Kinetics, Selectivity, and Deactivation in the Aldol Condensation of Acetaldehyde on Anatase Titanium Dioxide

Metal oxide catalysts are active for aldol condensation reactions of aldehydes and ketones but typically exhibit rapid deactivation. In this study, aldol condensation with dehydration of acetaldehyde to produce crotonaldehyde was catalyzed by oxidized TiO2 anatase. Particular emphasis was placed on determination of product selectivities and reaction kinetics as a function of both conversion and the extent of catalyst deactivation. Condensation of acetaldehyde is rapid and selective on anatase TiO2, with turnover frequencies exceeding 0.03 s(-1) crotonaldehyde selectivities approaching 100%. The main side reactions generating volatile products are: (a) hydrogenation of the reactant and product aldehydes, (b) a secondary cross-esterification between crotonaldehyde and acetaldehyde to form ethyl crotonate, and (c) secondary condensations involving crotonaldehyde. Catalyst deactivation was observed to affect both the rate of reaction and the product selectivities, particularly at low values of time-on-stream. Secondary condensations that deposit nonvolatile organic species on the catalyst surface are responsible for the initial deactivation of the catalyst. The catalyst mass increase during the course of reaction was observed to be directly proportional to the rate of deactivation. Selectivity patterns were impacted by deactivation in a manner that could not be explained solely by the changing conversion levels associated with the deactivation process. We conclude that deactivation during aldol condensation on the anatase polymorph of titania alters both activity and selectivity of the active sites. As a result, care must be taken to account for catalyst deactivation when comparing both catalytic activities and selectivities.