V. Van Speybroeck

Catalytic Performance of Vanadium MIL-47 and Linker-Substituted Variants in the Oxidation of Cyclohexene: A Combined Theoretical and Experimental Approach

M. Vandichel, S. Biswas, K. Leus, J. Paier, J. Sauer, T. Verstraelen, P. Van der Voort, M. Waroquier, V. Van Speybroeck
ChemPlusChem
79 (8), 1183–1197
2014
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Abstract 

The epoxidation of cyclohexene has been investigated on a metal–organic framework MIL-47 containing saturated V+IV sites linked with functionalized terephthalate linkers (MIL-47-X, X=OH, F, Cl, Br, CH3, NH2). Experimental catalytic tests have been performed on the MIL-47-X materials to elucidate the effect of linker substitution on the conversion. Notwithstanding the fact that these substituted materials are prone to leaching in the performed catalytic tests, the initial catalytic activity of these materials correlates with the Hammett substituent constants. In general, substituents led to an increased activity relative to the parent MIL-47. To rationalize the experimental findings, first-principles kinetic calculations were performed on periodic models of MIL-47 to determine the most important active sites by creating defect structures in the interior of the crystalline material. In a next step these defect structures were used to propose extended cluster models, which are able to reproduce in an adequate way the direct environment of the active metal site. An alkylperoxo species V+VO(OOtBu) was identified as the most abundant and therefore the most active epoxidation site. The structure of the most active site was a starting basis for the construction of extended cluster models including substituents. They were used for quantifying the effect of functionalization of the linkers on the catalytic performance of the heterogeneous catalyst MIL-47-X. Electron-withdrawing as well as electron-donating groups have been considered. The epoxidation activity of the functionalized models has been compared with the measured experimental conversion of cyclohexene. The agreement is fairly good. This combined experimental–theoretical study makes it possible to elucidate the structure of the most active site and to quantify the electronic modulating effects of linker substituents on the catalytic activity.

Synthesis, Structural Characterization, and Catalytic Performance of a Vanadium-Based Metal-Organic Framework (COMOC-3)

Y-Y Liu, K. Leus, M. Grzywa, D. Weinberger, K. Strubbe, H. Vrielinck, R. Van Deun, D. Volkmer, V. Van Speybroeck, P. Van der Voort
European Journal of Inorganic Chemistry
(16) 2819 - 2827
2012
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Abstract 

A vanadium 2,6-naphthalenedicarboxylate, VIII(OH)(O2C–C10H6–CO2)·H2O, denoted as COMOC-3as (COMOC = Center for Ordered Materials, Organometallics and Catalysis, Ghent University), has been synthesized under hydrothermal conditions by means of both a solvothermal and a microwave synthesis procedure. The structure shows the topology of an aluminium 2,6-naphthalenedicarboxylate, the so-called MIL-69 (MIL = Materials of the Institute Lavoisier). After calcination at 250 °C in air, the VIII center was oxidized to VIV with the structure of VIVO(O2C–C10H6–CO2) (COMOC-3). The oxidation process was verified by cyclic voltammetry and EPR spectroscopy. The crystallinity was investigated by variable-temperature XRD. The title compound is stable against air and moisture. The catalytic performance of COMOC-3 was examined in the liquid-phase oxidation of cyclohexene. COMOC-3 exhibited similar catalytic performance to MIL-47 [VO(O2C–C6H4–CO2)]. The compound is reusable and maintains its catalytic activity through several runs.

New V-IV-Based Metal-Organic Framework Having Framework Flexibility and High CO2 Adsorption Capacity

Y-Y Liu, S. Couck, M. Vandichel, M. Grzywa, K. Leus, S. Biswas, D. Volkmer, J. Gascon, F. Kapteijn, J.F.M. Denayer, M. Waroquier, V. Van Speybroeck, P. Van der Voort
Inorganic Chemistry
52 (1), 113-120
2013
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Abstract 

A vanadium based metal–organic framework (MOF), VO(BPDC) (BPDC2– = biphenyl-4,4′-dicarboxylate), adopting an expanded MIL-47 structure type, has been synthesized via solvothermal and microwave methods. Its structural and gas/vapor sorption properties have been studied. This compound displays a distinct breathing effect toward certain adsorptives at workable temperatures. The sorption isotherms of CO2 and CH4 indicate a different sorption behavior at specific temperatures. In situ synchrotron X-ray powder diffraction measurements and molecular simulations have been utilized to characterize the structural transition. The experimental measurements clearly suggest the existence of both narrow pore and large pore forms. A free energy profile along the pore angle was computationally determined for the empty host framework. Apart from a regular large pore and a regular narrow pore form, an overstretched narrow pore form has also been found. Additionally, a variety of spectroscopic techniques combined with N2 adsorption/desorption isotherms measured at 77 K demonstrate that the existence of the mixed oxidation states VIII/VIV in the titled MOF structure compared to pure VIV increases the difficulty in triggering the flexibility of the framework.

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
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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
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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
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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
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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

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