Selective Adsorption of Dibenzothiophenes on Activated Carbons with


Jun 8, 2009 - To whom correspondence should be addressed. Telephone: (212) 650-6017. Fax: (212) 650-6107. ... Metal species contribute to an increase ...
4 downloads 14 Views 4MB Size


Energy & Fuels 2009, 23, 3737–3744

3737

Selective Adsorption of Dibenzothiophenes on Activated Carbons with Ag, Co, and Ni Species Deposited on Their Surfaces Mykola Seredych and Teresa J. Bandosz* Department of Chemistry, The City College of New York, 160 ConVent AVenue, New York, New York 10031 ReceiVed March 23, 2009. ReVised Manuscript ReceiVed May 26, 2009

Adsorption of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) from simulated diesel fuel with 20 ppm total concentration of sulfur was investigated on three polymer-derived carbon-containing silver, cobalt, and nickel species. The initial and exhausted carbons were characterized using adsorption of nitrogen, thermal analysis, XRD, SEM, and TEM. The selectivities for DBT and 4,6-DMDBT adsorption were calculated with reference to naphthalene. The results showed a good adsorption capacity of the carbons studied. The adsorbents remove DBT and 4,6-DMDBT selectively from model diesel fuel. Metal species contribute to an increase in the number of active centers, which enable adsorption of DBT and 4,6-DMDBT on the surface. That surface would not be used to the same extent when only dispersive forces were involved. Nanosize metallic silver particles not only attract DBT and 4,6-DMDBT but also catalyze their oxidation.

Introduction One of the goals of the U.S. EPA is to decrease the content of sulfur in diesel oil to 30 ppm. It promotes investigation of new techniques of separations because such a low concentration cannot be achieved using hydrodesulfurization (HDS). HDS is not able to remove aromatic heterocyclic sulfur compounds to the required levels. Among those compounds, dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) are considered as refractory (resisting ordinary methods of treatment) species. Recently investigated methods have been oxidation with hydrogen peroxide in the presence of titanium oxides1,2 or reaction-enhanced adsorption on undisclosed reactive adsorbent such as that used in the S-Zorb process of ConocoPhilips†.3 In the latter method, a proprietary adsorbent attracts sulfur and withdraws it from DBT or 4,6-DMDBT, and an aromatic hydrocarbon is released back to the system.4 Another adsorptionbased process, with some potential of feasibility, employs π-electron interactions where sulfur-containing aromatic compounds interact with metals on the catalyst support (MCM alumina, activated carbon, zeolites).5 Recent studies indicated that activated carbons exhibit a good performance as adsorbents of dibenzothiophenic compounds.4-15 * To whom correspondence should be addressed. Telephone: (212) 6506017. Fax: (212) 650-6107. E-mail: tba[email protected] † FCC Network News. April 2002, Vol. 2. http://www.thefccnetwork. com/pdf/newsletters/newsletter2.pdf. (1) Shirashiri, Y.; Hara, H.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 1999, 38, 1589–1595. (2) Matsuzawa, S.; Tanaka, J.; Sato, S.; Ibusuki, T. J. Photochem. Photobiol., A 2002, 149, 183–189. (3) Gislason, J. Oil Gas J. 2001, 99, 72–76. (4) Song, Ch. Catal. Today 2003, 86, 211–263. (5) Herna´ndez-Maldonado, A. J.; Qi, G.; Yang, R. T. Appl. Catal., B 2005, 61, 212–218. (6) Salem, A. B. S. H.; Hamid, H. S. Chem. Eng. Technol. 1997, 20, 342–347. (7) Haji, S.; Erkey, C. Ind. Eng. Chem. Res. 2003, 42, 6933–6937. (8) Jiang, Z.; Liu, Y.; Sun, X.; Tian, F.; Sun, F.; Liang, Ch.; You, W.; Han, Ch.; Li, C. Langmuir 2003, 19, 731–736.

