Biphasic catalytic conversion of fructose by continuous hydrogenation of HMF over a hydrophobic ruthenium catalyst

Article abstract of DOI:10.1002/cssc.201301270

The production of chemicals directly from sugars is an important step in biomass conversion. Herein, tetrahydro-2,5-furandimethanol (THFDM), obtained from fructose, is formed by using a combination of acid and hydrophobic Ru/SiO₂ in a water/cyclohexane biphasic system. Two key factors enable the high selectivity towards THFDM: modifying the hydrogenation catalyst so that it has hydrophobic properties, and the continuous hydrogenation of generated 5-(hydroxymethyl)furfural in the cyclohexane phase. Moreover, the selectivity towards THFDM is found to depend strongly on the acid catalyst used. Divide and conquer: A method for direct catalytic conversion of fructose to tetrahydro-2,5-furandimethanol (THFDM) via 5-(hydroxymethyl)furfural (HMF) is reported. High selectivity towards THFDM is achieved by using a catalyst combination of acid and a hydrophobic ruthenium catalyst (Ru/SiO₂-TM) in a water/cyclohexane biphasic system by continuous hydrogenation of generated HMF. The use of the hydrophobic Ru/SiO₂-TM is the key, as it prevents hydrogenation of fructose to mannitol and sorbitol in the water phase.

Full text of DOI:10.1002/cssc.201301270

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DOI: 10.1002/cssc.201301270
Biphasic Catalytic Conversion of Fructose by Continuous
Hydrogenation of HMF over a Hydrophobic Ruthenium
Catalyst
Yanliang Yang,[a, b] Zhongtian Du,[a] Jiping Ma,[a] Fang Lu,[a] Junjie Zhang,[a, b] and Jie Xu*[a]
The production of chemicals directly from sugars is an impor-
tant step in biomass conversion. Herein, tetrahydro-2,5-furandi-
methanol (THFDM), obtained from fructose, is formed by using
a combination of acid and hydrophobic Ru/SiO2 in a water/cy-
clohexane biphasic system. Two key factors enable the high se-
lectivity towards THFDM: modifying the hydrogenation catalyst
so that it has hydrophobic properties, and the continuous hy-
drogenation of generated 5-(hydroxymethyl)furfural in the cy-
clohexane phase. Moreover, the selectivity towards THFDM is
found to depend strongly on the acid catalyst used.
We postulate that the key for this one-step process is the
separation of sugar and hydrogenation catalyst, to prevent hy-
drogenation of the sugar. Herein, we use a biphasic system to
achieve this goal (Scheme 1). The dehydration of fructose to
Diminishing fossil resources are driving chemists to find ways
to make full use of renewable biomass for the production of
fuels and chemicals.[1,2] 5-(Hydroxymethyl)furfural (HMF), ob-
tained from the dehydration of sugars, is viewed as one of the
most important platform compounds.[3] Converting HMF into
valuable chemicals by means of oxidation, hydrogenation, and
etherification is a subject of growing interest.[4–6] Among these
chemicals, especially tetrahydro-2,5-furandimethanol (THFDM),
derived from the hydrogenation of HMF, is a valuable molecule
that can be used as solvent or monomer.[7] Recent research has
revealed that THFDM can be converted to 1,6-hexanediol, an
important monomer in the plastics industry, in very high
yield.[2c,8]
Scheme 1. Conversion of fructose into THFDM in a water/cyclohexane bi-
phasic system.
HMF was catalyzed by Amberlyst-15 in the aqueous phase. The
generated HMF was then hydrogenated to THFDM over hydro-
phobic Ru/SiO2-TM (Ru/SiO2 modified by trimethylchlorosilane)
in the organic phase. This strategy avoids the hydrogenation
of fructose to mannitol and sorbitol under H2. Simultaneously,
the rehydration of HMF to levulinic acid (LA), occurring in
water in the presence of acid, can also be avoided. Moreover,
the low concentration of HMF can be maintained by continu-
ous hydrogenation of the generated HMF, thus decreasing the
degradation reaction. Thereby the selective direct transforma-
tion of fructose into THFDM is realized.
Previous studies have reported the hydrogenation of HMF
to THFDM over Ni, Cu, Pt, Pd, or Ru, Ni–Pd bimetallic cata-
lysts.[9] Considering that HMF is not stable and not easy to be
separated, purified, or stored, it is desirable to directly obtain
THFDM from sugars via one-pot, one-step dehydration and hy-
drogenation processes. However, this is challenging because
the sugars could be hydrogenated to sugar alcohols before
they dehydrate to HMF. As far as we are aware, so far no
report has described the direct conversion of sugars to
THFDM.
