Highly active and selective catalytic transfer hydrogenolysis of α-methylbenzyl alcohol catalyzed by supported Pd catalyst

Article abstract of DOI:10.1016/j.catcom.2011.04.009

A novel supported Pd catalyst was synthesized by using plant tannin grafted collagen fiber as the supporting matrix. The as-prepared Pd catalyst was subsequently used for the catalytic transfer hydrogenolysis of α-methylbenzyl alcohol. Due to the fibrous

Full text of DOI:10.1016/j.catcom.2011.04.009

Catalysis Communications 12 (2011) 1177–1182
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Catalysis Communications
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Short Communication
Highly active and selective catalytic transfer hydrogenolysis of α-methylbenzyl
alcohol catalyzed by supported Pd catalyst
a,b,
Hui Mao a, Xuepin Liao a,b, , Bi Shi
⁎
⁎
a
Department of Biomass Chemical and Engineering, Sichuan University, Chengdu, 610065, PR China
National Engineering Laboratory of Clean Technology for Leather Manufacture, Sichuan University, Chengdu, 610065, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 March 2011
Received in revised form 29 March 2011
Accepted 11 April 2011
Available online 17 April 2011
A novel supported Pd catalyst was synthesized by using plant tannin grafted collagen fiber as the supporting
matrix. The as-prepared Pd catalyst was subsequently used for the catalytic transfer hydrogenolysis of α-
methylbenzyl alcohol. Due to the fibrous morphology of collagen fiber and the well dispersed Pd
nanoparticles anchored by tannins, the as-prepared Pd catalyst showed superior activity for the catalytic
transfer hydrogenolysis of α-methylbenzyl alcohol when compared with the Pd catalyst supported on
inorganic oxides. Moreover, the as-prepared Pd catalyst can be reused at least 6 times without significant loss
of activity and selectivity, suggesting a satisfied reusability.
Keywords:
Catalytic transfer hydrogenolysis
Collagen fiber
Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
Plant tannin
Palladium nanoparticles
Activity
Selectivity
1. Introduction
suitable interaction, and the metal species are also need to be highly
dispersed on the surface of supporting matrix. As a consequence, the
Catalytic transfer hydrogenolysis (CTH) is an important synthetic
technique, which has been widely used in biomass conversion [1–3],
pharmaceutical production [4,5] and organic synthesis [6–8]. By using
organic compound as hydrogen donor, CTH can be conveniently
carried out under mild conditions, which exhibits distinct advantages
over the conventional hydrogenolysis methods that need to be
performed with molecular hydrogen at high pressure. In addition,
CTH is available for various functional groups including alcohol, halo,
nitro and benzyl [9–13], thus providing an environmental benign
synthetic route for a large number of organic compounds.
In general, noble metals, such as palladium [6,7], platinum [14] and
nickel [15], have been employed as the catalysts in CTH, and those
metals were often immobilized onto insoluble matrices to improve
their reusability. Inorganic oxides with high specific surface area are
commonly used as the supporting matrices, including active carbon,
γ-alumina and silica etc. [16]. Unfortunately, those immobilized metal
species often suffered from a reduced catalytic activity due to their
poor distribution with aggregation. Furthermore, the metal species
may also be leached into the solvent during reaction owing to the
weak association with supporting matrices, which inevitably leads to
a decreased activity when the catalyst was recycled. Ideally, the metal
species should be stably immobilized onto the supporting matrix via
design of supporting matrix becomes the crucial factor for preparing
noble metal catalyst with high activity and stability for CTH reaction.
