Polymer degradation to fuels over microporous catalysts as a novel tertiary plastic recycling method

Polymer degradation to fuels over microporous catalysts as a noveltertiary 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 cat-

alysts as well as clays and their pillared analogues—was studied in a semi-batch reactor. Over all catalysts the liquid productsformed had a boiling point distribution in the range of motor engine fuels, which increases considerably the viability of the methodas a commercial recycling process. From the zeolites, ZSM-5 resulted mostly in gaseous products and almost no coking due to itsshape 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 processand 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 claysoffered practically the same performance as fresh samples, but their original clays’ performance deteriorated after removal of theformed 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 secondhalf of the twentieth century was accompanied by adrastic increase in plastic product use. This unavoidablyhad a huge impact on the environment, as it caused arapid increase in plastic waste and hence a large strainon existing disposal methods, landfill and incineration.Landfill space is becoming scarce and expensive, aproblem exacerbated by the fact that plastic waste ismore voluminous than other waste type. Incinerationon the other side, to recover energy, produces toxicgaseous products, which only shifts a solid waste prob-lem to an air pollution one.

Polymer recycling becomes an increasingly betteralternative to those methods. Thermal degradation

methods to gas and liquid products [1,2] show variousadvantages compared with other polymer recyclingmethods. Pure thermal degradation of plastic wastethough requires high temperatures and produces heavyproducts that need further processing for their qualityto be upgraded.

The presence of catalyst on the other hand reduces theprocess temperature and forms hydrocarbon productsin the motor fuel boiling point range, which eliminatesthe need for further upgrading process steps [3,4].

In such a catalytic cracking system, mainly zeoliteshave been used so far as acidic solid catalysts [3–10].Previous studies [3] have suggested that the initial poly-olefin degradation occurs mainly on the external surfaceof the catalyst. Only smaller fragments formed by thisinitial cracking can then enter the zeolite pore structure,where the majority of the active sites are located, toundergo further reactions. Because of the strong zeoliticacidity, severe overcracking takes place resulting intothe formation of small molecules that are collectedmainly in the gaseous fraction, increasing its yield.

0141-3910/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0141-3910(03)00272-6

Polymer Degradation and Stability 83 (2004) 267–279

www.elsevier.com/locate/polydegstab

* Corresponding author. Tel.: +44-20-7679 3810; fax: +44-20-

7383 2348.

E-mail address: [email protected] (G. Manos).

http://www.sciencedirect.com



http://www.elsevier.com/locate/polydegstab/a4.3d

mailto:[email protected]

Hence the yield to liquid fuel decreases, which weconsider as the most saleable product.

We recently introduced pillared clays as well as theiroriginal analogues as catalysts for a polymer catalyticdegradation process [11,12]. Clay based catalysts pos-sess much milder acidity than zeolites [13–15] and havea bimodal pore structure including mesopores [16] aswell as micropores. Plastic catalytic cracking over thesecatalysts results in much less degree of overcracking andhigher liquid yield [11].

In the search of further catalysts for improving theyield to liquid fuel in the plastic catalytic cracking, inthis work we tested two commercial cracking catalysts,containing 20 and 40% ultrastable Y zeolite respec-tively. Containing only a small amount of zeolite,cracking catalysts are less acidic and should form moreliquid hydrocarbons than their parent zeolite. The testof commercial cracking catalysts is important on theother hand, as one of the options of commercialisingthis polymer recycling method is to co-feed polymerwaste to existing refinery crackers. In this case the suit-ability of commercial cracking catalysts to degradepolymer waste is vital. Focus of these initial studies wasto test the suitability of such catalysts for convertingpolymer into fuel. Together with these catalysts weadded in our experimental programme a ZSM-5 zeoliteand a mixture of US-Y/ZSM-5 (50%–50% in mass),since ZSM-5 additives are increasingly important incommercial Fluid Catalytic Cracking (FCC) units [17].

Additionally to the studies using zeolites and com-mercial cracking catalysts, we continued the studiesusing pillared clays and their original analogues [11,12].Having confirmed the superiority of clay-based catalystscompared with zeolites regarding higher formation ofliquid hydrocarbons and lower catalyst coking, we focusedon the performance of coked and regenerated clay sam-ples, as the regenerability of clays and pillared clays is animportant issue [11,12] and has so far hindered the com-mercialisation of processes using clay based catalysts.

