Robust, Transferable, and Physically Motivated Force Fields for Gas Adsorption in Functionalized Zeolitic Imidazolate Frameworks

We extend our existing methodology for generating physically motivated, tailored ab initio force fields via symmetry-adapted perturbation theory (SAPT). The revised approach naturally yields <i>transferable</i> atomic exchange, charge penetration, and dispersion parameters, facilitating the creation of versatile, optimized force fields; this approach is general, applicable to a wide array of potential applications. We then employ this approach to develop a force field, “ZIF FF”, which is tailored to accurately model CO₂/N₂ adsorption in zeolitic imidazolate frameworks (ZIFs). In conjunction with our previous “SYM” force field used to model adsorbate–adsorbate interactions, we compute adsorption isotherms for both CO₂ and N₂ in nine different ZIFs, yielding results that are in excellent accord with the corresponding experimental results. We find that ZIF FF accurately predicts isotherms for three different topologies of ZIFs (RHO, SOD, GME) and reproduces gas adsorption trends for varying functionalization across an isoreticular series of ZIFs of the GME topology. Because ZIF FF is free of empirical parameters, it presents the opportunity for computationally screening novel ZIFs that have not yet been synthesized and/or characterized

Microscopic Origins of Enhanced Gas Adsorption and Selectivity in Mixed-Linker Metal–Organic Frameworks

We use molecular simulations to study the gas adsorption properties of metal–organic framework (MOF) materials composed of mixtures of linker groups, focusing on the prototypical MTV-MOF-5 systems. While MOF functionalization is well-known to influence gas uptake, we show that the absolute gas uptake is frequently not merely a sum of linear contributions from its constituent functionalities but rather there exists a synergistic <i>enhancement</i> that arises due to cooperative adsorbate–linker interactions involving multiple functionalities. In certain mixed-linker MOFs, such cooperativity yields increased gas uptake over any possible corresponding pure “parent” compound. Considering a model system based on ZIF-8, we are able to clearly demonstrate the microscopic origin of this synergy, arising from the strong, simultaneous interactions of multiple linker groups with a single adsorbate. We also provide a concrete example of a mixed-linker MOF that exhibits gas adsorption superior to that of any of its pure parent compounds. We conclude that such cooperativity should be a fairly general phenomenon and suggest some design guidelines that can be exploited to synthesize synergistically enhanced mixed MOFs

Microscopic Origins of Enhanced Gas Adsorption and Selectivity in Mixed-Linker Metal–Organic Frameworks

We use molecular simulations to study the gas adsorption properties of metal–organic framework (MOF) materials composed of mixtures of linker groups, focusing on the prototypical MTV-MOF-5 systems. While MOF functionalization is well-known to influence gas uptake, we show that the absolute gas uptake is frequently not merely a sum of linear contributions from its constituent functionalities but rather there exists a synergistic <i>enhancement</i> that arises due to cooperative adsorbate–linker interactions involving multiple functionalities. In certain mixed-linker MOFs, such cooperativity yields increased gas uptake over any possible corresponding pure “parent” compound. Considering a model system based on ZIF-8, we are able to clearly demonstrate the microscopic origin of this synergy, arising from the strong, simultaneous interactions of multiple linker groups with a single adsorbate. We also provide a concrete example of a mixed-linker MOF that exhibits gas adsorption superior to that of any of its pure parent compounds. We conclude that such cooperativity should be a fairly general phenomenon and suggest some design guidelines that can be exploited to synthesize synergistically enhanced mixed MOFs

Ab Initio, Physically Motivated Force Fields for CO₂ Adsorption in Zeolitic Imidazolate Frameworks

We present an entirely ab initio methodology, based on symmetry adapted perturbation theory (SAPT), for constructing force-fields to study CO₂ adsorption in nanoporous zeolitic imidazolate frameworks (ZIFs). Our approach utilizes the SAPT energy decomposition to generate physically motivated force fields for the CO₂-ZIF interaction, with explicit terms representing exchange, electrostatic, induction and dispersion interactions. Each of these terms is fit to the corresponding term in the SAPT energy decomposition, yielding a force field entirely free of empirical parameters. This approach was utilized to construct force fields describing the CO₂ interaction with both ZIF-8 and ZIF-71. In conjunction with our existing CO₂–CO₂ force field, parametrized in a consistent manner, we validate our force fields using grand canonical Monte Carlo simulations and obtain good agreement with the corresponding experimental CO₂ adsorption isotherms. Furthermore, the explicit correspondence between force field terms and fundamental interaction types (dispersion, electrostatics, and induction) allows for an analysis of the underlying physics controlling ZIF gas adsorption that is far more direct and well-defined than with the generic force fields that had been previously utilized to study these systems. As our force fields are free from empirical parameters, these results demonstrate the potential for computationally screening novel ZIFs for flue gas separation applications with near quantitative accuracy

Mechanistic Insights into Solution-Phase Oxidative Esterification of Primary Alcohols on Pd(111) from First-Principles Microkinetic Modeling

We present an ab initio microkinetic model for the oxidative esterification of 1-propanol to methyl propionate over Pd(111). The model fully accounts for solvation of solution-phase species and added catalytic base and provides key insights into the factors that limit the activity of unpromoted Pd aerobic oxidation catalysts. In particular, we find that the activity is limited by the large steady-state surface H coverage, which destabilizes other adsorbed intermediates via lateral interactions, and substantial barriers governing the formation of O–H bonds, which is required for the reduction of O₂ and removal of H byproducts from the catalyst surface

