Graphene-Based Membranes for CO2/CH4 Separation: Key Challenges and Perspectives

High-Performance Mixed-Matrix Membranes Using a Zeolite@MOF Core-Shell Structure Synthesized via Ion-Exchange-Induced Crystallization and Post-Synthetic Conversion

Strategic design of nanostructures, such as the core-shell configuration, offers a promising avenue to harness the desired properties while mitigating the inherent limitations of individual materials. In our pursuit of synergizing the advantages of two distinct porous materials, namely, zeolites and metal-organic frameworks (MOFs), we aimed to develop the zeolite@MOF core-shell structures. To synthesize this targeted material while minimizing undesirable side reactions, we devised an innovative approach involving ion-exchange-induced crystallization and post-synthetic conversion. This method enabled the exclusive growth of a MOF on the zeolite surface. Specifically, we successfully crafted a CaA@ZIF-8 core-shell structure, employing it in the fabrication of mixed-matrix membranes for CO2 separation. Within this core-shell configuration, the ZIF-8 in the shell played a crucial role in enhancing the filler-polymer interfaces, leading to the development of defect-free membranes. Simultaneously, the CaA zeolite core exhibited a highly selective transport of CO2. The synergistic effects resulted in a membrane incorporating a CaA@ZIF-8 core-shell filler, which demonstrated a high CO2 permeability of 1142 Barrer and a CO2/CH4 selectivity of 43.3, significantly surpassing the established upper limits for polymeric membranes. Our findings underscore the potential of core-shell structures composed of microporous materials for achieving the coveted properties necessary for high-performance gas separation membranes.

Recent Progress in Mixed-Matrix Membranes for Hydrogen Separation

Membrane separation is a compelling technology for hydrogen separation. Among the different types of membranes used to date, the mixed-matrix membranes (MMMs) are one of the most widely used approaches for enhancing separation performances and surpassing the Robeson upper bound limits for polymeric membranes. In this review, we focus on the recent progress in MMMs for hydrogen separation. The discussion first starts with a background introduction of the current hydrogen generation technologies, followed by a comparison between the membrane technology and other hydrogen purification technologies. Thereafter, state-of-the-art MMMs, comprising emerging filler materials that include zeolites, metal-organic frameworks, covalent organic frameworks, and graphene-based materials, are highlighted. The binary filler strategy, which uses two filler materials to create synergistic enhancements in MMMs, is also described. A critical evaluation on the performances of the MMMs is then considered in context, before we conclude with our perspectives on how MMMs for hydrogen separation can advance moving forward.

A Process for Carbon Dioxide Capture Using Schiff Bases Containing a Trimethoprim Unit

Environmental problems associated with the growing levels of carbon dioxide in the atmosphere due to the burning of fossil fuels to satisfy the high demand for energy are a pressing concern. Therefore, the design of new materials for carbon dioxide storage has received increasing research attention. In this work, we report the synthesis of three new Schiff bases containing a trimethoprim unit and the investigation of their application as adsorbents for carbon dioxide capture. The reaction of trimethoprim and aromatic aldehydes in acid medium gave the corresponding Schiff bases in 83%–87% yields. The Schiff bases exhibited surface areas ranging from 4.15 to 20.33 m2/g, pore volumes of 0.0036–0.0086 cm3/g, and average pore diameters of 6.64–1.4 nm. An excellent carbon dioxide uptake (27–46 wt%) was achieved at high temperature and pressure (313 K and 40 bar, respectively) using the Schiff bases. The 3-hydroxyphenyl-substituted Schiff base, which exhibited a meta-arrangement, provided the highest carbon dioxide uptake (46 wt%) due to its higher surface area, pore volume, and pore diameter compared with the other two derivatives with a para-arrangement.

Graphene-based Membranes for H2 Separation: Recent Progress and Future Perspective

Hydrogen (H2) is an industrial gas that has showcased its importance in several well-known processes such as ammonia, methanol and steel productions, as well as in petrochemical industries. Besides, there is a growing interest in H2 production and purification owing to the global efforts to minimize the emission of greenhouse gases. Nevertheless, H2 which is produced synthetically is expected to contain other impurities and unreacted substituents (e.g., carbon dioxide, CO2; nitrogen, N2 and methane, CH4), such that subsequent purification steps are typically required for practical applications. In this context, membrane-based separation has attracted a vast amount of interest due to its desirable advantages over conventional separation processes, such as the ease of operation, low energy consumption and small plant footprint. Efforts have also been made for the development of high-performance membranes that can overcome the limitations of conventional polymer membranes. In particular, the studies on graphene-based membranes have been actively conducted most recently, showcasing outstanding H2-separation performances. This review focuses on the recent progress and potential challenges in graphene-based membranes for H2 purification.

