oxidation using a zinc-substituted polyoxometalate as catalyst under homogeneous and heterogeneous (MIL-101(Cr) encapsulated) conditions

Fuel Processing Technology 131 (2015) 78–86

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Desulfurization of model diesel by extraction/oxidation using a zinc-substituted polyoxometalate as catalyst under homogeneous and heterogeneous (MIL-101(Cr) encapsulated) conditions Diana Julião a, Ana C. Gomes b, Martyn Pillinger b, Luís Cunha-Silva a, Baltazar de Castro a, Isabel S. Gonçalves b, Salete S. Balula a,⁎ a b

REQUIMTE, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, Porto 4169-007, Portugal Department of Chemistry, CICECO, University of Aveiro, Campus Universitário de Santiago, Aveiro 3810-193, Portugal

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Article history: Received 25 July 2014 Received in revised form 21 October 2014 Accepted 22 October 2014 Available online 25 November 2014 Keywords: Oxidative desulfurization Model diesel oil Ionic liquid extraction Polyoxometalate MIL-101

a b s t r a c t A composite material comprising the chromium terephthalate metal-organic framework MIL-101 and the tetrabutylammonium (TBA) salt of a zinc-substituted polyoxotungstate anion, TBA4.2H0.8[PW11Zn(H2O)O39] (denoted PW11Zn), has been prepared by an impregnation method and characterized by powder X-ray diffraction, scanning electron microscopy, N2 adsorption/desorption, thermogravimetric analysis, FT-IR, FT-Raman and 31P magic-angle spinning (MAS) NMR spectroscopies. The characterization data reveal that the polyoxometalate was homogeneously encapsulated within the cages of the support without affecting its crystalline structure and morphology. Both the homogeneous catalyst PW11Zn and the composite material PW11Zn@MIL-101 exhibit high activity in the extractive and catalytic oxidative desulfurization of a model oil containing dibenzothiophene, 1-benzothiophene and 4,6-dimethyldibenzothiophene. Complete desulfurization of the model oil could be achieved within 2 h by using the ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate as extraction solvent, aqueous 30% H2O2 as oxidant and a reaction temperature of 50 °C. When acetonitrile was used as extraction solvent instead of the IL, the heterogeneous catalyst PW11Zn@MIL-101 could be easily recovered and reused several times without leaching or loss of activity. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Crude oil naturally contains sulfur in the form of sulfides, thiols, thiophenes, substituted benzo- and dibenzothiophenes, etc [1]. If the sulfur is not removed during the refining process, it will contaminate transportation fuels, leading to the emission of sulfur oxides (SOx) upon combustion, which are responsible for smog, acid rain, respiratory disorders in humans and poisoning of catalytic converters for exhaust emission treatment. Regulations for sulfur content in fuels have therefore become increasingly stringent, with "sulfur-free" (b10 ppm S) diesel and gasoline being mandatory in Europe since 2009 [2,3]. At present, the conventional industrial process for removing sulfur-containing compounds from fuels is hydrodesulfurization (HDS), which requires high temperatures, high pressures and high hydrogen consumption [3]. Although HDS is very efficient in removing thiols, sulfides and disulfides, it is not effective in removing heterocyclic sulfur-containing compounds such as dibenzothiophene (DBT) and its derivatives, especially 4,6dimethyldibenzothiophene (4,6-DMDBT). Therefore, to produce ultralow sulfur fuels, new complementary processes have been investigated ⁎ Corresponding author: Dr Salete S. Balula Tel: + 351 220402576, Fax: + 351 220402659. E-mail address: [email protected] (S.S. Balula).

http://dx.doi.org/10.1016/j.fuproc.2014.10.030 0378-3820/© 2014 Elsevier B.V. All rights reserved.

[3,4]. One of the most promising methods is oxidative desulfurization (ODS) [4,5]. In this process, the refractory sulfur compounds are oxidized to sulfoxides and sulfones with subsequent removal of these products by extraction or adsorption [5–12]. Ionic liquids (ILs) have proved to be successful as sustainable extraction solvents for the removal of oxidized sulfur compounds from fuels and model fuel oils, replacing volatile and flammable organic solvents [6,8,9,13–18]. The best results may be obtained through the combination of ODS and extractive desulfurization with ILs in one system [6,8,9,15,17–21]. The IL may even function simultaneously as both extractant and catalyst [16]. The application of H2O2 as oxidant and polyoxometalates (POMs) as catalysts has led to effective ODS systems [20–25]. Hydrogen peroxide is usually the preferred oxidant not only due to its high oxygen content but also because of its environmental compatibility [26]. A lot of work has been performed on the oxidation of various organic substrates using H2O2 as oxidant and POMs as catalysts [27–30]. POMs possess a unique ensemble of properties that make them interesting for diverse applications. The stability of POMs combined with their structural flexibility to incorporate various elements in their framework make them attractive catalysts. To date, POMs have proved to be among the best catalysts for ODS processes [6–8,20–25,31–35]. One of the main characteristics that limits the use of POMs as catalysts is their generally good solubility, which complicates their recovery and reuse. In recent years

