to steam. High CO conversions resulted in the H₂ and CO partial pressures in the reaction chamber moving in opposite directions, thus favouring H₂/CO separation to treble (5–15) from 150 to 250 °C. On the other hand, the separation of H₂/CO₂ remained relatively low (2–4) as the driving force for diffusion or partial pressure of these gases remained equal in the reaction chamber irrespective of the extent of conversion. Below approximately 40% CO conversion, the MR is ineffective as the H₂ driving force for permeation was so low that H₂/CO selectivity was below unity. Operating under equilibrium limited conversion (space velocities 7500 h⁻¹) conditions, very high conversions in excess of 95% were observed and there were no significant advantages of the MR performance over the packed bed reactor (PBR). However, for higher throughputs (space velocities 38000 and 75000 h⁻¹) conversion is affected by the reaction rate, and relatively enough H₂ is removed from the reactor through the membrane. Increasing temperature to 250 °C as a function of the space velocity (75000 h⁻¹) allowed for the CO conversion in the MR to shift up to 12% as compared to the PBR.     

In situ determination of the adsorption characteristics of a zeolite membrane

A methodology based on adsorption-branch porosimetry is described for in situ measurement of the adsorption of condensable gases within the pore structure of inorganic membranes. The method is applied to the study of n-hexane and p-xylene adsorption in a high-silica, MFI zeolite membrane. The results, interpreted in terms of a simple model for competitive adsorption effects on the permeance of a non-adsorbing gas, yield Langmuir adsorption constants and Henry’s law constants for n-hexane and p-xylene that are in excellent agreement with measurements on bulk materials. The method is proposed for the fundamental study of fouling characteristics of inorganic membranes, especially in cases where a true bulk surrogate is not available.     

Exchange and correlation energy in density functional theory

Several different versions of density functional theory (DFT) that satisfy Hohenberg–Kohn theorems are characterized by different definitions of a reference or model state determined by an N‐electron ground state. A common formalism is developed in which exact Kohn–Sham equations are derived for standard Kohn–Sham theory, for reference‐state density functional theory, and for unrestricted Hartree–Fock (UHF) theory considered as an exactly soluble model Hohenberg–Kohn theory. A natural definition of exchange and correlation energy functionals is shown to be valid for all such theories. An easily computed necessary condition for the locality of exchange and correlation potentials is derived. While it is shown that in the UHF model of DFT the optimized effective potential (OEP) exchange satisfies this condition by construction, the derivation shows that this condition is not, in general, sufficient to define an exact local exchange potential. It serves as a test to eliminate proposed local potentials that are not exact for ground states. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 521–525, 2000     

Analytical gradients for subsystem density functional theory within the slater‐function‐based amsterdam density functional program

We present a new implementation of analytical gradients for subsystem density‐functional theory (sDFT) and frozen‐density embedding (FDE) into the Amsterdam Density Functional program (ADF). The underlying theory and necessary expressions for the implementation are derived and discussed in detail for various FDE and sDFT setups. The parallel implementation is numerically verified and geometry optimizations with different functional combinations (LDA/TF and PW91/PW91K) are conducted and compared to reference data. Our results confirm that sDFT‐LDA/TF yields good equilibrium distances for the systems studied here (mean absolute deviation: 0.09 Å) compared to reference wave‐function theory results. However, sDFT‐PW91/PW91k quite consistently yields smaller equilibrium distances (mean absolute deviation: 0.23 Å). The flexibility of our new implementation is demonstrated for an HCN‐trimer test system, for which several different setups are applied. © 2016 Wiley Periodicals, Inc.     

Isoelectronic changes in X-alpha theory using a Z-transition state approach

Using a simple geometrical interpretation based on the Z-perturbation theory and the Hellman-Feynman theorem a Z-transition method is proposed for the calculation of total electronic energy difference of ions with respect to the isoelectronic neutral atoms. The method is particularly useful within the X-alpha like models wherein the negative ions are known to be inaccessible.     

A network model for the permeability of condensable vapours through mesoporous media

The paper aims at introducing a discrete (network) approach to modelling transport of condensable vapours in mesoporous structures. Such models possess the potential for improving the understanding of the mechanisms responsible for the observed transport behaviour. The basic elements of a typical pore network representing a mesoporous medium are summarized and the current state concerning the simulation of the related phenomena is given. The main processes involved (adsorption, mass diffusion, surface flow, capillary condensation) are simulated over the entire range of relative pressure. Finally, the effects of material structural parameters (average pore radius, pore size distribution and standard deviation, pore connectivity) and other relevant factors (relative pressure, temperature, resistance to surface flow, total pressure drop) on vapour permeability are presented and the future research directions are pointed out.     

