A Theoretical Study Of Propylene Epoxidation Over TS-1 Catalyst

TS-1 zeolite exhbits good catalytic performance for selective oxidation.Propylene epoxidation catalyzed by TS-1 in the presence of hydrogen peroxide?H₂O₂?solution is an eco-friendly industrial process,and has been widely studied experimentally in recent decades.In this thesis,we performed density functional theorey?DFT?calculations to systematically investigate reaction mechanisms for TS-1/H₂O₂ catalyzed propylene epoxidation and important factores that impact the epoxidation property.Main conclusions are summarized as follows:1.Tripodal?1I?,Tripodal?2I?and Ti/defect site models have been identified by DFT calculations,and the optimized geometrical parameters are in agreement with EXAFS characterization data.Formation of these three sites are exothermic or mildly endothermic,indicating that they are all likely to be formed inside TS-1.2.Propylene epoxidation can proceed either through stepwise or concerted pathway over Tripodal?1I?,Tripodal?2I?and Ti/defect sites.The stepwise mechanism involves two elementary steps:?1?the dissociation of H₂O₂ to form a Ti-OOH intermediate,and?2?the epoxidation of propylene to produce propylene epoxide.While in the conceted mechanism,H₂O₂ is adsorbed at the Ti center without dissociation and directly reacts with propylene molecule for epoxidization.By comparison of all the pathways occurred over the three active sites,we found that the stepwise mechanism via a unique 5MR-Ti-??¹-OOH?intermediate over the Ti/defect site is energetically the most favorable.The silanol nest and the produced water can generate consecutive hydrogen bond network which largely stabilizes the Ti-OOH intermediate and the transition states,and thus resulting in the significant reduction of the activation barriers.3.144T and 143T layered cluster models containing typical 10MR channels of TS-1 were constructed to model Tripodal?2I?and Ti/defect sites.The confinement effect of TS-1 pore structure on propylene epoxidation was inestigated based on these large cluster models.It was found that the confinement of zeolite could impact the adsorption stability of guest molecules as wells as reaction barriers.B3LYP and?B97XD,two functionals were considered for calculation,and significant differences in adsorption energies and activation barriers were observed,implying that long-range dispersion corrections have important influence on the description of guest molecule adsorption and reaction inside zeolite framework.4.An explicit methanol molecule was included into the 143T layered model,the hydrogen-bonding promotes the enhancement of CH₃OH-H₂O₂ co-adsorption strength but has slight effect on propylene adsorption.The activation barrier of Ti-OOH species formation is decreased owing to the hydrogen transfer among H₂O₂ and Ti-OH species mediated by methanol,whereas the activation barrier of epoxidation step becomes increased due to the repulsion effect from surrounding TS-1 framework atoms.It turns out that the rate-determining step changes from H₂O₂ dissociation to propene epoxidation and the effective barrier has been sigmificantly decreased.By comparion of the results calculated by B3LYP and?B97XD functionals,it was found that the adsorption heats obtained by?B97XD are all higher than B3LYP functional.