Selective hydrogenolysis of sorbitol to ethylene glycol and propylene glycol on ZrO2-supported bimetallic Pd-Cu catalysts

Article abstract of DOI:10.1016/S1872-2067(15)60892-0

Sorbitol is one of the key building blocks in catalytic conversion of biomass, and its selective hydrogenolysis to ethylene glycol and propylene glycol provides a viable and sustainable route towards the synthesis of the two glycols. Herein, the hydrogenolysis of biomass-derived sorbitol was studied on Pd-modified Cu/monoclinic zirconia (Pd-Cu/ZrO₂) catalysts with a wide range of Cu/Pd atomic ratios in the presence of La(OH)₃. The bimetallic Pd-Cu/ZrO₂ catalysts showed superior activities and selectivities to the two target glycols, compared with the monometallic Cu/ZrO₂ and Pd/ZrO₂ catalysts under identical conditions. At nearly 100% sorbitol conversion, a combined selectivity of 61.7% to ethylene glycol, propylene glycol, and glycerol was obtained on Pd-Cu/ZrO₂ (Cu/Pd=5) at 493 K under 5.0 MPa H₂. Pd-Cu/ZrO₂ was also stable and recyclable, in contrast to Cu/ZrO₂, which suffered severe deactivation because of agglomeration of Cu particles during sorbitol hydrogenolysis. Clearly, the presence of Pd improved not only the activity and selectivity of the Cu catalyst, but also the hydrothermal stability. Characterization of these catalysts by X-ray diffraction, diffuse-reflectance infrared Fourier transform spectroscopy of CO adsorption, and H₂ temperature-programmed reduction suggests that the Cu particles deposited on the Pd surfaces with close contact and strong interaction between the two metals, most likely involving electron transfer from Pd to Cu. Such structural and electronic effects are proposed as the critical contributors to the significant promoting effect of Pd on the activity and stability of Pd-Cu catalysts in sorbitol hydrogenolysis. These findings provide useful information for design of new Cu-based catalysts with higher efficiency and stability for selective hydrogenolysis of polyols and other biomass-derived reactants under hydrothermal conditions.

Full text of DOI:10.1016/S1872-2067(15)60892-0

Chinese Journal of Catalysis 36 (2015) 1552–1559
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c h n j c
Article (Special Issue for Excellent Research Work in Recognition of Scientists Who Are in Catalysis Field in China)
Selective hydrogenolysis of sorbitol to ethylene glycol and
propylene glycol on ZrO2-supported bimetallic Pd-Cu catalysts
Yuqing Jia, Haichao Liu*
Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Stable and Unstable Species, College of Chemistry
and Molecular Engineering, Peking University, Beijing 100871, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Sorbitol is one of the key building blocks in catalytic conversion of biomass, and its selective hydro-
genolysis to ethylene glycol and propylene glycol provides a viable and sustainable route towards
the synthesis of the two glycols. Herein, the hydrogenolysis of biomass-derived sorbitol was studied
on Pd-modified Cu/monoclinic zirconia (Pd-Cu/ZrO2) catalysts with a wide range of Cu/Pd atomic
ratios in the presence of La(OH)3. The bimetallic Pd-Cu/ZrO2 catalysts showed superior activities
and selectivities to the two target glycols, compared with the monometallic Cu/ZrO2 and Pd/ZrO2
catalysts under identical conditions. At nearly 100% sorbitol conversion, a combined selectivity of
61.7% to ethylene glycol, propylene glycol, and glycerol was obtained on Pd-Cu/ZrO2 (Cu/Pd=5) at
493 K under 5.0 MPa H2. Pd-Cu/ZrO2 was also stable and recyclable, in contrast to Cu/ZrO2, which
suffered severe deactivation because of agglomeration of Cu particles during sorbitol hydrogenoly-
sis. Clearly, the presence of Pd improved not only the activity and selectivity of the Cu catalyst, but
also the hydrothermal stability. Characterization of these catalysts by X-ray diffraction, dif-
fuse-reflectance infrared Fourier transform spectroscopy of CO adsorption, and H2 tempera-
ture-programmed reduction suggests that the Cu particles deposited on the Pd surfaces with close
contact and strong interaction between the two metals, most likely involving electron transfer from
Pd to Cu. Such structural and electronic effects are proposed as the critical contributors to the sig-
nificant promoting effect of Pd on the activity and stability of Pd-Cu catalysts in sorbitol hydrogen-
olysis. These findings provide useful information for design of new Cu-based catalysts with higher
efficiency and stability for selective hydrogenolysis of polyols and other biomass-derived reactants
under hydrothermal conditions.
Received 3 May 2015
Accepted 19 May 2015
Published 20 September 2015
Keywords:
Selective hydrogenolysis
Sorbitol
Ethylene glycol
Propylene glycol
Bimetallic palladium-copper catalyst
© 2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
different catalytic routes [5,6]. One such example is selective
hydrogenolysis of sorbitol and xylitol to ethylene glycol and
There is an increasing interest worldwide in utilizing re-
newable biomass for sustainable production of liquid fuels and
chemicals [1–4]. As such, biomass-derived sorbitol and xylitol
have been intensively studied as the key platform molecules,
especially for synthesis of various value-added chemicals via
propylene glycol [7–18], the two important glycols that are
currently manufactured from multistep reactions of petrole-
um-derived ethylene and propylene [19,20].
Selective hydrogenolysis of sorbitol and xylitol to ethylene
glycol and propylene glycol has been explored on Ru-, Ni-, and
* Corresponding author. Tel/Fax: +86-10-62754031; E-mail: hcliu@pku.edu.cn
This work was supported by the National Basic Research Program of China (973 Program, 2011CB201400) and the National Natural Science Founda-
tion of China (21173008, 21373019, 21433001).
DOI: 10.1016/S1872-2067(15)60892-0 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 36, No. 9, September 2015

