Iron porphyrins immobilised on silica surface and encapsulated in silica matrix: a comparison of their catalytic activity in hydrocarbon oxidation

Abstract

In this work, different iron porphyrins, either immobilised on silica surface or encapsulated in silica matrix (FePES), have been used as catalyst in hydrocarbon oxidation by PhIO or H 2O 2, and results have been compared. Such study has aimed at understanding the relationship between the catalytic results and the following properties of the catalytic reaction sites: (i) the coordination environment around the central iron, which can change substrate binding; (ii) the nature of the support, since polarity can affect substrate accessibility to the active site; (iii) the FeP nature and the microenvironment it create. We have observed that all systems are able to oxidise ( Z)-cyclooctene and cyclohexane, and better product yields are obtained with the supported systems. In the case of FePES, high cyclohexanol and epoxide yields are obtained with the electronegatively substituted Im-[FeTFPP]ES. The low yields obtained with the cationic FePES can be explained by the polar environment of the FeP active site, which hinders the oxygen rebound mechanism necessary for the hydroxylation of the inert cyclohexane. As for the supported systems, commercial silica leads to high epoxidation and hydroxylation yields, showing that cationic iron porphyrins are efficient catalysts even when immobilised on a simple support. The use of the clean oxidant H 2O 2 for olefin epoxidation in cases of the 2- and 4- N-methyl-pyridyl substituted FePs in heterogeneous systems is reported for the first time in this paper, and the results are comparable with literature data on electron-deficient FePs in homogeneous systems. The best catalyst is [Fe(TF4TMAPP)] 5+ on the SiSH and SiO 2 supports rendering 80% and 86% epoxide yields with H 2O 2, respectively. All materials have been characterised by BET analysis, UV–vis and EPR spectroscopies.