Hydrogenation of cinnamaldehyde to hydrocinnamaldehyde over Pd nanoparticles deposited on nitrogen-doped mesoporous carbon

Article abstract of DOI:10.1039/c6ra04154j

Palladium nanoparticles deposited on nitrogen-doped mesoporous carbon (NMC) were synthesized by simple ultrasonic-assisted method. This novel Pd-NMC catalyst was highly active and selective for the hydrogenation of cinnamaldehyde (CA) to hydrocinnamaldehyde (HCA) at room temperature (30 °C) under low H₂ pressure. The nitrogen-free mesoporous carbon (MC) and activated carbon (AC) were also employed as the support for Pd in the liquid-phase hydrogenation of CA. The incorporation of nitrogen into carbon matrix remarkably enhanced the catalytic activity and CC bond hydrogenation selectivity (HCA selectivity of 93% with 100% CA conversion for Pd-NMC) in CA hydrogenation compared to the catalysts with no nitrogen (HCA selectivity of 66 and 47% for Pd-MC and Pd-AC, respectively). Moreover, Pd-NMC catalyst demonstrated an excellent recyclability without any loss in activity and HCA selectivity when it was reused for six times. The superior catalytic performance of Pd-NMC catalyst in CA hydrogenation is attributed to the small size of Pd nanoparticles due to presence of high nitrogen content (11.6 wt%) and mesoporous nature of NMC support.

Full text of DOI:10.1039/c6ra04154j

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Hydrogenation of cinnamaldehyde to
hydrocinnamaldehyde over Pd nanoparticles
Cite this: RSC Adv., 2016, 6, 44333
deposited on nitrogen-doped mesoporous carbon†
Atul S. Nagpure, Lakshmiprasad Gurrala, Pranjal Gogoi
*
and Satyanarayana V. Chilukuri
Palladium nanoparticles deposited on nitrogen-doped mesoporous carbon (NMC) were synthesized by
simple ultrasonic-assisted method. This novel Pd-NMC catalyst was highly active and selective for the
hydrogenation of cinnamaldehyde (CA) to hydrocinnamaldehyde (HCA) at room temperature (30 ꢀC)
under low H2 pressure. The nitrogen-free mesoporous carbon (MC) and activated carbon (AC) were also
employed as the support for Pd in the liquid-phase hydrogenation of CA. The incorporation of nitrogen
into carbon matrix remarkably enhanced the catalytic activity and C]C bond hydrogenation selectivity
(HCA selectivity of 93% with 100% CA conversion for Pd-NMC) in CA hydrogenation compared to the
catalysts with no nitrogen (HCA selectivity of 66 and 47% for Pd-MC and Pd-AC, respectively). Moreover,
Pd-NMC catalyst demonstrated an excellent recyclability without any loss in activity and HCA selectivity
when it was reused for six times. The superior catalytic performance of Pd-NMC catalyst in CA
hydrogenation is attributed to the small size of Pd nanoparticles due to presence of high nitrogen
content (11.6 wt%) and mesoporous nature of NMC support.
Received 15th February 2016
Accepted 28th April 2016
DOI: 10.1039/c6ra04154j
www.rsc.org/advances
carbon nanobers.4a Conversion of CA was 100% at 98% HCA
selectivity when the reaction was carried out at 80 ꢀC for nearly
1. Introduction
30 h. Liu et al. reported 91.3% HCA selectivity at 98.6% CA
conversion over Pd supported on multiwalled carbon nanotubes
under 40 bar H2 and 148 bar CO2 pressure at 60 ꢀC.4b Arai et al.
obtained 87% HCA selectivity and 100% CA conversion using
Pd/C catalyst under 40 bar H2 and 80 bar CO2 pressure.4c
Amadou et al. achieved 90% HCA selectivity at nearly 100% CA
conversion over Pd supported on nitrogen-doped carbon
nanotubes at 80 ꢀC in 8 h.4d Begin et al. used few-layer graphene
supported Pd catalyst to obtained 92% HCA selectivity and
almost 100% CA conversion at 80 ꢀC.4e Giambastiani et al.