Yuqing Jia et al. / Chinese Journal of Catalysis 36 (2015) 1552–1559
1553
Cu-based catalysts in the presence of base additives. Sun et al.
hydrogenolysis.
[7] studied the xylitol hydrogenolysis on Ru/C with Ca(OH)2 at
473 K under 4.0 MPa H2, reaching ∼61% combined selectivity
to ethylene glycol and propylene glycol with near-complete
conversion of xylitol. Using CaO as a promoter, Zhou et al. [8]
reported 85.7% sorbitol conversion and 60.8% combined se-
lectivity to ethylene glycol, propylene glycol, and glycerol on
carbon nanofiber-supported Ru catalyst at 493 K under 8.0
MPa H2. Alternatively, Ye et al. [9] obtained 55%–60% selectiv-
ity to ethylene glycol and propylene glycol with >90% sorbitol
conversion on Ce-promoted Ni/Al2O3 catalysts with a small
amount of Ca(OH)2. Chen et al. [10] carried out sorbitol hydro-
genolysis on a co-precipitated Ni-MgO catalyst, and observed
80.8% combined selectivity to ethylene glycol, propylene gly-
col, and glycerol at 67.8% sorbitol conversion at 473 K under
4.0 MPa H2. Sun et al. [11] found that Ni-CeO2/C and Ni-CaO/C
were efficient for xylitol hydrogenolysis, and achieved 69.5%
combined selectivity to ethylene glycol, propylene glycol, and
glycerol at nearly 100% xylitol conversion on Ni-CaO/C with-
out additional bases. These studies show that the Ru- and
Ni-based catalysts exhibit good activities and selectivities to the
two target glycols from sorbitol and xylitol hydrogenolysis
[7–12,15,17]. However, these materials also catalyze cleavage
of C–C bonds to form undesirable by-products, especially me-
thane [7,9], as also reported for glycerol hydrogenolysis
[20,21].
For this purpose, a variety of oxides including Al2O3 [22],
Ga2O3 [25], and B2O3 [23] have been employed to stabilize
copper particles in glycerol hydrogenolysis. However, these
catalysts still undergo deactivation over a prolonged reaction
time. Recently, Kim et al. [26] found that addition of Pd en-
hances the activity of CuCr2O4 in glycerol hydrogenolysis via
hydrogen spillover. Jin et al. [27] reported
a
gra-
phene-supported Cu-Pd catalyst with excellent recyclability in
glycerol conversion to lactic acid. These reports show the po-
tential of the noble metals to increase the activity and stability
of Cu catalysts.
In the present work, we studied Cu/monoclinic zirconia
(Cu/ZrO2) catalysts for sorbitol selective hydrogenolysis in the
presence of La(OH)3. The effect of noble metals including Pd, Pt,
and Ru on the catalytic property of the Cu particles was exam-
ined, and Pd-Cu/ZrO2 exhibited high activities and selectivities
with enhanced reusability. The promoting effect of Pd on the Cu
catalysts is discussed on the basis of the complementary char-
acterization by X-ray diffraction (XRD), diffuse-reflectance in-
frared Fourier transform spectroscopy of CO adsorption
(CO-DRIFT), and H2 temperature-programmed reduction
(H2-TPR).
2. Experimental
Different from the aforementioned Ru and Ni catalysts, Cu is
less active toward C–C bond cleavage in polyol hydrogenolysis
[13,14,22,23]. This promising feature of Cu has been confirmed
by Huang et al. [13,14], who recently studied xylitol hydrogen-
olysis on Cu-SiO2 catalysts. They observed close to 100% con-
version and 70.4% combined selectivity to ethylene glycol and
propylene glycol at 473 K under 6.0 MPa H2 in the presence of
Ca(OH)2. However, sintering of Cu particles and deactivation
occurred during xylitol hydrogenolysis, even in the presence of
Ca(OH)2 [14], which can normally prevent the agglomeration
and leaching of non-noble metal particles [8,11,24]. Moreover,
Cu-based catalysts generally exhibit activities that are inferior
to those of Ru and Ni catalysts. Therefore, it is of great im-
portance to design Cu catalysts that show improved efficiency
and high stability under the hydrothermal conditions of polyol
2.1. Catalyst preparation
ZrO2-supported Pd, Ru, and Pt catalysts were prepared by
incipient wetness impregnation of monoclinic ZrO2 (Alfa Aesar)
with aqueous solutions of [Pd(NH3)4](NO3)2 (Sigma-Aldrich, St.
Louis MO, United States), RuCl3·nH2O (GR, Sinopharm Chemical,
Beijing, China), and H2PtCl6·nH2O (GR, Sinopharm Chemical),
respectively. The impregnated samples were dried at room
temperature for 12 h and then at 393 K overnight. Afterward,
the samples were calcined in flowing air at 673 K for 3 h. The
loading of the supported noble metal was 1 wt%.
Cu/ZrO2 and bimetallic M-Cu/ZrO2 (M=Pd, Ru, or Pt) cata-
lysts were prepared by an impregnation method, similar to that
described above for ZrO2-supported noble metals. The ZrO2
support and the calcined M/ZrO2 samples were impregnated
Haichao Liu (College of Chemistry and Molecular Engineering, Peking University) received the
Catalysis Award for Young Scientists from The Catalysis Society of China in 2012. Professor
Haichao Liu received his B.S. in 1990 and M.S. degree in 1993 from Sichuan University, and his
Ph.D degree in 1996 from Research Institute of Petroleum Processing. From 1997 to 2003, he did
postdoctoral research at The University of Tokyo and The University of California at Berkeley. At
the end of 2003, he joined the faculty of Peking University, where he is now Changjiang Distin-
guished Professor of Chemistry. His research interests currently focus on molecular catalysis and
sustainable energy chemistry with emphasis on design of new catalysts and control of reaction
pathways for selective conversion of biomass and its derivatives to fuels and especially chemicals,
etc. Some of his recent progresses include the novel approaches to catalytic deploymerization of
cellulose to polyols, the controlled conversion of cellulose to propylene glycol and acetol, and the
selective synthesis of important chemicals or intermediates exemplified by lactic acid and acetic
anhydride respectively from aerobic oxidation of glycerol and ethanol. He has coauthored about
100 peer-reviewed papers and 30 patents.

