Hydroformylation of formaldehyde catalyzed by cobalt-rhodium bimetallic systems

Abstract

The hydroformylation of formaldehyde to glycolaldehyde and its hydrogenation to ethylene glycol on using either Co-Rh bimetallic carbonyl clusters or mixtures of cobalt and rhodium derivatives as catalyst precursors has been investigated. The effect on activity and selectivity of varying the Co/Rh ratio of the Rh 4(CO) 12-CoCl 2·6H 2O system has also been studied. A limiting molar selectivity in C 2-oxygenated products (glycolaldehyde and ethylene glycol) of ca. 65% has been obtained in the most favorable conditions. Attempts to drive the reaction toward ethylene glycol in a one-batch process led to significantly lower selectivities in C 2 species, due to concomitant hydrogenation of formaldehyde to methanol. The latter represented the major competitive reaction in all the experiments. Under the adopted experimental conditions, all the investigated systems deactivate with time, owing to reduction to [Rh(CO) 4] − and/or [Co(CO) 4] −; this is further accelerated by an increase of the reaction temperature, probably because of formation of methylamines from decomposition of the dimethyl formamide used as reaction solvent. The most relevant effect on selectivity of the Co(II) salts in this system seems to be due to their oxidizing behavior toward rhodium carbonyl anionic species and their consequent capacity to buffer for some time the rhodium in a yet unknown oxidation state x (probably +1 > x > −0.2), for which hydroformylating rather than hydrogenating behavior prevails.