ABSTRACT. KEYWORDS Polypropylene, pyrolysis, superacid, cracking INTRODUCTION. THAILAND E_mail:

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1 Catalytic Pyrolysis of Polypropylene Waste Film Using Sulfated Zirconia as Catalysts On-line Number 66 Janthima Supphanam, Sirirat Jitkarnka *, and Sujitra Wongkasemjit* * The Petroleum and Petrochemical College, Chulalongkorn University, Patumwan, Bangkok 133, THAILAND E_mail: sirirat.j@chula.ac.th ABSTRACT Plastic bags are greatly involved in modern lifestyle, especially in food and goods packaging, creating environmental concerns on waste treatment due to their low biodegrability. A large number of plastic bags with other municipal wastes are landfilled and burned into atmosphere. Pyrolysis of these plastics is an alternative for utilizing plastic waste because it can produce valuable raw materials that can be used in petroleum and petrochemical industries. Due to high energy consumption, catalysts are also employed in plastic pyrolysis to reduce costs of operation. The pyrolysis of commercial polypropylene film was studied in a semi-batch reactor. The reaction was carried out at 5 C for 1 hour under nitrogen flow. Sulfated zirconia was employed as a catalyst. The result showed that the cracking activity increased with the catalyst to polymer ratio and the amount of sulfate loading. Liquid product was the most dominant pyrolyzed product. Moreover, the addition of catalysts resulted in gasoline production, and the acidity caused higher kerosene and gas oil production. KEYWORDS Polypropylene, pyrolysis, superacid, cracking INTRODUCTION Polypropylene (PP) is one of the most widely used plastics and has become indispensable materials in contemporary life. One of the most important applications of PP is film, for example, garbage bag, shopping bag, woven bag, and food packaging. An undesirable consequence of this widespread use of PP films is that it creates a lot of waste materials and then environmental problems. PP films are usually disposed by either landfilling or incineration. In both cases, the films are totally destroyed due to their small volumes which are unattractive for recycle industry. Pyrolysis, the thermal decomposition of a substance to form lower molecular weight compounds in absence of oxygen is a viable alternative to landfill and incineration for utilizing waste plastic film. In this way, plastic waste materials can be converted into monomers (Ballice, et al., 22), fuels (Uddin, et al., 1998), and valuable chemicals (Zhao, et al., 1996). Furthermore, the products of pyrolysis can also be used as a source of raw materials, replacing natural gas or petroleum. The effects of temperature (Ishihara, et al., 1993), and kinetics (Dawood and Miura, 21) on product yields and distribution have been investigated on thermal pyrolysis of PP. These studies proved their great possibility to produce chemical feed stock. However, the process was energy excessive, causing the concern about the economics of investment. Moreover, product selectivity can not be controlled and manipulated. Catalytic conversion of plastic wastes has been more attractive in the past decade. The discovered advantages of catalysts are: (1) reduction of 1

2 operation temperature (Zhao, et al., 1996), (2) acceleration of the degradation rate (Uddin, et al., 1998; Dawood and Miura, 22), (3) improvement of controllable product selectivity and yields (Uddin, et al., 1998; Cardona and Corma, 2; Dawood and Miura, 22), (4) lower range of desirable hydrocarbon products (Hwang, et al., 22; Kim, et al., 22) and (5) less residue production (Uddin, et al., 1998; Dawood and Miura, 22; Kim, et al., 22). Most researchers often used zeolites as acid catalysts, for example, ZSM-5 (Zhibo, et al., 1996), HY zeolite (Zhao, et al., 1996), HNZ (Kim, et al., 21), and FCC catalyst (Cardona and Corma, 2), in their studies