A review of recent advances in metal-organic frameworks materials for zero-energy passive adsorption of chemical pollutants in indoor environments

Approximately 75-90 % of a person's lifetime is spent inside increasingly airtight buildings, where indoor pollutant levels typically exceed those outdoors. Poor indoor air quality can lead to allergies, respiratory diseases, and even cancer, and can also reduce the longevity of buildings. Passive adsorption materials play a crucial role in reducing indoor pollutants. This review highlights the latest advances in using Metal-organic Frameworks (MOFs) as passive adsorption materials for indoor pollutant capture and outlines the principles for developing high-performance adsorbents. It provides a comparative analysis of the development and performance of MOFs and composite adsorbent materials, highlighting their respective advantages and limitations in indoor pollutant adsorption technology. The article proposes strategies to address these challenges and offers a comprehensive review of current practical adsorption devices. Finally, aiming to advance commercialization of MOFs, the anticipated development of indoor pollutant adsorption technology is discussed in this paper.

New insight into impact of humidity on direct air capture performance by SIFSIX-3-Cu MOF

Removal of CO2 from air is one of the key human challenges in battling global warming. SIFSIX-3-Cu is a promising metal-organic framework (MOF) suggested for carbon capture even at low CO2 concentrations. However, the impact of humidity on its performance in direct air capture (DAC) is poorly understood. To evaluate the MOF performance for DAC application under humid conditions, we investigate the adsorption of H2O, CO2, and N2 using density functional theory (DFT), grand canonical Monte Carlo (GCMC), and molecular dynamics (MD) simulations. The simulation results show a higher tendency of SIFSIX-3-Cu towards H2O adsorption rather than CO2 (and N2). The results agree with the adsorption isotherms for the pure compounds from the Sips model. The extended Sips model shows 1.34 mmol g-1 CO2 adsorption at the atmospheric pressure and 298 K for the CO2/N2 mixture containing 400 ppm CO2, and low CO2 adsorption (less than 0.75 mmol g-1) at a low relative humidity (RH) of 20%. This finding highlights the efficiency of SIFSIX-3-Cu for DAC in dry air and the negative impact of humidity on the CO2 selective adsorption. Therefore, we suggest to consider the impairing of humidity effects when designing a SIFSIX-3-Cu-based CO2 separation process and removal of any water vapor before introduction of the air to SIFSIX-3-Cu.

Coarse-grained force field for ZIF-8: A study on adsorption, diffusion, and structural properties

Metal-organic frameworks (MOFs) are revolutionizing a spectrum of industries, from groundbreaking gas storage solutions to transformative biological system applications. The intricate architecture of these materials necessitates the use of advanced computational techniques for a comprehensive understanding of their molecular structure and prediction of their physical properties. Coarse-grained (CG) simulations shine a spotlight on the often-neglected influences of defects, pressure effects, and spatial disorders on the performance of MOFs. These simulations are not just beneficial but indispensable for high-demand applications, such as mixed matrix membranes and intricate biological system interfaces. In this work, we propose an optimized CG force field tailored for ZIF-8. Our work provides a deep dive into sorption isotherms and diffusion coefficients of small molecules. We demonstrate the structural dynamics of ZIF-8, particularly how it responds to pressurization, which affects its crystal structure and leads to local changes in aperture size and area. Emphasizing the game-changing potential of CG simulations, we explore the characteristics of amorphization in ZIF-8. Through computational exploration, we aim to bridge the knowledge gap, enhancing the potential applications of nanoporous materials for various applications.

DFT-Quality Adsorption Simulations in Metal-Organic Frameworks Enabled by Machine Learning Potentials

