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Ind. Eng. Chem. Res. 2000, 39, 1198-1202
Catalytic Degradation of High-Density Polyethylene over Different Zeolitic Structures George Manos,*,† Arthur Garforth,‡ and John Dwyer§ Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, United Kingdom, Environmental Technology Centre, Department of Chemical Engineering, and Centre for Microporous Materials, Department of Chemistry, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 1QD, United Kingdom
The catalytic degradation of high-density polyethylene to hydrocarbons was studied over different zeolites. The product range was typically between C3 and C15 hydrocarbons. Distinctive patterns of product distribution were found with different zeolitic structures. Over large-pore ultrastable Y, Y, and β zeolites, alkanes were the main products with less alkenes and aromatics and only very small amounts of cycloalkanes and cycloalkenes. Medium-pore mordenite and ZSM-5 gave significantly more olefins. In the medium-pore zeolites secondary bimolecular reactions were sterically hindered, resulting in higher amounts of alkenes as primary products. The hydrocarbons formed with medium-pore zeolites were lighter than those formed with large-pore zeolites. The following order was found regarding the carbon number distribution: (lighter products) ZSM-5 < mordenite < β < Y < US-Y (heavier products). A similar order was found regarding the bond saturation: (more alkenes) ZSM-5 < mordenite < β < Y < US-Y (more alkanes). Dependent upon the chosen zeolite, a variety of products was obtained with high values as fuel, confirming catalytic degradation of polymers as a promising method of waste plastic recycling. 1. Introduction Between the various methods of plastic waste recycling,1 thermal and/or catalytic degradation of plastic waste to gas and liquid products are 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. This way waste plastics could be regarded as a cheap source of material. Because pure thermal degradation demands relatively high temperatures and its products require further processing for their quality to be upgraded, catalytic degradation of plastic waste offers considerable advantages.1-10 It occurs at considerably lower temperatures1 and forms hydrocarbons in the gasoline range,1 eliminating the necessity of further processing. This paper reports on the catalytic degradation of high-density polyethylene (hdPE) over several zeolite structures, ultrastable Y, Y zeolite, β zeolite, mordenite, and ZSM-5. This study continued previous work1 on the influence of catalyst quantity, mechanism of the initiation of the degradation, pattern of gas and liquid formation, and temperature effects on the degradation behavior of high-density polyethylene, carried out over ultrastable Y zeolite. 2. Experimental Section The acidic forms of the following zeolites were used in this study: (1) Ultrastable Y zeolite (Si/Al ) 5.7, * To whom correspondence should be addressed. Tel.: +4420-7679 3810. Fax: +44-20-7383 2348. E-mail: g.manos@ ucl.ac.uk. † University College London. ‡ Environmental Technology Centre, Department of Chemical Engineering, University of Manchester Institute of Science and Technology. § Centre for Microporous Materials, Department of Chemistry, University of Manchester Institute of Science and Technology.
framework ratio determined by solid-state NMR), (2) Y zeolite (Si/Al ) 2.5), (3) β zeolite (Si/Al ) 25), (4) mordenite (Si/Al ) 20), and (5) ZSM-5 (Si/Al ) 20). The only polymer used was unstabilized high-density polyethylene (hdPE) in powder form. Previous work using thermal gravimetric analysis (TGA)1 has shown that there was no significant change in the degradation pattern for polymer-to-catalyst mass ratios below 2. Therefore, the mass ratio of polymer to zeolite in this study has been kept constant and equal to 2. The reaction was carried out in a semibatch Pyrex reactor. The experimental apparatus for catalytic degradation of polymers was the same as that used in previous work.1 It consisted of the semibatch reactor, heated by a furnace, with a programmable temperature controller. The heating rate in all experiments of this study was 5 K/min, with 633 K being the final reactor temperature. Nitrogen was purged through the reactor at 50 mLN/ min, controlled by a mass flow controller, to remove the volatile reaction products from the reactor. Liquid products were collected in a cooling trap placed in an ice bath (273 K) and connected to the reactor outlet. Liquid products were sampled and weighed at different reaction times using a syringe. The liquid products were analyzed by a gas chromatograph coupled to a mass spectrometer (GC-MS) (ATI-Unicam Automass 120, Quadrupole), using a CP-Sil PONA fused silica capillary column (100-m × 0.25-mm i.d.). Gaseous products were collected in valved Tedlar gas bags, every 10 min. The gas products were analyzed by a gas chromatograph (GC) (Varian 3400) equipped with thermal conductivity (TCD) and flame ionization (FID) detectors, using a PLOT-Al2O3/KCl (50-m × 0.32-mm i.d.) fused silica capillary column. A calibration cylinder containing 1% C1-C5 hydrocarbons in nitrogen (Linde Gas Ltd, U.K.) was used to help identify and quantify the gaseous products.
