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Journal of Analytical and Applied Pyrolysis 89 (2010) 313–317

Contents lists available at ScienceDirect

Journal of Analytical and Applied Pyrolysis

journa l homepage: www.e lsev ier .com/ locate / jaap

hermal-catalytic degradation of polyethylene over silicoaluminophosphateolecular sieves—A thermogravimetric study

icha Singhal, Chhavi Singhal, Sreedevi Upadhyayula ∗

epartment of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

r t i c l e i n f o

rticle history:eceived 27 March 2009ccepted 14 September 2010vailable online 7 October 2010

a b s t r a c t

The present work is aimed at recycling plastic wastes economically and efficiently, for which pure highdensity polyethylene (HDPE) has been initially selected for the investigations. Thermogravimetric tech-nique has been used to investigate, analyze and compare the thermal and catalytic degradation of HDPE.The catalytic degradation was investigated over the medium pore silicoaluminophosphate, SAPO-11

eywords:igh-density polyethyleneegradationAPO-11hermogravimetry

molecular sieve. The thermogravimetric evaluation was performed using 2–30 wt% catalyst, and theapparent activation energies for the thermal and catalytic polymer degradation were estimated usingvarious iso-conversional methods. The apparent activation energy was found to be lower when SAPO-11was used compared to the direct thermal degradation of HDPE. The activation energy and coke levelsare comparable to the medium pore zeolite ZSM-5 and lower than the values obtained over large pore

ture.

ctivation energy zeolites reported in litera

. Introduction

Polymer recycling has been suggested as the only sustainableolution to the problem of huge, rapidly increasing amounts oflastic wastes [1–5]. The destruction of wastes by incineration isrevalent, but is expensive and often generates problems withnacceptable emissions. It is also undesirable to dispose wastelastics by landfill due to high costs and poor biodegradability.n alternative strategy is chemical recycling, which has attracteduch interest recently. This aims at converting waste polymers into

asic petrochemicals to be used as hydrocarbon feedstock or fuelil for a variety of downstream processes [6].

The main chemical recycling methods are the thermal and cat-lytic degradation of waste plastics. Catalytic degradation of plasticaste offers considerable advantages compared to pure thermalegradation as the latter demands relatively high operating tem-eratures typically more than 500 ◦C and even up to 900 ◦C [6].

The catalytic degradation of polymeric materials has beeneported for a large range of model catalysts, including amor-hous silica–alumina, zeolites Y, mordenite and ZSM-5 [7–12],he family of mesoporous MCM-41 materials [13–15] and a few

ilicoaluminophosphate molecular sieves [16,17]. Catalytic activ-ty is closely related to the amount of acid sites, pore size andlso shape of the catalyst [18–20]. Silicoaluminophosphate (SAPO)olecular sieves represent an important class of adsorbents and

∗ Corresponding author. Tel.: +91 11 26591083; fax: +91 11 26591120.E-mail address: sreedevi@chemical.iitd.ac.in (S. Upadhyayula).

165-2370/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.jaap.2010.09.007

© 2010 Elsevier B.V. All rights reserved.

catalytic materials generated by the introduction of silicon intoits aluminophosphate framework [21]. However, little work hasbeen reported on the use of the medium pore SAPOs in polymerdegradation. The medium pore SAPOs are attractive for catalyticapplications due to the presence of specific acid sites in its structure[22–24] which can convert the polymer into useful hydrocarbons.The medium pore SAPO-11 has a one-dimensional 10-memberedring system with a pore opening of 6.3 × 3.9 A [25].

The catalytic effect of molecular sieves on polymer decomposi-tion can be evaluated using different techniques and reactors, butthermogravimetric analysis is one of the most frequently used tech-niques [26–29]. This technique can be used to derive the effect ofthe polymer structure, composition and also detailed kinetic dataon the degradation process to be studied [30].

