5-7 OCTOBERISFR2011

93

TOLEDO (SPAIN)

HZSM-5 CATALYST DEACTIVATION IN THE CRACKING OF POLYOLEFINS IN A
CONICAL SPOUTED BED REACTOR
P. Castaño, G. Elordi, J. Bilbao and M. Olazar
University of the Basque Country, Department of Chemical Engineering, 48080-644 Bilbao, Spain.
e-mail: pedro.castano@ehu.esAbstract
In the pyrolysis-cracking of polyolefins, the catalyst suffers deactivation by coke. We have studied the deactivation of aHZSM-5 catalyst in terms of activity decay, deterioration of the catalytic properties and coke composition. The reactionswere performed in a conical spouted bed reactor by continuously feeding the solid plastic –high density polyethylene(HDPE) and polypropylene (PP)– for 30 h, atmospheric pressure and 500 ºC. Samples of the spent catalyst werecollected from the reactor at different times-on-stream and then analysed using TG-TPO, N2 and NH3 adsorption, Raman,FTIR, UV-vis, 13C NMR spectroscopies, and submicron imaging: TEM, polarized-light-, FTIR- and Raman-microscopies).
Combining the kinetic results with the deterioration of the catalyst and the properties of coke, we have explained theformation and the character of the coke.

Keywords: catalyst deactivation, coke; MFI, BEA, FAU zeolite; pyrolysis, cracking, plastics

1. Introduction
The pyrolysis of plastics, particularly of polyolefins, is aninteresting pathway for obtaining fuels or recovering theoriginal monomers [1,2]. Using an acid catalyst in thepyrolysis reactor leads to more efficient operation, andenhanced selectivity toward the formation of C 2-C4
olefins when an HZSM-5 zeolite is used, or gasoline
when a HY zeolite is used [3]. The conical spouted bedreactor has a cyclic movement of the particles, which
improves the melting of the plastic, and thehomogeneous coating of the catalyst. Thus, the physicalsteps do not have a role on the product yields andselectivities.
The main obstacle for the scale-up of the cracking ofplastics is the deactivation of the catalyst caused by coke
deposition [4]. In this work we have studied thedeactivation of acid catalysts used for the pyrolysis of
HDPE and PP in terms of activity decay, degradation of
the catalytic properties and coke composition.

2. Materials and Methods
The catalyst was prepared through wet extrusion
agglomerating 25 wt% of zeolite (HZSM-5 with
SiO2/Al2O3= 30, Zeolyst International), bentonite (Exaloid,30 wt%) and α-Al2O3 (Martinswerk, 45 wt%). The
agglomeration is intended for giving extra resistanceagainst attrition in the fluidized bed, and for increasing
the volume of macro and mesopores of the catalyst. The
particles were obtained after wet extrusion, drying (120°C, 24 h), crushing-sieving (0.6-1.2 mm), and calcination(550 °C, 3 h). The reactor is a conical spouted bed of 3 Lcapacity. Conditions: 500 ºC, 1 bar. The bed consists of30 g of catalyst, and plastic (HDPE or PP) wasinteriores libro.indd 93

continuously fed (1 g min-1) through the top of thereactor, up to 1.6 kg [3]. Samples of the deactivatedcatalyst were taken at different values of time-on-stream.
The analyses of hydrocarbon products were conductedwith on-line and off-line chromatographs.
The methods used for characterizing the catalyst and thecoke were: TG-TPO, N2 and NH3 adsorption, Raman,FTIR, UV-vis, 13C NMR spectroscopies, and submicron
imaging: TEM, polarized-light-, FTIR- and Ramanmicroscopies.

