Hydrolysis of crystalline cellulose to glucose in an autoclave containing both gaseous and liquid water

Article abstract of DOI:10.1039/c4ra02396j

Hydrolysis of cellulose was investigated in an autoclave. The yield of glucose produced from crystalline cellulose was dependent on the amount of water added to the autoclave. The highest glucose yield of 17% was obtained after 5 h reaction at 190 °C in t

Full text of DOI:10.1039/c4ra02396j

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Hydrolysis of crystalline cellulose to glucose in an
autoclave containing both gaseous and liquid
water†
Cite this: RSC Adv., 2014, 4, 26838
Takashi Harada,* Yuji Tokai, Akira Kimura, Shigeru Ikeda and Michio Matsumura
Hydrolysis of cellulose was investigated in an autoclave. The yield of glucose produced from crystalline
cellulose was dependent on the amount of water added to the autoclave. The highest glucose yield of
ꢀ
1
7% was obtained after 5 h reaction at 190 C in the presence of a small amount of liquid water in the
autoclave. When hydrolysis was carried out in saturated vapor pressure in the autoclave in which
cellulose was separated from liquid water, nearly the same yield of 15% was obtained. However, when
cellulose was soaked in water, the yield was decreased to about 5%. The results suggested that water
vapor plays an important role in hydrolysis of the b-1,4-glycosidic bonds of cellulose. In addition,
hydrolysis of the b-1,4-glycosidic bonds and degradation of produced glucose under hydrothermal
conditions were investigated by using cellobiose (a dimer of glucose) and authentic glucose, respectively.
Received 19th March 2014
Accepted 9th June 2014
DOI: 10.1039/c4ra02396j
www.rsc.org/advances
or other useful compounds in a closed reaction system. Onda
et al. carried out hydrolysis of cellulose over sulfonated acti-
Introduction
Cellulose is the most abundant source of biomass on earth and vated carbon and obtained glucose in a yield (moles of glucose
1
is a valuable renewable energy resource. Glucose, which can be produced/moles of the glucose units in cellulose) of 41% and
ꢀ
8
obtained by hydrolysis of cellulose, is expected to become one of selectivity of more than 90% aer reaction at 150 C for 24 h.
the main feedstocks for various chemicals and fuels in the Suganuma et al. reported hydrolysis using amorphous carbon
2
future. However, efficient hydrolysis of cellulose to glucose by bearing SO
H, COOH, and OH functional groups as catalysts
3
an environmentally friendly process remains a challenge and obtained glucose in a yield of 4% and water soluble b-1,4-
ꢀ
9
because of the robust crystalline structure of cellulose, which is glucan in a yield of 64% aer reaction at 100 C for 3 h.
composed of glucose linked by b-1,4-glycosidic bonds and Kobayashi et al. reported hydrolysis of cellulose into glucose
hydrogen bonds between the glucose units. The conguration over mesoporous carbon (CMK-3) catalysts on which transition
of the b-1,4-glycosidic bonds in the crystal structure inhibits metals had been deposited. They obtained glucose at the
ꢀ
10
water molecules and also catalysts from approaching the bonds. highest yield of 34% with 10 wt%Ru/CMK-3 at 230 C.
This limitation makes hydrolysis of cellulose much slower than
In these reaction systems, ball milling was widely used as a
3
that of starches. Various approaches have been proposed to pretreatment of crystalline cellulose to break up the crystalline
circumvent this difficulty. They include reactions using cata- structure. Once the crystalline structure is destroyed, it becomes
4
5
lysts such as homogeneous mineral acids, enzymes, and het- easier for reactants and catalysts to access the b-1,4-glycosidic
6
7
eropoly acids, reactions in ionic liquids. However, these bonds of the glucose units of cellulose. Using cellulose that had
processes have drawbacks including difficulty in separation, been treated by ball milling with preparing K26 carbon powder
high cost, low reaction rate and low glucose selectivity.
