M. Waroquier

The Gradient Curves Method: An improved strategy for the derivation of molecular mechanics valence force fields from ab initio data

T. Verstraelen, D. Van Neck, P.W. Ayers, V. Van Speybroeck, M. Waroquier
LECTURE SERIES ON COMPUTER AND COMPUTATIONAL SCIENCES
Volume 7A-B, page 576 -+
2006
P1

Abstract 

A novel force-field parameterization procedure[1] is proposed that surmounts well-known difficulties of the conventional least squares parameterization. The multidimensional ab initio training data are first transformed into individual one-dimensional data sets, each associated with one term in the force-field model. In the second step conventional methods call be used to fit each energy term separately to its corresponding data set. The first step call be completed without any knowledge of the analytical expressions for the energy terms. Moreover the transformed data sets dictate the form of these expressions, which makes the method very suitable for deriving valence force fields. During the transformation in the first step, continuity and least-norm criteria are imposed. The latter facilitate the intuitive physical interpretation of the energy terms that are fitted to the transformed data sets, a prerequisite for transferable force fields. Benchmark parameterizations have been performed oil three small molecules, showing that the new method results in physically intuitive energy terms, exactly when a conventional parameterization would suffer from parameter correlations, i.e. when the number of redundant internal coordinates in the force-field model increases.

Universal descriptors for zeolite topology and acidity to predict the stability of butene cracking intermediates

P. Cnudde, M. Waroquier, V. Van Speybroeck
Catalysis Science & Technology
13, 4857-4872
2023
A1

Abstract 

The influence of pore topology and acid strength on the adsorption of (iso)butene in Brønsted acid zeolites is investigated using a combination of static calculations and ab initio molecular dynamics simulations at operating conditions. The nature and lifetime of the adsorbed intermediates – a physisorbed alkene, a chemisorbed carbenium ion or an alkoxide – is assessed for a series of one-dimensional and three-dimensional zeolite topologies as well as metal substituted aluminophosphates with varying acid site strength. While alkoxides are elusive intermediates at high temperature, irrespective of the pore dimensions or acidity, the carbenium ion stabilization is highly correlated with the zeolite confinement and acid site strength. The impact of both topology and acidity can be nicely predicted by identifying universal descriptors such as the dispersion component of the isobutene adsorption energy (topology) and the ammonia adsorption energy (acidity). It is shown that the isobutene adsorption energies and protonation barriers follow clear linear correlations with these descriptors. Our findings yield essential insight into the reactivity differences for frameworks with a different topology and acidity. The activity of a zeolite for alkene conversion can for a large part be ascribed to variations in adsorption strength and its protonation ability.

Experimental and theoretical evidence for promotional effect of acid sites on the diffusion of alkenes through small-pore zeolites

P. Cnudde, E. A. Redekop, W. Dai, N.G. Porcaro, M. Waroquier, S. Bordiga, M. Hunger, L. Li, U. Olsbye, V. Van Speybroeck
Angewandte Chemie int. Ed.
60(18): 10016-10022
2021
A1

Abstract 

The diffusion of saturated and unsaturated hydrocarbons is of fundamental importance for many zeolite‐catalyzed processes. Transport of small alkenes in the confined pores of narrow pore zeolites can become hindered, resulting in a significant impact on the ultimate product selectivity and separation. Herein, intracrystalline light olefin/paraffin diffusion through the 8‐ring windows of zeolite SAPO‐34 is characterized by a complementary set of first‐principle molecular dynamics simulations, PFG‐NMR experiments and pulse‐response Temporal Analysis of Products measurements, yielding information at different length and time scales. Our results clearly show a promotional effect of the presence of Brønsted acid sites on the diffusion rate of ethene and propene, whereas transport of alkanes is found to be insensitive to the presence of acid sites. The enhanced diffusivity of unsaturated hydrocarbons is ascribed to the formation of favorable π‐H interactions with acid protons, as confirmed by IR spectroscopy measurements. The acid site distribution is proven to be an important design parameter for optimizing product distributions and separations.

