Innovation in a chemical reaction process using a supercritical water microreaction system: Environmentally friendly production of ε-caprolactam

Article abstract of DOI:10.1039/b206539h

Our microreaction system using supercritical water solutions achieves nearly 100% yield and 100% selectivity for ε-caprolactam production at reaction times shorter than 1 s.

Full text of DOI:10.1039/b206539h

Innovation in a chemical reaction process using a supercritical water
microreaction system: environmentally friendly production of
e-caprolactam
Yutaka Ikushima,* Kiyotaka Hatakeda,ab Masahiro Sato, Osamu Sato and Masahiko Araic
ab
a
ab
a
Supercritical Fluid Research Center, National Institute of Advanced Industrial Science and Technology,
4
-2-1 Nigatake, Miyagino-Ku, Sendai 983-8551, Japan. E-mail: y-ikushima@aist.go.jp
b
c
CREST, Japan Science and Technology Corporation (JST), Honcho, Kawaguchi 332-0012, Japan
Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University,
Sapporo 060-8628, Japan
Received (in Cambridge, UK) 5th July 2002, Accepted 19th August 2002
First published as an Advance Article on the web 2nd September 2002
Our microreaction system using supercritical water solu-
tions achieves nearly 100% yield and 100% selectivity for e-
caprolactam production at reaction times shorter than 1 s.
reaction times were adjusted to be shorter than 1 s. After the
reaction, the solution was quenched rapidly to room tem-
perature in order to prevent pyrolysis. In order to effect such a
rapid heating and to prevent temperature changes from
occurring upon the addition of the substrate, not only was the m-
system filled with glass wool lagging material, but the relatively
The Beckmann rearrangement of cyclohexanone oxime into e-
caprolactam is one of the most industrially important acid-
catalyzed reactions, which is the starting monomer for the
production of nylon 6. However, its practical production has
suffered from serious disadvantages of using environmentally
damaging catalysts such as highly concentrated sulfuric acid
and of forming large quantities of valueless by-products such as
ammonium sulfate, about twice as much as the product by
weight. The use of heterogeneous catalysts as an environmen-
tally friendly alternative has been limited so far, because of
problems such as the low selectivity to e-caprolactam as well as
short catalyst life.1
faster flow rate of scH O was kept at > 33 that of the substrate
2
solution. The fluctuations in temperature were controlled to
within ±0.1 K. Detailed analysis of the reaction mixture was
2
performed by GC, LC-MS and NMR. The IR spectra of scH O
8
were measured by high-pressure and high-temperature FTIR.
As shown in Table 1, the e-caprolactam was obtained in a
selectivity of 99% or above when the reaction was performed
with the scH O m-reaction system. Although a very small
2
amount (maximally about 1%) of 6-aminocaproic acid was
produced as a by-product, it can be easily dehydrated into the
desired e-caprolactam. When the mixture was heated up to 643
2
Supercritical water (scH O) should be a useful replacement
2
1
for organic solvents because water is the most environmentally
acceptable and inexpensive solvent, and its physicochemical
properties can be changed widely with density. However, the
K at a slower rate of 6 K s in a batchwise operation, we
obtained only 1.9% yield (ratio of e-caprolactam formed per
mole of cyclohexanone oxime) at a reaction time of 180 s at 673
K and 40 MPa, in which cyclohexanone, as the hydrolysis
product of cyclohexanone oxime, was predominantly formed.
Hence, the hydrolysis of cyclohexanone oxime in the hot water
region on the way to the supercritical state is presumed to be
responsible for the low yields. Hot water without the m-reactor
did not show any reactivities below 573 K at 40 MPa, but the
introduction of the m-reaction system in the same region of hot
water reaction led to an increase in the yield at a shorter reaction
ionic product (K
w
) for scH
water, and there is almost no research work on the use of
scH
O as a catalyst for acid-catalyzed organic syntheses.3
According to the K concept, such reactions have been
2
O is much lower than that of liquid
2
2
w
4
undertaken in hot water below 573 K, but the reaction rates are
relatively slow. We tried the Beckmann rearrangement using
5
scH
2
O in a batchwise operation; however, the observed yields
were very far from satisfactory for practical purposes. Contrary
to the conventional wisdom that acid-catalyzed reactions will be
time of 0.913 s. On the other hand, in the sH O m-reaction
2
difficult to rapidly proceed in scH
acceleration of the organic synthesis by using scH
2
O, we have still predicted
O, based on
system at 673 K and 40 MPa the yield was increased greatly up
2
to 83% even at a short reaction time of 0.625 s, its yield being
9
our finding on the significant reduction of the strength of
comparable or superior to those promoted by conc. H
2
SO
4
or
6
10
hydrogen bonding near the critical point. We demonstrate that
B
2
O
3
/Al
can prevent the hydrolysis from taking place in hot water and
thereby can bring out more adequately the function of scH
itself as acid. e-Caprolactam production by our scH O m-
reaction system has been thus proved to be very selective and
rapid, though a relatively higher K for hot water is generally
2 3
O catalyst. We recognize that the m-reaction system
our microreaction (m-reaction) system can achieve high se-
lectivity in satisfactory yield for e-caprolactam production even
in the absence of any acids.
2
O
2
The scH O m-reaction system can heat up reacting species
2
very quickly to the supercritical state and then can quench them
rapidly to room temperature after the reaction, and can exclude
the influence of the region of hot water on the way to the
supercritical state. Furthermore, the decrease in linear dimen-
sions provides very large surface to volume ratios as well as
very good heat- and mass-transfer conditions, which avoid hot
spots inside the reactor, leading to the possibilities of eliminat-
w
ing unwanted side reactions and of promoting desired reac-
7
tions. As shown in Fig. 1, the scH
2
O solution heated up to
around 773 K was sent to the entrance of a quick-heating part
union tee made of Hastelloy C-276) through a 1/16 inch
Hastelloy C-276 tube, while a stream of ambient cyclohexanone
(
2
1
oxime aqueous solution (0.35 mol kg ) passing through a 1/16
inch Hastelloy C-276 tube was struck against the high-speed
flow of the scH
48 K within 0.05 s. The mixture was then introduced into a 50
mL reactor (250 mm i.d.) made of Hastelloy C-276, in which the
2
O in the union tee, and was found to heat up to
6
Fig. 1 A microreaction system for supercritical water, which has been
applied to the rapid and selective production of e-caprolactam.
2
208
CHEM. COMMUN., 2002, 2208–2209
This journal is © The Royal Society of Chemistry 2002

