Theoretical Investivation On The Active Centers And Olefin Epoxidation In Titanosilicate/H₂O₂/H₂O System

Epoxides are an important organic synthesis intermediates,resulting in an important role in organic synthesis for the epoxidation reactions.Various types of titanosilicate/H₂O₂/H₂O zeolite systems have been applied as catalysts for the epoxidation reaction of olefins,and exhibited a pivotal role due to its environmentally friendly characteristics such as mild reaction conditions,high atomic utilization,and by-products such as water.The catalytic reaction mechanism and the structures of the active intermediate have also attracted researchers' attention.In this paper,we address on the characterization of the active Ti-hydroperoxo intermediates in Ti-containing zeolites TS-1,Ti-Beta,and Ti-YNU-1 by means of the theoretical calculations on the IR-Raman,and UV-Vis spectroscopic features,as well as the activation barriers in olefin epoxidation reaction.The epoxidation reactions of olefins on the active centers were investigated in order to further confirm the active center.In addition,the epoxidation mechanisms were discriminated from the atomic level,electronic structure,and other aspects.The cis/trans-selectivity mechanisms on different zeolites were also investigated.The main results are as follows:1.The formation energies,the optimized geometric parameters,IR-Raman vibrational frequencies,and UV-Vis spectra of the Ti-hydroperoxo intermediates in TS-1,Ti-Beta,and Ti-YNU-1 zeolites have been systematically investigated,and the calculated geometric parameters and spectroscopic properties have been compared with the existing experimental data.The results indicated that the O?–O? stretching vibration frequency is about 837 cm-1 in the IR-Raman spectra for Ti-?²?OOH?and Ti-?²?OOH?-H₂O species,while the transition wavelength is about 385 nm for Ti-?¹?OOH?and Ti-?¹?OOH?-H₂O in the UV-Vis spectra.In summary,we can deduce that as titanosilicate zeolites contact with H₂O₂ aqueous solution,the initial Ti-hydroperoxo intermediates is Ti-?¹?OOH?species,which can take a solvent ligand to form Ti-?¹?OOH?-H₂O,or convert into the more stable Ti-?²?OOH?-H₂O that works as the actual catalytic active center for the olefin epoxidation reactions.2.The epoxidation reactions of propylene over different active centers in TS-1 were selected to elucidate the catalytic performance of Ti-hydroperoxo intermediates.It was found that Ti-?¹?OOH?-H₂O has special structure and large molecular volume and can not attached by propylene molecular due to the spacial hindrance inside the zeolite channel.It should be excluded from the real active center.The Ti-?²?OOH?species at T8 and T10 sites led to the activation barriers of lower than 25 k J/mol for propylene epoxidation,which stability is also much lower than that of Ti-?²?OOH?-H₂O,in other word,it cannot exist in an aqueous solution,and can not be the real active center.At the active center of Ti-?¹?OOH?there are two mechanisms for the propylene epoxidation.One is that the transition state structure is transformed to resemble that for Ti-?²?OOH?-H₂O species with one water molecule desorpted from the five-membered ring and coordinated to the Ti center.In this process,the activation barrier is about 60 k J/mol and similar to that on Ti-?²?OOH?-H₂O active center.In another process,the transition state still retains the the five-membered ring of Ti-?¹?OOH?with an activation barrier up to 100 k J/mol.Comprehensive analysis shows that the Ti-?²?OOH?-H₂O species has moderate stability and catalytic activity,and is proposed as the most reasonable active center for epoxidation reaction.3.The mechanism of the cis-selectivity of cis/trans-2-hexene epoxidation in TS-1 zeolite was investigated.The results indicated that the transition state structures for cis-and trans-2-hexene epoxidation over Ti8-?2?OOH?-H₂O active center are different.For cis-2-hexene molecule,it has to twist its propyl group and adopts a torsion conformation,whereas the trans-2-hexene molecule extends its propyl group into the zigzag channels.There are stronger vd W interactions between the cisconfiguration and the zeolite frameworks due to the confinement effect in the zeolite pore and the channels—this increases the stability of the transition state structure and reduces the activation barrier.Dynamically,cis-2-hexene reacts faster than trans-2-hexene with the cis/trans ratio of the rate constant of 2.0:1,demonstrating the cis-selectivity in the TS-1 zeolite.4.The mechanisms of the 1-hexene epoxidation,solevent effects,and cis/trans-selectivity over different active centers in Ti-YNU-1 zeolite were studied.The NBO calculation analysis shows that the adsorption of CH₃CN results in the atomic charges of O? to be less negative,and also facilitates the desorption of the product.The desorption processes are endothermic with much higher desorption energies than the corresponding activation energies.Therefore,desorption of the products are suggested as the rate-determing step.Moreover,we also studied the epoxidation reaction mechanisms of cis/trans-2-hexene over the Ti-hydroperoxo intermediates in Ti-YNU-1 zeolite.The calculated activivation barriers of epoxidation are cis-2-hexene > trans-2-hexene at T3 site.This is also the first time that the theoretical calculations are used to reveals the mechanism of the trans-selectivity of Ti-YNU-1 acting differently from TS-1.