Particle size effects in the selective hydrogenation of cinnamaldehyde over supported palladium catalysts

Article abstract of DOI:10.1039/c6ra17000e

This work investigated the catalytic performance of palladium catalysts in the selective hydrogenation of α,β-unsaturated aldehydes, and especially the effect of Pd particle size on the hydrogenation of cinnamaldehyde (CAL). An unsupported nanosized Pd catalyst and a series of supported Pd catalysts using supports of activated carbon, SiO₂, TiO₂, γ-Al₂O₃, SiC, and graphene oxide were prepared and evaluated in the selective hydrogenation of CAL. Varied sizes of Pd particles could be obtained directly by using different Pd precursors and indirectly by introducing an inactive Ag metal over a γ-Al₂O₃ supported palladium catalyst. Over a reducible TiO₂ support, the Pd particle size can also be controlled indirectly by the effect of strong metal-support interaction (SMSI). Combined with their TEM observations and catalytic tests, density functional theory (DFT) calculations have confirmed that smaller Pd particles favored CC-centered adsorption of CAL leading to a higher selectivity to hydrogenation of the CC bond, forming hydrocinnamaldehyde (HCAL), while on larger ones, the CC-centered adsorption would be partly substituted by CO-centered adsorption resulting in a lowered selectivity to HCAL but an increased selectivity to CO bond hydrogenation, forming hydrocinnamyl alcohol (HCOL). It is clarified that the size dependence of the catalytic selectivity originates from the strong dependence of CC/CO-centered adsorption on Pd particle size. Finally, tests using solvents with different Pd-solvent interactions and α,β-unsaturated aldehydes with remarkable steric effect variations were applied to further regulate the adsorption between palladium and substrate to alter the catalytic activity and selectivity.

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Particle Size Effects in the Selective Hydrogenation of
Cinnamaldehyde over Supported Palladium Catalysts
Feng Jiang,a,b Jian Cai, a Bing Liu,a Yuebing Xu a and Xiaohao Liu *, a
Received 00th January 20xx,
Accepted 00th January 20xx
Dedicated to Professor Makoto Tokunaga on the occasion of his 50th birthday
DOI: 10.1039/x0xx00000x
This work investigated the catalytic performance of palladium catalysts in the selective hydrogenation of α,β-unsaturated
aldehydes, and especially the effect of Pd particle size on the hydrogenation of cinnamaldehyde (CAL). An unsupported
nanosized Pd catalyst and a series of supported Pd catalysts using supports of activated carbon, SiO2, TiO2, γ-Al2O3, SiC, and
graphene oxide were prepared and evaluated in the selective hydrogenation of CAL. Varied sizes of Pd particles could be
obtained directly by using different Pd precursors and indirectly by introducing an inactive Ag metal over a γ-Al2O3
supported palladium catalyst. Over a reducible TiO2 support, the Pd particle size can also be controlled indirectly by the
effect of strong metal-support interaction (SMSI). Combined with their TEM observations and catalytic tests, the density
functional theory (DFT) calculations have confirmed that smaller Pd particles favored C=C-centered adsorption of CAL
leading to a higher selectivity to hydrogenation of C=C bond, forming hydrocinnamaldehyde (HCAL), while on larger ones,
the C=C-centered adsorption would be partly substituted by C=O-centered adsorption resulting in a lowered selectivity to
HCAL but an increased selectivity to C=O bond hydrogenation, forming hydrocinnamyl alcohol (HCOL). It’s clarified that the
size dependence of the catalytic selectivity originates from the strong dependence of C=C/C=O-centered adsorption on Pd
particle size. Finally, the tests of using solvents with different Pd-solvent interactions and α,β-unsaturated aldehydes with
remarkable steric effect variations were applied to further regulate the adsorption between palladium and substrate to
alter the catalytic activity and selectivity.
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of C=C and C=O bonds.10-13 Hydrogenation of the C=C bond
only is more desired for forming hydrocinnamaldehyde (HCAL),
an important intermediate for synthesis of pharmaceuticals in
the treatment of HIV.9 Many noble metals can be used as an
active component for the activation of C=C and C=O bonds,
and the selectivity to C=C bond follows the sequence of
Os<Ir<Pt<Ru<Pd.1, 10 The Pd-based catalysts are considered to
be one of the most promosing systems for the selective
hydrogenation of the C=C bond.
1. Introduction
Catalytically selective hydrogenation of α,β-unsaturated
aldehydes is a very important and also challenging reaction in
both academic research and industrial application, for example,
for producing flavors and pharmaceuticals.1-8 Many efforts
have been devoted to catalyst design for purposefully
regulating its catalytic activity and the selectivities to disired
products.Thermodynamically, hydrogenation of both C=C bond
and C=O bond of an α,β-unsaturated aldehyde is feasible,
while selective hydrogenation of the C=C bond only, leaving
the C=O group intact for the production of saturated
aldehydes, is quite important for catalytic transformation.
Several methods have been applied to adjust the
selectivity to C=C bond, including using bimetallic catalysts,9, 11-
15 changing the support,4, 16 using different reaction media,10, 17,
18
modifying the surface of active metal,19, 20 controlling the
size of metal particles,21-28 and so on. Among them, particle
size seems to be a significant and straight-forward factor
regulating the catalytic activity and selectivity to C=C bond.
