Direct synthesis of hybrid layered double hydroxide-carbon composites supported Pd nanocatalysts efficient in selective hydrogenation of citral

Article abstract of DOI:10.1039/c5ra03201f

This present study reports a facile one-pot strategy for the direct synthesis of hybrid layered double hydroxide (LDH)-carbon composites supported palladium nanocatalysts by the in situ reduction of PdCl₄^2--intercalated MgAl-LDH combined with amorphous carbon under mild hydrothermal conditions. The results demonstrated that most of the Pd(II) species intercalated in the interlameller space of MgAl-LDH could be reduced in situ to metallic Pd⁰ species, and simultaneously, the hybrid structure of the LDH-C composites facilitated the formation of uniform Pd nanoparticles with small diameter, as well as the strong metal-support interactions. Furthermore, with the decreasing proportion of the LDH component in LDH-C composites, the average diameter of Pd nanoparticles decreased progressively and the metal-support interactions were weakened. The as-formed supported Pd nanocatalyst with Pd loading of 5.5 wt% was found to show a superior catalytic activity in the liquid-phase selective hydrogenation of citral than other supported Pd nanocatalysts, while the one with the Pd loading of 2.7 wt% yielded a much higher yield of citronellal (～80.0%) at 100% conversion. The catalytic performance of Pd nanocatalysts was proposed to be mainly related to both the metal-support interactions and the compositions of hybrid LDH-C composite supports.

Full text of DOI:10.1039/c5ra03201f

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PAPER
Direct synthesis of hybrid layered double
hydroxide–carbon composites supported Pd
nanocatalysts efficient in selective hydrogenation
of citral†
Cite this: RSC Adv., 2015, 5, 33199
Ruirui Han, Chunshi Nan, Lan Yang, Guoli Fan and Feng Li*
This present study reports a facile one-pot strategy for the direct synthesis of hybrid layered double
hydroxide (LDH)–carbon composites supported palladium nanocatalysts by the in situ reduction of
2ꢀ
PdCl
4
-intercalated MgAl–LDH combined with amorphous carbon under mild hydrothermal conditions.
The results demonstrated that most of the Pd(II) species intercalated in the interlameller space of MgAl–
0
LDH could be reduced in situ to metallic Pd species, and simultaneously, the hybrid structure of the
LDH–C composites facilitated the formation of uniform Pd nanoparticles with small diameter, as well as
the strong metal–support interactions. Furthermore, with the decreasing proportion of the LDH
component in LDH–C composites, the average diameter of Pd nanoparticles decreased progressively
and the metal–support interactions were weakened. The as-formed supported Pd nanocatalyst with Pd
loading of 5.5 wt% was found to show a superior catalytic activity in the liquid-phase selective
hydrogenation of citral than other supported Pd nanocatalysts, while the one with the Pd loading of
Received 20th February 2015
Accepted 24th March 2015
2
.7 wt% yielded a much higher yield of citronellal (ꢁ80.0%) at 100% conversion. The catalytic
DOI: 10.1039/c5ra03201f
performance of Pd nanocatalysts was proposed to be mainly related to both the metal–support
interactions and the compositions of hybrid LDH–C composite supports.
www.rsc.org/advances
approaches, complex pretreatments of pristine carbon mate-
rials are required, such as oxidation by strong acid or func-
1
. Introduction
Over the last two decades, well-dispersed supported noble- tionalization, which usually raise costs as well as cause serious
metal (e.g. Pt, Pd, and Au) catalysts have been explored widely environmental pollution.
1
–4
in the eld of heterogeneous catalysis. In general, the nature
of the supports is one of the most important factors in the aldehydes is an important step in the production of ne
Nowadays, the selective hydrogenation of a,b-unsaturated
5
–7
17–21
formulation of high-performance supported catalysts,
chemicals.
