Gas Adsorption Separation Research Articles

Inherent porosity of Zeolitic Imidazolate Framework-62 melt leading to formation of the porous melt-quenched glass

Porous glasses produced through melt-quenching of some selective metal organic frameworks like Zeolitic Imidazolate Framework-62 (ZIF-62) and ZIF-4 belong to the advanced functional materials because of their inherent porosity, ease of processing, high gas adsorption capacity and gas separation selectivity. We have delineated thermal induced modifications in the porosity features (pore size, size-distribution and pore density) of crystalline ZIF-62 from room temperature (RT) to its melt-state (> melting point, Tm) followed by its quenching, back to RT, carrying out the depth sensitive positron annihilation lifetime spectroscopy (PALS) measurements in-situ at varying temperatures (RT−Tm). On heating under vacuum, the pores' size as well as size-distribution of crystalline ZIF-62 increases up to ∼473 K as a consequence of removal of entrapped solvent molecules and nonuniform thermal expansion. At higher temperatures (∼473 −573 K), a reduction in pores’ size and size-distribution is observed due to the loss of long range ordering and volume collapse. On melting, ZIF-62 turns into a porous liquid having ∼1.4 times larger pores compared to its crystalline form. The quenching of this porous melt is fully irreversible, and results in the formation of a porous glass having the pores larger than its crystalline counterpart. The in-situ PALS investigation provides the first experimental evidence of inherent porosity in ZIF-62 melt existing at high temperature that has been predicted before through molecular dynamics simulation of the ZIFs-based melts.

Adsorption characteristics and separation behavior of binder and binderless zeolite LiX Pellets: O2, N2, and CO2

In adsorptive gas separation process design, optimal adsorbent selection acts as a critical factor in process efficiency. This study investigated the adsorption equilibria and kinetics of O2, N2, and CO2 (weak, medium and strong affinity, respectively) and the breakthrough performance of four commercial zeolite LiX pellets (one binderless and three binder zeolite LiX pellets) because the performance of pelletized zeolites becomes different by adding binders and pelletizing methods. The adsorption isotherms at 293–323 K and up to 100 kPa were correlated with the dual-site Langmuir and Sips models. For all adsorbents, the order of adsorption amount and isosteric heat of adsorption was CO2 ≫ N2 > O2. However, differences among the four zeolite pellets were also observed, showing the highest values of the binderless zeolite LiX pellet. Furthermore, the adsorption capacity was 20–25 % smaller for CO2 and 40–50 % smaller of N2 and O2 at 100 kPa compared to pure zeolite LiX powder. According to kinetic analysis using a non-isothermal diffusion model, the adsorption rate of N2 was faster than that of CO2 for all adsorbents, showing the order difference in the low-pressure range. The kinetic difference among the zeolite pellets resulted from the effects of heat of adsorption, heat dissipation, and thermal resistance. The breakthrough performance using N2 and O2 was mainly controlled by the adsorption capacity, showing almost constant breakthrough pattern regardless of the samples. This study suggests that detailed evaluation of pelletized adsorbent is essential before designing adsorptive processes because performance of adsorbent powder would lead to under-design of the process. (255 words)

Influence of pore length non-uniformity on transport properties of random nanopore networks

The modeling of fluid transport in nanoporous materials has long attracted attention because of the numerous industrial applications of such materials, particularly in membrane-based separations, and others such as adsorptive gas separation and storage, catalysis, and the emerging area of nanofluidics. While effective medium theory has been widely used in modelling this transport, a critical approximation that all pores have the same length is commonly made, whose adequacy is unknown. Here we apply effective medium theory to a randomly overlapping capillary model having an inherent pore length distribution related to the pore radius distribution, and compare the network transport properties with those of the commonly used uniform pore length random network. We find that the consideration of nonuniform pore length distribution leads to significantly higher effective diffusivity and lower associated apparent tortuosity, mediated by the lower mean pore length of large pores due to their higher probability of overlap. The decrease in pore length with an increase in pore radius and the competing effects of adsorption and diffusion, lead to more complex behaviour of the transport properties with variation in temperature and relative standard deviation of the pore radius distribution. Our results clearly demonstrate the importance of considering nonuniformity of pore length, as the conventional uniform pore length approximation leads to significantly different and therefore misleading results.

