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Catalysis Communications 19 (2012) 115–118
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Direct conversion of cellulose into sorbitol using dual-functionalized catalysts in
neutral aqueous solution
Joung Woo Han, Hyunjoo Lee ⁎
Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 120-749, Republic of Korea
a r t i c l e
i n f o
Article history:
Received 3 November 2011
Received in revised form 23 December 2011
Accepted 23 December 2011
Available online 30 December 2011
Keywords:
Biomass
Cellulose
Sorbitol
Heterogeneous catalyst
Dual-functionalized catalyst
a b s t r a c t
Direct and selective conversion of cellulose into sorbitol was carried out over dual-functionalized catalysts
containing both sulfonate groups and Ru nanoparticles. A high sorbitol yield of 71.1% was obtained in a neutral aqueous solution without an aid of liquid phase acid at an intermediate reaction temperature of 165 °C
via synergy of sulfonate groups and Ru sites on the catalyst surface. No deactivation was observed even
after 5th repeated reactions. The effect of reaction time, temperature, and the Ru amount was also evaluated
for the sorbitol production. The cellulose was decomposed by simultaneous hydrolysis and hydrogenation
producing cello-oligomers with partially hydrogenated end groups.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Recently, biomass has drawn a lot of attention as an environmentally friendly and sustainable resource for the production of fuel and
chemical products. Because the CO2 generated by the consumption of
biomass-derived chemicals can be recovered through the growth of
more biomass, biomass is called as a ‘carbon-neutral’ resource. Unlike edible types of biomass such as starch or oil, which has raised ethical issues,
cellulosic biomass cannot be digested by humans and is very abundant in
nature [1]. Therefore, cellulose is the most promising natural resource for
the conversion into more valuable chemicals.
Cellulose can be converted into sugar alcohols [2–9], oxygenated
bio-oil [10], and hydrocarbons [11] by various chemical transformations. Especially, sorbitol, which is the hydrogenated form of glucose,
was targeted in this work because it is a good model system to study
both hydrolysis and hydrogenation. Sorbitol is widely used as a
sweetener, a moisture controller in cosmetics, and in medical applications. It also has been studied as resource for the production of hydrogen, alkane, and value-added chemicals such as ethylene glycol and
propylene glycol [12–14].
Since Yan et al. presented the conversion of cellobiose into sorbitol
over an acidic Ru colloidal solution [2], several groups have also
reported the conversion of cellulose into sorbitol. Fukuoka et al.
reported that cellulose can be cracked into sorbitol and mannitol
⁎ Corresponding author. Tel.: + 82 2 2123 5759; fax: + 82 2 312 6401.
E-mail address: [email protected] (H. Lee).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.12.032
with yields of 25% and 6%, respectively, over Pt/γ-Al2O3 at 190 °C
[3]. C. Luo et al. reported a 38% of cellulose conversion and a 22% of
sugar alcohol yield obtained over a Ru/C catalyst in hot water at
245 °C [4]. Deng et al. reported the cracking of cellulose into sorbitol
with a 69% sorbitol yield, which is, to the best of our knowledge, the
highest value to date, over a Ru/CNT catalyst after the cellulose has
been pretreated with phosphoric acid at a reaction temperature of
185 °C [5]. Generally, the conversion of cellulose into sorbitol necessitates both acid and metal catalysts, for hydrolysis and hydrogenation,
respectively. Liquid-phase mineral acids or heteropolyacids have been
used together with metal catalysts containing Pt or Ru [8,9]. These
acids, however, are difficult to recover and can cause corrosion of the reactor. In addition, a large amount of waste sludge is formed in the acid
neutralization process. Water at temperatures above 190 °C, which
can donate an H+ ion reversibly [4,6], was used for hydrolysis as well.
But it is better to avoid such high reaction temperatures to minimize
the energy input.
