Pure Gas Permeation Research Articles

Elucidating the Molecular Mechanisms by which Porous Polymer Networks Affect Structure, Aging Propensity, and Selectivity of Microporous Glassy Polymer Membranes using a Multiscale Approach.

Microporous glassy polymer membranes suffer from physical aging, which adversely affects their performance in the short time frame. We show that the aging propensity of a model microporous polymer, poly(1-trimethylsilyl-1-propyne) (PTMSP), can be effectively mitigated by blending with as little as 5 wt % porous polymer network (PPN) composed of triptycene and isatin. The aging behavior of these materials was monitored via N2 pure gas permeability measurements over the course of 3 weeks, showing a 14% decline in PTMSP blended with 5 wt % PPN vs a 41% decline in neat PTMSP. Noteworthy, PPNs are 2 orders of magnitude cheaper than the porous aromatic frameworks previously used to control PTMSP aging. A variety of experimental and computational techniques, such as Positron Annihilation Lifetime Spectroscopy (PALS), free volume measurements, cross-polarization/magic angle spinning (CP/MAS) 13C NMR, transport measurements and molecular dynamics (MD) simulations were used to uncover the molecular mechanisms leading to enhanced aging resistance. We show that partial PTMSP chain adsorption into the PPN porosity reduces the PTMSP local segmental mobility, leading to improved aging resistance. Permeability coefficients were broken into their elementary sorption and diffusion contributions, to elucidate the mechanism by which the reduced PTMSP local segmental mobility affects selectivity in gas separation applications. Finally, we demonstrate that in these systems, where both chemical and physical interactions take place, transport coefficients must be corrected for thermodynamic nonidealities to avoid erroneous interpretation of the results.

Enhancing CO2 Transport Across the PEG/PPG-Based Crosslinked Rubbery Polymer Membranes with a Sterically Bulky Carbazole-Based ROMP Comonomer.

A series of poly(ethylene glycol)-block-poly(propylene glycol) (PEG/PPG)- and 5,6-di(9H-carbazol-9-yl)isoindoline-1,3-dione (2CZPImide)-based crosslinked rubbery polymer membranes, denoted as PEG/PPG-2CZPImide (x:y), are prepared from the norbornene-functionalized PEG/PPG oligomer (NB-PEG/PPG-NB) and 2-(bicyclo[2.2.1]hept-5-en-2-ylmethyl)-5,6-di(9H-carbazol-9-yl)isoindoline-1,3-dione (2CZPImide-NB) via ring-opening metathesis polymerization (ROMP). The molar ratio (x:y) of the NB-PEG/PPG-NB (x) to 2CZPImide-NB (y) monomers is varied from 10:1 to 6:1. X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), and pure gas permeability studies reveal that the comonomer 2CZPImide-NB successfully increases the d-spacing among the crystalline PEG/PPG segments, hence enhancing the diffusivity of gases through the membranes. The synthesized membranes exhibit good CO2 separation performance, with CO2 permeabilities ranging from 311.1 to 418.1 Barrer and CO2/N2 and CO2/CH4 selectivities of 39.4-52.0 and 13.4-16.0, respectively, approaching the 2008 Robeson upper bound. Moreover, PEG/PPG-2CZPImide (6:1), displaying optimal CO2 permeability and CO2/N2 and CO2/CH4 selectivities, shows long-term stability against physical aging and plasticization resistance up to 20 days and 10 atm, respectively.

Ionic liquid fluoropolymer membranes for the separation of R-410A: Understanding the effect of ionic liquid on membrane characteristics and separation performance

