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Metal decorated montmorillonite as a catalyst for the degradation of polystyrene
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Metal decorated montmorillonite as a catalyst for the degradation of polystyrene Jasmin Shah a,∗, Muhammad Rasul Jan a, Adnan a,b a b
Institute of Chemical Sciences, University of Peshawar, Peshawar, Khyber Pakhtunkhwa, Pakistan Institute of Chemical Sciences, University of Swat, Swat, Khyber Pakhtunkhwa, Pakistan
a r t i c l e
i n f o
Article history: Received 17 October 2016 Revised 17 July 2017 Accepted 18 July 2017 Available online xxx Keywords: Decorated montmorillonite Decorated catalysts Polystyrene degradation Catalytic activity
a b s t r a c t Benefiting a renewable source of precious hydrocarbons and getting rid of a huge environmental pollutant like polystyrene is the cry of the day. Waste polystyrene (PS) represents a source of valuable chemical products like styrene and other aromatics. Therefore, metals decorated montmorillonite (Mt) catalysts were prepared using Mg, Zn, Al, Cu and Fe and were evaluated for the tertiary recycling of waste polystyrene. The metal decorated catalysts characterized by N2 adsorption/desorption, XRD & SEM techniques. 20% Fe/Mt and 5% Al/Mt were found with high activity for the production of liquid products and good selectivity for the production of low molecular weight aromatic products like styrene, toluene and ethylbenzene, etc. The yield of toluene, ethylbenzene, styrene and α -methylstyrene was 8.49 wt.%, 5.13 wt.%, 49.28 wt.% and 2.80 wt.%, respectively using 5% Al/Mt catalyst and it was 10.15 wt.%, 6.42 wt.%, 50.93 wt.% and 2.31 wt.%, respectively using 20% Fe/Mt catalyst. The results showed that among the metal decorated Mt catalysts used, 5% Al/Mt and 20% Fe/Mg were found to be the most effective catalyst for selective conversion into aromatics. The products contained styrene monomer as a major along with toluene, ethylbenzene and α -methylstyrene value added products. Thus, it was possible to demonstrate the feasibility of catalytic degradation as an alternative technology for the chemical recycling of waste PS. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction The ongoing growths of population and rapid industrialization have led to the refusal of huge quantities of solid wastes on daily basis [1]. Municipal solid waste comprises a large proportion of styrene based plastics [2]. Plastics are recycled using five different ways, i.e., disposing by landfill, mechanical recycling, biological recycling, thermal incineration and chemical recycling [3]. Like other plastic, polystyrene (PS) is also non-biodegradable and their disposal depletes landfill sites. The incineration of these wastes produces toxic gases causing different health problems, including cancer, therefore, prohibited by public opinion [2,4–8]. Mechanical recycling is expensive and often full of impurities while biological recycling is much slow process and it is only practiced for degradable plastics. All the mentioned methods are not suitable for waste management. The best option is chemical recycling or tertiary recycling to get maximum benefit of the plastics waste and convert it into valuable hydrocarbons [3]. The tertiary recycling is achieved by three different means; (i) depolymerization,
∗
Corresponding author. E-mail address: [email protected] (J. Shah).
(ii) partial oxidation and (iii) degradation or cracking or pyrolysis. Depolymerization cannot be applied to more than 70% of municipal solid wastes [9]. Partial oxidation is the direct combustion of polymer waste with high calorific value for energy recovery [9]. Therefore, the third method which includes catalytic degradation is the best option to be used because it does not require high temperature or costly conditions as needed for thermal degradation, biodegradation or photodegradation [9–11]. Proper selection or development of a catalyst enables us to get improved and selective products that not only yield desirable products, but also decrease the production cost [12]. Mostly, for the degradation of PS solid acid and base catalysts are used. The solid acid catalysts include zeolites, silica–alumina (SiO2 /Al2 O3 ), FCC, MCM-41, HZSM-5, mordenite, clinoptilolite, etc., while the solid base catalysts include magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO), titanium dioxide (TiO2 ), potassium oxide (K2 O), iron oxide (Fe2 O3 ), chromium oxide (Cr2 O3 ), cobalt oxide (Co3 O4 ), zinc oxide (ZnO) and copper oxide (CuO), etc. These catalysts have been reported with high yield of liquid products having styrene monomer as the major product, but the reaction conditions used were economically not feasible and the selectivity of component products were also low with maximum number of undesirable products [13–21]. In order to increase the yield of
http://dx.doi.org/10.1016/j.jtice.2017.07.026 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: J. Shah et al., Metal decorated montmorillonite as a catalyst for the degradation of polystyrene, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.07.026
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liquid products with good selectivity of desirable products, novel impregnated catalysts were used for the degradation of PS [22]. Some researchers have degraded PS with modified or promoted catalysts but limited data is available with impregnated catalysts in the literature. Acid-treated halloysite clays were used by Tae et al., [23], they found that ethylbenzene was increased with the increase of contact time and acidity of catalyst, where high temperature increased styrene monomer. Better activity was found by Xie et al., [24] with modified silicon mesoporous molecular sieve as catalyst. Kim et al., [25] used modified alumina support with Fe, K, Ba, Zn and Mg catalysts for the degradation of PS and found Fe-K/Al2 O3 the best catalyst with a significant amount of liquid. The use of modified, or treated catalysts have brought significant increase in the activity and selectivity of the products, therefore, suggests the need for more advance catalysts (supported catalysts) more specifically impregnated catalyst. Impregnated catalysts can be prepared with the selection of proper active metal center (a precursor) over a suitable support [26] like silica, alumina, and activated carbon, etc. Mineral clays like montmorillonite (Mt) in the field of catalysis have got much interest and have been used for various purposes due to their high porosity, exchangeable cations and swellable properties. Mt is one of the important clay minerals that has widely used as supporting material [27–30]. In the present work a series of metal decorated catalysts were prepared over Mt support with Mg, Zn, Al, Cu and Fe as the active metal centers. The aim of the current study was the improvement of catalytic activity in terms of liquid products as well as the selectivity of component products. 2. Experimental 2.1. Materials The polystyrene (PS) samples used have an average molecular weight (Mw) 20 0,0 0 0 g/mol purchased from Sigma-Aldrich. Montmorillonite (Mt) clay was obtained from a local research laboratory. MgCl2 ·6H2 O, ZnCl2 and FeCl3 ·6H2 O were purchased from Merck KGaA 64271, Darmstadt, Germany. AlCl3 ·6H2 O and CuCl2 ·2H2 O were purchased from BDH Laboratory Supplies, Poole, BH151TD, England. 2.2. Catalyst preparation Different concentrations of salts of Mg, Zn, Al, Cu and Fe, i.e., (MgCl2 ·6H2 O, ZnCl2 , AlCl3 ·6H2 O, CuCl2 ·2H2 O and FeCl3 ·6H2 O) were calculated as 5%, 10%, 15%, 20% and 25% on the basis of metal weight for a known weight of Mt. The relevant salts were impregnated over Mt support using the wet impregnation method. The active metal center salt was dissolved in appropriate amount of water and was poured into the slurry of Mt support and stirred the mixture for 1 h at 60 °C, filtered, washed with distilled water and dried in oven at 110 °C for 6 h. The catalyst sample was then calcined at 300 °C for 4 h and cooled in a desiccator. The dried sample was ground to powder and screened to a particle size ≤445 μm. 2.3. Catalyst characterization Surface Area Analyzer NOVA2200e Quantachrome, USA was used for the determination of BET surface area of the prepared catalysts using N2 adsorption/desorption at 77.4 K. The morphology of catalysts was determined using Scanning Electron Microscope (SEM) JSM5910, JEOL, Japan instrument. X-ray diffraction (XRD) patterns were taken using a JDX-3532 JEOL (Japan) diffractometer ˚ at 40 KV and with monochromatic Cu-Kα radiation (λ = 1.5418 A) 30 mA in the 2θ range of 10–80° with 1.03° per minute.