as a catalyst, Kobayashi et al. obtained high conversion of
ꢀ
Among the various approaches, hydrolysis of cellulose into cellulose and high yields of glucose (20% at 180 C and 57% at
ꢀ
glucose using solid catalysts has been attracted much attention 230 C) under hydrothermal conditions without mineral acid.
due to easy separation of the catalysts from reaction solutions, They attributed the effect of ball milling to good physical
11
their high reusability, and relatively mild reaction condi- contact between cellulose and the carbon catalyst. Although
8–10
tions.
development of catalysts for hydrolysis of cellulose into glucose mechanical energy to signicantly decrease the crystallinity of
Considerable efforts have been directed toward the ball milling is an easy process, it takes a long time and large
12
cellulose, which is a serious drawback for utilization of natural
resources in a large scale by environmentally friendly methods.
Concerning chemicals and solvents, pure water is the most
ideal solvent and reactant for hydrolysis of cellulose because
water is the most environmentally friendly solvent and is able to
dissolve low-molecular-weight products, such as glucose,
Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama,
Toyonaka 560-8531, Japan. E-mail: harada@chem.es.osaka-u.ac.jp; Fax: +81-6-
6
†
1
850-6699; Tel: +81-6-6850-6697
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra02396j
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cellobiose and 5-hydroxymethylfurfural at high concentrations.
Efficient hydrolysis of crystalline cellulose can be achieved
using hot-compressed water in a supercritical state without
catalyst, although the process requires severe reaction condi-
13
tions. Jollet et al. reported 35% conversion of cellulose aer
ꢀ
1
processing for 24 h at 190 C under 5 MPa of H
2
, although the
4
yield was very low (2%). In some studies on enzymatic
hydrolysis of biomass materials including crystalline cellulose,
15
the materials were treated with water vapor beforehand. The
results gave us an idea that water vapor may have a favorable
effect on the hydrolysis of crystalline cellulose. In this study, we
tried to nd conditions to hydrolyze cellulose, especially crys-
talline cellulose, using only water as the reagent and investi-
gated the role of water vapor in the hydrolysis.
Experimental
Fig. 1 XRD patterns of cellulose (a) before the ball-milling treatment,
(b) after the treatment for 3 days, and (c) after the treatment for 7 days.
A typical reaction procedure for hydrolysis of cellulose was as
follows. About 324 mg of crystalline cellulose (MERCK, micro-
crystalline) was dispersed into a desired amount of distilled
water (0.3–10 mL). The mixture was transferred into a 100 mL
Teon-lined steel autoclave. The autoclave was sealed and then
placed in a hydrothermal synthesis reactor (Hiro Co., Japan)
and attached to the rotary drive sha in the heater unit. The
Hydrolysis of crystalline cellulose and amorphous cellulose
ꢀ
was carried out at 160 and 190 C for 5 h in an autoclave con-
taining 5 mL of water. Since the expected amounts of water
ꢀ
vaporized in the autoclave at 160 and 190 C were about 16.6
mmol (0.32 mL) and 33.3 mmol (0.62 mL), respectively, as dis-
cussed later, cellulose was soaked in liquid water in the auto-
clave during hydrolysis. The yields of glucose obtained from
ꢀ
reaction was carried out at 160 and 190 C for several hours with
rotation at 20 rpm. Aer the reaction process, the autoclave was
removed from the reactor and cooled to room temperature. The
reaction solution was transferred into a sample tube and 15 mL
of distilled water was added to the reaction solution. Then the
mixture was separated to solid and liquid parts by centrifuga-
tion. The solid part was dried and weighed, and a portion of the
liquid part was sampled with a syringe and subjected to liquid
chromatographic analysis using a Hitachi HPLC (with a Rezex
RPM-Monosaccharide Pb + 2 (8%) column) equipped with a
refractive index detector. Ball-milling treatment was conducted
at room temperature to decrease the crystallinity of cellulose.