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many strategies have been developed to immobilize homogenous POMs in various supports to form new catalytically active and easily recoverable materials [7,20–23,29–33,36–41]. There are different kinds of materials that can be used as solid supports: mesoporous silica, TiO2 and Al2O3 nanocomposites and microporous materials such as zeolites and metal-organic frameworks (MOFs). MOFs possess structures with large, regular, accessible cages and channels that may act as nanoreactors since they can accommodate catalytically active molecules. The three-dimensional porous MIL-101(Cr) exhibits high porosity combined with high thermal and chemical stability that make it an excellent candidate to support catalytic species [21,39,40,42–45]. A few papers have been published on the immobilization of catalytically active POMs in MIL-101(Cr) (POMs@MIL-101) and the study of the resultant materials as heterogeneous catalysts in various oxidative reactions [21, 38,40,42,43,46–49]. Only recently have POMs@MIL-101 been identified as promising catalysts for ODS processes [21,34,40,46,49]. The present work reports the preparation of a novel composite formed by the incorporation of the zinc-substituted polyoxotungstate anion [PW11Zn(H2O)O39]5− (PW11Zn) into MIL-101(Cr) nanocages (PW11Zn@MIL-101). The catalytic performance of this heterogeneous catalyst was investigated in the desulfurization of a model oil containing the refractory sulfur compounds most commonly found in diesel. Desulfurization performance of the homogeneous PW11Zn and heterogeneous PW11Zn@MIL-101 are compared. The efficiency of the extraction solvent in the ODS process was also evaluated using an ionic liquid (1-butyl-3methylimidazolium hexafluorophosphate) and an organic polar solvent (acetonitrile). Remarkable influence was detected mainly for the stability of the heterogeneous catalyst PW11Zn@MIL-101. 2. Experimental 2.1. Materials All the reagents used in the preparation of the composite material, namely, tetrabutylammonium bromide (Alfa Aesar, 98%), sodium tungstate (Aldrich), sodium phosphate (Aldrich), zinc acetate (M&B), chromium(III) nitrate nonahydrate (Aldrich, 99%), benzene-1,4-dicarboxylic acid (Aldrich, 98%) and hydrofluoric acid (Aldrich, 40–45%) were used as received. The chemicals used for desulfurization experiments, namely, dibenzothiophene (Aldrich, 98%), 1-benzothiophene (Fluka, 95%), 4,6dimethyldibenzothiophene (Aldrich, 97%), n-octane (Prolabo), acetonitrile (Panreac), 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF6, Aldrich, 97%), tetradecane (Aldrich, 99%) and hydrogen peroxide (30%, Aldrich) were also used as received. 2.2. Synthesis and characterization of catalysts 2.2.1. Instrumentation For the preparation of MIL-101(Cr) was performed using a CEM Discover microwave oven. Elemental analysis for tungsten was performed by ICP-MS on a Varian 820-MS, and C, N and H analyses were executed on a Leco CHNS-932, both performed at the University of Santiago. Thermogravimetric analysis was performed under air with a heating rate of 10 °C min−1 using a TGA-50 Shimadzu thermobalance (CICECO, University of Aveiro). Powder X-ray diffraction analyses were collected at ambient temperature on a PANalytical Empyrean instrument equipped with a PIXcel 1D detector (45 kV and 40 mA) (CICECO, University of Aveiro). Intensity data were collected by the step-counting method (step 0.02°), in continuous mode, in the range ca. 3.5 ≤ 2θ ≤50°. Scanning electron microscopy (SEM) images were obtained using a high resolution FEI Quanta 400 FEG ESEM instrument (CEMUP, University of Porto). The energy-dispersive X-ray spectroscopy (EDS) studies and SEM mapping images were recorded with the same microscope working at 10 keV and using an EDAX Genesis X4M microanalysis system. Nitrogen adsorption–desorption isotherms were recorded at − 196 °C using a Micromeritics Gemini V 2380 surface analyzer (University of

