Unprecedentedly High Selective Adsorption of Gas Mixtures in <i>rho</i> Zeolite-like Metal−Organic Framework: A Molecular Simulation Study

We report a molecular simulation study for the separation of industrially important gas mixtures (CO2/H2, CO2/CH4, and CO2/N2) in rho zeolite-like metal-organic framework (rho-ZMOF). Rho-ZMOF contains a wide-open anionic framework and charge-balancing extraframework Na+ ions. Two types of binding sites for Na+ ions are identified in the framework. Site I is in the single eight-membered ring, whereas site II is in the α-cage. Na+ ions at site I have a stronger affinity for the framework and thus a smaller mobility. The binding sites in rho-ZMOF resemble those in its inorganic counterpart rho-zeolite. CO2 is adsorbed predominantly over other gases because of its strong electrostatic interactions with the charged framework and the presence of Na+ ions acting as additional adsorption sites. At ambient temperature and pressure, the CO2 selectivities are 1800 for the CO2/H2 mixture, 80 for the CO2/CH4 mixture, and 500 for the CO2/N2 mixture. Compared with other MOFs and nanoporous materials reported to date, rho-ZMOF exhibits unprecedentedly high selective adsorption for these gas mixtures. This work represents the first simulation study to characterize extraframework ions and examine gas separation in a charged ZMOF. The simulation results reveal that rho-ZMOF is a promising candidate for the separation of syngas, natural gas, and flue gas

Molecular Screening of Metal−Organic Frameworks for CO₂ Storage

We report a molecular simulation study for CO2 storage in metal−organic frameworks (MOFs). As compared to the aluminum-free and cation-exchanged ZSM-5 zeolites and carbon nanotube bundle, IRMOF1 exhibits remarkably higher capacity. Incorporation of Na+ cations into zeolite increases the capacity only at low pressures. By variation of the metal oxide, organic linker, functional group, and framework topology, a series of isoreticular MOFs (IRMOF1, Mg-IRMOF1, Be-IRMOF1, IRMOF1-(NH2)4, IRMOF10, IRMOF13, and IRMOF14) are systematically examined, as well as UMCM-1, a fluorous MOF (F-MOF1), and a covalent−organic framework (COF102). The affinity with CO2 is enhanced by addition of a functional group, and the constricted pore is formed by interpenetration of the framework; both lead to a larger isosteric heat and Henry’s constant and subsequently a stronger adsorption at low pressures. The organic linker plays a critical role in tuning the free volume and accessible surface area and largely determines CO2 adsorption at high pressures. As a combination of high capacity and low framework density, IRMOF10, IRMOF14, and UMCM-1 are identified from this study to be the best for CO2 storage, even surpass the experimentally reported highest capacity in MOF-177. COF102 is a promising candidate with high capacity at considerably low pressures. Both gravimetric and volumetric capacities at 30 bar correlate well with the framework density, free volume, porosity, and accessible surface area. These structure−function correlations are useful for a priori prediction of CO2 capacity and for the rational screening of MOFs toward high-efficacy CO2 storage

Unraveling the Energetics and Dynamics of Ibuprofen in Mesoporous Metal−Organic Frameworks

A computational study is reported to investigate the microscopic behavior of ibuprofen (IBU) in two mesoporous metal−organic frameworks (MOFs), MIL-101 and UMCM-1. Both host structures possess remarkably large pore volumes and surface areas. The predicted maximum loading of IBU is in good agreement with experimental measurement and approximately four times greater than that in silica MCM-41. The lowest-energy conformation of IBU in MIL-101 is preferentially located near the metal−oxide. From the highest-occupied molecular orbitals (HOMOs) and band gap, a coordination bond is found to form between the carboxylic group of IBU and the exposed metal site of MIL-101. IBU exhibits a stronger binding energy and a smaller mobility in MIL-101 than in UMCM-1. These factors are crucial for the delayed release of IBU from MIL-101, which was observed experimentally. This work unravels the energetics and dynamics of IBU in MOFs at the molecular level and provides useful insight into the interactions of drug with host MOFs

Effect of Pore Topology and Accessibility on Gas Adsorption Capacity in Zeolitic−Imidazolate Frameworks: Bringing Molecular Simulation Close to Experiment

