Pitch and Polyethylene Terephthalate

Apr 1, 2009 - Blends of two bituminous coals and a coal-tar pitch (CTP) with ... matrix of the chars produced at 800 °C in a horizontal oven was stud...
8 downloads 0 Views 5MB Size
Download PDF
Energy & Fuels 2009, 23, 2675–2683

2675

Activated Carbons from Coal/Pitch and Polyethylene Terephthalate Blends for the Removal of Phenols from Aqueous Solutions Ewa Lorenc-Grabowska, Grazyna Gryglewicz,* and Jacek Machnikowski Department of Polymer and Carbonaceous Materials, Faculty of Chemistry, Wrocław UniVersity of Technology, Gdan´ska 7/9, 50-344 Wrocław, Poland

Maria-Antonia Dı´ez and Carmen Barriocanal Instituto Nacional del Carbo´n (INCAR), CSIC, Apartado 73, 33080 OViedo, Spain ReceiVed December 2, 2008. ReVised Manuscript ReceiVed March 5, 2009

Blends of two bituminous coals and a coal-tar pitch (CTP) with polyethylene terephthalate (PET) were evaluated as precursors of activated carbons (ACs). The intensity of the interactions between the raw materials, coal/CTP and PET during copyrolysis was closely observed by means of thermogravimetric analysis. In addition, the homogeneity of the carbon matrix of the chars produced at 800 °C in a horizontal oven was studied by polarized light optical microscopy. Activated carbons were prepared from single components and their blends (1:1 w/w) by subjecting them to carbonization up to 800 °C in a horizontal oven and then activation with steam at 800 °C to 50% burnoff. The porous structure of the ACs was determined by sorption of N2 at 77 K and of CO2 at 273 K. The PET-containing blends produced microporous activated carbons with a maximum BET surface area of nearly 1100 m2 g-1 and a maximum micropore size distribution of 0.6-0.8 nm in the case of the AC from the CTP/PET blend. The addition of PET to a bituminous coal was compared with the preoxidation of coal P in air as a way to reduce thermoplasticity and to promote the development of the porous structure. The modification of bituminous coals by PET appeared to be more effective than conventional coal preoxidation treatment. The resultant ACs were tested by measuring their effectiveness in removing phenols from an aqueous solution. The adsorption of p-chlorophenol (PCP) by the ACs prepared from the PET-containing blends was slightly higher than for the commercial activated carbon. The ability to adsorb PCP was found to be related to the volume of the super-micropores.

1. Introduction The increasing amount of polyethylene terephthalate (PET) in municipal plastic wastes due to the widespread use of plastic bottles and containers has led to a policy of recycling this polyester into useful products. Thermal degradation in an inert or reductive atmosphere has been studied for a variety of plastics as one of the possible routes of recycling, mainly with the aim of producing energy and chemical raw materials. The main components of municipal plastic wastes, polyolefins and polystyrene decompose totally into volatile products.1 In contrast, some solid residue is produced on heat-treating PET with terephthalic acid, acetaldehyde, and carbon oxides, which are the major volatile products evolved.2 For this reason, of the six major thermoplastics present in municipal wastes, PET-derived char has been reported to be the most promising precursor of activated carbons (ACs) and has been a subject of great interest in recent years.3-8 The ACs obtained from the pyrolysis and activation of PET have been tested for applications such as * Corresponding author. E-mail: [email protected]; fax: 00-48-713206506; phone: 00-48-713206398. (1) Nomura, S.; Kato, K.; Nakagawa, T.; Komaki, I. Fuel 2003, 82, 1775–1782. (2) Murty, M. V. S.; Rangarajan, P.; Grule, E. A.; Bhattacharyya, D. Fuel Process. Technol. 1996, 49, 75–90. (3) Laszlo, K.; Bota, A.; Nagy, L. G. Carbon 2000, 38, 1965–1976. (4) Laszlo, K.; Szu´cs, A. Carbon 2001, 39, 1945–1953. (5) Bota, A.; Laszlo, K.; Nagy, L. G.; Copitzky, T. Langmuir 1997, 13, 6502–6509.

