Selective hydrogenation of benzonitrile in multiphase reaction systems including compressed carbon dioxide over Ni/Al2O3 catalyst

Article abstract of DOI:10.1016/j.molcata.2013.07.017

Selective hydrogenation of benzonitrile was studied with Ni/Al ₂O₃ in compressed CO₂, hexane-CO₂, ethanol-CO₂ and H₂O-CO₂ systems. The phase behavior and the effect of CO₂ on the conversion of benzonitrile and the yield of benzylamine were discussed. The reaction rate was retarded in solventless, hexane and ethanol, but accelerated in water in the presence of compressed CO₂. The decrease in reaction rate was mainly ascribed to the formation of carbamate from benzylamine and the intermediate 1-aminodibenzylamine reacting with CO₂, and it precipitated out to coat on the surface of catalyst in solventless and hexane, and the dilution effect of compressed CO₂ in ethanol. But the yield of benzylamine was increased in solventless, hexane and ethanol due to the following nucleophilic addition of benzylamine and benzylimine to N-benzylidenebenzylamine was inhibited. Although, the reaction rate increased in water for the enhanced solubility of H₂ and benzonitrile in H₂O and decreased mass-transfer resistance in the presence of compressed CO₂, the yield of benzylamine decreased because of the acidic nature of H₂O-CO ₂ favorites the elimination of NH₃ and the formation of NH₄HCO₃. Accordingly, the possible reaction pathway of benzonitrile hydrogenation was proposed for all the studied systems.

Full text of DOI:10.1016/j.molcata.2013.07.017

Accepted Manuscript
Title: Selective hydrogenation of benzonitrile in multiphase
reaction systems including compressed carbon dioxide over
Ni/Al2O3 catalyst
Author: Haiyang Cheng Xiangchun Meng Chaoyong Wu
Xiaoyu Shan Yancun Yu Fengyu Zhao
PII:
S1381-1169(13)00279-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2013.07.017
MOLCAA 8817
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
3-3-2013
15-6-2013
28-7-2013
Please cite this article as: H. Cheng, X. Meng, C. Wu, X. Shan, Y. Yu, F. Zhao, Selective
hydrogenation of benzonitrile in multiphase reaction systems including compressed
carbon dioxide over Ni/Al2O3 catalyst, Journal of Molecular Catalysis A: Chemical
(2013), http://dx.doi.org/10.1016/j.molcata.2013.07.017
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Selective hydrogenation of benzonitrile in multiphase reaction systems including compressed
carbon dioxide over Ni/Al2O3 catalyst
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Haiyang Cheng a,b, Xiangchun Meng c, Chaoyong Wu a,b, Xiaoyu Shan c,
Yancun Yu a,b, Fengyu Zhao a,b,*
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a State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, PR China
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bLaboratory of Green Chemistry and Process, Changchun Institute of Applied Chemistry, Chinese
Academy of Sciences, Changchun 130022, PR China
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c
School of Chemical Engineering, Changchun University of Technology, Changchun 130012, PR
China
* Corresponding author. Tel. & fax: +86 431 8526 2410; E-mail address: zhaofy@ciac.jl.cn.
Abstract
Selective hydrogenation of benzonitrile was studied with Ni/Al2O3 in compressed CO2,
hexane–CO2, ethanol–CO2 and H2O–CO2 systems. The phase behavior and the effect of CO2 on
the conversion of benzonitrile and the yield of benzylamine were discussed. The reaction rate was
retarded in solventless, hexane and ethanol, but accelerated in water in the presence of compressed
CO2. The decrease in reaction rate was mainly ascribed to the formation of carbamate from
benzylamine and the intermediate 1-aminodibenzylamine reacting with CO2, and it precipitated
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out to coat on the surface of catalyst in solventless and hexane, and the dilution effect of
compressed CO2 in ethanol. But the yield of benzylamine was increased in solventless, hexane and
ethanol due to the following nucleophilic addition of benzylamine and benzylimine to
N-benzylidenebenzylamine was inhibited. Although, the reaction rate increased in water for the
enhanced solubility of H2 and benzonitrile in H2O and decreased mass-transfer resistance in the
presence of compressed CO2, the yield of benzylamine decreased because of the acidic nature of
H2O–CO2 favorites the elimination of NH3 and the formation of NH4HCO3. Accordingly, the
possible reaction pathway of benzonitrile hydrogenation was proposed for all the studied systems.
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Keywords: benzonitrile, hydrogenation, nickel, compressed carbon dioxide, multiphase reaction
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1. Introduction
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Aromatic primary amines are important chemical compounds commonly used as
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intermediates for the pharmaceutical, agrochemical and textile industries. The heterogeneous
catalytic hydrogenation of aromatic nitriles is an important process to produce aromatic primary
amines. However, during the hydrogenation of aromatic nitriles, several byproducts such as
secondary amine and enamine are usually formed, resulting in a loss of yield toward aromatic
primary amines [1, 2]. For the selective hydrogenation of nitriles to primary amines, the rate and
the product composition are primarily depending on the type of catalyst. Raney Ni, Raney Co,
supported Ni, Pd, Pt, Ru, and Rh are suitable catalysts [2]. Supported Ni catalysts are often used
because of their low cost [3], and moderate activity and selectivity to primary amines [4-7]. In the
gas phase hydrogenation of benzonitrile with 5% Ni on various supports, it was reported that the
activity of the catalysts was in an order of Al2O3>TiO2>SiO2-Al2O3>SiO2 [8]. In addition, the type
of solvent, reactant concentration, reaction conditions and additives (e.g. ammonia) also play
important roles [1, 2].