Table 1 BET Surface area for metal decorated catalysts using Mt as supporting material. Surface area (m2 /g) Catalysts Mt 20% 20% 05% 15% 20%
Mg/Mt Zn/Mt Al/Mt Cu/Mt Fe/Mt
BET
BJH
116.21 96.14 78.25 102.20 49.86 69.53
489.23 412.08 297.6 264.56 54.6 77.53
˚ Pore size (A)
Pore volume (cc/g)
115.39 121.84 102.38 118.96 78.73 77.72
1.25 1.26 0.68 1.23 0.09 0.13
2.4. Catalyst activity Polystyrene degradation experiments were conducted at atmospheric pressure in a Pyrex glass reactor (height 22 cm and internal diameter 7 cm) set in a heating assembly operates up to 10 0 0 °C. PS (5 g) was degraded with the mixture of appropriate amount of relevant catalysts (weight by weight blend) for a constant period of time. The degradation experiments were run with virgin PS and heterogeneous catalysts without the use of any solvent and other additives. The degradation products, i.e., liquids and gases were collected after condensation and measured. The residue left in the reactor was weighted and the products were expressed in terms of wt.% of the PS degraded. 2.5. Analysis GC/MS analysis of the degraded products was performed with Shimadzu QP2010 Plus GC/MS. The instrument was fitted with 30 m capillary column, 0.25 mm internal diameter having DB-5MS (95% dimethylpolysiloxane, 5% polyphenyl) fused silica capillary column with 0.25 μm film thickness. Helium gas (99.99%) was used as carrier gas with a flow rate of 1.5 mL/min, sample injection volume was 1 μL and the injector port was provided with split mode (1:50). The injection temperature was 300 °C and the oven temperature was programmed as follow; initial oven temperature 35 °C (5 min), ramped at 5 °C/min to 100 °C (1 min), then ramped at 10 °C/min to 150 °C (10 min) and finally ramped at 2.5 °C/min to 290 °C (10 min). The ion source temperature used was 280 °C and interface temperature was 290 °C. The spectra were compared with GC/MS updated library for the identification of component products. 2.6. Catalyst activity studies for the degradation of PS The catalytic activity of catalysts was tested in two phases as in our previous work [31]. In the first phase the Mt support was used as a catalyst and was employed in the process of PS degradation with the effect of degradation temperature, reaction time and feed to catalyst ratio. The selection criterion for optimum reaction conditions was based on maximum production of liquid products as well as total products. In the second phase, the prepared metal decorated catalysts over Mt support were evaluated. The catalytic activities of metal decorated catalysts were tested with the effect of percentage (%) of the impregnated metal using the same reaction conditions as used for Mt support alone. 3. Results and discussions 3.1. Catalyst characterization Surface area, pore volume and size were calculated for all the prepared metal decorated catalysts over Mt support in comparison to Mt support (Table 1). The BET and BJH surface area of all
Please cite this article as: J. Shah et al., Metal decorated montmorillonite as a catalyst for the degradation of polystyrene, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.07.026
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Fig. 1. SEM micrograph of (a) Mt, (b) 20% Mg/Mt, (c) 20% Zn/Mt, (d) 05% Al/Mt, (e) 15% Cu/Mt and (f) 20% Fe/Mt.
the metal decorated catalysts were decreased as compared to Mt support, this indicates the successful impregnation of the precursor active center over the surface and in the pores of support causing to block the pores ultimately decreasing the pore size and pore volume [31]. However, the pore size and pore volume were increased in the cases of 20% Mg/Mt and 5% Al/Mt catalysts as compared to Mt support. The increase in the case of pore size might be due the interaction of precursor active center molecules over the surface of pores, which also indicated the formation of new layers and/or pores and/or channels. The SEM image of Mt support is depicted in Fig. 1(a), the image present highly porous structure with particle size ranging from 10 to 20 μm. Fig. 1(b) presents the morphology of 20% Mg/Mt, the image reveals a huge condense structure, the surface seems having pores throughout the whole structure. The surface morphology of 20% Zn/Mt is shown in Fig. 1(c), the Zn/Mt have relatively uniform particles of 2–3 μm in size. Fig. 1(d) indicates 5% Al/Mt
catalyst with irregular shape particles having size 1–4 μm, the bright metallic view of the particles indicates successful impregnation of the precursor component. Fig. 1(e) presents the morphology of 20% Cu/Mt catalyst, which indicate agglomerated particles with smaller size than that of Mt support. The picture shows 0.6 μm (600 nm) to 5 μm particles with adequate dispersion of precursor active metal center (Cu). Fig. 1(f) corresponds to 20% Fe/Mt, the surface of the catalysts also presents the agglomeration of uniform particle cling to each other at the sides of the particle thus developing porous channels and channels throughout the surface of the catalyst. The XRD diffraction patterns of both supports and metal decorated catalysts were taken. It can be seen from Fig. 2 that the composition of Mt has changed after impregnation of precursor active metals. Metal decorated catalysts using XRD spectrum for montmorillonite support are shown in Fig. 2(a), the diffractogram for Mt at 19.51°, 25.24°, 26.29°, 29.41°, 47.38° and 57.07°
Please cite this article as: J. Shah et al., Metal decorated montmorillonite as a catalyst for the degradation of polystyrene, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.07.026
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Fig. 2. XRD diffractogram of (a) Mt support, (b) 20% Mg/Mt, (c) 20% Zn/Mt, (d) 5% Al/Mt, (e) 15% Cu/Mt and (f) 20% Fe/Mt.
indicates bredigite (Ca14 Mg2 (SiO4 )8 according to ICDD Card No. 360399, the patterns at 20.62°, 25.24° and 57.07° also shows sodium magnesium silicate (Na2 MgSiO4 ) (ICDD Card No. 471499). The peaks at 19.51°, 23.77° and 29.41° show montmorillonite15A (CaO2 (Al,Mg)Si4 O10 (OH)2 ) according to ICDD Card No. 130135. The XRD diffraction patterns for 20% Mg/Mt are shown in Fig. 2(b). The major diffraction peaks at 21.58°, 29.63° and 31.87° indicate Na3 Mg3 Ca5 Al19 Si117 O272 according to ICDD Card No. 491831 and, at 20.95°, 22.48°, 31.03°, 42.19° and 49.42° show Na2 Al2 Si2 .5 O9 .6 ·2H2 O (ICDD Card No. 380237) with some relevant mineral. Fig. 2(c) shows XRD patterns for 20% Zn/Mt, it indicates sodium zinc oxide (Na10 Zn4 O9 ) at 22.03°, 32.38° and 34.93° according to ICDD Card No. 520058; the pattern also shows sodium zinc silicate (Na2 ZnSiO4 ) at 19.87° and 26.71° (ICDD Card No. 370407). The XRD diffractogram also indicates sodium magnesium zinc silicate (Na2 Mg3 Zn2 Si12 O30 ) at 29.50°, 32.38° and 63.46° according to ICDD Card No. 480418. The XRD patterns for 5% Al/Mt catalyst is shown in Fig. 2(d), it shows the major peaks for silicon oxide (Si34 O68 ) at 26.65°, 27.64°, 35.17° and 45.40° according to ICDD Card No. 520144. The patterns also show at 31.75° and 62.02° sodium aluminum silicate (Na1.15 Al1.15 Si0.85 O4 and Na1.75 Al1.75 Si0.25 O4 ) according to ICDD Card Nos. 490 0 07 and 490 0 04, respectively. The catalyst also contains a small amount of magnesium aluminum oxide (MgAl2 O3 ) and magnesium aluminum silicate (Mg3 Al2 (SiO4 )3 according to ICDD Card No. 211152 and 150742. The presence of these substances indicates interaction of Al with Mt support. Fig. 2(e) corresponds to 15% Cu/Mt and its pattern shows Ca2 CuO2 Cl2 according to ICDD Card No. 480319 at peaks 29.02°, 32.62° and 61.63°, the patterns shows Al2 CuMg at 45.34°, 54.76° and 61.63° according to ICDD Card No. 280014, it shows Ca2 Al2 O5 according to ICDD Card No. 521722 at peaks 26.62°, 34.09°, 61.63°, it also shows monoclinic CaCu at peaks 16.24°, 32.62°, 34.09°, 45.34° and 57.55° with ICDD Card No. 411276 and monoclinic CuCl2 with ICDD Card No. 10185. The XRD pattern indicates that 20% Fe/Mt is amorphous as shown in Fig. 2(f).
