Oxidative Desulfurization of Gas Oil Catalyzed by (TBA) - American

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Oxidative Desulfurization of Gas Oil Catalyzed by (TBA)4PW11Fe@PbO as an Efficient and Recoverable Heterogeneous Phase-Transfer Nanocatalyst Mohammad Ali Rezvani,* Sahar Khandan, and Negin Sabahi Department of Chemistry, Faculty of Science, University of Zanjan, Post Office Box 451561319, Zanjan, Iran ABSTRACT: In this paper, a tetra(n-butyl)ammonium salt of ironIII-substituted phosphotungstate@lead oxide composite, (TBA)4PW11Fe@PbO, was successfully synthesized by the thermal decomposition method as a nanocatalyst for oxidative desulfurization (ODS) of gas oil. The incorporation of the materials was confirmed by Fourier transform infrared spectroscopy (FTIR), ultraviolet−visible (UV−vis), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and 31P nuclear magnetic resonance (NMR) characterization methods. To evaluate the catalytic activity of (TBA)4PW11Fe@PbO, the ODS process was carried out using a CH3COOH/H2O2 oxidizing agent at 60 °C. From the attained results, the total sulfur and mercaptan content of real gas oil were reduced with 97% efficiency after 2 h. Furthermore, the various comparative experiments were performed to investigate the capability of (TBA)4PW11Fe@PbO in ODS of prepared model fuel. Results were indicated that the kinetics of sulfur oxidation fitted the pseudo-first-order kinetic model. The probable mechanism was proposed via the electrophilic mechanism through the formation of a peroxometalate intermediate complex with phasetransfer properties. After 5 oxidation runs, the heterogeneous nanocatalyst was separated and recovered easily.

1. INTRODUCTION The various types of sulfur compounds present in petroleum products emit SOx gases during combustion. These hazardous air pollutants are not only causing corrosion but also contributing to acid rain, photochemical smog, and human diseases.1−3 Also, the strict regulations regarding environmental safety and improvement of transportation fuel quality are the most pressing issues of the world recently. In this respect, a lot of substantial research efforts have been conducted to reduce the sulfur levels in refined petroleum products to less than 10 ppmw.4,5 Hydrodesulfurization (HDS) is an often used traditional refining process, which has been proven as a robust method for lowering thiol-, mercaptan-, sulfide-, and disulfidecontaining impurities.6 Nevertheless, it is less effective in the elimination of refractory heterocyclic organosulfur molecules, such as thiophene (Th), benzothiophene (BT), dibenzothiophene (DBT), and their sterically hindered derivatives. Additionally, the requirement of high-temperature and highpressure operating conditions with large quantities of hydrogen imposes high costs and energy consumption.7 Therefore, some affordable and energy-saving alternative methods have been developed to produce the ultraclean fuels in many countries. Among them, oxidative desulfurization (ODS) has been widely recognized as one of the most competitive systems for deep desulfurization under moderate conditions.7,8 This process has been implemented in two stages: (i) conversion of sulfur compounds to the corresponding sulfoxide or sulfone molecules using an oxidizing agent and (ii) subsequent removal of the oxidized materials from the fuel by means of different methods, such as extraction or adsorption.9 Given the importance of the choosing oxidant, organic peracids, which are produced in situ by the reaction of H2O2 and short-chain carboxylic acids (formic acids or acetic acids), are employed as an effective oxidizing system in the organic phase.10,11 © 2017 American Chemical Society

