Hydrocarbon oxidation by iron-porphyrin immobilized on SBA-15 as biomimetic catalyst: role of silica surface

We have immobilized iron porphyrins on SBA-type mesoporous silica matrixes for application in hydrocarbon oxidation. Understanding the role the silica surface has in this process is crucial when developing biomimetic catalysts with improved activity. More specifically, we immobilized 5,10,15,20-tetrakis(pentafluorophenyl) porphyrin iron(III) chloride (FeP) on SBA-15 structures modified with (3-aminopropyl) triethoxysilane (APTES), designated FeP-APSBA. We prepared a second class of catalyst via reaction of FeP-APSBA with 1,1,1,3,3,3-hexamethyldisilazane (HMDS), so that non-polar Si-(CH3)(3) (TMS) groups partially replaced surface Si-OH groups and yielded the FeP-APSBA-TMS catalyst. Goniometric measurements of contact angles with different solvents confirmed that surface polarities changed as expected; these measurements also allowed determination of surface free energies. FTIR, Si-29 CP-MAS NMR, atomic absorption spectrometry and UV-Vis spectroscopies, SAXS, thermal analyses, scanning/transmission electron microscopy (SEM/TEM), and N-2 adsorption/desorption isotherms helped to characterize the prepared materials. Hydrocarbon oxidations enabled us to correlate surface changes with catalyst structure and activity. Surprisingly, the Si-OH groups played an important part in the adsorption of iodosylbenzene molecules, used as oxygen donors during the oxidation reactions. In addition, the TMS end-capped groups negatively affected catalytic performance. In conclusion, the catalytic assays demonstrated the high catalytic potential of iron porphyrins immobilized on SBA-15 and attested to their biomimetic behavior.