V. Van Speybroeck

Accelerated living cationic ring-opening polymerization of a methyl ester functionalized 2-oxazoline monomer

P.J.M. Bouten, D. Hertsen, M. Vergaelen, B. Monnery, M.A. Boerman, H. Goossens, S. Catak, J.C.M. van Hest, V. Van Speybroeck, R. Hoogenboom
Polymer Chemistry
6, 514-518
2015
A1

Abstract 

Kinetic studies on the homo- and copolymerization of 2-methoxycarboxyethyl-2-oxazoline (MestOx) with 2-methyl-2-oxazoline (MeOx) and 2-ethyl-2-oxazoline (EtOx) were performed. For the homopolymerisation of MestOx an increased propagation rate constant was observed compared to MeOx and EtOx while the copolymerization of MestOx with MeOx or EtOx unexpectedly revealed slower incorporation of MestOx. Density functional theory (DFT) calculations show that nearby MestOx residues in the living chain can activate both the oxazolinium chain end and the attacking monomer, stabilizing the propagation transition state, leading to faster homopolymerisation of MestOx. These effects also accelerate incorporation of both monomers in the copolymerisations. However, since MeOx is shown to be more nucleophilic than MestOx, the incorporation order is reversed in the copolymerisations.

Open Access version available at UGent repository

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
2015 (5), 22334–22342
2015
A1

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

Complex reaction environments and competing reaction mechanisms in zeolite catalysis: insights from advanced molecular dynamics

K. De Wispelaere, B. Ensing, A. Ghysels, E.J. Meijer, V. Van Speybroeck
Chemistry - A European Journal
21 (26), 9385-9396
2015
A1

Abstract 

The methanol to olefins process is a show case example of complex zeolite-catalyzed chemistry. At real operating conditions, many factors such as framework flexibility, adsorption of various guest molecules and competitive reaction pathways, affect reactivity. In this paper we show the strength of first principle molecular dynamics techniques to capture this complexity by means of two case studies. Firstly, the adsorption behavior of methanol and water in H-SAPO-34 at 350 °C is investigated. Hereby we observed an important degree of framework flexibility and proton mobility. Secondly, we studied the methylation of benzene by methanol via a competitive direct and stepwise pathway in the AFI topology. Both case studies clearly show that a first principle molecular dynamics approach enables to obtain unprecedented insights into zeolite-catalyzed reactions at the nanometer scale.

Active site engineering in UiO-66 type metal-organic frameworks by intentional creation of defects: a theoretical rationalization

M. Vandichel, J. Hajek, F. Vermoortele, D. De Vos, M. Waroquier, V. Van Speybroeck
CrystEngComm
17 (2), 395-406
2015
A1

Abstract 

The catalytic activity of the Zr-benzenedicarboxylate (Zr-BDC) UiO-66 can be drastically increased in the absence of part of the linkers, as it removes the full coordination of the framework metal ions and leads to open sites. As a result, metal centers become more accessible and thus more active for Lewis acid catalysed reactions. Addition of modulators (MDL) to the synthesis mixture can create more linker deficiencies (Vermoortele et al., J Am Chem Soc, 2013, 135, 11465) and can lead to a significant increase of the catalytic activity due to the creation of a larger number of open sites. In this paper, we rationalize the function of the modulators under real synthesis conditions by the construction of free energy diagrams. The UiO-66 type materials form a very appropriate test case as the effect of addition of modulators hydrochloric acid (HCl) and trifluoroacetate (TFA) has been intensively investigated experimentally for the synthesis process and post-synthetic thermal activation. In synthesis conditions, direct removal of BDC linkers requires a high free energy but replacement of such linker by one or more TFA species might occur especially at high TFA:BDC ratios in the reaction mixture. The presence of HCl furthermore enhances the creation of defect structures. Post-synthesis activation procedures at higher temperatures lead to a substantial removal of the species coordinated to the Zr bricks creating open metal sites. A mechanistic pathway is presented for the dehydroxylation process of the hexanuclear Zr cluster. For the citronellal cyclization, we show that the presence of some residual TFA in the structure may lead to faster reactions in complete agreement with experiment. Hirshfeld-e partial charges for the Zr ions have been computed to investigate their sensitivity to substituent effects; a strong correlation with the experimental Hammett parameters, and with the rates of the citronellal cyclization is found. The theoretical rationalization may serve as a basis for detailed active site engineering studies.

Open Access version available at UGent repository

Active site engineering in UiO-66 type Metal Organic Frameworks by intentional creation of defects : A theoretical rationalization

M. Vandichel, J. Hajek, F. Vermoortele, M. Waroquier, D. De Vos, V. Van Speybroeck
CrystEngComm
First published online 15 Sep 2014
2014
A1
Published while none of the authors were employed at the CMM

