Oxidative Desulfurization of Model Diesel Using Ionic Liquid 1-Octyl

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Oxidative desulfurization of model diesel using ionic liquid 1-octyl-3-methylimidazolium hydrogen sulfate: an investigation of ultrasonic irradiation effect on performance Mahdieh Safa, Babak Mokhtarani, Hamid Reza Mortaheb, and Kurosh Tabar Heidar Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01708 • Publication Date (Web): 02 Nov 2016 Downloaded from http://pubs.acs.org on November 7, 2016

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Oxidative desulfurization of model diesel using ionic liquid 1-octyl-3-methylimidazolium hydrogen sulfate: an investigation of ultrasonic irradiation effect on performance Mahdieh Safa,a Babak Mokhtarani*a ,Hamid Reza Mortaheb**a and Kurosh Tabar Heidara Chemistry and Chemical Engineering Research Center of Iran, P.O. Box 14335-186, Tehran, Iran. KEYWORDS: Oxidative desulfurization, Brønsted acidic ionic liquid, Ultrasonic irradiation, Model oil

ABSTRACT: A Brønsted acidic ionic liquid (IL), 1-octyl-3-methylimidazolium hydrogen sulfate ([Omim][HSO4]), was prepared and utilized as the extractant and catalyst to study the oxidative desulfurization of dibenzothiophene (DBT) in n-decane as the model oil. The effects of alkyl chain length of IL cation, temperature, H2O2/DBT molar ratio (O/S), IL/oil mass ratio, initial S-content, and sulfur species on the sulfur removal of the model oil were investigated. Complete removal of DBT was observed by [Omim][HSO4], O/S molar ratio of 5, and IL/oil mass ratio of 1:2 after 70 min at 25 ˚C. The order of observed oxidizing reactivity for different sulfur

species

was:

DBT

>

benzothiophene

(BT)

>

1 Environment ACS Paragon Plus

thiophene

(TH)

>

4,6-

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dimethyldibenzothiophene (4,6-DMDBT). The IL could be reused six times without a significant decrease in the desulfurization activity. Kinetics of oxidative desulfurization for DBT by [Omim][HSO4] was found to be pseudo-first-order with an apparent rate constant of 0.0734 min-1 (at 298 K) and the apparent activation energy of 24.51 kJ/mol. Ultrasound-assisted oxidative desulfurization (UAOD) process was also applied and represented a high desulfurization performance for the model oil in a fast reaction. The effects of various parameters including irradiation time, settling time, O/S molar ratio, and IL/model oil mass ratio on the UAOD process were studied. The complete sulfur removal efficiency could be reached after 3 min of ultrasonic irradiation with an ultrasonic power of 30 W, ultrasonic frequency of 20 kHz, O/S molar ratio of 5, and IL/oil mass ratio of 1:2. It was observed that the application of ultrasonic irradiation allows desulfurization process to be performed in a shorter time. The sulfur removal of real diesel was 77.2% in the ODS process, and 76.3% in the UAOD process under the optimal conditions.

1. INTRODUCTION Deep sulfur removal of transportation fuels has gained considerable attentions worldwide because combustion of sulfur-containing fuels imposes hazardous effects on environment and human health.1 Since hydrodesulfurization (HDS) as a traditional desulfurization technology has some limitations in reducing refractory aromatic sulfur compounds such as benzothiophene (BT) and dibenzothiophene (DBT), many complementary techniques including bioprocess2, 3, extraction with ionic liquids (ILs)4-8, adsorption9, 10 and oxidation11-13 have been proposed.14


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Recently, oxidative desulfurization (ODS) method in combination with IL extraction has been extensively investigated because of its moderate operation conditions and high sulfur removal efficiency. Moreover, using ILs as alternatives to volatile organic solvents in extractive desulfurization processes is proven to be effective due to their advantages such as wide liquid range, non-flammability, and high thermal stability.15-17 In the ODS process, various researchers have employed ILs as extractants along with catalysts such as organic acids or transition metal salts.18-21 The drawback of the process is separation of catalyst and its partial solubility in oil.22 A series of polyoxometalate-based23-26, Lewis22,

27-29

, and Brønsted acidic30-33 ILs have been

recently applied in the oxidative desulfurization of fuel oils as the extractant/catalyst. Lu and co-workers (2014) employed a protic ionic liquid, 1-methyl-2-pyrrolidonium formate ([Hnmp][HCOO]), as a new extractant and catalyst for oxidative desulfurization of a model diesel. DBT removal could be reached to 99% under the conditions of 50 ˚C, IL/model oil volume ratio = 1:10, O/S molar ratio = 5, and in 3 h. The IL could be recycled five times without any significant decrease in its activity.34 Chen et al. (2014) synthesized a Brønsted-Lewis acidic IL, 1-methyl-2-pyrrolidonium zinc chloride ([Hnmp]Cl/ZnCl2), and used in oxidative desulfurization of a model diesel and FCC diesel fuel. The S-content of the model diesel could be reduced from 500 to 1 ppm (99.9%) at 75 ˚C, IL/oil mass ratio = 1:3, and O/S molar ratio = 8. The sulfur removal of less than 38% in one stage and 83% after five stages could be attained for the real FCC diesel fuel.35 A Brønsted acid IL, 1-butyl-3-methyl-imidazolium trifluoroacetic acid ([Bmim][TFA]), was prepared and used for oxidative desulfurization of thiophene as the extractant and catalyst by