Nanoporous materials such as metal-organic frameworks (MOFs) have been extensively studied for their potential for adsorption and separation applications. In this respect, grand canonical Monte Carlo (GCMC) simulations have become a well-established tool for computational screenings of the adsorption properties of large sets of MOFs. However, their reliance on empirical force field potentials has limited the accuracy with which this tool can be applied to MOFs with challenging chemical environments such as open-metal sites. On the other hand, density-functional theory (DFT) is too computationally demanding to be routinely employed in GCMC simulations due to the excessive number of required function evaluations. Therefore, we propose in this paper a protocol for training machine learning potentials (MLPs) on a limited set of DFT intermolecular interaction energies (and forces) of CO2 in ZIF-8 and the open-metal site containing Mg-MOF-74, and use the MLPs to derive adsorption isotherms from first principles. We make use of the equivariant NequIP model which has demonstrated excellent data efficiency, and as such an error on the interaction energies below 0.2 kJ mol-1 per adsorbate in ZIF-8 was attained. Its use in GCMC simulations results in highly accurate adsorption isotherms and heats of adsorption. For Mg-MOF-74, a large dependence of the obtained results on the used dispersion correction was observed, where PBE-MBD performs the best. Lastly, to test the transferability of the MLP trained on ZIF-8, it was applied to ZIF-3, ZIF-4, and ZIF-6, which resulted in large deviations in the predicted adsorption isotherms and heats of adsorption. Only when explicitly training on data for all ZIFs, accurate adsorption properties were obtained. As the proposed methodology is widely applicable to guest adsorption in nanoporous materials, it opens up the possibility for training general-purpose MLPs to perform highly accurate investigations of guest adsorption.

The influence of force fields on the structure and dynamics of water confined in ZIF-8 from atomistic simulations

The complexity of modeling flexible crystals, such as ZIF-8, mainly stems from the handling of intramolecular interactions. Numerous force fields have been proposed in the literature to describe the interactions between atoms in ZIF-8. We employ seven force fields to examine the structure and dynamic behavior of water molecules confined in ZIF-8, with the aim of investigating the impact of force fields on simulation results. Various structural characterization methods consistently indicate that the choice of different force fields has quantitative effects but no qualitative effects on the structural characteristics of confined water. Additionally, the force fields do not impact the qualitative description of the diffusion mechanism. Both mean-square displacement and van Hove autocorrelation function reveal two characteristic movements of water molecules diffusing in ZIF-8: a short-time intra-cavity hopping process and a long-time inter-cavity hopping process. However, the framework flexibility is found to play a crucial role in determining the order of spatial arrangement and local structure, self-diffusion coefficient and reorientational dynamics of confined water. Specifically, the DREIDING force field gives rise to an unrealistic stiff framework, enhancing the order of spatial arrangement and diminishing the local ordered structure of confined water. Meanwhile, it results in much slower translational and reorientational dynamics. Hence, the general DREIDING force field cannot be considered for providing a quantitative description of the water structure and dynamics.

The Effect of Surface Composition on the Selective Capture of Atmospheric CO2 by ZIF Nanoparticles: The Case of ZIF-8

We performed theoretical studies of CO₂ capture in atmospheric conditions by the zeolitic imidazolate framework-8 (ZIF-8) via classical Monte Carlo (MC) simulations with Metropolis sampling and classical molecular dynamics (MD) simulations in the NVT and NPT ensembles and different thermodynamic conditions. The ZIF-8 framework was described by varying unit cell dimensions in the presence of pure gases of CO₂, N₂, O₂, Ar, and H₂O steam as well as binary mixtures of CO₂:N₂ and CO₂:H₂O in s 1:1 concentration. Different chemical compositions of the framework surface was considered to provide an accurate treatment of charge and charge distribution in the nanoparticle. Hence, surface groups were represented as unsaturated zinc atom (Zn⁺²), 2-methylimidazole (mImH), and deprotonated 2-methylimidazole (mIm^-). Force field reparameterization of the surface sites was required to reproduce the interactions of the gas molecules with the ZIF-8 surface consistent with quantum mechanics (QM) calculations and Born-Oppenheimer molecular dynamics (BOMD). It was observed that ZIF-8 selectively captures CO₂ due to the negligible concentrations of N₂, O₂, Ar, and H₂O. These molecules spontaneously migrate to the inner pores of the framework. At the surface, there is a competitive interaction between H₂O, CO₂, and N₂, for the positively charged ZIF-8 nanoparticle with a large binding energy advantage for water molecules (on average -62, -15, and -8 kcal/mol respectively). For the neutral ZIF-8 nanoparticle, the water molecules dominate the interactions due to the occurrence of hydrogen bond with the imidazolate groups at the surface. Simulations of binary mixtures of CO₂/water steam and CO₂/N₂ were performed to investigate binding competition between these molecules for the framework positively charged and neutral surfaces. It was found that water molecules drastically block the interaction between CO₂ molecules and the framework surface, decreasing CO₂ capture in the central pore, and CO₂ molecules fully block the interaction between N₂ molecules and the framework. These findings show that CO₂ capture by ZIF-8 is possible in atmospheric environments only upon dehydration of the atmospheric gas. It further shows that ZIF-8 capture of CO₂ from the atmospheric environment is dependent on thermodynamic conditions and can be increased by decreasing temperature and/or increasing pressure.

