Reductive amination of triacetoneamine with n-butylamine over Cu-Cr-La/γ-Al2O3

Article abstract of DOI:10.1007/s10562-011-0699-7

A series of Cu-based catalysts were prepared and examined for the reductive amination of triacetoneamine with n-butylamine, thereinto, Cu-Cr-La/γ- Al₂O₃ showed excellent results. The catalysts were studied by XRD, XPS, H₂-TPR and NH₃-TPD. It was found that doped Cr remarkably enhanced the activity of Cu/γ-Al₂O₃ due to better dispersion of Cu⁰, which is believed to be the active site for the reductive amination. Additionally, it was obvious that introduction of La to Cu-Cr/γ-Al₂O₃ led to a higher selectivity and longer lifetime. The reaction parameters were optimized and N-butyl-2,2,6,6-tetramethyl-4-piperidinamine was obtained in a yield of 94%. Graphical Abstract: The addition of Cr does not impact obviously on the total acidity of Cu₂₀/γ-Al₂O₃. However, the La addition to Cu₂₀Cr₅/γ-Al₂O₃ not only decreases the total acidity but also reduces the strength of acid sites. The introduction of La was found to result in a higher selectivity and longer lifetime. The conversion of TAA and selectivity of TEMPBA remained around 99.7 and 94.5% in 120 h.[Figure not available: see fulltext.][Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-011-0699-7

Catal Lett (2011) 141:1703–1708
DOI 10.1007/s10562-011-0699-7
Reductive Amination of Triacetoneamine with n-Butylamine Over
Cu–Cr–La/c-Al2O3
•
Meng Sun Xiaobao Du Huabang Wang
•
•
•
Zhiwei Wu Yang Li Ligong Chen
•
Received: 13 July 2011 / Accepted: 8 September 2011 / Published online: 21 September 2011
Ó Springer Science+Business Media, LLC 2011
Abstract A series of Cu-based catalysts were prepared
and examined for the reductive amination of triacetone-
amine with n-butylamine, thereinto, Cu–Cr–La/c-Al2O3
showed excellent results. The catalysts were studied by
XRD, XPS, H2-TPR and NH3-TPD. It was found that
doped Cr remarkably enhanced the activity of Cu/c-Al2O3
due to better dispersion of Cu0, which is believed to be the
active site for the reductive amination. Additionally, it was
obvious that introduction of La to Cu–Cr/c-Al2O3 led
to a higher selectivity and longer lifetime. The reaction
parameters were optimized and N-butyl-2,2,6,6-tetra-
methyl-4-piperidinamine was obtained in a yield of 94%.
or palladium as catalysts [6, 7]. However, the batch process
in autoclave is a tedious process with high energy con-
sumption, environmental pollution and much waste of
labor. There are only a few reports concerning about the
production of 4-alkylaminopiperidines in a fixed-bed
reactor [8, 9], but suffered from expensive catalysts or poor
yield. These results encouraged us to develop a continuous
process for the production of TEMPBA.
In this work, a continuous process for TEMPBA from
triacetoneamine and n-butylamine in a fixed-bed reactor
over Cu-based catalysts was established. TEMPBA was
obtained in 94% yield over Cu–Cr–La/c-Al2O3, which
exhibited excellent properties. The effects of Cr and La
doped in Cu/c-Al2O3 were studied by XRD, XPS, H2-TPR
and NH3-TPD.
Keywords Reductive amination Á Triacetoneamine Á
N-butylamine Á Cu–Cr–La/c-Al2O3
1 Introduction
2 Experimental
N-butyl-2,2,6,6-tetramethyl-4-piperidinamine (TEMPBA,
b.p. 249.7 °C) obtained by the reductive amination of
triacetoneamine (TAA) and n-butylamine is a key inter-
mediate of hindered amine light stabilizers (HALS), which
have dominated light stabilizers market for several decades
due to their excellent light stabilization [1, 2]. It is well
known that reductive amination of carbonyl compounds is
an important method for the production of amines [3–5].
Up to now, the reductive amination of TAA with n-butyl-
amine to TEMPBA in autoclave has been widely described
in many studies in the presence of cobalt, nickel, platinum
2.1 Materials and Catalysts
Triacetoneamine (m.p. 35–39 °C, b.p. 205 °C) was
obtained from Beijing Tiangang Auxiliary Co. Ltd, Bei-
jing, China. Commercially available solvents and reagents
were used without further purification.
