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volume ratios in these reactors resulted in higher reaction
selectivity. We also conrmed that continuous ow photo-
oxygenation in the TIT-R was applicable to mass production and
that the reaction could be carried out under natural sunlight
instead of an articial LED lamp. However, further efforts to
improve this mass-producible and environmentally friendly
process are needed, and an advanced TIT-R with a solar tracker
system is in development.
Acknowledgements
We gratefully acknowledge nancial support from the Chung-
nam National University (research fund in 2011) and the
National Research Foundation (NRF-2012-1012596).
Fig. 2 The photooxygenation of b-pinene under an LED lamp and
sunlight. Solutions of 5 mmol b-pinene and 5 wt% MB were prepared in
5 mL MeOH. The reaction was carried out for 30 min under the LED
lamp and natural sunlight in the MC-MR.
Notes and references
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with that of the MC-MR and the batch system. The use of the
TIT-R for photooxygenation of (ꢀ)-citronellol produces a
67.9-fold greater DOP than the batch system and 270.0-fold
greater DOP than the MC-MR. Improvements of 33.9-fold and
295.3-fold for a-terpinene, 17.4-fold and 277.6-fold for b-pinene,
56.9-fold and 273.8-fold for a-pinene, and 20.3-fold 298.6-fold
for d-limonene are observed with the TIT-R in comparison with
the batch system and the MC-MR, respectively. The productivity
obtained with the TIT-R has potential for a feasible scale-up
process and could lead to a green industrial photooxygenation
process.
´
F. Vilela, Chem. Commun., 2013, 49, 11158; (e) F. Levesque
In previous reports, various articial light sources, including
tungsten lamps and LEDs,6a,11,15 were introduced to maximize the
efficiency of reactions. However, as photooxygenation is a repre-
sentative green processes, we replaced the articial light source
with natural sunlight to further increase the eco-friendliness of
this reaction (Fig. 2). Solutions of 2.5 mmol b-pinene and 5 wt%
MB prepared in 5 mL MeOH were injected into the MC-MR with
slug lengths of 0.5 mm, and the system was kept for 30 min at an
average temperature of 28.5 ꢁC without any articial temperature
control. Experiments were conducted from 11:00 AM to 01:30 PM
(>22.5ꢁ solar elevation angle) to obtain similar reaction conditions,
and a convex lens (diameter: 270 mm) was used to focus the
sunlight. Even though the conversions of 68.28% (3 bar) and
11.77% (1 bar) obtained under sunlight are signicantly lower
than those of 99.9% (3 bar) and 48.06% (1 bar) obtained under the
LED lamp, the results indicate the potential for an ideal green
photooxygenation process under a natural light source.
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Conclusions
In summary, we tested two continuous ow reactors for the
photooxygenation of monoterpenes isolated from trees, and
both reactors gave excellent yield and selectivity. Because higher
oxygen concentrations in the liquid media reduces the genera-
tion of activated vinyl radical and accelerates the generation of 10 J. H. Park, C. Y. Park, H. S. Song, Y. S. Huh, G. H. Kim and
singlet oxygen, the introduction of higher contact-area-to-
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4236 | RSC Adv., 2015, 5, 4233–4237
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