This paper reports about the performance in polymercatalytic cracking of zeolite-based catalysts, includingUS-Y, ZSM-5, a 50–50% (in mass) US-Y/ZSM-5 mix-ture and two commercial cracking catalysts containing20 and 40% US-Y respectively, as well as clay-basedcatalysts, two pillared clays and their original clays.

2. Experimental

2.1. Materials

The model polymer feed was unstabilised linear lowdensity polyethylene (LLDPE) in powder form (averageparticle size, 100 mm) kindly provided by BASF AGwith a density of 0.928 g/cm3 and an average molarmass 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 particlesize 1 mm).
(b) ZSM-5 zeolite (Si/Al ratio: 22.5, average
particle size 1 mm).
(c) An equimass mixture of the two above sam-
ples, US-Y and ZSM-5 (50%–50% mass).
(d) Two commercial cracking catalysts, named 1
and 2, containing 20 and 40% US-Y respec-tively (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 sapo-
nite (particle size <160 mm).
(d) AZA, a pillared derivative of the montmor-
illonite Zenith-N (particle size <160 mm).
The preparation method of pillared clays as well asfurther details on the composition of the clay catalystsare described elsewhere [11,12].

2.2. Equipment

The experimental apparatus for catalytic degradationof LLDPE consisted of a semi-batch Pyrex reactor inwhich the reaction took place, heated by two semi-circleinfrared heating elements for fast heating, connected to aprogrammable temperature controller. For each experi-mental 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 removethe volatile reaction products from the reactor. Prior tothe reaction the reactor was purged with nitrogen inorder to remove any oxygen. The amounts of polymerand dry catalyst were 2 and 1 g, respectively. The massratio of polymer to catalyst in this study has been keptconstant and equal to 2, as previous work using thermalgravimetric analysis (TGA) [3] has shown, that therewas no significant change in the degradation pattern forpolymer to catalyst mass ratios below 2. The addition ofextra catalyst does not enhance further the polymerdegradation rate [3].

Liquid products were collected in condensers placedin an ice bath (273 K) and analysed by GC equippedwith a flame ionisation detector (FID) using a J&W

268 K. Gobin, G. Manos / Polymer Degradation and Stability 83 (2004) 267–279

Scientific DB-Petro capillary column (100 m�0.25mm�0.5 mm). Using a two-way valve, collection ofsamples at various reaction times and temperatures waspossible. A number of liquid samples were analysed alsoby solution NMR using a two channel spectrometerwith two 5 mm probes (Advance 500) in the NMRlaboratory of the Chemistry Department at UCL, inorder to compare the ratio of unsaturated hydrocarbonproducts to saturated ones.

2.3. Experimental calculations in the semi-batch reactor-equipment

The conversion to volatile products was calculated asthe fraction of the initial mass of polymer reacted toform volatile products. The yield to liquid products wascalculated as the mass of liquid collected divided by theinitial amount of polymer and represents the fraction oforiginal polymer converted to liquid products. Liquidyield values were estimated at various reaction times asthe use of a two-way valve enabled the collection ofvarious liquid samples during the reaction. The cokeyield was calculated by dividing the mass of unvolati-lised polymer on the catalysts by the original mass ofpolymer and hence: Coke Yield=1�Conversion. In allbut one cases, unvolatilised polymer represented cokeformed on the catalyst. Visual inspection at the end ofexperimental runs revealed coked catalysts to be theonly phase present in the reactor and no remnant poly-mer mass. In only one case, using regenerated Zenith-Nclay catalyst, the presence of polymeric remnants toge-ther with the catalyst was visually obvious.

The boiling point distribution of each liquid fractionwas used to represent the liquid product distribution.That was possible as the employed non-polar capillarycolumn separated the components of a mixture accord-ing to their volatility/boiling point. The boiling pointdistribution has been estimated as follows:

A calibration mixture containing normal alkanes,pentane to eicosane (C5–C20) was prepared and used toassign each retention time observed from the chroma-togram to a boiling point. The whole sample for analy-sis was divided in intervals between the boiling points ofthe normal alkanes of the calibration mixture.

The mass fraction corresponding to each interval wascalculated from the sum of the area fractions of allcomponents in this interval. The mass fraction of eachcomponent is set equal to the area fraction [18] a factthat was confirmed using the calibration mixture. Toeach interval the probability density function value wasthen calculated as being equal to the mass fraction ofthis interval divided by the temperature interval width�T. Hence the probability density function is expressedas% /K. In the graphs of the boiling point distributioneach interval is represented by its middle value. Allcomponents with retention times smaller than this of

n-pentane were assigned to a group corresponding tothe boiling point interval between n-butane and n-pen-tane (272.7–309.2 K).

