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Current Analytical Chemistry, 2011, 7, 110-116
Catalytic Conversion of Thermal Decomposition Products of Halogen Containing Polymers Studied by Pyrolysis-GC-MS Marianne Blazsó* and János Bozi Institute of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Budapest, Hungary Abstract: Feedstock recycling of plastics wastes by pyrolysis is adequate for obtaining valuable oil. However, thermal decomposition products of the halogen containing components of the plastics are contaminating the pyrolysis oil. In this work, pyrolysis-gas chromatography-mass spectrometry combined with online catalytic conversion has been applied to study the activity of Na-zeolite catalysts for dehalogenation of chloro- and bromo-hydrocarbons in pyrolysis oil of poly(vinylbenzyl chloride), polychloroprene, polychlorostyrene and polybromostyrene. GC-MS analysis of the products helped to recognize catalytic reactions of halogenated alkyl, alkenyl and aromatic hydrocarbons over three sodium zeolites, two of faujasite (X and Y) and one of beta structure. It was concluded that chlorine is completely eliminated from alkyl and alkenyl chloride compounds over all the three investigated zeolites. However, chlorine and bromine substituent of aromatic rings split off only partially: 6, 15 and 17% dechlorination and 14, 31 and 29% debromination of chloro- and bromobenzenes have been observed over X, Y and zeolite, respectively.
Keywords: Catalytic dehalogenation, GC-MS, Organic chloro compounds, Pyrolysis-gas chromatography, Sodium-zeolite, Halogen containing polymers. INTRODUCTION Plastic containing wastes are typically multi-component mixtures thus their handling and utilisation require special techniques. Thermal decomposition in an inert atmosphere (pyrolysis) proved to be an adequate method for extracting valuable part of these complex wastes. Thermal fragments of plastic components are accumulated in the pyrolysis oil that may be used as fuel of high energy content. Research on the pyrolytic recycling of plastic waste often focuses on the possible halogenated aromatic hydrocarbon content of the pyrolysis oil because brominated and chlorinated organic compounds may be the source of serious pollution upon combustion [1]. One of the most reasonable sources of chlorinated compounds is hydrogen chloride thermally eliminated from PVC that may react with thermal decomposition products of other polymer components of plastic waste. Chlorinated products of the co-pyrolysis of PVC with various polymers have been detected and identified [2, 3], moreover, their catalytic elimination has been also more or less succeeded [4, 5]. Several works have been published on the analysis and attempts for removal of brominated compounds present in the pyrolysis oils of plastics waste, originated from flame retardant compounds and polymers [6, 7]. Dehydrohalogenation of organic compounds in air or dissolved in water can be effectively completed with the help of various noble metal catalysts [8] but dehalogenation of pyrolysis oil of plastics waste requires other type of catalysts and methods because
of the higher halogen content and different system. Hydrogen chloride or bromide gas is separated and/or trapped without difficulty, but the elimination of organic halogencontaining compounds from pyrolysis oil by various catalysts is far not so straightforward. Y and X type zeolite catalysts have sufficient thermal stability in application converting hot pyrolysis vapours, moreover their low price, and commercial availability are satisfactory for economical waste handling. Cracking activity of zeolites is widely studied also for plastics’ pyrolysis oil, however, their applicability for dehalogenation of different halogenated organic compounds is not well explored. Typical catalysts used in polymer cracking for feedstock recycling of polymer wastes are H-zeolites [9]. In our earlier work it has been demonstrated that Na-zeolite is able to reduce considerably the bromophenol, dibromophenol, and tetrabromobromobisphenol A content of pyrolysis oil of electronic waste [10], furthermore the same activity has been observed in our laboratory eliminating tribromochloro- and dibromochlorobisphenol A. Nevertheless, the question arises whether sodium zeolite is also active in dehalogenation of other halogenated compounds than phenols. In this work GC-MS analysis of catalytically converted pyrolysis oils of halogen containing polymers has been carried out in order to recognize catalytic reactions of halogenated alkyl, alkenyl and aromatic hydrocarbons over sodiumzeolites. METHODS
*Address correspondence to this author at the Institute of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1525 Budapest Pf.17, Hungary; Tel: +36 1 438 1148; Fax: +36 1 438 1147; E-mail:
[email protected] 1573-4110/11 $58.00+.00
Pyrolysis-gas chromatography-mass spectrometry (Pyrolysis-GC/MS) combined with online catalytic conversion [11] has been applied using a Pyroprobe 2000 pyrolyser (Chemical Data System, USA) equipped with a platinum coil © 2011 Bentham Science Publishers Ltd.
