catalytic pyrolysis of tyres: influence of catalyst temperature

Catalytic pyrolysis of tyres: influence of catalyst temperatureq

Paul T. Williams*, Alexander J. Brindle

Department of Fuel and Energy, The University of Leeds, Leeds LS2 9JT, UK

Received 10 October 2001; revised 3 May 2002; accepted 14 June 2002; available online 17 July 2002

Abstract

Two stage pyrolysis–catalysis of used tyres was undertaken to upgrade the derived oil to a highly aromatic oil suitable to be used as a

chemical feedstock rather than a liquid fuel. The tyres were pyrolysed in a fixed bed reactor and the evolved pyrolysis gases were passed

through a secondary fixed bed reactor containing zeolite catalyst. The pyrolysis reactor was maintained at 500 8C and the influence of catalyst

temperature between 430 and 600 8C on the yield and composition of the derived oils was examined. Two zeolite catalysts were examined; a

Y-type zeolite catalyst and zeolite ZSM-5 catalyst of differing pore size and surface activity. The influence of the catalyst was to reduce the

yield of oil with a consequent increase in the gas yield and formation of coke on the catalyst. Single ring aromatic hydrocarbons, benzene,

toluene and xylenes present in the oils showed a marked increase in the presence of the catalyst. Naphthalene and alkylated naphthalenes

were also analysed and showed a similar marked increase in the concentration when a catalyst was present. The Y-type zeolite catalyst of

larger pore size and higher surface activity was found to produce higher concentrations of aromatic compounds compared to the ZSM-5

catalyst. Increasing the catalyst temperature resulted in significant changes in the concentration of benzene, toluene, xylenes, naphthalene

and the alkylated naphthalenes. q 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Tyres; Pyrolysis; Catalysis; Chemicals

1. Introduction

Pyrolysis has been proposed as a viable recycling

technology to treat the very large tonnages of used tyres

generated each year throughout the world. Pyrolysis, the

thermal degradation of the tyre in the absence of oxygen,

generates an oil, char, gas and residual steel product, all of

which have the potential to be recycled [1–8]. The process has

further advantages in that the product gas has a high calorific

value and can be used to provide the energy requirements of

the process plant [3,4]. The steel may be readily recycled back

into the steel industry, the oil may be combusted as a substitute

furnace or boiler fuel [7] and the char may be used as a low

grade activated carbon or carbon black [2,3].

However, there is some resistance to the use of the char

and oil as recycled products due to their perceived lower

quality and variability of properties. Therefore, there has

been interest in process developments which enhance the

economic value of the char and oil. For example, the char

has been successfully upgraded using physical activation in

the presence of either steam or carbon dioxide to produce an

activated carbon with surface area over 600 m2 g21 [2,3]. In

addition, the oil has been investigated for its potential as a

chemical feedstock rather than a fuel and has been shown to

contain certain single ring aromatic chemicals of economic

potential [8]. The identification of processes which lead to

greatly increased concentrations of such high value single

ring aromatic hydrocarbons in the oil would significantly

increase the economic viability of the pyrolysis of tyres.

In this paper, used tyres were pyrolysed in a fixed bed

reactor and the derived gases passed directly to a second

catalytic reactor containing a zeolite catalyst. Two types of

zeolite catalyst and the influence of the temperature of the

catalyst were investigated to determine their influence on

the yield and composition of the derived oils and gases.

2. Materials and methods

2.1. Tyre and catalyst characteristics

The used tyres were shredded, crumbed and sieved to

produce a size range of 1.0–1.4 mm. The used tyres were

obtained from passenger cars. The tyres had a typical

volatile content of 62.2 wt%, fixed carbon of 29.4 wt%, ash

content of 7.1 wt% and a moisture content of 1.3 wt% for

0016-2361/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.

PII: S0 01 6 -2 36 1 (0 2) 00 1 96 -5

Fuel 81 (2002) 2425–2434

www.fuelfirst.com

q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com

* Corresponding author. Fax: þ44-1132-440-572.

E-mail address: [email protected] (P.T. Williams).

http://www.fuelfirst.com

http://www.fuelfirst.com

the tyre rubber used. Two zeolite catalysts were used, Y-

zeolite (CBV-400) of pore size 7.8 A and a Si/Al ratio of 5.4

representing a higher acidity catalyst, and zeolite ZSM-5 of

pore size 5.6 A and Si/Al ratio of 40 representing a lower

acidity catalyst. The catalysts were used in their extrudate

form with dimensions of 1 mm diameter £ 5 mm length.

