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Catalytic Cracking of Polyethylene over Clay Catalysts. Comparison with an Ultrastable Y Zeolite George Manos,*,† Isman Y. Yusof,‡ Nikos Papayannakos,§ and Nicolas H. Gangas§ Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, U.K., Chemical Engineering Research Centre, School of Applied Science, South Bank University, London, U.K., and Department of Chemical Engineering, National Technical University, Athens, Greece
The catalytic cracking of polyethylene has been studied over two natural clays and their pillared analogues with a view toward assessing their suitability in a process for recycling plastic waste to fuel. Although these clays were found to be less active than US-Y zeolite around 600 K, at slightly higher process temperatures, they were able to completely decompose polyethylene. Their yields to liquid products were around 70%, compared to less than 50% over US-Y zeolite. Moreover, the liquid products obtained over the clay catalysts were heavier. Both of these facts are attributed to the milder acidity of clays, as the very strong acidity characterizing zeolites leads to overcracking. Furthermore, this milder acidity leads to significantly lower occurrence of hydrogen-transfer secondary reactions compared to US-Y zeolite, and as a consequence, predominantly alkenes were the products over the clay catalysts. An additional advantage of these catalysts is the considerably lower amount of coke formed. 1. Introduction The use of plastic products has closely followed the growth of the welfare level in modern societies. Consequently, the amount of plastic waste has dramatically increased, causing a serious environmental problem. The currently prevailing practices in solid waste management are landfill disposal and, to a much lesser extent, incineration, with the latter practice enabling recovery of the energy content of the waste. Both practices lead, however, to serious environmental problems when applied also to plastic wastes, because (i) plastic goods, by being more voluminous than organic wastes, quickly exhaust available landfill space, which is, in any case, becoming progressively more scarce and expensive; and (ii) incineration of plastics produces toxic gaseous compounds and shifts the solid waste problem to one of air pollution, which is already prohibited and is becoming increasingly politically unacceptable in many countries. However, in times of accelerated depletion of natural resources, plastic waste presents a cheap source of raw materials, and hence, its recycling becomes a necessity. Among the various methods of polymer recycling, thermal and/or catalytic degradation of plastic waste to gas and liquid products1-10 is the most promising to be developed into a commercial polymer recycling process. The products of such a process could be utilized as fuels or chemicals. Because pure thermal degradation demands relatively high temperatures and its products require further processing to upgrading their quality, catalytic degradation of plastic waste offers considerable advantages.1 It occurs at considerably lower temperatures and forms hydrocarbons in the gasoline range, eliminating the necessity of further processing. * Corresponding author. Tel.: +44-20-7679 3810. Fax: +4420-7383 2348. E-mail:
[email protected]. † University College London. ‡ South Bank University. § National Technical University.
In such a recycling process, the most valuable product is obviously liquid fuel. Although gaseous products are useful too, as their burning can contribute to the energy demand of an endothermic polymer cracking process, excess gas production is not desirable. Gaseous products are considered low value because of their transportation costs. Consequently, the target of a commercially viable recycling process should be an increase of the liquid product yield. So far, mainly zeolite-based catalysts have been tried for the catalytic cracking of plastics,1-10 but their utilization is obstructed by the formation of small product molecules that consequently decrease the liquid product yield. This is attributed to the very strong acidity of the zeolitic sites, which brings about severe cracking of the plastic molecules. In a search for catalysts suitable for plastic catalytic cracking, we considered smectites, i.e., expanding layer clays, as possible candidates in view of their known catalytic potential.11 The layer structure of smectites offers the possibility of molding a porous network that is, in many aspects, different from that of zeolites. Specifically, intercalated molecular moieties can prop apart the clay layers, thus creating a two-dimensional network of interconnected micropores with dimensions larger than those of the zeolitic pores. As a result, cationic or neutral species of quite appreciable size can then be inserted in the interlayer space via cation exchange or sorption, respectively. Along this latter line, of particular relevance to the present study is the polymer melt intercalation that has been employed for making nanocomposites out of high-molecular-weight polymers and layered clays.12 With a view toward shedding some light on the role of pore structure in the catalytic cracking of plastics, pillared clays13 have also been considered in this exploratory study, because their network of voids is, at least conceptually, less flexible than that of their parent expanding clays. Specifically, pillared clays are derived from expanding smectites by propping their layers apart with nanometer-sized pillars of metal oxides. Because
