EXAFS investigation of the morphology of immobilized Rh(PPh3)3Cl on phosphinated MCM-41

Rh(PPh3)3Cl was immobilized on a (OEt)3Si(CH2)3PPh2 modified MCM-41 to form a stable hydrogenation catalyst. The morphology of the catalyst was characterized by X-ray absorption spectroscopy. The EXAFS results indicates that the immobilized Rh catalyst has an average Rh–Os (Os is a surface oxygen) and Rh–Si bond distances of 2.01 Å and 3.15 Å, respectively, suggesting that monomeric Rh complex bonds to the MCM-41 surface. However, the structure of the immobilized Rh complex is not retained after hydrogenation of cyclohexene at 75 °C under 150 psi of H2. After the reaction, the presence of Rh–Rh contribution with average bond distance of 2.68 Å and coordination number of 1.3 and the missing contribution with bond distance of 3.85 Å suggest that the reaction induces the coalescence of Rh complex to form dimeric Rh species. However, formation of trace amount of Rh metal clusters cannot be ruled out because rather small characteristic peak for the missing Rh–Rh shell may be covered by noise.

EXAFS analysis shows the immobilized Rh(PPh3)3Cl on phosphinated MCM-41 bonds to the MCM-41 surface through Rh–Os bonding in addition to the Rh–P bonding.Figure optionsDownload high-quality image (60 K)Download as PowerPoint slide