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 studied using molecular oxygen and neutral aqueous solution. The performance of Pt/alumina and Pt/C catalysts could be improved by addition of small quantities of tetraalkylammonium hydroxides, trialkyl-, triaryl-, and cycloaliphatic amines. The optimum modifier:Pt-s molar ratio is around 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 pK(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 (bifunctional catalysis). The influence of supported N-bases on the selectivity of Pr is a function of their chemical structure. The best results, 95% selectivity at 30% conversion and about 40% selectivity enhancement in the whole investigated conversion range, was obtained with Pt/C acid hexamethylenetetramine. Electrochemical model studies revealed that hexamethylenetetramine is adsorbed on Pt and not oxidized during reaction. Molecular modeling suggests that the preferential oxidation at C-1 is due to complex formation by ii-bonding between hexamethylenetetramine 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 oxidative dehydrogenation. (C) 1996 Academic Press, Inc.