Iron(III) porphyrin anchored onto organosilylated multiwalled carbon nanotubes as an active catalyst for epoxidation reactions under mild conditions

The immobilization of an efficient iron(III) porphyrin catalyst onto multiwalled carbon nanotubes (MWCNT) is reported through a quaternization reaction between the dimethylamine-tetrafluorophenyl groups of the metalloporphyrin and the bromoalkyl moieties attached on the nanotubes surface. Prefunctionalization of MWCNT considered its direct reaction with 4-hydroxyaniline to introduce hydroxyphenyl groups on the nanotube sidewalls, followed by silylation with a bromoalkylorganosilane. The preparation of the catalysts was performed in conventional and microwave heating, and the obtained materials were characterized by XPS, TGA, FTIR, and XRD. Although the two procedures led to similar functionalization degree, the microwave-assisted synthesis allowed the anchoring reaction to be performed in 1 h, reducing significantly the reaction time relatively to conventional heating. For both materials, the catalyst loading was 76 μmol g−1 based on the iron content obtained by XPS. The materials were tested as recyclable catalysts in the epoxidation of cis-cyclooctene by hydrogen peroxide as a green oxidant at room temperature, and in ethanol as a cheap and environmental compatible solvent with no need of other additives. A 95 % conversion and 100 % selectivity towards the epoxide were obtained after 5 h of reaction time, and the reutilization of the catalysts, upon three cycles, was more efficient when the rate of H2O2 addition was 0.5 molar equivalents h−1.