Highly Efficient Hydrogenation of Nitrocyclohexane to Cyclohexanone Oxime over NiTi-Layered Double Hydroxide: The Key Role of Oxygen Vacancies

Abstract

Highly efficient selective hydrogenation of nitrocyclohexane (NCH) to cyclohexanone oxime (CHO) is important for the production of caprolactam. In this work, NiTi catalysts (NiTi-IMP, NiTi-HDT, and NiTi-LDH) were prepared by different methods and applied in NCH hydrogenation to CHO. It was found that NiTi-LDH with rich oxygen vacancies (Ov) induces a strong metal−support interaction (SMSI) between Ni and TiO 2 , hence favoring the dispersion of Ni nanoparticles, promoting the reduction of NiO and the formation of much more metallic Ni, and facilitating the electron transfer between Ni and Ti. In situ DRIFTS reveals the mechanism for NCH hydrogenation on Ov-rich NiTi-LDH, showing that Ov benefits the enhancement of NCH conversion and suppresses overhydrogenation to the byproducts of cyclohexanone (CHone) and cyclohexylamine (CHA). Moreover, density functional theory (DFT) calculations illustrate that Ov facilitates the adsorption of NCH and desorption of CHO, favors the α-H transfer of nitrosocyclohexane (N−CHH) to CHO, and significantly reduces the overall reaction energy barrier of NCH to CHO. Under optimum conditions, NiTi-2 with the largest amount of Ov gives 90.71% selectivity to CHO at 99.84% NCH conversion. This study provides an efficient and economical catalyst for the preparation of CHO from NCH, and it is valuable for promoting the industrial production of caprolactam toward resource conservation and environment friendliness.