Tuning catalytic selectivity of liquid-phase hydrogenation of furfural via synergistic effects of supported bimetallic catalysts

Article abstract of DOI:10.1016/j.apcata.2015.05.006

Bimetallic catalysts supported over TiO₂-ZrO₂ binary oxides were prepared by co-impregnation methods and used for catalyzing liquid-phase hydrogenation of furfural. Highly selective hydrogenation catalysts can be developed based on bimetallic synergistic effect. The coexistence of small proportion of palladium with supported nickel species greatly improves the catalytic performance and transfer the reaction selectivity from partial hydrogenation to total hydrogenation. The catalyst with Ni-Pd mole ratio of 5:1 shows the best performance. The yield of tetrahydrofurfuryl alcohol (THFA) reaches 93.4%. Ni-Pd synergistic effect is interpreted through XPS measurement and a hydrogen-transfer mechanism is proposed. Pt-Re bimetallic catalyst is an excellent partial hydrogenation catalyst for furfural conversion. Furfural can be totally converted and the selectivity of partial hydrogenation product (FA) reaches 95.7%. When rhenium oxide species are located on the Pt surface, the hydrogen species on Pt are transferred to adsorbed C=O bond to achieve selective hydrogenation.

Full text of DOI:10.1016/j.apcata.2015.05.006

Applied Catalysis A: General 500 (2015) 23–29
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Tuning catalytic selectivity of liquid-phase hydrogenation of furfural
via synergistic effects of supported bimetallic catalysts
Bingfeng Chen, Fengbo Li∗, Zhijun Huang, Guoqing Yuan∗
Beijing National Laboratory of Molecular Science, Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190,
PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Bimetallic catalysts supported over TiO2–ZrO2 binary oxides were prepared by co-impregnation methods
and used for catalyzing liquid-phase hydrogenation of furfural. Highly selective hydrogenation catalysts
can be developed based on bimetallic synergistic effect. The coexistence of small proportion of palladium
with supported nickel species greatly improves the catalytic performance and transfer the reaction selec-
tivity from partial hydrogenation to total hydrogenation. The catalyst with Ni–Pd mole ratio of 5:1 shows
the best performance. The yield of tetrahydrofurfuryl alcohol (THFA) reaches 93.4%. Ni–Pd synergistic
effect is interpreted through XPS measurement and a hydrogen-transfer mechanism is proposed. Pt–Re
bimetallic catalyst is an excellent partial hydrogenation catalyst for furfural conversion. Furfural can be
totally converted and the selectivity of partial hydrogenation product (FA) reaches 95.7%. When rhenium
Received 7 January 2015
Received in revised form 28 March 2015
Accepted 9 May 2015
Available online 16 May 2015
Keywords:
Bimetallic catalysts
Synergistic effect
Hydrogenation
Furfural
oxide species are located on the Pt surface, the hydrogen species on Pt are transferred to adsorbed C
bond to achieve selective hydrogenation.
O
Catalytic selectivity
© 2015 Published by Elsevier B.V.
1. Introduction
is how to control its reaction route and hydrogenation degree
through using highly selective catalyst.
Limited fossil fuel resources and global warming issues stim-
ulate great research efforts in sustainable production of fuel and
chemicals from renewable biomass in recent decades. Furfural
from acidic hydrolysis of hemicellulose is the most promising
biomass platform chemical that can be manufactured through
large-scale industrial process [1]. Through catalytic hydrogenation,
furfural can be converted to furfuryl alcohol (FA) and tetrahy-
drofurfuryl alcohol (THFA). Furfuryl alcohol is widely used to
manufacture foundry resins, synthetic fiber, farm chemicals, adhe-
sives and fine chemicals such as vitamin and lysine [2–5]. THFA
is considered as a green solvent and used in agricultural appli-
cations, printing inks, and industrial and electronics cleaners [6].
