A MOLECULAR APPROACH TO HETEROGENEOUS CATALYSIS - ETHYLENE HYDROFORMYLATION CATALYZED BY SILICA-SUPPORTED [RH-12(CO)(30)](2-) CLUSTER ANION- INFLUENCE OF THE COUNTERCATIONS LI-+, ZN2+(, NA+, K)

Cluster-derived rhodium catalysts, prepared from salts of the cluster anion [Rh-12(CO)(30)](2-) supported on silica, have been studied in et hylene hydroformylation. A catalyst pre-treatment was carried out in A r at 393 K before the admission of the hydroformylation mixture. The s electivity for propanal formation depends on the alkaline countercatio n in the following order: K greater than or equal to Li greater than o r equal to Na. The Zn-promoted catalyst is the most selective. The cor responding value for the unpromoted system, prepared from neutral Rh-4 (CO)(12), was about 20% lower. In situ infrared studies suggest the ox idative disruption of the metal framework of the physisorbed Rh-12 clu ster to take place in isolated Rh-I(CO)(2) species. In catalytic condi tions a reductive agglomeration of Rh(I) into metallic particles occur red. 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 t o generate the active metallic phase.