Catalytic degradation of high density polyethylene over nanocrystalline HZSM-5 zeolite

Polymer Degradation and Stability 91 (2006) 3330e3338 www.elsevier.com/locate/polydegstab

Catalytic degradation of high density polyethylene over nanocrystalline HZSM-5 zeolite J.F. Mastral, C. Berrueco, M. Gea, J. Ceamanos* Department of Chemical and Environmental Engineering, C.P.S, University of Zaragoza, Marı´a de Luna 3, 50018 Zaragoza, Spain Received 20 December 2005; received in revised form 31 May 2006; accepted 16 June 2006 Available online 15 September 2006

Abstract High density polyethylene (HDPE) was catalytically degraded using a laboratory fluidised bed reactor in order to obtain high yield of gas fractions at mild temperatures, between 350 and 550  C. The catalyst used was nanocrystalline HZSM-5 zeolite. High yields of butenes (25%) were found in the gas fractions, which were composed mainly of olefins. Waxes were wholly composed of linear and branched paraffins, with components between C10 and C20. The effects of both temperature and polymer to catalyst ratio on the product yield were studied. Gas conversion was dramatically decreased when the operation temperature was low (below 450  C) or when the polymer to catalyst ratio was greatly increased (9.2). Gas and wax compositions significantly altered over 500  C, showing that a part of the HDPE was degraded thermally, increasing the olefin concentration in the waxes. The same variation was observed in the experiments carried out at high polymer to catalyst ratios, obtaining a 50% olefinic concentration in the waxes. The differences observed in product distributions can be attributed to both thermal and catalytic degradations. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: High density polyethylene; Zeolite; Catalyst; Pyrolysis; Product distribution

1. Introduction Huge amounts of plastic wastes are generated every year due to the increasing variety of uses of plastics. Consumption of polymers in Europe between 2000 and 2002 increased by

Abbreviations: ATOS, pillared clay derivative of a saponite (particle size < 160 mm); AZA, pillared clay derivative of a montmorillonite (ZenithN) (particle size < 160 mm); FID, flame ionization detector; GC/MS, gas chromatograph/mass spectrum detector; HDPE, high density polyethylene; HDPE/ Cat, ratio of grams of HDPE fed per gram of catalyst in the reactor; HUSY, ultrastable zeolite Y (Hþ, acid form), or Faujasite, structure FAU; HZSM-5, zeolite Socony Mobil e five (Hþ, acid form), structure MFI; LDPE, low density polyethylene; LLDPE, linear low density polyethylene; MCM-41, mesoporous aluminosilicate in the hexagonal phase; MOR, mordenite (Hþ, acid form), structure MOR; NZ, natural zeolite, clinoptilolite type; SAHA, silicaealumina, commercial name Synclyst 25; TCD, thermal conductivity detector; TPD, temperature programmed desorption. * Corresponding author. Tel.: þ34 976762160; fax: þ34 976761879. E-mail address: [email protected] (J. Ceamanos). 0141-3910/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2006.06.009

6%, resulting in a 5% increase in plastic residues which were mainly eliminated by landfill and incineration. This causes severe environmental problems because of chemical inertness and pollutants [1]. The tertiary or chemical recycling of plastics has become the most attractive method for the environmentally friendly recovery of these solid residues. Polyethylene, considering all types, is the most widely generated plastic residue. Being an addition polymer, it cannot be recycled by solvolysis. Consequently the processes studied in previous work have been directed towards gasification and pyrolysis. Due to the gaseous, oil and solid contaminants’ generation during gasification and the partial consumption of the polymer treated, pyrolysis is the most common method tested by previous authors. However, thermal pyrolysis of polyethylene requires high temperatures (above 650  C) to generate considerable gaseous fractions [2]. Moreover, the product distribution is very wide. Because of the energy and the separation required, pyrolysis cannot be considered economically viable.

