Semiamorphous Fe-BDC: The missing link between the highly-demanded iron carboxylate MOF catalysts

Abstract

Porous Fe carboxylates are amongst the most promising MOF-based materials due to their low price, low toxicity, metal environments (including open metal sites) and remarkable (meso)porosity variety. Fe-MOFs based on the cluster [Fe3O(X)(solvent)2]6+, that is, MIL-101(Fe), MIL-100(Fe) and semiamorphous Fe-BTC, are of particular interest. These three materials are quite related each other: (i) MIL-100(Fe) and MIL-101(Fe) have the same zeolitic topology MTN and two types of mesocavities, whereas (ii) Fe-BTC and MIL-100(Fe) form an unprecedented semiamorphous / fully-crystallized pair, having in common the metal cluster, the composition, one mesocavity, etc but without becoming a nano- / micro-crystalline pair. This work describes the room-temperature synthesis, characterization and catalytic performance in the aerobic cyclohexene oxidation of the semiamorphous Fe-BDC, which together with MIL-101(Fe) would form the second semiamorphous / crystallized pair in MOFs. Unfortunately, Fe-BDC could not be prepared in water as solvent, but in either ethanol or in N,N-dimethylformamide. It possesses relatively high textural properties (above 500 m2g-1) and key common features with MIL-101(Fe): XRD reflections at the same 2θ positions, similar thermal stability, almost equal linker conformations, etc. Fe-BDC became quite active in the solvent-free aerobic oxidation of cyclohexene under mild conditions, surpassing the activity performance of the well-known commercial Fe-BTC catalyst in the same reaction under the same mild conditions.