J. Wieme

Unravelling thermal stress due to thermal expansion mismatch in metal-organic frameworks for methane storage

J. Wieme, V. Van Speybroeck
Journal of Materials Chemistry A
2020
A1

Abstract 

Thermal stress is present in all systems undergoing temperature changes during their operation. Metal-organic frameworks (MOFs) are a class of porous, crystalline materials ideally suited for a wide range of adsorption-based technologies. The release and consumption of the heat of adsorption instigate temperature fluctuations and thermal stress in these materials that could induce disruptive volume changes. To bring these materials to engineering applications, it is of utmost importance to understand their thermal expansion behavior and the overall induced thermal stress due to thermal expansion mismatch with other components. In this work, we focus on a large group of MOFs known to have promising methane adsorption properties and predict their thermal expansion coefficients based on force field molecular dynamics simulations. Negative thermal expansion (NTE) behavior is predicted for all studied MOFs, and the magnitude of the NTE coefficients is found to be positively correlated with the degree of porosity of the frameworks. Finally, as a proxy for the thermal stress, the thermal pressure coefficient is calculated, which is found to be in the range between polymers and ceramics. Variations within the operating temperature range of MOFs are therefore expected to result in a relatively low thermal stress.

Atomistic insight in the flexibility and heat transport properties of the stimuli-responsive metal-organic framework MIL-53(Al) for water-adsorption applications using molecular simulations

A. Lamaire, J. Wieme, A.E.J. Hoffman, V. Van Speybroeck
Faraday Discussions
2020
A1

Abstract 

To exploit the full potential of metal-organic frameworks as solid adsorbents in water-adsorption applications, many challenges remain to be solved. A more fundamental insight into the properties of the host material and the influence water exerts on them can be obtained by performing molecular simulations. In this work, the prototypical flexible MIL-53(Al) framework is modelled using advanced molecular dynamics simulations. For different water loadings, the presence of water is shown to affect the relative stability of MIL-53(Al), triggering a phase transition from the narrow-pore to the large-pore phase at the highest considered loading. Furthermore, the effect of confinement on the structural organisation of the water molecules is also examined for different pore volumes of MIL-53(Al). For the framework itself, we focus on the thermal conductivity, as this property plays a decisive role in the efficiency of adsorption-based technologies, due to the energy-intensive adsorption and desorption cycles. To this end, the heat transfer characteristics of both phases of MIL-53(Al) are studied, demonstrating a strong directional dependence for the thermal conductivity.

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

J. Wieme, S. Vandenbrande, A. Lamaire, V. Kapil, L. Vanduyfhuys, V. Van Speybroeck
ACS Applied Materials & Interfaces
11 (42), 38697-38707
2019
A1

Abstract 

Thermal engineering of metal-organic frameworks (MOFs) for adsorption-based applications is very topical in view of their industrial potential, especially 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 material classes. 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.

Pillared-layered metal-organic frameworks for mechanical energy storage applications

J. Wieme, S.M.J. Rogge, P.G. Yot, L. Vanduyfhuys, S.-K. Lee, J.-S. Chang, M. Waroquier, G. Maurin, V. Van Speybroeck
Journal of Materials Chemistry A
7 (39), 22663-22674
2019
A1

Abstract 

Herein we explore the unique potential of pillared-layered metal–organic frameworks of the DMOF-1 family for mechanical energy storage applications. In this work, we theoretically predict for the guest-free DMOF-1 a new contracted phase by exerting an external mechanical pressure of more than 200 MPa with respect to the stable phase at atmospheric pressure. The breathing transition is accompanied by a very large volume contraction of about 40%. The high transition pressures and associated volume changes make these materials highly promising with an outstanding mechanical energy work. Furthermore, we show that changing the nature of the metal allows to tune the behavior under mechanical pressure. The various phases were revealed by a combination of periodic density-functional theory calculations, force field molecular dynamics simulations and mercury intrusion experiments for DMOF-1(Zn) and DMOF-1(Cu). The combined experimental and theoretical approach allowed to discover the potential of these materials for new technological developments.

