Catalytic Degradation of High-Density Polyethylene on an Ultrastable

The catalytic degradation of high-density polyethylene (hdPE) over ultrastable Y zeolite in a .... indicating that no solid-state reactions took place...
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Ind. Eng. Chem. Res. 2000, 39, 1203-1208

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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 George Manos,*,† Arthur Garforth,‡ and John Dwyer§ Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, United Kingdom, and 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 (hdPE) over ultrastable Y zeolite in a semibatch reactor was studied at different heating rates and reaction temperatures. Catalytic degradation of the polymer occurred at much lower temperatures than pure thermal degradation. When gel permeation chromatography was used to determine the molar mass distribution, it was found that solid state reactions occur only in the presence of a catalyst. These reactions change the polymer structure well before the formation of significant amounts of volatile products. The pattern of formation of gaseous and liquid products was studied and found to follow the temperature increase. After the system reached its final temperature, the reaction rate of formation of volatile products decreased rapidly. The product range was typically between C3 and C15. Isobutane and isopentane were the main gaseous products. The liquid product fraction was alkane-rich, as alkenes rapidly undergo bimolecular hydrogen transfer reactions to give alkanes as secondary products. 1. Introduction The huge amount of waste plastics gives rise to serious environmental concerns. Plastic does not degrade and remains in municipal refuse tips for decades. During the past few years recycling of plastics has been recognized as a necessity. Polymer recycling methods can be grouped as follows: (1) Mechanical reprocessing of the used plastics to form new products. This method has found very limited application, as it is not generally applicable, because of the low quality of the new products and the need for pure waste plastic streams. (2) Incineration of the plastics to recover energy. This method produces toxic gaseous compounds and only shifts a solid waste problem to one of air pollution. In many countries incineration of plastic waste is forbidden or politically unacceptable. (3) Thermal and/or catalytic degradation of plastic waste to gas and liquid products, which can be utilized as fuels or chemicals. These methods seem to be the most promising to be developed into a cost-effective commercial polymer recycling process to solve the acute environmental problem of plastic waste disposal. The products of pure thermal degradation show a wide product distribution of carbon numbers requiring further processing for their quality to be upgraded.1,2 * 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.

Catalytic degradation, on the other hand, yields a much narrower product distribution of carbon atom number with a peak at lighter hydrocarbons.3-13 In these studies acidic catalysts were used: amorphous silica-alumina,4-6 zeolites,4,6,7,11-13 zeolite-based commercial FCC catalysts,9 MCM mesoporous materials,10 and superacidic zirconia.8 This paper reports on the catalytic cracking of highdensity polyethylene (hdPE) over ultrastable Y zeolite (US-Y). The aim of this work was to perform a systematic study of the mechanism of the initial breakdown of the polymer macromolecules, the product distribution, and temperature effects. 2. Experimental Section (a) Materials. Acidic US-Y zeolite, in powder form, was used as the catalyst in this study. The framework Si/Al ratio was 5.7, estimated by solid-state MAS NMR. Unstabilized hdPE in powder form was used as the model feed. (b) Thermal Gravimetric Analysis (TGA) Equipment. Thermal analysis equipment (TA Instruments SDT 2960 simultaneous TGA-DTA) was used to study the mass change of a polymer sample with temperature in the presence of different amounts of catalyst. In these experiments hdPE powder was mixed with US-Y powder in various ratios and each mixture (ca. 5-10 mg) charged into the sample basket of the TGA equipment. Samples were subjected to a constant heating rate of 5 K/min in a flow of nitrogen (100 mLN/min). (c) Semibatch Reactor. The reaction was carried out in a semibatch Pyrex reactor. The experimental apparatus for catalytic degradation of hdPE consisted of the semibatch reactor, heated by an electric furnace, which was connected to a programmable temperature controller. The reactor was purged with nitrogen at 50

10.1021/ie990513i CCC: $19.00 © 2000 American Chemical Society Published on Web 03/25/2000

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Figure 1. TGA graphs of hdPE at various polymer-to-US-Y zeolite ratios. Heating rate, 5 K/min; nitrogen flow, 50 mLN/min.