In this study, dual-functionalized catalysts were developed by depositing Ru nanoparticles on carbon supports treated with sulfuric
acid. The sulfonate groups were shown to be effective for the hydrolysis of cellulose to form glucose [15–17]. Ru nanoparticles, which
act as catalysts for hydrogenation, were deposited on these carbon
supports by chemical reduction method to preserve the surface sulfonate groups. The conventional calcination method would destroy the
surface functional groups. The prepared dual-functionalized catalysts
produced sorbitol directly from cellulose via a one-pot process with a
high yield, and the reaction proceeded in a neutral aqueous solution
at an intermediate temperature.
116
J.W. Han, H. Lee / Catalysis Communications 19 (2012) 115–118
2. Experimental section
a)
b)
50nm
2nm
2.1. Catalyst preparation
AC-SO3H was prepared by treating 300 mg of activated carbon
(AC, NORIT) with 30 ml of concentrated sulfuric acid (96%, Duksan)
in a three-neck flask with mild stirring (500 rpm) and refluxing at
200 °C. After 24 h, the AC-SO3H was filtered and washed with plenty
of water until the filtrate pH became 7. Then, the washed AC-SO3H
was dried in a convection oven at 80 °C. To obtain 10 wt.% Ru/
AC-SO3H, 100 mg of dried AC-SO3H, 43.8 mg of Ru(acac)3 (Aldrich,
97%) and 20 ml of ethylene glycol (Aldrich, 99%) were put in a
three-neck flask. The flask was heated to 240 °C for 1.5 h with vigorous stirring and refluxing. Then, the solution was washed with ethyl
alcohol (Duksan, 99.9%), filtered, and dried in a convection oven. Pt/
AC-SO3H, Pd/AC-SO3H, and Ni/AC-SO3H were prepared by using the
same method except the metal precursors (Pt(acac)2 (Aldrich, 97%),
Pd(acac)2 (Aldrich, 99%) and Ni(acac)2 (Aldrich, 95%) used instead).
The prepared catalysts were analyzed by TEM (Philips Tecnai 20, acceleration voltage: 200 kV) and XPS (Thermo VG Escalab 220i-XL).
c)
d)
S 2p
Ru 3p3/2
2.2. Hydrogenation reactions of cellulose
The cellulose hydrogenation reactions were carried out in a stainless steel autoclave reactor (Hanwoul Eng). 50 mg of ball-milled cellulose, 20 mg of the prepared catalyst, and 12 ml of water were
introduced into the reactor, and the reactor was purged with 50 bar
of hydrogen gas. Then, the reactor was heated to 165 °C with stirring
at 450 rpm. After the reaction, the product solution was centrifuged,
and the supernatants were filtered. The filtered water-soluble products were analyzed by high-performance liquid chromatography
(HPLC; Young Lin YL-9100 series) with an ELSD detector (Schambeck
SFD ZAM3000). The columns used in this study were an Agilent
Zorbax-NH2 column (mobile phase; acetonitrile:water = 3:1, 1 ml/
min, 40 °C) and a Waters Sugar-Pak1 column (mobile phase; water,
0.4 ml/min, 85 °C). The recyclability of the catalysts was tested after
removing the unreacted solid cellulose by dissolving in sulfuric acid
at room temperature under otherwise the same reaction condition.
This dissolution process did not affect the sulfonate group on the catalyst. Ru and S content in the catalysts showed little difference before
and after the dissolution process.
3. Results and discussion
Fig. 1. (a) TEM, (b) HR-TEM images, and XPS spectra for (c) S 2p and (d) Ru 3p3/2 of
10 wt.% Ru/AC-SO3H catalysts.
In the cases in which no catalyst was used (entry 1), or activated
carbon (entry 2) was used, no sugar alcohols were observed, although
conversions were approximately 40%. The cellulose appeared to be
converted into cello-oligomers that dissolved in the water or into gaseous products such as CO and CO2. (For the detailed results, see Table
S1 and Fig. S2) When sulfonated activated carbon was used (entry 3),
a yield of 11.4% glucose was obtained, but no sugar alcohols were observed due to the absence of hydrogenation. In contrast, when Ru
supported on activated carbon (Ru/AC) was used without acidic
sites (entry 4), a high conversion of 79.2% was observed, but no
sugar alcohols were obtained. In the absence of acidic groups, proper
Table 1
Conversion of the cellulose to sugar alcohols over various solid catalystsa.
