Article pubs.acs.org/EF
Oxidative Desulfurization of Diesel Using Vanadium-Substituted Dawson-Type Emulsion Catalysts F. Banisharif,† M. R. Dehghani,*,† and J. M. Campos-Martin‡ †
School of Chemical Engineering, Thermodynamics Research Laboratory, Iran University of Science and Technology, Narmak, Tehran, Iran ‡ Grupo de Energía y Química Sostenibles (EQS), Instituto de Catálisis y Petroleoquímica, CSIC, Marie Curie, 2 Cantoblanco, 28049 Madrid, Spain ABSTRACT: In this study, we aimed at investigating the catalytic oxidative desulfurization (CODS) of the model diesel oil and real diesel oil using the vanadium-substituted Dawson-type emulsion catalyst ([cetrimonium]6+xP2W18−xVxO62 (x = 1, 3, 5)). Among all prepared samples, [cetrimonium]11P2W13V5O64 showed the best results in CODS of model diesel oil under determined conditions (10 g/L catalyst and O/S mole ratio = 4). The Taguchi method was then applied to optimize the catalyst dosage, hydrogen peroxide dosage, and the reaction temperature in CODS using the best emulsion catalyst. Then formic acid and acetic acid were used as a co-oxidant to improve the oxidation ability. Under optimum conditions, a mixture of H2O2/formic acid (1:1), in the presence of [cetrimonium]11P2W13V5O62 could remove 98% of dibenzothiophene and 82% of benzothiophene. Finally, under optimum conditions of CODS, 90% of total sulfur was removed from a real diesel sample. It is worth mentioning that we could recycle [cetrimonium]11P2W13V5O64, eight times without a significant decrease in catalyst activity.
1. INTRODUCTION Sulfur in fuel is considered as a source of air pollution; in this regard, many governments around the world enact environmental legislations to decrease the sulfur content of fuels to ultralow levels.1 Usually, meeting restricted environmental conditions causes major operational and economical challenges for petroleum refineries.2 Hydrodesulfurization (HDS) is one of the most conventional method for sulfur reduction; unfortunately, this process requires severe conditions such as high pressure and temperature, which increase capital expenditure (CAPEX) and operating expenditure (OPEX) of the unit. In this regard during recent years, oxidative desulfurization (CODS) has been considered as an alternative technology for deep desulfurization of fuel.3,4 ODS generally consists of oxidation of organosulfur compounds in fuel using appropriate oxidizing agents in the presence of a suitable catalyst. Different oxidizing agents such as H2O2,5−7 ozone8 and tert-butyl hydroperoxide9 have been used in ODS. H2O2 is the mostly selected oxidant not only due to oxidizing ability but also because of its environmental compatibility.5 To improve the performance of oxidative desulfurization, different combinations of H2O2/acid have also been studied. Previous works showed that organic acids, such as formic acid and acetic acid have higher efficiency compared to inorganic acids, such as H2SO4 in ODS.8−12 In the presence of mixed oxidant (organic acid + hydrogen peroxide, the sulfur content was reduced to 5 ppmw under the mild conditions (temperature lower than 100 °C and atmospheric pressure). Formation of acid such as peroxyacetic acid and peroxyformic acid in the presence of H2O2 and their solubility in oil phase are the main reason for their high ODS performance,13 whereas the high consumption of oxidant is a major limitation for industrialization. In this regard, during last years, scientists have focused on oxidation in the presence of oxidizing agents © 2017 American Chemical Society
and catalysts which is named catalytic oxidative desulfurization (CODS). Previous studies on CODS systems revealed that catalysts like polyoxometalates(POMs), V2O5/Al2O3, Ni/Al−Si show good activities in the oxidation of DBT and benzothiophene (BT).5,8−13 Among different catalysts, POMs have received much more attention because of their acidic and redox properties and being environmental friendly.5,10−14 Keggintype ([XM12O40](3+a)−; X= P, Si; M = W, Mo; a = 1, 2) and Dawson-type ([X2M18O64]6−; X = P, Si; M = W, Mo) are the most common types of POMs. 15 Previous researches demonstrated that the acidity and catalytic activity of Dawson-type POMs are higher than Keggin-type POMs. The acidity of Dawson-type POMs are in the order of H6P2W18O62 > H6Si2 W18O 62 > H 6P 2Mo18O 62 > H 6Si2 Mo 18 O62 .16−18 Furthermore, one of the most significant subclasses of POMs is the vanadium(V)-substituted POMs. The introduction of vanadium to POM frame is beneficial