Easy synthesis of bimetal PtFe-containing ordered mesoporous carbons and their use as catalysts for selective cinnamaldehyde hydrogenation

Article abstract of DOI:10.1039/c3nj40946e

This work focuses on the preparation and characterization of bimetal PtFe-containing ordered mesoporous carbon (PtFe-OMC) materials with different Pt/Fe ratios and their use as the catalysts in selective cinnamaldehyde hydrogenation. The carbon materials were synthesized by means of an easy one-pot organic-organic self-assembly strategy using H₂PtCl ₆·6H₂O and Fe(NO₃)₃· 9H₂O as the metal precursors. All the samples were characterized by nitrogen adsorption-desorption, X-ray diffraction, X-ray photoelectron spectroscopy, temperature programmed reduction, and transmission electron microscopy, and evaluated for catalytic cinnamaldehyde hydrogenation at various temperatures and H₂ pressures. The results showed that the Pt-Fe alloy nanoparticles were highly dispersed in the OMC matrix, and the presence of Fe resulted in charge-transfer, and thus, enhanced greatly the selective cinnamaldehyde hydrogenation of the Pt catalyst towards cinnamyl alcohol.

Full text of DOI:10.1039/c3nj40946e

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Easy synthesis of bimetal PtFe-containing ordered
mesoporous carbons and their use as catalysts for
selective cinnamaldehyde hydrogenation
Cite this: NewJ.Chem., 2013,
37, 1350
Zhi Liu,*a Xiuli Tan,a Jia Lib and Cong Lva
This work focuses on the preparation and characterization of bimetal PtFe-containing ordered
mesoporous carbon (PtFe-OMC) materials with different Pt/Fe ratios and their use as the catalysts in
selective cinnamaldehyde hydrogenation. The carbon materials were synthesized by means of an easy
one-pot organic–organic self-assembly strategy using H2PtCl6Á6H2O and Fe(NO3)3Á9H2O as the metal
precursors. All the samples were characterized by nitrogen adsorption–desorption, X-ray diffraction,
X-ray photoelectron spectroscopy, temperature programmed reduction, and transmission electron
microscopy, and evaluated for catalytic cinnamaldehyde hydrogenation at various temperatures and H2
pressures. The results showed that the Pt–Fe alloy nanoparticles were highly dispersed in the OMC
matrix, and the presence of Fe resulted in charge-transfer, and thus, enhanced greatly the selective
cinnamaldehyde hydrogenation of the Pt catalyst towards cinnamyl alcohol.
Received (in Montpellier, France)
18th October 2012,
Accepted 1st February 2013
DOI: 10.1039/c3nj40946e
www.rsc.org/njc
active components can be directly introduced into the carbon
framework to prepare metal-containing ordered mesoporous
1. Introduction
In recent years, ordered mesoporous carbon (OMC), a novel
nanostructured carbon material, has been widely investigated
as a catalyst support, as an activated carbon, for liquid phase
reactions due to its excellent stability under hydrothermal
conditions, accessible porosity and high specific surface area
for dispersing active components.1,2 In particular, OMC offers
great advantages over conventional activated carbon owing to
its uniform, tailored and well-defined pore size distribution,
flexible framework composition, and efficient mass transfer of
the reactant and the product molecules.3 Up to now, there are
two typical approaches for fabricating OMCs. One is a nano-
casting method with ordered mesoporous silica as the hard
template,4,5 and the other is an organic–organic self-assembly
strategy (OSS) with surfactants as the soft template and
phenolic resin as the carbon source.6,7 Compared to the nano-
casting method, which is expensive, low yielding and indust-
rially unfeasible, the OSS has recently become a popular
method to synthesize OMC due to its simplified procedure,
easy mass production and abilities to produce various forms of
OMCs (e.g., films, powders, monoliths, etc.). More significantly,
carbon (M–OMC) during the self-assembly process. Different
from the traditional impregnation, deposit or precipitation, such
M–OMCs synthesized with the OSS not only yields intimate
contact and strong interaction between the metal species and
the carbon matrix, but it also makes the metal particles highly
dispersed in the carbon matrix, which potentially gives rise to
superior catalytic performance. For example, Zhang et al. have
synthesized Ir–OMC and MoC–OMC, which showed high cata-
lytic activities for hydrazine decomposition.8,9 Using the same
OSS, Wang et al. have fabricated Fe–OMC with a Fe/resorcinol
ratio of 0.1, which possess a specific activity for the catalytic wet
peroxide oxidation of a phenol solution with H2O2.10 In our
previous work,11,12 we have prepared Pt–OMC and Cu–OMC
with the OSS and found that they exhibited much superior
performance for toluene hydrogenation and cinnamaldehyde
hydrogenation, respectively, compared to the traditional
impregnated counterparts.
