점토광물에 의한 폴리프로필렌의 촉매분해

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

화학공학의 이론과 응용 제8권 제1호 2002년

반종호, 윤직현, 박대원

부산대학교 화학공학과

The Catalytic Degradation of Polypropylene over Clay Mineral Catalysts

J.H. Van, J.H. Yoon, D.W. Park

Department of Chemical Engineering, Pusan National University

Introduction

Polymer wastes can cause serious pollution problems. Until recently, they have usually been discharged or incinerated. However, waste plastics, especially thermoplastics, can be regarded as being an abundant source of chemicals and energy. Furthermore, recycling of thermoplastics from waste products can contribute to the solution of pollution problems. Though several methods have been proposed for recycling of waste plastics, it is generally accepted that material recovery is not a long-term solution to the present problem, and that energy or chemical recovery is a more attractive one. One approach to chemical recovery is to employ inert gas pyrolysis to produce gasoline-like materials. In this method, the waste plastics are thermally or catalytically degraded into gases and oils, which can be used as sources in fuels or chemicals [1].

In the thermal degradation of polyolefins, many hydrocarbons having a wide distribution of carbon atom numbers are formed. In contrast, the oils produced by the catalytic degradation are known to contain a relatively narrow distribution of hydrocarbons. Also, compared to thermal degradation, catalytic degradation has an advantage of a lower temperature of degradation. An excellent summary on the catalytic recycle of polymers was reported by Uemichi [2]. The most commonly used catalysts in the catalytic degradation of polymers are solid acids (zeolite, silica-alumina) and activated carbon.

However, very few works have been performed on the investigation of halloysite and clinoptilolite as a catalyst for this process. Clinoptilolite has a pore structure of a monoclinic framework consisting of a 10-membered ring (7.6×3.0 Å) and an 8-membered ring (3.3×4.6 Å), and is a silica-rich member of the heulandite family. Halloysite is a clay material having  pore size of 7∼10 Å.

The objective of this study is to examine the performance of the natural zeolite and clay material in the catalytic degradation of PP. Special focus will be placed on the effect of acid treatment of the catalyst on the distribution of liquid hydrocarbon products.

Experimental

Polypropylene (PP), in powder form, is an isotactic sample obtained by Honam Petrochemical Co. (in Yeacheon, Korea) as grade FC-150 (Melt Index=8.0g/10min, Density=0.90 g/cm3) with an average molecular weight (Mw) of 329700 and Mn=54300. The plastic samples of 60-150 mesh size were used for this study. Several types of solid acid catalysts such as natural halloysite (NH; Al2O3□2SiO2□4H2O : 75∼95 wt.%, Illite 5∼20 wt.%, Kaolinite 0∼5 wt.%, occurring in South-East area of Korea), natural clinoptilolite zeolite (NZ; occurring in Pohang area of Korea), aluminum pillared montmorillonite (Mont.; reagent of Fluka) were evaluated through the degradation experiments of PP. NH and NZ were ion-exchanged three consecutive times with 1M NH4Cl solution at 70∼80 ℃ for 20 h. The zeolites exchanged with NH4+ were dried at 125 ℃ for overnight, and then calcined in air at 400 ℃ to obtain the proton(H+)-exchanged catalysts like HH and HNZ. Also, to study dealumination effects, HNZ and HH were modified by hydrochloric acid (35 % HCl ; 1N, 4N) at 70∼80 ℃ for 24 h. Drying and calcination procedure for the latter two catalysts was the same as that for the preparation of HNZ and HH. Each acid treated catalyst was denoted as 1N-HH, 4N-HH, 1N-HNZ or 4N-HNZ.

The catalytic degradation of the plastics was carried out in a semi-batch reactor where nitrogen is continuously passed with a flow rate of 30 mL/min. A mixture of 3.0 g of the plastics and 0.3 g of the catalyst was loaded inside a pyrex vessel of 30 mL and heated at a rate of 30 ℃/min up to the desired temperature. The distillate from the reactor was collected in a cold trap (-5 ℃) over a period of 2 h. The degradation of the plastic gave off gases, liquids and residues. The residue means the carbonaceous compounds remaining in the reactor and deposited on the wall of the reactor. The amount of coke deposit on the catalyst was calculated by measuring the desorbed amount of carbon dioxide during temperature programmed oxidation of used catalysts. The gases were analyzed by an on-line GC (HP 5890) with a Porapak Q column. The condensed liquid samples were analyzed by GC-MS (Micromass Co., Auto spec.) with a capillary column (HP-5MS) and GC (HP6890) with a capillary column (HP-1MS). The physical properties and the composition of the liquids were also measured by a PONA (Paraffins-Olefins-Naphthenes-Aromatics) analyzer (DHA 2520).

Results and Discussion

First, chemical composition of acid treated clay minerals are presented in Table 1. As the acid treatment time became longer, the silica/alumina mole ratio increased, and at the same time the surface area and pore volume were decreased by the dealumination. Table 2 lists the gaseous and liquid products, and residues obtained in the thermal and catalytic degradation of PP at 400 ℃ for 2 h. The amount of gaseous products was calculated by subtracting the weight of liquid products, residues and catalyst from the total weight of PP sample and fresh catalyst initially loaded to the reactor. The residue means the wax-like material collected on the reactor wall and on the catalyst, and its amount was measured by dissolving it with n-hexane. We also observed the formation of carbon deposit (coke) on the used catalyst. The main degradation product was liquid oil. The analysis of gaseous products in this experiment showed the following relative amounts of each component; ethane (6.6 wt.%), propylene (29.7 wt.%), propane (5.3 wt.%), butene (46.6 wt.%), butane (7.5 wt.%) and pentane (4.3 wt.%).

