College of Chemical Engineering, Nanjing Forestry University, 210037, Nanjing, P. R. China.
School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States.
J Phys Chem B. 2021 Apr 15;125(14):3690-3699. doi: 10.1021/acs.jpcb.0c11252. Epub 2021 Apr 2.
Defect engineering leads to an effective manipulation of the physical and chemical properties of metal-organic frameworks (MOFs). Taking the common missing linker defect as an example, the defective MOF generally possesses larger pores and a greater surface area/volume ratio, both of which favor an increased amount of adsorption. When it comes to the self-diffusion of adsorbates in MOFs, however, the missing linker is a double-edged sword: the unsaturated metal sites, due to missing linkers, could interact more strongly with adsorbates and result in a slower self-diffusion. Therefore, it is of fundamental importance to evaluate the two competing factors and reveal which one is dominating, a faster self-diffusion due to larger volume or a slower self-diffusion owing to strong interactions at unsaturated sites. In this work, via Monte Carlo and molecular dynamics simulations, we investigate the behavior of isopropyl alcohol (IPA) in the Zr-based UiO-66 MOFs, with a specific focus on the missing linker effects. The results reveal that unsaturated Zr sites bind strongly with IPA molecules, which in return would significantly reduce the self-diffusion coefficient of IPA. Besides this, for the same level of missing linkers, the location of defective sites also makes a difference. We expect such a theoretical study will provide an in-depth understanding of self-diffusion under confinement, inspire better defect engineering strategics, and promote MOF based materials toward challenging real-life applications.
缺陷工程导致对金属有机骨架（MOF）的物理和化学性质的有效操纵。以常见的缺失配体缺陷为例，有缺陷的 MOF 通常具有更大的孔和更大的表面积/体积比，这两者都有利于增加吸附量。然而，就 MOF 中吸附物的自扩散而言，缺失的配体是一把双刃剑：由于缺失的配体，不饱和金属位可能与吸附物更强地相互作用，导致自扩散更慢。因此，评估这两个竞争因素并揭示哪个因素占主导地位是至关重要的，即由于体积较大而导致更快的自扩散，还是由于不饱和位的强相互作用而导致更慢的自扩散。在这项工作中，我们通过蒙特卡罗和分子动力学模拟研究了异丙醇（IPA）在基于 Zr 的 UiO-66 MOFs 中的行为，特别关注缺失配体的影响。结果表明，不饱和 Zr 位与 IPA 分子强烈结合，这反过来又会显著降低 IPA 的自扩散系数。除此之外，对于相同程度的缺失配体，缺陷位的位置也会产生影响。我们期望这种理论研究将为受限条件下的自扩散提供深入的理解，激发更好的缺陷工程策略，并推动基于 MOF 的材料朝着具有挑战性的实际应用方向发展。
