aromatic chemicals from the catalytic pyrolysis of scrap tyres

Journal of Analytical and Applied Pyrolysis67 (2003) 143–164 www.elsevier.com/locate/jaap

Aromatic chemicals from the catalytic pyrolysisof scrap tyres

Paul T. Williams *, Alexander J. BrindleDepartment of Fuel and Energy, The Uni�ersity of Leeds, Leeds LS2 9JT, UK

Received 1 November 2001; accepted 2 April 2002

Abstract

Scrap tyres were pyrolysed in a fixed bed reactor and the evolved pyrolysis gases werepassed through a secondary catalytic reactor. The main objective was to maximise theconcentration of single ring aromatic compounds, which are of known higher commercialvalue. Three types of zeolite catalyst were examined of different surface acidity and pore size.The influence of catalyst to tyre ratio on the yield and composition of the derived oils wasexamined. The results showed that the influence of the catalyst was to reduce the yield of oilwith a consequent increase in the gas yield. Coke formation on the catalyst amounted toapproximately 4 wt.%. However, there was a dramatic increase in the concentration ofcertain single ring aromatic compounds in the derived oils after catalysis. For example,toluene reached a maximum value in the oil of 24 wt.%, benzene 5 wt.%, m/p-xylenes 20wt.% and o-xylene 7 wt.%. The yield in terms of conversion of the mass of tyre to mass ofindividual chemical were, 7.7 wt.% toluene, 1.4 wt.% benzene, 6.4 wt.% m/p-xylenes and 2.2wt.% o-xylene, representing a very significant potential increase in the value of the derivedoils. The yield of aromatic hydrocarbons in the derived oils were related to the differentproperties of the three catalysts such as pore size, which influenced selectivity, and thesilica/alumina ratio which influenced the number of catalytically active sites on the catalystsurface.© 2002 Elsevier Science B.V. All rights reserved.

Keywords: Pyrolysis; Tyres; Recycling; Catalysis; Chemicals

* Corresponding author. Fax: +44-1132-440-572.E-mail address: [email protected] (P.T. Williams).

0165-2370/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0165 -2370 (02 )00059 -1

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144 P.T. Williams, A.J. Brindle / J. Anal. Appl. Pyrolysis 67 (2003) 143–164

1. Introduction

With the increasing emphasis on the environment and sustainability, recycling ofwaste rather than disposal is the preferred treatment route for many waste streams.Such wastes include scrap tyres, which have the potential to be readily recycledsince large tonnages are generated and they may be easily collected as a separatewaste stream. For example, approximately 150 million scrap tyres are produced inNorth America and 180 million in the European Union [1]. Pyrolysis is a recyclingtechnology, which is receiving renewed interest. Pyrolysis of tyres produces an oil,char and gas product all of which have the potential for recycling [1–13]. The charcan be used as a low-grade carbon black, as a solid fuel or may be upgraded toactivated carbon [5,6]. The gas has sufficient calorific value that it may be used toprovide process energy [8]. The yield of oil from the process can be high, of theorder of 58 wt.%, and because the oil is a liquid, it has the advantages of the easeof handling, storage and transport and hence the product does not have to be usedat or near the recycling plant. The pyrolytic oil has a high calorific value, of theorder of 42 MJ kg−1, and has been successfully combusted [11]. However, of mostinterest, the oils have been shown to be highly aromatic and contain concentrationsof valuable chemicals such as, benzene, toluene, xylenes and limonene which areused in the chemical industry [7,8,12,13]. The importance of such high valuechemicals suggests that identification of new processes which lead to increasedconcentrations of such chemicals would greatly enhance the economic viability ofthe pyrolysis of tyres.

In this paper, scrap tyres have been pyrolysed in a fixed bed reactor and thederived pyrolysis gases passed directly to a secondary catalyst reactor maintained at500 °C. The derived oils have been characterised for their content of aromaticcompounds in relation to catalyst/feed ratio. Three types of zeolite catalyst wereinvestigated.

2. Materials and methods

The tyre material was in the form of finely shredded crumbed tyre derived frompassenger cars. The shredded tyre was made up of a range of sizes from powder(�0.1 mm) to granules several mm across and was sieved to produce a size rangeof 1.0–1.4 mm. Typical analysis of the tyre, on a steel- and fabric-free basis aregiven in Table 1.

The catalysts were used ‘as received’ in their extrudate form. The extrudate formmade for ease of handling and a typical extrudate dimension was 1 mm in diameterby 5 mm in length. Three zeolite catalysts were used, Y-zeolite (CBV-400) of largerpore size (7.8 A� ) and high acidity, Y-zeolite (CBV-780) of larger pore size (7.8 A� )and lower acidity and zeolite ZSM-5 of smaller pore size (5.6 A� ) and lower acidity.Details of the catalysts are shown in Table 2. The influence of catalyst/tyre feedratio was examined, in that, Y-zeolite (CBV-400) was examined at catalyst/feedratios of 0.25, 0.5, 1 and 1.5. Y-zeolite (CBV-780) was studied at catalyst/feed ratios

145P.T. Williams, A.J. Brindle / J. Anal. Appl. Pyrolysis 67 (2003) 143–164

Table 1Typical composition of the scrap tyre feedstock rubber

Proximate analysis (%)Elemental composition (%)

62.2C 86.4 Volatiles29.4Fixed carbon8.0H

7.1N 0.5 Ash1.3MoistureS 1.7

3.4O

of 0.25, 0.5, 1 and 1.75. ZSM-5 was investigated for catalyst/feed ratios of 0.5, 1,1.5 and 2. All other parameters such as carrier gas flow rate, reactor temperatureand catalyst temperature were kept constant.

