Catalytic Oxidative Desulfurization of Fuel by H2O2 In Situ Produced

Mar 8, 2012 - Wenfeng Li , Yingjie Li , Yong Chen , Qingnan Liu , Yingzhou Lu , Hong Meng , and Chunxi Li. Energy & Fuels 2017 31 (9), 9035-9042...
4 downloads 0 Views 459KB Size
Download PDF
Article pubs.acs.org/IECR

Catalytic Oxidative Desulfurization of Fuel by H2O2 In Situ Produced via Oxidation of 2-Propanol Hong-Xing Zhang,†,‡ Jia-Jun Gao,†,‡ Hong Meng,‡ Ying-zhou Lu,‡ and Chun-Xi Li*,†,‡ †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ‡ College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ABSTRACT: A novel integrated process is proposed for the catalytic oxidative desulfurization of fuel oil, in which the oxidant H2O2 is in situ generated by oxidizing 2-propanol with oxygen, and its feasibility is evaluated in terms of the S-conversion of 3-methylthiophene (3-MT), benzothiophene (BT), and dibenzothiophene (DBT) in octane under varying conditions. The catalysis of [π-C5H5NC16H33]3[PW4O16] is found to be much superior to H3PW12O40 and [(C4H9)4N]3[PW12O40] due to its good dispersivity in oil and adsorptivity for S-compounds. Some influencing factors for the S-conversion were studied, viz., time, temperature, various S-compounds, and the amount of 2-propanol, initiator, oxygen, and catalyst. All factors that favor the production of the 2-propanol radicals affect the desulfurization rate remarkably. Both BT and DBT can be removed efficiently at mild conditions (1.4 MPa O2, 90 °C) in 6 h with S-conversion above 96%, and the resulting sulfones can be separated via settling or filtration.

1. INTRODUCTION The limitation of the S-content in fuel oils has become stricter for the abatement of air pollution and acid rain.1 The current 50 ppm is already a challenge for most refineries, let alone reducing the S-content further to 10 ppm.2 The current technology of hydrodesulfurization (HDS) is effective for aliphatic and acyclic S-compounds, but less effective for the treatment of benzothiophene (BT), dibenzothiophene (DBT), and their derivatives.3 Thus, it is necessary to develop alternative ultradeep desulfurization processes such as adsorption desulfurization (ADS),4−6 extraction desulfurization (EDS),7−9 and oxidative desulfurization (ODS)10−12 and in situ hydrodesulfurization.13 Among them, the ODS process seems a promising one for deep desulfurization technology because it can be carried out under mild conditions with low cost of operation. In the ODS process, these organosulfur compounds are oxidized to their corresponding sulfoxides and/or sulfones. Afterward, these highly polarized products can be removed by a number of separation processes including solvent extraction, adsorption, and distillation, etc.14 Among the various types of oxidants used in the ODS process, H2O2 is more attractive because it is environmentally friendlier.10 However, its industrial application in an ODS process has been hindered by some intrinsic obstacles, e.g., higher cost, explosiveness in bulk storage, high content of water, and thus immiscible to oil.15 To solve this problem, an integrated process of the oxidation and H2O2 generation has been proposed here. With respect to the in situ production of H2O2, the following strategies can be considered: (1) the anthraquinone process,16 which however can introduce new contaminants to the fuels, e.g., anthraquinone and N- and P-containing solvents; (2) direct synthesis of H2O2 from the H2/O2 mixture,17 which is explosive and risky;18 and (3) synthesis of H2O2 through oxidation of 2-propanol by oxygen,19 and the byproduct acetone could be recycled back to 2-propanol by hydrogenation.20 The advantage of this integrated © 2012 American Chemical Society

process is that the H2O2 needed can be formed in situ and used instantly.21 In the present work, the third integrated process was first used in the desulfurization study of model oil. And a reactioncontrolled phase-transfer catalyst [π-C5H5NC16H33]3[PW4O16]22 was adopted in the present ODS process. This catalyst is insoluble in both water and organic solvent but becomes oilsoluble in the presence of H2O2; when the H2O2 is used up, it precipitates and thus is easily recycled.22 On the basis of the insight into 2-propanol oxidation and ODS process with H2O2, the present integrated process for DBT oil, as shown in Figure 1, is assumed to be a serial processes involving the following courses:

Figure 1. Reaction mechanism of the integrated ODS process.

