Production of Aromatic Hydrocarbons by Catalytic Degradation of

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Production of Aromatic Hydrocarbons by Catalytic Degradation of Polyolefins over H-Gallosilicate Kazuhiko Takuma, Yoshio Uemichi,* Masatoshi Sugioka, and Akimi Ayame Department of Applied Chemistry, Muroran Institute of Technology, Mizumoto, Muroran 050-8585, Japan

Low- and high-density polyethylene and polypropylene have been degraded in a fixed-bed flow reactor system with and without H-gallosilicate catalyst at 375-550 °C to investigate the product distribution and the catalyst stability. The thermal degradation of the polyolefins mainly produced waxy hydrocarbons, with the yield largely depending on the polymer type. On the other hand, the catalytic degradation over the gallosilicate yielded lighter hydrocarbon mixtures that were rich in valuable aromatic components, mostly benzene, toluene, and xylenes. The product distribution was influenced little by the structure of the polymers to be degraded. This can be explained by a mechanism involving frequent skeletal isomerization of the decomposed fragments. The unsaturated fragments, which were the most abundant and thereby the most important reaction intermediates, rapidly isomerized on the acidic gallosilicate, and the resulting isomers were distributed in thermodynamically equilibrated concentrations. The catalytic degradation of polyolefins thus proceeds through similar intermediates regardless of the structure of the degrading polymers, leading to almost the same product distributions. The gallosilicate exhibited a stable catalytic activity for the degradation of polyolefins when reused, because of a very low yield of coke deposited on the catalyst surface. Introduction Waste plastics are increasingly generated all over the world and have mostly been disposed of by landfill and incineration. However, they are now not the management methods that can gain social acceptance for environmentally serious reasons such as low biodegradability, lack of available space for landfilling, and emission of toxic chemicals. It is, hence, desired to develop an alternative technology to recycle or reuse plastic waste. Plastics can be recycled by three different methods: mechanical (primary) recycling, energy recovery (secondary recycling), and chemical (tertiary or feedstock) recycling. Because mechanical recycling by simply remelting and shaping waste plastics usually results in a low-quality product, its application is highly limited. Energy recovery through combustion means a loss of potential for use as resources and may contribute to an environmental pollution caused by the generation of harmful materials. Because of these difficulties, chemical recycling that converts the plastics into fundamental chemicals or fuels is currently growing in importance as a promising solution to solve the problems relating to waste plastics. An effective way to decompose plastic waste depends, in principle, on the type of the plastics to be recycled. Condensation polymers such as poly(ethylene terephthalate) (PET) and nylon can be broken down into their monomer units by several depolymerization processes (pyrolysis, hydrolysis, methanolysis, and glycolysis).1-3 Unfortunately, vinyl polymers such as polyolefins are quite difficult to decompose into monomers because of random scission of the carbon-carbon bonds of the polymer chains, and therefore the thermal degradation of polyolefins produces many kinds of

components with broadly comparable yields. Such a mixture of hydrocarbons can be used as fuel, but its quality is low. To improve the product quality, catalytic cracking and reforming have been investigated extensively. Successful results have been obtained by using solid catalysts such as silica-alumina,4-6 HZSM-57-10 and REY11-13 zeolites, carbon-based materials,14 and mesoporous materials,15-17 which can effectively convert polyolefins into liquid fuels. The fuel recovery from plastic waste is regarded as a significant option available nowadays. The degradation of polyolefins into petrochemical raw materials with activated carbon catalysts was reported in the 1980s.18,19 Recently, we have developed a new technology for the chemical recycling of low-density polyethylene (LDPE) by selective conversion into aromatic hydrocarbons using H-gallosilicate.20 Furthermore, the effect of the reactor type on the product composition21 and the reaction mechanism of aromatic formation22 have also been investigated. It has thus become possible to reuse polyethylene waste chemically. High-density polyethylene (HDPE) and polypropylene (PP) are also major constituents of plastic waste, as well as LDPE, and waste streams usually contain their mixtures. Hence, it is of great significance to study the degradation behavior of individual polyolefins and their mixtures. The objective of the present study is to extend the previous work to include studies on the catalytic degradation of structurally different polyolefins over H-gallosilicate. The yield and composition of the products and the stability of the H-gallosilicate catalyst have been examined. These work would be helpful to support the applicability of the newly developed technology for chemical recycling of polyolefins. Experimental Section

* To whom correspondence should be addressed. E-mail: [email protected]. Tel and Fax: +81-143-465724.

