NMR study of structural changes of alkyl-phosphonium decanoate ionic liquid induced by water and butyric acid extraction

Ionic liquids (ILs) have received substantial attention in basic and applied research in the last two decades. This work is focused on deeper understanding of carboxylic acids extraction by ILs with potential in recovery of platform chemicals from diluted streams in biorefineries. Results of an NMR study of structural changes of a system composed of trihexyl-(tetradecyl)-phosphonium decanoate, butyric acid (BA) and water are presented. H-1, C-13 and P-31 NMR were used for the determination of system composition, its dynamic behavior and for the study of interactions between components of the system with different water and BA loadings. The hypothesis of water being extracted by IL in separate regions (polar domains) was postulated. Water extraction goes through a continuous series of intermediate structures until a relatively stable structure is achieved. Extraction of BA with water saturated IL proceeds via two different mechanisms: replacement of water molecules in polar domains by BA molecules in early stages of the extraction, and co-extraction of BA and water molecules at higher BA concentrations with the change of IL structure to a less rigid one. From dynamic study (selfdiffusion coefficients) it follows that components of IL systems do not form tight complexes, but water and BA molecules can move partly independently of IL ions. (C) 2020 Elsevier B.V. All rights reserved.