In the present work, thermogravimetric analysis has been car-ried out for both thermal degradation and catalytic degradationover medium pore SAPO-11 catalyst and the activation energyfor the overall degradation of high-density polyethylene wasestimated using thermogravimetric (TG) curves generated at vary-ing heating rates. Activation energy calculated from Vyazovkin’sadvanced isoconversional method is compared with that calcu-lated using the popular Ozawa iso-conversional method basedon Doyle approximation [31,32], Coats–Redfern, as well as Gor-bachev’s approximations [33].

The isoconversional methods are based on the assumptions thatthe reaction rate at a constant conversion depends only on the tem-perature, and the reaction model is independent of the heatingrate. However, the conventional linear isoconversional methodsmay lead to significant error in the determination of activation

314 R. Singhal et al. / Journal of Analytical and A

eccimtr[

2

2

fd0

Sisat

u

2

Tg1wmoc

omfTm

dp

Fig. 1. FT-IR spectrum of SAPO-11 catalyst.

nergy when significant variation of the activation energy withonversion occurs which is eliminated using the advanced iso-onversional method. Further, it also eliminates the uncertaintyn evaluating the kinetic parameters as generated by model-fitting

ethods. It allows the activation energy to be determined as a func-ion of conversion degree without previous assumptions on theeaction model function by using multiple heating rate experiments34,35].

. Experimental

.1. Materials

The extrusion grade high density polyethylene was obtainedrom Gas Authority of India (GAIL), India in powder form. The pow-er has a melt flow index 0.26 g/10 min (ASTM D1238) and density.95 g/cm3 (ASTM D1505).

SAPO-11 catalyst, in powder form, was purchased from M/süd-Chemie Inc., India. The BET surface area of SAPO-11 catalysts 168 m2/g and the micropore volume is 0.0596 cm3/g. The FT-IRpectrum of the catalyst is shown in Fig. 1. The peaks at 715, 545nd 468 cm−1 are the characteristic bands of SAPO-11 similar tohat reported in literature [36].

The particle size range of both polymer and catalyst samplessed for the analysis are in the range of 75–300 �m.

.2. Thermogravimetric experimental procedure

Thermogravimetric (TG) experiments were performed on ahermal Advantage TG instrument (SDT Q600 series) under nitro-en environment. The nitrogen flow rate was maintained at00 ml/min (STP). Both dynamic as well as isothermal experimentsere carried out with pure polymer as well as polymer/catalystixtures at a specific catalyst to polymer mass ratio of 1:3. 20 mg

f sample was used for each run. For dynamic condition, runs wereonducted at four different heating rates, 5, 10, 20 and 30 ◦C/min.

In order to evaluate the experimental results and the estimationf the activation energy, the original TG curve that records the totalass of polymer and catalyst was transformed to the polymer mass

ractional curve. Hence, the catalyst mass was subtracted from the

G mass reading and the result was divided by the initial polymerass.The amount of coke deposited on the catalysts after the degra-

ation of HDPE was determined by reheating the catalyst in theresence of air to 600 ◦C at 10 ◦C/min in the thermobalance. The

pplied Pyrolysis 89 (2010) 313–317

coke deposited on catalyst is expressed as loss in mass of cokedcatalyst per unit mass of HDPE.

2.3. Methodology for the determination of activation energy

The kinetic rate model of polymer degradation reaction can bedescribed by the following equation:

d˛

dt= ko exp

(−E

RT

)f (˛) (1)

where t is time (min), T is the temperature (K), ˛ is the conversionof reaction, f(˛) is the reaction model, and, ko, the pre-exponentialfactor (1/min) and E, activation energy (kJ/mol) are the Arrheniusparameters. R is the universal gas constant (8.314 J/mol K).

For non-isothermal conditions at a constant heating rate, Eq. (1)becomes,

ˇd˛

dT= ko exp

(−E

RT

)f (˛) (2)

where ˇ(=dT/dt) is the heating rate (K/min).Activation energy was calculated by Ozawa isoconversional

method [31,32], Vyazovkins advanced isoconversional method[34,35] and the equations based on Coats–Redfern and Gorbachevapproximation [33].

