Understanding CO2Dynamics in Metal-Organic Frameworks with Open Metal Sites

Multilayer Graphtriyne Membranes for Separation and Storage of CO2: Molecular Dynamics Simulations of Post-Combustion Model Mixtures

The ability to remove carbon dioxide from gaseous mixtures is a necessary step toward the reduction of greenhouse gas emissions. As a contribution to this field of research, we performed a molecular dynamics study assessing the separation and adsorption properties of multi-layered graphtriyne membranes on gaseous mixtures of CO₂, N₂, and H₂O. These mixtures closely resemble post-combustion gaseous products and are, therefore, suitable prototypes with which to model possible technological applications in the field of CO₂ removal methodologies. The molecular dynamics simulations rely on a fairly accurate description of involved force fields, providing reliable predictions of selectivity and adsorption coefficients. The characterization of the interplay between molecules and membrane structure also permitted us to elucidate the adsorption and crossing processes at an atomistic level of detail. The work is intended as a continuation and a strong enhancement of the modeling research and characterization of such materials as molecular sieves for CO₂ storage and removal.

Physicochemical Understanding of the Impact of Pore Environment and Species of Adsorbates on Adsorption Behaviour

For a better design of adsorbents, it is important to know the intermolecular interaction among adsorbates and host material, leading to improved guest selectivity and uptake capacity. In this study, we demonstrate the influence of the interaction among adsorbates and substrate, controlled by the pore environment and species of adsorbates, on the adsorption behaviour. We report the unique CO₂ adsorption behaviour of MOF-205 due to distinct pore geometry. The precise analysis through gas-adsorption crystallography with molecular simulation shows that capillary condensation of CO₂ in MOF-205 occurs preferentially in the large dodecahedral pore rather than the small tetrahedral pore, because the interaction of CO₂ with MOF-205 framework is weaker than that among CO₂ molecules, while Ar and N₂ are sequentially filled into two different pores of MOF-205 according to their size. Comparison of the materials with different pore environments reveals that the relative strength of the adsorbate-adsorbate and adsorbate-substrate interaction gives rise to different shapes of isotherms.

Two-dimensional diamine-linked covalent organic frameworks for CO2/N2 capture and separation: theoretical modeling and simulations

Two-dimensional covalent organic frameworks (2D-COFs) with diamine-based linkers have been designed and investigated for CO2/N2 gaseous mixture adsorption and separation via a systematic theoretical study by combining density functional theory (DFT) calculations and force field-based molecular dynamics (MD) simulations. We explored the adsorption sites and adsorption energies of CO2/N2 on 2D-COFs. The gas uptake capacity, adsorption isotherms, permeability, and selectivity were simulated based on an improved formulation of force fields for mixture separation in post-combustion conditions. This theoretical approach provided atomistic understanding and quantitative description of intermolecular interactions governing the physisorption dynamics of the considered systems. The results suggest that 2D-COFs investigated in this study are competitive with other 2D materials for carbon capture and separation and can be considered as alternative molecular sieving materials offering efficient and rapid separation and adsorption of different molecules.

Effect of Larger Pore Size on the Sorption Properties of Isoreticular Metal-Organic Frameworks with High Number of Open Metal Sites

Four isostructural CPO-54-M metal-organic frameworks based on the larger organic linker 1,5-dihydroxynaphthalene-2,6-dicarboxylic acid and divalent cations (M=Mn, Mg, Ni, Co) are shown to be isoreticular to the CPO-27 (MOF-74) materials. Desolvated CPO-54-Mn contains a very high concentration of open metal sites, which has a pronounced effect on the gas adsorption of N2 , H2 , CO2 and CO. Initial isosteric heats of adsorption are significantly higher than for MOFs without open metal sites and are slightly higher than for CPO-27. The plateau of high heat of adsorption decreases earlier in CPO-54-Mn as a function of loading per mole than in CPO-27-Mn. Cluster and periodic density functional theory based calculations of the adsorbate structures and energetics show that the larger adsorption energy at low loadings, when only open metal sites are occupied, is mainly due to larger contribution of dispersive interactions for the materials with the larger, more electron rich bridging ligand.

