Combined ExtractionâOxidation System for Oxidative Desulfurization

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Combined extraction-oxidation system for oxidative desulfurization (ODS) of a model fuel. Yajie Tian, Yue Yao, Yanhui Zhi, Lijun Yan, and Shuxiang Lu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502396b • Publication Date (Web): 15 Jan 2015 Downloaded from http://pubs.acs.org on January 20, 2015

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Combined extraction-oxidation system for oxidative desulfurization (ODS) of a model fuel. Yajie Tian,† Yue Yao,† Yanhui Zhi,† Lijun Yan*,‡, Shuxiang Lu*,† †

School of Materials Science and Chemical Engineering, Tianjin University of

Science and Technology, Tianjin 300457, People’s Republic of China ‡

Petrochemical Research Institute, PetroChina Co. Ltd., Beijing 100195, People’s

Republic of China ABSTRACT: An efficient extraction-oxidation catalytic system for deep desulfurization of a model fuel was explored. First, Mo/γ-Al2O3 catalysts were prepared using an impregnation method and were characterized using X-ray diffraction, temperature-programmed reduction and N2 physical adsorption isotherms. Acetonitrile, methanol, N,N-dimethylformamide, N-methylpyrrolidone and H2O were added to investigate the influences of different extracting agents for extraction-oxidation desulfurization. Acetonitrile showed synergistic action for dibenzothiophene oxidation and a comparatively low dissolving capacity for aromatic compounds. Under n(H2O2)/n(S) molar ratio of 2.3 and v(oil)/v(acetonitrile)=3:1, benzothiophene, dibenzothiophene and 4,6-dimethyldibenzothiophene were almost completely removed in the presence of the 16 wt.% Mo/γ-Al2O3 catalyst over 40 min at 333 K. A reaction pathway based on extraction-oxidation was proposed in which the sulfur compounds were transferred to the extracting phase before oxidizing to form sulfones in the extracting phase. The extraction process was rapid relative to the oxidation process. The advantages of the extraction-oxidation catalytic system are that a high sulfur removal can be achieved under lower n(H2O2)/n(S) molar ratio and that the stability of the catalyst is significantly improved. 1. Introduction 1

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Driven by environmental considerations, the deep removal of sulfur-containing compounds from fuel oils has attracted increasing attention. New and more stringent specifications for sulfur in fuel have been established in different countries, such as sulfur concentrations of less than 10 ppmw are projected in Europe and the sulfur level in diesel fuel should not exceed 15 ppmw according to the U.S. guidelines.1,2 Hydrodesulfurization (HDS) is a conventional process that is highly efficient for removing thiols, mercaptan and disulfides from gasoline, diesel and other intermediate distillates. However, aromatic sulfur

compounds,

such

as

dibenzothiophene

and

its

derivatives

(such

as

4,6-dimethyldibenzothiophene), that are present at high proportions in fuel oils are minimally desulfurized by HDS due to their steric hindrance.3 To produce ultra-clean oils, severe operating conditions, such as high temperatures, low space velocities, high hydrogen pressures and highly active catalysts, are required.4,5 Therefore, many new techniques for ultra-deep desulfurization have been explored, such as oxidation,6-9 extraction,10,11 adsorption,12,13 biodesulfurization,14 and desulfurization using ionic liquids.15,16 Among these techniques, oxidative desulfurization (ODS) is considered to be one promising new method for the deep desulfurization of fuel oil because sulfur compounds, which are the most difficult to eliminate by HDS, are the most reactive components in ODS.17 Among many kind of catalysts, the supported molybdenum catalysts are active and widely used for ODS.18-21 Hydrogen peroxide is usually used as a promising oxidizing reagent because it is cheap, nonpolluting, is not strongly corrosive and is commercially available. 22 In general, ODS requires two steps, the organic sulfur compounds are oxidized to their corresponding sulfones, and these products are removed by extraction, adsorption, distillation or decomposition.23,24 To achieve good mixing between the hydrogen peroxide, catalyst and oil, strong 2


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stirring or ultrasound is necessary. 25 It is difficult to create continuous processes for industrial applications because of drawbacks involving product separation and process economics. Meanwhile, catalysts are apt to rapid deactivation due to metal leaching and sulfone adsorption. 26 In recent years, the development of reactive separation processes has received considerable attention.27-29 The combination of separation and reaction in a single unit can improve conversion and lead to the simultaneous separation of the product. Extraction combined with reaction bypasses the chemical reaction equilibrium limitation and reduces the number of unit operations required. Sengupta et al.30 used titanium-beta catalysts as catalysts for oxidative desulfurization coupled with extraction and concluded that simultaneous oxidation-extraction processes resulted in a significant amount of sulfur removal. In this work, a combined extraction-oxidation system for oxidative desulfurization (ODS) of a model fuel was performed in a batch reactor with Mo/γ-Al2O3 as catalysts. Different types of extracting agents, including acetonitrile, methanol, N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP) and H2O, were studied to determine the effects of extracting agents for ODS. Different reaction conditions (i.e., n(H2O2)/n(S), v(oil)/v(acetonitrile)) were evaluated based on the removal of dibenzothiophene. For aromatic compounds in real fuels have analogous structures with aromatic sulfides, xylene was selected as a model aromatic compound to study its influence on extraction-oxidation desulfurization. In addition, the stability of the catalyst in the extraction-oxidation catalytic system was determined. Finally, a reaction pathway of combined extraction-oxidation system for oxidative desulfurization was proposed. 2. Experimental 2.1. Materials 3