Materials. The polyolefin samples used in the present study were all obtained from Aldrich Chemical Co. They

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Figure 2. Yield of products obtained from thermal degradation of polyolefins at 525 °C.

Figure 1. Schematic diagram of a fixed-bed flow reactor system.

were HDPE of F ) 0.95 g/cm3, LDPE of F ) 0.915 g/cm3, and PP of F ) 0.90 g/cm3. Powdered H-gallosilicate (Si/ Ga ) 25) obtained from N. E. CHEMCAT was used as a catalyst. It was pressed into a disk, crushed, and sieved to give particle sizes ranging from 16 to 32 mesh. The catalyst thus obtained was calcined in air at 550 °C and further treated in situ in a helium stream at the same temperature for 1 h just prior to use. Procedures. The downflow tubular reactor shown in Figure 1 was used for the degradation of polyolefins. The details of the apparatus and experimental procedures have been described elsewhere.4 Briefly, the polyolefin melt, heated at 270 °C, under a helium atmosphere in the melter was pressed out continuously for 15 min by pressurized helium (0.11-0.15 MPa) into the reactor loaded with the H-gallosilicate catalyst through a capillary heated at 310 °C. The degradation was carried out at a temperature of 375-550 °C and a space time, W/F (W ) mass of catalyst and F ) mass flow rate of polyolefin), of 1-25 g of catalyst‚min/g of polyolefin. Unless otherwise noted, W/F was set at 10 g of catalyst‚min/g of polyolefin; that is, W ) 0.2 g and F ) 0.02 g/min. Helium was flowed as a carrier gas at a rate of 10 mL/min. The helium flow and the reaction temperature were further kept for 15 min after the run to purge components remaining in the reactor and on the catalyst surface. The spent catalyst was taken out of the reactor at room temperature, and the amount of coke deposited on the catalyst surface was determined from the increase in the catalyst weight before and after the reaction. The catalyst was repacked into the cleaned reactor for the subsequent run. The degradation reaction for 15 min was thus repeated to investigate the deactivation behavior of the catalyst. Glass beads of 2 mm diameter instead of catalyst were packed in the reactor in thermal degradation of polyolefins. The volatile products were first collected in a Ushaped trap cooled at -196 °C; after the experiment, the gaseous fraction was separated from the liquid at 0 °C, collected in a Teflon gas sample bag, and measured by a calibrated syringe, while the liquid fraction col-

lected in the trap was measured by weighing. In the thermal degradation, waxy or greaselike compounds, which adhered to the inner wall of the outlet of the reactor, were produced together with gaseous and liquid fractions. The wax was also measured by weighing. Product Analysis. The gaseous and liquid samples were analyzed on a Shimadzu GC-17A chromatograph equipped with a flame ionization detector and a 60 m OV-1 capillary column. The identification of the products was performed on a GC-MS spectrometer (HP 5989B). The degradation products were classified into gas (C1-C4 hydrocarbons), liquid aliphatics (C5 and higher fractions except wax), aromatics, wax, and coke. The gas and liquid yields were corrected by adding C4 components dissolved in the liquid sample to the gas fraction and C5 and C6 components found in the gas sample to the liquid fraction. Results and Discussion Thermal Degradation Products. The yields of products obtained from the thermal degradation of polyolefins at 525 °C are shown in Figure 2. For all of the polyolefins, the products almost exclusively consisted of aliphatic hydrocarbons, and aromatics were detected only in trace quantities. The yield of the products largely depended on the structure of the degrading polymers, although wax was always produced dominantly because of very low degradation rates under the present reaction conditions. The more the branching on the polymer backbone, the higher were the yields of lighter hydrocarbons. This can be explained on the basis of thermal stability of the polyolefins. HDPE having the least branches was most stable to give the lowest gas and liquid yields, while the highest yields were shown by PP, which is the most branching and thereby the most reactive. The distributions of the gaseous and liquid products as a function of carbon number are shown in Figure 3. The thermal degradation of both polyethylenes produced hydrocarbon mixtures that were mostly composed of linear components and were distributed in wide ranges of carbon numbers with comparative yields, indicating a very low selective degradation. PP thermally decomposed into branched hydrocarbons, the distribution of which quite differed from that obtained from HDPE or LDPE, and showed peaks at carbon numbers of 3, 5, 9, 12, 15, and 18. It is obvious that any fraction cannot be recovered with a high yield from the thermal degrada-

