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Catalytic Cracking of a Polyolefin Mixture over Different Acid Solid Catalysts David P. Serrano,*,† Jose´ Aguado,† and Jose´ M. Escola Department of Chemical Engineering, Faculty of Chemistry, Complutense University of Madrid, Madrid 28040, Spain
Catalytic cracking of a polyolefin mixture consisting of polypropylene and both low- and highdensity polyethylene has been studied at 400 °C over a variety of acid solids as catalysts. The highest activities were obtained over HMCM-41, n-HZSM-5 zeolite, with nanometer crystal size, and HBeta zeolite. The high surface area and large pores present in HMCM-41 are responsible for the high conversions obtained with this catalyst. Likewise, in the case of n-HZSM-5, the presence of a high external surface area enhances its cracking activity, because the zeolite external acid sites are not sterically hindered for the conversion of the bulky polyolefin molecules. Significant differences are observed in the product distribution: n-HZSM-5 shows the highest selectivity toward C1-C4 gaseous hydrocarbons (50 wt %), HBeta leads mainly to liquid hydrocarbons in the range C5-C12 (60 wt %), whereas HMCM-41 yields both C5-C12 (54 wt %) and C13-C 30 (32 wt %) fractions. A certain loss of activity of these catalysts has been observed after one cycle of regeneration. For HMCM-41, this phenomenon is caused by both dealumination and particle aggregation that take place during the regeneration treatment. Introduction The growth of welfare levels in modern societies during the past decades has brought about a huge increase in the production of all kinds of commodities. Plastics have been one of the materials with the fastest growth because of their wide range of applications: household, agriculture, construction, packaging, etc. The whole output of plastics in Western Europe in 1996 was estimated around 25.9 Mtons, whereas the amount of plastic wastes collected in the same year was close to 16.9 Mtons.1 The preferred alternatives for the management of these wastes in Western Europe were deposition in landfills (75%) and incineration with energy recovery (14%), although these percentages change significantly from one country to another.1 Incineration is strongly questioned in many countries because of the possible emissions of different pollutants (dioxins, furans, PCBs, etc.), which are toxic at very low concentrations. On the other hand, the space available for landfills is decreasing progressively, whereas the deposition of plastic wastes there implies an undesired loss of raw materials due to the progressive depletion of natural resources.2,3 Different alternatives are currently being considered for reducing the environmental impact of plastic wastes. Plastic items start to be manufactured by thinking of their necessary future recycling. Thus, the easy identification of their components starts to be favored through ecolabels.4 Recycling of plastic wastes may proceed through two main alternatives: mechanical and feedstock recycling. Mechanical recycling is aimed at the reintroduction of the plastic materials in the consumption cycle, although usually in secondary applications because goods made of recycled plastics are of lower quality. On the contrary, feedstock recycling involves * To whom correspondence should be addressed. E-mail:
[email protected]. † Current address: Rey Juan Carlos University, c/Tulipa ´n s/n, Mo´stoles 28933, Spain.
the transformation of the plastic wastes into valuable chemicals or fuels, which exhibit properties similar to those of their counterparts prepared by conventional methods.5 Feedstock recycling of polyolefins into fuels by catalytic cracking has been studied by numerous authors.6-8 The different types of polyolefins account roughly for 70% of the total plastic wastes. The catalysts used in those works have been mainly acid solids (zeolites, alumina, amorphous silica-alumina, etc.), although basic solids (BaO, K2O, etc.) and metal-loaded activated carbons have also been tested for polystyrene depolymerization9 and polyethylene degradation,10 respectively. Among the zeolitic catalysts, it has been proved that HZSM-5 promotes the cracking of polyethylene with high yields into both gaseous and aromatic hydrocarbons,11,12 whereas REY zeolite leads to a mixture of hydrocarbons with boiling points within the range of commercial gasolines in the catalytic conversion of a heavy oil feed, obtained by a previous polyethylene thermal