Water Stability and Competition Effects Toward CO2Adsorption on Metal Organic Frameworks

Probing the stability of metal-organic frameworks by structure-responsive mass spectrometry imaging

The widespread application of metal-organic frameworks (MOFs) is seriously hindered by their structural instability and it is still very challenging to probe the stability of MOFs during application by current techniques. Here, we report a novel structure-responsive mass spectrometry (SRMS) imaging technique to probe the stability of MOFs. We discovered that intact CuBTC (as a model of MOFs) could generate the characteristic peaks of organic ligands and carbon cluster anions in laser desorption/ionization mass spectrometry, but these peaks were significantly changed when the structure of CuBTC was dissociated, thus enabling a label-free probing of the stability. Furthermore, SRMS can be performed in imaging mode to visualize the degradation kinetics and reveal the spatial heterogeneity of the stability of CuBTC. This technique was successfully applied in different application scenarios (in water, moist air, and CO2) and also validated with different MOFs. It thus provides a versatile new tool for better design and application of environment-sensitive materials.

A metal-OH group modification strategy to prepare highly-hydrophobic MIL-53-Al for efficient acetone capture under humid conditions

A series of highly-hydrophobic MIL-53-Al (MIL = Materials of Institut Lavoisier) frameworks synthesized via decoration of the Al-OH groups by alkyl phosphonic acid were developed as adsorbents for removing acetone from humid gas streams. The newly prepared materials were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), N₂ adsorption-desorption and thermogravimetric analysis (TGA). Their adsorption behaviors toward acetone vapor under dry and wet conditions were studied subsequently. Results showed that alkyl phosphonic acid was successfully grafted into MIL-53-Al skeleton through coordinating interaction with Al³⁺ generating MIL-53-Al@C_x (x = 12, 14, 18). The MIL-53-Al@C_x exhibited similar crystal structure and thermal stability to parent MIL-53-Al. Furthermore, the modified materials showed significantly enhanced hydrophobicity. The water vapor uptake of MIL-53-Al@C₁₄ decreased by 72.55% at 75% relative humidity (RH). Dynamic adsorption experiments demonstrated that water vapor had almost no effect on the acetone adsorption performance of MIL-53-Al@C₁₄. Under the condition of 90% RH, the acetone adsorption capacity of MIL-53-Al@C₁₄ was 102.98% higher than that of MIL-53-Al. Notably, MIL-53-Al@C₁₄ presented excellent adsorption reversibility and regeneration performance in 10 adsorption-desorption cycles. Taken together, the strategy of metal-OH group modification is an attractive way to improve the acetone adsorption performance over metal-organic frameworks (MOFs) under humid conditions. Besides, MIL-53-Al@C₁₄ would be deemed as a promising candidate for capturing acetone in high moisture environment.

Effective Magnetic MOFs Adsorbent for the Removal of Bisphenol A, Tetracycline, Congo Red and Methylene Blue Pollutions

A magnetic metal−organic frameworks adsorbent (Fe₃O₄@MIL-53(Al)) was prepared by a typical solvothermal method for the removal of bisphenol A (BPA), tetracycline (TC), congo red (CR), and methylene blue (MB). The prepared Fe₃O₄@MIL-53(Al) composite adsorbent was well characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), and fourier transform infrared spectrometer (FTIR). The influence of adsorbent quantity, adsorption time, pH and ionic strength on the adsorption of the mentioned pollutants were also studied by a UV/Vis spectrophotometer. The adsorption capacities were found to be 160.9 mg/g for BPA, 47.8 mg/g for TC, 234.4 mg/g for CR, 70.8 mg/g for MB, respectively, which is superior to the other reported adsorbents. The adsorption of BPA, TC, and CR were well-fitted by the Langmuir adsorption isotherm model, while MB followed the Freundlich model, while the adsorption kinetics data of all pollutants followed the pseudo-second-order kinetic models. The thermodynamic values, including the enthalpy change (ΔH°), the Gibbs free energy change (ΔG°), and entropy change (ΔS°), showed that the adsorption processes were spontaneous and exothermic entropy-reduction process for BPA, but spontaneous and endothermic entropy-increasing processes for the others. The Fe₃O₄@MIL-53(Al) was also found to be easily separated after external magnetic field, can be a potential candidate for future water treatment.

