The reductive amination of cyclohexanone with 1,6-diaminohexane over alumina B modified Cu-Cr-La/γ-Al2O3

Article abstract of DOI:10.1016/j.catcom.2012.01.004

The reductive amination of cyclohexanone with 1,6-diaminohexane over alumina B modified Cu-Cr-La/γ-Al₂O₃ was investigated. The experimental results indicated that the doped alumina B remarkably increased the selectivity of N, N′-dicy

Full text of DOI:10.1016/j.catcom.2012.01.004

Catalysis Communications 20 (2012) 58–62
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Catalysis Communications
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Short Communication
The reductive amination of cyclohexanone with 1,6-diaminohexane over alumina B
modified Cu–Cr–La/γ-Al2O3
Meng Sun a, Xiaobao Du a, Xiangjin Kong b, Liang Lu a, Yang Li a, Ligong Chen a,
⁎
a
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China
b
School of Pharmaceutical Science and Technology, Tianjin University, Tianjin 300072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The reductive amination of cyclohexanone with 1,6-diaminohexane over alumina B modified Cu–Cr–La/γ-
Al2O3 was investigated. The experimental results indicated that the doped alumina B remarkably increased
the selectivity of N, N′-dicyclohexyl-1,6-diaminohexane. The catalysts were studied by BET, XRD, TEM and
NH3-TPD, the introduction of alumina B to Cu–Cr–La/γ-Al2O3 reduced the strong acid capacity of the catalyst,
which inhibited the generation of cyclohexane and 2-cyclohexylidene-cyclohexanone, and facilitated the de-
sorption of amino compounds. The reaction parameters, including reaction temperature and molar ratio of
starting materials, were optimized and a 92.6% selectivity of N, N′-dicyclohexyl-1,6-diaminohexane was
obtained under optimized conditions.
Received 14 November 2011
Received in revised form 30 December 2011
Accepted 6 January 2012
Available online 16 January 2012
Keywords:
Reductive amination
Cyclohexanone
Alumina B
© 2012 Elsevier B.V. All rights reserved.
Cu–Cr–La/γ-Al2O3
1. Introduction
2. Experimental
Secondary amines are a key class of intermediates of industrial
chemicals [1–3]. One of the most useful methods for the preparation
of secondary amines is the reductive amination of aldehydes or ke-
tones with primary amines in the presence of hydrogen and a hetero-
geneous metal catalyst [4–11], and all these works are about the
reductive amination of carbonyl compound with monoamine. In in-
dustry, the reductive amination of carbonyl compound with primary
diamine, such as 1,6-diaminohexane, was rarely reported and suf-
fered from low selectivity of N, N′-dicyclohexyl-1,6-diaminohexane
and tedious process [12–14]. Thus an attempt was made to establish
a continuous process for the reductive amination of cyclohexanone
with 1,6-diaminohexane.
Since Cu–Cr–La/γ-Al2O3 was developed and demonstrated to be
effective for the reductive amination of triacetoneamine with n-buty-
lamine in our previous work [15], it thus was first employed for the
reaction of cyclohexanone with 1,6-diaminohexane. In this work, in
order to reduce the acidic centers of catalyst, the alumina B (basic alu-
mina, pH>8) was added to the Cu–Cr–La/γ-Al2O3 and the obtained
catalysts were applied to this reaction. The modified catalysts were
studied by XRD and NH3-TPD, and the reaction parameters were
also investigated.
2.1. Materials and catalysts
All the chemicals were reagent grade and were used without
further purification. The Cu20Cr5La5/γ-Al2O3-ABm (AB: alumina B,
m: content of alumina B in the catalyst) was prepared by kneading
the metal carbonates with the mixture of pseudo-boehmite and alu-
mina B. The detailed procedures for the preparation of metal car-
bonates and the catalyst were described in our previous work [15].
Take Cu20Cr5La5/γ-Al2O3-AB35 (the content of alumina B in the catalyst
was 35 wt.%) as an example, 18.24 g dried metal carbonates were
kneaded sufficiently with a mixture of 16.54 g pseudo-boehmite and
10.50 g alumina B, accompanied with deionized water as adhesive, fol-
lowed 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 use.