CO2 permeation properties of poly(ethylene oxide)-based segmented block copolymers

This paper discusses the gas permeation properties of poly(ethylene oxide) (PEO)-based segmented block copolymers containing monodisperse amide segments. These monodisperse segments give rise to a well phase-separated morphology, comprising a continuous PEO phase with dispersed crystallised amide segments. The influence of the polyether phase composition and of the temperature on the permeation properties of various gases (i.e., CO₂, N₂, He, CH₄, O₂ and H₂) as well as on the pure gas selectivities were studied in the temperature range of −5 °C to 75 °C. The CO₂ permeability increased strongly with PEO concentration, and this effect could partly be explained by the dispersed hard segment concentration and partly by the changing chain flexibility. By decreasing the PEO melting temperature the low temperature permeabilities were improved. The gas transport values were dependant on both the dispersed hard segment concentration and the polyether segment length (length between crosslinks). The gas selectivities were dependant on the polyether segment length and thus the chain flexibility.     

Electron correlations in Kohn–Sham exchange‐only theory

We provide an interpretation for the “exchange” energy and potential of Kohn–Sham exchange‐only theory, or equivalently that of the optimized potential method (OPM), which shows that in addition to contribution due to the Pauli exclusion principle, there is a kinetic component to these properties. The interpretation is in terms of a conservative field R ^OPM( r ), which is a sum of two fields, one representative of Pauli electron correlations and the other of kinetic effects. The OPM exchange potential is derived via the differential virial theorem to be the work done to move an electron in the field R ^OPM( r ). The OPM exchange energy is then expressed via the integral virial theorem in terms of this field. A similar interpretation for the energy and potential may also be derived directly from the OPM integral equation. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71:473–480, 1999     

Gas permeation parameters and other physicochemical properties of a polymer of intrinsic microporosity: Polybenzodioxane PIM-1

A detailed study of gas permeation, thermodynamic properties and free volume was performed for a novel polymer of intrinsic microporosity (PIM-1). Gas permeability was measured using both gas chromatographic and barometric methods. Sorption of vapors was studied by means of inverse gas chromatography (IGC). In addition, positron annihilation lifetime spectroscopy (PALS) was employed for investigation of free volume in this polymer. An unusual property of PIM-1 is a very strong sensitivity of gas permeability and free volume to the film casting protocol. Contact with water in the process of film preparation resulted in relatively low gas permeability (P(O₂) = 120 Barrer), while soaking with methanol led to a strong increase in gas permeability (P(O₂) = 1600 Barrer) with virtually no evidence of fast aging (decrease in permeability) that is typical for highly permeable polymers. For various gas pairs (O₂/N₂, CO₂/CH₄, CO₂/N₂) the data points on the Robeson diagrams are located above the upper bound lines. Hence, a very attractive combination of permeability and selectivity is observed. IGC indicated that this polymer is distinguished by the largest solubility coefficients among all the polymers so far studied. Free volume of PIM-1 includes relatively large microcavities (R = 5 Å), and the results of the PALS and IGC methods are in reasonable agreement.     

Influence of the transport direction on gas permeation in two-layer ceramic membranes

Experimental and theoretical results of studying gas permeation through porous membranes are presented. In order to mimic an asymmetric membrane two porous ceramic disks with different pore radii were arranged in series. Besides the possibility to perform conventional permeation measurements, the applied experimental setup permits the determination of the pressure at the interface between the two discs. To predict the performance of the asymmetric structure, in preliminary experiments structure parameters were determined for both membranes separately. For the same total pressure difference across the two-disk arrangement, different interlayer pressures and fluxes were predicted and detected experimentally depending on the flow direction.     

Transport of organic vapors through poly(1-trimethylsilyl-1-propyne)

Poly(1-trimethylsilyl-1-propyne) [PTMSP], a high-free-volume glassy polymer, has the highest gas permeability of any known synthetic polymer. In contrast to conventional, low-free-volume, glassy polymers, PTMSP is more permeable to large, condensable organic vapors than to permanent gases. The organic-vapor/permanent-gas selectivity of PTMSP based on pure gas measurements is low. In organic-vapor/permanent-gas mixtures, however, the selectivity of PTMSP is much higher because the permeability of the permanent gas is reduced dramatically by the presence of the organic vapor. For example, in n-butane/methane mixtures, as little as 2 mol% n-butane (relative n-butane pressure 0.16) lowers the methane permeability 10-fold from the pure methane permeability. The result is that PTMSP shows a mixed-gas n-butane/methane selectivity of 30. This selectivity is the highest ever observed for this mixture and is completely unexpected for a glassy polymer. In addition, the gas mixture n-butane permeability of PTMSP is considerably higher than that of any known polymer, including polydimethylsiloxane, the most vapor-permeable rubber known. PTMSP also shows high mixed-gas selectivities and vapor permeabilities for the separation of chlorofluorocarbons from nitrogen. The unusual vapor permeation properties of PTMSP result from its very high free volume - more than 20% of the total volume of the material. The free volume elements appear to be connected, forming the equivalent of a finely microporous material. The large amount of condensable organic vapor sorbed into this finely porous structure causes partial blocking of the small free-volume elements, reducing the permeabilities of the noncondensable permanent gases from their pure gas values.