Synthesis and use of carvedilol metal complexes as carbon dioxide storage media

Abstract

The consequences of increased fossil fuel consumption on the environment presents a challenge. Carbon dioxide capture is a useful technique to reduce global warming. Therefore, three carvedilol metal (nickel, cobalt, and copper) complexes were synthesized as potential carbon dioxide storage media. The structural and textural properties of metal carvedilol complexes have been established using various techniques. The metal complexes have mesoporous structures in which pore size was approximately 3 nm. Particle size ranged from 51.0 to 393.9 nm with a relatively small surface area (6.126–9.073 m2/g). The carvedilol metal complexes have either type-III or IV nitrogen adsorption–desorption isotherm. The complexes showed reasonable capacity towards carbon dioxide uptake (up to 18.21 cm3/g) under the optimized condition (40 bar and 323 K).

Graphical Abstract

A Combined Experimental and First-Principle Calculation (DFT Study) for In Situ Polymer Inclusion Membrane-Assisted Growth of Metal-Organic Frameworks (MOFs)

A simple yet effective strategy was developed to prepare a metal-organic framework- (MOF-) based asymmetric membrane by depositing the Zeolitic imidazolate framework-8 (Zif-8) layer on the aminosilane-functionalized surface of a polymer inclusion membrane via an in situ growth process. During the extraction of the ligand molecules from the source to stripping compartment, metal ions react with ligand, and layers of Zif-8 were gradually grown onto aminosilane-modified polymer inclusion membrane (PIM). The properties of the surface-grown Zif-8 nanocrystalline layer were well characterized by powder X-ray diffraction, adsorption-desorption analysis, and scanning electron microscopy. The potential use of these Zif-8-supported PIM membranes for the separation of gases N2, CH4, and CO2 was evaluated at two temperatures (25 and 50°C) and pressures (1, 3, and 5 bar), by comparing the permeability and selectivity behavior of these membranes with neat PIM. The gas permeability of both pure PIM (PCO2=799.2 barrer) and PIM-co-MOF (PCO2=675.8 barrer) increases with the temperature for all three gases, and the permeation rate order was CO2 > CH4 > N2. The results showed that the presence of a layer of Zif-8 on the surface of the polymer inclusion membranes can get a slightly reduced permeability (~21%) but an enhanced selectivity of up to ~70% for CO2/CH4 and ~34% for CO2/N2. In the case of both membrane types, the ideal permselectivity decreases with the temperature, but this decrease was slightly more pronounced for the case of PIM-co-MOF. To understand more details about the electronic structure and optical and adsorption properties of Zif-8 and M+Zif-8 (M=N2,CH4,and CO2) compounds, the periodic plane-wave density functional theory (DFT) calculations were used. The electronic band structures and density of states for pure Zif-8 showed that this compound is metallic. Also, using DFT, the formation energy of M+Zif-8 compounds was calculated, and we showed that the CO2+Zif-8 composition is more stable than other compounds. This result suggests that the tendency of the Zif-8 compound to absorb the CO2 molecule is higher than that of other molecules. Confirming these results, DFT optical calculations showed that the affinity of the CO2+Zif-8 composition to absorb infrared light was greater than that of the other compounds.

A New Industrial Technology for Mass Production of Graphene/PEBA Membranes for CO2/CH4 Selectivity with High Dispersion, Thermal and Mechanical Performance

Polyether block amide (PEBA) nanocomposite membranes, including Graphene (GA)/PEBA membranes are considered to be a promising emerging technology for removing CO2 from natural gas and biogas. However, poor dispersion of GA in the produced membranes at industrial scale still forms the main barrier to commercialize. Within this frame, this research aims to develop a new industrial approach to produce GA/PEBA granules that could be used as a feedstock material for mass production of GA/PEBA membranes. The developed approach consists of three sequential phases. The first stage was concentrated on production of GA/PEBA granules using extrusion process (at 170-210 °C, depending on GA concentration) in the presence of Paraffin Liquid (PL) as an adhesive layer (between GA and PEBA) and assisted melting of PEBA. The second phase was devoted to production of GA/PEBA membranes using a solution casting method. The last phase was focused on evaluation of CO2/CH4 selectivity of the fabricated membranes at low and high temperatures (25 and 55 °C) at a constant feeding pressure (2 bar) using a test rig built especially for that purpose. The granules and membranes were prepared with different concentrations of GA in the range 0.05 to 0.5 wt.% and constant amount of PL (2 wt.%). Also, the morphology, physical, chemical, thermal, and mechanical behaviors of the synthesized membranes were analyzed with the help of SEM, TEM, XRD, FTIR, TGA-DTG, and universal testing machine. The results showed that incorporation of GA with PEBA using the developed approach resulted in significant improvements in dispersion, thermal, and mechanical properties (higher elasticity increased by ~10%). Also, ideal CO2/CH4 selectivity was improved by 29% at 25 °C and 32% at 55 °C.

