Degradation of polystyrene using clinoptilolite catalysts

Journal of Analytical and Applied Pyrolysis 64 (2002) 71–83

www.elsevier.com/locate/jaap

Degradation of polystyrene using clinoptilolite catalysts Seung-Yup Lee, Jik-Hyun Yoon, Jong-Ryeul Kim, Dae-Won Park * Department of Chemical Engineering, Pusan National Uni6ersity, Pusan 609 -735, Republic of Korea Received 14 March 2001; accepted 6 August 2001

Abstract Several solid acids such as silica–alumina, HZSM-5, HY, mordenite and clinoptilolite (natural and synthesized) are screened for their performances in the catalytic degradation of polystyrene (PS). The clinoptilolites showed good catalytic activity for the degradation of PS with very high selectivity to aromatic liquids. The effects of catalyst acidity, reaction temperature and contact time on the distribution of aromatics are discussed. The increase of contact time and surface acidity enhanced the production of ethylbenzene. High degradation temperature favored the selectivity to styrene monomer. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Polystyrene; Catalytic degradation; Clinoptilolite catalyst; Aromatics

1. Introduction Plastic waste disposal has been recognized as a worldwide environmental problem. Even the manufacture of bio- and photodegradable plastics cannot solve the problem because these plastics have the limitation of long-term degradation and cause a different kind of environmental problem by the stabilizers introduced in their preparation [1]. Therefore, in recent years, increased attention has been paid to the recycling of synthetic polymer waste. This can contribute to solving pollution problems and the reuse of cheap and abundant waste products. * Corresponding author. Tel.: + 82-51-510-2399; fax: + 82-51-512-8563. E-mail address: [email protected] (D.-W. Park). 0165-2370/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 0 1 ) 0 0 1 7 1 - 1

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Though several methods have been proposed for recycling waste plastics, it is generally accepted that material recovery is not a long-term solution to the present problem, and that energy or chemical recovery is a more attractive one [2–4]. In this method, the waste plastics are thermally or catalytically degraded into gases and oils, which can be used as resources in fuels or chemicals. However, in the thermal degradation of polyolefin many hydrocarbons having a wide range distribution of the carbon atom number are formed. In contrast, the oils produced by catalytic degradation are known to contain a relatively narrow distribution of hydrocarbons [5,6]. The most commonly used catalysts in the catalytic degradation of polymers are solid acids and base [7 – 11]. Plastic wastes for the catalytic degradation processes are mainly limited to polyolefinic wastes (polyethylene and polypropylene) and polystyrene (PS). In contrast to polyethylene and polypropylene, PS can be thermally depolymerized to obtain the monomer styrene with a high selectivity. Zhang et al. [12] obtained a styrene yield of 70 wt.% by PS degradation at 350 °C using a semi-batch reactor with a continuous flow of nitrogen. On the contrary, Audisio et al. [13] reported very low selectivity (B 5 wt.%) to the styrene in PS degradation with solid acids such as silica – alumina and HY or REY zeolites at 350 °C. The main products in their study were benzene, ethylbenzene and cumene. Recently, Ukei et al. [11] reported that solid bases, especially BaO, were more effective catalysts than solid acids (HZSM-5 and silica –alumina) for the degradation of PS to styrene monomer and dimer at 350 °C. In our previous works, we reported that the natural clinoptilolite zeolite (occurring in the Youngil area of Korea) was a good catalyst for the degradation of polypropylene [10] and polyethylene [14]. The purpose of this study is to evaluate the performance of the natural and synthesized clinoptilolite in the catalytic degradation of PS. The effect of catalyst acidity, degradation temperature and contact time on the PS degradation are investigated.

2. Experimental

2.1. Material and catalysts PS, in powder form, was supplied by LG Chemical Co. (Grade 50IS, Mn= 98 000 –99 000, melt index= 7.5 g 10 min − 1, density=1.03 g cm − 3). PS samples of 60–150 mesh size were used for this study. Several types of solid acid catalysts such as silica –alumina (SA, Aldrich), mordenite (JRC-M20), Y-zeolite (JRC-Y5.5), ZSM-5 (RC-Penta), natural clinoptilolite zeolite (NZ) and synthesized clinoptilolite (SCLZ) were evaluated through the degradation experiments of PS. SCLZ was synthesized according to the previously reported method [15]. Na-form of all the zeolites were ion-exchanged three consecutive times with 1 M NH4Cl solution for 20 h. The zeolites exchanged with NH+ 4 were dried at 110 °C for 6 h, and then calcined in air at 400 °C to obtain the proton (H+)-exchanged zeolites like HZSM-5, HSCLZ and HNZ.

