Kinetic study of liquid-phase glycerol hydrogenolysis over Cu/SiO2 catalyst

• Glycerol hydrogenolysis kinetics was studied over a Cu/SiO2 catalyst.
• The reaction selectively leads to the formation of 1,2-propanediol.
• Over the conditions studied a kinetically regime was ensured.
• A power law type kinetic model successfully fitted the experimental data.
• The results provided hints for the mechanistic steps and their kinetic importance.

The kinetics of glycerol hydrogenolysis reaction to 1,2-propanediol was studied over a stable 18 wt%Cu supported on commercial silica gel catalyst in a batch reactor. This catalyst is extremely selective to 1,2-propanediol (up to 95%). The second largest product formed is 1,3-propanediol. The reaction kinetics was investigated over a temperature range of 453–513 K, hydrogen concentration in liquid phase of 4.52–93.8 mmol/L (by varying H2 pressure between 2 and 8 MPa), initial glycerol concentration 20–40 v/v/% and catalyst weight 0.05–0.35 g. Glycerol hydrogenolysis reaction proceeds via a two-step dehydration–hydrogenation mechanism with different intermediates (acetol and 3-hydroxypropionaldehyde) formed which sequentially lead to 1,2- and 1,3-propanediol production. In the kinetic regime studied, the overall reaction rate showed an almost zero order dependence on glycerol initial concentration and nearly first order on hydrogen concentration in liquid phase. The formation rate of 1,2-propanediol also seems to not to be affected of the initial glycerol concentration, depending (almost first order) on H2 concentration. Furthermore, the formation rate of the 1,3-propanediol, is strongly dependent on glycerol concentration and presents variable order to H2 from 0.94 (nearly first order) at lower pressures to zero at higher pressures. The activation energies calculated for the overall (96.8 kJ/mol) and the two parallel reaction routes (1,2-propanediol = 94.3 and 1,3-propanediol = 135.3 kJ/mol) demonstrate the selective formation of the 1,2-propanediol at lower temperatures.

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