Effects of modification of Pd/SiO2 with Rh on catalytic activity for ethene hydroformylation

Abstract

Effects of modification of Pd/SiO₂ on the catalytic activity for ethene hydroformylation have been studied on catalysts with a high Pd dispersion [76%, denoted by Pd(H)/SiO₂] and with a low Pd dispersion [13%, Pd(L)/SiO₂]. In the case of modification with Rh(CH₃CO₂)₃, the rate of propionaldehyde formation increased with an increase in the amount of Rh(CH₃CO₂)₃, introduced. The activity on Pd(H)/SiO₂ modified with Rh(CH₃CO₂)₃ at Rh/Pd = 1.0 was found to be very high, and corresponded to about four times that on unmodifed Pd(H)/SiO₂ or to more than 50 times that on Rh/SiO₂ containing the equivalent amount of Rh alone. By modification with Rh(CH₃CO₂)₃, the activity for ethane formation was observed to be lower than that on the corresponding unmodified Pd/SiO₂ catalyst, resulting in the enhancement of selectivity to propionaldehyde. However, the effects of the modification of Pd/SiO₂ by RhCI₃ on catalytic behaviour were found to be complicated compared with those modified by Rh(CH₃CO₂)₃. With respect to the activity for propionaldehyde formation, the maximum appeared at ca. Rh/Pd = 0.1 for Pd(H)/SiO₂, and at ca. Rh/Pd = 0.5 for Pd(L)/SiO₂. The catalytic activity on Pd(H)/SiO₂, which was modified with RhCl₃ at Rh/Pd = 1.0, was found to be comparable to that on Pd(L)/SiO₂ with the modification at Rh/Pd = 1.0. In addition, the activities on such modifed catalyst were comparable to that on the catalyst prepared by the deposition of RhCI₃ and PdCI₂ from a solution containing both. The variation observed in the modification effects was interpreted by the difference between the solubility of Pd particles in the solutions containing RhCl₃ or Rh(CH₃CO₂)₃, where the former was much higher than the latter. X-Ray diffraction (XRD) results showed remarkable decreases in the size of large Pd particles during the modification of the catalysts with a solution containing HCI. The remarkable increase in the activity for propionaldehyde formation observed for Pd(H)/SiO₂ modified with Rh(CH₃CO₂)₃, is probably due to the formation of a Rh–Pd bimetallic structure at the surfaces of metal particles.