V. Van Speybroeck

Host-guest and guest-guest interactions between xylene isomers confined in the MIL-47(V) pore system

A. Ghysels, M. Vandichel, T. Verstraelen, M. van der Veen, D. De Vos, M. Waroquier, V. Van Speybroeck
Theoretical Chemistry Accounts
131 (7) 1234-1246
2012
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Abstract 

The porous MIL-47 material shows a selective adsorption behavior for para-, ortho-, and meta-isomers of xylenes, making the material a serious candidate for separation applications. The origin of the selectivity lies in the differences in interactions (energetic) and confining (entropic). This paper investigates the xylene–framework interactions and the xylene–xylene interactions with quantum mechanical calculations, using a dispersion-corrected density functional and periodic boundary conditions to describe the crystal. First, the strength and geometrical characteristics of the optimal xylene–xylene interactions are quantified by studying the pure and mixed pairs in gas phase. An extended set of initial structures is created and optimized to sample as many relative orientations and distances as possible. Next, the pairs are brought in the pores of MIL-47. The interaction with the terephthalic linkers and other xylenes increases the stacking energy in gas phase (−31.7 kJ/mol per pair) by roughly a factor four in the fully loaded state (−58.3 kJ/mol per xylene). Our decomposition of the adsorption energy shows various trends in the contributing xylene–xylene interactions. The absence of a significant difference in energetics between the isomers indicates that entropic effects must be mainly responsible for the separation behavior.

Open Access version available at UGent repository

Mechanistic insight into the cyclohexene epoxidation with VO(acac)(2) and tert-butyl hydroperoxide

M. Vandichel, K. Leus, P. Van der Voort, M. Waroquier, V. Van Speybroeck
Journal of Catalysis
294, 1-18
2012
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Abstract 

The epoxidation reaction of cyclohexene is investigated for the catalytic system vanadyl acetylacetonate (VO(acac)2) with tert-butyl hydroperoxide (TBHP) as oxidant with the aim to identify the most active species for epoxidation and to retrieve insight into the most plausible epoxidation mechanism. The reaction mixture is composed of various inactive and active complexes in which vanadium may either have oxidation state +IV or +V. Inactive species are activated with TBHP to form active complexes. After reaction with cyclohexene, each active species transforms back into an inactive complex that may be reactivated again. The reaction mixture is quite complex containing hydroxyl, acetyl acetonate, acetate, or a tert-butoxide anion as ligands, and thus, various ligand exchange reactions may occur among active and inactive complexes. Also, radical decomposition reactions allow transforming V+IV to V+V species. To obtain insight into the most abundant active complexes, each of previous transformation steps has been modeled through thermodynamic equilibrium steps. To unravel the nature of the most plausible epoxidation mechanism, first principle chemical kinetics calculations have been performed on all proposed epoxidation pathways. Our results allow to conclude that the concerted Sharpless mechanism is the preferred reaction mechanism and that alkylperoxo species V+IVO(L)(OOtBu) and V+VO(L1)(L2)(OOtBu) species are most abundant. At the onset of the catalytic cycle, vanadium +IV species may play an active role, but as the reaction proceeds, reaction mechanisms that involve vanadium +V species are preferred as the acetyl acetonate is readily oxidized. Additionally, an experimental IR and kinetic study has been performed to give a qualitative composition of the reaction mixture and to obtain experimental kinetic data for comparison with our theoretical values. The agreement between theory and experiment is satisfactory.

Open Access version available at UGent repository

Diastereoselective aldol reaction of zincated 3-chloro-3-methyl-1- azaallylic anions as key-step in the synthesis of 1,2,3,4- tetrasubstituted 3-chloroazetidines

S. Mangelinckx, B. De Sterck, F. Colpaert, S. Catak, J. Jacobs, S. Rooryck, M. Waroquier, V. Van Speybroeck, N. De Kimpe
Journal of Organic Chemistry
77 (7), 3415–3425
2012
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Abstract 

Zincated 3-chloro-3-methyl-1-azaallylic anions undergo a stereoselective aldol addition across aromatic aldehydes and subsequent mesylation to produce syn alpha-chloro-beta-mesyloxyketimines, which were isolated in 80-84% yield and high diastereomeric excess (dr > 97/3) after purification via flash chromatography. The syn alpha-chloro-beta-mesyloxyketimines were further stereoselectively reduced to give stereochemically defined 3-aminopropyl mesylates, which were cyclized to 1,2,3,4-tetrasubstituted 3-chloroazetidines containing three contiguous stereogenic centers. DFT calculations on the key aldol addition revealed the presence of a highly ordered bimetallic six-membered twist-boat-like transition state structure with a tetra-coordinated metal cyclic structure. DFT calculations revealed that chelation of both zinc and lithium cations in the transition state structure leads to the experimentally observed high syn diastereoselectivity of aldol reactions.

