The High Activity Of Pd Catalyst Used In Glyoxal Selective Oxidation Of Glyoxylic Acid Was Generated

Glyoxylic acid is an important intermediate for organic synthesis and fine chemical production, such as vanillin, ethyl vanillin, and allantoin etc. It has also been widely used in the perfume, pharmaceutical, paper, leather, dyes, plastics, pesticides, paints and food additives, biochemistry etc. Methods of industrial production of glyoxylic acid mainly include nitric acid or ozone oxidation of glyoxal, oxalic acid reduction, hydrogenation of maleic anhydride. However, the glyoxal nitric acid oxidation process usually causes severe environmental pollution. The discovery and application of efficient catalysts will promote the air oxidation of glyoxal to glyoxylic acid. Air oxidation of glyoxal by using oxygen in the air as the oxidant, the metal as a catalyst could overcome the traditional shortcomings of preparation including difficult separation. Meanwhile, the reaction could be carried out under mild reaction conditions. More importantly, the environmental pollution could be greatly decreased. Obviously, such a production route is more suitable for the industrial applications.This paper focuses on improving the activity, the selectivity to glyoxylic acid and the durability of the catalysts in liquid phase oxidation of glyoxal under air flow by using different additives, supports and depositing methods. Moreover, the correlation of the catalystic performances to the catalyst composition and/or structure has also been investigated.1. Catalyst preparation(1) B-doped uniformly dispersed Pd/C catalyst was prepared by impregnation of active carbon support with PdCl2 under ultrasonication, followed by reduction with sodium borohydride at room temperature.(2) P-doped uniformly dispersed Pd/C catalyst was prepared by impregnation of active carbon support with PdCl2 under ultrasonication, followed by reduction with sodium hypophosphite at 90 oC.(3) Pd/C catalysts with metal-dopants were prepared by co-impregnation of active carbon support with PdCl2 and the other metallic salt, followed by reduction with sodium hypophosphite at 90 oC.(4) P-doped Pd/CNTs catalyst was prepared by impregnation of CNTs with PdCl2 under ultrasonication, followed by reduction with sodium hypophosphite at 90 oC.2. Activity test and examination of the structure-efficiency relationshipLiquid phase oxidation of glyoxal was carried out in a self-designed glass reactor with controllable temperature, catalyst amount, glyoxal concentration, air flow rate, pH value, stirring speed, and the rate to add sodium hydroxide. The product analysis was performed on a high performance liquid chromatography for determining the glyoxal conversion and the selectivity to glyoxylic acid. According to the catalytic activity data, and the detailed characterizations using TEM, XRD, XPS and BET etc., the following conclusions could be drawn.(1) The P-doped Pd/C catalyst obtained by reduction with sodium hypophosphite exhibited higher activity and selectivity than those obtained by reduction with hydrogen, sodium borohydride and formaldehyde reduction, which could be mainly attributed to the increased dispersion of Pd active sites.(2) The Bi-Pd/C catalyst obtained by reduction with sodium hypophosphite displayed higher activity and selectivity than the undoped Pd/C catalyst, which could be mainly ascribed to the enhanced dispersion of Pd active sites. Meanwhile, the interaction between Pd active sites and Bi(III) dopants was also beneficial for substrate adsorption.(3) Pd/CNT catalyst showed higher selectivity to glyoxylic acid than the Pd/C catalyst, possibly owing to unique carbon nanotube structure, which might effectively inhibit the deep oxidation to produce oxalic acid and formic acid.