(1) Oxidation of 2-propanol to produce H2O2: According to the literature,23 the oxidation of 2-propanol by oxygen is a free radical reaction, which includes a series of steps, Received: Revised: Accepted: Published: 4868

January 5, 2012 March 7, 2012 March 8, 2012 March 8, 2012 dx.doi.org/10.1021/ie300035c | Ind. Eng. Chem. Res. 2012, 51, 4868−4874

Industrial & Engineering Chemistry Research

Article

85% H3PO4 (0.29 g, 2.5 mmol) dissolved in 1 mL of deionized water at room temperature. And then the whole solution was diluted with 20 mL of deionized water and stirred for 30 min. To the resultant solution, a solution of hexadecylpyridinum chloride (1.8 g, 5 mmol) in dichloromethane (40 mL) was added dropwise with stirring vigorously. And the solution was stirred for more than 60 min. The organic phase was then separated by a separatory tunnel, dried with Na2SO4, filtered, and distilled at 60 °C at atmospheric pressure. The obtained pale yellow solid product was further dried under vacuum at 50 °C and ground into powder for use. [(C4H9)4N]3[PW12O40] was synthesized according to Zhang et al.27 2.3. Catalyst Characterization. The C, H, and N elementary analyses were performed on an Elementar Vario EL elemental analyzer. The infrared spectra (FTIR) of the catalysts were recorded on a Nicolet Nexus 8700 FTIR spectrometer with samples prepared as KBr disks in the 400− 4000 cm−1 range. 2.4. Integrated ODS Process. Model diesel oils were prepared by dissolving DBT, BT, or 3-MT in n-octane, respectively, with their initial S-content all being 1000 ppm. In a typical desulfurization experiment, the catalyst, AIBN, 2-propanol, and model oil were added in turn to a 300 mL oxygen bomb which was then sealed off and filled with oxygen. The oxygen pressure in the bomb was controlled by an oxygenator which was equipped with a pressure gauge. The bomb was then placed in a thermostatic water bath at a specific temperature from 50 to 90 °C and stirred vigorously with magnetic stirrer. After the reaction, the resulting solution was filtered and the filter liquor was analyzed by gas chromatography−flame ionization detector (GC-FID; Shimadzu, GC-2010) equipped with a SE-54 capillary column (5% phenyl poly(dimethylsiloxane) as stationary phase; 30 m × 0.25 mm i.d. × 0.25 μm film thickness; Lanzhou Institute of Chemical Physics, China) for DBT and BT, and an AT.FFAP capillary column (poly(ethylene glycol) modified with nitroterephthalic acid as stationary phase; 30 m × 0.53 mm i.d. × 1.0 μm film thickness; Lanzhou Institute of Chemical Physics) for 3-MT. The GC operating conditions were as follows: the temperatures for injector, detector, and oven were 340, 250, and 250 °C, respectively, for DBT, and 250, 250, and 200 °C, respectively, for BT. For 3-MT, the temperatures for injector and detector were 180 and 200 °C, and the oven temperature was programmed from 60 (hold for 3 min) to 80 °C (hold for 2 min) at 2.5 °C·min−1. The injection volume was 0.4 μL for all samples. 2.5. Adsorption Experiments. Adsorption experiments were carried out to determine the adsorption capacity of catalysts for DBT from oil at 70 °C and atmospheric pressure with magnetic stirrer. In the adsorption experiments, 10 g of 1000 ppm DBT oil and 1 g of catalysts were loaded in the bomb. After stirring for 6 h, the resulting suspension were centrifuged and the amount of DBT adsorbed on the catalysts were gained by measuring the decrease of the sulfur concentration in the treated oil using GC.

viz., chain initiation, chain propagation, and chain termination, whereby some active intermediates such as 2-propanol radicals and peroxo-2-propanol radicals, and H2O2 molecules can be produced. The overall reaction is as (CH3)2 CHOH + O2 → (CH3)2 CO + H2O2

(1)

(2) Oxidation of the catalyst [π-C5H5NC16H33]3[PW4O16] to its oil-soluble peroxide form [π-C5H5NC16H33]3[PO4{W(O)2(O2)}4] in the presence of H2O2,22 resulting in the active oxygen being brought into the oil phase. (3) Oxidation of DBT to its corresponding sulfoxide (DBTO) and then to sulfone (DBTO2) by [PO4{W(O)2(O2)}4]3−, meanwhile [PO4{W(O)2(O2)}4]3− is reduced to [PO4{WO3}4]3−:22,24 According to the research of Torres-Garciá and co-workers,14 the oxidation of DBT with polyoxometalates (POM) as catalyst consists of two consecutive stages, the first one leading to the formation of sulfoxide and the second one yielding sulfone. And each stage actually occurs in two separated steps, i.e., addition and elimination, shown as follows:14

stage 1: POO + DBT → POO−DBT → PO + DBTO stage 2: POO + DBTO → POO−DBTO → PO + DBTO2 where POO represents the peroxometalate and PO is the reduced form. Therefore, the direct mechanism yielding DBTO2 was found to be ruled out. For the oxidation products, Torres-Garciá et al.14 and Can Li et al.25 recently clarified that the final oxidized products of DBT and BT are the corresponding sulfones, while the oxidation of thiophene yields benzaldehyde, benzoic acid, and sulfuric acid due to the unstability of thiophene sulfone. As the number of substitute groups on thiophene increase, the corresponding sulfone becomes more stable and thus can be detected.25 (4) Reoxidation of [PO4{WO3}4]3− to its peroxide form as in step 2 forming a reaction loop: After a period of time, high sulfur conversion can be obtained for the thiophenic S-compounds by integrated ODS process.