In some cases, more than one samples were collectedwith the same catalyst. The total boiling point distribu-tion for each catalyst was calculated as their weightedaverage using the following equation:

Total Boiling Point Distribution: X toti ¼

P

j Xij LFð Þj

where Xitot is the total probability density function (%/

�T) value of group i in the overall liquid sample. Xij isthe probability density function (%/�T) value of groupi in liquid sample j. (LF)j is the mass fraction of liquidsample j.

This was repeated for all 16 groups and a total dis-tribution curve for the overall sample was constructed.

3. Results and discussion

3.1. Zeolite-based catalysts

3.1.1. Conversion, yield to liquid products and coke yieldExperiments in the absence of any catalyst showed a

polymer conversion below 5% and no liquid formation,Table 1, confirming that liquid formation during allexperimental runs was wholly due to catalytic reactions.

Two experiments with US-Y zeolite, which was usedas the reference catalyst through the whole study, havebeen carried out to test reproducibility of the experi-mental method. The liquid yield vs. time is presented inFig. 1 for both runs. Although the temperature profileswere not identical, the reproducibility was satisfactory.The slightly higher reactivity in one of the experimentsat 20 min could be explained by its higher reactiontemperature. It is also obvious that the final values werealmost the same although the duration of the experi-ments differed by five minutes. With US-Y that is a veryactive catalyst due to its strong acidity, plastic degradesfast and after some reaction time only coke has been leftthat cannot form any liquid products.

Table 1 shows the overall conversion, the yield toliquid products and the coke yield for US-Y, ZSM-5,their mixture (50–50 wt.%) and the two commercialcracking catalysts used in this study.

All zeolite-based catalysts used showed conversionvalues above 90%. It should be emphasised here thatthe final conversion value does not necessarily indicatethe activity level of each catalyst, as the temperatureincreased during the reaction run. A more detailed pic-ture of the liquid yield vs. time is used in followingparagraphs to assess and compare the activity of eachcatalyst used. The low coking level of ZSM-5 is reflectedon the higher conversion value achieved by this catalystin comparison to US-Y. ZSM-5 is well known for its

K. Gobin, G. Manos / Polymer Degradation and Stability 83 (2004) 267–279 269

low coking tendency [19]. This can be attributed to theshape selectivity properties of its relatively small porestructure that does not allow the growth of large cokemolecules. The coke yield value for the US-Y/ZSM-5 mix-ture is intermediate between the individual values, indicat-ing 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 formationof smaller molecules collected in the gaseous fraction[4]. The contribution from ZSM-5 is obvious in its mix-ture with US-Y where the liquid yield is very near to theone observed over pure ZSM-5. Obviously the presenceof ZSM-5 caused a lot of products formed on US-Y to

undergo further cracking in its structure. A moredetailed run of liquid yield vs. time (Fig. 2) shows thatover US-Y the reaction was faster than over ZSM-5,obviously due to the higher acidity of US-Y. Themixture US-Y/ZSM-5 degraded the polymer with a rateintermediate of the individual catalysts.

With both commercial cracking catalysts similarresults were obtained; coke yield around 5%, conver-sion around 95% and yield to liquid products around70%. Surprisingly the liquid yield vs. time graph (Fig. 3)does not show significant difference between the perfor-mance of both catalysts albeit the difference in zeolitecontent at a factor of 2. The catalyst containing 40%US-Y shows a slightly higher activity as reflected in aslightly faster liquid formation (Fig. 3). However, overthe 20% containing catalyst temperature compensatedfor this slight activity difference leading to same levels ofpolymer degradation at a slightly higher temperature. Itis clear as stated in previous studies [3,4] that the wholeperformance of such catalytic systems is not only amatter of catalyst activity but further process phenom-ena play an important role, like mixing of polymer andsolid catalyst. Currently further studies with the twocracking catalysts are being carried out to clarify theactivity level of each catalyst in polymer cracking reac-tions as well as the effect of other variables.