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Aveiro). The support MIL-101 and the composite PW11Zn@MIL-101 were degassed for 2.5 h at 120 °C prior to analysis. The Brunauer– Emmett–Teller (BET) method was utilized to calculate the specific surface areas (SBET) using adsorption data in a relative pressure range from 0.005 to 0.2. The total volumes were estimated from the adsorbed amount at a relative pressure P/P0 of 0.98. Transmission FT-IR spectra were measured as KBr pellets on a Unican Mattson 7000 spectrophotometer equipped with a DTGS CsI detector (resolution 4 cm−1, 128 scans) at CICECO, University of Aveiro. FT-Raman spectra were recorded on a Bruker RFS 100 spectrometer with a Nd:YAG coherent laser (λ = 1064 nm) and a laser power of 200 mW (CICECO, University of Aveiro). 31P NMR spectra were recorded for liquid solutions using a Bruker Avance III 400 spectrometer at 161.9 MHz at ambient temperature. Solid-state 31P high power decoupled (HPDEC) NMR spectra were recorded on a 9.4 T Bruker Avance spectrometer at 161.9 MHz, using a 6.5 μs 1H 90° pulse, a spinning rate of 12 kHz, and a recycle delay of 60 s. GC-FID was carried out with a Varian V3800 chromatograph to monitor catalytic reactions and a Bruker 430-GC to follow the reactions. In both experiments, hydrogen was the carrier gas (55 mL s− 1) and fused silica SPB-5 Supelco capillary columns (30 m × 0.25 mm i.d.; 25 μm film thickness) were used. 2.2.2. Zinc-substituted polyoxometalate The tetrabutylammonium (TBA) salt TBA 4.2 H 0.8 [PW 11 Zn(H2 O) O39] · 5H2O (PW11Zn) was prepared by following published procedures [50,51]. Elemental and thermogravimetric analysis, vibrational spectra (FT-IR and FT-Raman), NMR and powder XRD data confirmed the successful preparation of PW 11 Zn. Anal. Calcd (%) for C67.2H164N4.2O45PW11Zn (3869.83): C, 20.86; H, 4.27; N, 1.52. Found: C, 20.95; H, 4.18; N, 1.49. TGA showed a mass loss of 3.0% up to 150 °C (calcd: for loss of 5H2O: 2.3%; for loss of 6H2O, 2.8%). 31P (161.9 MHz, CD3CN, 298 K): δ = − 10.65 ppm. FT-IR (cm− 1): ν = 2960 (s), 2936 (s), 2873 (s), 1642 (m), 1484 (s), 1380 (m), 1057 (s), 953 (vs), 887 (s), 816 (vs), 728 (sh), 592 (m), 514 (m). FT-Raman (cm−1): ν = 2936 (s), 2872 (s), 1454 (m), 1319 (w), 1131 (w), 1057 (w), 983 (vs), 976 (vs), 904 (m), 515 (w), 374 (w). 2.2.3. Solid support MIL-101(Cr) The porous MOF MIL-101(Cr) was prepared by an adaptation of the original method described by Férey et al. [52]. A mixture containing chromium(III) nitrate nonahydrate (0.40 g, 1.00 mmol), benzene-1,4-dicarboxylic acid (0.166 g, 1.00 mmol) and hydrofluoric acid (50 μL) in water (5 mL) was stirred at room temperature for 10 min to obtain a homogeneous suspension, which was then transferred to a reaction vessel and subjected to microwave-assisted heating at 483.2 K for 2 h. After slow cooling (inside the oven), the resultant material was isolated by filtration and purified through a double DMF treatment followed by a double ethanol treatment. FT-IR (cm−1): ν = 3465 (sh), 3311 (vs), 1555 (vs), 1508 (s), 1439 (s), 1396 (s), 1149 (w), 1099 (w), 1022 (m), 924 (w), 874 (m), 833 (m), 812 (m), 758 (w), 742 (m), 702 (m), 677 (w), 594 (s), 515 (s). FT-Raman (cm−1): 3082 (m), 2976 (w), 2934 (m), 1615 (vs), 1493 (w), 1460 (s), 1148 (m), 1044 (w), 872 (s), 812 (w), 633 (w). 2.2.4. Composite material PW11Zn@MIL-101 The composite material PW11Zn@MIL-101 was prepared by using a method previously described by our group [21]. A solution of PW11Zn in MeCN (10 mM, 9 mL) was added to MIL-101(Cr) (0.4 g), and the mixture was stirred at room temperature for 72 h. The solid was filtered, washed thoroughly with MeCN and dried in a desiccator over silica gel. Anal. Found (%): W, 21%; loading of PW11Zn = 0.104 mmol g−1. Selected FT-IR (cm−1): 2976 (sh), 1624 (vs), 1510 (s), 1405 (vs), 1168 (w), 1092 (w), 1053 (w), 1018 (w), 956 (m), 891 (m), 833 (s), 748 (m), 715 (w), 669 (m), 592 (s), 388 (m). Selected FT-Raman (cm−1): 3082 (m), 2935 (m), 1615 (vs), 1462 (s), 1148 (m), 986 (m), 974 (m), 872 (s), 812 (w), 633 (w), 218 (w), 168 (w).

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3. Results and discussion 3.1. Catalyst synthesis and characterization

2.3. Oxidative desulfurization process (ODS)

The zinc-substituted polyoxotungstate salt TBA4.2H0.8[PW11Zn(H2O) O39] · 5H2O (PW11Zn) and the MOF MIL-101(Cr) were prepared by following published procedures or modified versions thereof [50–52]. A composite of MIL-101 and PW11Zn was prepared by impregnation of the polyoxometalate from a solution in acetonitrile in a manner similar to that performed previously to encapsulate TBA3PW12O40 (PW12) [21]. The resultant material, denoted as PW11Zn@MIL-101, had a PW11Zn loading of 0.104 mmol g−1 on the basis of the W content determined by ICP-MS. FT-IR, FT-Raman and 31P MAS NMR spectroscopic studies confirmed that the structures of MIL-101, and the POM were retained in the composite PW11Zn@MIL-101. Thus, the vibrational spectra of the composite exhibit the characteristic bands of both the MIL-101 support and the Keggin structure of the phosphotungstate anion (Figs. 1 and 2). In particular, the FT-IR spectrum (Fig. 1) contains bands at 1053 (νas(PO4)), 956 (νas(W = O)) and 891 cm−1 (νas(W–O–W)), which match those exhibited by non-encapsulated PW11Zn (1057, 953 and 887 cm−1). Similarly, the Raman spectrum contains the typical νs(W = O) bands of PW11Zn at 974/986 cm−1 (cf. 976/983 cm−1 for PW11Zn) in addition to those expected for MIL-101 (Fig. 2). The 31P MAS NMR spectrum of PW11Zn@MIL-101 presents one somewhat broad resonance centered at –11.8 ppm (cf. –11.9 ppm for PW11Zn, with a weak shoulder at –12.6), further confirming that the integrity of the Keggin structure was maintained after immobilization in the cages of MIL-101 (Fig. 3). The powder XRD patterns of the support MIL-101(Cr), the composite material PW11Zn@MIL-101 and the TBA salt PW11Zn are compared in Fig. 4. The pattern for MIL-101(Cr) is in very good agreement with the expected pattern [21,52,53], thus confirming the formation of the MOF. Most of the diffraction peaks characteristic of MIL-101 are observed for the composite at the same 2θ values, thus indicating that

