Selective hydrogenation of nitroarenes to amines by ligand-assisted Pd nanoparticles: influence of donor ligands on catalytic activity

Article abstract of DOI:10.1039/C8NJ05932B

Ligand-assisted synthesis of metal nanoparticles has advanced significantly; however, the influence of donor groups on the catalytic performances of such heterogeneous systems has not been systematically explored. Here, we have synthesized and characterized three ligand-based silica-supported palladium nanocatalysts via an impregnation-reduction method through anchorage of palladium onto silica gel functionalized with amine, phosphine and thiol. TEM images of the amine- and phosphine-based materials showed formation of uniformly distributed palladium nanoparticles (Pd NPs) with fine particle sizes, whereas the thiol-based material showed formation of palladium nanowires (Pd NWs) of irregular sizes. To investigate the influence of the donor ligands, selective hydrogenation of 4-chloronitrobenzene (4-CNB) to 4-chloroaniline (4-CAN) was carried out. Under similar experimental conditions, the catalytic activity decreased in the order of phosphine > amine > thiol. A maximum yield of 98% and selectivity of 100% was achieved with the phosphine-based catalyst using molecular hydrogen as a reducing agent. A diverse range of nitroarenes were efficiently converted to their corresponding amines.

Full text of DOI:10.1039/C8NJ05932B

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Selective hydrogenation of nitroarenes to amines
by ligand-assisted Pd nanoparticles: influence of
donor ligands on catalytic activity†
Cite this:DOI: 10.1039/c8nj05932b
a
a
b
Gauravjyoti D. Kalita,
Podma P. Sarmah,
Pallab Kr. Saikia,b Lakshi Saikia
a
and Pankaj Das
*
Ligand-assisted synthesis of metal nanoparticles has advanced significantly; however, the influence of
donor groups on the catalytic performances of such heterogeneous systems has not been systematically
explored. Here, we have synthesized and characterized three ligand-based silica-supported palladium
nanocatalysts via an impregnation-reduction method through anchorage of palladium onto silica gel
functionalized with amine, phosphine and thiol. TEM images of the amine- and phosphine-based
materials showed formation of uniformly distributed palladium nanoparticles (Pd NPs) with fine particle
sizes, whereas the thiol-based material showed formation of palladium nanowires (Pd NWs) of irregular
sizes. To investigate the influence of the donor ligands, selective hydrogenation of 4-chloronitrobenzene
(4-CNB) to 4-chloroaniline (4-CAN) was carried out. Under similar experimental conditions, the catalytic
activity decreased in the order of phosphine 4 amine 4 thiol. A maximum yield of 98% and selectivity
of 100% was achieved with the phosphine-based catalyst using molecular hydrogen as a reducing agent.
A diverse range of nitroarenes were efficiently converted to their corresponding amines.
Received 22nd November 2018,
Accepted 3rd February 2019
DOI: 10.1039/c8nj05932b
rsc.li/njc
as the major side reaction resulting in a decrease in selectivity of
4-CAN.11,12 To improve the product selectivity, many attempts
Introduction
Over the past few decades, the selective hydrogenation of nitro- have been made which includes tuning the steric and electronic
arenes to amines has become a subject of apex priority both properties of the ancillary ligands in homogeneous catalysts,13,14
in industrial and academic domains. Among various amines, controlling the sizes and shapes in the case of metal nanocata-
chloroanilines (CANs) have received particular attention because lysts15–19 or engineering surface properties in the case of sup-
of their ardent necessity as intermediates in pharmaceuticals, ported heterogeneous catalysts;20–27 however, the side reaction of
dyes, pigments and fine chemical industries.1–5 Until now, the hydrodechlorination could not be fully circumvented. Moreover,
most widely acceptable protocol for large-scale production of the majority of the above systems use reducing agents, like
CANs is the Bechamp’s process which involves Fe-promoted hydrazine derivatives,15 sodium borohydrides,28–30 hydrosilanes31
´
stoichiometric reduction of nitro-compounds in acidic media.6–8 etc., that have serious environmental concerns.32 Thus, recently
However, from economic and environmental perspectives, this much attention has been given towards designing more promising
protocol is no longer viable as it produces a significant amount recyclable hydrogenation catalysts that ensure selective reduction of
of toxic waste apart from low selectivity and poor yield. In 4-CNB keeping intact the C–Cl bond using molecular hydrogen as a
recent years, liquid-phase catalytic hydrogenation employing reducing agent. Nevertheless, there exist a few successful examples
noble metals like Pt, Pd, Rh, Ru, etc. has emerged as a suitable of such catalysts, like Ru/Fe3O4;33 Ru/CNT,16 Au/ZrO2,19 Pd/N-
9,10
Although significant CMK-332 etc. Although, a few of the aforementioned systems
´
alternative to the Bechamp’s process.
