Density Functional Theory Research On Mechanisms and Rate-Determining States of Cyclohexanone Ammoximation Over Titanium Silicalite-1

Density functional theory research on mechanisms and rate-determining states of cyclohexanone ammoximation over Titanium Silicalite-1

Yanying Qi  Feng Xin©~

School of Chemical Engineering and Technology, Department of Chemical Engineering, Tianjin University, Tianjin 300072, China

Abstract

The concept of "rate-determining steps" has been of great significance in deriving reaction kinetics from reaction mechanism for many decades. Nevertheless, more and more articles have got an agreement on identifying the rate-determining states more useful than the rate-determining steps for both theoretical and practical researches. The rate-determining states provide the possibility of finding an appropriate catalyst or improving the existed ones. For an environmental friendly reaction of cyclohexanone ammoximation on titanium silicalite-1 (TS-1), the mechanisms should be confirmed from molecular simulation, and then its kinetics will be established by the rate-determining states and experiments, which is useful for improving the catalyst performance.

In this paper, the pathways of catalytic ammoximation of cyclohexanone in TS-1 and H₂O₂ system was simulated in a cluster model of TS-1 containing a defect Ti site by using Dmol3 in Material Studios software. Density functional theory (DFT) were utilized with exchange functional of Becke's 1988 plus Lee-Yang-Parr's 1988 correlation energy function (BLYP), which was more suitable to this system compared to other functions in Dmol3. The used atomic orbital basis sets were Double Numerical plus polarization (DNP), which included a polarization p-function on all hydrogen atoms and a polarization d-function on non-hydrogen atoms. All of the energies were calculated with zero-point energy (ZPE) correlation. For searching transition state, we chose the method of complete linear synchronous transit and quadratic synchronous transit (complete LST/QST). For acquiring more accurate transition state, we used TS confirmation tool, which began by approximating the Intrinsic Reaction Path (IRP) with QST and then performed subsequent refinements.

In the beginning, we compared our results of the formation of Ti-OOH with some experimental and computational data to correct the method and model. As a result, a novel active center was proposed, where TS-1 had Ti(¦Ç1-OOH) defect active center with a ligand of NH₃. The transition states of all these reaction pathways were explored. A whole catalytic cycle was determined. In the imine mechanism, NH₃ and cyclohexanone reacted firstly to form an intermediate complex C₆H₁₀-OH-NH₂ that was transformed to imine with a hydrogen transfer from the nitrogen to the oxygen. The formation of imine was rate-determining step in the above mechanism. While in the hydroxylamine mechanism, we compared the energies of different reaction routes in the catalytic and non catalytic reactions of hydroxylamine and cyclohexanone. In the end, Campbell's degree of rate control was utilized to obtain the rate-determining transition states and intermediates.

Keywords:

degree of rate control£»cyclohexanone ammoximation£»rate-determining states£»DFT

*Corresponding author. E-mail: xinf@tju.edu.cn

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