Selective hydrogenation of cinnamaldehyde (CMA) to the
corresponding alcohols is of great importance in the fine
chemical industry. Also, it is considered as a good model
reaction for correlating the catalytic behaviors of the micro-
structures of heterogeneous catalysts.13 Generally, CMA is
hydrogenated to cinnamyl alcohol (CMO) and hydrocinnam-
aldeyde (HCMA), depending on whether the CQC double bond
is hydrogenated or the CQO one. CMO and HCMA can both be
a Institute of Chemistry for Functionalized Materials, Faculty of Chemistry and
Chemical Engineering, Liaoning Normal University, Dalian 116029, P. R. China.
E-mail: zhiliu@lnnu.edu.cn; Fax: +86 411 82156858; Tel: +86 411 82156989
b College of Chemistry, Jilin Normal University, Siping 136000, P. R. China
c
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from the CO hydrogenation reaction, which presented much
better activity and selectivity than that supported on the OMC
without modified metal particles and conventional catalysts
supported on silica. Enlightened by this work, as well as the
potential of bimetal PtFe-based catalyst, herein we further
broaden the variety of bimetal-containing OMCs and prepared
bimetallic PtFe containing-OMCs with different ratios of Pt/Fe
using the OSS followed by a direct carbonization process. The
Scheme 1 Possible reaction pathways in the hydrogenation of CMA.
further hydrogenated to hydrocinnamyl alcohol (HCMO) as the selective hydrogenation of CMA to CMO was used as a probe
final product (Scheme 1). CMO is an important intermediate in reaction to evaluate the catalytic performance of the PtFe–OMCs.
the manufacture of perfumes and pharmaceuticals. Over the The results showed that the Pt–Fe alloy nanoparticles prepared
past few years, noble metal (Pt, Ir, Ru and Au)-based catalysts with the OSS were highly dispersed in the mesoporous carbons,
have used for the hydrogenation of CMA.14–20 Unfortunately, and the bimetallic PtFe–OMC presented a higher selectivity
they usually exhibit moderate selectivities towards CMO, there- toward CMO than those of its monometallic counterparts in the
fore bimetallic catalysts obtained by the addition of a second hydrogenation of CMA.
metal with electronic or structural modification of the noble
one are often required for improving the activity and/or
selectivity towards unsaturated alcohols. Such a modification
would usually give rise to a ‘‘synergetic effect’’ between the two
2. Experimental section
2.1. Chemicals
metals. For example, the differences in the lattice constants Pluronic F127 (EO106PO70EO106) was supplied by Sigma-Aldrich
between Pt and 3d transition metals (Fe, Co, and Ni) can lead to and has an average molecular weight of 12 600. Resorcinol (R),
the formation of strained PtM bimetallic structures.21–24 In formaldehyde (37 wt%) (F) and ethanol were purchased from
these examples, due to the diverse catalytic application of Shenyang Chemical Preparation Corp. H2PtCl6Á6H2O and
bimetal PtFe, various bimetallic PtFe-based catalysts have Fe(NO3)3Á9H2O were purchased from Tianjin Kermel Chemical
recently been exploited with different approaches. Goodwin Reagent Co. Ltd. All the reagents were used as received without
et al. reported an investigation into the impact of Fe promotion further purification. Distilled water was used in all experiments.
on Pt/g-Al2O3 using isotopic transient kinetic analysis for the
selective oxidation of CO in hydrogen.25 Alexeev and Amiridis
2.2. Synthesis of PtFe–OMC
et al. utilized Pt5Fe2(COD)2(CO)12 to prepare cluster-derived In a typical synthesis, 1.65 g of resorcinol was dissolved in a
PtFe/SiO2 catalysts for the oxidation of CO, which gave rise to solution containing 1.25 g of F127 and 10 g of ethanol/water
a greater degree of metal dispersion and a larger fraction of (1/1 vol%) under stirring until a brown solution was formed.
