Promoting Effect of Sulfur Compounds on the Degradation of

The liquefaction of low-density polyethylene was studied in several solvents with or without sulfur as an additive. With the addition of molecular sul...
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Energy & Fuels 2002, 16, 1314-1320

Promoting Effect of Sulfur Compounds on the Degradation of Polyethylene Terutaka Shiro, Toshiyuki Kanno, Kyohei Aratani, Yasuhiro Katsura, Na-oki Ikenaga, and Toshimitsu Suzuki* Department of Chemical Engineering, Faculty of Engineering, Kansai University, Suita, Osaka 564-8680, Japan Received March 28, 2002

The liquefaction of low-density polyethylene was studied in several solvents with or without sulfur as an additive. With the addition of molecular sulfur in the liquefaction of polyethylene in 1-methylnaphthalene, the conversion increased from 77.8% to 100%, and the oil yield increased from 62.4% to 78.7% at 673 K, as compared to the conversion and yield results of the run in the absence of sulfur. The molecular weight distributions of the oil fraction, obtained in the presence of sulfur, shifted to the lower molecular weight side as compared to that obtained without sulfur. At an elevated temperature, molecular sulfur would generate sulfur radical, which abstracts hydrogen from polyethylene to initiate the chain reactions involved in the degradation of polyethylene. The promoting effect of sulfur on the degradation of polyethylene was affected by the chemical structure of the solvent. The promoting effect of sulfur was significant in aromatic and non-hydrogen donating compounds such as 1-methylnaphthalene, naphthalene, and toluene. However, in tetralin no promoting effect of sulfur was observed because the sulfur radical would abstract hydrogen from the solvent and polyethylene competitively.

1. Introduction The amount of waste plastics is increasing all over the world. About 88% of waste plastics are disposed of in landfills or by combustion.1 Landfill and combustion is no longer acceptable for the disposal of plastics because of serious environmental concerns and the low weight-to-volume ratio of plastics.2,3 Therefore, the recycling of waste plastics has received significant worldwide attention. Among various recycling methods, chemical recycling into chemical feedstocks and fuel is the most promising. Although polymers, prepared by condensation polymerization, such as polyethylene terephthalete (PET) and nylon, can be hydrolyzed into their monomers, polyolefins prepared by the addition polymerization are very difficult to depolymerize into their respective monomer units, because of random scission of the C-C bonds of the polymer chains.4-6 Therefore, the thermal or catalytic degradation of polyolefin, especially polyethylene (PE), into fuel has been examined by many researchers. Various solid acids, such as silica-alumina and zeolite, have been extensively used as catalysts in the degradation of * Corresponding author. Fax: +81-6-6388-8869. E-mail: tsuzuki@ ipcku.kansai-u.ac.jp. (1) Naki, M. J. Jpn. Inst. Energy 2001, 80, 600. (2) Ballice, L.; Yuksel, M.; Saglam, M. Energy Fuels 2000, 14, 612. (3) Kaji, M. J. Jpn. Inst. Energy 1996, 75, 778. (4) Masuda, T.; Kuwahara, Mukai, S. R.; Hashimoto, K. Chem. Eng. Sci. 1999, 54, 2773. (5) Anuado, J.; Sotelo, J. L.; Serrano, D. P.; Calles, J. A.; Escola, J. M. Energy Fuels 1997, 11, 1225. (6) Takuma, K.; Uemichi, Y.; Ayame, A. Appl. Catal. A 2000, 192, 273.

