D. Lesthaeghe

Theoretical study on the alteration of fundamental zeolite properties by methylene functionalization

D. Lesthaeghe, G. Delcour, V. Van Speybroeck, G.B. Marin, M. Waroquier
Microporous and Mesoporous Materials
96 (1-3), 350-356
2006
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Abstract 

Following the recent boost of papers reporting synthesis of organic functionalized microporous and mesoporous materials, a detailed theoretical study was performed to probe the effect of organic functionalizations on certain fundamental properties in organosilicas from a microscopic viewpoint. The simplest functionalization of a bridging methylene unit was modeled in a zeolite MFI-type framework to serve as a model system for more complex organic moieties and other structures. Calculated adsorption energies for H2O and NH3 in methylenesilica reveal that the methylene functionalization increases the strength of the interaction of both probe molecules with the zeolite framework. Investigation of the combination of an ion-exchanged aluminum site containing a CH2-bridge demonstrates how the methylene moiety creates a steric obstruction for adsorbed alkali metal ions such as Li, Na and K, resulting in a weaker bond between these ions and the aluminum site. Finally, a study of proton mobility from a Brønsted acid site to a neighboring methylene bridge reveals that the acid proton will most likely migrate from the basic oxygen bridge to the methylene substitution. This implies that the combination of methylene moieties with aluminum impurities will lead to terminally bound methyl groups and cleavage of the hybrid organic–inorganic lattice.

Understanding the failure of direct C-C coupling in the zeolite-catalyzed methanol-to-olefin process

D. Lesthaeghe, V. Van Speybroeck, G.B. Marin, M. Waroquier
Angewandte Chemie int. Ed.
45 (11), 1714-1719
2006
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Abstract 

You are the weakest link, goodbye! Many individual steps of the direct mechanisms in the methanol-to-olefin process are tied together in an integrated scheme, allowing a simple identification of the weakest links. Calculations show that a combined pathway from methanol directly to ethylene does not exist and no CC bond can be formed directly.

Bifunctional acid-base catalyzed reactions in zeolites from the HSAB viewpoint

K. Hemelsoet, D. Lesthaeghe, V. Van Speybroeck, M. Waroquier
Chemical Physics Letters
419 (1-3), 10-15
2006
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Abstract 

The applicability of the hard and soft acids and bases principle is investigated for the interaction of 5T zeolite clusters with probe molecules such as chloromethane, methanol and olefins. The reactions are intermediately hard–hard and, therefore, mainly charge-controlled. This is confirmed by the success of the atomic charges and the electrostatic interaction energy at the acid site as correct descriptors of regio-selectivity and reactivity sequences. Both acid and basic reactive sites can be clearly indicated using frontier orbitals. Moreover, an excellent correlation is found between the activation hardnesses and the energy barriers at the absolute zero.

What role do oxonium ions and oxonium ylides play in the ZSM-5 catalysed methanol-to-olefin process?

D. Lesthaeghe, V. Van Speybroeck, G.B. Marin, M. Waroquier
Chemical Physics Letters
417(4-6), 309-315
2006
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Abstract 

The adsorption properties and possible rearrangements of several proposed oxonium ylides and oxonium ions in protonated ZSM-5 are studied using the 2-layered ONIOM(B3LYP/6-31+g(d,p):HF/3-21g) approach. We show that both methyl oxonium methylide and dimethyl oxonium methylide are highly energetic species and unlikely to be intermediates in the formation of a carbon–carbon bond as the zeolite lattice does not offer supplementary stabilisation. The trimethyl oxonium and ethyldimethyl oxonium ions, however, are distinctly stabilised by the surrounding ZSM-5 framework, which does not impose steric constraints on further intermolecular reactions.

DFT Investigation of Alkoxide vs Alkylammonium Formation in Amine-Substituted Zeolites

D. Lesthaeghe, V. Van Speybroeck, G.B. Marin, M. Waroquier
Journal of Physical Chemistry B
109 (16), 7952–7960
2005
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Abstract 

Density functional theory (DFT) cluster calculations were used to describe bifunctional acid−base properties of amine-substituted zeolites containing a Brønsted acid site. Preliminary results (J. Am. Chem. Soc. 2004, 126, 9162) indicated that efficient use of both functional groups might lead to a substantial lowering of activation barriers. In this paper, comparison is made between the alkoxide formation in zeolites containing only oxygen bridges and alkylammonium formation on the bridging NH groups in amine-functionalized zeolites for various guest species, such as methanol, ethene, and chloromethane. The amine functionalization only lowers barriers for SN2 type reactions with otherwise highly strained transition states, as is the case for chloromethane. In these new materials more basic sites are introduced into the zeolite framework, enabling optimal linear SN2 type transition states incorporating various T sites.

Efficient Use of Bifunctional Acid−Base Properties for Alkylammonium Formation in Amine-Substituted Zeolites

D. Lesthaeghe, V. Van Speybroeck, M. Waroquier
JACS (Journal of the American Chemical Society)
126 (30), 9162–9163
2004
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Abstract 

The formation of alkylammonium groups in amine-doped zeolites is studied using density functional theory on small clusters representing the chemically active site. The presence of both strong Lewis base and Brønsted acid sites leads to a significant lowering of reaction barriers as opposed to alkoxide formation in full-oxygen zeolites. Furthermore, amine-substituted zeolites suggest novel reaction pathways that are not solely centralized around the aluminum substitution but in which two tetrahedral sites are involved, maximizing use of the zeolitic acid site and its surroundings. An investigation of the proton mobility in these yet to be synthesized materials demonstrates the need for minimizing the amount of Al−NH−Si bridges, as to prevent protonation of the amine group.

