High-rate nanofluidic energy absorption in porous zeolitic frameworks.

Optimal mechanical impact absorbers are reusable and exhibit high specific energy absorption. The forced intrusion of liquid water in hydrophobic nanoporous materials, such as zeolitic imidazolate frameworks (ZIFs), presents an attractive pathway to engineer such systems. However, to harness their full potential, it is crucial to understand the underlying water intrusion and extrusion mechanisms under realistic, high-rate deformation conditions. Here, we report a critical increase of the energy absorption capacity of confined water-ZIF systems at elevated strain rates. Starting from ZIF-8 as proof-of-concept, we demonstrate that this attractive rate dependence is generally applicable to cage-type ZIFs but disappears for channel-containing zeolites. Molecular simulations reveal that this phenomenon originates from the intrinsic nanosecond timescale needed for critical-sized water clusters to nucleate inside the nanocages, expediting water transport through the framework. Harnessing this fundamental understanding, design rules are formulated to construct effective, tailorable and reusable impact energy absorbers for challenging new applications.

Chlorination of a Zeolitic-Imidazolate Framework Tunes Packing and van der Waals Interaction of Carbon Dioxide for Optimized Adsorptive Separation.

Molecular separation of carbon dioxide (CO2) and methane (CH4) is of growing interest for biogas upgrading, carbon capture and utilization, methane synthesis and for purification of natural gas. Here, we report a new zeolitic-imidazolate framework (ZIF), coined COK-17, with exceptionally high affinity for the adsorption of CO2 by London dispersion forces, mediated by chlorine substituents of the imidazolate linkers. COK-17 is a new type of flexible zeolitic-imidazolate framework Zn(4,5-dichloroimidazolate)2 with the SOD framework topology. Below 200 K it displays a metastable closed-pore phase next to its stable open-pore phase. At temperatures above 200 K, COK-17 always adopts its open-pore structure, providing unique adsorption sites for selective CO2 adsorption and packing through van der Waals interactions with the chlorine groups, lining the walls of the micropores. Localization of the adsorbed CO2 molecules by Rietveld refinement of X-ray diffraction data and periodic density functional theory calculations revealed the presence and nature of different adsorption sites. In agreement with experimental data, grand canonical Monte Carlo simulations of adsorption isotherms of CO2 and CH4 in COK-17 confirmed the role of the chlorine functions of the linkers and demonstrated the superiority of COK-17 compared to other adsorbents such as ZIF-8 and ZIF-71.

Charting the Metal-Dependent High-Pressure Stability of Bimetallic UiO-66 Materials.

In theory, bimetallic UiO-66(Zr:Ce) and UiO-66(Zr:Hf) metal-organic frameworks (MOFs) are extremely versatile and attractive nanoporous materials as they combine the high catalytic activity of UiO-66(Ce) or UiO-66(Hf) with the outstanding stability of UiO-66(Zr). Using in situ high-pressure powder X-ray diffraction, however, we observe that this expected mechanical stability is not achieved when incorporating cerium or hafnium in UiO-66(Zr). This observation is akin to the earlier observed reduced thermal stability of UiO-66(Zr:Ce) compounds. To elucidate the atomic origin of this phenomenon, we chart the loss-of-crystallinity pressures of 22 monometallic and bimetallic UiO-66 materials and systematically isolate their intrinsic mechanical stability from their defect-induced weakening. This complementary experimental/computational approach reveals that the intrinsic mechanical stability of these bimetallic MOFs decreases nonlinearly upon cerium incorporation but remains unaffected by the zirconium: hafnium ratio. Additionally, all experimental samples suffer from defect-induced weakening, a synthesis-controlled effect that is observed to be independent of their intrinsic stability.

Light Olefin Diffusion during the MTO Process on H-SAPO-34: A Complex Interplay of Molecular Factors.