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
<5 0 -
US-Y
92 55 8
ZSM-5
99 39 <1
US-Y and ZSM-5
96 42 4
Cracking Catalyst 1 (20% US-Y)
94 68 6
Cracking Catalyst 2 (40% US-Y)
95 72 5
Fig. 1. Liquid yield and temperature vs. time during catalytic degradation of LLDPE in two experiments using US-Y zeolite.


The formation of liquid fuel on the other side wassignificantly enhanced with the cracking catalysts com-pared with US-Y. This is an indication that less over-cracking took place over the commercial crackingcatalysts, due to their significantly lower acidity.

These results confirm the suitability of FCC catalystsfor the catalytic cracking of plastic waste. They increasesignificantly the commercial potential of a recyclingprocess based on catalytic degradation, as cracking cat-alysts could cope with the conversion of plastic wasteco-fed into a refinery FCC unit.

3.1.2. Product distributionFor all the catalysts above, the liquid samples col-

lected were analysed by gas chromatography and theproduct distribution of the liquid hydrocarbon fractionwas presented in the form of a boiling point distributioncurves.

Fig. 4 shows the boiling point distribution of all threesamples obtained with US-Y zeolite at different reactiontimes, including the total boiling point distribution. Allshow a peak at the corresponding octane boiling point,indicating the high quality of the fuel. An interestingfeature is the observed shift towards less volatile hydro-carbons from the first collected liquid sample to later

samples similarly to results obtained by pillared clays[12]. Obviously the first sample formed at lower tem-peratures is expected to contain a higher proportion oflower boiling components. Reactions at lower tempera-ture on the other side are expected to lead into scissionof smaller chain fragments, while larger fragments thatdemand higher activation energies are broken away athigher temperatures. Furthermore solid phase cross-linking reactions change the nature of the polymerreactant, making it more difficult to degrade.

A comparison between the boiling point distributionsfor US-Y, ZSM-5 and their mixture is shown in Fig. 5.These were similar to each other with the majority ofthe liquid being in the gasoline boiling range.

With US-Y, the majority of the liquid was in theboiling range C6–C10. The distribution over ZSM-5shows a sharp peak corresponding to C8 and lighterproducts than US-Y due to its smaller pore size. Theliquid fraction obtained over US-Y/ZSM-5 mixturecontained mostly C8–C18, showing surprisingly a higherfraction of heavy components than both of their con-stituent catalysts. Obviously only light liquid productsformed by cracking over USY underwent further crack-ing in the ZSM-5 structure increasing the gaseousfractions as mentioned above while heavy liquid

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.


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.


hydrocarbons did not. This has caused a shift in thedistribution of the liquid fraction towards heavier com-ponents. Overall the distributions obtained with zeolitecatalysts, highlight the quality of the liquid fuel pro-duced by catalytic cracking of plastic waste and itssuitability for blending in a refinery gasoline pool.

The boiling point distributions of the liquid productsproduced over the commercial cracking catalysts,presented in Fig. 6, were similar to each other. Com-pared with US-Y however they show clearly a flatterpattern. The liquid formed over both cracking catalystscontained a lower amount of light hydrocarbons and ahigher amount of heavy hydrocarbons than the liquidformed over US-Y. Obviously the lower cracking activ-

ity of cracking catalysts compared with US-Y, resultedinto the formation of heavier products and this fractioncaused the overall liquid yield to increase.

3.2. Clay-based catalysts

3.2.1. Conversion, yield to liquid products and coke yieldTable 2 summarises the main results obtained with clay-

based catalysts. So far clays were not successfully intro-duced in a commercial catalytic process, as they sufferfrom poor regenerability. During regeneration via cokeburning the structure collapses due to the high tempera-tures developed. Although pillaring of the structure hasbeen introduced to combat this problem, pillared clays

Fig. 6. Boiling point distribution during catalytic degradation of LLDPE over the commercial cracking catalysts.

Fig. 5. Boiling point distribution of liquid fractions obtained over US-Y, ZSM-5 and an equimass US-Y/ZSM-5 mixture.


experience similar difficulties that hamper their commer-cialisation. As regeneration is a key issue, as far as clay-based catalysts are concerned, we included in the list besideeach fresh clay sample, the regenerated catalyst, more thanonce in some cases, as well as the coked catalyst.

Very high conversion values above 98% wereobtained with all fresh and regenerated samples(Table 2) due to low coking taking place with theexception of regenerated Zenith-N. The mild acidity ofclays does not catalyse strong coke formation resultingin high conversion values.

