Surface-exposed Pd nanoparticles supported over nanoporous carbon hollow tubes as an efficient heterogeneous catalyst for the C[sbnd]C bond formation and hydrogenation reactions

Article abstract of DOI:10.1016/j.molcata.2016.09.037

Designing uniformly dispersed Pd nanoparticles over nanoporous carbon supports is very demanding in the context of heterogeneous catalysis. However in most of the cases cluster/agglomerated Pd particles are formed over carbon matrixes, which lack sufficient stability and formation of a sustainable passive layer that can prevent the direct contact between the active metal sites with the reactants. Herein we report the in-situ preparation of surface-exposed Pd nanoparticle over N-doped carbon hollow tubes i.e. Pd@CHT, which showed high catalytic activity compared with agglomerated Pd on carbon. The simplicity in the preparation of Pd@CHT via one step direct carbonization of hypercrosslinked polymer tubes followed by reduction in the presence of NaBH₄ can offer huge potential in liquid phase heterogeneous catalysis. High dispersibility of the catalyst in the reaction medium, good stability and reusability of Pd@CHT is observed for the Sonogashira, cyanation and hydrogenation reactions for the synthesis of a wide range of value added fine chemicals, suggesting its future potential in heterogeneous catalysis.

Full text of DOI:10.1016/j.molcata.2016.09.037

Journal of Molecular Catalysis A: Chemical 425 (2016) 147–156
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Surface-exposed Pd nanoparticles supported over nanoporous carbon
hollow tubes as an efficient heterogeneous catalyst for the C C bond
formation and hydrogenation reactions
∗
Arindam Modak, Asim Bhaumik
Indian Association for the Cultivation of science, Jadavpur, Kolkata, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 August 2016
Received in revised form
Designing uniformly dispersed Pd nanoparticles over nanoporous carbon supports is very demanding in
the context of heterogeneous catalysis. However in most of the cases cluster/agglomerated Pd particles are
formed over carbon matrixes, which lack sufficient stability and formation of a sustainable passive layer
that can prevent the direct contact between the active metal sites with the reactants. Herein we report the
in-situ preparation of surface-exposed Pd nanoparticle over N-doped carbon hollow tubes i.e. Pd@CHT,
which showed high catalytic activity compared with agglomerated Pd on carbon. The simplicity in the
preparation of Pd@CHT via one step direct carbonization of hypercrosslinked polymer tubes followed
by reduction in the presence of NaBH4 can offer huge potential in liquid phase heterogeneous catalysis.
High dispersibility of the catalyst in the reaction medium, good stability and reusability of Pd@CHT is
observed for the Sonogashira, cyanation and hydrogenation reactions for the synthesis of a wide range
of value added fine chemicals, suggesting its future potential in heterogeneous catalysis.
© 2016 Elsevier B.V. All rights reserved.
2
8 September 2016
Accepted 30 September 2016
Available online 1 October 2016
Keywords:
Carbon hollow tubes
N-doped carbon
Pd nanoparticles
C
C coupling
Hydrogenation reaction
1
. Introduction
Over the past decade considerable progress has been made in
catalysis is exciting as far as the efficiency of the process, but its’
preparation, characterization, active sites isolation is very difficult
task. Also, because of high surface mobility of single atoms, they
possess enough surface energy for agglomeration, which actually
precludes its single atom stage and subsequently becomes agglom-
erated nanoparticles [14].
However, unlike oxides as support, nanoparticles or single
atoms supported on hollow carbon spheres/N-doped carbon nano-
tubes (CNT)showed considerableimprovementin stabilityof active
sites, owing to significant interaction between supported carbon
and metal species [15–17]. This means that carbons containing
unique hollow nanostructure are better support for nanoparticles,
because of the advantages of two distinguishable surfaces i.e. hol-
low interior and the exterior, which decreases mobility of the active
sites and hence prevents agglomeration, sintering etc. Therefore
there remains a great significance for the utilization of N-doped
hollow carbon tubes as support for metal particles, in contrast to
solid amorphous carbon.
the synthesis and applications of nanoporous carbon ranging from
nanometer to micrometer level [1–3]. It is to be believed that
nanoporous carbon derived from high surface area porous poly-
mers with/without activating agent is quite simple and elegant than
templated methods involving zeolite, silica etc. [4–6]. However, soft
porous polymers are generally suffered from huge loss in porosity
and morphology during carbonization at high temperature, which
is supposed to be disadvantageous for their application. Therefore,
it would be challenging to retain the shape of porous polymers after
its carbonization at high temperature in order to generate porous
carbon. Carbon hollow tubes (CHT) are among such nanoporous
carbon, which are demanding for several applications, but is devoid
of any suitable and facile preparation methods [7].
