Selective Adsorptive Separation of CO2/CH4 and CO2/N2 by a Water Resistant Zirconium–Porphyrin Metal–Organic Framework

Challenges and insights in CO2 adsorption using N-doped carbons under realistic conditions

N-doped carbon was synthesized from an N-rich precursor, chitosan, and systematically compared with materials reported in the literature to evaluate its potential for CO2 capture in post-combustion scenarios and biogas upgrading under realistic conditions. The synthesized material, with 5.2 at.% nitrogen and a high surface area of 1000 m2 g-1, exhibited moderate CO2 uptake (2.6 mmol g-1) and an ideal CO2/N2 selectivity (SCO2/N2) of 20.4 under post-combustion conditions (15% vol. CO2, 1 bar, 298 K). For equimolar CO2/CH4 mixtures (50% vol. CO2), the material showed an ideal CO2/CH4 selectivity (SCO2/CH4) of 3.3 under the same conditions. When exposed to humidity, the material adsorbed CO2 efficiently at low relative humidity (RH) levels, but suffered a significant loss of capacity at 30% RH, attributed to water-induced site blocking. A comparison with other N-doped/enriched materials from the literature revealed difficulties in correlating CO2 adsorption and selectivity with specific material properties, such as surface area and nitrogen content. This underscores the complexity of defining universal design principles for CO2 capture.

Solvent Regulation in Layered Zn-MOFs for C2H2/CO2 and CO2/CH4 Separation

The development of alternative adsorptive separation technologies is extremely significant for the separation of C2H2/CO2 and CO2/CH4 in the chemical industry. Emerging metal-organic frameworks (MOFs) have shown great potential as adsorbents for gas adsorption and separation. Herein, we synthesized two layered Zn-MOFs, UPC-96 and UPC-97, with 1,2,4,5-tetrakis(4-carboxyphenyl)-3,6-dimethylbenzene (TCPB-Me) as a ligand via the solvent regulation of the pH values. UPC-96 with a completely deprotonated ligand was obtained without the addition of acid, exhibiting two different channels with cross-sectional sizes of 11.6 × 7.1 and 8.3 × 5.2 Å2. In contrast, the addition of acid led to the partial deprotonation of the ligand and afforded UPC-97 two types of channels with cross-sectional sizes of 11.5 × 5.7 and 7.4 × 3.9 Å2. Reversible N2 adsorption isotherms at 77 K confirmed their permanent porosity, and the differentiated single-component C2H2, CO2, and CH4 adsorption isotherms indicated their potential in C2H2/CO2 and CO2/CH4 separation.

Molecular Imprinting as a Tool for Exceptionally Selective Gas Separation in Nanoporous Polymers

The alarming rise in atmospheric CO2 levels, primarily driven by fossil fuel combustion and industrial processes, has become a major contributor to global climate change. Effective CO2 capture technologies are urgently needed, particularly for the selective removal of CO2 from industrial gas streams, such as flue gas and biogas, which often contain impurities like N2 and CH4. In this study, we report the design and synthesis of novel molecularly imprinted polymers (MIPs) using 4-vinylpyridine (4VP) and methacrylic acid (MAA) as functional monomers, and thiophene (Th) and formaldehyde (HC) as molecular templates. The MIPs were specifically engineered to create selective molecular cavities within a nanoporous polymer matrix for the efficient capture of CO2. By adjusting the molar ratios of the template to functional monomers, we optimized the molecular imprinting process to enhance CO2 selectivity over N2 and CH4. The resulting MIPs exhibited outstanding performance, with a maximum CO2/N2 selectivity of 153 at 25 bar and CO2/CH4 selectivity of 25.3 at 1 bar, significantly surpassing previously reported porous polymers and metal-organic frameworks (MOFs) under similar conditions. Furthermore, we conducted heat of adsorption studies, which revealed the strong and selective interaction of CO2 with the imprinted cavities, confirming the superior adsorption properties of the synthesized MIPs. The study demonstrates that molecular imprinting can effectively enhance both CO2 capture capacity and selectivity, providing a cost-efficient and scalable solution for industrial CO2 separation and purification processes.

