Controllable synthesis of bis[3-(dimethylamino)propyl]amine over Cr and Co double-doped Cu/Γ-Al2O3

Article abstract of DOI:10.1016/j.catcom.2018.03.031

The continuous synthesis of bis[3-(dimethylamino)propyl]amine (BPA) by hydrogenation of 3-(dimethylamino)-propionitrile (PN) in the presence of 3-(N,N-dimethylamino)propylamine (PA) over copper-based catalysts was investigated. It was found that Cu-Cr

Full text of DOI:10.1016/j.catcom.2018.03.031

CatalysisCommunications111(2018)64–69
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Catalysis Communications
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Short communication
Controllable synthesis of bis[3-(dimethylamino)propyl]amine over Cr and
Co double-doped Cu/γ-Al2O3
⁎
⁎
Chenhui Lina,b,1, Jiayi Lia,b,1, Haotian Guoa,b, Xingchun Wua,b, Bowei Wanga,b, , Xilong Yana,b,
a
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, PR China
b
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The continuous synthesis of bis[3-(dimethylamino)propyl]amine (BPA) by hydrogenation of 3-(dimethylamino)-
propionitrile (PN) in the presence of 3-(N,N-dimethylamino)propylamine (PA) over copper-based catalysts was
investigated. It was found that Cu-Cr2O3-CoO/γ-Al2O3 yielded satisfactory results with almost 100% PN con-
version and 88.5% BPA selectivity (H2 pressure: 1 Mpa, temperature: 100 °C, PN/PA ratio: 2:1). All catalysts
were characterized by XRD, XPS, H2-TPR, and TEM, the obtained results indicated that Cr could improve the
catalytic activity. Moreover, Co dispersed around Cu particles promoted the catalytic nature, which could further
enhance the activity and stability of the catalyst.
Nitrile hydrogenation
Secondary amine
Cu catalysts
Co-doped catalyst
1. Introduction
abundant metal catalyst. Nickel, cobalt, and copper catalysts were
prepared and employed for the continuous synthesis of BPA, in which
The hydrogenation of nitriles to amines is a valuable reaction to
many areas of chemical industry, for example pesticides, fungicide, and
pharmaceuticals [1–4]. Generally, a mixture of primary, secondary, and
tertiary amines was obtained by the hydrogenation of nitriles, de-
pending on the employed catalysts and reaction conditions [5–8]. The
hydrogenation of nitriles is quite complicated, which consists of several
parallel and competitive reactions (Scheme 1) [7,9]. A high selectivity
to a certain amine product is usually desired because this eliminates the
need for costly product separation processes. Even though, many simple
secondary amines have been synthetized with high selectivity by
homogeneous or heterogeneous hydrogenations of nitriles [10,11].
However, some specific one such as bis[3-(dimethylamino) propyl]
amine (BPA) (Fig. 1) is seldom reported. BPA is wildly used as poly-
urethane catalyst and curing agent for epoxy resin [12,13]. Bayer re-
ported its batch-type synthesis by catalytic hydrogenation of 3-(di-
methylamino)-propionitrile (PN) over Pd/Al2O3 [14]. Under, 10 Mpa
H2, 81.5% BPA was obtained with PN/PA molar ratio 1:1 at 80–90 °C.
Besides, Krupka reported that hydrogenation of PN over 5%Pd/γ-Al2O3
gave 82.16% of BPA yield at similar conditions [15]. However, there
are still many problems in above synthesis process, such as the scarcity
of noble metal Pd, low efficiency of batch production and the poor
selectivity.
Cu-Cr2O3-CoO/γ-Al2O3 exhibited the best catalytic performance. All
catalysts were characterized by XRD, XPS, H2-TPR, TEM, and FT-IR, to
investigate the structure-activity relationship. The influences of doping
with Cr and Co elements on the dispersion and stability of Cu were
investigated, the process of forming byproducts was also investigated.
2. Experimental
All catalysts were prepared by coprecipitation-kneading method. A
solution of metal nitrates and a solution of Na2CO3 were simultaneously
added dropwise into a pseudo-boehmite suspension. After coprecipita-
tion, the mixture was filtered and washed with distilled water. Mixed
with nitric acid, the dried powder was molded into bars. To obtain the
target catalysts, the dried bars were calcined and reduced in a hydrogen
steam. The specific preparation and characterization of catalysts, as
well as catalytic reaction are described in the Supporting information.
