Characterization and gas permeation properties of polyimide/ZSM-5 zeolite composite membranes containing ionic liquid

Realizing high performance gas filters through nano-particle deposition

We have studied the separation of a mixture of hydrogen and methane in equal proportions, using a thin film comprised of 10 layers of nanoparticles deposited layer-wise using our "two-point sticking algorithm" which simulates controlled agglomeration of such nanoparticles. We simulate the process of gas separation using LAMMPS. We have studied the scenario where nanoparticles act like hard spheres, maintaining their shape and size, similar to what has been demonstrated by experiments involving self-assembled nanoparticle thin films. We consider the pressure dependence of the results by working at 3 different initial pressures, 0.1 × P₀, 0.5 × P₀ and P₀, where P₀ is the atmospheric pressure. Three different diameters of the nanoparticles, namely 3 nm, 6 nm and 9 nm, are considered, and therefore the overall thickness of the membranes considered ranges from 30 nm to 90 nm. We obtained perm-selectivity values that are significantly higher than the Robeson line for hydrogen-methane gas separation, indicating the novelty and therefore the significant applications of this work. We find that while the permeance of hydrogen remains more or less steady with a ten-fold increase of pressure, the corresponding fall in methane's permeance is very sharp. The fall in methane's permeance with increasing pressure is more pronounced the smaller the nanoparticles of the membrane being used. This results in an even higher selectivity at higher pressure for smaller nanoparticle based membranes.

Preparation of biomass-based gas separation membranes from biochar residue obtained by depolymerization of lignin with ZSM-5 to promote a circular bioeconomy

Reuse of biochar residues after lignin degradation will not only save costs but also reduce the pollution, protect and improve the environment. In this study, biochar residue (BR) after peanut shell lignin selective depolymerization on ZSM-5 were recycled, and characterized by Scanning Electron Microscopy, Surface area & pore size distribution analyzers, Thermogravimetric Analysis. Subsequently, a series of hybrid matrix membranes were prepared using ethyl cellulose as the matrix and biochar residue after depolymerization under different reaction conditions as the filler. The separation performance of BR/EC membranes for CO₂/CH₄ mixed gas and CO₂/N₂ mixed gas was measured. The results showed that the gas separation membranes prepared with biochar residue (3 h, 300 °C) as filler had good gas separation characteristics. The resulting mixed-matrix membrane exhibited a permeability of 66.00 Barrer for CO₂ and selectivities of 9.97 for CO₂/CH₄. Meanwhile, the resulting mixed-matrix membrane exhibited a permeability of 79.53 Barrer for CO₂ and selectivities of 20.01 for CO₂/N₂. Both exceed the upper limit of known pure EC membranes. Therefore, the use of biochar residue after ZSM-5 depolymerization as a filler for gas separation membranes is a feasible way. Furthermore, the membrane is well stabilized, proving its good potential for industrial applications.

Poly(ethylene glycol) Diacrylate Iongel Membranes Reinforced with Nanoclays for CO2 Separation

Despite the fact that iongels are very attractive materials for gas separation membranes, they often show mechanical stability issues mainly due to the high ionic liquid (IL) content (≥60 wt%) needed to achieve high gas separation performances. This work investigates a strategy to improve the mechanical properties of iongel membranes, which consists in the incorporation of montmorillonite (MMT) nanoclay, from 0.2 to 7.5 wt%, into a cross-linked poly(ethylene glycol) diacrylate (PEGDA) network containing 60 wt% of the IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][TFSI]). The iongels were prepared by a simple one-pot method using ultraviolet (UV) initiated polymerization of poly(ethylene glycol) diacrylate (PEGDA) and characterized by several techniques to assess their physico-chemical properties. The thermal stability of the iongels was influenced by the addition of higher MMT contents (>5 wt%). It was possible to improve both puncture strength and elongation at break with MMT contents up to 1 wt%. Furthermore, the highest ideal gas selectivities were achieved for iongels containing 0.5 wt% MMT, while the highest CO2 permeability was observed at 7.5 wt% MMT content, due to an increase in diffusivity. Remarkably, this strategy allowed for the preparation and gas permeation of self-standing iongel containing 80 wt% IL, which had not been possible up until now.