On the other hand, metal nanoparticles and/or cluster of few
atoms that are stabilized on various solid supports like silica, zeo-
lites, porous polymers, metal oxides (TiO , Alumina etc.), graphene,
possesses significant applications in catalysis, hydrogenation reac-
tions, energy conversion, fuel cell etc. [8–14]. Although single atom
Although activated carbon is well known for metal supported
catalysis, but it suffers from diffusion limitation due to its inher-
ent microporosity, as well as less tensile strength and presence
of impurity substances further direct its disadvantageous perfor-
mances for catalytic applications [11]. Compared to commercially
available CNT and activated carbon, carbon hollow tubes may
provide good dispersibility of Pd NPs at the catalyst surface
2
∗ Corresponding author.
E-mail addresses: msab@iacs.res.in, abhaumik68@yahoo.co.in (A. Bhaumik).
http://dx.doi.org/10.1016/j.molcata.2016.09.037
1
381-1169/© 2016 Elsevier B.V. All rights reserved.

1
48
A. Modak, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 425 (2016) 147–156
together with its mesoporosity, which are very demanding for
achieving high turnover numbers in heterogeneous catalysis [18].
Molecular-defined homogeneous Pd complexes are less accept-
able in industrial process due to almost no chance for repetitive
recycling, which is obviously disadvantageous. In this regard, Pd
immobilized heterogeneous catalysts comprising porous carbon,
silica, porous organic polymers (POPs) etc. shows huge prospect
and potential [19–22]. But the catalyst preparation methods are
quite lengthy and complex, which is disadvantageous. Of course
those catalysts are devoid of distinct shape and morphology.
Very recently we have reported the facile synthesis of
micrometer long CHT from amine functionalized porous polymer
tubes, which are stable for carbonization and retains interesting
tube-shaped morphology with N-functionality [23]. Due to the
uct of hydrogenation reactions were analyzed from Agilent 7890 B
GC having HP-5 column (30 m × 0.32 mm × 0.25 ␮m) and a FID. Par-
ticle size has been calculated from powder X-ray diffraction using
Debye-Scherer equation, as given below in Eq. (1) [24].
D = K␭/(␤ cos ␪)
(1)
where D is the mean size of crystallites (nm), K is crystallite shape
factor, having good approximation is 0.9, ␭ is the X-ray wavelength,
B is the full width at half the maximum (FWHM) in radians of the
X-ray diffraction peak and ␪ is the Braggs’ angle (deg.) Angle degree
to radian transformation: (degree*22)/(7*180).
2.1. Synthesis of Pd@CHT-H2/Pd@CHT-N2
−1
availability of 2 atomic% nitrogen (1.42 mmol g ) in our CHT, it can
be a very useful support for grafting of reactive metal nanoparticles’
at its surface. In order to accomplish this, we synthesized Pd@CHT
after immobilization of palladium nanoparticles of different sizes
employing both its inner and outer hollow space, which are stabi-
lized by N-functional sites like pyridine, pyrrole or imine types, as
present in CHT [23]. Nevertheless, the utility of surface oxidation of
chemically inert carbon nanotubes with oxidizing agents for better
interaction with doped transition metal nanoparticles is harmful
and such requirement is not necessary in making Pd@CHT because
of the presence of surface active nitrogen containing species in
CHT. Furthermore, nano-dimensional carbon tubes with smaller
diameter prevents the dispersion of active metal centers at both
surfaces. This motivates us to explore the opportunity in making
macro size carbon tubes with diameter ranging from ∼50–100 nm.
In consequence, we foresee this research possibly has fundamental
importance not only in nanoparticle-based solid catalysis, but also
could provide an ample scope for developing nanoreactors, exploit-
ing the huge space of hollow interiors, which is so far has not been
explored. High activity and well durability of Pd@CHT has been
explored here in the Sonogashira and cyanation reactions as well
as in hydrogenation of aromatic olefins, alkynes and nitroaromatics.
0.005 g Pd(OAc)2 was dispersed in 20 mL glass vial containing
5 mL distilled water and stirred until the solution becomes yellow.
Next, 0.1 g PP-3 was mixed with the solution and stirred for 5 h. The
mixture was centrifuged and washed with water, ethanol followed
◦
by drying at 60 C. The dried sample was directly carbonized in a
◦
◦
−1
Quartz tube at 600 C (2 C min ) under continuous flow of N2 or
H2 for 2 h. Weight loss of the black carbonized product was found
to be ∼30%. It produced Pd@CHT-H2 (2.1 wt% Pd) and Pd@CHT-N2
(0.8 wt% Pd).
2
.2. Synthesis of Pd@CHT-NaBH /Pd@CHT-MeOH
4
0
.005 g Pd(OAc)2 was dispersed in 20 mL glass vial contain-
ing 5 mL distilled water and stirred until it becomes yellow. Next,
.05 g as produced PP-3-600 was mixed with yellow Pd(OAc) solu-
0
2
tion and stirred for 5 h. After that the mixture was cooled in ice
water and aqueous solution of NaBH4 was added slowly. Finally,
after 30 min stirring, the mixture was centrifuged and thoroughly
◦
washed with water, ethanol, acetone and dried at 60 C. It produced
Pd@CHT-NaBH (5 wt% Pd). The preparation procedure of Pd@CHT-
4
MeOH is exactly similar with that of Pd@CHT-NaBH , except that
4
the reduction of Pd(OAc)2 has been done in 5 mL MeOH, instead of
aqueous NaBH . It produced Pd@CHT-MeOH (3 wt% Pd).
