Conversion of Polyethylene into Gasoline-Range Fuels by Two-Stage

Centre for Microporous Materials, Department of Chemistry, UMIST, P.O. Box 88, ... A two-stage catalytic degradation of polyethylene using amorphous ...
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Ind. Eng. Chem. Res. 1999, 38, 385-390

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Conversion of Polyethylene into Gasoline-Range Fuels by Two-Stage Catalytic Degradation Using Silica-Alumina and HZSM-5 Zeolite Yoshio Uemichi,* Junko Nakamura, Toshihiro Itoh, and Masatoshi Sugioka Department of Applied Chemistry, Muroran Institute of Technology, Mizumoto, Muroran, 050-8585 Japan

Arthur A. Garforth Environmental Technology Centre, Department of Chemical Engineering, UMIST, P.O. Box 88, Manchester M60 1QD, U.K.

John Dwyer Centre for Microporous Materials, Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, U.K.

A two-stage catalytic degradation of polyethylene using amorphous silica-alumina and HZSM-5 zeolite catalysts in series has been developed for converting the polymer into high-quality gasoline-range fuels. Compared with the one-stage degradation over each catalyst, the twostage method provides some advantages. They are an improved gasoline yield and a high octane number despite low aromatics content. Significant results were obtained when silica-alumina and HZSM-5 were used in a weight ratio of 9:1 as upper and lower catalysts, respectively, in a flow reactor. The reverse sequence of catalysts showed no advantage. It was suggested that large pores and moderate acidity of the silica-alumina loaded in the upper layer operated favorably to catalyze the degradation of polyethylene into liquid hydrocarbons. The resulting oils showed low quality, and they were transformed into high-quality gasoline on the strongly acidic sites of the HZSM-5 loaded in the lower layer at the expense of oil yield. Increases in concentration of isoparaffins and aromatics contributed to the upgrading. Introduction Recently, plastics recycling has received much attention all over the world because of serious environmental problems caused by waste plastics as well as their potential for use as resources. Landfill and incineration have not gained social acceptance as the methods for disposing of the waste, and they are becoming legally restricted because of strong pollution concerns. A chemical method that converts waste plastics into chemical resources or fuels is of great interest as an alternative because it provides a viable means of contributing to solution of the problems. Because the thermal degradation of polyolefins, the main components of waste plastics, is usually a low selectivity reaction, successful application of catalysts to their conversion processes would be a key step toward the development of recycling technologies. There have been a number of investigations in the recent past concerning the use of solid acid catalysts for the degradation of polyolefins into fuel oils. Amorphous silica-alumina (SA) has extensively been used as catalyst in the degradation of polyolefins. The present authors have reported the activity and deactivation behaviors of the catalyst1 and the detailed product distribution from degradation of polyethylene,2 and other researchers3,4 have studied the effects of acid strengths and amounts on the product distribution. Recent work has significantly centered on such zeolite catalysts as HZSM-51,5-8 and REY,9-11 which are usually more active than SA. More recently there has been * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +81-143-46-5724.

interest in mesoporous materials such as MCM-4112,13 and KFS-16.14 The investigation on the plastic conversion has further been extended to include studies on coprocessing of plastics and petroleum heavy oil or coal, where FCC15,16 or NiMo/Al2O317 catalysts were employed. Despite these research efforts, product selectivity in the catalytic degradation of polyolefins is not high; various fuel fractions such as gas, gasoline, diesel, and heavy oil are concurrently formed. It is very important to study how the yield of particular fuel fractions can be maximized from the practical point of view. We believe it is reasonable to obtain the gasoline fraction from the degradation of polyolefins when solid acid catalysts are used because acidic catalysts are essentially suitable for producing branched and aromatic hydrocarbons with high octane numbers. However, the gasoline yields so far reported in the degradation of polyolefins are not sufficiently high, mostly less than 50%. The gasoline fraction containing high-boiling-point components such as a diesel fraction must be fractionated for its practical use, and the gasoline with low octane number must be upgraded by a subsequent reforming, which lead to complicated and costly operations. Therefore, it is of great importance to improve product selectivity in the degradation of polyolefins to obtain gasoline with high octane number in a good yield. In addition, reduction of the aromatic concentration in gasoline is also a recent requirement.18 In the present work, a two-stage catalytic degradation method has been proposed, where amorphous SA and HZSM-5 zeolite catalysts are placed in sequence in a flow reactor, to successfully convert polyethylene into