Recent Advances in Polymer-Inorganic Mixed Matrix Membranes for CO2 Separation

Since the second industrial revolution, the use of fossil fuels has been powering the advance of human society. However, the surge in carbon dioxide (CO2) emissions has raised unsettling concerns about global warming and its consequences. Membrane separation technologies have emerged as one of the major carbon reduction approaches because they are less energy-intensive and more environmentally friendly compared to other separation techniques. Compared to pure polymeric membranes, mixed matrix membranes (MMMs) that encompass both a polymeric matrix and molecular sieving fillers have received tremendous attention, as they have the potential to combine the advantages of both polymers and molecular sieves, while cancelling out each other's drawbacks. In this review, we will discuss recent advances in the development of MMMs for CO2 separation. We will discuss general mechanisms of CO2 separation in an MMM, and then compare the performances of MMMs that are based on zeolite, MOF, metal oxide nanoparticles and nanocarbons, with an emphasis on the materials' preparation methods and their chemistries. As the field is advancing fast, we will particularly focus on examples from the last 5 years, in order to provide the most up-to-date overview in this area.

A Review on Ionic Liquid Gas Separation Membranes

Ionic liquids have attracted the attention of the industry and research community as versatile solvents with unique properties, such as ionic conductivity, low volatility, high solubility of gases and vapors, thermal stability, and the possibility to combine anions and cations to yield an almost endless list of different structures. These features open perspectives for numerous applications, such as the reaction medium for chemical synthesis, electrolytes for batteries, solvent for gas sorption processes, and also membranes for gas separation. In the search for better-performing membrane materials and membranes for gas and vapor separation, ionic liquids have been investigated extensively in the last decade and a half. This review gives a complete overview of the main developments in the field of ionic liquid membranes since their first introduction. It covers all different materials, membrane types, their preparation, pure and mixed gas transport properties, and examples of potential gas separation applications. Special systems will also be discussed, including facilitated transport membranes and mixed matrix membranes. The main strengths and weaknesses of the different membrane types will be discussed, subdividing them into supported ionic liquid membranes (SILMs), poly(ionic liquids) or polymerized ionic liquids (PILs), polymer/ionic liquid blends (physically or chemically cross-linked 'ion-gels'), and PIL/IL blends. Since membrane processes are advancing as an energy-efficient alternative to traditional separation processes, having shown promising results for complex new separation challenges like carbon capture as well, they may be the key to developing a more sustainable future society. In this light, this review presents the state-of-the-art of ionic liquid membranes, to analyze their potential in the gas separation processes of the future.

Facilitated Transport Membranes With Ionic Liquids for CO2 Separations

In recent years, significant development milestones have been reached in the areas of facilitated transport membranes and ionic liquids for CO2 separations, making the combination of these materials an incredibly promising technology platform for gas treatment processes, such as post-combustion and direct CO2 capture from air in buildings, submarines, and spacecraft. The developments in facilitated transport membranes involve consistently surpassing the Robeson upper bound for dense polymer membranes, demonstrating a high CO2 flux across the membrane while maintaining very high selectivity. This mini review focuses on the recent developments of facilitated transport membranes, in particular discussing the challenges and opportunities associated with the incorporation of ionic liquids as fixed and mobile carriers for separations of CO2 at low partial pressures (<1 atm).

Preparation and Characterization of a PVDF Membrane Modified by an Ionic Liquid

In order to enhance the hydrophobicity of polyvinylidene fluoride (PVDF) porous membranes, the blending of PVDF with a hydrophobic ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) was carried out. The modified PVDF membranes with [Bmim][PF6] were fabricated through a non-solvent induced phase inversion using lithium chloride as a porogen in the PVDF casting solution. The effects of [Bmim][PF6] on the membrane characteristics were investigated. FT-IR analysis indicates that the IL is successfully retained by the PVDF membrane. Thermogravimetric analysis reveals that the optimum temperature of the modified membrane is below 300°C. Scanning electron microscopy pictures show that modified membranes have more homogeneous and larger diameter pores with a mean pore size of 0.521µm and porosity of 78%. By measuring the IL leaching during the membrane fabrication, it was found that the modified membrane does not lose IL. Atomic force microscopy shows that the roughness of the modified membrane surface increases slightly, but the contact angle of the modified membrane increases significantly from 88.1° to 110.1°. The reason for this is that the fluorine-containing IL has a low surface energy, which can enhance the hydrophobicity of the membrane. Finally, by comparing modified membranes with different IL concentrations, we draw a conclusion that the modified membrane with an IL concentration of 3 wt-% has the best properties of pore size, porosity, and hydrophobicity.

Functionalized ionic liquid membranes for CO2 separation

It is imperative to develop efficient, reversible and economic technologies for separating CO₂ which mainly comes from flue gas, natural gas and syngas.

