The shape effect of La2O2CO3 in Pd/La2O2CO3 catalyst for selective hydrogenation of cinnamaldehyde

Article abstract of DOI:10.1016/j.apcata.2017.06.039

Controllably adjusting the morphology of nanocatalysts is known as an effective way to improve catalytic performance. Pd/La₂O₂CO₃ catalysts with different morphologies (nanorods and nanoparticles) were successfully synthes

Full text of DOI:10.1016/j.apcata.2017.06.039

Accepted Manuscript
Title: The shape effect of La2O2CO3 in Pd/La2O2CO3 catalyst
for selective hydrogenation of cinnamaldehyde
Authors: Fei Wang, Zhongnan Zhang, Xuejiao Wei, Qihua
Fang, Xingmao Jiang
PII:
S0926-860X(17)30296-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.06.039
APCATA 16301
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
3-5-2017
29-6-2017
30-6-2017
Please cite this article as: Fei Wang, Zhongnan Zhang, Xuejiao Wei, Qihua
Fang, Xingmao Jiang, The shape effect of La2O2CO3 in Pd/La2O2CO3
catalyst for selective hydrogenation of cinnamaldehyde, Applied Catalysis A,
Generalhttp://dx.doi.org/10.1016/j.apcata.2017.06.039
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The Shape Effect of La2O2CO3 in Pd/La2O2CO3 Catalyst
for Selective Hydrogenation of Cinnamaldehyde
Fei Wanga,*, Zhongnan Zhanga, Xuejiao Weib, Qihua Fanga, Xingmao Jianga,c,
*
a
Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou
University, Changzhou 213164, PR China.
b Department of chemical and Engineering, Changzhou Institute of Technology, Changzhou 213022,
PR China.
c Key Laboratory for Green Chemical Process of Ministry of Education, Hubei Key Laboratory for
Novel Reactor and Green Chemistry Technology, School of Chemical Engineering and Pharmacy,
Wuhan Institute of Technology, Wuhan 430073, China
* Corresponding author: E-mail: wangfei@cczu.edu.cn; jxm@cczu.edu.cn; jxm@wit.edu.cn Fax: +86-
519-8633-0263; Tel: +86-519-8633-0253
Graphical Abstract
H
H
H
H
H
H
Pd
Pd
Pd
Pd
La3+
La3+
O2+
La3+
La3+
O2+
O2+
La3+
La3+
La3+
La3+
O2+
O2+
O2+
La3+
La3+
O2+
O2+

Highlights


Pd/La2O2CO3-NRs catalysts were successfully synthesized via hydrothermal method.
Pd/La2O2CO3-NRs catalyst exhibited better catalytic performance than Pd/La2O2CO3-NPs
catalyst for selective hydrogenation of CAL to HCAL.


Support shape can modify surface basicity, which resulting in enhanced hydrogenation activity.
Pd/La2O2CO3-NRs catalyst is a highly efficient catalyst for selective hydrogenation of CAL to
HCAL.
Abstract
Controllably adjusting the morphology of nanocatalysts is known as an effective way to improve
catalytic performance. Pd/La2O2CO3 catalysts with different morphologies (nanorods and
nanoparticles) were successfully synthesized via simple hydrothermal method and evaluated for
selective hydrogenation of cinnamaldehyde. The highly catalytic activity (86 % HCAL selectivity
at 100 % CAL conversion at 3 h) occurred over Pd/La2O2CO3-NRs with La2O2CO3 nanorods as
support. Remarkably, Pd/La2O2CO3-NRs catalyst not only showed better catalytic performance,
nearly 10 times higher than Pd/La2O2CO3-NPs catalyst, but also exhibited excellent cycling stability.
CO2-TPD showed that the intense desorption peaks at about 560 K represented the medium-strength
basic sites (La3+-O2− pairs) with a total amount of 0.13 mmol g-1 on the La2O2CO3-NPs and 0.21
mmol g-1 on the La2O2CO3-NRs. These results indicated that support morphology has a significant
effect on catalytic activity and can modify surface basicity, which resulting in enhanced
hydrogenation activity.
Keywords: Palladium; La2O2CO3 nanorods; Cinnamaldehyde hydrogenation; Shape effect;
Basicity
1. Introduction
Selective hydrogenation of α, β-unsaturated aldehydes to the corresponding unsaturated alcohols is
highly attractive because it is an essential step in the synthesis of value-added fine chemicals [1].
For example, selective hydrogenation of cinnamaldehyde (CAL) route was shown in Scheme 1,