The key to this strategy is the design of a hydrophobic hy-
drogenation catalyst. Therefore, we began our study with the
preparation of a hydrophobic hydrogenation catalyst of Ru/
SiO2-TM according to our previous work.[10] Direct introduction
of Ru nanoparticles onto the surface of hydrophobic SiO2 is
not easy When the H2 reduction method is employed, elevated
.
temperatures that can destroy the hydrophobic components
of organic groups are inevitable. As for the chemical reduction
method, the hydrophobic surface of SiO2 prevents close con-
tact with the ruthenium precursor of RuCl3. As a result the
ruthenium cannot be successfully loaded onto the surface of
SiO2. Based on the above considerations, we first prepared hy-
drophilic Ru/SiO2 using an impregnation method, and then
modified it with trimethylchlorosilane (TMCS) using a post-
grafting method.
[a] Y. Yang, Dr. Z. Du, Dr. J. Ma, Dr. F. Lu, J. Zhang, Prof. Dr. J. Xu
Dalian National Laboratory for Clean Energy
State Key Laboratory of Catalysis
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
457 Zhongshan Road, Dalian 116023 (PR China)
Fax: (+86)411-84379245
E-mail: xujie@dicp.ac.cn
[b] Y. Yang, J. Zhang
The crystalline Ru/SiO2 phase was examined by X-ray
powder diffraction (XRD). The XRD pattern showed six peaks at
2q=388, 428, 448, 588, 698, and 788, which can be assigned to
(100), (002), (101), (102), (110), and (103) diffractions of Ru, re-
University of Chinese Academy of Sciences
Beijing 100039 (PR China)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201301270.
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spectively (Supporting Information, Figure S3b). This proves
that the ruthenium precursor was completely reduced to
ruthenium metal. The Ru/SiO2 texture was characterized by
transmission electron microscopy (TEM), revealing that the size
of Ru nanoparticles was about 2 nm (Figures S1a and S2a).
There were no obvious differences between TEM images and
XRD patterns of Ru/SiO2 and Ru/SiO2-TM, revealing that the
crystalline phase and texture of Ru/SiO2 were retained after the
post-grafting procedure (Figures S1, S2, and S3). Figure 1
shows the Fourier-transform infrared (FT-IR) spectra of as-syn-
reflects the hydrophilic or hydrophobic property of the tested
materials. The unmodified Ru/SiO2 gave a water contact angle
of 188, while the as synthesized Ru/SiO2-TM catalyst had
a water contact angle of 1378. This indicates that Ru/SiO2-TM is
very hydrophobic, a consequence of the introduction of
methyl on the Ru/SiO2-TM catalyst as revealed by FT-IR. Be-
cause of its hydrophobic properties, the Ru/SiO2-TM remained
in the oil phase and did not enter into water phase either
before or after the reaction (Scheme 2; Figure 2b and Support-
ing Information, Figure S4), while the Ru/SiO2 maintained in
the water phase (Figure 2a).
Scheme 2. Products of fructose conversion in water/cyclohexane biphasic
system.
The hydrophobicity of a hydrogenation catalyst is very im-
portant for the direct conversion of fructose to THFDM. Table 1
shows the different product distributions resulting from the
different catalytic behaviors of hydrophilic Ru/SiO2 and hydro-
phobic Ru/SiO2-TM using fructose as substrate. Fructose was
hydrogenated to sorbitol and mannitol with 80% total selectiv-
ity over Ru/SiO2 (entry 1). No THFDM was detected under
Figure 1. FT-IR spectra of SiO2, Ru/SiO2, and Ru/SiO2-TM.
thetized catalysts. All of the materials show absorption peaks
at n˜1103, 811, and 472 cmÀ1, which can be attributed to SiÀOÀ these conditions. When Ru/SiO2-TM was used as catalyst, the
Si absorption band,[11b] and at 1636 and 3445 cmÀ1, which can
be attributed to the OÀH absorption band.[11a] However, differ-
ent from SiO2 and Ru/SiO2, the Ru/SiO2-TM shows an absorp-
tion peak at n˜2963 cmÀ1 which can be attributed to the CÀH
stretching vibration of methyl groups.[11c] This result demon-
strates the successful introduction of methyl on the Ru/SiO2-
TM catalyst.
selectivity of THFDM was 61% at 56% fructose conversion
while the selectivity of sugar alcohols was less than 1%
(entry 2). This difference is caused by the hydrophilic–hydro-
phobic properties of the catalyst. The solubilities of fructose in
water and cyclohexane are 3750 gLÀ1 and less than 0.001 gLÀ1,
respectively. Therefore, no fructose could be detected in the
cyclohexane phase of the H2O/cyclohexane biphasic system.
