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Preparation of Organic Sulfur Adsorbent from Coal for Adsorption of Dibenzothiophene-type Compounds in Diesel Fuel Cigdem Shalaby,† Xiaoliang Ma,† Anning Zhou,†,‡ and Chunshan Song*,† Clean Fuels and Catalysis Program, EMS Energy Institute, and Department of Energy and Mineral Engineering, The PennsylVania State UniVersity, 209 Academic Projects Building, UniVersity Park, PennsylVania 16802, USA, and Department of Chemistry and Chemical Engineering, Xian UniVersity of Science and Technology, Xian 710054, China ReceiVed December 27, 2008. ReVised Manuscript ReceiVed March 16, 2009
High-performance organic sulfur adsorbents (OSA) have been prepared from coal by chemical activation for selective adsorption of the refractory sulfur compounds, such as 4-methyl dibenzothiophene and 4,6-dimethyldibenzothiophene, in diesel fuel. The performance of the prepared OSAs for adsorptive desulfurization (ADS) was evaluated in batch and flow adsorption systems at room temperature using a model diesel fuel. It was found that coal rank and preparation conditions, including activation agents (NaOH, KOH, and NaOH + KOH) and their ratio to coal, activation temperature, and time have significant impacts on the yield and ADS performance of the OSAs. The high-performance OSAs can be prepared from different ranks of coal by using NaOH + KOH as an activation agent with an activating-agent-to-coal ratio of 3.5. The yield of OSA increased in the order of lignite < high volatile bituminous coal < medium volatile bituminous coal < anthracite. The OSA-A, which was derived from an anthracite with the highest yield (68 wt %) by the activation at 650 °C for 1 h, gave the best ADS performance among the OSAs from all coal samples tested. The sulfur adsorption capacity of OSA-A reached 0.281 mmol-S/g-A at an equilibrium sulfur concentration of 50 ppmw in the model diesel fuel, which was 155% higher than a commercial coal-derived activated carbon and 35% higher than the best commercial activated carbon among all commercial activated carbons examined in our laboratory. The higher ADS capacity of OSA-A can be attributed to its significantly higher density (2.77 µmol/m2) of the adsorption sites on the surface as determined by Langmuir adsorption isotherm, which is related to its oxygen-containing functional groups on the carbonaceous surface as revealed by temperatureprogrammed desorption analysis.
1. Introduction Deep desulfurization of liquid hydrocarbon fuels has become an increasingly important subject worldwide. The sulfur content in the transportation fuels is a very serious environmental concern, because the sulfur in fuel is converted to toxic SOx in combustion process and contributes to acid rains. The SOx also poisons the catalysts used in the exhaust gas treatment system on vehicles for reducing NOx emission. Thus, since 2006, the US EPA has issued a Tier II regulation that mandates large refineries to reduce the sulfur content to less than 30 ppm by weight (ppmw) for gasoline and to less than 15 ppmw for highway diesel. On the other hand, liquid hydrocarbon fuels are also promising fuels for producing H2 for automotive, portable, and resident fuel cells due to their high energy density, availability, and easy handling for transportation and storage.1-3 Since sulfur in the fuels poisons the reforming and water-gasshift catalysts in fuel processor and the electrode catalysts in fuel cells, the sulfur content in the liquid hydrocarbon fuels needs to be reduced at least to less than 1 ppmw for PEMFC and 10 ppmw for SOFC.1 Hydrodesulfurization (HDS) is the conventional process in petroleum refineries to reduce the sulfur * Corresponding author: e-mail:
[email protected]. † The Pennsylvania State University. ‡ Xian University of Science and Technology. (1) Song, C. S. Catal. Today 2003, 86, 211–263. (2) Ma, X. L.; Sun, L.; Song, C. S. Catal. Today 2002, 77, 107–116. (3) Song, C. S.; Ma, X. L. Appl. Catal. B. EnV., 2003, 41, 207–238.
in liquid hydrocarbon fuels, but it is difficult or very costly to reduce the sulfur in diesel fuel to less than 1 ppmw. Using adsorbents to selectively remove the sulfur compounds from liquid hydrocarbon fuels is one of the promising approaches for producing ultra clean fuels.1-4 As the diesel fuel contains not only sulfur compounds, but also a large number of aromatic compounds that have aromatic skeleton structure similar to the coexisting sulfur compounds, a great challenge in development of an effective ADS process is to develop an adsorbent that can selectively adsorb the sulfur compounds. Many adsorbent materials, such as the reduced metals,5-11 metal oxides,12-14 metal chlorides,15,16 zeolite-based materials,4,17-19 (4) Hernandez-Maldonado, A. J.; Yang, R. T. Catal. ReV. 2004, 46 (2), 111–150. (5) Ma, X. L.; Sprague, M.; Song, C. S. Ind. Eng. Chem. Res. 2005, 44, 5768–5775. (6) Kim, J. H.; Ma, X. L.; Zhou, A. N.; Song, C. S. Catal. Today 2006, 111, 74–83. (7) Ma, X. L.; Velu, S.; Kim, J. H.; Song, C. S. Appl. Catal. B: EnV. 2005, 56, 137–147. (8) Bailey, G. W.; Swan, G. A. US Patent, 4,634,515, 1987. (9) Bonville, L. J.; DeGeorge, C. L.; Foley, P. F.; Garow, J.; Lesieur, R. R.; Perston, J. L.; Szydlowski, D. F. US Patent, 6,159,256, 2000. (10) Lesieur, R. R.; Teeling, C.; Sangiovanni, J. J., Boedeker, L. R.; Dardas, Z. A.; Huang, H.; Sun, J.; Tang, X.; Spadaccini, L. J. US Patent, 6,454,935, 2002. (11) Fukunaga, T.; Katsuno, H.; Matsumoto, H.; Takahashi, O.; Akai, Y. Catal. Today 2003, 84, 197–200. (12) Watanabe, S.; Ma, X.; Song, C. Prepr., DiV. Fuel Chem., Am. Chem. Soc. 2004, 49 (2), 511–513.
