A Theoretical Study On Vinyl Acetate Synthesis From Gas-phase Ethylene Acetoxylation Catalyzed By Pd-based Catalysts

As an important chemical raw material,vinyl acetate(VA)plays a significant role in the national economy.VA synthesis from gas-phase ethylene acetoxylation guadually becomes the predominant technology for VA production.Pd-based catalysts are adopted by most of the industrial production equipment of VA synthesis from gas-phase ethylene acetoxylation and its catalytic mechanism has attracted the wide attention of many researchers.At present,just a few investigations have been carried out on Pd(100)and the efforts made on Pd/Au(100)are even scarcer.Besides,no consensus has been reached on which mechanism is dominant on Pd(100)and Pd/Au(100)which are said to be of the highest activity for VA synthesis.In this work,both Density Functional Theory(DFT)calculations and kinetic Monte Carlo(kMC)simulation were employed to study the electronic properties and the mechanism of VA synthesis from gas-phase ethylene acetoxylation on Pd(100)and Pd/Au(100)to clarify the improvement mechanism of Au to Pd-based catalysts and elucidate the reaction mechanism of VA synthesis from gas-phase ethylene acetoxylation on Pd(100)and Pd/Au(100).With DFT method,the electronic properties of pure Pd(100)and Pd/Au(100)with diagonal Pd atoms on the surface were investigated.The results show that after Pd alloys with Au,the electrons transfer from Pd atoms to Au atoms;meanwhile,the electrons of d band of Pd atoms become more localized,which improves the reactivity of Pd.With DFT method,the adsorption and co-adsorption of the species involved in VA synthesis from gas-phase ethylene acetoxylation on Pd(100)and Pd/Au(100)were examined systematically to identify the most stable adsorption configuration of relevant species.After the comparison of the adsorption and coadsorption on Pd(100)and Pd/Au(100),it is found that the addition of Au impairs the adsorption of species on the Pd metal surface.With DFT method,the reaction mechanism of VA synthesis from gas-phase ethylene acetoxylation was studied.It was found that both surface Os and OHs can consistently reduce the activation energy of ethylene dehydrogenation,acetic acid dehydrogenation and hydrogenated vinyl acetate(VAH)dehydrogenation on Pd(100)to some extent with only one exception that OH somehow increases the activation energy of VAH dehydrogenation.The surface Os and OHs kinetically facilitate ethylene dehydrogenation,acetic acid dehydrogenation and VAH dehydrogenation on Pd/Au(100).The rate-limiting step of the Samanos pathway on Pd(100)surface and Pd/Au(100)surface is the VAH dehydrogenation.The rate-limiting step of the Moiseev pathway on Pd(100)surface is the coupling of vinyl with acetate,while that on Pd/Au(100)is the ethylene dehydrogenation.Comparison of the activation energies shows that Moiseev mechanism is preferred on Pd/Au(100).However,which mechanism is preferred on Pd(100)can't be determined solely by the activation energies.Through kMC simulation,it was found that VA synthesis from gas-phase ethylene acetoxylation proceeds through Moiseev mechanism both on Pd(100)and Pd/Au(100).The addition of Au into Pd can prohibit ethylene dehydrogenation while it can promote acetic acid dehydrogenation,which eventually is beneficial for VA synthesis as a whole.The addition of Au into Pd can further promote the conversion and selectivity of VA synthesis from gas-phase ethylene acetoxylation.When the reaction network is analyzed,besides the energetics of each elementary reaction,the surface coverage of each species and the occupancy of the surface sites on the catalyst should be taken into consideration as well.This work can add to the understanding of the improvement mechanism of Au to Pd-based catalysts used in VA synthesis from gas-phase ethylene acetoxylation and provide significant theoretical guidance for further understanding the reaction mechanism of VA synthesis from gas-phase ethylene acetoxylation.