The methanol-to-olefins process over H-SAPO-34 is characterized by its high shape selectivity toward light olefins. The catalyst is a supramolecular system consisting of nanometer-sized inorganic cages, decorated by Brønsted acid sites, in which organic compounds, mostly methylated benzene species, are trapped. These hydrocarbon pool species are essential to catalyze the methanol conversion but may also clog the pores. As such, diffusion of ethene and propene plays an essential role in determining the ultimate product selectivity. Enhanced sampling molecular dynamics simulations based on either force fields or density functional theory are used to determine how molecular factors influence the diffusion of light olefins through the 8-ring windows of H-SAPO-34. Our simulations show that diffusion through the 8-ring in general is a hindered process, corresponding to a hopping event of the diffusing molecule between neighboring cages. The loading of different methanol, alkene, and aromatic species in the cages may substantially slow down or facilitate the diffusion process. The presence of Brønsted acid sites in the 8-ring enhances the diffusion process due to the formation of a favorable π-complex host-guest interaction. Aromatic hydrocarbon pool species severely hinder the diffusion and their spatial distribution in the zeolite crystal may have a significant effect on the product selectivity. Herein, we unveil how molecular factors influence the diffusion of light olefins in a complex environment with confined hydrocarbon pool species, high olefin loadings, and the presence of acid sites by means of enhanced molecular dynamics simulations under operating conditions.

Thermal Engineering of Metal-Organic Frameworks for Adsorption Applications: A Molecular Simulation Perspective.

Thermal engineering of metal-organic frameworks for adsorption-based applications is very topical in view of their industrial potential, in particular, since heat management and thermal stability have been identified as important obstacles. Hence, a fundamental understanding of the structural and chemical features underpinning their intrinsic thermal properties is highly sought-after. Herein, we investigate the nanoscale behavior of a diverse set of frameworks using molecular simulation techniques and critically compare properties such as thermal conductivity, heat capacity, and thermal expansion with other classes of materials. Furthermore, we propose a hypothetical thermodynamic cycle to estimate the temperature rise associated with adsorption for the most important greenhouse and energy-related gases (CO2 and CH4). This macroscopic response on the heat of adsorption connects the intrinsic thermal properties with the adsorption properties and allows us to evaluate their importance.

Modeling the Structural and Thermal Properties of Loaded Metal-Organic Frameworks. An Interplay of Quantum and Anharmonic Fluctuations.

Metal-organic frameworks show both fundamental interest and great promise for applications in adsorption-based technologies, such as the separation and storage of gases. The flexibility and complexity of the molecular scaffold pose a considerable challenge to atomistic modeling, especially when also considering the presence of guest molecules. We investigate the role played by quantum and anharmonic fluctuations in the archetypical case of MOF-5, comparing the material at various levels of methane loading. Accurate path integral simulations of such effects are made affordable by the introduction of an accelerated simulation scheme and the use of an optimized force field based on first-principles reference calculations. We find that the level of statistical treatment that is required for predictive modeling depends significantly on the property of interest. The thermal properties of the lattice are generally well described by a quantum harmonic treatment, with the adsorbate behaving in a classical but strongly anharmonic manner. The heat capacity of the loaded framework-which plays an important role in the characterization of the framework and in determining its stability to thermal fluctuations during adsorption/desorption cycles-requires, however, a full quantum and anharmonic treatment, either by path integral methods or by a simple but approximate scheme. We also present molecular-level insight into the nanoscopic interactions contributing to the material's properties and suggest design principles to optimize them.

Ab Initio Evaluation of Henry Coefficients Using Importance Sampling.

