Transient Response of Propionaldehyde Formation during CO/H2/C2H4 Reaction on Rh/SiO2

Article abstract of DOI:10.1006/jcat.1995.1026

The transient response of propionaldehyde formation during ethylene hydroformylation over Rh/SiO2 has been studied by transient isotopic methods combined with in situ infrared spectroscopy at 0.1-0.5 MPa and 503 K.The transient methods used in this study involved pulsing ¹³CO into the CO feed flow and switching from CO to ¹³CO flow.The C2H5¹³CHO response to the ¹³CO step input was found to be equivalent to the integration of the C2H5¹³CHO response to the pulse input with respect to time.The pulse method allows the economical use of costly isotope to obtain the transient information that is commonly acquired from the step method.Analysis of the transient response reveals that the propionaldehyde may be formed via (i) the insertion of CO into absorbed ethyl species to form the acyl intermediate, (ii) hydrogenation of the acyl intermediate to produce adsorbed propionaldehyde, and (iii) desorption of adsorbed propionaldehyde.Increasing the total reaction pressure (i.e., increasing all partial pressures in the same ratios) increases the rate constant for hydrogenation of the acyl intermediate which has been identified as the rate-determining step for propionaldehyde formation at 0.1 MPa.Increasing the pressure also increases the coverage, but decreases the residence time of intermediates leading to propionaldehyde.Steady-state rate mesurements show that increasing reaction pressure decreased the overall activation energy and increased both rate and selectivity for propionaldehyde.The increase in the rate constant for hydrogenation of acyl intermediate can be related to the decrease in the overall activation energy for propionaldehyde formation.Rate constant analysis of the propionaldehyde response shows that the rate constant for propionaldehyde formation exhibits a sharp single distribution.