PE.4,7-15 Sakata et al. reported that mesoporous silica (KFS-16) afforded the higher yield of liquid products and KFS-16 deactivated much more slowly.8 Sharratt et al.,9,11,12 Aguado et al.,10 Park et al.,13 and Uemichi et al.14 investigated the activity of zeolites, such as ZSM5, USY, mordenite, and MCM-41, for the degradation of polyethylene using fluidized-bed and fixed-bed reactors. They indicated that ZSM-5 exhibited the highest activity among these catalysts. We have investigated the co-liquefaction of Yallourn coal with PE as one of the recycling methods of waste plastics and reported that the synergistic effect of coprocessing afforded higher conversion and oil yield.16 In the liquefaction of PE with Fe(CO)5-S as a catalyst, with an increase in the S/Fe molar ratio from 2 to 4, the conversion and the oil yield increased. From these (7) Songip, A. R.; Masuda, T.; Kuwahara, T.; Hashimoto, K. Energy Fuels 1994, 8, 131. (8) Sakata, Y.; Uddin, M. A.; Muto, A.; Kanada, Y.; Kiozumi, K.; Murata, K. J. Anal. Appl. Pyrolysis 1997, 43, 15. (9) Sharratt, P. N.; Lin, Y.-H.; Garforth, A. A.; Dwyer, J. Ind. Eng. Chem. Res. 1997, 36, 5118. (10) Aguado, J.; Sotelo, J. L.; Serrano, D. P.; Calles, J. A.; Escola, J. M. Energy Fuels 1997, 11, 1225. (11) Lin, Y.-H.; Sharratt, P. N.; Garforth, A. A.; Dwyer, J. Energy Fuels 1998, 12, 767 (12) Garforth, A. A.; Lin, Y.-H.; Sharratt, P. N.; Dwyer, J. Appl. Catal. A: Gen. 1998, 169, 331. (13) Park, D. W.; Hwang, E. Y.; Kim, J. R.; Choi, J. K.; Kim, Y. A.; Woo, H. C. Polym. Degrad. Stab. 1999, 65, 193. (14) Uemichi, Y.; Nakamura, J.; Itoh, T.; Sugioka, M.; Garforth, A. A.; Dwyer, J. Ind. Eng. Chem. Res. 1999, 38, 385. (15) Luo, G.; Suto, T.; Yasu, S.; Kato, K. Polym. Degrad. Stab. 2000, 70, 97. (16) Kanno, T.; Kimura, M.; Ikenaga, N.; Suzuki, T. Energy Fuels 2000, 14, 612.

10.1021/ef020081f CCC: $22.00 © 2002 American Chemical Society Published on Web 08/13/2002

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results, the addition of sulfur to the reaction system seemed to promote the degradation of PE. Sulfur is known to generate sulfur radicals (‚Sn‚) by the dissociation of the S-S bond.17-19 In the co-liquefaction reaction, sulfur radical seems to initiate the degradation of PE. The degradation of plastics using molecular sulfur has been reported by several investigators. Sivakumar et al. reported that the addition of molecular sulfur in the liquefaction of PE decreased the boiling point ranges of the oil product to lower temperature.20 Ibrahim et al. investigated thermal degradation of commingled plastics (CP) containing PE and polypropylene under hydrogen pressure.21 They concluded that molecular sulfur is an effective material for lowering the depolymerization temperature of CP by about 100 K. Murakata et al. reported that the yield of oligomeric styrene increased greatly when sulfur compounds were added during the degradation of polystyrene.22 However, the detailed mechanism of the sulfur-promoted degradation of polyolefins is still open to discussion. The objective of the present work is to reveal the role of sulfur in the degradation of PE. Various sulfur compounds were used as additives under hydrogen or nitrogen atmosphere in order to clarify the mechanisms of sulfur-assisted degradation. To clarify the degradation mechanism of PE, n-hexadecane was used as a model compound of PE and degraded in the presence of sulfur. 2. Experimental Section 2.1. Materials. Low-density polyethylene (LDPE) (specific gravity, 0.88-0.94; melting point, 365-390 K) was purchased from General Science Co. and was ground to pass through a 60 mesh screen. 1-Methylnaphthalene (1-MN), naphthalene, tetralin (TL), n-hexadecane, toluene, 1-decene, carbon disulfide, dimethyl disulfide, and sulfur were purchased from commercial sources and used without purification. 2.2. Procedures. The degradation reaction of polyethylene and the separation of products were carried out according to the previous paper.16 A 2.0 g of feed, PE or n-hexadecane, was charged into a 50 mL magnetically stirred autoclave together with a solvent and an additive. Hydrogen or nitrogen was charged to 5.0 MPa at a room temperature, and the autoclave was heated to a required temperature (573-698 K) which was maintained for 60 min. At the end of each run, the reactor was cooled rapidly room temperature by air blowing. Gaseous products were then collected in a gasbag after measuring the volume of gaseous materials by using a gas buret. Liquid and solid products were separated by sequential solvent extraction using tetrahydrofuran (THF), toluene, and hexane under ultrasonic irradiation. We denote the hexane-soluble fraction as oil, the toluene-soluble and hexane-insoluble fraction as asphaltene (AS), the THF-soluble and toluene-insoluble fraction as preasphaltene (PA), and the THF-insoluble fraction as residue. All fractions except the oil fractions were dried at 343 K under vacuum overnight and weighed. The conversion, the yields of oil, gas, AS, and PA were calculated as follows: (17) Farmer, E. H.; Shipley, F. W. J. Chem. Soc. 1947, 1519. (18) Fairbrother, F.; Gee, G.; Merrall, G. T. J. Polym. Sci. 1955, 16, 459. (19) Horton, A. W. J. Org. Chem. 1949, 14, 761. (20) Sivalumar, P.; Jung, H.; Tierney, J. W.; Wender, I. Fuel Process. Technol. 1996, 49, 219. (21) Ibrahim, M. M.; Seehra, M. S. Energy Fuels 1997, 11, 926. (22) Murakata, T.; Saito, M.; Hirai, H.; Sato, S. J. Appl. Polym. Sci 1998, 70, 2299.

