Ming Yang, Kumar Nitin, Siegel Donald J
Department of Physics, University of Michigan, 1440 Randall Laboratory, 450 Church Street, Ann Arbor, Michigan 48109-1040, United States.
Mechanical Engineering Department, Materials Science & Engineering, Applied Physics Program, and University of Michigan Energy Institute, University of Michigan, 2250 G.G. Brown Laboratory, 2350 Hayward Street, Ann Arbor, Michigan 48109-2125, United States.
ACS Omega. 2017 Aug 24;2(8):4921-4928. doi: 10.1021/acsomega.7b01129. eCollection 2017 Aug 31.
The high surface areas and tunable properties of metal-organic frameworks (MOFs) make them attractive materials for applications in catalysis and the capture, storage, and separation of gases. Nevertheless, the limited stability of some MOFs under humid conditions remains a point of concern. Understanding the atomic-scale mechanisms associated with MOF hydrolysis will aid in the design of new compounds that are stable against water and other reactive species. Toward revealing these mechanisms, the present study employs van der Waals-augmented density functional theory, transition-state finding techniques, and thermodynamic integration to predict the thermodynamics and kinetics of water adsorption/insertion into the prototype compound, MOF-5. Adsorption and insertion energetics were evaluated as a function of water coverage, while accounting for the full periodicity of the MOF-5 crystal structure, that is, without resorting to cluster approximations or structural simplifications. The calculations suggest that the thermodynamics of MOF hydrolysis are coverage-dependent: water insertion into the framework becomes exothermic only after a sufficient number of HO molecules are coadsorbed in close proximity on a Zn-O cluster. Above this coverage threshold, the adsorbed water clusters facilitate facile water insertion via breaking of Zn-O bonds: the calculated free-energy barrier for insertion is very low, 0.17 eV at 0 K and 0.04 eV at 300 K. Our calculations provide a highly realistic description of the mechanisms underlying the hydrolysis of MOFs under humid working conditions.
金属有机框架材料（MOFs）具有高比表面积和可调节的性质，使其成为催化以及气体捕获、存储和分离应用中的有吸引力的材料。然而，一些MOFs在潮湿条件下稳定性有限仍是一个关注点。了解与MOF水解相关的原子尺度机制将有助于设计对水和其他活性物质稳定的新化合物。为了揭示这些机制，本研究采用范德华增强密度泛函理论、过渡态寻找技术和热力学积分来预测水吸附/插入原型化合物MOF-5的热力学和动力学。在考虑MOF-5晶体结构的完整周期性的情况下，即不借助簇近似或结构简化，评估了吸附和插入能量随水覆盖率的变化。计算表明，MOF水解的热力学与覆盖率有关：只有在足够数量的HO分子在Zn-O簇上紧密共吸附之后，水插入框架才变为放热过程。高于此覆盖率阈值，吸附的水簇通过Zn-O键的断裂促进水的容易插入：计算得出的插入自由能垒非常低，在0 K时为0.17 eV，在300 K时为0.04 eV。我们的计算为潮湿工作条件下MOFs水解的潜在机制提供了高度逼真的描述。
