Supramolecular Interactions between Finite Tapes of Water Molecules and Hydrated Metal Ions To Produce Infinite Two-Dimensional Cationic Layers of Water Molecules

A supramolecular self-assembly of finite tapes of lattice water molecules with the coordinated water molecules of hydrated metal ions forms 2D cationic layers of water molecules in the coordination complex [Ni­(H₂O)₆]­[Ni­(Pydi)₂(H₂O)₂]·4H₂O (Pydi = 2,5-pyridinedicarboxylate). It reveals yet another unique mode of the cooperative association of water molecules. The anionic layers have a well-known 2D, nonporous metal–organic framework formed by the assistance from H-bonding of coordinating water molecules

Supramolecular Interactions between Finite Tapes of Water Molecules and Hydrated Metal Ions To Produce Infinite Two-Dimensional Cationic Layers of Water Molecules

A supramolecular self-assembly of finite tapes of lattice water molecules with the coordinated water molecules of hydrated metal ions forms 2D cationic layers of water molecules in the coordination complex [Ni­(H₂O)₆]­[Ni­(Pydi)₂(H₂O)₂]·4H₂O (Pydi = 2,5-pyridinedicarboxylate). It reveals yet another unique mode of the cooperative association of water molecules. The anionic layers have a well-known 2D, nonporous metal–organic framework formed by the assistance from H-bonding of coordinating water molecules

Enhancement of the Water Adsorptivity of Metal–Organic Frameworks upon Hybridization with Layered Double Hydroxide Nanosheets

Efficient water adsorbents with improved hydrostability can be synthesized by the hybridization of metal–organic framework (MOF) compounds with exfoliated layered double hydroxide (LDH) 2D nanosheets. The self-assembly between copper benzene tricarboxylate (Cu-BTC) MOF nanocrystals and exfoliated Mg–Al-LDH nanosheets leads to the nanoscale mixing of the MOF and LDH components, as well as to the prevention of the formation of aggregated secondary MOF particles. In the resulting nanohybrids, spherical Cu-BTC nanocrystals with small particle sizes of ∼5–10 nm are uniformly anchored on the surface of Mg–Al-LDH 2D nanosheets with the dimensions of several hundred nanometers. At the optimal composition, the surface area of the resulting nanohybrid becomes greater than that of pristine Cu-BTC, which is attributable to the suppression of the self-aggregation of MOF nanocrystals and to the formation of the mesoporous stacking structure of the LDH nanosheets. Of prime importance is that both the water adsorption ability and the hydrostability of Cu-BTC become notably improved upon hybridization with LDH nanosheets. The present study clearly demonstrates that exfoliated LDH nanosheets can be used as an effective hybridization matrix for exploring novel efficient MOF-based hybrid water adsorbents

N₂ Capture Performances of the Hybrid Porous MIL-101(Cr): From Prediction toward Experimental Testing

The purification of nitrogen-containing gas mixtures, natural/shale gas, and dry air calls for economically viable adsorptive separation processes involving an adsorbent with a higher affinity for N₂ over hydrocarbons and oxygen. This led to the discovery of a new class of unprecedented N₂-selective metal–organic frameworks (MOFs) with coordinatively unsaturated chromium­(III) sites, e.g., MIL-100­(Cr) (MIL: Materials of Institut Lavoisier). Following this preliminary study, here grand canonical Monte Carlo simulations identified MIL-101­(Cr), an analogue of MIL-100­(Cr), as another N₂-selective adsorbent from mixtures of both CH₄–N₂ (natural gas purification) and O₂–N₂ (air purification). This prediction was further compared to single gas adsorption and breakthrough separation experiments. It was evidenced that only the more energetic coordinatively unsaturated chromium sites released using an activation temperature of 523 K are responsible for the N₂-selective behavior of MIL-101­(Cr). The separation mechanisms were then elucidated at the molecular-level, and this emphasized the central role played by the concentration of coordinatively unsaturated chromium­(III) sites in MIL-101­(Cr) that can be controlled by the activation temperature of the sample

N₂ Capture Performances of the Hybrid Porous MIL-101(Cr): From Prediction toward Experimental Testing

The purification of nitrogen-containing gas mixtures, natural/shale gas, and dry air calls for economically viable adsorptive separation processes involving an adsorbent with a higher affinity for N₂ over hydrocarbons and oxygen. This led to the discovery of a new class of unprecedented N₂-selective metal–organic frameworks (MOFs) with coordinatively unsaturated chromium­(III) sites, e.g., MIL-100­(Cr) (MIL: Materials of Institut Lavoisier). Following this preliminary study, here grand canonical Monte Carlo simulations identified MIL-101­(Cr), an analogue of MIL-100­(Cr), as another N₂-selective adsorbent from mixtures of both CH₄–N₂ (natural gas purification) and O₂–N₂ (air purification). This prediction was further compared to single gas adsorption and breakthrough separation experiments. It was evidenced that only the more energetic coordinatively unsaturated chromium sites released using an activation temperature of 523 K are responsible for the N₂-selective behavior of MIL-101­(Cr). The separation mechanisms were then elucidated at the molecular-level, and this emphasized the central role played by the concentration of coordinatively unsaturated chromium­(III) sites in MIL-101­(Cr) that can be controlled by the activation temperature of the sample

