Molecular regulatory mechanism of alkyl side chain length on the thermal decomposition of imidazolium nitrate ionic liquid: theoretical calculation and experimental analysis

Abstract

This study aimed at the thermal safety hazards of imidazole nitrate ionic liquids under high-temperature working conditions. The influence mechanism of the length of alkyl side chains on their thermal decomposition characteristics was systematically explored.

In this study, three ionic liquids 1-ethyl-3-methylimidazolium nitrate, 1-propyl-3-methylimidazolium nitrate and 1-hexyl-3-methylimidazolyl nitrate, were systematically studied at multiple scales from macroscopic to microscopic, based on thermal analysis experiments and density functional theory calculation. The results show that as the alkyl side chain extends from ethyl to hexyl, the thermal stability of the ionic liquid shows a significant downward trend. The initial decomposition temperatures were 1-ethyl-3-methylimidazolium nitrate (266.5 °C), 1-propyl-3-methylimidazolium nitrate (275.5 °C), and 1-hexyl-3-methylimidazolyl nitrate (173.0 °C). The apparent activation energy decreased from 210.51 kJ/mol (1-ethyl-3-methylimidazolium nitrate) to 168.59 kJ/mol (1-hexyl-3-methylimidazolyl nitrate). In addition, the adiabatic temperature rise, heat release and maximum adiabatic pressure all significantly decrease with the growth of side chains, which are 1-ethyl-3-methylimidazolium nitrate (600.79 °C, 1756.14 J/g, 15.5 MPa), 1-propyl-3-methylimidazolium nitrate (498.75 °C,1603.56 J/g, 6.55 MPa), 1-hexyl-3-methylimidazolyl nitrate (356.26 °C, 1055.95 J/g, 5.57 MPa), respectively. The thermal runaway risk parameters show a gradient decreasing law. At the microscopic level, the electrostatic potential distribution confirms the energy-containing characteristics of the material. The highest occupied molecular orbital - lowest unoccupied molecular orbital energy gap analysis reveals the law that the molecular dynamic stability decreases with the growth of the alkyl chain, which were agree with the macroscopic thermal behavior. This study analyzed the influence mechanism of the length of alkyl side chains on the thermal decomposition characteristics from both macroscopic and microscopic dimensions. Providing a theoretical basis for the molecular design of this type of ionic liquid and the safety strategy for high-temperature applications.