Selective Oxidation of Glyoxal to Glyoxalic Acid by Air over Mesoporous Silica Supported Pd Catalysts

A series of mesoporous silica (KIT-6, MCM-41 and SBA-15) supported Pd catalysts were successfully synthesized and applied for selective oxidation of glyoxal. All of these catalysts exhibited significantly higher activity than commercial Pd/C. Among them, Pd/KIT-6 exhibited the best activity and selectivity with 41.3% glyoxal conversion and 57.0% selectivity to glyoxalic acid. The better performance of Pd/KIT-6 was attributed to its three-dimensional mesoporous structure. The three-dimensional mesoporous structure of KIT-6 could enhance Pd dispersion, providing sufficient accessible active sites which improved the conversion of glyoxal. Meanwhile, the better mass transfer capability of Pd/KIT-6 allowed glyoxalic acid to leave the catalyst easily, reducing the probability of over-oxidation. The ratio of kI (rate constant of initial oxidation reaction) to kII (rate constant of over-oxidation) was compared among three catalysts. The kI/kII of Pd/KIT-6 (0.50) was higher than that of Pd/MCM-41 (0.39) and Pd/SBA-15 (0.34), which reflected its best selectivity from kinetic aspect.