Energy Fuels 2010, 24, 3394–3400 Published on Web 02/02/2010
: DOI:10.1021/ef901525b
Carbon Adsorbents from Polycarbonate Pyrolysis Char Residue: Hydrogen and Methane Storage Capacities† Laura Mendez-Li~ n an, F. Javier L opez-Garz on, Marı´ a Domingo-Garcı´ a, and Manuel Perez-Mendoza* Departamento de Quı´mica Inorg anica, Facultad de Ciencias, University of Granada, 18071 Granada, Spain Received December 14, 2009. Revised Manuscript Received January 13, 2010
The pyrolysis of bisphenol A polycarbonate (PC) can be a solution to chemical recycling of this widely used polymer, which, once it is used, losses its value. The char residue produced in the pyrolysis constitutes more than a quarter of the weight of the PC pyrolyzed and is considered one of the drawbacks of the recycling process. The hypothesis that such char can become a potential good precursor for selective carbon adsorbents, attending to the high carbon content and regular structure of the polymer, is worth considering. Therefore, this work analyses the physical and chemical activation of such char to produce selective high-capacity carbon adsorbents for acting as gas storage solutions. According to an exhaustive textural characterization, the activated carbons prepared are eminently microporous, with a significant volume of narrow micropores, which make them efficient for the adsorption of light energy-carrier gases, such as methane and hydrogen. The methane and hydrogen uptakes measured are in the highest end of those reported for physisorption storage processes and seem to be only related to micropore textural parameters. Surprisingly, a certain amount of very narrow micropores (totally inaccessible to nitrogen molecules at 77 K and showing diffusion restrictions for CO2 adsorption at 273 K, hence, very difficult to characterize) are already present in the char and prove to be effective in hydrogen adsorption.
cleaner energies that can help the transition from oil-based economies to more sustainable sources.5-9 This work presents a study on the preparation of carbon adsorbents, especially for methane and hydrogen adsorption, from the bisphenol A polycarbonate pyrolysis solid residue. Bisphenol A polycarbonate, commonly known as polycarbonate (PC), is one of the most widely used engineering plastics because its shows very good mechanical and thermal properties. It is used for manufacturing a large amount of products in some different applications, such as bottles, packages, and other containers, mobile phones, computers and plugs, lights in motor vehicles, medical connexions and tubes, eyeglasses, security glasses, police shields, and layers for greenhouses. Another usage of PC that is becoming very important nowadays is the use in biopolymer processing. It is not cost-effective to convert PC, but it is a more problematic issue to recycle it. The thermal cracking of this polymer usually produces 5-15 wt % solid residue, becoming a problem for a potential chemical recycling process.1,10 Although typical char residue usages include activated carbon and carbon black processing,11 to the best of our knowledge, the possible use of the PC pyrolysis residue to produce an added-value material, such as a carbon adsorbent, has not yet been explored.
1. Introduction The recovery and recycling of plastic solid waste (PSW) is a priority of the highest order in industrialized countries. Many processes related to tertiary (chemical recycling) and quaternary (energy recovery) treatment schemes are worthy of additional research efforts to find effective solutions to accumulation and landfilling problems.1-4 Finding these solutions urgently has become a necessity. Pyrolysis processes appear to be a robust strategy to recover the energy and combustible gases out of the polymer waste, although some disadvantages, such as the handling of the chars produced, are still unresolved1 and need to be tackled immediately. Similarly, finding suitable storage media for highly energetic gases, such as methane and principally hydrogen, has become crucial in the development of † This paper has been designated for the special section Carbon for Energy Storage and Environment Protection. *To whom correspondence should be addressed. Telephone: þ34958241000 ext. 20425. Fax: þ34-958248526. E-mail:
[email protected]. (1) Al-Salem, S. M.; Lettieri, P.; Baeyens, J. Recycling and recovery routes of plastic solid waste (PSW): A review. Waste Manage. 2009, 29, 2625–2643. (2) Smolders, K.; Baeyens, J. Thermal degradation of PMMA in fluidised beds. Waste Manage. 2004, 24, 849–857. (3) Zia, K. M.; Bhatti, H. N.; Bhatti, I. A. Methods for polyurethane and polyurethane composites, recycling and recovery: A review. React. Funct. Polym. 2007, 67, 675–692. (4) Howard, G. T. Biodegradation of polyurethane: A review. Int. Biodeterior. Biodegrad. 2002, 49, 245–252. (5) Matranga, K. R.; Myers, A. L.; Glandt, E. D. Storage of naturalgas by adsorption on activated carbon. Chem. Eng. Sci. 1992, 47, 1569– 1579. (6) Parkyns, N. D.; Quinn, D. F. Natural gas adsorbed on carbon. In Porosity in Carbons; Patrick, J. W., Eds.; Edward Arnold: London, U.K., 1995; pp 291-325. (7) Lozano-Castello, D.; Alcaniz-Monge, J.; de la Casa-Lillo, M.; Cazorla-Amoros, D.; Linares-Solano, A. Advances in the study of methane storage in porous carbonaceous materials. Fuel 2002, 81, 1777–1803.