All catalysts used in this study were prepared by copre-
cipitation-kneading method. For example, Cu20Cr5La5/
c-Al2O3 was prepared as follows. Under mechanical
stirring, a mixture of 22.81 g Cu(NO3)2Á3H2O, 11.54 g
Cr(NO3)3Á9H2O and 4.68 g La(NO3)3Á6H2O in 200 mL
H2O, and a solution of 16.32 g sodium carbonate in
200 mL H2O were added dropwise into a beaker containing
200 mL H2O at ambient temperature over 1 h and aged for
another 1 h. During the process, the pH of the system was
maintained around 8. The precipitate was then filtered,
M. Sun Á X. Du Á H. Wang Á Z. Wu Á Y. Li Á L. Chen (&)
School of Chemical Engineering and Technology, Tianjin
University, Tianjin 300072, People’s Republic of China
e-mail: lgchen@tju.edu.cn
123

1704
M. Sun et al.
washed with distilled water, dried for 6 h at 110 °C and
pulverized. The dried samples were kneaded vigorously
with a mixture of 33.07 g pseudo-boehmite and 50 mL
2wt% nitric acid, followed by molding to bars by an
extruder. After dried in air for 6 h at 110 °C, the bars were
calcined for 4 h at 500 °C and reduced at 220 °C in a
hydrogen stream (1.0 MPa) for 4 h before later use.
temperature in the catalyst zone was measured using a
thermocouple located in the center of the tube and was
regulated by a PID cascade controller. A solution of TAA
and n-butylamine (molar ratio of 1:1) in 1,4-dioxane
(concentration: 14.7%) was stirred for 10 h at ambient
temperature, and then dosed into the reactor at a flow rate
of 0.3 mL/min by a syringe pump. The GC-MS (Polaris Q,
Thermo Finngan, America) used to confirm the compo-
nents of the reaction mixture was performed on a HP-5
capillary column (30 m 9 0.25 mm, 0.2 lm film thick-
ness) equipped with an ion trap MS detector. The compo-
sition of the reaction mixture was determined by GC with a
30 m SE-54 capillary column.
2.2 Catalysts Characterization
X-ray diffraction (XRD) was carried out on a Rigaka
D/max 2500 X-ray diffractometer with Cu-Ka radiation
(40 kV, 100 mA) in the range of 5–95°. The mean diam-
eter of Cu crystals was calculated from XRD patterns using
Scherrer equation. X-ray photoelectron spectroscopy (XPS)
was performed with a PHI 1600 spectroscope using a Mg-
Ka X-ray source for excitation. The catalyst samples were
removed from the reactor under the protection of nitrogen
before XPS analysis. H2-temperature programmed reduc-
tion (H2-TPR) was measured using a micromeritics 2910
apparatus equipped with a thermal conductivity detector
(TCD). The calcined samples were heated from ambient
temperature to 600 °C at a rate of 10 °C/min in a mixture
of 5% H2/N2. NH3-temperature programmed desorption
(NH3-TPD) was performed with a TP-5000 instrument
with a TCD.
3 Results and Discussion
3.1 Catalyst Selection
For reductive amination, hydrogenation/dehydrogenation
catalysts such as transition metal based catalysts are often
employed. Supported palladium, platinum [9, 10], nickel
[11, 12], cobalt [11, 13, 14], copper [15–18] catalysts have
been reported to be effective for the reductive amination of
ketones or alkanols with alkylamine. Taking the costs of
catalysts into consideration, the noble metals were not
selected for this reaction. Thus, Ni20/c-Al2O3, Co20/
c-Al2O3 and Cu20/c-Al2O3 were prepared and examined for
this reaction. The reaction mixture was determined by
GC-MS, the main and side reactions occurring during the
reductive amination are depicted in Scheme 1. TEMPBA
as the main product, 2,2,6,6-tetramethylpiperidin-4-ol
2.3 Catalytic Reaction
The catalytic reaction was carried out in a tubular, fixed-
bed reactor with an inner diameter of 15 mm and a length
of 660 mm, which was loaded with 40.0 mL catalysts. The
NH
O
N
Cat.
H2
+
NH2
N
H
N
H
N
H
TAA
TEMPBA
O
OH
Cat.
H2
N
H
N
H
TMP
OH
Cat.
