Permeability of dense (homogeneous) cellulose acetate membranes to methane, carbon dioxide, and their mixtures at elevated pressures

A Strategical Improvement in the Performance of CO2/N2 Gas Permeation via Conjugation of L-Tyrosine onto Chitosan Membrane

Rubbery polymeric membranes, containing amine carriers, have received much attention in CO₂ separation because of their easy fabrication, low cost, and excellent separation performance. The present study focuses on the versatile aspects of covalent conjugation of L-tyrosine (Tyr) onto the high molecular weight chitosan (CS) accomplished by using carbodiimide as a coupling agent for CO₂/N₂ separation. The fabricated membrane was subjected to FTIR, XRD, TGA, AFM, FESEM, and moisture retention tests to examine the thermal and physicochemical properties. The defect-free dense layer of tyrosine-conjugated-chitosan, with active layer thickness within the range of ~600 nm, was cast and employed for mixed gas (CO₂/N₂) separation study in the temperature range of 25-115 °C in both dry and swollen conditions and compared to that of a neat CS membrane. An enhancement in the thermal stability and amorphousness was displayed by TGA and XRD spectra, respectively, for the prepared membranes. The fabricated membrane showed reasonably good CO₂ permeance of around 103 GPU and CO₂/N₂ selectivity of 32 by maintaining a sweep/feed moisture flow rate of 0.05/0.03 mL/min, respectively, an operating temperature of 85 °C, and a feed pressure of 32 psi. The composite membrane demonstrated high permeance because of the chemical grafting compared to the bare chitosan. Additionally, the excellent moisture retention capacity of the fabricated membrane accelerates high CO₂ uptake by amine carriers, owing to the reversible zwitterion reaction. All the features make this membrane a potential membrane material for CO₂ capture.

Control of zeolite framework flexibility for ultra-selective carbon dioxide separation

Molecular sieving membranes with uniform pore size are highly desired for carbon dioxide separation. All-silica zeolite membranes feature well-defined micropores, but the size-exclusion effect is significantly compromised by the non-selective macro-pores generated during detemplation. Here we propose a template modulated crystal transition (TMCT) approach to tune the flexibility of Decadodecasil 3 R (DD3R) zeolite to prepare ultra-selective membranes for CO₂/CH₄ separation. An instantaneous overheating is applied to synchronize the template decomposition with the structure relaxation. The organic template molecules are transitionally converted to tight carbon species by the one-minute overheating at 700 °C, which are facilely burnt out by a following moderate thermal treatment. The resulting membranes exhibit CO₂/CH₄ selectivity of 157~1,172 and CO₂ permeance of (890~1,540) × 10⁻¹⁰mol m⁻² s⁻¹ Pa⁻¹. The CO₂ flux and CO₂/CH₄ mixture selectivity reach 3.6 Nm³ m⁻² h⁻¹ and 43 even at feed pressure up to 31 bar. Such strategy could pave the way of all-silica zeolite membranes to practical applications.

All-silica zeolite membranes are highly desired for natural gas upgrading but the size-exclusion effect is compromised by defects generated during high-temperature detemplation. Here, the authors develop a strategy to fabricate ultra-selective DD3R zeolite membranes via tuning the zeolite flexibility under rapid template decomposition.

The prospect of synthesis of PES/PEG blend membranes using blend NMP/DMF for CO2/N2 separation

Carbon dioxide (CO2) emissions have been the root cause for anthropogenic climate change. Decarbonisation strategies, particularly carbon capture and storage (CCS) are crucial for mitigating the risk of global warming. Among all current CO2 separation technologies, membrane separation has the biggest potential for CCS as it is inexpensive, highly efficient, and simple to operate. Polymeric membranes are the preferred choice for the gas separation industry due to simpler methods of fabrication and lower costs compared to inorganic or mixed matrix membranes (MMMs). However, plasticisation and upper-bound trade-off between selectivity and permeability has limited the gas separation performance of polymeric membranes. Recently, researchers have found that the blending of glassy and rubbery polymers can effectively minimise trade-off between selectivity and permeability. Glassy poly(ethersulfone) (PES) and rubbery poly(ethylene) glycol (PEG) are polymers that are known to have a high affinity towards CO2. In this paper, PEG and PES are reviewed as potential polymer blend that can yield a final membrane with high CO2 permeance and CO2/nitrogen (N2) selectivity. Gas separation properties can be enhanced by using different solvents in the phase-inversion process. N-Methyl-2-Pyrrolidone (NMP) and Dimethylformamide (DMF) are common industrial solvents used for membrane fabrication. Both NMP and DMF are reviewed as prospective solvent blend that can improve the morphology and separation properties of PES/PEG blend membranes due to their effects on the membrane structure which increases permeation as well as selectivity. Thus, a PES/PEG blend polymeric membrane fabricated using NMP and DMF solvents is believed to be a major prospect for CO2/N2 gas separation.