Conversion of PN and selectivity of BPA were defined as:
PNout
PNin
⎛
⎞
⎟
⎜
Conversion (%) = 1 −
× 100
⎝
⎠
BPAout
Selectivity(%) =
× 100
1 − PNout − PAout
In this work, we tried to establish a continuous synthetic method for
BPA by catalytic hydrogenation of PN in a fixed-bed reactor over earth-
PNin was the concentration of PN in mixture of all reactants, while
⁎
Corresponding authors at: School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, PR China.
E-mail addresses: bwwang@tju.edu.cn (B. Wang), yan@tju.edu.cn (X. Yan).
These authors contributed equally to this work.
1
https://doi.org/10.1016/j.catcom.2018.03.031
Received 13 December 2017; Received in revised form 20 March 2018; Accepted 27 March 2018
Availableonline29March2018
1566-7367/©2018ElsevierB.V.Allrightsreserved.

C. Lin et al.
CatalysisCommunications111(2018)64–69
aggregation of copper oxide [18]. For the unreduced Cu-Cr2O3/γ-
Al2O3 sample, the main peak shifted to 179 °C. The obvious low
reduction temperature peak was probably related to the reduction of
highly-dispersed CuO [19], and the disappearance of shoulder peak
indicated that the existence of Cr improves the dispersion of CuO
particles, which agreed well with the results of XRD. As for the
unreduced Cu-Cr2O3-CoO/γ-Al2O3 sample, the main peak further
shifted to lower temperature (146 °C), which meant that Co might
promote the dispersion of CuO or change its reduction. In con-
sideration of the similar Cu particle sizes with Cu-Cr2O3/γ-Al2O3
from XRD, properly the latter was the main reason. But it still
needed further verification by TEM. In addition, the new shoulder
peak may be ascribed to the reduction of Co3+ to Co2+ [20], while
the second broad peak around 300–500 °C was assigned to the re-
duction of the remaining Co2+ to Co0 [21,22]. Furthermore, no
peaks corresponding to the reduction of Cr species were identified.
Thus, under our experimental conditions, only the Cu specie was
reduced to metallic state, and the Cr and Co species still maintained
the oxidation state.
XPS. To further explore the chemical state of the elements at the
surface of catalysts, XPS analysis of three catalysts was carried out,
and the results are presented in Fig. S1 (Supporting Information).
The binding energies of Cu 2p3/2 for Cu/γ-Al2O3, Cu-Cr2O3/γ-Al2O3,
and Cu-Cr2O3-CoO/γ-Al2O3 were 932.4 eV, 932.2 eV, and 932.3 eV,
respectively. These were mainly ascribed to Cu 2p3/2 peaks of Cu0
[23]. This agreed well with the results of XRD pattern. The binding
energy of Cr 2p3/2 located at 576.7 eV, indicating the presence of
Cr3+ species as Cr2O3 or CuCr2O4 [19,24]. As for Co species in Cu-
Cr2O3-CoO/γ-Al2O3, the peaks at 780.6 eV was assigned to Co 2p3/2
with the shake-up satellite at 785.9 eV, while the peak at 796.3 eV
was assigned to Co 2p1/2 with the satellite peak at 802.7 eV [25].
The peaks and their shake-up satellites confirmed the presence of
Co2+ in the high-spin state [26]. The absence of the characteristic
peak at 778.1 eV indicated the nonexistence of Co metal. This result
declared that Co species exits as CoO phase, which is consistent with
H2-TPR curve [20].
Scheme 1. Reaction pathways for the hydrogenation of nitriles.
PNout, PAout, and BPAout were the concentrations of PN, PA, and BPA in
reaction fluid (including all products and unreacted reactants), re-
spectively. They were determined by GC with area normalization
method.
TEM. To further investigate the influences of Cr and Co on Cu/γ-
Al2O3 catalyst, these catalysts were characterized by TEM. The TEM
image of fresh Cu-Cr2O3-CoO/γ-Al2O3 catalyst is described in Fig. 4.
