Selective hydrogenolysis of biomass-derived xylitol to ethylene glycol and propylene glycol on supported Ru catalysts

Article abstract of DOI:10.1039/c0gc00571a

The selective hydrogenolysis of biomass-derived xylitol to ethylene glycol and propylene glycol was carried out on different catalysts in the presence of Ca(OH)₂. The catalysts included Ru supported on activated carbon (C) and, for comparison, on metal oxides, Al₂O₃, TiO ₂, ZrO₂ and Mg₂AlO_x as well as C-supported other noble metals, Rh, Pd and Pt, with similar particle sizes (1.6-2.0 nm). The kinetic effects of H₂ pressures (0-10 MPa), temperatures (433-513 K) and solid bases including Ca(OH)₂, Mg(OH)₂ and CaCO₃ were examined on Ru/C. Ru/C exhibited superior activities and glycol selectivities than Ru on TiO₂, ZrO₂, Al₂O₃ and Mg₂AlO_x, and Pt was found to be the most active metal. Such effects of the metals and supports are attributed apparently to their different dehydrogenation/ hydrogenation activities and surface acid-basicities, which consequently influenced the xylitol reaction pathways. The large dependencies of the activities and selectivities on the H₂ pressures, reaction temperatures, and pH values showed their effects on the relative rates for the hydrogenation and base-catalyzed reactions involved in xylitol hydrogenolysis, reflecting the bifunctional nature of the xylitol reaction pathways. These results led to the proposition that xylitol hydrogenolysis to ethylene glycol and propylene glycol apparently involves kinetically relevant dehydrogenation of xylitol to xylose on the metal surfaces, and subsequent base-catalyzed retro-aldol condensation of xylose to form glycolaldehyde and glyceraldehyde, followed by direct glycolaldehyde hydrogenation to ethylene glycol and by sequential glyceraldehyde dehydration and hydrogenation to propylene glycol. Clearly, the relative rates between the hydrogenation of the aldehyde intermediates and their competitive reactions with the bases dictate the selectivities to the two glycols. This study provides directions towards efficient synthesis of the two glycols from not only xylitol, but also other lignocellulose-derived polyols, which can be achieved, for example, by optimizing the reaction parameters, as already shown by the observed effects of the catalysts, pH values, and H₂ pressures.

Full text of DOI:10.1039/c0gc00571a

Cutting-edge research for a greener sustainable future
www.rsc.org/greenchem
Volume 13 | Number 1 | January 2011 | Pages 1–212
ISSN 1463-9262
COVER ARTICLE
Liu et al.
Selective hydrogenolysis of biomass-
derived xylitol to ethylene glycol and
propylene glycol on supported Ru
catalysts
1463-9262(2011)13:1;1-Y

Dynamic Article Links
Green Chemistry
Cite this: Green Chem., 2011, 13, 135
www.rsc.org/greenchem
PAPER
Selective hydrogenolysis of biomass-derived xylitol to ethylene glycol and
propylene glycol on supported Ru catalysts
Jiying Sun and Haichao Liu*
Received 18th September 2010, Accepted 5th November 2010
DOI: 10.1039/c0gc00571a
The selective hydrogenolysis of biomass-derived xylitol to ethylene glycol and propylene glycol
was carried out on different catalysts in the presence of Ca(OH)2. The catalysts included Ru
supported on activated carbon (C) and, for comparison, on metal oxides, Al2O3, TiO2, ZrO2 and
Mg2AlOx as well as C-supported other noble metals, Rh, Pd and Pt, with similar particle sizes
(1.6–2.0 nm). The kinetic effects of H2 pressures (0–10 MPa), temperatures (433–513 K) and solid
bases including Ca(OH)2, Mg(OH)2 and CaCO3 were examined on Ru/C. Ru/C exhibited
superior activities and glycol selectivities than Ru on TiO2, ZrO2, Al2O3 and Mg2AlOx, and Pt was
found to be the most active metal. Such effects of the metals and supports are attributed
apparently to their different dehydrogenation/hydrogenation activities and surface acid-basicities,
which consequently influenced the xylitol reaction pathways. The large dependencies of the
activities and selectivities on the H2 pressures, reaction temperatures, and pH values showed their
effects on the relative rates for the hydrogenation and base-catalyzed reactions involved in xylitol
hydrogenolysis, reflecting the bifunctional nature of the xylitol reaction pathways. These results
led to the proposition that xylitol hydrogenolysis to ethylene glycol and propylene glycol
apparently involves kinetically relevant dehydrogenation of xylitol to xylose on the metal surfaces,
and subsequent base-catalyzed retro-aldol condensation of xylose to form glycolaldehyde and
glyceraldehyde, followed by direct glycolaldehyde hydrogenation to ethylene glycol and by
sequential glyceraldehyde dehydration and hydrogenation to propylene glycol. Clearly, the relative
rates between the hydrogenation of the aldehyde intermediates and their competitive reactions
with the bases dictate the selectivities to the two glycols. This study provides directions towards
efficient synthesis of the two glycols from not only xylitol, but also other lignocellulose-derived
polyols, which can be achieved, for example, by optimizing the reaction parameters, as already
shown by the observed effects of the catalysts, pH values, and H2 pressures.
polyester fibers and resins, etc. Industrial production of ethylene
glycol and propylene glycol is currently based on the multi-step
1. Introduction
The use of renewable biomass provides a viable route to
transformation of petroleum-derived ethylene and propylene via
both alleviate the shortage of fossil fuels and reduce the CO2
their epoxide intermediates. In comparison with ethylene and
emissions.1–3 In this context, biomass-derived polyols emerge
propylene, xylitol and sorbitol are not only renewable, but also
as promising building blocks for liquid fuels and value-added
structurally analogous to ethylene glycol and propylene glycol
chemicals from biomass.4–10 Among them, xylitol and sorbitol
with adjacent hydroxyl groups. These features render xylitol and
are especially noticeable because they are readily available from
sorbitol to be favorable feedstocks, in term of sustainability and
abundant lignocellulose, and can be converted to different prod-
energy efficiency, for the synthesis of the two glycols by catalytic
ucts such as ethylene glycol, propylene glycol and glycerol.11,12
hydrogenolysis.
