镧改性提高ZSM-5分子筛水热稳定性

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.

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

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.