Efficient construction of free energy profiles of breathing metal–organic frameworks using advanced molecular dynamics simulations

In order to reliably predict and understand the breathing behavior of highly flexible metal–organic frameworks from thermodynamic considerations, an accurate estimation of the free energy difference between their different metastable states is a prerequisite. Herein, a variety of free energy estimation methods are thoroughly tested for their ability to construct the free energy profile as a function of the unit cell volume of MIL-53(Al). The methods comprise free energy perturbation, thermodynamic integration, umbrella sampling, metadynamics, and variationally enhanced sampling. A series of molecular dynamics simulations have been performed in the frame of each of the five methods to describe structural transformations in flexible materials with the volume as the collective variable, which offers a unique opportunity to assess their computational efficiency. Subsequently, the most efficient method, umbrella sampling, is used to construct an accurate free energy profile at different temperatures for MIL-53(Al) from first principles at the PBE+D3(BJ) level of theory. This study yields insight into the importance of the different aspects such as entropy contributions and anharmonic contributions on the resulting free energy profile. As such, this thorough study provides unparalleled insight in the thermodynamics of the large structural deformations of flexible materials

Quantum tunneling rotor as a sensitive atomistic probe of guests in a metal-organic framework

Quantum tunneling rotors in a zeolitic imidazolate framework ZIF-8 can provide insights into local gas adsorption sites and local dynamics of porous structure, which are inaccessible to standard physisorption or x-ray diffraction sensitive primarily to long-range order. Using in situ high-resolution inelastic neutron scattering at 3 K, we follow the evolution of methyl tunneling with respect to the number of dosed gas molecules. While nitrogen adsorption decreases the energy of the tunneling peak, and ultimately hinders it completely (0.33 meV to zero), argon substantially increases the energy to 0.42 meV. Ab initio calculations of the rotational barrier of ZIF-8 show an exception to the reported adsorption sites hierarchy, resulting in anomalous adsorption behavior and linker dynamics at subatmospheric pressure. The findings reveal quantum tunneling rotors in metal-organic frameworks as a sensitive atomistic probe of local physicochemical phenomena.MMC Laboratory is supported by the ERC Consolidator Grant (PROMOFS Grant Agreement No. 771575) and EPSRC Awards (Grants No. EP/N014960/1 and No. EP/R511742/1). We thank ISIS Facility for the awarded OSIRIS beamtime (Grants No. RB1410426, No. RB1510529, and No. RB1610180), DOIs 10.5286/ISIS.E.RB1410426, 10.5286/ISIS.E.RB1510529, and 10.5286/ISIS.E.RB1610180, as well as the Cryogenics, and Pressure & Furnaces teams for their exemplary support. M.R.R. acknowledges the U.S. DOE Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division (Separation Sciences). This work is further supported by the Fund for Scientific Research Flanders (FWO) through a Ph.D. fellowship for A.L. (Grant No. 11D2220N) and a postdoctoral fellowship for S.M.J.R. (Grant No. 12T3522N). Financial support for F.F.-A. from the Spanish Ministry of Science and Innovation (Grant No. PID2020-114506GBI00 funded by MCIN/AEI/10.13039/501100011033 and Grant No. TED2021-129457B-I00 funded by MCIN/AEI/10.13039/501100011033 and the European Union NextGenerationEU/PRTR) as well as the Basque Government (Grant No. PIBA-2021-0026) is gratefully acknowledged. We also acknowledge the financial support received from the IKUR Strategy under the collaboration agreement between Ikerbasque Foundation and the Materials Physics Center on behalf of the Department of Education of the Basque Government.Peer reviewe

Strongly Reducing (Diarylamino)benzene-Based Covalent Organic Framework for Metal-Free Visible Light Photocatalytic H2O2 Generation

