Rh(PPh(3))(3)Cl was immobilized on a (OEt)(3)Si(CH(2))(3)PPh(2) 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-O(s) (O(s) is a surface oxygen) and Rh-Si bond distances of 2.01 angstrom and 3.15 angstrom, 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 degrees C under 150 psi of H(2). After the reaction, the presence of Rh-Rh contribution with average bond distance of 2.68 angstrom and coordination number of 1.3 and the missing contribution with bond distance of 3.85 angstrom 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. (C) 2010 Elsevier B.V. All rights reserved.