Assembly of cyclic hydrocarbons from ethene and propene in acid zeolite catalysis to produce active catalytic sites for MTO conversion

M. Vandichel, D. Lesthaeghe, J. Van der Mynsbrugge, M. Waroquier, V. Van Speybroeck
Journal of Catalysis
271 (1), 67-78
2010
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Abstract 

The formation of cyclic hydrocarbons from smaller building blocks such as ethene and propene is investigated in protonated ZSM-5, using a 2-layered ONIOM(B3LYP/6-31+g(d):HF/6-31+g(d)) approach and an additional Grimme-type van der Waals dispersion correction term to account for the long-range dispersion interactions. These cyclic species form precursors for active hydrocarbon pool species and play a key role in activating the acidic zeolite host for successful methanol-to-olefin (MTO) conversion. Starting from trace amounts of ethene and propene that are formed during an initial induction period or during the active phase, dimerization reactions allow for rapid chain growth. The products of these reactions can be neutral alkenes, framework-bound alkoxide species or intermediate carbenium ions, depending on the zeolite environment taken into account. On the basis of rate constants for successive reaction steps, a viable route toward cyclization is proposed, which starts from the formation of a framework-bound propoxide from propene, followed by dimerization with an additional propene molecule to form the 2-hexyl carbenium ion which finally undergoes ring closure to yield methylcyclopentane. This cyclic species in turn forms a precursor for either an active hydrocarbon pool compound or for deactivating coke deposit.

Open Access version available at UGent repository

A Complete Catalytic Cycle for Supramolecular Methanol-to-Olefins Conversion by Linking Theory with Experiment

D.M. McCann, D. Lesthaeghe, P.W. Kletnieks, D.R. Guenther, M.J. Hayman, V. Van Speybroeck, M. Waroquier, J.F. Haw
Angewandte Chemie int. Ed.
47 (28), 5179-5182
2008
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Abstract 

Le Tour de MTO: A complete working catalytic cycle for the conversion of methanol to olefins in HZSM-5 is reported in full consistency with both experimental and theoretical observations. This particular route includes carbon-atom scrambling into a methylbenzene ring, NMR-observed cationic intermediates, and the production of isobutene.

Full Theoretical Cycle for both Ethene and Propene Formation during Methanol-to-Olefin Conversion in H-ZSM-5

D. Lesthaeghe, J. Van der Mynsbrugge, M. Vandichel, M. Waroquier, V. Van Speybroeck
ChemCatChem
3 (1), 208-212
2011
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Abstract 

The methanol-to-olefin (MTO) process, catalyzed by acidic zeolites such as H-ZSM-5, provides an increasingly important alternative to the production of light olefins from crude oil. However, the various mechanistic proposals for methanol-to-olefin conversion have been strongly disputed for the past several decades. This work provides theoretical evidence that the experimentally suggested ‘alkene cycle’, part of a co-catalytic hydrocarbon pool, offers a viable path to the production of both propene and ethene, in stark contrast to the often- proposed direct mechanisms. This specific proposal hinges on repeated methylation reactions of alkenes, starting from propene, which occur easily within the zeolite environment. Subsequent cracking steps regenerate the original propene molecule, while also forming new propene and ethene molecules as primary products. Because the host framework stabilizes intermediate carbenium ions, isomerization and deprotonation reactions are extremely fast. Combined with earlier joint experimental and theoretical work on polymethylbenzenes as active hydrocarbon pool species, it is clear that, in zeolite H-ZSM-5, multiple parallel and interlinked routes operate on a competitive basis.

29Si NMR and UV-Raman Investigation of Initial Oligomerization Reaction Pathways in Acid-Catalyzed Silica Sol-Gel Chemistry

A. Depla, D. Lesthaeghe, T.S. van Erp, A. Aerts, K. Houthoofd, F. Fan, C. Li, V. Van Speybroeck, M. Waroquier, C. Kirschhock, J.A. Martens
Journal of Physical Chemistry C
115 (9), 3562–3571
2011
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Abstract 

The initial molecular steps of the acid-catalyzed silica sol−gel process de-parting from tetraethylorthosilicate (TEOS) were investigated by in situ 29Si NMR and UV−Raman spectroscopy. The use of a substoichiometric H2O:TEOS molar ratio (r-value 0.2−1.2) slowed the silicate oligomerization reaction and allowed unraveling the initial steps of silica condensation. Molecular modeling confirmed Raman signal and 29Si NMR shift assignment. A comprehensive listing of all Raman and 29Si NMR assignments is provided, including unique Raman assignments of cyclosilicates and the linear tetramer. The combination of experiment and modeling allowed an analysis of the reaction kinetics. The derived kinetic model and the experimental observation both revealed that the H2O:TEOS molar ratio had a strong influence on the reaction kinetics but not on the reaction pathways. The multianalytical approach led to development of an oligomerization scheme. As dominant oligomerizations, chain growth, cyclodimerization, and branching were identified. Under the investigated conditions, chains did not grow longer than pentamer, and ring sizes were limited to 6-rings. Chains of 4 Si atoms and 4-rings were abundant species. Branched rings and chains were formed by attachment of dimers and trimers. Gelation proceeded from branched 4-rings and branched chains with limited hydroxyl functionalities.

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