Approaches to the synthesis of heterogenised metalloporphyrins

Porphyrins and different metalloporphyrins have been synthesised and heterogenised on robust inorganic solids such as silica gel, purely siliceous MCM-41, and delaminated zeolites ITQ-2 and ITQ-6. Two different strategies for preparation of these catalysts have been explored, mainly based on immobilisation of a functionalisated porphyrin (route A) or heterogenisation of an aromatic aldehyde and consecutive porphyrin formation (route B). Route A yields stable materials and allows introduction of a larger quantity of metalloporphyrin onto the support (contents ranged from 0.3 to 0.4mmolg−1) whereas route B was unsuccessful and no evidence was found to support the porphyrin formation. A multitechnique approach is employed for characterisation of samples and their catalytic behaviour has been tested in the electroreduction of oxygen. A cyclic voltammetry investigation, at varying scan rate, was carried out in an attempt to elucidate the net reaction for the oxygen reduction. The synthesised metalloporphyrins are active for electrocatalytic reduction of oxygen by a two-electron mechanism, producing hydrogen peroxide. The comparison between homogeneous and heterogenised catalysts confirms that heterogenisation avoids the catalyst desorption (lost of activity) from the electrode. Catalytic activity is directly related with the content of metalloporphyrin in heterogenised materials that are addressable electronically.