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Yuqing Jia et al. / Chinese Journal of Catalysis 36 (2015) 1552–1559
with aqueous solutions of Cu(CH3COO)2·H2O (GR, Sinopharm
Chemical). The impregnated samples were dried at ambient
temperature until most of the water was evaporated and then
at 393 K overnight. Afterward, the samples were calcined in
flowing air at 623 K for 2 h, and then reduced in a 20% H2/N2
(Beijing Haipu) flow at 673 K for 2 h. The resulting Cu/ZrO2 and
M-Cu/ZrO2 catalysts were denoted as xCu/ZrO2 and
1M-xCu/ZrO2, respectively, where x represents a Cu loading of x
wt% in the catalyst and 1 M indicates 1 wt% noble metal in the
catalyst. Pd-Cu/ZrO2 catalysts with different Cu/Pd atomic ra-
tios of 1.5, 3.0, 5.0, 8.0, and 10.0 were prepared by adjusting the
amount of Cu(CH3COO)2 in the impregnation solutions while
the Pd loading was kept constant at 1 wt%.
subtracting the background spectrum of each sample.
2.3. Sorbitol hydrogenolysis reaction
Sorbitol hydrogenolysis reactions were carried out in a
stainless steel autoclave (100 mL) with vigorous stirring at 750
r/min to avoid the mass-transfer effect. Typically, 50 g of 10
wt% sorbitol (98%, Alfa Aesar) aqueous solution, the specified
amount of Cu catalyst, and 0.2 g of La(OH)3 (AR, Sinopharm
Chemical) were loaded into the autoclave. The reaction was
maintained at 493 K under 5.0 MPa H2 for 4 h. The catalyst and
liquid phase were separated by filtration after the reaction. The
reactants and products in the liquid phase were analyzed by
high-performance liquid chromatography (Shimadzu LC-20A)
using Bio-Rad Aminex HPX-87H and HPX-87C columns con-
nected to refractive index detectors. Gas products, mainly me-
thane, were analyzed by gas chromatography (Shimadzu 2010,
HJ-OV-101 column) with flame ionization detection. Activities
in the sorbitol reaction were calculated by the molar sorbitol
conversion per mole of metal loaded per hour (h−1), and prod-
uct selectivities were reported on a carbon basis.
2.2. Catalyst characterization
XRD patterns of the catalysts were collected on a Rigaku
D/MAX-2400 diffractometer with Cu Kα radiation (λ = 0.15406
nm) in the range of 20°–60° at a scanning rate of 2°/min. The
operation voltage and current were 40 kV and 100 mA.
H2-TPR experiments were conducted on a TP-5080 flow
unit (Tianjin Xianquan). A catalyst portion of about 50 mg (25
mg for 1Pd-6Cu/ZrO2) was placed in a quartz cell, and heated
from 295 to 1073 K at 10 K/min in a 5% H2/N2 (Beijing Haipu,
certified mixture) flow (30 mL/min). H2 consumption was
quantified by thermal conductivity detector signals, calibrated
by reduction of CuO powder under identical conditions.
3. Results and discussion
3.1. Activity and selectivity of bimetallic Pd-Cu catalysts in sor-
bitol hydrogenolysis
CO-DRIFTS experiments were carried out on a Bruker Ten-
sor 27 spectrometer equipped with liquid nitrogen-cooled
HgCdTe detector and a diffuse-reflectance reaction chamber
with CaF2 windows. The spectral resolution was 4 cm−1 and
100 scans were applied for each measurement. The
pre-reduced catalyst was added to the chamber and treated in
situ in a 5% H2/N2 flow at 673 K for 1 h in case of any surface
oxidation. The sample was then flushed with N2 for 30 min at
493 K before cooling to room temperature under N2 flow. The
background spectrum of the sample was recorded in N2 at
room temperature. Afterward, the sample was exposed to a 5%
CO/He (Beijing Haipu, certified mixture) flow (20 mL/min) for
30 min. The chamber was then purged with N2 for 20 min be-
fore the spectrum was recorded. All spectra were obtained by
Table 1 shows the activities (normalized to total metal at-
oms) and selectivities for sorbitol hydrogenolysis on monome-
tallic Cu/ZrO2 and Pd/ZrO2 and bimetallic Pd-Cu/ZrO2,
Pt-Cu/ZrO2, and Ru-Cu/ZrO2 catalysts in the presence of
La(OH)3 at 493 K under 5.0 MPa H2. 1Pd-3Cu/ZrO2 exhibited an
activity of 20.3 h−1, which was 2–3 times greater than those of
monometallic 3Cu/ZrO2 (6.5 h−1) and 1Pd/ZrO2 (8.7 h−1) in the
kinetic controlled regime. The lower activity of 1Pd/ZrO2 is
consistent with the finding in glycerol hydrogenolysis by Xia et
al. [28] that monometallic Pd0.04/Mg5.56Al2O8.56 is less active
than bimetallic Pd0.04Cu0.4/Mg5.56Al2O8.56. 1Pd-3Cu/ZrO2 was
also more selective for the target glycols (ethylene glycol and
propylene glycol) than the two monometallic catalysts. Includ-
ing glycerol, the combined selectivity to the three polyol prod-
Table 1
Activity and product selectivity in sorbitol hydrogenolysis on monometallic Cu/ZrO2 and Pd/ZrO2 catalysts and various bimetallic Cu-based catalysts.
Selectivity a (carbon basis, %)
Catalyst
Activity (h−1)
Ethylene glycol Propylene glycol
Glycerol
8.4
Lactic acid C4 polyols
C5 polyols C6 polyols
1Pd-3Cu/ZrO2
3Cu/ZrO2
1Pd/ZrO2
1Pt-3Cu/ZrO2
1Ru-3Cu/ZrO2
1Pd/ZrO2 + 3Cu/ZrO2
20.3
6.5
8.7 c
10.7
7.2
16.3
14.1
12.6
16.3
21.1
14.2
36.8
7.9
16.7
4.9
6.0
6.2
6.9
5.8
5.0
6.6
1.1
3.1
9.6
1.7
2.4
1.6
7.6
13.0
9.4
8.1
9.7
26.2(4.2) b
4.7
2.4
3.3
4.4
23.8
35.7
31.0
34.9
7.8
10.3
9.0
12.2 d(6.9) e
7.0
10.4
Reaction conditions: 493 K, H2 5.0 MPa, 10 wt% sorbitol aqueous solution 50 g, catalyst 0.2 g, La(OH)3 0.2 g, 4 h.
a C4 polyols including 1,2-butanediol, erythritol, and threitol; C5 polyols including arabitol and xylitol; C6 polyols including mannitol and iditol.
b Data in parenthesis represents acetol selectivity.
c 1.0 g catalyst was used.
d Physical mixture of 0.2 g 1Pd/ZrO2 and 0.2 g 3Cu/ZrO2.
e Data in parenthesis represents a theoretical activity of the mixture estimated from the weighted average activity based on its two individual com-
ponents tested separately.