2.2. Pyrolysis–catalysis reactors

The crumbed tyre was pyrolysed in a fixed bed reactor

constructed of stainless steel and was 10 cm diameter £ 15

cm high and could contain up to 200 g of tyre sample. The

reactor was externally heated via an external electrical

heater, and heating rate and final temperature were

controlled and temperatures monitored throughout the

reactor using thermocouples. A continuous purge of inert

nitrogen was introduced to the reactor via a perforated ring

located at the base of the reactor to sweep the evolved gases

through the reactor. The used tyre was contained in a sample

cage, which fitted into the reactor and was removable,

allowing the sample to be introduced and post-reaction solid

product to be removed easily. The temperature programme

for the pyrolysis of the tyres in the reactor was fixed

throughout the experiment at a heating rate of 10 8C min21

to the final tyre pyrolysis temperature of 500 8C and held at

that temperature for 1 h or until pyrolysis was complete.

The pyrolysis gases exiting the pyrolysis reactor were

passed directly to a heated secondary catalyst reactor. The

catalyst reactor was constructed of stainless steel and was

5 cm diameter £ 10 cm high and tapered at each end to aid

gas flow. Up to 100 g of zeolite catalyst could be used, held

in the catalytic reactor by a perforated support plate to allow

easy exit of the catalytically cracked vapours. The influence

of catalyst temperature was examined at 430, 470, 500, 530

and 600 8C for each of the two zeolite catalysts. The

temperature of the catalyst was monitored throughout the

experiment. The catalyst was heated to the desired

experimental temperature and held at that temperature for

2 h to activate the catalyst prior to the commencement of the

pyrolysis of the tyre sample. All other parameters such as

carrier gas flow rate, tyre pyrolysis reactor temperatures

(10 8C min21 heating rate and 500 8C final temperature) and

catalyst loading (catalyst/tyre ratio of 1:1) were kept

constant. For the experiments where no catalyst was used,

the zeolite was replaced with inert ceramic beads of similar

size to the catalyst, to determine the extent of thermal

cracking of the pyrolysis gases as distinct from catalytic

reactions. The ceramic beads were also heated to 500 8C.

The evolved gases from the pyrolysis and pyrolysis–

catalysis experiments were condensed in a series of glass

condensers cooled by a solid CO2/acetone mixture. Non-

condensable gases, which passed through the condensation

system, were sampled throughout the experiment using gas

syringes and were analysed off-line by packed column gas

chromatography. Repeatability experiments carried out on

the reactor system showed excellent repeatability and mass

closures for the system were always close to 100 wt%. Fig. 1

shows a schematic diagram of the experimental system.

2.3. Gas analysis

Analysis of the non-condensable gases was via separate

analyses of hydrocarbon and non-hydrocarbon gases using

packed column gas chromatography. Hydrocarbons from C1

Fig. 1. Schematic diagram of the pyrolysis–catalysis reactor.

P.T. Williams, A.J. Brindle / Fuel 81 (2002) 2425–24342426

to C4 were analysed using a Pye-Unicam gas chromatograph

with flame ionisation detector, glass column 2.2 m £ 6 mm

packed with n-octane Porasil C of 80–100 mesh size and

nitrogen carrier gas. Nitrogen, hydrogen, oxygen and carbon

monoxide were analysed with a Pye-Unicam series 204 gas

chromatograph with a 1.8 m £ 6 mm stainless steel column

packed with 5 A sized silica molecular sieve, argon carrier

gas and thermal conductivity detector. Carbon dioxide was

analysed using a Gow-Mac Spectra gas chromatograph with

a 1.8 m £ 6 mm stainless steel column packed with 100–

120 mesh silica gel, helium carrier gas and thermal

conductivity detector. The total mass of gas evolved was

calculated from the measured concentrations and molecular

masses of the gases and compared well with the gaseous

product yield calculated by difference.

2.4. Oil analysis

The oils derived from the pyrolysis/catalysis of used

tyres were found to be highly aromatic and were diluted in

either hexane or methylene chloride to appropriate standard

volumes. The diluted oils were analysed directly by gas

chromatography with flame ionisation detection for quanti-

fication and identification purposes and also by coupled gas

chromatography/mass spectrometry for confirmation of

identifications. Chemical species were also identified

using retention times and retention indices. Single ion

monitoring (SIM) was also carried out to confirm the

identification of compounds and also to examine the

samples for a series of substituted compounds, for example,

naphthalene and its mono-, di-, and trimethyl derivatives.

