MUF-16: A Robust Metal-Organic Framework for Pre- and Post-Combustion Carbon Dioxide Capture

Investigating the Effect of Trace Levels of Manganese Ions During Solvothermal Synthesis of Massey University Framework-16 on CO2 Uptake Capacity

The effects of impurities on reaction precursors for metal-organic framework (MOF) synthesis have not been studied in extensive detail. The impact of these impurities can be an important factor while considering scale-up of these materials. In this work, we study the apparently positive impact of the presence of manganese ions for the synthesis of a Co-based MOF, Massey University Framework-16 (MUF-16). The presence of a trace amount of manganese in the reaction mixture led to consistently high CO2 uptake across multiple batches. Characterization including X-ray diffraction, scanning electron microscopy, Fourier transform infrared-attenuated total reflectance, ultraviolet-visible spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure spectroscopy led us to hypothesize that the differences in CO2 adsorption among materials with differing synthesis routes arise from variations in the local environment around the cobalt metal center. Aided by density functional theory calculations, we speculate that manganese ions get inserted into the structure during crystallization and act as catalysts for ligand substitution, improving the possibility for octahedral coordination of cobalt with the ligand, thus leading to Co-based pristine structures with higher CO2 uptakes.

Composite of MIL-101(Cr) with a Pyrrolidinium-Based Ionic Liquid Providing High CO2 Selectivity

Capturing CO₂ selectively from flue gas and natural gas addresses the criteria of a sustainable society. In this work, we incorporated an ionic liquid (IL) (1-methyl-1-propyl pyrrolidinium dicyanamide, [MPPyr][DCA]) into a metal organic framework (MOF), MIL-101(Cr), by wet impregnation and characterized the resulting [MPPyr][DCA]/MIL-101(Cr) composite in deep detail to identify the interactions between [MPPyr][DCA] molecules and MIL-101(Cr). Consequences of these interactions on the CO₂/N₂, CO₂/CH₄, and CH₄/N₂ separation performance of the composite were examined by volumetric gas adsorption measurements complemented by the density functional theory (DFT) calculations. Results showed that the composite offers remarkably high CO₂/N₂ and CH₄/N₂ selectivities of 19,180 and 1915 at 0.1 bar and 15 °C corresponding to 1144- and 510-times improvements, respectively, as compared to the corresponding selectivities of pristine MIL-101(Cr). At low pressures, these selectivities reached practically infinity, making the composite completely CO₂-selective over CH₄ and N₂. The CO₂/CH₄ selectivity was improved from 4.6 to 11.7 at 15 °C and 0.001 bar, yielding a 2.5-times improvement, attributed to the high affinity of [MPPyr][DCA] toward CO₂, validated by the DFT calculations. These results offer broad opportunities for the design of composites where ILs are incorporated into the pores of MOFs for high performance gas separation applications to address the environmental challenges.

An Upper Bound Visualization of Design Trade-Offs in Adsorbent Materials for Gas Separations: CO2 , N2 , CH4 , H2 , O2 , Xe, Kr, and Ar Adsorbents

The last 20 years have seen many publications investigating porous solids for gas adsorption and separation. The abundance of adsorbent materials (this work identifies 1608 materials for CO₂ /N₂ separation alone) provides a challenge to obtaining a comprehensive view of the field, identifying leading design strategies, and selecting materials for process modeling. In 2021, the empirical bound visualization technique was applied, analogous to the Robeson upper bound from membrane science, to alkane/alkene adsorbents. These bound visualizations reveal that adsorbent materials are limited by design trade-offs between capacity, selectivity, and heat of adsorption. The current work applies the bound visualization to adsorbents for a wider range of gas pairs, including CO₂ , N₂ , CH₄ , H₂ , Xe, O₂ , and Kr. How this visual tool can identify leading materials and place new material discoveries in the context of the wider field is presented. The most promising current strategies for breaking design trade-offs are discussed, along with reproducibility of published adsorption literature, and the limitations of bound visualizations. It is hoped that this work inspires new materials that push the bounds of traditional trade-offs while also considering practical aspects critical to the use of materials on an industrial scale such as cost, stability, and sustainability.

