Phosphotungestovanadate immobilized on PVA as an efficient and reusable nano catalyst for oxidative desulphurization of gasoline

Accepted Manuscript Title: Phosphotungestovanadate immobilized on PVA as an efficient and reusable nano catalyst for oxidative desulphurization of gasoline Author: Mohammad Ali Rezvani Mina Oveisi Mokhtar Ali Nia asli PII: DOI: Reference:

S1381-1169(15)30086-8 http://dx.doi.org/doi:10.1016/j.molcata.2015.09.010 MOLCAA 9626

To appear in:

Journal of Molecular Catalysis A: Chemical

Received date: Revised date: Accepted date:

18-6-2015 20-8-2015 14-9-2015

Please cite this article as: Mohammad Ali Rezvani, Mina Oveisi, Mokhtar Ali Nia asli, Phosphotungestovanadate immobilized on PVA as an efficient and reusable nano catalyst for oxidative desulphurization of gasoline, Journal of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2015.09.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Phosphotungestovanadate immobilized on PVA as an efficient and reusable nano catalyst for oxidative desulphurization of gasoline Mohammad Ali Rezvani *, Mina Oveisi, Mokhtar Ali Nia asli

Department of Chemistry, Faculty of Science, University of Zanjan, 451561319, Zanjan, Iran

*Corresponding author; E-mail: [email protected] Tel: +98 241 5152477 Fax: +98241 5152617.

Graphical abstract

Highlights    

Vanadium containing polyoxometalate was supported on polyvinylalcohol (PVA) via sol-gel method in mild temperature. Catalytic activity tested on ODS of actual gasoline and model sulfur compounds. (N(tBu)4)4HPW10V2O40-PVA nanocomposite was shown be able to scavenge mercaptans (with high yield) in gasoline by H2O2 as an oxidant. Activity of nanocomposite is much higher than unsupported polyoxometalates.

Abstract

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In this manuscript, phosphotungestovanadate (N(tBu)4)4HPW10V2O40 (PWV) as a Keggin-type polyoxometalate was synthesized and immobilized on poly vinyl alcohol (PVA) via sol-gel method. The materials characterized by FT-IR, XRD, UV-Vis, SEM and

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P-NMR spectroscopy. Catalytic

activity of synthesized nanocomposite was tested on oxidative desulphurization of actual gasoline and results are compared with that of model sulphur compounds. This Keggin-type supported catalyst was shown to be able to have oxidative desulphurization of model sulphur compounds and actual gasoline with high yield. The addition of acetic acid enhanced the conversion. The advantages of this method lie in its mild condition, low cost, large scale, simplicity and environmentally friendly route.

Keywords: Synthesis; Nanocomposite; Immobilization; Oxidative desulphurization; Gasoline.

1. Introduction Polyoxometalates (POMs) as metal–oxygen anionic cluster can undergo to a reversible reduction without changing its structure [1]. They have numerous applications in a multitude of areas ranging from catalysis to pharmacology and remain of continuous interest, especially in the areas of design and synthesis of functional nanomaterials [2-5]. In addition, POM as an electron shuttle is known to resist oxidation and to catalyze redox reactions [6]. Several researchers indicate that the polyoxoanion acts as a molecular acceptor by interactions with the nitrogen and oxygen atoms of the organic donor [7-11]. Two main types of POMs are known, based on their chemical composition: 2

isopoly and heteropoly anions. Heteropoly anion (HPA), especially the Keggin-type, as strong acids and multielectron oxidants have attracted much attention [12,13]. Their acid strength is mostly higher than many conventional solid acids [14]. Further catalytically important subclasses of the Keggin compounds are the mixed-addenda vanadium (V) substituted HPAs with the general formula H3+nPM12–nVnO40 (M = Mo and W; n = 1 to 6). The major limitations of HPAs are low surface areas and solubility in polar media which makes difficult the recycling and reuse of them. So, many attempts have been performed to immobilize POM on various high surface area solid materials. The immobilized HPA catalysts are important since they can easily be recovered from reaction mixtures and reused. Therefore, researchers have focused on immobilizing HPAs onto different acidic or neutral materials such as SiO2, TiO2, zeolites, acidic ion-exchange resins, clays, active carbons, polymers and polyaniline–graphene composite [12-18]. The architecture and topology of entangled POM-based coordination polymers have been of particular interest due to their potential applications ranging from catalysis and adsorption [19-23]. A major source of pollution is sulphur containing fuel oils which damage the environment in some different ways. Besides produces the precursors of the acid rain (SOx), their combustion in automobile engines also irreversibly poisons noble metal catalysts of the exhaust systems [24], an effective process that been done in this field is oxidative desulphurization that operates at mild reaction conditions. In continuation of our group researches on the syntheses and application of polyoxometalate in organic reactions [25-28], we hereby report the applicability of PWV/PVA for efficient desulphurization of gasoline to preparation of ultra-clean fuels. The catalytic performance of heterogeneous catalysts was tested on oxidation desulphurization (ODS) of gasoline and model sulphur compounds that exist in gasoline such as benzothiophene (BT) and thiophene (T), with acetic acid/hydrogen peroxide as the oxidizing reagent. Various oxidants have been used in ODS, such as

