Influence of the Operating Variables on the Catalytic Conversion of a

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Influence of the Operating Variables on the Catalytic Conversion of a Polyolefin Mixture over HMCM-41 and Nanosized HZSM-5 J. Aguado,*,† D. P. Serrano,† J. L. Sotelo,‡ R. Van Grieken,† and J. M. Escola† ESCET, Rey Juan Carlos University, 28933 Mo´ stoles, Spain, and Chemical Engineering Department, Faculty of Chemistry, Complutense University of Madrid, 28040 Madrid, Spain

A mesoporous HMCM-41 type material and a nanosized HZSM-5 (n-HZSM-5) zeolite have been investigated as catalysts for the conversion of a plastic mixture consisting of polypropylene and both low- and high-density polyethylenes. The effects of the plastic/catalyst mass ratio (P/C) and the temperature have been studied using a batch reactor. Both materials present a high activity for the conversion of the polyolefin mixture, leading to almost total plastic conversion as the temperature is increased from 375 to 450 °C or as the P/C ratio is varied from 200 to 4. The product distributions obtained with these two catalysts are completely different, which is related to the prevailing cracking mechanism. n-HZSM-5 zeolite, with high external surface area and strong acid sites, promotes end-chain scission reactions of the polymers, leading to light hydrocarbons, with around 80-90% of the products in the range C3-C6. In contrast, heavier products (C5-C12 and C13-C22) are obtained over HMCM-41, indicating that random scission reactions are predominant as a result of the large pores and mild acidity of this material. Changes in the reaction temperature and/or the P/C ratio over the HMCM-41 catalyst allowed for optimization of the selectivity toward gasoline and middle distillate fractions up to values close to 90%. Introduction During the past few decades, plastic wastes have become an environmental problem in developed societies as the growth in plastic consumption has been accompanied by a progressive increase in plastic residues. The different alternatives for the management of plastic wastes currently being applied, such as landfilling and incineration, are far from being widely accepted by the population because of their related pollution problems. Likewise, mechanical recycling of plastic wastes is limited by the low quality of the recycled plastic mixture. In recent years, feedstock recycling has arisen as a promising alternative aimed at the conversion of plastic wastes into raw chemicals or fuels.1,2 Thus, catalytic cracking of polyolefins over acidic solids (zeolites, MCM-41, SiO2-Al2O3, etc.) has been proposed as an interesting solution and is the subject of research by numerous authors.3-7 Catalytic conversion has proven to be a versatile procedure because, depending on the catalyst, the reactor type, and the operating conditions, the polyolefin cracking can be addressed toward different interesting products (olefinic gases, gasolines, heavy oils, etc.).8-10 Among the different catalysts tested for the conversion of polyolefinic plastics, zeolites are one of the most extensively studied.5-7,11 However, the catalytic cracking of polyolefins over conventional zeolites is limited by strong steric hindrances, due to both the bulky nature of the polyolefins and the small size of the zeolite micropores (at best ∼0.75 nm).12 This hindrance can be avoided by using larger-pore catalysts, such as meso* To whom correspondence should be addressed. E-mail: [email protected]. Tel.: 34-91-664-74-02. Fax: 34-91-6647490. Address: c/ Tulipan s/n, 28933, Mostoles, Spain. † Rey Juan Carlos University. ‡ Complutense University of Madrid.

porous MCM-41,13 or zeolites with small crystal sizes (e100 nm)14,15 that exhibit a high proportion of external surface acid sites. Thus, a significant decrease in the crystal size of ZSM-5 zeolite has led to an enhanced activity in the cracking of both polyolefins and lube oils.15 Another alternative recently reported is the combination in series of catalysts with different pore sizes: the catalyst with the larger pores promotes the cracking of the bulky polymer chains, whereas the lower-pore-size catalyst favors the conversion and reforming of the partially cracked product. This approach was demonstrated to be successful in the catalytic cracking of polyethylene in a two-stage process over amorphous SiO2-Al2O3 and HZSM-5 zeolite catalysts, leading to an improved yield and octane number for the gasoline thus obtained.16 The great majority of the previous works study the cracking of pure polymers, whereas those dealing with plastic mixtures are scarce, even though mixtures represents the most realistic situation in the management of plastic wastes. Thus, we earlier investigated the catalytic cracking of a standard polyolefin mixture made up of 25% high-density polyethylene, 46.5% lowdensity polyethylene, and 28.5% polypropylene over different acid solids (zeolites, silica-alumina, and HMCM-41).17 These polyolefin proportions are similar to those found throughout the plastic waste stream in Western Europe. The segregated nature of this polyolefin mixture was evidenced by the presence of a polypropylene phase that did not mix with the polyethylene regions. Additionally, the low activity obtained in the cracking of the mixture over a conventional HZSM-5 zeolite (3 µm crystal size) suggested that polypropylene plays a controlling role in the catalytic degradation of the plastic mixture. This polymer can be only cracked over the acid sites located on the external zeolite surface, as its cross diameter is too large to allow it to