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All of the purchased and obtained materials were used without further treatment. The γ-Al2O3 was purchased from CNOOC Tianjin Chemical Research & Design Institute, China, and the (NH4)6Mo7O24·4H2O (99 +%) was purchased from the Tianjin Fuchen Chemical Company Co., China. Analytical standards of thiophene (Th, liquid 99 +%), benzothiophene (BT, powder 99 +%), dibenzothiophene (DBT, powder 99 +%) and 4,6-dimethyldibenzothiophene (4,6-DMDBT, powder 99 +%) were purchased from Adamas Reagent Co., Ltd., Shanghai, China, and were used as model sulfides. n-Octane (99 +%) was obtained from the Tianjin Bodi Chemical Company Co., Ltd., China, and was used as a model oil. Hydrogen peroxide (30 wt.%, Tianjin Jiangtian Technology Company, Ltd., China) was used as an oxidant. Analytical standards of acetonitrile, methanol, DMF, NMP and xylene, were purchased from the Tianjin Jiangtian Technology Company Ltd., China. 2.2. Catalyst preparation The Mo/γ-Al2O3 catalysts were prepared using γ-Al2O3 as an alumina support and by using incipient wetness impregnation with (NH4)6Mo7O24·4H2O as a molybdenum precursor. A typical synthesis procedure is described here. For 10 wt.% Mo as an example, 3.0 g of γ-Al2O3 (80-120 mesh size) was mixed with 5 mL aqueous solution containing 0.6495 g (NH4)6Mo7O24·4H2O. The mixture was left in an open vessel at 303 K for 12 h to allow the excess water to evaporate. Next, the substrate was dried at 373 K for 12 h and calcined at 773 K for 4 h in air. The catalyst was obtained. 2.3. Characterization The Mo/γ-Al2O3 catalysts were characterized by X-ray diffraction (XRD) using a Bruker D8-Focus diffractometer (Germany) running at 40 kV and 40 mA and using Cu Kα radiation. The scan speed and scan step were 5°/min and 0.02°, respectively. The scanning angles 2θ ranged from 10° to 80°. The N2 physical adsorption isotherms were measured using a Quantachrome Autosorb-1 physical 4


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adsorption apparatus (USA) at 77 K. The samples were outgassed under vacuum at 573 K for 4 h before performing the measurements. The surface areas of the catalysts were calculated using the BET (Brunauer-Emmett-Teller) equation. In addition, the total pore volume was calculated from the amount of N2 adsorption. Fourier transform infrared (FT-IR) spectra were recorded over a range of 4000-400 cm-1 on a Thermo Nicolet Nexus 470 spectrometer (Bruker, Germany) using a KBr disc. Temperature-programmed reduction (H2-TPR) was carried out from 323 K to 1273 K using freshly calcined catalysts and a Quantachrome TPR/TPD analyzer. Prior to analysis, the catalysts were pretreated at 573 K for 30 min by passing them through a helium stream. The tests were carried out by using a heating rate of 10 K/min under flowing 9.7% H2 in Ar. The consumption of H2 was measured using a thermal conductivity detector (TCD). Molybdenum content of the synthesized samples was measured by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) analysis collected in a Leeman Prodigy-H system. 2.4. Oxidative desulfurization The sulfur compounds were dissolved in n-Octane to produce model fuel with a sulfur concentration of 320 ppmw. The desulfurization experiments involving extraction only, oxidation only and simultaneous extraction-oxidation were performed in a 50 mL round-bottom flask fitted with a magnetic stirrer and a condenser (Figure S1 provided in the Supporting Information). In the oxidation experiment, 0.2 g of catalyst, 15 mL of model fuel (320 ppmw S) and 5 mL of extractant (or not) were added into the reactor and stirred at 450 rpm. When thermal equilibrium was reached, 25μL H2O2 (n(H2O2/n(S) = 2.3) was introduced. In the extraction experiment, the extracting solvent and model fuel were mixed together by stirring for 30 min. Next, the contents of the flasks were left undisturbed to achieve phase separation (Figure S2 provided in the Supporting Information). All of the samples that 5


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contained DBT in two phases were collected for analysis in a GC-HP6890 (Agilent Technologies, USA) equipped with an HP-5 capillary column (30 m × 0.25 mm, 0.32 μm film thickness) and a flame ionization detector. The analysis conditions were as follows: injection port temperature, 553 K; detector temperature, 593 K; chromatographic column temperature program, 373 K, holding for 2 min, 373 K to 573 K at 20 K/min, holding for 10 min (carrier gas, nitrogen; injection volume of sample, 1 μL). The removal of the sulfides was calculated as follows:

X

C0  C t  100% C0

where C0 is the initial concentration of sulfur in the model fuel and Ct is the concentration of sulfur in the oil phase after the reaction began for a certain amount of time. 3. Results and discussion 3.1. Characterization of the catalysts Figure 1 presents the XRD patterns of the γ-Al2O3-supported Mo catalysts under various loadings. The X-ray diffraction patterns of all of the samples exhibited broad peaks that corresponded to the microcrystallites of γ-Al2O3.31 A trace of microcrystallite Al2(MoO4)3 could be observed when molybdenum loading exceeded 10% at 2θ = 14.1°, 23.4° and 28.0° (JCPDS 23-0764). Crystalline MoO3 was only present at high loading rates of 30% and 40% Mo/γ-Al2O3, and it appeared at 2θ = 23.0°. The absence of MoO3 species and some diffraction peaks of Al2(MoO4)3 in 10%, 16% and 26% Mo catalysts indicate the strong interaction of molybdenum with Al2O3 resulting in well dispersed molybdenum oxide.32 These species are likely present as an amorphous phase or as too small crystallites to be detected by XRD analysis. Crystalline structures of MoO3 and Al2(MoO4)3 become obviously with the increase of molybdenum loading in catalyst. The crystallite of Al2(MoO4)3 presented peaks at 2θ = 14.1°, 15.5°, 21.0°, 22.2°, 23.4°, 26.3°, 28.0° and 30.8° which was also 6


