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TOLEDO (SPAIN)CATALYTIC CRACKING OF POLYETHYLENE OVER HIERARCHICAL H-ZSM-5
ZEOLITES WITH DIFFERENT PROPORTION OF MICRO- AND MESOPOROSITY
D. P. Serrano1,2, J. Aguado3, J. M. Escola3, A. Peral1
1Department of Chemical and Energy Technology, ESCET, Universidad Rey Juan Carlos, C/ Tulipán s/n, 28933 Móstoles,Madrid, Spain
2Madrid Institute for Advanced Studies on Energy, IMDEA Energía, C/ Tulipán s/n, 28933 Móstoles, Madrid, Spain
3Department of Chemical and Environmental Technology, ESCET, Universidad Rey Juan Carlos, C/ Tulipán s/n, 28933

Móstoles, Madrid, Spaine-mail: david.serrano@urjc.esAbstract
A variety of H-ZSM-5 zeolites with hierarchical porosity, that is, containing both the characteristic zeolite micropore system
and additional mesoporosity were prepared according to the method of crystallization from silanized seeds. These materials
differing in crystallinity and textural properties were compared with nanocrystalline H-ZSM-5 samples, and tested in thecatalytic cracking of low density polyethylene (LDPE). The selected catalysts range from fully crystalline according to XRDand FTIR to amorphous ones. As crystallinity diminishes, microporosity decreases, BET surface area increases (up to 1085m2g-1) and secondary mesoporosity appears, with pore diameters centered at 20 and 400 Å. Cracking reactions were carriedout employing really mild conditions: 340 ºC and plastic/catalyst mass ratio of 100. Turnover frequencies values (TOF) from0.03 to 0.90 were attained, the most active materials being those combining high crystallinity and acid sites fully accessible
through the additional mesopores generated (20-70 Å). The selectivity depends on the catalyst employed but the major
products were C1-C5 hydrocarbons, percentages always higher than 50% being attained. The amount of middle distillates(>C13) was always below 2%.

Keywords: catalytic cracking, hierarchical zeolite, ZSM-5, silanized seeds.

1. Introduction

2. Materials and Methods

Among the number of alternatives existing in the field offeedstock recycling, catalytic cracking over different solid
catalysts has attracted special attention in the last years[1]. Particularly, it is possible to obtain valuablehydrocarbon mixtures, with remarkable activities andvery high selectivities towards light olefins employing
nanocrystalline H-ZSM-5 zeolite, due to its strong acidity
and high proportion of external surface area. Moreover,hierarchical nanocrystalline H-ZSM-5, containing abimodal pore system with an important mesoporosity
additional to the conventional zeolite micropore system,has been synthesized according to the strategy of
crystallization from silanized seeds [2] and has proven its
higher activity in the catalytic cracking of polyolefins withrespect to the traditional nanocrystalline H-ZSM-5 [3].
These results were ascribed to the good acid properties
and higher proportion of external surface area
(intercrystal mesoporosity) of the hierarchical zeolites. In
this work, a number of hierarchical H-ZSM-5 zeolites,
containing different proportions of zeolite micropores andadditional mesopores, have been prepared. Thesematerials have been compared with two nanocrystalline
H-ZSM-5 samples, and tested in the catalytic cracking of
low density polyethylene (LDPE), with the aim toascertain the importance of the mesoporosity in thecatalytic activity of the hierarchical catalysts.

Synthesis methods published elsewhere were applied inorder to prepare the hierarchical [2] and nanocrystalline[4] H-ZSM-5 zeolites. The experimental conditions weremodified and adjusted in order to obtain zeolites withdifferent physicochemical and textural properties. The
samples were characterized by means of conventional
techniques: XRD, FTIR, N2 adsorption-desorption
isotherms at 77 K, Ar adsorption-desorption isotherms at87 K, TEM, NMR, ICP-AES, TPD, TG and elementalanalysis.

interiores libro.indd 83

The cracking reactions of low density polyethylene(LDPE) were carried out in a stainless steel batch reactor
provided with a screw stirrer. In a typical experiment, amixture of 10 g of plastic and 0.10 g of catalyst (P/C =100) were loaded into the reactor. Next, the reactor isheated up to the reaction temperature (340 ºC) that wasmaintained for 2 h. The volatile products are swept fromthe reactor by a continuous nitrogen flow. Subsequently,
liquid and gaseous products are separated in acondenser cooled by an ice/water mixture and collected
for GC analysis. Conversions have been calculated
taking into account only the products that leave thereactor with the N2 stream, the remaining residue being
not considered as a reaction product. The activity was

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6th International Symposium on Feedstock Recycling of Polymeric Materials