2. EXPERIMENTAL SECTION 2.1. Materials. Tetrabutylammonium bromide and azodiisobutyronitrile (AIBN), both with AR grade, were purchased form Tianjin Fuchen Chemical Reagent Co., Ltd. Tungstic acid (H2WO4, AR grade) was purchased from Sinopharm Chemical Reagent Co., Ltd. Phosphoric acid, hydrogen peroxide, and 2-propanol (IPA), all with AR grade, were purchased from Beijing Chemical Works. Hexadecylpyridinum chloride (AR grade) was purchased from Aladdin Chemistry Co., Ltd. Dichloromethane, trichloromethane, and n-octane, all with AR grade, were purchased from Beijing Chemical Co., Ltd. Dibenzothiophene (DBT, 99%), benzothiophene (BT, 97%), and 3-methylthiophene (3-MT, 99%) were purchased from Acros Organics. All reagents were used as received. 2.2. Preparation of Catalysts. [π-C 5H5NC16H33]3[PW4O16] was prepared according to the procedure described by Sun et al.26 A yellow suspension of tungstic acid (2.5 g, 10 mmol) in 10 mL of 30% H2O2 aqueous solution was stirred for 60 min at 60 °C. To the resulting milky solution was added

3. RESULTS AND DISSCUSSION 3.1. Characterization of the Catalysts. The elementary analysis results are as follows: Anal. Calcd for [(C4H9)4N]3[PW12O40]: C, 15.99; H, 3.02; N, 1.17. Found: C, 16.10; H, 3.14; N, 1.08. Anal. Calcd for [π-C5H5NC16H33]3[PW4O16]: C, 39.09; H, 5.94; N, 2.17. Found: C, 38.19; H, 5.87; N, 2.10. It is noted that the element percentages between the calculated and 4869

dx.doi.org/10.1021/ie300035c | Ind. Eng. Chem. Res. 2012, 51, 4868−4874

Industrial & Engineering Chemistry Research

Article

thiophenic ring of DBT, which suggests a concentrating of DBT in the vicinity of the catalyst and thus a high oxidation rate. In contrast, H3PW12O40 and [(C4H9)4N]3[PW12O40] show negligible adsorbance for DBT, and thus the concentration of DBT near the catalyst is the same as that in the bulk oil. On the other hand, the catalyst [π-C5H5NC16H33]3[PW4O16] can be changed from insoluble to oil-soluble at its peroxide form [(C4H9)4N]3[PO4{WO(O2)2}4] in the presence of H2O2, and thus homogeneously dispersed into oil, which in return facilitates the oxidation of DBT. [(C4H9)4N]3[PW12O40] is however neither soluble to 2-propanol nor soluble to oil and thus suspends in the reaction system as a heterogeneous catalyst with the lowest catalytic activity due to the limited accessibility to DBT. In comparison with [(C4H9)4N]3[PW12O40], H3PW12O40 as a neat inorganic catalyst can be partly dissolved in the reaction system due to the presence of 2propanol, leading to a little higher sulfur conversion of about 35% in 6 h. Considering the unique properties of [πC5H5NC16H33]3[PW4O16], i.e., a reaction-controlled phasetransfer catalyst with definite adsorptivity for the thiophenic S-compounds, it is chosen here as a representative catalyst for the present ODS process in the following study. 3.3. Reaction Time on Sulfur Conversion. The influence of reaction time on the sulfur conversion of DBT in n-octane was studied in the time range from 1 to 8 h, and the results are presented in Figure 3. As seen from the figure, the sulfur

the found values are very close, which indicates that the cation is integrated with the anion accurately. IR spectra of the three catalysts, i.e., [π-C5H5NC16H33]3[PW4O16], H3PW12O40, and [(C4H9)4N]3[PW12O40] are displayed in Figure 2. It is noted that the IR spectrum of

Figure 2. IR spectra of [π-C5H5NC16H33]3[PW4O16] (spectrum A), H3PW12O40 (spectrum B), and [(C4H9)4N]3[PW12O40] (spectrum C).

[π-C5H5NC16H33]3[PW4O16] is similar to that reported in the literature,26 implying that the structure of the catalyst is consistent with its formula. And the peroxo band is not found at the position of ν(O−O) = 840 cm−1; the peroxo group is likely destroyed by the long time treatment at higher temperature.28 As shown in Figure 2, the IR spectra of both [(C4H9)4N]3[PW12O40] and H3PW12O40 show characteristic skeletal vibrations of the Keggin structure; i.e., ν(P−O) = 1080 cm−1, ν(WO) = 977−980 cm−1, ν(W−Ob−W) = 897−901 cm−1, and ν(W−Oc−W) = 808−816 cm−1.27 These data suggest that the Keggin structure of H3PW12O40 is still retained in [(C 4 H 9 ) 4 N] 3 [PW 12 O 40 ] but is not present in [πC5H5NC16H33]3[PW4O16]. 3.2. Different Catalysts on Sulfur Conversion. The catalytic performance of three catalysts, i.e., H3PW12O40, [(C4H9)4N]3[PW12O40], and [π-C5H5NC16H33]3[PW4O16], were studied in the present integrated ODS process in terms of the sulfur conversion of DBT under the same conditions. The results are presented in Table 1 along with adsorptivity of the catalysts for DBT and the dissolubility of the catalysts in different solvents. As seen from Table 1, the catalytic activity of the three catalysts follows the order of [π-C5H5NC16H33]3[PW4O16] ≫ H3PW12O40 > [(C4H9)4N]3[PW12O40] with their sulfur conversion being 98.90, 34.87, and 18.65%, respectively. This result may be closely associated with the adsorption ability of the catalysts for DBT from oil and the dissolubility of the catalysts in the reaction system from the point of view of reaction kinetics since other reaction conditions are the same. On one hand, [π-C5 H5 NC 16 H33]3[PW 4O16] shows the strongest adsorption for DBT among the three catalysts, being 13.94 mg of DBT/(g of catalyst), due to the specific π−π interaction between the pyridinium ion of the catalyst and the