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enter into the zeolite micropores.12 Likewise, some steric hindrance can also arise in LDPE cracking because of the presence of branching in the chains of this polymer compared to the more linear chains of HDPE. However, the synthesis and use of HZSM-5 with nanometer crystal sizes allowed these limitations to be overcome through the presence of a high proportion of acid sites on the external zeolite surface.17 In this work, the highest activities for the conversion of a polyolefin mixture were obtained over HBeta and nanosized HZSM-5 zeolites, as well as mesoporous HMCM-41. The good performance of these two zeolites was ascribed to both their small crystal sizes and their high acid strengths, whereas in the case of the HMCM-41, it was related to both its high surface area and the presence of uniform mesopores with 2.4 nm diameters. The present work reports the influence of the main operating variables (plastic/catalyst mass ratio and temperature) on the activity and product distribution obtained during the cracking of a polyolefin mixture over the nanosized HZSM-5 and HMCM-41 catalysts. These catalysts were selected because of their high activities and their selectivities toward completely different products (gaseous and liquid fractions, respectively). Experimental Section Materials. The polyolefins used were low-density polyethylene (LDPE, Mw ) 416 000), high-density polyethylene (HDPE, Mw ) 188 000), and polypropylene (PP, Mw ) 450 000, isotacticity index ) 93%). These polymers were provided by Repsol. A standard mixture of polyolefins consisting of 25% HDPE, 46.5% LDPE, and 28.5% PP was prepared by homogeneous mixing of previously ground polymer particles (with sizes in the range 0.5-1 mm). Catalyst Preparation. The catalysts used were an HMCM-41 type material and a nanosized HZSM-5 zeolite (n-HZSM-5). They were synthesized in our laboratory according to procedures described elsewhere.13,18 The as-synthesized catalysts were calcined in static air at 550 °C for 12 h, and were obtained directly in their acid forms after calcination. The catalysts were ground and sieved to obtain a particle size lower than 0.074 mm; as in previous experiments, it was concluded that larger particles lead to reduced cracking activities because of diffusional limitations. Catalyst Characterization. The structural parameters and symmetry of both the n-HZSM-5 zeolite and HMCM-41 samples were determined by X-ray diffraction (XRD) using a Philips X’PERT MPD diffractometer with Cu KR radiation. The morphology and size of the catalyst crystals and particles were determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM micrographs were taken with a JEOL JSM-6400 instrument working at 35 kV. The SEM samples were prepared by dispersion in acetone; stirring in an ultrasonic bath; and deposition on a brass holder, being covered with a gold layer. The TEM micrographs were taken with a JEOL JEM 2000 FX microscope working at 200 kV. The TEM samples were prepared by dispersion in acetone, stirring in an ultrasonic bath, and deposition on a coal grid. The textural properties of the two catalysts were obtained from nitrogen adsorption-desorption isotherms at 77 K. The isotherms were measured using a Micromeritics ASAP 2010 apparatus with samples that

had been previously outgassed at 200 °C under vacuum for 5 h. Surface areas were obtained through application of the BET equation. The pore size of the HMCM-41 sample was determined through application of the BJH method to the adsorption branch of the isotherm, assuming a cylindrical pore shape and using the JuraHarkins equation to obtain the thickness of the adsorbed layer. The external surface area was calculated through application of the t-plot method to a selected zone of the adsorption branch of the isotherm. The Si/Al atomic ratios of the two catalysts were determined from X-ray fluorescence (XRF) measurements with a Phillips PW 1404 spectrometer. The acid properties were tested by ammonia temperatureprogrammed desorption (TPD) using a Micromeritics 2900 TPD apparatus. The samples were previously outgassed by heating from ambient temperature to 560 °C under a He flow of 50 NmL/min at a rate of 15 °C/ min, with the final temperature being kept constant for 30 min. Subsequently, the samples were cooled to 180 °C and treated with an ammonia flow of 30 NmL/min for 30 min. The physisorbed ammonia was eliminated by flowing He at 180 °C for 90 min, whereas the chemically bonded ammonia was determined by recording its desorption upon increasing the temperature to 550 °C at a heating rate of 15 °C/min and holding this temperature constant for 30 min. During these treatments, the ammonia evolved in the effluent He stream was measured continuously with a thermal conductivity detector. Cracking Experiments. The cracking reactions were carried out in a batch reactor made up of a Pyrex tube with a 26 mm width and a 65 mm height. In a typical experiment, 1.6 g of the polyolefin mixture and the corresponding amount of catalyst are loaded into the reactor and heated to obtain a macroscopically homogeneous mixture. The whole reaction system is swept by a continuous flow of nitrogen (25 NmL/min). The reactor is placed inside an electric oven and heated to the reaction temperature at a rate of 25 °C/min. The temperature is controlled to within (1 °C by a thermocouple in direct contact with the reaction mixture. The products leaving the reactor with the nitrogen stream are passed through a condenser cooled by an ice/water mixture, which allows the liquid products to be separated, accumulated, and weighed, whereas the gaseous products are finally collected in a gas bag. The conversion of the plastic mixture is calculated as the sum of the liquid and gaseous products obtained during the reaction time (0.5 h), while the product remaining inside the reactor, consisting of partially cracked polymers, is regarded as residue. Product Analyses. The compositions of both the gaseous and liquid products were determined by GC analysis. The gaseous products were analyzed in a Hewlett-Packard 5880 A GC equipped with a TCD detector, using a 6 m length 1/8 in. diameter stainless steel column with Porapak Q as the stationary phase. The liquid products were analyzed in a Perkin-Elmer 8310 GC equipped with a FID detector using a 25 m BP-5 capillary column. Results and Discussion Catalyst Properties. The main physicochemical properties of the two catalysts investigated in this work are summarized in Table 1. HMCM-41 is a mesoporous material with a pore size of 2.4 nm and high surface

