Theoretical Study On Catalytic Reduction Of NO_x By Transition Metal-substituted Keggin-type Polyoxometalates

Polyoxometalates are rich and typical family of metal-oxygen clusters,which made up of early transition metal connected by oxygen bridge.This compounds have wide application prospects in quite diverse fields such as catalysis,medicine and material sciences because of a series of excellent properties including thermal stability,acidity,redox,etc.As an advanced research method,quantum chemical calculation have significant functions to guide and predict the success of experiment synthesis and material preparation in field of POMs chemistry.Because of the limitation of computational resources in early stage,the theoretical study of POMs were majorly focused on the various static properties such as electronic configurations,redox,catalytic activity and optics,and rare study aimed at the dynamic theoretical level of reactions.In recent years,with the rapid development of computation technology,the theoretical research on the catalytic dynamic process and reaction mechanism have been widely concerned in the field of POMs chemistry.In this thesis,quantum chemistry calculations have been carried out to investigate the reaction mechanism of NO_x reduction catalyzed by mono-transition-metal substituted Keggin–type polyoxometalates.The present work has been focus on the following two aspects:?1?Reaction mechanism corresponding to reduction of NO to N₂O catalyzed by mono–transition–metal–substituted Keggin–type polyoxometalates?POMs?has been studied by using Density Functional Theory?DFT?method.The results reveal that,Compared with Fe,Co,Zn-substituted POM complexes,a Mn-substituted POM complex possesses good feature for activation of NO molecule because of considerable absorption energy and significant charge transfer from metal center to NO molecule.And the effective interaction between NO ligand and the Mn center mainly comes from an overlap of the?^*orbital of the NO molecule with d_xzz and d_z22 orbital of Mn center.Three possible reaction pathways for reduction of NO to N₂O catalyzed by Mn-substituted POM complex have been considered based on a dimer mechanism.The calculated free energy profile indicates that the reaction pathway undergoing a cis-?NO?₂conformation is the favorable routes because of a low free energy barrier of 4.24 kcal mol^-1.?2?DFT calculations were employed to explore possible reaction mechanism about the catalytic cycle of epoxidation of alkenes by N₂O catalyzed by Mn–substituted polyoxometalates.The results show that in the presence of N₂O pressure,the formation of the active catalytic species[PW₁₁O₃₉Mn^V=O]^4-involves a ligand–substituted reaction about replacement of the aqua ligand with N₂O to generation of POM/N₂O adduct[PW₁₁O₃₉Mn^IIION₂]^4-and dissociation of N₂from this adduct.The calculated free energy indicates that the ligand–substituted reaction is endergonic both in gas phase or various solvents.The partial optimization method reveals that the dissociation of N₂ from[PW₁₁O₃₉Mn^IIIO–N₂]^4-involves crossing of the quintet state with a low-lying triplet state.Due to the high reactivity,the high–valent Mn^V–oxo species,[PW₁₁O₃₉Mn^V=O]^4-,may react with the excess N₂O and alkenes.Thus,two alternative reaction pathways corresponding to activation of N₂O and epoxidation of alkenes have been considered in this work.The calculated free energy profile indicates that epoxidation of alkenes pathway is the favorable routes.Finally,a complete catalytic cycle for this reaction has been proposed.The rate–determining step in this catalytic cycle is the dissociation of N₂ from the low–valent POM/N₂O adduct according to our DFT–M06L calculations.