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.