However, owning to the varied hydrogenation substrates, such
as acrolein and crotonaldehyde, the influence of particle size
could be different and even contradictory over the same active
metal, Ag.21, 22 Consequently, different active metals, such as
Ag, Au and Pt, usually exhibit varied adsorption modes of the
same α,β-unsaturated aldehyde thereby yields varied particle
size effects on selectivity to C=C bond due to the different
electronic properties.22 As for cinnamaldehyde, some
Particularly, cinnamaldehyde (CAL) hydrogenation is
a
popularly reported model reaction for the selective activation
a.The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education,
School of Chemical and Material Engineering, Jiangnan University
No.1800 Lihu Avenue, Wuxi, Jiangsu Province, 214122, China
E-mail: liuxh@jiangnan.edu.cn
b.Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology,
Changzhou University, 213164, China.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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researchers report that a higher selectivity to C=C bond could SMSI effect were also applied to tailor the particle size of Pd
be achieved over smaller particles of Pt, Ru and Rh active metal indirectly. Further, the DFT calcinations were also
metals.24, 25 For Au metal, however, smaller clusters are prone carried out to explain the relationship between Pd particle size
to favor C=O adsorption over C=C adsorption, which is not in and adsorption modes, the results of which is well in
agreement with the above mentioned Pt, Ru and Rh metals.26 accordance with our experimental results. Finally, other
Therefore, there is an incentive to search for the particle size factors including using solvents with distinct palladium-solvent
effect of Pd metal in CAL hydrogenation. There are a few interaction and α,β-unsaturated aldehydes with remarkable
articles can be found in the literature using Pd as the catalysts differences in steric effect were studied to tailor the
for selective hydrogenation.8, 29-34 However, most researches adsorption of substrate and then catalytic activity and
concern with the application of novel preparation methods selectivity.
and materials in this reaction, such as preparing Pd@MOF
catalysts,30 and using N-doped reduced graphene oxides29 as
2. Experimental and Computational Details
supports. The particle size effect of Pd metal in selective
hydrogenation of CAL is still needed to be clarified, both
experimentally and theoretically.
2.1 Materials
Palladium acetate [Pd(OAc)2], palladium(II) acetylacetonate
[Pd(acac)2], palladium nitrate hydrate [Pd(NO3)2], tetraammine
dichloropalladium monohydrate [Pd(NH3)4Cl2], silver nitrate
(AgNO3), oleylamine (OAm, >70%), borane-morpholine
complex (MB, >95%), cinnamaldehyde (CAL, >95%), 1-
octadecene (ODE, >90%), α-methylcinnamaldehyde (95%), and
β-phenylcinnamaldehyde (98%) were purchased from Aladdin.
P25 TiO2 (surface area = 51 m2 g-1) was obtained from Sigma-
Aldrich. Methanol, acetone, n-hexane, p-xylene, cyclohexane,
tetrahydrofuran, pyridine, N,N-dimethylaniline, acrolein, and
N,N-dienthylaniline were purchased from Sinopharm Chemical
Reagent Co., Ltd. All of the regents used in this study were
analytical grade and used without further purification.
Experimentally, Pd particles with different sizes are
supposed to be obtained directly and indirectly from multiple
approaches. On the one hand, varied Pd particles should be
obtained directly by adjusting preparation conditions, such as
changing metal precursor, preparation solvent, metal loading,
and so on.35-37 On the other hand, smaller particles are
supposed to be achieved indirectly through dividing or
modifying larger particle. In our previous research, Ag metal is
inactive in the hydrogenation of CAL. It has been reported to
has a strong surface enrichment over Pd metal.38-41 Thus, the
surface concentration of Pd atoms decreased as the increase
of Ag/Pd ratio suggesting a promising method to alter particle
size. So far, there are few researches about the application of
Ag-Pd bimetallic catalysts in the selective hydrogenation of
CAL. The strong metal-support interaction (SMSI) effect, has
been reported to be due to the migration of partially reduced
metal oxide species onto the surface of metal particles. Under
proper treatment conditions, a porous layer could be observed
covering the active metal particles when TiO2 was used as the
support.42, 43 Owning to this layer, the leaching of active metal
particles has been effectively prevented. However, the
application of this effect in tuning particle size has not been
founded yet. It’s assumed that this porous layer may also have
some effect on tuning the particle size of active metal.
Theoretically, the density functional theory (DFT) is an efficient
tool to calculate the adsorption energy on varied adsorption
modes thereby elaborating the adsorption between active
metal and substrates.22, 44 In addition, the active metal-solvent
interaction strongly depends on the solvent characters, which
might result in significant impacts on particle size effect by
affecting the adsorption between substrate and active metal
indirectly.
2.2 Catalyst preparation
(1) Supported and unsupported Pd catalysts
Supported Pd catalysts were prepared by an incipient wetness
impregnation method. Typically, desired amounts of Pd(OAc)2
solution were impregnated to various supports including
activated carbon (AC), γ-Al2O3, SiO2, SiC, and TiO2. After
impregnation, the samples were dried slowly in vacuum at
40 °C and then at 120 °C in an oven for 12 h. Then, these dried
samples were calcined in N2 at 300 °C for 5 h and finally
reduced in H2 first at 120 °C for 1 h and then at 250 °C for 0.5 h
at a temperature ramping rate of 2 °C min-1. All catalysts
containing the same Pd loading equal to 0.5 wt%. The finally
obtained samples were denoted as Pd/AC, Pd/γ-Al2O3, Pd/SiO2,
Pd/SiC, and Pd/TiO2, respectively.
Unsupported Pd catalyst was prepared by an oleylamine-
mediated synthesis method as reported by Sun.45 Briefly,
Pd(acac)2 of 0.1 g was mixed with 8 mL ODE and 10 mL OAm,
and the mixture was heated to 100 °C under nitrogen
protection to form a near colorless solution. MB of 0.2 g
dissolved in 2 mL OAm was injected into the above solution.
Then, this solution was heated to 130 °C and kept at this
temperature for 20 min. After the solution was cooled down to
room temperature, ethanol (20 mL) was added and the
product was separated by centrifugation. The product was
dispersed in n-hexane and denoted as Pd NPs. The average size
of Pd particles detected by TEM is 4.1 nm (Figure S1).
