Ind. Eng. Chem. Res. 2008, 47, 5353–5361
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Solvent Effect in Homogeneous and Heterogeneous Reactions To Remove Dibenzothiophene by an Oxidation-Extraction Scheme L. F. Ramı´rez-Verduzco,*,†,‡ J. A. De los Reyes,‡ and E. Torres-Garcı´a† Programa de InVestigacio´n y Desarrollo Tecnolo´gico de Procesos y Reactores, Instituto Mexicano del Petro´leo, Eje Central La´zaro Ca´rdenas 152, C.P. 07730 Me´xico, D.F., Me´xico, and Departamento de Ingenierı´a de Procesos e Hidra´ulica, DiVisio´n de Ciencias Ba´sicas e Ingenierı´a, UniVersidad Auto´noma MetropolitanasIztapalapa, AVenida Michoaca´n y La Purı´sima, Col. Vicentina, 09340 Me´xico, D.F., Me´xico
In this work a study of dibenzothiophene removal by an oxidation-extraction scheme is presented. Experiments were carried out to observe the role that the solvent plays during the process, as well as the oxidizing agent and catalyst. The oxidation was carried out with hydrogen peroxide in the presence of a catalyst of tungsten supported on zirconia (WOx-ZrO2). A dibenzothiophene + n-hexadecane model mixture was employed to simulate a diesel fuel. Methanol, ethanol, acetonitrile, and γ-butyrolactone were used as extraction solvents. Dibenzothiophene (DBT) was removed more efficiently by γ-butyrolactone with respect to other solvents. The highest reactivity was achieved when γ-butyrolactone was used during DBT oxidation with and without catalyst. When oxidation was carried out without catalyst, the oxidant behavior of the mixture could be explained in terms of the dissociation of hydrogen peroxide to produce strong oxidant species such as perhydroxyl ions (HO2-) by the influence of the aprotic solvents. Finally, when a catalyst was used during the oxidation, there was an additional oxidation contribution through the formation of surface peroxo-metal intermediates (W-O-O-H). 1. Introduction Recently, the oxidation-extraction process has been investigated as an attractive alternative to obtain diesel fuel with ultralow sulfur content,1–3 particularly in the context of the new severe regulations adopted in the world. For example, the Environmental Protection Agency (EPA) has called for the production and use of more environmentally friendly transportation fuels with lower sulfur contents.4 The oxidation-extraction process consists in sulfur transformation to obtain the corresponding sulfoxides and sulfones followed by their elimination using polar solvents. The extraction is carried out easily because sulfoxides and sulfones are substantially more polar than sulfides.5,6 Currently, fundamental research has been reported to eliminate sulfur compounds, employing a wide variety of solvents, catalysts, and oxidizing agents.7–13 The nature of the compounds that are participating in the oxidation-extraction scheme gives their global behavior. The WOx-ZrO2 catalyst was used by Chen et al.14 in the oxidative dehydrogenation of propane, which involves a ratedetermining C-H bond activation step using the lattice oxygen atoms. Catalysts using tungsten(VI) complexes can activate hydrogen peroxide (H2O2) effectively. Moreover, a catalyst with tungsten generallyworksappropriatelyinaqueoussystems.Peroxo-tungsten species are formed when tungsten complexes are combined with H2O2. The understanding and improvement of the performance of such oxidation reactions by peroxo-tungsten species intermediates have been a constant activity since 50 years ago.15 The peroxo-tungsten system behaves closely to the organic peracids, sharing their range of applications. Additionally, it has the advantage that the catalyst can be recovered in opposition to nonrecoverable homogeneous catalysts like peracids. * To whom correspondence should be addressed. † Instituto Mexicano del Petro´leo. ‡ Universidad Auto´noma MetropolitanasIztapalapa.
WOx-ZrO2 is a versatile catalyst. It can catalyze both acid and redox reactions. Lewis acid sites on the catalyst surface permit enhancing dramatically the electrophilicity of H2O2 toward their reaction with weakly nucleophilic dibenzothiophene (DBT). Recently, Figueras et al.16 studied the oxidation of DBT with H2O2 over zirconia-supported W catalyst. They found that catalysts containing highly dispersed tetrahedral W cations were more active than WO3 clusters. They also found that the reactivity was higher when the reactions were performed in acetonitrile compared to alcohol solvents. The effect of the aprotic/protic solvent character was not discussed by the authors. Moreover, the information on the solvent extraction capacity was not included. In a previous work,17 the catalytic oxidation of diesel fuel by peracids was studied. In this work, the extraction and reactivity of sulfur compounds were higher when γ-butyrolactone was used. Although this information is useful to finding better operating conditions for the oxidation-extraction process, a fundamental study is needed in order to know specific details about the chemical and physical issues related to this particular kind of reaction. Also, a fair amount of work has been published regarding the oxidant capacity of oxidizing agents, including H2O2,7,4,8,10 HNO3,18 NO2,19 O2,20 O3,21 tert-butyl hydroperoxide (tBuOOH),22 air,23 etc. In this work, H2O2 was selected to carry out the oxidation reactions, due to its high oxidative capacity. Also, it can be activated easily by WOx-ZrO2 catalysts. There are two parallel reactions when H2O2 is used to oxide the DBT compound. These reactions are shown in Figure 1. As can be seen, the reaction subproducts are oxygen and water, which are environmentally friendly. The right choice of solvent is a matter of paramount relevance; the main aspects that should be observed are cost, boiling point, viscosity, density, polarity, dielectric constant, and the capacity to dissolve a certain substrate. These properties have a direct influence on the solvent performance (e.g.,
10.1021/ie701692r CCC: $40.75 2008 American Chemical Society Published on Web 07/02/2008
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Figure 1. Parallel chemical reactions occurring in DBT oxidation to obtain DBTO2.
