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Kinetics of mixing-assisted oxidative desulfurization of dibenzothiophene in toluene using phosphotungstic acid/H2O2 system: Effects of operating conditions Mark Daniel de Luna, Meng-Wei Wan, Lucille R. Golosinda, Cybelle Morales Futalan, and Ming-Chun Lu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01773 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 25, 2017

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Kinetics of mixing-assisted oxidative desulfurization of dibenzothiophene in toluene using phosphotungstic acid/H2O2 system: Effects of operating conditions Mark Daniel de Lunaa, Meng-Wei Wan,b Lucille R. Golosindac, Cybelle M. Futaland, MingChun Lub* a

Department of Chemical Engineering, University of the Philippines, 1101 Diliman, Quezon City, Philippines

b

Department of Environmental Resources Management, Chia-Nan University of Pharmacy and Science, Tainan 71710, Taiwan

c

Environmental Engineering Program, National Graduate School of Engineering, University of the Philippines, 1101 Diliman, Quezon City, Philippines

d

National Research Center for Disaster-Free and Safe Ocean City, Busan, 604714 Republic of Korea

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Abstract Mixing-assisted oxidative desulfurization of model fuel that contains dibenzothiophene using phosphotungstic acid (HPW) as catalyst and H2O2 as oxidant was evaluated. Characterization analysis of HPW shows that the average crystallite size is 82.39 nm with a disintegrated structure and morphology. The effect of operating parameters such as mixer speed (5,000 to 10,000 rpm), PTA:HPW ratio (0.5:1 to 5:1) and temperature (25°C to 60°C) on the sulfur conversion of DBT was examined. Results show that the sulfur conversion increases with increasing temperature and mixer speed, and lower amount of PTA. The highest sulfur removal and rate constant of 100.0% and 0.1528 min-1 were attained under the following conditions: 1:1 ratio, 40°C and 10,000 rpm. The pseudo-first order equation and Arrhenius equation were applied in order to determine the kinetic rate constant and activation energy of HPW in the oxidation of DBT in a MAOD system. High correlation coefficient values (R2 ≥ 0.93) indicate that pseudo-first order has the goodness of fit in describing the experimental kinetic data. Moreover, the activation energy of HPW derived from the Arrhenius equation was 42.60 kJ/mol.

Keywords: Arrhenius equation, Desulfurization, Dibenzothiophene, First order kinetics, Phosphotungstic acid


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1.0 INTRODUCTION Total refinery products are composed of 75% to 80% of diesel, gasoline and nontransportation fuels, that would all undergo desulfurization processes.1-2 For the past decades, refineries and researches have developed selective and efficient processes and upgraded existing technologies in producing fuels with very low sulfur content in order to meet new environmental regulations with stricter requirements.1,3-4 Moreover, high deposition of sulfur and nitrogen compounds into Asian regions such as South Korea, China, Bangladesh, Japan, India, Laos, Myanmar, North Korea and Thailand was recorded in early 2000s.5 In 2017, regulations of the US Environmental Protection Agency implemented sulfur content of ultra-low sulfur diesel and federal gasoline to be 15 ppmw and 10 ppmw, respectively.6 Moreover, EU regulations restricted the total sulfur content of diesel and gasoline to less than 10 ppmw.7-8 The 2016 Euro IV standards in the Philippines restrict the total sulfur content in diesel to below 50 ppm. The sulfur compounds contained in fuel oils are considered to be detrimental during refining processes and commercial use. Catalyst deactivation during oil processing and corrosion problems in pipeline, pumping and refining equipment are some inherent problems caused by sulfur compounds during refining. In fuel combustion, formation of particulate matter and sulfates contributes to air pollution and formation of acid rain. Moreover, exhaust gasses containing sulfur compounds cause damage to the emission control technology utilized for suspended particulate matter and nitrogen oxides.9-10 Crude oil and refinery streams utilize catalytic hydrodesulfurization (HDS) technology to convert organosulfur compounds into hydrocarbons and H2S in the presence of noble catalysts (Ni-Mo/Al2O3 or Co-Mo/Al2O3) at high pressure (20 to 100 atm using H2) and temperature (300 to 340°C) in middle distillate fuels. Consequently, the Claus process is applied in order to 3 ACS Paragon Plus Environment

