High yield of renewable hexanes by direct hydrolysis-hydrodeoxygenation of cellulose in an aqueous phase catalytic system

Article abstract of DOI:10.1039/c4ra14304c

In aqueous phosphoric acid, cellulose was efficiently converted into hexanes using a Ru/C catalyst combined with layered compounds or silica-alumina materials. In this process, the direct production of hexanes from cellulose can be improved by suppressing the formation of isosorbide, which makes it difficult to yield hexanes by further hydrodeoxygenation. As the co-catalyst, layered compounds showed a significant inhibition effect on the formation of isosorbide from sorbitol due to the steric restrictions of sorbitol dehydration within the interlayers of layered compounds. Typically, layered LiNbMoO₆ played a great role in promoting the production of hexanes directly from cellulose and a promising yield (72% carbon mol) of hexanes was obtained. In addition, the protonic acid, H₃PO₄, offered efficient catalysis for the hydrolysis of cellulose and the dehydration of the sorbitol hydroxyl moiety.

Full text of DOI:10.1039/c4ra14304c

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High yield of renewable hexanes by direct
hydrolysis–hydrodeoxygenation of cellulose in an
aqueous phase catalytic system†
Cite this: RSC Adv., 2015, 5, 11649
Yong Liu,ab Lungang Chen,a Tiejun Wang,a Xinghua Zhang,a Jinxing Long,a Qi Zhanga
a
*
and Longlong Ma
In aqueous phosphoric acid, cellulose was efficiently converted into hexanes using a Ru/C catalyst
combined with layered compounds or silica–alumina materials. In this process, the direct production of
hexanes from cellulose can be improved by suppressing the formation of isosorbide, which makes it
difficult to yield hexanes by further hydrodeoxygenation. As the co-catalyst, layered compounds showed
a significant inhibition effect on the formation of isosorbide from sorbitol due to the steric restrictions of
sorbitol dehydration within the interlayers of layered compounds. Typically, layered LiNbMoO6 played a
great role in promoting the production of hexanes directly from cellulose and a promising yield (72%
carbon mol) of hexanes was obtained. In addition, the protonic acid, H3PO4, offered efficient catalysis for
the hydrolysis of cellulose and the dehydration of the sorbitol hydroxyl moiety.
Received 11th November 2014
Accepted 11th December 2014
DOI: 10.1039/c4ra14304c
www.rsc.org/advances
conversion of sorbitol and xylitol into hexanes and pentane in
the aqueous phase, respectively, low-cost Ni/HZSM-5 catalysts
have been developed by our group.22–24 Furthermore, the
transformation of sugar and sugar polyols into n-alkanes over
iridium-based catalysts with high yield was also achieved by
Tomishige's group.25 The iridium-catalyzed and the B(C6F5)3-
Introduction
To compensate for the decline of fossil fuel energy sources,
biomass has been developed as a sustainable and renewable
alternative of carbon sources for production of chemicals and
fuels.1–13 Cellulose is the most low-cost but substantial raw
material of biomass for its annual net yield of over 0.72 trillion
tons, which presents it as an attractive carbon source.14–16
However, cellulose is insoluble in water and other conventional
solvents because of its robust structure composed of b-1,4-
glycosidic bonds in D-glucose and the abundance of hydrogen
bonds between the hydroxyl groups. This feature brings a
challenge for researchers to directly convert cellulose into
chemicals or fuels, particularly hydrocarbons such as alkanes,
which require the process of lowering the high oxygen content.
Therefore, direct utilization of cellulose still needs further
investigation.10,17,18
To transform cellulose into alkanes, hydrodeoxygenation of
cellulose-derived water-soluble carbohydrates and sugar alco-
hols was widely investigated. Recently, signicant progress for
production of light gasoline alkanes from cellulose-derived
sorbitol has been made by Huber and co-workers.19–21 They
developed an aqueous-phase reforming method to transform
sorbitol into light alkanes over platinum-based catalysts. For
catalyzed hydrosilylative reduction of carbohydrates to hexanes
26,27
´
has been exploited by the group of Gagne.
However, the feedstock of the aforementioned approaches is
either carbohydrates or sugars/polyols, which require stepwise
reactions from cellulose. The production of alkanes from
biomass through a cascade reaction could reduce the energy
conversion efficiency of the entire process. Hence, high
economic efficiency necessitates direct conversion of cellulose
to hexanes. Unfortunately, little focus has been devoted to this
direct conversion. However, more recently, some reports on the
direct transformation of cellulose into alkanes have been
reported. Huber's group,28 Sekine's group29 and Takahara's
group30 have reported this direct conversion, but all of them
gave a low yield of hexanes. Tomishige used the same catalytic
system of Ir–Re/SiO2 + HZSM-5 to convert cellulose into
hexanes, which obtained a good yield of hexanes with the
addition of a considerable amount of n-dodecane.31 The
biphasic system was also used by Sels's group to transform
cellulose into straight-chain alkanes over modied Ru/C and
H4SiW12O40.32
aCAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion,
Chinese Academy of Sciences, Guangzhou 510640, P. R. China. E-mail: mall@ms.giec.
ac.cn; Fax: +86-20-8707673; Tel: +86-20-8705-7673
bUniversity of Chinese Academy of Sciences, Beijing, 100049, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra14304c
Although, a good yield of alkanes can be achieved from
cellulose in a biphasic system or organic phase, the mono-phasic
aqueous system is also necessary to explore efficient conversion
of cellulose to hydrocarbons from the viewpoint of more
economic and more environmentally friendly reaction media.
