An efficient catalytic process for converting methane into methanol could have far-reaching economic implications. Iron-containing zeolites (microporous aluminosilicate minerals) are noteworthy in this regard, having an outstanding ability to hydroxylate methane rapidly at room temperature to form methanol. Reactivity occurs at an extra-lattice active site called α-Fe(ii), which is activated by nitrous oxide to form the reactive intermediate α-O; however, despite nearly three decades of research, the nature of the active site and the factors determining its exceptional reactivity are unclear. The main difficulty is that the reactive species-α-Fe(ii) and α-O-are challenging to probe spectroscopically: data from bulk techniques such as X-ray absorption spectroscopy and magnetic susceptibility are complicated by contributions from inactive 'spectator' iron. Here we show that a site-selective spectroscopic method regularly used in bioinorganic chemistry can overcome this problem. Magnetic circular dichroism reveals α-Fe(ii) to be a mononuclear, high-spin, square planar Fe(ii) site, while the reactive intermediate, α-O, is a mononuclear, high-spin Fe(iv)=O species, whose exceptional reactivity derives from a constrained coordination geometry enforced by the zeolite lattice. These findings illustrate the value of our approach to exploring active sites in heterogeneous systems. The results also suggest that using matrix constraints to activate metal sites for function-producing what is known in the context of metalloenzymes as an 'entatic' state-might be a useful way to tune the activity of heterogeneous catalysts.

Two distinct [Cu-O-Cu](2+) sites with methane monooxygenase activity are identified in the zeolite Cu-MOR, emphasizing that this Cu-O-Cu active site geometry, having a ∠Cu-O-Cu ∼140°, is particularly formed and stabilized in zeolite topologies. Whereas in ZSM-5 a similar [Cu-O-Cu](2+) active site is located in the intersection of the two 10 membered rings, Cu-MOR provides two distinct local structures, situated in the 8 membered ring windows of the side pockets. Despite their structural similarity, as ascertained by electronic absorption and resonance Raman spectroscopy, the two Cu-O-Cu active sites in Cu-MOR clearly show different kinetic behaviors in selective methane oxidation. This difference in reactivity is too large to be ascribed to subtle differences in the ground states of the Cu-O-Cu sites, indicating the zeolite lattice tunes their reactivity through second-sphere effects. The MOR lattice is therefore functionally analogous to the active site pocket of a metalloenzyme, demonstrating that both the active site and its framework environment contribute to and direct reactivity in transition metal ion-zeolites.

A novel catalyst design for the conversion of mono- and disaccharides to lactic acid and its alkyl esters was developed. The design uses a mesoporous silica, here represented by MCM-41, which is filled with a polyaromatic to graphite-like carbon network. The particular structure of the carbon-silica composite allows the accommodation of a broad variety of catalytically active functions, useful to attain cascade reactions, in a readily tunable pore texture. The significance of a joint action of Lewis and weak Brønsted acid sites was studied here to realize fast and selective sugar conversion. Lewis acidity is provided by grafting the silica component with Sn(IV), while weak Brønsted acidity originates from oxygen-containing functional groups in the carbon part. The weak Brønsted acid content was varied by changing the amount of carbon loading, the pyrolysis temperature, and the post-treatment procedure. As both catalytic functions can be tuned independently, their individual role and optimal balance can be searched for. It was thus demonstrated for the first time that the presence of weak Brønsted acid sites is crucial in accelerating the rate-determining (dehydration) reaction, that is, the first step in the reaction network from triose to lactate. Composite catalysts with well-balanced Lewis/Brønsted acidity are able to convert the trioses, glyceraldehyde and dihydroxyacetone, quantitatively into ethyl lactate in ethanol with an order of magnitude higher reaction rate when compared to the Sn grafted MCM-41 reference catalyst. Interestingly, the ability to tailor the pore architecture further allows the synthesis of a variety of amphiphilic alkyl lactates from trioses and long chain alcohols in moderate to high yields. Finally, direct lactate formation from hexoses, glucose and fructose, and disaccharides composed thereof, sucrose, was also attempted. For instance, conversion of sucrose with the bifunctional composite catalyst yields 45% methyl lactate in methanol at slightly elevated reaction temperature. The hybrid catalyst proved to be recyclable in various successive runs when used in alcohol solvent.