Cellulose powder (20 g) was put into an alumina bottle con-
taining alumina balls and the bottle was rotated for 7 days at
ꢀ
crystalline cellulose were less than 0.2% at 160 C and less than
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1
1.0% at 190 C. However, when amorphous cellulose was used,
the yields of glucose were increased to 7.8% and 33.5% at
ꢀ
temperatures of 160 and 190 C, respectively. The favorable
effect by amorphousization is consistent with the report by
12
Zhao et al. Although hydrolysis of cellulose can be accelerated
by amorphousization, activation by the process of ball milling
does not seem to be a good solution due to its long process time
and the mechanical energy needed.
To nd conditions to increase the yield of glucose from
crystalline cellulose, we simply changed the amount of water
added to the autoclave. Fig. 2 shows the results for hydrolysis of
400 rpm. XRD patterns were recorded using a Rigaku MiniFlex
ꢀ
ꢀ
crystalline cellulose carried out at 160 C and 190 C for 5 h in
the autoclave with increase in the amount of water added.
Interestingly, the yield showed strong dependence on the
amount of water added. The glucose yields reached the maxima
when about 2 mL of water was added to the autoclave: the
X-ray diffractometer (CuKa, Ni lter).
Results and discussion
ꢀ
ꢀ
Although the target of our present study is hydrolysis of crys- maximal yields were 0.2% and 17% at 160 C and 190 C,
talline cellulose, we also studied hydrolysis of amorphous respectively. Cellobiose, a dimer of glucose linked by a b-1,4-
cellulose as the reference because amorphous cellulose is glycosidic bond, and 5-hydroxymethylfurfural (5-HMF), a
8,10,11
frequently used in this eld.
Typical XRD patterns of decomposed product from glucose, were the major by-products
cellulose before and aer ball-milling treatment are shown in of the reaction solution. The yields of cellobiose and 5-HMF
ꢀ
ꢀ
ꢀ
ꢀ
Fig. 1: the peaks at 2q ¼ 16 , 22 and 34 are assigned to (101), were 0.3% and 1.6%, respectively, aer reaction for 5 h at 190 C
002) and (040) facets, respectively. These peaks became weaker in an autoclave containing 2 mL of water.
and broader with processing time of ball milling. They were
The amount of vaporized water in the autoclave can be
(
hardly observed aer processing for 7 days, indicating that the estimated using Teten's equation, which correlates saturated
crystalline phase was all converted to the amorphous phase. vapor pressure Psat(T) at a temperature (T) by
The cellulose without the processing and aer treatment for 7
7
.5T/(T+273.3)
days are hereaer called crystalline cellulose and amorphous
cellulose, respectively.
P
sat(T) ¼ 0.611 ꢁ 10
,
(1)
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Hence, the highest yields of glucose are expected to be obtained
under conditions in which a large number of gaseous water
molecules can make access to the b-1,4-glycosidic bonds of
cellulose.
Fig. 3 shows the time course of conversion of crystalline
cellulose and yield of glucose when the reactions were carried
ꢀ
ꢀ
out at 160 C (Fig. 3a) and 190 C (Fig. 3b) by addition of 2 mL of
water, at which the yields of glucose showed the maxima
ꢀ
(
Fig. 2). In the case of reaction at 160 C, glucose yield reached
the highest value of 16% at 45 h and the conversion reached
ꢀ
4
0% at about 50 h. When the reaction was carried out at 190 C,
the reaction rate was enhanced and it took less than 7 h for the
conversion to reach 40%. The conversion increased monoto-
nously with reaction time, reaching about 60% at 10 h. The yield
ꢀ
of glucose at 190 C reached 22% at 8 h and then leveled off or
started to decrease gradually. Leveling off of the yield of glucose
was also observed for the reaction at 160 C as shown in Fig. 3a.
