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Chemistry Letters Vol.36, No.9 (2007)
Oxidative Degradation of Nylon-6 and Nylon-6,6 with Nitrogen Dioxide
in Supercritical Carbon Dioxide
Naohisa Yanagihara,ꢀ1 Naoto Abe,1 Hironori Takama,1 Yuichiro Shimamura,1 and Masaaki Yoshida2
1Department of Biosciences, Faculty of Science and Engineering, Teikyo University, 1-1 Toyosatodai, Utsunomiya 320-8551
2Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, 7-2-1 Yoto, Utsunomiya 321-8585
(Received May 31, 2007; CL-070589; E-mail: yanagi@koala.mse.teikyo-u.ac.jp)
Nylon-6 and nylon-6,6 were subjected to oxidative degrada-
tion with NO2 in supercritical CO2. It was found that valuable
short-chain ꢀ,!-diacid mixture could be obtained in good yield
under relatively mild conditions.
of nylon sample10 (Aldrich) was placed into the reactor followed
by the introduction of appropriate amounts of liquid NO2 and
liquid CO2. The reactor was then heated to the appropriate
reaction temperature using an oil bath. Finally, the reaction mix-
ture was extracted with acetone, and the extract was analyzed by
NMR, GC-MS, and GLC.
Supercritical fluids (SCF), and supercritical CO2 (scCO2) in
particular, have recently attracted much attention as the fourth
phase for performing chemical processes, in addition to gas,
liquid, and solid phases. The physicochemical characteristics
of SCF, such as miscibility with other gases, high mixing rates
and relatively weak molecular association, make them unusual
media for chemical reactions.1 Among the SCF, scCO2 seems
to be the most attractive because of its low cost, low critical tem-
perature (31 ꢁC) and pressure (7.4 MPa), and its non-toxic and
non-flammable nature. Furthermore, CO2 separation from the re-
action mixture is energy-efficient, hence the product is directly
obtained by simple pressure reduction. Thus, scCO2 gets prom-
ise of an environmentally acceptable solvent for a wide range of
chemical, analytical, and materials processes, including extrac-
tion and materials processing,2 SCF chromatography,3 polymer-
ization,4 catalytic reactions,5 etc.
When the final product was firstly subjected to analysis by
1H NMR, neither unchanged polymers nor nitro compounds
were detected by NMR. Subsequently, esterification of the
product by diazomethane, followed by GC-MS analysis revealed
that a mixture of dimethyl esters of ꢀ,!-diacids such as succinic
acid (SA), glutaric acid (GA), and adipic acid (AA) was the
predominant product.
Attention was subsequently focused on the yields11 of the
ꢀ,!-diacids, by varying experimental conditions such as reac-
tion temperature, time, pressure, and quantity of NO2. As can
be seen in Figure 1, in both cases of nylon-6 and -6,6, the
total yield of the acids increases with increasing temperature.
In the case of nylon-6, both the amount of GA and SA
increase continuously, as temperature rises. In contrast, for
nylon-6,6 the amount of AA is almost unchanged over the range
of temperature up to 140 ꢁC.
On the other hand, even though landfill space is limited, the
production of polymeric materials keeps rising every year in the
world. Therefore, it becomes increasingly important to develop
new techniques for reducing the amount of material lost to land-
fills.6 Among three categories of recycling techniques, e.g., ther-
mal recycling, material recycling, and chemical recycling, it
seems that chemical recycling has not received much attention,
although this technique may furnish materials that are reusable
as fuel or chemicals from the break down of polymeric waste.
Moreover, it should be mentioned that almost all of chemical re-
cycling being done today focuses on the use of olefinic addition
polymers. As far we know, little has been done, especially in the
recycling of condensation polymers by the use of SCF. Mono-
merization of nylon-67 and nylon-6,68 have been done in sub-
and/or supercritical H2O. In each case, a severe experimental
condition (300–400 ꢁC and 28–30 MPa) was required to create
monomerization, because of the use of sub- and/or supercritical
water. Pifer and Sen have reported the oxidative degradation of
nylon-6,6 by a mixture of NO and O2.9 In their study, the method
is too time-consuming (170 ꢁC, 16 h), which may reflect the low
oxidation power of the oxidant.
As shown in Figure 2, increasing the reaction time leads to
lowering the total yield of the diacids for both nylons. Moreover,
for the two nylons, it should be emphasized that the amount of
AA decreases drastically as the reaction time increases, but that
of SA increases with the progression time.
Results from studying the effect of the NO2 quantity are giv-
en in Figure 3. It is noteworthy that the overall yield goes
through a maximum at 2.0 g of NO2. Beyond the maximum,
the yield of AA tends to decrease, whereas no change is observed
in the yield of SA. Finally, it was found that there was little
pressure dependence on the oxidative degradation, even though
the total pressure of the reaction was varied from 8 to 15 MPa,
keeping the other conditions constant during the experiments.
100
80
60
40
20
0
100
80
60
40
20
0
Succinic Acid
Glutaric Acid
Adipic Acid
Succinic Acid
Glutaric Acid
Adipic Acid
(A)
(B)
Herein, we describe a new oxidative degradation procedure
that can convert nylons into valuable organic compounds in
a relatively mild condition (140 ꢁC, 1 h), applying scCO2 as a
solvent combined with NO2, which is expected to be a more
powerful oxidant than that applied on the previous study.9
All the reactions were conducted in batch in a 50-cm3 high-
pressure stainless steel reactor with a magnetic stirrer. Ca. 0.5 g
60 80 100 120 140
60
80 100 120 140
Temperature/°C
Temperature/°C
Figure 1. Effect of temperature on the yield of ꢀ,!-diacids
obtained by the oxidative degradation of nylons. (A) nylon-6;
(B) nylon-6,6 (sample: 0.5 g; 1 h; 10 MPa; NO2: 2.0 g).
Copyright Ó 2007 The Chemical Society of Japan
Yanagihara, Naohisa
