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 C,H2,0 and other aldehydes C,H2,0 with 2 < m < II, ketones CmH2m0 with 3 c m < n if the alkane is branched and finally COz and HzO. 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 > GC > 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.The photo-oxidation of various organic molecules (except alkanes) in the presence of ultra-violet irradiation and a solid oxide surface is a phenomenon which has been observed for many ~e a r s , l -~ the partial photocatalytic oxidation of alkanes (propane, isobutane, n-butane) was recorded only recently.6-10 In the present paper the previous work is extended to alkanes from C, to C8 and the reactivities of various types of carbon atom are considered. A tentative mechanism for this photo-oxidation is proposed.
EXPERIMENTALThe catalyst is a non-porous anatase prepared by the flame reactor method l1 which has already been described in detail.' The differential photocatalytic fixed bed reactor was also described previously ** l o as well as the gas chromatographic methods for the analysis of effluenfs.l2 A mercury vapour lamp (Philips HKP 125 W) was used throughout.
RESULTS A N D DISCUSSIONThe photocatalytic activity of Ti02 powder at room temperature in the oxidation of propane (in the mixture 30 % propane, 20 % oxygen and 50 % helium at atmospheric pressure, with a flow rate of 20 cm3 min-l) as a function of the mass of the catalyst, spread out in the reactor as a thin homogeneous layer, is represented in fig. 1. The areas of chromatographic peaks for acetone, propanal and ethanal and the height of the peak for C 0 2 first increase with the mass of the catalyst and then level off, showing that with the increasing thickness (or the mass) of the catalytic bed, only catalyst layers irradiated by U.V. light are active. The same relationship was observed for the photo-oxidation of isobutane on this catalyst.