to change POM’s reactivity from acid-dominated to redox-dominated, as shown by the oxidation of some organic compounds.18−20 Dawsontype POM contains 12 axial sites and 6 polar sites,15 usually the vanadium is located in a polar site of [P2W18O62]6−, which leads to [P2W18‑xVxO62](6+x)‑.21 The vanadium substituted for tungstate strongly affects the terminal and bridge bonds on the plane formed by WO6 octahedral sharing edges,20 this substitution increases reaction activity. To remain the structure of Dawson-type POM unaffected, the maximum number of vanadium substituted for tungstate cannot be more than 5 in the frame of phosphotungstate.21−23 Received: October 26, 2016 Revised: March 6, 2017 Published: March 21, 2017 5419
DOI: 10.1021/acs.energyfuels.6b02791 Energy Fuels 2017, 31, 5419−5427
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example, [cetrimonium]11P2W13V5O62 was synthesized as follows: The (3.6 g) H11P2W13V5O62 was dissolved in distilled water (25 mL) appropriately followed by adjusting the pH (pH = 4.4) with a solution of HCl (2 mol/L). Then 4.05 g of cetrimonium (CTAB) was dissolved in 40 mL of ethanol and was added dropwise. A red precipitate [CTA]11P2W13V5 O62 was immediately formed. After continuous stirring for 2 h, the resulting solid was filtered off by centrifuge and dried at 70 °C in vacuum oven overnight. 2.3. Characterization of Catalyst. Transmission FT-IR spectra of emulsion catalysts were measured as 3% KBr pellets on a Shimadzu8400S FT-IR spectrometer. The chemical analysis (P, V, W) of each sample was determined by inductively coupled plasma spectroscopy (ICP) (ICPS S7000, Shimadzu). An elemental combustion system (ECS), 4010 CHNS-O elemental analyzer Costech Analytical Technologies, Inc., Italy, was also used to determine the amount of C, H, and N in each sample. The UV−vis diffuse reflectance spectroscopy (UV−vis DRS) spectrum was recorded on a Shimadzu UV-2101 PC spectrometer equipped with a diffuse-reflectance to distinguish the electronic properties of the center-metal ions. BaSO4 was used as the internal standard. The scanning patterns were recorded at 190−800 nm in a step-scan mode with a step of 2 nm. To determine the crystalline phase, the X-ray diffraction (XRD) experiments were performed on a Philips PW-1800 diffractometer using Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 30 mA. The XRD spectrum of catalyst samples were measured over an angular range of 5° < 2θ < 60° for all of samples, with a step size (2θ) of 0.04° and a count time of 2 s per step. Solid state 31P magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were recorded by NMR spectrometer Bruker Avance II-9,4 T magnet (400 MHz) operating at 162 MHz with a MAS probe-head using 4 mm ZrO2 rotors spun at 10 kHz. X-ray photoelectron spectroscopy (XPS) was run by using VG Escalab 200 R with Al/Mg X-ray source. The pass energy was 20 eV, and the base pressure of analysis chamber was not greater than 1 × 10−8 Pa. The spectra were calibrated against the reference binding energy of C 1s (284.8 eV). 2.4. Catalytic Oxidative Desulfurization Procedure of Model Fuel. CODS was performed on a model fuel containing representative refractory sulfur compounds in diesel (approximately 500 ppmw DBT and 500 ppmw BT in isooctane). The solution was heated to a fixed temperature. The determined amounts of emulsion catalyst and the oxidant were added simultaneously at a magnetic stirring speed of 1000 r/min. The sample was taken out periodically and cooled down to stop the reaction. The emulsion catalyst in the sample was separated by centrifugation. The performance of reaction was analyzed by a gas chromatography (Agilent, 7890A) coupled with a FID detector using a capillary column (HP-5, 30m × 0.32 mm × 0.25 μm). 2.5. Catalytic Oxidation Desulfurization of Real Diesel. The oxidation of diesel was conducted same as model fuel under the optimum conditions determined by Taguchi method. After the oxidation was finished, the oxidized sulfur compounds in the diesel sample were extracted by using acetonitrile (vol ratio solvent:oil = 1:2) at 25 °C. The oil phase was separated by decantation. The sulfur content in real diesel before and after reaction was determined using X-ray fluorescence spectrometer (ASTM D4294 method). 2.6. Taguchi Experimental Approach. To investigate and optimize the effect of various parameters (Table 2) on the CODS process, Taguchi experimental design approach was followed by an orthogonal array (L16), as shown in Table 3.