Hybridization of a Metal–Organic Framework with a Two-Dimensional Metal Oxide Nanosheet: Optimization of Functionality and Stability

An effective way to improve the functionalities and stabilities of metal–organic frameworks (MOFs) is developed by employing exfoliated metal oxide 2D nanosheets as matrix for immobilization. Crystal growth of zeolitic imidazolate framework-8 (ZIF-8) nanocrystals on the surface of layered titanate nanosheets yields intimately coupled nanohybrids of ZIF-8-layered titanate. The resulting nanohybrids show much greater surface areas and larger pore volumes than do the pristine ZIF-8, leading to the remarkable improvement of the CO₂ adsorption ability of MOF upon hybridization. Of prime importance is that the thermal- and hydrostabilities of ZIF-8 are significantly enhanced by a strong chemical interaction with the robust titanate nanosheet. A strong interfacial interaction between ZIF-8 and the layered titanate is verified by molecular mechanics simulations and spectroscopic analysis. The universal applicability of the present strategy for the coupling of MOFs and metal oxide nanosheets is substantiated by the stabilization of Ti-MOF-NH₂ via the immobilization on exfoliated V₂O₅ nanosheets. The present study underscores that hybridization with metal oxide 2D nanosheets provides an efficient and universal synthetic route to novel MOF-based hybrid materials with enhanced gas adsorptivity and stability

Syngas Purification by Porous Amino-Functionalized Titanium Terephthalate MIL-125

The adsorption equilibrium of carbon dioxide (CO₂), carbon monoxide (CO), nitrogen (N₂), methane (CH₄), and hydrogen (H₂) was studied at 303, 323, and 343 K and pressures up to 7 bar in titanium-based metal–organic framework (MOF) granulates, amino-functionalized titanium terephthalate MIL-125­(Ti)_NH₂. The affinity of the different adsorbates toward the adsorbent presented the following order: CO₂ > CH₄ > CO > N₂ > H₂, from the most adsorbed to the least adsorbed component. Subsequently, adsorption kinetics and multicomponent adsorption equilibrium were studied by means of single, binary, and ternary breakthrough curves at 323 K and 4.5 bar with different feed mixtures. Both studies are complementary and aim the syngas purification for two different applications, hydrogen production and H₂/CO composition adjustment, to be used as feed in the Fischer–Tropsch processes. The isosteric heats were calculated from the adsorption equilibrium isotherms and are 21.9 kJ mol^–1 for CO₂, 14.6 kJ mol^–1 for CH₄, 13.4 kJ mol^–1 for CO, and 11.7 kJ mol^–1 for N₂. In the overall pressure and temperature range, the adsorption equilibrium isotherms were well-regressed against the Langmuir model. The multicomponent breakthrough experimental results allowed for validation of the adsorption equilibrium predicted by the multicomponent extension of the Langmuir isotherm and validation of the fixed-bed mathematical model. To conclude, two pressure swing adsorption (PSA) cycles were designed and performed experimentally, one for hydrogen purification from a 30/70% CO₂/H₂ mixture (hydrogen purity was 100% with a recovery of 23.5%) and a second PSA cycle to obtain a light product with a H₂/CO ratio between 2.2 and 2.4 to feed to Fischer–Tropsch processes. The experimental cycle produced a light stream with a H₂/CO ratio of 2.3 and a CO₂-enriched stream with 86.6% purity as a heavy product. The CO₂ recovery was 93.5%

How Water Fosters a Remarkable 5-Fold Increase in Low-Pressure CO₂ Uptake within Mesoporous MIL-100(Fe)

The uptake and adsorption enthalpy of carbon dioxide at 0.2 bar have been studied in three different topical porous MOF samples, HKUST-1, UiO-66­(Zr), and MIL-100­(Fe), after having been pre-equilibrated under different relative humidities (3, 10, 20, 40%) of water vapor. If in the case of microporous UiO-66, CO₂ uptake remained similar whatever the relative humidity, and correlations were difficult for microporous HKUST-1 due to its relative instability toward water vapor. In the case of MIL-100­(Fe), a remarkable 5-fold increase in CO₂ uptake was observed with increasing RH, up to 105 mg g^–1 CO₂ at 40% RH, in parallel with a large decrease in enthalpy measured. Cycling measurements show slight differences for the initial three cycles and complete reversibility with further cycles. These results suggest an enhanced solubility of CO₂ in the water-filled mesopores of MIL-100­(Fe)