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in aqueous phosphoric acid media at 503 K (Table 1). A
commercial Ru/C catalyst was chosen for the conversion of
cellulose due to the fact that Ru metal possesses excellent
activity for C–O cleavage.38 However, without the addition of
phosphoric acid and the co-catalyst, a single Ru/C catalyst
exhibited outstanding hydrogenolysis activity, which broke the
C–O bonds and the C–C bonds. As a result, 91% carbon of
cellulose was converted to C1–C4 alkanes and the yield of the
desired hexanes was only 1.9% with complete conversion of
cellulose (entry 1). With the presence of phosphoric acid (0.59
M), the side reaction of C–C cleavage was greatly inhibited. The
yield of C1–C4 alkanes was decreased to 6.9% and the yield of
desired hexanes was increased to 23.3% (entry 2). In the liquid
phase, a large amount (42.1%) of isosorbide was found. In the
acidic media, it was easy to yield isosorbide by dehydration
from sorbitol, which was formed in the hydrolysis hydro-
deoxygenation of cellulose. In the absence of phosphoric acid,
the Ru/C catalyst combined with single co-catalyst MCM-41 or
HNbMoO6 was tested in aqueous solution. The addition of
MCM-41 gave the complete conversion of cellulose (entry 3),
whereas the yield of hexanes was very low (7.9%). The main
products were C1–C4 alkanes (75.6%). The addition of
HNbMoO6 greatly inhibited the conversion of cellulose (entry
11). The main products were small polyols (mainly ethylene
glycol and propylene glycol), which indicated the cracking of the
C–C bond was still easily performed by Ru/C, whereas the
desired reaction of C–O bond cracking was suppressed.
Scheme 1 Direct catalytic conversion of cellulose into hexanes over
Ru/C combined with LiNbMoO6 in aqueous phosphoric acid.
Water is the cheapest and most convenient media for the reac-
tion, which suggests that the use of an aqueous phase system for
cellulose conversion is attractive. Therefore, it is necessary to
develop an efficient and environmentally friendly reaction system
to directly convert cellulose into hydrocarbons in aqueous media.
In addition, some mild acids have been proposed as environ-
mentally friendly acids for hydrolysis. For example, phosphoric
acid can be utilized as a nutrient and fertilizer for microorgan-
isms and plants.33,34 However, former studies suggested cellulose
or cellulose-derived products (glucose or sorbitol) are easily
converted into isosorbide.35–37 Isosorbide is hardly transformed
into hexanes, as revealed in this study. Herein, we report a direct
selective conversion of cellulose into hexanes by a Ru/C catalyst
combined with layered compounds in low concentration phos-
phoric acid aqueous media. A promising yield (72% C) of hexanes
was achieved (Scheme 1), whereas the yield of isosorbide was less
than 3%. Hexanes directly produced from cellulose over this
catalytic system bring a special environmentally friendly advan-
tage for the development of sustainable and renewable hydro-
carbon fuels.
Interestingly, when the Ru/C catalyst combined with MCM-
41 in the aqueous phosphoric acid media, the yield of
hexanes was remarkably increased (entries 4–7). By increasing
the concentration of phosphoric acid, the yield of hexanes could
be promoted from 27.6% to 48.6%, whereas the yield of iso-
sorbide was still high (20%–30%). In this catalytic system, C–C
cracking reactions such as formation of C1–C4 alkanes from
cellulose were promisingly suppressed, and the yield of iso-
sorbide was lower than that of the same reaction conditions
without the addition of MCM-41. It is certain that the increased
concentration of phosphoric acid promoted the reaction rate of
cellulose hydrolysis, as well as favoured the cleavage of C–O
bonds with the aid of the protonic acid, which resulted in the
dehydration of alcoholic hydroxyls. Moreover, the presence of
Experimental section
0.2 g Ru/C, 0.8 g substrate, and 40 mL of different concentra-
tions of aqueous phosphoric acid were loaded into a 100 mL
stainless autoclave. Different amounts of co-catalyst were added
according to different reaction requirements. The reactor was
purged with pure H2 and pressurized with 6 MPa H2. Then, the
reactor was heated to 503 K at a stirring rate of 450 rpm. The
starting time of the reaction was when the desired temperature
was reached.
The reuse experiments for the aqueous phase catalytic
system were carried out without the separation of Ru/C and
LiNbMoO6. The process is described below. The conversion was
tested for 24 h at 503 K, the autoclave was cooled in ice water
and the samples were collected. The next test was carried out by
loading 0.8 g fresh cellulose into the autoclave. Aer the
completion of reaction, we used ethanol to separate the solid
mixture and the Ru/C catalyst was suspended in the ethanol
solution, whereas the used LiNbMoO6 remained at the bottom.
The regeneration of LiNbMoO6 was performed by simple
calcination of the used catalyst at 903 K for 36 h.