cracking.13 However, zeolite catalysts are microporous materials having a maximum pore size of roughly 0.75 nm. This fact is responsible for the appearance of steric hindrances in the cracking of bulky polymeric molecules, as has been found in the catalytic cracking of polypropylene over HZSM-5 zeolite.14 This problem can be solved using catalysts of larger pores or zeolites with smaller crystal size. The first approach has demonstrated to be successful in the catalytic cracking of polypropylene over HMCM-41, which is a material of uniform mesoporosity. The large pore size (∼2.5 nm) and medium acid strength distribution of this catalyst allow polyolefinic plastics to be degraded with high activity and selectivity to gasoline and middle distillates.15,16 The second approach has been carried out using a HZSM-5 sample with crystal size in the nanometer range (n-HZSM-5), as well as REY zeolite with crystals around 0.1 µm length.17,18 In both cases the presence of a high proportion of external acid sites
10.1021/ie9906363 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/13/2000
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promotes the initial steps of the polymer cracking, whereas a faster intracrystalline diffusion favors the reforming of the primary cracking products. In contrast with the number of works published on the conversion of pure polyolefins, studies dealing with the catalytic cracking of polyolefin mixtures are scarce. However, the processing of plastic mixtures is usually required in the management of plastic wastes due to the high cost and identification problems still present in the operations of sorting and classification by resin. The present work tries to fill this gap by showing the results obtained in the catalytic cracking of a standard mixture of polyolefins over a number of catalysts: zeolites (n-HZSM-5, HZSM-5, HBeta, and HY), amorphous silica-alumina, and mesoporous HMCM-41. The polyolefin mixture consists of 46.5 wt % low-density polyethylene (LDPE), 25 wt % high-density polyethylene (HDPE), and 28.5 wt % polypropylene (PP), a proportion very close to the one usually found in the mixed plastics present in municipal solid wastes (MSW). These catalysts were selected in order to study the influence of different variables (acid strength distribution, crystal size, and pore structure) on the activity and product distribution obtained in the conversion of the polyolefin mixture. Experimental Section Catalyst Preparation. (a) HZSM-5 Zeolite. The preparation of ZSM-5 zeolite was carried out by crystallization of ethanol-containing gels at 170 °C for 24 h following a procedure described elsewhere.19 The acid form of the zeolite was obtained by an ion-exchange treatment with a 0.6 M HCl aqueous solution at 25 °C for 4 h (1 g of zeolite/40 g of HCl solution), followed by calcination in static air at 550 °C for 5 h. (b) n-HZSM-5 Zeolite. The synthesis of ZSM-5 with crystal size in the nanometer range (n-HZSM-5) was carried out according to a procedure recently reported using tetrapropylammonium hydroxide (TPAOH) as template.20 Tetraethyl orthosilicate (Aldrich, 98 wt %) and aluminum isopropoxide (Aldrich, 98 wt %) were used as silica and aluminum sources, respectively. The crystallization took place under autogenous pressure at 170 °C for 72 h. The zeolite obtained was separated by centrifugation, washed with deionized water, dried at 110 °C for 12 h, and finally calcined under static air for 7 h at 550 °C. (c) HBeta Zeolite. The synthesis of this material was carried out from an amorphous xerogel by a wetness impregnation procedure. First, a silica gel was prepared by hydrolysis of tetraethyl orthosilicate with a 0.05 M aqueous HCl solution for 45 min (H2O/Si(C2H5O)4 molar ratio ) 4). Gelification of this mixture was caused by dropwise addition of tetraethylammonium hydroxide (TEAOH, 20% aqueous solution). The gel was washed with deionized water and dried in an oven for 12 h. Subsequently, the so-obtained xerogel was impregnated with a solution consisting of aluminum isopropoxide in 20 wt % aqueous TEAOH. Their respective amounts were fixed to obtain a final mixture with a Si/Al atomic ratio of 60 and a SiO2/TEAOH molar ratio of 3.6. The wetness-impregnated solid was introduced in an autoclave and maintained at 135 °C for 72 h under autogenous pressure. Thereafter, the produced zeolite was separated by centrifugation, dried in an oven at 110 °C for 12 h, and finally calcined under static air at 550 °C for 5 h.