Energy & Fuels, Vol. 16, No. 5, 2002 1315 conversion ) (weight of charged PE - residue23)/ (weight of charged PE) × 100 oil yield ) (conversion - gas yield - AS yield - PA yield) gas yield ) (weight of C1 + C2 + C3)/ (weight of charged PE) × 100 AS yield ) (weight of AS)/(weight of charged PE) × 100 PA yield ) (weight of PA)/(weight of charged PE) × 100 The reproducibility of duplicate runs was in the range of 3% in conversion and oil yield. 2.3. Product Analyses. Gaseous products were analyzed using a gas chromatograph (Shimadzu, GC-14B) equipped with a FID detector employing a Porapack Q column (3.0 mm i.d. × 3.1 m) for hydrocarbon (C1-C3), and a gas chromatograph (Shimadzu, GC-8A) equipped with a TCD detector using an activated carbon column (3.0 mm i.d. × 3.0 m) for hydrogen. Liquid products were analyzed using a gas chromatograph (Shimadzu, GC-14A) equipped with a FID detector employing a diethylene glycol adipate polyester (DEGA) column (3.0 mm i.d. × 3.1 m) and a gas chromatograph (Shimadzu, GC-14A) equipped with a FID detector using a CBP1 capillary column (0.53 mm i.d. × 25 m). Hydrogen sulfide concentration in the recovered gas was analyzed using a gas detector indicator tube (Gastec Corp., GV-100S). Liquid products were characterized using a gas chromatograph-mass spectrometer (Shimadzu, GC-MSQP2000A) employing a CBP1 capillary column (0.25 mm i.d. × 30 m). A Fourier transform infrared (FT-IR) spectrometer (JEOL, JIR-7000) was used to detect double bonds in the products. 100 scans were accumulated for each spectrum at a resolution of 4.0 cm-1. The molecular weight distributions of oil fractions were measured using a gel permeation chromatograph equipped with a Hitachi L-6000 pump, with a Shodex HF-2002 (25 mm i.d. × 500 mm) column, and with a mobile phase THF (3.5 mL/min), equipped with a VAREX model ELSD IIA evaporative light scattering detector. Sulfur contained in products was quantitatively analyzed with an X-ray fluorescent spectrometer (Shimadzu, SXF-1200). The sulfurto-carbon ratio was calculated based on the calibration curve obtained by measurement of several known proportions of a mixture of activated carbon and elemental sulfur.

