Energy Fuels 2010, 24, 3352–3360 Published on Web 02/19/2010
: DOI:10.1021/ef9015087
Adsorption of Dibenzothiophenes on Nanoporous Carbons: Identification of Specific Adsorption Sites Governing Capacity and Selectivity† Mykola Seredych and Teresa J. Bandosz* Department of Chemistry, The City College of New York, 160 Convent Avenue, New York City, New York 10031 Received December 10, 2009. Revised Manuscript Received February 8, 2010
Adsorption of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) from simulated diesel fuel with 20 ppmw total concentration of sulfur was investigated on polymer-derived carbon and its oxidized counterpart. The initial and exhausted carbons were characterized using the adsorption of nitrogen, thermal analysis, potentiometric titration, X-ray fluorescence (XRF), mass spectrometry (MS), and scanning electron microscopy (SEM). The selectivities for DBT and 4,6-DMDBT adsorption were calculated with reference to naphthalene. Both the capacity and selectivity for DBT and 4,6-DMDBT removal from model diesel fuel increase with an increase in the volume of pores similar in sizes to DBT molecules, which is about 7 A˚. The specific geometry of those pores in relation to the sizes of DBT and 4,6DMDBT causes their adsorption to be stronger than that of arenes. Functional groups present on the surface in larger pores (about 100 A˚) contribute to adsorption via polar interactions. The oxidative effect of the carbon surface was also evidenced by the presence of sulfoxide in the surface reaction products. alumina,13,14 zeolites,15,16 and activated carbons have been investigated.14,17-35 Activated carbons have been shown as good adsorbents of dibenzothiophenic compounds.14,17-35 Their performance is enhanced after oxidation.24-26,31 Introduction of oxygen functional groups on the surface results in a specific adsorption of dibenzothiophenic species via oxygen-sulfur interactions.25 This along with the acid-base interactions of slightly basic thiophenes with acidic groups increases not only the capacity but also selectivity.24,25 The latter is an important factor because diesel fuel also contains arenes, which compete with DBTs for adsorption sites. An improvement in the selectivity of adsorption was also found when copper27,33 or iron oxides33 were present on the surface of activated carbons.
Introduction Removal of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT), which are considered as refractory (resisting ordinary methods of treatment) species, to a low level of 30 ppm [current Environmental Protection Agency (EPA) requirements] from diesel fuel is not a trivial task. Various methods including hydrodesulfurization on Co/ Ni catalysts,1-6 oxidation/photo-oxidation,7-10 and adsorption11,12 have been explored. Among them, the latter seems to be very promising, especially when removal to a very low level, less than 30 ppm, is considered. As adsorbents, activated † This paper has been designated for the special section Carbon for Energy Storage and Environment Protection. *To whom correspondence should be addressed. Telephone: (212) 650-6017. Fax: (212) 650-6107. E-mail:
[email protected]. (1) Topsoee, H.; Clausen, B. S.; Massoth, F. E. In Hydrotreating Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer: New York, 1996; Vol. 11, pp 65-69. (2) Song, C.; Reddy, K. M. Appl. Catal., A 1999, 176, 1–10. (3) Song, T.; Zhang, Z.; Chen, J.; Ring, Z.; Yang, H.; Zheng, Y. Energy Fuels 2006, 20, 2344–2349. (4) Rabarihoela-Rakotovao, V.; Brunet, S.; Perot, G.; Diehl, F. Appl. Catal., A 2006, 306, 34–44. (5) Smit, T. S.; Johnson, K. H. Catal. Lett. 1994, 28, 361–372. (6) Michaud, P.; Lemberton, J. L.; Perot, G. Appl Catal., A 1998, 169, 343–353. (7) Shirashiri, Y.; Hara, H.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 1999, 38, 1589–1595. (8) Shirashiri, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 1999, 38, 3300–3309. (9) Matsuzawa, S.; Tanaka, J.; Sato, S.; Ibusuki, T. J. Photochem. Photobiol., A 2002, 149, 183–189. (10) Tao, H.; Nakazato, T.; Sato, S. Fuel 2009, 88, 1961–1969. (11) Gislason, J. Oil Gas J. 2001, 99, 72–76. (12) Song, Ch. Catal. Today 2003, 86, 211–263. (13) Hern andez-Maldonado, A. J.; Qi, G.; Yang, R. T. Appl. Catal., B 2005, 61, 212–218. (14) Kim, J. H.; Ma, X.; Zhu, A.; Song, Ch. Catal. Today 2006, 111, 74–83. (15) Hoguet, J.-C.; Karagiannakis, G. P.; Valla, J. A.; Agrafiotis, C. C.; Konstandopoulos, A. G. Int. J. Hydrogen Energy 2009, 34, 4953– 4962. (16) Yang, R. T.; Hernandez-Maldonado, A. J.; Yang, F. H. Science 2003, 301, 79–81.