Mass transfer resistance between the aqueous phase, containing the oxidizer and catalyst, and the oil phase is a barrier and limits the overall efficiency of CODS. Using a phase transfer agent can improve the mass transfer between phases. Surfactant-based catalysts can be considered as a new approach for phase transfer agent. Surfactant-based catalyst is a combination of a POM anion and a quaternary ammonium cation.24−26 Recently, Lü et al. utilized emulsion catalyst [(C 1 8 H 3 7 ) 2 N(CH 3 ) 2 ] 3 [PW 1 2 O 4 0 ] and [(C 1 8 H 3 7 ) 2 N(CH3)2]3Co(OH)6Mo6O18·3H2O to reduce the sulfur contents of diesel oil from 500 mg/g to less than 30 mg/g under mild conditions. They could recycle it with 100% selectivity.24,25 Most studies on usage of POM in desulfurization is limited to Keggin-type POM as a homogeneous and heterogeneous catalysts, whereas a few studies paid attention to application of Dawson-type POM for desulfurization.8−28 So, in this work we aim at studying the capability of vanadium-substituted Dawsontype emulsion catalyst, [cetrimonium]6+xP2W18−xVxO62 (x = 1, 3, 5), as an amphiphilic catalyst for the first time. Finally, the operation conditions to achieve ultralow-sulfur diesel oil are presented using the Taguchi method. Taguchi is one of the best robust designs that utilizes an orthogonal array to handle any given system by a set of independent factors over specific levels to reduce test error and to increase reproducibility.29
2. EXPERIMENTAL SECTION 2.1. Chemical and Reagent. Benzothiophene (BT), dibenzothiophene (DBT) (representative refractory sulfur compounds in a real diesel oil), and H2O2 (aqueous solution, 30 wt %) were purchased from Sigma-Aldrich. Other reagents and solvents used in this work are available commercially and were used as received. Typical actual diesel (density = 0.8365 g/mL at 15 °C, total sulfur content ∼0.01 wt %) was used and details are shown in Table 1.
Table 1. Properties of Diesel Real Sample entry 1 2 2 3 6
properties total sulfur specific gravity @ 60 °F density @ 15 °C water content by distillation distilled
IBP 10% 20% 50% 90% FBP
unit
method
result
wt % [−]
ASTM D4294 ASTM D1298
0.0107 0.8365
g/mL vol %
ASTM D1298 ASTM D4006
0.8361 0.025
°C °C °C °C °C °C
ASTM ASTM ASTM ASTM ASTM ASTM
157.8 194.6 213.6 268.6 353.9 384.9
D86 D86 D86 D86 D86 D86
2.2. Preparation of Emulsion Catalyst. H6+xP2W18−xVxO62 (x = 1, 3, and 5) heteropolyacids were prepared according to the similar method reported in the literature.30 For example, H11P2W13V5O62 was prepared as follows: 5.8 g of NH4VO3 and 3.28 g of Na3PO4 were dissolved with 100 mL of deionized water at 40 °C. Thereafter, 3.315 g of Na2WO4 was added and solution was heated to 96 °C, after adjusting the pH (pH = 4.4) by adding 5 mL of H2SO4 (1 mol/L). After 8 h of refluxing, the solution was allowed to cool slowly to 25 °C. Then, 150 mL of diethyl ether was added. The solution was divided into three phases by fully shaking and standing for 1 h. Then an oily red phase at the bottom of decanter, which consists of the mixture of heteropolyacid and diethyl ether was separated. Finally, after evaporation of diethyl ether at 200 °C, a powdered vanadiumsubstituted heteropolyacid was obtained. Emulsion catalysts, [cetrimonium]6+xP2W18−xVxO62 (x = 1, 3, and 5), were synthesized utilizing available procedures for POMs,31 For
Table 2. Selected Factors and Assigned Levels level
5420
