Oxidative Desulfurization Using in-Situ-Generated Peroxides in Diesel

Jul 22, 2014 - In this work, we explored a two-step oxidative desulfurization (ODS) approach using in-situ-generated peroxides in diesel by light irra...
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Oxidative Desulfurization Using in-Situ-Generated Peroxides in Diesel by Light Irradiation Wei Zhang, Jing Xiao,* Xun Wang, Guang Miao, Feiyan Ye, and Zhong Li* Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education and School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, People’s Republic of China S Supporting Information *

ABSTRACT: In this work, we explored a two-step oxidative desulfurization (ODS) approach using in-situ-generated peroxides in diesel by light irradiation. The supported catalysts were prepared by incipient wetness impregnation and characterized by N2 adsorption test, X-ray diffraction, and X-ray photoelectron spectroscopy. Kinetic curves for peroxide generation by light irradiation and self-decomposition over a MoO3/SiO2 catalyst were measured. Catalytic activities of the catalysts for ODS were tested. Results showed that (a) the efficiency of peroxide generation in diesel under a mercury lamp was much higher than that under a xenon lamp at the same light intensity and can be enhanced at a higher temperature, (b) with in-situ-generated peroxides in diesel by light irradiation, the ODS conversion of catalysts followed the order of MoO3/SiO2 > V2O5/SiO2 > WO3/SiO2 and the conversion reached 75.6% using the MoO3/SiO2 catalyst at the reaction temperature of 45 °C at the O/S ratio of 8, and (c) accompanying the main ODS reaction with hydroperoxides over the MoO3/SiO2 catalyst in diesel, the competing side reaction of peroxide self-decomposition occurred and its kinetics increased dramatically with the reaction temperature. The overall ODS conversion may be affected by the diffusion of bulky refractory sulfur compounds in diesel on the catalyst, which can be enhanced by increasing the pore size of the MoO3/SiO2 catalyst. The two-step oxidative desulfurization approach provides a viable path to achieve clean diesel effectively under mild conditions without using costly hydrogen.

1. INTRODUCTION

ODS involves the oxidation of sulfur compounds by an oxidant to the corresponding sulfoxides/sulfones, followed by the removal of these sulfoxides/sulfones by extraction, adsorption, or distillation. 8,9 Studied oxidants include H2O2,10,11 organic peroxides,9,12 air/oxygen,7,13,14 etc. H2O2 is often used as an industrial oxidizing agent because of a high amount of active oxygen by mass unit (47%) and giving only water as a byproduct.15 The main challenge using H2O2 as an ODS oxidant is the slow reaction rate because of mass-transfer limitation16 in the biphasic oxidation reaction and the subsequent energy-consuming biphasic separation process. The microemulsion approach has been proposed and studied by Li et al.11 to improve mass transfer. Chang et al.12 studied ODS with cumene hydroperoxide on MoO3/SiO2 modified with alkaline earth metals and achieved a high dibenzothiophene conversion of 95% from model fuel. The use of organic peroxides as ODS oxidants enables a simple fixed-bed reactor system; however, the major drawback is the increased cost in handling and storage of the liquid oxidants.7 To overcome the issues of liquid oxidants, employing gasphase oxidant oxygen/air has attracted great attention because of its low cost, abundant availability, ease to be separated, etc. Ma et al.13 proposed to use molecular oxygen as a direct ODS oxidant, but its selectivity to sulfur compounds was not clearly addressed. Xiao et al.2 recently reported the oxidation of sulfur compounds in commercial ultralow-sulfur diesel (ULSD) to sulfoxides over TiO2−CeO2 mixed oxides with air under