J.F. Mastral et al. / Polymer Degradation and Stability 91 (2006) 3330e3338

In the last 10 years, researchers have been testing the use of different catalysts in the pyrolysis of HDPE and low density polyethylene (LDPE) [3e9]. Previous works show that there are many solids that can reduce the temperature and increase the selectivity of the process, obtaining similar yields of the gas fraction than those obtained in pure thermal pyrolysis. These can be grouped into zeolitic, mesoporous and clay catalysts. Zeolitic materials are often used in the catalytic degradation of polyethylene and other addition polymers. Their acidity and structure are suitable for obtaining high gas conversions at relatively low temperatures, between 350 and 500  C. Seo et al. [10] tested different types of zeolites such as HZSM-5, ultrastable zeolite (HUSY) and mordenite (MOR) for HDPE pyrolysis at 450  C, obtaining higher gas yields over HZSM-5(powder) (63.5%) than over HUSY(powder) (27.0%) and MOR(pellet) (18.5%). This extraordinary yield can be explained by the properties of HZSM-5. Most zeolites, including HZSM-5, show an excellent catalytic efficiency on cracking, isomerization and aromatization due to a strong acidity and a crystalline microporous structure. HZSM-5 catalyst has an MFI structure with inter˚ channels. Therefore, the inisecting 5.4  5.6 and 5.1  5.5 A tially cracked fragments can diffuse through its pores and react further in the cavities created at the intersection of the two channels, yielding more gaseous products. HZSM-5 presents an excellent stability due to its particular structure, which prevents the formation of coke. Furthermore, the opportunity of pore blocking is low due to the intersecting connection of the channels [11]. On the other hand, HUSY presents larger ˚ ) than HZSM-5 and large supercages in the pore size (7.4 A channel systems. This structure leads to lower gas yields and easy deactivation by coke deposition. Mordenite has a bottleneck pore structure consisting of a parallel two-dimensional structure with intersecting channels of two different sizes, which result in rapid deactivation. These results show that degradation activity of zeolites depends not only on pore size and acidity, but also on their shape [12]. Park et al. [13] tested many more zeolites (Beta, natural zeolite (NZ(clinoptilolite type)), HUSY and HZSM-5), also obtaining the greatest gas yield from degradation over HZSM-5. However, their activity and selectivity depend on other factors, the most important being the acidity, due to the role of acid sites in the degradation mechanism. Bro¨nsted acidity can be increased by substituting the Naþ counter-ion by Hþ protons. The acid form is thus frequently used in the cracking processes. TPD analysis showed that HZSM-5 and Beta zeolites had the highest acidity, unlike HUSY and natural zeolite which had medium and low acidities, respectively [14,15]. Lewis acidity can be increased by changing the Si:Al ratio. In fact, Park et al. [13] proved that the smaller the Si:Al ratio, the higher the production of gases. The distribution of Si and Al atoms in the structure can also affect the acidity, but only the strength of the acid sites [16]. The pore size of zeolites works as a molecular sieve obstructing the free diffusion of large or wide molecules into the internal surface of the catalyst. This can be a problem in HDPE pyrolysis if the crystal size is not very small. Serrano

3331

et al. [15] carried out experiments of pyrolysis of a polyolefin mixture (46.5% LDPE, 25% HDPE, 28.5% PP) over two different types of HZSM-5, one with a 75 nm crystal size and the other with 3 mm. The results showed 85% and 10% gas conversion, respectively. HZSM-5 with a crystal size in the nanometer range has enough external cracking activity to generate molecules small enough to pass through the pores and overcrack leading to higher gas yields. The last factor to consider is deactivation. Both pore size and acidity accelerate or inhibit coke deposition, which is the main cause of deactivation [17,18]. Although HZSM-5 has strong acidity, and therefore a high degree of activity, its pore size inhibits coke formation, making it highly resistant to deactivation [19]. The large pores of HUSY and MOR make deactivation easy in these cracking processes [20,21]. Mesoporous materials such as aluminiumesilicate MCM41 and silicaealumina SAHA, studied by different authors [19,22e24], did not give adequate results due to their lower acidity and wide pore size distribution. The most recently studied catalysts are clays and pillared clays. Gobin and Manos [20] tested linear low density polyethylene (LLDPE) degradation over saponite, montmorillonite and their pillared derivatives (ATOS, AZA). Despite the high liquid conversion, they showed a strong regenerability after coke deposition during the experiments. Due to their wide distribution of pore size and their weak acid sites, they could not produce higher gaseous fractions than other zeolites such as HUSY and HZSM-5. In the work presented here nanocrystalline HZSM-5 was selected in order to study the catalytic pyrolysis of polyethylene. One problem identified in the previous works [19,21,25] is the poor contact between the catalyst and the polymer, so this study has been carried out in a fluidised bed reactor which favours isothermal control and mass transfer. The influence of temperature and polymer to catalyst ratio on the yields to the different fractions and on the product distribution has been analysed. 2. Materials High density polyethylene was obtained from HOECHST (HOSTALEN GH 4765), with a density of 0.958 g/cm3, a softening point of 80  C and a mean particle size of 3 mm. The catalyst used in the experiments was zeolite HZSM-5 with crystal size in the nanometer range; provided in powder form by Su¨d Chemie Corporation. The physical properties were as follows: ˚ pore size), Si:Al raMFI structure (5.3  5.6/5.1  5.5 A tio ¼ 35, crystal size ¼ 100 nm, BET surface ¼ 397 m2/g and hydrogen is used as the cation in the structure. The material was sieved to obtain the adequate particle size, and was previously activated by calcination at 550  C under static air. The composition of the reaction bed was 90% (by weight) silica sand and 10% catalyst. Several fluidisation runs were performed at ambient temperature and pressure to select the suitable particle size range and to optimise the fluidising N2 flow rates to be used during the experiments. The particle sizes were 75e150 mm for silica and 150e350 mm for HZSM-5 in order to avoid segregation.

3332

J.F. Mastral et al. / Polymer Degradation and Stability 91 (2006) 3330e3338

3. Laboratory scale pyrolysis The experimental system used in this work is shown in Fig. 1. The study was carried out using a stainless steel, fluidised bed reactor 4.8 cm in diameter and 23 cm in height. The reactor was externally heated using an electrical ring furnace. The bed temperature was measured by a K type thermocouple inside the bed. Catalytic pyrolysis of HDPE was tested at different temperatures (350, 400, 450, 500 and 550  C) and with different polymer to catalyst ratios (HDPE/Cat ¼ 0.93, 1.41, 9.20 and N(Thermal)). High density polyethylene was gravity fed, via a gate valve, to the fluidised bed. HDPE was dropped directly into the bed in five batches of 4 g every 10 min, the time required for the complete degradation of the polymer [20]. For experiments in which the HDPE degradation rate was too slow (HDPE/ Cat ratio ¼ 0.93 or 350  C), it was necessary to vary the quantity of every batch or the number of batches (Tables 1 and 2). Complete fluidisation of the bed was secured with a continuous N2 flow of 30 ml N/min which entered at the bottom of the reactor. The product stream was passed through a cyclone to remove any particulate matter. The resulting vapours were passed through a cool water heat exchanger at 20  C, where some heavy hydrocarbons were condensed. The remaining components were condensed using an iceeNaCl bath at 10  C. The exit flow was divided into two fractions. One fraction was made to pass through silica gel in order to retain waxy products carried by the gas in aerosol form, and subsequently emitted to the atmosphere. The other was collected online in 3 l Tedlar bags for analysis during the experiment, one being used in every batch charged in the reactor.