Computer simulation of the early stages of self-assembly and thermal decomposition of ZIF-8

We employ all-atom well-tempered metadynamics simulations to study the mechanistic details of both the early stages of nucleation and crystal decomposition for the benchmark metal–organic framework (MOF) ZIF-8. To do so, we developed and validated a force field that reliably models the modes of coordination bonds via a Morse potential functional form and employs cationic and anionic dummy atoms to capture coordination symmetry. We also explored a set of physically relevant collective variables and carefully selected an appropriate subset for our problem at hand. After a rapid increase of the Zn–N connectivity, we observe the evaporation of small clusters in favor of a few large clusters, which leads to the formation of an amorphous highly connected aggregate. [Formula: see text] and [Formula: see text] complexes are observed with lifetimes in the order of a few picoseconds, while larger structures, such as four-, five-, and six-membered rings, have substantially longer lifetimes of a few nanoseconds. The free ligands act as “templating agents” for the formation of sodalite cages. ZIF-8 crystal decomposition results in the formation of a vitreous phase. Our findings contribute to a fundamental understanding of MOF’s synthesis that paves the way to controlling synthesis products. Furthermore, our developed force field and methodology can be applied to model solution processes that require coordination bond reactivity for other ZIFs besides ZIF-8.

Adsorption and diffusion of the H2/CO2/CO/MeOH/EtOH mixture into the ZIF-7 using molecular simulation

The adsorption and diffusion of synthesis gas components (methanol, ethanol, H₂, CO_2, and CO molecules) in ZIF-7 by grand canonical Monte Carlo and molecular dynamics simulation were investigated. The initial diffusion coefficient at the beginning of the process depends on the kinetic diameter of the guest molecules. Also, the diffusion coefficient at equilibrium conditions probably depends on the interaction between the guest molecules with the ZIF-7 framework. The radial distribution function results indicate that the distribution of guest molecules in the framework is affected by the interaction between the guest molecules. These results indicate that the CO, CO₂, and H₂ guest molecules are adsorbed on both the Zn metal atom and the organic linker (especially the C₁ atom). In contrast, the organic linker is the most favorable adsorption site for methanol and ethanol guest molecules. In addition, the diffusion coefficient of guest molecules in binary mixtures is related to the attraction or repulsion between the guest molecules.

The role of surface chemistry on CO2 adsorption in biomass-derived porous carbons by experimental results and molecular dynamics simulations

Biomass-derived porous carbons have been considered one of the most effective adsorbents for CO2 capture, due to their porous structure and high specific surface area. In this study, we successfully synthesized porous carbon from celery biomass and examined the effect of external adsorption parameters including time, temperature, and pressure on CO2 uptake in experimental and molecular dynamics (MD) simulations. Furthermore, the influence of carbon's surface chemistry (carboxyl and hydroxyl functionalities) and nitrogen type on CO2 capture were investigated utilizing MD simulations. The results showed that pyridinic nitrogen has a greater tendency to adsorb CO2 than graphitic. It was found that the simultaneous presence of these two types of nitrogen has a greater effect on the CO2 sorption than the individual presence of each in the structure. It was also revealed that the addition of carboxyl groups (O=C-OH) to the carbon matrix enhances CO2 capture by about 10%. Additionally, by increasing the simulation time and the size of the simulation box, the average absolute relative error for simulation results of optimal structure declined to 16%, which is an acceptable value and makes the simulation process reliable to predict adsorption capacity under various conditions.

Effect of ZSM-5 zeolite porosity on catalytic cracking of n-heptane

The selectivity of ethylene and propylene in cracking of n-heptane is connected to the micropore to mesopore ratioof ZSM-5 structure.