For example, industrially important unsaturated
because the metal–support interactions can play a signicant geraniol/nerol isomers and citronellal can be produced by
role in the activity and selectivity of the desired products. selectively hydrogenating the C]O functional group and
Recently, carbon materials (e.g. carbon nanotubes, carbon aer- conjugated C]C bond of citral (3,7-dimethyl-2,6-octadienal),
2
2–25
ogels, and activated carbon) as excellent catalyst supports have respectively.
attracted increasing attention,
Furthermore, the consecutive hydrogenation
8–10
due to their high specic of the abovementioned intermediates generates citronellol and
surface area, excellent electrical properties, and high mechan- nally, 3,7-dimethyloctanol. In this regard, palladium, plat-
ical stability. In order to achieve excellent catalytic performance, inum, and ruthenium are tested extensively in the above-
26,27
the enhancement of the metal dispersion and the catalyst mentioned hydrogenations.
Among them, palladium, which
stability by controlling the metal–support interfacial structure is favors the hydrogenation of the C]C bond, appears to be the
28–33
highly desired. At present, the most widely used preparation most selective in forming saturated aldehydes.
As a result,
routes for carbon materials-supported catalysts are incipient with the aim of the production of desired products, the design
1
1,12
13–15
wetness impregnation,
deposition–precipitation,
and of low-cost and efficient supported noble-metal catalysts has
16
ion exchange. In most of the cases of the abovementioned become an important issue in terms of economic and envi-
ronmental sustainability.
In
addition,
layered
double
O) are a class of highly ordered
two-dimensional layered clays consisting of positively charged
hydroxides
(LDHs,
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical
Technology, P. O. BOX 98, Beijing, 100029, P. R. China. E-mail: lifeng@mail.buct.edu. [M1ꢀx
cn; Fax: +86-10-64425385; Tel: +86-10-64451226
Electronic supplementary information (ESI) available: Additional experimental
2
+
3+
x+ nꢀ
M
x 2
(OH) ] (A )x/n$mH
2
†
34,35
layers with charge-balancing anions between them.
LDHs
results. See DOI: 10.1039/c5ra03201f
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ꢂ
can accommodate a large number of tunable divalent and for 10 h. The resultant slurry was dried overnight at 70 C to
trivalent metal species either in the form of metal cations within obtain the Pd/AC sample.
the layers or in the form of metal complexes in the interlayer.
More interestingly, highly dispersed metal nanoparticles (NPs)
2
.3. Characterization
on metal oxide matrixes may be attained by controlled calcina-
35–38
tion and reduction processes for LDH precursors.
We have Powder X-ray diffraction (XRD) patterns of the samples were
explored the generation and stabilization of highly dispersed obtained on a Shimadzu XRD-6000 diffractometer using Cu Ka
nickel NPs via either the reduction of NiAl–LDH precursors radiation (l ¼ 0.15418 nm).
supported on carbon nanotubes or the self-reduction of hybrid
Elemental analysis was performed using a Shimadzu ICPS-
composites of NiAl–LDH and amorphous carbon at high 7500 inductively coupled plasma atomic emission spectros-
3
9–41
temperatures.
copy (ICP-AES) aer the samples were dissolved in dilute nitric
ꢂ
The abovementioned results have stimulated us to extend acid at 150 C. The carbon content was determined by elemental
our work using wonderful LDH materials as noble-metal cata- microanalysis (Elementar Vario analyzer).
lyst precursors. In this study, a series of hybrid LDH–C
Transmission electron microscopy (TEM) and high-
composites supported Pd nanocatalysts were directly synthe- resolution transmission electron microscopy (HRTEM) obser-
sized via a facile one-step hybridization-reduction route under vations were carried out on a JEOL JEM-2010 electron micro-
mild hydrothermal conditions, which involved the assembly of scope at an accelerating voltage of 200 kV. The size distribution
2
ꢀ
composites of PdCl4 -intercalated MgAl–LDH and amorphous of the particles was determined from more than 200 individual
carbon through the carbonization of glucose and simultaneous metal particles. The surface area-weighted diameter of particles
reduction of Pd(II) species, without further chemical modica- (D) was calculated by the following equation:
tion of the supports. The as-formed supported Pd nanocatalysts,
in which Pd NPs and hybrid LDH–C composites could form a
unique triple-junction structure, were found to be more active
or more selective toward the liquid-phase hydrogenation of
citral to citronellal compared with the traditional activated
carbon supported Pd catalyst. A correlation between the struc-
tural characteristics of catalysts and the catalytic performance
was investigated.