advancements have been made in terms of waste minimization seem promising, tedious synthetic protocols,34 high metal loading,35
and yield increment, the hydrogenation of 4-CNB to the corres- drastic reaction conditions,36 etc. are some of the limiting factors for
ponding 4-CAN is always accompanied by hydrodechlorination their industrial scale applications. Hence, development of new
catalytic systems that induce efficient reduction of nitroarenes
using molecular H2 as reducing agent under mild condition is
still more appealing.
a Department of Chemistry, Dibrugarh University, Dibrugarh, Assam, 786004, India.
E-mail: pankajdas@dibru.ac.in
b Materials Sciences and Technology Division, CSIR-North East Institute of Science
It is noteworthy to mention that ligand-assisted synthesis of
metal nanoparticles (MNPs) has received immense importance
and Technology, Jorhat, Assam, 785006, India
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj05932b because of their potential applications in catalysis.34–39 Compared to
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conventional solid supported methods, the ligand-assisted
method has the distinct advantages of controlled growth of
well-distributed nanostructures with fine particle sizes and
reduced aggregation.40–43 Besides this, it also presents scope
for maneuvering the shape and size of the MNPs, improves
impregnation of metal precursors on the support and prevents
metal leaching into the liquid phase through higher metal–
support interaction. Consequently, ligand-assisted MNPs have
found a wide range of applications in hydrogenation,40,44 cross
coupling,45 etc. Recently, in a perspective review article, Rossi
et al. highlighted the role of ligands in various MNP based
catalysis.46 In an elegant work, the same group described a
method to synthesize Pd NPs bound to amino and ethylenene-
diamino functionalized silica where the size of the Pd NPs
(1–6 nm) can be tuned by changing the donor ligand bound to
the support.42 In another significant work, Perez-Ramırez
et al.44 demonstrated the critical impact of the ligand shell on
the selective catalytic behavior of Pd NPs supported on TiSi2O6
for the direct hydrogenation of alkynes. Although the role of the
ligand in MNP based catalysis is well documented, systematic
investigations on the effect of donor ligands in MNP based
heterogeneous catalysis, particularly for selective reductions of
nitroarenes to corresponding amines, have not been reported
so far. In this work, we have reported syntheses of three Pd
nanocatalysts supported on amine-, phosphine- and thiol-
functionalized silica gel and investigated their catalytic poten-
tials in the hydrogenation of nitrobenzenes to corresponding
amines using molecular hydrogen as a reducing agent.
´
´
Fig. 1 TEM images with corresponding size histograms (based on particle
count 200) for the Pd NPs: (a) Pd-AP@SiO2 and (b) Pd-DPPE@SiO2; HRTEM
images of (c) Pd-AP@SiO2 and (d) Pd-DPPE@SiO2.
spherical in shape and homogeneously distributed throughout
the support matrix. The particle size distribution histograms
showed a size averaged at 5–6 nm in both the materials [Fig. 1a
and b inset]. The HRTEM images for both Pd-AP@SiO2 and
Pd-DPPE@SiO2 (Fig. 1c and d respectively) clearly showed that
the reticular planes extended throughout the NPs without any
stack or fault.
On the other hand, the thiol based Pd-MP@SiO2 showed
distinct nanowire (NW) like morphologies of irregular shape
and size (Fig. 2). The distinct shapes of the Pd NWs could be
seen from the HRTEM image of Pd-MP@SiO2 (Fig. 2b) where
the reticular planes extend along the direction of the NWs with
a narrow size distribution of diameter around 10 nm [Fig. 2a
(inset)]. EDX spectra of all the materials invariably showed the
presence of Pd (Fig. S4, ESI†).
