Adsorption of water can have an important effect on chemical and physical processes in porous materials, such as zeolites and metal–organic frameworks (MOFs). However, the molecular simulation of ...water adsorption brings many challenges, especially the slow simulation speed. In this study, we examined the hydrophobic MOF ZIF-8 as a representative adsorbent to discover the adsorption mechanism of water in hydrophobic MOFs. Based on the mechanistic insights obtained, we proposed and investigated several advanced Monte Carlo algorithms including energy-bias moves and continuous fractional component Monte Carlo (CFC MC) and were able to accelerate the simulation speed by a factor of 6.7 over the conventional grand canonical Monte Carlo algorithm. The insights obtained from this work may also help improve the molecular simulation efficiency for studies of water adsorption in other hydrophobic materials.
A new software package, RASPA, for simulating adsorption and diffusion of molecules in flexible nanoporous materials is presented. The code implements the latest state-of-the-art algorithms for ...molecular dynamics and Monte Carlo (MC) in various ensembles including symplectic/measure-preserving integrators, Ewald summation, configurational-bias MC, continuous fractional component MC, reactive MC and Baker's minimisation. We show example applications of RASPA in computing coexistence properties, adsorption isotherms for single and multiple components, self- and collective diffusivities, reaction systems and visualisation. The software is released under the GNU General Public License.
The development of new microporous materials for adsorption separation processes is a rapidly growing field because of potential applications such as carbon capture and sequestration (CCS) and ...purification of clean‐burning natural gas. In particular, new metal‐organic frameworks (MOFs) and other porous coordination polymers are being generated at a rapid and growing pace. Herein, we address the question of how this large number of materials can be quickly evaluated for their practical application in carbon dioxide separation processes. Five adsorbent evaluation criteria from the chemical engineering literature are described and used to assess over 40 MOFs for their potential in CO2 separation processes for natural gas purification, landfill gas separation, and capture of CO2 from power‐plant flue gas. Comparisons with other materials such as zeolites are made, and the relationships between MOF properties and CO2 separation potential are investigated from the large data set. In addition, strategies for tailoring and designing MOFs to enhance CO2 adsorption are briefly reviewed.
Looking for holes: How can new microporous materials, such as metal–organic frameworks (MOFs, see picture), be quickly evaluated for their practical application in CO2 separation processes? Five adsorbent evaluation criteria are used to assess over 40 MOFs for their potential in natural gas purification, landfill gas separation, and capture of CO2 from power‐plant flue gas. Comparisons with other materials such as zeolites are made.
Computational high-throughput screening using molecular simulations is a powerful tool for identifying top-performing metal–organic frameworks (MOFs) for gas storage and separation applications. ...Accurate partial atomic charges are often required to model the electrostatic interactions between the MOF and the adsorbate, especially when the adsorption involves molecules with dipole or quadrupole moments such as water and CO2. Although ab initio methods can be used to calculate accurate partial atomic charges, these methods are impractical for screening large material databases because of the high computational cost. We developed a random forest machine learning model to predict the partial atomic charges in MOFs using a small yet meaningful set of features that represent both the elemental properties and the local environment of each atom. The model was trained and tested on a collection of about 320 000 density-derived electrostatic and chemical (DDEC) atomic charges calculated on a subset of the Computation-Ready Experimental Metal–Organic Framework (CoRE MOF-2019) database and separately on charge model 5 (CM5) charges. The model predicts accurate atomic charges for MOFs at a fraction of the computational cost of periodic density functional theory (DFT) and is found to be transferable to other porous molecular crystals and zeolites. A strong correlation is observed between the partial atomic charge and the average electronegativity difference between the central atom and its bonded neighbors.
As metal–organic frameworks move towards practical application, data for an expanded range of physical properties are needed. Molecular-level modelling and data science can play an important role.
Density functional theory is used to study the tunability of trigonal prismatic SBUs found in metal–organic frameworks (MOFs) such as MIL-100, MIL-101, and PCN-250/MIL-127 of chemical composition M3+ ...2M2+(μ3-O)(RCOO)6 for the partial oxidation of methane to methanol. We performed a combinatorial screening by varying the composition of the trimetallic node (M1 3+)2(M2 2+) (where M1 and M2 = V, Cr, Mn, Fe, Co, and Ni) and calculated the reaction pathway on both M1 and M2 sites. The systematic replacement of metals in the trimetallic cluster allowed us to study the influence of spectator atoms on the catalytic activity of a specific metal site in the cluster toward the N2O activation and C–H bond activation steps of the reaction. In the screening, we identified the top-performing node compositions with predicted barriers lower than those already reported for experimentally tested MOFs with trigonal prismatic SBUs. This work demonstrates the opportunity to tune the catalytic activity of MOFs for redox reactions by changing their metal node composition.
Competitive coadsorption of water is a major problem in the deployment of adsorption-based CO2 capture. Water molecules may compete for adsorption sites, reducing the capacity of the material, and ...dehumidification prior to separating CO2 from N2 increases process complexity and cost. The development of adsorbent materials that can selectively adsorb CO2 in the presence of water would be a major step forward in the deployment of CO2 capture materials in practice. In this study, large-scale computational screening was carried out to search for metal–organic frameworks (MOFs) with high selectivity toward CO2 over H2O. Calculating framework charges for thousands of MOFs is a significant challenge, so initial screening used a fast, but approximate, charge calculation method. On the basis of the initial screening, 15 MOFs were selected, and Monte Carlo simulations were carried out to compute the adsorption isotherms for these MOFs using more accurate framework charges calculated by density functional theory. A detailed investigation was performed on the effect of using different methods for calculating partial charges, and it was found that electrostatic interactions contribute the majority of the adsorption energy of H2O in the selected MOFs.
The surface area is one of the most important quantities for characterizing novel porous materials. The BET analysis is the standard method for determining surface areas from nitrogen adsorption ...isotherms and was originally derived for multilayer gas adsorption onto flat surfaces. Metal−organic frameworks (MOFs) are a relatively new class of crystalline, porous materials that have been shown to exhibit very large BET surface areas. These materials are microporous and possess surfaces that are far from flat. In some MOFs, adsorption occurs through a pore-filling mechanism rather than by layer formation. Thus, it is unclear whether BET surface area numbers reported for these materials are truly meaningful. Given the standard practice of reporting BET surface areas for novel porous materials, a critical test of the BET method is much needed. In this work, grand canonical Monte Carlo simulations were used to predict adsorption isotherms for nitrogen in a series of MOFs. The predicted isotherms were used as pseudoexperimental data to test the applicability of the BET theory for obtaining surface areas of microporous MOFs. BET surface areas calculated from the simulated isotherms agree very well with the accessible surface areas calculated directly from the crystal structures in a geometric fashion. In addition, the surface areas agree well with experimental reports in the literature. These results provide a strong validation that the BET theory can be used to obtain surface areas of MOFs.
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