The leveling off of the yield or the decrease in the yield of
glucose is probably due to the degradation and/or polymeriza-
Fig. 2 Yield of glucose produced by hydrolysis of crystalline cellulose
ꢀ
as a function of water added to the autoclave (100 mL). The hydrolysis
ꢀ
ꢀ
was carried out for 5 h at (a) 160 C and (b) 190 C. The plots for (a) are
enlarged by 20 times. Cellulose: 324 mg, water: 2 mL. It was estimated
from eqn (1) that water was fully vaporized in the autoclave when the tion of glucose produced. With regard to cellobiose, its yield
ꢀ
amounts of water added were less than 0.32 mL and 0.62 mL for cases reached 0.3% aer reaction for 6 h at 190 C and then started to
(
a) and (b), respectively. When the amounts of water added were larger
decrease gradually. In contrast, the yield of 5-HMF increased
than these critical values, the exceeding parts remained as liquid.
monotonously with reaction time and reached 3.3% aer
ꢀ
reaction for 10 h at 190 C (see Fig. S2†). This monotonous
increase of 5-HMF suggests that it is produced via glucose. To
ꢀ
16
where T is given in C and vapor pressure is in kPa. Using this conrm the degradation of glucose, we put reagent-grade
equation, the amounts of water vaporized in the Teon-lined glucose into the Teon-lined tube and heated it at 160 and
ꢀ
ꢀ
ꢀ
tube (100 mL) at 160 C and 190 C were estimated to be 190 C in the autoclave containing 2 mL of water. The results
6.6 mmol (0.32 mL) and 33.3 mmol (0.62 mL), respectively. showed that glucose is gradually decomposed/polymerized
The yield of glucose was decreased when the amount of under the conditions of hydrolysis of cellulose, as shown in
1
water added was less than 0.6 mL, as shown in Fig. 2. This is Fig. 4. Polymerization of glucose or the products from it was
probably because all of the water is vaporized and the pressure thought to have occurred because the residue and solution aer
of water in the system becomes low with decrease in the amount the reaction were strongly colored. It is worth noting that the
of water added. The yield of glucose was also decreased with an reaction rate was independent of the amount of water added to
increase in the amount of water added when the amount was the autoclave as long as the amount was larger than 1 mL; under
more than 2 mL. When amorphous cellulose was used instead that condition, glucose was dissolved in liquid water.
of crystalline cellulose, decrease in the yield of glucose with
In order to obtain information about hydrolysis of the b-1,4-
increase in the amount of water (more than 2 mL) was not glycosidic bond of cellulose, we investigated hydrolysis of
observed (see Fig. S1 in ESI†).
cellobiose in the autoclave containing 2 mL of water. The initial
It is thought that crystalline cellulose has specic interac-
tions with liquid or vaporized water. More specically, the fact
that the maximum yield of glucose was obtained by adding 2 mL
of water (Fig. 2) suggests that too much liquid water has a
harmful effect on the hydrolysis, which will be discussed later.
This fact also suggests that a small amount of liquid water is
effective for enhancing the yield because the highest yields of
glucose were obtained when a small amount of liquid water was
expected to be present in the autoclave. A small amount of
liquid water may be useful for removing polymerized products
from the surface of cellulose bers.
The harmful effect of a large amount of liquid water on the
production of glucose suggests that hydrolysis of crystalline
cellulose is carried out mostly by water vapor and that liquid
water prevents contact of b-1,4-glycosidic bonds with water
vapor. In the case of liquid water, hydrolysis becomes difficult
because the intermolecular hydrogen bonds between cellulose
Fig. 3 Yield of glucose and conversion of cellulose by hydrolysis of
crystalline cellulose at (a) 160 C and (b) 190 C. Cellulose: 324 mg,
water: 2 mL.
ꢀ
ꢀ
3,17
molecules and hydrogen bonds with water are strengthened.
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Fig. 6 shows the amount and yield of glucose produced
from crystalline cellulose as a function of the amount of
cellulose added to the autoclave that contained 2 mL of water.