r 2010 American Chemical Society
(17) Alhamed, Y. A.; Bamufleh, H. S. Fuel 2009, 88, 87–94. (18) Kumagai, S.; Ishizawa, H.; Toida, Y. Fuel 2010, 89, 365–371. (19) Wang, Q.; Liang, X.; Qiao, W.; Liu, C.; Liu, X.; Zhan, L.; Ling, L. Fuel Process. Technol. 2009, 90, 381–387. (20) Wang, Q.; Liang, X.; Qiao, W.; Liu, C.; Liu, X.; Zhang, R.; Ling, L. Appl. Surf. Sci. 2009, 255, 3499–3506. (21) Jayne, D.; Zhang, Y.; Haji, S.; Erkey, C. Int. J. Hydrogen Energy 2005, 30, 1287–1293. (22) Salem, A. B. S. H.; Hamid, H. S. Chem. Eng. Technol. 1997, 20, 342–347. (23) Haji, S.; Erkey, C. Ind. Eng. Chem. Res. 2003, 42, 6933–6937. (24) Jiang, Z.; Liu, Y.; Sun, X.; Tian, F.; Sun, F.; Liang, Ch.; You, W.; Han, Ch.; Li, C. Langmuir 2003, 19, 731–736. (25) Ania, C. O.; Bandosz, T. J. Langmuir 2005, 21, 7752–7759. (26) Zou, A.; Ma, X.; Song, Ch. J. Phys. Chem. B 2006, 110, 4699– 4707. (27) Ania, C. O.; Bandosz, T. J. Carbon 2006, 44, 2404–2412. (28) Seredych, M.; Bandosz, T. J. Langmuir 2007, 23, 6033–6041. (29) Velu, S.; Watanabe, S.; Ma, X.; Song, Ch. Prepr. Symp.-Am. Chem. Soc., Div. Fuel Chem. 2003, 526. (30) Yu, G.; Lu, S.; Chen, H.; Zhu, Z. Carbon 2005, 43, 2285–2294. (31) Yang, Y.; Ku, H.; Ying, P.; Jiang, Z.; Li, C. Carbon 2007, 45, 3042–3059. (32) Deliyanni, E.; Seredych, M.; Bandosz, T. J. Langmuir 2009, 25, 9302–9312. (33) Seredych, M.; Bandosz, T. J. Fuel Process. Technol. 2010, in press. (34) Seredych, M.; Lison, J.; Jans, U.; Bandosz, T. J. Carbon 2009, 47, 2491–2500. (35) Jeon, H.-J.; Ko, C. H.; Kim, S. H.; Kim, J.-N. Energy Fuels 2009, 23, 2537–2543.