economic viability, recovery, extraction capacity, and easy transportation through the industrial equipment). It is well-known that reactions can be influenced by solvents. In this context the protic/aprotic character of solvents is the main aspect to be considered. For example, Clereci et al.24 found that the activity of TS-1 catalyst was enhanced by methanol and other protic solvents for the epoxidation of propylene. In contrast, a high reactivity of Ti-beta catalyst was founded by Corma et al.25 when acetonitrile was used for the oxidation of 1-hexene. The fundamental research on the oxidation-extraction scheme that has been published in the open literature is rather scarce. The oxidation of DBT with H2O2 in the presence of 12tungstophosphoric acid (TPA) in the octane/acetonitrile biphasic system was studied by Yazu et al.13 These authors found a better removal of the DBT compound when a simple extraction was combined with the oxidation, and also when the polar solvent volume was increased. However, no further explanations were provided by these authors. On the other hand, the oxidation of benzothiophene (BT) and DBT with H2O2 in a three-phase S-L1-L2 system was reported by Hulea et al.4 In this work, only methanol and acetonitrile were used as representative protic and nonprotic solvents. The authors found that the solvent performance depends strongly on the sulfone solubility and the hydrophilic/hydrophobic catalyst character. However, the chemical contribution toward the activation of the hydrogen peroxide is unclear. According to the issues previously described, an objective of this work was to investigate the role of the solvent in the reactivity oxidation of DBT with H2O2 using a WOx-ZrO2 catalyst. Two protic solvents (methanol and ethanol) and two aprotic solvents (acetonitrile and γ-butyrolactone) were used to study the protic/aprotic character. The aim of this paper was the study of the synergic effect among solvent, oxidizing agent, and catalyst. Individual experiments were carried out: single extraction and oxidation-extraction with and without catalyst. Additionally, the catalyst was characterized by X-ray diffraction (XRD) and Raman spectroscopy. It is important to show that regardless of the extensive studies in the sulfur compound oxidation by hydrogen peroxide, little attention has been paid to establishing the relationship among mixture components (catalyst, solvent, and oxidizing agent) with catalytic properties. 2. Experimental Section 2.1. Materials. n-Hexadecane (n-C16; 99 mol %), 99 mol % γ-butyrolactone (GBL), 98% dibenzothiophene (DBT), and 97% dibenzothiophene sulfone (DBTO2) were purchased from Aldrich; 99.9 mol % methanol (MeOH), 99.9 mol % ethanol (EtOH), 99.9 mol % acetonitrile (MeCN) were obtained from
Teqsiquim; and 30 wt % hydrogen peroxide (H2O2) was provided by J. T. Baker, respectively, and used with no further purification. 2.2. Catalyst Preparation and Characterization. The WOx-ZrO2 catalyst was prepared in the same way as described previously.26,27 Briefly, the WOx-ZrO2 catalyst was prepared by impregnation of ZrO2-x(OH)2x with ammonium metatungstate [(NH4)6(H2W12O40) · nH2O] solution (Strem Chemicals, 99.9%), maintaining the pH at 10. The mixture was stirred and heated in order to evaporate the excess water and then calcined in air atmosphere during 3 h at 1073 K. The catalyst was synthesized with 20 wt % tungsten. High-surface-area ZrO2-x(OH)2x (320 m2/g) was prepared by hydrolysis of 0.5 M zirconyl chloride solution (ZrOCl2 · 8H2O; Aldrich, >98 wt %, Hf 0.5 wt %) with NH4OH solution (Baker, 28%) at pH 10. The nitrogen physisorption was performed using AUTOSORB-1 equipment, measuring the quantity of gas adsorbed onto or desorbed from the solid surface at some equilibrium vapor pressure by the static volumetric method. Specific surface area was obtained using the BET equation. The Raman spectra were recorded at room temperature using a Jobin Yvon Horiba (T64000) spectrometer, equipped with a confocal microscope (Olympus, BX41). The exciting line at 514.5 nm of an Ar+ laser was focused using a long-distance 10× objective. The size of the laser spot was typically 50 µm with a power of 10 mW. The spectrometer is equipped with a CCD detector which is Peltier cooled to 243 K to reduce thermal noise. The X-ray diffraction (XRD) patterns were recorded at 0.003 deg · s-1 scanning rate with Cu KR radiation and a Ni filter using a Siemens diffractometer (Model D500). Diffraction intensity was measured in the 2θ between 10° and 70° at 30 kV and 20 mA. The diffraction patterns were compared with the results reported by the Joint Committee of Powder Diffraction Standards (JCPDS-ICDD 25-668) database. 2.3. Extraction Experiment. The extraction was carried out in a liquid-liquid equilibrium cell. The temperature was controlled by a heat circulating bath (PolyScience) with a (0.1 K resolution. The extraction temperature was set at 303 K and measured with a Fluke digital thermometer that was calibrated previously. The equilibrium cell was loaded with the model mixture (DBT into the n-C16 solvent) and the extraction solvent in a 1:1 volume ratio. The liquid-liquid system was stirred at 500 rpm for 1 h, and then the mixture was allowed to rest afterward for 3 h; finally a sample of each phase was taken to be analyzed by gas chromatography (GC). The GC analysis was performed using an Alltech Econo capillary column (5% phenyl-95% methylpolysiloxane; 30 m × 0.25 mm i.d., 250 µm film thickness). The carrier gas was nitrogen. Initially, the oven temperature was held at 403 K. Subsequently, the temperature was raised to 418 K at 5 K/min, and finally, the temperature was changed to 603 K at 45 K/min.
