Dynamic and LHHW Kinetic Analysis of Heterogeneous Catalytic Hydroformylation

Heterogeneous ethylene hydroformylation on a 4 wt% Rh/SiO 2 catalyst was studied using a steady-state pulse transient method coupled with in situ infrared spectroscopy. Four independent quantities, including the rates of propionaldehyde and ethane formation and the surface coverages of adsorbed CO and adsorbed acyl species, were measured at steady state as a function of the partial pressures of the reactants. The coverage of intermediates during ethylene hydroformylation was determined from the dynamic response of (C 2H 5 13CHO to a 13CO pulse input. The coverage of adsorbed CO was measured by in situ IR spectroscopy. The rate laws for C 2H 5CHO and C 2H 6 formation and the isotherm equations for adsorbed C 2H 5CO and adsorbed CO were derived using the Langmuir-Hinshelwood-Hougen-Watson (LHHW) approach from a proposed mechanism with the hydrogenation of adsorbed C 2H 5CO as the rate-determining step for propionaldehyde formation and the hydrogenation of adsorbed C 2H 5 as the rate-determining step for ethane formation. The high degree of fitting of rate and coverage data to the derived rate law and isotherm equations suggests that the LHHW model describes the surface reaction with high accuracy. Although the assumptions for the Langmuir isotherm do not account for the interactions between adsorbates, the LHHW equations and the proposed mechanism satisfactorily describe the kinetics, reaction pathways, and rate-limiting steps for the formation of ethane and propionaldehyde. This study demonstrates that the measurement of coverage of adsorbates by isotopic tracer pulsing and in situ infrared spectroscopy provides direct experimental evidence to confirm a postulated mechanism and rate law.