3352
pubs.acs.org/EF
Energy Fuels 2010, 24, 3352–3360
: DOI:10.1021/ef9015087
Seredych and Bandosz
They took part in the activation of oxygen or redox reactions, leading to the oxidation of DBT and 4,6-DMDBT, which improved the selectivity via stronger interactions of the oxidized species with polar groups of the carbon surface.32,34 When the adsorption on carbons is analyzed, one has to remember that significant assets of those adsorbents are their high volume of pores and large surface area. It has been suggested recently that adsorption of DBT is strictly affected by the volume of pores with sizes less than 7 A˚ [34]. On the other hand, in our recent reports, we have shown that both surface porosity and acidity/polarity are important for desulfurization from model diesel fuel of low sulfur content.32,34,36 Although mainly the volume of pores similar in size to DBT and 4,6-DMDBT molecules governs the capacity, the performance can be significantly enhanced when the oxygen groups are present in larger pores. They attract DBT and 4,6DMDBT via specific interactions, and thus, the adsorbent surface is used more efficiently. On the other hand, the density of groups should not be too high. When the groups are placed on a carbon surface in a large quantity, they interfere with the most optimal orientation of adsorbate on the surface and thus negatively affect the adsorption capacity.32,36 The objective of this study is to evaluate the performance of the polymer-derived carbons with specific surface chemistry originating from oxygen and sulfur heteroatoms in desulfurization from model diesel fuel containing only 20 ppm of sulfur. An important novelty is a relatively homogeneous pore structure on the level of small micropores. Moreover, the adsorbents have a significant volume of mesopores, which have high probability to contain functional groups and also help in the transport of the adsorbates to the small pores. Model diesel fuel besides 10 ppm of sulfur from DBT and 10 ppm of sulfur from 4,6-DMDBT also contains equimolar concentrations of arenes to test the selectivity.
for 3 h to remove solvent and arenes to better analyze the mechanism of DBT and 4,6-DMDBT interactions with the carbon surface. The boiling points of decane, hexadecane, naphthalene, 1-methylnaphthalene, DBT, and 4,6-DMDBT are 174, 287, 218, 240, 332, and 340-350 C, respectively.38,39 The exhausted samples after adsorption are designed with the letter S, and those after 300 C are designed with the letter H. Thus, the samples are referred to as CS, CSO, CS-S, CSO-S, CSH, and CSO-H. The subsamples were treated with furan to extract the adsorbed/formed species. A total of 0.1 g of exhausted sample in 5 mL of furan was shacked for 4 days. The sample was filtrated, and the filtrate was used for further mass spectroscopy analyses. These samples are designated with the letters EX. Methods. Adsorption of DBT and 4,6-DMDBT. To carry out the adsorption from liquid phase, MDF was prepared. Model fuel contained the same molar concentrations of DBT, 4,6DMDBT, naphthalene (Nap), and 1-methylnaphthalene (1MNap) in a mixture of decane and hexadecane (1:1). All compounds were obtained from Sigma-Aldrich Co. and used as received. The molar concentration of each compound in fuel was 2.35 10-7 mol/mL. The corresponding total sulfur concentration was 20 ppmw. The adsorption process was carried out in the dynamic conditions and at ambient temperature and pressure. Dry carbons (at 120 C overnight) with granule sizes of 0.4250.212 mm were packed into a 60 mm polyethylene column with a 4 mm inside diameter. The height of the carbon bed was approximately 55 mm, with a volume of 0.70 mL. The mass of the carbon bed was 0.267 and 0.251 g for CS and CSO, respectively. Model fuel was passed into the column with adsorbent from the top by a peristaltic pump (MasterFlex C/L) with a flow rate of 8.3 mL/h. Effluent was collected periodically in 5 mL fractions, and the concentration of arenes and thiophenes was determined. Then, on the basis of the mass of the adsorbent, concentration, and the flow rate, the breakthrough curves were constructed and the breakthrough capacities were calculated. The concentration of arenes and thiophenes in the effluent