10.1021/ie990512q CCC: $19.00 © 2000 American Chemical Society Published on Web 03/25/2000
Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000 1199 Table 1. Conversion and Selectivity to the Liquid Fraction of the Catalytic Degradation of High-Density Polyethylene on Different Zeolites; Overall Reaction Time: 160 min zeolite
Si/Al
conversion (%)
liquid selectivity (%)
US-Y Y β mordenite ZSM-5
5.7 2.5 25 20 20
92 87 94 97 99.5
50 41 48 45 39
3. Results and Discussion (a) Conversion and Selectivity to Liquid Products. The overall conversion and the selectivity to the liquid fraction for the different zeolites are listed in Table 1. The conversion to volatile products was calculated as the fraction of the initial mass of polymer reacted to volatile products (gas or liquid). The selectivity to liquid products was calculated as the mass of the liquid collected divided by the mass of reacted polymer to volatile products and represents the liquid fraction of the volatile products. No polymeric residue was observed in any of the experiments because of the long reaction times (160 min) and relatively high reaction temperature. The only residue in the reactor was coke deposited on the catalyst. The conversion values hence can be used as a measure for the coke built on the catalyst as the percentage of the original polymer (coke yield ) 1 conversion). Indeed, the very high conversion in the case of ZSM-5 reflects the very little coking occurring on such a zeolite. These values have been confirmed by gravimetric estimation of coke content. Gravimetric analysis of used ZSM-5 has confirmed that the coke amount was less than 1% of the catalyst, or less than 0.5% of the polymer, as the polymer-to-zeolite ratio was 2:1. Gravimetric estimation of coke on the used β zeolite sample gave 12.2% of catalyst, that is, 6.1% of polymer, which is in good agreement with the value calculated from the conversion, that is, 6%. As the coke molecules are quite bulky, their formation was hindered in zeolites with medium pores such as ZSM-5 (pore size, 0.54 × 0.56 nm and 0.51 × 0.55 nm).11 Furthermore, the presence of supercages (2.5 nm) in the Y and US-Y frameworks accounted for the lower conversions obtained in comparison to the β zeolite and mordenite, whose structures contain no cages and therefore less coke was formed.11 The selectivity values in our experiments were in the range of 40-50%. The highest amount of liquid was produced over US-Y and the lowest on ZSM-5. The reaction rate of the formation of gaseous and liquid fractions have been plotted against the time and the results are shown in Figures 1 and 2, respectively. From Figure 1 it becomes obvious that the gas production on the different zeolites took place at different rates. On β zeolite and ZSM-5 the gas production happened with a higher intensity than that of the liquid one. In the same zeolites the gas and liquid product formations occurred for shorter periods, indicating faster degradation. With Y zeolite the degradation started relatively early (Figures 1 and 2), but it took place at a lower rate and for a longer time, allowing more coke to be formed. (b) Product Distribution. In Table 2 the products of the catalytic degradation in gas and liquid fractions are listed, grouped in alkanes, alkenes, cycloproducts (cycloalkanes, cycloalkenes), and aromatics. Gaseous Fraction. Insignificant amounts of methane and ethane or ethene were produced with all
Figure 1. Formation rates of the gaseous fraction vs reaction time over various zeolites.