Environmental legislation such as the American Innovation and Manufacturing Act will phase out today's generation of high-global warming hydrofluorocarbon refrigerants over the next two decades. Energy-efficient technologies are needed to separate these often azeotropic and complex mixtures. Membrane technology and extractive distillation with ionic liquids as entrainers have been individually developed for the separation of azeotropic refrigerant mixtures. This study provides details for the separation of R-410A through combining the benefits of glassy, amorphous polymers with ionic liquids to form ionic liquid polymer membranes. Two different classes of ionic liquid polymer membranes are considered: polyvinyl acetate composite membranes with 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, and 1-hexyl-3-methylimidazolium chloride, and Hyflon AD 60 with 1-ethyl-3-methylimidazolium bis(pentafluoroethylsulfonyl)imide. The membranes are characterized with FTIR, helium pycnometry, differential scanning calorimetry, and a high-pressure view-cell to characterize the effect of ionic liquid on polymer properties. The Hyflon AD 60 composite membranes were evaluated for the separation of R-410A through pure-gas permeability, selectivity, solubility, and diffusivity measurements, which show exceptional selectivity for separation of R-410A. Results provide insights on the effect of ionic liquid on the separation of R-410A and indicate that improved separation performance of glassy polymers can be achieved with the addition of ionic liquid.

Molecular design of free volume structure in thermally rearranged polybenzoxazole membranes for gas separation studied by positron annihilation

AbstractIn this work, 2,2‐bis(3‐amino‐4‐hydroxyphenyl) hexafluoropropane (APAF) was used as the fixed diamine monomer, and 4,4′‐(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 4,4′‐oxydiphthalic anhydride (ODPA), 3,3′,4,4′‐biphenyltetracarboxylic diandhydride (BPDA) and pyromellitic dianhydride (PMDA) were selected as four different dianhydride monomers, respectively, to prepare the thermally rearranged polybenzoxazole (TR‐PBO) gas separation membranes with different structures via a thermal rearrangement reaction at 450°C. The microstructure of TR‐PBO membranes was investigated using X‐ray diffraction and positron annihilation spectroscopy. All the TR‐PBO membranes exhibit bimodal free volume structure, containing ultramicropores (<0.7 nm) and micropores (<2 nm) with size of 3.6–4.6 and 7.6–8.4 Å, respectively. As the steric hindrance of the bridging groups in the dianhydride monomers increases, the radius and number of pores in the membranes increases, leading to an increase in fractional free volume (FFV). The pure gas permeation test shows a positive correlation between gas permeability and FFV in the TR‐PBO membranes, indicating that an increase in FFV is favorable for the diffusion of gas molecules. Particularly, the ultramicropores play a decisive role on the gas selectivity. All the TR‐PBO membranes exhibit excellent gas permeaselectivity. Among them, the H2/CH4 separation performance of APAF‐6FDA and APAF‐ODPA membranes approaches the 2008 Robeson upper bound, which is due to the high number of both micropores and ultramicropores. Our results demonstrate that the free volume structure of polymer membranes can be designed by selecting monomers with different structures and utilizing thermal rearrangement reactions, allowing for the preparation of gas separation membranes with high permeability and selectivity.

Comparison of the effect of topology type and linker composition of zeolitic imidazolate framework fillers on the performance of mixed matrix membranes in CO2/N2 separation