P
3
n
i
i
d
i
i
i
D ¼ P
(1)
2
n
d
i
where n
i
is the number of particles of diameter d
i
.
Low temperature N
2
adsorption–desorption isotherms of the
samples were obtained on a Micromeritics ASAP 2020 sorp-
tometer apparatus. The total specic surface areas were evalu-
ated by the multipoint Brunauer–Emmett–Teller (BET) method.
The mesopore size and volume were determined by the Barrett–
Joyner–Halenda (BJH) method according to desorption
isotherms.
2. Materials and methods
2.1. Materials
All reagents were of analytical grade and purchased from Bei-
2 2
The Pd dispersion was determined by H –O titration on a
jing Chemical Reagent Ltd. without further purication.
Micromeritics ChemiSorb 2720. About 100 mg of sample was
loaded in a U-shaped quartz reactor. Before titration, the
ꢂ
sample was heated at 150 C for 2 h in a stream of He gas
2.2. Synthesis of supported Pd nanocatalysts
ꢀ
1
(
40 mL min total ow) and then cooled to room temperature.
PdCl42 -intercalated MgAl–LDH precursors were synthesized by
ꢀ
ꢂ
The sample was then exposed to H /He at 120 C for 60 min to
2
a
coprecipitation method.
Mg(NO $6H O (0.12 M), Al(NO
6 mM) dissolved in decarbonated deionized water was titrated
by an alkali solution of NaOH (0.18 M) with vigorous agitation
under N atmosphere at room temperature until the pH value of
A
salt solution containing
ensure the saturated adsorption of hydrogen on its surface.
3
)
2
2
3
)
3
$9H O (0.06 M) and H PdCl
2
2
4
Subsequently, the sample was purged by He gas for 60 min.
Pulses of oxygen were introduced until complete saturation of
the sample was achieved. The chemisorbed oxygen was then
titrated by hydrogen. The dispersion of Pd (Dis(%)) on the
(
2
the solution was adjusted to 10.0. The suspension was aged at
42
catalysts was calculated by the following equation:
ꢂ
7
0 C for 24 h, and then centrifuged, washed and redispersed in
H
T
ꢀ3
2
ꢀ
2 ꢃ M ꢃ V ꢃ 10
deionized water for 5 cycles. Finally, the resulting PdCl4
-
Disð%Þ ¼
ꢃ 100
(2)
3
ꢃ 22:4 ꢃ W ꢃ P
intercalated MgAl–LDH precipitate was transferred and
ꢀ1
H
T
dispersed into a Teon-lined autoclave with 60 mL of glucose where M is the formula weight of Pd (MPd ¼ 106.42 g mol ), V
2
+
3+
solution ([C
6
H
12
O
6
ꢂ
] ¼ x[Mg + Al ]; x ¼ 0.5, 1.5, 2.5 and 3.5) is the volume of H
2
used for the titration of O
2
(mL), W is the
and aged at 150 C for 10 h. The resultant suspension was mass of sample (g), and P is the weight percentage of Pd in the
centrifuged and washed 3 times with deionized water. Finally, catalyst as determined by ICP-AES.
ꢂ
the obtained solid was dried at 70 C and denoted as Pd/C
x
–
X-ray photoelectron spectroscopy (XPS) was recorded on a VG
LDH. For comparison, an activated carbon (AC) supported Pd ESCALAB 2201 XL spectrometer with a monochromatic Al Ka
catalyst with the Pd loading of 2.9 wt% also was prepared. AC X-ray radiation (1486.6 eV photons). Binding energies were
(
0.5 g) was added to a Teon-lined autoclave, in which H PdCl
calibrated based on the graphite C 1s peak at 284.6 eV.
2
4
aqueous solution (20 mL) and methanol (60 mL) were placed.
The autoclave was then tightly sealed and maintained at 100 C hexane at 25 C were used to estimate the surface base
The adsorption isotherms of phenol dissolved in cyclo-
ꢂ
ꢂ
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properties of the solids. The amount of phenol adsorbed by the
solids was measured by UV spectroscopy (lmax ¼ 271.6 nm).
