(92e) Mechanistic Insights into the Water Induced Degradation Pathway of Metal Organic Frameworks Using Reaction Force Field.

Metal-Organic Frameworks (MOFs) are a class of highly organized three dimensional porous materials formed by interconnecting the metal nodes and organic linkers leading to unique properties such as large surface area and varying porosity. Recently MOFs are gaining importance for carbon capture and it is crucial to understand and evaluate the performance of these materials. One of the evolving technology would be adsorption based separation of carbon dioxide from flue gas using these porous materials. The challenge with these MOFs for both wet/dry flue gas based carbon capture would be maintaining their selectivity and stability for longer time during the adsorption cycles. It is inevitable that unless an ideal dry equipment was used to obtain dry flue gas, water vapor competes with the CO2 for the same adsorption sites that might cause the substantial change in the porosity of the sorbent leading to the reduction in overall capacity of the material. This study focuses on deriving the mechanistic insights of reaction involving MOFs having Zinc as metal centers and carboxylic acid functionalized organic linkers with single water molecule using Metadynamics. Reactivity of the MOF with water would be estimated by determining the free energy surface (FES) land scape using a set of control variables driving MOF-water hydrolysis reaction. The energy leading for the formation of MOF hydrolyzed product estimated with the FES helps us to bring out the considerable energy penalty representing the water stability of MOF. Our atomistic reactive force field (ReaxFF) based Metadynamics simulations found that MOFs possessing larger pores with high aromaticity in linker are comparatively more stable than the small pored MOFs, as the water could accommodate within the adequate sized pores and cause hydrolysis reaction leading to the dissemination of metal and linker bond forming hydrolyzed products. The slower reaction in MOFs with large pores would be due to the effective hydrophobic nature of the linker. Additionally, zeolitic imidazolate frameworks(ZIFs), a sub class of MOFs having strong covalent Zn-N bond were explored and noticed that the ZIFs are mostly stable than the iso-reticular MOFs with Zn-O bond. The FES landscape having deep or shallow wells could be explored through a control variable during the hydration reaction of MOF with water, tracking its evolution showed up peaks of high energy confirming the stable behavior of the MOFs. Metadynamics estimated FES energy can set a criterion in designing the stable MOFs for carbon capture even in humid conditions.