L. Vanduyfhuys

Thermodynamic insight into stimuli-responsive behavior of soft porous crystals

L. Vanduyfhuys, S.M.J. Rogge, J. Wieme, S. Vandenbrande, G. Maurin, M. Waroquier, V. Van Speybroeck
Nature Communications
9, 1, 204
2018
A1

Abstract 

Knowledge of the thermodynamic potential in terms of the independent variables allows to characterize the macroscopic state of the system. However, in practice, it is difficult to access this potential experimentally due to irreversible transitions that occur between equilibrium states. A showcase example of sudden transitions between (meta)stable equilibrium states is observed for soft porous crystals possessing a network with long-range structural order, which can transform between various states upon external stimuli such as pressure, temperature and guest adsorption. Such phase transformations are typically characterized by large volume changes and may be followed experimentally by monitoring the volume change in terms of certain external triggers. Herein, we present a generalized thermodynamic approach to construct the underlying Helmholtz free energy as a function of the state variables that governs the observed behaviour based on microscopic simulations. This concept allows a unique identification of the conditions under which a material becomes flexible.

Open Access version available at UGent repository
Gold Open Access

Elucidating the Vibrational Fingerprint of the Flexible Metal-Organic Framework MIL-53(Al) Using a Combined Experimental/Computational Approach

A.E.J. Hoffman, L. Vanduyfhuys, I. Nevjestic, J. Wieme, S.M.J. Rogge, H. Depauw, P. Van der Voort, H. Vrielinck, V. Van Speybroeck
Journal of Physical Chemistry C
122, 5, 2734-2746
2018
A1

Abstract 

In this work mid-infrared (mid-IR), far-IR, and Raman spectra are presented for the distinct (meta)stable phases of the flexible metal-organic framework MIL-53(Al). Static density functional theory (DFT) simulations are performed allowing for the identification of all IR active modes, which is unprecedented in the low-frequency region. A unique vibrational fingerprint is revealed, resulting from aluminum-oxide backbone stretching modes, which can be used to clearly distinguish the IR spectra of the closed- and large-pore phases. Furthermore, molecular dynamics simulations based on a DFT description of the potential energy surface enable to determine the theoretical Raman spectrum of the closed- and large-pore phases for the first time. An excellent correspondence between theory and experiment is observed. Both the low-frequency IR and Raman spectra show major differences in vibrational modes between the closed- and large-pore phases indicating changes in lattice dynamics between the two structures. In addition, several collective modes related to the breathing mechanism in MIL-53(Al) are identified. In particular, we rationalize the importance of the trampoline-like motion of the linker for the phase transition.

Open Access version available at UGent repository
Gold Open Access

Efficient Construction of Free Energy Profiles of Breathing Metal-Organic Frameworks Using Advanced Molecular Dynamics Simulations

R. Demuynck, S.M.J. Rogge, L. Vanduyfhuys, J. Wieme, M. Waroquier, V. Van Speybroeck
Journal of Chemical Theory and Computation (JCTC)
13 (12), 5861-5873
2017
A1

Abstract 

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.

Open Access version available at UGent repository
Gold Open Access

Mechanical properties of a gallium fumarate metal-organic framework: a joint experimental-modelling exploration

P. Ramaswamy, J. Wieme, E. Alvarez, L. Vanduyfhuys, J.-P. Itié, P. Fabry, V. Van Speybroeck, C. Serre, P.G. Yot, G. Maurin
Journal of Materials Chemistry A
5 (22), 11047-11054
2017
A1

Abstract 

A gallium analogue of the commercially available Al-fumarate MOF A520 - recently identified as isotypic to MIL-53(Al)-BDC - has been synthesized and further characterized in its hydrated and dehydrated forms. The structural response under applied mechanical pressure of this MIL-53(Ga)-FA solid was investigated using advanced experimental techniques coupled with computational tools. Hg porosimetry and high-pressure X-Ray Powder Diffraction (XRPD) experiments evidenced that the pristine dehydrated large pore form undergoes an irreversible structure contraction upon an applied pressure of 85 MPa with an associated volume change of ca. 14% which makes this material promising for mechanical energy storage applications, in particular as a shock absorber. The breathing behavior was further rationalized performing a series of periodic Density Functional Theory (DFT) calculations with the construction of an energy profile as a function of volume for both MIL-53(Ga)-FA and its Aluminum analogue. As such we could fully unravel the microscopic origin of the difference in pressure-induced behavior for the aluminum and gallium fumarate based materials.

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

S.M.J. Rogge, J. Wieme, L. Vanduyfhuys, S. Vandenbrande, G. Maurin, T. Verstraelen, M. Waroquier, V. Van Speybroeck
Chemistry of Materials
28 (16), 5721-5732
2016
A1

Abstract 

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.

