Chemical and kinetic study of acetophenone hydrogenation over Pt/Al2O3: Application of BTEM and other multivariate techniques to quantitative on-line FTIR measurements

Abstract

The heterogeneous catalytic hydrogenation of acetophenone (Aceph) over Pt/Al2O3 in d8-toluene/h8-toluene at 273 K was performed in semibatch mode, using a recycle configuration and on-line quantitative Fourier transform infrared (FTIR) spectroscopy measurements. Based on the d8-toluene solvent used and the multivariate analysis applied, further details of the reaction chemistry were elucidated, including the following: (i) Solvent activation occurred during reaction, leading to observable hydrogenation and H-D exchange, and (ii) 1-phenylethanol (Phel), cyclohexyl methyl ketone (CMK), and cyclohexylethanol (Che) were observable products, and little H-D exchange occurred. The on-line FTIR measurements, with sensitivity on the order of 10-5 mol / L, also lead to the following kinetic observations: (i) Water had a strong inhibiting effect on the hydrogenation rates, and (ii) very interesting short time-scale kinetic behavior occurred after some perturbations. The latter included rapid initial hydrogenations on fresh catalyst (due to the presence of spillover hydrogen) and observable adsorption-desorption of other reactants. The reaction rates obtained from the well-defined experiments in h8-toluene were fit to a number of Langmuir-Hinshelwood-Hougen-Watson (LHHW) models, in which the effects of solvent and water were included. A model involving a pairwise addition of adsorbed dissociated hydrogen to the adsorbed substrate provided the best fit of the data. The regression of the kinetic data suggested that water made a statistically significant contribution to the competitive adsorption on the catalyst surface. In more general terms, the present contribution suggests the utility of detailed on-line liquid-phase spectroscopy together with multivariate techniques for exploratory studies of heterogeneous catalytic systems. © 2006 Elsevier Inc. All rights reserved.