Methane Activation over Zn-Modified MFI Zeolite: NMR Evidence for Zn−Methyl Surface Species Formation

Abstract

The early stages of methane activation over Zn-modified H-MFI catalysts obtained by high-temperature reaction with zinc vapor have been studied by 13 C MAS NMR in situ. Methane 99.9% enriched with 13 C was used as labeled reactant. The spectroscopic data pointed to formation of zinc methyl species at ambient temperature just after methane adsorption onto the zeolite sample. The results suggest that methane activation occurs via dissociative adsorption over acid-base Zn-O pairs involving Zn 2+ cations and negatively charged oxygen atoms of the zeolite framework. The nature of the sites responsible for the dissociation is discussed.

1H and 13C MAS NMR studies of light alkanes activation over MFI zeolite modified by Zn vapour

“…The analysis of the literature data shows that the chemical shifts of these lines are characteristic for the methyl groups directly bonded to metal atom [9,10,23,24]. The electropositive metal cations induce significant shielding of carbon nuclei attached to them, which results in pronounced shift of the corresponding NMR lines to higher field with respect to those in organic compounds.…”

“…The role of Zn-O sites in alkanes activation was thus established in detail. On the contrary, as far as our knowledge concerned no NMR data were available until recently [10] on alkanes activation over Zn 2+ cations, which are considered to be the most efficient in light alkanes aromatization [3].…”

Methane Interaction with Zn2+-Exchanged Zeolite H-ZSM-5: Study of Adsorption and Mobility by One- and Two-Dimensional Variable-Temperature 1H Solid-State NMR

“…Methane (≥ 99.0 % purity) was purchased from Aldrich Chemical Company Inc. and was used without further purification. 4 The hydrogen form of the zeolite (H-ZSM-5) was prepared from the parent ammonium form by calcination at 773 K for 5 h. The silicon-to-aluminum ratio of 13 was estimated for H-ZSM-5 zeolite based on 29 Si MAS NMR analysis 18 and confirmed by chemical analysis data. No extra framework aluminum atoms were observed in the 27 Al MAS NMR spectrum of the hydrated H-ZSM-5 zeolite.…”

“…[4][5][6][7][8][9] The initial stages of C 1 -C 2 alkanes activation by Zn-modified zeolites have been studied in detail. Theoretical studies predicted the light alkane activation to occur via heterolytic dissociation of C-H bond over Zn δ+ -O δ-pairs.…”

“…[10][11][12][13][14][15] Later on, these theoretical results were confirmed experimentally by IR and NMR spectroscopic methods. [4][5][6][8][9][16][17] With regard to the molecular adsorption of methane on Zn-modified zeolites, which precedes the alkane dissociation, only theoretical studies are presented in the literature. 11,14 Physisorption of methane has been considered using different active sites possible for zinc cation, the appropriate values of the energies of methane adsorption have been calculated.…”