Mechanochemical Synthesis of a Carboxylated Carbon Catalyst and Its Application in Cellulose Hydrolysis

Article abstract of DOI:10.1002/cctc.201501422

A carbon catalyst containing a high density of carboxyl groups was prepared by solvent-free mechanochemical oxidation of activated carbon by using persulfate salts as the oxidant. The mechanochemical oxidation preferentially oxidized the carbon to introduce carboxyl groups without incorporation of sulfonated groups. The material exhibited hydrophilic behavior and was easily dispersed in water. Upon mix-milling, the oxidized carbon showed good catalytic activity for the hydrolysis of cellulose, even at a low catalyst loading. Glucose was obtained in 85 % yield from mix-milled cellulose in the presence of a trace amount of HCl after a reaction time of 20 min.

Full text of DOI:10.1002/cctc.201501422

DOI: 10.1002/cctc.201501422
Communications
Mechanochemical Synthesis of a Carboxylated Carbon
Catalyst and Its Application in Cellulose Hydrolysis
[
a]
Abhijit Shrotri, Hirokazu Kobayashi, and Atsushi Fukuoka*
[
6,7]
A carbon catalyst containing a high density of carboxyl groups
was prepared by solvent-free mechanochemical oxidation of
activated carbon by using persulfate salts as the oxidant. The
mechanochemical oxidation preferentially oxidized the carbon
to introduce carboxyl groups without incorporation of sulfo-
nated groups. The material exhibited hydrophilic behavior and
was easily dispersed in water. Upon mix-milling, the oxidized
carbon showed good catalytic activity for the hydrolysis of cel-
lulose, even at a low catalyst loading. Glucose was obtained in
clude the dehydration of alcohols,
the oxidation of alco-
[
8,9]
[10]
hols,
esterification,
the electrochemical synthesis of
[
11]
[12]
H O , and the reduction of NOx. More recently, oxygenated
2
2
carbon catalysts have also emerged as useful catalysts for the
hydrolysis of renewable polysaccharides such as cellulose and
[
13–21]
hemicellulose.
The active sites on the oxygenated carbon
catalyst are made up of carboxyl, ketonic, and phenolic func-
[
22]
tional groups. These functional groups can be introduced by
oxidation of the carbon material by using chemical and ther-
[
23]
8
5% yield from mix-milled cellulose in the presence of a trace
mal methods. Often, only one functional group catalyzes the
desired reaction. For example, the oxidative dehydrogenation
amount of HCl after a reaction time of 20 min.
[
5]
of hydrocarbons is catalyzed by ketonic sites, whereas the
[
24]
dehydration of alcohols occurs on the carboxylic acid sites.
Carbon is an inexpensive and widely available material that
serves as a versatile catalyst and catalyst support for various
Alternatively, vicinal functional groups can work together to
enhance catalytic activity. This behavior is observed in the
carbon catalyst used for the hydrolysis of b(1!4) glycosidic
[
1,2]
chemical reactions.
Whereas carbon materials are extensive-
[
14,25]
ly used as supports for metal catalysts, their use in the field of
carbocatalysis has recently gained more attention (Scheme 1).
The use of an oxygenated carbon catalyst for the dehydrogen-
ation of hydrocarbons to the corresponding olefins is well
bonds in cellulose and hemicellulose.
In this case, vicinal
carboxyl–carboxyl and carboxyl–phenolic groups work synerg-
istically through formation of a hydrogen bond with the
glucan chain followed by attack on the glycoside bond. There-
fore, it is essential to develop new oxidation methods that can
selectively introduce oxygenated functional group in high den-
sity to control catalytic activity. Selective introduction of car-
boxyl groups is most attractive, as the acidic carbon catalyst
can act as a substitute for other acid catalysts.
[
3–5]
known.