Performance Analysis of Blended Membranes of Cellulose Acetate with Variable Degree of Acetylation for CO2/CH4 Separation

The separation and capture of CO₂ have become an urgent and important agenda because of the CO₂-induced global warming and the requirement of industrial products. Membrane-based technologies have proven to be a promising alternative for CO₂ separations. To make the gas-separation membrane process more competitive, productive membrane with high gas permeability and high selectivity is crucial. Herein, we developed new cellulose triacetate (CTA) and cellulose diacetate (CDA) blended membranes for CO₂ separations. The CTA and CDA blends were chosen because they have similar chemical structures, good separation performance, and its economical and green nature. The best position in Robeson’s upper bound curve at 5 bar was obtained with the membrane containing 80 wt.% CTA and 20 wt.% CDA, which shows the CO₂ permeability of 17.32 barrer and CO₂/CH₄ selectivity of 18.55. The membrane exhibits 98% enhancement in CO₂/CH₄ selectivity compared to neat membrane with only a slight reduction in CO₂ permeability. The optimal membrane displays a plasticization pressure of 10.48 bar. The newly developed blended membranes show great potential for CO₂ separations in the natural gas industry.

Molecular Simulations of MOF Membranes and Performance Predictions of MOF/Polymer Mixed Matrix Membranes for CO2/CH4 Separations

Efficient separation of CO₂ from CO₂/CH₄ mixtures using membranes has economic, environmental and industrial importance. Membrane technologies are currently dominated by polymers due to their processing abilities and low manufacturing costs. However, polymeric membranes suffer from either low gas permeabilities or low selectivities. Metal organic frameworks (MOFs) are suggested as potential membrane candidates that offer both high selectivity and permeability for CO₂/CH₄ separation. Experimental testing of every single synthesized MOF material as membranes is not practical due to the availability of thousands of different MOF materials. A multilevel, high-throughput computational screening methodology was used to examine the MOF database for membrane-based CO₂/CH₄ separation. MOF membranes offering the best combination of CO₂ permeability (>10⁶ Barrer) and CO₂/CH₄ selectivity (>80) were identified by combining grand canonical Monte Carlo and molecular dynamics simulations. Results revealed that the best MOF membranes are located above the Robeson's upper bound indicating that they outperform polymeric membranes for CO₂/CH₄ separation. The impact of framework flexibility on the membrane properties of the selected top MOFs was studied by comparing the results of rigid and flexible molecular simulations. Relations between structures and performances of MOFs were also investigated to provide atomic-level insights into the design of novel MOFs which will be useful for CO₂/CH₄ separation processes. We also predicted permeabilities and selectivities of the mixed matrix membranes (MMM) in which the best MOF candidates are incorporated as filler particles into polymers and found that MOF-based MMMs have significantly higher CO₂ permeabilities and moderately higher selectivities than pure polymers.

Structural, Thermal, and Gas-Transport Properties of Fe3+ Ion-Exchanged Nafion Membranes

The physical and gas-transport properties of Fe³⁺ cross-linked Nafion membranes were examined. Wide-angle X-ray diffraction results revealed a lower crystallinity for Nafion Fe³⁺ but showed essentially no changes in the average chain spacing upon cation exchange of Nafion H⁺. Raman and Fourier transform infrared spectroscopy techniques qualitatively measured the strength of the ionic bond between the Fe³⁺ cations and sulfonate anions. Thermal gravimetric analysis indicated that the incorporation of Fe³⁺ adversely affected the thermal stability of Nafion due to the catalytic decomposition of perfluoroalkylether side chains. Gas sorption isotherms of Nafion Fe³⁺ measured at 35 °C up to 20 atm exhibited a linear sorption uptake for O₂, N₂, and CH₄ following Henry's law and slight concave behavior for CO₂. Pure-gas permeation results showed reduced gas permeability but higher permselectivities compared to Nafion H⁺ with αN₂/CH₄ = 4.0, αCO₂/CH₄ = 35, and αHe/CH₄ = 733 attributable to the strong physical cross-linking effect of Fe³⁺ that caused chain stiffening with enhanced size-sieving behavior. Gas mixture permeation experiments using 1:1 molar CO₂/CH₄ feed demonstrated reduced CO₂ plasticization for Nafion Fe³⁺. At 10 atm CO₂ partial pressure, CO₂/CH₄ selectivity decreased to 28 from the pure-gas value of 35, which was a significant improvement compared to the performance of a Nafion H⁺ membrane.

Preparation and Characterization of a Bioartificial Polymeric Material: Bilayer of Cellulose Acetate-PVA

A new bioartificial polymeric material consisting of a bilayer of cellulose acetate and poly(vinyl alcohol) was successfully obtained by casting method. The material was characterized by Fourier transform infrared spectroscopy, contact angle, scanning electron microscopy, differential scanning calorimetry, gas permeability, water vapor permeability, and mechanical properties. The characterization indicates that two distinct and well-differentiated surfaces were achieved without detriment to the bulk properties. The interaction between natural and synthetic polymers indeed enhanced the gas permeability as well as the water vapor permeability in comparison to the original components, although mechanical properties were not substantially boosted by the combination of both. Moreover, beyond the interface, there were no detected interactions between the polymers as can be evidenced by the presence of a uniqueTgin the bilayer. The amalgamation of the relatively good mechanical properties with the two differentiated surfaces and the improvement of the permeability properties could indicate the potential of the material for being used in medicine.

Membrane applications for biogas production and purification processes: an overview on a smart alternative for process intensification

Biogas is the result of a complex conversion process that takes place because of the metabolic activity of various types of bacteria.