Fig. 3. Effect of degradation temperature using Mt as catalyst (Reaction conditions; reaction time 30 min, feed to catalyst ratio 1:0.2).
3.2. Catalyst activity studies for the degradation of PS In the first phase, using Mt support as catalysts for the degradation of PS was used. The effect of degradation temperature was optimized from 250 to 500 °C for the degradation of PS at constant reaction conditions of 30 min reaction time and feed to catalyst ratio of 1:0.2, the results are shown in Fig. 3. As can be seen from the figure, the degradation processes of PS were in three steps. The first degradation step in the temperature range of 250– 350 °C was relatively slow. At 250 °C, the yield of liquid products was 0 wt.% and then a gradual increase was recorded in the yield of liquid products up to 350 °C. In the second step, from 350 to 400 °C, the yield of liquid products was abruptly increased up to
Please cite this article as: J. Shah et al., Metal decorated montmorillonite as a catalyst for the degradation of polystyrene, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.07.026
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Fig. 4. Effect of reaction time using Mt as catalyst (Reaction conditions: degradation temperature 450 °C, feed to catalyst ratio 1:0.2).
87.60 ± 0.20 wt.%. The same has been already reported in our previous work for other catalysts [32]. The abrupt increase in the yield of liquid products indicates the propagation step of the degradation process of PS. In the 3rd step, a slight change was recorded from 400 to 450 °C but with further increase in the degradation temperature, no increase in the yield of liquid products was observed. This slight increase in the yield of liquid products might be due to the condensation of gaseous molecules and their reaction to form substitutes of the pre-existing products. A maximum was obtained at 450 °C with a significant amount of liquid products, i.e. 89.93 ± 0.31 wt. % with the total conversion of 100%. The heating time varied from 30 to 150 min using degradation temperature 450 °C and feed to catalyst ratio 1:0.2, the results are shown in the Fig. 4. The yield of liquid products increased from 30 to 60 min and then observed a decrease in the yield of liquid products due to increase in residence time [25]. Further increase in the residence time (90 min or above) might cause the cracking of preexisting products into further smaller fragments inside the reactor resulting low yield of liquid products. The maximum yield of liquid products was 91.33 ± 0.31 wt.% obtained with a 60 min reaction time. The degradation of PS into maximum liquid products was also optimized for feed to catalyst ratio. The feed to catalyst ratio was varied from 1:0.1 to 1:0.5 using optimized reaction condition, i.e. degradation temperature 450 °C and reaction time 60 min (Fig. 5). The results show that increase in the amount of catalyst has no effect on the yield of liquid products; however, the yield of liquid products was 92.40 ± 0.87 with 100% total conversion and was relatively high using the feed to catalyst ratio 1:0.2. Therefore, 1:0.2 feed to catalyst ratio was selected for further catalytic degradation studies. In the second phase of catalytic activity studies, different metals decorated catalysts over Mt were used for the degradation of PS. The effect of the percentage of impregnated metal varied from 5 to 25% using the optimized reaction conditions of the 1st phase for Mt support as a catalyst, i.e., degradation temperature 450 °C, reaction time 60 min, feed to catalyst ratio 1:0.2 (Fig. 6). The yield of liquid products increased with the increase of active metal percentage over Mt support except for aluminum decorated catalysts (Al/Mt). While in Al/Mt, the yield of liquid products decreased with the increase of active metal percentage and then kept constant with further increase in the percentage of impregnated metal. The decrease of liquid products with the increase of active metal percentage, i.e., Al metal increased the Lewis character of the catalysts
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Fig. 5. Effect of feed to catalysts ratio using Mt as catalysts (Reaction conditions: degradation temperature 450 °C, reaction time 30 min).
Fig. 6. Effect of percentage of the impregnated metal (Mg, Zn, Al, Cu and Fe) over Mt support for maximum liquid products (Reaction conditions: degradation temperature 450 °C, reaction time 30 min and feed to catalyst ratio 1:0.2).
yielding more gaseous products ultimately decreasing the yield of liquid products [33,34]. The yield of liquid products obtained with 5% impregnated Al metal was 89.60 ± 0.20 wt.%. In cases of Mg/Mt, Zn/Mt and Fe/Mt, maximum yield of liquid products was 84.53 ± 0.42, 84.53 ± 0.12 and 88.87 ± 0.42 wt.%, respectively using 20% impregnation of the respective metals (Mg, Zn and Fe). However, 89.20 ± 0.20 wt. % maximum yield of liquid product was obtained with 15% Cu/Mt decorated catalyst. The yield of liquid products using Mt and metal decorated catalysts over Mt was compared with thermal degradation reported in our previous work [22], i.e. 78.07 wt.% in 150 min reaction time at 500 °C degradation temperature. The current metal decorated Mt catalysts were found to increase the yield of liquid products significantly in a short reaction time and low degradation temperature as compared to that of thermal degradation. Comparison of the reaction conditions and contents of products of the degradation of PS with optimized metal decorated catalysts over Mt is given in Table 2.
Please cite this article as: J. Shah et al., Metal decorated montmorillonite as a catalyst for the degradation of polystyrene, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.07.026
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J. Shah et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–8 Table 2 Comparison of reaction condition and yield of products using catalytic degradation.
Reaction conditions Temperature (°C) Time (min) Feed to catalyst ratio
Mt
20% Mg/Mt
20% Zn/Mt
5% Al/Mt
15% Cu/Mt
20% Fe/Mt
450 60 1:0.2
450 30 1:0.3
450 120 1:0.3
450 60 1:0.2
450 30 1:0.3
450 30 1:0.2
84.53 15.30 – 10 0.0 0
84.53 15.40 – 10 0.0 0
89.60 2.40 8.00 92.00
89.20 9.07 1.73 98.27
88.87 11.20 – 10 0.0 0
Content of products (wt.%) Liquid yield 92.40 Gas yield 7.60 – Residue Total conversion 10 0.0 0
Table 3 GC/MS analysis of products formed by PS degradation using Mt support as catalyst and metal decorated Mt catalysts. Product S.No. name 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Benzene Toluene Ethylbenzene Styrene α -Methylstyrene Naphthalene Benzene, 1,1 -(1,2-ethanediyl)bis Benzene, 1,1 -(1-methyl-1,2-ethanediyl)bis Benzene, 1,1 -(1,3-propanediyl)bis Phenanthrene Benzene, 3-butynyl Benzene, (1-methyl-3-butenyl) 2-Phenylnaphthalene p-Terphenyl 1,2-propanediol, 3-benzyloxy-1,2-diacetyl 1,1 :3,1 -Terphenyl, 5 -phenyl Other hydrocarbons
Composition (wt.%) Mt
20% Mg/Mt
20% Zn/Mt
5% Al/Mt
15% Cu/Mt
20% Fe/Mt
0.04 1.59 0.49 45.42 0.70 0.01 0.75 0.31 0.81 0.07 18.35 0.72 0.10 0.04 19.00 0.30 3.70
0.36 5.90 5.48 46.62 2.22 0.30 1.79 1.38 0.70 0.28 4.86 0.50 1.59 0.22 0.92 0.50 9.33
0.55 7.16 3.28 46.16 2.46 0.72 4.41 2.06 0.48 0.55 7.87 0.72 1.25 0.49 0.62 0.25 5.56
0.81 8.49 5.13 49.28 2.80 0.62 3.56 1.69 0.41 0.52 5.58 0.55 1.84 0.53 0.39 0.41 7.00
0.82 9.76 6.61 48.46 2.75 0.00 2.47 1.12 0.32 0.56 2.68 0.32 2.80 0.51 0.16 0.36 9.51
0.79 10.15 6.42 50.93 2.31 0.67 2.36 1.14 0.00 0.54 2.60 0.00 2.81 0.63 0.00 0.72 6.75