Conversion from Heterometallic to Homometallic Metal-Organic Frameworks

Two new heterometallic metal-organic frameworks (MOFs), LnZnTPO 1 and 2, and two homometallic MOFs, LnTPO 3 and 4 (Ln=Eu for 1 and 3, and Tb for 2 and 4; H₃ TPO=tris(4-carboxyphenyl)phosphine oxide) were synthesized, and their structures and properties were analyzed. They were prepared by solvothermal reaction of the C₃ -symmetric ligand H₃ TPO with the corresponding metal ion(s) (a mixture of Ln³⁺ and Zn²⁺ for 1 and 2, and Ln³⁺ alone for 3 and 4). Single-crystal XRD (SXRD) analysis revealed that 1 and 3 are isostructural to 2 and 4, respectively. TGA showed that the framework is thermally stable up to about 400 °C for 1 and 2, and about 450 °C for 3 and 4. PXRD analysis showed their pore-structure distortions without noticeable framework-structure changes during drying processes. The shapes of gas sorption isotherms for 1 and 3 are almost identical to those for 2 and 4, respectively. Solvothermal immersion of 1 and 2 in Tb³⁺ and Eu³⁺ solutions resulted in the framework metal-ion exchange affording 4 and 3, respectively, as confirmed by photoluminescence (PL), PXRD, IR, inductively coupled plasma atomic emission spectroscopy (ICP-AES), and energy-dispersive X-ray (EDX) analyses.

Solid-State NMR Spectroscopy: A Powerful Technique to Directly Study Small Gas Molecules Adsorbed in Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have shown great potential in gas separation and storage, and the design of MOFs for these purposes is an on-going field of research. Solid-state nuclear magnetic resonance (SSNMR) spectroscopy is a valuable technique for characterizing these functional materials. It can provide a wide range of structural and motional insights that are complementary to and/or difficult to access with alternative methods. In this Concept article, the recent advances made in SSNMR investigations of small gas molecules (i.e., carbon dioxide, carbon monoxide, hydrogen gas and light hydrocarbons) adsorbed in MOFs are discussed. These studies demonstrate the breadth of information that can be obtained by SSNMR spectroscopy, such as the number and location of guest adsorption sites, host-guest binding strengths and guest mobility. The knowledge acquired from these experiments yields a powerful tool for progress in MOF development.

A Multifaceted Study of Methane Adsorption in Metal-Organic Frameworks by Using Three Complementary Techniques

Methane is a promising clean and inexpensive energy alternative to traditional fossil fuels, however, its low volumetric energy density at ambient conditions has made devising viable, efficient methane storage systems very challenging. Metal-organic frameworks (MOFs) are promising candidates for methane storage. In order to improve the methane storage capacity of MOFs, a better understanding of the methane adsorption, mobility, and host-guest interactions within MOFs must be realized. In this study, methane adsorption within α-Mg₃ (HCO₂ )₆ , α-Zn₃ (HCO₂ )₆ , SIFSIX-3-Zn, and M-MOF-74 (M=Mg, Zn, Ni, Co) has been comprehensively examined. Single-crystal X-ray diffraction (SCXRD) experiments and DFT calculations of the methane adsorption locations were performed for α-Mg₃ (HCO₂ )₆ , α-Zn₃ (HCO₂ )₆ , and SIFSIX-3-Zn. The SCXRD thermal ellipsoids indicate that methane possesses significant mobility at the adsorption sites in each system. ² H solid-state NMR (SSNMR) experiments targeting deuterated CH₃ D guests in α-Mg₃ (HCO₂ )₆ , α-Zn₃ (HCO₂ )₆ , SIFSIX-3-Zn, and MOF-74 yield an interesting finding: the ² H SSNMR spectra of methane adsorbed in these MOFs are significantly influenced by the chemical shielding anisotropy in addition to the quadrupolar interaction. The chemical shielding anisotropy contribution is likely due mainly to the nuclear independent chemical shift effect on the MOF surfaces. In addition, the ² H SSNMR results and DFT calculations strongly indicate that the methane adsorption strength is linked to the MOF pore size and that dispersive forces are responsible for the methane adsorption in these systems. This work lays a very promising foundation for future studies of methane adsorption locations and dynamics within adsorbent MOF materials.

Electronic metal–organic framework sensors

This review provides an overview on the different types of electronic MOF sensors used for the detection of molecules in the gas/vapour phase and how to assess their performances.

Polarizable Force Fields for CO2 and CH4 Adsorption in M-MOF-74

The family of M-MOF-74, with M = Co, Cr, Cu, Fe, Mg, Mn, Ni, Ti, V, and Zn, provides opportunities for numerous energy related gas separation applications. The pore structure of M-MOF-74 exhibits a high internal surface area and an exceptionally large adsorption capacity. The chemical environment of the adsorbate molecule in M-MOF-74 can be tuned by exchanging the metal ion incorporated in the structure. To optimize materials for a given separation process, insights into how the choice of the metal ion affects the interaction strength with adsorbate molecules and how to model these interactions are essential. Here, we quantitatively highlight the importance of polarization by comparing the proposed polarizable force field to orbital interaction energies from DFT calculations. Adsorption isotherms and heats of adsorption are computed for CO2, CH4, and their mixtures in M-MOF-74 with all 10 metal ions. The results are compared to experimental data, and to previous simulation results using nonpolarizable force fields derived from quantum mechanics. To the best of our knowledge, the developed polarizable force field is the only one so far trying to cover such a large set of possible metal ions. For the majority of metal ions, our simulations are in good agreement with experiments, demonstrating the effectiveness of our polarizable potential and the transferability of the adopted approach.

