Liquid phase hydrogenation of adiponitrile over directly reduced Ni/SiO2 catalyst

Article abstract of DOI:10.1016/j.catcom.2015.10.021

Liquid phase hydrogenation of adiponitrile (ADN) to 6-aminocapronitrile (ACN) and hexamethylenediamine (HMD) was investigated on Ni/SiO₂ catalysts prepared under different conditions. In this reaction, the highly reactive imine intermediate forms condensation byproducts by reacting with the primary amine products (ACN and HMD). A highly dispersed Ni/SiO₂ catalyst prepared by the direct reduction of Ni(NO₃)₂/SiO₂ was found to suppress the condensation reactions by promoting the hydrogenation of adsorbed imine, and it gave excellent hydrogenation activity and primary amine selectivity. Addition of NaOH increased the primary amine selectivity to 79% at the ADN conversion of 86%.

Full text of DOI:10.1016/j.catcom.2015.10.021

Catalysis Communications 73 (2016) 80–83
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Liquid phase hydrogenation of adiponitrile over directly reduced
Ni/SiO catalyst
2
a
a,b
a,
a
Zekun Jia , Bin Zhen , Minghan Han ⁎, Chengqiang Wang
a
Department of Chemical Engineering, Tsinghua University, Beijing 100084, China
College of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Liquid phase hydrogenation of adiponitrile (ADN) to 6-aminocapronitrile (ACN) and hexamethylenediamine
HMD) was investigated on Ni/SiO catalysts prepared under different conditions. In this reaction, the highly re-
active imine intermediate forms condensation byproducts by reacting with the primary amine products (ACN
and HMD). A highly dispersed Ni/SiO catalyst prepared by the direct reduction of Ni(NO /SiO was found to
suppress the condensation reactions by promoting the hydrogenation of adsorbed imine, and it gave excellent
hydrogenation activity and primary amine selectivity. Addition of NaOH increased the primary amine selectivity
to 79% at the ADN conversion of 86%.
Received 28 August 2015
Received in revised form 13 October 2015
Accepted 14 October 2015
Available online xxxx
(
2
2
)
3 2
2
Keywords:
Adiponitrile hydrogenation
Ni/SiO
-Aminocapronitrile
Hexamethylenediamine
2
© 2015 Elsevier B.V. All rights reserved.
6
1
. Introduction
The selective hydrogenation of adiponitrile (ADN) to 6-
that improving the hydrogenation activity of the catalyst would be an
effective way to suppress the byproducts.
In this work, a highly dispersed Ni/SiO
2
catalyst prepared by
aminocapronitrile (ACN) is the key step in the butadiene
hydrocyanation route for the manufacture of caprolactam [1–4]. It is cat-
alyzed by Raney Ni catalysts [5,6] prepared by removing the aluminum
from binary Al–Ni alloys using an aqueous alkaline solution. This catalyst
preparation process consumes a lot of energy and causes environmental
pollution. It is also well known that Raney type catalysts have low
mechanical strength, which leads to much catalyst consumption and
they are pyrophoric and cannot be exposed to air. Due to these
drawbacks, many attempts have been made to replace Raney Ni by
a supported Ni catalyst where the main challenge is to achieve a
satisfactory ACN selectivity at high ADN conversion [7,8].
the direct reduction of a Ni(NO precursor [12] was used to promote
3 2
)
the hydrogenation of the imine intermediate, thus increasing both the
catalytic activity and primary amine selectivity. The influence of the
addition of NaOH was also studied.
2. Experimental
2.1. Catalyst preparation
The preparation of the supported metal catalyst by incipient wetness
impregnation comprised three steps: impregnation, calcination and
reduction. In this work the Ni/SiO catalyst prepared by the direct
reduction (DR) of Ni(NO /SiO in H was denoted as NiDR/SiO . For
comparison, another catalyst prepared by calcination and reduction
CR) was denoted as NiCR/SiO . They were prepared as follows. The
loading amount of nickel was 20 wt.%.
NiDR/SiO : First, SiO (Alfa Aesar) was soaked in an aqueous solution
of Ni(NO (98%, Alfa Aesar) at room temperature. Then, the mixture
was treated by ultrasonic waves 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.
NiCR/SiO : The impregnated and dried Ni(NO /SiO was calcined at
50 °C for 4 h and then reduced in H flow. The other steps were the
same as those for NiDR/SiO . Before reduction, the calcined (C) catalyst
was denoted as Ni /SiO
A general reaction scheme for the hydrogenation of ADN is given
in Fig. 1 [9–11]. ADN is selectively hydrogenated to ACN and then to
hexamethylenediamine (HMD) which is used for producing Nylon-
2
3
)
2
2
2
2
6
,6. During the sequential hydrogenation, a highly reactive imine inter-
(
2
mediate 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 the side reactions. From the chemistry, it can be seen
2
2
3 2
)
2
2
)
3 2
2
4
2
⁎
Corresponding author.
2
E-mail address: hanmh@tsinghua.edu.cn (M. Han).
C
2
.
http://dx.doi.org/10.1016/j.catcom.2015.10.021
566-7367/© 2015 Elsevier B.V. All rights reserved.
1

