甲烷氧化偶联反应催化剂碱性与反应性能的相关性

CORRELATION BETWEEN BASICITY AND CATALYTIC PERFORMANCE OF CATALYSTS FOR THE OXIDATIVE COUPLING OF METHANE

在催化剂甲烷氧化偶联反应性能研究的基础上，利用ＣＯ２－ＴＰＤ技术考察了不同的碱金属化合物－Ｌａ２Ｏ３／ＢａＣＯ３催化剂的表面碱性．结果表明，ＢａＣＯ３的协同作用、碱金属化合物的添加都增大了催化剂表面碱性的强度，也增加了碱性位的数量．催化剂表面碱性与催化剂反应性能之间存在着良好的相关性，强碱性位的出现和增强相应于催化剂活性与选择性的提高．对催化剂表面碱性位在甲烷氧化偶联反应中的作用、碱金属化合物的添加效应也进行了探讨．

Most of the effective catalysts for the oxidative coupling of methane (OCM) exhibit strong basic character. In previous studies, it has been found that the addition of proper alkali compounds to La 2O 3/BaCO 3 catalysts can improve the performance of the catalysts. This paper studies the surface basicity of those catalysts by CO 2 TPD with the emphasis on the correlation between the basicity and reaction performance of the catalysts. The experimental results reveal that, with the coordinate effect between La 2O 3 and BaCO 3, stronger basic sites form on La 2O 3/BaCO 3 catalyst surface. The addition of alkali compounds to La 2O 3/BaCO 3 catalysts greatly reduces the weak or medium basic sites and intensifies the strong basic sites on the catalyst surface. The alkali metals affect the surface character of the catalysts through geometric effect as well as more important electronic effect. It is found that there exists a good correlation between the surface basicity and catalytic performance. A catalyst showing large desorption peaks of CO 2 at higher temperatures in TPD measurement tends to give higher activity and selectivity. The addition of alkali compounds to La 2O 3/BaCO 3 catalysts enhances the surface basicity of the catalysts, especially increases the amount of strong basic sites, while the activity and selectivity of the catalysts are improved. The Na 2O La 2O 3/BaCO 3 catalyst has more and stronger basic sites, and it is also most active and selective in the OCM reaction. The strong basic sites on the catalyst surface play an important role in the selective activation of methane. They provide sites for gas oxygen to be adsorbed, i.e., oxygen is adsorbed on the basic sites and converted to surface active oxygen species to activate methane selectively. The more and stronger basic sites lead to more selective oxygen species, which accelerate the formation of methyl radicals and inhibit the deep oxidation of C 1 and C 2+ products.