不同硅/铝比ZSM-5分子筛对烷烃和环烷烃催化裂解性能的影响

Effect of Si/Al Ratio on the Catalytic Performance of ZSM-5 Zeolites in Alkane and Cycloalkane Cracking

以硅/铝摩尔比(n(SiO_2)/n(Al_2O_3)=24的ZSM-5分子筛为母体,通过酸处理脱铝制备了具有不同硅/铝摩尔比(50、85、110、140)的ZSM-5分子筛,通过X射线衍射(XRD)、扫描电镜(SEM)、氨气程序升温脱附(NH_3-TPD)、吡啶红外吸附(Py-FTIR)、N_2吸附-脱附等手段对其进行表征,考察其应用于正辛烷和乙基环己烷催化裂解反应的性能差异。结果表明,正辛烷和乙基环己烷的转化率与ZSM-5分子筛硅/铝比存在较好的对应关系,即硅/铝比越低、酸量越高,转化率越高;但ZSM-5分子筛硅/铝比低、酸量过多会导致非选择性副反应发生,降低目的产物低碳烯烃收率和选择性。不同硅/铝比ZSM-5分子筛在正辛烷和乙基环己烷催化裂解反应中显示出不同的催化性能,对于相同碳数的烷烃正辛烷和乙基环己烷,由于其分子结构不同,所适宜的硅/铝比不同;在相同硅/铝比分子筛条件下,环烷烃乙基环己烷的总体反应活性低于相对应的直链烷烃正辛烷;正辛烷在ZSM-5-85分子筛上具有更优异的催化裂解反应性能,乙基环己烷在ZSM-5-50分子筛上具有更优异的催化裂解反应性能。

Taking ZSM-5 zeolite(n(SiO 2)/n(Al 2O 3)=24)as matrix,the acid-treated ZSM-5 zeolites with different n(SiO 2)/n(Al 2O 3)were prepared by post synthesis treatment.Various ZSM-5 zeolites were characterized by means of XRD,SEM,NH 3-TPD,Py-FTIR and N 2 physical adsorption,and their catalytic performances in light hydrocarbons cracking were evaluated.It is found that the conversion of n-octane and ethylcyclohexane is in good relation to the ratio of SiO 2/Al 2O 3.Under lower n(SiO 2)/n(Al 2O 3),the larger amount of acid sites favors the higher conversion.But,when the n(SiO 2)/n(Al 2O 3)is too low,the nonselective side reactions are happened,thereby causing the decreasing of light olefins yield and selectivity.ZSM-5 zeolites with different n(SiO 2)/n(Al 2O 3)show different catalytic performance in hydrocarbon catalytic cracking.With respect to the same carbon number of octane and ethylcyclohexane,the appropriate n(SiO 2)/n(Al 2O 3)needed is different,due to the structure difference among various molecules.Under the same n(SiO 2)/n(Al 2O 3),the total reactivity of ethylcyclohexane is lower than that of octane.ZSM-5-85 and ZSM-5-50 shows the most excellent catalytic performance in the cracking of n-octane and ethylcyclohexane,respectively.