Effect of modifiers on the stability of 1‑butyl‑3-methylimidazolium-based ionic liquids

•Ionic liquids (ILs) can degrade up to 80 wt.% during biorefining processes.•During ILs degradation volatile and non-volatile compounds are formed.•Additives to ILs reduce the amount of volatile degradation compounds.•Additives to ILs increase the diversity of non-volatile compounds.•Addition of water increases the “greenness” of ionic liquids. The distinctive capabilities of 1‑butyl‑3-methylimidazolium (bmim) cation-based ionic liquids (ILs) to dissolve lignocellulosic biomass offer the potential for the development of novel green bioprocessing technologies based on their utilization as green solvents. The limited range of thermal stability of ILs necessitates the use of various co-solvents to maintain solubility. In the present study, an original approach to determine the degree of degradation of alkylimidazolium ILs was proposed, based on the application of gravimetric analysis method to determine the content of volatile degradation products and high-performance liquid chromatography-high-resolution mass spectrometry (HPLCHRMS) to determine the total degree of degradation. The proposed approach, as well as the combination of HPLCHRMS with gas chromatography-mass spectrometry (GC–MS), was employed to comprehensively characterize the impact of additives and impurities (water, dimethyl sulfoxide, hydrochloric and acetic acids, diethylamine) on the degradation of bmim acetate, chloride, and methyl sulfate during thermal treatment under typical biomass dissolution conditions (150 °C, 24 h). The presence of additives or impurities in the composition of ILs has been found to predominantly contribute to a decrease in the number of volatile compounds formed, while increasing the variety and relative content of non-volatile degradation products. The use of protic solvents and acids results in a decrease in the overall degree of degradation of ionic liquids and in the fraction of volatile compounds formed. This allows for the classification of binary mixtures based on these additives as "green" solvents at temperatures below 150 °C.