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of the cross-linking of the pillars to the clay sheets, the interlayer space can neither further expand upon intercalation nor collapse upon calcination up to about 800 K. The lateral dimension of the micropores thus formed in the interlayer space varies from a few to several nanometers. Therefore, although quite large molecules can be accommodated in this network of voids, its spatial constraints bestow pillared clays with high selectivity. The particular clays chosen for the present study were two types of smectites: (i) a montmorillonite, a mineral with mainly Mg substituting for Al in its octahedral layer; and (ii) a saponite, a mineral with mainly Al substituting for Si in its tetrahedral layer. This difference in structural composition furnishes their Al-pillared forms with different acidic14 and catalytic behavior.15 The consideration of pillared clays as possible catalysts is further supported by the fact that the acidity, a crucial parameter for coke formation, of these materials is weaker in strength as well as smaller in intensity than that of zeolites. Moreover, in pillared clays, the major sources for Bro¨nsted and Lewis sites are associated with the clay lattice hydroxyls and the pillars, respectively.16 This differentiates further these materials from their parent clays, as well as from other catalysts. Hence, this paper reports on the catalytic cracking of polyethylene over a saponite and a montmorillonite, as well as over their aluminum-oxide-pillared derivative materials. An ultrastable Y zeolite (US-Y) was also tested in this study to compare its catalytic performance with that of the clays. 2. Experimental Section (a) Materials. The starting saponite (C-27 Saponite) was provided by Tolsa S.A. (Madrid, Spain), and according to the supplier, the main impurity present was sepiolite (10 wt %). Its surface area was 45 ( 5 m2/g. The starting montmorillonite (Zenith-N) was provided by Silver & Barite Ores Mining Co. (Greece). According to the supplier, the main phases present in this bentonite are 85% montmorillonite (a dioctahedral smectite), 5% feldspars, 3% calcite, 2.5% quartz, 2% illite, and 2% christobalite. This sample had a surface area of 165 ( 10 m2/g. A process described elsewhere17 was applied for pillaring the starting with aluminum oxide pillars. X-ray diffraction and nitrogen adsorption was used to characterize the obtained pillared derivatives of the raw saponite and montmorillonite, ATOS and AZA, respectively. Both of these pillared clays are replicas of the reference materials prepared for the needs of the Concerted European Action on Pillared Layered Structures (EC project BREU CT-91-0462; 1991-1995). For AZA, the specific surface area was determined to be 220 ( 10 m2/g, and the d001 spacing to be 1.82 ( 0.02 nm. For ATOS, the specific surface area was measured as 230 ( 15 m2/g, and the d001 spacing as 1.82 ( 0.03 nm. In addition to these clays and their pillared derivatives, an acidic ultrastable Y zeolite (US-Y) (surface area ) 590 ( 25 m2/g) was also tested in this study to establish a comparison reference. All of these materials were used in powder form; US-Y average particle size ) 1 mm, clay particle average particle size < 160 mm. Finally, unstabilized linear low-density polyethylene (lldPE) and unstabilized high-density polyethylene
(hdPE), kindly provided by BASF AG, also in powder form, were used as the model feed. (b) Semibatch Reactor Equipment. The reaction was carried out in a semibatch Pyrex reactor. The experimental apparatus for catalytic degradation of polyethylene was similar to that used in previous studies,1 with the main difference being the use of an infrared heater. The experimental rig consisted of the semibatch reactor, heated by two-semicircular infrared heating elements that were connected to a programmable temperature controller. The infrared heaters, which allow very fast heating of the polymer sample, replaced the electric heating furnace described in previous work.1 The reactor was purged with nitrogen at 50 mLN/min, determined by a mass flow controller, to remove the volatile reaction products from the reactor. At the beginning of the experiment, the catalyst sample was weighed, charged into the reactor, and dried under a nitrogen stream at 473 K. The reactor was then cooled, sealed, and weighed to determine the water loss and the exact amount of catalyst. The polymer sample was added afterward at a mass ratio 2:1, and the reactor was weighed again to determine the exact polymer amount. The mass ratio of polymer to catalyst in this study was kept constant and equal to 2, as previous work using thermal gravimetric analysis (TGA)1 has shown that there