Furfural molecule has several active functional groups. There are
many possible conversion routes during catalytic hydrogenation,
including partial hydrogenation to FA, total hydrogenation to THFA
and various possible side reactions such as heat-driven oligomer-
ization, hydrogenolysis of side-substituents to 2-methylfuran,
decarbonylation to furan and rearrangement to levulinic acid, and
furan ring opening [7]. The core issue of furfural hydrogenation
The performance of heterogeneous hydrogenation catalysts is
directly related to active metals and supports [8]. Supported noble
and non-noble metals have been reported for hydrogenation of
FR. Conventional production of THFA is based on a two-step strat-
egy: hydrogenation of furfural FA over Cu–Cr catalyst and further
hydrogenation of FA to THFA over supported noble catalysts.
Direct hydrogenation of furfural to THFA has been achieved over
Ni–Pd/SiO2 [9], Ni/SiO2 [10] and Pd/MFI [11] catalysts. The catalysts
for hydrogenation of furfural to FA are supported Ni, Co, Cu, Ru, Pd
and their bimetallic catalysts [2,12,13]. Pt-based catalysts are rarely
used for producing FA from furfural due to complex side reactions
(hydrogenolysis of the C O bond, decarbonylation, total hydro-
genation and furan ring opening, etc.) [14]. It has not been reported
that the hydrogenation degree can be adjusted through synergis-
tic effects of bimetallic catalytic system. It is of great significant to
achieve both selective hydrogenation and total hydrogenation of
furfural through a well-define bimetallic mechanism.
Physicochemical properties of the catalyst’s support markedly
influence its performance for liquid-phase catalytic hydrogena-
tion of unsaturated aldehydes and ketones [8]. TiO2–ZrO2 mixed
oxides exhibit high surface area, profound surface acid-base prop-
erties, a high thermal stability, and strong mechanical strength [15].
TiO2–ZrO2 mixed oxides are used as catalysts and catalysts sup-
ports for various reactions [16]. TiO2–ZrO2 supported Pt species are
∗
Corresponding authors. Tel.: +86 10 62634920; fax: +86 10 62559373.
E-mail addresses: lifb@iccas.ac.cn (F. Li), yuangq@iccas.ac.cn (G. Yuan).
http://dx.doi.org/10.1016/j.apcata.2015.05.006
0926-860X/© 2015 Published by Elsevier B.V.

24
B. Chen et al. / Applied Catalysis A: General 500 (2015) 23–29
the most selected catalysts for hydrogenation conversion such as
naphthalene hydrogenation [17], selective hydrogenation of fur-
fural [12], and selective hydrogenation of crotonaldehyde [18].
TiO2–ZrO2 mixed oxides are used as supports of bimetallic catalysts
for selective furfural hydrogenation.
In this work, Ni–Pd over TiO2–ZrO2 was selected as model
catalytic system to investigated synergistic effects of bimetallic
catalysts upon selectivity of liquid-phase furfural hydrogenation.
Raney Ni and Pd/C were used as benchmarked catalysts. The
bimetallic catalysts with various ratio of Ni to Pd were screened
and the experimental results showed marked synergistic effects.
The corresponding mechanism was revealed by XPS measurement.
In further investigation, the selectivity of Pt-M/TiO2–ZrO2 bimetal
catalysts (M = Re, Sn, In) for furfural hydrogenation was explored
and a different trend of bimetallic synergistic effect was observed.
2. Experiment
2.1. Materials and chemicals
Titanium butoxide, Zirconium (IV) butoxide solution (80 wt%
in n-butanol), dodecyl amine (DDA), nickel nitrate hexahydrate,
furural, ethanol, toluene, dioxane, and 2-propanol were purchased
from Sinopharm Chemical Reagent. H2PtCl6, KReO4, PdCl2, Pd/C and
Raney Ni were purchased from Strem. SnCl4·5H2O and In (SO3CF3)3
were obtained from Alfa Aesar. All the reagents were used as
received except furfural, which was used after vacuum distillation.
