Fluorinated silica gels doped with TPAP as effective aerobic oxidation catalysts in dense phase carbon dioxide

Hybrid organic-inorganic fluorinated silica glasses doped with the ruthenium species TPAP (tetra-n-propylammonium perruthenate) are effective catalysts for the aerial oxidation of benzyl alcohol to benzaldehyde in dense phase CO2. Moderate silica fluorination (10%) by short-chain fluoroalkyl-containing monomers in the sol-gel polycondensation with TMOS affords highly active catalysts which at 75 °C and 220 bar selectively dehydrogenate the alcohol with oxygen as primary oxidant. Both the activity and the stability of the fluorinated materials vary with the degree of fluorination and the nature of the fluoroalkyl residue attached to the silica polymeric network. An explanation of the behaviour of doped sol-gel oxides in supercritical carbon dioxide is proposed which is thought to be of general validity for future practical applications to heterogeneously catalysed aerobic oxidations eliminating the current need for both organic solvents and stoichiometric oxidants.