Evolution of Products Generated during the Dynamic Pyrolysis of

Energy & Fuels 2008, 22, 2917–2924

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Evolution of Products Generated during the Dynamic Pyrolysis of LDPE and HDPE over HZSM5 A. Marcilla,* M. I. Beltrań, and R. Navarro Departamento de Ingenierıá Quı´mica, Facultad de Ciencias, UniVersidad de Alicante, Apartado 99, Alicante 03080, Spain ReceiVed April 2, 2008. ReVised Manuscript ReceiVed July 8, 2008

The catalytic pyrolysis of low-density polyethylene (LDPE) and high-density polyethylene (HDPE) over the HZSM5 catalyst in a batch reactor has been studied, and an analysis of the evolution of the products generated with temperature is presented. As expected, the catalyst favored the formation of gaseous compounds. The composition of the products obtained depended upon the polyethylene structure, especially at the beginning of the degradation process. Initially during the catalytic cracking of LDPE, olefins were the major compounds obtained in both the gaseous and condensed phases. As the temperature was increased, the composition of the gases remained constant, whereas in the condensed fraction, the olefins yield decreased, resulting in aromatics being the compounds predominantly generated. With HDPE, a greater quantity of paraffins and aromatics formed at the beginning of the decomposition process in the gaseous and condensed products, respectively, as compared to LDPE. As the temperature was increased, however, a more similar composition to that for LDPE was obtained.

The thermal and catalytic pyrolyses probably constitute the most promising methods of plastic waste recycling yet to be developed. Plastic wastes can be converted to valuable chemicals or fuels by these processes. Some authors have centered their efforts on analyzing the composition of the products derived from the thermal and catalytic pyrolysis of different polymers. For products evolved from polyethylene, the most common plastic found in landfilling, a wide carbon number distribution, typically C1-C40, and product type distribution are obtained. It is generally agreed that linear hydrocarbons, i.e., paraffins, 1-olefins, and diolefins, are the main products obtained, with their relative concentrations varying with the experimental conditions involved.1,2 In contrast to the thermal process, the catalytic pyrolysis of plastics demands less energy, and an advantageous narrower carbon number distribution for the products generated has been extensively reported.3 Zeolites have been found to be effective in producing gasoline-sized hydrocarbon molecules in the cracking of polyethylene, with the typical product range varying from C3 to C15 hydrocarbons, but the product distribution heavily depends upon the catalyst employed.4-7

The HZSM5 zeolite in particular finds industrial application in benzene alkylation, xylene isomerization, toluene disproportionation, and pyrolysis of plastic wastes.7 Its structure contains two types of intersecting channels (straight and sinusoidal), both formed by rings of 10 oxygen atoms, making it a medium-pore zeolite.8 The catalytic cracking of polyethylene over HZSM5 has been studied by different authors.5,9-11 The products obtained in the polyethylene catalytic decomposition largely depend upon the reactor type and the experimental conditions, although a great selectivity up to certain compounds is generally obtained in the presence of HZSM5. These compounds could have various industrial applications. For example, light olefins as propene, 2-methyl-2-butene, and isobutene and paraffins as propane, which are obtained in great quantities in the catalytic pyrolysis of polyethylene over HZSM5,6 could be used as feedstock for alkylation and isomerization units from which high octane gasoline components and other chemicals are produced. Another important application for the light olefins is in the production of oxygenates used as octane-boosting additives in gasoline.12,13 On the other hand, household consumption of propane is as a cooling agent or liquid gas.14 Most studies dealing with the products evolved from the degradation processes take a global view of the process and,

