Synthesis of 1,4-diaminocyclohexane in supercritical ammonia

Article abstract of DOI:10.1006/jcat.1999.2410

The amination of 1,4-cyclohexanediol in supercritical ammonia has been studied in a continuous fixed-bed reactor at 135 bar. An unsupported cobalt catalyst stabilized by 5 wt% Fe afforded the main reaction products 4-aminocyclohexanol and 1,4-diaminocyclohexane with a cumulative selectivity of 97% at 76% conversion. Excess of ammonia and short contact time favored the desired reactions. At low and high conversions the amination selectivity decreased due to the formation of dimers and oligomers and degradation products. Recycling of the unreacted diol and amino alcohol intermediate can provide an alternative economic process for the synthesis of 1,4-diaminocyclohexane.

Full text of DOI:10.1006/jcat.1999.2410

Journal of Catalysis 182, 289–291 (1999)
Article ID jcat.1999.2410, available online at http://www.idealibrary.com on
PRIORITY COMMUNICATION
Synthesis of 1,4-Diaminocyclohexane in Supercritical Ammonia
A. Fischer, T. Mallat, and A. Baiker1
Laboratory of Technical Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zurich, Switzerland
Received November 23, 1998; revised January 4, 1999; accepted January 4, 1999
tivity, catalyst lifetime, and separation) of heterogeneous
catalytic processes. Advances made in this field have been
discussed in a recent review (13).
The amination of 1,4-cyclohexanediol in supercritical ammo-
nia has been studied in a continuous fixed-bed reactor at 135 bar.
An unsupported cobalt catalyst stabilized by 5 wt% Fe afforded
the main reaction products 4-aminocyclohexanol and 1,4-diamino-
cyclohexane with a cumulative selectivity of 97% at 76% conver-
sion. Excess of ammonia and short contact time favored the desired
reactions. At low and high conversions the amination selectivity
decreased due to the formation of dimers and oligomers and degra-
dation products. Recycling of the unreacted diol and amino alcohol
intermediate can provide an alternative economic process for the
c
In a previous study we found that the application of
supercritical ammonia (scNH3) as a solvent and reactant
affords remarkable selectivity improvement in the amina-
tion of 1,3-propanediol, compared with the subcritical pres-
sure procedure (14). The selectivity improvement could be
traced to the suppression of hydrogenolysis (degradation)-
type side reactions. In the present study we describe the
amination of 1,4-cyclohexanediol (Scheme 1) under super-
critical conditions, using an iron-stabilized cobalt catalyst
in a continuous high-pressure reactor.
synthesis of 1,4-diaminocyclohexane.
1999 Academic Press
Key Words: amination; cobalt–iron; 1,4-cyclohexanediol; 1,4-
diaminocyclohexane; supercritical ammonia.
At present, diaminocyclohexanes are manufactured by
the catalytic hydrogenation of aromatic amines such as
p-phenylenediamine (15, 16). Considering the availability,
oxidation stability, and toxicology of the reactant, the am-
ination of cyclohexanediol is an attractive alternative. 1,4-
Diaminocyclohexane is an important chemical applied as
chain extender in polyurea elastomers (17), ingredient in
lubricants (18), agent in the synthesis of ZSM-35 (19), and
component of antitumor agent platinum complexes (20).
To our knowledge direct amination starting from 1,4-cyclo-
hexanediol has not been reported so far.
INTRODUCTION
Amination of aliphatic alcohols on a metal catalyst pro-
vides economic access to a multitude of amines (1–4). How-
ever, the yields and selectivities are usually rather low in the
synthesis of aliphatic diamines from the corresponding di-
ols and ammonia.
The metal-catalyzed synthesis of aliphatic amines from
the corresponding alcohol includes dehydrogenation of the
alcohol to a carbonyl compound, condensation with ammo-
nia to form an imine or enamine, and hydrogenation to the
amine (5, 6). Each intermediate and the product amine can
take part in condensation, decarbonylation, disproportion-
ation, and hydrogenolysis side reactions (7–10). The synthe-
sis of a diamine from the corresponding diol requires the
repetition of all three steps which increases the by-product
formation. In addition, the bifunctional intermediates have
the tendency to undergo oligomerization reactions (6, 11,
12). A further difficulty is that the intermediate and prod-
uct amines are significantly more reactive than ammonia.
Application of supercritical fluids provides interesting op-
portunities for improving the efficiency (conversion, selec-
EXPERIMENTAL
For the preparation of the 95 wt% Co–5 wt% Fe catalyst,
the metal nitrates (molar ratio of 20/1; total metal ni-
trates: 0.18 mol) were dissolved in 500 ml water. One hun-
dred grams of an aqueous solution containing 20 wt%
(NH4)2CO3 was added at room temperature over a 1-h pe-
riod until a pH of 7 was reached. The suspension was stirred
for 2 h and filtered. After careful washing, the precipitate
was dried at 100 C, calcined at 400 C for 2 h, and activated
by hydrogen reduction at 335 C for 4 h. Structural prop-
erties of the reduced catalyst were BET surface area of
12 m2 g 1, specific pore volume of 0.1 cm3 g 1, and mean
pore diameter of 43 nm. A detailed characterization of the
catalyst will be presented elsewhere (21).
1 To whom correspondence should be addressed. Fax: ++41–1–
632 11 63. E-mail: baiker@tech.chem.ethz.ch.
289
0021-9517/99 $30.00
c
Copyright
1999 by Academic Press
All rights of reproduction in any form reserved.

