Theoretical Study Of Ethylene Epoxidation And The Optimal Design Of Ag-based Catalvsts

Ethylene oxide(EO)is an important petrochemical product,which has been widely used in fine chemicals industries.The ethylene epoxidation reaction producing ethylene oxide is easy to over-oxidize to form acetaldehyde,and then completely oxidize to carbon dioxide and water,which greatly reduces the yield of ethylene oxide.Therefore,it is significant to improve the selectivity of ethylene epoxidation and prevent complete oxidation to increase the yield of ethylene oxide,which can greatly improve the economic efficiency.At present,silver,as the active element,is widely used in the ethylene epoxidation,however the selectivity of pure silver catalysts can only achieve 50%.Meanwhile,the reaction mechanism of ethylene epoxidation is not clear,which bring great inhibition to the design of high-performance silver-based catalysts.In this work,the reaction mechanism of silver-based catalysts has been studied by using the quantum chemical calculation method.The effects of crystal surface,promoter and alloying on the EO selectivity and the reaction mechanism of silver-based catalyst have been investigated.The main research contents and innovations are as follows:1.The effect of crystal surface on selectivity to EOThe ethylene epoxidation on different crystal surfaces has been studied by quantum chemical calculation.We study four silver crystal surfaces,including(111),(110),(100)and(211)surfaces.(i)The effects of different crystal surfaces on the adsorption of reactants,intermediates and productsWe study the adsorption of ethylene,atomic and molecular oxygen,OMC(oxametallacycle)intermediates,ethylene oxide and acetaldehyde on four silver surfaces((111),(110),(100)and(211)).The adsorption of ethylene oxide on four surfaces is weak.Thus,ethylene oxide is more likely to desorb from the catalyst surface rather than to isomerize to acetaldehyde.However,the adsorption of oxygen atoms and OMC(oxametallacycle)intermediates on four surfaces is significantly different.(ii)The effect of crystal surfaces on the selectivity of ethylene epoxidationIt is well known that the ethylene epoxidation reaction consists of two steps,including the formation of OMC intermediates and the isomerization of OMC intermediates.After determining the optimal adsorption configurations of reactants,intermediates and products,we calculate the energy barriers of the formation and the isomerization of OMC intermediates to produce ethylene oxide and acetaldehyde.Ag(100)surface is found to be most selective toward ethylene oxide.We also study the adsorption strength of OMC intermediates on different silver crystal surfaces.It is found that the adsorption strength of OMC intermediates and the configuration of transition state in the isomerization process of OMC intermediates can significantly affect the selectivity to EO.(iii)Synergistic effect of silver crystal surface and promoter on selectivityWe systematically study the effect of Re(or Cs,Cu)promoted Ag surfaces and find that the charge redistribution caused by the same promoter on different crystal surfaces is different,which result in a significant change in the adsorption strength of OMC intermediates.Hence,the selectivity to EO is different on promoted Ag surfaces.2.The effect of crystal surfaces on diffusion of oxygen atomThe surface and downward diffusion of oxygen atom on different crystal surfaces are studied by quantum calculation method,which quantify the formation tendency of surface and subsurface oxygen atom on different crystal faces.Thus,further understanding of selectivity to EO on different crystal surfaces can be obtained through exploring the activation effect of different crystal surfaces on oxygen atoms diffusion.(i)Adsorption of surface and subsurface oxygen on different crystal surfacesWe study the adsorption of surface,the first subsurface and second subsurface oxygen atom on Ag(111),(110),(100)and(211)surfaces.Surface and the second subsurface oxygen are stable on four silver surfaces while the first subsurface oxygen only exist on Ag(111)and(110)surfaces.(ii)Diffusion of surface and subsurface oxygen atom on different crystal surfacesWe study the surface diffusion of the surface oxygen,the first subsurface oxygen,the second subsurface oxygen atom,as well as the downward diffusion of surface oxygen and the first subsurface oxygen atom.The surface diffusion of surface oxygen atom is the most facile on Ag(111)surface while the downward diffusion of oxygen atom is promoted on Ag(110)surface.3.The promotion effects of Cl on ethylene epoxidationWe carry out a systematic research on Cl-modified Ag surfaces for ethylene epoxidation by density functional theory.We discover that Cl promoter enhances the selectivity of Ag-catalyzed ethylene epoxidation by activating oxygen species,which can be classified into three aspects:(i)The effect on electron redistribution of oxygen speciesWe study the ethylene epoxidation on Ag surface with Cl and without Cl.The presence of Cl can significantly improve the selectivity of ethylene epoxidation.Cl induces the electron redistribution of surface oxygen atom involved in the reaction,which makes it more electrophilic and accordingly enhance the selectivity to EO.(ii)The effect on oxygen diffusionWe study the surface and downward diffusion of oxygen on Ag catalysts with Cl and without Cl.Cl has a stimulative effect on the surface and downward diffusion of oxygen atom.Introduction of Cl results in the facile formation process of high-oxygen coverage surface and subsurface oxygen atom.(iii)The synergistic effect with subsurface oxygenBy comparing the selectivity to EO on pure Ag surface,Cl-promoted Ag surface,Ag surface with subsurface oxygen,and Cl-promoted Ag surface with subsurface oxygen,we find that Cl can synergize with subsurface oxygen atom to further improve the selectivity to EO.4.The selectivity enhancement mechanism of metal dopants in silver-based alloysThe kinetics of ethylene epoxidation on the surface of gold,silver,copper,palladium,platinum,nickel,iridium,osmium,ruthenium and rhodium models have been studied by quantitative calculation.According to the micro dynamic model of the reactant and the scale relationship between the adsorption energy,the activity contour map was established.Meanwhile,we describe the trend of the adsorption rate and selectivity to EO.GCH3*and GO*are used as selectivity descriptors.Through the analyses of the transition states of in the process of ethylene oxide and acetaldehyde formation through the isomerization of OMC intermediates,the selectivity can be related to the bonding affinity toward oxygen and carbon(EM-O and EM-C)of the reaction site.Adding the extrinsic metal into silver catalyst can effectively modify the descriptor GO*,while GCH3*is almost unaffected.By introducing metal dopants with moderate EM-O,the GO*value can be adjusted to the optimum range,thus improving the selectivity.