Effect of anions in ionic liquids on microstructure and oxidation characteristics of lignite

There is an urgent need for a green and efficient inhibitor to inhibit coal spontaneous combustion (CSC). Ionic liquids (ILs) have led to a growing interest because of eco-friendliness, sustainability, and excellent designable properties. Herein, we report some ILs, which are composed of [EMIM]+ cations and different anions (Cl–, [BF4]–, [OAC]–, [OTf]–). Anions show the characteristics of increased chain length and enhanced electronegativity of acid radical. The effect of the chain volume of anions and the strength of acidity on the microstructure of coal were measured by analyzing and comparing the changes of element morphology, Fourier Transform infrared spectroscopy structure parameters, and microcrystalline in this work. The results illustrate that with the increase of the anionic chain volume and the electronegativity of acid radical, the ILs appear the outstanding ability to destroy the lamellar diameter microcrystals and the structural abundance of aliphatic hydrocarbons of coal. With the enhancement of the electronegativity of anionic acid radical, the degree of reduction of CO and COO functional groups of coal and the ability of desulfurization rate are enhanced. The thermodynamic process demonstrates that the dry cracking temperature and ignition temperature can increase by 19.3 °C and 10.0 °C, respectively, resulting in coal may be inhibited during the oxidation. In additional, the thermal kinetic behaviors of samples were investigated by Starink models. Results indicated that the apparent activation energy of the inhibited samples can be increased from 51.96 kJ mol−1 of Un-tc to [EMIm][BF4]-tc 60.52 kJ mol−1 in the stages of thermal decomposition and combustion. The regular destruction characteristics of anions and cations on coal molecules to suggest hopeful prospects ILs as a novel and designable materials in the field of prevention CSC fire.