View Article Online
Table 1 The yields of e-caprolactam production under various conditions
Reaction type
T/K
P/MPa
Reaction time/s
Selectivity (%)
Yield (%)
7
B
.69 m conc. H
2
SO
4
(ref. 9)
383
573
623
523
573
673
623
648
673
693
648
648
648
648
0.1
2.6 3 10
0.1
40
40
40
40
40
40
40
5400
5.65
72.0
72.0
95.3
0
9.5
1.9
38.9
80.0
83.0
42.1
63.5
49.6
99.3
99.5
24
2 3
O
(20%)/Al
2
O
3
(ref. 10)
75.0
95.7
0
high silica MFI zeolite (ref. 11)
hot water
hot water m-reaction
scH
scH
scH
scH
scH
scH
scH
3600
180.0
0.913
180.0
0.802
0.728
0.625
0.506
0.667
0.603
0.728
0.728
c
100.0
99.3
99.6
98.6
98.6
99.2
99.7
99.8
99.3
99.5
c
2
2
2
2
2
2
2
O
O m-reaction
O m-reaction
O m-reaction
O m-reaction
O m-reaction
O m-reaction
30
25
40
40
a
HCl scH
H
2
O m-reaction
2
b
2
SO
4
scH O m-reaction
a
HCl concentration: 7.74 3 1025 M (293 K, 0.1 MPa). b
concentration: 2.46 3 1024 M (293 K, 0.1 MPa). c Batchwise operation.
H
2
SO
4
4
considered to serve in the place of conventional acids. We
further quantify the reaction efficiency, in which the rate of
reaction is defined as the number of moles of e-caprolactam
marked decrease in the dimer structures, and at or just above the
critical temperature part of the dimer structure likely undergoes
breakdown to H O and OH .
3
+
2
2
1 21
formed per kilogram of solvent per unit time (in mol kg
h
).
We further found that our m-reaction approach enables the
production of e-caprolactam in more satisfactory yield by the
addition of a very small amount of acid. As shown in Table 1,
the addition of hydrochloric acid and sulfuric acid can increase
the yield up to nearly 100% even at reaction times shorter than
The rate of reaction with the scH
and 40 MPa reaches a high value of 549.5 mol kg
2
O m-reaction system at 673 K
2
1
21
h
and is
over 17000 times larger than in the batchwise operation, and
what is more surprising, it is about 1100, 700, and 250 times
2
1
9
+
larger than those obtained in 7.69 m (mol kg ) H
2
SO
4
and in
1 s. To investigate the influence of increment [H ] on the
10
the vapor phase over B
2
O
3
(20%)/Al
2
O
3
and high silica MFI
reactivities, we estimated the hydrogen ion concentrations of
aqueous hydrochloric and sulfuric acids at 648 K and 40 MPa
based on SueAs model13 and XiangAs equation, respectively.
zeolite,11 respectively, which have been employed or are
14
currently used for industrial production of e-caprolactam.†
2
5
It is further noted that the yield in scH
2
O becomes maximized
The concentrations of 7.74 3 10
M (293 K, 0.1 MPa)
2
4
at the near critical temperature at 40 MPa (Table 1). K
monotonously decreases with increasing temperature in the
near-critical region at 40 MPa, and the maximized yield is still
w
concept alone. The
change of hydrogen bonding is presumed to be a key factor for
clarifying the specific temperature dependence. We observed
w
hydrochloric and 2.46 3 10 M (293 K, 0.1 MPa) sulfuric acid
5
2
26
solutions correspond to 6.75 3 10 m and 7.55 3 10 m,