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Figure 3. Carbon number distributions of the gaseous and liquid products obtained from thermal degradation of polyolefins at 525 °C.

Figure 5. Composition of aromatic hydrocarbons produced from catalytic degradation of polyolefins at 400 and 525 °C.

Figure 4. Yield of products obtained from catalytic degradation of polyolefins at 400 and 525 °C.

tion of polyolefins around 500 °C. Moreover, we can conclude that the product distribution obtained from thermal degradation of mixed polyolefins is significantly influenced by the composition of the mixtures, as pointed out already.23 Because plastics in municipal solid waste usually exist as mixtures, although industrial plastic waste is predominantly of a single type, it is difficult to predict a likely composition of their derived products. Such products may only be used as low-quality fuels. Catalytic Degradation Products. Figure 4 shows the yields of products derived from degradation of polyolefins over the gallosilicate at 400 and 525 °C. The catalyst greatly promoted the degradation of polyolefins, mostly giving gas and liquid fractions with negligible yields of wax and very low coke yields. The product distributions were quite different from those obtained thermally and were characterized by highly selective formation of aromatic hydrocarbons with total yields of 64-69 wt %. Moreover, the individual polyolefins and their mixture produced similar yield patterns, despite

their different carbon structures. It is not significant that there was a very slight difference in the yield of products obtained at 525 °C, while the yield was almost the same at a low temperature of 400 °C. Figure 5 shows that benzene, toluene, and xylenes (hereafter abbreviated to BTX) accounted for most of the aromatics produced, and the aromatic composition was independent of the types of polyolefins but was dependent on the temperature. The detailed effect of the reaction temperature on the composition of the products obtained from PP degradation over H-gallosilicate is shown in Figure 6, where the products were subdivided into several constituents. It is found that the higher the temperature, the higher the yield of BTX and the lower the other aromatics yield, suggesting that, upon an increase in the temperature, dehydrocyclization occurred more frequently and the resulting aromatics reacted further through dealkylation and/or transalkylation to yield lighter aromatics. The C5 and higher aliphatics were produced in negligible amounts above 500 °C, and hence the fraction recovered as liquid at higher temperatures showed a very simplified composition mostly consisting of BTX. The disappearance of liquid aliphatics leads to an easy operation of the process for BTX purification. Similar results were also observed for LDPE and HDPE degradation. The carbon number distributions of the catalytic degradation products obtained at 400 and 525 °C are shown in parts a and b of Figure 7, respectively. Again, the distributions at the higher temperature were similar, and those at the lower temperature almost completely overlapped each other. The degradation products were distributed in carbon atom numbers of less than 14, with two maxima at 4 and 8 at 400 °C and 3 and 7 at 525 °C. It is not surprising that the yields of C4 and C5 fractions decreased at the higher temperature without a major increase in the selectivity toward C1-C3 fractions, because the decrease was responsible for their conversion into aromatics.22

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Figure 6. Effect of reaction temperature on the yield of products in catalytic degradation of PP.