In this paper, a wide range of Pd catalysts including
unsupported Pd NPs and several activated carbon, SiO2, γ-
Al2O3, TiO2, SiC, and graphene oxide supported Pd catalysts
have been prepared in order to get
a preliminary
understanding of the catalytic performance of Pd catalysts in
selective hydrogenation of CAL. On basis of that, γ-Al2O3 and
TiO2 supported Pd catalysts were chosen for the further study
of particle size effect. In addition to tuning the particle size
directly by changing the precursor and reduction temperature,
the other two methods including introducing Ag metal and
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The Pd/GO (Graphene Oxide) was prepared as the follow particle size distribution diagram and estimation of the
procedure: 5 mg Pd NPs was dissolved in 50 mL acetone and average particle size, at least 100 particles were manually
sonicated for 15 min. GO of 1 g was also dissolved in another counted.
50 mL acetone and sonicated for 15 min as well. Then, mix the
two mixtures and sonicate for another 1 h. The product was 2.4 Catalytic tests
then centrifugated and washed with ethanol twice before they
Selective hydrogenation of cinnamaldehyde was performed in
were dried under N2.
an autoclave (100 mL) with a fixed stirring rate of 800 rpm at a
(2) Pd/γ-Al2O3 catalysts prepared with different precursors
hydrogen pressure of 2 MPa and a temperature of 80 °C.
Paraxylene was typically used as a media for the reaction. In
detail, the vessel was charged with 40 g of paraxylene, 10 g of
The Pd/γ-Al2O3 catalysts were prepared with different Pd
precursors, including Pd(NO3)2, Pd(NH3)4Cl2 and Pd(OAc)2, by
the above mentioned incipient wetness impregnation method.
For Pd(NO3)2 and Pd(NH3)4Cl2 two precursors, the Pd/γ-Al2O3
catalysts were prepared in their aqueous solutions, while for
Pd(OAc)2 precursor, the catalyst were done in methanol
solution. The finally obtained samples were denoted as 0.5wt%
Pd-NO3/γ-Al2O3, 0.5wt% Pd-Cl/γ-Al2O3, 0.1wt% Pd-Ac/γ-Al2O3
and 0.5wt% Pd-Ac/γ-Al2O3, respectively.
cinnamaldehyde and
a desired amount of catalyst. The
autocalve was purged with hydrogen three times before the
pressure was adjusted to a given initial pressure (2 MPa) at
room temperature. The mixture was then stirred and the
temperature was elevated. The reaction time was recorded as
soon as the temperature reached 80 °C. In recycling tests, the
Pd/γ-Al2O3 catalyst was collected by centrifugation, and
recycled into the next batch test without drying.
(3) AgPd/γ-Al2O3 bimetallic catalysts
After reaction, the reaction mixture was cooled, collected
and analyzed by a gas chromatograph equipped with an FID
detector and a capillary column KF-FFAP (60 m × 0.32 mm ×
0.5 μm). Conversion and product selectivity were calculated
based on the carbon balance with calibration curves.
The bimetallic catalysts were prepared by a co-impregnation
method using AgNO3 and Pd(OAc)2 methanol solution to
achieve varied Ag/Pd atomic ratios from 0.1 to 20 with the
fixed Pd loading of 0.5 wt%. The impregnated samples were
dried, calcined and reduced as the above mentioned
preparation procedure of Pd/γ-Al2O3 catalysts. Finally, the
2.5 Computational details
obtained catalysts were denoted as AgPd-x/γ-Al2O3, where x All the density functional theory (DFT) calculations were
was referred to the atomic ratio of Ag/Pd. In addition, Ag/γ- performed using the Vienna ab initio simulation package
Al2O3 catalyst was also prepared by the same method with an (VASP).46, 47 The projector-augmented wave method was used
Ag loading of 0.5 wt% as a reference catalyst.
to represent core-valence interactions.48 Valence electrons
were described by a plane wave basis with an energy cutoff of
400 eV. The generalized gradient approximation with the
(4) Pd/TiO2 catalysts for SMSI effect
The preparation method of Pd/TiO2 catalyst is similar as that of Perdew-Burke-Ernzerhof functional was used to model
a reported Co/TiO2 catalyst.42 Prior to preparation of the electronic exchange and correlation.49 Gaussian smearing with
catalysts, a commercial support P25 TiO2 was calcined at a width of 0.05 eV was used to improve convergence of states
750 °C (4 °C/min) for 4 h. A methanol solution of Pd(OAc)2 near the Fermi level. The Brillouin zone was sampled at the
precursor was impregnated with TiO2 support by an incipient Gamma point. Optimized structures were obtained by
wetness impregnation method. The loading of Pd was minimizing the forces on each ion until they were less than
designed as 0.1 wt%. After the impregnation, the catalyst was 0.05 eV/Å.
dried slowly in vacuum at 40 °C and then 100 °C in an oven for The adsorption energy was defined as:50, 51
12 h followed by calcined in air at 300 °C (1 °C min-1) for 2 h.
Eads = E(adsorbate+surface) - E(adsorbate) - E(surface)
Before selective hydrogenation of CAL, the Pd/TiO2 catalysts where E(adsorbate+surface) is the total energy of the
were directly reduced in H2 at 250, 450 and 600 °C for 2 h and adsorbate interacting with the surface; E(adsorbate) and
respectively labeled as Pd/TiO2-250R, Pd/TiO2-450R, and E(surface) are the energies of the free adsorbate in gas phase
Pd/TiO2-600R. For comparison, the Pd/TiO2-600C-600R catalyst and the bare surface, respectively.
