Role of Structural Defects in the Adsorption and Separation of C3 Hydrocarbons in Zr-Fumarate-MOF (MOF-801)

An
effective separation of propylene/propane mixtures is one of
the most important processes in the petrochemical industry. Incidentally,
this separation is challenging due to the extensive similarities between
both gases in terms of physicochemical properties such as, but not
only limited to, boiling point, kinetic diameter, and molecular weight.
A drive to switch to energy efficient processes, like adsorption or
membrane separation, has highlighted several microporous metal organic
frameworks as promising materials. In this work, we present a combined
numerical and experimental investigation on propane and propylene
adsorption in Zr-fumarate-MOF (also known as MOF-801), a small pore
isoreticular analogue of UiO-66. Here, we demonstrate how the presence
of structural defects can completely change the sorptive properties
and separation performance of the Zr-fumarate-MOF, with the loss of
sieving effects and a reversal of selectivity toward propane, as well
as enhanced capacity and diffusion rates for C3-sized hydrocarbons.
Extensive GCMC simulations performed on mixed defective supercells
show that a ratio of missing-cluster defects of around 1/8 can best
account for the experimental results. Furthermore, analysis of low-frequency
phonon spectra is used to explain gaseous diffusion in the original
pristine material. Finally, the thermodynamic preference for propane
over propylene is confirmed through column breakthrough experiments,
suggesting the potential applicability of the Zr-fumarate-MOF in this
challenging separation.