ꢀ
The reaction was carried out at 190 C for 4 h. The amount of
glucose produced increased with increase in the amount of
cellulose added, which is attributed to the increased surface
area of crystalline cellulose. However, the yield of glucose was
low when the amount of cellulose was small (less than 100
mg). The small amount of cellulose was probably completely
soaked in liquid water and had less chance to be exposed to
water vapor; the amount of liquid water was estimated to be
about 1 mL, as stated above. Hence, this result also suggests
the importance of water vapor for hydrolysis of crystalline
cellulose.
Here a question arises as to whether hydrolysis of crystalline
cellulose proceeds in the absence of liquid water, if water vapor
is supplied to the surface of the cellulose ber. To answer the
ꢀ
ꢀ
Fig. 4 Degradation of glucose at (a) 160 C and (b) 190 C. Glucose:
24 mg, water: 2 mL.
3
question, hydrolysis of crystalline cellulose was carried out at
ꢀ
1
90 C by exposing cellulose only to water vapor in the auto-
production rate of glucose from cellobiose was much higher at
clave. In the experiment, cellulose was put in a small beaker
placed away from liquid water inside the autoclave, which was
not rotated. As a result, we obtained glucose in a yield of about
5%, which was nearly the same as the highest yield obtained
about 17%) by using 2 mL of water, as shown in Fig. 2. In this
ꢀ
ꢀ
190 C than at 160 C, as shown in Fig. 5. The apparent
induction periods are probably due to the gradual increase in
the actual temperature of the autoclave to the set temperature
and not due to the essential properties of the reaction. The
production of glucose from cellobiose is more efficient than
that from crystalline cellulose probably because water can easily
1
(
arrangement, naturally, the amount of liquid water added
hardly affected the yield of glucose, if the amount was large
enough to obtain saturated vapor pressure, as shown by the
broken line in Fig. 7. This tendency is in contrast to the case in
which cellulose was not separated from liquid water in the
autoclave, which was not rotated, as shown by the dotted line;
the yield of glucose started to decrease sharply when the
amount of water exceeded 1 mL. For reference, the result
obtained without separating cellulose from liquid water is
shown by the solid line, which is the same as Fig. 2b; the
autoclave was rotated. In this case, the rotation may have
enhanced the contact between cellulose and water vapor as long
as the amount of liquid water added was not large (or less
than 10 mL).
ꢀ
access the b-1,4-glycosidic bond of cellobiose. At 190 C, the
yield of glucose started to decrease aer 3 h reaction due to the
degradation of glucose.
The effect of water vapor and liquid water on crystallinity of
cellulose was investigated. XRD patterns of cellulose aer pro-
ꢀ
cessing at 190 C for 4 h and 8 h in the autoclave containing 2 mL
of water were almost the same as the XRD pattern of original
cellulose (see Fig. S3†). During the processing, some of the
cellulose was converted to glucose. This result suggests that the
crystallinity of cellulose was not affected by the processing and
that the hydrolysis took place at the surface of crystalline cellu-
18
lose bers. Similar results have been reported by Xiao et al.
Fig. 6 Amount and yield of glucose produced from crystalline cellu-
ꢀ
ꢀ
Fig. 5 Hydrolysis of cellobiose at (a) 160 C and (b) 190 C. Cellobiose: lose as a function of crystalline cellulose added to the autoclave by
ꢀ
3
24 mg, water: 2 mL.
hydrolysis for 4 h at 190 C. Water: 2 mL.
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Fig. 7 Yield of glucose produced by hydrolysis of crystalline cellulose
as a function of water added. The hydrolysis was carried out at for 5 h
ꢀ
at 190 C under the following conditions; (a) cellulose was separated
from liquid water, (b) cellulose was in contact with liquid water in the
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1
1
All of the results suggest the importance of water vapor for
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Hydrolysis of crystalline cellulose to glucose is generally diffi-
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ꢀ
1
60 and 190 C and found that water vapor plays an important
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than the amount needed to obtain saturated vapor pressure.
Even when cellulose was separated from liquid water, almost
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