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The injector temperature and pressure were 553 K and 83.4 kPa, respectively. A quantitative analysis was performed by the internal standard method. Sulfolane and n-tetradecane were used as the internal standards for the polar and nonpolar phases, respectively. 2.4. Oxidation-Extraction with and without Catalyst. The oxidation-extraction experiments without catalyst were carried out using 75 mL of the model mixture and 75 mL of solvent. A Robinson-Mahoney reactor was used in order to perform the chemical reaction. The reactor was made of Pyrex glass with three baffles to avoid the vortex effect. A mechanical stirrer was used to minimize the external mass transfer. The reaction temperature was set at 303 K and 8.9 mL of 30 wt % H2O2 was added to the reactor at the initial time reaction. Samples of each phase were taken and analyzed by GC. In the case of the reactions with catalyst, a three phase S-L1-L2 system was formed when 0.1 g of WOx-ZrO2 catalyst was added to the reactor. Experiments at 303 and 333 K were performed. Additionally, the dissociation of the H2O2 was measured through the oxygen released. The oxygen pressure was measured by an U manometer connected to the reactor. The oxygen pressure was converted to a molar concentration using the ideal gas equation. Finally, the H2O2 concentration was determined by a stoichiometric balance. 2.5. Conversion of DBT and Kinetic Equations. During the reaction experiments performed in this work, DBT could be found in both polar and nonpolar phases; thus the conversion of DBT (xDBT) was calculated using eq 1: xDBT )
nDBT,initial - (nDBT,nonpolar + nDBT,polar) nDBT,initial
(1)
where nDBT,initial is the number of moles of the initial DBT in the nonpolar phase; nDBT,nonpolar and nDBT,polar are the number of the moles of the DBT after t minutes of reaction in nonpolar and polar phases, respectively; and the sum of the terms between parentheses in eq 1 represents the total consumption of DBT. The integral method was used to corroborate that the reaction data fitted well to a pseudo-first-order reaction, according to the results reported by several authors.4,16 Also, the apparent constant velocity was derived by the integral method. The power laws of reactions A and B that are shown in Figure 1 are given by eqs 2 and 3: -rDBT ) k1CDBT
(2)
-rH2O2 ) k2
(3)
where k1 and k2 are the intrinsic constant rate velocities, CDBT is the DBT concentration in mol · m3, and -rDBT and -rH2O2 are the rate velocities of DBT and H2O2 consumption, respectively, expressed in mol · kgcatal-1 · s-1. The order of the kinetic reaction for the H2O2 decomposition was zero, due to a linear tendency between H2O2 concentration and time. The extraction capacity is defined by eq 4: ε)
CDBT,initial - CDBT,nonpolar × 100 CDBT,initial
(4)
where ε is the extraction capacity, CDBT,initial is the initial concentration of DBT in the nonpolar phase expressed in mol · m-3, and CDBT,nonpolar is the concentration of DBT in the nonpolar phase at the equilibrium extraction time expressed in mol · m-3. Both CDBT,initial and CDBT,nonpolar were determined by chromatography with an experimental error of (0.3 mol · m-3.
Figure 2. Solvent extraction capacity (ε) as a function of polarity, (0) dipole moment (µ), and (O) dielectric constant (Λ). T ) 303 K; CDBT,initial ) 36 mol · m-3. Table 1. Solvent Extraction Capacity (ε) at 303 K and Characteristics of the Solvents Used in the Oxidation of DBT on the WOx-ZrO2 catalyst solvent
ε
protic (P) or aprotic (A) nature
µ (D)
Λ
bp (K)
F at 298 K (kg · m-3)
MeOH EtOH MeCN GBL
31 37 51 84
P P A A
2.87 1.66 3.44 4.12
32.7 24.5 37.5 39.0
337.8 351.5 354.8 479.2
786.6 785.0 776.6 1125.4
Then, an accuracy of (0.7% for the extraction capacity was estimated. 3. Results and Discussion 3.1. Extraction. The solvent extraction capacity to remove DBT from n-C16 had the order GBL > MeCN > EtOH > MeOH, as can be seen in Table 1. This behavior relates to the polarity and solubility. Table 1 also shows the characteristics of polar solvents used in this work. Figure 2 shows that the solvent extraction capacity increases with solvent polarity, expressed by dipole moment (µ) or dielectric constant (Λ). It is well-known that a high dipole moment or dielectric constant permits the separation of charges. Consequently, the solvents with high polarity are suitable for the extraction of ionic solids, as well as the polar and polarizable molecules. Figure 2 shows a linear tendency between the polarity and extraction capacity of the MeOH, MeCN, and GBL solvents. However, there is an exception in the case of EtOH, which was a better option to extract DBT than MeOH. In this case the solubility might be more important than polarity. The additional -CH2- group of the EtOH solvent could dissolve more efficiently the hydrocarbon section of the DBT molecule than MeOH did. The solid–liquid equilibrium was determined with the solvents that showed the highest extraction capacities. The results are shown in Figure 3. In this figure the solubility (expressed by the DBT concentration) and temperature were in the ranges 0.0-0.02 mol · m-3 and 228.0-354.8 K, respectively. There was the same order between extraction capacity and solubility for the MeCN and GBL solvents. For example, the solubility of DBT in MeCN at 298 K is 2.72 × 10-7 mol · m-3, while the solubility of DBT in GLB is 1.46 × 10-6 mol · m-3 (these values were obtained by interpolation of data reported
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[S:](l) + H2O2(aq) h [Sδ-···Hδ+-OOH](solv) h [S-H](solv)+ + HO2(solv)-
(5)
[S:](l) + H2O(aq) h [Sδ-···Hδ+-OH](solv) h [S-H](solv)+ + OH(solv)- (6) Two strong oxidant species (hydroxide, HO-, and perhydroxyl, HO2-, ions) might be formed during the dissociation process. This fact suggests the presence of reversible equilibria given by OH- + HO2- h O2- + H2O
(7)
OH- + HO2- h O2 + H2O + 2e-
(8)
3OH- h HO2- + H2O + 2e-
(9)
and Figure 3. Solid-liquid equilibrium for the (O) MeCN(1) + DBT(2) and (0) GBL(1) + DBT(2) binary systems.