Effect analysis of temperature and initial water content on the adsorption and separation of VOCs by ZIF‐8 based on molecular simulation

AbstractVolatile organic compounds (VOCs) have seriously polluted the atmospheric environment and caused harmful effects on human beings, so it is imperative to control oil vapor emissions. Metal organic framework materials, as one of the porous materials, have a good application prospect in the field of gas adsorption separation. In this paper, the effects of temperature and initial water content (IWC) on the adsorption properties of methane (CH4), ethane (C2H6), ethylene (C2H4), propane (C3H8), and propylene (C3H6) by ZIF‐8 were studied by combining the large gauge Monte Carlo (GCMC) and ideal adsorption solution theory. The results show that the higher the temperature is, the lower the adsorption capacity is, the higher the threshold pressure is, and the single molecule interaction energy changes little with temperature. But pore volume is still the main influencing factor. The IWC also reduces the saturated adsorption capacity. Preloaded water molecules can enhance the electrostatic interaction at low pressure, thus affecting the adsorption heat and interaction energy of single molecules. Overall, these findings provide valuable mechanistic insights into the effects of temperature and IWC on the adsorption properties of ZIF‐8 for VOCs.

Optimizing Trace Acetylene Removal from Acetylene/Ethylene Mixture in a Flexible Metal-Organic Framework by Crystal Downsizing.

Improving the gas separation performance of metal-organic frameworks (MOFs) by crystal downsizing is an important but often overlooked issue. Here, we report three different-sized flexible ZUL-520 MOFs (according to the crystal size from large to small, the three samples are, respectively, named ZUL-520-0, ZUL-520-1, and ZUL-520-2) with the same chemical structure for optimizing trace acetylene (C2H2) removal from acetylene/ethylene (C2H2/C2H4) mixture. The three differently sized activated ZUL-520 (denoted as ZUL-520a) exhibited almost identical C2H2 uptake of 4.8 mmol/g at 100 kPa, while the C2H2 uptake at 1 kPa increased with a downsizing crystal. The C2H2 uptake of activated ZUL-520-2 (denoted as ZUL-520-2a) at 1 kPa was ∼55% higher than that of activated ZUL-520-0 (denoted as ZUL-520-0a). The adsorption isotherms and adsorption kinetics validated that gas adsorptive separation is governed not only by adsorption thermodynamics but also by adsorption kinetics. In addition, all three different-sized ZUL-520a MOFs showed high C2H2/C2H4 selectivity. Grand canonical Monte Carlo (GCMC) simulations and dispersion-corrected density functional theory (DFT-D) computations illustrated a plausible mechanism of C2H2 adsorption in MOFs. Importantly, breakthrough experiments demonstrated that ZUL-520a can effectively separate the C2H2/C2H4 (1/99, v/v) mixture and the C2H4 productivity obtained by ZUL-520-2a was much higher than that by ZUL-520-0a. Our work may provide an easy but powerful strategy for upgrading the performance of gas adsorptive separation in MOFs.

Tunable Gas Admission via a "Molecular Trapdoor" Mechanism in a Flexible Cationic Metal-Organic Framework Featuring 1D Channels.

Achieving high gas selectivity is challenging when dealing with gas pairs of similar size and physiochemical properties. The "molecular trapdoor" mechanism discovered in zeolites holds promise for highly selective gas adsorption separation but faces limitations like constrained pore volume and slow adsorption kinetics. To address these challenges, for the first time, a flexible metal-organic framework (MOF) featuring 1D channels and functioning as a "molecular trapdoor" material is intoduced. Extra-framework anions act as "gate-keeping" groups at the narrowest points of channels, permitting gas admissions via gate opening induced by thermal/pressure stimuli and guest interactions. Different guest molecules induce varied energy barriers for anion movement, enabling gas separation based on distinct threshold temperatures for gas admission. The flexible framework of Pytpy MOFs, featuring swelling structure with rotatable pyridine rings, facilitates faster gas adsorption than zeolite. Analyzing anion properties of Pytpy MOFs reveals a guiding principle for selecting anions to tailor threshold gas admission. This study not only overcomes the kinetic limitations related to gas admission in the "molecular trapdoor" zeolites but also underscores the potential of developing MOFs as molecular trapdoor adsorbents, providing valuable insights for designing ionic MOFs tailored to diverse gas separation applications.