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Table 1. Physicochemical Properties of the Catalysts

catalyst

BET external pore surface surface particle size area area size aciditya Tmaxa (nm) (m2 g-1) (m2 g-1) (nm) Si/Al (mmol g-1) (°C)

n-HZSM-5 0.55 HMCM-41 2.4 a

430 1164

81 73

60 100

23.9 45.2

0.380 0.215

462 338

From ammonia TPD data.

Figure 1. Conversions obtained in the catalytic cracking of the polyolefin mixture over HMCM-41 and n-HZSM-5 using different P/C ratios (T ) 400 °C, t ) 30 min).

area. On the other hand, zeolite HZSM-5 is a microporous solid with pore diameters of around 0.55 nm. In contrast with the micrometer size of the crystals in conventional HZSM-5, the sample used in this work presents very small crystals with an average size of around 60 nm and is thus named n-HZSM-5 (nanometer-size HZSM-5). This fact provides this material with remarkable properties for the conversion of bulky molecules, mainly the presence of a high proportion of external surface, which accounts for approximately 20% of the total zeolite surface area. Regarding the acid properties, the higher acidity of the n-HZSM-5 sample, measured by ammonia TPD, compared to the HMCM41 material agrees well with its higher Al content. The maximum of ammonia desorption is registered at a higher temperature for the zeolite as a consequence of the higher strength of the acid sites in this material compared to the HMCM-41 catalyst. Effect of the Plastic/Catalyst Ratio on the Catalytic Activity. To study the effect of the plastic/catalyst ratio on the cracking of the polyolefin mixture, different experiments were carried out at 400 °C with the two catalysts varying this parameter in the range P/C ) 4-200. The conversions thus obtained are depicted and compared in Figure 1. As expected, for both catalysts, the plastic conversion increases with the catalyst loading in the reaction mixture. It can be observed that n-HZSM-5 is always more active than HMCM-41, except for the experiments with P/C ) 4, which result in almost complete conversion over both materials. The differences in activity between the two catalysts are especially remarkable at high P/C ratios. Thus, for P/C ) 200, the catalytic cracking over HMCM-41 is practically negli-

gible, leading to a conversion of around 3%, which is very close to the result obtained in a reference blank reaction of thermal cracking (2.4%). Instead, n-HZSM-5 shows a conversion of roughly 18% for the same P/C ratio, despite the extremely low amount of catalyst used. Likewise, over n-HZSM-5, complete conversion of the polyolefin mixture is reached for P/C ) 10, whereas it occurs over HMCM-41 solely for P/C ) 4. These data indicate seemingly that n-HZSM-5 is always more active than HMCM-41 as a catalyst. However, these results might be misleading because the aluminum contents of the two catalysts are very different. Catalytic cracking reactions over aluminosilicates proceeds through carbocationic mechanisms catalyzed by acid sites directly related to the aluminum atoms. Likewise, these acid sites can be Bro¨nsted or Lewis in nature, which present different catalytic behaviors. As shown in Table 1, the aluminum content of the nHZSM-5 catalyst is almost twice that of HMCM-41, which must clearly contribute to its superior activity. However, the results in Figure 1 for P/C ) 100 and 200 indicate that the activity exhibited by the zeolite is more than twice the activity found for HMCM-41. Moreover, it can be concluded that this difference in intrinsic activity is even more pronounced, because, in the case of the zeolite, only a fraction of the acid sites, those located on the external surface of the crystals, must be really active in the initial steps of the polyolefin cracking. In this way, as concluded in previous works, the access of the bulky polyolefin molecules, especially polypropylene, to the HZSM-5 internal acid sites is hindered by the small size of the zeolite micropores (0.55 nm). In addition to the Al content, the higher activity of n-HZSM-5 compared to HMCM-41 can be related to the higher strength of the zeolite acid sites compared to those of the mesoporous material, as denoted by the ammonia TPD measurements. Another factor that might also influence the catalytic activity is the fact that most of the acidity in the HZSM-5 zeolite originates in Bro¨nsted acid sites, whereas both Bro¨nsted and Lewis acid sites have been identified in HMCM-41.19 Nevertheless, it must be pointed out that, despite its lower conversion compared to n-HZSM-5, the activity shown by the HMCM-41 catalyst for the cracking of the polyolefin mixture is also remarkable, mainly when compared with the behavior previously observed for other catalytic systems, such as amorphous silicaalumina and different types of zeolites, under the same reaction conditions (400 °C, 0.5 h).12,17 Effect of the Plastic/Catalyst Ratio on the Product Distribution. To investigate the effect of the P/C ratio on the selectivity and product distribution, the different compounds obtained by cracking of the polyolefin mixture were grouped into the following fractions according to their carbon atom numbers: C1-C4 paraffins, C2-C4 olefins, C5-C12 (gasoline), and C13-C22 and C23-C40 (gasoil fractions). In addition, the proportion of C6-C9 aromatics present in the gasoline range is also taken into account in this discussion. Figure 2A shows the product distribution by groups obtained over the n-HZSM-5 catalyst for varying P/C ratios. It can be observed that the main products are hydrocarbons within the gasoline range, with a selectivity that varies randomly around a mean value of 50% as the P/C ratio is changed in the range of 4-200. However, although the overall gasoline selectivity remains almost constant, significant changes are observed


