Effective depolymerization of nylon-6 in wet supercritical hydrocarbons

Article abstract of DOI:10.1246/cl.2011.370

Treatment of nylon-6 with supercritical toluene in the presence of small amounts of water resulted in an effective conversion of polyamide to give ε-caprolactam in quantitative yield. The presence of a small amount of water is critical for the progress of the reaction; completely anhydrous conditions failed to achieve depolymerization. ε-Caprolactam was readily isolated after the removal of toluene under reduced pressure. The present method can serve as a useful treatment for the effective chemical recycling of waste plastics. The combined use of hydrocarbon and water is a new technique to control the reactivity of hightemperature water.

Full text of DOI:10.1246/cl.2011.370

doi:10.1246/cl.2011.370
3
70
Published on the web March 2, 2011
Effective Depolymerization of Nylon-6 in Wet Supercritical Hydrocarbons
Kouji Kaiso,1,2 Tsunemi Sugimoto, Kohichi Kashiwagi, and Akio Kamimura*
2
2
1
1
Department of Applied Molecular Bioscience, Graduate School of Medicine, Yamaguchi University,
Ube, Yamaguchi 755-8611
2
Ube Laboratory, Ube Industries, Co., Ltd., Ube, Yamaguchi 755-8633
(
Received January 5, 2011; CL-110010; E-mail: ak10@yamaguchi-u.ac.jp)
O
Treatment of nylon-6 with supercritical toluene in the
O
sub- or supercritical
hydrocarbon
presence of small amounts of water resulted in an effective
conversion of polyamide to give ¾-caprolactam in quantitative
yield. The presence of a small amount of water is critical for the
progress of the reaction; completely anhydrous conditions failed
to achieve depolymerization. ¾-Caprolactam was readily isolated
after the removal of toluene under reduced pressure. The present
method can serve as a useful treatment for the effective chemical
recycling of waste plastics. The combined use of hydrocarbon
and water is a new technique to control the reactivity of high-
temperature water.
NH
N
3
70°C, 1 h
H
n
1
Scheme 1.
Table 1. Depolymerization of nylon-6 in hydrocarbon­water
mixture
Water
wt %
100
Time 1; yield Tcd
Pce
Entry Solvent
b
/
/h
/%
/°C /MPa
a
1
2
3
4
5
6
7
8
9
0
Water
1 (19.7) 67
374.0 22.1
318.6 4.1
Development of an efficient recycling method for waste
plastics is an important issue worldwide. Among many strategies
for plastic recycling, the most ideal way is monomer recycling,
in which waste plastics are converted into corresponding
0.0001 2 (6.5)a 15
Toluene
Toluene
Hexane
Hexane
Heptane
Octane
a
c
10
1 (12.9) 97 (93)
0.0001 2
27
91
234.3 3.0
10
10
10
1
1
1
1
1
1
1
1
monomers that are converted into new plastics. An advantage
92
90
93
96
94
74
267.1 2.7
295.6 2.5
374.1 1.8
385.1 1.8
402.7 1.7
of this strategy is the preservation of depleting petroleum
resources. To establish a monomer recycling method, an efficient
depolymerization method is required. Plastics usually decom-
pose when exposed to high temperatures in the presence of acids
or bases. However, these conditions can be harsh enough to
destroy polymers as well as monomeric materials during the
reaction. To enhance the efficiency of depolymerization, less
severe and selective methods are necessary.
Cyclooctane 10
Dodecane
Tridecane
10
10
1
11
a
Liq. Paraffin 10
Internal pressure (MPa) of the reaction vessel in parentheses.
GC yield. Isolated yield. Critical temperature of the solvent.
b
c
d
e
Polyamides such as nylon-6 are among the most widely
used polymers in our daily lives, and the monomer recycling
Critical pressure of the solvent.
2
of nylon is of interest to many chemists. Recently, we have
reaction also offered unsatisfactory results (Entry 2). Converse-
ly, the presence of 10 wt % of water dramatically improved the
conversion of nylon and the yield of ¾-caprolactam (1) increased
to 97% (Entry 3). The isolation of 1 was quite simple. Removal
of the solvents under reduced pressure resulted in nearly pure
¾-caprolactam (1) in 93% yield. Isolated 1, for example,
displayed a sharp melting point at 67.8­69.2 °C, which was
sufficiently close to the literature data. As long as we checked,
there seemed to be no side products derived from toluene. A
similar improvement was also observed when hexane was
employed for the reaction (Entries 4 and 5). Thus, the addition
of water to hydrocarbon was very effective in enhancing the
reaction rate of the depolymerization reaction. The same
improvement was also observed with the use of other hydro-
carbon solvents (Entries 6­10). It should be noted that the
supercritical phase seemed unnecessary for the depolymerization
of the polyamide (Entries 8­10). However, liquid paraffin did
not affect the depolymerization reaction (Entry 11). Thus, the
present modification provides a useful and selective method
for the depolymerization of nylon-6 into a sufficiently pure
monomer. No further reaction for 1 progressed under the present
conditions.
successfully developed a useful depolymerization of nylon-6 in
supercritical secondary alcohols.3 Depolymerization was also
4
achieved in ionic liquids under high-temperature conditions.
During the course of our exploration to develop more efficient
techniques for depolymerization, we have focused on the use of
wet supercritical hydrocarbons because dilution of water with
inert hydrocarbon may weaken the excessive reactivity of
supercritical water. In this paper, we report high-temperature
wet hydrocarbons as novel media for the depolymerization of
nylon-6. The presence of a specific amount of water is a key
factor for the effective conversion of nylon-6 into ¾-caprolactam
in good yield with excellent purity. The combined use of
hydrocarbon and water is a new method to control the reactivity
of high-temperature water.
The depolymerization reaction of nylon-6 was examined
using several types of hydrocarbon solvent. The reactions were
carried out at 370 °C for 1 h in an autoclave (Scheme 1). These
results are summarized in Table 1.
Treatment of nylon-6 in supercritical water resulted in its
smooth consumption; however, the yield of 1 was rather poor
5
6
(
Entry 1). The use of anhydrous supercritical toluene for the
Chem. Lett. 2011, 40, 370­371
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