3ꢀ
PO4 brought a suppression effect of retroaldol cleavage of
glucose to some extent, as suggested in former studies.39,40
Therefore, the addition of phosphoric acid was necessary in the
current study. For the co-catalysts, other silica–alumina mate-
rials or solid acids, such as HZSM-5, SBA-15 and g-Al2O3, were
also tested (entries 8–10) with the presence of Ru/C and 0.59 M
phosphoric acid, and 36.6%, 29.7% and 35.3% yield of hexanes
were obtained, respectively. Previous studies suggested the
MCM-41 and HZSM-5 were effective for hydrodeoxygenation of
carbohydrates and sugar polyols to produce hydrocarbons.22,25
In the current study, MCM-41 and HZSM-5 also showed higher
hexane yields than SBA-15 and g-Al2O3. The effect of HZSM-5
has been suggested by Tomishige's group who found that
HZSM-5 was efficient in producing ve-membered-ring cycle
Other details are given in the ESI.†
Results and discussion
Conversion of cellulose
Hydrolysis hydrodeoxygenation of cellulose was investigated by ethers such as tetrahydrofuran through an internal dehydration
a Ru/C catalyst combined with or without different co-catalysts reaction.25 However, in the current catalytic system, a large
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Table 1 Hydrolysis hydrodeoxygenation of cellulose to yield hexanes over Ru/C with co-catalyst in aqueous phosphoric acida
Yieldb [%]
Co-
Con.
[H3PO4] [M] [%]
Carbon balance
[%]
C1–C4
Glu/
Entry catalyst
Hex Pent alkanes cel
Sor
Sorb Iso
Other Hex/total alkanesc
1
2
3
4
5
6
7
8
—
—
—
0.59
—
>99.9 95.4
>99.9 93.6
>99.9 98.3
>99.9 93.5
>99.9 91.1
>99.9 91.2
>99.9 93.9
>99.9 93.4
>99.9 94.5
>99.9 95
1.9
23.3 12.4
1.1 91
6.9
<0.1
<0.1
0.1
<0.1 <0.1
1.2 <0.1
2.0
54.7
8.2
1.6
7.3 42.1 <0.1
MCM-41
MCM-41
MCM-41
MCM-41
MCM-41
HZSM-5
g-Al2O3
SBA-15
HNbMoO6
HNbMoO6 0.59
HNbMoO6 0.42
LiNbMoO6 0.42
LiNbMoO6 0.21
LiNbMoO6 0.12
7.9 13.4 75.6
27.6 12.1 12.8
32.8 11.8 11.9
40.1
48.6
36.6 16.4
35.3 14.4
29.7 13.8
2.4
44.7
65.9
70.2
72
<0.1 <0.1
1.3 <0.1
0.21
0.42
0.59
0.85
0.59
0.59
0.59
—
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
2.3
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
0.5
2.0
1.8
1.2
1.1
0.8
4.2
9.3 29.7 <0.1
4.5 28.2 <0.1
2.9 28.0 <0.1
2.3 23.7 <0.1
1.5 32.1 <0.1
1.5 31.9 <0.1
52.6
58.1
67.9
72.8
62.0
62.0
58.0
27.6
71.6
85.3
87.0
87.6
85.1
76.8
84.3
88.1
85.9
8.9 10.1
9.6
8.6
6.0
7.2
7.7
5.1
9
10
11
12
13
14
15
16
17
18d
19e
20f
2.7 10.3 30.8 <0.1
40
97.3
1.2
0.1
1.5
0.9
0.2
0.2
1.5
0.6
1.1
1.2
2.3
2.4
3.6
1.0
0.9
0.7
8.9
0.5
1.4
1.6
1.4
6.2 19.2
4.8 14.2
>99.9 86.5
>99.9 88.4
>99.9 89.2
>99.9 88.6
91.4 85.7
90.5 84.5
70.5 89.1
94.5 87.3
80.1 88.1
7.5 10.2
6.9
5.9
5.9
4.9
5.5
3.1
3.9
2.8
4.5
4.6
4.3
3.8
8.9
4.7
4.4
5.1
4.4
3.9
3.0
7.8
4.8
3.5
2.5
1.9
49.6
47.7
41.9
61.6
48.2
LiNbWO6
0.21
2.1 11.2
LiNbMoO6 0.21
LiNbMoO6 0.21
LiNbMoO6 0.21
5.4
5.2
5.1
4.7
4.4
5.1
0.2
0.6
a
Reaction conditions: 0.8 g microcrystalline cellulose, 0.2 g Ru/C, 0.4 g co-catalyst for entries 3–10 or 0.2 g layered compound for entries 11–17, 40
mL H2O, 503 K, 6 MPa H2, 24 h. b Hex ¼ n-hexane and 2-methylpentane; Pent ¼ n-pentane; Glu/cel ¼ glucose and cellobiose; Sor ¼ sorbitol; Sorb ¼
sorbitans, include 1,4-sorbitan and 2,5-sorbitan; Iso ¼ isosorbide; other ¼ 1-hexanol, 1-pentanol, 1,6-hexanediol, ethylene glycol, and propylene
c
d
glycol. Hex/total alkanes ¼ hexanes/total alkanes. Reaction conditions: 1.6 g microcrystalline cellulose, 0.2 g Ru/C, 0.2 g layered compound,
e
40 mL H2O, 503 K, 6 MPa H2, 24 h. Reaction conditions: 1.6 g microcrystalline cellulose, 0.4 g Ru/C, 0.4 g layered compound, 40 mL H2O, 503
K, 6 MPa H2, 24 h. f Reaction conditions: 4 g microcrystalline cellulose, 0.5 g Ru/C, 0.5 g layered compound, 40 mL H2O, 503 K, 6 MPa H2, 24 h.
