Additive-free vapour-phase hydrogenation of benzonitrile over MgO-supported Ni catalysts

Article abstract of DOI:10.1007/s11164-020-04113-y

Abstract: MgO-supported Ni catalysts were selectively catalysed by the hydrogenation of benzonitrile to benzylamine in continuous flow at atmospheric pressure without any additives. Ni/MgO catalysts were synthesized by co-precipitation method with NaOH as

Full text of DOI:10.1007/s11164-020-04113-y

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-020-04113-y
Additive‑free vapour‑phase hydrogenation of benzonitrile
over MgO‑supported Ni catalysts
A. Nagu1,2 · K. Vasikerappa · P2. Gidyonu · Ch. Prathap1,2 · M. Venkata Rao2 ·
1,2
1,2
2
K. S. Rama Rao · B. David Raju
Received: 11 November 2019 / Accepted: 25 February 2020
©
Springer Nature B.V. 2020
Abstract
MgO-supported Ni catalysts were selectively catalysed by the hydrogenation of
benzonitrile to benzylamine in continuous ꢀow at atmospheric pressure without
any additives. Ni/MgO catalysts were synthesized by co-precipitation method with
NaOH as the precipitating agent. All the synthesized catalysts were characterized
by various physiochemical techniques, namely XRD, H -TPR, BET surface area,
2
CO -TPD, SEM, and TEM. Among them, 10% Ni/MgO catalyst showed the best
2
activity with 98% conversion of benzonitrile, 60% selectivity to benzylamine at
4
73 K. The high dispersion of Ni and strong metal support interactions with MgO
are key factors for the catalytic performance. 10Ni/MgO catalyst was found to be
consistent throughout the time-on-stream studies of 36 h which clearly explains the
excellence of the catalytic system.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4
-020-04113-y) contains supplementary material, which is available to authorized users.
*
B. David Raju
david@iict.res.in
1
2
CSIR-Academy of Scientiꢁc and Innovative Research (CSIR-AcSIR), New Delhi, India
Catalysis and Fine Chemicals Division, CSIR-Indian Institute of Chemical Technology,
Hyderabad 500007, India
Vol.:(012
1
3456789)
3

A. Nagu et al.
Graphic abstract
Ni/MgO catalysts are active in gas phase hydrogenation of benzonitrile to ben-
zylamine at atmospheric pressure.
Keywords Benzonitrile · Benzylamine · Hydrogenation · Ni · MgO
Introduction
The chemo-selective hydrogenation of aromatic nitriles to amines is an imperative
reaction due to the wide utility of amines in various industries, especially pharma-
ceutical and textile industries [1-4]. From the bibliographic standpoint, numerous
studies were carried out for the hydrogenation of nitriles over both homogeneous
and heterogeneous metal catalysts [5, 6]. Comparatively, homogeneous catalysts
are eꢂective in activity and selectivity, but involve tedious workup procedures,
metal particle contamination and diꢃculty in recovery and reuse of catalyst and in
addition, use of base additives create further problems [4]. For instance, Ru(cod)
methylallyl ]-DPPF active catalyst at 50 bar of H pressure in t-BuOK and toluene
2
2
solvent give 99% selectivity to benzylamine [7]. 10Pd–C is an active catalyst under
6
bar of H pressure in NaH PO and dimethyl carbonate–water dual solvent [8].
2
3 3
2
4
RuCl (PPh ) catalyst showed the best results under a H pressure of 50 bar in 24 h
2
2
[
9]. With the economic and environmental barriers, use of homogeneous catalysts
can be abandoned. K. Lévay et al. published a detailed review on selective hetero-
geneous catalytic hydrogenation of nitriles to primary amines. They pointed out that
the selective formation of primary amines is a challenging task. The selection of
metal, metal support interactions, and reaction parameters are crucial for achieving
1
3

