Reactivity descriptors of diverse copper-oxo species on ZSM-5 zeolite towards methane activation

Methane to Methanol Transformation on Cu2+/H-ZSM-5 Zeolite. Characterization of Copper State and Mechanism of the Reaction

Cu-modified zeolites provide methane conversion to methanol with high selectivity under mild conditions. The activity of different Cu-sites for methane transformation is still under discussion. Herein, ZSM-5 zeolite has been loaded with Cu2+ cations (1.4 wt % Cu) as characterized by UV-vis DRS, EPR, EXAFS, and 1H MAS NMR. It is inferred that Cu2+ cations, attached to the cation-exchange Al-O--Si sites of the zeolite framework, can exist in the form of either isolated or paired Cu2+ sites. The transformation of methane to methanol on Cu2+/H-ZSM-5 has been verified by the observation of the methoxy species formation with 13C MAS NMR and FTIR spectroscopy. The related mechanisms have been analyzed by DFT calculations. The calculations show that the paired Cu2+ sites enable heterolytic C-H bond dissociation via the "alkyl" pathway resulting in methylcopper species, which however are not detected experimentally due to further rapid transformation to surface methoxy species through methyl radical formation and recombination with Si-O-Al site. Based on the obtained data, it has been concluded that methane transformation to methanol on paired Cu2+ sites, having no extra-framework oxygen ligand, is possible in Cu-modified zeolites. The pathways of Cu2+ cations regeneration with O2 and H2O have been experimentally explored.

Partial Oxidation of Methane to Methanol on the M-O-Ag/Graphene (M = Ag, Cu) Composite Catalyst: A DFT Study

Partial oxidation of methane (CH₄) to methanol (CH₃OH) remains a great challenge in the field of catalysis due to its low selectivity and productivity. Herein, Ag-O-Ag/graphene and Cu-O-Ag/graphene composite catalysts are proposed to oxidize methane (CH₄) to methanol (CH₃OH) by using the first-principles calculations. It is shown that reactive oxygen species (μ-O) on both catalysts can activate the C-H bond of CH₄, and in addition to CH₄ activation, the catalytic activity follows the order of Ag-O-Ag/graphene (singlet) > Ag-O-Ag/graphene (triplet) ≈ Cu-O-Ag/graphene (triplet) > Cu-O-Ag/graphene (singlet). For CH₃OH* formation, the catalytic activity follows the order of Cu-O-Ag/graphene (triplet) > Ag-O-Ag/graphene (triplet) > Ag-O-Ag/graphene (singlet) > Cu-O-Ag/graphene (singlet). It can be inferred that the introduction of Cu not only reduces the use of noble metal Ag but also exhibits a catalytic effect comparable to that of the Ag-O-Ag/graphene catalyst. Our findings will provide a new avenue for understanding and designing highly effective catalysts for the direct conversion of CH₄ to CH₃OH.

Ab initio molecular dynamics free energy study of enhanced copper (II) dimerization on mineral surfaces

Understanding the adsorption of isolated metal cations from water on to mineral surfaces is critical for toxic waste retention and cleanup in the environment. Heterogeneous nucleation of metal oxyhydroxides and other minerals on material surfaces is key to crystal growth and dissolution. The link connecting these two areas, namely cation dimerization and polymerization, is far less understood. In this work we apply ab initio molecular dynamics calculations to examine the coordination structure of hydroxide-bridged Cu(II) dimers, and the free energy changes associated with Cu(II) dimerization on silica surfaces. The dimer dissociation pathway involves sequential breaking of two Cu2+-OH- bonds, yielding three local minima in the free energy profiles associated with 0-2 OH- bridges between the metal cations, and requires the design of a (to our knowledge) novel reaction coordinate for the simulations. Cu(II) adsorbed on silica surfaces are found to exhibit stronger tendency towards dimerization than when residing in water. Cluster-plus-implicit-solvent methods yield incorrect trends if OH- hydration is not correctly depicted. The predicted free energy landscapes are consistent with fast equilibrium times (seconds) among adsorbed structures, and favor Cu2+ dimer formation on silica surfaces over monomer adsorption.

