Marijke H. Groothaert

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. 18O2 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-(μ-oxo)dicupric site. Spectroscopically validated electronic structure calculations show polarization of the low-lying singly-occupied molecular orbital of the [Cu2O]2+ core, which is directed into the zeolite channel, upon approach of CH4. 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-(μ-oxo)dicupric core is highly activated for H atom abstraction from CH4. The oxygen intermediate of Cu-ZSM-5 is now the most well defined species active in the methane monooxygenase reaction.

A reinterpretation of the EPR spectra of Cu(II) in zeolites A, Y and ZK4, based on ab initio cluster model calculations

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 different Al contents, as observed for the different 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” for a position of both their oxygens in the first Cu(II) coordination sphere. The electronic spectra and EPR g-tensors of all optimized cluster models were calculated by means of the CASPT2 method (multiconfigurational perturbation theory based on a complete-active-space reference wavefunction), with inclusion of spin–orbit coupling. These calculations pointed to the appearance of two distinct EPR-signals in connection with the two different 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 different 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.

Identification of Cu(II) coordination structures in Cu-ZSM-5, based on a DFT/ab initio assignment of the EPR spectra

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∥ = 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 α, a six-ring with bridging T-site, containing 2 lattice Als. The axially symmetric species with g∥ = 2.26–2.28, present at medium copper loadings, is assigned to a square-planar 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∥ = 2.26–2.28 signal at the highest Cu/Al ratios is explained by the coordination of ELO to Cu(II) in these sites with one Al, yielding an EPR silent species.

The Coordination of CuII in Zeolites − Structure and Spectroscopic Properties

Zeolites loaded with transition metal ions are promising heterogeneous catalysts. Knowledge about the location and structure of the metal centers is of paramount importance for the understanding of the catalytic potential of these materials. In this work, the spectroscopic studies of the coordination of CuII in zeolite A, ZK4, X, Y and mordenite are reviewed. Experimentally, diffuse reflectance spectroscopy (DRS) and electron spin resonance (ESR) spectroscopy have been applied to study the coordination of CuII in zeolites. Ab initio calculations on model clusters, representing the CuII sites in zeolites, are used for the interpretation of the experimental data. The combination of experimental spectroscopic information with theoretical results leads to a new and profound insight into the CuII−zeolite interaction.

Study of the coordination of Cu2+ in zeolite Y: Interaction with water and ammonia

The ligand field spectrum of Cu(II ) exchanged zeolite Y, obtained after saturation with H 2 O and NH 3 and during the gradual desorption of these ligands, was measured by diVuse reflectance spectroscopy (DRS ). DFT and ab-initio calculations on several model clusters were performed to interpret the spectra. The structure of the model clusters was optimized by means of density functional theory (DFT ), using the B3LYP functional. The electronic spectra of the models were calculated using multiconfigurational perturbation theory based on a CASSCF wavefunction (CASPT2) and compared with the DRS spectra. Firstly, several [Cu(NH 3 ) y (H 2 O) x ]2+ complexes were studied. It was shown that in fully hydrated Cu(II )Y, a [Cu(H 2 O) 6 ]2+ complex can be present in the cages of the zeolite. In Cu(II )Y, saturated with NH 3 , the [Cu(NH 3 ) 4 ]2+ complex is present, but the Cu2+ center in this complex must still be coordinated to one or two lattice oxygens in the zeolite. Secondly, calculations were performed on large cluster models, representing the adsorption complexes of one H 2 O or one NH 3 ligand on Cu2+ in the six-ring sites in zeolite Y. The ligand field spectrum of partially dehydrated and deammoniated Cu(II )Y shows d‐d transitions at a lower energy than the spectrum of fully dehydrated Cu(II )Y, which is confirmed by the CASPT2 results of the six-ring clusters.

The siting of Cu(II) in mordenite: a theoretical spectroscopic study

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)+. The calculations indicate also that in fully dehydrated mordenite, the twisted eight-ring (site A) is not occupied by Cu(II).

Bis(μ-OXO)dicopper as intermediate in the catalytic decomposition of No over Cu-ZSM-5

Abstract Novel spectroscopic methods were elaborated to tackle the intriguing question of the active site in Cu-ZSM-5 catalyzing the decomposition of NO. First, a DFT/ ab initio approach was developed, allowing to assign the experimental EPR spectra of Cu-ZSM-5 to representative Cu-zeolite structures. Second, an optical fiber UV-vis set-up was optimized, permitting to monitor the events taking place on the catalyst under reaction conditions. The computational study showed that both EPR signals result from bare Cu(II) ions, i.e. without coordinated extra-lattice oxygen ligands. Studying the NO decomposition activity in function of the Cu/Al ratio of the samples, indicated a sharp increase in TOF between Cu/Al = 0.2 and 0.3. Concomitantly, at the latter Cu/Al ratio, an EPR silent species is formed that is characterized by an intense band at 22700 cm 1 in UV-vis. EXAFS identified it as a dimeric Cu species with CuCu distance of 2.87 . Combining all spectroscopic data and comparing them with the well-characterized copper centers in enzymes and synthetic model complexes led to the identification of the bis(μ-oxo)dicopper core, i.e. [Cu 2 (μ-O) 2 ] 2+ . The operando UV-vis approach assigned the bis(μ-oxo)dicopper core as a key intermediate in the NO decomposition reaction, allowing the smooth formation and desorption of O 2 .

Local Site Deformations in Zeolites by the Coordination of Cu(II)

The electronic and ESR spectra of Cu(II)-exchanged zeolites were interpreted by means of ab initio calculations. The Cu(II) coordination in the crystal sites was studied by partial geometry optimizations of Cu(II) clusters using DFT. The corresponding Cu(II) ligand field spectrum and g-factors were calculated using multiconfigurational perturbation theory (CASPT2), and compared with experiment. It was shown that Cu(II) induces strong lattice deformations in the cation sites to achieve a four-fold planar coordination. The Cu(II) spectroscopic features are believed to be influenced by both the geometry and the Si/A1 distribution of the cation site.

15-P-11-A theoretical/spectroscopic study of the coordination of transition metal ions in zeolites

Publisher Summary This chapter presents a theoretical or spectroscopic study of the coordination of transition-metal ions in zeolites. Knowledge about the coordination of transition-metal ions in zeolites is important for the understanding of the catalytic properties of these materials. Electron spin resonance (ESR) and diffuse reflectance spectroscopy (DRS) provide spectroscopic signatures for Cu 2+ in different zeolites. High-level theoretical calculations are used for the assignment of the spectroscopic signals to specific coordination modes.