4
2
. Experimental
2.3. Procedure for the Sonogashira coupling
PP-3 and PP-3-600 has been synthesized following our recently
reported strategy [23]. Palladium acetate, NaBH4 and all other
reagents and solvents required for catalysis were purchased from
Sigma Aldrich, USA and Loba Chemie India. N2 sorption analy-
sis has been carried out at 77 K using Quantachrome Instrument,
A 50 mL round bottom flask was charged with aryl iodide
(
1 mmol), phenyl acetylene (1.5 equivalent), Cs CO3 (1.5 equiva-
2
lent) and mixed with 5 mL DMF and 0.005 g Pd@CHT. The resulting
◦
◦
mixture was subjected to reflux in N2 atmosphere at 80 C under
Autosorb-1, where all samples was degassed at 120 C for 4 h before
continuous stirring. The progress of the reaction was monitored
by TLC. After the reaction was over, the catalyst was separated by
simple filtration and washed extensively with ethyl acetate. Later,
the filtrate was extracted with ethyl acetate and washed with brine
the measurement. Pore size distribution (PSD) was determined
using N2 adsorption at 77 K on carbon as reference using non
local density functional theory (NLDFT). Brunauer–Emmett–Teller
(
BET) surface area was calculated over the entire pressure region
solution. The organic part thus obtained was dried by Na SO4 and
2
from ∼0.05 to ∼0.18 P/P . Thermogravimetric analyses (TGA) were
0
◦
−1
the solvent was then removed by rotary evaporation under reduced
pressure. The final product was purified by either recrystallization
or column chromatography methods, before analyzing results.
measured by thermal analyzer (TA-SDT Q-600) at 10 C min heat-
ing rate under flowing N2 of 0.5 mL/min. Transmission electron
microscopy (TEM) was obtained from Hitachi HT-7700 with accel-
eration voltage at 100 kV. The samples were dispersed in ethanol by
sonication and placed onto an ultrathin carbon film supported on a
copper grid. Powder X-ray diffraction (PXRD) was done on a Rigaku
D/Max2500 PC diffractometer with Cu K␣ radiation (␭ = 1.5418 Å)
2.4. Procedure for the cyanation reactions
In this case, aryl iodide (1 mmol), K [Fe(CN) ] (0.8 mmol),
4
6
◦
◦
over the 2ꢀ range of 5–70 at a scan speed of 5 per min at room
0.005 g Pd@CHT and triethylamine (1.5 mmol) were mixed in a
quartz tube having dimensions 18 cm × 4 cm. The mixture was
homogenized on addition of DMF/DMF-water mixture. Next, the
tube was flash frozen with liquid nitrogen and evacuated to create a
vacuum inside and immediately sealed with a Bunsen flame burner.
temperature. A Bruker DPX-300 NMR spectrometer was used to
1
13
measure H, C NMR of catalytic products in liquid state. The metal
content was determined by PLASAM-SPEC-II inductively coupled
plasma atomic emission spectrometry (ICP). X-ray photoelectron
spectroscopy (XPS) was recorded on a VG ESCALAB MK2 apparatus
by using AlK␣ (hꢁ = 1486.6 eV) as the excitation light source. Particle
sizes were determined from Nano Measurer 1.2 software. The prod-
◦
The sealed tube was immersed in an oil bath at 110 C and heated
under stirring for 12–24 h. At the end, the tube was cooled at room
temperature and the reaction was monitored by TLC. The organic

A. Modak, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 425 (2016) 147–156
149
to investigate the effect of activation agent in generation of Pd
nanoparticles and these materials are designated as Pd@CHT-
MeOH and Pd@CHT-NaBH . As observed from the TEM images,
4
Fig. 1(c and d) and size distribution patterns (Fig. 2c and d), MeOH
mediated formation of Pd@CHT shows much larger particles ∼7 nm
(
30%) with almost 10% distribution of ∼30–50 nm large particles,
which could be inferior to catalysis. Nevertheless, the possibility
of having atomic Pd deposition on CHT can be ruled out in either
synthesis condition, because it requires promoter for stabilization
and moreover they are too small for detection in conventional TEM
measurements [14].