reported 97% HCA selectivity and 100% CA conversion by using
Chemoselective hydrogenation of a,b-unsaturated carbonyl
compounds into their corresponding unsaturated alcohols and
saturated carbonyls over heterogeneous catalysts has drawn
increasing attention for both scientic and economic reasons.1
As a representative of a,b-unsaturated aldehyde, cinnamalde-
hyde (CA) is particularly vital as it can be selectively hydroge-
nated to hydrocinnamaldehyde (HCA) or cinnamyl alcohol
(CAL), depending on whether C]C bond is hydrogenated or the
C]O bond (Scheme 1). HCA and CAL are very important
intermediates for the synthesis of many ne chemicals,
perfumes and pharmaceuticals.2 HCA was found to be essential
intermediate in the preparation of a drug used in the treatment
of HIV.2
Supported Pt,3a–e Au,3f Ru,3g–h Ir,3i–j Co3k and Cu3l–m catalysts
were reportedly active for the selective hydrogenation of CA to
CAL. Various supported metal catalysts were employed for the
hydrogenation of CA to get HCA with high selectivity.4
Mahmoud et al. achieved 100% CA conversion and 75% HCA
selectivity over Pd/SiO2 catalyst.2b Ledoux et al. studied selective
hydrogenation of CA to HCA over Pd catalyst supported on
Pd/g-Al2O3 catalyst at 100 C in 3 h.4f Zhang et al. studied CA
ꢀ
hydrogenation over SiO2 supported nickel phosphide catalyst
and achieved 93% HCA selectivity and 78% CA conversion at
Catalysis & Inorganic Chemistry Division, CSIR-National Chemical Laboratory, Dr.
Homi Bhabha Road, Pune-411008, India. E-mail: sv.chilukuri@ncl.res.in; Fax: +91-
20-25902633; Tel: +91-20-25902019
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra04154j
Scheme 1 Possible reaction pathways for hydrogenation of CA.
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120 ꢀC.4g Marchi et al. reported 59.5% HCA selectivity with 60% Pd-NMC catalyst used in this investigation showed admirable
CA conversion by using Cu/SiO2 catalyst at 120 C.4h However, catalytic activity (100% CA conversion) with high HCA selectivity
ꢀ
obtaining high HCA selectivity at high conversion of CA, (93%) under low H2 pressure at room temperature (30 ꢀC).
particularly at ambient reaction conditions and reusability of Furthermore, Pd-NMC catalyst displayed an excellent reus-
the catalyst are still important issues that need to be addressed. ability in the hydrogenation of CA to HCA.
The high activity and selectivity of catalysts in CA hydroge-
nation is
a challenging task due to intricate reaction
mechanisms involved including dissociative/non-dissociative,
competitive adsorption as well as formation of coke, side reac-
tions, adsorption of solvents, etc.1c,2a,5 Although, many studies
have been reported for the selective hydrogenation of CA to
HCA, a more efficient catalyst system that is active even at room
temperature and under low H2 pressure is highly desirable.
Recent research efforts on carbon materials demonstrate
their exceptional features (chemical and thermal stability, high
surface area, etc.), as a new class of solid supports for a variety of
heterogeneous catalysts.6 However, noble metals (e.g., Pd, Pt, Rh
and Ru) deposited on carbon materials easily leach out during
catalyst evaluation because of weak interaction between carbon
surface and the metal nanoparticles. Moreover, the catalytic
properties of porous carbons do not always satisfy all the
requirements of a catalyst support. Hence, modication of
carbons is essential in the majority of cases.7 Usually,
single-walled carbon nanotubes cannot efficiently anchor Pd
nanoparticles without generation of defects.8 In order to rein-
force the interaction between metal and the support, carbons
are generally oxidized with HNO3 to introduce defects.9
Nitrogen-doped carbon materials have attracted worldwide
attention due to their outstanding performance in various
applications, as catalyst supports,10 for applications in super-
capacitors,11 for metal-free oxygen reduction reaction in case of
fuel cell cathodes,12 hydrogenation reaction,13 etc. Doping with
electron-rich nitrogen atoms in the carbon architecture can
enhance electrical, chemical and functional properties.14
Nitrogen-doped carbon materials show several features that are
signicantly different from their undoped counterparts. The
presence of nitrogen atoms in the carbon matrix could also alter
the electronic and chemical interactions with the deposited
metal nanoparticles, which can really modify the overall
catalytic activity as well as product selectivity.4d
2. Results and discussion
2.1. Structural characteristics of the catalysts
The NMC support was prepared by a colloidal silica nanocasting
route (Scheme S1, ESI†), which involved the mixing of
melamine–phenol–formaldehyde polymer sol with colloidal
silica to obtain composite hydrogel.15 Its subsequent carbon-
ization in N2 atmosphere at 800 ꢀC and silica dissolution by
treatment with NaOH gave the NMC with disordered meso-
pores. The elemental analysis showed that the resulting NMC
has a high nitrogen content of 11.6 wt% (Table 1 and Table S1†).