entry
factor
1
2
3
4
1 4 5
O/S mole ratio catalyst dosage (g/L) temp (°C)
2 2.5 50
4 5 60
6 7.5 70
8 10 80
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2918 cm−1 belong to the vibrations of the CTA cation. The band at 1468 cm−1 is referred to deformation vibrations of the CH that belongs to CH3 and CH2. The asymmetric stretching vibration of CH is found at 2918 cm−1 for emulsion catalysts. The symmetric stretching vibrations of C H at 2850 cm−1 result from the CH3 and CH2 in the CTA cations. FT-IR results show that the quaternary ammonium cations and polytungstophosphate anions combine with each other effectively by electrostatic interactions.32,33 the strong and broad FT-IR bands around 3442 cm−1 for OH vibrations are indicative of the hydrogen bonding networks between polyoxometalate anion and quaternary ammonium countercation. The XRD patterns of emulsion catalysts are shown in Figure 1. The main positions (2θ) of the XRD peaks, which are typical features of the Dawson-type structure, appear in the region 16− 22°, 25−30°, and 35−42° in the XRD spectra of synthesized emulsion catalysts and The positions and relative intensities of all diffraction peaks match well with those from the powder diffraction file, cards 37-0570 and 41-0369.17 In Figure 1a ([CTA]7P2W17VO62), the X-ray diffraction peaks with d space values of 12.51, 10.1, 8.62, 6.51, 5.19, 4.32, 3.71, 3.24, 2.98, 2.35, 2.22, 2.01, and 1.86, in Figure 1b, ([CTA]9P2W15V3O62) with d space values of 14.57, 12.07, 10.57, 5.82, 4.79, 4.34, 3.72, 2.95, 2.79, 2.50, 2.32, and 1.86, and in Figure 1c, ([CTA]11P2W13V5O62) with d space values of 15.10, 12.95, 10.38, 4.31, 4.09, 3.74, 2.94, 2.53, 2.46, 2.25, 2.03, 1.87, and 1.58 are not associated with pure [P2W18]6− (d space 12.92, 9.90, 9.56, 3.61, 3.52, 3.31, and 2.95).17 So, it can be concluded that tungstate has been substituted by vanadium. The intensity of XRD peaks in the region 16−22° and 25−30° are reduced by the increase the number of vanadium in the frame of POM. The peaks in the region 35−42° is omitted by the increase the number of vanadium in the frame of POM. The XRD spectra of prepared samples show that the crystal structure of samples changes from crystalline to amorphous phase by the increase the number of vanadium substitution. 31 P MAS NMR spectra of emulsion catalysts in the solid state are shown in Figure 2. In comparison to [P2W18]6− P MAS NMR, which has a line with a resonance at −13.75 ppm, substitution of vanadium creates a peak for the vanadiumsubstituted Dawson-type emulsion catalysts at δ = −13.5 ppm (typical of vanadium-substituted Dawson-type POM).33 The peak with resonance δ = −13.5 ppm becomes intense as the amount of vanadium in the frame of POM increases. Meanwhile, the peak around δ = −10.42 ppm, related to the substitution of tungstate by vanadium,33 is omitted as the number of vanadium in frame of POM increases. Mentioned facts and figures by FT-IR, XRD and 31P MAS NMR proves formation of vanadium-substituted Dawson-type emulsion catalysts (W, P). The UV spectra (Figure 3) of the heteropolyacids and the catalysts show absorption bands at 198, 214, 278, and 378 nm. The bands at 198 (charge transfer from O to P), 214 (charge transfer from O to P), 278, and 378 nm (the incorporation of
Table 3. L16 Orthogonal Array Experiment Designed Used for Optimization the Oxidative Desulfurization of Model Fuel removal (%) no.