Our previous research showed that collagen fiber was able to react
with plant tannins to prepare adsorbents, which were highly effective
for the adsorptive recovery of Pd2+ and Pt2+ from aquoues solutions
[17]. Moreover, Pd2+ and Pt2+ absorbed by the adsorbent can be
desorped using diluted acid solution, indicating that the interaction of
Pd2+/Pt2+ with tannin is mild. In addition, the fibrous morphology of
collagen fiber can considerably decrease the mass transfer resistance,
resulting fast adsorption rate. Based on unique characteristics of the
fibrous adsorbent, we proposed herein a feasible strategy for synthesis
of highly active and recoverable heterogenous Pd catalyst for CTH
reaction, that is the use of plant tannins grafted collagen fiber as the
supporting matrix to prepare heterogenous supported Pd nanoparti-
cles catalyst. In this supporting matrix, the phenolic hydroxyls of plant
tannins act as the anchors of Pd nanoparticles, thus ensuring a high
dispersion and stability of Pd nanoparticles. Additionally, the low
mass transfer resistance nature of the fibrous supporting matrix
should be also beneficial for the CTH reaction performance, resulting
in a higher catalytic activity. At present invesigation, α-Methylbenzyl
alcohol (MBA) was employed as model compound to study the
activity and selectivity of the as-preapred Pd catalyst in CTH reaction
of alcohol (C―OH bond). As shown in Scheme 1, the target product in
CTH of MBA was ethylbenzene (EB) while acetophenone (AP) may
also be formed as the by-product. We therefore systematically
investigated the effects of various experimental conditions on the
⁎
Corresponding authors. Tel.: +86 28 85400382; fax: +86 28 85400356.
E-mail addresses: xpliao@scu.edu.cn (X. Liao), shibi@scu.edu.cn (B. Shi).
1566-7367/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.04.009

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H. Mao et al. / Catalysis Communications 12 (2011) 1177–1182
provided by the plant of forest product in Guangxi province (China).
Pd/SiO2, and Pd/Al2O3 were prepared by conventional impregnation
methods (see supporting information).
2.2. Preparation and characterization of Pd nanoparticles supported on
black wattle tannin grafted collagen fiber (Pd–BT–CF)
3.0 g of BT was dissolved in 100.0 mL of deionized water, and then
5.0 g of CF was added. The resultant mixture was stirred at 298 K for
2.0 h. Then, 50.0 mL of glutaraldehyde solution (2.0%), used as
bifunctional cross-linking agent, was added into the above mixture
at pH 6.5, and the reaction proceeded at 318 K for 6.0 h. Subsequently,
the product was filtrated, fully washed with deionized water and
dried in vacuum at 308 K for 12.0 h, and then the black wattle tannin
grafted collagen fiber (BT–CF) supporting matrix was obtained. Based
on ultraviolet measurement, the grafting degree of BT on BT–CF was
determined to be 60% by weight.
Scheme 1. The disproportionation-type reaction of MBA during the CTH process.
activity and selectivity of the as-prepared Pd catalyst in CTH of MBA,
such as water content in reaction medium, type of hydrogen donor
and reaction temperature. For comparison, Pd/C, Pd/SiO2, and Pd/
Al2O3 catalysts were also used in CTH of MBA under the same
conditions.
2. Experimental
Subsequently, 1.0 g of BT–CF was suspended in 100.0 mL of PdCl2
solution, of which the concentration of Pd2+ was 2.0×10−3 mol/L.
After the solution pH was adjusted to 4.5, the mixture was stirred at
303 K for 8.0 h, allowing the chelating adsorption of Pd2+ on BT–CF.
Then, the mixture was filtrated and fully washed with deionized
water. The collected intermediate product was reduced by 20.0 mL of
0.1 M NaBH4 aqueous solution, filtrated, and successively washed
with deionized water and ethanol. The loading amount of Pd on BT–CF
was determined to be 2.0% in weight.
2.1. Materials
PdCl2, formic acid, NaBH4, and other chemicals were all analytical
reagents and purchased from Sigma-Aldrich Corporation, which were
used as received without further purification. Pd/C (2.0%) and MBA
(97%) was purchased from Alfa Aesar. Collagen fiber (CF) was
purchased from Institute of Chemical Industry of Forest Product
(China). Black wattle tannin (BT), a typical condensed tannin, was
Fig. 1. The molecular structure of BT, and the proposed preparation mechanism of the Pd–BT–CF catalyst.

H. Mao et al. / Catalysis Communications 12 (2011) 1177–1182
1179
The obtained Pd–BT–CF was then characterized by Scanning
Electron Microscopy (SEM, JEOL LTD JSM-5900LV), X-ray Photoelec-
tron Spectroscopy (XPS, Kratos XSAM-800, UK) and Transmission
Electron Microscopy (TEM, Tecnai G2 F20 S-TWIN, U.S.).