phenol, polycyclic aromatic hydrocarbons (PAHs), and dibenzothiophene adsorption.9-13 The microporous material produced at 50% burnoff has a high surface area (near 1200 m2/g) and amphoteric surface nature due to the presence of oxygencontaining groups of a basic character. The phenol adsorption capacity of activated carbons produced from waste PET from aqueous solutions has been found to be similar to that of commercial sorbents, and the pH of the solution and the chemistry of the carbon surface are considered to be of major importance for the adsorption efficiency.10-13 However, one of the drawbacks of using PET for this purpose is the relatively low pyrolysis yield (below 20%), a factor that may affect the economic viability of the process. A possible way to overcome this drawback and at the same time preserve (6) Fernandez-Morales, I.; Almazan-Almazan, M. C.; Perez-Mendoza, M.; Domingo-Garcia, M.; Lopez-Garzon, F. J. Microporous Mesoporous Mater. 2005, 80, 107–115. (7) Nakagawa, K.; Mukai, S. R.; Suzuki, T.; Tamon, H. Carbon 2003, 41, 823–831. (8) Tamon, H.; Nakagawa, K.; Suzuki, T.; Nagano, S. Carbon 1999, 37, 1643–1645. (9) Parra, J. B.; Ania, C. O.; Arenillas, A.; Rubiera, F.; Pis, J. J. Appl. Surf. Sci. 2004, 238, 304–308. (10) Ania, C. O.; Parra, J. B.; Pis, J. J. Fuel Process. Technol. 2002, 77-78, 337–343. (11) Laszlo, K. Colloids Surf. 2005, 265, 32–39. (12) Laszlo, K.; Podkoscielny, P.; Dabrowski, A. Appl. Surf. Sci. 2006, 252, 2752–5762. (13) Ania, C. O.; Parra, J. B.; Arenillas, A.; Rubiera, F.; Bandosz, T. J.; Pis, J. J. Appl. Surf. Sci. 2007, 253, 5899–5903.

10.1021/ef801045k CCC: $40.75  2009 American Chemical Society Published on Web 04/01/2009

2676

Energy & Fuels, Vol. 23, 2009

the positive features of PET char as an AC precursor is to coprocess PET with coal or coal tar pitch. The cotreatment of bituminous coals with PET has been studied until now in relation to its effect on coking properties.14,15 PET is a strong modifier of the coking behavior of coal. A 10 wt % addition of PET waste completely destroys the thermoplastic properties of bituminous coals, as detected by a Gieseler plastometer. The extent of the reduction of the coal fluidity when less than 5 wt % of PET is added depends on its rank and thermoplastic properties. When higher amounts of PET (>5 wt %) are added to a coal, its coking ability is totally destroyed, independently of its rank and thermoplasticity.14,15 Moreover, the ultra-micropore volume of the resultant char becomes larger as the amount of this polymer in the blend increases.16 Consequently, the coprocessing of coal with PET can be considered as a viable alternative to air oxidation, which is the pretreatment commonly used in the activated carbon industry to eliminate the thermoplasticity of bituminous coals.15 Coal-tar pitch is a mixed solvent with a high boiling point, which is aromatic in nature and capable of dissolving most polymers, including polyethylene terephthalate17 and polyacrylonitrile (PAN).18 The copyrolysis of PAN with pitch has also been studied as a way to produce nitrogen-enriched activated carbons.18,19 The enhanced residue yield and the considerable reduction in the size and order of the optical texture components of the resultant chars proves that strong interactions of PAN with pitch take place during copyrolysis. Thus, it can be inferred that an activated carbon with a similar porous structure could be produced by using char from blends containing 25% of PAN and pure polymer.19 The aim of this work was to evaluate the effect of adding PET to a bituminous coal or a coal-tar pitch on the characteristics of the resultant chars and activated carbons. This study has been undertaken for a double purpose: (i) to find an alternative way to chemically recycle PET waste and (ii) to develop activated carbons with properties suitable for practical applications, for example, the removal of organic impurities from water. 2. Experimental Section 2.1. Materials. The raw materials used for the production of activated carbons were: two bituminous coals of different rank but of similar caking ability (OG and P), a coal-tar pitch (CTP), polyethylene terephthalate (PET), oxidized coal Pox, and blends of the two coals and the pitch with PET. For purposes of comparison, a commercial activated carbon CWZ-22 produced by Gryfskand (Poland) was tested for its ability to adsorb phenols. The coals and pitch were ground to the appropriate size before being used. PET was used in the form of 2-4 mm granulates. The oxidized coal Pox was prepared by treating coal P in an air flow at 160 °C for 6 h. Blends OG/PET, P/PET, and CTP/PET (1:1 w/w) were carefully prepared as is normal practice in the preparation of blends of solids, making sure that the two solids are randomly distributed. The carbon, hydrogen, nitrogen, and sulfur contents of the samples were determined in a LECO CHNS-932 microanalyzer and (14) Vivero, L.; Barriocanal, C.; Alvarez, R.; Diez, M. A. J. Anal. Appl. Pyrolysis 2005, 74, 327–336. (15) Diez, M. A.; Barriocanal, C.; Alvarez, R. Energy Fuels 2005, 19, 2304–2316. (16) Barriocanal, C.; Diez, M. A.; Alvarez, R. J. Anal. Appl. Pyrolysis 2005, 73, 45–51. (17) Polaczek, J.; Pielichowski, J.; Lisicki, Z. Fuel 1987, 66, 1556– 1557. (18) Grzyb, B.; Machnikowski, J.; Weber, J. V.; Koch, A.; Heintz, O. J. Anal. Appl.Pyrolysis 2003, 67, 77–93. (19) Machnikowski, J.; Grzyb, B.; Machnikowska, H.; Weber, J. V. Microporous Mesoporous Mater. 2005, 82, 113–120.