Table 1 Series A: experimental conditions and product yields (wt% of feed) for pyrolysis experiments at different temperatures Experiment

A1

A2

A3

A4

A5

Temperature ( C) Number of batches gBATCH (g) HDPE/Cat Qfluidization (ml N/s) tTOTAL (min) Gas yield (%) Wax yield (%) Solid residue (%)

350 3

400 5

450 5

500 5

550 5

4 0.60 30

4 1.33 30

4 1.23 30

4 1.41 35

4 1.33 30

100 16.77 10.90 72.33

115 60.46 13.08 26.46

42 77.88 22.12 0.00

35 78.48 21.52 0.00

90 69.12 30.88 0.00

HPPlot/Al2O3, 50 m  0.53 mm  15 mm, with a flame ionization detector (FID). The concentration of H2 was determined by the TCD detector whilst the rest of the hydrocarbons from C1 to C7 were detected by the FID. Waxes were collected and analysed by GC/MS. They were first dissolved in tetrahydrofuran (THF). The equipment included an HT-5 aluminium clad column, 25 m  0.32 mm  0.1 mm (non polar), with a Tmax of 450  C. Helium was used as a carrier gas with a flow rate of 1 ml N/min. Samples of 1 ml were injected in splitless mode in order to increase the quantity of dissolved wax injected. The high temperature column enabled oven temperatures of 400  C to be reached ensuring that hydrocarbons up to C60 could be analysed. The temperature programme was as follows: 40  C initial temperature followed by a heating rate of 5  C/min up to 180  C, then an increase up to 350  C at a heating rate of 10  C/min, and finally 10 min at 350  C. The ion trap detector had a mass range from 32 to 800 amu and was linked to a computer with a WILEY library.

4. Analysis The gas fraction was analysed by gas chromatography using two HP 5890 series II. One of them was equipped with a Porapack N column, 2.8 m  3 mm packed with 80/ 100 mesh, and a molecular sieve 0.9 m  3 mm packed with 45/60 mesh, using a thermal conductivity detector (TCD). The second chromatograph included a semi-capillary column

5. Results 5.1. Product yields HDPE catalytic pyrolysis was carried out at five different temperatures (350, 400, 450, 500, and 550  C) in order to study

Fig. 1. Scheme of the experimental cracking installation.

J.F. Mastral et al. / Polymer Degradation and Stability 91 (2006) 3330e3338 Table 2 Series B: experimental conditions and product yields (wt% of feed) for pyrolysis experiments at 500  C and at different polymer to catalyst (PE/Cat) ratios Experiment

B1

B2

B3

B4

HDPE/Cat Number of batches gBATCH (g) %ZeolyteBED Qfluidization (ml N/s) tTOTAL (min) Gas yield (%) Wax yield (%) Solid residue (%)

0.93 7 2 10 35

1.41 5 4 10 35

9.2 5 4 1.35 30

N(Thermal) 3 4 0 30

76 80.13 19.87 0.00

35 78.48 21.52 0.00

56 46.86 53.14 0.00

100 33.16 66.84 0.00

the effect of the temperature on the yields of the pyrolysis products. Table 1 shows the yields to the different fractions obtained and the experimental conditions in every test. At lower temperatures, polymer cracking is not able to form lighter molecules, thus little gaseous product is yielded. Moreover, a solid residue is produced, corresponding to the partially pyrolysed HDPE, too heavy to escape from the catalytic bed. This residue tends to react with wax compounds while the temperature is increased, and finally disappears in the experiment at 450  C (Fig. 2). The higher the temperature, the greater the amount of gases collected. The most outstanding growth in gas yield is observed when the experimental temperature increases from 350 to 400  C. The gaseous fraction increases in the range from 17% to 60%, dramatically reducing solid residue conversion. At the higher temperatures studied (over 500  C), the gas yield suddenly decreases while waxy products increase. Although HDPE catalytic cracking conversion depends on the physical properties of HZSM-5 (BET surface, crystal size, acidity.), similar results were obtained in previous works. Herna´ndez et al. [26] degraded HDPE at different temperatures in a fluidised bed reactor using HZSM-5 zeolite. The highest increase in gas yield was obtained between 300 and 400  C. They reported a decrease in gas production at higher temperatures, over 600  C. Van Grieken et al. [19] used a stirred batch reactor and obtained similar results.

Yield (%wt.)

