酯加氢反应中影响羧基活化的因素

通过浸渍法制备4%Ru-9%La/γ-Al2O3催化剂,采用X射线衍射(XRD),X射线光电子能谱(XPS)和透射电子显微镜(TEM)对其结构进行表征.将该催化剂用于丙酸甲酯的加氢反应,分别考察了溶剂、无机盐添加剂、底物空间因素及电子因素对酯加氢反应的影响.发现以水为溶剂以及添加Co(NO3)2对丙酸甲酯加氢都显示出明显的促进作用,底物的转化率及丙醇的选择性随之增加.这是由于水及适量Co2+的引入能极化底物分子的C=O键,有利于活化氢对羧基碳原子的进攻,改善催化剂的性能.此外,底物分子中吸电子基团能提高羧基碳原子的正电性,也有利于加氢反应的进行;而增大底物分子空间位阻,不利于底物在催化剂上的吸附,催化反应速率下降.

Activation Factors for the Carboxyl Group in the Hydrogenation of Carboxylic Esters

We prepared a 4%Ru-9%La/γ-Al₂O₃ catalyst by impregnation method and characterized it using X-ray diffraction(XRD) ,X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The catalyst was used for the hydrogenation of methyl propionate. The effects of solvent, inorganic salt additive, steric as well as electronic factors of the substrate on the hydrogenation of a carboxylic ester were investigated. We found that both water and the Co(NO₃)₂ additive obviously improved the hydrogenation of methyl propionate, the conversion of the substrate and the selectivity for propanol. The promotional effects of water and Co²⁺ are attributed to polarization of the C=O bond in the carboxyl group of the substrate molecule by the formation of a hydrogen bond between water and the carboxylic group and the coordination of Co²⁺ to the carboxylic group. This is favorable for an attack on the carbon atom of the carboxyl group by the activated hydrogen. Similarly, the electron -withdrawing group in the substrate molecule also caused the high positive charge of carbon in the carboxyl group. The highly positive charged carbon is beneficial for the hydrogenation reaction. In addition, an increase in the steric hindrance of the substrate molecules was not favorable for the adsorption of the substrate on the catalyst and, therefore, the reaction rate decreased.