r 2010 American Chemical Society
(8) United States Department of Energy (Energy Efficiency and Renewable Energy). Hydrogen, fuel cells, and infrastructure technologies program. Multi-year research, development, and demonstration plan (http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/). (9) Benard, P.; Chahine, R. Modeling of adsorption storage of hydrogen on activated carbons. Int. J. Hydrogen Energy 2001, 26, 849–855. (10) Ciliz, N. K.; Ekinci, E.; Snape, C. E. Pyrolysis of virgin and waste polypropylene and its mixtures with waste polyethylene and polystyrene. Waste Manage. 2004, 24, 173–181. (11) Al-Salem, S. M.; Lettieri, P.; Baeyens, J. Kinetics and product distribution of end of life tyres (ELTs) pyrolysis: A novel approach in polyisoprene and SBR thermal cracking. J. Hazard. Mater. 2009, 172, 1690–1694.
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: DOI:10.1021/ef901525b
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The hypothesis that such char can be a good precursor to produce selective carbon adsorbents for gas energy storage lies in the regular molecular structure of the parent polymer, which can be an adequate raw material to obtain uniform porosity and, hence, high selectivity for light-gas adsorption. In fact, previous results on the preparation of active carbons from other synthetic organic polymers have shown that some of them can behave as molecular sieves.12,13 Therefore, different activations of the bisphenol A polycarbonate char have been studied, trying to preserve a narrow and homogeneous porosity but increasing the adsorption capacity toward methane and hydrogen, to find potential applications as energy storage media. The use of highcapacity adsorbents for adsorption of energy carrier gases has lately drawn a lot of attention from the scientific community, because finding effective storage solutions remains the key point for developing cleaner energy economies as alternatives for hydrocarbon-based fuel technologies. In fact, the targets for such change set as minimum criteria8 are still very far from being achieved. Therefore, all of the prepared new carbons have been texturally characterized in depth, and their performance as methane and hydrogen adsorbents has been measured. The aim is to find the most adequate preparation strategies and to establish the relationships between the textural properties and the adsorption capacities for those energy gases.
