Catalytic conversion of ethene to butadiene or hydrogenation to ethane on HY zeolite-supported rhodium complexes: Cooperative support/Rh-center route.

Rh(CH) species grafted on the HY zeolite framework significantly enhance the activation of H that reacts with CH ligands to form CH. While in this case, the simultaneous activation of CH and H and the reaction between these species on zeolite-loaded Rh cations is a legitimate hydrogenation pathway yielding CH, the results obtained for Rh(CO)(CH)/HY materials exposed to H convincingly show that the support-assisted CH hydrogenation pathway also exists. This additional and previously unrecognized hydrogenation pathway couples with the conversion of CH ligands on Rh sites and contributes significantly to the overall hydrogenation activity. This pathway does not require simultaneous activation of reactants on the same metal center and, therefore, is mechanistically different from hydrogenation chemistry exhibited by molecular organometallic complexes. We also demonstrate that the conversion of zeolite-supported Rh(CO) complexes into Rh(CO)(CH) species under ambient conditions is not a simple CO/CH ligand exchange reaction on Rh sites, as this process also involves the conversion of CH into C hydrocarbons, among which 1,3-butadiene is the main product formed with the initial selectivity exceeding 98% and the turnover frequency of 8.9 × 10 s. Thus, the primary role of zeolite-supported Rh species is not limited to the activation of H, as these species significantly accelerate the formation of the C hydrocarbons from CH even without the presence of H in the feed. Using periodic density functional theory calculations, we examined several catalytic pathways that can lead to the conversion of CH into 1,3-butadiene over these materials and identified the reaction route via intermediate formation of rhodacyclopentane.