80

5.2. Gas composition As is shown in Table 3, the gas yield is composed mainly of olefins. Catalytic cracking of HDPE over nanocrystaline HZSM-5 samples proceeds with high activity and very

Gas Yield (%) Wax Yield (%) Solid Residue (%)

100 GasYield (%) Wax Yield (%)

80

60

40

20

0 350

A high increase in gas production was observed between 340 and 400  C, and the solid residue disappeared at over 380  C. According to these results, it seems that the catalytic effect is reduced at the highest temperatures. This fact can be explained by considering the deactivation of the catalyst. Studies of deactivation of zeolites by TGA showed that the activity of HZSM-5 is affected by temperature due to structural changes. Apparently, high temperature affects the strong acid centres, mainly due to a dealumination process, although this effect appears at higher temperatures (around 600  C) [26]. Miskolczi et al. [27] observed that the difference between non-catalytic and thermo-catalytic crackings became less significant with increasing temperature. They indicated that it could be a consequence of the coking of the catalyst surfaces at higher temperature, which may block the active sites of the catalyst. There could therefore be an interaction between catalytic and thermal processes, and this is discussed below. Table 2 and Fig. 3 show the results of the experiments carried out with different polymer to catalyst ratios. No solid residue was produced due to the operation temperature (500  C). The lowest production of gas corresponds to HDPE thermal cracking (HDPE/Cat ¼ N(Thermal)). Increasing the catalyst content favours gas generation and minimizes wax yield. However, when the ratio is decreased from 1.41 to 0.93, a very small gas fraction variation, from 78.48% to 80.13%, is observed. This was demonstrated by thermogravimetric studies made in a cycled spheres reactor. Schirmer et al. [25] reported that small variations in zeolite to HDPE ratios when the ratio was small (around 1) could not significantly reduce the degradation temperature of HDPE. However, ratios greater than 7 produced a strong decrease in gas production.

Yield (%wt.)

100

3333

60

40

20

400

450

500

550

Temperature (ºC) Fig. 2. Influence of the pyrolysis temperature on product distribution (Series A, HDPE/Cat z 1.3).

0 0.93

1.41

9.20

(Thermal)

ratio PE/Cat Fig. 3. Influence of the catalyst ratios on product distribution (Series B, T ¼ 500  C).

J.F. Mastral et al. / Polymer Degradation and Stability 91 (2006) 3330e3338

3334

Table 3 Series A: gas composition (wt% of feed) for HDPE catalytic pyrolysis (HDPE/ Cat z 1.3) at different temperatures Experiment

A1

A2

A3

A4

A5

Temperature ( C)

350

400

450

500

550

CH4 C2H6 C2H4 C2H2 C3H8 C3H6 C4

0.00 0.00 0.00 0.00 0.00 3.22 5.27

0.00 0.00 0.18 0.00 1.59 9.80 18.31

0.00 0.00 0.79 0.00 0.44 18.30 24.88

0.00 0.00 0.78 0.00 0.26 18.17 23.85

0.00 0.10 0.91 0.00 0.47 16.59 22.46

C4H10 cis-C4H8/trans-C4H8 i-C4H8 C4H8 (1-butene) C4H6 (1,3-butadiene) C5eC7

0.19 2.09 2.35 0.49 0.15 9.27

0.39 7.68 7.98 2.28 0.36 30.3

0.36 10.19 10.63 3.74 0.32 35.17

0.21 10.30 9.64 3.84 0.07 35.99

0.20 9.37 8.84 3.95 0.31 28.68

high selectivity towards olefinic gases [2]. Some fractions are influenced by temperature. Although propylene is the most important product whatever be the temperature, its yield decreases over 500  C. The same phenomenon is observed in butene production, which shows an important concentration of iso-butene compared to 1-butene production (Fig. 4). Neither methane nor hydrogen is detected in this range of temperatures. Small amounts of other products such as ethene appear and show an increase in their yield when the temperature changes from 400 to 450  C (0.18 g/gHDPE at 400  C and 0.79 g/gHDPE at 450  C). Another remarkable fact is the production of ethane which starts at the highest temperatures (550  C). Lower temperatures produce gases rich in heavy (C5eC7) compounds. The higher the temperature, the lighter the compounds produced. Temperatures between 450 and 550  C produce high concentrations of C1eC4 fractions, which are mainly represented by propylene and butenes. On the other hand, the fraction of heavier compounds (C5eC7) shows

Alkane (C21-C40) Branched Alkene

Alkane (C10-C20) Alkene

Branched Alkane Cycled and aromatic

100

% Area

80 60 40 20 0 350

400

450

500

550

Temperature (ºC) Fig. 4. Area percentage of compound types detected in wax analysis from Series A experiments carried out at HDPE/Cat z 1.3 and at different temperatures.

high generation rates (9.27 g/gHDPE) at the lowest temperature tested (350  C). These results are in agreement with those of other authors. Garforth et al. [22] studied the degradation of HDPE over HZSM-5 in a fluidised bed reactor and obtained olefins at different temperatures (290e430  C). Propylene, cis/transbutene and iso-butene were the compounds produced in the greatest quantities at each temperature in that interval. In the formation of C5eC7 fraction, similar trends were observed, their yields being reduced by increasing the temperature and leading to lighter fractions (C1eC4). Bagri and Williams [4] described a great variation in ethane production when the temperature was near 550  C. They confirmed a significant methane production at that temperature and an increase at 600  C. The gas distribution is clearly influenced by the HDPE/ Cat ratio (Table 4). Although high HDPE to zeolite proportions (HDPE/Cat ¼ 9.2/N(Thermal)) increase the production of the heaviest fractions (C5eC7), the lightest hydrocarbons (methane and ethane) are detected in that conditions. These compounds are not produced in experiments with low HDPE/Cat ratios, where substantial amounts of butenes and propylene are formed. Small variations in the HDPE/Cat ratio can change the gas composition. Butene production increases from 23.8 g/gHDPE to 44.5 g/gHDPE by reducing this ratio from 1.41 to 0.93. On the other hand, the C5eC7 fraction decreases from 36.0 g/gHDPE to 25.6 g/gHDPE. As the ratio is increased, the gas distribution becomes similar to that of the gaseous products produced by thermal pyrolysis (infinite ratio). Methane and ethane, which are produced in thermal pyrolysis experiment, are not detected at low ratios (0.93 and 1.41), but they are produced at a ratio of 9.2. The zeolite effect is again shown in the production of all butenes. Thermal pyrolysis produces the same quantity of cis/ trans-butene, iso-butene and 1-butene while cis/trans-butene and iso-butene reach twice the level of production of 1butene when HZSM-5 is added (Fig. 5).