A negative value
was obtained by the calcined Pd/TiO2 sample experienced a corresponds to exothermic adsorption, with more negative
second calcination at 600 °C and then reduced at 600 °C.
value corresponding to stronger adsorption.
2.3 Catalyst characterization
3. Results and discussion
The morphology, particle size and composition of the samples
were evaluated by means of Field Emission Scanning Electron
Microscope FEI Tecnai F20 equipped with the X-ray energy
dispersive (EDX) system operating at 200 kV. TEM and STEM
samples were prepared by dropwise addition of the dilute
catalyst suspension on a copper-coated carbon TEM grid
followed by the evaporation of the solvent. To prepare the
3.1 CAL hydrogenation over several supported and
unsupported Pd NPs
In general, cinnamaldehyde has two unsaturated functional
groups, C=C bond and C=O bond. The hydrogenation can
proceed on either C=C or C=O bond or both. As shown in
Scheme 1, the C=C bond hydrogenation yield a saturated
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aldehyde, hydrocinnamaldehyde (HCAL), while that of C=O 3.2 Influence of Pd precursor on particle size and CAL
bond results in an unsaturated alcohol, cinnamyl alcohol (COL), hydrogenation
and the complete hydrogenation product is a saturated alcohol,
For the selected Pd/γ-Al2O3 catalyst, the Pd particle size can be
hydrocinnamyl alcohol (HCOL).
definitely varied by changing Pd precursors. As shown in Figure
1, under the same Pd loading of 0.5 wt%, the catalysts
prepared by Pd(OAc)2 showed the smallest Pd particle size
(1.8 nm) and much narrower size distribution (±0.4 nm) while
those prepared by Pd(NH3)4Cl2 and Pd(NO3)2 showed largest
(5.7±1.8 nm) and second largest (2.5±1.0 nm) Pd particle sizes,
respectively, and obvious broad size distributions. The
precursor containing chlorine yields much bigger particle sizes
than other precursors, which is in accordance with the results
of Piqueras’s.35 Further, the particle size became slightly
smaller from 1.8 to 1.4 nm as the Pd loading decreased from
0.5 to 0.1 wt%.
Scheme 1. Reaction pathways of the cinnamaldehyde hydrogenation.
The catalytic performance of supported selective
hydrogenation catalysts is profoundly influenced by the
surface properties of supports.52,
53
In order to get an
integrated understanding about the catalytic performance of
Pd catalysts in this reaction, several supports with totally
varied surface characters including activated carbon, SiO2, γ-
Al2O3, TiO2, SiC, graphene oxide were used to prepare the
supported Pd catalysts. Meanwhile, the unsupported Pd NPs
also have been prepared and tested for the title reaction in an
autoclave reactor. As shown in Table 1, a greatly high
selectivity to HCAL over 70% could be obtained for all the
prepared Pd catalysts, suggesting a good catalytic performance
of Pd in selective hydrogenation of C=C bond. Under the same
reaction conditions, however, there are also considerable
differences in hydrogenation ability of CAL over varied
palladium catalysts. Over SiC and activated carbon supported
Pd catalysts, the conversions of CAL were very low (35.5%,
31.8%), and the selectivities to HCAL (84.0%, 87.1%) were not
very promising, either. For unsupported and TiO2, grephene
oxide supported Pd catalysts, though the catalytic activity
increased giving CAL conversions ranging from 61% to 65%, the
selectivity to HCAL was still lower than 87%. In contrast, the γ-
Al2O3 and SiO2 supported Pd catalysts could convert CAL
completely under the same reaction conditions. Particularly,
the Pd/γ-Al2O3 catalyst yielded the highest HCAL selectivity of
Figure 1. STEM images and Pd particle size distributions of (a) 0.1wt% Pd-Ac/γ-
Al2O3, (b) 0.5wt% Pd-Ac/γ-Al2O3, (c) 0.5wt% Pd-NO3/γ-Al2O3, and (d) 0.5wt% Pd-
Cl/γ-Al2O3.
Table 2. Effect of precursor and Pd loading on the particle size and catalytic
performance in selective hydrogenation of CALa.
92.2% making it suitable for our further research on selective Entry Precursor
hydrogenation of CAL.
Size of Pd CAL
Selectivity, %
HCAL HCOL
particle,
nmb
1.3
Conversion, %
Table 1. Catalytic results over different supported and unsupported
palladium catalysts in selective hydrogenation of CALa.
c
1
2
3
4
5
6
Pd(OAc)2
Pd(OAc)2
Pd(NO3)2
100
95.7
92.2
92.1
77.5
95.9
92.4
4.3
7.8
7.9
22.5
4.1
7.6
d
d
1.8
100
Catalystsb
CAL Conversion, %
Selectivity, %
HCAL
92.2
2.5
62.9
41.9
99.7e
93.1e
HCOL
7.8
d
Pd(NH3)4Cl2
c
Pd(OAc)2
5.7
Pd/γ-Al2O3
Pd/SiO2
Pd/SiC
100
1.3
100
81.6
18.4
16.0
12.9
27.3
20.2
13.8
d
Pd(OAc)2
1.8
35.5
31.8
61.7
64.0
64.3
84.0
aReaction conditions: CAL 10 g, Pd 0.5 mg, 80 °C,
bDetermined from STEM images. Pd loading of 0.1 wt%. dPd loading of 0.5
wt%. e Reaction for 2 h.
6
h, H2
2
MPa.
Pd/AC
87.1
c
Pd/TiO2
Pd/GO
72.7
79.8
The catalytic results of the obtained Pd/γ-Al2O3 catalysts
were shown in Table 2. Obviously, the Pd particle size has
significant influence on both CAL conversion and HCAL
Pd NPs
86.2
aReaction conditions: CAL 10 g, Pd 0.5 mg, 80 °C, 6 h, H2 2 MPa. bPd loading
were fixed as 0.5 wt%.
