A. Onda et al. / Catalysis Communications 12 (2011) 421–425
423
Table 2
The CHN measurements of the AC before and after the starch reaction.
C / wt.%
H / wt.%
Treatments
AC
AC
AC
68
87
87
3.7
1.8
1.8
After the starch reaction at 393 K for 24 h.
After the hydrothermal treatment at 393 K for 24 h.
Before both treatments.
on the AC. From these results, probably, the other 68 C-% was
adsorbed on AC.
In contrast, the AC-SO3H catalyst gave glucose in 69 C-% yield and
oligosaccharides, such as maltose and maltotoriose, in 29 C-% yield,
but it gave almost no amounts of water soluble by-products. As shown
in Fig. 3a, the glucose yields increased with increasing the reaction
time over the AC-SO3H catalyst, and the glucose yield was over 90 C-%
and the yield of WSOCs was about 95 C-% for 70 h. The AC-SO3H
catalyst selectively converted starch into glucose, and the adsorption
amount of glucose on the AC-SO3H catalyst was negligible. A molar
ratio of produced glucose to sulfo groups in the AC-SO3H was about
10, which indicated that the sulfo groups catalytically accelerated the
hydrolysis of starch.
Fig. 1. XRD patterns of the activated-carbon (a), the AC-SO3H (b), the Pt/AC (c), the Pt/
AC-SO3H (d), and the used Pt/AC-SO3H (e).
As shown in Fig. 2, the Pt/AC-SO3H catalyst also gave high yields of
the hydrolysis products, which indicated that the Pt/AC-SO3H catalyst
had sulfo groups to hydrolyze of starch, as well as the AC-SO3H
catalyst. However, a main product was gluconic acid, and its yield was
40 C-%. The minor amounts of degradation products, such as acetic
acid and formic acid, were also formed. We have carried out the
catalytic reactions with changing the amounts of catalysts. As shown
in Fig. 4b, the yield of gluconic acid increased linearly with increasing
the amount of catalysts up to 100 mg over the Pt/AC-SO3H catalyst, as
well as the yield of glucose over the AC-SO3H catalyst (Fig. 4a). The
active sites for the glucose oxidation into gluconic acid would be
platinum particles because the catalysts without platinum, such as
AC-SO3H and AC, gave no amount of gluconic acid. After reaction, the
used Pt/AC-SO3H catalyst was characterized. The amounts of platinum
in the used Pt/AC-SO3H was 4.0 wt% determined by ICP and the
average platinum particle size was estimated to be 4.5 nm by XRD
(Fig. 1), which were almost the same as those before the reaction. In
addition, no leaches of platinum and sulfuric ions from Pt/AC-SO3H
catalysts were observed in the resultant solution by ICP and ion
chromatograph, respectively. The acid properties evaluated by the
titration method were also not changed by the reactions of starch. The
catalytic active sites, such as Pt particles and sulfo groups, were stable
during the reaction under the hot aqueous solution, which was due to
the hydrothermally pre-treatment of the Pt/AC-SO3H catalyst at
473 K.
the surface area, which might be due to the increase of adsorbed
water with increasing the amount of acidic surface functional groups
on the sulfonated catalysts and the decrease of porosity. In conclusion,
the Pt/AC-SO3H with strong acidic sulfo groups, platinum fine
particles, and high surface area was prepared by the impregnation
and sulfonation method.
3.2. The catalytic conversions of starch
The reaction results at 393 K for 24 h are summarized in Fig. 2.
After the reactions, all solutions were clear and colorless. A blank
reaction without catalyst gave almost no amounts of glucose and
oligosaccharides but gave the corresponding amount of water soluble
organic compounds (WSOCs) with the introduced starch, based on
carbon %. The WSOCs was mainly unreacted starch. Although the AC
catalyst had acidic surface functional groups, such as carboxylic acid
group, it showed a low catalytic activity for the starch hydrolysis. The
AC catalyst gave about 1 C-% yield of glucose and 31 C-% of other water
soluble organic compounds which were mainly unreacted starch. The
other 68 C-% was disappeared from the water phase. As shown in
Table 2, the C and H chemical composition of the AC treated under the
hydrothermal condition without starch was almost the same as that of
the fresh AC. In contrast, the C/H ratio of the AC catalyst after the
starch reaction was lower than that of the fresh AC. The decrease of C/
H ratio was corresponded to the adsorption of about 75 C-% of starch
Fig. 3b shows the changes in product yields during the starch
hydrolysis using the Pt/AC-SO3H catalyst at 393 K. Although the yields
of oligosaccharides, glucose, and gluconic acid increased linearly with
the increase of the reaction time within 9 h, the yield of oligosacchar-
ides decreased in the reaction time longer than 9 h and the yield of
glucose decreased in the reaction time longer than 18 h. In contrast,
gluconic acid was sequentially produced and became to a main
product at 24 h. Molar ratios of gluconic acid to total S atoms and total
Pt atoms in the catalyst were about 8 and 9, respectively, which would
be indicated that the sulfo groups and platinum particles in the
catalyst catalytically accelerated the hydrolysis of starch into glucose
and the oxidation of glucose into gluconic acid. The catalyst was
separated by the filtration, and then it was repeatedly used. Although
a small amount of the catalyst was lost experimentally for each run,
gluconic acid and glucose were formed as main products even in the
third run with a little decreasing of the formation rate (Fig. 5).
The pH values before and after the starch reaction over the Pt/AC-
SO3H catalyst were nearly neutral of about 6 and about 3, respectively.
The decrease of the pH value was mainly due to the production of
gluconic acid. In the filtrated solution after the reaction using the Pt/
Fig. 2. The reactions of starch at 393 K for 24 h. Starch 45 mg, water 5.0 mL, acatalyst
50 mg. bthe mixed catalyst 100 mg (It was consisted of 50 mg of Pt/AC and 50 mg of AC-
SO3H).