5-7 OCTOBER

ISFR

2011

79

TOLEDO (SPAIN)

POLYSTIRENE HYDROCRACKING IN SOLUTIONOVER BIFUNCTIONAL CATALYSTS
E. Fuentes, A. Salbidegoitia, M.P. González-Marcos and J.R. González-Velasco
Basque Country University, Bilbao, CP 49940, Spaine-mail: edwingustavo.fuentes@ehu.es

Abstract
Polystyrene hydrocracking in solution was successfully studied in batch reactor at 648-685 K, under H2 at 3-6 MPa, 1200rpm, 0-50 wt.% of PS, 0-240 min, over bifunctional catalysts and in the absence of catalyst. Comparing thermal andcatalytic processes, different reaction mechanisms can be observed to be involved in PS degradation depending on typeof acidic sites. Metallic and Lewis sites promote hydrogenation-dehydrogenation reactions which initiate the degradation,
followed by E-scission to become radicals and end-chain E-scission to produce low molecular weight hydrocarbons;
unfortunately they also terminate PS degradation by radical hydrogenation. Over Brønsted centres, a phenyl group is
attacked and released to produce benzene and radicals that propagate with end-chain E-scission. Metallic and Brønstedsites also promote ring opening reactions which decrease aromatic content in final product. With respect to operationconditions, high H2 pressure and temperature contribute both to PS degradation and ring opening reactions.

Keywords: polystyrene, hydrocracking, catalyst, Brønsted, Lewis, platinum

1. Introduction
Nowadays, polymer degradation is being studied in termsof environmental solution to waste plastics and recovery
of such an energetic source. Many studied processes,
such a pyrolysis, catalytic and thermal cracking [1], could
have applications in recovery of monomer. However,obtaining fuels needs ulterior hydro-conversion to reducearomatic fraction in the products [2]. The aim of this study
is to obtain a fraction of fuels with high quality throughhydrocracking.

2.

Materials and Methods

Hydrocracking reactions were performed with 30 wt.%
PS in decalin and 0.3% of catalyst. The reactor washeated at 648-685 K for 60 to 240 min, under H 2 at 6
MPa and 1200 rpm. Gas and liquid products wereanalyzed by GC-FID and GPC-IR. In Table 1, somecharacteristics of the catalyst used are presented.

3.

reactions. PS conversion above 50 % is reached in thefirst 60 min. Imperfections in original PS chains arethought to be responsible for initial conversions.
Pressure: Increasing H2 pressure slightly favors PS
degradation; moreover, high pressures promote ring
opening reactions and reduce catalyst deactivation.
Temperature: High temperature has a positive effect inconversion of PS, additionally encouraging ring opening
and cracking reactions. However, undesired
dehydrogenation reactions are also favored by increasing
temperature.
Catalyst: Figure 1 shows PS and decalin conversion, anddistribution of products for both thermal and catalyticprocess. Catalysts with high acidity and low
Brønsted:Lewis ratio, Pt/Al2O3 and Pt/H-Y, are observed
to produce lower PS conversion than thermal process.
On the other hand, Pt/H-Bea, that present less acidity but
the highest B/L ratio, reaches complete conversion ofResults and Discussion

Time on stream: High rate of PS degradation is observed
at low reaction times for both thermal and catalytic
Table 1. Catalyst characterization
SBETPt(m2·g-1)(% wt)Total

WeakStrong

Pt/Al2O3221

0.3550180370-Pt/H-Y661

0.5600340260

1.1

Pt/H-Bea

447

0.5

310

150

190

1.6

Catalyst

interiores libro.indd 79

Acidity (PmolNH3·g-1)

B/L

Figure 1. Conversion and product distribution of PShydrocracking in solution at 673 K, 6 MPa H2, 1200 rpm
and 240 min. P I A N O Oligomers.