10.1021/ie980341+ CCC: $18.00 © 1999 American Chemical Society Published on Web 12/17/1998

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Figure 1. Schematic diagram of a fixed-bed flow reactor system.

desired gasoline-like products. These catalysts were selected because of their excellent activities and stabilities.1 Experimental Section Materials. Low-density polyethylene (UBEC 180N of 3 mm pellets) was supplied from Ube Ind. Ltd. and used without further treatment. Powdered NaZSM-5 (Si/Al ) 11) supplied from Tosoh Co. was ion-exchanged by treating it with hydrochloric acid, washed with deionized water until no Cl- was detected, and dried at 110 °C for 3 h. HZSM-5 zeolite thus obtained was pressed into a disk and then crushed to 16-32 mesh granules. Commercially available SA (13.5 wt % Al2O3, N631L obtained from Nikki Chemical Co.) in a cylindrical form was crushed to granules of 16-32 mesh. These catalysts were calcined in air at 500 °C for 3 h and further treated in situ in a helium stream at the same temperature for 1 h just prior to use. Apparatus and Procedures. The fixed-bed tubular flow reactor apparatus shown in Figure 1, in which melter, capillary, reactor, and trap were vertically set up, was used for the degradation of polyethylene. The detailed experimental procedures have been given elsewhere.1 In brief, polyethylene melt heated at 310 °C under a helium atmosphere in the melter was pressed out by pressurized helium (0.11-0.15 MPa) into the reactor through a capillary heated at 340 °C. The degradation reaction of polyethylene was carried out at a temperature of 375-425 °C and a space time, W/F (W ) mass of catalyst, F ) mass flow rate of polyethylene (PE)), of 4-18 g of catalyst‚min/g of PE for 15 min. For each run, 0.2-0.3 g of catalyst was loaded into the reactor. In the two-stage catalytic degradation, the catalyst bed was divided into two parts by a thin (about 1 mm) plug of glass wool, and SA and HZSM-5 catalysts were packed into upper and lower layers separately at different weight ratios. The degradation products were first trapped at -196 °C, and after the reaction the gaseous sample was separated from the liquid at 0 °C, collected in a gas bag, and measured by a calibrated syringe. In this study, the degradation products were

Figure 2. Changes in the yield, GC-RON, and aromatics and benzene contents of the liquid products with the SA/HZSM-5 weight ratio at 400 °C and W/F ) 7 g of catalyst‚min/g of PE (upper catalyst, SA; lower catalyst, HZSM-5): (a) liquid yield and GCRON; (b) aromatics and benzene contents.

classified into four groups: gas (C1-C4), liquid (C5 and higher hydrocarbons except wax), wax (greaselike hydrocarbons), and coke (carbonaceous deposit on the catalyst surface). The wax rarely and slightly adhered to the wall of the outlet of the reactor. It was taken out by using a microspatula and determined by weighing. The amount of coke deposited was determined from the increase in catalyst weight before and after the reaction. The gas and liquid yields were corrected by adding C4 components dissolved in the liquid sample to the gas fraction and C5 and C6 components found in the gas sample to the liquid fraction. Product Analysis. The gaseous and liquid samples collected were analyzed on a Shimadzu GC-17A chromatograph equipped with a flame ionization detector and a 60 m OV-1 capillary column. The column temperature was initially held at 35 °C for 10 min, then programmed at 5 °C/min to 280 °C, and held at the final temperature for 60 min. On the basis of the GC analysis, the research octane number (GC-RON) of the liquid products was determined according to ref 11. Results and Discussion Degradation over Individual and Sequential Catalysts. The yield and some properties of the liquid products obtained from the degradation of polyethylene at 400 °C in individual and sequential catalyst systems are shown in Figure 2, where 0 and 100 wt % of HZSM-5 correspond to the one-stage degradations over SA and HZSM-5, respectively. The quality of the liquid product was evaluated in terms of research octane number and aromatics and benzene contents, which are the important characteristics of gasoline. Under the operating conditions, the product hydrocarbons consisted of gas and liquid, and no wax was formed. The liquid products were mostly distributed in the gasoline-range hydrocarbons of C5-C12.