Powder X-ray diffraction pattern of these materials as shown
in Fig. 3 clearly distinguishes the diffraction pattern of Pd@CHT
from that of CHT. In addition to the presence of amorphous car-
◦
◦
◦
◦
bon peak at 2ꢀ = 20 , several other peaks at 40.2 , 43.9 ,47.3 etc
are seen, which corresponds to different facets of Pd crystal par-
ticle, formed on carbon tubes [25]. Again, the particle size of each
Pd@CHT catalyst that is measured through the TEM images, showed
good correlation with the XRD results, where average particle size
of Pd@CHT-MeOH, Pd@CHT-N , Pd@CHT-H , Pd@CHT-NaBH are
2
2
4
1
0.5 nm, 4 nm, 3.1 nm and 2.9 nm respectively (as calculated by
using Debye-Scherer equation, given in Eq. (1)).
Scheme 1. Synthesis of palladium nanoparticles immobilized carbon hollow tubes
by Pd(OAc)2, through activation with H2, N2, Methanol and water.
N2 adsorpion-desorption isotherm of Pd@CHTs has been given
in Fig. 4, which depicts the presence of microporous characteris-
tics at low P/P0 region as observed from the Type I characteristics
product was collected by extraction with diethyl ether, purified by
column chromatography for characterization purposes.
of the isotherm. However at higher P/P , the isotherm increases
0
because of the mesoporosity arises due to inter-particle pores.
Brunauer–Emmett–Teller surface area of Pd@CHTs falls within
2
−1 −1
2
2.5. Hydrogenation reaction procedure
(380–400 m g ), lower than 450 m g for CHT, which is basi-
3
−1
cally due to decrease of pore volume to 0.14 cm g
from
−1
for CHT, suggesting that palladium loading partially
3
In a typical hydrogenation reaction, 0.005 g Pd@CHT was added
0.2 cm g
to 0.1 g substrate in a reaction glass vial fitted with a magnetic
stirring bar and a septum cap. The reaction vial was then placed
into a 300 mL steel Parr autoclave. The autoclave was flushed with
hydrogen three times to remove air inside the autoclave and finally
blocks the microporous carbon tubes, which agrees well with the
PSD, as shown in the inset of Fig. 4. PSD after Pd loading shows the
complete blocking of pores <2 nm from the deposition of 3–7 nm
Pd particles and concomitant increase of interparticle mesoporosity
at ∼4 nm, which is commonly observed with other porous materi-
als [26]. X-ray photoelectron spectroscopy (XPS) indicates reduced
Pd(0) state was formed on CHT owing to the presence of charac-
teristics peaks at (334.4–334.8) eV and (339.8–340.2) eV, which are
assigned to Pd 3d5/2 and Pd 3d3/2 electrons respectively (Fig. 5) [27].
It is noteworthy that very small and broad peak was seen at 337.3 eV
and 342.7 eV due to Pd(II) 3d5/2 and Pd(II) 3d3/2 respectively, which
suggests that almost complete reduction of Pd(II) takes place at the
surface of CHT. Surprisingly, a characteristics small shift (+0.5 eV) in
the peak position was seen in Fig. 5, which could be due to the differ-
ence in Pd nanoparticles size, a phenomenon commonly observed
with metal nanoparticles on various support [28]. Further charac-
terizations and catalysis in a diverse range of organic reactions are
carried out over Pd@CHT-NaBH4 owing to its high yield, compar-
atively easier preparation and better dispersion of smaller Pd NPs
over the support. On the other hand, in-situ reduction of Pd salt
through H2 activation is dangerous and it creates huge loss of the
catalyst when separated from the experimental set up. Also, the
◦
pressurized to 10 atm, which was kept at 40 C oil bath. After the
reaction, the autoclave was placed into a water bath and cooled to
room temperature. Finally, the remaining hydrogen was carefully
discharged and the products were analyzed by a GC instrument.
3
. Results and discussions
3.1. Synthesis and characterization
N-doped nanoporous carbon (CHT) support has been prepared
from the corresponding PP-3 polymer [23]. High BET surface area
2
−1
(
530 m g ), good stability and very large diameter (∼50–100 nm)
of the carbon tube could be attractive for immobilization of Pd
nanoparticles by exploiting both the exterior and hollow interior
of the tubes. Thus, Pd@CHT has been prepared through the immo-
bilization of Pd(OAc)2 in four different ways (Scheme 1). Pd(OAc)2
has been initially impregnated into PP-3 polymer and centrifuged,
◦
which was later carbonized under H or N at 600 C for 2 h, result-
2
2
◦
ing in Pd@CHT-H2 and Pd@CHT-N . It is to be believed that during
formation of Pd nanoparticles at high temperature viz. 600 C with
2
impregnation step, Pd(OAc)2 may be hydrolyzed to Pd(II) oxide
species (Pd-O) at the surface of the CHT. This could be further
reduced to Pd nanoparticles during reduction in either H2 or N2
atmosphere.
TEM images suggest the formation of palladium nanoparti-
cles with average sizes of about ∼3–5 nm, distributed uniformly
throughout the wall and also inside the hollow cavity (Figs. 1 and 2a
and b). This result suggests that in-situ carbonization together with
reduction could be effective in developing Pd nanoparticles deco-
rated CHT.
