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

In this work, the novel acidic catalyst based on protic ionic liquid (1-metylimidazoluim triflate, [Hmim][OTf]) and ZnO was developed. First, analysis of the structure was developed using various techniques like, HRMS, FT-IR and Raman spectroscopy, NMR and X-ray absorption spectroscopy. As a result, an alternative structure and mechanism of the formation of ionic liquid/ZnO adduct [Hmim][OTf]‑ZnO was proposed. The obtained liquid system was created based of molar ration [Hmim][OTf] : ZnO 3:1 and next demonstrated as an effective acidic catalyst and solvent in one for alternative plasticisers synthesis. In particular, [Hmim][OTf]-ZnO was used as catalyst (10 mol%) for esterification of succinic acid, adipic acid and lactic acid with two alcohols 1-butanol and 2-etylhexanol. The use of 2-ethyl-1-hexanol causes full conversion of succinic acid after 1 h at 130 °C. The synergistic effect of ionic liquid and ZnO was demonstrating via comparison of the catalytic efficiency of the components using separately. [Hmim][OTf]-ZnO was used in six consecutive reaction cycles without significant drop of efficiency. It has been demonstrated how structural studies can provide a foundation for the rational design and development of acidic catalyst, consequently leading to a more environmentally friendly and economically feasible process.