Figure 2. Product distributions obtained in the catalytic cracking of the polyolefin mixture over n-HZSM-5 using different P/C ratios (T ) 400 °C, t ) 30 min): (A) selectivity by groups, (B) selectivity by carbon atom number. Table 2. Selectivity toward Aromatic Hydrocarbons in the Cracking of the Polyolefin Mixture over n-HZSM-5 at Different P/C Ratiosa selectivity (%) P/C ratio trimethyl(w/w) benzene toluene ethylbenzene xylenes benzenes 200 100 50 10 4 a

0.33 0.60 0.81 0.98 1.03

0.41 0.34 0.87 3.46 5.32

0.25 0.28 0.54 1.28 1.74

0.91 0.99 3.39 6.90 9.19

0.64 0.44 0.23 0.81 2.28

T ) 400 °C, t ) 30 min.

in the content of aromatic hydrocarbons in this fraction. The aromatic hydrocarbon selectivity increases gradually with the amount of catalyst present in the reaction mixture until reaching a value of 23% for P/C ) 4, which accounts for almost one-half of the gasoline produced in this experiment. The selectivities corresponding to the different aromatics present in the gasoline fraction are shown in Table 2. The major aromatic compounds are toluene and xylenes, which reach selectivities of 5.32 and 9.19%, respectively, for P/C ) 4, with benzene, ethylbenzene, and trimethylbenzene also being detected in significant amounts. It is also interesting to note that the evolutions of gaseous paraffins and gaseous olefins show opposite trends. The selectivity toward C1-C4 paraffins increases continuously as the P/C ratio is decreased, reaching a maximum of 27% for P/C ) 4. Instead, the selectivity toward gaseous olefins shows a decreasing trend with the P/C ratio. The maximum production of gaseous olefins (34% selectivity) is obtained for P/C ) 100. Finally, the selectivities toward the gasoil fractions are very low, with values that are always below 5%, suggesting that a very intense cracking process has occurred over the n-HZSM-5 zeolite. Figure 2B illustrates the selectivity by carbon atom number obtained in the conversion of the polyolefinic

mixture over n-HZSM-5 for two different P/C ratios (100 and 10). These distributions indicate that, over this zeolite, the polyolefin cracking leads mainly to light products. Thus, the maximum of the distribution is placed at C4, with a selectivity for this fraction varying in the range of 25-31% for different P/C ratios. Significant amounts of C3, C5, and C6 hydrocarbons are also formed, the latter being the main components of the gasoline fraction (C5-C12). From these results, it can be concluded that the predominant cracking pathway with this catalyst is based on end-chain scission reactions. This fact is probably related to the strong acidity of the n-HZSM-5 zeolite, although it might be also due to a preferential contact of the zeolite external surface with the end of the polymer chains. Likewise, for the experiment carried out with a higher amount of catalyst (P/C ) 10), a second relative maximum is observed at C8, with a selectivity of around 8%. This second peak was previously observed in the cracking of pure polyolefins over different acid catalysts and was assigned to oligomerization reactions starting from the primary cracking products (C3-C5 hydrocarbons). Likewise, it must be pointed out that, regardless of the P/C ratio, the product distribution is quite narrow, with very low selectivities toward compounds heavier than C15. Moreover, a reduced number of hydrocarbon fractions accounts for most of the product distribution. Thus, in the experiment carried out with P/C ) 100, the overall selectivity toward products in the range of C3-C6 is 86%. This result is highly remarkable because, in most of the previous literature results on the catalytic conversion of polyolefins, the product formed is a complex mixture of hydrocarbons, which limits it to being used just as fuels and not as valuable raw chemicals. The narrow product distribution obtained over n-HZSM-5, as well as the presence of a high proportion of olefins at high P/C ratios and of aromatics at low P/C ratios, opens the possibility of using this

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Figure 3. Product distributions obtained in the catalytic cracking of the polyolefin mixture over HMCM-41 using different P/C ratios (T ) 400 °C, t ) 30 min): (A) selectivity by groups, (B) selectivity by carbon atom number.