The tyre pyrolysis reactor was of a fixed bed design and a schematic diagram isshown in Fig. 1. The reactor was of stainless steel 10 cm diameter×15 cm high andcould contain up to 200 g of tyre sample. Heating was via an external electricalheater and temperatures were controlled to give the sample a ramped heating rateof 10 °C min−1 to the final tyre pyrolysis temperature of 500 °C and held at thattemperature for 1 h or until pyrolysis was complete. The pyrolysis temperature of500 °C was selected to ensure complete pyrolysis of the tyre and to be compatiblewith the catalyst temperature. Temperatures were monitored throughout the reac-tor. A continuous purge of inert nitrogen was introduced to the reactor via aperforated ring located at the base of the reactor. The nitrogen flow rate gave amaximum gas residence time of approximately 30 s. The tyre was contained in asample cage which fitted into the reactor, flush with the reactor walls. The samplecage was removable allowing the sample to be introduced and post-reaction solidproduct to be removed easily. Gases exited the reactor via an exit tube placed abovethe sample cage.

The pyrolysis gases were passed directly to a heated secondary catalyst reactor.The catalyst reactor could hold up to 100 g of zeolite catalyst which rested on aperforated support plate to allow easy exit of the catalytically cracked vapours. Thetemperature of the catalyst was monitored throughout the experiments. The cata-lyst was heated to 500 °C and held at that temperature for 2 h to activate thecatalyst prior to the commencement of the pyrolysis of the tyre sample. For theexperiments where no catalyst was used, the zeolite was replaced with inert ceramicbeads, to determine the extent of thermal cracking of the pyrolysis gases as distinct

Table 2Physical and chemical properties of catalysts used

Pore size (A� )Catalyst Si:Al ratio

ZSM-5 40 5.67.85.4Y-Zeolite (CBV-400)

40Y-Zeolite (CBV-780) 7.8


Fig. 1. Schematic diagram of the pyrolysis and catalysis reactors.

from catalytic reactions. The gases after reaction were passed to the condensationsystem consisting of a series of glass condensers cooled by solid CO2/acetone. Gaseswere sampled throughout the experiment from a sample point after the condensa-tion system using gas syringes and were analysed off-line by packed column gaschromatography.

The gases sampled using the gas syringes were analysed for hydrocarbon andnon-hydrocarbon gases. Hydrocarbons from C1 to C4 were analysed using aPye-Unicam gas chromatograph with flame ionisation detector, glass column 2.2m×6 mm packed with n-octane Porasil C of 80–100 mesh size and nitrogen carriergas. Nitrogen, hydrogen, oxygen and carbon monoxide were analysed with a PyeUnicam series 204 gas chromatograph with a thermal conductivity detector. Thecolumn used was made of stainless steel being 1.8 m×6 mm dimensions with argonas the carrier gas. The silica packing was 5 A� sized molecular sieve. Carbon dioxidewas analysed using a Gow-Mac Spectra gas chromatograph with a 1.8 m×6 mmstainless steel column packed with 100–120 mesh silica gel, helium carrier gas andthermal conductivity detector.

Size exclusion chromatography (SEC) was used to determine the change inaverage molecular weight of the pyrolysis oils and determine the influence ofcatalysis and feedstock/catalysis ratio. SEC relies on the separation of molecules ina sample by size and shape and thereby produces an apparent molecular weight. For


this work a 30×0.78 cm column packed with silica beads of an average pore sizeof 50 A� , giving the column a molecular weight exclusion limit of 3000 Da Sampleswere injected with a Rheodyne injection valve with a 20 �l sample loop. The eluantused was tetrahydrofuran with dual ultra-violet and refractive index detection. Theultra-violet data only is presented here since it gave better detection levels, superiorsignal to noise ratios and inherent stability. The SEC was calibrated with a rangeof standards, including polystyrene standards with molecular weights of 480, 1050,1350, 1770 and 2550 and aromatic and polycyclic aromatic hydrocarbons (PAH)including benzene (MW of 78), naphthalene (MW of 128), anthracene (MW of178), chrysene (MW of 228) and benzanthracene (MW of 228).

Because of the complexity of the oils derived from the pyrolysis of tyres [8], theyrequired some form of preliminary fractionation to enable identification andquantification of individual species. The fractionation in this work took the form ofopen tubular column chromatography and subsequent analysis of the fractionsusing gas chromatography and coupled gas chromatography/mass spectrometry.The liquid column chromatography consisted of silica gel (40 �m grain size,supplied by Porasil) pre-dried in an oven at 105 °C for 12 h to give consistentadsorption sites. The silica gel was packed into 15 ml borosilicate glass columns andthe pyrolysis oil applied to the top of the column. The column was then eluted withpentane, benzene, ethyl acetate and methanol to produce, aliphatic, aromatic,hetero-atom and polar fractions, respectively.

Fractions were analysed by gas chromatography with flame ionisation detection.The gas chromatograph was a Fisons 8000 with on-column injection. The columnused was a non-polar polydimethylsiloxane HT-5 capillary column 30 m in lengthby 0.32 mm diameter giving 70 000 theoretical plates/m. The carrier gas used washelium and this was set to a flow rate of 2 ml min−1 at 40 °C. The temperatureprogramme of the oven was 40 °C for 2 min followed by a ramp rate of5 °C min−1 to 380 °C, at which point the oven was held isothermally for 20 minto ensure elution of all low volatility material. The following standard compoundswere selected for quantification of one ring aromatic species within the oil: benzene,toluene, ethylbenzene, p-, o-xylene, limonene, and 1,3,5-trimethylbenzene. To ob-tain a calibration curve the standards were diluted to the range 50–1000 ppm.Retention indices and the use of the standards were used for identification andquantification of species. Identification of compounds was also aided by coupledgas chromatography/mass spectrometry. The system used as a Finnigan MATGCQ with a 30 m by 0.32 mm BP-5 column and oven temperature conditionsidentical to those used with the gas chromatograph with flame ionisation detection.

In addition to single ring aromatic compounds, the alkyl substituted aromaticcompounds such as ethylbenzenes, ethyltoluenes and monomethyl-, dimethyl-,trimethyl- and tetramethyl-substituted benzenes were also analysed in the benzenefraction. Again appropriate standards and the use of gas chromatography withflame ionisation detection and gas chromatography/mass spectrometry were used toidentify and quantify species using the same instrumentation as before.

PAH from two ring to five rings were also analysed in the oils. Identification waswith the use of appropriate standards, retention indices and gas chromatography


and flame ionisation detection and with gas chromatography/mass spectrometry.Single ion monitoring (SIM) was also carried out to confirm the identification ofcompounds and also to examine the samples for a series of substituted compounds,for example, naphthalene and its mono methyl-, dimethyl-, and trimethyl-deriva-tives. Identical columns and gas chromatographic temperature programming wasused as before.