3.3. Catalytic selectivity studies for the degradation of PS The parent liquids obtained from thermo-catalytic degradation of PS using metal decorated catalysts over Mt were analyzed for their component products and selectivity studied by GCMS. The results of GC/MS analysis were expressed in terms of wt.% of PS samples given in Table 3. During thermal assisted degradation processes, different reactions occurring produced large number of compounds. Nature of the reaction and products formation depend upon the nature of polymer material used, pre-existing products formed, temperature provided, residence time, and amount, nature and shape of the catalyst [31,35–37]. The aim of our selectivity studies was to decrease the total number of interactions and to increase the amount of low molecular weight aromatic hydrocarbons, especially styrene monomer, and yield of limited number of products [31]. As reported earlier Mt and metal decorated catalysts over Mt are acidic in nature, which are reported to initiate the degradation process of PS with the attack of an associated Brönsted acid proton on to the phenyl ring of PS. The carbocation produced then undergo β -scission and a hydrogen transfer. The production of styrene and α -methylstyrene are shown in Scheme 1 from polymer ion (A). These reactions are abundant in the degradation of PS. Therefore, high amount of styrene is produced [13,31,38]. The yield of styrene was almost the same with 5% Al/Mt, 15% Cu/Mt and 20% Fe/Mt that was slightly more than that of Mt, 20% Mg/Mt and 20% Zn/Mt. The polymer ion (B) is attacked by a proton and forming another cation, which then undergo β -scission producing a toluene as shown in Scheme 2. The yield of toluene was in the following order with different catalysts, Mt > 20% Mg/Mt > 20% Zn/Mt > 5% Al/Mt > 15% Cu/Mt > 20% Fe/Mt. The decrease in toluene might
be due to its further degradation to smaller volatile hydrocarbons [33,34]. Some intermolecular hydrogen transfer also takes place, which results in the formation of ethylbenzene and benzene, (1-methylethyl). The mechanism is given in Scheme 3. In the current study the yield of ethylbenzene was the same with 15% Cu/Mt and 20% Fe/Mt. It was slightly low with 20% Mg/Mt and 5% Al/Mt and it was the lowest with Mt as catalyst. The hydrogen of acidic catalysts attacks the branched aromatic ring giving rise a cation π -complex, which upon β -scission convert to σ -complex (secondary cation) and release benzene, shown in Scheme 4. The yield of benzene was also in the following order with different catalysts, Mt > 20% Mg/Mt > 20% Zn/Mt > % Al/Mt > 15% Cu/Mt > 20% Fe/Mt. After styrene, toluene and ethylbenzene, the yield of α methylstyrene, benzene, 3-butynyl were high. As can be seen from Table 3, the major components of degradation were styrene monomeric and dimeric products. This has been already reported in the literature [15,23,39]. Other hydrocarbons like styrene trimers and oligomers were formed up to a limited extent. The GC/MS comparative analysis revealed 20% Fe/Mt catalyst with desired product selectivity. The yield of toluene (10.15 wt.%), ethylbenzene (6.42 wt.%), styrene monomer (50.93 wt.%), α methylstyrene (2.31 wt.%) and 2-phenylnaphthalene (2.81 wt.%) was maximum with 20% Fe/Mt catalysts. After 20% Fe/Mt; 5% Al/Mt was also found with good catalytic selectivity. The yield of products was 8.49%, 5.13%, 49.28%, 2.80% and 1.25% for toluene, ethylbenzene, styrene monomer, α -methylstyrene and 2-phenylnaphthalene, respectively. The yield of component products was compared with thermal degradation reported in our previous work [22]. In the case of which benzene, 1,1 -(1,3propanediyl)bis (1.97 wt.%); benzene, 3-butynyl (17.56 wt.%) and
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Scheme 1. Reaction steps for the production of styrene and α -methylstyrene.
Scheme 2. Reaction steps for the production of toluene through β -scission reaction.
Scheme 3. Reaction steps for the formation of ethylbenzene and benzene,(1-methylethyl).
Scheme 4. Reaction steps for the formation of benzene through β -scission reaction.
benzene, (1-methyl-3-butenyl) (1.16 wt.%) were relatively high molecular weight aromatic hydrocarbons and were maximum as compared to low molecular weight hydrocarbons. The compound 1,2-propanediol, 3-benzyloxy-1,2-diacetyl (10.10 wt.%) was abundant and unwanted might have formed due to the interac-
tion of residue molecules and/or atmospheric oxygen [10]. These unwanted and high molecular weight aromatic hydrocarbons were decreased using metal decorated catalysts over Mt specifically with 20% Fe/Mt catalyst.
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4. Conclusions Metal decorated catalysts over Mt were used for the degradation of PS in comparison to Mt support as catalysts and thermal degradation. The surface area of metal decorated catalysts over Mt significantly decreased, which indicates interaction of active metal in the pores with successful impregnation. The catalysts were also characterized using SEM. The catalysts were tested for catalytic activity and selectivity of products. The increase of reaction time decreases the liquid products. The presence of catalysts decreased the number of products formed. Among the metal decorated catalysts 5% Al/Mt and 20% Fe/Mt was found with high activity and good selectivity toward low molecular weight aromatic hydrocarbons. Acknowledgments The authors acknowledge the financial support from the Higher Education Commission (HEC) of Pakistan. References [1] Islam MR, Parveen M, Haniu H, Islam MR. Innovation in pyrolysis technology for management of scrap tire: a solution of energy and environment. Int J Environ Res Sci Dev 2010;1:89–96. [2] Chauhan RS, Gopinath S, Razdan P, Delattre C, Nirmala GS, Natarajan R. Thermal decomposition of expanded polystyrene in a pebble bed reactor to get higher liquid fraction yield at low temperatures. Waste Manage. (Oxford) 2008;28:2140–5. [3] Miskolczi N, Bartha L, Angyal A. High energy containing fractions from plastic wastes by their chemical recycling. Macromol Symp 2006;245-246:599–606. [4] Kronholm J, Vastamäki P, Räsänen R, Ahonen A, Hartonen K, Riekkola M-L. Thermal field-flow fractionation and gas chromatography−mass spectrometry in determination of decomposition products of expandable polystyrene after reactions in pressurized hot water and supercritical water. Ind Eng Chem Res 2006;45:3029–35. [5] Lee C-G, Kim J-S, Song P-S, Choi G-S, Kang Y, Choi M-J. Decomposition characteristics of residue from the pyrolysis of polystyrene waste in a fluidized-bed reactor. Korean J Chem Eng 2003;20:133–7. [6] Karaduman A, S¸ ims¸ ek EH, Çiçek B, Bilgesü AY. Flash pyrolysis of polystyrene wastes in a free-fall reactor under vacuum. J Anal Appl Pyrol 2001;60:179–86. [7] Liu Y, Qian J, Wang J. Pyrolysis of polystyrene waste in a fluidized-bed reactor to obtain styrene monomer and gasoline fraction. Fuel Process Technol 20 0 0;63:45–55. [8] Blazsó M. Recent trends in analytical and applied pyrolysis of polymers. J Anal Appl Pyrol 1997;39:1–25. [9] Panda AK, Singh RK, Mishra DK. Thermolysis of waste plastics to liquid fuel: a suitable method for plastic waste management and manufacture of value added products—A world prospective. Renewable Sustainable Energy Rev 2010;14:233–48. [10] Hussain Z, Khan KM, Hussain K. Microwave-metal interaction pyrolysis of polystyrene. J Anal Appl Pyrol 2010;89:39–43. [11] Beyler C.L. and Hirschler M.M. ASTM E 176, “Standard terminology of fire standards”. Annual book of ASTM standards, vol. 4.07. West Conshohocken, PA, USA: American Society for Testing and Materials. [12] Sokolowski J, Rokicki G, Marczewski M, Szewczyk K. Thermal-catalytic recycling of Polyolefins and polystyrene. Magazine Tech. Chemia R 2008;105:311–21. [13] Ukei H, Hirose T, Horikawa S, Takai Y, Taka M, Azuma N, et al. Catalytic degradation of polystyrene into styrene and a design of recyclable polystyrene with dispersed catalysts. Catal Today 20 0 0;62:67–75.