product as a source of valuable raw chemicals for the petrochemical industry. The enhancement observed in the gaseous paraffin selectivity and the corresponding abatement in gaseous olefins upon decreasing the P/C ratios can be related to the increase in the production of C6-C9 aromatics. At high P/C ratios, the predominant reaction that takes place is the cracking of the polyolefin molecules, mainly on the external surface of the zeolite. At lower P/C ratios, the higher catalyst concentration present in the reaction mixture promotes the development of secondary reactions from the primary cracking products, such as the oligomerization and aromatization of gaseous olefins.20 The formation of aromatics from olefins, mainly C3 and C4 fractions, involves several sequential steps: olefin oligomerization, cyclization, and dehydrogenation by hydrogen transfer. The H/C atomic ratio of the original raw plastic mixture is close to 2, whereas an aromatic compound always has a significantly lower H/C ratio. The hydrogen atoms released in the aromatization reactions are transferred and used in the saturation of gaseous olefins, which is in agreement with the observed increase in the selectivity toward gaseous paraffins at low P/C ratios. This mechanism also explains the aromatics distribution observed when the catalyst concentration is increased, with toluene and xylenes as the main components. Toluene (C7) can be formed by oligomerization of C3 and C4 olefins, whereas xylenes (C8) are the products expected from the oligomerization-cyclization-dehydrogention of two C4 olefins. In regards to the product distribution obtained over the HMCM-41 catalyst, Figure 3A shows the selectivity by groups versus the P/C ratio. The results corresponding to P/C ) 200 are very different from those obtained for the rest of the P/C ratios, with a high amount of gases and no production of gasoil fractions. This is probably due to the low conversion obtained in this experiment, which is practically identical to that ob-

tained in thermal cracking. However, for P/C ) 100, heavier products (C13-C22 gasoils) are obtained, whereas the selectivities toward gaseous paraffins and olefins drop to 6 and 14%, respectively. Further decreases in the P/C ratio cause slight variations in the gaseous fractions, with a trend toward increasing in the case of paraffins and a decreasing for olefins. These changes, although much less accentuated, are qualitatively similar to those observed with the n-HZSM-5 zeolite. Likewise, the major product obtained over the HMCM-41 catalyst is the gasoline fraction, regardless of P/C ratio. In this case, the gasoline selectivity increases with the catalyst concentration from roughly 50% (P/C ) 100) to 64% (P/C ) 10). The aromatic content of the gasoline is very low compared to that produced over n-HZSM-5, remaining almost constant within the range 4-6% for varying values of the P/C ratio. Another remarkable difference between the two catalysts is the high proportion of the C13-C22 gasoil fraction obtained over the HMCM-41 material, which evolves from around 28% (P/C ) 100) to 18% (P/C ) 4). Figure 3B compares the selectivity by carbon atom number obtained in the catalytic cracking of the polyolefin mixture over HMCM-41 for P/C ratios of 100 and 10. In both experiments, the selectivity maximum is observed for the C4 fraction (13-15%), although with a value very close to the maximum of the C5 fraction and significantly smaller than the maximum obtained over the n-HZSM-5 zeolite (around 30%). A second relative maximum, located at C8 and C9, is also appreciated in the distribution shown in Figure 3B for P/C ratios of 10 and 100, respectively. Another interesting point is that, in the gasoline and gasoil regions, a wide product distribution is observed, with significant selectivities even for products as heavy as the C25 fraction. Increasing the HMCM-41 loading causes a shift of the product distribution toward light hydrocarbons, as the cracking of the heaviest fractions is favored. Therefore, HMCM41 can be considered a suitable catalyst for the conver-


sion of polyolefins into gasoline and middle distillate fractions with low aromatic contents. The product distribution and selectivity observed over HMCM-41 can be explained on the basis of its medium acid strength and its uniform mesoporous structure. In this case, end-chain cracking reactions take place to a much lesser extent than for the n-HZSM-5 zeolite. The medium acid strength of HMCM-41 leads mainly to the formation of heavy products by random scission along the polymer chains, whereas the large diameter of its pores (2.4 nm) allows these primary cracking products to leave the catalyst without undergoing excessive secondary cracking reactions.21 Nevertheless, with increasing catalyst concentration, some overcracking is surmised to occur, which would explain the observed enhancement in the gasoline selectivity and the decrease in the gasoil proportion. Finally, the lower acid strength of HMCM-41 compared to n-HZSM-5 zeolite is probably the reason for the lower aromatization observed on the former. If the results obtained in the cracking of the plastic mixture are compared with the conversions of individual polymers over HMCM-41 and HZSM-5 catalysts, which were reported in earlier works,12,15 the same general trends are observed: the production of light hydrocarbons with the zeolite catalyst and the formation of heavier products with the HMCM-41 material. The main difference is related to the use of nanosized HZSM-5 instead of a conventional HZSM-5 sample. In the last case, a controlling effect of the polypropylene molecules has been observed during the cracking of the polymer mixture, leading to conversions much lower than those expected from both the cracking of the individual polymers and the plastic mixture composition. The results obtained in the present work confirm that this controlling effect does not exist when nHZSM-5 is used as the catalyst. The presence of a high number of external acid sites in this material allows the PP molecules to be degraded at a rate comparable to the degradation rates of LDPE and HDPE, i.e., the plastic mixture is converted over n-HZSM-5 with activities similar to those corresponding to the individual polymers. Effect of the Temperature on the Catalytic Activity. The influence of the reaction temperature has been investigated with both catalysts by changing this variable in the range of 375-450 °C for a P/C ratio of 100. Figure 4 illustrates the variation of the polyolefin conversion with increasing reaction temperature. These results confirm the higher activity of the n-HZSM-5 zeolite, as it leads to higher conversions at all temperatures. Total conversion of the plastic mixture is obtained over the zeolite at 450 °C despite the low catalyst concentration used in these experiments. An interesting parameter derived from these results is the threshold temperature at which these catalysts start to exhibit significant catalytic activity for the cracking of the polyolefin mixture. In the case of HMCM-41, the threshold temperature is around 400 °C, whereas for the n-HZSM-5 zeolite, a conversion higher than 20% is obtained at temperatures as low as 375 °C. It must be noted that TGA experiments on the pure polyolefins indicate the following threshold temperatures for the thermal degradations of the pure polyolefins: 395 °C (PP), 404 °C (LDPE), and 412 °C (HDPE).1 The coincidence between the threshold temperatures corresponding to thermal degradation and HMCM-41