10.1021/ef801135t CCC: $40.75 2009 American Chemical Society Published on Web 04/17/2009
Organic Sulfur Adsorbent from Coal
and carbon materials6,20-29 have been tested for deep desulfurization of liquid hydrocarbon streams. Among them, the carbonbased materials have attracted great attention due to their higher surface area, receptivity for modification, and widely available sources. Our previous work6,20,21 on ADS of liquid hydrocarbon fuels has shown that some carbon-based materials show excellent performance for ADS, especially high adsorptive selectivity for removing 4,6-dimethyldibenzothiophene (4,6-DMDBT), which is considered as one of the most refractory sulfur compounds in diesel fuels due to the steric hindrance of the two methyl groups at the 4 and 6 positions in hydrotreating process. However, most currently available commercial carbon-based adsorbents were designed and produced for gas-phase adsorption or liquid-phase adsorption in an aqueous solution, such as wastewater treatment, metal recovery, and removal of impurities in chemicals. In most liquid-phase applications, the carbon-based adsorbents were used for adsorption of the compounds with relatively weaker polarity from a polar fluid phase, such as adsorption of organics in wastewater. In distinct contrast, the ADS of diesel fuel addresses the selective removal of the sulfur compounds with very weak polarity from a nonpolar fluid phase (liquid hydrocarbons). Consequently, the currently available commercial carbon-based adsorbents may not be suitable for ADS of the liquid hydrocarbon fuels. On the other hand, coal is among the most popular source material for producing carbon-based adsorbents because of its carbonaceous nature, inherent microstructure, surface chemical properties, abundant resource, and lower cost. Development of the carbon-based adsorbents from coal for ADS of liquid hydrocarbon fuels could not only produce ultraclean hydrocarbon fuels for environmental protection and fuel cell applications, but also increase the premium carbon product from coal. Carbon-based adsorbents from coal can be produced by physical or chemical activation methods. The chemical activation has some significant advantages in comparison with the physical activation, such as low activation temperature and high yield of the activated carbons with high surface area. Alkali hydroxides have been considered as efficient activation agents in the chemical activation for producing the carbon materials with high (13) Turk, B. S.; Gupta, R. P. Prepr. Am. Chem. Soc. DiV. Fuel Chem. 2001, 46 (2), 392–393. (14) (a) Song, C. S. Catal. Today 2002, 77, 17–50. (b) Sun, F. X.; Ma, X. L.; Song, C. S. Prepr. Pap. Am. Chem. Soc., DiV. Petr. Chem. 2008, 53 (2), 154–156. (15) Ma, X. L.; Sakanishi, K.; Isoda, I.; Mochida, I. Fuel 1997, 76, 329–339. (16) Wang, Y. H.; Yang, R. T. Langmuir 2007, 23, 3825–3831. (17) Velu, S.; Ma, X. L.; Song, C. S. Ind. Eng. Chem. Res. 2003, 42, 5293–5304. (18) Hernandez-Maldonado, A. J.; Yang, R. T. Ind. Eng. Chem. Res. 2003, 42, 3103–3110. (19) Yang, R. T.; Hernandez-Maldonado, A. J.; Yang, F. H. Science 2004, 301, 79–81. (20) Zhou, A. N.; Ma, X. L.; Song, C. S. J. Phys. Chem. B 2006, 110, 4699–4707. (21) Zhou, A. N.; Ma, X. L.; Song, C. S. Appl. Catal. B: EnViron. 2009, 87, 190–199. (22) Sano, Y.; Choi, K.; Korai, Y.; Mochida, I. Energy Fuels 2004, 18, 644–651. (23) Sano, Y.; Choi, K. H.; Korai, Y.; Mochida, I. Appl. Catal. B. EnV. 2004, 49, 219–225. (24) Haji, S.; Erkey, C. Ind. Eng. Chem. Res. 2003, 42, 6933–6937. (25) Seredych, M.; Bandosz, T. J. Langmuir 2007, 23, 6033–6041. (26) Ania, C. O.; Parra, J. B.; Arenillas, A.; Rubiera, F.; Bandosz, T. J.; Pis, J. J. Appl. Surf. Sci. 2007, 253, 5899–5903. (27) Ania, C. O.; Bandosz, T. J. Carbon 2006, 44, 2404–2412. (28) Ania, C. O.; Bandosz, T. J. Energy Fuels 2006, 20, 1076–1080. (29) Ania, C. O.; Bandosz, T. J. Langmuir 2005, 21, 7752–7759.
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surface area and abundant functional groups on the surface.30-44 Lillo-Ro´denas et.al. found that low-rank coals are more reactive than high-rank coals, and KOH is a more reactive agent than NaOH in activation.32 The objective of the present study is to develop an organic sulfur adsorbent (OSA) from coal with high performance for selective adsorption of sulfur compounds from diesel fuel for producing ultra clean fuel. Such an OSA could become a premium carbon product from coal. The effects of coal rank and activation conditions, including activation agent, temperature, and time, on the ADS performance of the adsorbents prepared from different coals were examined. The adsorption capacity of OSAs for ADS of a model diesel was measured in a batch adsorption system, and the results were used as an index to determine the best activation condition. The physiochemical properties of some prepared adsorbents were further characterized by N2 adsorption and temperature-programmed desorption (TPD) for understanding the structure-performance relationship. 2. Experimental Section Coal Samples and Activation Agents. Four representative ranks of coal samplessDECS-6, DECS-11, DECS-21, and DECS30swith the particle sizes
