Evaluation of the thermal stability and pyrolysis mechanism of 1-ethyl-3-methylimidazolium dicyanamide and 1-Butyl-3-methylimidazolium dicyanamide by STA, DSC, TG-FTIR

Abstract

As a common ILs anion, dicyandiamide ILs is widely used in deep desulfurization and denitrogenation of fuel oil. In this study, two types of dicyandiamide salts containing cations with different carbon chain lengths were selected, 1-Ethyl-3-methylimidazolium dicyanamide ([EMIM]DCN) and 1-Butyl-3-methylimidazolium dicyanamide ([BMIM]DCN) were selected to investigate. However, under air conditions, the thermal effects of the two ILs are opposite. Therefore, the relationship between carbon chain length and the thermal stability and pyrolysis mechanism of ILs deserves further research. The thermal stability, exothermic properties and pyrolysis mechanism of the two substances under air conditions using simultaneous thermogravimetric analyzer measurements (STA), differential scanning calorimetry (DSC) and fourier transform infrared spectroscopy (FTIR).Through thermogravimetric experiments, it was found that the apparent activation energies of [EMIM]DCN and [BMIM]DCN were 120 kJ mol−1 and 101 kJ mol−1, respectively. This explain that the carbon chain length is inversely proportional to the thermal stability of ILs. DSC shows that [BMIM]DCN has three exothermic stages and emits a total of 2075.67 J g−1 heat, while [EMIM]DCN has one endothermic stage and one exothermic stage, releasing 343.44 J g−1 heat, compared to [BMIM]DCN, [EMIM]DCN is clearly more secure. Through FTIR experiments, it was found that due to the longer and less stable carbon chain of [BMIM]DCN, it is more prone to oxidative exothermic generation of CO2, thus [BMIM]DCN exhibits exothermic behavior in the first stage of DSC experiments. The carbon chain of [EMIM]DCN is relatively short and stable, so it fails to oxidize to generate sufficient CO2 in the first stage and needs to absorb heat to generate isocyanates. In the third stage, due to the increase in temperature, the carbon atoms of [EMIM]DCN and [BMIM]DCN are oxidized and converted into CO2, releasing enormous heat, which explains the huge exothermic phenomenon in the final DSC experiment.