(174ak) Mechanisms of Xylene Isomer Oxidation By Non-Thermal Plasma Via Paired Experiments and Simulations

Mechanisms of Xylene Isomer Oxidation by
Non-Thermal Plasma via Paired Experiments and Simulations

Xylene is a
widely used solvent and industrial chemical, but it is also considered to be a
volatile organic compound (VOC) pollutant. Meanwhile, non-thermal plasma (NTP)
is a potential method for remediating VOC contaminants, especially aromatic
hydrocarbons. During NTP degradation of xylene, the different oxidation
mechanisms of three isomers, p-xylene, o-xylene and m-xylene, have attracted
lots of attention but not been studied at the molecular level. In this study,
the individual degradation rates of xylene isomers in a NTP system are
measured. The results show the oxidation degradation rates have the following
order: o-xylene > p-xylene / m-xylene, and there is no big difference in
oxidation rates of p-xylene and m-xylene. Molecular dynamics (MD) simulations
with an applied external electric field were adopted to mimic the NTP
atmosphere. Oxidation process of xylene isomers were simulated and recorded
step by step. The oxidation rates from the simulations were calculated, the
order of which is in a good agreement with the experimental results. The
oxidation pathways of xylene isomers were analyzed more thoroughly to explain
the rate differences. A unique oxidation pathway (Route c in Figure 1) that
couples the synergistic effects of hydrogen atom supersaturation
with a common ortho-position ring cleavage mechanism
is put forward to explain why o-xylene degrades more quickly.
To understand the dependence of the oxidation pathway on external electric
fields, parallel cases of the MD simulated degradation process for all three
xylene isomers were also conducted with no applied voltage. The results
demonstrate that the applied external electric field further increases the
occurrence of faster ring cleavage pathways for o-xylene, which is a secondary
reason why o-xylene oxidizes more quickly. As a result, the external electrical field is
found to have two effects: one is to speed up the oxidation rate directly, and
the other is to alter the oxidation pathways of xylene isomers.
The results from this work suggests that external electric fields may have a
significant influence on other similar chemical reactions potentially.

Figure 1.
Reaction time and probability distribution for different ring cleavage pathways
of xylene isomers and o-xylene oxidation pathways observed during ReaxFF
simulations.

References

[1]Tianyu Shou, Nan Xu, Yihan Li, Guojin
Sun, Matthew T. Bernards, Yao Shi, Yi He, Plasma Chem.
Plasma Process., DOI: https://doi.org/10.1007/s11090-019-09986-5.