Carbon Capture Materials in Post-Combustion: Adsorption and Absorption-Based Processes

Global warming and climate changes are among the biggest modern-day environmental problems, the main factor causing these problems is the greenhouse gas effect. The increased concentration of carbon dioxide in the atmosphere resulted in capturing increased amounts of reflected sunlight, causing serious acute and chronic environmental problems. The concentration of carbon dioxide in the atmosphere reached 421 ppm in 2022 as compared to 280 in the 1800s, this increase is attributed to the increased carbon dioxide emissions from the industrial revolution. The release of carbon dioxide into the atmosphere can be minimized by practicing carbon capture utilization and storage methods. Carbon capture utilization and storage (CCUS) has four major methods, namely, pre-combustion, post-combustion, oxyfuel combustion, and direct air capture. It has been reported that applying CCUS can capture up to 95% of the produced carbon dioxide in running power plants. However, a reported cost penalty and efficiency decrease hinder the wide applicability of CCUS. Advancements in the CCSU were made in increasing the efficiency and decreasing the cost of the sorbents. In this review, we highlight the recent developments in utilizing both physical and chemical sorbents to capture carbon. This includes amine-based sorbents, blended absorbents, ionic liquids, metal-organic framework (MOF) adsorbents, zeolites, mesoporous silica materials, alkali-metal adsorbents, carbonaceous materials, and metal oxide/metal oxide-based materials. In addition, a comparison between recently proposed kinetic and thermodynamic models was also introduced. It was concluded from the published studies that amine-based sorbents are considered assuperior carbon-capturing materials, which is attributed to their high stability, multifunctionality, rapid capture, and ability to achieve large sorption capacities. However, more work must be done to reduce their cost as it can be regarded as their main drawback.

Insights into the thermodynamic-kinetic synergistic separation of propyne/propylene in anion pillared cage MOFs with entropy-enthalpy balanced adsorption sites

Propyne/propylene (C₃H₄/C₃H₆) separation is an important industrial process yet challenged by the trade-off of selectivity and capacity due to the molecular similarity. Herein, record C₃H₄/C₃H₆ separation performance is achieved by fine tuning the pore structure in anion pillared MOFs. SIFSIX-Cu-TPA (ZNU-2-Si) displays a benchmark C₃H₄ capacity (106/188 cm³ g^-1 at 0.01/1 bar and 298 K), excellent C₃H₄/C₃H₆ IAST selectivity (14.6-19.3) and kinetic selectivity, and record high C₃H₄/C₃H₆ (10/90) separation potential (36.2 mol kg^-1). The practical C₃H₄/C₃H₆ separation performance is fully demonstrated by breakthroughs under various conditions. 37.8 and 52.9 mol kg^-1 of polymer grade C₃H₆ can be produced from 10/90 and 1/99 C₃H₄/C₃H₆ mixtures. 4.7 mol kg^-1 of >99% purity C₃H₄ can be recovered by a stepped desorption process. Based on the in situ single crystal analysis and DFT calculation, an unprecedented entropy-enthalpy balanced adsorption pathway is discovered. MD simulation further confirmed the thermodynamic-kinetic synergistic separation of C₃H₄/C₃H₆ in ZNU-2-Si.

An Amine-Functionalized Ultramicroporous Metal-Organic Framework for Postcombustion CO2 Capture

Among the most promising methods by which to capture CO₂ from flue gas, the emission of which has accelerated global warming, is energy-efficient physisorption using metal-organic framework (MOF) adsorbents. Here, we present a novel cuprous-based ultramicroporous MOF, Cu(adci)-2 (adci^- = 2-amino-4,5-dicyanoimidazolate), which was rationally synthesized by combining two strategies to design MOF physisorbents for enhanced CO₂ capturing, i.e., aromatic amine functionalization and the introduction of ultramicroporosity (pore size <7 Å). Synchrotron powder X-ray diffraction and a Rietveld analysis reveal that the Cu(adci)-2 structure has one-dimensional square-shaped channels, in each of which all affiliated ligands, specifically NH₂ groups at the 2-position of the imidazolate ring, have the same orientation, with a pair of NH₂ groups therefore facing each other on opposite sides of the channel walls. While Cu(adci)-2 exhibits a high CO₂ adsorption capacity (2.01 mmol g^-1 at 298 K and 15 kPa) but a low zero-coverage isosteric heat of adsorption (27.5 kJ mol^-1), breakthrough experiments under dry and 60% relative humidity conditions show that its CO₂ capture ability is retained even in the presence of high amounts of moisture. In a Monte Carlo simulation and a radial distribution analysis, the preferential CO₂ binding site of Cu(adci)-2 was predicted to be between two ligands, forming a sandwich-like structure and implying that its CO₂ adsorption properties originate from the enhancement of Lewis base-acid and London dispersion interactions due to the amino groups and ultramicroporosity, respectively.