3. Results and Discussion 3.1. Effect of Sulfur on the Liquefaction of Polyethylene. Figure 1 shows the results of the liquefaction of PE using various sulfur compounds under nitrogen and hydrogen atmospheres. In the run without an additive under a nitrogen atmosphere, the conversion reached 100% and the oil yield was 82.2%. When hydrogen sulfide and sulfur were added to the reaction system, the conversions were 100% and the oil yields were 84.2 and 83.0%, respectively. The effect of additives was not observed under a nitrogen atmosphere. Under a hydrogen atmosphere without sulfur, the conversion and the oil yield decreased to 77.8% and 62.4%, respectively, as compared to the reaction under a nitrogen atmosphere. (23) The residue is not exactly unreacted PE and contains recombined PE fragments, but the separation procedure used here is typical to coal liquefaction experiments in small scale and shows the amounts of degraded products.

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Figure 1. Effect of sulfur compounds on the product distribution in the liquefaction of PE. Upper: Reaction in N2 atmosphere. Bottom: Reaction in H2 atmosphere. Reaction temperature: 673 K. Initial pressure: 5.0 MPa. Reaction time: 60 min. PE: 2.0 g, 1-MN: 4.0 g. Molecular sulfur: 4.0 mmol.

The thermal degradation reaction of PE is explained by a radical chain mechanism.24,25 In this mechanism, free radicals produced by the dissociation of the C-C bond of PE (R-CH2‚) abstract the hydrogen of PE to produce hydrocarbon radical (R′-CH‚-R′′), which leads to β-scission. Under a hydrogen atmosphere these free radicals were capped by hydrogen atom to suppress the degradation of PE. The effect of the addition of sulfur on the degradation of PE, however, promoted the degradation of PE under a hydrogen atmosphere, with the conversion and the oil yield reaching 100% and 78.7%, respectively. Addition of hydrogen sulfide and dimethyl disulfide slightly increased the conversions to 81.3% and 86.4%, but the oil yields were not affected as compared to the run without an additive. Figure 2 shows the molecular weight distribution patterns of oil fractions observed by gel permeation chromatography. In Figure 2, the area of each curve is proportional to the weight of the oil fraction. The molecular weight distribution curves of the oil fraction obtained under a nitrogen atmosphere shifted to the low molecular weight side, as compared to those under a hydrogen atmosphere. With the addition of sulfur compounds, the molecular weight distribution shifted to a lower molecular weight side, although the conversion and the oil yield did not change with the addition of sulfur compounds. The degree of diminishment in the molecular weight distribution pattern is most significant with molecular sulfur. Under a hydrogen atmosphere, similar results were observed. The addition of sulfur not only increased the conversion and the oil yield, but also decreased the molecular weight of the oil fraction. Effect of the reaction temperature on the liquefaction of PE was examined and the results are shown in Figure (24) Guy, L.; Fixari, B. Polymer 1999, 40, 2845. (25) Horvat, N.; Ng, F. T. T. Fuel 1999, 78, 459.

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Figure 2. Gel permeation chromatograms of oil fractions in the liquefaction of PE. Upper traces: N2 atmosphere. Bottom traces: H2 atmosphere. Conditions are the same as those in caption to Figure 1.

Figure 3. Effect of reaction temperature on the conversion and oil yield in the liquefaction of PE. Initial H2 pressure: 5.0 MPa. Reaction time: 60 min. PE: 2.0 g. 1-MN: 4.0 g. Sulfur: 4.0 mmol. (0) Conversion. (9) Conversion with sulfur. (O) Oil yield. (b) Oil yield with sulfur.

3. The conversion and the oil yield increased with increases in the reaction temperature. The promoting effect of sulfur was clearly observed at 623-673 K. At 623 K, the conversion increased by 17.9%, and the oil yield increased by 4.9%. The promoting effect of sulfur was the highest at 673 K, and the conversion and the oil yield increased by 22.2% and 16.3%, respectively, as compared to the run without sulfur. At 573 K, the conversion was low both with and without sulfur, but increases in both the conversion and the oil yield were observed. At 698 K, the dissociation reaction of the C-C bonds in PE proceeded rapidly without sulfur, conse-

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Figure 4. Effect of the amount of sulfur on the conversion and yield in the liquefaction of PE. Reaction temperature: 673 K. Initial H2 pressure: 5.0 MPa. Reaction time: 60 min. PE: 2.0 g. 1-MN: 4.0 g. (0) Conversion. (O) Oil yield. (4) Gas yield.