The PXRD of the synthesized Pd NPs showed two prominent
reflections at 2y = 40.16, 39.91, 39.67 and 46.66, 46.40 and 46.151
for Pd-AP@SiO2, Pd-DPPE@SiO2 and Pd-MP@SiO2 respectively
(Fig. 3). The peaks are assigned to the (111) and (200) planes for
Results and discussion
Structural and textural properties
The Pd NPs were synthesized via anchorage of Na2PdCl4 with
silica gel functionalized with 2-diphenyphosphinoethyl (DPPE@
SiO2), 3-aminopropyl (AP@SiO2) and 3-mercaptopropyl (MP@SiO2)
through incipient wet-impregnation in water followed by reduc-
tion with hydrazine solution and the materials prepared were
designated as Pd-DPPE@SiO2, Pd-AP@SiO2 and Pd-MP@SiO2
respectively (Scheme 1).
The TEM images of Pd-DPPE@SiO2 (Fig. 1a) and Pd-AP@SiO2
(Fig. 1b) show the formation of Pd NPs of uniform size, mostly
Fig. 2 (a) TEM image of Pd NWs for Pd-MP@SiO2 with corresponding size
histograms (inset based on site count 100); (b) HRTEM image of the Pd NWs
with the extended reticular nanowires represented in white.
Scheme 1 Synthesis of Pd NPs supported on ligand-grafted silica gel via
the wet-impregnation method.
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Fig. 3 Powder X-ray diffraction patterns for (a) Pd-AP@SiO2, (b) Pd-DPPE@
Fig. 4 N2-sorption isotherms for the Pd NP incorporated silica-based
SiO2 and (c) Pd-MP@SiO2 materials.
materials: (a) Pd-DPPE@SiO2, (b) Pd-AP@SiO2 and (c) Pd-MP@SiO2.
fcc Pd NPs. A broad peak corresponding to the silica phase was
observed at around 2y = 22.41 for all the materials which is
consistent with the parent support. This indicates that the meso-
porous structure of the silica matrix remained intact upon Pd
loading. The N2 adsorption–desorption isotherms were measured
for the materials at 77 K and the physical parameters were
compared with those for the parent supports. The N2-sorption
measurements of the functionalized silica materials showed an
isotherm with the characteristic type IV hysteresis loop for
mesoporous materials. A prominent decrease in the surface
area compared to the neat supports was observed (Table 1). This
decrease in surface area may be attributed to the generation of
Pd NPs/NWs within the pores of the silica matrices (Fig. 4).
The binding energies of the Pd 3d state for all the synthe-
sized materials are shown in the XPS spectrum [Fig. 5(a)–(c)].
The bands observed at 335.2 and 340.8 eV for Pd-DPPE@SiO2
and 335.5 and 341.1 eV for Pd-MP@SiO2 are attributed to Pd 3d5/2
and 3d3/2 states, respectively, coherent with the value for Pd(0)
species.43,47 This indicates the complete reduction of Pd(+2)
to Pd(0). A slight increase in the binding energies (+0.1 eV for
Pd-DPPE@SiO2 and +0.4 eV for Pd-MP@SiO2) from that of metallic
Pd was observed. This may be attributed to the interaction of
P- and S-donor atoms on the ligand-functionalized silica matrices
which induces a small positive charge on the metal surface.
On the other hand, for Pd-AP@SiO2 [Fig. 5(c)], the bands at
Fig. 5 XPS spectra for the materials: (a) Pd-MP@SiO2, (b) Pd-DPPE@SiO2
and (c) Pd-AP@SiO2 showing Pd 3d5/2 and 3d3/2 states.
334.6 and 339.4 eV exhibit a slight decrease in BE (À0.5 eV),
which is due to the strong sigma donor capability of N-donor
sites inducing a small negative charge on the metal surface.
To study the thermal behavior of the synthesized materials
thermogravimetric analysis was carried out under N2 atmo-
sphere. The samples were observed to be stable up to 350 1C.
