Gas Permeation Performance Research Articles

Hydroxyl-Functionalized Polymers of Intrinsic Microporosity and Dual-Functionalized Blends for High-Performance Membrane-Based Gas Separations.

Membrane technology has shown significant growth during the past two decades in the gas separation industry due to its energy-savings, compact and modular design, continuous operation, and environmentally benign nature. Robust materials with higher permeability and selectivity are key to reduce capital and operational cost, pushing it forward to replace or debottleneck conventional energy-intensive unit operations such as distillation. Recently designed ladder polymers of intrinsic microporosity (PIM) and polyimides of intrinsic microporosity (PIM-PI) with pores <20 Å have demonstrated excellent gas permeation performance. Here, a series of plasticization-resistant PIM-based membrane materials is reported, including the first example of a hydroxyl-functionalized triptycene- and Tröger's base-derived ladder PIM and two PIM-PI homopolymers and a series of dual-functionalized polyimide blends containing hydroxyl- and carboxyl-functionalized groups. Specifically, 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)-based PIM-PI blends demonstrated extremely high selectivity for a variety of industrially important applications. An optimized polyimide blend containing ─OH and ─COOH groups showed permselectivity values of 136 for CO2/CH4, 11.4 for O2/N2 and 636 for H2/CH4. Such extreme size-sieving capabilities are attributed to physical crosslinking induced by strong hydrogen bonding forming tightly structured polymer networks. The study provides a new general strategy for developing plasticization resistant, robust, and highly-selective PIM-based membrane materials.

Enhancing CO2 transport with plasma-functionalized ionic liquid membranes

The ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate treated with radiofrequency plasma is proposed for functionalization and immobilization on polyethersulfone supports to form supported ionic liquid membranes for CO2 separation. The effects of treatment time and transmembrane pressure difference on CO2 permeance were evaluated. The best gas permeation performance was obtained with a treatment time of 10 min and the transmembrane pressure difference was 0.25 MPa. Characterization of the materials by Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy and nuclear magnetic resonance spectroscopy demonstrates that the IL is grafted with carboxyl groups and deprotonated through plasma treatment. A preliminary mechanism for the plasma treatment and facilitated transport of CO2 has been proposed on this basis.

Influence of diamine moieties on the gas permeation performances of polyimide: perspectives from experiment and simulation

Influence of diamine moieties on the gas permeation performances of polyimide: perspectives from experiment and simulation

Effect of monomer moieties on the aggregate structure and gas transport properties of polyimides: Insights from experiments and simulations

AbstractTFDB‐based PIs (BTDA‐TFDB, PMDA‐TFDB and 6FDA‐TFDB) were prepared by the polymerization of 2,2′‐bis(trifluoromethyl) benzidine (TFDB) with three different dianhydrides, which contained flexible carbonyl bridged unit, rigid‐rod moiety without bridged unit, and bulky CF3 bridged unit in dianhydride moieties, respectively. The influence of dianhydride moieties on the aggregate structure and gas permeation performances of polyimide was systematically investigated in relation to their chain packing and free volumes, using the methods of molecular simulation, positron annihilation measurement and X‐ray diffractogram. Changing the dianhydride from benzophenonetetracarboxylic dianhydride (BTDA) to pyromellitic dianhydride (PMDA) and 4,4′‐(hexafluoroisopropylidene) diphthalic anhydride (6FDA), the gas permeation coefficients of polyimide increased, and the gas permeation selectivity (O2/N2, CO2/N2, H2/N2 and CO2/CH4) decreased. With the introduction of flexible carbonyl bridged unit into dianhydride moiety, the high rotational mobility favored tight chains packing and decreased free volumes, which in turn led to the lowest permeation coefficients of BTDA‐TFDB. The bulky CF3 bridged unit in dianhydride moiety greatly restricted chain packing and increased free volumes, thus resulting in the highest permeation coefficients of 6FDA‐TFDB. From BTDA‐TFDB to PMDA‐TFDB and 6FDA‐TFDB, the permeation selectivity decreased. This was mostly ascribed to the reduction of diffusive selectivity, which was resulted from the increase in free volumes.