Yuqing Jia et al. / Chinese Journal of Catalysis 36 (2015) 1552–1559
1555
ucts was 61.5% on 1Pd-3Cu/ZrO2, which was much higher than
those on 3Cu/ZrO2 and 1Pd/ZrO2, being 45.0% and 38.8%,
respectively. Other products included lactic acid, sorbitol epi-
mers (mannitol and iditol) as well as insignificant C5 and C4
polyols (arabitol, xylitol, 1,2-butanediol, erythritol, and
threitol). Notably, no methane was detected in the gas phase of
these reactions. Using 3Cu/ZrO2, acetol (4.2%) was also de-
tected together with a high selectivity to lactic acid (16.7%),
reflecting the inferior hydrogenation activity of Cu particles
[13,14]. The selectivity to lactic acid decreased significantly to
7.9%, and no acetol was observed on 1Pd-3Cu/ZrO2. Such
change in the selectivities shows that the addition of Pd im-
proved the hydrogenation activity of the Cu catalyst, most likely
because of its excellent H2 activation ability. Consequently, the
formation of propylene glycol was favored from the glycer-
aldehyde-derived intermediate, which would otherwise un-
dergo metal-catalyzed partial hydrogenation to acetol or
base-catalyzed benzilic acid rearrangement to lactic acid [7].
For comparison, the effects of Ru and Pt on the Cu catalyst
were also examined under identical conditions (Table 1).
However, the activity only increased slightly from 6.5 h−1 on
3Cu/ZrO2 to 7.2 h−1 on 1Ru-3Cu/ZrO2 and 10.7 h−1 on
1Pt-3Cu/ZrO2, which are much lower than that observed for
1Pd-3Cu/ZrO2. The combined selectivity to ethylene glycol,
propylene glycol, and glycerol was 56.5% and 55.3%, respec-
tively, on 1Ru-3Cu/ZrO2 and 1Pt-3Cu/ZrO2, which were also
lower than that on 1Pd-3Cu/ZrO2. Clearly, Pd is superior to Ru
and Pt for promoting the activity and selectivity of the Cu cata-
lyst in sorbitol hydrogenolysis. Although the reason for the
different effects of the three noble metals needs to be clarified,
it may be related to their different interactions with Cu. The
interaction of Pd with Cu is confirmed by the results with a
physical mixture of 1Pd/ZrO2 and 3Cu/ZrO2 (Table 1). The
activity of this physical mixture was 12.2 h−1, which is much
lower than that on 1Pd-3Cu/ZrO2 (20.3 h−1), implying that the
promoting effect of Pd is not just a result of the coexistence of
Cu and Pd, but requires interaction of the two metals.
and 20.3 h−1, irrespective of the Cu/Pd ratios, at 493 K under
5.0 MPa H2. Similarly, with Cu/Pd ratio increasing from 1.5 to
5.0, the combined selectivities to ethylene glycol, propylene
glycol, and glycerol remained essentially constant (around
61.5%–62.8%) and was still as high as 57.3% at the high Cu/Pd
ratio of 10.0. Moreover, these Pd-Cu/ZrO2 catalysts retained
such high selectivities at high sorbitol conversions. For exam-
ple, a 61.7% combined selectivity to ethylene glycol, propylene
glycol, and glycerol was achieved at ~100% conversion on
1Pd-3Cu/ZrO2 at 493 K under 5.0 MPa H2.
For the Cu-based catalysts, the ease with which the Cu parti-
cles aggregate under hydrothermal conditions to deactivate the
catalyst is a significant problem. Thus, the recyclability of the
