Theoretical study of methane adsorption on Zn(II) zeolites

Abstract

The adsorption of methane on Zn-exchanged zeolites was studied theoretically by density functional theory (DFT). Diverse types of active sites were employed, such as: a Zn(II) cation, which was placed on different rings (4T and 5T), a ZnO (in a tetramer configuration) cluster and a [Zn–O–Zn]²⁺ dimer complex. The Zn(II) cation is the most exposed to probe molecules when situated on the “ 4T ring” of zeolites. In this position, the cation activates strongly two IR bands of methane (ν₁ and ν₃). This activation is not observed when Zn(II) sits on larger rings. The assignment of the band shifts found in the experimental IR spectra of methane adsorption, which were associated with the cation position in the zeolite framework, had to be revised following the results for the calculated shifts. Larger shifts are associated with the cationic position at small rings. Methane prefers to adsorb 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 dissociative adsorption is more favorable. In the last two cases, the dissociation is in good agreement with the “alkyl path”, described previously in the literature. The [Zn–O–Zn]²⁺ site appears to be one of the active forms of the Zn cation on zeolites with a low Si/Al ratio. This site is more thermodynamically stable than both Zn(II) on 5T and (ZnO)₄. In the absence of Brønsted sites, (ZnO)₄ does not decompose into Zn(II) or [Zn–O–Zn]²⁺. Owing to its size, this oxide cluster may only be stable in special positions inside the pores. Different adsorption configurations and loadings were tested in order to understand the large shift observed experimentally for the ν₁ band of methane. The largest shifts calculated here were found for 3-fold physisorption on ZnO and Zn(II) (“4T ring”), as well as in dissociative adsorption on the [Zn–O–Zn]²⁺ site. None of these models, however, could describe this large experimental shift. The same results were also observed when these systems were embedded in a solvent cage, which mimicked the framework environment around the active site.