Plasma-assisted synthesis of ZIF-8 membrane for hydrogen separation

Well-intergrown MOF membranes were challenging to construct using the in-situ approach, due to the poor heterogeneous nucleation of MOF on the supports. Here, we described a technique to manufacture ZIF-8 membranes devoid of induced seeds using support modified by O2 plasma for the first time. O2 plasma can increase the number of oxygen-containing functional groups (–OH) on the support surface that could absorb zinc ions from synthesizing solution and offer heterogeneous nucleation sites for the formation of ZIF-8 membranes. The as-synthesized membrane exhibits outstanding gas permeation performance with a high hydrogen permeance of about 2.22 × 10−7 mol·m−2·s−1·Pa−1 and a high H2/CH4 ideal selectivity of 25.91. The separation factor of the equimolar H2/CO2 and H2/CH4 mixtures reached 8.72 and 31.03 at 298 K and 0.1 MPa, respectively. In addition, this new technique also allowed for the synthesis of ZIF-67 membranes, and the prepared membrane had an H2/CH4 ideal selectivity of 14.92. This work develops a new route for the efficient fabrication of MOF membranes.

Mixed matrix membranes incorporated with zeolite AlPO-18 for CO2/CH4 separation

Mixed Matrix Membranes (MMMs) have garnered a lot of interest in carbon dioxide (CO2) and methane (CH4) gas separation due to their advantages over pure polymeric membranes. In the present work, various Aluminophosphate-18 (AlPO-18) loadings was incorporated into Polysulfone (PSf) matrix to fabricate MMMs for CO2 gas permeation and separation. The MMMs were subjected to characterization including Fourier Transform Infrared (FTIR), X-ray Diffraction (XRD) and Field Emission Scanning Electron Microscope (FESEM) to study the spectroscopic, crystallographic and morphological characteristics of the developed membranes. CO2 and CH4 gas permeation were conducted to evaluate the effect of different AlPO-18 loading on the gas permeation and separation performance of the membrane. FTIR and XRD analysis proved the incorporation of zeolite AlPO-18 particles on the polymeric matrix. Notable improvements in gas permeation and separation were obtained for MMMs incorporated with 1 wt% AlPO-18 (1AlPO-18/PSf MMMs), with the ideal selectivity increasing by 22% in comparison to the pristine PSf membrane.

PPO-ZIF MMMs possessing metal-polymer interactions for propane/propylene separation

An assertive formation of ZIF-based mixed matrix membranes (MMMs) with polyphenylene oxide (PPO), a high permeability polymer as a host, is presented. The interfacial interactions between the filler particles and polymer matrix are established by DSC and XPS analyses. The ZIF loading could be achieved up to 40% without hampering the stability of the resulting MMMs. These membranes were evaluated for pure gas permeability, specifically aiming at C3H6/C3H8 separation, a highly desired application in industry. The ZIF-PPO hybrids display promising pure gas as well as mixed gas permeation performance. The 40% ZIF-8 and ZIF-67 loaded membrane display promising C3H6/C3H8 selectivity of 27.5 and 25, with a permeability of 12 and 13 barrer, respectively. The enhanced selectivity is attributed to the absence of defects eliminated due to metal-polymer interactions. The permeation study of a 30% ZIF-8 loaded membrane while varying transmembrane pressure and long-time exposure (150 h) of propylene at 60 psi indicated the excellent stability of the membrane. The sorption analysis further confirmed the molecular sieving characteristics of the ZIF@PPO MMMs. The mixed gas permeation performance showed promising results of high permeability as well as maintaining selectivity over a wide range of compositions.