Pd-Cu/ZrO2catalysts was examined. Fig. 1(a) shows repre-
sentative results for 1Pd-3Cu/ZrO2 in the kinetic controlled
regime of sorbitol hydrogenolysis at 493 K under 5.0 MPa H2.
During the recycling experiments, the used catalyst was col-
lected by simple filtration and washed thoroughly with deion-
ized water. No significant decline in the sorbitol conversion
(maintained at ~34%) was observed after five successive cy-
cles. The selectivities to ethylene glycol, propylene glycol, and
lactic acid decreased slightly from 16.3%, 36.8%, and 7.9% in
the first run, to 14.5%, 35.4%, and 3.8% in the second run, re-
spectively. Concurrently, the selectivity to glycerol increased
slightly from 8.4% to 12.6%. Then the selectivities remained
essentially unchanged in the following cycles. These results
demonstrate that 1Pd-3Cu/ZrO2 is stable and reusable under
the reaction conditions employed in this work. For comparison,
the reusability of 3Cu/ZrO2 was also tested under identical
conditions. As shown in Fig. 1(b), the selectivities to ethylene
glycol, propylene glycol, glycerol and lactic acid remained es-
sentially unaltered on Cu/ZrO2 for the five successive cycles.
However, the sorbitol conversions decreased sharply from
28.7% in the first run to 20.4% in the second run, and continu-
ously to 16.5% in the fifth run, showing the instability of the
monometallic Cu/ZrO2 catalyst under hydrothermal conditions,
even in the presence of La(OH)3 base. Taken together, these
results demonstrate that the introduction of Pd into Cu/ZrO2
improves not only the activity and selectivity of the Cu parti-
cles, but also their stability, most likely as a result of the strong
interaction of Pd with Cu and the consequent effect on the
structural and electronic properties of the Cu particles.
Notably, the promoting effect of Pd was observed over a
wide range of Cu/Pd atomic ratios of 1.5, 3.0, 5.0, 8.0, and 10.0
on Pd-Cu/ZrO2. As shown in Table 2, compared at ~30% sor-
bitol conversion in the kinetic controlled regime, the activities
(normalized to total metal atoms) varied slightly between 17.8
Table 2
Activity and product selectivity in sorbitol hydrogenolysis on bimetallic Pd-Cu/ZrO2 catalysts with different Cu/Pd ratios.
Selectivity a (carbon basis, %)
Propylene glycol Glycerol Lactic acid C4 polyols
Cu/Pd
ratio
Catalyst
Activity (h−1)
Ethylene glycol
16.1
C5 polyols
1.3
C6 polyols
6.0
1Pd-0.9Cu/ZrO2
1Pd-1.8Cu/ZrO2
1Pd-3.0Cu/ZrO2
1Pd-4.8Cu/ZrO2
1Pd-6.0Cu/ZrO2
1Pd-3.0Cu/ZrO2
3/2
3/1
5/1
8/1
10/1
5/1
17.8
20.3
20.3
20.0
18.6
38.3
37.8
36.8
36.2
33.4
37.0
7.4
9.1
8.4
8.0
9.2
8.8
4.2
4.9
7.9
8.0
10.0
3.6
6.3
6.8
6.0
6.3
6.7
6.4
15.9
16.3
15.5
14.7
1.0
6.5
1.1
7.6
0.8
7.9
1.3
0.3
10.0
2.0
b
—
15.9
Reaction conditions: 493 K, H2 5.0 MPa, 10 wt% sorbitol aqueous solution 50 g, La(OH)3 0.2 g, 4 h, ca. 30% sorbitol conversion obtained by varying
catalyst amount.
a C4 polyols including 1,2-butanediol, erythritol, and threitol; C5 polyols including arabitol and xylitol; C6 polyols including mannitol and iditol.
b At ~100% sorbitol conversion.