With further increase in temperature, significant loss of weight
corresponding to the decomposition of alkylsilane moieties
(ligand part) of the functionalized silica was observed [Fig. S3
(ESI†)]. The total organic contents were also measured from the
TGA analysis and were found to be 8, 9 and 5 wt% for the thiol,
amine and phosphine-functionalized samples, respectively. The
elemental dot mapping images for Pd-DPPE@SiO2 invariably
demonstrated a uniform distribution of Pd throughout the sup-
port matrix as determined from Fig. 6. The amount of Pd present
on the synthesized materials was estimated by ICP-AES analysis
and found to be 0.84, 0.52 and 0.51 wt% for Pd-DPPE@SiO2,
Table 1 A comparative study of the surface area, pore volume and metal
content of the Pd NPs with their respective neat supports
a
Sample
SBET [m2 gÀ1
]
Pore volumeb [cc gÀ1
]
Pdc [wt/wt%]
DPPE@SiO2
AP@SiO2
MP@SiO2
Pd-DPPE@SiO2 411.3
Pd-AP@SiO2
Pd-MP@SiO2
500.9
550.3
500.0
0.476
0.882
0.634
0.263
0.823
0.445
—
—
—
0.84
0.52
0.51
482.0
231.7
a
b
BET surface area. Total pore volume was measured at P/P0 = 0.98.
c
Metal content of the Pd NPs as measured from ICP-AES analysis.
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Table 3 Hydrogenation of 4-CNB to 4-CAN with Pd-DPPE@SiO2 as the
catalyst under liquid phase conditionsa
Entry Pd cont.b (wt%) Substrate (mmol) Time (h) Conv.c [%] Sel.c [%]
1
2
3
4
5
0.84
0.84
0.84
0.84
1.65
0.5
0.5
0.5
1
0.5
1
2
2
0.5
90
95
98
86
80
100
83
90
83
73
0.5
a
Reaction conditions: catalyst (50 mg), solvent (4.5 mL), H2 pressure
b
(10 bar), 400 rpm, and 50 1C. Pd loading on support in wt% deter-
mined by ICP-AES analysis. Determined by GC analysis. [Pd cont. =
c
palladium content, Conv. = conversion, and Sel. = selectivity].
Fig. 6 EDS elemental mapping analysis for palladium in the as prepared
observed that at a reaction time of 2 h, the catalyst showed
a maximum conversion of 98% (Table 3, entry 3) with 90%
selectivity for 4-CAN. On decreasing the reaction time the
conversion decreases (entries 2 and 1). A maximum selectivity
of 100% was achieved when the reaction was continued for
30 min with a conversion of 90% (entry 1).
Likewise, varying the Pd loading on the support from 0.84 to
1.65 wt% led to a direct decrease in conversion as well as
selectivity (entry 5). Furthermore, on increasing the catalyst :
substrate ratio, decreases in conversion as well as selectivity
were observed (entry 4). From this screening, it was amply clear
that with increase in reaction time, palladium mole content and
catalyst: substrate ratio, there is always a noticeable decrease in
both conversion as well as selectivity. This is understandably due
to the increase in the extent of concomitant hydrodechlorination
of the preformed 4-CAN.
As anticipitated, lesser duration (entry 1) afforded signifi-
cantly reduced formations of undesired aniline from 4-CAN by
C–Cl bond scission. The temperature optimisation studies using
Pd-DPPE@SiO2 revealed that the reaction is largely temperature
dependent (Fig. 7a). With increase in temperature, both conver-
sion and selectivity for 4-CAN increase up to a temperature of
50 1C and, thereafter, they gradually decrease owing to high rates
of hydrodechlorination. To understand the effect of H2 pressure
on hydrogenation of 4-CNB to the corresponding amine, the
reaction was carried out at varying hydrogen pressures (Fig. 7b).
The conversion increased sharply from 2 bar H2 (60%) and
attained completion at 12 bar H2. The selectivity was marked
by a steady increase from 2 bar H2 (58%) to 8 bar H2 (66%)
Pd-DPPE@SiO2 material.
Pd-AP@SiO2 and Pd-MP@SiO2 respectively (Table 1). This indicates
that the phosphine-functionalized ligands stabilize the Pd NPs
more efficiently compared to the amine and thiol-functionalized
counterparts.
