In light of the enormous energy footprint of the Haber-Bosch process (1-2% of global energy consumption), alternative green routes of generating ammonia (NH3) are needed. The electrochemical reduction of NO3-...
Copper-exchanged zeolites are useful materials for step-wise methane-to-methanol conversion (MMC). However, methanol yields on copper-exchanged zeolites are often modest, spurring interest in the development of active-site species that are activated at moderate temperatures, afford greater yields, and provide excellent methanol selectivities. Ultraviolet–visible (UV–vis) spectroscopy is a major tool for characterizing the active-sites and their evolution during the step-wise MMC process. However, computation of the UV–vis spectra of the copper-oxo active sites using Tamm–Dancoff time-dependent density functional theory (TDA-DFT) can be quite problematic. This has led to utilization of expensive methods based on multireference approaches, Green functions, and the Bethe–Salpeter equation. In this work, we examined the optical spectra of [CuO]+, [Cu2O]2+, [Cu2O2]2+, and [Cu3O3]2+ species implicated in MMC in zeolites. For the larger species, we examined how agreement with experimental data is improved with increasingly larger cluster models. For [CuO]+, we compared TDA-DFT against restricted active space 2nd-order perturbation theory, RASPT2. We found that signature peaks for [CuO]+ have multireference behavior. The excited states have many configuration state functions with a double excitation character. These effects are likely responsible for the poor utility of conventional TDA-DFT methods. Indeed, we obtain good agreement with experimental data and RASPT2 after accounting for 2h/2p excitations within TDA-DFT with a previously described configuration interaction singles and doubles, CIS(D)-style scheme. This was the case for [CuO]+, [Cu2O]2+, as well as a [Cu2O2]2+ species. Using a long-range corrected double-hybrid, ωB2PLYP, we provide for the first time computational evidence for the experimental UV–vis spectrum of the [Cu3O3]2+ active site motif.
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.
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