2.2. Catalysts characterization
The specific surface area was determined by the BET method
with N2 adsorption–desorption measurements at liquid nitrogen tem-
perature using a NOVA 2000e analyzer (Quantachrome, US). X-ray dif-
fraction (XRD) was carried out on a Rigaka D/max 2500 X-ray
diffractometer with Cu–Kα radiation (40 kV, 100 mA) in the range
of 5–95°. The mean diameter of Cu crystals was calculated from
XRD patterns using Scherrer equation. Transmission electron micro-
scopic measurements (TEM) were carried out using a JEOL electron
⁎
Corresponding author. Tel.: +86 22 27891034; fax: +86 22 27406314.
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.01.004

M. Sun et al. / Catalysis Communications 20 (2012) 58–62
59
microscope (JEM-2010), with an accelerating voltage of 200 keV.
NH3-temperature programmed desorption (NH3-TPD) was per-
formed with a TP-5000 instrument with a thermal conductivity de-
tector (TCD).
determined by GC–MS. N, N′-dicyclohexyl-1,6-diaminohexane, the
desired product, was detected as the main product. N-cyclohexyl-
1,6-diaminohexane (N), cyclohexane and 2-cyclohexylidene-
cyclohexanone (Dianon) were detected as major by-products.
According to the above results, the reaction pathway was deduced
and shown in Scheme 1. N-cyclohexyl-1,6-diaminohexane was rea-
sonably obtained by the reductive amination of 1,6-diaminohexane
with 1 eq cyclohexanone (r2). Cyclohexane was considered to be
generated from the hydrogenation–dehydration–hydrogenation of
cyclohexanone (r3). Dianon could be formed from the aldol conden-
sation of cyclohexanone itself (r4). Chary et al. [16] and Reichle [17]
have reported that the strong acidic sites are responsible for cyclo-
hexanol dehydration and cyclohexanone condensation. Additionally,
it was also reported that strong acidic sites restraint the desorption
of amino compounds from the surface of catalyst [18], and that may
be the reason why only solvent was detected in the first 15 h reaction
mixture. Thus, even Cu–Cr–La/γ-Al2O3 was proved effective for the
reductive amination of triacetoneamine with n-butylamine, it also
should be improved for the reaction of cyclohexanone with 1,6-
diaminohexane.
2.3. Catalytic reductive amination
The reductive amination of cyclohexanone with 1,6-diaminohexane
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. A solution of cyclohexanone and 1,6-diaminohexane in 1,4-
dioxane was stirred for 10 h at ambient temperature, and then dosed
into the reactor at a flow rate of 0.2 mL/min by a syringe pump. The
GC–MS (Polaris Q, Thermo Finngan, America) used to confirm the com-
ponents of the reaction mixture was performed on a HP-5 capillary col-
umn (30 m×0.25 mm, 0.2 μm film thickness) equipped with an ion
trap MS detector. The composition of the reaction mixture was deter-
mined by GC with a 30 m SE-54 capillary column.
3. Results and discussion
According to the analysis of the above, the strong acid sites on the
surface of catalyst were confirmed responsible for the side reactions.
Thus, the alumina B was chosen to add to the Cu–Cr–La/γ-Al2O3 to de-
crease the catalyst acidity and the prepared catalysts were employed to
the reductive amination of cyclohexanone with 1,6-diaminohexane,
and the results are shown in Table 1.
All catalysts exhibited satisfied conversion of cyclohexanone,
which means the conversion was completely depending on the
metal components but not the support. With the addition of alumina
B, the selectivity of N, N′-dicyclohexyl-1,6-diaminohexane increased
from 53.1% to 79.1% and thereafter it decreased slightly. There was
no cyclohexane and dianon detected when the content of alumina B
reached to 50%, the reason is that the low acidity of catalysts can
inhibit the dehydration of cyclohexanol and the aldol condensation
of cyclohexanone. Nevertheless, the selectivity of N-cyclohexyl-1,6-
diaminohexane almost showed no change with the addition of alumi-
na B, which indicated that N-cyclohexyl-1,6-diaminohexane selectiv-
ity was impacted mainly by reaction conditions. In addition, all the
3.1. Catalyst modification
As what mentioned before, Cu–Cr–La/γ-Al2O3 was employed for
the reaction of cyclohexanone with 1,6-diaminohexane and expected
a high yield of N, N′-dicyclohexyl-1,6-diaminohexane. The first 15 h re-
action mixture was collected and detected, unexpectedly, only 1,4-
dioxane was obtained with this catalyst. We inferred that the amino
compounds were possibly adsorbed tightly on the surface of catalyst
due to the strong base–acid interreaction between amino compounds
and catalyst. However, the similar phenomenon was not observed in
the reductive amination of triacetoneamine with n-butylamine over
Cu–Cr–La/γ-Al2O3. The relatively weak interreaction of monoamine
and catalyst than that between diamine and catalyst, and the steric hin-
drance of amino group in N-butyl-2,2,6,6-tetramethyl-4-piperidinamine
were the reasons why the product could be obtained smoothly.