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oxidize H2S into elemental sulfur.1,9,11-13 Moreover, the use of HDS as an industrial method entails high capital costs that are attributed to extensive utilization of hydrogen, severe operating conditions and lower catalyst efficiency.14 Organosulfur compounds (OSCs) are classified into two categories: aliphatic compounds (thiols, sulfides, mercaptans, disulfides) and heterocyclic compounds (thiophene, dibenzothiophene, alkyl derivatives). Aliphatic OSCs are easily separated from fuel using HDS due to their high reactivity while heterocyclic OSCs are difficult to remove due to steric hindrance and their ineffective adsorption onto the catalyst surface.1-9,15 Ultrasound-assisted oxidative desulfurization (UAOD) is an innovative technology that utilizes transition metal catalyst, ultrasound and phase transfer catalysis under atmospheric pressure and ambient temperature.10,16-17 The removal of heterocyclic OSCs is attained through selective oxidation, solid adsorption and/or solvent extraction.18 However, the up-scale application for industries of the technology has been challenging due to high capital costs and the method being energy intensive.19 An alternative technology called mixing-assisted oxidative desulfurization (MAOD) was introduced by Lu et al. (2014). Mass transfer limitations that exist in the biphasic MAOD system can be overcome through the application of a high-shear mixer. The formation of finely dispersed bubbles and enhanced fluid-to-fluid interfacial area are made possible due to the application of energy using the mixer .20 Several studies on MAOD that employ high-shear mixers have been carried out where various catalysts and oxidant systems were utilized such as commercial ferrate (VI), phosphotungstic acid/H2O2 system, sodium phosphotungstate/H2O2 system, silicotungstic acid/H2O2 system and phosphomolybdic acid/H2O2 system .19,21-23 Based on the study of Choi et al. (2016), the kinetic rate constant (0.4008 min-1) using MAOD is better in comparison to that of a UAOD (0.3802 min-1) in a sodium phosphotungstate/H2O2 system at 70°C.21 In addition, phosphotungstic acid showed the highest


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catalytic activity and activation energy over phosphomolybdic acid and silicotungstic acid in the sulfur conversion of BT and DBT using MAOD system.22 Furthermore, Lu et al. (2014) investigated the sulfur conversion of DBT using UAOD and MAOD system. Results showed that 99.0% conversion of DBT was attained by both systems in 10 min.19 Heteroployacids (HPAs), characterized by their Keggin structure and molecular formula of [XM12O40]n-, have distinct structure that is composed of a central atom (where X = P5+, As5+, Ge4+ or Si4+ and M = W6+ or Mo6+) and surrounded by polyanions. The acidity of HPAs vary, which highly depends on the central atom and their structural form.24-25 HPAs are well-known for their high catalytic capability due to high thermal stability, superacidity, remarkable sensitivity to electricity and light, impressive redox properties and intrinsic resistance to oxidative decomposition.26-27 Currently, increasing attention to HPAs such as phosphotungstic acid (HPW) has been gained due to their performance in organic catalysis, wherein they are known as “green” industrial catalysts.28-30 Moreover, HPW can be easily prepared and is distinguished by its strong Brønsted acidity that can catalyze many organic reactions that require acid sites.25 Most importantly, HPW has been utilized as a catalyst in the oxidative desulfurization of BT and DBT and results were satisfactory.19,21-23,31-32 In the previous study of Choi et al. (2016), the effect of temperature and reaction time on the removal of DBT and BT using phosphotungstic acid/H2O2 system in a MAOD set-up was evaluated.22 In continuation of the study of Choi et al. (2016), the present work investigated the sulfur conversion of model oil (DBT dissolved in n-toluene) using commercial grade HPW as catalyst and H2O2 as oxidant in a MAOD system. The surface morphology and functional groups of HPW were determined using scanning electron microscopy (SEM), x-ray powder diffractogram (XRD) and Fourier transform infrared spectra (FT-IR). The effect of operating 5 ACS Paragon Plus Environment

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parameters such as mixer speed, amount of PTA and reaction temperature on the sulfur conversion of DBT by HPW was examined. Experimental data was fitted using the pseudo-first order equation, where the kinetic rate constant and activation energy under varying operating conditions were determined. 2.0 EXPERIMENTAL SECTION 2.1 Materials All chemicals and materials were utilized without further purification and are of analytical grade. Dibenzothiophene (C12H8S, 99% purity) was acquired from Alfa Aesar (Taiwan). Phosphotungstic acid (H3PW12O40.14H2O, 98% purity), ethanol and hydrogen peroxide (H2O2) were procured from Shimakyu’s Pure Chemical. Tetraoctylammonium bromide or TOAB (C32H68BrN, 98% purity) and toluene (C7H8) were obtained from Merck Chemical (USA). 2.2 Instrumentation Sulfur analysis was carried out using a gas chromatograph (Agilent Gas Chromatograph 7890A, California, USA) equipped with sulfur chemiluminescence detector (SCD Agilent 355) and fused silica capillary HP-5 MS column with thickness of 0.25-µm film (J&W Scientific, USA).