The oils derived from pyrolysis/catalysis of tyres were found to be highlyaromatic and did not require pre-fractionation via open tubular liquid chromatog-raphy. The oils were therefore dissolved in either hexane or methylene chloride andanalysed directly with gas chromatography and coupled gas chromatography/massspectrometry.

3. Results and discussion

3.1. Product mass balance

The slow pyrolysis of the tyre in the static batch reactor was undertaken at500 °C in the absence of a catalyst and also with secondary catalysis of thepyrolysis gases at 500 °C with three zeolite catalysts, Y-zeolite (CBV-400), Y-zeo-lite (CBV-780) and zeolite ZSM-5. Figs. 2–4 show the mass balance data for thepyrolysis and catalytic pyrolysis of the tyres in relation to tyre/catalyst ratio. Thepyrolysis in the absence of catalysis is shown as zero catalyst/feed ratio andproduced an oil yield of 55.8 wt.%, gas yield of 6.1 wt.% and char yield of 38.1wt.%. The coke yield also showed a value of 0.2 wt.% deposited at zero catalystaddition. This deposition was due solely to thermal cracking effects since thecatalyst had been replaced by inert ceramic beads. Very similar product yieldsresults have been reported earlier by the authors for a larger (3 kg) fixed bed, batchpyrolysis of tyres reactor [5,6]. Also, other workers using a variety of reactors havefound similar high yields of oil, for example, Benallal et al. [10] using a 19 kg h−1

Fig. 2. Effect of Y-zeolite (CBV-400) catalyst/feed ratio on the mass balance of gas, oil, char and cokeyields.


Fig. 3. Effect of Y-zeolite (CBV-780) catalyst/feed ratio on mass balance of gas, oil, char and cokeyields.

pilot unit yielded 50 wt.% oil at 510 °C and 2–20 kPa, which corrected for steeland fibres, represented approximately 57 wt.% oil. Kawakami et al. [4] obtained anoil yield of 53 wt.% from the pyrolysis of tyres in a rotary kiln at between 540 and640 °C. Roy and Unsworth [9] used a vacuum pyrolysis unit to pyrolyse tyres at500 °C, the reactor system would rapidly remove pyrolysis gases from the hot zoneof the reactor. Rapid removal of gases prevents secondary oil cracking and cokingreactions and tends to maximise the yield of oil. Their results showed a maximumoil yield of 57 wt.%, char yield of 38 wt.% and gas yield of 4 wt.%, very similar tothe results reported here for the continuously nitrogen purged fixed bed reactor,where pyrolysis gases are also rapidly removed from the reactor.

The presence of a catalyst served to reduce the oil yield and increase the gasyield, with formation of coke on the catalyst. As the catalyst/tyre feed ratio wasincreased there were further decreases in the yield of oil and increase in gas andcoke on the catalyst. The char yield remained the same throughout the pyrolysis/catalysis experiments as expected since the pyrolysis of the tyres was identical foreach experiment. The comparison of the three catalysts can be made for example,at an equivalent catalyst/oil feed of 1:1.5. Y-zeolite (CBV-400) showed a reduction

Fig. 4. Effect of ZSM-5 catalyst/feed ratio on mass balance of gas, oil, char and coke yields.


Table 3Effect of Y-zeolite (CBV-400) catalyst/feed ratio on the yields of individual gaseous compounds (wt.%)

0.5 10.25 1.5Catalyst/feed ratio 0

1.4 2.51.3 3.20.8Methane0.30.3 0.8 1.3 1.4Ethane0.80.3 1.2 2.3 2.9Propane

4.0 5.92.5 6.2Isobutane 0.10.30.1 0.4 1.0 0.9Butane1.11.2 0.8 1.2 1.3Ethene

2.1 1.92.1 1.6Propene 0.50.2Butene 0.10.1 0.10.41.9 2.22.2 1.61.9Butadiene

0.20.2 0.1 0.2 0.1Isobutene0.2 0.4Hydrogen 0.50.2 0.2

�0.1 0.1�0.1 0.10.1Carbon monoxide0.20.3 0.3 0.2 0.2Carbon dioxide

in oil yield of 24 wt.% that is from 55.8 to 32 wt.% and gas yield increase by 14 to20 wt.% and coke deposition from 0.2 to 10.1 wt.%. Oil yield for the Y-zeolite(CBV-780) was 33.4 wt.%, gas yield was 18.3 wt.% and coke found on the catalystwas nearly 2 wt.% lower for CBV-780 than the (CBV-780) form at 8.3 wt.%. At acatalyst/feed ratio of 1.5 zeolite ZSM-5 produced a gas yield of 19.8 wt.%, an oilyield of 35.8 wt.% and a coke yield of 6.7 wt.%. The formation of coke on the threecatalysts was significant. A variety of reactions lead to coke from a variety of feedmolecules. All of the classes of compounds are involved to a greater or lesser extentin the formation of coke, for example, aromatic hydrocarbons, naphthenes, alkenesand alkanes. Aromatic species and alkenes have a greater predisposition to beinginvolved in pathways to coke formation because of their ability to easily involvethemselves in hydrogen transfer and cyclisation reactions [14]. The tyre pyrolysisoils have been shown to contain significant concentrations of alkenes and aromaticspecies [8].

