11254
Ind. Eng. Chem. Res. 2010, 49, 11254–11259
Desulfurization of Gasoline over Nanoporous Nickel-Loaded Y-Type Zeolite at Ambient Conditions Majid Dastanian and Fakhry Seyedeyn-Azad* Chemical Engineering Department, Engineering Faculty, UniVersity of Isfahan, Isfahan, Iran
Desulfurization of heavy straight-run gasoline (HSRG) was accomplished over Ni(II)-Y zeolite. Na-Y zeolite, a nanoporous adsorbent, was synthesized and ion-exchanged with NH4NO3 to obtain NH4-Y zeolite. The obtained material was then converted to H-Y zeolite by calcination. Ni-Y zeolite was prepared by solidstate ion exchange (SSIE) of H-Y zeolite using Ni(NO3)2 · 6H2O. The breakthrough curve for desulfurization of HSRG containing about 140 ppmw of sulfur compounds was obtained in a batch reactor at ambient conditions. The effects of temperature, Ni content in the zeolite framework, and aging of the zeolite on the desulfurization process were investigated. Ni-Y zeolite exhibited a high capability for the desulfurization of gasoline at ambient conditions. 1. Introduction Removal of sulfur compounds from commercial gasoline and diesel engine exhaust gases is a major concern because sulfur compounds are the main source of SOx emissions.1 In 1998, the European Union for the first time mandated the regulations leading to a gross reduction level of sulfur.2 These regulations have been implemented since the year 2000. Similar regulations were enacted in the United States and elsewhere soon thereafter. Conventional processes and new approaches for desulfurization have been discussed in several recent reviews.3–5 Hydrodesulfurization (HDS) is a conventional method for eliminating sulfur compounds from commercial fuels. In the HDS process, removal of sulfur-containing compounds is achieved by employing catalysts such as Co-Mo/Al2O3 or Ni-Mo/Al2O3 at elevated temperatures (>300 °C) and pressures (20-100 atm H2).6 In such a process, organic sulfur compounds in the liquid fuels are converted to H2S through certain reactions using the catalysts that are later removed from the system. After completion of the HDS process, the remaining sulfur compounds in current commercial gasoline and diesel fuels are thiophenic sulfur compounds.3 They are mainly benzothiophene, dibenzothiophene, and their alkylated derivatives. The leastreactive derivatives are the dibenzothiophenes with methyl groups at the 4- and 6-positions. The reason for their low reactivity is the steric hindrance created by these compounds.7 In the case of gasoline, it is necessary to maintain the octane number by preserving olefins during the HDS process, making it more difficult to reach ultradeep sulfur removal. This is because performing deep desulfurization is followed by the saturation of olefinic compounds, resulting in a loss of about 10 in the octane number.8 Therefore, the reduction of the sulfur content of liquid fuels to 10 ppm by weight of sulfur (ppmw) by existing HDS technology is very difficult.9 For this purpose, an improved catalyst, increased reactor size, or more severe operating conditions such as higher temperature and higher H2 pressure are required to produce such low-sulfur fuels.9 Further reduction of the sulfur level, to near 0 ppm, is also very important, as the presence of even a trace of sulfur poisons fuelreforming catalysts, as well as electrode catalysts.1 Attempts have been made to develop new adsorbents for the deep removal of thiophenic compounds from commercial fuels * To whom correspondence should be addressed. E-mail: fsazad@ eng.ui.ac.ir or
[email protected].
such as gasoline, diesel, and jet fuels.3,5 Development of a new process for the desulfurization of liquid fuels under ambient conditions without using H2 is also one of the major goals in fuel processing for liquid-hydrocarbon-fuel-based fuel cell systems.3,5 However, these applications would depend on the development of a highly selective adsorbent with a high sulfur removal capacity at ambient conditions, as the current adsorbents are not very efficient for this purpose. Recently, zeolites have attracted increasing attention because they provide a very high surface area per unit mass. It has been shown that the introduction of different cations into the zeolite structure leads to different catalytic and adsorption properties of the zeolites.10 Adsorption desulfurization is a new and highly efficient method for removing sulfur compounds from transportation fuels in the presence of aromatic and olefinic compounds. In addition, adsorption desulfurization can be accomplished at relatively lower pressure and temperature and even at ambient conditions. Velu et al.11 reported that ion-exchanged NH4-Y zeolites with transition metals such as Cu, Ni, Zn, Pd, and Ce showed selective adsorption of organic sulfur from a model jet fuel (S content ) 507 ppmw). Ce-exchanged Y-type zeolite with 38 wt % Ce loading exhibited a higher sulfur adsorption capacity (10 mg of sulfur/g of adsorbent) than the other ion-exchanged zeolites. They reported that the sulfur content was reduced to almost 19 ppm using Ce-Y zeolite. This study suggests that transition metal ions have a strong tendency for the adsorption of organic sulfur compounds. During the past decade, Yang and co-workers have been developing π-complexation-based sorbents for many applications, including desulfurization12–19 and olefin/paraffin, diene/ olefin, and aromatics/aliphatics separations.20–29 For most of the mentioned applications, preparation of the metal-loaded zeolites was based on the ion-exchange method in the aqueous phase. However, ion exchange in the liquid phase is usually limited by zeolite selectivity for the new cationic species that have been introduced into the structure, passage of solvated cations through the zeolite pores, and hydrolysis of the available cations in the ion-exchange solution.30 Several studies on the solid-state ion-exchange (SSIE) preparation method for loading metal ions on zeolites have been reported.6,31–33 It is noted that the SSIE method is convenient as a preparation procedure when compared to the conventional ion-exchange method in the liquid phase. Beyer and Karge used
10.1021/ie100941s 2010 American Chemical Society Published on Web 10/11/2010
Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010 Table 1. HSRG Specifications carbon number (%)
carbon distribution
percentage
aromatics
13.2
naphthenes
30.8
olefins
