Physical Properties Of Propylene/Propane Mixtures In Zeolitic Imidazolate Frameworks By Dftb And Molecular Dynamics Simulation

Abstract

Zeolitic imidazolate frameworks (ZIFs) are fascinating porous materials which were widely utilized for gas adsorptions, separation and storage due to its high thermal and chemical stability. In this work, adsorptions of propane and propylene in ZIF−8 and ZIF−67 pores were studied under the periodic boundary condition based on the SCC−DFTB method. Six and eleven adsorption configurations of propane and propylene molecules can be observed in the ZIF−8 pore, whereas only four adsorption configurations in the ZIF−67 pore were found. Based on the most stable configurations, the adsorption energies of propane both ZIF−8 pore and ZIF−67 pore are lower than propylene. These results agree with abilities of ZIF−8 and ZIF−67 pores to separate propane/propylene mixture based on the principle of gas–solid partition. Due to adsorption abilities of ZIF−8 and ZIF−67 pores which can adsorb propane better than propylene, ZIF−8 and ZIF−67 pores, therefore, differentiate diffusion flux of propane and propylene via the hexagonal aperture. Activation energies of propane and propylene via the hexagonal aperture lead to the separation factor of the propylene/propane. As two transition–state structures of propylene interacting with methyl and ethylene groups of imidazolate belonging to the hexagonal aperture were found. In addition, diffusion of propane and propylene in the ZIF–8 were investigated using molecular dynamics simulation based on the NVE ensemble. The self–diffusivity coefficient of propane, propylene and propane/propylene mixtures were within the ranges of 1.19 x 10-9–6.68 x 10-9 m2/s, 3.49 x 10-10–8.29 x 10-10 m2/s and 1.68 x 10-9–9.82 x 10-10 m2/s, respectively. The results suggest that ZIFs could be an excellent material for the separation of propylene/propane.