Theoretical Calculation Studies On The Coordination Effect Of The Solvent Molecules With Framework Ti(?)

Ti-MWW zeolite has unique pore structure and shows superior catalytic properties.It has been used for the selective oxidations of different organic compounds with hydroperoxide as oxidant.Investigation of the structure of active center and the electronic structural properties may provide theoretical instruction for improving the catalytic performance.Density functional theory(DFT)has been widely applied to the theoretical calculation on zeolite catalysts,which can provide the microscopic information that can not be obtained experimentally.In this work,DFT calculations have been performed to investigate the coordination of framework Ti center in Ti-MWW zeolite with different solvent molecules and gust molecules.The effects of ten different functionals on the calculated results are systematically investigated.On the basis,the influence of the adsorption of solvents and gust molecues on the geometric strucutes and electronic structural proeperties of framework Ti center have been studied.The main results are as follows:1.DFT calculations were performed to study the interaction of Ti-MWW zeolite with H2O molecule.Ten different exchange-correlation functionals were selected,and their cauculation results have been compared.The functionals are B3LYP,B3PW91,PBE,BP86,M06-L,M06,M06-2X,M06-HF and DFT-D(B3LYP-D,B97-D)including dispersion correction.After geometry optimization,the adsorption energy was calculated,and corrected with BSSE.The geometric structures of the adsorption complexes were examined.The results indicated that DFT-D(B3LYP-D,B97-D)functionals with dispersion correction improve the evaluation of the weak interaction.The functionals of M06-2X and M06-HF overestimate the intermolecular weak interaction,resulting in the calculated adsorption energies higher than that by other methods.The B3LYP-D and M06-L functionals were proved to be appropriate methods.2.DFT was used to calculate the interaction of five different kinds of solvent molecules with the framework titanium center.M06-L functional was proved to be appropriate method with dispersion correction than B3LYP functional.It was found that the Ti species at the T3 site adsorbed the solvent molecules more strongly,since the dispertion effect between the Ti species and solvent molecules are stronger,which have more contribution to the adsorption energies,although there is steric hindrance which can lead to larger molecules difficult to approach.C4H9OH molecule has strong dispersion interaction with the framework atoms surrounding the Ti center,CH3CN molecule is a weak electron donor.The dispersion effect of solvent molecules with the framework atoms of zeolite and the electron donor ability together determines the coordination between the Ti species and the ligand molecules.The stronger the dispersion between the solvent molecules and the framework atoms,as well as the stronger the nucleophilicity of the solvent molecules lead to the stronger complexing adsorption..3.DFT was used to calculate the interaction of piperidine molecule with the framework Ti in the interlayer of Ti-MWW zeolite.Because the basic piperidine molecule can interact with the Lewis acid Ti centre,a strong acid-base adsorption complex can be produced.The Ti species with open structure contains Ti-O-Si-OH group that the electron induced effect of the hydroxyl group enhances the electropositive of Ti center,resulting in the intermolecular interaction to be improved.