Pt–Fe interactions compared to the PtFe/SiO2 samples prepared Afterwards, 1.25 g of formaldehyde (37 wt%) was added drop-
by co-impregnation of monometallic salt precursors.26 wise to the above solution. After an additional 5 h of stirring, a
Watanabe et al. prepared a Na-type mordenite supported desired amount of a mixture solution of H2PtCl6Á6H2O and
Pt–Fe catalyst by the conventional ion-exchange method. Fe(NO3)3Á9H2O at different mole ratios of Pt/Fe was added. The
Catalytic tests for preferential CO oxidation showed that the acid, formed from the ionization in the solution, was utilized as
pretreatment temperature was vital for the reactivity and the catalyst for RF polymerization. The mixture was kept
selectivity.27 Yan et al. synthesized a highly active and chemo- standing until it turned cloudy and began to separate into
selective assembled Pt/C(Fe) catalyst by an adsorption protocol, two layers. This two-phase mixture was left alone for 4 days.
which exhibited a high turnover frequency and selectivity for Subsequently, the upper layer was discarded while the lower
the complete conversion of o-chloronitrobenzene.28 Hu et al. polymer-rich phase was stirred overnight until a sticky mono-
applied microwave-irradiated ethyl glycol reduction and citric- lith was formed. Finally, the monolith was cured at 85 1C for
acid-assisted NaBH4 reduction strategies to prepare Pt–Fe/C 2 days and pyrolyzed under a N2 atmosphere up to 800 1C for
catalysts possessing different catalytic interface hydrogen iso- 3 h at a ramping rate of 1 1C minÀ1. The Pt loading was fixed at
tope separation activities.29 Yang et al. reported multi-walled 1.0 wt% and the Fe loadings were varied from 0.5 to 3.0 wt%.
carbon nanotubes supported on PtFe bimetallic catalysts pre- The resulting composites were named 1PtxFe-OMC, where x
pared using a microwave-assisted polyol reduction method. indicates the Fe weight content (wt%) in the PtFe–OMC. In
The catalyst exhibited high activity and selectivity in the hydro- addition, monometallic composites of 1.0 wt% of Pt and 3.0 wt%
genation of CMA.30,31 In addition, carbon-supported bimetallic of Fe were also prepared for comparison under the same condi-
PtFe catalysts have been found to have good electrocatalytic tions as those for the PtFe–OMCs. The denotations and chemical
performances in fuel cells.32–36 More recently, Ding et al. have compositions of these samples are shown in Table 1.
significantly developed the OSS for the preparation of M–OMC
2.3. Characterizations
from monometallic salt precursors and have successfully
synthesized highly dispersed bimetallic FeMn–OMC using Textural characterizations were carried out by adsorption of N2
bimetallic salt precursors.37 The bimetal-containing OMC was at À196 1C on a Micromeritics ASAP 2010 apparatus. Before
used as a support of Rh-based catalysts for ethanol synthesis measurement, the samples were degassed firstly at 110 1C for
c
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Table 1 Textural properties of the Pt–OMC and the PtFe–OMCs
Average Pt crystalline
size determined
À1) (nm) (nm) by XRD (nm)
SBET
Vp
Dp
a0
Samples
(m2
g
À1) (cm3
g
OMC
1Pt-OMC
1Pt0.5Fe–OMC 619
1Pt1Fe–OMC 612
1Pt1.5Fe–OMC 606
1Pt2Fe–OMC 597
1Pt3Fe–OMC 593
640
624
0.57
0.54
0.53
0.53
0.51
0.49
0.48
4.4 10.2 —
4.5 10.3 2.8
4.5 10.4 3.2
4.8 10.8 3.4
5.1 11.2 3.6
5.3 11.5 3.6
5.5 11.6 3.8
2 h and then at 300 1C for 3 h under vacuum. The powder X-ray
diffraction (XRD) patterns were collected with a D/Max-bb diff-
ractometer using a Cu Ka radiation source (l = 0.15432 nm).
