In situ assembly of ultrafine Mn3O4 nanoparticles into MIL-101 for selective aerobic oxidation

Abstract

Nanosize metal oxides generally possess high catalytic activity, but they tend to agglomerate into larger particles during a reaction. Therefore, the preparation of nanosize metal oxides with high stability is a crucial challenge. A novel and facile approach involving impregnation followed by a double solvent method was developed to directly encapsulate ultrafine Mn₃O₄ nanoparticles (NPs) into the nanocages of metal–organic frameworks (MOFs). A series of MIL-101 encapsulated Mn₃O₄ NPs were prepared, in which the contents of Mn₃O₄ ranged from 3.2% to 33%. Ultrafine Mn₃O₄ NPs with a particle size of about 3 nm have been successfully embedded in the nanocages of MIL-101 with a uniform distribution. The MIL-101 encapsulated Mn₃O₄ NPs with a Mn₃O₄ content of 15% exhibits the highest conversion of benzyl alcohol (38.7%) and a >99% selectivity to benzaldehyde. Furthermore, after being repeatedly used 10 times, its catalytic activity is hardly changed. When the content of Mn₃O₄ NPs was further increased, the catalytic activity of the catalyst decreases, due to aggregated Mn₃O₄ particles with a large size which formed outside the MIL-101 matrix.