Fundamental investigation on the development of composite membrane with a thin ion gel layer for CO2 separation

A composite membrane with a thin defect-free ion gel layer was developed in this study. The ion gel layer containing an interpenetrating polymer network (IPN) and high ionic liquid (IL) content (80–90 wt%) was prepared on a poly(dimethylsiloxane) gutter layer by spin coating. The thickness of the IPN ion gel layer was reduced from 20 μm to 600 nm by increasing the dilution ratio of the ion gel precursor solution, and the CO2 permeance of the composite membrane was increased from 45 to 613 GPU. By increasing the IL content of the IPN ion gel layer to 90 wt%, the prepared composite membrane exhibited the CO2 permeance of 778 GPU and CO2/N2 selectivity of 15. The CO2 permeance and CO2/N2 permselectivity of the IPN ion gel layer alone having 90 wt% IL were respectively estimated to be 1860 GPU and 27. The excellent gas permeation performance proves that IPN ion gel is a good optional material as a selective layer of a composite membrane for efficient CO2 separation.

Tuning the Gas Separation Performances of Smectic Liquid Crystalline Polymer Membranes by Molecular Engineering.

The effect of layer spacing and halogenation on the gas separation performances of free-standing smectic LC polymer membranes is being investigated by molecular engineering. LC membranes with various layer spacings and halogenated LCs were fabricated while having a planar aligned smectic morphology. Single permeation and sorption data show a correlation between gas diffusion and layer spacing, which results in increasing gas permeabilities with increasing layer spacing while the ideal gas selectivity of He over CO2 or He over N2 decreases. The calculated diffusion coefficients show a 6-fold increase when going from membranes with a layer spacing of 31.9 Å to membranes with a layer spacing of 45.2 Å, demonstrating that the layer spacing in smectic LC membranes mainly affects the diffusion of gasses rather than their solubility. A comparison of gas sorption and permeation performances of smectic LC membranes with and without halogenated LCs shows only a limited effect of LC halogenation by a slight increase in both solubility and diffusion coefficients for the membranes with halogenated LCs, resulting in a slightly higher gas permeation and increased ideal gas selectivities towards CO2. These results show that layer spacing plays an important role in the gas separation performances of smectic LC polymer membranes.

Tunning CO2 Separation Performance of Ionic Liquids through Asymmetric Anions.

This work aims to explore the gas permeation performance of two newly-designed ionic liquids, [C2mim][CF3BF3] and [C2mim][CF3SO2C(CN)2], in supported ionic liquid membranes (SILM) configuration, as another effort to provide an overall insight on the gas permeation performance of functionalized-ionic liquids with the [C2mim]+ cation. [C2mim][CF3BF3] and [C2mim][CF3SO2C(CN)2] single gas separation performance towards CO2, N2, and CH4 at T = 293 K and T = 308 K were measured using the time-lag method. Assessing the CO2 permeation results, [C2mim][CF3BF3] showed an undermined value of 710 Barrer at 293.15 K and 1 bar of feed pressure when compared to [C2mim][BF4], whereas for the [C2mim][CF3SO2C(CN)2] IL an unexpected CO2 permeability of 1095 Barrer was attained at the same experimental conditions, overcoming the results for the remaining ILs used for comparison. The prepared membranes exhibited diverse permselectivities, varying from 16.9 to 22.2 for CO2/CH4 and 37.0 to 44.4 for CO2/N2 gas pairs. The thermophysical properties of the [C2mim][CF3BF3] and [C2mim][CF3SO2C(CN)2] ILs were also determined in the range of T = 293.15 K up to T = 353.15 K at atmospheric pressure and compared with those for other ILs with the same cation and anion’s with similar chemical moieties.

Investigation on the effects of air gap distance on the formation of cellulose triacetate hollow fiber membrane for CO2 and CH4 gases permeation

