OXIDATION OF ALKANES WITH HYDROGEN-PEROXIDE CATALYZED BY IRON SALTS OR IRON-OXIDE COLLOIDS IN REVERSE MICROEMULSIONS

Catalytic oxidation of alkanes occurs in reverse (water in oil) microemulsions stabilized by an anionic surfactant (Aerosol OT(R)). The catalytically active microemulsion is generated by mixing two reverse microemulsions containing respectively an aqueous solution of iron (Fe(II) or Fe(III)) and an aqueous solution of H2O2 (30%) dispersed in liquid alkanes (continuous oil phase). This microemulsion system is active in the C-H bond oxidation of various liquid alkanes (cycloalkanes, decalin, tetralin, alkylbenzenes) into ketones and secondary and tertiary alcohols at room temperature. The selective oxidation of secondary carbons into ketones and tertiary carbons into alcohols is interpreted by the intervention of iron(V) oxene intermediates rather than radical Fenton-type mechanisms. Polynuclear aggregates such as colloidal hydrous ferric oxides generated by hydrolysis within the aqueous microdroplets are assumed to be the catalytically active species. In fact, we demonstrate that previously prepared iron oxide colloids (Fe2O3 and FeOOH) similarly catalyze the oxidation of alkanes with hydrogen peroxide in reverse microemulsions. The low interfacial water-hydrocarbon tension and the very high interfacial area in liquid-liquid dispersions greatly favour the catalytic process.