290
FISCHER, MALLAT, AND BAIKER
amines. To compensate this reactivity difference, a rela-
tively large NH3/R-OH molar ratio is necessary to obtain
acceptable selectivities to primary amines. Accordingly, all
reactions were carried out at 165 C and 135 bar with a mo-
lar ratio NH3/R-OH = 60/1. A small amount of hydrogen in
the feed (1–5 mol% ) was sufficient to prevent the undesired
dehydrogenation reactionsand the formation ofnitrilesand
carbonaceous deposits.
Figure 1 illustrates the influence of temperature on the
amination of 1,4-cyclohexanediol (1) in supercritical am-
monia over a 95% Co–5% Fe catalyst. The conversion
indicates the consumption of 1 via dehydrogenation to
4-hydroxycyclohexanone or acid–base catalyzed side reac-
tions. This metal-catalyzed step has been found to be rate
determining in the amination of simple aliphatic alcohols
(23).
The formation of the amination products 4-amino-
cyclohexanol (2, Scheme 1) and 1,4-diaminocyclohexane
(3) shows the typical course of a consecutive reaction se-
ries. A maximum yield of 32% for the amino alcohol (2)
was achieved at 165 C, and a maximum yield of 54% for
the diamine (3) at 185 C.
The selectivity to the diamine (3) could be further im-
proved by varying the contact time. Some examples are
shown in Table 1. The two sets of experiments at 165 and
195 C depict the same tendency: at higher contact times
the by-products formed by hydrogenolysis dehydration and
dimerization/oligomerization became dominant. At 165 C
the catalyst was rather selective and the amount of by-
products did not exceed 3% up to 40,000 gs mol 1. Con-
trary to the expectation, there was only a small change in
SCHEME 1. Important reactions occurring during the amination
of 1,4-cyclohexanediol with ammonia. The intermediates and products
shown were identified by GC–MS.
The apparatus comprised the dosing system for the
ammonia–alcohol mixture (ISCO D500 syringe pump) and
hydrogen (mass flow meter), the high-pressure continu-
ous fixed-bed reactor, and a gas/liquid separator. The re-
actor was constructed of Inconel-718 tubing of 13-mm in-
ner diameter and 38-ml volume. The temperature in the
reaction zone was measured with a thermocouple located
in the center of the tube and was regulated by a PID cas-
cade controller. The total pressure in the reactor system
was set by a Tescom backpressure regulator. Standard reac-
tion conditions were 8.0 g catalyst, 165 C, 135 bar, 40,000 gs
mol 1 contact time (WHSV: 1.63 g g 1 h 1), and molar ratio
of the reactants R-OH/NH3/H2 = 1/60/2.
The liquid products were analyzed using a gas chromato-
graph (HP-5890A with FID detector and HP-1701 column)
and were identified by GC–MS analysis.
The following reactant purities were quoted by the man-
ufacturer: 1,4-cyclohexanediol > 98% (Fluka), ammonia
99.98% (Pan-Gas), hydrogen 99.999% (Pan-Gas), and ni-
trogen 99.995% (Pan-Gas).
The existence of a single fluid phase under the experi-
mental conditions used (130 bar, 200 C) has been corrobo-
rated by independent visual tests with two diols in similarly
diluted mixtures (1.5 mol% diol). The critical data for am-
monia are TC = 132.4 C and Pc = 114.8 bar (22).
RESULTS AND DISCUSSION
As stated in the Introduction, the activity of ammonia is
usually lower than that of the intermediate and product
FIG. 1. Influence of conversion on the amination of 1,4-cyclo-
hexanediol over a 95% Co–5% Fe catalyst; standard conditions.

1,4-DIAMINOCYCLOHEXANE SYNTHESIS
291
TABLE 1
aminocyclohexane (8), was also favored. The degradation
products were partly further aminated (with ammonia or
some amines).
Influence of the Contact Time on the Amination of 1,4-Cyclo-
hexanediol (1, in Scheme 1) over a 95% Co–5% Fe Catalyst under
Otherwise Standard Conditions
CONCLUSIONS
Yield (% )
Contact time
Temperature
( C)
Conversion
(% )
An alternative catalytic route for the synthesis of 1,4-
diaminocyclohexane has been shown that is based on the
amination of 1,4-cyclohexanediol over a Co–Fe catalyst in
scNH3. The amination affords 67% yield at almost com-
plete conversion. The efficiency of the reaction can be fur-
ther improved by recycling the unreacted diol and amino
alcohol intermediate, reducing the amount of by-products
to ca. 3% . The high chemical efficiency combined with the
engineering advantages of continuous operation and easy
separation from the supercritical solvent and reactant am-
monia provides a good basis for industrial application of
the process.
1
[gs mol
]
2
3
By-products
30,000
40,000
60,000
20,000
30,000
40,000
60,000
165
165
165
195
195
195
195
70
76
93
21
32
9
6
3
46
42
29
67
55
54
52
3
2
55
26
42
43
47
99
100
100
100
3
1
the diamine yield at 195 C, despite of the complete con-
version of diol. The amount of by-products barely changed
when doubling the contact time from 30,000 to 60,000 gs
mol 1. Beside dimers and oligomers, degradation products
such as 7 and 8 (Scheme 1) could be identified in the prod-
uct mixture. Figure 2 illustrates the cumulative selectivity
to the amino alcohol (2) and diamine (3) as a function of
conversion, on the basis of the data in Fig. 1. The amination
selectivity has an optimum of 97% at 76% diol conversion.
The lower selectivity at low and high conversion is due to
the formation of dimers (4–6), oligomers, and degradation
products. At low conversion the dimer (5) was the main by-
product. At high conversion (and longer contact times) 2
and 3 were transformed to dimers (mainly 6) and insoluble
oligomers, and the generation of degradation products, e.g.,
ACKNOWLEDGMENT
Financial support from Lonza Ltd, Visp, Switzerland, is kindly acknowl-
edged.
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FIG. 2. Influence of temperature on the cumulative selectivity to
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