Scheme 1. Illustration of reaction routes of furfural hydrogenation.
were referenced to the C1s line at 284.8 eV from adventitious car-
bon. The Eclipse V2.1 data analysis software supplied by the VG
ESCA-Lab200I-XL instrument manufacturer was used to manipu-
late the acquired spectra. Transmission electron microscopy (TEM)
was performed on a JEOL 2010 TEM equipped with an attachment
for local energy dispersion analysis (EDX). The accelerating voltage
was 200 kV, and the spot size was 1.0 nm.
2.2. Catalyst preparation
TiO2–ZrO2 mixed oxide support (1:1, mole ratio) was prepared
by the sol–gel method reported by our group [19]. All cata-
lysts were prepared by co-impregnation or impregnation of metal
precursors onto TiO2–ZrO2 binary oxide support. The calculated
amount of TiO2–ZrO2 was impregnated with a certain concentra-
tion aqueous solution of metal precursors (such as PdCl2(CH3CN)2,
Ni (NO3)2·6H2O, H2PtCl6, KReO4 SnCl4·5H2O and In (SO3CF3)3) and
kept in an oven at 383 K overnight. Then these solids were calcined
at 723 K in air for 3 h. These catalysts were activated under flowing
diluted hydrogen (H2/Ar: 10%) at 673 K for 3 h before testing.
3. Results and discussions
3.1. Hydrogenation of furfural over Ni–Pd bimetallic catalysts
Scheme 1 shows reaction route map of liquid-phase hydrogena-
tion of furfural in alcohol solvents. There are two target molecules:
partial hydrogenation product (FA) and total hydrogenation prod-
uct (THFA). Cu–Cr mixed oxides are the classic catalysts for partial
hydrogenation of furfural. However, leached toxic Cr species pose
serious environmental issues. Nickel-based catalysts are consid-
ered as potential replacement. The main drawback of nickel
hydrogenation catalyst is that active zero-valent nickel species are
highly sensitive to oxygen. It is difficult to keep nickel catalysts in
their primary activated states. Fresh commercial Raney Ni shows
low catalytic activity and poor selectivity of FA (Entry 1 in Table 1).
Nickel over TiO2–ZrO2 binary oxide has improved catalytic activ-
ity for partial hydrogenation of furfural and the yield of FA reaches
69.2% (Entry 2 in Table 1). The reaction over the supported Pd cat-
alyst mainly produces total hydrogenation product. The yield of
THFA is 62.2% for furfural hydrogenation over commercial Pd/C and
the value increases to 78.6% for palladium over TiO2–ZrO2 binary
oxide (Entry 3, 4 in Table 1).
When nickel and palladium precursors are introduced onto
TiO2–ZrO2 through co-impregnation, Ni–Pd/TiO2–ZrO2 bimetal-
lic catalyst is obtained. The coexistence of nickel and palladium
lead to marked synergistic effects and enhance the catalytic per-
formance significantly (Entry 5–8 in Table 1). Ni–Pd bimetallic
catalysts show higher hydrogenation activity than anyone of the
two mono-metallic catalysts. The yield values of hydrogenation
products are above 90% and the percentage of byproducts is very
low. Though adjusting the mole ratio of nickel to palladium, the cat-
alytic selectivity is finely tuned to only one type of hydrogenation
2.3. Catalytic activity measurements
Catalytic hydrogenation of furfural was performed in a 100 ml
stainless autoclave equipped with a pressure gauge, a magnetic stir-
rer, and an electric temperature controller. The catalyst (200.0 mg),
the solvent (8.5 ml) and furfural (1.5 ml) were introduced into the
reactor. The sealed autoclave was flushed with H2 three times,
pressurized with H2 to 5.0 MPa, and started to stir. After the des-
ignated temperature was reached, the reaction began. At the end
of reaction, the autoclave was cooled to ambient temperature and
slowly depressurized. The conversion and product composition
were analyzed by GC and GC–MS. GC was performed on a GC-
2014 (SHIMADZU) equipped with a high-temperature capillary
column (MXT-1, 30 m, 0.25 mm ID) and a FID detector. GC–MS was
performed on a GCT Premier/Waters instrument equipped with a
capillary column (DB-5MS/J&W Scientific, 30 m, 0.25 mm ID).