on catalytic decomposition of PP. Acid-treated natural zeolites, using phosphoric and boric acids, were also examined on the pyrolysis of PP in a semi-batch reactor (Hwang, et al., 22). In contrast, no works have been performed using solid superacid catalyst to study the effect on the degradation process of polypropylene. In this study, solid superacid catalysts were investigated for the influence of the catalyst on the pyrolysis of PP films. MATERIALS AND METHODS Commercial polypropylene films were purchased from B.C. TRADING LTD. PART. Zirconium oxide purchased from Riedel-deHaën. was used as a catalyst and catalyst supports. Ammonium sulfate was purchased from Asia Pacific Specialty Chemical Limited. Pyrolyzed gas and liquid products were characterized via Gas Chromatography (GC) equipped with FID. Small pieces (approx.1mm 2 ) of.5 gram PP film sample were loaded into a semi-batch reactor with or without catalyst. In a typical run, after the reactor was set up, air remaining in the reactor was purged with nitrogen gas at a flow rate of 3 ml/min for half an hour. The reactor was then heated from room temperature to 5 C at a heating rate of 1 C/min held at 5 C for 1 hour. (See the reactor and reactor system in Figure 1). The degradation products were classified into three groups: gases (products which were not condensable at ice-nacl cooling temperature), liquid hydrocarbons and residues. The amount of gaseous products was calculated by subtracting the weight of liquid products, residues and catalyst from the total weight of PP film sample and fresh catalyst initially loaded to the reactor. The derived oils from four glass condensers were bulked and diluted with a suitable solvent, CS 2, to a Nitrogen P-I known concentration, with an approximate ratio of 1:1, and then analyzed by Varian CP-38 SIMDIST GC, equipped with FID, using a 15 m.25 mm.25 µm WCOT fused silica capillary. The oven temperature program, according to ASTM D 2887, was:.1 min at 3 o C; ramp rate 2 o min -1 ; 8.5 min at 32 o C. Some selected pyrolysis experiments were repeated to ensure reproducibility. The solid superacids SO 4 2- /ZrO 2 was easily prepared by impregnating ZrO 2 with (NH 4 ) 2 SO 4 solution followed by calcination at 55 C for 2 hours in a furnace. The amount of SO 4 2- in SO 4 2- /ZrO 2 was varied from to 8 wt%. P-I T-E Sample Heating Blocks and Heater T-E T-C Heating Wire T-C Ice-NaCl Condenser Gas Cylinder Screw-down valve filter T-E T-C Gas sampling Bag Figure 1. Schematic diagram of reactor system. Check val Mass flow controller Thermoco Temperatu Controlle 2

3 RESULTS AND DISCUSSION Effect of Catalyst to Polymer Ratio on Pyrolyzed Products The catalyst employed in this section was 4% SO 4 2- /ZrO 2. A total of.5 gram of polypropylene (PP) film sample and catalyst were mixed together and loaded at the bottom of the reactor. The ratio of catalyst to polymer (C/P) was varied from. to 1.. Product yield Product yield from pyrolysis of commercial PP film with 4% SO 4 2- /ZrO 2 at various catalyst to polymer (C/P) ratios are shown in Figure 2. No residue production is observed. The liquid fraction is the most dominant degradation products. Moreover, it is found that the gas yield decreases and the liquid yield increases as the C/P ratio increases from approximately 5 wt.% (C/P =.) to 71 wt.% (C/P = 1.). Product Yield (%wt) C/P =. C/P =.17 C/P =.5 C/P = 1. Gas Product Composition Gaseous products were identified as methane, ethylene, ethane, propylene, propane, C 4, C 5 and C 6 hydrocarbons as shown in Figure 3. Propylene, C 4, C 5 and C 6 hydrocarbons are mainly produced from pyrolysis of commercial PP film at various catalyst to polymer (C/P) ratios. As the C/P ratio increases, propylene decreases from 23 wt.% to 11 wt.%, C 4 hydrocarbons increases from 7 wt.% to 24 wt.