Additive-free vapour-phase hydrogenation of benzonitrile…
selectivity to primary amine [6]. Production of benzylamine from hydrogenation of
benzonitrile over heterogeneous catalysts such as supported metal catalysts is con-
sidered as an atom-eꢃcient route. However, hydrogenation of benzonitrile has been
established at higher H pressure (6–100 bar) [8, 10–16] over supported metal (Pt
2
[
15], Rh [15] Pd [10–17], and Ni [8, 18]) catalysts [19]. Pd-MCM-41 showed excel-
lent activity with 91% conversion of nitrile and 90% selectivity of benzylamine at a
partial pressure of H and CO of 20 and 100 bar, respectively [15]. Alumina-sup-
2
2
ported Pd catalyst is active towards hydrogenation of benzonitrile to benzylamine at
2
1
0 bar of H pressure in 2-propanol solvent with 95% selectivity of benzylamine [10,
2
1]. Even with heterogeneous catalysts, pressurized H operations and formation of
2
by-products are still to be solved [12]. Various supported Cu [4], Ni [10, 11] and Pd
[
[
[
8, 12] metal catalysts were reported for the titled reaction at eco-friendly conditions
20]. Due to the low cost of nickel, supported Ni catalysts have been frequently used
21]. A Ni/Mo is one of the catalysts with 100% conversion and 99% selectivity in
methanol solvent at 15 bar of hydrogen pressure [22]. RANEYs Ni catalyst showed
9
5% selectivity of benzylamine and 100% conversion of nitriles [23]. It is reported
that the acidic sites favour the formation of tertiary and secondary amines, whereas
basic sites selectively produce primary amines [4, 24]. Hence, use of basic supports
to get selective production of primary amines is highly helpful. The abundant litera-
ture on benzonitrile to benzylamine conꢁrming that the catalysts used involve: (1)
use of expensive metal catalysts and solvents, (2) base additive requirement, (3) high
H pressure operations, (4) poor activity and selectivity, etc. leads to environmental
2
and economic issues. At this juncture, development of inexpensive and eco-friendly
catalysts with high activity and selectivity under mild operating conditions is an
urgent requirement. Herein, Ni/MgO catalysts have been prepared, characterized,
and used for the hydrogenation of benzonitrile (BN) to benzylamine (BA) under
atmospheric pressure and obtained good activity and selectivity, which are deline-
ated sequentially.
Experimental
Catalyst preparation
Ni/MgO catalysts with various Ni loadings of 5Ni/MgO, 10Ni/MgO, 15Ni/MgO,
and 20Ni/MgO (in wt%) have been prepared by co-precipitation method [25]. Ni
(
NO ) ·6H O (M/s. Alfa Aesar, USA 99.9%) and Mg (NO ) ·6H O (M/s. Loba Che-
3 2 2 3 2 2
mie, India Assay 99.5%) were used as precursors of Ni/MgO catalysts. Precursors
were mixed together in 1000 mL distilled water, and the mixed solution precipitated
at pH 9.0 by the addition of 0.5 M aqueous solution of NaOH (M/s. Loba Chemie,
India). The precipitation is ꢁltered and washed ten times with 2000 mL of distilled
water in order to remove the sodium ions, dried at 393 K for 12 h. Finally, the dried
sample is calcined in static air at 773 K for 5 h and prior to the reaction the catalysts
were reduced in H ꢀow at 873 K for 4 h. The prepared catalysts were designated
2
as 5Ni/MgO, 10Ni/MgO, 15Ni/MgO, and 20Ni/MgO where the preꢁxed numerical
value indicates the weight percentage of Ni loading on the MgO support.
1
3