Integration of Eq. (2) and rearranging, leads to∫ ˛

0

d˛

f (˛)= g(˛) = ko

ˇ

∫ T

T0

exp(−E

RT

)dT = ko

ˇ

E

Rp(

E

RT

)(3)

Using Doyle approximation for p, we get the popular Ozawaequation for determining the activation energy as,

log ˇ = logkoE

g(˛)R− 2.315 − 0.4567

E

RT(4)

The activation energy can then be calculated from the slope ofplots of log ˇ vs. 1/T which is a straight line assuming g(˛) to beconstant for a given value of ˛.

E = −(Gradient)R

0.4567(5)

This estimated activation energy is independent of the kineticreaction model. However, the above equation was derived assum-ing constant activation energy which introduces error in estimatingE, if the latter varies with ˛. This error is eliminated in the advancedisoconversional technique proposed by Vyazovkin [34,35].

Similar to the other integral isoconversional methods, thismethod is based on the assumption that the reaction model, g(˛) isindependent of the heating program, T(t). For a set of experimentscarried out at different heating rates the activation energy can bedetermined at any particular value of ˛ by finding the value of E forwhich the objective function ˚(E˛) is minimized [34], where

˚(E˛) =n∑

i=1

n∑j /= i

J[E˛, Ti(t˛)]J[E˛, Tj(t˛)]

(6)

and

j[E˛, Ti(t˛)] =∫ t˛

0

exp( −E˛

RTi(t)

)dt (7)

The above integral isoconversional method uses the regularintegration from 0 to t˛ as a result of which each value of E˛ becomesaveraged over the region from 0 to ˛, and the whole E˛ dependenceundergoes an undesirable flattening. For this reason, Vyazovkin

proposed a modified Vyazovkin method based on the integrationover small time intervals as follows [35]:

j[E˛, Ti(t˛)] =∫ t˛

t˛−�˛

exp( −E˛

RTi(t)

)dt (8)

R. Singhal et al. / Journal of Analytical and Applied Pyrolysis 89 (2010) 313–317 315

F

na

b∫

∫

3

pmAtacTit

Fr

Table 1Average activation energy values from different methods for degradation of HDPEwith and without catalyst.

Method Thermalcrackinga, Eavg

(kJ/mol)

Catalyticcrackingb, Eavg

(kJ/mol)

Ozawa method 292.62 178.87Vyazovkin method (�˛ = 0) 308.23 166.33Modified Vyazovkin method (�˛ = 0.1) 269.16 165.80Modified Vyazovkin method (�˛ = 0.05) 267.53 167.65Modified Vyazovkin method (�˛ = 0.01) 263.4 164.12Coats-Redfern approximation 294.17 175.78

the �˛ value is decreased, accuracy increases. Initial low value of Eat very low ˛ is attributed to the fact that polymer degradation

ig. 2. TG curves for thermal degradation of HDPE alone at different heating rates.

The temperature integral (J) can be evaluated using directumerical integration using trapezoidal rule or several popularpproximations [33].

The equations based on Coats–Redfern approximation and Gor-achev approximations for the integral are as follows:

T

0

exp(

− E

RT

)dT = RT2

E

(1 − 2RT

E

)exp

(− E

RT

)(9)

T

0

exp(

− E

RT

)dT = RT2

E

(1

1 + (2RT/E)

)exp

(− E

RT

)(10)

. Results and discussion

The thermal as well as catalytic degradation of high densityolyethylene (HDPE) was studied by conducting kinetic experi-ents at four different heating rates of 5, 10, 20 and 30 ◦C/min.lthough one thermogravimetric (TG) curve is enough to describe

he kinetic behaviour of the system by several models, manyuthors agreed that reliable kinetic analysis requires a set of TG

urves examined at different heating rates [37]. Figs. 2 and 3 showG curves obtained from thermal and catalytic degradation exper-ments with varying heating rates. As expected, TG curves shiftedo higher temperatures as the heating rate increased from 5 to

ig. 3. TG curves for degradation of HDPE over SAPO-11 catalyst at different heatingates.

