27 research outputs found
Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites
A direct, catalytic conversion of benzene to phenol would have wide-reaching economic impacts. Fe zeolites exhibit a remarkable combination of high activity and selectivity in this conversion, leading to their past implementation at the pilot plant level. There were, however, issues related to catalyst deactivation for this process. Mechanistic insight could resolve these issues, and also provide a blueprint for achieving high performance in selective oxidation catalysis. Recently, we demonstrated that the active site of selective hydrocarbon oxidation in Fe zeolites, named α-O, is an unusually reactive Fe(IV)=O species. Here, we apply advanced spectroscopic techniques to determine that the reaction of this Fe(IV)=O intermediate with benzene in fact regenerates the reduced Fe(II) active site, enabling catalytic turnover. At the same time, a small fraction of Fe(III)-phenolate poisoned active sites form, defining a mechanism for catalyst deactivation. Density-functional theory calculations provide further insight into the experimentally defined mechanism. The extreme reactivity of α-O significantly tunes down (eliminates) the rate-limiting barrier for aromatic hydroxylation, leading to a diffusion-limited reaction coordinate. This favors hydroxylation of the rapidly diffusing benzene substrate over the slowly diffusing (but more reactive) oxygenated product, thereby enhancing selectivity. This defines a mechanism to simultaneously attain high activity (conversion) and selectivity, enabling the efficient oxidative upgrading of inert hydrocarbon substrates
Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites
A direct, catalytic conversion of benzene to phenol would have wide-reaching economic impacts. Fe zeolites exhibit a remarkable combination of high activity and selectivity in this conversion, leading to their past implementation at the pilot plant level. There were, however, issues related to catalyst deactivation for this process. Mechanistic insight could resolve these issues, and also provide a blueprint for achieving high performance in selective oxidation catalysis. Recently, we demonstrated that the active site of selective hydrocarbon oxidation in Fe zeolites, named α-O, is an unusually reactive Fe(IV)=O species. Here, we apply advanced spectroscopic techniques to determine that the reaction of this Fe(IV)=O intermediate with benzene in fact regenerates the reduced Fe(II) active site, enabling catalytic turnover. At the same time, a small fraction of Fe(III)-phenolate poisoned active sites form, defining a mechanism for catalyst deactivation. Density-functional theory calculations provide further insight into the experimentally defined mechanism. The extreme reactivity of α-O significantly tunes down (eliminates) the rate-limiting barrier for aromatic hydroxylation, leading to a diffusion-limited reaction coordinate. This favors hydroxylation of the rapidly diffusing benzene substrate over the slowly diffusing (but more reactive) oxygenated product, thereby enhancing selectivity. This defines a mechanism to simultaneously attain high activity (conversion) and selectivity, enabling the efficient oxidative upgrading of inert hydrocarbon substrates
Combined MCD/DFT/TDDFT Study of the Electronic Structure of Axially Pyridine Coordinated Metallocorroles
A series of metallocorroles were investigated by UVâvis and MCD spectroscopies to investigate similarities and differences in the electronic structure and spectroscopy of the closed- and open-shell metallocorroles. Similar to the free-base H3tpfc, inspection of the MCD Faraday B-terms for all of the macrocycles presented here revealed that a ÎHOMO \u3c ÎLUMO condition is present for each complex, which results in an unusual sign-reversed sequence for ÏâÏ* transitions in their MCD spectra
Advances in the synthesis, characterisation, and mechanistic understanding of active sites in Fezeolites for redox catalysts