* To whom correspondence should be addressed. Telephone: (+34)-96590-3400, ext. 3789. Fax: (+34)-96-590-3826. E-mail: antonio.marcilla@ ua.es. (1) Mertinkat, J.; Kirsten, A.; Predel, M.; Kaminsky, W. J. Anal. Appl. Pyrolysis 1999, 49, 87–95. (2) Kaminsky, W. J. Anal. Appl. Pyrolysis 1985, 8, 439–448. (3) Catalytic Cracking. Catalysts, Chemistry, and Kinetics; Wojciechowski, B. W., Corma, A. Eds.; Chemical Industries/25, Marcel Dekker, Inc.: New York, 1986. (4) Hernańdez, M. R.; Garcıá, A. N.; Marcilla, A. J. Anal. Appl. Pyrolysis 2007, 78, 272–281. (5) Marcilla, A.; Beltrań, M. I.; Navarro, R. Appl. Catal., A 2007, 333, 57–66. (6) Marcilla, A.; Beltrań, M. I.; Navarro, R. Thermal and catalytic pyrolysis of polyethylene over HZSM5 and HUSY zeolites in a batch reactor under dynamic conditions. Appl. Catal., B 2008, in press.

(7) Shape SelectiVe Catalysis in Industrial Applications; Chen, N. Y., Garwood, W. E., Dwyer, F. G. Eds.; Marcel Dekker, Inc.: New York, 1996. (8) Ullmann’s Encyclopedia of Industrial Chemistry; Elvers, B., Hawkins, S. Eds.; VCH: New York, 1996. (9) Hernańdez, M. R.; Go´mez, A.; Garcıá, A. N.; Agullo´, J.; Marcilla, A. Appl. Catal., A 2007, 317, 183–194. (10) Sakata, Y.; Uddin, M. A.; Muto, A. J. Anal. Appl. Pyrolysis 1999, 51, 135–155. (11) Mastral, J. F.; Berrueco, C.; Gea, M.; Ceamanos, J. Polym. Degrad. Stab. 2006, 91, 3330–3338. (12) Bortnovsky, O.; Sazama, P.; Wichterlova, B. Appl. Catal., A 2005, 287, 203–213. (13) den Hollander, M. A.; Wissink, M.; Makkee, M.; Moulijn, J. A. Appl. Catal., A 2002, 223, 85–102. (14) Quı´mica y Tecnologıá del Petro´leo y del Gas; E´rij, V., Ra´sina, M., Rudin, M. Eds.; Mir Moscu´: Moscow, Russia, 1988.

1. Introduction

10.1021/ef800229d CCC: $40.75  2008 American Chemical Society Published on Web 08/28/2008

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Figure 1. Scheme of the batch reactor system.

therefore, provide no information regarding the sequence in which the products are formed with respect to temperature or time. This kind of information may reveal how to assess the process to make it more valuable. Accordingly, White et al.15 employed the thermogravimetry/gas chromatography/mass spectrometry (TG/GC/MS) technique to analyze the products evolved during the catalytic degradation of polypropylene over HZSM5 at different conversions. Polypropylene cracking by the HZSM5 catalyst yields olefins as primary volatile products, but as conversion increases, a significant increase in the yield of aromatics is observed. In a similar way, these authors16 used a repetitive-injection GC/MS evolved gas analyzer to study the volatiles from the catalytic cracking of polyethylene over the HZSM5 catalyst after 5 min intervals and at a 2 °C min-1 heating rate. Paraffins were detected initially, and olefins were produced at somewhat higher temperatures. Aromatics were formed at temperatures 30-40 °C higher than those required for olefin production. In a semibatch reactor, Manos et al.17 observed that C3-C5 paraffins were evolved at slightly lower temperatures than the corresponding C3-C5 olefins during the catalytic degradation of high-density polyethylene (HDPE) over HZSM5. Garforth et al.18 employed a fluidized-bed reactor, and they stated that the HZSM5 product streams remained virtually unchanged throughout the degradation of HDPE. In a recent study,19 we applied the thermogravimetry/Fourier transform infrared spectroscopy (TG/FTIR) technique to analyze the gases evolved during the catalytic pyrolysis of low-density polyethylene (LDPE) and HDPE over HZSM5. Spectra showed a slight decrease in the yield of shorter and/or more branched hydrocarbons when the temperature increased. This paper reports on the catalytic pyrolysis processes of LDPE and HDPE in the presence of the HZSM5 catalyst. The aim of this study is to analyze in depth the evolution of the composition and amount of gaseous and condensed products obtained in the dynamic catalytic pyrolysis of these polymers (15) Negelein, D. L.; Lin, R.; White, R. L. J. Appl. Polym. Sci. 1998, 67, 341–348. (16) Hesse, N. D.; Lin, R.; Bonnet, E.; Cooper, J.; White, R. L. J. Appl. Polym. Sci. 2001, 82, 3118–3125. (17) Manos, G.; Garforth, A.; Dwyer, J. Ind. Eng. Chem. Res. 2000, 39, 1198–1202. (18) Garforth, A. A.; Lin, Y. H.; Sharratt, P. N.; Dwyer, J. Appl. Catal., A 1998, 169, 331–342. (19) Marcilla, A.; Beltrań, M. I.; Navarro, R. J. Anal. Appl. Pyrolysis 2006, 76, 222–229.