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NO2, O3, H2O2 and solid oxidizing agents [29]. Among these oxidants, H2O2 is mostly chosen as an oxidant, as only water is produced as a byproduct. Peracids produced in situ from organic acids catalysts and H2O2 are reported to be very effective for rapid oxidation of sulphur compounds in fuel oils under mild conditions [29]. This quaternary ammonium Keggin type tungstovanadophosphate PWV, which has lipophilic cation can better act as phase transfer agent and better transfer the peroxometal anion into organic phase. A literature review showed that desulphurization of gasoline using the immobilized phosphotungestovanadate (N(tBu)4)4HPW10V2O40 (PWV) onto poly vinyl alcohol (PVA) was not studied [29-31]. In this manuscript, PWV as a Keggin-type polyoxometalate was synthesized and immobilized onto PVA. The synthesized nanocomposite (PWV/PVA) used as a catalyst for oxidative desulphurization of thiophenes (Th), benzothiophenes (BT) and actual gasoline. The chemical characterization of this compound nanocomposite was accomplished by X-ray diffraction (XRD), Fourier Transform infrared spectra (FT-IR), Scanning Electron Microscopy (SEM),

31

P-NMR and

UV-Vis techniques.

2. Experimental 2.1. Materials All solvents and reagents which are used in synthesized procedure are available commercially and were used as received, unless otherwise indicated. Model compounds and chemicals, including thiophene (Th) and benzothiophene (BT) and solvent (n-heptane) for experiments and analysis and hydrogen peroxide (30 vol.%) were obtained from Aldrich Chemical Company. The products characterized by analytical instrument FT-IR, UV-vis, XRD and SEM. Several heteropolyoxometalate catalysts were prepared according to the literature 4

procedure [25, 26]. Typical actual gasoline (density 0.7968 g/mL at 15 °C, total sulphur content 0.424 wt.%) was used.

2.2. Preparation of catalysts 2.2.1.

Synthesis

of

Tetrabutylammonium

divanadodecatungstophosphate

((n-

C4H9)4N)4H[PW10V2O40] The tetrabutylammonium (TBA) salts TBA4HPW10V2O40 was synthesized according to published literature procedures [30]. In summary, sodium metavanadate (NaVO3, 6.1 g, 50 mmol) was dissolved in 50 mL of boiling water and mixed with a 50 mL aqueous solution of di-sodium hydrogen phosphate (Na2HPO4, 1.77g, 12.5 mmol). After the resulting solution was cooled to room temperature, concentrated sulphuric acid (5 mL, 85 mmol) was added to give a red solution. Then, 50 mL aqueous solution of sodium tungstate dehydrate (Na2WO4·2H2O, 41.25 g, 150 mmol) was added to the red solution with vigorous stirring. By addition of solid potassium carbonate, the pH is adjusted between 6 and 7. By addition of solid potassium chloride (2.2 g, 40 mmol) an orange potassium salt (2.5 g, 45 mmol) is precipitated and recrystallized in water. The remaining solid was dissolved in 55mL of warm distilled water and aqueous solution of tetrabutylammonium bromide (Bu4NBr) (1.8 mmol) was added drop-wise, with vigorous stirring. The solid formed was filtered off, recrystallized with acetonitrile and ether, and air dried (Scheme 1). The resultant (((nC4H9)4N)4H[PW10V2O40]) solid is designated as TBA-PVW. 2.2.2. Preparation of nanocatalyst by immobilization of PWV on PVA Preparation of [(n-C4H9)4N)]4HPW10V2O40-PVA via sol–gel method under oil-bath condition is as follows : In 25 mL of boiling distilled water, 0.1 g PVA was dissolved and the temperature was fixed

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at 65 °C. Then 0.1 g of PWV was added to this solution with stirring, resulting in a yellow solution and the mixture was vigorously stirred for 2 h at 65 ºC. After the reaction continuing for 2 h at 65 ºC, the color of precipitate not changed. After this time yellow gelatinous solution was observed. This jelly was placed in a drying oven at 50 °C for 2 h (Scheme 1).

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Scheme 1. Chart of synthesis of nanocomposite.

