A molecular approach to heterogeneous catalysis. Ethylene hydroformylation catalysed by silica-supported [Rh 12(CO) 30] 2− cluster anion: influence of the countercations Li +, Na +, K +, Zn 2+

Abstract

Cluster-derived rhodium catalysts, prepared from salts of the cluster anion [Rh 12(CO) 30] 2− supported on silica, have been studied in ethylene hydroformylation. A catalyst pre-treatment was carried out in Ar at 393 K before the admission of the hydroformylation mixture. The selectivity for propanal formation depends on the alkaline countercation in the following order: K≥Li≥Na. The Zn-promoted catalyst is the most selective. The corresponding value for the unpromoted system, prepared from neutral Rh 4(CO) 12, was about 20% lower. In situ infrared studies suggest the oxidative disruption of the metal framework of the physisorbed Rh 12 cluster to take place in isolated Rh I(CO) 2 species. In catalytic conditions a reductive agglomeration of Rh(I) into metallic particles occurred. Remarkable effects on activity and/or selectivity are induced on promoted systems only upon a catalyst pre-treatment in O 2 at 723 K. A simple exposure to the hydroformylation mixture at 393 K was enough to generate the active metallic phase.