The oxidative desulfurization studies were performed using model oils containing the refractory sulfur compounds commonly found in fuels: dibenzothiophene (DBT), 1-benzothiophene (1-BT) and 4,6dimethyldibenzothiophene (4,6-DMDBT). The model oils were prepared by dissolution of DBT or a mixture of the different sulfur compounds in n-octane (approximately 500 ppm or 0.0156 mol dm−3 of each one). The processes were performed using equal volumes of the model oil and an immiscible extracting solvent, forming a biphasic liquid–liquid system. The ODS system was optimized using PW11Zn as catalyst and a model oil of DBT in n-octane. Several parameters were evaluated, such as nature of extracting solvent (1-butyl-3 methylimidazolium hexafluorophosphate, (BMIPF6) or acetonitrile), amount of H2O2 (15, 30, 60 or 90 μL) and amount of PW11Zn (3, 5, 9 or 12 μmol). All experiments were carried out under air (atmospheric pressure) in a closed borosilicate 5 mL reaction vessel equipped with a magnetic stirrer and immersed in a thermostated oil bath at 50 °C. The sulfur content in the model oil was quantified periodically by GC analysis. Tetradecane was used as standard, and in a typical procedure, an aliquot (15 μL) of the standard solution was added to the aliquot removed from the model oil phase of the ODS system (15 μL). The same methodology of sulfur quantification was performed using the homogeneous (PW11Zn) and heterogeneous (PW11Zn@MIL-101) catalytic systems. When BMIPF6 was used as extraction solvent and PW11Zn was used as homogeneous catalyst, the extraction phase containing the catalyst could be recycled for consecutive cycles after removing the sulfur components by washing this phase with 1:1 (v/v) ethyl acetate and diethyl ether. The composite PW11Zn@MIL-101 could also be recycled by filtering the solid after each ODS cycle, washing with dichloroethane and acetone (for the oil/BMIPF6 system) or with MeCN (for the oil/ MeCN system) and drying at room temperature.

Fig. 2. FT-Raman spectra of (a) PW11Zn, (b) the support MIL-101(Cr), and the composite PW11Zn@MIL-101 before (c) and after (d) a catalytic run with the multicomponent oil/ BMIPF6 ODS system.

Fig. 1. FT-IR spectra of (a) PW11Zn, (b) the support MIL-101(Cr), and the composite PW11Zn@MIL-101 before (c) and after (d) a catalytic run with the multicomponent oil/ BMIPF6 ODS system.

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Fig. 3. 31P MAS NMR spectra of (a) PW11Zn and (b) the composite PW11Zn@MIL-10 1.

the crystalline structure of the support was maintained upon incorporation of the POM. Some differences in the relative intensities of the diffraction peaks are evident when the two patterns are compared. Similar changes were reported previously for POM-modified MIL-101 and were attributed to the interaction of the clusters with the MIL-101 framework and/or changes in the symmetry of the clusters in the cages of the MOF [52,54,55]. The preservation of the main characteristics of the crystalline structure of the solid support MIL-101 after the

