Heterogeneous oxidative desulfurization of diesel fuel catalyzed by mesoporous polyoxometallate-based polymeric hybrid

Journal of Hazardous Materials 333 (2017) 63–72

Contents lists available at ScienceDirect

Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat

Heterogeneous oxidative desulfurization of diesel fuel catalyzed by mesoporous polyoxometallate-based polymeric hybrid Huawei Yang, Bin Jiang, Yongli Sun, Luhong Zhang, Zhaohe Huang, Zhaoning Sun, Na Yang ∗ School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• PIL was assembled with commercial POMs to form the interlinked mesoporous catalysts. • The PIL/POM hybrid was used as catalysts for oxidative desulfurization of fuel. • The catalyst showed high efficiency and durability in real diesel treatment. • The mechanism of H2 O2 activation by H2 W12 O42 10− was put forward.

a r t i c l e

i n f o

Article history: Received 24 November 2016 Received in revised form 2 March 2017 Accepted 8 March 2017 Available online 10 March 2017 Keywords: Poly (ionic liquid) Polyoxometallates Oxidative desulfurization H2 W12 O42 10−

a b s t r a c t In this work, the simple preparation of novel polymer supported polyoxometallates (POMs) catalysts has been reported. Soluble task-specific cross-linked poly (ionic liquid) (PIL) was prepared with N, N- dimethyl- dodecyl- (4- vinylbenzyl) ammonium chloride and divinylbenzene as co-monomers. The as-prepared cationic PILs were assembled with different commercial POMs to form the interlinked mesoporous catalysts, and the formation mechanism was provided. The catalytic oxidation activities of the catalysts were closely related to the formation pathway of their corresponding peroxide active species. The catalyst with H2 W12 O42 10− as counterion, which exhibited the best activity in the oxidation of benzothiophene (BT) and dibenzothiophene (DBT) to sulfones in model oil with hydrogen peroxide (H2 O2 , 30 wt%) as oxidant, was characterized by different techniques and systematically studied for its sulfur removal performance. As for the oxidative desulfurization of a real diesel, it was observed that almost all of the original sulfur compounds could be completely converted, and the catalyst could be reused for at least eight cycles without noticeable changes in both catalytic activity and chemical structure. In the end, a catalytic mechanism was put forward with the assistant of Raman analysis. © 2017 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author. E-mail address: [email protected] (N. Yang). http://dx.doi.org/10.1016/j.jhazmat.2017.03.017 0304-3894/© 2017 Elsevier B.V. All rights reserved.

Sulfur compounds in fuel are converted into SOx when combusted. Furthermore, SOx in automobile exhaust degrades the catalytic converter performance resulting in increased NOx emission which is another important air pollutant [1]. In recent years, ultra-low-sulfur diesel (ULSD) was required in many countries

64

H. Yang et al. / Journal of Hazardous Materials 333 (2017) 63–72

by mandating stringent legislation to cut the S-content down to 10 ppm. At present, the traditional hydrodesulfurization (HDS), a conventional commercial technology, is wildly employed in the purification of fuels. Although HDS can remove various Scompounds such as thiols, sulfides and disulfides, when the deep desulfurization of diesel fuel is required, HDS is less effective due to the low hydrogenation activity to heterocyclic thiophenic compounds [2]. Further enhancing HDS conversion needs huge equipment investments and high operating costs [3]. Therefore, alternative or supplementary technologies with the advantages of low energy consumption are envisaged and oxidative desulfurization (ODS) is considered as one of the most promising methods to obtain ULSD [4,5]. In a typical ODS process, S-compounds in fuel are oxidized to their corresponding sulfones which can be subsequently removed by extraction, adsorption, distillation or decomposition [6,7]. Among the commonly employed oxidant, such as hydrogen peroxide (H2 O2 ), organic peroxide, dioxygen etc, H2 O2 is the most extensively adopted oxidant due to its easy preparation, no unwanted by-product and relatively high oxidative activity [8]. Various catalysts based on organic acids [9,10], metallic oxide [11–13], polyoxometallates (POMs) [14] and metal complexes [15,16] have been explored and led to good results. It should be noted that W and Mo-based POMs are the most commonly used catalysts for oxidation with H2 O2 due to their inherent redox properties [17]. The activation of H2 O2 with POMs can produce effective and selective polyoxoperoxo complexes for the oxidation of refractory thiophenic compounds [6]. However, the use of H2 O2 aqueous solution in ODS with heterogeneous catalysts makes the reaction a multi-phase system, in which the oxidant, sulfur compounds and catalysts are contained in different phases [8]. Consequently, sulfur removal can be significantly enhanced by the combination of POMs and phase transfer agent (PTA) [18–21]. It was frequently reported that the activated peroxide species can be transferred into the organic phase, thus making the catalysis bear homogeneous property [22]. However, the loss of catalyst is also inevitable [20]. To solve this problem, the grafting of phase transfer catalyst into porous materials can avoid the direct loss [23,24]. Recent advances in poly (ionic liquid) (PIL) bring various concept to the polymer supported phase transfer catalysts [25]. PIL possesses various advantages including tunable solubility, designable structure, and chemical and thermal stability which make them suitable for catalytic application [26,27]. Exchangeable counterion allows PIL to be a supporter of POM, and the introduction of monomers with different functional groups will enhance the functionality. To the best of our knowledge, PIL supported catalyst has not been reported for ODS application. In the research work of other fields, porous PIL materials were prefabricated by crosslinking polymerization, and then polyanions were exchanged to the surface to provide different catalytic effects [28,29]. In our opinions, the surface grafting does not bring an enough stabilizing effect to POM. POM can still be reconstructed or dissociated, especially in such a complex reaction environment of diesel [30]. In this work, towards the preparation of polymer supported phase transfer catalyst with better stability, a simple direct precipitation method which is based on the ionic selfassembly of polymerized cations and polyanions in solvent was adopted. Herein, crosslinking PIL was synthesized with N,Ndimethyl-dodecyl-(4-vinylbenzyl) ammonium chloride (DDVAC) and divinylbenzene (DVB) as co-monomers. Novel catalysts were then prepared by the precipitation method of the PIL and different POMs. Mesoporous structure and interlinked framework of the catalysts were formed, and extremely high POM loading was achieved. The desulfurization performances of the catalysts were first preliminarily evaluated for the oxidation of BT and DBT in model oil. The influences of crosslinking degree of cations and anion species were

