INFRARED STUDIES OF THE INTERACTIONS OF C2H4 AND H2 WITH RH+(CO)2 AND CO ADSORBED ON RHCL3/SIO2 AND RH(NO3)3/SIO2

Interactions of C2H4/H2 with Rh+(CO)2/SiO2 and CO adsorbed on RhCl3/SiO2 and Rh(NO3)3/SiO2 and the activity of these catalysts forethylene hydroformylation have been studied by infrared spectroscopy. Chemisorption of CO on RhCl3/SiO2 and Rh(NO3)3/SiO2 at 298 K results in the formation of linear CO on Rh+ (or Rh+2) and Rh+3 sites as well as gem-dicarbonyl. Temperature-programmed decomposition studies show that thermal stability of these adsorbed CO species decreases in the following order: linear CO on Rh+ of RhCl3/SiO2 > Rh+(CO)2 on Rh(NO3)3/SiO2 > Rh+(CO)2 and linear CO on Rh+3 on RhCl3/SiO2. Linear CO on Rh+ sites is more active than Rh+(CO)2, on either Rh(NO3)3/SiO2 or RhCl3/SiO2 at 298 K toward C2H4 and H2 leading to the formation of propionaldehyde. Gem-dicarbonyl is observed as a dominant CO species adsorbed on RhCl3/SiO2 while both gem-dicarbonyl and linear CO on Rh0 sites are found on Rh(NO3)3/SiO2 during ethylene hydroformylation at 393 K and 1MPa. RhCl3/SiO2 shows higher hydroformylation selectivity than Rh(NO3)3/SiO2. RhCl3/SiO2 is less susceptible to reduction by the reactant mixture than Rh(NO3)3/SiO2, resulting in higher hydroformylation selectivity. Increasing the reaction temperature from 393 to 513 K results in the reduction of RhCl3 and Rh(NO3)3 to reduced Rh crystallites. Both catalysts remain active for hydroformylation and exhibit similar hydroformylation selectivities at 513 K. However, their selectivities for hydroformylation are significantly lower than those at 393 K. The low hydroformylation selectivity is related to the reduced Rh crystallite, which is highly active for hydrogenation. © 1993 Academic Press, Inc.