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Current Analytical Chemistry, 2011, Vol. 7, No. 2
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and quartz sample tube, coupled to an Agilent 6890 GC5973 MSD (Agilent Technologies, USA) instrument. Typically 0.1 - 0.2 mg polymer was pyrolysed at 500oC for 20 s, and about 0.5 mg of solid catalyst has been placed into the sample holder at both ends of the quartz tube. Helium carrier gas at a flow rate of 20 mL min-1 purged the pyrolysis chamber held at 280oC that was split prior to be introduced into the GC column. The GC separation was carried out on a HP-5MS capillary column (30 m 0.25 mm 0.25m, Agilent Technologies, USA). After 1 min of isotherm period at 50oC the oven temperature was programmed to 300oC at 10oC min-1 heating rate and held at 300oC for 8 min. The temperature of the transfer line of GC/MS and the source of the mass spectrometer were 280 and 230oC, respectively. The mass spectrometer was operating in electronimpact mode (EI) at 70eV. The spectra were obtained over a mass range of 14-600 u. The identification of GC/MS peaks has been carried out by using mass spectral library, mass spectrometric identification principles and gas chromatographic retention relations. The reproducibility of pyrolysis-gas chromatograms (pyrograms) has been evaluated from three parallel experiments. The deviation of the peak areas of a pyrogram are generally higher than that of a GC-MS analysis because of small variation of pyrolysis parameters out of control. Similar reproducibility of pyrolysis-catalysis gas chromatograms has been observed when the catalyst microbeds were properly tight and the pyrolysis vapors could not escape without passing through them. The studied pure polymers were, poly(vinylbenzyl chloride) (60/40 of 3- and 4-isomers), polychloroprene, poly(4chlorostyrene), poly(4-bromostyrene) (Aldrich-Chemie) and the catalysts were 13X (Linde) ( faujasite structure with 14.7 wt% Na), NaY (faujasite structure with 10.7 wt% Na, and Na zeolite (Grace Davison) (beta structure with 2.2 wt% Na). RESULTS AND DISCUSSION 1. Conversion of Pyrolysis Products of Poly(Vinylbenzyl Chloride) The studied poly(vinylbenzyl chloride) was a mixture of poly(3-chloromethylstyrene) and poly(4-chloromethylstyrene). Their general formula is [-(CH2-CH(C6H4-CH2Cl)-]n (1). The thermal decomposition of these substituted styrene polymers leads mainly to 3- and 4-chloromethylstyrene (2) (monomer) by depolymerization and to 3- and 4-chloromethyltoluene (3) formed similarly as toluene from polystyrene [12]. The pyrogram obtained at 500°C displayed in the bottom of Fig. (1) indicates that in addition to depolymerization dechlorination also occurs to some extent resulting in HCl, 3- and 4-methylstyrene (4), and 1,3- and 1,4-dimethylbenzene (5). Boinon et al. [13] also detected HCl release in the first stage of a two steps mass loss from this polymer by thermogravimetry. The peak area values of the GC-MS total ion chromatogram are displayed in Table 1. (Note the 1,3- and 1,4- isomers of dimethylbenzene are not separated on the given GC column, and the pyrogram revealed that the sample contains a few percent of poly(vinylbenzyl bromide) (2*) as well).
6
11 C2-5 HCl
5
8
7
Naß
10 9
11 HCl
7
6 8 5 7
NaY
10
5
9
4
6 8 HCl
13X 9
2
2 HCl
4
2 * GC retention time / min 3
5
2
4
6
8
10
12
14
16
18
GC retention time / min
Fig. (1). Pyrolysis gas chromatogram of poly(vinylbenzyl chloride) at 500°C (bottom chromatogram), gas chromatogram of pyrolysis products converted over 13X, over NaY, and over Na zeolite. Peak labels correspond to numbers associated with compounds drawn in Fig. (2). Bold underlined numbers indicate halogenated compounds.
With the help of 13X zeolite nearly all chloromethyl substituted compounds have been converted into methyl substituted ones, thus less than 1% monomer (2) and no chloromethyltoluene (3) appear in the corresponding chromatogram in Fig. (1). Conversion routes of these compounds are supposed and drafted in Fig. (2) on the basis of the GCMS analysis of products. The dechlorinated compounds were also altered: vinyl group of methylstyrene (4) has been saturated leading to ethyltoluene (7), and one methyl group has been cleaved from dimethylbenzene (5) and etyltoluene (7) producing toluene (6) and ethylbenzene (8). On the other hand, benzene (11) naphthalene (10) and methylnaphthalenes (9) appear among the products indicating some degree of cracking and aromatisation activity of this zeolite. The chromatogram of the pyrolysis oil converted over NaY zeolite displayed in Fig. (1) and the data in Table 1 show that full dehalogenation has been achieved: chloromethylstyrene (2) and -toluene (3) peaks disappeared. Furthermore, extensive saturation of vinyl groups and split of alkyl groups result in ethyltoluene (7), toluene (6) and ethylbenzene (8), respectively. Formation of naphthalene (10) and benzene (11) is also enhanced over NaY compared to that over 13X attributed to cracking and aromatization. The products converted over Na zeolite are similar to those over NaY, but the relative amount of benzene (11) is considerably higher and C2-C5 hydrocarbons also appear (see Fig. (1) and Table 1), indicating a stronger cracking activity.
