Direct conversion of cellulose into C6 alditols over Ru/C combined with H+-released boron phosphate in an aqueous phase

Article abstract of DOI:10.1039/c4ra10834e

Non-edible cellulose has attracted considerable attention to be converted into valuable platform chemicals. Direct transformation of cellulose to C₆ alditols (mannitol and sorbitol) provides a sustainable route. A productive approach is presented in this work using a Ru/C catalyst combined with H⁺-released boron phosphate in an aqueous phase. A yield of C₆ alditols as high as 74.9% with a 91% conversion of cellulose is achieved. By adding silica-alumina materials, the yield of C₆ alditols can be improved to 93.5% with complete conversion. The acid sites gradually released from boron phosphate under hydrothermal conditions could promote the hydrolysis of cellulose without significant degradation of glucose. Furthermore, the interaction of boric acid with C₆ alditols may form borate-polyol complexes, which can enhance the stability of the C₆ alditols to avoid further hydrogenolysis and dehydration of the C₆ alditols formed. Due to the adsorption ability of the substrate, the addition of silica-alumina materials with a high content of silica leads to improved performance. This journal is

Full text of DOI:10.1039/c4ra10834e

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Direct conversion of cellulose into C6 alditols over
Ru/C combined with H+-released boron phosphate
in an aqueous phase†
Cite this: RSC Adv., 2014, 4, 52402
a
a
a
a
a
Yong Liu,ab Lungang Chen, Tiejun Wang, Ying Xu, Qi Zhang, Longlong Ma,
*
Yuhe Liaoab and Ning Shiab
Non-edible cellulose has attracted considerable attention to be converted into valuable platform chemicals.
Direct transformation of cellulose to C6 alditols (mannitol and sorbitol) provides a sustainable route. A
productive approach is presented in this work using a Ru/C catalyst combined with H+-released boron
phosphate in an aqueous phase. A yield of C6 alditols as high as 74.9% with a 91% conversion of
cellulose is achieved. By adding silica–alumina materials, the yield of C6 alditols can be improved to
93.5% with complete conversion. The acid sites gradually released from boron phosphate under
hydrothermal conditions could promote the hydrolysis of cellulose without significant degradation of
glucose. Furthermore, the interaction of boric acid with C6 alditols may form borate–polyol complexes,
which can enhance the stability of the C6 alditols to avoid further hydrogenolysis and dehydration of the
C6 alditols formed. Due to the adsorption ability of the substrate, the addition of silica–alumina materials
with a high content of silica leads to improved performance.
Received 16th August 2014
Accepted 29th September 2014
DOI: 10.1039/c4ra10834e
www.rsc.org/advances
For cellulose-derived chemicals, C6 alditols are the most
important platform molecules to produce alkanes10 and high-
value chemicals.11 Over a metal-acid catalytic system, C6 aldi-
Introduction
Lignocellulosic biomass is considered the most abundant and
cheapest carbon source.1–4 Among its compositions, cellulose is
the most abundant, which exists as a polymer consisting of
repeating units of glucose linked by b-(1,4)-glycoside bonds.
Through deoxygenation of cellulose or cellulose-derived carbo-
hydrates or sugar alcohols, biofuel and chemicals can be ach-
ieved for commercial application.5,6 However, its insolubility
limits the possibility of cleavage of the cellulose chain, and
therefore hampers its usability.5 Hence, pretreatment has been
proposed as an efficient method to improve its solubility in
water. For instance, an alkali–urea system can dissolve cellulose
in a low concentration through disruption of the hydrogen
bond networks in cellulose at a low temperature of 261 K.7 In
addition, ball-milled pretreatment can lower the degree of
crystallinity of cellulose through mechanical interactions has
been studied by researchers.8,9 However, those approaches are
questionable because they use high concentrations of alkali
solution to dissolve cellulose and the indispensible high energy
cost for ball-milled pretreatment.
tols can be achieved from the one-pot conversion of cellulose,
which is widely considered as a promising approach that avoids
the two-step process involving a pre-treatment step. The oper-
ation of cellulose depolymerization is oen takes place in acidic
media or catalyzed by an acid catalyst. Metals are responsible
for the hydrogenation reaction of the carbonyl groups in the
cellulose-derived carbohydrates. Inorganic acids like H2SO4,12,13
15
HCl,12,14 and H3PO4,12,14 or heteropoly acids like H5BW12O40
,
13,16
H4SiW12O40,13,15,16 and H3PW12O40
combined with metal-
supported catalysts have been proved as catalyst systems used
to transform cellulose to C6 alditols. In addition, even hot water
could be used as the acid source for the hydrolytic hydrogena-
tion process.17 However, these catalysts require long reaction
times, which is a barrier for their commercial application.
