乙苯脱氢氧化铁系催化剂的活性相及钾的助催作用

ACTIVE PHASE AND ROLE OF POTASSIUM IN POTASSIUM-PROMOTED IRON OXIDE CATALYST FOR DEHYDROGENATION OF ETHYLBENZENE

与机械混合法相比,用KOH水溶液浸渍法制成的K_2O-Fe_2O_3催化剂性能较佳。负载型多元催化剂的性能与C-64I相近。SEM、XPS和EDAX证实,经使用后,上述负载型多元催化剂表面形态明显改变,表面铁/钾原子比上升。连续升温XRD证实,在乙苯脱氢通常所采用的温度区间,载于表面的KOH会迅速和氧化铁相互作用生成K_2Fe_2O_4。本文认为K_3Fe_2O_4可能是活性相并据此对实验结果作出较为合理的解释。

Fe_2O_3-K_2O binary catalysts prepared by high temperature calcination of the relevant mixtures and by KOH solution impregnation showed different catalytic behaviour in the reaction of dehydrogenation of ethylbenzene. The latter had longer stability period of the highest activity and shorter induction period. An impregnated multi-component catalyst, which was particularly prepared in our laboratory and had similar chemical composition to a well-known commercial catalyst(G-64I),showed that both the conversion of ethylbenzene and the selectivity to styrene were similar to G-64I although the impregnation amount of KOH corresponded to the fact that this impregnated catalyst had about 54 monolayers of K_2O on its surface. These facts indicated that the potassium promotor operated specifically on the surface of the iron oxide and that Fe_3O_4 was not the active phase of the catalyst as some authors had suggested. It was proved by using SEM, EDAX and XPS that,the Fe/K atom ratio on the surface of this impregnated catalyst increased obviously while the surface morphlogical structure changed simultaneously after this catalyst was operated for about 24 hrs.These suggested that a strong interaction between potassium promotor and iron oxide had occured on the surface and that the product resulting from the interaction should be the active phase of the catalyst. The XRD study at different temperatures showed that the potassium promotor would reacted rapidly with iron oxide, thus forming K_2Fe_2O_4 in the temperature range 580—660℃,which was the usual operation temperature in the commercial production of ethylbenzene dehydrogenation to styrene. So, this paper suggested that the K_2Fe_2O_4 might be the active phase of the catalyst. Our experimental results could be satisfactorily explained according to this suggestion.