Preparation Of Promoted Palladium Nanoparticle Catalyst And Its Application For Benzyl Alcohol Partial Oxidation

Partial oxidation of alcohols is an important way to prepare high value-added industrial products,and is an indispensable step in the production of many fine chemical products,such as the partial oxidation of benzyl alcohol to benzaldehyde.Benzaldehyde is the simplest aromatic aldehyde,and it is also an important organic chemical basic raw material.The products are used in many fields including food,medicine,cosmetics,coatings and dyes.The process of partial oxidation of benzyl alcohol to benzaldehyde uses air,molecular oxygen,hydrogen peroxide,etc.as oxidants,and low-load noble metal or non-precious metal nanoparticles as catalysts.Because of its high product purity and green production process,it has attracted more and more researchers’attention.The preliminary work have shown that the supported Pd-based catalysts show good catalytic activity for the partial oxidation of benzyl alcohol,but the product selectivity needs to be further improved.In this thesis,Pd/MCF and Au-Pd/MCF catalysts were prepared by a one-step synthesis method and modified by the introduction of promoter metal/metal oxide through the impregnation-high temperature treatment process.The effects of different metals/metal oxides on the surface aggregation state and chemical properties of Pd/Au-Pd nanoparticles were investigated,and the reaction results were correlated to investigate their influence on the performance of partial oxidation reactions.Using different promoters（Mo,Ni,V,Au,In and Sn）to modify Pd/MCF to prepare M-Pd/MCF catalyst.The characterization results show that Mo,Ni,V and Pd cannot form a good alloy structure,while Au,In,and Sn can form a good alloy structure with Pd.As the degree of alloying increases,the number of Pd nanoparticles decreases significantly,but exists in the structure of M-Pd alloy and the surface Pd⁰ species increase.The reaction evaluation results show that the relationship between the partial oxidation reaction activity of benzyl alcohol and the alloy of M-Pd conforms to the volcano type principle.With the increase in the degree of alloying between Pd and the promoter metal M,the reaction activity showed a trend that first increased and then decreased.The selectivity of benzaldehyde decreased with the increase of the degree of alloying.In summary,Au-Pd and In-Pd can form a certain degree of alloy structure,which can promote the dispersion of Pd nanoparticles,improve the electronic effect on the surface Pd atoms,and improve the reaction activity through structural effects and electronic effects;while Sn-Pd is easier to form a stable alloy structure,resulting in a decrease in reactivity;Mo,Ni,and V additives that are not easy to form an alloy structure mainly act as structural additives to cover up the strong oxidation active sites,and when the reaction activity is slightly reduced,the selectivity of the product benzaldehyde is improved.Regarding the increase in the degree of alloying,the selectivity of the product benzaldehyde is reduced.This can be explained by the high desorption energy of benzaldehyde on the surface.The benzaldehyde cannot be effectively desorbed and the oxidation reaction continues,resulting in low selectivity.The Au-Pd/MCF alloy catalyst showed high benzyl alcohol reaction activity,but the benzaldehyde selectivity was not high,while the Ni modified Pd/MCF catalyst showed better benzaldehyde selectivity.Therefore,the promoter metal Ni is used to modify the bimetallic Au-Pd/MCF catalyst and applied to the oxidation of benzyl alcohol.Compared with the bimetallic gold palladium catalyst,Ni@Au-Pd/MCF greatly improves the selective of benzaldehyde.Under the condition that the reaction activity is not affected,the selectivity of the main product benzaldehyde can reach 82%.Characterization results show that after reduction at 300°C,Au-Pd on the surface exists in the form of alloy structure,while the promoter Ni exists in the state of Ni OOH and Ni O;the addition of Ni modulates the aggregation state of surface Pd species,transforming from highly dispersed nanoclusters to nanoparticles;confirming that Ni exists in Ni O and Ni OOH,and there is no electron transfer between Ni and active metal Au and Pd,indicating that Ni mainly acts as a structural promoter.Ni OOH/Pd₃Au（111）,Ni O/Pd₃Au（111）planes are beneficial to the adsorption and activation of benzyl alcohol and oxygen,and the desorption energy of the main product benzaldehyde is smaller than that of Pd₃Au（111）planes on the bimetallic catalyst.The product is easy to desorb,so the selectivity of benzaldehyde on the Ni modified catalyst is greatly improved.