Low-angle diffractions (2y = 0.1–51) and wide-angle diffractions
(2y = 10–801) were recorded at a scanning speed of 11 minÀ1
and 51 minÀ1, respectively. Transmission electron microscopy
(TEM) images were obtained on a JEOL 2000EX electron micro-
scope operating at an accelerating voltage of 120 kV. The exact
Pt or Fe contents in these samples were determined by an IRIS
Intrepid II XSP instrument. X-ray photoelectron spectroscopy
(XPS) was conducted on PHI Quantum 2000 equipped with
an Al Ka radiation source. All binding energies (BE) were
calibrated with graphitic carbon C 1s peak at BE of 284.5 eV
as a reference. The hydrogen temperature programmed reduction
(H2-TPR) experiments were performed on a Micromeritics
AutoChem II 2920 instrument. Prior to the measurements,
the samples were pretreated in a He flow at 200 1C for 1 h.
After cooling to room temperature under He, the gas was
switched to 10% H2/He and the sample bed was heated to
800 1C at a ramp of 10 1C minÀ1. The consumption of H2 was
determined with a thermal conductivity detector.
Fig. 1 N2 adsorption–desorption isotherms (A) and the corresponding pore size
distributions (B) of the OMC, 1Pt–OMC and the PtFe–OMC samples.
size distributions were very narrow, centering at around 4.5 nm
for the Pt–OMC and around 5 nm for the PtFe–OMCs. From the
textural parameters listed in Table 1, it can be seen that there
2.4. Catalytic measurements
The CMA hydrogenation reactions using the PtFe–OMCs were were gradual decreases in the surface area (SBET) and pore
carried out in a batch reactor with a stirring speed of 400 rmp. volume (VP) of the PtFe–OMCs, which is probably due to partial
After the addition of 0.2 g catalyst, 3 ml CMA, and 100 ml pore blocking occurring with a rise in the Fe content or/and the
isopropanol, the reaction was monitored by taking 1.0 ml ravaged long-range order structure of the OMC in the presence
samples from the reaction mixture periodically to determine of Fe. On the other hand, both the average pore sizes (DP) and
the conversion and selectivity. The reaction products were the cell parameters (a0) of the PtFe–OMCs had an increase
analyzed by a gas chromatograph (Agilent 6890) equipped with compared to those of the 1Pt–OMC, showing that a higher Fe
a flame ionization detector and a 0.25 mm  30 m FFAP loading could make the pore diameter become larger, suggest-
capillary column. In comparison with the OSS, a commercial ing that the presence of Fe could limit the shrinkage of the
Norit activated carbons (NACs) were used as supports for mesostructure of carbons during pyrolysis. This was likely to be
preparing 1Pt1.5Fe/NAC catalysts, which were synthesized by due to the increasing consumption of carbon as Fe(III) nitrate
an incipient wetness impregnation method using H2PtCl6Á6H2O was reduced to a lower valence Fe species by the surrounding
and Fe(NO3)3Á9H2O as the precursors.
carbon, which could accelerate the development of a carbon
structure and enlarge the pore size.
Fig. 2A presents the low-angle XRD patterns of the OMC,
1Pt–OMC and the PtFe–OMC samples. For these samples, there
was a strong peak at 2y = 0.5–11, indexed as a reflection of (100)
3. Results and discussion
3.1. Structural features
Fig. 1 shows the nitrogen adsorption–desorption isotherms and for long-range structural ordering with 2D hexagonal symmetry,
the corresponding pore size distributions of the OMC, which was in accordance with the nitrogen adsorption–
1Pt–OMC and PtFe–OMCs samples. It could be seen that they desorption results. Moreover, the (100) peaks seemed to shift
all exhibited the type-IV isotherm with well-defined H1 hyster- toward the lower angles with a rise of the Fe content, indicating
esis loops, characteristic of a mesoporous structure. The pore that the incorporation of a higher amount of Fe led to larger cell
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detailed components of the PtFe–OMCs and the interactions
between the Fe and Pt atoms, we conducted XPS to probe the
chemical state of the Pt and Fe atoms in the carbon framework.
Fig. 3 depicts the XPS spectra of the 1Pt–OMC and PtFe–OMCs.