mLN/min, determined by a mass flow controller, to remove the volatile reaction products from the reactor. At the beginning of the experiment the zeolite 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 zeolite. The polymer sample was added afterward at a mass ratio of 2:1 and the reactor was weighed again to determine the exact polymer amount. 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 heating rate to the required reaction temperature. 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. 3. Results and Discussion (a) Influence of the Polymer-to-Catalyst Ratio Using TGA. Initial experiments were carried out on different polymer-to-catalyst mass ratios using TGA equipment. The results presented in Figure 1 are derived from the original TGA curves. The mass of polymer and water was normed by dividing the polymer mass during the experimental run by the initial polymer mass. The normed mass was plotted against the tem-

perature in Figure 1. In this figure 100% represents the full polymer mass. Because the amount of zeolitic water varied with the polymer-to-catalyst ratio, the various curves in this figure start from different values above 100%. At temperatures up to 400 K the mass loss observed was due to the removal of zeolitic water. In the absence of catalyst, the polymer (hdPE) degradation pattern showed a very steep decrease at about 773 K, while in the presence of a catalyst, the polymer degradation occurred at much lower temperatures and a more gradual rate. Even with the smallest catalyst amount (polymer:catalyst ) 9:1), the degradation commenced at a much lower temperature than that in the absence of a catalyst. As more catalyst was added, the reaction proceeded at enhanced rates. At polymer-tocatalyst mass ratios of 1:2, 1:1, and 2:1 the polymer degradation curves were very similar. These results indicated the possible existence of a limiting step over the whole reaction process. It is reasonable to assume that large macromolecules had to react on the external surface of the zeolite catalyst first, which could be the limiting reaction step. Smaller cracked fragments diffused subsequently into the zeolite pores and underwent further reactions. It seemed that the addition of more zeolite above a specific quantity, corresponding to a polymer-to-catalyst ratio between 1:1 and 2:1, did not increase the overall degradation rate. The melted polymer resided in the voids of the zeolitic bed. When the amount of polymer was high (high polymer-to-catalyst ratio), the polymer filled these voids fully and the excessive polymer mass was not in contact with zeolite. The more zeolite was added, the more polymer was in contact with it and more polymer participated in the initial degradation step. This was true to a point when the added zeolite was not in contact with plastic anymore. In the last case the excessive zeolite did not contribute to the initial degradation step of the large macromolecules. (b) Initial Degradation Steps. Molar Mass Distribution. The previous TGA experiments showed that, in the absence of a catalyst, hdPE was decomposed very rapidly at around 773 K, without any weight loss at lower temperatures, indicating that no volatile products were formed until then. TGA experiments were not conclusive though on the existence of any solid-state reaction that would have changed the polymer structure, as they would not necessarily lead to mass change and would be undetectable by the TGA equipment. This question of solid-state reactions was an important one because homogeneous solid-state reactions could be the initial step in the heterogeneously catalyzed process. To clarify the mechanism of the first degradation step, homogeneous or heterogeneous, a series of experiments were carried out at relatively mild temperatures in the semibatch reactor. Each polymer sample was recovered at the end of these experiments and its molar mass distribution has been determined by gel permeation chromatography, carried out by RAPRA Technology Ltd., Shawburry, Shropshire, U.K. The results are shown in Figure 2. Curve number 1 shows the molar mass distribution of the original hdPE sample. Curves number 2 and 3 show the molar mass distribution of samples heated in the absence of a catalyst over 4 h at 413-423 K (curve number 2) and 6 h total heating time to a maximum temperature of 573 K (curve number 3). The molar mass distribution curves of the samples exposed at higher

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Figure 2. Molar mass distribution of fresh hdPE and heated hdPE in the absence and presence of US-Y zeolite.