Moreover, even with the energy-cost of ball milled cellulose or
acid-impregnated cellulose, a long reaction time is still a
requirement for high yields of C6 alditols. Besides, when the
acid functionality is too dominant in the hydrolytic hydroge-
nation of cellulose, a build-up of free glucose and hexitols
causes a loss of selectivity through glucose degradation and
hexitol dehydration. For metal catalysts, Ni,18,19 Pt,20–23 Ru,16,24–27
and Ir28,29 were studied for the hydrolytic hydrogenation of
cellulose. Among the studied metals, ruthenium possesses the
highest activity during hydrogenation, which could shorten the
reaction time and lower the reaction temperature. But it still
casts the negative effect on the process by performing the
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: +86-20-8705-7673; 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/c4ra10834e
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hydrogenolysis reaction on the C6 alditol products formed.30 the phosphoric acid solution with stirring. The dried white
Hence, it is necessary to seek for an effective catalyst system powder was calcined at 1273 K for 5 h. Then, the calcined
with a suitable balance between the acid and metal functions to powder was washed with hot deionized water at 333 K and dried
avoid the C–C bond cleavage of the desired C6 alditol products at 373 K overnight. MCM-41, HZSM-5, Hb, HMOR, USY, and g-
on the ruthenium metal, and also avoid the dehydration of the Al2O3 were milled and then calcined at 823 K for 4 h. They were
desired C6 alditols as well as glucose degradation on the acidic used aer the milling process.
sites. Moreover, to achieve a high yield of C6 alditols from
Zirconium phosphate (ZrP) was synthesized by precipitation
cellulose, the improved thermal stability of C6 alditols is of a ZrOCl2$8H2O solution in NH4H2PO4 solution.40 Aer being
indispensable for a faster and more complete conversion of washed with deionized water, the white precipitate was dried at
cellulose at higher reaction temperatures.
373 K for 12 h and calcined at 673 K in air for 4 h.
Boron phosphate is regarded as an acidic catalyst, which
Tantalum phosphate (TaP) was prepared based on previous
contains both a Brønsted acidic center and Lewis acidic center reports.41,42 Tantalum pentaethoxide as the tantalum source was
its surface.31–35 It has been used for the dehydration reaction of added to a tartaric acid solution using ethanol as the media.
alcohols36 and isomerization reactions.37 But it has never been Aer the ethanol was evaporated, distilled water was added.
used in the hydrolytic hydrogenation of cellulose. Herein, we Then, a diammonium hydrogen phosphate solution was added
report a boron phosphate-Ru/C catalytic system to directly with stirring to get a homogeneous solution. A certain amount
convert cellulose into C6 alditols. With a gradual increase in the of hexadecyltrimethyl ammonium bromide was added with
concentration of H+ arising from the slow hydrolysis of boron vigorous stirring for 0.5 h. The white precipitate was sealed in a
phosphate in the aqueous phase, the gradually enhanced acidic Teon autoclave for 24 h at 403 K. The precipitate was washed
media can catalyze the transformation of cellulose to cellulose with distilled water and ethanol, and dried at 343 K. Before
oligomers. This can avoid the instant arrival of a dominant being used, the resulting TaP was calcined at 823 K for 6 h.
acidic functionality, which causes glucose degradation and
Ag3PO4 (AgP) was synthesized by dropwise addition of an
hexitol dehydration. Over the Ru/C catalyst, the cellulose olig- Na2HPO4 aqueous solution into a AgNO3 aqueous solution.43
omers were transformed to C6 alditols. More importantly, the Aer ltration and washing with distilled water, the golden
boric acid and C6 alditols produced can form borate–polyol yellow precipitate of AgP was dried in air at 343 K overnight.
complexes, which prevent further hydrogenolysis and dehy-
Zn3(PO4)2 (ZnP) was synthesized using ZnCl2 as the zinc
dration due to their larger steric hindrance. Hence, a high yield source. A ZnCl2 aqueous solution was added dropwise into a
of C6 alditols can be obtained over the boron phosphate-Ru/C H3PO4 solution with stirring. The mixed solution was neutral-
catalytic system.
ized with NH3$H2O and a white precipitate of ZnP was formed.
