Composite hollow fiber membranes for CO2 capture

Composite hollow fibers membranes were prepared by coating poly(phenylene oxide) (PPO) and polysulfone (PSf) hollow fibers with high molecular polyvinylamine (PVAm). Two procedures of coating hollow fibers outside and respective inside were investigated with respect to intrinsic PVAm solution properties and hollow fibers geometry and material.The influence of operating mode (sweep or vacuum) on the performances of membranes was investigated. Vacuum operating mode gave better results than using sweep because part of the sweep gas permeated into feed and induced an extra resistance to the most permeable gas the CO₂. The composite PVAm/PSf HF membranes having a 0.7–1.5 μm PVAm selective layer, showed CO₂/N₂ selectivity between 100 and 230. The selectivity was attributed to the CO₂ facilitated transport imposed by PVAm selective layer. The CO₂ permeance changed from 0.006 to 0.022 m³(STP)/(m² bar h) in direct correlation with CO₂ permeance and separation mechanism of the individual porous supports used for membrane fabrication. The multilayer PVAm/PPO membrane using as support PPO hollow fibers with a 40 nm PPO dense skin layer, surprisingly presented an increase in selectivity with the increase in CO₂ partial pressure. This trend was opposite to the facilitated transport characteristic behaviour of PVAm/porous PSf. This indicated that PVAm/PPO membrane represents a new membrane, with new properties and a hybrid mechanism, extremely stable at high pressure ratios. The CO₂/N₂ selectivity ranged between 20 and 500 and the CO₂ permeance from 0.11 to 2.3 m³(STP)/(m² bar h) depending on the operating conditions.For both PVAm/PSf and PVAm/PPO membranes, the CO₂ permeance was similar with the CO₂ permeance of uncoated hollow fiber supports, confirming that the CO₂ diffusion rate limiting step resides in the properties of the relatively thick support, not at the level of 1.2 μm thin and water swollen PVAm selective layer. A dynamic transfer of the CO₂ diffusion rate limiting step between PVAm top layer and PPO support was observed by changing the feed relative humidity (RH%). The CO₂ diffusion rate was controlled by the PPO support when using humid feed. At low feed humidity the 1.2 μm PVAm top layer becomes the CO₂ diffusion rate limiting step.     

Graphene Oxide Membranes with Heterogeneous Nanodomains for Efficient CO2 Separations

Achieving high membrane performance in terms of gas permeance and carbon dioxide selectivity is an important target in carbon capture. Aiming to manipulate the channel affinity towards CO₂ to implement efficient separations, gas separation membranes containing CO₂‐philic and non‐CO₂‐philic nanodomains in the interlayer channels of graphene oxide (GO) were formed by intercalating poly(ethylene glycol) diamines (PEGDA). PEGDA reacts with epoxy groups on the GO surface, constructing CO₂‐philic nanodomains and rendering a high sorption capacity, whereas unreacted GO surfaces give non‐CO₂‐philic nanodomains, rendering low‐friction diffusion. Owing to the orderly stacking of nanochannels through cross‐linking and the heterogeneous nanodomains with moderate CO₂ affinity, a GO‐PEGDA500 membrane exhibits a high CO₂ permeance of 175.5 GPU and a CO₂/CH₄ selectivity of 69.5, which is the highest performance reported for dry‐state GO‐stacking membranes.     

Carbon Molecular Sieve Membrane Preparation by Economical Coating and Pyrolysis of Porous Polymer Hollow Fibers

Dip coating and pyrolysis processes are used to create multi‐layer asymmetric carbon molecular sieve (CMS) hollow fiber membranes with excellent gas separation properties. Coating of an economical engineered support with a high‐performance polyimide to create precursor fibers with a dense skin layer reduces material cost by 25‐fold compared to monolithic precursors or ceramic supports. CMS permeation results with CO₂/CH₄ (50:50) mixed gas feed show attractive CO₂/CH₄ selectivity of 58.8 and CO₂ permeance of 310 GPU at 35 °C.     

