MIL-101(Cr)-NO2 as efficient catalyst for the aerobic oxidation of thiophenols and the oxidative desulfurization of dibenzothiophenes

A series of MIL-101(Cr)-X functionalized with electron withdrawing (NO2, SO3H or Cl) or electron donor (NH2 or CH3) groups has been tested for the solvent-free oxidative coupling of thiophenol to disulfides. No byproducts were observed. A relationship between the catalytic activity of these MOFs with the substituent meta Hammet constant on the terephthalate ligand and with their redox potential was found, MIL-101(Cr)-NO2 being the most active catalyst. NO2-substituted MIL-101 is also more efficient than the parent MIL-101(Cr) to promote the aerobic desulfurization of dibenzothiophenes in n-dodecane or commercial Diesel as solvent. No byproduct formation was observed. Mechanistic studies reveal that MIL-101(Cr)-NO2 is acting as heterogeneous catalyst in thiophenol oxidation and as radical initiator for the aerobic desulfurization. For both reactions, the catalyst can be reused without deactivation, maintaining its crystallinity and with negligible metal leaching.