The white precipitate was washed with distilled water and dried
at 343 K overnight.
Experimental section
Materials
A Nicolet iS50 FT-IR (Fourier transform infrared spectrom-
eter) spectrophotometer (Thermo Scientic, USA) was used to
verify the borate–polyol complexes. The instrument was equip-
ped with a MCT (mercury cadmium telluride) detector. The
Hitachi S-4800 instrument was used to analyze the morpho-
logical information (scanning electron microscopy) of BP. The
XRD information of BP was obtained using X0 Pert Pro MPD
equipment. The surface area and pore properties were analyzed
by a Quantachrome instrument.
Microcrystalline cellulose (MCC) and tantalum(V) ethoxide
(C10H25O5Ta) were bought from Alfa Aesar. Ru/C (5 wt%) and
Tungstosilicic acid (H4SiW12O40, SiW) were purchased from
Aladdin. Phosphoric acid (H3PO4), boric acid (H3BO3), and
ethanol were supplied by Tianjin Fuyu Fine Chemical Co., Ltd.
MCM-41(pure silica), HZSM-5 (Si/Al ¼ 38), Hb (Si/Al ¼ 38),
HMOR (Si/Al ¼ 25), USY (Si/Al ¼ 9), and g-Al2O3 were purchased
from Catalyst Plant of NanKai University. Na2HPO4, ammonium
dihydrate phosphate (NH4H2PO4), and hexadecyl trimethyl
ammonium bromide (CH3(CH2)15N(Br)(CH3)3) were bought
from Tianjin FuChen Chemical Reagents Factory. Ammonium
hydroxide (NH3$H2O) was purchased from Guangzhou Chem-
ical Reagent Factory. Silver nitrate (AgNO3), zinc chloride
(ZnCl2), kaolin, and tartaric acid (C4H6O6) were purchased from
Tianjin Damao Chemical Reagent Factory. RuCl3 was purchased
from Shaanxi Kaida Chemical Engineering Co., Ltd. Zirconium
oxychloride octahydrate (ZrOCl2$8H2O) was supplied by Sino-
pharm Chemical Reagent Co., Ltd.
Cellulose conversion experiment
In a typical procedure, Ru/C (0.1 g), cellulose (0.8 g), co-catalyst
(0.5 g), and water (40 mL) were introduced into a 100 mL
stainless steel reactor. The reactor was puried with pure
hydrogen for six cycles and pressurized with 5 MPa of H2 (room
temperature, RT). The autoclave was heated to 463 K with stir-
ring and kept at this temperature for 1 h. The autoclave was
cooled to RT by placing it into an ice–water bath aer the
reaction and liquid samples were collected. To improve the
yield of C6 alditols, 0.2 g zeolite or solid acid as an additive was
added.
Catalyst preparation and characterization
A Waters e2695 high performance liquid chromatography
The boron phosphate catalyst (BP) was synthesized based on (HPLC) apparatus was used to qualitatively analyze the aqueous
previous reports.38,39 A stoichiometric mixture of boric acid and samples. The HPLC was equipped with an Inertsustain C18
phosphoric acid was dried at 323 K. Boric acid was dissolved in column to quantify the polyols and a Shodex Sugar SH1011
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column to detect the sugars. The mobile phase was water and
0.005 M H2SO4, respectively. The gas phase samples were
analyzed using an Agilent 7890A instrument.
Results and discussion
The effect of various acids and the reaction conditions on the
hydrolytic hydrogenation of cellulose
Fig. 1 shows the performance of different acids and the Ru/C
catalyst for the transformation of cellulose to alditols at 463 K
for 1.0 h. ZrP shows a low cellulose conversion and the yield of
alditols is about 46%, though our previous studies suggested
ZrP can efficiently convert ball-milled cellulose into C6 aldi-
tols.44 For the SiW–Ru/C system, though it has been reported as
an effective system to transform cellulose to hexitols,13 the 1.0 h
reaction time is too short to get a high yield and selectivity for C6
alditols. For solid acid phosphates, the use of ZnP and AgP
could cause the hydrogenolysis reaction to produce a 16.6%
yield of ethylene glycol and 8.5% yield of propylene glycol.