3.2. Gas composition

Tables 3–5 show the gas composition of the derived gases from the pyrolysis andcatalytic pyrolysis of tyres and in relation to the catalyst/tyre feed ratio. The gascomposition where the catalyst was replaced by inert ceramic beads showed that themain gases from pyrolysis of tyres were butadiene, ethene and methane. Other gasespresent were C2–C4 alkanes and alkenes and also carbon monoxide and carbondioxide. The addition of a catalyst to the process resulted in a marked increase intotal gas yield as shown by Figs. 2–4. The individual gases showed somewhatdifferent trends in relation to increasing catalyst/tyre feed ratio. For both theY-zeolite catalysts, the alkane gases, methane, ethane, propane, isobutane andbutane and also hydrogen all showed a steady increase in concentration withincreasing ratio. The alkene gases, in general showed a decrease with catalysis and


Table 4Effect of Y-zeolite (CBV-780) catalyst/feed ratio on the yields of individual gaseous compounds (wt.%)

Catalyst/feed ratio 0.50 1 1.750.25

1.6 1.80.8 2.91.0Methane0.6 0.9 1.1Ethane 0.3 0.41.0 1.50.7 2.2Propane 0.3

1.90.1 3.0 5.0 5.6Isobutane0.20.1 0.3 0.4 0.7Butane

1.2 1.21.1 1.5Ethene 1.22.8Propene 2.90.5 2.22.90.3 0.30.2 0.10.1Butene

2.21.9 2.0 1.8 1.4Butadiene0.20.2 0.2 0.1 0.1Isobutene

0.3 0.40.2 0.7Hydrogen 0.20.1Carbon monoxide 0.10.1 �0.10.10.2 0.2 0.20.3Carbon dioxide 0.2

further decrease with increasing catalyst/feed ratio, although some trends were notclear cut. The ZSM-5 catalyst showed some similarities with the Y-zeolite catalystsin that the same gases were found to increase with catalyst/feed ratio; but the actualrate and yields of increase were very different. The yields of isobutane were muchlower when using ZSM-5, less than a third of that of either variant of Y-zeoliteacross a range of equivalent catalyst/feed ratios. Methane yields were less than halfthe yield for the Y-zeolites at the same catalyst/feed ratio. Ethane production wassignificantly less whilst ethene yields were significantly more than Y-zeolite. Pro-pane and propene yields were much higher when using ZSM-5 as opposed to theY-zeolites; propene yields in particular reached 4.7 wt.% at a catalyst/feed ratio of1. This was nearly 2 wt.% more than the yield of the same compound usingY-zeolite (CBV-780).

Table 5Effect of ZSM-5 catalyst/feed ratio on the yields of individual gaseous compounds (wt.%)

Catalyst/feed ratio 0.50 1 1.5 2

0.8 1.40.60.8Methane 1.20.3 1.1Ethane 0.5 1.00.7

3.4 4.12.3Propane 0.3 1.21.4 1.6Isobutane 1.60.1 0.9

0.3 0.4Butane 0.60.1 0.6Ethene 1.2 1.7 2.5 3.6 3.8Propene 0.5 4.3 4.7 4.9 4.1

0.20.40.5Butene 0.50.12.8 2.6 2.1 1.3Butadiene 1.9

0.2 0.3Isobutene 0.4 0.2 0.20.2 0.9Hydrogen 0.70.40.2

0.10.10.1 0.2Carbon monoxide 0.10.2 0.2Carbon dioxide 0.10.3 0.1


Table 6Effect of catalyst/feed ratio on the mean molecular weight of the pyrolysis–catalysis oils (Da)

Catalyst Catalyst/feed ratio

0.5 1.0 1.5 1.75 2.00.0 0.25

121 104 89321 –Y-Zeolite (CBV-400) –197Y-Zeolite (CBV-780) 203 142 103 – 95 –321

–321 215 167 123 – 109Zeolite-ZSM-5

Catalytic cracking has been shown to yield large amounts of C3–C6 products [14]which would account for the increase in yields of methane, ethane, propane, butaneand isobutane. Also, isobutane can be formed as a primary or secondary stableproduct by reaction of straight chain alkanes whereas isobutene was calculated tobe a primary unstable product [15]. Hydrogen has been shown to be a secondaryreaction product during the formation of aromatic compounds from carbocationreactions and alkenes [16]. It has also been shown that in some cases molecularhydrogen may be formed as a primary stable product from branched alkenes [17].Butadiene was found to increase with the addition of catalyst then decrease as thecatalyst/feed ratio was increased. This suggested that butadiene was being con-sumed into forming other products with the addition of further catalyst or that theincreased catalyst loading hindered the creation of butadiene by cracking itsprecursors to different products or promoting different reaction pathways.

3.3. Oil composition

The oils derived from tyre pyrolysis and tyre pyrolysis with catalysis wereanalysed for their average molecular weight using SEC and the results are shown inTable 6. The initial molecular weight range of the oils was from a nominal 60 Dato over 1000 Da with a mean molecular weight of 321 Da With the addition of acatalyst in the secondary reactor, the mean molecular weight of the oils was shiftedto lower values. For example, at a catalyst/feed ratio of 1 the mean molecularweight was 104 Da for the Y-zeolite (CBV-400), 103 for the Y-zeolite (CBV-780)and 167 for the ZSM-5 catalyst. It was clear that the Y-zeolites had a more markedeffect on the molecular weight range of the oils than the ZSM-5 catalyst. The majordifference between the two catalyst types being the pore size, with the Y-zeoliteshaving a larger pore size. The defined pore size of the catalysts were, 5.6 A� for theZSM-5 catalyst and 7.8 A� for the Y-zeolite catalysts. Consequently, reactants whichare too large to enter the catalyst pores will remain unconverted unless they canreact on the catalyst surface. For example, the maximum size of product which canpass into and out of the ZSM-5 type catalysts are of size equivalent to a C10

molecule such as naphthalene. Therefore, the larger molecular weight material inthe uncatalysed pyrolysis oil vapours would first be required to thermally decom-pose before catalytic reactions can occur due to the specific pore size of the


catalysts. The larger molecular weight material may also decompose on the surfaceof the catalyst followed by pore catalysed reactions.

The oils were analysed in detail for their composition of certain single ringaromatic hydrocarbons since these are known to be of high economic value. Theresults for Y-zeolite (CBV-400) are shown in Fig. 5. As the catalyst/feed ratio wasincreased from 0 to 1.5, benzene increased 25 fold from 0.2 to 5.2 wt.%, tolueneincreased over 20 fold from 1.1 to 24.3 wt.%, m/p-xylene increased over 15 foldfrom 1.3 to 20.2 wt.% and o-xylene increased from 0.2 to 7 wt.%. Conversely, therewas a complete loss of limonene from 3.6 wt.% to 0. The total concentration of thefive aromatic hydrocarbons, benzene, toluene, m-, p- and o-xylene in the oil was56.7 wt.% at a catalyst/feed ratio of 1.5, representing a very high concentration ofthese chemicals. It should also be noted that the total yield of oil under theseprocess conditions was 32 wt.%. Therefore, for each tonne of tyre rubber processedunder these conditions, 16.6 kg of benzene, 77.7 kg toluene, 64.6 kg m/p-xylene and22.4 kg o-xylene would be produced, a significant economic gain.