quently, the addition of sulfur did not affect the conversion and the oil yield. Hereafter, further investigation was carried out at 673 K. Figure 4 shows the effect of the amount of sulfur on the liquefaction of PE. The conversion increased to 95.6% with an addition of 0.5 mmol of sulfur. With an increase in the amount of sulfur to 1.0 mmol, the conversion reached 100%. However, the oil yield increased in proportion to the amount of sulfur added. Only small amounts of gases were produced, but the gas yield increased in proportion to the amount of sulfur. The gas yield free from hydrogen sulfide was 0.8% in the absence of an additive, and 1.2% when 4.0 mmol was added. The functional groups in the oil fraction were examined using FT-IR spectrometry. The results are shown in Figure 5. To avoid contamination by 1-MN, the reaction was carried out without solvent. The degradation products of PE are 1-alkene, nalkane, and other branched products according to the chemical structure of PE. The characteristic adsorptions of alkene appear at 1640 cm-1 (CdC double bond), 991 cm-1, 908 cm-1 (terminal vinyl), 964 cm-1 (transCHdCH double bond), and 887 cm-1 (vinylidene). By increasing the amount of sulfur from 0.5 mmol to 10 mmol, the adsorptions at 1640 cm-1, 991, 908, and 887 cm-1 gradually decreased. The decrease in these adsorptions indicates the decrease in the terminal CdC double bonds. Similar results were obtained in the presence of hydrogen sulfide. 1-Decene was reacted with hydrogen sulfide in 1-MN in order to examine whether selective decreases in terminal CdC double bonds do proceed. In the absence of hydrogen sulfide, only 13% of 1-decene was converted into decene isomers. On the other hand, 35% of 1-decene was converted into isomeric decenes, in the presence of hydrogen sulfide. Hydrogen sulfide could react through addition reaction to the terminal CdC double bond to lead to isomerization. From this result, the decrease in the terminal alkene on the degradation of PE would be

Figure 5. IR spectrum of oil fractions obtained by the liquefaction of PE with sulfur. Reaction temperature: 673 K. Initial H2 Pressure: 5.0 MPa. Reaction time: 60 min. PE: 2.0 g. Sulfur: 0-10.0 mmol.

responsible for the isomerization of the primary products of 1-alkenes. 3.2. Effect of Solvent on the Liquefaction of Polyethylene. The effect of a type of solvent on the liquefaction of PE was examined and results are shown in Figure 6. In naphthalene, the conversion and the oil yield increased by 16.8% and 18.5%, respectively, as compared to the run without sulfur, indicating that naphthalene and 1-MN exhibited similar behaviors. However, the conversion and the oil yield in the absence of sulfur were the same as that in the presence of sulfur in TL. The addition of sulfur did not affect the degradation of PE. In using hexadecane as a solvent without sulfur, the conversion was 100%, because at this temperature (673 K) the decomposition of hexadecane proceeds together with the degradation of PE to give a larger amount of free radicals than the run in 1-MN. However, no promoting effect was observed, and the conversion decreased to 90.0%. From these results, it is evident that the effect of sulfur on the degradation of PE was dependent on the chemical structure of the solvent, and the promoting effect was observed only for the runs in aromatic non-hydrogen donating solvents such as 1-MN or naphthalene. Table 1 shows the conversion of solvent and the amount of hydrogen sulfide produced in the reaction of sulfur with a solvent or solvent and PE under a nitrogen atmosphere. The conversion of 1-MN and naphthalene was calculated based on the amount of each solvent which was recovered after the reaction, and the conver-