The porous texture of all samples was studied by N2 and CO2 adsorption at 77 and 273 K, respectively, using an automatic adsorption system (Micromeritics ASAP 2020). The specific surface areas have been calculated applying the Brunauer-EmmettTeller (BET) method, while micropore volumes and other textural parameters have been obtained by Dubinin-Radushkevich (DR) analysis. The mean pore size (L0) has been determined applying the empirical equations proposed by Dubinin14 [L0 (nm) = 24/E0], in the case of E0 < 20 kJ mol-1, and Stoeckli et al.15,16 [L0 (nm) = 10.8/(E0 - 11.4)], for E0 > 20 kJ mol-1. The Rs method17 has been applied to the N2 adsorption isotherms to obtain further conclusions on the microporosity of the adsorbents. We used Vulcan 3G as reference material (SBET = 72 m2 g-1) because it allows for analysis at low relative pressures. The subtracting pore effect (SPE) method has been used to obtain the surface area of the microporous carbons, SSPE, from the Rs plot, according to the method proposed by Kaneko et al.18 This method can be applied when the Rs plot has a linear region at Rs values close to 0.5, which can be extrapolated to the origin. 2.3. Methane and Hydrogen Adsorption. The CH4 and H2 adsorption capacities of selected samples were measured in a highpressure volumetric adsorption system (up to 3 MPa) built for this purpose. The equipment consists in a stainless-steel volumetric adsorption apparatus equipped with mass flow and backpressure controllers to set the desired gas pressure in the system and two MKS Baratron absolute pressure transducers to accurately measure the pressure under and over the atmospheric pressure. For each run, approximately 0.5 g of the sample was degassed at 423 K for at least 12 h down to 10-5 mbar. CH4 adsorption isotherms were measured at 273 K, and the H2 adsorption isotherms were carried out at 77 and 273 K. The adsorbed amount was calculated using the Peng-Robinson equation of state. 2.4. Molecular Simulation and Pore Size Distribution (PSD) Analysis. The PSD of the samples was analyzed following the method proposed by Davies and Seaton.19 For this purpose, a kernel of simulated N2 (77 K) adsorption isotherms in single-size slit pore models (from 0.3 to 4.2 nm) was generated by Monte Carlo simulation in the Grand Canonical ensemble (GCMC). Basically, a succession of molecular configurations characteristic of equilibrium are generated in the model pore by trial configurations consisting of moving, adding, and deleting molecules and accepting each trial probabilistically according to the appropriate Boltzmann factor.20,21 The model pores consist of a rectangular unit cell formed by three-layered planar walls of graphitic carbon with hexagonal structure, where periodic boundary conditions in the x and y directions are applied.22
2. Experimental Section 2.1. Sample Preparation. Batches of 10 g of PC were placed in a tubular reactor and heated at 5 °C min-1 to 950 °C for 1 h in inert atmosphere (100 cm3 min-1 N2 flow). The temperature chosen for carbonization was based on former results for other polymers12 and in thermogravimetric analysis. A 26% carbon yield was obtained after the pyrolysis. The char prepared (C950) was ground and sieved to 0.2-0.6 mm particle size. Physical activation of the char was carried out at 950 °C for 1, 4, or 8 h in 100 cm3 min-1 CO2 flow. The sample was heated to the activation temperature under an inert atmosphere (100 cm3 min-1 N2 flow, 5 °C min-1) and then switched to the CO2 flow. After the activation time, the sample was cooled to ambient temperature in N2 flow. Three samples were obtained and labeled as A950-1, A950-4, and A950-8 (where the A accounts for “activated”, the first value indicates the activation temperature, and the second value indicates the residence time, in hours, under CO2 flow). For the chemical activation, the char was mixed directly with the appropriate amount of KOH (powder) to obtain three different KOH/C ratios (1:1, 4:1, and 6:1). The mixture was treated at 600 and 800 °C for 1 h in an inert atmosphere (100 cm3 min-1 N2). The activated carbons thus prepared were washed with water to remove the excess of the activation agent. The ash content of the samples was lower than 1% in all cases. Samples were labeled similarly as before: AK600-1, AK600-4, AK600-6, AK800-1, AK800-4, and AK800-6. Now the “AK” refers to KOH activation, and the second value indicates the KOH/C ratio (all of these samples had the same residence time of 1 h). The activation conditions, burnoff, and ash content are shown in Table 1. 2.2. Characterization. The bulk density of the carbons was measured by mercury pycnometry at atmospheric pressure using a Quantachrome Autoscan mercury porosimeter. Prior to the measurement, the sample was evacuated to 0.1 mbar. The bulk density data are also compiled in Table 1.
