Theoretical and experimental correlations of gas dissolution, diffusion, and thermodynamic properties in determination of gas permeability and selectivity in supported ionic liquid membranes

Abstract

Supported ionic liquid membranes (SILMs) has the potential to be a new technological platform for gas/organic vapour separation because of the unique non-volatile nature and discriminating gas dissolution properties of room temperature ionic liquids (ILs). This work starts with an examination of gas dissolution and transport properties in bulk imidazulium cation based ionic liquids [C nmim][NTf2] (n = 2.4, 6, 8.10) from simple gas H 2, N 2, to polar CO 2, and C 2H 6, leading to a further analysis of how gas dissolution and diffusion are influenced by molecular specific gas-SILMs interactions, reflected by differences in gas dissolution enthalpy and entropy. These effects were elucidated again during gas permeation studies by examining how changes in these properties and molecular specific interactions work together to cause deviations from conventional solution–diffusion theory and their impact on some remarkably contrasting gas perm-selectivity performance. The experimental perm-selectivity for all tested gases showed varied and contrasting deviation from the solution–diffusion, depending on specific gas-IL combinations. It transpires permeation for simpler non-polar gases (H 2, N 2) is diffusion controlled, but strong molecular specific gas-ILs interactions led to a different permeation and selectivity performance for C 2H 6 and CO 2. With exothermic dissolution enthalpy and large order disruptive entropy, C 2H 6 displayed the fastest permeation rate at increased gas phase pressure in spite of its smallest diffusivity among the tested gases. The C 2H 6 gas molecules “peg” on the side alkyl chain on the imidazulium cation at low concentration, and are well dispersed in the ionic liquids phase at high concentration. On the other hand strong CO 2-ILs affinity resulted in a more prolonged “residence time” for the gas molecule, typified by reversed CO 2/N 2 selectivity and slowest CO 2 transport despite CO 2 possess the highest solubility and comparable diffusivity in the ionic liquids. The unique transport and dissolution behaviour of CO 2 are further exploited by examining the residing state of CO 2 molecules in the ionic liquid phase, which leads to a hypothesis of a condensing and holding capacity of ILs towards CO 2, which provide an explanation to slower CO 2 transport through the SILMs. The pressure related exponential increase in permeations rate is also analysed which suggests a typical concentration dependent diffusion rate at high gas concentration under increased gas feed pressure. Finally the strong influence of discriminating and molecular specific gas-ILs interactions on gas perm-selectivity performance points to future specific design of ionic liquids for targeted gas separations.