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Polymer degradation to fuels over microporous catalysts as a novel tertiary plastic recycling method
Polymer Degradation and Stability 83 (2004) 267–279 www.elsevier.com/locate/polydegstab
Polymer degradation to fuels over microporous catalysts as a novel tertiary plastic recycling method Karishma Gobin, George Manos* Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, UK Received 20 April 2003; received in revised form 10 June 2003; accepted 15 June 2003
Abstract The catalytic degradation of polyethylene over various microporous materials—zeolites, zeolite-based commercial cracking catalysts as well as clays and their pillared analogues—was studied in a semi-batch reactor. Over all catalysts the liquid products formed had a boiling point distribution in the range of motor engine fuels, which increases considerably the viability of the method as a commercial recycling process. From the zeolites, ZSM-5 resulted mostly in gaseous products and almost no coking due to its shape selectivity properties. Commercial cracking catalysts fully degraded the polymer resulting in higher liquid yield and lower coke content than their parent ultrastable Y zeolite. This confirmed the suitability of such catalysts for a polymer recycling process and its commercialisation potential, as it confirmed the potential of plastic waste being co-fed into a refinery cracking unit. Clays, saponite and Zenith-N, a montmorillonite, and their pillared analogues were less active than zeolites, but could fully degrade the polymer. They showed enhanced liquid formation, due to their mild acidity, and lower coke formation. Regenerated pillared clays offered practically the same performance as fresh samples, but their original clays’ performance deteriorated after removal of the formed coke. Although performance of the regenerated saponite was satisfactory, with the regenerated Zenith the structural damage was so extensive that plastic was only partly degraded. # 2003 Elsevier Ltd. All rights reserved. Keywords: Polymer degradation; Catalytic cracking; Fuel; Polymer recycling
1. Introduction The dramatic growth of welfare levels in the second half of the twentieth century was accompanied by a drastic increase in plastic product use. This unavoidably had a huge impact on the environment, as it caused a rapid increase in plastic waste and hence a large strain on existing disposal methods, landfill and incineration. Landfill space is becoming scarce and expensive, a problem exacerbated by the fact that plastic waste is more voluminous than other waste type. Incineration on the other side, to recover energy, produces toxic gaseous products, which only shifts a solid waste problem to an air pollution one. Polymer recycling becomes an increasingly better alternative to those methods. Thermal degradation
* Corresponding author. Tel.: +44-20-7679 3810; fax: +44-207383 2348. E-mail address: [email protected] (G. Manos). 0141-3910/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0141-3910(03)00272-6
methods to gas and liquid products [1,2] show various advantages compared with other polymer recycling methods. Pure thermal degradation of plastic waste though requires high temperatures and produces heavy products that need further processing for their quality to be upgraded. The presence of catalyst on the other hand reduces the process temperature and forms hydrocarbon products in the motor fuel boiling point range, which eliminates the need for further upgrading process steps [3,4]. In such a catalytic cracking system, mainly zeolites have been used so far as acidic solid catalysts [3–10]. Previous studies [3] have suggested that the initial polyolefin degradation occurs mainly on the external surface of the catalyst. Only smaller fragments formed by this initial cracking can then enter the zeolite pore structure, where the majority of the active sites are located, to undergo further reactions. Because of the strong zeolitic acidity, severe overcracking takes place resulting into the formation of small molecules that are collected mainly in the gaseous fraction, increasing its yield.
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Hence the yield to liquid fuel decreases, which we consider as the most saleable product. We recently introduced pillared clays as well as their original analogues as catalysts for a polymer catalytic degradation process [11,12]. Clay based catalysts possess much milder acidity than zeolites [13–15] and have a bimodal pore structure including mesopores [16] as well as micropores. Plastic catalytic cracking over these catalysts results in much less degree of overcracking and higher liquid yield [11]. In the search of further catalysts for improving the yield to liquid fuel in the plastic catalytic cracking, in this work we tested two commercial cracking catalysts, containing 20 and 40% ultrastable Y zeolite respectively. Containing only a small amount of zeolite, cracking catalysts are less acidic and should form more liquid hydrocarbons than their parent zeolite. The test of commercial cracking catalysts is important on the other hand, as one of the options of commercialising this polymer recycling method is to co-feed polymer waste to existing refinery crackers. In this case the suitability of commercial cracking catalysts to degrade polymer waste is vital. Focus of these initial studies was to test the suitability of such catalysts for converting polymer into fuel. Together with these catalysts we added in our experimental programme a ZSM-5 zeolite and a mixture of US-Y/ZSM-5 (50%–50% in mass), since ZSM-5 additives are increasingly important in commercial Fluid Catalytic Cracking (FCC) units [17]. Additionally to the studies using zeolites and commercial cracking catalysts, we continued the studies using pillared clays and their original analogues [11,12]. Having confirmed the superiority of clay-based catalysts compared with zeolites regarding higher formation of liquid hydrocarbons and lower catalyst coking, we focused on the performance of coked and regenerated clay samples, as the regenerability of clays and pillared clays is an important issue [11,12] and has so far hindered the commercialisation of processes using clay based catalysts. This paper reports about the performance in polymer catalytic cracking of zeolite-based catalysts, including US-Y, ZSM-5, a 50–50% (in mass) US-Y/ZSM-5 mixture and two commercial cracking catalysts containing 20 and 40% US-Y respectively, as well as clay-based catalysts, two pillared clays and their original clays.
2. Experimental 2.1. Materials The model polymer feed was unstabilised linear low density polyethylene (LLDPE) in powder form (average particle size, 100 mm) kindly provided by BASF AG with a density of 0.928 g/cm3 and an average molar mass of 117 kg/mol.
The catalyst samples used included: 1. Zeolite-based catalysts (a) US-Y zeolite (original Si/Al ratio: 2.5, framework Si/Al ratio: 5.7, average particle size 1 mm). (b) ZSM-5 zeolite (Si/Al ratio: 22.5, average particle size 1 mm). (c) An equimass mixture of the two above samples, US-Y and ZSM-5 (50%–50% mass). (d) Two commercial cracking catalysts, named 1 and 2, containing 20 and 40% US-Y respectively (average particle size 100 mm). 2. Clay-based catalysts (a) Saponite, with small amount of impurities, mainly sepiolite (particle size < 160 mm). (b) Zenith-N, a montmorillonite (85%), 5% feldspars, 3% calcite, 2.5% quartz, illite 2% and christobalite 2% (particle size < 160 mm). (c) ATOS, a pillared derivative of the raw saponite (particle size < 160 mm). (d) AZA, a pillared derivative of the montmorillonite Zenith-N (particle size < 160 mm). The preparation method of pillared clays as well as further details on the composition of the clay catalysts are described elsewhere [11,12]. 2.2. Equipment The experimental apparatus for catalytic degradation of LLDPE consisted of a semi-batch Pyrex reactor in which the reaction took place, heated by two semi-circle infrared heating elements for fast heating, connected to a programmable temperature controller. For each experimental run, the actual profile of reactor temperature vs. time is presented in the same graphs as the liquid yield. The reactor was purged with nitrogen 50 mlN/min, determined by a mass flow controller in order to remove the volatile reaction products from the reactor. Prior to the reaction the reactor was purged with nitrogen in order to remove any oxygen. The amounts of polymer and dry catalyst were 2 and 1 g, respectively. The mass ratio of polymer to catalyst in this study has been kept constant and equal to 2, as previous work using thermal gravimetric analysis (TGA) [3] has shown, that there was no significant change in the degradation pattern for polymer to catalyst mass ratios below 2. The addition of extra catalyst does not enhance further the polymer degradation rate [3]. Liquid products were collected in condensers placed in an ice bath (273 K) and analysed by GC equipped with a flame ionisation detector (FID) using a J&W
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Scientific DB-Petro capillary column (100 m0.25 mm0.5 mm). Using a two-way valve, collection of samples at various reaction times and temperatures was possible. A number of liquid samples were analysed also by solution NMR using a two channel spectrometer with two 5 mm probes (Advance 500) in the NMR laboratory of the Chemistry Department at UCL, in order to compare the ratio of unsaturated hydrocarbon products to saturated ones.
