Point Defects Control Guest Molecule Diffusion in the 1D Pores of Zn(tbip)

Molecular diffusion plays a critical role in metal-organic
frameworks
(MOFs) within the application of kinetic chemical separations. We
carefully study in this work the unexpected role of point defects
for short-chain alkanes diffusing in Zn­(tbip), an MOF with rigid one-dimensional
(1D) channels. Inside a defect-free Zn­(tbip), guest molecules are
expected to follow single-file diffusion along 1D channels. It has
been found previously by Heinke et al. that these parallel 1D channels
are connected for molecular diffusion. Our density functional theory
(DFT) calculations suggest that linker vacancy defects could arise
under experimentally relevant conditions by removing a pair of linkers.
Further climbing-image nudged elastic band (cNEB) DFT calculations
indicate that hopping of short-chain alkanes between adjacent 1D channels
over defect windows can occur at moderate temperatures. In addition
to providing insights into connected adjacent 1D channels in Zn­(tbip),
Heinke et al. also inferred that most 1D pores are blocked from a
microkinetic model. We also explored the influence of hydrolyzed linker
created by the formation of linker removal inside 1D pores. Our DFT
calculations show that the linkers can effectively block the pores
and the linker diffusion in 1D channels is slow. Our results, for
the first time, offer a mechanistic explanation of the unexpected
molecular diffusion behavior in this MOF with 1D channels.