which can produce cinnamal alcohol (COL), hydrocinnamaldehyde (HCAL), hydrocinnamal
alcohol (HCOL) [2]. Formed HCAL through hydrogenation of the C=C bond were found as an
essential organic intermediate for the preparation of drugs in the treatment of HIV [3].
OH
H2
H2
COL
OH
O
HCOL
CAL
O
H2
H2
HCAL
Scheme 1 Reaction pathways for selective hydrogenation of CAL
Many metals such as Ni [4], Cu [5], Os [6], Ir [7], Pt [8], Ru [9], Au [10], Pd [11] and so on, have
been proved as highly efficient catalysts for the activation of C=C and C=O bonds. Among them,
the Pd-based catalysts are considered to be one of the most promising active center for selective
oxidation [12], Suzuki coupling reaction [13] and especially the selective hydrogenation of the C=C
bond [14]. Recently, Liao et al. [15] reported a high-performance bimetallic PdAu catalyst with
mesoporous silica nanoparticles as prepared by an organic impregnation-hydrogen reduction
approach, which showed up to four times higher activity than that of Pd/MSN (without Au as a
promoter) and eight times higher activity than that of commercial Pd/C catalyst. Liu et al. [16] also
found that smaller Pd particles preferred a higher selectivity to C=C bond hydrogenation, forming
HCAL, while on larger ones, the selectivity to C=O bond hydrogenation increased yielding a higher
selectivity to COL. Except adding other metal as promoter and controlling the size of metal particles,
support effect seems to be an important factor adjusting the catalytic activity and selectivity to C=C
bond [17]. Metal oxide nanomaterials as the catalyst supports were reported to be able to improve
the activity and selectivity to COL [18]. Previous reports [19] demonstrate that the shape of the
metal oxide particle at the nanometre scale is essentially important for obtaining highly active and
selective performance through selective exposure of the reactive facets and that morphological
control could greatly promote the catalytic performance by enriching the surface active sites. Up to
date, there are very few reports about the shape effect of support on the catalytic performance of
hydrogenation of CAL. Previously, we have reported [20] that copper nanoparticles dispersed rod-

shaped La2O2CO3 efficiently catalyzed transfer dehydrogenation of primary aliphatic alcohols with
an aldehyde yield of up to 97 %. These results show that the (110) planes of the La2O2CO3 support
not only provided a substantial number of medium-strength basic sites for activating the alcohol but
also directed the selective dispersion of copper particles approximately 4.5 nm in size on their
surfaces.
In this study, we extended the application of the La2O2CO3 nanorods support to selective
hydrogenation of CAL. Herein, we reported an active and stable catalyst, Pd nanoparticles dispersed
on La2O2CO3 nanorods. The resulting catalyst displayed obviously enhanced catalytic activity for
hydrogenation of CAL to HCAL. Compared with Pd nanoparticles dispersed on La2O2CO3
nanoparticles catalyst, this enhancement can be rationalized by basicity of La2O2CO3.
2. Experimental section
2.1 Materials synthesis
The La(OH)3 nanorods were prepared by a hydrothermal method from our previous work [20]. 1.0
g of La(NO3)3·6H2O was dissolved in 20 mL of water, and the mixture was added to 60 mL of
aqueous 5.0 M KOH solution, yielding a white precipitate. The slurry was transferred into an
autoclave (100 mL), heated to 453 K and maintained at that temperature for 12 h. The solid product
was washed with water and dried at 323K under vacuum, giving La(OH)3 nanorods. Calcination of
this lanthanum hydroxide precursor at 773K for 4 h in air produced La2O2CO3 nanorods. La(OH)3
nanoparticles were prepared by simple precipitation method and other conditions were same as
La(OH)3 nanorods.
The Pd/La2O2CO3 catalyst was prepared by a deposition-precipitation method. A certain amount
Pd(NO3)3 was dissolved in 120 ml of water, and 0.5 g of La2O2CO3 nanorods (or nanopaticles) was
dispersed into the solution at room temperature. Aqueous 0.1 M NaOH solution was added dropwise
to the mixture. Addition of NaOH used to adjust the pH=8-9 and then the precipitate was aged for
2 h at room temperature. The resulting solid was washed with water, dried at 383 K overnight and
calcined at 723 K for 4 h in air, forming PdO/La2O2CO3. 200 mg of this catalyst precursor was
treated with pure hydrogen (40 ml min–1) at 573 K for 1 h in a fixed-bed reactor, yielding the
Pd/La2O2CO3 catalyst.

