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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:
[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
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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 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.
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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
