锂离子交换ZSM–5型分子筛中水分子吸附特性的分子模拟

MOLECULAR SIMULATION OF CHARACTERISTICS FOR WATER ADSORPTION ON ZSM–5 TYPE ZEOLITE DOPED BY LITHIUM ION

用巨正则系综 Monte Carlo(grand canonical assemblage Monte Carlo, GCMC)模拟方法计算了水在 HZSM–5 型分子筛中的吸附热,计算结果与文献报道的实验结果吻合得较好。模拟了水在经锂离子(Li+)交换后的 ZSM–5 型分子筛上的吸附等温线。在此基础上,进一步预测了水在不同硅铝摩尔(下同)比的 LinZSM–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显著影响水的吸附量和等温吸附曲线,随着硅铝比的减小,分子筛中的自由体积减小,对水的吸附量增加。水分子的主要吸附位置是在 Li+ 和铝原子的周围,LinZSM–5 型分子筛中存在着 1 个主要的吸附位点。Li+等金属阳离子的交换能有效降低水的吸附热。在吸附量相同的条件下,水的吸附热随着硅铝比的升高而降低。

The grand canonical assemblage Monte Carlo （GCMC） method was used to calculate the adsorption heat of HZSM-5 zeo-lite. The results are in good agreement with the experimental data reported in the literature. Then the adsorption isotherms of water by Li^＋ exchanged ZSM-5 zeolite were simulated. Based on these facts, adsorption of water on LinZSM-5 zeolite with various molar ratios n（Si）/n（Al） is predicted. The simulation results indicate that the n（Si）/n（Al） ratio of the zeolite framework greatly affects the water adsorption and adsorption isotherms. The adsorption amount decreases as the n（Si）/n（Al） ratio increases, and the free volume is even greater. The adsorbed water molecules are located around Li cations and AI atoms on the zeolite framework and a main sorption site for water in LinZSM-5 zeolite. The ZSM-5 zeolite modified by Li cations can decrease the isosteric heat effectively. At the same loading, the adsorption heat decreases as the n（Si）/n（Al） ratio increases.