Gas Transport Performance Research Articles

Cross-Linked Thermally Rearranged Poly(benzoxazole-co-imide) Membranes Prepared from ortho-Hydroxycopolyimides Containing Pendant Carboxyl Groups and Gas Separation Properties

The incorporation of small amounts of 3,5-diaminobenzoic acid (DABA) comonomer (usually 5–20 mol %) into an ortho-hydroxypolyimide (HPI) precursor constitutes a new methodology to easily improve the gas transport performance of thermally rearranged (TR) polymer membranes. Thermogravimetric analysis (TGA) profiles of HPI with DABA copolyimides (HPID) suggests that there is an overlap between the temperature ranges of two processes: the degradation of DABA carboxyl groups and the actual thermal rearrangement process. During thermal treatment at 450 °C, carboxyl pendant groups in DABA degrade while a more rigid biphenyl cross-linked structure is formed either following or at the same time as the rearrangement of imide to benzoxazole. Mixed-gas CO2/CH4 (1:1) experiments proved this new series of cross-linked TR poly(benzoxazole-co-imide) (XTR-PBOI) membranes as superior materials for CO2/CH4 separation applications, with enhanced permeability and selectivity, as well as high resistance to plasticization up to 40 bar.

Thermally Rearranged Poly(benzoxazole-co-imide) Membranes with Superior Mechanical Strength for Gas Separation Obtained by Tuning Chain Rigidity

Thermally rearranged polybenzoxazoles (TR-PBO) are some of the most promising materials for gas separation because of their microporous and bimodal cavities that offer high gas transport performance. However, the brittleness of fully converted TR-PBO membranes has impeded their widespread industrial implementation. In this study, we prepared novel, thermally rearranged poly(benzoxazole-co-imide) membranes (TR-PBOI) with improved mechanical strength and good gas separation performance. These membranes are based on two commercially available TR-able diamines and two non-TR-able diamines with various compositions and different polymer rigidities. TR-PBOI membranes with the appropriate ratio of PBO and PI displayed a high fractional free volume and therefore exceptional gas separation properties (CO2 permeability over 300 barrer and CO2/N2 ideal selectivity above 20); both these values were higher than those of the corresponding original TR-PBO membranes. Furthermore, a substantial improvement in the mechanic...

PEG-imbedded PEO membrane developed by a novel highly efficient strategy toward superior gas transport performance.

Low-molecular-weight poly(ethylene glycol) (PEG) is deliberately incorporated into synthesized swellable poly(ethylene oxide) (PEO) membranes via a facile post-treatment strategy. The membranes exhibit both larger fractional free volume (FFV) and a higher content of CO2-philic building units, resulting in significant increments in both CO2 permeability and CO2/H2 selectivity. The separation performance correlates nicely with the microstructure of the membranes. This study may provide useful insights in the formation and mass transport behavior of highly efficient polymeric membranes applicable to clean energy purification and CO2 capture, and possibly bridge the material-induced technology gap between academia and industry.

Post-synthetic Ti exchanged UiO-66 metal-organic frameworks that deliver exceptional gas permeability in mixed matrix membranes.

Gas separation membranes are one of the lowest energy technologies available for the separation of carbon dioxide from flue gas. Key to handling the immense scale of this separation is maximised membrane permeability at sufficient selectivity for CO2 over N2. For the first time it is revealed that metals can be post-synthetically exchanged in MOFs to drastically enhance gas transport performance in membranes. Ti-exchanged UiO-66 MOFs have been found to triple the gas permeability without a loss in selectivity due to several effects that include increased affinity for CO2 and stronger interactions between the polymer matrix and the Ti-MOFs. As a result, it is also shown that MOFs optimized in previous works for batch-wise adsorption applications can be applied to membranes, which have lower demands on material quantities. These membranes exhibit exceptional CO2 permeability enhancement of as much as 153% when compared to the non-exchanged UiO-66 mixed-matrix controls, which places them well above the Robeson upper bound at just a 5 wt.% loading. The fact that maximum permeability enhancement occurs at such low loadings, significantly less than the optimum for other MMMs, is a major advantage in large-scale application due to the more attainable quantities of MOF needed.

Temperature-dependent gas transport performance of vertically aligned carbon nanotube/parylene composite membranes.