(14) Dubinin, M. M. Generalization of the theory of volume filling of micropores to nonhomogeneous microporous structures. Carbon 1985, 23, 373–380. (15) Stoeckli, F. Characterization of microporous carbons by adsorption and immersion techniques. In Porosity in Carbons; Patrick, J. W., Eds.; Edward Arnold: London, U.K., 1995; pp 67-92. (16) Stoeckli, F.; Guillot, A.; Slasli, A. M.; Hugi-Cleary, D. The comparison of experimental and calculated pore size distributions of activated carbons. Carbon 2002, 40, 383–388. (17) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd; Academic Press: New York, 1982. (18) Kaneko, K.; Ishii, C.; Ruike, M.; Kuwabara, H. Origin of superhigh surface-area and microcrystalline graphitic structures of activated carbons. Carbon 1992, 30, 1075–1088. (19) Davies, G. M.; Seaton, N. A.; Vassiliadis, V. S. Calculation of pore size distributions of activated carbons from adsorption isotherms. Langmuir 1999, 15, 8235–8245. (20) Frenkel, D.; Smit, B. Understanding molecular simulations: From algorithms to applications, 2nd; Academic Press: New York, 2001. (21) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Clarendon Press: Gloucestershire, U.K., 1989. (22) Perez-Mendoza, M.; Schumacher, C.; Suarez-Garcia, F.; Almazan-Almazan, M. C.; Domingo-Garcı´ a, M.; L opez-Garz on, F. J.; Seaton, N. A. Analysis of the microporous texture of a glassy carbon by adsorption measurements and Monte Carlo simulation. Evolution with chemical and physical activation. Carbon 2006, 44, 638–645.
(12) Fernandez-Morales, I.; Almazan-Almazan, M. C.; PerezMendoza, M.; Domingo-Garcı´ a, M.; L opez-Garz on, F. J. PET as precursor of microporous carbons: Preparation and characterization. Microporous Mesoporous Mater. 2005, 80, 107–115. (13) Villar-Rodil, S.; Suarez-Garcia, F.; Paredes, J. I.; MartinezAlonso, A.; Tascon, J. M. D. Activated carbon materials of uniform porosity from polyaramid fibers. Chem. Mater. 2005, 17, 5893–5908.
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Table 1. Activation Conditions, Burnoff, Ash Content, and Bulk Density activation agent
sample C950 A950-1 A950-4 A950-8 AK600-1 AK600-4 AK600-6 AK800-1 AK800-4 AK800-6
CO2 CO2 CO2 KOH KOH KOH KOH KOH KOH
activation temperature (°C)
activation time (h)
950 950 950 600 600 600 800 800 800
1 4 8 1 1 1 1 1 1
KOH/C ratio
BO (%)
ash content (%)
bulk density (g cm-3)
1 4 6 1 4 6
17.6 56.2 78.0 5.2 22.1 23.8 12.3 35.1 52.6
nil nil nil nil 0.50 0.18 0.00 0.55 0.10 0.59
1.5033 1.3281 0.8545 0.5205 1.3523 1.2040 0.9968 1.2502 0.8921 0.7759
Table 2. Textural Parameters from N2 (77 K) and CO2 (273 K) Adsorption Data N2 adsorption sample C950 A950-1 A950-4 A950-8 AK600-1 AK600-4 AK600-6 AK800-1 AK800-4 AK800-6
CO2 adsorption
SDR V0 E0 L0 Vmic(Rs) SEXT SSPE VPSD SDR V0 E0 L0 SBET (m2 g-1) (m2 g-1) (cm3 g-1) (kJ mol-1) (nm) (cm3 g-1) (m2 g-1) (m2 g-1) (cm3 g-1) (m2 g-1) (cm3 g-1) (kJ mol-1) (nm) 1.7 656 1301 1927 594 1123 1365 320 1674 2096
758 1543 2150 680 1341 1626 370 2028 2456
0.27 0.55 0.76 0.24 0.48 0.58 0.13 0.72 0.87
24.2 15.9 14.6 23.5 15.8 14.8 20.8 14.7 14.2
0.84 1.60 1.76 0.89 1.62 1.74 1.16 1.75 1.81
0.26 0.58 0.71 0.24 0.48 0.60 0.14 0.73 1.00
13.1 47.1 235.1 0.3 1.2 2.1 0.8 1.1 1.4
1245 1534 1065 1248 331 1569 1844
0.25 0.48 0.69 0.23 0.42 0.51 0.12 0.63 0.77
638 859 1044 1166 713 916 945 732 1400 1355
0.23 0.31 0.38 0.42 0.26 0.33 0.34 0.27 0.51 0.49
28.9 28.7 26.0 24.4 29.9 28.3 27.3 28.9 25.4 24.5
0.62 0.62 0.74 0.83 0.58 0.64 0.68 0.62 0.77 0.82