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The multiphase system containing compressed CO2 has been paid much attention recently
[9-12]. The pressurization of liquid reaction mixtures with CO2 will be effective for accelerating
the rate of reaction, modifying the product selectivity, and making it easy to separate and recycle
the catalyst. Different types of multiphase systems containing compressed CO2 could be designed
and applied for various chemical transformations [10]. For example, Eckert et al. investigated
CO2-expanded ethanol for the heterogeneous hydrogenation of benzonitrile and phenylacetonitrile
with NiCl2/NaBH4. The yield of primary amines increased to 98% because the primary amines
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were protected by CO2 and their further hydrogenation to the secondary amines were effectively
suppressed. They also studied CO2-expanded tetrahydrofuran for homogeneous hydrogenation of
the same nitriles with RhH(P-i-Ph3)3, in which, the primary amines are separated in situ in the
form of solid carbamic acid and/or ammonium carbamates while the catalysts remain in the
solution [13]. The hydrogenation of nitrile was performed in supercritical CO2 over Pd/MCM-41
and Ni/MCM-41, respectively. With Pd/MCM-41, the selectivity to benzylamine was 91% at 90%
conversion. It is possible to tune the selectivity from primary amine to secondary amine by
varying the CO2 pressure. With Ni/MCM-41, dibenzylamine was produced as the main byproduct
with a poor selectivity to benzylamine of 37.5% [14]. The use of large amount of noble metal
catalyst makes the chemical process too expensive to apply in the large scale industrial utilization.
Therefore, development of efficient non–noble metal catalysts like Ni-based catalyst and green
catalytic reaction system is a big challenge to the researchers.
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In this work, the hydrogenation of benzonitrile was studied with Ni/Al2O3 in compressed CO2,
hexane–CO2, ethanol–CO2 and H2O–CO2 systems. For comparison, the reaction was also
examined in solventless system, hexane, ethanol and water. The phase behavior and the functional
roles of CO2 were discussed, and the possible reaction pathway was proposed in multiphase
systems containing compressed CO2.
2. Experimental
2.1. Chemicals, catalyst preparation and characterization
Benzonitrile, hexane, ethanol, benzaldehyde, benzyl alcohol, benzylamine, dibenzylamine,
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tribenzylamine, Ni(NO3)2·6H2O and Al(NO3)3 are of analytical grade and used without further
purification. Deionized water and gases of H2 (99.999%), N2 (99.999%), CO2 (99.999%) and NH3
(99.999%) were used as delivered. N-benzylidenebenzylamine was prepared by condensing of
benzaldehyde and benzylamine in ethanol at room temperature for 24 h, and obtained through
rotary evaporation, and analyzed with a gas chromatograph and gas chromatography/mass
spectrometry.
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Alumina-supported Ni catalyst was prepared by co-precipitation using Ni(NO3)2·6H2O and
Al(NO3)3 with a Ni/Al atomic ratio of 1/1. After dried at 120 oC for 12 h, the sample was calcined
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at 450 C for 5 h under air in Muffle oven. Before each hydrogenation run, a certain amount of
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samples was reduced under H2 flow at 580 C for 2 h. The Ni loading in the calcined samples of
Ni/Al2O3 was measured by inductively coupled plasma optical emission spectrometry (ICP-OES,
iCAP6300, Thermo USA). The structural properties of catalysts were examined by X-ray
diffraction (XRD, Philips PW1710 BASED) and transmission electron microscopy (TEM,
JEM-2000EX). Temperature-programmed reduction (TPR) was carried out by heating 50 mg
catalyst sample under H2/N2 at 10 oC·min-1 up to 800 oC.
2.2. Hydrogenation of benzonitrile
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The hydrogenation of benzonitrile was carried out at 80 C in a 50 ml autoclave. The reactor
was charged with 2 ml benzonitrile (19.6 mmol) and a catalyst sample, flushed with N2, and
heated with a water bath. After introduction of 4 MPa H2, liquid CO2 was flowed into the reactor
with a high-pressure liquid pump (Jasco SCF-Get) to the desired pressure when necessary. The
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reaction was conducted with a continuously stirring stirred with a magnetic stirrer. The reaction
was carried out with a stirring speed of 1200 rpm, at which the mass transport limitation was
confirmed to remove completely for that the reaction rate did not change after the stirring speed
was increased up to 800 rpm. After reaction, the reactor was cooled with an ice-water bath,
depressurized carefully. Then the reaction mixture was dissolved with about 30 ml ethanol and the
liquid products were centrifuged and analyzed with a gas chromatograph (GC-Shimadzu-2010,
FID, Capillary column, Rtx-5 30 m × 0.25 mm × 0.25 μm), and identified by gas
chromatography/mass spectrometry (GC/MS, Agilent 5890). The extent of error for the
conversion and yield was in a range of ±2%. The carbon balance was calculated based on reactant
and all the detected products such as benzylamine, N-benzylidenebenzylamine, dibenzylamine,
tribenzylamine, benzaldehyde and benzylalcohol with external standard method, the data was near
to 99.8%. The reaction proceeds without adding any solvent except for the reactants, naming
“solventless system”. Hydrogenation reactions in ethanol, hexane, water, ethanol–CO2,
hexane–CO2 and H2O–CO2 were conducted with the similar procedures. For some cases, the
carbamic acid and/or ammonium carbamates were formed and precipitated as white solid such as
in dense phase CO2 and hexane–CO2. The reaction results are reproducible at same reaction
conditions.