[14] Lee KH. Composition of aromatic products in the catalytic degradation of the mixture of waste polystyrene and high-density polyethylene using spent FCC catalyst. Polym Degrad Stab 2008;93:1284–9. [15] Lee S-Y, Yoon J-H, Kim J-R, Park D-W. Degradation of polystyrene using clinoptilolite catalysts. J Anal Appl Pyrol 2002;64:71–83. [16] García RA, Serrano DP, Otero D. Catalytic cracking of HDPE over hybrid zeolitic–mesoporous materials. J Anal Appl Pyrol 2005;74:379–86. [17] 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 2002;225:167–76. [18] Achilias DS, Roupakias C, Megalokonomos P, Lappas AA, Antonakou EV. Catalytic cracking of polyethylene over all-silica MCM-41 molecular sieve. J Hazard Mater 2007;149:536–42. [19] Marcilla A, García-Quesada JC, Sánchez S, Ruiz R. Study of the catalytic pyrolysis behaviour of polyethylene–polypropylene mixtures. J Anal Appl Pyrol 2005;74:387–92. [20] Corma A, Iborra S, Miquel S, Primo J. Catalysts for the production of fine chemicals: production of food emulsifiers, monoglycerides, by glycerolysis of fats with solid base catalysts. J Catal 1998;173:315–21. [21] Pinto F, Costa P, Gulyurtlu I, Cabrita I. Pyrolysis of plastic wastes: 2. effect of catalyst on product yield. J Anal Appl Pyrol 1999;51:57–71. [22] Shah J, Jan MR, Adnan. Catalytic activity of metal impregnated catalysts for degradation of waste polystyrene. J Ind Eng Chem 2014;20:3604–11. [23] Tae J-W, Jang B-S, Kim J-R, Kim I, Park D-W. Catalytic degradation of polystyrene using acid-treated halloysite clays. Solid State Ionics 2004;172:129–33. [24] Xie C, Liu F, Yu S, Xie F, Li L, Zhang S, et al. Study on catalytic pyrolysis of polystyrene over base modified silicon mesoporous molecular sieve. Catal Commun 2008;9:1132–6. [25] Kim J-S, Lee W-Y, Lee S-B, Kim S-B, Choi M-J. Degradation of polystyrene waste over base promoted Fe catalysts. Catal Today 2003;87:59–68. [26] Liu X, Khinast JG, Glasser BJ. A parametric investigation of impregnation and drying of supported catalysts. Chem Eng Sci 2008;63:4517–30. [27] Farias M, Martinelli M, Rolim GK. Immobilized molybdenum acetylacetonate complex on montmorillonite K-10 as catalyst for epoxidation of vegetable oils. Appl Catal A 2011;403:119–27. [28] Fatimah I, Wang S, Narsito, Wijaya K. Composites of TiO2 -aluminum pillared montmorillonite: synthesis, characterization and photocatalytic degradation of methylene blue. Appl Clay Sci 2010;50:588–93. [29] Olszewska D. Characterization of ZrO2 -acid activated montmorillonite doped with Cu, Ni or Mn ions. Appl Clay Sci 2011;53:353–8. [30] Yin W-P, Shi M. Nitration of phenolic compounds by metal-modified montmorillonite KSF. Tetrahedron 2005;61:10861–7. [31] Adnan, Shah J, Jan MR. Polystyrene degradation studies using Cu supported catalysts. J Anal Appl Pyrol 2014;109:196–204. [32] Shah J, Jan MR, Adnan. Tertiary recycling of waste polystyrene using magnesium impregnated catalysts into valuable products. J Anal Appl Pyrol 2015;114:163–71. [33] Serrano DP, Aguado J, Escola JM, Rodríguez JM. Catalytic conversion of polystyrene over HMCM-41, HZSM-5 and amorphous SiO2 –Al2 O3 : comparison with thermal cracking. J Anal Appl Pyrol 2005;74:353–60. [34] Lopez-Urionabarrenechea A, de Marco I, Caballero BM, Laresgoiti MF, Adrados A. Catalytic stepwise pyrolysis of packaging plastic waste. J Anal Appl Pyrol 2012;96:54–62. [35] Shah J, Jan MR, Adnan. Catalytic activity of aluminum impregnated catalysts for the degradation of waste polystyrene. Int J Chem Nucl Metal Mater Eng 2014;8:83–9. [36] Serrano DP, Aguado J, Escola JM. Catalytic conversion of polystyrene over HMCM-41, HZSM-5 and amorphous SiO2 –Al2 O3 : comparison with thermal cracking Appl. Appl Catal, B 20 0 0;25:181–9. [37] Shah J, Jan MR, Adnan. Conversion of waste polystyrene through catalytic degradation into valuable products. Korean J Chem Eng 2014;31:1389–98. [38] Audisio G, Bertini F, Beltrame PL, Carniti P. Catalytic degradation of polymers: part III—degradation of polystyrene. Polym Degrad Stab 1990;29:191–200. [39] Adnan, Shah J, Jan MR. Thermo-catalytic pyrolysis of polystyrene in the presence of zinc bulk catalysts. J Taiwan Inst Chem Eng 2014;45:2494–500.
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Metal decorated montmorillonite as a catalyst for the degradation of polystyrene Jasmin Shah a,∗, Muhammad Rasul Jan a, Adnan a,b a b
Institute of Chemical Sciences, University of Peshawar, Peshawar, Khyber Pakhtunkhwa, Pakistan Institute of Chemical Sciences, University of Swat, Swat, Khyber Pakhtunkhwa, Pakistan
a r t i c l e
i n f o
Article history: Received 17 October 2016 Revised 17 July 2017 Accepted 18 July 2017 Available online xxx Keywords: Decorated montmorillonite Decorated catalysts Polystyrene degradation Catalytic activity
a b s t r a c t Benefiting a renewable source of precious hydrocarbons and getting rid of a huge environmental pollutant like polystyrene is the cry of the day. Waste polystyrene (PS) represents a source of valuable chemical products like styrene and other aromatics. Therefore, metals decorated montmorillonite (Mt) catalysts were prepared using Mg, Zn, Al, Cu and Fe and were evaluated for the tertiary recycling of waste polystyrene. The metal decorated catalysts characterized by N2 adsorption/desorption, XRD & SEM techniques. 20% Fe/Mt and 5% Al/Mt were found with high activity for the production of liquid products and good selectivity for the production of low molecular weight aromatic products like styrene, toluene and ethylbenzene, etc. The yield of toluene, ethylbenzene, styrene and α -methylstyrene was 8.49 wt.%, 5.13 wt.%, 49.28 wt.% and 2.80 wt.%, respectively using 5% Al/Mt catalyst and it was 10.15 wt.%, 6.42 wt.%, 50.93 wt.% and 2.31 wt.%, respectively using 20% Fe/Mt catalyst. The results showed that among the metal decorated Mt catalysts used, 5% Al/Mt and 20% Fe/Mg were found to be the most effective catalyst for selective conversion into aromatics. The products contained styrene monomer as a major along with toluene, ethylbenzene and α -methylstyrene value added products. Thus, it was possible to demonstrate the feasibility of catalytic degradation as an alternative technology for the chemical recycling of waste PS. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction The ongoing growths of population and rapid industrialization have led to the refusal of huge quantities of solid wastes on daily basis [1]. Municipal solid waste comprises a large proportion of styrene based plastics [2]. Plastics are recycled using five different ways, i.e., disposing by landfill, mechanical recycling, biological recycling, thermal incineration and chemical recycling [3]. Like other plastic, polystyrene (PS) is also non-biodegradable and their disposal depletes landfill sites. The incineration of these wastes produces toxic gases causing different health problems, including cancer, therefore, prohibited by public opinion [2,4–8]. Mechanical recycling is expensive and often full of impurities while biological recycling is much slow process and it is only practiced for degradable plastics. All the mentioned methods are not suitable for waste management. The best option is chemical recycling or tertiary recycling to get maximum benefit of the plastics waste and convert it into valuable hydrocarbons [3]. The tertiary recycling is achieved by three different means; (i) depolymerization,
∗
Corresponding author. E-mail address: [email protected] (J. Shah).
