Gas Separation Applications Research Articles

Investigation of the effect of functionalized carbon nanotube and graphene oxide in guar gum‐based mixed matrix membrane for gas separation application

AbstractThe present study deals with preparing mixed matrix membranes (MMMs), a new polysaccharide‐based natural polymer used as a matrix with functionalized carbon nanotubes (FCNTs) and graphene oxide (GO) used as an inorganic filler. This work identified the effect of the inorganic fillers (FCNTs or GO) with naturally occurring polymer for gas separation. The incorporation of fillers improves the gas separation performance of MMMs. In GG/FCNTs MMMs, the selectivities of CO2/N2 and CO2/H2 were enhanced by 55.24% and 57.89%, respectively. Moreover, in GG/GO MMMs, the selectivities of CO2/N2 and CO2/H2 were improved by 99.50% and 50%, respectively. The membrane was characterized by scanning electron microscopy (SEM) and Fourier‐transform infrared spectroscopy (FTIR). The SEM analysis of GG/GO MMMs reveals layered structure, and GG/FCNTs MMMs create passages to transport gases. The Universal testing machine (UTM) is used to analyze the mechanical properties of pristine and modified membranes.

The Production of Industrial-Grade Oxygen from Air by Pressure Swing Adsorption

Oxygen, an odorless and colorless gas constituent of the atmosphere, is a vital gas component for the Earth, as it makes up 21% of the composition of the air we breathe. Apart from the importance of oxygen for human breathing, its highly pure form is demanding for industrial applications. As such, several technologies have been established to increase the oxygen purity from 21% to somewhat higher than 95%. One of the competitive technologies for producing this high-purity oxygen from the air is through pressure swing adsorption (PSA), which has the advantages of low cost and energy while being highly efficient. Also, PSA is a simple and flexible system due to its ability to start up and shut down more rapidly since its operation occurs at ambient temperature, which is enabled through the use of adsorbents to bind and separate the air molecules. The enhancement of the PSA’s performances was reported through the modification of PSA step cycles and material (zeolite) tailoring. A simplified complete set of a mathematical model is included for modelling the PSA system, aiming to ease the experimental burden of the process design and optimization of an infinite modification of PSA step cycles. Finally, some technological importance of oxygen production via PSA, particularly for onboard oxygen generation system and oxy-enriched incineration of municipal solid waste, was discussed. Continuous development of PSA will make significant contributions to a wide range of chemical industries in the near future, be it for oxygen production or other gas separation applications.

Thermodynamic effects of the interaction of multiple solutes and dodecahedral-cage deformation on the semi-clathrate hydrate formation with CH4-CO2

The accurate prediction of thermodynamic equilibrium hydrate formation pressure (Peq) is crucial to the industrial application of hydrate-based gas separation. This study evaluated Peq of CH4-CO2 gas mixtures with different TBAB solutions at different temperatures. Unlike previously reported models, the proposed model accounted for the effects of interaction of multiple solutes and the deformation of dodecahedral (D)-cages on Peq, thereby minimizing deviations in predictions. The effects of the interaction of multiple solutes on Peq can be attributed to gas dissolution and the presence of electrolytes. For accurate prediction, the proposed model incorporated the aforementioned effects based on the electrolyte-cubic plus association equation of state and the parameters for interaction among guest molecules in cages Furthermore, the proposed model can quantitatively describe the effects of gas dissolution, the presence of electrolytes and deformation of D-cages on the Peq values for corresponding systems.

Development of melamine-based covalent organic framework-MOF pearl-like heterostructure integrated poly(ether-block-amide) for CO2/CH4 separation

Regarding gas separation application, covalent organic frameworks (COFs) are one of the supreme ones because of their numerous merits, such as ordered pores, excellent compatibility with organic particles, and a diverse range of functionality. Unfortunately, the pore size of these nanomaterials is wider than most gas sizes, so there is an explicit limitation to using them in the gas separation field. Herein, the confined growth of metal-organic frameworks (MOFs) in a facile way in melamine-based COF pores (mCOFs) was proposed. There is some one-dimensional channel in two-dimensional COFs; therefore, there is a perfect possibility for confined growth of MOF in those channels with pearl-like structures. After forming these COF-MOF nanohybrid particles (CXMs), a polyether block amide (Pebax) membrane with excellent size sieving and high molecular transport was fabricated. The permeability and selectivity rates of neat Pebax, mCOF/Pebax, and CXM/Pebax membranes were compared systematically. It was found CXM/Pebax membrane had much better CO2/CH4 selectivity, and long-term stability was enhanced simultaneously. The CO2/CH4 selectivity reached 44.2 ± 1.3 for the optimized membrane elucidating 2.3 folds improvement compared to the neat membrane and surpassing Robeson's upper bounds. The long-term tests elucidated that the optimized membrane kept 94% of its initial CO2/CH4 selectivity, 25% higher than the unmodified one.