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2.2. Apparatus and procedure The catalytic degradation of PS was carried out in a semi-batch reactor where nitrogen is continuously passed with a flow rate of 60 ml min − 1. The gas flow was controlled by mass flow controller (Brooks MFC 5850E). The blending of the PS and the catalyst was performed in a ball mill. A mixture of 3.0 g of PS and 0.3 g of the catalyst was loaded inside a Pyrex vessel of 30 ml and heated at a rate of 30 °C min − 1 up to the desired temperature. The distillate from the reactor was collected in a cold trap over a pre-determined time.

2.3. Analysis The bulk structure of the natural zeolites was confirmed by an X-ray diffractomer (XRD, MAC Science Co., M18XHF) analysis with Cu–Ka radiation. The composition of the natural zeolites was determined by atomic absorption spectroscopy (Perkin–Elmer AAS 5100). The acidic properties of the catalysts were determined by a conventional temperature programmed desorption (TPD) experiment of ammonia in the temperature range of 100– 700 °C at a constant heating rate of 5 °C min − 1. The specific surface area and pore size of the catalysts were measured by a Brunauer – Emmett –Teller (BET) apparatus (Micromeritics ASAP 2010). The degradation of the PS gave off gases, liquids and residues. The residues means the carbonaceous compounds remaining in the reactor and deposited on the wall of reactor. The residues adhering to the reactor wall were dissolved in n-hexane and the amount was measured by a chemical balance (OHAUS Corp. Explorer). The amount of gases products was calculated by subtracting the sum of weights for liquids, residues and catalyst, from the total weight of PS sample and fresh catalyst initially loaded to the reactor. The gases were analyzed by an on-line GC (HP 5890) with a Porapak Q column. The condensed liquid samples were analyzed by GC–MS (Micromass Co., Auto spec.) with a capillary column (HP-5MS). The physical properties and the composition of the liquids were also measured by a PONA (Paraffins –Olefins – Naphthenes – Aromatics) analyzer (DHA 2520). The amount of coke deposit on the catalyst was calculated by measuring the desorbed amount of carbon dioxide, formed by the reaction of coke and oxygen, during temperature programmed oxidation (TPO) of used catalysts. In TPO, the used catalyst was first treated by helium at 150 °C, then it was cooled down to 40 °C before passing a mixed gas of 1 vol.% O2 and the balance helium. The temperature was increased up to 600 °C with a heating rate of 10 °C min − 1, and the produced CO2 peaks are measured.

3. Result and discussion

3.1. Characterization of catalysts Table 1 shows the physicochemical properties of the catalysts used in this study.

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Table 1 Physical properties of the catalysts used in the experiments Catalyst

SA Y5.5a M20a ZSM-5 NZ SCLZ a

Structure

Amorphous FAU MOR MFI Clinoptilolite type Clinoptilolite type

Pore size (A, )

60–100 7.4 6.5×7.0 5.3×5.6, 5.1×5.5 7.6×3.0, 3.3×4.6 7.6×3.0, 3.3×4.6

Composition (wt.%) SiO2

Al2O3

Na2O

Fe2O3

86.0 67.1 86.4 86.7 65.9 64.8

13.0 20.4 7.3 5.7 14.8 25.2

– 12.4 5.0 3.1 1.4 7.7

– 0.04 0.04 0.04 1.5 –

The numbers represent the Si/Al ratios of the catalysis.