(ii) partial oxidation and (iii) degradation or cracking or pyrolysis. Depolymerization cannot be applied to more than 70% of municipal solid wastes [9]. Partial oxidation is the direct combustion of polymer waste with high calorific value for energy recovery [9]. Therefore, the third method which includes catalytic degradation is the best option to be used because it does not require high temperature or costly conditions as needed for thermal degradation, biodegradation or photodegradation [9–11]. Proper selection or development of a catalyst enables us to get improved and selective products that not only yield desirable products, but also decrease the production cost [12]. Mostly, for the degradation of PS solid acid and base catalysts are used. The solid acid catalysts include zeolites, silica–alumina (SiO2 /Al2 O3 ), FCC, MCM-41, HZSM-5, mordenite, clinoptilolite, etc., while the solid base catalysts include magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO), titanium dioxide (TiO2 ), potassium oxide (K2 O), iron oxide (Fe2 O3 ), chromium oxide (Cr2 O3 ), cobalt oxide (Co3 O4 ), zinc oxide (ZnO) and copper oxide (CuO), etc. These catalysts have been reported with high yield of liquid products having styrene monomer as the major product, but the reaction conditions used were economically not feasible and the selectivity of component products were also low with maximum number of undesirable products [13–21]. In order to increase the yield of
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liquid products with good selectivity of desirable products, novel impregnated catalysts were used for the degradation of PS [22]. Some researchers have degraded PS with modified or promoted catalysts but limited data is available with impregnated catalysts in the literature. Acid-treated halloysite clays were used by Tae et al., [23], they found that ethylbenzene was increased with the increase of contact time and acidity of catalyst, where high temperature increased styrene monomer. Better activity was found by Xie et al., [24] with modified silicon mesoporous molecular sieve as catalyst. Kim et al., [25] used modified alumina support with Fe, K, Ba, Zn and Mg catalysts for the degradation of PS and found Fe-K/Al2 O3 the best catalyst with a significant amount of liquid. The use of modified, or treated catalysts have brought significant increase in the activity and selectivity of the products, therefore, suggests the need for more advance catalysts (supported catalysts) more specifically impregnated catalyst. Impregnated catalysts can be prepared with the selection of proper active metal center (a precursor) over a suitable support [26] like silica, alumina, and activated carbon, etc. Mineral clays like montmorillonite (Mt) in the field of catalysis have got much interest and have been used for various purposes due to their high porosity, exchangeable cations and swellable properties. Mt is one of the important clay minerals that has widely used as supporting material [27–30]. In the present work a series of metal decorated catalysts were prepared over Mt support with Mg, Zn, Al, Cu and Fe as the active metal centers. The aim of the current study was the improvement of catalytic activity in terms of liquid products as well as the selectivity of component products. 2. Experimental 2.1. Materials The polystyrene (PS) samples used have an average molecular weight (Mw) 20 0,0 0 0 g/mol purchased from Sigma-Aldrich. Montmorillonite (Mt) clay was obtained from a local research laboratory. MgCl2 ·6H2 O, ZnCl2 and FeCl3 ·6H2 O were purchased from Merck KGaA 64271, Darmstadt, Germany. AlCl3 ·6H2 O and CuCl2 ·2H2 O were purchased from BDH Laboratory Supplies, Poole, BH151TD, England. 2.2. Catalyst preparation Different concentrations of salts of Mg, Zn, Al, Cu and Fe, i.e., (MgCl2 ·6H2 O, ZnCl2 , AlCl3 ·6H2 O, CuCl2 ·2H2 O and FeCl3 ·6H2 O) were calculated as 5%, 10%, 15%, 20% and 25% on the basis of metal weight for a known weight of Mt. The relevant salts were impregnated over Mt support using the wet impregnation method. The active metal center salt was dissolved in appropriate amount of water and was poured into the slurry of Mt support and stirred the mixture for 1 h at 60 °C, filtered, washed with distilled water and dried in oven at 110 °C for 6 h. The catalyst sample was then calcined at 300 °C for 4 h and cooled in a desiccator. The dried sample was ground to powder and screened to a particle size ≤445 μm. 2.3. Catalyst characterization Surface Area Analyzer NOVA2200e Quantachrome, USA was used for the determination of BET surface area of the prepared catalysts using N2 adsorption/desorption at 77.4 K. The morphology of catalysts was determined using Scanning Electron Microscope (SEM) JSM5910, JEOL, Japan instrument. X-ray diffraction (XRD) patterns were taken using a JDX-3532 JEOL (Japan) diffractometer ˚ at 40 KV and with monochromatic Cu-Kα radiation (λ = 1.5418 A) 30 mA in the 2θ range of 10–80° with 1.03° per minute.
Table 1 BET Surface area for metal decorated catalysts using Mt as supporting material. Surface area (m2 /g) Catalysts Mt 20% 20% 05% 15% 20%
Mg/Mt Zn/Mt Al/Mt Cu/Mt Fe/Mt
BET
BJH
116.21 96.14 78.25 102.20 49.86 69.53
489.23 412.08 297.6 264.56 54.6 77.53
˚ Pore size (A)
Pore volume (cc/g)
115.39 121.84 102.38 118.96 78.73 77.72
1.25 1.26 0.68 1.23 0.09 0.13
2.4. Catalyst activity Polystyrene degradation experiments were conducted at atmospheric pressure in a Pyrex glass reactor (height 22 cm and internal diameter 7 cm) set in a heating assembly operates up to 10 0 0 °C. PS (5 g) was degraded with the mixture of appropriate amount of relevant catalysts (weight by weight blend) for a constant period of time. The degradation experiments were run with virgin PS and heterogeneous catalysts without the use of any solvent and other additives. The degradation products, i.e., liquids and gases were collected after condensation and measured. The residue left in the reactor was weighted and the products were expressed in terms of wt.% of the PS degraded. 2.5. Analysis GC/MS analysis of the degraded products was performed with Shimadzu QP2010 Plus GC/MS. The instrument was fitted with 30 m capillary column, 0.25 mm internal diameter having DB-5MS (95% dimethylpolysiloxane, 5% polyphenyl) fused silica capillary column with 0.25 μm film thickness. Helium gas (99.99%) was used as carrier gas with a flow rate of 1.5 mL/min, sample injection volume was 1 μL and the injector port was provided with split mode (1:50). The injection temperature was 300 °C and the oven temperature was programmed as follow; initial oven temperature 35 °C (5 min), ramped at 5 °C/min to 100 °C (1 min), then ramped at 10 °C/min to 150 °C (10 min) and finally ramped at 2.5 °C/min to 290 °C (10 min). The ion source temperature used was 280 °C and interface temperature was 290 °C. The spectra were compared with GC/MS updated library for the identification of component products. 2.6. Catalyst activity studies for the degradation of PS The catalytic activity of catalysts was tested in two phases as in our previous work [31]. In the first phase the Mt support was used as a catalyst and was employed in the process of PS degradation with the effect of degradation temperature, reaction time and feed to catalyst ratio. The selection criterion for optimum reaction conditions was based on maximum production of liquid products as well as total products. In the second phase, the prepared metal decorated catalysts over Mt support were evaluated. The catalytic activities of metal decorated catalysts were tested with the effect of percentage (%) of the impregnated metal using the same reaction conditions as used for Mt support alone. 3. Results and discussions 3.1. Catalyst characterization Surface area, pore volume and size were calculated for all the prepared metal decorated catalysts over Mt support in comparison to Mt support (Table 1). The BET and BJH surface area of all
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Fig. 1. SEM micrograph of (a) Mt, (b) 20% Mg/Mt, (c) 20% Zn/Mt, (d) 05% Al/Mt, (e) 15% Cu/Mt and (f) 20% Fe/Mt.