3. Results and Discussion
Figure 1 shows the evolution of product yields with timeon-stream (which is proportional to the amount of plastic
fed into the reactor) during the pyrolysis of PP. The
products can be lumped into waxes (C12+), light olefins(C2-4), light paraffins (C2-4), PONs (paraffins, olefins andnapthenes) C5-11, aromatics C5-11, and coke. As observedin Figure 1, the yield of waxes increases over time-on
stream (at the expense of the decrease in light paraffins,and aromatics). This result is linked to the catalyst
deactivation. The yield of olefins peaks (at weight ofplastic of 800 g) due to the fact that olefins areintermediates from the cracking of waxes and paraffins,in the formation of aromatics and coke.
Figure 2 shows the degradation of the physical propertiesof the catalyst, surface area (SBET), and surface ofmicropores (Smicropore), upon a pyrolysis run of HDPE
(similar deactivation as Figure 1). The deterioration of theproperties has an initial fast evolution (weigh of plastic <200 g, time-on-stream < 200 min) and reaches steadydeactivation. This result is consistent with the evolution
with time-on-stream of the yield of wax observed in

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6th International Symposium on Feedstock Recycling of Polymeric Materials

Figure 1. Furthermore, the number of active sites (acidityof the catalyst, which is not shown here) is also reduced
in identical manner as the surface area.

t-I

1.4

70

t-II

zPEzPP
Type II

1.260
Light olefins
Light paraffins
PON C530

0.8

20600

0.6
0.2

500

0.4

400

0.1

10300

0.2
0.0
0.000200 400 600 800 1000 1200 1400 1600

Weight of plastic (g)

0.5

1.0

1.5

Temperature (‫؛‬C)

Aromatics
Wax C12+

dW/dt

40

Type I-2Yields (wt. %)Coke (10 gcoke/gcat)

1.050

2.0Time (h)0.00200

400

600

800

1000

1200

Weight of plastic (g)Figure 1. Evolution of the yields of different lumps withthe amount of PP fed into the reactor.

Figure 3. Evolution of the coke-species with the amountof plastic (both HDPE and PP) fed into the reactor.

4. Conclusions

250-1

Area (m gcat )225
200
175150

SBET

125
10075

Smicropore

5025020040060080010001200

Weight of plastic (g)Figure 2. Evolution of the surface area (SBET) andmicropore area (Smicropore) during the pyrolysis of HDPE.
The content of coke have been measured using thermogravimetric analysis of coke combustion. The use oftemperature programed oxidation (TPO) ramp show two
peaks of combustion, as it is shown in the sub-graph ofFigure 3. This result led us to deconvolute the profile ofcombustion into two peaks: (1) type I coke, with thecombustion peak at ca. 450 ºC; and (2) type II coke, withthe combustion peak at ca. 550 ºC. Together with theresults of Raman, FTIR and UV-vis spectroscopies, wehave assigned type I coke to aromatic species with many
aliphatic chains, external to the micropores of the zeolite;
type II coke is made up of mainly aromatic structures
deposited inside the micropores of the zeolite, and it is
the main cause responsible for the deactivation. Figure 3
shows that there is a very fast initial formation of type II
coke and then a steady increase, together with type IIcoke.

interiores libro.indd 94

The conical spouted bed reactor together with HZSM-5
catalyst are suitable for the catalytic pyrolysis ofpolyolefins, giving way to high yields of olefins and low
amounts of aromatic components in the liquid productfraction. The operation in continuous mode is onlyaffected by the catalyst deactivation. It is noteworthy thatthe partially deactivated HZSM-5 catalyst maintains its
activity almost constant, with a slight change in productdistribution due to coke deposition. Coke can be divided
into type I coke (of aromatic-aliphatic nature) on themesopores of the catalyst; and type II coke (of aromaticnature) inside the micropores of the catalyst, which ismain cause responsible for the degradation of theproperties of the catalyst and the deactivation.

References[1] J. Aguado, D.P. Serrano, Feedstock Recycling ofPlastic Waste, The Royal Society of Chemistry:
Cambrige (1999).
[2] S.M. Al-Salem, P. Lettieri, J. Baeyens, Waste
Manage. 29 (2009) 2625-2643.
[3] G. Elordi, M. Olazar, G. Lopez, P. Castaño, J. Bilbao,
Appl Catal B: Environ. 102 (2011) 224-231.
[4] A. Marcilla, M.I. Beltrán, R. Navarro, Appl Catal A:
Gen. 333 (2007) 57-66.
[5] P. Castaño, G. Elordi, M. Olazar, A.T. Aguayo, B.
Pawelec, J. Bilbao, Appl Catal B: Environ. doi:
10.1016/j.apcatb.2011.02.024 (2011).

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