3
71
Figure 1. Depolymerization in toluene­water (9:1) mixture.
T = 250 ( ), 280 ( ), 300 ( ), and 350 °C ( ).
Figure 2. Effect of the amounts of water.
We have successfully demonstrated that the use of super-
critical hydrocarbon containing water dramatically improves
the depolymerization of nylon-6. It should be noted that the
combined use of water and hydrocarbon effectively controlled
reactivity and provided selective depolymerization. The present
method, which consisted of a simple manipulation to produce a
high yield of 1 with excellent purity, will prove viable for plastic
recycling chemistry. Further investigation and application of the
present method is now underway in our laboratory.
To clarify the reaction profiles, we next examined temper-
ature dependence of the reactions. We chose a 90:10 toluene­
water system as a model and examined the reactions at several
temperatures for 1 h. The results are summarized in Figure 1.
The reaction performed at 350 °C for 1 h produced a liquid­
liquid two-phase mixture with no observed precipitates of
oligomeric polyamides. It should be noted that the yield of 1 did
not change when the reaction time was prolonged to 3 h. This
indicated that compound 1 remained under these reaction
conditions for several hours. Conversely, the reaction rate
became much slower when the temperature was set to 280 °C or
below; the yield of 1 only reached around 60% after 3 h. Under
these conditions, the reaction mixture was a heterogeneous
mixture that contained solid material. This observation clearly
indicates that oligomeric polyamides remained in the reaction
mixture. The oligomer was isolated by filtration. The mean
molecular weight of the oligomer was estimated to be about
References and Notes
1
Technology for Feedstock Recycling of Plastic Wastes, ed. by
Research Association of Feedstock Recycling of Plastics, CMC
Books, Tokyo, Japan, 2005.
2
a) S. R. Shukla, A. M. Harad, D. Mahato, J. Appl. Polym. Sci.
2
006, 100, 186. b) M. Goto, M. Sasaki, T. Hirose, J. Mater. Sci.
2006, 41, 1509. c) T. Iwaya, M. Sasaki, M. Goto, Polym.
Degrad. Stab. 2006, 91, 1989. d) L. Meng, Y. Zhang, Y. Huang,
M. Shibata, R. Yosomiya, Polym. Degrad. Stab. 2004, 83, 389.
e) O. Sato, Y. Ikushima, Kobunshi Ronbunshu 2001, 58, 533.
f) M. Goto, M. Umeda, A. Kodama, T. Hirose, S. Nagaoka,
Kobunshi Ronbunshu 2001, 58, 548. g) H. Bockhorn, A.
Hornung, U. Hornung, J. Weichmann, Thermochim. Acta 1999,
¹
1
1300 g mol based on GPC analysis. It should be noted that the
oligomer gave ¾-caprolactam (1) when treated at 350 °C under
the same reaction conditions.
We next investigated the effects of water content. The
results are illustrated in Figure 2. When the reaction was carried
