P. Van der Voort

Quantification of silanol sites for the most common mesoporous ordered silicas and organosilicas: total versus accessible silanols

M. Ide, M. El-Roz, E. De Canck, A. Vicente, T. Planckaert, T. Bogaerts, I. Van Driessche, F. Lynen, V. Van Speybroeck, F. Thibault-Starzyk, P. Van der Voort
Physical Chemistry Chemical Physics (PCCP)
15, 642-650
2013
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Abstract 

IR and NMR spectroscopy were used to determine the silanol content in the most common mesoporous ordered silicas: MCM-41, MCM-48, SBA-15 and SBA-16. In addition, a spray dried MCM-41 and an ethene bridged PMO are investigated. The results are compared with a commercial chromatographic silica (Nucleosil). The complete distribution of surface and bulk silanols, and of isolated, geminal and vicinal silanols for all these materials is presented. A distinction is made between the total silanol number and the reachable or surface silanol content. The latter is determined by controlled reactions with simple silanes. All mesoporous ordered silicas, and especially the thick walled SBA-type materials and the PMO contain a surprisingly high amount of total silanol sites, albeit that up to 90% if these silanols are buried inside the walls and are not reachable for small silanes.

Ti-functionalized NH2-MIL-47: an effective and stable epoxidation catalyst

K. Leus, G. Vanhaelewyn, T. Bogaerts, Y-Y Liu, F. Esquivel, F. Callens, G.B. Marin, V. Van Speybroeck, H. Vrielinck, P. Van der Voort
Catalysis Today
208, 97-105
2013
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Abstract 

In this paper, we describe the post-functionalization of a V-containing Metal-organic framework with TiO(acac)2 to create a bimetallic oxidation catalyst. The catalytic performance of this V/Ti-MOF was examined for the oxidation of cyclohexene using molecular oxygen as oxidant in combination with cyclohexanecarboxaldehyde as co-oxidant. A significantly higher cyclohexene conversion was observed for the bimetallic catalyst compared to the non-functionalized material. Moreover, the catalyst could be recycled at least 3 times without loss of activity and stability. No detectable leaching of V or Ti was noted. Electron paramagnetic resonance measurements were performed to monitor the fraction of V-ions in the catalyst in the +IV valence state. A reduction of this fraction by ∼17% after oxidation catalysis is observed, in agreement with the generally accepted mechanism for this type of reaction.

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.

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

A coordinative saturated vanadium containing metal organic framework that shows a remarkable catalytic activity

K. Leus, I. Muylaert, V. Van Speybroeck, G.B. Marin, P. Van der Voort
Studies in Surface Science and Catalysis
175, 329-332
2010
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Abstract 

A completely saturated Metal Organic Framework, MIL-47 was synthesized and tested for its catalytic performance in the oxidation of cyclohexene with tert-butyl hydroperoxide as oxidant. The catalyst was compared to several reference catalysts: namely VAPO-5, supported VOx/SiO2 and the homogeneous catalyst VO(acac)2. MIL-47 shows a remarkable catalytic activity and preserves its crystalline structure and surface area after a catalytic run. Furthermore MIL-47 exhibits a very high activity in successive runs.

The remarkable catalytic activity of the saturated metal organic framework V-MIL-47 in the cyclohexeneoxidation

K. Leus, I. Muylaert, M. Vandichel, G.B. Marin, M. Waroquier, V. Van Speybroeck, P. Van der Voort
Chemical Communications
46, 5085-5087
2010
A1

Abstract 

The remarkable catalytic activity of the saturated metal organic framework MIL-47 in the epoxidation of cyclohexene is elucidated by means of both experimental results and theoretical calculations.

Open Access version available at UGent repository

The coordinatively saturated vanadium MIL-47 as a low leaching heterogeneous catalyst in the oxidation of cyclohexene

K. Leus, M. Vandichel, Y-Y Liu, I. Muylaert, J. Musschoot, H. Vrielinck, F. Callens, G.B. Marin, C. Detavernier, Y.Z. Khimyak, M. Waroquier, V. Van Speybroeck, P. Van der Voort
Journal of Catalysis
285 (1) 196-207
2012
A1

Abstract 

A Metal Organic Framework, containing coordinatively saturated V+IV sites linked together by terephthalic linkers (V-MIL-47), is evaluated as a catalyst in the epoxidation of cyclohexene. Different solvents and conditions are tested and compared. If the oxidant TBHP is dissolved in water, a significant leaching of V-species into the solution is observed, and also radical pathways are prominently operative leading to the formation of an adduct between the peroxide and cyclohexene. If, however, the oxidant is dissolved in decane, leaching is negligible and the structural integrity of the V-MIL-47 is maintained during successive runs. The selectivity toward the epoxide is very high in these circumstances. Extensive computational modeling is performed to show that several reaction cycles are possible. EPR and NMR measurements confirm that at least two parallel catalytic cycles are co-existing: one with V+IV sites and one with pre-oxidized V+V sites, and this is in complete agreement with the theoretical predictions.

Open Access version available at UGent repository

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