Application of MOFs in The Field of Adsorption And Separation

Metal-organic frameworks (MOFs) materials are a new type of zeolite-like porous materials with high porosity, high adsorption capacity and good thermal stability, which have attractive application potential in gas adsorption separation, photoelectric performance, catalysis, sensors and other aspects. Computational chemistry can not only break through the limitations of traditional methods, but also provide theoretical basis for the design of the best adsorption materials and the determination of ideal operating circumstances, and realize the shift from the main experience to the quantitative and directional preparation, thus saving a lot of complicated experimental research. As a result, it is important to carry out theoretical research on methane adsorption in MOFs materials. In this research, the adsorption properties, gas separation, seawater desalination and chiral drug separation in MOFs materials were systematically studied by molecular simulation method. This research can provide a new design approach for the synthesis and structural design of MOFs materials, and expand their applications in other fields

Two metal-organic frameworks based on tetracarboxylic ligand exhibiting high performance for gas adsorption and separation

Developing porous materials for purification of acetylene is of great significance, but remains challenging. Currently, the use of metal-organic frameworks (MOFs) for gas adsorption separation has received substantial research interest. Herein, based on the tetracarboxylic acid ligand H4TPTA ([1,1':3′,1″-terphenyl]-4,4′,4″,6′-tetracarboxylic acid), two MOFs (ErTPTA and CdTPTA) constructed by trinuclear cluster secondly building units (SBUs) have been successfully synthesized. They are three dimensional (3D) framework with 1D rhomboid channels, demonstrating a (4,8)-connected scu network. A series of gas adsorption tests were performed and the both compounds exhibited outstanding gas selective adsorption performance. Accordingly, the ideal adsorption solution theory (IAST) is used to simulate the adsorption selectivity, the selectivity of CdTPTA for CO2/CH4 is 11.85 (0.5:0.5) and 17.02 (0.05:0.95), and for C2H2/CH4 is 11.81 (0.5:0.5); the selectivity of ErTPTA for C2H2/CO2 is 4.45 (0.5:0.5).

Nanoporous Fluorinated Covalent Organic Framework for Efficient C2H2/CO2 Separation with High C2H2 Uptake

Effective C2H2/CO2 separation is regarded as a crucial procedure in the C2H2 industry yet extremely challenging because of their similar physical and chemical properties. Covalent organic frameworks (COFs) have become a promising platform for gas adsorption separation, but they still suffer from unsatisfactory C2H2 adsorption capacity and selectivity. Herein, we report a nanoporous fluorine-functioned COF (TpPa-F) for C2H2/CO2 separation, which was synthesized by a mechanochemical approach with a F-containing precursor (2-fluoro-1,4-benzenediamine). A superior C2H2 adsorption capacity of 117 cm3/g (4.78 mmol/g) and a C2H2/CO2 selectivity of 3.3 at 298 K and 1 bar were achieved, which surpass most of the reported COF adsorbents in the literature. Notably, TpPa-F exhibited an extraordinary thermal stability of up to around 673 K and showed chemical robustness in organic or acidic/basic solutions. Theoretical calculations reveal the hydrogen bond interaction of C≡C–H···F, which contributes to the high C2H2 uptake and separation selectivity. This work provides a promising strategy of fluorine functionalization for enhancing the ability to recognize and separate small gas molecules in a large channel.

Direct air capture of CO2 by metal cation‐exchanged LTA zeolites: Effect of the charge‐to‐size ratio of cations