out in anhydrous toluene, the depolymerization did not progress
well, and insoluble solids remained in the reaction mixture. GC
analysis determined the yield of 1 to be only 8%. The results
changed completely when 1.25 wt % of water was added to the
reaction mixture, increasing the yield of 1 to 83%. The yield of 1
rose as the amount of water increased, and the maximum yield
was observed when 10 wt % of water was added to the reaction
mixture. Although depolymerization occurred smoothly, the
yield of ¾-caprolactam gradually decreased when more than
3
37, 97. h) S. Czernik, C. C. Elam, R. J. Evans, R. R. Meglen,
L. Moens, K. Tatsumoto, J. Anal. Appl. Pyrolysis 1998, 46, 51.
i) A. Ballistreri, D. Garozzo, M. Giuffrida, G. Impallomeni, G.
Montaudo, Polym. Degrad. Stab. 1989, 23, 25. j) H. Ohtani, T.
Nagaya, Y. Sugimura, S. Tsuge, J. Anal. Appl. Pyrolysis 1982,
4, 117. k) I. Lüderwald, F. Merz, M. Rothe, Angew. Makromol.
Chem. 1978, 67, 193. l) N. Ogata, Bull. Chem. Soc. Jpn. 1961,
3
4, 1201.
3
4
A. Kamimura, Y. Oishi, K. Kaiso, T. Sugimoto, K. Kashiwagi,
ChemSusChem 2008, 1, 82.
a) A. Kamimura, S. Yamamoto, Org. Lett. 2007, 9, 2533. b) A.
Kamimura, S. Yamamoto, Polym. Adv. Technol. 2008, 19, 1391.
c) S. Yamamoto, A. Kamimura, Chem. Lett. 2009, 38, 1016.
General procedure for the depolymerization of nylon-6:
10% water was present in the reaction mixture. Thus, 10 wt %
water was determined to be the optimum reaction condition for
the present depolymerization.
5
¹1
It is not easy to explain the role of water in the reaction;
however, we hypothesized the following. The presence of water
enhanced the hydrolysis rate of nylon-6. This reaction forms
monomeric 6-aminocapronic acid, which readily cyclizes to
yield ¾-caprolactam. Water may also react with these monomers
to give undesirable side products.2b However, the limited water
content under the present conditions likely suppresses the side
reaction efficiently. As a result, ¾-caprolactam became the sole
product of the reaction and the yield of 1 increased to more than
Nylon chip (1.2 g, Mw = 22000 g mol ) was put in a mixture
of toluene­water (16 g, 90:10). The reaction vessel was sealed
and heated at 370 °C for 1 h. The reaction vessel was cooled by
water. Toluene and water were removed under reduced pressure
to give ¾-caprolactam (1) in 93% yield (1.099 g); mp 67.8­
1
6
9.2 °C; H NMR (270 MHz, CDCl3): ¤ 1.73 (m, 6H), 2.47 (t,
J = 5.5 Hz, 2H), 3.21 (dd, J = 5.9, 9.9 Hz, 2H), 6.2­6.5 (br,
13
1
H); C NMR (67.5 MHz, CDC13): ¤ 23.1, 29.6, 30.5, 36.7,
4
2.7, 179.4.
6
For example: Aldrich Handbook of Fine Chemicals, 2009,
90%.
p. 600.
Chem. Lett. 2011, 40, 370­371
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/