Determination Of Reaction Mechanisms And Rate-determining States For Cyclohexanone Ammoximation Reaction Over Titanium Silicalite-1by Molecular Simulation

The concept of “rate-determining step” has been of great significance in derivingreaction kinetics from reaction mechanism for many decades. Nevertheless, more andmore researchers have got an agreement on identifying the rate-determining state tobe more useful than the rate-determining step for both theoretical and practicalresearchs. The rate-determining states provide the possibility of finding an appropriatecatalyst or improving the existed ones. For an environmental friendly reaction ofcyclohexanone ammoximation on titanium silicalite-1, because of oxime’s significantvalue, it has drawn extensive attentions. Although various experiments have beencarried out, there has not been a general agreement about the mechanism to date.In this paper, the pathways of catalytic ammoximation of cyclohexanone in TS-1and H2O2system was simulated in a cluster model of TS-1containing a defect Ti siteby utilizing density functional theory (DFT). Considering the special reactionenvironment, a novel Ti(η1-OOH) defect active center with a ligand of NH3wasproposed, which was more stable than the conventional TiOOH or TiOOH-H2O.In hydroxylamine route, ammonia attacked at the Obcoordinated to the titaniumof the active center and led to the formation of the complex intermediate TS-1-O-NH3,which then transformed to hydroxylamine further. Hydroxylamine was desorbed fromthe catalyst and reacted with cyclohexanone firstly formed the complex，andthen oxime. With regard to the imine mechanism, the reactions with and withoutcatalyst participating were studied. It was found that the reaction route was same,which imine formed firstly by two steps of hydrogen atom transfer, and then iminereacted with the active center to form oxime. The reaction activation energy withTS-1participation was less, and we concluded that the catalyst play an important role.The activation energies and rate constants for all elementary reactions wereestimated. The results revealed that the step from intermediate to imine and the step ofoxime formation in imine route had the biggest energy barrier of183.353kJ/mol and151.607kJ/mol respectively; while the step from intermediate to hydroxylamine andthe step from intermediate to oxime in hydroxylamine route owned biggest energybarrier of136.785kJ/mol and153.064kJ/mol respectively. Degree of TOF control was calculated by AUTOF software, it was found that the transition states in the step fromintermediate to hydroxylamine and the step from intermediate to imine were TOFdetermining states.The effects of NH3, H2O2and H2O adsorption on the geometrical, vibrationalfrequency and energetic properties of TS-1were studied by QM/MM method. Fromour calculations, the distance of Ti-O bond and Ti-Si bond was increased afteradsorption, and a blue shift of960cm-1band has been observed. It revealed that theinteraction between NH3and catalyst was the strongest among the three molecules.Compared with the adsorption of the one molecule, the co-absorption of NH3andH2O2gave out more energy.