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lent performance for the oxidation of sulfides in water, com-
pared with in organic solvents.[21]
The time course of the product distribution was recorded
for the photocatalytic oxidation of 1a in methanol over the
4 wt% C60/g-C3N4 catalyst. During the reaction process, the
concentration of 1a gradually decreased, whereas the concen-
tration of 1b gradually increased (Figure 6). No intermediates
or other byproducts were observed during the reaction pro-
cess. After light irradiation for 6 h, 1a was quantitatively con-
verted into the target product of 1b.
In our catalytic system, a quantitative conversion of 1a was
observed in water after 6 h, but the reaction in water pro-
duced an almost equal mixture of 1b and methyl phenyl sul-
fone (Table 2, entry 5). Pleasingly, methanol was found to be
the best solvent because 1a was quantitatively transformed
into 1b after 6 h in methanol under visible-light irradiation
(Table 2, entry 6). Therefore, methanol was the best solvent for
the photocatalytic oxidation of 1a, and could stabilize the oxi-
dation product of 1b to prevent further oxidation. Compound
1b in water could be further oxidized into methyl phenyl sul-
fone; this was possibly because 1b could easily dissolve in
water, as a result of its polarity, to promote further oxidation.
In addition, the huge difference between ethanol and metha-
nol in the activity of the C60/g-C3N4 catalyst was possibly be-
cause the hydrophilic C60/g-C3N4 catalyst could be much better
dispersed in methanol. According to the experimental results,
the 4 wt% C60/g-C3N4 catalyst was the best choice for the pho-
tocatalytic oxidation of sulfides.
To obtain more information on the photocatalytic oxidation
of 1a, additional experiments were also performed. For com-
parison, a control experiment was also conducted in the ab-
sence of catalyst under the same reaction conditions (Table 1,
entry 6); the oxidation of 1a did not occur, which suggested
that the reaction was promoted by the catalyst. In addition, a
low conversion of 20.2% was observed in the presence of g-
C3N4 with a high selectivity of 98.1% to 1b (Table 1, entry 7).
However, the single use of C60 produced a very low conversion
of 1a (Table 1, entry 8). The conversion was not increased if C60
was added with g-C3N4 (Table 1, entry 13 vs. entry 8), which in-
dicated that the merely mixing with C60 had little effect on the
catalytic efficiency of g-C3N4. These results suggested that C60
and g-C3N4 had a synergetic effect on the photocatalytic oxida-
tion of 1a. The dispersion of C60 on the surface of g-C3N4, to
give rise the hybrid C60/g-C3N4 catalyst, might enhance the ab-
sorption of visible light (Figure 4) and promote the electron-
transfer ability (Figure 9, below), resulting in enhanced photo-
catalytic activity. In addition, no reaction took place in the dark
or under a nitrogen atmosphere (Table 1, entries 9 and 10).
These results show that the C60/g-C3N4photocatalyst, light ir-
radiation, and oxygen are all crucial factors in the photocatalyt-
ic oxidation of sulfides into sulfoxides. Although there has
been an example of photocatalytic oxidation of sulfide into
sulfoxide with g-C3N4, this catalytic system required the use of
isobutyraldehyde as an additive.[24] To the best of our knowl-
edge, this is the first example of the successful oxidation of
sulfide to sulfoxide with a metal-free heterogeneous catalyst
without any additive under mild conditions (258C, 1 atm
(=101325 Pa) O2). Clearly, it is more sustainable to perform
chemical reactions without additives. Photocatalytic oxidation
in air resulted in a relatively low conversion efficiency, com-
pared with that if the system was filled with oxygen (Table 1,
entry 4 vs. entry 11); this was due to the low concentration of
oxygen in the reaction solution. By prolonging the reaction
time to 10 h, compound 1b was attained in a high yield of
94.4% in air (Table 1, entry 12).
Figure 6. Time course of the products distribution for the photocatalytic oxi-
dation of 1a (MPS) into 1b (MPSO).
Substrate scope
The substrate scope of the developed method was extended
to a series of structurally diverse sulfides, and the reactions
were performed in methanol over the 4 wt% C60/g-C3N4 cata-
lyst. As shown in Table 3, this method with the 4 wt% C60/g-
C3N4 catalyst exhibits a good tolerance for a wide range of sul-
fides. The sulfides were successfully oxidized into the corre-
sponding sulfoxides with good to excellent yields. For all
cases, the selectivity of the sulfoxides reached nearly 100%.
However, the activity of the substrates was greatly influenced
by their structures. The electronic properties of the substitu-
ents in the aryl sulfides played an important role in the sub-
strate activity (Table 3, entries 1–3 vs. 4–6).
Generally, the substrates with electron-donating groups
demonstrate higher activity than those substrates with elec-
tron-withdrawing groups; the latter require longer reaction
times to give high yields of the corresponding sulfoxides
(Table 3, entries 1–3 vs. 4–6). The substrates with electron-do-
nating groups more easily donate electrons to sulfur atoms
than the substrates with electron-withdrawing groups, which
would be beneficial for the oxidation reactions. Additionally,
the steric hindrance of the groups also showed a great influ-
ence on the substrate activity (Table 3, entries 1 vs. 7, 5 vs. 6,
and 8 vs. 9). The large steric hindrance of the substrates
caused them difficulties in accessing the active sites. For exam-
ple, diphenyl sulfide, with two aromatic rings connected by a
sulfur atom, has a high electron density on the sulfur atom,
but it demonstrated the lowest activity of all tested substrates
(Table 3, entries 8 and 9), owing to the greatest steric hin-
ChemSusChem 2018, 11, 1 – 10
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