Effects of ZnO Nanoparticle on the Gas Separation Performance of Polyurethane Mixed Matrix Membrane

Polyurethane (PU)-ZnO mixed matrix membranes (MMM) were fabricated and characterized for gas separation. A thermogravimetric analysis (TGA), a scanning electron microscope (SEM) test and an atomic-force microscopy (AFM) revealed that the physical properties and thermal stability of the membranes were improved through filler loading. Hydrogen Bonding Index, obtained from the Fourier transform infrared spectroscopy (FTIR), demonstrate that the degree of phase separation in PU-ZnO 0.5 wt % MMM was more than the neat PU, while in PU-ZnO 1.0 wt % MMM, the phase mixing had increased. Compared to the neat membrane, the CO₂ permeability of the MMMs increased by 31% for PU-ZnO 0.5 wt % MMM and decreased by 34% for 1.0 wt % ZnO MMM. The CO₂/CH₄ and CO₂/N₂ selectivities of PU-ZnO 0.5 wt % were 18.75 and 64.75, respectively.

Pebax-based composite membranes with high gas transport properties enhanced by ionic liquids for CO2 separation

A series of composite membranes with high gas transport properties enhanced by IL and ZIF-8 have been developed. The influence of ionic liquid and ZIF-8 addition on gas separation performance were systematically investigated.

Ionic Liquids as the MOFs/Polymer Interfacial Binder for Efficient Membrane Separation

Obtaining strong interfacial affinity between filler and polymer is critical to the preparation of mixed matrix membranes (MMMs) with high separation efficiency. However, it is still a challenge for micron-sized metal organic frameworks (MOFs) to achieve excellent compatibility and defect-free interface with polymer matrix. Thin layer of ionic liquid (IL) was immobilized on micron-sized HKUST-1 to eliminate the interfacial nonselective voids in MMMs with minimized free ionic liquid (IL) in polymer matrix, and then the obtained IL decorated HKUST-1 was incorporated into 4,4'-(hexafluoroisopropylidene)diphthalic anhydride-2,3,5,6-tetramethyl-1,3-phenyldiamine (6FDA-Durene) to fabricate MMMs. Acting as a filler/polymer interfacial binder, the favorable MOF/IL and IL/polymer interaction can facilitate the enhancement of MOF/polymer affinity. Compared to MMM with only HKUST-1 incorporation, MMM with IL decorated HKUST-1 succeeded in restricting the formation of nonselective interfacial voids, leading to an increment in CO₂ selectivity. The IL decoration method can be an effective approach to eliminate interfacial voids in MMMs, extending the filler selection to a wide range of large-sized fillers.

Ionic liquid-based materials: a platform to design engineered CO2 separation membranes

This review provides a judicious assessment of the CO₂ separation efficiency of membranes using ionic liquid-based materials and highlights breakthroughs and key challenges in this field.

Ether-functionalized ionic liquid based composite membranes for carbon dioxide separation

The cellulose acetate and ether-functionalized pyridinium-based ionic liquid composite membranes has been designed to improve CO₂ separation performance.

Enhanced CO2 permeability of membranes by incorporating polyzwitterion@CNT composite particles into polyimide matrix

In this study, polyzwitterion is introduced into a CO2 separation membrane. Composite particles of polyzwitterion coated carbon nanotubes (SBMA@CNT) are prepared via a precipitation polymerization method. Hybrid membranes are fabricated by incorporating SBMA@CNT in polyimide matrix and utilized for CO2 separation. The prepared composite particles and hybrid membranes are characterized by transmission electron microscopy (TEM) with element mapping, field emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR) spectra, differential scanning calorimetry (DSC) and an electronic tensile machine. Water uptake and water state of membranes are measured to probe the relationship among water uptake, water state and CO2 transport behavior. Hybrid membranes show significantly enhanced CO2 permeability compared to an unfilled polyimide membrane at a humidified state. A hybrid membrane with 5 wt % SBMA@CNT exhibits the maximum CO2 permeability of 103 Barrer with a CO2/CH4 selectivity of 36. The increase of CO2 permeability is attributed to the incorporation of the SBMA@CNT composite particles. First, SBMA@CNT form interconnected channels for CO2 transport due to the facilitated transport effect of the quaternary ammonium in repeat unit of pSBMA. Second, SBMA@CNT improve water uptake and adjust water state of membrane, which further increases CO2 permeability. Meanwhile, the variation of CO2/CH4 selectivity is dependent on the bound water portion in the membrane. A gas permeation test at a dry state and a pressure test are conducted to further probe the membrane separation performance.