Gorbachev approximation 294.11 175.72

a Without catalyst.b With catalyst.

30 ◦C/min for all samples. This is due to the shorter time require-ments for the sample to reach a given temperature at increasedheating rate. In thermal degradation of HDPE, no significant decom-position occurred until 400 ◦C, however, in presence of SAPO-11catalyst degradation started before 250 ◦C.

The average values of E, calculated via different isoconversionalmethods are reported in Table 1. Fig. 4 shows the dependenceof activation energies on ˛ calculated from various methods forboth thermal and catalytic degradation. The average activationenergy for the thermal degradation of HDPE was in agreement withprevious literature values (250–320 kJ mol−1) [16,26]. The catalystSAPO-11 as expected significantly reduced the activation energy ascompared with thermal process, thereby providing evidence for itspotential in degradation of HDPE. The E values predicted by Ozawamethod, Coats–Redfern and Gorbachev approximations are foundto be almost similar. The modified Vyazovkin method which incor-porates integral over small integral limits yields more accurate Evalue than other methods.

Fig. 5 shows the variation of activation energy with ˛ for differ-ent �˛ values computed from the modified Vyazovkin method.It is observed from the figure that activation energy is a strongand increasing function of conversion for ˛ ≤ 0.1, then a weak andalmost constant function for 0.1 ≤ ˛ ≤ 0.9. It can be noted that as

initiates at the weak links. Moreover, the degradation of longerchain hydrocarbons is comparatively easier due to the increasingease of formation of carbocations on the interior carbons of lin-

Fig. 4. Variation of activation energy with HDPE conversion from different isocon-versional methods.

316 R. Singhal et al. / Journal of Analytical and Applied Pyrolysis 89 (2010) 313–317

Fig. 5. Variation of activation energy with HDPE conversion for different �˛ values.

Fo

ebsb

mhuAodacrH

TCa

Fig. 7. TG curve of coked catalyst after degradation of HDPE to 600 ◦C.

Table 3Comparison of coke levels, determined after degradation of HDPE to 600 ◦C for SAPO-11 and catalysts reported in literature.

Catalyst Pore size (nm) Coke level (wt%)

SAPO-11 0.39 × 0.63 1.3

with increased temperatures. These plots also show that reactionrates increase significantly in the presence of catalyst (SAPO-11)as it was observed that in the presence of catalyst the completeconversion of HDPE was obtained within 30 min at 400 ◦C com-

ig. 6. Comparison between present work and literature reported data for variationf E with ˛ for thermal degradation of HDPE alone.

ar molecules [38]. Therefore, as the carbon chain length reducesy random scission reactions, the activation energy increases untilhort chained hydrocarbons are left after which activation energyecomes constant.

The comparison of present work with literature data for ther-al degradation is shown in Fig. 6. The curves show similar trend,

owever, some differences are noted due to difference in samplessed and the method employed for calculation of activation energy.raujo et al. [16] reported a weak dependence of activation energyn conversion for ˛ ≥ 0.1. The initial strong and increasing depen-ence is observed in all the cases due to the reasons mentioned

bove, for ˛ ≤ 0.1. Table 2 lists the average activation energies foratalytic degradation of HDPE over different catalysts. SAPO-11esults in more rapid degradation of HDPE compared to SAPO-37,AlMCM-41. The difference in the activation energies over these

able 2omparison of activation energies for degradation of HDPE with SAPO-11 and cat-lysts reported in literature.

Catalyst E (kJ/mol)

SAPO-11 164.12SAPO-37 [16] 220HA1MCM-41 (30 wt%) [42] 263.68HZSM-5 [26] 118

HZSM-5(17) [26] 0.55 × 0.51 0.7HUSY [26] 0.74 10.6HY [26] 0.74 9.4

catalysts may be attributed to the differences in their pore size andgeometry, pore distribution, acid sites distribution and acid sitestrength.