2
.4. Selective hydrogenation of citral
The liquid phase hydrogenation of citral was carried out in a
00 mL stainless steel autoclave. At the beginning of the
2
experiment, 100 mL of citral solution (58.4 mM in isopropanol)
was charged with catalyst, in which the citral/Pd ratio was kept
ꢀ
1
at ca. 0.30 mol g through out this study. Aer that, air was
ushed out of the reactor with nitrogen at room temperature,
and the reactor was sealed and ushed with 2.0 MPa H at least
ve times. The reactor was then submerged into a water bath at

2

ꢂ
1
00 C for 1 h under atmospheric pressure. H was fed into the
2
reactor at 1.0 MPa and the reaction was initiated by stirring at a
speed of 900 rpm. Aer reaction, the autoclave was cooled with
an ice-water bath, and depressurized carefully. Finally, the
liquid products were analyzed by an Agilent GC7890A gas
chromatograph equipped with a ame ionization detector and
Fig. 1 XRD patterns of Pd/C0.5–LDH (a), Pd/C1.5–LDH (b), Pd/C2.5
LDH (c) and Pd/C3.5–LDH (d).
–
ꢂ
HP-5 capillary column. The injector temperature was 250 C,
ꢂ
and the detector column temperature increased from 100 C to
ꢂ
ꢂ
ꢀ1
1
50 C with a ramp rate of 5 C min .
by glucose serving as a reducing agent, thereby leading to the
2ꢀ
decrease in the proportion of PdCl
4
anions in the interlameller
3
. Results and discussion
space. Based on the Scherrer formula, X-ray line broadening
analyses from the (111) diffraction of the metallic Pd yield
average coherent diffraction domain sizes of Pd NPs in the range
of 5.6–7.4 nm for the supported Pd samples (Table 1), which are
smaller than that in the Pd/AC comparison sample (about
3.1. Characterization of catalysts
The XRD patterns of the supported Pd samples are shown in
Fig. 1. It is found that all the samples present the typical char-
acteristic diffractions for (003) and (006) crystal planes of two
types of hydrotalcite-like materials, which correspond to the
different basal spacings and higher order diffractions.
each case, two characteristic lines indexed as (110) and (113)
11.2 nm). Small crystallite size of Pd NPs may be associated with
the highly hybrid structure of LDH–C composites facilitating the
uniform distribution of Pd NPs over the LDH–C matrixes.
TEM analyses were conducted to investigate the morphology
of the supported Pd samples (Fig. 2). In each case, spheroidal Pd
NPs with narrow particle size distributions are uniformly
distributed over the support regardless of the compositions of
the supports and the Pd loadings, and plate-like LDH crystal-
lites and amorphous carbon interweave together. Correspond-
ingly, Pd NPs and the LDH–C composite support form a unique
triple-junction structure. Furthermore, it is observed that the
average crystallite size of Pd NPs decreases slightly with the
decreasing Pd loading. Therefore, larger Pd NPs, around 7.2 nm
with narrow size distribution, exist in the Pd/C0.5–LDH (inset in
Fig. 2a). With the decreasing Pd loading from 5.5 to 1.6 wt%, the
average crystallite size of Pd NPs slightly decreases to about
4
3,44
In
ꢂ
planes of LDH phase also appear at high 2q of about 61–62 . The
intensity of the (003) plane at 2q of around 11.2 decreases
ꢂ
gradually with increasing amounts of glucose added in the
course of hybridization, whereas that of another (003) plane at
ꢂ ꢂ ꢂ
0.6 increases. The diffractions at 11.2 and 10.6 arise from
1
the basal spacings of approximately 0.847 and 0.771 nm,
respectively, which are in good agreement with the reported
values for carbonate-intercalated MgAl–LDH (CO
PdCl
strates that the proportion of interlayer PdCl
decreases with the amount of glucose added, and charge-
2
ꢀ
3
–LDH) and
2
ꢀ
2ꢀ
45
4
-intercalated MgAl–LDH (PdCl
4
–LDH). It demon-
2
ꢀ
4
complex anions
2
ꢀ
balancing CO
3
anions easily enter the interlameller space
because of its high affinity to hydroxyl groups on the layers
through hydrogen bonding. Notably, in each case, the charac-
teristic (111), (200) and (220) diffractions of metallic Pd (JCPDS
5.7 nm (inset in Fig. 2b–d). This is consistent with that inferred
from the XRD results.