2.4. Catalyst characterization
Powder X-ray diffraction (XRD) patterns was measured on a
Rigaku Rotaflex diffractometer equipped with a rotating anode
˚
and a Cu-K␣ radiation source (40 kV, 200 MA; ꢀ = 1.54056 A). XPS
data were obtained with an ESCALab220i-XL electron spectrom-
eter from VG Scientific using 300 W Al-K␣ radiations. The base
pressure was approximately 3 × 10−9 mbar. The binding energies

B. Chen et al. / Applied Catalysis A: General 500 (2015) 23–29
25
Table 1
Screening experimental results of furfral hydrogenation over supported nickel and
palladium catalysts.
Entry
Catalysts
Hydrogenation
Others (%)c
product yield (%)
FA
THFA
1
2
3
4
Raney Nia
Ni/TiO2–ZrO2 (5.0 wt%)
Pd/C (5.0 wt%)
29.0
69.2
14.1
3.8
8.3
26.4
62.2
78.6
23.3
3.13
11.5
16.1
Pd/TiO2–ZrO2 (5.0 wt%)
5
6
7
8
Ni–Pd (7:1)b
Ni–Pd (5:1)
Ni–Pd (3:1)
Ni–Pd (1:1)
39.1
2.1
3.3
59.8
93.4
86.4
84.1
1.1
3.9
10.3
9.7
1.6
a
Raney Ni (11.2 mg; ca. 0.17 mmol Ni).
Ni–Pd/TiO2–ZrO2 (Ni load: 5 wt%, 0.2 g; 0.17 mmol Ni), the value in brackets is
b
mole ratio of nickel to palladium.
c
Acetals and oligomers.
Reaction conditions: furfural (1.5 ml), ethanol (8.5 ml), catalysts (0.2 g), pressure of
hydrogen (5.0 MPa), reaction temperature: 403 K, reaction time: 8 h. FA: furfuryl
alcohol; THFA: tetrahydrofurfuryl alcohol.
product. Addition of small proportion of palladium directly
transfers the catalytic performance of supported nickel from par-
tial hydrogenation catalyst to total hydrogenation catalyst. The
bimetallic catalyst with Ni–Pd mole ratio of 5:1 shows the best
performance. The yield of THFA reaches 93.4% and the percentage
of useless byproducts is only 3.9%. These findings have the practi-
cal significance that supported nickel species can be transformed
into highly active hydrogenation catalysts through being doped
with a small proportion of palladium and the resulting catalysts
even show higher activity than the supported palladium catalysts
with the same metal load. The increase of palladium load of Ni–Pd
bimetallic catalyst leads to gradual decrease of THFA yield.
Fig. 1. (a) Reaction curves on time of furfural hydrogenation over Ni–Pd
(5:1)/TiO2–ZrO2catalyst. (b) Experimental results of furfural hydrogenation over
Ni–Pd (5:1)/TiO2–ZrO2catalyst in various solvents.
Ni–Pd (5:1)/TiO2–ZrO2 is a highly active hydrogenation cata-
lyst. After the reaction runs for 1.5 h, 97.3% conversion of furfural
is achieved and the hydrogenation products include FA (39.8%) and
THFA (38.4%) (Fig. 1a). FA is further hydrogenated to THFA. Solvent
effect also influences catalytic activity and selectivity of liquid-
phase hydrogenation of furfural [20,21]. Fig. 1b shows experimen-
tal results of furfural hydrogenation in various solvent. The reaction
is carried out over Ni–Pd/TiO2–ZrO2 (Ni/Pd = 5) at 403 K and under
5 MPa of H2. The selected solvents include ethanol, 2-propanol,
1,4-dioxane, and toluene. The selectivity of THFA shows the
following sequence: ethanol > 1,4-dioxane > 2-propanol > toluene.