CO2 removal is essential for the sweetening process of sour natural gas in order to meet the desired specifications for natural gas to be utilized and transported. CO2 separation from natural gas stream can be achieved through the application of membrane technology which will enable natural gas to be suitable for commercial use. Cellulose triacetate (CTA) is a well-known polymer material for CO2 removal due to its high affinity towards CO2. As compared to flat sheet membranes, hollow fiber configuration is preferable due to its large surface to unit volume ratio and self-supporting characteristics, therefore making it a superior choice compared to other configurations. In this work, fabrication of hollow fiber membranes by employing CTA as a polymer for the separation of CO2 and CH4 gases is focused. During fabrication, the air gap distance was varied from 1 cm to 7 cm. Following that, the morphology of the resulting fibers was investigated by using a scanning electron microscope (SEM). Next, the CO2 and CH4 gas permeation, and CO2/CH4 gas pair selectivity of the resultant HFMs were assessed using a custom-built gas permeation test rig. In addition, post-treatment of HFM with polydimethylsiloxane (PDMS) was also carried out. Then, the gas permeation and gas pair selectivity of the resultant hollow fiber membranes before and after the PDMS coating were compared. The optimum gas permeation performance of CTA HFM was obtained at an air gap distance of 3 cm and a feed pressure of 3 bar as it yielded the highest CO2 permeance of 78.77 GPU and CO2/CH4 gas pair selectivity of 2.80. Whereas for PDMS coated CTA HFM, fibers spun at air gap distance of 1 cm demonstrated the highest CO2 permeance of 14.85 GPU and CO2/CH4 selectivity of 2.54 at feed pressure of 9 bar.

Thin film nanocomposite membranes of superglassy PIM-1 and amine-functionalised 2D fillers for gas separation

Nanosheets of reduced holey graphene oxide functionalised with amines may be used to tailor the long-term gas permeation performance of a polymer of intrinsic microporosity.

Localized conversion of ZnO nanorods for fabricating Metal-Organic framework MAF-5 membranes for hydrogen separation

For the first time, a homologous metal oxides induced approach was used to fabricate MAF-5 membrane for small molecule separation systems. A continuous and dense MAF-5 membrane was well formed on a porous ceramic substrate by the localized conversion of a thinlayer of ZnO nanorods. The ZnO nanorods layer acting as an intermediate plays a multifunctional role in the formation of the high-quality MAF-5 membrane: (1) modifying the porous substrate to make it more smooth and even; (2) inducing the nucleation and growth of MAF-5 membrane; (3) connecting the MAF-5 membrane and substrate tightly to avoid the membrane peeling off. The as-prepared membrane was characterized by various techniques and proved that the membrane is continuous and high-quality. The excellent gas permeation performance for the MAF-5 membrane was achieved for single gases. The ideal selectivities were as high as 80.2, 87.1 and 105.7 for H2/CO2, H2/N2 and H2/CH4, respectively, which are far higher than their corresponding Knudsen diffusion values. It is therefore believed that the MAF-5 membrane is of potential application in the field of some small-molecular gas separation through a molecular sieving effect.

CO2/CH4 separation properties of PES mixed matrix membranes containing Fullerene-MWCNTs hybrids

In this work fullerene (Fullerene) nanoparticles, carboxylated multi-walled carbon nanotubes (MWCNTs), and fullerene- carboxylated multi-walled carbon nanotubes hybrid (Fullerene-MWCNTs), were used as the dispersed phase in polyethersulfone (PES) matrix to form mixed matrix membranes (MMMs). Low molecular weight para-nitroaniline (pNA) was used as the functionalization agent. Nanoparticles were specified using Fourier Transform Infrared (FTIR) spectroscopy and Transmission Electron Microscopy (TEM). The membranes were formed by the wet phase inversion method and their properties were determined by Differential Scanning Colorimetry (DSC) and Field Emission Scanning Electron Microscopy (FESEM). Gas permeation and separation performance of the membrane samples were also tested via a constant pressure system utilizing pure CO2 and CH4 gasses. The permeation test results showed that the PES/pNA-MWCNTS 0.5 wt% sample has the highest selectivity (42.05) among other samples. Also, this sample showed the desired amount of CO2 permeance (12.86 GPU).