Comparison of the ionic conductivity properties of microporous and mesoporous MOFs infiltrated with a Na-ion containing IL mixture

IL@MOF (IL: ionic liquid; MOF: metal-organic framework) materials have been proposed as a candidate for solid-state electrolytes, combining the inherent non-flammability and high thermal and chemical stability of the ionic liquid with the host-guest interactions of the MOF. In this work, we compare the structure and ionic conductivity of a sodium ion containing IL@MOF composite formed from a microcrystalline powder of the zeolitic imidazolate framework (ZIF), ZIF-8 with a hierarchically porous sample of ZIF-8 containing both micro- and mesopores from a sol-gel synthesis. Although the crystallographic structures were shown to be the same by X-ray diffraction, significant differences in particle size, packing and morphology were identified by electron microscopy techniques which highlight the origins of the hierarchical porosity. After incorporation of Na0.1EMIM0.9TFSI (abbreviated to NaIL; EMIM = 1-ethyl-3-methylimidazolium; TFSI = bis(trifluoromethylsulfonyl)imide), the hierarchically porous composite exhibited a 40% greater filling capacity than the purely microporous sample which was confirmed by elemental analysis and digestive proton NMR. Finally, the ionic conductivity properties of the composite materials were probed by electrochemical impedance spectroscopy. The results showed that despite the 40% increased loading of NaIL in the NaIL@ZIF-8micro sample, the ionic conductivities at 25 °C were 8.4 × 10-6 and 1.6 × 10-5 S cm-1 for NaIL@ZIF-8meso and NaIL@ZIF-8micro respectively. These results exemplify the importance of the long range, continuous ion pathways contributed by the microcrystalline pores, as well as the limited contribution from the discontinuous mesopores to the overall ionic conductivity.

Spectroscopy, microscopy, diffraction and scattering of archetypal MOFs: formation, metal sites in catalysis and thin films

A comprehensive overview of characterization tools for the analysis of well-known metal–organic frameworks and physico-chemical phenomena associated to their applications.

Density Functional Theory Studies on Zeolitic Imidazolate Framework-8 and Ionic Liquid-Based Composite Materials

The identification of suitable density functional methods for predicting the properties of nanoporous composite materials is highly significant in the field of chemical and material sciences. The stability of the composite materials depends on the nature of bonding and dispersive interaction at the interface. Thus, we have studied the effect of dispersion correction in the incorporation of hydrophobic and hydrophilic ionic liquids (ILs) into zeolitic imidazolate framework-8 (ZIF-8) nanostructures using the density functional theory (DFT)-based approaches. These structures were analyzed employing selected methods (Becke-Lee-Yang-Parr and Perdew-Burke-Ernzerhof) with dispersion correction (D2 or D3) and different basis sets (such as double-zeta valence polarized (DZVP), triple-zeta valence polarized (TZVP), and triple-zeta valence doubly polarized (TZV2P)) for the understanding of microscopic features of IL@ZIF-8 nanopores. It is found that the result obtained from DFT-D2/TZVP is more reliable for the prediction of the experimental crystal structure as well as stability and spectral information of the complexes. Furthermore, the microscopic analysis of geometries reveals that ILs are highly dispersed and stabilized at the nanopores of ZIF-8, particularly the ZIF-8 structure is highly preferable for the hydrophobic group in ILs. It is found that fluorine-containing anions are highly dispersed on the ZIF-8 surface compared to the nonfluorinated anion (i.e., [BMIM]⁺[Cl]^-). This is confirmed from the adsorption energies (E _ads), charge transfer, electron density analyses, and IR spectral analysis. These findings can provide more insights into the stability of composite materials, which are suitable for applications of catalytic conversion at the confined state, gas storage, and separation techniques.