Abstract 

The catalytic activity of the Zr-benzenedicarboxylate (Zr-BDC) UiO-66 can be drastically increased in the absence of part of the linkers, as it removes the full coordination of the framework metal ions and leads to open sites. As a result, metal centers become more accessible and thus more active for Lewis acid catalysed reactions. Addition of modulators (MDL) to the synthesis mixture can create more linker deficiencies (Vermoortele et al., J Am Chem Soc, 2013, 135, 11465) and can lead to a significant increase of the catalytic activity due to the creation of a larger number of open sites. In this paper, we rationalize the function of the modulators under real synthesis conditions by the construction of free energy diagrams. The UiO-66 type materials form a very appropriate test case as the effect of addition of modulators HCl and trifluoroacetate (TFA) has been intensively investigated experimentally for the synthesis process and post-synthetic thermal activation. In synthesis conditions, direct removal of BDC linkers requires a high free energy but replacement of such linker by one or more TFA species might occur especially at high TFA:BDC ratios in the reaction mixture. The presence of HCl furthermore enhances the creation of defect structures. Post-synthesis activation procedures at higher temperatures lead to a substantial removal of the species coordinated to the Zr bricks creating open metal sites. A mechanistic pathway is presented for the dehydroxylation process of the hexanuclear Zr cluster. For the citronellal cyclization, we show that the presence of some residual TFA in the structure may lead to faster reactions in complete agreement with experiment. Hirshfeld-e partial charges for the Zr ions have been computed to investigate their sensitivity to substituent effects; a strong correlation with the experimental Hammett parameters, and with the rates of the citronellal cyclization is found. The theoretical rationalization may serve as a basis for detailed active site engineering studies.

Binary and Ternary Po-containing Molecules Relevant for LBE Cooled Reactors at Operating Temperature

A. Van Yperen-De Deyne, K. Rijpstra, M. Waroquier, V. Van Speybroeck, S. Cottenier
Journal of Nuclear Materials
458, 288-295
2015
A1

Abstract 

Quantum-chemical calculations at several levels of theory were used to assess the stability at different temperatures of a set of 13 binary and ternary Po-containing molecules that could possibly be formed in an environment with lead, bismuth, oxygen and water. The conclusions are that especially PoPb, PbPoO and PoOH and to a lesser extent Po2 and PoO are stable. These small molecules are therefore likely to be found near the Lead-Bismuth eutectic (LBE) coolant at operational temperatures in a heavy liquid metal cooled fission reactor. In contrast, Po3 and PoBi are unlikely to be present under the assumed conditions. Several stability criteria, such as the dissociation into free atoms or into molecular fragments at realistic Po-concentrations or in the thermodynamic limit are discussed at different temperatures. The results obtained with a medium level of theory (Density Functional Theory, PBE0 with relativistic effective core potentials) show good qualitative correspondence with calculations performed at a much higher level of theory (Multi Reference Configuration Interaction, with spin–orbit coupling and scalar relativistic Hamiltonian).

Open Access version available at UGent repository

QuickFF: toward a generally applicable methodology to quickly derive accurate force fields for Metal-Organic Frameworks from ab initio input

L. Vanduyfhuys, S. Vandenbrande, T. Verstraelen, R. Schmid, M. Waroquier, V. Van Speybroeck
Journal of Computational Chemistry
2015
A1
Published while none of the authors were employed at the CMM

Aliovalent doping of CeO2: DFT study of oxidation state and vacancy effects

D.E.P. Vanpoucke, P. Bultinck, S. Cottenier, V. Van Speybroeck, I. Van Driessche
Journal of Materials Chemistry A
2 (3), 13723-13737
2014
A1

Abstract 

The modification of CeO2 properties by means of aliovalent doping is investigated within the ab initio density functional theory framework. Lattice parameters, dopant atomic radii, bulk moduli and thermal expansion coefficients of fluorite type Ce1-xMxO2-y (with M = Mg, V, Co, Cu, Zn, Nb, Ba, La, Sm, Gd, Yb, and Bi) are presented for 0.00 < x < 0.25. The relative stability of the doped systems is discussed, and the influence of oxygen vacancies is investigated. It is shown that oxygen vacancies tend to increase the lattice parameter, and strongly decrease the bulk modulus. Defect formation energies are correlated with calculated crystal radii and covalent radii of the dopants, and are shown to present no simple trend. The previously observed inverse relationship between the thermal expansion coefficient and the bulk modulus in group IV doped CeO2 [J. Am. Ceram. Soc., 2014, 97(1), 258] is shown to persist independent of the inclusion of charge compensating vacancies.

Open Access version available at UGent repository

Quasi-1D physics in Metal-Organic Frameworks: MIL-47(V) from first principles

D.E.P. Vanpoucke, J. Jaeken, S. De Baerdemacker, K. Lejaeghere, V. Van Speybroeck
Beilstein Journal of Nanotechnology
5, 1738–1748
2014
A1

Abstract 

The geometric and electronic structure of the MIL-47(V) metal-organic framework (MOF) is investigated by using ab initio density functional theory (DFT) calculations. Special focus is placed on the relation between the spin configuration and the properties of the MOF. The ground state is found to be antiferromagnetic, with an equilibrium volume of 1554.70 Å3. The transition pressure of the pressure-induced large-pore-to-narrow-pore phase transition is calculated to be 82 MPa and 124 MPa for systems with ferromagnetic and antiferromagnetic chains, respectively. For a mixed system, the transition pressure is found to be a weighted average of the ferromagnetic and antiferromagnetic transition pressures. Mapping DFT energies onto a simple-spin Hamiltonian shows both the intra- and inter-chain coupling to be antiferromagnetic, with the latter coupling constant being two orders of magnitude smaller than the former, suggesting the MIL-47(V) to present quasi-1D behavior. The electronic structure of the different spin configurations is investigated and it shows that the band gap position varies strongly with the spin configuration. The valence and conduction bands show a clear V d-character. In addition, these bands are flat in directions orthogonal to VO6 chains, while showing dispersion along the the direction of the VO6 chains, similar as for other quasi-1D materials

Open Access version available at UGent repository

Pages

Subscribe to RSS - V. Van Speybroeck