Supported Ni catalyst for liquid phase hydrogenation of adiponitrile to 6-Aminocapronitrile and hexamethyenediamine

Article abstract of DOI:10.3390/molecules23010092

Supported Ni catalysts prepared under different conditions, for liquid phase hydrogenation of adiponitrile (ADN) to 6-aminocapronitrile (ACN) and hexamethyenediamine (HMD), were investigated. The highly reactive imine intermediate can form condensation byproducts with primary amine products (ACN and HMD), which decreased the yield coefficient of primary amines. The catalysts support, condition of catalyst preparation and dosage of additive were studied to improve the yield. A highly dispersed Ni/SiO₂ catalyst prepared by the direct reduction of Ni(NO₃)₂/SiO₂ suppressed the condensation reactions by promoting the hydrogenation of adsorbed imines, and it gave the improved hydrogenation activity of 0.63 mol·kg_cat^?1·min^?1 and primary amine selectivity of 94% when NaOH was added into the reactor.

Full text of DOI:10.3390/molecules23010092

molecules
Article
Supported Ni Catalyst for Liquid Phase
Hydrogenation of Adiponitrile to
6
-Aminocapronitrile and Hexamethyenediamine
1
1
1,2
and Minghan Han 1,*
Chengqiang Wang , Zekun Jia , Bin Zhen
1
Department of Chemical Engineering, Tsinghua University, Beijing 100084, China;
thuwcq@163.com (C.W.); jiazekun1991@163.com (Z.J.); zhenbin@tjut.edu.cn (B.Z.)
College of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, China
Correspondence: hanmh@tsinghua.edu.cn; Tel.: +86-10-6278-1469
2
*
Received: 25 November 2017; Accepted: 30 December 2017; Published: 4 January 2018
Abstract: Supported Ni catalysts prepared under different conditions, for liquid phase hydrogenation of
adiponitrile (ADN) to 6-aminocapronitrile (ACN) and hexamethyenediamine (HMD), were investigated.
The highly reactive imine intermediate can form condensation byproducts with primary amine products
(
ACN and HMD), which decreased the yield coefficient of primary amines. The catalysts support, condition
of catalyst preparation and dosage of additive were studied to improve the yield. A highly dispersed
Ni/SiO catalyst prepared by the direct reduction of Ni(NO ) /SiO suppressed the condensation reactions
2
3 2
2
by promoting the hydrogenation of adsorbed imines, and it gave the improved hydrogenation activity of
.63 mol·kgcat− ·min and primary amine selectivity of 94% when NaOH was added into the reactor.
1
−1
0
Keywords: supported Ni catalyst; adiponitrile hydrogenation; 6-aminocapronitrile; hexamethyenediamine
1
. Introduction
The hydrogenation of ADN to ACN and HMD is an important industrial process [
1–4], which is
mainly catalyzed by Raney Ni catalysts [ ]. Although the skeleton structure of Raney Ni catalysts
5,6
favors a high catalytic activity, it causes low mechanical strength and bad regeneration capability.
In addition, a safety hazard also exists as the Raney Ni catalysts are flammable when exposed to air.
Due to these drawbacks, many researchers have been working on developing supported Ni catalysts
to replace Raney Ni. The use of a supported Ni catalyst can dramatically reduce the cost of the catalyst,
making the process easy to operate and improving the safety of the hydrogenation reaction. The main
challenge is that so far, the supported Ni catalysts are inferior in terms of catalytic activity and primary
amine selectivity [7,8].
The reaction scheme for the hydrogenation of ADN is given in Figure 1 [9–11]. ADN is hydrogenated
to ACN and then to HMD. During the sequential hydrogenation, a highly reactive imine intermediate
is formed, which has a strong tendency towards intermolecular condensation with the primary amine
products (ACN and HMD) and intramolecular cyclization to undesired condensation byproducts and
hexamethyleneimine (HMI). The formation of the primary amines has to compete with these side reactions.
From the chemistry, it can be seen that increasing the hydrogenation activity of the catalyst would be an
effective way to suppress the byproducts.
In this work, supported Ni catalysts were studied from several aspects, such as the type of
support, conditions of catalyst preparation and dosage of additive, in order to increase the yield of
primary amines. A highly dispersed Ni/SiO catalyst prepared by the direct reduction of a Ni(NO )
2
3 2
precursor [12] was found able to promote the hydrogenation of the imine intermediate so that its
performance was close to that of Raney Ni.
Molecules 2018, 23, 92; doi:10.3390/molecules23010092
www.mdpi.com/journal/molecules