Using PDMS coated TFC-RO membranes for CO2/N2 gas separation: Experimental study,modeling and optimization

Thin film composite (TFC) reverse osmosis (RO) membranes are semipermeable membranes that are utilized in water purification or water desalination systems. Discarding these membranes after end-of-life leads to environmental problems. Reusing old TFC-RO membranes is one way to solve this problem. For this reason, in this study, used TFC-RO membranes were coated with polydimethylsiloxane (PDMS) for CO₂/N₂ gas separation application. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was utilized to confirm the crosslinking of coated PDMS. The morphology of PDMS/TFC-RO membranes was characterized using scanning electron microscopy (SEM). The parameters that can affect performance of prepared membranes (N₂ permeance and CO₂/N₂ selectivity) are concentration of PDMS solution, coating time, solvent evaporation time and curing temperature and time. Given that the used membranes don't have uniform surfaces, the first step of this study was to investigate the effect of the above mentioned factors on virgin membranes using fractional factorial design (FFD) of experiments. The results obtained showed that PDMS concentration is the most significant factor that has a negative effect on N₂ permeance and positive effect on CO₂/N₂ selectivity. The reported CO₂/N₂ selectivity of PDMS membranes was 11–12, but this selectivity for prepared PDMS/TFC-RO membranes was in the range of 6.7–22.5. After determining optimum conditions, the gas separation performance of PDMS coated used TFC-RO membrane under these conditions was finally determined. The results showed that the used membranes had a better performance than virgin membranes.     

Facilitated transport of CO2 through polyethylenimine/poly(vinyl alcohol) blend membrane

Polyethylenimine (PEI)/poly(vinyl alcohol) (PVA) blend membranes were prepared for the facilitated transport of CO₂. The polymeric carrier PEI was retained in the blend membrane by the entanglement with PVA chains. The CO₂ permeance decreased with an increase in CO₂ partial pressure in feed gas, whereas the N₂ permeance was nearly constant. This result clearly showed that only CO₂ was transported by the facilitated transport mechanism. The CO₂ and N₂ permeabilities increased monotonically with the PEI weight percent in the blend membrane, whereas the selectivity of CO₂ over N₂ showed a maximum. The selectivity increased remarkably with increasing heat-treatment temperature of the membrane. The highest selectivity obtained reached more than 230 when the CO₂ partial pressure was 0.065 atm. The prepared membrane was stable.     

Preparation of CO_2 selective composite membranes using Pebax/CuBTC/PEG-ran-PPG ternary system

Three phase Pebax~? MH 1657/PEG-ran-PPG/CuBTC(polymer/liquid/solid) was successfully deposited as a selective layer on a porous Polysulfone(PSF) support. In fact, the beneficial properties of PEG(high selectivity) with those of PPG(high permeability, amorphous) have been combined with superior properties of mixed matrix membrane(MMMs). The membranes were characterized by DSC, TGA and SEM, while CuBTC was characterized by CO_2 and CH_4adsorption test. Statistically based experimental design(central composite design, CCD) was applied to analyze and optimize the effect of PEG-ran-PPG(10–50 wt%) and CuBTC(0–20 wt%) mass contents on the CO_2 permeance and CO_2/CH_4 ideal selectivity. Based on the regression coefficients of the obtained models, the CO_2 permeance was notably influenced by PEG-ran-PPG,while CuBTC has the most significant effect on the CO_2/CH_4 ideal selectivity. Under the optimum conditions(PEG-ran-PPG: 32.76 wt% and CuBTC: 20 wt%), nearly 620% increase in the CO_2 permeance and43% enhancement in the CO_2/CH_4 ideal selectivity was observed compared to the neat Pebax. The effect of pressure(3, 9 and 15 bar) on the pure and mixed gas separation performance of the composite membranes was also investigated. The high solubility of CO_2 in the membranes resulted in the enhancement of CO_2 permeability with increase in gas pressure.     

Temperature and pressure effects on CO2 and CH4 permeation through MFI zeolite membranes

Single gas and mixture permeances of CO₂ and CH₄ were measured as functions of pressure and temperature through three MFI zeolite membranes that have different fractions of their permeances through non-zeolite pores. The effect of pressure on CO₂ permeance, which was different for each membrane, was fit by a modified surface diffusion model. The differences in the pressure behavior of the membranes are attributed to pores with viscous and Knudsen flow. Membranes with the largest permeation through non-zeolite pores have the lowest CO₂/CH₄ mixture selectivity. The highest CO₂/CH₄ mixture selectivity is 5.5 at room temperature and decreases with temperature because of a decrease in competitive adsorption. Although increasing pressure at constant pressure drop increases the apparent CO₂/CH₄ selectivity, the ratio of the CO₂ and CH₄ fluxes decreases.     

