PARTⅤ SOLVOLYSISCATALYTIC HYDROTHERMAL DEPOLYMERIZATIONAND KINETICS OF NYLON 6Jinyang Chen*,1 Guiyang Liu,1 Lujiang Jin,1 Pei Ni,2 Zhi Li,1 Yi Xu,1 Jianqiu Zhang,1Haibo He,3 Junping Dong3
1

School of Environmental and Chemical Engineering, Shanghai University,Shanghai 200444, China
2State Key Laboratory for Mineral Deposit Research, Institute of Geo-Fluid, School of Earth
Science and Engineering, Nanjing University, Nanjing 210093, China
3
School of Sciences, Shanghai University, Shanghai 200444, China
E-mail: chenjy@shu.edu.cn
Abstract: Depolymerization of nylon6 to ε-caprolactam with environmental-friendly
heteropolyacid catalyst at temperatures between 553K and 603K, under different pressures of6.4-12.8 MPa in water medium. The products of depolymerization were analysed qualitatively
and quantitatively by means of FT-IR, LC/MS and HPLC. The results indicated that the main
depolymerized product was ε-caprolactam. Remnants solid product was undepolymerizednylon6 and the liquid products contained a little 6-aminocaproic acid and oligomer. The
highest yield of ε-caprolactam was 77.96% at 573K under 85min.Compared with no catalyst,the depolymerization rate and yield of ε-caprolactam was high under catalytic condition.
Temperature and time also had important effect on depolymerization reaction. Catalytic
depolymerization kinetic at different reaction conditions was studied,where the reaction
activation energy was evaluated to be 77.38kJ/mol.
Keywords: nylon6; heteropolyacid catalyst; depolymerization reaction; ε-caprolactam;
kinetics
Plastic products are widely used in human life and waste plastics have caused serious
pollution. Recently, feedstock recycling is becoming a promising route for the management ofwaste plastic, and the pyrolysis to convert waste plastic into chemical intermediates gets more
and more attention. Direct pyrolysis in inert atmospheric conditions is batch, low yield, poor
quality, long time, second pollution of dust, exhaust air, sewage, and solid waste, and highcarbonization to block tubing, thus, its industrial application is very discommodious. Now it isknown that high temperature and pressure water presents some unique properties such as
dissolution of non-polar organic, low dielectric properties and high diffusibility, and highconcentrations of H+ and OH- ions from dissociation of water.[1-7] Pyrolysis of waste plastic- 255 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
in high temperature and pressure water can avoid disadvantage of direct pyrolysis and shows
good prospect for recycling waste polymer to produce chemicals.[8]Nylon 6 (PA 6) is an important engineering plastic and widely used, so treatment ofwaste PA 6 becomes more and more urgent. As a polymer of acyl and amido link, it is foundthat PA 6 can hydrolyze at acyl-amido bond to produce caprolactam ultimately in hightemperature and pressure water.[9,10] With increasing temperature, although hydrolysis rateof PA 6 increases, but product of caprolactam also decomposes quickly. Therefore, we study
phosphotungstic heteropoly acid (HPA) as catalyzer to accelerate hydrolysis of PA6 andincrease yield of caprolactam in comparatively mild hydrothermal condition. HPA as an acidcatalyzer presents strong acidity, high hydrothermal stability, and low acid corrosion to mostmetal owing to passivation layer forming.[11,12]
The hydrothermal vessel used is a volume of 18ml and heated with electric salt bath ofmix of NaNO3 and KNO3. The samples of nylon:water:catalyzer is about 1:15:0.03(Wt/Wt/Wt) and total volume of all samples put in vessel is 12 ml. After reaction, thehydrothermal vessel was put in ice water to cool to room temperature, and then opened to takeproduct for analysis. Liquid products are neutralized by 0.1mol/L NaOH dissolution before
measurement and were analyzed by MS and HPLC. The MS instrument is Hybrid
Quadrupole-TOF system, Applied Biosystems Co. of USA made. The measurement uses ESIof 4.5kV and 300°C, GS1 of 45.00, GS2 of 30.00, CUR of 10.00µA, and range (m/z）90-1000.
The HPLC is Aglient 1100 system, Aglient Co. of USA. The measurement uses
chromatographic column of Zorbax SB- C18 (5µm, 250 mm ×4.6 mm), UV-Vis detector of225nm, temperature of column 30°C, mobile phase water: acetonitrile=80:20+0.1% (V/V)
acetic acid, velocity of flow 0.1ml/min, sample size 5µl.
Typical MS spectrum of liquid products is shown in Fig. 1 and corresponding
compounds of main peaks is given in Tab. 1. The result clearly shows that the main product iscaprolactam, and in addition, there are some 6- aminocaproic acid, dimer, trimer, tetramer,pentamer and other oligomer of caprolactam and 6- aminocaproic acid.
Fig. 2 and 3 give degradation rate of PA 6 and yield of caprolactam of different
temperature with time. With increasing temperature, the time of total degradation of PA 6
decreases. For the 603 K, it is only 25 min that almost all PA 6 is degraded, whereas, the timefor 553, 573, and 583K are 100, 70, and 55 min, respectively. The result shows that hightemperature benefits the degradation of PA 6.
At comparatively short time, yield of caprolactam present same change with degradation,that is it also increases with increasing time. Whereas, with increasing time to somecomparatively long time, yield of caprolactam presents transition point. For example, as forthe highest temperature of 603 K, the highest yield of caprolactam is 77.52% at 70 min, andwith increasing time, at 85 min the yield decreases to 75.32% on the contrary. The result
indicates that caprolactam will decompose with time at high temperature. Therefore, as for theprocess of hydrothermal degradation of PA 6 to produce caprolactam, temperature and time
- 256 -PARTⅤ SOLVOLYSIS