Catalytic activity
To explore the catalytic activities of the Pd NPs, hydrogenation
of nitroarenes to their corresponding amines was carried out
with 4-CNB as the model substrate using ethanol as solvent.
Initial screening reveals that under similar experimental condi-
tions, among the three catalysts, Pd-DPPE@SiO2 (Pd = 0.84 wt%)
exhibited superior catalytic performance, and the results are
summarized in Table 2. It is almost established that MNPs with
smaller particle sizes usually exhibit better catalytic performance
due to their preferentially exposed surface area; however, in the
present study, even though the particle sizes are more or less
similar, the materials Pd-DPPE@SiO2 and Pd-AP@SiO2 showed
contrasting catalytic activities. Higher conversion and selectivity
were observed in the case of the former compared to the latter
(Table 2, entries 1 vs. 2). This result clearly suggests that the
donor groups have some implications on catalytic performance.
The efficiency order of the catalysts follows the trend: Pd-
DPPE@SiO2 4 Pd-AP@SiO2 4 Pd-MP@SiO2. Given the distinct
selectivity response for 4-CAN, Pd-DPPE@SiO2 was chosen for
further comprehensive catalyst optimization protocols.
To further optimize the reaction conditions such as reaction
time, temperature, metal loading, catalyst : substrate ratio, hydro-
gen pressure, etc. for better catalytic performance, a number of
runs were performed with Pd-DPPE@SiO2 (Table 3). It was
Table 2 Hydrogenation of 4-CNB to 4-CAN with Pd NPs as catalysts
under liquid phase conditionsa
Entry Nanocatalyst
Catalyst amountb (mg) Conv.c [%] Sel.c [%]
1
2
3
Pd-DPPE@SiO2 50
98
66
Trace
90
35
100
Pd-AP@SiO2
Pd-MP@SiO2
81
82
a
Reaction conditions: solvent (4.5 mL), H2 pressure (10 bar), 400 rpm,
b
Fig. 7 Plots of temperature (left) and H2 pressure (right) optimization studies for
the Pd-DPPE@SiO2 catalyst (0.02 mmol) with 4-CNB as the model substrate
(0.5 mmol) and ethanol (4.5 mL) as the solvent for 2 hours.
and 50 1C, 2 h. Varying amounts of catalyst were added to ensure
equal Pd content on the support (0.0039 mmol). Determined by GC
c
analysis. [Conv. = conversion, and Sel. = selectivity].
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Table 4 Hydrogenation of nitroarenes to aromatic amines with Pd-DPPE@SiO2 as catalyst under liquid phase conditionsa
Entry
1
Substrate
Product
Time (h)
0.5
Conv.b [%]
Sel.b [%]
85
100
83
2
3
2
2
90
98 (fresh), 80 (1st cycle),
78 (2nd cycle), 74 (3rd cycle)
90 (fresh), 78 (1st cycle),
72 (2nd cycle), 70 (3rd cycle)
4
5
2
74
100
98
2
2
90
69
6
100
a
b
Reaction conditions: substrate (0.5 mmol), solvent (4.5 mL), H2 pressure (10 bar), catalyst (50 mg), 400 rpm, and 50 1C. Determined by GC
analysis. [Conv. = conversion, and Sel. = selectivity].
followed by a steep rise observed at 10 bar H2 corresponding to performed with Pd-DPPE@SiO2 as the catalyst under a pressure
a maximum selectivity of 90%. As anticipated, the selectivity for of 10 bar H2 and temperature of 50 1C for a period of 15 minutes.
4-CAN decreased significantly after reaching the maximum of The catalyst was separated from the reaction mixture and the
10 bar H2. This is because of undesired hydrodechlorination which products were analyzed using GC which exhibited a yield of
begins to predominate after 10 bar H2. Hence, after careful investi- 47%. The filtrate was further allowed to react for an hour under
gation of a wide range of experimental parameters, a reaction the same reaction conditions, which eventually showed no
temperature of 50 1C coupled with a H2 pressure of 10 bar increase in product yield. This indicates that the Pd NPs were
seemed best suited for further systematic optimization (Table 4). well intact in the silica matrix. The ICP-AES analysis for the
The liquid phase hydrogenation of different nitroarenes to sample after the third cycle exhibited a negligible loss (0.02%)
amines (Scheme 2) was carried out under the optimized con- in content of Pd.