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observed by Wang et al.33 The N2 physical adsorption results (shown in Table 1) indicated that the support possessed a high surface area of 284 m2/g. With the increase of the molybdenum content, the surface areas of the catalysts decreased. Meanwhile, as the molybdenum content increased, the total pore volume gradually decreased. Figure 1 should be placed here Table 1 should be placed here The catalysts were also characterized by H2-TPR, as shown in Figure 2. Compared with the γ-Al2O3, the Mo/γ-Al2O3 catalyst yielded a tri-peak pattern. The first peak (between 723 and 823 K) was characterized in all of the catalysts, and it indicated a strong interaction of molybdenum containing species with Al2O3. This peak is generally associated with the reduction of Mo6+ to Mo4+ in dispersed polymeric molybdenum structures.34 In the Al2O3-supported 26% and 30% Mo catalysts, a second peak was observed between 823 and 923 K. This temperature window represents the dispersed MoO 3 phase at higher loadings because of the decreased interaction between MoO3 with Al2O3 as the loading rate increases which was also observed from XRD analysis of 30% Mo/γ-Al2O3.35 The third peak for the Al2O3-supported catalysts represented the Al2(MoO4)3 phase.36 Figure 2 should be placed here 3.2. Catalytic evaluation In this work, DBT was chosen as a typical sulfide to investigate the catalytic performance of Mo/γ-Al2O3 using hydrogen peroxide as the oxidant in a combined extraction-oxidation system with acetonitrile as the extractant. The effect of the molybdenum content in the catalyst on eliminating sulfur was evaluated at 333 K 7


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with an n(H2O2)/n(DBT) molar ratio of 2.3 (Table S1 is provided in the Supporting Information). The removal of DBT was enhanced from 68.33% to 99.6% over 60 min as the molybdenum content increased from 5% to 16%. However, catalysts with a molybdenum content greater than 16 wt.% showed a decreasing trend for DBT elimination. Generally, the catalytic oxidation activity depends on the structures of these materials and on the amount of molybdenum adsorbed by the support. At low molybdenum loading (10 or 16 wt.%), the Mo/γ-Al2O3 presented well dispersed molybdenum containing species including Al2(MoO4)3 and undetermined molybdenum oxide. With the increase of molybdenum loading, structure of MoO3 and Al2(MoO4)3 become obviously. The combination of XRD and H2-TPR along with DBT oxidation data indicated that interaction of molybdenum species with the support is vital for oxidation activity. Aubry et al. have reported that MoO42- reacts catalytically with H2O2 in basic aqueous solution to form singlet oxygen in quantitative yield through the intermediary of a diperoxomolybdate specie.37,38 In addition, Wang et al. have proposed a peroxidic oxidation mechanism of DBT on MoO3 catalyst with t-BuOOH as oxidant.18 MoO3 could also act as active phase that generates peroxide for oxidation of sulfur compounds.39 Based on the results of literatures and our research, Al2(MoO4)3 could be considered as the active phase in ODS. Meanwhile, there may exist molybdenum species including high dispersed MoO3 in 16 wt.% Mo/γ-Al2O3 which could also contribute to sulfur oxidation. Because hydroperoxymolybdate group can be formed by MoO3 and singlet oxygen can be formed by Al2(MoO4)3 for oxidation of sulfur compounds.18,37-39 The effects of different catalyst doses on the elimination of DBT are shown in the Supporting Information (Figure S3). These results indicated that the removal of DBT increased with weight, ranging from 0.05 g to 0.2 g. In addition, 0.2 g or 0.3 g of Mo/γ-Al2O3 could yield nearly the same sulfur elimination. Increasing catalyst quantity enhanced the removal of sulfur because the 8


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concentration of the catalytically active species increased. However, further increases in the concentrations of the active species had negligible effects on the sulfur removal. The phenomenon could be related that the surface density of H2O2 adsorbed on the catalysts decreased as the catalyst dosage increased, which in turn restricted the reaction activity.40 The temperature effect was studied in the 303-353 K range (Figure S3 in the Supporting Information). The sulfur elimination increased between 303 K and 343 K and then declined when the temperature was increased to 353 K. Higher temperatures resulted in a decrease in the amount of sulfur eliminated, most likely due to the decomposition of H2O2 or the catalyst deactivation. Figure 3 displays the oxidation of DBT under different n(H2O2)/n(DBT) ratios with 16% Mo/γ-Al2O3 as the catalyst at 333 K. The results indicated that an increase in n(H2O2)/n(DBT) in the range of 2.3-4.6 did not significantly influence the elimination of sulfur. It should be noted that the DBT could be completely removed when an n(H2O2)/n(DBT) ratio of 2.3 was used in the extraction-oxidation system. Nevertheless, when the DBT is completely converted to DBTO2, the stoichiometric ratio of n(H2O2)/n(DBT) is 2. Figure 3 should be placed here 3.3 Extractant evaluation The effects of the different extractants on the elimination of DBT during the extraction-oxidation process were evaluated (Table 2). The use of polar solvents improved the DBT elimination relative to oxidation only. Mo/γ-Al2O3 is hydrophilic in nature due to the presence of the surface hydroxyl (-OH) groups that made it reside in polar phase.41 According to the theory of “like dissolves like”, a portion of the sulfides could be extracted by the extracting agent and come in contact with the catalyst. In addition, the primary oxidation products (sulfones) were highly polarized compounds that easily dissolved in the extractant. According to Ramí rez-Verduzco et al.,42 the order of sulfur elimination during the extraction 9


