(147a) 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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