0
HRTEM measurements were further carried out to reveal the
microstructure of the Pd NPs. As for the representative
Pd/C2.5–LDH sample (Fig. 3), the lattice image displays an
interplanar spacing of 0.225 nm, corresponding to the (111)
ꢂ
88-2335) appear at about 40.1, 46.7 and 67.9 , respectively.
Furthermore, no diffractions related to graphite or other forms
of carbon appear in the XRD patterns, which is suggestive of the
presence of noncrystalline carbon.
In the course of the synthesis of supported Pd samples, the
aromatization and carbonization of glucose under hydrothermal
conditions can generate amorphous carbon, and thus the
resultant carbon may be assembled with LDH crystallites.
Moreover, with the increasing amount of glucose, more inter-
0
plane of metallic Pd . It is noted that the Pd NPs show a poor
contrast with the support matrix, which is suggestive of the
presence of the strong metal–support interactions (SMSI). In
contrast, severe agglomeration of large globular particles is
found in the Pd/AC comparison sample (see Fig. S1†), and the
size distribution of particles with an average crystallite size of
4
6
2ꢀ
0
layer PdCl
4
anions are reduced in situ to metallic Pd species
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Table 1 Analytical, structural and catalytic activity data obtained for different samples
a
Content (wt%)
d
e
i
k
S
BET
V
p
Specic basicity
TOF
b
c
2
ꢀ1
3
ꢀ1
f
g
h
ꢀ1
j
ꢀ1 Pdꢀ1
2
ꢀ1
Sample
Pd Mg Al
C
D
111 (nm) (m g ) (cm g ) dav (nm) D (nm) Dis (%) (mmol gcat
)
r (mmol s
g
) (ꢃ 10 s
)
Pd/C0.5–LDH 5.5 24.4 20.6 39.2 7.4
Pd/C1.5–LDH 3.9 15.9 14.7 55.5 6.9
Pd/C2.5–LDH 2.7 10.5 9.7 67.8 6.0
Pd/C3.5–LDH 1.6 8.6 6.9 72.3 5.6
122
90
61
46
—
0.38
0.40
0.36
0.38
—
11.5
20.9
23.8
29.2
—
7.2
6.8
6.3
5.7
8.7
25.6
37.7
43.3
54.6
23.3
0.209
0.133
0.086
0.065
—
406.1
380.6
295.0
227.4
238.2
16.9
10.7
7.2
4.4
10.8
Pd/AC
2.9 —
—
87.5 11.2
a
b
c
Determined by ICP-AES. Determined by elemental analysis. Crystallite size of metallic Pd based on XRD line broadening of (111) plane for Pd
d
e
f
g
h
metal. BET surface area. Total pore volume. Average pore size. Particle size of Pd NPs obtained by TEM analysis. Degree of Pd dispersion,
determined by H –O pulse chemisorption. Concentration of total basic site, determined by adsorption of phenol. Initial reaction rate of citral
hydrogenation. Turnover frequency of hydrogenation of citral, which was given as the overall rate of citral conversion normalized by the number of
active sites over 15 min.
i
j
2 2
k
Fig. 3 Typical HRTEM images (a and b) of Pd/C2.5–LDH.
Correspondingly, the electronic interaction between the
metallic Pd and the LDH–C support leads to the formation of
electron-decient Pd species. With the decrease in the propor-
tion of MgAl–LDH component in LDH–C composite support,
0
the binding energies of Pd 3d5/2 and Pd 3d3/2 for Pd species
exhibit a small shi to the lower energy side from 335.9
and 340.9 eV for Pd/C0.5–LDH to 335.2 and 340.4 eV for
Fig. 2 TEM images of Pd/C
x
–LDH samples: (a) Pd/C0.5–LDH, (b) Pd/
Pd/C3.5–LDH, respectively. This may be ascribed to the reduced
C
1.5–LDH, (c) Pd/C2.5–LDH and (d) Pd/C3.5–LDH. Insets in (a–d) show
0
interphase interaction between the surface Pd species and
histograms of the size distributions of Pd NPs.
positively charged lattices of MgAl–LDH platelets, which mainly
originates from the decrease in the proportion of MgAl–LDH
component in the composite support. In addition, a small peak
in the range of 335.0–338.0 eV is assigned to Pd(II) species in the
about 8.7 nm covers a broad range of around 3–11 nm. Such
poor dispersion of Pd particles on the AC support suggests the
weak metal–support interactions.