Refining details of the structural and electronic properties of the CuB site in pMMO enzyme through sequential molecular dynamics/CPKS-EPR calculations

This work investigated the structural and electronic properties of the copper mononuclear site of the PmoB part of the pMMO enzyme at the molecular level. We propose that the Cu_B catalytic site in the soluble portion of pMMO at room temperature and under physiological conditions is a mononuclear copper complex in a distorted octahedral arrangement with the residues His³³, His¹³⁷, and His¹³⁹ on the equatorial base and two water molecules on the axial axis. Our view was based on the molecular dynamics results and DFT calculations of the electronic paramagnetic resonance parameters and comparisons with experimental EPR data. This new proposed model for the Cu_B site brings additional support concerning the recent experimental evidence, which pointed out that a saturated coordination sphere of the copper ion in the Cu_B center is an essential factor that makes it less efficient than the Cu_C site in the methane oxidation. Therefore, according to the Cu_B site model proposed here, an additional step involving a displacement of at least one water molecule of the copper coordination sphere by the O₂ molecule prior to its activation must be necessary. This scenario is less likely to occur in the Cu_C center once this one is buried in the alpha-helices, which are part of the pMMO structure bound to the membrane wall, and consequently located in a less solvent-exposed region. In addition, we also present a simple and efficient sequential S-MD/CPKS protocol to compute EPR parameters that can, in principle, be expanded for the study of other copper-containing proteins.

Advances in Catalytic Applications of Zeolite-Supported Metal Catalysts

Zeolites possessing large specific surface areas, ordered micropores, and adjustable acidity/basicity have emerged as ideal supports to immobilize metal species with small sizes and high dispersities. In recent years, the zeolite-supported metal catalysts have been widely used in diverse catalytic processes, showing excellent activity, superior thermal/hydrothermal stability, and unique shape-selectivity. In this review, a comprehensive summary of the state-of-the-art achievements in catalytic applications of zeolite-supported metal catalysts are presented for important heterogeneous catalytic processes in the last five years, mainly including 1) the hydrogenation reactions (e.g., CO/CO₂ hydrogenation, hydrogenation of unsaturated compounds, and hydrogenation of nitrogenous compounds); 2) dehydrogenation reactions (e.g., alkane dehydrogenation and dehydrogenation of chemical hydrogen storage materials); 3) oxidation reactions (e.g., CO oxidation, methane oxidation, and alkene epoxidation); and 4) other reactions (e.g., hydroisomerization reaction and selective catalytic reduction of NO_x with ammonia reaction). Finally, some current limitations and future perspectives on the challenge and opportunity for this subject are pointed out. It is believed that this review will inspire more innovative research on the synthesis and catalysis of zeolite-supported metal catalysts and promote their future developments to meet the emerging demands for practical applications.

Critical Role of Al Pair Sites in Methane Oxidation to Methanol on Cu-Exchanged Mordenite Zeolites

Cu-exchanged aluminosilicate zeolites have been intensively studied for the selective oxidation of methane to methanol via a chemical looping manner, while the nature of active Cu-oxo species for these catalysts is still under debate. This study inquired into the effects of Al distribution on methane oxidation over Cu-exchanged aluminosilicate zeolites, which provided an effective way to discern the activity difference between mononuclear and polynuclear Cu-oxo species. Specifically, conventional Na+/Co2+ ion-exchange methods were applied to quantify isolated Al and Al pair (i.e., Al−OH−(Si−O)1–3−Al−OH) sites for three mordenite (MOR) zeolites, and a correlation was established between the reactivity of the resultant Cu-MOR catalysts and the portions of the accessible framework Al sites. These results indicated that the Cu-oxo clusters derived from the Al pair sites were more reactive than the CuOH species grafted at the isolated Al sites, which is consistent with in situ ultraviolet-visible spectroscopic characterization and density functional theory calculations. Further theoretical analysis of the first C–H bond cleavage in methane on these Cu-oxo species unveiled that stabilization of the formed methyl group was the predominant factor in determining the reactivity of methane oxidation.

DFT study of H2 adsorption at a Cu-SSZ-13 zeolite: a cluster approach

In this work the H2 adsorption at a Cu(i)-SSZ-13 exchanged zeolite was theoretically investigated. A systematic cluster approach was used and different density functionals (B3LYP, B3LYP-D3(BJ), M06L, PBE, PBE-D3(BJ) and ωB97XD) and a def2-SVP basis set were benchmarked. In order to select the best approach to the H2 adsorption over a Cu(i)-SSZ-13 cluster with 78 atoms (16 T-sites), two main tasks were performed: (1) a comparison between theoretical and experimental structures and (2) a comparison between theoretical and experimental adsorption enthalpies. By employing the most suitable functional - the ωB97X-D - the H2 interaction with the zeolite structure was studied by means of NBO, NCI, AIM and DLPNO-CCSD(T)/LED analyses.

