P. Van der Voort

Heterogeneous Ru(III) oxidation catalysts via ‘click’ bidentate ligands on a Periodic Mesoporous Organosilica support

S. Clerick, E. De Canck, K. Hendrickx, V. Van Speybroeck, P. Van der Voort
Green Chemistry
18, 6035–6045
2016
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Abstract 

A 100% monoallyl ring-type Periodic Mesoporous Organosilica (PMO) is prepared as a novel, versatile and exceptionally stable catalytic support with a high internal surface area and 5.0 nm pores. Thiol-ene ‘click’ chemistry allows straightforward attachment of bifunctional thiols (-NH2, -OH, -SH) which, exploiting the thioether functionality formed, give rise to ‘solid’ bidentate ligands. [Ru(acac)2(CH3CN)2]PF6 is attached and complex formation on the solid is studied via theoretical calculations. All resulting solid catalysts show high activity and selectivity in alcohol oxidation reactions performed in green conditions (25°C/ water). The PMO catalysts do not leach Ru during reaction and are thus easily recuperated and re-used for several runs. Furthermore, oxidation of poorly water-soluble (±)-menthol illustrates the benefits of using hydrophobic PMOs as catalytic supports.

Facile synthesis of cooperative acid-base catalysts by clicking cysteine and cysteamine on an ethylene-bridged periodic mesoporous organosilica

J. Ouwehand, J. Lauwaert, D. Esquivel, K. Hendrickx, V. Van Speybroeck, J.W. Thybaut, P. Van der Voort
European Journal of Inorganic Chemistry
2016, 13-14, 2144-2151
2016
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Abstract 

A Periodic Mesoporous Organosilica (PMO) containing ethylene bridges was functionalized in order to obtain a series of cooperative acid-base catalysts. A straightforward, single-step procedure was devised to immobilize cysteine and cysteamine on the support material via a photoinitiated thiol-ene click reaction. Likewise, PMO materials capped with hexamethyldisilazane (HMDS) were used to support both compounds. This resulted in different materials, where the amine site was promoted by carboxylic acid groups, surface silanol groups or both. The catalysts were tested in the aldol reaction of 4-nitrobenzaldehyde and acetone. It was found that silanol groups have a stronger promoting effect on the amine than the carboxylic acid group. The highest turnover frequency (TOF) was obtained for an amine functionalized material which only contained silanol promoting sites. The loading of the active sites also has a significant effect on the activity of the catalysts, which was rationalized based on a computational study.

Systematic study of the chemical and hydrothermal stability of selected "stable" Metal Organic Frameworks

K. Leus, T. Bogaerts, J. De Decker, H. Depauw, K. Hendrickx, H. Vrielinck, V. Van Speybroeck, P. Van der Voort
Microporous and Mesoporous Materials
226, 110-116
2016
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Abstract 

In this work, the hydrothermal and chemical stability towards acids, bases, air, water and peroxides of Metal Organic Frameworks, that are commonly considered to be stable, is presented. As a proof of stability both the crystallinity and porosity are measured before and after exposure to the stress test. The major part of the MOFs examined in this study showed a good hydrothermal stability except for the UiO-67, NH2-MIL-101 (Al) and CuBTC material. The chemical stability towards acids and bases show a similar tendency and an ordering can be proposed as: MIL-101(Cr)>NH2-UiO-66>UiO-66>UiO-67>NH2-MIL-53>MIL-53(Al)>ZIF-8>CuBTC>NH2-MIL-101(Al). In the tests with the H2O2 solution most materials behaved poorly, only the UiO-66 and NH2-UiO-66 framework showed a good stability.