Table 4 Series B: gas composition (wt% of feed) for HDPE catalytic pyrolysis at 500  C and at different polymer to catalyst ratios (HDPE/Cat) Experiment

B1

B2

B3

B4

PE/Cat

0.93

1.41

9.2

N(Thermal)

CH4 C2H6 C2H4 C2H2 C3H8 C3H6 C4

0.00 0.00 0.93 0.00 0.35 9.72 44.54

0.00 0.00 0.78 0.00 0.26 18.17 23.85

0.05 0.13 0.30 0.00 0.25 5.85 8.60

0.26 0.29 0.40 0.09 0.24 2.58 4.24

C4H10 cis-C4H8/trans-C4H8 i-C4H8 C4H8 (1-butene) C4H6 (1,3-butadiene)

0.06 18.16 17.54 8.84 0.00

0.21 10.30 9.64 3.84 0.07

0.23 3.71 3.20 1.68 0.00

0.31 1.73 1.06 1.07 0.06

C5eC7

25.64

35.99

34.08

25.72

J.F. Mastral et al. / Polymer Degradation and Stability 91 (2006) 3330e3338 Alkane (C10-C20) Alkene

Alkane (C21-C40) Branched Alkene

Propene i- Butene

Branched Alkane Cycled and aromatic

3335 Butane 1-Butene

cis/trans Butene 1,3-Butadiene

20

100

18 16

Yield (% wt.)

% Area

80 60 40

14 12 10 8 6 4

20

2 0

0 0.93

1.41

(Thermal)

9.20

0.93

5.3. Wax composition Waxy products are composed of linear hydrocarbons (high molecular weight paraffins) between decane and eicosene. As is shown in Fig. 6, this composition is invariant at middlerange temperatures (350e500  C). An important change in the composition is observed when the temperature is raised to 550  C. The waxes produced are then composed of a mixture of heavy and light linear hydrocarbons (C10eC20 and C21e C40). Some olefins, branched paraffins and aromatics are also detected. Varying the HDPE/Cat ratio determines the wax composition greatly (Fig. 7). Low ratios (HDPE/Cat ¼ 0.93, 1.41) produce light and linear hydrocarbons (between decane and eicosene). Increasing the ratio (HDPE/Cat ¼ 9.2) generates waxes richer in olefinic hydrocarbons, branched paraffins and aromatics. This composition is quite similar to that obtained in the absence of catalysts (thermal degradation). Waxes rich in olefins and aromatics were produced in the experiment carried out at 500  C and HDPE/Cat ¼ N(Thermal).

Propene 1-Butene

iso-butilene

cis/trans Butene C5-C7

40 35

Yield (% wt.)

30 25 20 15 10 5 0 350

400

450

500

550

Temperature (ºC) Fig. 6. Evolution of main gases obtained with pyrolysis temperature (Series A, HDPE/Cat z 1.3).

9.2

(Thermal)

Ratio (HDPE/Cat)

Ratio HDPE/Cat Fig. 5. Area percentage of compound types detected in wax analysis from Series B experiments carried out at 500  C and at different polymer to catalyst ratios.

1.41

Fig. 7. Evolution of main gases obtained with polyethylene/catalyst ratio (Series B, T ¼ 500  C).

Some typical chromatograms are presented in Fig. 8. Catalytic pyrolysis waxes (experiments at ratios of 0.93 and 1.41) show wide peaks which represent linear alkanes. Little branched alkane content is quantified and no alkene is detected. Many differences are shown in the waxes from the runs carried out with a ratio of 9.2. Peaks representing alkenes appear near the wide linear alkane peaks. This chromatogram is quite similar to those obtained from the thermal pyrolysis experiments previously carried out by other authors.

6. Discussion It is generally believed that thermal degradation occurs by a radical mechanism, generating many oligomers by hydrogen transfer from the tertiary carbon atom along the polymer chain. However, catalytic degradation of HDPE is known to proceed by a carbenium ion mechanism [13,28e30]. The initial step is considered to occur either by the abstraction of the hybrid ion (by Lewis acid sites) from the HDPE molecule or by the addition of a proton (by Bro¨nsted acid sites) to the CeC bonds of the HDPE molecule or thermally degraded olefins. The cracking of polymer carbocation proceeds there after step-by-step into lower molecular weight hydrocarbons. These carbenium and carbonium mechanisms require the presence of strong acid sites. However, a direct relationship between the acid strength and the measured activities is not observed. On the contrary, a direct relationship is clearly observed between the activity, the external surface area/crystal size of the zeolite samples and the pore shape, which confirms that steric and/or internal diffusion hindrances are present for the catalytic cracking of the bulky polymer molecules [12,15]. As expected, the catalysts having small crystals present high external surface area. The external acid sites are not limited by steric or diffusion problems. Thus, they are essential to promote the initial steps of the polyolefin cracking. Accordingly, bulky molecules which are not able to enter the zeolite micropores can be cracked on the external surface. Moreover, given the small crystal size the primary products of