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incorporation of PW11Zn was also confirmed by SEM and EDS. As exemplified in Fig. 5, SEM images of both MIL-101 and PW11Zn@MIL-101 reveal identical morphologies (size and shape) consisting of octahedral shaped nanocrystals ranging in size from 50 to 400 nm. Furthermore, EDS mapping of Cr, Zn and W in the composite showed a homogeneous distribution of the elements across the particles, thus indicating a good dispersion of the POM in the MOF. Fig. 6 shows the nitrogen adsorption–desorption isotherms of MIL101(Cr) and the composite PW11Zn@MIL-101 measured at − 196 °C. In agreement with previous reports [43,52,56], the curves show features typical of both Type I and Type IV isotherms. The specific surface area of MIL-101(Cr), estimated by applying the BET method, was 3410 m2 g−1, while the total pore volume was estimated to be 1.95 cm3 g−1 at a relative pressure of P/P0 ~ 0.98. The BET specific surface area and total pore volume of PW11Zn@MIL-101 were found to be lower compared to assynthesized MIL-101, which is to be expected from the mass increase due to the introduction of the TBA salt PW11Zn and the pore space occupied by the POM clusters and TBA cations. Nevertheless, considerable BET surface area and pore volume of 1880 m2 g−1 and 1.09 cm3 g−1, respectively, were preserved. On the basis of the PW11Zn loading of 0.10 mmol g−1, the mass of chromium terephthalate MOF in the composite is equivalent to ca. 0.6 g. This quantity of as-synthesized MOF (i.e., before POM encapsulation) would have surface area and pore volume values of 2050 m2 and 1.2 cm3, respectively. The slightly lower values observed for the composite are an indication that the cages of the host are partially occupied by PW11Zn species. Indeed, on the basis of an estimate of 2000 Å3 for the volume occupied by one PW11Zn(H2O)O539− anion (dimension of Keggin anion ca. 11 Å [57]) and four tetrabutylammonium cations (volume of ca. 300 Å3 for each cation), the volume occupied by PW11Zn species in the composite (at 0.104 mmol g−1) is calculated as 0.12 cm3 g− 1, which is consistent with the observed difference (1.2–1.09 = 0.11 cm3 g−1). Thermogravimetric analysis (TGA) was performed for MIL-101(Cr) and the composite PW11Zn@MIL-101 (Fig. 7). In agreement with previous reports [52,58], the TGA curve of the as-synthesized MOF essentially comprises two main weight loss steps up to 400 °C. The first one (48.7%) takes place in the range 25–180 °C and corresponds to the removal of guest water and organic solvent (ethanol/DMF) molecules in the large cages. A small loss of ca. 3.0% centered around 245 °C was attributed by Liu et al. to departure of guest molecules from the middlesized cages [58]. The third step, comprising an abrupt mass loss of 25.1% in the range 325–380 °C (DTGmax = 360 °C), corresponds to the elimination of OH/F groups and the decomposition of the framework. The TGA curve for the composite PW11Zn@MIL-101 is similar to that for the parent MOF except that the initial weight loss up to 100 °C is less (32.1%), and the final decomposition step is shifted by 15 °C to higher temperature (DTGmax = 375 °C). Also, the relative weight loss of 37.4% for this last step (= 100 × (mass loss between 325 and 400 °C)/(residual mass at 325 °C)) is lower than that for the parent MOF (57.4%), as would be expected from the presence of the polyoxotungstate anions in the composite. 3.2. Application in oxidative desulfurization process

Fig. 4. Powder XRD patterns of (a) PW11Zn, (b) the support MIL-101(Cr) and the composite PW11Zn@MIL-101 before (c) and after (d) a catalytic run with the multicomponent oil/ BMIPF6 ODS system.

The oxidative desulfurization studies were performed using model oils containing the refractory sulfur-containing compounds commonly found in diesel oil: dibenzothiophene (DBT), 1-benzothiophene (1-BT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) in n-octane with a concentration of approximately 500 ppm for each compound. The ODS processes were carried out in a biphasic liquid–liquid system with equal volume of model oil and an extraction solvent (Scheme 1): acetonitrile (MeCN) or the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF6). Initially, the catalyst was added to the extraction solvent and afterwards the model oil was added. The biphasic system was heated at 50 °C for 10 min and an aliquot was removed from the oil phase to quantify the initial extraction of the sulfur compounds

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Fig. 5. SEM images of (a) the support MIL-101(Cr) and the composite material PW11Zn@MIL-101 before (b) and after (c) a catalytic run with the multicomponent oil/BMIPF6 ODS system. Representative SEM and corresponding EDS mapping images are shown for PW11Zn@MIL-101.

from the oil to the extractant phase. After this, the oxidant H2O2 was added to the system to initiate catalytic oxidation of the sulfur compounds present in the extractant to sulfoxides and/or sulfones. The decrease in the concentration of thiophene compounds in the extractant phase causes a continuous transfer of these compounds from the oil to the extractant. The catalytic performances of the TBA salt of PW11Zn (homogeneous catalyst) and the composite catalyst PW11Zn@MIL101(Cr) were compared. When the parent material MIL-101(Cr) was used as catalyst in the ODS process, no desulfurization of oil was detected after the initial extraction step. Besides, in the absence of catalyst, the sulfur compound concentration in the model oil remained constant after the initial extraction step.

3.2.1. Optimization of ODS process Initially, an optimization of experimental parameters was performed. The influence of using acetonitrile or the ionic liquid (BMIPF6) as extraction solvent was investigated. Also, the influence of the amounts of the oxidant H2O2 and homogeneous catalyst PW11Zn were analyzed. These experiments were performed using a DBT model oil. The influence of the extraction solvent was investigated for the ODS process performed at 50 °C using 5 μmol of PW11Zn and 0.26 mmol of H2O2. Fig. S1 in the supporting information presents the results for the catalytic oxidation of DBT using either the oil/MeCN or oil/BMIPF6 ODS systems. Desulfurization of the DBT model oil is higher in the presence of BMIPF6. In fact, after the first hour of the ODS process, 98% of the

Fig. 6. N2 adsorption (filled symbols)–desorption (blank symbols) isotherms at −196 °C of MIL-101(Cr) (■,□) and the composite PW11Zn@MIL-101 (●,○).

Fig. 7. TGA curves of MIL-101(Cr) (− − −) and the composite PW11Zn@MIL-101 (−−).

