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针对MER型沸石骨架和骨架外阳离子进行分子模拟研究。采用密度泛函理论(DFT)计算MER型沸石骨架中Al替代Si的替代能,确定了沸石骨架中Al替代Si的位置顺序,其优先位置为T_2;确定了骨架最小Si/Al为1.67。在此基础上,应用巨正则蒙特卡洛(GCMC)方法模拟MER型沸石骨架外阳离子的位置,并用DFT计算其结合能。结果表明,骨架外K~+、Na~+及NH_4~+有3种吸附位置,K~+优先占据弯曲的八元环;吸附位置中,K~+与NH_4~+优先吸附在Ⅱ位,其次是Ⅰ位,最后是Ⅲ位;在低Si/Al时,Na~+优先吸附在Ⅱ位和Ⅰ位,随Si/Al增加,Na~+优先吸附在Ⅲ位;从结合能的角度,沸石对K~+的吸附能力最强,其次是Na~+,最后是NH_4~+;Ⅲ位阳离子最容易交换,其次是Ⅰ位,最难交换的是Ⅱ位;阳离子与骨架的结合能越大,MER型沸石骨架外阳离子与骨架原子O和Al的平均距离越小。
Molecular modeling of MER zeolites and extracortical cations. The substitution energies of Al substitution for Si in MER zeolite framework were calculated by density functional theory (DFT), and the order of Al substitution for Si in zeolite framework was determined. The priority order was T_2. The minimum Si / Al of framework was 1.67. On this basis, the position of cations outside the framework of MER zeolite was simulated by GCM method, and its binding energy was calculated by DFT. The results showed that there were three adsorption sites for K ~ +, Na ~ + and NH_4 ~ + outside the framework, and K + preferentially occupied the bent eight - membered ring. In adsorption sites, K ~ + and NH_4 ~ At the low Si / Al, Na ~ + preferentially adsorbed on the sites Ⅱ and Ⅰ, and Na ~ + preferentially adsorbed on the sites Ⅲ with the increase of Si / Al. From the perspective of the binding energy, Zeolite has the strongest adsorption capacity for K ~ +, followed by Na ~ +, and finally NH_4 ~ +. The cation in the Ⅲ position is the most easily exchangeable, followed by the Ⅰ position and the most difficult exchangeable site is the Ⅱ position. Large, MER zeolite skeleton outside the cation and skeleton atoms O and Al the smaller the average distance.