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Figure 7. Sulfur contents of the products in the liquefaction of PE with sulfur. Conditions are the same as those in caption to Figure 6. Figure 6. Effect of a type of solvent on the product distribution in the liquefaction of PE. Reaction temperature: 673 K. Initial H2 pressure: 5.0 MPa. Reaction time: 60 min. PE: 2.0 g. Solvent: 4.0 g. Sulfur: 4.0 mmol. Table 1. Results of the Reaction of Sulfur with Various Solventsa H2S (mmol) solvent

solvent

solvent and PE

1-MNb Naphthaleneb TLc n-hexadecane

2.56 1.98 2.60 2.55

2.32 2.60 2.63 2.44

solvent conversion (%) solvent

solvent and PE

8.9 26.7 8.2 15.0

12.9 16.7 8.5 19.8

a Reaction temperature: 673 K. Initial nitrogen pressure: F 5.0 MPa. Reaction time: 60 min. Sulfur: 4.0 mmol. Solvent: 4.0 g. PE: F 2.0 g. b Conversion was caluculated from the amount of racted solvent. c Conversion to naphthalene

sion of TL was calculated on the basis of the the amount of TL converted to naphthalene. In naphthalene, the amount of hydrogen sulfide produced in the run without PE amounted to 1.98 mmol and the conversion was 26.7%. On the other hand, 2.60 mmol of hydrogen sulfide was produced and the conversion decreased to 16.7% with PE. The increase in hydrogen sulfide and the decrease in conversion indicate that abstraction of hydrogen atom from PE proceeded. In 1-MN, the amount of hydrogen sulfide produced slightly decreased with the addition of PE, since hydrogen sulfide might have reacted with alkyl radicals from PE as mentioned below (in section 3.3, reaction mechanism, and Scheme 1). Consequently, the degradation of PE was promoted in a solvent such as 1-MN, naphthalene. On the other hand, the amount of hydrogen sulfide produced was almost constant in TL and hexadecane. It is possible that the sulfur radical abstracted hydrogen from both PE and solvent in TL and hexadecane, and as a result, no promoting effect was observed. Figure 7 shows the amounts of sulfur contained in residue, PA, AS, and oil. In all cases, most of the added sulfur was converted into hydrogen sulfide. In 1-MN and naphthalene, relatively large amounts of sulfur were contained in oil. In TL, the amounts of sulfur in the liquid and the solid products were smaller

than those obtained in 1-MN, because TL rapidly hydrogenated sulfur radicals produced in the first step to give hydrogen sulfide. The smallest sulfur contents contained in oil were observed with hydrogen sulfide as an additive. Hydrogen sulfide seems to generate radicals such as HS‚, which recombine with alkyl radical produced from the degradation of PE. 3.3. Reaction Mechanisms in the Degradation of Polyethylene with Sulfur. To understand reaction mechanism of PE degradation in the presence of sulfur, hexadecane was reacted in toluene, as a model reaction. Table 2 shows the carbon number distribution in the degradation products and the distribution of 1-alkenes, n-alkane, and the isomers of 1-alkenes in respective number of carbon atoms. The conversion of hexadecane in the absence of sulfur was only 9.22%. By contrast, its conversion in the presence of sulfur increased to 20.4%, indicating the promoting effect of sulfur in toluene, similar to the case of PE shown in Figure 1. In the absence of sulfur, the ratios of 1-alkenes and n-alkanes were nearly unity for C3-C13 hydrocarbons. In the presence of sulfur, n-alkane increased to 65-90% of degraded hydrocarbon of the same carbon numbers; consequently, the proportion of 1-alkene decreased to about 15%. In addition, several other products were observed. These products were identified by means of GC/MS and together with hydrogen reduced products with palladium carbon catalyst. They were found to be internal alkenes. In the presence of sulfur, sulfur radical abstracts hydrogen from PE to produce hydrogen sulfide. The effect of hydrogen sulfide on the degradation of hexadecane was examined. The conversion was 15.4%, and the promoting effect was observed by the addition of hydrogen sulfide. The distribution obtained in the run with hydrogen sulfide was similar to that obtained in the run with sulfur. The proposed degradation mechanisms of polyethylene in solvent are shown in Scheme 1. Since the bond dissociation energy of S-S bond (266 kJ/mol) is lower than those of C-C (341 kJ/mol) and C-H bonds (413 kJ/mol), initiation by means of the dissociation of the S-S bond could produce sulfur radical (1), followed by the reaction with PE or solvent. In non-hydrogen

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Scheme 1. The Degradation Mechanism of Polyethylene with Sulfur in Various Solvents

Table 2. Result of the Degradation of n-Hexadecane in Toluene under Nitrogen Atmospherea additive:

none

H2S

sulfur

conversion (%):