112 Current Analytical Chemistry, 2011, Vol. 7, No. 2
Table 1.
Blazsó and Bozi
GC-MS Peak Area % of the Pyrolysis Product Components of poly(vinylbenzyl chloride) Changed Over Na-zeolites Zeolite Type Compound None
13X
NaY
Na
3-chloromethylstyrene (2)
21.4 ±0.5
0.9 ±0.2
-
-
4-chloromethylstyrene (2)
6.0 ±0.5
-
-
-
3-chloromethyltoluene (3)
2.9 ±0.3
-
-
-
4-chloromethyltoluene (3)
0.8 ±0.2
-
-
-
3-methylstyrene (4)
7.8 ±1.4
5.7 ±0.9
1.5 ±0.4
0.8 ±0.05
4-methylstyrene (4)
8.1 ±1.5
2.9 ±0.9
0.5 ±0.4
-
1,3- and 1,4-dimethylbenzene (5)
4.4 ±0.5
14.4±0.5
12.1 ±0.5
7.5 ±0.3
toluene (6)
0.7 ±0.1
15.2±0.5
22.3 ±0.4
23.8 ±0.4
3- and 4- ethyltoluene (7)
2.4 ±0.4
20.0 ±1.0
12.9 ±0.6
5.6 ±0.3
ethylbenzene (8)
-
5.5 ±0.2
9.6 ±0.4
4.7 ±0.2
methylnaphthalene (9)
-
2.6 ±0.2
3.6 ±0.2
2.7 ±0.2
naphthalene (10)
-
1.1 ±0.1
2.0 ±0.1
3.3 ±0.1
benzene (11)
-
2.0 ±0.1
6.4 ±0.1
11.3 ±0.2
C3-C5 hydrocarbons
-
-
-
14.2 ±0.05
1,2- dimethylbenzene
-
-
-
2.1 ±0.1
Numbers in bracket correspond to those indicating the compounds in Fig. (2).
2. Conversion of Pyrolysis Products of Polychloroprene Polychloroprene (12) (or Neoprene) used for special rubber goods. Its thermal decomposition is seemingly analogous with that of polyisoprene [14] leading to monomer and cyclic dimer. Chlorine is bonded to an olefin carbon in the molecule of chloroprene (13) (monomer) and of (1-chloro-5-(1chlorovinyl)cyclohex-1-ene (14) and 1-chloro-4-(1-chlorovinyl)cyclohex-1-ene (15)) (cyclic dimers). The pyrolysisgas chromatogram of polychloroprene is displayed in the bottom of Fig. (3).
Fig. (2). Catalytic conversion of the main thermal decomposition products of poly(vinylbenzyl chloride): chloromethyltoluene (3) and chloromethylstyrene (2) over Na zeolite; c: dechlorination and H addition, d: split of aliphatic substituent from aromatic carbon, e: H addition, f: cracking and aromatisation.
The chromatograms of the converted pyrolysis products are displayed in Fig. (3) and the peak area values of the GCMS total ion chromatograms are listed in Table 2. These results suggest that converting pyrolysis products of polychloroprene over sodium zeolite a hydrogen chloride molecule is eliminated from the chlorovinyl group of chloroprene and of the cyclic dimer while the cyclohexene ring is aromatised as illustrated by the reaction scheme in Fig (4). This process results in approximately similar proportions of 3and 4-chloro-1-ethylbenzene (16 and 17) over 13X as that of dimers (14 and 15) prior to conversion. However, over NaY the relative amount of the isomer (16) is increased as the data show in the corresponding columns in Table 2. Over Na zeolite the two isomers of chloro-ethylbenzene are formed in more balanced amounts and the third isomer also appears (2chloro-1-ethylbenzene (18)) indicating a powerful isomerisation activity of this zeolite. Simultaneous cracking and dehydrochlorination of cyclic dimers lead partly to light alkenes partly to a series of aro-
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Current Analytical Chemistry, 2011, Vol. 7, No. 2
dium zeolites and that of cracking and isomerisation increases in the order of 13x < NaY
