Selective oxidation of methyl α-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 selective oxidation of-methyl alpha-D-glucopyranoside to sodium methyl alpha-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 alpha-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)

Relaxation processes during the selective oxidation of aqueous ethanol with oxygen on a platinum catalyst

The observed loss of activity at constant conditions during the selective oxidation of ethanol with oxygen in a continuous stirred-tank reactor with carbon-supported platinum can be described by a model considering reversible transformations between three oxidizing species on the catalyst. One of these species is much more reactive toward ethanol and can be considered as a reaction intermediate in the selective oxidation of the latter. The model is also able to simulate the relaxation of the catalyst potential when the reaction is performed with a platinum foil in an electrochemical cell. The loss of activity as well as the relaxation of the catalyst potential can be attributed to changes in the degree of coverage by the two less reactive forms of oxygen. The latter should be considered as an extrinsic relaxation in contrast to the establishment of the steady-state degree of coverage by the reaction intermediates in the selective oxidation of ethanol, i.e., the intrinsic relaxation

The montmorillonite catalysed production of phenol and acetone from cumene hydroperoxide.

The kinetics of the montmorillonite F-20 catalyzed prodn. of phenol and acetone from cumene hydroperoxide has been investigated. Batch expts. were carried out at initial cumene hydroperoxide concns. of 50-500 mol m-3 at 303 K in cumene with dried catalyst in the absence and presence of 5-30 mol m-3 of typical feed impurities such as 2-phenyl-2-propanol, dicumyl peroxide, acetophenone and .alpha.-methylstyrene. A reaction network is presented. Regression anal. of the exptl. data, using a multi-response Marquardt algorithm, allowed the data to be adequately described by a model based on a Langmuir-Hinshelwood mechanism. The large inhibiting effect of water could be best incorporated by a neg. order dependence of 2.13 0.08 on the water concn. in the acid catalyzed steps

Promoter effects in the Pt-catalysed oxidation of propylene glycol

Oxidation of aqueous solutions of propylene glycol has been performed at 333 K, pH = 8 using Pb-, Bi- and Sn-promoted Pt/graphite catalysts. Oxidation of propylene glycol towards pyruvic acid proceeds via two pathways, one via hydroxyacetone, the second via lactic acid. Experiments, performed under oxygen mass transfer limitations to avoid de-activation by over-oxidation showed significant effects of the promoters on both activity and selectivity. The addition of Pb and Bi enhances oxidation of lactic acid resulting in higher yields of pyruvic acid. The addition of Sn leads to high selectivities for both hydroxyacetone and pyruvic acid. The effect of Sn can be explained by the formation of a Sn–diol complex, where Sn is in the Sn(IV) state