Mechanistic Revelations: Surface Hydrogen-Mediated Selectivity Control in Aldol Condensation on TiO₂(101)

The aldol condensation of butyraldehyde to 2-ethyl-2-hexenal is a critical step in synthesizing 2-ethylhexanol and 2-ethylhexanoic acid. Although TiO₂ is a promising replacement for traditional base catalysts, the reaction mechanisms of the main and side pathways and the selectivity factors remain elusive. To address this, the Density Functional Theory (DFT) was utilized to elucidate the reaction network and microscopic mechanisms of the aldol condensation on the anatase TiO₂(101) surface. The initial activation of butyraldehyde is dominated by the α-H dissociation pathway with a barrier of 0.45 eV rather than the direct enolization pathway that has a higher barrier of 2.69 eV. Following this step, the optimal pathway proceeds sequentially through C-C coupling, β-O hydrogenation, and dehydration. Among these, β-O hydrogenation is the rate-determining step, with an energy barrier of 1.00 eV. The dehydration step follows a stepwise pathway involving initial hydrogen elimination followed by OH removal with a barrier of 0.59 eV, lower than the 1.01 eV barrier of the concerted elimination pathway. A common feature in the side reactions is that the formations of both butyl butyrate and 4-heptanone are governed by hydrogen dissociation, with rate-determining barriers of 2.70 eV for the aldehyde-H dissociation in butyl butyrate formation and 1.04 eV for the β-H dissociation in 4-heptanone formation. Overall, the main reaction is limited by β-O hydrogenation, whereas the side reactions are controlled by H-dissociation. This mechanistic understanding suggests that tuning the surface hydrogen concentration is key to enhancing selectivity, providing a theoretical basis for developing advanced TiO₂ catalytic processes.