Mechanism of CH₄Activation on a Monomeric Zn²⁺-Ion Exchanged in MFI-Type Zeolite with a Specific Al Arrangement: Similarity to the Activation Site for H₂

Abstract

In this work, we used both experimental and density functional theory (DFT) calculation methods to examine the mechanism of CH 4 activation taking place on the Zn 2+ ion exchanged MFI-type zeolite (ZnMFI). The heterolytic dissociation of CH 4 on ZnMFI around 300 K was observed experimentally, causing the appearance of IR bands at 3615, 2930, and 2892 cm −1 . The first band can be assigned to the OH stretching vibration associated with the formation of the Brønsted acid site and the latter to the C−H stretching modes ascribable to the −[ZnCH 3 ] + species. Combining the IR spectroscopy with a DFT calculation, it is apparent that the heterolytic C−H bond dissociation of CH 4 has an activation energy of 15 kJ mol −1 and takes place on a monomeric Zn 2+ at the M7S2 site. The M7S2 site has a specific Al arrangement in MFI and exhibits a pronounced reactivity for the H−H bond cleavage of H 2 , even at room temperature. In addition, to our knowledge, we are the first to succeed in explaining the dissociation process of CH 4 by applying natural bond orbital (NBO) and interaction localized orbital (ILO) analyses to the present system; the donation interaction from the CH 4 −σ(C−H) orbital to the Zn−4s orbital triggers the cleavage of the C−H bond of CH 4 under mild conditions.

Framework Effects on Activation and Functionalisation of Methane in Zinc‐Exchanged Zeolites

“…Neither FER or MOR frameworks, modified with zinc, have been previously reported to activate methane. Zn/CH 4 /ZSM‐5‐IE, prepared by aqueous ion exchange (IE) of Zn 2+ ions, also is able to activate methane, as demonstrated by the 13 C NMR signal observed at −19 ppm (Figure b), in line with observations reported by other groups . It should be noted that the conditions required to observe C−H activation of methane with the IE sample (Zn/CH 4 /ZSM‐5‐IE) were based on those reported in the literature but are substantially different to those required using samples prepared by CVD with zinc vapour.…”

Reactivity of Methane and Benzene over Metal/MFI Zeolite Analyzed with Temperature‐Programmed Reaction Technique

“…Rh 1 O 5 species on MFI zeolite promoted methane partial oxidation . In addition those, it has been reported that metal species on zeolite (mainly MFI) had quite unique activities for methane‐related reactions, i. e., aromatization of methane, combustion of methane, selective catalytic reduction of NOx with methane, and methane activation at low temperature . Therefore, metal species supported on zeolite (or MFI zeolite) possibly have the ability for methane activation.…”

Synthesis of an unexpected [Zn2]2+ species utilizing an MFI-type zeolite as a nano-reaction pot and its manipulation with light and heat

“…[12][13][14] The final number shown in the sample notation indicates the ion exchange capacity that was evaluated by assuming one Zn 2+ -ion exchange per two protons in MFI.…”

“…Previously, our studies confirmed that this B3LYP-optimized MFI zeolite model comprised of 10-MR in cavities reproduced well the experimentally observed data. [12][13][14]22,[24][25][26][27][28][29][30][31] Taking our previous studies into consideration, we used the M7 site for the Al substitution position; 32 and we defined the Al configuration as the [M7-S2] arrangement. [12][13][14]30 To optimize fully the larger MFI geometry with a 10-MR cavity, we used the 6-311G(d) basis set for the zinc atom and the 6-31G (d) basis set for the Al atom, as well as four O atoms that were connected to the substituted Al atom.…”

“…[12][13][14]22,30 The reason why we chose this model is because it had been confirmed previously that the calculation using this model provides a complete description of the electronic states of a monomeric Zn 2+ ion embedded in MFI in the activation processes of H 2 or CH 4 activation at temperatures around 300 K, as well as the formation of Zn + ions. [12][13][14]30 The resulting structural parameters of the optimized [Zn 2 ] 2+ -(M7-S2) model were found to be 1.92 and 2.32 Å for the Zn-O and Zn-Zn distances, respectively, which are well consistent with the structural data obtained from our EXAFS analysis (Table 1). In addition, the UV-vis spectrum reproduced by the time-dependent density functional theory (TD-DFT) calculation using the [Zn 2 ] 2+ -(M7-S2) model showed qualitative agreement with the experimental spectrum (Fig.…”