discussed. The catalyst with H2 W12 O42 10− performed the best in the current work. Thus, more detailed assessments about its structure characteristics and catalytic performances were carried out. The optimized catalyst and reaction conditions were then applied to the ODS of a real diesel. The repeatability of the catalyst in real diesel treatment was also investigated. In the end, a catalytic mechanism was put forward with the assistant of Raman analysis. 2. Experiments 2.1. Materials and chemicals 4-Vinylbenzyl chloride and N,N-dimethyl dodecylamine were purchased from Tianjin Heowns Biochem LLC. 2,2 -Azobis(2methylpropionitrile) (AIBN), hydroquinone, acetone, ethanol and methanol were purchased from Tianjin Jiangtian Chemical Technology Co., Ltd. BT, DBT, DVB, n-dodecane, phosphotungstate acid (H3 PW12 O40 ), silicotungstic acid (H4 SiW12 O40 ), ammonium heptamolybdate tetrahydrate ((NH4 )6 Mo7 O24 ·4H2 O), ammonium paratungstate hydrate ((NH4 )10 H2 W10 O42 ·xH2 O) and ammonium metatungstate hydrate ((NH4 )6 H2 W12 O40 ·xH2 O) were purchased from Aladdin Reagent Co. Ltd. Hydrogenated diesel was provided by SINOPEC Tianjin Petrochemical Co., Ltd. 2.2. Synthesis of dimethyl-dodecyl-4-vinylbenzyl ammonium chloride (DDVAC) DDVAC was prepared according to the literature with some modification [31]. Simply, 4-Vinylbenzyl chloride (0.2 mol), hydroquinone (0.2 g) and 150 ml ethanol were charged into a flask and vigorously stirred at −20 ◦ C. N,N-dimethyldodecylamine (0.2 mol) which was dissolved in 100 ml ethanol was slowly added into the flask by use of a dropping funnel. The mixture was stirred at −20 ◦ C for 2 h after dropping and further stirred at 25 ◦ C for 48 h. After the reaction, the solvent was removed using rotary evaporation at 30 ◦ C. The obtained solid product was extracted with hot acetone twice, then filtered and dried. 1 H NMR spectrum of DDVAC is shown in Fig. S1. 2.3. Synthesis of the cross-linked polymer In a typical procedure, 15 mmol DDVAC was charged into a Schlenk flask and dried under vacuum overnight. Then, AIBN (3.5% mol. eq.), certain amount of DVB (1/5, 1/10 or 1/15 mol. eq.), and 15 ml ethanol were charged into the flask. The flask was degassed with five vacuum/nitrogen cycles, and subsequently immersed in a preheated water bath at 65 ◦ C. The polymerization was allowed to proceed under the protection of N2 for 24 h, before the reaction was cooled to room temperature. The polymer was precipitated in pure water, then filtered, washed with water and dried under vacuum at 60 ◦ C for 12 h. The content of chloride ion was determined by the silver nitrate titration method in a methanol-H2 O mixed solution with K2 CrO4 as an indicator. 1 H NMR and FT-IR spectra of the cross-linked polymer (1/10) are illustrated in Figs. S2 and S3 . 2.4. Preparation of PILs based catalysts The PIL based catalysts were prepared through a direct precipitation method [32]. In short, the polymer (1.5 mmol based on the content of chloride ion) was dissolved into 100 ml methanol, and certain amount of POM was dissolved into 50 ml DI water. The dosages of different POMs are listed in Table S1. The anion exchange reaction was done by slowly dropping the POM aqueous solution into the PIL solution by use of a syringe under magnetic stirring at 60 ◦ C, and further stirred for another 2 h. The mixture was then

H. Yang et al. / Journal of Hazardous Materials 333 (2017) 63–72

moved to a hydrothermal reaction vessel and heated at 100 ◦ C for 24 h. The white precipices were then separated, and washed with DI water (50 ml, twice) and methanol (50 ml, twice) successively by centrifugalization. The obtained catalysts were dried under vacuum at 80 ◦ C for 24 h.

65

Model oil was prepared by dissolving BT or DBT in n-dodecane to reach 1000 ppm S-content. The ODS process was conducted in a 50 ml jacked reactor equipped with a circulator bath, a magnetic stirrer and a reflux tube. Certain amounts of catalysts, model oil and H2 O2 aqueous solution were added into the reactor in sequence, and vigorously stirred at 400 r/min. Samples were taken from the oil phase at set intervals and analyzed by GC-FID (Agilent 7890A; HP-5, FID: Agilent) for the evolution of S-compounds concentration. The ODS of real diesel with the S-content of 559.7 ppm was also conducted. The desulfurization procedure of real diesel was similar to that of model oil. Species and concentration of S-compounds were analyzed by GC-FPD (Lunan GC-7820; HP-5; FPD: Lunan). The total sulfur content was analyzed with an elemental analyzer (Analytical jena, multi EA 5000). More information about chromatography is given in Table S2.

