ZIF-67, a Co-substituted ZIF-8 structure,
is investigated as a
candidate for the industrially highly demanding propylene/propane
separation, with the use of computational techniques for the first
time. A new force field for the ZIF-67 framework based on density
functional theory calculations is reported along with a recently developed
force field for ZIF-8. The new force field is validated through comparison
with structural data for ZIF-67 from the literature. Molecular dynamics
simulations are reported for ZIF-67, showing a dramatic increase of
propylene/propane corrected diffusivities ratio when compared to ZIF-8,
implying a huge improvement in the separation of the mixture. The
sieving mechanism of ZIF frameworks is investigated, and the results
yield a dependency of the swelling motion of the gates from the bonding
of the metal atom with its surrounding atoms. The presence of Co in
the modified framework results in a tighter structure with a smaller
oscillation of the gate opening, which leads to a narrower aperture.
The results from the simulations and experiments in ZIF-67 place this
new structure at the top of the candidates for propylene/propane separation.
ZIF-8
is a strong candidate for propane/propylene separation, which
is regarded as one of the most industrially demanding. Molecular simulation
of this separation must account for the flexibility of the structure,
which enables the adsorption and diffusion of molecules with kinetic
diameter larger than the apertures of the pores. Moreover, this simulation
requires modeling subtle changes since the strong sieving effect upon
the mixture depends on the very small differences between propane
and propylene molecular sizes (∼0.2 Å). In this work,
a new force-field for the ZIF-8 structure has been developed from
DFT calculations in simplified structures. The new parameter set reproduces
structural properties in very good agreement with the experimental
measurements reported in literature. Molecular dynamics simulations
and the Widom test particle insertion method were then employed for
the calculation of diffusivities, activation energies and adsorption
properties of propane and propylene. The results are in agreement
with experiments and demonstrate that the sieving of such a mixture
is a kinetic driven separation process.
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