The surface/near-surface chemical states of the samples were
determined by XPS (Fig. 4). In the ne spectrum of the Pd 3d
core level for Pd/C0.5–LDH, the peaks at about 335.9 and
2ꢀ
form of PdCl4 anions intercalated in the interlameller space
of MgAl–LDH–LDH, as evidenced by the abovementioned XRD
results. Clearly, the peak area of Pd(II) species is much smaller
0
than that of Pd species, indicating that most of the Pd(II) are
reduced under hydrothermal conditions and Pd species exist
340.9 eV are assigned to Pd 3d5/2 and Pd 3d3/2, respectively,
0
mainly in the form of metallic Pd species.
47,48
which are characteristic of metallic Pd species.
noted that the binding energies of Pd species at the 3d region
are higher than the reported values for Pd species (about 335.3
eV at 3d5/2 and 340.5 eV at 3d3/2) in the literature.
fore, as for Pd/C0.5–LDH, the charge transfer from the surface
metallic Pd to the support probably occurs because the higher
Pd loading (ꢁ5.5 wt%) may facilitate a more widespread
distribution of Pd species over the LDH–C double support.
It can be
0
0
3.2. Catalytic performance of supported Pd nanocatalysts
49–51
There-
The liquid-phase catalytic hydrogenation of citral is a typical
three-phase catalytic reaction. It is found that the external
particle mass transfer limitation in the present hydrogenation
system is negligible at an agitation speed of above 900 rpm.
x
Moreover, Weisz–Prater numbers corresponding to Pd/C –LDH
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remarkable difference in the catalytic performance. The
conversions of citral over Pd/C0.5–LDH and Pd/C1.5–LDH
increase rapidly within 30 min, and reach approximately 100%
aer 90 and 120 min, respectively, whereas the conversions of
citral over Pd/C2.5–LDH and Pd/C3.5–LDH reach 95.7% and
84.6% aer 90 min. The selectivity toward geraniol and nerol
remains rather low (<10%) during citral hydrogenation, sug-
gesting that the C]C/C]O adsorption competition of citral
molecules mainly facilitates the C]C bond on the as-formed
supported Pd catalysts under the present experimental condi-
tions. As for Pd/C0.5–LDH, the hydrogenation of the conjugated
C]C bond in citral to citronellal is performed rapidly within the
initial reaction time of 15 min, and simultaneously the consec-
utive hydrogenation of citronellal to citronellol also occurs.
Within the reaction time, most citronellal converts gradually
into citronellol over the Pd/C0.5–LDH through the hydrogenation
of the C]O bond. Finally, citronellol becomes the dominant
product when the reaction is extended to a longer time than 150
min. With the decreasing proportion of LDH component in the
composite supports, the hydrogenation of the conjugated C]C
bond in citral is dominant at total reaction times, thus leading to
the increase in the selectivity toward citronellal (Fig. 6B–D). It is
Fig. 4 Pd 3d XPS of Pd/C0.5–LDH (a), Pd/C1.5–LDH (b), Pd/C2.5–LDH
(
c), Pd/C3.5–LDH (d).
catalysts were calculated to be lower than 0.15 for both seen from Table 1 that Pd/C0.5–LDH catalyst yields the highest
ꢀ1
ꢀ1
hydrogen and citral, which indicate the absence of the pore initial conversion rate of citral (406.1 mmol s
transport diffusion limitations.
g
Pd ), along
Therefore, the intrinsic with the highest TOF value of 0.169 s . With the increasing
activity of Pd/C –LDH catalysts could be actually determined carbon content in the catalysts, the initial hydrogenation rate
ꢀ
1
52,53
x
under the present reaction conditions.
