锂/铌镁催化剂中碳酸锂的熔点与甲烷氧化偶联反应的活化和失活的关系

研究了铌掺杂的Li/MgO甲烷氧化偶联催化剂的反应性质及铌的助剂作用。铌的引入使得该催化剂上甲烷氧化偶联反应的活化温度降低50℃以上, 使此温度降到了催化剂中碳酸锂的熔点附近。试验观察到部分催化剂上甲烷氧化偶联反应的活性曲线在碳酸锂的熔点附近有一转折, 这一转折现象的出现与否及程度取决于制备条件。在碳酸锂的熔点附近, 含有铌的催化剂得到活化, 观察到无稀释气体时的反应引燃现象, 即温度增加几度活性便达到最大值。当在比碳酸锂熔点稍高的温度下且不稀释时反应, 含铌催化剂活性很高但很快失活, 在稍低于此熔点下则不失活, 但活性较低。这些试验结果表明, 含铌催化剂的活化与失活均与催化剂中的碳酸锂的相变化有关。试验还观察到了在稍高于碳酸锂的熔点下做寿命试验时, 甲烷氧化偶联反应的振荡现象。

The role of Li2CO3 melting in the activation and deactivation of Li/Nb/MgO catalyst for the reaction of methane oxidative coupling

The promotion effect of niobium on the Li/MgO catalyst for methane oxidative coupling has been investigated in some detail. Doping with Nb into the catalyst lowers the activation temperature for the coupling reaction for some 50℃. The lowered temperature falls into the range of Li2CO3 melting. Catalytic performance, DSC and TGA measurement reveal that the coupling reaction on Yb doped catalyst activates as the temperature reaches the range of Li2CO3 partly melting, and the catalyst starts to losing weight when the temperature runs over the metling point. Doping with Nb lowers and scatters the DSC peak of Li2CO3 melting in the catalyst. The activation of the catalyst is very sensitive to the melting process, that there appears a shift in the activity-temperature curve around the melting point for some catalyst, and that there is a light off around the melting temperature for the coupling reaction when measured at CH4/O2=5 and with no dilution. In the ageing experiment with high methane oxygen concentration, when the temperature is a little bit lower than the melting point, there is no deactivation observed for 65 hours, however, when the temperature is adjusted to a little bit higher than the melting point, the activity reaches a very high level at the beginning but starts to deactivate quickly. When the catalyst is aged at constant temperature a little bit above the melting point, the oscillation of the coupling reaction was observed. These results indicate that both the activation and the deactivation of the Nb doped catalyst is related to the melting of the Li2CO3 in the catalyst.