ditions of 10 bar H2 and 50 1C and the results are presented in
Recyclability tests were performed for all the catalysts and the
Table 4. It was observed that both the o- and m-isomers of CNB results are demonstrated in Fig. 9. Gradual loss in activity and
showed similar conversions and selectivities (entries 1 and 2). selectivity were observed on subsequent recycling for all the cata-
Usually, substrates with electron donating groups like –CH3 and lysts. Compared to the catalysts, Pd-AP@SiO2 and Pd-MP@SiO2, the
–OCH3 exhibited good-to-excellent conversions with excellent loss became more prominent in the case of Pd-DPPE@SiO2,
selectivities (entries 4 and 5). Likewise, the catalyst showed particularly in the first recycle. The decrease in the reactivity
excellent selectivity for 8-nitroquinoline giving exclusively after the first recycle may be due to the blockage of some of the
8-aminoquinoline with moderate conversion (entry 6).
active pore sites. The morphology of the catalyst after the fourth
Heterogeneity of the catalyst was also checked using the hot cycle was examined using TEM (Fig. 8). The micrograph showed
filtration technique with 4-CNB as a substrate. The reaction was that the Pd NPs remained intact within the silica matrix
maintaining their homogeneity without any agglomeration
and exhibiting good size distribution like the fresh one; thus
indicating that the catalyst can be used for still more catalytic
runs. Strategically speaking, the materials Pd-AP@SiO2 and
Pd-MP@SiO2 do not stand out as possible catalysts for the said
reaction due to incompetence in either conversion or selectivity.
On the other hand, Pd-DPPE@SiO2 exhibits superior catlytic
results on both grounds, thereby making it the ideal candidate
Scheme 2 Hydrogenation of nitroarenes to corresponding amines.
for liquid-phase nitroarene hydrogenation (Fig. 9).
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The kinetic study for conversion of 4-CNB to 4-CAN by
Pd-DPPE@SiO2 was carried out by monitoring the reaction for
different time intervals and a plot of conversion versus time is
given in Fig. 10 which follows a parabolic curve. A linear fit of
pseudo-first order was observed as is evidenced from the plot of
ln(C0/Ct) versus time, where C0 and Ct are the concentrations
of 4-CNB at time 0 and t, respectively. From the slope of the
plot, the rate constant was calculated and found to be 2.486 Â
10À4 À1. As stated earlier, there are several reports on hydro-
s
genation of 4-CNB to 4-CAN; however, few reports exist with
regards to heterogeneous Pd NP-based catalysts with molecular
H2 as a reducing agent; and, to understand the actual standing of
our catalyst with respect to the reported ones we have performed a
literature survey (Table 5). We are delighted to see that our catalyst
is much more superior in comparison to other reported catalysts
[entry 1 vs. entries (2–6)].
Fig. 8 TEM image of Pd-DPPE@SiO2 after the 4th run with the corres-
ponding size histogram (inset).
Conclusion
In summary, we have presented a facile one step protocol for syn-
thesising Pd NPs supported on three donor ligand-functionalized
silica gels and investigated their potential as catalysts for selective
hydrogenation of nitroarenes. The influences of the donor ligands
on the structural morphology and catalytic properties were dis-
tinctly visible. Among the three ligands, the phosphine-based
catalyst, Pd-DPPE@SiO2, was the most active and demonstrated
excellent activity (98%) and selectivity (100%). Our catalyst could
be easily recovered from the reaction mixture by simple filtration
and recycled up to four times without compromising much with
the activity and selectivity.
Fig. 9 Recyclability tests for the Pd NPs corresponding to both conver-
sion and selectivity.
Experimental
Materials and method
2-Diphenylphosphinoethyl-functionalized silica gel (DPPE@SiO2;
0.7 mmol gÀ1 loading; 200–400 US mesh), 3-aminopropyl-
functionalized silica gel (AP@SiO2; 1 mmol gÀ1 loading;
200–400 US mesh), 3-mercaptopropyl-functionalized silica gel,
(MP@SiO2; 1.2 mmol gÀ1 loading; 200–400 US mesh) and PdCl2
were purchased from Sigma-Aldrich. All solvents, substrates
and other chemicals were purchased from Acros Organics
(purity 498%) and Merck (purity 499%). PXRD analysis were
carried out in an Ultima IV Powder X-ray diffractometer using
Fig. 10 Conversion vs. time curves for formation of 4-CAN from 4-CNB
(left) and plot of ln(C0/Ct) vs. time (right) catalysed by Pd-DPPE@SiO2.