2.2 Structure analyses
X-ray powder diffraction (XRD) patterns were recorded on a D/MAX 2500/PC powder
diffractometer (Rigaku) using a CuKα radiation source operated at 40 kV and 200 mA from 5 ° to
70 ° (in 2 θ) with the scanning rate of 20 ° min-1. X-ray photoelectron spectroscopy (XPS) was
operated using Thermo Scientific Escalab 250 instrument under ultrahigh vacuum with a pressure
close to 2 × 10-9 mbar. Transmission electron microscope (TEM, JEOL JEM-2100) was applied for
the detailed microstructure and composition information. The surface area was measured by the
surface area and pore size analyzer (Autosorb-iQ2-MP). The palladium content in the prepared
catalysts was determined by an inductively coupled plasma atomic emission spectrometry (ICP-
AES).
Temperature programmed desorption of CO2 on the samples was carried out with a U-shape quartz
tubular reactor. The sample (100 mg) was exposed to a mixture of 30 vol.% CO2/He (30 ml min–1)
for 0.5 h at 423 K. After being purged with He (30 ml min–1) for 2 h, the sample was heated to 1073
K at a rate of 5 K min–1; the amount of CO2 desorbed was monitored by an on-line mass spectrometer.
2.3 Catalytic test
Selective hydrogenation of CAL was conducted in a stainless steel autoclave (50 ml). Typically, the
reaction mixture contained 1.0 mLCAL, 20 mL ethanol (solvent) and 100 mg Pd/La2O2CO3 catalyst.
During the hydrogenation, catalysts were suspended in solution by violent stirring. The reaction was
performed at 333-363 K for a certain period (0.5–3 h) under 1.0 MPa H2 pressure. At the end of
the reaction, the reactor was cooled to the ambient temperature, and the catalysts were separated
from the reaction mixture. The catalysts were recycled and reused in the next reaction. The product
of each hydrogenation reaction was analyzed by Haixin GC-950 gas chromatography (HP-5
capillary column).
3. Results and discussion
3.1. Structural characteristics of the catalysts
The phase purity of the products was examined by X-ray diffraction (XRD) measurement performed
on Rigaku X-ray diffractometer with Cu Kα radiation. As shown as Fig. 1, the peaks at about 23 °,