Sulfur in diesel converts to sulfur dioxide during combustion, which leads to aerosol of sulfuric acid that causes acid rain and is harmful to the stratospheric zone. Moreover, sulfur in diesel poisons catalysts for the exhaust gas treatment in vehicles and causes the release of CO, NOx, and particulate matter.1,2 Therefore, the environmental restrictions regarding the emissions of pollutants from the refinery are set, which essentially require diesel desulfurization, despite the high cost to meet them.3 Moreover, the issues of diesel desulfurization are becoming more serious because the quality of crude oils available to the refineries is declining. It has become a tough challenge for the refineries to produce a high quality of diesel fuel, such as low-sulfur diesel fuel, from the low quality of feedstock.4 Therefore, diesel desulfurization has become a more important and urgent subject in refinery and research communities worldwide. Desulfurization in a refinery is achieved by conventional catalytic hydrotreating processes, which is energy-intensive (operating temperature over 300 °C and hydrogen pressure at 20−100 atm5) and cost-ineffective to remove refractory sulfur compounds.6 Among the alternative desulfurization technologies, oxidative desulfurization (ODS) attracts great attention because it operates at mild conditions coupled with no hydrogen consumption. Moreover, the refractory sulfur compounds, such as 4,6-dimethyldibenzothiophene, with the least reactivity for hydrotreating, show increased oxidative activity under certain conditions.7 Therefore, ODS has potential to complement hydrotreating for clean fuel production. © 2014 American Chemical Society

Received: May 2, 2014 Revised: July 2, 2014 Published: July 22, 2014 5339

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98%), 4-methyldibenzothiophene (4-MDBT, Sigma-Aldrich, 96%), and 4,6-dimethyldibenzohiophene (4,6-DMDBT, Sigma-Aldrich, 97%) into n-hexane (Sigma-Aldrich, 99%). The model fuel-2 (MDF-2) with a peroxide concentration of 1280 ppm was prepared by mixing a model hydroperoxide compound [cumene hydroperoxide (CHP), Aladdin, 80%] with n-hexane solvent. 2.3. Light Irradiation of Fuel and ODS Experiments. Light irradiation of fuel was carried out in a quartz photoreactor, with the schematic diagram as shown in Figure 1. Diesel was loaded in the flask

ambient conditions. Indirect ODS approaches using air/oxygen as a mild oxidant were proposed. Rao et al.17 studied ODS using molecular oxygen in the presence of sacrificial aldehyde. Oxygen first oxidizes aldehyde to form peracid, which further serves as an oxidant for sulfur compound oxidation. To avoid the use of additional sacrificial agents, Sundararaman et al.7,14 proposed a two-step ODS approach using hydroperoxides generated in situ by catalytic air oxidation, followed by oxidation of sulfur compounds to corresponding sulfones with in-situgenerated hydroperoxides, which addressed a new direction for ODS using oxygen/air. Given that peroxides are the primary products of hydrocarbon auto-oxidation in petroleum induced by light,18 in situ peroxides can be generated in diesel in the presence of air via photolysis reported by Xiao et al.19 Moreover, photolysis, especially ultraviolet (UV) photolysis, has shown effectiveness in treating contaminated water and air at both bench- and commercial-scale levels,20 while the issues, such as decreased efficiency, intensity distribution, mixing effect, lamp surface contamination, etc., in large-size reactors should be carefully considered for a bulk process. In this work, we explored the ODS approach using in-situ-generated peroxides in diesel by light irradiation. In the first step, the fuel is mixed with air under light irradiation to generate peroxides in situ, which further oxidize sulfur compounds in fuel to oxidized sulfur compounds employing an ODS catalyst in the second step. The main purpose of this work is to investigate the activities of supported catalysts MoO3/SiO2, V2O5/SiO2, and WO3/SiO2 for ODS with peroxides generated in situ and clarify the effect of kinetics of hydroperoxide self-decomposition during ODS on overall ODS conversion. A commercial diesel with 320 ppm S was used in this study. The kinetics of peroxide generation and selfdecomposition were monitored using a peroxide photometer. The effect of light irradiation using mercury and xenon lamps on peroxide generation in diesel is investigated. Effects of supported catalysts and operating conditions on ODS conversion using in-situ-generated peroxides are further discussed and reported here.

