This work reports on the capability of the O2-activated Cu-ZSM-5 and Cu-MOR zeolites to selectively convert methane into methanol at a temperature of 398 K. A strong correlation between (i) the activity and (ii) the intensity of the 22 700 cm-1 UV-vis band, assigned to the bis(mu-oxo)dicopper core, is found (i) as a function of the reaction temperature, (ii) as a function of the Cu loading of the zeolite, and (iii) in comparison to other Cu materials. These three lines of evidence firmly support the key role of the bis(mu-oxo)dicopper core in this selective, low-temperature hydroxylation of methane.

Driven by the depletion of crude oil, the direct oxidation of methane to methanol has been of considerable interest. Promising low-temperature activity of an oxygen-activated zeolite, Cu-ZSM-5, has recently been reported in this selective oxidation and the active site in this reaction correlates with an absorption feature at 22,700 cm ؊1 . In the present study, this absorption band is used to selectively resonance enhance Raman vibrations of this active site. 18 O2 labeling experiments allow definitive assignment of the observed vibrations and exclude all previously characterized copper-oxygen species for the active site. In combination with DFT and normal coordinate analysis calculations, the oxygen activated Cu core is uniquely defined as a bent mono-( -oxo)dicupric site. Spectroscopically validated electronic structure calculations show polarization of the low-lying singly-occupied molecular orbital of the [Cu 2O] 2؉ core, which is directed into the zeolite channel, upon approach of CH 4. This induces significant oxyl character into the bridging O atom leading to a low transition state energy consistent with experiment and explains why the bent mono-( -oxo)dicupric core is highly activated for H atom abstraction from CH 4. The oxygen intermediate of Cu-ZSM-5 is now the most well defined species active in the methane monooxygenase reaction. density functional theory ͉ dicopper(II)-oxo ͉ oxygen activation ͉ resonance Raman spectroscopy ͉ zeolite

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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...

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/O 2 intermediates in biological, homogeneous, and heterogeneous catalysts exhibit unique spectral features that reflect novel geometric and electronic structures that make significant contributions to reactivity. This review considers how the respective intermediate electronic structures overcome the spin forbidden nature of O 2 binding, activate O 2 for electrophillic aromatic attack and H-atom abstraction, catalyze the 4 e-reduction of O 2 to H 2 O, and discusses the role of exchange coupling between Cu ions in determining reactivity.Our focus has been on the use of spectroscopic methods to elucidate active sites in catalysis. In the area of Cu/O 2 chemistry, this has mostly involved studies on metalloenzymes, however these have led to parallel studies in Cu coordination chemistry and now to studies on Cu sites in zeolites. There are five main topics in Cu/O 2 biological, homogeneous and heterogeneous reactivity that will be the scope of this overview. First is the spin-forbidden, reversible binding of dioxygen by the coupled binuclear Cu site in hemocyanin. Next, we will consider O 2 activation by coupled binuclear copper sites for electrophilic attack on phenolic substrates in tyrosinase and related model complexes. We will then consider Hatom abstraction from relatively weak C-H bonds (~85 kcal/mol) by the "non-coupled" binuclear Cu enzymes and how the difference in magnetic "exchange" coupling can control reactivity. We will then move to the four e-reduction of O 2 to H 2 O by the multi-copper oxidases at a trinuclear Cu cluster, a structural motif originally defined to be present by MCD spectroscopy. 1, 2 Finally, we will focus on O 2 activation for H-atom abstraction from the strong C-H bond of methane (~105 kcal/mol) which in biology is accomplished by methane monooxygenases (MMO) but can now be achieved in the active sites of zeolites . The copper-oxygen intermediates in these systems have unique spectroscopic features that we have shown to reflect novel geometric and electronic structures that make key contributions to reactivity. I. Reversible O 2 Binding: Coupled binuclear Cu SitesHemocyanin (Hc) functions as an extracellular oxygen transport protein in arthropods and mollusks. 3 Deoxy-Hc contains 2 Cu(I) ions that reversibly bind O 2 to form the binuclear cupric site in oxy-Hc. Thus, 2e − are transferred to O 2 reducing it to the peroxide level. As will be discussed below, oxy-Hc has unique spectral features, and to understand these we first consider "normal" peroxide-Cu(II) bonding. 4 O 2 is a triplet that has two unpaired electrons in the doubly degenerate π * orbitals. Reduction of O 2 to peroxide leads to a fully occupied π * HOMO. As shown in Fig. 1A, upon binding O 2 2− end-on to a Cu(II), one π * orbital is stabilized due to σ bonding with the d 9 Cu(II) half occupied d orbital, which is in turn destabilized. This leads to the characteristic EPR Correspondence to: Edward I. Solomon. NIH Public Access Author ManuscriptFaraday Discuss. Author manuscript; available in PMC 2012 Januar...

Zeolites containing transition metal ions (TMI) often show promising activity as heterogeneous catalysts in pollution abatement and selective oxidation reactions. In this paper, two aspects of research on the TMI Cu, Co and Fe in zeolites are discussed: (i) coordination to the lattice and (ii) activated oxygen species. At low loading, TMI preferably occupy exchange sites in six-membered oxygen rings (6MR) where the TMI preferentially coordinate with the oxygen atoms of Al tetrahedra. High TMI loadings result in a variety of TMI species formed at the zeolite surface. Removal of the extra-lattice oxygens during high temperature pretreatments can result in auto-reduction. Oxidation of reduced TMI sites often results in the formation of highly reactive oxygen species. In Cu-ZSM-5, calcination with O 2 results in the formation of a species, which was found to be a crucial intermediate in both the direct decomposition of NO and N 2 O and the selective oxidation of methane into methanol. An activated oxygen species, called α-oxygen, is formed in Fe-ZSM5 and reported to be the active site in the partial oxidation of methane and benzene into methanol and phenol, respectively. However, this reactive α-oxygen can only be formed with N 2 O, not with O 2 . O 2 activated Co intermediates in Faujasite (FAU) zeolites can selectively oxidize α-pinene and epoxidize styrene. In Co-FAU, Co III superoxo and peroxo complexes are suggested to be the active cores, whereas in Cu and Fe-ZSM-5 various monomeric and dimeric sites have been proposed, but no consensus has been obtained. Very recently, the active site in Cu-ZSM-5 was identified as a bent [Cu-O-Cu] 2+ core (Proc. Natl. Acad. Sci. USA 2009, 106, 18908-18913). Overall, O 2 activation depends on the interplay of structural factors such as type of zeolite, size of the channels and cages and chemical factors such as Si/Al ratio and the nature, charge and distribution of the charge balancing cations. The presence of several different TMI sites hinders the direct study of the spectroscopic features of the active site. Spectroscopic techniques capable of selectively probing these sites, even if they only constitute a minor fraction of the total amount of TMI sites, are thus required. Fundamental knowledge of the geometric and electronic structure of the reactive active site can help in the design of novel selective oxidation catalysts.

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