Structural and photophysical properties of various polypyridyl ligands: A combined experimental and computational study

L. De Bruecker, J. Everaert, P. Van der Voort, C.V. Stevens, M. Waroquier, V. Van Speybroeck
ChemPhysChem
21 (22), 2489–2505
2020
A1

Abstract 

Covalent triazine frameworks (CTFs) with polypyridyl ligands are very promising supports to anchor photocatalytic complexes. Herein, we investigate the photophysical properties of a series of ligands which vary by the extent of the aromatic system, the nitrogen content and their topologies to aid in selecting interesting building blocks for CTFs. Interestingly, some linkers have a rotational degree of freedom, allowing both a trans and cis structure, where only the latter allows anchoring. Therefore, the influence of the dihedral angle on the UV‐Vis spectrum is studied . The photophysical properties are investigated by a combined computational and experimental study. Theoretically, both static and molecular dynamics simulations are performed to deduce ground‐ and excited state properties based on density functional theory (DFT) and time‐dependent DFT. The position of the main absorption peak shifts towards higher wavelengths for an increased size of the π‐system and a higher π‐electron deficiency. We found that the position of the main absorption peak among the different ligands studied in this work can amount to 271 nm; which has a significant impact on the photophysical properties of the ligands. This broad range of shifts allows modulation of the electronic structure by varying the ligands and may help in a rational design of efficient photocatalysts.

Gold Open Access

Ab initio enhanced sampling kinetic study on MTO ethene methylation reaction

S. Bailleul, K. Dedecker, P. Cnudde, L. Vanduyfhuys, M. Waroquier, V. Van Speybroeck
Journal of Catalysis
388, 38-51
2020
A1

Abstract 

The methylation reaction of ethene with methanol over the Brønsted acidic ZSM-5 catalyst is one of theprototype reactions within zeolite catalysis for which experimental kinetic data is available. It is one ofthe premier reactions within the methanol-to-olefins process and has been the subject of extensive the-oretical testing to predict the reaction rates. Herein, we apply, for the first time, first principle moleculardynamics methods to determine the intrinsic reaction kinetics taking into account the full configurationalentropy. As chemical reactions are rare events, enhanced sampling methods are necessary to obtain suf-ficient sampling of the configurational space at the activated region. A plethora of methods is availablewhich depend on specific choices like the selection of collective variables along which the dynamics isenhanced. Herein, a thorough first principle molecular dynamics study is presented to determine thereaction kinetics via various enhanced MD techniques on an exemplary reaction within zeolite catalysisfor which reference theoretical and experimental data are available.

Green Open Access

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

P. Cnudde, R. Demuynck, S. Vandenbrande, M. Waroquier, G. Sastre, V. Van Speybroeck
JACS (Journal of the American Chemical Society)
142 (13), 6007-6017
2020
A1

Abstract 

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.

Ab Initio Study of Poly(vinyl chloride) Propagation Kinetics: Head-to-Head versus Head-to-Tail Additions

K. Van Cauter, V. Van Speybroeck, M. Waroquier
ChemPhysChem
8, 541-552
2007
A1

Abstract 

The relative importance of head-to-head versus head-to-tail addi-tions during the free-radical polymerization of vinyl chloride isdetermined by ab initio methods for different chain lengths ofthe polymer. First, a level of theory study is performed to deter-mine cost-effective methods for the ab initio description of thepropagation kinetics of vinyl chloride. The study includes the fol-lowing DFT-based methods: B3LYP, B3PW91, BHandH, BHandH-LYP, BLYP, BP86, MPW1K and MPW1PW91, in combination withdouble or triple zeta basis sets 6-31G(d) and 6-311GACHTUNGTRENNUNG(d,p). Also,the more recently developed BMK and MPW1K functionals are in-cluded. The influence of diffuse functions is tested by comparisonwith the basis sets 6-31+G(d) and 6-311++GACHTUNGTRENNUNG(3df,2p). The best-performing methods are B3LYP, B3PW91 and MPW1K combinedwith the 6-31+G(d) basis set. The converged probability of head-to-head propagation (2 per 1000 monomer units) is put into rela-tion with the experimental concentrations of defect structures. Acomparison is made with the head-to-head (HH) content of fluo-rine-substituted polymers and poly(vinyl acetate). The ab initiocalculations correctly predict the relative sequence of HH contentamong the various polymers.

