An unexpected bifunctional acid base catalysis in IRMOF-3 for Knoevenagel condensation reactions

In this work, we have reexamined the catalytic properties of zinc aminoterephthalate IRMOF-3 for the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate. The active sites for this reaction were previously proposed to consist in the amino groups of the aminoterephthalate linkers [1]. We have now found that the reaction can also be catalyzed by MOF-5, which has the same crystalline structure than IRMOF-3, but in which the aminoterephthalate ligand is replaced by terephthalate, thus lacking the amino groups that are present in IRMOF-3. It has been found that the catalytic activity of MOF-5 is related with the presence of defects and/or occluded metal oxides. Different preparation routes of MOF-5 have been followed that result in different concentration of defects and different reactivity. These defects sites can also be present in IRMOF-3 and contribute to the catalytic activity. Therefore, depending on the preparation conditions and on the concentration of defects, in the case of IRMOF-3 we can pass from a simple, monofunctional base to a (unintentional) bifunctional catalyst. (C) 2012 Elsevier Inc. All rights reserved.