86
J. Gascon et al. / Journal of Catalysis 261 (2009) 75–87
ilar as observations in the homogeneous phase [25,26]. This is
attributed to the hydrogen bond acceptor power of DMSO, sta-
bilizing the protonated form of the amino group, increasing the
rate of proton transfer [4] and promoting in this way in the for-
mation of the benzaldimine intermediate species. The faster re-
action in EtOH than in DMF differs, however, from the trend in
solvent polarity. This suggests that besides the increased rate of
proton transfer, the different solvation power and consequently in
the partition of the reactants at the solid–liquid interface plays a
role [23]. The amphiprotic properties of ethanol may induce the
electrophilic polarization of the carbonyl group of benzaldehyde
(due to its moderated acid character) and enhance the reaction
rate [26]. A nearly apolar solvent like toluene seems to affect neg-
atively the catalytic performance of the MOFs, in clear contrast
with results reported for other solid supported basic catalysts [4].
In the latter case the hydrophilic character of the support deter-
mines the performance of supported homogeneous catalysts [3,4];
the higher the hydrophobicity of the porous material, the less the
effect of the solvent and the faster the diffusion of the products.
Since IRMOF-3 is a hydrophilic solid (Zn atoms are not fully coor-
dinated), it is not surprising that the polarity of the solvent has a
large impact on the performance of the catalyst. Indeed, the results
suggest that the affinities of benzaldehyde and ethyl cyanoacetate
for toluene are higher than for other solvents and that this dis-
tribution between the solvent and the solid makes the reaction
slower.
limitations make this amino-MOF unsuited for the studied reac-
tion.
The behavior of the IRMOF-3 catalysts demonstrates that the
basicity of the aniline-type amino group is enhanced when in-
corporated inside the IRMOF-3 structure. The catalysts are stable
under the studied reaction conditions and could be reused with-
out losing activity.
Acknowledgments
Senter Novem is gratefully acknowledged for financial support
through the project EOSLT-04008 (Lange termijn EOS-onderzoeks-
programma). The X-ray facilities of the Department of Materials
Science and Engineering of the Delft University of Technology is
acknowledged for the XRD analyses. Prof. Em. dr. ir. H. van Bekkum
is gratefully acknowledged for fruitful discussions.
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5. Conclusions
In this work, metal-organic frameworks with non-coordinated
amino groups are shown to be active solid basic catalysts in
the Knoevenagel condensation of benzaldehyde with two methy-
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The activity of the IRMOF-3 depends on the specific surface
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