intensity countare both important effect to yield of caprolactam and determination of appropriate time andtemperature is very important. According to the relation of yield of caprolactam withtemperature and time, the optimum condition of 3% HPA as catalyzer is the temperature of573K, time of 85 min, and caprolactam can attain highest yield of 77.96%.
30 114.1
2826242220
18161412245.2
358.3
10227.2
86471.4
4584.5
2 132.1
0100 150 200 250 300 350 400 450 500 550 600 650 700m/z amu

Fig. 1 Typical MS spectrum of liquid products (573K and 40min).
Tab. 1 Corresponding compounds shown in MS spectrum
m/z

compound

114.1
132.1
227.1
245.2
340.2
358.0
471.4
584.5

caprolactam
6- aminocaproic acidcaprolactam dimer
6- aminocaproic acid dimer
caprolactam trimer
6- aminocaproic acid trimer
6- aminocaproic acid tetramer
6- aminocaproic acid pentamer
80

yield of caprolactam/%100

degradation rate/%

8060553K
573K
583K
603K

4060553K
573K
583K
603K

402020000204060

reaction time/min80100120020406080100120

reaction time/minFig. 2 Degradation rate with reaction time Fig. 3 Yield of caprolactam with reaction time- 257 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
As for the kinetics of polymer degradation, it can be treated as pseudo first order reaction.
Thus, the equation of PA 6 degradation kinetics can be expressed as following 1 as pseudo
first order reaction.
-d[PA 6]/dt=K[PA 6][HPW]m[H2O]n(1)Owing to excess of water and constant of HPW, then the equation 1 can be simplified asequation 2.
-d[PA 6]/dt=K' [PA 6](2)
If the initial content of PA 6 is expressed as [nylon6]0, and the content of some time isexpressed as [nylon6]t, then, integral of equation 2 can be expressed as following equation 3.
ln [nylon6]0/[nylon]t= K't(3)
As for PA 6, it is solid, thus its content can be expressed as weight. Therefore, theexpression of [nylon6]0 /[nylon6]t can be replaced as M0/Mt. M0 and Mt are weights initial andsome time, respectively. Then the equation 3 can be changed as ln(M0/Mt)= K't. Fig. 4 shows
relation between ln(M0/Mt) and reaction time and the line fitting rate constant K' of different
temperature is given in Tab. 2.
Tab. 2 Constant of catalytic degradation at different temperature
Temperature/K

553K

573K

583K

-1

0.0485

0.09650.1142

K'/min603K
0.2003

The relation of rate constant K' and temperature is linearly fitted is shown in Fig. 5.
According to Arrhenius equation, the line fitting relation of lnK' and 1000/T gives thatreaction activation energy and pre-exponential factor A are 77.38 kJ/mol and 1.05×106,
respectively.
5-1.0603K
583K
573K
553K

-1.5

y=-9.3075x+13.8631
R2=0.9899-2.0

3lnK'ln( M 0/Mt)

42-2.5

-3.0

1-3.5

00

10

2030

40

50

60

70

80

90

100

-4.0
1.65

1.70

Fig. 4 Relation of ln(M0/Mt) with reaction time

1.80

1.85

Fig. 5 Relation of rate constants with temperature

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- 258 -1.75

1000/T

reaction time/min

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