%, whereas, other gas components remain fairly constant. In 1999, using silica-alumina and ZSM-5, Sakata et al. reported that the decrease of C 3 components with the consequent increase of C 4 components in the gaseous products was a special feature of the solid acid catalyst on degradation of polypropylene. Ishihara et al., (1993) proposed the mechanism of C 3 and C 4 formation in catalytic decomposition of polypropylene in the presence of silica-alumina catalyst with 13% alumina content. They suggested that the gases were produced from the chain-ends of the liquid fractions. They also Methane Ethylene Ethane Propylene Propane C4 C5 C6 studied the effect of catalyst to polymer ratio on gas composition. The reaction was carried out at 22 C. Nitrogen was passed through the reactor at 12 ml/min for 1 hour. The polymer and catalyst were mixed % Volume Residue Gas Liquid Figure 2. Product yield from catalytic pyrolysis of commercial PP film with 4% SO 2-4 /ZrO 2 at various catalyst to polymer ratios. C/P =. C/P =.17 C/P =.5 C/P = 1. Figure 3. Gas composition from catalytic pyrolysis of commercial PP film with 4% sulfated zirconia at various catalyst to polymer ratios on the basis of volume. 3

4 together at different C/P ratios of.5-6. before being introduced into the reactor. They found that the main component was isobutane (C 4 ), and the yields of all components were essentially the same at all C/P ratios. For our case, SO 4 2- /ZrO 2 was used in the catalytic degradation at different conditions. The selectivity of C 3 (propylene) over C 4 components decreases with increasing C/P ratio as shown in Figure 4. It is known that thermal degradation occurs by radical mechanism. However, catalytic degradation of PP is known to proceed by carbenium ion mechanism (Audisio, et al., 1984). At C/P ratio of., propylene can be highly produced as compared to other ratios through radical mechanism since no catalyst was used. But when increasing the amount of catalyst, propylene tends to decrease with a consequent increase in C 4 components due to catalytic degradation according to the mechanism proposed by Ishihara, et al. (1993). Liquid Product Composition Compositions of the liquid products from pyrolysis of commercial PP film with 4% sulfated zirconia at various catalyst to polymer (C/P) ratios as compared to thermal degradation are shown in Figure 5. For thermal degradation, the liquid products are distributed over a wide range of carbon numbers (C 9 to C 49 ) equivalent to boiling point ranges of 17 to 6 C. In the case of using sulfated zirconia at the catalyst with the polymer ratio of.17, the distribution of carbon number shifts to heavier hydrocarbons. The further increase of catalyst to polymer ratio from.17 to.5 and 1. resulted in an increase of lighter hydrocarbons. Oil Fractions of Liquid Product Figure 6 shows the distribution of liquid products at various C/P ratios. The fractions are defined according to the boiling points as gasoline ( o C), kerosene ( o C), gas oil ( o C), light vacuum gas oil ( o C), and heavy vacuum gas oil ( o C). Selectivity of C 3 /C 4 % Mass C/P Ratio In the thermal degradation (C/P =.), there is no gasoline production. On the other hand, gasoline can be produced when the catalyst was introduced. It indicates that using catalyst has a positive effect on gasoline Figure 4. Selectivity ratio of C 3 /C 4 from catalytic pyrolysis of commercial PP film with 4% sulfated zirconia at various catalyst to polymer ratios Carbon Number C/P =. C/P =.17 C/P =.5 C/P = 1. Figure 5. Carbon number distribution of liquid products from pyrolysis with various catalyst to polymer ratio. 4