J.F. Mastral et al. / Polymer Degradation and Stability 91 (2006) 3330e3338

3336

(a) Tridecane Tetradecane

Abundance

Dodecane

Pentadecane Hexadecane Heptadecane

Undecane

Hexadecane Octadecane Nonadecane

C11

C12

C13

n.d

n.d

n.d

n.d

n.d

C14

C15

n.d.

n.d

C16

C17

C18

C19

C20

n.d.: not determined.

Abundance

Abundance

(b) Dodecane Methyl- undecene Branched undecane Cycle

Dodecene

C12

C10

C15

C20

Fig. 8. Chromatogram of waxes obtained in the experiments carried out at: (a) 500  C and PE/Cat ¼ 1.41; (b) 500  C and PE/Cat ¼ 9.2.

degradation are favoured in nanocrystalline HZSM-5, like the one used in this study. Some researchers indicate that 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 leading to the formation of a wider variety of hydrocarbons, although limited and controlled by the HZSM-5 shape-selectivity [30]. In the case of nanocrystalline HZSM-5, it is expected that the presence of a high external surface affects negatively its shape-selectivity. However, the obtained results indicate that, in spite of their high external surface, the end-chain cracking is the predominant mechanism. Only when the catalyst activity is very high, oligomerization, cyclization and aromatization reactions are important enough to promote the formation of heavier products. Catalytic cracking of high density polyethylene that starts at the active sites on the external surface of the zeolite continues on the strong acid sites inside the pores [26,30]. The cracking could occur in the channels, due to the strong interaction existing between the adsorbed carbenium ions and the zeolite surface, though the reaction is less energetically

favourable due to the formation of primary carbenium ions. The stability of these chemical species increases as alkyl-substitutes are added to the carbon atom. Thus, carbenium ions formed on acid sites will isomerize in order to stabilize, explaining the low selectivity to terminal alkenes (1-butene). The porous structure of zeolites makes a restrictive selection of molecules by size. Big molecules formed at the external surface cannot enter the pores and crack on the external acid sites. Moreover, the small pores prevent bimolecular reactions such as protolytic cracking or oligomerization from taking place. The smaller the pore diameter, the stronger the interaction between the adsorbed carbenium ions and the zeolite surfaces, resulting in a more direct cracking. The smaller the pore size, the greater the extent of suppression of the hydrogen transfer reaction of alkenes. The hydrogen transfer reactions, which involve the formation of bulky bimolecular reaction intermediates, are mainly controlled by steric constraints, by the space available inside the micropores of the zeolites [11]. The most important mechanism in the pores is b-scission producing light olefinic compounds. Gases composed of light olefins result from catalytic cracking taking place in the internal pores. The stability of terminal carbenium ions

J.F. Mastral et al. / Polymer Degradation and Stability 91 (2006) 3330e3338

and the b-scission mechanism are the two factors that obstruct ethylene formation and favour propylene and butene yields. On the other hand, acid sites on the external surface are not restrictive. Paraffin producing reactions such as hydrogen transfer or alkylation can take place. Waxes composed of heavier and paraffin hydrocarbons are mainly produced in the acid sites of the external surface. Branched and linear paraffins are the result of bimolecular reactions which are not hindered by the structure. 6.1. Temperature effect Temperature has a direct effect on gas, wax and solid yields. The higher the temperature, the more important are the b-scission reactions. In these conditions, light molecules are formed that can enter the pores and overcrack. As a result of this phenomenon a very sharp increase of the gas yield between 350 and 450  C is observed. This is not, however, a constant tendency. When the temperature is too high, thermal cracking can take place and polyethylene degrades in both ways: thermally and catalytically. The gas yield decreases at 550  C due to the higher production of wax by thermal degradation at this temperature. It is well known that thermal degradation of HDPE is a chain reaction mechanism in which free radicals are consumed and produced [2,31]. Typical reactions of these species are b-scission, isomerization, hydrogen transfer, oligomerization and DielseAlder addition. There are no pore restrictions in thermal cracking, so temperature is the only parameter that determines all the thermodynamic equilibria leading to the final conversion of each compound produced. Aromatics, olefins and branched olefins are significant components of the wax. Various previous works report the influence of catalyst deactivation on product yields at high temperatures. Herna´ndez et al. [26] attributed the decrease in gas yield operating at an HDPE/Cat ratio of 5 and at 600  C to temperature deactivation. High temperatures can distort the structure of the zeolite, favouring dealumination and sintering inside the crystal which would directly alter the acidity and consequently its activity. Other researchers [27] indicate that the different activities observed at different temperatures could be a consequence of the coking of the surfaces at higher temperatures, which may block the active sites. In these conditions, HDPE would be mainly thermally degraded. Nevertheless, the temperatures tested are not too high. Products obtained at 550  C show an influence of thermal cracking which is clearly seen in the composition of olefin waxes. It is thus possible that the effect of the thermal pyrolysis process is more significant than zeolite deactivation by dealumination, which is not reported by some authors at 550  C [4,29]. Gas and wax compositions do not vary over a wide range of temperatures (350e 500  C). However, the influence of thermal cracking at 550  C changes both distributions. Aromatics and olefins are formed in the wax fraction, and gases contain light paraffins. Cracking activity decreases producing heavier gaseous fractions (C5eC7) and waxy linear paraffins (C21eC40).

