An unexplored O2-involved pathway for the decarboxylation of saturated carboxylic acids by TiO2 photocatalysis: an isotopic probe study.
An unexplored O2-involved pathway for the decarboxylation of saturated carboxylic acids by TiO2 photocatalysis: an isotopic probe study.
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DOI：
10.1002/chem.201001704
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发表时间：
2010-10
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通讯作者：
Bo Wen;Yue Li;Chuncheng Chen;Wanhong Ma;Jincai Zhao
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作者：
Bo Wen;Yue Li;Chuncheng Chen;Wanhong Ma;Jincai Zhao
本文系统地研究了TiO（2）催化剂对水中饱和羧酸（从C（2）到C（5））的好氧脱羧反应。发现酸的C（1）-C（2）键的断裂以释放CO（2）顺序进行（即，C（5）酸顺序形成C（4）产物，然后C（3），依此类推）。作为模型反应，用同位素标记的H（2）（18）O跟踪丙酸脱羧生成乙酸的反应。生成的乙酸中约42%的氧原子来自双氧（16）O（2）。通过漫反射红外光谱测量（DRIFTS），我们证实在初始羧基截止之前生成了中间体丙酮酸;在1772 cm（-1）处出现吸收峰证明了该中间体当H（2）（18）O取代H（2）（16）O时，该峰位移至1726 cm（-1）。因此，选择了异辛酸作为另一个模型分子，观察其在H（2）（16）O和（18）O（2）气氛下的脱羧反应。在丙酮酸转化率接近100%的条件下，丙酮酸的α-酮氧保留在乙酸的羧基上，生成的乙酸中约有24%的新氧原子来自分子氧。其余约76%的氧原子由H（2）O通过空穴/OH自由基氧化提供。在导带电子的存在下，O（2）可以独立地完成这种C（1）-C（2）键的断裂，从而以约100%的选择性生成乙酸，如在黑暗中进行的电化学实验所证实的。更重要的是，O（2）参与脱羧反应的比例沿着增加，表明非取代酸与α-酮酸的差异。这也表明O（2）依赖的脱羧反应与空穴/OH自由基促进的脱羧反应竞争，并且取决于TiO（2）表面缺陷，在这些缺陷处Ti（4c）位点可用于同时配位底物和O（2）。
The aerobic decarboxylation of saturated carboxylic acids (from C(2) to C(5)) in water by TiO(2) photocatalysis was systematically investigated in this work. It was found that the split of C(1)-C(2) bond of the acids to release CO(2) 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 H(2)(18)O. As much as ≈42% of oxygen atoms of the produced acetic acids were from dioxygen ((16)O(2)). 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 C=O stretch of α-keto group of pyruvic acid) and the shift of this peak to 1726 cm(-1) when H(2)(16)O was replaced by H(2)(18)O. Consequently, pyruvic acid was chosen as another model molecule to observe how its decarboxylation occurs in H(2)(16)O under an atmosphere of (18)O(2). With the α-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 H(2)O 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 ≈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 α-keto acids. This also suggests that the O(2)-dependent decarboxylation competes with hole/OH-radical-promoted decarboxylation and depends on TiO(2) surface defects at which Ti(4c) sites are available for the simultaneous coordination of substrates and O(2).