Although the cellulose conversion is relatively high using the
TaP–Ru/C catalytic system, the selectivity of C6 alditols is 54%,
which lead to a low yield. However, the BP–Ru/C catalytic system
exhibits a high cellulose conversion (91%) with 82.1% selectivity
for C6 alditols aer 1.0 h.
The effect of reaction time was performed using the BP–Ru/C
catalytic system, and the results given in the Fig. 2. By
increasing the reaction time from 0.5 h to 2 h, the substrate
cellulose was completely converted. Aer 1 h reaction time at
463 K, the yield of C6 alditols achieved was 74.9% with a 91%
cellulose conversion. Surprisingly, further prolongation of
reaction time didn't cause any noticeable loss of C6 alditols.
Aer 4 h, the yield of C6 alditols only decreased from 74.9% to
72.9%, suggesting the stability of the C6 alditols formed is
relatively high at 463 K using the current catalytic system. But
we have already noted that reaction conditions requiring a long
reaction time and high reaction temperatures will lead to side
reactions, which were demonstrated in our previous study.44
Specically, C6 alditols underwent a further hydrogenolysis
Fig. 2 The effect of reaction time on the yield of C6 alditols over the
BP–Ru/C catalytic system. Reaction conditions: 0.5 g acid, 0.8 g MCC,
0.1 g Ru/C, 40 mL H2O, 5 MPa of H2 (RT), 463 K.
reaction to produce other polyols, such as ethylene glycol and
propylene glycol, or a dehydration reaction to yield sorbitan or
isosorbide. In fact, sorbitol dehydration reactions can be occur
at high temperatures of 523–573 K without the addition of any
acid catalysts,45 indicating the hydrolytic hydrogenation of
cellulose to produce C6 alditols should be performed for a short
reaction time at relatively low temperatures. Meanwhile,
through characterization of the liquid products, negligible
cellobiose or glucose was observed in the reaction process,
which agrees with the former study that Ru/C shows high
hydrogenation activity that outruns the hydrolysis rate of
cellulose over an acid catalyst.46 This also suggests a partial
inhibition effect on the hydrogenolysis of C6 alditols is ascribed
to the Ru/C when enclosed by glucose or other intermediates.
Furthermore, the effect of reaction temperature was carried
out under the optimized conditions, and the results shown in
Fig. 3 The effect of reaction temperature on the yield of C6 alditols
over the BP–Ru/C catalytic system. Reaction conditions: 0.5 g BP, 0.8
Fig. 1 Conversion of cellulose to alditols over different acids and Ru/ g MCC, 0.1 g Ru/C, 40 mL H2O, 5 MPa of H2 (RT), 1 h. (Green: the yield
C. Reaction conditions: 0.5 g acid, 0.8 g MCC, 0.1 g Ru/C, 40 mL H2O, of C6 alditols; blue: the yield of gas phase products; magenta: the yield
5 MPa of H2 (RT), 463 K, 1 h.
of small polyols and dehydration products).