multiple-charged polymerized cation and polyanion will lead to huge ionic bond network. H2 W12 O42 10− -based polymeric catalysts are selected to represent a panoramic view of this kind of material. The TEM image of the catalyst synthesized with the DVB to DDVAC molar ratio of 1/15 is shown in Fig. 1a. The as-prepared material possesses interlinked mesoporous structure which is different from orderly arranged mesopore channels. Its skeletons are a kind of nanowire with the diameter around 30 nm, and the skeletons are interconnected to form cross-linked framework. With the DVB to IL molar ratio increasing to 1/10, more branches and mesopores as well as smaller skeleton diameter (20–25 nm) are attained, which might be more beneficial for its catalytic performance [33]. However, when the DVB to IL molar ratio was further increased to 1/5, the obtained catalyst bears uneven distributed skeleton diameter and disordered crosslinking, which could be attributed to the bigger obstacle and more random interconnection in the arrangement of polymer molecules caused by the high polymer crosslinking degree. Fig. 1d illustrates the SEM image of the catalyst. It can be observed that the catalysts in bulk are stacked together due to the nonrigid property [34]. In fact, the TEM images can better reveal the structure specialty of the catalyst presented in the real reaction system. As the nanostructure of the catalyst shown in Fig. 1e and Fig. S4, the skeleton possesses a thin layer structure, and the H2 W12 O42 10− isopolyanion could be identified by its darker color. It could be seen that the polyanions are firmly surrounded by organic cations and uniformly distribute in the skeleton. The possible formation mechanism of the mesoporous structure is provided as follows. Firstly, it should be noted that the polyanion can play the role of crosslinking agent through the ionic assembly and bring curing effect to the material. In the preparation process, the polymeric cations were cross-linked by the POM anions, and the formed skeletons can keep growing with the continuous reaction between POM and PIL. Meanwhile, due to the steric effect of long carbon chain, the tight arrangement of the polymerized cations could only performed in two-dimensional direction, thus forming the thin layer skeleton structure and the width of the fully formed skeleton is also restricted. In addition, different skeletons could also be connected, thus forming the interlinked mesoporous structure. As a comparative experiment, a catalyst (Fig. 1f) was also prepared with the same precursor as the catalyst shown in Fig. 1b but no hydrothermal synthesis step. It could be observed that its cross-linking structure had been initially formed but was quite different from the structure shown in Fig. 1b. There are some very narrow nanowires which are owing to the incomplete growth of the skeletons, while the wide ones are a kind of scattered structure due to the inadequate crosslinking of PIL. The semi-finished material shown in Fig. 1f could demonstrate that the formation process of the nanowire skeletons is a gradually growing process through ionic self-assembly.

3. Results and discussion

3.2. Preliminary study of catalytic activity

3.1. Preparation of the PIL/POM hybrid

The desulfurization performances of the catalysts with different POM cluster prepared in this work were primarily evaluated. Under the same conditions, desulfurization experiments were carried out using BT- and DBT-containing n-dodecane as model oil with H2 O2 as the oxidant, and the results are listed in Table 1. The PIL-based catalysts with Mo8 O26 4− and H2 W12 O42 10− as anions (entry 3 and 5) both showed efficient desulfurization performance by achieving high BT and DBT conversion rate. However, sulfur removal with the other three catalysts (entry 1, 2 and 4) could barely be observed. The W or Mo-based Keggin-type heteropolyanions combined with phase transfer cations have been frequently reported for good catalytic performance in ODS [6]. Moreover, when they grafted onto different ionic liquid modified supports,

2.5. Characterization 1H

NMR spectra of monomer and polymer were recorded on a VARIAN INOVA 500 MHz spectrometer. The Fourier transform infrared spectroscopy (FT-IR) of samples was recorded on a Bio-Rad FTS 6000 FT-IR spectrometer. X-ray photoelectron spectroscopy (XPS) data were obtained with an electron spectrometer (ULVACPHI, PHI 5000 VersaProbe). Thermograms were recorded using the TGA instrument (Mettler Toledo TGA-DSC 1) with a weighing precision of ±0.1% at a heating rate of 10 ◦ C/min in the temperature range between 50 and 650 ◦ C under a nitrogen atmosphere. The surface morphologies of the catalysts were characterized by field emission scanning electron microscopy (SEM, Hitachi S-4800). Transmission electron micrographs (TEM) were taken with a Transmission Electron Microscope (JEOL JEM-2100F). Catalyst sample and H2 O2 aqueous solution were characterized by a Raman Microscope and Spectrometer (Renishaw, inVia Reflex) with the excitation laser at 785 nm. The reaction system was also characterized by in-situ Raman with the procedure provided as follows. Firstly, the catalyst was spreaded in the circular groove of the sample table. After a drop of H2 O2 aqueous solution was dripped onto the catalyst, the microscope was quickly aimed at the wetted part of the catalyst, and the reaction time was measured with a stopwatch. Then, Raman curve was taken after the reaction began for a certain amount of time. 2.6. Desulfurization and analysis

The PIL/POM catalysts were prepared as the route illustrated in Scheme 1. Firstly, cross-linked copolymers were synthesized with DDVAC and DVB as co-monomer. For the DVB ratio to DDVAC was only ranging from 1/15 to 1/5, the polymerized products were soluble in many polar solvents due to the low crosslinking degree. When the PIL reacted with POM in methanol-water solvent, the chlorine ions of PIL could be exchanged by the polyanion of POM [32], and stable precipitates formed due to the large lattice energy between organic cation and polyanion. Meanwhile, one polyanion can combine with multiple polymerized cations through ionic bond, and vice versa. Consequently, the combination of

66

H. Yang et al. / Journal of Hazardous Materials 333 (2017) 63–72

Scheme 1. Synthesis route of the POM-based polymeric catalysts.

Fig. 1. TEM images of PIL- H2 W12 O42 10− catalysts with DVB to DDVAC molar ratio of 1/15 (a), 1/10 (b) and 1/5 (c). (d) SEM image of PIL- H2 W12 O42 10− (1/10). (e) TEM image of PIL- H2 W12 O42 10− (1/10). (f) TEM image of PIL- H2 W12 O42 10− (1/10) prepared without the hydrothermal synthesis step. Table 1 DBT conversion by use of catalysts a . entry 1 2 3 4 5 6 7 a

anion species 3−

PW12 O40 SiW12 O40 4− Mo8 O26 4− H2 W12 O40 6− H2 W12 O42 10− H2 W12 O42 10− H2 W12 O42 10−

DVB/IL

BT conversion (%)

DBT conversion (%)

1/10 1/10 1/10 1/10 1/10 1/5 1/15

<1 <1 53.6 <1 68.9 59.1 58.3

4.6 4.3 82.3 3.5 95.5 85.1 92.1

Reaction conditions: T = 30 ◦ C, mcatal = 25 mg, moil = 10 g, H2 O2 /S = 4/1, initial S-content = 1000 ppm, t = 90 min.

