DFT Study of the Mechanism of Selective Hydrogenation of Acetylene by Rhodium Single-Atom Catalyst Supported on HY Zeolite

The selectivity of acetylene hydrogenation by the Rh single-atom catalyst (SAC) supported on HY zeolite was investigated using density functional theory (DFT) and a 5/83T quantum mechanics/molecular mechanics (QM/MM) embedded cluster model. The calculated activation barrier (Delta G not equal) for the oxidative addition of dihydrogen to the Rh metal center (15.9 kcal/mol) is lower in energy than that for the sigma-bond metathesis of dihydrogen to the Rh-C bond (22.7 kcal/mol) and the Rh-O bond (28.4 kcal/mol). The activation barriers of the oxidative addition of subsequent dihydrogen molecules are significantly higher than that of the oxidative addition of the first dihydrogen molecule. These findings align with the experimentally observed activity and selectivity of the atomically dispersed Rh catalyst supported on HY zeolite. Natural bond orbital (NBO), molecular orbital (MO) and fuzzy bond order analyses were used to examine the interaction between the Rh metal center and acetylene versus ethylene ligands. The occupancies of the Rh lone pairs, pi-bonding and pi*-antibonding orbitals of acetylene and ethylene are consistent with the expected stronger interaction between the Rh metal center and acetylene compared to ethylene on the HY zeolite support.