In order to quantify and identify various sulfur compounds, the initial column

temperature was raised to 200°C for 60 s, followed by gradual increase in temperature at a rate of 20/min up to 280°C and was kept constant for 60 s. 2.3 Characterization The surface and internal morphology of HPW were analyzed with a scanning electron microscopy (S-3000N Hitachi) using a tungsten filament under a vacuum of 1.33 x 10-6 mBar 6 ACS Paragon Plus Environment

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and running at 20.0 kV. XRD analysis was carried out using X-ray powder diffractometer (D8 Bruker) using Cu anode that running at 40 kV and scanning speed of 0.3°/s at a wide range angle (2θ values between 2°-80°). MDI Jade 5.0 software (Materials Data, Inc.) was utilized in the analysis of the XRD data in order to calculate for the crystallite size. The HPW catalyst was analyzed using Fourier transform infrared spectroscopy (FTIR, Jusco FTIR-410) using disc composed of 1:10 ratio of sample to KBr. 2.4 Batch MAOD The stock solution of model diesel fuel with initial concentration of 500 ppm was prepared by dissolving DBT into n-toluene using a 1-L volumetric flask under ambient temperature. Desulfurization experiments were carried out in a 200-mL glass jar reactor where equal amounts of model diesel and H2O2 were mixed along with tetraoctylammonium bromide (PTA) and catalyst. The mixture was heated under varying temperature (25°C to 60°C), mixer speed (5,000 to 10,000 rpm) and amount of PTA (5:1, 3:1, 1:1, 0.5:1) from 3 to 50 min using a high-shear mixer (Heidolph RZR 2020 Silentcrusher). An aliquot of 1 mL sample was obtained at predetermined time intervals and was allowed to cool down. Samples were centrifuged for 10 min, where the synthetic oil contained within the organic phase was drawn out and subjected for further analysis using GC-SCD. Desulfurization of model fuel was computed using Eq. 1: SF =

Ct C0

(1)

where SF is the fraction of sulfur concentration, Ct and C0 are the sulfur concentration at any time t after oxidation (ppm) and initial sulfur concentration (ppm), respectively.


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The sulfur conversion (%S) was calculated using Eq. 2:

 S − St % S =  0  S0

  × 100 

(2)

where S0 and St refer to the initial sulfur concentration (ppm) and final sulfur concentration (ppm) at time t, respectively. 3.0 RESULTS AND DISCUSSION 3.1 Structural characterization In Fig. 1, the FT-IR spectra of HPW are illustrated where the Keggin structure can be characterized by the following strong bands: 1084 cm-1 that is attributed to the stretching of the asymmetric vibrations of P-O of the central PO4 tetrahedral,27 989 cm-1 that is assigned to the stretching vibration modes at terminal bands of the W=O located at the exterior of WO6 octahedron, 899 cm-1 that refers to the W-Ob-W edge sharing and 786 cm-1 that is assigned to bands at W-Oc-W edge sharing and symmetric stretching frequency of Si-O-Si bands.27-28 Based on the wide-angle XRD pattern (Fig. 2), there are several intense diffraction peaks observed for HPW: 8.2°, 9.6°, 17.5°, and 48° that are all found within 2θ = 8 to 60°. The crystallite size was determined using the Jade code, where average crystallite size of HPW is 82.39 nm. The surface morphology of HPW is illustrated at Fig. 3 under different magnifications (150X and 300X), wherein a more disintegrated structure is displayed. HPW illustrates a morphology composed of large and small grain sizes. 3.2 Effect of mixer speed


