Molecular Simulation Of Transesterification Catalyzed By Basic Single Atom Catalyst

As a substitute for non-renewable energy,biodiesel has good lubricity,flammability and degradability.However,the existing homogeneous catalysts used in industrial biodiesel preparation are difficult to be separated from the reaction solution,which requires a lot of cost to purify the product oil.Compared with homogeneous catalyst,heterogeneous catalyst is easy to separate and has higher catalytic efficiency and reusability.However,its specific surface area is small,and the catalyst cannot contact with the reaction solution internally.It is difficult to give full play to the effect in the reaction.Therefore,the single-atom catalyst with high surface free energy and high seed utilization rate is introduced into the preparation process of biodiesel.The single-atom catalyst combine the advantages of homogeneous and heterogeneous catalysts to further improve the activity of the catalyst.Alkaline catalysts have high activity and high conversion efficiency in transesterification.Therefore,alkaline metal atoms are selected as active centers to construct alkaline monatomic catalysts.In this paper,single-atom catalysts model with γ-Al2O3,graphene and FAU zeolite molecular sieve as the support are established.Based on density functional theory and molecular dynamics calculation,the reaction mechanism of different catalysts in transesterification process is studied,and the doping energy released during the loading process of metal atoms is calculated.The most stable configuration of the catalyst is obtained,and the adsorption of reactant molecules on the catalyst is discussed by analyzing the properties of adsorption energy,Mulliken charge layout and state density.(1)The effect of γ-A12O3(110)doped by Sr and Zn atoms on the adsorption of methanol molecules is studied.The surface models of γ-Al2O3(110)doped by Sr single atom,Zn single atom and Sr-Zn double atoms are established.The most stable surface is obtained by calculating the doping energy.The configuration of the catalyst surface for adsorption of methanol is optimized and the corresponding adsorption energy is calculated.The results show that Sr atom tended to replace Al3c-1 coordination aluminum,and methanol adsorption at this site is the most stable.Zn atom tend to replace Al3c-1,but methanol is more stable at Al4c-2 top site.When Sr and Zn atoms doping γ-Al2O3(110)surface simultaneously,Sr/Al4c-2 and Zn/Al4c-4 is the most stable adsorption sites.In this case,the adsorption energy of methanol on the catalyst surface is the highest,which is-305.60 kJ/mol.The number of electrons obtained by oxygen atom on methanol increases.Hydrogen atom on the hydroxyl group bond better with the catalyst surface.(2)A single-atom catalyst model is established with Ca,Sr and Zn as the active atoms and defective graphene as the carrier.The adsorption energies of methanol on different catalysts are studied by density functional theory,and the influence of actual operating parameters such as temperature and methanol to oil molar ratio on the adsorption system is studied by molecular dynamics.The results show that the doping of nitrogen and oxygen atoms and the addition of defects effectively promote the adsorption of methanol on the catalyst surface.The adsorption energy of oxygen doping is better than that of nitrogen doping,and the adsorption energy of the optimal structure is-232.62 kJ/mol.When the temperature is 338 K and the methanol to oil molar ratio is 18:1,the interaction energy of the adsorption system is 297.60 Kcal/mol.(3)The optimal configuration and adsorption mechanism of methanol and methyl formate doped with Na,K,Ca,Sr and Zn atoms are studied using FAU zeolite as the carrier.The results show that Zn atom has the best modification effect,and Na atom has the worst modification effect.Methyl formate and methanol molecules have the highest adsorption energy on Zn/X catalyst,which maximum values are-169.16 kJ/mol and-188.12 kJ/mol,respectively.However,the addition of Zn atom leads to serious deformation of zeolite X structure and break its stability.The activation effect of Ca/X catalyst on reactant molecules is second only to that of Zn/X,but its stability is better and the leaching rate of active atoms is lower.During the adsorption of methanol,charge transfer occurs between H atom and O atom on the hydroxyl group.Electrons on methanol are transferred to the catalyst,and H atom interacts with O atom on the catalyst.During the adsorption of methyl formate,O atom on the carbonyl group and active metal atom have overlapping peaks at the same energy,and there is a bonding trend,indicating that the catalyst has a good adsorption effect on methanol and methyl formate.