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Figure 3. Change in the ratio of liquid loss/octane gain with the proportion of HZSM-5 at 400 °C and W/F ) 7 g of catalyst‚min/g of PE.

When SA was used singly, gasoline-range fuel oil was obtained in an excellent yield, but its research octane number was rather lower than 90 of commercial gasoline. This suggests that the catalytic performance of SA with moderate acidity was appropriate to crack polyethylene into C5-C12 hydrocarbons but was insufficient for reforming reactions, through which the resulting hydrocarbons were transformed into high-quality gasoline components. On the other hand, a low liquid yield and a high octane number were observed in the one-stage degradation over HZSM-5. The zeolite catalyst was highly active for the carbon-carbon bond scission because of its strong acidity to yield gaseous products preferentially, and consequently the liquid yield was low. Cyclization reactions also frequently occurred on the strong acid sites of the zeolite to produce a large amount of aromatics, which are the components greatly contributing to the high octane number of the product. However, a high concentration of aromatics is currently stipulated as an undesirable property for gasoline for environmental reasons. In particular, benzene removal is a matter of concern. Although HZSM-5 is a promising catalyst for the degradation of polyethylene, the yield and quality of the product should be improved to give a more environmentally acceptable gasoline. To achieve better selectivity to high-quality gasoline in the degradation of polyethylene, a combination of SA and HZSM-5 by two-bed arrangement was examined. SA and HZSM-5 were respectively loaded as upper and lower catalysts at weight ratios of 9:1, 8:2, 7:3, and 5:5. As shown in Figure 2, the liquid yield decreased and the research octane number and the aromatics and benzene contents increased with increasing proportions of HZSM-5. Figure 3 shows the change in the ratio of decrease in liquid yield to increase in octane number, liquid loss/octane gain, with proportions of HZSM-5. The ratio was lowered with increasing HZSM-5. Its lowering is a favorable event, but it accompanied a significant increase in concentration of aromatics and benzene, which must be avoided to produce clean fuel oils. It is thus shown that HZSM-5 should be used in reducing amounts. The purpose of this study is to obtain liquid product that meets the following: the first is to consist of C5-C12, not containing the C13+ fraction, the second is to have an octane number higher than 90, and the third is to contain aromatics as low as possible. A good compromise between the yield and the quality of the liquid product was made when SA and HZSM-5 were loaded as the upper and lower catalysts at a weight ratio of 9:1, respectively. The catalyst combination is designated by SA (9)/Z (1) and was used in further studies.

Figure 4. Effects of reaction temperature on the yield and quality of the liquid obtained over HZSM-5 and SA (9)/Z (1) at W/F ) 7 g of catalyst‚min/g of PE: (a) liquid yield, (b) GC-RON, (c) aromatics content, (d) benzene content.

Other catalyst sequences caused no marked advantage, as will be described later. Effect of Reaction Conditions. Because the yield and the quality of the liquid product largely depend on the operating conditions, the effects of reaction temperature and W/F on them were investigated to elucidate the versatility of the two-stage catalytic degradation process. The results obtained were compared with those observed in the one-stage degradation over HZSM-5. Figure 4 shows the effects of reaction temperature on the liquid yield and quality. Similar tendencies were observed for both catalyst systems. In Figure 4a, the liquid yield decreased with increasing reaction temperature because of more formation of gaseous products. The octane number and the aromatics content increased with reaction temperature, as shown in parts b and c of Figure 4, respectively. Here, their temperature dependencies were almost the same, indicating that the octane number and the aromatics content are closely related to one another. The benzene content also increased with the temperature (Figure 4d). Figure 5 shows that W/F influenced the liquid yield and quality, as well as reaction temperature. Over both catalysts, an increase in W/F caused a decrease in the liquid yield (Figure 5a) and increases in the octane number (Figure 5b) and the aromatics (Figure 5c) and benzene (Figure 5d) contents.