the metal decorated catalysts were decreased as compared to Mt support, this indicates the successful impregnation of the precursor active center over the surface and in the pores of support causing to block the pores ultimately decreasing the pore size and pore volume [31]. However, the pore size and pore volume were increased in the cases of 20% Mg/Mt and 5% Al/Mt catalysts as compared to Mt support. The increase in the case of pore size might be due the interaction of precursor active center molecules over the surface of pores, which also indicated the formation of new layers and/or pores and/or channels. The SEM image of Mt support is depicted in Fig. 1(a), the image present highly porous structure with particle size ranging from 10 to 20 μm. Fig. 1(b) presents the morphology of 20% Mg/Mt, the image reveals a huge condense structure, the surface seems having pores throughout the whole structure. The surface morphology of 20% Zn/Mt is shown in Fig. 1(c), the Zn/Mt have relatively uniform particles of 2–3 μm in size. Fig. 1(d) indicates 5% Al/Mt
catalyst with irregular shape particles having size 1–4 μm, the bright metallic view of the particles indicates successful impregnation of the precursor component. Fig. 1(e) presents the morphology of 20% Cu/Mt catalyst, which indicate agglomerated particles with smaller size than that of Mt support. The picture shows 0.6 μm (600 nm) to 5 μm particles with adequate dispersion of precursor active metal center (Cu). Fig. 1(f) corresponds to 20% Fe/Mt, the surface of the catalysts also presents the agglomeration of uniform particle cling to each other at the sides of the particle thus developing porous channels and channels throughout the surface of the catalyst. The XRD diffraction patterns of both supports and metal decorated catalysts were taken. It can be seen from Fig. 2 that the composition of Mt has changed after impregnation of precursor active metals. Metal decorated catalysts using XRD spectrum for montmorillonite support are shown in Fig. 2(a), the diffractogram for Mt at 19.51°, 25.24°, 26.29°, 29.41°, 47.38° and 57.07°
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Fig. 2. XRD diffractogram of (a) Mt support, (b) 20% Mg/Mt, (c) 20% Zn/Mt, (d) 5% Al/Mt, (e) 15% Cu/Mt and (f) 20% Fe/Mt.
indicates bredigite (Ca14 Mg2 (SiO4 )8 according to ICDD Card No. 360399, the patterns at 20.62°, 25.24° and 57.07° also shows sodium magnesium silicate (Na2 MgSiO4 ) (ICDD Card No. 471499). The peaks at 19.51°, 23.77° and 29.41° show montmorillonite15A (CaO2 (Al,Mg)Si4 O10 (OH)2 ) according to ICDD Card No. 130135. The XRD diffraction patterns for 20% Mg/Mt are shown in Fig. 2(b). The major diffraction peaks at 21.58°, 29.63° and 31.87° indicate Na3 Mg3 Ca5 Al19 Si117 O272 according to ICDD Card No. 491831 and, at 20.95°, 22.48°, 31.03°, 42.19° and 49.42° show Na2 Al2 Si2 .5 O9 .6 ·2H2 O (ICDD Card No. 380237) with some relevant mineral. Fig. 2(c) shows XRD patterns for 20% Zn/Mt, it indicates sodium zinc oxide (Na10 Zn4 O9 ) at 22.03°, 32.38° and 34.93° according to ICDD Card No. 520058; the pattern also shows sodium zinc silicate (Na2 ZnSiO4 ) at 19.87° and 26.71° (ICDD Card No. 370407). The XRD diffractogram also indicates sodium magnesium zinc silicate (Na2 Mg3 Zn2 Si12 O30 ) at 29.50°, 32.38° and 63.46° according to ICDD Card No. 480418. The XRD patterns for 5% Al/Mt catalyst is shown in Fig. 2(d), it shows the major peaks for silicon oxide (Si34 O68 ) at 26.65°, 27.64°, 35.17° and 45.40° according to ICDD Card No. 520144. The patterns also show at 31.75° and 62.02° sodium aluminum silicate (Na1.15 Al1.15 Si0.85 O4 and Na1.75 Al1.75 Si0.25 O4 ) according to ICDD Card Nos. 490 0 07 and 490 0 04, respectively. The catalyst also contains a small amount of magnesium aluminum oxide (MgAl2 O3 ) and magnesium aluminum silicate (Mg3 Al2 (SiO4 )3 according to ICDD Card No. 211152 and 150742. The presence of these substances indicates interaction of Al with Mt support. Fig. 2(e) corresponds to 15% Cu/Mt and its pattern shows Ca2 CuO2 Cl2 according to ICDD Card No. 480319 at peaks 29.02°, 32.62° and 61.63°, the patterns shows Al2 CuMg at 45.34°, 54.76° and 61.63° according to ICDD Card No. 280014, it shows Ca2 Al2 O5 according to ICDD Card No. 521722 at peaks 26.62°, 34.09°, 61.63°, it also shows monoclinic CaCu at peaks 16.24°, 32.62°, 34.09°, 45.34° and 57.55° with ICDD Card No. 411276 and monoclinic CuCl2 with ICDD Card No. 10185. The XRD pattern indicates that 20% Fe/Mt is amorphous as shown in Fig. 2(f).
Fig. 3. Effect of degradation temperature using Mt as catalyst (Reaction conditions; reaction time 30 min, feed to catalyst ratio 1:0.2).
3.2. Catalyst activity studies for the degradation of PS In the first phase, using Mt support as catalysts for the degradation of PS was used. The effect of degradation temperature was optimized from 250 to 500 °C for the degradation of PS at constant reaction conditions of 30 min reaction time and feed to catalyst ratio of 1:0.2, the results are shown in Fig. 3. As can be seen from the figure, the degradation processes of PS were in three steps. The first degradation step in the temperature range of 250– 350 °C was relatively slow. At 250 °C, the yield of liquid products was 0 wt.% and then a gradual increase was recorded in the yield of liquid products up to 350 °C. In the second step, from 350 to 400 °C, the yield of liquid products was abruptly increased up to
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Fig. 4. Effect of reaction time using Mt as catalyst (Reaction conditions: degradation temperature 450 °C, feed to catalyst ratio 1:0.2).
87.60 ± 0.20 wt.%. The same has been already reported in our previous work for other catalysts [32]. The abrupt increase in the yield of liquid products indicates the propagation step of the degradation process of PS. In the 3rd step, a slight change was recorded from 400 to 450 °C but with further increase in the degradation temperature, no increase in the yield of liquid products was observed. This slight increase in the yield of liquid products might be due to the condensation of gaseous molecules and their reaction to form substitutes of the pre-existing products. A maximum was obtained at 450 °C with a significant amount of liquid products, i.e. 89.93 ± 0.31 wt. % with the total conversion of 100%. The heating time varied from 30 to 150 min using degradation temperature 450 °C and feed to catalyst ratio 1:0.2, the results are shown in the Fig. 4. The yield of liquid products increased from 30 to 60 min and then observed a decrease in the yield of liquid products due to increase in residence time [25]. Further increase in the residence time (90 min or above) might cause the cracking of preexisting products into further smaller fragments inside the reactor resulting low yield of liquid products. The maximum yield of liquid products was 91.33 ± 0.31 wt.% obtained with a 60 min reaction time. The degradation of PS into maximum liquid products was also optimized for feed to catalyst ratio. The feed to catalyst ratio was varied from 1:0.1 to 1:0.5 using optimized reaction condition, i.e. degradation temperature 450 °C and reaction time 60 min (Fig. 5). The results show that increase in the amount of catalyst has no effect on the yield of liquid products; however, the yield of liquid products was 92.40 ± 0.87 with 100% total conversion and was relatively high using the feed to catalyst ratio 1:0.2. Therefore, 1:0.2 feed to catalyst ratio was selected for further catalytic degradation studies. In the second phase of catalytic activity studies, different metals decorated catalysts over Mt were used for the degradation of PS. The effect of the percentage of impregnated metal varied from 5 to 25% using the optimized reaction conditions of the 1st phase for Mt support as a catalyst, i.e., degradation temperature 450 °C, reaction time 60 min, feed to catalyst ratio 1:0.2 (Fig. 6). The yield of liquid products increased with the increase of active metal percentage over Mt support except for aluminum decorated catalysts (Al/Mt). While in Al/Mt, the yield of liquid products decreased with the increase of active metal percentage and then kept constant with further increase in the percentage of impregnated metal. The decrease of liquid products with the increase of active metal percentage, i.e., Al metal increased the Lewis character of the catalysts
5
Fig. 5. Effect of feed to catalysts ratio using Mt as catalysts (Reaction conditions: degradation temperature 450 °C, reaction time 30 min).