This trend is roughly consistent with the order of sol-
which are embedded in the mixed oxide matrix and give novel
catalytic properties of TiO2–ZrO2 binary oxide (Fig. 2). The amor-
phous TiO2–ZrO2 mixed oxide (Ti/Zr, 1:1) has a high surface area of
243 m2/g and the average pore size is 1.66 nm [19]. Fig. 3a shows
TEM image of porous structures of Ni–Pd (5:1)/TiO2–ZrO2 bimetal
catalyst. Energy-dispersive X-ray analysis (EDX) was employed to
confirm the existence of nickel and palladium species over the sup-
port. EDX spectrum of Ni–Pd/TiO2–ZrO2 (Fig. 3b) shows peaks of Ni,
Pd, Ti, Zr, O and Cu. The signals of copper are from supporting grid
for TEM sample preparation.
vent polarity: ENT (ethanol, 0.654) > ENT (2-propanol, 0.546) > ENT
>
(1,4-dioxane, 0.164) > ENT (toluene, 0.099) (ENT solubility parame-
ter of the solvent polarity) [22]. When alcohols are used as solvents
for aldehyde hydrogenation, acetals are typical byproducts [23].
As comfirmed by GC–MS, 2-furaldehyde diisopropyl acetal and
2-furaldehyde diethyl acetal are detected after furfual is hydro-
genated in 2-propanol and ethanol. TiO2–ZrO2 binary oxides have
surface acidity by charge imbalance of Ti–O–Zr bonding [24,25].
TiO2–ZrO2 with equal proportion of Ti and Zr shows the highest
surface acid strength [16]. Acetalization is correlated with surface
acid of catalyst support. This side reaction has also been reported
for liquid hydrogenation of furfural using Ir/Nb2O5 in alcohol [26].
The formation of hemiacetal byproducts in has been reported for
Ir/TiO2-catalyzed furfural hydrogenation in ethanol [27].
TiO2–ZrO2 mixed oxides not only have the advantages of both
TiO2 as active catalyst or support and ZrO2 as acid-base sites, but
also exhibit novel catalytic properties imparted by the formation
of active intermediate states. TiO2–ZrO2 mixed oxide (Ti/Zr, 1:1) is
calcined at 823 K and the crystalline ZrTiO4 is formed. The materials
are mixtures of ZrTiO4 nanocrystals and amorphous mixed oxides.
The size of ZrTiO4 nanocrystals is in the range from 5 nm to 10 nm,
3.2. Tuning selectivity via synergistic effects of supporting Ni–Pd
bimetallic catalysts
The results in Table 1 reveal marked synergistic effects between
nickel and palladium for catalytic liquid-phase hydrogenation of
furfural. A highly active and selective catalyst can be developed
through doping supported nickel with palladium of one fifth of
nickel load. Ni–Pd synergistic effect has been found in catalytic
hydrogenation of nitrobenzene over Ni–Pd bimetallic catalyst sup-
ported by Ti containing mesoporous silica. The optimal molar ratio
of nickel to palladium is 1.5 [28]. EDX spectrum has demonstrated
the coexistence of nickel and palladium ove TiO2–ZrO2. However,
there is no detailed information about the chemical states of nickel
species and palladium species. To reveal the mechanism of Ni–Pd
bimetallic synergistic effect, XPS measurement is performed to
exam the chemical states of supported nickel species and palla-
dium species. Fig. 4a shows the XPS survey spectrum of Ni–Pd
(5:1)/TiO2–ZrO2 bimetallic catalyst. Typical elements are detected.