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Figure 6. Product distributions in the polyethylene degradation over SA (9)/Z (1) and HZSM-5 at 375 °C and W/F ) 7 g of catalyst‚ min/g of PE.

Figure 7. Relationship between aromatics content and GC-RON of the liquid obtained over HZSM-5 and SA (9)/Z (1) at 375 °C.

Figure 5. Effects of W/F on the yield and quality of the liquid obtained over HZSM-5 and SA (9)/Z (1) at 375 °C: (a) liquid yield, (b) GC-RON, (c) aromatics content, (d) benzene content.

It is concluded from Figures 4 and 5 that the production of high-quality gasoline is favored by low reaction temperatures and space times. The yield and quality of the liquid product obtained from the one-stage degradation over HZSM-5 were improved to some extent by choosing the favorable conditions. Nevertheless, the liquid yield was still low (never exceeded 50%) and the aromatics content remained high. The limitations shown by HZSM-5 were overcome by the combined use of SA and HZSM-5. The liquid yield obtained over SA (9)/Z (1) was always higher than that obtained when HZSM-5 was singly used. In addition, the aromatics and benzene contents markedly decreased by the catalyst combination. The two-stage catalytic degradation system has thus been proven to be effective for obtaining highquality gasoline selectively. Figure 6 shows the product distributions obtained at a low reaction temperature of 375 °C and a low W/F of 7 g of catalyst‚min/g of polyethylene. A high liquid yield of 58.8 wt % was obtained over SA (9)/Z (1), and then C13+ components were not detected. The liquid contained 25.2% of aromatics and 0.9% of benzene. These aromatic concentrations almost satisfied the new standard for gasoline.18 A small amount of coke was deposited on the sequential catalyst. Its deposition mostly occurred on SA because of a higher coking activity of the catalyst than of HZSM-5.1 We used the two catalysts at the same temperature in this work, the first part of

a series of polyethylene degradation by catalyst combination. However, the same temperature of operation is not essential, and different temperatures or separate reactors for each catalyst would rather be highly useful. From a practical point of view, the relatively shorter catalyst life of SA, which resulted from more coke deposition, will need its operation under conditions different from those adopted for HZSM-5. Discovery or development of more stable catalysts which can take the place of SA may allow a simple operation like this work. Relationship between Aromatics and GC-RON. As described already, the quality of the gasoline largely depends on the concentration of aromatics, and so the relationships between the octane number and the aromatics content for the liquid products obtained over HZSM-5 and SA (9)/Z (1) are shown in Figure 7. The octane number increased almost linearly with increasing aromatics content. A distinct difference was observed between the two catalyst systems, and a desirable result was shown by SA (9)/Z (1) which gave higher octane numbers despite lower aromatics contents. This resulted from more formation of isoparaffins, which have high octane numbers and are the most favorable components of gasoline, over SA (9)/Z (1). Structural isomerization that forms isoparaffins is presumed to occur readily on HZSM-5, but the zeolite would also greatly accelerate their consecutive reactions such as cracking and aromatization which lead to a decreased liquid yield and a high aromatics content. Therefore, HZSM-5 is preferred to be used in a reduced amount to depress the consecutive reactions. It should be noted that the formation of a proper amount of aromatics is necessary to yield isoparaffins; aromatization involved in the degradation of polyethylene plays very important roles to produce high-quality gasoline. The H/C atomic ratio of polyethylene is 2. This means that it is theoretically difficult to yield isoparaffins (H/C > 2) very

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Figure 8. Two-stage degradation of polyethylene at different catalyst sequences.