The reaction resultants were flushed out from the catalyst bed
by large amount of 1,4-dioxane, and the obtained solution was
Scheme 1. The main and side reaction processes for the reductive amination.

60
M. Sun et al. / Catalysis Communications 20 (2012) 58–62
Table 1
Reductive amination of cyclohexanone with 1,6-diaminohexane over various catalysts.
Catalyst
Conversion/%
Selectivity/%
Cyclo-hexanol
Cyclo-hexane
Dianon
Na
N, Nb
Cu20Cr5La5/γ-Al2O3
99.8
99.8
100
17.3
11.7
8.1
8.2
4.7
0
7.3
2.4
0
14.1
12.8
12.8
13.6
53.1
68.4
79.1
78.5
c
Cu20Cr5La5/γ-Al2O3-AB17.5
Cu20Cr5La5/γ-Al2O3-AB35
Cu20Cr5La5/γ-Al2O3-AB52.5
100
7.9
0
0
Reaction conditions: temperature=120 °C; hydrogen pressure=4.0 MPa; flow rate=0.2 mL/min; cyclohexanone: 1,6-diaminohexane (mole)=2:1; concentration in 1,4-dioxane=
20.0 wt.%.
a
N N-cyclohexyl-1,6-diaminohexane.
N, N N, N′-dicyclohexyl-1,6-diaminohexane.
AB alumina B.
b
c
Table 2
90.0° in the XRD curves could be assigned to the crystals of elementary
copper. There was no evidence for the copper crystalline was affected
BET results for the prepared catalysts.
by the addition of alumina B. Moreover, calculation from the main diffrac-
tion line (43.3°) on the XRD patterns using Scherrer equation, the
mean diameters of Cu0 particle in all these four catalysts showed no ob-
vious difference. These results demonstrated that the doped alumina B
had no influence to the active species. Furthermore, no obvious peaks
corresponding to Cr, La or any their combinations were picked out from
the XRD curves, we suggest that these components are either too small
to be detected or in the amorphous phases.
Catalyst
Surface area
(m2/g)
Average pore
diameter (Å)
Total pore volume
(cm3/g)
Cu20Cr5La5/γ-Al2O3
200.0
208.7
224.5
233.9
81.27
87.23
103.10
108.70
0.31
0.37
0.42
0.52
Cu20Cr5La5/γ-Al2O3-AB17.5
Cu20Cr5La5/γ-Al2O3-AB35
Cu20Cr5La5/γ-Al2O3-AB52.5
components could be detected in the first 3 h reaction mixture in case
of Cu20Cr5La5/γ-Al2O3-AB35 and Cu20Cr5La5/γ-Al2O3-AB52.5. To better
understand the effects of alumina B to Cu20Cr5La5/γ-Al2O3, the cata-
lysts were characterized by XRD and NH3-TPD.
3.2.3. TEM
TEM images for Cu20Cr5La5/γ-Al2O3 and Cu20Cr5La5/γ-Al2O3-AB35
are shown in Fig. 2. The images showed that the size distribution of
copper particles was similar in the both samples. The dispersion and
average size of copper particles in Cu20Cr5La5/γ-Al2O3 were not so
far different from that of Cu20Cr5La5/γ-Al2O3-AB35, and the structural
properties of active species was not influenced by the addition of alu-
mina B, which agreed well with the XRD results.
3.2. Catalysts characterization
3.2.1. Textural properties of catalysts
The specific surface areas and pore structural parameters of all the
four catalysts are summarized in Table 2. As can be seen, the surface
area of Cu20Cr5La5/γ-Al2O3 is smaller than that of modified by alumi-
na B but the surface areas are still in the similar range due to the small
changes. As for the modified catalysts, the surface area, average pore
diameter and total pore volume increase with the increase of alumina
B amount in the prepared catalysts.