A. Nagu et al.
Characterizations of catalysts
X-ray diꢂraction (XRD) patterns have been recorded (after ex-situ reduction
of the catalysts) on Ultima-IV (M/s. Rigaku Corporation, Japan), XRD unit
was operated at 40 kV and 40 mA equipped with Ni-ꢁltered Cu Kα radiation
(
λ = 1.54056 Å), and a 2θ value ranges from 20° to 80° at a scanning rate of
−
1
4
° min . N physisorption studies have been carried out by using a Quadrasorb-
2
SI (M/s. Quantachrome Instruments Corporation, USA) at liquid nitrogen tem-
perature. Prior to the measurement, the sample was out-gassed at 473 K for 3 h.
Temperature-programmed reduction (TPR) studies were acquired on a homemade
equipment containing a quartz reactor with electrical heating and a thermal con-
ductivity detector (TCD)-equipped gas chromatograph connected at the outlet of
the reactor. About 50 mg of catalyst was placed at the centre of the quartz reac-
tor between two plugs of quartz; wool was pre-treated at 573 K for 1 h in Ar
−
1
ꢀ
ꢀ
ow (60 mL min ). Then the catalyst was exposed to 5% H balance Ar gas
2
ow for 1 h at 373 K followed by being raised to a temperature up to 1073 K at
−
1
a heating rate of 10 K min . The H concentration was monitored by standard
2
GC software. The total basicity of the catalysts has been measured by tempera-
ture-programmed desorption (TPD) of CO using an AUTOSORB iQ automated
2
gas sorption analyzer (M/s. Quantachrome Instruments, USA); 0.0.1 g of cata-
−
1
lyst has been loaded and pre-treated in a He gas ꢀow (99.8%, 50 mL min ) at
5
23 K for 1 h. After pre-treated catalysts were saturated with 10% CO -helium
2
at 323 K for 1 h, it was followed by ꢀushing with helium gas for 1 h to ꢀush oꢂ
the physisorbed CO at the same temperature. Then temperature to 1200 K at
2
−
1
1
0 K min and desorbed CO was monitored with the inbuilt TCD in a ꢀow of
2
−
1
helium (99.8%, 30 mL min ). SEM and TEM analyses were studied on scanning
electron microscope (M/s. JEOL, Switzerland) and transmission electron micro-
scope (M/s JEOL, Switzerland) operating between 160 and 180 kV, respectively.
Before TEM analysis, the catalyst sample ultrasonication in ethanol and a drop
was placed on a copper grid (carbon coated) and solvent was evaporated in air at
3
50 K for 5 h.
Catalyst activity test
Gas-phase selective hydrogenation of benzonitrile was studied in a ꢁxed-bed
quartz reactor (14 mm id and 300 mm length) at atmospheric pressure in the
temperature range of 453–513 K; 1 g of catalyst was sandwiched at the mid-
dle of quartz reactor in between two quartz wool layers, with dilution of quartz
−
1
grains, and catalyst was reduced at 873 K under the H ꢀow rate of 40 mL h
2
for 4 h. Reaction temperature was ꢁxed, and benzonitrile was fed at a ꢀow rate
−
1
of 1 mL h by using a syringe feed pump (M/s. B. Braun, Germany) along
−
1
with H (99.5% pure) ꢀow of 30 mL min . The product mixture was col-
2
lected in an ice-cooled trap at every interval and product mixture identiꢁed by
an FID-equipped gas chromatograph, GC-17A (Equity-5 capillary column
1
3

