Flame retardancy of anions in imidazolium-based ionic liquids on Xinjiang lignite

Ionic liquids (ILs) is a new type inhibitor,which can destroy the active functional groups in coal and fundamentally solve the problem of coal spontaneous combustion. In this study,four kinds of imidazolium-based ILs with different anions were synthesized via two-step method,which was 1-butyl-3-methylimidazolium fluoride (BMIM F,I),1-butyl-3-methylimidazolium trifluoroacetate (BMIM CF₃COO,II),1-butyl-3-methylimidazolium tetrafluoroborate (BMIM BF₄,III) and 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM PF₆,IV).They were characterized by nuclear magnetic resonance (NMR) and fouier transform infrared spectroscopy (FTIR).The results showed that the absorption peaks of NMR and FTIR were the same as targeted ILs. Four ILs were stirred with Xinjiang lignite at 50℃ over 8 h. Then the spontaneous combustion process of coal samples treated by ILs were researched via thermogravimetric technology. The results demonstrated that the dry cracking temperature point,ignition point and the maximum mass loss rate point of coal samples treated by ILs increased. Moreover,the flame retardant capacities of four anions were determined to bePF₆^->BF₄^->CF₃COO^->F^-,which was the same as the variation law of the number of fluorine atoms and the radius of ions in anions from large to small. The reason is that the more fluorine atoms amounts and ionic radius of anion,the stronger inductive effect on -OH,binding ability to the H⁺and destruction ability of anion. Wherein the flame retardant capacity of IV which containingPF₆^-was greatest,it increased autogenous ignition temperature from 349.6℃ to 380.2℃,made the maximum mass loss ratepoint move from 440.0℃ to 480.0℃,and reduced the weight loss percentage from 92.3% to 80.7%. FTIR showed that the contents of hydroxyl functional groups of coal samples treated by ILs were lower than that of raw coal,which indicated that ILs could inhibit coal spontaneous combustion by destroying hydroxyl functional groups in coal and the sequence of destructive capacity was IV> III> II> I. The study results provide a theoretical guidance for the design of ILs with better flame retardant effect.