drops rapidly by 56.0% over the Pd/C3.5–LDH catalyst, indicative
Fig. 5 illustrates the main reactions that probably occur of a great decline in the activity. For the as-formed supported Pd
during the hydrogenation of citral. Usually, the catalytic catalysts, the increase in the carbon content gives rise to the
hydrogenation of citral can generate a variety of products. The improvement in the selectivity to citronellal, probably due to the
C]O bond in citral may be hydrogenated to produce unsatu- reduced activation of the C]O bond.
rated alcohols (geraniol and nerol), while the conjugated C]C
In addition, compared with Pd/C2.5–LDH, the Pd/AC
bond can be reduced to citronellal. Subsequently, consecutive comparison catalyst with a similar Pd loading presents a lower
hydrogenation of the abovementioned intermediates can initial reaction rate (Fig. S2†). As shown in Fig. 7, the selectivity
produce
citronellol
and
nally
saturated
alcohol towards citronellal can be improved by the appropriate addition
of carbon component in catalysts. Among all the catalysts,
(3,7-dimethyloctanol).
The dependence of product compositions on time in the Pd/C2.5–LDH exhibits the highest citronellal selectivity of about
liquid-phase hydrogenation of citral over different catalysts at 87.4% at a reaction time of 45 min (100% conversion of citral).
ꢂ
1
00 C is presented in Fig. 6, whereas the initial reaction rates
x
In the present catalyst system, the supported Pd/C –LDH
and turnover frequencies (TOF) are summarized in Table 1. It is nanocatalysts present a slight difference in the particle size. It
found that the different supported Pd catalysts exhibit demonstrates that there is no size dependence for hydrogena-
tion activity. Normally, it is believed that the higher catalytic
activity is ascribed to a larger amount of accessible active sites
on the catalyst surface. Correspondingly, the dispersion of
active metal species has a signicant effect on the catalytic
activity. As listed in Table 1, with the increasing proportion of
carbon component in composite supports, the supported Pd
catalysts present a decrease in the Pd loading and an
improvement in the metal dispersion. Although the metal
dispersion of Pd/C0.5–LDH is lower than other catalysts, it
exhibits the highest catalytic activity. It suggests that the metal
dispersion is not the only important factor for the reactivity of
supported Pd catalysts, and there are other factors affecting the
catalyst activity.
As for the hydrogenation catalyzed by metals, the nature of
adsorption is the key factor affecting the catalytic activity.
Fig. 5 Reaction paths for liquid-phase hydrogenation of citral.
According to the abovementioned XPS results, the electron
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Fig. 6 Catalytic performance in citral hydrogenation as a function of reaction time over different catalysts: Pd/C0.5–LDH (A), Pd/C1.5–LDH (B),
Pd/C2.5–LDH (C), Pd/C3.5–LDH (D).
0
transfer from the metallic Pd species to the support demon- intimate contact with metallic Pd sites can easily interact with
strates the presence of SMSI. With the increasing proportion of the C]O group in the citral molecule through hydrogen
MgAl–LDH component in the composite supports, the SMSI bonding, which probably favors the activation of the oxygen
is enhanced gradually. Correspondingly, in the case of electronic doublet of the carbonyl function and thus the
Pd/C0.5–LDH, the lower electron density on the metal surface improvement of the reactivity of the carbonyl group. As for
facilitates the adsorption of citral with the electron donating Pd/C0.5–LDH, a larger amount of MgAl–LDH in the hybrid
C]C groups, by the lowered repulsive forces existing between d composite support inevitably increases the amount of exposed
5
4,55
orbitals of Pd and reactants,
leading to a higher activity for hydroxyl groups on the lattices of the plate-like LDH crystallites,
56
the hydrogenation of the C]C bond.
