2 research outputs found
Atmospheric Oxidation Mechanism of Furfural Initiated by Hydroxyl Radicals
Furfural
is emitted into the atmosphere because of its potential applications
as an intermediate to alkane fuels from biomass, industrial usages,
and biomass burning. The kinetic and mechanistic information on the
furfural chemistry is necessary to assess the fate of furfural in
the atmosphere and its impact on the air quality. Here we studied
the atmospheric oxidation mechanisms of furfural initiated by the
OH radicals using quantum chemistry and kinetic calculations. The
reaction of OH and furfural was initiated mainly by OH additions to
C<sub>2</sub> and C<sub>5</sub> positions, forming R2 and R5 adducts,
which could undergo rapid ring-breakage to form R2B and R5B, respectively.
Our calculations showed that these intermediate radicals reacted rather
slowly with O<sub>2</sub> under the atmospheric conditions because
the additions of O<sub>2</sub> to these radicals are only slightly
exothermic and highly reversible. Alternatively, these radicals would
react directly with O<sub>3</sub>, NO<sub>2</sub>, HO<sub>2</sub>/RO<sub>2</sub>, etc. Namely, the atmospheric oxidation of furfural would
unlikely result in ozone formation. Under typical atmospheric conditions,
the main products in OH-initiated furfural oxidation include 2-oxo-3-pentene-1,5-dialdehyde,
5-hydroxy-2(5<i>H</i>)-furanone, 4-oxo-2- butenoic acid,
and 2,5-furandione. These compounds will likely stay in the gas phase
and are subject to further photo-oxidation
Atmospheric Oxidation of Furan and Methyl-Substituted Furans Initiated by Hydroxyl Radicals
The
atmospheric oxidation mechanism of furan and methylfurans (MFs) initiated
by OH radicals is studied using high-level quantum chemistry and kinetic
calculations. The reaction starts mainly with OH addition to the C2/C5-position,
forming highly chemically activated adduct radical R2*/R5*, which
would either be stabilized by collision or promptly isomerize to R2B*/R5B*
by breaking the C2-O/C5-O bond and then isomerize to other conformers
of R2B/R5B by internal rotations. Under the atmospheric conditions,
the ring-retaining radical R2/R5 would recombine with O<sub>2</sub> and be converted to a 5-hydroxy-2-furanone compound and a compound
containing epoxide, ester, and carbonyl functional groups, while the
ring-opening radicals R2B/R5B would react with O<sub>2</sub> and form
unsaturated 1,4-dicarbonyl compounds. RRKM-ME calculations on the
fate of R2*/R5* from the addition of OH and furans predict that the
fractions of R2B/R5B formation, i.e., the molar yields of the corresponding
dicarbonyl compounds, are 0.73, 0.43, 0.26, 0.07, and 0.28 for furan,
2-MF, 3-MF, 2,3-DMF, and 2,5-DMF, respectively, at 298 K and 760 Torr
when using the RHF-UCCSD(T)-F12a/cc-pVDZ-F12 reaction energies and
barrier heights. The predicted yields for dicarbonyl compounds agree
reasonably with recent experimental measurements. Calculations here
also suggest high yields of ring-retaining 5-hydroxy-2-furanone compounds,
which might deserve further study