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Fang et al. (2014). The complete sulfur removal was obtained under the conditions of 70 ˚C, and IL/model oil volume ratio =1:35 with 2 mL of H2O2 in 30 min.36 The

iron-based

IL,

1-methyl-3-(trimethoxysilylpropyl)-imidazolium

tetrachloroferrate

([Pmim][FeCl4]) was grafted onto mesoporous SBA-15 and used in oxidative desulfurization of model oil by Ding and co-workers (2014). The DBT removal was 94.3% in the optimum conditions of 30 ˚C and O/S molar ratio = 5 in 90 min with [Omim][BF4] as a reaction media.37 Liang et al. (2012) investigated the oxidative catalytic activity of an acidic IL, 1-octylbenzimidazole acetate ([Otbim][CH3COO]), in desulfurization of thiophene in a model oil. The sulfur removal of 87.5% was obtained under the optimal conditions of 70 ˚C and in oxidation time of 180 min. The IL could be reused for 5 times without a remarkable decrease in its activity.38 Zhu et al. (2013) reported the ionic liquid, N-butyl-pyridinium tetrachloroferrate ([BPy][FeCl4]), representing a high catalytic activity in the oxidative desulfurization process. The

sulfur

removals of DBT, BT, and 4,6-DMDBT were obtained as 95.3%, 75.0%, and 54.8%, respectively, in 10 min. After the reaction, the IL was easily separated from the model oil by applying an external magnetic field.27 A

peroxotungsten

anions-based

ionic

liquid,

1-Hexadecyl-3-methylimidazolium

peroxotungstate ([Hdmim][PyW]), was prepared and used for oxidative desulfurization of DBT by Chen (2015). The DBT removal was 99% at 50 ˚C. The catalyst could be recycled at least ten times with unnoticeable loss in its activity.39 Although the ODS processes by these ILs show high sulfur removal efficiencies, some of them require large amounts of ILs, high temperatures, or long reaction times.


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Since consumption of IL and the reaction time both directly affect the process costs, optimizing the process and finding appropriate conditions to decrease these parameters are worthy tasks. In the present work, an (HSO4)-based IL is applied in the ODS process. The aim is to reduce IL consumption by changing the chain length of IL’s alkyl cation and employing ultrasound to increase the rate of reaction. The reason for choosing this IL is that previous studies have proven the ability of hydrogen sulfate anion for the ODS process. The desulfurization capabilities of ILs with imidazolium as the cation having different anions were found to be decreased in the following order: TFA− > HSO4− > carboxylate group (COO−) > Tetrachloroaluminate (AlCl4−) > Acetoxy group (AcO−). This implies that ILs with a stronger acidity have better desulfurization performances.30, 36, 40 In order to increase the ODS reaction rate and eliminate the mass transfer limitations, many researchers have investigated the ultrasound-assisted oxidative desulfurization (UAOD) process.41-46 The studies have shown that using ultrasound irradiation in the ODS process under mild conditions leads to a considerable increase in the reaction rate and reduction of reaction time.47 This is due to generated cavitation by ultrasonic waves and formation, growth, and the collapse of the bubbles in the liquid. This phenomenon provides a microenvironment with extreme local conditions (high temperature and pressure) creating active intermediates and increasing the reaction rates. Cavitation also creates intense microturbulence that leads to fine emulsion between the phases. The emulsion-like dispersion with very small droplets improves the interfacial area available for the reaction, increases the mass transfer rate in the reaction, and enhances the effective local concentrations of reactive species. 43, 48, 49 However, UAOD processes using ionic liquids were only studied by yen et al. in a complicated process comprising model oil, oxidant, IL, phase transfer agent, and additional catalyst.50


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To the best of our knowledge, this is the first report on ultrasound-assisted oxidative desulfurization with a halogen-free IL without adding an additional catalyst. A Brønsted acidic IL, [Omim][HSO4], was synthesized and utilized as a dual effect extractant/catalyst agent in the oxidative desulfurization of a model oil. Hydrogen peroxide was used as the oxidant in the desulfurization system. There are two parallel reaction pathways: nonproductive decomposition of H2O2 and oxidative desulfurization reaction. Decomposition of H2O2 introduces more water to the system and reduces the concentration of the oxidant that is unfavorable for desulfurization. Increasing oxidant content and temperature may result in oxidizing more sulfur compounds while an excessive amount of H2O2 and high reaction temperature may accelerate decomposition of H2O2. Thus, selecting the optimal amount of H2O2 and operational temperature have important influences on the desulfurization system.34, 39 The effects of parameters such as reaction time, temperature, the molar ratio of O/S, the mass ratio of IL/oil, initial S-content, chain length of IL’s alkyl cation, different sulfur species, and IL recycling on desulfurization performance were investigated. The kinetics of DBT oxidative desulfurization was also studied. The effects of parameters including molar ratio of O/S, mass ratio of IL/oil, irradiation time, and settling time on the UAOD process were also investigated. In addition, the solubility of model oil in the ionic liquid as well as real diesel fuel reactivity in the ODS and UAOD processes were investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. Dibenzothiophene (DBT, 99%), thiophene (TH, 99%), benzothiophene (BT, ≥ 98.0 %), n-decane (99%), hexadecane (99%), hydrogen peroxide (aqueous solution, 30 wt. %),