Z. Jia et al. / Catalysis Communications 73 (2016) 80–83
81
Fig. 1. Mechanism of ADN hydrogenation.
2
.2. Catalyst characterization
The XRD patterns of the catalysts are shown in Fig. 2. Ni
NiCR/SiO
tallites. Louis et al. [14] investigated the Ni/SiO
C
/SiO
2
and
2
gave very sharp peaks, which indicated large NiO and Ni crys-
catalyst and found out
The specific surface area and pore structure of the support and two
2
catalysts were determined by N
instrument. The crystalline phase of the catalysts was characterized by
a Bruker Advance D8 X-ray diffractometer with a Cu Kα radiation
2 2
source. The H uptake of the catalysts was determined by H chemisorp-
2
adsorption with a Quadrasorb SI
the existence of nickel phyllosilicates, which were located at the surface
of silica and acted as the anchoring sites for the NiO and Ni particles. Cal-
cination led to the decomposition of the nickel phyllosilicates and the
migration of NiO, which resulted in large Ni particles after reduction.
tion on a Quantachrome ChemBET Pulsar TPR/TPD instrument. The
active Ni area was calculated assuming H/Nisurface = 1 and a surface
2
In the case of NiDR/SiO , NiO was reduced to Ni before the decomposi-
tion of the anchoring sites. The broader diffraction peaks of the Ni
phase indicated smaller Ni crystallite sizes [15]. In this moderate reduc-
tion process, the Ni species were not completely reduced to the metallic
−
20
2
area of 6.5 × 10
m per Ni atom [13]. A JEM-2010 transmission
electron microscope (TEM, JEOL Ltd., Tokyo, Japan) were employed to
examine the Ni particle size of the catalysts.
state and some NiO reflexes were present in the XRD pattern of NiDR
SiO . However, the H uptake by NiDR/SiO (76.2 μmol/g) was five
times that of NiCR/SiO (15.4 μmol/g), corresponding to the active nickel
/
2
2
2
2
.3. Catalytic reaction
2
2
areas of 6.0 and 1.2 m /g (Table 1). The NiDR/SiO
excellent H chemisorption ability due to its superior Ni dispersion.
The TEM images of the two catalysts are presented in Fig. 3. The sizes
of most of the Ni particles on NiDR/SiO were around 10 nm, while those
on NiCR/SiO were much larger, which was in accordance with the XRD
and H uptake results.
2
catalyst exhibited
The hydrogenation of ADN was carried out in a stainless steel auto-
2
clave equipped with a temperature control system and magnetic stirrer.
Typically, 5 g ADN (98%, Alfa Aesar), 80 mL of methanol (N99.5%, Beijing
Chemical Works), 5 g pre-reduced catalyst and 0.1 g NaOH (N96.0%, Bei-
jing Chemical Works) were added into the reactor. The hydrogenation
conditions were: 80 °C, 3 MPa, 500 rpm. The products were sampled on-
line and were analyzed by a gas chromatograph (GC 7890F, Techcomp
Instrument Company) 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 secondary and tertiary amines with a
high molecular weight which were not eluted in the GC. Dimethyl
phthalate was used as the internal standard to determine the content
of ADN, ACN and HMD. The conversion of ADN and the selectivity to
ACN and HMD were calculated as:
2
2
2
3.2. Catalytic hydrogenation of ADN
Fig. 1 showed that ADN was selectively hydrogenated to ACN and
then to HMD. The change in the concentration of ACN with time showed
the presence of a consecutive reaction mechanism and the maximum
ACN yield occurred at a particular reaction time [11]. Since both ACN
and HMD are useful products, their selectivity should be a target used
to evaluate the catalytic performance.
Table 2 shows the results from the hydrogenation of ADN over NiDR
SiO and NiCR/SiO . As seen in Group 3 and 4, NiDR/SiO exhibited
superior hydrogenation activity with 86% ADN conversion in 80 min
compared to NiCR/SiO with 82% ADN conversion after a reaction time
/
moles of converted ADN
moles of ADN feedstock
2
2
2
ADN conversion ¼
ACN selectivity ¼
HMD selectivity ¼
ꢀ 100%:
ꢀ 100%:
ꢀ 100%:
ð1Þ
ð2Þ
ð3Þ
2
of 360 min. The selectivity to the primary amine products (ACN and
HMD) over NiDR/SiO was also higher than that over Ni /SiO because
moles of ACN
moles of converted ADN
2
CR
2
of less condensation byproduct formation. HMI was formed on both
catalysts, but its selectivity over NiDR/SiO was higher than that over
NiCR/SiO , which was different from the selectivity to the condensation
2
moles of HMD
moles of converted ADN
2
byproducts.
Table 1
3
. Results and discussion
.1. Catalyst characterization
Table 1 summarized the physical properties of SiO
Surface area, pore structure and metallic Ni surface area of SiO
2 2 2
, NiCR/SiO and NiDR/SiO .
Sample
S
/
BET
Pore volume
Pore size
/nm
Surface area of Ni
3
m2 g−1
/cm
3
g
−1
/m g
2
−1
2
and two
SiO
NiCR/SiO2
NiDR/SiO
2
245
188
203
0.9
0.7
0.7
17.8
17.7
17.4
2
1.2
6.0
2
catalysts. The SiO had a specific surface area of 245 m /g and a pore
volume of 0.9 cm /g, which decreased after the introduction of Ni.
3
2