Mixed matrix membranes (MMMs) combine the high separation performance of porous materials with the processibility of polymers and so possess potential for carbon capture from CO2-containing gas streams. Zeolitic imidazolate frameworks (ZIFs) are promising candidates as molecular-sieve fillers in MMMs due to their tunability and ease of synthesis. We have compared four ZIFs, all as nanoparticles of similar sizes (ca. 400 nm), as MMM fillers, to investigate the effects of ZIF structure and chemistry on MMM performance of pure gas (CO2, N2) permeation under the same conditions. The chosen ZIFs include two that exhibit strong CO2 adsorption (hybrid ZIF-7/COK-17 and ZIF-94) and two that have higher pore volumes but weaker CO2 interactions (ZIF-8 and a hybrid ZIF-11/ZIF-71). The hybrid ZIF-7/COK-17 and ZIF-94 are structurally related to ZIF-7 (rhombohedral sod topology) and ZIF-8 (cubic sod), respectively, via partial or complete substitution of benzimidazole or 2-methylimidazole by 4,5-dichloroimidazole or 4-methyl-5-imidazolecarboxaldehyde, while the hybrid ZIF-11/ZIF-71 has the rho topology but the same composition as the ZIF-7/COK-17 hybrid. In the first part of the comparative study, MMMs based on two types of commercial polymers, Matrimid®5218 and PEBAX-MH1657, were prepared containing the ZIF-7/COK-17 hybrid and also with ZIF-94. ZIF-94 shows much better compatibility with the polymers, forming homogeneous dispersions at all loadings attempted (≤35 % wt%) whereas the hybrid shows inhomogeneity above 12 wt% in each case. At 12 wt% loading, both fillers show an increase in CO2 permeability at 1.2 bar and 293 K compared to the pure membrane (in PEBAX, this increases from 49.5 to 60 and 68 Barrer) which is the result of increased solubility compensating for decreased diffusivity, and this improvement in permeability continues to increase at the higher levels of loading possible with ZIF-94. ZIF-7/COK-17 in PEBAX show higher selectivity, achieving a calculated CO2/N2 selectivity up to 70. Further investigation of CO2 and N2 permeation on MMMs with the four ZIFs at 12 wt% in PEBAX-MH1657 showed a clear distinction between the ZIF-94 and ZIF-7/COK-17 MMMs (which show higher membrane solubilities but lower diffusivities) compared to ZIF-8 and ZIF-11/ZIF-71 MMMs. At the loading chosen, the CO2 permeability increase achieved by the four ZIFs over PEBAX-MH1657 increases in the order ZIF-11/-71, ZIF-7-COK-17 (ca. 60 Barrer) < ZIF-94 (68) < ZIF-8 (81), reflecting the complex interplay between CO2 solubility (increasing with interaction strength) and diffusivity (increasing with available cage and window size). The calculated CO2/N2 selectivity is highest for the hybrid ZIF-7/COK-17 membrane (70), which is attributed to molecular sieving effects in the rhombohedral sod structure.

A semi-rigid co-poly(imide) derived from an isomeric mixture of monomers. Assessing gas transport properties in self-standing polymer membrane

Chemical structure and morphology of polymers are directly related with the membrane separation performance. Poly(imide)s (PIs) are the most widely used polymers in the preparation of membranes for gas separation applications; thus, research on the structural design of polymers is of great interest to develop new membranes. In the present work, we reported the synthesis, characterization, and measurement of the gas transport properties of a new co-poly(imide)s (PI-D2a-D2b-6FDA) prepared from a mixture of isomeric diamines. The co-poly(imide) synthetic route involved several steps, starting by a bromination reaction, followed by a double nucleophilic aromatic substitution giving an isomeric mixture of precursors, which suffered a Suzuki-Miyaura C–C cross-coupling reaction followed by the reduction of nitro groups to give two new isomeric diamines. Finally, diamines simultaneously reacted with the dianhydride 6FDA to obtain PI-D2a-D2b-6FDA. The co-poly(imide) had a Mn of 47.7 kDa and a Mw of 74.0 kDa with a PDI of 1.6. The sample exhibited a 10% weight loss at 540 °C, Tg of 280 °C, BET surface area of 110 m2 g−1, and wide-angle X-ray diffraction (WAXD) interchain d-spacing at 9.5 Å and 6.3 Å. Tensile strength, elongation at break and Young's modulus were 109.6 MPa, 6.66% and 2.18 GPa, respectively. co-Poly(imide) was soluble in various polar aprotic organic solvents such as DMSO, NMP, DMF, DMAc, THF, and chloroform, forming a self-standing dense film whose gas transport properties were measured. Pure gas permeability coefficients for H2, CO2, O2, N2, and CH4, were 47.28, 24.04, 4.35, 1.02, and 0.76 (Barrer), respectively, which follows a decreasing order by the increasing kinetic diameters of the respective gases. Ideal gas selectivities H2/N2, O2/N2, CO2/CH4, and CO2/N2 were 46.4, 4.3, 31.6, and 23.6, respectively. These gas transport properties were compared with the commercial polymer Matrimid®, showing higher gas permeability coefficients than Matrimid®.