A novel composite membrane consisting of vertically aligned carbon nanotubes (CNTs) and parylene was successfully fabricated. Seamless filling of the spaces in CNT forests with parylene was achieved by a low-pressure chemical vapor deposition (CVD) technique and followed with the Ar/O2 plasma etching to expose CNT tips. Transport properties of various gases through the CNT/parylene membranes were explored. And gas permeances were independent on feed pressure in accordance with the Knudsen model, but the permeance values were over 60 times higher than that predicted by the Knudsen diffusion kinetics, which was attributed to specular momentum reflection inside smooth CNT pores. Gas permeances and enhancement factors over the Knudsen model firstly increased and then decreased with rising temperature, which confirmed the existence of non-Knudsen transport. And surface adsorption diffusion could affect the gas permeance at relatively low temperature. The gas permeance of the CNT/parylene composite membrane could be improved by optimizing operating temperature.

Fabrication and modification of cellulose acetate based mixed matrix membrane: Gas separation and physical properties

[Cellulose acetate (CA)-blend-multi walled carbon nano tubes (MWCNTs)] mixed matrix membranes (MMMs), [CA/polyethylene glycol (PEG)/MWCNTs] and [CA/styrene butadiene rubber (SBR)/MWCNTs] blend MMMs were prepared by solution casting method for gas separation applications using Tetrahydrofuran (THF) as solvent. Both raw-MWCNTs (R-MWCNTs) and functionalized carboxylic-MWCNTs (C-MWCNTs) were used in membrane preparation. The MWCNTs loading ratio and pressure effects on the gas separation performance of prepared membranes were investigated for pure He, N2, CH4 and CO2 gases. Results indicated that utilizing C-MWCNT instead of R-MWCNTs in membrane fabrication has better performance and (CO2/CH4) and (CO2/N2) selectivity reached to 21.81 and 13.74 from 13.41 and 9.33 at 0.65wt% of MWCNTs loading respectively. The effects of PEG and SBR on the gas transport performance and mechanical properties were also investigated. The highest CO2/CH4 selectivity at 2bar pressure was reached to 53.98 for [CA/PEG/C-MWCNT] and 43.91 for [CA/SBR/C-MWCNT] blend MMMs at 0.5wt% and 2wt% MWCNTs loading ratio respectively. Moreover, increase of feed pressure led to membrane gas permeability and gas pair selectivity improvement for almost all prepared membranes. The mechanical properties analysis exhibited tensile modules improvement with increasing MWCNTs loading ratio and utilizing polymer blending.

Synthesis and characterization of Thermally Rearranged (TR) polymers: influence of ortho-positioned functional groups of polyimide precursors on TR process and gas transport properties

Aromatic polyimides bearing various ortho-functional groups (i.e., acetate group and pivalic acetate group) were prepared via acetylation of a poly(hydroxyimide) containing ortho-positioned hydroxy groups using acetic anhydride or pivalic anhydride. The completeness of acetylation was confirmed by 1H NMR and FTIR. Chemically derivatized polyimides were used as precursors for an imide-to-benzoxazole thermal rearrangement (TR) process. The influence of various ortho-functionalities on the TR process and gas transport properties of the resulting TR polymers was studied. Differing from the –OH groups in a poly(hydroxyimide), the acetate groups of acetylated polyimide precursors degrade at elevated temperatures, and the degradation process interplays with imide-to-benzoxazole conversion. The acidic degradation product, as detected by 1H NMR, is suspected to have some catalytic effect on the TR process, which along with the protecting function of the acetate groups, resulted in a lower onset TR conversion temperature, the ability to conduct the TR process in air, and a higher TR conversion level. Gas permeation properties greatly depend on the ortho-functionality of polyimide precursors as well. The precursor films containing larger functional groups are much more permeable with comparable gas selectivities. Similarly, the resulting TR polymers formed from polyimides with larger leaving groups also showed much higher gas permeabilities despite similar degrees of TR conversion. The incorporation of bulkier functional groups in the TR precursors provides an effective way to significantly improve the gas transport performance, particularly the gas permeabilities of both the polyimide precursors and the resulting TR polymers.