Figure 4. Conversions obtained in the catalytic cracking of the polyolefin mixture over n-HZSM-5 and HMCM-41 at different temperatures (P/C ) 100, t ) 30 min).

catalytic conversion suggests that, with this catalyst, the contribution of thermal cleavage cannot be discarded during the first scission reactions of the polymer chains. In contrast, the difference between the threshold temperatures corresponding to thermal treatment and polyolefin cracking over n-HZSM-5 indicates the participation of the zeolite even in the first cracking steps. Effect of the Temperature on the Product Distribution. The product distribution by groups obtained over the n-HZSM-5 zeolite with increasing temperature is shown in Figure 5A; a slight trend toward the formation of heavier products can be observed. The overall selectivity toward gases (C1-C4 paraffins and C2-C4 olefins) drops from 52% at 375 °C to 44% at 450 °C. Likewise, the selectivity toward gasoline remains constant at around 50%, while the formation of gasoil fractions is favored at high temperatures (6% at 450 °C). The proportion of aromatic hydrocarbons is also enhanced with temperature, with a 2-fold increase from 400 °C (2.7%) to 425 °C (5.4%). The selectivity by carbon atom number over this catalyst is shown in Figure 5B for the experiments carried out at 400 and 450 °C, indicating that a reduction in the C4 and C5 fractions takes place as the temperature is increased. Thus, selectivity toward C4 decreases from 32% (400 °C) to 24% (450 °C), whereas the corresponding to C5 selectivity drops from 25% to roughly 16%. However, the C3 selectivity remains almost constant (16-18%), regardless of the temperature. The selectivity by groups obtained in the catalytic cracking of the polyolefin mixture at different temperatures over HMCM-41 is shown in Figure 6A. These results exhibit also a trend toward heavier products at increasing reaction temperature, as was previously observed over n-HZSM-5. In the case of the experiment at 375 °C, the product distribution is very similar to that obtained in thermal cracking under the same conditions, which is probably due to the low catalytic activity of HMCM-41 at that temperature. The selectivities toward gaseous olefins and paraffins decrease with increasing temperature. Likewise, the proportion of gasoline undergoes a slight decline with the temperature, whereas

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Figure 5. Product distributions obtained in the catalytic cracking of the polyolefin mixture over n-HZSM-5 at different temperatures (P/C ) 100, t ) 30 min): (A) selectivity by groups, (B) selectivity by carbon atom number.

Figure 6. Product distributions obtained in the catalytic cracking of the polyolefin mixture over HMCM-41 at different temperatures (P/C ) 100, t ) 30 min): (A) selectivity by groups, (B) selectivity by carbon atom number.

its content in aromatic hydrocarbons grows to 8% at 450 °C. It is interesting to note that the selectivity toward gasoil fractions is enhanced with the temperature, reaching an overall value slightly higher than 40% at 450 °C. The selectivities by carbon atom number obtained over HMCM-41 for two temperatures (400 and 450 °C) using P/C ) 100 are shown in Figure 6B. A wide distribution, with a maximum at C4-C5, is observed in

the experiment carried out at 400 °C. However, at 450 °C, a significant reduction in those products takes place, with the maximum selectivity being shifted to hydrocarbons in the C8-C9 fractions. These results suggest that an increase in temperature favors the oligomerization of C4 and C5 olefins, leading to a wider product distribution. The evolution of the product selectivities toward the formation of heavier hydrocarbons observed over both


Figure 7. Reaction pathways in the conversion of the polyolefin mixture.

