Alumina and Tungstophosphoric Acid Loaded Mesoporous Catalysts

Article pubs.acs.org/IECR

Alumina and Tungstophosphoric Acid Loaded Mesoporous Catalysts for the Polyethylene Degradation Reaction Buğcȩ Aydemir and Naime A. Sezgi* Department of Chemical Engineering, Middle East Technical University, Ankara-06800, Turkey ABSTRACT: Polyethylene degradation over alumina and tungstophosphoric acid (TPA) containing ordered mesoporous catalysts was studied in a semibatch reactor. In the noncatalytic degradation reaction, selectivity of C3 and C4 hydrocarbons was quite high. However, significant ethylene and C4 hydrocarbon selectivity was observed in the catalytic degradation reaction depending on the type of catalyst used. Liquid product distribution in the noncatalytic degradation reaction indicated the presence of hydrocarbons, being greater than C18. In the catalytic degradation, chemical composition of the liquid product was hydrocarbons in the range of C5−C14. With an increase in the TPA loading, the amount of lighter hydrocarbons in the carbon number range of C5−C12 was slightly increased due to a decrease in the catalysts’ surface area with loading. Both catalysts are materials with promising catalytic properties for the production of liquid feedstocks from polymeric wastes.

1. INTRODUCTION Plastic materials are widely used throughout the world due to their low price, high capacity of production, and simple processing techniques. Consumption of these materials has been continuously increasing, which also has the potential to create critical environmental problems. Landfilling, incineration, and mechanical and chemical recycling are disposal methods. In the landfilling disposal method, plastic wastes are buried. They are generally nonbiodegradable, and landfilled plastic waste will be degraded after hundreds of years have passed. Therefore, a great amount of space is required, and available free space is running out everyday. In the incineration method, plastic waste is burned. It is especially popular in countries in which land is a limited resource. Depending on the nature of plastic waste, highly toxic chemicals evolve in the effluent gas. Therefore, it is harmful for human health, and extra cleaning treatment units for the effluent gas are required. A recycling method reduces energy usage and the raw material consumption. Mechanical recycling of plastic waste is the simplest and a relatively cheap disposal method compared to the other methods. However, there is a certain loss of material quality with some degradation of its properties. Pyrolysis of polymers (chemical recycling) seems to be the most promising in terms of environmental safety and valuable chemical recovery like raw material and fuel-oil. Pyrolysis of plastic materials can be categorized as catalytic and noncatalytic pyrolysis. The catalytic pyrolysis has advantages compared to the noncatalytic one.1−3 In the presence of catalyst, pyrolysis starts at lower temperature and residence time for the reaction is low.4 Lower temperature and residence time causes a decrease in energy consumption. The catalytic pyrolysis also provides higher quality hydrocarbon products with narrow distribution of carbon numbers.4 Gaseous and liquid products of catalytic degradation are potential fuels and raw materials for the petrochemical industry. In the noncatalytic pyrolysis, products have wide distribution of molecular weights, requiring further treatment to obtain higher quality hydrocarbons.5,6 © XXXX American Chemical Society

Zeolites, amorphous aluminosilicates, and mesostructured materials have been used as catalyst for the cracking reaction of polymeric materials.2,3,7 Polymeric molecules are bulky and remarkable in size; therefore, catalysts’ textural properties are very important. Polymer should have access to the active sites, and the degradation reaction takes place over these active sites. Mesostructured catalysts like MCM-41 and SBA-15 are preferred in the degradation reaction compared to microporous catalysts because their mesopores allow easier access of the polymeric molecules within the catalysts’ pores. However, MCM-41 and SBA-15 do not show considerable catalytic activity if suitable acidic medium is not incorporated into their structure.8−10 Therefore, aluminum and tungstophophoric acid (TPA) are introduced into the structure for better catalytic activity. There are very few studies in the literature which use heteropoly acid loaded SBA-15 and aluminum containing MCM-41 as the solid catalysts in the polyethylene pyrolysis reaction.11 In the present study, the catalytic performance of alumina and tungstophosphoric acid loaded mesoporous catalysts and the product distributions were investigated in the polyethylene pyrolysis reaction.