Effect of zeolitic imidazolate frameworks on the gas transport performance of ZIF8-poly(1,4-phenylene ether-ether-sulfone) hybrid membranes

This work reports the permeability of hydrogen, nitrogen, oxygen, carbon dioxide, methane, ethane and ethylene across ZIF-8/poly(1,4-phenylen ether-ether-sulfone) hybrid membranes containing 0, 0.10, 0.20 and 0.30 weight fractions of ZIF-8. The glass transition temperature is independent on the filler content suggesting that the filler does not affect the polymer chains mobility. The study was carried out at 1 bar in the range of 283–313 K and the diffusion coefficient of the gases was obtained in this range of temperature by means of the lag time method. The permeability and diffusion coefficients obey Arrhenius behavior and the activation energies associated with this processes were obtained from the slopes of the respective plots. The performance of the membranes as determined by the closeness of their selectivity to the Robeson's upper bounds increases as the ZIF-8 content of the hybrid membranes increases. The membrane with 30% (w/w) content exhibits at 303 K pretty good selective properties for O 2/N 2 and H 2/N 2. The selectivity for C 2H 4/C 2H 6 is of the order of 3 with P(C 2H 4) ≅ 3 Barrer. Extrapolation of the permeability data for H 2/N 2 at high temperatures gives a selectivity that coincides with that predicted by the Robeson limit for this pair of gases.

Synthesis and characterization of poly (ethylene oxide) containing copolyimides for hydrogen purification

Reverse selective membranes comprising poly(ethylene oxide) (PEO) containing copolyimides (PEO-PI) with variations of acid dianhydrides and diamines have been synthesized for hydrogen purification. The reverse selectivity of the membranes decimate the energy required for hydrogen recompression process. Factors including PEO content, PEO molecular weight, and fractional free volume (FFV) that would affect the gas transport performance have been investigated and elucidated in terms of degree of crystallinity, phase separation in the PEO domain as well as inter-penetration between the hard and soft segments. In mixed gas tests of CO2 and H2 mixtures, a highly condensable CO2 out compete H2 for the sorption sites in hard segment and diminishes H2 permeability. Thus the CO2/H2 selectivity in the mixed gas tests is much higher than that in pure gas tests. Mixed gas permeation tests at 35 °C and 2atm show that the best reverse selective membranes have a CO2 permeability of 179.3 Barrers and a CO2/H2 permselectivity of 22.7. The physical properties of PEO-PIs have also been characterized by FTIR, DSC, GPC, WAXS, AFM and tensile strain tests.

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

Abstract 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 O 2 /N 2 and CO 2 /CH 4 . 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, H 2 , O 2 , N 2 , and CO 2 ) with increasing filler content shows that the trend is related to the true silica volume and void volume fraction.

Effect of the prominent catalyst layer surface on reactant gas transport and cell performance at the cathodic side of a PEMFC

The cell performance enhancement of a proton exchange membrane fuel cell (PEMFC) has been numerically investigated with the prominence-like form catalyst layer surface of the same composition at the cathodic half-cell of a PEMFC. The geometries of the prominence-like form catalyst layer surface are assigned as one prominence, three prominences, and five prominences catalyst layer surfaces with constant distance between two prominences in the same gas diffusion layer (GDL) for the purpose of investigating the cell performance. To confine the current investigation to two-dimensional incompressible flows, we assume that the fluid flow is laminar with a low Reynolds number 15. The results indicate that the prominence-like form catalyst layer surface can effectively enhance the local cell performance of a PEMFC.

Copolymers of Intrinsic Microporosity Based on 2,2′,3,3′‐Tetrahydroxy‐1,1′‐dinaphthyl

A series of new copolymers with high molecular weight and low polydispersity, prepared from tetrahydroxydinaphthyl, tetrahydroxyspirobisindane, and tetrafluoroterephthalonitrile monomers, prevent efficient space packing of the stiff polymer chains and consequently show intrinsic microporosity. One copolymer, DNPIM-33, has an excellent combination of properties with good film-forming characteristics and gas transport performance, and exhibits higher selectivity than the corresponding spirobisindane-based homopolymer PIM-1 for gas pairs, such as O(2) /N(2) , with a corresponding small decrease in permeability. This work demonstrates that significant improvements in properties may be obtained through development of copolymers with intrinsic microporosity (CoPIMs) that extends the spectrum of high-molecular-weight ladder structures of poly(dibenzodioxane)s.