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The effect of mixer speed on the sulfur conversion was investigated since conversion is highly dependent on both formation of emulsion and its stability. In Fig. 4, results show that after 10 min, the removal of sulfur was 41.02%, 45.46%, 47.46% and 49.58% for mixer speed at 5000, 6000, 8000 and 10,000 rpm, respectively. Moreover, the following sulfur conversion was attained at 30 min reaction time: 92.0% at 5000 rpm, 94.41% for 6000 rpm, 97.32% for 8000 rpm and 99.01% for mixer speed of 10000 rpm. Mixer speed from 5000 to 8000 rpm attained low sulfur removal when compared to mixer speed at 10,000 rpm. It was observed that increasing the mixer speed from 5000 to 10,000 rpm causes a corresponding increase in the sulfur conversion. However, there was only a slight increase observed in sulfur conversion where an increase of less than 7.0% was attained when mixing speed was doubled from 5,000 to 10,000 rpm. The slight increase in sulfur removal can be attributed to the occurrence of back-mixing at higher mixer speed that would lead for smaller droplets to coalesce and form droplets with larger diameters.33 Kinetic rate constants were determined using pseudo-first order equation (Eq. 3 and 4): −

dC = kC dt

(3)

C = kt C0

(4)

− ln

where k is the pseudo-first order kinetic rate constant (min-1), C0 (ppm) and C (ppm) refer to the initial sulfur concentration at zero time and sulfur concentration at time t (min), respectively. The plot of –ln(C/C0) against reaction time is illustrated in Fig. 4b. The data shows goodness of fit when the pseudo-first order equation is utilized in describing the experimental data based on high values of the correlation coefficient (R2 ≥ 0.93). 9 ACS Paragon Plus Environment

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From Table 1, the kinetic rate constants were observed to increase from 0.0891 to 0.1528 min-1 as the mixer speed was increased from 5000 to 10,000 rpm. The performance of a MAOD system is dependent on the quantity and size of droplets formed that would indicate the available contact area for mass transfer to occur.34 As the mixer speed is increased, more energy is applied to the system that result to stronger shear forces, which further intensifies the dispersion in order to create droplets that are finer. This implies there is a larger interfacial mass surface area available per unit volume for the transfer of peroxometal complex from aqueous to organic phase to occur. In addition, surface renewal was enhanced since there is higher turbulence present at 10,000 rpm, which causes lower mass transfer resistance.33 Based on the high sulfur conversion and kinetic rate constants, mixer speed of 10,000 rpm has the least mass transfer resistance due to higher turbulence created and high local energy dissipation that created droplets of smaller sizes. 3.3 Effect of amount of PTA Figure 5 illustrates the effect of varying PTA amount (PTA:HPW ratio of 5:1, 3:1, 1:1 and 0.5:1) on the sulfur conversion of DBT in a MAOD system. In order to discuss the effect of the PTA:HPW ratio on the sulfur conversion of DBT in an MAOD system, the mechanism of oxidative desulfurization using HPW as catalyst, TOAB as PTA and H2O2 as oxidant was provided. 22 The desulfurization in a MAOD system is biphasic in nature, where an aqueous phase and organic phase exist. The first step in the reaction scheme can be summed up in Eq. 5: k1

W (O )n + H 2 O2 →W (O2 )n + H 2 O −

−

(5)

where in the aqueous phase, excess H2O2 causes the metal precursor W(O)n contributed by HPW to undergo peroxidation and to disaggregate to form anionic peroxometal complex, [W(O2)n]-. 10 ACS Paragon Plus Environment

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PTA such as TOAB generates Q+, which is an ammonium cation, that reacts with the peroxometal complex and facilitates its transfer into the organic phase (Eq. 6). k2

W (O2 )n ( aq ) + Q(+aq ) ↔ W (O2 )n Q + ( organic ) −

−

(6)

k3

The presence of [W(O2)-nQ+] in the organic phase enables the oxidation of DBT into sulfoxides and sulfones, as shown in Eq. 7 and 8. k4

W (O2 )n Q + + DBT → Sulfoxide + W (O )n Q + −

−

k5

Sulfoxide + W (O2 )n Q + → Sulfone + W (O )n Q + −

−

(7)

(8)