Photocatalytic reduction of molecular oxygen is a promising route toward sustainable production of hydrogen peroxide (H2O2). This challenging process requires photoactive semiconductors enabling solar energy driven generation and separation of electrons and holes with high charge transfer kinetics. Covalent organic frameworks (COFs) are an emerging class of photoactive semiconductors, tunable at a molecular level for high charge carrier generation and transfer. Herein, we report two newly designed two-dimensional COFs based on a (diarylamino)benzene linker that form a Kagome (kgm) lattice and show strong visible light absorption. Their high crystallinity and large surface areas (up to 1165 m(2)center dot g(-1)) allow efficient charge transfer and diffusion. The diarylamine (donor) unit promotes strong reduction properties, enabling these COFs to efficiently reduce oxygen to form H2O2. Overall, the use of a metal-free, recyclable photocatalytic system allows efficient photocatalytic solar transformations.DFG, 390540038, EXC 2008: Unifying Systems in Catalysis "UniSysCat"EC/H2020/665501/EU/[PEGASUS]², giving wings to your career./PEGASUS-2EC/H2020/834134/EU/Water Forced in Hydrophobic Nano-Confinement: Tunable Solvent System/WATUSOEC/H2020/647755/EU/First principle molecular dynamics simulations for complex chemical transformations in nanoporous materials/DYNPO

Reliably Modeling the Mechanical Stability of Rigid and Flexible Metal–Organic Frameworks

ConspectusOver the past two decades, metal–organic frameworks (MOFs) have matured from interesting academic peculiarities toward a continuously expanding class of hybrid, nanoporous materials tuned for targeted technological applications such as gas storage and heterogeneous catalysis. These oft-times crystalline materials, composed of inorganic moieties interconnected by organic ligands, can be endowed with desired structural and chemical features by judiciously functionalizing or substituting these building blocks. As a result of this reticular synthesis, MOF research is situated at the intriguing intersection between chemistry and physics, and the building block approach could pave the way toward the construction of an almost infinite number of possible crystalline structures, provided that they exhibit stability under the desired operational conditions. However, this enormous potential is largely untapped to date, as MOFs have not yet found a major breakthrough in technological applications. One of the remaining challenges for this scale-up is the densification of MOF powders, which is generally achieved by subjecting the material to a pressurization step. However, application of an external pressure may substantially alter the chemical and physical properties of the material. A reliable theoretical guidance that can presynthetically identify the most stable materials could help overcome this technological challenge.In this Account, we describe the recent research the progress on computational characterization of the mechanical stability of MOFs. So far, three complementary approaches have been proposed, focusing on different aspects of mechanical stability: (i) the Born stability criteria, (ii) the anisotropy in mechanical moduli such as the Young and shear moduli, and (iii) the pressure-versus-volume equations of state. As these three methods are grounded in distinct computational approaches, it is expected that their accuracy and efficiency will vary. To date, however, it is unclear which set of properties are suited and reliable for a given application, as a comprehensive comparison for a broad variety of MOFs is absent, impeding the widespread use of these theoretical frameworks.Herein, we fill this gap by critically assessing the performance of the three computational models on a broad set of MOFs that are representative for current applications. These materials encompass the mechanically rigid UiO-66­(Zr) and MOF-5­(Zn) as well as the flexible MIL-47­(V) and MIL-53­(Al), which undergo pressure-induced phase transitions. It is observed that the Born stability criteria and pressure-versus-volume equations of state give complementary insight into the macroscopic and microscopic origins of instability, respectively. However, interpretation of the Born stability criteria becomes increasingly difficult when less symmetric materials are considered. Moreover, pressure fluctuations during the simulations hamper their accuracy for flexible materials. In contrast, the pressure-versus-volume equations of state are determined in a thermodynamic ensemble specifically targeted to mitigate the effects of these instantaneous fluctuations, yielding more accurate results. The critical Account presented here paves the way toward a solid computational framework for an extensive presynthetic screening of MOFs to select those that are mechanically stable and can be postsynthetically densified before their use in targeted applications

MicMec: Developing the Micromechanical Model to Investigate the Mechanics of Correlated Node Defects in UiO-66

