CarbonCarbon Bond Formation via Aldolization of Acetaldehyde on Single Crystal and Polycrystalline TiO2 Surfaces

The aldol condensation of acetaldehyde. CH3CHO, to form crotonaldehyde. CH3CHCHCHO, and crotyl alcohol, CH3CHCHCH2OH, takes place on single-crystal surfaces of TiO2 (rutile), even under ultrahigh-vacuum conditions. Both the {011}-faceted TiO2(001) surface (which nominally exposes only fivefold coordinated cations) and the {114}-faceted (011) surface (which exposes four-, five-, and sixfold coordinated cations) are active for this bimolecular reaction. This observation is in contrast to the sharp activity difference between these two surfaces for carboxylate ketonization and suggests that aldol condensation does not exhibit a strong dependence on surface structure. The principal reaction observed in TPD and XPS experiments to compete with aldolization of acetaldehyde was reduction to ethanol; Cannizzaro disproportionation to acetate plus ethoxides and reductive coupling to butene were minor pathways. The aldolization selectivity increased somewhat as the surface heterogeneity increased from the {011}-faceted TiO2(001) surface, to the {114}-faceted (001) surface, and to polycrystalline TiO2 (anatase) powder. This selectivity variation likely reflects the influence of surface heterogeneity on the activity for the various competing reactions, especially hydrogenation; the aldol coupling reaction, although bimolecular, appears to be relatively insensitive to surface structure.