bonate at 950 °C produces a char (C950) containing basically the rests of the aromatic structures, with a high carbon content (∼93% according to elemental analysis) and low crystallinity. Textural parameters of the char C950 are compiled in Table 2. Nitrogen adsorption at 77 K is nearly negligible on this sample in all of the range of partial pressures (Figure 1). Despite the lack of nitrogen adsorption, CO2 does adsorb on C950 (Figure 2), although long times were necessary to reach equilibrium. Therefore, porosity does exist, although not accessible to nitrogen molecules at 77 K. This can be related to the presence of constrictions that hinder the access of the N2 molecules to the porous structure of the material.28 This effect is common and has been observed in other carbons prepared using organic polymers as precursors12 but to a lower extent. The long times necessary to reach CO2 adsorption equilibrium (around 10 h for each pressure), despite the higher kinetic energy of CO2 molecules at the adsorption temperature (273 K), suggest restricted diffusion and seem to confirm the low accessibility of gas molecules to the porous texture. Similar results have been reported recently for other carbon materials,29 although showing much shorter equilibration times. At these pressure and temperature conditions, CO2 is known to adsorb only in micropores smaller than 1 nm12,30 and the use of a model that explicitly takes into account the
The van der Waals interactions between gas molecules and adsorbent are represented by Lennard-Jones potentials. Lorentz-Berthelot combination rules were applied to obtain the interaction parameters between different Lennard-Jones sites. Nitrogen molecules are considered as two-center molecules. The parameters and bond lengths used can be found elsewhere.23,24 The carbon atoms of the two graphitic surface layers of the pore walls are represented explicitly, while the middle layer is considered a plane of “smeared-out” carbon atoms with the density of graphite, as proposed by Steele,23 to save computation time. The Peng-Robinson equation of state was used to relate the pressure of the system to the chemical potential. Once the set of simulated isotherms for different pore sizes has been generated, the adsorption integral equation can be solved by numerical inversion to obtain a representative PSD that fits the experimental data.19
3. Results and Discussion According to the literature,25-27 during the thermal degradation in a nitrogen atmosphere, bisphenol A polycarbonate losses the carbonate, isopropylidene groups, and the hydrogen of the benzene rings at the first stage. In following stages, the degradation of aromatic carbon occurs. Therefore, the pyrolysis of bisphenol A polycar(23) Steele, W. A. Physical interaction of gases with crystalline solids. 1. Gas-solid energies and properties of isolated adsorbed atoms. Surf. Sci. 1973, 36, 317–352. (24) Murthy, C. S.; Singer, K.; Klein, M. L.; Mcdonald, I. R. Pairwise additive effective potentials for nitrogen. Mol. Phys. 1980, 41, 1387– 1399. (25) Li, X. G.; Huang, M. R. Thermal degradation of bisphenol A polycarbonate by high-resolution thermogravimetry. Polym. Int. 1999, 48, 387–391. (26) Jang, B. N.; Wilkie, C. A. The thermal degradation of bisphenol A polycarbonate in air. Thermochim. Acta 2005, 426, 73–84. (27) Rivaton, A.; Mailhot, B.; Soulestin, J.; Varghese, H.; Gardette, J. L. Comparison of the photochemical and thermal degradation of bisphenol-A polycarbonate and trimethylcyclohexane-polycarbonate. Polym. Degrad. Stab. 2002, 75, 17–33.