When the hydrophilic catalyst Ru/SiO2 was used, the hydroge-
nation of fructose proceeded smoothly in water. When the hy-
drophobic catalyst Ru/SiO2-TM was used, the catalyst was in
the cyclohexane phase where no fructose is present, excluding
the possibility of fructose hydrogenation. As a result, no sugar
alcohols were detected. This finding is also supported by com-
paring the conversion–time profiles of fructose dehydration to
HMF and fructose to THFDM (Figure 3). The conversion–time
The introduction of methyl on the Ru/SiO2-TM catalyst was
further evidenced by contact-angle measurements of H2O
droplets, as shown in Figure 2. The surface water contact angle
Table 1. The conversion of fructose over Ru/SiO2 and Ru/SiO2-TM.[a]
Entry
Catalyst
Conversion [%]
Selectivity [%]
1
2–8
9
Others
1
2
Ru/SiO2
Ru/SiO2-TM
99
56
–
61
–
25
80
<1
20
13
[a] Reaction conditions: Ru/SiO2 or Ru/SiO2-TM (30 mg), Amberlyst-15
(40 mg), fructose aqueous solution (3 mL, fructose 1 mmol), cyclohexane
(6 mL), H2 (4 MPa), 1308C, 4 h.
Figure 2. Contact angles of a water droplet on a) Ru/SiO2, b) Ru/SiO2-TM,
and the distribution of each component in reaction liquid.
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SiO2 and Ru/SiO2-TM, respectively. The selectivity towards
THFDM almost tripled at an HMF concentration of 2.1 gLÀ1
(29% for Ru/SiO2 and 33% for Ru/SiO2-TM).
Figure 5 shows the product selectivity versus fructose con-
version. When the reaction was carried out in the presence of
Amberlyst-15 (Figure 5b), dehydration of fructose to HMF
mainly occurred. A high HMF selectivity of 70% was obtained
at a lower fructose conversion of 17%. With the fructose con-
Figure 3. Conversion-time profiles of fructose dehydration to HMF and fruc-
!
tose conversion to THFDM. Reaction conditions: : Amberlyst-15 (40 mg),
fructose aqueous solution (3 mL, fructose 1 mmol), cyclohexane (6 mL), H2
&
(4 MPa),1308C. : Ru/SiO2-TM (30 mg).
profiles were almost the same, regardless of whether Ru/SiO2-
TM was added or not. This indicates that the Ru/SiO2-TM is not
involved in the conversion of fructose in water, because other-
wise the conversion should be much higher than the conver-
sion obtained without Ru/SiO2-TM catalyst.
The significance of continuous hydrogenation of generated
HMF was investigated in a systematic manner. HMF is known
to be an unstable molecule that easily degrades to acids and
humins in the presence of water and acid catalyst.[3] Thus,
having lower concentrations of HMF may be favorable for ach-
ieving high THFDM selectivities because it could slow down
the degradation reaction in water. Figure 4 shows the relation-
ship between the selectivity of THFDM and the HMF concen-
tration. The THFDM selectivity increases when decreasing the
concentration of HMF either on hydrophilic Ru/SiO2 or on hy-
drophobic Ru/SiO2-TM. Selectivities of 9% and 11% THFDM are
obtained at HMF concentrations of 42 gLÀ1 catalyzed by Ru/
!
&
^
Figure 5. Products selectivity ( : THFDM, : HMF, : levulinic acid) versus
fructose conversion over a) Amberlyst-15 and Ru/SiO2-TM, b) Amberlyst-15.
Reaction conditions: Amberlyst-15 (40 mg), Ru/SiO2-TM (30 mg), fructose
aqueous solution (3 mL, fructose 1 mmol), cyclohexane (6 mL), H2 (4 MPa),
1308C.
version increased, the selectivity of HMF decreased rapidly
from 70% to 21%. At the same time, the selectivity towards
levulinic acid increased from 18% to 45% (Figure 5b). This evi-
dences that HMF is unstable in water and easily degrades to
levulinic acid in the presence of the solid acid Amberlyst-15.
When Amberlyst-15 and Ru/SiO2-TM were used at the same
time, a reaction cascade was formed. The dehydration of fruc-
tose to HMF was first carried out in aqueous phase. Then the
generated HMF was continuously hydrogenated to THFDM in
the organic phase, and THFDM was the main product (Fig-
ure 5a). The selectivity towards THFDM decreased much
slower from 65% to 55% with conversion increasing from 18%
to 84%. Furthermore, the HMF selectivity was less than 1%
during the whole reaction process, indicating that the concen-
tration of HMF was kept at a very low level. This is a favorable
property to achieve high selectivity towards THFDM, as we
mentioned above.
Because the hydrophobicity of Ru/SiO2-TM plays a key role
in the cascade reaction, the stability of its hydrophobicity was
further examined. Recycling experiments are summarized in
Figure 6. The water contact angle of about 1378 of Ru/SiO2-TM
was maintained in all five cycles, showing that the hydropho-
bicity of Ru/SiO2-TM was maintained. The conversion de-
Figure 4. THFDM selectivity versus HMF concentration (gLÀ1). Reaction con-
!
