PLATINUM CATALYSTS MODIFIED BY ADSORBED AMINES - A NEW METHOD OF ENHANCING RATE AND SELECTIVITY OF L-SORBOSE OXIDATION

The Pt-catalyzed oxidation at C-1 of unprotected L-sorbose to 2-keto-L -gulonic acid, an intermediate in vitamin C synthesis, has been studie d using molecular oxygen and neutral aqueous solution. The performance of Pt/alumina and Pt/C catalysts could be improved by addition of sma ll quantities of tetraalkylammonium hydroxides, trialkyl-, triaryl-, a nd cycloaliphatic amines. The optimum modifier:Pt-s molar ratio is aro und 0.1, which corresponds to an amine:sorbose molar ratio of 1:1700. Rate enhancement (by a factor of up to 4.6) can be obtained when the p K(a) is around 10 or higher, and this correlation depends very little on the chemical structure of the amine. Rate acceleration is proposed to be connected with the hydration of the intermediate aldehyde (bifun ctional catalysis). The influence of supported N-bases on the selectiv ity of Pr is a function of their chemical structure. The best results, 95% selectivity at 30% conversion and about 40% selectivity enhanceme nt in the whole investigated conversion range, was obtained with Pt/C acid hexamethylenetetramine. Electrochemical model studies revealed th at hexamethylenetetramine is adsorbed on Pt and not oxidized during re action. Molecular modeling suggests that the preferential oxidation at C-1 is due to complex formation by ii-bonding between hexamethylenete tramine and sorbose. The adsorption of this complex on Pt results in a tilted position of the reactant in which only C-1 is exposed to oxida tive dehydrogenation. (C) 1996 Academic Press, Inc.