Chapter 4
Downloaded by UNIV ILLINOIS URBANA on October 30, 2013 | http://pubs.acs.org Publication Date (Web): December 14, 2011 | doi: 10.1021/bk-2011-1088.ch004
Ultra-Deep Desulfurization of Ultra-Low Sulfur Diesel over Nickel-Based Sorbents in the Presence of Hydrogen for Fuel Cell Applications C. Sentorun-Shalaby, X. L. Ma,* and C. S. Song* Clean Fuels and Catalysis Program, EMS Energy Institute, and Department of Energy & Mineral Engineering, The Pennsylvania State University, 209 Academic Projects Building, University Park, Pennsylvania 16802 *E-mails:
[email protected] (Ma);
[email protected] (Song)
This study explored the addition of H2 to improve the sorption performance of the nickel-based sorbents in ultra-deep desulfurization of ultra-low sulfur diesel (ULSD). The desulfurization of ULSD over Raney Nickel and Ni20/SBA-15 ˚ and was conducted in a fixed-bed sorption system at 220C ambient pressure in the absence and presence of H2. The ADS performances of the prepared nickel-based sorbent and commercial Raney Nickel in the absence and presence of H2 were evaluated and compared. Addition of very small amount of H2 into the sorption system significantly increases the adsorptive capacity of the SBA-15-supported nickel-based sorbent for desulfurization of ULSD. The improvement of the sorption performance is through accelerating the C-S bond cracking of the adsorbed sulfur compounds to release the corresponding hydrocarbon part from the surface, and thus to provide more exposed nickel atoms on the surface to interact with other sulfur compounds. Each kilogram of the prepared Ni20/SBA-15 sorbent is able to treat about 240 L of the commercial ULSD with sulfur content of 14.5 ppmw to get a desulfurized fuel with sulfur content less than 1 ppmw in the presence of H2.
© 2011 American Chemical Society In Production and Purification of Ultraclean Transportation Fuels; Hu, Y., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
Downloaded by UNIV ILLINOIS URBANA on October 30, 2013 | http://pubs.acs.org Publication Date (Web): December 14, 2011 | doi: 10.1021/bk-2011-1088.ch004
Introduction The commercial ultra-low sulfur diesel (ULSD) is a preferred liquid hydrocarbon fuel for automotive, portable, resident, and military fuel cells due to its high energy density, availability, safety, and/or ease for delivery and storage. The current sulfur level of the commercial ULSD is less than 15 ppmw. However, even at this sulfur level, the sulfur content in ULSD is still too high for fuel cell applications, as the sulfur compounds in the fuel and H2S produced from them in the fuel processor poison the catalysts used in reforming and water-gas-shift processes and fuel cell stacks (1–4). In our previous studies (5, 6), we have developed the high-performance nickel-based sorbent by loading nickel on the mesoporous molecular sieves, such as MCM-48 and SBA-15 for adsorptive desulfurization (ADS) of the commercial ULSD for fuel cell applications. The high breakthrough capacity for ADS of the ULSD at a breakthrough sulfur level of 1 ppmw has been achieved. However, we found that each sulfur-containing molecule was adsorbed approximately on the twenty exposed nickel atoms. This value is much lower than the stoichiometry (from 0.48 to 1.09 for different crystal faces of nickel) of the S atoms per exposed Ni atom when the nickel surface is saturated by sulfur (7), indicating that the majority of the exposed Ni atoms still remain intact after the breakthrough. Our further investigation found that more than 95 % of the sulfur compounds were adsorbed on the surface without occurrence of the C-S bond cracking to release the corresponding hydrocarbon part from the surface. The results suggest that introducing hydrogen to the nickel surface may accelerate the hydrogenolysis of the adsorbed sulfur compounds and the release of corresponding hydrocarbon part, and thus, provide more accessible nickel atoms to interact with other sulfur compounds. In the present study, we explored the ultra-deep desulfurization of ULSD on the nickel-based sorbents in the presence of hydrogen to improve the ADS performance of the nickel-based sorbents. The ultra-deep desulfurization of ULSD over Raney Nickel and SBA-15 supported nickel-based sorbents was conducted in a fixed-bed sorption system in the absence and presence of H2 at atmospheric pressure. The ADS performance of the nickel-based sorbents in the absence and presence of H2 was evaluated and compared.
Experimental Nickel-Based Sorbents The mesoporous-molecular-sieve-supported nickel sorbent, Ni20/SBA-15, was prepared by loading 20 wt % of Ni on SBA-15 using an incipient wetness impregnation (IWI) method with ultrasonic aid. The SBA-15 was synthesized according to the procedure reported by Wang et al. (8, 9) based on the method initially reported by Zhao et al. in 1998 (10). Typically, a homogeneous mixture, which was composed of triblock copolymer Pluronic of P123 (MW = 5800, 56 In Production and Purification of Ultraclean Transportation Fuels; Hu, Y., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
Downloaded by UNIV ILLINOIS URBANA on October 30, 2013 | http://pubs.acs.org Publication Date (Web): December 14, 2011 | doi: 10.1021/bk-2011-1088.ch004
Aldrich) and tetraethyl orthosilicate (TEOS) in hydrochloric acid, was stirred at 40˚C for 20 h, and then further treated at 100˚C for 24 h. The solid product was filtered and washed with plenty of water, dried in an oven at 100˚C, and subsequently calcined at 550˚C for 6 h under an air flow (100 ml/min). The mesoporous-molecular-sieve-supported nickel sorbent was prepared using an incipient wetness impregnation (IWI) method. Tetrahydrofuran (THF) was used as a solvent to prepare the Ni(NO3)2 solution. The nickel sorbent preparation method is given in Figure 1. The desired amount of Ni(NO3)2·6H2O was dissolved in THF, and the solution was slowly added into the support material at room temperature with ultrasonic aid in a VWR-Model 75T ultrasonic bath. After adding the solution, the mixture was kept in the ultrasonic bath for 3 h at room temperature. The mixture was then dried in an oven at 100˚C overnight. The dried samples were then reduced in situ in a fixed-bed reactor under a pure hydrogen gas flow at 550˚C for 4 h before use.
Figure 1. The nickel sorbent preparation method.
All chemicals that were used in the preparation of the sorbents, including Pluronic (P123) (Mn: 5800), TEOS with a purity of 98%, THF with a purity of 99%, hydrochloric acid solution (HCl) with a purity of 37%, and nickel nitrate hexahydrate (Ni(NO3)2·6H2O), were purchased from Sigma–Aldrich (Allentown, PA), and were used as received without further purification. A commercial Raney Nickel®2800 was also used in this study, which was purchased from Sigma-Aldrich. Some physical properties of the nickel-based sorbents and SBA-15 used in this study are listed in Table 1. The nitrogen adsorption–desorption at −196˚C was conducted using a Micromeritics ASAP2020 instrument. Samples were degassed for 3 h at 400˚C under vacuum (P