5 production. At C/P ratio of.5, it gives the maximum amount of gasoline as compared to other ratios. As the ratio increased, from.17 to 1., the amounts of kerosene, gas oil, and light vacuum gas oil are dramatically increased with a consequent decrease in heavy vacuum gas oil. 1 8 Gasoline Kerosene Gas oil Light vacuum gas oil Heavy vacuum gas oil % Mass C/P Ratio Figure 6. Liquid fractions from catalytic pyrolysis of commercial PP film with 4% sulfated zirconia at various catalyst to polymer ratios. Effect of Amount of Sulfate Loaded on Zirconia on Pyrolyzed Products Catalyst to polymer ratio at.17 was selected to study the effect of amount of sulfate loaded on zirconia on the catalytic pyrolysis of commercial PP film. The amount of sulfate was varied from -8 %. Product Yield Figure 7 shows the product yield obtained from the catalytic pyrolysis of commercial PP film using various amounts of sulfate loaded on zirconia. Pyrolysis in the absence of catalyst produced negligible residue, a yield of 5.3 wt. % gas and 49.7 wt. % liquid. As the amount of sulfate loaded on zirconia increases, the liquid yield increases with a consequent decrease in gas yield. Product Yield (%wt) w/o catalyst + % sulfate + 2% sulfate + 4% sulfate + 6% sulfate + 8% sulfate Residue Gas Liquid Figure 7. Product yield from catalytic pyrolysis of commercial PP film using sulfated zirconia at various amounts of sulfate loading. 5

6 Gas Product Composition Figure 8 shows the gas composition obtained from the catalytic pyrolysis of commercial PP film using various amounts of sulfate loaded on zirconia. On volume basis, it is found that propylene and C 5 hydrocarbons are dominating gases. Moreover, it is evidenced that propylene yield obtained from thermal degradation was higher than catalytic degradation, whereas C 4 components are lower, and other gas components remain fairly constant. Ethylene is in low concentrations. Furthermore, propylene decreases with increasing amount of sulfate loaded on catalyst while C 4 hydrocarbons increase. This trend was consistent with that of gas composition when increasing catalyst to polymer ratio. Liquid Product Composition In order to study the effect of amount of sulfate loaded on zirconia on pyrolyzed liquid product, the distribution of liquid components from loading with different amounts of sulfate are compared in Figure 9. The distribution of liquid products shift to lower number of carbon atoms when higher amount of sulfate was loaded. This result implies that sulfate loading enhances the degradation of heavier liquid hydrocarbons into lighter liquid hydrocarbons. Oil Fractions of Liquid Product The fractions of oils yielded from samples at various amount of sulfate loaded on zirconia are shown in Figure 1. Heavy vacuum gas oil is the most dominant degradation product. Gasoline is observed for all samples, especially at % and 8% sulfate. However, the highest amount of heavy vacuum gas oil is produced with % sulfate. The increase in the amount of sulfate loading gave higher amount of kerosene and causes a decrease in heavy fractions. Therefore, sulfate loading enhances the catalytic degradation of polypropylene film into high valuable products. % Volume % Mass w/o catalyst + % sulfate + 2% sulfate + 4% sulfate + 6% sulfate + 8% sulfate Methane Ethylene Ethane Propylene Propane C4 C5 C6 Figure 8. Gas composition from catalytic pyrolysis of commercial PP film using sulfated zirconia at various amounts of sulfate loading on the basis of volume Carbon Number + % sulfate + 2% sulfate + 4% sulfate + 6% sulfate + 8% sulfate Figure 9. Carbon number distribution of liquid products from catalytic pyrolysis of commercial PP film using sulfated zirconia at various amounts of sulfate loading. 6

