Selective oxidation of methyl alpha-D-glucoside on carbon supported platinum II. Assessment of the Arrhenius and Langmuir parameters

In order to estimate the Arrhenius and Langmuir parameters of the selective oxidation of methyl (Yn-glucoside on a platinum catalyst, the temperature was varied from 293 to 333 K. The initial reaction kinetics can be described adequately over this temperature range by a Langmuir-Hinshelwood rate equation which is based on two parallel reaction paths: one involving adsorbed methyl cr-D-glucoside and dominating at low pH, the other involving the adsorbed methyl cr-n-glucoside anion and dominating at high pH. Both reaction paths contain a rate-determining step consisting of a surface reaction involving the corresponding methyl cr-n-glucoside species and chemisorbed oxygen. The apparent activation energies amount to 50.5 f 6.4 kJ mol-’ and 110.5 t 10.6 kJ mol-‘. The assessment of the Arrhenius parameter estimates within the framework of the transition state theory showed that these rate-determining steps lead to physically meaningful estimates. Langmuir adsorption coefficients account for the reversible dissociative chemisorption of oxygen and for the reversible associative adsorption of methyl o!-n-glucoside and methyl Lu-n-glucoside anion. A standard adsorption enthalpy for oxygen of 60.0 + 10.5 kJ mol-‘, for methyl a-D-glucoside of 17.6? 8.3 kJ mol-’ and for the methyl ol-n-glucoside anion of 61 f 54 kJ mol-’ were estimated. During the oxidation at a pH of eight oxygen adatoms are the most abundant surface species, followed by adsorbed methyl cy-D-glucoside. With increasing temperature the degree of oxygen coverage decreases from 0.8 to 0.6, while the degree of coverage with methyl a-Dglucoside increases from 0.08 to 0.13. At a pH of 10 the surface coverage with oxygen decreases from 0.6 at 293 K to 0.5 at 333 K and the surface coverage with methyl cy-D-glucoside anion from 0.5 to 0.3.