After oxidation, the peroxometal complex dissociates with PTA, where it is reduced to its original form and transferred back to the aqueous phase. The effect of PTA amount in MAOD system on the sulfur conversion of DBT was investigated in Fig. 5. In an oxidative desulfurization system, the quantity of PTA will define the emulsion droplet stability and catalyst reactivity. The higher concentration of PTA available would favor the activation of larger molecules of cyclic sulfur to the catalytic center that would form stable emulsion droplets and higher oxidation rates. However, results in this study showed the opposite trend, where increasing the amount of PTA from 0.5:1 to 5:1 (PTA:HPW) in the system, the sulfur conversion was observed to decrease. After 10 min reaction time, sulfur conversion was 44.80% for 0.5:1, 100.0% for 1:1, 43.20% for 3:1 and 49.80% for 5:1. The highest sulfur conversion was attained using PTA:HPW ratio of 1:1, where the sulfur conversion based on amount of PTA can be arranged in the order: 1:1 > 0.5:1 > 3:1 > 5:1. Results showed that the highest sulfur conversion was achieved for the following: 100.0% for 0.5:1 after 40 min,


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100.0% for 1:1 after 10 min, 69.40% for 3:1 after 50 min and 50.10% for 5:1 after 50 min. The efficiency of oxidation of DBT and its conversion into dibenzothiophene sulfone (DBTO) are dependent on the presence and concentration of the peroxometal complex in the organic phase. When the amount of PTA was increased from 0.5:1 to 5:1, there is higher amount of quaternary ammonium cation (Q+) present. Based on the study of Mei et al., (2003), the excess H2O2 in the aqueous phase may undergo several reactions such as undergoing thermal decomposition and direct transfer of excess H2O2 by Q+ into the organic phase. The H2O2-PTA complex is polar in nature, which would have an adverse effect on the transfer of the complex itself to the organic medium. 10 In addition, direct formation of H2O2-PTA complexes at high amount of PTA present would yield less concentration of unreacted H2O2 to cause the peroxidation and formation of the anionic peroxometal complexes. This would indicate that there would be reduction in the oxidation of DBT since there would be less [W(O2)-nQ+] complexes available in the organic phase that could be utilized for oxidation. Kinetic studies show that PTA amount of 0.5:1 can be aptly describe using the pseudofirst order equation (R2 = 0.95). Meanwhile, low values of correlation coefficient (0.12 ≤ R2 ≤ 0.62) for 3:1 and 5:1 were obtained. The calculated kinetic rate constants were observed to decrease from 0.0836 to 0.001 when PTA amount was increased from 0.5:1 to 5:1, respectively. Higher amount of Q+ present in the aqueous system would indicate lower amount of excess H2O2 available to cause the disaggregation of W(O)n into peroxometal complex. 3.4 Effect of temperature To determine the optimum temperature, the oxidation reaction was investigated with varying reaction temperature from atmospheric temperature atm (25°C), 40°C, 50°C, and 60°C using PTA:HPW ratio of 1:1, mixer speed of 6000 rpm, C0 of 600 ppm, HPW of 0.05 g and PTA of 12 ACS Paragon Plus Environment

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0.05 g. In Fig 6a, the sulfur conversion of DBT was observed to increase with increasing temperature. The oxidation of DBT can be arranged in the order of: 60°C > 50°C > 40°C > 25°C. This implies that sulfur conversion was favorable at higher temperature, which assists in the acceleration of DBT oxidation. Higher temperature causes a greater amount of metallic peroxides to be produced and its corresponding oxidative capacity towards heterocyclic sulfur compounds would increase correspondingly.14,36 At temperature of 60°C, 100% sulfur conversion was achieved in 40 min. However, longer time of 60 min was needed in order to achieve optimum sulfur conversion of 100%, 95.40% and 93.00% for 50°C, 40°C and 25°C, respectively. Under varying temperature, ln(C/C0) was plotted against reaction time in Fig. 6b. The correlation coefficient values obtained were high (0.86 ≤ R2 ≤ 0.99), which implies that pseudofirst order equation is suitable in describing the DBT oxidation under MAOD system. Table 1 summarizes the kinetic rate constants of DBT under varying temperature, where highest rate constant of 0.1023 min-1 was attained at 60°C, which translates to high sulfur conversion (100.0%) in a short amount of time (40 min). The kinetic rate constant was observed to increase from 0.0219 to 0.1023 min-1 with increasing temperature from 25°C to 60°C, respectively. As the temperature was increased by 35°C, the kinetic rate constant increased by 4.67 times. The activation energy of the oxidation of DBT is calculated using Eq. 9:

ln k = ln A −

Ea 1 R T

(9)

where A, Ea, T and R are the pre-exponential factor, activation energy (kJ/mol), temperature (K) and universal gas constant (8.314x10-3 kJ/mol•K), respectively.37