From the detailed comparison of the yield and composition of the products, the structure of the degrading polyolefin was found to have no essential effect on the product distribution observed with the H-gallosilicate catalyst. This was further confirmed from the catalytic degradation of a mixture of LDPE and PP (1:1 by weight). The derived products were the same as those obtained from the individual plastics, as shown in Figures 4, 5, and 7b. It is therefore reasonable to expect that we can always obtain aromatics from the degradation of polyolefins with a desired yield and composition, even if the types of polyolefins and the composition of their mixtures are changed, which are the cases usually observed for the actual waste plastics. The catalytic degradation of polyolefins using H-gallosilicate thus overcomes several disadvantages observed in the thermal degradation. This is highly preferred to industrialize the process. Reaction Mechanism. To clarify the reaction mechanism of how the three polyolefins with different structures behave quite similarly in the catalytic degradation over H-gallosilicate, some experiments were carried out at 525 °C and W/F values of 1, 2, 10, 15, and 25 g of catalyst‚min/g of polyolefin. The effect of W/F on the product distribution for PP degradation is shown in Figure 8. The yield of aromatics steeply increased in the lower W/F range, while that of liquid aliphatics decreased with increasing W/F. These changes were due to the transformation of the aliphatics into aromatics. A further gradual increase in the yield of aromatics with W/F was primarily responsible for the oligomerization and subsequent aromatization of the C4 fractions. The formation of aromatics from gaseous components occurred above 450 °C.22 Similar W/F dependences were also observed in the degradation of LDPE and HDPE. Table 1 shows the distributions of butane, butene, xylene, and trimethylbenzene isomers obtained at W/F values of 1 and 10 g of catalyst‚min/g of polyolefin,

Figure 7. Carbon number distributions of the gaseous and liquid products obtained from catalytic degradation of polyolefins at 400 and 525 °C.

together with their thermodynamically equilibrated concentrations.24-26 The isomers were checked because of their sufficient formation to be analyzed, although butenes and xylenes were not separated completely. The percentages of n-butane and isobutane depended on W/F and polyolefin type and were not in agreement with the thermodynamic equilibrium values, suggesting that the skeletal isomerization of paraffins proceeded slowly. Butenes were always distributed at the equilibrium concentrations. This is reasonable, because olefins are well-known to be highly reactive on acidic catalyst to undergo skeletal and double-bond isomerization very rapidly. Xylene isomers were detected at the same percentages as the equilibrium ones in the degradation of both the polyolefins at a W/F of 10 g of catalyst‚min/g of polymer but not at the low W/F of 1 g of catalyst‚ min/g of polymer. This suggests that the isomerization of xylenes is slower than that of butenes but is faster than that of butanes. The fact that trimethylbenzene isomers were never obtained in equilibrium concentrations indicates that their isomerization proceeded under shape-selective catalysis by the gallosilicate; the percentage of the 1,2,4 isomer, the dimension of which is the smallest, in trimethylbenzenes exceeded the equilibrium value. On the basis of the above results, the degradation of polyolefins can be explained by postulating a frequent

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Table 1. Isomer Distributions of the Products at 525 °C percentage isomer isobutane n-butane 1-butene and isobutene trans-2-butene cis-2-butene m-xylene and p-xylene o-xylene 1,3,5-trimethylbenzene 1,2,4-trimethylbenzene 1,2,3-trimethylbenzene a

thermal LDPE PP 0 100 84 9 7

W/Fa )

1

LDPE

PP

74 26 61 23 16 80 20 4 93 3

76 24 61 23 16 80 20 4 93 3

47 53 100 0 0

W/Fa ) 10 LDPE PP 40 60 62 22 16 76 24 7 88 5

48 52 62 22 16 76 24 9 85 6

thermodynamic equilibrium 32 68 58 25 17 77 23 22 68 10

Units of g of catalyst‚min/g of polyolefin.