29 °, and 44° for La2O2CO3-NPs sample can be indexed to (110), (130), and (200) phases,
corresponding to a monoclinic structure for La2O2CO3 (JCPDS No. 48-1113), while the peaks at
about 26 °, 27 °, 31 °, and 45° for La2O2CO3-NRs sample can be indexed to (100), (101), (103), and
(110) phases, corresponding to hexagonal phase of La2O3CO3 (JCPDS card 37-0804) [21]. After
loading Pd, an almost identical XRD pattern was obtained for Pd/La2O2CO3-NPs comparing with
La2O2CO3-NPs. Moreover, Pd/La2O2CO3-NRs also showed same structure as La2O2CO3-NRs,
indicating the crystalline structure and the average size of the crystalline domain of the support were
well maintained in the Pd samples. No peaks due to metallic Pd were detected, owing to the fact
that the Pd particle sizes were very small (<5 nm).
Pd/La2O2CO3-NRs
(103)
(101)
(100)
(110)
La2O2CO3-NRs
Pd/La2O2CO3-NPs
(130)
30
(110)
(200)
La2O2CO3-NPs
60 70
10
20
40
50
2 Theta(degree)
Fig. 1 XRD patterns of La2O2CO3-NPs, La2O2CO3-NRs, Pd/La2O2CO3-NRs and Pd/La2O2CO3-NPs
samples.
The shapes of the obtained samples were analyzed by TEM (Fig. 2). The as-obtained La2O2CO3-
NRs were 200–300 nm in length and about 15-20 nm in diameter (Fig. 2a). In their HRTEM images
(Fig. 2b), the Pd nanoparticles were dispersed effectively on the surface of the La2O2CO3-NRs
synthesized at 180 °C for 12 h and are not agglomerated. And supported Pd particles in
Pd/La2O2CO3-NRs catalysts exhibit similar polyhedral shapes, and the Pd nanoparticles on the
La2O2CO3-NRs have crystallite sizes between 3.0 and 3.5 nm. This result was in accordance with
XRD results. On the other hand, the La2O2CO3-NPs exhibited irregular shapes and their
agglomeration can be observed (Fig. 2c). Fig. 2d showed that the Pd particle size distribution reflect
the uniform distribution with an average particle size of 3.0–3.5 nm as same as Pd/La2O2CO3-NRs

catalysts.
a
c
b
d
Fig. 2 TEM and HRTEM images of La2O2CO3-NRs (a), Pd/La2O2CO3-NRs (b) and Pd/La2O2CO3-
NPs (c, d) samples
Table 1 summarizes the physical and chemical properties of the Pd/La2O2CO3-NRs and
Pd/La2O2CO3-NPs samples. Surface area of Pd/La2O2CO3-NRs sample was 54.2 m2 g-1, while
surface area of Pd/La2O2CO3-NPs was only 20.6 m2 g-1. To investigate the base characteristics of
catalysts, which are crucial to the selective hydrogenation of CAL, the CO2 adsorption of
Pd/La2O2CO3-NPs and Pd/La2O2CO3-NRs materials with different support morphologies were
studied (Fig. 3). Desorption peaks located at 323-423 K, 423-673 K, 673-1073 K are attributed to
the weak, medium-strength and strong basic sites, respectively [22]. The amount of CO2 desorbed
from these base sites were calculated and displayed in Table 1. It can be noted that surface of
Pd/La2O2CO3-NRs exposed more amount of weak and medium-strength basic sites (totally 0.21
mmol g-1) than Pd/La2O2CO3-NPs (0.13 mmol g-1). These results indicated that support morphology
can modify surface basicity.
Table 1 Physical and chemical properties of Pd/La2O2CO3-NPs and Pd/La2O2CO3-NRs samples.
Amount of basicityc (mmol g-1)
Pd
contenta
(wt%)
0.99
b
SBET
Sample
Weak basic
Medium-strength
basic sites
0.11
(m2 g-1)
Total
sites
0.02
0.05
Pd/La2O2CO3-NPs
Pd/La2O2CO3-NRs
20.6
54.2
0.13
0.21
0.98
0.16

a Determined by ICP-AES. b BET surface area. c Estimated from the CO2-TPD profiles in Fig. 3.
Pd/La2O2CO3-nanorods
Pd/La2O2CO3-nanoparticles
50
100 150 200 250 300 350 400 450
Temperature (degree)
Fig. 3 CO2-TPD profiles of Pd/La2O2CO3-NRs and Pd/La2O2CO3-NPs samples.
350 348 346 344 342 340 338 336 334 332 330
Binding Energy (eV)
Fig. 4 Pd 3d XPS spectra of of Pd/La2O2CO3-NRs sample.
XPS was used to check the oxidation state of the reduced and heat-treated Pd/La2O2CO3-NRs
catalysts. As shown as Fig. 4, the Pd spectrum showed two weak peaks associated with Pd 3d3/2 and
Pd 3d5/2 at around 340.9 and 335.1 eV, respectively. These values clearly indicate the existence of
Pd in metallic form on the surface of La2O2CO3-NRs support (B.E. value of Pd 3d5/2 in metallic
form is 335.2 eV) [23]. Weak peaks and some burrs probably owes to small size and high-dispersity
of Pd nanoparticles.
3.2 Catalytic activity in hydrogenation of CAL

