Styrene hydroformylation over modified Rh/SiO2•Al2O3 catalysts

The hydroformylation of styrene over Rh/SiO2 . Al2O3 has been studied in the presence of the chiral diphosphines (-)-Chiraphos and (-)-DIOP. FTIR studies of the solid-liquid interface show that these molecules react with the surface rhodium gem-dicarbonyl complex to yield Rh(I)(CO)(P-P), which remains attached to the solid. FTIR and P-31 CP MAS NMR measurements, indicate that the optical modifiers are also adsorbed on the support and to a lesser extent, on the metal surfaces. Leaching of rhodium is greatly reduced by controlling the water content of the solvent and the degree of hydration of the catalyst surface. Catalytic activity is significantly decreased in the presence of the diphosphines although high levels of both chemo and regio selectivity towards the chiral aldehyde are maintained. Optical yields are moderate (maximum e.e. 9.0%) and drop to zero when the catalysts are reused. Styrene conversion is recovered after recycling of the catalyst. The participation of metallic particles in the hydroformylation process is discussed in relation with the changes that occur under reaction conditions. (C) 2000 Elsevier Science B.V. All rights reserved.