We present a new algorithm that allows for an efficient evaluation of the Henry coefficient of a guest molecule inside a porous material, which permits to use ab initio energy calculations. The Widom insertion method, which is currently used to compute these Henry coefficients, typically requires millions of energy evaluations. Our new methodology reduces this number by more than 1 order of magnitude, enabling the use of an ab initio potential energy surface. The methodology we propose is reminiscent of the well-known importance sampling technique which is frequently used in Monte Carlo integrations. First, a conventional Widom insertion simulation is performed using a force field. In the second step, the Widom results are used to select a limited number of configurations and only for these configurations the ab initio evaluation of the energy is required. Finally, by appropriately reweighting the latter energies, an accurate estimation of the ab initio Henry coefficient is possible at a moderate computational cost. We apply our methodology to the adsorption of CO2 in Mg-MOF-74, a prototypical case where interactions of a polar guest molecule with unsaturated metal sites dominate the adsorption mechanism. In this case generic force fields such as UFF or Dreiding are inappropriate and the use of ab initio methods is indispensable. In a second case study, we compute Henry coefficients of methane in UiO-66 using different levels of theory. We pay particular attention to the influence of the dispersion corrections and the role of many-body effects. For the final example, we qualitatively investigate adsorption features for a series of functionalized UiO-66 frameworks. Overall the cases we present show that accurate computations of Henry coefficients is extremely challenging, as different levels of theory provide strongly varying results. At the same time ab initio calculations have added value compared to force fields, as they provide a physically more sound description of the adsorption mechanism and in some cases clearly improve correspondence with experiment.

Extension of the QuickFF force field protocol for an improved accuracy of structural, vibrational, mechanical and thermal properties of metal-organic frameworks.

QuickFF was originally launched in 2015 to derive accurate force fields for isolated and complex molecular systems in a quick and easy way. Apart from the general applicability, the functionality was especially tested for metal-organic frameworks (MOFs), a class of hybrid materials consisting of organic and inorganic building blocks. Herein, we launch a new release of the QuickFF protocol which includes new major features to predict structural, vibrational, mechanical and thermal properties with greater accuracy, without compromising its robustness and transparent workflow. First, the ab initio data necessary for the fitting procedure may now also be derived from periodic models for the molecular system, as opposed to the earlier cluster-based models. This is essential for an accurate description of MOFs with one-dimensional metal-oxide chains. Second, cross terms that couple internal coordinates (ICs) and anharmonic contributions for bond and bend terms are implemented. These features are essential for a proper description of vibrational and thermal properties. Third, the fitting scheme was modified to improve robustness and accuracy. The new features are tested on MIL-53(Al), MOF-5, CAU-13 and NOTT-300. As expected, periodic input data are proven to be essential for a correct description of structural, vibrational and thermodynamic properties of MIL-53(Al). Bulk moduli and thermal expansion coefficients of MOF-5 are very accurately reproduced by static and dynamic simulations using the newly derived force fields which include cross terms and anharmonic corrections. For the flexible materials CAU-13 and NOTT-300, the transition pressure is accurately predicted provided cross terms are taken into account. © 2018 Wiley Periodicals, Inc.

Influence of a Confined Methanol Solvent on the Reactivity of Active Sites in UiO-66.

UiO-66, composed of Zr-oxide bricks and terephthalate linkers, is currently one of the most studied metal-organic frameworks due to its exceptional stability. Defects can be introduced in the structure, creating undercoordinated Zr atoms which are Lewis acid sites. Here, additional Brønsted sites can be generated by coordinated protic species from the solvent. In this Article, a multilevel modeling approach was applied to unravel the effect of a confined methanol solvent on the active sites in UiO-66. First, active sites were explored with static periodic density functional theory calculations to investigate adsorption of water and methanol. Solvent was then introduced in the pores with grand canonical Monte Carlo simulations, followed by a series of molecular dynamics simulations at operating conditions. A hydrogen-bonded network of methanol molecules is formed, allowing the protons to shuttle between solvent methanol, adsorbed water, and the inorganic brick. Upon deprotonation of an active site, the methanol solvent aids the transfer of protons and stabilizes charged configurations via hydrogen bonding, which could be crucial in stabilizing reactive intermediates. The multilevel modeling approach adopted here sheds light on the important role of a confined solvent on the active sites in the UiO-66 material, introducing dynamic acidity in the system at finite temperatures by which protons may be easily shuttled from various positions at the active sites.

Methane Adsorption in Zr-Based MOFs: Comparison and Critical Evaluation of Force Fields.