Other reactions catalyzed by oxidized carbon in-
Conventional oxidation methods that rely on brute force oxi-
dation with the use of concentrated acids or high tempera-
tures are not selective, and the final catalyst contains all three
functional groups in different proportions. Moreover, the use
of strong mineral acids to introduce large amounts of oxygen-
ated functional groups also introduces unwanted functional
groups. For example, the synthesis of graphite oxide by
Hummer’s method requires oxidation with KMnO in the pres-
4
[
26]
ence of concentrated H SO . Evidently, the use of sulfuric
2
4
acid introduces sulfonic groups, which can act as unstable cat-
[
27]
alytic sites. Persulfate salts such as ammonium persulfate are
[
28]
reported to favor the formation of carboxylic groups. How-
ever, the oxidation intensity of persulfate salt solutions is lower
than that of other systems. Moreover, a sulfuric acid solution is
essential for oxidation reaction in which persulfates are used,
and this can cause sulfonation to a small degree.
Scheme 1. Examples of organic transformations catalyzed by different func-
tional groups on an oxygenated carbon catalyst.
[
5,21,24]
[
a] Dr. A. Shrotri, Dr. H. Kobayashi, Prof. Dr. A. Fukuoka
Institute for Catalysis
In this paper, we report a mechanochemical approach for
the oxidation of carbon to prepare a carbon catalyst consisting
of a high density of carboxyl groups and its application as
a catalyst for the hydrolysis of cellulose. The catalyst was pre-
pared by mechanochemical oxidation of activated carbon by
using persulfate salts as the oxidant in a planetary ball mill
without using any solvent. In a typical catalyst preparation ex-
periment, steam activated carbon was milled with the solid ox-
Hokkaido University
Kita 21 Nishi 10, Kita Ku, Sapporo
Hokkaido 001-0021 (Japan)
Fax: (+81)11-706-9139
E-mail: fukuoka@cat.hokudai.ac.jp
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cctc.201501422.
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Communications
idizing agent in a planetary ball mill for 5 h. The resulting mix-
ture was washed with 5% HCl to protonate the carboxylate
ions and was then washed with copious amounts of water to
remove HCl and the remaining salts of the oxidizing agent.
Two oxidizing agents were used for this study, Oxone (potassi-
um peroxymonosulfate KHSO ; KPS) and ammonium persulfate
5
(
NH ) S O (APS).
4 2 2 8
The elemental compositions of the catalysts used in this
study are shown in Table 1. The oxygen content of the parent
steam activated carbon (AC) was 4.6%. After mechanochemical
oxidation, the oxygen content of the activated carbon milled
with KHSO4 (AC-M-KPS) increased to 24.4%. Similarly, the
oxygen content of the activated carbon milled in the presence
of (NH ) S O (AC-M-APS) increased to 26.6%. The sulfur con-
4
2
2
8
tent in AC-M-KPS was reduced from 0.1% to beyond the de-
tection limit, and the sulfur content increased marginally in
AC-M-APS (0.3%), as did the nitrogen content (0.0 to 0.6%).
This suggests that there was little or no sulfonation of the
carbon during oxidative milling. The presence of nitrogen in
AC-M-APS indicates that sulfur may be present as unwashed
impurities in the form of (NH ) S O or its corresponding re-
4
2
2
8
duced salt. In contrast, ball milling of activated carbon in the
absence of an oxidizing agent (AC-M) caused a slight increase
in the oxygen content (4.6 to 7.8%). The O in air can act as
2
a potential oxidant during ball milling, as carbon can react
[29]
with O2 under ball-milling conditions.
Milling of carbon
under all conditions increased the ash content of the catalyst.
This was a result of the corrosion of the milling media owing
to mechanical abrasion. X-ray diffraction of the catalyst con-
firmed the presence of a-alumina in all of the milled samples
(
Figure S1, Supporting Information). The ash content in AC-M
Figure 1. XPS analysis of the carbon catalysts.
was very high (18.5%), as carbon alone could not dampen the
mechanical force causing severe abrasion.
