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

Abstract

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.

Proton Conductivity of Composite Polyelectrolyte Membranes with Metal‐Organic Frameworks for Fuel Cell Applications

“…As a result, they have been used in the preparation of composite membranes and films for gas and liquid separation . Among them, ZIF‐8 (a representative Zn‐based ZIF material) has been widely studied as the intricate Zn−N coordination in ZIF‐8 enables a large pore (11.6 Å) for low resistance transport (high permeability) of molecules through the ZIF‐8 pore and a small nominal pore aperture (0.34 nm) excellent for molecular exclusion ( Figure ) …”

Alternatives to Cryogenic Distillation: Advanced Porous Materials in Adsorptive Light Olefin/Paraffin Separations

“…Although showing an excellent kinetic selectivity for propylene and propane, ZIF‐8 shows a much lower kinetic selectivity (≈5) for ethylene and ethane according to the analysis of gas uptake kinetics by in situ infrared microscopy (IRM) and pulsed field gradient nuclear magnetic resonance (PFG NMR) . This was ascribed to the relatively smaller sizes of the C2 gases and the sizable entropic selectivity of ZIF‐8 limited by the zeolitic pore shape …”

“…[155] This was ascribed to the relatively smaller sizes of the C2 gases and the sizable entropic selectivity of ZIF-8 limited by the zeolitic pore shape. [156] Recently, Peng et al reported two ultramicroporous MOFs, namely Zn(ox) 0.5 (trz) (ox = oxalate, trz = 1,2,4-triazole) and Zn(ox) 0.5 (atrz) (atrz: 3-amino-1,2,4-triazole), for kinetic propylene/propane separation. [157] These MOFs are constructed by pillaring of Zn-triazolate layers with the oxalate groups, and exhibit 1D zig-zag channels along the crystallographic a-axis with narrow necks of diameters of 2.6 and 2.9 Å for Zn(ox) 0.5 (trz) and Zn(ox) 0.5 (atrz), respectively.…”

Metal–Organic Framework Membranes: From Fabrication to Gas Separation

“…The dissimilarity in separation efficiency observed between the single gas and the mixed gas measurements were attributed to the difference in experimental conditions. Later on, the transport properties of ethane/ethylene separation in the same (ZIF-8) membrane were studied by James et al at different temperatures ranging from 25 to 100 °C and different pressures [161]. Figure 61.…”