It can be clearly seen, both in the 1Pt–OMC and in the PtFe–
OMCs in Fig. 3A, that the BE of the Pt 4f, centered at 71.1 eV
(Pt 4f7/2) and 74.5 eV (Pt 4f5/2), are indicative of metallic
Pt(0).40,41 In addition, there are no obvious changes in the Pt
4f peak position. In contrast, the BE of Fe 2p in Fig. 3B showed
that all the PtFe–OMC samples presented dual characteristic
peaks of Fe 2p1/2–Fe 2p3/2 at 720.4 and 707.2 eV, indexed as Fe3+
in Fe2O3.42,43 A large variation in the Fe 2p peak position was
found with the increasing Fe content in the PtFe–OMCs. The Fe
2p3/2 BE was found to shift gradually from 707.2 eV for
1Pt0.5Fe–OMC to 707.8 eV for 1Pt3Fe–OMC. The +0.6 eV BE
shift was likely due to the strengthened Pt–Fe bonding, i.e., the
alloying of Pt and Fe. It was reported that the bonding between
Fe and Pt could result in a charge transfer from Fe to Pt, which
led to a shift of the Fe 2p BE to higher values.44 However, taking
into account the fact that the BE of Pt 4f was not affected by the
incorporation of Fe, the +0.6 eV BE shift did not necessarily
mean the electronic effects of Fe on Pt. It was also probably due
Fig. 2 Low (A)- and wide (B)-angle XRD patterns of the OMC, 1Pt–OMC and
PtFe–OMC samples.
parameters, which is in agreement with the results of N2-sorp-
tion. In the wide-angle XRD patterns (Fig. 2B), a broad diffraction
peak at about 2y = 251 appeared, which is characteristic of
amorphous carbon. In addition, four pronounced peaks at 2y
= 39.81, 46.21, 67.81, and 81.31 corresponding to (111), (200),
(220) and (311) were observed respectively, which revealed the
presence of crystalline metallic Pt with a face-centered cubic
structure.38 It was clear seen that all Pt diffraction peaks in the
PtFe–OMCs were shifted from that of metallic Pt to a higher
angle of the Pt3Fe alloy. No metallic Fe, Fe2O3 or other Fe oxide
peaks were observed, possibly suggesting that Fe was completely
involved in the formation of the Pt–Fe alloy after pyrolysis at
800 1C. Because the Pt (220) peak was isolated from the diffrac-
tion peaks of the OMC, the average Pt particle sizes for all these
samples were calculated from the Scherrer equation based on
the (220) peak.39 The corresponding results are summarized in
Table 1 also. The particle size of 1Pt–OMC, 1Pt0.5Fe–OMC,
1Pt1.0Fe–OMC, 1Pt1.5Fe–OMC, 1Pt2.0Fe–OMC and 1Pt3.0Fe–
OMC was 2.8, 3.2, 3.4, 3.6, 3.6, and 3.8 nm, respectively.
XPS is an effective technique to determine the valence of the
active species in catalysts. In order to further understand the Fig. 3 XPS spectra of Pt 4f (A) and Fe 2p (B) for 1Pt–OMC and the PtFe–OMC samples.
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to the widening of the signal caused by the presence of a higher the 1Pt3Fe–OMC sample occurred at temperatures as high as
content Fe2O3. Therefore, it was concluded that Fe was not 149 (a) and 234 1C (b). Recently, Zhang et al. developed a two-
completely influencing the chemical state of Pt. For the sample step method for the preparation of highly dispersed and
with a low Fe content (0.5 wt% of Fe), there would be a close thermally stable Au–Ag and Au–Cu nanoparticles on a silica
interaction between Fe and Pt, whereas for the sample with a support. This method is particularly useful in designing a
high Fe content (2.0 or 3.0 wt% of Fe), a second contribution at core–shell-like structure. After support-functionalization, metal
higher BE appeared due to the effect of Fe2O3.
precursor deposition and H2-reduction treatment, Au core and
H2-TPR is an ideal tool for examining the reducibility and Ag or Cu shell catalysts were formed, whose shell can protect
interaction degree between the metal species and support of a the core from oxidation. Such a structure gave rise to an
solid catalyst. Fig. 4 reveals the H2-TPR profiles of the bime- obviously synergistic effect when they were used as catalysts
tallic PtFe–OMC samples as well as the 3Fe–OMC and 1Pt–OMC for CO oxidation.47–49 In the present OSS synthesis procedure,
samples. For the pure 3Fe–OMC sample, there were two major the formation of metal nanoparticles originates from the
reduction peaks (a and b) centered at 227 1C and 253 1C, and a thermal reduction with the carbon itself as the reducing agent,
minor reduction peak (g) centered at 627 1C. The a and b were together with the formation of a mesoporous carbon structure.