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Figure 1. Mechanism of ADN hydrogenation.
2
. Results and Discussion
2
.1. Catalyst Support
Since the catalyst support has an important influence on the catalytic performance, the effect of
different supports was investigated. Many oxides can be used as the support, among which Al O
2
3
and SiO are the most widely used. The characterization result of the specific surface area and pore
2
structure of the Al O , SiO and Ni/Al O , Ni/SiO used are shown in Table 1. The evaluation result
2
3
2
2
3
2
in Table 2 showed the Ni/SiO was better than Ni/Al O in terms of the selectivity of primary amines
2
2
3
and condensation byproducts.
Table 1. Specific surface area and pore structure of the supports and the catalysts.
−1
3
−1
Sample
Al O
Specific Surface Area/m2g
Pore Volume/cm ·g
Average Pore Size/nm
242
175
245
190
0.64
0.41
0.94
0.72
9.9
9.5
17.8
17.4
2
3
Ni/Al O3
2
SiO2
Ni/SiO2
Table 2. Evaluation reaction result of Ni/SiO and Ni/Al O .
2
2
3
Selectivity/%
Catalytic Activity/mol·kgcat−1·min−1
Catalysts
ACN HMD HMI
Condensation Byproducts
Ni/SiO2
0.25
0.29
50
43
4
4
9
8
37
45
Ni/Al O3
2
Using the bifunctional mechanism [13], it is believed that in a nitrile hydrogenation system,
the hydrogenation reactions are catalyzed by metal sites while condensation reactions are catalyzed
by acid sites. This would indicate that the acidic properties of the support are crucial to the catalytic
performance. A stronger acidity favors the conversion of nitrile but decreases the selectivity to primary
amines. The NH -TPD data for Al O and SiO and their corresponding catalysts are presented in
3
2
3
2
◦
Figure 2a. Al O has a large NH desorption peak in the temperature range from 100 to 500 C while
2
3
3
SiO does not. After the introduction of the Ni, Ni/Al O still showed an obvious NH desorption
2
2
3
3
peak in contrast to Ni/SiO as shown in Figure 2b. The acid sites can absorb imine intermediate and
2
primary amine and cause them to react with each other so that condensation products are produced,
which lead to the decreased selectivity of the target products.

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(
a)
(b)
Figure 2. NH -TPD of (a) Supports and (b) Catalysts.
3
Therefore, SiO is the more suitable support for the hydrogenation of ADN to ACN and HMD,
2
and SiO was used as the support in the following study.
2
2
.2. Direct Reduction Method
The supported Ni catalyst was prepared by the wetness incipient impregnation method,
which consists of three steps: impregnation, calcination and reduction. After calcination and reduction,
the Ni(NO ) loaded on the supports became metal particles with catalytic activity. The calcination
3
2
and reduction are key steps and have important influence on the catalytic performance.
Table 3 shows mean particle sizes of Ni estimated by the two independent methods, TEM and
XRD analysis [14]. Figure 3 showed the TEM images of two catalysts. The particle size of nickel on the
NiDR/SiO is around 7 nm, while that on the Ni /SiO is much larger. XRD analysis results of the
2
CR
2
catalysts show the similar information, 6.4 and 17.7 nm, respectively.
Table 3. Ni particle size of NiCR/SiO and Ni /SiO from TEM and XRD analysis.
2
DR
2
1
2
Sample
Ni Particle Size(a) /nm
Ni Particle Size(b) /nm
NiCR/SiO2
NiDR/SiO2
19
7
17.7
6.4
1
Estimated by TEM analysis. 2 Calculated by XRD analysis according to Scherrer equation.
(
a)
(b)
Figure 3. TEM images of (a) NiCR/SiO and (b) NiDR/SiO2.
2