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2.3. Visual observation of the phase behavior
The observation of the phase state was carried out in an 85 ml high-pressure view cell. The
observations were made with a similar volumetric ratio of benzonitrile to the reactor volume as
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used in the hydrogenation runs. In the typical process, benzonitrile (3.4 ml) was added and the
reactor was heated up to 80 oC, H2 (4 MPa) and then liquid CO2 was introduced into the reactor to
the required pressure. At each pressure the mixture was stirred for several minutes, and then the
stirring was stopped and the phase behavior was observed by the naked eye through the
transparent sapphire windows.
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Fig. 1. Lift: XRD patterns of the Ni/Al2O3 samples (a) precursor dried at 120 C, (b) calcined at
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450 C for 5 h under air, (c) reduced at 580 C with H2 for 2 h, () NiAl(CO3)(OH)3, () NiO,
() Ni, and TPR of calcined Ni/Al2O3 samples (inset). Right: TEM of reduced Ni/Al2O3 sample.
3. Results and discussion
3.1. Catalyst characterization
The XRD patterns of Ni/Al2O3 are presented in Fig. 1. The co-precipitated intermediate
exhibited the characteristic diffraction peaks of nickel aluminum carbonate hydroxide, i.e.,
NiAl(CO3)(OH)3 (PDF No. 48-0593), but its crystalline was somewhat poor (Fig. 1a). After
calcination, the sample showed a broad NiO diffraction, but no Al2O3 and NiAl2O4 spinel were
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identified, indicating that Al species were likely to be well mixed in the bulk of NiO (Fig. 1b) [15].
In addition, only one main reduction peak centered at 555 oC was found from the TPR profile of
calcined Ni/Al2O3 sample, indicating the NiO species was homogeneously dispersed (Fig. 1 inset).
For the sample reduction at 580 oC for 2 h, it showed sharp peaks of Ni diffraction and the average
particle size of Ni was ca. 7.3 nm as determined from the Ni (111) facet by equation (Fig. 1c). The
peaks of NiO diffraction disappear, indicating the most NiO was reduced to the metallic Ni. Based
on the TPR, the amount of hydrogen consumption of the reduction peaks at 360-790 oC was 0.332
mmol as estimated by using nano CuO as reference, it was nearly to the amount required for full
reduction of NiO on Ni/Al2O3, which was also confirmed by the XRD analysis. TEM image
indicated the average particle size of Ni particles was 6.9 nm (Fig. 1d), which is accordant with
the XRD results. The loading of Ni was 41.4% as measured by ICP-OES, which is close to the
theoretical value of 46.7%.
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Scheme 1. Proposed reaction pathways of benzonitrile hydrogenation in CO2, hexane–CO2 and
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ethanol–CO2.
3.2. Hydrogenation of benzonitrile in different media
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As described in scheme 1, generally, benzonitrile is hydrogenated to benzylimine 2, and it is
then hydrogenated to benzylamine 3 and condensed with benzylamine 3 to 1-amino
dibenzylamine 5; 1-amino dibenzylamine 5 can be transformed to benzylamine 3 as well as
deaminated to N-benzylidenebenzylamine 6 and then hydrogenated to dibenzylamine 7. In the
presence of CO2, benzylamine 3 is transformed to carbamate 4, carbamate 4 cannot act as a
nucleophile to react with benzylimine 2, so the nucleophilic addition between benzylamine 3 and
benzylimine 2 is inhibited [13, 14].
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Fig. 2. Hydrogenation of benzonitrile in (a) solventless systems, (b) 10 MPa CO2. Reaction
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conditions: 2 ml benzonitrile, 0.1 g Ni/Al2O3, 4 MPa H2, 80 C, () conversion of benzonitrile,
yield of () N-benzylidenebenzylamine, () benzylamine, () dibenzylamine.
3.2.1. Hydrogenation of benzonitrile in compressed CO2, hexane–CO2, ethanol–CO2
Fig. 2 shows the evolution of product distribution along with the process of benzonitrile
hydrogenation over Ni/Al2O3 catalyst in the absence and presence of compressed CO2. The rate of
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benzonitrile conversion in the absence of CO2 was faster than that in 10 MPa CO2, complete
conversion presented at 10 h for the former, but it presented at 15 h for the latter case. And in both
the systems, N-benzylidenebenzylamine was produced as the predominant intermediate product at
the beginning of reaction. In the absence of CO2, N-benzylidenebenzylamine was converted to
both benzylamine and dibenzylamine with the extension of reaction and the yield of benzylamine
and dibenzylamine reached 74.6% and 23.7% at complete conversion of
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N-benzylidenebenzylamine after reaction for 25
h
(Fig. 2a). In 10 MPa CO2,
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N-benzylidenebenzylamine was converted to benzylamine with the extension of reaction, and the
yield of benzylamine reached 96.5% at the end of reaction, but dibenzylamine did not change
anymore and its yield was only 2.8% (Fig. 2b). Therefore, the product distribution changed largely
and the benzylamine was yielded much more in the presence of CO2, and it is about 20% higher
than that in the solventless conditions.
