OXIDATION OF METHYL AND N-OCTYL ALPHA-D-GLUCOPYRANOSIDE OVER GRAPHITE-SUPPORTED PLATINUM CATALYSTS - EFFECT OF THE ALKYL SUBSTITUENT ON ACTIVITY AND SELECTIVITY

The oxidation of methyl and n-octyl alpha-D-glucopyranoside to methyl and n-octyl alpha-D-glucopyranosiduronate with molecular oxygen over a graphite-supported platinum catalyst was investigated. An increase of the length of the n-alkyl substituent from methyl to n-octyl. resulte d in a ten-fold decrease of the catalyst activity and an increase of t he selectivity at pH 8.0 and 323 K. The selectivity decreased with inc reasing pH. The lower activity for a longer n-alkyl substituent is att ributed to steric effects upon adsorption on the platinum surface and not to internal diffusion limitations. A tentative reaction scheme is presented, which describes the formation of side products through oxid ation of secondary hydroxyl groups, ring cleavage and hydrolysis. Majo r side products are mono-and di-carboxylates with 2, 4, and 6 carbon a toms and mono-carboxylates, resulting from the oxidation of the alkyl substituent. C-C-Bond cleavage mainly occurs between C-2 and C-3 or C- 4 and C-5, the former being less important for a longer alkyl substitu ent. The higher selectivity for a longer alkyl substituent is attribut ed to its protecting ability against hydrolysis and the exposition of neighboring hydroxyl groups to the platinum surface. (C) 1997 Elsevier Science Ltd.