thereby enhancing the catalytic activity for C]O hydrogenation
Moreover, elemental analysis reveals that the content of and thus the selectivity towards citronellol aer a longer period
carbon in Pd/C0.5–LDH is about 39.2 wt%, demonstrating a of reaction time. With the decreasing proportion of MgAl–LDH
dominant MgAl–LDH component in the support, as evidenced by component, an improvement of the selectivity towards citronellal
the abovementioned TEM results (Fig. 2). Furthermore, the basic can be observed due to the reduced reactivity of the carbonyl
properties of the surfaces of the catalysts were investigated by group. In the case of the Pd/AC comparison catalyst, the relatively
studying the adsorption isotherms for phenol from cyclohexane low activity should originate from the presence of bulky, large Pd
ꢂ
solution at 25 C. As shown in Fig. 8, the overall number of basic particles, as well as the low metal dispersion. Moreover, large-
sites, calculated on the basis of the data from adsorption sized Pd NPs do not favor the adsorption of the C]C bond
isotherms in accordance with the Langmuir equation, decreases due to the insufficiency of low-coordinated surface sites, thus
with the decreasing proportion of MgAl–LDH component in resulting in a decline in the selectivity toward citronellal.
0
composite supports (Table 1). As for MgAl–LDH, the basic sites
In the present catalyst system, surface active Pd species and
mainly come from hydroxyl groups on the lattice of LDH. support composition are two key factors affecting the citral
Therefore, hydroxyl groups on the surface of the support in hydrogenation. For the as-formed supported Pd catalysts, the
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Fig. 7 Conversion and selectivity for the hydrogenation of citral over
different catalysts at a reaction time of 45 min.
Fig. 9 The reusability of the Pd/C2.5–LDH catalyst at a reaction time of
45 min.
SMSI leads to the formation of electron-decient metal species,
and thus induces an enhanced oriented chemisorption of the of C]C/C]O bonds in citral molecules, thus inducing a
C]C bond in the citral molecule on the surface of Pd particles, negative effect on the activity. Correspondingly, the selectivity
while the LDH component makes the effective chemical to citronellal is improved over catalysts with the higher carbon
adsorption of citral on the support easier. Therefore, content. Especially, a high citronellal yield of about 80.0% is
Pd/C0.5–LDH catalyst remains very active for citral hydrogena- achieved over the Pd/C2.5–LDH at a conversion of 100% (Fig. 6).
tion and highly selective towards citronellol aer a long reac-
Reusability is one of the most attractive hallmarks of cata-
tion time due to the consecutive hydrogenation of citronellal to lysts, because a promising catalyst must exhibit fairly stable
citronellol. With the decreasing LDH content in the catalysts, catalytic performance in multiple successive runs. The reus-
the reduced SMSI and hydrogen bonding between the C]O ability of Pd/C2.5–LDH was investigated in our study. In the
group in the citral molecule and surface hydroxyl groups on the cycling tests, the reaction mixture solution was centrifuged to
lattices of plate-like LDH crystallites inhibit the hydrogenation remove the catalyst. The collected catalyst was washed with
ethanol and deionized water, and dried for 24 h, then reused in
the next reaction cycle. It is found that the catalytic property of
Pd/C2.5–LDH can be well recovered with no detectable loss aer
recycling 5 times (Fig. 9). Elemental analysis by ICP-AES further
reveals that the Pd leaching loss is only about 2.1 wt% for the
initial amount of Pd in the Pd/C0.5–LDH aer recycling 5 times,
as a result of the unique triple-junction structure of the
as-formed supported Pd nanocatalysts.
4. Conclusions
In summary, we have developed a new method to directly
synthesize hybrid LDH–C composites supported Pd nano-
catalysts for the liquid-phase selective hydrogenation of citral.
Supported Pd nanocatalysts were prepared via a simple in situ
ꢀ
reduction of composites of PdCl
4
-intercalated MgAl–LDH and
amorphous carbon, which were assembled through a carbon-
ization process of glucose. With the increase in the LDH
component in LDH–C composite supports, the SMSI was
enhanced gradually, which resulted in the enhancement of the
adsorption of the C]C bond in the citral molecule on the active
Fig. 8 Adsorption isotherms for phenol from cyclohexane solution on
different catalysts: Pd/C0.5–LDH (a), Pd/C1.5–LDH (b), Pd/C2.5–LDH (c),
Pd/C3.5–LDH (d).
0
Pd sites and, thus enhanced the catalytic activity. In contrast,
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