(28) Rodriguez-Reinoso, F.; Linares-Solano, A. Microporous structure of activated carbons as revealed by adsorption methods. In Chemistry and Physics of Carbon; Thrower, P. A., Eds.; Marcel Dekker: New York, 1989; Vol. 21. (29) Rios, R. V. R. A.; Silvertre-Albero, J.; Sepulveda-Escribano, A.; Molina-Sabio, M.; Rodriguez-Reinoso, F. Kinetic restrictions in the characterization of narrow microporosity in carbon materials. J. Phys. Chem. C 2007, 111, 3803–3805. (30) Vishnyakov, A.; Ravikovitch, P. I.; Neimark, A. V. Molecular level models for CO2 sorption in nanopores. Langmuir 1999, 15, 8736– 8742.
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Figure 1. N2 adsorption isotherms at 77 K of char C950 and (a) physically and (b) chemically activated samples.
Figure 2. CO2 adsorption isotherms at 273 K of char C950 and (a) physically and (b) chemically activated samples.
filling of the micropores seems to be more correct to obtain adsorption parameters than the standard BET method. Therefore, the DR equation has been applied to CO2 adsorption data to obtain micropore volumes and other textural parameters, which are also collected in Table 2. The V0 and E0 values indicate that the sample presents a very narrow porosity far from being negligible. It is noticeable the lack of wider or accessible pores, where nitrogen molecules undoubtedly would have been adsorbed. Therefore, the char has a limited but interesting porosity. To improve the adsorption behavior of the char, different physical (CO2) and chemical (KOH) activations have been studied following the methods described in the Experimental Section. Ideally, these treatments should increase the adsorption capacity but preserve the narrow and homogeneous character of the porous network. In contrast to the original sample C950, all of the activated carbons adsorb N2 at 77 K up to very different extents, depending upon the degree of activation (Table 2). The whole set of adsorption isotherms are shown in Figure 1. Both physically and chemically activated samples present isotherms characteristic of microporous solids, with a widening of the “knee” of the isotherm as the activation progresses (indicative of a more heterogeneous character). In contrast to samples obtained by physical activation, no capillary condensation at high relative pressures can be observed for the samples prepared with KOH. Moreover, more marked “knees” are evident, which can be attributed to more homogeneous porous networks, even for the samples with the highest degree of activation of this series. Figure 2 shows CO2 adsorption isotherms for all of the samples, a more appropriate probe to obtain information on the narrow micropore system. It is evident that physical activation (Figure 2a) has a small effect on the CO2 adsorption and all of the physically activated samples
present similar CO2 isotherms. Therefore, the volume of pores in which CO2 adsorption is taking place (pore size < 1.0 nm)12,30 is not being effectively increased by physical activation treatments. That is not the case of chemical activation (Figure 2b): chemically activated carbons present clearly increasing CO2 adsorption capacities as the activation progresses. The microporosity of the samples has been further characterized using the classical Rs plots and also Monte Carlo simulation of N2 adsorption at 77 K. Modeling the adsorption on ideal single-sized slit-shaped pores to obtain a kernel of theoretical isotherms allows us to obtain a PSD that fits the experimental data by solving the adsorption integral equation. The detailed analysis of the Rs plots obtained from the N2 adsorption isotherms (panels a and b of Figure 3) leads to similar conclusions as those drawn out from the isotherm analysis. All of the carbons are essentially microporous, with only samples A950-4 and A950-8 presenting wider pores (as shown by the cooperative swing in the 0.5-1 range) and important external surface areas. For the rest of the samples, small external surface areas are obtained and the DR and Rs total micropore volumes are practically coincident (Table 2). Panels a and b of Figure 4 compile the PSDs obtained for the physically and chemically activated samples, respectively. From Figure 4a, it is evident that physical activation generates narrow microporosity (1 nm) and also small mesopores. Nevertheless, the micropore volume of all of these samples is considerable. Meanwhile, KOH activation seems to develop an important number of ultramicropores (