3337

6.2. HDPE/Cat ratio effect The ratio of polymer to catalyst has a very important effect on the composition and yield of every fraction produced. Low ratios result in a high concentration of acid sites per gram of polymer fed, and thus high gas conversion [24]. Waxes obtained in the experiments at ratios of 0.27 and 1.41 are composed of linear paraffins, typical of the catalytic pyrolysis over HZSM-5. Increasing the ratio directly affects the way in which the polymer degrades. The higher the HDPE/Cat proportion, the greater the HDPE being thermally degraded. Gas products from the experiments carried out at an HDPE/Cat ratio of 9.2 and N(Thermal) contain methane, which is not formed by catalytic pyrolysis (b-scission mechanism). Moreover, the gas fraction generated is relatively low whilst the wax fraction is important due to the low temperature (500  C). It is generally accepted that the thermal pyrolysis mechanism begins to dominate HDPE cracking at 500  C. The composition of waxes at a 9.2 ratio is rich in olefins. In any case, increasing the catalyst content can reduce the thermal effect, and this shows that the temperature is not so high as to have any influence over the predominance of the catalytic pathway. However, as has been explained above, experiments undertaken at 550  C show the influence of thermal cracking in spite of the high polymer to catalyst ratio (1.14). 7. Conclusions Catalytic pyrolysis of HDPE is a possible lower cost alternative to other polymer recycling technologies. The use of nanocrystalline HZSM-5 allows greater yields of gas fractions at mild temperatures and a higher selectivity to the products obtained than those achieved by thermal cracking. First, polyethylene is cracked over the acid sites on the external surface of the zeolite. A part of the compounds produced can then diffuse into the internal active surface through the zig-zag pores and overcrack giving lighter compounds. Due to the size restriction of the pores, gases are produced mainly in the pores while waxes are the result of external cracking. The temperature affects the gas and wax yields. Gas production increases as the temperature rises. However, when the temperature exceeds 500  C, some HDPE is cracked thermally, increasing the wax yield and varying the wax and gas compositions. If the catalytic pyrolysis temperature is too low (350e400  C), the polymer is not fully cracked and a solid residue is produced in the reaction bed. The gases are mainly composed of olefins. Cracking reactions (b-scission) control the reaction mechanism inside the pores. Due to their small dimensions, HZSM-5 pores inhibit bimolecular reactions, which are a source for paraffin production. Terminal olefins are not produced in significant quantities because of the strong effect of isomerization reactions. The waxes obtained have very high concentrations of compounds between C10 and C20. This composition does not change over a wide temperature range (350e500  C). The paraffin production mechanism controls the process (b-scission þ hydrogen

3338

J.F. Mastral et al. / Polymer Degradation and Stability 91 (2006) 3330e3338

transfer). As with the gas distribution, the wax composition changes at temperatures exceeding 500  C. Typical thermal pyrolysis products such as heavy olefins, cycles and aromatics are detected at 550  C. The polymer to catalyst ratio also affects the product yields. This influence is especially noticeable when the ratio is high. In these cases, the number of acid sites per gram of polyethylene is not sufficient to degrade all the polymer, resulting in a lower gas production and altering the product distribution. It has been demonstrated that thermal pyrolysis (HDPE/ Cat ¼ N(Thermal)) produces lower gas yields than catalytic pyrolysis, and that this fraction is largely composed of heavier compounds (C5eC7). The chain reaction mechanism and radical species involving the thermal degradation of polyethylene produce olefinic gases, as in the catalytic mechanism, but they also allow methane formation which had not previously been produced. The most important compositional change is observed in the waxes which increase their olefinic and cycle contents as the HDPE/Cat ratio increases. The results obtained show that the most suitable temperature and HDPE/Cat ratio for the process are 450  C and 1.41, respectively. In these conditions no solid residue is formed and the wax obtained does not contain olefins and cyclics which are environmentally and operationally dangerous. Acknowledgements The authors express their gratitude to C.I.C.Y.T. (Project PPQ2002-01625) for providing financial support for this work. References [1] Arandes JM, Bilbao J, Lo´pez D. Reciclado de residuos pla´sticos. Rev Iberoam Polim 2004;5(1):28e45. [2] Mastral JF, Esperanza E, Berrueco C, Juste M, Ceamanos J. Fluidized bed thermal degradation products of HDPE in an inert atmosphere and in airenitrogen mixtures. J Anal Appl Pyrolysis 2003;70(1):1e17. [3] Arandes JM, Abajo I, Ferna´ndez I, Azkoiti MJ, Bilbao J. Effect of HZSM-5 zeolite addition to a fluid catalytic cracking catalysts. Study in a laboratory reactor operating under industrial conditions. Ind Eng Chem Res 2000;39:1917e24. [4] Bagri R, Williams PT. Catalytic pyrolysis of polyethylene. J Anal Appl Pyrolysis 2002;63:29e41. [5] Lin YH, Sharratt PN, Garforth AA, Dwyer J. Deactivation of US-Y zeolite by coke formation during the catalytic pyrolysis of high density polyethylene. Thermochim Acta 1997;294:45e50. [6] Marcilla A, Beltra´n M, Conesa JA. Catalyst addition in polyethylene pyrolysis. Thermogravimetric study. J Anal Appl Pyrolysis 2001;58e 59:117e26. [7] Puente de la G, Klocker C, Sedran U. Conversion of waste plastics into fuels. Recycling polyethylene in FCC. Appl Catal B Environ 2002;36:279e85. [8] Uemichi Y, Nakamura J, Itoh T, Sugioka M, Garforth AA, Dwyer J. Conversion of polyethylene into gasoline-range fuels by two stage catalytic degradation using silicaealumina and HZSM-5 zeolite. Ind Eng Chem Res 1999;38:385e90. [9] Walendziewski J, Steininger M. Thermal and catalytic conversion of waste polyolefines. Catal Today 2001;65:323e30.

