Na^+/CaO催化剂上的甲烷氧化偶联

OXIDATIVE COUPLING OF METHANE OVER Na+/CaO CATALYSTS

研究了不同碱金属离子对CaO的促进作用,发现以Na^+的添加效果最好。在此基础上,研究了不同含钠化合物对CaO的促进作用,并用脉冲反应技术研究Na^+/CaO催化剂表面氧物种的特性及其作用。CaO表面上存在非选择性氧化的氧物种。Na^+对CaO的修饰作用是抑制非选择性氧化。当表面上的非选择性氧化的氧物种消耗后,体相的晶格氧会向表面迁移,以补充消耗掉的表面氧物种。消耗掉的表面氧物种也可由气相氧补充。CH_4脉冲和混合气脉冲说明仅靠[Na^+O^-]中心不足以使甲烷转化成C_2产物,必须有气相氧的参与才能使甲烷转化成C_2产物。

The activity and selectivity of various alkali ions of Li,Na and K doped CaO catalysts for the oxidative coupling of CH4 were evaluated.The incorporation of alkali ions into CaO led to an obvious increase in selectivity for C2H6 and C2H4,and the effect was in the sequence of Li+>Na+>K+,but a decrease in activity for Li+ and K+ ions.These results were explained in terms of Lunsf ord's proposal of the [M+O-] active sites,which is responsible for the oxidative coupling of CH4 over alkali ions/alkaline earth metal oxides.The results also suggested that for the sake of the formation of active sites,it is necessary that the radius sizes between the alkali cation and the substrate cation should be matched as good as possible.Various compounds containing Na+ were also doped on CaO and tested.5mol% Na(Na2CO3,NaOH and Na4P2O7)/CaO catalysts gave a C2 yield of 17-20%.The pulse reaction of CH4,O2 and the mixture of CH4 and O2 over Na+/CaO catalysts revealed that (1) under the reaction temperature,some kinds of oxygen species on CaO surface are only active for unselective oxidation of CH4;these oxygen species once reacted with CH4 and consumed,can be regenerated by the adsorption oa..oxygen molecules from gaseous phase or can be made up by the migration of the lattice oxygen in bulk onto the surf ace.( 2 )Na+ can heavily suppress the unselective oxygen species on the surface,and when incorporation of 5mol% Na+ onto CaO surface shows no reactivity to methane at all.(3 )when oxygen presents in gaseous phase,or the pulse contains CH4 and O2,5mol% Na+/CaO shows a high selectivity for C2He and C2H4.These results give a definite evidence that both [M+O-] active centre and gaseous oxygen play key roles in the oxidative coupling of CH4 over Na+/CaO catalysts.