selectively in the degradation of polyethylene provided that a reaction which supplies hydrogen to olefinic intermediates scarcely occurs. Aromatization is the best reaction that releases hydrogen necessary to saturate the olefins, but an excess formation of aromatics should be avoided from the reasons mentioned above. Coking is also the reaction that results in a hydrogenation of the olefins, but it was not significant in this work, as can be expected from a little deposition of coke (Figure 6). Role of Upper and Lower Catalysts. To elucidate the role of upper and lower catalysts in the two-stage degradation of polyethylene, several catalyst combinations were examined. The first combination consisted of SA followed by HZSM-5 in sequence in the reactor at a weight ratio of 9:1, respectively, which was described above. The second combination, Z (1)/SA (9), used a sequence in the reverse order. The third combination employed SA poisoned by sodium (Na-SA), designated Na-SA (9)/Z (1). The sodium poisoning was carried out by treating SA first with a NH4OH solution and then with a Na2CO3 solution to reduce proton acidity of the catalyst without any change of pore structure. The acid amount of the catalyst thus obtained was about onefourth of that of nonpoisoned SA.19 Figure 8 shows the results obtained at different catalyst systems. When the catalyst sequence was reversed, for Z (1)/SA (9), no advantage by the catalyst combination appeared; the octane number was almost the same low level as that for SA. This indicates that the catalyst sequence is of primary importance and SA and HZSM-5 should be placed in that order, i.e., SA (9)/Z (1), to obtain high-quality gasoline selectively. It seems that large pores of SA are essential to facilitate the diffusion of the decomposed fragments involved in the reaction. The pores of HZSM-5 are probably too tight to permit their easy passage. To examine the influence of acidity of SA catalyst loaded in the upper layer, the sequential catalyst of NaSA (9)/Z (1) was prepared by replacing SA with Na-SA with less acidity and was used at 400 °C. Because of the low cracking activity of Na-SA, the sequential catalyst produced a significant amount of wax and hence the liquid yield was rather low. It is therefore suggested that the upper catalyst needs to have not only large pores but also an acidity enough to convert polyethylene into gasoline-range hydrocarbons. The octane number

Figure 9. Reaction scheme for two-stage catalytic degradation.

of the liquid product obtained over Na-SA (9)/Z (1) was also low, indicating that the reforming reactions on HZSM-5 did not occur frequently. This is probably because, due to an insufficient decomposition of polyethylene in the Na-SA layer, HZSM-5 mainly catalyzed cracking of considerably large fragments and reforming was a minor reaction. We can therefore speculate that the reforming reactions over HZSM-5 take place frequently only when the fragments that take part in the steps are rather small, which is the case of SA (9)/Z (1). Reaction Scheme for Two-Stage Degradation. Figure 9 shows a possible reaction scheme for the twostage degradation of polyethylene over SA (9)/Z (1). Because thermal degradation of polyethylene occurs at around 400 °C,20,21 it is reasonable to assume that polyethylene thermally decomposed into high-molecularweight fragments, mostly olefins,22 before it reached the SA layer. The decomposed fragments were allowed to diffuse into large pores of SA catalyst and cracked into gasoline-range hydrocarbons on the acidic sites. The acid strength of SA seems to be enough to crack the olefinic fragments, because the formation of carbenium ion intermediates by the protonation of olefins easily occurs even on moderately or weakly acidic sites of SA. However, because reforming reactions such as structural isomerization and aromatization did not occur so much in the SA layer, the liquid product showed low qualities, a low octane number (Figure 2), and a high olefin content, as gasoline. The fractions passed through the SA layer were transformed into high-octane-number components in the HZSM-5 layer. Strong acidity of HZSM-5 enabled a frequent occurrence of reforming reactions, but the zeolite simultaneously promoted overcracking and thereby the liquid yield decreased. These mechanistic considerations reasonably explain the importance of the catalyst sequence shown in Figure 8 and make it possible to predict a less effect of composite catalyst prepared by a physical mixture of SA and HZSM-5 than the sequential catalyst, which will be reported elsewhere. There have recently been a few reports on the potential of mesoporous silicate/aluminosilicate with uniform pores as catalysts for the degradation of polymers.12-14 Although the pores of SA are distributed