2.3. Catalytic test 2.3.1. Oxidative desulphurization of model compound (simulated gasoline) The experiments of oxidative desulphurization were performed in a round bottom flask equipped with stirrer. For oxidative desulphurization of simulated gasoline, BT and thiophene was selected as model sulphur compound. To make a stock solution with a sulphur content of 500 ppm, BT or thiophene was dissolved in n-heptane. In the typical run, the water bath was first heated and stabilized to a certain temperature (25–40 °C). Into the flask, 6 mL acetic acid/H2O2 (peracetic acid, acetic acid/hydrogen peroxide mole ratio of 1/2) was added and mixed with 50 mL of the model sulphur compound (Th or BT). To initiate the reaction, 0.1 g of the PWV-PVA as a nano catalyst was added to the flask. The flask was immersed in a heating bath (25–40 °C) and stirred at 500 rpm for 2 h. The biphasic mixture was separated by decantation. The BT content in both phases was determined. Product identification was achieved by using GC-MS (Varian cp-1200 quadrupole MS), HP5890 Series II with 5972 Series MS detector. The results indicated a perfect match of the mass spectrum of the product with the standard benzothiophene sulfone. Also the total sulphur concentration of the simulated gasoline before and after reaction was determined using a Tanaka Scientific RX-360 SH X-ray fluorescence spectrometer (ASTM D-4294 method). Results are showed in Table 1. Conversion of BT was calculated to evaluate the catalytic activity of PWV-PVA by using Equation (1): 7

% Conversion = [(Co-Ct)/Co] × 100%

(1)

in which Co is initial concentration of BT, and Ct is final concentration of BT.

Table 1. Effect of different catalysts in the ODS of gasoline, Thiophene and BT. Conversion (%) Entry Catalyst Ref. Thiophene BT Actual gasoline 1 (N(tBu)4)5PV 2W10O 40/PVA 98 98 97 In this work 2 (N(tBu)4)5PV2W10O40 92 91 91 [30] 3 (N(But)4)5PMo10V2O40 89 88 88 [30] 4 H5PV2W10O40 84 83 83 [30] 5 (NH4)5PV2W10O40 83 82 81 [30] 6 K5PV2W10O40 82 81 80 [25] 7 H3PW12O40 80 80 78 [26] 8 H4SiW12O 40 79 80 77 [26] Conditions for desulphurization: 50 mL of model gasoline (500 ppm S) or 50 mL of gasoline, 0.1 g catalyst, 6 mL acetic acid/H2O 2, 10 mL extraction solvent, time 2 h and temperature 35 °C.

2.3.2. Oxidative desulphurization (ODS) of gasoline: For ODS of actual gasoline, 50 mL of gasoline fuel was added to two-necked round flask. The temperature of solution fixed at 65°C. Then 0.1 g of PWV-PVA added to the solution and strongly stirred by a magnetic stirrer. Through 2 h, a mixture of acetic acid: H2O2 in ratio 1:2 was added drop wise in it while it was stirring vigorously. After the oxidation was finished, the mixture was cooled down to room temperature and the oxidized sulfur in the gasoline fuel was extracted with acetonitrile at room temperature. The acetonitrile/gasoline ratio used was 1/5 by volume. The oil phase was separated and weighed to calculate recovery percentage of actual gasoline (for three times reaction: %97, %96 and %94). The total sulphur and mercaptan sulphur content in actual gasoline before and after reaction were determined using X-ray fluorescence ASTM D-4294 and D-3227 standard test method. This standard test method (ASTM D-4294) provides precise and rapid measurement of total

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sulphur in petroleum and petroleum products with a minimum of sample preparation. A typical analysis time is 1 to 5 min per sample. Results are showed in Table 2.

Table 2. Oxidative desulphurization of gasoline by PWV-PVA. Entry Properties of gasoline Unit Method Before ODS After ODS a 1 Total Sulphur by X-Ray Wt.% ASTM D 4294 0.424 0.014 2 Mercaptans ppm ASTM D 3227 82 4 3 Density by hydrometer @ 15 0C g/ml ASTM D 1298 0.7968 0.7965 4 Salt (ptb) ASTM D 3230 18 16 5 Water Content by distillation vol. % ASTM D 4006 Nil. Nil. IBP 47.3 46.8 0 C FBP 208.8 208.5 10 69.6 68.7 6 Distillation ASTM D 86 50 117.7 117.2 Vol% 90 189.2 188.1 95 206.3 205.8 a Condition for desulphurization: 50 mL of gasoline, 0.1 g nano catalyst, 3 ml oxidant, 10 mL of extraction solvent, time = 2 h, and temperature = 35 °C.

2.4. Characterization methods Fourier transform infrared (FT-IR) spectra were recorded by Thermo-Nicolet-is 10 in the range of 400–4000 cm-1. UV–visible spectra were studied

with

a double beam termo-heylos

spectrophotometry in the range of 200-500 nm. X-ray diffraction (XRD) patterns was accomplished by D8 advance Brucker and radiation Cu kα (λ = 1.54 nm) in the range of 0º-60º (2θ). Scanning electron microscope (SEM) images were obtained on a LEO 1455 VP with an accelerating voltage of 10.00 kV. 31P-NMR spectra were recorded by Bruker Ultra-shield spectrometer.