9.22

15.36

20.44

carbon number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 17+d

distribution (%) distribution (%) distribution (%) C mmol n-alkane 1-alkene alkeneb C mmol n-alkane 1-alkene alkene C mmol n-alkane 1-alkene alkene c 0.12 0.19 0.16 0.28 0.42 0.49 0.50 0.51 0.66 0.57 0.59 0.64 0.38 0.34 0.70

76.8 55.7 58.1 49.9 47.1 50.9 50.0 51.1 42.9 51.0 49.6 50.3 20.2 63.3

23.2 44.3 41.9 50.1 52.9 49.1 49.1 48.9 57.1 49.0 50.4 49.7 79.8 36.7

0.0 0.0 0.0 0.0 0.0 0.0 0.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.10 0.23 0.16 0.27 0.39 0.51 0.53 0.68 0.80 0.85 0.87 0.91 0.34 0.25 0.32

86.3 67.2 77.2 69.4 77.6 76.6 77.1 75.9 66.2 66.5 64.2 66.3 36.8 70.6

13.7 32.8 22.8 30.6 22.4 23.4 22.9 17.7 19.7 19.9 25.0 28.3 57.0 29.4

0.0 0.0 0.0 0.0 0.0 0.0 0.0 6.5 14.1 13.6 10.8 5.4 6.2 0.0

0.20 0.31 0.28 0.45 0.65 0.97 1.00 1.20 1.35 1.35 1.33 1.45 0.88 0.33 2.18

89.1 54.1 80.8 82.2 87.6 78.0 83.0 70.5 65.8 68.3 65.7 65.7 38.8 68.5

10.9 45.9 19.2 17.8 12.4 14.6 17.0 11.2 17.0 15.7 18.5 28.0 43.5 27.9

0.0 0.0 0.0 0.0 0.0 7.3 0.0 18.3 17.2 16.0 15.8 6.3 17.7 3.6

a n-Hexadecane: F 2.0 g (8.85 mmol). Toluene: 4.0 g (43.4 mmol). Sulfur: 4.0 mmol. Nitrogen preesure: F 5.0 MPa.Reaction temperature: 425 °C. Reaction time: 60 min. b Internal alkene. c Not detected. d Sum of more than carbon number 17.

donating solvents such as toluene, 1-MN, and naphthalene, sulfur radical reacts mainly with PE to produce hydrocarbon radical (2), which leads to β-scission (3). Hydrogen sulfide reacts with alkyl radical to produce n-alkane and HS‚ radical (4), which promote the deg-

radation of PE. In addition, hydrogen sulfide produced during the reaction seems to promote the isomerization of 1-alkene to produce internal alkene by the addition reaction (5, 6). As a result, an increase in n-alkane and the production of internal alkene was observed. In TL

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and hexadecane, no promoting effect on the degradation of PE was observed, since both the dehydrogenation of solvent and the abstraction of hydrogen atom in PE by sulfur radical proceeded competitively (7, 8). It was found that the addition of sulfur was effective for the degradation of PE in aromatic non-hydrogen donating solvent. Conclusion The addition of sulfur compounds promoted the degradation of PE and, among various sulfur compounds, molecular sulfur exhibited the greatest effect. With the addition of molecular sulfur in the liquefaction of PE in 1-MN, the conversion increased by 22% and the oil yield increased by 16% at 673 K, as compared to the run in the absence of sulfur. Molecular weight

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distributions of the oil fraction, obtained in the presence of sulfur, shifted to the lower molecular weight side as compared to that obtained without sulfur. The promoting effect of sulfur on the degradation of PE was affected by the chemical structure of the solvent. The promoting effect of sulfur was high in aromatic and non-hydrogen donating compounds, such as 1-MN, naphthalene, and toluene. In these solvents, the generated sulfur radical could produce hydrocarbon radical by abstraction of the hydrogen atom from PE, which would lead to the degradation of PE. In addition, hydrogen sulfide, produced during the reaction, promoted the degradation of PE, and caused the isomerization of 1-alkene into internal alkene and an increase in n-alkane. EF020081F