Synergistic Effect of Deep Eutectic Solvent and Zinc Oxide in Transesterification of Dimethyl Terephthalate for the Synthesis of Alkyl Terephthalate Esters

A deep eutectic solvent (DES), composed of choline chloride and urea, demonstrated exceptional solubility properties, allowing the dissolution of 0.8 wt % of zinc oxide. This liquid system was used as both solvent and catalyst (DES 300 mol %; ZnO 7.8 mol %) for the transesterification of dimethyl terephthalate (DMTP) with various alcohols (1-butanol, 2-butanol, 2-ethylhexanol, 1-hexanol, and 1-octanol). For example, the use of 1-butanol caused full conversion of DMTP after 24 h at 120 °C. The synergistic effect of the DES and ZnO was demonstrated via a comparison with the catalytic efficiency of the components used separately. The advantage of the catalytic system is the lack of miscibility of the product with DES-ZnO, resulting in the formation of a biphasic system during the reaction, the shifting of the reaction equilibrium being the main driving force for this process. DES-ZnO was used in six consecutive reaction cycles without any significant drop in efficiency.