3.2.4. NH3-TPD
In the present investigation, NH3-TPD is used to compare the acidity
distinction between Cu20Cr5La5/γ-Al2O3 and Cu20Cr5La5/γ-Al2O3-AB35
and the TPD curves are shown in Fig. 3. The Cu20Cr5La5/γ-Al2O3 catalyst
exhibited a typical double-peak. The two desorption peaks at the lower
temperature (229 °C) and the higher temperature (470–529 °C) corre-
spond to the weak and strong acid sites on the catalyst surface, respec-
tively. As for Cu20Cr5La5/γ-Al2O3-AB35, the higher temperature peak
could not be observed obviously. We deduced that the alumina B neu-
tralized the strong acid sites on the catalyst surface. Furthermore, the
area of a specific peak can be used to estimate the amount of ammonia
desorbed from the sample, and can be taken as a standard to quantify
the acidity of the sample [19,20]. The peak areas for the two catalysts
are listed in Table 3, it can be found that the total peak area decreased
from 5499 in curve (a) to 2421 in curve (b). Combination with the
both points, it is clearly that the addition of alumina B dramatically de-
creased the acidity of Cu20Cr5La5/γ-Al2O3. As shown in Table 1, the selec-
tivity of cyclohexane and dianon decreased from 8.2% and 7.3% to 4.7%
and 2.4% with the addition of alumina B, and no cyclohexane and dianon
was detected when the content of alumina B reached to 50%. It means
that the addition of alumina B to Cu20Cr5La5/γ-Al2O3 can decrease the
selectivity of cyclohexane and dianon, namely, the lower acidity is re-
sponsible for inhibiting the dehydration of cyclohexanol and aldol con-
densation of cyclohexanone. What's more, all the components could
be detected in the first 3 h reaction mixture in case of Cu20Cr5La5/γ-
Al2O3-AB35, which indicates that the low acidity is the reason why the
3.2.2. XRD
The XRD curves for the reduced Cu20Cr5La5/γ-Al2O3, Cu20Cr5La5/γ-
Al2O3-AB17.5, Cu20Cr5La5/γ-Al2O3-AB35 and Cu20Cr5La5/γ-Al2O3-AB52.5
are shown in Fig. 1. The typical diffraction lines at 43.3°, 50.4°, 74.2° and
#
# Cu
#
#
#
(d)
(c)
(b)
(a)
products could be desorbed easily from Cu20Cr5La5/γ-Al2O3-AB35
.
0
20
40
60
80
100
3.3. The effect of reaction temperature
Two-Theta (degree)
The effect of reaction temperature on the reductive amination of cy-
Fig.1. XRD curves for reduced (a) Cu20Cr5La5/γ-Al2O3, (b) Cu20Cr5La5/γ-Al2O3-AB17.5
,
(c) Cu20Cr5La5/γ-Al2O3-AB35 and (d) Cu20Cr5La5/γ-Al2O3-AB52.5
.
clohexanone with 1,6-diaminohexane over Cu20Cr5La5/γ-Al2O3-AB35

M. Sun et al. / Catalysis Communications 20 (2012) 58–62
61
Fig. 2. TEM images for (a) Cu20Cr5La5/γ-Al2O3 and (b) Cu20Cr5La5/γ-Al2O3-AB35
.
was investigated and the results are shown in Fig. 4. As can be seen, the
conversion of cyclohexanone was independent on the temperature in
the range 120 °C–210 °C and remained around 100%. In response
to the growing temperature, the selectivity of N, N′-dicyclohexyl-1,6-
diaminohexane slightly increased and a highest selectivity of 88.3%
was achieved at 180 °C, and thereafter decreased to 75.3% at 210 °C,
while the selectivity to cyclohexanol decreased insistently. The reason
for the results was deduced. The hydrogenation of cyclohexanone on
the surface of catalyst was irreversible at low temperature but revers-
ible at higher temperature (r3 in Scheme 1). At low temperature
(≤120 °C), cyclohexanol was generated from the hydrogenation of cy-
clohexanone and followed by dehydration-hydrogenation to form
cyclohexane. However, at the higher temperature (≥150 °C), the
dehydrogenation of generated cyclohexanol to cyclohexanone
preferentially occurred, and this part of cyclohexanone can further
react with 1,6-diaminohexane to form N, N′-dicyclohexyl-1,6-diamino-
hexane. The similar phenomena were also reported in some other stud-
ies [21–24]. Thus, higher temperature was favorable for the production
of N, N′-dicyclohexyl-1,6-diaminohexane, but more N-cyclohexyl-1,6-
diaminohexane was generated in the temperature range 180 °C–
210 °C, 180 °C was chosen as the reaction temperature.
