(643a) Thermal Stability of Imidazolium-Based Ionic Liquids: The Effects of Alkyl Chain Length, Environment, and Heating Conditions

Ionic liquids are room-temperature molten salts that have found increasing use as solvents for polymer and biomaterial processing. This study investigates a series of 1-alkyl-3-methylimidazolium ionic liquids, which have been found to effectively dissolve lignocellulosic biomass. This family of ionic liquids has received significant attention in the literature. Despite this, the thermal stability and degradation behavior of these solvents is not well understood. Furthermore, there have been only a few investigations into the effect of structural variations such as varying the length of their alkyl chains.

In the present study the thermal stabilities of these ionic liquids were investigated with the ethyl, propyl, butyl, hexyl, and decyl alkyl chains, and the chloride and acetate cations. The ionic liquids were studied under both isothermal conditions from 100 to 200 ^oC over sixteen hours, and under dynamic heating conditions with heating rates from 5 to 20 ^oC/min up to 650 ^oC. It was found that the ionic liquids with longer alkyl chains generally displayed decreased thermal stability.

The effect of the environment on the stability of ionic liquids is also significant, and was investigated with 1-ethyl- and 1-butyl-3-methylimidazolium acetate in mixtures with ethanol, methanol, water, and N,N-Dimethylformamide (DMF) at elevated temperatures and pressures, up to 150 ^oC and 350 bar. A custom high-pressure view cell was used in conjunction with a UV-vis spectrophotometer to capture the degradation of the ionic liquid-solvent mixtures over time. It was found that in protic solvents, including ethanol, methanol, and water degradation was suppressed as reflected by the UV absorption patterns. In contrast DMF, an aprotic solvent, was found to significantly accelerate thermal degradation.

This research is supported by NSF Grant CBET-1509390.