High hydrogen permeable ZIF-8 membranes on double modified substrates

Heterogeneous nucleation on the substrate surface is an advantage for inducing the synthesis of a thin and continuous membrane. Here, high hydrogen permeable ZIF-8 membranes were fabricated by combining the dopamine functionalized substrate and seed-induced growth method. In addition to providing abundant functional groups, polydopamine also can chelate metal ions (Zn2+) to accelerate the nucleation on the substrate surface. Furthermore, polydopamine can anchor the seeds for promoting the heterogeneous nucleation on the substrate surfaces, and prevent the seeds from entering into the substrate porous. The characterization results by the X-ray diffraction and scanning electron microscopy showed a continuous and thin ZIF-8 layer was synthesized on the modified substrate, resulting in a low gas permeance resistance. For the single gas permeation performance, the membrane showed a marked hydrogen permeance of 1.8 × 10−6 mol·m−2·s−1·Pa−1, and the idea selectivity for H2/CO2, H2/N2, H2/CH4 and H2/C3H8 is 4.1, 8.5, 11.3 and 294, respectively. The H2 permeance of the obtained membrane is about one order of magnitude higher than that of similar substrate supported membrane. The membrane also demonstrated a comparable separation performance for mixed gas separation.

On the Order and Orientation in Liquid Crystalline Polymer Membranes for Gas Separation.

To prevent greenhouse emissions into the atmosphere, separations like CO2/CH4 and CO2/N2 from natural gas, biogas, and flue gasses are crucial. Polymer membranes gained a key role in gas separations over the past decades, but these polymers are often not organized at a molecular level, which results in a trade-off between permeability and selectivity. In this work, the effect of molecular order and orientation in liquid crystals (LCs) polymer membranes for gas permeation is demonstrated. Using the self-assembly of polymerizable LCs to prepare membranes ensures control over the supramolecular organization and alignment of the building blocks at a molecular level. Robust freestanding LC membranes were fabricated that have various, distinct morphologies (isotropic, nematic cybotactic, and smectic C) and alignment (planar and homeotropic), while using the same chemical composition. Single gas permeation data show that the permeability decreases with increasing molecular order while the ideal gas selectivity of He and CO2 over N2 increases tremendously (36-fold for He/N2 and 21-fold for CO2/N2) when going from randomly ordered to the highly ordered smectic C morphology. The calculated diffusion coefficients showed a 10-fold decrease when going from randomly ordered membranes to ordered smectic C membranes. It is proposed that with increasing molecular order, the free volume elements in the membrane become smaller, which hinders gasses with larger kinetic diameters (Ar, N2) more than gasses with smaller kinetic diameters (He, CO2), inducing selectivity. Comparison of gas sorption and permeation performances of planar and homeotropic aligned smectic C membranes shows the effect of molecular orientation by a 3-fold decrease of the diffusion coefficient of homeotropic aligned smectic C membranes resulting in a diminished gas permeation and increased ideal gas selectivities. These results strongly highlight the importance of molecular order and orientation in LC polymer membranes for gas separation.

Polyazole polymers membranes for high pressure gas separation technology

Aromatic (fluorinated and non-fluorinated) polyoxadiazole and polytriazole polymers were synthesized through one step polycondensation method in the lab followed by gas permeation and purification performance of the membranes prepared by casting their polymer solution from chloroform or NMP solvents. The gas permeability and selectivity values were found mainly depending on the bulky fluorinated groups (hexafluoroisopropylidene, HFA) on the main polymer stem and a bulky aniline derivatives (4-bromoaniline & 4-aminophenol) groups as a side chain of the polymer stem. No plasticization effect was observed for the membranes under a high range of pure CO2 feed pressure (100–800 psi). The polymers synthesis process is simple and reproducible, easy solubility in most of the common organic solvents and easily scalable for commercial production. The polymer membrane materials developed in this research work, showed a considerable high permeability for CO2 and helium gases and a very low or negligible permeability for other gases such as methane (CH4), nitrogen (N2) and ethane (C2H6) at a very high operating feed pressure (up to 800 psi). Based on the preliminary results, these polymers membranes can be considered as promising materials for gas separation technology especially for helium recovery and CO2 gas removal with a high selectivity. This work represents the testing of dense membranes under different experimental conditions of feed pressure (up to 800 PSI) at room temperature to investigate their mechanical and chemical stability for potential industrial application.