after adsorption of DBT and 4,6-DMDBT in the presence of Nap and 1-MNap were determined by a Waters 2690 liquid chromatograph equipped with a Waters 996 photodiode array detector. For samples with sulfur organic compounds, separation was conducted using the following conditions: Lichrospher RP-18 column (10 nm, 5 μm, 4.0 125 mm, EM Separations, Gibbstown, NJ) and a guard column (4.0 4.0 mm) of the same material. In this case, a gradient method was used, which started at 90% methanol [high-performance liquid chromatography (HPLC) grade] and 10% distilled water (Milli-Q water) as the mobile phase for 10 min, then changed to 100% methanol over 1 min, held for 15 min, and changed back to 90% methanol and 10% water for 5 min to re-equilibrate. The flow rate was 1.0 mL/ min, and the injection volume was 10 μL. To determine the concentration of DBT and 4,6-DMDBT, a wavelength of 231 nm was chosen, and a wavelength of 220 nm was chosen to analyze Nap and 1-MNap. Thermal Analysis. Thermal gravimetry (TG) curves were obtained using a TA Instruments thermal analyzer. The samples (initial or exhausted) were exposed to an increase in the temperature of 10 C/min up to 1000 C, while the nitrogen or air flow rate was held constant at 100 mL/min. The ash content was evaluated from the residual after heating in air. Characterization of Pore Structure of Adsorbents. Nitrogen isotherms were measured at -196 C using an ASAP 2010 (Micromeritics). Prior to each measurement, all samples were
Experimental Section Materials. Poly(sodium 4-styrene sulfonate) of the following structure was used as a nanoporous carbon precursor:
The carbonization was performed as that described in ref 37. Briefly, the powdered polymer was carbonized at 800 C for 40 min under nitrogen, in a horizontal furnace. The flow of nitrogen was 300 mL/min, and the heating rate was 50 C/ min. The sample was then washed in a Soxhlet extractor with deionized water to remove the excess of water-soluble species. Finally, it was dried in air at 120 C for 24 h. The carbon is referred to as CS. The CS carbon was oxidized at 350 C for 3 h in air, in a horizontal furnace. The heating rate was 50 C/ min. This material is referred to as CSO. The subsamples of both carbons after adsorption of sulfur from model diesel fuel (MDF) were heated at 300 C in nitrogen
(38) Weast, R. C. Handbook of Chemistry and Physics, 67th ed.; CRC Press: Boca Raton, FL, 1986; pp 261-380. (39) Torrisi, S., Jr.; Remans, T.; Swain, J. Process Technol. Catal. 2002, 1–4.
(36) Seredych, M.; Deliyanni, E.; Bandosz T. J. Fuel 2010, doi: 10.1016/j.fuel.2009.09.032. (37) Petit, C.; Kante, K.; Bandosz, T. J. Carbon 2010, 48, 654–667.
3353
Energy Fuels 2010, 24, 3352–3360
: DOI:10.1021/ef9015087
Seredych and Bandosz
Figure 1. Breakthrough curves for the total amount of sulfur (A) per gram of adsorbent and (B) per unit volume of the adsorbent bed. C-II is polymer-derived carbon described in ref 34.
outgassed at 120 C until the vacuum 10 -5 Torr was reached. Approximately 0.20-0.25 g of sample was used for these analyses. The surface area, SBET, was calculated from the Brunauer-Emmett-Teller (BET) method, whereas for the volume of pores smaller than 10 A˚, V10 A˚, micropore volume, Vmic, and mesopore volume, Vmes, the density functional theory (DFT) approach was applied.40,41 The total pore volume was calculated from the last point of the isotherm. Pore size distributions were determined using nonlocal density functional theory (NLDFT).42 Elemental Analysis. The content of sulfur was evaluated using X-ray fluorescence (XRF, SPECTRO model 300T benchtop analyzer, ASOMA Instruments, Inc.) based on the calibration curve performed for the carbon samples with the internal standard of sulfur. The instrument has a titanium target X-ray tube and a high-resolution detector. Surface pH. A 0.1 g sample of dry carbon powder was added to 5 mL of distilled water, and the suspension was stirred overnight to reach equilibrium. Then, the pH of the suspension was measured. Scanning Electron Microscopy (SEM). SEM images were obtained at Zeiss Supra 55 VP. The samples were outgassed until vacuum 2 10-6 Torr was reached. The accelerating voltage was 15.00 kV. Scanning was performed in situ on a