Figure4.MasstransferofDBTandH2O2 schemeduringtheoxidation-extraction process in a biphasic L1-L2 system.
in Figure 3). Then, it was possible to dissolve 5.4 times more DBT in GBL than in MeCN, just before saturation at room temperature (established when DBT crystal precipitation can be visualized). On the other hand, the extraction capacity was 1.6 times more in GBL than in MeCN. Therefore, the order of extraction capacity and solubility was GBL > MeCN in both cases. 3.2. Oxidation-Extraction without Catalyst. A representative scheme of the mass transfer in the biphasic L1-L2 system is shown in Figure 4. Initially, the DBT compound is transferred from n-C16 to the polar solvent, where it reacts with H2O2 to produce DBTO2 and H2O. H2O2, H2O, and DBTO2 are practically nonmiscible in the n-C16 phase. The results of the DBT conversion are shown in Table 2. The highest conversion was achieved when aprotic (GBL and MeCN) solvents were used. Although the global oxidation reaction of the DBT compound with H2O2 in a polar solvent has been accepted in the literature,4,16 the details of the reaction mechanism are unclear, due to the number of simultaneous complex processes that could be occurring in the mixture. However, from our chemical point of view, this process might be occurring according to a complexation mechanism between the aprotic solvent and H2O2 or H2O compounds, which promote a dissociation effect. Both reactions could be represented by the following equations:
These chemical equilibria could explain the oxidant performance of the binary mixture (H2O2 + nonprotic solvent), even in the absence of a solid catalyst. Once the HO2-/OH- species are formed (which are stronger oxidants than O2), they might be diffused and react directly with the organosulfur compounds to be converted into sulfoxide or sulfone. The strong tendency to produce water versus the dissociation tendency for the obtained perhydroxyl ions is a limitation to obtaining a high concentration of these oxidant species.28 However, it is well-known that the half-life of these species in aprotic solvents and alcohols is considerably longer in contrast to the low stability in water. Thus, the selection of an appropriate solvent involves two main issues: First, the dissociation effect has not been observed in protic solvents. Second, there are azeotropes between MeOH or EtOH and water. It is useful to remember that water is a secondary product of DBT oxidation with H2O2, and it is also present in the H2O2 solution. 3.3. Oxidation-Extraction with Catalyst. DBT conversions achieved through the oxidation-extraction experiments performed with and without catalyst are compared in Table 2. In Table 2 an increase in DBT conversion of 1.6-1.7 times was obtained by using the catalyst when the experiments were performed at 303 K. A major contribution of the catalyst was found when the experiments were carried out at 333 K, as can be seen in Figure 5. This figure shows the conversion of DBT as a function of time. In these experiments only GBL was used as a polar solvent. After 4 h of chemical reaction the conversion with catalyst was 3.0 times higher than the conversion without catalyst. The apparent kinetic constant (k1,app) was 1.2 × 10–5 and 11.5 × 10–5 m3 · kgcatal-1 · s-1 at 303 and 333 K, respectively, when catalyst and GBL were used for the DBT oxidation. Therefore, the catalytic activity was increased 9.6 times while the temperature was increased 30 K; this is a typical behavior expected from catalytic processes. Table 2. DBT Mole Conversion after 3 h of Oxidation-Extraction Processa solvent
xDBT without catalyst after 3 h
xDBT with catalyst after 3 h
MeOH EtOH MeCN GBL
0.036 0.044 0.055 0.102
0.058 0.075 0.092 0.164
a Reaction conditions: 7.5 × 10-5 m3 of n-C16; 7.5 × 10-5 m3 of polar solvent; CDBT,initial ) 36 mol · m-3; T ) 303 K; H2O2/DBT mole ratio ) 16; Qcatal ) 0.5 kgcatal · m-3.
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Figure 5. DBT conversion as a function of time (O) without catalyst and (0) with catalyst. Reaction conditions: 7.5 × 10-5 m3 of n-C16; 7.5 × 10-5 m3 of polar solvent; CDBT,initial ) 36 mol · m-3; T ) 333 K; H2O2/DBT mole ratio ) 16; Qcatal ) 0.5 kgcatal · m-3.