New functional materials, such as mixed matrix membranes and metal–organic framework (MOF) monoliths, outperform traditional materials in gas separation and storage applications, among other pressing challenges. However, while most engineered materials nowadays exhibit spatial heterogeneities on different length scales, available simulation techniques to date cannot capture this spatial complexity fully. Herein, we present the MicMec implementation of the micromechanical model we introduced earlier as a systematic coarse-graining approach to routinely access these larger length scales and characterize the mechanical properties of these materials. We thoroughly discuss the key components of our open-source code and validate it both on an analytical system and a case study on correlated reo defects in the UiO-66 MOF. We reveal that the time step that can be reached in micromechanical simulations is 2 to 3 orders of magnitude larger than for atomistic simulations, while still capturing well the macroscopic mechanical properties of the spatially disordered UiO-66 material. It is our hope that the MicMec implementation discussed here may provide a complementary tool to existing atomistic and coarse-grained software and aid the computational design of new materials for pressing applications

MOFs for long-term gas storage: exploiting kinetic trapping in ZIF-8 for on-demand and stimuli-controlled gas release

In this study, we investigate the potential of metal-organic frameworks (MOFs) for long-term gas storage under ambient conditions. Specifically, we selected a MOF ZIF-8 (with a 0.34 nm large pore aperture), which exhibits a temperature- and pressure-regulated gating effect, and loaded it with sulphur hexafluoride (with a kinetic diameter of 0.55 nm). By optimising the loading conditions, we were able to achieve up to 33 wt% SF6 loading into the pores of ZIF-8. Although MOFs featuring gating effects are known to adsorb gases larger than the pore openings, herein, by applying high pressure (and optionally elevated temperature), kinetic trapping of the gas guest was also achieved. When investigating the gas release under ambient conditions, three MOF samples of different crystal sizes (ca. 45 nm, 1.5 & mu;m and 5 & mu;m) were examined. Remarkably, for the largest crystals, more than 86% of the initially loaded gas remained trapped in the pores even after being exposed to air for 100 days under ambient conditions. Our findings indicate that the extremely slow release of SF6 is due to the high activation energy for the guest diffusion through the narrow pore opening in ZIF-8, which was supported by both ab initio-based computational studies and experimental data including modulated thermogravimetric analysis. On the other hand, we also showed that the gas could be released on-demand by applying an elevated temperature or by exposing the MOF to an acidic environment, which opens possibilities for facile gas micro- and nano-dosing applications

OGRe: Optimal Grid Refinement Protocol for Accurate Free Energy Surfaces and Its Application in Proton Hopping in Zeolites and 2D COF Stacking

While free energy surfaces are the crux of our understanding of many chemical and biological processes, their accuracy is generally unknown. Moreover, many developments to improve their accuracy are often complicated, limiting their general use. Luckily, several tools and guidelines are already in place to identify these shortcomings, but they are typically lacking in flexibility or fail to systematically determine how to improve the accuracy of the free energy calculation. To overcome these limitations, this work introduces OGRe, a Python package for optimal grid refinement in an arbitrary number of dimensions. OGRe is based on three metrics that gauge the confinement, consistency, and overlap of each simulation in a series of umbrella sampling (US) simulations, an enhanced sampling technique ubiquitously adopted to construct free energy surfaces for hindered processes. As these three metrics are fundamentally linked to the accuracy of the weighted histogram analysis method adopted to generate free energy surfaces from US simulations, they facilitate the systematic construction of accurate free energy profiles, where each metric is driven by a specific umbrella parameter. This allows for the derivation of a consistent and optimal collection of umbrellas for each simulation, largely independent of the initial values, thereby dramatically increasing the ease-of-use toward accurate free energy surfaces. As such, OGRe is particularly suited to determine complex free energy surfaces with large activation barriers and shallow minima, which underpin many physical and chemical transformations and hence to further our fundamental understanding of these processes