Figure 1. Schematic diagram of the light irradiation of the fuel system. and irradiated with a built-in 400 W high-pressure mercury lamp/ xenon lamp (Leman, China). A constant air flow at 10 mL/min was bubbled into 70 g of diesel fuel. The treated fuels were sampled periodically, and peroxide concentration was monitored with a Milwaukee MI490 peroxide photometer. ODS experiments were carried out in a 50 mL batch reactor equipped with a temperature controller, condenser, and magnetic stirrer for 2 h. The weight ratio of catalyst/fuel was 1:50. The ODS products in liquid fuel were analyzed using a gas chromatograph/mass spectrometer (GCMS-QP 2010, Shimadzu) equipped with capillary DB-1 (30 m × 0.25 mm × 1 μm), and verified as sulfones (see Figure S1 of the Supporting Information, consistent as reported in the literature7). Highly polar sulfones (dipole of DBT sulfone as 5.454 D20) in treated fuel after ODS were removed by an excess amount of silica adsorbent SiO2-2 (weight ratio of fuel/silica was 8.5) completely, and its adsorption to the barely polar initial refractory sulfur compounds (dipole of 4,6-DMDBT as 0.764 D19) is negligible. The sulfur conversions in real diesel were calculated on the basis of the decrease in the concentration of the total sulfur compounds before and after silica adsorption of oxidized fuels using an Antek 9000 series total sulfur analyzer. The sulfur conversions in MDF were calculated on the basis of the decrease in the concentration of initial model sulfur compounds analyzed using a high-performance liquid chromatograph (HPLC) equipped with an ODS C18 column and an UV detector (Dalian Elite Analytical Instrument Co., China). The detailed procedures and measurement program were reported elsewhere.23 2.4. Catalyst Characterization. Textural properties of adsorbents were measured by N2 adsorption/desorption at 77 K using an ASAP2020 analyzer (Micromeritics). The surface area and pore volume were calculated using the Brunauer−Emmett−Teller (BET) method and the amount of nitrogen adsorbed at P/P0 = 0.95, respectively. The average pore size was evaluated from the desorption branch of the isotherm using the Barret−Joyner−Halenda (BJH) model. Prior to each measurement, all samples were outgassed under vacuum at 150 °C for 8 h. Powder X-ray diffraction (XRD) patterns of the catalysts were obtained on a Bruker D8 Advance powder diffractometer using Cu Kα radiation (λ = 0.154 nm) operated at 40 mA and 40 kV at a scanning range of 10−80° following Joint Committee on Powder Diffraction Standards (JCPDS).

2. EXPERIMENTAL SECTION 2.1. Materials. Silica supports SiO2-1 (prepared in the lab by a sol−gel method21), SiO2-2 (Sigma-Aldrich), and SiO2-3 (Qingdao Sea Chemicals) with different textural properties (as listed in Table 1)

Table 1. Textural Properties of Different SiO2 Supports support

SBET (m2/g)

pore size (nm)

Vtotal (cm3/g)

SiO2-1 SiO2-2 SiO2-3

696.2 450.0 344.9

2.80 6.00 10.27

0.37 0.92 0.94

were used as the supporting materials. Supported ODS catalysts were prepared by an incipient wetness impregnation method assisted with ultrasound. 22 (NH 4 ) 6 Mo 7 O 24 ·4H 2 O (Sigma-Aldrich, 99.98%), (NH4 ) 10 W 12 O 41 ·5H 2 O (Sigma-Aldrich, 99.99%), and NH 4 VO 3 (Sigma-Aldrich, 99%) were used as metal oxide precursors. A total of 10 wt % of Mo/W/V was loaded on a silica support (SiO2-2). The catalysts were then dried overnight and calcined at 400 °C for 4 h. MoO3 (Sigma-Aldrich, 99.5%) was used as a reference. The model compounds were purchased from Sigma-Aldrich and used as such without further purification. 2.2. Fuels. The commercial diesel with 320 ppm by weight of sulfur (ppmw S) was provided by Guangdong Petrochemical Corporation, China. The model fuel-1 (MDF-1) was prepared by dissolving a mixture of 100 ppmw S of dibenzothiophene (DBT, Sigma-Aldrich,

3. RESULTS AND DISCUSSION 3.1. Effect of Light Irradiation on Peroxide Generation in Diesel. Figure 2 shows the amount of peroxides generated 5340

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Figure 2. Amount of peroxides generated in diesel as a function of light-irradiation time using a (a) mercury lamp and (b) xenon lamp at different temperatures (reaction conditions: air flow rate, 150 mL/ min; fuel volume, 70 mL).

Figure 3. Effect of the O/S ratio on sulfur conversion [reaction conditions: catalyst, MoO3/SiO2-2; T, 55 °C; fuel/catalyst ratio (w/ w), 50].

