Theoretical Study:Selective Hydrogenation Of ?,?-unsaturated Aldehydes Catalyzed By Pt Based Metals

?,?-unsaturated alcohols?COL?is an important chemical raw materials.It is widely used in many fields such as flavors and medicines.The?,?-unsaturated aldehydes?CAL?have been attracted attention because they are relatively cheap and easy to obtain,and the catalytic hydrogenation conforms to the green chemical concept.However,the products of the catalytic hydrogenation of CAL are complicated.When Pt is used as a catalyst in the experiment,it shows good selectivity for COL by controlling the particle size of the catalyst,the type of additive and supporter,etc.Therefore,the Dmol³ module of Materials Studio software was used to investigate,the adsorption with different configurations and incomplete hydrogenation mechanism catalyzed by Pt and its alloys,based on the density functional theory?DFT?.The main research contents and conclusions are as follows:We investigated the adsorption and the mechanism for selective hydrogenation of cinnamaldehyde?CAL?on Pt?111?surface and Pt₁₄ cluster.The results illustrate that the synergistic adsorption of CAL molecule with C=O and C=C bonds is most stable at the hcp position on Pt?111?surface,and the adsorption energy is 92.42 kJ/mol.Whereas most stable adsorption of CAL appears on the Pt₁₄ cluster with C=C bond,the adsorption energy is 135.84 kJ/mol.The adsorption of CAL on the Pt₁₄ cluster is stronger than that on Pt?111?surface.The charge distribution of the adsorbed molecules and the density distribution of the p-orbital electron states in the stable adsorption configuration show that during the adsorption process part of the electrons transfer from the cinnamaldehyde to the catalyst,and the main reason for the adsorption is the interaction of the p-orbital of the cinnamaldehyde molecule with the metal d orbital.The values of reaction barriers show that CAL preferentially hydrogenate C=O on the Pt?111?surface and Pt₁₄ cluster,forming COL,the hydrogenation of O atom takes the priority.Both Pt surface and cluster have good selectivity for hydrogenation of CAL to COL.On the Pt?111?surface,the first step of hydrogenation is the controlled rate reaction with an activation energy of 65.02 kJ/mol.However,on the Pt₁₄ cluster,the controlled rate of hydrogenation is the second step with an activation energy of 86.63 kJ/mol,higher than the Pt?111?plane.Therefore,the catalytic activity of Pt for the adsorption and hydrogenation of CAL is closely related to its structure.The adsorption of CAL by Pt clusters is stronger,while Pt?111?is more favorable for the formation of COL.NiPtPt?111?and PtNiPt?111?planes were constructed to simulate Ni-Pt alloys with Ni as the terminal metal and Pt as the terminal metal.The mechanism of adsorption and incomplete hydrogenation of CAL on on Ni-Pt?111?surface of two different structures were studied and compared with pure metal Pt?111?.NiPtPt?111?has a strong positive charge on the surface and has the strongest adsorption effect on CAL.The reaction energy barrier show that CAL has a high energy barrier for the hydrogenation of O₁ on the NiPtPt?111?surface,and preferentially hydrogenate the C₄ atom to produce the MS₄intermediate,and then hydrogenate C₃ and O₁ to produce phenylpropanal?HCAL?and phenylpropenol?ENOL?.CAL is relatively easy to hydrogenate C=O to obtain COL by mechanism R12 on PtNiPt?111?.It is consistent.with the optimal hydrogenation mechanism on Pt?111?surface The activation energy required for each elementary reaction is lower than that of Pt?111?surface.The doping of Ni changes the catalytic performance for hydrogenating CAL.The Ni-Pt alloy with Pt atom as the terminal metal is more advantageous for the formation of COL.The adsorption behavior of crotonaldehyde and cinnamaldehyde with PtNiPt?111?surface and the activation energy of each elementary reaction in incomplete hydrogenation were investigated.The results show that both molecules are synergistically adsorbed on the active sites of PtNiPt?111?surface with C=O and C=C bonds.Among them,the optimal adsorption energy of crotonaldehyde is 62.32 kJ/mol,and the optimal adsorption energy of cinnamaldehyde is 99.85 kJ/mol.The charge distribution analysis revealed that part of the electrons of CAL molecules were transferred to the PtNiPt?111?surface after adsorption.The p-electron of the large?bond in the benzene ring makes the adsorption of cinnamaldehyde with the metal surface stronger.The energy barrier of the two hydrogenation pathways of both molecules C=C,C=O and 1,4-hydrogenation show that the dominant pathways are all mechanisms R12,R43 and R14,and the reaction energy barrier of mechanism R12lowest.The lowest energy barrier indicates that PtNiPt?111?has better selectivity for unsaturated alcohols.That is to say,PtNiPt?111?has better selectivity for hydrogenating CAL to produce COL.