3. Results and discussion 3.1. Characterization of synthesized catalysts

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FT-IR spectra of free PWV, PVA, and PWV-PVA support are studied for structural characterization purposes (Fig. 1). The strong peaks at 2934 cm−1, 2872 cm−1 (Fig.1) were mainly attributed to alkyl groups(C-H stretching vibration). The broad band in the 3500–3700 cm−1 regions can be attributed to the symmetrical stretching vibration modes of the adsorbed water molecules and band in 3404 cm-1 is due to O-H. The structure of Keggin type heteropolyanion consists of a PO 4 tetrahedron that surrounded by four W 3O9 groups formed by the edge sharing octahedron. This structure gives rise to different types of oxygen atoms, being responsible for the fingerprint FT-IR bands of the heteropolyanion between 750–1100 cm−1 (Fig.1). The four distinct bands belonging to the Keggin structure were centred at 1095, 961, 888, and 803 cm−1 [33]. Earlier reports assigned these peaks that appeared at about 778– 785 and 850–890 cm−1 to W-O-W. The peaks around 950–998 cm−1 and 1070–1090 cm−1 were associated with M=O and P-O bonds, respectively, and explicitly indicate the neat structure of Keggin-type heteropolyanion. The band at 1095 cm−1 was assigned to the symmetric stretching vibration of PO 4 tetrahedron, by comparison the peaks of PWV with PWV/PVA, clarified the maximum displacement (red shift) is related to (W-Od) of PWVPVA because it has more interaction with PVA (Table 3). However, a slight shift is evidenced as a result of PWV interaction with functional groups of the support. In the FT-IR spectrum of PWV-PVA, the peaks slightly overlapped with PVA and the intensity of the peak was lower.

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Figure 1. FT-IR spectra of PWV (a), PVA (b) PWV-PVA (c).

Table 3. FT-IR related to Kegging PWV and PWV-PVA. Compound

ν (cm-1) (W-Oc c-W)

ν (cm-1) (W-O b b-W)

ν (cm-1) (W-Od d)

ν (cm-1) (P-Oa a)

PV2W10O40-PVA

803.32

888.46

961.70

1095.07

PV2W10O40

803.53

889.22

960.61

1095.67

a

Oa: oxygen bonded with phosphorus atom. octahedral edge-sharing oxygen.

b

Ob: octahedral corner-sharing oxygen.

c

Oc:

Fig. 2 shows the UV-vis spectra of pure PVA, PWV and PWV-PVA as comparative data to confirm the changes on pure PWV after introducing PVA. The UV–vis spectra of PVA (a), PWV (b) and PWV-PVA (c) (Fig. 2), showed strong absorption peak at 241 nm which assigned to oxygen-to11

tungsten charge-transfer transition of PWV, 245 nm for PWV-PVA and 270 nm for PVA which indicated PWV-PVA blue shift towards PVA and red shift towards PWV. Excitation of the oxygen to metal charge transfer band of PWV in near UV light region results in the charge transfer from an O -2 to W +6, forming the O -1 and W +5.

Figure 2. UV–vis spectra of PWV-PVA (a), PWV (b) and PVA (c).

The surface morphology of PWV-PVA nanocomposite was investigated by SEM. A typical SEM picture of the as prepared PWV-PVA catalyst is shown in Fig. 3. From Fig. 3(a), the blank PVA film is observed to be relatively flat surface and Fig. 3(b) shows polyoxometalate consist of very small agglomerated nanoparticle. This picture (Fig. 3 c,d) shows non regular morphology. Nanocomposite Keggin-type polyoxometalate modified PVA is created and the estimated particle sizes are achieved nano-scale.

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Figure 3. SEM images of PWV-PVA. The images of a): PVA film (3 µm), b): PWV (1µm), c): PWV/PVA (1 µm) and d): PWV/PVA (300 nm).

The powder nanostructures were investigated by X-ray diffraction (XRD) measurement [34]. The patterns were collected in the scanning range 6° ≤ 2θ ≤ 60° are shown in Fig. 4. The pattern, observed for pure PVA, shows several distinct crystalline sharp peaks at 2θ = 20.654, 23.520 and 25.19 degrees [35]. The XRD patterns of PWV-PVA shows the pure PVA with less intensity and some peaks are no longer detectable since they are of small intensity in the XRD pattern of PWV13

PVA. The peaks for pure PWV at 6-10, 16-23 and 25-30 degrees indicated kegging type of polyoxometalate [36]. By comparison of PWV and PWV-PVA, the peaks of PWV are still observed after the loading although the intensity of the diffraction peak becomes broader and weaker than pure PWV. In addition, the most peaks for PWV-PVA attributed to the crystal of PWV are observed this indicates the highly dispersed PWV on the surfaces of PVA support, when PWV is bound to the PVA surface, PWV-PVA, all of signals corresponding to PWV and the final pattern matched to PWV, which is most likely due to PWV forming on the PVA surface and thus the majority of the observed signals are due to the crystal phases of PWV, which is in good agreement with the results of SEM. Scherer equation, was used to calculate the nano crystallite size by XRD radiation of wavelength from measuring full width at half maximum of peaks in radian located at any 2θ in the pattern. The mean crystal size obtained was around 22.5 nm.

