Role of water on ozonation of cinnamaldehyde to benzaldehyde under Ca(OH)2 catalysis: A combined in situ DRIFTS and DFT study

Highlights: • The water effect in the ozonation was clarified: promotion first and then inhibition. • Promotion came from the transformation of CaO to Ca(OH)₂, having higher activity. • The competitive adsorption between H₂O and O₃ was weak due to the adsorption energy. • H₂O inhibited the ozonation by promoting ozonide decomposition and ·OH formation. The role of water on cinnamaldehyde ozonation to benzaldehyde under Ca(OH)₂ catalysis was studied by in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) and Density Function Theory (DFT). According to the thermogravimetry results, the water, generated in the ozonation, reacted with CaO to form Ca(OH)₂ and then existed on the Ca(OH)₂ surface in the form of adsorbed and interlayer water. The experiment results showed that lab-made and pre-processed Ca(OH)₂ had better catalytic activities because their specific surface area values were 3.59 and 4.46 times larger than CaO. DFT results indicated that the competitive adsorption between water and ozone was weak due to the difference of their average adsorption energy difference. The major effect of water was to promote ozonide decomposition and ·OH formation. At the presence of water, the DRIFTS signals of physisorption ozone and ozonide disappeared within 20 min, but the signals of oxygen atoms and O₂^– had about 12% and 33% increasing, respectively. As the water content increased in the system and the key intermediates, ozonides, and ·OH, changed, the ozonation mechanism was changed from the Criegee mechanism to the free radical mechanism, resulting in a significant decrease in the selectivity of benzaldehyde.