(d) HY Zeolite. The synthesis of this material was performed according to a published procedure,21 wherein the silica and aluminum sources were Cab-O-Sil and sodium aluminate, respectively. The crystallization was carried out at 100 °C for 48 h, and the zeolite obtained was activated by ion exchange with a 1 M NH4Cl aqueous solution under stirring at ambient temperature for 19 h (1 g of zeolite/67 g of NH4Cl solution). The exchanged zeolite was washed with deionized water, dried at 110 °C for 12 h, and calcined at 450 °C for 3 h under static air. (e) Amorphous SiO2-Al2O3. This catalyst was prepared following a sol-gel procedure described elsewhere,14 using aluminum isopropoxide and tetraethyl orthosilicate as aluminum and silica sources, respectively. (f) MCM-41. This material was prepared according to a method previously described,16 using hexadecyltrimethylammonium chloride as the surfactant, and aluminum isopropoxide and tetraethyl orthosilicate as aluminum and silica sources, respectively. The synthesis was carried out at room conditions and in just 3 h. The obtained material was washed with deionized water, dried in an oven at 110 °C for 12 h, and finally calcined at 550 °C in static air for 12 h, which led directly to the acid form of MCM-41, with ion-exchange treatments not being necessary. Catalyst Characterization. The Si/Al atomic ratio of the different catalysts was determined by X-ray fluorescence (XRF) with a Philips PW 1404 spectrometer. The crystallinity and purity of the zeolites and MCM-41 samples were confirmed by X-ray diffraction (XRD). Thereby, the spectra were recorded on a Philips X’PERT MPD diffractometer using Cu KR radiation. Nitrogen adsorption-desorption isotherms at 77 K were performed using a Micromeritics ASAP 2010 apparatus. Previously, all of the samples were subjected to a standard outgassing procedure at 200 °C under vacuum for 5 h. Surface areas were obtained by application of the Brunauer-Emmett-Teller (BET) procedure. Pore size distributions of both MCM-41 and amorphous silica-alumina were determined by the BJH method, applied to the adsorption branch of the isotherm. In these calculations, cylindrical pore geometry was assumed and the Jura-Harkins equation was used to obtain the thickness of the adsorbed layer. The external surface area and the micropore volume were calculated by application of the t-plot method to a selected zone of the isotherm. Ammonia temperature desorption measurements (TPD) of the catalysts were carried out in a Micromeritics 2900 TPD apparatus. The samples were outgassed under a He flow (50 NmL/min), with a heating rate of 15 °C/min from room temperature up to 560 °C. This final temperature was maintained for 30 min and, subsequently, the samples were cooled to 180 °C and treated with an ammonia flow (30 NmL/min) for 30 min. The physisorbed ammonia was removed from the sample by flowing He at 180 °C for 90 min. The chemically bonded ammonia was determined by increasing the temperature up to 550 °C with a heating rate of 15 °C/ min, remaining at this temperature for 30 min. The ammonia present in the effluent stream was monitored continuously with a thermal conductivity detector (TCD). Scanning electron micrographs (SEM) were taken with a JEOL JSM-6400 microscope working at 35 kV. The samples were previously dispersed in acetone under
Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000 1179 Table 1. Physicochemical and Textural Properties of the Catalysts catalyst
n-HZSM-5
HZSM-5
HBeta
HY
HMCM-41
SiO2-Al2O3
Si/Al pore size (nm) BET surface area (m2 g-1) external surface area (m2 g-1)a pore volume (cm3 g-1)b micropore volume (cm3 g-1)a crystal size (µm) acidity (mequiv of NH3 g-1)c Tmax (°C)c
24 0.55 430 81 0.48 0.15 0.075 0.38 462
31 0.55 361 7 0.18 0.17 3 0.52 470
39 0.64 613 25 0.35 0.24 0.20 0.32 422
3 0.74 583 9 0.26 0.23 0.50 0.37 300
45 2.4 1164 73 0.79 s 0.2-2 0.22 338
36 2-12 261 s 0.97
a
s 0.24 302
Obtained by application of the t-plot method. b Measured at p/p0 ) 0.995. c From ammonia TPD.