is no significant change in the degradation pattern for polymer-to-catalyst mass ratios below 2. Before the actual experiment, the reactor was purged with a high nitrogen stream at room temperature to remove the oxygen from the reactor. The reactor was then heated at a determined temperature program. In this study, a temperature program was chosen that increased the sample temperature stepwise after the sample reached 413 K, i.e., the melting point of hdPE, as, until this point, no reaction took place.1 Thereafter, the program set the temperature first to 573 K for 5 min, then to 633 K for the next 5 min, and finally to 673 K for another 5 min. This thermal treatment was found to be sufficient for the full degradation of the polymer samples with all of the catalysts used. We refer to the carbonaceous deposits on the catalysts as coke. Another advantage of the employed temperature program was that each catalytic experiment covered the range of temperatures in about 15 min, thus enabling a quick evaluation of the performance of the various catalysts tested. Liquid products were collected in a cooling trap placed in an ice bath (273 K) and connected to the reactor outlet through a two-way valve. The two-way valve enabled collection of various liquid samples in different cooling traps during the experiment. Three liquid samples were collected, one for each stage of the temperature program. The liquid products were analyzed on a Shimadzu 8A gas chromatograph equipped with a flame ionization detector (FID) using a CP-Sil PONA fused-silica capillary column (100m × 0.25 mm id). The gaseous products were collected in valved gas sampling bags every 5 min, one for each stage of the temperature program. The gas products were analyzed on a Shimadzu 8A gas chromatograph equipped with a FID detector using a PLOT-Al2O3/KCl (3m × 3 mm id) packed column. 3. Results and Discussion (a) Conversion and Selectivity. Tables 1 and 2 give the results obtained over the various catalysts tested
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Table 1. Conversion, Selectivity and Yield to Liquid Fraction, and Coke Yield and Concentration for the Catalytic Cracking of lldPE over the Catalysts Used in This Study catalyst US-Y ATOS AZA C-27 Saponite Zenith-N total conversion (%) selectivity to liquid (%) yield to liquid (%) coke yield (%) coke concentration (%)
89 50 44 11 23
98 73 72 2 4
96 75 72 4 8
98 70 68 2 5
99 76 75 1 2
Table 2. Conversion, Selectivity and Yield to Liquid Fraction, and Coke Yield and Concentration for the Catalytic Cracking of hdPE over US-Y Zeolite and ATOS Pillared Clay catalyst total conversion (%) selectivity to liquid (%) yield to liquid (%) coke yield (%) coke concentration (%)
US-Y
ATOS
86 50 43 14 28
96 70 67 4 9
with lldPE and hdPE, respectively, for the overall conversion to volatile products, the liquid fraction selectivity, the yield to liquid, the coke yield, and the coke concentration. The conversion was calculated as the mass fraction of the original polymer sample that reacted to yield volatile products. The mass of the reacted polymer was measured by the difference in weight of the reactor before and after the experiment. The rest of the original polymer was converted to coke. We refer to the carbonaceous deposits on the catalysts as coke. No further treatment of the coke has taken place. The selectivity to liquid fraction was calculated as the mass of the liquid products condensed at 273 K divided by the volatilized mass of the polymer. The yield to liquid products was calculated by multiplying the conversion by the selectivity and represented the fraction of the original polymer sample converted to liquid products. The coke yield was calculated by dividing the mass of coke formed on the catalyst by the original mass of polymer, and the coke concentration by dividing the coke mass by the mass of dry catalyst. The coke yield represented the fraction of the original polymer converted to coke and was simply equal to 1 - conversion. The coke concentration in our experiments was, in all cases, twice the coke yield, as the catalyst-to-polymer ratio was 1:2. For all clay and pillared clay catalysts tested, the selectivity was much higher (>70%) than that with the US-Y zeolite (50%). Although the conversion showed the same pattern, it is noted that the lower conversion values observed over US-Y are not due to lower activity, but simply reflect the fact that the coke formation on all clay catalysts is lower than that on US-Y. As will be shown later, the latter catalyst is much more active than the former. However, over all of the clay catalysts, the combined effects of their higher selectivity and conversion resulted in a significantly higher yield to liquid products, around 70% compared with less than 50% over US-Y. Figure 1 shows the overall conversion values achieved with each catalyst for the three stages of the temperature program. These conversion values were calculated as the sum of the masses of the liquid and gaseous
Figure 1. Overall conversion of catalytic cracking of lldPE for each time/temperature interval over US-Y zeolite, pillared clays, and their original clays.
