DIRECT OXIDATION OF L-SORBOSE TO 2-KETO-L-GULONIC ACID WITH MOLECULAR-OXYGEN ON PLATINUM-BASED AND PALLADIUM-BASED CATALYSTS

The selective oxidation of the C1 hydroxyl group of L-sorbose to a car boxylic group without protection of the four other hydroxyl functions was investigated. The reactions were performed in slightly alkaline aq ueous solutions with molecular oxygen over various alumina- and carbon -supported Pt and Pd catalysts. Optimum reaction conditions were 50 de grees C, pH 7.3, and a catalyst:reactant ratio of 1:4 (wt/wt). The low er the pH and the temperature, the higher the selectivity toward 2-ket o-L-gulonic acid. Catalyst deactivation was also found to increase wit h lower pH and temperature. A 5 wt% Pt/alumina catalyst showed the bes t catalytic performance (67% selectivity at 58% conversion). Promotion with Pi or Pb had a detrimental effect on selectivity for 2-keto-L-gu lonic acid. Electrochemical measurements indicated that the reaction o ccurs in a rather narrow potential range, which corresponds to a moder ate oxygen coverage of Pt or Pd. Four types of catalyst deactivation p rocesses were identified, based on XPS and ICP-AES analysis and on the in situ determination of the oxidation state by monitoring the cataly st potential during reaction. A significant chemical poisoning of the active noble metal sites occurred during the initial, destructive adso rption of L-sorbose and during the oxidation reaction. The successive contamination of active sites resulted in overoxidation (too high oxyg en coverage of Pt or Pd). The partially oxidized promoters and noble m etals were corroded and dissolved in the presence of 2-keto-L-gulonic acid, resulting in an irreversible deactivation.(C) 1994 Academic Pres s, Inc.