J. Anal. Appl. Pyrolysis 74 (2005) 353–360
www.elsevier.com/locate/jaap
Influence of nanocrystalline HZSM-5 external surface on
the catalytic cracking of polyolefins
D.P. Serrano *, J. Aguado, J.M. Escola, J.M. Rodrı́guez
Rey Juan Carlos University, Escuela Superior de Ciencias Experimentales y Tecnologı́a (ESCET), c/Tulipán s/n, 28933 Móstoles, Spain
Received 23 June 2004; accepted 9 November 2004
Available online 7 April 2005
Abstract
Catalytic cracking of both LDPE and HDPE has been studied using three different samples of nanocrystalline HZSM-5 zeolite as catalysts.
The zeolite samples were synthesized under different conditions in order to obtain materials with different crystal sizes in the nanometer range
(10–60 nm) and high external surface areas (78–242 m2/g). The plastic cracking reactions were carried out at 340 8C for 2 h using a plastic/
catalyst mass ratio of 100. Despite these really mild conditions, high activities were observed over the three catalysts. The plastic conversion
and the turnover frequency increased strongly with the amount of external surface area of the catalysts, showing the importance of the nonsterically hindered external acid sites for the conversion of the bulky plastic molecules. The main products obtained in the cracking reactions
over the three catalysts are light hydrocarbons (C3–C5 fraction) with a considerable amount of olefins, which indicates the possibility of
addressing the zeolite selectivity towards hydrocarbons useful as raw chemicals. These products are formed via an end-chain cracking
pathway of the polymer molecules over the zeolite acid sites. The occurrence of this mechanism has been confirmed by DSC analysis of the
LDPE and HDPE residues remaining within the reactor after the catalytic tests, showing they still consist of plastic molecules with melting
temperatures similar to those of the raw polymers.
# 2005 Elsevier B.V. All rights reserved.
Keywords: LDPE; HDPE; Plastic cracking; Nanocrystalline HZSM-5
1. Introduction
The worldwide production of plastics has grown from
virtually 0 to almost 100 million tonnes per year in the past
half century. Between 1991 and 2002, the per capita
consumption of plastics increased in Western Europe from
64 to 95 kg/inh/year, an average growth of 3% per year [1].
Although policies demanding recycling and greater levels of
resource conservation are being introduced, land-filling still
remains as the preferred choice of plastic waste management
in Europe. However, this alternative presents a variety of
constraining factors related to the volume that plastics
occupy in relation to their weight and the loss of natural
resources that this practice represents. Another choice is
material recycling, in which plastics are regranulated and
reused, but it is limited by the necessity of collecting pure
* Corresponding author. Tel.: +34 91 488 70 88; fax: +34 91 664 74 90.
E-mail address: d.serrano@escet.urjc.es (D.P. Serrano).
0165-2370/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jaap.2004.11.037
and high quality plastics, as well as by product market
limitations. Therefore, feedstock recycling of plastic wastes
has appeared as a reliable option for converting plastic
residues into useful materials [2].
The simplest method of plastic feedstock recycling is
thermal cracking which proceeds with low yields towards
the corresponding raw monomers giving a complex mixture
of products with low value as raw chemicals and mainly
useful as fuel [3–5]. However, by plastic cracking over
different solid catalysts it is possible to obtain products with
higher commercial value and with a wider number of
applications [6–8].
Nanocrystalline HZSM-5 zeolite has demonstrated to be
a successful catalyst in the degradation of polyolefins
(HDPE, LDPE and PP) into light hydrocarbon mixtures [9].
The high activity and the high selectivity towards gases
obtained over this catalyst can be explained as a
consequence of two factors: the strong acidity of the zeolite
and the considerable amount of fully accesible acid sites
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D.P. Serrano et al. / J. Anal. Appl. Pyrolysis 74 (2005) 353–360
located on the external surface of the nanocrystalline zeolite
samples. Catalytic cracking over acid solids takes place
through the formation of carbenium and carbonium ions.