Figure 2. Formation rates of the liquid fraction vs reaction time over various zeolites. Table 2. Product Distribution of the Catalytic Degradation of High-Density Polyethylene on Different Zeolites; Overall Reaction Time: 160 min weight percent alkanes zeolite
alkenes
Si/Al gas liquid gas liquid
US-Y 5.7 Y 2.5 β 25 mordenite 20 ZSM-5 20
35 44 38 20 21
44 35 34 12 4
10 15 13 35 40
5 3 13 30 30
cycloproducts aromatics liquid
liquid
1 0 0 1 1
5 3 2 2 4
catalysts. Higher alkanes, especially isobutane and isopentane, were the main gaseous products with USY, Y, and β zeolite, whereas alkenes were the main products with mordenite and ZSM-5. Figure 3 shows the change of the mole fractions of C4-C5 hydrocarbons in the gas fraction with time on the Y zeolite, as a typical run with large-pore zeolites. The amounts of alkenes were lower than those of alkanes. Between the alkanes, isoparaffins were formed in higher amounts than those of their normal isomers. This fact meant that the octane number of the mixture was high, as isoparaffins show a considerably higher octane number than normal paraffins. With mordenite and ZSM-5, alkenes were the major gaseous products. A typical run of the mole fractions of C3-C5 hydrocarbons in the gas fraction is plotted in Figure 4 for the case of ZSM-5. For mordenite the total ratio of butenes to butanes was 1.6 and pentenes to pentanes 2.0. For ZSM-5 the total ratio of olefins to paraffins with the same carbon number was even higher: 1.9 for C4 and 3.5 for C5. In the product distribution between alkanes, with mordenite, isobutane
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Figure 3. Mole fractions of C4-C5 gas products vs time on Y zeolite.
Figure 4. Mole fractions of C3-C5 gas products vs time on ZSM5.
Figure 5. Carbon number distribution of liquid alkanes on various zeolites.
Figure 6. Carbon number distribution of liquid alkenes on various zeolites.
and isopentane were still the major products, while with ZSM-5, propane was produced in the highest amount (Figure 4). Liquid Fraction. The liquid samples collected in the cooling trap were analyzed by GC-MS using a PONA capillary column and the components have been grouped in alkanes, alkenes, cycloproducts (cycloalkanes and cycloalkenes), and aromatics. Figures 5 and 6 present the carbon number distribution of the liquid alkanes and alkenes respectively for each zeolite of this study. The
concentration is expressed as the weight percentage of the total liquid amount in each case. The total percentages of each of these groups on each zeolite are presented in Table 2. Alkanes were the main liquid product group on the Y-type large-pore zeolites (US-Y and Y) with significantly smaller amounts of alkenes and aromatics. Cycloproducts were present in very small amounts. The distribution of the alkanes showed a peak at the octanes and the heaviest paraffin found was C15. The percentage of the heavier alkanes was significantly lower with the Y zeolite than that with US-Y. On the third large-pore zeolite (β zeolite), alkanes were predominantly the major products, but significantly higher amounts of alkenes than Y-type zeolites were produced. Not too surprisingly, the liquid product distribution with mordenite and ZSM-5 was completely different, as was the one with the gas products. Alkenes were the main liquid products with a peak at C6. The difference between alkenes and alkanes was higher with ZSM-5 than with mordenite. The product range was narrower and the products were lighter than those over the largepore catalysts. Less than 2% of the products were heavier than C10 and octanes predominated in the alkane fraction. From polyethylene cracking we would expect alkenes to be formed in a high percentage. With US-Y and Y zeolite, however, the product distribution showed alkanes to be formed in excess. The reason for this could be the strong adsorption of olefins on zeolites. Hence, alkenes were likely to stay longer in the zeolitic structure, undergo secondary reactions, and only a small amount exit the zeolitic structure. In our noncontinuous experimental setup, this effect was profound. Over β zeolite, a zeolite with a higher silicon content, olefins were less strongly adsorbed, undergoing therefore further reactions to a lesser extent. This was reflected in their higher percentage in the product distribution. This trend was confirmed with mordenite and ZSM-5. ZSM-5 belongs to a zeolite structure family known for its hydrophobicity and generally weak adsorption of polar molecules. This zeolite showed the highest amount of olefins between the catalysts of this study. In addition to that, secondary bimolecular reactions are sterically hindered in medium-pore zeolites,12-16 like mordenite and ZSM-5. Therefore, these zeolites showed significantly higher amounts of alkenes than the β zeolite. The distribution of the volatile products was also reflected on the amount of coke formed. Over ZSM-5, where practically no coke was formed, the volatile products showed a lower degree of bond saturation. In the cases of significant amounts of coke, coke was the component that carried a lot of hydrogen deficiency, allowing volatile products to have a higher degree of saturation. Excess of paraffins over olefins has been reported by other studies9,10 and hydrogen redistribution seems to be the key for explaining this.12 In the cases where alkenes are strongly adsorbed and/or secondary hydrogen transfer reactions are favored, a high percentage of them was converted to coke. Clear trends were observed with the different pore structures. The heaviest hydrocarbons were formed with large-pore zeolites (Y, US-Y, β). The presence of large supercages in the Y-type structures should account for this as well as for the higher coking. Significantly lighter products have been formed with medium-pore zeolites (mordenite, ZSM-5), with ZSM-5
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smaller the pore channels, the stronger the sterically hindered bimolecular secondary reactions. This effect is shown when comparing the zeolite structures with high silicon content, β, mordenite, and ZSM-5. Alkenes had a higher chance to react further in the order β > mordenite > ZSM-5 and hence the observed product distribution. A comparison of the carbon number distribution of the total products (Figure 7) highlights the shift to lighter products when medium-pore zeolites, mordenite and ZSM-5, were used, in the following order: Figure 7. Total carbon number distribution on different zeolites.
