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

Abstract

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.

Conversion of levulinic acid into chemicals: Synthesis of biomass derived levulinate esters over Zr-containing MOFs

“…In a previous study, we pointed out that the presence of structural defects and also of phase impurities in the zinc terephthalate IRMOF-3 may have a critical effect on the reaction rate of acid-catalyzed reactions (Llabrés i Xamena et al, 2012). Similarly, the Lewis acid properties of UiO-type materials arise (at least partially) from the formation of crystalline defects associated to linker deficiencies, as already established in previous works by the groups of Lamberti (Valenzano et al, 2011) and De Vos (Vermoortele et al, 2012).…”

Origin of highly active metal–organic framework catalysts: defects? Defects!

“…lower coordination number) and hydroxyl terminated surface can be applied for the definition of MOF defects which are relevant to explain the observed, yet "unexpected" catalytic results. 30 …”

Zirconium-containing metal organic frameworks as solid acid catalysts for the esterification of free fatty acids: Synthesis of biodiesel and other compounds of interest

“…Thus, UiO-66-NH 2 would be acting as a bifunctional acid-base catalyst for the esterification reaction, as shown in Scheme 2. In this dual activation, strong individual acid or basic sites are no longer needed to attain high activities and selectivities, but rather the reaction is better performed with mild acid-base pairs in close proximity [35][36][37][38], such as those present in UiO-66-NH 2 . Thus, our proposal to explain the higher activity of UiO-66-NH 2 with respect to UiO-66 is that the Zr Lewis acid sites (which are present in both UiO-66 and UiO-66-NH 2 as revealed by FTIR spectroscopy of adsorbed cyclohexanone) can adsorb the fatty acid, thus increasing the electrophilic character of the carboxylic carbon atom.…”