锂—锰复氧化物催化剂上甲烷氧化偶联反应的研究

METHANE OXIDATIVE COUPLING OVER LITHIUM-MANGANESE COMPLEX OXIDES

本文研究了具有不同Li/Mn比的锂-锰复氧化物催化剂对甲烷氧化偶联反应的催化性能,并用X射线粉末衍射(XRD)、光电子能谱(XPS)和程序升温还原(TPR)等方法对催化剂进行了表征。结果表明:当Li/Mn=2形成亚锰酸锂Li_2MnO_3化合物时,样品的催化活性明显上升。随锂含量的增加,产物中C_2选择性上升;甲烷转化率在Li/Mn比为2.6-2.8时出现极大值.在780℃,烷氧比为2时,可获得19％的C_2收率。XRD和TPR的结果说明,催化活性与样品中锰的价态有关,Mn^(4+)的催化活性明显大于Mn^(3+)。适当过量的锂可使锰稳定在正四价并覆盖催化剂表面的完全氧化活性位,从而抑制深度氧化作用,因而提高了催化剂的活性及选择性。催化剂表面O_(18)结合能为531.9eV的氧物种的存在有利于C_2选择性的提高。

Methane oxidative coupling over lithium-manganese complex oxides with various Li/Mn ratio has been studied. The catalysts have been characterized by XRD, TPR and XPS techniques. The formation of Li_2MnO_3 at Li/Mn ratio of 2 drastically increases the catalytic activity of the samples. C_2 selectivity in the product increases with increasing Li content. When Li/Mn ratio reaches 2.6-2.8, a maximum methane conversion is achieved. At 780℃, methane-to-oxygen ratio of 2, a 19% C_2 yield is obtained over the catalyst with Li/Mn ratio of 2.8. XRD and TPR results reveal that the catalytic performance is related to the oxidation state of manganese. The catalytic activity of Mn^(4+) is higher than that of Mn^(3+). Proper excessive content of Li stabilizes Mn4+, covers the surface complete oxidation sites of the sample and also compresses the deep oxidation reaction. thus increases the catalytic cativity and selectivity. The surface oxygen species with binding energy at 531.9eV seems responsible to the C_2 selectivity.