Vibrational fingerprint of the absorption properties of UiO-type MOF materials

A. Van Yperen-De Deyne, K. Hendrickx, L. Vanduyfhuys, G. Sastre, P. Van der Voort, V. Van Speybroeck, K. Hemelsoet
Theoretical Chemistry Accounts
135, 4, 102
2016
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Abstract 

The absorption properties of UiO-type metal–organic frameworks are computed using TD-DFT simulations on the organic linkers. A set of nine isoreticular structures, including the UiO-66 and UiO-67 materials and functionalized variants, are examined. The excitation energies from a static geometry optimization are compared with dynamic averages obtained from sampling the ground-state potential energy surface using molecular dynamics. The vibrational modes that impact the excitation energy are identified. This analysis is done using a recently proposed tool based on power spectra of the velocities and the excitation energies. The applied procedure allows including important factors influencing the absorption spectra, such as the periodic framework, linker variation and dynamical effects including harmonic and anharmonic nuclear motions. This methodology allows investigating in detail the vibrational fingerprint of the excitation energy of advanced materials such as MOFs and gives perspectives to tailor materials toward new light-based applications.

Open Access version available at UGent repository

Understanding Intrinsic Light Absorption Properties of UiO- 66 Frameworks: A Combined Theoretical and Experimental Study

K. Hendrickx, D.E.P. Vanpoucke, K. Leus, K. Lejaeghere, A. Van Yperen-De Deyne, V. Van Speybroeck, P. Van der Voort, K. Hemelsoet
Inorganic Chemistry
54, 22, 10701-10710
2015
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Abstract 

A combined theoretical and experimental study is performed in order to elucidate the effects of linker functional groups on the photoabsorption properties of UiO-66-type materials. This study, in which both mono- and di-functionalized linkers (with X= -OH, -NH2, -SH) are studied, aims to obtain a more complete picture on the choice of functionalization. Static Time-Dependent Density Functional Theory (TD-DFT) calculations combined with Molecular Dynamics simulations are performed on the linkers and compared to experimental UV/VIS spectra, in order to understand the electronic effects governing the absorption spectra. Di-substituted linkers show larger shifts compared to mono-substituted variants, making them promising candidates for further study as photocatalysts. Next, the interaction between the linker and the inorganic part of the framework is theoretically investigated using a cluster model. The proposed Ligand-to-Metal-Charge Transfer (LMCT) is theoretically observed and is influenced by the differences in functionalization. Finally, computed electronic properties of the periodic UiO-66 materials reveal that the band gap can be altered by linker functionalization and ranges from 4.0 down to 2.2 eV. Study of the periodic Density of States (DOS) allows to explain the band gap modulations of the framework in terms of a functionalization-induced band in the band gap of the original UiO-66 host.

Fine-tuning the theoretically predicted structure of MIL-47(V) with the aid of powder X-ray diffraction

T. Bogaerts, L. Vanduyfhuys, D.E.P. Vanpoucke, J. Wieme, M. Waroquier, P. Van der Voort, V. Van Speybroeck
CrystEngComm
17, 8612–8622
2015
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Abstract 

The structural characterization of complex crystalline materials such as metal organic frameworks can prove a very difficult challenge both for experimentalists as for theoreticians. From theory, the flat potential energy surface of these highly flexible structures often leads to different geometries that are energetically very close to each other. In this work a distinction between various computationally determined structures is made by comparing experimental and theoretically derived X-ray diffractograms which are produced from the materials geometry. The presented approach allows to choose the most appropriate geometry of a MIL-47(V) MOF and even distinguish between different electronic configurations that induce small structural changes. Moreover the techniques presented here are used to verify the applicability of a newly developed force field for this material. The discussed methodology is of significant importance for modelling studies where accurate geometries are crucial, such as mechanical properties and adsorption of guest molecules.

The enantioselectivity of the manganese-salen complex in the epoxidation of unfunctionalized olefins and the influence of grafting

T. Bogaerts, S. Wouters, P. Van der Voort, V. Van Speybroeck
Journal of Molecular Catalysis A: Chemical
Vol. 406, 106-113
2015
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Abstract 

Jacobsen’s complexes are famous for their usability for enantioselective epoxidations. However, the applicability of this catalytic system has been severely limited by several practical problems such as deactivation and separation after reaction. Grafting of Jacobsen-type complexes on solid supports is an attractive way to overcome these problems but led to a decrease in selectivity. A combined theoretical and experimental approach is presented to unravel the factors governing enantioselectivity. The importance of different substituents was determined by analyzing the transition state for the oxygen transfer using the full system as a model. An analysis of the asymmetric complex has shown an inherent tendency for a decreased selectivity due to the lack of specific bulky groups. Experimentally an immobilized Jacobsen catalyst on a metal organic framework (MIL-101) was synthesized which confirms the computational tendencies but the decrease in selectivity is limited, indicating that the MIL-101(Cr) is a suitable carrier for this complex.