H2
-H2O
N
H
N
N
H
H
TMP
HTMP
Scheme 1 The reaction process of the reductive amination
123

Reductive Amination of Triacetoneamine
1705
Table 1 Results of reductive amination of TAA with n-butylamine
over various catalysts
Table 2 Results of reductive amination of TAA with n-butylamine
over the prepared catalysts
Catalyst
TAA
conversion (%)
Selectivity (%)
Catalyst
TAA
conversion
(%)
Selectivity (%)
HTMPa
TMPb
TEMPBAc
HTMP
TMP
TEMPBA
Ni20/c-Al2O3
Co20/c-Al2O3
Cu20/c-Al2O3
82.2
87.6
85.7
3.1
0.5
3.7
9.7
15.7
5.1
80.9
70.5
86.4
Cu20/c-Al2O3
85.7
96.4
99.7
99.8
99.7
99.9
3.7
1.9
2.1
0
5.1
3.7
86.4
89.7
92.5
91.8
94.5
85.6
Cu20Cr5/c-Al2O3
Cu20Cr5Fe5/c-Al2O3
Cu20Cr5Mn5/c-Al2O3
Cu20Cr5La5/c-Al2O3
Cu20Cr5Zn5/c-Al2O3
3.0
7.5
Reaction conditions: temperature = 120 °C, hydrogen pressure =
4.0 MPa, flow rate of the solution of TAA and n-butylamine =
0.3 mL/min, TAA: n-butylamine (mole) = 1:1, concentration of
solution = 14.7%
0
2.3
1.2
12.6
a
Reaction conditions: temperature = 120 °C, hydrogen pressure =
4.0 MPa, flow rate of the solution of TAA and n-butylamine =
0.3 mL/min, TAA: n-butylamine (mole) = 1:1, concentration of
solution = 14.7%
HTMP, 2,2,6,6-tetramethylpiperidine
b
TMP, 2,2,6,6-tetramethylpiperidin-4-ol
c
TEMPBA, N-butyl-2,2,6,6-tetramethyl-4-piperidinamine
(TMP) and 2,2,6,6-tetramethylpiperidine (HTMP) were
detected. TMP was considered to be generated from the
direct hydrogenation of TAA, and HTMP would be affor-
ded by the dehydration-hydrogenation of TMP.
105
Conversion of TAA
Selectivity of TEMPBA
The results of the reductive amination of triacetone-
amine with n-butylamine are summarized in Table 1. All
catalysts exhibited satisfied conversion of TAA, but it is
obvious that Cu20/c-Al2O3 displayed much better selec-
tivity to TEMPBA than both Ni20/c-Al2O3 and Co20/
c-Al2O3. Therefore, copper was chosen as the main cata-
lyst. However, the catalytic activity of Cu20/c-Al2O3
decreased quickly with time-on-stream, the conversion of
TAA decreased from 85.7% to 45.3% in 20 h. The color of
reaction mixture also turned into greenish from light yel-
low. The mixture was detected by atomic absorption
spectroscopy and Cu was checked out. So, we suggest that
the lost of copper may be one reason for the deactivation.
What’s more, Shiau and Lee [19] reported that the sintering
of copper could cause the deactivation of Cu/c-Al2O3. The
used Cu20/c-Al2O3 catalyst was characterized by XRD
(Fig. 2b) and the mean diameter of Cu0 particle calculated
100
95
90
85
80
0
20
40
60
80
100
120
Reaction time (h)
Fig. 1 Service life of Cu20Cr5La5/c-Al2O3
results, especially in the selectivity of TEMPBA, which
imply that the doped La can improve the catalytic selec-
tivity remarkably.
˚
by Scherrer equation was 875 11 A, which is much
˚
larger than 412 9 A in the fresh one (Fig. 2c). Thus, it
can be deduced that the lost of copper together with the
sintering of copper led to the deactivation of Cu20/c-Al2O3.
In order to improve its stability and increase the activity
and selectivity, Cr was introduced into Cu20/c-Al2O3 as
auxiliary catalyst, and Fe, Mn, La, Zn were doped in Cu-
Cr/c-Al2O3 as the third component. The obtained catalysts
were then applied to the reductive amination of triace-
toneamine with n-butylamine, and the results are summa-
rized in Table 2.
3.2 Lifetime of Catalyst
The reaction parameters, including the reaction tempera-
ture and the hydrogen pressure, were optimized. Then the
catalytic process was performed under optimum reaction
conditions for 120 h and the results are shown in Fig. 1.
During this period, the catalyst exhibited excellent stabil-
ity. The conversion of TAA and selectivity of TEMPBA
remained around 99.7 and 94.5%, respectively. Therefore,
the catalyst could be economically used in large-scale
production. XRD, XPS, H2-TPR and NH3-TPD were used
to characterize these catalysts for further understanding of
the results.