The metal load of palladium is relatively low and Pd 3d XPS peaks
are overlapped with strong Zr 3p peaks. These make Pd 3d XPS

26
B. Chen et al. / Applied Catalysis A: General 500 (2015) 23–29
Fig. 3. (a) TEM image of Ni–Pd (5:1)/TiO2–ZrO2. (b) EDX spectrum of the selected
sample of Ni–Pd (5:1)/TiO2–ZrO2.
for Ni/TiO2–ZrO2 (A in Fig. 5). The proportion has small variation
during catalytic hydrogenation process (B in Fig. 5). This results
in its low catalytic activity for furfural hydrogenation. For Ni–Pd
(5:1)/TiO2–ZrO2, the proportion of active zero-valent nickel species
has a marked increase (approximately 20.2%) (C in Fig. 5). The addi-
tion of Pd increases the reducibility of supported nickel species and
similar phenomenon has been observed for Ni–Pd/Al2O3 bimetal-
lic catalyst [32]. It is noteworthy that the coexistence of nickel and
palladium makes supported nickel species highly dynamic dur-
ing hydrogenation process. The proportion of zero-valent nickel
species sharply increases to 42.9% after one-run furfural hydro-
genation testing of Ni–Pd (5:1)/TiO2–ZrO2 (D in Fig. 5). Active
zero-valent nickel species are in situ formed during hydrogenation
process. This directly leads to greatly improved catalytic perfor-
mance. The presence of palladium is the crucial factor in situ
forming zero-valent nickel species. A hydrogen-transfer mecha-
nism is proposed to interpret Ni–Pd synergistic effect (Scheme 2).
Supported nickel species and palladium species should achieve a
well-doped dispersion state. Palladium nanoparticles are kept in
small size and uniformly embedded with nickel species. Molecu-
lar hydrogen is adsorbed and activated by palladium species and
adjacent nickel species are facilely reduced to active zero-valent
sites. This conversion significantly increases the surface concen-
tration of catalytic active sites for furfural hydrogenation. However,
such positive synergistic effect of Ni–Pd might be spoilt by lose of
the required doping dispersion state. As revealed by experimental
results (Table 1), the addition of palladium metal load has a negative
effect upon the catalytic performance. Further increase of palla-
dium proportion from 20% to 50% makes the catalytic performance
more like monometallic palladium catalyst.
Fig. 2. (a) TEM image of ZrTiO4 microcrystals embedded in TiO2–ZrO2 (1:1; 823 K)
mixed oxide matrix. (b) Powder XRD pattern of TiO2–ZrO2 (1:1; 823 K) mixed oxide.
peaks hardly suitable for interpreting Ni–Pd synergistic effect. Fit-
ting of the XPS envelope of Zr 3p results in identification of Pd
3d peaks (Fig. 4b). However, the overlap with strong Zr 3p signals
makes binding energy measurement not quite accurate.
Ni 2p XPS peaks have complicated patterns, but more useful
chemical information about the synergistic effect. XPS measure-
ment and calculation are performed over four selected model
samples: Ni/TiO2–ZrO2 (A, reduced by hydrogen flow and stored in
argon), Ni/TiO2–ZrO2 (B, separated A after one-batch hydrogena-
tion, dried and stored in pure hydrogen), Ni–Pd (5:1)/TiO2–ZrO2
(C, reduced by hydrogen flow and stored in argon), and Ni–Pd
(5:1)/TiO2–ZrO2 (D, separated C after one-batch hydrogenation,
dried and stored in pure hydrogen) (Fig. 5). Fitting of Ni 2p XPS
envelope results in identification of two chemical states of nickel
species and their satellite peaks. Ni 2p3/2 at binding energy of
851.4 eV and Ni 2p1/2 (851.4 + 17.3 eV) are attributed to zero-valent
nickel species. Ni 2p3/2 standard value of bulk nickel is 852.6 eV
[29]. Nickel species supported over TiO2–ZrO2 show a little shift
(1.2 eV) in their Ni 2p3/2 binding energies (BE) from the standard
value. This may be attributed to the strong interaction between
nickel species and TiO2–ZrO2 support [30]. Ni 2p3/2 at binding
energy of 854.9 eV and Ni 2p1/2 (854.9 + 18.3 eV) are indexed to
electro-deficient nickel species (Ni2+) [31]. There are some broad
bands following Ni 2p3/2 and Ni 2p1/2 that are caused by satellite
peaks. After being activated by hydrogen at relatively high temper-
ature, the proportion of active zero-valent nickel species is very low

B. Chen et al. / Applied Catalysis A: General 500 (2015) 23–29
27
Fig. 4. (a) XPS survey spectrum of Ni–Pd (5:1)/TiO2–ZrO2. (b) Fitting of XPS
envelopes of Zr 2p and Pd 3d.