PP-3 as porous organic solid is found to be quite demotivating in
catalysis. Therefore, we choose Pd@CHT-NaBH as solid support for
4
liquid phase organic transformations.
It is worth mentioning that unlike commercially available car-
bon nanotubes, our Pd@CHT-NaBH4 showed high dispersibility in
neat water (Fig. S1), which is meaningful for its catalytic appli-
cations. In addition to this, high thermal stability was observed
◦
at 480 C (weight loss ca. ∼5%) (Fig. S2), signifying the merit of
Pd@CHT-NaBH . Based on such perspective, our Pd@CHTs has been
4
tested for several C C cross coupling reactions like Sonogashira and
cyanation reaction (Table 1). In this regard we observed almost
comparable activity with Pd@CHT-H2 and Pd@CHT-NaBH4 for
Furthermore, we have changed the synthesis method of Pd@CHT
while replacing H /N by MeOH and aqueous NaBH , in order
2
2
4

1
50
A. Modak, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 425 (2016) 147–156
Fig. 1. Transmission electron microscopic images of carbon hollow tubes (CHT) after Pd immobilization through (a) H2 activation, (b) N2 treatment, (c) with Methanol, (d)
with aqueous NaBH4.
Fig. 2. Size distribution of Pd particles on CHT through (a) H2 activation, (b) N2 treatment, (c) with Methanol, (d) with aqueous NaBH4.
Sonogashira coupling products (80–85% conversion), but Pd@CHT-
MeOH showed comparatively much lower conversion i.e. ∼65%.
Similar results are also observed for the cyanation reaction also.
This anomaly could be due to the formation of large agglomerated
Pd cluster, stick with the surface of the tube, which possibly lacks
direct contact with substrates, as have been evidenced from TEM
images, given in Fig. 1. Again, when comparing our results with
other Pd-based solid support viz. tubes/fiber shape Pd/CNT [29],
Pd/PANI [30] (PANI: polyaniline), we observe almost comparable
activity for the cross-coupling products. It is pertinent to mention
that Pd/carbon film SBA-15 [31], Pd/MCM-41 [32] showed much
lower activity to cross coupling products; possibly they do not have
surface active sites like N, which could impart better interaction
with deposited nanoparticles.
3.2. Heterogeneous catalysis of Sonogashira reaction
Pd catalyzed Sonogashira cross coupling reaction is promising
2
for C(sp) C(sp ) bond formation between aryl halide and termi-
nal alkyne, which shows profound application in pharmaceuticals,

A. Modak, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 425 (2016) 147–156
151
Table 1
a
Screening of several Pd catalysts for cross coupling reaction.
Entry
Catalyst
Reaction (% conv.)
Reaction (% Conv.)
1
2
3
4
5
6
7
8
Pd@CHT-H2
Pd@CHT-N2
Pd@CHT-MeOH
Pd@CHT-NaBH4
Pd/CNT
Pd/PANI
Pd/carbon film SBA-15
Pd/MCM-41
80 (Scheme A)
78 (Scheme A)
65 (Scheme A)
>85 (Scheme A)
>90 (Suzuki) [24]
>90 (Suzuki) [25]
63 (Suzuki) [26]
>30 Sonogashira [27]
64 (Scheme B)
65 (Scheme B)
60 (Scheme B)
>76 (Scheme B)
–
–
–
–
Fig. 3. Powder X-ray diffraction pattern of CHT (Pink), Pd@CHT-N2 (blue), Pd@CHT-
NaBH4 (black), Pd@CHT-H2 (orange), Pd@CHT-MeOH (red). (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version
of this article.)
For Scheme A, 1 mmol aryl halides, 1.5 mmol terminal alkynes and for Scheme B,
1
mmol aryl halides, 0.8 mmol K4[Fe(CN)6], 1.5 mmol Et3N has been considered.
a
Pd@CHT having 2.3 × 10−6 mol Pd has been used.
could be demotivating in catalysis [20,21,27,36]. On the other hand,
nanotubes, nanofibers shows interesting phenomenon for encap-
sulation of metal NPs through utilization of their cylindrical space
by means of support-active site interaction [37,38], which could be
promising in solid supported sonogashira reaction.
Thus, Pd@CHT-NaBH4 catalyzed Sonogashira reaction has been
carried out with aryl halides including aryl chlorides as substrates in
presence of Cs CO3 as base and tertiary butyl ammonium bromide
2
(
TBAB) as phase transfer catalyst in either DMF or H O/DMF mixed
2
solvent system. Generally, activation of aryl chloride requires high
catalyst loading, elevated reaction temperature, prolonged reaction
time with CuI as cocatalyst and phosphine-based ligand as catalysts
[39]. However, Pd@CHT-NaBH4 with ∼5 wt% Pd showed high effi-
ciency in sonogashia reaction which is devoid of any Cu-based salt.