Preparation and characterisation of heat‐resistant fluorinated co‐polyimides and their gas transport characteristics

A series of fluorinated co-polyimide films based on the 6FDA-FTPPA/BAFL backbone were prepared from 4-(3-fluoro-4-(trifluoromethyl)phenyl)-2,6-bis(4-aminophenyl)pyridine (FTPPA),commercial 9,9-bis(4-aminophenyl)fluorene (BAFL), and 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), with varying ratios of FTPPA:BAFL units (3:1, 2:1, 1:1, 1:2, and 1:3). Co-polyimides not only exhibit high thermal stability ( Tg &gt; 357°C), good optical properties (λcutoff &gt; 327 nm) and hydrophobicity (contact angle &gt;91.4°), but also favorable solubility properties in both high and low boiling organic solvents. The fractional free volume of the polymers was simulated using molecular mechanics and molecular dynamics and correlated with experimental gas separation data. The presence of bulky groups led to high FFVs (&gt;17.8%) and the formation of large microcavities with diameters ranging from 5.48 to 5.72 Å, which efficiently increased gas permeability. In pure gas permeation experiments, the permeability coefficients of CO2 and He for the 6FDA-FTPPA/BAFL (3:1) film were 41.48 and 79.38 bar, respectively, which were superior to those of commercial Matrimid and Kapton in terms of gas permeability. In especial, the 6FDA-FTPPA/BAFL (1:2) and 6FDA-FTPPA/BAFL (1:3) in co-polyimide membranes exhibited the most attractive separation performance for O2/N2 gas pairs, approaching the 1991 Roberson upper limit.

Impact of Concentration Polarization Phenomena on Gas Separation Processes with High-Performance Zeolite Membranes: Experiments vs. Simulations.

Polarization phenomena play a key role in membrane separation processes but remain largely unexplored for gas separations, where the mass transfer resistance is most often limited to the membrane. This assumption, which is commonly used today for the simulation of membrane gas separations, has to be reconsidered when high-performance materials, showing a very high permeance and/or selectivity, are used. In this study, a series of steady-state separation performances experimentally obtained on CO2/CH4 mixtures with a zeolite membrane are compared to the predictions of a dedicated 1D approach, recently derived and validated through CFD simulations. Polarization effects are shown to generate a significant negative impact on the separation performances, both in terms of the productivity and separation efficiency. The 1D model predictions, based on pure gas permeance data and without any adjustable parameters, are in very good agreement with the experimental data. This fast and efficient modeling approach can easily be implemented in simulation or process synthesis programs for the rigorous evaluation of membrane gas separation processes, when high-performance materials are used.

Tailoring the CH4/CO2/N2 separation performance of ultrapermeable polymeric composite membranes by altering the concentration of Pd/g-C3N4

Effectively tailoring the interfacial interaction between the nanoadditives and polymer matrix to develop defect-free membrane is the key parameter to fabricate mixed matrix membranes (MMMs) with improved gas separation performance. Here, MMMs were successfully fabricated by using Pd/g-C3N4 as a nanoadditive for the separation of CO2/CH4 and CO2/N2 gas mixtures. The g-C3N4 with palladium provides more compatible structure to the MMMs. The addition of Pd/g-C3N4 nanoadditive introduces additional functional groups into the membrane matrix which induces the permeability of the molecules. The Pd/g-C3N4:PSf MMMs exhibited better thermal, mechanical stability and gas permeability than the bare polysulfone (PSf) membrane. The MMM loaded with 300 mg of the nanoadditive exhibited greater permeability than unfilled membrane which is nearly 5 times for CH4, 4 times for N2 and 10 times for CO2 at 2 bar pressure with selectivity of 5.25 for CO2/CH4 and 2.15 for CO2/N2 mixtures. The permeability of the CO2 molecules is higher than the N2 and CH4 molecules because of its smaller kinetic diameter. The Pd/g-C3N4 nanoadditive exhibited microporous nature with presence of nitrogen-rich functional groups which develops CO2-philic functional groups in MMMs. These findings emphasize the unique properties of Pd/g-C3N4 nanoadditive in MMMs towards pure and mixed gas permeability and separation. This simplistic approach to modulate PSf membrane via Pd/g-C3N4 addition promotes the practical design of highly effective gas separation membranes.