Figure 3. Influence of reaction time on sulfur conversion. Reaction conditions: DBT oil = 10 g; 2-propanol = 2.5 g; mass ratio m(AIBN)/ m(2-propanol) = 4%; mole ratio n(catalyst)/n(S) = 1:50; catalyst = [π-C5H5NC16H33]3[PW4O16]; initial gage pressure = 1.4 MPa; T = 70 °C.

conversion increases steadily from 23.07 (1 h) to 59.26% (4 h), followed by a dramatic increase from 59.26 (4 h) to 98.90% (6 h), and then levels off. Obviously, the sulfur conversion rate from 4 to 6 h is much faster than that in the preceding 4 h, which may be largely attributed to the presence of a long induction period for the oxidation of 2-propanol.29 In the induction period, the oxidation rate of 2-propanol is slow,

Table 1. Sulfur Conversion of Different Catalysts along with Their Adsorptivity for DBT and Dissolubility in Different Solvents Dissolubility of Catalyst in Solvent catalyst

S-conversiona (%)

DBT adsorptionb (mg of DBT/(g of catalyst))

IPA

octane

IPA + octane + H2O2

H3PW12O40 [(C4H9)4N]3[PW12O40] [π-C5H5NC16H33]3[PW4O16]

34.87 18.65 98.90

0.41 0.04 13.94

soluble insoluble insoluble

insoluble insoluble insoluble

partly soluble insoluble soluble

a

Reaction conditions: DBT oil = 10 g; 2-propanol = 2.5 g; mass ratio of m(AIBN)/m(2-propanol) = 4%; mole ratio of n(catalyst)/n(S) = 1:50; initial gauge pressure = 1.4 MPa; T = 70 °C; t = 6 h. bAdsorption conditions: DBT oil = 10 g; catalyst = 1 g; T = 70 °C; t = 6 h. 4870

dx.doi.org/10.1021/ie300035c | Ind. Eng. Chem. Res. 2012, 51, 4868−4874

Industrial & Engineering Chemistry Research

Article

which results in a lower production rate of H2O2, and accordingly a slower sulfur conversion rate. After the induction period, the production of 2-propanol radicals, i.e., (CH3)2C•OH, and thus the oxidation of 2-propanol as well as the follow-up oxidation of S-compounds become faster, and a deep sulfur conversion of 99.69% can be achieved in 8 h. In view of the marginal increase of sulfur conversion in the longer period of time, 6 h is suitable and fixed in the following study. 3.4. Reaction Temperature on Sulfur Conversion. To evaluate the role of the reaction temperature on the sulfur conversion, the integrated ODS process were carried out at five different temperatures, 50, 60, 70, 80, and 90 °C, and the results are displayed in Figure 4. As shown in the figure, the Figure 5. Influence of AIBN amount on sulfur conversion. Reaction conditions: DBT oil = 10 g; 2-propanol = 2.5 g; mole ratio n(catalyst)/n(S) = 1:50, catalyst = [π-C5H5NC16H33]3[PW4O16]; initial gauge pressure = 1.4 MPa; T = 70 °C; t = 6 h.

reaction time by a self-initiation process. The results indicate that the initiation of free radicals in the induction period is a controlling process due to the high activation energy for the cleavage of the C−H bond in the tertiary carbon of 2-propanol, and the more the isopropyl free radicals are formed via initiation of AIBN, the higher is the generation rate of H2O2 and thus the higher is the sulfur conversion. Finally, 4% AIBN can be regarded as an optimal usage in the present integrated ODS process. 3.6. Pressure of O2 on Sulfur Conversion. Oxygen is the oxidant of 2-propanol for the in situ production of H2O2 in the present process, its pressure in the gas phase, and accordingly the dissolved O2 in the oil phase may influence the sulfur conversion remarkably. To investigate this effect, several experiments were done at different initial gauge pressures in the range from 0 to 1.8 MPa, and the results are displayed in Figure 6. As seen from Figure 6, the sulfur conversion increases

Figure 4. Influence of reaction temperature on sulfur conversion. Reaction conditions: DBT oil = 10 g; 2-propanol = 2.5 g; mass ratio m(AIBN)/m(2-propanol) = 4%; mole ratio n(catalyst)/n(S) = 1:50; catalyst = [π-C5H5NC16H33]3[PW4O16]; initial gage pressure = 1.4 MPa; t = 6 h.