Figure 6 shows the change in the number of DBT and DBTO2 moles as a function of time, in a typical oxidation-extraction experiment with different solvents. The purpose of these experiments was to study the effect of the protic/aprotic solvent character for both the extraction and oxidation processes, when the experiments were carried out in the presence of the WOx-ZrO2 catalyst at 333 K. In Figure 6, the curves with square and triangular symbols represent the change of the DBT concentration in the nonpolar and polar phases, respectively. Once DBTO2 was formed in the surface of catalyst, it was transferred to the polar phase where it remained due to high polar affinity. The curves with the circles represent the variation of the DBTO2 concentration in the polar phase. Before the process began, the DBT was in the nonpolar phase and was transferred to the polar phase quickly, that is, in the first seconds after the process began. The amount of DBT transferred was higher when aprotic solvents (MeCN or GBL) than when protic solvents (MeOH and EtOH) were used. Quantitatively, the initial concentration decreased to 20, 31, 37, and 73% when MeOH, EtOH, MeCN, and GBL were used. On the other hand, the DBT concentration of DBTO2 only was increased to 3.3 and 6.0 mol · m-3 when MeOH and EtOH were used as solvents. The limited production of DBTO2 might be due to the low mass transfer of the DBT compound and the inhibitory effect over the active sites by hydroxyl species. A constant decrease in the DBT concentration is observed in Figure 6c,d. This behavior could be related to the major extraction capacity and more efficient consumption of DBT by chemical reaction. The chemical reaction is a combination of homogeneous and heterogeneous contributions. In the homogeneous reaction oxidizing species (hydroxide and perhydroxyl ions) are formed when solvent and hydrogen peroxide interact. In the heterogeneous reaction W-O-O-H species are formed on the surface of the catalyst. Summarizing, the order of the solvent extraction capacity was as follows: GBL > MeCN > EtOH > MeOH. The order of the solvents at which a higher quantity of the DBTO2 compound was obtained was the following: GBL > MeCN > EtOH > MeOH. Figure 7 shows the solvent effect on the DBT oxidation by H2O2 over WOx-ZrO2 catalyst, where the DBT conversion is plotted as a function of time for the reactions carried out at 333 K. A pseudo-first-order was obtained for the DBT oxidation
with H2O2 by the integral method. Figure 8 shows the plot of the L ) (1/Qcatal)(-ln(CDBT/CDBT,initial)) versus time. The slope represents the apparent constant velocity (k1,app). The values of the apparent constant velocity (k1,app) and the correlation coefficients (R) from the regression data are summarized in Table 3. It can be seen from these results that the protic/aprotic character of solvent influenced strongly the rate of DBT oxidation. There was a higher activity when aprotic solvents (MeCN and GBL) were used compared to the protic solvents (MeOH and EtOH). This behavior might also be related to the hydrophilic and acidic character of the WOx-ZrO2 catalyst. MeOH and EtOH molecules could be strongly adsorbed by WOx-ZrO2 through the hydroxyl groups, and could hinder the DBT transfer to the catalyst active sites. In contrast, MeCN and GBL have a favorable effect due to the fact that there is no direct interaction of the solvents over the WOx-ZrO2 catalyst.4 The hydrogen peroxide is consumed in a parallel reaction of decomposition, which is favored at high temperatures, as can be seen in Figure 9. Table 4 summarizes the k2,app data at different temperatures during the H2O2 decomposition. Figure 9 shows the corresponding Arrhenius plot. The activation energy and preexponential factor obtained in this work were 68.1 kJ · mol-1 and 4.42 × 108 mol · kgcatal-1 · s-1, respectively, which were derived from Figure 9. Craig29 reported a value of 75.3 kJ · mol-1 for the activation energy in the absence of catalyst for the H2O2 decomposition. Then, it is possible to estimate that the catalyst WOx-ZrO2 can reduce the activation energy by 9.6%. 3.4. Catalytic Characterization and Reaction Mechanism Proposal. After the calcination of the catalyst at 1073 K, the BET surface area, pore size, and pore volume of WOx-ZrO2 were estimated, obtaining values of 47 m2 · g-1, 5.6 nm, and 0.12 cm3 · g-1, respectively. The surface area of pure zirconia before calcination was 320 m2 · g-1. As expected, the presence of WOx species stabilizes the surface area against the tetragonal crystalline structure of ZrO2 nanometric particles, presumably by an oxolation restriction and a growing intercrystallite inhibition (sinterization) of material.27 The volume and diameter of a DBT molecule were estimated to be 0.19 nm3 and 0.7 nm, respectively. Therefore, the pores within the catalysts are large enough to minimize the intraparticle diffusion resistance, as can be seen when the dimensions of catalyst and DBT molecule are compared. It is well-known that high surface area (50 m2 · g-1 or higher), high pore volume (0.1 cm3 · g-1 or higher), and high pore size (50 Å or higher) are desirable textural properties to promote effective catalytic processes. Figure 10 compares the XDR patterns of WOx-ZrO2 before and after the oxidation-extraction process. Highly crystalline material was obtained after calcination at 1073 K, as can be seen in Figure 10a. It is well-known that the fraction of tetragonal zirconia is increased with tungsten content, and this tetragonal phase becomes dominant at roughly 10 wt % W loading, as shown in previous work.27,30 In this work, a catalyst at 20 wt % tungsten was used for the reaction test. The presence of reflections at 23.1°, 23.6°, and 24.5° in Figure 10 is associated with the formation of an orthorhombic phase by a local recrystallization process of the tungsten oxide, which suggests that WOx species have been segregated as a new crystalline phase on the ZrO2 nanoparticle surface. Big clusters are formed when WOx loading is increased. The influence of ZrO2 surface decreases on the formed structure. The polytungstate species tend to generate crystals of WO3, which have been identified only in the samples that were loaded at concentrations
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Figure 6. DBT and DBTO2 mole variation as a function of time during the oxidation-extraction process. Polar solvent: (a) MeOH; (b) EtOH; (c) MeCN; (d) GBL. Legend: (0) moles of DBT in n-C16 phase; (4) moles of DBT in polar phase; (O) moles of DBTO2 in polar phase. Reaction conditions: 7.5 × 10-5 m3 of n-C16; 7.5 × 10-5 m3 of polar solvent; Qcatal ) 0.5 kgcatal · m-3; CDBT,initial ) 32.7 mol · m-3; T ) 333 K; H2O2/DBT mole ratio ) 16.
Figure 7. Solvent effect on DBT conversion during the oxidation-extraction process with catalyst (O) MeOH, (]) EtOH, (0) MeCN, and (4) GBL. Reaction conditions: 7.5 × 10-5 m3 of n-C16; 7.5 × 10-5 m3 of polar solvent; Qcatal ) 0.5 kgcatal · m-3; CDBT,initial ) 32.7 mol · m-3; T ) 333 K; H2O2/DBT mole ratio ) 16.
higher than 15 wt % tungsten. This structural behavior induces an important change not only in the physical-chemical properties, but also in the chemical affinity and corresponding catalytic activity.31 There were no significant changes in the crystalline structure; the peaks associated with the monoclinic zirconia 2θ ) 25.2°
Figure 8. L versus time plot at 333 K to derive the pseudofirst order by integration method: (O) MeOH; (]) EtOH; (0) MeCN; (4) GBL.
and 31.4°, tetragonal zirconia 2θ ) 30.3°, and WO3 crystallite 2θ ) 23.2°, 23.6°, and 24.5°, were retained in both samples. This certainly represents no a massive structure modification of the oxide. The main modifications should be found on WOx and WO3 surfaces. There was an additional contribution to the oxidation reaction when it was accompanied by the WOx-ZrO2 catalyst. When H2O2 and catalyst were loading together, then a surface
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Figure 9. Arrhenius plot for the H2O2 decomposition.
