V. Van Speybroeck

Structure–performance descriptors and the role of Lewis acidity in the methanol-to-propylene process

I. Yarulina, K. De Wispelaere, S. Bailleul, J. Goetze, M. Radersma, E. Abou-Hamad, I. Vollmer, M. Goesten, B. Mezari, E.J.M. Hensen, J. S. Martínez-Espín, M. Morten, S. Mitchell, J. Perez-Ramirez, U. Olsbye, B.M. Weckhuysen, V. Van Speybroeck, F. Kapteijn, J. Gascon
Nature Chemistry
10 (8), 804-812
2018
A1

Abstract 

The combination of well-defined acid sites, shape-selective properties and outstanding stability places zeolites among the most practically relevant heterogeneous catalysts. The development of structure–performance descriptors for processes that they catalyse has been a matter of intense debate, both in industry and academia, and the direct conversion of methanol to olefins is a prototypical system in which various catalytic functions contribute to the overall performance. Propylene selectivity and resistance to coking are the two most important parameters in developing new methanol-to-olefin catalysts. Here, we present a systematic investigation on the effect of acidity on the performance of the zeolite ‘ZSM-5’ for the production of propylene. Our results demonstrate that the isolation of Brønsted acid sites is key to the selective formation of propylene. Also, the introduction of Lewis acid sites prevents the formation of coke, hence drastically increasing catalyst lifetime.

DOI 

http://dx.doi.org/10.1038/s41557-018-0081-0

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

DOI 

http://dx.doi.org/10.1016/j.cpc.2018.09.020

Use of 3-Hydroxy-4-(trifluoromethyl)azetidin-2-ones as Building Blocks for the Preparation of Trifluoromethyl-Containing Aminopropanes, 1,3-Oxazinan-2-ones, Aziridines, and 1,4-Dioxan-2-ones

H.D. Thi, G. Le Nhat Thuy, S. Catak, V. Van Speybroeck, T. Van Nguyen, M. D'Hooghe
Synthesis-Stuttgart
50, 1439-1456
2018
A1

Abstract 

3-Hydroxy- 4-(trifluoromethyl)azetidin- 2-ones were efficiently synthesized from the corresponding 3-benzyloxy-β-lactams and successfully transformed into 3-chloro- 4-(trifluoromethyl)azetidin-2- one building blocks. The latter chlorides were shown to be eligible precursors for the construction of novel CF 3 -containing aminopropanes, 1,3-oxazinanes, 1,3-oxazinan-2- ones and aziridines. In addition, 3-hydroxy- 4-(trifluoromethyl)azetidin- 2-ones proved to be interesting substrates for the synthesis of novel 3-[2,2,2- trifluoro-1-(arylamino)ethyl]-1,4- dioxan-2- ones via intramolecular cyclization of 3-(2- hydroxyethoxy)-β-lactam intermediates.

DOI 

http://dx.doi.org/10.1055/s-0036-1591537

Understanding Brønsted-Acid Catalyzed Monomolecular Reactions of Alkanes in Zeolite Pores by Combining Insights from Experiment and Theory

J. Van der Mynsbrugge, A. Janda, L.-C. Lin, V. Van Speybroeck, M. Head-Gordon, A.T. Bell
ChemPhysChem
19 (4), 341-358
2018
A1

Abstract 

Acidic zeolites are effective catalysts for the cracking of large hydrocarbon molecules into lower molecular weight products required for transportation fuels. However, the ways in which the zeolite structure affects the catalytic activity at Brønsted protons are not fully understood. One way to characterize the influence of the zeolite structure on the catalysis is to study alkane cracking and dehydrogenation at very low conversion, conditions for which the kinetics are well defined. To understand the effects of zeolite structure on the measured rate coefficient (kapp), it is necessary to identify the equilibrium constant for adsorption into the reactant state (Kads-H+) and the intrinsic rate coefficient of the reaction (kint) at reaction temperatures, since kapp is proportional to the products of Kads-H+ and kint. We show that Kads-H+ cannot be calculated from experimental adsorption data collected near ambient temperature, but can, however, be estimated accurately from configurational-bias Monte Carlo (CBMC) simulations. Using monomolecular cracking and dehydrogenation of C3-C6 alkanes as an example, we review recent efforts aimed at elucidating the influence of the acid site location and the zeolite framework structure on the observed values of kapp and its components, Kads-H+ and kint.