The search for nanoporous materials that are highly performing for gas storage and separation is one of the contemporary challenges in material design. The computational tools to aid these experimental efforts are widely available, and adsorption isotherms are routinely computed for huge sets of (hypothetical) frameworks. Clearly the computational results depend on the interactions between the adsorbed species and the adsorbent, which are commonly described using force fields. In this paper, an extensive comparison and in-depth investigation of several force fields from literature is reported for the case of methane adsorption in the Zr-based Metal-Organic Frameworks UiO-66, UiO-67, DUT-52, NU-1000, and MOF-808. Significant quantitative differences in the computed uptake are observed when comparing different force fields, but most qualitative features are common which suggests some predictive power of the simulations when it comes to these properties. More insight into the host-guest interactions is obtained by benchmarking the force fields with an extensive number of ab initio computed single molecule interaction energies. This analysis at the molecular level reveals that especially ab initio derived force fields perform well in reproducing the ab initio interaction energies. Finally, the high sensitivity of uptake predictions on the underlying potential energy surface is explored.

Alternating Copolymer of Double Four Ring Silicate and Dimethyl Silicone Monomer-PSS-1.

A new copolymer consisting of double four ring (D4R) silicate units linked by dimethylsilicone monomer referred to as polyoligosiloxysilicone number one (PSS-1) was synthesized. The D4R building unit is provided by hexamethyleneimine cyclosilicate hydrate crystals, which were dehydrated and reacted with dichlorodimethylsilane. The local structure of D4R silicate units and dimethyl silicone monomers was revealed by multidimensional solid-state NMR, FTIR and modeling. On average, D4R silicate units have 6.8 silicone linkages. Evidence for preferential unidirectional growth and chain ordering within the PSS-1 copolymer was provided by STEM and TEM. The structure of PSS-1 copolymer consists of twisted columns of D4R silicate units with or without cross-linking. Both models are consistent with the spectroscopic, microscopic and physical properties. PSS-1 chains are predicted to be mechanically strong compared to silicones such as PDMS, yet more flexible than rigid silica materials such as zeolites.

The Monomer Electron Density Force Field (MEDFF): A Physically Inspired Model for Noncovalent Interactions.

We propose a methodology to derive pairwise-additive noncovalent force fields from monomer electron densities without any empirical input. Energy expressions are based on the symmetry-adapted perturbation theory (SAPT) decomposition of interaction energies. This ensures a physically motivated force field featuring an electrostatic, exchange-repulsion, dispersion, and induction contribution, which contains two types of parameters. First, each contribution depends on several fixed atomic parameters, resulting from a partitioning of the monomer electron density. Second, each of the last three contributions (exchange-repulsion, dispersion, and induction) contains exactly one linear fitting parameter. These three so-called interaction parameters in the model are initially estimated separately using SAPT reference calculations for the S66x8 database of noncovalent dimers. In a second step, the three interaction parameters are further refined simultaneously to reproduce CCSD(T)/CBS interaction energies for the same database. The limited number of parameters that are fitted to dimer interaction energies (only three) avoids ill-conditioned fits that plague conventional parameter optimizations. For the exchange-repulsion and dispersion component, good results are obtained for all dimers in the S66x8 database using one single value for the associated interaction parameters. The values of those parameters can be considered universal and can also be used for dimers not present in the original database used for fitting. For the induction component such an approach is only viable for the dispersion-dominated dimers in the S66x8 database. For other dimers (such as hydrogen-bonded complexes), we show that our methodology remains applicable. However, the interaction parameter needs to be determined on a case-specific basis. As an external validation, the force field predicts interaction energies in good agreement with CCSD(T)/CBS values for dispersion-dominated dimers extracted from an HIV-II protease crystal structure with a bound ligand (indinavir). Furthermore, experimental second virial coefficients of small alkanes and alkenes are well reproduced.

Thermodynamic Insight in the High-Pressure Behavior of UiO-66: Effect of Linker Defects and Linker Expansion.