Catalytic cycle of the partial oxidation of methane to methanol over Cu-ZSM-5 revealed using DFT calculations

Density functional theory (DFT) calculations were performed to investigate the catalytic cycle of methane conversion to methanol over both [Cu2(O2)]2+ and [Cu2(μ-O)]2+ active sites in the Cu-ZSM-5 catalyst. The [Cu2(O2)]2+ site is found to be active for the partial oxidation of methane to methanol, and although it has a higher energy barrier in the methane activation step, it involves a very low energy barrier in the methanol formation step (36.3 kJ mol-1) as well as a lower methanol desorption energy (52.5 kJ mol-1). As the [Cu2(O2)]2+ active site is also thermodynamically stable, it may play an important role during methane conversion to methanol. Furthermore, the methane activation step follows the homolytic route and the heterolytic route for the [Cu2(O2)]2+ and [Cu2(μ-O)]2+ active sites, respectively, whereas the methanol formation step follows the direct radical rebound mechanism and the indirect rebound mechanism, respectively. Our calculations further indicate that the electronic properties of the reactive O atoms in the active site influence their reactivity toward methane oxidation. More specifically, the higher the spin density and the more negative the charge of the reactive O atom at the active site are, the lower the energy barrier for methane activation will be; and the more negative the charge of the hydroxyl group in the reaction intermediate during the partial oxidation of methane to methanol is, the higher energy barrier of the methanol formation step will be in the triplet state. Furthermore, we used a larger cluster model to predict the mechanism of the methane activation step and the effect of atomic charge of the O atom at the [Cu2(μ-O)]2+ and [Cu2(O2)]2+ active sites on the energy barriers of partial oxidation of methane to methanol, and the conclusions drawn employing the larger cluster model are consistent with those drawn using the smaller double-5T-ring cluster model. In addition, different from the traditional mechanism for methane activation at [Cu2(O2)]2+, which consists of two transition states, we find that the partial oxidation of methane at [Cu2(O2)]2+ can also occur via a single step by direct insertion of one of the O atoms at the active site into the C-H bond of methane.

Reaction Mechanism for Methane-to-Methanol in Cu-SSZ-13: First-Principles Study of the Z2[Cu2O] and Z2[Cu2OH] Motifs

As transportation continues to increase world-wide, there is a need for more efficient utilization of fossil fuel. One possibility is direct conversion of the solution gas bi-product CH4 into an energy-rich, easily usable liquid fuel such as CH3OH. However, new catalytic materials to facilitate the methane-to-methanol reaction are needed. Using density functional calculations, the partial oxidation of methane is investigated over the small-pore copper-exchanged zeolite SSZ-13. The reaction pathway is identified and the energy landscape elucidated over the proposed motifs Z2[Cu2O] and Z2[Cu2OH]. It is shown that the Z2[Cu2O] motif has an exergonic reaction path, provided water is added as a solvent for the desorption step. However, a micro-kinetic model shows that neither Z2[Cu2O] nor Z2[Cu2OH] has any notable activity under the reaction conditions. These findings highlight the importance of the detailed structure of the active site and that the most stable motif is not necessarily the most active.

Applications of Zeolites to C1 Chemistry: Recent Advances, Challenges, and Opportunities

C1 chemistry, which is the catalytic transformation of C1 molecules including CO, CO₂ , CH₄ , CH₃ OH, and HCOOH, plays an important role in providing energy and chemical supplies while meeting environmental requirements. Zeolites are highly efficient solid catalysts used in the chemical industry. The design and development of zeolite-based mono-, bi-, and multifunctional catalysts has led to a booming application of zeolite-based catalysts to C1 chemistry. Combining the advantages of zeolites and metallic catalytic species has promoted the catalytic production of various hydrocarbons (e.g., methane, light olefins, aromatics, and liquid fuels) and oxygenates (e.g., methanol, dimethyl ether, formic acid, and higher alcohols) from C1 molecules. The key zeolite descriptors that influence catalytic performance, such as framework topologies, nanoconfinement effects, Brønsted acidities, secondary-pore systems, particle sizes, extraframework cations and atoms, hydrophobicity and hydrophilicity, and proximity between acid and metallic sites are discussed to provide a deep understanding of the significance of zeolites to C1 chemistry. An outlook regarding challenges and opportunities for the conversion of C1 resources using zeolite-based catalysts to meet emerging energy and environmental demands is also presented.