The NZ was characterized in a previous work [16]. It is a silica-rich member of the clinoptilolite family. Its pore structure is characterized by two main channels parallel to the c-direction, one formed by a 10-member ring (7.6× 3.0 A, ) and the other by an eight-member ring (3.3× 4.6 A, ). BET analysis on pore volume distribution showed that most of the pores in HNZ were micropores of about 5 A, , and it had practically no mesopores. Silica– alumina has a wide-range mesopore with surface area of 504 m2 g − 1. ZSM-5 has Si/Al ratio of 22 and surface area of 379 m2 g − 1. Y5.5 has a faujasite structure with pores of about 7.4 A, and shows very high surface area (870 m2 g − 1). M20 has Si/Al ratio of 20 and its surface area is 399 m2 g − 1. Table 2 shows surface area and pore volume of the clinoptilolite catalysts. Proton exchanged clinoptilolite catalysts (HNZ, HSCLZ) show much higher surface areas and pore volumes than NZ and SCLZ. The HSCLZ has higher surface area and pore volume than HNZ. The total pore volume of pores between 4.5 and 9.1 A, for HNZ and HSCLZ is 0.107 and 0.174 cm3 g − 1, respectively. The bulk structures of NZ and SCLZ and their proton exchanged forms (HNZ, HSCLZ) were analyzed by XRD as shown in Fig. 1. All of these catalysts were identified to be clinoptilolite. There is little change in the structure of the clinopilolite when NZ and SCLZ are protonated, except the disappearance of some small peaks probably due to the elimination of some impurities such as Na, K, Ca and Fe.

Table 2 BET surface areas of natural and synthesized zeolites of clinoptilolite type Catalyst

NZ

HNZ

SCLZ

HSCLZ

BET S.A. (m2 g−1) Cumulative pore volume (cm3 g−1)a

74 0.031

271 0.107

40 0.014

448 0.174

a

Cumulative pore volume of pores between pore diameters of 4.5–9.1 A, .

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Fig. 1. X-ray diffraction patterns for the clinoptilolite zeolites.

Fig. 2 shows NH3-TPD profiles of the clinoptilolite catalysts. The temperature of the peak maximum (Tmax) reflects the acid strength, although the re-adsorption of desorbed ammonia may shift the temperature to a higher one [17]. SCLZ has only weak acidic sites (Tmax =220 – 250 °C). NZ has weak and medium acidic sites (Tmax =480–510 °C) with a small number of strong acidic sites. However, HNZ and HSCLZ have a much higher number of acidic sites with a wide distribution of acid strength. The number of medium and strong acid sites (Tmax = 610–650 °C) increased considerably with these protonated catalysts compared to NZ and SCLZ.

3.2. Screening of catalysts PS, alone or together with catalysts in the reactor, was degraded at 400 °C for 2 h. Table 3 lists the gases, liquids, residues and cokes on the catalyst from the degradation experiments. In all cases, the liquid oils were the main products. The

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Fig. 2. NH3-TPD chromatograms of the clinoptilolite zeolites.

HNZ and HSCLZ showed comparable performances with other catalysts in producing liquid oils without producing many residues. Table 4 shows distribution of the liquid products by hydrocarbon type in the PS degradation. Almost all of liquid products formed in the PS degradation were aromatics. The main products, aromatic hydrocarbons, are identified in detail as shown in Table 5. Thermal degradation shows the highest selectivity of styrene (52.2%) and the lowest selectivity of ethylbenzene (6.6%). Thermal degradation of PS starts with a random initiation to form polymer radicals [18], the main products being styrene and its corresponding dimers and trimers. The catalytic degradation over amorphous SA exhibits lower selectivity of styrene (36.1%) and higher selectivity of ethylbenzene (22.6%) and benzene (1.9%) compared to the degraded aromatic products for HNZ (styrene= 48.1%, ethylbenzene=12.8%), HSCLZ (styrene=47.7%, ethylbenzene= 16.2%) and HZSM-5 (styTable 3 Mass balances in the degradation of PS at 400 °C for 2 h Catalyst