An ethynyl-modified interpenetrated metal-organic framework for highly efficient selective gas adsorption.

An ethynyl-modified interpenetrated MOF material with lvt topology, [Cu2(BTEB)(NMF)2]·NMF·8H2O (compound 1, H4BTEB = 4,4',4'',4'''-(benzene-1,2,4,5-tetrayltetrakis(ethyne-2,1-diyl))tetrabenzoic acid, NMF = N-Methylformamide), was successfully synthesized by using an alkynyl-functionalized H4BTEB organic ligand under solvothermal conditions. Structural analysis shows that compound 1, consisting of a tetradentate carboxylic acid ligand and classical [Cu2(CO2)4] paddle-wheel structure building units, has a rare 4-connected lvt topology with dual interpenetrating structure, which can improve the framework stability, as well as the gas adsorption capacity and selectivity due to the restricted pore channel. According to the study of gas adsorption performance, compound 1 with a larger surface area, boasts a superior adsorption capacity for small gas molecules. Also, ideal adsorption solution theory (IAST) computational simulation shows that compound 1 has good gas adsorption selectivity for C3H8/CH4, indicating its potential application in gas separation.

The Mechanical Property of 2D Materials and Potential Application in Gas Separation

The Mechanical Property of 2D Materials and Potential Application in Gas Separation

Fabrication and Characterization of Nanoscale Metal-Organic Frameworks (MOFs) for Gas Storage and Separation

In recent years, Metal-Organic Frameworks (MOFs) have emerged as a promising class of materials for gas storage and separation applications due to their high surface area, tunable pore size, and chemical functionality. In this study, we report the successful fabrication and characterization of nanoscale MOFs for enhanced gas storage and separation performance. We synthesized a series of MOFs with varying metal nodes and organic linkers, and systematically investigated their structural, thermal, and chemical stability. Advanced characterization techniques, including X-ray diffraction, scanning electron microscopy, and gas adsorption isotherms, were employed to elucidate the structural and morphological features of the synthesized MOFs. The gas storage capacities of the MOFs were evaluated for hydrogen, methane, and carbon dioxide, revealing a significant enhancement in storage capacity compared to bulk MOFs. Furthermore, we investigated the gas separation performance of the MOFs for CO2/CH4 and CO2/N2 mixtures, demonstrating high selectivity and separation efficiency. The results of this study provide valuable insights into the design and fabrication of nanoscale MOFs for gas storage and separation applications, and pave the way for the development of next-generation materials for clean energy and environmental applications.

Zeolites and metal-organic frameworks for gas separation: the possibility of translating adsorbents into membranes.

Zeolites and metal-organic frameworks (MOFs) represent an attractive class of crystalline porous materials that possesses regular pore structures. The inherent porosity of these materials has led to an increasing focus on gas separation applications, encompassing adsorption and membrane separation techniques. Here, a brief overview of the critical properties and fabrication approaches for zeolites and MOFs as adsorbents and membranes is given. The separation mechanisms, based on pore sizes and the chemical properties of nanochannels, are explored in depth, considering the distinct characteristics of adsorption and membrane separation. Recommendations for judicious selection and design of zeolites and MOFs for gas separation purposes are emphasized. By examining the similarities and differences between the roles of nanoporous materials as adsorbents and membranes, the feasibility of zeolites and MOFs from adsorption separation to membrane separation is discussed. With the rapid development of zeolites and MOFs towards adsorption and membrane separation, challenges and perspectives of this cutting-edge area are also addressed.

Recent advances in the chemistry and applications of fluorinated metal-organic frameworks (F-MOFs).

