Mechanisms of Noncatalytic Organic Chemical Reactions in Super- and Subcritical Water

系统地研究了水热条件下的非催化反应。最近公认的是，在不从外部添加任何催化剂的情况下，热水诱导在强酸或强碱存在下在环境条件下进行的化学反应。对于水热条件下在水中的非催化反应性，一种普遍的解释是，由于水的电离常数增加，H^+或OH^-浓度增加，反应被催化。我们的重点是从动力学上证明，水的未离解形式是高温水的非催化反应性的原因。通过专注于一个简单的反应，涉及醚键，我们确定的反应路径诱导的未解离形式的水的速率常数。通过改变氧鎓离子（H^+）浓度，分别测定了1，4-丁二醇脱水的水诱导和酸催化速率常数。在动力学水平上发现，在从中等温度到超临界温度的宽范围内，在有效酸浓度为10^-10 ^&lt;+6&gt; M时，未离解形式的水促进反应<-4>。进一步研究了醛的反应。在400℃和0.5 g/cm ^3（37 MPa）的超临界水中，乙醛在无催化剂条件下发生康尼扎型缩合反应，生成乙醇和乙酸。未检测到醛醇型缩合产物，并且缩合与分解成甲烷竞争。乙醇产生过量的乙酸，和二氧化碳出现作为一个产品，只有在超临界水的存在下。该反应行为通过考虑乙醛分解提供一氧化碳并导致形成甲酸作为乙醛的还原中间体来解释。

Noncatalytic reactions at hydrothermal conditions are investigated systematically. It is well recognized recently that without adding any catalysts from outside, hot water induces a chemical reaction that proceeds at ambient conditions in the presence of a strong acid or base. A widespread explanation to the noncatalytic reactivity in water at hydrothermal conditions is that the reaction is catalyzed by the increased concentration of H^+ or OH^- due to the enhanced ionization constant of water. Our point is to kinetically prove that the undissociated form of water is responsible for the noncatalytic reactivity of high-temperature water. By focusing on a simple reaction involving ether bonding, we determine the rate constant for the reaction path induced by the undissociated form of water. The water-induced and acid-catalyzed rate constants are separately determined for the dehydration of 1,4-butanediol by varying the oxonium ion (H^+) concentration. It is found on the kinetic level over a wide range of temperature from moderate to supercritical that the undissociated form of water promotes the reaction at an effective acid concentration of 10^<-4>-10^<+6> M. The reaction of aldehyde was further investigated. In supercritical water at 400℃ and 0.5 g/cm^3 (37 Mpa), acetaldehyde was found to be transformed without catalysts into ethanol and acetic acid through a Cannizzarotype disproportionation reaction. No aldol-type condensation products were detected, and the disproportionation competed with the decomposition into methane. Ethanol was generated in excess to acetic acid, and carbon dioxide appeared as a product only in the presence of supercritical water. This reaction behavior is explained by considering that carbon monoxide is provided by the acetaldehyde decomposition and leads to the formation of formic acid as a reducing intermediate for acetaldehyde.