Ethylene glycol and propylene glycol are important commod-
Hydrogenolysis of xylitol and sorbitol to ethylene glycol and
ity chemicals. They are widely used as functional fluids such as
propylene glycol requires cleavage of specific C–C and C–O
antifreezes and coolants, and as monomers in the synthesis of
bonds, which generally involves the use of Ni or Ru-based
catalysts and basic promoters.13–21 Clark et al.13 reported that
sorbitol can be hydrogenolyzed to ethylene glycol, propylene
Beijing National Laboratory for Molecular Sciences, State Key
Laboratory for Structural Chemistry of Stable and Unstable Species,
College of Chemistry and Molecular Engineering, Green Chemistry
Center, Peking University, Beijing, 100871, China. E-mail: hcliu@
pku.edu.cn; Fax: +86-10-6275-4031; Tel: +86-10-6275-4031
glycol and glycerol in yields of 16%, 17% and 40%, respectively,
at 488 K and 14 MPa H2 in water on kieselguhr supported
Ni catalysts with Ca(OH)2 as a promoter. Using more basic
NaOCH3, Tanikella et al.14 achieved high yields of ethylene
This journal is
The Royal Society of Chemistry 2011
Green Chem., 2011, 13, 135–142 | 135
©

glycol (45%) and propylene glycol (33%) from xylitol in
methanol on Ni/Al2O3 and Ni/SiO2 at 548 K and 27.6 MPa H2.
Dubeck et al.15 found that on sulfur-modified Ru/C catalysts,
sorbitol hydrogenloysis forms the three products of ethylene
glycol, propylene glycol and glycerol with a combined selectivity
of as high as 96% in the presence of Ca(OH)2 at 513 K
and 17 MPa H2. Such a high selectivity has been attributed
tentatively to the electron transfer from Ru to S, which possibly
leads concurrently to the deactivation of direct C–C and C–
O hydrogenolysis and the formation of hydroxyl groups on
Ru as the active sites for C–C bond cleavage via retro-aldol
condensation, etc.16 The modification of Ni catalysts was also
explored in the patents, as exemplified by Ni–Re/C17 catalysts,
exhibiting improved performances in hydrogenolysis of sorbitol
and xylitol. For instance, on 2.5% Ni–2.5% Re/C, xylitol
hydrogenolysis is 89% selective to the three products at a
~50% conversion in the presence of KOH under relatively mild
conditions (473 K and 8.5 MPa H2). Structural effects of the C
supports have also been examined.18,19 Recently, Zhou et al.20,21
reported that mesoporous carbon nanofibers are superior as the
Ru catalyst supports to the commercial activated carbons, which
can facilitate the mass transfer and electronic modification of
the Ru particles, and thus lead to a significant increase in the
selectivity to ethylene glycol, propylene glycol and glycerol from
26.1% to 51.3% at similar sorbitol conversions (70–85%) under
the identical conditions (493 K and 8 MPa H2).
These previous efforts have demonstrated the feasibility of
the hydrogenolysis route in the synthesis of ethylene glycol and
propylene glycol. However, these studies apparently encounter
low activities or selectivities for the target glycols, or harsh
reaction conditions (e.g. high temperatures or H2 pressures).
Addressing these problems requires the development of im-
proved catalysts, which is dependant unequivocally on the
understanding of the key fundamental issues including the effect
of catalyst structures and functions and the reaction mechanism.
However, these issues remain still unclear, likely as a consequence
that most of the studies to date have been reported in patents
with focus mainly on optimization of the catalyst compositions
and reaction parameters.
Here, we report a detailed study of xylitol hydrogenolysis on
supported Ru catalysts. Our choice of xylitol is based on the fol-
lowing consideration. Relative to more extensively studied sor-
bitol, xylitol possesses a shorter chain and is more structurally
advantageous in understanding the hydrogenolysis mechanism
in term of formation of ethylene glycol and propylene glycol,
the dominant products from both xylitol and sorbitol. On the
other hand, xylitol is derived from xylose that is not as efficient
as glucose in production of ethanol fuel via fermentation, and
thus other outlets of xylitol need to be explored including
its hydrogenolysis discussed here. We examine the effects of
supports and basic promoters on the activity and selectivity
of the Ru catalysts. The supports include activated carbon
(C), ZrO2, TiO2, Al2O3 and Mg2AlOx with a wide range of
acid-base and redox surface properties, and the bases include
Ca(OH)2, Mg(OH)2 and CaCO3 resulting in different pH values
in water. We compare the catalytic activity and selectivity of
Ru/C and other supported noble metal catalysts, Rh/C, Pd/C
and Pt/C, in xylitol hydrogenolysis, and also examine the effects
of reaction parameters (e.g. pH, H2 pressure and temperature).
These detailed studies are aimed to probe the effects of acid-
basicity and dehydrogenation/hydrogenation property of the
catalysts on the reaction pathways and to elucidate the reaction
mechanism for xylitol hydrogenolysis.
