ABSTRACT
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.
ABSTRACT
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).
