Mechanism of [BMIM][BF4] inhibiting coal groups activity from experiments and DFT calculations

Abstract

The interaction between ionic liquid and coal (IL-coal) plays a theoretically pivotal role in revealing the internal mechanism of ILs inhibiting coal spontaneous combustion (CSC). This can determine the degree of reaction or inhibitory ability between IL-coal molecules. First, the activity characteristics of FTIR structural parameters from molecular surfaces affected by [BMIM][BF4] were obtained. In addition, the optimal structures of [BMIM][BF4] with methyl, hydroxyl, ether bond, methylene, and carboxyl groups were determined based upon Density Functional Theory. The activity characteristics of the IL-coal complexes were investigated from the aspects of charge population and transfer, bond length change, and electrostatic potential distribution. Finally, the types and relationships of the forces between the five structures of the IL-coal complexes and the stability and activity order were explored. The results demonstrated that [BMIM][BF4] could possibly reduce the structural abundance, side chain length, and oxygen-containing groups of aliphatic hydrocarbons. The interaction between anion [BF4]− and active groups was more stable than that between cation [BMIM]+ and active groups. Meanwhile, the dissolution and destruction of coal active groups by ILs were not completely determined by the number of hydrogen bonds formed between ILs and coal active groups, but by the strength of hydrogen bonds formed between anions and coal active groups, and the effect of anions was stronger than that of cations. The order of oxidation capacity of the IL-coal complexes was: [BMIM][BF4]–CH2– > [BMIM][BF4]–O– > [BMIM][BF4]–COOH > [BMIM][BF4]–CH3 > [BMIM][BF4]–OH. The van der Waals effect mainly presented complexes. Despite this, there were hydrogen bonds between the complexes formed by hydroxyl groups, but the hydrogen bond force showed exceptionally slight effects.