Figure 10. X-ray diffraction for the WOx–ZrO2 system (at 20 wt % W, calcined at 1073 K in air for 3 h): (a) before reaction and (b) after reaction. t ) tetragonal zirconia phase; m ) monoclinic zirconia phase; W ) orthorhombic WO3 phase. Table 3. Intrinsic Velocity Constant (k1,app) for DBT Conversion Using Different Solvents at 333 K solvent
105k1,app (m3 · kgcatal-1 · s-1)
R
MeOH EtOH MeCN GBL
1.28 2.78 7.47 11.50
0.9976 0.9860 0.9995 0.9991
Table 4. Intrinsic Velocity Constant (k2,app) for the H2O2 Decomposition Reaction at Different Temperatures 103k2,app (mol · kgcatal-1 · s-1)
313 K
323 K
333 K
343 K
1.89
4.25
9.28
18.50
peroxo-tungsten (W-O-O-H) species was formed. The validity of this approach was supported by Raman spectroscopic results. Figure 11 shows the Raman spectra of WOx-ZrO2 sample before and after H2O2 addition. The focus is on the signals corresponding to the surface tungsten-oxo species,
which are expected in the 700-1060 cm-1 range, for the ν(W-O-W) and ν(WdO) stretching modes.32,33 In the original catalyst sample, before H2O2 addition (Figure 11a), the bands at 715 and 804 cm-1 are characteristic of the WO3 crystallite clusters in octahedral coordination, while the bands around 985 cm-1 can be assigned to the symmetric stretch mode of ν(W)O) bonds at the borders, which are related to the surface polytungsten oxide.34–37 However, the Raman spectra of the sample obtained after H2O2 addition (see Figure 11b) were completely different from the corresponding spectra of the original sample. Two fundamental bands at 715 and 804 cm-1 were shifted to higher wavenumbers, where their relative intensities became much lower, and the resolution of Raman bands was revealed with difficulty. This shift might indicate a lower symmetry and a coordination modification in tungsten atoms. The presence of a new band at 850 cm-1 was related to the formation of surface peroxo-tungstate species assigned to ν(O-O). The bands at 834 and 930 cm-1 can be attributed to the ν(sym) and ν(asym) vibration modes of (WO4)2- anions in Td symmetry. This observation was in agreement with other phase crystalline patterns, in which tungsten cations were located in tetrahedral sites, for example in the Na2WO4 cubic phase, where Raman bands were present at 929 and 837 cm-1.38,39 An additional three shoulder bands around 900, 912, and 947 cm-1 could be assigned to the ν(W-O-Zr), ν(W-O-W), and ν(WdO) modes, respectively. Indirectly, their positions might reveal the strength of support interactions. This observation suggests that the oxygen atoms may be physisorbed onto WOx species located over the support, and the formation of perhydroxyl species is promoted during the catalytic reaction with coadsorbed H2O or OH groups. Similar observations have been proposed for other systems.40–43 The bands at 834 and 930 cm-1 indicate the presence of WO42- ion; it contains oxo groups which withdraw electrons strongly enough to activate the peroxo group. This is in accord with the observation given recently by Wong et al.44 Under the analysis of these considerations, it has been postulated that the formation, stability, and direct reaction of the surface peroxo-tungsten (W-O-O-H) species have a direct contribution in the oxidation process to convert DBT to DBTO2. A proposed mechanism could be given from Raman evidence, where the WdO group interacts with H2O2 and DBT to form water and dibenzothiophene sulfoxide (DBTO). This could be represented by the following elementary reactions: WdO + H2O2 f HO-W-OOH
(10)
HO-W-OOH f W(O2) + H2O
(11)
W(O2) + DBT f WdO + DBTO
(12)
where W(O2) is the peroxo species. The corresponding sulfone (DBTO2) is formed when the W(O2) species interacts with the DBTO intermediate, as can be seen by means of eq 13. W(O2) + DBTO f WdO + DBTO2
(13)
Figure 12 shows a mass transfer scheme of the oxidation-extraction process (in the presence of catalyst). A sequence description is given below. First, DBT moves from the n-C16 phase to the polar phase through an L1-L2 interface (a thermodynamic equilibrium is established in the interface). Then both DBT and H2O2 are transferred from the polar phase to the catalyst through an L2-S interface. It is important to mention that H2O2 will
5360 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008
Figure 11. Raman spectra for the WOx–ZrO2 system (at 20 wt % W, calcined at 1073 K in air for 3 h). (a) WOx–ZrO2 spectra before H2O2 addition and (b) after H2O2 addition.
or H2O is combined with the aprotic solvents. The presence of these species in the mixture explains the oxidant performance, even in the absence of solid catalysts. The catalyst structure had no massive modification, and only the WOx and WO3 species on the surface could be changed during the oxidation-extraction process, as can be proved by X-ray results. There was an additional contribution of the catalyst to the oxidation process when a peroxo-metal intermediate (W-O-O-H) was formed on the surface of the catalyst, as can be revealed by Raman results. The results given in this work showed that there is a synergic effect among the catalyst, solvent, and oxidizing agent, during the oxidation-extraction process to remove DBT from n-C16. Efforts are being made to establish a rigorous theoretical model that can describe the kinetic behavior of DBT oxidation with H2O2 in the presence of solvent and catalyst. Figure12.MasstransferofDBTandH2O2 schemeduringtheoxidation-extraction process in a three-phase S-L1-L2 system.