High activity of cross coupling products were achieved between
aryl iodides/bromides/chlorides with phenyl acetylene, produced
Fig. 4. N2 adsorption (filled triangles) desorption (open circles) isotherm at 77 K
for CHT (blue), Pd@CHT-H2 (black), Pd@CHT-N2 (gray, +30), Pd@CHT-MeOH (red,
+
50), Pd@CHT-NaBH4 (pink, +80); (pore size distribution was shown in inset). (For
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
8
5%–65% yield of final product (diphenyl acetylene derivatives),
as mentioned in Table 2, where we observed high activity of
both activated (Table 2, Entry 2, 3) and deactivated aryl halides
(Table 2, Entry 4, 6). Furthermore, sterically hindered 2-iodo ben-
zoic acid shows ∼70% conversion of the desired cross coupling
product (Table 2, Entry 7). All these eventually demonstrate the
efficacy of Pd@CHT-NaBH4 regarding formation of di-substituted
alkynes through the sonogashira coupling route, which is higher
than Pd/MCM-41 (40% yield), Pd/Al O3 (60% yield), but compara-
2
ble with Pd/activated carbon (>75%), Pd/CNT (>80%) as support. For
control experiments without any Pd-salt, obviously CHT shows no
conversion, suggesting the merit of Pd dispersed CHT.
3
.3. Cyanation reaction
Fig. 5. XPS spectra of Pd@CHT-H2 (red), Pd@CHT-MeOH (black), Pd@CHT-N2 (grey),
Pd@CHT-NaBH4 (blue). (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
Inspired by the notable performance of Pd@CHT in the Sono-
gashira cross-coupling reactions, we later applied it for cyanation
reaction in order to make aromatic nitriles, which have significance
in valuable medicinal products, dyes, herbicides, agrochemicals,
natural products etc. [40,41]. Because of the easy conversion of
nitrile to other functional groups like carboxylic acids, aldehydes,
amines, ketones etc., one step direct conversion of halides to nitriles
demands merit for the perspective of the synthesis of diverse chem-
icals. However, the traditional method uses extremely toxic KCN as
cyanation agent together with phosphine-based ligands and multi-
step synthesis procedure, which is really disadvantageous [42,43].
In this context, only a few catalytic systems could meet the require-
dyes, natural products etc. [33,34]. However, the original sono-
gashira reaction requires degassed organic solvent, inert condition,
strong base, prolonged reaction time, phosphine containing cat-
alysts etc., which is complicated and hazardous since there is a
possibility of having contaminated products. Therefore, substantial
improvement based on the development of mild protocol consider-
ing heterogeneous catalysts has been carried out [35]. But, most of
them is devoid of any distinct morphology and has been associated
with agglomerated surfaces or contains some solid spheres, which

1
52
A. Modak, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 425 (2016) 147–156
Table 2
Sonogashira cross-coupling of aryl halides with terminal alkynes.
a
−1
)
Entry
1
R
X
Product
Yieldb (%)
85,65c
TOF (h
c
c
44, 18c
53, 24c
4-H
−I, −Br, −Cl
2
4-NO2
−I, −Br, −Cl
82, 76c
3
4
5
6
4-CO2H
4-OCH3
4-COCH3
4-CH3
−I, −Br
−I, −Br
−I, −Br
−Br
88
80
82
85
36, 27
31
30.3
26.5
7
2-CO2H
−I
70
25
a
−6
1
mmol aryl halides, 1.5 mmol terminal alkynes, 5 mL DMF, 5 mg Pd@CHT-NaBH4 ((2.3 × 10 mol Pd), 10–18 h.
b
c
Isolated yields on the basis of NMR (Supporting information).
Chloro halides, 0.5 mmol tertiary butyl amonium bromide (TBAB), 24 h.
Table 3
Cyanation of aryl halides with Pd@CHT.a
Yieldb (%)
85
TOF (h
−1
)
Entry
1
R
Product
−
4-H
37
2
3
4
4-CH3
4-NO2
4-OCH3
4-NH2
82
88
80
80
31
20
24
20
5
a
−6
◦
1
mmol aryl halides, 0.8 mmol K4[Fe(CN)6], 1.5 mmol Et3N, (1:1), 5 mg Pd@CHT (2.3 × 10 mol Pd), DMF/Water, 100 C for 12–24 h.
b
Yields obtained on the basis of NMR results (Supporting information).
ment of environmentally friendly condition for making aromatic
nitrile, which used either K [Fe(CN) ] or the combination of ammo-
[K4 Fe(CN) ] in aqueous DMF medium under sealed tube condi-
tion, which shows good conversion of different substrates varying
from electron rich to electron poor systems (Table 3).