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process using a 1:1 volume ratio of solvent:diesel fuel at 293 K was DMF > γ-butyrolactone > 2-ethoxyethanol > acetonitrile. In our study, regarding the extraction only of DBT, the order of sulfur elimination was NMP (76.13%) > DMF (67.24%) > acetonitrile (28.56%) > methanol (13.93%) >> H2O (0). A high Nernst partition coefficient (KN = CDBT,E/CDBT,O, CDBT,O: sulfur content in the oil phase, CDBT,E: sulfur content in the extractant phase) for DBT extraction was observed using NMP (3.19) and DMF (2.05) as extractants. Higher KN values represent better desulfurization efficiencies. Nevertheless, in extraction-oxidation combined systems, the sequence of DBT elimination decreased as follows: acetonitrile (99.6%) > methanol (89.13%) > NMP (76.27%) > DMF (67.45%) >> H2O (0) at 333 K. A comparison of the results for extraction only and extraction-oxidation showed that the conversion of DBT only resulted from the extraction process when NMP and DMF were used as extractants. Almost no oxidation activity was detected when comparing extraction only and extraction-oxidation using the two aforementioned solvents, as shown in Table 2. Tests for DBT adsorption on Mo/γ-Al2O3 under different organic solvents were performed and are shown in Figure 4. In these tests, different organic solvents containing DBT (320 ppmw) and 0.2 g of catalyst were mixed together by stirring for 60 min until the adsorption equilibrium was achieved. After phase separation achieved, concentration of DBT in organic solvents was detected by GC-FID. The reduced amount of DBT in solvent was deemed as “adsorbance” on catalyst. In this case, the DBT in the DMF and NMP exhibited less adsorbance than the DBT in methanol and acetonitrile on Mo/γ-Al2O3, which likely contributed to the lack of DBT oxidation that was observed when NMP and DMF were used as the solvent. In fact, the adsorption equilibrium is established by the relative chemical potential of the solvated DBT compared with that of the adsorbed DBT. The NMP and DMF reduced the chemical potential of the solvated DBT and could drive the equilibrium away from adsorption. Aromatic compounds in real fuels have structures that are 10


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analogous to DBT. If these aromatic compounds are also easily extracted by the solvent, this could decrease the oil recovery. Xylene dissolved in n-Octane (10% volume ratio) was selected as a model aromatic compound to study the extraction capacities of different solvents (analysed by GC). As shown in Figure 5, compared with DMF and NMP, the acetonitrile and methanol resulted in lower xylene dissolution, which could reduce fuel loss. Figure 4 should be placed here Figure 5 should be placed here Table 2 should be placed here The addition of acetonitrile and methanol could significantly improve the elimination of DBT when using a 3:1 volume ratio of model oil:solvent. In addition, a higher DBT removal was achieved when using acetonitrile as an extracting agent compared with using methanol. It is possible that the oxidation reaction occurred through a nucleophilic path in which the nucleophile was more reactive in the solvents with higher dielectric constants than in the solvents with lower dielectric constants (ɛacetonitrile = 37.5 > ɛmethanol = 32).43,44 No conversion of DBT was observed when using H2O as an extractant. Almost no DBT was extracted into H2O because H2O molecules are more polar than DBT. Thus, H2O is totally ineffective if the DBT cannot move into the same phase as the catalyst. Acetonitrile is an appropriate solvent for synergistic action in the extraction-oxidation of DBT and has a comparatively low dissolving capacity for aromatic compounds. Because the aromatic content of real fuels may compete with the aromatic sulfur compounds for active sites on the catalyst surface, different volume ratios of xylene were added to the model fuel to determine the effects of aromatic compounds in the reaction. As shown in Table 3, xylene does not influence the relative reactivity of the sulfur compounds toward oxidation. In addition, Filippis et al.45 11


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observed that a mixture of xylene and DBT resulted in a high conversion of DBT when using H2O2-formic acid as the oxidant. Table 3 should be placed here Different v(oil)/v(acetonitrile) ratios for DBT removal were recorded to understand the effects of the extraction agents (Figure S4 provided in the Supporting Information). Compared with oxidation only, the addition of an extracting agent significantly improved the conversion of DBT. An increase in the acetonitrile content produced a slight increase in the sulfur elimination. Meanwhile, the removal of DBT was large when a v(oil)/v(acetonitrile) ratio of 3 was used. The effects of temperature on the extractive desulfurization of model fuel containing DBT with a model fuel to acetonitrile ratio of 3:1 (v/v) by varying the temperature from 313 to 343 K are shown in Figure S5 and are provided in the Supporting Information. At equilibrium and at different temperatures, CDBT,E was a linear function of CDBT,O. A mild decreasing trend was observed for KN with increasing temperature during the extraction process. A similar trend was also reported elsewhere.46 The variations of CDBT,E/CDBT,O with time were studied during extraction-oxidation and are shown in Figure 6. The CDBT,E/CDBT,O ratio was maintained near the equilibrium constant at 333 K (KN = 0.4025, v(oil)/v(acetonitrile) = 3/1) during the reaction for different initial DBT concentrations. Thus, the extraction process could be considered a rapid step compared with the oxidation reaction, and the entire reaction rate could be determined by the oxidation process. Figure 6 should be placed here 3.4 Oxidation of different sulfides The removal of the typical organic sulfur compounds when using acetonitrile as the extractant at 333 K and an n(H2O2)/n(S) ratio of 2.3 is shown in Figure 7. The extraction-oxidation combined 12