Pebax‐Based Membrane Filled with Two‐Dimensional Mxene Nanosheets for Efficient CO2 Capture

Mixed matrix membranes (MMMs) made from inorganic fillers and polymers is a kind of promising candidate for gas separation. In this work, two‐dimensional MXene nanosheets were synthesized and incorporated into a polyether‐polyamide block copolymer (Pebax) matrix to fabricate MMM for CO₂ capture. The physicochemical properties of MXene nanosheets and MXene/Pebax membranes were studied systematically. The introduction of MXene nanosheets provided additional molecular transport channels and meanwhile enhanced the CO₂ adsorption capacity, thereby enhancing both the CO₂ peremance and CO₂/N₂ selectivity of Pebax membrane. The optimized MXene/Pebax membrane with a MXene loading of 0.15 wt % displayed a high separation performance with a CO₂ permeance of 21.6 GPU and a CO₂/N₂ selectivity of 72.5, showing potential application in CO₂ capture.     

MIL-101(Cr) Microporous Nanocrystals Intercalating Graphene Oxide Membrane for Efficient Hydrogen Purification

Graphene oxide (GO) is a promising two-dimensional building block for fabricating high-performance gas separation membranes. Whereas the tortuous transport pathway may increase the transport distance and lead to a low gas permeation rate, introducing spacers into GO laminates is an effective strategy to enlarge the interlayer channel for enhanced gas permeance. Herein, we propose to intercalate CO₂-philic MIL-101(Cr) metal-organic framework nanocrystals into the GO laminates to construct a 2D/3D hybrid structure for gas separation. The interlayer channels were partially opened up to accelerate gas permeation. Meanwhile, the intrinsic pores of MIL-101 provided additional transport pathways, and the affinity of MIL-101 to CO₂ molecules resulted in higher H₂/CO₂ diffusion selectivity, leading to a simultaneous enhancement in gas permeance and separation selectivity. The MIL-101(Cr)/GO membrane with optimal structures exhibited outstanding and stable mixed-gas separation performance with H₂ permeance of 67.5 GPU and H₂/CO₂ selectivity of 30.3 during the 120-h continuous test, demonstrating its potential in H₂ purification application.     

Continuous Porous Aromatic Framework Membranes with Modifiable Sites for Optimized Gas Separation

Continuous microporous membranes are widely studied for gas separation, due to their low energy premium and strong molecular specificity. Porous aromatic frameworks (PAFs) with their exceptional stability and structural flexibility are suited to a wide range of separations. Main-stream PAF-based membranes are usually prepared with polymeric matrices, but their discrete entities and boundary defects weaken their selectivity and permeability. The synthesis of continuous PAF membranes is still a major challenge because PAFs are insoluble. Herein, we successfully synthesized a continuous PAF membrane for gas separation. Both pore size and chemistry of the PAF membrane were modified by ion-exchange, resulting in good selectivity and permeance for the gas mixtures H₂/N₂ and CO₂/N₂. The membrane with Br^? as a counter ion in the framework exhibited a H₂/N₂ selectivity of 72.7 with a H₂ permeance of 51844 gas permeation units (GPU). When the counter ions were replaced by BF₄^?, the membrane showed a CO₂ permeance of 23058 GPU, and an optimized CO₂/N₂ selectivity of 60.0. Our results show that continuous PAF membranes with modifiable pores are promising for various gas separation situations.     

