D. Lesthaeghe

The effect of confined space on the growth of naphthalenic species in an H-SSZ-13 catalyst: a molecular modeling study

K. Hemelsoet, A. Nollet, M. Vandichel, D. Lesthaeghe, V. Van Speybroeck, M. Waroquier
ChemCatChem
1 (3), 373-378
2009
A1

Abstract 

Methylation reactions of naphthalenic species over the acidic microporous zeolite with chabazite topology have been investigated by means of two-layered ab initio computations. Large cluster results combined with van der Waals contributions provide thermodynamic and kinetic results of successive methylation steps. The growth of fused bicyclic species is important as these can act as hydrocarbon pool species within the methanol-to-olefin (MTO) process, but ultimately leads to the deactivation of the catalyst. The influence of the confined space of the zeolite pore on the resulting transition state or product shape selectivity is investigated in detail.

Theoretical Insights on Methylbenzene Side-Chain Growth in ZSM-5 Zeolites for Methanol-to-Olefin Conversion

D. Lesthaeghe, A. Horré, M. Waroquier, G.B. Marin, V. Van Speybroeck
Chemistry - A European Journal
15 (41), 10803–10808
2009
A1

Abstract 

The key step in the conversion of methane to polyolefins is the catalytic conversion of methanol to light olefins. The most recent formulations of a reaction mechanism for this process are based on the idea of a complex hydrocarbon-pool network, in which certain organic species in the zeolite pores are methylated and from which light olefins are eliminated. Two major mechanisms have been proposed to date—the paring mechanism and the side-chain mechanism—recently joined by a third, the alkene mechanism. Recently we succeeded in simulating a full catalytic cycle for the first of these in ZSM-5, with inclusion of the zeolite framework and contents. In this paper, we will investigate crucial reaction steps of the second proposal (the side-chain route) using both small and large zeolite cluster models of ZSM-5. The deprotonation step, which forms an exocyclic double bond, depends crucially on the number and positioning of the other methyl groups but also on steric effects that are typical for the zeolite lattice. Because of steric considerations, we find exocyclic bond formation in the ortho position to the geminal methyl group to be more favourable than exocyclic bond formation in the para position. The side-chain growth proceeds relatively easily but the major bottleneck is identified as subsequent de-alkylation to produce ethene. These results suggest that the current formulation of the side-chain route in ZSM-5 may actually be a deactivating route to coke precursors rather than an active ethene-producing hydrocarbon-pool route. Other routes may be operating in alternative zeotype materials like the silico-aluminophosphate SAPO-34.

Theoretical evaluation of zeolite confinement effects on the reactivity of bulky intermediates

D. Lesthaeghe, V. Van Speybroeck, M. Waroquier
Physical Chemistry Chemical Physics (PCCP)
11 (26), 5222-5226
2009
A1

Abstract 

Zeolites provide a unique setting for heterogeneous Brønsted acid catalysis, because the effects of the surrounding framework on fundamental reaction kinetics go well beyond what would be expected for a mere reaction flask. This aspect becomes very pronounced when bulky molecules form key intermediates for the reaction under study, which is exactly when the interaction between the framework and the intermediate is maximal. We will use the example of methanol-to-olefin conversion (MTO), and, more specifically, the constant interplay between the inorganic host framework and the organic hydrocarbon pool co-catalyst, to illustrate how zeolite confinement directly influences catalytic reaction rates. Theoretical calculations are used to isolate and quantify these specific effects, with the main focus on methylbenzenes in ZSM-5, as the archetypical MTOcatalyst. This review intends to give an overview of recent theoretical insights, which have proven to provide an ideal complementary tool to experimental investigations. In addition, we will also introduce the role of zeolite breathing in activating a catalytic cycle.

MFI Fingerprint: How Pentasil-Induced IR Bands Shift during Zeolite Nanogrowth

D. Lesthaeghe, P. Vansteenkiste, T. Verstraelen, A. Ghysels, C. Kirschhock, J.A. Martens, V. Van Speybroeck, M. Waroquier
Journal of Physical Chemistry C
112 (25), 9186-9191
2008
A1

Abstract 

Silicalite-1 zeolite exhibits a characteristic pentasil framework vibration around 540−550 cm−1. In the initial stages of zeolite synthesis, however, this band is observed at much higher wavenumbers: literature shows this vibration to depend on particle size and to shift over 100 cm−1 with increasing condensation. In this work, the pentasil vibration frequency was derived from theoretical molecular dynamics simulations to obtain the correct IR band assignments for important nanoparticles. The IR spectroscopic fingerprint of oligomeric five-ring containing precursors proposed in the literature was computed and compared with experimental data. Our theoretical results show that, while isolated five-membered rings show characteristic vibrational bands around 650 cm−1, the combination of five-membered rings in the full MFI-type structure readily generates the bathochromic shift to the typical pentasil vibration around 550 cm−1. As opposed to what was previously believed, the IR band does not shift gradually as nanoparticle size increases, but it is highly dependent on the specific way structural units are added. The most important feature is the appearance of an additional band when double five-membered rings are included, which allows for a clear distinction between the key stages of early zeolite nucleation. Furthermore, the combination of the simulated spectra with the experimental observation of this spectral feature in nanoparticles extracted from silicalite-1 clear solutions supports their structured nature. The theoretical insights on the dependency of pentasil vibrations with the degree of condensation offer valuable support toward future investigations on the genesis of a zeolite crystal.

