On-line spectroscopic studies and kinetic measurements of liquid-phase heterogeneous catalytic systems

Abstract

A versatile and compact experimental apparatus for the on-line spectroscopic study of liquid-phase catalytic systems was designed and characterized with respect to gas-liquid mass transfer, liquid-solid mass transfer and intra-particle diffusion. The utility of the system was demonstrated by two semi-batch differential heterogeneous catalytic reactions in d8-toluene/h8-toluene, the racemic hydrogenation of acetophenone (Aceph) and 2-butanone over Pt/AI2O3. The use of very sensitive on-line FTIR measurements and multivariate analysis techniques to obtain spectral data lead to the following observations: (i) solvent activation during reaction leads to observable hydrogenation and H-D exchange in both catalytic systems; (ii) very interesting short time-scale behavior occurred after some perturbations; and (iii) water had a strong inhibiting effect on the hydrogenation rates. The second observation included rapid initial hydrogenation of Aceph on the fresh catalyst and observable unusual spectral changes caused by hydrogen bonding between water and 2-butanone. A Langmuir-Hinshelwood-Hougen-Watson (LHHW) model was used to fit the kinetic data of acetophenone hydrogenation to 1-phenylethanol (Phel) in h8-toluene, at 0 °C and near 1 bar hydrogen pressure, with the effects of water and solvent included. A model involving a pair-wise addition of adsorbed dissociated hydrogen to the adsorbed substrate provided the best fit of the data. The regression of the kinetic data suggested that there was a statistically significant contribution of water to the competitive adsorption on the catalyst surface. In general, the present contribution suggests the utility of on-line liquid-phase spectroscopy together with multivariate techniques for exploratory studies of heterogeneous catalytic systems.