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Based from Fig. 7, the Arrhenius plot of –ln k against 1/T showed high value of correlation coefficient (R2 = 0.9972). This implies that the Arrhenius plot is suitable in order to attain the activation energy of the DBT oxidation using a MAOD system. The activation energy obtained from the Arrhenius equation is 42.60 kJ/mol. The activation energy and rate constant of previous studies in a MAOD system using different catalysts including HPW were compared to the results obtained in this study. As listed in Table 2, this study attained a lower activation energy when compared to other catalysts such as sodium phosphotungstate. Moreover, kinetic rate constant obtained in this study is the third highest after sodium phosphotungstate and HPW utilized in the study of Choi et al. (2016). 21-22 The lower catalytic performance of phosphomolybdic acid and silicotungstic acid can be attributed to formation of unstable peroxometal complexes such as {PO4[WO(O2)2]4}-3 and {SiO4[WO(O2)2]4}-4.38 The lower value of the kinetic rate constant of HPW obtained in this work is due to lower reaction temperature of 40°C while in the study of Choi et al. (2016), the operating temperature was 70°C. Moreover, the lower value of activation energy for sodium phosphotungstate indicates the ease of catalytic oxidation occurring for a sodium phosphotungstate/H2O2 system when compared to a HPW/H2O2 system. 4.0 Conclusion In this study, the oxidative desulfurization of model fuel using H2O2 as oxidant, HPW as catalyst and TOAB as PTA in a MAOD system was investigated. Characterization of HPW determined that the average crystallite size is 82.39 nm where surface morphology displays a disintegrated structure. Sulfur conversion was observed to be the highest using the following parameters: 1:1 ratio for PTA:HPW, temperature of 60°C and mixer speed of 10,000 rpm. From the experiment, it shows increasing the amount of PTA is detrimental to the oxidation of DBT. 14 ACS Paragon Plus Environment

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At 35 min reaction time, sulfur conversion was observed to decrease from 94.20% to 52.00% with increasing amount of PTA from 0.5:1 to 5:1. The highest sulfur conversion of 100% was attained after 10 min using 1:1 ratio for PTA:HPW. As the amount of PTA was increased in the MAOD system, quaternary ammonium cation would tend to react directly with excess H2O2 and facilitate its transfer into the organic medium. This indicates there would be less concentration of H2O2 available to react with HPW and would lead to less peroxometal complexes being formed, which are needed for the oxidation of DBT. Hence, higher amount of PTA in the system would yield lower oxidizing capacity of the system and lower sulfur conversion. Among the reaction temperatures, 60°C showed the highest oxidation of sulfur, which is similar to the findings of previous researches that have concluded 60°C to be the optimum temperature. As the temperature was increased from 25°C to 60°C, a corresponding increase in sulfur conversion was observed from 53.60% to 100.0% at 40 min reaction time. High temperature causes favorable formation of metallic peroxides, which indicates high oxidizing capacity and high sulfur conversion. When mixer speed was varied from 5000 to 10,000 rpm, an increase in the sulfur conversion and kinetic rate constant were observed. This is due to application of higher shear forces that would create finer droplets, which means higher interfacial area available for the mass transfer of peroxometal complexes from aqueous to organic phase to occur. Kinetic study shows that pseudo-first order equation is suitable in describing the experimental data under varying mixer speed, temperature and amount of PTA due to high correlation coefficient values obtained (R2 ≥0.86) with the exception of 3:1 and 5:1 ratio. Moreover, kinetic rate constants were observed to increase with increasing temperature, mixer speed and decreasing PTA amount. Under the following parametric conditions: 10,000 rpm 15 ACS Paragon Plus Environment

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mixer speed, 1:1 PTA ratio and 40°C, the highest kinetic rate constant and sulfur conversion of 0.1528 min-1 and 100.0% were attained, respectively. Therefore, sulfur conversion of DBT using H2O2 as oxidant and HPW as catalyst in a MAOD system is favorable at higher mixer speed and temperature, as well as PTA:HPW ratio of 1:1.