Adsorption in Pyrene-Based Metal-Organic Frameworks: The Role of Pore Structure and Topology

Pore topology and chemistry play crucial roles in the adsorption characteristics of metal-organic frameworks (MOFs). To deepen our understanding of the interactions between MOFs and CO2 during this process, we systematically investigate the adsorption properties of a group of pyrene-based MOFs. These MOFs feature Zn(II) as the metal ion and employ a pyrene-based ligand, specifically 1,3,6,8-tetrakis(p-benzoic acid)pyrene (TBAPy). Including different additional ligands leads to frameworks with distinctive structural and chemical features. By comparing these structures, we could isolate the role that pore size, the presence of open-metal sites (OMS), metal-oxygen bridges, and framework charges play in the CO2 adsorption of these MOFs. Frameworks with constricted pore structures display a phenomenon known as the confinement effect, fostering stronger MOF-CO2 interactions and higher uptakes at low pressures. In contrast, entropic effects dominate at elevated pressures, and the MOF's pore volume becomes the driving factor. Through analysis of the CO2 uptakes of the benchmark materials ─some with narrower pores and others with larger pore volumes─it becomes evident that structures with narrower pores and high binding energies excel at low pressures. In contrast, those with larger volumes perform better at elevated pressures. Moreover, this research highlights that open-metal sites and inherent charges within the frameworks of ionic MOFs stand out as CO2-philic characteristics.

Low-Hydrophilic HKUST-1/Polymer Extrudates for the PSA Separation of CO2/CH4

HKUST-1 is an MOF adsorbent industrially produced in powder form and thus requires a post-shaping process for use as an adsorbent in fixed-bed separation processes. HKUST-1 is also sensitive to moisture, which degrades its crystalline structure. In this work, HKUST-1, in the form of crystalline powder, was extruded into pellets using a hydrophobic polymeric binder to improve its moisture stability. Thermoplastic polyurethane (TPU) was used for that purpose. The subsequent HKUST-1/TPU extrudate was then compared to HKUST-1/PLA extrudates synthesized with more hydrophilic polymer: polylactic acid (PLA), as the binder. The characterization of the composites was determined via XRD, TGA, SEM-EDS, and an N2 adsorption isotherm analysis. Meanwhile, the gas-separation performances of HKUST-1/TPU were investigated and compared with HKUST-1/PLA from measurements of CO2 and CH4 isotherms at three different temperatures, up to 10 bars. Lastly, the moisture stability of the composite materials was investigated via an aging analysis during storage under humid conditions. It is shown that HKUST-1's crystalline structure was preserved in the HKUST-1/TPU extrudates. The composites also exhibited good thermal stability under 523 K, whilst their textural properties were not significantly modified compared with the pristine HKUST-1. Furthermore, both extrudates exhibited larger CO2 and CH4 adsorption capacities in comparison to the pristine HKUST-1. After three months of storage under atmospheric humid conditions, CO2 adsorption capacities were reduced to only 10% for HKUST-1/TPU, whereas reductions of about 25% and 54% were observed for HKUST-1/PLA and the pristine HKUST-1, respectively. This study demonstrates the interest in shaping MOF powders by extrusion using a hydrophobic thermoplastic binder to operate adsorbents with enhanced moisture stability in gas-separation columns.

Water-stable metal-organic frameworks (MOFs): rational construction and carbon dioxide capture

Metal-organic frameworks (MOFs) are considered to be a promising porous material due to their excellent porosity and chemical tailorability. However, due to the relatively weak strength of coordination bonds, the stability (e.g., water stability) of MOFs is usually poor, which severely inhibits their practical applications. To prepare water-stable MOFs, several important strategies such as increasing the bonding strength of building units and introducing hydrophobic units have been proposed, and many MOFs with excellent water stability have been prepared. Carbon dioxide not only causes a range of climate and health problems but also is a by-product of some important chemicals (e.g., natural gas). Due to their excellent adsorption performances, MOFs are considered as a promising adsorbent that can capture carbon dioxide efficiently and energetically, and many water-stable MOFs have been used to capture carbon dioxide in various scenarios, including flue gas decarbonization, direct air capture, and purified crude natural gas. In this review, we first introduce the design and synthesis of water-stable MOFs and then describe their applications in carbon dioxide capture, and finally provide some personal comments on the challenges facing these areas.

Trajectory in biological metal-organic frameworks: Biosensing and sustainable strategies-perspectives and challenges

The ligand attribute of biomolecules to form coordination bonds with metal ions led to the discovery of a novel class of materials called biomolecule-associated metal-organic frameworks (Bio-MOFs). These biomolecules coordinate in multiple ways and provide versatile applications. Far-spread bio-ligands include nucleobases, amino acids, peptides, cyclodextrins, saccharides, porphyrins/metalloporphyrin, proteins, etc. Low-toxicity, self-assembly, stability, designable and selectable porous size, the existence of rigid and flexible forms, bio-compatibility, and synergistic interactions between metal ions have led Bio-MOFs to be commercialized in industries such as sensors, food, pharma, and eco-sensing. The rapid growth and commercialization are stunted by absolute bio-compatibility issues, bulk morphology that makes it rigid to alter shape/porosity, longer reaction times, and inadequate research. This review elucidates the structural vitality, biocompatibility issues, and vital sensing applications, including challenges for incorporating bio-ligands into MOF. Critical innovations in Bio-MOFs' applicative spectrum, including sustainable food packaging, biosensing, insulin and phosphoprotein detection, gas sensing, CO2 capture, pesticide carriers, toxicant adsorptions, etc., have been elucidated. Emphasis is placed on biosensing and biomedical applications with biomimetic catalysis and sensitive sensor designing.