2. Experimental
Activated carbon (C) supported noble metal catalysts, 4 wt%
Ru/C, 4 wt% Pd/C, 8 wt% Pt/C and 4 wt% Rh/C, were prepared
by incipient wetness impregnation of C (AR, Beijing Dali Fine
Chemical, dried at 383 K in air for 12 h) with acetone solutions of
RuCl3·nH2O (GR, Sinopharm Chemical), PdCl2 (AR, Shenyang
Research Institute of Nonferrous Metals) with several drops of
aqueous HCl solution, H2PtCl6·6H2O (AR, Beijing Chemical)
and RhCl3·nH2O (Rh 38.5–45.5%, Alfa Aesar), respectively.
After evaporation of acetone, the resulting powders were dried
at 383 K in air overnight, and then reduced in a flowing 20%
H2/N2 at 673, 523, 473 and 673 K, respectively.
Metal oxide-supported Ru catalysts were prepared using
the method similar to that of Ru/C above. Monoclinic ZrO2
(m-ZrO2) was prepared hydrothermally in the way reported
previously by Li et al.22 Mg2AlOx (Mg/Al molar ratio of 2/1)
was prepared by urea hydrolysis reaction under hydrothermal
conditions. Briefly, Teflon inner vessel containing an aqueous
solution of 0.16 M Al(NO3)3, 0.33 M Mg(NO3)2, and 1.6 M
urea in a stainless steel jacket was heated at 393 K for 12 h. The
resulting white precipitates were filtered and washed thoroughly
with de-ionized water. g-Al2O3 (Condea Chemie Gmbh), TiO2
(Alfa Aesar), m-ZrO2 and Mg2AlOx were calcined at 673 K
under an air flow before impregnation of them with acetone
solutions of RuCl3.
The catalysts were characterized by X-ray diffraction (XRD)
and transmission electron microscopy (TEM). The XRD pat-
terns were obtained on a Rigaku D/MAX-2400 diffractometer
˚
using Cu Ka1 radiation (l = 1.5406 A) operated at 40 kV
and 100 mA. The TEM images were taken on a Philips Tecnai
F30 FEGTEM operated at 300 kV. Samples were prepared by
uniformly dispersing the catalysts in ethanol and then placing
them onto carbon-coated copper grids. The average size of metal
particles and their size distributions were calculated by averaging
of more than 300 particles randomly distributed in the TEM
images.
Xylitol hydrogenolysis reactions were carried out in a Teflon-
lined stainless steel autoclave (100 ml) at a stirring speed of
800 rpm. In a typical run, 40 g of 10 wt% xylitol (99%, Alfa
Aesar) aqueous solution, 0.1 g of metal catalyst and at least
0.26 g of Ca(OH)2 were introduced to the autoclave. Afterwards,
the reactor was purged with H2 (>99.99%, Beijing Huayuan)
three times, and pressurized with H2 to 4.0 MPa and then heated
to 473 K, which was kept constant during the reaction. The
reactant and liquid products, after silylation with hexamethyld-
isilazane (HMDS) and trimethylchlorinesilane (TMSCl) (both
≥98.0%, Sinopharm Chemical) in pyridine (AR, Shantou Xilong
Chemical), were analyzed by gas chromatography (Shimadzu
2010 GC) using a capillary column OV-101 (30 m ¥ 0.25 mm ¥
0.33 mm) and a flame ionization detector. The reactant and liquid
products were also analyzed by high-performance liquid chro-
matography (Shimadzu LC-20A) using a PRP-X300 column at
304 K and a PDA detector.
136 | Green Chem., 2011, 13, 135–142
This journal is
The Royal Society of Chemistry 2011
©

Table 1 Activities and selectivities in xylitol hydrogenolysis on different catalysts under basic conditionsa
Selectivity (on a carbon basis, %)
Catalyst
Activitiy (h-1)
Ethylene glycol
Propylene glycol
Glycerol
Threitol
Arabitol
Lactic acid
Formic acid
Ru/C
184.4
87.8
66.7
162.5
145.2
63.0
32.4
24.2
30.0
17.5
19.5
30.0
26.4
25.0
24.9
20.5
24.0
9.0
9.6
9.6
8.0
13.2
4.8
3.4
7.2
10.5
2.3
3.2
2.3
4.3
trace
trace
trace
trace
3.4
n.d.
n.d.
8.3
16.8
22.8
28.3
25.2
36.5
28.4
23.3
15.5
2.3
2.1
3.7
1.6
2.2
0.9
4.6
2.0
Ru/ZrO2
Ru/TiO2
Ru/Al2O3
Ru/Mg2AlOx
Pd/C
Rh/C
Pt/C
7.9
6.9
29.0
30.0
23.3
n.d.b
n.d.
n.d.
61.2
754.5
a Reaction conditions: 473 K, 4.0 MPa H2, 0.02–0.20 g catalysts, 0.26–0.39 g Ca(OH)2, 40 g 10 wt% xylitol aqueous solution, 1 h, ~20% xylitol
conversion. b Not detected.
(Table 1). In contrast, the selectivity to lactic acid increased
from 16.8% on Ru/C to 36.5% on Ru/Mg2AlOx (Table 1). Such
3. Results and discussion
Table 1 shows the activity and selectivity of xylitol hydrogenol-
ysis at 473 K and 4.0 MPa H2 on different supported Ru
catalysts in the presence of Ca(OH)2. The supports include
activated carbon (C), TiO2, m-ZrO2, Al2O3, and Mg2AlOx with
a wide range of surface properties such as acid-basicity and
reducibility in favor of examining the support effects. C is a
known favorable support in polyol hydrogenolysis, and it is inert
and stable under hydrothermal conditions even at high pressures
and temperatures.23 TiO2 and m-ZrO2 are also hydrothermally
stable, but they are amphoteric oxides with redox functions
that can catalyze alcohol dehydrogenation and dehydration.22,23
Mg2AlOx and Al2O3 are unreducible oxides with basic and acid
properties, respectively. The five Ru catalysts were characterized
by XRD and TEM. The XRD patterns (not included here)
show only the diffraction features of the respective supports.