Metal-organic frameworks are a class of porous crystalline materials based on the ordered connection of metal centers or metal clusters by organic linkers with comprehensive functionalities. The interest in these materials is rapidly moving towards their application in industry and real life. In this context, cheap and sustainable synthetic strategies of MOFs with tailored structures and functions are nowadays a topic widely studied from different points of view. In this review, fluorinated MOFs (F-MOFs) and their applications are investigated. The principal aim is to provide an overview of the structural features and the main application of MOFs containing fluorine atoms both as anionic units or as coordinating elements of more complex inorganic units and, therefore, directly linked to the structural metals or as part of fluorinated linkers used in the synthesis of MOFs. Herein we present a review of F-MOFs reported in the recent literature compared to benchmark compounds published over the last 10 years. The compounds are discussed in terms of their structure and properties according to the aforementioned classification, with an insight into the different chemical nature of the bonds. The application fields of F-MOFs, especially in sustainability related issues, such as harmful gas sorption and separation, will also be discussed. F-MOFs are compounds containing fluorine atoms in their framework and they can be based on: (a) fluorinated metallic or semi-metallic anionic clusters or: (b) fluorinated organic linkers or (c) eventually containing both the building blocks. The nature of a covalent C-F bond in terms of length, charge separation and dipole moment sensibly differs from that of a partly ionic M-F (M = metal) one so that the two classes of materials (points a and b) have different properties and they find various application fields. The study shows how the insertion of polar M-F and C-F bonds in the MOF structure may confer several advantages in terms of interaction with gaseous molecules and the compounds can find application in gas sorption and separation. In addition, hydrophobicity tends to increase compared to non-fluorinated analogues, resulting in an overall improvement in moisture stability.

Modular metallotecton for engineering permanently porous frameworks: supernumerary role of ancillary ion.

The formation of robust supramolecular frameworks built from hetero-polytopic metal complexes and interacting with different ancillary ions remains a long-standing and underexplored desire. Herein, the secondary sphere interaction chemistry of [Ru(5-oxido-6-hydroxy-1,10-phenanthroline)a(5,6-dihydroxy-1,10-phenanthroline)(3-a)]-(a-2) (1) (a = 1, 3) coordination ion is reported, where the π-conjugated phenanthroline ligands are functionalized with catecholate groups used as H-bond donors and ligands. The deprotonation of the catechols is found to control the overall charge stoichiometry in 1, acting as a metallotecton to interact with anions of different basicity (Cl- in 1.Cl and Br- in 1.Br) as well as with Li+ cations (in 1.Li+). These interactions lead to the formation of 2D porous honeycomb networks without any significant alteration in the molecular packing. This implies that the self-assembly process is controlled by complementary intermolecular non-covalent interactions making the choice of the ancillary ion insignificant. The robust porous structure of the frameworks is established by uptake of D2O and I2 molecules within the microporous channels. This work demonstrates that supramolecular frameworks appear as flexible candidates for applications in gas sorption, separation and chemical sensing.

Post-synthetic modifications of covalent organic frameworks (COFs) for diverse applications.

Covalent organic frameworks (COFs) are porous and crystalline organic polymers, which have found usage in various fields. These frameworks are tailorable through the introduction of diverse functionalities into the platform. Indeed, functionality plays a key role in their different applications. However, sometimes functional groups are not compatible with reaction conditions or can compete and interfere with other groups of monomers in the direct synthetic method. Also, pre-synthesis of bulky moieties in COFs can negatively affect crystal formation. To avoid these problems a post-synthetic modification (PSM) approach is a helpful tactic. Also, with the assistance of this strategy porous size can be tunable and stability can be improved without considerable effect on the crystallite. In addition, conductivity, hydrophobicity/ hydrophilicity, and chirality are among the features that can be reformed with this method. In this review, different types of PSM strategies based on recent articles have been divided into four categories: (i) post-functionalization, (ii) post-metalation, (iii) chemical locking, and (iv) host-guest post-modifications. Post-functionalization and chemical locking methods are based on covalent bond formation while in post-metalation and host-guest post-modifications, non-covalent bonds are formed. Also, the potential of these post-modified COFs in energy storage and conversion (lithium-sulfur batteries, hydrogen storage, proton-exchange membrane fuel cells, and water splitting), heterogeneous catalysts, food safety evaluation, gas separation, environmental domains (greenhouse gas capture, radioactive element uptake, and water remediation), and biological applications (drug delivery, biosensors, biomarker capture, chiral column chromatography, and solid-state smart nanochannels) have been discussed.

Synthesis and characterization of a high-performance bio-based Pebax membrane for gas separation applications

Synthesis of biobased membranes for gas separation applications. The Pebax® Rnew® 30R51 membrane holds potential for CO2 separation. Increasing the polymer concentration enhanced the gas selectivity to 91 and 26 for CO2/N2 and CO2/CH4, respectively.

