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

(Graph Presented) Zeolitic imidazolate framework-8 or ZIF-8 membranes have shown great promise in separating propylene/propane (C₃) mixtures; however far fewer works have analyzed ethylene/ethane (C₂) transport behavior in ZIF-8 membranes. This work studies C₂ 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°C, the permeances of ethylene and ethane show a negative correlation with temperature attributable to activation energies of diffusion (E_d) 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 E_d values are observed for C₂ molecules in ZIF-8 due to pore flexibility. C₂ diffusive selectivity is limited in ZIF-8 due to the similar size of C₂ 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 C₂ molecules in mixture with hydrogen decreases H₂ permeability and inverts the H₂ temperature dependency of permeation from adsorption controlled to diffusion controlled. In single and binary C₂ 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 C₂ molecules. The ZIF-8 structure is amenable to adsorption/pressure induced distortions which greatly affect C₂ permeation behavior.