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 (ΔG≠) for the oxidative addition of dihydrogen to the Rh metal center (15.9 kcal/mol) is lower in energy than that for the σ-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, π-bonding and π*-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.

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

Rh(C₂H₄)₂ species grafted on the HY zeolite framework significantly enhance the activation of H₂ that reacts with C₂H₄ ligands to form C₂H₆. While in this case, the simultaneous activation of C₂H₄ and H₂ and the reaction between these species on zeolite-loaded Rh cations is a legitimate hydrogenation pathway yielding C₂H₆, the results obtained for Rh(CO)(C₂H₄)/HY materials exposed to H₂ convincingly show that the support-assisted C₂H₄ hydrogenation pathway also exists. This additional and previously unrecognized hydrogenation pathway couples with the conversion of C₂H₄ 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)₂ complexes into Rh(CO)(C₂H₄) species under ambient conditions is not a simple CO/C₂H₄ ligand exchange reaction on Rh sites, as this process also involves the conversion of C₂H₄ into C₄ 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^-3 s^-1. Thus, the primary role of zeolite-supported Rh species is not limited to the activation of H₂, as these species significantly accelerate the formation of the C₄ hydrocarbons from C₂H₄ even without the presence of H₂ in the feed. Using periodic density functional theory calculations, we examined several catalytic pathways that can lead to the conversion of C₂H₄ into 1,3-butadiene over these materials and identified the reaction route via intermediate formation of rhodacyclopentane.

Highly Active Gas Phase Organometallic Catalysis Supported Within Metal-Organic Framework Pores

Metal-organic frameworks (MOFs) can act as a platform for the heterogenization of molecular catalysts, providing improved stability, allowing easy catalyst recovery and a route toward structural elucidation of the active catalyst. We have developed a MOF, 1, possessing vacant N,N-chelating sites which are accessible via the porous channels that penetrate the structure. In the present work, cationic rhodium(I) norbornadiene (NBD) and bis(ethylene) (ETH) complexes paired with both noncoordinating and coordinating anions have been incorporated into the N,N-chelation sites of 1 via postsynthetic metalation and facile anion exchange. Exploiting the crystallinity of the host framework, the immobilized Rh(I) complexes were structurally characterized using X-ray crystallography. Ethylene hydrogenation catalysis by 1·[Rh(NBD)]X and 1·[Rh(ETH)₂]X (X = Cl and BF₄) was studied in the gas phase (2 bar, 46 °C) to reveal that 1·[Rh(ETH)₂](BF₄) was the most active catalyst (TOF = 64 h^-1); the NBD materials and the chloride salt were notably less active. On the basis of these observations, the activity of the Rh(I) bis(ethylene) complexes, 1·[Rh(ETH)₂]BF₄ and 1·[Rh(ETH)₂]Cl, in butene isomerization was also studied using gas-phase NMR spectroscopy. Under one bar of butene at 46 °C, 1·[Rh(ETH)₂]BF₄ rapidly catalyzes the conversion of 1-butene to 2-butene with a TOF averaging 2000 h^-1 over five cycles. Notably, the chloride derivative, 1 [Rh(ETH)₂]Cl displays negligible activity in comparison. XPS analysis of the postcatalysis sample, supported by DFT calculations, suggest that the catalytic activity is inhibited by the strong interactions between a Rh(III) allyl hydride intermediate and the chloride anion.

C–C coupling at a zeolite-supported Rh(i) complex. DFT search for the mechanism

DFT modelling suggests a metallacycle mechanism for the dimerization of ethene over a faujasite-supported Rh(i) complex, rationalizing the experimental selectivity.

Intra-electron transfer induced by protonation in copper-containing nitrite reductase

Electron transfer between two Cu sites in the enzyme induced by protonation of remote catalytic residues.

Molecular Rhodium Complexes Supported on the Metal-Oxide-Like Nodes of Metal Organic Frameworks and on Zeolite HY: Catalysts for Ethylene Hydrogenation and Dimerization

Metal-organic frameworks (MOFs) with nodes consisting of zirconium oxide clusters (Zr₆) offer new opportunities as supports for catalysts with well-defined, essentially molecular, structures. We used the precursor Rh(C₂H₄)₂(acac) (acac is acetylacetonate) to anchor Rh(I) complexes to the nodes of the MOF UiO-67 and, for comparison, to the zeolite dealuminated HY (DAY). These were characterized experimentally by measurement of catalytic activities and selectivities for ethylene hydrogenation and dimerization in a once-through flow reactor at 298 K and 1 bar. The catalyst performance data are complemented with structural information determined by infrared and extended X-ray absorption fine structure spectroscopies and by calculations at the level of density functional theory, the latter carried out also to extend the investigation to a related MOF, NU-1000. The agreement between the experimental and calculated structural metrics is good, and the calculations have led to predictions of reaction mechanisms and associated energetics. The data demonstrate a correlation between the catalytic activity and selectivity and the electron-donor tendency of the supported rhodium (as measured by the frequencies of CO ligands bonded as probes to the Rh(I) centers), which is itself a measure of the electron-donor tendency of the support.

Atomically dispersed supported metal catalysts: perspectives and suggestions for future research

Catalysts consisting of metal atoms that are atomically dispersed on supports are gaining wide attention because of the rapidly developing understanding of their structures and functions and the discovery of new, stable catalysts with new properties.