Theyssen Nils, Hou Zhenshan, Leitner Walter
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany.
Chemistry. 2006 Apr 12;12(12):3401-9. doi: 10.1002/chem.200501385.
The oxidation of cycloalkanes or alkylarenes with molecular oxygen and acetaldehyde as sacrificial co-reductant occurs efficiently in compressed (supercritical) carbon dioxide (scCO2) under mild multiphase conditions. No catalyst is required and high-pressure ATR-FTIR online measurements show that a radical reaction pathway is heterogeneously initiated by the stainless steel of the reactor walls. For secondary carbon atoms, high ketone to alcohol ratios are observed (3.5-7.9), most probably due to fast consecutive oxidation of alcoholic intermediates. Since C--C scission reactions are detected only to a very small extent, tertiary carbon atoms are transformed into the corresponding alcohols with high selectivity. Detailed analysis of the product distributions and other mechanistic evidence suggest that acetaldehyde acts not only as the sacrificial oxygen acceptor, but also as an efficient H-atom donor for peroxo and oxo radicals and as a crucial reductant for hydroperoxo intermediates. In comparison to other inert gases such as compressed N2 or Ar, the use of carbon dioxide was shown to increase the yields of alkane oxygenates under identical reaction conditions.
在温和的多相条件下，环烷烃或烷基芳烃与分子氧以及作为牺牲共还原剂的乙醛在压缩（超临界）二氧化碳（scCO2）中能高效发生氧化反应。该反应无需催化剂，高压衰减全反射傅里叶变换红外光谱（ATR-FTIR）在线测量表明，自由基反应途径是由反应器壁的不锈钢非均相引发的。对于仲碳原子，观察到较高的酮与醇比例（3.5 - 7.9），这很可能是由于醇中间体的快速连续氧化所致。由于仅在非常小的程度上检测到碳 - 碳键断裂反应，叔碳原子能以高选择性转化为相应的醇。对产物分布的详细分析及其他机理证据表明，乙醛不仅作为牺牲性氧受体，还作为过氧自由基和氧自由基的有效氢原子供体以及氢过氧中间体的关键还原剂。与其他惰性气体如压缩氮气或氩气相比，在相同反应条件下，使用二氧化碳能提高烷氧基化物的产率。
