4
X. Li et al. / Catalysis Communications xxx (2015) xxx–xxx
100
80
60
40
20
0
The acid contents and acidities of three ionic hybrids were determined
by acid base titration and Hammett indicator method according to the
methods in literatures [26,27]. The acidity (denoted as Hammett acidi-
ties function (H0), Hammett indicator: p-nitrotoluene) of S2SiIH,
S2PIH and S4SiIH is −3.51, −3.42 and −3.94, respectively. The acid
content of S2SiIH, S2PIH and S4SiIH is 1.12, 0.98 and 1.37 mmol g−1 re-
spectively. These results indicated that they all exhibited strong
Brønsted acidity and high acid content, and these properties are crucial
for them as effective catalysts in BV oxidation. Among them, S4SiIH ex-
hibit the highest acidity and acid content for its four sulfonic acid groups
S2SiIH S2PIH S4SiIH
on organic cation. The reason why S2SiIH shows higher acidity and acid
−3
content than S2PIH lie in its counter anion is HSiW12
O
.
40
1
2
3
4
5
6
7
8
9
10
3.2. BV oxidation catalyzed by S2SiIH, S2PIH and S4SiIH
Run number
Initially, the nonstrained cyclohexanone was chosen as a model sub-
strate to investigate the catalytic activity of three ionic hybrids and the
effect of catalyst amount, reaction temperature and time. Under opti-
mum conditions, these results are listed in Table 1. Three ionic hybrids
all exhibited high catalytic activity for cyclic ketone and aromatic ketone
oxidations. For cyclic ketones the corresponding lactones were obtained
with yields of 69% to 88%. Based on this success, we continued to carry
out the BV oxidation of acyclic ketones, the results are listed in Table 1
(Entries 4–5). For aromatic ketones the corresponding esters are obtain-
ed with yields of 25% and 42%, but for aliphatic ketones no correspond-
ing esters were obtained. These results indicated that this approach only
has generality for oxidation of cyclic ketones and aromatic ketones. In
order to reasonably explain for these good results, the performances
of single SO3H-functionalized Brønsted acidic ionic liquids BAILs-1–2
[28] (Fig. 1), H4SiW12O40, H3PW12O40, Amberlyst-15 [8] and supported
sulfonic acid (silica sulfate) [29] as catalysts in cyclohexanone oxidation
under the same reaction conditions were investigated, these results are
listed in Table 2. These results indicated that the combining of sulfonic
group with heteropoly acid anion is essential for the high catalytic activ-
ity. Based on these experimental results, the high catalytic performance
of S2SiIH, S2PIH and S4SiIH in BV oxidation could be attributed to the
following facts: 1) strong Brønsted acidity and high acid content,
which not only enhance the addition activity of carbonyl group on sub-
strate ketones by protonation [30], but also made H2O2 to be activated;
2) active persulfonic acid was generated in situ. Three ionic hybrids
showed core–shell structure, the outer layer is a multi-sulfonic acid
group functionalized organic cations, which was oxidized by the activat-
ed H2O2, and generated more active persulfonic acid intermediate;
3) amphiphilic surface. The structure of SO3H-functionalized long
chain alkylammonium for organic cation endowed three ionic hybrids
with excellent amphiphilic surface, which caused the reaction between
the ketone and H2O2 aqueous to be carried out smoothly; 4) Porous
structure. The BET surface areas of S2SiIH, S2PIH and S4SiIH are 10.71,
110.24, and 10.53 m2 g−1 respectively, though still not high but its cat-
alytic activity for oxidation cyclohexanone is much higher than the
BAILs-1–2, H4SiW12O40 and H3PW12O40 homogeneous catalytic system.
These features suggest a bulk-type catalysis mode for three ionic hy-
brids (not a surface-type), in terms of mass transfer, the ionic hybrid's
good catalytic performances are mostly because of the amphiphilic
Fig. 2. Recycling of S2SiIH, S2PIH and S4SiIH in the oxidation of cyclohexanone.
environment in bulk areas of the solid hybrid, which are suitable for
transportation of both hydrophobic and hydrophilic species; 5) The co-
operative catalytic effect of organic cation and heteropolyacid anion.
Being polyoxometalate anions, they were enclosed tightly by the coun-
ter organic cations, the active persulfonic acid was generated in situ to-
gether with the active H2O2 that also oxidized them into peroxo
intermediate, and two generated active peroxide species together oxi-
dized ketone into the corresponding lactone or ester. With the increas-
ing of sulfonic acid group, the more active peroxide species were
obtained, so three ionic hybrids, Amberlyst-15 and silica sulfate gave
better results than BAILs-1–2, H4SiW12O40 and H3PW12O40. Since oxida-
tion reaction was performed in aqueous solution, with the increasing of
acidity and acid content, the hydrolysis rate of obtained lactones in-
creased. Take the conversion and selectivity into account, appropriate
acidity and acid content of catalytic system are crucial for the high
yield. Compared to S2PIH and S4SiIH, Amberlyst-15 and SiO2–SO3H,
the reason for S2SiIH gave the best result that might ascribe to its
enough and suitable acidity and acid content.
In order to investigate the recoverability and recyclability of S2SiIH,
S2PIH and S4SiIH, the oxidations of cyclohexanone were chosen as
model reactions. Three ionic hybrids were regenerated by simple wash-
ing with distilled water, diethyl ether and dried at 85 °C for 8 h. The re-
cycle test results showed that there is only a marginal decrease in yield
after they were reused for ten times (Fig. 2). For the purpose of compar-
ison, the TG pattern, IR spectra, acidity and acid content of recycled ionic
liquids were also determined, and it has been found that there are on
obvious differences between the recycled and fresh samples. These re-
sults indicate that three ionic hybrids are stable in the reaction system.
4. Conclusion
In conclusion, we have developed a simple, efficient and eco-friendly
approach for BV oxidation using three novel multi-SO3H functionalized
S2SiIH, S2PIH and S4SiIH as recyclable catalyst under solvent-free
conditions. Several noteworthy features of this approach, including
the convenient preparation of three ionic hybrids with high purity and
low cost, using commercial available 35% aqueous H2O2, simple workup,
high yield, mild reaction conditions, S2SiIH, S2PIH and S4SiIH can be
reused after simple treatment, etc. These advantages make this method-
ology become a green alternative for BV oxidation.
Table 2
Catalytic performance of various catalysts for BV oxidation of cyclohexanone with H2O2.
Entry
Catalyst
Solubility in reaction
Yields (%)
1
2
3
4
5
6
BAILs-1
BAILs-2
H3PW12O40
H4SiW12O40
Amberlyst-15
SiO2–SO3H a
Soluble
Soluble
Soluble
Soluble
Insoluble
Insoluble
25
22
18
13
45
52
Acknowledgments
The authors are grateful for the financial support from Natural
Science Foundation of Fujian Province (No. 2013J01053), Research Pro-
ject of Education Department of Fujian Province (No. JA13252), Re-
search Project of Fuzhou City (No. 2012-G-138), and Research Project
of Minjiang University (No. MJK14005).
Reaction conditions: temperature 50 °C; time 3 h ketone:H2O2:Cat = 1:2.5:0.05 (mol).
a
SiO2–SO3H was prepared according to the approach described in literature [29].
Please cite this article as: X. Li, et al., Solvent-free Baeyer–Villiger oxidation with H2O2 as oxidant catalyzed by multi-SO3H functionalized