O/S mole ratio
catalyst dosage (g/L)
temp (oC)
DBT
BT
total sulfur
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
2 2 2 2 4 4 4 4 6 6 6 6 8 8 8 8
2.5 5 7.5 10 2.5 5 7.5 10 2.5 5 7.5 10 2.5 5 7.5 10
50 60 70 80 60 50 80 70 70 80 50 60 80 70 60 50
56.56 65.44 74.25 69.69 85.94 69.38 88.75 84.38 81.13 78.75 81.25 82.50 81.13 88.13 89.06 78.00
30.56 36.00 48.89 71.58 59.03 54.44 77.78 72.22 63.89 68.61 77.78 70.50 67.78 75.00 76.39 66.67
41.50 48.39 59.56 70.78 70.36 60.73 82.40 77.34 71.14 72.88 79.24 75.55 73.39 80.53 81.72 71.43
3. RESULTS AND DISCUSSION 3.1. Characterization of Dawson-Type Emulsion Catalyst. The elementary analyses proves that the content of each element in the prepared samples showed acceptable agreement with calculated values before preparation. The results are as follows: [CTA]7P2W17VO62. Calculated: C, 25.66; H, 4.73; N, 1.58; P, 1.00 ; W, 50.28; V, 0.82. Analytical results: C, 24.37; H, 4.51; N, 1.48; P, 0.96; W, 49.26; V, 0.79. [CTA]9P2W15V3O62. Calculated: C, 31.46; H, 5.79; N, 1.92; P, 0.95; W, 42.31; V, 2.35. Analytical results: C, 29.89; H, 5.53; N, 1.82; P, 0.92; W, 41.77; V, 2.27. [CTA]11P2W13V5O62. Calculated: C, 36.75; H, 6.77; N, 2.26; P, 0.91; W, 35.05; V, 3.74. Analytical results: C, 34.91; H, 6,46; N, 2.12; P, 0.88; W, 33.94; V, 3.62. The FT-IR spectroscopy is widely used to drive finger printing and structural interpretation.32,33 The FT-IR of the prepared vanadium-substituted emulsion catalysts are illustrated in Table 4. The characteristic peaks in the range 700−1100 cm−1 indicate that the four compounds possess Dawson structure.15,32,33 The peak near 1090 cm−1 is related to the antisymmetric stretching vibration of PO, and the peaks of near 912 and 960 cm−1 are associated with the antisymmetric stretching vibration of WO and VO. The vibration band of VO is covered by a WO band. Meanwhile, the spectral band of PO moves to lower frequency gradually along with the increase of the vanadium content. The peaks close to 912 and 780 cm−1 are associated with the antisymmetric stretching vibration of MOM.15,32,33 The former is the bridge bond of total octahedral angle; the latter is the bridge bond of total octahedral edge. In addition, other peaks at 1468, 2850, and
Table 4. FT-IR Absorption Frequencies (cm−1) of Emulsion Catalyst (M = W, V) sample
υas(MOM)
υas(MO)
υas(MO)
υas(PO)
υas(OH)
υas(CH)
[CTA]7P2W17VO62 [CTA]9P2W15V3O62 [CTA]9P2W13V5O62
780 786 778
912 908 912
960 960 958
1091 1090 1089
3442 3442 3442
1468, 2850, 2918 1468, 2850, 2918 1468, 2850, 2918
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Figure 1. Powder X-ray diffraction patterns of emulsion catalyst (d space of main peak mentioned in each diagram).
results.33 These bands have been shifted lower (attributed to Dawson-type anion [P2W18−xVxO62](6+x)−, x = 0, 1, 3, 5) in the
vanadium into the Dawson-type ion) are attributed to the Dawson-type structure and in agreement with the reported 5422
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Figure 2. P MAS NMR of emulsion catalysts.