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Fig. 3. It is obvious that the cellulose conversion increased as the temperature, the acidity continued to increase. It is clear
the reaction temperature was increased. When the temperature that the hydrolysis process of cellulose was accompanied with
increased to 483 K, the substrate was completely converted, but the hydrolysis of BP. In addition, BP was found fully transferred
the yield of by-products also increased. Specically, the yield of into the aqueous phase in the cellulose conversion experiments,
gas phase products, which are mainly methane, ethane, and which wasn't observed at atmosphere pressure. Hence, this
pentane sharply increased from 8.2% to 40.3%, and in the phenomenon would signicantly increase the acidity of the
liquid phase, the yield of dehydration products (sorbitan and reaction system to improve the cellulose hydrolysis rate, which
isosorbide) and small polyols (ethylene glycol and propylene is the rate-determinate step for the hydrolytic hydrogenation of
glycol) were increased to 7.8%. It's clear that the selectivity of C6 cellulose.46,52 We should note that the BP hydrolysis process is
alditols dramatically decreased from 83.3% to 43.1% since the gradual and that H+ was slowly released from BP. Since cellulose
higher reaction temperature led to an increase in by-products. is probably adsorbed by BP on its surface, the gradually released
This observation could be explained by the fact that the C–O H+ can be directly used for the hydrolysis of cellulose, which
and C–C cracking reactions of polyols can be easily performed essentially avoids the diffusion process. Moreover, with the
in an acidic media at high temperatures. It was found that the increasing acidity of the mixed solution, cellulose can be
C6 alditols produced can be dehydrated to sorbitan (mono- partially dissolved. This dissolution of cellulose in water can
molecular dehydration) and isosorbide (bimolecular dehydra- further enhance the rate of cellulose hydrolysis via trans-
tion) in the absence of a catalyst at high temperatures.47 The formation to smaller oligomer fractions. Through breaking the
yield of dehydration products could be promoted in an acidic glycoside bonds of soluble oligosaccharides, which possess a
media.48 The small polyols might be formed by a retro-aldol low threshold of access to glycoside bonds, a high yield of C6
condensation of carbohydrates, which has been studied by alditols can therefore be achieved in a short reaction time. In
Zhang's group.49 The increased yield of light alkanes was a fact, previous research has conrmed this acid effect on the
consequence of the accelerated hydrocracking of C6 alditols at hydrolytic hydrogenation of cellulose to produce polyols.53
high temperatures.50
We have noticed that the BP catalyst was synthesized using
To detect the leaching phenomenon of BP aer the reaction, H3BO4 (HB) and H3PO4 (HP), and this hydrolysis process may
3ꢀ
ion chromatography was used and the result showed that PO4
cast a negative impact on the hydrolytic hydrogenation of
and boric acid existed in the liquid phase (See Fig. S1†). Further cellulose. To rule out this hypothesis, comparative experiments
investigation on the pH of the aqueous phase suggested the were performed by substituting BP with boric acid and/or
acidity of the solution was increased aer the reaction. This phosphoric acid under reaction conditions with an equivalent
phenomenon disagrees with the former reports, which illus- molar ratio of boron and phosphorus. The cellulose conversion
trated the BP catalyst prepared using 1273 K calcination process decreased to 68.5% by replacing the BP catalyst with HB, and
could not be dissolved in water.51
the yield of C6 alditols was sharply lowered from 74.9% to 42.8%
Since the pH of the mixed solution aer reaction was (Table 1, entry 1). Compared with the inorganic acid HB, the
decreased, we measured the pH of water upon the addition of cellulose conversion was increased from 68.5% to 91.9% over
BP at different temperatures and atmosphere pressure as shown HP and Ru/C (Table 1, entry 2). However, the selectivity for C6
in Fig. 4. By elevating the temperature from 293 K to 463 K, the alditols decreased, which is a consequence of the fact that the
pH of the mixture decreased to pH ¼ 1.95. By further increasing more dominant acid functionality derived from HP would cause
glucose degradation or hexitol dehydration.12 With a mixture of
HB and HP, both the selectivity of C6 alditols and the yield of C6
alditols (Table 1, entry 3) were improved when compared with
the use of HB or HP as the acid functionality. This observation
may be a result that the borate species and polyols form borate–
polyol complexes, which suppress the dehydration reaction of
C6 alditols to form sorbitan or isosorbide.54 In addition, the
observed results show that the addition of HB can inhibit the
C–C cleavage reaction of C6 alditols to yield small polyols. These
results can be veried by entries 3, 5 and 6 in Table 1. When
different ratios of HB/HP were used for the tests, the higher the
ratio of HB/HP, the lower the yield of small polyols and gas
phase products were observed along with a decrease in cellulose
conversion. With a decreasing ratio of HB/HP, the selectivity for
C6 alditols decreased, while the yield of small polyols was
increased. It is indicated that the presence of a dominant
amount of HP could cause more side reactions such as dehy-
dration and C–C bond hydrogenolysis of C6 alditols, though it is
effective for cellulose hydrolysis. The addition of HB at a suit-
able content can effectively optimize the balance between
Fig. 4 The pH evolution of the liquid phase upon the addition of 0.5 g
BP in 40 mL distilled water at different temperatures and atmosphere
pressure.
hydrolysis and side reactions to improve the yield of C6 alditols.