Polyoxometalates (POMs) as well-known metal−oxygen inorganic compounds have received extensive attention in the catalysis area over the past decade.12−14 Among the different varieties of their structures, transition-metal-monosubstituted polyoxoanion in the primary Keggin structure, [XM11TMO39]n− (X = P, Si, or B; M = W or Mo; and TM = transition metal), is favored for the synthesis of POM-based catalysts as a result of their variable structures, tunable solubility, reversible redox transformations, and high catalytic activity.15−17 The bulk POMs with water-soluble properties have been widely investigated in catalytic homogeneous reactions.15 However, these systems suffered from difficult separation and regeneration of the catalyst. To overcome these undesirable obstacles, designed heterogeneous catalysts are explored by deposition of POMs onto suitable supports or preparation of their insoluble salts.18,19 Nowadays, the incorporation of POMs and metal oxide supports, such as TiO2,20 ZrO2,21 and Al2O3,22 have been developed as a promising strategy to enhance the catalytic activity of the polyoxoanion clusters. Lead oxide (PbO) particles are attracting interest for catalyzing the organic reactions,23 graphite oxidation,24 and dye degradation.25 The oxidative properties, simple preparation, and recoverability of PbO particles make them appropriate candidates for supporting POMs in the heterogeneous oxidation reaction of sulfur-containing compounds. As a part of our ongoing efforts for developing the application of POMs to promote the quality of fuels,26−29 we report the design of a phase-transfer-type (TBA)4PW11Fe@ PbO nanocatalyst for ODS of real gas oil and prepared model Received: April 4, 2017 Revised: April 20, 2017 Published: April 24, 2017 5472

DOI: 10.1021/acs.energyfuels.7b00948 Energy Fuels 2017, 31, 5472−5481

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Energy & Fuels

Figure 1. FTIR of (a) PbO, (b) (TBA)4PW11Fe, and (c) (TBA)4PW11Fe@PbO. 2.3. Synthesis of the PbO Support. In a typical synthesis, 1.90 g (10 mmol) of C6H8O7·H2O was dissolved in 15 mL of distilled water as a capping agent. After the complete dissolution of the above solid substance, this solution was added dropwise to 15 mL of aqueous solution containing 3.90 g (10 mmol) of Pb(NO3)2 under magnetic stirring at 70 °C for 45 min to form a milky white gel. At last, the obtained gel was aged and dried at 80 °C for 2 h, and then the dried sample was calcined at 600 °C for 2 h. 2.4. Synthesis of the ((n-C4H9)4N)4[PW11Fe(H2O)O39]@PbO Nanocatalyst. ((n-C4H9)4N)4[PW11Fe(H2O)O39]@PbO was prepared as follows: synthesized (TBA)4PW11Fe (0.10 g) was dissolved in 5 mL of boiling distilled water and then added slowly to the Pb(NO3)2 and C6H8O7·H2O solution during the preparation of the PbO support. The resulting mixture was maintained at 70 °C under stirring for 45 min to obtain a uniform and homogeneous solution. The following steps were accomplished the same as the PbO production. After the c a l ci n a t i o n s t e p , th e r e m a i n i n g or a n g e po w d e r ( ( n C 4 H 9 ) 4 N) 4 [PW 1 1 Fe(H 2 O)O 3 9 ]@PbO was designated as (TBA)4PW11Fe@PbO nanocatalyst. 2.5. ODS Process of Prepared Model Fuel. In this investigation, a certain amount of the aromatic sulfur compounds (ASCs), such as BT, DBT, 4-MDBT, and 4,6-DMDBT, was dissolved in n-heptane as a model fuel to evaluate the catalytic performance of (TBA)4PW11Fe@ PbO nanocatalyst in sulfur oxidation reactions. The sulfur concentration of each ASC was 500 ppm (by weight). At first, the water bath was heated to 25, 40, 50, and 60 °C in separate experiments. Then, 50 mL of prepared fuel sample in a closed round-bottom flask, equipped with a magnetic stirrer, was heated to the reaction temperature. Afterward, 6 mL of CH3COOH/H2O2 (volume ratio of 1:1) and 0.1 g of nanocatalyst were added slowly to the reaction vessel. The ODS process was continued under stirring conditions (500 rpm). After the passage of 2 h, the above mixture was cooled to room temperature and 10 mL of acetonitrile was added to extract the oxidized ASCs. The formed immiscible liquids (n-heptane and water phases) were