The carbon catalysts were analyzed by X-ray photoelectron
spectroscopy (XPS), and the C1s region between binding ener-
gies of 291 and 281 eV is shown in Figure 1. Deconvolution of
the spectra revealed five peaks. The most prominent peak cen-
tered at a binding energy of 284.6 eV was assigned to the pol-
yaromatic carbon present in the C=C bonds (Table 2). The
weak and overlapping peak at a binding energy of 283.7 eV
was assigned to the CÀH bonds. The peak at a binding energy
of 286 eV was assigned to the CÀO bonds of alcohols and
epoxy bridges, and that at a binding energy of 287 eV was as-
signed to the C=O bond in the carbonyl group. The increase in
the relative areas for these two peaks was low (À1% for AC-M-
KPS and 4% for AC-M-APS), which suggests that mechano-
chemical oxidation disfavors the formation of these functional
groups. The peak centered at a binding energy of 288.9 eV
was assigned to carbon in the form of OÀC=O as carboxylic
acid, lactone, or acid anhydride. Calculation of the atomic ratio
based on the peak area revealed that 12 and 13% of the
carbon atoms were present in the OÀC=O environment in AC-
M-KPS and AC-M-APS, respectively. Combining the elemental
analysis data with the XPS data, we calculated the surface den-
sity of carboxyl groups in the AC-M-KPS sample to be
Table 1. Elemental composition and surface properties of the carbon catalysts before and after mechanochemical treatment.
[
a]
[c]
Catalyst
C
H
[%]
N
[%]
S
[%]
Ash
[%]
O
pH in NaCl
solution
Surface area
[b]
2
À1
[%]
[%]
[m g
]
[
[
[
d]
[e]
AC
AC-M
AC-M-KPS
AC-M-APS
AC-A-KPS
92.1
72.4
69.1
68.9
86.3
0.8
1.2
1.6
1.7
0.8
nd
nd
nd
0.1
0.1
2.4
18.5
4.9
4.1
2.5
4.6
6.5
5.8
3.1
3.3
4.9
963
710
406
453
749
d]
d]
7.8
24.4
26.6
10.4
[d]
nd
0.3
0.6
[d]
[f]
[g]
nd
na
[
0
a] Oxygen content was calculated by subtracting the wt% of C, H, N, S, and ash from 100 wt%. [b] pH of a suspension containing the catalyst (50 mg) in
.1m NaCl solution (40 mL). [c] Surface area measured by BET approximation of the N adsorption isotherms. [d] Not detected. [e] Mainly consisted of
quartz. [f] Catalyst was prepared by aqueous-phase oxidation of AC by using KPS. [g] Not analyzed.
2
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Communications
[
35]
the carboxylate structure.
The broad band at n˜ =1300–
Table 2. Composition of carbon atoms present in different environments
À1
[
a]
1000 cm was a result of multiple overlapping bands resulting
based on the area of peaks in Figure 1.
[
32,33]
from n(CÀO) bonds in different environments.
Catalyst
C=C
%]
CÀO
C=O
[%]
OÀC=O
The textural properties of the carbon materials were also of
interest, as they are heterogeneous catalysts. By analyzing the
nitrogen adsorption isotherms of the catalysts (Figure S2), the
Brunauer–Emmett–Teller (BET) surface areas of all the materials
were determined, as shown in Table 1. Carbon milled in the ab-
sence of an oxidizing agent showed a surface area of
[
[%]
[%]
AC
AC-M
AC-M-KPS
AC-M-APS
78
17
3
0
[
b]
[b]
[b]
[b]
[b]
[b]
[b]
76 (À2)
16 (À1)
4 (+1)
3 (0)
5 (+2)
2 (+2)
12 (+12)
13 (+13)
[
b]
b]
[b]
[b]
[b]
65 (À13)
59 (À19)
16 (À1)
[
19 (+2)
[
a] Composition calculated by taking the percentage of area of the fitted
2
À1
7
10 m g , which is slightly smaller than that of the parent ma-
peaks with respect to the total area under the curve. [b] Change in com-
position relative to that of the AC.
2
À1
terial (963 m g ). Mechanochemical oxidation reduced the
2
À1
area to 453 and 406 m g for AC-M-APS and AC-M-KPS, re-
spectively. The BET surface area is decreased by only one half
and may have a limited influence during the catalytic reac-
tions, especially in the aqueous phase, which is used for the
hydrolysis of cellulose. The oxidized carbon catalyst is expected
to be hydrophilic, and therefore, the available contact area in
water may be larger than the BET surface area owing to swel-
ling effects. We analyzed the adsorption of water on the sur-
face of the untreated and oxidized carbon catalysts at 298 K
À1
6
.8 mmolg . Notably, this amount is higher than that in oxi-
dized carbons prepared by conventional oxidation meth-
[30,31]
ods.