Fabrication and Characterization of Novel Pebax2533/POSS-FS Nanocomposite Membranes for CO2 Removal

In this work, novel mixed matrix membranes (MMMs) were fabricated by incorporation of octatrimethylsiloxy polyhedral oligomeric silsesquioxane (POSS) and fumed silica (FS) as nanoparticle fillers into poly(ether-block-amides) (Pebax2533) as polymer matrix in order to carbon capture and remove acid gas from natural gas. Fourier transforms infrared (FTIR) spectroscopy proved the chemical structure of the nanocomposites and field-emission scanning electron microscopy (FESEM) unveiled the appropriate filler dispersion and adhesion between fillers and polymer. Thermogravimetric analysis (TGA) showed that all the samples have reasonable thermal stability which is enhanced by nanoparticles. The permeability and selectivity of pure CO2, CH4, and N2 gases were measured in the 4-10bar pressure difference range. The incorporation of studied nanoparticles caused improve performance including the permeability and selectivity relative to the neat Pebax. In 4bar and 0.1wt% POSS, CO2 permeability raised by 87%, while the highest increase in CO2/CH4 selectivity is at 2wt% POSS. The gas transport and free volume were also interpreted by densitometry showing that the addition of the fillers had a positive effect on the free volume and chain packing of the polymeric membranes.

Investigation of the Gas Permeation Properties Using the Volumetric Analysis Technique for Polyethylene Materials Enriched with Pure Gases under High Pressure: H2, He, N2, O2 and Ar.

Polyethylene (PE) is widely used as a gas-sealing material in packing films and gas transport pipes. A technique for evaluating the permeability of water-insoluble gases has recently been developed. This technique is a volumetric analysis that is used to calculate the gas permeability by measuring the gas uptake and diffusivity. With this technique, we investigated the permeability of pure gases, such as H2, He, N2, O2 and Ar, enriched under high pressure up to 9 MPa in low-density polyethylene (LDPE), ultrahigh molecular weight polyethylene (UHMWPE) and high-density polyethylene (HDPE). The gas uptake showed a linear pressure-dependent behavior that followed Henry's law, and the diffusivity was independent of the pressure. Furthermore, the logarithmic diffusivity values of the five gases linearly decreased as their molecular kinetic diameters increased. The logarithmic solubility values linearly increased as the critical temperatures of the gases increased. The calculated permeability results were correlated with the volume fraction of the amorphous phase and the fractional free volume. This result newly showed that the amorphous phase was directly correlated to the fractional free volume.

Enhancing the performance of a modified poly(ether-b-amide) blend membrane by PAMAM dendritic polymer for separation of CO2/CH4

Novel dense membranes were fabricated by blending poly(ether-b-amid), Pebax 1657, as the matrix and a poly(amidoamine) dendritic polymer (PAMAM), as the modifier, to evaluate CO2/CH4 gases separation. The study investigated the effects of PAMAM loading, operating pressure, and temperature on gas permeability and CO2/CH4 pair-gas ideal selectivity. All the membranes were characterized using various analytical techniques, including ATR-FTIR, XRD, SEM, FESEM, DSC, AFM, Tensile Analysis, density, Aging, Stability pure gas permeation, and gas solubility tests. FTIR results confirmed the interaction of PEO and PA segments with PAMAM increased polymer crystallinity, which is in agreement with DSC, XRD, and Tensile Analysis results. The membrane containing 3 wt% PAMAM showed the highest FFV which along with facilitated transport of CO2 by PAMAM, resulted in the highest CO2 permeability equal to 56.9 Barrer at 2 bar and 30 °C (more than 30% higher than the neat Pebax membrane at the same condition). Furthermore, the addition of 1 wt% PAMAM to Pebax increased the CO2/CH4 selectivity to 22.8 at 8 bar and 30 °C (more than 78% higher than the neat Pebax membrane at the same condition). Increasing PAMAM loading and applied pressure resulted in the enhancement of gases solubility, and a better ideal selectivity was obtained, correspondingly, Also higher solubility of CO2 compared with CH4 was confirmed by solubility measurements. It was revealed that gas permeability in the membranes was primarily governed by gas solubility rather than diffusivity. The results demonstrate that the modifying Pebax membrane by PAMAM enhances the separation performance with a negligible change against aging and good stability, which confirms potential applications of Pebax/PAMAM membranes in gas separation processes.

