Table 1 The ketalization of cyclohexanone and 1,2-ethanediol to cyclohexanone ethylene ketal
Entry
Molar ratio ethanediol : cyclohexanone : catalyst
Solvent
Yield
Ref.
1
2
3
2000 : 2000 : 1
900 : 900 : 1
84.5 : 65 : 1
Cyclohexane
Toluene
Toluene
96%
B96%
69%
This work
7
22
Furthermore, the TGA curve and FTIR spectra provide
evidence that the Brønsted acidity sites of TiP nanospheres are
still stable even after calcination at 400 1C (see Fig. S3 and S4w).
The thermogravimetry curve gives a total weight loss of
ca. 22.4% recorded from room temperature to 1000 1C. The
first step before 250 1C in the TGA curve is related to the loss of
free adsorptive water, crystal water and hydrogen of AOT which
was proved by NMR analysis. The next step from 250 to 350 1C
corresponds to the partial release of structural water from the
condensation of P–OH groups and loss of carbon of AOT based
on the white sample obtained at 350 1C. FTIR spectra of the TiP
nanospheres without and with adsorbed pyridine as probe are
shown in Fig. S4.w Two bands at 1540 cmÀ1 and 1490 cmÀ1
indicated that Brønsted acid sites are present on the surface of
the TiP nanoparticles to react with pyridine.21
In conclusion, we have synthesized uniform multi-
component TiP nanospheres by a facile approach. The open
framework nanospheres with orderly open gradient nanopores
have uniform distribution and good dispersion together with a
strong acidic surface and large surface area even after high
temperature treatment. Moreover, as a solid Brønsted acid,
the TiP nanocatalyst has proved to be an extremely active,
selective, and stable catalyst for ketalization reactions.
The authors are grateful to Ms Ee Ling Goh (NMR lab) for
her assistance in solid NMR analysis and Mr Hailong Hu
from PAP for his help in FETEM operation.
Notes and references
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Based on the above discussion, we can draw conclusions
concerning the formation mechanism of multicomponent TiP
nanospheres. The stable NaTi2(PO4)3 crystal was first
obtained after the addition of titanium precursors, owing to
its poor solubility. Then, an amorphous Ti(HPO4)2 phase
starts to grow with NaTi2(PO4)3 as crystal seeds using the
acidifying surfactant AOT as template to form core–shell
structure TiP nanospheres. Although the core–shell structure
is difficult to distinguish due to the closely related components,
we could detect the cores from the solution with faint turbidity
at the early stage and the core–shell-like structure by heating
up to 600 1C (see Fig. S5w).
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The stable core–shell TiP nanospheres may find many
potential applications as a solid Brønsted acid catalyst due
to the open framework with uniform distribution and good
dispersion in polar solutions. We used TiP nanospheres
as catalyst to catalyze the ketalization of cyclohexanone and
1,2-ethanediol to cyclohexanone ethylene ketal, an important
reaction in organic synthesis.22,23 The results are shown in
Table 1. The molar ratio of cyclohexanone to TiP nanocatalyst
(based on Ti atom) was 2000 : 1, and the yield could reach 96%
in 3 h in cyclohexane at refluxing temperature. This activity is
much higher compared to the one reported in ref. 22, which
demonstrates a ketal yield of 69% at a cyclohexanone to
catalyst molar ratio of 65 : 1, and the one reported in ref. 7,
which shows a similar yield with a lower reactant/catalyst
molar ratio of 900 : 1 in toluene at refluxing temperature. The
nanocatalyst also displays very high stability; no obvious
decay in catalytic activity was observed when the catalyst
was recycled five times. Moreover, as shown in Fig. S6,w the
TiP nanocatalyst has very good dispersion in the polar phase
and TiP nanocatalyst could precipitate on the bottom of the
reactor after the reaction. Therefore, the product could be
directly taken out and the recycling of TiP nanocatalyst is very
easy without the use of any other process. To our knowledge,
it shows the best catalytic activity among all the reported TiP
catalysts.
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ꢀc
This journal is The Royal Society of Chemistry 2010
1672 | Chem. Commun., 2010, 46, 1670–1672