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selectivity. With the increase of Pd particle size, more CAL
Since the hydrogenation can proceed on both C=C and
couldn’t be totally converted in the same reaction period of 6 C=O bond, clarifying the reaction pathway is important for a
h and less HCAL was formed in products. For the 0.5wt% Pd- deeper understanding of this reaction. Noting that only HCAL
Cl/γ-Al2O3 catalyst, especially, only 41.9% of CAL conversion (C=C bond hydrogenation product) and HCOL (complete C=C
with 77.5% of HCAL selectivity was obtained after 6 h. In and C=O bonds hydrogenation product) were detected in the
comparison, for the 0.1wt% Pd-Ac/γ-Al2O3 catalysts that final product while none of the COL via hydrogenation of C=O
containing the smallest Pd particles,
a near 100% CAL bond was detected. Similar observations can also be found
14
conversion with more than 95% of HCAL selectivity could be over SiO2
and carbon54 supported palladium catalysts
obtained even after 2 h of reaction period.
indicating that the hydrogenation of C=C bond is highly
preferred over C=O bond in selective hydrogenation of CAL. It
can also be seen in Table 2 that the selectivities to HCAL and
HCOL remained stable with further increase in the reaction
time even as the CAL was completely consumed (0.1wt% Pd-
Ac/γ-Al2O3 catalyst, Entries 1 and 5). This result suggests that
HCAL can’t be further hydrogenated, and the saturated alcohol
HCOL is the product of further hydrogenation of the C=C bond
in COL rather than the C=O bond in HCAL. In other words, if
C=C bond in CAL was first hydrogenated, the C=O bond could
not be further done. Such findings indicate a very poor
hydrogenation ability of Pd catalyst on oliphatic aldehydes. To
confirm this, the hydrogenation of mixed hexene-1 and
hexaldehyde with n-heptane as the solvent were carried out at
a constant H2 pressure of 2 MPa and a reaction temperature of
150 °C using Pd/γ-Al2O3 and Pd/SiC as catalysts, respectively.
After 2 h of reaction, there was no alcohol detectable in the
Figure 2. CAL conversion versus product selectivity over 0.1wt% Pd-Ac/γ-Al2O3
and Pd/SiC catalysts. Reaction conditions: CAL 10 g, 0.1wt% Pd-Ac/γ-Al2O3 200 mg
(Pd/SiC 100 mg), 80 °C, H2 2 MPa.
products with only hexene-1 hydrogenated product of
n-
hextane, solvent and the unreacted hexaldehyde in the final
mixture. It’s proved that palladium does not hydrogenate
aliphatic aldehydes in our reaction conditions. Further, if
hydrogen donor was added in the reaction system or changing
the electronic properties of the palladium on the surface, the
hydrogenation of aliphatic aldehydes over a Pd catalyst could
also be done.55-57 In contrast, the hydrogenation of aromatic
aldehydes is much easier on conventional Pd catalysts.58-61
Therefore, in our research, if the C=C bond in CAL was not first
hydrogenated, the C=O bond in CAL which was vinylogue with
benzaldehyde could be done yielding COL as the product.
Further, the hydrogenation rate of COL to HCOL was quite fast,
which was about 30 times faster than that of CAL to HCAL on
supported Pd catalyst.62 In other words, once the
hydrogenation proceeded on C=O bond which would inevitably
lead to further hydrogenation of C=C bond. So the final
products conatined both the C=C bond hydrogenation product
HCAL and the complete C=O and C=C bond hydrogenation
product HCOL which came from the further hydrogenation of
COL. The selectivities to HCAL and HCOL are determined by
which bond the first hydrogenation step taking place, C=C
bond or C=O bond.
The TEM observations and catalytic results indicated that
particle size could influence not only CAL conversion but also
product distribution. Thus, the selectivities were obtained at
different conversion level. To eliminate the influence of CAL
conversion on product selectivity, the relationship between
conversion and selectivity were studied over Pd/γ-Al2O3 and
Pd/SiC catalyst. As shown in Figure 2, the product selectivity
was independent on conversion within the conversion ranging
from 10% to 100% for both catalysts. In addition, although the
average reaction rate decreased sharply from 295 to 68 molCAL
gPd-1h-1 as the reaction proceed from 10 to 250 minutes, the
selectivity to HCAL was maintained a very high level of above
94% (Table S1). In other words, the product selectivity was
independent on both conversion and reaction rate. Therefore,
it can be confirmed that the varied selectivity in Table 2 is not
due to the varied conversions but the different particle size of
Pd. The hydrogenation of CAL is quite structure sensitive. A
smaller Pd particle size can be greatly benificial to not only the
catalytic acivity but also the selective hydrogenation of the C=C
bond in CAL.
In addition, although the supported palladium catalysts
show preferred selectivity to HCAL, the value of 95.9% in our
work is superior compared with the literature reported values
3.3 Influence of Ag metal on the particle size and CAL
hydrogenation
(
Table S2). Moreover, the average reaction rate is higher than
the corresponding values of literature reported in the same
reaction period as well, especially the one which also use γ-
Al2O3 as the support (Table S2). Except for the support effects,
the much smaller Pd particle size in the present research is
responsible for the higher activity and selectivity to C=C bond
hydrogenation.
In addition to obtain the varied particle size directly, other
methods were employed to tune the particle size indirectly.