The gas chromatograph with flame ionisation detection was

a Fisons 8000 with on-column injection. The column used

was a non-polar polydimethylsiloxane HT-5 capillary

column 30 m in length by 0.32 mm diameter. The carrier

gas used was helium with an oven temperature programme

of 40 8C for 2 min, ramp rate of 5 8C min21 to 380 8C and

held for 20 min at 380 8C. The coupled gas chromatogra-

phy/mass spectrometry system was a Finnigan MAT GCQ

with a 30 m £ 0.32 mm BP-5 column and oven temperature

conditions identical to those used with the gas chromato-

graph with flame ionisation detection. A range of standard

compounds were selected for quantification of aromatic

species within the oil, including benzene, toluene, ethyl-

benzene, p-xylene, o-xylene, 1,3,5-trimethylbenzene,

naphthalene and a series of mono-, di- and tri-methly-

naphthalenes. To obtain a calibration curve for quantifi-

cation the standards were diluted to the range 50–

1000 ppm.

The oil derived from the pyrolysis of used tyres in the

absence of a catalyst contained a wide variety of species of

higher complexity compared to the pyrolysis–catalysis oils.

Therefore, for the analysis of these higher molecular weight

species such as naphthalene and its derivatives, the oil

required preliminary fractionation prior to analysis by gas

chromatography. The oil was fractionated using open

tubular column chromatography with a 40 mm grain size

Porasil silica gel packing, pre-dried in an oven at 105 8C for

12 h to give consistent adsorption sites. The silica gel was

packed into 15 ml borosilicate glass columns and the

pyrolysis oil applied to the top of the column. The column

was then eluted with pentane, benzene, ethyl acetate and

methanol to produce aliphatic, aromatic, hetero-atom and

polar fractions, respectively. The benzene fraction was then

analysed to determine the concentration of naphthalene and

its derivatives.

3. Results and discussion

3.1. Product yield

Table 1 shows the product yield and gas composition

from the pyrolysis of used tyres in the absence of a catalyst

but with the catalytic reactor containing inert ceramic beads.

The yield of oil was high at 55.8 wt% with a char yield of

38.1 wt% and gas yield of 6.1 wt%. The mass closure shown

in Table 1 was a normalised figure. The char and oil data

were recorded by weight difference and the mass of gas

calculated from the appropriate gas concentration and

molecular mass. Consequently, gas components not

analysed such as C5 gases and sulphurous gases would not

contribute to the mass of calculated gas. Similar results for

product yield have been reported by several other workers

investigating the pyrolysis of tyres using a variety of

reactors [2–6,9,10]. There was also some formation of

carbonaceous coke on the ceramic beads of approximately

0.2 wt%. This deposition was due solely to thermal cracking

effects of the pyrolysis gases on the ceramic bead surface

since the catalyst had been replaced by inert ceramic beads.

Figs. 2 and 3 show the product yield for the pyrolysis–

catalysis of used tyres in relation to catalyst temperature for

the Y-zeolite (CBV-400) and zeolite ZSM-5 catalysts,

respectively. It was apparent that increasing the catalyst bed

temperature had the effect of markedly increasing gas

production at the expense of oil yield. For example, for the

Y-zeolite (CBV-400) catalyst the total gas yield rose from

16.3 to 21.8 wt% and oil yield fell from 38.7 to 32.2 wt% as

the catalyst bed temperature was increased from 430 to

600 8C. These data represent a decrease in oil yield from the

original uncatalysed pyrolysis which produced an oil yield

of 55.8 wt% and a gas yield of 6.1 wt%. Char yield, as

expected remained constant at around 38.0 wt%, since the

pyrolysis of the tyres was the same for each experiment. The

formation of carbonaceous coke on the catalyst increased

Table 1

Product yield from the pyrolysis of used tyres at 500 8C

Oil yield (wt%) Char yield (wt%) Gas yield (wt%)

55.8 38.1 6.1

P.T. Williams, A.J. Brindle / Fuel 81 (2002) 2425–2434 2427

marginally from 7.2 to 8 wt% for the Y-zeolite (CBV-400)

catalyst. The data suggests that catalyst bed temperature was

an important factor during catalysis. When the zeolite ZSM-

5 catalyst was used, the oil yield was higher and gas yield

lower compared to the Y-zeolite (CBV-400) catalyst. For

example, there was an initial decrease in oil yield from

55.8 wt% in the absence of a catalyst to 42.9 wt% oil yield

followed by a further decrease to 34.6 wt% as the catalyst

temperature was increased from 430 to 600 8C. In addition,

the gas yield increased from an uncatalysed gas yield of

6.1–15.1 wt% at 430 8C catalyst temperature increasing to

20.0 wt% at 600 8C catalyst temperature. The zeolite ZSM-

5 catalyst showed a significant increase in coking at higher

temperatures, rising from 4.0 wt% at 430 8C to 7.6 wt% at

600 8C. This was in contrast to Y-zeolite (CBV-400) which

showed a higher coke content at 430 8C of 7.2 wt%. Tyre

pyrolysis oils have been shown to contain high concen-

trations of alkenes and aromatic species [5]. Both these

groups of chemical species have been shown in catalyst

studies to be precursors for the formation of carbonaceous

coke on zeolite catalysts through their ability to easily

involve themselves in hydrogen transfer and cyclisation

reactions [11].

