Abstract:

The COSMO-SAC model was used to study the vapor-liquid equilibrium of the ammonia solution in the presence of different ionic liquids (ILs). The effects of the hydrophilicity, acidity, alkalinity, anion and cation types, and functional group modification of the ionic liquids on the relative volatility of ammonia to water were analyzed and discussed. It was found that the vapor-liquid equilibrium of the ammonia-water solution can be strongly affected by ILs due to their diverse properties. The ability to escape of ammonia from the aqueous solution could be generally improved when the interaction energy of water and ILs is higher than that of ammonia and water. The stronger the anion’s hydrophilicity and the ability of forming hydrogen bond is, or the stronger the interaction energy between water and anion is, or the weaker the interaction energy between ammonia and cation is, the better the ability to improve the separation of ammonia and water is. Meanwhile, the greater the difference between the interaction energy of water and ILs and that of ammonia and water, the higher the value of relative volatility of ammonia to water is. In addition, the improvement was observed at low concentration of ammonia when the interaction energy of water and ILs is lower than that of ammonia and water. In general, the anions affect the relative volatility of ammonia to water more than the cations. Among the investigated anions, chloride ([Cl]^-) and acetate ([Ac]^-) are the best for the separation of ammonia and water. For methyl imidazole cations ([C _n mim]⁺, n=2, 4, 6, 8), the longer the alkyl chain is, the more unfavorable the separation of ammonia is, but at the low concentrations of ammonia, the grafting of the amino group (—NH₂) on the [C₂mim]⁺ will improve the separation effect of ammonia.

Key words: ionic liquids, COSMO-SAC, ammonia, water, vapor-liquid equilibria, simulation

摘要：

采用COSMO-SAC模型研究了不同离子液体存在下氨水溶液的汽液相平衡，探讨了离子液体的亲水性、酸碱性、阴阳离子种类以及功能基团修饰等对氨的相对挥发度的影响。研究发现，不同性质的离子液体均会影响氨水系统的汽液相平衡。一般地，如果水与离子液体相互作用能高于氨与水的相互作用能，离子液体将有利于氨的逸出。当阴离子亲水性和形成氢键的能力越强，或者水与阴离子相互作用能越强，或者氨与阳离子相互作用能越弱，则离子液体越能促进氨水分离。水/离子液体之间的相互作用能与氨/水之间的相互作用能差值越大，离子液体越能提高氨的相对挥发度。当水与离子液体相互作用能低于氨与水的相互作用能时，离子液体也能促进低浓度下的氨水分离。阴离子要比阳离子更能影响氨的相对挥发度，其中氯离子([Cl]^-)、醋酸根离子([Ac]^-)型离子液体对促进氨水分离的效果更佳。对于甲基咪唑类阳离子([C _n mim]⁺，n=2、4、6、8)，烷基链越长，越不利于氨的分离，但在[C₂mim]⁺上嫁接胺基(—NH₂)将会改善低浓度下氨水的分离效果。

关键词: 离子液体, COSMO-SAC, 氨气, 水, 汽液平衡, 模拟