Dynamic and kinetic modeling of isotopic transient responses for CO insertion on Rh and Mn–Rh catalysts

Abstract

Isotopic transient tracer techniques have been employed to study heterogeneous hydroformylation on Rh/SiO 2 and Mn–Rh/SiO 2. Pulse injection of D 2 and 13 CO allowed tracing of the deuterium and CO incorporation pathway into the aldehyde product. The d 1- and d 2-propionaldehyde responses showed a double-peak, or two-hump, response to the D 2 pulse, while C 2H 5 13CHO showed a single-hump response to the 13 CO pulse. Analysis of the product responses to the D 2 pulse in CO/H 2/C 2H 4 and CO/H 2/C 2H 4/C 2H 5CHO suggests that the first hump of the d 1- and d 2-propionaldehyde responses was due to rapid H/D exchange with adsorbed propionaldehyde via enol intermediates. The decay of the second hump was due to reaction of adsorbed acyl with spillover hydrogen/deuterium. The C 2H 5 13CHO response was due to CO insertion followed by acyl hydrogenation. Compartment modeling of the product responses from the 13 CO and D 2 pulse inputs allowed determination of residence times of adsorbed intermediates, surface coverages of adsorbed intermediates, and the elementary rate constants for acyl hydrogenation and CO insertion. Elementary rate constants for acyl hydrogenation determined from this study were consistent with the value calculated by transition state theory (TST). The addition of Mn promoter to Rh/SiO 2 increased coverages of ∗ CO , ∗ C 2H 5 , and ∗ C 2H 5CO and shifted the rate-limiting step for propionaldehyde formation. Acyl hydrogenation is the rate-limiting step on Rh/SiO 2 while CO insertion and acyl hydrogenation are both kinetically significant on Mn–Rh/SiO 2.