(478h) A Quantum Chemical and Molecular Dynamic Study On Cyclohexanone Ammoximation Over TS-1

Cyclohexanone oxime is an essential precursor of the caprolactam for the nylon-66 manufacturing. Because of the significant value, the oxime production by TS-1 catalyzing cyclohexanone has drawn extensive attentions. With regard to the mechanism of this reaction, various experiments have been made. There are two main reaction routes, imine mechanism and hydroxylamine mechanism. However, this reaction occurs so rapidly that the experimental detecting techniques can barely detect the intermediates. In addition, the exact activation barrier of the rapid elementary reaction cannot be acquired by experiments either. In terms of that, we employed the computational methods to investigate the reaction mechanisms and the activate barrier of every elementary reaction in our research. The results are very useful to guide the future experiment researches.  

In this paper, the mechanisms were simulated in a cluster model of TS-1 containing a non-defect Ti site by using DMol³. Density functional theory were utilized with exchange functional of Becke’s 1988 plus Lee-Yang-Parr’s 1988 correlation energy function (BLYP), which was more suitable for this system compared to other functions in DMol³. The used atomic orbital basis sets were Double Numerical plus polarization, which included a polarization p-function on all hydrogen atoms and a polarization d-function on non-hydrogen atoms. There were no frozen orbitals for all atoms. The geometry optimization was not stopped until the maximum energy change was less than 1×10^-5 atomic units. The SCF tolerance was 1×10^-6 atomic units. All energy was calculated with zero-point energy (ZPE) correlation. For searching transition state, we utilized complete linear synchronous transit and quadratic synchronous transit (LST/QST) method. Vibrational analysis was performed to make sure the transition state true by keeping only one imaginary vibrational frequency, whose normal mode corresponds to the reaction coordinate and all other eigenvalues must be real. For searching more accurate transition state, we also used TS confirmation method which begins by approximating the Intrinsic Reaction Path (IRP) with QST and then performs subsequent refinements.

According to the characteristics of the reaction, a novel active site of TiOOH(η2) with a ligand of ammonia water was proposed. The newly-found active center was more stable than the conventional one by lower 3.3kcal/mol of overall energy. The results based on the transition state theory revealed that both mechanisms could take place, and in hydroxylamine path, NH₃ reacted with active site to form O-NH₄⁺ firstly,  then to hydroxylamine. Hydroxylamine can react with cyclohexanone without TS-1. As for the activation energy and rate constant of elementary steps, the results were that in imine mechanism the adsorption of cyclohexanone and the formation of imine were energy controlling steps, whose energy barriers were 56.61kcal/mol and 76.5kcal/mol respectively; in the hydroxylamine mechanism, the formation of hydroxylamine with energy barrier 65.19kcal/mol was the energy determining step. The difference between the activation energy in two mechanisms was discussed, and it was found that the hydroxylamine mechanism was the energy-preferred pathway in cyclohexanone ammoximation. The relationships between rate constants and temperature were discussed. It revealed that the imine mechanism was more sensitive to temperature, and superior to hydroxylamine mechanism in higher temperature.