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Adsorptive removal of dibenzothiophene and dibenzothiophene sulfone over mesoporous materials Yawei Shi, Guozhu Liu, and Xiangwen Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03673 • Publication Date (Web): 13 Feb 2017 Downloaded from http://pubs.acs.org on February 14, 2017
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Industrial & Engineering Chemistry Research
Adsorptive removal of dibenzothiophene and dibenzothiophene sulfone over mesoporous materials Yawei Shi, Guozhu Liu*, and Xiangwen Zhang*
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin) , Tianjin University, Tianjin 300072, China
ABSTRACT:
Dibenzothiophene
(DBT,
a
typical
sulfur
compound)
and
dibenzothiophene sulfone (DBTO2, oxidation product of DBT) were chosen as the model compounds and their adsorption over a mesoporous silica (MCM-41) and a mesoporous carbon (CMK-3) have been conducted under exact the same conditions. The adsorption amounts of DBTO2 over both adsorbents were enhanced compared to DBT adsorption amounts, which clearly demonstrated that a combination of oxidation and adsorption is better than direct adsorption to achieve a better desulfurization performance. Besides, it was found that CMK-3 performed better for DBT removal compared to MCM-41, but the adsorption amount of DBTO2 over MCM-41 exceeded the corresponding value over CMK-3 (1.60 mgS/g vs. 0.88 mgS/g in para-xylene). Moreover, model fuels with varied hydrocarbon compositions were employed to investigate the impact of arene amounts. Finally, a jet fuel and a diesel fuel were oxidized and treated with MCM-41, resulting in 85.6% and 81.4% sulfur removals
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respectively. In contrast, when the fuels were directly treated with MCM-41, the corresponding sulfur removals were only 19.8% and 40.9%. The results with real fuels further verified the necessity of the oxidation process. A simple washing method was found to be effective for regeneration of the adsorbent.
KEYWORDS: MCM-41, CMK-3, adsorption, desulfurization, sulfone
INTRODUCTION
Sulfur compounds present in liquid hydrocarbon fuels may cause detrimental impact on the environment and human health1. The traditional hydrodesulfurization (HDS) process is widely employed and performs well in the conversion of thiols, sulfides and disulfides, but it suffers from high operating and capital costs when faced with refractory thiophenic compounds2-5. As an alternative, oxidative desulfurization (ODS) is attracting more and more attention6-9. During this process, the refractory thiophenic compounds are easily converted to corresponding sulfones by all kinds of oxidants under mild reaction conditions. The resulting sulfones can be further removed by adsorption or extraction10-13.
Adsorption, which is a well-known process for removal of pollutants such as heavy metal ions14-18 and hazardous coloring agents19-22, is widely used for reasons including simplicity of operation, ease of design and universal in application23. Various conventional adsorbents, including diatomitete24, alumina25-27, silica28,
29
,
activated carbon26-29, etc., have been tested for the removal of sulfones from oxidized
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transportation fuels. Recently, mesoporous materials have drawn increasing attention thanks to their large surface areas and controlled pore size distributions. Two typical mesoporous materials were studied by Nanoti et al.30 for sulfone removal from an oxidized diesel fuel. The adsorption amounts over them were found to be at least two times higher than commercial silica gel and activated carbons30. Pristine or modified mesoporous materials were also widely used as catalysts or supports for oxidative desulfurization of transportation fuels31-36. In these reports, the resulting sulfones were simultaneously adsorbed and thus removed from the liquid fuels.
Although mesoporous materials have been widely employed for sulfone removal, these materials were also used and found to be good adsorbents for direct adsorptive adsorption of pristine sulfur compounds37-44. Thus, it is important to compare the two processes and figure out whether the oxidation process is necessary. However, simply comparing the results reported in previous works is difficult because the experimental conditions used by various researchers are always different, and all the conditions including adsorption temperature, adsorbent dosage and initial sulfur concentration will influence the performance of the adsorbent. To the best of our knowledge, no work has been done to compare the adsorption behaviors of both pristine and oxidized sulfur compounds over mesoporous materials under identical conditions. Besides, arenes always present in real fuels45, and the amount of arenes may vary among different fuels. Although it is well known that hydrocarbon composition of the fuel is an important factor for adsorption of pristine sulfur compounds (such as DBT), no work has been done to investigate the effect of arenes on adsorption of sulfones (such
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as DBTO2).
In the current work, we selected DBT (a typical sulfur compound) and DBTO2 (oxidation product of DBT) as the model compounds and performed the adsorption of them over two mesoporous materials (CMK-3 and MCM-41) under exact the same conditions. The adsorption behaviors of both DBT and DBTO2 over the two adsorbents were found to be significantly different. Furthermore, model fuels with varied hydrocarbon compositions were employed to investigate the impact of arene amounts on the adsorption of both DBT and DBTO2. The desulfurization of real fuels and the regeneration of the adsorbents were also inspected.
EXPERIMENTAL SECTION
Materials
DBT and DBTO2 were obtained from J&K Chemical Ltd. (Beijing, China). N-octane and para-xylene were purchased from Tianjin Jiangtian Chemical Industry (Tianjin, China). Commercial MCM-41 and CMK-3 were obtained from Nanjing Xianfeng Nano Com. Ltd. The adsorbents were dried at 120°C overnight before use. Model fuels were prepared by dissolving DBT or DBTO2 into para-xylene or mixtures of para-xylene and n-octane. The initial sulfur concentration was fixed at 100 ppmwS in all cases unless otherwise stated. A RP-3 jet fuel and a real diesel fuel were obtained from Beihang University and PetroChina Dagang Petrochemical Co., respectively.
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Characterization
The morphologies of the samples were characterized by a transmission electron microscopy (TEM, Tecnai G2 F20). Texture properties of the adsorbents were assessed from nitrogen adsorption−desorption isotherms measured at −196 °C on a Micromeritics ASAP 2020. The specific surface area (SBET) was calculated by the Brunauer−Emmett−Teller (BET) method. The multi-point method was adopted when calculating surface areas, and the pressure range was carefully selected to ensure positive C value and large correlation coefficient (>0.9999). For the pore size distributions and the volumes of pores smaller than 1 nm (V