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Table 1 shows the results of the hydrogenation of benzonitrile in hexane and hexane–10 MPa
CO2. The rate of benzonitrile conversion in hexane was faster than that in hexane–10 MPa (entries
1, 5). N-benzylidenebenzylamine was also produced as the predominant intermediate product at
the beginning of the reaction in these both systems. In hexane, when both benzonitrile and
N-benzylidenebenzylamine were completely transformed after 24 h, the yield of benzylamine and
dibenzylamine was 72.8% and 26.6%, respectively (entry 4). When 10 MPa CO2 was introduced
into hexane, the yield of benzylamine increased to 94.4%, and the yield of dibenzylamine was
5.5%, it increased slightly during the reaction process (entries 5-8). It suggested also that the
presence of compressed CO2 could prohibit the formation of dibenzylamine.
Fig. 3 shows the evolution of products in ethanol and ethanol–10 MPa CO2. The
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hydrogenation rate of benzonitrile in ethanol was faster than that in ethanol–CO2.
N-benzylidenebenzylamine was also produced as the predominant intermediate product at the
beginning of reaction. In ethanol, when both of the hydrogenation of benzonitrile and
N-benzylidenebenzylamine were completed, the yield of benzylamine and dibenzylamine was
reached to 82.3% and 16.3%, respectively (Fig. 3a). However, when 10 MPa CO2 was introduced
into ethanol, the yield of benzylamine increased to 95.6%, and the yield of dibenzylamine was
2.3% and changed lightly during the reaction process (Fig. 3b). It is similar to the cases above
discussed for the solventless and hexane, that the presence of compressed CO2 retarded the rate of
benzonitrile conversion, inhibited the formation of dibenzylamine and enhanced the yield of
benzylamine.
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Table 1 Hydrogenation of benzonitrile in hexane and hexane–CO2
Conv.
(%)
100
100
100
100
72.0
100
100
100
Yield (%)
BA
Entry
Solvent
t (h)
Rate (h-1)a
BBA
46.4
15.0
11.2
0.6
DBA
9.7
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3
4
5
6
7
8
5 ml hexane
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43.9
59.4 (26.1)
– (9.8)
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15
24
6
66.1
18.9
14.8
26.6
3.9
74.0
– (8.8)
72.8
– (5.4)
5 ml hexane +
10 MPa CO2
42.0
26.1
7.9
26.1
21.4 (7.8)
– (7.8)
16
24
30
69.8
4.1
86.6
5.5
– (6.4)
0.1
94.4
5.5
– (5.6)
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Reaction conditions: 2 ml benzonitrile, 0.1 g Ni/Al2O3, 80 oC, 4 MPa H2. BBA:
N-benzylidenebenzylamine, BA: benzylamine, DBA: dibenzylamine.
a
the reaction rate of benzonitrile hydrogenation, which was given as the overall rate of
benzylamine conversion normalized by the number of active sites over the specified time; values
in parentheses indicate rate of the hydrogenation of benzonitrile into benzylamine. The number of
active sites was calculated using the average Ni diameters (d) via the following equation:
Dispersion = 6(vm/am)/d, where vm and am are equal to 10.95 Å3 and 6.51 Å2 respectively, for Ni.
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Fig. 3. Hydrogenation of benzonitrile in (a) 5 ml ethanol, (b) 5 ml ethanol–10 MPa CO2. Reaction
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conditions: 2 ml benzonitrile, 0.1 g Ni/Al2O3, 4 MPa H2, 80 C, () conversion of benzonitrile,
yield of () N-benzylidenebenzylamine, () benzylamine, () dibenzylamine.
3.2.2. Hydrogenation of benzonitrile in H2O and H2O–CO2
Water is the most abundant, cheap, safe, and environmentally benign solvent in nature and
the study of organic reaction in/on water is an important theme of current research from the point
of green chemistry [16-18]. The pressurization of H2O with CO2 could make it become more
effective in separating and recycling of catalyst, accelerating the rate of reaction, and modifying
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the product selectivity [19-23]. Herein, the combination of H2O and CO2 was first used for the
hydrogenation of benzonitrile to study the effect of H2O and CO2 in this reaction, and the results
are listed in Table 2. The conversion of benzonitrile in H2O–10 MPa CO2 (97.0%) was higher than
that in H2O (82.0%) (entries 1, 5). N-benzylidenebenzylamine was also produced as the
predominant intermediate product at the beginning of reaction, and then it was transformed to
benzylamine and dibenzylamine. The yield of benzylamine in H2O–10 MPa CO2 (51.2%) was
lower than that in H2O (62.1%) but the yield of dibenzylamine in former (37.0%) was little higher
than that in H2O (34.9%) under the complete conversion of benzonitrile and
N-benzylidenebenzylamine (entries 4, 8). In addition, tribenzylamine was produced in H2O–CO2
but not in H2O. The results obtained herein were totally different from the above three systems,
that the presence of compressed CO2 could accelerate the reaction rate and benefit the formation
of dibenzylamine and tribenzylamine in water; it is contradiction to the results obtained in both
hexane and ethanol. This will be discussed from the phase behavior and the reaction pathway later.