Lorenc-Grabowska et al. the oxygen content by direct determination using a LECO VTF900 coupled to the microanalyser. The proximate and coking properties (swelling index, Roga index and Gieseler fluidity) of the two coals were determined according to standard procedures described elsewhere.20 The vitrinite reflectance (Ro) was measured in nonpolarized reflected light in oil immersion using a wavelength of 546 nm under a Zetopan microscope. The softening point of the coal-tar pitch was measured in accordance with the DIN 51920 standard method, using a Mettler Toledo FP90. The coking value was determined by heating 1 g of pitch sample at 550 °C for 2.5 h, following the ISO 6998 standard. The solubility parameters of the pitch (quinoline and toluene insoluble, QI and TI, respectively) were determined in accordance with ASTM standard procedures. 2.2. Thermal Analysis of Raw Materials. The thermogravimetric analysis of the individual components and the binary blends was carried out using a TA Instrument SDT2960 simultaneous analyzer. The sample to be analyzed (ca. 12 mg) was sized to CTP. This trend reflects the extent of anisotropic development in the char/coke precursor. The oxidation of coal P increases porosity development noticeably on char activation, with SBET reaching 602 and 812 m2 g-1 and VT ) 0.28 and 0.34 cm3 g-1 for P and Pox, respectively. Nevertheless, the blending of coal P with PET at a 1:1 weight ratio is a more effective way of improving char properties for AC production than oxidation with air at a relatively high temperature. The char from the P-PET blend at 50 wt % burnoff gives rise to a porous material with a BET surface area of 994 m2 g-1 and a total pore volume of 0.40 cm3 g-1 (Table 5). The addition of PET to coal OG results in a minor enhancement of porosity development, with coal-tar pitch appearing to be the most promising material for blending with PET. Indeed, the AC from the CTP/PET blend exhibits a much more developed porosity than that prepared from any of the coal/PET blends (SBET ) 1082 m2 g-1 and VT ) 0.44 cm3 g-1). As discussed above, the extent of the interactions between CTP and PET during copyrolysis, estimated on the basis of pyrolysis behavior, char yield, and microtexture, is much higher than for the coal/PET blends. This shows the importance of chemical and physical interactions between blend components during copyrolysis for producing a char with a microtexture capable of developing porosity on activation. All the ACs are microporous solids with a micropore volume (VDR) contribution to the total pore volume VT of around 0.8. The CTP/PET activated carbon is also clearly distinguishable

ActiVated Carbons from Coal/Pitch and PET Blends

Energy & Fuels, Vol. 23, 2009 2681

Figure 5. Pore size distribution of activated carbons determined from N2 adsorption using the DFT method.

