On the role of CO formation during the aerobic oxidation of alcohols on Pd/Al2O3: an in situ attenuated total reflection infrared study
On the role of CO formation during the aerobic oxidation of alcohols on Pd/Al2O3: an in situ attenuated total reflection infrared study
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DOI：
10.1016/j.jcat.2005.05.019
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发表时间：
2005-08
影响因子：
7.3
通讯作者：
C. Keresszegi;D. Ferri;T. Mallát;A. Baiker
中科院分区：
文献类型：
--
作者：
C. Keresszegi;D. Ferri;T. Mallát;A. Baiker
研究了六种不同的伯醇和仲醇、脂肪族和芳香族醇在环己烷中、在空气存在和不存在的情况下脱氢和氧化脱氢生成相应的羰基化合物的反应。通过催化固液界面的衰减全反射红外 (ATR-IR) 光谱和连续流反应器中流出物的 GC 分析来检查反应网络。 5 wt% Pd/Al2O3 催化剂位于用作微型反应器的 ATR-IR 池中。研究表明，所有由伯醇（1-辛醇、苯甲醇和肉桂醇）形成的醛在 Pd 上脱羰，而酮（2-辛酮、环己酮和苯乙酮）则稳定。即使在分子氧存在的情况下，IR 指示的脱羰反应以及 GC 检测到的氢化和氢解型副反应证实了表面 Pd0 位点的存在和醇氧化的经典脱氢机制。此外，在苯甲醇氧化过程中，CO和苯甲醛脱羰形成的苯基自由基的轻松去除明确证明了Pd表面上同时存在吸附的氧和氢。相对较高的苯甲醇氧化速率归因于从金属表面更快地去除强烈吸附的副产物，部分原因是该醇在所有研究的反应物中具有最高的极性。苯甲醇增加了水在非极性溶剂中的溶解度，从而最大限度地减少了催化剂表面积聚的水副产物的负面影响。
Dehydrogenation and oxidative dehydrogenation of six different primary and secondary, aliphatic and aromatic alcohols to the corresponding carbonyl compounds were studied in cyclohexane, in the presence and absence of air. The reaction network was examined by attenuated total reflection infrared (ATR-IR) spectroscopy of the catalytic solid–liquid interface and by GC analysis of the effluent in a continuous-flow reactor. The 5 wt% Pd/Al2O3catalyst was located in an ATR-IR cell that served as a tiny reactor. The studies revealed that all aldehydes formed from primary alcohols (1-octanol, benzyl and cinnamyl alcohol) decarbonylated on Pd, whereas the ketones (2-octanone, cyclohexanone, and acetophenone) were stable. The decarbonylation reactions indicated by IR and the hydrogenation and hydrogenolysis-type side reactions detected by GC even in the presence of molecular oxygen corroborated the presence of surface Pd0sites and the classic dehydrogenation mechanism of alcohol oxidation. Moreover, the facile removal of CO and the phenyl radical formed from decarbonylation of benzaldehyde during benzyl alcohol oxidation unambiguously proved the simultaneous presence of adsorbed oxygen and hydrogen on the Pd surface. The relatively high rate of benzyl alcohol oxidation is attributed to the faster removal of strongly adsorbed by-products from the metal surface and partly to the highest polarity of this alcohol among all of the reactants investigated. Benzyl alcohol increases the solubility of water in the apolar solvent and thus minimizes the negative effect of water coproduct that accumulates on the catalyst surface.