as a function of temperature. Bibliography regarding the sequence in which the different product families are formed in the degradation process at different conversion degrees of initial polymer is limited. This study could be of great interest because it gives additional information concerning the cracking mechanism. 2. Experimental Section 2.1. Materials. Low- and high-density polyethylene (LDPE 780R from Dow Chemical and HDPE HD3560UR from BP) were employed. The densities are 0.923 and 0.935 g cm-3, and melt flow indexes are 20 and 6 g 10 min-1 for LDPE and HDPE, respectively, as provided by suppliers. Both polymers were supplied in powder form, with a maximum particle size of 500 µm each. HZSM5 zeolite was used and provided by GRACE-Davison. Brunauer-Emmett-Teller (BET) surface area and micropore volume from nitrogen adsorption-desorption isotherms at 77 K were 341 m2 g-1 and 0.16 cm3 g-1, respectively. The Si/Al atomic ratio obtained by X-ray fluorescence (XRF) was 22.2. Data obtained from temperature-programmed desorption (TPD) of ammonia indicate that HZSM5 desorbs 1.15 mmol of NH3 g-1 at 199 °C (weak acid sites) and 0.88 mmol of NH3 g-1 at 416 °C (strong acid sites). 2.2. Equipment and Experimental Procedure. Figure 1 shows a scheme of the reactor system employed. It consists of a quartz batch reactor (30.7 mm internal diameter and 470.0 mm height), which is in an upright position. The reactor outlet was heated to 300 °C to prevent condensation of the evolved products. Around 600 mg of polyethylene and 60 mg of catalyst (ratio of 10:1) were placed in a crucible in the middle of the reactor. A thermocouple was placed very close to the sample to monitor its actual temperature. A nitrogen flow rate of 150 mL min-1 (STP) was used, and the system was purged at room temperature for 30 min prior to all experiments. The temperature was varied from 30 to 550 at 5 °C min-1. Under these conditions, all of the polyethylene was converted to gaseous or liquid products, and no solid residue apart from the coke deposited on the catalyst was observed after each experiment. Two valves were placed in the stream after the reactor, as shown in Figure 1. In this way, the flow direction could be changed, which allowed for collection of the products in 5 min intervals (corresponding to 25 °C temperature increments). The stream from the reactor was passed through a cooling trap system, which was placed in an ice/NaCl bath, and the uncondensed products remaining were collected in 1 L valved Tedlar bags. 2.3. Analysis of Products. The gaseous fractions were analyzed by an Agilent 6890N gas chromatograph (GC) equipped with a