not be present in the nonpolar phase at the beginning, due to the fact that H2O2 is nonmiscible in this phase. After that, chemical reactions are carried out on the catalytic surface to obtain water, oxygen, and the corresponding sulfone. Finally, the sulfone is transferred from the catalytic surface to the polar phase; both the sulfone and water remain in the polar phase, due to their polar affinity. In this scheme, the interface of the WOx-ZrO2 catalyst is covered by the polar solvent, because WOx-ZrO2 has a rather high hydrophilic character. 4. Conclusions Τhe γ-butyrolactone was the solvent that can remove the highest quantity of DBT. Also, dibenzothiophene oxidation reactivity is highest in γ-butyrolactone, even when reactions are carried out both with and without catalyst. Strong oxidant species (hydroxide, HO-, and perhydroxyl, HO2-, ions) could be formed by a dissociation effect when H2O2
Acknowledgment The authors are grateful to Instituto Mexicano del Petro´leo and Universidad Auto´noma Metropolitana, for the financial support of this research. Literature Cited (1) Babich, I. V.; Moulijn, J. A. Science and Technology of Novel Processes for Deep Desulfurization of Oil Refinary Streams: A Review. Fuel 2003, 82, 607. (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. (3) Ito, E.; Veen, R. V. On Novel Processes for Removing Sulphur from Refinary Streams. Catal. Today 2006, 116, 446. (4) Hulea, V.; Fajula, F.; Bousquet, J. Mild Oxidation with H2O2 over Ti-Containing Molecular Sieves. A Very Efficient Method for Removing Aromatic Sulfur Compounds from Fuels. J. Catal. 2001, 198, 179. (5) Gosling, C.; Molinari, D.; Gatan, R.; Cavana, A. The Role of Oxidative Desulfurization in an Effective ULSD Strategy. NPRA 2004, AM04-48, 1.
Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5361 (6) Liotta, F. J.; Han, Y. Z. Production of Ultra-Low Sulfur Fuels by Selective Hydroperoxide Oxidation. NPRA 2003, AM-03-23, 1. (7) Drago, R. S.; Burns, D. S. Molybdate and Tungstate Doped Porous Carbons as Hydrogen Peroxide Activation Catalysts for Sulfide Oxidations. J. Catal. 1997, 166, 377. (8) Dumitriu, E.; Guimon, C.; Cordoneanu, A.; Casenave, S.; Hulea, T.; Chelaru, C.; Martinez, H.; Hulea, V. Heterogeneous Sulfoxidation of Thioetheres by Hydrogen Peroxide over Layered Double Hydroxides as Catalysts. Catal. Today 2001, 66, 529. (9) Kong, L.; Li, G.; Wang, X. Mild Oxidation of Thiophene over TS1/H2O2. Catal. Today 2004, 341, 93–95. (10) 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. (11) Shiraishi, Y.; Tachibana, K.; Hirai, T. Komasawa, I. Desulfurization and Denitrogenation Process for Light Oils Based on Chemical Oxidation Followed by Liquid-Liquid Extraction. Ind. Eng. Chem. Res. 2002, 41, 4362. (12) Te, M.; Fairbridge, C.; Ring, Z. Oxidation Reactivities of Dibenzothiophenes in Polyoxometalate/H2O2 and Formic Acid/H2O2 Systems. Appl. Catal., A 2001, 219, 267. (13) Yazu, K.; Yamamoto, Y.; Furuya, T.; Miki, K.; Ukegawa, K. Oxidation of Dibenzothiophenes in an Organic Biphasic System and Its Application to Oxidative Desulfurization of Light Oil. Energy Fuels 2001, 15, 1535. (14) Chen, K.; Bell, A. T.; Iglesia, E. Kinetics and Mechanism of Oxidative Dehydrogenation of Propane on Vanadium, Molybdenum and Tungsten Oxides. J. Phys. Chem. B 2000, 104, 1292. (15) Sanderson, W. R. Cleaner Industrial Processes using Hydrogen Peroxide. Pure Appl. Chem. 2000, 72, 1289. (16) Figueras, F.; Palomeque, J.; Lorindant, S.; Feche, C.; Essayem, N.; Gelbard, G. Influence of the Coordination on the Catalytic Properties of Sopported W Catalysts. J. Catal. 2004, 226, 25. (17) Ramı´rez-Verduzco, L. F.; Murrieta-Guevara, F.; Garcı´a-Gutie´rrez, J. L.; Saint Martı´n-Castan˜on, R.; Martı´nez-Guerrero, M.; Montiel-Pacheco, M.; Mata-Dı´az, R. Desulfurization of Middle Distillates by Oxidation and Extraction Process. Pet. Sci. Technol. 