100
80
60
40
20
0
100
80
60
40
20
0
a
b
Pd/La2O2CO3-NPs
Pd/La2O2CO3-NRs
Pd/La2O2CO3-NPs
Pd/La2O2CO3-NRs
0.5
1.0
1.5
2.0
2.5
3.0
0.5
1.0
1.5
2.0
2.5
3.0
Reaction time (h)
Reaction time (h)
Fig. 5 (a) CAL conversion and (b) HCAL selectivity as a function of reaction time over
Pd/La2O2CO3-NPs (blue curve) and Pd/La2O2CO3-NRs (red curve) samples. Reaction conditions:
CAL (1 mL); catalyst (100 mg); solvent (ethanol, 20 mL); H2 pressure (0.1 MPa); temperature
(80 °C); stirring speed (800 rpm).
Catalytic tests in selective hydrogenation of CAL were performed over Pd/La2O2CO3 catalysts with
the different morphologies support of nanorods, nanoparticles and the results were presented in Fig.
5. The catalytic activity of the two Pd/La2O2CO3 catalysts increased with extending the reaction
time. However, The CAL conversion over the Pd/La2O2CO3-NRs sample using nanorods as support
was close to 100ꢀ% for 3 h, whereas the Pd/La2O2CO3-NPs using nanoparticles as support sample
showed only around 18 % CAL conversion for 3 h. We also compared the TOFs of HCAL produced
by these two catalysts. Remarkably, Pd/La2O2CO3-NRs catalyst showed better catalytic
performance (0.3ꢀs−1), nearly 10 times higher than Pd/La2O2CO3-NPs catalyst (0.04 s−1). As shown
as Table 2, the obtained activities for the Pd/La2O2CO3-NRs catalysts were compared with previous
reports to demarcate superiority of the present catalysts systems [24-28]. D. Srinivas et al. [29]
reported that alkali ions facilitate adsorption of CAL through polarization of the carbonyl of CAL.
This enhanced adsorption of CAL molecules and electronic effects are the causes for more efficient
activity of Pd/La2O2CO3 catalysts in presence of alkali solution. Combining CO2-TPD results, the
more medium-strength basic sites on Pd/La2O2CO3-NRs sample using nanorods as support is, the
more adsorption of CAL is on its surface, which resulting in enhanced reaction activity. It can be
clearly seen that more medium-strength basic sites were favorable for the catalytic performance.
Although more CAL was absorbed on La2O2CO3-NRs support by C=O bond, the hydrogenation
activity of Pd for C=O bond was much weaker than for C=C bond [30]. Thus, the main product was

still HCAL by the hydrogenation C=C of CAL over Pd/La2O2CO3 catalyst. These results suggested
that increase of basic sites could promote hydrogenation rate, but product selectivity was not
affected in presence of Pd. The enhanced catalytic activity may result from the morphology effect
of La2O2CO3 supports.
Table 2 Comparison of the activity reported of catalysts for the CAL hydrogenation.
Pd content
(wt%)
0.98
0.99
5.0
Reaction
CAL
HCAL
Catalysts
Ref.
time (h) conversion (%) selectivity (%)
Pd/La2O2CO3-NRs
Pd/La2O2CO3-NPs
Pd/MWNTs
3
3
100
17
86
96
81
90
91
82
93
This work
This work
25
8
100
79
24
25
26
27
28
Pd/CeO2-ZrO2
Pd/ZIF-8
2.0
1.0
6
100
100
100
Pd@TNTs
2.5
7
Pd/NMC
2.05
4
100
90
80
70
60
50
40
30
20
10
0
80
70
60
50
40
30
20
10
0
HCAL
HCOL
COL
1
2
3
4
Recycle number
Fig. 6 Recyclability study of Pd/La2O2CO3-NRs catalyst in the CAL hydrogenation. Reaction
conditions: CAL (1 mL); catalyst (100 mg); solvent (ethanol, 20 mL); H2 pressure (0.1 MPa);
temperature (80 °C); stirring speed (800 rpm).
The recyclability of Pd/La2O2CO3-NRs catalyst was investigated through a four-run recycling test
in selective hydrogenation of CAL. As shown as Fig. 6, recycling tests verified that the conversion
of CAL decreases slightly from 93 % to 82 % while the selectivity of HCAL keeps constant at a
very high level of 75 %, which indicating that Pd/La2O2CO3-NRs catalyst exhibited excellent
cycling stability. Based on XRD and TEM results (Fig. 7), the loss of catalytic activity may be