good results were also led to [23]. Nevertheless, they hardly have catalytic activity in the polymer matrix. In addition, the catalyst with H2 W12 O40 6− , a Keggin-type isopolyanion, also suffered from no activity. Comparing all the five catalysts (entry 1 − 5), it could be concluded that whether or not a catalyst has a catalytic activity is not affected by the charge number of polyanion. So, we hold

that the differences in active species formed by different POMs with H2 O2 as well as the formation pathway are the main factors. on previous reports, the activation of Based H2 O2 with PW12 O40 3− produces a serious of polyoxoperoxo complexes, including dissociated [(PO4 ){WO(O2 )2 }2 {WO(O2 )2 (H2 O)}]3− [(PO4 ){WO(O2 )2 }4 ]3− ,

H. Yang et al. / Journal of Hazardous Materials 333 (2017) 63–72

67

and [(PO3 (OH)){WO(O2 )2 }2 ]2− [30,35,36]. However, for the catalyst prepared in the current work, PW12 O40 3− is firmly embedded in the skeleton of the polymeric material through the ionic assembly, so the dissociation of PW12 O40 3− to produce activated species is almost impossible. Hence, the fixation effect on polyanion is suspected to be reason for the low activities of all the three Keggintype catalysts. On the contrary, for Mo8 O26 4− , the active species were reported to be formed through the exchange of terminal oxygen to peroxo group via the nucleophilic attack of H2 O2 [37]. In spite of the firmly embedding in the polymer matrix, the naked terminal oxygen enables Mo8 O26 4− to possess good catalytic activities. In addition, the catalytic oxidative desulfurization using H2 W12 O42 10− based amphiphilic catalyst was once reported, but the active species were not investigated [38]. Therefore, detailed work is essential to gain insight into the reaction mechanism. On the other hand, higher DBT reactivity was obtained than that of BT, which is attributed to the electron donating effect of aromatic moieties on the electron density of sulfur atom [39]. The oxidation products of both BT and DBT using PIL- H2 W12 O42 10− as catalyst were analyzed by GC–MS, whose spectra are shown in Figs. S5 and S6 . The influence of DVB/IL molar ratio in the polymerization process was also investigated upon the catalytic performance (Table 1, entry 5, 6 and 7). As is indicated in Fig. 1, the catalyst with DVB/IL molar ratio of 1/10 possesses more developed porous structure, thus exhibiting better catalytic performance than that with DVB/IL molar ratio of 1/15. When the DVB/IL molar ratio is up to 1/5, the loading capacity of the obtained polymer will be greatly weakened. Consequently, the PIL- H2 W12 O42 10− catalyst with DVB/IL molar ratio of 1/10 is the best choice and studied only in the following work.

3.3. Characterization of catalysts The composition and chemical state of the elements in the catalyst were analyzed by XPS. As is shown in the survey spectrum (Fig. 2a), the catalyst was composed of C, N, O and W. A very high surface element C content of 78.49 atm.% owing to the carbon skeleton suggests the good affinity towards oil phase. Surprisingly, element O which is mainly derived from polyanion, also accounts for 15.54 atm.% in the surface. The results ensure the good exposure of catalytic sites. As is shown in Fig. 2b, the O 1s profile could be separated into multiple peaks, which probably induces the coexistence of naked and encapsulated bridging oxygen or terminal oxygen on H2 W12 O42 10− [40,41]. Besides, the W 4f and C 1s profiles are shown in Fig. S7. The two peaks centered at 35.8 and 38.0 eV are attributed to W 4f7/2 and W 4f5/2 of W6+ state (Fig. S7a) [42,43]. The C 1s profile (Fig. S7b) is separated into four peaks at binding energy of 284.6, 285.6, 286.6 and 288.8 eV. The strongest peak at 284.6 eV can be assigned to C C bond in the long carbon chain [44], the peak at 285.6 eV indicates the existence of benzene ring (C C C) [45], and the peaks at 286.6 and 288.8 eV are attributed to the quaternary ammonium group (C N and N-C-Ph) [46]. The thermogravimetric analysis (TGA) of the PIL (1/10) and the corresponding PIL-H2 W12 O42 10− was conducted under the air atmosphere, as is illustrated in Fig. 3. The initial decomposition temperature of the preformed PIL (1/10) is near 170 ◦ C, and its weight loss is close to 100% at 600 ◦ C. Consequently, the organic moieties of PIL- H2 W12 O42 10− were also completely decomposed at 600 ◦ C, and the residual could be assigned to WO3 which account for 52.1% of total mass. According to the WO3 content, we then calculated the monomer to W12 O42 12− molar ratio. When the monomer to H2 W12 O42 10− molar ratio is 7/1 (chemical formula as DDVAC7 DVB7/10 (NH4 )3 H2 W12 O42 ), the WO3 content is 51.7%. A CHN elemental analysis was then performed (shown in Table S3), the result of which also supports the chemical formula of the

Fig. 2. XPS spectra of PIL (1/10) − H2 W12 O42 10− . (a) Survey spectrum (b) O1s profile.

Fig. 3. TGA curves of PIL(1/10) and PIL(1/10)-H2 W12 O42 10− .

68

H. Yang et al. / Journal of Hazardous Materials 333 (2017) 63–72

Fig. 4. Influence of reaction temperature on DBT conversion. Reaction conditions: mcatal = 25 mg, moil = 10 g, H2 O2 /S = 4/1, content = 1000 ppm.

initial

S-

Fig. 5. Influence of H2 O2 /S molar ratio on DBT conversion. Reaction conditions: T = 50 ◦ C, mcatal = 25 mg, moil = 10 g, content = 1000 ppm.