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Yuqing Jia et al. / Chinese Journal of Catalysis 36 (2015) 1552–1559
40
35
30
25
20
15
10
5
40
30
25
20
15
10
5
35
30
25
20
15
10
5
(a)
(b)
35
30
25
20
15
10
5
PG
PG
EG
EG
Gly
LA
Gly
LA
0
0
0
0
1
2
3
4
5
1
2
3
4
5
Number of cycles
Number of cycles
Fig. 1. Sorbitol conversions and selectivities to ethylene glycol (EG), propylene glycol (PG), glycerol (Gly), and lactic acid (LA) in five successive reac-
tion cycles of sorbitol hydrogenolysis on 1Pd-3Cu/ZrO2 catalyst (a) and 3Cu/ZrO2 catalyst (b). Reaction conditions: 493 K, 5.0 MPa H2, 10 wt% sorbi-
tol aqueous solution 50 g, 1Pd-3Cu/ZrO2 0.2 g or 3Cu/ZrO2 0.6 g, La(OH)3 0.2 g in the first run and La(OH)3 0.1 g in runs 2–5, 4 h.
alysts, especially their Cu particles, La(OH)3 base was replaced
3.2. Interaction between Cu and Pd in Pd-Cu/ZrO2 catalysts
with MgO-Al2O3 (Mg/Al atomic ratio=3/1) in sorbitol hydro-
genolysis. This allowed a clear spectral window to observe the
peak of Cu at around 2θ = 43.4°, which would otherwise be
obscured by the intense peaks (2θ = 43.0°, 43.9°) of lanthanum
carbonate hydroxide (LaCO3OH), formed by reaction of
La(OH)3 with the CO2 formed in polyol hydrogenolysis [7,14].
As presented in Fig. 3, a sharp peak appeared at 2θ = 43.4° for
the used 3Cu/ZrO2 catalyst, showing agglomeration of the Cu
particles during sorbitol hydrogenolysis under the hydrother-
mal conditions applied in this work. The Cu particle size was
estimated to be 30 nm by the Scherrer equation. This phenom-
enon is in line with previous findings [22,23]. Such a dramatic
increase in the size of Cu particles after the reaction can explain
the sharp decrease in the activity of 3Cu/ZrO2 during the recy-
cling experiments (Fig. 1(b)). For the representative
1Pd-3Cu/ZrO2 catalyst, however, no characteristic Cu peak was
detected after the hydrogenolysis reaction. Such stability of the
Cu particles suggests a strong interaction between Pd and Cu,
which efficiently prevents the agglomeration of Cu particles
under the hydrothermal conditions, and consequently leads to
Figure 2 shows the XRD patterns of 1Pd/ZrO2, Cu/ZrO2, and
Pd-Cu/ZrO2 catalysts after reduction in H2 at 673 K in the 2θ
range of 35°–50°. The characteristic diffraction peaks of the
ZrO2 support were present for all the samples. No peaks corre-
sponding to Pd were detected for the 1Pd/ZrO2 and
Pd-Cu/ZrO2 catalysts, suggesting that the Pd particles were
highly dispersed on ZrO2 with such a low Pd loading of 1 wt%
used in this work. Similarly, no characteristic peaks of metallic
Cu were observed on fresh 3Cu/ZrO2 and Pd-Cu/ZrO2 catalysts,
even with a high Cu/Pd atomic ratio of 10.0, implying high dis-
persion of Cu on ZrO2 for these fresh monometallic and bime-
tallic catalysts. For catalysts with higher Cu content, a weak
diffraction signal for Cu (2θ = 43.4°) was detected for mono-
metallic 6Cu/ZrO2 catalyst, but not for 1Pd-6Cu/ZrO2. This re-
sult suggests that the addition of Pd can improve the dispersion
of Cu particles on ZrO2, especially for Cu catalysts with high Cu
loadings.
To examine the stability of the Cu/ZrO2 and Pd-Cu/ZrO2 cat-
 Cu0
m-ZrO2