The TEM images reveal that these catalysts possessed similar
mean diameters of Ru particles (ca. 1.4–1.8 nm) with narrow
size distributions, as shown in Fig. 1 and 2.
On the five Ru catalysts, ethylene glycol, propylene glycol
and glycerol were formed as the major products in the presence
of Ca(OH)2. Lactic acid was also detected with a significant
amount. Other products mainly included CH4, CO2 and some
unidentified compounds possibly from condensation of xylitol
or its derivatives. The carbon balance was generally better than
90%. The activities (normalized per Ru atom) and selectivities
strongly depend on the identity of the supports. As shown
in Table 1, the activity was 184.4 h-1 on Ru/C, which was
greater than on the four metal oxide-supported Ru catalysts,
especially Ru/ZrO2 (87.8 h-1) and Ru/TiO2 (66.7 h-1) at similar
xylitol conversions (~20%). Such support effect on the activity,
although the underlying reason needs to be clarified, might be
related to their effects on the Ru electronic property as well
as to the difference in the adsorption of xylitol, its derivatives
or products on these supports. As for the selectivities, Ru/C
is more selective, relative to the other four catalysts, to ethylene
glycol and propylene glycol with a combined selectivity of 57.3%
at ca. 20% xylitol conversion (Table 1). Including glycerol, the
selectivity to the sum of the three C2 and C3 polyols reached
66.9% on Ru/C, which was similar to the result on Ru/TiO2
(62.0%), but much higher than those on Ru/ZrO2, Ru/Al2O3
and Ru/Mg2AlOx, being 54.3%, 39.7% and 32.2%, respectively
changes in these selectivities appear to be consistent with the
known difference in the basicity of the five supports, i.e. C < TiO2
< ZrO2 < Al2O3 < Mg2AlOx, apparently reflecting the effects of
the basicity on the xylitol reaction pathways, as discussed later.
Among the five supports examined above, C is clearly the
most preferable one, leading to superior activity and selectivity
to the target products for xylitol hydrogenolysis. Using C as
support, three other noble metals, Pd, Rh and Pt, were also
examined for comparison with Ru/C. The three catalysts were
also characterized by TEM, and possessed metal particle sizes
of 1.7–1.9 nm (Fig. 3), similar to the size for Ru/C (1.7 nm).
In the presence of Ca(OH)2, the three catalysts were also
active and selective for xylitol conversion to ethylene glycol and
propylene glycol, and the combined selectivities (48.3–59.0%)
were comparable with the value of Ru/C (57.3%) under the
identical conditions. Pd/C and Rh/C were less active, but
Pt/C offered a four times greater activity than Ru/C. The
superior activity of Pt/C was also observed by Ukisu et al.24
in isopropanol dehydrogenation and by Davis et al.25 in glycerol
hydrogenolysis under the basic conditions.
The recyclability of these catalysts were examined in the
kinetic controlled regime (at ca. 33% xylitol conversion), and
the representative results on Ru/C are shown in Fig. 4. No
significant decline in the xylitol activities and selectivities was
observed after recycling the Ru/C catalyst over six times. This is
consistent with the TEM characterization result of this catalyst
(Fig. 2), demonstrating that the mean diameters of the Ru
nanoparticles and their size distributions remained essentially
unaltered (1.7 0.4 vs. 2.0 0.3 nm) after the six successive
runs. These results reveal that the Ru/C catalyst is stable and
recyclable under the reaction conditions in this work.
Fig. 4 displays the change of selectivities for the four major
C2 and C3 products, ethylene glycol, propylene glycol, glycerol
and lactic acid, as a function of the xylitol conversions on
Ru/C at 473 K and 4.0 MPa H2. The selectivities to propylene
glycol increased slightly from 24.0 to 28.4% with increasing the
xylitol conversion from 15.0 to 98.0%, while the ethylene glycol
selectivities remained essentially constant (ca. 33%). Such trend
leads to the increase in the combined selectivity to 61.2% for the
target glycols at nearly 100% xylitol conversion, corresponding
to a 60% yield, showing the potential advantage of the Ru/C
This journal is
The Royal Society of Chemistry 2011
Green Chem., 2011, 13, 135–142 | 137
©

Fig. 2 TEM micrographs and histograms of Ru particle size distribu-
tion of Ru/C before reaction and after six reaction cycles (scale bar =
20 nm). (a) Before reaction, 1.7 0.4 nm; (b) after six reaction cycles,
2.0 0.3 nm.
from the same intermediates. The observed slight increase in
the propylene glycol selectivities indicates that glycerol can also
convert via its secondary reactions to propylene glycol. Such
discussion on the glycerol conversion is consistent with the
results reported previously20,25–28 and for our separate glycerol
reaction. Glycerol hydrogenolysis on Ru/C in the presence of
Ca(OH)2 at 473 K and 4.0 MPa H2 formed lactic acid and
propylene glycol with 67.4% and 24.6% selectivities, respectively.