fuel. In typical oxidation reactions, the mixture of CH3COOH/ H2O2 is used as an oxidant and CH3CN is applied for extracting the polar oxidized sulfur compounds. Moreover, the influence of the main affecting factors on the ODS efficiency, kinetic parameters, and probable reaction mechanism is discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals and solvents were commercially available and used as received. Benzothiophene (BT), dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT), 4,6-dimethyldibenzothiophene (4,6-DMDBT), n-heptane, hydrogen peroxide (H2O2, 30 vol %), acetic acid (CH3COOH, 99.7%), acetonitrile (CH3CN), sodium tungstate dihydrate (Na2WO4·2H2O), disodium hydrogen phosphate (Na2HPO4), iron(III) nitrate nonahydrate [Fe(NO3)3·9H2O], and tetrabutylammonium bromide (TBAB) were purchased from Sigma-Aldrich. Lead nitrate [Pb(NO3)2] and citric acid monohydrate (C6H8O7·H2O) were obtained from Merck. Typical real gas oil was used with the following specification: density of 0.8256 g/mL at 15 °C and total sulfur content of 0.8738 wt %. 2.2. Synthesis of ((n-C4H9)4N)4[PW11Fe(H2O)O39]. The tetra(nbutyl)ammonium salt of ironIII-substituted phosphotungstate was synthesized by the following procedure: 3.29 g (10 mmol) of Na2WO4·2H2O was dissolved in 20 mL of distilled water. To the solution, 0.13 g (0.91 mmol) of Na2HPO4 and 0.49 g (1.2 mmol) of Fe(NO3)3·9H2O were added. The pH of the solution was adjusted to 4.5 under stirring, and the mixture was heated to 80−85 °C. After that, an aqueous solution of 1.45 g (4.5 mmol) of TBAB in 5 mL of distillated water was added dropwise to the above solution. The mixture was magnetically stirred to form a white precipitate. Finally, the precipitate ((n-C4H9)4N)4[PW11Fe(H2O)O39] [hereinafter referred to as (TBA)4PW11Fe] was recovered by filtration, washed with ether, and air-dried. 5473


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Figure 2. UV−vis spectra of (a) PbO, (b) (TBA)4PW11Fe, and (c) (TBA)4PW11Fe@PbO. (NMR) spectra were recorded on Bruker Ultra Shield 250 MHz. The total sulfur and mercaptan contents in gas oil before and after treatment were determined using X-ray fluorescence with a TANAKA X-ray fluorescence spectrometer RX-360 SH.

separated by a separation funnel and decantation technique. The synthesized heterogeneous nanocatalyst was separated and regenerated from the reaction system using simple filtration. The total sulfur concentration after oxidation treatment was determined using the Xray fluorescence spectrometer according to ASTM D4294 and ASTM D3227. The ASC removal efficiency was calculated using eq 1, in which Ai is the initial concentration and Af is the final concentration of ASCs after oxidation treatment.

⎡ A ⎤ ASC removal efficiency (%) = ⎢1 − f ⎥ × 100 Ai ⎦ ⎣

3. RESULTS AND DISCUSSION 3.1. Material Characterizations. The identification of specific chemical bands and functional groups of the synthesized samples was characterized using FTIR to confirm their successful incorporation. FTIR spectra of (a) PbO, (b) (TBA)4PW11Fe, and (c) (TBA)4PW11Fe@PbO nanocatalyst are depicted in Figure 1. The clear intense adsorption peak around 470 cm−1 is attributed to the Pb−O stretching vibrations, and also the appearance of the band at 683 cm−1 is assigned to the asymmetric bending vibrations of Pb−O−Pb bands (Figure 1a). In the spectrum of the prepared PbO support using a citric acid capping agent, the bands at 1732 and 2922 cm−1 revealed the stretching vibrations of CO and antisymmetric stretching vibrations of C−H, respectively.30 The bending and stretching vibrations of O−H in water molecules are observed at 1417 and 3385 cm−1. According to Figure 1b, the unique characteristic peaks at 793, 887, 962, and 1068 cm−1 are caused by the stretching modes involving edgesharing W−Oc−W, corner-sharing W−Ob−W, terminal W Od, and P−O bonds in the Keggin-type [PW11Fe(H2O)O39]4− anions, respectively.18 Furthermore, the peaks at 1383 and 1483 cm−1 are attributed to C−H scissoring vibrations of CH3−N+. The absorption bands at 2873 and 2961 cm−1 are ascribed to the symmetric and asymmetric stretching modes of −CH2 of the TBA cation.31 Figure 1c indicates the infrared spectra patterns of (TBA)4PW11Fe@PbO. The existence of peaks in the region of 700−1000 cm−1 demonstrated the presence of (TBA)4PW11Fe after supporting on PbO. The broad peak at 3360 cm−1 corresponded to the water molecule coordinated to the ironIII-substituted phosphotungstate center, which overlapped the stretching vibrations of the −NH functional group. The observed peak shifts in the FTIR spectra of the synthesized nanocatalyst compared to the pure materials suggested the preparation of the (TBA)4PW11Fe@PbO composite.