 Cu0
 m-ZrO2

(8)
(7)

(3)
(2)
(1)
(6)
(5)
(4)
(3)
(2)
(1)

35
40
45
50
35
40
45
50
2/(o )
2/(o )
Fig. 3. XRD patterns of 3Cu/ZrO2 (1), 1Pd-3Cu/ZrO2 (2), a physical
mixture of 1Pd/ZrO2 and 3Cu/ZrO2 (3) after sorbitol hydrogenolysis
reaction at 493 K under 5.0 MPa H2.
Fig. 2. XRD patterns of freshly reduced samples. (1) 1Pd/ZrO2; (2)
1Pd-0.9Cu/ZrO2; (3) 1Pd-1.8Cu/ZrO2; (4) 1Pd-3Cu/ZrO2; (5)
1Pd-4.8Cu/ZrO2; (6) 1Pd-6Cu/ZrO2; (7) 3Cu/ZrO2; (8) 6Cu/ZrO2.

Yuqing Jia et al. / Chinese Journal of Catalysis 36 (2015) 1552–1559
1557
the observed stable sorbitol conversions on 1Pd-3Cu/ZrO2
during the five successive cycles (Fig. 1(a)).
closely interacts with the ZrO2 support [32]. The peak at ~563
K corresponds to the reduction of ZrO2 [33]. Such improved
reduction of ZrO2 may be caused by the promoting effect of PdO
on the mobility of the lattice oxygen on the ZrO2 surface, or
because of hydrogen spillover from Pd to ZrO2, as generally
found on other ZrO2-supported noble metal catalysts [33,34].
For 3Cu/ZrO2, two major reduction peaks were observed at
around 423 and 435 K. The first peak was assigned to the high-
ly dispersed CuO phase, which closely interacts with ZrO2,
whereas the second peak was ascribed to the reduction of CuO,
which has weaker interaction with ZrO2 [35]. Compared with
1Pd/ZrO2 and 3Cu/ZrO2, only a broader single peak appeared
at 384 K for 1Pd-3Cu/ZrO2, which increased from 376 to 415 K
as the Cu loading increased from 1.8 to 6.0 wt% (i.e., the Cu/Pd
atomic ratio from 3.0 to 10.0). Apparently, the presence of Pd
enhanced the reducibility of the Cu species, which is indicative
of the close contact and strong interaction between Cu and Pd
in the Pd-Cu catalysts.
The reduction features of the Pd-Cu catalysts could be also
associated with the effect of H2 spillover, which is derived from
the dissociation of H2 to H atoms on Pd, and their subsequent
spillover to the Cu surfaces [29,36,37]. To examine such an
effect, the H2-TPR profile for the 1:1 (wt/wt) physical mixture
of 1Pd/ZrO2 and 3Cu/ZrO2 was collected. As shown in Fig. 5,
this sample presents two peaks at 378 and 432 K, correspond-
ing to the reduction of PdO and CuO species, respectively. The
presence of Pd indeed improved the reducibility of the Cu spe-
cies as a result of the H2 spillover. However, the reduction fea-
ture is different from those for bimetallic Pd-Cu catalysts,
which further confirms the strong interaction between Pd and
Cu in Pd-Cu/ZrO2 catalysts. Such a difference can account for
the observed difference in their activities. As shown in Table 1,
the physical mixture of 1Pd/ZrO2 (0.2g) and 3Cu/ZrO2 (0.2g)
had an activity of 12.2 h−1, obviously higher than the weighted
average activity (6.9 h−1), which is derived from the activities
for the two individual components (0.2 g 1Pd/ZrO2 and 0.2 g
3Cu/ZrO2) tested separately. Such enhanced activity of the
physical mixture might be a result of Cu particle growth being
inhibited by the addition of 1Pd/ZrO2 via hydrogen spillover
Figure 4 shows the DRIFT spectra of adsorbed CO on the
monometallic Pd and Cu catalysts and the bimetallic Pd-Cu
catalysts after treatment in H2 at 673 K. For the monometallic
1Pd/ZrO2 catalyst, a band at 2079 cm−1 was assigned to the
linear adsorption of CO on the Pd surfaces. Other spectral fea-
tures at 1800–2000 cm−1, the broad band at around 1958 cm−1
,
and a shoulder at 1855 cm−1 were attributed respectively to
bridging and multi-bonded CO species on Pd [29]. These bands
were dramatically attenuated for 1Pd-1.8Cu/ZrO2, and only an
extremely weak band of linearly adsorbed CO on the Pd sites
was observed for 1Pd-6Cu/ZrO2 with a high Cu/Pd ratio of
10.0. Such phenomena, especially the decrease in the intensi-
ties of the bridging and multi-bonded CO bands on the Pd sites
with increasing Cu/Pd ratios, are consistent with previous
findings for bimetallic Pd-Cu catalysts [29,30], and are likely
the result of dilution or coverage of the Pd ensembles with the
Cu sites. Concurrent with such change, a new intense band ap-
peared at around 2106 cm−1 for the bimetallic catalysts, resem-
bling the single peak observed for monometallic 3Cu/ZrO2 cat-
alyst in terms of both intensity and shape. A band in the range
of 2100–2110 cm−1 has been previously assigned to the linear
adsorption of CO on metallic Cu0 sites on Cu/ZrO2 catalysts
[31]. Therefore, this intense band can be also tentatively as-
signed to linearly adsorbed CO species on the metallic Cu0 sites
of Pd-Cu/ZrO2. Based on these results, we envisaged that Cu
was deposited on the Pd particles for the Pd-Cu/ZrO2 catalysts,
and most of the Pd surfaces were covered with Cu at the high
Cu/Pd ratios, such as 1Pd-6Cu/ZrO2. Taken together with the
observed stability of the Pd-Cu catalysts and their superior
activity in sorbitol hydrogenolysis, such close contact between
Cu and Pd particles should lead to strong interaction between
them for these Pd-Cu catalysts, as indeed evidenced from their
H2-TPR results (see below).
Figure 5 shows the H2-TPR profiles for monometallic
Pd/ZrO2 and Cu/ZrO2 and bimetallic Pd-Cu/ZrO2 after treat-
ment in air at 623 K. For 1Pd/ZrO2, the main peak centered at
around 374 K was assigned to the reduction of PdO, which
2106
(5)
(4)
(6)
(5)
(4)
(3)
(3)
(2)
(2)
(1)
1958
(1)
1855
2079
300
400
500
600
700
2300
2200
2100
2000
1900
1800
Wavenumber (cm1)
Temperature (K)
Fig. 4. DRIFT spectra of CO adsorption on 1Pd/ZrO2 (1),
1Pd-1.8Cu/ZrO2 (2), 1Pd-3Cu/ZrO2 (3), 1Pd-6Cu/ZrO2 (4) and
3Cu/ZrO2 (5).
Fig. 5. H2-TPR profiles of 1Pd/ZrO2 (1), 1Pd-1.8Cu/ZrO2 (2),
1Pd-3Cu/ZrO2 (3), 1Pd-6Cu/ZrO2 (4), the 1:1 (wt/wt) physical mixture
of 1Pd/ZrO2 and 3Cu/ZrO2 (5) and 3Cu/ZrO2(6).