amount of by-product isosorbide was formed through the phosphoric acid was optimized to 0.21 M, the highest yield of
dehydration of sorbitol. Compared with sorbitol, we assume hexanes was obtained (72%). With lowering the concentration
that isosorbide is difficult to undergo ring-open reaction to yield of phosphoric acid to 0.12 M, the yield of hexanes decreased to
hexanes by further hydrodeoxygenation. Therefore, the inhibi- 49.6%. This suggests that phosphoric acid is indispensable for
tion of formation of isosorbide from sorbitol is crucial for conversion of cellulose because the C–O cleavage reaction was
improving the production of hexanes. Surprisingly, the prom- catalyzed by the protonic acid. However, the amount of phos-
ising yield of hexanes was obtained by the combination of the phoric acid also affects the hydrodeoxygenation ability of Ru/C
Ru/C catalyst and HNbMoO6 in aqueous phosphoric acid media and the dehydration of sorbitol, which results in the formation
(entries 12 and 13). Specically, a 44.7% yield of hexanes was of small polyols and isosorbide. Therefore, a certain balance
obtained in 0.59 M phosphoric acid. By decreasing the phos- between the co-catalyst and phosphoric acid concentration
phoric acid concentration to 0.42 M, the yield of hexanes was should be maintained. Furthermore, the layered compound,
signicantly promoted to 65.9%. More importantly, the yield of LiNbWO6, also possesses activity for the conversion of cellulose
the by-product isosorbide was considerably lower (less than 5% to hexanes (47.7%, entry 17). However, tungsten species are also
C) than that of MCM-41 or HZSM-5. It is clear that the formation effective in producing ethylene glycol from oligosaccharides or
of isosorbide is greatly inhibited due to the presence of glucose.48–50 In this catalytic system with the LiNbWO6, 11.2% C
HNbMoO6. Moreover, the total yield of C–C cracking products yield of small polyols (10.1% C in ethylene glycol and 1.1%C in
in the gas phase was approximately 11.4%, which was lower propylene glycol) was obtained from cellulose as the main by-
than that of 0.59 M phosphoric acid. In the liquid phase, the product in the aqueous phase.
side reactions to produce small polyols through C–C cracking
In addition, the amount of substrate cellulose was enlarged
were suppressed with only 4.8% carbon in the form of small over the same catalytic system. Although the conversion of
polyols. This result indicates that high concentration of phos- cellulose decreased to 70.5% (entry 18), the 41.9% yield of
phoric acid catalyses the C–C cracking reaction in the presence hexanes was still obtained. When increasing the amount of Ru/
of layered compounds. LiNbMoO6 is the precursor of the tested C catalyst and layered compound, the yield of hexanes reached
HNbMoO6, which is also a layered compound.41–47 Interestingly, 61.6% (entry 19). Entry 20 suggested that the concentrated
using LiNbMoO6 as the co-catalyst, higher yields of hexanes cellulose could be efficiently converted into hexanes with a yield
were observed (entries 14–16). When the concentration of of 48.2%.
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Table 2 Evolution of species in the process of cellulose conversion over Ru/C with MCM-41 or LiNbMoO6 in aqueous phosphoric acida
Yield [%]
C1–C4
Entry Co-catalyst t [h] Con. [%] Carbon balance [%] Hex Pent alkanes Glu/cel Sor Sorb Iso
Other Hex/total alkanes
1
2
3
4
5
6
7
8
MCM-41
MCM-41
MCM-41
MCM-41
MCM-41
MCM-41
MCM-41
LiNbMoO6
LiNbMoO6
LiNbMoO6
LiNbMoO6
0.5
1
3
6
12
18
24
0.5
1
3
6
52
95.4
93.3
93.8
87.9
83.1
89.1
93.9
90.0
90.1
85.2
81.5
90.7
89.8
88.6
1.2 2.9
18.8 3.6
29.9 5.3
31.8 6.5
34.4 8.2
41.6 9.1
48.6 9.6
5.7 0.6
24.8 1.1
39.1 1.8
45.5 2.3
63.4 3.8
70.8 4.3
3.0
3.5
4.1
5.4
7.6
8.2
8.6
0.5
1.8
2.2
2.7
2.9
3.5
4.3
0.5
0.3
0.1
<0.1
<0.1
<0.1
<0.1
1.1
3.4 13.9 24.7 <0.1
2.5 16.5 25.5 <0.1
2.7 10.2 32.3 <0.1
16.9
72.6
76.1
72.8
68.5
70.6
72.8
83.8
89.5
90.7
90.1
90.4
90.1
87.6
75.8
90.2
94.1
>99.9
>99.9
>99.9
20.0
41.1
60.1
71.8
85.1
95.2
>99.9
3.1
2.9
1.9
1.1
4.1
4.2
3.2
2.5
1.5
0.9
0.2
7.7 28.2 <0.1
3.8 26.1 <0.1
2.9 25.4 <0.1
2.3 23.7 <0.1
2.8
2.1
1.8
1.4
1.2
1.0
0.7
3.2 <0.1
3.1 <0.1
3.1 <0.1
9
0.2
10
11
12
13
14
<0.1
<0.1
<0.1
<0.1
<0.1
3.0
2.9
3.3
3.0
1.1
1.5
1.7
2.5
LiNbMoO6 12
LiNbMoO6 18
LiNbMoO6 24
72
5.9
a
Reaction conditions: 0.8 g microcrystalline cellulose, 0.2 g Ru/C, 0.4 gMCM-41 or 0.2 g LiNbMoO6, 40 mL H2O, 503 K, 6 MPa H2. For MCM-41,
[H3PO4] ¼ 0.59 M; for LiNbMoO6, [H3PO4] ¼ 0.21 M.