Addition of a noble metal Ag which is inactive for CAL
hydrogenation into the 0.5wt% Pd-Ac/γ-Al2O3 catalyst could
further affect the Pd particle size. As shown in Figure 3, the Ag
metal tended to alloy with the Pd metal and the formed Ag-Pd
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particles grew bigger, being about 6.6±2.0 nm as the Ag/Pd
ratio was 1.0. For the AgPd-1.0/γ-Al2O3 catalyst, the surface
Ag/Pd ratio was confirmed to be 3.3 by STEM-EDX analysis of
an isolated particle and the ratio was much greater than the
bulk one, suggesting a surface enrichment of Ag in the Ag-Pd
alloy. One can image that the enrichment of Ag on the alloy
surface would greatly lower the number of Pd active sites for
this reaction, but on the other hand, it could isolate one big
alloy particle into many smaller Pd particles which favored the
hydrogenation of C=C bond, forming HCAL (Scheme 2).
Figure 4. CAL conversion and product selectivity over AgPd/γ-Al2O3 catalysts.
Reaction conditions: CAL 10 g, catalyst 100 mg, 80 °C, 6 h, H2 2 MPa.
3.4 Influence of SMSI on the particle size and CAL
hydrogenation
Over a reducible support of TiO2, the size of Pd particles can be
tunable directly by treatments of varying reduction
temperatures and indirectly by SMSI effect. For a Pd/TiO2
catalyst calcined at 300 °C, the size of Pd particle on it
increased from 2.3 to 2.8 and further to 3.8 nm (Figure 5),
when the catalyst was reduced in H2 at 250, 450 and 600 °C,
respectively. As expected, both the CAL conversion and HCAL
selectivity decreased obviously with the increasing of Pd
particle size (Table 3). This result shows a good agreement
with the observed relationship between the Pd particle size
Figure 3. (A) STEM image of AgPd-1.0/γ-Al2O3 and Pd particle size distribution in
the sample; (B) STEM-EDX spectrum of the single isolated particle “1”.
As shown in Figure 4, the CAL conversion did decrease
linearly from 100% to 51.6% as the Ag/Pd ratio increased from
0 to 1.0, and the catalysts showed no activity at the Ag/Pd
ratio of 20 and higher, while the HCAL selectivity unchanged
and remained a very high level of 92% as long as the AgPd/γ-
Al2O3 catalyst was still active for the reaction. The Pd atoms
are supposed to be totally covered by Ag atom when the
Ag/Pd ratio is higher than 3.0.39 Therefore, there is only Ag
atoms on the alloy surface as the ratio increasing to 20.
Consequently, there is no reaction over the AgPd-20/γ-Al2O3
catalyst. These results also suggest that the smaller Pd particle
size is the key for the high selectivity to HCAL, while the
number of exposed Pd sites is responsible for the level of CAL
conversion.
and catalytic performance of Pd/γ-Al2O3 catalyst in Table 2
.
Besides, analogous variations of activity and selectivity could
also be observed over Pd/γ-Al2O3 catalysts treated by
increasing calcination/reduction temperature (Table S3, S4).
However, the selectivity to HCAL over Pd/TiO2 catalyst is
always lower than over Pd/γ-Al2O3 catalyst.
Table 3. Effect of calcination and reduction treatments on the particle size
and catalytic performance in selective hydrogenation of CALa.
Sample
Size of Pd CAL
Selectivity, %
particle,
nmb
2.3
conversion,
HCAL
HCOL
%
Pd/TiO2-250R
100
99.9
31.6
13.9
71.0
61.9
43.2
67.9
29.0
38.1
56.8
32.1
Pd/TiO2-450R
2.8
Pd/TiO2-600R
3.8
Pd/TiO2-600C-600R
4.0
aReaction conditions: CAL 10 g, catalyst 250 mg, 80 °C, 3 h, H2 2 MPa.
bDetermined from TEM images.
Scheme 2. Catalyst models of the prepared supported Pd catalysts.
Further, as the prepared Pd/TiO2 catalyst experienced a
second calcination at 600 °C before its reduction at 600 °C, the
size of the Pd particle (Figure 5d) was observed to increase
slightly to 4.0 nm and the CAL conversion further decreased
from 31.6% to 13.9% in the same reaction period, but the
selectivity to HCAL unexpectedly recovered to 67.9%,
compared to those obtained over Pd/TiO2-600R catalyst. After
carefully observing the TEM image of Pd/TiO2-600C-600R
catalyst (Figure 5f), there was a thin layer covered the Pd
6 | J. Name., 2012, 00, 1-3
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particle. Such layer was not observed over the Pd/TiO2-600R activating C=O bond. Obviously, the effect of the formor is
catalyst (Figure 5e). This layer is believed to be TiOx and stronger, which results in a much higher selectivity to HCAL of
formed by migration of partially reduced TiO2.42 The SMSI 67.9% than that of the Pd/TiO2-600R catalyst (43.2%)
effect is also responsible for this phenomenon. Because of this possessing the similar particle size of Pd but no such layer. In
layer, the available number of active Pd sites decrease. general, the SMSI effect can be used to decorate the active
However, the layer may be porous, which divides the large metal nanoparticles thereby tailoring the particle size. These
particle into many smaller ones benefiting the selective relusts also confirmed the preferred hydrogenation of C=C
hydrogenation of C=C bond in CAL (Scheme 2).
bond on smaller particle over palladium catalyst.
3.5 DFT calculations for CAL adsorption
Our experimental results have confirmed the superior activity
and C=C bond selectivity on smaller Pd particles from different
aspects. To find out the reason for this phenomenon, DFT
calculations were performed to elucidate the effect of Pd
particle size on the selectivity and activity theoretically. A Pd4
nanocluster was used to represent the smaller Pd particle, and
the slab model of Pd(111) surface was used to represent the
larger Pd particle. The calculated structures of the CAL
molecule adsorbed on Pd4 nanocluster and Pd(111) surface
and the corresponding adsorption energies are shown in
Figure 6.
