J. Tuteja et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 3 (2012)
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5
One-Pot Synthesis of 5-Hydroxymethyl-2-furaldehyde
(HMF) from Lactose. Lactose has poor digestibility,
373 K
383 K
393 K
therefore it is widely utilized in the dairy industry and it has
much potential to serve as a chemical or energy feedstock.47
The lactose is a disaccharide of glucose and galactose.
These aldohexoses give fructose and tagatose respectively
on isomerization with base catalyst, followed by dehydration
with acid catalyst to give HMF (Schemes 1a and 1b). Table 3
shows the results in one-pot conversion of lactose, glucose, and
galactose using Amberlyst-15 and HT catalysts. It is clear that
HMF yield obtained from glucose is much higher than that
from galactose in all reaction temperatures. The reason behind
this can be explained from the stereochemical configuration of
galactose. The keto isomer of galactose is tagatose which is a
C-4 epimer of fructose and there is an inefficient dehydration
of tagatose into HMF. Comparing the reaction temperatures,
glucose shows the maximum yield at 373 K while galactose
and lactose produces HMF in good yield at 383 K. Addition-
ally, both hydrolysis of lactose and formation of HMF were
enhanced with increasing temperature. These results indicate
that the sequential reaction as hydrolysis of lactose by acid,
isomerization of galactose and glucose by base, and then
dehydration by acid took place in the reaction. Moreover, we
can easily see (See details in Table S1) that the amount of
galactose and tagatose left in the reaction mixture is more than
the amount left of glucose and fructose clearly indicating that
under the same reaction conditions galactose and tagatose are
less reactive than glucose and fructose.
One-Pot Synthesis of Furans from Sugar Mixtures.
Natural biomass contains a mixture of sugars like arabinose,
galactose, xylose, and many other mono- and disaccha-
rides.48,49 For example, rhamnose is commonly bound to other
sugars in nature,50 therefore it is necessary to develop a method
for the conversion of rhamnose to MF in the presence of other
sugars, therefore conversion of a mixture of sugars was also
carried out and it was observed that this method is applicable
for the formation of different furans in a single pot.
First, one-pot conversion of a mixture of arabinose and
rhamnose was tested (Table 4, Entry 1) with the same acid-
base catalyst pair Amberlyst-15 and HT in DMF at the
optimum time and temperature for both the sugars i.e., at
383 K for 6 h to obtain 33.1 and 32.5% yield of furfural and
MF respectively. Thereafter the conversion of a mixture of all
three sugars (arabinose, rhamnose, and lactose) were exam-
ined (Entry 2), with near about 29-33% yield of furfural,
HMF, and MF. The yield of furans obtained from a mixture
of sugars is slightly less than the yield obtained by one-pot
conversion of a particular single sugar. It is proven from
these results that this system can be used for conversion of
monosaccharides, disaccharides and for mixtures of mono- and
disaccharides.
0
0
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4
6
8
10
12
14
Time/h
Figure 4. Time course in the one-pot synthesis of MF from
rhamnose at different temperatures. Reaction conditions:
Rhamnose (0.55 mmol), Amberlyst-15 (0.1 g), HT (0.2 g),
DMF (3 mL), 500 rpm, N2 flow (3 mL min¹1).
Figure 4 shows the time course for the one-pot formation of
MF from L-rhamnose at a different temperature range of 373-
393 K. The yield of MF increases with increase in temperature
for 3 h, but long reaction time shows quite interesting behavior.
The yield increases up to a maximum yield of 24.0% MF in
10 h, thereafter it started decreasing in the case of 373 K, while
the same tendency with the maximum yield 38.6% in 6 h
reaction and following decreasing was obtained at 383 K.
The reaction progress at 393 K is different from 373 and
383 K, and a gradual decrease in yield was observed from 3 h
reaction. Accordingly, appearances of the peak tops were
shifted to shorter time with increase of the reaction temper-
ature. In the one-pot synthesis including isomerization and
successive dehydration, increasing the activity of dehydration
enhances the yield of product owing to the enhancement of
isomerization.41 From the view point of thermodynamics, the
dehydration reaction is an exothermic reaction. Therefore,
long reaction at high temperature leads to good yield of the
product, whereas the side reactions for undesired by-products
like humins, oligomers, etc., and decomposition of the product
are also enhanced by high temperature and/or long reaction.
According to these aspects, we considered that reaction for 6 h
at 383 K showed the highest activity for the one-pot synthesis
of MF from rhamnose.
The aldo-sugars have been reported earlier to isomerize to
keto-sugars in the presence of solid base, for instance glucose
to fructose. These isomerizations were detected as twin peaks
in the HPLC chromatogram with the retention time as: glucose
(11.4) and fructose (12.3); arabinose (13.2) and ribulose (13.6);
galactose (12.1) and tagatose (11.8 min).40-42 Similarly, a twin
peak is observed in the case of rhamnose at 12.3 and 12.8 min
(See Supporting Information, Figure S3), where the former is
likely attributed to rhamnulose, a keto isomer of rhamnose,
while the latter is observed for rhamnose. Rhamnulose is
commonly known as 6-deoxyfructose, hence it was supposed
that the retention time for fructose and rhamnulose are found to
be similar.46 According to these results, we believed that the
one-pot synthesis of MF from rhamnose also enhanced the
pathway via the isomerization of rhamnose to rhamnulose over
base catalyst and successive dehydration from rhamnulose to
MF over acid catalyst.
Thereafter the next step was to check the recyclability of the
catalysts for conversion of mixtures of sugars. The acid and
base catalysts were simply recovered by decantation, washed
with DMF (10-12 mL), dried in vacuo overnight, and recycled
for further reactions. It was found that the catalysts could be
reused at least three times (shown in Figure 5) for the synthesis
of 2 or 3 furans simultaneously, without any significant loss
in the catalytic activity.
Tuteja, Jaya