2.3. CTH of MBA
The CTH reaction of MBA was carried out in a 50.0 mL round-
bottomed flask equipped with water-cooled condenser and magnetic
stirrer. The flask was added with 10.0 mL of ethanol, an appropriate
amount of water, 1.5 mmol of MBA, a certain amount of hydrogen donor
and 50.0 mg of palladium catalyst (containing 10.0 μmol Pd). During the
reaction, the flask was placed in the oil bath and maintained at the given
temperature under atmosphere pressure. The reaction mixture was
analyzed at regular intervals using a gas chromatograph equipped with a
flame ionization detector and a capillary column (ECTM-WAX,
30.0 m×0.25 mm×0.25 mm). For comparison, the CTH of MBA was
also performed using commercial Pd/C (2.0%) catalyst under the same
conditions. The turnover frequency (TOF) of the catalysts was calculated
Substrate reactedðmolÞ
as:
.
PdðmolÞ × tðhÞ
3. Results and discussion
3.1. Preparation and characterization of the Pd–BT–CF catalyst
The molecular structure of BT is shown in Fig. 1a. it can be seen that
there are large numbers of phenolic hydroxyls located at the B rings of
BT, which are able to chelate with many metal ions with empty d
orbits, like Pd2+[18]. On the other hand, the A rings of BT have high
nucleophilic reaction activity, and thus, BT can be covalently bound
with the amino groups of CF using glutaraldehydes as the cross-
linking agents (Fig. 1b). Additionally, the hydrogen bonds formed
between BT and the amino groups of CF can also improve the stability
of BT–CF. Subsequently, the obtained BT–CF was used for the
adsorption of Pd2+, and further reduced to Pd nanoparticles by
NaBH4, and finally, the Pd–BT–CF catalyst was obtained (Fig. 1c–d).
Due to the electron donating/accepting interactions of phenolic
hydroxyls of BT with Pd nanoparticles, the Pd nanoparticles should
be stably anchored by the BT grafted onto CF, which can prevent the
aggregation of Pd nanoparticles and improve its stability during
catalytic reaction process.
To confirm the proposed preparation mechanism, XPS analyses of
BT–CF, Pd2+–BT–CF and Pd–BT–CF were carried out. In Fig. 2a, the O
1s spectrum of BT–CF shows two peaks at 533.0 eV and 531.6 eV,
respectively. The peak at 533.0 eV is assigned to the oxygen in
―C―O―H bond (mainly attributed to phenolic hydroxyls of BT)
while the peak 2 at 531.6 eV is related to the oxygen in ―C O bond
[19,20]. After the adsorption of Pd2+ on BT–CF, the O 1s spectrum of
Pd2+–BT–CF exhibits a new peak at 534.1 eV (Fig. 2b), which should
belong to the electron donating–accepting interaction of Pd2+ with BT
when considering the strong chelating ability of BT towards transition
metal ions. After the reduction of Pd2+, another new peak with high
binding energy of 537.3 eV is observed at the O 1s spectrum of Pd–BT–
CF (Fig. 2c). This new peak is likely to be associated with the
stabilization interaction of hydroxyl oxygens of BT to the formed Pd
nanoparticles. Consequently, it can be concluded that the BT-CF
support is able to stabilize the Pd nanoparticles by its phenolic
hydroxyls. In addition, the Pd 3d XPS analysis of Pd–BT–CF revealed
that about 63.9% of Pd2+ has been reduced to elemental Pd (data is
not shown here).
Fig. 2. The O 1s XPS spectra of BT–CF (a), Pd2+–BT–CF (b) and Pd–BT–CF (c).
catalytic activity of the Pd–BT–CF catalyst for CTH of MBA can be
expected. Further TEM analysis revealed that the Pd nanoparticles
with an average diameter of 4.0–6.0 nm were well dispersed in the
Pd–BT–CF catalyst (Fig. 3b), which confirmed the anchoring effect of
BT towards the Pd nanoparticles. Based on the above XPS analysis of
Pd 3d spectrum of Pd–BT–CF, the observed Pd nanoparticles in Fig. 3b
should be Pd(0) nanoparticles and/or Pd(0)-Pd(II) hybrid
nanoparticles.
