Isotope tracing study to directly reveal the roles of H2O’s oxygen and hydrogen atoms in photocatalytic oxidation of gaseous aromatic alcohols to aromatic products

Abstract

H₂O shows a significant contribution to form oxygen- and hydrogen-containing products during photocatalytic oxidation of aromatic hydrocarbons. However, it was unclear what the roles of atoms in H₂O in subsequent oxidation of these products. In this study, the photocatalytic oxidation of two aromatic alcohols, benzyl alcohol and phenethyl alcohol, on TiO₂ (79 % anatase and 21 % rutile) were investigated in mixed H₂O (or H₂¹⁸O, D₂O) and O₂ system. Aromatic aldehydes, acids and esters were main aromatic products, while modified TiO₂ and isotope-containing H₂O did not change the product species. Aromatic aldehydes showed ≤ 0.19 % and ≤ 1.58 % of ¹⁸O- and D-labeling rates, suggesting hardly involvement of H₂O in their formation. Aromatic acids exhibited a ¹⁸O-labeling rate range of 28.12 %-40.33 % and the highest D-labeling rate of 0.76 %, while a maximum enhancement of ¹⁸O-labeling rate was 0.11 % in the –C=O group of the acids, indicating that H₂O selectively provided O atom in the formation of –OH group of aromatic acids. The ¹⁸O- and D-labeling rates of aromatic esters were in the range of 9.01 % to 15.95 % and 1.73 % to 8.08 %, suggesting that both oxygen and hydrogen atoms from H₂O contributed to aromatic ester formation. Further analyzing mass fragmentation confirmed that H₂O separately contributed its hydrogen and oxygen atoms to the –C=O and R–O groups of the esters. Based on the isotope tracing results, the contribution roles of H₂O’s hydrogen and oxygen atoms in oxidation of aromatic alcohols to corresponding aromatic products were proposed. The findings in this study help to clarify the role of H₂O in the oxidation process of aromatic alcohols at the atomic level and comprehensively reveal the oxidation mechanism of the aromatics.