Additive-free vapour-phase hydrogenation of benzonitrile…
(
30 m × 0.53 mm × 5.0 μm), M/s. Shimadzu Instruments, Japan) and conꢁrmed by
GC–MS, QP-201O (EB-5MS capillary column (30 m × 0.25 mm × 0.25 μm) M/s.
Shimadzu Instruments, Japan).
Moles of reactant − Moles of reactant
in
out
Conversion (%) =
Selectivity (%) =
× 100
Moles of reactantin
Moles of product formed
× 100
Moles of reactant converted
Results and discussion
Brunauer–Emmett–Teller (BET) surface areas of prepared catalysts are shown in
Table 1. The surface area of MgO is more than that of Ni/MgO catalysts. As metal
content increases, the surface area of catalysts is decreased due to the pore blockage
of the support. The results are in accordance with results reported in the literature
[26–28].
X-ray diꢂraction (XRD) patterns of calcined Ni/MgO catalysts, NiO, and MgO
support (prepared by the same method) are depicted in Fig. 1. Support MgO and
NiO not only exhibited similar diꢂraction patterns but also showed nearby char-
acteristic peaks because both NiO and MgO have similar ionic radii and the same
rock salt structure [29, 30]. All Ni/MgO catalysts exhibited the same XRD pat-
terns before the reduction. There are no distinguishable peaks of NiO identiꢁed in
Ni/MgO catalysts. It should be observed that support MgO and NiO have similar
patterns without doublet structure, indicating that NiO–MgO mixed oxide formed
during the calcination [31]. This NiO–MgO mixed oxide acts as sources of small
0
Ni particles [32–34]. The peaks located at 2θ of 36.93°, 43.04°, 62.47°, 74.97°,
and 78.91° in the XRD patterns of Ni/MgO catalysts contribute to the forma-
tion of NiO–MgO mixed oxide. Figure 2 shows the XRD patterns of reduced Ni/
Table 1 Physico-chemical properties of the catalysts
a
b
c
d
Catalyst
Surface area
Cystallite size of Ni
Hydrogen consumption Total basic-
2
−1
−1
(
m g
)
(111) nm
(µmol g
)
ity (µmol
−1
CO g
)
2
MgO
118
98
88
76
62
–
–
478
463
448
410
378
5
1
1
2
Ni/MgO
5.2
7.2
9.5
12
214
253
288
358
0Ni/MgO
5Ni/MgO
0Ni/MgO
a
Calculated from BET
Calculated from XRD
Calculated from TPR
b
c
d
Calculated from TPD of CO2
1
3

A. Nagu et al.
Fig. 1 XRD patterns of calcined Ni/MgO catalysts, NiO, and MgO support
Fig. 2 XRD patterns of reduced Ni/MgO catalysts
MgO catalysts at 873 K for 4 h. After reduction, three metallic nickel diꢂrac-
tion peaks were detected at 44.49 (111), 51.89 (200), and 76.38 (220) (JCPDF
8
7-0712). The peaks around 2θ = 42.88°, 36.99°, 62.28°, 76.18°, and 78.98° cor-
respond to crystalline-phase MgO or solid solution of NiO–MgO on 2θ scale. The
increase in crystallite size of metallic Ni (111) with increase in nickel loading and
obtained results are shown in Table 1.
1
3

Additive-free vapour-phase hydrogenation of benzonitrile…
Fig. 3 H -TPR patterns of Ni/MgO catalysts
2
Temperature-programmed reduction (H -TPR) patterns of Ni/MgO are depicted
2
in Fig. 3. It can be inferred from Fig. 3 that the TPR patterns of the 5Ni/MgO and
1
0Ni/MgO catalysts exhibited two reduction peaks with T at 761, 1083, and 729,
max
1
096 K, respectively. On the other hand, for 15Ni/MgO and 20Ni/MgO catalysts,
three reduction peaks with T
696, 1026, 1098 K and 669, 983, 1093 K, respec-
max
tively, were also observed. The reduction peaks attributed to the reduction of NiO
2
+
located on the MgO surface; in the outermost layer of the MgO structure, some Ni
ions possess square–pyramidal coordination and the NiO–MgO (MgNiO ) mixed
2
oxide lattice [35], respectively. The defect sites of MgO are responsible for strongly
adsorbed nickel species on MgO [26]. Zecchina et al. reported that the NiO–MgO
2
+
mixed oxides reduced in the temperature range 671–1093 K are mainly the Ni ions
2
+
located on the surface, and at temperature higher than 1093 K Ni located in the
2
+
bulk could be reduced [31]. Therefore, metallic Ni was originated from Ni ions
presented over surface of NiO–MgO mixed oxide in our catalysts. NiO–MgO mixed
oxide solution is achieved during the calcination [30].
Temperature-programmed desorption of CO (CO -TPD) is a well-known tech-
2
2
nique to determine the total basicity of solid catalysts; the respective CO -TPD pro-
2
ꢁ
les of MgO and Ni/MgO catalysts are displayed in Fig. 4. All the catalysts dis-
played a similar distribution of basic sites with T in the range of 430 K, 650 K,
max
and 873 K. The desorption peaks in the temperature range of 373–523 K represent
the presence of weak basic sites, and 523–773 K and 773–973 K indicate the pres-
ence of moderate and strong basic sites, respectively. As metal loading increases, the
moderate basic sites are increased. Alternatively, the strong basic sites are decreased.
The morphological features of the 10Ni/MgO catalyst were studied on scan-
ning electron microscopy (SEM) and transmission electron microscopy (TEM)
techniques. The images of SEM and TEM are displayed in Fig. S1 and Fig. 5.
1
3

