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.