The strength and nature (Brönsted or Lewis) of the acid sites
affect both the activity and the products obtained in the
cracking: the stronger the sites, the lighter hydrocarbon
mixture obtained [10,11]. However, due to the bulky nature
of the polymer macromolecules, another factor playing a key
role for catalyst activity is the accesibility of the acid sites.
Thus, several works have been reported in the last years on
the polymer cracking over mesoporous materials [12,13] or
nanosized zeolites [9,14]. Although in some of the works the
amount of external surface of the catalyst is considered as
one of the main factors affecting the catalyst activity, the
influence of this variable has not been studied as deeply as
the strength of the acid sites [15,16].
In this work, cracking of low-density polyethylene
(LDPE) and high-density polyethylene (HDPE) has been
investigated over a number of nanocrystalline HZSM-5
samples having different crystal sizes. Thereafter, the
relationship between the amount of external acid sites,
fully accesible for the polymer macromolecules, and the
zeolite activity is discussed.
measurements were carried out in a JEOL 2000 electron
microscope operating at 200 kV.
The incorporation of aluminum into the zeolite framework was checked by 27Al–MAS–NMR spectra of the
calcined catalysts. The spectra were obtained at 104.26 MHz
using a Bruker MSL-400 spectrometer equipped with a
Fourier transform unit. The external standard reference was
[Al(H2O)63+] and all measurements were carried out at
ambient temperature with a spinning frequency of 4000 cps
and time intervals of 5 s between successive accumulations.
The acid properties of the catalysts were determined by
ammonia temperature-programmed desorption (TPD) in a
Micromeritics 2910 (TPD/TPR) equipment. Previously, the
samples were outgassed under a He flow (50 N ml min1) by
heating with a rate of 15 8C min1 up to 560 8C and
remaining at this temperature for 30 min. After cooling to
180 8C, an ammonia flow of 35 N ml min1 was passed
through the sample for 30 min. The physisorbed ammonia
was removed by flowing He at 180 8C for 90 min. The
chemically adsorbed ammonia was determined by increasing the temperature up to 550 8C with a heating rate of
15 8C min1, this temperature being maintained constant for
30 min. The ammonia concentration in the effluent He
stream was monitored by a thermal conductivity detector.
2. Experimental
2.2. Plastics
2.1. Catalysts
The raw polyolefins used in this work are pure lowdensity polyethylene (LDPE, MW = 416,000) and highdensity polyethylene (HDPE, MW = 188,000) provided by
REPSOL-YPF.
The catalysts used in this work were synthesized in our
laboratory according to procedures published elsewhere
[17,18], using tetrapropylammonium hydroxide (TPAOH)
as structure-directing agent, and tetraethylorthosilicate
(TEOS) and aluminum isopropoxide (AIP) as silica and
aluminum sources, respectively. The molar composition of
the obtained synthesis gel was as follows: Al2O3:60 SiO2:11
TPAOH:900 H2O. After aging for 40 h, the crystallization
took place under autogenous pressure at 170 8C (sample A)
or under atmospheric pressure at 90 8C (samples B and C).
The zeolites obtained were separated by centrifugation,
washed with deionized water, dried at 110 8C for 12 h and
finally calcined under static air for 5 h at 550 8C.
The crystallinity of the samples was checked by X-ray
diffraction (XRD) in a Philips X’PERT MPD diffractometer
using Cu Ka radiation. The Si/Al atomic ratio of the catalysts
was determined by induced coupled plasma spectroscopy
(ICP) with a Varian VISTA-AX CCD apparatus. Nitrogen
adsorption–desorption isotherms at 77 K of the calcined
catalysts were obtained with a Micromeritics ASAP 2010
sorptometer. Previously, the samples were outgassed under
vacuum at 200 8C for 5 h. Surface areas were calculated by
means of the BET equation. Micropore volume and external
surface area of the catalysts were obtained by application of
the t-plot method. Cylindrical pore geometry was assumed for
the calculations and the Jura–Harkins equation was used to
determine the thickness of the adsorbed layer. TEM
2.3. Catalytic cracking experiments
The experimental installation used in the present study is
depicted in Fig. 1 and it consists of a stirred batch reactor
provided with a helicodal stirrer. In a typical experiment, a
mixture of 10 g of plastic and 0.1 g of catalyst (plastic/
catalyst mass ratio = 100) were loaded into the reactor.