As shown in Table 2, further studies were focused on the decreased, and the amount of sorbitol and isosorbide slightly
evolution of other species in the process of cellulose conversion. increased. In the h run, a hexane yield of 52.8% was
In the case of MCM-41, the formation of glucose/cellobiose and obtained. This result was comparable to the result of HNbMoO6
sorbitol were detected initially, and a considerable amount of (Table 1, entries 12 and 13). In this catalytic system, HNbMoO6
dehydration product (sorbitans and isosorbide) was formed might be formed by the proton-exchange reaction of LiNbMoO6
from sorbitol (entry 1). However, 5-hydroxymethylfurfural in the presence of phosphoric acid (Fig. S12†). More impor-
(HMF) was not detected in the process. It might be caused by tantly, detectable Ru leaching aer the h run was the reason
the fast hydrogenation of HMF or negligible yield.32 By for the decreased performance. Furthermore, the catalytic
increasing the reaction time (entries 1–7), hexanes were grad- performance of h reused LiNbMoO6 could be recovered to
ually formed as the main products with the complete conver- that of the fresh LiNbMoO6 by calcination, and a high hexanes
sion of cellulose. Aer 24 h, a promising hexanes yield of 48.6% yield of 71.5% was also obtained. Those results suggested that
was obtained (entry 7), although the yields of C–C cracking the LiNbMoO6 and Ru/C are essentially reusable, and
products and pentane gradually increased to 9.6% and 8.6%, LiNbMoO6 can be regenerated by simple calcination.
respectively. The sorbitans were drastically decreased from
16.5% to 3.8% (entries 2–7), whereas isosorbide remained at
approximately the same amount (about 26% C). For the dehy-
dration products of sorbitol, isosorbide was robust to be
transformed into hexanes. Even aer 24 h, a high isosorbide
Hydrodeoxygenation of glucose, sorbitol, and isosorbide
We did not nd the existence of HMF or its derived products
(such as 2,5-dihydroxymethylfuran or 2,5-dihydroxymethylte-
trahydrofuran) in the liquid phase (Fig. S8 and S9†), and this
yield (23.7%) was detected in the aqueous phase (entry 7). What
might be a result of negligible yield or the fast hydrogenation of
intrigued us was that LiNbMoO6 showed excellent activity for
HMF. On the contrary, large amounts of glucose, sorbitol and
isosorbide were detected in the liquid phase when the yield of
conversion of cellulose to hexanes (entries 8–14). In this cata-
lytic system, the yield of isosorbide was maintained at a low
hexanes was relatively low. It might suggest that these inter-
level (below 3.5%) along with the conversion process of cellu-
mediates, particularly isosorbide, had a signicant inuence on
the yield of hexanes. Hence, in the following tests, we deliber-
ately used glucose, sorbitol, and isosorbide to perform extended
lose. It is evident that the LiNbMoO6 has an excellent suppres-
sion effect on the formation of isosorbide compared with MCM-
41, although cellulose was not readily converted with the
tests over the Ru/C and co-catalyst LiNbMoO6 in aqueous
LiNbMoO6 as the co-catalyst (entry 14). When the reaction time
phosphoric acid.
was 24 h, the yield of hexanes reached 72%. In gaseous phase, a
Glucose can be formed by the hydrolysis of cellulose and
smaller amount of C1–C4 alkanes and pentane and 2.5% yield
cellobiose.46,51,52 Sorbitol can be obtained from the hydrogena-
of small polyols were obtained.
