P. Cnudde

Mechanistic insight into the framework methylation ofH-ZSM-5 for varying methanol loading and Si/Al ratiousing first principles molecular dynamics simulations

S. A. F. Nastase, P. Cnudde, L. Vanduyfhuys, K. De Wispelaere, V. Van Speybroeck, C.R.A. Catlow, A. J. Logsdail
ACS Catalysis
10, 15, 8904-8915
2020
A1
Gold Open Access

Cation−π Interactions Accelerate the Living Cationic Ring-Opening Polymerization of Unsaturated 2-Alkyl-2-oxazolines

E. Van den Broeck, B. Verbraeken, K. Dedecker, P. Cnudde, L. Vanduyfhuys, T. Verstraelen, K. Van Hecke, V. V. Jerca, S. Catak, R. Hoogenboom, V. Van Speybroeck
Macromolecules
53, 10, 3832-3846
2020
A1

Abstract 

Cation–dipole interactions were previously shown to have a rate-enhancing effect on the cationic ring-opening polymerization (CROP) of 2-oxazolines bearing a side-chain ester functionality. In line with this, a similar rate enhancement—via intermolecular cation−π interactions—was anticipated to occur when π-bonds are introduced into the 2-oxazoline side-chains. Moreover, the incorporation of π-bonds allows for facile postfunctionalization of the resulting poly(2-oxazoline)s with double and triple bonds in the side-chains via various click reactions. Herein, a combined molecular modeling and experimental approach was used to study the CROP reaction rates of 2-oxazolines with side-chains having varying degrees of unsaturation and side-chain length. The presence of cation−π interactions and the influence of the degree of unsaturation were initially confirmed by means of regular molecular dynamics simulations on pentameric systems. Furthermore, a combination of enhanced molecular dynamics simulations, static calculations, and a thorough analysis of the noncovalent interactions was performed to unravel to what extent cation−π interactions alter the reaction kinetics. Additionally, the observed trends were confirmed also in the presence of acetonitrile as solvent, in which experimentally the polymerization is performed. Most intriguingly, we found only a limited effect on the intrinsic reaction kinetics of the CROP and a preorganization effect in the reactive complex region. The latter effect was established by the unsaturated side-chains and the cationic center through a complex interplay between cation−π, π–π, π–induced dipole, and cation–dipole interactions. These findings led us to propose a two-step mechanism comprised of an equilibration step and a CROP reaction step. The influence of the degree of unsaturation, through a preorganization effect, on the equilibration step was determined with the following trend for the polymerization rates: n-ButylOx < ButenOx < ButynOx ≥ PentynOx. The trend was experimentally confirmed by determining the polymerization rate constants.

Open Access version available at UGent repository
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.

Effect of zeolite topology and reactor configuration on the direct conversion of CO2 to light olefins and aromatics

A. Ramirez Galilea, A. Dutta Chowdhury, A. Dokania, P. Cnudde, M. Caglayan, I. Yarulina, E. Abou-Hamad, L. Gevers, S. Ould-Chikh, K. De Wispelaere, V. Van Speybroeck, J. Gascon
ACS Catalysis
9, 6320-6334
2019
A1

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

Effect of temperature and branching on the nature and stability of alkene cracking intermediates in H-ZSM-5

P. Cnudde, K. De Wispelaere, J. Van der Mynsbrugge, M. Waroquier, V. Van Speybroeck
Journal of Catalysis
345, 53-69
2017
A1

Abstract 

Catalytic cracking of alkenes takes place at elevated temperatures in the order of 773–833 K. In this work, the nature of the reactive intermediates at typical reaction conditions is studied in H-ZSM-5 using a complementary set of modeling tools. Ab initio static and molecular dynamics simulations are performed on different C4single bond C5 alkene cracking intermediates to identify the reactive species in terms of temperature. At 323 K, the prevalent intermediates are linear alkoxides, alkene π-complexes and tertiary carbenium ions. At a typical cracking temperature of 773 K, however, both secondary and tertiary alkoxides are unlikely to exist in the zeolite channels. Instead, more stable carbenium ion intermediates are found. Branched tertiary carbenium ions are very stable, while linear carbenium ions are predicted to be metastable at high temperature. Our findings confirm that carbenium ions, rather than alkoxides, are reactive intermediates in catalytic alkene cracking at 773 K.

Open Access version available at UGent repository

On the stability and nature of adsorbed pentene in Brønsted acid zeolite H-ZSM-5 at 323 K

J. Hajek, J. Van der Mynsbrugge, K. De Wispelaere, P. Cnudde, L. Vanduyfhuys, M. Waroquier, V. Van Speybroeck
Journal of Catalysis
340, 227 - 235
2016
A1

Abstract 

Adsorption of linear pentenes in H-ZSM-5 at 323 K is investigated using contemporary static and molecular dynamics methods. A physisorbed complex corresponding to free pentene, a π-complex and a chemisorbed species may occur. The chemisorbed species can be either a covalently bonded alkoxide or an ion pair, the so-called carbenium ion. Without finite temperature effects, the π-complex is systematically slightly more bound than the chemisorbed alkoxide complex, whereas molecular dynamics calculations at 323 K yield an almost equal stability of both species. The carbenium ion was not observed during simulations at 323 K. The transformation from the π-complex to the chemisorbed complex is activated by a free energy in the range of 33–42 kJ/mol. Our observations yield unprecedented insights into the stability of elusive intermediates in zeolite catalysis, for which experimental data are very hard to measure.

Open Access version available at UGent repository

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