J. Phys. Chem. B 2006, 110, 4699-4707
4699
Liquid-Phase Adsorption of Multi-Ring Thiophenic Sulfur Compounds on Carbon Materials with Different Surface Properties Anning Zhou,†,‡ Xiaoliang Ma,† and Chunshan Song*,† Clean Fuels and Catalysis Program, The Energy Institute, and Department of Energy & Geo-EnVironmental Engineering, The PennsylVania State UniVersity, 209 Academic Projects Building, UniVersity Park, PennsylVania 16802, and Department of Chemistry and Chemical Engineering, Xian UniVersity of Science and Technology, Xian 710054, China ReceiVed: September 5, 2005; In Final Form: January 11, 2006
This work examines the effects of structural and surface properties of carbon materials on the adsorption of benzothiophene (BT), dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) in the presence of 10 wt % of aromatics in liquid alkanes that simulate sulfur compounds in diesel fuels. The equilibrium-adsorption capacity varies significantly, from 1.7 to 7.0 mg-S/g-A. The results show that different carbon materials have significantly different sulfur-adsorption capacities and selectivities that depend not only on textural structure but also on surface functional groups. The adsorption of multi-ring sulfur compounds on carbon materials was found to obey the Langmuir isotherm. On the basis of adsorption tests and the characterization of carbon materials by BET and XPS, the oxygencontaining functional groups on the surface appear to play an important role in increasing sulfur-adsorption capacity. The adsorption-selectivity trend of the carbon materials for various compounds increases in the order of BT < naphthalene < 2-methylnaphthalene < DBT < 4-MDBT < 4,6-DMDBT, regardless of carbon material types. This selectivity trend for sulfur compounds is dramatically different and almost opposite from that previously observed for adsorption over nickel-based adsorbents. The regeneration of spent activated carbons was also conducted by solvent washing. The high-adsorption capacity and selectivity for methyl DBTs indicate that certain activated carbons are promising adsorbents for selective adsorption for removing sulfur (SARS) as a new approach to ultra deep desulfurization of diesel fuels.
1. Introduction Ultra deep sulfur removal has become a very important and active research subject worldwide in the last 10 years1-9 because the production and use of more environmentally friendly transportation fuels have been attracting increasing attention in many countries. The regulation of sulfur contents in liquid transportation fuels is becoming more and more stringent. The maximum allowable sulfur content in highway diesel fuel will be reduced to less than 15 ppmw by 2006 in the US from the current maximum of 500 ppmw and less than 10 ppmw by 2010 in the EU from 50 ppmw in 2005.1,2 Further regulation for lower sulfur content is expected in the future,1 and this trend is continuing worldwide, including in many developing countries. In addition to transportation needs for internal combustion engines, liquid hydrocarbon fuels are preferred for fuel processing on-board and on-site portable and mobile fuel-cell applications because of their higher-energy density, ready availability, and safety during storage and transportation. However, for proton-exchange membrane fuel cells (PEMFCs) and solid oxide fuel cells (SOFCs), the sulfur content in liquid hydrocarbon fuels needs to be reduced to less than 1 ppmw and
