Room-temperature prepared bimetallic nanocrystalline MOF-74 as catalysts in the aerobic oxidation of cyclohexene

Abstract

MOF-74 is probably the MOF material that best combine three key features for catalytic applications: compositional versatility, the presence of open metal sites and the possibility of becoming nanocrystalline, thus avoiding or minimizing any reactants/products diffusion problems within their pores. This work describes, for the first time, the room-temperature preparation of bimetallic nanocrystalline MOF-74 materials and their catalytic performance in the solvent-free oxidation of cyclohexene using oxygen as oxidant. Although the preparation of oligo-metallic MOF-74 has been already published and even certain catalytic and adsorption metallic synergism has been made clear, this work shows that the room temperature synthesis approach provides some further possibilities for controlling the metal distribution by two different strategies: (i) changing the order of metal addition and/or (ii) by synthesis time. The nanocrystalline Me-MOF-74 catalysts based on metal with redox behavior (Mn, Co, Cu) are significantly active in the aerobic oxidation of cyclohexene, whereas Zn is rather an inhibitor. In spite of such inhibitor role of Zn-MOF-74, the bimetallic Zn-M2-MOF-74 samples (M2 = Mn, Co, Cu) show catalytic performance similar to that of the monometallic counterparts M2-MOF-74, and therefore their turnover is considerably increased. The system Cu-Mg-MOF-74 was selected to study effect of different metal distribution within the crystal on their catalytic activity: Longer synthesis time and consecutive addition of Cu and then Mg (the reverse order did not lead to pure MOF-74 phase) favors the catalytic activity of the resultant materials. Finally, the CoMn combination of two of the three most active metals (samples CuMn, CoMn and CuCo) lead to a bimetallic catalyst with higher catalytic performance than the corresponding monometallic samples.