Copper dioxygen (bio)inorganic chemistry.

Cu/O2 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 O2 binding, activate O2 for electrophilic aromatic attack and H-atom abstraction, catalyze the 4 e- reduction of O2 to H2O, and discusses the role of exchange coupling between Cu ions in determining reactivity.

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(2)O](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(2)O](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(2)O](2+). This study defines the mechanism of oxo-active site formation in Cu-ZSM-5.

Transition-metal ions in zeolites: coordination and activation of oxygen.

Zeolites containing transition-metal ions (TMIs) often show promising activity as heterogeneous catalysts in pollution abatement and selective oxidation reactions. In this paper, two aspects of research on the TMIs Cu, Co, and Fe in zeolites are discussed: (i) coordination to the lattice and (ii) activated oxygen species. At low loading, TMIs preferably occupy exchange sites in six-membered oxygen rings (6MR), where the TMIs preferentially coordinate with the O atoms of Al tetrahedra. High TMI loadings result in a variety of TMI species formed at the zeolite surface. Removal of the extralattice O atoms during high-temperature pretreatments can result in autoreduction. 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 alpha-O, 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 alpha-O can only be formed with N(2)O, not with O(2). O(2)-activated Co intermediates in faujasite (FAU) zeolites can selectively oxidize alpha-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. U.S.A. 2009, 106, 18908-18913). Overall, O(2) activation depends on the interplay of structural factors such as the type of zeolite and sizes of the channels and cages and chemical factors such as the 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 structures of the reactive active site can help in the design of novel selective oxidation catalysts.

A [Cu2O]2+ core in Cu-ZSM-5, the active site in the oxidation of methane to methanol.

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)O(2) 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-(mu-oxo)dicupric site. Spectroscopically validated electronic structure calculations show polarization of the low-lying singly-occupied molecular orbital of the [Cu(2)O](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-(mu-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.

Selective oxidation of methane by the bis(mu-oxo)dicopper core stabilized on ZSM-5 and mordenite zeolites.

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.