Wu Jian Yang, Liu Qing Lin, Xiong Ying, Zhu Ai Mei, Chen Yu
Department of Chemical and Biochemical Engineering, Xiamen University, China.
J Phys Chem B. 2009 Apr 2;113(13):4267-74. doi: 10.1021/jp805923k.
The COMPASS (condensed-phase optimized molecular potentials for atomistic simulation studies) force field with two sets of partial atomic charges of water was used to simulate adsorption and diffusion behavior of water/methanol and water/ethanol mixtures in zeolite 4A at 298 K. The adsorption of alcohol first increased and then decreased with increasing pressure, whereas the adsorption of water increased progressively until an adsorption equilibrium was reached. Both the adsorbed molecules and the zeolite framework were treated as a fully flexible model in MD simulations. The simulation results show that the effects of the size and steric hindrance of the diffusing molecules on diffusivity are significant. The diffusivity of water, methanol, and ethanol molecules decreases by 1 order of magnitude in the order of water > methanol > ethanol. The diffusivity of water molecules depends on the mass fraction and the partial charges of water in zeolite 4A. The ethanol and methanol molecules have restricted motion through the alpha-cages, whereas the water molecules can easily pass through the alpha-cages window at low feed alcohol concentrations. And the extent of hydrogen bonding increased with increasing water concentration.
采用具有两组水的部分原子电荷的COMPASS（用于原子模拟研究的凝聚相优化分子势）力场，在298 K下模拟水/甲醇和水/乙醇混合物在4A沸石中的吸附和扩散行为。醇的吸附量随压力增加先增大后减小，而水的吸附量则逐渐增加直至达到吸附平衡。在分子动力学模拟中，吸附分子和沸石骨架均被视为完全柔性模型。模拟结果表明，扩散分子的尺寸和空间位阻对扩散系数的影响显著。水、甲醇和乙醇分子的扩散系数按水>甲醇>乙醇的顺序降低1个数量级。水分子的扩散系数取决于4A沸石中水的质量分数和部分电荷。乙醇和甲醇分子在α笼中的运动受到限制，而在低进料醇浓度下，水分子可以很容易地通过α笼窗口。并且氢键的程度随水浓度的增加而增加。