The reaction activities and selectivities strongly depend on the
reaction parameters of H2 pressure, temperature, and pH value
in the aqueous reaction solutions. Table 2 shows the effects of
the pH value on xylitol hydrogenolysis on Ru/C at 473 K and
4.0 MPa H2. In the neutral solution, i.e. without addition of any
bases, xylitol converted predominantly to arabitol and adonitol
(totally 80%) together with small amounts of glycerol, threitol
and erythritol on the Ru catalyst. Only trace amounts of ethylene
glycol and propylene glycol were formed. Arabitol and adonitol
are epimers of xylitol. It has been reported that polyol epimer-
ization involves polyol dehydrogenation to the corresponding
carbonyl intermediates and their subsequent hydrogenation.29
Together with the result of the control experiment showing no
detectable xylitol conversion in the absence of Ru/C (with or
without Ca(OH)2), the predominance of the xylitol epimers
implies that xylitol hydrogenolysis on Ru/C proceeds primarily
by its dehydrogenation.16,30–32 The detection of C4 polyols,
threitol and erythritol, is indicative that xylitol dehydrogenates
at its primary hydroxyl groups to form the corresponding
aldopentose (i.e. xylose) rather than ketopentoses. The reason
is that hydrogenolysis of primary alcohols tends to occur by
cleavage of their C–C bonds at the end of the chains via their
aldehyde intermediates, while C–O bonds in secondary alcohols
are more susceptible than C–C bonds to cleavage, forming
dehydroxylated products on the metal catalysts.33,34
Fig. 1 TEM micrographs and histograms of Ru particle size distribu-
tion for different Ru catalysts (scale bar = 20 nm). (a) Ru/Al2O3, 1.6
0.3 nm; (b) Ru/ZrO2, 1.7 0.2 nm; (c) Ru/Mg2AlOx, 1.4 0.3 nm; (d)
Ru/TiO2, 1.8 0.4 nm.
catalyst for selective hydrogenolysis of xylitol. These results
reflect the stability of the two glycols under the conditions in
this work, as indeed verified by their separate hydrogenolysis
reactions under the identical conditions (not shown here).
The non-zero selectivities for the two glycols observed as the
conversions approached the zero level indicate that they are
formed as primary products directly from xylitol. In contrast,
the selectivities to glycerol declined dramatically from 14.6 to
less than 1.0% concurrently with an increase in the lactic acid
selectivity from 10.5 to 28.0%, as the conversion increased from
15.0 to 98.0%. Such concurrent changes in the lactic acid and
glycerol selectivities indicate that lactic acid is formed via sec-
ondary glycerol reactions or they may be formed competitively
138 | Green Chem., 2011, 13, 135–142
This journal is
The Royal Society of Chemistry 2011
©

Table 2 Effects of solid bases (i.e. pH values) on activities and selectivities in xylitol hydrogenolysis on Ru/Ca
Selectivity (on a carbon basis, %)
Base
pHb
Activity (h-1) Ethylene glycol Propylene glycol Glycerol Threitol Erythritol Arabitol Adonitol
Lactic acid
—
CaCO3
Mg(OH)2 10.7 116.2
Ca(OH)2 12.3 184.4
7.0
9.4
815
67.8
trace
21.0
24.5
32.4
trace
18.4
21.0
24.9
3.5
8.3
4.2
3.7
2.3
2.9
67.8
14.9
11.7
trace
12.6
trace
trace
n.d.
n.d.
2.2
4.1
15.5
16.8
9.6
trace
trace
trace
16.8
a Reaction conditions: 473 K, 4.0 MPa H2, 0.02–0.20 g Ru/C (4 wt% Ru), 40 g 10 wt% xylitol aqueous solution, 1 h, ~20% xylitol conversion.
b Measured at 298 K.
Fig. 4 Activities and selectivities to ethylene glycol, propylene glycol
and glycerol for the six reaction cycles of xylitol hydrogenolysis on
Ru/C. Reaction conditions: 473 K, 4.0 MPa H2, 0.15 g Ru/C (4 wt%
Ru), 0.33 g Ca(OH)2, 40 g 10 wt% xylitol aqueous solution, 1 h.
effects of the bases on xylitol reaction although the underlying
reason is not clear.
The addition of the solid bases and consequent increase in the
pH values led to a significant change in the dominant products
from the xylitol epimers to C2 and C3 products (Table 2). The
selectivities to ethylene glycol, propylene glycol and lactic acid
monotonically increased to 32.4, 24.9 and 16.8%, respectively,
with increasing the pH values from 7.0 to 12.3; the combined
Fig. 3 TEM micrographs and histograms of metal particle size
distribution for Pd/C, Pt/C and Rh/C (scale bar = 20 nm). (a) Rh/C,
1.7 0.3 nm; (b) Pd/C, 1.8 0.3 nm; (c) Pt/C, 1.9 0.5 nm.
selectivity to arabitol and threitol concurrently decreased to
nearly zero. Glycerol was also detected with selectivities of
about 16% at pH 9.4 and pH 10.7, which further decreased
to 9.6% at pH 12.3. But the combined selectivities to the three
C3 products were always greater at higher pH values. Such pH
effects on the selectivities clearly show the direct involvement of
the bases in the C–C bond cleavage of xylitol, which affects the
reaction pathways of hydrogenation of the xylose intermediate
to C4–C5 products and their degradation to the C2–C3 products,
and ultimately the product selectivities. Obviously, appropriate
pH values are required for efficient xylitol conversion to the
C2–C3 products, as indeed was found in Table 2 and further
confirmed by the following experiment. The use of stronger
bases such as NaOH and KOH increased the pH values to 13 or
above, leading to complex products due to the rapid reactions
of xylitol or its derivatives with OH- in the solutions (not shown
Upon addition of CaCO3, Mg(OH)2 and Ca(OH)2 bases, the
pH values in the reaction solutions increased to 9.4, 10.7 and
12.3, respectively (measured at 298 K), which kept constant
during the reaction due to the known solubility equilibrium of
these solid bases slightly soluble in the solutions. The activities
decreased sharply by 4–10 fold in comparison with the activity
in the absence of the bases. Such phenomenon is different from
that observed by Davis and coworkers in the hydrogenolysis
of glycerol that Ru/C is more active in the basic solutions.25
However, in the presence of the solid bases, the activities
increased almost linearly from 67.8 to 184.4 h-1 with increasing
the pH values from 9.4 to 12.3. These results show the dual
This journal is
The Royal Society of Chemistry 2011
Green Chem., 2011, 13, 135–142 | 139
©

here), consistent with the finding by Zhao et al. in sorbitol
hydrogenolysis.20 The result reflects the finding by Norkus et al.35
that xylitol can be fully deprotonated in the solution of pH 13,
as characterized by 13C NMR.