tion of glucose53 and the hydrolysis–hydrogenation of cellulose
over noble-metal catalysts.54–56 With the addition of acid, the
Reuse experiments of aqueous phase catalytic system were
carried out to investigate the durability of the catalyst. As listed
ether bonds of cellulose are susceptible to hydrolysis and form
in Table 3, the second run also gave a promising result. With the
oligosaccharides or glucose. In addition, sorbitans (mono-
increase of reuse number, the yield of hexanes gradually
molecular dehydration) and isosorbide (bimolecular
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Table 3 Reuse of Ru/C and LiNbMoO6 in the cellulose conversiona
Yield [%]
Co-
Con. Carbon balance
C1–C4
Glu/
Usage times catalyst
[H3PO4] [M] [%]
[%]
Hex Pent alkanes cel
Sor Sorb Iso Other Hex/total alkanes
1
2
3
4
LiNbMoO6 0.21
>99.9 88.6
>99.9 91.1
>99.9 85.4
>99.9 86.3
90.1 89.1
>99.9 88.7
72
5.9
4.3
8.2
8.4
8.2
8.5
4.5
<0.1
<0.1
<0.1
<0.1
0.2
0.2 0.7
0.1 0.8
0.2 0.9
0.5 0.4
0.8 1.3
0.3 0.8
3.0 2.5
3.5 0.6
4.8 0.8
4.9 1.5
5.2 1.6
3.1 2.6
87.6
82.6
80.6
77.6
74.2
87.3
LiNbMoO6 0.21
LiNbMoO6 0.21
LiNbMoO6 0.21
LiNbMoO6 0.21
LiNbMoO6 0.21
71.1 6.8
63.4 6.9
61.3 9.5
52.8 9.9
71.5 5.9
5
6b
<0.1
a
Reaction conditions: 0.8 g microcrystalline cellulose, 0.2 g Ru/C, 0.2 g LiNbMoO6, [H3PO4] ¼ 0.21 M, 40 mL H2O, 503 K, 6 MPa H2, 24 h. The yields
of aqueous products were calculated by subtracting the yields of former test. b Layered compound was calcined at 903 K for 36 h. Fresh Ru/C and
phosphoric acid solution were added.
dehydration) can be formed from sorbitol under acidic condi- produce sorbitans and isosorbide in the aqueous phase at high
tions. It is possible that some reversible reactions can be per- reaction temperature.37 Typically, when protonic acid was
formed in these three tested feedstocks.
added to the reaction system, the isosorbide yield could be
As listed in Table 4, the results of the glucose conversion improved to a higher level.36 Without adding LiNbMoO6, only
showed the evolution of reaction species at various times. 17.2% C yield of hexanes was obtained (entry 8). Although, the
Within 24 h reaction time, the yields of the cyclic ethers (sor- yield of hexanes was increased by adding LiNbMoO6, the yield
bitans and isosorbide) were high in the absence of LiNbMoO6 of 26.7% C was still very low. However, the yield of isosorbide
(entries 8). Not only does it suggest that glucose is easily was kept at a high level with/without LiNbMoO6. This might be
hydrogenated into sorbitol over Ru/C, but it also reveals that the a result of the fact that isosorbide is a more stable intermediate
formed sorbitol is readily transformed into dehydration prod- that is readily formed from dehydration of sorbitol catalysed by
ucts. Aer 24 h, the yield of desired hexanes was only 14.9% due protonic acid in the aqueous phase. For the production of
to the formation of large amount of stable isosorbide. In the desired hexanes from sorbitol hydrogenolysis, it showed that
case of LiNbMoO6 (entries 1–7), the formation of isosorbide was sorbitol could not be efficiently converted into hexanes over the
greatly suppressed within 24 h, and the yield of isosorbide tested catalytic system.
decreased to below 14.5%. The initial formed sorbitans
To demonstrate the intrinsically robust feature of isosorbide,
decreased from 13.1% to 0.7%. The results suggest that the it was used as the substrate in the same catalytic system. As
addition of co-catalyst LiNbMoO6 shows an excellent suppres- listed in Table 6, the yield of hexanes obtained from isosorbide
sion effect on the formation of isosorbide. Therefore, a higher was very low aer 24 h. It should be noted that the isosorbide
yield of hexanes (54.2%) can be obtained (entry 7).
could not be efficiently converted into hexanes with or without
As listed in Table 5, using sorbitol as the substrate, large the presence of LiNbMoO6. Furthermore, the isosorbide
amounts of sorbitans and isosorbide were detected initially. conversion was very low, particularly when LiNbMoO6 was used
Their yields reached 48.9% and 40.2%, respectively (entry 1). (only 14.1% isosorbide conversion). This indicates the high
Aer 24 h, the high isosorbide yield of 40.2% over the yield of hexanes obtained from cellulose over LiNbMoO6 does
LiNbMoO6 was still obtained (entry 7). This might be caused by not result from the fact that LiNbMoO6 has high reactivity for
the fact that sorbitol can undergo stepwise dehydration to isosorbide conversion to produce hexanes. We assume
Table 4 Evolution of species in the process of glucose conversion over Ru/C with LiNbMoO6 in aqueous phosphoric acida
Yield [%]
C1–C4
alkanes
Hex/total
alkanes
Entry
Co-catalyst
t [h]
Con. [%]
Carbon balance [%]
Hex
Pent
Sor
Sorb
Iso
Other
1
2
3
4
5
6
7
8
LiNbMoO6
LiNbMoO6
LiNbMoO6
LiNbMoO6
LiNbMoO6
LiNbMoO6
LiNbMoO6
—
0.5
1
3
45.1
57.1
60.5
72.5
91.1
98.3
99.2
92.9
85.5
87.6
98.5
90.1
90.1
88.1
89.9
90.6
10.2
17.1
32.8
35.9
50.8
53.8
54.2
14.9
0.3
0.9
1.4
3.3
5.5
7.6
8.4
14.1
0.5
1.2
1.5
4.9
6.1
7.8
9.1
11.9
3.9
3.7
2.3
2.1
1.8
0.9
0.2
4.2
13.1
12.2
7.7
3.9
2.5
1.0
0.7
5.2
10.1
14.2
13.8
14.1
13.9
13.8
14.1
39.1
0.4
0.7
0.1
<0.1
<0.1
<0.1
<0.1
<0.1
92.7
89.1
91.9
81.4
81.4
77.7
75.6
36.4
6
12
18
24
24
a
Reaction conditions: 0.8 g glucose, 0.2 g Ru/C, 0.2 g LiNbMoO6, 40 mL H2O, 503 K, 6 MPa H2, [H3PO4] ¼ 0.21 M.