One of the drawbacks in the application of activated carbons is the lack of selectivity. Owing to the hydrophobic nature of their surfaces, other aromatic hydrocarbons present in diesel fuel can be also adsorbed in significant quantities. Thus far, it has been established that oxidation of carbons enhances their capacity for desulfurization.8-10,16 This enhancement is a result of specific adsorption of dibenzothiophenic compounds via oxygen-sulfur or sulfur-sulfur interactions.9 Oxidation has also a positive effect on increasing the selectivity for DBT and 4,6-DMDBT retention via specific acid-base interactions owing to the slightly basic nature of thiophenic compounds.8,9 An improvement in the selectivity of adsorption was also found when copper was present on the surface of activated carbons.11 Moreover, the compounds being able to activate oxygen or supply oxygen for redox reactions were shown as beneficial for oxidation of adsorbed DBT and 4,6-DMDBT on activated carbons.17 The objective of this study is to evaluate the performance of carbons with silver, cobalt, and nickel species deposited on the surface. That performance is analyzed from the point of view of the amount adsorbed, selectivity of adsorption, and the activity of the surface of activated carbon in selective adsorption of DBT and 4,6-DMDBT. In this study, a model diesel fuel (MDF) with 10 ppm of sulfur from DBT and 10 ppm of sulfur from 4,6-DMDBT and with other aromatic compounds is used. (9) Ania, C. O.; Bandosz, T. J. Langmuir 2005, 21, 7752–7759. (10) Zou, A.; Ma, X.; Song, Ch. J. Phys. Chem. B 2006, 110, 4699– 4707. (11) Ania, C. O.; Bandosz, T. J. Carbon 2006, 44, 2404–2412. (12) Seredych, M.; Bandosz, T. J. Langmuir 2007, 23, 6033–6041. (13) Velu, S.; Watanabe, S.; Ma, X.; Song, Ch. Prepr. Pap.-Am. Chem. Soc., DiV. Fuel Chem. 2003, 526. (14) Yu, G.; Lu, S.; Chen, H.; Zhu, Z. Carbon 2005, 43, 2285–2294. (15) Kim, J. H.; Ma, X.; Zhu, A.; Song, Ch. Catal. Today 2006, 111, 74–83. (16) Yang, Y.; Ku, H.; Ying, P.; Jiang, Z.; Li, C. Carbon 2007, 45, 3042–3059. (17) Deliyanni, E.; Seredych, M.; Bandosz, T. J. Langmuir, submitted for publication, 2009.

10.1021/ef900254k CCC: $40.75  2009 American Chemical Society Published on Web 06/08/2009