initial

S-

catalyst. In addition, the TGA analysis was also performed to the catalysts prepared with more excessive amount of (NH4 )10 H2 W12 O42 . As shown in Fig. S8, the WO3 proportion was almost unchanged. The results prove that the H2 W12 O42 10− could achieve a maximum degree of assembly with polymeric cation. But due to the steric hindrance, it only combined with repeat units at an average value of seven. 3.4. Effect of reaction conditions Fig. 4 illustrates the effect of temperature on desulfurization efficiency under the condition of H2 O2 /S = 4:1, mcatalyst = 25 mg and initial S-content = 1000 ppm. When the temperature is increased from 30 to 60 ◦ C, the DBT conversion at 60 min increased significantly from 73.1% to 100%. The sulfur removal was limited by kinetics at low temperature, while it can also be restricted by the possible kinetic competition of the decomposition of H2 O2 at high temperature. With the reaction temperature at 60 ◦ C, the DBT conversion efficiency was pretty high before 15 min, but then reduced. Therefore, 50 ◦ C was chosen as the optimistic reaction temperature. The effect of H2 O2 dosage was also investigated at T = 50 ◦ C (Fig. 5). Due to the unproductive decomposition of H2 O2 , only 81.8% sulfur removal was obtained with the H2 O2 /S ratio of 2 at 120 min. With an increase of H2 O2 /S ratio from 2 to 4, the sulfur removal at 60 min increased significantly from 72.1% to 98.3%, which showed a remarkable influence of H2 O2 /S ratio on the desulfurization efficiency. However, when the H2 O2 /S ratio was higher than 4, the reaction rate was no longer sensitive to the H2 O2 /S ratio with sufficient H2 O2 dosage. The results indicated that the activation of H2 O2 is not a rate control step in this reaction. Considering the limited improvement with a further increase to 6, H2 O2 /S = 5 was selected as the optimal value. 3.5. Investigation of real diesel fuel The ODS performance of the catalyst was evaluated using a real diesel with S-content of 559.7 ppm. The sulfur compounds in the original diesel are composed of BT, DBT and their derivatives, and a detailed composition is given in Table S4 and Fig. S9. Speciation of sulfur compounds via the reaction time between 0–60 min was detected by GC-FPD and is given in Fig. 6.

Fig. 6. GC-FPD chromatograms of diesel after the reaction began for a certain amount of time. Reaction conditions: mcatal = 25 mg, mdiesel = 10 g, H2 O2 /S = 5:1, T = 50 ◦ C.

In the chromatogram of original diesel, peaks between 5 and 13 min belong to BT and its derivatives, while the peaks in 13–24 min are assigned to DBT and its derivatives. It was observed that the original peaks shifted to the heavier direction with increasing reaction time, which is attributed to the conversion of thiophenic compounds to sulfones. Almost all the BT and its derivatives were completely converted at 60 min. Though the peaks of DBT derivatives and oxidized products were overlapped, they should have been completely converted considering the high reactivity of DBT. The oxidized diesel was extracted using methanol two times (methanol/oil ratio was 1/1 by volume), and 6.35 ppm sulfur-content was achieved. Leaching experiments were performed to evaluate the absence of W in the homogeneous phase. The oxidized diesel was analyzed by an atomic absorption spectrometer. The absence of W elements

H. Yang et al. / Journal of Hazardous Materials 333 (2017) 63–72

69

Fig. 7. Reuse of the catalyst in ODS. (left: GC-FPD chromatograms of diesel after each repeated cycles; middle: local enlarged GC-FPD chromatograms; right: C3 BT conversion in each cycles). Reaction conditions: mcatal = 25 mg, mdiesel = 10 g, H2 O2 /S = 5:1, T = 50 ◦ C, t = 60 min.

highlighted that the H2 W12 O42 10− was not released during the reaction. 3.6. Reuse of the catalyst In order to accurately test its durability and recycling performance, repeated experiments have been conducted in the ODS of real diesel. After each reaction run, the reaction mixture was let stand at room temperature. After removing the oxidized diesel carefully, fresh diesel fuel and H2 O2 aqueous solution were then added to start a new cycle. Samples were taken at the end of each cycle and analyzed by GC-FPD. In the original diesel, C3BT accounted for the most content in sulfides. Thus, it was selected as standard for the evaluation of S-removal. As is indicated in Fig. 7, the catalyst could at least be repeated for eight runs with an almost unchanged catalytic activity. After the repeated experiments, the reused catalyst was dispersed in ethanol, then filtered to remove the residual diesel and dried under vacuum. The analysis of FT-IR was carried out to investigate the changes of the catalyst before and after reaction. As is shown in Fig. 8, two peaks centered at 1102 and 1383 cm−1 appeared, which were attributed to the overlap of O S O stretching vibration peaks of different sulfones [47]. The characteristic peaks of W O and W-O at 977, 935, 882, 813, 710, 499 cm−1 could easily be distinguished [48], and no shifting was observed. The result strongly suggests that the polyoxometallate-based polymeric hybrid is a durable and recyclable catalyst in the ODS of real diesel. 3.7. Catalytic mechanism In the current work, the catalyst PIL- H2 W12 O42 10− showed excellent catalytic activity and stability in ODS of real diesel. As is analyzed, the polyanions acted as crosslinking agents in the polymeric catalysts due to the formed large ionic bond network, and were firmly embedded in the skeletons. A possible hindering effect for the dissociation of polyanion may be the reason to

Fig. 8. FT-IR analysis of the fresh and reused catalyst.

explain the extremely low catalytic activity of Keggin-type catalysts. It also indirectly reveals the reason for the good stability of the catalysts. In order to explore the catalytic mechanism of PIL-H2 W12 O42 10− , In Situ Raman Spectroscopy has been employed to investigate the changes of catalyst in hydrogen peroxide system. For the original catalyst (Fig. 9a), peaks at 960, 644, 562, 344, 236 and 182 cm−1 are well consistent with the reported ones [49], which reconfirmed the species form of H2 W12 O42 10− . Peaks at 960 and 344 cm−1 are assigned to the stretching vibration and bending vibration of W O respectively, while peak at 644 cm−1 is attributed to W O W vibration. Shown in Fig. 9d, the O O stretching vibration of H2 O2 locates at 879 cm−1 . After H2 O2 aqueous solution was added to the catalysts, new peaks of stretching vibration of W O (562 cm−1 ) and O O (854 cm−1 ) appeared and kept strengthening

70

H. Yang et al. / Journal of Hazardous Materials 333 (2017) 63–72

Fig. 9. Raman spectra of the reaction system. (a) PIL- H2 W12 O42 10− (b) PIL- H2 W12 O42 10− + H2 O2 H2 W12 O42 10− + H2 O2 at 2 min (d) H2 O2 aqueous solution.

at

1 min

(c)