2. EXPERIMENTAL SECTION 2.1. Catalysts and Materials. The aluminum loaded MCM-41 and TPA containing SBA-15 type catalysts with different Al/Si and W/Si ratios were synthesized by impregnation of aluminum and TPA into hydrothermally synthesized MCM-41 and SBA-15 using aluminum isopropoxide and tungstophosphoric acid hydrate as the aluminum and acid sources, respectively. Details of the catalyst preparation are published elsewhere.9 Their properties are given in Table 1. Special Issue: NASCRE 3 Received: February 27, 2013 Revised: April 27, 2013 Accepted: April 30, 2013

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dx.doi.org/10.1021/ie400634m | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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thermometer readout was inserted to this location in order to measure the exact reaction temperature. This porous glass dispersed the gas and prevented backflow of the polymer melt. The reactor was purged with nitrogen gas and adjusted with a rotameter, to sweep out the reaction products from the reactor. The nitrogen gas entered from the reactor base through 1/4 in. tubing. The stainless steel tube between the reactor and the condenser was wrapped with the heating tape to prevent the condensation in the tubing, and its temperature was measured using thermocouple connected to a controller and kept at the reaction temperature during the experiment. The swept gas products were passed through the condenser, and the nonvolatile products were collected inside the glass vessels surrounded with water cooling jackets. The noncondensable gaseous products were collected in the gas balloon, and the gas products were taken at several time intervals by an injector and analyzed using a gas chromatograph (GC) (Varian 3800) equipped with a thermal conductivity detector and Propac Q column (6 ft × 1/8 in., Discovery Sciences). The nonvolatile products were also analyzed using the same GC with a flame ionization detector and J&W Scientific HP-5 capillary column (30 m × 0.320 mm × 0.25 μm). In all catalytic pyrolysis experiments, a powder mixture of 1 g of polyethylene and 0.5 g of the synthesized catalyts was put into the reactor, and they were well mixed.

Table 1. Physical Properties of the Catalysts sample ID Al-0.03 Al-0.25 SBA0.1 SBA0.25 SBA0.40

surface area “multipoint” BET (m2/g)

pore volume BJH desorption (cc/g)

BJH desorption average pore diameter (nm)

Al or W/ Si ratio (EDS)

1227 1146 749

1.01 0.97 1.19

2.44 2.44 6.5

0.03 0.18 0.096

230

0.36

6.6

0.25

212

0.31

6.5

0.40

The alumina loaded MCM-41 and TPA impregnated SBA-15 catalysts were named as Al-X and SBA-X, respectively. X in the notation stands for the Al or W/Si ratio of the synthesized material. The polyethylene(PE) used in the degradation reaction was provided from Sigma-Aldrich Co, having Mw value of 4000 with Brookfield Thermoset viscosity value of 1.500 poise at 25 °C. Its density is 0.92 g/mL at 25 °C, and it has a polydispersity index of 2.35. 2.2. Polyethylene Catalytic Pyrolysis System. Polyethylene pyrolysis experiments were carried out at atmospheric pressure under nitrogen gas with a flow rate of 60 mL/min at different reaction temperatures. Figure 1 shows a schematic diagram of the catalytic pyrolysis system. The experimental apparatus consisted of a semibatch Pyrex reactor which is heated by an electric furnace. The reactor was mainly made of three parts. The bottom part of the reactor was the spiral part to increase the contact time of the flowing gas for efficient heating. In this spiral portion, tiny glass particles within the spirals helped increase the surface area. Above the spiral part, there was a special porous glass, where the polymer and catalysts were put. This was the place where the reaction took place; therefore, a thermocouple connected to a digital

3. RESULTS AND DISCUSSION Different amounts of aluminum and TPA loaded MCM-41 and SBA-15 type materials were synthesized and characterized using a variety of techniques, including nitrogen physisorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS). Details of the characterization results are published elsewhere.9 EDS results showed that aluminum and TPA were successfully incorporated into the structure of the catalyst. For both alumina

Figure 1. Schematic diagram of the experimental equipment. B



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loading but it remained constant with TPA amounts in the SBA-15 catalyst. However, at 430 °C, it increased with an increase in aluminum and TPA loading. In the aluminum loaded catalysts, the liquid product yield was higher than TPA containing ones. In the SBA-0.4 catalyst, the liquid yield increased with an increase in temperature as in the other catalyst (Table 3). In the noncatalytic PE degradation reaction, acetylene, ethane, ethylene, propylene, propane, and n-butane were observed as gasous products. In the presence of TPA loaded catalysts, i-butane was formed in addition to the gaseous products formed in the absence of catalyst. In the Al containing catalysts, i-butane was also formed but formation of propane was not observed (Figure 2). Both catalysts produced a high

