Study On The Structure-activity Relationship Of Direct Oxidative Esterification Of Methacrolein Catalyzed By Pd And Au

Carboxylic acid ester plays significant roles in fine organic chemical raw materials,in which methyl methacrylate（MMA）is a critical intermediate for the synthesis of plexiglass.The traditional production process is the oxidation of olefin aldehyde into enoic acid and then reacts with alcohol to produce MMA,however,the process still suffers from the disadvantages of low product selectivity,long process and high cost.In this regard,direct oxidative esterification of alcohols and aldehydes is a new green and environment-friendly process for MMA preparation,in which the preparation of catalysts with high catalytic performance is the key,Pd-and Au-based catalysts have attracted wide attention,however,there are still some ambiguous problems,such as the understanding of surface active sites and surface structure,the synergistic effect between active components and promoter components,and the interaction between active components and support,which extremely limit the continuous improvement of catalysts for direct oxidative esterification of aldehydes and alcohols.In this paper,density functional theory（DFT）and microkinetic analysis are applied to study the direct oxidative esterification of methacrolein（MAL）to form MMA,the structure-activity relationship between Pd-and Au-based catalysts modulated by promoters（second metal and supports）and catalytic performance（stability and catalytic activity）is revealed.Using DFT method,twelve kinds of crystal facet structure of Pd catalysts modulated by promoters Pb and Bi models are constructed,taking the activation mechanism of the product H₂O as the probe reaction,the（111）crystal facet of trimetallic Pd-Pb-Bi catalysts with dimer Pd active units and d-band center far from Fermi level can promote H₂O activation and inhibit H₂O aggregation,resulting in strong ability of anti-deactivation and high stability;Through the Mulliken charge and d-band center analysis of the twelve kinds of crystal facets,a method for screening the minimum active units of different crystal facets is proposed:the minimum active unit is determined according to the average charge of the crystal facet;An important parameter for judging the stability of crystal facet structure is obtained:d-band center.The reaction mechanism of direct oxidative esterification of MAL on the selected highly stable trimetallic Pd-Pb-Bi catalysts is studied,the favorable path for MMA formation via MAL+（CH₃O）→CH₂C（CH₃）CO+（H）+CH₃O→MMA+（H）is determined;Through the microkinetic analysis,it is clarified that the presence of Pb and Bi in trimetallic Pd-Pb-Bi catalysts not only promot the formation of MMA,but also inhibit the formation of by-products;The order of catalytic performance from strong to weak is Pd-Pb-Bi>Pd-Pb>Pd.By introducing the active component of Pd into Au catalysts,the bimetallic Pd-Au catalysts with different ratios models are constructed,taking H₂O and CH₃OH activation as well as MMA formation as the probe reaction,it is clear that the second metal Pd modulated Au can greatly improve the catalytic performance,the microscopic reason is that the second metal Pd modulation can regulate the reduction of the surface average charge and the d-band center near the Fermi level of Au catalysts;It is found that the Pd₄Au（111）catalyst has good catalytic performance;Pd₁Au₂₀/γ-Al₂O₃ catalyst is prepared by homogeneous deposition precipitation method,the MAL conversion is 99.9%and the selectivity is 99.7%,the experimental results verify the accuracy of theoretical calculation results.Pd_n（Au_n）（n=1-4）,Pd₃Au₁,Pd₂Au₂ and Pd₁Au₃ supported onγ-Al₂O₃（110）models are constructed,the strong interaction and high stability betweenγ-Al₂O₃（110）and Pd₄,Au₄,Pd₃Au₁e,Pd₂Au₂b and Pd₁Au₃e are selected;Taking MMA formation as the probe reaction,the order of catalytic performance from strong to weak is Pd₂Au₂e/γ-Al₂O₃（110）>Pd₄/γ-Al₂O₃（110）>Pd₁Au₃b/γ-Al₂O₃（110）>Au₄/γ-Al₂O₃（110）>Pd₃Au₁a/γ-Al₂O₃（110）;It is clear that the microcosmic reason for the great improvement of the catalytic performance of Pd₂Au₂e/γ-Al₂O₃（110）catalyst is that the charge of Pd₂Au₂e is moderate,and the d orbital of Pd（Au）interacts strongly with the s,p orbitals of Al and O inγ-Al₂O₃.Pd_n（Au_n）（n=1-3）and Pd₁Au₁ supported on perfect graphene（GR）,single defect graphene（SGR）,double defect graphene（DGR）and graphyne（GDY）models are constructed,the strong interaction of GR,SGR,DGR and GDY with single atom Pd₁（Au₁）and bimetallic Pd₁Au₁ is determined;The diffusion of H₂O is studied by GCMC and MD method,and MMA formation was studied by DFT method,it is clear that the bimetallic Pd₁Au₁ supported on carbon material has better stability and catalytic activity than monometallic Pd₁ or Au₁;Pd₁Au₁/GDY catalyst is selected to has the best catalytic performance.The idea of regulating the catalytic performance of the supported catalysts is proposed to adjust the moderate charge of metal cluster and have a strong interaction with the support.