Methane‐to‐methanol conversion (MMC) can be facilitated with high methanol selectivities by copper‐exchanged zeolites. There are however two open questions regarding the use of these zeolites to facilitate the MMC process. The first concerns the possibility of operating the three cycles in the stepwise MMC process by these zeolites in an isothermal fashion. The second concerns the possibility of improving the methanol yields by systematic substitution of some copper centers in these active sites with other earth‐abundant transition metals. Quantum‐mechanical computations can be used to compare methane activation by copper oxide species and analogous mixed‐metal systems. To carry out such screening, it is important that we use theoretical methods that are accurate and computationally affordable for describing the properties of the hetero‐metallic catalytic species. We have examined the performance of 47 exchange‐correlation density functionals for predicting the relative spin‐state energies and chemical reactivities of six hetero‐metallic [M‐O‐Cu]2+ and [M‐O2‐Cu]2+, (where MCo, Fe, and Ni), species by comparison with coupled cluster theory including iterative single, double excitations as well as perturbative treatment of triple excitations, CCSD(T). We also performed multireference calculations on some of these systems. We considered two types of reactions (hydrogen addition and oxygen addition) that are relevant to MMC. We recommend the use of τ‐HCTH and OLYP to determine the spin‐state energy splittings in the hetero‐metallic motifs. ωB97, ωB97X, ωB97X‐D3, and MN15 performed best for predicting the energies of the hydrogen and oxygen addition reactions. In contrast, local, and semilocal functionals do poorly for chemical reactivity. Using [Fe‐O‐Cu]2+ as a test, we see that the nonlocal functionals perform well for the methane CH activation barrier. In contrast, the semilocal functionals perform rather poorly. © 2018 Wiley Periodicals, Inc.
There is recent interest in organometallic complexes of the trans-uranium elements. However, preparation and characterization of such complexes are hampered by radioactivity and chemotoxicity issues as well as the airsensitive and poorly understood behavior of existing compounds. As such, there are no examples of small-molecule activation via redox reactivity of organometallic trans-uranium complexes. This contrasts with the situation for uranium. Indeed, a multimetallic uranium(III) nitride complex was recently synthesized, characterized, and shown to be able to capture and functionalize molecular nitrogen (N 2 ) through a four-electron reduction process, N 2 → N 2 4− . The bis-uranium nitride, U−N−U core of this complex is held in a potassium siloxide framework. Importantly, the N 2 4− product could be further functionalized to yield ammonia (NH 3 ) and other desirable species. Using the U−N−U potassium siloxide complex, K 3 U−N−U, and its cesium analogue, Cs 3 U−N−U, as starting points, we use scalar-relativistic and spin−orbit coupled density functional theory calculations to shed light on the energetics and mechanism for N 2 capture and functionalization. The N 2 → N 2 4− reactivity depends on the redox potentials of the U(III) centers and crucially on the stability of the starting complex with respect to decomposition into the mixed oxidation U(IV)/ U(III) K 2 U−N−U or Cs 2 U−N−U species. For the trans-uranium, Np and Pu analogues of K 3 U−N−U, the N 2 → N 2 4− process is endoergic and would not occur. Interestingly, modification of the Np−O and Pu−O bonds between the actinide cores and the coordinated siloxide framework to Np−NH, Pu−NH, Np−CH 2 , and Pu−CH 2 bonds drastically improves the reaction free energies. The Np−NH species are stable and can reductively capture and reduce N 2 to N 2 4− . This is supported by analysis of the spin densities, molecular structure, long-range dispersion effects, as well as spin−orbit coupling effects. These findings chart a path for achieving small-molecule activation with organometallic neptunium analogues of existing uranium complexes.