loaded MCM-41 and TPA impregnated SBA-15 type catalysts, Type IV adsorption/desorption isotherms were observed.9 The surface areas of the synthesized catalysts were in the range of 212−1227 m2/g (Table 1). Their average pore diameters were 2.44 and 6.54 nm. These pore diameter values supported the mesoporosity of these materials. The surface area of the aluminum loaded MCM-41 catalyst was higher than that of the TPA loaded SBA-15 one because the average pore diameter of TPA loaded ones was approximately 2.7 times higher than that of aluminum impregnated ones. The surface area of the SBA type materials decreased significantly with an increase in TPA loading. The blocking of material’s micropores and filling of the corona that surrounds the mesopores of SBA-15 by TPA species might cause this significant decrease.4 However, in the MCM-41 type catalysts, such a significant decrease with the aluminum loading was not observed. Polyethylene starts to degrade at a temperature of 410 °C and ends up at 500 °C. TGA analysis showed that, in the presence of both types of catalysts, the degradation temperature shifted to a lower temperature range.9 Aluminum species was located tetrahedrally and octahedrally in the MCM-41 structure. In the Al-0.3 sample, the intensity of peak corresponding to tetrahedrally coordinated aluminum species was higher compared to the octahedrally coordinated one. Both type of catalysts had Brönsted acid sites in addition to Lewis acid sites. The noncatalytic and catalytic polyethylene pyrolysis experiments were carried out isothermally under nitrogen atmosphere in the pyrolysis system (Figure 1). The noncatalytic and catalytic pyrolysis products were hydrocarbon gases, liquids, and solid residue. The catalytic polyethylene degradation reaction yields are given in Tables 2 and 3. No formation of

Figure 2. Selectivity of gaseous products in the presence of catalyst at 430 °C for 15 min.

Table 2. Product Yield in the Catalytic Degradation of PE at 390 and 430 °C for 15 min 390 °C catalyst

gas (wt %)

no catalyst Al-0.03 Al-0.25 SBA-0.1 SBA-0.25 SBA-0.40

63.8 72.9 90.2 87.9 88.4

amount of i-butane, which is an indication of catalytic degradation of PE liquid products. In other words, these catalysts changed the distribution of gaseous products. The Al loaded MCM-41 catalysts were n-butane selective. n-Butane selectivity increased with an increase in both temperature and aluminum amount (Figures 2 and 3). In TPA loaded catalyst, while ethylene selectivity decreased with an increase in temperature, n-butane selectivity increased with temperature. However, this trend was not observed in the SBA-0.40 catalyst. Propane selectivity increased with temperature in the SBA-15 catalysts. In the Al loaded catalysts, selectivities of acetylene and

430 °C

liquid (wt %)

gas (wt %)

liquid (wt %)

36.2 27.1 9.8 12.1 11.6

41.5 81.9 53.7 81.1 76.2 69.2

58.5 18.1 46.3 18.9 23.8 30.8

Table 3. Product Yield in the Degradation of PE over the SBA-0.40 Catalyst at Different Temperatures for 15 min temperature (°C)

gas (wt %)

liquid (wt %)

390 410 430 460

88.4 71.5 69.2 58.0

11.6 28.5 30.8 42.0

solid residue was observed in the presence and absence of catalyst in the studied temperature range. The gas product yield increased with use of both catalysts in the pyrolysis of PE. In other words, the catalytic degradation favored gaseous production. The liquid product obtained in the presence of catalyst was light, flowing liquid, which might be an indication of the presence of lower molecular weight hydrocarbons. In both catalysts, the liquid product yield increased with an increase in temperature except the Al-0.03 catalyst. At 390 °C, the liquid product yield decreased with an increase in aluminum

Figure 3. Selectivity of gaseous products in the presence of catalyst at 390 °C for 15 min. C



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ethane were very low compared to the TPA containing ones. For the SBA-0.4 catalyst, selectivity of gaseous products as a function of temperature is shown in Figure 4. The SBA-0.4

Figure 6. Selectivity of the liquid products in the presence of catalyst at 390 °C for 15 min.

Figure 4. Selectivity of gaseous products in the presence of the SBA0.4 catalyst at different temperatures for 15 min.

had a positive effect on the activity of the catalyst for degradation of PE. The increase in the amount of acidity caused a decrease in the PE degradation temperature and decomposition of heavier hydrocarbons to lighter ones due to initiation step of the PE degradation reaction proceeding over Brönsted acid sites causing protolysis. The amount of heavy hydrocarbons formed using TPA containing catalysts was higher than that of heavy hydrocarbons formed using aluminum loaded ones. This might be due to the fact that the TPA catalysts had smaller surface area compared to the aluminum catalysts. In other words, the heavier hydrocarbons did not further degrade to the lighter hydrocarbons because they did not stay in the pore of the catalyst. The effect of temperature on selectivity of the liquid products for the SBA-0.4 catalyst is shown in Figure 7. At 390 °C, the highest selectivity was C14. The C14 selectivity decreased with temperature, except a temperature value of 430 °C. However, the amount of lighter hydrocarbons (C5−C12) increased with an increase in temperature. This showed that the increased in temperature enhanced the production of lower molecular weight hydrocarbons. The effect of TPA loading on the liquid product distribution of the PE degradation reaction is shown in Figure 8. With an increase in the TPA loading, the amount of lighter hydrocarbons in the carbon number range of C5−C12 was slightly increased. The increase in the amount of lighter hydrocarbons with an increase in the TPA loading was not high due to a decrease in the catalysts’ surface area with TPA loading.