A study of the permeation barrier of nanoporous Graphene

Nanoporous Graphene, due to its single atom thickness and good mechanical strength is an emerging candidate for efficient and reliable gas separation applications. The permeation of the gas molecules through the pores can be seen as an activated process, whereby the gas molecules have to overcome a threshold barrier before it can pass through the pore. Here, we study and compare the Single Point Energy of the CO2 molecule as it approaches a nanoporous Graphene sheet comprising of 6 Carbon atoms removed contiguously to create what is known as the Type A nanopore. The distance of the single molecule from the pore is varied continuously for 2 configurations of the gas molecule, one parallel and other perpendicular to the nanopore, positioned vertically above the centre of the 6-pore, by means of ab-initio simulations based on Density Functional Theory. The permeation barrier of the CO2 molecule is estimated. It is found that orientation of incident molecule along with the pore geometry, plays an important role in gas permeation and penetration barriers, and thereby in all applications that crucially depend upon the rate of gas permeation, like gas separation for example.

3D ceramic structure fabrication by Stereolithography and physicochemical characterization

In this study, a methodology for 3D printing of Silica (SiO2) structures for potential gas separation applications (either as membranes or as substrates for selective layers deposition) utilizing Stereolithography (SLA) is presented. The porosity, morphology and crystallinity of the 3D printed samples structure were optimized by the parametric study of the calcination profile of the printed ceramic SiO2 resin; the printed material was calcined at 450, 600 and 1250 °C by different residence times at different temperature steps. The microstructure of the 3D printed samples was characterized with Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). Structural characterization (crystallinity and average crystal size) was performed by X-Ray Diffraction (XRD). The porosity and the specific surface area of the 3D printed samples were determined through the Brunauer–Emmett–Teller (BET) technique, which revealed that the surface area was gradually increased by increasing final calcination temperature. Permeability tests with flat sheet printed samples were performed in a constant pressure (isobaric) gas permeation system to quantify pure gases’ (e.g., CO2, N2, O2, Ar) permeances. Permselectivities of binary gaseous mixtures were obtained for printed samples calcined under different temperature profiles. The outcomes open new opportunities in 3D printing of versatile ceramic structures which can be exploited either as substrates for gas separation ceramic membranes or as membrane contactors for gas–liquid applications.

Revisiting Competitive Adsorption of Small Molecules in the Metal–Organic Framework Ni-MOF-74

To precisely evaluate the potential of metal-organic frameworks (MOFs) for gas separation and purification applications, it is crucial to understand how various molecules competitively adsorb inside MOFs. In this paper, we combine in situ infrared spectroscopy with ab initio calculations to investigate the mechanisms associated with coadsorption of several small molecules, including CO, NO, and CO2 inside the prototypical structure Ni-MOF-74. Surprisingly, we find that the displacement of CO bound inside Ni-MOF-74 (binding energy of 53 kJ/mol) is readily driven by CO2 exposure, even though CO2 has a noticeably weaker binding energy of only 41 kJ/mol; meanwhile, the significantly more strongly binding NO molecule (90 kJ/mol) is not able to easily displace bound CO inside Ni-MOF74. These results show that single-phase binding energies of a molecule inside the MOF cannot completely describe their interaction with the MOF in the presence of other guest molecules. We unveil many crucial factors, such as the kinetic barrier, partial pressure, secondary binding sites, and guest-host/lateral interactions that control the coadsorption process and, combined with the binding energy, are better descriptors of the behavior and adsorption of gas mixtures inside MOFs.

A comprehensive review on zeolite-based mixed matrix membranes for CO2/CH4 separation

Biogas consisting of carbon dioxide/methane (CO2/CH4) gas mixtures has emerged as an alternative renewable fuel to natural gas. The presence of CO2 can decrease the calorific value and generate greenhouse gas. Hence, separating CO2 from CH4 is a vital step in enhancing the use of biogas. Zeolite and zeolite-based mixed matrix membrane (MMM) is considered an auspicious candidate for CO2/CH4 separation due to thermal and chemical stability. This review initially addresses the development of zeolite and zeolite-based MMM for the CO2/CH4 separation. The highest performance in terms of CO2 permeance and CO2/CH4 selectivity was achieved using zeolite and zeolite-based MMM, which exhibited CO2 permeance in the range of 2.0 × 10− 7–7.0 × 10− 6 mol m− 2 s− 1 Pa− 1 with CO2/CH4 selectivity ranging from 3 to 300. Current trends directed toward improving CO2/CH4 selectivity via modification methods including post-treatment, ion-exchanged, amino silane-grafted, and ionic liquid encapsulated of zeolite-based MMM. Those modification methods improved the defect-free and interfacial adhesions between zeolite particulates and polymer matrices and subsequently enhanced the CO2/CH4 selectivity. The modifications via ionic liquid and silane methods more influenced the CO2/CH4 selectivity with 90 and 660, respectively. This review also focuses on the possible applications of zeolite-based MMM, which include the purification and treatment of water as well as biomedical applications. Lastly, future advances and opportunities for gas separation applications are also briefly discussed. This review aims to share knowledge regarding zeolite-based MMM and inspire new industrial applications.

