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

Abstract

Regulation of the catalytic selectivity of rhodium for the industrially important hydrogenation of 1,3-butadiene to <i>n</i>-butenes has been achieved by controlling the structure of essentially molecular rhodium species bound to supports. The selectivity for <i>n-</i>butene formation increases as the nuclearity of the metal species decreases from several Rh atoms to one, but these catalysts form the undesired product <i>n</i>-butane, even at low diene conversions. The <i>n</i>-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 H₂ dissociation to avoid butane formation via primary reactions and also favoring the bonding of 1,3-butadiene over butenes to limit secondary reactions giving butane. With this catalyst, selectivities to <i>n-</i>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 1 bar) than others that are selective for this reaction