Tunable catalytic properties of multi-metal–organic frameworks for aerobic styrene oxidation

The Cu and Co mixed metal–organic frameworks (MOFs) MOF-74(Cu/Co) with different Cu/Co ratios were prepared via a facile one-pot synthesis method and then characterized by means of various techniques. Whatever the Cu/Co ratios in the synthesized MOFs were, both Cu2+ and Co2+ could be uniformly distributed in MOF-74. The synthesized MOF-74(Cu/Co) materials were used as catalysts in styrene oxidation with O2 under solvent-free and mild reaction conditions. The results show that the catalytic properties of MOF-74(Cu/Co) can be tuned by varying the Cu/Co ratio in MOF-74. MOF-74(Cu) has a rather low catalytic activity but an absolute selectivity for benzaldehyde while MOF-74(Co) possesses a relatively higher catalytic activity, leading to the formation of benzaldehyde, styrene epoxide, and phenylacetaldehyde as well as to polystyrene. It is surprising that the incorporation of Cu2+ in MOF-74(Co) can completely inhibit the polymerization of styrene while the incorporation of Co2+ in MOF-74(Cu) can significantly enhance the conversion of styrene to benzaldehyde, styrene epoxide, and phenylacetaldehyde. In addition, by increasing the Co2+ amount in MOF-74(Cu/Co), the conversion of styrene and the selectivity for styrene epoxide and phenylacetaldehyde are increased while the selectivity for benzaldehyde is decreased. Moreover, the catalytic properties of MOF-74(Cu/Co) were compared with those of the physical mixtures of MOF-74(Cu) and MOF-74(Co), demonstrating the synergetic effects of Co2+ and Cu2+ in MOF-74(Cu/Co) on the catalytic oxidation of styrene. The current work reveals a strategy to incorporate multiple metal cations into MOFs for enhancing a specific catalytic property or achieving a new catalytic property.