The needle-like γ-Al2O3 was clearly observed, and many black dots
were assigned to Cu metal particles, which were dispersed uniformly
on the support. Similar morphology was also observed on Cu/γ-
Al2O3 and Cu-Cr2O3/γ-Al2O3 (Fig. S2). For Cu-Cr2O3/γ-Al2O3, Cu
particles were also dispersed well on the support, and the particle
size was smaller. However, Cu particles aggregated severely on Cu/
γ-Al2O3 catalyst. This agreed well with the XRD results. EDS analysis
in the STEM image revealed that Cr was homogeneously dispersed
on the whole catalyst. It has been verified in our previous work that
γ-Al2O3 support modified by doping with Cr can enhance the dis-
persion of Cu metal particles [19,27]. Besides, the EDS map of Co
demonstrates that the CoO particles were strictly dispersed around
the Cu metal particles. Even though, CoO didn't further enhance the
dispersion of Cu, it changed the circumstance of Cu. That's why the
peak of Cu-Cr2O3-CoO/γ-Al2O3 in H2-TPR decreased 33 °C compared
with Cu-Cr2O3/γ-Al2O3. The CoO beside Cu particles played an
important role in the catalytic performances.
3. Results and discussion
3.1. Catalyst characterization
XRD. The X-ray diffraction patterns of supported Cu catalysts were
measured and the results are shown in Fig. 2. All catalysts showed the
characteristic diffraction peaks of γ-Al2O3 (located at 2θ = 37.6°, 45.8°,
and 66.7°) and the metallic Cu phase (located at 43.3°, 50.4°, and
74.1°). It was obvious that the Cu diffraction peaks of Cu-Cr2O3/γ-Al2O3
and Cu-Cr2O3-CoO/γ-Al2O3 were much weaker than Cu20/γ-Al2O3.
According to the Scherrer equation, the crystal sizes of Cu metal were
estimated to be 23.1, 14.0, and 13.7 nm for Cu/γ-Al2O3, Cu-Cr2O3/γ-
Al2O3, and Cu-Cr2O3-CoO/γ-Al2O3, respectively (Table S1). It was de-
monstrated that Cr enhanced the dispersion of Cu, which has been
verified in previous works [16,17]. No obvious diffraction peaks of
chromium and cobalt compounds were observed in the XRD patterns,
possibly attributed to their amorphous or small size.
H2-TPR. Fig. 3 showed the H2-TPR profiles for the precursors of
three samples. The first sharp peak, for Cu/γ-Al2O3, with the
maxima at 210 °C was ascribed to the reduction of well-dispersed
CuO to Cu0. The shoulder peak at 240 °C was corresponding to the
Fig. 1. Structures of PA, PN and BPA.
65

C. Lin et al.
CatalysisCommunications111(2018)64–69
Fig. 2. XRD patterns of (a), fresh Cu/γ-Al2O3; (b) and (c), used and fresh Cu-Cr2O3/γ-Al2O3; (d) and (e), used and fresh Cu-Cr2O3-CoO/γ-Al2O3.
3.2. Catalytic performance
was up to 52.2% and 82.8% based on PN, the best among these three
catalysts. The conversion of PN over Ni/γ-Al2O3 was impressive
(94.4%). However, the selectivity of BPA was not satisfactory (56.0%),
and some other byproducts were detected by GC–MS. Thus we spec-
ulate that PN is easily split into dimethylamine and acrylonitrile under
catalytic conditions due to retro-Michael reaction [28], and then to
form other byproducts (N,N,N'N′-tetramethyl-1,3-propanediamine and
N,N-dimethyl-N′-propyl-1,3-propanediamine, Scheme S1). A detailed
discussion on the reaction pathways for formation of byproducts in PN
hydrogenation to BPA was given in previous study [15].
Even though Cu/γ-Al2O3 exhibited a good catalytic performance,
the activity and selectivity still did not meet the demand of practical
production. So, the catalysts were modified by Cr and Co, and the two
catalysts Cu-Cr2O3/γ-Al2O3 and Cu-Cr2O3-CoO/γ-Al2O3 were applied to
The catalytic hydrogenation of PN is quite complicated, compared
with the hydrogenation of normal aliphatic nitriles. Besides, the se-
lectivity among 3-(N, N-dimethylamino)propylamine (PA, primary
amine), bis[3-(dimethylamino) propyl]amine (BPA, secondary amine),
and tris(N,N-dimethylaminopropyl)amine (TPA, tertiary amine) is
hardly controlled.
Firstly, Cu/γ-Al2O3, Ni/γ-Al2O3, and Co/γ-Al2O3 were prepared and
employed for this catalytic hydrogenation. We chose PA and PN as
reactants, to restrain the formation of TPA. The results are shown in
Table 1. It's surprised that copper displayed a higher activity than co-
balt which barely catalyzed the hydrogenation of PN. Cu/γ-Al2O3 ex-
hibited a moderate catalytic activity. The yield and selectivity of BPA
Fig. 3. TPR profiles of unreduced catalysts (a) Cu/γ-Al2O3, (b) Cu-Cr2O3/γ-Al2O3, (c) Cu-Cr2O3-CoO/γ-Al2O3.