Gold Open Access

Structure–Mechanical Stability Relations of Metal-Organic Frameworks via Machine Learning

P.Z. Moghadam, S.M.J. Rogge, A. Li, C.-M. Chow, J. Wieme, N. Moharrami, M. Aragones-Anglada, G. Conduit, D.A. Gomez-Gualdron, V. Van Speybroeck, D. Fairen-Jimenez
Matter
1 (1), 219-234
2019
A1

Abstract 

Assessing the mechanical stability of metal-organic frameworks (MOFs) is critical to bring these materials to any application. Here, we derive the first interactive map of the structure-mechanical landscape of MOFs by performing a multi-level computational analysis. First, we used high-throughput molecular simulations for 3,385 MOFs containing 41 distinct network topologies. Second, we developed a freely available machine-learning algorithm to automatically predict the mechanical properties of MOFs. For distinct regions of the high-throughput space, in-depth analysis based on in operando molecular dynamics simulations reveals the loss-of-crystallinity pressure within a given topology. The overarching mechanical screening approach presented here reveals the sensitivity on structural parameters such as topology, coordination characteristics and the nature of the building blocks, and paves the way for computational as well as experimental researchers to assess and design MOFs with enhanced mechanical stability to accelerate the translation of MOFs to industrial applications.

Gold Open Access

On the importance of anharmonicities and nuclear quantum effects in modelling the structural properties and thermal expansion of MOF-5

A. Lamaire, J. Wieme, S.M.J. Rogge, M. Waroquier, V. Van Speybroeck
Journal of Chemical Physics
150 (9), 094503
2019
A1

Abstract 

In this article, we investigate the influence of anharmonicities and nuclear quantum effects (NQEs) in modelling the structural properties and thermal expansion of the empty MOF-5 metal-organic framework. To introduce NQEs in classical molecular dynamics simulations, two different methodologies are considered, comparing the approximate, but computationally cheap, method of generalised Langevin equation thermostatting to the more advanced, computationally demanding path integral molecular dynamics technique. For both methodologies, similar results were obtained for all the properties under investigation. The structural properties of MOF-5, probed by means of radial distribution functions (RDFs), show some distinct differences with respect to a classical description. Besides a broadening of the RDF peaks under the influence of quantum fluctuations, a different temperature dependence is also observed due to a dominant zero-point energy (ZPE) contribution. For the thermal expansion of MOF-5, by contrast, NQEs appear to be only of secondary importance with respect to an adequate modelling of the anharmonicities of the potential energy surface (PES), as demonstrated by the use of two differently parametrised force fields. Despite the small effect in the temperature dependence of the volume of MOF-5, NQEs do however significantly affect the absolute volume of MOF-5, in which the ZPE resulting from the intertwining of NQEs and anharmonicities plays a crucial role. A sufficiently accurate description of the PES is therefore prerequisite when modelling NQEs.

The impact of lattice vibrations on the macroscopic breathing behavior of MIL-53(Al)

A.E.J. Hoffman, J. Wieme, S.M.J. Rogge, L. Vanduyfhuys, V. Van Speybroeck
Zeitschrift für Kristallographie - Crystalline Materials
234 (7-8), 529-545
2019
A1

Abstract 

The mechanism inducing the breathing in flexible metal-organic frameworks, such as MIL-53(Al), is still not fully understood. Herein, the influence of lattice vibrations on the breathing transition in MIL-53(Al) is investigated to gain insight in this phenomenon. Through solid-state density-functional theory calculations, the volume-dependent IR spectrum is computed together with the volume-frequency relations of all vibrational modes. Furthermore, important thermodynamic properties such as the Helmholtz free energy, the specific heat capacity, the bulk modulus, and the volumetric thermal expansion coefficient are derived via these volume-frequency relations using the quasi-harmonic approximation. The simulations expose a general volume-dependency of the vibrations with wavenumbers above 300 cm−1 due to their localized nature. In contrast, a diverse set of volume-frequency relations are observed for vibrations in the terahertz region (< 300 cm−1) containing the vibrations exhibiting collective behavior. Some terahertz vibrations display large frequency differences over the computed volume range, induced by either repulsion or strain effects, potentially triggering the phase transformation. Finally, the impact of the lattice vibrations on the thermodynamic properties is investigated. This reveals that the closed pore to large pore phase transformation in MIL-53(Al) is mainly facilitated by terahertz vibrations inducing rotations of the organic linker, while the large pore to closed pore phase transformation relies on two framework-specific soft modes.

