Preparation And Application Of Au-Pd Bimetallic Catalyst For Benzyl Alcohol Oxidation

As a multipurpose chemical intermediate,benzaldehyde has a wide range of applications in pharmaceutical engineering,dye engineering,perfume and fine chemical production.Selective oxidation of benzyl alcohol to benzaldehyde is valued in industry using catalytic oxidation technique.Meanwhile,because of its simple reaction mechanism and controllable by-products,the reaction is often studied as a model reaction in the laboratory.However,the current stage of research work related to benzaldehyde production from benzyl alcohol requires the addition of inorganic bases as co-catalysts to facilitate the key steps of the reaction.However,the alkaline environment tends to lead to deep oxidation of the product to form a large amount of by-product benzoic acid,which causes difficulties in subsequent separation,corrodes the equipment and reduces the economic efficiency.Therefore,it is of great practical importance to design a green and sustainable multiphase catalyst to achieve high selectivity of benzaldehyde in an alkali-free environment.Exploring the conformational relationship of the catalyst in this process is of great practical significance.In this paper,a series of catalysts consisting of gold–palladium bimetallic nanoparticles anchored on carbon nanotubes were synthesized by colloidal method and investigated for the base-free,selective aerobic conversion of benzyl alcohol to benzyl aldehyde under mild conditions.The structure and morphology of Au-Pd nanoparticles with different molar ratio were characterized by transmission electron microscopy,energy dispersive X-ray spectroscopy and X-ray diffraction to analyze the surface chemistry,structure and properties of the catalysts.The results of benzyl alcohol reaction shows that bimetallic catalyst shows much higher activity than monometallic Au or Pd counterparts,indicating the synergistic effect generated by the Au_xPd_y/CNT catalysts.In summary,all catalysts were prepared successfully to synthesize Au-Pd bimetallic alloy particles.When the similar ratio of the two metals was obtained（Au/Pd from 1.5/1 to 1/1.5）,an Au-Pd@Pd O structure with Au-Pd alloy nanoparticles as the core and Pd O nanofragments as the surface was formed.Since the molar of Pd excess,because of insufficient Au to restrain the surface Pd,the excessive Pd overflowed the lattice and was partialy oxidized to Pd O,which was the main reason to reduce the catalytic activity.The Au Pd/CNT resulted in the highest catalytic effect of benzyl alcohol oxidation with 91%for conversion and 92%for selectivity 91%(80°C,5 h,n _{Bn OH}/n_（Au+Pd）=187).The analysis results indicate that the metal structure and the electronic properties of the nanocrystals could be adjusted by controlling the Au/Pd ratios,consequently engineering the catalytic properties.The Au Pd/CNT catalyst shows excellent selective oxidation performance as well as good catalyst stability.Subsequently,the partial deactivation of the catalysts and the decrease in the activity of the Pd-rich catalysts after the reaction were investigated.The reason was the Pd O coverage on the surface of bimetallic nanoparticles to hinder the active site.Then the series catalysts were subjected to different atmospheric heat-treatments to study the microscopic modulation of the nanoparticles and improve the problem of lower catalytic activity.The result shows that the thermal atmosphere of hydrogen has a much higher catalytic activity than the two-step heat treatment（calcination with oxygen followed by hydrogen reduction）.Reaction results show that the conversion of Au Pd₂/CNT increases from 27%for oxygen calcination to 68%for hydrogen reduction,while the two-step heat treatment only increases to 50%.That might be due to the deactivation of the large particle size caused by multiple high temperature treatments.The conclusion shows that the heat-treatment with hydrogen can micro-regulate the structure and chemical composition of the nanoparticles.This moderation could also improve the catalytic performance and recovery the deactivated catalysts.This process provides an idea for the subsequent research in the preparation of highly active catalysts using trace metals.The achievement has the prospect of high efficiency and greening in the selective oxidation of benzyl alcohol.