3.3. Catalytic selectivity of furfural hydrogenation over Pt-M
bimetallic catalysts (M: Re, In, Sn)
Table 2 lists experimental results of furfural hydrogenation over
TiO2–ZrO2 supported Pt-M bimetallic catalysts. The hydrogena-
tion over Pt/TiO2–ZrO2 leads to 49.6% of furfural conversion and
product mixtures of FA (5.6%), THFA (49.6%), and ring-opening
products (44.8%). Ring-opening products, including ethyl levuli-
nate ester and 5,5-diethoxylpentan-2-one, are produced through
hydrogenation of furfural and the following alcoholysis of fur-
furyl alcohol. These products can be explained by the bifunctional
catalyst Pt/TiO2–ZrO2: hydrogenation activity and acidity of sup-
port. Similar results have been observed by our group using the
Fig. 5. Fitting of Ni 2p XPS envelopes of four selected model samples: Ni/TiO2–ZrO2
(A, reduced by hydrogen flow and stored in argon), Ni/TiO2–ZrO2 (B, separated
A
after one-batch hydrogenation, dried and stored in pure hydrogen), Ni–Pd
(5:1)/TiO2–ZrO2 (C, reduced by hydrogen flow and stored in argon), and Ni–Pd
(5:1)/TiO2–ZrO2 (D, separated C after one-batch hydrogenation, dried and stored
in pure hydrogen).
Table 2
Experimental results of TiO2–ZrO2 supported Pt-M (M: Re, In, Sn) catalysts in selec-
tive hydrogenation of furfural.
TiO2–ZrO2 supported
catalysts
FR conv. (%)
Selectivity (%)
FA
5.6
57.6
95.7
74.9
47.8
THFA
Acetal
Others
Pt (2 wt.%)
Re (1 wt.%)
Pt (2 wt.%)–Re (1 wt.%)
Pt (2 wt.%)–In (2 wt.%)
Pt (2 wt.%)–Sn (2 wt.%)
49.6
44.6
100
73.3
98.3
49.6
–
1.3
7.1
35.4
–
29.1
3.0
14.5
1.7
44.8
13.3
–
3.5
15.1
Reaction conditions: catalyst (0.2 g), furfural (1.5 ml), ethanol (8.5 ml), H2 (5.0 MPa),
reaction temperature: 403 K, reaction time: 8 h. FR: furfural; FA: furfuryl alcohol;
THFA: Tetrahydrofurfuryl alcohol; Acetal: 2-(diethoxymethyl)furan.
Scheme 2. Proposed mechanism of nickel–palladium synergistic effect.

28
B. Chen et al. / Applied Catalysis A: General 500 (2015) 23–29
bond [40]. When metal oxide species are located on the Pt surface,
the hydrogen species on Pt are transferred to adsorbed C O bond
to achieve selective hydrogenation. Pt–Re catalyst exhibits he best
performance with yield of furfuryl alcohol 95.7%.