3.2. Gas composition

Table 2 shows the analysis of the product gases (wt%) for

hydrocarbon and non-hydrocarbon gases for the uncatalysed

pyrolysis of used tyres and for the pyrolysis–catalysis of

tyres for the Y-zeolite (CBV-400) and zeolite ZSM-5

catalysts at 500 8C. The mass of gas is calculated from the

appropriate gas concentration and molecular mass. Direct

comparison of the uncatalysed gas composition may be

made with the catalysed pyrolysis since the uncatalysed

experiments were carried out with inert ceramic beads

placed in the catalyst reactor and the temperatures used were

500 8C for the catalyst and ceramic bed experiments to

determine gas composition. The gases from the uncatalysed

pyrolysis of used tyres were shown to consist of mainly

ethane, ethene, propane, propene, butane, butene and high

concentrations of butadiene. Hydrogen, carbon monoxide

and carbon dioxide were also detected in significant

concentrations. The influence of the presence of the

catalysts in the reactor system produced a large increase in

gas yield. As such, all the gases showed a marked increase in

concentration in the presence of the zeolite catalyst,

however, carbon monoxide and carbon dioxide showed a

decrease with both catalysts. Venuto and Habib[11] have

shown that zeolite catalysts produce increased concen-

trations of hydrogen and hydrocarbon gases in the C3–C6

range during catalytic reaction. There were some differences

Fig. 2. Effect of catalyst bed temperature on mass balance of gas, oil, char and coke yields using Y-zeolite (CBV-400) as catalyst.

Fig. 3. Effect of catalyst bed temperature on mass balance of gas, oil, char

and coke yields using ZSM-5 as catalyst.

Table 2

Gas composition for the uncatalysed and catalysed pyrolysis of used tyres at

500 8C

Gas Pyrolysis

(wt%)

Y-zeolite

(wt%)

Zeolite ZSM-5

(wt%)

Methane 0.81 2.45 0.79

Ethane 0.30 1.28 0.65

Propane 0.32 2.28 2.28

Isobutane 0.14 5.85 1.39

Butane 0.11 1.00 0.43

Ethene 1.16 1.19 2.53

Propene 0.51 1.87 4.71

Butene 0.13 0.14 0.46

Butadiene 1.92 2.23 2.61

Isobutene 0.19 0.19 0.36

Hydrogen 0.16 0.39 0.37

Carbon monoxide 0.09 0.11 0.09

Carbon dioxide 0.32 0.15 0.13


between the two catalysts used. For example, the alkene

gases—methane, ethane, butane and isobutane were much

higher in concentration for the Y-zeolite (CBV-400)

compared to the zeolite ZSM-5 catalyst. However, the

alkene gases—ethene, propene and butene were lower for the

Y-zeolite (CBV-400) compared to the zeolite ZSM-5

catalyst. Clearly, the different properties of the two catalysts

in terms of their porosities and surface acidities influenced

the catalytic formation of the different gases.

3.3. Oil composition

Table 3 shows the analysis of the oil derived from the

uncatalysed pyrolysis of used tyres undertaken at 500 8C

and with ceramic beads acting as an inert blank placed in the

catalyst reactor heated to 500 8C. The pyrolysis oils derived

from tyre are very complex containing a wide variety of

mainly alkanes, and alkenes and also aromatic compounds,

particularly polycyclic aromatic hydrocarbons [5–8]. The

single ring aromatic concentrations found in the uncatalysed

tyre pyrolysis oils and shown in Table 3 are in low

concentration. The results therefore suggest that although

chemicals such as benzene, toluene, xylenes, indane and

naphthalene are present in the oils, their concentrations are

not sufficiently high to enable their extraction from the oil

and utilisation as a chemical feedstock a viable economic

proposition. However, in the presence of the Y-zeolite

(CBV-400) and zeolite ZSM-5 catalysts, there was a marked

increase in the concentration of aromatic hydrocarbons. Of

particular interest were the increases in benzene, toluene

and m-, p-, and o-xylenes shown in Figs. 4 and 5 for the

Y-zeolite (CBV-400) and zeolite ZSM-5 catalysts, respect-

ively. The data representing the concentrations in the oil.

