Construction of Oxygen Vacancy-Rich TiO₂ Nanocrystals for Boosting the Ammonolysis of Caprolactam to 6-Aminocapronitrile

Abstract

Hexamethylene diamine, an important chemical intermediate for polyamides, can be synthesized through the two-step route of caprolactam (CPL) ammonolysis to 6-aminocapronitrile (ACN), followed by hydrogenation. This method has received increasing attention from academia and industry. However, studies on the catalyst structure-performance correlation in CPL ammonolysis are still sporadic. In this work, a series of anatase TiO₂ with different oxygen vacancy concentrations was prepared by chemical reduction using NaBH₄. The oxygen vacancy on TiO₂ surface, presented as Ti³⁺ sites, substantially enhances the adsorption and activation of NH₃, which are demonstrated as the key steps in ammonolysis. Owing to the synergistic effect of Ti³⁺ and Ti⁴⁺ species, the CPL conversion rate and ACN selectivity of 85 and 97%, respectively, are achieved within 250 h. Density functional theory calculations showed that the intermediates on oxygen vacancy-rich TiO₂ had a more favorable adsorption energy compared to those on intact TiO₂, which is in good agreement with the experimental results.