3738

Energy & Fuels, Vol. 23, 2009

The results are discussed in terms of chemistry and porosity of the adsorbents’ surfaces. Experimental Section Materials. Poly(4-styrenesulfonic acid-co-maleic acid) sodium salt (PS) was used as an organic precursor. The carbon obtained from PS was synthesized using the procedure described by Hines and co-workers18 and Ania and Bandosz.11 First, the polymer was treated with appropriate amounts of either AgNO3, Ni(NO3)2, or Co(NO3)2 to impose ion exchange, and then carbonization was carried out. It was done in a horizontal furnace with nitrogen as an inert gas (300 mL/min flow rate). The samples were heated at a rate of 50 °C/min and held at 800 °C (final carbonization temperature) for 40 min. The excess of water-soluble sodium salts was removed by washing with distilled water, until neutral pH a leachate was reached. The samples obtained from silver, cobalt, and nickel salts are designated as C-Ag, C-Co, and C-Ni, respectively. The subsamples of carbons after adsorption of sulfur from MDF were heated at 250 °C in nitrogen for 3 h to remove weakly adsorbed compounds and naphthalene to better analyze the mechanism of DBT and 4,6-DMDBT interactions with the carbon surface. The boiling points for decane, hexadecane, naphthalene, N-methylnaphthalene, DBT, and 4,6-DMDBT are 174, 287, 218, 240, 332, and 340-350 °C, respectively.19,20 The exhausted samples after heating are referred to with letter S (without heating) and H. Thus, C-Ag-S-H is exhausted C-Ag carbon heated at 250 °C for 3 h. For the sake of comparison, the initial carbons were also heated at 250 °C in nitrogen. They are referred to as C-Ag-H, C-Co-H, and C-Ni-H. Even though our carbons were not activated in the classical meaning of this word, we refer to them as “activated” since the self-activation occurred during carbonization of the polymer as described elsewhere.18 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, 4,6-DMDBT, naphthalene (Nap), and 1-methylnaphthalene (1-MNap) in a mixture of decane and hexadecane (1:1). All compounds were obtained from Sigma-Aldrich Co. and used as received. 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. Adsorbents with granule sizes of 0.425-0.212 mm were packed into a 60-mm polyethylene column with 4-mm inside diameter. The height of the carbon bed was approximately 55 mm with volume of 0.70 mL. Activated carbons were dried overnight at 120 °C before contact with fuel. Model fuel was passed into the column with adsorbent from the top by 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 experiment in the presence of arenes in diesel fuel is referred as E1 and without arenes as E2. The concentration of arenes and thiophenes in effluent after adsorption of DBT and 4,6-DMDBT in the presence of Nap and 1-MNap (in experiment E1) was 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 mm × 125 mm, EM Separations) and a guard column (4.0 mm × 4.0 mm) of the same material. In this (18) Hines, D.; Bagreev, A.; Bandosz, T. J. Langmuir 2004, 20, 3388– 3397. (19) Weast, R. C. Handbook of Chemistry and Physics, 67th ed.; CRC Press: Boca Raton, FL, 1986; pp 261-380. (20) Torrisi, S., Jr.; Remans, T.; Swain, J. Process Technol. Catal. 2002, 1–4.

Seredych and Bandosz case, a gradient method was used, which started at 90% methanol (HPLC, grade) and 10% distilled water (Milli-Q water) as mobile phase for 10 min, then changed to 100% methanol over 1 min, held 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. In experiments E2, the concentration of DBT and 4,6-DMDBT as total sulfur concentration was determined using UV spectroscopy. We prepared first the calibrations solution with total concentration of sulfur (equimolar with regards to both compounds) between 1 and 20 ppmw of sulfur. Then the absorbance was measured at 313 nm. On the basis of the intensity of the absorbance of the effluent, the concentration of sulfur and then the breakthrough curves were constructed and the capacities calculated. Thermal Analysis. Thermal gravimetry curves were obtained using a TA Instruments thermal analyzer. The samples (initial or exhausted) were exposed to an increase in 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). Before each measurement, all samples were 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, S, was calculated from the BET method, whereas for the volume of pores smaller than 1 nm, V<1 nm, micropore volume, Vmic, and mesopore volume, Vmes, the DFT approach was applied.21,22 The total pore volume was calculated from the last point of the isotherm. Dubinin-Radushkevich method was used to calculate the characteristic energy, Eo.23 Elemental Analysis. The content of various elements in the samples was evaluated using inductively coupled plasma (ICP) atomic spectroscopy. 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 suspension was measured. XRD. X-ray diffraction measurements were conducted using a standard powder diffraction procedure. Adsorbents were ground with methanol in a small agate mortar. The mixture was smearmounted onto the zero-background quartz window of a Phillips specimen holder and allowed to air-dry. Samples were analyzed by Cu KR radiation generated in a Phillips XRG 300 X-ray diffractometer. A quartz standard slide was run to check for instrument wander and to obtain accurate location of 2Θ peaks. SEM with EDAX. Scanning electron microscopy (SEM) images were obtained at Zeiss Supra 55 VP with a secondary electron detector with an energy-dispersive analysis of X-rays (EDAX). The samples were outgassed until vacuum 2 × 10-6 Torr was reached. TEM. Transmission electron microscopy (TEM) was performed on a Zeiss EM 902 instrument. The microscope had a line resolution of 0.34 nm and a point resolution of 0.5 nm and operated in normal, diffraction, and low dose modes at 50 or 80 kV. Analyses were done after the samples were resuspended in ethanol.