Reference to Figs. 2 and 3 enable the concentration of each

component to be determined in relation to the original

feedstock tyre. Benzene, toluene and xylenes have major

uses as chemical feedstocks. For example, benzene is a

feedstock for the production of plastics, resins, fibres,

surfactants, dyestuffs and pharmaceuticals and toluene is

used in the production of pesticides, dyestuffs, surfactants

and solvents. Similarly o-xylene is used to produce

plasticisers, dyes and pigments, m-xylene derivatives have

applications in the polyester resin and fibre industries and p-

xylene derivatives are used in the production of polyester

fibres [12]. Direct comparison of the data in Table 3 for the

uncatalysed pyrolysis of used tyres may be made with the

concentrations of benzene, toluene, m-, p-, and o-xylene for

the 500 8C Y-zeolite (CBV-400) and zeolite ZSM-5 catalyst

temperature in Figs. 4 and 5. For example for the Y-zeolite

(CBV-400), from a benzene concentration in the uncata-

lysed oil of 0.2 wt% the concentration increased to 4.0 wt%,

for toluene the increase was from 1.1 to 21.2 wt%, and the

three xylenes increased from a total of 1.5 to 19.5 wt%. The

total concentration of benzene, toluene, m-, p- and o-xylene

in the oil at 500 8C catalyst temperature for the Y-zeolite

(CBV-400) catalyst was 44.7 wt% representing a very high

concentration of these chemicals. It should also be noted

that the total yield of oil under these process conditions was

36 wt%. Therefore, for each tonne of tyre rubber processed

under these conditions, 14.4 kg of benzene, 76.3 kg toluene

and 70.2 kg of m-, p-, and o-xylene would be produced, a

potentially significant economic gain. The zeolite ZSM-5

catalyst showed a smaller but still very significant increase

in concentration of benzene, toluene and the xylenes

compared to the uncatalysed pyrolysis of used tyres.

The influence of the temperature of the catalyst on the

concentration of benzene, toluene and xylenes is also shown

in Figs. 4 and 5. For the Y-zeolite (CBV-400) catalyst,

benzene increased from 2.8 to 10.2 wt% as the catalyst bed

temperature was increased from 430 to 600 8C. Toluene

increased from 16.3 to 21.7 wt% and the xylenes decreased

with temperature, m/p-xylene falling from 14 to 8.5 wt%

and o-xylene decreased from 4.8 to 2.7 wt%. However, no

significant reduction was seen in xylene yield until 530 8C,

similarly toluene concentration appeared to level off after

500 8C.

The Y-zeolite (CBV-400) catalyst produced significantly

higher concentrations of benzene, toluene and xylenes

compared to the zeolite ZSM-5 catalyst. The Y-zeolite

(CBV-400) had a pore size of 7.8 A and a Si/Al ratio of 5.4

and the zeolite ZSM-5 a pore size of 5.6 A and Si/Al ratio of

40. The lower silica/alumina ratio results in an increase in

the surface acidity of the catalyst by increasing the relative

surface concentration of aluminium [11,13]. The surface

acidity provides the catalytic activity of the catalyst.

Zeolites behave as solid acid catalysts because they can

have strongly acidic protons uniformly distributed through-

out the internal volume of the catalyst channels. By

changing the silica/alumina ratio it is possible to vary both

the number and strength of the acid sites. Consequently, the

increased formation of aromatic compounds, particularly

Table 3

Aromatic hydrocarbons in oil from the pyrolysis of used tyres at 500 8C

Aromatic hydrocarbons wt%

Benzene 0.20

Toluene 1.10

m/p-Xylenes 1.30

o-Xylene 0.20

Ethylbenzene 0.15

3-Ethyltoluene 0.27

4-Ethyltoluene 0.30

1,3,5-Trimethylbenzene 0.13

2-Ethyltoluene 0.15


Indane 0.20


4-Ethyl-1,2-dimethylbenzene 0.00

5-Ethyl-1,3-dimethylbenzene 0.00

1,2,3,5-Tetramethylbenzene 0.00

1,2,4,5-Tetramethylbenzene 0.00

Total 1.39


benzene, toluene and xylenes found with the Y-zeolite

(CBV-400) catalyst was a result of the higher available

acidic catalytic sites due to the lower Si/Al ratio compared

to the ZSM-5 catalyst. Other workers have also shown that

decreasing the silica/alumina ratio and thereby increasing

surface activity results in a significant increase in the yield

of aromatic hydrocarbons produced [14,15].