DOI 

http://dx.doi.org/10.1002/cphc.201701084

Theoretical Insight into the Regioselective Ring-Expansions of Bicyclic Aziridinium Ions

E. Birsen Boydas, G. Tanriver, M. D'Hooghe, H-J. Ha, V. Van Speybroeck, S. Catak
Organic & Biomolecular Chemistry
16 (5), 796-806
2018
A1

Abstract 

Transient bicyclic aziridinium ions are known to undergo ring-expansion reactions, paving the way to functionalized nitrogen-containing heterocycles. In this study, the regioselectivity observed in the ring-expansion reactions of 1-azoniabicyclo[n.1.0]alkanes was investigated from a computational viewpoint to study the ring-expansion pathways of two bicyclic systems with different ring sizes. Moreover, several nucleophiles leading to different experimental results were investigated. The effect of solvation was taken into account using both explicit and implicit solvent models. This theoretical rationalization provides valuable insight into the observed regioselectivity and may be used as a predictive tool in future studies.

DOI 

http://dx.doi.org/10.1039/C7OB02253K

On the intrinsic dynamic nature of the rigid UiO-66 metal-organic framework

J. Hajek, C. Caratelli, R. Demuynck, K. De Wispelaere, L. Vanduyfhuys, M. Waroquier, V. Van Speybroeck
Chemical Science
9 (10), 2723-2732
2018
A1

Abstract 

UiO-66 is a showcase example of an extremely stable metal–organic framework, which maintains its structural integrity during activation processes such as linker exchange and dehydration. The framework can even accommodate a substantial number of defects without compromising its stability. These observations point to an intrinsic dynamic flexibility of the framework, related to changes in the coordination number of the zirconium atoms. Herein we follow the dynamics of the framework in situ, by means of enhanced sampling molecular dynamics simulations such as umbrella sampling, during an activation process, where the coordination number of the bridging hydroxyl groups capped in the inorganic Zr6(μ3-O)4(μ3-OH)4 brick is reduced from three to one. Such a reduction in the coordination number occurs during the dehydration process and in other processes where defects are formed. We observe a remarkable fast response of the system upon structural changes of the hydroxyl group. Internal deformation modes are detected, which point to linker decoordination and recoordination. Detached linkers may be stabilized by hydrogen bonds with hydroxyl groups of the inorganic brick, which gives evidence for an intrinsic dynamic acidity even in the absence of protic guest molecules. Our observations yield a major step forward in the understanding on the molecular level of activation processes realized experimentally but that is hard to track on a purely experimental basis.

Open Access version available at UGent repository
Green Open Access

DOI 

http://dx.doi.org/10.1039/C7SC04947A

How chain length and branching influence the alkene cracking reactivity on H-ZSM-5

P. Cnudde, K. De Wispelaere, L. Vanduyfhuys, R. Demuynck, J. Van der Mynsbrugge, M. Waroquier, V. Van Speybroeck
ACS Catalysis
8, 9579 − 9595
2018
A1

Abstract 

Catalytic alkene cracking on H-ZSM-5 involves a complex reaction network with many possible reaction routes and often elusive intermediates. Herein, advanced molecular dynamics simulations at 773 K, a typical cracking temperature, are performed to clarify the nature of the intermediates and to elucidate dominant cracking pathways at operating conditions. A series of C4-C8 alkene intermediates are investigated to evaluate the influence of chain length and degree of branching on their stability. Our simulations reveal that linear, secondary carbenium ions are relatively unstable, although their lifetime increases with carbon number. Tertiary carbenium ions, on the other hand, are shown to be very stable, irrespective of the chain length. Highly branched carbenium ions, though, tend to rapidly rearrange into more stable cationic species, either via cracking or isomerization reactions. Dominant cracking pathways were determined by combining these insights on carbenium ion stability with intrinsic free energy barriers for various octene β-scission reactions, determined via umbrella sampling simulations at operating temperature (773 K). Cracking modes A (3° → 3°) and B2 (3° → 2°) are expected to be dominant at operating conditions, whereas modes B1 (2° → 3°), C (2° → 2°), D2 (2° → 1°) and E2 (3° → 1°) are expected to be less important. All β-scission modes in which a transition state with primary carbocation character is involved have high intrinsic free energy barriers. Reactions starting from secondary carbenium ions will contribute less as these intermediates are short living at the high cracking temperature. Our results show the importance of simulations at operating conditions to properly evaluate the carbenium ion stability for β-scission reactions and to assess the mobility of all species in the pores of the zeolite.

Open Access version available at UGent repository
Gold Open Access

DOI 

http://dx.doi.org/10.1021/acscatal.8b01779

Extension of the QuickFF force field protocol for an improved accuracy of structural, vibrational, mechanical and thermal properties of Metal Organic Frameworks

L. Vanduyfhuys, S. Vandenbrande, J. Wieme, M. Waroquier, T. Verstraelen, V. Van Speybroeck
Journal of Computational Chemistry
39 (16), p. 999-1011
2018
A1

Abstract 

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 transparant 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 is 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.

Open Access version available at UGent repository
Gold Open Access

DOI 

http://dx.doi.org/10.1002/jcc.25173

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