Aluminum formate, Al(HCOO)3: An earth-abundant, scalable, and highly selective material for CO2 capture

A combination of gas adsorption and gas breakthrough measurements show that the metal-organic framework, Al(HCOO)3 (ALF), which can be made inexpensively from commodity chemicals, exhibits excellent CO2 adsorption capacities and outstanding CO2/N2 selectivity that enable it to remove CO2 from dried CO2-containing gas streams at elevated temperatures (323 kelvin). Notably, ALF is scalable, readily pelletized, stable to SO2 and NO, and simple to regenerate. Density functional theory calculations and in situ neutron diffraction studies reveal that the preferential adsorption of CO2 is a size-selective separation that depends on the subtle difference between the kinetic diameters of CO2 and N2. The findings are supported by additional measurements, including Fourier transform infrared spectroscopy, thermogravimetric analysis, and variable temperature powder and single-crystal x-ray diffraction.

Interlayer Expansion in a Layered Metal-Organic Framework Enhances CO2 Capture and CO2/N2 Separation

Developing efficient CO 2 adsorbent materials and technologies is significant to reduce the increasing greenhouse gases concentration in the atmosphere. Herein, a layered MOF with a porous kagomé lattice (kgm), which owned three phases (kgm-1, kgm-2, and kgm-3) via interlayer expansion, was evaluated as a promising CO 2 capture and separation material by using grand canonical Monte Carlo simulations. Results showed that the interlayer expansion provided additional pore volume, which played a considerable role in CO 2 adsorption and separation. The CO 2 adsorption capacity and CO 2 /N 2 selectivity followed the sequence kgm-3 > kgm-2 > kgm-1, and kgm-3 exhibited an excellent CO 2 adsorption capacity of 8.7 mmol g -1 at 1 bar with a CO 2 /N 2 selectivity of 130.3 at 20 bar and 298 K. Gas distribution analysis showed that CO 2 and N 2 are adsorbed only in the channels in kgm-1, whereas they could be adsorbed between layers in kgm-2 and kgm-3 due to the interlayer expansion. The adsorption heat and interactions between CO 2 and frameworks were analyzed to elucidate the effect of interlayer expansion. Results of this work highlighted that appropriate interlayer expansion can be an effective approach for framework adsorbents to improve CO 2 capture ability and separation performance at the same time.

Comprehensive Pore Tuning in an Ultrastable Fluorinated Anion Cross-Linked Cage-Like MOF for Simultaneous Benchmark Propyne Recovery and Propylene Purification

Propyne/propylene (C₃ H₄ /C₃ H₆ ) separation is an important but challenging industrial process to produce polymer-grade C₃ H₆ and recover high-purity C₃ H₄ . Herein, we report an ultrastable TiF₆ ^2- anion cross-linked metal-organic framework (ZNU-2) with precisely controlled pore size, shape and functionality for benchmark C₃ H₄ storage (3.9/7.7 mmol g^-1 at 0.01/1.0 bar and 298 K) and record high C₃ H₄ /C₃ H₆ (10/90) separation potential (31.0 mol kg^-1 ). The remarkable C₃ H₄ /C₃ H₆ (1/99, 10/90, 50/50) separation performance was fully demonstrated by simulated and experimental breakthroughs under various conditions with excellent recyclability and high productivity (42 mol kg^-1 ) of polymer-grade C₃ H₆ from a 1/99 C₃ H₄ /C₃ H₆ mixture. A modelling study revealed that the symmetrical spatial distribution of six TiF₆ ^2- on the icosahedral cage surface provides two distinct binding sites for C₃ H₄ adsorption: one serves as a tailored single C₃ H₄ molecule trap and the other boosts C₃ H₄ accommodation by cooperative host-guest and guest-guest interactions.

Advanced strategies in Metal-Organic Frameworks for CO2 Capture and Separation

The continuous carbon dioxide (CO₂ ) gas emissions associated with fossil fuel production, valorization, and utilization are serious challenges to the global environment. Therefore, several developments of CO₂ capture, separation, transportation, storage, and valorization have been explored. Consequently, we documented a comprehensive review of the most advanced strategies adopted in metal-organic frameworks (MOFs) for CO₂ capture and separation. The enhancements in CO₂ capture and separation are generally achieved due to the chemistry of MOFs by controlling pore window, pore size, open-metal sites, acidity, chemical doping, post or pre-synthetic modifications. The chemistry of defects engineering, breathing in MOFs, functionalization in MOFs, hydrophobicity, and topology are the salient advanced strategies, recently reported in MOFs for CO₂ capture and separation. Therefore, this review summarizes MOF materials' advancement explaining different strategies and their role in the CO₂ mitigations. The study also provided useful insights into key areas for further investigations.