Variation of the H2 pressure led to similar effects on the re-
action pathways and selectivities. At similar xylitol conversions
(~25%), the activities increased, as expected, with increasing
the H2 pressure in the range 0–10 MPa on Ru/C at 473 K
and pH 12.3 (in the presence of Ca(OH)2). Under inert N2
atmosphere, the major products were lactic acid (54.9%) together
with small amounts of propylene glycol and ethylene glycol.
With increasing the H2 pressure, the selectivities for the two
glycols increased sharply and reached the maximum values of
30.0 and 35.3%, respectively, at 2.0 MPa H2. Concurrently, the
selectivities to lactic acid sharply decreased from 54.9 to 23.1%
as the H2 pressures increased from 0 to 2.0 MPa, which then
gradually decreased to 15.0% at 10.0 MPa. The selectivities
for glycerol, arabitol and threitol increased monotonically with
increasing the H2 pressure inthe range 0–10MPa. Suchchange in
the selectivities for the C3 products indicates that they are formed
via the same intermediates, and higher H2 pressures facilitate the
hydrogenation reactions and the formation of propylene glycol
and glycerol at the expense of lactic acid. The gradual increase
in the selectivities for arabitol and threitol with the H2 pressure
apparently reflects the enhanced rates at higher H2 pressures for
hydrogenation of the xylose intermediate, which competes with
its C–C bond cleavage to ultimately form the C2 and C3 products.
Xylitol hydrogenolysis forms C2 and C3 products by breaking
its central C2–C3 bond through the xylose intermediate, for
example, following the retro-aldol condensation mechanism,
as proposed by Wang et al.32 This mechanism should lead to
glycoaldehyde and glyceraldehyde intermediates with theoretical
selectivities of 40% and 60%, respectively, as indeed was found
in Fig. 6. The combined selectivities for the C3 products, lactic
acid, propylene glycol and glycerol, were approximately 60% at
the H2 pressures of below 2.0 MPa, near the theoretical value
(60%) from xylitol. The selectivities to the C2 products (mainly
ethylene glycol) were however lower than 40%, reflecting the
known instability of glycoaldehyde intermediate under basic
and H2-poor conditions especially at the high temperature
of 473 K. Above 2.0 MPa H2, as the hydrogenation steps
were accelerated, the ethylene glycol selectivities increased; the
selectivity ratio of ethylene glycol to the C3 products approached
the theoretical value of 2/3 (Fig. 6). This cleavage site selectivity,
together with the observed pH effects, confirms the involvement
of the retro-aldol condensation as the dominating C–C bond
cleavage route in xylitol hydrogenolysis.
Fig. 5 Dependence of products selectivities on xylitol conversion for
Ru/C. Reaction conditions: 473 K, 4.0 MPa H2, 0.10–0.40 g Ru/C
(4 wt% Ru), 0.26–0.60 g Ca(OH)2, 40 g 10 wt% xylitol aqueous solution,
1–6 h.
Fig. 7 shows the activities and selectivities for xylitol hy-
drogenolysis as a function of the reaction temperature on
Ru/C at similar xylitol conversions (~25%). The activities
increased dramatically from 20.8 to 981.1 h-1 with increasing
the temperature from 433 to 513 K. The selectivities to ethylene
glycol and propylene glycol increased with increasing the
temperature, and reached the maximum values between 473
and 493 K. In contrast, the selectivities to glycerol and to
the sum of arabitol and threitol monotonically decreased in
the range 433–513 K. Lactic acid selectivities remained almost
constant (around 17%) between 433–473 K, which however
sharply increased to 31.2% at 513 K, showing its favorable
Fig. 6 Effects of H2 pressure on activities and selectivities for xylitol
hydrogenolysis on Ru/C at ca. 25% conversion. Reaction conditions:
473 K, 0.10 g Ru/C (4 wt% Ru), 0.26 g Ca(OH)2, 40 g 10 wt% xylitol
aqueous solution, 1 h.
formation at higher temperatures, as previously found in glycerol
reactions.26 The combined selectivities to the C3 products,
propylene glycol, glycerol and lactic acid, continuously increased
with the temperature, which indicates that the decline in the
propylene glycol selectivities above 493 K is due to the favored
140 | Green Chem., 2011, 13, 135–142
This journal is
The Royal Society of Chemistry 2011
©

Scheme 1
Fig. 7 Effects of temperature on activities and selectivities for xylitol
hydrogenolysis on Ru/C at ca. 25% conversion. Reaction conditions: 4.0
MPa H2, 0.02–0.20 g Ru/C (4 wt% Ru), 0.26 g Ca(OH)2, 40 g 10 wt%
xylitol aqueous solution, 1–4 h.