Open Access version available at UGent repository

Efficient Approach for the Computational Study of Alcohol and Nitrile Adsorption in H-ZSM-5

J. Van der Mynsbrugge, K. Hemelsoet, M. Vandichel, M. Waroquier, V. Van Speybroeck
Journal of Physical Chemistry C
116 (9), 5499-5508
2012
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Abstract 

Since many industrially important processes start with the adsorption of guest molecules inside the pores of an acidic zeolite catalyst, a proper estimate of the adsorption enthalpy is of paramount importance. In this contribution, we report ab initio calculations on the adsorption of water, alcohols and nitriles at the bridging Brønsted sites of H-ZSM-5, using both cluster and periodic models to account for the zeolite environment. Stabilization of the adsorption complexes results from hydrogen bonding between the guest molecule and the framework, as well as from embedding, i.e. van der Waals interactions with the pore walls. Large-cluster calculations with different DFT-methods, in particular B3LYP(-D), PBE(-D) M062X(-D) and ωB97X-D, are tested for their ability to reproduce the experimental heats of adsorption available in literature. (J. Phys. Chem. B 1997, 101, 3811-3817) A proper account of dispersion interactions is found to be crucial to describe the experimental trend across a series of adsorbates of increasing size, i.e. an increase in adsorption enthalpy by 10-15 kJ/mol for each additional carbon atom. The extended-cluster model is shown to offer an attractive alternative to periodic simulations on the entire H-ZSM-5 unit cell, resulting in virtually identical results for the final adsorption enthalpies. Comparing calculated stretch frequencies of the zeolite acid sites and the adsorbate functional groups with experimental IR-data additionally confirms the cluster approach provides an appropriate representation of the adsorption complexes.

Open Access version available at UGent repository

The influence of a polyamide matrix on the halochromic behaviour of the pH-sensitive azo dye Nitrazine Yellow

L. Van der Schueren, K. Hemelsoet, V. Van Speybroeck, K. De Clerck
Dyes and Pigments
94 (3), 443-451
2012
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Abstract 

It is of great interest to introduce pH-sensitive dyes into fibrous materials since this may result in flexible sensor systems. However, to date, the effect of a textile matrix on the halochromic properties of dyes is still unknown which severely limits their further development. Therefore, this paper focuses on an in-depth study of the halochromism of the azo pH-indicator dye Nitrazine Yellow in solution and incorporated in polyamide textile matrices with different structures. Based on both experimental spectroscopic data and computational calculations, an azo hydrazone tautomerism was found to be responsible for the halochromism of Nitrazine Yellow in solution. The hydrazone tautomer was most stable in neutral pH while the deprotonated dye molecule was believed to be an azo tautomer, resulting in a bathochromic shift with increasing pH. This tautomerism was, moreover, also present in the polyamide matrices. However, the equilibrium was clearly affected by the polymeric environment resulting in a shift and broadening of the dynamic pH-range. The polyamide type and textile structure influenced the halochromic response due to different interactions and accessibility of the dye. In conclusion, the halochromism of Nitrazine Yellow is present in all studied systems and is always based on an azo hydrazone tautomerism but the polyamide matrix causes distinct alterations in the tautomeric equilibrium.

Solvent-controlled selective transformation of 2-Bromomethyl-2-methylaziridines to functionalized aziridines and azetidines

S. Stankovic, H. Goossens, S. Catak, M. Tezcan, M. Waroquier, V. Van Speybroeck, M. D'Hooghe, N. De Kimpe
Journal of Organic Chemistry
77, 3181-3190
2012
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Abstract 

The reactivity of 2-bromomethyl-2-methylaziridines toward oxygen, sulfur, and carbon nucleophiles in different solvent systems was investigated. Remarkably, the choice of the solvent has a profound influence on the reaction outcome, enabling the selective formation of either functionalized aziridines in dimethylformamide (through direct bromide displacement) or azetidines in acetonitrile (through rearrangement via a bicyclic aziridinium intermediate). In addition, the experimentally observed solvent-dependent behavior of 2-bromomethyl-2-methylaziridines was further supported by means of DFT calculations.

Open Access version available at UGent repository

Experimental and theoretical IR study of methanol and ethanol conversion over H-SAPO-34

K. Hemelsoet, A. Ghysels, D. Mores, K. De Wispelaere, V. Van Speybroeck, B.M. Weckhuysen, M. Waroquier
Catalysis Today
177 (1), 12-24
2011
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Abstract 

Theoretical and experimental IR data are combined to gain insight into the methanol and ethanol conversion over an acidic H-SAPO-34 catalyst. The theoretical simulations use a large finite cluster and the initial physisorption energy of both alcohols is calculated. Dispersive contributions turn out to be vital and ethanol adsorbs stronger than methanol with approximately 14 kJ mol(-1). Calculated IR spectra of the alcohols and of formed aromatic cations upon conversion are also analyzed and support the peak assignment of the experimental in situ DRIFT spectra, in particular for the C-H and C=C regions. Theoretical IR spectra of the gas phase compounds are compared with those of the molecules loaded in a SAPO cluster and the observed shifts of the peak positions are discussed. To get a better understanding of these framework-guest interactions, a new theoretical procedure is proposed based on a normal mode analysis. A cumulative overlap function is defined and enables the characterization of individual peaks as well as induced frequency shifts upon adsorption. (C) 2010 Elsevier B. V. All rights reserved.

Open Access version available at UGent repository

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
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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.

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