This as-synthesized NMC was used as a support for loading of
Pd by a modied ultrasonic-assisted method (Scheme S2,
ESI†).13 A similar procedure was adopted for the preparation of
activated carbon (AC) and mesoporous carbon (MC) supported
Pd catalysts. The X-ray diffraction (XRD) pattern of NMC,
Pd-NMC, Pd-AC and Pd-MC are shown in Fig. 1a. A broad
diffraction peak at around 2q ¼ 25.2ꢀ and a weak peak around
43.7ꢀ were observed for NMC, that correspond to the (002) and
(100) crystal planes of graphite lattice, respectively.15 The XRD
pattern of Pd-NMC catalyst was similar to that of NMC and does
not contain any diffraction peaks related to the metallic Pd,
suggesting formation of highly dispersed Pd nanoparticles. The
diffraction peaks of Pd in Pd-AC were weak. But, signicantly
sharp diffraction peaks of Pd at 2q ¼ 40.1, 46.7, 68.1 and 82.1ꢀ
were detected in Pd-MC, which were assigned to (111), (200),
(220) and (311) crystalline planes of metallic Pd, respectively. As
shown in Fig. 1b, NMC, Pd-NMC and Pd-MC samples show type
IV adsorption–desorption isotherms with a H2 hysteresis loop,
corresponding to the typical mesoporous structure of the
materials, while Pd-AC exhibits type II adsorption–desorption
isotherm, related to the microporous nature of the sample. The
other textural parameters and physico-chemical properties of
the catalysts are summarized in Table 1.
Herein, we report a highly efficient and chemoselective Pd
catalyst supported on nitrogen-doped mesoporous carbon
(NMC) for the liquid-phase hydrogenation of CA to HCA. The
Raman spectroscopy was employed to study the graphitic
nature of the carbon (Fig. 1c). It consists of characteristic D- and
G-bands of disordered graphitic carbon materials, at 1335 and
Table 1 Textural properties and Pd metal characteristics of the catalysts
Pd contenta BET surface Micropore surface Average pore
Nitrogen
Average Pd particle Pd metal
Pd metal surface
Catalyst (wt%)
area (m2 gꢁ1) areab (m2 gꢁ1
)
diameterc (nm) contentd (wt%) sizee (nm)
dispersionf (%) areag (m2 gꢁ1
)
NMC
Pd-NMC 2.05
Pd-MC 1.97
—
844
791
712
103
85
116
663
12.4
12.2
13.0
3.8
11.6
11.5
0
—
—
—
2.4
7.7
4.6
54.0
16.8
28.2
4.15
1.29
2.16
Pd-AC
1.94
1020
0
a
b
c
Determined by ICP-OES. Determined by t-plot method. Calculated from desorption branch of N2 sorption isotherm by BJH method.
d
e
f
Determined using elemental analysis. Calculated based on TEM analysis. Estimated on the basis of average Pd particle size calculated from
g
TEM analysis and using equation described by Isaifan et al. (ref. 17). Calculated assuming Pd metal particles as hemispherical in shape with
the at side on the support.
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vibration of C]C bonds of benzenoid ring.14c,d The peaks
detected at 1381 and 1235 cmꢁ1 in case of NMC and Pd-NMC
corresponds to the stretching modes of C]N and C–N bonds,
respectively.14c,d These peaks (1381 and 1235 cmꢁ1) were not
seen in Pd-MC, which shows the absence of nitrogen in the Pd-
MC. The peak at 2925 cmꢁ1 is related to the stretching vibration
of C–H bonds.14d The broad peak in the range of the 3150–3560
cmꢁ1 detected in NMC and Pd-NMC were attributed to the
stretching vibration of O–H and N–H bonds.14c,d Whereas, the
peak (3150–3560 cmꢁ1) observed in Pd-MC were assigned to the
O–H bonds stretching frequency.
In X-ray photoelectron spectroscopy (XPS) study of NMC,
carbon, oxygen and nitrogen were detected while only carbon
and oxygen were seen in MC (Fig. 2a). The N 1s spectra of NMC
(Fig. 2b) are curve-tted into three peaks with the binding
energy values at 398.2, 400.4 and 401.8 eV that correspond to
pyridinic N (N1), pyrrolic N (N2) and graphitic N (N3), respec-
tively.15 The Pd 3d XPS spectra of Pd-MC (Fig. 2c) and Pd-NMC
(Fig. 2d) catalysts consist of two peaks related to Pd 3d5/2 and
Pd 3d3/2. The peaks around 336.1 and 341.2 eV were assigned to
Fig. 1 (a) XRD pattern, (b) N2 adsorption–desorption isotherm, (c)
Raman spectra and (d) FT-IR spectra of NMC and Pd catalysts.