Relation between structure and gas transport properties of polyethylene oxide networks based on crosslinked bisphenol A ethoxylate diacrylate

Abstract Poly(ethylene oxide) (PEO) networks prepared from the photopolymerization of bisphenol A ethoxylate diacrylate (BPA–EDA) have been investigated as a function of crosslinker molecular weight and copolymer composition. Dynamic mechanical and dielectric methods have been used to elucidate the thermal relaxation characteristics of the polymers as a function of network composition and architecture, and these properties were related to measured gas transport for CO 2 separations. Copolymerization strategies involving the insertion of flexible PEG side chains along the network backbone proved effective in enhancing network free volume and increasing permeability. The gas transport performance of rubbery amorphous membranes based on the n =15 BPA–EDA crosslinker (i.e., crosslinker encompassing 30 ethylene oxide repeat units between crosslinks) compared favorably to model polymers synthesized from poly(ethylene glycol) diacrylate.

Control of pore characteristics in carbon molecular sieve membranes (CMSM) using organic/inorganic hybrid materials

Microporous carbon membranes have shown extraordinary gas transport performance with thermal and chemical stability. The permeation characteristics of carbon membranes can be simply controlled by pyrolysis conditions. However, the results from a regulation of the factors give trade-off relationship. To overcome the trade-off relationship, carbon–silica membranes derived from poly(imide siloxane) composed of continuous polyimide matrix and porous siloxane domains were reported. To obtain higher permeability without a severe loss in selectivity, mesoporous alumina particles were introduced to the carbon–silica membrane. Poly(imide siloxane)–alumina composites were prepared as precursors and pyrolyzed at 600°C. The morphologies were observed using FE-SEM with energy dispersive spectroscopy. Pore characteristics were investigated by nitrogen adsorption/desorption experiments. The alumina-loaded carbon membrane showed a high volume of adsorbed nitrogen, indicating high surface area and pore volume. Gas permeation properties were measured at room temperature by the time-lag method for small gas molecules. The carbon–silica–alumina membranes pyrolyzed at 600°C had CO 2 permeability of 1765 Barrer (10 −10 cm 3 (STP) cm cm −2 s cm Hg) and a CO 2/N 2 selectivity of 28, while the carbon–silica membranes had CO 2 permeability of 610 Barrer and CO 2/N 2 selectivity of 34.

Highly Hydrothermally Stable Microporous Silica Membranes for Hydrogen Separation

Fluorocarbon-modified silica membranes were deposited on gamma-Al2O3/alpha-Al2O3 supports by the sol-gel technique for hydrogen separation. The hydrophobic property, pore structure, gas transport and separation performance, and hydrothermal stability of the modified membranes were investigated. It is observed that the water contact angle increases from 27.2+/-1.5 degrees for the pure silica membranes to 115.0+/-1.2 degrees for the modified ones with a (trifluoropropyl)triethoxysilane (TFPTES)/tetraethyl orthosilicate (TEOS) molar ratio of 0.6. The modified membranes preserve a microporous structure with a micropore volume of 0.14 cm3/g and a pore size of approximately 0.5 nm. A single gas permeation of H2 and CO2 through the modified membranes presents small positive apparent thermal activation energies, indicating a dominant microporous membrane transport. At 200 degrees C, a single H2 permeance of 3.1x10(-6) mol m(-2) s(-1) Pa(-1) and a H2/CO2 permselectivity of 15.2 were obtained after proper correction for the support resistance and the contribution from the defects. In the gas mixture measurement, the H2 permeance and the H2/CO2 separation factor almost remain constant at 200 degrees C with a water vapor pressure of 1.2x10(4) Pa for at least 220 h, indicating that the modified membranes are hydrothermally stable, benefiting from the integrity of the microporous structure due to the fluorocarbon modification.

Relation between network structure and gas transport in crosslinked poly(propylene glycol diacrylate)

A series of crosslinked poly(propylene oxide) rubbers was prepared by UV photopolymerization of poly(propylene glycol) diacrylate (PPGDA) in the presence of varying amounts of poly(propylene glycol) methyl ether acrylate (PPGMEA). The polar ether oxygen linkages in the resulting copolymers interact favorably with CO 2, imparting a high selectivity for CO 2 over light, non-polar gases as required for CO 2 separation applications. The introduction of mono-functional PPGMEA in the polymerization reaction mixture resulted in the insertion of short side branches along the copolymer network and a corresponding reduction in the effective crosslink density; the concentration of propylene oxide (PO) segments in the networks ranged from 60 to 85 wt.%, depending upon the initial reaction composition. The effect of PPGMEA content on the mass density, free volume, and viscoelastic relaxation properties of the polymer networks was studied, and these results were related to the gas transport performance of the rubbery films. Permeability measurements (35 °C) are reported for H 2, N 2, CH 4, CO 2, C 2H 6, and C 2H 4; solubility and diffusivity data are presented for CH 4, CO 2, C 2H 6, and C 2H 4. The physical and gas transport characteristics of the crosslinked PPGDA polymers were compared with those obtained for rubbery networks based on poly(ethylene glycol) diacrylate.