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6th International Symposium on Feedstock Recycling of Polymeric Materials

PS. These results suggest that different reaction
mechanisms are involved over each catalyst.
Polymer degradation is known to follow 3 steps: initiation,propagation and termination. Firstly, in the thermalprocess, initiation occurs mainly by breakage of C-C
bonds to obtain radicals; then, in propagation, hydrogentransfer and E-scission reactions take place, to produce
monomers, dimers and trimers; and recombination,radical addition and disproportionation are termination
reactions [3]. Secondly, over Pt/AL2O3, which is abifuntional catalyst with metallic and acidic Lewis centres,
initiation is associated to C-H bond fission andE-scission; propagation occurs by alkyl and chain-end
fission reactions and undesired termination reactions likeradical hydrogenations. Finally, over Pt/H-Y and PT/HBea, in addition to metallic and Lewis centers, there areacidic Brønsted centers, and other initiation reactions canbe included, described as an attack to a proton on thephenyl group followed by releasing of benzene toproduce a cation and E-scission to produce radicals [4].
With respect to decalin conversion, decalin is observed
to be thermally stable but, in the presence of a catalyst,
both metallic and acidic sites are responsible forconversion of decalin. However, we can clearly see thatmetallic and Lewis sites promote hydrogenationdehydrogenation reactions, while metallic and Brønsted
centers promote ring opening and cracking reactions.
Ring opening reactions are widely desired because they
contribute to reduce aromatic content in final products.
Through detail hydrocarbon analysis, the following lumps
are used to present the product distributions: paraffins(P), isoparaffins (I), aromatics (A), naphthenes (N),olefins (O) and oligomers. The results shown in Figure 1,
support the reaction mechanisms above mentioned.
Firstly, oligomers and a highly aromatic fraction areobtained in the thermal process. Then, oligomers,
aromatic products, from PS degradation and decalin
dehydrogenation, and naphthenes, which are obtainedby aromatic lump hydrogenation, occur over Pt/Al2O3.
Finally, over Pt/H-Y and Pt/H-Bea, oligomers from PS
degradation, aromatics from PS and decalin ring
opening; naphthenes, as a product of hydrogenation ofaromatics; olefins, paraffins and isoparaffins, as a
product of ring opening and cracking reactions of decalin
and naphthenes derived from PS, are produced.
PS concentration: Reactions with concentration up to 50
wt.% in PS were carried out over Pt/H-Bea. It isnoteworthy that, in all cases, PS degradation wascomplete before 240 min.
Yield and product distribution by number of carbons andtype of hydrocarbons are shown in Figure 2. C1-C3hydrocarbons are observed to be paraffins formed bycracking reactions, and their yield increases with PS
concentration. C4-C5 are isoparaffins and paraffins,probably produced by ring opening of single ringaromatic compounds. C6-C8 is an aromatic fraction that
increases with PS initial concentration and naphthenes
from hydrogenation reactions; benzene and cyclohexane

interiores libro.indd 80Figure 2. Product distribution of PS hydrocracking insolution over Pt/H-Bea at 673 K, 6 MPa H2, 1200 rpm
and 240 min reaction time.
in C6 are probably obtained by PS degradation overBrønsted centers, whereas toluene, ethylbenzene andtheir hydrogenated products in C7 and C8 are formed
over Lewis centers. In fraction C9-C12 there arearomatics, naphthenes and isoparaffins, mainly produced
by decalin reaction. Finally, in C13+ there are aromaticcompounds, which are associated with oligomers fromPS degradation; this fraction increases with PS content.

4. Conclusions
Differences in PS conversion and product distribution canbe explained by changes in the reaction mechanisms of
PS degradation in the absence and in the presence ofacidic catalysts. Acidic catalyst with high Brønsted:Lewis
ratio are observed to be favorable to PS degradation
and, moreover, this kind of catalysts produce a hugevariety of products, because they promote further
reactions such as hydrogenation-dehydrogenation, ring
opening and cracking. This fact contributes to improve
the quality of the product.
Increasing PS initial concentration does not have a huge
impact in the reaction because, until 50 wt.% of PS, PS
was observed to be successfully converted into C1-C8hydrocarbons. The increase of initial viscosity may cause
problems in mass and heat transfer, but the initial highreaction rates compensate this fact.

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liquid to an environmentally favorable gasoline range
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93 (2008) 561-584.
[4] H. Ukei, T. Hirose, S. Horikawa, Y. Takai, M. Takaand N. Azuma. Catalytic degradation of polystyrene intostyrene and a design of recyclable polystyrene withdispersed catalysts. Catalysis Today 62 (2000) 67-75.

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