Metalloporphyrins are the most widely studied catalysts for homogeneous selective oxidation and hydroxylation of hydrocarbons. An important area of research into their catalytic properties is associated with immobilization of these species onto a solid support, in order to make the catalyst easier to handle and to separate from the reaction medium, as well as possibly stabilize and/or modify the catalytic performance. However, the conventional microporous supports, including zeolites, fail as possible host structures, due to the incompatibility between the small pore size and large dimensions of catalytically active macromolecules. Discovery of mesoporous molecular sieves with well-defined pore sizes of 2 to 50 nm opened new opportunities in the area of immobilization of organometallic macrocycles. Our recent works demonstrated that Al,Si-mesoporous molecular sieves represent an extremely interesting class of supports for cationic metalloporphyrins, allowing for a strong, electrostatic binding of the species and, what is of immense importance in catalysis, for tailoring of the catalyst selectivity. The talk will present the examples of catalytic oxidation of cyclic alkenes and alkanes by metalloporphyrins supported on mesoporous aluminosilicate solids of HMS, MCM-41, FSM and SBA-15 types. In particular, it will be demonstrated that the catalyst selectivity is modified by constrained space around the metalloporphyrin centre located within the internal system of nanosized pores. Catalytic results imply that a simple reaction, such as oxidation of cyclohexene over metalloporphyrins supported on mesoporous aluminosilicates, may be used to probe the spatial distribution of Al sites within the silica framework. Quantum chemical calculations, shedding light on the mechanism of the observed steric effects, will be presented.

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