(1)

2.6. ODS Process of Gas Oil Fuel. In the same manner as the ODS of the ASCs, after heating the water bath, 50 mL of real gas oil fuel was added to the round-bottom flask and its temperature was maintained constant at 60 °C during the experiment. Subsequently, 6 mL of CH3COOH/H2O2 and 0.1 g of (TBA)4PW11Fe@PbO were added to the vessel. The mixture was vigorously stirred by a magnetic stirrer for 2 h. When the oxidation process has been completed, the flask was cooled to room temperature and then 10 mL of polar CH3CN was used to extract the polar oxidized sulfur compounds from gas oil. In the separation step, the oil phase was separated by decantation. The total sulfur and mercaptan contents in gas oil before and after the ODS test were determined using X-ray fluorescence. The ODS efficiency was expressed by the following eq 2, where Si and Sf correspond to the initial and final concentrations of the total sulfur content in gas oil, respectively:

⎡ S⎤ ODS efficiency (%) = ⎢1 − f ⎥ × 100 Si ⎦ ⎣

(2)

2.7. Characterization Methods. Fourier transform infrared spectroscopy (FTIR) studies were performed on a Thermo Nicolet iS 10 spectrometer, using KBr disks in the range of 400−4000 cm−1. Ultraviolet−visible (UV−vis) spectra were measured with a doublebeam Thermo Heylos spectrometer in the range of 200−400 nm. Measurements were performed using quartz cuvettes. Powder X-ray diffraction (XRD) analysis was collected between 2θ of 5° and 80° at room temperature on a Bruker D8 advance powder X-ray diffractometer with a Cu Kα (λ = 0.154 nm) radiation source. The surface morphologies were examined by scanning electron microscopy (SEM) by LEO 1455 VP equipped with an energy-dispersive X-ray spectroscopy (EDX) apparatus. 31P nuclear magnetic resonance 5474


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Figure 3. XRD patterns of (a) PbO, (b) (TBA)4PW11Fe, and (c) (TBA)4PW11Fe@PbO.

The surface morphology of the PbO support and (TBA)4PW11Fe@PbO catalyst was accurately determined using the SEM technique. As depicted in Figure 4a, the SEM images of pure PbO presented well-dispersed spherical particles. After deposition of (TBA)4PW11Fe clusters on PbO, the uniform morphology of the support is particularly changed to a rugged surface with a large number of cavities (panels b and c of Figure 4). It can be suggested that the porous surface of the synthesized catalyst gives unique features to enhance its catalytic activity. Each of the cavities is an appropriate site for scavenging the sulfur-containing compounds. The EDX pattern revealed the existence of elemental C, O, Fe, W, and Pb in the (TBA)4PW11Fe@PbO composite structure (Figure 5). The observed intense peak is represented around 2−3 keV, indicating the formation of the catalyst from a high amount of Pb (84.07 wt %), which is even in good agreement with the results of XRD and SEM studies. The successful preparation of the nanocatalyst was further investigated using 31P NMR spectroscopy. After the dissolution of (TBA)4PW11Fe and (TBA)4 PW11Fe@PbO solids in dimethyl sulfoxide (DMSO), the main single peak was located at −11.55 and −11.35 ppm (panels a and b of Figure 6). These peaks were ascribed to the central phosphorus in the tetrahedral PO4 unit of ironIII-substituted phosphotungstate. As seen, there was no significant shift in the position of the peaks, which verifies the presence of the identical phosphorus atom in the (TBA)4PW11Fe structure before and after supporting on PbO. 3.2. ODS Results of Real Gas Oil and Prepared Model Fuel. For investigation of the capability of the (TBA)4PW11Fe@PbO nanocatalyst, the ODS process was performed on typical real gas oil and prepared model fuel under the mentioned conditions in the Experimental Section. The attained results after oxidation treatment were reported in