Figure 5. (a) Pd/TiO2-250R, (b) Pd/TiO2-450R, (c, e) Pd/TiO2-600R, and (d, f)
Pd/TiO2-600C-600R.
There are some researches about the effect of SMSI on
63-65
hydrogenation of α,β-unsaturated aldehydes with Pt,23,
Ir,66, 67 Rh,68 Au and Ag used as the active metals, while
that of Pd metal can be hardly found. Over TiO2 support, most
studies disclose that the SMSI effect would leads to a higher
selectivity to C=O bond owning to the TiOx species at the
interface of active metal and TiO2 support. In CAL
hydrogenation, a reduced selectivity to C=C bond could be
observed over Pt/TiO2-SiO2 catalysts owning to this effect.23
However, in our present study, this effect obviously increased
the selectivity to C=C bond. This may be mainly attributed to
the totally different calcination/reduction conditions leading to
69
70
Figure 6. CAL adsorption on Pd4 nanocluster through (a) C=C bond and (b)
C=O bond; CAL adsorption on Pd(111) surface through (c) C=C bond and (d)
C=O bond. Eads is the corresponding adsorption energy.
As can be seen from Figure 6, for the Pd4 nanocluster, the
adsorption of CAL through C=C bond (Figure 6a) is stronger
than that through C=O bond (Figure 6b), indicating that
smaller Pd particle favors the C=C-centered adsorption and
hydrogenation which leads to the selectivity toward HCAL; on
the contrary, for Pd(111) surface, the adsorption of CAL
through C=O bond (Figure 6d) is stronger than that through
C=C bond (Figure 6c), indicating that larger Pd particle favors
the C=O-centered adsorption and hydrogenation which leads
to the selectivity toward COL and then to HCOL. In other words,
the proportion of C=C-centered adsorption declined gradually
as Pd particle grew bigger. It can also be seen from Figure 6
that the adsorption of CAL on smaller Pd particle is stronger
than that on bigger particle for both C=C and C=O bond
suggesting a higher catalytic activity of hydrogenation over
smaller Pd particles. These results are in agreement with the
a
different level of SMSI effect. In the literature, the
pretreatment condition is relatively mild below 200 °C. There
is no TiOx layer formed over the active metal particle and only
TiOx species exist at the interface region resulting in a
negeligible decoration of the particle size. The generated TiOx
species favored activating the C=O bond and consequently
leads to a lower selectivity to HCAL. It‘s not hard to understand
that the selectivity to HCAL over Pd/TiO2 catalyts (Table 3) was
lower than that over Pd/γ-Al2O3 catalyts (Table 2) even if the
size of Pd nanoparticle was at the same level. When the Pd
particle was covered by a porous TiOx layer, on the one hand,
it will create samller Pd particles benefiting the activation of
C=C bond; on the other hand, the TiOx species would also favor
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experimental results which show that smaller Pd particle has and O atom was weaker than that of N atom. There may be a
higher HCAL selectivity and higher CAL conversion, and larger compete adsorption between tetrahydrofuran (THF) solvent
Pd particle has higher HCOL selectivity and lower CAL and CAL reactant leading to a declined CAL conversion of
conversion. Further, Figure 6 also suggests that on smaller 61.6%. Meanwhile, the adsorbed THF was harmful for the lying
particle, the CAL molecule favors lying on the palladium of CAL molecule on palladium, leading to a slightly diminished
surface, while on the larger particle, the adsorption of CAL selectivity to C=C bond (90.6%). For N,N-dimethylaniline
tends to be vertical due to the enhanced repulsive force solvent, the large substituents including one phenyl and two
between the large rigid phenyl ring and the flat surface. In fact, methyl groups hindered the coordination of N and Pd atoms
aromatic rings that are chemisorbed on a flat surface must lie thereby making the adsorption of CAL on Pd surfaces available.
at a distance exceeding 0.3 nm owning to the energy barrier to So, a 45.4% conversion of CAL was achieved when N,N-
the surface.71 Compared with the C=C-centered adsorption, dimethylaniline was used as solvent. Also due to the steric
the C=O-centered adsorption is more beneficial for the less effect, the selectivity to HCAL (88.7%) is lower than that of p-
steric hindrance on larger particle thereby giving a higher xylene as solvent. When another solvent N,N-diethylaniline
selectivity to HCOL. Therefore, it can be concluded that the containing even larger substituents was used, the adsorption
particle size of Pd highly determined the adsorption mode of of CAL on Pd atoms was more fovourable due to the weaker
CAL thereby changing the catalytic selectivity and activity.
coordination between Pd and N atoms giving a higher CAL
conversion of 56.2%. The larger steric effect was responsible
for the lower selectivity to HCAL (82.4%) as well. In general,
3.6 Solvent effect on CAL hydrogenation
Both the experimental data and DFT calculations have proved the adsorption of CAL could be altered by using slovents with
that the smaller Pd particle preferred C=C-centered adsorption varied Pd-solvent interactions thereby changing the catalytic
thereby yielded a higher selectivity to HCAL. Since this reaction activity and selectivity.
is carried out with reaction media, a question can be promoted
3.7 Selective hydrogenation over different α,β-unsaturated
that whether the C=C-centered adsorption of CAL will be
aldehydes
affected by adjusting the adsorption between Pd particle and
reaction media over the smaller Pd particles. Therefore, Moreover, this superior adsorption of C=C bond over C=O
several solvents with remarkable differences were used in the bond on Pd particles in Figure 6 is also suitable for other α,β-
selective hydrogenation of CAL to search for the influence of unsaturated aldehydes. On the same Pd catalyst, the
Pd-solvent interaction on the catalytic performance of smaller adsorption of substrates could also be altered by changing the
Pd particles. The results are listed in Table 4
.
substituents on C=C bond.