Table 1. Physicochemical and textural properties.
Sample

Si/Al

1
2
3
4
5
6
7
8
9
10

36
29
34
31
36
35
33
28
17
33

Tmax
(ºC)
391
395
385
363
333
361
321
302
265
269

SBET
(m2/g)
420
409
553
559
675
798655
792
888
1085

SZMP(m2/g)
346
352
311
293
233
180
161
57
38
19SSP(m2/g)
7457
242
266
442
618
494
735
850
1066

All the later samples were tested in the catalytic crackingof LDPE employing really mild conditions (340 ºC andP/C=100), and the results obtained are depicted in Fig. 1.
The highest conversion (86%) and turnover frequency(TOF) values (0.90 s-1) are attained employing thesample 3, decreasing these parameters along the seriesof hierarchical H-ZSM-5 samples (3-10). The twointeriores libro.indd 841001,0900,9800,8700,7600,6500,5400,4300,3200,2-1

Ten H-ZSM-5 zeolites with different textural properties
were synthesized and tested in the catalytic cracking ofLDPE. Table 1 summarizes the main physicochemical
properties of the calcined samples. According to XRDand FTIR data (not shown), the crystallinity decreases
from sample 1 to sample 10, being samples 9 and 10
practically amorphous materials. This fact is inagreement with the reduction in the surface of the zeolite
micropores (SZMP), observed in Table 1, from sample 1 to
10. Moreover, as the cristallinity (and SZMP values) ofthese samples diminishes, the values of BET surface
area (SBET) and secondary porosity (SSP = SBET - SZMP)
increase. Consequently, the decrease of XRD and FTIR
crystallinity of the samples is accompanied by the paralleldevelopment of mesoporosity. While both nanocrystalline
samples 1 and 2 show mainly a strong maximum
centered at 5.5 Å in the pore size distribution,corresponding to the dimension of the micropores ofZSM-5 zeolites, hierarchical H-ZSM-5 zeolites (from 3 to8) exhibit also mesoporosity within 20-70 Å. Finally, themicropore peak practically disappear in amorphous
samples 9 and 10, arising however a huge peak centered
at 400 Å, which could be assigned to interparticle
mesoporosity. As inferred from Table 1, all the samples
were prepared with Si/Al atomic ratios in the range 1736, according to ICP analyses, and practically the wholeof aluminium was incorporated into its framework (from27Al
MAS-NMR experiments, not shown). The
temperature maximum (Tmax) of the NH3 desorption(calculated from NH3 TPD measurements) shows aroughly decreasing trend from sample 1 (~290) to sample
10 (~260 ºC). Consequently, the generation of secondary
porosity implies a decrease in the acid strength of theacid sites.

TOF (s )3. Results and Discussion

nanocrystalline samples (1 and 2) show conversion andTOF values around 25-50% and 0.30-0.45 s-1,
respectively, clearly higher than those obtained with thesamples 8 to10. The selectivity patterns obtained are inline with the typically obtained over the H-ZSM-5 zeolite,
which is known to yield mainly light hydrocarbons (belowC13). The major products are C1-C5 hydrocarbons, withpercentages in the range 55-65% for samples 1 to 7, andhigher than 80% for samples 8 to 10. The proportion ofhydrocarbons heavier than C13 is always below 2%.

Conversion (%wt)

calculated as (mass of plastic converted) (mass ofaluminium in the catalysts)-1 s-1.

0,1

100

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100,0

SampleFig. 1. Catalytic cracking of LDPE over the calcinedsamples (T = 340ºC, t = 2 h, P/C = 100).

4. Conclusions
The catalytic performance of the different samples isexplained in terms of the compromise between the acidstrength and the accessibility of the acid sites(development of mesoporosity). Hence, the intermediate
activity of the two nanocrystalline H-ZSM-5 samples isdue to the presence of fully crystalline acid sites but notveryaccessible
throughtheirinterparticularmesoporosity. In contrast, the remarkable activity ofhierarchical H-ZSM-5 sample 3 can be ascribed to thepresence of highly crystalline acid sites fully accessible
through its mesopores (20-70 Å), enabling the bulky
molecules of LDPE to diffuse. Likewise, the progressive
loss of strength of the acid sites along the series ofhierarchical H-ZSM-5 materials (4-10) explains theobserved decrease of the activity, despite theprogressive development of secondary porosity.

References[1] J. Sheirs, W. Kaminsky. Feedstock recycling andpyrolysis of waste plastics: converting waste plastics into
diesel and other fuels. John Wiley & Sons, Chichester(2006).
[2] D.P. Serrano, J. Aguado, J.M. Escola, J.M.
Rodríguez, A. Peral. Chem. Mater. 18 (2006) 2462–2464.
[3] D.P. Serrano, J. Aguado, J.M. Escola, J.M.
Rodríguez, A. Peral. J. Catal. 276 (2010) 152-160[4] R. Van Grieken, J.L. Sotelo, J.M. Menendez, J.A.
Melero. Microporous Mesoporous Mater. 39 (2000) 135–147.

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