Thermal decomposition of phosphonium salicylate and phosphonium benzoate ionic liquids

Abstract

The thermal stability of ionic liquids is important for their use in a variety of applications. Here, reactive molecular dynamics simulations and thermogravimetric analyses were used to explore the thermal decomposition mechanisms of phosphonium salicylate and phosphonium benzoate. Experiments performed at different heating rates indicated that the decomposition temperatures of phosphonium salicylate and phosphonium benzoate were comparable, but phosphonium benzoate was less stable than phosphonium salicylate under isothermal high temperature conditions. The lower thermal stability of the benzoate compared to the salicylate was reproduced in reactive molecular dynamics simulations. The simulations also showed that cation chain length had little effect on thermal stability. The simulations revealed that thermal decomposition for both phosphonium salicylate and phosphonium benzoate occurred through many different pathways that could be broadly categorized as proton-transfer, association, and dissociation reactions. The phosphonium benzoate underwent more of these reactions and exhibited a wide range of reaction pathways in each category than the phosphonium salicylate. Multiple possible mechanisms were explored to explain this difference and it was found that the dominant factor was the presence of the hydroxyl group in salicylate that affects the ability of oxygen atoms to take part in proton-transfer reactions that are the first step of all subsequent reactions. These findings demonstrate that even subtle differences in anion chemistry may significantly affect the thermal stability of ionic liquids, suggesting avenues for tuning these properties through molecular design.