Development of high performance carbon molecular sieve membranes via tuning the side groups on PI precursors

In the current study, three 6FDA1/BPDA1-based polyimides (PIs) containing different diamine moieties (DMB, TCDB, TFDB) were selected to prepare CMS membranes (CMSMs), providing a systematic study of PIs with different side group functionalization. The resultant CMSMs were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectra, and CO2 absorption measurements. Pure gas (N2, O2, CH4, CO2) and mixed gas (CO2/CH4) permeability tests were also conducted. The resultant CMSMs exhibited attractive gas separation performance. Compared to DMB-based PI, TFDB-based with trifluoromethyl (-CF3) side groups exhibited ultra-high permeability (PCO2 = 14938 Barrer, PO2 = 3146 Barrer) and moderate selectivity (αCO2/CH4 = 20.3, αO2/N2 = 4.5), while CMSMs prepared from PI with chlorine functionalization showed higher gas permeability (PCO2 = 6031 Barrer, PO2 = 1311 Barrer) and selectivity (αCO2/CH4 = 46.2, αO2/N2 = 6.5). The results indicated that side group functionalization on PIs exerts a considerable influence on the gas permeation properties of CMSMs.

The Effect of Concentration on PES/NMP System on Flat Sheet Membrane Fabrication and its Performance for CO2 Gas Separation

Carbon dioxide (CO2) capture utilising membrane technology have become the interest of research due to its low carbon footprint, feasible fabrication process and scalability in its operation. In this study, anisotropic polyethersulfone (PES) membrane was fabricated at various concentration ranging from 20 wt% to 35 wt% without the use of any additives. This study revealed that the finger-like structure disappeared with increased polymer dope concentration which was associated with increased viscosity of the dope solution. Moreover, the surface porosity of the membrane also virtually reduced with increased PES concentration as observed with the SEM images. The pure gas permeation test was also consistent with the observed morphology of the membrane. Membrane made with 20 wt% of PES dope solution exhibits the highest gas permeance which was 154.9 GPU at 2 bar while the CO2/nitrogen (N2) and CO2/methane (CH4) ideal selectivity was close to that of Knudsen’s selectivity value. With increased PES concentration, the CO2 gas permeance reduced drastically accompanied by enhancement on the CO2/N2 and CO2/ CH4 ideal selectivity. The critical concentration of PES dope solution obtained by plotting the PES dope concentration against viscosity was 29.4 wt%. With critical concentration of the dope solution, the CO2 permeance was recorded to be 8.1 GPU while the CO2/N2 and CO2/CH4 ideal selectivity were recorded to be 2.13 and 1.48, respectively at the pressure of 2 bar.

Gas separation properties of PIM-1 films treated by elemental fluorine in liquid perfluorodecalin

Gas transport properties of PIM-1 films treated by elemental fluorine in a liquid perfluorodecalin were investigated. The investigation included an estimation of fluorine content in a fluorinated layer, pure and mixed gas permeability, diffusion and solubility coefficients measurement, ideal selectivity assessment and analysis of fluorination time effect on gas transport properties. An increase of treatment duration resulted in an increase of concentration of fluorine in the fluorinated layer as well as a depth of fluorination. The depths of fluorination linearly increased with square of fluorination time indicating a diffusion of fluorine in the fluorinated layer to be a restrictive stage of kinetics of fluorination. The effect of the drop in gas permeability was demonstrated to be dependent on the kinetic size of the penetrant. The comparison of liquid-phase and gas-phase regimes was performed, and it was shown that sharp decrease in gas permeability is observed for gas-phase regime at short times of fluorination already, whereas this decrease is more gradual for liquid-phase fluorination. According to a Barrie method, the gas transport parameters of the fluorinated layer of the modified PIM-1 films were calculated, which turned out to be much lower than those for the virgin PIM-1 polymer and slightly lower than the gas transport parameters for dry Nafion. The reduction of the gas permeability coefficients is mostly induced by a sharp decrease in the gas diffusion coefficients. According to combination of permeability and selectivity, the fluorinated PIM-1 samples show excellent results exceeding the 2008 Robeson upper bounds for H2/CH4, H2/CO2, H2/N2, He/CH4 and others gas pairs. Mixed gas experiments were also conducted for hydrogen-containing mixtures and fluorinated PIM-1 samples have demonstrated good separation characteristics for the model binary mixtures attaining separation factors as high as 115–143 (H2/CH4), 4.4–6.7 (H2/CO2) and 49–146 (He/CH4), which makes these materials promising for helium recovery from natural gas and hydrogen recovery from various catalytic and biohydrogen production mixtures. Ten-months aging of the PIM-1 sample fluorinated during 30 min showed a decline of permeability by 25 and 40%, respectively and a slight decrease of ideal selectivity.

