An in-situ infrared study of the formation of n- and iso-butyraldehyde from propylene hydroformylation on Rh/SiO[sub 2] and sulfided Rh/SiO[sub 2]

In situ infrared (IR) technique has been employed to study the reaction of adsorbed CO on Rh/SiO[sub 2] and S-Rh/SiO[sub 2] with C[sub 3]H[sub 6] and H[sub 2], and to investigate the effect of sulfur on the n- and iso-butyraldehyde selectivities during steady-state propylene hydroformylation. CO adsorption on Rh/SiO[sub 2] results in the formation of linear CO and bridged CO adsorbed on Rh[sup 0] sites, and the gemdicarbonyl on Rh[sup +] sites. CO adsorption on S-Rh/SiO[sub 2] results in a high wavenumber Rh[sup +](CO) species in addition to linear CO adsorbed on Rh[sup 0] and the gem-dicarbonyl adsorbed on Rh[sup +]. Sulfur decreases the rate of CO adsorption and inhibits the formation of bridged CO on Rh/SiO[sub 2]. Pulse CO chemisorption on Rh/SiO[sub 2] and S-Rh/SiO[sub 2] reveals that equilibrium constant for CO adsorption on S-Rh/SiO[sub 2] is smaller than that on Rh/SiO[sub 2] at 303 K. The analogy between homogeneous and heterogeneous hydroformylation suggests that enhancement of the isobutryaldehyde formation by adsorbed sulfur could be due to the spacious environment of the protruding Rh[sup +] ion sites ion the S-Rh/SiO[sub 2] allowing isomerization of n-propyl groups before CO insertion. 56 refs., 11 figs., 3 tabs.