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Figure 4. XRD of PWV (a), PVA (b) and PWV-PVA (c). Nuclear magnetic resonance (NMR) of the different active nuclei constituting POM is considered to be a very powerful method to clear their molecular structures both in solution and in the solid state. The

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P NMR spectrum of PWV and PWV-PVA in DMSO at ~25 °C was a clear single line 15

spectrum at -14.797 ppm due to the internal phosphorus atom, thereby confirming the compound’s purity, as shown in Fig. 5. The signal exhibited a shift from the signals of PWV, suggesting the insertion of PWV and PWV-PVA.

Figure 5. 31P-NMR spectrum of PWV (a) and PWV/PVA (b) dissolved in DMSO.

3.2. Desulphurization process According to data from the results of Table 2, total sulphur content (Entry 1) and content of mercaptans (Entry 2) were much lower, while other properties of gasoline showed in Table 2 16

remained unaffected. From the results obtained in this research, it was demonstrated that the nanocomposite PWV-PVA, can catalyze the oxidative desulphurization reaction in 2 h and can reduce total sulphur content (wt.%) of actual gasoline from 0.424 wt.% to 0.014 wt.% and also, reduce content of mercaptans from 82 ppm to 4 ppm. 3.3. Effect of the catalyst structure Tables 1and 4, and Figure 6 show the results of oxidative desulphurization of thiophene, BT and actual gasoline with acetic acid/H2O2 as the oxidant using different catalysts. The amount of each catalyst was constant throughout the series. Blank experiment was performed in the absence of catalyst. Under these conditions, % conversion was very low (22% in 2 h) at 35 °C (Table 4, entry 9). The results show that the catalytic activity of PWV-PVA nanocomposite has presented much higher than other unsupported POMs. The vanadium-substituted Keggin type polyoxometalate catalyst was very active systems for the ODS of gasoline. The PWV with a phase transfer or emulsion catalyst comprising a quaternary ammonium salt-based polyoxometalate, it is shown very active system for ODS. This quaternary ammonium Keggin type which has lipophilic cation can better act as phase transfer agent and better transfer the peroxometal anion into organic phase. That is, the oxidation reactivity of the catalysts depends on the type of countercation: ((C4H9)4N) + > NH4+ > K+. PWV is more effective than (N(tBu)4)4HPMo10V2O40 in oxidative desulphurization of model sulphur compounds (BT). It may be different in tungsten and molybdenum reduction potentials. The results in Table 1 and 4 show that oxidative desulphurization of gasoline ability decreases generally in the order: V- > W- > Mo-containing heteropolyanions, which means that vanadium-containing heteropoly compounds are the strongest oxidants.

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Table 4. Effect of different catalysts in the ODS of gasoline a % remove of total sulphur % remove of mercaptans Entry Catalyst With H2O 2 Without With H2O2 Without H2O2 H2O2 1 (N(tBu)4)4HPV2W10O40/PVA 97 45 95 40 2 (N(tBu)4)4HPV2W10O40 91 40 89 36 3 (N( tBu)4)4HPMo10V2O 40 88 38 88 35 4 H5PV2W10O40 83 37 82 32 5 (NH4)4HPV2W10O40 81 41 80 31 6 K5PV2W10O40 80 34 79 30 7 H3PW12O40 78 33 77 30 8 H4SiW12O 40 77 16 77 28 9 None 22 20 a Condition for desulphurization: 50 mL of gasoline, 0.1 g catalyst, 6 mL oxidant, 10 mL of extraction solvent, time = 2 h, and temperature = 35 °C.

Figure 6. Effect of different catalyst on oxidative desulphurization of BT. 1- None, 2- H4SiW12O40 3H3PW12O40

4-

K5PV2W10O40

5-

(N(But)4)4HPMo10V2O40

(N(tBu)4)4HPV2W10O40/PVA.

18

6-

(N(tBu)4)4HPV2W10O40

7-

3.4. Effect of Catalyst dosage Another factor that should be concerned is the catalyst dosage. It was found from the results of Table 5 and Figure 7 that the catalyst dosage does have a marked influence on the process efficiency. Under otherwise identical conditions, without catalyst, 24% of the thiophene, 23% of the benzothiophene and 23% of actual gasoline is removed from the n-heptane phase in 120 min by oxidation; % conversion of actual gasoline in the presence of PWV-PVA were found to be 68%, 88% and 97%, corresponding to catalyst amount of 0.06, 0.08 and 0.1 gr respectively. Desulphurization efficiency increased rapidly with the increase of catalyst dosage. The results are summarized in Table 5.