As expected, the addition of Cr into Cu20/c-Al2O3 gives
rise to an increase in the conversion of TAA. However, the
selectivity of TEMPBA was not enhanced obviously. The
Cu–Cr/c-Al2O3 doped Fe, Mn, La, Zn showed different
results. Thereinto, Cu–Cr-La/c-Al2O3 exhibited the best
123

1706
M. Sun et al.
3.3 Characterization of Catalysts
(a) demonstrate that Cu presents as CuO in the unreduced
Cu20Cr5La5/c-Al2O3. The typical diffraction lines at 43.38,
50.48, 74.28 and 90.08 in the XRD curves of (b), (c),
(d) and (e) could be assigned to the crystals of elementary
copper. It can be found that the addition of Cr and La made
Cu crystals smaller and dispersed better. Calculation from
the main diffraction line (43.38) on the XRD patterns using
Scherrer equation, the mean diameter of Cu0 particle
3.3.1 XRD
The XRD curves for the unreduced Cu20Cr5La5/c-Al2O3,
reduced Cu20/c-Al2O3, reduced Cu20Cr5/c-Al2O3 and
reduced Cu20Cr5La5/c-Al2O3 are shown in Fig. 2. The
diffraction lines labeled with ‘‘?’’ in the XRD curve of
˚
decreases from 412 9 A in the reduced Cu20/c-Al2O3 to
˚
˚
211 6 A and 194 6 A in the reduced Cu20Cr5/c-Al2O3
and reduced Cu20Cr5La5/c-Al2O3, respectively. The intro-
duction of Cr led to better dispersion of active species,
which can supply more active centers and prevent the
sintering of copper. The doped La to Cu20Cr5/c-Al2O3
made this action further enhanced. The more active centers
may be the reason for the higher activity of Cu20Cr5/c-
Al2O3 and Cu20Cr5La5/c-Al2O3 compared with Cu20/c-
Al2O3, which showed in Table 2. No obvious peaks cor-
responding to Cr, La or any their combinations were picked
out from the XRD curves, but it is noticeable that the doped
La made the selectivity of TEMPBA enhanced very obvi-
ously. We suggest that these components are either too
small to be detected or in the amorphous phases.
# Cu
+ CuO
#
#
#
#
(e)
(d)
(c)
(b)
+
+
+
+
+
(a)
0
20
40
60
80
100
3.3.2 XPS
Two-Theta (degree)
The XPS spectrum of reduced Cu20Cr5La5/c-Al2O3 is
shown in Fig. 3. The peak at 931.99 eV, the binding energy
of the Cu 2P3/2 level (Fig. 3a), indicates that Cu exists in
Fig. 2 XRD curves for (a) unreduced Cu20Cr5La5/c-Al2O3, (b) used
Cu20/c-Al2O3, (c) reduced Cu20/c-Al2O3, (d) reduced Cu20Cr5/
c-Al2O3 and (e) reduced Cu20Cr5La5/c-Al2O3
3.0x105
Fig. 3 XPS spectra of the
reduced Cu20Cr5La5/c-Al2O3
1.8x104
2.5x105
Energy: 576.24
Cr2p
1.8x104
1.7x104
1.6x104
2.0x105
1.5x105
1.0x105
5.0x104
0.0
1.5x104
(b)
1000
800
600
400
200
0
600 595 590 585 580 575 570 565 560
Binding energy (eV)
Binding energy (eV)
2.3x104
2.2x104
2.1x104
2.1x104
2.8x104
2.6x104
2.4x104
2.2x104
2.0x104
Energy: 835.49
La3d
Energy: 931.99
Cu2p
(c)
(a)
970
960
950
940
930
920
910
860 855 850 845 840 835 830 825 820
Binding energy (eV)
Binding energy (eV)
123

Reductive Amination of Triacetoneamine
1707
Cu20Cr5La5/c-Al2O3 mainly as Cu0 species. This assump-
tion could be supported by the typical diffraction lines of
the elementary copper crystals in the XRD pattern of the
reduced Cu20Cr5La5/c-Al2O3. The binding energy of the Cr
2p3/2 level (Fig. 3b) at 576.24 eV could be assigned to
Cr3?2p3/2. Cr3? may exist as Cr2O3 or CuCr2O4, which
could not be identified by our XRD patterns. The binding
energy for the La 3d5/2 level at 835.49 eV originates from
La3? is probably present as La2O3 or LaAlO3, which also
could not be identified by XRD patterns.