Furthermore the considerably lower acidity of claysand their pillared analogues, compared with US-Y,resulted in higher values of yield to liquid products.Low clay acidity did not support severe overcracking ofprimary products to small gaseous molecules. Theregenerated Zenith was the only sample of this study,where a polymeric residue was obtained at the end ofthe experiment. The reason for the incomplete plasticdegradation is considered to be the non-regenerabilityof Zenith-N. Obviously the structure was extensivelydamaged during the coke burning stage at 823 K.

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
99 83 1
Regenerated Saponite
99 72 1
Twice Regen.Saponite
99 80 1
Coked Saponite
94 70 6
Atos

Fresh ATOS
94 62 6
Regenerated ATOS
96 64 4
Coked ATOS
95 82 5
Zenith-N

Fresh Zenith-N
98 68 2
Regenerated Zenith-N
63 34 37
AZA

Fresh AZA
99 75 1
Regenerated AZA
99 71 1
Coked AZA
98 71 2
Fig. 7. Liquid yield and temperature vs. time during catalytic degradation of LLDPE over saponite samples; fresh, coked and regenerated.


The high conversion values over the clay-based cata-lysts, in comparison with US-Y do not in any way indi-cate higher activity than US-Y but simply reflect on thelower coking levels and the graphs of liquid yield vs.time clear this. The rate of liquid formation is slowerover clay catalysts than zeolites. Clays and pillared claysneed higher temperatures than zeolites to reach thesame levels of reaction rates.

Fig. 7 shows the liquid yield vs. time for all the sapo-nite samples; fresh, coked, regenerated and twice regen-erated. Although regenerated samples showed similarfinal values of conversion and liquid yield, the liquidyield graph shows clearly that the liquid formation rateis lower than in a fresh saponite sample. This behaviourleads to the conclusion that although mainly intact theclay structure undergoes irreversible partial damage

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.


with each regeneration cycle. Nevertheless slightlyhigher temperatures compensates for any loss of activityof the regenerated samples achieving the same finalresult and enhancing the life time of a saponite catalystin a plastic recycling process.

In contrast to saponite, which performed well, Zenith-N could not even be regenerated. Obviously the extentof the structural damage of the used Zenith sample aftercoke burning was high and as a result, a low liquid yieldand liquid formation rate were observed over it as

shown in Fig. 8. At the end of this experimental runpolymeric remnants were visually present together withthe coked catalyst.

In Fig. 9, the yield is presented of the liquid fractionformed over various ATOS samples: fresh, regeneratedand coked. The conclusion can be safely drawn thatATOS is completely regenerable, in agreement withprevious results [17]. Similar behaviour was observedwith the second type of pillared clay used, AZA, asFig. 10 shows.

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.


In general, with the pillared clays, the performancebetween fresh and regenerated samples was similar butwith the original clays the deviation was stronger.

As for the coked catalysts, with exception of Zenith-N, although the rate of reaction was in all cases con-siderably lower than the fresh sample, it is worth men-tioning that as the temperature increased to the finaltemperature the coked catalysts also achieved high finalconversion values. In the case of coked ATOS evenhigher final value of the liquid yield was reached thanthe fresh sample. The lower acidity of the coked sampleresulted not only in slower degradation rate but lessovercracking and therefore higher liquid yield. The factthat even coked samples could degrade the plastic

indicates that clay based catalysts lifetime is longer thanjust one batch cycle.

3.3. Product distribution

The boiling point curves for saponite and AZA includ-ing the regenerated catalysts are presented in Figs. 11 and12 respectively as examples of liquid formation over clay-based catalysts together with the equivalent distributionover US-Y for comparison. They show peaks at highertemperatures than the liquids formed over zeolites astheir acidity is milder. Comparing the various samples ofthe same type, less volatile components were observedwith regenerated saponite than fresh saponite and evenless volatile with coked saponite. Clearly a shift towardsheavier hydrocarbons was observed with regenerationcycles, indicating the structural changes occurring, whichresulted to slight loss of activity.

Although mostly heavier hydrocarbons were formedwith clay-based catalysts than zeolites, most of theseproducts were in the boiling range of motor fuels. Thiscombined with the fact that the liquid yield has sig-nificantly increased, confirms the potential of the clay-based catalysts for a commercial recycling process.