66

C. Lin et al.
CatalysisCommunications111(2018)64–69
Fig. 4. TEM, STEM images and element maps of Cu-Cr2O3-CoO/γ-Al2O3.
test their catalytic properties. It was obvious that Cr promoted the
catalytic activity. The conversion of PN increased from 63.4% to 87.0%.
Besides, Co further enhanced the catalytic activity. PN was completely
converted when Cu-Cr2O3-CoO/γ-Al2O3 was applied. Actually, Cu-
Cr2O3-FeOx/γ-Al2O3 and Cu-Cr2O3-LaOx/γ-Al2O3 trimetallic catalysts
were also prepared and tested their catalytic performances. However,
compared with Cu-Cr2O3/γ-Al2O3, they didn't exhibit much promotion.
Meanwhile, the selectivity over all catalysts didn't change much during
the runs. As we know, a better metal dispersion can promote the cat-
alytic activity [29,30]. Cu-Cr2O3/γ-Al2O3 exhibited a higher activity
than Cu20/γ-Al2O3, because Cr promoted the dispersion of Cu. Cu-
Cr2O3-CoO/γ-Al2O3 exhibited the highest activity, though its Cu
dispersion was demonstrated to be similar with Cu-Cr2O3/γ-Al2O3. It
was mainly due to the CoO dispersed around Cu, which enhanced its
activity. Meanwhile, it was found that the reaction between PA and PN
proceeded with molar ratio about 1:1 over both Cu/γ-Al2O3 and Cu-
Cr2O3/γ-Al2O3. However, different from Cu/γ-Al2O3 and Cu-Cr2O3/γ-
Al2O3, the conversion of PA over Cu-Cr2O3-CoO/γ-Al2O3 was low. It
could be explained by the pathways for the hydrogenation of nitrile
(Scheme 1). Primary amine barely formed over former two catalysts.
While the hydrogenation and the addition of imine were competitive
reactions over Cu-Cr2O3-CoO/γ-Al2O3. Even though the selectivity of
BPA was up to 96.5%, PA didn't convert much, which resulted in the
low yield of BPA (53.2%).
Table 1
Catalytic results over transition metal catalysts.a
Other byproductsb (%)
Conversion (%)
Selectivity (%)
b
b
b
Catalysts
PNout (%)
PAout (%)
BPAout (%)
Ni/γ-Al2O3
2.8
48.2
18.3
6.5
7.6
6.2
0
39.6
48.5
18.7
6.7
7.4
6.3
32.3
1.9
25.3
1.3
94.4
3.6
56.0
59.2
82.8
80.7
81.3
80.1
96.5
Co/γ-Al2O3
Cu/γ-Al2O3
52.2
70.0
69.0
70.1
53.2
10.8
16.7
16.0
17.4
2.0
63.4
87.0
85.1
87.3
100
Cu-Cr2O3/γ-Al2O3
Cu-Cr2O3-LaOx/γ-Al2O3
Cu-Cr2O3-FeOx/γ-Al2O3
Cu-Cr2O3-CoO/γ-Al2O3
44.8
a
Reaction conditions: PA: PN (mole), 1:1; temperature, 100 °C; hydrogenation, 1.0 Mpa; LHSV, 0.15 h−1; Solvent, toluene; Concentration, 20 wt%.
Data were percentages of peak areas in GC, and were average of three and more runs.
b
67

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CatalysisCommunications111(2018)64–69
Fig. 5. Time on stream performances. ■ conditions:
PN:PA, 1:1; temperature, 100 °C; LHSV, 0.15 h−1. ▲
conditions: PN:PA, 2:1; temperature, 100 °C; LHSV,
0.20 h−1; catalysts, Cu-Cr2O3-CoO/γ-Al2O3.