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The H uptake was 76.2
corresponding to the active nickel areas of 30.0 and 6.0 m /g [15]. We can conclude that NiDR/SiO2
µ
mol/g by NiDR/SiO while that by Ni /SiO was 15.4
µ
mol/g,
2
2
CR
2
2
had a superior Ni dispersion of 4.50% while that of NiCR/SiO is 0.90%, which is in accordance with
2
◦
the TEM and the XRD analysis. Ni(NO ) /SiO catalyst precursor would be totally reduced at 700 C
3
2
2
from H -TPR profiles. The H -chemisorption data showed the H adsorption capacity of a catalyst
2
2
2
◦
partially reduced at 450 C. This means that H -chemisorption data here cannot be used to calculate
2
the mean particle size accurately.
Figure 4 showed the XRD patterns of the catalysts. The characteristic diffraction peaks of NiO
in Ni /SiO are obvious, which indicated that the nickel mainly existed in the form of nickel oxide.
C
2
After the reduction in H , Ni /SiO showed a very sharp Ni peak while Ni /SiO showed both
2
CR
2
DR
2
NiO and Ni diffraction peaks. Louis et al. [16] have studied supported Ni catalyst and found existence
of nickel phyllosilicates, which were located at the surface of catalyst precursor after nickel nitrate
impregnated onto silica. The nickel phyllosilicates act as anchoring sites for metallic Ni particles
and suppress the formation of large Ni particles. However, calcination before reduction leads to
the formation of larger metal particles and to their aggregation. This is probably due to the partial
decomposition of nickel phyllosilicates during calcination, the breaking of the anchoring sites for
impregnated nickel, which favors nickel migration. Direct reduction of Ni(NO ) /SiO was a moderate
3
2
2
reduction process, which sacrificed the reduction degree but gave a superior Ni dispersion.
Figure 4. XRD pattern of SiO , Ni /SiO , Ni /SiO and NiDR/SiO2.
2
C
2
CR
2
As described above, the two catalysts prepared by the different methods had different
microstructures, which probably caused their catalytic performance for hydrogenation of ADN to be
different. Table 4 shows the evaluation result of the catalysts. Ni /SiO exhibited a better hydrogenation
DR
2
−1
−1
−1 −1
min . The selectivity
activity of 0.56 mol
·kgcat
·
min compared to NiCR/SiO with 0.25 mol
to the primary amine products over Ni /SiO was higher than that over Ni /SiO , which, respectively,
·
kgcat
·
2
DR
2
CR
2
were 79% and 54%. Meanwhile, the selectivity to condensation byproducts were inhibited over NiDR/SiO2,
which decreased from 37% to 3%. In the sequential reaction, shown in Figure 1, the highly reactive
imine intermediate can further react along three pathways: further hydrogenation, intramolecular
condensation to HMI, and intermolecular to condensation byproducts. The three pathways are parallel,
that is, the formation of primary amine competes with the side reactions. NiDR/SiO exhibited a
2
superior hydrogenation activity, which promoted the hydrogenation and improved the selectivity to ACN
and HMD.