sample powder. Extraction/Mass Spectroscopy. The samples for analysis were extracted by treating 0.1 g of exhausted carbon with 5 mL of furan at room temperature for 4 days. The extract was directly injected into a mass spectrometer Q-TRAP 4000 (Applied Biosystems). The mass spectra were collected for m/z from 20 to 500. The mass spectrometer was operated in positive-ion mode; enhanced product ion (EPI) was used for the data acquisition. The mass spectra parameters for the analysis were as follows: an ion spray voltage of 5500 V (highest sensitivity), collision energy and collision energy spread of 10 and 40 V, respectively, and declustering potential of 80 V. Nitrogen was used as curtain gas (value of 25 psi) and collision gas (set to high). Potentiometric Titration. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm). The instrument was set at the mode where the equilibrium pH is collected. Subsamples of the initial and
exhausted materials (∼0.100 g) were added to NaNO3 (0.01 M, 50 mL) and placed in a container maintained at 25 C overnight for equilibrium. During the titration, to eliminate the influence of atmospheric CO2, the suspension was continuously saturated with N2. The suspension was stirred throughout the measurements. Volumetric standard NaOH (0.1 M) was used as the titrant. The experiments were performed in the pH range of 3-10. Each sample was titrated with base after acidifying the sample suspension. The surface properties were evaluated first using potentiometric titration experiments.43,44 Here, it is assumed that the population of sites can be described by a continuous pKa distribution, f(pKa). The experimental data can be transformed into a proton-binding isotherm, Q, representing the total amount of protonated sites, which is related to the pKa distribution by the following integral equation: Z ¥ qðpH, pKa Þf ðpKa ÞdpKa QðpHÞ ¼ -¥
The solution of this equation is obtained using the numerical procedure,43,44 which applies regularization combined with non-negativity constraints. On the basis of the spectrum of acidity constants and the history of the samples, the detailed surface chemistry was evaluated.
Results and Discussion The breakthrough curves for the total amount of sulfur adsorbed on the carbons studied are presented in Figure 1. On the x axis, either the amount of treated fuel per gram of adsorbent (Figure 1A) or per unit volume of an adsorbent bed (Figure 1B) is plotted. While the latter is important from the point of view of the practical performance, the former data will be used in our analysis to link the performance with the surface properties. Also, for comparison, the performance of the highly porous carbon derived from poly(4-styrenesulfonic acid-co-maleic acid) sodium salt, described in ref 34, is presented. As seen, the differences exist owing to the differences in the densities of carbons. The best capacity per unit volume of the bed is found for CSO, but per gram of the materials, C-II is able to treat more fuel than the other two carbons. For C-II, the surface area, volume of micropores,
(40) Lastoskie, C.; Gubbins, K. E.; Quirke, N. J. Phys. Chem. B 1993, 97, 4786–4796. (41) Olivier, J. P. J. Porous Mater. 1995, 2, 9–17. (42) Jagiello, J.; Olivier, J. P. J. Phys. Chem. C 2009, 113, 19382– 19385.
(43) Jagiello, J.; Bandosz, T. J.; Schwarz, J. A. Carbon 1994, 32, 1026– 1028. (44) Jagiello, J. Langmuir 1994, 10, 2778–2885.
3354
Energy Fuels 2010, 24, 3352–3360
: DOI:10.1021/ef9015087
Seredych and Bandosz Table 1. Adsorption Capacity for Each Component of MDF (in mmol/ g), Selectivity Factors, and Total Sulfur Adsorbed (in mg of S/g) sample
Nap
capacity at Ba capacity at 0.7b capacity at Sc selectivity at B selectivity at 0.7 selectivity at S
0.006 0.020 0.037 1.00 1.00 1.00
0.006 0.015 0.037 1.00 0.75 1.00
CS 0.015 0.046 0.073 2.50 2.30 1.97
0.011 0.046 0.073 1.83 2.30 1.97
0.82 2.97 4.68
capacity at B capacity at 0.7 capacity at S selectivity at B selectivity at 0.7 selectivity at S
0.016 0.045 0.059 1.00 1.00 1.00
0.016 0.040 0.059 1.00 0.89 1.00
CSO 0.083 0.144 0.168 5.19 3.20 2.85
0.073 0.124 0.168 4.56 2.76 2.85
4.99 8.57 10.77
a
1-MNap DBT 4,6-DMDBT total sulfur
At the breakthrough point. b At C/Co = 0.7. c At the saturation.