As for the unreduced Cu20Cr5La5/c-Al2O3, a slight shift of
the main reduction peak towards lower temperature
(223 °C) compared with that of the unreduced Cu20Cr5/
c-Al2O3 and a shoulder peak were detected. We suggest
that the existence of La may further improve the dispersion
of Cu particles and therefore promote the reducibility of the
catalyst. That is why Cu–Cr–La/c-Al2O3 showed the best
reaction results at the same reduction temperature. These
results agree well with the XRD results, and also prove that
the reduction temperature at 220 °C is suitable. It should be
noted that a new broad peak was also observed at
648–796 °C, this can probably ascribed to the combination
of La with the aluminum support. This combination was
claimed to inhibit the sintering of high surface area of
c-Al2O3 [25], which is one of the reasons for the long
lifetime of Cu20Cr5La5/c-Al2O3.
3.3.3 H2-TPR
The reducibility of copper species in the calcined catalysts
was investigated by H2-TPR experiments and the results
are described in Fig. 4. The unreduced Cu20/c-Al2O3 curve
displays two signals. The one at Tmax of 292 °C is corre-
sponding to the typical bulk-like CuO reduction peak,
another at Tmax of 733 °C presumably due to the reduction
of aluminate CuAl2O4 which is much more difficult to
reduce [19–22]. For the unreduced Cu20Cr5/c-Al2O3, the
reduction temperature of main peak obviously shifts to
234 °C from 292 °C, and the higher temperature peak at
733 °C is disappeared. The peak at 234 °C can probably be
attributed to the reduction of highly-dispersed copper
oxide, which could be reduced completely at the relatively
low temperature [23, 24]. The disappearance of the peak at
733 °C suggests that the addition of chromium can sig-
nificantly inhibit the formation of aluminate. These results
indicate that the chromium addition can not only improve
the dispersion of Cu particles but can also prevent copper
from reacting with aluminum support to form aluminate.
3.3.4 NH3-TPD
In this present investigation, NH3-TPD is used to measure
the strength of acidity and distribution of acidic sites of the
catalysts. The TPD curves of reduced Cu20/c-Al2O3, reduced
Cu20Cr5/c-Al2O3 and reduced Cu20Cr5La5/c-Al2O3 are
shown in Fig. 5. All the reduced catalysts exhibit a typical
double-peak. The two desorption peaks at the lower tem-
perature (229 °C) and the higher temperature (470–529 °C)
correspond to the weak and strong acid sites on the catalyst
surfaces, respectively [26]. Normally, the area of a specific
peak can be used to estimate the amount of ammonia des-
orbed from the sample, and can be taken as a standard to
quantify the acidity of the sample [27, 28]. The peak areas
for the two temperature peaks are listed in Table 3. It can be
223
234
229
470
(c)
529
(c)
292
(b)
(a)
(b)
(a)
0
200
400
600
800
1000
100
200
300
400
500
600
700
800
Temperature
Temperature
Fig. 4 H2-TPR curves for (a) unreduced Cu20/c-Al2O3, (b) unreduced
Cu20Cr5/c-Al2O3 and (c) unreduced Cu20Cr5La5/c-Al2O3. Operating
conditions: 10 °C/min, 5% H2 in N2, with total flow rate of 20 mL/min
Fig. 5 NH3-TPD curves for (a) reduced Cu20/c-Al2O3, (b) reduced
Cu20Cr5/c-Al2O3 and (c) reduced Cu20Cr5La5/c-Al2O3
123

1708
M. Sun et al.
La was found to result in a higher selectivity and longer
lifetime. N-butyl-2,2,6,6-tetramethyl-4-piperidinamine was
obtained in 94% yield over Cu20Cr5La5/c-Al2O3 and this
process could be applicable for large-scale industrial
production.
Table 3 Temperature programmed desorption of NH3 for various
catalysts
Catalyst
Tades
Peak
areac
(a.u.)
Tbdes
Peak
areac
(a.u.)
Total
peak
areac
(a.u.)
(°C)
(°C)
Acknowledgment The authors would like to express their thanks
for the financial support provided by the National Natural Science
Foundation of China (Grant No. 20976123).
Cu20/c-Al2O3
Cu20Cr5/c-Al2O3
Cu20Cr5La5/c-Al2O3
a
229
229
229
4161
4052
3095
529
529
470
4329
4320
2404
8490
8372
5499
Lower temperature peak for weak acid sites
b
c
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