Unprecedented Reversible Redox Process in the ZnMFI—H₂ System Involving Formation of Stable Atomic Zn⁰

Abstract

In its element: Zn(2+) at the M7 site of MFI-type zeolites activates H(2), via ZnH and OH species, and leads to Zn(0) species. The Zn(0) species returns to its original state, a Zn(2+) ion, upon evacuation of the zeolite at 873 K (see picture). The formation of the Zn(0) species is supported by DFT calculations.

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.…”

Evolution of Zn Species on Zn/HZSM‐5 Catalyst under H2 Pretreated and its Effect on Ethylene Aromatization

“…Oda et al. found that Zn 2+ ions in Zn‐ion‐exchanged MFI‐type zeolite were converted to stable Zn 0 species through a reaction with H 2 at 300 °C, and the atomic Zn 0 species could further react with AlOH to form paramagnetic Zn + species with the ultraviolet light excitation . Tamiyakul et al .…”

“…However, ZnO crystals formed on Zn/H-ZSM5 prepared by impregnation were reduced to Zn 0 metal and eluted from the catalyst bed as a Zn vapor under typical propane aromatization conditions. Oda et al found that Zn 2 + ions in Zn-ion-exchanged MFI-type zeolite were converted to stable Zn 0 species through a reaction with H 2 at 300°C, [12] and the atomic Zn 0 species could further react with AlOH to form paramagnetic Zn + species with the ultraviolet light excitation. [13] Tamiyakul et al [14] showed that the ZnOH + species were converted to ZnH + by dehydroxylation after subjecting the Zn/H-ZSM5 to H 2 flow at 150°C.…”