Molecular Simulation Research On Mechanisms Of Adsorption,Reaction And Diffusion Of Cyclohexanone Ammoximation Over Titanium Silicalite-1

Molecular sieve, applied in catalyzed reaction widely, is a kind of environmental friendly material and frequently used in the field of"Green chemistry". Titanium Silicalite-1 (TS-1) catalytic ammoximation of cyclohexanone in the liquid phase, developed by Enichem Co., is one of the typical processes in molecular sieve catalysis, and is advantageous from the environmental point of view and meets the requirement of"Green Chemical Engineering". Notwithstanding several studies on this process, there is not a general agreement about the catalytic reaction mechanism, and the theoretical bases for existing kinetic models are also absent.In this paper, the mechanism of catalytic ammoximation of cyclohexanone with H2O2 to cyclohexanone oxime over TS-1 catalyst was investigated by the method of molecular simulation. The adsorption,reaction and diffusion mechanisms were discussed respectively. The formation of active complex was studied by the ab-initio method based on the establishment of proper cluster model of TS-1. According to the characters of this reaction system, a novel active complex was proposed by incorporated ammonia water in the catalyst model as a ligand of Ti atom. The newly-developed active complex was found to be more stable than the conventional one by lower 3.3kcal/mol of overall energy.After the establishment of active complex model, the interaction between reactant and the catalyst was estimated by calculating the overall energy when reactants adsorbed on different positions of the active complex. The final adsorption states of reactants were found at the minimum of calculated energy profiles. Then the reaction pathways of cyclohexanone ammoximation were investigated by transition state theory based on the final sorption states. The results revealed that the imine mechanism and hydroxylamine mechanism were both existed in the reaction. The intermediate, imine and hydroxylamine, were all attainable from perspectives of quantum chemistry. Only one transition state could be found during imine formation which caused by the interaction between adsorbed ammonia and cyclohexanone. Meanwhile, two transition states could be found consecutively during hydroxylamine formation by the interaction between the ammonia water and the reaction active center. One intermediate structure, which represented O-NH4+, was also found and it was generated after the first transition state formed. Then the intermediate could subsequently formed hydroxylamine. The formation of cyclohexanone oxime was also attainable from both imine and hydroxylamine with only one transition state found in each of the procedures. It also could be found that the hydroxylamine and cyclohexanone could generate cyclohexanone oxime without zeolite participating.The activation energy and rate constant of each elementary reaction were also estimated by chemical dynamics. The results indicated that the formation of imine and the adsorption of cyclohexanone were the rate control steps in the imine mechanism procedure with the activation energy barrier of 61.5kcal/mol and 46.61kcal/mol respectively. The formation of hydroxylamine was the rate control step in the hydroxylamine mechanism with the activation energy barrier of 65.19kcal/mol. The energy changes were compared, and it was found that the hydroxylamine mechanism was the energy-preferred pathway in cyclohexanone ammoximation, which means the hydroxylamine mechanism was the main pathway to produce oxime during the reaction.At last, the diffusion mechanism of cyclohexanone in ZSM-5 zeolite with MFI topology was studied by Molecular Dynamics and Kinetic Monte Carlo. The calculate energy profiles and diffusivity coefficients shows that the potential energy between cyclohexanone and zeolite framework was the main factor of rate control while duffusion and the repulsion was the major factor for the interaction among cyclohexanones.