New scaling relations to compute atom-in-material polarizabilities and dispersion coefficients: part 1. Theory and accuracy

Polarizabilities and London dispersion forces are important to many chemical processes. Force fields for classical atomistic simulations can be constructed using atom-in-material polarizabilities and C n (n = 6, 8, 9, 10…) dispersion coefficients. This article addresses the key question of how to efficiently assign these parameters to constituent atoms in a material so that properties of the whole material are better reproduced. We develop a new set of scaling laws and computational algorithms (called MCLF) to do this in an accurate and computationally efficient manner across diverse material types. We introduce a conduction limit upper bound and m-scaling to describe the different behaviors of surface and buried atoms. We validate MCLF by comparing results to high-level benchmarks for isolated neutral and charged atoms, diverse diatomic molecules, various polyatomic molecules (e.g., polyacenes, fullerenes, and small organic and inorganic molecules), and dense solids (including metallic, covalent, and ionic). We also present results for the HIV reverse transcriptase enzyme complexed with an inhibitor molecule. MCLF provides the non-directionally screened polarizabilities required to construct force fields, the directionally-screened static polarizability tensor components and eigenvalues, and environmentally screened C6 coefficients. Overall, MCLF has improved accuracy compared to the TS-SCS method. For TS-SCS, we compared charge partitioning methods and show DDEC6 partitioning yields more accurate results than Hirshfeld partitioning. MCLF also gives approximations for C8, C9, and C10 dispersion coefficients and quantum Drude oscillator parameters. This method should find widespread applications to parameterize classical force fields and density functional theory (DFT) + dispersion methods.

On the Efficient Separation of Gas Mixtures with the Mixed-Linker Zeolitic-Imidazolate Framework-7-8

A recently reported modification of the zeolitic-imidazolate framework-8 (ZIF-8) with partial replacement of the 2-methylimidazolate (mIm) linker with benzimidazolate (bIm), namely ZIF-7-8, is investigated with molecular simulations using a first-time reported force field. The size of the ZIF-7-8 aperture, which governs the gas-separation efficiency of this material and which has not been estimated before for this modification, is smaller than that of the original ZIF-8. The diffusivities of CO₂, N₂, and CH₄ estimated through transition state theory calculations result in remarkably high diffusion selectivities for CO₂/CH₄ and CO₂/N₂ mixtures. This performance enhancement is investigated in terms of structural flexibility in the form of the aperture motion through extensive estimation of the effective diameter, the total effective area, and the motion of the aperture linkers, of both ZIF-8 and ZIF-7-8. Both apertures exhibit an oscillation through the rotation of the linkers, which are adjusted according to the size of the penetrant molecules the moment they pass through it. Finally, a subsequent analysis reveals that there is strong dependency of the separation performance on the bIm-to-mIm ratio: below 33% bIm incorporation, the appearance of ZIF-8-alike wide apertures decreases dramatically the size-based selectivity of the mixtures in ZIF-7-8.

Biporous Metal-Organic Framework with Tunable CO2/CH4 Separation Performance Facilitated by Intrinsic Flexibility

In this work, we report the synthesis of SION-8, a novel metal-organic framework (MOF) based on Ca(II) and a tetracarboxylate ligand TBAPy^4- endowed with two chemically distinct types of pores characterized by their hydrophobic and hydrophilic properties. By altering the activation conditions, we gained access to two bulk materials: the fully activated SION-8F and the partially activated SION-8P with exclusively the hydrophobic pores activated. SION-8P shows high affinity for both CO₂ ( Q_st = 28.4 kJ/mol) and CH₄ ( Q_st = 21.4 kJ/mol), while upon full activation, the difference in affinity for CO₂ ( Q_st = 23.4 kJ/mol) and CH₄ ( Q_st = 16.0 kJ/mol) is more pronounced. The intrinsic flexibility of both materials results in complex adsorption behavior and greater adsorption of gas molecules than if the materials were rigid. Their CO₂/CH₄ separation performance was tested in fixed-bed breakthrough experiments using binary gas mixtures of different compositions and rationalized in terms of molecular interactions. SION-8F showed a 40-160% increase (depending on the temperature and the gas mixture composition probed) of the CO₂/CH₄ dynamic breakthrough selectivity compared to SION-8P, demonstrating the possibility to rationally tune the separation performance of a single MOF by manipulating the stepwise activation made possible by the MOF's biporous nature.

