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بخشی از مقاله انگلیسیعنوان انگلیسی:Mesoporous-molecular-sieve-supported nickel sorbents for adsorptive desulfurization of commercial ultra-low-sulfur diesel fuel~~en~~

Abstract

A high-performance nickel-based sorbent was developed by loading nickel on a mesoporous molecular sieve, MCM-48, for adsorptive desulfurization (ADS) of commercial ultra low sulfur diesel (ULSD) for fuel cell applications. The prepared sorbents were characterized by the N2 adsorption–desorption, X-ray diffraction (XRD), H2 chemisorption, and transmission electron microscope (TEM), and the ADS performance was evaluated in a fixed-bed flow sorption system at 220 °C using a commercial ULSD with a sulfur content of 14.5 ppmw. Effects of the ultrasonic aid in incipient wetness impregnation (IWI), nickel loading amount and support materials on the sorbent performance were examined. It was found that the incipient wetness impregnation with the ultrasonic aid improved significantly the ADS performance of the sorbent by increasing the dispersion of nickel on the surface. Using MCM-48 as a support with 20 wt% nickel loading (Ni20/MCM-48) can lead to an excellent nickel-based sorbent with a breakthrough capacity of 2.1 mg-S/g-sorb for ADS of the ULSD at a breakthrough sulfur level of 1 ppmw. The alkyl dibenzothiophenes are likely adsorbed on the sorbent surface directly through an interaction between the sulfur atom and the exposed nickel atoms, and a part (6%) of the adsorbed alkyl dibenzothiophenes react further with the surface nickel to release the corresponding hydrocarbons. The desulfurization reactivity of the alkyl dibenzothiophenes is dependent on not only the number, but also the size of the alkyl substituents at the 4- and 6-positions of alkyl dibenzothiophenes. Graphical abstract

۱ Introduction

Ultra-deep desulfurization of transportation fuels, such as diesel, gasoline, and jet fuel, has attracted a great deal of attention because of not only the stringent fuel specifications for the transportation fuels, but also the severe requirement of liquid hydrocarbon fuels with sulfur content less than 1 ppmw for fuel cell applications [1–۶]. The current commercial ultra-low sulfur diesel (ULSD) with sulfur content less than 15 ppmw is a preferred fuel for the on-site and on-board fuel cell applications due to its high energy density, availability, safety and ease for production, delivery and storage by using the existing infrastructures. However, even in ULSD, in which the sulfur content is usually more than 10 ppmw, the sulfur content is still too high to be directly fed to the fuel processor to produce hydrogen for fuel cell applications [4,7,8] especially for the proton exchange membrane fuel cell (PEMFC) [5], as the sulfur compounds and H2S produced from them in the fuel processor poison the reforming and water–gas-shift catalysts as well as the fuel cell stacks.

Currently, the sulfur removal from various liquid hydrocarbon streams is conducted by the catalytic hydrodesulfurization (HDS) process at 300–۴۰۰ C and 3–۶ MPa hydrogen pressure with high hydrogen consumption in refineries. According to our previous study [2,9], if reducing the sulfur content in the current commercial ULSD from 15 ppmw to less than 1 ppmw by using the current hydrotreating technology, the catalyst bed volume or the catalyst activity must be about 68% higher than that currently used in refineries, as the remaining sulfur compounds in the commercial ULSD are the most refractory sulfur compounds. As is well known, the increase in both volume of the high-temperature and highpressure reactor and the catalyst amount is very costly. Working at high temperature and high pressure also limits the usage of the HDS process in the on-site and on board desulfurization due to the complication and safety of the process. Therefore, it is desired to develop a novel technology for ultra-deep desulfurization of ULSD for fuel cell applications.

Many new approaches for ultra-deep desulfurization of liquid hydrocarbon fuels have been reported in the literature [1–۳]. Among them the adsorptive adsorption on the nickel-based sorbents is promising and has attracted a great deal of attention due to the high capacity and selectivity without using hydrogen gas [10–۱۹]. The nickel-based sorbent, such as Raney–Nickel, has been used for desulfurization in organic synthesis for a long time [10,20,21].Ma et al. reported that the nickel-based sorbent was very efficient in selective removal of some sulfur compounds, such as thiophene, benzothiophene and their alkyl substituted derivatives, from gasoline [11,13]. Velu et al. studied the desulfurization of the light JP-8 with 380 ppmw of sulfur at 220 C on a Ni/SiO2–Al2O3 sorbent with Ni loading of 55 wt% [12], and got a breakthrough capacity of 13.5 mg-S/g-sorbent at a breakthrough sulfur level of 30 ppmw. Kim et al. found that the lower sorption selectivity of the Ni/SiO2–AlO2 sorbent for 4,6-DMDBT than DBT, and pointed out that the alkyl groups at 4- and/or 6-positions have strong steric hindrance toward the sorption [14]. A study reported later by Hernndez et al. [18] in desulfurization of a model fuel over a Ni/SiO2–Al2O3 sorbent also supports the finding by Kim et al. [14]. In order to improve the sorption performance of the nickelbased sorbents, Ko et al. and later Park et al. reported using the SBA-15-supported nickel sorbent with different nickel loadings for desulfurization of a commercial diesel fuel with 240 ppmw of sulfur [15,16]. They found that the SBA-15-supported nickel sorbent with 30 wt% of Ni loading gave the best breakthrough capacity of 1.7 mg-S/g-sorbent at a breakthrough sulfur level of 10 ppmw. However, when using the same SBA-15-supported nickel sorbent for desulfurization of a commercial ULSD with 11.7 ppmw of sulfur, they found that the breakthrough capacity was only 0.47 mg-S/gsorbent. Thus, the capacity of the nickel based sorbent needs to be improved for the practical application in the on-board and on-site desulfurization of ULSD for fuel cell systems.

The objective of the present study is to improve the ADS performance of the nickel-based sorbents for ULSD by increasing the dispersion of nickel in the nickel-based sorbent, and get better insight into the ADS mechanism of the refractory sulfur compounds on the sorbent. Different mesoporous molecular sieves (SBA-15 and MCM-48) and ultrasonic aid technique were used to increase the nickel dispersion. The ADS performance of the prepared sorbents was evaluated in a fixed-bed flow sorption system at 220 C using a commercial ULSD with sulfur content of 14.5 ppmw. Effects of the ultrasonic aid in the incipient wetness impregnation (IWI), nickel loading amount and support materials on the ADS performance of the sorbents were examined. The prepared sorbents were characterized by the N2 adsorption–desorption at 196 C, X-ray diffraction (XRD), H2 chemisorption, and transmission electron microscope (TEM), and the results were correlated with their ADS performance. The ADS selectivity and mechanism for the sulfur compounds on the nickel-based sorbent were also discussed on the basis of the detailed analysis of the sulfur compounds and the formed hydrocarbons in the treated fuels.

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