Open Access version available at UGent repository

Unraveling the thermodynamic criteria for size-dependent spontaneous phase separation in soft porous crystals

S.M.J. Rogge, M. Waroquier, V. Van Speybroeck
Nature Communications
10, 4842
2019
A1

Abstract 

Soft porous crystals (SPCs) harbor a great potential as functional nanoporous materials owing to their stimuli-induced and tuneable morphing between different crystalline phases. These large-amplitude phase transitions are often assumed to occur cooperatively throughout the whole material, which thereby retains its perfect crystalline order. Here, we disprove this paradigm through mesoscale first-principles based molecular dynamics simulations, demonstrating that morphological transitions do induce spatial disorder under the form of interfacial defects and give rise to yet unidentified phase coexistence within a given sample. We hypothesize that this phase coexistence can be stabilized by carefully tuning the experimental control variables through, e.g., temperature or pressure quenching. The observed spatial disorder helps to rationalize yet elusive phenomena in SPCs, such as the impact of crystal downsizing on their flexible nature, thereby identifying the crystal size as a crucial design parameter for stimuli-responsive devices based on SPC nanoparticles and thin films.

Gold Open Access

A Supramolecular View on the Cooperative Role of Brønsted andLewis Acid Sites in Zeolites for Methanol Conversion

S. Bailleul, I. Yarulina, A.E.J. Hoffman, A. Dokania, E. Abou-Hamad, A. Dutta Chowdhury, G. Pieters, J. Hajek, K. De Wispelaere, M. Waroquier, J. Gascon, V. Van Speybroeck
JACS (Journal of the American Chemical Society)
141 (37), 14823-14842
2019
A1

Abstract 

A systematic molecular level and spectroscopic investigation is presented to show the cooperative role of Brønsted acid and Lewis acid sites in zeolites for the conversion of methanol. Extra-framework alkaline-earth metal containing species and aluminum species decrease the number of Brønsted acid sites, as protonated metal clusters are formed. A combined experimental and theoretical effort shows that postsynthetically modified ZSM-5 zeolites, by incorporation of extra-framework alkaline-earth metals or by demetalation with dealuminating agents, contain both mononuclear [MOH]+ and double protonated binuclear metal clusters [M(μ-OH)2M]2+ (M = Mg, Ca, Sr, Ba, and HOAl). The metal in the extra-framework clusters has a Lewis acid character, which is confirmed experimentally and theoretically by IR spectra of adsorbed pyridine. The strength of the Lewis acid sites (Mg > Ca > Sr > Ba) was characterized by a blue shift of characteristic IR peaks, thus offering a tool to sample Lewis acidity experimentally. The incorporation of extra-framework Lewis acid sites has a substantial influence on the reactivity of propene and benzene methylations. Alkaline-earth Lewis acid sites yield increased benzene methylation barriers and destabilization of typical aromatic intermediates, whereas propene methylation routes are less affected. The effect on the catalytic function is especially induced by the double protonated binuclear species. Overall, the extra-framework metal clusters have a dual effect on the catalytic function. By reducing the number of Brønsted acid sites and suppressing typical catalytic reactions in which aromatics are involved, an optimal propene selectivity and increased lifetime for methanol conversion over zeolites is obtained. The combined experimental and theoretical approach gives a unique insight into the nature of the supramolecular zeolite catalyst for methanol conversion which can be meticulously tuned by subtle interplay of Brønsted and Lewis acid sites.

Open Access version available at UGent repository
Gold Open Access

Dynamic interplay between defective UiO-66 and protic solvents in activated processes

C. Caratelli, J. Hajek, E.J. Meijer, M. Waroquier, V. Van Speybroeck
Chemistry - A European Journal
25 (67), 15315-15325
2019
A1

Abstract 

UiO‐66, composed by Zr‐oxide inorganic bricks [Zr6(μ3‐O)4(μ3‐OH)4] and organic terephthalate linkers, is one of the most studied metal–organic frameworks (MOFs) due to its exceptional thermal, chemical, and mechanical stability. Thanks to its high connectivity, the material can withstand structural deformations during activation processes such as linker exchange, dehydration, and defect formation. These processes do alter the zirconium coordination number in a dynamic way, creating open metal sites for catalysis and thus are able to tune the catalytic properties. In this work, it is shown, by means of first‐principle molecular‐dynamics simulations at operating conditions, how protic solvents may facilitate such changes in the metal coordination. Solvent can induce structural rearrangements in the material that can lead to undercoordinated but also overcoordinated metal sites. This is demonstrated by simulating activation processes along well‐chosen collective variables. Such enhanced MD simulations are able to track the intrinsic dynamics of the framework at realistic conditions.

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