&
ditions: : Ru/SiO2 or : Ru/SiO2-TM (30 mg), Amberlyst-15 (40 mg), HMF
aqueous solution (3 mL, HMF 1 mmol), cyclohexane (6 mL), H2 (4 MPa),
1308C, 4 h.
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Figure 6. Recycle experiments. Reaction conditions: Amberlyst-15 (40 mg),
Ru/SiO2-TM (30 mg), fructose aqueous solution (3 mL, fructose 1 mmol), cy-
clohexane (6 mL), H2 (4 MPa), 1308C, 4 h.
Figure 7. The conversion of fructose over different acids. Reaction condi-
tions: Acid (40 mg), fructose aqueous solution (3 mL, fructose 1 mmol), cy-
clohexane (6 mL), H2 (4 MPa), 1308C, White bar: no Ru/SiO2-TM, Striped bar:
Ru/SiO2-TM (30 mg). HY: HY-zeolite, Si/Al=5, Nb2O5-P: H3PO4-treated niobic
acid. H2SO4: 1 mo%. Selectivity was measured at a conversion of ca. 15%.
creased after the first use, but this decrease was not obvious
in the following cycles. Literature reports have found that Am-
berlyst-15 is not so stable at high temperature.[12] The reaction
was performed at a temperature close to the limit for Amber-
lyst-15. As a result, the conversion showed a decrease after the
first run due to temperature-sensitive acid sites being dimin-
ished. The remaining acid sites were more stable, thus the de-
crease in conversion was not obvious in the following cycles.
The selectivity of THFDM followed the behavior of fructose
conversion. There was no obvious loss in THFDM selectivity
after the first cycle. The fructose conversion of 41% with
THFDM selectivity of 49% was observed even in the fifth cycle.
The effects of acids were further studied by using different
kinds of acid catalysts. The selectivity towards HMF was mea-
sured over different acids at around 15% fructose conversion
by controlling the reaction time. Then the selectivity of THFDM
was tested by adding Ru/SiO2-TM adopting the same reaction
time (Figure 7 and Table S2). When HY-zeolite was used, the se-
lectivity towards HMF was only 20%, correspondingly, the se-
lectivity of THFDM was just 15%. When the acid was changed
to SO42À/ZrO2 and then Nb2O5-P, the selectivity of HMF in-
creased and that of the THFDM increased accordingly. When
liquid H2SO4 was used, a high selectivity of 83% HMF was ach-
ieved. Accordingly, the selectivity of THFDM was 75%. It is
clear that the selectivity of THFDM is closely related to the se-
lectivity of HMF on the same acids.
thus an example of the selective production of valuable chemi-
cals directly from fructose.
Experimental Section
Catalytic reactions were performed in a 60 mL stainless steel auto-
clave equipped with a magnetic stirrer, a pressure gauge, and au-
tomatic temperature control apparatus. The reactor was connected
to a hydrogen cylinder for reaction pressure. In a typical experi-
ment, an aqueous solution of fructose (3 mL, 1.00 mmol), cyclohex-
ane (6 mL), Ru/SiO2-TM (30.0 mg), and Amberlyst-15 (40.0 mg) were
loaded into the reactor. After sealing and purging with H2 for 4
times to exclude air, the autoclave was heated to the desired tem-
perature (1308C), and then H2 (4 MPa) was charged into the reac-
tor. After reaction, the autoclave was cooled. The organic and
aqueous phases were separated and analyzed by GC using the in-
ternal standard method. The aqueous phase was also analyzed by
HPLC for the quantification of fructose, sorbitol, and mannitol
using the external standard method. The catalyst was washed with
alcohol 3 times and then with cyclohexane 3 times for the cycle
experiment. More experimental details can be found in the Sup-
porting Information.
Acknowledgements
In summary, the direct conversion of fructose to THFDM by
a combination of acid and hydrophobic Ru/SiO2-TM in a water/
cyclohexane biphasic system is reported. The selectivity of
THFDM is closely related to the selectivity of HMF on the acids.
The hydrophobicity of the hydrogenation catalyst, which en-
sures that it remains in the organic phase, is indispensable and
is stable under the examined reaction conditions. This can pro-
hibit direct hydrogenation of fructose to mannitol and sorbitol
in water. The other key factor is that the generated HMF is
continuously hydrogenated to THFDM. This allows a lower
concentration of HMF in the water phase and thus prohibits its
rehydration to LA in the presence of acid catalyst. This work is
This work was supported by the National Natural Science Foun-
dation of China (21233008, 21103174, and 21303183).
Keywords: catalysis · fructose · hydrogenation · hydrophobic ·
tetrahydro-2,5-furandimethanol
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Published online on March 18, 2014
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