In Fig. 6a and b, we have given the kinetic aspect of C C bond
formation reaction considering the entire region between 0% and
100% conversion, which shows the decrease in iodobenzene (as
open circles) and subsequent increase of the product (by closed cir-
6
4
6
nium bicarbonate, DMF system, instead of KCN [44,45]. However
such catalysts work under homogeneous condition and thereby
prevention of any contaminated product is not completely ruled
out under such circumstances. Heterogeneous catalysts can con-
tribute significantly in this context [46–48]. We used Pd@CHT for
direct conversion of aryl iodides to aryl cyanides in presence of
cles) through Pd@CHT-NaBH . Furthermore, no induction period is
4

A. Modak, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 425 (2016) 147–156
153
Fig. 6. Reaction kinetics of (a) sonogashira reaction between iodo benzene, phenyl acetylene and (b) cyanation reaction of iodo benzene, open circles denotes remaining
concentration of iodobenzene; closed circle suggests the product concentration; Plot of natural logarithm of remained iodobenzene concentration vs. time for (c) sonogashira
(
d) cyanation reaction.
Table 4
observed for the activation of Pd nanoparticles exposed on carbon
surface.
Solvent free hydrogenation of aromatic alkenes, alkynes with various Pd-based
support.
a
Under this condition, our model reaction exhibits apparently
first order behavior (Fig. 6c and d) with respect to iodobenzene,
consistent with the zero order dependence of phenyl acetylene for
sonogashira and [K Fe(CN) ] for cyanation purposes. The rate con-
4
6
stant (kobs) and life time (t1/2) of the model sonogashira reaction,
−
1
were found to be kobs = 0.105 ± 0.005 h , t1/2 = 6.6 h. Accordingly,
−1
kobs = 0.101 ± 0.005 h , t = 6.93 h were calculated for cyanation
1/2
reaction. All these result suggests that Pd@CHT catalyzed C C bond
formation is slightly faster for sonogashira reaction than cyanation.
In this regard, the first order reaction could be attributed to the ini-
tial oxidative addition of iodo benzene with surface exposed Pd
nanoparticles as rate determining step, followed by the addition of
Entry
Catalyst
Conversion (%)
Selectivity (%)
1
2
3
4
5
Pd@CHT-H2
Pd@CHT-N2
Pd@CHT-MeOH
Pd@CHT-NaBH4
Pd/CNT (5 wt% Pd)
>90
>90
>65
>99
>86
>95
>95
>95
>99
>98
either phenyl acetylene/[K4 Fe(CN) ] with the Pd-intermediates in
a rapid step to yield the corresponding cross coupling product.
6
a
Pd@CHT refers to 2.3 × 10−6 mol Pd.
3
.4. Hydrogenation reaction
Nevertheless, hollow carbon tube morphology has special effect
for the confinement of small Pd NPs by utilization of both inner
and outer diameter of hollow tubes, therefore we hope Pd@CHT
could be suitable for hydrogenation of olefins compared with other
traditional heterogeneous support (polymers/clay/minerals etc.).
It is worthy to mention Pd@CHT-NaBH4 shows good conversion
(>99%) and high selectivity (>99%) for hydrogenation of unsatu-
After successful accomplishment of C C bond formation reac-
tions, we later investigated the scope of Pd@CHT-NaBH4 for liquid
phase hydrogenation reaction because catalytic hydrogenation
represents the fundamental methodology in catalysis that spans
from academics to industrial process [49]. We initially investi-
gated hydrogenation of aromatic alkenes, n-alkene, alkynes to
saturated aromatics, which is meaningful in petroleum refining
purposes [50,51]. Previously, Pd(II)-based heterogeneous support
rated aromaticolefins, alkynes under solvent free conditionat 1 atm
◦
H , 40 C (Table 4), which expands the scope to investigate with
2
various substrates (Table 5). Cycloalkenes, trans-stilbene, styrene,
n-alkenes were hydrogenated to its corresponding alkanes with
>99% conversion and >99% selectivity at both higher (10 atm) and
ꢀ
are reported as hydrogenation catalysts, e.g. Pd(II) supported 2,2 -
dihydroxybenzophenone-SBA-15 [52], Ta -alkyl-based catechol
V
functionalized porous organic polymer [53]. However, the syn-
thesis of the ligand matrix is tedious and expensive. Apart from
molecular heterogeneous system, nanoparticles (Pd/Rh) supported
on sepiolite type clay mineral Si12Mg O (OH) (OH ) ·8H O [54],
lower H pressure (5 atm), as given in Table 5. Furthermore, at 1 atm
2
H2 pressure, styrene shows >63% conversion/2 h along with >99%
selectivity in ethylbenzene (Table 5, entry 8), which signifies that
Pd@CHT-NaBH4 could also work at atmospheric H2 pressure.
However, Pd@CHT-NaBH4 is not active for ring hydrogenation
reaction, as has been evidenced from the poor yield of 9,10 dihy-
droanthracene (Table 5, entry 6). Regarding alkyne hydrogenation
8
30
6
2
4
2
Attapulgite type natural mineral i.e. (H O) (Mg,Al,Fe)5 (OH)
2
2
4
Si O ·4H O [55], metal hydrides [56], porous glass beads [57],
8
20
2
showed good selectivity in cylohexene hydrogenation reaction.

