V. Van Speybroeck

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.

The electronegativity equalization method and the split charge equilibration applied to organic systems: Parametrization, validation, and comparison

T. Verstraelen, V. Van Speybroeck, M. Waroquier
Journal of Chemical Physics
131 (4), 044127
2009
A1

Abstract 

An extensive benchmark of the electronegativity equalization method (EEM) and the split charge equilibration (SQE) model on a very diverse set of organic molecules is presented. These models efficiently compute atomic partial charges and are used in the development of polarizable force fields. The predicted partial charges that depend on empirical parameters are calibrated to reproduce results from quantum mechanical calculations. Recently, SQE is presented as an extension of the EEM to obtain the correct size dependence of the molecular polarizability. In this work, 12 parametrization protocols are applied to each model and the optimal parameters are benchmarked systematically. The training data for the empirical parameters comprise of MP2/Aug-CC-pVDZ calculations on 500 organic molecules containing the elements H, C, N, O, F, S, Cl, and Br. These molecules have been selected by an ingenious and autonomous protocol from an initial set of almost 500 000 small organic molecules. It is clear that the SQE model outperforms the EEM in all benchmark assessments. When using Hirshfeld-I charges for the calibration, the SQE model optimally reproduces the molecular electrostatic potential from the ab initio calculations. Applications on chain molecules, i.e., alkanes, alkenes, and alpha alanine helices, confirm that the EEM gives rise to a divergent behavior for the polarizability, while the SQE model shows the correct trends. We conclude that the SQE model is an essential component of a polarizable force field, showing several advantages over the original EEM.

Reversibility from DFT-Based Reactivity Indices: Intramolecular Side Reactions in the Polymerization of Poly(vinyl chloride)

F. De Vleeschouwer, A. Toro-Labbe, S. Gutierrez-Oliva, V. Van Speybroeck, M. Waroquier, P. Geerlings, F. De Proft
Journal of Physical Chemistry A
113 (27), 7899-7908
2009
A1

Abstract 

A detailed investigation of the kinetic irreversibility−reversibility concept is presented on the basis of the analysis of four side reactions occurring in the polymerization of poly(vinyl chloride), the intramolecular 1,5- and 1,6-backbiting and 1,2- and 2,3-Cl shift side reactions. Density functional theory-based reactivity indices combined with an analysis of the reaction force are invoked to probe this concept. The reaction force analysis is used to partition the activation and reaction energy and characterize the behavior of reactivity indices along the three reaction regions that are defined within this approach. It has been observed that in the reactant and product regions mainly geometric rearrangements take place, whereas in the transition state region changes in the electronic bonding pattern occur; here most changes of the electronic properties are observed. The kinetic irreversibility−reversibility of the reactions is confirmed and linked to the differences in the Fukui function and dual descriptor of the radical centers associated with the initial and final species.

Levofloxacin ozonation in water: Rate determining process parameters and reaction pathway elucidation

B. De Witte, H. Van Langenhove, K. Hemelsoet, K. Demeestere, P. De Wispelaere, V. Van Speybroeck, J. Dewulf
Chemosphere
76 (5), 683-689
2009
A1

Abstract 

Ozonation of the quinolone antibiotic levofloxacin was investigated with focus on both the levofloxacin degradation rate and degradation product formation. Degradation was about 2 times faster at pH 10 compared to pH 3 and 7 explained by direct ozonation at the unprotonated , one of the tertiary amines of the piperazinyl substituent. H2O2 concentration (2–100 μM) had only limited effect. Liquid chromatography – high resolution mass spectrometry revealed degradation at the piperazinyl substituent and the quinolone moiety, with the relative importance of both pathways being strongly affected by changes in pH. Levofloxacin N-oxide concentrations reached up to 40% of the initial levofloxacin concentration during ozonation at pH 10. Degradation at the quinolone moiety resulted in isatin and anthranilic acid type metabolites, probably formed through reaction with hydroxyl radicals. Ab initio molecular orbital calculations predicted radical attack mainly at C2 of the quinolone moiety. This is the carbon atom with the largest Fukui function. Reaction with ozone is expected to mainly occur at , characterized by the largest negative charge.