Table 5. Effect of catalyst dosage in the ODS of gasoline and simulated gasoline a Conversion (%) Entry Amount of catalyst (g) Thiophene Benzothiophene Actual gasoline 1 0 (none) 24 23 23 2 0.02 39 38 37 3 0.04 48 47 46 4 0.06 69 68 68 5 0.08 89 88 88 6 0.1 98 98 97 7 0.11 98 97 97 8 0.12 98 97 97 a Condition for desulphurization: 50 ml of gasoline, 6 ml oxidant, 10 ml of extraction solvent, time = 2 h, and temperature = 35 °C, catalyst= PWV-PVA

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Figure 7. Effect of catalyst dosage on oxidative desulphurization of BT.

3.5. Influence of quaternary ammonium on catalytic activity Countercation with quaternary ammonium salts with lipophilic cation can act as phase transfer agent and can transfer the peroxometal anion into organic phase. An amphiphilic catalyst with a proper quaternary ammonium cation can form metastable emulsion droplets in gasoline with an aqueous H2O2 solution, demonstrating high oxidative activity, and can be separated after reaction through centrifugation. For example, [N(C4H9)4]+ is a proper quaternary ammonium cation (Tables 1 and 4). We tested other cations and found that [N(C4H9)4]+ forms metastable emulsion droplets more readily than NH4+ and K+ in gasoline. Hydrogen peroxide cannot dissolve in oil phase of actual gasoline. 20

Therefore, oil is unable to absolutely access to acetic acid: H2O2 as oxidant in the ODS process, which reduces ODS speed, and leads to an improper ODS and raises the amount of oxidant (acetic acid: H2O2). Phase transfer catalyst can improve the poor contact between gasoline (oil phase) and an acetic acid: H2O2 (liquid phase). This type of polyoxometalate as a catalyst can expedite the reaction between two reactants in two mutually exclusively soluble solvents (solid - liquid two-phase system or liquid - liquid two-phase system). During the ODS, the actual reactants are transferred from one phase to another by this type of polyoxometalate. Consequently, the reaction can be conducted at high rate. 3.6. Effect of temperature on the oxidative desulphurization of gasoline or simulated gasoline Influence of reaction temperature on the ODS of simulated gasoline and actual gasoline was investigated under the same conditions by PWV-PVA as a catalyst and acetic acid/H2O2 as oxidant. The results are shown in Table 6 and Figure 8. Four temperatures were tested (25◦, 30◦, 35◦ and 40◦C). The results show that yields of products are a function of temperature. The conversion of sulphur in model compound and in actual gasoline increased with temperature and time. The conversion of sulphur in simulated fuel at 35 ºC is higher than that at 30 ºC. 97% conversion of sulphur was obtained at 35 ºC in 120 min. Table 6. Effect of different temperatures in the ODS of gasoline and simulated gasoline a Conversion (%) Entry Temperature (°C) Thiophene Benzothiophene Actual gasoline 1 25 75 71 71 2 30 89 86 88 3 35 98 98 97 4 40 98 98 97 a Condition for desulphurization: 50 ml of gasoline, 0.1 g catalyst, 6 ml oxidant, 10 ml of extraction solvent, time = 2 h.

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Figure 8. Effect of different temperature on oxidative desulphurization of BT.

3.7. Effect of different oxidative system on the oxidative desulphurization Effect of oxidative system on the oxidative desulphurization of gasoline was studied (Table 7). Hydrogen peroxide, KMnO4 and K2Cr2O5 were selected as oxidizing agents which were used in the presence of organic or inorganic acids such as; acetic acid, oxalic acid, benzoic acid, H2SO4 and H2CO3 to acidify the system. The results in Table 6 showed that in inorganic acids, H2SO4 and H2CO3, oxidation reactivity is lower than organic acids. In actual gasoline, H2SO4 and H2CO3 cannot dissolve; percent sulphur removal of gasoline in inorganic acid/H2O2 was lower than organic acid/H2O2 system.

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Table 7. Effect of different oxidation system in the ODS of gasoline and simulated gasolinea Entry

Oxidant

Acid

% remove of % remove of mercaptans total sulphur 1 H2O2 acetic acid 95 97 2 H2O2 oxalic acid 89 97 3 H2O2 benzoic acid 87 92 4 H2O2 H2SO4 85 91 5 H2O2 H2CO3 84 79 6 H2O2 -78 78 7 KMnO 4 formic acid 86 85 8 KMnO 4 oxalic acid 75 74 9 KMnO 4 H2SO4 74 73 10 KMnO 4 -73 74 11 K2Cr2O5 formic acid 84 76 12 K2Cr2O5 oxalic acid 76 74 13 K2Cr2O5 H2SO4 73 74 14 K2Cr2O5 -74 73 a Condition for desulphurization: 50 ml of gasoline, 0.1 g PWV-PVA, 6 ml oxidant, 10 ml of extraction solvent, time = 2 h, and temperature = 35 °C.

