Oxidations of hydrocarbons are a class of reactions of extreme importance to the chemical industry. Conventional oxidation methods require harsh conditions, stoichiometric amounts of oxidant and produce the reduced oxidant as waste material. The increased need for "greener" technologies, based on oxidation by molecular oxygen or aqueous hydrogen peroxide at low temperature calls for a different approach and stimulates the development of a new generation of catalysts.
Thiol-functionalized cobalt porphyrins can be used as a model system for the investigation of the activity in homogeneous and heterogeneous biomimetic oxidation catalysis. Reactions using porphyrins or equivalent under homogeneous conditions have been used for decades with great success, although with some general drawbacks, such as the need of separation from the product, or the poisoning of the catalyst after some reaction cycles, probably due to di- or polymerization.
In the present study, the thiol-functionalized cobalt porphyrins (CoTPP-L) were grafted onto a Si(100) wafer surface modified with di-sulfide-pyridin groups. The properties of the obtained layers were characterized on the molecular level by means of X-ray photoelectron spectroscopy (XPS) and Atomic Force Microscopy (AFM). The immobilisation of the molecules on the derivatised Si surface takes place covalently through the thiol-linkers by the formation of thiolate bonds. The oxidation of hydroquinone into benzoquinone was run on this modified surface. The poisoning of the catalyst seems to be inhibited, even 600 hours after start. Thus the grafting of the porphyrins onto the Si surface is sufficiently strong to prevent their polymerisation. The measured activity is ca 2 order of magnitude higher than the one observed in the case of the homogeneous catalysis. A comparison with the results obtained with CoTPP-L grafted onto a gold surface [1] is made, and interpreted in the light of the molecule-substrate interaction through the Co metal atom.
[1] S. Berner, S. Biela, G. Ledung, A. Gogoll, J.E. Bäckvall, C. Puglia and S. Oscarsson, J. of Catal. 244 (2006) 86.
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