Humidity of reactant fuel on the cell performance of PEM fuel cell with baffle-blocked flow field designs

The objective of this work is to examine the effects of humidity of reactant fuel at the inlet on the detailed gas transport and cell performance of the PEM fuel cell with baffle-blocked flow field designs. It is expected that, due to the water management problem, the effects of inlet humidity of reactant fuel gases on both anode and cathode sides on the cell performance are considerable. In addition, the effects of baffle numbers on the detailed transport phenomena of the PEM fuel cell with baffle-blocked flow field are examined. Due to the blockage effects in the presence of the baffles, more fuel gas in the flow channel can be forced into the gas diffuser layer (GDL) and catalyst layer (CL) to enhance the chemical reactions and then augment the performance of the PEMFC systems. Effect of liquid water formation on the reactant gas transport is taken into account in the numerical modeling. Predictions show that the local transport of the reactant gas, the local current density generation and the cell performance can be enhanced by the presence of the baffles. Physical interpretation for the difference in the inlet relative humidity (RH) effects at high and low operating voltages is presented. Results reveal that, at low voltage conditions, the liquid water effect is especially significant and should be considered in the modeling. The cell performance can be enhanced at a higher inlet relative humidity, by which the occurrence of the mass transport loss can be delayed with the limiting current density raised considerably.

Reactant gas transport and cell performance of proton exchange membrane fuel cells with tapered flow field design

The objective of this work is to examine the reactant gas transport and the cell performance of a proton exchange membrane fuel cell (PEMFC) with a tapered flow channel design. It is expected that, with the reduction in the channel depth along the streamwise direction, the reactant fuel gas in the tapered channel can be accelerated as well as forced into the gas diffuser layer to enhance the electrochemical reaction and thus augment the cell performance. The effects of liquid water formation on the reactant gas transport are taken into account in the present study. Numerical predictions show that the cell performance can be enhanced with the fuel channel tapered, and the enhancement is more noticeable at a lower voltage. The results also reveal that the liquid water effect in general influences the cell performance and the effect becomes significant at lower voltages.

Analysis of the gas transport performance through PDMS/PS composite membranes using the resistances-in-series model

The gas transport performance through the polydimethylsiloxane (PDMS)/polysulfone (PS) composite hollow fiber membranes is investigated. A resistances-in-series model based on the Henis–Tripodi’s model and the theory of the boundary layer for gas separation in composite membranes is presented, in which the resistance of the boundary layer and the resistance of membrane have been taken into account. This model can be used to predict the membrane performance for refinery dry gas separation, which is replaced by the mixed gas of 40% H 2/10% N 2/50% CH 4 (volume fraction) in the experiment. Then the influences of feed flow rate, feed pressure, and the partial pressure of hydrogen in permeate gas on gas permeance and selectivity of membranes have been considered and analyzed. Although the present work focused on PDMS/PS composite membranes, this model also can be applied to the other so-called “resistance model” type membranes.

Gas transport properties of CoAlPO4-5/PC membranes

The transport phenomena of oxygen and nitrogen across a pure polycarbonate (PC) and CoAlPO4-5/PC membranes were studied. Various CoAlPO4-5 membranes with different cobalt content were added to polycarbonate membranes to improve the gas transport performance. Oxygen and nitrogen isotherms were studied. Solubility of oxygen and nitrogen was greatly increased by adding CoAlPO4-5 to the membranes, which also resulted in a higher solubility ratio of oxygen to nitrogen. It might be that a pinhole of the membrane caused the increase in diffusivity and a decrease in selectivity when excess CoAlPO4-5 was added. The results also showed that CoAlPO4-5 with a higher cobalt content would more effectively increase the gas solubility, but make only a minor change in the solubility ratio of oxygen to nitrogen. It was found that the increase in oxygen to nitrogen selectivity was mainly due to an increased diffusivity ratio of oxygen to nitrogen when CoAlPO4-5 was added into the membranes. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 89–95, 2000