AbstractDirect air capture of CO2 (DAC) has been increasingly recognized as a promising carbon‐negative technology. The challenge in deploying energy‐efficient DAC lies in effective sorbent materials. In this research, we comprehensively investigated the DAC behavior of LTA zeolites exchanged with different metal cations (Na+, K+, Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Y3+, La3+, Ce3+, Eu3+, Tb3+, and Yb3+) by both static single‐component gas adsorption and dynamic mixture gas adsorptive separation tests. We found that a large charge‐to‐size ratio of cations is critical to imparting a high DAC capacity of LTA zeolites, which is ascribed to the enhanced electrostatic interaction and/or π‐back bonding toward CO2. Meanwhile, a detrimental effect is associated with an excessively large charge‐to‐size ratio, that is, a significant “shielding effect” of (pre‐) adsorbed contaminants (e.g., H2O and CO2) on cations (e.g., Mn2+ and Mg2+) reduce the accessible CO2 capacity. Ca‐LTA featuring Ca2+ with an appropriate charge‐to‐size ratio exhibits the highest DAC capacity (350 ppm CO2 in the air, 1.20 mmol/g) with fast kinetics and good reusability. These results provide valuable insights for the design of zeolites‐based physisorbents for DAC.

Halogen-modified metal-organic frameworks for efficient separation of alkane from natural gas.

As a rich green energy source, natural gas is widely used in many fields such as the chemical industry, automobile energy, and daily life. However, it is very challenging to separate and recover C2H6 and C3H8 from natural gas. Metal-organic frameworks (MOFs) as an emerging type of multi-pore porous materials show huge potential in gas adsorption separation. Herein, we report pillar-layered MOFs, Ni (BDC)(DABCO)0.5 (DMOF-X), modified by halogen atoms (F, Cl, Br), and investigate their CH4/C2H6/C3H8 separation performance. The experimental results show that DMOF-Cl exhibited a extremely high adsorption capacity for C3H8 and C2H6. Under the conditions of 298 K and 100 kPa, the adsorption capacities for C3H8 and C2H6 on DMOF-Cl are as high as 6.23 and 4.94 mmol g-1, which are superior to the values for most of the porous materials that have been reported. In addition, DMOF-Cl also shows high C3H8/CH4 (5: 85, V/V) and C2H6/CH4 (10: 85, V/V) separation selectivities, with values of 130.9 and 12.5, respectively. Finally, DMOF-Cl also demonstrated great potential as an adsorbent for separating C3H8/C2H6/CH4.

A Computational Study of the Adsorptive Separation of Methane and Hydrogen in Zeolite Templated Carbons

Combustion of conventional energy sources produces pollutants such as SOx, NOx, and CO; the use of hydrogen and methane can eliminate these harmful emissions. In fuel cell technology and other uses, hydrogen must be refined by extracting methane from the methane/hydrogen combination, produced via dry or steam reforming. This study investigates the adsorption and separation capabilities of recently discovered zeolite-templated carbons (ZTCs) for binary mixtures consisting of hydrogen and methane. To assess the adsorption and separation performances of these carbon-based nanostructures, grand canonical Monte Carlo (GCMC) simulations were used. The simulation results revealed that AFY (|(C6H15N)3(H2O)7|[Co3Al5P8O32]) and RWY (|(C6H18N4)16| [Ga32Ge16S96]) structures could be viable alternatives for applications involving adsorptive gas separation based on selectivity and the CH4 uptake capacity. The selectivity of AFY was calculated to be 176, while its capacity to uptake CH4 was found to be 2.57 mmol/g, the selectivity of RWY was calculated to be 132, and its CH4 uptake was 3.49 mmol/g.

CO2-Selective Capture from Light Hydrocarbon Mixtures by Metal-Organic Frameworks: A Review

CO2 represents a typical impurity in light hydrocarbon feedstocks, which affects the quality of subsequent chemical products. Owing to their highly similar nature, industrial separation requires large amounts of energy. Adsorptive gas separation based on porous materials is considered an efficient alternative, as it can offer faster kinetics, higher selectivity, long-term stability and more energy-efficient regeneration. For the adsorption separation method, preferential CO2 capture from gas mixtures in one step is more energy-efficient for direct purification than light hydrocarbons, saving about 40% energy by eliminating energy-intensive post-regeneration processes such as countercurrent vacuum blowdown. Therefore, CO2-selective adsorbents are more sought-after than light hydrocarbon-selective adsorbents. Metal-organic frameworks (MOFs) have been demonstrated as outstanding physisorbents for CO2 capture due to their configurable channels for CO2 recognition, structural flexibility and large specific surface area. Many highly selective CO2 adsorption behaviors of MOFs have been reportedly achieved by precise modulation of pore size, pore chemistry or structural flexibility. In this review, we discuss the emerging development of MOFs for CO2-selective capture from different light hydrocarbon mixtures. The challenges of CO2 recognition and the strategies employed to achieve CO2 selectivity over light hydrocarbon mixtures by MOFs are summarized. In addition, the current challenges and prospects in the field of MOFs for CO2 capture are discussed and elaborated.