7 1 8 Gasoline Kerosene Gas oil Fuel oil Heavy vacuum gas oil % Mass Sulfate Loading (wt.%) Figure 1. Liquid fractions from catalytic pyrolysis of commercial PP film using sulfated zirconia at various amounts of sulfate loading. CONCLUSIONS Catalyst to polymer (C/P) ratio was varied from. to 1. using 4% of sulfate loaded on zirconia. The result showed that an increase of C/P ratio caused a decrease of gas with a consequent increase in liquid yield especially a lighter liquid fraction. As the C/P ratio was increased, there was a decrease in selectivity of C 3 /C 4 since propylene tended to decrease with a consequent increase in C 4 components. This result showed a special feature of the solid acid catalyst degradation of polypropylene according to carbenium ion mechanism. Gasoline can be slightly produced when the catalyst was introduced. C/P ratio of.17 was selected to study the effect of amount of sulfate loaded on zirconia on pyrolysed products. As the amount of sulfate increased, the liquid yield increased with a consequent decrease in gas yield. In addition, changes of some gas components which were propylene and C 4 components were observed as similarly as in the case of various C/P ratios. In brief, the higher the acidity, the higher C 4 hydrocarbons were produced. Furthermore, kerosene and light vacuum gas oil were continuously increased as the amount of sulfate increased and caused the decrease in the heavy fraction. ACKNOWLEDGEMENTS This work was supported by THE ASAHI GLASS FOUNDATION Oversea Research Grant 23, and partially funded by Postgraduate Education and Research Programs in Petroleum and Petrochemical Technology (PPT Consortium). 7

8 REFERENCES Audisio, G., J. Silvani ; Catalytic thermal degradation of polymer : degradation of polypropylene, Journal of Analytical and Applied Pyrolysis. 7, 83-9 (1984) Ballice, L., and R. Reimert ; Classification of volatile products from the temperature-programmed pyrolysis of polypropylene (PP), atactic-polypropylene (APP) and thermogravimetrically derived kinetics of pyrolysis, Chemical Engineering and Processing, 41, (22) Cardona, S.C. and A. Corma ; Tertiary recycling of polypropylene by catalytic cracking in a semibatch stirred reactor, Applied Catalysis B: Environmental. 25, (2) Dawood, A. and K. Miura ; Catalytic pyrolysis of γ-irradiated polypropylene (PP) over HY-zeolite for enhancing the reactivity and the product selectivity, Polymer Degradation and Stability. 72, (22) Dawood, A. and K. Miura ; Pyrolysis kinetics of γ-irradiated polypropylene, Polymer Degradation and Stability. 73, (21) Hwang, E.Y., J.R. Kim, J.K. Choi, H.C. Woo, and D.W. Park ; Performance of acid treated natural zeolites in catalytic degradation of polypropylene, Journal of Analytical and Applied Pyrolysis. 62, (22) Ishihara, Y., H. Nanbu, K. Saido, T. Ikemura, T. Takesue, and T. Kuroki ; Mechanism of gas formation in catalytic decomposition of polypropylene. Fuel. 72, (1993). Kim, J.R., J.H. Yoon, and D.W. Park ; Catalytic recycling of the mixture of polypropylene and polystyrene, Polymer Degradation and Stability. 76, (22) Kim, J.R., Y.A. Kim, J.H. Yoon, D.W. Park, and H.C. Woo ; Catalytic degradation of polypropylene: effect of dealumination of clinoptilolite catalyst Polymer Degradation and Stability. 75, (22) Sakata, Y., M.A. Uddin, and A. Muto, Degradation of polyethylene and polypropylene into fuel oil by using solid acid and non-acid catalysts, Journal of Analytical and Applied Pyrolysis. 51, (1999) Uddin, M.A., Y. Sakata, A. Muto, Y. Shiraga, K. Koizumi, Y. Kanada, and K. Murata ; Catalytic degradation of polyethylene and polypropylene into liquid hydrocarbons with mesoporous silica, Microporous and Mesoporous Materials. 21, (1998) Zhao, W., S. Hasegawa, J. Fujita., F. Yoshii, T. Sasaki, K. Makuuchi, J. Sun, and S.I. Nishimoto ; Effects of zeolites on the pyrolysis of polypropylene, Polymer Degradation and Stability. 53, (1996) Zhao, W., S. Hasegawa, J. Fujita, F. Yoshii, T. Sasaki, K. Makuuchi, J. Sun, and S.I. Nishimoto ; Effects of irradiation on pyrolysis of polypropylene in the present of zeolites, Polymer Degradation and Stability. 53, (1996) Zhibo, Z., S. Nishio, Y. Morioka, A. Ueno, H. Ohkita, Y. Tochihara,, T. Mizushima,, and N. Kakuta ; Thermal and chemical of waste polymers, Catalysis Today. 29, (1996) 8