Rationally designed oxidation catalysts: Functionalized metalloporphyrins encapsulated in transition metal-doped mesoporous silica (abstract)

Catalytic oxidation by metalloporphyrins plays an important role in the conversion of both saturated and unsaturated hydrocarbons to valuable fine chemicals. The advantages in the development of metalloporphyrin systems that mimic cytochrome P-450 mono-oxygenases include substrate specificity, chemoselectivity and high catalytic activity under mild reaction conditions. Substantial efforts have been devoted to the development of an effective supported metalloporphyrin heterogeneous catalyst system that prevents self-oxidation of the active centers and allows for facile recovery of the catalyst. Mesoporous materials with a well-defined pore structure have recently attracted a great deal of research attention in catalytic applications. In this study, a rational design strategy has been established to prepare heterogeneous metalloporphyrin oxidation catalysts encapuslated in a hexagonally-packed mesoporous molecular sieve, providing (1) superior catalytic performance through a rational design of the matrix structure, and (2) improved metalloporphyrin fixation through a tailored interaction between the catalyst and support. A series of mesoporous silicas known as MCM-41 has been synthesized with selective dopants for the metalloporphyrins. The microstructure of the materials was found to depend strongly on processing parameters, such as dopant concentration, aging time and temperature. In the case of Nb-doped silica (Nb/Si-TMS8,Nb/Si-TMS9), the formation of covalent bonding between the surface-exposed niobium sites and the functionalized groups of the iron porphyrin is crucial for immobilizing the iron complex within the mesoporous structure.