Recent theoretical insights into the role of the zeolite framework on methanol-to-olefin conversion

D. Lesthaeghe, V. Van Speybroeck, M. Waroquier
Studies in Surface Science and Catalysis
174, Part A, 741-744
2008
P1

Abstract 

The two major conflicting proposals in the methanol-to-olefin process are thoroughly investigated and compared using hybrid multi-level modeling techniques. This investigation leads to the absolute rejection of the intensively studied direct mechanisms and provides a successful alternative catalytic cycle as well as additional insight into the hydrocarbon pool proposal.

The Rise and Fall of Direct Mechanisms in Methanol-to-Olefin Catalysis:  An Overview of Theoretical Contributions

D. Lesthaeghe, V. Van Speybroeck, G.B. Marin, M. Waroquier
Industrial & Engineering Chemistry Research
46 (26), 8832-8838
2007
A1

Abstract 

Over the past 30 years, the methanol-to-olefin (MTO) process on acidic zeolites has been subject to a vast number of studies from both industrial and academic researchers, leading to numerous controversies regarding the most probable reaction mechanism. Improvement of computational facilities during the past decade led to a sudden boost of theoretical contributions that, when considered individually, all seemed to provide reasonable evidence for partial pathways of the commonly proposed direct mechanisms. Not only the reactions suggested by experimental studies were investigated, but in addition novel potential routes were discovered by theoreticians as well. However, when all of the individual reactions scattered throughout the literature were recently combined, theoretically obtained rate coefficients turned out to show the exact opposite, that is, the complete failure of the direct mechanisms to produce ethylene from methanol only. In this paper, we give a detailed overview of the theoretical contributions that initially supported the direct mechanism proposal, but which finally culminated in its demise.

Refinement of the supramolecular concept in methanol-to-olefin catalysis

D. Lesthaeghe, V. Van Speybroeck, G.B. Marin, M. Waroquier
Studies in Surface Science and Catalysis
170, 1668-1676
2007
P1

Abstract 

The supramolecular character of methanol-to-olefin conversion in acidic zeolites is thoroughly investigated from a theoretical viewpoint. State-of-the-art modeling techniques have not only led to an absolute rejection of the intensively studied direct mechanisms, but have also provided additional insights into the alternative hydrocarbon pool proposal. The role of various external factors such as zeolite topology on the formation of crucial carbenium ions is discussed and the establaished supramolecular picture is refined.

Zeolite Shape-Selectivity in the gem-Methylation of Aromatic Hydrocarbons

D. Lesthaeghe, B. De Sterck, V. Van Speybroeck, G.B. Marin, M. Waroquier
Angewandte Chemie int. Ed.
46 (8), 1311-1314
2007
A1

Abstract 

The kind of olefins obtained from methanol in zeolites is strongly dependent on specific combinations of the intermediate organic hydrocarbon-pool species and zeolite topology (see picture). If the cage is too large, neutral species are favored over reactive cations. If the cage is too small, transition-state-shape selectivity poses severe limitations on the reactivity of bulkier species.

Global DFT-Based Reactivity Indicators:  An Assessment of Theoretical Procedures in Zeolite Catalysis

K. Hemelsoet, D. Lesthaeghe, V. Van Speybroeck, M. Waroquier
Journal of Physical Chemistry C
111 (7), 3028-3037
2007
A1

Abstract 

The dependence of global reactivity descriptors on electronic structure method as well as basis set is investigated for typical reactions in zeolite catalysis. This research is especially focused on hard−hard interactions between small probe molecules (such as chloromethane, methanol, ethylene, and propene) and different zeolite clusters containing both oxygen and amine functionalities. The performance of novel hybrid metafunctionals (such as BMK and MPWB1K) on crucial reactivity predictors is assessed through comparison with both Hartree−Fock and B3-LYP results. For the complex bifunctional zeolite systems, we find accurate results using any of the DFT functionals, in conjunction with a basis set of at least double-ζ quality further augmented with both polarization and diffuse functions. Reactivity sequences, based on global softness differences as well as activation hardness values, are generally found to be independent of the level of theory whenever a DFT functional is used.

Pagina's

Abonneren op RSS - D. Lesthaeghe