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and

di

ethyl

ether

(99.7%)

were

purchased

from

Merck

Company.

4,6-

Dimethyldibenzothiophene (4,6-DMDBT, ≥99%) was procured from Haohua Industry Company Ltd., China. A real diesel fuel with initial sulfur content of 746 ppm was supplied from Tehran refinery.

2.2. Apparatus. ODS experiments were performed using a high-speed mechanical stirrer. The UAOD experiments were carried out by a 20 kHz ultrasonic processor with nominal power of 200W manufactured by BANDELIN Company (Model HD 3200, Berlin, Germany) equipped with an ultrasonic probe (ϕ 6 mm, 137 mm length high-grade titanium probe solid). The sulfur content was determined by gas chromatograph (GC; Varian, cp 3800) with a flame ionization detector (FID) and capillary column (Chrompack, 30 m × 0.25 mm × 1.2 µm). Hexadecane was used as the internal standard. The analysis conditions were as follows: nitrogen as the carrier gas; injector temperature, 280 ˚C; detector temperature, 280 ˚C; column temperature, 2 min at 200 ˚C, then heated up to 280 ˚C for 15 minutes with a heating rate of 10.0 ˚C.min-1.

The

sulfur

content

of

real

diesel

fuel

was

analyzed

using

Rigaku

X-ray fluorescence spectrometer (USA). 2.3. Preparation and characterization of ionic liquids. The ILs [Bmim][HSO4] and [Omim][HSO4] were synthesized according to the published procedure by a dropwise addition of concentrated sulfuric acid (9.52 ml, 98%, 171.7 mmol) to a cooled solution of 1-butyl-3methylimidazolium chloride or 1-ocyl-3-methylimidazolium chloride (30 g, 171.7 mmol) in anhydrous methylene chloride.51 The mixture was refluxed for 24h and the produced HCl was neutralized by an aqueous NaOH solution. At the end of the reaction, the solution was cooled to the room temperature and the solvent was removed by a rotary evaporator. The product was


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dried under vacuum at 75 ˚C for 3 h. The physical properties of ILs are reported in Table 1. The water content in the ILs was measured by a 684 Karl Fischer coulometer. The halide contents of the ILs were measured by Ion chromatography. The densities of ILs were measured using an Anton Paar DMA-5000 digital densitometer with an experimental uncertainty of less than + 510-5 g.cm-3. The structures of ILs were identified by 1H NMR. [Omim][HSO4] 1H NMR (500 MHz, DMSOd6): δ (ppm) 9.39 (1H, s), 7.58 (1H, s), 7.44 (1H, s), 5.99 (1H, s), 4.26 (2H, J = 7.0 Hz, t), 4.02 (3H, s), 1.88-1.85 (2H, m), 1.30-1.27 (10H, m), 0.89 (3H, J = 7.0 Hz, t), [Bmim][HSO4] 1H NMR (500 MHz, DMSO-d6) δ (ppm) 9.14 (1H, s), 7.77 (1H, s), 7.70 (1H, s), 7.37 (1H, s), 4.16 (2H, J = 7.2 Hz, t), 3.84 (3H, s), 1.78-1.75 (2H, m), 1.27-1.25 (2H, m), 0.90 (3H, J = 7.2 Hz, t).

2.4. Oxidative desulfurization (ODS) and ultrasound-assisted oxidative desulfurization (UAOD). The model oil was prepared by dissolving 500 ppm DBT in n-decane. The ODS experiments were performed in 10-ml glass vials. The temperature was maintained by an oil bath (+1˚C). The mixture of IL and the model oil was stirred vigorously at 900 rpm for 20 min to reach thermodynamic equilibrium. The phases were split after settling for 5 min. A certain amount of 30 wt% H2O2 was poured into the mixture. The mixture was then stirred again for 45 minutes to perform the oxidation reaction. The samples of upper oil phase were periodically withdrawn and their sulfur content was determined using the gas chromatograph. The UAOD experiment was carried out using a certain amount of model oil, IL ([Omim][HSO4]) and the solution of 30 wt% H2O2, which were mixed in the glass vial by inserting the ultrasonic probe into the mixture under the following conditions: 25 kHz ultrasound frequency, 30 W instrument power, and at the room temperature. The mixture was then settled for 35 min to be


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split into two phases. The phases were separated by decantation and the sample of upper phase was analyzed by the gas chromatograph.