n-pentane were assigned to a group corresponding to the boiling point interval between n-butane and n-pentane (272.7–309.2 K). In some cases, more than one samples were collected with the same catalyst. The total boiling point distribution for each catalyst was calculated as their weighted average using the following equation: P Total Boiling Point Distribution: Xitot ¼ Xij ðLFÞj j
2.3. Experimental calculations in the semi-batch reactorequipment
where Xitot is the total probability density function (%/ T) value of group i in the overall liquid sample. Xij is the probability density function (%/T) value of group i in liquid sample j. (LF)j is the mass fraction of liquid sample j. This was repeated for all 16 groups and a total distribution curve for the overall sample was constructed.
The conversion to volatile products was calculated as the fraction of the initial mass of polymer reacted to form volatile products. The yield to liquid products was calculated as the mass of liquid collected divided by the initial amount of polymer and represents the fraction of original polymer converted to liquid products. Liquid yield values were estimated at various reaction times as the use of a two-way valve enabled the collection of various liquid samples during the reaction. The coke yield was calculated by dividing the mass of unvolatilised polymer on the catalysts by the original mass of polymer and hence: Coke Yield=1Conversion. In all but one cases, unvolatilised polymer represented coke formed on the catalyst. Visual inspection at the end of experimental runs revealed coked catalysts to be the only phase present in the reactor and no remnant polymer mass. In only one case, using regenerated Zenith-N clay catalyst, the presence of polymeric remnants together with the catalyst was visually obvious. The boiling point distribution of each liquid fraction was used to represent the liquid product distribution. That was possible as the employed non-polar capillary column separated the components of a mixture according to their volatility/boiling point. The boiling point distribution has been estimated as follows: A calibration mixture containing normal alkanes, pentane to eicosane (C5–C20) was prepared and used to assign each retention time observed from the chromatogram to a boiling point. The whole sample for analysis was divided in intervals between the boiling points of the normal alkanes of the calibration mixture. The mass fraction corresponding to each interval was calculated from the sum of the area fractions of all components in this interval. The mass fraction of each component is set equal to the area fraction [18] a fact that was confirmed using the calibration mixture. To each interval the probability density function value was then calculated as being equal to the mass fraction of this interval divided by the temperature interval width T. Hence the probability density function is expressed as% /K. In the graphs of the boiling point distribution each interval is represented by its middle value. All components with retention times smaller than this of
3. Results and discussion 3.1. Zeolite-based catalysts 3.1.1. Conversion, yield to liquid products and coke yield Experiments in the absence of any catalyst showed a polymer conversion below 5% and no liquid formation, Table 1, confirming that liquid formation during all experimental runs was wholly due to catalytic reactions. Two experiments with US-Y zeolite, which was used as the reference catalyst through the whole study, have been carried out to test reproducibility of the experimental method. The liquid yield vs. time is presented in Fig. 1 for both runs. Although the temperature profiles were not identical, the reproducibility was satisfactory. The slightly higher reactivity in one of the experiments at 20 min could be explained by its higher reaction temperature. It is also obvious that the final values were almost the same although the duration of the experiments differed by five minutes. With US-Y that is a very active catalyst due to its strong acidity, plastic degrades fast and after some reaction time only coke has been left that cannot form any liquid products. Table 1 shows the overall conversion, the yield to liquid products and the coke yield for US-Y, ZSM-5, their mixture (50–50 wt.%) and the two commercial cracking catalysts used in this study. All zeolite-based catalysts used showed conversion values above 90%. It should be emphasised here that the final conversion value does not necessarily indicate the activity level of each catalyst, as the temperature increased during the reaction run. A more detailed picture of the liquid yield vs. time is used in following paragraphs to assess and compare the activity of each catalyst used. The low coking level of ZSM-5 is reflected on the higher conversion value achieved by this catalyst in comparison to US-Y. ZSM-5 is well known for its
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Fig. 1. Liquid yield and temperature vs. time during catalytic degradation of LLDPE in two experiments using US-Y zeolite.
Table 1 Conversion, liquid yield and coke yield during catalytic cracking of LLDPE over zeolite-based catalysts Catalyst
Conversion (%)
Yield to liquid product (%)
Coke yield (%)
No Catalyst US-Y ZSM-5 US-Y and ZSM-5 Cracking Catalyst 1 (20% US-Y) Cracking Catalyst 2 (40% US-Y)
<5 92 99 96 94 95
0 55 39 42 68 72
8 <1 4 6 5
low coking tendency [19]. This can be attributed to the shape selectivity properties of its relatively small pore structure that does not allow the growth of large coke molecules. The coke yield value for the US-Y/ZSM-5 mixture is intermediate between the individual values, indicating an equivalent contribution of each mixture constituent. ZSM-5 showed the lowest yield to liquid products (39%) due to its smaller pores that caused the formation of smaller molecules collected in the gaseous fraction [4]. The contribution from ZSM-5 is obvious in its mixture with US-Y where the liquid yield is very near to the one observed over pure ZSM-5. Obviously the presence of ZSM-5 caused a lot of products formed on US-Y to
undergo further cracking in its structure. A more detailed run of liquid yield vs. time (Fig. 2) shows that over US-Y the reaction was faster than over ZSM-5, obviously due to the higher acidity of US-Y. The mixture US-Y/ZSM-5 degraded the polymer with a rate intermediate of the individual catalysts. With both commercial cracking catalysts similar results were obtained; coke yield around 5%, conversion around 95% and yield to liquid products around 70%. Surprisingly the liquid yield vs. time graph (Fig. 3) does not show significant difference between the performance of both catalysts albeit the difference in zeolite content at a factor of 2. The catalyst containing 40% US-Y shows a slightly higher activity as reflected in a slightly faster liquid formation (Fig. 3). However, over the 20% containing catalyst temperature compensated for this slight activity difference leading to same levels of polymer degradation at a slightly higher temperature. It is clear as stated in previous studies [3,4] that the whole performance of such catalytic systems is not only a matter of catalyst activity but further process phenomena play an important role, like mixing of polymer and solid catalyst. Currently further studies with the two cracking catalysts are being carried out to clarify the activity level of each catalyst in polymer cracking reactions as well as the effect of other variables.
