An Unexplored O₂-Involved Pathway for the Decarboxylation of Saturated Carboxylic Acids by TiO₂ Photocatalysis: An Isotopic Probe Study

摘要

The aerobic decarboxylation of saturated carboxylic acids (from C₂ to C₅) in water by TiO₂ photocatalysis was systematically investigated in this work. It was found that the split of C1C2 bond of the acids to release CO₂ proceeds sequentially (that is, a C₅ acid sequentially forms C₄ products, then C₃ and so forth). As a model reaction, the decarboxylation of propionic acid to produce acetic acid was tracked by using isotopic-labeled H₂18O. As much as ≈42 % of oxygen atoms of the produced acetic acids were from dioxygen (16O₂). Through diffuse reflectance FTIR measurements (DRIFTS), we confirmed that an intermediate pyruvic acid was generated prior to the cut-off of the initial carboxyl group; this intermediate was evidenced by the appearance of an absorption peak at 1772 cm−1 (attributed to CO stretch of -keto group of pyruvic acid) and the shift of this peak to 1726 cm−1 when H₂16O was replaced by H₂18O. Consequently, pyruvic acid was chosen as another model molecule to observe how its decarboxylation occurs in H₂16O under an atmosphere of 18O₂. With the -keto oxygen of pyruvic acid preserved in the carboxyl group of acetic acid, ≈24 % new oxygen atoms of the produced acetic acid were from molecular oxygen at near 100 % conversion of pyruvic acid. The other ≈76 % oxygen atoms were provided by H₂O through hole/OH radical oxidation. In the presence of conduction band electrons, O₂ can independently accomplish such C1C2 bond cleavage of pyruvic acid to generate acetic acid with ≈100 % selectivity, as confirmed by an electrochemical experiment carried out in the dark. More importantly, the ratio of O₂ participation in decarboxylation increased along with the increase of pyruvic acid conversion, indicating the differences between non-substituted acids and -keto acids. This also suggests that the O₂-dependent decarboxylation competes with hole/OH-radical-promoted decarboxylation and depends on TiO₂ surface defects at which Ti_4c sites are available for the simultaneous coordination of substrates and O₂.