products formed during each interval divided by the initial mass of polymer. Although a significant amount of polymer was converted during the first stage at 573 K with US-Y, over the clay-based catalysts, essentially no conversion took place. As expected, US-Y was a more active catalyst, as its acidity is much higher than that of the clay-based catalysts. The stronger and more numerous external zeolitic acid sites must have accelerated the initial breakdown of the original macromolecules and, thereby, the overall degradation reaction. This higher acidity leads, however, to higher coke formation and stronger cracking, which, in turn, furnishes smaller molecules and a lower liquid fraction. Although almost no conversion took place in the first degradation stage (573 K/5 min) over clay-based catalysts, after the second heating stage, roughly all of the polymer had been already converted. Hence, pillared clays, as well as their original clays, were found to be less active than zeolites, but temperature compensated for this, leading to full polymer degradation at a slightly higher temperature. Surprisingly, the starting clays performed very similarly to their pillared derivatives, a similarity that is also depicted in the product distribution and indicates that the acidity of the starting clays was sufficient to degrade the polyethylene. Hence, it appears that, under the conditions of the present catalytic tests, the aluminum oxide pillars do not contribute to the polymer decomposition activity. To clarify this apparent inactivity of the pillars, a similar study is currently being carried out to test the performance of pillared clays after acid pretreatment resulting in different levels of final acidity. From studies using spent commercial cracking catalysts,18 it seems that, at least at similar levels of acidity, no significant changes occur in the reaction system. From another viewpoint, one should note that ATOS and AZA have an interlayer spacing of about 2 nm, i.e., a gallery height of about 1 nm. In contrast, for the conditions of the conducted experiments, the starting clays should have a collapsed interlayer spacing, i.e., a gallery height of e0.2 nm. This large difference in the gallery heights of the clays and their pillared derivatives should have an effect on the catalytic performance.15 The absence of such an effect might be related to polymer clay intercalation, which starts at temperatures just above the polymer melting point. The intercalated polymer molecules prop apart the clay layers as other inorganic and organic moieties do when adsorbed by a smectite.13,19 Figures 2 and 3 present the yields to the liquid and gas formation, respectively, obtained from each catalyst during each stage of the temperature program. The data shown indicate that, for the second stage (at 633 K for
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Figure 2. Liquid formation during the catalytic cracking of lldPE over US-Y zeolite, pillared clays, and their original clays.
Figure 4. Boiling point distribution of liquid products of catalytic cracking of lldPE over ATOS, T-saponite, and US-Y zeolite.