Table 3 shows product distribution and physical properties of liquids for various catalysts at 400 ℃ for 2 h of PP degradation. Liquid products from catalytic degradation of PP contained quite a large amount of iso-paraffins, olefins, and aromatics, which constitute gasoline fraction. The formation of gasoline fraction seems to be advantageous for the utilization of the liquid products. The liquid products over NZ had much lower amount of olefins than HNZ. Therefore, the oligomers found in the case of NZ can be considered to be cracked mainly to olefins and iso-paraffin in the PP degradation over the HNZ.

Table 4 shows the distribution of aromatics products for the hydrocarbons of carbon number C6∼C13. The aromatics from the thermal degradation contained mainly dimethylethylbenzene and ethylbenzene. However, for NZH and NZ catalysts tested, the amount of ethyltoluene was the highest. The amount of aromatics in HNZ decreased as ethyltoluene > propylbenzene > trimethylbenzene. Thermal degradation showed somewhat higher amount of high carbon number aromatics, such as dimethylethylbenzene and diethylbenzene, than the futher dealuminated catalysts did. C11 and higher aromatics were produced only in limited amounts except HH catalyst. Therefore, one can assume that aromatics were mainly formed by the dehydrocyclization of the C6 to C10 decomposed fragments, which were mainly alkylpentane, alkylhexane, alkylheptane, and their unsaturated derivatives. Aromatics can also be produced by the dealkylation of higher carbon number aromatics. However, this step is presumed to be multi-stage reactions consisting of dehydrocyclization to five-membered ring compounds and their ring expansion [3].

It is well known that acidic property is necessary to form the carbenium ion for the catalytic degradation of PP. The acid amounts of HCl treated catalysts were much smaller than that of HNZ or HH, but the portions of medium acid site for these catalysts were higher. The distribution of liquid products for the dealuminated catalysts shifted to lower carbon number hydrocarbons and lighter aromatics are produced with these catalysts. The dealuminated catalysts had a little smaller portion of micropores than the non-treated ones. From this fact, it may be concluded that the catalytic degradation of PP did not only depend on the proper distribution of acid sites but also on the pore structure. The work of Sakata et al. [4] supports this supposition. They reported high degradation acidity of a mesoporous silica (KFS-16), which was comparable to that of silica-alumina, even though KFS-16 had no acidic site at all on the surface. High degradation temperature offered lighter hydrocarbons by accelerating the cracking reactions.

화학공학의 이론과 응용 제8권 제1호 2002년

Acknowledgement

The authors wish to acknowledge the financial support of the Korea Science and Engineering Foundation (2000-1-30700-008-3).

References

1. Ohkita H, Nishiyama R, Tochihara T, Mizushima T. Ind Eng Chem Res

1993;32(12):3112.

2. Uemichi Y. Catalysis(Japan) 1995;37(4):286.

3. Uemichi Y, Kakino Y, Kanazuka T. J. Anal. Appl. Pyrol. 1989;16:229.

4. Sakata Y, Uddin MA, Muto A, Kanada Y, Koizumi K, Murata K. J Anal Appl Pyrol

1997;43:15

Table 1. Relative compositional characteristics of the catalysts used in this study.

Sample	NZ	HNZ	1N-HNZ	4N-HNZ	HH	1N-HH	4N-HH	Mont.
SiO2 (wt.%)	73.3	71.6	82.1	84.1	53.3	58.4	91.8	64.3
Al2O3(wt.%)	15.5	14.4	8.7	6.6	33.7	29.0	3.5	23.5
Si/Ala	8	8	15	22	3	4	45	5
a: silica / alumina mole ratio

Table 2. Yield (wt.%) of product in the catalytic degradation of PP at 400 ℃ for 2 h.

Catalyst	Gas (wt.%)	Liquid (wt.%)	Residue (wt.%)
None	13.7	75.3	11.0
NZ	31.0	68.3	0.7
HNZ	25.7	74.3	0
NH	34.3	54.6	11.1
HH	26.4	64.1	9.5
Mont.	24.5	58.2	17.3

Table 3. Distribution (wt.%) of liquid product in the catalytic degradation of PP at 400 ℃ for 2 h.

Catalyst	n-paraffins	iso-paraffins	olefins	naphthenes	aromatics
NZ	0.79	17.45	29.21	7.99	21.55
HNZ	0.62	18.86	51.0	9.78	19.74
NH	1.78	16.36	52.04	9.26	20.57
HH	2.12	17.23	49.76	10.23	20.66
Mont.	3.15	18.39	58.10	10.45	9.92

Table 4. Composition of aromatics (wt.%) formed in the degradation of PP at 400 ℃ for 2 h.

Aromatics	Thermal	NZ	HNZ	NH	HH	Mont
Benzene	0	0.18	2.22	3.62	3.63	6.77
Toluene	0.29	0	0	1.88	1.71	2.15
Ethylbenzene	21.68	5.94	7.61	4.72	3.84	15.23
Xylenes	2.59	7.03	8.69	23.01	20.76	18.19
Propylbenzene	9.43	15.53	14.88	3.41	3.16	14.35
Ethyltolunes	1.22	22.63	22.5	4.88	4.57	16.41
Trimethylbenzene	0.67	12.79	11.06	1.11	0.89	6.35
Butylbenzene	0	4.75	3.47	0.92	0.48	0.13
Methylpropylbenzene	1	11.87	9.84	22.26	14.57	4.27
Diethylbenzene	0.75	5.47	4.76	7.23	0.83	3.54
Dimethylethylbenzene	53.68	9.51	8.42	14.31	19.27	7.26
Tetramethylbenzenes	1.88	2.41	4.3	4.57	6.63	3.05
Methylindan	2.07	1.56	1.85	3.86	13.91	0
Naphthalene	4.76	0.36	0.37	4.22	5.75	2.3