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Table 1 Conversion of cellulose over different acids and Ru/Ca
Yield/%
C6 alditols
Entry
Acid
Cellulose conversion/%
selectivity/%
C6 alditols
Glucose
Cellobiose
Isosorbide
Small polyolsb
Othersc
1
HB
HP
HB + HP
BP
HB + HP
HB + HP
68.5
91.9
83.9
91.3
77.8
87.9
62.5
42.9
75.6
82.1
59.1
58.2
42.8
39.4
61.5
74.9
46.0
51.1
2.9
1.2
1.1
ND
1.9
1.6
1.5
0.5
0.6
ND
1.1
0.8
ND
7.5
1.5
ND
3.9
5.1
ND
6.9
1.1
ND
3.3
4.8
12.9
24.5
15.1
13.3
19.4
22.7
2
3d
4
5e
6f
a
Reaction conditions: 463 K, 5 MPa of H2 (RT), 1 h, 0.1 g Ru/C, 0.8 g cellulose, H2O 40 mL, an equivalent molar ratio of boron and phosphorus was
added according to the different acids used. ND: not detected. b Small polyols: ethylene glycol, propylene glycol. c Others are the gas phase products,
which mainly include methane, ethane, and propane. d The same amount of HB and HP (molar ratio ¼ 1 : 1) was used to synthesize 0.5 g BP. e The
molar ratio of HB–HP is 3 : 1 (maintained the same amount of H+ mol as entry 3). f The molar ratio of HB–HP is 1 : 3 (which maintained the same
amount of H+ mol as entries 3 and 5).
This might be caused by the steric hindrance of the borate– cellulose conversion process. In the current catalytic system, the
polyol complexes formed, which blocks the contact of Ru with formation of borate–polyol complexes would have an enhanced
the C–C bonds. Besides, HB is the weaker acid and is less steric hindrance to prevent any further hydrogenolysis and
effective towards the dehydration of C6 alditols. However, even dehydration of the C6 alditols formed.
with an increased selectivity of C6 alditols by using a mixture of
It should be noted that the separation of C6 alditols from the
HB and HP, the yield of C6 alditols was still inferior to the 74.9% mixture is a necessity. Recent literature suggests the recovery of
achieved using the BP acid catalyst. These results indicated that H3PO4 from the solution can be performed using anion
the BP catalyst is better than the relative inorganic acids.
As noted above, the superiority of the BP catalyst is ascribed
to the balance between the acidic functionality released from BP
and the metal functionality. Most importantly, the gradual
release of H+ provides a favorable environment to catalyze the
hydrolysis of cellulose, and when simultaneously combined
with the Ru-catalyzed hydrogenation, the in situ catalysis of
released H+ and hydrogen can give rise to the production of C6
alditols. In this process, the hexitol dehydration and glucose
degradation occurring with a dominant acidic functionality
could be signicantly avoided and reduced the yield loss of C6
alditols. Moreover, when the BP completely released the H+ in
the aqueous phase, the formed HB from BP may play an
important role to suppress the dehydration and C–C bond
cleavage reactions of C6 alditols by forming borate–polyol
complexes, which is another key factor for the improvement in
yield and stability of the C6 alditols.
FT-IR characterization was performed to verify the presence
of the borate–polyol complexes. As shown in Fig. 5, the addition
of HB into a sorbitol aqueous solution resulted in the appear-
ance of a CH2 bend and the enhanced intensity of the CH2 bend
at 1460 cmꢀ1, which was caused by the electron withdrawing
effect of the borate complex. By increasing the concentration of
HB, the peak at 1460 cmꢀ1 became stronger. Moreover, the
peaks belonging to the C–O groups (1200 to 900 cmꢀ1) were
shied towards lower frequencies upon increasing the
concentration of boric acid in the solution of sorbitol. It's clear
that the boric acid can coordinate with sorbitol. In contrast, HP
doesn't give this shi effect in the sorbitol solution (See
Fig. S5†). In fact, borate minerals were found to form complexes
with cis-diols of organic molecules, such as ribose, arabinose,
and xylose.55 Hence, the complexation with borate minerals
actually stabilizes the products of C6 alditols formed during the
Fig. 5 FT-IR spectra of: (a) 1 wt% H3BO3 aqueous solution, (b) 5 wt%
H3BO3 aqueous solution, (c) 25 wt% sorbitol aqueous solution, (d) 1
wt% H3BO3 – 25 wt% sorbitol aqueous solution, and (e) 5 wt% H3BO3 –
25 wt% sorbitol aqueous solution.