with OH- ions. The glyceraldehyde intermediate undergoes
dehydration readily to pyruvaldehyde, followed by its hydro-
genation to propylene glycol or by its base-catalyzed benzilic
acid rearrangement to lactic acid, as found previously by Davis,
Liu and their coworkers.25,26 The formation of the C3 products
from the same glyceraldehyde intermediate can account for
the observed effects of the H2 pressure on their selectivity
trends, i.e. the concurrent increase in the propylene glycol and
glycerol selectivities and decrease in the lactic acid selectivities
with increasing the H2 pressure (Fig. 6). The hydrogenation
of glyceraldehyde to glycerol is reversible and glycerol can
convert ultimately to propylene glycol and lactic acid especially
at the higher xylitol conversions, as reflected by the observed
concurrent increase in the propylene glycol and lactic acid
selectivities and decrease in the glycerol selectivities as xylitol
conversion increases (Fig. 5). Such glycerol reaction is consistent
with the previous studies on glycerol conversion,20,25–28 and it
was also verified by our separate experiment in this work, as
described above.
In addition, the xylitol dehydrogenation step appears to
be prerequisite for its hydrogenolysis according to Scheme 1.
This step may dictate the overall hydrogenolysis rate. For
verification, xylose hydrogenolysis was examined under the
standard conditions of 473 K and 4.0 MPa H2 as well as at
lower temperatures (e.g. 373 K) in the presence of Ca(OH)2. It
was found that xylose readily converted to a complex mixture
of products, such as xylitol, arabitol, and various organic acids,
even at 373 K or lower (e.g. 313K), reflecting the high reactivity
of sugars with H2 on Ru surface and with bases in the reaction
solutions.36 However, ethylene glycol, propylene glycol and
glycerol were always detected with a combined selectivity of less
than 30% under a wide range of reaction conditions. This finding
additionally implies that xylose, although it is the intermediate
for xylitol hydrogenolysis, is less effective than xylitol for direct
synthesis of ethylene glycol and propylene glycol, which is
consistent with the conclusion reported by Mu¨ller et al.37
formation of lactic acid from the same intermediates at higher
temperatures. Obviously, lower temperatures favor the direct
hydrogenation of the xylose intermediate while its degradation
reactions prevail at higher temperatures, and lead to efficient
synthesis of the target glycols at appropriate temperatures.
Taken together, the observed effects of the catalyst and
reaction parameter show the directions toward efficient xylitol
hydrogenolysis by tuning of catalysts and reaction parameters,
and in turn of the reaction pathways involving the formation
of the xylose intermediate and its subsequent transformations.
These effects have shed light on the xylitol hydrogenolysis
mechanism, as discussed below.
Concerning the mechanism, although there is no clear de-
scription on xylitol hydrogenolysis, it has been proposed that
polyols (e.g. sorbitol) are hydrogenolyzed generallyinvolving two
key steps, i.e. dehydrogenation of polyols to the corresponding
carbonyl intermediates on metal catalysts, and their subsequent
C–C bond cleavage with bases, most likely via the retro-
aldol condensation. Such proposition was verified by Wang
et al.,32 based on their study on hydrogenolysis of different 1,3-
diol model compounds on Raney Cu and Ni catalysts in the
presence of NaOH. In combination with the current mechanistic
understandings, we propose the reaction pathways for xylitol
hydrogenolysis under the conditions employed in this work.
As depicted in Scheme 1, the first step is dehydrogenation of
xylitol to the corresponding xylose intermediate on the metal
catalysts as discussed by others.16,30–32 Subsequently, the xylose
intermediate are either hydrogenated on the metal catalysts to
the xylitol epimers (i.e. arabitol and adonitol) and C4 polyols
or undergo the base-involved retro-aldol condensation to yield
glycoaldehyde and glyceraldeyde, which are then hydrogenated
to ethylene glycol and glycerol, respectively, or in parallel react
This journal is
The Royal Society of Chemistry 2011
Green Chem., 2011, 13, 135–142 | 141
©

Clearly, these proposed reaction pathways demonstrate that
while the rate of xylitol hydrogenolysis appears to be limited by
its primary dehydrogenation step, the final product distributions
are controlled by the competition between the hydrogenation
steps on the metal surfaces and the base-catalyzed reactions.
Such understanding of these pathways enables one to improve
the efficacy of xylitol hydrogenolysis into the target glycols, for
example, by optimizing the reaction parameters, as revealed,
at least partially, from the effects of the catalyst, pH, and H2
pressure.
3 P. Gallezot, Catal. Today, 2007, 121, 76–91.
4 E. L. Kunkes, D. A. Simonetti, R. M. West, J. C. Serrano-Ruiz, C. A.
Ga¨rtner and J. A. Dumesic, Science, 2008, 322, 417–421.
5 P. L. Dhepe and A. Fukuoka, ChemSusChem, 2008, 1, 969–
975.
6 C. Luo, S. Wang and H. Liu, Angew. Chem., Int. Ed., 2007, 46, 7636–
7639.
7 M. Schlaf, Dalton Trans., 2006, 4645–4653.
8 A. Behr, J. Eilting, K. Irawadi, J. Leschinski and F. Lindner, Green
Chem., 2008, 10, 13–30.
9 Y. Nakagawa, Y. Shinmi, S. Koso and K. Tomishige, J. Catal., 2010,
272, 191–194.
10 A. Villa, G. M. Veith and L. Prati, Angew. Chem., Int. Ed., 2010, 49,
4499–4502.
11 J. J. Bozell and G. R. Petersen, Green Chem., 2010, 12, 539–554.
12 B. Blanc, A. Bourrel, P. Gallezot, T. Haas and P. Taylor, Green Chem.,
2000, 2, 89–91.