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Table 2 Hydrogenation of benzonitrile in H2O and H2O–CO2
t
(h)
1
Conv.
(%)
Yield (%)
Entry Solvent
Rate (h-1)
BBA
79.6
39.2
19.7
0.8
BA
–
DBA TBA BAE BAL
1
2
3
4
5
5 ml
H2O
82.0
100
0.1
–
–
2.3
0.1
–
–
146.0 (–)
– (36.3)
– (31.9)
– (9.2)
2
40.8
53.8
62.1
7.8
19.2
23.8
34.9
22.8
0.7
2.7
2.2
3
100
–
12
1
100
–
–
97.0
51.7
4.5
9.6
0.6 172.7 (13.9)
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5 ml
H2O +
10 MPa
CO2
2.5
5
100
100
43.8
13.8
14.5
42.4
30.9
33.7
9.8
9.6
0.5
–
0.4
0.5
– (10.3)
– (15.1)
8
6.5
100
0.9
51.2
37.0
9.8
–
1.1
– (14.0)
1
2
Reaction conditions: 2 ml benzonitrile, 0.1 g Ni/Al2O3, 80 oC, 4 MPa H2. BBA:
N-benzylidenebenzylamine, BA: benzylamine, DBA: dibenzylamine, TBA: tribenzylamine, BAE:
Benzaldehyde, BAL: Benzyl alcohol.
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3.3. Discussion
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The presence of compressed CO2 could retard the rate of benzonitrile conversion in
solventless, hexane and ethanol, but accelerate the rate in water. This could be explained from the
phase behavior. Fig. 4 shows the photographs of the states of reactant mixture during the reaction
in different reaction systems. In solventless system, the liquid benzonitrile slightly expanded with
the introduction of CO2 and the maximum volume was obtained at 11 MPa CO2, and a solid phase
presented with the introduction of CO2 (Fig. 4a and Fig. S1, S2). In hexane, the liquid benzonitrile
and hexane gradually expanded with the introduction of CO2 and the maximum volume was
obtained at 11~12 MPa CO2, then liquid mixture volume decreased and finally formed a single gas
phase at 13.4 MPa CO2. The product benzylamine completely transformed from liquid to solid in
the presence of CO2 and precipitated out from solution, even though the solution transformed into
a single gas phase at 11 MPa (Fig. 4b and Fig. S3, S4). In ethanol, the liquid benzonitrile and
ethanol gradually expanded with the introduction of CO2 and transformed into a single gas phase
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at 15.6 MPa CO2. The product benzylamine and ethanol gradually expanded with the introduction
of CO2 also and the maximum volume was obtained at 8~11 MPa CO2, then liquid mixture
volume decreased from 12 to 18 MPa CO2. However, the product benzylamine transformed from
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liquid to solid and precipitated out from ethanol when the reactor cooled from 80 C to room
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temperature, depressurizing of CO2. (Fig. 4c and Fig. S5, S6). In water, benzonitrile was
immiscible with water, the liquid benzonitrile and water expanded with the introduction of CO2
and the maximum volume was obtained at 10~14 MPa CO2 (Fig. 4d and Fig. S7). The formed
benzylamine can be miscible with water, the volume of the liquid mixture of benzylamine and
water changed slightly with the introduction of CO2, and benzylamine transformed from liquid to
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solid when the reactor cooled from 80 C to room temperature, depressurizing of CO2 (Fig. S8).
However, dibenzylamine and tribenzylamine were immiscible with water. When CO2 was
introduced into dibenzylamine, the phase behavior didn’t change, even the temperature was cooled
to room temperature and waiting for 24 h, but the viscosity of dibenzylamine increased obviously
which indicated the formation of dibenzylamine carbamate (photographs were not show). So when
the product benzylamine, dibenzylamine and tribenzylamine were mixed with water, two liquid
phases were formed, and the phase behavior didn’t change with the introduction of CO2, but the
product benzylamine transformed from liquid to solid when the reactor cooled from 80 oC to room
temperature, depressurizing of CO2 and waiting for 2 h (Fig. S9). With the introduction of 10 MPa
CO2, the liquid phase of benzonitrile in solventless system, hexane, ethanol and water were
expanded; the product benzylamine was transformed to carbamate and precipitated out in CO2 and
hexane–CO2 but dissolved in ethanol–CO2 and H2O–CO2 under the reaction conditions, but they
precipitated out in ethanol–CO2 and H2O–CO2 also after cooling and depressurizing.
15
Page 15 of 26

1
2
In this work, the presence of compressed CO2 may affect the reaction rate from (1) increasing
the solubility of H2 in the liquid phase (positive); (2) forming carbamate and/or coating on the
surface of catalyst (negative); (3) the dilution effect (negative). With the introduction of CO2, the
positive effect of the increasing of H2 solubility was more significant than the negative one of
dilution effect, but they might be counteracted each other in some cases, and so the reaction rate of
benzonitrile to benzylamine decreased due to the negative effect of the precipitation of carbamate
in solventless and hexane. However, in ethanol and water, the formed carbamate is soluble in the
solution. When 10 MPa CO2 was introduced into ethanol (entries 3 in Table S1), the solution was
largely expanded for large amount of CO2 dissolved in it, therefore the reaction rate decreased due
to the significant dilution effect of CO2. However, H2O could not be expanded due to low
solubility of CO2 in it, and so the reaction rate in water was increased with introduction of CO2,
which is benefited from the enhanced concentration of gaseous reactant H2.