The SEM image of the Pd–BT–CF catalyst is shown in Fig. 3a. It is
clearly observed that the fibrous morphology of collagen fiber was
well preserved after the grafting of BT and the anchoring of Pd
nanoparticles. According to the literature [21], fibrous materials
usually offer better mass transfer characteristics, and thus higher
3.2. CTH of MBA
3.2.1. Effect of water content
In general, the mixture of water and ethanol is often used as the
solvent in CTH because the use of water is able to promote the

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H. Mao et al. / Catalysis Communications 12 (2011) 1177–1182
Fig. 3. SEM and TEM images of the Pd–BT–CF catalyst.
ionizability of hydrogen donor [22], which therefore results in the
formation of more active hydrogen species. As a consequence, we first
investigated the effect of water content on the CTH of MBA. As shown
in Fig. 4, the increase of water content always promotes the
conversion of MBA, which reaches the maximum when the volume
ratio of water to ethanol was 2:10. Considering the fact that HCOOH is
soluble in pure ethanol, the promoted conversion of MBA should be
due to the promoted dissolution of HCOOH by water but not the
increased solubility of HCOOH in water/ethanol mixture. On the other
hand, the selectivity to EB first increases with the increase of water
content (from 0 to 20%), and then gradually decreases along with the
further increase of water content from 20% to 100%. These facts
suggested that the use of water can promote the conversion of MBA,
but excessive use of water leads to the decrease in selectivity to EB.
Therefore, the content of water was fixed at 20% (v/v) in the following
experiments.
CTH of MBA. Table 1 summarized the activity and selectivity of the
Pd–BT–CF catalyst in CTH of MBA using different hydrogen donors. By
using HCOOH as the hydrogen donor, the Pd–BT–CF catalyst exhibits
the highest activity and selectivity while much lower conversion and
selectivity to EB were observed when formate salts were employed.
Feng et al. suggested that the dissociative chemisorption of HCOO−on
Pd metals and the protonation of MBA by H+ are the key factors in
CTH of MBA [16]. Hence, HCOOH can provide both HCOO−and H+ and
ensures the highest activity and selectivity in CTH of MBA. This
suggestion is consistent with our experiments. It should also be noted
that due to the well dispersion of Pd nanoparticles as well as the
fibrous morphology of the catalyst, the TOF value of the Pd–BT–CF
catalyst (316.6 mol mol−1 h−1) was much higher than those of
conventional supported catalyst using inorganic oxides as the
supporting matrices (Entry 1–4). For example, the average TOF of
the Pd–BT–CF catalyst was almost 5-folds higher than that of the
commercial Pd/C catalyst (60.0 mol mol−1 h−1) when HCOOH was
used as the hydrogen donor. Compared with the commercial Pd/C
catalyst, the superior activity of the as-prepared Pd catalyst directly
supports our claimed strategy that the fibrous morphology of collagen
fiber and the well dispersed Pd nanoparticles anchored by tannins can
improve the activity of the supported Pd catalyst for the catalytic
transfer hydrogenolysis of α-methylbenzyl alcohol.
3.2.2. Effect of hydrogen donor
From the perspective of green chemistry, HCOOH and formate salts
are more environmental benign hydrogen donors than other organic
compounds such as 2-propanol and hydrazine [23]. Hence, HCOOH
and formate salts were used as the hydrogen donors to perform the
100
80
100
80
Conversion of MBA
Selectivity to EB
Selectivity to AP
60
40
20
0
Conversion of MBA
Selectivity to EB
Selectivity to AP
60
40
20
0
20
30
40
50
60
70
80
0
20
40
60
80
100
Reaction TemperatureoC
Water Content (%,v/v)
Fig. 4. Effect of water content on the CTH of MBA.
Fig. 5. Effect of reaction temperature on CTH of MBA.

H. Mao et al. / Catalysis Communications 12 (2011) 1177–1182
1181
Pd-BT-CF
Pd-C
Table 2
Effect of molar ratio of HCOOH to MBA on the CTH of MBA .
⁎
300
250
200
150
100
50
Entry
Molar ratio of
HCOOH/MBA
Conversion
(%)
Average TOF
Selectivity
(%)
(mol mol−1 h−1
)
EB
AP
1
2
3
4
5
1.0
1.5
2.0
3.0
4.0
91.2
96.7
99.2
99.7
100
291.2
308.6
316.6
318.2
319.1
87.5
95.8
99.0
99.2
99.5
12.5
4.2
1.0
0.8
0.5
⁎
Reaction conditions: 1.5 mmol MBA, 50.0 mg Pd–BT–CF catalyst, 10.0 mL ethanol,
2.0 mL H2O, 60 °C and 30 min.
