EXAFS and FTIR characterization of tetrarhodium carbonyl clusters attached on tris-(hydroxymethyl)phosphine grafted silica catalytically active for olefin hydroformylation

Abstract

The structure of the cluster framework in Rh4(CO)12 attached on tris-(hydroxy-methyl)phosphine (P(CH2OH)3,THP) modified SiO2 was studied by EXAFS and IR spectroscopies in conjunction with their catalytic activities and selectivities for the olefin hydroformylation reaction. Rh4(CO)12 was attached on THP/SiO2 by phosphine substitution with remaining Rh4 framework (Rh4/THP/SiO2), which gives the IR bands at 2068, 2044 cm−1 and 1870, 1840 cm−1 due to the linear and bridge CO of the Rh4 clusters. EXAFS study suggested that Rh4 clusters on THP/SiO2 were coordinated by two adjacent phosphine ligands of THP/SiO2 and that the Rh4 framework was fairly distorted. The Rh-Rh distance of Rh4/THP/SiO2 was 0.05 A longer than that of Rh4(CO)12 crystal. Rh4/THP/SiO2 exhibited the selective formation of propanal in the gas phase hydroformylation of ethene under mild conditions (>98% of propanal formation in 300–373 K, 40 kPa). In contrast, the attached Rh6(CO)16, [Rh(CO)2Cl]2 on THP/SiO2 and Rh4(CO)10(THP)2 on SiO2 were not active for the hydroformylation, which implies that the distorted Rh4 clusters attached on THP/SiO2 are catalytically active and selective towards aldehydes in the olefin hydroformylation.