An in-Situ Infrared Study of the Formation of n- and iso-Butyraldehyde from Propylene Hydroformylation on Rh/SiO2 and Sulfided Rh/SiO2

In situ infrared (IR) technique has been employed to study the reaction of adsorbed CO on Rh/SiO2 and S-Rh/SiO2 with C3H6 and H2, and to investigate the effect of sulfur on the n- and isobutyraldehyde selectivities during steady-state propylene hydroformylation. CO adsorption on Rh/SiO2 results in the formation of linear CO and bridged CO adsorbed on Rh0 sites, and the gem-dicarbonyl on Rh+ sites. CO adsorption on S-Rh/SiO2 results in a high wavenumber Rh+(CO) species in addition to linear CO adsorbed on Rh0 and the gem-dicarbonyl adsorbed on Rh+. Sulfur decreases the rate of CO adsorption and inhibits the formation of bridged CO on Rh/SiO2. Pulse CO chemisorption on Rh/SiO2 and S-Rh/SiO2 reveals that the equilibrium constant for CO adsorption on S-Rh/SiO2 is smaller than that on Rh/SiO2 at 303 K. Exposure of adsorbed CO to C3H6 at 303 K does not result in the formation of the aldehyde on both Rh/SiO2 and S-Rh/SiO2. Exposure of adsorbed CO to C3H6 and H2 on Rh/SiO2 causes a decrease in the intensity of the linear CO band and a slight increase in the intensity of the aldehyde band. A prominent decrease in the Rh+(CO) band is observed on S-Rh/SiO2 with a simultaneous increase in the aldehyde band on exposure of adsorbed CO to C3H6 and H2. The linear CO on Rh+ is more active for CO insertion than linear CO on Rh0 at 303 K. The Rh+(CO) is removed from the surface of S-Rh/SiO2 at temperatures above 363 K in the presence of C3H6/H2; however, the species remains stable at 513 K in the absence of H2. Sulfided Rh/SiO2 exhibits a higher CO insertion selectivity, a lower hydrogenation activity, and a lower n-/iso-butyraldehyde selectivity than Rh/SiO2 during steady-state propylene hydroformylation at 513 K and 0.1-1 MPa. The analogy between homogeneous and heterogeneous hydroformylation suggests that enhancement of the isobutyraldehyde formation by adsorbed sulfur could be due to the spacious environment of the protruding Rh+ ion sites on the S-Rh/SiO2 allowing isomerization of n-propyl groups before CO insertion.