metallic Pd (Pdo), corresponding to Pdo 3d5/2 and Pdo 3d3/2
,
respectively.10,13 Whereas, the peaks at 337.8 and 342.7 eV are
attributed to the Pd in +2 oxidation state (PdO) and hence
assigned to Pd2+ 3d5/2 and Pd2+ 3d3/2, respectively.10,13 According
to XPS data, the Pdo percentage in Pd-NMC and Pd-MC were 86
and 65%, respectively. These results clearly suggest that the
incorporation of nitrogen in the carbon framework resulted in
the increased proportions of metallic Pd.10,13
The mesoporous structure of NMC and MC can be further
conrmed by transmission electron microscopy (TEM) (Fig. 3),
which revealed that both samples consists of randomly
distributed spherical mesopores with disordered amorphous
carbon structure. It can be seen that the Pd nanoparticles with
an average size of 2.4 nm were homogeneously distributed
throughout the NMC support (Fig. 4a and b). On the other hand,
Pd nanoparticles were found to be dispersed unevenly and large
agglomerated Pd nanoparticles were detected on the surface of
MC and AC (Fig. 4c–f). The average particle sizes of Pd nano-
particles in Pd-MC and Pd-AC were found to be 7.7 and 4.6 nm,
respectively. Hence, it can be concluded that the nitrogen
dopant signicantly contributes for the stabilization of Pd
nanoparticles, thus leading to smaller Pd particles.16
1590 cmꢁ1, respectively. The D-band reects defects in carbons
whereas the G-band indicates the graphitic structure of
carbons.15 The intensity ratios of the D-band to the G-band
(ID/IG, calculated from integral area of the peaks) are 1.68,
1.75, 2.38 and 2.45 for Pd-AC, Pd-MC, NMC and Pd-NMC,
respectively. The higher ID/IG ratios for nitrogen containing
samples compared to nitrogen-free samples (Pd-AC and Pd-MC)
shows that the incorporation of nitrogen in carbon frameworks
led to more disordered structures.15
Fourier transform infrared (FT-IR) spectroscopy studies were
carried out to identify the functional groups present in NMC,
Pd-NMC and Pd-MC samples (Fig. 1d). The peak observed at
1558 cmꢁ1 in all the samples was assigned to stretching
The average particle size of Pd nanoparticles (calculated based
on TEM analysis) on Pd-NMC, Pd-AC and Pd-MC catalysts were
used to calculate dispersion (%) of Pd assuming spherical shape of
particles and using the formula described by Isaifan et al.17
Fig. 2 (a) XPS survey of MC and NMC. (b) High resolution XPS spectra
of N 1s for NMC. (c) XPS spectra of Pd 3d for Pd-MC. (d) XPS spectra of
Pd 3d for Pd-NMC.
Fig. 3 TEM image of the (a) MC and (b) NMC.
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selectivity data over Pd-NMC catalyst at 30 ꢀC. The chemical and
physical properties (polarity and H2 solubility) of the solvents
used are also given in Table 2. Under the reaction conditions
studied, mainly HCA and 3-phenyl propanol (PPL) products
were observed. The CA conversion was found to be relatively
high in ethanol and 2-propanol compared to other solvents. On
the other hand, lower CA conversion was seen in cyclohexane
and toluene, possibly due to the poor solubility of reactants,
including H2 in these solvents (entry 2 and 3, Table 2). It is
interesting to note that HCA selectivity was in the range of 81.2
to 93% in all the solvents. When 2-propanol was employed as
solvent; comparatively high catalytic activity was witnessed. The
100% CA conversion was achieved within 3 h along with 93%
HCA selectivity at a TOF value of 987 hꢁ1 (entry 5, Table 2).
Although, 2-propanol is known as a hydrogen transfer reagent,
the results suggested that the effect of transfer hydrogenation
was nil under the reaction conditions used (entry 6, Table 2). In
case of water as solvent, low reactivity (TOF ¼ 229 hꢁ1) with
23.2% CA conversion and 81.2% HCA selectivity was noticed
(entry 7, Table 2). The inferior catalytic activity in water may be
due to the poor solubility of H2 in water (Table 2).19 In this
particular case, probably H2 solubility along with solvent
polarity are the vital factors. In view of the higher apparent
activity and superior HCA selectivity (93%), 2-propanol was
chosen as the solvent for further investigations.
Fig. 4 TEM images and the Pd nanoparticles size distribution for Pd-
NMC (a and b), Pd-AC (c and d) and Pd-MC (e and f), respectively.
