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[摘要]：The aerobic decarboxylation of saturated carboxylic acids (from C-2 to C-5) in water by TiO2 photocatalysis was systematically investigated in this work. It was found that the split of C-1-C-2 bond of the acids to release CO2 proceeds sequentially (that is, a C-5 acid sequentially forms C-4 products, then C-3 and so forth). As a model reaction, the decarboxylation of propionic acid to produce acetic acid was tracked by using isotopic-labeled (H2O)-O-18. As much as approximate to 42% of oxygen atoms of the produced acetic acids were from dioxygen (O-16(2)). Through diffuse reflectance FTIR measurements (DRIFT'S), 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 C=O stretch of alpha-keto group of pyruvic acid) and the shift of this peak to 1726 cm(-1) when (H2O)-O-16 was replaced by (H2O)-O-18. Consequently, pyruvic acid was chosen as another model molecule to observe how its decarboxylation occurs in (H2O)-O-16 under an atmosphere of O-18(2). With the alpha-keto oxygen of pyruvic acid preserved in the carboxyl group of acetic acid, approximate to 24% new oxygen atoms of the produced acetic acid were from molecular oxygen at near 100% conversion of pyruvic acid. The other approximate to 76% oxygen atoms were provided by H2O through hole/OH radical oxidation. In the presence of conduction band electrons, O-2 can independently accomplish such C-1-C-2 bond cleavage of pyruvic acid to generate acetic acid with approximate to 100% selectivity, as confirmed by an electrochemical experiment carried out in the dark. More importantly, the ratio of O-2 participation in decarboxylation increased along with the increase of pyruvic acid conversion, indicating the differences between non-substituted acids and alpha-keto acids. This also suggests that the O-2-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 O-2. |
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