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Fig. 2. Liquid yield and temperature vs. time during catalytic degradation of LLDPE over US-Y, ZSM-5 and an equimass US-Y/ZSM-5 mixture.
The formation of liquid fuel on the other side was significantly enhanced with the cracking catalysts compared with US-Y. This is an indication that less overcracking took place over the commercial cracking catalysts, due to their significantly lower acidity. These results confirm the suitability of FCC catalysts for the catalytic cracking of plastic waste. They increase significantly the commercial potential of a recycling process based on catalytic degradation, as cracking catalysts could cope with the conversion of plastic waste co-fed into a refinery FCC unit. 3.1.2. Product distribution For all the catalysts above, the liquid samples collected were analysed by gas chromatography and the product distribution of the liquid hydrocarbon fraction was presented in the form of a boiling point distribution curves. Fig. 4 shows the boiling point distribution of all three samples obtained with US-Y zeolite at different reaction times, including the total boiling point distribution. All show a peak at the corresponding octane boiling point, indicating the high quality of the fuel. An interesting feature is the observed shift towards less volatile hydrocarbons from the first collected liquid sample to later
samples similarly to results obtained by pillared clays [12]. Obviously the first sample formed at lower temperatures is expected to contain a higher proportion of lower boiling components. Reactions at lower temperature on the other side are expected to lead into scission of smaller chain fragments, while larger fragments that demand higher activation energies are broken away at higher temperatures. Furthermore solid phase crosslinking reactions change the nature of the polymer reactant, making it more difficult to degrade. A comparison between the boiling point distributions for US-Y, ZSM-5 and their mixture is shown in Fig. 5. These were similar to each other with the majority of the liquid being in the gasoline boiling range. With US-Y, the majority of the liquid was in the boiling range C6–C10. The distribution over ZSM-5 shows a sharp peak corresponding to C8 and lighter products than US-Y due to its smaller pore size. The liquid fraction obtained over US-Y/ZSM-5 mixture contained mostly C8–C18, showing surprisingly a higher fraction of heavy components than both of their constituent catalysts. Obviously only light liquid products formed by cracking over USY underwent further cracking in the ZSM-5 structure increasing the gaseous fractions as mentioned above while heavy liquid
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Fig. 3. Liquid yield and temperature vs. time during catalytic degradation of LLDPE over commercial cracking catalysts 1 and 2.
Fig. 4. Boiling point distribution of the three liquid samples obtained during catalytic degradation of LLDPE over US-Y.
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Fig. 5. Boiling point distribution of liquid fractions obtained over US-Y, ZSM-5 and an equimass US-Y/ZSM-5 mixture.
Fig. 6. Boiling point distribution during catalytic degradation of LLDPE over the commercial cracking catalysts.
hydrocarbons did not. This has caused a shift in the distribution of the liquid fraction towards heavier components. Overall the distributions obtained with zeolite catalysts, highlight the quality of the liquid fuel produced by catalytic cracking of plastic waste and its suitability for blending in a refinery gasoline pool. The boiling point distributions of the liquid products produced over the commercial cracking catalysts, presented in Fig. 6, were similar to each other. Compared with US-Y however they show clearly a flatter pattern. The liquid formed over both cracking catalysts contained a lower amount of light hydrocarbons and a higher amount of heavy hydrocarbons than the liquid formed over US-Y. Obviously the lower cracking activ-
ity of cracking catalysts compared with US-Y, resulted into the formation of heavier products and this fraction caused the overall liquid yield to increase. 3.2. Clay-based catalysts 3.2.1. Conversion, yield to liquid products and coke yield Table 2 summarises the main results obtained with claybased catalysts. So far clays were not successfully introduced in a commercial catalytic process, as they suffer from poor regenerability. During regeneration via coke burning the structure collapses due to the high temperatures developed. Although pillaring of the structure has been introduced to combat this problem, pillared clays
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Table 2 Conversion, liquid yield and coke yield during catalytic cracking of LLDPE over clay-based catalysts Catalyst
Conversion (%)
Yield to liquid product (%)
Coke yield (%)
Saponite Fresh Saponite Regenerated Saponite Twice Regen.Saponite Coked Saponite
99 99 99 94
83 72 80 70
1 1 1 6
Atos Fresh ATOS Regenerated ATOS Coked ATOS
94 96 95
62 64 82
6 4 5
Zenith-N Fresh Zenith-N Regenerated Zenith-N
98 63
68 34
2 37
AZA Fresh AZA Regenerated AZA Coked AZA
99 99 98
75 71 71
1 1 2
experience similar difficulties that hamper their commercialisation. As regeneration is a key issue, as far as claybased catalysts are concerned, we included in the list beside each fresh clay sample, the regenerated catalyst, more than once in some cases, as well as the coked catalyst. Very high conversion values above 98% were obtained with all fresh and regenerated samples (Table 2) due to low coking taking place with the exception of regenerated Zenith-N. The mild acidity of clays does not catalyse strong coke formation resulting in high conversion values. Furthermore the considerably lower acidity of clays and their pillared analogues, compared with US-Y, resulted in higher values of yield to liquid products. Low clay acidity did not support severe overcracking of primary products to small gaseous molecules. The regenerated Zenith was the only sample of this study, where a polymeric residue was obtained at the end of the experiment. The reason for the incomplete plastic degradation is considered to be the non-regenerability of Zenith-N. Obviously the structure was extensively damaged during the coke burning stage at 823 K.
Fig. 7. Liquid yield and temperature vs. time during catalytic degradation of LLDPE over saponite samples; fresh, coked and regenerated.
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Fig. 8. Liquid yield and temperature vs. time during catalytic degradation of LLDPE over zenith-N samples; fresh and regenerated.
Fig. 9. Liquid yield and temperature vs. time during catalytic degradation of LLDPE over ATOS samples; fresh, coked and regenerated.
The high conversion values over the clay-based catalysts, in comparison with US-Y do not in any way indicate higher activity than US-Y but simply reflect on the lower coking levels and the graphs of liquid yield vs. time clear this. The rate of liquid formation is slower over clay catalysts than zeolites. Clays and pillared clays need higher temperatures than zeolites to reach the same levels of reaction rates.
Fig. 7 shows the liquid yield vs. time for all the saponite samples; fresh, coked, regenerated and twice regenerated. Although regenerated samples showed similar final values of conversion and liquid yield, the liquid yield graph shows clearly that the liquid formation rate is lower than in a fresh saponite sample. This behaviour leads to the conclusion that although mainly intact the clay structure undergoes irreversible partial damage
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Fig. 10. Liquid yield and temperature vs. time during catalytic degradation of LLDPE over AZA samples; fresh, coked and regenerated.