Pebax/TSIL blend thin film composite membranes for CO_2 separation

In this study a thin film composite (TFC) membrane with a Pebax/Task-specific ionic liquid (TSIL) blend selective layer was prepared. Defect-free Pebax/TSIL layers were coated successfully on a polysulfone ultrafiltration porous support with a polydimethylsiloxane (PDMS) gutter layer. Different parameters in the membrane preparation (e.g. concentration, coating time) were investigated and optimized. The morphology of the membranes was studied by scanning electron microscopy (SEM), while the thermal properties and chemical structures of the membrane materials were investigated by thermo-gravimetric analyzer (TGA), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). The CO₂ separation performance of the membrane was evaluated using a mixed gas permeation test. Experimental results show that the incorporation of TSIL into the Pebax matrix can significantly increase both CO₂ permeance and CO₂/N₂ selectivity. With the presence of water vapor, the membrane exhibits the best CO₂/N₂ selectivity at a relative humidity of around 75%, where a CO₂ permeance of around 500 GPU and a CO₂/N₂ selectivity of 46 were documented. A further increase in the relative humidity resulted in higher CO₂ permeance but decreased CO₂/N₂ selectivity. Experiments also show that CO₂ permeance decreases with a CO₂ partial pressure increase, which is considered a characteristic in facilitated transport membranes.     

Superior gas separation performance of dual-layer hollow fiber membranes with an ultrathin dense-selective layer

A concept demonstration has been made to simultaneously enhance both O₂ and CO₂ gas permeance and O₂/N₂ and CO₂/CH₄ selectivity via intelligently decoupling the effects of elongational and shear rates on dense-selective layer and optimizing spinning conditions in dual-layer hollow fiber fabrication. The dual-layer polyethersulfone hollow fiber membranes developed in this work exhibit an O₂/N₂ selectivity of 6.96 and an O₂ permeance of 4.79 GPU which corresponds to an ultrathin dense-selective layer of 918 Å at room temperature. These hollow fibers also show an impressive CO₂/CH₄ selectivity of 49.8 in the mixed gas system considering the intrinsic value of only 32 for polyethersulfone dense films. To our best knowledge, this is the first time to achieve such a high CO₂/CH₄ selectivity without incorporating any material modification. The above gas separation performance demonstrates that the optimization of dual-layer spinning conditions with balanced elongational and shear rates is an effective approach to produce superior hollow fiber membranes for oxygen enrichment and natural gas separation.     

High pressure gas permeability of microporous carbon membranes

Microporous carbon membranes are prepared, characterised structurally and tested in terms of high pressure CO₂ permeability at temperatures around the critical. A maximum in the permeance versus relative pressure curve is observed in close analogy to the case of mesoporous membranes. This weakens considerably as the temperature is increased above the critical. The results offer significant input for an improved understanding and theoretical modelling of the process and may be potentially useful for the identification of the optimal pressure and temperature conditions for efficient gas separations.     

Appealing sheath-core spun high-performance composite carbon molecular sieve membranes

Carbon molecular sieve (CMS) membranes are attractive candidates to meet requirements for challenging gas separations. The added ability to maintain such intrinsic properties in an asymmetric morphology with a structure that we term a “Pseudo Wheel+Hub & Spoke” asymmetric form offers new opportunities. For CMS membrane, specifically, the structure provides both selective layer support and low flow resistance even for high feed pressures and fluxes in CO₂ removal from natural gas. This capability is unavailable to even rigid glassy polymers due to the much higher modulus of CMS materials. Combining precursor asymmetric hollow fiber formation and optimized pyrolysis creates a defect free CMS proof-of-concept membrane for this application. Facile formation of the sheath-core spun precursor with a 6FDA-DAM sheath and Matrimid® core also avoids the need to seal defects before or after the carbonization of the precursors. The composite CMS membrane shows CO₂/CH₄ (50 : 50) mixed gas feed with an attractive CO₂/CH₄ selectivity of 64.3 and CO₂ permeance of 232 GPU at 35 °C. A key additional benefit of the approach is reduction in use of the more costly high performance 6FDA-DAM in a composite sheath-core CMS membrane with the “Pseudo Wheel+Hub & Spoke” structure.     

Cross‐Linking between Sodalite Nanoparticles and Graphene Oxide in Composite Membranes to Trigger High Gas Permeance,Selectivity, and Stability in Hydrogen Separation

Thin membranes (900 nm) were prepared by direct transformation of infiltrated amorphous precursor nanoparticles, impregnated in a graphene oxide (GO) matrix, into hydroxy sodalite (SOD) nanocrystals. The amorphous precursor particles rich in silanols (Si?OH) enhanced the interactions with the GO, thus leading to the formation of highly adhesive and stable SOD/GO membranes via strong bonding. The cross‐linking of SOD nanoparticles with the GO in the membranes promoted both the high gas permeance and enhanced selectivity towards H₂ from a mixture containing CO₂ and H₂O. The SOD/GO membranes are moisture resistance and exhibit steady separation performance (H₂ permeance of about 4900 GPU and H₂/CO₂ selectivity of 56, with no degradation in performance during the test of 50 h) at high temperature (200 °C) under water vapor (4 mol %).     