PIL-

(Fig. 9b and c) [50]. The results indicated that the peroxide species with coordinated peroxide groups were formed [51]. In addition, it was observed that the vibration peaks of W O at 960 and 344 cm−1 were shifted to 978 and 322 cm−1 respectively. The H2 W12 O42 10− anion comprises twelve corner- or edgesharing octahedrons, in which there are six octahedrons containing double terminal carbonyl oxygen [38]. When the peroxide species of H2 W12 O42 10− is formed, one of the double terminal oxygen can be replaced by a coordinate peroxide group. However, the electron withdrawing effect of coordinate peroxide is weaker than that of terminal carbonyl oxygen. As a result, the positive charge density of W atom increases, thus leading to both the polarity increase and the bond length decrease of the remaining W O. Hence in the Raman spectra, the stretching vibration peak of WO shifted to high wavenumber, while its bending vibration peak moved to low wavenumber. Similar phenomena was also observed in some previous literatures. For H2 WO4 , the stretching vibration and bending vibration peaks of WO were located at 947 and 330 cm−1 respectively [52], while the corresponding peaks of H2 WO4 + H2 O2 system were shifted to 962 and 314 cm−1 [30]. Consequently, it can be concluded that the peak shift was caused by the inductive effect of the formed coordinated peroxide group on another W O bond in the same WO6 octahedron unit. Based on some previous reports [33,37,38] and results in this work, the possible catalytic mechanism of H2 W12 O42 10− was illustrated in Scheme 2. The peroxide species which could be expressed as H2 W12-n O42-3n {WO2 (O2 )}n 10− are formed through nucleophilic attack of H2 O2 on bridging oxo ligands on the surface of the catalyst and a subsequent loss of H2 O molecule. As is indicated by Fig. 9, the formation rate of peroxide species is very fast. Therefore, the reaction rate of peroxide species to sulfur is a key factor which affects the oxidation efficiency of sulfides. During the reaction process, the long carbon chain and highly interlinked mesoporous structure could bring significant phase transfer property, hence making the catalysis quite efficient.

Scheme 2. Supposed mechanism of the ODS reaction.

POM in solvent. The polymerized cations can be cross-linked by the polyanions at a maximum degree to form abundant mesopores and interlinked framework. The microstructure of the catalysts could also be adjusted by the variation of IL monomer to DVB molar ratio. The catalysts prepared from different POMs were primarily investigated in the ODS of model oil. As is clearly evidenced by the experimental results, whether or not a catalyst possess catalytic activity is closely related to the formation pathway of activated species. The firmly embedding of polyanion in the polymer matrix is responsible for the inactive catalysis of Keggin-type catalysts, but maybe contribute to the stability of polyanion [53]. The catalysts with H2 W12 O42 10− as counterion showed excellent catalytic activity. The different behavior of the catalysts with various DVB to DDVAC molar ratio was confirmed by TEM analyses. The chemical composition of both cationic and anionic components could be revealed by XPS, TG, IR and Raman. As for the ODS of a real diesel, the conversion of sulfur compounds was close to 100%, and a further solvent extraction led to ultra-deep sulfur removal. After being reused for eight cycles, the catalyst maintained an almost unchanged activity. The leaching of W species was also proved to be undetectable. The mechanism of H2 O2 activation by this catalyst was investigated with the assistant of Raman analysis. The active species of H2 W12 O42 10− was proved to be H2 W12-n O42-3n {WO2 (O2 )}n 10− which is formed through nucleophilic attack of H2 O2 on bridging oxo ligands. In summary, this work provides efficient and stable polymer supported POM catalysts, and reveals some unique properties of the PIL-based catalysts prepared through the ionic self-assembly method.

4. Conclusion

Acknowledgement

In this paper, a series of mesoporous POM-based polymeric catalysts have been developed through ionic self-assembly of PIL and

We are grateful for the financial support from National Key R&D Program of China (No. 2016YFC0400406).

H. Yang et al. / Journal of Hazardous Materials 333 (2017) 63–72

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2017.03. 017.

[25] [26] [27]

[28]