Fig. 11. Boiling point distribution during catalytic degradation of LLDPE over saponite samples; fresh, coked and regenerated.
with each regeneration cycle. Nevertheless slightly higher temperatures compensates for any loss of activity of the regenerated samples achieving the same final result and enhancing the life time of a saponite catalyst in a plastic recycling process. In contrast to saponite, which performed well, ZenithN could not even be regenerated. Obviously the extent of the structural damage of the used Zenith sample after coke burning was high and as a result, a low liquid yield and liquid formation rate were observed over it as
shown in Fig. 8. At the end of this experimental run polymeric remnants were visually present together with the coked catalyst. In Fig. 9, the yield is presented of the liquid fraction formed over various ATOS samples: fresh, regenerated and coked. The conclusion can be safely drawn that ATOS is completely regenerable, in agreement with previous results [17]. Similar behaviour was observed with the second type of pillared clay used, AZA, as Fig. 10 shows.
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Fig. 12. Boiling point distribution during catalytic degradation of LLDPE over AZA samples; fresh, coked and regenerated.
Table 3 Ratio of olefinic to aliphatic hydrogens in the liquid fractions formed during catalytic cracking of LLDPE over various catalysts, determined by solution NMR Catalyst US-Y Sample 2 (5–20 min, 610–670 K) Cracking Catalyst 1 Sample 3 (20–25 min, 652–677 K) Cracking Catalyst 1 Sample 4 (25–30 min, 652–677 K) Cracking Catalyst 2 Sample 3 (20–25 min, 652–680 K) ATOS Sample 3 (20–25 min, 650–685 K)
Olefinic Hydrogens/ Aliphatic Hydrogens Ratio 1.37 5.34 5.52 4.36 10.33
In general, with the pillared clays, the performance between fresh and regenerated samples was similar but with the original clays the deviation was stronger. As for the coked catalysts, with exception of ZenithN, although the rate of reaction was in all cases considerably lower than the fresh sample, it is worth mentioning that as the temperature increased to the final temperature the coked catalysts also achieved high final conversion values. In the case of coked ATOS even higher final value of the liquid yield was reached than the fresh sample. The lower acidity of the coked sample resulted not only in slower degradation rate but less overcracking and therefore higher liquid yield. The fact that even coked samples could degrade the plastic
indicates that clay based catalysts lifetime is longer than just one batch cycle. 3.3. Product distribution The boiling point curves for saponite and AZA including the regenerated catalysts are presented in Figs. 11 and 12 respectively as examples of liquid formation over claybased catalysts together with the equivalent distribution over US-Y for comparison. They show peaks at higher temperatures than the liquids formed over zeolites as their acidity is milder. Comparing the various samples of the same type, less volatile components were observed with regenerated saponite than fresh saponite and even less volatile with coked saponite. Clearly a shift towards heavier hydrocarbons was observed with regeneration cycles, indicating the structural changes occurring, which resulted to slight loss of activity. Although mostly heavier hydrocarbons were formed with clay-based catalysts than zeolites, most of these products were in the boiling range of motor fuels. This combined with the fact that the liquid yield has significantly increased, confirms the potential of the claybased catalysts for a commercial recycling process. Another question we answer in this paper is the chemical nature of the liquid products. In previous work using GC–MS [3,4], it was found out that the main products over US-Y zeolite, were paraffins that are considered to be produced mainly by secondary reactions. Olefins, considered to be primary cracking products, are strongly adsorbed on the US-Y framework undergoing secondary reactions leading to more paraffins
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and coke. Over ZSM-5 on the other hand secondary bimolecular reactions are sterically hindered by its small channel size with the result the main product group to be olefins [4]. Solution H NMR was used in this study to characterise the chemical nature of the liquid products, as olefinic hydrogen atoms show separate distinctive peaks than paraffinic hydrogens. Table 3 shows for some of the used catalysts the ratio of olefinic hydrogens to aliphatic (olefinic plus paraffinic) hydrogens, which ratio is indicative of the saturation degree. With ATOS this ratio (0.103) is much higher than the one with US-Y zeolite (0.014). Due to the milder acidity of ATOS, secondary reactions have been limited with the result of a much higher presence of primary products alkenes in the sample. Cracking catalysts produced intermediate figures (0.044–0.053). Due to the presence of US-Y, secondary reactions occurred, but did not progress to the same degree as with pure US-Y. It should be emphasised that the ratio of olefinic hydrogens to aliphatic hydrogens is much lower than the actual molar ratio of alkenes to the sum of alkanes and alkenes, as only hydrogens of the double bond contribute to the olefinic NMR peak, not all hydrogen atoms of the alkene molecule.
Clay-based catalysts have been proven less active than zeolites, but still were able to completely degrade polyethylene after a slight increase in the process temperature. However, they proved superior to zeolites as far as the formation of liquid hydrocarbon fuel was concerned. This was attributed to the weaker acidity of the clay-based catalysts, due to which overcracking to small molecules was surpressed. For this reason heavier hydrocarbons were present in the liquid products formed. Clay based catalysts showed a mixed picture regarding regenerability. Regenerated pillared clays, after combustion of the formed coke, showed practically the same behaviour with their fresh counterparts regarding conversion and yield, as well as product distribution. Their original counterparts however showed a deterioration in their ability to degrade polyethylene, especially Zenith-N. Finally using solution NMR the chemical character of the liquid products was studied. The following order in product saturation was found: (more alkenes) Clay catalysts > Cracking catalysts > US-Y (more alkanes). The much higher amount of alkenes over the clay catalysts than US-Y zeolite and cracking catalysts indicated the significantly lower degree in which secondary reactions took place, due to the milder acidity of clays vis-a´-vis the strong acidity of the US-Y zeolite. The milder acidity explains also the lower coking levels obtained with the clay-based catalysts.
4. Conclusions Acknowledgements The catalytic degradation of plastic waste has the potential to be developed to a commercial polymer recycling process, as it produces at relatively low degradation temperatures liquid hydrocarbons in the boiling range of motor engine fuels. More specifically the first part of this work using zeolite-based catalysts found the following: 1. US-Y was the most active catalyst but produced the highest amount of coke, due to its strong acidity. 2. The presence of ZSM-5 increased the yield to gaseous products and decreased the coke content, due to their small pores and hence shape selective properties. 3. Commercial cracking catalysts were very active degradation catalysts. They resulted into higher liquid yields and lower coke content, due to lower acidity levels. 4. With all catalysts the produced liquid fraction had a boiling point distribution in the boiling range of motor engine fuels. Heavier hydrocarbons were formed with commercial cracking catalysts due to their lower acidity. Lighter hydrocarbons were formed with ZSM-5 due to its smaller pore size.
Financial support by the Engineering and Physical Sciences Research Council (EPSRC) for a PhD studentship to K.G. is acknowledged. We would like to thank Professors N. Papayannakos and N.H. Gangas, National Technical University of Athens for kindly providing the clay and pillared clay samples and Dr. Paul O’Connor, Akzo-Nobel for kindly providing the two commercial cracking catalyst samples.
