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a
Table 3 Acetoin formation using supported catalysts
This work was supported by the Institute of Bioengineering
and Nanotechnology (Biomedical Research Council), Biomass-
to-Chemicals Program (Science and Engineering Research
Council), Agency for Science, Technology and Research, Singapore.
The authors acknowledge Dr Su Seong Lee for providing the
2
CuO/SiO (SBA-16) catalyst.
E
Precat. (mol%) Base (mol%) Temp. (1C) Time (h) Yield (%)
1
2
2 (1)
2 (reused)
1 (1)
1 (1)
1 (1)
1 (1)
Reused
1 (1)
Reused (1st)
Reused (2nd)
2 (1)
K
K
2
CO
CO
3
(1)
(1)
80
80
4
4
2
2
2
1
2
2
2
2
3
24
24
2
95
o10
95
95
95
98
36
98
95
55
69
88
46
83
42
Notes and references
2
3
b
3
4
5
6
7
8
9
1
1
1
1
1
1
SB1 (5)
SB2 (5)
SB3 (5)
SB4 (1)
120
120
120
120
120
120
120
120
120
120
120
120
120
1
T. Werpy, G. Petersen, A. Aden, J. Bozell, J. Holladay, J. White,
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b
b
c
c
2
3
T. Lu, X. Li, L. Gu and Y. Zhang, ChemSusChem, 2014, DOI: 10.1002/
cssc.201402396.
For recent reviews and opinions of bioethanol as a renewable resource
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SB5 (1)
d
0
1
2
3
4
5
SB5 (1)
SB4(1)
2 (1)
Reused
2 (1)
(
b) OECD/FAO, OECD/FAO Agricultural Outlook 2011–2020, OECD Publish-
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d
d
SB3 (5)
Reused
2
(d) C. Angelici, B. M. Weckhuysen and P. C. A. Bruijnincx, ChemSusChem,
a
2013, 6, 1595. For recent successful examples of the ethanol upgrading to
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Oppermann and A. Steinb u¨ chel, J. Bacteriol., 1994, 176, 469 and refer-
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Reaction conditions: a mixture of precatalyst, base and 2 ml acet-
b
aldehyde was stirred in a sealed tube. 200 mg of ion exchanger resin
was used in the presence of 300 mg of 4 Å MS. 8 ml acetaldehyde was
added. By-products were detected.
c
d
4
(1 mol%) in combination with bases SB4 and SB5, excellent acetoin
yields (98%) were achieved even without any pretreatment and
drying additives [entries 6 and 8]. The catalyst could be recycled
together with solid bases, although there were different extents of
deactivations observed for the recycled catalysts [entries 6–10,
Table 3]. These promising results may be attributed to the equili-
brium between 5 and 6 as we proposed in Scheme 1, which allows
the reactive carbene species 6 to go back to the catalytic cycle. In
addition, the combination of solid catalyst 2 and solid bases has also
been tested for the current reaction [entries 11–15, Table 3]. The
combination of 2 with the weak solid base SB5 showed rather low
activities [entry 11] even at the first time use. As 2 was used together
with stronger solid bases SB4 and SB3, good catalytic activities were
observed [entries 12 and 14] at the first time use. The catalysts were
also recyclable, but the activities of recycled catalysts obviously
dropped [entries 13 and 15].
6
7
(
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1 Some other commercially available and synthesized NHC precata-
lysts were also screened [Table S1 in ESI†].
1
1
In conclusion, a highly efficient and robust formation of acetoin 12 CuO/SiO
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2
SBA-16 catalyst was provided by Dr Su Seong Lee, IBN.
from acetaldehyde has been achieved under the catalysis of the
thiazolylidene NHC, which was formed in situ by deprotonation
of the commercially available thiazolium salt with a base. High
yields and selectivities were achieved with extremely low precatalyst
loadings (as low as 0.045 mol%). This reaction system is sufficiently
tolerant toward ethanol and moisture, rendering it the practical key
step of the ethanol to the C4 chemical upgrading process. The
possibility of catalyst separations and recycling has been successfully
demonstrated by designing the novel heterogeneous NHC catalysts
based on precatalyst 1: the supported NHCs, which were generated
by ad hoc deprotonation/immobilization of 1 with a solid base (ion-
exchanger), demonstrated great potential as recyclable NHC catalysts
for the acetion reaction. To further improve the sustainability of this
method, the structural optimizations of above-mentioned hetero-
geneous catalysts are ongoing in our laboratory.
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2310 | Chem. Commun., 2014, 50, 12308--12310
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