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system presented a high catalytic activity for different organic sulfides. The order of reactivity for the sulfur compounds was the same as the electron density of these compounds, which was 4, 6-DMDBT > DBT > BT > Th. The same order was observed by García-Gutiérrez in a Mo/Al2O3-H2O2 system (4, 6-DMDBT > DBT > BT > 2,5-DMT > Th).20 In addition, the extraction-oxidation combined system that used acetonitrile as the extracting agent and Mo/γ-Al2O3 as the catalyst exhibited a higher desulfurization efficiency for different sulfur compounds compared with a previous study (as shown in Table 4). 9,30, 47-50 Figure 7 should be placed here Table 4 should be placed here 3.5 Catalyst stability Finally, we carried out catalyst reuse experiments. The filtered catalysts were dried at room temperature after reaction and were reused without further treatment. Table 5 shows the removal of DBT using a series of Mo/γ-Al2O3. The catalysts used in oxidation only without acetonitrile gradually lost their catalytic activity as the use time increased. However, in the combined extraction-oxidation system, the spent catalysts showed a high catalytic activity that was nearly the same as that observed when fresh catalyst was used. The leaching of molybdenum by the catalysts was tested by ICP-OES. The results were 5.40%, 1.42%, 0.24% and 0.12% during the third, fourth, fifth and sixth uses of Mo/γ-Al2O3, respectively. This result indicated that the unsteady Mo species could be leached during the reaction and that the influence of molybdenum loss could be ignored. In contrast, the FT-IR spectra of the different catalysts are presented in Figure 8. The S-O double bond appeared in the spent catalyst during the single-oxidation process, which indicated the existence of DBTO2 adsorption.51 After adding acetonitrile into the reaction system, the S-O double bond disappeared from the spectrum. Thus, the 13


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adsorption of the product (DBTO2) could be considered the main factor that caused a decrease in the catalytic activity. Jia et al. 50 suggested that the catalytic activity could be recovered though methanol washing at 333 K in ODS using Mo/γ-Al2O3 as a catalyst. Similarly, in the extraction-oxidation system, the sulfone products could be washed from the catalyst in the presence of the extractant during oxidation, which renewed the catalytic activity. These measurements showed that there was no difference between the fresh and spent catalysts, which indicated that no catalytic activity was lost, despite the observed Mo leaching. Table 5 should be placed here Figure 8 should be placed here 3.6 Reaction Pathway The reaction mixture applied herein is a typical system consisting of a polar solvent (acetonitrile) and a nonpolar phase (n-Octane). In this system, DBT preferentially resides in the nonpolar phase, whereas the oxidants and catalyst primarily reside in the polar phase. Based on our results, a plausible reaction pathway is presented in Scheme 1. In the first step, organic sulfides were extracted into the extractant solution. Then, the sulfides reacted with H2O2, which was catalyzed by the catalyst in the second step. The liquid-liquid extraction and the liquid-solid reaction occurred simultaneously in the combined extraction-oxidation system. With the generation of sulfone products, the sulfide extraction equilibrium was disturbed. More sulfides were transferred to the extracting agent with the aid of reduced sulfide concentrations in the extractant until the end of the reaction. Scheme 1 should be placed here 4. Conclusions This study presents an approach for deep desulfurization of a model fuel using a combined 14


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extraction-oxidation system. The proposed reaction system exhibits a high oxidation activity for sulfur compounds. In addition, the system simplifies reaction procedures and extends the catalyst life significantly. Mo/γ-Al2O3 catalysts are useful for oxidizing sulfur compounds when hydrogen peroxide is used as the oxidant. High oxidation activities were obtained for the various sulfur compounds in the model fuel at 333 K (over 0.2 g of 16 wt.% Mo/γ-Al2O3 catalyst). An analysis of the catalyst by XRD and H2-TPR revealed that Al2(MoO4)3 and high dispersed Mo containing species could possess catalytic activity for oxidation of DBT. The use of extracting agents increased the conversion of DBT. In addition, NMP, DMF and H2O showed no synergistic actions within the extraction-oxidation system. Overall, acetonitrile and methanol were demonstrated to be optimal extractants. Acetonitrile is more appropriate as an extractant due to its synergistic action in the oxidation of DBT and its comparatively low dissolving capacity for xylene, which is used as an aromatic compound. Xylene does not influence the relative reactivity of the sulfur compounds toward oxidation. When using an n(H2O2)/n(S) molar ratio of 2.3 and a v(oil)/v(extractant) ratio of 3/1, BT, DBT and 4,6-DMDBT were almost completely removed. In the combined extraction-oxidation system, the catalytic life of Mo/γ-Al2O3 was prolonged because the produced sulfones could be washed away from the catalyst surfaces in the presence of the extractant. This study demonstrates that extraction processes and oxidation processes occur within the system and that the extraction process is a rapid step. Thus, the combined extraction-oxidation system is feasible and effective for deep oxidative desulfurization of a model fuel. ACKNOWLEDGMENTS 15


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Special thanks are extended to the Tianjin Research Program of Application Foundation and Advanced Technology (Nos. 14JCZDJC40600); Tianjin College Science & Technology Developing Fund (Nos. 20140502); the National Undergraduate Innovation and Entrepreneurship Training Project (Nos. 201410057043) for providing funding and support for this research. ASSOCIATED CONTENT Supporting Information More detailed information describing the reaction vessel under stirring and the phase conditions after the reaction is included in this section. In addition, more information regarding the following results is included: the oxidative removal of DBT when using the different Mo-loading catalysts, the effects of catalyst dosage and reaction temperature on the removal of DBT, the effects of v(oil)/v(acetonitrile) on the removal of DBT and extraction equilibrium of DBT in acetonitrile at different temperatures and different initial concentrations of DBT. This information is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Shuxiang Lu). [email protected] (Lijun Yan) Notes The authors declare no competing financial interests. References (1) Hasan, Z.; Jeon, J.; Jhung, S. H. J. Hazard. Mater. 2012, 205-206, 216-221. (2) Zhu, W. S.; Li, H. M.; Jiang, X.; Yan, Y. S.; Lu, J. D.; He L. M.; Xia, J. X. Green Chem. 2008, 10, 16


