DFT Study On The Adsorption Behavior Of Substrate In Heterogeneous Reductive Amination And Hydrogenation Reactions

Density functional theory calculations have been widely used in the catalysis field.Through calculation,not only can the existing experimental data and phenomena be verified and explained,but also the experimental results can be predicted.In the heterogeneous metal catalytic reactions,the selection of the metal components and the support in a metal catalyst has important influence on the final reaction efficiency,and the difference of the metal components can result in a remarkable difference of the final yield.As we all know,the adsorption and activation of the reaction substrate is an important step in the heterogeneous catalytic reaction process.In this thesis,in order to explore the influence of subsrate's adsorption behavior on reaction activity of the same reaction catalyzed by different metal catalyst,the reductive amination of 5-hydroxymethylfurfural and furfuryl alcohol and the hydrogenation of aromatic-ring-containing diisooctyl phthalate were selected as the template reactions to research the effects of metal components and supports on the reaction activity of these two reactions,respectively.Then,the adsorption behavior of the reactants on the metals and the supports in the reaction process was simulated by density functional theory calculations to further study their impact on the catalytic activity.Reductive amination of 5-hydroxymethylfurfural and furfuryl alcohol were catalyzed by different catalysts including Raney Co,Raney Ni,Pd/C,Ru/C and PtJC catalysts.For furfuryl alcohol,only the primary amine was generated over Raney Co and Raney Ni catalysts with the yields of 0.99%and 11.0%,respectively.After optimization,the yield of the furfuryl amine over Raney Ni was ca 70.8%at 180 ? in 60 h.Although 5-hydroxymethylfurfural was completely converted and the yields of 2,5-furandimethylamine were 14.7%and 56.9%on Raney Co and Raney Ni,respectively,the yields of 2,5-furandimethylamine catalyzed by other catalysts were not satisfactory.The order of metal catalysts for reductive amination of 5-hydroxymethylfurfural is:Ru<Pt<Pd<Co<Ni.The adsorption energies of ammonia and hydrogen atoms on the surface of slab of different metals were calculated through density functional theory,and the difference of adsorption energies between ammonia and hydrogen atoms was calculated.The maximum difference of the adsorption energies was 0.7720 eV on Pt(111),while the smallest one was 0.2938 eV on Ni(111)with a descending order of:Pt>Rh>Pd>Ru>Co>Ni.It can be seen that the smaller the difference between the adsorption energies of the two species on the metal surface,the better the metal catalysts favors the reaction.Ni/Al2O3,Ni/TiO2 and Ni/SiO2 catalysts were prepared by the incipient wetness impregnation method for reductive amination of 5-hydroxymethylfurfural.The highest yield of 2,5-furandimethylamine(21.5%)was achieved over Ni/Al2O3 on the same reaction conditions.When the amount of catalyst increases from 0.20 g to 0.8 g,the yield can increase from 21.5%to 70.4%at 160 ? in 12 h.The effect of the supports on 2,5-furandimethylamine yield has a descending order of Ni/Al2O3>Ni/TiO2>Ni/SiO2.The surfaces of lattice strurcture of Al2O3,SiO2 and TiO2 was therefore constructed according to the experiments and literature for DFT calcualtions on their adsorption energies with 5-hydroxymethylfurfural.The results showed that the larger the adsorption energy of 5-hydroxymethylfurfural molecules on the surface of the support(Al2O3>TiO2>SiO2),the better the reductive amination activity of the corresponding metal catalyst.The yield of 1,2-cyclohexanedicarboxylate from diisooctyl over fresh Pd/C was 98.5%.Unfortunately,a very low yield of 33.5%was obtained at the second cycle of the Pd/C catalyst,indicating Pd/C deactived very quilkly during such hydrogenation reaction.The thermogravimetric analysis showed that the catalyst was deactivated due to the adsorption of some unknown substances.Further experiments by changing the metal and support showed that the unknown substances were adsorbed on metal other than support.It was confirmed that Pd deactivation was due to the adsorption of a substance on the metal surface.The adsorption energies of phthalic acid molecules on the Pd and Ni surfaces at different angles were calculated by density functional theory.The results show that the adsorption energy of phthalic acid on the Ni metal surface is bigger than that on the Pd surface,so the Pd/C inactivation is not caused by the adsorption of phthalic acid or dioctyl phthalate on the Pd surface.