Figure 3. UV spectra of emulsion catalysts and their mother sources.
spectra of the catalysts. This shift can be related to the intermolecular electronic interactions between the CTA cation and tungestovanadophosphoric anion.33 Vanadium-substituted Dawson-type heteropolyacid and emulsion catalyst were analyzed by XPS. The XPS signal shapes of V2p and W4f of all acid and emulsion catalyst are the same. Meanwhile, they are similar to the signal shape illustrated in Figure 4a,b. The binding energies of V2p3/2 (∼517 eV) and W4f7/2 (∼35 eV) are attributed to V5+ and W6+. The position of peaks of V2p3/2 and W4f7/2 of vanadium-substituted Dawsontype heteropolyacid are the same. Considering this, we can
Figure 4. XPS spectra for the V2p3/2 and W4f7/2. The spectra for H6+xP2W18−xVxO62 (x = 1, 3, 5) and [CTA]11P2W13V5O62 are the nearly the same.
justify that tungstate and vanadium are at the higher oxidation level in [P2W18−xVxO62](6+x)− and introduction of vanadium in 5423
DOI: 10.1021/acs.energyfuels.6b02791 Energy Fuels 2017, 31, 5419−5427
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desulfurization was determined via analysis of variance (ANOVA). ANOVA data were computed for the 95% confidence interval among the studied parameters, the results are illustrated in Table 6. The error term contains information about the following variables for results: uncontrollable factors, factors that are not considered in the tests, and test error (20.84%). The F-ratio is the criterion for distinguishing important factors from those with less importance. According to the Fisher tables29 at a 95% confidence level, the degree of freedom (DOF) of error equals 6 and the DOF of each factor equals 3, for individual factors give an F-value of 8.94. Table 6 illustrates that the F-values of all factors are greater than the value from the Fischer table. This means that the variance of all factors is important for variance of error at 95% confidence level and shows that experiments are reliable. PF obtained for each factor shows that the significant order of parameters is as follows: hydrogen peroxide dosage (53.506%) > catalyst dosage (15.327%) > temperature (10.327%). This determines that factor hydrogen peroxide dosage (amount of oxidant) has the utmost effect on the CODS. In Figure 5, the average effects of various parameters are presented. These values have been obtained by using experiments’ results presented in Table 3. Figure 5 shows that the highest desulfurization level may achieve at O/S mole ratio = 8 (Figure 5a), catalyst dosage = 7.5 g/L (Figure 5b), and temperature = 80 °C (Figure 5c). To test these proposed criteria, the CODS experiment was conducted under these conditions. At these conditions, 91% of DBT and 76% of BT were removed. These results are nearly similar to the results obtained by test number 15. So, the designed experiments could be reputational and the predicted optimum conditions were validated. The results also show that a temperature increase from 60 to 80 °C, has a negligible effect on BT and DBT removal. Though we expected an increase in oxidation rate. The following justification can be considered for this matter: there are two parallel H2O2-involved reactions in the CODS, including BT and DBT oxidation and H2O2 thermal decomposition. As teh temperature is raised, the rate of oxidation is increased. This phenomenon is observable in the range 50−60 °C whereas, after that, thermal decomposition of H2O2 becomes the dominant reaction; consequently, the rate of oxidation decreases. In this regard, T = 60 °C has been selected as the optimum temperature in the following experiments. In the case of the O/S mole ratio, it is obvious that there is a direct relation between oxidation efficiency and H2O2 dosage. Meanwhile, addition of more H2O2 introduces more water into the reaction system, composed of two phases of water and isooctane, which meaningfully affects the reaction environment. When the amount of H2O2 is increased, the mass transfer efficiency is decreased to some extent and, consequently, the catalytic activity decreases. In the next steps, we try to decrease O/S mole ratio.