3.2.5. Reusability of the Pd–BT–CF catalyst
0
As shown in Fig. 6, the catalytic activity of the Pd–BT–CF catalyst
was almost unchanged after six times repeated application (the TOF
value was a little changed from 318.2 to 310.9 mol mol−1 h−1),
exhibiting satisfied reusability of the Pd–BT–CF catalyst. On the
contrary, the reusability of commercial Pd/C catalyst should be
improved, of which the TOF in sixth time was only about 59.5% of
the fresh catalyst (from 60.0 to 35.7 mol mol−1 h−1). We believed
that the anchoring effect of plant tannins to the Pd nanoparticles
should be the main reason for the high stability of this novel catalyst.
1
2
3
4
5
6
Catalysts Cycles
Fig. 6. Recycling of the Pd–BT–CF catalyst in the CTH of MBA.
3.2.3. Effect of molar ratio of HCOOH to MBA
As mentioned above, the HCOOH provides both HCOO−and H+ in
CTH of MBA, thus the molar ratio of HCOOH to MBA should influence
the conversion of MBA as well as the selectivity to EB. We therefore
investigated the effect of molar ratio of HCOOH to MBA in CTH of MBA.
The corresponding results are summarized in Table 2. As expected, the
conversion of MBA and the selectivity to EB are both considerably
increased with the increase of molar ratio of HCOOH to MBA, which
roughly reaches the maximum when the molar ratio is 2.0. When the
molar ratio is 4.0, the conversion, TOF and selectivity of EB were 100%,
319.1 and 99.5%, respectively.
4. Conclusions
Highly active and selective heterogeneous Pd catalyst was
prepared by using collagen fiber grafted plant tannin as the
supporting matrix. Due to the presence of plant tannin on collagen
fiber, the supported Pd nanoparticles were stably anchored and well
dispersed. Moreover, the fibrous morphology of collagen fiber
ensured a high catalytic activity due to its nature of low mass transfer
resistance. The as-preapred can be used for the catalytic transfer
hydrogenolysis of α-methylbenzyl alcohol, which exhibited the
distinct advantages of high activity, selectivity and reusability.
3.2.4. Effect of reaction temperature
The effect of reaction temperature on the catalytic activity of the
Pd–BT–CF catalyst was also investigated, and the corresponding
results are shown in Fig. 5. It can be seen that the increase of reaction
temperature accelerates the reaction rate in CTH of MBA. When the
temperature is increased from 20 °C to 60 °C, the conversion of MBA is
significantly increased from 21.1% to 99.2%. As for the selectivity to EB,
it is also steadily increased along with the increase of temperature in
the temperature range of 20–60 °C. Therefore, the reaction temper-
ature should not be lower than 60 °C in order to obtain high
conversion and selectivity.
Acknowledgements
We acknowledge the financial supports provided by the National
Natural Science Foundation of China (20776090) and theFoundation
for the Author of National Excellent Doctor Dissertation of PR China
(FANEDD200762). We also give thanks to the Test Central for the TEM
analyses.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
doi:10.1016/j.catcom.2011.04.009.
Table 1
⁎
Effect of hydrogen donors on the CTH of MBA .
References
Entry
Hydrogen
donor
Conversion
(%)
Average TOF
Selectivity (%)
(mol mol−1 h−1
)
EB
AP
1
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5a
6a
HCOOH
HCOOH
HCOOH
HCOOH
HCOOLi
HCOONa
HCOOK
HCOONH4
99.2
18.8
3.4
16.4
9.8
8.1
17.3
21.5
316.6
60.0
10.9
52.3
31.3
25.9
55.2
68.6
99.0
98.7
65.4
76.7
39.9
57.6
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⁎
Reaction conditions: 1.5 mmol MBA, 3.0 mmol hydrogen donor, 10.0 mL ethanol,
2.0 mL H2O, 60 °C and 30 min.
a
50.0 mg Pd–BT–CF (10 μmol) was used as the catalyst.
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d
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