conjunct, where peak a was the small particle reduction of This procedure has been proved to be very successful in the
Fe2O3 to Fe3O4 and accordingly peak b belonged to the rela- preparation of highly dispersed and well homogenized metal
tively large particle reduction of Fe2O3 to Fe3O4, while the g was nanoparticles. Based on these results, the present bimetallic
attributed to the further reduction of Fe3O4 to Fe(0).25,45,46 This PtFe nanoparticles presumably had a similar core–shell (Pt core
implied the presence of an Fe2O3 phase in the matrix of the and Fe shell) alloy structure with a high dispersion degree in
OMC frameworks. For the bimetallic PtFe–OMC samples, the the OMC matrix, which would potentially give rise to a superior
temperatures of a and b were lower than those of 3Fe–OMC, catalytic performance.
suggesting that the presence of Pt decreased the reduction
Fig. 5 illustrates the TEM images of the OMC, 1Pt–OMC and
temperature required for Fe2O3 due to hydrogen spillover.25 the PtFe–OMC samples. The images are in good agreement with
Meanwhile, they were independent and shifted to a higher the low-angle XRD patterns. The well-ordered mesostructures
temperature with the increase of the Fe content from 0 to with arrays of long 1-D channels can be clearly observed,
3.0 wt%. No peak corresponding to the reduction of a PtOx implying that the ordered mesoporous structures remained
species was found in the bimetallic PtFe–OMC samples. These after the incorporation of Pt and Fe. By carefully examining
results were consistent with the XPS analysis. On the other the TEM images of these samples, it was clearly observed that
hand, for the pure 1Pt–OMC catalyst, the reduction occurred the spherical Pt–Fe alloy nanoparticles with sizes of 2–4 nm
below 40 1C, which can be ascribed to the reduction of the PtOx were highly dispersed in the channels or on the walls of the
species formed by the oxidation of outmost surface Pt upon OMC, which was similar to the calculations based on the
exposure to air. Evidently, in the bimetallic PtFe–OMC samples, Scherrer equation from the wide-angle XRD characterizations.
the presence of Fe2O3 prevented the Pt(0) from oxidation by Additionally, the observed pore size of 1Pt–OMC, 1Pt0.5Fe–OMC,
exposure to air, which further verified the alloying of Pt and Fe. 1Pt1.0Fe–OMC, 1Pt1.5Fe–OMC, 1Pt2.0Fe–OMC and 1Pt3.0Fe–OMC
Moreover, the higher the amount of Fe2O3, the more difficult it was 4.5, 4.6, 4.7, 4.8, 5.2, 5.4, and 5.7 nm, respectively, which is
will be to reduce. For example, the reduction of the Fe2O3 of in accordance with the results of N2-sorption, and further
confirms that limitation of the shrinkage of the mesostructure
of carbons during pyrolysis.
3.2. Catalytic hydrogenation of CMA
Transition metals have been employed widely as catalysts for
the hydrogenation of organic compounds. Additionally, they
have been used as co-catalysts in a large variety of chemical
reactions.50–52 The addition of a transition metal to some noble
metals can considerably modulate both the activity and the
selectivity of the catalysts. Table 2 lists the catalytic properties
of the CMA hydrogenation over the Pt–OMC and the PtFe–OMC
catalysts with different content of Fe. In the present work, three
hydrogenation products of CMA, CMO, HCMA and HCMO were
detected. Furthermore, acetals were also observed, showing
that the condensation of isopropanol and CMA occurred. On
the other hand, products from cracking or decarbonylation
were not found. It was found that the conversion of CMA over
the PtFe–OMC catalysts increased with a rise in the Fe content
from 65.4% (Fe 0.5 wt%) to 90.3 (Fe 3.0 wt%). Also, the addition
of Fe to the Pt–OMC catalyst could obviously improve the
Fig. 4 H2-TPR profiles of different PtFe–OMC samples as well as 1Pt–OMC and
3Fe–OMC samples.