With SAPO-11 catalyst, the amount of coke formed was foundto be 1.3 wt% (determined from TG curve, Fig. 7). The coke levels ofSAPO-11 are compared with values over other catalysts reported inliterature as shown in Table 3. It can be seen that HUSY and HY showhigher coking which may be attributed to their large pores andsupercage structure. SAPO-11 coke levels are almost comparablewith HZSM-5.

Figs. 8 and 9 show results from the isothermal TG experimentsfor thermal and catalytic degradation of HDPE. It is clearly evidentfrom the graphs that the degradation time decreased considerably

Fig. 8. Variation of HDPE degradation with time for isothermal conditions.

R. Singhal et al. / Journal of Analytical and A

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ig. 9. Variation of HDPE degradation with time over SAPO-11 catalyst for isother-al conditions.

ared to 24% conversion without the catalyst even after 100 mint the same temperature. Similarly, at 425 ◦C, 100% conversion waschieved with SAPO-11 in 7 min, while in the absence of catalyst5% conversion was achieved in 100 min. Thus, the presence of cat-lysts reduces the time required to achieve a given conversion athe specified temperature.

. Conclusion

TGA provides a useful and convenient tool for the investiga-ion of the thermal and catalytic degradation of polymers. SAPO-11eems to show promise in the catalytic degradation of HDPE cat-lyst, since, significant reduction in the time and temperatureequired to achieve a particular HDPE conversion was observed. Theatalyst (SAPO-11) reduced the average apparent activation energyy around 100 kJ/mol as compared to thermal degradation of HDPElone. This shows that SAPO-11 possesses good catalytic activityor the degradation of HDPE and can be utilized for the degradationf HDPE into the fuel range hydrocarbons.

cknowledgement

The authors are grateful to Gas Authority of India limited, India

or providing high-density polyethylene.

eferences

[1] G. Manos, A.A. Garforth, J. Dwyer, Ind. Eng. Chem. Res. 39 (2000) 1203–1208.

[[

pplied Pyrolysis 89 (2010) 313–317 317

[2] W. Kaminsky, B. Schlesselmann, C. Simon, J. Anal. Appl. Pyrolysis 32 (1995)19–27.

[3] S.F. Sodero, F. Berruti, L.A. Behie, Chem. Eng. Sci. 51 (1996) 2805–2810.[4] M.L. Mastellone, F. Perugini, M. Ponte, U. Arena, Polym. Degrad. Stab. 76 (2002)

479–487.[5] I. Fortelny, D. Michálková, Z. Krulis, Polym. Degrad. Stab. 85 (2004) 975–979.[6] Y.H. Lin, M.H. Yang, T.T. Wei, C.T. Hsu, K.J. Wu, S.L. Lee, J. Anal. Appl. Pyrolysis

87 (2010) 154–162.[7] P.N. Sharratt, Y.H. Lin, A.A. Garforth, J. Dwyer, Ind. Eng. Chem. Res. 36 (1997)

5118–5124.[8] Y.H. Lin, P.N. Sharratt, A.A. Garforth, J. Dwyer, Energy Fuels 12 (1998) 767–

774.[9] Y. Uemichi, J. Nakamura, T. Itoh, M. Sugioka, A.A. Garforth, J. Dwyer, Ind. Eng.

Chem. Res. 38 (1999) 385–390.10] G. Luo, T. Suto, S. Yasu, K. Kato, Polym. Degrad. Stab. 70 (2000) 97–102.11] S.Y. Lee, J.H. Yoon, J.R. Kim, D.W. Park, Polym. Degrad. Stab. 74 (2001) 297–305.12] A. Dawood, K. Miura, Polym. Degrad. Stab. 76 (2002) 45–52.13] A. Marcilla, A. Gómez, A.N. García, M.M. Olaya, J. Anal. Appl. Pyrolysis 64 (2002)

85–101.14] A. Marcilla, A. Gómez, A. Reyes-Laberta, A. Giner, Polym. Degrad. Stab. 80 (2003)