Table 5 Hydrogenation of 4-CNB to 4-CAN with Pd-DPPE@SiO2 as catalyst under liquid phase conditionsa
Catalyst
Catalyst (w/w%)b
Conditions
Conv.
Prod. (s) (Sel. %)c
Ref.
Pd-DPPE@SiO2
Pd/Al2O3
Pd/Al2O3
Pd/ZnO
Pd/-Fe2O3-PR
Pd/C
0.84
0.35
1.2
4.7
3
50 1C, 10 bar, 30 min
100 1C, 11 bar, —
100 1C, 1 bar, —
180 1C, 1 bar, 180 min
80 1C, 10–40 bar, 300 min
100 1C, 1 bar, 115 min
90
100
100
92
100
100
4-CAN (100)
AN (56), NB (44)
AN (52), NB (48)
4-CAN (95.9) AN (4)
4-CAN (96.2), NB (3.1)
4-CAN (95.92), AN (4.08)
Present worka
11
47
32
36
34
2
a
b
Reaction conditions: substrate (0.5 mmol), solvent (4.5 mL), H2 pressure (10 bar), catalyst (50 mg), 400 rpm, 50 1C, and 2 h. Metal content of the
c
Pd NPs as measured from ICP-AES analysis. Determined by GC analysis; 4CAN = 4chloronitrobenzene; AN = aniline; NB = nitrobenzene. [Conv. =
conversion, Prod. = product, Sel. = selectivity, Ref. = references].
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CuKN as source (l = 1.54 Å) with a scan rate of 2y = 41 minÀ1
.
the reactor vessel was cooled to room temperature and depres-
X-ray photoelectron spectroscopy (XPS) analysis was carried out surized. The reaction mixture was separated from the catalyst
using a FEI PHI 5000 Versa Prob II system. The thermogravi- by simple filtration and analyzed using GC.
metric measurements were carried out using a Perkin Elmer
General procedure for recyclability tests
TGA-DTA analyzer (Model: STA 8000) taking a-Al2O3 as the
reference sample. Samples weighing 5.0 mg were heated from
35 to 800 1C under nitrogen atmosphere at a rate of 20 1C minÀ1
The catalysts for the recyclability tests were recovered from the
reaction mixture after fresh runs by simple filtration followed by
washing and drying. All the recyclability tests were performed
maintaining the same stoichiometric ratios.
.
Surface area measurements were carried out at liquid nitrogen
temperature in a Quantachrome Autosorb-iQ analyser using N2
gas as adsorbent. Surface area calculations were carried out from
the isotherm using the BET model and pore size distributions
were calculated using the BJH model. Samples were degassed at
100 1C for 2 h before performing the experiment. Transmission
electron microscopy (TEM) images were recorded in a JEOL,
JEM-2100 transmission electron microscope with an accelerating
voltage of 200 keV. The ICP-AES analysis was performed in an
ACROS simultaneous ICP spectrometer equipped with an R. F.
generator (1.6 KW, 27.12 MHz), with wavelengths in the range of
130–770 nm. The hydrogenation reactions were carried out in a
reactor made of Hastelloy (model: 5500, make: Parr Instruments,
USA). Gas chromatographic analysis of the reactants and pro-
ducts was performed in a Perkin Elmer Clarus 480 GC system
equipped with a capillary column (L 30 m, I.D. 0.25 mm) fitted
with an FID.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
DST, New Delhi is gratefully acknowledged for financial support
(Project No: EMR/2015/000021). The UGC, New Delhi is also
acknowledged for the SAP-DRS-I grant to the Department of
Chemistry, Dibrugarh University. CSIR-IIP, Dehradun and IIT,
Kanpur are greatly acknowledged for various analytical services.
Notes and references
Synthesis of Na2PdCl4
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