temperatures in the absence of a catalyst were identical to the curve of the original polymer sample. This implied that the polymer structure has not been changed, indicating that no solid-state reactions took place. These experiments did not yield even trace amounts of volatile products. However, all the samples heated in the presence of the US-Y catalyst (polymer-to-catalyst mass ratio of 2:1) showed a deviation from the original polymer molar mass distribution in the region of lower molar masses. In the first experiment, the polymer/US-Y zeolite sample was exposed at 378 K for 120 min and then for 30 min at 418 K. No volatile products were initially observed, but traces of isobutane and isopentane were detected when the temperature was raised to 418 K. Although these conditions were much milder than those in the equivalent experiment with pure polymer (curve number 2), the molar mass distribution, curve number 4 in Figure 2, was different than the one of the original polymer. In the second experiment the mixture of hdPE and US-Y was exposed at significantly higher temperatures, from ambient temperature to 443 K with 5 K/min and then to 518 K with 1 K/min. The experiment was stopped when the final temperature was reached and the total experiment duration was 3 h. The sample of this experiment showed much larger deviation in the molar mass distribution, although the conditions were again milder than those at which the pure polymer was exposed (curve number 3). In this experiment the first liquid drops of hydrocarbon products (≈5 mg) were collected when the reactor temperature was 478 K. These experiments concluded that no reaction occurred in the absence of a catalyst. The presence of a

catalyst was necessary to initiate solid-state reactions that changed the polymer structure, mainly by the breaking of chains of low molar mass to smaller chains. In the catalytic process the first gaseous products, even if only in traces, were formed at about 413 K and the first liquid products slightly above 473 K. Differential thermal analysis (DTA) runs had revealed that 413 K was the melting point of the polymer sample. (c) Conversion and Selectivity. The overall conversion of the polymer to volatile products, the liquid fraction selectivity, and yield for the various reaction conditions are listed in Table 1. The conversion was calculated as the mass fraction of the original polymer sample, which 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 either unvolatilized and of high molar mass or was converted to coke. The selectivity to a 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 conversion by selectivity and represented the fraction of the original polymer sample converted to liquid products. Table 1 also shows the overall averaged reaction rate and the averaged reaction rate to liquid fraction. They have been calculated as the mass of the polymer converted to volatile products (initial mass of polymer minus final mass) and the mass of the collected liquid products, respectively, during the whole experiment divided by the dried catalyst mass and the total time of the experiment. Obviously, these reaction rates changed

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Table 1. Conversion, Selectivity to Liquid Fraction, Yield to Liquid Fraction, Averaged Reaction Rate, Overall and to Liquid Products at Various Experimental Conditions (Heating Rate and Final Temperature)

heating rate final temperature

conversion (%)

selectivity to liquids (%)

liquid yield (%)

overall averaged reaction rate (mg of polymer/ (g of catalyst min))

100 K/min to 529 K 100 K/min to 570 K 100 K/min to 649 K 10 K/min to 561 K 2 K/min to 559 K

54 75 92 76 70

47 44 50 51 50

26 33 46 39 35

6.8 9.7 12.0 9.5 8.5

Figure 3. Conversion vs time at various reaction conditions.

with time and we examine that later in this paper, but because the duration of all experiments was the same (185 min), the averaged values can be used to compare the overall performance under different conditions. As expected, the conversion increased with the reactor temperature. In a comparison of the experiments at a heating rate of 100 K/min, the following was found: the higher the final temperature, the greater the conversion of polymer to volatile products. Not too surprisingly, the conversion values were very similar in the experiments with similar final reactor temperatures (559-570 K), but different heating rates, because of the relatively long duration of the experiments. This allowed the polymer samples to be exposed at the final temperature for a long enough time. The only experiment where the polymer sample was exposed at the final temperature for considerably less time was the one with the lowest heating rate of 2 K/min. This could count for the slightly lower conversion. A run of the conversion versus time on the other side revealed a more detailed picture. Figure 3 shows the change of conversion with time under various conditions. As it was not possible to weigh the reactor during the experiment, the total mass of collected gas and liquid was used to calculate the conversion. The sum of the collected gas and liquid mass was reasonably assumed to represent the converted polymer, as mass balances of >90% were obtained. In Figure 3 the temperature effect on the conversion to volatile products is shown very clearly. In the experiment with the highest heating rate and reactor temperature, 100 K/min to 649 K, the conversion reached a plateau. In all other experiments no plateau was reached, indicating a slow catalytic degradation. Similar results were obtained from the thermogravimetric experiments, where the pure thermal degradation, although at a much higher temperature, occurred very rapidly, while the catalytic one happened more gradually. These results indicated that the homogeneous pure thermal degradation was a kinetically controlled reaction, albeit requiring much higher activation, while the catalytic one occurred at lower temperature, but was limited by a slow initial macromolecule breaking reaction step.

averaged reaction rate to liquid (mg of liquid/ (g of catalyst min)) 3.2 4.3 6.0 4.9 4.2

Figure 4. Arrhenius diagram of the reaction rate to volatile products (overall) and to liquid products.

