Interactions of C2H6 and C2H4 with the homologous series [CnMIM][BF4] ionic liquids at high pressure studied by in situ ATR-FTIR spectroscopy

It is believed that ionic liquids (IL) can be a promising sorbent for separating a mixture of olefins and paraffins due to their unique properties. Understanding the interaction between IL and sorbed gases plays a key role in the development of selective separation processes. In this work, for the first time, in situ ATR-FTIR spectroscopy was used to study the interaction of light hydrocarbons (C2H6, C2H4) and a series of 1-alkyl-3-methylimidazolium tetrafluoroborate [CnMIM][BF4] (n = 2, 4, 6, 8, 10, 12) ILs. The spectra of liquid ethane and ethylene were obtained, as well as the spectra of IL under the pressure of hydrocarbons. An increase in the length of the alkyl chain of the cation leads to an increase in the solubility of both ethane and ethylene. The data obtained demonstrate a significant interaction between ethylene and IL, which leads to a greater dissolution of ethylene relative to ethane. A blue-shift of the bands of sorbed ethylene relative to liquid ethylene is observed. At the same time, an increase in the length of the alkyl chain of the cation leads to a shift in the wavenumber of sorbed ethylene towards lower wavenumbers, which is the result of a decrease in the interaction between IL and ethylene and the dissolution of ethylene in the free volume of IL. In addition, it has been found that it is possible for two hydrogen atoms of ethylene to interact to form a hydrogen bond between the anion of the IL and ethylene.