Table 5. Selective hydrogenation of different α,β-unsaturated aldehydes
a
Table 4. Influence of reaction solvent on the catalytic performance in
hydrogenation of cinnamaldehydea.
over Pd/γ-Al2O3
.
O
O
O
O
Entry
Solvent
CAL Conversion, %
Selectivity, %
H
H
H
CH3
HCAL
92.2
92.0
91.3
90.6
---
HCOL
H
1
2
3
4
1
2
3
4
5
6
7
P-xylene
100
7.8
8.0
8.7
9.4
---
Entry Substrates
Conversion
, %
Selectivity, %
Cyclohexane
n-Hexane
99.9
Saturated Saturated
100
aldehyde
100
alcohol
0
Tetrahydrofuran
Pyridine
61.6
undetectable
45.4
1
2
3
4
acrolein
cinnamaldehyde
100
N,N-dimethylaniline
88.7
11.3
40.5
86.9
13.1
19.0
---
N,N-dienthylaniline
56.2
82.4
17.6
α-methylcinnamaldehyde 10.0
β-phenylcinnamaldehyde
81.0
aReaction conditions: 0.5wt% Pd-Ac/γ-Al2O3 100 mg, CAL 10 g, 80 °C, 6 h, H2
undetectable ---
2 MPa.
aReaction conditions: 0.5wt% Pd-Cl/γ-Al2O3 20 mg, CAL 1 g, 80 °C, 6 h, H2
2
MPa.
From entry 1 to 2, we can see that although the p-xylene
has a “π” system, the interaction between Pd atom and “π”
Table 5 shows the hydrogenation results of different α,β-
system was very weak similar as the interaction between Pd unsaturated aldehydes with varied steric effects. It’s clear that
atom and cyclohexane because CAL was completely converted both the catalytic activity and selectivity to saturated aldehyde
and the product selectivity were equal (92%) for both solvents. decreased sharply as the increase of the steric hinderance of
The catalytic performance was also negligibly influenced when C=C bondˊs substituents in the substrates. There were even on
the cyclic hexane was replaced by the linear hexane. However, reaction detectable in the hydrogenation of
β-
if the cyclic media containing hetero-atom like O and N was phenylcinnamaldehyde for the two large rigid phenyl ring
used, the hydrogenation of CAL became harder and even made the adsorption of substrate on the Pd surfaces hardly
terminated. The N atom in pyridine which has a lone pair of possible. As the substituents on C=C bond became smaller, like
electrons could strongly coordinate with Pd atoms thereby methyl or H atom, the adsorption of substrate was available
inhibit the adsorption of Pd and CAL molecule resulting in an and thereby showed an increasing conversion of 10.0% and
undetectable CAL conversion. The coordination of Pd atom 40.5% over α-methylcinnamaldehyde and cinnamaldehyde,
8 | J. Name., 2012, 00, 1-3
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respectively. Consequently, the selectivity to C=C bond and indirectly by introducing Ag metal and SMSI effect.
hydrogenation also increased from 81.0% to 86.9% as the Together with their TEM images, their catalytic results
methyl group in α-methylcinnamaldehyde was replaced by H confirmed that smaller Pd particles preferred
a higher
atom. Particularly, in the hydrogenation of acrolein, the selectivity to C=C bond hydrogenation, forming HCAL, while on
saturated aldehyde was the only product detected in the final larger ones, the selectivity to C=O bond hydrogenation
mixtures. Even if the substrate was changed to crotonaldehyde, increased yielding a higher selectivity to HCOL. The DFT
the saturated aldehyde was still the only hydrogenation calculations were well in accordance with the experimental
product over common Pd catalysts.72, 73 This is also can be results, which revealed the favored C=C-centered adsorption
explained by the steric effect discussed above. On these of CAL on smaller Pd particles, and the preferred C=O-centered
substrates, the large phenyl ring or methyl are substituated by adsorption of CAL on a flat palladium surface. In addition to
H atoms or methyl group, thus the steric hindrance is streamly controlling particle size, the adsorption of substrate can also
lowed resulting in only C=C-centered adsorption in those be regulated by using solvents with varied palladium-solvent
substrates. And the C=O bond can not be further interaction and substrates of significant differences in steric
hydrogenated for the poor catalytic ability of Pd metal over effect. These results may provide new insights into tailoring
aliphatic aldehyde. Therefore, only the hydrogenation of C=C particle size of active metal over supported catalysts as well as
bond can be achieved over acrolein or crotonaldehyde on altering product distribution in selective hydrogenation.
common Pd catalysts.
3.8 Catalyst recyclability
Acknowledgements
The recyclability of Pd/γ-Al2O3 catalyst was investigated
This work is financially supported by the Postdoctoral Sciene
through a ten-run recycling test in selective hydrogenation of
Foundation of Jiangsu Province (1501022B), the Jiangsu Key
CAL. As shown in Figure 7, the conversion of CAL decreases
Laboratory of Advanced Catalytic Materials and Techonolgy
gradually from 100% to 70% while the selectivity of HCAL
(BM2012110), the National Natural Science Foundation of
keeps constant at a very high level of 90% with the recycle
China (21576119), and the Natural Science Foundation of
time increasing from 1 to 10. Since the selectivity is quite
Jiangsu Province (BK20151125).
sensitive to the particle size of Pd, it proposes that there is
negligible aggregation of Pd particle during the repeated
hydrogenation reactions. The loss of catalytic activity may be
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Graphical Abstract
Palladium particles of different sizes obtained directly and indirectly by various methods were studied to clarify the
particle size effect in the selective hydrogenation of cinnamaldehyde (CAL).
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