(69.3%) by light irradiation than CHP (82.0%) can be noted, which could be due to a lower average reactivity of in-situgenerated peroxides by light irradiation than CHP for ODS. Another possibility is that accompanying in situ peroxide generation, a certain amount of polar byproducts, such as phenolic compounds, carboxylic acids, ketones, and alcohols,29,30 is generated, which could be adsorbed on the MoO3/SiO2 catalyst during ODS, partially occupying/poisoning ODS catalytic sites and, thus, causing a lower ODS conversion. It should be mentioned that the optimal O/S ratio of 8 is greater than the theoretic stoichiometric value of 2 (to oxidize 1 mol of sulfur compound to its corresponding sulfone). The side reaction of peroxide self-decomposition over MoO3/SiO2 may consume a greater amount of peroxides, which will be further discussed later in this work. Figure 4 shows ODS kinetics of different sulfur compounds in MDF-1 over the MoO3/SiO2 catalyst. The ODS selectivity of

in diesel as a function of light-irradiation time using a mercury lamp and a xenon lamp separately at different temperatures. It was observed that the concentration of in-situ-generated peroxides increases almost linearly with irradiation time in the experimental range, and the reaction rates increase with irradiation temperatures. Peroxides were generated in diesel by light-irradiation bubbling air, which could be attributed to radical chain reactions induced by initiator compounds, i.e., photosensitive polyaromatic compounds24 in fuel via photolysis.25 The mechanism for the initiating peroxide generation by light irradiation could possibly go through a variety of pathways, including initiation through an excited hydrocarbon/oxygen collision complex, which rearranges to hydroperoxides,26 through the singlet oxygen route for peroxide formation, where the transport of oxygen into oil film can be the limiting step,27 etc. Under a mercury lamp, as shown in Figure 2a, by increasing the temperature from 30 to 70 and 110 °C, the peroxide generation rates increase from 2.2 to 4.2 and 12.5 mmol kg−1 h−1, respectively. Under a xenon lamp, as shown in Figure 2b, the peroxide generation rates increase from 0.7 to 1.1 and 2.6 mmol kg−1 h−1, respectively. It indicated that the peroxide generation was much faster under a mercury lamp than a xenon lamp at the same light intensity (400 W; see Figure S2 of the Supporting Information), suggesting that peroxides can be generated more effectively under UV rather than visible-light and infrared irradiation. This can be due to a stronger absorption of UV than visible light and infrared by diesel, indicated by UV−vis absorption spectroscopy of diesel fuel, as shown in Figure S3 of the Supporting Information. Therefore, from the perspective of industrial applications, employing UV irradiation at a higher temperature provides a feasible approach for improving the reaction kinetics of peroxide generation. It should be mentioned that, from the perspective of industrial applications, the peroxide generation rate in fuel by light irradiation was slow, and therefore, introducing a photocatalyst may improve the efficiency of photolysis, which will be further studied in the future work. 3.2. Catalytic ODS Using in-Situ-Generated Peroxides. Figure 3 shows the effect of the O/S ratio using in-situgenerated peroxides on sulfur conversion of diesel fuel. The conversion reached a maximum of 69.3% at an O/S ratio of 8. ODS using model CHP at an O/S ratio of 8 was also carried out, with the sulfur conversion measured as 82.0%. In addition, the result confirmed that in-situ-generated peroxides by light irradiation can serve as the oxidant for ODS over the MoO3/ SiO2 catalyst, similar to the model CHP compound,28 and insitu-generated peroxides by catalytic air oxidation with CuO.7 A lower ODS conversion with in-situ-generated peroxides

Figure 4. ODS kinetics of different sulfur compounds in MDF-1 [reaction conditions: catalyst, MoO3/SiO2-2; T, 55 °C; fuel/catalyst ratio (w/w), 50].

different sulfur compounds follows the order of DBT > 4MDBT > 4,6-DMDBT, consistent with that reported in the literature.28 Given the fact that the electron density on the S atom of DMDBT (5.760) is higher than DBT (5.758),32 which could be more susceptible to electrophilic addition of the ODS reaction,31 other factors, such as steric hindrance effect of methyl groups on 4 and 6 positions of DMDBT occurring in hydrotreating,6 may play a role on the lower ODS reactivity. Figure 5 shows ODS conversion over different silicasupported catalysts. ODS conversion follows the order of MoO3/SiO2 > V2O5/SiO2 > WO3/SiO2 (SiO2-2). Table 2 lists textural properties of these mesoporous SiO2-supported catalysts. In comparison to the textural properties of SiO2-2 supporting material, as listed in Table 1, the BET surface area and total pore volume were decreased and pore sizes were increased. It may be ascribed to partial pore collapse of silica supports during catalyst preparation, calcination in particular, 5341

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Figure 5. ODS conversion over different silica-supported catalysts at 55 °C [reaction conditions: O/S ratio, 8; fuel/catalyst ratio (w/w), 50].

