The Theoretical Study On Synthesis Of Acetic Acid And Methyl Formate From Methane And Carbon Dioxide On Ni Doped MgO(100)

The synthesis of acetic acid from methane and carbon dioxide is known as atomic economic reaction,which can provide a feasible solution to the current environmental degradation under Greenhouse Effect and produce certain economic value.In this paper,density functional theory(DFT)is used to explore the reaction mechanism of catalytic synthesis of acetic acid and methyl formate from methane and carbon dioxide on Ni-doped MgO(100)surface by Materials Studio.The results show that the reaction process is mainly divided into the adsorption and activation of reactants and the subsequent product formation.The adsorption activation stage mainly includes the adsorption of reactants and the dehydrogenation of methane.The CH3*,H*and CO2*produced after activation react along different paths to produce acetic acid or methyl formate.The contents of the study are as follows:1.The adsorption of reactants on Ni4-MgO(100)surface and the dehydrogenation of methane are studied.The adsorption structure of methane and carbon dioxide and the optimal dehydrogenation activation path of methane were determined.There are many stable adsorption sites of carbon dioxide on the surface of Ni4-MgO(100),including the undoped part of MgO(100),and the edge and center of Ni4.And the only stable adsorption site of methane is the center of Ni4.The energy barriers of the four H gradual dissociation processes on methane are 3.20,1.98,1.64 and 1.42 eV respectively when only methane is adsorbed on the Ni4-MgO(100)surface.While in the case of co-adsorption of carbon dioxide and methane on Ni4,the energy barriers of the four H gradual dissociation processes on methane are 2.45,3.02,0.45 and 1.24 eV respectively,and the latter is more favorable for methane to lose the first H.There is adsorption competition between methane and carbon dioxide at the center of Ni4.After the adsorption of methane at the center of Ni4,add sufficient carbon dioxide to complete the adsorption of the two molecules at their respective active sites,which can make it easier for methane to lose the first H and prevent the CH3*from losing more H to form CH3*,CO2*,H*.In this process,the energy barrier is 2.45 eV,and the apparent activation energy is 1.91 eV,and the heat absorption is 0.43 eV.2.Four reaction pathways for the formation of acetic acid from CH3*,H*and CO2*produced by the activation of methane and carbon dioxide on Ni4-MgO are determined,and the structure and energy changes of each stable state and transition state in the reaction process are described.There are many ways to combine CH3*,H*and CO2*on Ni4-MgO(100),leading to the formation of acetic acid molecules in four ways:a.CH3*and CO2*are coupled,and the energy barrier of this process is 1.35 eV.The formed Bi-CH3COO*is adsorbed on Ni4,and then protonated with H*,forming acetic acid adsorbed on Ni4-MgO(100).In this process,the energy barrier of 0.86eV is overcome;b.CH3*and CO2*are coupled on Ni4 to form Mono-CH3COO*,the energy barrier of this process is 1.40 eV.And then Mono-CH3COO*forms acetic acid adsorbed on Ni4-MgO(100)with H*,the energy barrier of this process is 0.65 eV;c.H*and CO2*form cis-COOH*,the energy barrier of this process is 2.89 eV,and then binds with CH3*to form adsorbed acetic acid with energy barrier of 2.63 eV;d.H*and CO2*form trans-COOH*,the energy barrier of this process is 1.77 eV,and then binds with CH3*to form adsorbed acetic acid,with energy barrier of 2.17 eV.Finally,the adsorbed acetic acid molecule absorbs 1.69 eV energy to complete desorption and becomes a gaseous product.Among the four pathways,path a is favorable,and the rate-determining step of the whole reaction process is methane dehydrogenation,the apparent activation energy is 1.91 eV,and the reaction absorbs 0.13 eV heat.3.Four reaction pathways for the formation of methyl formate from CH3*,H*and CO2*on Ni4-MgO(100)are determined,the structure and energy changes of each stable state and transition state in the reaction process are described.The four paths to produce methyl formate include:a.CH3*and CO2*form cis-CH3O(O)C*with an energy barrier of 1.41 eV,cis-CH3O(O)C*combines with H*to form cis-CH3O(O)CH*with an energy barrier of 1.53 eV,cis-CH3O(O)CH*absorbs 0.95 eV energy to desorb to form a gaseous product;b.CH3*and CO2*form trans-CH3O(O)C*with an energy barrier of 1.41 eV,trans-CH3O(O)C*combines with H*to form trans-CH3O(O)CH*with an energy barrier of 1.53 eV,trans-CH3O(O)CH*absorbs 0.95 eV energy to desorb to form a gaseous product;c.H*and CO2*form HO(O)C*with an energy barrier of 2.76 eV,HO(O)C*combined with CH3*to form cis-CH3O(O)CH*with an energy barrier of 2.34 eV,cis-CH3O(O)CH*absorbis 1.22 eV and desorbing to form a gaseous product;d.H*and CO2*form HO(O)C*with an energy barrier of 2.76 eV,HO(O)C*combined with CH3*to form trans-CH3O(O)CH*with an energy barrier of 1.77 eV,trans-CH3O(O)CH*absorbis 1.82 eV and desorbing to form a gaseous product.Among the four pathways,path a is favorable,and the rate-determining step of the whole reaction process is methane dehydrogenation,the apparent activation energy is 1.91 eV,and the reaction absorbs 1.05 eV heat.