Subsequently, the reactor is heated with a rate of 6 8C min1
up to the reaction temperature (340 8C) that was maintained
for 2 h. The volatile products are swept from the reactor by a
continuous nitrogen flow (35 N ml min1). Subsequently,
the liquids are condensed in an ice trap and the gases are
collected in a gas-bag. The analyses of both liquid and
gaseous fractions were carried out by gas chromatography in
a Varian 3800 GC using a 100 m length 0.25 mm i.d.
Chrompack capillary column. Conversions have been
calculated taking into account only the products that leave
the reactor with the N2 stream (Cn, n 40), the remaining
residue being not considered as a reaction product. The
turnover frequency was calculated as (grams of plastic
converted) (grams of aluminum in the catalyst)1 (s)1.
The mass balances were closed in every experiment within a
5 wt.% error. After the different catalytic tests, the residues
remaining within the reactor were characterized by
D.P. Serrano et al. / J. Anal. Appl. Pyrolysis 74 (2005) 353–360
355
Fig. 1. Scheme of the experimental cracking installation.
differential scanning calorimetry (DSC) using a DSC822e
METTLER-TOLEDO instrument. The samples were heated
from 20 to 250 8C with a rate of 10 8C min1.
3. Results and discussion
3.1. Catalysts
The purity of the nanocrystalline HZSM-5 samples used
as catalysts in this work has been checked by XRD analysis,
Fig. 2. XRD spectra of the as-synthesized nanocrystalline HZSM-5 (samples A, B and C) compared to a micrometer crystal size HZSM-5 (sample
D).
shown in Fig. 2. As a reference, it has been also included the
spectra corresponding to a micrometer crystal size ZSM-5
zeolite (sample D). The peaks present in the XRD patterns
correspond with the main diffractions of the MFI zeolitic
structure and no broad bottom reflection is observed
showing the absence of amorphous material in the samples.
The intensity and broadness of the peaks vary from one
sample to another. Thus, broader and less intense diffraction
peaks were attained continuously on going through the
series as follows: sample D ! A ! B ! C. According to
the Scherrer law, this fact can be considered as a result of the
decrease in the crystal size.
27
Al–MAS–NMR spectra of the three catalysts before
calcination (not shown) present only a distinct peak at
52 ppm corresponding to tetrahedral aluminum incorporated
into the zeolite framework. In the calcined samples, another
fairly small peak, placed at 0 ppm and assigned to
octahedral (extraframework) aluminum appears. These
results indicate that all the aluminum is incorporated with
tetrahedral coordination into the zeolite structure and only a
negligible amount of aluminum is extracted from the
framework after calcination.
Nitrogen adsorption–desorption isotherms at 77 K and
TEM micrographs of the three nanocrystalline HZSM-5
samples are shown in Fig. 3. All of them are formed by
crystals with sizes within the nanometer range. However,
meaningful differences can be appreciated among them.
While in sample A the crystals exhibit cubic-like
morphology and size within the 40–70 nm range, samples
B and C are formed by much smaller crystals (10–20 nm)
which are aggregated in larger units with size around
200 nm. Since the external surface area of zeolite HZSM-5
is enhanced when the crystal size decreases [18], the three
samples possess high external surface areas. In the case of
sample C, the external surface area is 242 m2/g, which is a
really high value and accounts for about 50% of the total
surface area in this sample. These great differences among
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D.P. Serrano et al. / J. Anal. Appl. Pyrolysis 74 (2005) 353–360
Fig. 3. N2 adsorption isotherms at 77 K and TEM micrographs of the nanocrystalline HZSM-5 samples.
the crystal size of the three catalyst samples are originated
by the variation introduced in the zeolite synthesis
conditions. Samples B and C have been crystallized at a
quite lower temperature (908) compared to sample A (1708),
which favours the formation of smaller nanocrystals.