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Table 5 Evolution of species in the process of sorbitol hydrogenolysis over Ru/C with LiNbMoO6 in aqueous phosphoric acida
Yield [%]
C1–C4
Entry
Co-catalyst
t [h]
Con. [%]
Carbon balance [%]
Hex
Pent
alkanes
Glu
Sorb
Iso
Other
Hex/total alkanes
1
2
3
4
5
6
7
8
LiNbMoO6
LiNbMoO6
LiNbMoO6
LiNbMoO6
LiNbMoO6
LiNbMoO6
LiNbMoO6
—
0.5
1
3
94.1
97.7
98.1
98.5
98.9
99.3
99.5
93.5
99.5
>99.9
95.2
91.1
93.9
89.1
82.8
95.8
3.7
6.8
7.6
13.5
22.9
24.8
26.7
17.2
0.1
0.4
0.5
1.6
1.9
2.1
2.5
9.8
0.2
0.4
0.5
1.1
1.8
2.5
2.8
7.8
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
48.9
40.7
38.9
30.6
25.1
18.1
10.2
1.4
40.2
49.1
45.8
42.9
41.2
40.9
40.2
54.8
0.5
0.3
0.1
<0.1
<0.1
<0.1
<0.1
<0.1
92.5
89.5
88.4
83.3
86.1
84.4
83.4
49.4
6
12
18
24
24
a
Reaction conditions: 0.8 g sorbitol, 0.2 g Ru/C, 0.2 g LiNbMoO6, 40 mL H2O, 503 K, 6 MPa H2, [H3PO4] ¼ 0.21 M.
isosorbide is difficult to deoxygenate due to its two tetrahy- hydrolysis and hydrogenation processes. Compared with
drofuranic moieties, which enhance steric hindrance. Hence, it sorbitol as the substrate, the sorbitol gradually released from
limits the interaction of isosorbide and the catalysts. Moreover, cellulose or glucose might be efficiently intercalated into the
isosorbide is widely and successfully applied as a monomer for space of the layered LiNbMoO6, which actually reduces the
medical applications and polymers for its high stability, which chance of isosorbide forming. In contrast, a large amount of
in turn enhances the products' stability.57–59 This also suggests sorbitol in the solution would be easily transformed into iso-
isosorbide is robust to hydrodeoxygenation. Therefore, the sorbide rather than intercalated into the interlayers of
formation of isosorbide should be suppressed in the direct LiNbMoO6. Therefore, cellulose as the substrate had a higher
conversion of cellulose to alkanes under the current catalytic yield of hexanes than glucose and sorbitol.
system.
In the current catalytic system, the presence of LiNbMoO6
The effect of layered compound LiNbMoO6
plays a critical role in determining the rate of the dehydration
Because the addition of LiNbMoO6 gave the highest yield of
reaction of sorbitol. Table 6 shows that the formation of iso-
hexanes with negligible isosorbide in the conversion of cellu-
sorbide is the key barrier for increasing the hexane yield. The
lose for the current catalytic system, further investigation was
results from Table 1,4 and 5 indicate that the formation of
focused on the effect of this co-catalyst. Textural properties of
isosorbide could be signicantly inhibited by the addition of
MCM-41 and layered compounds LiNbMoO6, HNbMoO6, and
LiNbMoO6, regardless of whether cellulose, glucose or sorbitol
LiNbWO6 are given in the ESI (Table S1 and Fig. S1†). The
morphologies of layered compounds were characterized by
scanning electron microscope (SEM), in which the layered
structures of the compounds were clearly observed. FT-IR and
was used as the substrate. With the decrease of isosorbide yield,
the yield of hexanes sharply increased. Moreover, it should be
noted that the suppression effect of isosorbide formation also
depended on the substrates with a descending order of cellulose
NH3-TPD of the layered compounds are given in the ESI
> glucose > sorbitol. This can be explained by the fact that
(Fig. S10 and S11†). HNbMoO6 gave a peak around 1033 K which
sorbitol can be gradually released from the hydrogenation of
glucose. Similarly, sorbitol yield from cellulose required
was attributed to very strong acid sites, conrmed by peaks at
3386 cmꢀ1 and 1618 cmꢀ1, which were ascribed to O–H and H–
Table 6 Evolution of species in the process of isosorbide hydrogenolysis over Ru/C with LiNbMoO6in aqueous phosphoric acida
Yield [%]
C1–C4
Entry
Co-catalyst
t [h]
Con. [%]
Carbon balance [%]
Hex
Pent
alkanes
Glu
Sor
Sorb
Other
Hex/total alkanes
1
2
3
4
5
6
7
LiNbMoO6
LiNbMoO6
LiNbMoO6
LiNbMoO6
LiNbMoO6
LiNbMoO6
LiNbMoO6
0.5
1
3
8.9
9.1
86.5
85.7
94.1
97.4
95.1
91.0
95.7
1.5
3.0
4.1
5.5
5.8
6.9
7.4
0.1
0.2
0.3
0.3
0.4
0.8
1.4
0.1
0.1
0.2
0.4
0.5
0.9
1.3
1.1
0.2
2.1
2.2
3.2
2.5
1.5
0.9
0.2
2.8
2.1
1.8
1.4
1.2
1.0
0.7
<0.1
<0.1
<0.1
1.1
1.5
1.7
88.2
90.9
89.1
88.7
86.6
80.2
73.3
10.2
11.5
12.3
13.4
14.1
<0.1
<0.1
<0.1
<0.1
<0.1
6
12
18
24
2.5
a
Reaction conditions: 0.8 g isosorbide, 0.2 g Ru/C, 0.2 g LiNbMoO6, 40 mL H2O, 503 K, 6 MPa H2, [H3PO4] ¼ 0.21 M.
