Theoretical Calculations On Oxidation Of Dibenzothiophene Derivatives Over Active Hydrogen Peroxide Species In Ti-MOR Zeolite

Ti-MOR?modenite?zeolites have transparent straight 12-MR channels.The mesoporous and hierarchical Ti-MOR possess open channel structures which is effective in catalyzing the larger sulfur compounds,showing high catalytic activity and selectivity.In this work,density functional theory?DFT?calculations were performed to investigate the Ti?IV?species in zeolite framework,the structure of Ti-peroxo active intermediate,and to study the oxidative desulfurization?ODS?mechanism of dibenzothiophene derivatives over Ti-MOR zeolite.The results are as follows:1.The location and structure of the Ti?IV?species at four different T?tetrahedral?sites in Ti-MOR zeolite were studied.The cluster models including 7 shells of framework atoms around the central T atom were used.For different structures the geometric optimization and frequency calculation have been carried out.By analysis of the Ti/Si substitution energies and geometric parameters,together with the comparison between th acalclulated IR vibrational frequencies and the experimental IR spectra,it was confirmed that the framework Ti special vibration peak at 963cm^-1 can be ascribed to the[Ti?OSi?₄]species at the T6 site in Ti-MOR zeolite.The framework Ti special peak of[Ti?OSi?₃OH]species,the hydrolyzed product shifts to higher wave number.2.The 200T cluster models around the T1 and T6 sites including the whole channel were constructed.The ONIOM method was used to study the structures of Ti-peroxo active intermediates in Ti-MOR.In the presence of H₂O and H₂O₂,four possible Ti-peroxo active intermediates were constructed,including Ti-?²?OOH?,Ti-?¹?OOH?,Ti-?²?OOH?-H₂O and Ti-?¹?OOH?-H₂O at the T1 and T6 sites.According to the calculated formation energies and exploration of the local stereospecific blockade,it is concluded that the six-coordinating Ti6-?²?OOH?-H₂O species may be the most possible Ti-peroxo active intermediate.3.Using Ti6-?²?OOH?-H₂O as the active center,the mechanisms of ODS for DBT,4-MDBT,and 4,6-DMDBT were studied.The result indicated that oxidation activity are as4,6-DMDBT>4-MDBT>DBT.The reaction is a kind of autocatalytic process.Sulfoxide compounds,the first step oxidation product are more likely to take place in further oxidation to form sulfone compounds.4.The effects of solvent molecules participating in the oxidation were investigated by the coordination with active centers.By taking Ti6-?²?OOH?-H₂O,Ti6-?²?OOH?-CH₃OH,Ti6-?²?OOH?-CH₃CN and Ti6-?²?OOH?as active centers,the ODS's solvents of dibenzothiophene was studied.The results indicated that the catalytic activities of different active centers are as Ti6-?²?OOH?>Ti6-?²?OOH?-H₂O>Ti6-?²?OOH?-CH₃OH>Ti6-?²?OOH?-CH₃CN?The lower the?*?O_?-O_??orbital energy level of the Ti-peroxo active center is,the higher the catalytic activity will be.