B. Van de Voorde

Mechanistic studies of aldol condensations in UiO-66 and UiO-66-NH2 metal organic frameworks

J. Hajek, M. Vandichel, B. Van de Voorde, B. Bueken, D. De Vos, M. Waroquier, V. Van Speybroeck
Journal of Catalysis
331, 1-12


A full mechanistic investigation is proposed for the industrially important cross-aldol condensation reaction of heptanal with benzaldehyde on the UiO-66 and the amino-functionalized UiO-66-NH2 metal–organic frameworks to form jasminaldehyde. Several experimental studies indicate that the activity for the aldol condensation reaction can be increased by proper functionalization of the material, e.g. by introducing an additional basic amino site and thus creating a bifunctional acid–base catalyst for the aldol condensation. The precise molecular level origin for this behavior is to date unclear. Herein state-of-the-art Density-Functional Theory (DFT) calculations have been performed to unravel the mechanism of the cross- and self-aldol condensations of benzaldehyde and propanal. To this end free energy calculations have been performed on both extended cluster and periodic models. It is found that the mechanism on both catalysts is essentially the same, although a slightly stronger adsorption of the reactants and slightly lower barriers were found on the amino functionalized material, pointing toward higher initial activities. New experiments were performed to confirm these observations. It is indeed found that the initial activity toward cross-aldol condensation on the amino functionalized material is higher, although after about 40 min of reaction both materials become equally active. Our results furthermore point out that the basic amino groups may promote side reactions such as imine formation, which is induced by water. The study as presented can assist to engineer materials at the molecular level toward the desired products.

Open Access version available at UGent repository

Synthesis modulation as a tool to increase the catalytic activity of MOFs: the unique case of UiO-66(Zr)

F. Vermoortele, B. Bueken, G. Le Bars, B. Van de Voorde, M. Vandichel, K. Houthoofd, A. Vimont, M. Daturi, M. Waroquier, V. Van Speybroeck, C. Kirschhock, D. De Vos
JACS (Journal of the American Chemical Society)
135 (31), 11465–11468


The catalytic activity of the zirconium terephthalate UiO-66(Zr) can be drastically increased by using a modulation approach. The combined use of trifluoroacetic acid and HCl during the synthesis results in a highly crystalline material, with partial substitution of terephthalates by trifluoroacetate. Thermal activation of the material leads not only to dehydroxylation of the hexanuclear Zr cluster but also to post-synthetic removal of the trifluoroacetate groups, resulting in a more open framework with a large number of open sites. Consequently, the material is a highly active catalyst for several Lewis acid catalyzed reactions.

Electronic effects of linker substitution on Lewis acid catalysis with Metal-organic frameworks

F. Vermoortele, M. Vandichel, B. Van de Voorde, R. Ameloot, M. Waroquier, V. Van Speybroeck, D. De Vos
Angewandte Chemie int. Ed.
51(20), 4887-4890


Functionalized linkers can greatly increase the activity of metal–organic framework (MOF) catalysts with coordinatively unsaturated sites. A clear linear free-energy relationship (LFER) was found between Hammett σm values of the linker substituents X and the rate kX of a carbonyl-ene reaction. This is the first LFER ever observed for MOF catalysts. A 56-fold increase in rate was found when the substituent is a nitro group.

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