Figure 5. Boiling point distribution of liquid products of catalytic cracking of lldPE over AZA, Zenith-N, and US-Y zeolite. Figure 3. Gas formation during the catalytic cracking of lldPE over US-Y zeolite, pillared clays, and their original clays. Table 3. Boiling Point Distribution Intervals group
bp range (K)
middle bp (K)
∆T (K)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
309.2-341.9 341.9-371.6 371.6-398.8 398.8-424.0 424.0-447.3 447.3-469.1 469.1-489.5 489.5-508.6 508.6-526.7 526.7-543.8 543.8-560.0 560.0-575.2 575.2-589.5 589.5-603.1 603.1-617.0
325.6 356.8 385.2 411.4 435.7 458.2 479.3 499.1 517.7 535.3 551.9 567.6 582.4 596.3 610.1
32.7 29.7 27.2 25.2 23.3 21.8 20.4 19.1 18.1 17.1 16.2 15.2 14.3 13.6 13.9
5-10 min), cracking over US-Y favors gas formation, i.e., polymer degradation down to small molecules. At this stage, the catalytic activity over the clay-based catalysts has, however, been boosted without leading to excessive cracking. Hence, the main products were large enough molecules to be collected in the liquid fraction. (b) Product Distribution. Liquid Products. The product distributions of the liquid hydrocarbon fraction for the various catalysts are presented as boiling point distribution curves. This was possible because the employed nonpolar PONA capillary column separated the components of a mixture according to their volatilities/boiling points. The boiling point distribution curves were estimated as follows. We prepared a calibration mixture containing normal alkanes from hexane to eicosane, C6-C20, that we used to assign each observed retention time in the chromatogram to a boiling point. The whole sample for analysis was then divided into intervals between the boiling points of the normal alkanes of the calibration mixture (Table 3). The mass fraction corresponding to each interval was calculated from the sum of the area fractions of all components in this interval. To each interval, the probability density function value was calculated as the mass fraction of
this interval divided by the interval width ∆T, listed in Table 3. Hence, the probability density function is expressed in percent per Kelvin. In the graph of the boiling point distribution, each interval was represented by its middle value (see Table 3). Components with retention times smaller than that of n-hexane were allocated to an interval between n-pentane and nhexane (bp ) 309.2-341.9 K). The resulting boiling point curves are presented in Figure 4 for ATOS and Saponite and Figure 5 for AZA and Zenith-N, together with the curve for US-Y in both figures for comparison purposes. The distribution of the liquid produced over US-Y shows a peak at 400 K with a considerable amount in lighter fractions. The distributions of all of the liquid samples produced over the clays show peaks at higher temperatures, 440 K for ATOS and Saponite and 420 K for AZA and Zenith-N. More importantly, the opposite trend is observed than with US-Y. One notices that the liquid fraction produced over the clay and pillared clay catalysts is characterized by a significantly higher content of less volatile components. This fact is revealed more as a change in the shape of the entire distribution curve rather than simply as a shift in the peak from 400 K to 420-440 K. It is obvious that the lower cracking activity of the clay-based catalysts results not only in more liquid products, but also in heavier products. One should also notice that, although heavier hydrocarbons were produced over the clay-based catalysts than over US-Y zeolite, the overwhelming majority of these products were in the boiling point range of gasoline. This fact, together with the significant increase in the liquid yield, makes clay-based catalysts strong candidates for a commercial process. Gaseous Products. An overview of the yields to gaseous products for all of the catalysts used is shown in Figure 3. With all of the catalysts tested in this study, higher amounts of methane, ethane, and ethene were produced than in previous experiments with US-Y,1 presumably because of the higher-temperature treatment employed in the present study. US-Y zeolite clearly produced more gas products in the second and final temperature stages than the clay