Benzene, toluene and the xylenes have a major use as chemical feedstocks. Themajor industrial products from benzene are derivatives such as ethylbenzene,cyclohexane and cumene together with miscellaneous other derivatives. The deriva-tives are used as basic materials for the production of plastics, resins, fibres,surfactants, dyestuffs and pharmaceuticals, and long chain alkylbenzenes, which areused as feedstocks in the manufacture of surfactants [18]. Xylenes are also regardedas major industrial chemicals and have applications in the plastics industry.o-Xylene is used to produce phthalic anhydride which is used to produce plasticis-ers, dyes and pigments, m-xylene derivatives have applications in the polyester resinand fibre industries and p-xylene derivatives are used in the production of polyesterfibres [18]. Toluene has a wide range of applications as a chemical feedstock and isused for example, in the production of pesticides, dyestuffs, surfactants andsolvents.

For comparison with the Y-zeolite (CBV-400) catalyst, the Y-zeolite (CBV-780)catalyst which had an identical pore structure but with a higher silica/alumina ratiowas investigated to determine its influence on the production of single ring aromatic

Fig. 5. Effect of Y-zeolite (CBV-400) catalyst/feed ratio on yields of selected high value chemicals.


Fig. 6. Effect of Y-zeolite (CBV-780) catalyst/feed ratio on yields of selected high value chemicals.

hydrocarbons. Y-zeolite (CBV-780) had a silica/alumina ratio well above that ofY-zeolite (CBV-400); 40 compared to 5. Reducing the silica/alumina ratio from 40to 5 served to increase the surface acidity of the catalyst by increasing the relativesurface concentration of aluminium [14,19]. The acidity providing the catalyticactivity whilst the pore size provides the shape selectivity. Zeolites behave as solidacid catalysts because they can have strongly acidic protons uniformly distributedthroughout the internal volume of the catalyst channels. By changing the silica/alu-mina ratio it is possible to vary both the number and strength of the acid sites. Fig.6 shows the effect of increasing the catalyst/feed ratio to 1.75 for the Y-zeolite(CBV-780) catalyst. It was found that increasing the catalyst/feed ratio for theY-zeolite (CBV-780) catalyst increased the yields of high value aromatic hydrocar-bons to 5.1 wt.% for benzene, 20.9 wt.% for toluene, 18.1 wt.% for m/p-xylene and6.4 wt.% for o-xylene. As was the case for the Y-zeolite (CBV-400), increasedcatalyst/feed ratio eliminated production of limonene. The higher silica/aluminaratio and consequent lower surface acidity of the Y-zeolite (CBV-780) catalystproduced lower yields of the high value aromatic hydrocarbons at equivalentcatalyst/feed ratios. For example, at a catalyst/feed ratio equivalent to 1.5, fordirect comparison, the total yield of the five aromatic hydrocarbons, benzene,toluene, m-, p- and o-xylene for the CBV-780 form of Y-zeolite was 44.7 wt.%compared to 56.7 wt.% for CBV-400. It may be suggested therefore that thedecreased amount of available acidic catalytic sites because of increased silica/alu-mina ratio hindered the effective production of high value aromatic hydrocarbons.Dejaifve [16] also showed the influence of the silica/alumina ratio on the yields ofaromatic hydrocarbons using zeolite to catalyse methanol. As the silica/aluminaratio was decreased, and thereby increasing surface activity, there was a clear andsignificant increase in the yield of aromatic hydrocarbons produced. Kuehler alsofound that reducing the Si:Al ratio of zeolite from 850 to 40 radically enhanced theyield of aromatic hydrocarbons found in the product oil [20].

Fig. 7 shows the yield of benzene, toluene, m-, p- and o-xylene and limonene forthe catalytic pyrolysis of tyres for the zeolite-ZSM-5 catalyst in relation to


catalyst/feed ratio. The production of benzene, toluene, m-, p- and o-xylene wereagain markedly increased, although to a lesser extent than the Y-zeolite catalysts.The maximum yields obtained were 5.4 wt.% for benzene, 18.1 wt.% for toluene, 9.7wt.% for m/p-xylene and 3.3 wt.% for o-xylene at a catalyst/feed ratio of 2. Directcomparison of the three catalysts may be made at a catalyst/feed ratio of 1.5. At acatalyst/feed ratio of 1.5 for the ZSM-5 catalyst the overall yield of benzene,toluene, m-, p- and o-xylenes in the oil was 29.6 wt.% compared to 56.7 wt.% forY-zeolite (CBV-400) and 44.7 wt.% for Y-zeolite (CBV-780). The ZSM-5 andY-zeolite (CBV-780) had exactly the same silica/alumina ratio and therefore similaracidities and number of active catalytic sites. However, the yield of the aromatichydrocarbons shown in Fig. 7 were significantly lower than the yield for Y-zeolite(CBV-780) catalyst. This difference was most probably due to the smaller pore sizeof ZSM-5. Consequently, lower yields of benzene, toluene, m-, p- and o-xylenes areproduced with the ZSM-5 catalyst compared to the Y-zeolite (CBV-400) catalystdue to both its lower pore size and its lower acidity. A smaller pore size wouldentail less hydrocarbons being of sufficiently small enough size to enter the poresystem in the first place, that is, restricted reactant selectivity, therefore lesspyrolysis vapour experiences the zeolitic cracking and reformation mechanisms thatlead to the formation of aromatic hydrocarbons.

