# Cyanopyridinium-Based Ionic Liquids and Their Mixtures for Ethylene and Ethane Separation

## TL;DR

This paper explores how certain ionic liquids can separate ethylene and ethane gases, finding that the liquids do not strongly interact with ethylene.

## Contribution

The study introduces cyanopyridinium-based ionic liquids and evaluates their solubility properties for ethylene and ethane separation.

## Key findings

- Ethylene and ethane solubilities in cyanopyridinium ionic liquids were measured, showing minimal selectivity.
- Mixing ionic liquids with a diluent did not enhance ethylene-specific interactions.
- MD simulations showed stronger ethylene association with one cation isomer over another.

## Abstract

The solubility of ethane and ethylene was determined
in a series
of cyanopyridinium ionic liquids known to form charge-transfer complexes
with polyaromatic hydrocarbons to determine their potential to form
specific interactions with the unsaturated gas. The solubilities of
ethylene and ethane in 1-butyl-4-cyanopyridinium bis­(trifluoromethane)­sulfonimide
([C4
4CNPy]­[NTf2]) and 1-butyl-3-cyanopyridinium bis­(trifluoromethane)­sulfonimide
([C4
3CNPy]­[NTf2]) were measured using an isochoric saturation method. The
mole fraction solubility of ethane in the ionic liquids ranged from
6.0 × 10–3 to 7.2 × 10–3 and from 7.5 × 10–3 to 9.9 × 10–3 for ethylene in [C4
3CNPy]­[NTf2] and [C4
4CNPy]­[NTf2] at 0.1
MPa and 313 K, respectively. The small preferential solubility of
ethylene over ethane in the ionic liquids results in ideal ethylene
separation selectivities between 1.2 and 1.4, which is in the same
range as typical physisorbent ionic liquids of the same type and molecular
weight, indicating that there is no significant preferential interaction
between the ionic liquids and ethylene. The calculated thermodynamic
properties of solvation reveal that the solvation of both gases is
entropically driven. To promote cyanopyridinium–ethylene interactions
and decrease the possibility of steric constrictions to the interactions,
1-butyl-4-methylimidazolium bis­(trifluoromethane)­sulfonimide ([C4C1Im]­[NTf2]) was added as diluent to
[C4
4CNPy]­[NTf2]. This IL mixture was found to behave almost ideally based
on isothermal titration nanocalorimetry results. The solubility of
ethylene or ethane in the mixture was found to be the weighted average
of the corresponding solubilities in the two pure ionic liquids, still
indicating that no specific ethylene–ionic liquid interactions
were formed. Molecular dynamics (MD) simulations of the systems were
performed and revealed that the slightly higher ethylene solubility
in [C4
4CNPy]­[NTf2] is due to a slightly stronger association with this cation
compared to the 3-isomer.