EXAFS/FT-IR characterization of tetra-iridium carbonyl clusters bound to tris-(hydroxymethyl)phosphine grafted silica surface catalytically active for propene oxidation to acetone

Abstract Structure and catalytic activity of Ir 4 (CO) 12 bound to tris-(hydroxymethyl)phosphine (THP) grafted silica ( THP SiO 2 ) was investigated by means of EXAFS, FT-IR and kinetic studies. It was found that Ir 4 (CO) 12 was uniformly attached on THP SiO 2 by substitution of CO by THP (Ir 4 /THP/SiO 2 ). The tetra-iridium carbonyl cluster framework was remained during the substitution of THP ligands and two THP ligands coordinated to the iridium carbonyl clusters to form Ir 4 (CO) 10 (THP SiO 2 ) 2 . species. When Ir 4 /THP/SiO 2 was evacuated at 373 K, bridge CO was desorbed and Debye-Waller factor of IrIr contribution derived from EXAFS analysis was increased which suggested that the cluster framework was distorted by the evacuation at 373 K. The resulting sample evacuated at 373 K was an active catalyst for hydroformylation of ethene and partial oxidation of propene, while the Ir 4 /THP/SiO 2 without evacuation exhibited poor catalytic activities. The propene oxidation reaction proceed on the Ir 4 /THP/SiO 2 evacuated at 323–353 K under subatmospheric pressures to give acetone as a product in high selectivity. The ethene hydroformylation proceed on the evacuated Ir 4 /THP/SiO 2 at lower temperatures compared with other conventional iridium catalysts. EXAFS characterization and kinetic studies suggested that the catalytic activities were associated with the structural distortion of the iridium cluster framework due to surface attachment by the bidentate phosphine substitution.