catalysts with increasing the temperature can be explained by considering several simultaneous phenomena. First, higher temperatures of the reaction medium allow heavier products to vaporize and escape from the reactor with the effluent gaseous stream, i.e., higher temperatures involve shorter residence times of the heavy hydrocarbon fractions. Second, the contribution of thermal cracking reactions, having activation energies higher than that of catalytic cracking, cannot be regarded as negligible at high temperature. Thermal cracking of polyolefins is known to proceed through a free-radical chain mechanism, leading to a wide product distribution that includes considerable amounts of heavy hydrocarbons. Finally, the increase in temperature might modify the preferred pathway of the catalytic conversion. Thus, for the n-HZSM-5 zeolite, a decrease of the selectivity toward C4 and C5 hydrocarbons is observed on going from 400 to 450 °C, whereas the proportion of C3 remains almost constant. This behavior might be related to a greater extent of direct scission reactions at the ends of the polymer chains, rather than scission with previous rearrangement leading to branched C4 and C5 hydrocarbons. Likewise, over the HMCM-41 catalyst, an increase in temperature might promote the oligomerization of C4 and C5 olefins, which also causes a significant change in the product distribution. Main Reaction Pathways. On the basis of the results discussed earlier, the reaction scheme shown in Figure 7 is proposed to explain the catalytic conversion of the polyolefin mixture over the n-HZSM-5 and HMCM-41 catalysts. In the case of the n-HZSM-5 zeolite, the cracking reactions take place on the external surface of the crystals, yielding mainly light olefins as the primary products coming from an end-chain scission mechanism. Thus, the selectivities toward C3-C5 olefins obtained over n-HZM-5 are very high, typically in the range of 50-60 wt %. These olefins undergo subsequent oligomerization and cyclization reactions, leading to heavier aliphatic hydrocarbons. Likewise, hydrogen-transfer reactions from the aliphatic products to gaseous olefins lead to aromatic hydrocarbons and favor the transformation of olefins into paraffins. On the other hand, polyolefin cracking over the HMCM-41 catalyst occurs mainly within its mesopores, yielding waxes as the primary products, although a contribution from thermal degradation cannot be discarded, at least in the initial cracking steps. The mild acidity and large pore size of this catalyst promotes a random scission mechanism that is responsible for the formation of heavy products, such as the waxes that remain in the reactor. Further cracking of these waxes yields gasoil and gasoline fractions.

Accordingly, each type of catalyst leads to a different prevailing polyolefin cracking mechanism: end-chain scission over n-HZSM-5 because of its high acid strength and random scission over HMCM-41 as a consequence of its mesoporosity and weaker acidity. Nevertheless, a certain contribution of the alternative cracking pathway should not be completely neglected for each catalyst. Thus, random scission reactions can occur over the n-HZSM-5 zeolite, mainly on the external acid sites, whereas end-chain cracking reactions are also present to some extent over the HMCM-41 material. Conclusions The results obtained in the present research demonstrate that both n-HZSM-5 and HMCM-41 show promising properties for use as catalysts in the catalytic cracking of polyolefin mixtures (HDPE, LDPE, and PP). The presence of a high proportion of external acid sites in n-HZSM-5 and of both high surface area and uniform mesopores in HMCM-41 are the reasons explaining their remarkable catalytic activities for the conversion of bulky polymeric molecules. The n-HZSM-5 zeolite exhibits a cracking activity superior to that of HMCM-41, which cannot be ascribed only to its higher aluminum content; rather, the existence of strong acid sites in the zeolite must also be considered. The n-HZSM-5 zeolite presents significant catalytic activity for the conversion of the polyolefin mixture even at temperatures below the threshold temperature corresponding to polymer thermal degradation. The zeolite catalyst is involved in the first steps of the polyolefin cracking, whereas a contribution of thermal scission reactions cannot be discarded in the initial steps of the cracking over HMCM-41. Different cracking pathways predominate over the two catalysts. End-chain scission reactions take place over the n-HZSM-5 zeolite, probably because of its high acid strength. As a consequence, light hydrocarbons with a narrow product distribution are the main components obtained over this catalyst, whereas the production of heavy fractions, such as gasoil, is negligible. Increasing the catalyst concentration in the reaction medium leads to the formation of a high proportion of aromatics, mainly toluene and xylenes, through oligomerization, cyclization, and hydrogen-transfer reactions. On the other hand, both end-chain and random scission reactions occur over the HMCM-41 catalyst. Random scissions at any point along the polymer chains lead to a wide product distribution with a high proportion of heavy hydrocarbons. The product distributions obtained with these catalysts can be modified and optimized by changing the