The Role of Water in Carbon Dioxide Adsorption in Porphyrinic Metal-Organic Frameworks

Capturing and converting CO2 through artificial photosynthesis using photoactive, porous materials is a promising approach for addressing increasing CO2 concentrations. Porphyrinic Zr-based metal-organic frameworks (MOFs) are of particular interest as they incorporate a photosensitizer in the porous structure. Herein, the initial step of the artificial photosynthesis is studied: CO2 sorption and activation in the presence of water. A combined vibrational and visible spectroscopic approach was used to monitor the adsorption of CO2 into PCN-222 and PCN-223 MOFs, and the photophysical changes of the porphyrinic linker as a function of water concentration. A shift in CO2 sorption site and bending of the porphyrin macrocycle in response to humidity was observed, and CO2/H2O competition experiments revealed that the exchange of CO2 with H2O is pore-size dependent. Therefore, humidity and pore-size can be used to tune CO2 sorption, CO2 capacity, and light harvesting in porphyrinic MOFs, which are key factors for CO2 photoreduction.

Enhanced the Efficiency of Electrocatalytic CO2 -to-CO Conversion by Cd Species Anchored into Metal-Organic Framework

Metal-organic frameworks (MOFs) as a promising platform for electrocatalytic CO2 conversion are still restricted by the low efficiency or unsatisfied selectivity for desired products. Herein, zirconium-based porphyrinic MOF hollow nanotubes with Cd sites (Cd-PCN-222HTs) are reported for electrocatalytic CO2 -to-CO conversion. The dispersed Cd species are anchored in PCN-222HTs and coordinated by N atoms of porphyrin structures. It is discovered that Cd-PCN-222HTs have glorious electrocatalytic activity for selective CO production in ionic liquid-water (H2 O)-acetonitrile (MeCN) electrolyte. The CO Faradaic efficiency (FECO ) of >80% could be maintained in a wide potential range from -2.0 to -2.4 V versus Ag/Ag+ , and the maximum current density could reach 68.0 mA cm-2 at -2.4 V versus Ag/Ag+ with a satisfied turnover frequency of 26 220 h-1 . The enhanced efficiency of electrocatalytic CO2 conversion of Cd-PCN-222HTs is closely related to its hollow structure, anchored Cd species, and good synergistic effect with electrolyte. The density functional theory calculations indicate that the dispersed Cd sites anchored in PCN-222HTs not only favor the formation of *COOH intermediate but also hinder the hydrogen evolution reaction, resulting in high activity of electrocatalytic CO2 -to-CO conversion.

CO2 Separation by Imide/Imine Organic Cages

Two novel imide/imine-based organic cages have been prepared and studied as materials for the selective separation of CO2 from N2 and CH4 under vacuum swing adsorption conditions. Gas adsorption on the new compounds showed selectivity for CO2 over N2 and CH4 . The cages were also tested as fillers in mixed-matrix membranes for gas separation. Dense and robust membranes were obtained by loading the cages in either Matrimid® or PEEK-WC polymers. Improved gas-transport properties and selectivity for CO2 were achieved compared to the neat polymer membranes.

Ultrathin porous amine-based solid adsorbent incorporated zeolitic imidazolate framework-8 membrane for gas separation

A novel gas separation approach is proposed in this work by combining an amine-based solid adsorbent with a zeolitic imidazolate framework-8 (ZIF-8) membrane. This was achieved by incorporating the amine-based solid adsorbent during the fabrication of the ZIF-8 membrane on a macroporous substrate. An amine-based solid adsorbent was prepared using porous ZIF-8-3-isocyanatopropyltrimethoxysilane (IPTMS) and N-[(3-trimethoxysilyl)propyl]diethylenetriamine (3N-APS) amine compounds. The as-prepared porous amine-based solid adsorbent (denoted as ZIF-8-IPTMS-3N-APS) possessed excellent adsorptive CO₂/N₂ and CO₂/CH₄ separation performances. As the adsorbent needs to be regenerated, this could indicate that the CO₂ adsorption separation process cannot be continuously operated. In this work, an amine-based solid adsorbent was applied during the preparation of the ZIF-8 membranes owing to the following reasons: (i) gas separation by the membrane can be operated continuously; (ii) the amino group provides a heterogeneous nucleation site for ZIF-8 to grow; and (iii) the reparation of surface defects on the macroporous substrate can be performed prior to the growth of the ZIF-8 membrane. Herein, the ZIF-8 membrane was successfully fabricated, and it possessed excellent CO₂/CH₄, CO₂/N₂, and H₂/CH₄ separation performances. The 0.6 μm ultrathin ZIF-8 membrane demonstrated a high CO₂ permeance of 4.75 × 10^-6 mol m^-2 s^-1 Pa^-1 at 35 °C and 0.1 MPa, and ideal CO₂/N₂ and CO₂/CH₄ selectivities of 4.67 and 6.02, respectively. Furthermore, at 35 °C and 0.1 MPa, the ideal H₂/CH₄ selectivity of the ZIF-8 membrane reached 31.2, and a significantly high H₂ permeance of 2.45 × 10^-5 mol m^-2 s^-1 Pa^-1.

