Oxidation Of Glyoxal To Glyoxylic Acid By Oxygen Over V₂O₅/C Catalyst

Vanadium oxide catalyst supported on active carbon was prepared by incipient wetness impregnation method, and the precursor was obtained from oxalic acid aqueous solutions of NH4VO3. This catalyst was applied for the liquid phase oxidation of glyoxal to glyoxylic acid. The results showed that V₂O₅/C catalyst exhibited obvious activity for glyoxal oxidation and high selectivity of glyoxylic acid. Glyoxylic acid can be obtained without pH regulation during the reaction, and glyoxylic acid can not be oxidized further into oxalic acid over V₂O₅/C catalyst by oxygen. Formic acid and glyoxylic acid were the main oxidations of glyoxal.Absorbance ratio derivative method was developed for the simultaneous determination of glyoxal and glyoxylic acid. It was a new analytical means for speedy and simultaneous determination in the process of glyoxal catalytic oxidation. Glyoxylic acid and glyoxal were not disturbed by formic acid. Linearity for the determination of glyoxylic acid and glyoxal were found in the ranges of 1.47～7.33mg/L and 1.68～8.53mg/L respectively and their regression equations were also calculated as follows: D=0.01631ρ-0.0135, r=0.9956, n=5(for glyoxylic acid), and D=-0.5374ρ+0.0104, r=1, n=5(for glyoxal).The catalytic performance of V₂O₅/C catalyst was largely affected by V₂O₅ content and calcination temperature. The results of BET, XRD, and TEM showed that the conversion of glyoxal and yield of glyoxylic acid increased with the increase of V₂O₅ contents (w(V₂O₅)≤3wt%), meanwhile V₂O₅ particles were well dispersed on the active carbon. The conversion of glyoxal and yield of glyoxylic acid decreased when V₂O₅ content >3wt%, and the main reason was that V₂O₅ particles gathered on the surface of active carbon and the active surface area of the catalyst decreased. With increasing calcinations temperature, the crystallite of V₂O₅ increased with the increase of calcinations temperature. The optimal V₂O₅ content was 3wt% and calcinations temperature was 573K, the conversion of glyoxal was 25.82% and the yield of glyoxylic acid was 10.28% after 10h reaction. The conversion of glyoxal and yield of glyoxylic acid were reduced by 14.91% and 6.46% respectively after V₂O₅/C catalyst ran three times. The XRD spectra of fresh and used catalysts showed that V₂O₅ losses was responsible for deactivation of catalyst.Thermodynamic analysis for the reaction system showed that the oxidation of glyoxal to glyoxylic acid was exothermal and could carry through spontaneously. The mechanistic oxidation of glyoxal on V₂O₅/C was proposed, and the kinetic equation was developed by the model of Langmuir-Hinshelwood, surface chemical reaction could be the controlling step in the oxidation of glyoxal into glyoxylic acid. The reaction activation energy is 27.02kJ/mol, and the heat of glyoxal and glyoxylic acid adsorptions were -11.85kJ/mol and 17.88kJ/mol respectively.