Synthesis of silicalite-1 zeolite membranes for vapor-permeation separation of dichlorobenzene Isomers

Silicalite-1 zeolitic membranes have been successfully fabricated on the porous α-Al2O3 support by templated and template-free protocol, respectively, for vapor permeation separation of dichlorobenzene (DCB) isomers. After proving the high quality of the membranes by single gas permeation (He and SF6) performance, the vapor-permeation of DCB isomers over two types of the silicalite-1 membrane was then investigated. The separation results clearly indicated that under the lower partial pressure and higher temperature, the effect of DCB isomer adsorption on the permeance could be kept at a sufficiently low level and sharp selectivity become more important. Thus, high p-DCB selectivity could be achieved. Comparatively, the template-free silicalite-1 zeolite has a much higher p-DCB selectivity due to the relatively fewer inter-crystalline gaps. Under certain separation conditions, the highest selectivity of p-DCB for p-/m-DCB and p-/o-DCB binary systems could reach 165 and 113, respectively.

Modified Zeolite/Polysulfone Mixed Matrix Membrane for Enhanced CO2/CH4 Separation.

In recent years, mixed matrix membranes (MMMs) have received worldwide attention for their potential to offer superior gas permeation and separation performance involving CO2 and CH4. However, fabricating defect-free MMMs still remains as a challenge where the incorporation of fillers into MMMs has usually led to some issues including formation of undesirable interfacial voids, which may jeopardize the gas separation performance of the MMMs. This current work investigated the incorporation of zeolite RHO and silane-modified zeolite RHO (NH2–RHO) into polysulfone (PSf) based MMMs with the primary aim of enhancing the membrane’s gas permeation and separation performance. The synthesized zeolite RHO, NH2–RHO, and fabricated membranes were characterized by X-ray diffraction (XRD) analysis, Fourier transform infrared-attenuated total reflection (FTIR-ATR), thermogravimetric analysis (TGA) and field emission scanning election microscopy (FESEM). The effects of zeolite loading in the MMMs on the CO2/CH4 separation performance were investigated. By incorporating 1 wt% of zeolite RHO into the MMMs, the CO2 permeability and ideal CO2/CH4 selectivity slightly increased by 4.2% and 2.7%, respectively, compared to that of a pristine PSf membrane. On the other hand, a significant enhancement of 45% in ideal CO2/CH4 selectivity was attained by MMMs incorporated with 2 wt% of zeolite NH2-RHO compared to a pristine PSf membrane. Besides, all MMMs incorporated with zeolite NH2-RHO displayed higher ideal CO2/CH4 selectivity than that of the MMMs incorporated with zeolite RHO. By incorporating 1–3 wt% zeolite NH2-RHO into PSf matrix, MMMs without interfacial voids were successfully fabricated. Consequently, significant enhancement in ideal CO2/CH4 selectivity was enabled by the incorporation of zeolite NH2–RHO into MMMs.

Preparation and characterization of modified halloysite nanotubes—Pebax nanocomposite membranes for CO2/CH4 separation

In this study, halloysite nanotubes (HNTs) filler particles were treated in a basic solution with a pH of 11 for their better dispersion during incorporation in Pebax-1657 matrix by 1, 2, 5, 10, 15 wt.% loadings to prepare some HNTs-Pebax nanocomposite membranes. The treated HNTs were evaluated by XRD analysis and the prepared nanocomposite membranes were characterized by DSC, TGA, and FESEM analysis. Treatment of the HNTs in basic solution increased their porosity, pore volume, and specific surface area. CO2 adsorption capacity of the raw and the modified HNTs were measured as 45.7 and 48 mol/kg adsorbent and those of CH4 were evaluated as 34.6 and 32.4 mol/kg adsorbent, respectively. Gas permeation performance of the neat Pebax-1657 and the modified HNTs-Pebax nanocomposite membranes were investigated for CO2/CH4 separation. The 10 wt.% loaded raw HNTs membrane's CO2 permeability and ideal CO2/CH4 selectivity increased to 132.1 Barrer and 18.6 from those for the neat Pebax as 98.4 Barrer and 13.6 while those of 10 wt.% loaded modified HNTs were measured as 144.4 Barrer and 27.2, respectively. The modified HNTs-Pebax nanocomposite membranes’ separation performance considerably approached the Robeson upper bound limit of 2008.