3D Metal-Organic Frameworks Based on Co(II) and Bithiophendicarboxylate: Synthesis, Crystal Structures, Gas Adsorption, and Magnetic Properties

Three new 3D metal-organic porous frameworks based on Co(II) and 2,2′-bithiophen-5,5′-dicarboxylate (btdc²⁻) [Co₃(btdc)₃(bpy)₂]·4DMF, 1; [Co₃(btdc)₃(pz)(dmf)₂]·4DMF·1.5H₂O, 2; [Co₃(btdc)₃(dmf)₄]∙2DMF∙2H₂O, 3 (bpy = 2,2′-bipyridyl, pz = pyrazine, dmf = N,N-dimethylformamide) were synthesized and structurally characterized. All compounds share the same trinuclear carboxylate building units {Co₃(RCOO)₆}, connected either by btdc^2– ligands (1, 3) or by both btdc^2– and pz bridging ligands (2). The permanent porosity of 1 was confirmed by N₂, O₂, CO, CO₂, CH₄ adsorption measurements at various temperatures (77 K, 273 K, 298 K), resulted in BET surface area 667 m²⋅g⁻¹ and promising gas separation performance with selectivity factors up to 35.7 for CO₂/N₂, 45.4 for CO₂/O₂, 20.8 for CO₂/CO, and 4.8 for CO₂/CH₄. The molar magnetic susceptibilities χ_p(T) were measured for 1 and 2 in the temperature range 1.77–330 K at magnetic fields up to 10 kOe. The room-temperature values of the effective magnetic moments for compounds 1 and 2 are μ_eff (300 K) ≈ 4.93 μ_B. The obtained results confirm the mainly paramagnetic nature of both compounds with some antiferromagnetic interactions at low-temperatures T < 20 K in 2 between the Co(II) cations separated by short pz linkers. Similar conclusions were also derived from the field-depending magnetization data of 1 and 2.

Multiple Functions of Gas Separation and Vapor Adsorption in a New MOF with Open Tubular Channels

Separation or purification is one of the difficult problems in the petrochemical industry. To help solve the difficulty of separation or purification for C₂H₂/CO₂ and C₂H_n/CH₄ in the chemical industry, we synthesized a new metal-organic framework (MOF), [Ni(dpip)]·2.5DMF·H₂O (1), by a bipyridyl-substituted isophthalic acid ligand. The MOF includes two types of one-dimensional (1D) tubular channels with different sizes and porous environments. The unique tubular channels lead to not only remarkable gas sorption capacity of C₂H₄, C₂H₂, and CO₂, but also good selectivity for C₂H₂/CH₄, C₂H₂/CH₄, CO₂/CH₄, and C₂H₂/CO₂, as demonstrated by single-component sorption isotherm results, ideal adsorbed solution theory calculations, and dynamic breakthrough curves. Grand canonical Monte Carlo (GCMC) simulation reveals preferential adsorption sites in the MOF for CO₂, C₂H₂, and C₂H₄. The MOF also exhibits an obvious size-selective absorption effect on vapor molecules.

Preferential adsorption of ethane over ethylene on a Zr-based metal–organic framework: impacts of C–H⋯N hydrogen bonding

C–H⋯N interactions are more important than C–H⋯π interactions for ethane-selective adsorption.

Augmenting the Carbon Dioxide Uptake and Selectivity of Metal-Organic Frameworks by Metal Substitution: Molecular Simulations of LMOF-202

Metal organic frameworks (MOFs) are promising porous materials for the adsorption of CO_2. Here, we report the study of a luminescent MOF (LMOF), called LMOF-202. We have employed Grand Canonical Monte Carlo (GCMC) simulations to understand and explain the adsorption phenomena inside LMOF-202, and based on the phenomena happening at the molecular level, we have varied the metal ions in LMOF-202 to increase the CO₂ affinity and selectivity of the material. We show that the CO₂ adsorption capacity and selectivity can be increased by approximately 1.5 times at 1 bar and 298 K by changing the metal ion from Zn to Ba. We also report the feasibility of using this material to capture CO₂ from flue gas under realistic conditions (1 bar and 298 K). This work shows that LMOF-202 merits further consideration as a carbon capture adsorbent.