At a heating rate of 100 K/min and reactor temperature of 570 K, a steep initial conversion increase, which slowed considerably after the final temperature was reached, was observed. Comparing this conversion run with the one at a heating rate of 10 K/min and reactor temperature of 561 K, it was noted that at the lower heating rate a much slower initial degradation led to a slightly higher final conversion level. This could be explained by taking into consideration polymer structure changes during the reaction. In this reaction system, the structure and composition of the melted polymer reactants changed with time, as the results of the previous chapter demonstrated clearly. Different heating rates might influence the pattern of such polymer structural changes. Further research is needed to explore it. Regarding the liquid selectivity, we considered its values to be practically the same at all conditions. To examine this statement more thoroughly, we compared the activation energies of the overall reaction and the formation of liquid fraction, respectively. We plotted the logarithm of the reaction rate, overall and to liquid products, respectively, against the reciprocal reactor temperature (Arrhenius diagram) for the three experiments at high heating rates of 100 K/min. The plots are shown in Figure 4 and it is obvious that both lines have almost the same slope. This indicated that both activation energies were practically the same and the selectivity to liquid products could be considered independent of the temperature. This intriguing result could be explained by the fact that at all conditions the polymer sample temperature increased from the ambient to the final temperature with a finite rate, undergoing a very similar temperature pattern at the first stage of the experiments. This initial heating stage might determine the reaction pattern, including solid-state reactions, and result in a very similar pattern of final products. This statement is supported by the following results that showed that the formation rate of products has slowed considerably after the temperature reached its final maximum value.

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Figure 5. Formation rates of the gaseous fraction vs reaction time at various conditions.

Figure 7. Typical run of mole fractions of C4-C5 gases at the following conditions: reactor temperature, 649 K; heating rate, 100 K/min. Table 2. Liquid Product Distribution of the Catalytic Degradation of High-Density Polyethylene at Various Experimental Conditions (Heating Rate and Final Temperature) mass fraction of liquid (%) conditions 100 K/min to 649 K 100 K/min to 570 K 100 K/min to 529 K 2 K/min to 559 K

Figure 6. Formation rates of the liquid fraction vs reaction time at various conditions.

Next, we examine the reaction rates at different reaction times. In Figures 5 and 6 the formation rates of gaseous and liquid fractions are respectively presented against the time. These graphs reveal clearly the pattern of gas and liquid formation. Gases were produced first, while the peak of the liquid formation rate showed a significant lag. At its peak, the liquid formation rate exceeded the gas one. The lag and the excess of the liquid formation rate were higher at higher temperatures and higher heating rates. After this the formation of both fractions slowed considerably with time. The higher the temperature, the faster the decline. The higher the reactor temperature and/or the heating rate, the faster the conversion of the plastic. This supported the conclusion drawn before that the initial stage determined the pattern of reaction and product formation. (d) Product Distribution. Gaseous Products. At all reaction conditions, insignificant amounts of methane, ethane, and ethene were produced. Higher alkanes, especially isobutane and isopentane, were the main gaseous products. Significantly lower amounts of hexanes and heptanes were present and the majority of these products were collected in the liquid fraction. Figure 7 shows a typical time course of the mole fraction of C4-C5 hydrocarbons in the gas fraction, with nitrogen as the inert gas, on US-Y for the reactor temperature 649 K and heating rate 100 K/min. The pattern of the formation of these main gaseous products followed the overall pattern of gas formation shown in the previous section. The amounts of alkenes were much lower than those of alkanes and were formed much later. The reason for this was the stronger adsorption of alkenes on zeolites, especially high aluminous ones such as US-Y, and their conversion by secondary reactions. The amounts of isoparaffins were much higher than those of their normal isomers; total isobutane-to-normal butane ratio was between 7 and 11 and the isopentaneto-normal pentane ratio between 20 and 25. This fact