Performance optimization of polyetherimide‐zeolite 4A mixed matrix membranes for carbon dioxide/methane separation process using response surface methodology

AbstractThe performance optimization studies of zeolite 4A embedded polyetherimide mixed matrix membranes to separate carbon dioxide/methane by simultaneously considering the effect of process parameters on process responses were the focus of this study. Mixed matrix membranes were characterized and analyzed. The thermophysical characteristics of the synthesized membranes were assessed by different analytical equipment. The permeability of pure gases was determined at varying feed pressures (4 bar to 10 bar) to evaluate gas separation performance. Process optimization studies were accomplished by response surface methodology to find the relation of pressure and zeolite loading on carbon dioxide and methane permeability, and carbon dioxide/methane selectivity. The characterization results revealed that all membranes were dense in structure and has improved thermal stability. The spectrometry results confirmed the molecular interaction between polyetherimide and zeolite 4A filler. Gas permeability results showed a more than 90 % increase in carbon dioxide permeability compared to the nascent polyetherimide membrane. Similarly, selectivity of mixed matrix membranes was 45 % higher than polyetherimide membrane. The optimal operating conditions were found to be 20 wt. % zeolite loading and 6 bar pressure with overall desirability of 0.700. These membranes can find potential in various gas separation applications.

Design, synthesis and characterization of novel fluorinated polyimide membranes with bulky pendant for gas transport

AbstractThree novel fluorinated polyimides (PIs) were synthesized via one‐step solution polycondensation from a new diamine monomer, 4‐(3‐methoxy‐4‐(difluoromethoxy)phenyl)‐2,6‐bis(4‐aminophenyl)pyridine, and three commercially available aromatic dianhydrides. The solubility, thermal, optical, hydrophobic and gas transport properties of the obtained PIs were investigated. The results demonstrated that these polymers present good solubility in dimethylformamide, N‐methylpyrrolidone, dimethylsulfoxide, dimethylacrylamide, CHCl3, tetrahydrofuran and CH2Cl2 at room temperature or upon heating. Meanwhile, they displayed excellent thermal properties with glass transition temperature of 261–345 °C, weight loss temperatures at 10% of 518–523 °C as well as char yield of 60–62% at 800 °C in nitrogen atmosphere. Furthermore, the PI membranes have good hydrophobic properties with water contact angle value in the range 90.1–92.0°. Moreover, they show excellent optical properties, with cut‐off wavelength (λcut‐off) values of 368–381 nm and wavelength values at 80% transmittance (λ80%) of 421–462 nm. The pure gas permeabilities of He, CO2, O2 and N2 of the PI membranes were measured by the method of constant volume–variable pressure. The PI containing 4,4′‐(hexafluoroisopropyl)diphthalic anhydride moiety exhibits the largest molecular chain spacing and free volume, giving the best gas transport performance in this work. Molecular simulation results show that the introduction of fluorine‐containing structure and bulky pendant into polymer backbones is beneficial for gas transport. © 2023 Society of Industrial Chemistry.

Fabrication, Facilitating Gas Permeability, and Molecular Simulations of Porous Hypercrosslinked Polymers Embedding 6FDA-Based Polyimide Mixed-Matrix Membranes.