Figure 7. XRD patterns of different silica-supported MoO3 catalysts.

Table 2. Textural Properties of SiO2-Supported Catalysts catalyst

SBET (m2/g)

pore size (nm)

Vtotal (cm3/g)

MoO3/SiO2 V2O5/SiO2 WO3/SiO2

312.9 351.6 413.8

7.47 6.45 6.08

0.65 0.65 0.73

possibly because of its thin wall thickness and/or low thermal stability under high temperatures.33 The pore sizes of SiO2supported catalysts were noted to follow the order of MoO3/ SiO2 > V2O5/SiO2 > WO3/SiO2, consistent with the order of ODS conversion. In comparison to V2O5/SiO2 and WO3/SiO2 with higher surface areas (351.6 and 413.8 m2/g, respectively), bulk MoO3 with more than a magnitude of 2 lower surface area (1.7 m2/g) had a higher ODS reactivity (sulfur conversion of 47.1%), suggesting that MoO3 is a suitable active catalyst species for ODS. Figure 6 shows ODS conversion of different silica-supported MoO3 catalysts as a function of the reaction temperature. The

Figure 8. XPS spectra of different silica-supported MoO3 catalysts.

MoO3 catalyst. The doublet located at 235.85 and 232.65 eV can be assigned for Mo6+ species, while no peaks at 234.9 and 231.7 eV and 232.3 and 229.1 eV34 suggest that no Mo5+ and Mo4+ were present in all of the MoO3/SiO2 catalysts. Table 3 shows textural properties of supported MoO3 catalysts. The BET surface area of the three silica-supported Table 3. Textural Properties of Supported MoO3 Catalysts catalyst

SBET (m2/g)

pore size (nm)

Vtotal (cm3/g)

MoO3/SiO2-1 MoO3/SiO2-2 MoO3/SiO2-3

502.6 312.9 294.0

2.88 7.47 10.74

0.28 0.65 0.83

MoO3 catalysts follows the order of MoO3/SiO2-1 (502.6 m2/ g) > MoO3/SiO2-2 (312.9 m2/g) > MoO3/SiO2-3 (294.0 m2/ g), reversed with the trend of ODS conversion. The results further suggest that the surface area of silica-supported MoO3 catalysts is not the major factor governing the ODS reactivity. As seen in Figure 6, for the silica-supported MoO3 catalysts, ODS conversion first increases to the maximum value (75.6, 69.3, and 30.8% for MoO3/SiO2-3, MoO3/SiO2-2, and MoO3/ SiO2-1 catalysts, respectively) with the temperature and then decreases by further increasing the reaction temperature. Assuming that no competing side reactions occurred accompanying ODS, ODS conversion would increase with the reaction temperature, obeying the thermodynamic rules of a single ODS reaction. However, experimental results show that the ODS conversion did not increase continuously with the temperature. It can be ascribed to the presence of side reactions of hydroperoxide self-decomposition during the ODS reaction. These competing side reactions are strongly affected by the reaction temperature. Although catalytic ODS using hydroperoxides was reported in the literature, hydroperoxide self-decomposition has not

Figure 6. ODS conversion of different silica-supported MoO3 catalysts as a function of the reaction temperature [reaction conditions: O/S ratio, 8; fuel/catalyst ratio (w/w), 50].