The recent research developments on the active sites in Fe-zeolites for redox catalysis are discussed. Building on the characterisation of the α-Fe/α-O active sites in the beta and chabazite zeolites, we demonstrate a bottom-up approach to successfully understand and develop Fe-zeolite catalysts. We use the room temperature benzene to phenol reaction as a relevant example. We then suggest how the spectroscopic identification of other monomeric and dimeric iron sites could be tackled. The challenges in the characterisation of active sites and intermediates in NOX selective catalytic reduction catalysts and further development of catalysts for mild partial methane oxidation are briefly discussed.status: publishe
Initial Report on Molecular and Electronic Structure of Spherical Multiferrocenyl/tin(IV) (Hydr)oxide [(FcSn)12O14(OH)6]X2 Clusters
Two spherical organic-inorganic ferrocene-tin (hydr)oxide clusters of general formula [(FcSn)12O14(OH)6]X2 (Fc = ferrocenyl, X = nitroso-dicyanmethanide, DCO- and benzoylcyanoxime, PCO- anions) were prepared by the direct hydrolysis of Fc2SnCl2 or FcSnCl3 precursors in the presence of light- and thermally stable Ag(DCO) or Ag(PCO) salts. Molecular structures of FcSnCl3Py2 (1), Fc2SnCl2Py2 (2), [(FcSn)12O14(OH)6](DCO)2 (3), and [(FcSn)12O14(OH)6](PCO)2 (4) were investigated by X-ray crystallography. Density function theory (DFT) and time-dependent density functional theory (TDDFT) calculations were conducted on FcSnCl3Py2, Fc2SnCl2Py2, and [(FcSn)12O14(OH)6]2+ compounds in order to elaborate electronic structures and assign transitions in UV-vis spectra of these systems. The DFT and TDDFT calculations suggest that the organometallic substituents in the [(FcSn)12O14(OH)6]2+ core are rather isolated from each other
Coordination and activation of nitrous oxide by iron zeolites
Iron-containing zeolites are heterogeneous catalysts that exhibit remarkable activity in the selective oxidation of inert hydrocarbons and catalytic decomposition of nitrous oxide (N2O). The reduction of N2O is critical to both these functions, but experimental data tracking the iron active sites during N2O binding and activation are limited. Here, the N2O-ligated Fe(ii) active site in iron-exchanged zeolite beta is isolated and characterized by variable-temperature Mossbauer, diffuse reflectance UV-vis-NIR and Fourier transform infrared spectroscopy. N2O binds through the terminal nitrogen atom with substantial backbonding from the Fe(ii) centre at low temperature. At higher temperatures, the Fe-N2O interaction is weakened, facilitating isomerization to the O-bound form, which is competent in O-atom transfer. Density functional theory calculations show the geometric and electronic structure requirements for N2O binding and activation. A geometric distortion imposed by the zeolite lattice plays an important role in activating N2O. This highlights a mechanism for structural control over function in Fe-zeolite catalysts.
Nitrous-oxide-mediated oxidation reactions can be effectively promoted by iron-containing zeolites, although structural information on the interaction between oxidant and metal centre is limited. Here, the authors report the characterization of the N2O-ligated Fe(ii) active site in iron-exchanged zeolite beta
Tuning Copper Active Site Composition in Cu-MOR through Co-Cation Modification for Methane Activation
The industrial implementation of a direct methane-to-methanol
process
would lead to environmental and economic benefits. Copper zeolites
successfully execute this reaction at relatively low temperatures,
and mordenite zeolites in particular enable high methanol production.
When loaded to a Cu/Al ratio of 0.45, mordenite (Si/Al 5â9)
has been shown to host three active sites: two [CuOCu]2+ sites labeled MOR1 and MOR2 and a mononuclear [CuOH]+ site. Also at low copper loadings (Cu/Al < 0.20), mordenite has
been demonstrated to activate methane, but its active site has never
been reported. Here, we investigate Na+ mordenite with
varying copper loadings to better understand copper speciation in
mordenite. At low copper loadings, we uncover an unidentified active
site (âMOR3â) with a strong overlap with the [CuOH]+ siteâs spectroscopic signal. By changing the co-cation,
we selectively speciate more MOR3 relative to [CuOH]+,
allowing its identification as a [CuOCu]2+ site. Active
site identification in heterogeneous catalysts is a frequent problem
due to signal overlap. By changing cation composition, we introduce
an innovative method for simplifying a material to allow better analysis.