3
4
5
6
7
8
9
10
11
12
13
Before reaction
After reaction
Before reaction
After reaction
(a)
H2
CO2
BN
H2
CO2
BA·CO2
H2
BA
DBA
H2
BN
Solventless system
10 MPa CO2
14
16
Page 16 of 26

Before reaction
After reaction
Before reaction
After reaction
(b)
H2
CO2
BN
H2
BA
DBA
H2
CO2
BA·CO2
hexane
H2
BN
hexane
hexane
hexane
Hexane
Hexane–10 MPa CO2
1
Before reaction
After reaction
Before reaction After reaction Cooling
(c)
H2
BA
DBA
H2
CO2
BN
H2
CO2
BA·CO2
ethanol
H2
BN
ethanol
ethanol
ethanol
Ethanol
Ethanol–10 MPa CO2
2
Before reaction
After reaction
Before reaction After reaction Cooling
(d)
H2
CO2
H2
BA
DBA
H2O
H2
CO2
BN
BA·CO2
DBA·CO2
TBA
H2
BN
H2O
H2O
H2O
H2O
H2O–10 MPa CO2
3
4
Fi
o
g. 4. Phase behavior of the reaction mixture before and after reaction at 80 C in (a) solventless
system (conditions for after reaction: 2.42 ml benzylamine, 1.06 ml dibenzylamine and 1.6 MPa
H2) and 10 MPa CO2 (conditions for after reaction: 3.64 ml benzylamine and 1.6 MPa H2), (b) 8.5
ml hexane (conditions for after reaction: 2.42 ml benzylamine, 1.06 ml dibenzylamine and 1.4
MPa H2) and 8.5 ml hexane–10 MPa CO2 (conditions for after reaction: 3.64 ml benzylamine and
1.4 MPa H2), (c) 8.5 ml ethanol (conditions for after reaction: 2.42 ml benzylamine, 1.06 ml
dibenzylamine and 1.4 MPa H2) and 8.5 ml ethanol–10 MPa CO2(conditions for after reaction:
17
5
6
7
8
9
10
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2
3.64 ml benzylamine and 1.4 MPa H2), (d) 8.5 ml H2O (conditions for after reaction: 1.71 ml
benzylamine, 1.69 ml dibenzylamine and 1.4 MPa H2) and 8.5 ml H2O–10 MPa CO2 (conditions
for after reaction: 1.2 ml benzylamine, 1.53 ml dibenzylamine, 0.62 ml tribenzylamine and 1.4
MPa H2). Conditions for before reaction: 3.4 ml benzonitrile and 4 MPa H2 for all the systems.
3
4
5
6
N-benzylidenebenzylamine was produced as the predominant product at the beginning of
reaction in all the studied systems, then it transformed to benzylamine or/and dibenzylamine. It
has been reported that N-ethylethylimine (CH3CH=NCH2CH3) was produced as intermediate for
the hydrogenation of acetonitrile over the supported nickel catalysts [4, 6]. The yield of
benzylamine increased remarkable with the introduction of CO2 into solventless, hexane and
ethanol. With comparing to solventless system and ethanol, the yields of benzylamine were higher,
but the yields of N-benzylidenebenzylamine 6 were lower in 10 MPa CO2 and in ethanol–CO2 at
similar conversions (Fig. 2, 3). According to scheme 1, once benzylamine was produced, a part of
it was transformed to carbamate 4 by reacting with CO2 [13, 14], carbamate 4 cannot act as a
nucleophile to interact with benzylimine 2 [13], so the formation of intermediate
N-benzylidenebenzylamine 6 was inhibited with the introduction of CO2. However, a major part of
benzylamine was condensed with benzylimine 2 to 1-amino dibenzylamine 5 on the surface of
catalyst. In the absence of CO2, 1-amino dibenzylamine 5 was transformed to benzylamine 3 as
well as to N-benzylidenebenzylamine 6 and then hydrogenated to dibenzylamine 7. In the
presence of CO2, 1-amino dibenzylamine 5 could also be converted to carbamate 8, so the further
deamination of 1-amino dibenzylamine 5 to N-benzylidenebenzylamine 6 and then hydrogenation
to dibenzylamine 7 was inhibited, resulting in high yield of benzylamine in 10 MPa CO2,
18
7
8
9
10
11
12
13
14
15
16
17
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20
21
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1
2
hexane–CO2 and ethanol–CO2. These results could be further confirmed by the reaction of
N-benzylidenebenzylamine 6 as starting reactant (Table 3). In the presence of 4 MPa H2 and 4
MPa NH3, the conversion increased from 35.9% to 72.8% and the selectivity to benzylamine
increased from 87.0% to 94.9% when 10 MPa CO2 was introduced into hexane (entries 1, 2), and
the conversion increased from 91.3% to 94.8% and the selectivity to benzylamine increased from
61.6% to 97.1% when 10 MPa CO2 was introduced into ethanol (entries 3, 4). In the presence of
much excess NH3, N-benzylidenebenzylamine 6 was mainly transformed to 1-amino
dibenzylamine 5, it can be hydrogenated to benzylamine 3 and dibenzylamine 7, and converted to
carbamate 8 in the presence of CO2. The formation of carbamate 8 inhibited the deamination of
1-amino dibenzylamine 5 and the further hydrogenation to dibenzylamine 7, and the desired
product benzylamine can also be converted to carbamate 4. So the yield of benzylamine was high
in the presence of CO2.
