Coby J. CLARKE1, Husain BAAQEL2, Richard P. MATTHEWS2, Yiyan CHEN2, Kevin R. J. LOVELOCK3, Jason P. HALLETT2, Peter LICENCE1, Heather PARTLOW1
1University of Nottingham, Nottingham, United Kingdom
2Imperial College London, London, United Kingdom
3University of Reading, Reading, United Kingdom
Ionic liquids can dissolve large amounts of metal salts while remaining free-flowing liquids. Metal ionic liquids are highly tuneable because speciation can be controlled by the anions, cations, and ratios of the components.1 Catalysis remains the most popular industrial application of ionic liquids because of this excellent tunability. Thermal stability is particularly important for metal containing ionic liquids because exceeding the upper temperature limit can produce decomposition products that poison catalysts, react with reagents, and change the physical properties of the medium. Degraded solvents also need to be replaced or topped-up, which adds a further economic and environmental burden because ionic liquid production involves many steps, each with high energy requirements and waste streams of their own. When considering that solvents are used in supra-stoichiometric quantities, it is clear that small amounts of decomposition can have outsized impacts. Harmful decomposition products, such as HF, can also evolve from overheated ionic liquids, which adds a serious safety concern and provides further justification for studying decomposition pathways. Understanding thermal structure-property relationships is therefore particularly important for application of metal ionic liquids at elevated temperatures.
In this work, we discuss the thermal properties of several halometallate ionic liquids containing Zn, Co, In, Ni, Pt, and Ag, to understand how thermal properties change as a function of the metal.2 We also explore thermal properties of 9 halozincate ionic liquids with various amounts of metal, to show how speciation affects stability. Importantly, this helps us understand the intricate balance between chemical and physical properties.3 Although we focus on high temperature stability, we also discuss other industrially relevant thermal parameters, such as heat capacities and phase transitions. We use hyphenated and hybridised TGA techniques and ex situ analysis to gain a mechanistic insight of decomposition and show how thermal studies of metal ionic liquids are crucial for their adoption to industry. We also present life cycle analysis (LCA) data for the production of halometallate ionic liquids as a function of concentration, which suggests that simplicity really is key to a sustainable process.
1 J. Estager, J. D. Holbrey and M. Swad?ba-Kwa?ny, Chem. Soc. Rev., 2014, 43, 847–886.
2 C. J. Clarke, H. Baaqel, R. P. Matthews, Y. Chen, K. R. J. Lovelock, J. P. Hallett and P. Licence, Green Chem., 2022, 24, 5800–5812.
3 A. W. Taylor, S. Men, C. J. Clarke and P. Licence, RSC Adv., 2013, 3, 9436.