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Polymer degradation to fuels over microporous catalysts as a novel tertiary plastic recycling method Karishma Gobin, George Manos* Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, UK Received 20 April 2003; received in revised form 10 June 2003; accepted 15 June 2003
Abstract The catalytic degradation of polyethylene over various microporous materials—zeolites, zeolite-based commercial cracking catalysts as well as clays and their pillared analogues—was studied in a semi-batch reactor. Over all catalysts the liquid products formed had a boiling point distribution in the range of motor engine fuels, which increases considerably the viability of the method as a commercial recycling process. From the zeolites, ZSM-5 resulted mostly in gaseous products and almost no coking due to its shape selectivity properties. Commercial cracking catalysts fully degraded the polymer resulting in higher liquid yield and lower coke content than their parent ultrastable Y zeolite. This confirmed the suitability of such catalysts for a polymer recycling process and its commercialisation potential, as it confirmed the potential of plastic waste being co-fed into a refinery cracking unit. Clays, saponite and Zenith-N, a montmorillonite, and their pillared analogues were less active than zeolites, but could fully degrade the polymer. They showed enhanced liquid formation, due to their mild acidity, and lower coke formation. Regenerated pillared clays offered practically the same performance as fresh samples, but their original clays’ performance deteriorated after removal of the formed coke. Although performance of the regenerated saponite was satisfactory, with the regenerated Zenith the structural damage was so extensive that plastic was only partly degraded. # 2003 Elsevier Ltd. All rights reserved. Keywords: Polymer degradation; Catalytic cracking; Fuel; Polymer recycling
1. Introduction The dramatic growth of welfare levels in the second half of the twentieth century was accompanied by a drastic increase in plastic product use. This unavoidably had a huge impact on the environment, as it caused a rapid increase in plastic waste and hence a large strain on existing disposal methods, landfill and incineration. Landfill space is becoming scarce and expensive, a problem exacerbated by the fact that plastic waste is more voluminous than other waste type. Incineration on the other side, to recover energy, produces toxic gaseous products, which only shifts a solid waste problem to an air pollution one. Polymer recycling becomes an increasingly better alternative to those methods. Thermal degradation
* Corresponding author. Tel.: +44-20-7679 3810; fax: +44-207383 2348. E-mail address: [email protected] (G. Manos). 0141-3910/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0141-3910(03)00272-6
methods to gas and liquid products [1,2] show various advantages compared with other polymer recycling methods. Pure thermal degradation of plastic waste though requires high temperatures and produces heavy products that need further processing for their quality to be upgraded. The presence of catalyst on the other hand reduces the process temperature and forms hydrocarbon products in the motor fuel boiling point range, which eliminates the need for further upgrading process steps [3,4]. In such a catalytic cracking system, mainly zeolites have been used so far as acidic solid catalysts [3–10]. Previous studies [3] have suggested that the initial polyolefin degradation occurs mainly on the external surface of the catalyst. Only smaller fragments formed by this initial cracking can then enter the zeolite pore structure, where the majority of the active sites are located, to undergo further reactions. Because of the strong zeolitic acidity, severe overcracking takes place resulting into the formation of small molecules that are collected mainly in the gaseous fraction, increasing its yield.
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Hence the yield to liquid fuel decreases, which we consider as the most saleable product. We recently introduced pillared clays as well as their original analogues as catalysts for a polymer catalytic degradation process [11,12]. Clay based catalysts possess much milder acidity than zeolites [13–15] and have a bimodal pore structure including mesopores [16] as well as micropores. Plastic catalytic cracking over these catalysts results in much less degree of overcracking and higher liquid yield [11]. In the search of further catalysts for improving the yield to liquid fuel in the plastic catalytic cracking, in this work we tested two commercial cracking catalysts, containing 20 and 40% ultrastable Y zeolite respectively. Containing only a small amount of zeolite, cracking catalysts are less acidic and should form more liquid hydrocarbons than their parent zeolite. The test of commercial cracking catalysts is important on the other hand, as one of the options of commercialising this polymer recycling method is to co-feed polymer waste to existing refinery crackers. In this case the suitability of commercial cracking catalysts to degrade polymer waste is vital. Focus of these initial studies was to test the suitability of such catalysts for converting polymer into fuel. Together with these catalysts we added in our experimental programme a ZSM-5 zeolite and a mixture of US-Y/ZSM-5 (50%–50% in mass), since ZSM-5 additives are increasingly important in commercial Fluid Catalytic Cracking (FCC) units [17]. Additionally to the studies using zeolites and commercial cracking catalysts, we continued the studies using pillared clays and their original analogues [11,12]. Having confirmed the superiority of clay-based catalysts compared with zeolites regarding higher formation of liquid hydrocarbons and lower catalyst coking, we focused on the performance of coked and regenerated clay samples, as the regenerability of clays and pillared clays is an important issue [11,12] and has so far hindered the commercialisation of processes using clay based catalysts. This paper reports about the performance in polymer catalytic cracking of zeolite-based catalysts, including US-Y, ZSM-5, a 50–50% (in mass) US-Y/ZSM-5 mixture and two commercial cracking catalysts containing 20 and 40% US-Y respectively, as well as clay-based catalysts, two pillared clays and their original clays.
2. Experimental 2.1. Materials The model polymer feed was unstabilised linear low density polyethylene (LLDPE) in powder form (average particle size, 100 mm) kindly provided by BASF AG with a density of 0.928 g/cm3 and an average molar mass of 117 kg/mol.
The catalyst samples used included: 1. Zeolite-based catalysts (a) US-Y zeolite (original Si/Al ratio: 2.5, framework Si/Al ratio: 5.7, average particle size 1 mm). (b) ZSM-5 zeolite (Si/Al ratio: 22.5, average particle size 1 mm). (c) An equimass mixture of the two above samples, US-Y and ZSM-5 (50%–50% mass). (d) Two commercial cracking catalysts, named 1 and 2, containing 20 and 40% US-Y respectively (average particle size 100 mm). 2. Clay-based catalysts (a) Saponite, with small amount of impurities, mainly sepiolite (particle size < 160 mm). (b) Zenith-N, a montmorillonite (85%), 5% feldspars, 3% calcite, 2.5% quartz, illite 2% and christobalite 2% (particle size < 160 mm). (c) ATOS, a pillared derivative of the raw saponite (particle size < 160 mm). (d) AZA, a pillared derivative of the montmorillonite Zenith-N (particle size < 160 mm). The preparation method of pillared clays as well as further details on the composition of the clay catalysts are described elsewhere [11,12]. 2.2. Equipment The experimental apparatus for catalytic degradation of LLDPE consisted of a semi-batch Pyrex reactor in which the reaction took place, heated by two semi-circle infrared heating elements for fast heating, connected to a programmable temperature controller. For each experimental run, the actual profile of reactor temperature vs. time is presented in the same graphs as the liquid yield. The reactor was purged with nitrogen 50 mlN/min, determined by a mass flow controller in order to remove the volatile reaction products from the reactor. Prior to the reaction the reactor was purged with nitrogen in order to remove any oxygen. The amounts of polymer and dry catalyst were 2 and 1 g, respectively. The mass ratio of polymer to catalyst in this study has been kept constant and equal to 2, as previous work using thermal gravimetric analysis (TGA) [3] has shown, that there was no significant change in the degradation pattern for polymer to catalyst mass ratios below 2. The addition of extra catalyst does not enhance further the polymer degradation rate [3]. Liquid products were collected in condensers placed in an ice bath (273 K) and analysed by GC equipped with a flame ionisation detector (FID) using a J&W
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Scientific DB-Petro capillary column (100 m0.25 mm0.5 mm). Using a two-way valve, collection of samples at various reaction times and temperatures was possible. A number of liquid samples were analysed also by solution NMR using a two channel spectrometer with two 5 mm probes (Advance 500) in the NMR laboratory of the Chemistry Department at UCL, in order to compare the ratio of unsaturated hydrocarbon products to saturated ones.