due to its higher microporosity, nearly 0.9. In terms of mean micropore width Lo(N2), which ranges from 1.1 to 1.3 nm, all the ACs are similar. Figure 5 shows the pore size distribution determined from the N2 adsorption data using the DFT method. A common feature of most of the ACs is the concentration of pores in the 1.0-1.6 nm size range, which constitutes more than half of the micropore volume. The maximum size is centered between 1.2 and 1.4 nm, except for A-CTP/PET. The addition of PET to coal P results in the development of narrower micropores, the maximum pore size being in the 1.2-1.4 nm size range. The same pore size distribution is observed for A-Pox. The two activated carbons, A-P/PET and A-Pox, have narrower pores than those of A-P and A-PET. Of all the carbons prepared in this study, A-CTP/PET shows the most atypical distribution, with a predominance of narrow pores and a maximum size of about 0.8 nm (Figure 5). This correlates with the high VDR/VT ratio but not with the mean micropore size Lo(N2) (Table 5). This finding is very interesting from the point of view of application, especially in the field of methane or hydrogen storage. A shift to narrower pores, though to a lesser extent, is also a characteristic of the AC obtained from coal OG.

Table 6. Characteristics of the Ultra-microporous Structure of Activated Carbons Determined by the Adsorption of CO2 at 273 K activated carbon

VDR(CO2) (cm3 g-1)

Lo(CO2) (nm)

Smic (m2 g-1)

A-OG A-P A-CTP A-PET A-OG/PET A-P/PET A-Pox A-CTP/PET

0.096 0.114 0.109 0.146 0.112 0.161 0.130 0.180

0.58 0.59 0.57 0.64 0.60 0.65 0.64 0.66

331 386 382 456 373 495 406 545

Regarding ultra-microporosity (widths of 0.4-0.8 nm) calculated on the basis of the ACs CO2 adsorption isotherms, the volume and surface area of the narrow micropores are smaller than those of the precursor chars (Table 6). This proves that at this stage of activation there is a widening of the existing pores and that new narrow pores are also generated. In the case of the activated carbons the VDR(CO2) and Smic(CO2) values range between 0.096 and 0.180 cm3 g-1 and 331-545 m2 g-1, respectively, with a slight tendency to increase in the SBET of AC. The highest VDR(CO2) and Smic(CO2) values for A-CTP/PET agrees with the concentra-

2682

Energy & Fuels, Vol. 23, 2009

tion of narrow micropores in the DFT distribution based on the N2 adsorption results. 3.5. Effectiveness of the Activated Carbons for Removing of Phenols from Aqueous Solution. The adsorption capacity of ACs depends on various factors such as the surface area, pore size distribution, and surface functional groups of the adsorbent; the polarity, the solubility, and molecular size of adsorbate; the pH of the solution, and the presence of other ions in the solution. Both the electrostatic and nonelectrostatic interactions between the adsorbate molecules and the adsorbent surface should be considered in the adsorption process.25,26 The ACs produced from the PET-containing blends and individual components were tested according to their capacity to remove phenols from aqueous solutions. These compounds are common contaminants in water effluents in many industries. Figure 6

Figure 6. Relationship between the BET surface area and the extent of PCP adsorption. Solid symbol (9) corresponds to the commercial activated carbon used as a reference.

shows the amount of PCP adsorbed by the ACs after 24 h as a function of their BET surface area. The linear tendency (correlation coefficient r ) 0.936) observed for this relationship indicates that the interactions between the PCP molecules and the carbon surface are of a similar nature for all the ACs. A similar relation was found between the amount of PCP adsorbed and the total pore volume. Indeed, only dispersive forces seem to be involved when the ACs remove PCP from an aqueous solution in the conditions of the adsorption process used in this work. The pHPZC of the ACs was in the 7.21-8.13 range. The adsorption tests were performed at the pH of the solution, that is, between 5.1 and 6.2, whereas the pKa of PCP was 9.18. Thus, PCP occurs in an undissociated form, and the electrostatic interactions between the molecules of PCP and the charged carbon surface can be ignored. In such circumstances the extent of PCP adsorption is very much related to the porous texture of the activated carbon. To test the feasibility of using ACs prepared from the coal/PET and CTP/PET blends to remove phenols, the same adsorption test was carried out on a commercial activated carbon CWZ 22 (SBET ) 977 m2 g-1; VT: 0.491 cm3 g-1), which is widely used for water treatment. In the case of CWZ 22 the amount of PCP adsorbed is 274 mg g-1, which is lower than for the A-P/PET and A-CTP/PET activated carbons (291 and 370 mg g-1, respectively). Figure 7a shows the relationship between the amount of PCP adsorbed and the VDR(N2), which corresponds to the volume of micropores with a size in the range of 0.7-2.0 nm for which the correlation coefficient (r) was 0.941. A much poorer correlation was found between the amount of PCP adsorbed and the volume of micropores with a size 0.4-0.8 nm [VDR(CO2)], with the correlation coefficient being 0.840 (Figure 7b). This suggests (25) Moreno-Castilla, C. Carbon 2004, 42, 83–94. (26) Lorenc-Grabowska, E.; Gryglewicz, G. Dyes Pigments 2007, 74, 34–40.