Results and Discussion The breakthrough curves for all components of MDF are collected in Figure 1. Although there is no dramatic difference in the performance of the samples, a striking feature is a distinctive shape of the curves for DBT and 4,6-DMDBT measured on C-Co and their total overlapping. These curves (21) Lastoskie, C.; Gubbins, K. E.; Quirke, N. J. Phys. Chem. B 1993, 97, 4786–4796. (22) Olivier, J. P. J. Porous Mater. 1995, 2, 9–17. (23) Dubinin, M. M. Porous structure and adsorption properties of active carbons. In Chemistry and Physics of Carbon; Walker, P. L., Ed.; Dekker: New York, 1966; Vol. 2, pp 51-120.

SelectiVe Adsorption of DBTs on ActiVated Carbons

Figure 1. Breakthrough curves for all components of MDF.

are less steep than those for other samples, and this indicates slower kinetics and/or involvement of surface active centers in specific interactions with sulfur-containing species. For two other carbons, the breakthrough time for DBT at the C/Co ) 0.7 point is about 70 min longer than that for 4,6-DMDBT. Comparison of the curves obtained with arenes and without arenes and those presented per unit volume of the adsorbent bed and per gram of the adsorbent reveals that when no competitive adsorption takes place (no arenes) (Figure 2) the greatest differences in the samples’ performance are observed. Even though in both conditions C-Co has the longest breakthrough time when metal-containing carbons are compared,

Energy & Fuels, Vol. 23, 2009 3739

without arenes that time is almost twice as long as that for C-Ag. With arenes the difference is much less pronounced. This suggests that arenes decrease the specificity of surface interactions. The calculated capacities and selectivities with respect to naphthalene are collected in Table 1. These values are reported at the breakthrough point (B), when C/Co ) 0.7 (0.7), and at the saturation point (S). It is done to investigate the differences in the performance with the progress of adsorption. In Table 1, we also report total capacities in milligrams of sulfur per gram of the adsorbent for the experiments run in the presence of arenes (E1) and without arenes (E2). Whereas in the case of samples containing silver and nickel the presence of arenes in fuel practically does not affect the amount of sulfur removed from diesel fuel, for C-Co the differences are clearly seen, especially when the amounts adsorbed at the breakthrough are compared. When arenes are not present in the system, about 100 and 45% more sulfur is adsorbed on the surface of C-Co and C-Ni, respectively. This means that active centers on the C-Ag sample differ from those on C-Co and C-Ni and arenes do not compete with DBT and 4,6-DMDBT for their occupation. When the amounts adsorbed at the breakthrough are compared, there is practically no difference in the samples’ performance. With the progress of adsorption the differences become more visible and the highest amounts adsorbed of DBT and 4,6-DMDBT are found for C-Co. The amounts adsorbed on C-Ag and C-Ni are about 30% less than that on C-Co. Adsorption selectivities are similar for all samples. Values about 15% higher for 4,6-DMDBT than those for DBT can be linked to 15% greater molar mass of the former sample. This indicates that the same centers are involved in adsorption of those two compounds. The trends in the selectivities observed with the progress of adsorption are also similar for all samples. To analyze the adsorption process in detail, the effects of the surface features on the performance of carbons as DBT and 4,6-DMDBT adsorbents have to be discussed. Comparison of the results of chemical analyses presented in Table 2 clearly shows the differences between C-Ag and two other carbons. An excellent agreement between the content of silver and ash content suggests that the predominant species present on the surface of this sample is metallic silver. Reduction of silver oxide or salts during analysis is rather excluded since the determination of ash was done in the flow of air. On the other hand, on the surface of C-Co and C-Ni insoluble sulfides and oxides are likely deposited. The pH over 7 supports the hypothesis that the oxides are a significant part of an inorganic matter. The element maps presented in Figure 3 indeed prove that mainly metallic silver is present on the surface of C-Ag; however, a small amount of silver sulfide is also detected. In the case of C-Co, mainly cobalt oxide is detected with the very highly dispersed sulfur, which is in fact a part of the carbon matrix.18 In the case of C-Ni, nickel sulfide seems to be a predominant species. In all carbons, a significant amount of oxygen is present as a part of the carbon matrix. It is incorporated mainly in carboxylic and sulfonic groups.18,24 It is interesting that with such big differences in the metal contents the capacities for desulfurization are comparable. Thus, other factors such as dispersion and accessibility of pore space must also play a role in the adsorption of dibenzothiophenic compounds. (24) Seredych, M.; Lison, J.; Jans, U.; Bandosz, T. J. Carbon [Online early access]. DOI: 10.1016/j.carbon.2009.05.001. Published online May 10, 2009.