Entry Catalyst
TEM and XPS data of Ru/AC-SO3H are shown in Fig. 1. Fig. 1(a)
shows that the size of Ru nanoparticles was approximately 10 nm.
The high-resolution transmission electron microscopy (HR-TEM)
image in Fig. 1(b) clearly shows a Ru nanoparticle supported on the
carbon support. The X-ray photoelectron spectra (XPS) of Ru/
AC-SO3H in Fig. 1(c) and (d) show a large S 2p peak around 168 eV
and a Ru 3p3/2 peak around 462 eV, indicating the presence of sulfonate groups (−SO3H) and metallic Ru, respectively [7,18].
To reduce the crystallinity of the cellulose for enhancing the contact
between the catalysts and the cellulose, ball-milling was performed as a
pretreatment procedure. The detailed process is presented in the supplementary data. Fig. S1 shows that the crystallinity of the cellulose
was significantly reduced after the ball-milling. When the cellulose
obtained after 72 h of ball-milling was treated at 165 °C and 50 bar of
H2 for 24 h with the catalysts and water, the results shown in Table 1
were obtained. The cellulose conversion was calculated based on the
weight difference of solid cellulose before and after the reaction. The
solid cellulose separated from the solution phase after the reaction
was surely dried at 80 °C overnight before the weight measurement.
The yields of the products in the solution were measured using highperformance liquid chromatography (HPLC) calibrated for each sugar
alcohol and for glucose.
Product yield (%)
Sorbitol Mannitol Xylitol Glucose Othersb
1
2
3
4
5
6
d
7
e,f
8
9
10
a
b
c
d
e
f
No catalyst
Activated
carbon (AC)
AC-SO3H
10 wt.% Ru/
AC
10 wt.% Ru/
AC-SO3H
10 wt.% Ru/
AC-SO3H
Ru NP + ACSO3H
10 wt.% Pt/
AC-SO3H
10 wt.% Pd/
AC-SO3H
10 wt.% Ni/
AC-SO3H
Cellulose
conversion
(%)c
0
0
0
0
0
0
0
0
39.4
41.3
39.4
41.3
0
0
0
0
0
0
11.4
0
49.3
79.2
60.7
79.2
58.7
6.7
6.8
0
8.8
81.0
8.6
3.6
3.5
0
3.9
19.6
34.5
7.5
4.7
0
24.6
71.3
51.0
4.0
4.7
0
24.5
84.2
2.2
0.4
0.7
9.5
57.4
70.2
1.8
0.4
0.3
8.7
54.9
66.1
Ball-milled cellulose 50 mg, catalyst 20 mg, water 12 ml, 165 °C, 50 bar H2, 24 h.
Unidentified molecules including cello-oligomers and gaseous products.
Calculated by weight difference of solid cellulose before and after the reaction.
Reaction with microcrystalline cellulose without ball-milling.
The amount of the used Ru metal was the same as the other systems.
The method for Ru nanoparticle synthesis was adapted from Yan's work [2].
J.W. Han, H. Lee / Catalysis Communications 19 (2012) 115–118
Sorbitol Yield
Cellulose Conversion
Yield & Conversion (%)
100
80
100
80
80
60
60
40
40
20
20
120
1 40
160
Cellulose Conversion (%)
Sorbitol Yield (%)
a) 100
180
Temperature (oC)
b) 100
100
80
80
60
60
40
40
20
20
6
12
18
24
30
36
42
48
Cellulose Conversion (%)
Sorbitol Yield (%)
hydrolysis did not occur at relatively moderate temperature of 165 °C,
resulting in destructive dissociation.