Dynamic Pyrolysis of LDPE and HDPE oVer HZSM5

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The catalytic cracking experiments were duplicated, obtaining similar results. The average mass balances were 89.5% for LDPE-HZSM5 and 90.6% for HDPE-HZSM5. Condensation of some heavy products on reactor walls or the determination of the amount of volatile products evolved might explain the deviations observed. Table 1 shows the yield per gram of polyethylene of the gaseous and condensed products generated at regular intervals of 5 min each (corresponding to temperature intervals of approximately 25 °C) during the catalytic cracking of LDPE and HDPE over HZSM5. As can be observed, the presence of the catalyst favors the formation of volatile compounds, with a yield of gases close to 70% for both types of polyethylene. This feature of the HZSM5 catalyst is related to its acidity and structural characteristics because it has already been described elsewhere.6 The decomposition process with LDPE starts at slightly lower temperatures than that with HDPE. Similarly, the highest decomposition rate during the catalytic degradation of LDPE occurs between 382 and 435 °C, whereas in the case of HDPE, it occurs between 408 and 459 °C. This difference in behavior can be ascribed to the higher degree of branching of LDPE as compared to HDPE, which favors catalytic cracking because the polymer chain ends are able to penetrate into the zeolite pores, reaching acid sites located there.20 3.1. Analysis of the Evolution of Gaseous Products. Figures 2 and 3 show for the catalytic cracking of LDPE and HDPE, respectively, the evolution with temperature of the yield of gaseous products formed with respect to carbon number and grouped by compound type: 1-olefins, olefins (different to 1-olefins), n-paraffins, and iso-paraffins. Other compounds, such as alkynes, aromatics, and cyclics, were also detected. However,

their concentrations were lower than 1.2%, and hence, they are not shown. The scale corresponding to n-paraffins and isoparaffins has been shrunk to half the scale of 1-olefins and olefins to facilitate comparison. It can be observed that the selectivity for olefins and 1-olefins is noticeably higher than for the other compounds. In both systems, propene and olefins with 4 and 6 carbon atoms are the major compounds. All of the products follow a similar trend and present maximum yields in the same temperature range. The production of n-paraffins and iso-paraffins is higher for HDPE than for LDPE, with propane, n-butane, and isobutane being the most important compounds in these groups. Figures 4 and 5 represent the evolution of the composition (weight fraction) of the gases generated in the catalytic degradation as a function of the compound type and the carbon number, respectively. For LDPE (at the top in both figures), the weight fraction of all compound families remains nearly constant during the whole process (Figure 4). With respect to the carbon number (Figure 5), the weight fraction of all hydrocarbons remains nearly constant until the end of the process, when the weight fraction of the lightest hydrocarbons (C2-C4) decreases slightly, whereas the heaviest fraction (C6-C8) increases. In the catalytic cracking of HDPE (at the bottom in both figures), the composition as a function of the carbon number (Figure 5) follows the same trend as that observed for LDPE. On the other hand, the evolution of the products grouped by compound type (Figure 4) shows clear differences between the LDPE-HZSM5 and HDPE-HZSM5 systems, mainly at the beginning of the processes. As can be seen, in the case of HDPE, the weight fraction of n-paraffins is initially much higher than for LDPE, to the detriment of the olefins. As the temperature increases, the concentration of n-paraffins decreases, while that of olefins increases, and the composition of the gases obtained for both polymers becomes very similar. Previously,19 the TG/FTIR analysis of the evolution of the FTIR spectra of the gases evolved during the catalytic pyrolysis of LDPE and HDPE with HZSM5 showed that, although the spectra were very similar toward the end of the degradation process, the yield of shorter and/or more branched hydrocarbons was slightly higher for HDPE than for LDPE at the beginning of the process. 3.2. Analysis of the Evolution of Condensed Products. Figure 6 shows the results obtained for the condensed products in the catalytic cracking of LDPE with HZSM5. Because the carbon number distribution for the condensed products is larger than for gases, the carbon number is plotted on the horizontal axis in this figure. The compounds are grouped by type and the temperature range in which they were generated. Despite liquid production being low, aromatics and dienes have been included here because their relative quantities are high, especially in the case of aromatics. As can be observed, aromatics and olefins are the main compounds generated in the catalytic degradation of LDPE over HZSM5. Analysis of the condensed products obtained shows the predominant presence of compounds with a number of carbon atoms ranging between 7 and 13 (C7-C13). Similar distributions were obtained by other authors for the global composition of this system.10,21 Low-molecular-weight olefins, aromatics, dienes, and iso-paraffins are mainly obtained. Olefins are the major products up to 408 °C, while the formation of aromatics and dienes is favored as the temperature is increased to the point at which, in the fraction obtained at 408-435 °C,

(20) Marcilla, A.; Beltrań, M. I.; Hernańdez, F.; Navarro, R. Appl. Catal., A 2004, 278, 37–43.

