Functionalised ionic liquid mixtures for the enhanced separation of ethylene and ethane
Sam MCCALMONT1, John D. HOLBREY1, Nimal Q. GUNARATNE1, David WILKINS2, Margarida COSTA GOMES3, Leila MOURA1
1QUILL Research Centre, Queen's University Belfast, Belfast, United Kingdom
2School of Mathematics and Physics, Queen’s University Belfast, Belfast, United Kingdom
3Laboratoire de Chimie de l’ENS Lyon, Lyon, France
Separation of light olefins such as ethylene from their paraffin counterparts have been described as one of the seven chemical separations to change the world. Global annual production of light olefins exceeds 200 million tons, about 30 kg for each person on the planet. The current method for their separation is cryogenic distillation, one of the most energy-intensive processes in the industry. Alternative methods can focus on the olefin being selectively captured either through a physical interaction (physisorption) or chemical reaction (chemisorption). One class of alternative sorbents are ionic liquids (ILs). However, so far, IL physisorbents have not demonstrated sufficient efficiency in either selectivity or capacity to compete with current technologies. Complexation of ethylene through its double bond with silver and copper ions has been used for chemical separation of olefins and paraffins. However, other components of raw gas feeds, such as acetylene, can react with the silver and become explosive. This has prevented the uptake of these materials into large scale processes.
This work involves testing cyanopyridinium ionic liquids (shown in the image) as potential selective chemical absorbents for ethylene. These ILs have previously been shown to form liquid charge transfer complexes with electron-rich aromatics via π-cation interactions. We have examined the capacity of these materials to bind selectively with ethylene through similar π-cation association. The solubility of the ethylene and ethane in the cyanopyridinium ionic liquids was tested using a pressure-volume-temperature method. In this system, mixed gas measurements were also completed, mimicking industrial conditions. Gas solubility measurements have shown that the absorption capacity and gas separation selectivity is comparable to other physisorbent ionic liquids. Gas-liquid interactions were probed through NMR spectroscopy and no strong/specific interactions were observed with either of the ionic liquids, in agreement with gas solubility results. Molecular dynamic simulations allowed for the identification of the mechanisms of solvation and the preferential solvation sites for each gas in the different ionic liquids.
In an attempt to promote the specific interactions between cyanopyridinium and ethylene, these viscous ionic liquids were mixed with a less viscous ionic liquid, [C4C1Im][NTf2]. The mixture was studied using isothermal titration nanocalorimetry to determine the enthalpy of mixing. The enthalpy of mixing was only slightly positive, suggesting that the ionic liquids mixed ideally with no new interaction between them. The ethylene and ethane solubility was determined for the equimolar mixture of ionic liquids.
 D.S. Sholl and R.P. Lively, 2016, Nature, 532, 435–437.
 L. Moura, C. C. Santini and M. F. Costa Gomes, 2016, Oil Gas Sci. Technol. – Rev. d’IFP Energies Nouv., 71, 23.
 D.J. Safarik and R.B. Eldridge,1998, Industrial and Engineering Chemistry Research, 37, 2571–2581.
 C. Hardacre, J. D. Holbrey, C. L. Mullan, M. Nieuwenhuyzen, T. G. A. Youngs, D. T. Bowron and S. J. Teat, 2010, Phys. Chem. Chem. Phys., 532, 435–437.