n-pentane were assigned to a group corresponding to the boiling point interval between n-butane and n-pentane (272.7–309.2 K). In some cases, more than one samples were collected with the same catalyst. The total boiling point distribution for each catalyst was calculated as their weighted average using the following equation: P Total Boiling Point Distribution: Xitot ¼ Xij ðLFÞj j
2.3. Experimental calculations in the semi-batch reactorequipment
where Xitot is the total probability density function (%/ T) value of group i in the overall liquid sample. Xij is the probability density function (%/T) value of group i in liquid sample j. (LF)j is the mass fraction of liquid sample j. This was repeated for all 16 groups and a total distribution curve for the overall sample was constructed.
The conversion to volatile products was calculated as the fraction of the initial mass of polymer reacted to form volatile products. The yield to liquid products was calculated as the mass of liquid collected divided by the initial amount of polymer and represents the fraction of original polymer converted to liquid products. Liquid yield values were estimated at various reaction times as the use of a two-way valve enabled the collection of various liquid samples during the reaction. The coke yield was calculated by dividing the mass of unvolatilised polymer on the catalysts by the original mass of polymer and hence: Coke Yield=1Conversion. In all but one cases, unvolatilised polymer represented coke formed on the catalyst. Visual inspection at the end of experimental runs revealed coked catalysts to be the only phase present in the reactor and no remnant polymer mass. In only one case, using regenerated Zenith-N clay catalyst, the presence of polymeric remnants together with the catalyst was visually obvious. The boiling point distribution of each liquid fraction was used to represent the liquid product distribution. That was possible as the employed non-polar capillary column separated the components of a mixture according to their volatility/boiling point. The boiling point distribution has been estimated as follows: A calibration mixture containing normal alkanes, pentane to eicosane (C5–C20) was prepared and used to assign each retention time observed from the chromatogram to a boiling point. The whole sample for analysis was divided in intervals between the boiling points of the normal alkanes of the calibration mixture. The mass fraction corresponding to each interval was calculated from the sum of the area fractions of all components in this interval. The mass fraction of each component is set equal to the area fraction [18] a fact that was confirmed using the calibration mixture. To each interval the probability density function value was then calculated as being equal to the mass fraction of this interval divided by the temperature interval width T. Hence the probability density function is expressed as% /K. In the graphs of the boiling point distribution each interval is represented by its middle value. All components with retention times smaller than this of
3. Results and discussion 3.1. Zeolite-based catalysts 3.1.1. Conversion, yield to liquid products and coke yield Experiments in the absence of any catalyst showed a polymer conversion below 5% and no liquid formation, Table 1, confirming that liquid formation during all experimental runs was wholly due to catalytic reactions. Two experiments with US-Y zeolite, which was used as the reference catalyst through the whole study, have been carried out to test reproducibility of the experimental method. The liquid yield vs. time is presented in Fig. 1 for both runs. Although the temperature profiles were not identical, the reproducibility was satisfactory. The slightly higher reactivity in one of the experiments at 20 min could be explained by its higher reaction temperature. It is also obvious that the final values were almost the same although the duration of the experiments differed by five minutes. With US-Y that is a very active catalyst due to its strong acidity, plastic degrades fast and after some reaction time only coke has been left that cannot form any liquid products. Table 1 shows the overall conversion, the yield to liquid products and the coke yield for US-Y, ZSM-5, their mixture (50–50 wt.%) and the two commercial cracking catalysts used in this study. All zeolite-based catalysts used showed conversion values above 90%. It should be emphasised here that the final conversion value does not necessarily indicate the activity level of each catalyst, as the temperature increased during the reaction run. A more detailed picture of the liquid yield vs. time is used in following paragraphs to assess and compare the activity of each catalyst used. The low coking level of ZSM-5 is reflected on the higher conversion value achieved by this catalyst in comparison to US-Y. ZSM-5 is well known for its
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Fig. 1. Liquid yield and temperature vs. time during catalytic degradation of LLDPE in two experiments using US-Y zeolite.
Table 1 Conversion, liquid yield and coke yield during catalytic cracking of LLDPE over zeolite-based catalysts Catalyst
Conversion (%)
Yield to liquid product (%)
Coke yield (%)
No Catalyst US-Y ZSM-5 US-Y and ZSM-5 Cracking Catalyst 1 (20% US-Y) Cracking Catalyst 2 (40% US-Y)
<5 92 99 96 94 95
0 55 39 42 68 72
8 <1 4 6 5
low coking tendency [19]. This can be attributed to the shape selectivity properties of its relatively small pore structure that does not allow the growth of large coke molecules. The coke yield value for the US-Y/ZSM-5 mixture is intermediate between the individual values, indicating an equivalent contribution of each mixture constituent. ZSM-5 showed the lowest yield to liquid products (39%) due to its smaller pores that caused the formation of smaller molecules collected in the gaseous fraction [4]. The contribution from ZSM-5 is obvious in its mixture with US-Y where the liquid yield is very near to the one observed over pure ZSM-5. Obviously the presence of ZSM-5 caused a lot of products formed on US-Y to
undergo further cracking in its structure. A more detailed run of liquid yield vs. time (Fig. 2) shows that over US-Y the reaction was faster than over ZSM-5, obviously due to the higher acidity of US-Y. The mixture US-Y/ZSM-5 degraded the polymer with a rate intermediate of the individual catalysts. With both commercial cracking catalysts similar results were obtained; coke yield around 5%, conversion around 95% and yield to liquid products around 70%. Surprisingly the liquid yield vs. time graph (Fig. 3) does not show significant difference between the performance of both catalysts albeit the difference in zeolite content at a factor of 2. The catalyst containing 40% US-Y shows a slightly higher activity as reflected in a slightly faster liquid formation (Fig. 3). However, over the 20% containing catalyst temperature compensated for this slight activity difference leading to same levels of polymer degradation at a slightly higher temperature. It is clear as stated in previous studies [3,4] that the whole performance of such catalytic systems is not only a matter of catalyst activity but further process phenomena play an important role, like mixing of polymer and solid catalyst. Currently further studies with the two cracking catalysts are being carried out to clarify the activity level of each catalyst in polymer cracking reactions as well as the effect of other variables.
