The 5th ISFR (October 11-14, 2009, Chengdu, China)

PRODUCTION OF HYDROGEN AND CARBON NANOTUBESBY CATALYTIC PYROLYSIS OF WASTE POLYPROPYLENE
IN A TWO-STEP PROCESS
Jie Liu,* Zhiwei Jiang, Haiou Yu, Tao Tang
1

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun 130022, China.
E-mail: liujie@ciac.jl.cn

Abstract: The efficient conversion of virgin or waste polypropylene (PP) into multi-walledcarbon nanotubes (MWCNTs) and hydrogen without carbon oxides has been investigatedusing a two-step reaction system. The proposed process consisted of two steps: pyrolysis ofPP over HZSM-5 zeolite and montmorillonite in a screw kiln reactor and the following
catalytic decomposition of pyrolysis gases over the nickel catalysts in a moving-bed reactor
for producing MWCNTs and hydrogen simutaneously. The effects of pyrolysis temperature,pyrolysis catalysts, decomposition catalysts and polymer modifiers on the yields andmorphology of MWCNTs have been determined. MWCNTs yields increased with thepyrolysis temperature increasing from 550 to 750℃. Modification of the nickel catalyst withdoping of copper promoted the yield of MWCNTs and led to a production of MWCNTs withan attractive structure. These results have demonstrated that the coupled process is an efficientsystem for the continuous conversion of waste PP to hydrogen and nanocarbon materials.
Hydrogen is considered as an ideal alternative clean energy source. Catalytic
decomposition of hydrocarbons has been widely accepted as an attractive process for thegeneration of COx-free hydrogen and valuable carbon product. Previously reported workshowed that multi-walled carbon nanotubes (MWCNTs) can be synthesized using polymer ascarbon source in the presence of catalyst [1,2]. With increasing consumption of plastic
products during the past decades, the accumulated plastic waste has led to serious
environmental pollution. Recycling plastic waste (mostly polyolefin) to produce highvalue-added products has caught a great interest of scientists for economic and environmental
impacts in recent years. Our findings point out that polypropylene (PP) can be reorganized toproduce carbon nanotube and hydrogen over nickel catalyst [3] [4]. In this report we present asemi-continuous two-step process, in which PP or waste PP is firstly catalytically degradedover HZSM-5 zeolite in a screw kiln reactor and the produced hydrocarbons are used as afeedstock for producing MWCNTs and hydrogen simultaneously over the nickel catalyst in amoving-bed reactor.
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PARTⅡ GASIFICATION

Polymer Storage
Condensor

Productgas

Screw kiln reactor

Liquidcollector

Moving bed reactor
Residue storage

Fig. 1. Schematic diagram of the two-step reaction system.

As illustrated in Fig.1, the reaction system employed in this study is comprised of two
independent reactors: a screw kiln reactor to degrade PP into a mixture of many differenthydrocarbons (pyrolysis gas) and a following moving-bed reactor for producing MWCNTs
and hydrogen-rich gases (product gas) simultaneously. The speed and temperature of bothreactors can be adjusted as required. In a typical experiment, the moving bed reactor, theliquid collector and the residue storage were flushed with nitrogen gas for 20 min while thepyrolysis furnace and vapor deposition furnace were heated to 750 ℃ and 700 ℃,respectively. Thereafter the screw motor was switched on and the system was kept operating
until steady state has been reached. Plastic feed rate remained at 80 g/h. The product gases
were collected in a sample bag for 5 min and the other reaction products were collected for 30
min. 1.6 g of NiO catalyst was placed in the moving bed reactor (0.025 g/cm) whose speed is2 rpm. The influence of pyrolysis catalysts was investigated in this work.
The pyrolysis reaction has been carried out on three kinds of ZSM-5 zeolites with Si/Al
molar ratio of 25, 38 and 50. Fig. 2 shows the effect of Si/Al molar ratio of ZSM-5 zeolites on
the yields of various products in the two-step reaction. The amount of product gas is about 2times higher than that of pyrolysis gas, which can be attributed to more produced hydrogenfrom decomposition of the hydrocarbon mixture over nickel catalyst. According to calculation,
the volume of produced hydrogen increases markedly from 13.3 to 81.4 L. The hydrogenconcentration in the product gas is achieved at 73.7 vol% (Si/Al=25). With increasing Si/Al
ratio, yields of the product gas decrease drastically. This can be attributed to the reduced
pyrolysis gas volume and different compositions of the pyrolysis gas showed Fig. 3a, where
the concentration variation of methane and hydrogen are observed with increasing Si/Al ratio.
For example, the hydrogen content reduces from 21.4 to 19.5% whereas methane increases
from 37.4 to 41.2%. As shown in Fig. 3b, a higher concentration of hydrogen is observed for- 119 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
lower aluminum content. Hydrocarbons (C2-C5) can be cracked into methane, which is also
the carbon source for producing MWCNTs and hydrogen. The NiO catalyst is used without
pre-reduction since the pyrolysis gas contains hydrogen, which can reduce NiO to Ni in situ.
No CO and CO2 are formed during the above process.
4030Carbon nanotube
Liquid/wax
CokePyrolysis gas
Product gas

201008060
10400Gas released (L/100g PP)

Product yield (g/100g PP)

12020253850

Si/AlFig. 2 Product yield from the catalytic conversion of PP in the two-step reaction system overZSM-5 catalyst with different Si/Al molar ratio.

