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.

The catalytic activity of IRMOF-3 for the Knoevenagel reaction is reexamined in view of its possible (unintentional) acid–base bifunctional character. Acid sites at defect positions created during the synthesis could exist together with basic amino groups of the linker.Figure optionsDownload as PowerPoint slideHighlights
► IRMOF-3 is a highly active catalyst for the Knoevenagel condensation.
► MOF-5, lacking the amino groups of IRMOF-3, is also active for this reaction.
► The active centers of MOF-5 are acid sites at defects created during synthesis.
► The same defects are probably also present in IRMOF-3.
► IRMOF-3 can behave as an (unintentional) acid–base bifunctional catalyst.