Investigation on the Hydrocarbon and Oxygen-Containing Hydrocarbon Intermediates Leading to Crotonaldehyde Formation on TiO₂

Aldol condensation of CH₃CHO, forming crotonaldehyde (2-butenal, CH₃CH=CHCHO), readily occurs on TiO₂ at 35 °C. At a higher coverage or at an elevated temperature, the crotonaldehyde can be oxidized to crotonate. Adsorption and thermal reactions of CH₃CHBr₂, BrCH₂CH₂Br, BrCH₂CH₂OH, and ClCH₂CH₂OH on TiO₂ can produce crotonaldehyde, in contrast to CH₂=CHBr. CH₃CHBr₂ has the highest reactivity toward the crotonaldehyde formation among the halogenated compounds studied. The pathways of CH₃CHBr₂ + Ti-O-Ti → CH₃CHO + 2Ti-Br and BrCH₂CH₂Br + Ti-O-Ti → Ti-O-CH₂CH₂Br + Ti-Br → CH₃CHO +2 Ti-Br are proposed for the reactions of CH₃CHBr₂ and BrCH₂CH₂Br. The crotonaldehyde generated from the reactions of the four halogenated compounds on TiO₂ has lower C=O and C=C stretching frequencies as compared to those of the crotonaldehyde directly from its adsorption on TiO₂. This result is attributed to the presence of Br or Cl atoms near the crotonaldehyde adsorption sites and the change in the Ti ionic bonding environment. In addition, photoirradiation (325 nm) on ClCH₂CH₂OH on TiO₂ can enhance the crotonaldehyde formation.