水在Na_nZSM-5型分子筛中吸附的研究:分子模拟

Study on the Adsorption of Water on Na_nZSM-5 Type Zeolite: Molecular Simulation

利用分子动力学(MD)模拟退火的方法和巨正则系综MonteCarlo模拟方法(GCMC)研究了水在NanZSM-5型分子筛中的吸附行为,计算结果与文献中报道的实验结果吻合较好.在此基础上,进一步预测了水在不同硅铝比的NanZSM-5型分子筛中的吸附性质,计算结果显示:分子筛骨架上的硅铝比会显著影响水分子的吸附量和吸附等温线,随着硅铝比的降低,水的吸附量增加;水分子的吸附位置是在钠离子和铝原子的周围,平均每个钠离子周围吸附4个水分子,而当水的吸附量增大时,水分子与分子筛骨架上的氧原子之间发生了氢键作用;在吸附量相同的条件下,水的吸附热随着硅铝比的降低而升高.

Molecular dynamic （MD） quench method and grand canonical Monte Carlo method （GCMC） are used to study the adsorption of water on NanZSM-5 type zeolite. The simulated adsorption isotherm is in good agreement with the experimental data reported in literatures. Based on these facts, adsorption of water on NanSM-5 with various nS/nAl ratios is predicted. The simulation results indicate that nS/nAl ratio of zeolite framework affects the water adsorption and adsorption isotherms greatly and the adsorption amount decreases as the nS/nAl ratio increases, which can be explained by that the atomic charge of Na^＋ cation would influence greatly the polar water molecules due to the Coulomb force. The adsorbed water molecules are located around Na cations and Al atoms on the zeolite framework and the average number around Na cation and Al atom is four at lower loading. However, there are hydrogen bonds between the adsorbed water molecules and the oxygen atoms on the zeolite frameworks at higher loading. At the same loading, the isosteric heat decreases as the nS/nAl ratio increases.