Study on the esterification mechanism catalyzed by MIL-125 for the synthesis of 1,2-cyclohexanedicarboxylates

Di(2-ethylhexyl)-1,2-cyclohexane dicarboxylate (DEHCH) is expected to function as a non-toxic plasticizer for replacing di(2-ethylhexyl) phthalate (DEHP). During the esterification for synthesis of DEHCH, the di-esterification process is usually known as rate control step. To better understand this process, Density Functional Theory (DFT) was utilized to investigate the mechanism of mono(2-ethylhexyl)-1,2-cyclohexane dicarboxylate with 2-ethylhexanol catalyzed by tetraisopropyl titanate [Ti(OiPr)₄], MIL-125, MIL-125-NH₂, and hydrated MIL-125. The results indicate that Ti(OiPr)₄ possesses a single coordinatively unsaturated Ti site and the reaction follows the bimolecular nucleophilic substitution (SN₂) mechanism, with an energy barrier as high as 184.97 kJ·mol⁻¹. Given Ti's high oxygen affinity and the abundance of reaction sites in metal-organic frameworks (MOFs), MIL-125, was innovatively selected for investigation. Simulated results show that the μ₂-O bridging between the two coordinatively unsaturated Ti atoms acts as an additional Brønsted base site, enhancing the reactivity. Meanwhile, the generated μ₂-OH assists in dehydration, serving as the rate-determining step with an energy barrier of 101.11 kJ·mol⁻¹. To further improve the dehydration, the -NH₂ group functionalized on the organic ligand and the dissociated -OH group from product water were strategically utilized as distinct Brønsted base sites. In MIL-125-NH₂, the energy barrier for dehydration was reduced to 33.31 kJ·mol⁻¹. Notably, in hydrated MIL-125, 2-ethylhexanol remains unadsorbed, lowering the energy barrier for dehydration to 25.24 kJ·mol⁻¹. These findings suggest that MIL-125 can transform the adverse impact of product water into a beneficial factor, thereby MIL-125 is recommended to catalyze the esterification of 1,2-cyclohexanedicarboxylic acid with 2-ethylhexanol.