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plastic/catalyst ratio and/or the temperature. Thus, operating at high P/C ratios over n-HZSM-5 zeolite leads to C3-C6 hydrocarbons with a selectivity of around 85%, which can be very valuable as raw chemicals for the petrochemical industry when their high olefin contents are taken into account. In the case of HMCM-41, the production of the gasoil fractions is favored by increasing temperature, with an overall selectivity toward gasoline and gasoil fractions of 90% at 450 °C. Acknowledgment We acknowledge the Comisioń Interministerial de Ciencia y Tecnologıá of Spain (Project CICYT AMB-97/ 0530) and the Comunidad de Madrid (Strategic Group Project) for the financial support of this research. We are also grateful to REPSOL S. A. for supplying the polyolefin samples used in this work and to CEPSA Research Centre for the XRF analyses of the catalysts. Literature Cited (1) Aguado, J.; Serrano, D. P. Feedstock Recycling of Plastic Wastes; Royal Society of Chemistry: Cambridge, U.K. 1999. (2) Kaminsky, W.; Hartmann, F. New Pathways in Plastic Recycling. Angew. Chem., Int. Ed. Engl., 2000, 39 (2), 331. (3) Ohkita, H.; Nishiyama, R.; Tochihara, Y.; Mizushima, T.; Kakuta, N.; Morioka, Y.; Ueno, A.; Namiki, Y.; Tamifuji, 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) Aguado, J.; Serrano, D. P.; Romero, M. D.; Escola, J. M. Catalytic Conversion of Polyethylene into Fuels over Mesoporous MCM-41. Chem. Commun. 1996, 725. (5) Mordi, R. C.; Fields, R.; Dwyer, J. Gasoline Range Chemicals from Zeolite-Catalysed Thermal Degradation of Polypropylene. J. Chem. Soc., Chem. Commun. 1992, 374. (6) Lin, R.; White, R. L. Effects of Catalyst Acidity and HZSM-5 Channel Volume on the Catalytic Cracking of Polyethylene. J. Appl. Polym. Sci. 1995, 58, 1151. (7) Manos, G.; Garforth, A.; Dwyer, J. Catalytic Degradation of High-Density Polyethylene over Different Zeolitic Structures. Ind. Eng. Chem. Res. 2000, 39, 1198. (8) Songip, A. R.; Masuda, T.; Kuwahara, H.; Hashimoto, K. Test to Screen Catalysts for Reforming Heavy Oil from Wastes Plastics. Appl. Catal. B2 1993, 153. (9) Audisio, G.; Bertini, F.; Beltrame, P. G.; Carniti, P. Catalytic Degradation of Polyolefins. Makromol. Chem., Macromol. Symp. 1992, 57, 191.

(10) Sakata, Y.; Uddin, M. A.; Muto, A.; Kanada, Y.; Koizumi, K.; Murata, K. Catalytic Degradation of Polyethylene into Fuel Oil over Mesoporous Silica. J. Anal. Pyrol. 1997, 43, 15. (11) Aguado, J.; Serrano, D. P., Escola, J. M., Garagorri, E., Fernańdez, J. A. Catalytic Conversion of Polyolefins into Fuels over Zeolite Beta. Polym. Degrad. Stab. 2000, 69, 11. (12) 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. (13) Serrano, D. P.; Aguado, J.; Sotelo, J. L.; Van Grieken, R.; Escola, J. M.; Meneńdez, J. M. Catalytic Properties of MCM-41 for the Feedstock Recycling of Plastic and Lubricating Oil Wastes. Stud. Surf. Sci. Catal. 1998, 117, 437. (14) 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. (15) Sotelo, J. L.; Van Grieken, R.; Aguado, J.; Serrano, D. P.; Escola, J. M.; Meneńdez, J. M. Effect of the ZSM-5 Crystal Size on the Cracking Activity of Lube Oils and Polyolefins. In Proceedings of 12th International Zeolite Conference; Treacy, M. M. J., Markus, B. K., Bisher, M. E., Higgins, J. B., Eds.; Materials Research Society: Warrendale, PA, 1999; p 1441. (16) Uemichi, Y.; Nakamura, J.; Itoh, T.; Sugioka, M.; Garforth, A. A.; Dwyer, J. Conversion of Polyethylene into Gasoline Range Fuels by Two-Stage Catalytic Degradation Using Silica-Alumina and HZSM-5 Zeolite. Ind. Eng. Chem. Res. 1999, 38, 385. (17) Aguado, J.; Serrano, D. P.; Escola, J. M. Catalytic Cracking of a Polyolefin Mixture over MCM-41 and Several Acid Catalysts. Ind. Eng. Chem. Res. 2000, 39, 1177. (18) Van Grieken, R.; Sotelo, J. L.; Meneńdez, J. M.; Melero, J. A. Anomalous Crystallization Mechanism in the Synthesis of Nanocrystalline ZSM-5. Microporous Mesoporous Mater. 2000, 39, 135. (19) Corma, A.; Forne´s, V.; Navarro, M. T.; Pe´rez-Pariente, J. Acidity and Stability of MCM-41 Crystalline Aluminosilicates. J. Catal. 1994, 148, 569. (20) Takuma, K.; Uemichi, Y.; Ayame, A. Product Distribution from Catalytic Degradation of Polyethylene over H-Gallosilicate. Appl. Catal. A 2000, 192, 273. (21) Serrano, D. P.; Van Grieken, R.; Aguado, J.; Garcıá, R., Rojo, C.; Temprano, F. Study on the Initial Steps of the Polyethylene Cracking over Different Acid Catalysts. Stud. Surf. Sci. Catal. 2000, 130, 1589.

Received for review May 10, 2001 Revised manuscript received August 22, 2001 Accepted August 31, 2001 IE010420C

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