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.