The present study focuses on the characterization of the active sites for oxygen adsorption in both copper-free and copper-containing HZSM5 zeolites. FTIR, EPR, EXAFS and UV-Vis measurements offer insight into the initial state of the catalyst before oxygen adsorption. Both liquid and solid state ion exchanged samples contain a certain amount of Cu(ii) and Cu(i) ions in the alpha3, alpha4 and gamma6 position, their population ratio depending on the ion exchange temperature. They are accessible for interaction with the adsorbate, as the copper-oxygen spin exchange demonstrates. Both the sample magnetization and the EXAFS analysis indicate that 10-30% of the Cu(ii) exists in the form of oxygen bridged Cu-Cu pairs. UV-Vis measurements prove that two different antiferromagnetically coupled copper peroxide complexes are formed during the sample preparation process, the bis(mu-oxo)- and (mu-eta(2):eta(2)-peroxo) dimers. One of the complexes is susceptible to oxygen adsorption, which cleaves it irreversibly into two individual Cu(ii)-O(2)(-) units, while Cu(i) ions are oxidised to the same species. The Brønsted acid sites are also able to adsorb oxygen both at room and low temperatures. The presence of the different active sites may be an explanation for the high catalytic activity of the Cu/HZSM5 zeolite. The Brønsted sites near copper centers could protonate the peroxide complexes, leading to the in situ formation of hydrogen peroxide, a common oxidant. This peroxide would be a highly active species for catalytic reactions.

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.

The incorporation of manganese into the amorphous mesoporous material TUD-1 (n Si/n Mn = 89−8; Mn content:  1−11 wt %) was achieved by the direct hydrothermal method in the presence of triethanol amine. Nitrogen sorption revealed the mesoporous nature (d P = 4−9 nm) of MnTUD-1 and a decreasing specific surface area with increasing Mn content. FT-IR spectroscopy indicates the incorporation of Mn into the silica framework of TUD-1. As evident from DR-UV−vis and EPR spectroscopy, the Mn atoms in MnTUD-1 exist in the oxidation states +2 or +3. The fraction of Mn2+ is <1 % in the as-prepared samples and drops to zero upon further calcination in oxygen. With increasing Mn content, Mn2O3 as an extraframework species is detected both by XRD and by high-resolution transmission electron microscopy. According to EPR, a fraction of the Mn atoms, decreasing with the n Si/n Mn ratio, is antiferromagnetically coupled. The uncoupled, that is, isolated, Mn species in the framework play a predominant role in the catalytic activity of MnTUD-1 in the liquid-phase epoxidation of styrene or trans-stilbene with tert-butyl hydroperoxide as an oxidant. The MnTUD-1 catalysts are highly stable toward leaching of Mn, in contrast to MnO x supported on macroporous silica gel, and they are significantly more active than the previously reported catalysts MnMCM-41 or MnSBA-15.