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641-646. (3) Lu, H. Y.; Gao, J. B. Jiang, Z. X.; Jing, F.; Yang, Y. X.; Wang, G.; Li, C. J. Catal. 2006, 239 (2), 369-375. (4) Ko, N. H.; Lee, J. S.; Huh, E. S.; Lee, H.; Jung, K. D.; Kim H. S.; Cheong, M. Energy Fuels 2008, 22 (3), 1687-1690. (5) He, L. N.; Li, H. M.; Zhu, W. S.; Guo, J. X; Jiang, X.; Lu, J. D.; Yan, Y. S. Ind. Eng. Chem. Res. 2008, 47 (18), 6890-6895. (6) Campos-Martin, J. M.; Capel-Sanchez, M. C.; Perez-Presas, P.; Fierro, J. L. G. J. Chem. Technol. Biotechnol. 2010, 85 (7), 879-890. (7) Liu, G. Z.; Cao, Y. B.; Jiang, R. P.; Wang, L.; Zhang, X. W.; Mi, Z. T.; Energy Fuels 2009, 23 (12), 5978-5985. (8) Sundararaman, R.; Ma, X. L.; Song, C. S. Ind. Eng. Chem. Res. 2010, 49 (12), 5561-5568. (9) Zhang, J.; Wang, A. J.; Li, X.; Ma, X. H. J. Catal. 2011, 279 (2), 269-275. (10) Yu, G. R.; Li, X.; Liu, X. X.; Asumana, C.; Chen, X. C. Ind. Eng. Chem. Res. 2011, 50 (4), 2236-2244. (11) Mokhtar, W. N. A. W.; Bakar, W. A. W. A.; Ali. R.; Kadir, A. A. A. J Taiwan Inst Chem Eng. 2014, 45 (4), 1542-1548. (12) Kwon, J. M.; Moon, J. H.; Bae, Y. S.; Lee, D. G.; Sohn, H. C.; Lee, C. H. ChemSusChem. 2008, 1 (4), 307-309. (13) Peralta, D.; Chaplais, G.; Simon-Masseron, A.; Barthelet, K.; Pirngruber, G. D. Energy Fuels 2012, 26 (8), 4953-4960. (14) Torkamani, S.; Shayegan, J.; Yaghmaei, S.; Alemzadeh, I. Ind. Eng. Chem. Res. 2008, 47 (19), 17


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7476-7482. (15) Rodríguez-Cabo, B.; Rodrí guez, H.; Rodil, E.; Arce, A.; Soto, A. Fuel. 2014, 117, 882-889. (16) Zhou, M. D.; Meng, W. Y.; Li, Y.; Wang, Q.; Li, X. B.; Zang, S. L. Energy Fuels 2014, 28 (1), 516-521. (17) Han, X. H.; Wang, A. J.; Wang X. S.; Li, X.; Wang, Y.; Hu, Y. K. Catal Commun. 2013, 42, 6-9. (18) Wang, D.;Qian, E. W.; Amano, H.; Okata, K.; Ishihara, A.; Kabe T. Appl Catal., A. 2003, 253 (1), 91-99. (19) Li, L. C.; Zhu, Y. D.; Lu, X. H.; Wei, M. J.; Zhuang, W.; Yang, Z. H.; Feng X. Chem. Commun. 2012, 48, 11525-11527. (20) García-Gutiérrez, J. L.; Fuentes, G. A.; Hernández-Terán, M. E.; Murrieta, F.; Navarrete, J.; Jiménez-Cruz, F. Appl Catal., A. 2006, 305 (1), 15-20. (21) Prasad, V.V.D.N.; Jeong,K.; Chae,H.; C, Kim.; Jeong, S. Catal Commun. 2008, 9(10), 1966-1969. (22) Zhang, H. X.; Gao, J. J.; Meng, H.; Lu, Y. Z.; Li, C. X. Ind. Eng. Chem. Res. 2012, 51 (13), 4864-4874.

(23) Stanislaus, A.; Marafi, A.; Rana, M. S. Catal. Today. 2010, 153 (1-2), 1-68.

(24) Srivastav, A.; Srivastava,V. C. J. Hazard. Mater. 2009, 170 (2-3), 1133-1140. (25) Gonzalez, L. A.; Kracke, P.; Green, W. H.; Tester, J.W.; Shafer, L. M.; Timko, M. T. Energy Fuels 2012, 26 (8), 5164-5176. (26) Chica, A.; Corma, A.; Dómine, M. E. J. Catal. 2006, 242 (2), 299-308. (27) Pătruţ, C.; Bîldea, C. S.; Kiss, A. A. Chem Eng Process. 2014, 81, 1-12. (28) Tang, K. W.; Miao, J. B.; Zhou, T.; Liu, Y. B.; Song, L. T. Chem Eng Sci. 2011, 66 (3), 397-404. (29) Lu, S. X; Wang, L.; Wang, Y. Q.; Mi, Z. T. Chem. Eng. Technol. 2011, 34 (5), 823-830. 18