The nitrogen isotherms illustrated in Fig. 3 exhibit three
different adsorption zones. The first one, observed at very
low partial pressures, corresponds to the nitrogen adsorption
in the HZSM-5 micropore system. The micropore volume of
the zeolites decreases with the crystal size (from 0.159 cm3 /
g in sample A to 0.127 cm3/g in sample C). The second one,
at medium partial pressures, arises from the nitrogen
adsorption on the external surface, being much more
important in samples B and C. The third one is the steep
jump present at high relative pressures (P/P0 > 0.8),
describing a hysteresis cycle, which indicates the presence
of interparticular porosity.
The acid properties of the catalysts are summarised in
Table 1. Although all the samples were synthesized with
Table 1
Acid properties of the nanocrystalline HZSM-5 samples
Sample A
Sample B
Sample C
a
b
c
Si/Ala
Acidityb
(meq/g)
Tmaxb (8C)
Crystal
sizec (nm)
30
59
41
0.324
0.115
0.145
350
325
305
55
20
12
From ICP–AES measurements.
From ammonia TPD experiments.
From TEM micrographs.
D.P. Serrano et al. / J. Anal. Appl. Pyrolysis 74 (2005) 353–360
close Si/Al atomic ratios (30) in order to get a similar
content of acid sites, a different degree of aluminum
incorporation into the catalysts is observed. Si/Al atomic
ratios in the samples vary between 30 (sample A) and 59
(sample B). The catalyst acidity measured by the amount of
NH3 being desorbed in the TPD experiments also varies
from one sample to another. This variation agrees well with
the different aluminum content of the samples. The
temperature maxima for ammonia desorption of the
nanocrystalline HZSM-5 samples are placed between 305
and 350 8C. These differences can be explained either as a
result of their different acid strength or as a consequence of
their different crystal size. The temperature maxima
decrease with the crystal size can be assigned to the minor
diffusional constrains presented by smaller crystals during
the ammonia desorption step, since the intracrystalline
diffusion rate is proportional to De =R2C , where De is the
effective diffusion coefficient and RC the radius of the zeolite
crystal. Accordingly, a variation of the crystal size by a
factor of about 5 from sample A–C should lead to a rather
faster intracrystalline ammonia diffusion for the last sample.
Nevertheless, the existence of a weaker acid strength in the
zeolite samples as the external surface is increased, cannot
be completely neglected to explain those changes in the
ammonia desorption temperature maxima, since the nature
and acidic features of the external sites may be very different
from those of the internal ones.
3.2. Plastic cracking experiments
Catalytic cracking of the polyolefins (LDPE and HDPE)
has been carried out over the three zeolite samples in a
stirred batch reactor at 340 8C for 2 h. Table 2 summarizes
the results obtained in the catalytic degradation of both
polymers in terms of conversion and selectivity by
hydrocarbon groups.
It is remarkable that high activity is exhibited by the three
catalyst samples, leading to plastic conversion between c.a.
20 and 80%, in spite of the low temperature and the reduced
amount of catalyst used (plastic/catalyst mass ratio of 100).
Additional cracking experiments using other types of solid
acid catalysts, such as ordered mesoporous materials (Al–
Table 2
Catalytic cracking of HDPE and LDPE over different samples of nanocrystalline HZSM-5 zeolite
Conversion (%)
Selectivity by groups (wt.%)
C6–C12
>C13
LDPE cracking (T = 340 8C; P/C = 100; t = 2 h)
A
31.6
100
B
42.1
95
C
79.7
88
0
5
12
0
0
0
HDPE cracking (T = 340 8C; P/C = 100; t = 2 h)
A
18.8
100
B
27.2
100
C
48.7
100
0
0
0
0
0
0
C1–C5
357
SBA-15, Al–MCM-41) or standard HZSM-5 with micrometer crystal size, were carried out under the same reaction
conditions, showing that nor catalytic activity neither
thermal cracking is possible at such a reduced temperature
(340 8C) and with such a small amount of these catalysts.