adsorption, the mechanism may vary because the pores of increasing sizes will be involved.32,34,37 From the point of view of the number of moles adsorbed, there are practically no differences between DBT and 4,6-DMDBT retained on either the surface of CS or CSO. A weak indication of the preferable adsorption of DBT at the low surface coverage (at C/Co = 0.7) is found on the CSO sample. These results suggest that the slightly stronger basicity of 4,6-DMDBT does not play an enhancing role in the adsorption process. On the other hand, the modification of the CS surface via mild oxidation caused an increase of the capacity at the breakthrough point, at C/Co = 0.7, and at the saturation of about 7, 3, and 2.5 times, respectively. Interestingly, an enhancing effect becomes less pronounced with an increase in the progress of adsorption. This suggests that the oxidation of the surface causes the formation of high energy adsorption centers, which are preferably occupied by DBT and 4,6-DMDBT molecules. This hypothesis is formulated on the basis of an observed increase in the adsorption of arenes after oxidation, which is much less pronounced (about 2-fold) than that of DBTs. Oxidation also causes an increase in the selectivity of adsorption, which is once again more visible at the breakthrough point than at the saturation. To explain the changes in the capacity caused by mild oxidation applied to CS carbon, the surface of the adsorbents before and after exposure to MDF was analyzed. The protonbinding curves presented in Figure 3A show only small changes in surface chemistry. Heating in air apparently caused a decrease in surface acidity because the proton-binding curve is shifted toward slightly higher pH values. This is seen on the distributions of pKa collected in Figure 3B. Oxidation apparently resulted in a more heterogeneous surface with two new species revealed in the range of strong (pKa < 8) and weak (pKa > 8) acids. The number of moles of weak acids represented by the area of the peak at pKa of about 10 visibly increased. The number of groups represented as weak and strong acids is summarized in Table 2. In this table, we also present the surface pH values and the contents of sulfur detected using XRF. The number of groups increased about 50% after oxidation, and it is similar to that found on highly oxidized wood-based carbons.32 On the other hand, the pH value of the CS carbon is very small, which indicates the presence of strong acids. The X-ray photoelectron spectroscopy (XPS) study of this carbon presented elsewhere37 indicated that about 42.9% of sulfur is engaged in sulfonic acids.
Figure 2. Breakthrough curves for all components of MDF: (A) CS and (B) CSO.
and volume of pores smaller than 10 A˚ are 1286 m2/g, 0.486 cm3/g, and 0.315 cm3/g, respectively, as reported previously.34 While C-II exhibits a typical S shape of the breakthrough curves, on the curves for the CS and CSO, the change in the slope is noticed, especially for the former samples. This can be linked either to the change in the mechanism of adsorption, adsorption isotherms, or the mass-transfer resistances related to the diffusion processes. Breakthrough curves for each component of MDF are collected in Figure 2. A significant improvement in the performance with the oxidation is seen for arenes, DBT, and 4,6-DMDBT. As in Figure 1, the change in the slope, especially for CS is observed. For CSO, an increase in C/Co for arenes over 1 can be explained by their replacement by stronger adsorbed DBT and 4,6-DMDBT.14 The fact that the concentrations of arenes do not decrease to 1 during the duration of the experiment can be explained by the plateau in the concentration of DBT and 4,6-DMDBT. Their concentrations, although close to 1, never reached the complete saturation, which is likely caused by the diffusion problems. Calculated capacities for each component of MDF are collected in Table 1. For the sake of analysis, we list the capacities at the breakthrough point (B) when the first concentration of adsorbate in the effluent is detected, at C/Co equals to 0.7 (0.7), and at the saturation (S). This is performed assuming that, at the breakthrough, the highest energy adsorption centers will be occupied, and with the progress of 3355