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exchange membranes.56,57 Since the solubility of boric acid in materials exhibit much smaller amounts of gas phase products
water at room temperature is very low, it can be separated from (mainly methane, ethane, and propane) than that observed
the mixture by simply lowering the temperature. Aer ltration, without adding them. It is a fact that the Ru/C catalyst used as
we can obtain the C6 alditols solution.
the hydrogenolysis catalyst can also catalyse the conversion of
the C6 alditols to smaller alkanes.58 The C6 alditols produced
were adsorbed by the additives, therefore, the contact of Ru/C
with the C6 alditols was limited, which lowered the possibility
of the hydrogenolysis of the C6 alditols formed. Hence, the yield
of C6 alditols was improved.
The effect of silica–alumina materials
In the current study, another interesting observation was that
the yield of C6 alditols from the hydrolytic hydrogenation of
cellulose can be further improved using silica–alumina mate-
rials or solid acids as additives to be added into the catalytic
system. As shown in Table 2, using different additives, the
glucose, cellobiose, isosorbide and small polyols were detected
in negligible amounts or not at all, and the yield of C6 alditols
was different with the change of C1–C4 alkane yield. MCM-41
composed of pure silica gave the best results among the
tested silica–alumina materials with a 93.5% yield of C6 alditols.
By lowering the silica content of the additive, the C6 alditol yield
decreased. HZSM-5 and Hb with a Si/Al ratio of 38 gave a C6
alditols yield of 87.6% and 86.1%, respectively. The additives
with lower Si/Al ratios such as HMOR (Si/Al ¼ 25) and USY (Si/Al
¼ 9) showed a 82.7% and 80.1% yield of C6 alditols, respectively.
With the pure alumina additive g-Al2O3, the yield of C6 alditols
was almost the same as the results of the previous experiments
without the presence of an additive. It is obvious that the silica–
alumina materials can adsorb the oligomer intermediates
during the formation of the C6 alditols due to their affinity to
the hydrophilic surface of the silica–alumina material. This had
been pioneered by Sels' group who studied the sugar alcohols
produced from cellulose over Ru/HUSY.12 The adsorption test
was run by adding additives into the cellobiose, sorbitol, and
glucose solution, which veried the additives possess an affinity
to the cellulose-derived glucose and cellobiose, which can be
further hydrogenated to C6 alditols. Among the silica–alumina
materials tested, MCM-41 had the highest adsorption ability
towards glucose and the cellobiose molecules. On the contrary,
the surface of g-Al2O3 showed a reluctance towards the
adsorption of glucose and cellobiose. It might be a result from
the fact silica can adsorb the hydroxyl groups of the interme-
diates easier by itself than it does in the presence of alumina.
On the other hand, the presence of these silica–alumina
Fig. 6 FT-IR spectra of sorbitol with/without an additive (A and B share
the same color indicators for the different samples).
Table 2 The effect of additives combined with Ru/C and BP on the cellulose conversion and adsorption testsa
Adsorption/mg gꢀ1
Product yield/%
Entry
Additive
Cellulose conversion/%
C6 alditols
Others in aqueous phaseb
In gas phasec
Glucose
Cellobiose
Sorbitol
1
2
3
4
5
6
7
—
91.3
98.7
99.4
>99.9
>99.9
96.5
74.9
93.5
87.6
86.1
82.7
80.1
75.1
ND
0.5
1.3
3.1
3.9
2.5
4.7
13.3
3.6
8.1
8.3
7.9
8.6
9.3
—
—
—
MCM-41
HZSM-5
Hb
HMOR
USY
1.75
1.68
1.66
1.67
1.58
1.13
2.43
2.18
1.95
2.04
1.77
1.42
1.88
1.73
1.55
1.38
1.43
1.01
g-Al2O3
90.7
a
Reaction conditions: 0.8 g cellulose, 0.1 g Ru/C, 0.5 g BP, 0.2 g additive, 40 mL H2O, 5 MPa of H2 (RT), 463 K, 1 h. The adsorption test was carried
out at room temperature and atmosphere pressure overnight. b Others in the aqueous phase include isosorbide and small polyols. c In gas phase:
methane, ethane, and propane.