1
54
A. Modak, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 425 (2016) 147–156
Table 5
Solvent free hydrogenation of various alkenes, alkynes, by Pd@CHT-NaBH4.
a
◦
Conv./Sel.b (%)
>99, >99
−1
)
Entry
Substrate
Product
T/ C
PH2/MPa
1
t/h
0.5
TOF (h
1
40
40
1033
2
3
0.5
1
1
1
>99, >98
>99, >99
510
236
40
40
4
1
1
>99, >99
386
5
6
40
40
1
1
1
>99, >99
<25, 98
530
–
1.5
7
40
40
1
1.5
2
>99, >99
>63, >99
272
204
8
9
0.1
1-Octene
1-Octane
40
40
0.1
1
1
>95, >85
>99, >99
378
451
1
0
1
1.5
1
40
40
1
1
2
2
>99, >94
>95, >99
210
183
1
1
2
3
40
1
2
>98, >99
160
a
0
.1 g alkyne/alkene, 0.005 g Pd@CHT-NaBH4 (5 wt% Pd).
b
Conversion/Selectivity was determined on the basis of GC analysis of the products.
(
Table 5, entry 11–13), high selectivity (>94%) was observed for
the formation of substituted ethyl benzene compared with styrene
<4% conversion), regardless the presence of OMe, Me sub-
a solvent free condition. Owing to the importance of aniline in
fine chemical industry and synthesis of valuable dugs, pharma-
ceutical compounds, several heterogeneous catalysts have been
developed, which uses sodium borohydride, silanes, hydrazine
as reducing agent in THF/DMF/isopropanol/ethanol etc as solvent
[60–64]. Among several catalysts tested, Pd@CHT-NaBH4 showed
high conversion of nitrobenzene together with >99% selectivity
in solvent free condition, as given in Table 6. Also, the catalytic
activity of Pd@CHT-NaBH4 was preserved in ethanol, water/THF
(1:1), but slightly decreases when the temperature was raised
(
stituent. Eventually, all these results demonstrate high catalytic
efficiency of Pd@CHT, which might be because of the confinement
effect of the active sites, and also could be due to the adequate
space being provided from the hollow architecture of our support
to facilitate smooth diffusion of substrates and products [58,59].
Finally, Pd@CHT has been employed for the selective hydro-
◦
genation of nitrobenzene to anilines under H2 atm. at 60 C in

A. Modak, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 425 (2016) 147–156
155
Table 6
cycle is not observed. All these results indicated strong binding of
Pd NPs at the surface of N-functionalized CHT in Pd@CHT.
a
Solvent free hydrogenation of nitrobenzene with various Pd-based support.
4. Conclusion
Herein, we present a facile methodology for designing a
nanocatalyst based on supported Pd nanoparticles over N-doped
nanoporous carbon tubes. In viewing the potential of nanocarbon-
based solid support for liquid phase catalysis, we have developed
Pd@CHT, which showed excellent catalytic activity in C C bond
formation via Sonogashira and cyanation reactions, and also hydro-
genation of various aromatic alkenes, alkynes and nitro compounds
with molecular H2 as cheap and abundant source. High catalytic
efficiency of Pd@CHT has been well documented in this study,
which is much more efficient than Pd/CNT, Pd/activated carbon etc.
Robust nature and high catalytic efficiency of Pd@CHT can be very
beneficial for its future scope for a wide range of catalytic reaction
under heterogeneous conditions.
Entry
Catalyst
Conversion (%)
Selectivity (%)
1
2
3
4
Pd@CHT-H2 (R H)
Pd@CHT-N2 (R H)
Pd@CHT-MeOH (R H)
>85
>88
>60
>95
>95
>98
b
c
b
c
Pd@CHT-NaBH4
>99, >99 , >99
100, 100 , 100
b,c
(
(
R
R
H)
CH3, OMe, Cl)
(>99, >95, >89)
>75–78
(>95, >94, >90)
>95
5
Pd/CNT (5 wt% Pd)
a
1
mmol nitrobenzene, 5 mg Pd-catalyst (5 wt%) stirred under H2 balloon for 10 h
without any solvent.
b
Water:THF (1:1) by volume.
Reaction at 1 MPa H2 for 3.5 h.
c
Acknowledgements
Authors thank Prof. Qihua Yang, Dalian institute of chemical
physics, China for providing support in measuring TEM, XRD. We
also thank reviewers’ for their helpful comments. AB wishes to
thank DST, New Delhi, India for funding through DST-SERB project
grant.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2016.09.
0
37.
Fig. 7. Recycling efficiency of Pd@CHT for four cycles; hydrogenation refers to
styrene as substrate.
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