1558
Yuqing Jia et al. / Chinese Journal of Catalysis 36 (2015) 1552–1559
during sorbitol hydrogenolysis. This physical mixture also
possessed higher stability, as confirmed by its XRD pattern,
which showed no agglomeration of the Cu particles after sor-
bitol hydrogenolysis (Fig. 3). The combined selectivity of the
physical mixture to ethylene glycol, propylene glycol, and glyc-
erol was 56.1%, which is higher than that for 1Pd/ZrO2
(38.8%) and 3Cu/ZrO2 (45.0%). Furthermore, compared with
3Cu/ZrO2, the physical mixture showed a lower selectivity to
lactic acid (16.7% vs 9.0%) and no detectable formation of
acetol. These results show the physical mixture of 1Pd/ZrO2
and 3Cu/ZrO2 possessed higher hydrogenation activity than its
two individual components as a result of hydrogen spillover.
This is in accordance with the finding for hydrogenation of
nitrate using a physical mixture of Pd/C and Cu/C catalysts
[38]. However, as discussed in Section 3.1, the physical mixture
is much less efficient than the 1Pd-3Cu/ZrO2 catalyst in terms
of activity and selectivity in sorbitol hydrogenolysis with the
same amount of Pd and Cu.
Clearly, these results reveal that the observed superior ac-
tivity, selectivity, and stability of the bimetallic Pd-Cu catalysts
are caused by close contact between Pd and Cu particles and
their strong interaction, which involves electron transfer from
Pd to Cu [30,36]. This electronic effect consequently facilitates
sorbitol dehydrogenation, which is the rate-determining step in
the hydrogenolysis reaction, and subsequent hydrogenation
steps toward the formation of the target glycols on the Cu sur-
face [7].
of the Pd-Cu catalysts is considered to be directly related to the
close contact and strong interaction between the two metals,
most likely involving electron transfer from Pd to Cu. These
findings may be beneficial to the design of new Cu-based cata-
lysts with high efficiency and hydrothermal stability for differ-
ent reactions of polyols and other biomass-derived feedstocks.
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Graphical Abstract
Chin. J. Catal., 2015, 36: 1552–1559 doi: 10.1016/S1872-2067(15)60892-0
Selective hydrogenolysis of sorbitol to ethylene
22
glycol and propylene glycol on ZrO2-supported
bimetallic Pd-Cu catalysts
OH
20
18
Pd-Cu/m-ZrO2
HO
OH
HO
Yuqing Jia, Haichao Liu*
Peking University
6
4
2
The presence of Pd largely improves the activity and
stability of Cu/ZrO2 catalyst in selective sorbitol hy-
drogenolysis to ethylene glycol and propylene glycol
as a result of strong interaction between Pd and Cu.
Cu
Pd
m-ZrO2
Cu/m-ZrO2
1
2
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5
Number of cycles

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