(smallest pores) producing the lightest hydrocarbons. The order was the following:
(light products) ZSM-5 < mordenite < β < Y < US-Y (heavy products) Total Product Distribution. For the overall product distribution the results of the liquid and gas analysis had been combined. If ml is the total mass of liquid collected (estimated by weighing the liquid samples), mg is the total mass of gas collected (calculated from the composition of the gas samples and taking as reference the mass of inert nitrogen in the gas collection bag), cli is the mass fraction of compound i in the liquid fraction, and cgi is the mass fraction of compound i in the gas fraction, then the overall mass fraction of compound i, ctot i , is given by the equation g l ctot i ) (ci mg + ciml)/(mg + ml)
The total carbon number distributions are plotted in Figure 7. C3 hydrocarbons were the lightest products formed, with especially high amounts over ZSM-5. The same zeolite generated the lightest products. The heaviest hydrocarbons detected in any zeolite were C15. With US-Y, 79% of the total products were alkanes (Table 2) against 15% of alkenes, whose majority was collected in the gaseous fraction. Pentanes were the largest product group. Zeolites Y and β produced similar pictures in the total product distribution: 79% alkanes and 18% alkenes on Y zeolite; 72% alkanes and 26% alkenes on β zeolite. In all these three cases there was a very steep increase in the percentage from C3 to C4 alkanes, a peak of the distribution curve at C4-C5, and a gradual decrease on the right-hand side of the peak. In the cases of Y and β zeolites the distribution was sharper than that on US-Y, indicating that considerably lower amounts of the heavy products were formed. Besides the micropore zeolitic structure, US-Y zeolites have a secondary mesopore structure generated during the dealumination process.17 The formation of larger hydrocarbon molecules on US-Y could be attributed to these larger pores. In the experiments using mordenite and ZSM-5 we observed a shift to lighter products. The main group of products in both cases were alkenes: 65% over mordenite and 70% over ZSM-5, with pentenes being the largest component group over both catalysts. The reason for this product distribution pattern is the degree of secondary reactions occurring as explained above. In the large-pore zeolites, US-Y, Y, and β, olefins were able to undergo secondary reactions, whereas in the medium-pore zeolites, mordenite and especially ZSM-5, these reactions were limited sterically. The
(heavier products) US-Y > Y > β > mordenite > ZSM-5 (lighter products) Polyethylene molecules are very large and it is reasonable to assume that they cannot enter the zeolite micropores where the majority of the active acid sites are. The reaction pattern proposed for catalytic degradation of polymers on zeolites is as follows. The polymer macromolecules react on the active sites on the external surface of the zeolite crystallites. The products of this initial degradation, which are small enough to enter the zeolitic pores, diffuse into the zeolite crystals and react further on the internal active centers. The porous structure of the zeolite is very important for this process and the smaller the pore size, the smaller the product molecules tend to be (Figure 7). This mechanism is supported by previous research.1 Zeolites US-Y and Y have exactly the same structure. In the US-Y framework part of the Al has been extracted out of the framework, but still remains in a mesoporous system formed by the process. This way, very strong acidic sites are formed in US-Y.17 The strength and density of acid sites influence the hydrogen transfer reactions12,13,15,16 and that should explain the differences between the results obtained by the two catalysts. 4. Conclusions The product distribution of the catalytic degradation of high-density polyethylene on different zeolites showed the following characteristics: (1) Hydrocarbons from C3 to C15 were detected. (2) The structure of the zeolitic framework has shown a significant influence on the product distribution. (3) Alkanes were the major products with US-Y, Y, and β zeolites, whereas alkenes were the major products with mordenite and ZSM-5. (4) The following order was found in bond saturation degree:
