Cu-ZSM-5: A biomimetic inorganic model for methane oxidation

The present work highlights recent advances in elucidating the methane oxidation mechanism of inorganic Cu-ZSM-5 biomimic and in identifying the reactive intermediates that are involved. Such molecular understanding is important in view of upgrading abundantly available methane, but also to comprehend the working mechanism of genuine Cu-containing oxidation enzymes

Oxygen precursor to the reactive intermediate in methanol synthesis by Cu-ZSM-5

The reactive oxidizing species in the selective oxidation of methane to methanol in oxygen activated Cu-ZSM-5 was recently defined to be a bent mono(μ-oxo)dicopper(II) species, [Cu_2O]^(2+). In this communication we report the formation of an O_2-precursor of this reactive site with an associated absorption band at 29,000 cm^(-1). Laser excitation into this absorption feature yields a resonance Raman (rR) spectrum characterized by ^(18)O_2 isotope sensitive and insensitive vibrations, νO-O and νCu-Cu, at 736 (Δ^(18)O_2 = 41 cm^(-1)) and 269 cm^(-1), respectively. These define the precursor to be a μ-(η^2:η^2) peroxo dicopper(II) species, [Cu_2(O_2)]^(2+). rR experiments in combination with UV-vis absorption data show that this [Cu_2(O_2)]^(2+) species transforms directly into the [Cu_2O]^(2+) reactive site. Spectator Cu^+ sites in the zeolite ion-exchange sites provide the two electrons required to break the peroxo bond in the precursor. O_2-TPD experiments with ^(18)O_2 show the incorporation of the second ^(18)O atom into the zeolite lattice in the transformation of [Cu_2(O_2)]^(2+) into [Cu_2O]^(2+). This study defines the mechanism of oxo-active site formation in Cu-ZSM-5

Cu-ZSM-5: A biomimetic inorganic model for methane oxidation

The present work highlights recent advances in elucidating the methane oxidation mechanism of inorganic Cu-ZSM-5 biomimic and in identifying the reactive intermediates that are involved. Such molecular understanding is important in view of upgrading abundantly available methane, but also to comprehend the working mechanism of genuine Cu-containing oxidation enzymes

[Cu_2O]^(2+) active site formation in Cu-ZSM-5: geometric and electronic structure requirements for N_2O activation

Understanding the formation mechanism of the [Cu_2O]^(2+) active site in Cu-ZSM-5 is important for the design of efficient catalysts to selectively convert methane to methanol and related value-added chemicals and for N_2O decomposition. Spectroscopically validated DFT calculations are used here to evaluate the thermodynamic and kinetic requirements for formation of [Cu_2O](2+) active sites from the reaction between binuclear Cu(I) sites and N_2O in the 10-membered rings Cu-ZSM-5. Thermodynamically, the most stable Cu^I center prefers bidentate coordination with a close to linear bite angle. This binuclear Cu^I site reacts with N_2O to generate the experimentally observed [Cu_2O]^(2+) site. Kinetically, the reaction coordinate was evaluated for two representative binuclear Cu^I sites. When the Cu-Cu distance is sufficiently short (5.0 Å), N_2O binds in a "terminal" η^1-O fashion to a single Cu^I site of the dimer and the resulting E_a for N_2O activation is significantly higher (16 kcal/mol). Therefore, bridging N_2O between two Cu^I centers is necessary for its efficient two-electron activation in [Cu_2O]^(2+) active site formation. In nature, this N_2O reduction reaction is catalyzed by a tetranuclear Cu_Z cluster that has a higher E_a. The lower E_a for Cu-ZSM-5 is attributed to the larger thermodynamic driving force resulting from formation of strong Cu^(II)-oxo bonds in the ZSM-5 framework

[Cu_2O]^(2+) active site formation in Cu-ZSM-5: geometric and electronic structure requirements for N_2O activation