On flexible force fields for metal-organic frameworks: Recent developments and future prospects

Classical force field simulations can be used to study structural, diffusion, and adsorption properties of metal-organic frameworks (MOFs). To account for the dynamic behavior of the material, parameterization schemes have been developed to derive force constants and the associated reference values by fitting on ab initio energies, vibrational frequencies, and elastic constants. Here, we review recent developments in flexible force field models for MOFs. Existing flexible force field models are generally able to reproduce the majority of experimentally observed structural and dynamic properties of MOFs. The lack of efficient sampling schemes for capturing stimuli-driven phase transitions, however, currently limits the full predictive potential of existing flexible force fields from being realized. This article is categorized under: Structure and Mechanism > Computational Materials ScienceMolecular and Statistical Mechanics > Molecular Mechanics.

Molecular Mechanism of Swing Effect in Zeolitic Imidazolate Framework ZIF-8: Continuous Deformation upon Adsorption

Zeolitic imidazolate framework ZIF-8 displays flexibility of its structure by rotation of its imidazolate linker. This "swing effect" has been widely described in the literature, both experimentally and theoretically, as a bistable system where the linker oscillates between two structures: "open window" and "closed window". By using quantum chemistry calculations and first-principles molecular dynamics simulations, it is shown that the deformation upon adsorption is in fact continuous upon pore loading, with thermodynamics of packing effects being the reason behind stepped adsorption isotherms experimentally observed. Finally, we study a variant of ZIF-8 with a different linker, highlighting the influence of the linker and the balance of microscopic interactions on the framework's flexibility.

On the molecular mechanisms for the H2/CO2 separation performance of zeolite imidazolate framework two-layered membranes

Double-layered zeolitic imidazolate framework (ZIF) membranes were fabricated inside polyimide P84 hollow fibers by a step-synthesis conducted by microfluidic technology and applied to pre-combustion gas separation. Our hypothesis, based on the information provided by a combination of molecular simulation and experiments, is that a CO2 adsorption reduction on the surface of the ZIF-9 would enhance the molecular sieving effect of this ZIF-9 layer and therefore the selectivity in the H2/CO2 mixture separation of the entire membrane. This reduction would be achieved by means of a less-CO2-adsorptive methylimidazolate-based ZIF-67 or ZIF-8 layer coating the ZIF-9. ZIF-8/ZIF-9 and ZIF-67/ZIF-9 double-layered membranes were prepared and characterized by XRD, FTIR, SEM, FIB, TEM and EDS. This unprecedented strategy led to a H2/CO2 separation selectivity of 9.6 together with a 250 GPU H2 permeance at 150 °C, showing a significant improvement with respect to the pure ZIF-9 membrane. Double-layered membranes also showed higher apparent CO2 activation energies than single-layered membranes, attributable to a diminished adsorption.

Exploring the Flexibility of MIL-47(V)-Type Materials Using Force Field Molecular Dynamics Simulations

The flexibility of three MIL-47(V)-type materials (MIL-47, COMOC-2, and COMOC-3) has been explored by constructing the pressure versus volume and free energy versus volume profiles at various temperatures ranging from 100 to 400 K. This is done with first-principles-based force fields using the recently proposed QuickFF parametrization protocol. Specific terms were added for the materials at hand to describe the asymmetry of the one-dimensional vanadium-oxide chain and to account for the flexibility of the organic linkers. The force fields are used in a series of molecular dynamics simulations at fixed volumes but varying unit cell shapes. The three materials show a distinct pressure-volume behavior, which underlines the ability to tune the mechanical properties by varying the linkers toward different applications such as nanosprings, dampers, and shock absorbers.

Intermolecular Force Field Parameters Optimization for Computer Simulations of CH4in ZIF-8

The differential evolution (DE) algorithm is applied for obtaining the optimized intermolecular interaction parameters between CH4and 2-methylimidazolate ([C4N2H5]−) using quantum binding energies of CH4-[C4N2H5]−complexes. The initial parameters and their upper/lower bounds are obtained from the general AMBER force field. The DE optimized and the AMBER parameters are then used in the molecular dynamics (MD) simulations of CH4molecules in the frameworks of ZIF-8. The results show that the DE parameters are better for representing the quantum interaction energies than the AMBER parameters. The dynamical and structural behaviors obtained from MD simulations with both sets of parameters are also of notable differences.

Theoretical model estimation of guest diffusion in metal–organic frameworks (MOFs)

An analytical model was developed to efficiently and accurately estimate the guest diffusivity in flexible metal–organic frameworks (MOFs).