When all cages are assumed to be accessible, popular force fields such as universal force field (UFF) and DREIDING dramatically overpredicted gas adsorption capacity in two widely studied zeolitic−imidazolate frameworks (ZIFs), ZIF-68 and -69. Instead of adjusting the force-field parameters to match the experiments, herein we show that when the pore topology and accessibility are correctly taken into account, simulations with the standard force fields agree very well with the experiments. Careful inspection shows that ZIF-68 and -69 have two one-dimensional channels, which are not interaccessible to gases. The small channel consists of alternating small (HPR) and medium (GME) cages, while the large channel comprises the large (KNO) cages. Our analysis indicates that the small channel is not accessible to gases such as CO2. So when the cages in the small channel are intentionally blocked in our simulation, the predicted adsorption capacities of CO2, CH4 and N2 at room temperature from standard force-field parameters for the framework show excellent agreement with the experimental results. In the case of H2, all cages are accessible, so simulation results without cage-blocking show excellent agreement with experiment. Due to the promising potential of ZIFs in gas storage and separation, our work here shows that pore topology and accessibility should be carefully examined to understand how gases adsorb in ZIFs

Molecular Insight into Adsorption and Diffusion of Alkane Isomer Mixtures in Metal−Organic Frameworks

Adsorption and diffusion of alkane isomer mixtures (C4 and C5) are investigated in catenated and noncatenated metal−organic frameworks (IRMOF-13, IRMOF-14, PCN-6, and PCN-6′) using molecular simulations. Competitive adsorption between isomers is observed, particularly at high pressures, at which a linear isomer shows a larger extent of adsorption due to configurational entropy. An inflection is found in the isotherm as a consequence of sequential adsorption in multiple favorable sites. Compared with the noncatenated counterparts, IRMOF-13 and PCN-6 have a greater loading at low pressures because of the constricted pores and stronger affinity with adsorbate. However, the reverse is true at high pressures due to the smaller pore volume. Catenated MOFs exhibit larger adsorption selectivity for alkane mixtures than the noncatenated counterparts. Adsorption selectivity in the four MOFs is comparable to that in carbon nanotube and silicalite, though adsorption capacity is lower in the latter. It is found that diffusivity of alkane in MOFs decreases with the degree of branching because a slender isomer diffuses faster. With the presence of constricted pores, diffusivity in catenated MOFs is smaller than that in noncatenated counterparts. In IRMOF-14 and IRMOF-13 diffusivity decreases monotonically, while it initially increases and then decreases in PCN-6′. The diffusion selectivity in catenated IRMOF-13 and PCN-6 is larger than that in noncatenated IRMOF-14 and PCN-6′. This work provides insightful microscopic mechanisms for the adsorption and diffusion of alkane isomers in MOFs and reveals that both adsorption and diffusion selectivities can be enhanced by catenation

Multipurpose Metal–Organic Framework for the Adsorption of Acetylene: Ethylene Purification and Carbon Dioxide Removal

The separation of acetylene, ethylene, and carbon dioxide is a great challenge in view of their similar sizes and physical properties. Recently, adsorptive separations using porous metal–organic frameworks have risen to prominence. Here, we report a novel microporous metal–organic framework, termed MUF-17, that selectively adsorbs acetylene in the presence of ethylene or carbon dioxide. MUF-17 possesses one-dimensional zig-zag pores that are lined with amino and carboxylate groups, and coordinated water molecules. This pore surface is highly polar and has appropriate dimensions to interact optimally with guest acetylene molecules. Dispersion-corrected density functional theory calculations confirm the strong interactions between the framework and acetylene and illustrate the electrostatic basis for its lower affinity for other gases. The application of MUF-17 to gas separations was demonstrated by dynamic breakthrough measurements. It is a multipurpose adsorbent, removing trace quantities of acetylene from ethylene and sequestering bulk quantities in the presence of carbon dioxide. Its excellent performance fruitfully couples high selectivity with uptake capacity. Advantageously, MUF-17 is straightforward, robust, and inexpensive to prepare. Its recyclability and high stability render it a high-performance material for sustainable and energy-efficient separation processes

Understanding the High Solubility of CO₂ in an Ionic Liquid with the Tetracyanoborate Anion

The ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate, [emim][B(CN)4], shows greater CO2 solubility than several popular ionic liquids (ILs) of different anions including [emim]bis(trifluoromethylsulfonyl)imide [emim][Tf2N]. Herein, both classical molecular dynamics simulation and quantum mechanical calculations were used to understand the high solubility of CO2 in the [emim][B(CN)4] IL. We found that the solubility is dictated by the cation–anion interaction, while the CO2–anion interaction plays a secondary role. The atom–atom radial distribution functions (RDFs) between cation and anion show weaker interaction in [emim][B(CN)4] than in [emim][Tf2N]. A good correlation is observed between gas-phase cation–anion interaction energy with CO2 solubility at 1 bar and 298 K, suggesting that weaker cation–anion interaction leads to higher CO2 solubility. MD simulation of CO2 in the ILs showed that CO2 is closer to the anion than to the cation and that it interacts more strongly with [B(CN)4] than with [Tf2N]. Moreover, a higher volume expansion is observed in [emim][B(CN)4] than in [emim][Tf2N] at different mole fractions of CO2. These results indicate that [B(CN)4] as a small and highly symmetric anion is unique in giving a high CO2 solubility by interacting weakly with the cation and thus allowing easy creation of cavity for close contact with CO2

