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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In situ XAFS combined with UV-vis-near-IR spectroscopy are used to identify the active site in copper-loaded ZSM-5 responsible for the catalytic decomposition of NO. Cu-ZSM-5 was probed with in situ XAFS (i) after O(2) activation and (ii) while catalyzing the direct decomposition of NO into N(2) and O(2). A careful R-space fitting of the Cu K-edge EXAFS data is presented, including the use of different k-weightings and the analysis of the individual coordination shells. For the O(2)-activated overexchanged Cu-ZSM-5 sample a Cu.Cu contribution at 2.87 A with a coordination number of 1 is found. The corresponding UV-vis-near-IR spectrum is characterized by an intense absorption band at 22 700 cm(-1) and a relatively weaker band at 30 000 cm(-1), while no corresponding EPR signal is detected. Comparison of these data with the large databank of well-characterized copper centers in enzymes and synthetic model complexes leads to the identification of the bis(mu-oxo)dicopper core, i.e. [Cu(2)(mu-O)(2)](2+). After dehydration in He, Cu-ZSM-5 shows stable NO decomposition activity and the in situ XAFS data indicate the formation of a large fraction of the bis(mu-oxo)dicopper core during reaction. When the Cu/Al ratio of Cu-ZSM-5 exceeds 0.2, both the bis(mu-oxo)dicopper core is formed and the NO decomposition activity increases sharply. On the basis of the in situ measurements, a reaction cycle is proposed in which the bis(mu-oxo)dicopper core forms the product O(2) on a single active site and realizes the continuous O(2) release and concomitant self-reduction.

A theoretical study is presented of the EPR spectra of the dehydrated Cu(II)-A, Cu(II)-Y and Cu(II)-ZK4 zeolites. B3LYP-DFT geometry optimizations were performed on cluster models representing six-ring sites with di †erent Al contents, as observed for the di †erent zeolites. All calculated structures indicated a strong preference of the Cu(II) ion for coordination to oxygens bound to Al rather than Si, together with a striving for a planar four-fold oxygen coordination in the six-rings. Depending on the number and relative positions of the aluminiums in the ring two distinct four-fold coordination modes were distinguished, containing either only one or several aluminiums "" competing ÏÏ for a position of both their oxygens in the Ðrst Cu(II) coordination sphere. The electronic spectra and EPR g-tensors of all optimized cluster models were calculated by means of the CASPT2 method (multiconÐgurational perturbation theory based on a complete-active-space reference wavefunction), with inclusion of spinÈorbit coupling. These calculations pointed to the appearance of two distinct EPR-signals in connection with the two di †erent four-fold coordination modes. Based on the close correspondence between the calculated g-factors and the experimental EPR-signals of the three zeolites under investigation, a new interpretation of the latter signals is suggested. According to this new interpretation the occurrence of two EPR signals in zeolite Y as opposed to only one signal in zeolite A is connected to the higher Si/Al ratio in the former zeolite, rather than to a di †erent topology (as was suggested in earlier assignments of the spectra). Our new interpretation is corroborated by the experimental EPR signals obtained for Cu-ZK4 : with the same topology as zeolite A, but containing a Si/Al ratio closer to zeolite Y, two rather than one Cu(II) EPR signals were indeed observed. Finally, our calculations also indicate that, in six-rings containing more than one aluminium, Cu(II) is likely to undergo a hopping process at room temperature.

The coordination structures of Cu(II) exchanged into ZSM-5 were obtained by B3LYP-DFT geometry optimizations on cluster models, representing the cation sites. The EPR g-factors of the resulting cluster models were calculated by means of the CASPT2 method (multi-configurational perturbation theory), with the inclusion of spin-orbit coupling. In order to facilitate the confrontation of theoretical and experimental results, the EPR spectra of a selection of dehydrated Cu(II)-ZSM-5 samples are presented as well. The axially symmetric signal with g k ¼ 2.30-2.33, which is present over the whole range of copper loadings, is assigned to a five-fold or distorted three-fold Cu(II) coordination in site a, a six-ring with bridging T-site, containing 2 lattice Al's. The axially symmetric species with g k ¼ 2.26-2.28, present at medium copper loadings, is assigned to a squareplanar Cu(II) coordination in six-rings and a square-pyramidal Cu(II) coordination in five-rings, with both rings containing only one Al and no extra-lattice oxygen (ELO). The near absence of the g k ¼ 2.26-2.28 signal at the highest Cu/Al ratio's is explained by the coordination of ELO to Cu(II) in these sites with one Al, yielding an EPR silent species.

The role of the bis(µ-oxo)dicopper core, i.e., [Cu 2 (µ-O) 2 ] 2+ , in the decomposition of NO and N 2 O by the Cu-ZSM-5 zeolite has been studied with combined operando UV-vis monitoring of the catalyst and on-line GC analysis. An optical fiber was mounted on the outer surface of the quartz wall of the plug-flow reactor and collected the UV-vis diffuse reflectance spectra under true catalytic conditions.

The siting of Cu(II) in mordenite has been studied by ab initio calculations on large cluster models, representing the cation exchange sites in mordenite. Partial geometry optimizations, based on density functional theory (DFT), were performed to obtain the structure of the coordination environment of Cu(II) at the different sites. The ligand field spectra and EPR g-tensors of these clusters were then calculated by means of multiconfigurational perturbation theory (CASPT2). The calculated results were compared with experimental information, obtained by diffuse reflectance spectroscopy (DRS) and EPR. The calculations indicate that at low exchange levels Cu(II) is coordinated to oxygen six-rings in the main channel of mordenite, in the presence of two aluminiums. At higher loadings, six-or five-rings containing only one aluminium also become occupied, where Cu(II) is coordinated as a single ion, not as (Cu-OH) þ . The calculations indicate also that in fully dehydrated mordenite, the twisted eight-ring (site A) is not occupied by Cu(II).