catalyst was mainly ethylene selective. Ethylene selectivity increased with temperature up to 430 °C. After 430 °C, selectivity of n- and i-butane increased. With an increase in temperature, formation of propylene and propane gases decreased and at 460 °C the propylene and propane were not observed. In the catalytic thermal degradation, the liquid products were distributed over lower carbon numbers compared to the noncatalytic thermal degradation (Figure 5). In the aluminum

Figure 5. Comparison of the catalytic degradation liquid products with the noncatalytic degradation liquid products at 430 °C for 15 min.

4. CONCLUSIONS Both TPA loaded SBA-15 and aluminum containing MCM-41 catalysts improved the yield of gaseous products and provided better selectivity in the product distributions. Aluminum loaded MCM-41 and TPA impregnated SBA-15 catalysts were selective to higher (n-butane, i-butane, propylene) and lower (ethylene) molecular weight gaseous products. Both catalysts were active for the conversion of PE to lighter hydrocarbons which were in the carbon number range of C5−C14 depending on the degradation temperature. In other words, value added chemicals could be recovered from the plastic waste under suitable conditions. Use of these types of catalyst in the degradation of PE shifted degradation reaction temperature to a lower value.

loaded catalysts, the amount of heavier hydrocarbons (C13− C18) decreased with an increase in temperature and aluminum loading (Figures 5 and 6). A high amount of C5 which was not observed in the noncatalytic thermal degradation was formed, and the highest hydrocarbon amount was C8 at high temperature. However, at lower temperature, the highest one was C14 in the Al-0.3 catalyst while C8 was the highest hydrocarbon in the Al-0.25 catalyst. The Al/Si ratio in the Al0.25 catalyst was 8-fold of the Al/Si ratio in the Al-0.03 catalyst. In other words, the acid sites in the catalyst structure played an important catalytic role in the PE pyrolysis. Impregnation of aluminum into pure MCM-41 sample led the formation of Brönsted acid sites within the structure.9 The increase of acidity D



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Figure 7. Effect of temperature on selectivity of the liquid products for the SBA-0.4 catalyst.

Figure 8. Effect of TPA loading on selectivity of the liquid products for the SBA catalyst at 430 °C for 15 min.

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(4) Obalı, Z.; Sezgi, N. A.; Doğu, T. Catalytic degradation of polypropylene over alumina loaded mesoporous catalysts. Chem. Eng. J. 2012, 207−208, 421. (5) Isoda, T.; Nakahara, T.; Kusakabe, K.; Shigeharu, M. Catalytic cracking of polyethylene-liquefied oil over amorphous aluminosilicate catalysts. Energy Fuels 1998, 12, 1161. (6) Aguado, J.; Sotelo, J. L.; Serrano, D. P.; Calles, J. A.; Escola, J. M. Catalytic conversion of polyolefins into liquid fuels over MCM-41: Comparison with ZSM-5 and amorphous SiO2-Al2O3. Energy Fuels 1997, 11, 1225. (7) Serrano, D. P.; Aguado, J.; Escola, J. M. Catalytic conversion of polystyrene over HMCM-41, HZSM-5 and amorphous SiO2-Al2O3: Comparison with thermal cracking. Appl. Catal., B: Environ. 2000, 25, 181. (8) Obalı, Z.; Sezgi, N. A.; Doğu, T. Performance of acidic MCM-like aluminosilicate catalysts in pyrolysis of polypropylene. Chem. Eng. Commun. 2009, 196, 116. (9) Aydemir, B.; Sezgi, N. A.; Doğu, T. Synthesis of TPA impregnated SBA-15 catalysts and their performance in polyethylene degradation reaction. AIChE J. 2012, 58 (8), 2466. (10) Obalı, Z.; Sezgi, N. A.; Doğu, T. The synthesis and characterization of aluminum loaded SBA-type materials as catalyst for polypropylene degradation reaction. Chem. Eng. J. 2011, 176−177, 202.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +90 312 2102608. Fax: +90 312 2102600. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Middle East Technical University Research Fund (BAP-03-04-2008-08) for the financial support and Central Laboratory of Middle East Technical University for the analyses.

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REFERENCES

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(11) Jalil, P. A. Investigations on polyethylene degradation into fuel oil over tungstophosphoric acid supported on MCM-41 mesoporous silica. J. Anal. Appl. Pyrolysis 2002, 65, 185.

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Alumina and Tungstophosphoric Acid Loaded Mesoporous Catalysts

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