(more alkanes) US-Y > Y > β > mordenite > ZSM-5 (more alkenes) (5) The majority of alkanes were isoparaffins, having a high octane number; this speaks for an increased fuel quality. (6) A small amount of cycloproducts and aromatics were formed on all samples. This is an important advantage of the catalytic degradation, as environmental concern about aromatics grows and strict legislation for low levels of aromatics in fuels is under discussion. (7) There was a tendency to produce heavier products from ZSM-5 to US-Y, in the following order:
(lighter products) ZSM-5 < mordenite < β < Y < US-Y (heavier products)
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Future work should investigate other significant factors such as acid strength distribution and crystallite size in the same zeolite structure. Both factors are believed to be important. Acknowledgment This work was supported by the EPSRC (Engineering and Physical Sciences Research Council), Process Engineering Program, Swinton, U.K. (GR/J10730). BASF AG kindly provided the unstabilized high-density polyethylene sample. 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, and Temperature Effects. Ind. Eng. Chem. Res. 2000, 39, 1203. (2) Uemichi, Y.; Kashiwaya, Y.; Tsukidate, M.; Ayame, A.; Kanoh, H. Product Distribution in Degradation of Polypropylene over Silica-Alumina and CaX Zeolite Catalysts Bull. Chem. Soc. Jpn. 1983, 56, 2768. (3) Audisio, G.; Bertini, F.; Beltrame, P. L.; Carniti, P. Catalytic Degradation of Polyolefins. Makromol. Chem.-Macromol. Symp. 1992, 57, 191. (4) 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. (5) 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. (6) Shabtai, J.; Xiao, X.; Zmierczak, W. DepolymerizationLiquefaction of Plastics and Rubbers. 1. Polyethylene, Polypropylene, and Polybutadiene. Energy Fuels 1997, 11, 76. (7) 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. (8) 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. (9) Sharratt, P. N.; Lin, Y.-H.; Garforth, A. A.; Dwyer, J. Investigation of the Catalytic Pyrolysis of High-Density Polyethylene over a HZSM-5 Catalyst in a Laboratory Fluidised-Bed Reactor. Ind. Eng. Chem. Res. 1997, 36, 5118. (10) Garforth, A. A.; Lin, Y.-H.; Sharratt, P. N.; Dwyer, J. Production of Hydrocarbons by Catalytic Degradation of HighDensity Polyethylene in a Laboratory Fluidised-Bed Reactor. Appl. Catal. A 1998, 169, 331. (11) Guisnet, M.; Magnoux, P. Coking and Deactivation of Zeolites. Influence of the Pore Structure. Appl. Catal. 1989, 54, 1. (12) Dwyer, J.; Rawlence, D. J. Fluid Catalytic Cracking: Chemistry. Catal. Today 1993, 18, 487. (13) Abbot, J.; Wojciechowski, B. W. Hydrogen Transfer Reactions in the catalytic cracking of paraffins. J. Catal. 1987, 107, 451. (14) Abbot, J.; Wojciechowski, B. W. The Effect of Temperature on the Product Distribution and Kinetics of Reactions of NormalHexadecane on H-Y Zeolites. J. Catal. 1988, 109, 274. (15) 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. (16) Wojciechowski, B. W. The Reaction Mechanism of Catalytic Cracking: Quantifying Activity, Selectivity, and Catalyst Decay Catal. Rev.-Sci. Eng. 1996, 38, 101. (17) Beyerlein, R. A.; Choi Feng, C.; Hall, J. B.; Huggins, B. J.; Ray, G. J. Effect of Steaming on the Defect Structure and Acid Catalysis of Protonated Zeolites. Top. Catal. 1997, 4, 27.
Received for review July 13, 1999 Revised manuscript received December 20, 1999 Accepted January 11, 2000 IE990512Q