A. Nagu et al.
Fig. 4 CO -TPD of MgO and Ni/MgO catalysts
2
Fig. 5 a TEM image of reduced 10Ni/MgO and b TEM image of used 10Ni/MgO catalyst
SEM images of 10Ni/MgO catalyst show smooth surfaces of MgO in support
and MgO-supported Ni catalyst. TEM image of reduced 10Ni/MgO catalyst
indicated well-distributed Ni particles with an average particle size of 7.2 nm
which is consistent with the XRD results, and TEM image of used 10Ni/MgO
catalyst exhibited Ni particles with an average particle size of 7.5 nm.
1
3

Additive-free vapour-phase hydrogenation of benzonitrile…
Fig. 6 Inꢀuence of nickel loading for the hydrogenation of benzonitrile. Reaction conditions: catalyst
weight 1 g, feed 1 mL/h, temperature 473 K, pressure 1 atm, and H /BN=8 (molar ratio)
2
Activity studies
The vapour-phase hydrogenation of benzonitrile was studied over MgO-sup-
ported Ni catalysts, and the results are presented in Fig. 6. Scheme 1 represents
probable reaction path ways during the hydrogenation of benzonitrile. This reac-
tion involves hydrogenation, condensation, and hydrogenolysis reactions that
can yield benzylamine (BA), N-benzylidenebenzylamine (DBI), dibenzylamine
(
DBA), and toluene (TOL), respectively; ammonia (NH ) is eliminated during
3
condensation reaction and hydrogenolysis to toluene [9].
The conversion of BN and selectivity to BA is increased with nickel loading
from 5 to 10 wt% and reaches a maximum BN conversion of 98% with 60% selec-
tivity to BA at 473 K over 10Ni/MgO catalyst. Further with increase in nickel
loading from 10 to 20 wt%, there is a reverse trend in both conversions of BN
and selectivity to BA. These results can be attributed to the formation of larger
Ni particles over MgO in 15Ni/MgO and 20Ni/MgO catalysts that decrease the
activity and selectivity in the vapour-phase hydrogenation of BN. The activity
results are found to be in the order: 10Ni/MgO > 15Ni/MgO > 5Ni/MgO > 20Ni/
MgO.
The eꢂect of reaction temperature on the best catalyst 10Ni/MgO was conducted
at diꢂerent temperatures, viz. 453 473, 493, and 513 K, and the results are presented
in Fig. 7. As reaction temperature increased, the conversion of BN is enhanced while
the selectivity to BA is increased with reaction temperature up to 473 K; beyond
this, the selectivity to BA shows a downfall. The increase in reaction temperature
promotes the formation of other products through over-hydrogenation and hydrogen-
olysis reactions. The maximum selectivity to BA is 60% obtained at 473 K with 98%
conversion of BN.
1
3

