Theoretical Study On The Structure Of Active Site In Tetravalent Metal Incorporated MTW And Platinum Modified MOR Zeolites

Zeolite is a common type of porous materials,which are widely applied in catalysis due to their adjustable acidity.There is no acid site in pure silica zeolites,which is required to modify and introduce active sites that could be used as the solid acid catalyst for actual conditions.The main principle of modifying zeolite is introducing/tuning active centers,in which two most common modified methods in experiment are heteroatom incorporated in zeolite framework and active metal loaded in zeolitic channel.After modification,it is difficult to detect variably local structure and information of chemical bond by traditional experimental characterizations.Therefore,it is hard to relate the catalytical activity with corresponding structural properties.To clarify the‘structure-activity relationship’in zeolitic catalysts,theoretical calculations could well explain the experimental appearances at the atomic and molecular levels,accurately predict the framework structure and reaction mechanism.On the basis of the periodic density functional theory with D3 dispersion correction,we systematically construct MTW and MOR zeolitic models,then accurately predict the distribution of tetravalent metal heteroatoms in zeolitic framework and the evolution of platinum clusters with different sizes loaded in the zeolitic channel.Finally,the inherent laws among the different zeolitic properties,e.g.,acidity,energy,electronic property,and geometric structure.etc.,are explored.（1）Constructing the periodic MTW-type zeolite model,the thermodynamics of heteroatom（Ti,Zr,Ge,and Sn）incorporating zeolitic framework is simulated at the molecular level.The calculated substitution energy is defined to reflect the thermodynamic stability in incorporating process.It indicates that Ti and Zr are more easily incorporated into MTW than Ge and Sn,in which the most stable substitution energies of four elements correlate with their covalent radii（R²=0.95）,implying that the thermodynamics of incorporation could be designed by covalent radii.Incorporations of tetravalent metal result in the variation of cell volume.It is interesting that there is a good correlation between substitution energy and cell volume,indicating that the thermodynamics stability of incorporation could be judged by the cell volume.The Lewis acidity is measured by adsorption of ammonia probe molecule,as follows:Sn-MTW>Zr-MTW>Ti-MTW>Ge-MTW.This finding provides the basis of theoretical Lewis acidity and agrees with previous experiments.In addition,the relationship between structural deformation and substation energy is explored.It is found that relative mean square deviation of[MO₄]is related to substitution energy of Ti-,Ge-,and Sn-MTW;root-mean-square error of[MSi₄]is related to substitution energy of Zr-MTW.This section provides the theoretical basis for the incorporation of zeolitic framework,and builds the correction between incorporation of zeolitic framework and covalent radius,cell volume,Lewis acidity,and local structure.（2）Constructing the periodic H-type and dehydrated MOR zeolite models,based on that,we consider the distribution of platinum cluster with different size in MOR,as well as corresponding evolution in hydrogen and ammonia atmospheres.Firstly,we investigate the platinum clusters in gas phase systematically.Ab initio thermodynamics revealed that,with the help of ammonia,Pt ammonia complexes exist in the formation of Pt₄（NH₃）₄ with small size,and atomically dispersed Pt（NH₃）₂ species take the second place.However,platinum species will form hydride in hydrogen atmosphere,in which hydrogen serves as the stabilized hydrogen bridge inserted in Pt-Pt bond.H₂₈Pt₇ species with the largest size is stabilized when temperature is less than 665 K.The origin of the difference between hydrogen and ammonia atmospheres is the bonding properties:The covalent bonding between Pt and H promotes the stability of hydride in larger size,while Pt and NH₃ in small-sized platinum ammonia complex has the strong interaction of coordination bonding.Then we predict the distribution of Pt clusters in MOR zeolites,and obtain the structural information,e.g.,acidity,electronic structure,and cell volume,etc.On the basis of that,the evolution of Pt species in high hydrogen and ammonia coverage were investigated further.Aggregation reaction and corresponding change of Gibbs free energy are designed and calculated,respectively.We predict that Pt monomer could easily aggregate into larger size in hydrogen atmosphere,vs.aggregation is unfavorable in ammonia atmosphere.Furthermore,ab initio thermodynamics of Pt cluster within MOR main channel demonstrated ammonia atmosphere promoted the dispersion into small size less than four atoms,while the largest H₂₈Pt₇ was still stabilized in hydrogen atmosphere.Finally,multiple ammonia molecules were stepwise desorbed to simulate ammonia temperature programmed desorption（NH₃-TPD）test,in which the actual Lewis acidity is related with the size of Pt clusters and corresponding locations.This section reveals the influential mechanism of atmosphere on cluster evolution in zeolites at the molecular level.