2.2.2. Comparison of various supported Pd catalysts. To
study the effect of the catalyst support, the Pd-NMC catalyst was
compared with other catalysts including Pd-MC and Pd-AC
ꢀ
under identical reaction conditions (30 C and 5 bar H2 pres-
(see ESI†). The Pd dispersion values were: Pd-NMC, 54%; Pd-MC,
16.8% and Pd-AC, 28.2% (Table 1). The higher dispersion of Pd
in Pd-NMC conrms that nitrogen doping led to the homogeneous
distribution of Pd nanoparticles over NMC support.
sure) and the results are given in the Fig. 5. Among the different
catalysts studied, Pd-NMC exhibited relatively better hydroge-
nation activity affording 93% HCA selectivity with 100% CA
conversion within 3 h. The HCA selectivity for MC and
NMC-based Pd catalysts were almost stable, at 66 and 93%,
respectively. However, HCA selectivity for AC-based Pd catalyst
was decreased from 53 to 44% with increasing CA conversion.
MC supported Pd nanoparticles were not effective for this
reaction, resulting in low selectivity of desired product (HCA)
with low CA conversion. The reason for this may be poor Pd
dispersion and the larger size of Pd nanoparticles (7.7 nm).
Similarly, Pd-AC catalyst showed low HCA selectivity, in spite of
2.2. Catalytic activity in hydrogenation of CA to HCA
2.2.1. Effect of solvent. Solvent plays an important role in
heterogeneous catalytic hydrogenations and possibly may alter
the distribution of the products.18 Hydrogenation of CA was
carried out in polar–aprotic (acetonitrile), protic (ethanol and
2-propanol), non-polar (cyclohexane and toluene) and in water
solvents. Table 2 provides the CA conversion and products
Table 2 Hydrogenation of CA in different solvents over Pd-NMCa
Products selectivity (%)
Entry
Solvent
Solvent polarity index
H2 solubilityb (mmol Lꢁ1
)
CA conv. (%)
HCA
PPL
Othersc
TOFd (hꢁ1
)
1
2
3
4
5
6e
7
Acetonitrile
Cyclohexane
Toluene
5.8
0.2
2.4
5.2
3.9
3.9
9.0
—
71.2
43.5
57.3
89.7
100
—
88.3
91.1
87.5
83.0
93.0
—
11.7
8.9
12.5
12.2
4.9
0
0
0
4.8
2.1
—
0
702
429
565
885
987
—
3.72
2.75
3.43
3.90
3.90
0.81
Ethanol
2-Propanol
2-Propanol
Water
—
18.8
23.2
81.2
229
a
Reaction conditions: CA (7.5 mmol); catalyst (Pd-NMC, 25 mg); solvent (25 mL); H2 pressure (5 bar); time (3 h); temperature (30 ꢀC); stirring speed
(800 rpm). b H2 solubility values in various solvents is taken from ref. 19. c Other products include acetal and ethers. d TOF ¼ turnover frequency
e
(moles of CA converted per mole of surface Pd per unit hour). In the absence of H2.
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Fig. 5 (a) CA conversion and (b) HCA selectivity as a function of Fig. 6 Hydrogenation of HCA and CAL over (a) Pd-NMC and (b) Pd-AC
reaction time over different Pd catalysts. Reaction conditions: CA (7.5 catalyst as a function of reaction time. Reaction conditions: HCA (4
mmol); catalyst (25 mg); solvent (2-propanol, 25 mL); H2 pressure (5 mmol); CAL (4 mmol); catalyst (25 mg); solvent (2-propanol, 25 mL);
bar); temperature (30 ꢀC); stirring speed (800 rpm).
H2 pressure (5 bar); temperature (30 ꢀC); stirring speed (800 rpm).
having high surface area (1020 m2 gꢁ1, Table 1) and PPL was HCA in signicant concentration at close to ambient tempera-
obtained in considerable quantity. The high microporosity of ture. Whereas, the hydrogenation of CAL to PPL proceeded
AC (>60% of total surface area, Table 1) might be responsible for nearly 30 times faster than the hydrogenation of CA to CAL.