Another question we answer in this paper is the che-mical nature of the liquid products. In previous workusing GC–MS [3,4], it was found out that the mainproducts over US-Y zeolite, were paraffins that areconsidered to be produced mainly by secondary reac-tions. Olefins, considered to be primary cracking pro-ducts, are strongly adsorbed on the US-Y frameworkundergoing secondary reactions leading to more paraffins

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
Olefinic Hydrogens/
Aliphatic Hydrogens Ratio

US-Y Sample 2

(5–20 min, 610–670 K)

1.37

Cracking Catalyst 1 Sample 3

(20–25 min, 652–677 K)

5.34


(25–30 min, 652–677 K)

5.52


(20–25 min, 652–680 K)

4.36

ATOS Sample 3

(20–25 min, 650–685 K)

10.33


and coke. Over ZSM-5 on the other hand secondarybimolecular reactions are sterically hindered by its smallchannel size with the result the main product group tobe olefins [4].

Solution H NMR was used in this study to char-acterise the chemical nature of the liquid products, asolefinic hydrogen atoms show separate distinctive peaksthan paraffinic hydrogens. Table 3 shows for some ofthe used catalysts the ratio of olefinic hydrogens to ali-phatic (olefinic plus paraffinic) hydrogens, which ratio isindicative of the saturation degree.

With ATOS this ratio (0.103) is much higher than theone with US-Y zeolite (0.014). Due to the milder acidityof ATOS, secondary reactions have been limited withthe result of a much higher presence of primaryproducts alkenes in the sample.

Cracking catalysts produced intermediate figures(0.044–0.053). Due to the presence of US-Y, secondaryreactions occurred, but did not progress to the samedegree as with pure US-Y.

It should be emphasised that the ratio of olefinichydrogens to aliphatic hydrogens is much lower thanthe actual molar ratio of alkenes to the sum of alkanesand alkenes, as only hydrogens of the double bondcontribute to the olefinic NMR peak, not all hydrogenatoms of the alkene molecule.

4. Conclusions

The catalytic degradation of plastic waste has thepotential to be developed to a commercial polymerrecycling process, as it produces at relatively low degra-dation temperatures liquid hydrocarbons in the boilingrange of motor engine fuels.

More specifically the first part of this work usingzeolite-based catalysts found the following:

1. US-Y was the most active catalyst but produced
the highest amount of coke, due to its strongacidity.
2. The presence of ZSM-5 increased the yield to
gaseous products and decreased the coke con-tent, due to their small pores and hence shapeselective properties.
3. Commercial cracking catalysts were very active
degradation catalysts. They resulted into higherliquid yields and lower coke content, due to loweracidity levels.
4. With all catalysts the produced liquid fraction
had a boiling point distribution in the boilingrange of motor engine fuels. Heavier hydro-carbons were formed with commercial crackingcatalysts due to their lower acidity. Lighterhydrocarbons were formed with ZSM-5 due to itssmaller pore size.
Clay-based catalysts have been proven less active thanzeolites, but still were able to completely degrade poly-ethylene after a slight increase in the process tempera-ture. However, they proved superior to zeolites as far asthe formation of liquid hydrocarbon fuel was con-cerned. This was attributed to the weaker acidity of theclay-based catalysts, due to which overcracking to smallmolecules was surpressed. For this reason heavierhydrocarbons were present in the liquid productsformed. Clay based catalysts showed a mixed pictureregarding regenerability. Regenerated pillared clays,after combustion of the formed coke, showed practicallythe same behaviour with their fresh counterpartsregarding conversion and yield, as well as product dis-tribution. Their original counterparts however showed adeterioration in their ability to degrade polyethylene,especially Zenith-N. Finally using solution NMR thechemical 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 ofalkenes over the clay catalysts than US-Y zeolite andcracking catalysts indicated the significantly lowerdegree in which secondary reactions took place, due to themilder acidity of clays vis-a-vis the strong acidity of theUS-Y zeolite. The milder acidity explains also the lowercoking levels obtained with the clay-based catalysts.

Acknowledgements

Financial support by the Engineering and PhysicalSciences Research Council (EPSRC) for a PhD stu-dentship to K.G. is acknowledged. We would like tothank Professors N. Papayannakos and N.H. Gangas,National Technical University of Athens for kindlyproviding the clay and pillared clay samples and Dr.Paul O’Connor, Akzo-Nobel for kindly providing thetwo commercial cracking catalyst samples.

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Polymer degradation to fuels over microporous catalysts as a novel tertiary plastic recycling method