Theoretical study of methane adsorption on Zn(II) zeolites

The adsorption of methane on Zn-exchanged zeolites was studied theoreticall y by density functional theory (DFT). Diverse types of active sites were em ployed, such as: a Zn(ii) cation, which was placed on different rings (4T a nd 5T), a ZnO (in a tetramer configuration) cluster and a [Zn-O-Zn](2+) dim er complex. The Zn(ii) cation is the most exposed to probe molecules when s ituated on the "4T ring'' of zeolites. In this position, the cation activat es strongly two IR bands of methane (nu(1) and nu(3)). This activation is n ot observed when Zn(ii) sits on larger rings. The assignment of the band sh ifts found in the experimental IR spectra of methane adsorption, which were associated with the cation position in the zeolite framework, had to be re vised following the results for the calculated shifts. Larger shifts are as sociated with the cationic position at small rings. Methane prefers to adso rb physically in a 3-fold configuration on Zn(ii) at both 4T and 5T rings. This is not the case for ZnO and [Zn-O-Zn](2+) sites, in which the dissocia tive adsorption is more favorable. In the last two cases, the dissociation is in good agreement with the "alkyl path'', described previously in the li terature. The [Zn-O-Zn](2+) site appears to be one of the active forms of t he Zn cation on zeolites with a low Si/Al ratio. This site is more thermody namically stable than both Zn(ii) on 5T and (ZnO)(4). In the absence of Bro nsted sites, (ZnO)(4) does not decompose into Zn(ii) or [Zn-O-Zn](2+). Owin g to its size, this oxide cluster may only be stable in special positions i nside the pores. Different adsorption configurations and loadings were test ed in order to understand the large shift observed experimentally for the n u(1) band of methane. The largest shifts calculated here were found for 3-f old physisorption on ZnO and Zn(ii) ("4T ring''), as well as in dissociativ e adsorption on the [Zn-O-Zn](2+) site. None of these models, however, coul d describe this large experimental shift. The same results were also observ ed when these systems were embedded in a solvent cage, which mimicked the f ramework environment around the active site.