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FT-IR spectroscopy conrmed the adsorption of sorbitol on soluble oligomers and glucose. Also, the interaction of BP with
the surface of the additives (Fig. 6), and the peaks at 1224 cmꢀ1 cellulose can directly use the released H+, which eliminates the
and 1091 cmꢀ1 veried the Si–O–Si bonds of the silica–alumina considerable diffusion path. Meanwhile, the enhanced acidity
materials while the g-Al2O3 doesn't possess these frequencies from the H+ released helps to improve the solubility of cellulose,
(Fig. S6†).59 Aer immersion into the sorbitol solution over- which also accelerates the hydrolysis of cellulose. With the high
night, these additives were dried in vacuo and analyzed by FT-IR hydrogenation reactivity of Ru/C, the sugar or cellulose oligo-
spectroscopy. The sp3 C–H stretching frequency of sorbitol mers are rapidly converted into C6 alditols. Thanks to the
appears at 2932 cmꢀ1, which wasn't found in these additives. It borate–polyol complexes formed, the side reactions, such as the
was found that the sp3 C–H stretching frequency was shied. hydrogenolysis reaction and the dehydration reaction of the C6
Moreover, the peak at 1419 cmꢀ1 assigned to the CH2 bending alditols formed are suppressed. Hence, a high yield of C6 aldi-
region of sorbitol was shied to a lower frequency, which tols is achieved. Also, the addition of silica–alumina materials
suggests these additives can effectively adsorb the sorbitol can adsorb the cellobiose and glucose, which prevents the
molecules. These shis conrmed that the addition of these intermediates from degradation. Plus, the product sorbitol can
additives actually played an important role in damaging the be adsorbed by these additives, which stabilized the desired
hydrogen bonds between the free sorbitol molecules in solu- products. Therefore, the addition of silica–alumina materials
tion.60 Since the carbon atoms of sorbitol are surrounded by can further promote the yield of C6 alditols.
oxygen and hydrogen atoms, the hydroxyl groups are in direct
interaction with the additives. Hence, the intermolecular and
intramolecular hydrogen bonds of free sorbitol molecules in
Conclusions
solution were destroyed, which resulted in the CH2 bending
In summary, our novel reaction system provides a highly
productive approach to produce C6 alditols from the hydrolytic
hydrogenation of cellulose. Within 1 h of reaction time, the
untreated cellulose can be directly transformed to C6 alditols
frequency of sorbitol to be shied to the lower region and the
sp3 C–H stretching frequency was shied to a higher region. As
a result, the adsorption effect of the additives stabilizes the C6
alditols and prevents the C6 alditols from degradation. Hence,
with a yield of 74.9%. Compared with tests using boric acid and/
the C6 alditols yielded from the cellulose conversion can be
or phosphoric acid, the results suggested the better perfor-
enhanced by the adsorption of the C6 alditols intermediates and
mance should be ascribed to the slow release of H+ and the
the C6 alditols themselves onto the surface of additives.
negligible dehydration reaction and hydrogenolysis reaction of
the borate–polyol complexes formed. By introducing silica–
alumina materials as additives, especially those with high silica
content, into the catalytic system, a higher yield of C6 alditols
Reaction pathway
can be achieved with the highest yield (93.5%) of C6 alditols
produced using pure silica MCM-41.
Our catalytic system of Ru/C–BP shows exceptional performance
to yield C6 alditols from cellulose in a short time. We propose
the following pathway to rationalize the system (Scheme 1). The
use of hot water and the slow release of H+ from BP are
responsible for the breakdown of the cellulose chains to create
Acknowledgements
This work was carried out under the nancial support of the
National Basic Research Program of China (no. 2012CB215304),
the key program of Chinese Academy of Sciences (no. KGZD-
EW-304-2) and the Natural Science Foundation of Guangdong
Province (no. S2013040016417).
Notes and references
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52408 | RSC Adv., 2014, 4, 52402–52409
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