Incorporation of zinc into the protic imidazolium-based ionic liquid: A novel catalytic route to esters plasticizers

In this work, a novel acidic catalyst based on protic ionic liquid (1-metylimidazoluim triflate, [Hmim][OTf]) and ZnO was developed. First, analysis of the structure was developed by HRMS, FT-IR and Raman spectroscopy, NMR, and X-ray absorption spectroscopy. The obtained results revealed an alternative structure and mechanism of the formation of the ionic liquid/ZnO adduct ([Hmim][OTf])3Zn. The developed liquid system was created based on the molar ratio of ([Hmim][OTf])3Zn and was shown as an effective acidic catalyst and solvent in the alternative synthesis of plasticizers. In particular, ([Hmim][OTf])3Zn (10 mol%) displayed catalytic efficiency for the esterification of succinic acid, adipic acid, and lactic acid with two alcohols 1-butanol and 2-etylhexanol. For 2-ethyl-1-hexanol, full conversion of succinic acid was achieved after 1 h at 130 °C. The synergistic effect of the ionic liquid and ZnO was demonstrated by comparing the catalytic efficiency of the individual components. ([Hmim][OTf])3Zn was used in six consecutive reaction cycles without a significant decrease in catalytic efficiency. Additionally, structural studies provided the foundation for the rational design and development of an acidic catalyst, consequently leading to a more environmentally friendly and economically feasible process.