3. RESULTS AND DISCUSSION 3.1. Reaction mechanism. The mechanisms for ODS and UAOD processes are proposed in Scheme 1 and 2. In both processes, aromatic sulfur compounds are extracted first into the IL phase through π-π interaction between the aromatic S-compounds and the IL’s imidazolium ring. At the same time, the Brønsted acidic IL is oxidized to peracid by hydrogen peroxide. The extracted S-compounds are then oxidized to the corresponding sulfone compounds in the reaction with peracid. Since the sulfone compounds have higher polarities than the sulfur compounds, the equilibrium is broken and the sulfur compounds are extracted into the IL phase continuously until the IL is saturated.36, 39, 52 The ultrasonic irradiation provides acoustic cavitation by passing high intensity acoustic waves through the liquid and inducing a bulk pressure gradient. The acoustic cavitation leads to formation and growth of many microbubbles, which the their collapse creates local high temperatures (up to 5000 K) , high pressures (up to 1000 bar), and liquid jets with velocities up to 280 m.s-1.47 The cavitation in a system with two or more immiscible liquid phases causes formation of fine emulsions that increase the local concentration of reactive species and improve the mass transfer between reaction agents. It also creates active intermediates such as hydroxyl radicals that accelerate the reaction in the extreme local conditions of the temperature and pressure. This phenomenon is applied in the ODS process to increase the rate of oxidation in a biphasic system.41, 53


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3.2. Effect of time and alkyl chain length of IL cation on DBT removal. In order to study the effect of alkyl chain length of IL cation on extractive and oxidative desulfurization of the model oil, two ILs of [Omim][HSO4] and [Bmim][HSO4] were investigated. As seen in Figure 1, the initial extractive desulfurization efficiency by [Omim][HSO4] and [Bmim][HSO4] are 44.7% and 29.2%, respectively. The sulfur removal is increased sharply after a certain amount of H2O2 is added into the both systems. The equilibrium is attained after 45 and 90 min and the oxidative desulfurization efficiencies are 100 and 84.5% for [Omim][HSO4] and [Bmim][HSO4], respectively. [Omim][HSO4] represents higher sulfur removal efficiency and approaches to equilibrium in a shorter time than [Bmim][HSO4] under the same operating conditions. This may be ascribed to decreasing the coulombic interaction between ions and providing more spaces between the cation and anion of IL for accommodation of DBT by increasing the size of cation.54 The results are in agreement with the data in our last work and literature.8, 40, 50, 55, 56 A similar result was obtained by Gao et al. using [Bmim][HSO4] in oxidative desulfurization of DBT in n-octane (1000 ppm) where the removal efficiency was 99.6% in 90 min under the conditions of 25 ˚C, model oil/IL volume ratio = 2 (model oil/IL mass ratio≈ 1:1.1), and O/S molar ratio = 5.33

3.3. Effect of O/S molar ratio and temperature on ODS process. The desulfurization experiments by [Omim][HSO4] under various O/S molar ratios (2, 3, 4, 5, 6 and 7) were performed at 25 ˚C. Although, based on the stoichiometric reaction 2 mol of H2O2 is required to oxidize 1 mol of DBT to its corresponded sulfone, an excess amount of H2O2 was consumed to compensate the partial decomposition of H2O2 in the process. Table 2 indicates the effect of O/S molar ratio on DBT removal efficiency. By increasing the O/S molar ratio from 2 to 6, the sulfur removal efficiency is enhanced from 87 to 100%. However,


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when the O/S molar ratio is set to 7, the sulfur removal is decreased to 98%. It may be due to introducing water into the reaction environment and diluting the IL as the result of using an excess amount of H2O2.57 The results also show that the sulfur removal efficiencies with the O/S ratio of 5 and 6 are the same. The O/S molar ratio of 5 was then selected as the optimal value. The effect of temperature on extractive and oxidative desulfurization of DBT by [Omim][HSO4] is shown in Figure 2. As the temperature is increased from 25 to 80 ˚C, the desulfurization efficiency of extractive desulfurization is increased but the efficiency of the ODS process is decreased due to decomposition of H2O2. A similar result was observed in oxidative desulfurization process using [Bmim][HSO4], in which the temperature had a significant effect on the ODS efficiency whereas it was not an effective parameter in the extractive desulfurization.33

3.4. Effects of IL/model oil mass ratio and initial sulfur content on ODS process. The effect of IL/model oil mass ratio on desulfurization is illustrated in Figure 3. As the sulfur removal is increased from 17.6 to 100% by increasing IL/model oil mass ratio from 1:20 to 1:2, the IL/model oil mass ratio of 1:2 was chosen as the optimal ratio. Figure S1 shows the effect of initial sulfur content on DBT removal. The S-removal is slightly increased as the initial sulfur concentration is decreased from 1000 to 500 ppm at 25 ˚C.

