Ion-Exchanged OMS-1 Nanoparticles for Efficient Aerobic Oxidation Catalysis of Sulfides and Other Organics under Mild Conditions

Various metal cations can be inserted into/deinserted from octahedral molecular sieves such as todorokite-type manganese oxide (OMS-1) having large (3 × 3) tunnel structures. Even so, the synthesis of small nanoparticles of these materials and the subsequent ion exchange for such particles by a postsynthesis treatment remains challenging. This study devised an improved precursor crystallization method using Mg(MnO4)2 to synthesize mesoporous OMS-1 nanoparticles (Mg-OMS-1), employing KMnO4 as an inexpensive, readily available reagent. The introduction of various 3d transition metal cations into the OMS-1 channels using a facile ion-exchange process was also demonstrated. Ion-exchanged OMS-1 nanomaterials (M-OMS-1; M = Fe, Co, Ni, Cu, or Zn) retained the original tunnels and porous structure, with specific surface areas in the range of 248–332 m2 g–1. These surface areas were much larger than those of materials made using the conventional process and comparable to that of OMS-1 nanoparticles (249 m2 g–1) previously synthesized using Mg(MnO4)2. Cu-OMS-1 and Zn-OMS-1 showed high catalytic activity for the oxidation of thioanisole with only molecular oxygen as the oxidant, with a performance that was significantly improved compared with that of the original Mg-OMS-1. In particular, Cu-OMS-1 functioned as a heterogeneous catalyst and could be reused without loss of activity or changes in the OMS-1 structure. This material was also applicable to the sulfoxidation of various aromatic sulfides (based on nine examples) and other liquid-phase aerobic oxidations (three examples). Detailed analyses of the aerobic oxidation of sulfides with Cu-OMS-1 catalyst indicated that the reaction likely proceeded with substrate oxidation by surface lattice oxygen species. The introduction of Cu ions into the tunnel structure evidently improved the oxidizing ability of the OMS-1 nanoparticles.