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Fig. 5 TEM images of the OMC (A), 1Pt–OMC (B), 1Pt0.5Fe–OMC (C), 1Pt1Fe–OMC (D), 1Pt1.5Fe–OMC (E), 1Pt2Fe–OMC (F), and 1Pt3Fe–OMC (G) samples.
Table 2 Conversions and selectivities of CMA hydrogenation over 1Pt–OMC and
alloy, which favor interactions with the carbonyl group and,
thus, increase the rate of the hydrogenation to crotylalcohol.
Figueiredo et al. also found that in a mesoporous carbon
the PtFe–OMC catalysts. (Reaction conditions: 70 1C, 4.0 MPa H2 and 2 h reaction
time)
supported Fe promoted Pt catalyst, the electron transfer from
Fe to Pt and the creation of electrophilic sites were beneficial
for the adsorption of the carbonyl group in CMA.24 Considering
these results, as well as the XPS and H2-TPR analysis, it was
therefore expected that the best possible alloying of the 1Pt3Fe–
OMC catalyst, in this case, would exhibit the highest CMO
selectivity. Unfortunately, the selectivity of CMO was decreased
to 65.3% over the 1Pt3Fe–OMC catalyst. This result is contrary
to our intuition that the addition of much more Fe might
further promote the CMO selectivity of the Pt catalyst in the
Selectivities (%)
CMO HCMA HCMO Acetals
Catalysts
1Pt–OMC
1Pt0.5Fe–OMC 65.4
1Pt1Fe–OMC 71.7
1Pt1.5Fe–OMC 85.3
1Pt2Fe–OMC
1Pt3Fe–OMC
1Pt1.5Fe–NAC
Conversions (%)
94.1
51.4
71.2
79.1
87.7
76.0
65.3
43.7
10.5
7.4
4.5
3.7
3.6
4.9
16.3
24.2
16.0
12.3
6.1
13.6
21.2
33.5
13.9
5.4
4.1
2.5
6.8
8.6
6.5
87.4
90.3
57.3
selectivity of CMA to CMO from 51.4% to 87.7%, although the hydrogenation of CMA. Indeed, multistep reaction processes in
1Pt–OMC catalyst possessed the highest CMA conversion heterogeneous catalysis are always very complicated, and the
(94.1%). Our results for the 1Pt1.5Fe–OMC catalyst (85.3% degree of the rate control of the elementary step might experi-
CMA conversion with 87.7% CMO selectivity at 70 1C, ence some changes over different catalyst formulations.55,56 This
120 min and 4.0 MPa of H2) are much higher than those of issue therefore deserves to be further explored. In addition,
the mesoporous carbon-supported 5Pt0.8Fe catalyst (50% CMA among the PtFe–OMC catalysts, the 1Pt1.5Fe–OMC catalyst
conversion with 71% CMO selectivity at 75 1C, 140 min and exhibited the best selectivity of CMA to CMO (87.7%), while
16 bar of H2),24 and is comparable to those of the carbon the selectivity towards other products (HCMA, HCMO and
nanotube-supported 5Pt3Fe catalyst(79.5% CMA conversion acetals) was relatively low (3.7%, 6.1% and 2.5%, respectively).
with 89.1% CMO selectivity at 60 1C, 60 min and 10 atm of In contrast, the selectivity of the 1Pt1.5Fe–NAC catalyst to CMO
H2).31 Earlier, Lercher et al. reported that an Fe promoted was much lower than that of the 1Pt1.5Fe–OMC catalyst. On the
Pt/SiO2 catalyst, prepared by the impregnation method, dis- contrary, its selectivity to HCMO was the highest (33.5%), which
played a high activity and selectivity to CQO in the gas phase could be attributed to the abundant microporous texture of the
hydrogenation of crotonaldehyde.53 Wu et al. reported a boron commercial NAC (specific surface area 798 m2 gÀ1, micropore
nitride (BN) supported bimetallic PtFe catalyst prepared by the surface area 657 m2 gÀ1) as discussed in ref. 57. The pore size in
co-incipient wetness method. With the same Pt and Fe contents, the micro scale is more than 80% over the surface of the NAC.
