表面强酸分布——影响HZSM-5沸石对位选择性的主要因素

考察了交换剂和改性处理对HZSM-5沸石在甲苯乙基化反应中对位选择性的影响,检测了反应初级产物的组成,用原位TPD法研究了沸石表面酸中心的催化性质。结果表明,孔径和酸性分布都是影响沸石对位选择性的重要因素,但是表面酸性的调变对提高HZSM-5沸石的对位选择性更为重要。对-甲乙苯是甲苯乙基化反应的初级产物,在表面异构化反应后生成间-甲乙苯。烷基化和异构化两种反应活性中心在酸强度上的差异很小,仅仅是所处位置不同。由于改性处理同时抑制了这两类活性中心,使HZSM-5沸石在甲苯乙基化反应中的对位选择性上升而活性下降,表现出反向变化关系。

SURFACE STRONG ACID DISTRIBUTION:THE MAIN FACTOR AFFECTING para-SELECTIVITY OF HZSM-5 ZEOLITES

The effects of ion exchange agents and modification on the para-selcctivity of HZSM-5 zeolite in toluene ethylation were studied. The components of primary product of the reaction wero; detected. The catalytic properties of the acid sites were studied with in situ TPD method. The results showed that both the pore size and acid distribution were the important factors affecting the para-selectivity of HZSM-5 zeolite, and the latter was particularly important. p-Ethyltoluono (p-ET) was found to bo. the primary product of toluene ethylation, and m-ET and o-ET were its s condary isomerization products. Both active sites for ethylation and isomerization had little difference in their acidic strength, but their location was different. Both active sites were suppressed at the same time during modification, so the para-selectivity of HZSM-5 zeolite was increased for toluene ethylation, while the toluene conversion was decreased, resulting in converse change of the catalytic activity and para-selectivity of ZSM-5 zeolite.