Thermal SA HNZ HSCLZ HZSM-5 HM20 HY5.5

Products (wt.%) Gases

Liquids

Residues

Cokes

8.5 7.7 9.7 18.1 14.7 13.8 10.9

81.7 83.5 81.5 72.6 75.1 73.9 79.4

9.8 4.8 4.9 5.1 6.3 8.3 5.6

– 4.0 3.9 4.2 3.9 4.0 4.1

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Table 4 Distribution of liquid product (wt.%) by hydrocarbon type in the degradation of PS at 400 °C for 2 h Catalyst

n-Paraffins

Iso-paraffins

Olefins

Naphthenes

Aromatics

Thermal SA HNZ HSCLZ HZSM-5 HM20 HY5.5

0.01 0.2 0.04 0.02 0.02 0.03 0.03

0.3 1.6 0.5 0.1 0.2 0.1 0.1

0.2 0.6 0.1 0.03 0.3 0.1 0.1

0.3 0.8 0.2 0.1 0.2 0.2 0.2

99.2 96.8 99.1 99.7 99.3 99.5 99.5

rene =50.7%, ethylbenzene= 10.5%). The same trend on the distribution of styrene and ethylbenzene is also reported by Serrano et al. [19] for the catalytic degradation of PS over SA and HZSM-5. For the catalytic cracking with SA, they obtained a low selectivity towards styrene (below 11%) accompanied by a parallel enhancement in the selectivity to benzene and ethylbenzene. From this screening test of catalysts, one can see that the HNZ and HSCLZ used in this work show as good performance as HZSM-5, most popularly used in the catalytic degradation of plastics on a commercial scale. Therefore, the following studies are focused on the clinoptilolite catalysts for enlarging their applications by better understanding of their performance in PS degradation.

3.3. Degradation of PS o6er clinoptilolite catalysts 3.3.1. Effect of acidity The effect of acidity on the PS degradation was studied using NZ, HNZ, SCLZ and HSCLZ. Since the main liquid products of PS degradation are aromatics, the Table 5 Selectivity of aromatics formed in the degradation of PS at 400 °C for 2 h Aromatics

Thermal

NZ

HNZ

SCLZ

HSCLZ

HZSM-5

SA

Benzene Toluene Ethylbenzene Xylene Styrene Iso-propylbenzene a-Methylstyrene Trimethylbenzene Indane C10–C15 C16–C21 C22–C30

0.04 5.7 6.6 0.01 52.2 1.6 7.9 0.25 0.01 0.19 17.3 8.2

0.15 5.2 10.1 0.02 50.8 2.5 7.3 0.26 0.30 0.37 16.0 7.0

0.18 6.3 12.8 0.02 48.1 2.8 9.2 0.03 0.01 0.65 13.7 6.3

0.06 6.7 8.0 0.02 53.2 1.9 7.8 0.34 0.01 0.37 15.8 5.8

0.05 6.0 16.2 0.03 47.7 2.9 7.7 0.29 0.08 0.55 14.2 4.3

1.1 6.4 10.5 0.14 50.7 2.1 8.4 0.34 0.12 1.2 13.2 5.8

1.9 4.7 22.6 0.01 36.1 0.04 7.6 0.40 0.05 1.5 19.2 5.9

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differences in aromatics distributions are compared between the catalysts. Table 5 shows selectivity of different aromatics. The main product in C16 –C21 is styrene dimer (C16) and that in C22 – C30 is trimer (C25). One can see that ethylbenzene selectivity increases (from 10.1 to 12.8%) and styrene selectivity decreases (from 50.8 to 48.1%) from NZ to HNZ. From SCLZ to HSCLZ, one can also find a decrease of styrene selectivity (from 53.2 to 47.7%) and an increase of ethylbenzene selectivity (from 8.0 to 16.2%). This result indicates that the hydrogenation of degraded styrene can occur more easily in the protonated catalysts and that it leads to the increased production of ethylbenzene. Ogawa et al. [20] reported the hydrogenation of styrene monomer to ethylbenzene with silica–alumina catalyst at 300 °C. The protonated catalysts, HNZ and HSCLZ, showed lower selectivities to dimers and trimers than NZ and SCLZ, respectively. The acid catalyzed cracking of PS is of a carbenium nature [21,22]. The most likely reaction pathway involves the attack of proton associated with a Bro¨ nsted acid site to the aromatic rings of PS, due to the reactivity of its side phenyl groups towards electrophilic reagents. The resulting carbenium ion may undergo b-scission followed by a hydrogen transfer. The possible production pathways of benzene, styrene, a-methlystyrene, toluene, ethylbenzene, isopropylbenzene and indane derivatives are reported by Audisio et al. [13]. However, Nanbu et al. [23] reported that the protonated polymer backbone could also proceed through cross-linking reactions among adjacent polymeric chains or even inside the same polymer. Therefore, the cross-linking reactions and cracking reaction are competitive in PS degradation. Serrano et al. [19] reported that the cross-linking reactions were favored by strong Bro¨ nsted acidic sites, especially at low temperatures. When the cross-linking reactions are accelerated, the competing catalytic cracking may occur to a smaller extent. Silica-alumina (SA), having medium strength Lewis acidic sites with a relatively small number of Bro¨ nsted acid sites, does not seem to promote these cross-linking reactions very significantly, but accelerates hydrogenation reactions. This may be the reason why it produced a low amount of styrene and a high amount of ethylbenzene as shown Table 5. The increase of cracking in SA is also related to its mesopores where the degraded PS fragments can easily enter the pores for further cracking. Solid bases are also reported as good catalysts for the PS degradation [11]. The catalytic degradation on solid base may start with the formation of carboanions by the elimination of a hydrogen atom of PS. BaO showed higher styrene selectivity (76.4%) than thermal degradation (70.0%) in the runs performed at 350 °C for 3 h [11]. From our experimental results and literature findings, one can see that a controlled strategy for the PS degradation becomes possible. Base catalysts are good for styrene monomer production, while acid catalysts promote the production of ethylbenzene by the hydrogenation of styrene.