they found that the PtFe/BN catalyst possessed the highest These micropores were disadvantageous for the diffusion of
selectivity towards unsaturated alcohols compared with com- intermediate products of CMO from the catalyst, and resulted
mercial graphite and g-Al2O3 supported catalysts.54 Both attrib- in its successive hydrogenation to the saturated alcohol. On the
uted such a high selectivity to CQO to the formation of Pt–Fe other hand, as concluded by the previous reports, the M–OMCs
alloy nanoparticles and the existence of polar sites in the Pt–Fe synthesized using the OSS would possess intimate interfacial
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Table 3 Influence of the reaction temperature and H2 pressure on the conversion of CMA hydrogenation over the 1Pt1.5Fe–OMC catalyst
Selectivities (%)
CMO
HCMA
HCMO
Acetals
Temperature (1C)
H2 pressure (MPa)
Reaction time (h)
Conversions (%)
70
70
70
70
50
60
70
80
1.0
2.0
3.0
4.0
4.0
4.0
4.0
4.0
2
2
2
2
2.5
2.5
2.5
2.5
35.6
50.8
69.7
85.3
92.3
97.1
100
59.1
68.6
78.7
87.7
79.2
88.3
93.0
86.6
10.6
8.3
6.8
3.7
9.2
4.4
1.2
2.2
9.8
8.3
7.1
6.1
5.3
5.0
3.5
5.1
20.5
14.8
7.4
2.5
6.3
4.3
2.3
6.1
100
contact between the active metal particles and the carbon average particle sizes of 2–4 nm and a rather narrow size
support.8,9,11,12,58 Based on this point, the present Pt–Fe alloy distribution by means of a simple one-pot organic–organic
nanoparticles formed with the OSS were mainly imbedded in or self-assembly strategy. These carbon materials displayed well-
anchored on the carbon walls, which would not only produce dispersed Pt–Fe alloy nanoparticles embedded inside the pores
highly dispersed PtFe nanoparticles in the carbon matrix, but and walls of the mesoporous carbon. These bimetal-containing
could also create intimate contacts and strong interactions carbon materials were used as catalysts for CMA hydrogenation
between the Pt–Fe alloy nanoparticles and the carbon support. and exhibited a higher selectivity toward cinnamyl alcohol than
Furthermore, the ordered mesoporous structure of the OMC the monometal Pt-containing counterpart. These results should
made mass transfer easier compared to the commercial NAC deepen the understanding of the role of combining bimetals
and provided better accessibility to reactant molecules. The with OMC materials for catalytic reactions. The unique features
multi-functions of the bimetallic PtFe–OMC catalyst synthe- and advantages of the bimetal-containing OMC materials are
sized using the OSS should be responsible for the superior worth further exploration.
performance of the selective hydrogenation of CMA.
The reaction temperature and pressure are important factors
that can influence the catalytic activity and selectivity, especially
in a multiphase sequential reaction. According to the catalytic
Acknowledgements
performance of the PtFe–OMC catalysts, we selected the
1Pt1.5Fe–OMC catalyst to study the influence of temperature
and pressure on hydrogenation of CMA. Table 3 summarizes the
conversion and selectivity changes along with the varied reaction
temperatures under 4.0 MPa of H2 pressure and 2.5 h of reaction
time. It was noted that when the reaction temperature increased
from 50 1C to 70 1C, the conversion of CMA and the selectivity of
CMA to CMO over the 1Pt1.5Fe–OMC catalyst remarkably
increased, and exhibited the highest selectivity of CMA to CMO
(93.0%) at 70 1C. However, when the reaction temperature was
elevated to 80 1C, the selectivity of CMA to CMO decreased due to
complete hydrogenation and an increase in the yields of by-
products. Table 3 also summarizes the conversion and selectivity
changes along with the varied H2 pressure under 70 1C and 2 h of
reaction time. It was noted that when the H2 pressure increased
from 1.0 MPa to 4.0 MPa, the conversion of CMA and the
selectivity of CMA to CMO over the 1Pt1.5Fe–OMC catalyst
greatly increased, indicating that increasing the H2 pressure
was favorable for the hydrogenation for the CQO bond. The
increase of the H2 pressure could increase the solubility of H2 in
the liquid phase, which leads to an increased frequency of
collisions between hydrogen molecules and the surface of the
catalyst, thus enhancing the activity of the catalyst.
Financial support from the foundation of Liaoning Province
Educational Committee (no. 2008368) and doctoral program
foundation of Liaoning Province (no. 20091047) are gratefully
acknowledged.
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