These results confirm the exceptionally high activity of the
nanocrystalline HZSM-5 zeolite for the cracking of
polyolefinic plastics.
For the three catalyst samples, conversions and turnover
frequencies obtained in the catalytic cracking of LDPE were
higher than those of HDPE due to the enhanced reactivity
associated to the structure of these polymers. HDPE is
formed by linear macromolecules, whereas LDPE is
characterized by a certain degree of branching. The presence
of tertiary carbons in the LDPE molecules provides
favourable positions for the initiation of the polymer chain
cracking since their activation requires weaker conditions
than the secondary carbon activation does [19].
Since the origin of the cracking activity in zeolites is
related to the presence of acid sites, and taken into account
that the three nanocrystalline HZSM-5 samples used in this
work present different Si/Al ratios, the comparison of
activities has been also performed in terms of turnover
frequency (TOF), defined as the mass of plastic converted
referred to the aluminum mass in the catalyst and the time
unit. Fig. 4 shows the activity results obtained in terms of
both conversion and TOF. For both polymers a clear trend of
higher cracking activity as the external surface area of the
catalysts increases is observed. Thus, TOF values for sample
B are multiplied by a factor higher than 2 compared to
sample A, which closely resembles the variation of the
external surface area between both samples. Likewise, the
highest conversion and TOF values are obtained with sample
C, having a very high external surface area. It is remarkable
that this catalyst has led to HDPE and LDPE conversions of
around 50 and 80%, respectively. These values should be
considered as extremely high having in mind the mild
reaction conditions used in this work. If the TOF values are
estimated per unit of external surface area, the following
order of intrinsic activity is obtained for both polymers:
sample B > sample C > sample A. The highest activity of
sample B can be interpreted as a right combination of small
crystal size and high acid strength.
The relationship observed between the cracking activity
and the HZSM-5 external surface area can be interpreted
taken into account that the intracrystalline diffusion of the
polymer macromolecules within the zeolite pores is
strongly hindered and the acid sites placed on the external
surface are liable to crack the polyolefinic chains.
Accordingly, bulky molecules which are not able to enter
the zeolites micropores can be cracked on the external
surface of the zeolite. Moreover, because of the small
crystal size, the subsequent intracrystalline diffusion and
reaction within the zeolite micropores of the primary
products of the plastic degradation is also favoured in
nanosized HZSM-5.
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D.P. Serrano et al. / J. Anal. Appl. Pyrolysis 74 (2005) 353–360
Fig. 4. Conversion and TOF (s1) values obtained in the LDPE and HDPE
cracking over nanocrystalline HZSM-5 zeolites.
In regards to the product distribution shown in Table 2, it
is clearly seen that for both polymers the cracking leads
mainly to the formation of light hydrocarbons (C1–C5
fraction), with very high selectivities (between 88 and
100%). This fact is a result of the cracking mechanism
through the formation of a carbenium ion taking place on the
nanocrystalline HZSM-5 acid sites. The main reaction
pathway is end-chain scission, which yields light hydrocarbons as primary products. Other reactions that may take
place are oligomerization, cyclization and aromatization
processes leading to the formation of a wider variety of
hydrocarbons, although limited and controlled by the
HZSM-5 shape-selectivity [20].
In the case of nanocrystalline HZSM-5 samples, the
presence of a high amount of external surface is expected to
affect negatively to its shape-selectivity, hence the
appearance of a wider product distribution should be
expected compared to micrometer size HZSM-5 samples.