2004, 22, 129. (18) Williams, J. The Thermal Decomposition of Hydrogen Peroxide in Aqueous Solutions. Trans. Faraday Soc. 1928, 24, 245. (19) Long, R. B.; Caruso, F. A. Selective Separation of Heavy Oil using a Mixture of Polar and Non Polar Solvents. U.S. Patent 4,493,765, 1985. (20) Murata, S.; Murata, K.; Kidena, K.; Nomura, M. A. Novel Oxidative Desulfurization System for Diesel Fuels with Molecular Oxygen in the Presence of Cobalt Catalysts and Aldehydes. Energy Fuels 2004, 18, 116. (21) Otsuki, S.; Nonaka, T.; Qian, W.; Ishihara, A.; Kabe, T. Oxidative Desulfurization of Middle Distillate Using Ozone. J. Jpn. Pet. Inst. 1999, 42, 315. (22) Wang, D.; Qian, E. W.; Amano, H.; Okata, K.; Ishihara, A.; Kabe, T. Oxidative Desulfurization of Fuel Oil Part I. Oxidation of Dibenzothiophenes using Tert-Butyl Hydroperoxide. Appl. Catal., A 2003, 1253, 91. (23) Sampanthar, J. T.; Xiao, H.; Dou, J.; Nah, T. Y.; Rong, X.; Kwan, W. P. A Novel Oxidative Desulfurization Process to Remove Refractory Sulfur Compounds from Diesel Fuel. Appl. Catal., B 2006, 63, 85. (24) Clerici, M. G.; Bellussi, G.; Romano, U. Synthesis of Propylene Oxide from Propylene and Hydrogen Peroxide Catalyzed by Titanium Silicalite. J. Catal. 1991, 129, 159. (25) Corma, A.; Esteve, P.; Martı´nez, A. Solvent Effects during the Oxidation of Olefins and Alcohols with Hydrogen Peroxide on Ti-Beta Catalyst: The Influence of the Hydrophilicity-Hydrophobicity of the Zeolite. J. Catal. 1996, 161, 11. (26) Ramı´rez-Verduzco, L. F.; Torres-Garcı´a, E.; Go´mez-Quintana, R.; Gonza´lez-Pen˜a, V.; Murrieta-Guevara, F. Desulfurization of Diesel by
Oxidation/Extraction Scheme: Influence of the Extraction Solvent. Catal. Today. 2004, 98, 289. (27) Torres-Garcı´a, E.; Canizal, G.; Ramı´rez-Verduzco, L. F.; MurrietaGuevara, F.; Ascencio, J. A. Influence of Surface Phenomena in Oxidative Desulfurization withWOx/ZrO2 Catalysts. Appl. Phys. A: Mater. Sci. Process. 2004, 79, 2037. (28) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Manfred, M. AdVanced Inorganic Chemistry, 6th ed; Wiley: New York, 1999. (29) Craig, W. J. Formerly of Solvay Interox R & D. Clean Technology Monographs; RSC: Widnes, U.K., 1999. (30) Torres-Garcı´a, E.; Rosas, G.; Ascencio, J. A.; Haro-Poniatowski, E.; Pe´rez, R. Evidence of the Surface Layer in Tungstated Zirconia. Appl. Phys. A: Mater. Sci. Process. 2004, 79, 401. (31) Ibarren, A.; Rodrı´guez-Gattorno, G.; Ascencio, J. A.; Medina, A.; Torres-Garcı´a, E. Tailoring Chemical Hardness in WOx-ZrO2 System. Chem. Mater. 2006, 18, 5446. (32) Kim, D. S.; Ostromecki, M.; Wachs, I. E.; Kohler, S. D.; Ekerdt, J. G. Preparation and Characterization of WO3/SiO2 Catalysts. Catal. Lett. 1995, 33, 209. (33) Urlacher, C.; Mugnier, J. Waveguide Raman Spectroscopy Used for Structural Investigations of ZrO2 Sol-Gel Waveguiding Layers. J. Raman Spectrosc. 1996, 27, 785. (34) Zhao, B.; Xu, X.; Gao, J.; Fu, Q.; Tang, Y. Structure Characterization of WO3/ZrO2 Catalysts by Raman Spectroscopy. J. Raman Spectrosc. 1996, 27, 549. (35) Xie, S.; Iglesia, E.; Bell, A. Water-Assisted Tetragonal-to-Monoclinic Phase Transformation of ZrO2 at Low Temperatures. Chem. Mater. 2000, 12, 2442. (36) Boyse, R. A.; Ko, E. I. Crystallization Behavior of Tungstate on Zirconia and Its Relationship to Acidic Properties. J. Catal. 1997, 171, 191. (37) Engweiler, J.; Harf, J.; Baiker, A. WOx/TiO2 Catalysts Prepared by Grafting of Tungsten Alkoxides: Morphological Properties and Catalytic Behavior in the Selective Reduction of NO by NH3. J. Catal. 1996, 159, 259. (38) Campbell, N. J.; Dengel, A. C.; Edwards, C. J.; Griffith, W. P. Studies on Transition Metal Peroxo Complexes. Part 8. The Nature of Peroxomolybdates and Peroxotungstates in Aqueous Solution. J. Chem. Soc., Dalton Trans. 1989, 6, 1203. (39) Loridant, S.; Feche, C.; Essayem, N.; Figueras, F. WOx/ZrO2 Catalysts Prepared by Anionic Exchange: In Situ Raman Investigation from the Precursor Solutions to the Calcined Catalysts. J. Phys. Chem. B 2005, 109, 5631. (40) Laufer, W.; Meiers, R.; Holderich, W. Propylene Epoxidation with Hydrogen Peroxide over Palladium Containing Titanium Silicalite. J. Mol. Catal. A 1999, 141, 215. (41) Chiker, F.; Launay, F.; Nogier, J. P.; Bonardet, J. L. Green and Selective Epoxidation of Alkenes Catalysed by New TiO2-SiO2 SBA Mesoporous Solids. Green Chem. 2003, 5, 318. (42) Ikeue, K.; Ikeda, S.; Wantanabe, A.; Othani, B. Elucidation of the Local Structure of Active Titanium(IV) Sites on Silica-Based PhaseBoundary Catalysts for Alkene Epoxidation with Aqueous Hydrogen Peroxide. Phys. Chem. Chem. Phys. 2004, 6, 2523. (43) Robbins, M. D.; Henderson, M. A. The partial oxidation of isobutene and propene on TiO2(110). J. Catal. 2006, 238, 111. (44) Wong, S. T.; Hwang, C. C.; Mou, C. Y. Tungstated Zirconia Catalyzed Bromination of Phenol Red Under Nearly Neutral Solution. Appl. Catal., B 2006, 63, 1.
ReceiVed for reView December 11, 2007 ReVised manuscript receiVed April 16, 2008 Accepted May 14, 2008 IE701692R
