Photointeraction on the surface of titanium dioxide between oxygen and alkanes
Under the influence of ultra-violet illumination (210–390 nm) alkanes, both normal and branched from ethane to octane, are photo-oxidized at room temperature, mainly into ketones and aldehydes, on the surface of non porous particles of titanium dioxide (anatase).
Alkanes CnH2n+ 2 form: ketones CnH2nO and other aldehydes CmH2mO with 2 < m < n, ketones CmH2mO with 3 < m < n if the alkane is branched and finally CO2 and H2O. The mean selectivity into products of partial oxidation (ketones and aldehydes) is 76 % for n-alkanes, 80 % for isoalkanes, and 57 % for neoalkanes.
The reactivity of different types of carbon atom follows the sequence Ctert > Cquat > Csec > Cprim. The carbon atom which is preferentially attacked by oxygen is that which presents the highest electron density together with the least steric obstruction. A proposed mechanism of the oxidation of alkanes implies the formation of an alcoholic intermediate, on primary, secondary and tertiary carbon atoms, which in turn is oxidized into aldehyde or ketone. On secondary and tertiary carbon atoms this intermediate may be first dehydrated into an olefin which is finally oxidized to an aldehyde or/and ketone.