The UV−vis absorption spectra of synthesized samples are illustrated in Figure 2. The maximum absorption peak for pure PbO is shown around 200 nm. As seen in Figure 2b, the peak at 257 nm is considered as a ligand−metal charge transfer transition of O2− → W6+, where W atoms were positioned in W−O−W. In both (TBA)4PW11Fe and (TBA)4PW11Fe@PbO patterns, the absorption peaks around the wavelengths of 197− 200 nm served as the evidence for πp−πd electronic transitions of terminal WOd.32 The XRD patterns of the powder PbO, (TBA)4PW11Fe, and (TBA)4PW11Fe@PbO nanocatalyst are shown in Figure 3. On the basis of the obtained results, the main reflections of pure PbO are observed at 2θ values of 28.5°, 29°, 31.7°, 35.6°, 48.5°, 56°, and 59.6°, which associated with the (111), (002), (200), (210), (022), (222), and (311) crystal planes, respectively.23 The unique intense peaks of (TBA)4PW11Fe are displayed at the positions of 16°, 19°, 21.5°, 22.7°, 23.8°, 29.7°, 30.5°, and 31.4° in Figure 3b. The XRD pattern of the prepared nanocatalyst is mainly composed of the diffraction peaks of PbO, while the exaction of peaks in the range of 15−30° is reflected the presence of (TBA)4PW11Fe species. It can be concluded that the structures of materials remained intact after incorporation using the introduced producer method. The nanocrystallite size of (TBA)4PW11Fe@PbO is estimated to be about 62 nm by means of the Debye−Scherrer equation (eq 3)33 D=

Kλ β cos θ

(3)

where the value of D is the size of the crystal, K is a constant equal to 0.89, λ is the wavelength of the X-ray (1.5406 Å), β is the full width at half maximum (fwhm), and θ is half of the diffraction angle. 5475


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Figure 5. EDX pattern of (TBA)4PW11Fe@PbO.