3.8. General desulphurization process A model gasoline was made by adding Thiophene and BT into n-heptane solvent, with a total sulphur concentration of 500 mg/L. The organic sulphur compounds were mixed with acetic acid/H2O2 and PWV-PVA then the oxidation reaction takes place at 35 °C under atmospheric pressure. This is followed by a liquid extraction (acetonitrile) to obtain gasoline with low sulphur. In the ODS process, many oxidizing agents have been reported and H2O2 is the main one. Hydrogen peroxide is one of the most attractive oxidants, mainly because it is environmentally clean and easily handled. It should be noted that during the ODS process, the H2O2 was used in the presence of acetic acid (CH3COOH) as oxidants because acetic acid as an organic acid, reacts with H2O2 to in situ produce peracid (CH3COOOH), which can efficiency convert organic sulphur to sulfones without forming a substantial amount of residual product [25, 26]. Hydrogen peroxide in short-chain carboxylic acids (formic acid or acetic acid) is considered as the common oxidative desulphurization

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system for the gasoline. The mechanism of sulfide oxidation to sulfoxides using H2O2-organic acid is not studied sufficiently; however, the potential mechanism is a heterolytic electrophyl interaction where H+X- is a polar solvent. Based on this mechanism, hydrogen peroxide fist reacts with organic acid (acetic acid) quickly and generates peroxide acid, and then the acid reacts with nonpolar sulphur compounds and generates relative sulfone or sulfoxide. The role of the metal atoms in (N(tBu)4)4HPM10V2O40, M = V or Mo, is to form peroxo-metal species which are able to activate the H2O2 and peracid molecules. During the oxidative desulphurization process, the H2O2 can efficiency convert organic sulphur to sulfones without forming a substantial amount of residual product. PWV accepted the active oxygen from the oxidant H2O2 to form new oxoperoxo species mediate. The cation with carbon chain transferred oxoperoxo species to the substrates (Th or BT) and made the oxidation reaction accomplish completely. In summary, as it is observed in abstract graphic and Fig.9, firstly, the phosphotungstic compounds with Keggin structure will convert to polyoxoperoxo complexes [37]. The catalyst was reacted with H2O2 to form polyoxoperoxo specie, and then sulphur-containing compounds were oxidized to the corresponding sulfones with polyoxoperoxo.

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Figure 9. Probable mechanism for the formation of peroxo-metal species and using H2O2 as the oxidant for oxidation of BT, DBT and RSR.

3.9. Kinetics of BT oxidation Nanocatalyst PWV/PVA as potential catalyst in the oxidative desulfurization of BT from model fuel and gasoline was studied. The kinetics of oxidation of BT was studied under the condition of different temperature ranging 25, 30, 35 and 40 °C. It is noteworthy, with increasing reaction temperature from 25 to 35°C removal of BT from 70% to 98% in 2h. The following rate equation is

25

applied to model BT oxidation reaction. Rate equations can be applied to describe the relationship between C and t: ln C/C0= -kt

(2)

Here C0, C, and k explain the initial concentration of BT, the concentration of BT at certain time (t), and the rate constant, respectively [38]. The plot of C/C0 versus time was shown in Fig. 10. The rate constant increases with an increase in the reaction temperature for BT in Table 8 and experimental data were fitted to a pseudo-first-order rate equation Eq. (2).

Figure 10. Plots of C/C0 for the oxidation of BT with the PWV/PVA catalyst. (1): 25 °C, (2): 30 °C, (3): 35 °C and (4): 40 °C.

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Table 8. Pseudo-first-order rate constants and correlation factors of the BT. Temperature (oC) 25 30 35 40

BT Rate constant k (min-1) 8.0 × 10 -3 1.3 × 10 -2 2.5 × 10 -2 2.6 × 10 -2

orrelation factor R2 0.91 0.94 0.91 0.92

The reaction temperature dependence of reaction rates can be expressed with the Arrhenius equation (Eq. (3)), where A is the pre-exponential factor, is the activation energy, and R is the universal gas constant. It is common knowledge that chemical reactions occur rapidly at higher temperatures [38]. The Arrhenius plot is shown in Fig. 11 and the calculated Ea values for the oxidation of BT are 92kJ/mol. k = Ae (-Ea/RT)

(3)