sulfur conversion of DBT increases remarkably from 29.73 to 98.90% as the temperature increases from 50 to 70 °C, and then levels off. The results indicate that the reaction temperature is crucial for the present integrated ODS process, and 70 °C can be determined as the appropriate one considering the negligible enhancement on sulfur conversion and the increasing degradation rate of H2O2 at higher temperatures. In effect, the observed effect of temperature on the present process is a compromise result of temperature on different factors, e.g., optimal temperature for free radical initiator (AIBN), induction period, and oxidation of 2-propanol to generate H2O2, DBT oxidation by H2O2, and the decomposition rate of H2O2, etc. The behavior is in line with the general trend that higher temperature is helpful for increasing the reaction rate and shortening the induction period of a free radical reaction.30 Besides, 70 °C is also an optimal working temperature for both AIBN initiator31−33 and H2O2 oxidant in similar systems. 3.5. Amount of Initiator on Sulfur Conversion. It is known that the oxidation of 2-propanol is a free radical reaction with a relatively long induction period and use of free radical initiator can shorten the induction period and thus accelerate the oxidation of 2-propanol. Considering the temperature range studied here and the feasible work temperature of various free radical initiators, AIBN was used as initiator and the influence of its amount on the sulfur conversion was investigated. As seen from Figure 5, the sulfur conversion increases linearly with the amount of initiator used before 4% AIBN against the amount of 2-propanol and then increases very slightly. While the sulfur conversion in the absence of AIBN is less than 9% in 6 h

Figure 6. Influence of oxygen pressure on sulfur conversion. Reaction conditions: DBT oil = 10 g; 2-propanol = 2.5 g; mass ratio m(AIBN)/ m(2-propanol) = 4%; n(catalyst)/n(S) = 1:50; catalyst = [πC5H5NC16H33]3[PW4O16]; T = 70 °C; t = 6 h.

monotonically with the increasing pressure of O2 from 59.06% at 0 MPa to 98.90% at 1.4 MPa, reaching the asymptotic line hereinafter. Therefore, from the point of view of efficiency and safety, 1.4 MPa is adopted as the optimum pressure. It is shown that the influence of the pressure of oxygen on the sulfur removal is much lower than the approximately linear increase of the dissolved O2 in the liquid phase. This observation is consistent with the fact that the rate determining step for the 4871

dx.doi.org/10.1021/ie300035c | Ind. Eng. Chem. Res. 2012, 51, 4868−4874

Industrial & Engineering Chemistry Research

Article

oxidation of 2-propanol is the induction process rather than the reaction of 2-propanol radicals with O2, and thus increasing pressure of O2 can only slightly increase the sulfur conversions. 3.7. Amount of 2-Propanol on Sulfur Conversion. According to the stoichiometry of the reaction, the least mole ratio of H2O2/S is 2:1 for converting DBT to DBTO2, since 1 mol of 2-propanol can produce 1 mol of H2O2 in theory. However, considering the limited oxidation rate of 2-propanol to H2O2 even at high temperature,21 an excessive amount of 2-propanol was used for the efficient desulfurization. To find the optimal usage of 2-propanol, some experiments were done at different amounts of 2-propanol, i.e., 0, 1.0, 1.5, 2.0, 2.5, and 3.0 g added to 10 g of oil with the corresponding mole ratios of 2-propanol/S being 0, 53, 80, 106, 133, and 160, respectively. As shown from Figure 7, the sulfur conversion of DBT

Figure 8. Influence of catalyst amount on sulfur conversion. Reaction conditions: DBT oil = 10 g; 2-propanol = 2.5 g; mass ratio m(AIBN)/ m(2-propanol) = 4%; catalyst = [π-C5H5NC16H33]3[PW4O16]; initial gauge pressure = 1.4 MPa; T = 70 °C; t = 6 h.

well as the oil solubility of the catalyst in its oxidized form that in return facilitates the oxidation of DBT in comparison with the heterogeneous catalysis. Second, the sulfur conversion increases slowly with the usage of catalyst; for example, as the catalyst usage doubles gradually from 1:200 to 1:50 in terms of the mole ratio of S/catalyst, the sulfur conversion increases steadily from 75.83 to 85.29% and to 98.90%. This further supports the assumption that the reaction rate of the whole process is controlled by the production of 2-propanol radicals, and all other factors can only influence the sulfur conversion in a moderate way. And finally, the appropriate mole ratio of S/catalyst is determined as 1:50 for [π-C5H5NC16H33]3[PW4O16] in the present integrated ODS process, since further increases in mole ratio above 1:50 cannot significantly improve the sulfur conversion of DBT. 3.9. Oxidation of Different S-Compounds. To investigate the viability of the present process for other thiophenic S-compounds present in diesel fuel, the sulfur conversion of BT and 3-MT was also tested at 70 °C and compared with DBT under similar conditions with their initial S-content all being 1000 ppm, as presented in Figure 9. It is obvious from Figure 9