the frame of the heteropolyanion has no effect on the oxidation level of tungstate.32,33 Also, it can be concluded that the exchange of proton by CTA cation has no obvious effect on the oxidation level of vanadium and tungstate. 3.2. Effect of Catalyst Structure on the CODS. The effect of catalyst structure on the CODS of model fuel using H2O2 (30 wt %) as an oxidant was investigated. The amount of each catalyst was constant throughout the series (10 g/L). The O/S mole ratio was set at 4. The solution was heated to 60 °C using a magnetic stirrer at speed of 1000 r/min. The results shows that in the presence of emulsion catalyst, the DBT removal and BT removal is increased approximately up to 80%. The results have been presented in Table 5, it can be seen that Table 5. Effect of Structure of Emulsion Catalysta removal (%) sample
DBT
BT
total sulfur
without catalyst [CTA]7P2W17VO62 [CTA]9P2W15V3O62 [CTA]11P2W17V5O62
45.06 67.60 82.81 86.69
35.75 57.20 61.73 72.79
40.28 60.42 70.16 82.22
Oxidation reaction conditions: O/S mole ratio = 4, T = 60 °C, rpm = 1000, catalyst dosage = 10 g/L, time = 60 min.
a
the catalytic activity of [CTA]6+xP2W18−xVxO62 (x = 1, 3, 5) is in the order V5 > V3 > V1. In fact, Vn+ which is the most strongly oxidizing element and can be readily reduced to V(n−1)+ with the concomitant oxidation of an organic substrate, improves the catalytic activity of emulsion catalyst. Meanwhile, the substitution of W6+ by V5+ in the POM’s frame results in the generation of more reactive lattice oxygen associated with the W−O−V species.19,20 The similar trend was also reported in the literature on aerobic oxidation of tetrahydrothiophene using Keggin-type heteropolyacids H3+x[PW12−xVxO40] catalysts34 and CODS of DBT using Keggin-type emulsion catalyst, (TBA)3+x[PW12xVxO40] (x = 0, 1, 2, 3).35 3.3. Optimization of CODS Reaction Conditions. In this study, Taguchi method design was used for optimization of the CODS using [CTA]11P2W13V5O62. In this investigation, the effects of various process parameters, catalyst, H2O2 (mole ratio of H2O2(O):sulfur(S)) dosage and reaction temperature on the CODS of model fuel (DBT and BT in isooctane) were investigated. Table 3 presents the studied parameters at their corresponded levels. The oxidative desulfurization percent in each experiment is indicated in the last column of Table 3. The sample was taken after 60 min and put into an ice chamber to stop the reaction. The catalyst in the emulsion sample was separated by centrifugation. The average effect of each parameter at different levels showed that variation in the reaction conditions affects the desulfurization of DBT and BT. The relative significance of the parameters on catalytic oxidative
Table 6. Results of ANOVA for Optimization the Catalytic Oxidative Desulfurization of Model Fuel Using VanadiumSubstituted Emulsion Catalyst factor
DOF ( f)
sum of squares (S)
variance (V)
F-ratio (F)
pure sum (S′)
percent P (%)
O/S mole ratio catalyst dosage temperature others/error total
3 3 3 6 15
1193.745 403.513 300.023 172.533 2069.816
397.915 134.504 100.007 28.755 28.755
13.837 4.677 3.477
1107.478 317.246 213.756
53.506 15.327 10.327 20.84 100.00
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reduce the hydrogen peroxide (30 wt %) consumption, the mole ratio of H2O2 to total sulfur was set at 4 instead of 8. At this condition sulfur removal was improved nearly 7% and the highest level of desulfurization was achieved at a mole ratio of H2O2/formic acid = 1 (Table 7). With the optimum catalyst Table 7. Effect of Different Oxidant System in Oxidation Desulfurization of Different Sulfur Compoundsa removal % type of acid
mole ratio of acid to H2O2
DBT
BT
total sulfur
formic acid
1:4 1:2 1:1 1:4 1:2 1:1
90.65 94.80 97.65 88.70 90.80 96.90
74.79 78.01 81.80 74.30 76.52 80.23
81.47 85.08 88.48 80.36 82.53 87.25
acetic acid
a
Conditions of desulfurization: 7.5 g/L emulsion catalyst, time = 60 min, and temperature = 60 °C.
[CTA]11P2W13V5O62 and under the optimum conditions (T = 60 °C, mole ratio O/S = 4, H2O2/formic acid mole ratio = 1, 7.5 g/L catalyst), model fuel was desulfurized. Figure 6 demonstrates that the sulfur removal versus time. The maximum removal of DBT (∼98%) and BT (∼82%) were achieved approximately after 45 min.