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
http://pubs.acs.org/doi/abs/10.1021/acs.chemmater.6b01956

Open Access version available at UGent repository
Gold Open Access

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

T. Verstraelen, S. Vandenbrande, F. Heidar-Zadeh, L. Vanduyfhuys, V. Van Speybroeck, M. Waroquier, P.W. Ayers
Journal of Chemical Theory and Computation (JCTC)
12(8), 3894-3912
2016
A1

Abstract 

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-Sheffler scheme for dispersion interactions.

Exploring the flexibility of MIL-47(V)-type materials using force field molecular dynamics simulations

J. Wieme, L. Vanduyfhuys, S.M.J. Rogge, M. Waroquier, V. Van Speybroeck
Journal of Physical Chemistry C
120 (27), 14934-14947
2016
A1

Abstract 

The flexibility of three MIL-47(V)-type materials (MIL-47, COMOC-2 and COMOC-3) has been explored by constructing the pressure-versus-volume and free energy-versus-volume curves at various temperatures ranging from 100 K to 400 K. This is done with first-principles based force fields using the recently proposed QuickFF parameterization protocol. Specific terms were added for the materials at hand to describe the asymmetry of the one-dimensional vanadium-oxide chain and to account for the flexibility of the organic linkers. The force fields are used in a series of molecular dynamics simulations at fixed volumes, but varying unit cell shapes. The three materials show a distinct pressure-versus-volume behavior, which underlines the ability to tune the mechanical properties by varying the linkers towards different applications such as nanosprings, dampers and shock absorbers.

Open Access version available at UGent repository

A breathing zirconium metal-organic framework with reversible loss of crystallinity by correlated nanodomain formation

B. Bueken, F. Vermoortele, M.J. Cliffe, M.T. Wharmby, D. Foucher, J. Wieme, L. Vanduyfhuys, C. Martineau, N. Stock, F. Taulelle, V. Van Speybroeck, A.L. Goodwin, D. De Vos
Chemistry - A European Journal
2016, 22, 1-5
2016
A1

Abstract 

The isoreticular analogue of the metal–organic framework UiO-66(Zr), synthesized with the flexible trans-1,4-cyclohexanedicarboxylic acid as linker, shows a peculiar breathing behavior by reversibly losing long-range crystalline order upon evacuation. The underlying flexibility is attributed to a concerted conformational contraction of up to two thirds of the linkers, which breaks the local lattice symmetry. X-ray scattering data are described well by a nanodomain model in which differently oriented tetragonal-type distortions propagate over about 7–10 unit cells.

Is the error on first-principles volume predictions absolute or relative?

K. Lejaeghere, L. Vanduyfhuys, T. Verstraelen, V. Van Speybroeck, S. Cottenier
Computational Materials Science
117, 390-396
2016
A1

Abstract 

Many benchmarks of density-functional theory with respect to experiment suggest the error on predicted equilibrium volumes to scale with the volume. Relative volume errors are therefore often used as a decisive argument to select one exchange-correlation functional over another. We show that the error on the volume (after correcting for systematic deviations) is only approximately relative. A simple analytic model, validated by rigorous Monte Carlo simulations, reveals that a more accurate error estimate can be derived from the inverse of the bulk modulus. This insight is not only instrumental for the selection or design of suitable functionals. It also calls for a new attitude towards computational errors: to report computational errors on electronic-structure calculations, identify systematic deviations and distinguish between relative and absolute effects. (C) 2016 Elsevier B.V. All rights reserved.

Open Access version available at UGent repository

Vibrational fingerprint of the absorption properties of UiO-type MOF materials

A. Van Yperen-De Deyne, K. Hendrickx, L. Vanduyfhuys, G. Sastre, P. Van der Voort, V. Van Speybroeck, K. Hemelsoet
Theoretical Chemistry Accounts
135, 4, 102
2016
A1

Abstract 

The absorption properties of UiO-type metal–organic frameworks are computed using TD-DFT simulations on the organic linkers. A set of nine isoreticular structures, including the UiO-66 and UiO-67 materials and functionalized variants, are examined. The excitation energies from a static geometry optimization are compared with dynamic averages obtained from sampling the ground-state potential energy surface using molecular dynamics. The vibrational modes that impact the excitation energy are identified. This analysis is done using a recently proposed tool based on power spectra of the velocities and the excitation energies. The applied procedure allows including important factors influencing the absorption spectra, such as the periodic framework, linker variation and dynamical effects including harmonic and anharmonic nuclear motions. This methodology allows investigating in detail the vibrational fingerprint of the excitation energy of advanced materials such as MOFs and gives perspectives to tailor materials toward new light-based applications.

Open Access version available at UGent repository

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