In view of the low conversion of PA over Cu-Cr2O3-CoO/γ-Al2O3,
particles of Cu-Cr2O3/γ-Al2O3 after reaction (Fig. S3). It was demon-
strated that Co could effectually restrain the agglomeration of Cu metal
particles. Meanwhile, FTIR spectra (Fig. S4) showed that some amine
compounds were adsorbed on the surfaces of catalyst, which could
block the active sites of catalysts. This is possibly another reason why
catalysts deactivate, which happened in both catalysts. Actually, many
researchers have confirmed that adsorbed amines on the surfaces of
catalysts and the metal agglomeration were major factors for catalyst
deactivation in the hydrogenation of nitrile compounds [31–33]. The
compounds adsorbed on catalyst can be removed by washing with
solvents or high temperature [34,35]. But the agglomeration of metal is
irreversible, and we found the CoO dispersed around could suppress the
agglomeration of Cu metal.
and to obtain a high yield of BPA in final mixture, the molar ratio of PN
and PA was adjusted. The results are listed in Table S2. When the re-
actant ratio increased from 1:1 to 2:1, BPA yield increased to 75.6%.
With the increase of PN proportion, however, the other byproducts also
increased. Especially, when PN as the single reactant, BPA yield was
only 60.5%. It may be that the splitting reaction is in the dominant
position. Meanwhile, the yield of tertiary amine increased to 10.2%.
BPA yield was 76.5%, when the ratio was 4:1. But the byproducts were
15.2%, much higher than 9.8% in situation of 2:1. Consequently, 2:1
was the optimum reactant ratio.
As expected, Cr gave rise to an increase in the activity of catalyst. It
was mainly due to a better dispersion of Cu metal when Cr was in-
troduced [17,19,27]. Meanwhile, the selectivity of BPA didn't float
much, which meant that the Cr2O3 didn't change the catalytic nature of
Cu metal. Situation changed when catalyst was doped with Co. Over
Cu-Cr2O3-CoO/γ-Al2O3, PN was more inclined to be hydrogenated into
PA rather than reacted with PA. As for catalytic activity, Cu-Cr2O3-
CoO/γ-Al2O3 had a much higher conversion of PN than Cu-Cr2O3/γ-
Al2O3 under circumstance of the similar Cu metal dispersion. A possible
speculation is that the CoO could promote the catalytic performance as
a cocatalyst dispersed around the Cu metal particles.
Subsequently, Cu/γ-Al2O3, Cu-Cr2O3/γ-Al2O3, and Cu-Cr2O3-CoO/γ-
Al2O3, their catalytic lives were continuously investigated. Fig. 5 shows
the time on steam performances of these catalysts. It's obvious that
catalyst Cu/γ-Al2O3 and Cu-Cr2O3/γ-Al2O3 lost a substantial part of its
activity during the first 20 h of operation. While the conversion of PN
still remain 100% over Cu-Cr2O3-CoO/γ-Al2O3. To further investigate
the catalytic life of Cu-Cr2O3-CoO/γ-Al2O3, the conversion of PN was
controlled at a medium level by increasing the liquid hourly space
velocity (LHSV). After 60 h of reaction, only slight deactivation was
observed. The conversion of PN decreased from 82.8% to 73.8%. It was
obvious that Co could suppress the deactivation of catalyst to a large
extent. Hence, the catalysts after reaction were characterized by XRD,
TEM, and FT-IR to investigate the reasons.
4. Conclusion
Cu supported catalysts were applied to the synthesis of BPA. The
catalysts were modified by doping with transition metal elements Cr
and Co, and characterized by XRD, XPS, H2-TPR, TEM, FT-IR, and N2
adsorption-desorption. The results indicated that Cr contributed to the
dispersion of Cu metal particles, which improved the catalytic activity.
The CoO dispersed around Cu particles enhanced the hydrogenation
activity of Cu and changed the catalytic selectivity. More primary
amine was obtained. Metal agglomeration and adsorbed amines on the
surfaces of catalysts were major factors for catalyst deactivation. It was
confirmed that CoO, to a certain extent, could suppress the agglom-
eration of Cu particles. The yield and selectivity of BPA over Cu-Cr2O3-
CoO/γ-Al2O3 was 75.6% and 88.5% with PN:PA of 2:1. The selectivity
was even up to 96.5%, when PN:PA was 1:1.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.catcom.2018.03.031.
It was distinct that Cu diffraction peaks of used Cu-Cr2O3/γ-Al2O3
became much stronger (Fig. 2). However, only minor increase was
detected over Cu-Cr2O3-CoO/γ-Al2O3. According to the Scherrer equa-
tion, the crystal sizes of Cu metal were estimated to be 22.4 and
15.9 nm for Cu-Cr2O3/γ-Al2O3 and Cu-Cr2O3-CoO/γ-Al2O3, respec-
tively. TEM images also verified the severe agglomeration of Cu metal
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