This has implications for the study of Cu zeolites for methane-to-methanol
and NOx catalysis, but also for studying
and tuning heterogeneous catalysts in general
Spectroscopic Identification of the alpha-Fe/alpha-O Active Site in Fe-CHA Zeolite for the Low-Temperature Activation of the Methane C-H Bond
The formation of single-site α-Fe in the CHA zeolite topology is demonstrated. The site is shown to be active in oxygen atom abstraction from N2O to form a highly reactive α-O, capable of methane activation at room temperature to form methanol. The methanol product can subsequently be desorbed by online steaming at 200 °C. For the intermediate steps of the reaction cycle, the evolution of the Fe active site is monitored by UV-vis-NIR and Mössbauer spectroscopy. A B3LYP-DFT model of the α-Fe site in CHA is constructed, and the ligand field transitions are calculated by CASPT2. The model is experimentally substantiated by the preferential formation of α-Fe over other Fe species, the requirement of paired framework aluminum and a MeOH/Fe ratio indicating a mononuclear active site. The simple CHA topology is shown to mitigate the heterogeneity of iron speciation found on other Fe-zeolites, with Fe2O3 being the only identifiable phase other than α-Fe formed in Fe-CHA. The α-Fe site is formed in the d6r composite building unit, which occurs frequently across synthetic and natural zeolites. Finally, through a comparison between α-Fe in Fe-CHA and Fe-*BEA, the topology's 6MR geometry is found to influence the structure, the ligand field, and consequently the spectroscopy of the α-Fe site in a predictable manner. Variations in zeolite topology can thus be used to rationally tune the active site properties.status: publishe
Magnetic Circular Dichroism Spectroscopy of <i>meso</i>-Tetraphenylporphyrin-Derived Hydroporphyrins and Pyrrole-Modified Porphyrins
A large
set of free-base and transition-metal 5,10,15,20-tetraphenyl-substituted
chlorins, bacteriochlorins, and isobacteriochlorins and their pyrrole-modified
analogues were investigated by combined UVâvisible spectroscopy,
magnetic circular dichroism (MCD), density functional theory (DFT),
and time-dependent DFT (TDDFT) approaches and their spectral characteristics
were compared to those of the parent compounds, free-base tetraphenylporphyrin <b>1H</b><sub><b>2</b></sub> and chlorin <b>2H</b><sub><b>2</b></sub>. It was shown that the nature of the pyrroline
substituents in the chlorin derivatives dictates their specific UVâvis
and MCD spectroscopic signatures. In all hydroporphyrin-like cases,
MCD spectroscopy suggests that the ÎHOMO is smaller than the
ÎLUMO for the macrocycle-centered frontier molecular orbitals.
DFT and TDDFT calculations were able to explain the large broadening
of the UVâvis and MCD spectra of the chlorin diones and their
derivatives compared to the other hydroporphyrins and hydroporphyrin
analogues. This study contributes to the further understanding of
the electronic effects of replacing a pyrrole in porphyrins by pyrrolines
or other five-membered heterocycles (oxazoles and imidazoles)
Tuning Electron-Transfer Properties in 5,10,15,20-Tetra(1âČ-hexanoylferrocenyl)porphyrins as Prospective Systems for Quantum Cellular Automata and Platforms for Four-Bit Information Storage
Metal-free (<b>1</b>) and zinc (<b>2</b>) 5,10,15,20-tetraÂ(1âČ-hexanoylferrocenyl)Âporphyrins
were prepared using an acid-catalyzed tetramerization reaction between
pyrrole and 1âČ-(1-hexanoyl)Âferrocencarboxaldehyde. New organometallic
compounds were characterized by combination of <sup>1</sup>H, <sup>13</sup>C, and variable-temperature NMR, UVâvis, magnetic
circular dichroism, and high-resolution electrospray ionization mass
spectrometry methods. The redox properties of <b>1</b> and <b>2</b> were probed by electrochemical (cyclic voltammetry and differential
pulse voltammetry), spectroelectrochemical, and chemical oxidation
approaches coupled with UVâvisânear-IR and MoÌssbauer
spectroscopy. Electrochemical data recorded in the dichloromethane/TBAÂ[BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] system (TBAÂ[BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] is a weakly coordinating tetrabutylammonium
tetrakisÂ(pentafluorophenyl)Âborate electrolyte) are suggestive of â1e<sup>â</sup> + 1e<sup>â</sup> + 2e<sup>â</sup>â
oxidation sequence for four ferrocene groups in <b>1</b> and <b>2</b>, which followed by oxidation process centered at the porphyrin
core. The separation between all ferrocene-centered oxidation electrochemical
waves is very large (510â660 mV). The nature of mixed-valence
[<b>1</b>]<sup><i>n</i>+</sup> and [<b>2</b>]<sup><i>n</i>+</sup> (<i>n</i> = 1 or 2) complexes
was probed by the spectroelectrochemical and chemical oxidation methods.
Analysis of the intervalence charge-transfer band in [<b>1</b>]<sup>+</sup> and [<b>2</b>]<sup>+</sup> is suggestive of the
Class II (in RobinâDay classification) behavior of all mixed-valence
species, which correlate well with MoÌssbauer data. Density
functional theoryâpolarized continuum model (DFT-PCM) and time-dependent
(TD) DFT-PCM methods were applied to correlate redox and optical properties
of organometallic complexes <b>1</b> and <b>2</b> with
their electronic structures