原位ATR红外光谱研究超临界条件下酯交换反应过程与机理

在温度15℃~300℃和压力0.1MPa~25MPa下，采用原位ATR(Attenuated Total Reflectance)红外光谱技术研究高温高压条件下甲醇、乙醇和丙醇的分子间氢键和分子内化学键随温度和压力的变化及亚/超临界甲醇条件下醇油的混合与酯交换反应过程与机理。纯物质的红外光谱研究表明，在压力高于14MPa时，随温度由15℃升高到250℃，甲醇、乙醇和丙醇的分子间氢键减弱，减弱程度最大的温度为75℃~225℃；但温度升高对甲基的振动没有影响，当温度超过225℃后，甲醇的羟基振动峰发生明显分峰，而乙醇和丙醇的羟基振动峰未发现分峰变化；在整个温度和压力范围内，三油酸甘油酯的红外光谱图未发生明显变化。醇油混合与酯交换反应过程的红外研究表明，在14MPa时，当温度超过185℃后甲醇与三油酸甘油酯完全互溶，两者形成均相；当混合体系温度超过220℃时，甲醇与三油酸甘油酯开始发生酯交换反应。因此，超临界甲醇条件下的酯交换是均相反应，而且氢键变化不是导致酯交换反应的主要原因，高温高压条件下C－OH键振动形式的变化，即出现C⁺…O^-…H⁺振动使小分子醇的亲电性和亲核性均增强是导致超临界无催化酯交换反应快速进行的主要原因。

Reaction processes and mechanism of supercritical transesterification in situ ATR infrared spectrum

The changes in hydrogen bond and valence bonds of methanol, ethanol, and propanol with temperature and pressure, the mixed state of methanol/oil, and the reaction process and mechanism of transesterification in sub/supercritical methanol in the temperature range of 15–300°C and pressure range of 0.1–25 MPa were studied by using in situ attenuated total reflectance (ATR) infrared spectrum technology. The results showed that hydrogen bonds become weak for methanol, ethanol, and propanol in the temperature range of 15–250°C at a pressure of over 14 MPa, especially at the temperature of 75–225°C. But temperature almost had no influence on the vibrating peak of their −CH₃ bond. When temperature was over 225°C, the vibrating peaks of hydroxyl group of methanol got split, which could not be found for ethanol and propanol. Throughout the whole range of temperature and pressure, the obvious change in infrared spectrum of triolein was not found. At the same time, in situ ATR-FTIR also displayed that methanol and triglycerides were dissolved mutually to form a single phase when the temperature was higher than 185°C at high pressure of 14 MPa. For transesterification between methanol and triglycerides without catalyst at 14 MPa, the initiative reaction temperature is about 220°C, which is near subcritical temperature of methanol. Therefore, the transesterification in supercritical methanol occurs in homogeneous state, and the new vibration form of C^+… O^−… H⁺, which strengthens the electrophilicity and the nucleophilicity of small molecule alcohols, should be the main reason for accelerating supercritical transesterification.