(21) Sakata, Y.; Uddin, M. A.; Muto, A.; Kanada, Y.; Koizumi, K.; Murata, K. J. Anal. Appl. Pyrolysis 1997, 43, 15–25.

Table 1. Yield of Gaseous and Condensed Compounds after Temperature Intervals of 25 °C during the Catalytic Pyrolysis of LDPE and HDPE over HZSM5 Zeolite LDPE-HZSM5

HDPE-HZSM5

condensed condensed temperature gases products temperature gases products (°C) (mg/g PE) (mg/g PE) (°C) (mg/g PE) (mg/g PE) 275-301 302-326 327-352 353-381 382-407 408-435 436-461 462-488

5.6 18.2 71.4 141.9 216.6 210.4 16.8 7.8

5.6 11.1 19.5 41.8 61.3 30.6 11.1 5.6

305-330 331-356 357-384 385-407 408-431 432-459 460-485 486-512

4.8 8.8 24.1 73.9 248.5 319.7 24.4 9.6

3.4 6.7 10.1 20.2 57.1 40.3 10.1 6.7

flame ionization detector (FID), using a GS-Alumina column (30 m × 0.53 mm i.d.). Standard gaseous hydrocarbons (from Scott Specialty Gases) were used to identify and quantify the gases obtained. Different volumes of these standard gases, in the range of 10-250 µL, were injected into the GC, obtaining by the analytic procedure an average response factor for each compound. The amount of condensed products was obtained by the difference of the weight of the cooling traps before and after the experiment. The stainless-steel Dixon rings and the glass traps were washed with n-hexane to collect the condensed products, which were analyzed by a gas chromatograph coupled to a mass spectrometer (Agilent 6890N GC-MSD 5973N, HP-5MS column, 30 m × 0.25 mm i.d.). Different standards, representative of the mixture of products obtained in the pyrolysis experiments, were used.

3. Results


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Figure 2. Evolution of the yield of the gaseous products with respect to the temperature for the catalytic pyrolysis of LDPE with HZSM5.

Figure 3. Evolution of the yield of the gaseous products with respect to the temperature for the catalytic pyrolysis of HDPE with HZSM5.

the yield of aromatics resembles that of olefins. Aromatics constitute the major compounds in the final fraction (436-461 °C). On the other hand, the contribution of compounds with more than 16 carbon atoms is very small in all of the temperature ranges and relatively higher at low and high temperatures. At low temperatures, this behavior could be related to the evolution of catalytically cracked polymer chains, whereas as the temperature increases, the thermal cracking reactions might also be taking place [experiments without catalyst (data not shown) prove that only at temperatures higher than 360 °C such

compounds are detected22]. As can be observed, these compounds are n-paraffins and 1-olefins, with their number of carbon atoms ranging between 16 and 41, which are typical of compounds obtained by the thermal pyrolysis of polyethylene.6 Aguado et al.23 also observed a similar evolution of product selectivities toward the formation of heavier hydrocarbons with (22) Marcilla, A.; Beltrań, M. I.; Navarro, R. Evolution of products during the degradation of polyethylene in a batch reactor. Polym. Degrad. Stab. 2008, manuscript submitted. (23) Aguado, J.; Serrano, D. P.; Sotelo, J. L.; Van Grieken, R.; Escola, J. M. Ind. Eng. Chem. Res. 2001, 40, 5696–5704.



Figure 8 represents the evolution of the composition (weight fraction) of the condensed products grouped by compound type. It can be observed that in the presence of LDPE the weight fraction of olefins decreases as the temperature increases, which in turn increases the fraction corresponding to aromatics. Aromatics were also the major compounds obtained by other authors in the liquid fraction4,24-27 when the catalytic degradation of polyethylene was carried out at temperatures higher than 400 °C. The composition of the remaining compounds stays practically constant, and only n-paraffins tend to increase toward the end of the process. According to Okhita et al.,24 aromatization, which is favored at high temperatures, leaves many hydrogen atoms behind on the surface of solid acids, these hydrogen atoms are consumed by the hydrogenation of olefins, and consequently, the yield of n-paraffins increases toward the end of the process. On the other hand, with HDPE, the composition is practically independent of the temperature. Figure 4. Evolution of the composition (weight fraction) of the gaseous products obtained in the catalytic pyrolysis of LDPE and HDPE with HZSM5 grouped by compound type.

