Acetone and acetaldehyde oligomerization on TiO2 surfaces

Acetone undergoes aldol condensation and cyclization reactions on TiO2 to form mesitylene (1,3,5-trimethylbenzene) below 400 K. The reaction rate is slow on pure anatase TiO2, but on Degussa P25, a mixture of anatase and rutile, more than 20% of a monolayer of acetone forms mesitylene during temperature-programmed desorption or hydrogenation. Other C-5-C-9 hydrocarbon products also form on both oxidized and reduced TiO2, whereas hexene forms only on reduced TiO2. Acetaldehyde undergoes aldol condensation on both types of TiO2; acetaldehyde either desorbs or forms dimeric condensation products on anatase. However, on Degussa P25 TiO2, trimeric condensation products, higher molecular weight compounds, and coke also form. In addition, C5H8, C5H10, C6H10, and C9H14 form as secondary reaction products of aldol condensation. Surface concentrations of acetaldehyde and acetone are higher on Degussa P25 than on anatase TiO2. Degussa P25 has more sites that catalyze aldolization, and it has more acid sites. These condensation reactions, which take place at relatively low temperature, may be partly responsible for deactivation of Degussa P25 during photocatalytic oxidation. (C) 1999 Academic Press.