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(30) Maity, U.; Basu, J. K.; Sengupta, S. Fuel Process. Technol. 2014, 121, 119-124. (31) Riou, D.; Leligeny, H.; Pham, C.; Labbe, P.; Raveau, B. Acta Crystallogr., Sect. B: Struct. Sci. 1991, 47, 617-630. (32) Kouachi, K.; Lafaye, G.; Pronier, S.; Bennini, L.; Menad, S. J Mole Catal. A: Chemical., 2014, 395, 210-216. (33) Wang, B. W.; Ding, G. Z.; Shang, Y. G.; Lv, J.; Wang, H. Y.; Wang, E. D.; Li, Z. H; Ma, X. B; Qin, S. D.; Sun, Q. Appl Catal., A. 2012, 431-432, 144-150. (34) Al-Dalama, K.; Stanislaus, A. Thermochim. Acta, 2011, 520 (1-2), 67-74. (35) Atanasova, P.; López Cordero, R.; Mintchev, L.; Halachev, T.; López Agudo, A. Appl Catal., A 1997, 159 (1-2), 269-289. (36) Sundararaman, R.; Song, C. S. Ind. Eng. Chem. Res. 2014, 53 (5), 1890-1899. (37) Aubry, J. M.; Cazin, B. J. Org. Chem. 1988, 27 (12), 2013-2014. (38) Aubry, J. M.; Cazin, B.; Duprat, F. J. Org. Chem. 1989, 54, 726-728. (39) Ishihara, A.;Wang, D.; Dumeignil, F.; Amano, H.; Qian, E. W.; Kabe, T. Appl Catal., A. 2005, 279(1-2), 279-287. (40) Romero, A.; Santos, A.; Vicente, F. J. Hazard. Mater. 2009, 162 (2-3), 785-790. (41) Stencel, J. M.; Makovsky, L. E.; Sarkus, T. A.; Vries, J. D.; Thomas, R.; Moulijn, J. A. J. Catal. 1984, 90, 314-322. (42) Ramí rez-Verduzco, L. F.; Torres-Garcí a, E.; Gómez-Quintana, R.; González-Peña, V.; Murrieta-Guevara, F. Catal. Today. 2004, 98 (1-2), 289-294. (43) Garcí a-Gutiérrez, J. L.; Fuentes, G. A.; Hernández-Terán, M. E.; García, P.; Murrieta-Guevara, F.; Jiménez-Cruz, F. Appl Catal., A. 2008, 334 (1-2), 366-373. (44) Frost, A. A.; Pearson, R. G. Kinetics and Pathway, John Wiley & Sons, Inc., New York, 1961. 19


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(45) Filippis, P. D.; Scarsella, M. Energy Fuels, 2003, 17 (6), 1452-1455. (46) Kȩdra-Królik, K.; Fabrice, M.; Jaubert, J. Ind. Eng. Chem. Res. 2011, 50 (4), 2296-2306. (47) Gonzalez-Garcia, O.; Cedeno-Caero, L. Catal. Today. 2009, 148 (1-2), 42-48. (48) Yu, F. L.; Wang, R. Molecules. 2013, 18 (11), 13691-13704. (49) Zhang, J.; Wang, A. J.; Wang, Y. J.; Wang, H. Y.; Gui, J. Z. Chem Eng J. 2014, 245, 65-70. (50) Jia, Y. H.; Li, G.; Ning, G. L. Fuel Process. Technol. 2011, 92 (1), 106-111. (51) Timko, M. T.; Schmois, E.; Patwardhan, P.; Kida, Y.; Class, C. A.; Green, W. H.; Nelson, R. K.; Reddy, C. M. Energy Fuels 2014, 28 (5), 2977-2983.

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Figure Captions Figure 1. XRD patterns of the catalysts with different Mo loadings. Figure 2. H2-TPR profiles of alumina-supported Mo catalysts at various Mo loadings. a: γ-Al2O3; b: 5% Mo/γ-Al2O3; c: 10% Mo/γ-Al2O3; d: 16% Mo/γ-Al2O3; e: 26% Mo/γ-Al2O3; f: 30% Mo/γ-Al2O3. Figure 3. The effects of n(H2O2)/n(DBT) on the removal of DBT. Reaction conditions: temperature = 333 K; CDBT = 320 ppmw; catalyst dosage = 0.2 g; v(oil)/v(acetonitrile) = 3/1; Mo wt.%=16%. Figure 4. Adsorptions of DBT on Mo/γ-Al2O3 (16 wt.%) in different organic solvents. Reaction conditions: temperature = 333 K; CDBT = 320 ppmw; time = 60 min; v(oil)/v(extractant) = 3/1; catalyst dosage = 0.2 g. Figure 5. The extraction capacity of xylene using different solvents. Reaction conditions: temperature = 333 K; time = 30 min; v(oil)/v(extractant) = 3/1; initial xylene content in the model fuel = 10% (vol). Figure 6. Time-course variation of CDBT, E/CDBT,O for different initial DBT concentrations. Reaction conditions: temperature = 333 K; n(H2O2)/n(DBT) = 2.3; time = 120 min; catalyst dosage = 0.2 g; v(oil)/v(acetonitrile) = 3/1; Mo wt.%=16%. Figure 7. Removal of different organic sulfur compounds. Reaction conditions: temperature = 333 K; n(H2O2)/n(S) = 2.3; C0=320 ppmw; catalyst dosage = 0.2 g; v(oil)/v(acetonitrile) = 3/1; Mo wt.%=16%. Figure 8. The IR spectra of the samples: (A) fresh Mo/γ-Al2O3, (B) spent catalyst without extractant and (C) spent catalyst in the extraction-oxidation system. Scheme 1. The reaction pathway of the extraction-oxidation system.

21


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Figure 1. XRD patterns of the catalysts with different Mo loadings.

22


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Figure 2. H2-TPR profiles of alumina-supported Mo catalysts at various Mo loadings. a: γ-Al2O3; b: 5% Mo/γ-Al2O3; c: 10% Mo/γ-Al2O3; d: 16% Mo/γ-Al2O3; e: 26% Mo/γ-Al2O3; f: 30% Mo/γ-Al2O3.

