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 degrees 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 CICH2CH2OH 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 + 2 Ti-Br and (BrCHCHBr)-C-2-Br-2 + 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 C1CH(2)CH(2)OH on TiO2 can enhance the crotonaldehyde formation.