SELECTIVE OXIDATION OF METHYL ALPHA-D-GLUCOPYRANOSIDE WITH OXYGEN OVER SUPPORTED PLATINUM - KINETIC MODELING IN THE PRESENCE OF DEACTIVATION BY OVEROXIDATION OF THE CATALYST

A kinetic model is presented, which describes the platinum-catalyzed s elective oxidation of-methyl alpha-D-glucopyranoside to sodium methyl alpha-D-glucuronate with molecular oxygen in the presence of deactivat ion 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 t he rapid establishment of the steady-state degree of coverage by the r eaction intermediates at the platinum surface and the much slower reve rsible process of overoxidation. This clear distinction is reflected i n 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 met hyl alpha-D-glucopyranoside. The corresponding standard activation ent ropy and enthalpy amount to respectively -111 +/- 12 J mol(-1) K-1 and 51 +/- 4 kJ mol(-1). The standard reaction entropy for the transforma tion of oxygen atoms into subsurface oxygen amounts to -35 +/- 16 J mo l(-1) K-1 and the standard reaction enthalpy to -36 +/- 15 kJ mol(-1).