ODS conversion of these catalysts followed the order of MoO3/SiO2-3 > MoO3/SiO2-2 > MoO3/SiO2-1. The ODS conversion of MoO3/SiO2-3 catalyst reached a maximum of 75.6% at 45 °C. Figure 7 shows XRD patterns of silicasupported MoO3 catalysts as well as the bulk MoO3 catalyst. Characteristic peaks of crystalline molybdenum oxide species are present in the bulk MoO3 catalyst, consistent with being reported in the literature.14 In a sharp contrast, crystalline molybdenum oxide species are barely observed in the silicasupported MoO3 catalysts, suggesting that molybdenum oxides were highly dispersed on silica supports at Mo loading of 10 wt %. MoO3 particle sizes on the silica support were small, which is beyond the detection limit of the XRD technique.19 Figure 8 shows X-ray photoelectron spectroscopy (XPS) spectra of different silica-supported MoO3 catalysts as well as the bulk 5342

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the side-reaction rate of hydroperoxide self-decomposition become high and ODS conversion in diesel can be dominated by the kinetics of the competing pathways of ODS by hydroperoxides and hydroperoxide self-decomposition rather than the thermodynamics. Therefore, a maximal ODS conversion can be reached at an optimal reaction temperature balancing the two reactions, as shown in Figure 6. A trade-off between the two reactions in Figure 10 should be taken into consideration in catalyst design. When ODS conversion is under thermodynamics control, a catalyst with high catalytic activity for ODS and low catalytic activity for peroxide decomposition would be preferable. When ODS conversion is under kinetics control, a catalyst with low activation energy for ODS and high activation energy for peroxide decomposition would be ideal. At such circumstances, kinetics of two competing pathways could be affected by the diffusion rates of bulky refractory sulfur compounds reaching active sites of catalysts, which may be changed by the textural properties, in particular, the pore size of catalysts.33,35,36 For the three MoO3/SiO2 catalysts, the pore size follows the order of MoO3/SiO2-3 (10.74 nm) > MoO3/SiO2-2 (7.47 nm) > MoO3/SiO2-1 (2.88 nm) as listed in Table 3, consistent with the order of ODS conversion. For ODS over the MoO3/SiO2-1 catalyst with a pore size of 2.88 nm, a certain amount of polar sulfones strongly adsorbed over −OH sites on the silica support19 may contribute to steric hindrance and, thus, a low diffusion rate of bulky refractory sulfur compounds for a low ODS conversion. Nonetheless, in addition to textural properties, there may be some other reasons for different activities over SiO2-supported catalysts, such as band gap, dispersion, morphology, etc. Besides, ODS conversion of the bulk MoO3 catalyst is as high as 47.1% (Figure 5), even though a quite low BET surface area of 1.7 m2/g, which could also be due to a faster diffusion of sulfur compounds to readily access the active sites on the bulk MoO3 catalyst, possibly exposed on the outer surface. 3.3. Catalyst Regeneration. The ODS performance of the fresh and regenerated MoO3/SiO2-2 catalysts in multiple runs are shown in Figure 11. The regeneration of the spent MoO3/

been disclosed. In this work, hydroperoxide self-decomposition as a function of the reaction time was monitored for the first time. Figure 9 shows kinetics of hydroperoxide self-decom-

Figure 9. Kinetics of hydroperoxide self-decomposition in MDF-2 over the MoO3/SiO2-2 catalyst at different temperatures.

position in MDF-2 (with no sulfur compounds added) over the MoO3/SiO2 catalyst at different temperatures. In the absence of the catalyst, hydroperoxides in MDF-2 remained stable without decomposition even at 95 °C. Interesting, in sharp contrast, self-decomposition of hydroperoxides over the MoO3/SiO2 catalyst were observed at the temperature range of 35−95 °C. Peroxides can be decomposed completely in 60, 45, and 30 min over the MoO3/SiO2 catalyst at 55, 75, and 95 °C, respectively, suggesting that the decomposition rate increases sharply with the temperature. The results further inferred that the kinetics of hydroperoxide self-decomposition may play a negative role on ODS conversion at higher temperatures because of a fast self-decomposition of the oxidant. The consumption of oxidants (hydroperoxides) by the side reaction of hydroperoxide self-decomposition accompanying ODS also resulted in the requirement of a higher optimal O/S ratio of 8, instead of the stoichiometric ratio of 2, as illustrated in Figure 3. Figure S4 of the Supporting Information shows sulfur conversion and peroxide concentration in light-irradiated diesel (with 320 ppm S and 1280 ppm peroxides) as a function of the reaction time over the MoO3/SiO2-2 catalyst. It was observed that the ODS reaction reached equilibrium at around 60 min, coincident with the time of an almost complete consumption of peroxides in diesel, which was also similar as the time of an almost complete self-decomposition of CHP in MDF-2 at 55 °C, as shown in Figure 9. The results further suggested that the kinetics of hydroperoxide self-decomposition could play a significant role on ODS conversion. Figure 10 illustrates the

Figure 11. ODS performance of the fresh and regenerated MoO3/ SiO2-2 catalysts in multiple runs (regeneration conditions: calcined at 500 °C for 4 h).