The difference in the pore size between the two catalysts

provides the selectivity of the catalyst. The smaller pore size

restricts the size of hydrocarbons entering the pore system

of the catalyst and thereby undergo catalytic cracking and

reformation reactions which lead to the formation of the

aromatic hydrocarbons, that is, restricted reactant selectivity

[11]. The catalysts used in this work had a defined pore size

of 7.8 A for the Y-zeolite (CBV-400) and 5.6 A for the

zeolite ZSM-5 catalyst. Consequently, reactants that are too

large to enter the catalyst pores will remain unconverted

unless they can react on the catalyst surface. For example,

the maximum size of product which can pass into and out of

the ZSM-5 type catalysts are of size equivalent to a C10

molecule such as naphthalene [16]. Therefore, the larger

molecular weight material found in tyre pyrolysis oils [5]

would first be required to thermally decompose before

catalytic reactions can occur due to the specific pore size of

the catalysts. The larger molecular weight material may also

decompose on the surface of the catalyst followed by pore

catalysed reactions. Alternatively, the vapour phase may

contain smaller molecular weight material which reacts with

the catalysts, but in the absence of the catalyst condenses

and polymerises to form the larger range of chemical

species found in uncatalysed tyre pyrolysis oils which have

a molecular weight range of 50–1000 Da [17]. Conse-

quently, the lower yield of benzene, toluene and xylenes

found for the zeolite ZSM-5 catalyst was due to both a lower

surface acidity and also a smaller pore size.

Table 3 shows the yield of alkyl substituted aromatic

hydrocarbons in the uncatalysed tyre pyrolysis oil and

Tables 4 and 5 show the yield of certain alkyl substituted

aromatic hydrocarbons in the pyrolysis/catalysis oils in

relation to the catalyst temperature for the Y-zeolite (CBV-

400) and ZSM-5 zeolite catalysts, respectively. The data

representing the concentrations in the oil, reference to Figs.

2 and 3 enable the concentration of each component to be

determined in relation to the original feedstock tyre. For the

Y-zeolite (CBV-400) catalyst, there was a marked increase

in concentration of alkyl substituted aromatic hydrocarbons

when the catalyst was added, from 1.39 to 13.89 wt% at an

equivalent catalyst reactor temperature of 500 8C. For the

zeolite ZSM-5 catalyst the increase for the listed hydro-

carbons was from 1.39 wt% to a lower concentration of

7.97 wt%. For the Y-zeolite (CBV-400) catalyst, the total

concentration of the listed alkyl substituted aromatic

Fig. 4. Effect of catalyst bed temperature on yields of high value chemicals using Y-zeolite (CBV-400) as catalyst.

Fig. 5. Effect of catalyst bed temperature on yields of high value chemicals using ZSM-5 as catalyst.


hydrocarbons showed a decrease in the concentration for the

alkyl substituted aromatic compounds as the temperature of

the catalyst was increased from 430 to 600 8C. Ethylbenzene

which is a mono-alkyl substituted aromatic hydrocarbon

showed an increase in the concentration in the oil up to

500 8C followed by a decrease. The zeolite ZSM-5 catalyst

showed a lower influence of increasing catalyst temperature

giving a decrease in the total concentration from 7.46 to

6.33 wt% as the temperature of the catalyst was increased.

The decrease in the concentration of these alkyl

substituted aromatic compounds with increasing catalyst

temperature suggests that they may be losing the methyl

groups from the aromatic molecule to produce other

compounds, perhaps the single ring aromatic compounds

shown in Figs. 4 and 5, the formation of higher molecular

weight aromatic compounds or they may be involved in

coke forming reactions on the catalyst. Venuto and Habib

[11] have shown that alkyl substituted aromatic hydro-

carbons are involved in the formation of polycyclic

aromatic hydrocarbons and the formation of coke in relation

to catalyst reactions involving zeolite catalysts.

The oils derived from the uncatalysed and pyrolysis–

catalysis of used tyres were analysed in detail for their

content of naphthalene, methylnaphthalenes, dimethyl-

naphthalenes and trimethylnaphthalenes. The groups of

alkylated naphthalenes were identified using the selective

ion monitoring mode of the coupled gas chromatograph–

mass spectrometer. Figs. 6 and 7 show the naphthalene and

1-, and 2-methylnaphthalene concentrations in the pyrol-

ysis–catalysis oils for the Y-zeolite (CBV-400) and zeolite

ZSM-5 catalysts, respectively. Analysis of naphthalene and

1-, and 2-methylnaphthalene in the uncatalysed oils showed

them to be in low concentration at less than 0.1 wt% total.

The presence of a catalyst served to greatly increase the

concentration of naphthalene and the methylnaphthalenes,

the Y-zeolite (CBV-400) showing a much higher concen-

tration of these compounds compared to the zeolite ZSM-5

catalyst. For example, the maximum total concentration of

these compounds reached 15.2 wt% for the Y-zeolite (CBV-

400) catalyst (at 600 8C) compared to 4.5 wt% for the

zeolite ZSM-5 catalyst (at 530 8C).