Figure 7. Influence of 2-propanol amount on sulfur conversion. Reaction conditions: DBT oil = 10 g; mass ratio m(AIBN)/m(2propanol) = 4%; mole ratio n(catalyst)/n(S) = 1:50, catalyst = [πC5H5NC16H33]3[PW4O16]; initial gauge pressure = 1.4 MPa; T = 70 °C; t = 6 h.

increases with the mole ratio of 2-propanol/S and reaches 98.90% at 133, which can be ascribed to the increasing amount of H2O2 in situ produced by the oxidation of 2-propanol. However, further increasing the mole ratio from 133 to 160 can only result in a little higher sulfur conversion from 98.90 to 99.52%, and thus 133 is determined as the appropriate mole ratio of 2-propanol/S in the following experiments. It is also noted that, even in the absence of 2-propanol, a low sulfur conversion of 5.08% can be achieved due to the direct oxidation of DBT by the dissolved O2 under the catalysis of [πC5H5NC16H33]3[PW4O16]. This means that the present integrated ODS process is superior to the conventional ODS process with oxygen as oxidant 3.8. Amount of Catalyst on Sulfur Conversion. According to the desulfurization mechanism of the present process shown in Figure 1, [π-C5H5NC16H33]3[PW4O16] is used as a catalyst for the oxidation of DBT by H2O2, and thus its usage may have an important role on the sulfur conversion. This influence was studied at varying mole ratios of catalyst to sulfur, i.e., n(catalyst)/n(S), from 0:1 to 1:25. The results are presented in Figure 8, from which some conclusion can be made. First, the catalysis of [π-C5H5NC16H33]3[PW4O16] is remarkable as inferred from the high sulfur conversion of 75.83% with the least catalyst usage of 1:200 against 17.57% without catalyst with the other conditions remaining the same. The good catalytic performance may be associated with the intrinsic catalysis of the phosphotungstic anion [PW4O16]3− as

Figure 9. Comparison of the sulfur conversion of DBT, BT, and 3MT. Reaction conditions: oil = 10 g; 2-propanol = 2.5 g; mass ratio m(AIBN)/m(2-propanol) = 4%; mole ratio n(catalyst)/n(S) = 1:50, catalyst = [π-C5H5NC16H33]3[PW4O16]; initial gauge pressure = 1.4 MPa; T = 70 °C; t = 6 h.

that the oxidative reactivity of the S-compounds followed the order of DBT > BT > 3-MT, which is consistent with their electron density on the sulfur atom, being 5.758, 5.739, and ́ and co-workers14 5.697, respectively.34,35 Besides, Torres-Garcia’s 4872

dx.doi.org/10.1021/ie300035c | Ind. Eng. Chem. Res. 2012, 51, 4868−4874

Industrial & Engineering Chemistry Research

Article

between them is that the white crystals show specific absorption peaks at 1288 and 1167 cm−1 for sulfone and 1047 cm−1 for sulfoxide,36 implying that DBT is oxidized to DBTO and subsequently to DBTO2 in the present process. This result indicates that the solubility of the oxidation products of DBT in n-octane is limited, which is helpful for the facile separation of the oxidized S-compounds from the treated oil. Meanwhile, the mechanism of the integrated ODS process shown in Figure 1 is justified.

recently found that the reactivity of different S-compounds also correlates well with their electronic properties, such as electronic hardness, electroaccepting power, and electrodonating power. For the present process, the reactivities of DBT, BT, and 3-MT are also consistent with their electronic properties.14 Figure 10 compares the sulfur removal rate of 3-MT, BT, and

4. CONCLUSIONS In this work, a new integrated ODS process is established, in which the oxidant H2O2 is generated in situ by the oxidation of 2-propanol with molecular oxygen and then oxidizes the thiophenic S-compounds under the help of the reactioncontrolled phase-transfer catalyst [π-C5H5NC16H33]3[PW4O16]. Besides, the catalytic performance of [π-C5H5NC16H33]3[PW4O16] is much higher than H3PW12O40 and [(C4H9)4N]3[PW12O40]. For the whole process, the rate determining step is the induction period of the oxidation of 2-propanol, and all measures favoring the production of the 2-propanol radicals can improve the reaction rate markedly. The reactivity of different S-compounds follows the order DBT > BT > 3-MT, which is in accordance with their decreasing electron density on the sulfur atom and other electronic properties, such as electronic hardness, electroaccepting power, and electrodonating power. At relatively mild reaction temperature, viz., 70−90 °C, both BT and DBT can be removed efficiently within 6 h with sulfur conversion above 96%. And the resulting oxidized S-compounds can be separated easily via settling or filtration.

Figure 10. Influence of temperature on the sulfur conversion of DBT, BT, and 3-MT. Reaction conditions: oil = 10 g; 2-propanol = 2.5 g; ass ratio m(AIBN)/m(2-propanol) = 4%; mole ratio n(catalyst)/n(S) = 1:50; catalyst = [π-C5H5NC16H33]3[PW4O16]; initial gauge pressure = 1.4 MPa; t = 6 h.

