Tuned-Up Rhodium Sharpens Butadiene Hydrogenation

A fundamental study in tuning catalyst selectivity has solidified a new design strategy for making heterogeneous rhodium catalysts for butadiene hydrogenations (ACS Catal., DOI: 10.1021/cs300475c). The achievement by Dicle Yardimci, Pedro Serna, and Bruce C. Gates of the University of California, Davis, transforms rhodium, a catalyst metal traditionally considered unselective for this industrially important refinery reaction, into one that is highly selective. 1,3-Butadiene is a problematic by-product in the production of n-butenes by thermal cracking of petroleum—the butadiene must be removed to improve product quality and prevent catalyst deactivation. Chemists have been looking for highly selective catalysts to convert 1,3-butadiene to butenes under mild conditions without further converting the butenes to unwanted butane. Gates and coworkers prepared uniformly dispersed Rh(C₂H₄)₂ on a magnesium oxide solid support. They then tuned the selectivity by treating the complexes with hydrogen to form dimers and displacing the ethylene ligands by exposing the material to carbon monoxide. The selective form of the catalyst is (CO)₂RhRh(CO)₂, which suppresses butane formation and forms n-butenes with 99% selectivity at butadiene conversions up to 97%. This catalytic activity matches that of the best gold catalysts reported for this reaction, the researchers note, but it occurs under much milder conditions of 40 °C and atmospheric pressure.