Understanding the formation mechanism of the [Cu_2O]^(2+) active site in Cu-ZSM-5 is important for the design of efficient catalysts to selectively convert methane to methanol and related value-added chemicals and for N_2O decomposition. Spectroscopically validated DFT calculations are used here to evaluate the thermodynamic and kinetic requirements for formation of [Cu_2O](2+) active sites from the reaction between binuclear Cu(I) sites and N_2O in the 10-membered rings Cu-ZSM-5. Thermodynamically, the most stable Cu^I center prefers bidentate coordination with a close to linear bite angle. This binuclear Cu^I site reacts with N_2O to generate the experimentally observed [Cu_2O]^(2+) site. Kinetically, the reaction coordinate was evaluated for two representative binuclear Cu^I sites. When the Cu-Cu distance is sufficiently short (5.0 Å), N_2O binds in a "terminal" η^1-O fashion to a single Cu^I site of the dimer and the resulting E_a for N_2O activation is significantly higher (16 kcal/mol). Therefore, bridging N_2O between two Cu^I centers is necessary for its efficient two-electron activation in [Cu_2O]^(2+) active site formation. In nature, this N_2O reduction reaction is catalyzed by a tetranuclear Cu_Z cluster that has a higher E_a. The lower E_a for Cu-ZSM-5 is attributed to the larger thermodynamic driving force resulting from formation of strong Cu^(II)-oxo bonds in the ZSM-5 framework

Spectroscopy and redox chemistry of copper in mordenite

Copper-containing zeolites, such as mordenite (MOR), have recently gained increased attention as a consequence of their catalytic potential. While the preferred copper loadings in these catalytic studies are generally high, the literature lacks appropriate spectroscopic and structural information on such Cu-rich zeolite samples. Higher copper loadings increase the complexity of the copper identity and their location in the zeolite host, but they also provide the opportunity to create novel Cu sites, which are perhaps energetically less favorable, but possibly more reactive and more suitable for catalysis. In order to address the different role of each Cu site in catalysis, we here report a combined electron paramagnetic resonance (EPR), UV/Vis-NIR and temperature-programmed reduction (TPR) study on highly copper-loaded MOR. Highly resolved diffuse reflectance (DR) spectra of the CuMOR samples were obtained due to the increased copper loading, allowing the differentiation of two isolated mononuclear Cu^(2+) sites and the unambiguous correlation with extensively reported features in the EPR spectrum. Ligand field theory is applied together with earlier suggested theoretical calculations to determine their coordination chemistry and location within the zeolite matrix, and the theoretical analysis further allowed us to define factors governing their redox behavior. In addition to monomeric species, an EPR-silent, possibly dimeric, copper site is present in accordance with its charge transfer absorption feature at 22200 cm^(-1), and quantified with TPR. Its full description and true location in MOR is currently being investigated

Oxygen precursor to the reactive intermediate in methanol synthesis by Cu-ZSM-5

The reactive oxidizing species in the selective oxidation of methane to methanol in oxygen activated Cu-ZSM-5 was recently defined to be a bent mono(μ-oxo)dicopper(II) species, [Cu_2O]^(2+). In this communication we report the formation of an O_2-precursor of this reactive site with an associated absorption band at 29,000 cm^(-1). Laser excitation into this absorption feature yields a resonance Raman (rR) spectrum characterized by ^(18)O_2 isotope sensitive and insensitive vibrations, νO-O and νCu-Cu, at 736 (Δ^(18)O_2 = 41 cm^(-1)) and 269 cm^(-1), respectively. These define the precursor to be a μ-(η^2:η^2) peroxo dicopper(II) species, [Cu_2(O_2)]^(2+). rR experiments in combination with UV-vis absorption data show that this [Cu_2(O_2)]^(2+) species transforms directly into the [Cu_2O]^(2+) reactive site. Spectator Cu^+ sites in the zeolite ion-exchange sites provide the two electrons required to break the peroxo bond in the precursor. O_2-TPD experiments with ^(18)O_2 show the incorporation of the second ^(18)O atom into the zeolite lattice in the transformation of [Cu_2(O_2)]^(2+) into [Cu_2O]^(2+). This study defines the mechanism of oxo-active site formation in Cu-ZSM-5

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