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Table 4. Gaseous Components Distribution of the LldPE Cracking over Various Catalysts catalyst component
US-Y
ATOS
AZA
C-27 Saponite
Zenith-N
alkanes (%) alkenes (%)
71 29
29 71
37 63
34 66
30 70
catalysts, which resulted in much lower liquid yields, as shown in previous sections. The high amount of gaseous products formed over US-Y zeolite is evidently due to the overcracking that took place in the very acidic environment of this catalyst. The gaseous products were categorized as alkanes and alkenes, and the percentage of each group in the total gas product is shown in Table 4. Clearly, more alkenes were formed over clay catalysts than over US-Y zeolite. Previous studies1,2 have shown that the product distribution trends for the gaseous fraction correlate with the distribution for the liquid fraction, and this trend has been confirmed by GC-MS analysis in this study. A qualitative evaluation of the GC-MS results in the range C6-C10 shows that the liquid components with the clay catalysts were predominantly alkenes, whereas with US-Y, the liquid components were primarily alkanes. Similar differences were seen between US-Y and medium-pore zeolites, such as ZSM-5, with the latter producing significantly more alkenes than alkanes.2 In that study, the differences were attributed to geometric restriction effects, as the small pores of ZSM-5 and of medium-pore zeolites more generally strongly suppress the intermediate stages of bimolecular reactions. On the other hand, the reason clay-based catalysts furnish less saturated products, such as alkenes, which are the primary products of cracking reactions, is probably the milder clay acidity, which does not support hydrogentransfer secondary reactions.20,21 Moreover, because the presence of more alkenes in the volatile product distribution was accompanied by the formation of less coke for both types of catalysts (clays and ZSM-5), it is inferred that secondary hydrogen-transfer reactions shift the hydrogen deficiency from the volatile products to the coke formed on the catalyst. 4. Conclusions In view of the importance of catalytic recycling of plastic waste to fuel, two smectites, a saponite and a montmorillonite, as well as their Al-pillared derivatives, were tested for their performance in the catalytic cracking of polyethylene. Below about 600 K, the clay-based catalysts proved to be less active than zeolites. However, this picture is drastically changed with temperature, as the clay catalysts were able to completely decompose polyethylene after a small increase in the process temperature. Moreover, they proved superior than zeolites with respect to the formation of liquid hydrocarbon fuel. The yield to liquid products was around 70%, compared with less than 50% over US-Y zeolite. This higher liquid yield of clay-based catalysts is attributed to their weaker acidity, which does not sustain overcracking to small molecules and is also reflected in the liquid product distribution. Clay catalysts furnish hydrocarbon liquid products that are considerably heavier than those from US-Y. In addition, the former catalysts furnish products that, in the overwhelming majority, are in the gasoline
range. Doubtless, this latter fact is very important for a commercial catalytic recycling process and encourages further research on clay-catalyzed plastic degradation. Furthermore, the predominance of alkenes as products over the clay catalysts is due to the significantly lower occurrence of hydrogen-transfer secondary reactions because of the milder acidity of clays vis-a`-vis the strong acidity of US-Y zeolite. This differentiation probably also explains the lower coke formation over the clay-based catalysts. Therefore, the higher liquid yield and the lower coke yield found in the catalytic cracking of plastic waste over the clays and pillared clays tested make these materials promising candidates for a future commercial processes. Toward the development of such a process, work should also address the issue of regeneration of both classes tested, i.e., smectites and their pillared derivatives. Acknowledgment We thank Mr. Waly Winter from