Open Access version available at UGent repository

DOI 

10.1016/j.molcata.2015.05.020

Mechanistic investigation on the oxygen transfer with the manganese-salen complex

T. Bogaerts, S. Wouters, P. Van der Voort, V. Van Speybroeck
ChemCatChem
7 (17), 2711–2719
2015
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Abstract 

The most well-known application of salen complexes is the use of a chiral ligand loaded with manganese to form the Jacobsen complex. This organometallic catalyst is used in the epoxidation of unfunctionalized olefins and can achieve very high selectivities. Although this application was proposed many years ago, the mechanism of oxygen transfer remains a question until now. In this paper, the epoxidation mechanism is investigated by an ab initio kinetic modeling study. First of all a proper DFT functional is selected which yields the correct ordering of the various spin states. Our results show that the epoxidation proceeds via a radical intermediate. Starting from the radical intermediate, these results can explain the experiments with radical probes. The subtle influences in the transition state using the full Jacobsen catalyst explain the experimentally observed product distribution.

Au@UiO-66: a base free oxidation catalyst

K. Leus, P. Concepcion, M. Vandichel, M. Meledina, A. Grirrane, D. Esquivel, S. Turner, D. Poelman, M. Waroquier, V. Van Speybroeck, G. Van Tendeloo, H. Garcia, P. Van der Voort
RSC Advances
5 (29), 22334–22342
2015
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Abstract 

We present the in situ synthesis of Au nanoparticles within the Zr based Metal Organic Framework, UiO-66. The resulting Au@UiO-66 materials were characterized by means of N2 sorption, XRPD, UV-Vis, XRF, XPS and TEM analysis. The Au nanoparticles (NP) are homogeneously distributed along the UiO-66 host matrix when using NaBH4 or H2 as reducing agents. The Au@UiO-66 materials were evaluated as catalysts in the oxidation of benzyl alcohol and benzyl amine employing O2 as oxidant. The Au@MOF materials exhibit a very high selectivity towards the ketone (up to 100 %). Regenerability and stability tests demonstrate that the Au@UiO-66 catalyst can be recycled with a negligible loss of Au species and no loss of crystallinity. In situ IR measurements of UiO-66 and Au@UiO-66-NaBH4, before and after treatment with alcohol, showed an increase in IR bands that can be assigned to a combination of physisorbed and chemisorbed alcohol species. This was confirmed by velocity power spectra obtained from the molecular dynamics simulations. Active peroxo and oxo species on Au could be visualized with Raman analysis.

Open Access version available at UGent repository

Communication: DMRG-SCF study of the singlet, triplet, and quintet states of oxo-Mn(Salen)

S. Wouters, T. Bogaerts, P. Van der Voort, V. Van Speybroeck, D. Van Neck
Journal of Chemical Physics
140, 241103
2014
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Abstract 

We use CheMPS2, our free open-source spin-adapted implementation of the density matrix renormalization group (DMRG) [S. Wouters, W. Poelmans, P. W. Ayers, and D. Van Neck, Comput. Phys. Commun. 185, 1501 (2014)], to study the lowest singlet, triplet, and quintet states of the oxo-Mn(Salen) complex. We describe how an initial approximate DMRG calculation in a large active space around the Fermi level can be used to obtain a good set of starting orbitals for subsequent complete-active-space or DMRG self-consistent field calculations. This procedure mitigates the need for a localization procedure, followed by a manual selection of the active space. Per multiplicity, the same active space of 28 electrons in 22 orbitals (28e, 22o) is obtained with the 6-31G∗ , cc-pVDZ, and ANO-RCC-VDZP basis sets (the latter with DKH2 scalar relativistic corrections). Our calculations provide new insight into the electronic structure of the quintet.

Open Access version available at UGent repository

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