The Effect of High Pressure on MOF-5: Guest-Induced Modification of Pore Size and Content at High Pressure

A Molecular Insight into the Dehydration of a Metal-Organic Framework and its Impact on the CO2 Capture

Metal-organic frameworks (MOFs) are porous material formed by the self-assembly of metallic ligands and organic linkers. They are a good candidate for CO₂ gas capture because they have large surface areas and the metal or linker can be tuned to improve CO₂ uptake. In the quest for water and acid stable MOFs, a phosphonate-based organic linker has recently been designed by Glavinovic et al. (Chem. Eur. J. 2022, 28, e202200874). By combining ionic calcium nodes, water and methanol molecules, they formed a microporous network, CALF-37. This network has been shown to be robust and can maintain its pore shape even in absence of water molecules or by the inclusion of gas molecules, such as CO₂ . The network can be heated to release the water and methanol molecules and form a dehydrated MOF, which retains its shape with the imprinted pore within. Herein, we perform molecular dynamics (MD) simulations in order to provide insight into the CO₂ capture and sequestration ability of the CALF-37. We model the dehydration of the inactivated MOF (HCALF-37) in the absence and in the presence of methanol molecules by progressively withdrawing water molecules from the MOF networks. We determine the crystal structure of the intermediate states from HCALF-37 to CALF-37 and shed light on the critical role of water molecules in the mediation of metal-linker bonds. Our calculations also reveal that the favorable interactions between the CO₂ molecules and the aromatic core of the linkers and metallic ions are responsible for the efficient sequestration of the gas in the CALF-37.

Pressure and guest-mediated pore shape modification in a small pore MOF to 1200 bar

Guest-mediated pore-shape modification of the metal-organic framework, Sc₂BDC₃ upon adsorption of n-pentane and isopentane is examined from 50-1200 bar. Rotation of the BDC linker responsible for the change in pore shape occurs at much lower pressures than previously reported, with distinct adsorption behaviour observed between pentane isomers.

Covalent Organic Framework (COF-1) under High Pressure

COF-1 has a structure with rigid 2D layers composed of benzene and B3 O3 rings and weak van der Waals bonding between the layers. The as-synthesized COF-1 structure contains pores occupied by solvent molecules. A high surface area empty-pore structure is obtained after vacuum annealing. High-pressure XRD and Raman experiments with mesitylene-filled (COF-1-M) and empty-pore COF-1 demonstrate partial amorphization and collapse of the framework structure above 12-15 GPa. The ambient pressure structure of COF-1-M can be reversibly recovered after compression up to 10-15 GPa. Remarkable stability of highly porous COF-1 structure at pressures at least up to 10 GPa is found even for the empty-pore structure. The bulk modulus of the COF-1 structure (11.2(5) GPa) and linear incompressibilities (k[100] =111(5) GPa, k[001] =15.0(5) GPa) were evaluated from the analysis of XRD data and cross-checked against first-principles calculations.

Elucidation of the Structural Origins and Contrasting Guest-Host Interactions in CO2 -Loaded CdSDB and PbSDB Metal-Organic Frameworks at High Pressures

PbSDB and CdSDB (SDB=4,4'-sulfonyldibenzoate) are two structurally related SDB-based metal-organic frameworks (MOFs) that demonstrate promising potential for selective CO₂ adsorption capabilities. The structural stabilities and guest-host interactions between CO₂ and PbSDB or CdSDB frameworks at high pressures up to 13 GPa in situ were comparatively investigated by Raman spectroscopy, FTIR spectroscopy, and synchrotron X-ray diffraction. Although both empty frameworks exhibited high chemical stabilities upon compression, they show different pressure-induced modifications in crystallinity. This difference can be attributed to their different coordination topologies that result in near isotropic contraction of unit cells for the CdSDB framework but anisotropic for the PbSDB framework. Furthermore, the CO₂ -loaded PbSDB and CdSDB frameworks at high pressures show strongly contrasting guest-host interactions in terms of the pressure-regulated CO₂ adsorption sites. In both frameworks, pressure can highly efficiently promote the formation of new CO₂ adsorption sites and the enhancement of guest-host interactions. In the CO₂ -loaded PbSDB framework, in particular, the peculiar pressure-tuned CO₂ population was observed preferentially on one of the two adsorption sites in response to external compression. These unique guest-host interaction behaviors can also be unambiguously correlated to their different topological origins. These findings for the PbSDB and CdSDB frameworks provide in-depth understanding of the structure-property relationship, which is of fundamental importance for CO₂ storage application in SDB-based MOFs.

