Redox molecular sieves as heterogeneous catalysts for liquid phase oxidations

Abstract

Publisher Summary Catalytic oxidations in the liquid phase generally employ soluble metal salts or complexes as the catalyst. So-called redox molecular sieves, unlike conventional supported catalysts, possess a regular microenvironment with homogeneous internal structures, consisting of uniform, well-defined cavities, and channels. Confinement of the redox active site in these channels and/or cavities can endow the catalyst with unique activity, as a result of strong electrostatic interactions between acidic and basic sites on the internal surface, and the substrate or reaction intermediate analogous to interactions with acidic carboxyl and basic amino groups of amino acid residues in the active site of (redox) enzymes. Zeolitcs and zeotypes are crystalline oxides, comprising corner sharing TO4 tetrahedra (T = Si, AI, P, etc.), and consisting of a regular pore system with diameters of molecular dimensions; hence, the term molecular sieve. The framework of molecular sieves is not completely rigid and the incoming molecules are able to induce slight structural changes. Three types of redox molecular sieve can be distinguished on the basis of the method of synthesis: ion exchange, framework substitution, and encapsulation. Redox molecular sieves have one important advantage over other heterogeneous catalysts; it is possible to influence that substrate molecules approach the active site by a suitable choice of molecular sieve and solvent. Hence, by using a redox molecular sieve, it is possible to obtain much higher selectivities to primary oxidation products than with analogous homogeneous catalysts. Titanium(IV) silicalite (TS-1), the first example of a framework-substituted redox molecular sieve, catalyzes a variety of synthetically useful oxidations with 30% aqueous hydrogen peroxide under mild conditions. Similar heterolytic mechanisms can be envisaged for other nucleophilic substrates like ammonia, amines, sulfides, phenols, and alcohols. Based on these recent developments, ship-in-a-bottle catalysts appear to hold much promise, extending even to the design of chiral “ships” for enantioselective oxidation.