3.5. Effect of sulfur species on ODS process. In order to study the effect of different sulfur species on desulfurization process, model oil was prepared by dissolving DBT, TH, BT, and 4,6DMDBT in n-decane with a sulfur content of 250 ppm for each.


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Figure 4 illustrates the ODS efficiency of the different sulfur species by [Omim][HSO4] under the optimal conditions (T, 25 ˚C; O/S (mol/mol), 5; IL/oil (w/w), 1:2). The sulfur removals are decreased in the following order: DBT > BT > TH > 4,6-DMDBT. These results can be attributed to the electron density of the sulfur atom and steric hindrance of alkyl groups in the sulfur compounds. As reported in pervious works, the electron density on the sulfur atom was 5.758 for DBT, 5.739 for BT, 5.696 for TH, and 5.760 for 4,6-DMDBT.58 The desulfurization efficiencies of sulfur compounds DBT, BT, and TH are enhanced by increasing the electron density. However, the removal of 4,6-DMDBT is the lowest because in the extractive desulfurization systems, the steric hindrance of methyl group weakens the interaction energy between the sulfur compound and the IL.27, 33, 59, 60

3.6. Kinetic study of ODS process. Figure S2 shows the sulfur removal versus reaction time at 25, 45, and 60 ˚C. It can be seen that the slope of sulfur removal efficiency follows the order of 60 ˚C > 45 ˚C > 25 ˚C at the first 15 min, 45 ˚C > 60 ˚C > 25 ˚C from 15 to 35 min, and 45 ˚C ≥ 25 ˚C > 60 ˚C after 35 min. This implies that high reaction temperature can simultaneously accelerate the oxidative desulfurization and decomposing of H2O2. The reaction rate was determined by assuming a pseudo-first-order reaction as follows: ln(CA0/CA) = k.t

(1)

where, CA0 and CA are the sulfur contents at the initial and after t min, respectively, and k is the first-order rate constant (min−1). Figure 5 shows the plots of ln(CA0/CA) against t as straight lines with the slope of k. Using Arrhenius equation; Ea =RT2 d lnk/dT

(2)

where, the value of apparent activation energy for ODS reaction by [Omim][HSO4] was estimated as 24.51 kJ/mol (Figure S3). The apparent activation energies for DBT oxidation by


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different systems are listed in Table 3. The activation energy obtained in this work is lower than values in literature that represents a faster DBT oxidation reaction by [Omim][HSO4]. In a recent study, Zhang and co-workers (2012) obtained the activation energy of 40 kJ/mol (oxidative sulfur removal of 99.4%) by [Hnmp][BF4] as a Brønsted acidic ionic liquid (BAIL) that was lower than and activation energy of 76.8 kJ/mol (oxidative sulfur removal of 19% ) by [Bmim][BF4] as a non-Brønsted acidic ionic liquid (non-BAIL).61 Qiu et al. (2009) also showed the activation energy of DBT with conversion rate of 96% catalyzed by [HPMo][HTAC]2 (26.8 kJ/mol) was lower than that of [HPMo][DTAC]2 with conversion rate of 89% (31.4 kJ/mol).62

3.7. Effect of irradiation and settling time on UAOD process. The effect of irradiation time on sulfur removal efficiency was evaluated in the range of 2 to 10 min. The results shown in Figure 6 imply that the extraction efficiency approaches to its maximum value at the irradiation time of 3 min and decreases afterward. The main reason might be rising the temperature and decomposing of H2O2 or degradation of the IL by prolonging the ultrasonic irradiation time. The FT-IR spectra of fresh ionic liquid ([Omim][HSO4]) after 3 and 9 min ultrasound irradiation indicated no change in the structure of ionic liquid (Figure S4). Therefore, it is unlikely that the reduction in sulfur removal is due to ionic liquid degradation. The ultrasonic time of 3 min was determined to be enough for this process. Since the UAOD process is performed at a short time under intense mixing, the mixture needs to be settled for a certain time. Figure S5 indicates the effect of settling time on the phase separation of ultrasound-assisted oxidative desulfurization in different IL/oil mass ratios. The results imply that increasing settling time increases the sulfur removal efficiency. The settling time of 35 min was chosen to guarantee that two phases are entirely split.