Polymer materials derived from the SEAr reaction for gas separation applications

A set of linear polymers were synthesized utilizing an electrophilic aromatic substitution reaction (SEAr) between biphenyl and ketone containing electron-withdrawing groups (isatin, IS; N-methylisatin, MeIS; and 4,5-diazafluoren-9-one, DF). Optimization of the polycondensation reaction was made to obtain high molecular weight products when using DF, which has not previously been used for linear polymer synthesis. Due to the absence of chemically labile units, these polymers exhibited excellent chemical and thermal stability. Linear SEAr polymers were blended with porous polymer networks derived from IS and MeIS, and both neat/mixed materials were tested as membranes for gas separation. The gas separation properties of both pristine polymers and mixed matrix membranes were good, showing some polymer membrane CO2 permeability values higher than 200 barrer.

Poly(vinyl alcohol) mediated bifunctional ZIF-BN nanocomposite for the high-selectivity H2 separation membrane

We combined the advantages of the 2-methylimidazole zinc salt (ZIF-8) and hydroxylated boron nitride (OH-BN) materials to prepare a new bifunctional separation material, namely ZIF-BN. In this, spherical ZIF-8 materials fill the macroporous structure of OH-BN sheets by coordination bonds due to the edge effect. Furthermore, a series of ZIF-BN/PVA membranes were prepared using the vacuum-assisted filtration of the layer-by-layer for the gas separation study. The trace amounts of PVA connect adjacent ZIF-BN sheets through hydrogen bonds, without covering the pore structures of ZIF-8 materials. Based on this exclusive strategy for preparing membranes, the obtained membranes exhibit superior H2 separation performances. Compared with the pure PVA membrane, the H2 permeability, H2/CO2 selectivity, and H2/N2 selectivity of the ZIF-BN/PVA-8 membrane have increased (54, 13, & 12)-fold, to (600 Barrer, 97.2, & 173.6), respectively. This strategy provides a facile way to design new bifunctional membranes for gas separation applications.

Synthesis of a Highly Crystalline Amide‐Linked Covalent Organic Framework

Comprehensive SummaryThe dilemma for the synthesis of covalent organic frameworks (COFs) is crystallinity and stability. Herein, we present the synthesis of a single‐crystalline amide‐linked COF, and demonstrate its excellent chemical as well as thermal stabilities. This study will inspire the synthesis of a wide spectrum of highly crystalline and stable COFs, promote their structure‐property investigations and boost their applications in selective gas adsorption, storage and separation.

Aromatic Polyimide Membranes with tert-Butyl and Carboxylic Side Groups for Gas Separation Applications-Covalent Crosslinking Study.

A set of aromatic copolyimides was obtained by reaction of 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), and mixtures of the diamines 1,4-bis(4-amino-2-trifluoromethylphenoxy)-2,5-di-tert-butylbenzene (CF3TBAPB) and 3,5-diamino benzoic acid (DABA). These polymers were characterized and compared with the homopolymer derived from 6FDA and CF3TBAPB. All copolyimides showed high molecular weight values and good mechanical properties. The presence of carboxylic groups in these copolymers allowed their chemical crosslinking by reaction with 1,4-butanediol. Glass transition temperatures (Tg) were higher than 260 °C, showing the non-crosslinked copolyimides had the highest Tg values. Degradation temperature of crosslinked copolyimides was lower than their corresponding non-crosslinked ones. Mechanical properties of all polymers were good, and thus, copolyimide (precursor, and crosslinked ones) films could be tested as gas separation membranes. It was observed that CO2 permeability values were around 100 barrer. Finally, the plasticization resistance of the crosslinked material having a large number of carboxylic groups was excellent.