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O–H, respectively, of adsorbed water molecules on acid sites,
whereas other compounds did not have these indications. This
suggests that the acidity of the co-catalyst shows no responsi-
bility for the difference of performance in cellulose conversion
because the acidity of acidic media plays the dominant role.
Therefore, the intercalation effect of layered compounds should
be considered. It has been reported that the layered compounds
possessed the intercalation effect of substrates within the
interlayers.41–47 Based on the abovementioned results, we
assume that the substrate like cellulose-derived glucose and
sorbitol could be intercalated into the layered compounds, but
isosorbide as the substrate could not be effectively interca-
lated. To conrm the role of intercalation of substrates, the
connement of glucose, sorbitol, and isosorbide into the
narrow spaces between the layers was investigated by
immersing LiNbMoO6 in these substrate solutions (0.1 M) for
24 h at room temperature.
X-ray diffraction (XRD) has already been used as an approach
to conrm the intercalation of substrates within the interlayers
of layered compounds in previous studies.41–47 In the current
study, XRD experiments were also performed to analyse the
layered materials aer ltrating and drying in air at 353 K
(Fig. 1). The results suggested that glucose and sorbitol rather
than isosorbide could be intercalated into the interlayers of
LiNbMoO6, which was also supported by previous research.41–47
The peaks of LiNbMoO6 were shied to lower angles aer
immersion in glucose and sorbitol solution. Those distinct
Scheme 2 The efficient conversion of cellulose to hexanes over
LiNbMoO6 and Ru/C in aqueous phosphoric acid.
peaks shied to lower angles were observed in the case of
immersion in glucose solution. It indicated that the basal
spacing of LiNbMoO6 was expanded aer immersion in glucose
or sorbitol. In contrast, the peak did not shi in the case of
LiNbMoO6 aer immersion in isosorbide solution, which indi-
cated isosorbide could not be intercalated. It should be noted
that the dehydration of sorbitol to isosorbide must undergo
consecutive 1,4- and 2,6-cyclodehydration. Because sorbitol was
intercalated within the interlayers of LiNbMoO6, the terminal
OH moieties of sorbitol molecules formed tetrahydrofuranic
moieties with difficulty by dehydration due to steric restrictions.
However, for the sorbitol in the bulk solution, the formation of
isosorbide is readily achieved. Therefore, the great decrease of
isosorbide yield aer the addition of layered compounds is
probably attributed to the steric restrictions of sorbitol dehy-
dration within interlayers of layered compounds. The fasci-
nating ability of layered LiNbMoO6 to inhibit the formation of
isosorbide is responsible for the production of hexanes with
high yield from cellulose conversion in the current catalytic
system.
Based on the abovementioned results, the following route of
cellulose conversion is proposed for the inuence of
glucose, sorbitol and isosorbide. As shown in Scheme 2, with
the catalysis of phosphoric acid, cellulose is hydrolysed to
yield glucose or other oligomers, which could be adsorbed by
layered compounds. Moreover, glucose could be trans-
formed into sorbitol over Ru/C by hydrogenation. For the
glucose and/or sorbitol gradually released from the
substrate, they can readily, successfully intercalate into the
interlayers of the layered compounds. In the active interlayer
sites, consecutive C–O cleavage reaction of glucose and/or
sorbitol could be performed for the production of hexanes.
It is clear that some intermediates are reduced over Ru/C in
the bulk solution. In addition, the reduction of intermedi-
ates might occur between the interlayers of layered
compounds by hydrogen spillover from Ru/C onto the
layered compounds.60–62 For the consecutive dehydration of
sorbitol within interlayers, the dehydration of terminal OH
moieties of sorbitol molecules to form isosorbide must
undergo the formation of tetrahydrofuranic moieties, which
was hardly performed within the interlayers. Therefore, the
formation of isosorbide within the interlayers hardly
occurred. In the case of cellulose conversion, the formation
of negligible isosorbide was achieved.
Fig. 1 XRD patterns of LiNbMoO6, and LiNbMoO6 after immersion in
aqueous solutions of glucose, sorbitol, or isosorbide.
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The Ru/C catalyst combined with LiNbMoO6 was efficient in
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Acknowledgements
The authors gratefully thank the nancial supports of the
National Basic Research Program of China (no. 2012CB215304),
the key program of Chinese Academy of Sciences (no. KGZDEW-
304-2), the Guangdong Province-Chinese Academy of Sciences
´
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2012B090400042), and the Natural Science Foundation of China
(no. 51306189 & no. 51106108).
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