DBT at three different temperatures under fixed other conditions, i.e., 70, 80, and 90 °C. It is seen that raising the temperature is helpful for the sulfur removal rate of thiophenic compounds especially for BT and 3-MT that are hard to oxidize at lower temperatures. For example, the sulfur conversions of BT and 3-MT were increased steadily from 69.21 and 25.84% at 70 °C to 96.03 and 43.34% at 90 °C, respectively. Therefore, the present integrated ODS process is of high efficiency for the removal of condensed aromatic S-compounds such as DBT and BT, which are the key S-compounds of diesel oils and the most difficult ones to remove by the catalytic hydrogenation process. In contrast, the present process is less effective for 3-MT or its homologues due to its intrinsic attribute, i.e., the lower electron density on the sulfur atom and the electronic properties and thus lower activity to be oxidized. 3.10. Characterization of the Oxidized Product of DBT. Accompanying the oxidation process of DBT, white precipitate occurred at the bottom of the reactor, which was collected via filtration and then dissolved in trichloromethane. The resulting solution was evaporated to remove the solvent, and the white crystals obtained were characterized by IR. The IR spectra of the white crystals and DBT are presented in Figure 11. As shown from the figure, the biggest difference

■

AUTHOR INFORMATION

Corresponding Author

*Tel. and Fax: +86 10 64410308. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful for the support from the Fundamental Research Foundation of Sinopec (Grant No. X505015).

■

REFERENCES

(1) 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. (2) Song, C.; Ma, X. New design approaches to ultra-clean diesel fuels by deep desulfurization and deep dearomatization. Appl. Catal., B 2003, 41, 207−238. (3) Basfar, A. A.; Mohamed, K. A. Radiation-induced desulfurization of Arabian crude oil and straight-run diesel. Radiat. Phys. Chem. 2011, 80, 1289−1290. (4) Shan, J.; Chen, L.; Sun, L.; Liu, X. Adsorptive removal of thiophene by Cu-modified mesoporous silica MCM-48 derived from direct synthesis. Energy Fuels 2011, 25, 3093−3099. (5) Tang, X.; Meng, X.; Shi, L. Desulfurization of model gasoline on modified bentonite. Ind. Eng. Chem. Res. 2011, 50, 7527−7533. (6) Tang, X.; Qian, W.; Hu, A.; Zhao, Y.; Fei, N.; Shi, L. Adsorption of thiophene on Pt/Ag-supported activated carbons prepared by ultrasonic-assisted impregnation. Ind. Eng. Chem. Res. 2011, 50, 9363− 9367. (7) Nie, Y.; Li, C.; Sun, A.; Meng, H.; Wang, Z. Extractive desulfurization of gasoline using imidazolium-based phosphoric ionic liquids. Energy Fuels 2006, 20, 2083−2087.