82
Z. Jia et al. / Catalysis Communications 73 (2016) 80–83
Table 2
Catalytic performance of NiCR/SiO
2
and NiDR/SiO
2
catalysts.
Groupa Time Conversion Selectivity/%
/min /%
ACN HMD HMIb Condensation byproductsc
1
2
3
4
360
100
360
80
55
85
82
86
41
55
50
66
1
12
4
8
21
9
50
12
37
3
13
18
a
Group 1 catalyzed by NiCR/SiO
without NaOH addition. Group 3 catalyzed by NiCR/SiO
with 0.1 g NaOH addition.
Calibration factor for HMI and the internal standard was set at 1.
Calculated based on that the selectivity to all components was 100%.
2
without NaOH addition. Group 2 catalyzed by NiDR
/
SiO
2
2
with 0.1 g NaOH
addition. Group 4 catalyzed by NiDR/SiO
2
b
c
2 2
at the same time. On the other hand, since NiDR/SiO has a superior H
adsorption activity versus NiCR/SiO , it adsorbed more electron-rich im-
2
ines. The adsorbed imino C atom was subject to a nucleophilic attack by
an amino N atom, resulting in the formation of a secondary imine which
was further hydrogenated to HMI [11,13]. Consequently, the selectivity
2 C 2 2 2
Fig. 2. XRD pattern of SiO , Ni /SiO , NiCR/SiO and NiDR/SiO .
2 2
to HMI over NiDR/SiO was higher than that over NiCR/SiO .
It has been demonstrated that the condensation reactions occur
on the catalyst surface and not homogeneously in the liquid phase
16,17]. To minimize these, much work has been done on modifying
the catalysts, such as decreasing catalyst acidity by doping with K or
using alkaline MgO support [8,13,18,19]. Although these methods
achieved good primary amine selectivity, it was at the cost of decreasing
The effect of NaOH on the hydrogenation activity and product selec-
tivity was also investigated. NaOH is a general and effective additive
used in ADN hydrogenation catalyzed by Raney Ni in a solvent such as
methanol [20,21]. As seen in Table 2, the addition of NaOH improved
the catalytic activity and the primary amine selectivity of both catalysts.
Since amines are more strongly adsorbed on the catalyst surface than
nitriles, they retard the hydrogenation of nitriles [22]. NaOH benefited
the desorption of amines, which not only promoted nitrile hydrogena-
tion but also inhibited the condensation reactions between the amines
and imines. Therefore, when NaOH was introduced, the ADN conversion
[
2
the catalytic activity. In this work, the NiDR/SiO catalyst that exhibited a
superior catalytic performance to NiCR/SiO indicated that good hydro-
2
genation activity and a high primary amine selectivity were not incom-
patible. The imine adsorbed on the catalyst surface reacted by either
consecutive hydrogenation to primary amines or by condensation reac-
tions with the primary amines formed. These are two parallel reactions.
over NiCR/SiO
facile desorption of the primary amines helped decrease the selectivity
to the condensation byproducts from 50% to 37% over NiCR/SiO and
from 12% to 3% over NiDR/SiO
2
increased from 55% to 82% in the same reaction time. The
The highly dispersed Ni particles and excellent H
2
uptake ability of NiDR
/
2
SiO promoted the hydrogenation to inhibit the condensation reactions
2
2
.
4. Conclusion
The liquid phase hydrogenation of ADN was investigated on NiCR
/
SiO
2
and NiDR/SiO catalysts and the conclusions were:
2
(
2 2
1) NiDR/SiO exhibited superior hydrogenation activity to NiCR/SiO ,
which was due to its higher Ni dispersion and therefore smaller
Ni particles.
(
2) In the reaction, the imine intermediate reacted by three parallel
reactions: hydrogenation, intramolecular cyclization and inter-
2
molecular condensation. The highly reactive NiDR/SiO promoted
the hydrogenation of adsorbed imines, thus inhibiting the
condensation reactions and increasing the selectivity to primary
amines.
(
(
2
3) The formation of HMI was promoted on NiDR/SiO by the strong
adsorption between the Ni particles and electron rich imines.
4) The addition of NaOH increased the hydrogenation activity
and primary amine selectivity. The use of the combination of
2
NiDR/SiO and NaOH increased the primary amine selectivity to
7
9% at the ADN conversion of 86%.
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