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Table 4. Evaluation reaction result of NiCR/SiO and NiDR/SiO2.
2
Selectivity/%
Catalytic Activity/Mol·kgcat−1·min−1
Catalysts
ACN HMD HMI
Condensation Byproducts
NiCR/SiO2
NiDR/SiO2
0.25
0.50
50
66
4
13
9
18
37
3
The condensation byproducts were secondary and tertiary amines with a high molecular weight
which were not eluted and cannot be detected in the GC analysis. The product mixture was analyzed
by a mass spectrometer to confirm the existence of condensation byproducts. The result is shown in
Figure 5. The mass to charge ratio of protonated HMI, ACN and HMD are, respectively, m/z = 100,
1
13, 117. The peaks of m/z = 200~300 indicated the formation of C12 and C18 oligomers. These showed
clearly that the condensation byproducts from NiDR/SiO was decreased, which verified the GC
2
evaluation result.
(
a)
(b)
Figure 5. Mass spectra of (a) NiCR/SiO and (b) NiDR/SiO2.
2
NiDR/SiO had a higher H adsorption capacity from H uptake result, which would indicate
2
2
2
that amines and imine intermediates are more abundantly adsorbed on NiDR/SiO . The C atoms of
2
imines were subjected to nucleophilic attack by N atoms of the amines [17], leading to the formation
of a secondary imine which can be further hydrogenated to HMI. NiDR/SiO was more selective
2
for intramolecular side reaction. As seen in Table 4, the selectivity to HMI over NiCR/SiO was 9%
2
while that over NiDR/SiO was 18%. From this point, further developing of the supported Ni catalyst
2
prepared by the direct reduction method is still necessary. The conditions of catalyst preparation and
reaction conditions have to be optimized to inhibit the formation of HMI and improve the selectivity
to primary amines.
2
.3. Reduction Teamperature
The microstructure of supported Ni catalyst prepared by direct reduction method is affected by
reduction temperature significantly. The TEM images of catalysts reduced at different temperature
are shown in Figure 6. The mean particle sizes of Ni became larger with the increase of reduction
temperature obviously.

Molecules 2018, 23, 92
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(
a)
(b)
(
c)
(d)
◦ ◦
) 350 C; (b) 400 C; (c) 450 C;
◦
Figure 6. TEM images of the catalysts reduced at different temperatures: (
a
◦
(
d) 500 C.
The H -TPR profiles of Ni(NO ) /SiO were given in Figure 7. The TPR profiles of the impregnated
2
3 2
2
catalysts, performed without any previous calcination, exhibit three peaks of hydrogen consumption at
◦
3
00, 400, and about 550 C, attributed to the decomposition of nickel nitrate into NiO, the reduction of NiO
◦
and the reduction of nickel phyllosilicates. After TPR up to 700 C, the nickel is totally reduced. Before
complete reduction, the nickel phyllosilicates are probably located at the interface between silica and the
remaining nickel, for which they act as anchoring sites. During TPR, after nitrate decomposition, the NiO
particles located on the phyllosilicates are reduced into Ni at 400 C without any migration. Between 400
and 700 C, the increase in metal particle size is due not only to NiO migration induced by thermal effect
0
◦
◦
but also to the reduction of the phyllosilicates by the weakening of the anchoring strength.
The H -TPR profiles of Ni(NO ) /SiO are in accordance with the TEM analysis of the four
2
3 2
2
◦
catalysts reduced at 350, 400, 450 and 500 C, respectively.
Figure 7. H -TPR of catalyst precursors.
2

