Ma Dongge, Liu Anan, Lu Chichong, Chen Chuncheng
School of Science, Beijing Technology and Business University, 100048 Beijing, P. R. China.
Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, 100190 Beijing, P. R. China.
ACS Omega. 2017 Aug 2;2(8):4161-4172. doi: 10.1021/acsomega.7b00754. eCollection 2017 Aug 31.
Solid/liquid heterogeneous photocatalysis was often considered to occur on the active sites of a solid catalyst surface. Herein, we report that the selectivity of photocatalytic dehydrogenative oxidations of aliphatic primary alcohols in acetonitrile solution into corresponding aldehydes exhibits an anomalous relationship with adsorption behavior of the alcohols. By using Pt-loaded TiO photocatalyst in an inert atmosphere under UV light illumination, primary short-chain alcohols (SCAs) with strong adsorption were dehydrogenated into aldehydes in very poor selectivity, whereas weak-adsorbable long-chain alcohols (LCAs) were transformed into corresponding aldehydes with much higher selectivity. More than 20 examples of primary LCAs (C-C) were successfully transformed into their corresponding aldehydes with satisfactory selectivity and yield. Both solid-state magic-angle-spinning C NMR and attenuated total reflectance-Fourier transform infrared spectroscopy studies provided concrete differences in adsorption behaviors on the Pt-TiO photocatalyst surface between SCA ethanol and LCA -octanol. To further uncover the mechanism for different selectivities of SCAs and LCAs in photodehydrogenation, in situ electron paramagnetic resonance (EPR) experiments (at 8 K temperature) were employed to observe the oxidation features of photogenerated hole in the valance band of Pt-TiO (h ). The EPR experimental studies exhibited that unlike ethanol, either -octanol or solvent acetonitrile alone all could not scavenge photogenerated h on Pt-P25 photocatalyst and only -octanol dissolved in acetonitrile solvent could smoothly react with photoinduced hole. This indicated that selective oxidations of LCAs were achieved by solvent-delivered oxidation rather than directly destructive oxidation of photogenerated h . Our results may open an alternative way in selective dehydrogenative oxidation of various substrates sensitive to both dioxygen and high-temperature treatments.
固/液多相光催化通常被认为发生在固体催化剂表面的活性位点上。在此，我们报道了在乙腈溶液中，脂肪族伯醇光催化脱氢氧化为相应醛的选择性与醇的吸附行为呈现出反常关系。通过在惰性气氛下紫外光照射下使用负载铂的TiO光催化剂，吸附性强的短链伯醇（SCA）脱氢生成醛的选择性非常低，而吸附性弱的长链醇（LCA）转化为相应醛的选择性则高得多。20多个长链伯醇（C-C）的例子成功地以令人满意的选择性和产率转化为它们相应的醛。固态魔角旋转碳核磁共振和衰减全反射傅里叶变换红外光谱研究提供了SCA乙醇和LCA -辛醇在铂-二氧化钛光催化剂表面吸附行为的具体差异。为了进一步揭示SCA和LCA在光脱氢中选择性不同的机制，采用原位电子顺磁共振（EPR）实验（在8K温度下）观察铂-二氧化钛价带中光生空穴（h ）的氧化特征。EPR实验研究表明，与乙醇不同，单独的 -辛醇或溶剂乙腈都不能清除铂-P25光催化剂上的光生h ，只有溶解在乙腈溶剂中的 -辛醇能顺利地与光生空穴反应。这表明LCA的选择性氧化是通过溶剂传递氧化而不是光生h 的直接破坏性氧化实现的。我们的结果可能为对氧气和高温处理都敏感的各种底物的选择性脱氢氧化开辟一条替代途径。
