超临界水中醋酸锌水解反应的分子动力学模拟

Molecular Dynamics Simulations on Hydrolysis of Zinc Acetate in Supercritical Water

氧化锌(ZnO)是一种重要的化工原料,超临界水热合成法制备纳米ZnO的第一步是锌盐与碱或水发生水解反应生成Zn(OH)2,后者接着脱水生成ZnO.以Zn(CH3COO)2为原料,直接和超临界水(SCW)反应能够制备纳米级的ZnO颗粒,但对反应机理的探讨较少.本研究利用分子动力学模拟超临界条件下Zn(CH3COO)2水解反应过程中的结构和能量变化,发现Zn(CH3COO)2在SCW中容易聚集成无定形的团簇,1个Zn2+平均和5个CH3COO-和1个H2O配位,形成6配位的八面体结构.处于Zn(CH3COO)2团簇和SCW界面的Zn2+能够和更多的H2O配位.水解反应后系统的势能降低,同时伴随Zn(CH3COO)2团簇结构的改变.反应产物OH-分布在Zn(CH3COO)2团簇内部,富集Zn2+,而CH3COOH则分布在SCW中.本文的工作为超临界水热合成的反应过程提供了基本的理论依据.

Zinc oxide （ZnO） is a multifunctional material with wide applications in chemical engineering. Hydrothermal synthesis of ZnO under supercritical conditions from salt solutions containing zinc ions is an environmentally safe process. Two reaction steps are involved, zinc hydroxide sol formation and dehydration from the sol. However, little is known about the underlying mechanism. In this study, molecular dynamics simulations were performed to investigate the structural and thermodynamic changes in the zinc acetate hydrolysis process, i.e., Zn（CH^COO）2, in supercritical water （SCW）. Our results show that Zn（CH3COO）2 is prone to aggregate in SCW. On average, one Zn2＋ ion coordinates with five CH3COO- species and one H2O molecule, forming an octahedral configuration. However, more water molecules bind Zn2 ＋ at the SCW interface to form Zn（CH3COO）2 clusters. The total potential energy of each system decreases after the hydrolysis of Zn（CH3COO）2, suggesting that it is a thermally favorable process in SCW. The OH- reaction product incorporates into the amorphous Zn（CH3COO）2 cluster and CH3COOH is in the SCW phase. Our results provide a general theoretical framework for the Zn（CH3COO）2 hydrothermal synthesis in SCW.