Three-phase hydrogenation of d-glucose over a carbon supported ruthenium catalyst—mass transfer and kinetics

Abstract

The catalytic hydrogenation of d-glucose to d-sorbitol over a 5% Ru/C catalyst was studied in a semi-batch slurry autoclave operating at 373–403 K and 4.0–7.5 MPa hydrogen pressure. The d-glucose concentration was varied between 0.56 and 1.39 mol/l. The kinetic experiments were carried out in the absence of mass transport limitations, which was verified by using measured gas–liquid mass transfer coefficients and estimated diffusion and liquid–solid mass transfer coefficients. Many literature reports suffer from transport limitations. In the operating regime studied the reaction rate showed a first order dependency with respect to hydrogen. A shift in the order of d-glucose was observed. At low d-glucose concentrations (up to ca. 0.3 mol/l) the reaction showed a first order dependency, while at higher concentrations this changed to zero order behavior. No inhibition by sorbitol or mannitol was observed. The kinetic data were modeled using three plausible rate models based on Langmuir–Hinshelwood–Hougen–Watson (LHHW) kinetics assuming that the surface reaction is rate-determining. Model 1 involves non-competitive adsorption of hydrogen and d-glucose. Hydrogen adsorption is either molecular or dissociative, but due to the weak adsorption it results in both cases in a linear hydrogen pressure dependency, i.e. the same rate expression; Model 2 is based on competitive adsorption of molecular hydrogen and d-glucose; and Model 3 assumes competitive adsorption of dissociatively chemisorbed hydrogen and d-glucose. All three models described the data satisfactorily and further statistic discrimination between these models was not possible.