Gold Open Access

Protocol for Identifying Accurate Collective Variables in Enhanced Molecular Dynamics Simulations for the Description of Structural Transformations in Flexible Metal–Organic Frameworks

R. Demuynck, J. Wieme, S.M.J. Rogge, K. Dedecker, L. Vanduyfhuys, M. Waroquier, V. Van Speybroeck
Journal of Chemical Theory and Computation
14 (11), pp 5511–5526
2018
A1

Abstract 

Various kinds of flexibility have been observed in metal–organic frameworks, which may originate from the topology of the material or the presence of flexible ligands. The construction of free energy profiles describing the full dynamical behavior along the phase transition path is challenging since it is not trivial to identify collective variables able to identify all metastable states along the reaction path. In this work, a systematic three-step protocol to uniquely identify the dominant order parameters for structural transformations in flexible metal–organic frameworks and subsequently construct accurate free energy profiles is presented. Methodologically, this protocol is rooted in the time-structure based independent component analysis (tICA), a well-established statistical modeling technique embedded in the Markov state model methodology and often employed to study protein folding, that allows for the identification of the slowest order parameters characterizing the structural transformation. To ensure an unbiased and systematic identification of these order parameters, the tICA decomposition is performed based on information from a prior replica exchange (RE) simulation, as this technique enhances the sampling along all degrees of freedom of the system simultaneously. From this simulation, the tICA procedure extracts the order parameters—often structural parameters—that characterize the slowest transformations in the material. Subsequently, these order parameters are adopted in traditional enhanced sampling methods such as umbrella sampling, thermodynamic integration, and variationally enhanced sampling to construct accurate free energy profiles capturing the flexibility in these nanoporous materials. In this work, the applicability of this tICA-RE protocol is demonstrated by determining the slowest order parameters in both MIL-53(Al) and CAU-13, which exhibit a strongly different type of flexibility. The obtained free energy profiles as a function of this extracted order parameter are furthermore compared to the profiles obtained when adopting less-suited collective variables, indicating the importance of systematically selecting the relevant order parameters to construct accurate free energy profiles for flexible metal–organic frameworks, which is in correspondence with experimental findings. The method succeeds in mapping the full free energy surface in terms of appropriate collective variables for MOFs exhibiting linker flexibility. For CAU-13, we show the decreased stability of the closed pore phase by systematically adding adsorbed xylene molecules in the framework.

i-PI 2.0: A Universal Force Engine for Advanced Molecular Simulations

V. Kapil, M. Rossi, O. Marsalek, R. Petraglia, Y. Litman, T. Spura, B. Cheng, A. Cuzzocrea, R.H. Meißner, D. Wilkins, P. Juda, S.P. Bienvenue, J. Kessler, I. Poltavsky, S. Vandenbrande, J. Wieme, C. Corminboeuf, T. Kühne, D. Manolopoulos, T. Markland, J. Richardson, A. Tkatchenko, G.A. Tribello, V. Van Speybroeck, M. Ceriotti
Computer Physics Communications
236, 214-223
2019
A1

Abstract 

Progress in the atomic-scale modelling of matter over the past decade has been tremendous. This progress has been brought about by improvements in methods for evaluating interatomic forces that work by either solving the electronic structure problem explicitly, or by computing accurate approximations of the solution and by the development of techniques that use the Born-Oppenheimer (BO) forces to move the atoms on the BO potential energy surface. As a consequence of these developments it is now possible to identify stable or metastable states, to sample configurations consistent with the appropriate thermodynamic ensemble, and to estimate the kinetics of reactions and phase transitions. All too often, however, progress is slowed down by the bottleneck associated with implementing new optimization algorithms and/or sampling techniques into the many existing electronic-structure and empirical-potential codes. To address this problem, we are thus releasing a new version of the i-PI software. This piece of software is an easily extensible framework for implementing advanced atomistic simulation techniques using interatomic potentials and forces calculated by an external driver code. While the original version of the code was developed with a focus on path integral molecular dynamics techniques, this second release of i-PI not only includes several new advanced path integral methods, but also offers other classes of algorithms. In other words, i-PI is moving towards becoming a universal force engine that is both modular and tightly coupled to the driver codes that evaluate the potential energy surface and its derivatives.

http://arxiv.org/abs/1808.03824

Open Access version available at UGent repository
Green Open Access

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