References [1] C. Song, An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel, Catal. Today 86 (2003) 211–263. [2] A. Stanislaus, A. Marafi, M.S. Rana, Recent advances in the science and technology of ultra low sulfur diesel (ULSD) production, Catal. Today 153 (2010) 1–68. [3] M. Breysse, G. Djega-Mariadassou, S. Pessayre, C. Geantet, M. Vrinat, G. Pérot, M. Lemaire, Deep desulfurization: reactions, catalysts and technological challenges, Catal. Today 84 (2003) 129–138. [4] S. Otsuki, T. Nonaka, N. Takashima, W.H. Qian, A. Ishihara, T. Imai, T. Kabe, Oxidative desulfurization of light gas oil and vacuum gas oil by oxidation and solvent extraction, Energy Fuels 14 (2000) 1232–1239. [5] A.D. Bokare, W. Choi, Bicarbonate-induced activation of H(2)O(2) for metal-free oxidative desulfurization, J. Hazard. Mater. 304 (2016) 313–319. [6] H. Mei, B.W. Mei, T.F. Yen, A new method for obtaining ultra-low sulfur diesel fuel via ultrasound assisted oxidative desulfurization, Fuel 82 (2003) 405–414. [7] D. Piccinino, I. Abdalghani, G. Botta, M. Crucianelli, M. Passacantando, M.L. Di Vacri, R. Saladino, Preparation of wrapped carbon nanotubes poly(4-vinylpyridine)/MTO based heterogeneous catalysts for the oxidative desulfurization (ODS) of model and synthetic diesel fuel, Appl. Catal. B-Environ. 200 (2017) 392–401. [8] J.M. Fraile, C. Gil, J.A. Mayoral, B. Muel, L. Roldan, E. Vispe, S. Calderon, F. Puente, Heterogeneous titanium catalysts for oxidation of dibenzothiophene in hydrocarbon solutions with hydrogen peroxide: on the road to oxidative desulfurization, Appl. Catal. B-Environ. 180 (2016) 680–686. [9] B. Jiang, H. Yang, L. Zhang, R. Zhang, Y. Sun, Y. Huang, Efficient oxidative desulfurization of diesel fuel using amide-based ionic liquids, Chem. Eng. J. 283 (2016) 89–96. [10] H. Lu, S. Wang, C. Deng, W. Ren, B. Guo, Oxidative desulfurization of model diesel via dual activation by a protic ionic liquid, J. Hazard. Mater. 279 (2014) 220–225. [11] A. Bazyari, A.A. Khodadadi, A. Haghighat Mamaghani, J. Beheshtian, L.T. Thompson, Y. Mortazavi, Microporous titania-silica nanocomposite catalyst-adsorbent for ultra-deep oxidative desulfurization, Appl. Catal. B-Environ. 180 (2016) 65–77. [12] Z. Hasan, J. Jeon, S.H. Jhung, Oxidative desulfurization of benzothiophene and thiophene with WOx/ZrO2 catalysts: effect of calcination temperature of catalysts, J. Hazard. Mater. 205–206 (2012) 216–221. [13] S. Xun, W. Zhu, F. Zhu, Y. Chang, D. Zheng, Y. Qin, M. Zhang, W. Jiang, H. Li, Design and synthesis of W-containing mesoporous material with excellent catalytic activity for the oxidation of 4,6-DMDBT in fuels, Chem. Eng. J. 280 (2015) 256–264. [14] B. Zhang, Z. Jiang, J. Li, Y. Zhang, F. Lin, Y. Liu, C. Li, Catalytic oxidation of thiophene and its derivatives via dual activation for ultra-deep desulfurization of fuels, J. Catal. 287 (2012) 5–12. [15] A. Di Giuseppe, M. Crucianelli, F. De Angelis, C. Crestini, R. Saladino, Efficient oxidation of thiophene derivatives with homogeneous and heterogeneous MTO/H2O2 systems: a novel approach for oxidative desulfurization (ODS) of diesel fuel, Appl. Catal. B-Environ. 89 (2009) 239–245. [16] F. Li, C. Kou, Z. Sun, Y. Hao, R. Liu, D. Zhao, Deep extractive and oxidative desulfurization of dibenzothiophene with C5H9NO·SnCl2 coordinated ionic liquid, J. Hazard. Mater. 205–206 (2012) 164–170. [17] S. Wang, G. Yang, Recent advances in polyoxometalate-catalyzed reactions, Chem. Rev. 115 (2015) 4893–4962. [18] X. Zhang, Y. Shi, G. Liu, Direct preparation of [(CH3)3NC16H33]4Mo8O26 and its catalytic performance in oxidative desulfurization, Catal. Sci. Technol. 6 (2016) 1016–1024. [19] W. Zhu, P. Wu, Y. Chao, H. Li, F. Zou, S. Xun, F. Zhu, Z. Zhao, A novel reaction-controlled foam-type polyoxometalate catalyst for deep oxidative desulfurization of fuels, Ind. Eng. Chem. Res. 52 (2013) 17399–17406. [20] J. Zhang, A. Wang, X. Li, X. Ma, Oxidative desulfurization of dibenzothiophene and diesel over [Bmim]3PMo12O40, J. Catal. 279 (2011) 269–275. [21] H. Lu, W. Ren, W. Liao, W. Chen, Y. Li, Z. Suo, Aerobic oxidative desulfurization of model diesel using a B-type Anderson catalyst [(C18H37)2N(CH3)2]3Co(OH)6Mo6O18·3H2O, Appl. Catal. B-Environ. 138–139 (2013) 79–83. [22] Y. Zhou, Z. Guo, W. Hou, Q. Wang, J. Wang, Polyoxometalate-based phase transfer catalysis for liquid-solid organic reactions: a review, Catal. Sci. Technol. 5 (2015) 4324–4335. [23] J. Xiong, W. Zhu, W. Ding, L. Yang, Y. Chao, H. Li, F. Zhu, H. Li, Phosphotungstic acid immobilized on ionic liquid-modified SBA-15: efficient hydrophobic heterogeneous catalyst for oxidative desulfurization in fuel, Ind. Eng. Chem. Res. 53 (2014) 19895–19904. [24] N. Zohreh, S.H. Hosseini, A. Pourjavadi, R. Soleyman, C. Bennett, Immobilized tungstate on magnetic poly(2-ammonium ethyl acrylamide): A high loaded

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42] [43]

[44]

[45]

[46]

[47]

[48]

[49]