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DBT was oxidized and removed from the oil phase. Therefore, BMIPF6 was elected as the preferred extraction solvent. The efficiency of ILs as extraction solvents has already been reported in the literature for desulfurization systems [6,17,59–61], and particularly the efficiency of BMIPF6 in ODS systems has been recognized [7,8,62,63]. A possible reason for the superior catalytic oxidation in the presence of this IL compared with MeCN may be the immiscibility of the IL with the aqueous oxidant. The homogeneous catalyst PW11Zn is only present in the IL phase in the triphasic system oil/H2O2/BMIPF6. This fact may have a positive influence on catalyst stability since the partial decomposition of PW11Zn in the presence of H2O2 in MeCN medium was reported recently [14]. The influence of the PW11Zn catalyst amount was also investigated using the DBT oil and the triphasic system oil/H2O2/BMIPF6, at 50 °C with 0.26 mmol of H2O2. As shown in Fig. S2, a significant improvement in DBT oxidation was observed when the amount of homogeneous catalyst was increased from 3 to 5 μmol. However, when the amount of PW11Zn was higher than 5 μmol, no appreciable increase in DBT desulfurization occurred. The amount of H2O2 (30% aqueous) was subsequently varied in the range 15–90 μL using the same oil/H2O2/ BMIPF6 triphasic system at 50 °C with 5 μmol of PW11Zn. Surprisingly, the worst catalytic performance for DBT oxidation was found using the highest amount of oxidant (90 μL, 0.78 mmol) and the best performance was achieved using 30 μL (0.26 mmol), Fig. S3. In fact, the oxidant phase in the ODS process is localized between the BMIPF6 and oil phases. A high amount of oxidant phase may prevent an efficient contact of the oil phase with the IL phase containing the catalyst, thereby limiting the desulfurization process. After the ODS optimization performed using a single DBT model oil, the optimized experimental conditions were tested using the multicomponent oil containing a mixture of DBT, 1-BT and 4,6-DMDBT. Under these conditions, the amount of H2O2 oxidant needed to be increased to 60 μL (0.52 mmol) to achieve a complete desulfurization. In fact, a complete desulfurization of the multicomponent oil was achieved after 3 h when 60 μL of H2O2 was used, instead of the 6 h necessary to achieve the same result in the presence of 30 μL of oxidant (Fig. S4).

3.2.2. Homogeneous catalyst in oil/BMIPF6 ODS system The desulfurization of the multicomponent oil (DBT, 1-BT and 4,6DMDBT) was carefully studied using the homogeneous PW11Zn catalyst in the optimized ODS system: BMIPF6 as extraction solvent, 5 μmol of PW11Zn, 0.52 mmol H2O2, 50 °C. The initial extractions found for each refractory sulfur compound from the oil to the BMIPF6 phase were 70% for 1-BT, 46% for DBT and only 4.6% for 4,6-DMDBT. This behaviour was observed previously using similar oil/BMIPF6 ODS systems [21]. The propensity of these compounds to be transferred from the oil to the ionic liquid phase by simple mixing of the system at 50 °C is related to their size and geometry. After this step, the catalytic stage is initiated by addition of the oxidant H2O2. The unoxidized sulfur compounds

present in the extraction solvent are oxidized to sulfones and/or sulfoxides and consequently more sulfur compounds are transferred from the oil to the IL. Fig. 8 displays the kinetic profiles for the catalytic oxidation of each sulfur component. Complete oxidation and desulfurization were achieved after 3 h. The kinetic profiles of DBT and 4,6-DMDBT oxidation are similar; however, higher conversion of DBT than 4,6-DMDBT was achieved in shorter reaction time. In fact, after 2 h the desulfurization of DBT was complete and only 3 ppm of 4,6-DMDBT and 16 ppm of 1BT remained in the oil. The desulfurization ability of each sulfur compound depends on its reactivity. The oxidative reactivity order DBT N 4,6-DMDBT N 1-BT is frequently described in the literature and is related to the electronic density at the sulfur atom and some steric hindrance [6,21,24,64,65]. The success of the oil/BMIPF6 ODS system containing the homogeneous PW11Zn catalyst led us to investigate its recyclability. After each ODS cycle, the sulfur-free model oil was removed from the system and the oxidized and unoxidized sulfur compounds present in the BMIPF6 extraction phase were removed by stirring this phase with a 1:1 (v/v) mixture of diethyl ether and ethyl acetate for 10 min. This procedure was repeated until no sulfur compounds were detected in the IL. The catalyst PW11Zn present in the IL phase was not lost during this extraction procedure since no tungsten was detected in the used diethyl ether and ethyl acetate mixtures. After this, the IL was dried for several hours at 50 °C to remove residual water originating from the oxidant. A new ODS cycle was performed by adding the model oil phase and a new aliquot of oxidant. Fig. 9 presents the desulfurization profiles for three consecutive cycles. The efficiency of the ODS system increases from the first to the second and third cycles. Similar behaviour was observed previously when the same catalytically active species, PW11Zn, was encapsulated into silica nanoparticles [41]. This must be related with the mechanism involved in the oxidation of these sulfur compounds. The literature describes the formation of an active peroxo catalytic species between the oxidant and the terminal WVI = O bonds or the substituted metal ZnOH2 from PW11Zn. The peroxo active complex is responsible for the oxidation of the sulfur compounds [61,66,67]. The stability of the homogeneous catalyst was analyzed by 31P NMR. Fig. S5 in the supporting information displays the 31P NMR spectra of PW11Zn in CD3CN (single peak at −10.7 ppm), in a 1:1 CD3CN/BMIPF6 mixture (two single peaks at −10.1 and −10.5 ppm) and after the third ODS catalytic cycle (three single peaks at −10.1, −10.5 and −10.6 ppm). These results demonstrate that no decomposition of PW11Zn seems to have occurred in consecutive ODS cycles; the extra peak at −10.6 ppm peak must be related to the coordination of peroxo ligands to form the catalytically active species.

