V. Van Speybroeck

Synthesis, characterization and sorption properties of NH2-MIL-47

K. Leus, S. Couck, M. Vandichel, G. Vanhaelewyn, Y-Y Liu, G.B. Marin, I. Van Driessche, D. Depla, M. Waroquier, V. Van Speybroeck, J.F.M. Denayer, P. Van der Voort
Physical Chemistry Chemical Physics (PCCP)
14, 15562–15570
2012
A1

Abstract 

An amino functionalized vanadium-containing Metal Organic Framework, NH2-MIL-47 has been synthesized by a hydrothermal reaction in an autoclave. Alternatively, a synthesis route via microwave enhanced irradiation has been optimized to accelerate the synthesis. The NH2-MIL-47 exhibits the same topology as MIL-47, in which the V center is octahedrally coordinated. After an exchange procedure in DMF the V+III center is oxidized to V+IV, which is confirmed by EPR and XPS measurements. The CO2 and CH4 adsorption properties have been evaluated and compared to MIL-47, showing that both MOFs have an almost similar adsorption capacity and affinity for CO2. DFT- based molecular modeling calculations were performed to obtain more insight into the adsorption positions for CO2 in NH2-MIL-47. Furthermore our calculated adsorption enthalpies agree well with the experimental values.

Ab initio parametrized force field for the flexible metal-organic framework MIL-53(Al)

L. Vanduyfhuys, T. Verstraelen, M. Vandichel, M. Waroquier, V. Van Speybroeck
Journal of Chemical Theory and Computation (JCTC)
8 (9), 3217-3231
2012
A1

Abstract 

A force field is proposed for the flexible metal-organic framework MIL-53(Al), which is calibrated using density functional theory calculations on non-periodic clusters. The force field has three main contributions: an electrostatic term based on atomic charges derived with a modified Hirshfeld-I method, a van der Waals (vdW) term with parameters taken from the MM3 model and a valence force field whose parameters were estimated with a new methodology that uses the gradients and Hessian matrix elements retrieved from non-periodic cluster calculations. The new force field, predicts geometries and cell parameters that compare well with the experimental values both for the large and narrow pore phases. The energy profile along the breathing mode of the empty material reveals the existence of two minima, which confirms the intrinsic bistable behaviour of the MIL-53. Even without the stimulus of external guest molecules the material may transform from the large pore (lp) to the narrow pore (np) phase [Liu et al. JACS 2008, 120, 11813]. The relative stability of the two phases critically depends on the vdW parameters and MM3 dispersion interaction has the tendency to overstabilize the np phase.

Methylation of benzene by methanol: single-site kinetics over H-ZSM-5 and H-beta zeolite catalysts

J. Van der Mynsbrugge, M. Visur, U. Olsbye, P. Beato, M. Bjørgen, V. Van Speybroeck, S. Svelle
Journal of Catalysis
292, 201-212
2012
A1

Abstract 

Benzenemethylation by methanol is studied on acidic zeolitesH-ZSM-5 (MFI) and H-beta (BEA) to investigate the influence of the catalyst topology on the reaction rate. Experimental kinetic measurements at 350 °C using extremely high feed rates to suppress side reactions show that methylation occurs considerably faster on H-ZSM-5 than on H-beta. Theoretical rate constants, obtained from first-principles simulations on extended zeolite clusters, reproduce a higher methylation rate on H-ZSM-5 and provide additional insight into the various molecular effects that contribute to the overall differences between the two catalysts. The calculations indicate this higher methylation rate is primarily due to an optimal confinement of the reacting species in the medium pore material. Co-adsorption of methanol and benzene is energetically favored in H-ZSM-5 compared with H-beta, to the extent that the stabilizing host–guest interactions outweigh the greater entropy loss upon benzene adsorption in H-ZSM-5 vs. in H-beta.

Open Access version available at UGent repository

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
A1

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
A1

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
A1

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
A1

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

Pages

Subscribe to RSS - V. Van Speybroeck