W. Jeon et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 264–269
267
Fig. 4. Effect of catalyst amounts on productions of alginic acid monomer, furfural
Fig. 3. Yields of alginic acid monomer, furfural and formic acid obtained by
hydrothermal reaction of alginic acid at different reaction temperatures and times.
Reaction condition: alginic acid concentration (2 wt%), catalyst loading (600 mg),
and stirring speed (600 rpm).
and formic acid from alginic acid. Reaction condition: alginic acid concentration
◦
(
2 wt%), temperature (180 C), reaction time (30 min), and stirring speed (600 rpm).
the acid-catalyzed decomposition of furfural to formic acid at rel-
atively high reaction temperatures would be also ascribed to the
consumption of furfural. Therefore, it is essential to optimize the
reaction temperature and time in order to maximize the produc-
tion of furfural from alginic acid while suppressing the side reaction
of furfural to humin or formic acid.
defined as the sum of yields of mannuronic acid and guluronic acid.
At 140 and 160 C, the production of the monomer was noticeable
◦
during the initial stages of reaction, which demonstrates that the
cleavage of 1,4-glycosidic linkage of alginic acid by acid-catalyzed
hydrolysis was dominant at the mild reaction temperature. This
is well corresponding with the higher intensities of GPC peaks
at 27.5 min for the monomer-sized molecules produced at 140
3.3. Effects of catalyst amounts and reactant concentrations on
production of furfural
◦
and 160 C for 30 min, as discussed in Fig. 2. However, the yield
of the monomer decreased with the time-on-stream whereas the
yield of furfural gradually increased at 140 C due to the acid-
catalyzed conversion of the monomer to furfural. Moreover, the
increase in reaction temperature also caused the enhanced pro-
duction of furfural at the expense of monomer yield. For example,
Fig. 4 shows the yields of alginic acid monomer, furfural and
formic acid produced by hydrothermal treatment of alginic acid
at 180 C for 30 min with different amounts of Amberlyst-15. In a
◦
◦
blank test without the acid catalyst, 4.9 and 0.3 mol% of furfural
and alginic acid monomer were produced, respectively, demon-
strating that the acid catalyst is essential for both hydrolysis of
alginic acid and production of furfural from alginic acid. Although
there is no acid catalyst initially loaded in the reactor, protons
the furfural yield significantly increased from 3.1 to 14.1 mol% at
◦
1
80 C for 30 min while the monomer yield decreased from 7.7 to
.3 mol%, implying that the monomer produced by hydrolysis of
1
+
alginic acid was rapidly converted to furfural by the acid-catalyzed
reactions like decarboxylation and dehydration [16]. In addition to
the alginic acid monomer and furfural, formic acid was produced
by the hydrothermal reaction of alginic acid over Amberlyst-15.
The yield of formic acid was also strongly dependent on reaction
(H ), the Brønsted acid catalyst, can be produced by the dissocia-
◦
tive ionization of water molecule at 180 C, which catalyzed the
hydrothermal conversion of alginic acid to furfural to some extent
[22]. When 150 mg of Amberlyst-15 was added to the hydrother-
mal reaction, the yield of furfural almost doubled from 4.9 to
10.2 mol%. The increase in the furfural yield can be explained by
the fact that Amberlyst-15 has sulfonic acid groups grafted on
the catalyst surface, which play a role as an active site to pro-
mote the acid-catalyzed hydrothermal reactions, such as hydrolysis
and dehydration [23–26]. In addition to furfural, other products
were produced more abundantly over the doubled amount of the
catalyst. In particular, formic acid showed the maximum yield
(10.6 mol%) over 150 mg of Amberlyst-15. However, as the amount
of the catalyst increased from 150 to 300 mg, the formic acid yield
began to decrease whereas the furfural yield increased remark-
ably, which implies that Amberlyst-15 is a more selective catalyst
to furfural than formic acid under the hydrothermal conditions.
However, it was observed that the furfural yield was almost con-
stant (ca. 14 mol%) although the amount of Amberlyst-15 increased
from 300 to 900 mg, which demonstrates that the optimum ratios of
catalyst to reactant lie in between 0.5 and 1.5. On the other hand,
the yield of furfural significantly decreased over 1200 mg of the
catalyst with the production of humin, implying that the excess
amount of the acid catalyst more accelerated the polymerization
of furfural to humin than the production of furfural from alginic
acid. The decrease in the yields of both furfural and formic acid at
the higher catalyst loading is likely related to the increase in the
average molecular weights of hydrothermally treated alginic acid
◦
temperature and time with the maximum of 13.7 mol% at 200 C. A
feasible reaction pathway for the formic acid production would be
the hydrothermal conversion of alginic acid and its monomer, since
formic acid is commonly produced by the hydrothermal reaction of
alginic acid regardless of the kinds of catalysts [3,19,20]. According
to previous study, formic acid can be also produced from furfural
due to the hydrothermal instability of furfural over acid catalysts,
which is regarded as another possible route to yield formic acid
◦
[
21]. Based on the product distribution at 140 C within 30 min,
the former possibility would be more reasonable since formic acid
was already produced before furfural began to form. In addition, it
seems that the variation in the yield of formic acid is well matched
with that of the monomer whereas the furfural yield increases
independently, supporting the hypothesis about the production of
formic acid primarily from alginic acid monomer.
◦
At higher reaction temperatures than 140 C, it was observed
that the yields of alginic acid monomer, furfural and formic acid sig-
nificantly decreased, suggesting that all the products were unstable
under the hydrothermal condition at the reaction temperatures
◦
over Amberlyst-15. After the acid-catalyzed reactions over 160 C,
humin was generated as shown in Fig. S2. The production of humin
is expected to cause the decrease in the yields of the monomer,
furfural and formic acid. In addition to the production of humin,