2 research outputs found
Characterization and Dynamics of the Different Protonic Species in Hydrated 12-Tungstophosphoric Acid Studied by <sup>2</sup>H NMR
Solid
heteropolyacids (HPAs) are promising nonpolluting superacids
used as catalysts and solid proton conductors. The catalytic and conducting
properties of HPAs are very sensitive to the amount of hydration water
present in the system, as water molecules tend to interact with the
HPA acid protons to form [H<sub>3</sub>O]<sup>+</sup> and [H<sub>5</sub>O<sub>2</sub>]<sup>+</sup> ions. These ions constitute active species
that govern the catalytic reaction pathways and the proton migration
mechanism. Establishing the structure and mobility of protonic species
could yield important information concerning the functions of material
based on HPA hydrates. In this work, we have performed the analysis
of both <sup>2</sup>H NMR line shape and the evolution of <i>T</i><sub>1</sub>, <i>T</i><sub>2</sub> relaxation
with temperature for the deuterated analogue of the solid 12-tungstophosphoric
acid (TPA × <i>n</i>H<sub>2</sub>O) at different hydration
levels (0 < <i>n</i> < 6) in the wide temperature
range of 103–503 K. This allowed us to characterize in detail
the mobility of different protonic species, including acidic OH groups,
water molecules, and hydroxonium ions. Kinetic parameters of internal
and diffusional motions for different protonic species at different
hydration levels <i>n</i> of TPA × <i>n</i>H<sub>2</sub>O were derived
Methane Activation on Zn<sup>2+</sup>-Exchanged ZSM‑5 Zeolites. The Effect of Molecular Oxygen Addition
In relation to the reported methane
activation on Zn-modified zeolite
ZSM-5 at room temperature to afford the surface methoxy species by
Xu et al. (<i>Chem. Sci.</i> <b>2012</b>, <i>3</i>, 2932), the activation of methane on Zn<sup>2+</sup>-exchanged
H-ZSM-5 zeolite in the absence and the presence of molecular oxygen
has been studied with <sup>13</sup>C magic angle spinning (MAS) NMR
spectroscopy. It has been established that the methane activation
on zinc cationic sites under nonoxidative conditions occurs exclusively
by an “alkyl” pathway to form the surface zinc-methyl
species. The addition of the molecular oxygen (dioxygen) to methane
adsorbed on the Zn<sup>2+</sup>-exchanged H-ZSM-5 zeolite results
in the surface methoxy and other oxygen-containing species, such as
formate, acetaldehyde, and acetic acid. The formation of the surface
methoxy species occurs by the oxidation with molecular oxygen of zinc-methyl
species primarily formed on the zeolite surface. The Zn<sup>2+</sup>/ZSM-5 zeolite with full substitution of Brønsted acid sites
(BAS) by Zn<sup>2+</sup> cations offers zinc-methyl species from methane
at <i>T</i> ≥ 523 K, whereas Zn<sup>2+</sup>/H-ZSM-5
with partial substitution (60%) of BAS produces zinc-methyl at room
temperature. BAS promotes the formation and decomposition (by the
sample evacuation) of zinc-methyl species on Zn<sup>2+</sup>/H-ZSM-5
at room temperature. Zinc-methyl is readily oxidized by the dioxygen
additive to offer methoxy species already at room temperature. Thus,
it has been shown that pure methane forms only zinc-methyl species
upon its interaction with zinc cationic sites of Zn<sup>2+</sup>-exchanged
H-ZSM-5 zeolite, while the surface methoxide could be formed only
by the interaction of zinc-methyl with dioxygen that might be contained
in the reactive methane