Liquid phase hydroformylation of ring substituted styrenes catalyzed by Rh-B and Rh-Zn-B systems supported on silica

Rhodium based catalysts supported on silica have been prepared by low temperature reduction of the preadsorbed salts with respectively NaBH4 and Zn(BH4)(2). The catalyst surfaces were characterized by ESCA, XRD and FT-IR spectroscopy, A remarkable boron and zinc surface enrichment resulted from ESCA data in both catalysts. From the FT-IR data it appeared that the bridging CO chemisorption is substantially suppressed, as was observed by others in similar systems, probably because boron and zinc oxide molecules cover most of the catalyst surface blocking the multicenter rhodium sites. Both catalysts are active in the liquid phase ring substituted styrenes hydroformylation. The reaction resulted was truly heterogeneous because no leaching of catalyst was observed. The chemoselectivity was always very high, close to 100%, and regioselectivity appeared to be controlled by the nature of the ring substituents. Evidences were found that the heterogeneous reaction proceeds through elementary steps similar to those proposed for the homogeneous process.