Simulation Of Quantum Chemistry For Mechanisms And Turnover Frequencies Of Cyclohexanone Ammoximation Reaction

In this paper, the method of quantum chemistry computation was used to exploreand discuss the mechanism of cyclohexanone ammoximation. Two parallel catalyticmechanism paths of hydroxylamine formation and an imine mechanism path werefound. The exploration and research on specific mechanism path can not only find thecatalytic active sites, but also help understand how the active site plays its catalyticrole. These exploration and understanding has a huge inspiration towards theimprovement of the catalyst defects. We can even design a new catalyst with highercatalytic activity and a longer life with these knowledge.Firstly, diffusion corrected density functional method—M062X was used tosimulate the mechanisms of cyclohexanone ammoximation on the constructed12Tcluster model of titanosilicalite1. In the exploration of defective titanium centercatalyzing hydrogen peroxide and ammonia molecules to generate hydroxylaminemolecule, the relationship between the location of adsorbed water molecule andactivity of titanium oxygen active species generated by reacting hydrogen peroxideand ammonia molecules. Then great attention was focused on the simulation of themechanism paths of titanium oxygen active species oxidizing ammonia molecule togenerate hydroxylamine. In both two reaction paths obtained, titanium center playedan decisive role, besides that, the oxygen atom bonded to titanium and the silanols inthe defective site also played an important role. In the simulation of the desorbedhydroxylamine reacting with cyclohexanone to generate cyclohexanone oxime, thereaction mechanisms with water molecules or not were explored and I found that thewater molecules could greatly facilitate the reaction.In order to investigate the effects of larger framework of TS1towards thereaction barrier and the adsorption energy as well as more accurate reaction energypaths, the hydroxylamine mechanism was recalculated on a41T cluster model.Through the comparison between the result of12T and that of41T, I found that thesize of the TS1model had a great effect on the adsorption energy but comparativelysmall effect on the reaction barrier energy.41T cluster model was used to simulate the imine mechanism of cyclohexanoneammoximation. The effect of water molecules was investigated in the reactionbetween ammonia molecule and cyclohexanone to generate imine and I found that thewater molecules greatly facilitate the reaction. As to the following reaction between titanium oxygen active species and imine to generate cyclohexanone oxime, tworeasonable reaction paths were found and one of them was much better than the otherone.Energy span model was applied to calculate the turnover frequencies (TOF) of thetwo hydroxylamine mechanism paths on the12T cluster model and I found that theratio of the two TOF was almost one. I further calculated the TOF of one of the twohydroxlamne mechanism paths and the imine mechanism path and found the TOF ofhydroxlamne mechanism path was2.5times the imine mechanism path. The TOFincluding the effect of concentration was calculated on the hydroxlamne mechanismpath.