mainly due to the minor rupture of La2O2CO3 nanorods and aggregation of Pd nanoparticles.
(103)
(101)
a
b
(110)
(100)
used catalyst
fresh catalyst
10
20
30
40
50
60
70
80
2 Theta (degree)
Fig. 7 (a) XRD patterns of fresh and used Pd/La2O2CO3-NRs catalyst in the CAL hydrogenation.
(b) TEM image of used Pd/La2O2CO3-NRs catalyst in the CAL hydrogenation.
Based on the literature and our results, a brief reaction mechanism of CAL hydrogenation over
Pd/La2O2CO3-NRs catalyst was proposed, which is shown in Fig. 8. Firstly, the C=O group in CAL
was preferentially adsorbed and activated by La3+-O2- pairs (medium-strength basic sites) at
La2O2CO3 support. Meanwhile, hydrogen was dissociated on the surface of Pd nanoparticles. While
because the chemisorption of C=C bond over the Pd metal catalysts becomes more favorable than
C=O bond, the selective hydrogenation of C=C bond preferably occurs by the attack of the activated
hydrogen species and the resulting atomic hydrogen reacted with the activated C=C group, forming
HCAL. Hence, surface basicity accelerated activation of C=C bond and promote the reaction.
H
H
H
H
H
H
Pd
Pd
Pd
Pd
La3+
La3+
O2+
La3+
La3+
O2+
O2+
La3+
La3+
La3+
La3+
O2+
O2+
O2+
La3+
La3+
O2+
O2+
Fig. 8 Possible reaction mechanism of CAL hydrogenation on Pd/La2O2CO3-NRs catalyst.
4. Conclusion
In summary, we successfully synthesis La2O2CO3 nanomaterials with different morphologies
(nanorods and nanoparticles) and then Pd nanoparticles were uniformly dispersed on them by

deposition-precipitation method. The catalytic activities of Pd/La2O2CO3 catalysts for selective
hydrogenation of CAL were investigated. It has been observed that the Pd/La2O2CO3-NRs catalyst
exhibits higher hydrogenation rate (0.3ꢀs−1) and excellent cycling stability, whereas Pd dispersed on
La2O2CO3 nanoparticles (Pd/La2O2CO3-NPs) showed relatively low catalytic activities (0.04ꢀs−1).
This difference in catalytic activities was attributed to basicity of La2O2CO3 supports. CO2-TPD
showed that the intense desorption peaks at about 560 K represented the medium-strength basic
sites (La3+-O2− pairs) with a total amount of 0.13 mmol·g-1 on the La2O2CO3-NPs and 0.21 mmol·g-
1
on the La2O2CO3-NRs. These results suggested that increase of basic sites could obviously
promote hydrogenation rate, but product selectivity was not affected in presence of Pd. The
enhanced catalytic activity may result from the morphology effect of La2O2CO3 supports.
Consequently, the Pd/La2O2CO3-NRs catalyst is a highly efficient catalyst for selective
hydrogenation of CAL to HCAL.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (No.
21503023) and Advanced Catalysis and Green Manufacturing Collaborative Innovation Center,
Changzhou University.
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