Incorporation of Alkylamine into Metal-Organic Frameworks through a Brønsted Acid-Base Reaction for CO2 Capture

The escalating atmospheric CO₂ concentration is one of the most urgent environmental concerns of our age. To effectively capture CO₂ , various materials have been studied. Among them, alkylamine-modified metal-organic frameworks (MOFs) are considered to be promising candidates. In most cases, alkylamine molecules are integrated into MOFs through the coordination bonds formed between open metal sites (OMSs) and amine groups. Thus, the alkylamine density, as well as the corresponding CO₂ uptake in MOFs, are severely restricted by the density of OMSs. To overcome this limit, other approaches to incorporating alkylamine into MOFs are highly desired. We have developed a new method based on Brønsted acid-base reaction to tether alkylamines into Cr-MIL-101-SO₃ H for CO₂ capture. A systematic optimization of the amine tethering process was also conducted to maximize the CO₂ uptake of the modified MOF. Under the optimal amine tethering condition, the obtained tris(2-aminoethyl)amine-functionalized Cr-MIL-101-SO₃ H (Cr-MIL-101-SO₃ H-TAEA) has a cyclic CO₂ uptake of 2.28 mmol g^-1 at 150 mbar and 40 °C, and 1.12 mmol g^-1 at 0.4 mbar and 20 °C. The low-cost starting materials and simple synthetic procedure for the preparation of Cr-MIL-101-SO₃ H-TAEA suggest that it has the potential for large-scale production and practical applications.

An indirect generation of 1D MII-2,5-dihydroxybenzoquinone coordination polymers, their structural rearrangements and generation of materials with a high affinity for H2, CO2 and CH4

1D coordination polymers undergo transformations upon desolvation to yield sorbent materials.

Metal-Organic Frameworks via Emissive Metal-Carboxylate Zwitterion Intermediates

Pyridinemethanol-carboxylate esters form octahedral complexes with Zn(NO₃ )₂ in aqueous DMF that subsequently undergo hydrolysis at elevated temperatures to form metal-carboxylate zwitterions. In situ deprotonation of the hydroxy group leads to thermally robust, neutral MOFs. This stepwise synthesis can be controlled by temperature and is made possible by the subtle difference in reactivity of the functional groups.

Molecular basis for the high CO2 adsorption capacity of chabazite zeolites

CO2 adsorption in Li-, Na-, K-CHA (Si/Al=6,=12), and silica chabazite zeolites was investigated by powder diffraction. Two CO2 adsorption sites were found in all chabazites with CO2 locating in the 8-membered ring (8MR) pore opening being the dominant site. Electric quadrupole-electric field gradient and dispersion interactions drive CO2 adsorption at the middle of the 8 MRs, while CO2 polarization due to interaction with cation sites controls the secondary CO2 site. In Si-CHA, adsorption is dominated by dispersion interactions with CO2 observed on the pore walls and in 8 MRs. CO2 adsorption complexes on dual cation sites were observed on K-CHA, important for K-CHA-6 samples due to a higher probability of two K(+) cations bridging CO2. Trends in isosteric heats of CO2 adsorption based on cation type and concentration can be correlated with adsorption sites and CO2 quantity. A decrease in the hardness of metal cations results in a decrease in the direct interaction of these cations with CO2.

Can Metal-Organic Framework Separate 1-Butene from Butene Isomers?

The separation of 1-butene from the other isomers is an industrially important but challenging task because these isomers mainly differ only by the position of C═C double bond with many of their physical properties very similar. In this work, we propose using first-principles calculations that Fe-MOF-74 can be a promising candidate for the separation of 1-butene from all other isomers with high selectivity. We demonstrate that the underlying mechanism of this olefin separation is the steric interactions; that is, 1-butene with terminal double bond has the smallest steric interactions with the framework and therefore can approach the metal binding sites more closely for stronger π-complexation. This combined effect (π-complexation modulated by steric interactions) in MOFs with open metal sites can offer a promising design strategy for difficult separation of even longer olefin isomers by properly engineering the lengths and functional groups of the MOF linkers.