stirring in an ultrasonic bath. Subsequently, they were deposited over a brass sample holder and covered with gold. Transmission electron micrographs (TEM) were taken on a JEOL JEM 2000 FX microscope working at 200 kV. Sample preparation was carried out by dispersion in acetone, stirring in an ultrasonic bath, and deposition over a carbon-coated copper grid. High-resolution 27Al MAS NMR measurements were recorded at 104.26 MHz in a Bruker MSL-400 apparatus equipped with a Fourier transform unit. The spinning frequency was 4000 cps with time intervals of 5 s between successive accumulations. The measurements were carried out at room temperature with Al(H2O)63+ as an external standard reference. Materials. Thermal and catalytic cracking experiments were carried out using a standard mixture of polyolefins with the following composition: 46.5 wt % LDPE (Mw ) 416 000), 25 wt % HDPE (Mw ) 188 000), and 28.5 wt % PP (Mw ) 450 000, isotacticity index ) 93%). The polyolefins were ground and sieved to a particle size below 1 mm. Likewise, the catalysts were ground and sieved to a particle size below 0.074 mm. Apparatus and Procedures. Thermal and catalytic cracking tests were carried out in a Pyrex batch reactor (26 mm width and 65 mm height) under a continuous nitrogen flow. Initially, weights of 0.032 g of catalyst and 1.6 g of the polyolefin mixture were loaded into the reactor and mixed thoroughly. Afterward, the reactor was put into an oven and heated with a rate of 25 °C min-1 up to the reaction temperature (400 °C), keeping it constant for 30 min. This temperature was continually monitored by a thermocouple in direct contact with the reaction mixture. The gaseous products at the reaction temperature were swept out from the reactor by the nitrogen stream and separated into liquid and gaseous fractions in a condenser cooled by an ice/water mixture. The liquid fraction was collected from the bottom of the condenser and weighed. The effluent gas flow was measured at every moment by a soap meter placed after the condenser, being finally collected in a gas bag. The plastic conversion was defined as the sum of collected gaseous and liquid products with regard to the initially loaded polyolefin mixture. The solid remaining in the reactor was considered as a residue, not being included in the conversion. Product Analysis. The gaseous products were analyzed in a Hewlett-Packard 5880A GC equipped with a TCD, using a 6 m length and 1/8 in. steel column containing Porapak Q as the stationary phase. The liquid products were analyzed in a Perkin-Elmer 8310 GC equipped with a flame ionization detector (FID) and using a 25 m BP-5 capillary column. The identification of the products was done by comparison with the retention times of pure compounds.
Results and Discussion Catalyst Properties. The main physicochemical and textural properties of the catalysts are summarized in Table 1. All of them present Si/Al atomic ratios in the range 24-45 except for HY zeolite, which possesses a value of 3. In all cases the amount of acid sites detected by ammonia TPD is lower than that expected according to their Si/Al atomic ratio, assuming that only one ammonia molecule is adsorbed on each aluminum atom. This discrepancy is likely due to the presence of weak acid sites that desorb ammonia during the physidesorption treatment at 180 °C and whose amount increases in a large extension for HY zeolite. In addition, the existence of aluminum atoms located inside the pore walls and nonaccessible for ammonia molecules should be taken into account also in the case of HMCM-41 and amorphous silica-alumina to explain this diference.14 Both HZSM-5 and n-HZSM-5 present the acid sites of highest strength, with peak maxima of ammonia desorption placed at 470 and 462 °C, respectively. HBeta zeolite exhibits sites of intermediate acid strength (Tmax ) 422 °C) compared to those of HZSM-5 zeolite and the rest of the catalysts. Likewise, HY zeolite presents acid sites of medium acid strength similar to those of amorphous silica-alumina and HMCM-41, according to their temperature maxima of ammonia desorption: 300, 302, and 338 °C, respectively. In regards to the textural properties summarized in Table 1, the contrast between the two HZSM-5 samples is noteworthy: standard and nanometer crystal size HZSM-5. The decrease in the HZSM-5 crystal size from 3 to 0.075 µm causes significant variations in the textural properties, especially in the external surface area which varies from 7 to 81 m2 g-1. This result implies that for the n-HZSM-5 sample the external surface of the zeolite crystals accounts for around 19% of the total surface area, while in the case of the standard HZSM-5 this parameter is just 2% of the BET surface area. It is also remarkable that, although both HZSM-5 samples present similar micropore volume, the total pore volume corresponding to n-HZSM-5 pore is quite higher. The voids existing between the zeolite crystallites provide this sample with additional porosity, explaining its high total pore volume (0.48 cm3 g-1). On the other hand, HBeta zeolite shows also quite high BET and external surface areas (613 and 25 m2 g-1, respectively), whereas HY zeolite presents a slightly lower BET surface area (583 m2 g-1) and a quite inferior external surface area (9 m2 g-1). For these samples, the micropore volumes are in agreement with the expected values corresponding to their structures. The t-plot analysis shows that both HMCM-41 and amorphous silica-alumina do not possess micropores. Moreover, HMCM-41 presents a very high BET surface area (1164
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Figure 1. Conversions obtained in the thermal and catalytic cracking of the polyolefin mixture (T ) 400 °C, P/C ) 50, t ) 30 min).