3740

Energy & Fuels, Vol. 23, 2009

Seredych and Bandosz

Figure 2. Breakthrough curves for DBT and 4,6-DMDBT. (A) Per volume of adsorbent bed. (B) Per gram of adsorbent. (E1) With arenes. (E2) Without arenes. C-II is polymer-derived carbon described in ref 24. Table 1. Adsorption Capacity for Each Component of MDF, Selectivity Factors, and Total Sulfur Adsorbed with Arenes (E1) and without Arenes (E2) in the Fuel Nap 1-MNap DBT DMDBT total S (E2) total S (E1) (mg/g) (mg/g) (mg/g) (mg/g) (mg S/g) (mg S/g)

sample C-Ag breakthrough breakthrough-0.7 saturation selectivity-B selectivity-0.7 selectivity-S

1.71 3.45 6.58 1.00 1.00 1.00

1.90 3.61 6.00 1.11 1.05 0.91

9.52 16.53 21.85 5.57 4.79 3.32

10.97 17.03 25.10 6.42 4.94 3.82

3.42 5.85 7.73

3.30 5.44 7.58

C-Co breakthrough breakthrough-0.7 saturation selectivity-B selectivity-0.7 selectivity-S

1.76 4.06 6.18 1.00 1.00 1.00

1.95 4.37 6.85 1.11 1.08 1.11

9.91 20.38 27.82 5.63 5.02 4.50

11.41 23.48 32.05 6.48 5.78 5.19

7.06 9.01 11.94

3.44 7.08 9.66

C-Ni breakthrough breakthrough-0.7 saturation selectivity-B selectivity-0.7 selectivity-S

1.40 3.49 5.48 1.00 1.00 1.00

1.55 3.25 5.44 1.11 0.93 0.99

8.76 14.59 22.13 6.26 4.18 4.04

10.10 14.81 25.50 7.21 4.24 4.65

4.35 5.50 7.79

3.04 4.76 7.68

Table 2. Content of Metal (from ICP and EDAX), Sulfur, Oxygen, and Ash and the pH of the Samples Studied; Initial ) in and Exhausted ) exh sample

ICP

EDAX

sulfur content (wt %)

C-Ag C-Co C-Ni

46.0 9.0 9.5

35.2 11.1 7.3

0.6 5.6 4.2

metal content (%)

oxygen content (wt %)

ash content (wt %)

pHin/pHexh

3.7 8.4 7.9

46.0 16.4 13.5

6.57/6.23 8.10/6.60 7.46/6.32

When a texture of the carbon matrixes was compared on SEM images, all samples have similar features governed by the mechanism of carbonization where pores are formed due to the release of gases from decomposition of carboxylic and sulfonic