Figure 5. Evolution of the composition (weight fraction) of the gaseous products obtained in the catalytic pyrolysis of LDPE and HDPE with HZSM5 grouped by carbon number.

increasing temperature during the catalytic pyrolysis of polyolefin mixtures over nanosized HZSM5. Figure 7 shows the analogous results for the catalytic cracking of HDPE with HZSM5. A small quantity of intermediatemolecular-weight n-paraffins and low-molecular-weight aromatics are present in the first fraction (305-330 °C). As can be observed, these compounds are practically the only ones generated up to 384 °C. However, in the following fraction (385-407 °C), the yield of olefins has increased, especially for compounds with 13 carbon atoms, and the distribution of olefins widens up to compounds with 16 carbon atoms. In the fraction obtained between 408 and 431 °C, the lightest compounds (C8-C9) present the maximum yield. In all of the temperature ranges, the compounds obtained are mainly aromatics and olefins and a low proportion of iso-paraffins are also present, but unlike in the pyrolysis of LDPE, no dienes are obtained. For the condensed products generated between 460 and 485 °C and between 486 and 512 °C, the peak centered at olefins with 13 carbon atoms disappears and both aromatics and olefins exhibit a higher molecular weight than at lower temperatures. In the case of HDPE, the contribution of compounds with more than 16 carbon atoms is smaller than for LDPE.

4. Discussion Analysis of the evolution of the products generated in the catalytic cracking of polyethylene over HZSM5 shows that the type of polyethylene and temperature have a clear effect on the composition obtained. At the beginning of the process, the formation of gaseous n-paraffins and iso-paraffins is more favored with HDPE than LDPE, whose major gaseous compounds were olefins in the whole temperature range, although these differences lessen when the temperature increases. In the condensed products, aromatics are the most important products obtained when pyrolyzing HDPE, whereas with LDPE, at low temperatures, olefins are the major compounds. Nevertheless, with increasing temperature, the formation of aromatics is favored in the LDPE case, generating higher yields than those of olefins at the end of the pyrolysis process. This behavior could be explained considering that catalytic cracking of hydrocarbons over acid solids proceeds through a carbocationic mechanism. It is reasonable to assume that the thermal cracking could also contribute to the degradation process at the same time as the catalytic cracking breaking the polymeric chain and yielding smaller polymeric fragments. As observed above, in the catalytic degradation of both polyethylenes, narrow product distributions were mainly obtained, favoring especially the formation of gaseous olefins with 3, 4, and 6 carbon atoms. According to Aguado et al.23 this carbon atom distribution indicates that the predominant cracking pathway is based on end-chain scission reactions. In the case of LDPE, the presence of a higher number of chain ends than in HDPE favors the access to the active sites inside the pores of the HZSM5 zeolite,5,20 where β-scission to form light olefins is the predominant reaction.11 As the temperature of the process increases, these olefins (primary products) can undergo subsequent oligomerization and cyclization reactions, leading to heavier hydrocarbons.23 Likewise, hydrogen transfer reactions can occur, yielding aromatics from these compounds, which may explain the observed decrease in the olefin yield and the increase in the aromatic fraction in the condensed products obtained with (24) 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. Ind. Eng. Chem. Res. 1993, 32, 3112– 3116. (25) Takuma, K.; Uemichi, Y.; Ayame, A. Appl. Catal., A 2000, 192, 273–280. (26) Bagri, R.; Williams, P. T. J. Anal. Appl. Pyrolysis 2002, 63, 29– 41. (27) Seo, Y. H.; Lee, K. H.; Shin, D. H. J. Anal. Appl. Pyrolysis 2003, 70, 383–398.