[10] Seo YH, Lee KH, Shin DH. Investigation of catalytic degradation of high-density polyethylene by hydrocarbon group type analysis. J Anal Appl Pyrolysis 2003;70:383e98. [11] Zhu X, Liu S, Song Y, Xu L. Catalytic cracking of C4 alkenes to propene and ethane: influences of zeolites pore structures and Si/Al2 ratios. Appl Catal A Gen 2005;288:134e42. [12] Park JW, Kim JH, Seo G. The effect of pore shape on the catalytic performance of zeolites in the liquid-phase degradation of HDPE. Polym Degrad Stab 2002;76:495e501. [13] Park DW, Hwang EY, Kim JR, Choi JK, Kim YA, Woo HC. Catalytic degradation of polyethylene over solid acid catalysts. Polym Degrad Stab 1999;65:193e8. [14] Sahoo SK, Viswanadham N, Ray N, Gupta JK, Singh ID. Studies on acidity, activity and coke deactivation of ZSM-5 during n-heptane aromatization. Appl Catal A Gen 2001;205:1e10. [15] Serrano DP, Aguado J, Escola JM. Catalytic cracking of polyolefin mixture over different acid solid catalysts. Ind Eng Chem Res 2000;39: 1177e84. [16] Triantafillidis CS, Evmiridis NP. Performance of ZSM-5 as a fluid catalytic cracking catalyst additive: effect of total number of acid sites and particle size. Ind Eng Chem Res 1999;38:916e27. [17] Chen D, Rebo HP, Moljord K, Holmen A. Influence of coke deposition on selectivity in zeolite catalysis. Ind Eng Chem Res 1997;36:3473e9. [18] Guisnet M. ‘Coke’ molecules trapped in the micropores of zeolites as active species in hydrocarbon transformations. J Mol Catal A Chem 2002;182e183:367e82. [19] Van Grieken R, Serrano DP, Aguado J, Garcı´a R, Rojo C. Thermal and catalytic cracking of polyethylene under mild conditions. J Anal Appl Pyrolysis 2001;58e59:127e42. [20] Gobin K, Manos G. Thermogravimetric study of polymer catalytic degradation over microporous materials. Polym Degrad Stab 2004;86(2): 225e31. [21] Serrano DP, Aguado J, Escola JM, Garagorri E, Rodrı´guez JM, Morselli L, et al. Feedstock recycling of agriculture plastic film wastes by catalytic cracking. Appl Catal B Environ 2004;49(4):257e65. [22] Garforth AA, Lin YH, Sharratt PN, Dwyer J. Production of hydrocarbons by catalytic degradation of high density polyethylene in a laboratory fluidised-bed reactor. Appl Catal A Gen 1998;169:331e42. [23] Seddegi ZS, Budrthumal U, Al-Arfaj AA, Al-Amer AM, Barri SAI. Catalytic cracking of polyethylene over all-silica MCM-41 molecular sieve. Appl Catal A Gen 2002;225(1e2):167e76. [24] Takuma K, Uemichi Y, Ayame A. Product distribution from catalytic degradation of polyethylene over H-gallosilicate. Appl Catal A Gen 2000;192:273e80. [25] Schirmer J, Kim JS, Klemm E. Catalytic degradation of polyethylene using thermal gravimetric analysis and a cycled-spheres-reactor. J Anal Appl Pyrolysis 2001;60:205e17. [26] Herna´ndez MR, Garcı´a AN, Marcilla A. Study of the gases obtained in thermal and catalytic pyrolysis of HDPE in a fluidized bed reactor. J Anal Appl Pyrolysis 2005;73(2):314e22. [27] Miskolczi N, Bartha L, Dea´k G, Jo´ver B, Kallo´ D. Thermal and thermocatalytic degradation of high-density polyethylene waste. J Anal Appl Pyrolysis 2004;72:235e42. [28] Corma A, Planelles J, Sa´nchez FT. The role of different types of acid sites in the cracking of alkanes on zeolite catalysts. J Catal 1985; 93:30. [29] Kazansky VB. Adsorbed carbocations as transition states in heterogeneous acid catalyzed transformations of hydrocarbons. Catal Today 1999;51:419e34. [30] Serrano DP, Aguado J, Escola JM, Rodrı´guez JM. Influence of nanocrystalline HZSM-5 external surface on the catalytic cracking of polyolefins. J Anal Appl Pyrolysis 2005;74:353e60. [31] Di Blasi C. Transition between regimes in the degradation of thermoplastic polymers. Polym Degrad Stab 1999;64:359e67.