Fig. 6. Effect of percentage of the impregnated metal (Mg, Zn, Al, Cu and Fe) over Mt support for maximum liquid products (Reaction conditions: degradation temperature 450 °C, reaction time 30 min and feed to catalyst ratio 1:0.2).
yielding more gaseous products ultimately decreasing the yield of liquid products [33,34]. The yield of liquid products obtained with 5% impregnated Al metal was 89.60 ± 0.20 wt.%. In cases of Mg/Mt, Zn/Mt and Fe/Mt, maximum yield of liquid products was 84.53 ± 0.42, 84.53 ± 0.12 and 88.87 ± 0.42 wt.%, respectively using 20% impregnation of the respective metals (Mg, Zn and Fe). However, 89.20 ± 0.20 wt. % maximum yield of liquid product was obtained with 15% Cu/Mt decorated catalyst. The yield of liquid products using Mt and metal decorated catalysts over Mt was compared with thermal degradation reported in our previous work [22], i.e. 78.07 wt.% in 150 min reaction time at 500 °C degradation temperature. The current metal decorated Mt catalysts were found to increase the yield of liquid products significantly in a short reaction time and low degradation temperature as compared to that of thermal degradation. Comparison of the reaction conditions and contents of products of the degradation of PS with optimized metal decorated catalysts over Mt is given in Table 2.
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J. Shah et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–8 Table 2 Comparison of reaction condition and yield of products using catalytic degradation.
Reaction conditions Temperature (°C) Time (min) Feed to catalyst ratio
Mt
20% Mg/Mt
20% Zn/Mt
5% Al/Mt
15% Cu/Mt
20% Fe/Mt
450 60 1:0.2
450 30 1:0.3
450 120 1:0.3
450 60 1:0.2
450 30 1:0.3
450 30 1:0.2
84.53 15.30 – 10 0.0 0
84.53 15.40 – 10 0.0 0
89.60 2.40 8.00 92.00
89.20 9.07 1.73 98.27
88.87 11.20 – 10 0.0 0
Content of products (wt.%) Liquid yield 92.40 Gas yield 7.60 – Residue Total conversion 10 0.0 0
Table 3 GC/MS analysis of products formed by PS degradation using Mt support as catalyst and metal decorated Mt catalysts. Product S.No. name 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Benzene Toluene Ethylbenzene Styrene α -Methylstyrene Naphthalene Benzene, 1,1 -(1,2-ethanediyl)bis Benzene, 1,1 -(1-methyl-1,2-ethanediyl)bis Benzene, 1,1 -(1,3-propanediyl)bis Phenanthrene Benzene, 3-butynyl Benzene, (1-methyl-3-butenyl) 2-Phenylnaphthalene p-Terphenyl 1,2-propanediol, 3-benzyloxy-1,2-diacetyl 1,1 :3,1 -Terphenyl, 5 -phenyl Other hydrocarbons
Composition (wt.%) Mt
20% Mg/Mt
20% Zn/Mt
5% Al/Mt
15% Cu/Mt
20% Fe/Mt
0.04 1.59 0.49 45.42 0.70 0.01 0.75 0.31 0.81 0.07 18.35 0.72 0.10 0.04 19.00 0.30 3.70
0.36 5.90 5.48 46.62 2.22 0.30 1.79 1.38 0.70 0.28 4.86 0.50 1.59 0.22 0.92 0.50 9.33
0.55 7.16 3.28 46.16 2.46 0.72 4.41 2.06 0.48 0.55 7.87 0.72 1.25 0.49 0.62 0.25 5.56
0.81 8.49 5.13 49.28 2.80 0.62 3.56 1.69 0.41 0.52 5.58 0.55 1.84 0.53 0.39 0.41 7.00
0.82 9.76 6.61 48.46 2.75 0.00 2.47 1.12 0.32 0.56 2.68 0.32 2.80 0.51 0.16 0.36 9.51
0.79 10.15 6.42 50.93 2.31 0.67 2.36 1.14 0.00 0.54 2.60 0.00 2.81 0.63 0.00 0.72 6.75
3.3. Catalytic selectivity studies for the degradation of PS The parent liquids obtained from thermo-catalytic degradation of PS using metal decorated catalysts over Mt were analyzed for their component products and selectivity studied by GCMS. The results of GC/MS analysis were expressed in terms of wt.% of PS samples given in Table 3. During thermal assisted degradation processes, different reactions occurring produced large number of compounds. Nature of the reaction and products formation depend upon the nature of polymer material used, pre-existing products formed, temperature provided, residence time, and amount, nature and shape of the catalyst [31,35–37]. The aim of our selectivity studies was to decrease the total number of interactions and to increase the amount of low molecular weight aromatic hydrocarbons, especially styrene monomer, and yield of limited number of products [31]. As reported earlier Mt and metal decorated catalysts over Mt are acidic in nature, which are reported to initiate the degradation process of PS with the attack of an associated Brönsted acid proton on to the phenyl ring of PS. The carbocation produced then undergo β -scission and a hydrogen transfer. The production of styrene and α -methylstyrene are shown in Scheme 1 from polymer ion (A). These reactions are abundant in the degradation of PS. Therefore, high amount of styrene is produced [13,31,38]. The yield of styrene was almost the same with 5% Al/Mt, 15% Cu/Mt and 20% Fe/Mt that was slightly more than that of Mt, 20% Mg/Mt and 20% Zn/Mt. The polymer ion (B) is attacked by a proton and forming another cation, which then undergo β -scission producing a toluene as shown in Scheme 2. The yield of toluene was in the following order with different catalysts, Mt > 20% Mg/Mt > 20% Zn/Mt > 5% Al/Mt > 15% Cu/Mt > 20% Fe/Mt. The decrease in toluene might
be due to its further degradation to smaller volatile hydrocarbons [33,34]. Some intermolecular hydrogen transfer also takes place, which results in the formation of ethylbenzene and benzene, (1-methylethyl). The mechanism is given in Scheme 3. In the current study the yield of ethylbenzene was the same with 15% Cu/Mt and 20% Fe/Mt. It was slightly low with 20% Mg/Mt and 5% Al/Mt and it was the lowest with Mt as catalyst. The hydrogen of acidic catalysts attacks the branched aromatic ring giving rise a cation π -complex, which upon β -scission convert to σ -complex (secondary cation) and release benzene, shown in Scheme 4. The yield of benzene was also in the following order with different catalysts, Mt > 20% Mg/Mt > 20% Zn/Mt > % Al/Mt > 15% Cu/Mt > 20% Fe/Mt. After styrene, toluene and ethylbenzene, the yield of α methylstyrene, benzene, 3-butynyl were high. As can be seen from Table 3, the major components of degradation were styrene monomeric and dimeric products. This has been already reported in the literature [15,23,39]. Other hydrocarbons like styrene trimers and oligomers were formed up to a limited extent. The GC/MS comparative analysis revealed 20% Fe/Mt catalyst with desired product selectivity. The yield of toluene (10.15 wt.%), ethylbenzene (6.42 wt.%), styrene monomer (50.93 wt.%), α methylstyrene (2.31 wt.%) and 2-phenylnaphthalene (2.81 wt.%) was maximum with 20% Fe/Mt catalysts. After 20% Fe/Mt; 5% Al/Mt was also found with good catalytic selectivity. The yield of products was 8.49%, 5.13%, 49.28%, 2.80% and 1.25% for toluene, ethylbenzene, styrene monomer, α -methylstyrene and 2-phenylnaphthalene, respectively. The yield of component products was compared with thermal degradation reported in our previous work [22]. In the case of which benzene, 1,1 -(1,3propanediyl)bis (1.97 wt.%); benzene, 3-butynyl (17.56 wt.%) and
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Scheme 1. Reaction steps for the production of styrene and α -methylstyrene.
Scheme 2. Reaction steps for the production of toluene through β -scission reaction.
Scheme 3. Reaction steps for the formation of ethylbenzene and benzene,(1-methylethyl).
Scheme 4. Reaction steps for the formation of benzene through β -scission reaction.
benzene, (1-methyl-3-butenyl) (1.16 wt.%) were relatively high molecular weight aromatic hydrocarbons and were maximum as compared to low molecular weight hydrocarbons. The compound 1,2-propanediol, 3-benzyloxy-1,2-diacetyl (10.10 wt.%) was abundant and unwanted might have formed due to the interac-
tion of residue molecules and/or atmospheric oxygen [10]. These unwanted and high molecular weight aromatic hydrocarbons were decreased using metal decorated catalysts over Mt specifically with 20% Fe/Mt catalyst.
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