A. Nagu et al.
Fig. 7 Inꢀuence of temperature on the hydrogenation of benzonitrile on 10Ni/MgO catalyst. Reaction
conditions: catalyst weight 1 g, feed 1 mL/h, pressure 1 atm, H /BN=8 (molar ratio)
2
Fig. 8 Inꢀuence of hydrogen molar ratio on BN over 10Ni/MgO catalyst. Reaction conditions: tempera-
ture 473 K, pressure 1 atm, catalyst weight 1 g, feed 1 mL h−
1
The vapour-phase hydrogenation of BN was studied over 10Ni/MgO catalyst
at 473 K under diꢂerent BN/H mole ratios to understand the optimum H moles
2
2
needed to convert BN to BA (Fig. 8). In general, one mole of BN requires two moles
of hydrogen to produce one mole of BA. The studies were carried out using diꢂerent
BN/H molar ratios such as 1:4, 1:8, and 1:12. At initial BN/H molar ratio (1:4),
2
2
6
5% conversion of BN with 30% selectivity of BA and 29% of TOL is formed. With
1
3

Additive-free vapour-phase hydrogenation of benzonitrile…
Fig. 9 Time-on-stream study for the hydrogenation of benzonitrile on 10Ni/MgO catalyst. Reaction con-
ditions: catalyst weight 1 g, feed 1 mL/h, temperature 473 K, pressure 1 atm, H /BN=8 (molar ratio)
2
Scheme 1 Probable reaction scheme in the hydrogenation of BN. Benzonitrile (BN), benzylamine (BA),
benzylideneimine (BI), a-aminodialkylamine (BIBA), N-benzylidenebenzylamine (DBI), dibenzylamine
(
DBA), and toluene (TOL)
rise of the BN/H molar ratio (1:8), both conversion of BN and selectivity of BA are
2
raised. At 1:12 molar ratio, a downfall in the conversion of BN (78%) was noticed,
while the selectivity of BA increased to 68%. The selectivity of BA is increasing
with the BN/H molar ratio. There exists a competition between H , BN, and BA
2
2
1
3

A. Nagu et al.
Table 2 Comparison of Ni/MgO catalyst activity with reported catalytic systems
Catalyst
Reaction conditions
BN Conv.
BA Sel.
(%)
References
(
%)
Solvent
Temp. (K) Additive Pressure
of H
2
(
bar)
Pd–MCM-41
Ni/Mo
–
323
–
20
91
100
98
100
100
98
90
99
98
95
86
60
[14]
[21]
[9]
[10]
[16]
Present study
Methanol 378
NH /H O 41
3
2
t
RuCl (PPh ) Toluene
353
BuOK
50
10
15
1
2
3
Pd/Al O
2-Propanol 353
Ethanol
–
–
–
–
2
3
Pd/γ-Al O
343
473
2
3
Ni/MgO
to adsorb onto active sites. At low BN/H mole ratio, the available active sites are
2
more and the formed BA product could adsorb and participate in further hydrogen-
olysis reactions to form TOL so that the selectivity to BA is decreased. As the BN/
H molar ratio increases, the available active sites for BA to adsorb and participate
2
in reactions are less. Hence, the selectivity to BA is more at high BN/H molar ratio.
2
To explore the catalyst stability of 10Ni/MgO catalyst, it is subjected to time-
on-stream (TOS) studies, and results are presented in Fig. 9. On continuous study
of 36 h, the 10Ni/MgO catalyst is stable with consistent activity and selectivity
which shows the sturdiness of the catalyst. The catalytic activity of Ni/MgO catalyst
is compared with reported catalytic systems and presented in Table 2. It illustrates
that 10Ni/MgO catalyst is free from additives, noble metal catalysts, and pressurized
operations.
Conclusions
Ni/MgO catalysts were prepared by co-precipitation method and used for hydrogen-
ation of benzonitrile to benzylamine in continuous process at atmospheric pressure.
All the Ni/MgO catalysts are found to be active and selective for the production of
benzylamine from hydrogenation of benzonitrile. 10 wt% Ni/MgO catalyst exhibited
better performance with 98% conversion of benzonitrile and 60% selectivity to ben-
zylamine without using any additives and mild reaction conditions. The high disper-
sion of Ni and strong metal support interactions with MgO are key factors for the
catalytic performance. 10 wt% Ni/MgO catalyst is robust during 36-h life study.
Acknowledgements AN thanks CSIR-New Delhi, India, for the award of fellowship.
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