Fig. 11. Arrhenius plot for the oxidation of BT with the PWV/PVA catalyst. 27

3.10. Recycling of the catalyst At the end of the ODS of the model sulphur compounds and gasoline, the PWV-PVA was filtered and washed with dichloromethane. In order to determine whether the PWV-PVA would succumb to poisoning and lose its catalytic activity during the reaction, the reusability of the PWV-PVA was investigated. For this purpose, we carried out the desulphurization reaction of gasoline and model compounds in the presence of fresh and recovered PWV-PVA. The PWV-PVA recovered after the reactions were characterized, in order to check the PWV-PVA stability. Figure 12 and 13 illustrate XRD pattern and IR spectra of PWV-PVA after three catalytic cycles, respectively. Even after three runs for the reaction, the catalytic activity of PWV-PVA was almost the same as that of freshly used catalyst. The results are summarized in Table 9.

Table 9. Reuse of the catalyst on the ODS of thiophene a Entry Isolated yield (%) 1 97 2 96 3 95 a Conditions for desulphurization: 50 ml of model gasoline (500 ppm S), 0.1 gr catalyst, 6 mL acetic acid/H2O2, 10 mL extraction solvent, time 2 h and temperature 35 °C

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Figure 12. Comparison of XRD pattern of free PWV/PVA (a) after first run reuse (b) after 2 run reuse (c) after 3 rune reuse (d).

Figure 13. Comparison of IR spectrum of free PWV/PVA (a) after first run reuse (b) after 2 run reuse (c) after 3 rune reuse (d). 29

4. Conclusion A mixed-addenda vanadium (V) substituted Keggin type POM-based inorganic–organic compound was presented. The preparation condition was simple and mild. The inorganic–organic material presented here is unique in several aspects; this simple synthesis route demonstrates that the use of PVA molecule as organic electron donor reacting chemically with polyoxometalate anion (electron acceptor) can give rise to amorphous nanomaterials, which exhibit high catalytic activity. SEM images indicate that the surface is uniform, non-smooth and nanoparticles were made. FT-IR spectra and XRD patterns proved the formation of desired heteropolyacids Keggin structure onto PVA. The PWV-PVA nanoparticle was very active catalyst system for desulphurization of the models compound, while unmodified PWV showed much lower activity. This PWV/PVA/H2O2 system provides an efficient, convenient and practical method for oxidative desulphurization of gasoline and the advantages of this method are nontoxic, mild condition and environmentally friendly.

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Figures and Schematic Captions Figure 1. FT-IR spectra of PVA (a), PWV-PVA (b) PWV (c). Figure 2. UV–vis spectra of PWV-PVA (a), PWV (b) and PVA (c). Figure 3. SEM images of PWV-PVA. The images of a (10 µm), b (1µm), c and d (300nm) as for PWV-PVA. Figure 4. XRD of PWV (a), PVA (b) and PWV-PVA (c). Figure 5. 31P-NMR spectrum of PWV (a) and PWV/PVA (b) dissolved in DMSO. Figure 6. Effect of different catalyst on oxidative desulphurization of BT. 1- None, 2- H4SiW12O40 3H3PW12O40

4-

K5PV2W10O40

5-

(N(But)4)4HPMo10V2O40

6-

(N(tBu)4)4HPV2W10O40

7-

(N(tBu)4)4HPV2W10O40/PVA. Figure 7. Effect of catalyst dosage on oxidative desulphurization of BT. Figure 8. Effect of different temperature on oxidative desulphurization of BT. Figure 9. Probable mechanism for the formation of peroxo-metal species and using H2O2 as the oxidant for oxidation of BT, DBT and RSR. Figure 10. Plots of C/C0 for the oxidation of BT with the PWV/PVA catalyst. (1): 25 °C, (2): 30 °C, (3): 35 °C and (4): 40 °C. Figure 11. Arrhenius plots for the oxidation of BT with the PWV/PVA catalyst. Figure 12. Comparison of XRD pattern of free PWV/PVA (a) after first run reuse (b) after 2 run reuse (c) after 3 rune reuse (d). Figure 13. Comparison of IR spectrum of free PWV/PVA (a) after first run reuse (b) after 2 run reuse (c) after 3 rune reuse (d). Scheme 1. Chart of synthesis of nanocomposite.

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Table Captions Table 1. Effect of different catalysts in the ODS of gasoline, Thiophene and BT. Table 2. Oxidative desulphurization of gasoline by PWV-PVA. Table 3. FT-IR related to Kegging PWV and PWV-PVA. Table 4. Effect of different catalysts in the ODS of gasoline. Table 5. Effect of catalyst dosage in the ODS of gasoline and simulated gasoline. Table 6. Effect of different temperatures in the ODS of gasoline and simulated gasoline. Table 7. Effect of different oxidation system in the ODS of gasoline and simulated gasoline. Table 8. Pseudo-first-order rate constants and correlation factors of the BT. Table 9. Reuse of the catalyst on the ODS of thiophene

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