Molecules 2018, 23, 92
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Figure 8 presents the evaluation result of the four catalysts. The catalytic activity first decreased
and then increased with the increase of reduction temperature, which can be explained by the reduction
degree and Ni dispersion. With the increase of reduction temperature, the reduction degree of the
catalysts became higher while the Ni dispersion became lower because the nickel phyllosilicates were
◦
gradually decomposed. Ni/SiO reduced at 350 C had a high hydrogenation activity because of its
2
◦
◦
high Ni dispersion. When the reduction temperature was increased to 400 C and 450 C, the migration
and aggregation of Ni particles led to a lower metallic Ni dispersion, which caused the catalytic activity
◦
decrease. Although the Ni dispersion of Ni/SiO reduced at 500 C became even lower, the catalytic
2
activity became higher because of its higher reduction degree. In conclusion, Ni dispersion was the key
◦
◦
factor to affect the catalytic activity for the reduction temperature range from 350 C to 450 C while the
◦
◦
reduction degree was the key factor in the temperature range from 450 C to 500 C, which explained
the variation of catalytic activity.
Figure 8. Evaluation result of catalysts reduced at different temperatures.
◦
4
50 C was considered the optimum temperature for the reduction when both catalytic activity
and selectivity to HMI were taken into account.
2
.4. Additive
Most of the side reactions are deamination reactions as shown in Figure 1. Thermodynamically,
adding an alkaline substance as additive to the liquid reaction system would benefit the catalytic
activity and selectivity to primary amines [ 18]. In ADN hydrogenation, H and nitrile adsorbed on
5,
2
metallic Ni first and then they reacted with each other to imine intermediates and amines. Imines and
amines desorbed from the catalyst with difficulty [19], which would not only suppress the following
hydrogenation of nitriles, but also promote the condensation side reactions. The addition of an alkaline
substance increases the electron density of metallic Ni on the catalyst surface, which would benefit the
desorption of imine and amine [20].
NaOH was chosen as an additive to improve the NiDR/SiO catalyst. The evaluation result
2
is presented in Figure 9 and Table 5. The mass ratio NaOH/Ni was used as the independent
−
1
−1
variable. With NaOH/Ni increasing, the catalytic activity rose from 0.39 mol
·
kgcat
·
min to
min and the selectivity to condensation byproducts decreased from 12% to 1%
while that to HMI decreased from 21% to 5%. When the NaOH/Ni was raised to 0.5, the primary
−
1
−1
0.63 mol
·
kgcat
·

Molecules 2018, 23, 92
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selectivity was as high as 94%. Clearly, the effect of NaOH on the catalytic activity and product
selectivity was positive.
Table 5. Evaluation reaction result of the catalysts with different NaOH/Ni mass ratio.
Selectivity/%
Catalytic Activity/mol·kgcat−1·min−1
NaOH/Ni
ACN
HMD
HMI
Condensation Byproducts
0
0.39
0.50
0.56
0.63
55
66
70
70
12
13
21
24
21
18
7
12
3
2
0
0
0
.1
.3
.5
5
1
Figure 9. Evaluation result of catalysts with the use of different NaOH/Ni ratios.
The addition of NaOH improved the hydrogenation activity and improved the target products
selectivity significantly. After the introduction of NaOH, the performance of NiDR/SiO , reduced at
2
◦
4
50 C, is as excellent as Raney Ni, which is presented in Table 6.
Table 6. Evaluation reaction result of the NiDR/SiO and Raney Ni.
2
Selectivity/%
Catalytic Activity/mol·kgcat−1·min−1
Catalysts
ACN
HMD
HMI
Condensation Byproducts
Raney Ni
NiDR/SiO2
0.61
0.63
78
70
17
24
1
5
4
1
3
. Materials and Methods
3
.1. Catalyst Preparation
The supported Ni catalysts were prepared by incipient wetness impregnation, which included
three steps: impregnation, calcination and reduction. Ni/SiO catalyst prepared by the direct reduction
2
(
DR) of Ni(NO ) /SiO in H was denoted as Ni /SiO . For comparison, another catalyst prepared by
3 2 2 2 DR 2
calcination and reduction (CR) was denoted as NiCR/SiO . They were prepared as follows. The loading
2
amount of nickel was 20 wt.%.

