物理化学学报 >> 2011, Vol. 27 >> Issue (01): 52-58.doi: 10.3866/PKU.WHXB20110130

理论与计算化学 上一篇    下一篇

镧改性提高ZSM-5分子筛水热稳定性

李延锋1, 朱吉钦1, 刘辉1, 王鹏2, 田辉平2   

  1. 1. 北京化工大学化工资源有效利用国家重点实验室, 北京100029;
    2. 中国石油化工股份有限公司石油化工科学研究院, 北京100083
  • 收稿日期:2010-09-09 修回日期:2010-10-21 发布日期:2010-12-31
  • 通讯作者: 刘辉 E-mail:hliu@mail.buct.edu.cn
  • 基金资助:

    国家重点基础研究发展规划项目(973) (2010CB732301)资助

Increase in the Hydrothermal Stability of the La-Modified ZSM-5 Zeolite

LI Yan-Feng1, ZHU Ji-Qin1, LIU Hui1, WANG Peng2, TIAN Hui-Ping2   

  1. 1. State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China;
    2. Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, P. R. China
  • Received:2010-09-09 Revised:2010-10-21 Published:2010-12-31
  • Contact: LIU Hui E-mail:hliu@mail.buct.edu.cn
  • Supported by:

    The project was supported by the National Key Basic Research Program of China (973) (2010CB732301).

摘要:

基于12T 团簇模型, 利用密度泛函理论(DFT)研究了ZSM-5 分子筛的水解脱铝机理以及镧改性提高ZSM-5分子筛水热稳定性的机理. 对未改性分子筛水解脱铝机理的研究表明, 首先是第一个水分子吸附在分子筛表面的酸性位上, 对分子筛的Al—O键起弱化作用, 使Al—O键伸长; 接着第二个水分子吸附到分子筛表面,分别与第一个水分子和分子筛骨架形成氢键, 进一步弱化与其最邻近的Al—O键, 并引致该键断裂. 同样, 其它的三个Al—O键也被削弱并逐一断裂, 从而发生分子筛水解脱铝现象. 引入的镧物种与分子筛骨架的四个O原子成键, 将铝包埋, 增加了分子筛孔壁厚度, 增大了水分子攻击铝的空间位阻, 抑制了水分子对Al—O键的弱化, 从而延缓Al—O键的断裂, 提高分子筛的水热稳定性. 计算的水分子吸附能和水解能进一步证实镧的引入提高了ZSM-5分子筛的水热稳定性.

关键词: 分子筛, 密度泛函理论, 脱铝, 镧改性, 水热稳定性

Abstract:

We carried out a theoretical investigation into the mechanism of steam dealumination of the ZSM-5 zeolite and into the mechanism for the increase in the hydrothermal stability of the La-modified ZSM-5 zeolite. This was done using density functional theory (DFT) with 12T cluster models that simulated the local structures of the zeolite materials. We demonstrate that because of the hydrogen bond interaction between the first adsorbed water molecule and the ZSM-5 zeolite framework, the Al—O bond is weakened and elongated. As the second water molecule is adsorbed, the Al—O bond near the second water molecule is further weakened and eventually broken because of the hydrogen bond interaction between the second adsorbed water molecule and the ZSM-5 zeolite framework. As more water molecules are adsorbed, the other Al—O bonds are broken sequentially resulting in a dealumination of the ZSM-5 zeolite. The introduced lanthanum coordinates with four zeolite framework oxygen atoms, thickens the zeolite framework, moves over the framework Al atom, increases the steric hindrances and partially prevents polar water molecules from attacking the Al—O bond. These actions retard the weakening of the Al—O bonds and improve the hydrothermal stability of the ZSM-5 zeolite. The calculated adsorption and hydrolysis energies of the water molecule further confirm that the presence of lanthanum enhances the hydrothermal stability of the ZSM-5 zeolite.

Key words: Zeolite, Density functional theory, Dealumination, La-modification, Hydrothermal stability

MSC2000: 

  • O641