100

80

Conversion (%)

Scheme 1. Schematic representation of the ODS system.

83

60

DBT

40

1-BT 4,6-DMDBT

20

0 0

1

2

3

4

Time (h) Fig. 8. Kinetic profiles for the oxidation of each refractory sulfur compound catalyzed by the homogeneous catalyst PW11Zn (5 μmol) at 50 °C, using a multicomponent model oil (approximately 500 ppm of DBT, 1-BT and 4,6-DMDBT in n-octane), BMIPF6 as extraction solvent and H2O2 (0.52 mmol) as oxidant.

D. Julião et al. / Fuel Processing Technology 131 (2015) 78–86

Total sulfur in model oil (%)

100

60%); however, a major difference is evident in the catalytic stage (after the first 10 min, after the addition of the oxidant) of the desulfurization process. This may suggest that the ionic liquid has some participation in the oxidation of the sulfur compounds, probably behaving as a cocatalyst. In fact, some recent publications have shown that imidazolium ionic liquids can actively participate in the oxidation of various benzothiophene derivatives [17,68]. Thus, the best ODS system consisted of the BMIPF6 solvent and the solid PW11Zn@MIL-101 catalyst, for which complete desulfurization was achieved after 1 h. Under the same conditions using the homogeneous PW11Zn catalyst, complete desulfurization was achieved after 3 h.

80

1st cycle

60

2nd cycle 40

3rd cycle 20

0 0

1

2

3

4

5

6

Time (h) Fig. 9. Desulfurization of the multicomponent model oil (approximately 500 ppm of DBT, 1-BT and 4,6-DMDBT), combining the initial extraction step (at 10 min) and the catalytic stage, in the presence of the homogeneous catalyst PW11Zn (5 μmol), using the ionic liquid BMIPF6 as extraction solvent and H2O2 (0.52 mmol) as oxidant, at 50 °C.

3.2.3. Homogeneous versus heterogeneous ODS systems Despite the high stability observed for the homogeneous PW11Zn catalyst in the oil/BMIPF6 ODS system, immobilization of PW11Zn in a suitable solid support is desirable to form a robust heterogeneous catalyst that would permit straightforward removal from the extraction solvent. The MOF MIL-101(Cr) has proved to be an effective support to accommodate various catalytically active POMs in its tridimensional cavities [21,38,40,42,43,46–49]. In this work, the monosubstituted PW11Zn was incorporated into MIL-101(Cr) and the catalytic performances of the homogeneous PW11Zn and the solid composite PW11Zn@MIL-101 were compared. Fig. 10 shows the desulfurization profiles of a multicomponent oil using the homogeneous or composite catalysts in the presence of H2O2 as oxidant and either BMIPF6 or acetonitrile as extraction solvent. When acetonitrile was used, the desulfurization profiles using PW11Zn and PW11Zn@MIL-101 are similar, while with BMIPF6 slightly better performance was obtained using the composite PW11Zn@MIL-101 as catalyst. These results demonstrate that MIL-101(Cr) is a suitable support to form an active catalyst for ODS processes. Comparing both extraction solvents a superior desulfurization is achieved for both ODS catalytic systems when BMIPF6 is used. The initial extraction achieved in the presence of both solvents is similar (around

3.2.4. Stability and recyclability of the heterogeneous ODS system The recyclability of PW11Zn@MIL-101 was investigated to test the efficiency of this catalyst in consecutive ODS cycles using a multicomponent oil and either BMIPF6 or MeCN as extraction solvent. The solid catalyst was separated from the ODS system after each cycle by simple filtration, followed by washing with dichloroethane and acetone (for the oil/BMIPF6 system) or with MeCN (for the oil/MeCN system) and drying at room temperature. The reutilization in a new ODS cycle was performed maintaining the same experimental conditions between different cycles. Fig. 11A displays the reusability of PW11Zn@MIL-101 for three consecutive desulfurization processes for both oil/BMIPF6 and oil/MeCN systems. The initial extraction is maintained between the different cycles for both systems. A small decrease in desulfurization efficiency is noticed for the multicomponent oil/BMIPF6 ODS system;

(a) 100

Oil desulfurization (%)

84

80

2nd cycle 40

3rd cycle 20 0

PW11Zn@MIL-MeCN

Sulfur compound removed (%)

(b)

Desulfurization (%)

100

80

60

PW11Zn@MIL-MeCN 40

PW11Zn@MIL-BMIPF6

PW11Zn-BMIPF6 20

PW11Zn-MeCN

1st cycle

60

DBT

PW11Zn@MIL-BMIPF6

1-BT

4,6-DMDBT

100 80 60 40 20 0

1st cycle

2nd cycle

3rd cycle

0 0

1

2

3

4

5

6

Time (h) Fig. 10. Desulfurization profile of a multicomponent model oil using the homogeneous PW11Zn or heterogeneous PW11Zn@MIL-101 catalysts in the presence of the ionic liquid BMIPF6 or acetonitrile as extraction solvent, and H2O2 (0.52 mmol) as oxidant, at 50 °C.

