Tuning Catalytic Selectivity: Zeolite- and Magnesium Oxide-Supported Molecular Rhodium Catalysts for Hydrogenation of 1,3-Butadiene

Abstract

[EN] Regulation of the catalytic selectivity of rhodium for the industrially important hydrogenation of 1,3-butadiene to n-butenes has been achieved by controlling the structure of essentially molecular rhodium species bound to supports. The selectivity for n-butene formation increases as the nuclearity of the metal species decreases from several Rh atoms to one, but these catalysts form the undesired product n-butane, even at low diene conversions. The n-butene selectivity increases when the rhodium is selectively poisoned with CO ligands, and it is highest when the support is the electron-donor MgO and the rhodium is in the form of clusters that are well approximated as dimers. The electron-donor support is crucial for stabilization of the rhodium carbonyl dimer sites, as it limits the oxidative fragmentation of the clusters which is facilitated when the support is HY zeolite (a poor electron donor) that leads to decreased catalytic activity and selectivity. The selective MgO-supported rhodium carbonyl dimers suppress the catalytic routes that yield butane, limiting the activity for H-2 dissociation to avoid butane formation via primary reactions and also favoring the bonding of I,3-butadiene over butenes to limit secondary reactions giving butane. With this catalyst, selectivities to n-butene of >99% were achieved at 1,3-butadiene conversions as high as 97%. This selectivity matches that of any reported for this reaction, and the catalyst works under milder conditions (313 K and I bar) than others that are selective for this reaction.