South Bank University, School of Applied Science, Analytical Laboratory, for his help with the chromatographic analysis. Literature Cited (1) Manos, G.; Garforth, A.; Dwyer, J. Catalytic Degradation of High-Density Polyethylene on an Ultrastable Y Zeolite. Nature of Initial Polymer Reactions, Pattern of Formation of Gas and Liquid Products, Temperature Effects. Ind. Eng. Chem. Res. 2000, 39, 1198. (2) Manos, G.; Garforth, A.; Dwyer, J. Catalytic Degradation of High-Density Polyethylene on Different Zeolitic Structures. Ind. Eng. Chem. Res. 2000, 39, 1203. (3) Songip, A. R.; Masuda, T.; Kuwahara, H.; Hashimoto, K. Test to Screen Catalysts for Reforming Heavy Oil from Waste Plastics Appl. Catal. B: Environ. 1993, 2, 153 (4) Songip, A. R.; Masuda, T.; Kuwahara, H.; Hashimoto, K. Kinetic Studies for Catalytic Cracking of Heavy Oil from Waste Plastics over REY Zeolite Energy Fuels 1994, 8, 131 (5) Audisio, G.; Bertini, F.; Beltrame, P. L.; Carniti, P. Catalytic Degradation of Polyolefins Makromol. Chem. Macromol. Symp. 1992, 57, 191 (6) Ohkita, H.; Nishiyama, R.; Tochihara, Y.; Mizushima, T.; Kakuta, N.; Morioka, Y.; Ueno, A.; Namiki, Y,; Tanifuji, S.; Katoh, H.; Sunazyka, H.; Nakayama, R.; Kuroyanagi, T. Acid Properties of Silica-Alumina Catalysts and Catalytic Degradation of Polyethylene. Ind. Eng. Chem. Res. 1993, 32, 3112 (7) Ng, S. H.; Seoud, H.; Stanciulescu, M.; Sugimoto, Y. Conversion of Polyethylene to Transportation Fuels through Pyrolysis and Catalytic Cracking. Energy Fuels 1995, 9, 735 (8) Arandes, J. W.; Abajo, I.; Lopez-Valerio, D.; Fernandez, I.; Azkoiti, M. J.; Olazar, M.; Bilbao, J. Transformation of Several Plastic Wastes into Fuel by Catalytic Cracking. Ind. Eng. Chem. Res. 1997, 36, 4523 (9) Arguado, 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 (10) Sharratt, P. N.; Lin, Y.-H.; Garforth, A. A.; Dwyer, J. Investigation of the Catalytic Pyrolysis of High-Density Polyethylene over a ZSM-5 Catalyst in a Laboratory Fluidized-Bed Reactor. Ind. Eng. Chem. Res. 1997, 36, 5118. (11) Chitnis, S. R.; Sharma, M. M. Industrial applications of acid-treated clays as catalysts. React. Funct. Polym. 1997, 32, 93. (12) Giannelis, E. P.; Krishnamoorti, R.; Manias, E. PolymerSilicate Nanocomposites: Model Systems for Confined Polymers and Polymer Brushes. Adv. Polym. Sci. 1999, 138, 107 (13) Gil, A.; Gandia, L. M.; Vicente, M. A. Recent Advances in the Synthesis and Catalytic Applications of Pillared Clays. Catal. Rev.-Sci. Eng. 2000, 42, 145 (14) Bergaoui, L.; Mrad, I.; Lambert, J.-F.; Ghorbel, A. A Comparative Study of the Acidity toward the Aqueous Phase and Adsorptive Properties of Al13-Pillared Montmorillonite and Al13Pillared Saponite. J. Phys. Chem. B 1999, 103, 2897.
Ind. Eng. Chem. Res., Vol. 40, No. 10, 2001 2225 (15) Moreno, S.; Sun Kou, R.; Molina, R.; Poncelet, G. Al-, Al,Zr-, and Zr-Pillared Montmorillonites and Saponites: Preparation, Characterization, and Catalytic Activity in Heptane Hydroconversion. J. Catal. 1999, 182, 174. (16) He, M.-Y.; Liu, Z.; Min, E. Acidic and hydrocarbon catalytic properties of pillared clay. Catal. Today 1988, 2, 321. (17) Kaloidas, V.; Koufopanos, C. A.; Gangas, N. H.; Papayannakos, N. Scale-up studies for the preparation of pillared clays at 1 kg per batch level. Microporous Mater. 1995, 5, 97. (18) Cardona, S. C.; Corma, A. Tertiary Recycling of Polypropylene by Catalytic Cracking in a Semibatch Stirred Reactor. Use of Spent Equilibrium FCC Commercial Catalysts. Appl. Catal. B: Environ. 2000, 25, 151. (19) Grzybek, T.; Klinik, J.; Motak, M.; Papp, H.; Zyla, M. Montmorillonites Modified with Carbonaceous Deposits I. Formation Mechanism and Acidity J. Colloid Interface Sci. 2000, 227, 291.
(20) Gonzalez, F.; Pesquera, C.; Benito, I.; Herrero, E.; Poncio, C.; Casuscelli, S. Pillared Clays: Catalytic Evaluation in Heavy Oil Cracking Using a Microactivity Test. Appl. Catal. A: Gen. 1999, 181, 71. (21) Cumming, K. A.; Wojciechowski, B. W. Hydrogen Transfer, Coke Formation, and Catalyst Decay and Their Role in the Chain Mechanism of Catalytic Cracking. Catal. Rev.-Sci. Eng. 1996, 38, 101.
Received for review December 5, 2000 Revised manuscript received February 28, 2001 Accepted March 3, 2001 IE001048O
