The Synthesis And Catalytic Performance Of Au/GO Catalysts In Selective Oxidation Of Alcohols

The selective oxidation of alcohols is one of the major chemical reactions for producing carbonyl compounds, which play an important role in the synthesis of fine chemicals and intermediates. Recently, the selective oxidation of alcohols by molecular oxygen has received considerable attention because it is an environmentally friendly reaction route. Specially, supported gold catalysts have been widely applied since they exhibit higher selectivity and stability for the oxidation of primary alcohol. The catalytic performance of supported gold catalysts depends largely on the size and shape of the Au nanoparticles and the nature of the supports. Compared to other supports such as metal oxides, carbon materials have many special physical and chemical properties(such as high stability in acidic/basic media), which makes them good candidates for use as supports for Au nanoparticles in the application of selective aerobic oxidation of alcohols.In the present study, graphene oxide (GO), synthesized by pressurized oxidation, was used as supports for gold nanoparticles. And various Au/GO catalysts were prepared by deposited Au nanoparticles on the different thermally- and chemically-treated graphene oxide supports using sol-immobilization method. The surface chemistry and structural properties of GO supports and Au/GO catalysts were characterized by a series of analytical techniques including powder X-ray diffraction patterns (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and Raman spectroscopy. Meanwhile, the catalytic performance of Au/GO catalysts were examined and compared in the liquid phase aerobic oxidation of benzyl alcohol (BA). The main results are as follows:(1) Freshly prepared GO materials were treated at different temperatures from 200 "C to 700 ℃ and then the modified GO were used as catalyst supports. Based on TEM, XRD and XPS results, it shows that all the supported Au nanoparticles are in the metallic form with a similar size distribution and average size (～4.1nm). After thermal treatment, all the supports exhibit a similar crystalline structure as indicated by Raman results. However, both TPD and XPS results clearly show that the concentration and type of surface oxygen-containing groups change largely with varying pre-treatment temperatures, which leads to different catalytic activities of Au/GO catalysts in the liquid phase aerobic oxidation of benzyl alcohol. Furthermore, all studied Au/GO catalysts have a high selectivity to benzaldehyde (> 98%). When increasing pre-treatment temperature from 200 ℃ to 400 ℃, the amount of surface carboxylic acid decreases significantly while the relative concentrations of other oxygen functional groups are very similar. Accordingly, the BA conversion decreases slightly from 73% for Au/GO200 to 69% for Au/GO400, suggesting no significant effect of surface carboxylic acid on BA conversion. For Au/GO500 catalyst, it exhibits a much lower concentration of anhydrides groups and the BA conversion increases to 77%. For Au/GO700 catalyst, the surface amount of lactone, phenol and epoxy groups decreases largely with a BA conversion of 64%. The above results indicate that the presence of the surface anhydrides groups may have an adverse effect on the catalytic activity while the existence of surface lactone, phenol and epoxy groups has a promotion effect.(2) GO200 was used as starting material and was treated with NaOH, HNO3 and sodium borohydride (NaBEH4). Then Au nanoparticles were deposited on the chemically modified GO supports by sol-immobilization method, denoted as Au/GO200-NaOH, Au/GO200-HNO3 and Au/RGO (NaBH4) respectively. For Au/GO catalysts in the selective oxidation of benzyl alcohol, both NaOH-and HNO3-treatment of GO supports decrease BA conversion significantly while reduction by NaBH4 increases BA conversion (90%). Moreover, no obvious influence was observed on benzaldehyde selectivity for all the chemically-treated Au/GO catalysts (above 98%). The activity and Raman results show that Au/GO200-NaOH has a similar BA conversion (65%) and similar ID/IG ratio with Au/GO700. Meanwhile, XPS results indicated that the amount of oxygen functional groups on Au/GO700 is only about 56% of that on Au/GO200-NaOH. Furthermore, Au/GO200-HNO3 has a highest ID/IG ratio but shows a lowest BA conversion (56%). The above results suggest that, compared to the amount and type of surface oxygen functional groups, the ordered structure of GO supports may play a more important role in determining the catalytic performance of Au/GO catalysts.