the poor diffusion of the reactant molecules and the hydroge-
The catalytic activity of NMC supported Pd (2 wt%) is
nated products.4a Hence, hydrogenation reaction might have signicantly higher than that of Pd (5 wt%) supported on
proceeded slowly with low HCA selectivity. The presence of carbon nanobers. With less amount of Pd in the NMC sup-
acidic sites on AC could also modify the hydrogenation pathway ported catalyst, 100% CA conversion was achieved only in 3 h at
leading to the loss in selectivity.4a The superior catalytic 30 ꢀC, while it required nearly 30 h at 80 ꢀC over the carbon
performance and improved C]C hydrogenation selectivity nanobers supported catalyst.4a Activated carbon supported Pd
observed on N-doped Pd catalyst could be attributed to the (10 wt%) catalyst converted 100% CA with 87% HCA selectivity
following factors: (i) better Pd dispersion (54%) leading to under 40 bar H2 pressure.4c Few-layer graphene supported Pd
higher Pd metal surface area (4.15 m2 gꢁ1, Table 1) due to the (5 wt%) catalyst showed nearly 100% CA conversion and 92%
presence of nitrogen, (ii) appropriate metal–support interaction HCA selectivity at 80 ꢀC.4e Nitrogen-doped carbon nanotubes
between Pd and NMC support because of small Pd particle size, supported Pd (10 wt%) catalyst offered almost 100% CA
(iii) electronic activation of Pd nanoparticles by the nitrogen conversion with 90% HCA selectivity at 80 ꢀC in 8 h.4d Therefore,
present in the support,4d,13 as more Pdo seen from the XPS it can be noticed that the Pd catalyst of the present study
studies that might modify the adsorption mode of the CA and (Pd-NMC) is more efficient and the NMC support plays an
selectivity of the products,20,21 and (iv) mesoporosity of NMC important role in enhancing the catalytic activity.
support that facilities free diffusion of the reactants and
2.2.3. Effect of reaction temperature. Inuence of reaction
hydrogenated products.
temperature on the CA conversion and selectivity to various
Investigations were also conducted with Pd-NMC and Pd-AC products was studied by changing the temperature in the 30–60
catalysts using equimolar quantities of HCA and CAL as reac- ꢀC range over Pd-NMC catalyst at 5 bar H2 pressure (Fig. 7). CA
tants, which are C]C and C]O hydrogenated product of CA, conversion was 100% with an excellent HCA selectivity (93%)
respectively (Fig. 6). The HCA was not further hydrogenated to within 3 h at ambient temperature (30 ꢀC). At 40 and 50 ꢀC, the
PPL on both the catalysts under the reaction conditions used CA hydrogenation rate was accelerated without any loss in HCA
(30 ꢀC and 5 bar H2 pressure), which indicates that the selectivity. For example, 100% CA conversion was achieved in
re-adsorption of HCA molecule through the C]O bond did not 2.5 and 2 h at 40 and 50 ꢀC, respectively. It is important to note
occur on the Pd nanoparticles irrespective of the nature of the that the HCA selectivity remains nearly constant (93%) even
catalyst support. However, CAL was converted to PPL to a great aer 100% CA conversion was attained when the CA hydroge-
extent on both the catalysts. The results shown in Fig. 5 and 6 nation was carried out at lower temperatures (30, 40 and 50 ꢀC).
clearly suggest that on Pd-AC, the CA must have adsorbed in This leads to the conclusion that under the above reaction
such a way that almost simultaneous hydrogenation of the C]C conditions the hydrogenation of HCA to PPL does not take place
and C]O bonds takes place whereas on the Pd-NMC catalyst signicantly. Otherwise, one would expect HCA selectivity to
the C]C hydrogenated product (HCA) were quickly desorbed diminish with time. Notably, when the reaction temperature
ꢀ
from the active sites. These important observations point to the was further increased to 60 C, a drop in HCA selectivity (from
fact that all PPL obtained in CA hydrogenation under the 88–72%) was witnessed. This reduction in HCA selectivity was
reaction conditions studied actually formed via the formation of attributed to the enhanced hydrogenation rate of HCA to PPL at
CAL as an intermediate. Notably, CAL was not detected in the higher temperature. These observations are in good agreement
reaction mixture, suggesting that CAL formed during the course with the literature, which reports that the over Pd catalyst
of reaction must have rapidly converted to PPL. Similar obser- hydrogenation of HCA does not yield any detectable amount of
vations were reported by Mahmoud et al.2b in CA hydrogenation PPL at lower temperatures.2b And the hydrogenation of HCA to
over Pd/SiO2 catalyst. They found that PPL was not formed from PPL was increased at higher reaction temperature.
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Scheme 2 Possible reaction mechanism for the selective hydroge-
nation of CA to HCA over Pd-NMC catalyst.