In this Article, we present a molecular-level understanding of the experimentally observed loss of crystallinity in UiO-66-type metal-organic frameworks, including the pristine UiO-66 to -68 as well as defect-containing UiO-66 materials, under the influence of external pressure. This goal is achieved by constructing pressure-versus-volume profiles at finite temperatures using a thermodynamic approach relying on ab initio derived force fields. On the atomic level, the phenomenon is reflected in a sudden drop in the number of symmetry operators for the crystallographic unit cell because of the disordered displacement of the organic linkers with respect to the inorganic bricks. For the defect-containing samples, a reduced mechanical stability is observed, however, critically depending on the distribution of these defects throughout the material, hence demonstrating the importance of judiciously characterizing defects in these materials.

Minimal Basis Iterative Stockholder: Atoms in Molecules for Force-Field Development.

Atomic partial charges appear in the Coulomb term of many force-field models and can be derived from electronic structure calculations with a myriad of atoms-in-molecules (AIM) methods. More advanced models have also been proposed, using the distributed nature of the electron cloud and atomic multipoles. In this work, an electrostatic force field is defined through a concise approximation of the electron density, for which the Coulomb interaction is trivially evaluated. This approximate "pro-density" is expanded in a minimal basis of atom-centered s-type Slater density functions, whose parameters are optimized by minimizing the Kullback-Leibler divergence of the pro-density from a reference electron density, e.g., obtained from an electronic structure calculation. The proposed method, Minimal Basis Iterative Stockholder (MBIS), is a variant of the Hirshfeld AIM method, but it can also be used as a density-fitting technique. An iterative algorithm to refine the pro-density is easily implemented with a linear-scaling computational cost, enabling applications to supramolecular systems. The benefits of the MBIS method are demonstrated with systematic applications to molecular databases and extended models of condensed phases. A comparison to 14 other AIM methods shows its effectiveness when modeling electrostatic interactions. MBIS is also suitable for rescaling atomic polarizabilities in the Tkatchenko-Scheffler scheme for dispersion interactions.

QuickFF: A program for a quick and easy derivation of force fields for metal-organic frameworks from ab initio input.

QuickFF is a software package to derive accurate force fields for isolated and complex molecular systems in a quick and easy manner. Apart from its general applicability, the program has been designed to generate force fields for metal-organic frameworks in an automated fashion. The force field parameters for the covalent interaction are derived from ab initio data. The mathematical expression of the covalent energy is kept simple to ensure robustness and to avoid fitting deficiencies as much as possible. The user needs to produce an equilibrium structure and a Hessian matrix for one or more building units. Afterward, a force field is generated for the system using a three-step method implemented in QuickFF. The first two steps of the methodology are designed to minimize correlations among the force field parameters. In the last step, the parameters are refined by imposing the force field parameters to reproduce the ab initio Hessian matrix in Cartesian coordinate space as accurate as possible. The method is applied on a set of 1000 organic molecules to show the easiness of the software protocol. To illustrate its application to metal-organic frameworks (MOFs), QuickFF is used to determine force fields for MIL-53(Al) and MOF-5. For both materials, accurate force fields were already generated in literature but they requested a lot of manual interventions. QuickFF is a tool that can easily be used by anyone with a basic knowledge of performing ab initio calculations. As a result, accurate force fields are generated with minimal effort. © 2015 Wiley Periodicals, Inc.

Direct computation of parameters for accurate polarizable force fields.

We present an improved electronic linear response model to incorporate polarization and charge-transfer effects in polarizable force fields. This model is a generalization of the Atom-Condensed Kohn-Sham Density Functional Theory (DFT), approximated to second order (ACKS2): it can now be defined with any underlying variational theory (next to KS-DFT) and it can include atomic multipoles and off-center basis functions. Parameters in this model are computed efficiently as expectation values of an electronic wavefunction, obviating the need for their calibration, regularization, and manual tuning. In the limit of a complete density and potential basis set in the ACKS2 model, the linear response properties of the underlying theory for a given molecular geometry are reproduced exactly. A numerical validation with a test set of 110 molecules shows that very accurate models can already be obtained with fluctuating charges and dipoles. These features greatly facilitate the development of polarizable force fields.