For dual-functionalized catalysts containing 10 wt.% Ru and sulfonate groups (entry 5), sorbitol was obtained with a yield of 58.7% at a
cellulose conversion of 81%. Small amounts of mannitol and xylitol
were also observed. The yield calculated for all detected sugar alcohol
molecules (sorbitol + mannitol + xylitol) was very high, at 72.2%. No
glucose was observed. When microcrystalline cellulose was used directly without the ball-milling process (entry 6), the sorbitol yield
was much lower, at 8.6%. The colloidal Ru nanoparticles were mixed
with sulfonated carbon supports and used for the conversion of cellulose into sorbitol (entry 7). The sorbitol yield was relatively high, at
34.5%. However, it was difficult to separate the Ru nanoparticles
(~2 nm) dispersed in the solution from the products in this case. It
should be noted that all of the reactions were performed in neutral
aqueous solutions. The solution pH did not differ before and after
the reaction. The effect of different metals was also investigated. Pt/
AC-SO3H (entry 8) showed a comparable catalytic activity to Ru/
AC-SO3H. In the cases of Pd (entry 9) and Ni (entry 10), however,
the sorbitol yield was much lower.
The stability of the dual-functionalized catalysts (Ru/AC-SO3H)
was tested by repeating the reaction several times, as shown in
Fig. 2. No leached Ru was detected in the product solution. The S content in the catalyst also showed little difference. The cellulose conversion and the sorbitol yield showed no difference up to the 5th cycle.
The dual-functionalized catalysts exhibited no deactivation and
were stable for the conversion of cellulose into sorbitol after repeated
reactions.
The effect of the reaction temperature and time on the sorbitol
yield was investigated, as shown in Fig. 3. The cellulose conversion increased as the temperature increased, but the sorbitol yield showed a
maximum at 165 °C. The products were degraded at higher reaction
temperatures. The sorbitol yield increased with time, showing a maximum of 71.1% at 36 h. For longer reaction times, the sorbitol yield decreased, although the cellulose conversion approached 100%. The
sorbitol was degraded with longer reaction times. A conversion
mechanism of cellulose into sorbitol was also investigated. The products after 6, 12, 18, and 24 h of reaction were analyzed by liquid chromatography mass spectrometry (LC–MS). As shown in Fig. S3, the
peaks for cello-oligomers consisting of sorbitol and glucose units
(sorbitol–glucose (m/z: 367.27) and sorbitol–glucose–glucose (m/z:
529.27)) were clearly observed, in addition to sorbitol. The cellooligomer peaks were much bigger in the early stage of the reaction
but became smaller over time and then almost disappeared after
24 h. This observation indicates that the cellulose polymer undergoes
hydrolysis and hydrogenation simultaneously, as proposed in Scheme
117
54
Reaction Time (h)
Fig. 3. Effect of (a) reaction temperature (for 24 h), and (b) reaction time (at 165 °C)
for the conversion of cellulose into sorbitol.
S1, rather than sequentially, with total hydrolysis followed by hydrogenation with the scheme of cellulose → glucose → sorbitol.
Different amounts of Ru were loaded on the activated carbon supports, and the effect of the Ru amount on the sorbitol yield was also
studied as shown in Table S2. The use of 1 wt.% Ru/AC-SO3H yielded
only 7.3% sorbitol; 6.8% of glucose was also detected. In this case,
the amount of Ru seemed to be insufficient to fully hydrogenate the
products into sorbitol. Among 5, 10, and 15 wt.% Ru/AC-SO3H, the
10 wt.% case showed the highest sorbitol yield and the highest selectivity for sorbitol. These results indicate that a balanced Ru-to-S ratio
is important for the effective conversion of cellulose into sorbitol.
4. Conclusions
A dual-functionalized catalyst containing both acidic groups and
metal active sites for hydrolysis and hydrogenation, respectively,
was prepared. This catalyst was very effective for the direct and selective conversion of cellulose into sorbitol with a maximum yield of
71.1% in neutral aqueous solution and an intermediate temperature
of 165 °C. No deactivation was observed after repeated reactions.
60
Acknowledgment
40
This work was supported by the Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (20100009174) and the S-Oil company.
20
0
1st
2nd
3rd
4th
5th
Recycling number
Fig. 2. Recyclability of dual-functionalized catalysts (Ru/AC-SO3H) up to 5th repeated
reactions.
Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2011.12.032.
118
J.W. Han, H. Lee / Catalysis Communications 19 (2012) 115–118
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