Molecules 2018, 23, 92
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NiDR/SiO : First, SiO (Alfa Aesar, Haverhill, MA, USA) was soaked in an aqueous solution of
2
2
Ni(NO ) (98%, Alfa Aesar) at room temperature. Then, the mixture was treated with ultrasonic waves
3
2
◦
◦
for 2 h and dried at 80 C for 12 h. Finally, the catalyst precursor was reduced in H flow at 450 C for 8 h.
2
◦
NiCR/SiO : The impregnated and dried Ni(NO ) /SiO was calcined at 450 C for 4 h and
2
3 2
2
then reduced in H flow. The other steps were the same as those for Ni /SiO . Before reduction,
2
DR
2
the calcined (C) catalyst was denoted as Ni /SiO .
C
2
3
.2. Catalyst Characterization
The specific surface area and pore structure of support and catalyst were determined by N adsorption
2
with a Quadrasorb SI instrument. The crystalline phase of the catalysts was characterized by a Bruker
Advance D8 X-ray diffractometer with a Cu K radiation source. The H uptake of the catalysts was
determined by H chemisorption on a Quantachrome ChemBET Pulsar TPR/TPD instrument. The active
α
2
2
−20
2
Ni area was calculated assuming H/Nisurface = 1 and a surface area of 6.5
×
10
m per Ni atom.
A JEM-2010 transmission electron microscope was employed to examine the Ni particle size of catalysts.
Some product samples were analyzed by mass spectrometry (MS model: Q Exactive) to identify the relative
amount of condensation byproducts.
3
.3. Catalytic Reaction
The hydrogenation of ADN was carried out in a stainless-steel autoclave equipped with a
temperature control system and magnetic stirrer. Typically, 5 g ADN (98%, Alfa Aesar, Haverhill, MA,
USA), 80 mL of methanol (>99.5%, Beijing Chemical Works, Beijing, China), 5 g pre-reduced catalyst
and 0.1 g NaOH (>96.0%, Beijing Chemical Works) were added into the reactor. The hydrogenation
◦
conditions were: 80 C, 3 MPa, 500 rpm. The products were sampled online and analyzed by a gas
chromatograph (GC 7890F, Techcomp Instrument Company, Kwai Chung, Hong Kong) equipped
with a flame ionization detector and a KB-624 capillary column (30 m
×
0.32 mm
×
1.8 µm, Kromat).
The condensation byproducts were high molecular weight secondary and tertiary amines which were
not eluted in the GC. Phenylamine was used as the internal standard to determine the content of ADN,
ACN, HMD and HMI. The catalytic activity, the conversion of ADN and the selectivity to ACN and
HMD were calculated as:
moles of converted ADN
Catalytic Activity =
(1)
Time × mCat
moles of converted ADN
moles of ADN feedstock
ADN Conversion =
ACN Selectivity =
HMD Selectivity =
× 100%
× 100%
× 100%
(2)
(3)
(4)
moles of ACN
moles of converted ADN
moles of HMD
moles of converted ADN
4
. Conclusions
Supported Ni catalysts for the liquid phase hydrogenation of ADN were investigated. The following
conclusions are based on the results and discussion:
1
.
The support affects the catalytic performance of supported Ni catalysts by providing original
acid sites, which promote the condensation side reactions and decrease the selectivity to primary
amines. SiO is chosen as the support in this work providing less original acid sites.
2
2
.
The catalyst prepared by the direct reaction method, without a prior calcination step, gave smaller
Ni particles, this is, a higher Ni dispersion and H uptake capacity, which improved the catalytic
2
activity and inhibited the intermolecular condensation side reaction.

Molecules 2018, 23, 92
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3
4
.
.
The reduction temperature had an appreciable effect on the catalytic performance of NiDR/SiO2.
◦
450 C was regarded as a preferable temperature to suppress the intramolecular condensation
reaction to HMI.
The addition of NaOH into the reaction system improved the hydrogenation activity and
significantly improved the target products selectivity. When the NaOH/Ni mass ratio was
increased to 0.5, the selectivity to primary amines attained 94%.
Author Contributions: M.H., Z.J. and C.W. conceived and designed the experiments; Z.J. and C.W. performed
the experiments; Z.J. and C.W. analyzed the data; B.Z. and C.W. wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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