m2 g-1) and uniform structure of mesopores (2.4 nm), which are typical features of these types of materials. Finally, amorphous silica-alumina shows a low BET surface area (261 m2 g-1) with a wide distribution of mesopores in the range 2-12 nm. Catalytic Cracking of the Polyolefin Mixture. (i) Activity. Figure 1 shows the results obtained in the catalytic cracking of the polyolefin mixture at 400 °C over the different catalysts, as well as those corresponding to a thermal cracking experiment used as the reference. From the data shown in Figure 1, the following activity order is observed:
n-HZSM-5 > HBeta > HMCM-41 . SiO2-Al2O3 > HZSM-5 > HY > thermal conversion The highest conversion is obtained with the HZSM-5 zeolite of small crystal size (n-HZSM-5) with a value of around 84%. Both HBeta zeolite and HMCM-41 show also high conversions, 68 and 49%, respectively. On the contrary, the rest of the catalysts (SiO2-Al2O3, HZSM5, and HY) lead to really low conversions that reach at best 10 wt % in the case of the amorphous SiO2-Al2O3. These significant differences in activity are also observed when varying the reaction conditions. Increasing the reaction time or reducing the plastic/catalyst ratio leads to enhanced plastic conversions. Thus, if the amount of catalyst loaded in the reactor is multiplied by a factor of 2 (plastic/catalyst mass ratio ) 25), the conversion obtained over n-HZSM-5, HBeta, and HMCM41 is higher than 90%. However, in the same reaction conditions, the plastic conversion obtained over the other three catalysts is still below 20%, with a value of 17% being obtained over the amorphous silica-alumina catalyst. The activity order found for the different catalysts can be related with their respective physicochemical properties and the nature of the polyolefin mixture. It must be taken into account that catalytic cracking over acid solids takes place through the formation of carbenium and carbonium ions, which requires the presence of strong acid sites. However, a direct correlation between the acid strength, shown in Table 1, and the measured
activities is not observed. Thus, the behavior of the standard HZSM-5 is noteworthy because, despite presenting the highest content of strong acid sites according to the ammonia TPD measurements, it leads to a very low conversion of the plastic mixture. On the contrary, a direct relationship is clearly observed between the activity and the external surface area/crystal size of the zeolite samples, which confirms that steric and/or internal diffusion hindrances are present for the catalytic cracking of the bulky polymer molecules. As expected, the catalysts having small crystals present high external surface area. The external acid sites are not limited by steric or diffusional problems; hence, they are essential to promote the initial steps of the polyolefin cracking, which explains the high activity obtained over both n-HZSM-5 and HBeta zeolites. In the case of HMCM-41, the large pore size of this material (2.4 nm) allows the polymer molecules to have a better access to the acid sites. This fact, along with its medium acid strength distribution, explains the high activity of this catalyst. The turnover frequency (TOF) values, calculated as the weight of plastic converted per second and per the Al weight in the catalyst, obtained for n-HZSM5, HBeta, and HMCM-41 are 1.27, 1.63, and 1.38 s-1, respectively. These data suggest that the acidic sites on HBeta and HMCM-41 are actually more active than those on n-HZSM-5, despite their lower acid strength. The good results obtained over HBeta zeolite can be explained as a combination of its relatively high acid strength, large pore diameter (0.64 nm), and small crystal size (0.2 µm). The low activity obtained over zeolite HY may be initially related to its medium crystal size (0.5 µm), which implies the possible presence of intracrystalline diffusion control, in addition to the presence of a low proportion of external acid sites. However, we think that two additional reasons must be considered: the zeolite topology and its high aluminum content. First, its pore structure possesses micropores of 0.75 nm and large cavities of 1.3 nm size (supercages), which allow bulky coke precursors to be formed, leading to a fast deactivation of the catalyst.13 Second, it is known that over acid solids the coke formation reactions are dependent on the density of acid sites.22 Accordingly, the high aluminum content of zeolite HY (Si/Al ∼ 3) and the presence of supercages in its structure may enhance coke formation reactions, which results in a low activity for the cracking of the plastic mixture. Regarding the results obtained over the amorphous silica-alumina sample, it is remarkable that its conversion is roughly 5 times inferior than that of HMCM-41, although both materials possess acid sites with similar content and strength. Note that the silica-alumina sample presents also large pores (2-12 nm); hence, its low activity cannot be assigned to diffusional limitations. The major differences compared to HMCM-41 are the high surface area and pore uniformity of the latter, which are probably the factors leading to the high acivity observed over HMCM-41. Polypropylene Controlling Role. An interesting phenomenon can be appreciated if the behavior of both n-HZSM-5 and HZSM-5 in the cracking of the polyolefin mixture is compared to that observed for these samples in the cracking of pure polyolefins, reported in earlier works.14,16 In this case, we observed that both LDPE and HDPE could be cracked over the standard HZSM-5 sample with high activity. On the contrary, polypropyl-