First-Principles, Physically Motivated Force Field for the Ionic Liquid [BMIM][BF₄]

Molecular simulations play an important role in establishing structure–property relations in complex fluids such as room-temperature ionic liquids. Classical force fields are the starting point when large systems or long times are of interest. These force fields must be not only accurate but also transferable. In this work, we report a physically motivated force field for the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]­[BF₄]) based on symmetry-adapted perturbation theory. The predictions (from molecular dynamics simulations) of the liquid density, enthalpy of vaporization, diffusion coefficients, viscosity, and conductivity are in excellent agreement with experiment, with no adjustable parameters. The explicit energy decomposition inherent in the force field enables a quantitative analysis of the important physical interactions in these systems. We find that polarization is crucial and there is little evidence of charge transfer. We also argue that the often used procedure of scaling down charges in molecular simulations of ionic liquids is unphysical for [BMIM]­[BF₄]. Because all intermolecular interactions in the force field are parametrized from first-principles, we anticipate good transferability to other ionic liquid systems and physical conditions

Computational Characterization of Defects in Metal–Organic Frameworks: Spontaneous and Water-Induced Point Defects in ZIF‑8

Zeolitic imidazolate frameworks (ZIFs) are an important class of porous crystalline metal–organic framework (MOF) materials that have attracted widespread attention for applications ranging from gas adsorption and separation to catalysis. Although the bulk crystal structures of MOFs are typically well-characterized, comparatively little is known regarding MOF defect structures. Drawing on analogies with conventional silicon-based zeolites, we utilize computational methods to examine the structure and stability of putative point-defect structures (including vacancies, substitutions, and “dangling” linkers) within the prototypical ZIF-8 structure. Considering both postsynthetic (gas-phase) and synthetic (solution-phase) conditions, we find that several of the defect structures lie low in energy relative to the defect-free parent crystal, with barriers to defect formation that are large but surmountable under relevant temperatures. These results are consistent with prior experimental observations of ZIF stability and reactivity and suggest that defects may play an important role in influencing the long-term stability of MOFs under conditions that include exposure to water vapor and trace contaminants such as acid gases

Insights into the Stability of Zeolitic Imidazolate Frameworks in Humid Acidic Environments from First-Principles Calculations

Understanding the degradation mechanisms of zeolitic imidazolate frameworks (ZIFs) is crucial to improve their chemical stability and realize their potential industrial applications. Here, we conduct a comprehensive study utilizing dispersion-corrected density functional theory calculations to investigate the chemical stability of bulk ZIFs and their external surfaces under conditions of acid-gas exposure. We examine the influence of steric factors such as topology and ligand functionalization on the relative chemical stability of prototypical ZIFs (ZIF-2 and ZIF-8), including their hypothetical polymorphs. We find that defect formation is more thermodynamically and kinetically favorable at ZIF external surfaces versus the bulk, and that both topology and ligand functionalization impact defect formation. In addition, we provide a detailed mechanism for the reaction of ZIFs with sulfurous and sulfuric acids, of which the latter serves as a catalyst in potential degradation reactions of ZIFs. We also provide information about the adsorption strength of a range of acid gases to defective ZIF structures, which can inform potential strategies to regenerate ZIFs and/or achieve defect engineering in these materials

Evaluation of Force Field Performance for High-Throughput Screening of Gas Uptake in Metal–Organic Frameworks

High-throughput computational screening is an increasingly useful approach to identify promising nanoporous materials for gas separation and adsorption applications. The reliability of the screening hinges on the accuracy of the underlying force fields, which is often difficult to access systematically. To probe the accuracy of common force fields and to assess the sensitivity of the screening results to this accuracy, we have computed CO₂ and CH₄ gas adsorption isotherms in 424 metal–organic frameworks using <i>ab initio</i> force fields and evaluated the contribution of electrostatic, van der Waals, and polarization interactions on the predicted gas uptake and the adsorption site probability distributions. While there are significant quantitative differences between gas uptake predicted by standard (generic) force fields (such as UFF) and <i>ab initio</i> force fields, the force fields predict similar ranking of the MOFs, supporting the further use of generic force fields in high-throughput screening studies. However, we also find that isotherm predictions of standard force fields may benefit from significant error cancellation resulting from overestimation of dispersion and neglect of polarization; as such, caution is warranted, as this error cancellation may vary among different classes of materials

Conformational and Dynamic Properties of Poly(ethylene oxide) in an Ionic Liquid: Development and Implementation of a First-Principles Force Field

The conformational properties of polymers in ionic liquids are of fundamental interest but not well understood. Atomistic and coarse-grained molecular models predict qualitatively different results for the scaling of chain size with molecular weight, and experiments on dilute solutions are not available. In this work, we develop a first-principles force field for poly­(ethylene oxide) (PEO) in the ionic liquid 1-butyl 3-methylimidazolium tetrafluoroborate ([BMIM]­[BF₄]) using symmetry adapted perturbation theory (SAPT). At temperatures above 400 K, simulations employing both the SAPT and OPLS-AA force fields predict that PEO displays ideal chain behavior, in contrast to previous simulations at lower temperature. We therefore argue that the system shows a transition from extended to more compact configurations as the temperature is increased from room temperature to the experimental lower critical solution temperature. Although polarization is shown to be important, its implicit inclusion in the OPLS-AA force is sufficient to describe the structure and energetics of the mixture. The simulations emphasize the difference between ionic liquids from typical solvents for polymers