Energy Fuels 2010, 24, 3352–3360
: DOI:10.1021/ef9015087
Seredych and Bandosz
Figure 3. Potentiometric titration results: (A) proton-binding curves and (B) pKa distribution for the species present on the surface of carbons. Table 2. Numbers of Strong Acidic Groups (pKa < 8), Weak Acidic Groups (pKa > 8), All Groups (in mmol/g), Sulfur Content, and the Surface pH Values for the Samples Studied sample pKa < 8 pKa > 8 all groups sulfur content (wt %) CS CSO
0.299 0.361
0.247 0.433
0.546 0.794
7.1 4.4
pH 3.50 4.03
The pKa of those acids is beyond our PT experimental window. Oxidation along with a decrease in the content of sulfur and an increase in the number of acidic groups results in a decrease in surface acidity. Thus, the groups formed are likely oxygen-based and with low acidic strength. This is visible by an increase in the intensity of the peak at pKa of about 10. This increase might be linked the formation of phenols, which are known to appear on the surface when the oxidation in air is applied.45-47 A small amount of new carboxylic groups is also formed. Changes in surface chemistry are also seen on the differential thermogravimetry (DTG) curves measured in nitrogen presented in Figure 4. The peaks represent the weight loss as a result of decomposition of surface functional groups.45 The first peak centered at about 80 C is assigned to the removal of physically adsorbed water. For CS, the weight loss at about 250 C is associated with decomposition of sulfonic groups and the continuous weigh loss at higher temperatures is caused by the decomposition of various oxygen groups45,46 and also sulfur species. On the DTG curve for CSO, sulfonic groups are not present and only a small amount of carboxylic groups, which are supposed to decompose at temperatures less than 600 C, is revealed. On the other hand, the amount of oxygen in phenols, carbonyls, and basic groups, such as chromene, pyrone, and quinones, decomposing at T > 600 C, significantly increased. This suggests that, even though the oxidation did not dramatically increase the acidic groups, other oxygen groups not dissociating in our experimental window are formed in a significant quantity. They might attract DBT and 4,6-DMDBT via polar sulfur-oxygen interactions. In
Figure 4. Comparison of the DTG curves in nitrogen for the initial samples.
fact, an apparent lack of the sensitivity in the measured capacities for differences in basicity between DBT and 4,6DMDBT suggests that other factors than acid-base specific interactions govern the adsorption process. Those other factors must represent porosity17,19,21,25 or porosity and surface chemistry combined.20,32,34,36 The parameters of the porous structure calculated from nitrogen adsorption isotherms are summarized in Table 3. The mild oxidation applied increased the surface area of about 100%, with about 120% increase in the volume of micropores. The strongest effect is seen for pores smaller than 10 A˚, whose volume increased almost 3 times. Even though filling of those pores can explain the increase in the capacity at C/Co = 0.7 and at the saturation, this is not enough to account for the 7fold increase in the capacity at the breakthrough point, assuming that the total pore filling takes place. The importance of those small pores is seen in their almost complete blocking after the adsorption from MDF. Also, pores smaller than 20 A˚ are almost totally occupied in both samples. With regard to the mesopore involvement in the adsorption, a larger volume of about 0.120 cm3/g is occupied in the case of CSO compared to only about 0.035 cm3/g for CS. This
ao, (45) Figueiredo, J. L.; Pereira, M. F. R.; Freitas, M. M. A.; Orf~ J. J. M. Carbon 1999, 37, 1379–1389. (46) Biniak, S.; Szymanski, G.; Siedlewski, J.; Swiatkowski, A. Carbon 1997, 35, 1799–1810. (47) Bandosz, T. J.; Ania, C. O. In Activated Carbon Surfaces in Environmental Remediation; Bandosz, T. J., Ed.; Elsevier: Amsterdam, The Netherlands, 2006; pp 159-231.
3356
Energy Fuels 2010, 24, 3352–3360
: DOI:10.1021/ef9015087
Seredych and Bandosz
Table 3. Parameters of the Pore Structure Calculated from Nitrogen Adsorption Isotherms 2
sample
SBET (m /g)
S>10 A˚ (m2/g)
Vt (cm3/g)
Vmeso (cm3/g)
V