Tables 7–9 show the yield of alkyl substituted aromatic hydrocarbons in the tyrepyrolysis oil and the pyrolysis/catalysis oils in relation to the catalyst/feed ratio forthe Y-zeolite (CBV-400), Y-zeolite (CBV-780) and ZSM-5 zeolite catalysts, respec-tively. For the Y-zeolite (CBV-400) catalyst there was an initial marked increase inconcentration of alkyl substituted aromatic hydrocarbons when the catalyst wasadded, from 1.4 to 10.7 wt.%. The total concentration of the listed alkyl substitutedaromatic hydrocarbons further increased at the catalyst/feed ratio of 0.5, but afterthat the concentration decreased. Ethylbenzene which is a mono-alkyl substitutedaromatic hydrocarbon was the only compound to continue to increase in yield asthe catalyst/feed ratio was increased to the maximum ratio investigated of 1.5. Allthe other alkyl substituted aromatic hydrocarbons detected initially increased inyield up to 0.5 catalyst/feed ratio, then declined. 1,3,5-trimethylbenzene reached itsmaximum yield at a catalyst/feed ratio of 1 and by 1.5 yield had only reduced bya small amount.

Fig. 7. Effect of ZSM-5 catalyst/feed ratio on yields of selected high value chemicals.


Table 7Effect of Y-zeolite (CBV-400) catalyst/feed ratio on yields of alkyl substituted single ring aromatichydrocarbons (wt.%/wt.%)

Catalyst/feed ratio 0.50 1 1.50.25

1.6 2.20.2 2.61.0Ethylbenzene2.2 2.13-Ethyltoluene 1.70.3 1.61.2 1.00.9 0.84-Ethyltoluene 0.3

0.80.1 1.3 1.4 1.41,3,5-Trimethylbenzene0.60.2 0.7 0.6 0.52-Ethyltoluene

3.9 3.92.6 3.51,2,4-Trimethylbenzene 0.20.70.2 1.0 0.7 0.6Indane

0.7 0.50.6 0.30.01,2,3-Trimethylbenzene0.50.0 0.6 0.3 0.24-Ethyl-1,2-dimethylbenzene0.40.0 0.8 0.4 0.35-Ethyl-1,3-dimethylbenzene

0.5 0.30.4 0.21,2,3,5-Tetramethylbenzene 0.00.60.0 0.8 0.4 0.21,2,4,5-Tetramethylbenzene

15.3 13.8 12.3Total 11.51.5

The higher alkyl substituted aromatics were most affected by increasing thecatalyst/feed ratio. For example, the yields of 4-ethyl-1,2-dimethylbenzene, 5-ethyl-1,2-dimethylbenzene, 1,2,3,5-tetramethylbenzene and 1,2,4,5-tetramethylbenzenewere all found to initially increase with catalyst/feed ratio but were then reducedfrom their peak value by over 60% as the catalyst/feed ratio reached 1.5. Thissuggests that as the degree of alkyl substitution increased on the aromatic ring soto did the disappearance of that compound in the product yield at the highcatalyst/feed ratios. It should also be noted that the simple single ring aromatic

Table 8Effect of Y-zeolite (CBV-780) catalyst/feed ratio on yields of alkyl substituted single ring aromatichydrocarbons (wt.%/wt.%)

0.5 1 1.750 0.25Catalyst/feed ratio

1.0 1.91.30.2 2.2Ethylbenzene0.3 1.81.6 2.33-Ethyltoluene 2.30.3 1.1 0.90.94-Ethyltoluene 1.1

1.4 1.6 1.61,3,5-Trimethylbenzene 0.1 0.90.2 0.7 0.50.52-Ethyltoluene 0.7

1,2,4-Trimethylbenzene 4.24.54.52.90.21.1 0.80.8 0.90.2Indane

0.41,2,3-Trimethylbenzene 0.0 0.5 0.7 0.60.0 0.20.4 0.40.84-Ethyl-1,2-dimethylbenzene

0.7 0.30.40.45-Ethyl-1,3-dimethylbenzene 0.00.30.0 0.5 0.6 0.51,2,3,5-Tetramethylbenzene

0.70.0 0.40.7 1.01,2,4,5-Tetramethylbenzene

16.2 15.6 13.6Total 11.11.5


Table 9Effect of ZSM-5 catalyst/feed ratio on yields of alkyl substituted single ring aromatic hydrocarbons(wt.%/wt.%)

1 1.5 2Catalyst/feed ratio 0 0.5

0.7 0.80.2 0.8Ethylbenzene 0.63-Ethyltoluene 0.7 1.1 0.9 0.80.3

0.60.3 0.7 0.5 0.54-Ethyltoluene0.5 0.50.2 0.71,3,5-Trimethylbenzene 0.1

0.30.2 0.3 0.3 0.32-Ethyltoluene1.30.2 2.6 2.3 2.31,2,4-Trimethylbenzene

0.5 0.50.3 0.5Indane 0.20.31,2,3-Trimethylbenzene 0.30.0 0.3�0.10.3 0.30.1 0.20.04-Ethyl-1,2-dimethylbenzene

0.10.0 0.3 0.3 �0.15-Ethyl-1,3-dimethylbenzene0.10.0 0.3 0.3 0.31,2,3,5-Tetramethylbenzene

0.4 0.3 0.4�0.11,2,4,5-Tetramethylbenzene 0.0

8.0 7.3 7.2Total 1.5 4.2

species presented in Figs. 5–7 have minimal alkyl substitution and continue toincrease in yield at the higher catalyst/feed ratios. Whilst zeolite catalysts are knownto increase the yield of aromatic hydrocarbons, as evidenced by the data in Figs.5–7 and Tables 7–9, it appears that at higher catalyst/feed ratios, the alkylsubstituted aromatic hydrocarbons become involved in other reactions leading to adecrease in production. For example, the production of PAH or coke formation.Venuto and Habib [14] have shown that the degree of branching on an alkylsubstituted aromatic hydrocarbon affects its predisposition to be involved inreaction towards coke.

The Y-zeolite (CBV-780) catalyst was found to produce slightly higher yields ofalkyl substituted aromatic hydrocarbons than CBV-400 across the range of catalyst/feed ratios investigated (Table 8). The maximum yield of alkyl substituted singlering aromatics for CBV-780 was 16.1 wt.% at a catalyst/feed ratio of 0.5. Thiscompared to 15.0 wt.% for CBV-400 at a catalyst/feed ratio of 0.5 (Table 7). Asimilar trend was seen for both types of Y-zeolite in that yields of alkylatedaromatic hydrocarbons rapidly increased at low catalyst/feed ratios, reached amaximum and then declined at differing rates at the highest catalyst/feed ratios.Ethylbenzene yield was found to be lower for CBV-780 than CBV-400, whereas allother alkylated aromatic hydrocarbons were higher for CBV-780.