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Figure 3. The effects of n(H2O2)/n(DBT) on the removal of DBT. Reaction conditions: temperature = 333 K; CDBT = 320 ppmw; catalyst dosage = 0.2 g; v(oil)/v(acetonitrile) = 3/1; Mo wt.%=16%.

24


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Figure 4. Adsorptions of DBT on Mo/γ-Al2O3 (16 wt.%) in different organic solvents. Reaction conditions: temperature = 333 K; CDBT = 320 ppmw; time = 30 min; v(oil)/v(extractant) = 3/1; catalyst dosage = 0.2 g.

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Figure 5. The extraction capacity of xylene using different solvents. Reaction conditions: temperature = 333 K; time = 30 min; v(oil)/v(extractant) = 3/1; initial xylene content in the model fuel = 10% (vol).

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Figure 6. Time-course variation of CDBT, E/CDBT,O for different initial DBT concentrations. Reaction conditions: temperature = 333 K; n(H2O2)/n(DBT) = 2.3; time = 120 min; catalyst dosage = 0.2 g; v(oil)/v(acetonitrile) = 3/1; Mo wt.%=16%.

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Figure 7. Removal of different organic sulfur compounds. Reaction conditions: temperature = 333 K; n(H2O2)/n(S) = 2.3; C0=320 ppmw; catalyst dosage = 0.2 g; v(oil)/v(acetonitrile) = 3/1; Mo wt.%=16%.

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Figure 8. The IR spectra of the samples: (A) fresh Mo/γ-Al2O3, (B) spent catalyst without extractant and (C) spent catalyst in the extraction-oxidation system.

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Scheme 1. The reaction pathway of the extraction-oxidation system.

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Table Captions Table 1. BET surface area and the total pore volumes of the various catalysts. Table 2. The effects of different extractants on the removal of DBT. Table 3. The effects of xylene on the removal of DBT. Table 4. Similar research under different desulfurization conditions. Table 5. Removal of DBT over Mo/γ-Al2O3 with (or without) acetonitrile.

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Table 1. BET surface area and the total pore volumes of the various catalysts. Catalyst

Surface Area,

Total Pore Volume,

(m2/g)

(mL/g)

γ-Al2O3

284

0.5031

10% Mo/γ-Al2O3

252

0.4367

16% Mo/γ-Al2O3

231

0.3665

26% Mo/γ-Al2O3

216

0.2524

30% Mo/γ-Al2O3

83

0.2381

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Table 2. The effects of different extractants on the removal of DBT.

Extractant

Removal of DBT

Removal of DBT

KN

by extraction, (%)

by extraction-oxidation, (%)

None

-

74.22a

-

H2O

0

-

-

Methanol

13.93

89.13

0.16

Acetonitrile

28.56

99.61

0.40

DMF

67.24

67.45

2.05

NMP

76.13

76.27

3.19

Reaction conditions: temperature = 333 K; n(H2O2)/n(DBT) = 2.3; CDBT = 320 ppmw; catalyst dosage = 0.2 g; time = 60 min; v(oil)/v(extractant) = 3/1; Mo wt.%=16%. a

oxidation only followed by extraction: temperature = 333 K; n(H2O2)/n(DBT) = 2.3; CDBT = 320

ppmw; catalyst dosage = 0.2 g; time = 60 min; v(oil)/v(extractant) = 3/1; Mo wt.%=16%.

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Table 3. The effects of xylene on the removal of DBT. Xylene content in the model fuel,

Removal of DBT,

(vol, %)

(%)

1%

99.27

10%

97.84

20%

98.13

Reaction conditions: temperature = 333 K; n(H2O2)/n(DBT) = 2.3; CDBT = 320 ppmw; catalyst dosage = 0.2 g; time = 60 min; v(oil)/v(acetonitrile) = 3/1; Mo wt.%=16%.

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Table 4. Similar research under different desulfurization conditions.

Extracting

Catalyst

agent

acetonitrile

Mo/γ-Al 2O3

V(oil)/V(e

T

n(O)/

Reaction

Sulfur removal, (%)

Ref.

xtractant)

(K)

n(S)

time, (min)

DBT

BT

Th

4,6-DMDBT

3:1

333

2.3

40

>99

>99

≈70

>99

This

work

acetonitrile

Mo(P)/γ-Al2O3

1:1

333

11

60

98.7

64.2

0.2

99.7

20

DMSO

TS-1/Ti-beta

10:1

333

10

120

89

83

92

-

30

none

V2O3

-

333

13

120

≈55

-

-

≈35

47

none

Mo/γ-Al 2O3

-

333

4

60

>99

>99

99

50

none

MgAl-PMo12

-

333

20

180

>99

≈76

≈76

≈84

48

none

[Bmim]3PMo12

-

333

3

120

>99

≈85

-

≈95

9

-

333

3

60

>99

≈80

-

>99

49

O40/SiO2

none

[Bmim]3PW12O

40/SiO2

35


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Table 5. Removal of DBT over Mo/γ-Al2O3 with (or without) acetonitrile. Recycling times

n(H2O2)/n(S) = 2.3 Without acetonitrile

With acetonitrile

X (%)

X (%)

1

97.01

99.95

2

90.32

98.66

3

88.54

98.12

4

87.67

98.20

5

87.31

97.85

6

87.34

97.63

Reaction conditions: temperature = 333 K; n(H2O2)/n(DBT) = 2.3; CDBT = 320 ppmw; catalyst dosage = 0.2 g; time = 60 min; v(oil)/v(acetonitrile) = 3/1; Mo wt.%=16%.

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Combined ExtractionâOxidation System for Oxidative Desulfurization

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