H2CH4Gas composition (vol. %)40

C2H6C2H4353025C3H8C3H620C4-C515105(b)70

Gas composition (vol. %)

(a)

60

H2CH4504030

2002538Si/Al

50253850Si/Al

Fig. 3 Pyrolysis (a) and product gases (b) composition during two-step reaction over ZSM-5catalyst with different Si/Al molar ratio.

- 120 -PARTⅡ GASIFICATION(b)

10025385080

Weight (%)

Deriv. Weight (%/℃)

(a)

60

40

425385032120000100200300400500

Temperature (℃)

600700800300400500

600700Temperature (℃)

Fig. 4 TG-DTG analysis of obtained carbon products for different pyrolysis catalyst.
The TG-DTG curves of the synthesized MWCNTs using various Si/Al ratio pyrolysis
catalysts are shown in Fig. 4. The higher temperature at weight loss onset and maximum
weight loss reflect better graphitized MWCNTs. The formation of ideal and nonideal
graphitized carbon materials in the products is observed. Using different pyrolysis catalysts
with Si/Al molar ratio of 25, 38 and 50, the temperature at maximum weight loss is 615.1,601.4 and 590.9 ℃, respectively. The residual weight decreases from 8.7 to 5.5 wt% withincreasing Si/Al ratio of the pyrolysis catalysts, which indicates higher carbon yield for thelow Si/Al ratio.

Fig. 5 SEM photographs of carbon nanotubes obtained by pyrolysis of(a) virgin PP and (b) waste PP over ZSM-5 catalyst.
The waste PP as feedstock for producing MWCNTs and hydrogen is explored. The ash
residue content of waste PP (from waste car bumper) determined by calcination in nitrogen at
700 ℃ is 11.4 wt%. SEM-EDX results suggest that the char contain silicon, magnesium,calcium, aluminum, titanium, sodium, iron, carbon and chlorine, which are related to plastic
fillers probably in the form of calcium carbonate, talcum powder or coupling agent of organic
titanate. The extraction experiment indicates that the waste PP contains 11.8 wt% of- 121 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
compatibilizers which may be EVA, POE or EPDM from IR analysis of the extracted fraction.
It can be easily seen in Fig. 5 that MWCNTs with wider diameter distribution are observed
when adopting the waste PP as the feedstock, and more impurity such as amorphous carbon isalso found. When PP and waste PP are used as feedstock, the concentration of hydrogen in theproduct gas is 73.7 and 61.3 vol%, respectively, which suggests negative effect of the fillers
in the waste PP on the quality of produced carbon nanotubes and the product gas composition.
In summary, pure and waste polypropylene have been pyrolyzed and decomposed in atwo-step process for the simultaneous production of carbon nanotube and hydrogen. The
effect of Si/Al molar ratio of ZSM-5 catalyst on the productivity of MWCNTs and hydrogenhas been investigated. The pyrolysis catalyst affects the composition of the pyrolysis gas andtherefore determines the quality and quantity of the products. 65.1 L/h hydrogen is produced
and the volume concentration is 73.7% for 80 g/h PP feeding during the reaction. Highvalue-added carbon nanotubes and hydrogen without COx are produced from low cost wasteplastics. The two-step reaction system has been proved to be efficient for producing hydrogenand carbon nanotubes from waste PP, thus it is a potential technique.
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China
for the Outstanding Youth Fund (NO. 50525311), general project (NO. 20674087) and theJilin Bureau of Science and Technology for the key project (NO. 20060319).
References[1] T. Tang, X. C. Chen, X. Y. Meng, H. Chen, Y. P. Ding, Angew. Chem. Int. Ed. 2005, 44,1517.
[2] U. Arena, M. L. Mastellone, G. Camino, E. Boccaleri, Polymer Degrad. Stab., 2006, 91,763.
[3] R. J. Song, Z. W. Jiang, W.G. Bi, W.X. Cheng, J. Lu, B.T. Huang, T. Tang, Chem. Eur.
J., 2007, 13, 3234.
[4] Z. W. Jiang, R. J. Song, W. G. Bi, J. Lu, T. Tang, Carbon, 2007, 45, 449.

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