Molecular Insight into Adsorption and Diffusion of Alkane Isomer Mixtures in Metal−Organic Frameworks

Adsorption and diffusion of alkane isomer mixtures (C4 and C5) are investigated in catenated and noncatenated metal−organic frameworks (IRMOF-13, IRMOF-14, PCN-6, and PCN-6′) using molecular simulations. Competitive adsorption between isomers is observed, particularly at high pressures, at which a linear isomer shows a larger extent of adsorption due to configurational entropy. An inflection is found in the isotherm as a consequence of sequential adsorption in multiple favorable sites. Compared with the noncatenated counterparts, IRMOF-13 and PCN-6 have a greater loading at low pressures because of the constricted pores and stronger affinity with adsorbate. However, the reverse is true at high pressures due to the smaller pore volume. Catenated MOFs exhibit larger adsorption selectivity for alkane mixtures than the noncatenated counterparts. Adsorption selectivity in the four MOFs is comparable to that in carbon nanotube and silicalite, though adsorption capacity is lower in the latter. It is found that diffusivity of alkane in MOFs decreases with the degree of branching because a slender isomer diffuses faster. With the presence of constricted pores, diffusivity in catenated MOFs is smaller than that in noncatenated counterparts. In IRMOF-14 and IRMOF-13 diffusivity decreases monotonically, while it initially increases and then decreases in PCN-6′. The diffusion selectivity in catenated IRMOF-13 and PCN-6 is larger than that in noncatenated IRMOF-14 and PCN-6′. This work provides insightful microscopic mechanisms for the adsorption and diffusion of alkane isomers in MOFs and reveals that both adsorption and diffusion selectivities can be enhanced by catenation

Functionalizing Porous Aromatic Frameworks with Polar Organic Groups for High-Capacity and Selective CO₂ Separation: A Molecular Simulation Study

Porous aromatic frameworks (PAFs) were recently synthesized with the highest surface area to date; one such PAF (PAF-1) has diamond-like structure with biphenyl building blocks and exhibits exceptional thermal and hydrothermal stabilities. Herein, we computationally design new PAFs by introducing polar organic groups to the biphenyl unit and then investigate their separating power toward CO2 by using grand-canonical Monte Carlo (GCMC) simulations. Among these functional PAFs, we found that tetrahydrofuran-like ether-functionalized PAF-1 shows higher adsorption capacity for CO2 at 1 bar and 298 K (10 mol per kilogram of adsorbent) and also much higher selectivities for CO2/CH4, CO2/N2, and CO2/H2 mixtures when compared with the amine functionality. The electrostatic interactions are found to play a dominant role in the high CO2 selectivities of functional PAFs, as switching off atomic charges would decrease the selectivity by an order of magnitude. This work suggests that functionalizing porous frameworks with tetrahydrofuran-like ether groups is a promising way to increase CO2 adsorption capacity and selectivity, especially at ambient pressures

Molecular Insight into Adsorption and Diffusion of Alkane Isomer Mixtures in Metal−Organic Frameworks

Adsorption and diffusion of alkane isomer mixtures (C4 and C5) are investigated in catenated and noncatenated metal−organic frameworks (IRMOF-13, IRMOF-14, PCN-6, and PCN-6′) using molecular simulations. Competitive adsorption between isomers is observed, particularly at high pressures, at which a linear isomer shows a larger extent of adsorption due to configurational entropy. An inflection is found in the isotherm as a consequence of sequential adsorption in multiple favorable sites. Compared with the noncatenated counterparts, IRMOF-13 and PCN-6 have a greater loading at low pressures because of the constricted pores and stronger affinity with adsorbate. However, the reverse is true at high pressures due to the smaller pore volume. Catenated MOFs exhibit larger adsorption selectivity for alkane mixtures than the noncatenated counterparts. Adsorption selectivity in the four MOFs is comparable to that in carbon nanotube and silicalite, though adsorption capacity is lower in the latter. It is found that diffusivity of alkane in MOFs decreases with the degree of branching because a slender isomer diffuses faster. With the presence of constricted pores, diffusivity in catenated MOFs is smaller than that in noncatenated counterparts. In IRMOF-14 and IRMOF-13 diffusivity decreases monotonically, while it initially increases and then decreases in PCN-6′. The diffusion selectivity in catenated IRMOF-13 and PCN-6 is larger than that in noncatenated IRMOF-14 and PCN-6′. This work provides insightful microscopic mechanisms for the adsorption and diffusion of alkane isomers in MOFs and reveals that both adsorption and diffusion selectivities can be enhanced by catenation