Table 9 shows the alkyl substituted aromatic hydrocarbons for the pyrolysis/catalysis of tyres with ZSM-5 zeolite catalyst in relation to catalyst/feed ratio. Aswas the case for both the Y-zeolite types, there was a rapid increase in the yields ofalkyl substituted aromatic hydrocarbons followed by a gradual decline at higherratios. However, the product yields of the total and individual alkyl substitutedaromatic hydrocarbons were much lower than with the Y-zeolites. In addition, themaximum yield of the hydrocarbons was at a catalyst/feed ratio of 1 for the ZSM-5


Fig. 8. Effect of Y-zeolite (CBV-400) catalyst/feed ratio on yields of naphthalene and monomethylsubstituted naphthalenes.

and 0.5 for the Y-catalysts. The much lower yields of alkylated aromatic hydrocar-bons suggests that the smaller pore size of the ZSM-5 catalyst restricted the entryof pyrolysis intermediates into the zeolite internal surfaces.

Detailed analysis of the oils from pyrolysis and pyrolysis/catalysis of tyres wasundertaken for the presence of PAH. Fig. 8 shows the yield of naphthalene andmonomethyl naphthalenes, Fig. 9 shows the dimethylnapthalenes and Fig. 10 showsthe trimethylnaphthalenes present in the oils for pyrolysis and pyrolysis/catalysisfor the Y-zeolite (CBV-400) catalyst.

The uncatalysed tyre pyrolysis oil contained a wide variety of PAH which havebeen detailed before [8]. The PAH found in the tyre pyrolysis oil consisted largelyof naphthalene, fluorene and phenanthrene and their alkylated substituents.

In the presence of the Y-zeolite (CBV-400) the majority of PAH detectedbelonged to two ring naphthalene and the monomethyl-, dimethyl- and trimethyl-naphthalenes, due to the acidic catalytic reaction promoting sites and pore size ofthe CBV-400 catalyst. The addition of just a small amount of Y-zeolite (CBV-400)catalyst radically increased the yields of these naphthalene isomers from less than0.1 to over 4.0 wt.%. Further addition of catalyst sought to enhance this effect until

Fig. 9. Effect of Y-zeolite (CBV-400) catalyst/feed ratio on yields of dimethyl and ethyl substitutednaphthalenes.


Fig. 10. Effect of Y-zeolite (CBV-400) catalyst/feed ratio on yields of trimethyl substituted naphthalenes.

at a catalyst/feed ratio of 1.5 the three naphthalene compounds accounted for 15.6wt.% of the oil. The presence of the catalyst increased the amount of catalyticreactions and therefore promoted cyclisation reactions to PAH. Other researchershave shown that PAH is formed as a stable secondary product from the catalysisof hydrocarbons [14]. Naphthalenes possess a molecular diameter which would fitinside a zeolite pore system whereas many larger ring PAH species would be toobig. Therefore, during pyrolysis-catalysis multi-ring species would be promoted bycatalytic reactions but generally, only naphthalene type compounds would be ofsufficiently small enough molecular diameter to leave the catalyst. Larger ringspecies formed as a result of zeolite catalyst promoted condensation reactionswould remain in the pore system and eventually lead to coke.

Fig. 9 shows the dimethylnaphthalenes present in the oils in relation to thecatalyst/feed ratio for the Y-zeolite (CBV-400) catalyst. The addition of catalystdramatically increased the yields of the dimethylnaphthalene isomers to 4.5 wt.%and there was a further increase in concentration in the oil up to a catalyst/feedratio of 1, after which there was a decrease in concentration. The higher catalyst/feed ratios either helped prevent the formation of the dimethylnaphthalene isomersby promoting reaction pathways to other products from their precursors orincreasingly consumed the dimethylnaphthalenes formed as intermediates to alter-native end products.

A similar increase followed by a decrease in concentration in relation toincreasing catalyst/feed ratio for the Y-zeolite (CBV-400) catalyst was found for thetrimethylnaphthalenes, as shown in Fig. 10. There was an initial large increase inconcentration of the trimethylnaphthalenes at a catalyst/feed ratio of 0.25. This wasfollowed by another large increase up to a maximum yield of 3.2 wt.%. Theaddition of greater amounts of catalyst caused a significant reduction in the yieldsof trimethylnaphthalenes until at a catalyst/feed ratio of 1.5 the yield had morethan halved to 1.4 wt.%.

Figs. 8–10 show that the degree of alkylation on the naphthalene group ofcompounds affected how that naphthalene derivative was reduced in yield at thehigher catalyst/feed ratios. Taking the case of naphthalene, it has been shown inFig. 8 that this compound showed the largest increase in yield at the highest


catalyst/feed ratios. This was followed by the mono-methylnaphthalenes whichshowed a slight increase in yield at Y-zeolite (CBV-400) catalyst/feed ratios above0.5. Dimethylnaphthalenes showed a slight decrease in yield and trimethylnaph-thalenes were significantly reduced above a Y-zeolite (CBV-400) catalyst/feed ratioof 0.5.

The trends seen for the naphthalene group of compounds can be compared tothose seen for varying degrees of alkyl substituted aromatic hydrocarbons in Table7. It was shown that increasing alkyl groups on the aryl ring aided in the reductionin yield of that compound at higher catalyst/feed ratios using Y-zeolite (CBV-400).It may therefore be suggested that the degree of branching on the aromatic species,whether it be one or two rings affected the yield of that product at the highercatalyst/feed ratios. Increasing the amount of catalyst past a certain point promotedreactions that led to the formation of less branched products. This pointed to tworeactions that would achieve that effect. Trans or de-alkylation leading to theformation of aromatic species without alkyl groups or condensation reaction tolarger ring PAH species or coke.