respectively at 648 K and 40 MPa. Thus, the conversion into e-
caprolactam proceeds much more efficiently than expected by
not satisfactorily elucidated by the K
+
the estimated increment [H ] with the addition of the dilute
acids.
the IR spectra of H
spectra were normalized by the spectrum of H
0 MPa. Fig. 2 shows the temperature dependence of the
2
O up to 743 K at 40 MPa (not shown). All
2
O at 298 K and
4
Notes and references
relative intensities of each species. The complete disappearance
of the peak intensity assigned to the monomer and/or dimer
†
Although the small size of the m-reactors would seem to preclude
industrial scale synthesis, it has been demonstrated that only a 50 mL reactor
operating continuously in pure scH O at 40 MPa and 673 K could produce
2
1
structures around 3579–3756 cm above 698 K indicates that
the monomer and dimer concentrations above 698 K are small.
It is well known12 that the harmonic vibration of the bending
2
about 0.25 kg of e-caprolactam in a day.
mode, 2n
proton donor, n
2
, is resonant with the dimer frequency assigned to the
1 W. K. Bell and C. D. Chang, US Pat. 4,359,421, 1982.
2 W. L. Marshall and E. U. Franck, J. Phys. Chem. Ref. Data, 1981, 10,
295.
2
1
1
( = 3579 cm ), and the resonance peak should
2
1
appear around 3620 cm . As shown in Fig. 2, the Fermi
resonance peak increases gradually with increasing temperature
above 423 K up to the critical temperature, over which the
intensity suddenly decreases, accompanied not only by the
marked reduction of the bending mode above 653 K, but by a
3
4
5
6
7
8
9
T. Sato, G. Sekiguchi, T. Adschiri and K. Arai, Chem. Commun., 2001,
566.
For example, J. An, T. Bagnell, T. Gablewski and C. R. Strauss, J. Org.
Chem., 1997, 62, 2505.
1
O. Sato, Y. Ikushima and T. Yokoyama, J. Org. Chem., 1998, 63,
9
100.
Y. Ikushima, K. Hatakeda, N. Saito and M. Arai, J. Chem. Phys., 1998,
08, 5855.
1
S. J. Haswell, R. J. Middleton, B. O’Sullivan, V. Skelton, P. Watts and
P. Styring, Chem. Commun., 2001, 291.
Y. Ikushima, O. Sato, T. Yokoyama and M. Arai, Angew. Chem., Int.
Ed., 1999, 38, 2910.
L. G. Donaruma and W. Z. Heldt, Org. React., 1961, 11, 1.
1
0 T. Curtin, J. B. McMonagle and B. K. Hodnett, Appl. Catal. A: Gen.,
992, 93, 75.
1
1
1
1 H. Ichihashi and H. Sato, Appl. Cat. A: Gen., 2001, 221, 359.
2 Y. Taniguchi, Kinsekigai-Bunkoho, Gakkai Shuppan Center, Tokyo,
p.119.
1
3 K. Sue, Y. Hakuta, R. L. Smith, T. Adschiri and K. Arai, Proc. 13th Int.
Conf. on Properties of Water and Steam, NRC Research Press, Ottawa,
2000, p. 145.
Fig. 2 The temperature dependence of the relative intensities of the
frequencies assigned to species such as monomer and/or dimer structures
(
n
m+d), bending mode (n
b
), out of plane bending mode (n
L
), Fermi
14 T. Xiang, K. P. Johnston, W. T. Wofford and E. F. Gloyna, Ind. Eng.
Chem. Res., 1996, 35, 4788.
resonance (n ) and hydrogen bonding network (npoly) at 40 MPa.
R
CHEM. COMMUN., 2002, 2208–2209
2209