Fig. 11. (A) Desulfurization data for three consecutive ODS cycles catalyzed by PW11Zn@ MIL-101 using multicomponent oil/BMIPF6 (results obtained after 2 h) and oil/MeCN (results obtained after 6 h) systems at 50 °C, with H2O2 (0.52 mmol) as oxidant. (B) Desulfurization data obtained for DBT, 1-BT and 4,6-DMDBT after 2 h for three consecutive ODS cycles, catalyzed by PW11Zn@MIL-101 using the multicomponent oil/BMIPF6 system at 50 °C, with H2O2 (0.52 mmol) as oxidant.

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however, for oil/MeCN no catalyst deactivation or decrease in desulfurization was observed, and 96–97% sulfur removal was achieved after 6 h for the various consecutive cycles (Fig. 11A). Fig. 11B presents the desulfurization for each sulfur compound in each cycle for the oil/BMIPF6 system. While desulfurization of DBT is constant for the three consecutive ODS cycles, a decrease in desulfurization efficiency is evident from the second to the third cycle for the sulfur compounds more difficult to oxidize, i.e. 1-BT and 4,6-DMDBT. Analysis of the tungsten content in the solid catalyst removed and washed after three consecutive cycles was performed to evaluate the possibility of leaching of active species. In fact, an appreciable loss of PW11Zn was found when the catalyst PW11Zn@MIL-101 was used in the oil/BMIPF6 ODS system. After the third cycle, 51% of the initial 0.104 mmol g−1 of PW11Zn loaded in PW11Zn@MIL-101 was leached to the extraction solvent. Despite this, FT-IR and FT-Raman data for the solid recovered after the first run maintained the characteristic vibrations of encapsulated PW11Zn (Figs. 1 and 2), and powder XRD data and SEM images were consistent with retention of the crystalline structure and morphology of the MOF support (Figs. 4 and 5). Surprisingly, no leaching occurred using the composite PW11Zn@MIL-101 in the multicomponent oil/MeCN ODS system. Thus, when PW11Zn@MIL101 is used as catalyst in the oil/BMIPF6 ODS system, better desulfurization data are achieved in a shorter reaction time but at the same time some leaching of active species occurs. On the other hand, when PW11Zn@MIL-101 is used as catalyst in the oil/MeCN ODS system, total desulfurization only occurs after 6 h but in this system the catalyst seems to be more robust, suffering no leaching. Similar behaviour was observed previously by our group using other catalysts of the type POMs@MIL-101(Cr) in ODS [21,34,40]. Therefore, these results suggest that POMs@MIL-101(Cr) are efficient and robust heterogeneous catalysts for oxidative desulfurization of multicomponent oils containing various benzothiophene derivatives when MeCN is used as extraction solvent. When POMs are used as homogeneous catalysts the ionic liquid BMIPF6 is a more suitable extraction solvent than MeCN because it facilitates catalyst recycling and also promotes high degrees of desulfurization within short reaction times. 4. Conclusions In summary, effective desulfurization processes based on extraction using either an ionic liquid or acetonitrile and catalytic oxidation using either PW11Zn as homogeneous catalyst or the composite PW11Zn@ MIL-101 as heterogeneous catalyst have been developed. The reaction conditions, including the effect of the amounts of catalyst and H2O2, have been investigated in detail. The homogeneous catalyst is best paired with the IL BMIPF6, resulting in short desulfurization times (b 3 h) and excellent recyclability after solvent extraction of oxidized and unoxidized sulfur compounds from the BMIPF6 phase. Although the system BMIPF6/PW11Zn@MIL-101 presented the shortest reaction times for complete desulfurization of the model oil, the composite was found to be more stable when using acetonitrile as extraction solvent, allowing straightforward catalyst recovery by filtration and reuse without loss of activity. Acknowledgments This work was partly financed by FEDER (Fundo Europeu de Desenvolvimento Regional) through COMPETE (Programa Operacional Factores de Competitividade) and by national funds through the FCT (Fundação para a Ciência e a Tecnologia) within the projects FCOMP01-0124-FEDER-020658 (FCT ref. PTDC/EQU-EQU/121677/2010), FCOMP-01-0124-FEDER-013026 (FCT ref. PTDC/CTM/100357/2008), CICECO-FCOMP-01-0124-FEDER-037271 (FCT ref. PEst-C/CTM/LA0011/ 2013) and Requimte-FCOMP-01-0124-FEDER-037285 (FCT ref. PEst-C/ EQB/La0006/2013).

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