Oxygen activated Cu-ZSM-5 has been recently shown to selectively oxidize methane to methanol at low temperatures 1 by means of a mono(μ-oxo)dicopper(II) species, [Cu 2 O] 2+ . 2 The geometric and electronic structure of this reactive core was unambiguously assigned using resonance Raman (rR) spectroscopy and density functional theory (DFT) and represents a new species in inorganic chemistry. DFT calculations reproduced the low reaction barrier and kinetic isotope effect (KIE) measured experimentally and showed that the low barrier for H-atom abstraction from CH 4 reflects the strong [Cu 2 O-H] 2+ bond in the initial product and a frontier molecular orbital (FMO) that polarizes to an oxyl (O -· ) along the reaction coordinate. Interestingly, a binuclear Cu site has recently been demonstrated to be the reactive site in particulate methane monooxygenase (pMMO), an enzyme that also oxidizes methane to methanol. 3 In this study we observe an oxygen precursor to the formation of the [Cu 2 O] 2+ species in Cu-ZSM-5 and, using rR spectroscopy, define its structure as a side-on bridged μ-(η 2 -η 2 ) peroxo dicopper (II) Na-ZSM-5 (VAW, Si/Al=12) samples were ion-exchanged with aqueous solutions of varied Cu(II)-acetate concentrations. 4 The samples were initially calcined under O 2 at 450°C for 2h (5°C/min, 50ml/min), followed by He flow overnight (50ml/min). This treatment results in the auto-reduction of the Cu sites in Cu-ZSM-5. [5][6][7] Fiber optic UV-vis spectroscopy was used to monitor spectral changes of Cu-ZSM-5 at ambient and elevated temperatures, and rR measurements were performed to obtain the electronic and geometric structure information regarding the Cu/O 2 species in Cu-ZSM-5. MS was used to monitor the O-isotope distribution in O 2 -TPD experiments.When pre-reduced Cu-ZSM-5 (He at 450°C; Cu/Al=0.5) was exposed to O 2 at room temperature (RT) an absorption band at ∼29,000 cm -1 is rapidly formed ( Figure 1A). After ∼2 min in O 2 flow, the intensity increase of this absorption band levels off. This band is also observed in a Cu-ZSM-5 sample with Cu/Al=0.3 and is essentially absent in samples with Cu/Al<0.2 (see Figures S1A and B). After full formation of the 29,000 cm -1 band, the sample was flushed in He to remove excess O 2 at RT. Subsequent heating of Cu-ZSM-5 (Cu/Al=0.3) in He atmosphere resulted in the UV-vis spectral changes shown in Figure 1B. The rR spectum of the oxygen precursor species formed at RT obtained with laser excitation at 363.8 nm (27,473 cm -1 ) is shown in Figure 2A. Vibrational features are observed at 269 and 736 cm -1 that are not present using laser excitation outside of the 29,000 cm -1 band, proving that they are resonance enhanced by the species responsible for this absorption feature. When the RT treatment of the auto-reduced Cu-ZSM-5 sample is performed with isotope labeled 18 O 2 , the 736 cm -1 feature shifts to 695 cm -1 (Δ 18 O 2 =41 cm -1 ) while the 269 cm -1 feature is isotope insensitive. These vibrational frequencies and isotope perturbation pattern are character...

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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.

Understanding the formation mechanism of the [Cu2O](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 N2O decomposition. Spectroscopically validated DFT calculations are used here to evaluate the thermodynamic and kinetic requirements for formation of [Cu2O](2+) active sites from the reaction between binuclear Cu(I) sites and N2O 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 N2O to generate the experimentally observed [Cu2O](2+) site. Kinetically, the reaction coordinate was evaluated for two representative binuclear Cu(I) sites. When the Cu-Cu distance is sufficiently short (<4.2 Å), N2O can bind in a "bridged" μ-1,1-O fashion and the oxo-transfer reaction is calculated to proceed with a low activation energy barrier (2 kcal/mol). This is in good agreement with the experimental Ea for N2O activation (2.5 ± 0.5 kcal/mol). However, when the Cu-Cu distance is long (>5.0 Å), N2O binds in a "terminal" η(1)-O fashion to a single Cu(I) site of the dimer and the resulting E(a) for N2O activation is significantly higher (16 kcal/mol). Therefore, bridging N2O between two Cu(I) centers is necessary for its efficient two-electron activation in [Cu2O](2+) active site formation. In nature, this N2O reduction reaction is catalyzed by a tetranuclear CuZ 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.

Ternary Ag/Magnesia-silica catalysts were tested in the direct synthesis of 1,3-butadiene from ethanol. The influence of the silver content and the type of silica source on catalytic performance has been studied. Prepared catalysts were characterized by (29) Si NMR, N2 sorption, small-angle X-ray scattering measurements, XRD, environmental scanning electron microscopy with energy dispersive X-ray analysis (ESEM/EDX), FTIR spectroscopy of adsorbed pyridine and CO2 , temperature-programmed desorption of CO2 and UV/Vis diffuse reflectance spectroscopy. Based on these characterization results, the catalytic performance of the catalysts in the 1,3-butadiene formation process was interpreted and a tentative model explaining the role of the different catalytically active sites was elaborated. The balance of the active sites is crucial to obtain an active and selective catalyst to form 1,3-butadiene from ethanol. The optimal silver loading is 1-2 wt% on a MgO-silica support with a molar Mg/Si ratio of 2. The silver species and basic sites (Mg−O pairs and basic OH groups) are of prime importance in the 1,3-butadiene production, catalyzing mainly the ethanol dehydrogenation and the aldol condensation, respectively.