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Fig. 2. Liquid yield and temperature vs. time during catalytic degradation of LLDPE over US-Y, ZSM-5 and an equimass US-Y/ZSM-5 mixture.
The formation of liquid fuel on the other side was significantly enhanced with the cracking catalysts compared with US-Y. This is an indication that less overcracking took place over the commercial cracking catalysts, due to their significantly lower acidity. These results confirm the suitability of FCC catalysts for the catalytic cracking of plastic waste. They increase significantly the commercial potential of a recycling process based on catalytic degradation, as cracking catalysts could cope with the conversion of plastic waste co-fed into a refinery FCC unit. 3.1.2. Product distribution For all the catalysts above, the liquid samples collected were analysed by gas chromatography and the product distribution of the liquid hydrocarbon fraction was presented in the form of a boiling point distribution curves. Fig. 4 shows the boiling point distribution of all three samples obtained with US-Y zeolite at different reaction times, including the total boiling point distribution. All show a peak at the corresponding octane boiling point, indicating the high quality of the fuel. An interesting feature is the observed shift towards less volatile hydrocarbons from the first collected liquid sample to later
samples similarly to results obtained by pillared clays [12]. Obviously the first sample formed at lower temperatures is expected to contain a higher proportion of lower boiling components. Reactions at lower temperature on the other side are expected to lead into scission of smaller chain fragments, while larger fragments that demand higher activation energies are broken away at higher temperatures. Furthermore solid phase crosslinking reactions change the nature of the polymer reactant, making it more difficult to degrade. A comparison between the boiling point distributions for US-Y, ZSM-5 and their mixture is shown in Fig. 5. These were similar to each other with the majority of the liquid being in the gasoline boiling range. With US-Y, the majority of the liquid was in the boiling range C6–C10. The distribution over ZSM-5 shows a sharp peak corresponding to C8 and lighter products than US-Y due to its smaller pore size. The liquid fraction obtained over US-Y/ZSM-5 mixture contained mostly C8–C18, showing surprisingly a higher fraction of heavy components than both of their constituent catalysts. Obviously only light liquid products formed by cracking over USY underwent further cracking in the ZSM-5 structure increasing the gaseous fractions as mentioned above while heavy liquid
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Fig. 3. Liquid yield and temperature vs. time during catalytic degradation of LLDPE over commercial cracking catalysts 1 and 2.
Fig. 4. Boiling point distribution of the three liquid samples obtained during catalytic degradation of LLDPE over US-Y.
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Fig. 5. Boiling point distribution of liquid fractions obtained over US-Y, ZSM-5 and an equimass US-Y/ZSM-5 mixture.
Fig. 6. Boiling point distribution during catalytic degradation of LLDPE over the commercial cracking catalysts.
hydrocarbons did not. This has caused a shift in the distribution of the liquid fraction towards heavier components. Overall the distributions obtained with zeolite catalysts, highlight the quality of the liquid fuel produced by catalytic cracking of plastic waste and its suitability for blending in a refinery gasoline pool. The boiling point distributions of the liquid products produced over the commercial cracking catalysts, presented in Fig. 6, were similar to each other. Compared with US-Y however they show clearly a flatter pattern. The liquid formed over both cracking catalysts contained a lower amount of light hydrocarbons and a higher amount of heavy hydrocarbons than the liquid formed over US-Y. Obviously the lower cracking activ-
ity of cracking catalysts compared with US-Y, resulted into the formation of heavier products and this fraction caused the overall liquid yield to increase. 3.2. Clay-based catalysts 3.2.1. Conversion, yield to liquid products and coke yield Table 2 summarises the main results obtained with claybased catalysts. So far clays were not successfully introduced in a commercial catalytic process, as they suffer from poor regenerability. During regeneration via coke burning the structure collapses due to the high temperatures developed. Although pillaring of the structure has been introduced to combat this problem, pillared clays
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Table 2 Conversion, liquid yield and coke yield during catalytic cracking of LLDPE over clay-based catalysts Catalyst
Conversion (%)
Yield to liquid product (%)
Coke yield (%)
Saponite Fresh Saponite Regenerated Saponite Twice Regen.Saponite Coked Saponite
99 99 99 94
83 72 80 70
1 1 1 6
Atos Fresh ATOS Regenerated ATOS Coked ATOS
94 96 95
62 64 82
6 4 5
Zenith-N Fresh Zenith-N Regenerated Zenith-N
98 63
68 34
2 37
AZA Fresh AZA Regenerated AZA Coked AZA
99 99 98
75 71 71
1 1 2
experience similar difficulties that hamper their commercialisation. As regeneration is a key issue, as far as claybased catalysts are concerned, we included in the list beside each fresh clay sample, the regenerated catalyst, more than once in some cases, as well as the coked catalyst. Very high conversion values above 98% were obtained with all fresh and regenerated samples (Table 2) due to low coking taking place with the exception of regenerated Zenith-N. The mild acidity of clays does not catalyse strong coke formation resulting in high conversion values. Furthermore the considerably lower acidity of clays and their pillared analogues, compared with US-Y, resulted in higher values of yield to liquid products. Low clay acidity did not support severe overcracking of primary products to small gaseous molecules. The regenerated Zenith was the only sample of this study, where a polymeric residue was obtained at the end of the experiment. The reason for the incomplete plastic degradation is considered to be the non-regenerability of Zenith-N. Obviously the structure was extensively damaged during the coke burning stage at 823 K.
Fig. 7. Liquid yield and temperature vs. time during catalytic degradation of LLDPE over saponite samples; fresh, coked and regenerated.
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Fig. 8. Liquid yield and temperature vs. time during catalytic degradation of LLDPE over zenith-N samples; fresh and regenerated.
Fig. 9. Liquid yield and temperature vs. time during catalytic degradation of LLDPE over ATOS samples; fresh, coked and regenerated.
The high conversion values over the clay-based catalysts, in comparison with US-Y do not in any way indicate higher activity than US-Y but simply reflect on the lower coking levels and the graphs of liquid yield vs. time clear this. The rate of liquid formation is slower over clay catalysts than zeolites. Clays and pillared clays need higher temperatures than zeolites to reach the same levels of reaction rates.
Fig. 7 shows the liquid yield vs. time for all the saponite samples; fresh, coked, regenerated and twice regenerated. Although regenerated samples showed similar final values of conversion and liquid yield, the liquid yield graph shows clearly that the liquid formation rate is lower than in a fresh saponite sample. This behaviour leads to the conclusion that although mainly intact the clay structure undergoes irreversible partial damage
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Fig. 10. Liquid yield and temperature vs. time during catalytic degradation of LLDPE over AZA samples; fresh, coked and regenerated.