Adsorption Behavior of High Stable Zr-Based MOFs for the Removal of Acid Organic Dye from Water

Zirconium based metal organic frameworks (Zr-MOFs) have become popular in engineering studies due to their high mechanical stability, thermostability and chemical stability. In our work, by using a theoretical kinetic adsorption isotherm, we can exert MOFs to an acid dye adsorption process, experimentally exploring the adsorption of MOFs, their external behavior and internal mechanism. The results indicate their spontaneous and endothermic nature, and the maximum adsorption capacity of this material for acid orange 7 (AO7) could be up to 358 mg·g⁻¹ at 318 K, estimated by the Langmuir isotherm model. This is ascribed to the presence of an open active metal site that significantly intensified the adsorption, by majorly increasing the interaction strength with the adsorbates. Additionally, the enhanced π delocalization and suitable pore size of UiO-66 gave rise to the highest host–guest interaction, which further improves both the adsorption capacity and separation selectivity at low concentrations. Furthermore, the stability of UiO-66 was actually verified for the first time, through comparing the structure of the samples before and after adsorption mainly by Powder X-ray diffraction and thermal gravimetric analysis.

Neon-Bearing Ammonium Metal Formates: Formation and Behaviour under Pressure

The incorporation of noble gas atoms, in particular neon, into the pores of network structures is very challenging due to the weak interactions they experience with the network solid. Using high-pressure single-crystal X-ray diffraction, we demonstrate that neon atoms enter into the extended network of ammonium metal formates, thus forming compounds Ne_x [NH₄ ][M(HCOO)₃ ]. This phenomenon modifies the compressional and structural behaviours of the ammonium metal formates under pressure. The neon atoms can be clearly localised within the centre of [M(HCOO)₃ ]₅ cages and the total saturation of this site is achieved after ∼1.5 GPa. We find that by using argon as the pressure-transmitting medium, the inclusion inside [NH₄ ][M(HCOO)₃ ] is inhibited due to the larger size of the argon. This study illustrates the size selectivity of [NH₄ ][M(HCOO)₃ ] compounds between neon and argon insertion under pressure, and the effect of inclusion on the high-pressure behaviour of neon-bearing ammonium metal formates.

A Computational and Experimental Approach Linking Disorder, High-Pressure Behavior, and Mechanical Properties in UiO Frameworks

Whilst many metal-organic frameworks possess the chemical stability needed to be used as functional materials, they often lack the physical strength required for industrial applications. Herein, we have investigated the mechanical properties of two UiO-topology Zr-MOFs, the planar UiO-67 ([Zr6O4(OH)4 (bpdc)6], bpdc: 4,4'-biphenyl dicarboxylate) and UiO-abdc ([Zr6O4(OH)4 (abdc)6], abdc: 4,4'-azobenzene dicarboxylate) by single-crystal nanoindentation, high-pressure X-ray diffraction, density functional theory calculations, and first-principles molecular dynamics. On increasing pressure, both UiO-67 and UiO-abdc were found to be incompressible when filled with methanol molecules within a diamond anvil cell. Stabilization in both cases is attributed to dynamical linker disorder. The diazo-linker of UiO-abdc possesses local site disorder, which, in conjunction with its longer nature, also decreases the capacity of the framework to compress and stabilizes it against direct compression, compared to UiO-67, characterized by a large elastic modulus. The use of non-linear linkers in the synthesis of UiO-MOFs therefore creates MOFs that have more rigid mechanical properties over a larger pressure range.

A non-topological mechanism for negative linear compressibility

When exposed to high pressure, the framework material UTSA-16 expands in one direction as the result of distortions localised in soft Co(ii)-based tetrahedra, rather than topological flexing of the network.