Manganese porphyrins immobilized on magnetic SBA-15 mesoporous silica as selective and efficient catalysts for cyclic and linear alkane oxidation

This work describes the immobilization of neutral and cationic Mn porphyrins (MnP) on magnetic SBA-15 mesoporous silica. XRD patterns, TEM images and N-2 adsorption-desorption analyses showed that the prepared solids had high surface areas and homogeneous porosities, confirming the presence of magnetite (Mag), which is a key feature for fast and easy recovery and reuse of the catalysts. The cationic MnP1 and MnP2 interacted with the support by electrostatic attraction and the neutral MnP by covalent bonding (MnP3). The catalytic activity for oxidation of cis-cyclooctene, cyclohexane, adamantane, n-hexane and substrate mixture (cyclohexane/nhexane) by iodosylbenzene (PhIO) was investigated for all prepared solids. In heterogeneous catalysis, the solids produced similar or higher yields than the corresponding free MnP catalysts with selectivity for alcohol when alkanes were used as substrate. The best yields of the supported catalysts were 30% of cyclohexanol, 28% of 1-adamantanol, 10% of 2-hexanol, and 18% of cyclohexanol, for oxidations of cyclohexane, adamantane, nhexane, and cyclohexane/n-hexane mixture, respectively. The catalytic results and the catalyst characterization suggested that MnP2 is immobilized inside the pores of SBA-15. This conclusion was also confirmed further by the selectivity for the primary alcohol in the n-hexane oxidation observed for the catalyst solid prepared with the MnP2. The assembly resulting from the immobilization of MnP2 and the silica pores probably created an adequate environment for the approach of the linear substrate, enabling its oxidation at the terminal carbon.