alkanes alkenes cycloproducts aromatics 73 75 83 86

7 5 2 3

4 3 4 3

16 17 11 8

means that the octane number of the produced mixture was high because isoparaffins show a considerably higher octane number than normal paraffins. Increased amounts of iso-C4 products are known to be from the cracking of oligomeric species, raising the possibility of isomerization of carbenium ions prior to cracking.14 Liquid Products. In all cases the components have been grouped into four groups: alkanes, alkenes, cycloproducts (cycloalkanes and cycloalkenes), and aromatics. Table 2 shows a summary of the liquid products at various experimental conditions. Alkanes were the main liquid product group, with significantly smaller amounts of alkenes and aromatics. Cycloproducts were present in very small amounts. Higher reactor temperatures and heating rates favored the formation of slightly higher amounts of alkenes and aromatics, but the temperature conditions seemed to have very little influence on the pattern regarding bond saturation. It was rather a strong characteristic of the zeolite structure.13 There might be two reasons for the much lower amount of olefins. Alkenes stay longer in the US-Y zeolite pores, because of their stronger adsorption, and undergo secondary reactions.14 This is supported by degradation results on different zeolites.13 With the ZSM-5 zeolite13 the main products were alkenes. ZSM-5 is a highly siliceous zeolite with weaker adsorption of olefins. In this zeolite, alkenes were staying for shorter times than in US-Y, undergoing no secondary reactions. In addition, the smaller channels of ZSM-5 sterically hindered bimolecular secondary hydrogen transfer reactions.14 Both effects resulted in the presence of much higher amounts of alkenes in the products in the case of ZSM-5. These results compare favorably with experiments in a fluidized bed,11,12 where alkenes were predominantly formed. However, these experiments required high nitrogen flow to affect fluidization, about 10 times that used in this work. The high nitrogen flow caused the rapid desorption of olefins, before they underwent secondary reactions. Similar differences in product distribution were reported between experiments in a

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heating to reach the final temperature more quickly, for example, by using infrared heating elements is another possible method. 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 Figure 8. Carbon number distribution of liquid alkanes at various conditions.

continuous flow reactor and in situ NMR experiments, where the reaction has been carried out in batch mode.15 Figure 8 shows the carbon number distribution of the main hydrocarbon liquid products, alkanes, expressed as a percentage of the total liquid amount. The distribution of the alkanes showed a peak at C8 or C9, depending upon the conditions, and the heaviest paraffin found was C15. Milder conditions, a lower temperature or lower heating rate, favored shifting of the peak to higher carbon numbers. At high temperatures more hydrocarbons were present at low carbon numbers, C6-C7, confirming that at higher temperature, as expected, increased cracking took place. 4. Conclusions The catalytic degradation of high-density polyethylene on ultrastable Y zeolite has significantly reduced the degradation temperature compared with pure thermal degradation in the absence of a catalyst. The products of the catalytic degradation were hydrocarbons in the gasoline range; C15 was the heaviest detected product. Both of these facts confirmed that the catalytic degradation of plastic waste has the potential to be developed into a commercial polymer recycling method. The additional fact that the majority of the products were isoparaffins, suggesting high octane number, speaks for a high-quality fuel as a product of a catalytic degradation process. Furthermore, the work indicated the following: (1) The start of the degradation reaction was proven to be catalytic, with isobutane and isopentane in traces being the initial products. (2) These two alkanes were the main gaseous products. (3) Gaseous and liquid alkenes were formed in smaller amounts and later than alkanes, because of stronger adsorption on US-Y and their conversion via secondary reactions. (4) Cycloalkanes, cycloalkenes, and aromatics were present in the product in small quantities. (5) At higher temperatures slightly lighter hydrocarbons were formed. (6) The selectivity to liquid fraction was found to be around 50% and insensitive to temperature. For future work different ways of carrying out the reaction could be used, for example, use of a continuous reactor, where the polymer will reach a catalyst bed of constant temperature. Alternatively, different ways of

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Received for review July 13, 1999 Revised manuscript received December 20, 1999 Accepted January 11, 2000 IE990513I