The influence of catalyst/feed ratio on the concentration of naphthalene and themonomethyl-, dimethyl- and trimethylnapthalenes for the Y-zeolite (CBV-780)catalyst is shown in Figs. 11–13. The yield of naphthalene and monomethylnaph-thalenes increased from less than 0.1 to 14.9 wt.% as the catalyst (CBV-780) to feedratio was increased from 0 to 1.75, as was shown for the Y-zeolite (CBV-400)catalyst. In addition, the dimethylnaphtahlenes for the Y-zeolite (CBV-780) catalystshown in Fig. 12 showed a similar trend to those found with the Y-zeolite(CBV-400) catalyst. As the catalyst/feed ratio was increased from 0 there was arapid increase in the yields of dimethylnaphthalenes up to a maximum 7.6 wt.% ata catalyst/feed ratio of 0.5. The addition of more catalyst served only to reduce theyields of dimethylnaphthalenes so that at a catalyst/feed ratio of 1.75 totaldimethylnaphthalene yield had reduced to 7.3 wt.%. Fig. 13 shows the influence ofcatalyst/feed ratio on the concentration of trimethyl substituted naphthalenes in thederived pyrolysis/catalysis oils. The maximum yield of 3.7 wt.% was found for acatalyst/feed ratio of 0.5 followed by a decreased yield as the amount of catalyst

Fig. 11. Effect of Y-zeolite (CBV-780) catalyst/feed ratio on yields of naphthalene and monomethylsubstituted naphthalenes.


Fig. 12. Effect of Y-zeolite (CBV-780) catalyst/feed ratio on yields of dimethyl and ethyl substitutednaphthalenes.

was increased such that at a catalyst/feed ratio of 1.75 for the Y-zeolite (CBV-780)catalyst the trimethylnaphthalene yield had fallen to 2.3 wt.%. As was the case forthe Y-zeolite (CBV-400) catalyst, the degree of branching on the aromatic speciesaffected the yield of that product at the higher catalyst/feed ratios. Increasing theamount of catalyst past a certain point promoted reactions that led to theformation of less branched products.

Fig. 14 shows the yield of naphthalene and monomethyl naphthalenes, Fig. 15shows the dimethylnapthalenes and Fig. 16 shows the trimethylnaphthalenespresent in the oils for pyrolysis and pyrolysis/catalysis for the ZSM-5 zeolitecatalyst. The ZSM-5 catalyst having a smaller pore size than the Y-zeolite types. Aswas found for the Y-zeolite catalysts there was a large increase in the yields ofnaphthalene and monomethylnaphthalenes as the amount of catalyst was increased(Fig. 14) but these were found to be a fraction of the yields quantified when usingeither variant of Y-zeolite. For example, at a catalyst/feed ratio of 1.5, ZSM-5catalysed pyrolysis oils yield approximately 30% of total naphthalene andmonomethylnaphthalenes in comparison to the two types of Y-zeolite catalyst.

In addition, in contrast to the trends found with the dimethyl- and trimethyl-naphthalenes which showed an increase then decrease in concentration as the

Fig. 13. Effect of Y-zeolite (CBV-780) catalyst/feed ratio on the yields of trimethyl substitutednaphthalenes.


Fig. 14. Effect of ZSM-5 catalyst/feed ratio on yields of naphthalene and monomethyl substitutednaphthalenes.

catalyst/feed ratio was increased for the Y-zeolites, the ZSM-5 catalyst produced asteady increase in concentration as the catalyst/feed ratio was increased. The maindifference between the ZSM-5 catalyst and the Y-type zeolites, particularly theZSM-5 and Y-zeolite (CBV-780) catalysts which had identical silica/alumina ratioswas the pore size. Because of the reduced pore size, ZSM-5 catalyst displayed arestricted reactant selectivity and/or restricted transition state selectivity and/orrestricted product selectivity. This had the effect of reducing the effectiveness of thecatalytic activity of this particular zeolite which was demonstrated by the muchreduced dimethylnaphthalene yields at lower catalyst/feed ratios (Fig. 15). How-ever, at higher catalyst/feed ratios this reduced catalytic activity, and in particularcatalytically promoted condensation reactions, meant that the yields of dimethyl-naphthalenes and trimethylnaphthalenes (Fig. 16) continued to increase. ZSM-5showed the smallest coke yield at a catalyst/feed ratio of 1 when compared to eithervariant of Y-zeolite. This fact, alongside data collected for naphthalene formation,demonstrated that ZSM-5 did not promote as strongly reactions which led to PAH.

Fig. 15. Effect of ZSM-5 catalyst/feed ratio on yields of dimethyl and ethyl substituted naphthalenes.


Fig. 16. Effect of ZSM-5 catalyst/feed ratio on yields of trimethyl substituted naphthalenes.

Other authors have also indicated the ability of ZSM-5 to reduce the production ofPAH and coke [21].

4. Conclusions

Catalytic pyrolysis of scrap tyres using three different zeolite catalysts has beeninvestigated. The results showed that the influence of the presence of either catalystwas to reduce the yield of oil with a consequent increase in the gas yield and theformation of coke on the catalyst. The concentration of certain single ring aromaticcompounds in the derived oils after catalysis were markedly increased. For exam-ple, toluene reached a maximum value in the oil of 24 wt.%, benzene 5 wt.%,m/p-xylenes 20 wt.% and o-xylene 7 wt.%. The yield of aromatic hydrocarbons inthe derived oils were related to the different properties of the three catalyst such aspore size, which influenced selectivity and the silica/alumina ratio which influencedthe number of catalytically active sites on the catalyst surface. The lower pore sizeof the ZSM-5 compared to the Y-zeolite catalyst resulted in a lower production ofaromatic compounds. The Y-zeolite (CBV-400) with the lower silica/alumina ratioand therefore higher surface activity due to higher aluminium surface concentrationproduced an oil with a higher aromatic hydrocarbon content compared to theY-zeolite (CBV-780) with a higher silica/alumina ratio.

Acknowledgements

The authors gratefully acknowledge the support of the UK Engineering andPhysical Science Research Council for support for this work (Grant No. GR/L35331/01).

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aromatic chemicals from the catalytic pyrolysis of scrap tyres

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