233–240.15] Y.H. Lin, H.Y. Yen, Polym. Degrad. Stab. 89 (2005) 101–108.16] A.S. Araujo, V.J. Fernandes Jr., G.J.T. Fernandes, Thermochim. Acta 392 (2002)

55–61.17] G.J.T. Fernandes, V.J. Fernandes Jr., A.S. Araujo, Catal. Today 75 (2002) 233–238.18] H.J. Park, J.-H. Yim, J.-K. Jeon, J.M. Kim, K.-S. Yoo, Y.-K. Park, J. Phys. Chem. 69

(2008) 1125–1128.19] H.J. Park, Y.K. Park, J.I. Dong, J.K. Jeon, J.H. Yim, K.E. Jeong, Res. Chem. Interm.

34 (8–9) (2008) 727–735.20] D.P. Serrano, J. Aguado, J.M. Escola, J.M. Rodriguez, L. Morselli, R. Orsi, J. Anal.

Appl. Pyrolysis 68–69 (2003) 481–494.21] S.T. Wilson, B.M. Lok, E.M. Flanigen, Crystalline metallophosphate composi-

tions, US Patent 4,310,440 (1992).22] P. Liu, J. Ren, Y. Sun, J. Catal. 29 (2008) 379–384.23] R.B. Borade, A. Clearfield, J. Mol. Catal. 88 (1994) 249–265.24] L. Yang, Y. Aizhen, X. Qinhua, Appl. Catal. A 67 (1990) 169–177.25] X. Zhang, J. Wang, J. Zhong, A. Liu, J. Gao, Microporous Mesoporous Mater. 108

(2008) 13–21.26] A. Garforth, S. Fiddy, Y.H. Lin, A. Ghanbari-Siakhali, P.N. Sharratt, J. Dwyer,

Thermochim. Acta 294 (1997) 65–69.27] I.C. Neves, G. Botelho, A.V. Machado, P. Rebelo, Eur. Polym. J. 42 (2006)

1541–1547.28] A. Durmus, S. Naci Koc, G. Selda Pozan, A. Kasgöz, Appl. Catal. B 61 (2005)

316–322.29] A. Marcilla, A. Gómez-Siurana, D. Berenguer, Appl. Catal. A 301 (2006) 222–231.30] A. Marcilla, M.I. Beltrán, R. Navarro, J. Anal. Appl. Pyrolysis 76 (2006) 222–229.31] T. Ozawa, Bull. Chem. Soc. Jpn. 38 (1965) 1881–1886.32] C.D. Doyle, J. Appl. Polym. Sci. 5 (1961) 285–292.33] B. Saha, A.K. Maiti, A.K. Ghoshal, Thermochim. Acta 444 (2006) 46–52.34] S. Vyazovkin, J. Therm. Anal. 49 (1997) 1493–1499.35] S. Vyazovkin, J. Comput. Chem. 22 (2) (2001) 178–183.36] S. Zhang, S.L. Chen, P. Dong, Z. Ji, J. Zhao, K. Xu, Catal. Lett. 118 (2007) 109–117.37] J.D. Peterson, S. Vyazovkin, C.A. White, Macromol. Chem. Phys. 202 (2001)

775–784.38] B.W. Wojciechowski, A. Corma, Catalytic Cracking: Catalysts, Chemistry and

Kinetics, Marcel Dekker, Inc., New York, 1986.39] B. Saha, A.K. Ghoshal, Thermochim. Acta 451 (2006) 27–33.

41] R.E. Lyon, Thermochim. Acta 297 (1997) 117–124.42] D.A. Costa, J.G.A. Pacheco Filho, M. Embirucu, M.J.B. Souza, A.S. Araújo, H.

Oliveira, T.F. Gomes, 2nd Mercosur Congress on Chemical Engineering, 4th Mer-cosur Congress on Process Systems Engineering, Costa Verde, Brazil, 2005, pp.1–8.