Development of environmentally friendly catalytic reaction system for selective hydrocarbons oxidation

Research Project , Research Report 2002

Noritaka MIZUNO Dept. of Applied Chemistry, The Univ. of Tokyo

Abstract

Molecular metal-oxygen cluster compounds, namely polyoxometalates, show the strong acidity and redox properties. In addition, the compounds are thermally and oxidatively stable. These properties can be controlled to a great extent by changing the constituent elements. In this study, we attempt to construct the catalytically active sites for selective oxidation of hydrocarbons with “green” oxidant such as O2 and H2O2. A Keggin-type diiron-substituted silicotungstate, -[SiW10{Fe(OH)(OH2)}2O36]4-, has been used as catalysts for the oxygenation of alkenes in homogeneous reaction media using molecular oxygen as an oxygen donor. Various alkenes including internal C=C moiety were catalytically oxygenated with high turnover numbers and high selectivity to the corresponding epoxides. The oxidation reaction mainly proceeded with non-radical processes through an iron-oxygen intermediate. The catalyst is thus able to use molecular oxygen as an oxidant, and raises a prospect of using this type of inorganic catalyst for industrial epoxidation processes. In addition, a divanadium-substituted compound, -[SiW10{V(=O)( -OH)}2O36]4-, exhibited the unique catalytic activity for the epoxidation of alkenes with H2O2. A diruthenium-substituted compound exhibited the capability of methane oxidation with O2. In contrast to such dimetal-substituted compounds, mono and trimetal-substituted polyoxometalates, were not so effective catalysts for oxidation of alkenes and methane. Novel supramolecular porous crystalline materials were synthesized by complexation of Keggin-type polyoxotungustate, [XW12O40]n- (X: P(where n = 3), Si (n = 4), B (n = 5), Co (n = 6)), with a trinuclear chromium(III) complex cation, [CrIII3( 3-O)( -O2CH)6]+. These crstalline copounds showed the molecular sieving effect for water and polar organic compounds.

Key word:

Molecular oxygen, Hydrogen peroxide, Hydrocarbons, Selective oxidation, Polyoxometalates