Theoretical Study On The Mechanism Of Prussian Blue Analogues Catalyzed Epoxidation Of Styrene

As an important industrial intermediate,Styrene Oxide can be prepared by epoxidation of styrene.This synthesis process is convenient and environment-friendly.Bimetallic Prussian blue Analogues?PBAs,also known as Double Metal Cyanide?is a mixed valence state material with three-dimensional network structure.And Prussian blue Analogues have many unique electrical,magnetic and catalytic properties,especially in the catalysis of styrene epoxidation.However,the mechanism of this reaction on PBAs surface is unclear.Modern computational chemistry that based on density functional theory?DFT?provides an opportunity to study the catalytic mechanism at the molecular level.Therefore,in this paper,density functional theory is applied to research the reaction mechanism of styrene epoxidation under the cluster model and slab model,which is helpful to understand the catalytic nature of PBAs.The main contents include:Through establishing cluster models for different metal centers of iron/cobalt Prussian blue Analogues?FeCoPBAs?and cobalt/iron Prussian blue Analogues?CoFePBAs?,the different styrene epoxidation reaction paths at various metal centers were explored.The different all reaction paths include two stages:the first stage is that oxidant TBHP decomposes into t-butanol and oxygen adsorbed on the metal center;the second stage is that the oxygen adsorbed on the metal center reacts with styrene to produce Styrene Oxide.The first stage is mainly accomplished by transferring the hydrogen on the hydroxyl group of TBHP to the alkoxy group.The hydrogen transfer is in two ways:one is the direct transfer.The other is the indirect transfer via the ligand cyanide?includes path II and path III?.In the latter,the-O and-OtBu generated by the breakage of O-O bond in TBHP will adsorb on the metal center)after the hydrogen on the hydroxyl group transferring to the cyanide,afterwards the hydrogen which bound to the cyanide can be transferred to the alkoxy group immediately?path II?.Furthermore the oxygen generated after breaking O-O bond is directly combined with styrene which is the second stage of reaction,-O^tBu is always adsorbed on the metal center until the Styrene Oxide leaving metal center.The transfer of H from the cyanide to-O^tBu yields tert-butanol?path III?.The transition states and activation energies of three paths are investigated using the different metal centers of FeCoPBAs and CoFePBAs.It was found that the rate control steps are different and may occur in the hydrogen transfer between hydroxy and cyanide,the hydrogen transfer between cyanide and alkoxy group,and the reaction of oxygen and styrene.By comparing the activation energy of the speed control steps of each path,it was found that with the change of the catalytic center,the optimal path of the catalytic process is switched between path I and path II as the change of the catalytic center.In the first stage,it is showed that the activation energy is low when the catalytic site is Fe.In the second stage,the activation energy is low when the catalytic site is the metal that connects to the N-terminus of cyanogen?independent of the metal species?.In order to study the catalytic mechanism in a system which is closer to the real environment,the slab models of?111?,?-111?,?110?and?100?crystal surfaces about CoFePBAs?KFe[Co?CN?₆]?are established.And then,the following five substances in different metal centers of each crystal surface are established.The adsorption models are???.The analysis result shows that:a)The adsorption of O at the metal center on the unconventional?100?crystal surface is an unstable structure.And it cannot catalyze the reaction of O and styrene.b)The adsorption of O on ??? the Fe site of?-111?crystal surface is too strong to further combine with styrene.c)The adsorption strength of O andon the Fe and Co sites of?110?crystal surface and the Co sites of?111?crystal surface are moderate,which is favorable for the reaction,but the Styrene Oxide adsorbed in Co site of?111?crystal surface is a more stable structure,so it is more conducive to product formation.The transition states search of styrene epoxidation is performed on the Co site of?111?crystal surface and the Fe site of?-111?crystal surface.The activation energy of the rate control step was lower when Co is used as a catalytic site.Combined with the adsorption of Fe site and Co site on different crystal surfaces,we believe that the Co site on the?111?crystal surface is the active site for the styrene epoxidation catalyzed by CoFePBAs.