Synthesis and Use of Valsartan Metal Complexes as Media for Carbon Dioxide Storage

To address global warming through carbon dioxide storage, three valsartan metal complexes were synthesized in excellent yields (87–92%) through a reaction of the appropriate metal chloride (tin chloride, nickel chloride hexahydrate, or magnesium chloride hexahydrate) and excess valsartan (two mole equivalents) in boiling methanol for 3 h. The structures of the metal complexes were established based on the data obtained from ultraviolet-visible, Fourier transform infrared, and proton nuclear magnetic resonance spectra, as well as from elemental analysis, energy-dispersive X-ray spectra, and magnetic susceptibility. The agglomeration and shape of the particles were determined using field emission scanning electron microscopy analysis. The surface area (16.63–22.75 m²/g) of the metal complexes was measured using the Brunauer-Emmett-Teller method, whereas the Barrett-Joyner-Halenda method was used to determine the particle pore size (0.011–0.108 cm³/g), total average pore volume (6.50–12.46 nm), and pore diameter (6.50–12.47 nm), for the metal complexes. The carbon dioxide uptake of the synthesized complexes, at 323 K and 4 MPa (40 bar), ranged from 24.11 to 34.51 cm²/g, and the nickel complex was found to be the most effective sorbent for carbon dioxide storage.

Porous Aromatic Melamine Schiff Bases as Highly Efficient Media for Carbon Dioxide Storage

High energy demand has led to excessive fuel consumption and high-concentration CO2 production. CO2 release causes serious environmental problems such as the rise in the Earth’s temperature, leading to global warming. Thus, chemical industries are under severe pressure to provide a solution to the problems associated with fuel consumption and to reduce CO2 emission at the source. To this effect, herein, four highly porous aromatic Schiff bases derived from melamine were investigated as potential media for CO2 capture. Since these Schiff bases are highly aromatic, porous, and have a high content of heteroatoms (nitrogen and oxygen), they can serve as CO2 storage media. The surface morphology of the Schiff bases was investigated through field emission scanning electron microscopy, and their physical properties were determined by gas adsorption experiments. The Schiff bases had a pore volume of 0.005–0.036 cm3/g, an average pore diameter of 1.69–3.363 nm, and a small Brunauer–Emmett–Teller surface area (5.2–11.6 m2/g). The Schiff bases showed remarkable CO2 uptake (up to 2.33 mmol/g; 10.0 wt%) at 323 K and 40 bars. The Schiff base containing the 4-nitrophenyl substituent was the most efficient medium for CO2 adsorption and, therefore, can be used as a gas sorbent.

Synthesis of Novel Heteroatom-Doped Porous-Organic Polymers as Environmentally Efficient Media for Carbon Dioxide Storage

The high carbon dioxide emission levels due to the increased consumption of fossil fuels has led to various environmental problems. Efficient strategies for the capture and storage of greenhouse gases, such as carbon dioxide are crucial in reducing their concentrations in the environment. Considering this, herein, three novel heteroatom-doped porous-organic polymers (POPs) containing phosphate units were synthesized in high yields from the coupling reactions of phosphate esters and 1,4-diaminobenzene (three mole equivalents) in boiling ethanol using a simple, efficient, and general procedure. The structures and physicochemical properties of the synthesized POPs were established using various techniques. Field emission scanning electron microscopy (FESEM) images showed that the surface morphologies of the synthesized POPs were similar to coral reefs. They had grooved networks, long range periodic macropores, amorphous surfaces, and a high surface area (SBET = 82.71–213.54 m2/g). Most importantly, they had considerable carbon dioxide storage capacity, particularly at high pressure. The carbon dioxide uptake at 323 K and 40 bar for one of the POPs was as high as 1.42 mmol/g (6.00 wt %). The high carbon dioxide uptake capacities of these materials were primarily governed by their geometries. The POP containing a meta-phosphate unit leads to the highest CO2 uptake since such geometry provides a highly distorted and extended surface area network compared to other POPs.

Polyvinylamine Membranes Containing Graphene-Based Nanofillers for Carbon Capture Applications

In the present study, the separation performance of new self-standing polyvinylamine (PVAm) membranes loaded with few-layer graphene (G) and graphene oxide (GO) was evaluated, in view of their use in carbon capture applications. PVAm, provided by BASF as commercial product named Lupamin^TM, was purified obtaining PVAm films with two degrees of purification: Low Grade (PVAm-LG) and High Grade (PVAm-HG). These two-grade purified PVAm were loaded with 3 wt% of graphene and graphene oxide to improve mechanical stability: indeed, pristine tested materials proved to be brittle when dry, while highly susceptible to swelling in humid conditions. Purification performances were assessed through FTIR-ATR spectroscopy, DSC and TGA analysis, which were carried out to characterize the pristine polymer and its nanocomposites. In addition, the membranes′ fracture surfaces were observed through SEM analysis to evaluate the degree of dispersion. Water sorption and gas permeation tests were performed at 35 °C at different relative humidity (RH), ranging from 50% to 95%. Overall, composite membranes showed improved mechanical stability at high humidity, and higher glass transition temperature (T_g) with respect to neat PVAm. Ideal CO₂/N₂ selectivity up to 80 was measured, paired with a CO₂ permeability of 70 Barrer. The membranes’ increased mechanical stability against swelling, even at high RH, without the need of any crosslinking, represents an interesting result in view of possible further development of new types of facilitated transport composite membranes.