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Figure 6. Distribution of the condensed products evolved (grouped by compound type) as a function of the carbon number for the catalytic pyrolysis of LDPE with HZSM5.

this polyethylene. However, the behavior observed in the presence of HDPE is different to that of LDPE, especially at the beginning of the catalytic cracking process. HDPE is a more linear polymer and has a lower number of chain ends than

LDPE, and consequently, a higher difficulty for the polymer to access the pores could be attributed to HDPE.5 Therefore, catalytic cracking of HDPE could start at the active sites on the external surface of the zeolite. The acid sites located on the



Figure 7. Distribution of the condensed products evolved (grouped by compound type) as a function of the carbon number for the catalytic pyrolysis of HDPE with HZSM5.

external surface are not restrictive; therefore, bimolecular reactions, such as hydrogen transfer, alkylation, or cyclization reactions are more favored than within the pores of HZSM5, where these reactions are limited sterically, leading to the

formation of paraffins and aromatics. The paraffin corresponding olefin ratio (hydrogen transfer index) reflects the tendency of an olefin (product of the monomolecular reaction) toward the formation of paraffin (product of the bimolecular reaction) by


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of these reactions decreases when the temperature increases. This decrease can be attributed to the initially HDPE cracking occurring over the acid sites present on the external surface of HZSM5, where paraffins and aromatics are generated, and subsequently continues on the active sites inside the pores, where β-scission reactions are more important and the olefin yield is increased with respect to that of paraffins. Likewise, as has been commented previously, other reactions, such as oligomerization, cyclization, and condensation, can occur. Therefore, the formation of aromatics continues when increasing the temperature, because the surface in the pores of the zeolite is accessible to the products of the HDPE degradation. 5. Conclusions

Figure 8. Evolution of the composition (weight fraction) of the condensed products obtained in the catalytic pyrolysis of LDPE and HDPE with HZSM5 grouped by compound type.

Figure 9. Paraffin/olefin ratio of the gases obtained in the catalytic pyrolysis of LDPE and HDPE with HZSM5.

hydrogen transfer reactions. Figure 9 shows the evolution of the paraffin/olefin ratio with respect to the temperature in some gaseous compounds obtained in the catalytic pyrolysis of LDPE and HDPE with HZSM5. At the beginning of the degradation process, both polyethylenes show a high ratio, although as the temperature increases in the case of LDPE, a strong decrease can be observed, whereas a more gradual trend is followed with HDPE. These results suggest that bimolecular reactions are more favored in the HDPE than LDPE case, although the extension

The composition of the products evolved in the catalytic cracking of polyethylene over the HZSM5 zeolite depends upon the temperature and the type of polyethylene used, especially at the beginning of the degradation process. In the gaseous products, the proportion of all compounds remains constant until the end of the process, when the weight fraction of the heaviest compounds (C6-C8) begins to increase, while that of the lightest (C2-C4) displays the opposite trend. With respect to compound types, yields of olefins with 3, 4, and 6 carbon atoms are very high in all of the temperature ranges for LDPE. The formation of n-paraffins and iso-paraffins is more favored at the beginning of the degradation process with HDPE than with LDPE, although these differences disappear with increasing temperature. Analysis of the condensed products generated shows that, in the presence of HZSM5, the major compounds are aromatics and olefins. At the beginning of the LDPE decomposition, the major compounds are olefins, although when the temperature increases, the formation of aromatics is favored, thus reaching a higher yield than that of olefins toward the end of the process. However, in the case of HDPE, aromatics are the most important products generated during the entire process, and in contrast to what occurs with LDPE, no dienes are obtained and a low proportion of n-paraffins is observed. The differences observed between the catalytic cracking of LDPE and HDPE could be related to the different reaction cracking mechanism followed by each polyethylene at the beginning of the process. Acknowledgment. Financial support for this investigation has been provided by the Spanish “Comisioń de Investigacioń Cientı´fica y Tecnolo´gica” de la Secretarıá de Estado de Educacioń, Universidades, Investigacioń y Desarrollo and the European Community (FEDER refunds) (CICYT CTQ2004-02187), the Generalitat Valenciana (project ACOMP/2007/094), and the University of Alicante, Grupo de Procesado y Piro´lisis de Polı´meros (VIGROB099). EF800229D

Evolution of Products Generated during the Dynamic Pyrolysis of

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