es0c01090_si_001.pdf (2.02 MB)
Formation of Low-Volatile Products and Unexpected High Formaldehyde Yield from the Atmospheric Oxidation of Methylsiloxanes
journal contribution
posted on 2020-05-22, 16:05 authored by Zihao Fu, Hong-Bin Xie, Jonas Elm, Xirui Guo, Zhiqiang Fu, Jingwen ChenWith stricter regulation of atmospheric
volatile organic compounds
(VOCs) originating from fossil fuel-based vehicles and industries,
the use of volatile chemical products (VCPs) and the transformation
mechanism of VCPs have become increasingly important to quantify air
quality. Volatile methylsiloxanes (VMS) are an important class of
VCPs and high-production chemicals. Using quantum chemical calculations
and kinetics modeling, we investigated the reaction mechanism of peroxy
radicals of VMS, which are key intermediates in determining the atmospheric
chemistry of VMS. L2-RSiCH2O2• and D3-RSiCH2O2• derived
from hexamethyldisiloxane and hexamethylcyclotrisiloxane, respectively,
were selected as representative model systems. The results indicated
that L2-RSiCH2O2• and D3-RSiCH2O2• follow a novel Si–C–O
rearrangement-driven autoxidation mechanism, leading to the formation
of low volatile silanols and high yield of formaldehyde at low NO/HO2• conditions. At high NO/HO2• conditions, L2-RSiCH2O2• and D3-RSiCH2O2• react with NO/HO2• to form organic
nitrate, hydroperoxide, and active alkoxy radicals. The alkoxy radicals
further follow a Si–C–O rearrangement step to finally
form formate esters. The novel Si–C–O rearrangement
mechanism of both peroxy and alkoxy radicals are supported by available
experimental studies on the oxidation of VMS. Notably, the high yield
of formaldehyde is estimated to significantly contribute to formaldehyde
pollution in the indoor environment, especially during indoor cleaning.