【摘要】 用NH₃和CO₂的TPD方法观察了化学改性对分子筛催化剂表面酸碱性的影响,并定量讨论了甲苯烷基化反应选择性与酸碱度的关系。结果表明苯环上的反应发生在表面酸度较高而碱度极低的催化剂上,甲苯侧链上的烷基化反应发生在具有足够碱度的催化剂上,而且这些催化剂都还带有较弱的酸度以帮助吸附和稳定苯环分子。表面酸碱度均很低的分子筛上既不能发生苯环烷基化反应,也不能发生侧链上的烷基化反应。还以邻二甲苯的吸附速率的定量数据考察了分子筛孔道结构对反应选择性的影响。更多还原

【Abstract】 The reaction selectivity, acid-base properties, adsorption rate of o-xylene, and average crystal size of various modified zeolites have been determined and correlated. The surface acidity and basicity of zeolite catalysts were determined quantitatively by TPD technique with NH3 and CO2 respectively. It is found that the side-chain alkylation of toluene requires the zeolite has certain surface basicity as well as a little bit of acidity which promotes the adsorption of toluene, but the acidity must not be too high, otherwise the selectivity will decrease. On the ot her hand, the alkylation of benzene-ring requires only an appropria-tenumber of acidic sites on the surface of zeolite.Moreover, the shape selectivity of zeolites is also an important factor affecting the alkylation of toluene. A free length of diffusion L (average crystal size divided by relative rate of adsorption of o-xylene) is proposed to characterize the tortuosity of the channel structure of the zeolites. The results show that the longer the L is, the larger the diffusion resistance for o-xylene and the higher the p-xylene selectivity for the alkylation of toluene ring will be.更多还原