The high density of carboxyl groups is a result of
edge functionalization in favor of the other functional groups.
This is in contrast with harsh chemical oxidation methods such
as Hummer’s method for the synthesis of graphene oxide, in
which the formation of hydroxyl groups and epoxy bridges is
[
26]
favored. Consequently, the mechanochemical oxidation suc-
cessfully produced a carbon material bearing a high density of
carboxylic groups.
(Figure S3). At a low relative pressure (p/p =0.1) and despite
0
having a lower BET surface area, the mechanochemically oxi-
dized samples adsorbed an amount of water that was four
times (for AC-M-KPS) and three times (for AC-M-APS) higher
than that adsorbed by the non-oxidized catalyst. Furthermore,
the catalysts were easily dispersed in water and the AC-M-APS
and AC-M-KPS catalyst particles did not settle even after 1 h
(Figure S4). Thus, we can expect that the oxidized catalysts are
suitable for aqueous-phase reactions owing to the presence of
a large amount of carboxyl groups and good dispersion in
water.
We measured the pH of the dispersed catalyst in 0.1m NaCl
to evaluate the acidity of the oxidized catalyst (Table 1). AC
and AC-M showed high pH values of 6.5 and 5.8, respectively,
which is indicative of a low concentration of acidic groups on
the surface. The oxidized AC-M-APS and AC-M-KPS catalysts
showed pH values of 3.3 and 3.1, respectively. This pH is in ac-
cordance with the expected value for the presence of dense
carboxylic groups on the surface of the catalyst (phthalic acid,
pK =3.0). The FTIR spectra of the carbon catalysts are shown
The prepared catalysts were used in the hydrolysis of cellu-
lose in an aqueous-phase reaction. Contrary to traditional
belief that only strong acid catalysts can hydrolyze glycosidic
a
À1
in Figure 2. Prominent bands in the n˜ =1780–1680 cm region
appear in the mechanochemically oxidized carbons, and these
bands are indicative of n(C=O) in the OÀC=O bond. Typically,
vibrations of C=O stretching resulting from carboxylic acid, lac-
[
27,36–42]
bonds,
oxygenated carbon catalysts utilize weakly acidic
[
14,15,25]
functional groups as active sites.
Unlike sulfonic acid
tone, and anhydride groups fall in the range of n˜ =1740 to
species on carbon catalysts, the oxygenated species are much
more durable under the hydrothermal reaction conditions re-
À1 [32–34]
1
700 cm .
All four spectra show complex overlapping
À1
[16,17]
bands at n˜ =1650–1550 cm . These bands were assigned to
quired for the hydrolysis of polysaccharides.
[
32]
n(C=C) in the aromatic rings and d(OÀH) of adsorbed water.
Pretreatment of cellulose is required to reduce its crystallini-
À1
[43]
The broad band at n˜ =1420–1380 cm could be assigned
either to in-plane d(CÀH) in different C=CÀH structures or to
ty. Steric hindrance owing to the crystalline state and the
[
44]
strong intermolecular hydrogen bonds of cellulose
inhibit
the hydrolysis reaction. In addition, hydrolysis of cellulose in
the aqueous phase by using a solid acid catalyst takes place at
the solid–solid interface. The reaction is slow owing to the lim-
ited contact area between the catalyst and the solid cellulose.
Previously, our group reported that amorphization of cellulose
and facilitation of a good solid–solid contact can be simultane-
ously achieved by milling the catalyst and cellulose together in
[
14,21,45]
a
process called mix-milling.
Mix-milling produces
a solid–solid mixture of cellulose and the catalyst; this results
in a 13-fold increase in the hydrolysis rate relative to the rate
of the reaction in which the components are individually
milled, as this only amorphizes the cellulose. Consequently,
a high yield of soluble oligomers can be obtained even at low
[
45]
temperature.
In aqueous solution, soluble glucans easily
Figure 2. FTIR spectra of the carbon catalysts.
adsorb back onto the aromatic carbon surface through CHÀp
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[
46,47]
bonds and further hydrolysis occurs.
