ZIF-8 Membrane Ethylene/Ethane Transport Characteristics in Single and Binary Gas Mixtures

Zeolitic imidazolate framework-8 or ZIF-8 membranes have shown great promise in separating propylene/ propane (C-3) mixtures; however far fewer works have analyzed ethylene/ethane (c(2)) transport behavior in ZIF-8 membranes. This work studies C-2 permeation behavior, transport properties, and selectivity as a function of temperature and pressure in single and binary gas mixtures. In single and binary separation tests conducted from 25 to 100 degrees C, the permeances of ethylene and ethane show a negative correlation with temperature attributable to activation energies of diffusion (Ed) for ethylene and ethane (11.7 and 13.2 kJ/mol) that are lower than their respective heats of adsorption (16.2 and 17.1 kJ/mol). Low Ed values are observed for C-2 molecules in ZIP-8 due to pore flexibility. C-2 diffusive selectivity is limited in ZIF-8 due to the similar size of C-2 molecules which are both smaller than the effective,ZIF-8 pore aperture (low energetic selectivity) and sizable entropic selectivity is limited by the zeolitic pore shape. Binary selectivity is 20% lower than ideal selectivity due to cooperative adsorption, which enhances ethane adsorption in the presence of ethylene. The presence of relatively stronger adsorbing C-2 molecules in mixture with hydrogen decreases H-2 permeability and inverts the H-2 temperature dependency of permeation from-adsorption controlled to diffusion controlled. In single and binary C-2 pressure dependent experiments performed between 1 and 4 atm, starkly contrasting ethylene/ethane separation profiles are observed due to differences in single and binary adsorption isotherms for C-2 molecules. The ZIF-8 structure is amenable to adsorption/pressure induced distortions which greatly affect C-2 permeation behavior.