Amide solvents for extraction and separation of 1-hexene from n-heptane: Effect of the amount of methyl groups

•The influence of methyl group in amide extractants on olefin extraction was studied by LLE and quantum chemical calculations.•The olefin selectivity is determined by the difference in electrostatic interaction.•Dispersive and electrostatic interactions both contribute to the olefin distribution coefficients.•The amount and intensity of C-H⋯O interactions and N/C-H⋯π interactions are all affected by methyl groups in extractants.•The existence of the C–H⋯O and N–H⋯π interactions were verified by the infrared spectroscopy. In this work, the influence mechanism of methyl group in the structure of amide solvent on the extraction and separation of alkene (1-hexene or 1-octene) from n-heptane has been studied by a combination of liquid–liquid equilibrium (LLE) experiments and quantum chemical calculations. For LLE experiments, as the number of methyl groups in the extractant structure increases, its selectivity for alkene (1-hexene or 1-octene) gradually decreases, and the distribution coefficients of alkene (1-hexene or 1-octene) and n-heptane gradually increase. In the interaction between 1-hexene and extractant, the amount and intensity of C-H⋯O interactions, as well as the presence or absence of N/C-H⋯π interactions and their intensities, are all affected by the amount (0,1,2,3) of methyl group introduced into the amide solvent. The existence of the C-H⋯O and N-H⋯π interactions were verified by the infrared spectroscopy. With increasing methyl group amounts in the extractant structure, the dispersion interaction strengthens, while the contribution percentage of electrostatic interaction and ΔEelst/ΔE declines, leading to the increase of distribution coefficient and decrease of selectivity for alkene (1-hexene or 1-octene) respectively.