Cross-Linking between Sodalite Nanoparticles and Graphene Oxide in Composite Membranes to Trigger High Gas Permeance,Selectivity, and Stability in Hydrogen Separation

Thin membranes (900 nm) were prepared by direct transformation of infiltrated amorphous precursor nanoparticles, impregnated in a graphene oxide (GO) matrix, into hydroxy sodalite (SOD) nanocrystals. The amorphous precursor particles rich in silanols (Si−OH) enhanced the interactions with the GO, thus leading to the formation of highly adhesive and stable SOD/GO membranes via strong bonding. The cross-linking of SOD nanoparticles with the GO in the membranes promoted both the high gas permeance and enhanced selectivity towards H₂ from a mixture containing CO₂ and H₂O. The SOD/GO membranes are moisture resistance and exhibit steady separation performance (H₂ permeance of about 4900 GPU and H₂/CO₂ selectivity of 56, with no degradation in performance during the test of 50 h) at high temperature (200 °C) under water vapor (4 mol %).     

A Uniformly Oriented MFI Membrane for Improved CO2 Separation

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

Effects of cross-linkers with different molecular weights in cross-linked Matrimid 5218 and test temperature on gas transport properties

The poly(ethylene oxide) (PEO) was introduced by the cross-linking method in the commercial Matrimid 5218. The two kinds of membranes were prepared from the Matrimid 5218 and the cross-linkers poly(propylene glycol) block poly(ethylene glycol) block poly(propylene glycol) diamine (PPG/PEG/PPGDA) with different molecular weights. The cross-linking reaction process was monitored by FTIR. The cross-linked Matrimid 5218 membranes display excellent CO₂ permeability and CO₂/light gas selectivity. The effects of cross-linkers with different molecular weights on gel content, thermal properties and H₂, CO₂, N₂ and CH₄ gas transport properties were reported. The effect of temperature on gas transport properties was also reported, and the permeabilities of these materials as a function of temperature were compared with other gas membrane materials.     

Plasticization of ultra-thin polysulfone membranes by carbon dioxide

Plasticization of gas separation membranes by carbon dioxide permanently alters their performance and increases the possibility of membrane failure. This is amplified in ultra-thin composite membranes, where the active polymeric layer is less than 2 μm. Here, the plasticization influence of CO₂ is measured on ultra-thin polysulfone composite membranes for a range of active layer thicknesses, at four temperatures. The resulting permeability–pressure isotherms demonstrate plasticization occurs for all thicknesses at pressures lower than has been reported for dense membranes. These isotherms were quantitatively fitted with an expanded dual-sorption model that takes into account plasticization of the membrane. The plasticization potential of CO₂ for polysulfone was found to increase with reduced active layer thickness. Similarly, the plasticization potential of CO₂ was found to decrease with temperature. These results are consistent with similar research that shows that thin films behave differently to dense membranes.     

Tuning the Stacking Modes of Ultrathin Two-Dimensional Metal–Organic Framework Nanosheet Membranes for Highly Efficient Hydrogen Separation

Two-dimensional (2D) metal–organic framework (MOF) membranes are considered potential gas separation membranes of the next generation due to their structural diversity and geometrical functionality. However, achieving a rational structure design for a 2D MOF membrane and understanding the impact of MOF nanosheet stacking modes on membrane separation performance remain challenging tasks. Here, we report a novel kind of 2D MOF membrane based on [Cu₂Br(IN)₂]_n (IN=isonicotinato) nanosheets and propose that synergetic stacking modes of nanosheets have a significant influence on gas separation performance. The stacking of the 2D MOF nanosheets is controlled by solvent droplet dynamic behaviors at different temperatures of drop coating. Our 2D MOF nanosheet membranes exhibit high gas separation performances for H₂/CH₄ (selectivity >290 with H₂ permeance >520 GPU) and H₂/CO₂ (selectivity >190 with H₂ permeance >590 GPU) surpassing the Robeson upper bounds, paving a potential way for eco-friendly H₂ separation.