3.3.2. Effect of temperature The effect of temperature on the PS degradation was studied using HNZ and HSCLZ catalysts. Table 6 shows the mass balance in product distribution. The

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Table 6 Mass balances in the degradation of PS with HNZ and HSCLZ catalysts for 2 h Catalyst

HNZ

HSCLZ

Temperature (°C)

Products (wt.%)

350 375 400 425 450 400 425 450

Gases

Liquids

Residues

Cokes

5.3 7.2 9.7 8.9 6.3 18.1 12.9 9.7

9.0 61.4 81.5 84.9 89.7 72.6 79.1 85.5

78.2 26.3 4.9 3.0 2.2 5.1 4.1 3.0

7.5 5.1 3.9 3.2 1.8 4.2 3.9 1.8

amount of liquid products increases with temperature, while the residues and cokes decrease with temperature. At low temperatures the amount of residues for HNZ catalyst was high, 78.2 and 26.3 wt.% for 350 and 375 °C, respectively. But their amounts were very small at higher temperatures. This indicates that the competitive cross-linking reactions take place first, especially at low temperatures, therefore the cracking of the resulting cross-linked polymer becomes more difficult. Serrano et al. [19] reported a large amount of black residue without any appreciable amount of cracking product in the PS degradation at 350 °C for 30 min with HZSM catalyst. Table 7 shows distribution of the liquid products by hydrocarbon type for HNZ catalyst. Even though the amount of liquid products was very small at low temperatures, aromatics were the main liquid products. With the increase of temperature, the amount of aromatics increased slightly. The distribution of aromatic hydrocarbons is shown in Table 8. At 350 °C, indane was produced with an appreciable amount, and its amount decreased at high temperatures. The production of indane in PS degradation was also reported by other works [13,20]. It is noteworthy that the highest selectivity towards styrene (60.0%) is obtained at the highest temperature (450 °C), and that the highest selectivity to ethylbenzene (13.5%) at the lowest temperature (350 °C). In the sense of competing reactions between cross-linking and cracking mentioned above, high Table 7 Distribution of liquid product (wt.%) in hydrocarbon type with HNZ catalyst at different reaction temperatures for 2 h Temperature (°C)

n-Paraffins

Iso-paraffins

Olefins

Naphthalenes

Aromatics

350 375 400 425 450

0.22 0.10 0.02 0.04 0.04

1.96 1.04 0.63 0.52 0.47

0.28 0.33 0.39 0.09 0.17

0.39 0.15 0.07 0.22 0.21

97.1 98.4 98.9 99.1 99.1

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Table 8 Selectivity of aromatics formed in the degradation of PS with HNZ catalyst at different temperatures for 2 h Aromatics

350 °C

375 °C

400 °C

425 °C

450 °C

Benzene Toluene Ethylbenzene Xylene Styrene Iso-propylbenzene a-Methylstyrene Trimethylbenzene Indane C10–C15 C16–C21 C22–C30