3.4. The effect of molar ratio of starting materials
Due to the unavoidable consumption of cyclohexanone in catalytic
hydrogenation and the generation of N-cyclohexyl-1,6-diaminohexane,
the appropriate excessive cyclohexanone was considered necessary.
Thus, the molar ratio of cyclohexanone to 1,6-diaminohexane over
Cu20Cr5La5/γ-Al2O3-AB35 was studied and the results are shown in
Fig. 5. With the molar ratio increased from 2.0 to 2.2, the selectivity of
N, N′-dicyclohexyl-1,6-diaminohexane slightly increased from 88.3%
to 92.6%. With the further increase in molar ratio, the side reactions in-
creased, and the selectivity of N, N′-dicyclohexyl-1,6-diaminohexane
229
(a)
100
90
80
(b)
70
conversion
60
cyclohexanol
N
N, N
200
300
400
500
600
700
800
50
40
30
20
10
0
Fig. 3. NH3-TPD curves for (a) Cu20Cr5La5/γ-Al2O3 and (b) Cu20Cr5La5/γ-Al2O3-AB35
.
Table 3
Temperature programmed desorption of NH3 for various catalysts.
Catalyst
Tdesa/°C Peak
areab/a.u.
Tdesc/°C Peak
areab/a.u. areab/a.u.
Total peak
120
150
180
210
Cu20Cr5La5/γ-Al2O3
Cu20Cr5La5/γ-Al2O3-AB35 229
229
3095
2421
470
–
2404
–
5499
2421
Fig. 4. Influence of the temperature on the reductive amination over Cu20Cr5La5/γ-
Al2O3-AB35.Reaction conditions: hydrogen pressure=4.0 MPa; flow rate=0.2 mL/min;
cyclohexanone: 1,6-diaminohexane (mole)=2:1; concentration in 1,4-dioxane=
20.0 wt.%.
a
Lower temperature peak for weak acid sites.
The peak area was calculated by Gaussian fitting method.
Higher temperature peak for strong acid sites.
b
c

62
M. Sun et al. / Catalysis Communications 20 (2012) 58–62
4. Conclusion
100
90
80
70
60
50
40
30
20
10
0
Alumina B modified Cu20Cr5La5/γ-Al2O3 was certified suitable for
the reductive amination of cyclohexanone with 1,6-diaminohexane
in a continuous process. The addition of alumina B remarkably de-
creased the acidity of catalyst but had no influence to the active
spices. The low acidity could inhibit the dehydration of cyclohexanol
and the aldol condensation of cyclohexanone, furthermore, the low
acidity facilitated the desorption of amino compounds from the sur-
face of catalyst. The hydrogenation of cyclohexanone on the surface
of catalyst was irreversible at low temperature but reversible at
higher temperature, and 180 °C was selected as the reaction temper-
ature. The selectivity of N-cyclohexyl-1,6-diaminohexane was mainly
affected by molar ratio of starting materials and reaction tempera-
ture. The stability of the catalyst was tested for 300 h under optimum
reaction conditions, and the conversion of cyclohexanone and selec-
tivity of N, N′-dicyclohexyl-1,6-diaminohexane were higher than
99% and 92% over the entire reaction time. This process is promising
for large-scale industrial production.
conversion
cyclohexanol
N
N, N
2.0
2.1
2.2
2.3
2.4
Molar ratio (mol/mol)
Fig. 5. Influence of molar ratio of cyclohexanone to 1,6-diaminohexane.Reaction condi-
tions: temperature=180 °C; hydrogen pressure=4.0 MPa; flow rate=0.2 mL/min;
concentration in 1,4-dioxane=20.0 wt.%.
Acknowledgment
The financial support was provided by the National Natural Science
Foundation of China (grant no. 20976123).
100
95
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3.5. The stability of catalyst
Finally the stability of Cu20Cr5La5/γ-Al2O3-AB35 was tested with
time-on-stream for 300 h under optimum reaction conditions and
the results are shown in Fig. 6. Clearly no deactivation occurred in
the reductive amination at 180 °C. The conversion of cyclohexanone
and selectivity of N, N′-dicyclohexyl-1,6-diaminohexane were higher
than 99% and 92% over the entire reaction time, respectively. There-
fore, the catalyst is promising for pilot plant test.