Theoretical Study On Hydrodeoxygenation Of Phenolic Compounds Over Fe-Pd Bimetallic Catalysts

In this thesis,different types of Fe-Pd bimetallic catalyst surface models were constructed,and phenol,o-cresol and guaiacol were selected as three phenolic model compounds in this theroretical study.The adsorption,activation and hydrodeoxygenation（HDO）reaction mechanism of these phenolic reactants over Fe-based catalysts were systematically investigated by density functional theory（DFT）calculations,based on which the rate-limiting steps and key elementary reactions in HDO reactions were identified.The effects of Pd doping,surface structure,surface coverage of H^*species,and surface H₂O^*species on the performance of HDO reactions were studied.Through the calculation of reaction mechanism and kinetic barriers,key factors that affect the product selectivity and promote the cleavage of C_Ar-O bond to produce aromatic products were revealed.The types and nature of substituents of different phenolic reactants on the adsorption configuration,adsorption stability and reaction property were further discussed.The calculation results showed that on the monometallic Fe and Fe-Pd bimetallic surfaces,the horizontal adsorption of the three phenolic reactants through the interaction of the benzene ring with catalyst surface is much stronger than the vertical adsorption through substituent groups.Compared to Fe（110）and Fe（111）surfaces,the stepped Fe（211）surface is more conducive to the direct dehydroxylation followed by hydrogenation of phenol to produce benzene.The introduction of Pd has little effect on the adsorption of phenol,but Pd addition can modify the surface structure and electronic properties of Fe catalyst,which may influence the performance of HDO reactions.Fe（211）and Pd-doped Fe（211）are potential promising catalysts for the selective formation of aromatic products from the HDO of phenol.For o-cresol,the energy barriers of direct dehydroxylation and hydrogenation on Fe（211）are lower than those of demethylation and hydrogenation,leading to the facilitation of toluene production.Doping Pd further reduces the energy barrier of direct dehydroxylation reaction of o-cresol and increases the reaction rate of toluene formation by about 1.6×10³times.Guaiacol is prone to the demethylation reaction on Fe（211）to produce catechol,and the energy barriers are higher through demethoxylation to produce phenol and dehydroxylation to produce anisole.When Pd is introduced on Fe（211）surface,the energy barriers associated with demethylation,demethoxylation and dehydroxylation of guaiacol are all increased.However,the product selectivity of the hydrodeoxygenation reaction to catechol is still higher than that of phenol and anisole.Doping Pd on the Fe（211）surface can promote the activation and dissociation of H₂molecules,resulting in an increase of the coverage of surface active H^*species,reducing the adsorption enthalpies of key reactive species,and ultimately increasing the rate of HDO,which is consistent with the experimental results.Surface OH^*and H₂O^*species can be generated during HDO reactions of phenolic compounds,and they may influence the reaction performance through hydrogen transfer or hydrogen bonding interaction.The calculation results reveal that the OH^*and H₂O^*species on the Fe（211）surface influence the reaction by hydrogen bonding,which reduces the energy barriers of the direct dehydroxylation of phenol and the direct demethoxylation and demethylation of guaiacol,respectively.On the surface of Fe（211）substituted by Pd,when H₂O^*species exists and participates the reaction in the form of hydrogen transfer,it can further promote the hydrodeoxygenation of o-cresol to produce the target product toluene,and the conversion of guaiacol to catechol and phenol products.This thesis also investigated the effect of relative distance and position between Pd and phenolic substituent group on the fisrt elementary step of C_Ar-O bond cleavage（rate-limiting step）to form aromatic products.Differential charge density calculation and electron transfer analysis showed that the relative position of Pd has a certain influence on the electron transfer between the benzene ring of phenolic reactant and catalyst surface as well as the types of substituent groups that mainly interact with the catalyst surface,thereby affecting the interaction strength between the adsorbed species and catalyst surface and the stability of transition states,which in turn influence the kinetics of HDO reaction.