Catalytic oxidation of methane and light hydrocarbons at ambient conditions using diiron phthalocyanine complexes – CATOX-METHANE

Scientific background and objectives Selective low temperature oxidation of methane, the most abundant and the least reactive compound of natural gas, is one of the most difficult chemical problems of a great practical importance. Two main approaches have been explored to achieve a low temperature CH4 oxidation. Organometallic activation of CH4 on Pt or Pd centres occurs at harsh reaction conditions (e.g. 220°C, oleum, see Periana et al, Science, 1998, 280, 560). In contrast, methane monooxygenase enzymes (MMO) oxidize CH4 to methanol at physiological conditions (water, ambient temperatures). Considerable amount of research has been directed to the development of the chemical models of MMO. Complexes mimicking a structural organisation and some spectral features of MMO have been developed, but real functional chemical models able of oxidizing CH4 have never been described. An essential feature of MMO is a diiron active site and all research efforts in this area were concentrated on non-heme binuclear complexes. On the other side, mononuclear metalloporphyrin complexes have been used for C-H oxidation as models of cytochrome P-450. However, both approaches were so far unfruitful for creating of the catalytic systems able to oxidize methane. We propose a novel unexplored approach: using of binuclear porphyrin-like complexes as oxidation catalysts. Up to now, similar complexes have completely been neglected as catalysts and have even been considered as catalytically inactive forms. Very recently, we have discovered a stable binuclear iron phthalocyanine complex with remarkable catalytic properties. Using UV-vis, ESI-MS, EPR, Mössbauer, XANES and EXAFS techniques we showed activation of H2O2, a biologically and ecologically relevant oxidant. The first experiments showed a very strong oxidizing ability of diiron macrocyclic complex – H2O2 system that prompted us to test this system in oxidation of CH4. For the first time we showed the efficient oxidation of CH4 by H2O2 in pure water at near-room temperatures. This oxidation was unambiguously evidenced by the experiments using labelled 13CH4 and 18O compounds. The yield of products (formaldehyde and formic acid) attains 50 % on the H2O2 and turnover number attains 400 cycles. Many laboratories are unsuccessfully trying to achieve this notoriously difficult reaction. However, no any example of bio-inspired oxidation of methane is still available. Our finding represents the very first example of mild bio-inspired oxidation of CH4 and is of great significance for the development of topical area of chemistry. This research project is devoted to the development of these binuclear iron complexes. We believe that this finding is a significant breakthrough in the oxidation field and has a great potential for further development. Therefore, the objectives of this project are (i) investigation of the mechanism of the methane oxidation; (ii) determination of the structure and properties of active species (iii) application of catalytic system for other difficult-to-oxidise substrates, e.g. alkanes, aromatic compounds, etc. (iv) further development of this catalyst type by modification of its structure. Methodology The formation, stability and properties of elusive active diiron phthalocyanine intermediates will be studied by electrospray ionization mass spectrometry (ESI-MS), resonance Raman and Mössbauer spectroscopies, EPR, XANES and EXAFS techniques. A significant part of the project will be dedicated to the investigation of oxidation mechanisms. We will use specific diagnostic probes to distinguish between homolytic and heterolytic cleavages of O-O bond during the formation of the active species, to evaluate nucleophilic vs electrophilic character of formed active species, to detect the presence or absence of radical species in the catalytic reactions (spin traps), etc. Special attention will be paid to experiments with 18O labelled molecules. Expected results Interest in these systems stems not only from their obvious fundamental importance but also from a desire to use their great synthetic potential to develop novel clean and selective oxidation methods. The main expected results are identification and detailed characterization of oxo and/or peroxo diiron phthalocyanine complexes for the first time. Success of this project will result in new knowledge which will be of great interest to the bioinorganic and catalytic communities. A detailed mechanistic study will allow the determination of mechanisms of oxidation and provide a basis for optimization of these catalysts in terms of activity and selectivity. The investigation of oxidation mechanisms and diiron phthalocyanine based active species will not only bridge a gap between biochemistry and industrially directed catalysis, but will also be beneficial for the development of new industrially viable catalytic methods for clean oxidation.