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in diameters of 2-8 nm, its average pore size is 4.4 nm,1 the value of which belongs to mesopores. The use of SA as one component in the sequential catalyst is in line with the strategy of mesoporous materials. The degradation of polyethylene over solid acids proceeds through a complicated mechanism which involves various types of reactions such as cracking, isomerization, cyclization, hydrogen transfer, coking, etc., in which the molecular weight of the fragments participating in each process remarkably changes as the degradation proceeds. Therefore, it may be considerably difficult to develop a catalyst having all functions that are necessary to decompose polyethylene into useful products selectively. Catalyst combination would be an alternative, in which the individual catalysts play different roles and concertedly contribute to the overall performance of the combined catalyst. This work may offer an indication to a new catalyst or process development for plastics recycling. Conclusions It has been proven that a two-stage catalytic degradation of polyethylene is highly effective to obtain highquality gasoline selectively. The newly developed technique used amorphous SA and HZSM-5 zeolite as upper and lower layer catalysts, respectively, in a flow reactor. These catalysts are not new materials, but their combination behaved like a new catalyst having mesopores and acidity suitable for the reaction. Under our experimental conditions, most favorable results were observed when SA and HZSM-5 were used at a weight ratio of 9:1 and at a temperature of 375 °C and a W/F of 7 g of catalyst‚min/g of PE. The liquid then produced consisted of gasoline fraction (C5-C12) only, and its yield was 58.8%, the research octane number was 94, and the aromatics and benzene contents were 25.2 and 0.9%, respectively, which almost satisfy the recent aromatic requirements of gasoline. On the other hand, no significant results were obtained over individual catalysts; HZSM-5 gave a low gasoline yield and a high aromatics content, while SA produced low-quality liquid. In the two-stage catalytic degradation, SA loaded in the upper layer effectively catalyzed the decomposition of polyethylene into gasoline-range hydrocarbons because of its moderate acidity and large pore structure, and overcracking and reforming did not occur significantly. HZSM-5 in the lower layer mainly acted as a reforming catalyst, by which the fragments formed in the SA layer were transformed into high-quality gasoline. Strong acidity of the zeolite enabled frequent occurrence of the reforming reactions. By the concerted actions of the two catalysts, polyethylene was converted into an environmentally acceptable gasoline in a good yield. Acknowledgment The authors thank Dr. Y.-H. Lin and Mr. J. K. A. Dapaah for their helpful discussions. Literature Cited (1) Uemichi, Y.; Hattori, M.; Itoh, T.; Nakamura, J.; Sugioka, M. Deactivation Behaviors of Zeolite and Silica-Alumina Catalysts in the Degradation of Polyethylene. Ind. Eng. Chem. Res. 1998, 37, 867. (2) Uemichi, Y.; Ayame, A.; Kashiwaya, Y.; Kanoh, H. Gas Chromatographic Determination of the Products of Degradation

