Angewandte
Chemie
DOI: 10.1002/anie.201108565
Metal–Organic Frameworks
Electronic Effects of Linker Substitution on Lewis Acid Catalysis with
Metal–Organic Frameworks**
Frederik Vermoortele, Matthias Vandichel, Ben Van de Voorde, Rob Ameloot, Michel Waroquier,
Veronique Van Speybroeck,* and Dirk E. De Vos*
Metal–organic frameworks (MOFs) are hybrid porous mate-
a series of functionalized Zr terephthalate materials with
UiO-66 structure, the activity in Lewis acid catalyzed
[1]
[9]
rials which hold great promise in adsorptive separation,
where selectivity is controlled by channel size and shape,
[2]
reactions can be much more effectively tuned than in
inorganic porous solids, and a Hammett-type structure–
activity LFER is readily identified. To support this concept,
molecular modeling calculations were performed to calculate
rates for individual reactions at various modified sites.
In the cubic Zr terephthalate UiO-66, each Zr O (OH)
4
[
3a–b]
hardness or softness of exposed framework ions,
frame-
They are attracting
increasing interest in catalysis. Some catalytic MOFs, like
[3c]
work hydrophobicity, and so on.
[4]
[
5]
HKUST-1 and the Fe-trimesate MIL-100, have open metal
sites at structural nodes. In other cases, homogeneous
catalysts, such as Mn Schiff bases, are integrated in the
6
4
octahedron is surrounded by maximally 12 terephthalate
linkers, resulting in large octahedral and small tetrahedral
cages. UiO-66 materials were synthesized with eight benzene-
1,4-dicarboxylates (BDC-X; X = H, NH , CH , OCH , F, Cl,
[6]
struts between the nodes, so that molecular catalysts are
isolated in an inert porous matrix.
In homogeneous catalysis, steric and electronic ligand
effects are decisive for activity and stereoselectivity, and in
some cases quantitative linear free-energy relationships
2
3
3
Br, NO ). To test the effect of the substituents, the cyclization
2
of citronellal was selected (Scheme 1). The isopulegol product
(
LFER) have been found between characteristics like the
enantiomeric ratio and the stereo-electronic substituent
[7]
parameters. For heterogeneous catalysts, it is less obvious
to establish quantitative structure–activity relationships. For
some zeolites, the number and nature of the isomorphously
substituting cations affect the number and strength of the
Brønsted or Lewis acid sites; however, the effects are not
[
8]
always easily rationalized due to site heterogeneity.
Although the effect of cation variation on catalytic activity
Scheme 1. Cyclization of (+)-citronellal to isopulegol and its isomers.
[
4d]
has been shown for MOFs, there are no systematic studies
on the effects of framework substitution on the catalytic
activity of the structural ions. Here we demonstrate that the
concept of electronic modulation of the active site can be
transferred from the familiar paradigm of homogeneous
catalysis to catalytic frameworks of the MOF type. For
of this carbonyl-ene reaction is a precursor of synthetic
menthol. The selectivity for the isopulegol isomers depends
[10]
strongly on the Lewis acidity of the active site.
Before
reaction, each catalyst was activated at 493 K. For UiO-66
materials, this results in dehydration of the Zr O (OH)
6
4
4
[
9a]
[
*] F. Vermoortele, B. Van de Voorde, Dr. R. Ameloot, Prof. D. E. De Vos
Centre for Surface Chemistry and Catalysis
Katholieke Universiteit Leuven
cluster, with formation of Zr O . Conversion profiles for
6 6
reactions in toluene are given in Figure 1.
Kasteelpark Arenberg 23, 3001 Leuven (Belgium)
E-mail: dirk.devos@biw.kuleuven.be
M. Vandichel, Prof. M. Waroquier, Prof. V. Van Speybroeck
Center for Molecular Modeling, Universiteit Gent
Technologiepark 903, 9052 Zwijnaarde (Belgium)
E-mail: veronique.vanspeybroeck@ugent.be
[**] We are grateful to FWO, the Belspo (IAP 6/27), to KULeuven
(
CASAS Methusalem grant) and the Research Board of Ghent
University (BOF) for project support. Funding was also received
from the European Research Council under the European Com-
munity’s Seventh Framework Programme [FP7(2007-2013) ERC
grant agreement number 240483]. R.A. is a post-doctoral fellow of
FWO. The computational resources were provided by Ghent
University (Stevin Supercomputer Infrastructure).
Supporting information for this article (synthesis procedures,
characterization, experimental methods, and information on the
Figure 1. Conversion of citronellal over UiO-66-X versus time (toluene,
373 K, citronellal:Zr=10).
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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