groups of the polymer and migration of metals through that hot carbonaceous matrix.18 On the surface of the samples containing metals, spots with large agglomerates of inorganic phases are present. In the case of C-Co, the spheres of cobalt oxides assemble in the structure’s remaining walls. On the surface of C-Ni, amorphous flakes of nickel sulfides are seen. On the other hand, the silver-containing sample has only a few particles of silver seen on the external surface, which suggests that smaller particles are within the carbon porous network. Those metal particles are seen on TEM micrographs presented in Figure 4. For the cobalt-containing samples, the size of the metal species particles is between 20 and 100 nm. Particles 100 nm and smaller are seen in the case of C-Ag where also a big silver agglomerate is visible. In the case of the nickel-containing sample, uniform particles with sizes between 20 and 50 nm are visible. The metal species, as indicated from the EDAX element maps, are very well dispersed within the carbon matrix. The highest dispersion of metal species and/or their presence in the amorphous form is found for C-Co. On X-ray diffraction pattern for this sample, no peaks representing surface species are revealed (Figure 5). On the other hand, in the case of C-Ni and C-Ag metals in the zero oxidation state are detected.25,26 This was also the conclusion about the state of silver from the EDAX and other chemical analyses. The analysis of porous texture is presented in Table 3, where the surface area and pore volume calculated from nitrogen adsorption isotherms are listed. The highest surface area was found for C-Co and the lowest one for C-Ag. The high content of a nonporous metal has to contribute to that small surface of the latter sample. C-Ag seems to have the smallest pores as (25) Inagaki, M.; Okada, Y.; Miura, K.; Konno, H. Carbon 1999, 37, 329–334. (26) Chun, S.; Grudinin, D.; Lee, D.; Kim, S.-H.; Yi, G.-P.; Hwang, I. Chem. Mater. 2009, 21, 343–350.

SelectiVe Adsorption of DBTs on ActiVated Carbons

Energy & Fuels, Vol. 23, 2009 3741

Figure 3. EDAX element maps for the samples studied.

indicated from the value of characteristic energy. In fact, C-Ag and C-Co are very similar from the point of view of the pore volume in each category listed in Table 3. The highest volume

of micropores and the smallest volume of mesopores are found for C-Ni. Since after adsorption of DBT and 4,6-DMDBT from MDF the materials became practically nonporous owing to the

3742

Energy & Fuels, Vol. 23, 2009

Seredych and Bandosz

Figure 5. X-ray diffraction patterns.

Figure 4. TEM micrographs.

deposition of the adsorbates and solvent,24 the samples were heated at 250 °C in nitrogen to remove decane, hexadecane, and arenes. Even though significant changes in porosity of the initial carbons after heating at 250 °C were not expected, to ensure meaningful comparison the parameters of the porous structure of these samples are also listed in Table 3. As seen, the changes are rather small and the texture can be considered as not affected by this treatment. After removal of solvent, only 30% of the initial surfaces of C-Ag and C-Ni are accessible for the nitrogen molecule. For C-Co, that fraction is 40%. The biggest decreases are found in the volumes of small pores. Thus, pores smaller than 1 nm are occupied after adsorption of DBT and 4,6-DMDBT and only from 12 to 25% volume of micropores is available after adsorption. This indicates that those