0.43 2.1 13.5 0.04 38.4 7.7 5.4 0.33 2.9 7.9 14.7 6.6

0.31 4.0 13.2 0.02 47.0 5.1 6.6 0.17 0.90 1.9 14.3 6.5

0.18 6.3 12.8 0.02 48.1 2.8 9.2 0.03 0.01 0.65 13.7 6.3

0.37 6.8 10.3 0.01 54.8 0.02 7.8 0.04 0.01 0.55 13.5 5.8

0.35 5.6 8.4 0.02 60.0 0.02 6.3 0.36 0.15 0.60 13.0 5.2

temperatures will produce less styrene because of promoted cracking reactions. Since the experimental results show inverse distribution trends of styrene and ethylbenzene with temperature (high styrene production at higher temperature), it needs another explanation besides the two competing reactions. Styrene and ethylbenzene are known to be produced by different reaction pathways, as shown in Scheme 1 [13]. Therefore, at high temperatures one can consider that the reaction pathway I is dominant with respect to the pathway II. Further hydrogenation of the produced styrene can also occur, but this reaction seems less important than pathway I at high temperatures. The increase of styrene with temperature was also reported by Audisio et al. [13]. They explained this by the relatively high stability of the polymeric ion A in pathway I.

Scheme 1.

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Fig. 3. Changes of product distribution during the PS degradation with HNZ at 400 °C.

3.3.3. Effects of process time and contact time The distribution of products during the PS degradation is presented in Fig. 3 for the HNZ catalyst. At the beginning stage (up to 30 min), much of the residues are observed, probably due to the cross-linking reactions of the protonated polymer backbone. As time proceeds, the liquids and gaseous products increase by the cracking reactions. The formation of coke also increased with process time, but the amount was small compared to the other degradation products. Fig. 4 shows time variant production of principal aromatic compounds in the PS degradation over HNZ at 400 °C. Styrene is produced very fast and then its amount decreases with process time. Ethylbenzene starts to increase with the decrease of styrene production, due to the hydrogenation of styrene. C16 –C21 and C22 –C30 increase in the earlier stage, and their selectivities attained a constant value after 60 min. This means that the dimerization of the product styrene takes place on the HNZ catalyst. The dimerization of styrene is known to occur using solid acid catalysts [24,26]. This process is always accompanied by an appreciable extent of oligomerization. Toluene and a-methylstyrene also increase with the decrease of styrene. The production of benzene is the smallest among the principal products. The effect of contact time on the composition of aromatic products is studied with HNZ at four different nitrogen flow rates of 30, 60, 100 and 150 ml min − 1. Table 9 shows the distribution of aromatics. A decrease of styrene selectivity from 54.0 to 47.0% is observed with the increase of contact time by reducing the nitrogen flow rate from 150 to 30 ml min − 1. However, the selectivity of ethylbenzene increased with the decrease of contact time. With increasing contact time, the product styrene has more time to participate in the hydrogenation reaction to produce ethylbenzene.

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Fig. 4. Selectivity of main products in the conversion of PS with HNZ at 400 °C as a function of process time.

4. Conclusion The clinoptilolite catalysts (HNZ, HSCLZ) showed good catalytic performance for the degradation of PS with selectivity to aromatics more than 99%. Styrene is the major product, and ethylbenzene is the second most abundant one in the liquid Table 9 Selectivity of aromatics formed in the degradation of PS with HNZ catalyst for different flow rates at 400 °C Aromatics

Benzene Toluene Ethylbenzene Xylene Styrene Iso-propylbenzene a-Methylstyrene Trimethylbenzene Indane C10–C15 C16–C21 C22–C30

Nitrogen flow rate (ml min−1) 30

60

100

150

0.15 6.7 13.7 0.03 47.0 2.9 9.9 0.35 0.09 0.50 12.9 5.8

0.18 6.3 12.8 0.02 48.1 2.8 9.2 0.03 0.01 0.65 13.7 6.3

0.28 5.2 11.7 0.02 52.5 1.9 7.9 0.25 0.08 0.37 13.5 6.3

0.16 4.3 10.3 0.02 54.0 1.9 8.1 0.27 0.08 0.48 13.9 6.5

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product. The increase of acidity favored the production of ethylbenzene by promoting the hydrogenation reaction of styrene. Higher selectivity to styrene is observed at higher temperatures. An increase of contact time by reducing nitrogen gas flow rate enhanced the selectivity to ethylbenzene. Therefore, a designed operation including acidity of catalyst, reaction temperature and contact time will be necessary to control the product distribution between styrene monomer and ethylbenzene.