Novel polymers applied in economic membrane technologies are a perennial hot topic in the fields of natural gas purification and O2 enrichment. Herein, novel hypercrosslinked polymers (HCPs) incorporating 6FDA-based polyimide (PI) MMMs were prepared via a casting method for enhancing transport of different gases (CO2, CH4, O2, and N2). Intact HCPs/PI MMMs could be obtained due to good compatibility between the HCPs and PI. Pure gas permeation experiments showed that compared with pure PI film, the addition of HCPs effectively promotes gas transport, increases gas permeability, and maintains ideal selectivity. The permeabilities of HCPs/PI MMMs toward CO2 and O2 were as high as 105.85 Barrer and 24.03 Barrer, respectively, and the ideal selectivities of CO2/CH4 and O2/N2 were 15.67 and 3.00, respectively. Molecular simulations further verified that adding HCPs was beneficial to gas transport. Thus, HCPs have potential utility in fabrication of MMMs for facilitating gas transport in the fields of natural gas purification and O2 enrichment.

Solubility, Diffusivity, and Permeability of HFC-32 and HFC-125 in Amorphous Copolymers of Perfluoro(butenyl vinyl ether) and Perfluoro(2,2-dimethyl-1,3-dioxole)

R-410A, an azeotropic mixture composed of 50 wt % difluoromethane (HFC-32, CH2F2) and 50 wt % pentafluoroethane (HFC-125, CHF2CF3) used in residential and commercial air-conditioning applications, will be phased down due to the high global warming potential (GWP) of HFC-125. The HFC-32 can be reused in low-GWP blends containing hydrofluoroolefins (HFOs); however, incumbent separation technology, fractional distillation, cannot separate azeotropic mixtures. Membrane technology provides the opportunity to achieve a selective separation of azeotropic HFC refrigerant mixtures with lower energy consumption and capital requirements. This study explores the use of amorphous perfluoropolymers for the separation of R-410A. The permeability, solubility, and diffusivity of HFC-32 and HFC-125 were measured in copolymers of perfluoro(butenyl vinyl ether) (PBVE) and perfluoro(2,2-dimethyl-1,3-dioxole) (PDD). Pure gas permeability of HFC-32 and HFC-125 were measured using a static membrane apparatus and the pressure-rise method. Solubility measurements were obtained using a gravimetric microbalance, and diffusivity was calculated using a Fickian model. The results indicate that a high permeability and selectivity of HFC-32/HFC-125 can be obtained with a 50 wt % PBVE and 50 wt % PDD copolymer and that the separation is diffusion-driven over the entire range of compositions tested.

Mixed matrix membranes comprising 6FDA-based polyimide blends and UiO-66 with co-continuous structures for gas separations

Membrane gas separation is considered a highly promising alternative to conventional gas separation. Recently, mixed matrix membranes containing UiO-66 metal–organic frameworks (MOFs) have been demonstrated to achieve high gas separation performance, especially in CO2-related separation. To date, few studies have used morphological control strategies to enhance the gas transport properties of mixed matrix membranes. In this study, we fabricated mixed matrix membranes comprising porous UiO-66 nanoparticles and immiscible 6FDA-based polyimide blends with co-continuous structures. The selective location of UiO-66 nanoparticles within one polymer phase of the co-continuous polymer blend was observed. However, this co-continuous structure cannot be referred to as a double-percolative morphology since the percolative UiO-66 networks did not successfully form. The gas permeability and gas sorption properties of pure gases (H2, CO2, N2, O2, and CH4) in the dense membranes and mixed matrix membranes were measured using the constant-volume variable-pressure method and the dual-volume pressure decay method, respectively. On the basis of the solution–diffusion mechanism, the contributions of gas sorption and gas transport to the overall gas permeation properties were determined and examined. When the UiO-66 content reached 35 wt%, the CO2 permeability of the mixed matrix membranes was significantly increased from 16.5 Barrer to 104.7 Barrer (by 635 %) without scarifying the permeability selectivities of CO2/N2 (16.5 ± 1.5) and CO2/CH4 (31.7 ± 5.8). The results suggest that both the addition of porous UiO-66 nanoparticles and the co-continuous structures contributed to the strongly enhanced gas separation performance of the mixed matrix membranes.