Understanding the Effect of Water on CO2 Adsorption

Carbon capture from large sources and ambient air is one of the most promising strategies to curb the deleterious effect of greenhouse gases. Among different technologies, CO₂ adsorption has drawn widespread attention mostly because of its low energy requirements. Considering that water vapor is a ubiquitous component in air and almost all CO₂-rich industrial gas streams, understanding its impact on CO₂ adsorption is of critical importance. Owing to the large diversity of adsorbents, water plays many different roles from a severe inhibitor of CO₂ adsorption to an excellent promoter. Water may also increase the rate of CO₂ capture or have the opposite effect. In the presence of amine-containing adsorbents, water is even necessary for their long-term stability. The current contribution is a comprehensive review of the effects of water whether in the gas feed or as adsorbent moisture on CO₂ adsorption. For convenience, we discuss the effect of water vapor on CO₂ adsorption over four broadly defined groups of materials separately, namely (i) physical adsorbents, including carbons, zeolites and MOFs, (ii) amine-functionalized adsorbents, and (iii) reactive adsorbents, including metal carbonates and oxides. For each category, the effects of humidity level on CO₂ uptake, selectivity, and adsorption kinetics under different operational conditions are discussed. Whenever possible, findings from different sources are compared, paying particular attention to both similarities and inconsistencies. For completeness, the effect of water on membrane CO₂ separation is also discussed, albeit briefly.

Effect of diol isomer/water mixtures on the stability of Zn-MOF-74

The stability of metal-organic frameworks is a key factor in many applications in some fields that require working under harsh conditions. It is known that a large number of MOFs are vulnerable to humid air. It means that when they are exposed to water, a structural collapse of the crystal happens. In this work, Molecular Dynamics simulations using a reactive force field have been performed to study the stability of MOF-74 against the adsorption of catechol, resorcinol and hydroquinone in the presence of water. We reproduced the water instability of Zn-MOF-74 and we studied the resistance of the structure. Our simulations showed that the three isomers generate a volume change in the framework but the structural collapse does not happen. In contrast, for water-isomer mixtures, there is structural collapse. Not only do catechol, resorcinol and hydroquinone not behave as stabilizing agents but they do enhance the hydration effect on the structure.

Remarkable adsorption performance of MOF-199 derived porous carbons for benzene vapor

Volatile organic compounds (VOCs) are perceived as serious pollutants due to their great threat to both environment and human health. Recovery and removal of VOCs is of great significance. Herein, novel MOF-199 derived porous carbon materials (MC-T-n) were prepared by using MOF-199 as precursor, glucose as additional carbon source and KOH as activator, and then characterized. Adsorption performance of MC-T-n materials for benzene vapor was investigated. Isotherms of MC-T-n samples towards benzene and water vapor were measured. The adsorption selectivities of benzene/water were estimated by DIH (difference of the isosteric heats) equation. Results indicated that BET surface area and pore volume of MC-T-n materials reached separately 2320 m²/g and 1.05 m³/g. Benzene adsorption capacity of MC-T-n materials reached as high as 12.8 mmol/g at 25 °C, outperforming MOF-199 and some conventional adsorbents. Moreover, MC-T-n materials presented type-V isotherms of water vapor, suggesting their excellent water resistance. The isosteric heats of benzene adsorption on MC-500-6 were much greater than that of water adsorption, leading to a preferential adsorption for C₆H₆ over H₂O. The adsorption selectivity of C₆H₆/H₂O on MC-500-6 reached up to 16.3 superior to some previously reported MOFs. Therefore, MC-500-6 was a promising candidate for VOC adsorption and seperation. This study provides a strong foundation for MOF derived porous carbons as adsorbents for VOC removal.

Structure stability of HKUST-1 towards water and ethanol and their effect on its CO2 capture properties

Water and ethanol stabilities of the crystal structure of the Cu-based metal-organic framework (MOF) HKUST-1 have been investigated. Vapour (water and ethanol) sorption isotherms and cyclability were measured by a dynamic strategy. The ethanol sorption capacity of HKUST-1 at 303 K remained unchanged contrasting water sorption (which decreased along with the sorption experiment time). Considering the binding energy of each sorbate with the open Cu(ii) sites, obtained by the use of diffusion coefficients, we showed the superior crystal stability of the HKUST-1 framework towards ethanol. Finally, a small quantity of ethanol (pre-adsorbed) slightly enhanced CO₂ capture without crystal structure degradation.

Theoretical and experimental studies on three water-stable, isostructural, paddlewheel based semiconducting metal–organic frameworks

Theoretical and experimental studies were carried out on three isostructural, water stable, visible light absorbing MOFs, [M₂(TCS)(BPY)] (M = Co(1), Ni(2) and Cu(3); H₄TCS = tetrakis(4-carboxyphenyl) silane, BPY = 4,4′-bipyridine).