Figure 6. Conversion of DBT and BT over [CTA]11P2W13V5O62 (H2O2/formic acid, mole ratio acid:H2O2 = 1:1, catalyst dosage 7.5 g/ L, O/S mole ratio = 4, T = 60 °C).
3.5. Catalytic Oxidative Desulfurization of Real Diesel. The catalytic oxidation desulfurization of commercial diesel (containing 105 ppmw) was carried out under optimum conditions (O/S molar ratio = 4, H2O2/formic acid mole ratio = 1, T = 60 °C, 7.5 g/L catalyst). At this state, the total sulfur of real diesel oil was decreased from 105 to less than 10 ppmw. This result shows that emulsion catalyst can have high catalytic activity for all kinds of sulfur-containing compounds, which may be present in real diesel oil. This result is also interesting, because there is a competition among sulfur compounds, naphthenics, aromatics, and nitrogen compounds in CODS. 3.6. Reusability of Dawson-Type Emulsion Catalyst. The reusability of the catalyst was investigated to distinguish whether the catalyst would lose its catalytic activity during the reaction. For this purpose, the oxidative desulfurization of real
Figure 5. Average removal of DBT and BT correspond to the different levels of the studied parameters: (a) O/S mole ratio; (b) catalyst dosage (g/L); (c) temperature (°C).
3.4. Effect of Organic Acid on the Catalytic Oxidative Desulfurization. To improve the oxidative desulfurization of DBT and BT, formic acid and acetic acid were also used as cooxidants at different mole ratios to hydrogen peroxide. To 5425
DOI: 10.1021/acs.energyfuels.6b02791 Energy Fuels 2017, 31, 5419−5427
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Energy & Fuels
CODS was applied to a real diesel sample; the results revealed that the level of total sulfur of real sample can be lowered to 10 ppmw after 45 min.
and model fuel was repeated several times. The results are presented in Figure 7. The emulsion catalyst was separated by a
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AUTHOR INFORMATION
Corresponding Author
* Tel: +98 2177240496. Fax: +98 2173227772. E-mail address:
[email protected] (M.R. Dehghani). ORCID
M. R. Dehghani: 0000-0002-6138-8462 Notes
The authors declare no competing financial interest.
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REFERENCES
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Figure 7. Recycle performance of the best catalyst, [CTA]11P2W13V5O62: (a) model fuel; (b) real diesel oil (total sulfur). Reaction conditions: H2O2/formic acid, mole ratio of acid:H2O2 = 1:1, 7.5 g/L catalyst, O/S mole ratio = 4, time = 45 min, and T = 60 °C.
centrifuge after each run and reused without any treatment. It can be seen that in the case of model fuel, after eight runs the catalytic activity is almost the same as the fresh catalyst. However, in the case of real fuel after 4 runs the catalyst activity is declined. This phenomenon can be referred to as catalyst loss during recycling and blockage of catalyst surface.
4. CONCLUSION For the first time, vanadium-substituted Dawson-type emulsion catalysts ([CTA]6+xP2W18−xVxO62 (x = 1, 3, 5)) were synthesized and used for the catalytic oxidative desulfurization of the model fuel (500 ppmw DBT and 500 ppmw BT in isooctane) and real diesel oil. The results showed that the order of catalytic activity was V5 > V3 > V1. The effects of the catalyst dosage, O/S mole ratio and the reaction temperature on the CODS were investigated. The results showed that the hydrogen peroxide dosage and catalyst dosage were the most effective parameters among selected parameters. Then formic acid and acetic acid were used as a co-oxidant to improve the desulfurization efficiency. The results revealed that H2O2/ formic acid was more efficient than H2O2/acetic acid and improved the desulfurization efficiency ∼7%. Under the optimum conditions (H2O2/formic acid mole ratio = 1, T = 60 °C, O/S mole ratio = 4 and 7.5 g/L of the best emulsion catalyst), ∼98% of DBT and ∼82% of BT were removed from the model fuel at 45 min. Finally, the optimum condition of 5426
DOI: 10.1021/acs.energyfuels.6b02791 Energy Fuels 2017, 31, 5419−5427
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