Lorenc-Grabowska et al.

Figure 7. Relationships between the uptake of PCP and the volumes of (a) super-micropores [VDR(N2)] and (b) ultramicropores [VDR(CO2)]. Solid symbol (9) corresponds to the commercial activated carbon used as a reference.

that the super-micropores are mainly responsible for the adsorption of PCP on the carbon surface. It is widely accepted that the molecules of phenols are adsorbed in flat-on position on the carbon surface.27 The dispersion interaction between the π electrons in the benzene ring of the phenols and those in the rings of the graphene structure of the activated carbon are generally considered to play a substantial role in the adsorption process. By substituting a value in the cross-sectional area (σ) of a PCP molecule in flat orientation,28 an effective molecular diameter can be calculated from the equation πr2 ) σ, giving the value of 0.78 nm. Therefore, the volume of micropores with a width greater than 0.78 nm can be taken as a measure of the extent of PCP adsorption on the activated carbon surface provided that only dispersive forces govern the process. The results obtained show that blends of bituminous coal or coal-tar pitch with PET may be promising precursors for producing activated carbons that have a porous structure suitable for removing organic contaminants from water. 4. Conclusions It has been demonstrated that PET is a suitable additive for producing microporous AC from bituminous coals and coal-tar pitch by means of steam activation. Coal-tar pitch is modified by PET during heating to a greater extent than bituminous coal. The presence of PET in the copyrolysis system promotes the formation of a disordered carbon structure in the char. Because it modifies thermoplastic behavior, the addition of PET to a coal at a 1:1 w/w ratio enhances the development of porosity in the resultant ACs, depending on the coal rank. A less mature bituminous coal (Ro ) 1.03 vs 1.40%) mixed with PET gives rise to activated carbon with a more highly developed porosity with a SBET of nearly 1000 m2 g-1 and a total pore volume of 0.40 cm3 g-1. It was also found that coprocessing a bituminous coal with PET at a 1:1 weight ratio is more effective than the preoxidation of coal for enhancing the development of porosity. Activation of the CTP/PET blend produces an isotropic carbon material with a large surface area of over 1000 m2 g-1, with the contribution of narrow micropores (0.4-1.0 nm) being predominant. These findings offer an alternative recycling route for using PET to produce activated carbons, especially if PET is blended with low value pitchlike substances. The ACs produced from PET-containing blends are characterized by an ability to adsorb phenols similar to that of the (27) Radovic, L. R.; Moreno-Castilla, C.; Rivera-Utrilla, J. Chemistry and Physics of Carbon; Marcel Dekker: New York, 2001; Vol 27, pp 228405. (28) Caturla, F.; Martin-Martinez, J. M.; Molina-Sabio, M.; RodriquezReinoso, F.; Torregrosa, R. J. Colloid Interface Sci. 1988, 124, 528–34.

ActiVated Carbons from Coal/Pitch and PET Blends

commercial AC tested. The results obtained indicate that the uptake of p-chlorophenol from an aqueous solution is highly dependent on the volume of super-micropores. Acknowledgment. The authors thank the Ministerio de Asuntos Exteriores (Spanish Foreign Office) and the Polish Ministry of Scientific Research and Information Technology for the

Energy & Fuels, Vol. 23, 2009 2683 financial support of the Joint Research Action (2001 PL0030). The financial support provided by the Spanish Ministry of Education and Science through the research project CTM200403254 is also gratefully acknowledged.

EF801045K