Art of Architecture: Efficient Transport through Solvent-Filled Metal–Organic Frameworks Regulated by Topology

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.