pores are the most active in the desulfurization process. A decrease is also found in the volume of mesopores, and it is the most noticeable for C-Ag for which that volume decreased almost 30%. For two other samples, about 20% decrease is found. Details on the porosity are seen on pore size distributions presented in Figure 6. While C-Co and C-Ni have similar porosity with the pore sizes in the whole range between 0.5 and 100 nm, in the case of C-Ag the small mesopores with sizes between 2 and 6 nm do not exist and there is a high volume of pores between 30 and 60 nm. The volume of those pores is significantly reduced after DBT and 4,6-DMDBT adsorption. They are likely transformed into pores with sizes between 2 and 10 nm as a result of the deposition of DBT and 4,6-DMDBT and their surface reaction products. The pores smaller than 1 nm are totally filled up in C-Ag after the adsorption process. Those pores are also filled in the case of C-Ni-S-H. For this sample, pores with sizes between 1 and 3 nm are also occupied. Similar pore activity is also noticed for C-Co-S-H. These changes suggest that the whole surface is involved in the retention of DBT and 4,6-DMDBT. The smallest pores can be either filled by adsorbed molecules of DBT or 4,6-DMDBT or are blocked by the adsorbates and/or their surface reaction products deposited in the larger pores. Analysis of DTG curves run in nitrogen provides information about the surface chemistry of the initial samples and the amount of species deposited on their surfaces after adsorption from MDF. They are collected in Figure 7. On the initial samples, marked differences are seen between C-Ag and two other samples. For the silver-containing material, the DTG curve is relatively featureless with broad low intensity peaks at about 300 and 700 °C representing decomposition of oxygen functionalities.27 The weight loss patterns look differently for C-Co and C-Ni where quite significant weight losses are observed in the whole temperature range with peaks with maxima at about 300 and 500 °C and between 600 and 700 °C. As suggested previously,18 these peaks can represent removal of surface functional groups (at T < 550 °C) and reduction of sulfides and oxide species (at T > 550 °C). After adsorption from MDF, complex weight loss patterns are observed for all samples. The first very intense peak with maximum at about 200 °C represents removal of solvents with boiling points 174 and 287 °C for decane and hexadecane, respectively. Naphthalene and 1-methylnaphthalene are removed at more than 200 °C where a clear indication of the peak is ´ rfa˜o, J. J. M. (27) Figueiredo, J. L.; Pereira, M. F. R.; Freitas, M. M. A.; O Carbon 1999, 37, 1379–1389.

SelectiVe Adsorption of DBTs on ActiVated Carbons

Energy & Fuels, Vol. 23, 2009 3743

Table 3. Parameters of the Pore Structure Calculated from Nitrogen Adsorption Isotherms sample

SBET (m2/g)

SDFT (m2/g)

Vt (cm3/g)

Vmeso (cm3/g)

V<1 nm (cm3/g)

Vmic (cm3/g)

Vmic/Vt

Eo (kJ/mol)

C-Ag C-Ag-H C-Ag-S-H C-Co C-Co-H C-Co-S-H C-Ni C-Ni-H C-Ni-S-H

522 506 161 626 663 254 657 702 175

466 474 59 536 567 132 556 617 65

0.510 0.481 0.237 0.517 0.547 0.309 0.473 0.553 0.237

0.329 0.304 0.209 0.308 0.326 0.257 0.250 0.314 0.207

0.131 0.128

0.181 0.177 0.028 0.209 0.221 0.052 0.223 0.239 0.030

0.36 0.37 0.12 0.40 0.40 0.17 0.47 0.43 0.13

22.1 22.5 15.4 21.3 21.5 16.0 21.2 21.6 15.7

0.135 0.143 0.015 0.142 0.162 0.002

noticed for C-Co-S and C-Ni-S. They have boiling points at 218 and 243 °C, respectively.19 The peak representing removal of arenes is least pronounced for the C-Ag-S sample. This is in agreement with our finding that the presence of arenes does not affect the amount of DBT and 4,6-DMDBT adsorbed on this carbon. On the basis of the boiling points of the solvents and arenes, 250 °C was chosen as the temperature at which the majority of

solvent should be removed and only DBT and 4,6-DMDBT should stay on the surface.24 Their oxidation products, if any, should have higher boiling points.28,29 To properly compare the results, the initial samples were also heated at 250 °C, and they are referred to with the letter H. The surface pH measurements on these samples show no visible changes in their surface chemistry compared to that of the initial counterparts. The weight loss patterns after heating for the C-Ni and C-Co

Figure 6. Comparison of pore size distributions for samples before and after the desulfurization process.

Figure 7. DTG curves in nitrogen for the initial and exhausted samples after removal of the solvents.