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3.8. Effect of O/S molar ratio and IL/model oil mass ratio on UAOD process. Table 4 shows the effect of O/S molar ratio on ultrasound-assisted oxidative desulfurization of DBT from the model oil. It can be seen that the sulfur removal efficiency is enhanced by increasing the relative amount of H2O2. The removal efficiency of 100% is observed when O/S molar ratio is set to 5. For higher values of molar ratio, the removal efficiency is reduced. This decrement might be explained by dilution of the reaction system. A comparison between Table 2 and 4 indicates that the sulfur removals with O/S molar ratios of 2, 3, and 4 in the UAOD process were higher than those in the ODS process under the same operational conditions. This can be related to excessive decomposition of H2O2 in ODS process in the absence of ultrasonic irradiation. In fact, two competing reactions in ODS process may result in reduction of H2O2: nonproductive decomposition of H2O2 to water and molecular oxygen, and oxidation of acidic ionic liquid to form peracid catalyst. Homolytic cleavage of H2O2 by ultrasound to form hydroxyl free radicals result in lower decomposition and clean consumption of H2O2 in the reaction.63, 64 The effect of IL/model oil mass ratio on the sulfur removal efficiency was also investigated. The ratios were chosen from 1:20 to 1:2 for the UAOD process. The experiment results are presented in Figure 7. As depicted in the figure, by increasing the IL/oil ratio from 1:20 to 1:2, the sulfur removal efficiency of the model oil is increased from 26.6% to 100%.

3.9. Solubility of model oil in ionic liquid in ODS and UAOD processes. The solubility of oil in the IL is a key factor in selecting ionic liquid as an extractant because a remarkable solubility of the oil in the IL causes losing the fuel oil and increasing the process cost.65 The solubility of


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the model oil in [Omim][HSO4] was measured using gas chromatography. The ionic liquid phase was diluted with acetone and injected to the GC. The capillary column of the gas chromatograph was protected with a pre-column to avoid ionic liquid reaching the column. The results showed that 1.91 and 1.45 wt% of oil are dissolved in [Omim][HSO4] in ODS (25 ˚C) and UAOD processes, respectively.

3.10. Reusability of IL in ODS and UAOD process. The reusability of [Omim][HSO4] for ODS and UAOD processes was studied. At the end of each reaction, the IL phase was separated by decantation from the oil and washed three times with an equal volume of diethyl ether. Much amount of diethyl ether was firstly separated from the ionic liquid phase by decantation. The IL phase was distillated at 35 ˚C to remove the remained diethyl ether. Then, the residue water and H2O2 were removed under the vacuum at 55 ˚C. To remove the trace amount of water in the IL, it was dried at 100 ˚C under the vacuum. The regenerated IL was used for desulfurization of fresh model oil in the next run. The results in Table 5 indicate that [Omim][HSO4] can be recycled six times without any significant change in the activity of both processes.

3.11. Comparison of the catalytic activity of IL [Omim][HSO4] with different catalysts Table 6 indicates the comparison of oxidative desulfurization activity for [Omim][HSO4] in ODS and UAOD processes with that for some ionic liquids reported in previous works. As can be observed, our proposed desulfurization system particularly for the UAOD process, exhibited a high catalytic activity using a low amount of IL in mild conditions and in a short reaction time.


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3.12. Desulfurization of real diesel fuel in ODS and UAOD processes. Desulfurization efficiencies of the real diesel fuel were investigated in the ODS and UAOD processes using [Omim][HSO4]. The sulfur removal of the real diesel was 77.2% under optimum conditions in the ODS process (25 ˚C; IL/ oil (w/w), 1:2; O/S (mol/mol), 5 and time; 70 min), and 76.3% under the optimum conditions in the UAOD process (IL/ oil (w/w), 1:2; O/S (mol/mol), 5 and time; 3 min). The obtained sulfur removal values were lower than that of the model oil. It can be related to the presence of wide range of sulfur compounds such as thiols in the real diesel that are hardly eliminated with these methods.22, 28

4. CONCLUTION The Brønsted acidic ionic liquids (ILs), 1-octyl-3-methylimidazolium hydrogen sulfate ([Omim][HSO4]) and 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim][HSO4]) were synthesized and served as extractant and catalyst for oxidative desulfurization of DBT from the model oil. It was observed that [Omim][HSO4] can attain higher oxidative sulfur removal efficiencies (100%) than [Bmim][HSO4] (84.5%). The oxidation reaction of DBT by [Omim][HSO4] followed a pseudo-first-order kinetics with an apparent rate constant of 0.0734 min-1 (at 298 K) and the apparent activation energy of 24.51 kJ/mol. The ultrasound-assisted oxidative desulfurization (UAOD) process was also applied to the model oil and presented a higher and faster performance on desulfurization reaction. The effects of various parameters including irradiation time, O/S molar ratio, and IL/model oil mass ratio on the UAOD process were studied. The sulfur removal efficiency could reach to 100% after 3 min of ultrasonic irradiation using an ultrasonic power of 30 W, ultrasonic frequency of 20 kHz, O/S molar ratio of 5, and IL/oil mass ratio of 1:2. The IL could be recycled 6 times without significant loss in its


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Energy & Fuels

activity in the ODS and UAOD processes. The sulfur removal of real diesel was 77.2% in the ODS process, and 76.3% in the UAOD process under the optimum conditions.

ASSOCIATED CONTENT Supporting Information Effect of initial sulfur content on desulfurization process (Figure S1), Sulfur removal versus reaction time at different temperatures (Figure S2), Activation energy for DBT oxidation reaction (Figure S3), FT-IR spectra of fresh [Omim][HSO4], and after 3 and 9 min ultrasound irradiation (Figure S4), and Effect of settling time on UAOD process (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] ** E-mail: [email protected]

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.


