The adsorption of dioxygen to copper in CuHY zeolites has been studied by means of FTIR spectroscopy and model calculations at the quantum mechanical/molecular mechanics (QM/MM) level. Different Si/Al ratios, substitution patterns and adsorption sites within the cavities of the zeolite lead to a large number of different isomers to be studied. In addition, these parameters control the end-on vs. side-on adsorption of dioxygen. High-level multireference benchmark calculations for the singlet and triplet states of such adsorption complexes corroborate the use of density functional theory for the investigation of these systems. Comparison of the experimental and computed data allows for the identification of a preferred adsorption site and a small number of isomers which appear to be most relevant for the adsorption process. Redshifts of >250 cm(-1) are obtained for the vibrational frequencies of adsorbed O(2).

Adsorption of benzene to copper-exchanged Y zeolites has been studied employing Fourier transform infrared spectroscopy and QM/MM calculations. Experimentally, both Cu(I) and Cu(II)-Y zeolites were used as adsorbents. Benzene adsorption was observed on both copper and Brønsted acid sites, but the copper site is preferred. The FTIR spectra show the activation of the ν 13 in-plane ring stretching mode at 1471 and 1479 cm -1 for Cu and Brønsted sites, respectively. With respect to the gas-phase value, Cu sites cause a red shift of 13 cm -1 whereas Brønsted sites show a shift of 5 cm -1 only. Theoretical investigations were performed on complexes of benzene with Cu-exchanged Y zeolites at two different sites, SII and SIII, with different Si/Al ratios. An η 2 -coordination of benzene to copper was observed for all adsorption complexes. After adsorption, the 3-fold coordination of Cu to framework oxygens is reduced to 2-fold at site SII, whereas 2-fold coordination prevails at site SIII. Benzene binds with Cu more strongly at SIII than at SII. The calculated frequency shifts are slightly overestimated compared to experiment. Population analysis reveals that 3dπ back-donation from Cu dominates over pπ donation from benzene. Comparison between experimental and theoretical results allows for the prediction of the preferred benzene-copper adsorption complexes, which may be of importance in the catalytic phenol formation from benzene and dioxygen using copper-containing zeolites.

Possible precursor complexes for the aerobic oxidation of benzene to phenol in CuHY zeolites were studied by QM/MM calculations. Structures limited to just one copper center were determined, in which both, benzene and dioxygen are adsorbed to and activated by the same transition metal cation. Frequency analyses reveal that these complexes can be identified by the frequency shift of the O(2) stretching mode.

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