Acknowledgements The authors would like to thank the Ministry of Science and Technology, Taiwan (Contract No. NSC 104-2221-E-041-002), Engineering Research and Development for Technology - Philippine Department of Science and Technology (ERDT-DOST), and National Research Foundation (NRF) of Korea through Ministry of Education (No. 2016R1A6A1A03012812) for providing financial support for this research undertaking. References (1) Babich, I.V.; Moulijn, J.A. Science and technology of novel processes for deep desulfurization of oil refinery streams: a review. Fuel 2003, 82, 607-631. (2) Katzer, J.R.; Ramage, M.P.; Sapre, A.V. Petroleum refining: Poised for profound changes. Chem. Eng. Prog. 2006, 6, 41-51. (3) Ali, M.F.; Al-Malki, A.; El-Ali, B.; Martinie, G.; Siddiqui, M.N. Deep desulphurization of gasoline and diesel fuels using non-hydrogen consuming techniques. Fuel 2006, 85, 1354-1363. (4) Song, C. An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel. Catal. Today 2003, 86, 211-263.


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(13) Jalali, M.R.; Sobati, M.A. Intensification of oxidative desulfurization of gas oil by ultrasound irradiation: Optimization using Box-Behnken design (BBD). Appl. Therm. Eng. 2017, 111, 1158-1170. (14) Qiu, J.; Wang, G.; Zeng, D.; Tang, Y.; Wang, M.; Li, Y. Oxidative desulfurization of diesel fuel using amphiphilic quaternary ammonium phosphomolybdate catalysts. Fuel Process. Technol. 2009, 90, 1538-1542. (15) Sachdeva, T.O.; Pant, K.K. Deep desulfurization of of diesel via peroxide oxidation using phosphotungstic acid as phase transfer catalyst. Fuel Process. Technol. 2010, 91, 1133-1138. (16) Wan, M.W.; Yen, T.F. Enhance efficiency of tetraoctylammonium fluoride applied to ultrasound-assisted oxidative desulfurization (UAOD) process. App. Catal. A 2007, 319, 237-245. (17) Wan,

M.W.;

Yen,

T.F.

Portable

continuous

ultrasound-assisted

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desulfurization unit for marine gas oil. Energy Fuels 2008, 22, 1130-1135. (18) Etamadi, O.; Yen, T. Aspects of selective adsorption among oxidized sulfur compounds in fossil fuels. Energy Fuels 2007, 21, 1622-1627. (19) Lu, M.C.; Biel, L.C.C.; Wan, M.W.; De Leon, R.; Arco, S. The oxidative desulfurization of fuels with a transition metal catalyst: A comparative assessment of different mixing techniques. Int. J. Green Energy 2014, 11, 833-848. (20) Al Taweel, A.M.; Azizi, F.; Sirijeerachai, G. Static mixers: Effective means for intensifying mass transfer limited reactions. Chem. Eng. Process. Process Intensif. 2013, 72¸51-62.


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(21) Choi, A.E.S.; Roces, S.; Dugos, N.; Wan, M.W. Oxidation by H2O2 of benzothiophene and dibenzothiophene over different polyoxometalate catalysts in the frame of ultrasound and mixing assisted oxidative desulfurization. Fuel 2016, 180, 127-136. (22) Choi, A.E.S.; Roces, S.; Dugos, N.; Wan, M.W. Mixing-assisted oxidative desulfurization of model sulfur compounds using polyoxometalate/H2O2 catalytic system. Sustainable Environ. Res. 2016, 26, 184-190. (23) Choi, A.E.S.; Roces, S.; Dugos, N.; Futalan, C.M.; Wan, M.W. Optimization analysis of mixing-assisted oxidative desulfurization of model sulfur compounds using commercial ferrate(VI). Desalin. Water Treat. 2015, 3994, 1-8. (24) Gomes,

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(37) Li, S.W.; Li, J.R.; Gao, Y.; Liang, L.L.; Zhang, R.L.; Zhao, J.S. Metal modified heteropolyacid incorporated into porous materials for a highly oxidative desulfurization of DBT under molecular oxygen. Fuel 2017, 197, 551-561. (38) Te, M.; Fairbridge, C.; Ring, Z. Oxidation reactivities of dibenzothiophenes in polyoxometalate/H2O2 and formic acid/H2O2 systems. Appl Catal Gen 2001, 219, 267-280.