Fabrication of bimetallic Cu-Zn adsorbents with high dispersion by using confined space for gas adsorptive separation

Fabrication of bimetallic Cu-Zn adsorbents with high dispersion by using confined space for gas adsorptive separation

Valorization of shrimp shell biowaste for environmental remediation: Efficient contender for CO2 adsorption and separation

Over the years, single heteroatom-doped biowaste-derived activated carbons were studied for effective CO2 adsorption. However, binary or ternary heteroatoms-doping is equally important and could significantly affect the CO2 adsorption and flue gas (i.e., CO2/N2) separation. Herein, for the first time, shrimp shell-derived chitosan was used to design a series of ternary (N, S, O)-doped hierarchically porous carbons. The resultant carbons exhibit a large specific surface area (up to 2095 m2/g), micropore volume (up to 1.2647 cm3/g), and high heteroatoms content i.e., N up to 4.1 at. %, S up to 4.6 at. %, and O up to 13.4 at. %. Consequently, high CO2 uptake of 236.80 mg/g at 273 K/1 bar and an excellent CO2/N2 gas selectivity (84.3) was observed, attributed to the synergistic role of narrow micropores (<1 nm) and optimum heteroatom content. Furthermore, the stable CO2 adsorption-desorption cyclic behavior under flue gas conditions i.e., 15% CO2/85% N2 reveals the physisorption mechanism of CO2 adsorption and appears to be an energy-efficient regeneration process. Concluding, our work demonstrates a facile route of valorization of biowaste for environmental remediation to combat biowaste accumulation and mitigating atmospheric CO2 levels, simultaneously.

Microwave-assisted synthesis of Zr-based metal–organic framework (Zr-fum-fcu-MOF) for gas adsorption separation

A reliable strategy of Zr-based metal–organic framework (Zr-fum-fcu-MOF) was successfully achieved by microwave-assisted synthesis. Well-shaped and octahedral Zr-fum-fcu-MOF particles were obtained at 100 °C for 1.0 h with 150 equivalents of FA per ZrOCl2·8H2O. A clear difference for the adsorption behavior of iso-C4H10 (1.11 mmol/g) and n-C4H10 (3.56 mmol/g) was observed at 25 °C and 1.0 bar. Moreover, 3.16 mmol/g CO2 uptake was also obtained at 0 °C and 1.0 bar while the adsorption capacity of CH4 and N2 is 0.85 mmol/g and 0.21 mmol/g, respectively. This indicates that Zr-fum-fcu-MOF is a promising candidate for CO2 capture.

Ligand‐Conformer‐Induced Formation of Zirconium–Organic Framework for Methane Storage and MTO Product Separation

In pursuit of novel adsorbents with efficient adsorptive gas storage and separation capabilities remains highly desired and challenging. Although the documented zirconium-tricarboxylate-based metal-organic frameworks (MOFs) have displayed a variety of topologies encompassing underlying and geometry mismatch ones, the employed organic linkers are exclusively rigid and poorly presenting one type of conformation in the resultant structures. Herein, a used and semirigid tricarboxylate ligand of H3 TATAB was judiciously selected to isolate a zirconium-based spe-MOF after the preliminary discovery of srl-MOF. Single-crystal X-ray diffraction reveals that the fully deprotonated TATAB linker in spe-MOF exhibits two distinct conformers, concomitant with popular Oh and rare S6 symmetrical Zr6 molecular building blocks, generating an unprecedented (3,3,12,12)-c nondefault topology. Specifically, the spe-MOF exhibits structurally higher complexity, hierarchical micropores, open metal sites free and rich electronegative groups on the pore surfaces, leading to relatively high methane storage capacity without considering the missing-linker defects and efficient MTO product separation performance.