4. Conclusions
Supported noble metals, Ru, Pd, Rh and Pt efficiently catalyze
xylitol hydrogenolysis into ethylene glycol and propylene glycol
in the presence of Ca(OH)2. The activities and selectivities
strongly depend on the nature of the metals and their underlying
supports. Ru supported on C exhibits superior activities and
glycol selectivities than on TiO2, ZrO2, Al2O3 and Mg2AlOx,
and Pt is found to be the most active metal compared at similar
particle sizes, apparently as a result of the difference in their
dehydrogenation/hydrogenation activities and surface acid-
basicities, and the consequent effects on the reaction pathways
of xylitol hydrogenolysis. The activities and selectivities also
depend largely on the H2 pressures, reaction temperatures,
and pH values varied by using different solid bases, CaCO3,
Mg(OH)2 and Ca(OH)2, which influence the hydrogenation and
base-catalyzed steps involved in xylitol hydrogenolysis. Taken
together, it is apparent that xylitol hydrogenolysis to ethylene
glycol and propylene glycol proceeds by its kinetically relevant
dehydrogenation of xylitol to xylose on the metal surfaces,
and its subsequent base-catalyzed retro-aldol condensation to
form glycolaldehyde and glyceraldehyde, the intermediates for
the two glycols. The involved hydrogenation of the aldehyde
intermediates on the metal surfaces and their competitive
reactions with the bases clearly dictate the ultimate selectivities
to the two glycols.
13 I. T. Clark, Ind. Eng. Chem., 1958, 50, 1125–1126.
14 M. S. S. R. Tanikella, US Pat., 4 404 411, 1983.
15 M. Dubeck and G. G. Knapp, US Pat., 4 430 253, 1984.
16 C. Montassier, J. C. Me´ne´zo, L. C. Hoang, C. Renaud and J. Barbier,
J. Mol. Catal., 1991, 70, 99–110.
17 T. A. Werpy, J. G. Frye, A. H. Zacher and D. J. Miller, US Pat., 7
038 094, 2006.
18 S. P. Chopade, D. J. Miller, J. E. Jackson, T. A. Werpy, J. G. Frye and
A. H. Zacher, US Pat., 6 291 725, 2001.
19 G. Gubitosa and B. Casale, US Pat., 5 326 912, 1994.
20 L. Zhao, J. H. Zhou, Z. J. Sui and X. G. Zhou, Chem. Eng. Sci., 2010,
65, 30–35.
21 J. H. Zhou, M. G. Zhang, L. Zhao, P. Li, X. G. Zhou and W. K.
Yuan, Catal. Today, 2009, 147S, S225–S229.
22 W. Z. Li, H. Huang, H. J. Li, W. Zhang and H. C. Liu, Langmuir,
2008, 24, 8358–8366.
23 W. C. Ketchie, E. P. Maris and R. J. Davis, Chem. Mater., 2007, 19,
3406–3411.
24 Y. Ukisu and T. Miyadera, React. Kinet. Catal. Lett., 2004, 81, 305–
311.
25 E. P. Maris and R. J. Davis, J. Catal., 2007, 249, 328–337.
26 Y. Shen, S. Zhang, H. Li, Y. Ren and H. Liu, Chem. Eur. J., 2010, 16,
7368–7371.
27 S. Wang, Y. Zhang and H. Liu, Chem. Asian J., 2010, 5, 1100–1111.
28 T. Miyazawa, S. Koso, K. Kunimori and K. Tomishige, Appl. Catal.,
A, 2007, 329, 30–35.
29 L. Wright and L. Hartmann, J. Org. Chem., 1961, 26, 1588–1596.
30 D. K. Sohounloue, C. Montassier and J. Barbier, React. Kinet. Catal.
Lett., 1983, 22, 391–397.
31 E. Tronconi, N. Ferlazzo, P. Forzatti, I. Pasquon, B. Casale and L.
Marini, Chem. Eng. Sci., 1992, 47, 2451–2456.
32 K. Y. Wang, M. C. Hawley and T. D. Furney, Ind. Eng. Chem. Res.,
1995, 34, 3766–3770.
Acknowledgements
33 C. A. M. Abreu, N. M. Lima and A. Zoulalian, Biomass Bioenergy,
1995, 9, 487–492.
34 V. N. Ipatieff, W. W. Thompson and H. Pines, J. Am. Chem. Soc.,
1951, 73, 553–555.
35 E. Norkus, J. Vaicˇiu¯niene˙, T. Vuorinen, E. Gaidamauskas, J.
Reklaitis, A. S. Ja¨a¨skela¨inen and D. C. Crans, Carbohydr. Res., 2004,
339, 599–605.
This work was supported by the National Natural Science
Foundation of China (Grant No. 20825310, 20733009, 20973011
and 50821061) and National Basic Research Project of China
(Grant No. 2006CB806100).
36 J. M. de Bruijn, A. P. G. Kieboom and H. van Bekkum, Recl. Trav.
Chim. Pays-Bas, 1986, 105, 176–183.
37 C. Mu¨ller, P. Rimmelin, J. P. Hindermann, R. Kieffer and A. Kien-
nemann, in Studies in Surface Science and Catalysis, ed. M. Guisnet,
J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and
C. Montessier, Elsevier, Amsterdam, 1991, Vol. 59, pp. 237–244.
References
1 D. M. Alonso, J. Q. Bond and J. A. Dumesic, Green Chem., 2010, 12,
1493–1513.
2 G. W. Huber, S. Iborra and A. Corma, Chem. Rev., 2006, 106, 4044–
4098.
142 | Green Chem., 2011, 13, 135–142
This journal is
The Royal Society of Chemistry 2011
©