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Table 1. Kinetic rate constant derived from pseudo-first order equation using HPW at varying mixer speed, temperature and PTA. DBT kinetic rate constant (min-1)

Operating parameters Mixer speed

5,000 rpm

0.0891

6,000 rpm

0.1009

8,000 rpm

0.1277

10,000 rpm

0.1528

25°C

0.0219

40°C

0.0395

50°C

0.0616

60°C

0.1023

Amount of PTA

1:0.5

0.0836

(PTA:HPW)

3:1

0.0190

5:1

0.0010

Temperature


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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

Table 2. Summary of activation energy of HPW and other catalysts in the oxidation of DBT under a MAOD system. Catalyst

Kinetic

rate Activation energy,

Reference

constant (min-1)

Ea (kJ/mol)

HPW

0.1528

42.60

This study

HPW

0.3727

45.90

[22]

Sodium phosphotungstate

0.4008

30.90

[21]

Phosphomolybdic acid

0.0538

29.00

[22]

Silicotungstic acid

0.0108

28.30

[22]

45.60

[38]

Metal

modified Not provided

heteropolyacid


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Figures

% Transmission

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

Page 24 of 30

400

765

1,130 1,495 1,861 2,226 2,591 2,956 3,322 3,687

Wavenumber (cm-1) Figure 1. FT-IR spectra of HPW catalyst.


2.0 5.2 8.4 11.6 14.8 18.0 21.2 24.4 27.6 30.8 34.0 37.2 40.4 43.6 46.8 50.0 53.2 56.4 59.6 62.8 66.0 69.2 72.4 75.6 78.8

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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Intensity

Page 25 of 30

2θ Figure 2. Wide-angle XRD patterns of HPW.


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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

(a)

Page 26 of 30

(b)

Figure 3. SEM images of the HPW catalyst at (a) 150 magnification and (b) 300 magnification.


Page 27 of 30

5

1.0

5000 6000 8000 10000

0.8 0.6 0.4

2

b

5000, r =0.98 2 6000, r =0.97 2 8000, r =0.96 2 10000, r =0.93

4 - ln (C / Co)

a

C / Co

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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3 2 1

0.2

0

0.0 0

5

10

15

20

25

30

0

5

Reaction Time (min)

10

15

20

25

30

Reaction Time (min)

Figure 4. The effect of mixer speed on sulfur conversion of DBT in a MAOD system. (Operating conditions: temperature = 333 K; PTA = 0.05 g; HPW = 0.05 g; C0 = 600 mg/L).


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a

1.0

b

10 9

0.8 PTA:HPW 5:1 0.4 3:1 0.2 0.0 0

10

20 30 Reaction Time (min)

40

0.05:1 1:1 50

-ln (C/Co)

8

0.6 C/Co

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

Page 28 of 30

2

0.5:1, r =0.95 2 1:1, r =undefined 2 3:1, r =0.62 2 5:1, r =-0.12

7 6 2

1

0 10

20 30 40 Reaction Time (min)

50

Figure 5. The effect of amount of PTA on sulfur conversion of DBT in a MAOD system. (Operating conditions: temperature = 333 K; Mixer speed = 5,000 rpm; HPW = 0.10 g; C0 = 600 mg/L).


a 1.0 atm o 40 C o 50 C o 60 C

0.8 C / Co

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

0.6 0.4

b

atm, o 40 C, o 50 C, o 60 C,

3

- ln (C/Co)

Page 29 of 30

2

r = 0.86 2 r = 0.91 2 r = 0.99 2 r = 0.97

2

1

0.2 0

0.0 0

10

20

30

40

50

60

0

10

Reaction Time (min)

20

30

40

50

60

Reaction Time (min)

Figure 6. The effect of temperature on sulfur conversion of DBT in a MAOD system. (Operating conditions: PTA = 0.05 g; Mixer speed = 6,000 rpm; HPW = 0.05 g; C0 = 600 mg/L).


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ln (k) Linear Fit of Arhenius Equation ln (k) -2.5

ln (k)

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

Page 30 of 30

-3.0

y = a + b*x

Equation

-3.5

No Weighting

Weight Residual Sum of Squares

0.00232

Pearson's r

-0.9991

Adj. R-Square

0.99731 Value Intercept

ln (k)

Slope

Standard Error 13.09605

0.48399

-5121.81911

153.62377

-4.0 0.0030

0.0031

0.0032

0.0033

1/T (K) Figure 7. Arrhenius plot of oxidation of DBT in H2O2/HPW using a MAOD system.


Kinetics of Mixing-Assisted Oxidative ... - ACS Publications

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