and Preyssler were tested as a catalyst for the elimination of ASCs and total sulfur content of real gas oil under the same experimental conditions. The results were listed in Table 2. It was observed that each catalyst by itself decreased the concentration of sulfur compounds and their catalytic activity was followed in order of H3PW12O40 > H14[NaP5W30O110] > H6P2W18O62 > H6P2Mo18O62. As concluded, the POMs in the Keggin structure were proven to be the more efficient catalysts in the ODS reactions. Besides, the high catalytic properties of tungsten compared to molybdenum-based catalysts were caused by the strong Brønsted acidity of tungsten atoms.34 In this work, a poor sulfur removal percentage was attributed to the PbO support, while the superior catalytic performance was achieved by the synthesized (TBA)4PFeW11@PbO nanocatalyst. It was reflected that the supporting of (TBA)4PFeW11 on PbO led to an enhancement in the catalytic activity of unsupported (TBA)4PFeW11. 3.4. Effect of Different Sulfur Compounds on the ODS Process. The reactivity of the ASCs in the designed ODS process is also studied using the (TBA)4PW11Fe@PbO catalyst. As noted in Figure 7 and entry 1 of Table 2, among the different used model sulfur compounds, DBT showed high oxidative reactivity and was removed from n-heptane phase with 97% yield. On the basis of the results, the ASC reactivity was decreased in the following order: DBT > 4,6-DMDBT > 4MDBT > BT. According to the reported literature, the trend of electron density of the sulfur atom in these compounds was 4,6DMDBT (5.760) > 4-MDBT (5.759) > DBT (5.758) > BT (5.696).35 It could be speculated that the partial electron charge on the sulfur atom had an effect on the reactivity of thiophenic molecules, as expressed by Xiao et al.8 Another relevant concept was attributed to the steric hindrance of the CH3 groups, which caused the highest removal efficiency for the DBT substrate. As a result of the high oxidation reactivity of DBT, it was selected as a representative compound of ASCs in the following ODS runs. 3.5. Effect of the Nanocatalyst Dosage on the ODS Process. To assess the effect of nanocatalyst dosage on removal efficiency of sulfur compounds, various amounts of the (TBA)4PW11Fe@PbO catalyst were used. On the basis of blank experimental results, 20% of DBT and 17% of the sulfur content of real gas oil were reduced (entry 8 of Table 2). The results are presented in Figure 8. Also, it was found that the removal percentage of DBT and organosulfur compounds increased consecutively with an increase in the concentration

Figure 4. SEM images of (a) PbO and (b and c) (TBA)4PW11Fe@ PbO.

Table 1 and Figure 7. According to entry 1, the total sulfur content of the gas oil sample was reduced from 0.8738 to 0.0285 wt %. Also, the mercaptan sulfur compounds were much lowered to 9 ppm (entry 3). It should be pointed out that the main specifications of gas oil remained unchanged after ODS. The removal efficiency of BT, DBT, 4-MDBT, and 4,6DMDBT from model fuel was 93, 97, 94, and 95%, respectively (Figure 7). These results rendered the success of this catalytic system (TBA)4PW11Fe@PbO/CH3COOH/H2O2 to desulfurize the organosulfur molecules. 3.3. Effect of Different Catalysts on the ODS Process. The competitive desulfurization reactions were applied to investigate the effect of the catalyst structure on the removal efficiency of sulfur compounds. For this purpose, typical kinds of heteropolyoxometalate involving Keggin, Wells−Dawson, 5476


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Figure 6. 31P NMR spectra of (a) (TBA)4PW11Fe and (b) (TBA)4PW11Fe@PbO nanocatalyst.

Table 1. Results of ODS of Real Gas Oil entry 1 2 3 4 5 6 7 8 9 10 11

properties of gas oil total sulfur content (wt %) density at 15 °C mercaptans (ppm) flash point (°F) water content (vol %) cloud point (°C) color test kinematic viscosity at 50 °C (cSt) pour point (°C) distillation (IBP, °C) distillation (FBP, °C)

before ODS

after ODSa

after ODSb

0.8738 0.8256 265 139 0.025 −4 1.5 2.7

0.0285 0.8255 9 138 0.025 −4 1.5 2.6

0.0287 0.8255 10 138 0.025 −4 1.5 2.6

−9 157.5 387.3

−9 157.3 387.2

−9 157.4 387.1

a

Conditions for ODS: 50 mL of gas oil, 0.1 g of (TBA)4PW11Fe@PbO nanocatalyst, 6 mL of oxidant, 10 mL of extraction solvent, time of 2 h, and temperature of 60 °C. bODS of real gas oil using the reused nanocatalyst.