Fig. 11. Boiling point distribution during catalytic degradation of LLDPE over saponite samples; fresh, coked and regenerated.
with each regeneration cycle. Nevertheless slightly higher temperatures compensates for any loss of activity of the regenerated samples achieving the same final result and enhancing the life time of a saponite catalyst in a plastic recycling process. In contrast to saponite, which performed well, ZenithN could not even be regenerated. Obviously the extent of the structural damage of the used Zenith sample after coke burning was high and as a result, a low liquid yield and liquid formation rate were observed over it as
shown in Fig. 8. At the end of this experimental run polymeric remnants were visually present together with the coked catalyst. In Fig. 9, the yield is presented of the liquid fraction formed over various ATOS samples: fresh, regenerated and coked. The conclusion can be safely drawn that ATOS is completely regenerable, in agreement with previous results [17]. Similar behaviour was observed with the second type of pillared clay used, AZA, as Fig. 10 shows.
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Fig. 12. Boiling point distribution during catalytic degradation of LLDPE over AZA samples; fresh, coked and regenerated.
Table 3 Ratio of olefinic to aliphatic hydrogens in the liquid fractions formed during catalytic cracking of LLDPE over various catalysts, determined by solution NMR Catalyst US-Y Sample 2 (5–20 min, 610–670 K) Cracking Catalyst 1 Sample 3 (20–25 min, 652–677 K) Cracking Catalyst 1 Sample 4 (25–30 min, 652–677 K) Cracking Catalyst 2 Sample 3 (20–25 min, 652–680 K) ATOS Sample 3 (20–25 min, 650–685 K)
Olefinic Hydrogens/ Aliphatic Hydrogens Ratio 1.37 5.34 5.52 4.36 10.33
In general, with the pillared clays, the performance between fresh and regenerated samples was similar but with the original clays the deviation was stronger. As for the coked catalysts, with exception of ZenithN, although the rate of reaction was in all cases considerably lower than the fresh sample, it is worth mentioning that as the temperature increased to the final temperature the coked catalysts also achieved high final conversion values. In the case of coked ATOS even higher final value of the liquid yield was reached than the fresh sample. The lower acidity of the coked sample resulted not only in slower degradation rate but less overcracking and therefore higher liquid yield. The fact that even coked samples could degrade the plastic
indicates that clay based catalysts lifetime is longer than just one batch cycle. 3.3. Product distribution The boiling point curves for saponite and AZA including the regenerated catalysts are presented in Figs. 11 and 12 respectively as examples of liquid formation over claybased catalysts together with the equivalent distribution over US-Y for comparison. They show peaks at higher temperatures than the liquids formed over zeolites as their acidity is milder. Comparing the various samples of the same type, less volatile components were observed with regenerated saponite than fresh saponite and even less volatile with coked saponite. Clearly a shift towards heavier hydrocarbons was observed with regeneration cycles, indicating the structural changes occurring, which resulted to slight loss of activity. Although mostly heavier hydrocarbons were formed with clay-based catalysts than zeolites, most of these products were in the boiling range of motor fuels. This combined with the fact that the liquid yield has significantly increased, confirms the potential of the claybased catalysts for a commercial recycling process. Another question we answer in this paper is the chemical nature of the liquid products. In previous work using GC–MS [3,4], it was found out that the main products over US-Y zeolite, were paraffins that are considered to be produced mainly by secondary reactions. Olefins, considered to be primary cracking products, are strongly adsorbed on the US-Y framework undergoing secondary reactions leading to more paraffins
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and coke. Over ZSM-5 on the other hand secondary bimolecular reactions are sterically hindered by its small channel size with the result the main product group to be olefins [4]. Solution H NMR was used in this study to characterise the chemical nature of the liquid products, as olefinic hydrogen atoms show separate distinctive peaks than paraffinic hydrogens. Table 3 shows for some of the used catalysts the ratio of olefinic hydrogens to aliphatic (olefinic plus paraffinic) hydrogens, which ratio is indicative of the saturation degree. With ATOS this ratio (0.103) is much higher than the one with US-Y zeolite (0.014). Due to the milder acidity of ATOS, secondary reactions have been limited with the result of a much higher presence of primary products alkenes in the sample. Cracking catalysts produced intermediate figures (0.044–0.053). Due to the presence of US-Y, secondary reactions occurred, but did not progress to the same degree as with pure US-Y. It should be emphasised that the ratio of olefinic hydrogens to aliphatic hydrogens is much lower than the actual molar ratio of alkenes to the sum of alkanes and alkenes, as only hydrogens of the double bond contribute to the olefinic NMR peak, not all hydrogen atoms of the alkene molecule.
Clay-based catalysts have been proven less active than zeolites, but still were able to completely degrade polyethylene after a slight increase in the process temperature. However, they proved superior to zeolites as far as the formation of liquid hydrocarbon fuel was concerned. This was attributed to the weaker acidity of the clay-based catalysts, due to which overcracking to small molecules was surpressed. For this reason heavier hydrocarbons were present in the liquid products formed. Clay based catalysts showed a mixed picture regarding regenerability. Regenerated pillared clays, after combustion of the formed coke, showed practically the same behaviour with their fresh counterparts regarding conversion and yield, as well as product distribution. Their original counterparts however showed a deterioration in their ability to degrade polyethylene, especially Zenith-N. Finally using solution NMR the chemical character of the liquid products was studied. The following order in product saturation was found: (more alkenes) Clay catalysts > Cracking catalysts > US-Y (more alkanes). The much higher amount of alkenes over the clay catalysts than US-Y zeolite and cracking catalysts indicated the significantly lower degree in which secondary reactions took place, due to the milder acidity of clays vis-a´-vis the strong acidity of the US-Y zeolite. The milder acidity explains also the lower coking levels obtained with the clay-based catalysts.
4. Conclusions Acknowledgements The catalytic degradation of plastic waste has the potential to be developed to a commercial polymer recycling process, as it produces at relatively low degradation temperatures liquid hydrocarbons in the boiling range of motor engine fuels. More specifically the first part of this work using zeolite-based catalysts found the following: 1. US-Y was the most active catalyst but produced the highest amount of coke, due to its strong acidity. 2. The presence of ZSM-5 increased the yield to gaseous products and decreased the coke content, due to their small pores and hence shape selective properties. 3. Commercial cracking catalysts were very active degradation catalysts. They resulted into higher liquid yields and lower coke content, due to lower acidity levels. 4. With all catalysts the produced liquid fraction had a boiling point distribution in the boiling range of motor engine fuels. Heavier hydrocarbons were formed with commercial cracking catalysts due to their lower acidity. Lighter hydrocarbons were formed with ZSM-5 due to its smaller pore size.
Financial support by the Engineering and Physical Sciences Research Council (EPSRC) for a PhD studentship to K.G. is acknowledged. We would like to thank Professors N. Papayannakos and N.H. Gangas, National Technical University of Athens for kindly providing the clay and pillared clay samples and Dr. Paul O’Connor, Akzo-Nobel for kindly providing the two commercial cracking catalyst samples.
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