The influence of catalyst temperature showed that, for the

Y-zeolite (CBV-400) catalyst, the concentration of naphtha-

lene and methylnaphthalenes showed a significant increase

in concentration from 5.8 wt% at 430 8C to 15.2 wt% at

600 8C. The zeolite ZSM-5 showed an increase in

concentration up to 530 8C, followed by a decrease at

600 8C. The relative concentrations of the methylnaphtha-

lenes to the parent naphthalene concentration showed

different trends for two catalysts in relation to catalyst

temperature. For the Y-zeolite (CBV-400) catalyst, the

naphthalene concentration increased in concentration at a

greater rate than the methyl derivatives, with a relative ratio

of 1:2.9 at 430 8C to 1:0.8 at 600 8C. However, the zeolite

ZSM-5 catalyst showed an approximately constant ratio of

naphthalene/methylnapthalenes of 1:2.4 at all the tempera-

tures. This data suggesting a loss of the methyl group at

higher temperatures as was also shown for the alkylated

aromatic hydrocarbons in Tables 4 and 5.

The naphthalene compounds containing two methyl

groups—the dimethylnaphthalenes were analysed in the

uncatalysed pyrolysis oils and the pyrolysis–catalysis oils

and the results are shown in Figs. 8 and 9. The uncatalysed

pyrolysis oil from the pyrolysis of used tyres contained only

low concentrations of the dimethylnaphthalenes, totalling

less than 0.3 wt%. In the presence of the Y-zeolite and

ZSM-5 catalysts, the concentration of these compounds was

significantly increased, the Y-zeolite (CBV-400) showing a

higher increase than the ZSM-5 catalyst.

Increasing the temperature of the catalyst from 430 to

600 8C showed that for the Y-zeolite, there was an initial

increase to 470 8C followed by a decrease in concentration

as the temperature was further increased. For the zeolite

Table 4

Effect of catalyst bed temperature on yields of alkyl substituted single ring

aromatics using Y-zeolite (CBV-400) as catalyst (wt%)

Catalyst bed temperature (8C)

430 470 500 530 600

Ethylbenzene 1.91 1.87 2.20 2.06 1.44

3-Ethyltoluene 2.24 2.14 2.08 1.60 0.99

4-Ethyltoluene 1.12 1.06 1.02 0.82 0.61

1,3,5-Trimethylbenzene 1.75 1.59 1.43 1.08 0.56

2-Ethyltoluene 0.62 0.61 0.64 0.54 0.50


Indane 0.87 0.79 0.73 0.59 0.42


4-Ethyl-1,2-dimethylbenzene 0.42 0.38 0.31 0.23 0.16


1,2,3,5-Tetramethylbenzene 0.44 0.39 0.29 0.22 0.14


Total (%, w/w) 15.61 14.52 13.89 10.97 7.15

Table 5

Effect of catalyst bed temperature on yields of alkyl substituted single ring

aromatics using ZSM-5 as catalyst (wt%)

Catalyst bed temperature (8C)

430 470 500 530 600

Ethylbenzene 1.02 0.85 0.72 0.42 0.71

3-Ethyltoluene 1.28 0.99 1.14 0.87 0.74

4-Ethyltoluene 0.80 0.65 0.70 0.62 0.47


2-Ethyltoluene 0.30 0.29 0.34 0.33 0.32


Indane 0.38 0.40 0.52 0.44 0.46






Total 7.46 6.59 7.97 6.61 6.33


ZSM-5, the concentration of the dimethylnaphthalenes at

the lower catalyst temperatures were similar but there was

an increase in concentration at the higher catalyst

temperatures.

The uncatalysed pyrolysis oils and pyrolysis–catalysis

oils were analysed for naphthalene compounds containing

three methyl groups—the trimethylnaphthalenes. The

uncatalysed pyrolysis oils contained less than 0.1 wt%

total of the trimethylnaphthalenes. Figs. 10 and 11 show the

trimethylnaphthalene concentrations in the Y-zeolite (CBV-

400) and zeolite ZSM-5 catalysed pyrolysis oils in relation

to catalyst temperature, respectively. The presence of a

catalyst served to increase the formation of the trimethyl-

naphthalenes. The influence of increased catalyst tempera-

ture for the Y-zeolite (CBV-400) catalyst showed a

reduction in concentration from 3.2 to 0.7 wt% as the

temperature was increased from 430 to 600 8C. The zeolite

ZSM-5 catalysed pyrolysis oils showed an increase in

concentration of trimethylnaphthalenes to 500 8C followed

by a decrease to 1.9 wt% at 530 and 600 8C.