Thus, we performed
to 48% in the presence of AC-M-APS under the same reaction
conditions (Table 3, entry 8). The higher activity of AC-M-KPS
can be attributed to the presence of a larger number of acidic
functional groups on the catalyst surface, as demonstrated by
the lower pH value of the catalyst dispersed in 0.1m NaCl solu-
tion (Table 1) and its better hydrophilicity, which exposes the
active sites as shown by the adsorption of water at low p/p0
(Figure S3).
mix-milling of the mechanochemically oxygenated carbon and
cellulose prior to the hydrolysis reaction. As such, mix-milling
was not expected to alter the acidic properties of the catalyst.
This was confirmed by analyzing the pH of a solution prepared
by dispersing mix-milled cellulose containing 50 mg of AC-M-
KPS in 40 mL of 0.1m NaCl. This dispersion showed a pH of
3
5
.2, which was the same as that of the suspension made from
0 mg of the AC-M-KPS catalyst.
The reaction time was reduced substantially by increasing
the temperature. After only 20 min at 1808C in the presence of
AC-M-KPS, cellulose was completely converted into soluble
oligomers (Table 3, entry 9). Further hydrolysis of the oligomers
under these conditions required a longer time, which reduced
the selectivity because of the degradation of glucose. Soluble
oligomers can be rapidly converted into glucose in the pres-
Table 3 shows the results of the hydrolysis of cellulose in dis-
tilled water. AC-M-KPS was the most active catalyst, and it
gave a conversion of 98% and a glucose yield of 69% after
a reaction time of 16 h at 1458C and a cellulose to catalyst (S/
C) weight ratio of 6.5 (Table 3, entry 4). AC-M-APS also showed
good activity with a glucose yield of 67% under the same re-
action conditions (Table 3, entry 6). In contrast, a conversion of
[
14]
ence of a small amount of mineral acid (0.012% HCl). Under
these conditions, the oligomers underwent quick hydrolysis to
afford glucose in a high yield of 85% (Table 3, entry 10). This is
one of the highest glucose yields reported from cellulose by
using a carbon catalyst.
7
6% and a glucose yield of 40% were obtained in the pres-
ence of AC-M (Table 3, entry 3). The parent AC gave an even
lower conversion (56%) and a low yield of glucose (27%;
[
45]
Table 3, entry 2). Assuming a pseudo-first-order reaction, the
rate constant of hydrolysis for AC-M-KPS is five times larger
than that for AC. These results show that the presence of car-
boxyl groups imparted by mechanochemical oxidation pro-
motes the cellulose hydrolysis reaction. We also used AC oxi-
dized under aqueous conditions by using the same amount of
We also tested the durability of the catalyst under hydro-
thermal reaction conditions (1808C, 20 min, water). Cellobiose
was chosen as a substrate to test the recyclability of the cata-
lyst to avoid the need to produce a large amount of recycled
catalyst to repeat the mix-milling. After hydrolysis of cellobiose,
the yield of glucose was 33, 31, and 30% over three repeated
runs (Figure S5). Hence, we conclude that the activity of the
catalyst did not decrease for at least three runs under the reac-
tion conditions used for cellulose conversion.
KHSO to evaluate the effectiveness of the mechanochemical
5
oxidation. The aqueous-phase oxidation was not effective in in-
troducing a large amount of oxygenated functional groups, as
evident from the low oxygen content (10.4%) and the high pH
value (4.9) of the AC-A-KPS catalyst. Evidently, this catalyst did
not yield a high amount of glucose after mix-milling (33%;
Table 3, entry 5). Therefore, it can be inferred that mechano-
chemical oxidation is essential to produce an active carbon
catalyst bearing large amounts of carboxylic acid groups.
To evaluate the activity of the oxygenated carbon catalyst
further, the S/C ratio of the hydrolysis condition was increased
from 6.5 to 13. The yield of glucose was 61% in the reaction
with AC-M-KPS (Table 3, entry 7). The glucose yield decreased
In conclusion, we found that mechanochemical oxidation of
activated carbon with persulfate salts resulted in a highly oxy-
genated carbon catalyst without introducing sulfonic acid
groups. Analysis of the prepared catalyst by X-ray photoelec-
tron spectroscopy showed that the newly generated oxygenat-
ed functional groups were mainly present in the form of car-
boxyl groups. These catalysts were active for the hydrolysis of
cellulose to produce glucose. The carbon oxidized with KHSO5
showed the highest activity for the hydrolysis of cellulose at
[a]
Table 3. Conversions and yields of products after the hydrolysis of cellulose by using the carbon catalysts.