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Table captions Table 1. Physical properties of ILs. Table 2. Effect of O/S molar ratio on ODS process. Conditions: IL, [Omim][HSO4]; T, 25 ˚C; IL/model oil (w/w), 1:2; S-content, 500 ppm. Table 3. Apparent activation energies for different DBT oxidation systems. Table 4. Effect of O/S molar ration on UAOD process. Conditions: IL, [Omim][HSO4]; T, 25 ˚C ; IL/oil, 1:2. Table 5. ODS and UOAD of model oil by recovered [Omim][HSO4]. Table 6. Comparison of catalytic activities of IL [Omim][HSO4] with different catalysts. Scheme captions Scheme 1. The proposed mechanism for oxidative desulfurization of DBT using ionic liquid [Omim][HSO4]. Scheme 2. Schematic of supposed mechanism for oxidative desulfurization of model diesel using ionic liquid [Omim][HSO4] with and without ultrasound Figure captions Figure 1. Desulfurization of model oil by [Omim][HSO4] (•) and [Bmim][HSO4] (). Conditions: T, 25 ˚C; IL/model oil (w/w), 1:2; S-content, 500 ppm. Figure 2. Effect of temperature on desulfurization process. Conditions: IL, [Omim][HSO4]; O/S (mol/mol), 5; IL/model oil (w/w), 1:2; S-content, 500 ppm. Figure 3. Effect of IL/model oil mass ratio on desulfurization process. (IL/oil, 1:2 (•), 1:3 (), 1:6 (), 1:10 (▲), 1:20 ()), Conditions: IL, [Omim][HSO4]; T, 25 ˚C; O/S (mol/mol), 5; Scontent, 500 ppm.


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Figure 4. Effect of sulfur species on desulfurization process. (DBT (•), BT (), TH (), 4,6DMDBT (▲)), Conditions: IL, [Omim][HSO4]; T, 25 ˚C; O/S (mol/mol), 5; IL/oil, 1:2. Figure 5. First-order kinetics for oxidation of DBT by [Omim][HSO4] at different temperatures (25 ˚C (•), 45 ˚C (), 60 ˚C ()), conditions: IL, [Omim][HSO4]; initial S-content, 500 ppm; O/S, 5; IL/oil, 1:2. Figure 6. Effect of irradiation time on UAOD process. Conditions: IL, [Omim][HSO4]; O/S (mol/mol), 5; IL/oil, 1:2. Figure 7. Effect of IL/model oil mass ratio on UAOD process. Conditions: IL, [Omim][HSO4]; O/S (mol/mol), 5.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 1. The proposed mechanism for oxidative desulfurization of DBT using ionic liquid [Omim][HSO4].


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Scheme 2. Schematic of supposed mechanism for oxidative desulfurization of model diesel using ionic liquid [Omim][HSO4] with and without ultrasound


Energy & Fuels

100

Sulfur removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

60

40

20

0 0

10

20

30

40

50

60

70

80

90

100

110

t (min)

Figure 1. Desulfurization of model oil by [Omim][HSO4] (•) and [Bmim][HSO4] (). Conditions: T, 25 ˚C; IL/model oil (w/w), 1:2; S-content, 500 ppm.


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100 Extraction

Sulfur removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

oxidation

60

40

20

0 25

40

50

60

70

80

T (˚C)

Figure 2. Effect of temperature on desulfurization process. Conditions: IL, [Omim][HSO4]; O/S (mol/mol), 5; IL/model oil (w/w), 1:2; S-content, 500 ppm.


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100

Sulfur removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 60 40 20 0 0

10

20

30

40

50

t (min)

Figure 3. Effect of IL/model oil mass ratio on desulfurization process. (IL/oil, 1:2 (•), 1:3 (), 1:6 (), 1:10 (▲), 1:20 ()), Conditions: IL, [Omim][HSO4];T, 25 ˚C; O/S (mol/mol), 5; Scontent, 500 ppm.


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100

80

Sulfur removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

60

40

20

0 0

10

20

30

40

50

60

t (min)

Figure 4. Effect of sulfur species on desulfurization process. (DBT (•), BT (), TH (), 4,6DMDBT (▲)), Conditions: IL, [Omim][HSO4]; T, 25 ˚C; O/S (mol/mol), 5; IL/oil, 1:2.


Energy & Fuels

4.5 4 k= 0.2056 R² = 0.9935

3.5

k = 0.1448 R² = 0.984

3

ln(CA0/CA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.5 k= 0.0734 R² = 0.9488

2 1.5 1 0.5 0 0

5

10

15

20

25

30

35

40

45

t (min)

Figure 5. First-order kinetics for oxidation of DBT by [Omim][HSO4] at different temperatures (25 ˚C (•), 45 ˚C (), 60 ˚C()), conditions: IL, [Omim][HSO4]; initial S-content, 500 ppm; O/S, 5; IL/oil, 1:2.