3
4
5
6
7
8
9
10
11
12
13
14
Table 3 Hydrogenation of N-benzylidenebenzylamine
Sel. (%)
Entry
Solvent
CO2 (MPa)
Conv. (%)
BA
87.0
94.9
61.6
97.1
DBA
13.0
5.1
1
2
3
4
hexane
hexane
ethanol
ethanol
–
10
–
35.9
72.8
91.3
94.8
38.4
2.9
10
15
16
Reaction conditions: 1.92 g BBA, 0.1 g Ni/Al2O3, 5 ml solvent, 4 MPa H2, 4 MPa NH3, 80 oC, 16
h for hexane and 5 h for ethanol. BBA: N-benzylidenebenzylamine, BA: benzylamine, DBA:
19
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1
2
dibenzylamine.
3
The benzylimine 2 has not been confirmed by direct observation, which is attributed to its
4
high reactivity [24]. Although 1-amino dibenzylamine 5 has never been observed, it is assumed to
be the intermediate according to literature [24, 25]. Carbamate 4 may decompose to benzylamine,
and carbamate 8 and intermediate 5 may decompose to N-benzylidenebenzylamine 6, and
carbamate 9 to dibenzylamine 7 when it dissolve in ethanol or/and during the GC analysis.
The yield of benzylamine decreased with the introduction of CO2 into water. It is
contradiction to the results obtained in solventless, hexane and ethanol. The yield of
dibenzylamine in H2O–10 MPa CO2 was higher than that in H2O, and tribenzylamine was
produced in H2O–CO2 but not in H2O. The reaction pathways of benzonitrile hydrogenation in
H2O–CO2 are proposed in scheme 2. Benzylamine 3, 1-amino-dibenzylamine 5 and
dibenzylamine 7 are produced and could be converted to corresponding carbamate by reacting
with CO2 also. Benzaldehyde is formed from the hydrolysis of benzylimine 2 followed by the
elimination of NH3 [1]. 1-amino-dibenzylamine 5 deaminated to N-benzylidenebenzylamine 6 and
then hydrogenated to dibenzylamine 7. Benzylimine 2 condensed with dibenzylamine 7 to
gem-diamine 14, which hydrogenated and deaminated to tribenzylamine. In H2O–CO2 system, the
acidity of the solution increases in pH value by about 3 at a wide range of pressures and
temperatures [26, 27]. Herein, the acidic nature of H2O–CO2 (pH≈2.9) could accelerate the
elimination of NH3 and form NH4HCO3, so the amount of benzaldehyde, dibenzylamine and
tribenzylamine in H2O–CO2 is higher than that in H2O. In addition, tribenzylamine may be formed
from the condensation of dibenzylamine and benzaldehyde to carbinolamine 13 followed by the
20
5
6
7
8
9
10
11
12
13
14
15
16
17
18
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20
21
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Page 20 of 26

1
2
3
4
5
6
direct hydrogenolysis. The carbinolamine 13 and gem-diamine 14 cannot undergo H2O and NH3
elimination to an imine, because no hydrogen atom is available on the central N atom, so the
direct hydrogenolysis is considered to be responsible for the formation of a tertiary amine in this
reaction [2]. Benzaldehyde could be hydrogenated to benzyl alcohol and condensed with
benzylamine to N-benzylidenebenzylamine 6. Benzylamine cannot be transformed to
dibenzylamine in H2O–CO2 when benzylamine was used as reactant under the similar conditions.
7
8
Scheme 2. Proposed reaction pathways of benzonitrile hydrogenation in H2O–CO2.
9
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1
2
4. Conclusions
3
The selective hydrogenation of benzonitrile to benzylamine over Ni/Al2O3 was discussed in
4
multiphase systems containing compressed CO2. The presence of compressed CO2 retarded the
rate of benzonitrile conversion in solventless, hexane and ethanol, but accelerated the rate in water.
The produced benzylamine was converted to carbamate and precipitated out in CO2 and
hexane–CO2 but dissolved in ethanol–CO2 and H2O–CO2 under the reaction conditions. The
decreasing reaction rate with the introduction of CO2 in solventless and hexane could be mainly
ascribed to the product benzylamine was transformed to carbamate and precipitated out and coated
on the surface of catalyst, but the decreasing reaction rate in ethanol was mainly due to the
dilution effect with the introduction of CO2. The increasing reaction rate in water was ascribed to
the enhanced concentrations of gaseous reactant H2 and benzonitrile in H2O brought by CO2.