[
b]
Entry
Catalyst
S/C
T
t
[h]
Conv.
[%]
Glucose
[%]
Other sugars
[%]
Oligomers
[%]
Levoglucosan
[%]
5-HMF
[%]
Furfural
[%]
[
8C]
[
c]
1
2
3
4
5
6
7
8
9
none
AC
–
6.5
6.5
6.5
6.5
145
145
145
145
145
145
145
145
180
180
16
16
16
16
16
16
16
16
16
56
76
98
65
86
84
78
94
97
5.0
27
40
69
33
67
61
48
20
85
1.7
2.9
8.2
4.4
2.7
4.1
3.4
4.0
1.6
2.6
4.9
19
15
5.8
13
1.8
10
8.9
68
1.2
0.2
0.6
1.2
1.7
0.8
1.4
1.5
1.3
0.9
2.8
1.2
1.6
8.3
7.5
3.8
7.4
5.8
7.3
0.3
2.3
0.4
0.5
1.4
1.3
0.1
1.2
1.2
1.1
0.1
0.3
AC-M
AC-M-KPS
AC-A-KPS
AC-M-APS
AC-M-KPS
AC-M-APS
6.5
13.0
13.0
6.5
[
d]
AC-M-KPS
AC-M-KPS
0.33
0.33
[d,e]
1
0
6.5
[
a] Cellulose and catalyst were ball milled together before the reaction. The substrate containing cellulose (81 mg) and the remaining catalyst were dis-
persed in water (10 mL) in a glass reactor, and reaction was performed in an oil bath. 5-HMF=5-(hydroxymethyl)furfural. [b] Cellulose to catalyst weight
ratio. [c] Cellulose was milled in the absence of the catalyst. [d] The milled substrate containing cellulose (324 mg) and the catalyst (50 mg) were charged
in a high-pressure reactor along with water (40 mL). The reactor was heated to 1808C, and this temperature was maintained for 20 min before cooling.
[
e] Dilute HCl (0.012 wt%) was used instead of water.
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high cellulose to catalyst weight ratios of all the catalysts stud-
ied. A glucose yield of 85% was achieved in only 20 min at
Acknowledgements
1
808C in the presence of a trace amount of HCl (0.012%).
The authors would like to thank Dr. I. Ogino for helping with the
water adsorption experiments. This work was supported by fund-
ing from Japan Science and Technology Agency through the Ad-
vanced Low Carbon Technology Research and Development Pro-
gram (JST ALCA).
Experimental Section
Catalyst preparation
Steam activated carbon (AC) for the synthesis of the catalysts was
supplied by Ajinomoto Fine-Techno, commercially called BA. For
mechanochemical oxidation, AC (0.5 g) was milled in the presence
of (NH ) SO (4.6 g, Wako Pure Chemical Industry, Ltd.) or Oxone
Keywords: carbon catalysts · carboxylic acids · hydrolysis ·
mechanochemistry · oxidation
4
2
8
(
4.2 g, KHSO as triple salt, Tokyo Chemical Industry Co., Ltd.). The
4
mixture was ball milled in a 250 mL alumina pot with alumina balls
1.5 cm, 100 g) at 500 rpm for 5 h by using a Fritsch P-6 planetary
ball mill. After milling, the mixture was recovered and washed with
wt% HCl (2”) and water until the pH of the washed solution was
more than 5. Subsequently, the catalyst was dried at 110 8C for
6 h. Aqueous-phase oxidation was done by dispersing the AC
0.5 g) and KHSO (4.2 g) in water (50 mL) and stirring for 5 h. After
[
[
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2
Catalyst characterization
[
[
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The elemental composition of the catalyst was determined by
using a CE440 CHN analyzer from Exeter Analytical. The ash con-
tent was calculated by measuring the weight of residue left after
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6
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim