The Activity Of Noble Metal (Au, Pd, Pt) Catalysts For The Selective Oxidation Of Glyoxal To Glyoxalic Acid

Glyoxylic acid is the simplest a-oxocarboxylic acid whose chemical properties are very active. Glyoxylic acid is an important intermediate in chemical industry, such as pharmaceutical industries, fine chemical industries and agricultural chemicals. All of the industry production methods of glyoxylic acid because of the serious environmental pollution, high cost and the lower quality of the product, are replaced of the selective oxidation of glyoxal to glyoxalic acid with metal catalysts. At present, the yield and selectivity of glyoxylic acid still need to be improved, and the reaction mechanism is not very clear. In this paper, the performance of catalysts has been systematically investigated for selective oxidation of glyoxal to glyoxalic acid by using O2 as oxidant. The paper focuses on improving the catalytic activity for glyoxalic acid synthesis and analyzing the reasons for different selectivity of Au, Pd, Pt catalysts. The catalytic activity influence of active species, the structure catalysts and support were investigated by XRD, AAS, TEM, UV-Vis, XPS techniques and theory methods.Firstly, a series of Au, Pd, Pt loading on γ-Al2O3 catalysts were prepared by impregnation-reduction method, and the influence of catalyst activity for the oxidation of glyoxal was investigated. The results indicated that 3%Pt/y-Al2O3 catalyst showed optimum catalytic performance, with 45% glyoxal conversion,67% glyoxylic acid selectivity and the 30% glyoxylic acid yield at reaction time of 1h, pH of 7.7 and reaction temperature of 40℃Secondly, all calculations were based on density functional theory as implemented in CASTEP module of Materials Studio 8.0 software. The adsorption of glyoxal on Au(111), Pd(111) and Pt(111) was systematically studied for the first time by means of density functional calculations. The results showed that Pt was an excellent catalyst. Glyoxal was strongly chemical adsorbed on Pt catalyst. When the adsorption energy was maximum, the C-H bond was easier to break, and the C-C bond was stable. Glyoxal was easily to be oxidized into glyoxylic acid. The adsorption energy of glyoxylic acid was lower than glyoxal on the surface of Pt catalyst. Therefore, the degree of oxidation of glyoxylic acid was small, and the selectivity of glyoxylic acid was the best.In the end, Au-Pd, Au-Pt and Au@Pt catalysts loading on different supports were prepared by impregnation-reduction method. The core-shell catalysts were firstly introduced into the selective oxidation of glyoxal to glyoxalic acid. The influence of active species, structure and support on the performance for the oxidation of glyoxal was investigated. The results demonstrated that 1%Au@1%Pt/γ-Al2O3 catalyst showed optimum catalytic performance with 36% glyoxal conversion,83% glyoxylic acid selectivity and the 34% glyoxylic acid yield at the reaction time of 2.5 h, pH of 7.7 and reaction temperature of 40℃, and could be recycled for several times.