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ene degradation proceeded over this catalyst with very low conversion, which was related to the large cross section of this polymer, which hinders its access to the zeolite micropores (0.55 nm), as well as to the low proportion of external surface area of this zeolite. These conclusions were further supported by the results obtained in the PP cracking over the n-HZSM-5 sample. Under the same reaction conditions (P/C ) 36, T ) 400 °C, t ) 30 min), the conversion obtained in the cracking of pure polypropylene over n-HZSM-5 was 67%, whereas the value corresponding to HZSM-5 was 11.3%. This result confirmed the significant role of the acid sites located on the external surface of the zeolite crystals to promote the cracking of the PP molecules. In the present work, the low conversion obtained in the cracking of the polyolefin mixture over HZSM-5 resembles the value corresponding to the cracking of pure polypropylene over this sample. However, because of the presence of high proportions of both HDPE and LDPE in the raw plastic mixture (71.5 wt %), its degradation over the standard HZSM-5 should proceed in a higher extension. If the theoretical conversion of the plastic mixture over the HZSM-5 sample is estimated by taking into account the results obtained with the pure polyolefins,17 a value of 28% is obtained, clearly higher than the 8% conversion really observed. These results suggest that PP plays an important role controlling the rate of the plastic mixture cracking. This anomalous behavior can be explained by taking into account the segregated nature of the plastic mixture. In this way, a low degree of mixing among the particles of the three polyolefins present in the raw mixture is observed, even after being melted. Accordingly, the earlier results suggest that a preferential contact takes place between the HZSM-5 zeolite crystals and the PP domains existing in the reaction medium. This assumption has been confirmed by observing the residue remaining in the reactor after the reaction. The PP particles surround and isolate the HZSM-5 crystals in regards to the PE regions, and because the PP cracking over the standard HZSM-5 proceeds very slowly, the overall conversion of the plastic mixture is considerably reduced. This phenomenon is not so important in the case of n-HZSM-5 because the PP cracking is not hindered on this material because of the presence of a high proportion of external acid sites. Product Distribution. Figure 2 shows the selectivity by groups obtained with the three catalysts of higher conversion (n-HZSM-5, HBeta, and HMCM-41). It is observed that n-HZSM-5 zeolite leads to a high selectivity toward gaseous products (the sum of C1-C4 paraffins and C2-C4 olefins is roughly 50 wt %) with a high amount of valuable C2-C4 olefins (∼34%). Neither of the other two catalysts yields this high amount of gases, although HBeta zeolite presents also a significant gas proportion (35%). A coincidence among the three catalysts can be noticed because their selectivity toward C2C4 olefins is approximately twofold higher than that corresponding to C1-C4 paraffins. Likewise, the C5C12 selectivity is always superior to 45% for the three catalysts, with the 60% obtained over HBeta zeolite and the intermediate selectivity of 54% over HMCM-41 being remarkable. In this fraction, the selectivity toward aromatic hydrocarbons in the range C6-C9 presents values for these three catalysts below 3%, which is in agreement with the environmental trends to limit the presence of aromatic hydrocarbons in gasolines. On the
Figure 2. Product selectivity by groups obtained in the catalytic cracking of the polyolefin mixture over n-HZSM-5, HMCM-41, and HBeta (T ) 400 °C, P/C ) 50, t ) 30 min).
other hand, the proportion of middle distillates obtained in the catalytic cracking over both zeolites is almost negligible, because it never overcomes 4%. This fact does not occur in the catalytic cracking over HMCM-41, where an important selectivity toward the C13-C30 fraction is obtained (31.7%). The selectivity by carbon atom number obtained in the catalytic cracking of the polyolefin mixture over these three catalysts is shown in Figure 3. A high similarity in the atom carbon distributions corresponding to both zeolites can be observed. A major maximum is appreciated for all of the catalysts corresponding to the C4 fraction, although less pronounced for the HMCM-41 sample. A second relative maximum located in the C8 fraction can be detected for the three catalysts with a value of around 7%. It is remarkable that both HBeta and n-HZSM-5 zeolites show a negligible selectivity toward fractions superior to C12 (