Moisture-Stable Zn(II) Metal-Organic Framework as a Multifunctional Platform for Highly Efficient CO2 Capture and Nitro Pollutant Vapor Detection

A moisture-stable three-dimensional (3D) metal-organic framework (MOF), {(Me2NH2)[Zn2(bpydb)2(ATZ)](DMA)(NMF)2}n (1, where bpydb = 4,4'-(4,4'-bipyridine-2,6-diyl)dibenzoate, ATZ = deprotonated 5-aminotetrazole, DMA = N,N-dimethylacetamide, and NMF = N-methylformamide), with uncoordinated N-donor sites and charged framework skeleton was fabricated. This MOF exhibits interesting structural dynamic upon CO2 sorption at 195 K and high CO2/N2 (127) and CO2/CH4 (131) sorption selectivity at 298 K and 1 bar. Particularly, its CO2/CH4 selectivity is among the highest MOFs for selective CO2 separation. The results of Grand Canonical Monte Carlo (GCMC) simulation indicate that the polar framework contributes to the strong framework-CO2 binding at zero loading, and the tetrazole pillar contributes to the high CO2 uptake capacity at high loading. Furthermore, the solvent-responsive luminescent properties of 1 indicate that it could be utilized as a fluorescent sensor to detect trace amounts of nitrobenzene in both solvent and vapor systems.

Microwave-assisted ionothermal synthesis of a water-stable Eu-coordination polymer: a Ba2+ ion detector and fluorescence thermometer

Presented is a water-stable Eu-coordination polymer, namely [HMIm]Eu(DHBDC)₂, capable of detecting Ba²⁺ in aqueous solution and acting as a two-PL transition fluorescence thermometer.

Accelerated degradation of MOFs under flue gas conditions

The zero length column (ZLC) technique is used to investigate the stability of Mg- and Ni-CPO-27 metal–organic framework (MOF) crystals in the presence of water and humid flue gas. The design of the ZLC enables the stability test to be conducted over a considerably shorter time period and with lower gas consumption than other conventional techniques. A key advantage over other experimental methods for testing the stability of adsorbents is the fact that the ZLC allows us to quantify the amount adsorbed of every component present in the gas mixture. The developed protocol is based on a two-stage stability test. The samples were first exposed to a humid carbon dioxide and helium mixture in order to study the effect of water on the carbon dioxide adsorption capacity of the samples. In the second stage the samples were exposed to a flue gas mixture containing water. From the preliminary water stability test, the Ni-sample exhibited the highest tolerance to the presence of water, retaining approximately 85% of its pristine CO₂ capacity. The Mg-MOFs deactivated rapidly in the presence of water. The Ni-CPO-27 was then selected for the second stage of the protocol in which the material was exposed to the wet flue gas. The sample showed an initial drop in CO₂ capacity after the first exposure to the wet flue gas, followed by a stabilisation of the performance over several cycles.

Kinetic Stability of MOF-5 in Humid Environments: Impact of Powder Densification, Humidity Level, and Exposure Time

Metal-organic frameworks (MOFs) are an emerging class of microporous, crystalline materials with potential applications in the capture, storage, and separation of gases. Of the many known MOFs, MOF-5 has attracted considerable attention because of its ability to store gaseous fuels at low pressure with high densities. Nevertheless, MOF-5 and several other MOFs exhibit limited stability upon exposure to reactive species such as water. The present study quantifies the impact of humid air exposure on the properties of MOF-5 as a function of exposure time, humidity level, and morphology (i.e., powders vs pellets). Properties examined include hydrogen storage capacity, surface area, and crystallinity. Water adsorption/desorption isotherms are measured using a gravimetric technique; the first uptake exhibits a type V isotherm with a sudden increase in uptake at ∼50% relative humidity. For humidity levels below this threshold only minor degradation is observed for exposure times up to several hours, suggesting that MOF-5 is more stable than generally assumed under moderately humid conditions. In contrast, irreversible degradation occurs in a matter of minutes for exposures above the 50% threshold. Fourier transform infrared spectroscopy indicates that molecular and/or dissociated water is inserted into the skeletal framework after long exposure times. Densification into pellets can slow the degradation of MOF-5 significantly, and may present a pathway to enhance the stability of some MOFs.

Linearly bridging CO2 in a metal–organic framework

A very rare CO₂-coordinated metal–organic framework was structurally confirmed by single-crystal X-ray diffraction. The CO₂ ligand links two open Zn metal centers in an absolutely linear μ(O,O′) coordination mode with a CO distance of 1.107(4) Å. The new complex reported here is stable under ambient conditions and may provide a new strategy for CO₂ fixation.

Carbon dioxide capture in the presence of water vapour in InOF-1

InOF-1 confirms a significant 2-fold increase (∼11 wt%) in CO₂ capture under 20% relative humidity of water vapour.

Synthesis of an exceptional water-stable two-fold interpenetrated Zn(ii)-paddlewheel metal–organic framework

An exceptional water-stable two-fold interpenetrated Zn(ii)-paddlewheel metal–organic framework was obtained via the facile conversion of its non-interpenetrated isomer at ambient temperature, showing high CO₂ and CH₄ uptake capacities.