Rearrangement of α-pinene oxide to campholenic aldehyde over the trimesate metal-organic frameworks MIL-100, MIL-110 and MIL-96

The catalytic performance of porous metal-benzenetricarboxylates, such as MIL-100(Al, Fe and Cr), MIL-110(Al) and MIL-96(Al), was investigated with a combination of physicochemical and catalytic approaches in the rearrangement of α-pinene oxide to campholenic aldehyde (CA). The investigation of Lewis acidity was done by EPR and IR spectroscopy using 2,2′,6,6′- tetramethyl-1-piperidinyoxyl radical and benzonitrile as the probe molecules, respectively. For Al-BTCs, both these methods showed a decrease in the amount of Lewis acid sites as follows: MIL-100 > MIL-110 > MIL-96. The reaction rate and selectivity toward CA also decreased in the same order. A relatively good correlation between selectivity toward CA and the electronegativity of the metal ion was found for isostructural MIL-100(Al, Fe and Cr). The selectivity toward CA decreased in the order: MIL-100(Al) > MIL-100(Fe) > MIL-100(Cr). The high selectivity toward CA in the presence of MIL-100 has been suggested to originate from the unique structure of this material, which favors shape selectivity.