width, larger the interaction of the metal surface with the
conjugated C]C bond compared to the C]O bond.23 The width
of the metal d-band increases in the following order: Pd < Pt < Ir
z Os.23 Therefore, the chemisorption of C]C bond becomes
more favorable over the Pd metal catalysts. DFT calculations
have revealed that the maleic anhydride interacts with the Pd
(111) surface through the C]C bond.24 Over the Pd-NMC cata-
lyst, the C]C bond in CA molecule may get adsorbed almost
parallel to the Pd metal surface as a results of small particle size
of Pd nanoparticles (2.4 nm). Most probably, the close interac-
tion between C]C bond and the Pd metal surface may lead to
superior catalytic performance. Scheme 2 demonstrates the
proposed step sequence in the chemoselective hydrogenation of
CA. In the rst step, CA interacts with the Pd surface through
the C]C bond and H2 molecule is dissociated over electroni-
cally promoted Pd (due to presence of nitrogen as observed
through XPS studies). Subsequently, the selective hydrogena-
tion of C]C bond occurs by the attack of the activated hydrogen
species. As a result of the presence of mesoporous channels in
the NMC support, the formed saturated carbonyl (HCA) rapidly
leaves the catalyst surface and is replaced with a new reactant
(CA) molecule. This avoids further hydrogenation of HCA to
saturated alcohol (PPL).
Fig. 7 Effect of reaction temperature (a) 30 ꢀC, (b) 40 ꢀC, (c) 50 ꢀC and
(d) 60 ꢀC, on CA conversion and products selectivity as a function of
reaction time. Reaction conditions: CA (7.5 mmol); catalyst (Pd-NMC,
25 mg); solvent (2-propanol, 25 mL); H2 pressure (5 bar); stirring speed
(800 rpm).
2.2.4. Effect of H2 pressure. Inuence of H2 pressure on the
CA hydrogenation was studied by varying the pressure in the
range of 1–5 bar over Pd-NMC catalyst at 30 ꢀC (Fig. 8). The CA
conversion was improved from 40 to 100%, when the H2 pres-
sure was increased from 1 to 5 bar. But, the HCA selectivity was
not signicantly altered and remained nearly at 93%. Hence, 5
bar H2 pressure was considered as optimum to get 100% CA
conversion and 93% HCA selectivity in 3 h. However, even at 1
bar H2 pressure, similar conversion and selectivity can be
achieved on prolonging the reaction time (9 h).
2.2.6. Recyclability study. The recyclability of the Pd-NMC
catalyst in the CA hydrogenation was investigated by repeating
the reaction with the same catalyst without any regeneration or
activation (Fig. 9). Pd-NMC was found to be active even aer six
recycles without any signicant drop in activity and HCA
2.2.5. Possible reaction mechanism. There is a relationship
between metal d-band width and the product selectivity's in the
hydrogenation of a,b-unsaturated carbonyl compounds.2a,22 The
Huckel calculations suggest that the narrower the metal d-band
Fig. 9 Recyclability study of Pd-NMC catalyst in the CA hydrogena-
Fig. 8 Effect of H2 pressure on CA hydrogenation. Reaction condi- tion. Reaction conditions: molar ratio of CA to Pd (1600); solvent (2-
tions: CA (7.5 mmol); catalyst (Pd-NMC, 25 mg); solvent (2-propanol, propanol, 25 mL); H2 pressure (5 bar); time (3 h); temperature (30 ꢀC);
25 mL); temperature (30 ꢀC); time (3 h, # 9 h); stirring speed (800 rpm). stirring speed (800 rpm).
44338 | RSC Adv., 2016, 6, 44333–44340
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selectivity, which is a prerequisite for practical industrial 9.84 g of melamine (78 mmol) and another part of formaldehyde
applications. In order to gain more insight into the catalytic (12 g) were added with continuous stirring for 30 min. Then, 50 g
activity during recycle study, XRD and TEM analysis of spent of Ludox SM-30 sol (30 wt% SiO2) was added to the above solution
catalysts were carried out. XRD results showed that the catalyst to react for 1 h with consecutive agitation. The suspension was
did not undergo any change during the reaction (Fig. S1, ESI†). then transferred to sealed bottle and heated at 80 ꢀC for 3 days. The
The TEM image of the Pd-NMC aer six recycles exhibited an obtained gel was dried at 80 ꢀC and the material obtained was
average Pd nanoparticles size of 2.6 nm, almost the same as that grinded in mortar. This powder was carbonized in N2 ow at
of the fresh catalyst (Fig. S2, ESI†). These results demonstrate an 800 C for 3 h while heating at the rate of 5 ꢀC minꢁ1. The nal
ꢀ
excellent stability of the catalyst. The reaction mixture aer each NMC was obtained by dissolution of the silica in 2 M NaOH
recycle was analyzed using ICP-OES for the presence of Pd, due solution at 80 ꢀC for 12 h. The resulting material was washed with
to leaching. No Pd was observed, indicating strong interaction distilled water until pH is neutral and dried at 100 ꢀC for 10 h. The
between the Pd and NMC support. In addition, the amount of nitrogen-free mesoporous carbon (MC) was also prepared by using
Pd in the catalyst aer six recycles was similar to that of the above process without adding any melamine content.
starting catalyst.