Ligand‐Conformer‐Induced Formation of Zirconium–Organic Framework for Methane Storage and MTO Product Separation

AbstractIn pursuit of novel adsorbents with efficient adsorptive gas storage and separation capabilities remains highly desired and challenging. Although the documented zirconium‐tricarboxylate‐based metal–organic frameworks (MOFs) have displayed a variety of topologies encompassing underlying and geometry mismatch ones, the employed organic linkers are exclusively rigid and poorly presenting one type of conformation in the resultant structures. Herein, a used and semirigid tricarboxylate ligand of H3TATAB was judiciously selected to isolate a zirconium‐based spe‐MOF after the preliminary discovery of srl‐MOF. Single‐crystal X‐ray diffraction reveals that the fully deprotonated TATAB linker in spe‐MOF exhibits two distinct conformers, concomitant with popular Oh and rare S6 symmetrical Zr6 molecular building blocks, generating an unprecedented (3,3,12,12)‐c nondefault topology. Specifically, the spe‐MOF exhibits structurally higher complexity, hierarchical micropores, open metal sites free and rich electronegative groups on the pore surfaces, leading to relatively high methane storage capacity without considering the missing‐linker defects and efficient MTO product separation performance.

Synergies between adsorption and energy conversion technologies

The analogy between heat uptake in a thermal regenerator and selective uptake of gas components in an adsorbent bed has inspired development of alternative architectures for adsorptive gas separation processes. An experimental laboratory sorption enhanced reaction approach for low pressure Haber-Bosch ammonia synthesis successfully used a Stirling engine mechanism to generate coordinated pressure swings and flow reversals of a thermally coupled PSA cycle. Attempts to apply similar mechanisms to air separation and hydrogen purification (rapid cycle PSA applications) motivated the development of very high surface area laminated parallel passage adsorbers for process intensification. Further efforts to develop compact PSA equipment for hydrogen purification and biogas upgrading led to practicable multiport rotary valves, enabling great simplification of PSA systems using multiple adsorbers. Higher degrees of simplicity and compactness for PSA and TSA processes were subsequently achieved using the laminated structured adsorbent in rotary adsorbers, now being used for post-combustion CO2 capture. Prospective applications in the energy conversion field include hydrogen recovery from SOFC anode tail gas, debottlenecking of MCFC carbon capture from flue gas, and recovery of tritium from fusion power plant breeder blanket helium purge gas. A major opportunity is identified for sorption-enhanced ammonia synthesis in the context of green hydrogen technologies.

КОНИЧЕСКИЙ СПИРАЛЬНЫЙ ДАТЧИК ВИБРАЦИИ С УЛУЧШЕННЫМИ МЕТРОЛОГИЧЕСКИМИ ХАРАКТЕРИСТИКАМИ

Relevance Currently, vibration of parts of various machines is widely used to increase the intensity and efficiency of technological processes for extracting raw materials and semi-finished products (granulating, drying, dissolving and leaching, extracting, adsorption gas separation, etc.). In other cases, on the contrary, it is necessary to eliminate the harmful effect of vibration on production processes to slow down the movement of units and mechanisms, to damp vibrations in various ways (controlled shock absorption systems, electromagnetic dampers, vibration protection, etc.). For active control of oscillatory processes occurring in these machines, vibration sensors directly connected to them are used. The main element of various vibration sensors is a sensitive element that converts mechanical movement into an electromagnetic signal, which allows it to be further processed. As sensitive elements of electromechanical transducers of vibration sensors for measuring small vibrations of the surfaces of vibration objects, classical strain gauge, magnetoelectric, magnetostrictive, piezoelectric and frequency-pulse elements are used. The design and creation of new vibration sensors with improved metrological characteristics is an urgent task for electrical engineering and electromechanics. In this work, the basic formulas and graphs of the experimental output data of the metrological characteristics of the new design of the vibration sensor are obtained. Aim of research To investigate the metrological characteristics of a new design of a conical spiral vibration sensor. Research methods To determine the oscillatory processes, the methods of the theory of electrical circuits, magnetic fields, methods of mathematical analysis and the theory of electrical oscillatory processes in (beats, induction, etc.) folded circuits are used. Results A new design of a conical spiral vibration sensor has been developed and its metrological characteristics have been investigated.