Ethene Conversion at a Zeolite‐Supported Ir(I) Complex. A Computational Perspective on a Single‐Site Catalyst System

Abstract

Applying a quantum mechanics/molecular mechanics scheme involving DFT calculations, a model study of mechanisms for ethene transformations at zeolite‐supported Ir(I) complexes is presented and the results compared to those of recent experiments and previous work on the isostructural Rh(I) complexes. Starting from the 2‐ligand complex [Ir(C2H4)2]+, in the presence of H2, the ethene conversion mechanisms studied yield solely ethane while the dimerization to 1‐butene via either the Cossee‐Arlman (CA) mechanism or the metallacycle (MC) mechanism was determined to be kinetically too demanding. Therefore, turning to 3‐ligand models, the calculations showed that the diethyl complex [Ir(C2H4)(C2H5)2]+ strongly favors ethene hydrogenation over dimerization (via a CA mechanism), with crucial activation free energies of 27 kJ mol−1 and 113 kJ mol−1, respectively. The alternative route to dimerization via a MC mechanism is also not operative because the C−C coupling barrier is higher by 30 kJ mol−1 (in absolute terms) than the hydrogen activation in the CA mechanism. Thus, when Rh is substituted by Ir, the computational results allowed to rationalize the experimentally determined switching from ethene dimerization to ethane formation due to the significantly higher calculated barrier, by ∼50 kJ mol−1 relative to Rh, of C−C coupling in the Ir system. The present study illustrates the advantage of describing the active site in a single site catalysis system, yet it also highlights the potential complexity of such systems as revealed by comparing 2‐ to 3‐ligand models as well as models with different metal centers, Rh vs Ir, in the light of conversion rates via the energetic span concept.

Syngas Conversion over Co4Cluster Grafted on HZSM‐5 Zeolite: Mechanistic Insights from DFT Modeling

“…[42] We thoroughly established this ONIOM strategy for similar zeolite- supported metal systems catalyzing hydrocarbon transformation reactions. [20][21][22] The geometry optimizations in the gas phase were carried out with an ultrafine integration grid, tight SCF convergence criteria, and without any symmetry constraints. For the transition state (TS) search, we employed a constrained optimization (freezing the atoms involved in the reaction) followed by TS optimization using eigenvector following with the Berny algorithm.…”

“…The MM partition was treated with universal force field (UFF) [42] . We thoroughly established this ONIOM strategy for similar zeolite‐supported metal systems catalyzing hydrocarbon transformation reactions [20–22] . The geometry optimizations in the gas phase were carried out with an ultrafine integration grid, tight SCF convergence criteria, and without any symmetry constraints.…”

“…[19] Alternatively, the other class metal-atom/cluster@zeolite composites, [9,10] provide a great opportunity to understand the catalytic process over such a system from a computational perspective. [20][21][22][23] Very recently, Chen et al reported [24] an experimentally well-characterized isolated Zn grafted on ZSM-5 zeolite catalyst system (Zn@ZSM-5), demonstrating a selectivity of over 86 % towards ethane in the syngas conversion. The active center was determined as [ZnÀ OÀ Zn] + 2 , and C 2 species, such as acetyl (À COCH 3 ), and ethyl (À CH 2 CH 3 ), were identified as reaction intermediates that provided insights into the CÀ C coupling pathway leading to ethane formation.…”

“…Alternatively, the other class metal‐atom/cluster@zeolite composites, [9,10] provide a great opportunity to understand the catalytic process over such a system from a computational perspective [20–23] . Very recently, Chen et al.…”

Observations of Ethylene-for-CO Ligand Exchanges on a Zeolite-Supported Single-Site Rh Catalyst by X-ray Absorption Spectroscopy

“…43 This exchange process would require the coexistence of C 2 H 4 either on the Rh(CO) 2 or in close proximity to the Rh(CO) 2 on the zeolite, forming a Rh(CO) 2 (C 2 H 4 ) complex prior to the removal of the CO ligand. Alternatively, the breaking of Rh−O bonds could occur to facilitate the formation of related structures (a suggestion that is bolstered by electronic structure calculations for analogous zeolite-supported iridium complexes 44 ). The potential to form a short-lived Rh-(CO) 2 (C 2 H 4 ) intermediate is not unexpected, as the Ir analogue, Ir(CO) 2 (C 2 H 4 ), has been observed by FTIR spectroscopy and XAS for the equivalent ligand exchange (Scheme 1).…”