Figure 7. Effect of different sulfur compounds on ODS efficiency using the (TBA)4PW11Fe@PbO nanocatalyst.

time also had the same effect on the catalytic capability of the oxidation system. The highest yield of sulfur removal was 97%, which was attained at 60 °C after 2 h. 3.7. Kinetic Studies on the ODS Process. To clarify the kinetics of DBT and sulfur oxidation, the reaction kinetic parameters were examined using a pseudo-first-order model at the different temperatures, from 25 to 60 °C, in 2 h. The reaction rate constant k was calculated by plotting ln[S]t/[S]i or [S]t/[S]i against t as follows:

levels of the nanocatalyst in reaction medium as a result of the abundance of peroxometalate intermediate complexes. When the amount of nanocatalyst was further increased to 0.12 g, any noticeable change was not observed. Therefore, the favorable dosage of (TBA)4PW11Fe@PbO was 0.10 g, and the sulfur removal efficiency of 97% was obtained. 3.6. Effect of the Reaction Temperature and Time on the ODS Process. The influence of the reaction temperature on ODS was evaluated in the temperature range of 25−60 °C. From Figure 9, the reduction of the concentration of DBT and total sulfur content in the model fuel and real gas oil at the temperature of 25 °C occurred with 63 and 61% removal efficiency, respectively, while increasing the reaction temperature to 60 °C had a remarkable impact. Increasing the reaction

r=− [S]t

∫[S]

i

5477

d[S] = k[S] dt

(4)

[S]t d[S] = ln = −kt [S] [S]i

(5) DOI: 10.1021/acs.energyfuels.7b00948 Energy Fuels 2017, 31, 5472−5481

Article

Energy & Fuels Table 2. Effect of Different Catalysts on ODS of Different Types of Sulfur Compoundsa entry

catalyst

DBT

4-MDBT

4,6-DMDBT

BT

real gas oil

1 2 3 4 5 6 7 8

(TBA)4PFeW11/PbO (TBA)4PFeW11 H3PW12O40 H14[NaP5W30O110] H6P2W18O62 H6P2Mo18O62 PbO none

97 70 66 62 58 58 58 20

94 68 62 61 56 54 55 18

95 69 63 61 55 55 57 19

93 66 61 60 52 52 53 18

97 68 62 61 57 51 55 17

a

Conditions for ODS: 50 mL of real gas oil or model fuel, 0.1 g of (TBA)4PW11Fe@PbO nanocatalyst, 6 mL of oxidant, 10 mL of extraction solvent, time of 2 h, and temperature of 60 °C.

Figure 8. Effect of the nanocatalyst dosage on the removal efficiency of DBT and the sulfur content of gas oil.

[S]t = [S]i e−kt

(6)

where [S]i and [S]t are the initial concentration and concentration at time t, respectively. According to Figure 10 and eq 6, a linear relationship was observed between [S]t/[S]i and t parameters and the correlation coefficients were obtained close to unity (Table 3). It was indicated that the kinetic results fitted to the pseudo-first-order kinetic model. Further, the affiliation of k on the reaction temperature could be express by the well-known Arrhenius equation,29 in which A is the preexponential factor, Ea is the apparent activation energy, and R and T are the universal gas constant and reaction temperature, respectively (eq 7). The Ea value was calculated using the plot of ln k versus 1/T (Figure 11). The Ea values were 24.83 and 22.62 kJ/mol for oxidation of DBT and total sulfur content of gas oil, respectively. (7)

Figure 9. Effect of the reaction temperature and time on removal efficiency of (a) DBT and (b) sulfur content of gas oil using the (TBA)4PW11Fe@PbO nanocatalyst.

3.8. Proposed Mechanism of the ODS Process. The overall mechanism of sulfide oxidation can be explained by the formation of the peroxometalate intermediate complex in the presence of the (TBA)4PW11Fe@PbO nanocatalyst and CH3COOH/H2O2 in a biphasic system (Scheme 1). To commence, H2O2 reacts with CH3COOH quickly to generate the in situ formation of peracetic acid (CH3COOOH) as a supplier source of active oxygen (step 1). The terminal metal atoms (M = W or Fe) in (TBA)4PW11Fe accept active oxygen from CH3COOOH, and then the peroxometalate complex, including (TBA)4[PO4(MO(O2)2)4], is made (step 2). Also,

the water molecules emerge as a byproduct. The sulfur oxidation occurred via the electrophilic mechanism.8 The electron pairs on the sulfur atom can be attacked by the electrophilic active oxygen in the (TBA)4[PO4(MO(O2)2)4] structure to produce sulfoxide molecules (step 3). The conversion of sulfoxide to their corresponding sulfone can be performed by undergoing a further oxidation process (step 4). At the end, an organic extraction solvent CH3CN is used for extraction of the oxidized products. The high polarity of CH3CN makes it a good candidate for increasing the polarity of

k = Ae−Ea / RT

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Figure 11. Arrhenius plots for the oxidation of (a) DBT and (b) sulfur content of gas oil.