Figure 10. Competing pathways of main (A) and side (B) reactions over the MoO3/SiO2 catalyst with hydroperoxides in diesel.

SiO2 catalyst was performed via calcination at 500 °C for 4 h under an air flow rate of 50 cm3/min, the same as the calcination conditions for the fresh catalyst preparation. The ODS conversions of the regenerated MoO3/SiO2 catalyst in the first 4 cycles were 69.3, 68.5, 69.2, and 75.0%, respectively, suggesting that the MoO3/SiO2 catalyst can be well-regenerated by air oxidation and reactivated at 500 °C.

competing pathways of coexisting main (A) ODS with hydroperoxides and side (B) peroxide self-decomposition reactions in diesel over the MoO3/SiO2 catalyst. It is quite likely that, at lower temperature ranges, the side-reaction rate of hydroperoxide self-decomposition is low and ODS conversion is under thermodynamic control and governed by the ODS reaction activity. By further increasing reaction temperatures, 5343

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4. CONCLUSION Oxidative desulfurization using in-situ-generated peroxides in diesel by light irradiation was demonstrated and studied in this work. Peroxides can be effectively generated in diesel via lightirradiation purging air, which further acted as the oxidant for the subsequent catalytic ODS. The two-step ODS approach provides a viable path to achieve clean diesel effectively under mild conditions without using costly hydrogen. The rate of peroxide generation increases with the decrease in the wavelength of the light source and the increase in the reaction temperature, implying that the employment of UV irradiation of diesel at a higher temperature provides a feasible approach to improve the kinetics of peroxide generation in diesel. The O/S ratio was optimized to be 8, and the ODS reactivity followed the order of DBT > MDBT > DMDBT. With in-situgenerated peroxides in diesel by light irradiation, the ODS conversion of catalysts followed the order of MoO3/SiO2 > V2O5/SiO2 > WO3/SiO2, of which the MoO3/SiO2-3 catalyst with the largest pore size (10.74 nm) but the lowest SBET showed the highest ODS conversion of 75.6% at 45 °C. Additionally, the MoO3/SiO2 catalyst can be well-regenerated via air oxidation. Accompanying the catalytic ODS, the side reaction of peroxide self-decomposition occurred simultaneously over the MoO3/SiO2 catalyst, and its kinetics increased dramatically with the reaction temperature. The overall ODS conversion is jointly affected by the ODS reaction and side reaction. Improvement of the diffusion of bulky refractory sulfur compounds in diesel over the catalyst would be helpful to enhance ODS conversion, which can be realized by increasing the pore size of the MoO3/SiO2 catalyst.

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ASSOCIATED CONTENT

* Supporting Information S

Mass spectra of the (a) oxidation product of DBT after ODS over the MoO3/SiO2-2 catalyst from a model fuel adding CHP and (b) standard DBTO2 (Figure S1), spectra distribution of a 400 W (a) mercury lamp and (b) xenon lamp (Figure S2), UV−vis absorption spectroscopy of diesel fuel (Figure S3), and sulfur conversion and peroxide concentration in diesel as a function of the reaction time over the MoO3/SiO2-2 catalyst at 55 °C (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +86-20-87113501. Fax: +86-20-87113513. E-mail: [email protected]. *Telephone: +86-20-87113501. Fax: +86-20-87113513. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to acknowledge the research grants provided by the National Natural Science Foundation of China (21306054), the Guangdong Natural Science Foundation (S2013040014747), the Specialized Research Fund for the Doctoral Program of Higher Education (20130172120018), and the Fundamental Research Funds for the Central Universities (2013ZM0047). 5344

dx.doi.org/10.1021/ef500998v | Energy Fuels 2014, 28, 5339−5344