Art of Architecture: Efficient Transport through Solvent-Filled Metal- Organic Frameworks Regulated by Topology
R Wang and BC Bukowski and JX Duan and JY Sui and RQ Snurr and JT Hupp, CHEMISTRY OF MATERIALS, 33, 6832-6840 (2021).
Guest transport through metal-organic frameworks (MOFs) is a critical process in the application of MOFs for catalysis. Understanding the interplay between transport behavior and a MOF's structure is of fundamental importance to further tailor MOFs for optimal catalysis. Here, we investigated dye transport processes through two solvent-filled Zr-MOFs, NU-600 and NU-1008, which are compositionally the same but display different topologies, i.e., she and csq, respectively. Dye transport through individual MOF crystallites was monitored spatially and temporally by confocal fluorescence microscopy. In both MOF crystals, dye molecules permeated the external-surface barrier first, then diffused along channels. Transport in NU-600 is three dimensional due to orthogonal channels, while diffusion in NU-1008 is primarily one dimensional owing to parallelly aligned channels. Quantitatively, the diffusivity of dye molecules in NU-600 is smaller than in NU-1008, which is attributed to the narrower channels and tortuous pore network of NU-600. However, comparing crystals of the same volume, macroscopic uptake of dye in NU-600 is significantly more efficient than in NU-1008, highlighting that the she-net NU-600, which features intersecting channels, affords efficient pathways for substrate transport. Additionally, for NU-600 and NU-1008, the nanoscale topologies of the compounds qualitatively govern the resulting macroscopic crystallite morphologies, including aspect ratios. The morphology difference is crucial to conferring a transport efficiency advantage on NU-600. Atomistic simulations of solvated dye diffusion in the two MOFs indicate energetically favorable interaction between the linkers and dye. Molecular dynamics trajectories reveal that the dye molecule spends more time on the linkers in NU-600 relative to NU-1008, which supports the smaller diffusivity in NU-600 measured by experiments. In this work, we combined experiments and simulations to demonstrate the interplay between MOF structure and guest transport behavior both microscopically and macroscopically, which provides insights for selecting or designing MOF topologies to enhance guest transport through MOFs intended, for example, for chemical catalysis.
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