Then, the recovered catalyst was used for the subsequent process under similar oxidation conditions. The results of the ODS reaction using the reused nanocatalyst after the first run are reported in Table 1. It was found that these results were quite close to the results obtained with the fresh nanocatalyst. As shown in Figure 12, the DBT removal efficiency was dropped from 97 to 93% after 5 regeneration cycles, which can be ascribed to cover the active sites of the nanocatalyst with sulfur-containing substrates during the ODS period.

Figure 10. Plots of St/Si for the oxidation of (a) DBT and (b) sulfur content of gas oil.

Table 3. Pseudo-First-Order Rate Constants and Correlation Factors of ODS of Real Gas Oil and DBT rate constant, K (min−1)

correlation factor, R2

temperature (°C)

real gas oil

DBT

real gas oil

DBT

25 40 50 60

0.021 0.025 0.045 0.050

0.008 0.012 0.016 0.023

0.919 0.889 0.917 0.948

0.988 0.990 0.959 0.934

4. CONCLUSION In conclusion, the (TBA)4PW11Fe@PbO nanocatalyst has been successfully prepared by supporting Keggin-type (TBA)4PW11Fe on PbO particles via the thermal decomposition method. The catalytic activity of the nanocatalyst was carried out on prepared model fuel and real gas oil for removing organosulfur compounds. The comparative experimental results demonstrated that the desulfurization efficiency depended upon the structure of the catalyst, nature of the sulfur molecules, reaction temperature, and dosage of the nanocatalyst. On the basis of catalytic (TBA)4PW11Fe@PbO/ CH3COOH/H2O2 system, the oxidation reactivity of ASCs was decreased according to the following order: DBT > 4,6DMDBT > 4-MDBT > BT. After catalytic oxidation reactions, the total sulfur content of gas oil fuel could be reduced to 0.0285 wt %. Moreover, the oxidation of sulfur compounds followed the pseudo-first-order kinetic model. At the end, the heterogeneous nanocatalyst was reused up to 5 regeneration cycles. This work was introduced as a facile method for the

the water phase to improve the extraction of polar sulfone compounds. Synthesized (TBA)4PW11Fe@PbO with a quaternary ammonium cantercation can act as a phase-transfer agent to facilitate the transformation of the peroxometal anions into the oil phase. The hydrophobic properties of TBA play an important role in affinity of the catalyst to the sulfur compounds. On the other hand, the mass transfer across the interface of the water phase and oil phase is faced with the rate limitation. Therefore, a phase-transfer catalyst is used to increase the mass transfer in the emulsion systems.12,36 3.9. Regeneration of the Nanocatalyst. After following each catalytic run, the heterogeneous nanocatalyst [(TBA)4PW11Fe@PbO] was regenerated by simple filtration, washed with dichloromethane, and dried at 90 °C for 1 h. 5479


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Energy & Fuels

Scheme 1. Probable Mechanism for the Formation of Peroxometal Species Using CH3COOH/H2O2 and the (TBA)4PW11Fe@ PbO Phase-Transfer Nanocatalyst for Oxidation Sulfur Compounds

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Figure 12. Removal efficiency of DBT after each regeneration cycle.

synthesized phase-transfer nanocatalyst (TBA)4PW11Fe@PbO and its application in the ODS treatment to promote the quality of gas oil fuel.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +982-415-152-617. E-mail: [email protected]. ORCID

Mohammad Ali Rezvani: 0000-0001-6155-5147 Notes

The authors declare no competing financial interest.

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REFERENCES

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