3744

Energy & Fuels, Vol. 23, 2009

Seredych and Bandosz

Table 4. Comparison of the Amount of Sulfur Adsorbed (from DBT and 4,6-DMDBT) and the Weight Loss Represented by the Removal of Adsorbed Species sample

% DBT + 4,6-DMDBT adsorbed

mass loss (%)

C-Ni C-Co C-Ag

4.76 5.99 4.70

4.58 6.15 7.85

samples exposed to DBT and 4,6-DMDBT are quite similar to each other with one main peak between 250 and 450 °C with maximum about 400 °C. Taking into account the closeness of DBT and 4,6-DMDBT boiling points (332 and 340-350 °C19,20), the complex peak at maximum at about 400 °C represents the removal of these compounds. One has to remember that the desorption temperature can increase when the adsorbed species are removed from small pores where adsorption potential is high. The presence of an additional peak on the exhausted C-Ag sample suggests a different mechanism of adsorption than in the case of C-Co and C-Ni samples. Taking into account the shift to high temperature, that peak might represent the products of surface oxidation or surface polymerization of DBT and 4,6-DMDBT. As former ones, sulfoxide and/or sulfones are expected.17,24 Comparison of the weight loss represented by the above-mentioned peaks and the amount of DBT and 4,6-DMDBT adsorbed is presented in Table 4. While for C-Ni and C-Co a very good agreement between those values is found, in the case of C-Ag the weight loss is about 40% higher than those for other samples. One reason for this might be in the oxidation suggested above of DBT and 4,6-DMDBT and an incorporation of oxygen to their molecules. On the basis of the results presented, metallic silver catalyzes those reactions. Moreover, silver, even present in large pores as big crystals, has the affinity to attract sulfur from DBT and 4,6-DMDBT. Since the volume of large pores is affected to the greatest extent for C-Ag, the adsorption must take place in those pores and thus it cannot be considered as nonspecific. The source of oxidant can be in chemisorbed oxygen on the carbon surface. Comparison of the amounts adsorbed at breakthrough points and at saturation normalized for the pore volumes of the adsorbents is presented in Figure 8. No difference in the surface activity is noticed at the breakthrough points, suggesting that the physical adsorption in pores plays a main role at this stage of the process. With the progress of adsorption, the differences appear and the most active is the sample containing cobalt. C-Ag is least active. This might be linked to the catalytic effects (28) Babich, I. V.; Moulijn, J. A. Fuel 2003, 82, 607–631. (29) Yazu, K.; Makino, M.; Ukegawa, K. Chem. Lett. 2004, 33, 1306– 1307.

Figure 8. Comparison of the normalized capacity for DBT and 4,6DMDBT at the breakthrough point (B) and at the saturation point (S).

of cobalt species in the process of hydrodesulfurization.30 It is expected that with the progress of desulfurization larger pores start to be filled. It is more likely that the cobalt species are present in these pores and not in very small micropores, which are filled first by the adsorbates. Thus, the effect of these species on enhancing the amount adsorbed can be seen only with the higher surface coverage. Support for that involvement of cobalt and nickel species is a decrease in the pH after adsorption (Table 2). Conclusions The results presented in this article show a good adsorption capacity of carbon-containing metal species highly dispersed on the surface. The adsorbents remove DBT and 4,6-DMDBT selectively from model diesel fuel. Metal species contribute to an increase in the number of active centers. The location in larger pores enables adsorption of DBT and 4,6-DMDBT on the surface. That surface would not be used to the same extent when only dispersive forces were involved. The nanosize metallic silver particles not only attract DBT and 4,6-DMDBT but also catalyze their oxidation. The enhanced adsorption capacity on the cobalt-containing sample is linked to well-known activity of cobalt in hydrodesulfurization processes. Acknowledgment. We are grateful to Dr. Jorge Morales for help with SEM/EDAX analysis. The help of Mr. Jakub Lison in breakthrough experiments and Dr. Urs Jans with HPLC analyses is highly valued. EF900254K (30) Song, Ch.; Reddy, K. M. Appl. Catal., A 1999, 176, 1–10.