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Sulfur removal efficiecny (%)

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100 80 60 40 20 0 1

2

3

6

9

Irradiation time (min)

Figure 6. Effect of irradiation time on UAOD process. Conditions: IL, [Omim][HSO4]; O/S (mol/mol), 5; IL/oil, 1:2.


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Sulfur removal efficiency (%)

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100 80 60 40 20 0 1/2

1/3

1/6

1/10

1/20

IL/oil mass ratio (w/w)

Figure 7. Effect of IL/model oil mass ratio on UAOD process. Conditions: IL, [Omim][HSO4]; O/S (mol/mol), 5.


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Table 1. Physical properties of ILs. Mass Fraction Purity

Water Mass Fraction

Halide content

Density (25 ˚C)

(%wt.)

(%wt.)

(ppm)

(g.cm-3)

[Omim][HSO4]

98

0.0594

300

1.1512

[Bmim][HSO4]

98

0.0510

300

1.2822

Ionic liquid


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Table 2. Effect of O/S molar ratio on ODS process. Conditions: IL, [Omim][HSO4];T, 25 ˚C; IL/model oil (w/w), 1:2; S-content, 500 ppm. Oxidative sulfur Entry

O/S ratio (mol/mol) removal (%)

1

2

87

2

3

94

3

4

94.2

4

5

100

5

6

100

6

7

98

7

8

93.3


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Table 3. Apparent activation energies for different DBT oxidation systems. Arrhenius activation energy (kJ/mol)

Conditions of DBT oxidation [Omim][HSO4] IL-H2O2

24.51

[C43MPy]Cl/3ZnCl2 IL-H2O2

61.13

[C5H9NO]SnCl2 IL-H2O2-CH3COOH

32.5

[Bmim][PF6] IL-H2O2- H3PW12O40

51.2

[Bmim][BF4] IL- (H3PMo12O4026H2O, PMo12)- H2O2

53.8

[HPMo][HTAC]2-H2O2

26.8

H3PW12O40,NH2/MCM41-H2O2

24.2

HCOOH-H2O2

29.1


Ref. This work 6666 2929 6767 6868 62 69 7070

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Table 4. Effect of O/S molar ration on UAOD process. Conditions: IL, [Omim][HSO4]; T, 25 ˚C ; IL/oil, 1:2. Entry

O/S molar ratio (mol/mol)

Sulfur removal efficiency (%)

1

2

92.1

2

3

95.1

3

4

96.0

4

5

100


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Energy & Fuels

Table 5. ODS and UOAD of model oil by recovered [Omim][HSO4]. Sulfur removal efficiency (%) Regeneration time (unit) ODS

UAOD

1

97.8

96.9

2

95.9

95.2

3

95.1

94.7

4

93.9

91.9

5

92.5

91.5

6

91.8

89.9

7

88.7

85.8


Energy & Fuels

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Page 36 of 36

Table 6. Comparison of the catalytic activities of IL [Omim][HSO4] with different catalysts. Oxidative sulfur Ionic liquid

Model oil

Reaction conditions

Ref.

T, 25 ˚C; O/S, 5; IL/oil,1:2; t, 70 min.

This work

O/S, 5; IL/oil,1:2; ultrasound time, 3 min.

This work

T,50 ˚C;O/S,5;V(IL)/V(oil),1:10;t,180 min.

3434

T, 75 ˚C;O/S,8;IL/oil,1:3;t,20 min.

3939

Room Temp.; O/S, 5;V(IL)/V(oil),1:2;t,90

33 33

removal (%) DBT, n-decane, [Omim][HSO4]

100 (500 ppm) DBT, n-decane,

[Omim][HSO4]

100 (500 ppm) DBT, n-octane,

[Hnmp][HCOO]

99.0 (500 ppm) DBT, n-octane,

[Hnmp]Cl/ZnCl2

99.9 (500 ppm) DBT, n-octane,

[Bmim][HSO4]

99.6 (1000 ppm)

min.

DBT, n-octane,

T, 25 ˚C; O/S, 14; oil, 5 mL; IL, 0.702

[(C8H17)3CH3N]Cl/FeCl3

97.9 (500 ppm)

mmol; t, 60 min.

DBT, n-octane,

T, 40 ˚C; O/S, 8; oil, 5 mL; IL, 0.78 mmol;

[BPy][FeCl4]

[Otbim][CH3COO]

5959

2727

95.3 (500 ppm)

t, 10 min.

Thiophene, n-

T, 70 ˚C;V(IL)/V(H2O2),

heptane, (1500

87.5

3838

1:1.1;V(IL)/V(oil),1:1.05;t, 180 min.

ppm) DBT, n-octane, [Hnmp][H2PO4]

T, 50 ˚C; O/S, 16; V(IL)/V(oil), 1:1; t, 300 99.8

(500 ppm)

min.


7171

Oxidative Desulfurization of Model Diesel Using Ionic Liquid 1-Octyl

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