71

heterogeneous catalyst for selective oxidation of sulfides using H2O2, J. Ind. Eng. Chem. 44 (2016) 73–81. J. Yuan, D. Mecerreyes, M. Antonietti, Poly(ionic liquid)s: an update, Prog. Polym. Sci. 38 (2013) 1009–1036. J. Yuan, M. Antonietti, Poly(ionic liquid)s: polymers expanding classical property profiles, Polymer 52 (2011) 1469–1482. Q. Zhao, P. Zhang, M. Antonietti, J. Yuan, Poly (ionic liquid) complex with spontaneous micro-/mesoporosity: template-free synthesis and application as catalyst support, J. Am. Chem. Soc. 134 (2012) 11852–11855. X. Wang, Y. Zhou, Z. Guo, G. Chen, J. Li, Y. Shi, Y. Liu, J. Wang, Heterogeneous conversion of CO2 into cyclic carbonates at ambient pressure catalyzed by ionothermal-derived meso-macroporous hierarchical poly(ionic liquid)s, Chem. Sci. 6 (2015) 6916–6924. C. Gao, G. Chen, X. Wang, J. Li, Y. Zhou, J. Wang, Hierarchical meso-macroporous poly(ionic liquid) monolith derived from a single soft template, Chem. Commun. 51 (2015) 4969–4972. J. Gao, Y. Chen, B. Han, Z. Feng, C. Li, N. Zhou, S. Gao, Z. Xi, A spectroscopic study on the reaction-controlled phase transfer catalyst in the epoxidation of cyclohexene, J. Mol. Catal. A-Chem. 210 (2004) 197–204. P. Zarras, O. Vogl, Polycationic salts. 3. Synthesis, styrene based trialkylammonium salts and their polymerization1-21, 2, J. Macromol. Sci. A 37 (2000) 817–840. R.N. Biboum, F. Doungmene, B. Keita, P. de Oliveira, L. Nadjo, B. Lepoittevin, P. Roger, F. Brisset, P. Mialane, A. Dolbecq, I.M. Mbomekalle, C. Pichon, P. Yin, T. Liu, R. Contant, Poly(ionic liquid) and macrocyclic polyoxometalate ionic self-assemblies: new water-insoluble and visible light photosensitive catalysts, J. Mater. Chem. 22 (2012) 319–323. J. Dou, H.C. Zeng, Integrated networks of mesoporous silica nanowires and their bifunctional catalysis-sorption application for oxidative desulfurization, ACS Catal. 4 (2014) 566–576. Y. Leng, J. Wu, P. Jiang, J. Wang, Amphiphilic phosphotungstate-paired ionic copolymer as a highly efficient catalyst for triphase epoxidation of alkenes with H2O2, Catal. Sci. Technol. 4 (2014) 1293. N.M. Gresley, W.P. Griffith, A.C. Laemmel, H.I.S. Nogueira, B.C. Parkin, Studies on polyoxo and polyperoxo-metalates part 511Part 4: Ref. [4].: peroxide-catalysed oxidations with heteropolyperoxo-tungstates and −molybdates, J. Mol. Catal. A: Chem. 117 (1997) 185–198. D.C. Duncan, R.C. Chambers, E. Hecht, C.L. Hill, Mechanism and dynamics in the H3[PW12O40]-catalyzed selective epoxidation of terminal olefins by H2O2. formation, reactivity, and stability of {PO4[WO(O2)2]4}3, J. Am. Chem. Soc. 117 (1995) 681–691. J.L. García-Gutiérrez, G.A. Fuentes, M.E. Hernández-Terán, P. García, F. Murrieta-Guevara, F. Jiménez-Cruz, Ultra-deep oxidative desulfurization of diesel fuel by the Mo/Al2O3-H2O2 system: the effect of system parameters on catalytic activity, Appl. Catal. A-Gen. 334 (2008) 366–373. D. Hao, W. You, P. Liu, C. Li, Q. Wu, H. Lu, Deep desulfurization of diesel by selective oxidation using amphiphilic paradodecatungstate catalyst, phosphorus, sulfur silicon, Relat. Elem. 191 (2016) 30–34. J. Xiao, L. Wu, Y. Wu, B. Liu, L. Dai, Z. Li, Q. Xia, H. Xi, Effect of gasoline composition on oxidative desulfurization using a phosphotungstic acid/activated carbon catalyst with hydrogen peroxide, Appl. Energy 113 (2014) 78–85. M. Xu, H. Li, L. Zhang, Y. Wang, Y. Yuan, J. Zhang, L. Wu, Charge and pressure-tuned surface patterning of surfactant-encapsulated polyoxometalate complexes at the air–water interface, Langmuir 28 (2012) 14624–14632. Z. He, Y. Yan, B. Li, H. Ai, H. Wang, H. Li, L. Wu, Thermal-induced dynamic self-assembly of adenine-grafted polyoxometalate complexes, Dalton. Trans. 41 (2012) 10043–10051. T.H. Fleisch, G.J. Mains, An XPS study of the UV reduction and photochromism of MoO3 and WO3, J. Chem. Phys. 76 (1982) 780–786. W. Grünert, R. Feldhaus, K. Anders, E.S. Shpiro, G.V. Antoshin, K.M. Minachev, A new facility for inert transfer of reactive samples to XPS equipment, J. Electron Spectrosc. 40 (1986) 187–192. C. Yao, X. Li, K.G. Neoh, Z. Shi, E.T. Kang, Surface modification and antibacterial activity of electrospun polyurethane fibrous membranes with quaternary ammonium moieties, J. Membr. Sci. 320 (2008) 259–267. Z. Li, W. Wang, Y. Chen, C. Xiong, G. He, Y. Cao, H. Wu, M.D. Guiver, Z. Jiang, Constructing efficient ion nanochannels in alkaline anion exchange membranes by in-situ assembly of poly (ionic liquid) in metal-organic frameworks, J. Mater. Chem. A 4 (2016) 2340–2348. J. Steinkoenig, F.R. Bloesser, B. Huber, A. Welle, V. Trouillet, S.M. Weidner, L. Barner, P.W. Roesky, J. Yuan, A.S. Goldmann, C. Barner-Kowollik, Controlled radical polymerization and in-depth mass-spectrometric characterization of poly(ionic liquid)s and their photopatterning on surfaces, Polym. Chem. 7 (2016) 451–461. V.T. Aleksanyan, Y.M. Kimel’fel’d, S.M. Shostakovskii, A.I. L’vov, Vibrational spectra and structure of thiophene derivative sulfones, J. Appl. Spectrosc. 3 (1965) 264–268. S. Gao, J. Zhao, B. Zhou, K. Yu, Z. Su, L. Wang, Y. Yin, Z. Zhao, Y. Yu, Y. Chen, Four novel polytungstate compounds built up of paradodecatungstate clusters and transition-metal complexes/transition metal: synthesis, structure, and electrochemical properties, Inorg. Chim. Acta 379 (2011) 151–157. W.P. Griffith, P.J.B. Lesniak, Raman studies on species in aqueous solutions. Part III. Vanadates, molybdates, and tungstates, J. Chem. Soc. A (1969) 1066–1071.

72

H. Yang et al. / Journal of Hazardous Materials 333 (2017) 63–72

[50] X. Shi, J. Wei, Preparation, characterization and catalytic oxidation properties of bis-quaternary ammonium peroxotungstates and peroxomolybdates complexes, Appl. Organomet. Chem. 21 (2010) 172–176. [51] W.P. Griffith, T.D. Wickins, Studies on transition-metal peroxy-complexes. Part V. Peroxyoxalates, J. Chem. Soc. A (1967) 590–592.

[52] B. Ingham, S.V. Chong, J.L. Tallon, Layered tungsten oxide-based organic-inorganic hybrid materials: an infrared and Raman study, J. Phys. Chem. B 109 (2005) 4936–4940. [53] M. Vigolo, S. Borsacchi, A. Soraru, M. Geppi, B.M. Smarsly, P. Dolcet, S. Rizzato, M. Carraro, S. Gross, Engineering of oxoclusters-reinforced polymeric materials with application as heterogeneous oxydesulfurization catalysts, Appl. Catal. B-Environ. 182 (2016) 636–644.