Selective Oxidation of Methyl α-d-Glucopyranoside with Oxygen over Supported Platinum: Kinetic Modeling in the Presence of Deactivation by Overoxidation of the Catalyst

Abstract

A kinetic model is presented, which describes the platinum-catalyzed selective oxidation of methyl α-d-glucopyranoside to sodium methyl α-d-glucuronate with molecular oxygen in the presence of deactivation by overoxidation. Overoxidation is completely reversible and most adequately described by a reversible transformation of oxygen adatoms into inactive subsurface oxygen. A clear distinction is made between the rapid establishment of the steady-state degree of coverage by the reaction intermediates at the platinum surface and the much slower reversible process of overoxidation. This clear distinction is reflected in the rate equation, which can be written as the product of an initial rate and a deactivation function. The deactivation function is given as a function of the degree of coverage by inactive subsurface oxygen. The rate-determining step in the selective oxidation consists of the reaction between dissociatively chemisorbed oxygen and physisorbed methyl α-d-glucopyranoside. The corresponding standard activation entropy and enthalpy amount to respectively −111 ± 12 J mol-1 K-1 and 51 ± 4 kJ mol-1. The standard reaction entropy for the transformation of oxygen atoms into subsurface oxygen amounts to −35 ± 16 J mol-1 K-1 and the standard reaction enthalpy to −36 ± 15 kJ mol-1.