Figure 8. Effect of W/F on the yield of products obtained from catalytic degradation of PP at 525 °C.

occurrence of skeletal isomerization of decomposed fragments. The participation of the external acid sites of the gallosilicate in the initiation reaction may not be ignored. However, we believe that the primary decomposed fragments are mostly those which are formed thermally, because of the diffusional limitation of bulky reactant polymers to the acid sites of the micropore catalyst and because the reaction conditions are of sufficient severity to undergo radical initiation. The resulting decomposed fragments of LDPE and PP are different from each other in structure; mainly linear olefins are from the former and branched ones from the latter. The olefinic intermediates readily undergo skeletal isomerization whose equilibrium state is reached very quickly on a sufficiently acidic catalyst such as H-gallosilicate before their further cracking into lighter hydrocarbons and/or dehydrocyclization into aromatics. By the successive occurrence of the isomerizationcracking/aromatization steps, the products obtained from LDPE and PP degradation become progressively similar in composition and finally show no distinct difference. Because the isomerization reaction is favored at lower temperatures, it is reasonable that a more complete coincidence among the product distributions from the degradation of different polyolefins was ob-

Figure 9. Changes in the yield of products with cumulative time on stream in catalytic degradation of LDPE at 525 °C.

served at the low temperature of 400 °C rather than 525 °C, as mentioned above. Activated carbon-supported metals can catalyze the conversion of polyolefins into aromatic hydrocarbons, but the products derived from LDPE and PP degradation showed quite different compositions because of little occurrence of skeletal isomerization on the nonacidic carbon catalysts.19 Stability of the Catalyst. From a practical point of view, it is of great importance to examine the stability of the H-gallosilicate catalyst during polyolefin degradation. It was studied by repeating LDPE degradation for 15 min using the same catalyst. Figure 9 shows the changes in the yields of products with cumulative time on stream. The product yields were almost constant except for the initial period of the reaction, during which the aromatics yield slightly decreased. The product distributions obtained at different times on stream of 15, 30, and 180 min are shown in Figure 10 as a function of the carbon number. No essential change in the distribution was again observed after 15 min. The gallosilicate was thus found to be highly stable for the conversion of LDPE to aromatics. We previously examined the deactivation behavior of various types of catalysts in the LDPE degradation and reported that

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of Education, Science, Sports and Culture, Japan, through a Grant-in-Aid for Scientific Research (No. 12650772). Literature Cited

Figure 10. Carbon number distributions of products obtained from catalytic degradation of LDPE at 525 °C and different times on stream.

HZSM-5 showed no significant deactivation due to very little deposition of coke.4 The small-pore structure of HZSM-5 resists the formation of polyaromatics that are considered to be coke precursors. This is believed to be true for gallosilicate, which belongs to the same ZSM-5 families. Conclusions The catalytic degradation of LDPE, HDPE, and PP over H-gallosilicate yielded gaseous and liquid products selectively even under the reaction conditions that led to a major formation of wax in the absence of catalyst. The liquids obtained were mainly aromatic hydrocarbons dominated by benzene, toluene, and xylenes (BTX), and their yields increased as the reaction temperature was elevated. Almost the same product distributions were observed in the degradation of all of the polyolefins and their mixture despite the different structures of the polymers to be degraded. This indicates that the new technology to recycle waste plastics by using the Hgallosilicate catalyst can be applied to the degradation of not only industrial plastic waste of a single type but also polyolefin mixtures such as plastics in municipal solid waste. No significant influence of the polymer structure on the product composition was explained by a mechanism involving skeletal isomerization of the decomposed fragments. A frequent occurrence of the isomerization gave similar intermediates, from which similar products were formed. The gallosilicate showed a stable catalytic activity for the production of aromatics from polyolefin degradation when time on stream was prolonged. The wide applicability of the gallium catalyst to the degradation of various single polyolefins and their mixtures and its excellent lifetime performance enhance the potential benefit of the recently developed technology to recycle polyolefinic waste chemically by conversion into aromatic hydrocarbons. Acknowledgment The authors greatly acknowledge the financial support by the Hokkaido Foundation for The Promotion of Scientific and Industrial Technology and by the Ministry

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Received for review July 6, 2000 Revised manuscript received November 27, 2000 Accepted November 28, 2000

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