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。最近人们认识到，热水在没有添加任何外部催化剂的情况下，会在环境条件下发生化学反应，并在强酸或强碱存在的情况下进行。对水热条件下水中的非催化反应活性的一个广泛的解释是，由于水的电离常数增加，H^或OH^-的浓度增加而被催化。我们的观点是从动力学上证明水的非解离形式是高温水的非催化反应性的原因。通过一个涉及醚键的简单反应，我们确定了由未解离形式的水引起的反应路径的速率常数。通过改变氧离子浓度(H^)，分别测定了1，4-丁二醇脱水反应的水诱导速率常数和酸催化速率常数。在中等到超临界的较宽温度范围内，在动力学水平上发现，在有效酸浓度为10^&lt；-4&gt；-10^&lt；6&gt；M时，水的未解离形式促进了反应。在40 0℃和0.5g/cm3(37 Mpa)的超临界水中，乙醛在无催化剂的条件下发生了Cannizzar型歧化反应。没有检测到羟醛缩合产物，歧化反应与甲烷的分解竞争。乙醇的生成超过了醋酸，而二氧化碳只有在超临界水存在的情况下才会出现。考虑到一氧化碳是由乙醛分解提供的，并导致甲酸作为乙醛的还原中间体，对该反应行为进行了解释。

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.