The overall concentration of the identified single ring

aromatic compounds, alkylated single ring compounds and

the concentration of naphthalene and mono-, di- and

trimethylnaphthalenes represent the bulk of the species

present in the catalysed oils. For example, these group of

compounds comprise over 75 wt% for the CBV-400 zeolite

catalysed oil for the 600 8C temperature condition. Other

components identified in the catalysed oils were 3 and 4 ring

aromatic and other branched chain compounds.

Pyrolysis of tyres in the absence of a catalyst produces an

oil which is very complex and has fuel properties

intermediate between those of a light fuel oil and gas oil

[7]. The results reported in this paper have identified a

process route for the pyrolysis of scrap tyres, which produce

an oil product with significant concentrations of aromatic

compounds such as benzene, toluene, xylenes and naphtha-

lene in the derived pyrolysis oil. Previous work on the use of

post-pyrolysis zeolite catalysis of other feedstocks has

shown that aromatic compounds can be produced in the

derived oil, but generally in much lower product yields

[18–23]. For example, Mordi et al. [19] pyrolysed a number

of plastic types over a zeolite ZSM-5 catalyst and showed

that the use of a catalyst increased the aromatic content of

the derived products. Also, Williams et al. [18] pyrolysed

polystyrene and found that the influence of a zeolite catalyst

was to markedly increase the concentration of aromatic

hydrocarbons. Similar results were also reported by Bagri

and Williams [20] for the post-pyrolysis zeolite catalysis of

polyethylene. The application of post-pyrolysis zeolite

catalysis to biomass has been mostly to remove the high

oxygen content of the pyrolysis oils via the formation of

H2O, CO and CO2. The product oil is relatively high in

Fig. 7. Effect of catalyst bed temperature on yields of naphthalene and monomethylnaphthalenes using ZSM-5 as catalyst.

Fig. 6. Effect of catalyst bed temperature on yields of naphthalene and monomethylnaphthalenes using Y-zeolite (CBV-400) as catalyst.


aromatic hydrocarbons, but the yields of individual aromatic

compounds are very low [21–23]. Whilst, zeolite catalysis

has been coupled with pyrolysis for other feedstocks, the

application to scrap tyres in such detail as presented in this

paper represents new work. Consequently, the use of zeolite

catalysts as a post-pyrolysis process step has the potential to

improve the process economics of pyrolysis of scrap tyres

via the production of a higher value oil product which can be

used as a chemical feedstock rather than a liquid fuel.

4. Conclusions

Catalytic pyrolysis of scrap tyres using two different

zeolite catalysts has been investigated in relation to the

influence of the catalyst temperature on the yield and

composition of the products. The results showed that the

influence of the presence of either catalyst was to reduce the

yield of oil with a consequent increase in the gas yield and

the formation of coke on the catalyst. The concentration of

Fig. 8. Effect of catalyst bed temperature on yields of dimethyl and ethyl substituted naphthalenes using Y-zeolite (CBV-400) as catalyst.

Fig. 9. Effect of catalyst bed temperature on yields of dimethyl and ethylnaphthalenes using ZSM-5 as catalyst.

Fig. 10. Effect of catalyst bed temperature on yields of trimethylnaphthalenes using Y-zeolite (CBV-400) as catalyst.


certain single ring aromatic compounds in the derived oils

after catalysis were markedly increased, particularly

benzene, toluene and the m-, p- and o-xylenes. The two

catalysts used were, Y-zeolite (CBV-400) and zeolite ZSM-

5 catalyst, the Y-zeolite catalyst had a lower silica/alumina

ratio and therefore higher surface acidity and also possessed

a larger pore size than the ZSM-5 catalyst. The Y-zeolite

catalyst produced significantly higher concentrations of

benzene, toluene, xylenes, naphthalene and alkylated

naphthalenes compared to the ZSM-5 catalyst. The

influence of increasing the temperature of the catalyst

from 430 to 600 8C resulted in different responses for the

two catalysts. For the Y-zeolite, benzene and toluene

showed an increase and xylenes a decrease and zeolite

ZSM-5 showed a general increase in benzene, toluene and

the xylenes. The concentration of naphthalene, methyl-

naphthalenes, dimethylnaphthalenes and trimethylnaphtha-

lenes also showed differences between the two catalyst

types in relation to increasing catalyst temperature.

Acknowledgments

The authors gratefully acknowledge the support of the

UK Engineering and Physical Science for support for this

work (Grant No. GR/L35331/01).

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Fig. 11. Effect of catalyst bed temperature on yields of trimethylnaphthalenes using ZSM-5 as catalyst.


catalytic pyrolysis of tyres: influence of catalyst temperature

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