Acknowledgements The authors wish to acknowledge the financial support of the Korea Science and Engineering Foundation (2000-1-30700-008-3).

References [1] R.C. Mordi, R. Fields, J. Dwyer, J. Anal. Appl. Pyrol. 29 (1994) 45. [2] W. Zhao, S. Hasegawa, J. Fugito, F. Yoshii, T. Sasaki, K. Makuuchi, J. Sun, S. Nishimoto, Polym. Degrad. Stab. 53 (1996) 129. [3] W.C. McCaffrey, M.R. Kamal, D.G. Cooper, Polym. Degrad. Stab. 47 (1995) 133. [4] A.R. Songip, T. Masuda, H. Kuwahara, K. Hashimoto, Appl. Catal. B: Environ. 2 (1993) 153. [5] H. Ohkita, R. Nishiyama, Y. Tochihara, T. Mizushima, Ind. Eng. Chem. Res. 32 (12) (1993) 3112. [6] Y. Uemichi, Catal. (Jpn.) 37 (4) (1995) 286. [7] G. Audisio, A. Silvani, P.L. Beltrame, P. Carniti, J. Anal. Appl. Pyrol. 7 (1984) 83. [8] P.L. Beltrame, P. Carniti, G. Audisio, F. Bertini, Polym. Degrad. Stab. 26 (1989) 209. [9] Y. Uemichi, Y. Kakino, T. Kanazuka, J. Anal. Appl. Pyrol. 16 (1989) 229. [10] E.Y. Hwang, J.K. Choi, D.H. Kim, D.W. Park, H.C. Woo, Kor. J. Chem. Eng. 15 (4) (1998) 434. [11] H. Ukei, T. Hirose, S. Horikawa, Y. Takei, M. Taka, N. Azuma, A. Ueno, Catal. Today 62 (2000) 67. [12] Z. Zhang, T. Hirose, S. Nishio, Y. Morioka, N. Azuma, A. Ueno, H. Ohkita, M. Okada, Ind. Eng. Chem. Res. 34 (1995) 4514. [13] G. Audisio, F. Bertini, P.L. Beltrame, P. Carniti, Polym. Degrad. Stab. 29 (1990) 191. [14] D.W. Park, E.Y. Hwang, J.R. Kim, J.K. Choi, Y.A. Kim, H.C. Woo, Polym. Degrad. Stab. 65 (1999) 193. [15] S. Satokawa, K. Itabashi, Microporous Matter 8 (1997) 49. [16] H.C. Woo, K.H. Lee, J.S. Lee, Appl. Catal. A: General 134 (1996) 147. [17] N. Katada, H. Igi, J.H. Kim, M. Niwa, J. Phys. Chem. B 101 (1997) 5969. [18] H.G. Jellinek, J. Polym. Sci. 4 (1949) 13. [19] D.P. Serrano, J. Aguado, J.M. Escola, Appl. Catal. B: Environ. 25 (2000) 181. [20] T. Ogawa, T. Kuroki, S. Ide, J. Appl. Polym. Sci. 27 (1981) 857. [21] Y. Sakata, M.A. Uddin, A. Muto, J. Anal. Appl. Pyrol. 51 (1999) 135. [22] G. Audisio, F. Bertini, P.L. Beltrame, P. Carniti, Makromol. Chem. Macromol. Symp. 57 (1992) 191. [23] H. Nanbu, Y. Sakuma, Y. Ishihara, T. Takesue, T. Ikemura, Polym. Degrad. Stab. 19 (1987) 61. [24] A.R. Taylor, G.W. Keen, E.J. Eisenbraun, J. Org. Chem. 22 (1977) 3477. [25] H. Hasegawa, T. Higashimura, Polym. J. 11 (1979) 737. [26] A. Benito, A. Corma, H. Garcia, J. Primo, Appl. Catal. A: General 116 (1994) 127.