of Polyethylene over a Silica-Alumina Catalyst. J. Chromatogr. 1983, 259, 69. (3) Ohkita, H.; Nishiyama, R.; Tochihara, Y.; Mizushima, T.; Kakuta, N.; Morioka, Y.; Ueno, A.; Namiki, Y.; Tanifuji, S.; Katoh, H.; Sunazuka, H.; Nakayama, R.; Kuroyanagi, T. Acid Properties of Silica-Alumina Catalysts and Catalytic Degradation of Polyethylene. Ind. Eng. Chem. Res. 1993, 32, 3112. (4) Ochoa, R.; Woert, H. V.; Lee, W. H.; Subramanian, R.; Kugler, E.; Eklund, P. C. Catalytic Degradation of MediumDensity Polyethylene over Silica-Alumina Supports. Fuel Process. Technol. 1996, 49, 119. (5) Vasile, C.; Onu, P.; Barboiu, V.; Sabliovschi, M.; Moroi, G.; Ganju, D.; Florea, M. Catalytic Decomposition of Polyolefins III. Decomposition over the ZSM-5 Catalyst. Acta Polym. 1988, 39, 306. (6) Mordi, R. C.; Fields, R.; Dwyer, J. Gasoline Range Chemicals from Zeolite-Catalysed Thermal Degradation of Polypropylene. J. Chem. Soc., Chem. Commun. 1992, 374. (7) Sharratt, P. N.; Lin, Y.-H.; Garforth, A. A.; Dwyer, J. Investigation of the Catalytic Pyrolysis of High-Density Polyethylene over a HZSM-5 Catalyst in a Laboratory Fluidized-Bed Reactor. Ind. Eng. Chem. Res. 1997, 36, 5118. (8) Ding, W.; Liang, J.; Anderson, L. L. Thermal and Catalytic Degradation of High-Density Polyethylene and Commingled PostConsumer Plastic Waste. Fuel Process. Technol. 1977, 51, 47. (9) Beltrame, P. L.; Carniti, P.; Audisio, G.; Bertini, F. Catalytic Degradation of Polymers: Part IIsDegradation of Polyethylene. Polym. Degrad. Stab. 1989, 26, 209. (10) Songip, A. R.; Masuda, T.; Kuwahara, H.; Hashimoto, K. Test to Screen Catalysts for Reforming Heavy Oil from Waste Plastics. Appl. Catal. B 1993, 2, 153. (11) Songip, A. R.; Masuda, T.; Kuwahara, H.; Hashimoto, K. Production of High-Quality Gasoline by Catalytic Cracking over Rare-Earth Metal Exchanged Y-Type Zeolites of Heavy Oil from Waste Plastics. Energy Fuels 1994, 8, 136. (12) Garforth, A. A.; Fiddy, S.; Lin, Y.-H.; Ghanbari-Siakhali, A.; Sharratt, P. N.; Dwyer, J. Catalytic Degradation of HighDensity Polyethylene: An Evaluation of Mesoporous and Microporous Catalysts Using Thermal Analysis. Thermochim. Acta 1997, 294, 65. (13) Aguado, J.; Sotelo, J. L.; Serrano, D. P.; Calles, J. A.; Escola, J. M. Catalytic Conversion of Polyolefins into Liquid Fuels over MCM-41: Comparison with ZSM-5 and Amorphous SiO2Al2O3. Energy Fuels 1997, 11, 1225. (14) Sakata, Y.; Uddin, M. A.; Muto, A.; Kanada, Y.; Koizumi, K.; Murata, K. Catalytic Degradation of Polyethylene into Fuel Oil over Mesoporous Silica (KFS-16) Catalyst. J. Anal. Appl. Pyrolysis 1997, 43, 15. (15) Ng, S. H. Conversion of Polyethylene Blended with VGO to Transportation Fuels by Catalytic Cracking. Energy Fuels 1995, 9, 216. (16) Arandes, J. M.; Abajo, I.; Lopez-Valerio, D.; Fernandez, I.; Azkoiti, M. J.; Olazar, M.; Bilbao, J. Transformation of Several Plastic Wastes into Fuels by Catalytic Cracking. Ind. Eng. Chem. Res. 1997, 36, 4523. (17) Joo, H. K.; Curtis, C. W. Effect of Reaction Time on the Coprocessing of Low-Density Polyethylene with Coal and Petroleum Resid. Energy Fuels 1997, 11, 801. (18) Bell, A. T.; Manzer, L. E.; Chen, N. Y.; Weekman, V. W.; Hegedus, L. L.; Pereira, C. J. Protecting the Environment Through Catalysis. Chem. Eng. Prog. 1995, Feb, 26. (19) Uemichi, Y.; Ayame, A.; Yoshida, T.; Kanoh, H. Gasification of Polyethylene over Solid Catalysts (Part 4) Gasification over Sodium X Zeolite and Silica-Alumina in a Fixed Bed Tubular Flow Reactor. J. Jpn. Pet. Inst. 1980, 23, 35. (20) Fernandes, V. J., Jr.; Araujo, A. S.; Fernandes, G. J. T. Catalytic Degradation of Polyethylene Evaluated by TG. J. Therm. Anal. 1997, 49, 255. (21) Mordi, R. C.; Fields, R.; Dwyer, J. Thermolysis of LowDensity Polyethylene Catalyzed by Zeolites. J. Anal. Appl. Pyrolysis 1994, 29, 45. (22) Uemichi, Y.; Kashiwaya, Y.; Ayame, A.; Kanoh, H. Formation of Aromatic Hydrocarbons in Degradation of Polyethylene over Activated Carbon Catalyst. Chem. Lett. 1984, 41.

Received for review June 1, 1998 Accepted November 5, 1998 IE980341+