4.2.2. Synthesis of Pd-NMC. The 2 wt% Pd-NMC catalyst
was synthesized by a modied ultrasonic-assisted method
(Scheme S2, ESI†).13 Typically, 0.1 g of NMC was dispersed in
50 mL of deionized water in 100 mL round bottom ask by
ultrasonication (20 min). To it, 0.5 mL of aqueous PdCl2
solution (Pd content 4 mg mLꢁ1) was added under ultra-
sonication. This mixture was stirred at 80 ꢀC for 6 h and cooled
to room temperature. Then, aqueous NaBH4 (Pd/NaBH4 ¼ 1 : 4
mol molꢁ1) was added to the above solution drop-wise under
ultrasonication for 30 min. The solution was ltered and
washed with deionized water until no chloride ions were
detected (conrmed by AgNO3 test). The resulting 2 wt%
Pd-NMC catalyst was dried in an oven at 80 ꢀC for 10 h and used
for catalytic tests. Catalysts 2 wt% Pd-MC and 2 wt% Pd-AC were
also prepared by following above procedure.
3. Conclusions
Chemoselective liquid-phase hydrogenation of CA was investi-
gated over Pd supported on NMC, MC and AC catalysts. Catalyst
Pd-NMC showed admirable catalytic activity and high HCA
ꢀ
selectivity (93%) under mild reaction conditions (30 C, 5 bar
H2). It could be reused for over six cycles without any loss in
activity and selectivity. This good catalytic performance is
attributed to the presence of nitrogen in the carbon framework
and mesoporous nature of the support. The mesoporous
structure of NMC support helps better mass transfer of the
reactant molecules to the active sites and promotes the free
diffusion of the hydrogenated products. The incorporation of
nitrogen atoms in the carbon network not only leads to a very
stable and homogeneous dispersion of Pd nanoparticles but
also responsible for electronic and morphologic modications
of the active phase. The synthesis approach used in this study
(incorporation of heteroatoms in the catalyst support) may help
to design various other catalysts, which may be useful for the
production of numerous ne chemicals.
4.3. Characterization techniques
The physico-chemical characterizations of the samples were
carried out by using various techniques such as powder XRD, N2
sorption, FT-IR, Raman, ICP-OES, TEM and XPS. Further details
are provided in the ESI.†
4.4. Evaluation of catalysts
4. Experimental section
4.1. Chemicals
All the reactions were carried out using a 100 mL Parr auto-
clave (SS316). In a typical experiment, CA (7.5 mmol), 25 mL
of solvent and the desired amount of freshly reduced catalyst
was introduced into the reactor vessel. The reactor contents
were mixed thoroughly and the reactor was sealed, purged
two to three times with hydrogen and pressurized to required
hydrogen pressure. Aerward, the reaction was carried out at
required temperature with continuous stirring (800 rpm).
Liquid samples were withdrawn periodically and analyzed
using GC (Agilent 7890A) equipped with a ame ionization
detector having CP Sil 8 CB capillary column (30 m length,
0.25 mm diameter). Product identication was done using
authentic standards and by using GC-MS (Shimadzu,
GCMS-QP2010 Ultra; HP-5 column; 30 m length, 0.25 mm
diameter).
All chemicals used were reagent grade and employed without
further purication. Acetonitrile, cyclohexane, toluene, ethanol,
2-propanol, melamine, phenol, formaldehyde, NaOH, NaBH4,
AgNO3, CA (98%), HCA (98%), CAL (98%) and PPL (98%) were
procured from Loba Chemie, Mumbai. Whereas, Ludox SM-30
colloidal silica (30 wt% SiO2 in water), PdCl2$2H2O and AC
were purchased from Alfa Aesar.
4.2. Synthesis of catalysts
4.2.1. Synthesis of NMC. The NMC support was synthe-
sized via a colloidal silica assisted sol–gel process by using
melamine as a nitrogen source (Scheme S1, ESI†).15 In a typical
synthetic procedure, 3.67 g of phenol (39 mmol) and 6.33 g of
formaldehyde (37 wt%, 78 mmol) were added drop-wise to
50 mL of NaOH solution (0.2 M, 10 mmol) under stirring. The
Acknowledgements
mixture was stirred at room temperature for 20 min and then Atul S. Nagpure and Lakshmiprasad Gurrala acknowledge Council
heated in an oil bath at 70 ꢀC under stirring for 40 min. To it, of Scientic and Industrial Research (CSIR), New Delhi, for
This journal is © The Royal Society of Chemistry 2016
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providing senior research fellowships. Authors also acknowledge
nancial support from CSIR Network project CSC-0122.
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