However, this is not the trend of the results obtained in the
present work, since for both polymers the formation of light
hydrocarbons in a narrow range of carbon atom numbers
occurs. These results indicate that in spite of their high
external surface the end-chain cracking is the mechanism
Fig. 5. DSC analyses of the residues obtained with the different catalysts.
predominant over nanocrystalline HZSM-5. Only when the
catalyst activity is very high, oligomerization, cyclization
and aromatization reactions are important enough to
promote the formation of heavier products (selectivity
towards gases decreases to 88% over sample C when the
turnover frequency reaches a value of 0.7 s1).
Fig. 5 illustrates the DSC analyses of the different
residues, which remain in the reactor after the catalytic tests,
being compared with those corresponding to the raw
polymers. In all cases just one endothermic peak is observed
which accounts for the melting temperature. In the case of
the residues obtained from LDPE this peak is placed at
around 110–111 8C, whereas for the residues coming from
the HDPE degradation is located at 131–132 8C. The
melting temperatures of the different residues match
perfectly with that of the corresponding raw polymers.
Moreover, no peaks are observed at lower temperatures
indicating the absence of waxy products that, on the
contrary, would be expected as main products of the polymer
D.P. Serrano et al. / J. Anal. Appl. Pyrolysis 74 (2005) 353–360
359
4. Conclusions
Fig. 6. Selectivity by carbon atom number and composition of the C2–C5
fraction obtained in the catalytic cracking of HDPE and LDPE over the
nanocrystalline HZSM-5 sample C.
molecules cracking through a random scission pathway.
These results show the plastic nature of the residues obtained
with the different catalysts, which agrees well with the
occurrence of an end-chain cracking mechanism leading
mainly to gaseous olefins.
Fig. 6 compares the product distributions per carbon
atom number, as well as the hydrocarbon nature, obtained
in the catalytic cracking of both HDPE and LDPE over
sample C. The main products are hydrocarbons in the
range C3–C5, which account for about 98% of the overall
product distribution. In both cases, the largest fraction
corresponds with C4 hydrocarbons. It is noteworthy the
high amount of olefins obtained with both polymers
(around 70%), since they are of interest for being applied
as raw chemicals. Therefore, nanocrystalline HZSM-5
zeolite can be considered, not only as a very active catalyst
for the polyolefin cracking, but its high selectivity towards
gaseous olefins may favour the industrial application of
this process.
Catalytic cracking of LDPE and HDPE over nanocrystalline HZSM-5 samples proceeds with high activity and very
high selectivity towards olefinic gases due to the reduced
diffusional hindrances showed by small HZSM-5 nanocrystals, and the presence of a high amount of external surface
area.
The catalytic activity, expressed in terms of plastic
conversion and turnover frequency, depends directly on the
external surface area present in the nanocrystalline HZSM-5
samples. Thus, the highest HDPE and LDPE conversions
were obtained over sample C having an external surface area
of 242 m2/g, which accounts for about 50% of the total
surface area in this material. This activity must be
considered as really high, taken into account the mild
conditions used: a temperature of 340 8C and a plastic/
catalyst ratio of 100.
A high selectivity towards light hydrocarbons (C3–C5),
with a considerable amount of olefins, has been obtained in
the cracking of both HDPE and LDPE over the nanocrystalline HZSM-5 samples. Only for reactions with high
conversions, minor amounts of heavier hydrocarbons have
been detected in the reaction products. This result indicates
that an end-chain scission mechanism is predominant,
probably due to the high acid strength of the HZSM-5
zeolite. DSC analyses of the residues remaining in the
reactor after the catalytic tests confirm the occurrence of this
cracking pathway, since their melting temperatures match
perfectly with those of the raw LDPE and HDPE polymers,
whereas lower temperature peaks, that could be associated to
the presence of waxy fractions, are not detected.
Both the high activity and selectivity towards gaseous
olefins place this process as a promising alternative for the
chemical recycling of plastic wastes.
Acknowledgements
We want to thank ‘‘Ministerio de Ciencia y Tecnologı́a’’
(Project CICYT REN2002-03530) for the financial support
of this research. We also want to thank REPSOL-YPF for
providing the plastic samples used in this work.
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