A theoretical study on the solvated structural properties of various metalated 3-halo-1-azaallylic anions

B. De Sterck, V. Van Speybroeck, S. Mangelinckx, G. Verniest, N. De Kimpe, M. Waroquier
Journal of Physical Chemistry A
113 (22), 6375-6380
2009
A1

Abstract 

Metalated 3-halo-1-azaallylic anions are important building blocks for the preparation of a wide variety of heterocyclic and highly functionalized compounds. A theoretical description of the structural properties of halogenated 1-azaallylic anions in vacuo and in tetrahydrofuran (THF) solution is presented to gain insight into their reactivity behavior. The configurational flexibility of fluorinated and chlorinated 1-azaallylic anions is examined, and it is shown that these anions have far less configurational flexibility as compared with nonhalogenated analogues, with a strong preference to occur as Z/anti isomers. In addition, the driving force for transmetalation, that is, the replacement of the lithium cations with K+, Cu+, ZnCl+, CuCl+, or MgBr+ is studied. To obtain reliable results, the structures were modeled in THF using the combined implicit/explicit solvent approach resulting in different coordination numbers for lithium in the Z/anti and E/anti isomers. Calculations on dimerization energies show that coordination with THF is energetically preferred over aggregation.

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.

DFT Study on the Propagation Kinetics of Free-Radical Polymerization of α-Substituted Acrylates

I. Değirmenci, V. Aviyente, V. Van Speybroeck, M. Waroquier
Macromolecules
42 (8), 3033–3041
2009
A1

Abstract 

The kinetics of the free-radical propagation of methyl acrylate (MA), methyl methacrylate (MMA), ethyl α-fluoroacrylate (EFA), ethyl α-chloroacrylate (ECA), ethyl α-cyanoacrylate (ECNA), and methyl α-hydroxymethacrylate (MHMA) have been calculated using quantum chemical tools. Various DFT functionals such as BMK, BB1K, MPW1B95, MPW1K, and MPWB1K were used to model the relative propagation kinetics of the monomers. Among the methodologies used, MPWB1K/6-311+G(3df,2p)//B3LYP/6-31+G(d) was found to yield the best qualitative agreement with experiment. We explored chain length effects by examining addition reactions of monomeric, dimeric, trimeric, and tetrameric radicals to the monomers. We have also modeled the tacticity of the widely used monomers MA and MMA by considering all of the alternatives of attack of the radical in the 3D space around the monomer. This study has qualitatively confirmed the experimental syndiotactic/isotactic ratio of 66/3 for MMA. Finally, the kinetics of the initiation to polymerization for MA and MMA is also successfully reproduced.

Normal modes for large molecules with arbitrary link constraints in the mobile block Hessian approach

A. Ghysels, D. Van Neck, B.R. Brooks, V. Van Speybroeck, M. Waroquier
Journal of Chemical Physics
130 (8), 084107
2009
A1

Abstract 

In a previous paper [ Ghysels et al., J. Chem. Phys. 126, 224102 (2007) ] the mobile block Hessian (MBH) approach was presented. The method was designed to accurately compute vibrational modes of partially optimized molecular structures. The key concept was the introduction of several blocks of atoms, which can move as rigid bodies with respect to a local, fully optimized subsystem. The choice of the blocks was restricted in the sense that none of them could be connected, and also linear blocks were not taken into consideration. In this paper an extended version of the MBH method is presented that is generally applicable and allows blocks to be adjoined by one or two common atoms. This extension to all possible block partitions of the molecule provides a structural flexibility varying from very rigid to extremely relaxed. The general MBH method is very well suited to study selected normal modes of large macromolecules (such as proteins and polymers) because the number of degrees of freedom can be greatly reduced while still keeping the essential motions of the molecular system. The reduction in the number of degrees of freedom due to the block linkages is imposed here directly using a constraint method, in contrast to restraint methods where stiff harmonic couplings are introduced to restrain the relative motion of the blocks. The computational cost of this constraint method is less than that of an implementation using a restraint method. This is illustrated for the α-helix conformation of an alanine-20-polypeptide. © 2009 American Institute of Physics

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