Figure 11. IR spectra of DBT and its oxidative products. 4873

dx.doi.org/10.1021/ie300035c | Ind. Eng. Chem. Res. 2012, 51, 4868−4874

Industrial & Engineering Chemistry Research

Article

(8) Kedra-Królik, K.; Fabrice, M.; Jaubert, J. Extraction of thiophene or pyridine from n-heptane using ionic liquids. Gasoline and diesel desulfurization. Ind. Eng. Chem. Res. 2011, 50, 2296−23−6. (9) Nie, Y.; Li, C.; Wang, Z. Extractive desulfurization of fuel oil using alkylimidazole and its mixture with dialkylphosphate ionic liquids. Ind. Eng. Chem. Res. 2007, 46, 5108−5122. (10) Zhang, J.; Wang, A.; Li, X.; Ma, X. Oxidative desulfurization of dibenzothiophene and diesel over [Bmim]3PMo12O40. J. Catal. 2011, 279, 269−275. (11) De Filippis, P.; Liuzzo, G.; Scarsella, M.; Verdone, N. Oxidative desulfurization II: Temperature dependence of organosulfur compounds oxidation. Ing. Eng. Chem. Res. 2011, 50, 10452−10457. (12) Jia, Y.; Li, G.; Ning, G. Efficient oxidative desulfurization (ODS) of model fuel with H2O2 catalyzed by MoO3/γ-Al2O3 under mild and solvent free conditions. Fuel Process. Technol. 2011, 92, 106−111. (13) Muhammad, Y.; Li, C. Dibenzothiophene hydrodesulfurization using in situ generated hydrogen over Pd promoted alumina-based catalysts. Fuel Process. Technol. 2011, 92, 624−630. (14) Torres-García, E.; Galano, A.; Rodriguez-Gattorno, G. Oxidative desulfurization (ODS) of organosulfur compounds catalyzed by peroxo-metallate complexes of WOx−ZrO2: Thermochemical, structural, and reactivity indexes analyses. J. Catal. 2011, 282, 201−208. (15) Zhao, J.; Zhou, J.; Su, J.; Guo, H.; Wang, X.; Gong, W. Propene epoxidation with in-site H2O2 produced by H2/O2 non-equilibrium plasma. AIChE J. 2007, 53, 3204−3209. (16) Peng, G.; Fu, Z.; Yin, D.; Zhong, S.; Yang, Y.; Yu, N.; Yin, D. A promising coupled process of Pd/γ-Al2O3−NH4VO3 catalyzing the hydroxylation of benzene with hydrogen peroxide produced in situ by an anthraquinone redox route. Catal. Lett. 2007, 118, 270−274. (17) Song, H.; Li, G.; Wang, X.; Chen, Y. Characterization and catalytic performance of Au/Ti-HMS for direct generation of H2O2 and in situ-H2O2-ODS from H2 and O2: An in situ-reduction synthesis and a recycle study of catalyst. Microporous Mesoporous Mater. 2011, 139, 104−109. (18) Yalfani, M. S.; Contreras, S.; Medina, F.; Sueiras, J. E. Hydrogen substitutes for the in situ generation of H2O2: An application in the Fenton reaction. J. Hazard. Mater. 2011, 192, 340−346. (19) Aiken, J. E. Process for integrated co-production of hydrogen peroxide and epoxidized esters. U.S. Patent 2010/0,113,809, May 6, 2010. (20) Zajacek, J. G.; Jubin, J. C.; Crocco, G. L. Integrated process for epoxide production. U.S. Patent 5,384,418, Jan. 24, 1995. (21) Liang, X.; Mi, Z.; Wang, Y.; Wang, L.; Zhang, X.; Liu, T. An integrated process of H2O2 production through isopropanol oxidation and cyclohexanone ammoximation. Chem. Eng. Technol. 2004, 27, 176−180. (22) Xi, Z.; Zhou, N.; Sun, Y.; Li, K. Reaction-controlled phasetransfer catalysis for propylene epoxidation to propylene oxide. Science 2001, 292, 1139−1141. (23) Kunugi, T.; Matsuura, T. Isopropanol oxidizes this way. Hydrocarb. Process. 1965, 44, 116−122. (24) Liu, Y.; Murata, K.; Inaba, M.; Mimura, N. Direct epoxidation of propylene by molecular oxygen over Pd(OAc)2−[(C6H13)4N]3{PO4[W(O)(O2)2]4}−CH3OH catalytic system. Appl. Catal., B 2005, 58, 51−59. (25) Zhang, B.; Jiang, Z.; Li, J.; Zhang, Y.; Lin, F.; Liu, Y.; Li, C. Catalytic oxidation of thiophene and its derivatives via dual activation for ultra-deep desulfurization of fuels. J. Catal. 2012, 287, 5−12. (26) Sun, Y.; Xi, Z.; Cao, G. Epoxidation of olefins catalyzed by [π-C5H5NC16H33]3[PW4O16] with molecular oxygen and a recyclable reductant 2-ethylanthrahydroquinone. J. Mol. Catal. A: Chem. 2001, 166, 219−224. (27) Zhang, S. J.; Zhao, G. D.; Gao, S.; Xi, Z. W.; Xu, J. Secondary alcohols oxidation with hydrogen peroxide catalyzed by [n-C16H33N(CH3)3]3PW12O40: Transform-and-retransform process between catalytic precursor and catalytic activity species. J. Mol. Catal. A: Chem. 2008, 289, 22−27.

(28) Li, J.; Gao, S.; Li, M.; Zhang, R.; Xi, Z. Influence of composition of heteropolyphosphatotungstate catalyst on epoxidation of propylene. J. Mol. Catal. A: Chem. 2004, 218, 247−252. (29) Kuriacose, J. C.; Markham, M. C. Conversion of isopropyl alcohol to acetone on irradiated zinc oxide. J. Catal. 1962, 1, 498−507. (30) Scherzer, T.; Langguth, H. The effect of temperature on the induction period in the photoinitiated polymerization of tripropylene glycol diacrylate. Nucl. Instrum. Methods Phys. Res., Sect. B 2001, 185, 276−282. (31) Peng, C.; Scricco, J.; Li, S.; Fryd, M.; Wayland, B. B. Organocobalt mediated living radical polymerization of vinyl acetate. Macromolecules 2008, 41, 2368−2373. (32) Liu, K.; Kiran, E. Density and viscosity as real-time probes for progress of high-pressure polymerizations: polymerization of methyl methacrylate in acetone. Ind. Eng. Chem. Res. 2008, 47, 5039−5047. (33) Ye, S.; Cramer, N. B.; Stevens, B. E.; Sani, R. L.; Bowman, C. N. Induction curing of thiol-acrylate and thio-ene composite systems. Maromolecules 2011, 44, 4988−4996. (34) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W.; Ishihara, A.; Imai, T.; Kabe, T. Oxidative desulfurization of light gas oil and vacuum gas oil by oxidation and solvent extraction. Energy Fuels 2000, 14, 1232−1239. (35) Ma, X.; Sprague, M.; Song, C. Deep desulfurization of gasoline by selective adsorption over nickel-based adsorbent for fuel cell applications. Ind. Eng. Chem. Res. 2005, 44, 5768−5775. (36) Silverstein, R. M.; Webster, F. X.; Kiemle, D. J. Spectrometric Identification of Organic Compounds, 7th ed.; Wiley: New York, 2005.

4874

dx.doi.org/10.1021/ie300035c | Ind. Eng. Chem. Res. 2012, 51, 4868−4874