There is significant interest in improving methanol yields from methane in copper-exchanged zeolites. Interestingly, zeolites with proton, H + , precursors provide greater methanol yields and selectivities than zeolites from sodium, Na + , precursors. There is however no quantitative description of the origins of these differences. Here, we use the density functional theory to probe differences in the properties of copper-oxo species in the 8-membered ring of zeolite mordenite, MOR. We focus only on [Cu 3 O 3 ] 2+ , [Cu 2 O] 2+ , and two [Cu 2 O 2 ] 2+ species in H-MOR and Na-MOR. Our calculations show that these sites are activated at 345−490 °C, with the bis(μ-oxo) dicopper(III) [Cu 2 O 2 ] 2+ moiety being the most stable and [Cu 3 O 3 ] 2+ the least stable. [Cu 3 O 3 ] 2+ and [Cu 2 O] 2+ are capable of activating methane at 200 °C, with similar C−H activation barriers in H-MOR and Na-MOR zeolites. The fate of the methyl group formed from methane C−H activation differentiates the Na-MOR and H-MOR zeolites.Crucially, we show that rebound of the methyl group to an active-site μ-oxo atom favors over-oxidation. Alternatively, the methyl group can be stabilized via exchange with H + /Na + located at remote aluminates. Exchange with Na + does not provide as much stabilization as the μ-(OCH 3 ) intermediate, thus favoring over-oxidation. By contrast, Bro̷ nsted acid sites provide similar levels of stabilization to the μ-(OCH 3 ) intermediate. This is a path to methanol, rather than over-oxidation products. The discrepancy in the stabilizations provided by Na + and H + -aluminate sites is rooted in the electronic structure.
We have examined the performance of Multiconfiguration Pair-Density Functional Theory (MC-PDFT) for computing the ground-state properties of actinide species. Specifically, we focused on the properties of UN2 and various actinyl species. The properties obtained with MC-PDFT at the scalar-relativistic level are compared to Kohn-Sham DFT (KS-DFT); complete active space self-consistent field theory, CASSCF; coupled-cluster theory, CCSD(T) and CCSDT; as well as multireference perturbation theory (CASPT2). We examine the degree to which MC-PDFT improves over KS-DFT and CASSCF while aligning with CASPT2, CCSD(T), and CCSDT. All properties that we considered were for the CASPT2 electronic ground states. For structural parameters, MC-PDFT confers very little advantage over KS-DFT, especially the B3LYP density functional. For NpO23+, MC-PDFT and local KS-DFT functionals excessively favor the bent structure, whereas CCSDT and CASPT2 predict the bent and linear structures as isoenergetic. For this special case, hybrid KS-DFT functionals like PBE0 and B3LYP provide results closer to CASPT2 and CCSDT than MC-PDFT. On a more positive note, MC-PDFT is very close to CASPT2 and CCSD(T) for the redox potentials, energetics of redox chemical reactions, as well as ligand-binding energies. These are encouraging results since MC-PDFT is more affordable. The best MC-PDFT functional is ft-PBE. Our findings suggest that MC-PDFT can be used to study systems and excited states with larger strong electron correlation effects than were considered here. However, for the systems and properties considered here, KS-DFT functionals do well, justifying their usage as the bulwark of computational actinyl chemistry over the last two to three decades.
Here, we analyze changes in the optical spectra of activated copper-exchanged zeolites during methane activation with the Tamm–Dancoff time-dependent density functional theory, TDA-DFT, while using the ωB2PLYP functional. Two active sites, [Cu2O]2+ and [Cu3O3]2+, were studied. For [Cu2O]+, the 22 700 cm–1 peak is associated with μ-oxo 2p → Cu 3d/4s charge transfer. Of the [Cu2O]2+ methane C–H activation intermediates that we examined, only [Cu–O(H)(H)–Cu] and [Cu–O(H)(CH3)–Cu] have spectra that match experimental observations. After methane activation, the μ-oxo 2p orbitals lose two electrons and become hybridized with methanol C 2p orbitals and/or H 1s orbitals. The frontier unoccupied orbitals become more Cu 4s/4p Rydberg-like, reducing overlap with occupied orbitals. These effects cause the disappearance of the 22 700 cm–1 peak. For [Cu3O3]2+, the exact structures of the species formed after methane activation are unknown. Thus, we considered eight possible structures. Several of these provide a significant decrease in intensity near 23 000–38 000 cm–1, as seen experimentally. Notably, these species involve either rebound of the separated methyl to a μ-oxo atom or its remote stabilization at a Brønsted acid site in exchange for the acidic proton. These spectral changes are caused by the same mechanism seen in [Cu2O]2+ and are likely responsible for the observed reduced intensities near 23 000–38 000 cm–1. Thus, TDA-DFT calculations with ωB2PLYP provide a molecular-level understanding of the evolution of copper-oxo active sites during methane-to-methanol conversion.
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