Implementation of molecular simulation to understand the structural stability of metal‒organic frameworks

Abstract

Among porous materials, metal–organic frameworks (MOFs) possess remarkable surface area per gram of the solid material. In addition, the diversity of their building blocks and topologies offers tunability in design for various applications such as gas adsorption, molecular sieving, catalysis, and drug delivery, just to name a few. However, many MOFs suffer from stability issues, and they can easily amorphize by the action of mechanical pressure (or stress), temperature, or if got in contact with reactive species such as water. In view of that, the mechanical, thermal, and chemical stability aspects of MOFs are very important issues that must be addressed before introducing MOFs to industry. In this work, the stability of MOFs was investigated in silico, using molecular simulation methods. Firstly, the commonly encountered structural degradation of MOFs at the activation stage was studied and linked to properties of the frameworks. Hence, mechanical properties represented by the bulk, shear, and young moduli were found to be reliable metrics in predicting the likelihood of structural collapse upon des-solvation. Accordingly, based on the estimated mechanical attributes, a stability map was constructed that define the vulnerability region wherein MOFs might face difficulties at the activation stage. Also, anomalies MOFs that were reported as collapsed, but displayed decent mechanical strength, were isolated for the sake of repeating the activation process with improved techniques. Secondly, the thermal stability of MOFs was studied using a reactive molecular dynamics simulation. The results from simulation showed that the thermal degradation was initiated by breakage of the bridging bond that connects the organic linker with the metal node. Since, increasing the temperature promoted cooperative motions in the framework, which eventually deformed and broke the weak bridging bond. In particularly, motion of the organic linker contributed the most to deprivation of the structural stability. Moreover, incorporation of a methyl functional group on the linker further promoted the linker dynamics, and consequently exacerbated the overall stability. Therefore, synthesizing a MOF with rigid linkers could help in improving the stability of the framework. Lastly, this research showed that not all the distorted amorphous structures are deficient, rather in some cases the amorphous structure can have better adsorption capabilities. Hence, there are MOFs with non-accessible regions that can open through mechanical amorphization methods. Accordingly, partial amorphization of these MOFs can improve their surface area characteristics compared to the perfect crystalline counterparts. As results, more than 40% increase in the accessible surface area was attained upon shear deformation of one of the MOFs that were investigated. Overall, this dissertation provides a molecular understanding to the factors that affects the structural stability of MOFs.