While, by contrasting, the yield of benzylamine in solventless, hexane and ethanol increased in the
presence of compressed CO2, and it reached to above 94.4% in these solvents, which was ascribed
to the formation of carbamate could prevent the formation of dibenzylamine. However, the yield
of benzylamine decreased in water with the introduction of CO2, the acidic nature of H2O–CO2
should be the main factor which could accelerate the hydrolysis and the elimination of NH3, thus
reduced the yield of the main product.
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Acknowledgements
The authors gratefully acknowledge the financial supports from NSFC 21273222, 21202159
and 21143001, and from Jilin province, China 20100562.
22
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References
3
[1] C. Debellefon, P. Fouilloux, Catal. Rev. Sci. Eng. 36 (1994) 459-506.
[2] S. Gomez, J.A. Peters, T. Maschmeyer, Adv. Synth. Catal. 344 (2002) 1037-1057.
[3] M.S. Wainwright, Preparation of solid catalysts, Wiley-VCH, Weinheim, 1999.
[4] F.M. Cabello, D. Tichit, B. Coq, A. Vaccari, N.T. Dung, J. Catal. 167 (1997) 142-152.
[5] N.T. Dung, D. Tichit, B.H. Chiche, B. Coq, Appl. Catal. A: Gen. 169 (1998) 179-187.
[6] A.C. Gluhoi, P. Marginean, U. Stanescu, Appl. Catal. A: Gen. 294 (2005) 208-214.
[7] H. Chen, M.W. Xue, J.Y. Shen, Catal. Lett. 135 (2010) 246-255.
[8] C.V. Rode, M. Arai, M. Shirai, Y. Nishiyama, Appl. Catal. A: Gen. 148 (1997) 405-413.
[9] G.R. Akien, M. Poliakoff, Green Chem. 11 (2009) 1083-1100.
[10] M. Arai, S. Fujita, M. Shirai, J. Supercrit. Fluids 47 (2009) 351-356.
[11] S. Fujita, X.C. Meng, F.Y. Zhao, M. Arai, in: M.R. Belinsky (Eds.), Supercritical Fluids,
Nova Science Publishers, Inc., New York, 2010, pp. 609-631.
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
[12] H. Liu, T. Jiang, B. Han, S. Liang, Y. Zhou, Science, 326 (2009) 1250-1252.
[13] X.F. Xie, C.L. Liotta, C.A. Eckert, Ind. Eng. Chem. Res. 43 (2004) 7907-7911.
[14] M. Chatterjee, H. Kawanami, M. Sato, T. Ishizaka, T. Yokoyama, T. Suzuki, Green Chem. 12
(2010) 87-93.
[15] J.L. Chen, Y.D. Li, Z.Q. Li, X.X. Zhang, Appl. Catal. A: Gen. 269 (2004) 179-186.
[16] D. Dallinger, C.O. Kappe, Chem. Rev. 107 (2007) 2563-2591.
[17] A. Chanda, V.V. Fokin, Chem. Rev. 109 (2009) 725-748.
[18] R.N. Butler, A.G. Coyne, Chem. Rev. 110 (2010) 6302-6337.
23
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[19] B.M. Bhanage, Y. Ikushima, M. Shirai, M. Arai, Chem. Commun. (1999) 1277-1278.
[20] S. Fujita, S. Akihara, F.Y. Zhao, R.X. Liu, M. Hasegawa, M. Arai, J. Catal. 236 (2005)
101-111.
3
4
[21] S. Fujita, S. Akihara, M. Arai, J. Mol. Catal. A: Chem. 249 (2006) 223-229.
[22] X.C. Meng, H.Y. Cheng, S. Fujita, Y.C. Yu, F.Y. Zhao, M. Arai, Green Chem. 13 (2011)
570-572.
5
6
7
[23] H.W. Lin, C.H. Yen, C.S. Tan, Green Chem. 14 (2012) 682-687.
[24] Y.Y. Huang, W.M.H. Sachtler, Appl. Catal. A. Gen. 182 (1999) 365-378.
[25] J.J.W. Bakker, A.G. van der Neut, M.T. Kreutzer, J.A. Moulijn, F. Kapteijn, J. Catal. 274
(2010) 176-191.
8
9
10
11
12
[26] K.L. Toews, R.M. Shroll, C.M. Wai, N.G. Smart, Anal. Chem. 67 (1995) 4040-4043.
[27] C. Roosen, M. Ansorge-Schumacher, T. Mang, W. Leitner, L. Greiner, Green Chem. 9 (2007)
455-458.
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24
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Graphical abstract
CO2
CO2
Yield >94.4%
Yield 51.2%
H2, CO2
H2, CO2
H2, CO2
H2, CO2
Benzonitrile
CO2
H2O
CO2
Ni/Al2O3
Benzonitrile
Ethanol
CO2
Benzonitrile
Hexane
CO2
Benzonitrile
CO2
Ni/Al2O3
Ni/Al2O3
Ni/Al2O3
3
4
5
6
25
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3
4
5
6
7
8
9
Highlights
The addition of CO2 remarkably affected the reaction rate and product selectivity.
The decreased reaction rate is due to the formation of solid carbamate in the
presence of CO2.
The yield of benzylamine was increased to > 94.4% in the presence of CO2.
The presence of CO2 could prohibit the formation of dibenzylamine in organic
solvents.
 The acidic nature of H2O–CO2 benefited the elimination of NH3.
10
26
Page 26 of 26