4. Conclusions
In summary, a strategy based on bimetallic synergistic effect
was developed to tune the catalytic selectivity of liquid-phase
furfural hydrogenation. TiO2–ZrO2 binary oxide was synthesized
as the support for monometallic or bimetallic catalyst. Addition
of small proportion of palladium directly transfers the catalytic
performance of supported nickel from partial hydrogenation cat-
alyst to total hydrogenation catalyst. The bimetallic catalyst with
Ni–Pd mole ratio of 5:1 shows the best performance. The yield of
THFA reaches 93.4% and the percentage of useless byproducts is
only 3.9%. Ni–Pd synergistic effect was interpreted through the
hydrogen-transfer mechanism, which significantly increases the
surface concentration of catalytic active sites for furfural hydro-
genation. Pt–Re bimetallic catalyst especially converts furfural into
partial hydrogenation product (FA). Furfural is totally converted
and the selectivity of partial hydrogenation product (FA) reaches
95.7%. This excellent performance is achieved through synergistic
effect of supported rhenium oxide species and platinum species.
Fig. 6. Pt 4f XPS peaks of Pt (2 wt%)/TiO2–ZrO2 (A) and Pt (2 wt%)–Re
(1 wt%)/TiO2–ZrO2 (B); Re 4f XPS peaks of Pt (2 wt%)–Re (1 wt%)/TiO2–ZrO2 (C).
bifunctional catalyst Pt/ZrNbPO4 in the conversion of furfural [33].
The low selectivity has also been reported for furfural hydrogena-
tion over Pt/C [2] and Pt/Al2O3 catalysts [12]. The main products
of furfural hydrogenation over Re/TiO2–ZrO2 are FA (57.6%) and
acetal of furfural (29.1%). Pt–Re bimetallic catalyst shows excel-
lent catalytic activity and selectivity. Furfural is totally converted
and the selectivity of partial hydrogenation product (FA) reaches
95.7%. In and Sn are introduced as the second metal and show pos-
itive effects upon catalytic activity. Pt–Sn bimetallic catalyst has
high furfural conversion (98.3%), but the selectivity is relatively
poor. Pt–In bimetallic catalyst can convert 73.3% of furfural and
the selectivity of FA is 74.9%.
Ni–Pd bimetallic catalyst is highly active for total hydrogena-
tion of furfural and Pt–Re bimetallic catalyst especially converts
furfural into partial hydrogenation product (FA). Chemical states
of Pt–Re/TiO2–ZrO2 catalyst are measured by XPS (Fig. 6). Pt 4f7/2
XPS peak of Pt/TiO2–ZrO2 is at the binding energy of 70.77 eV and
the value measured over Pt–Re/TiO2–ZrO2 is 71.01 eV (Fig. 6A and
B). The tiny difference may be attributed to measurement error.
Supported platinum species of Pt/TiO2–ZrO2 and Pt–Re/TiO2–ZrO2
are kept in zero-valent state. Re 4f7/2 peak is detected at the bind-
ing energy of 45.85 eV, which is indexed to middle chemical states
between Re6+ and Re7+. It has been reported that Re 4f7/2 of ReO3
is at the BE of 45.2 eV [34] and Re 4f7/2 of Re2O7 is at the BE of
46.4 eV [35]. This finding implies that Re2O7 or ReO3 may be in situ
converted to catalytic active species for hydrogenation of furfural.
Similar results have been observed by Pascoe and Broadbent that
cinnamaldehyde and crotonaldehyde have been hydrogenated to
cinnamyl alcohol and crotyl alcohol by Re2O7 catalysts [36,37]. The
side-reaction of acetalization is correlated with surface acid of the
catalyst support. This phenomenon has been reported over other
catalysts in catalytic hydrogenation of furfural [38].
Acknowledgement
This work was financially supported by the National Natural Sci-
ences Foundation of China (nos. 21174148, 21403248, 21304101).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2015.05.
006
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