硼改性HZSM-5沸石对甲苯乙基化的择形催化作用

SHAPE SELECTIVITY FOR ETHYLATION OF TOLUENE OVER HZSM-5 ZEOLITE MODIFIED WITH BORON

采用XRD、XPS、BET吸附、脉冲中毒、TPD、IR等方法,探讨了硼改性后HZSM-5沸石分子筛孔结构和表面酸性的变化对甲苯和乙醇烷基化反应活性和选择性的影响。BHZSM-5沸石催化剂能在活性稍有降低的基础上使甲乙苯产率和对甲乙苯选择性大大提高。当烷基化剂乙醇含水量少于30%时,也得到相近的结果。处理剂H_3BO_3是以B_2O_3的形式分布于沸石晶内外的,使其晶面间距,比孔容、比表面减小,而Γ_(p-ET)/Γ_(m-Et)增大。表明沸石孔道变窄,对甲乙苯相对更易扩散。同时,B_2O_3也调变了HZSM-5沸石的表面酸性。外表面酸中心为B_2O_3所覆盖,以致脉冲中毒实验中,4-甲基喹啉基本不起作用。TPD谱中表征强酸中心的高温峰消失。IR谱中表征B酸中心、L酸中心的特征吸收带强度变弱,即B、L酸中心数同时减少。不是所有酸中心都是活性中心,只有处于一定大小空间、具有适当强度的酸中心才能满足对烷基化反应活性和选择性的要求。此外,ESR谱表明,L酸中心参与了甲苯烷基化过程。

The shape selectivity for alkylation of toluene with ethanol over BHZSM-5 zeolite catalyst was investigated. The effect of boron modification on the structure and acidity of HZSM-5 was studied using XRD,XPS, BET adsorption, pulse poisoning, TPD, IR and ESR techniques.It was found that the values of d-spacing, sp. volume and sp. surfaec area decreased while the value of relative sorption, increased after modification with boron. Therefore, one of the main factors leading to high para-selectivity is the narrowing of pores and channels. Another main factor is the disappearing of strong acid sites shown by NH_3TPD profile, expecially on the external surface as demonstrated by 4-methylquinoline pulse poisoning test. It seems that only those acid sites having certain steric space and suitable strengh catalyze the reaction of toluene ethylation.Lewis acid sites take parts also in the alkylation reaction.