(704i) Effects of Topology and Pore Size of Metal-Organic Frameworks on Their Adsorption and Transport Properties: Insights from Molecular Simulations

Metal-organic frameworks (MOFs), which have high crystallinity, large surface areas, well-defined porosities and tunable chemical environments, are emerging as next-generation materials for a broad range of applications related to adsorption, separation and catalysis. The structural features of MOFs, such as topology and pore sizes, can be controlled in a systematic way, which allows precise design for specific applications. In this work, we studied the effects of MOF topology and pore sizes on the adsorption and transport properties via atomistic molecular simulations and established structure-property relationships as design rules. First, we studied the adsorption behavior of ethanol and alkanes in a series of MOFs to explore the potential of MOFs as adsorbent materials in adsorption-based refrigeration systems, which are a promising energy-efficient alternative to traditional compression-based systems. Our results showed that certain MOFs can provide largely improved working capacities over traditional adsorbents and there exists a structure-property relationship between the MOF pore size and the working capacity of the system for each specific working condition. Molecular-level insights on the microscopic adsorption mechanism were also gained from the simulations. Next, we investigated the diffusion of alkanes in MOFs to understand how the structural parameters determine the diffusivity. We looked at a series of hierarchical MOFs that contain both micropores and mesopores with the same topology but varying pore sizes. In general, the introduction of mesopores in hierarchical porous materials was believed to enhance the mass transfer. However, from molecular simulations, we obtained a non-monotonic diffusivity-loading relationship in which the diffusivity first decreases then increases. From detailed analysis of the simulation results, we concluded that the diffusivity at different stages of loading is controlled by different structural factors—the size of the micropores and the mesopores determine the diffusivity at low and high loadings, respectively. We then designed another MOF with that topology and its micropores have the same size as in the MFI zeolite. The diffusivity-loading relationship of n-hexane in that MOF coincides with that in the hierarchical self-pillared pentasil (SPP) zeolite assembled by MFI units, which verified our conclusion. Together, our results shed light on the complex microscopic adsorption and transport behavior in MOFs and may guide the future design of functional porous materials.