Deep oxidative desulfurization of real fuel and thiophenic model fuels using polyoxometalate-based catalytic nanohybrid material

Journal Pre-proof Deep oxidative desulfurization of real fuel and thiophenic model fuels using polyoxometalate-based catalytic nanohybrid material Mohammad Ali Rezvani, Nasrin Khalafi

PII:

S2352-4928(19)30670-1

DOI:

https://doi.org/10.1016/j.mtcomm.2019.100730

Reference:

MTCOMM 100730

To appear in:

Materials Today Communications

Received Date:

23 August 2019

Revised Date:

10 October 2019

Accepted Date:

28 October 2019

Please cite this article as: Rezvani MA, Khalafi N, Deep oxidative desulfurization of real fuel and thiophenic model fuels using polyoxometalate-based catalytic nanohybrid material, Materials Today Communications (2019), doi: https://doi.org/10.1016/j.mtcomm.2019.100730

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Deep oxidative desulfurization of real fuel and thiophenic model fuels using polyoxometalate-based catalytic nanohybrid material

Mohammad Ali Rezvani* and Nasrin Khalafi

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

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*Corresponding author: E-mail: [email protected] ; Tel./Fax: +98 (24) 3305 2477

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Graphical Abstract

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Abstract In this paper, a new inorganic-organic nanohybrid material (Cu-SPOM@PbO@PVA) was constructed and used as a catalyst for oxidative desulfurization (ODS) treatment. The hybrid catalyst composed of Cu-substituted polyoxometalate (Cu-SPOM), lead oxide (PbO), and polyvinyl alcohol (PVA) was prepared via sol-gel method. The Cu-SPOM@PbO@PVA

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composite was characterized by various analytical techniques. The SEM results showed that the Cu-SPOM and PbO particles were well dispersed on the PVA matrix. Also, the mean particle size of the composite was in a range of 30-40 nm. According to the ODS results of real

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gasoline, the removal efficiency of sulfur content could reach 97% at 35 °C after 1 h. Under the same reaction conditions, the concentration of some of thiophenic compounds, including,

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thiophene (Th), benzothiophene (BT), and dibenzothiophene (DBT) was significantly reduced from the model fuels with the efficiency of 97, 98, and 98%, respectively. In addition, a series

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of ODS tests were carried out concerning desulfurization effect, kinetic study, and regeneration of the nanohybrid catalyst. The excellent catalytic activity of the Cu-SPOM@PbO@PVA

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composite suggested a fascinating new field in designing of inorganic-organic nanohybrid

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catalysts for ODS reactions.

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Keywords: nanohybrid catalyst, oxidative desulfurization, sandwich-type polyoxometalate, thiophenic model fuel, gasoline

1. Introduction

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Polyoxometalates (POM) as inorganic metal-oxide assemblies have widespread applications in medicine [1], material science [2], electrochemistry [3], magnetism [4], and catalysis [5]. Undoubtedly, catalysis is the most important and promising application of POMs in recent years due to their extraordinary tunable physical and chemical properties at molecularlevel by varying constituent elements [6,7]. Hence, the new generation which is called metalsubstituted polyoxometalates (MSPOMs) can be developed by incorporation of transition metal ions into their defect structures [8]. The sandwich-type of MSPOMs were well explored as both

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“homogeneous and heterogeneous catalysts” in chemical reactions [9-11]. In 2006, Haimov et al. reported the selective oxidation of 2-alkanols to 2-alkanones using sandwich-type zincsubstituted POM, [(ZnW9O34)2 ZnWZn2(H2O)2], as a highly efficient catalyst [12]. Vickers

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group evaluated the photocatalytic activity of [Co4(H2O)2(PW9O34)2]10- for light-driven water oxidation [13]. Also, Rezvani et al. came up with the synthesis and characterization of

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(Bu4N)7H3[P2W18Cd4(Br)2O68]-TiO2 nanocomposite in heterogeneous catalytic ODS of

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simulated gas oil [14]. It should be noticed that the POMs are often very soluble in aqueous media, which leads to their poor recycling and reusability [15]. In this regard, much effort has been dedicated to tackle this issue by immobilization of POMs on appropriate support materials

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such as metal oxide nanoparticles (NPs) [16], organic polymers [17], metal-organic frameworks [18], etc. Metal oxide NPs have received substantial research attention because of

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their large surface-to-volume ratio, less solubility, and catalytic properties [19]. Lead oxide

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(PbO) NPs is used for wide ranges of applications due to its oxidative properties, simple synthesis, and easy recycling [20]. Interestingly, the addition of metal oxide NPs to organic polymers as fillers offers novel hybrid materials with tailored properties and characteristics such as lightweight, flexibility, thermal and chemical stability [21,22]. Polyvinyl alcohol (PVA) is one of the well-known water-soluble synthetic polymer with excellent film forming,

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emulsifying, and adhesive properties. Since its biodegradability, PVA has received extensive attention as a support material for immobilizing metal oxides [23-25]. Transportation fuels produced by the petrochemical industry contain substantial amounts of organosulfur compounds (OSCs) [26]. The presence of OSCs in hydrocarbon fuels is undesirable from the environmental viewpoint. Mercaptans, with the generic formula of R-SH, are the most malodorous and highly corrosive sulfur compounds [27,28]. Further, combustion of mercaptans and other OSCs-containing fuels releases a number of hazardous chemicals into

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the environment, especially sulfur dioxide (SO2) as a source of acid rain [29]. Therefore, several techniques have been adopted to reduce sulfur content of liquid fuels such as catalytic hydrodesulfurization (HDS) [30], oxidative desulfurization (ODS) [31], extractive desulfurization

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(EDS) [32], azeotropic distillation [33], adsorptive desulfurization (ADS) [34], etc. Among the mentioned technologies, ODS has arisen to be the most extensively used method as it can be

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operated at very mild process conditions [35,36]. Also, the production of clean and ultra-clean

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(ultra-low sulfur) fuel using energy efficient and environmentally responsible approach is one of the key advantages of ODS. Indeed, OSCs are oxidized into their corresponding hexavalent sulfur (also known as sulfone) at temperatures below 70 °C and atmospheric pressure [37]. In

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consideration of the fact that the best ODS result can be achieved when using oxidant conjunction with a suitable catalyst, the hydrogen peroxide/acetic acid as an oxidant and POM-

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based catalyst are found as a highly efficient ODS system [38,39].

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Herein, for the first time, we report the design and fabrication of Cu-SPOM@PbO@PVA nanohybrid catalyst. Characterization studies were performed using FT-IR, XRD, SEM, and EDX analyses. Then, the catalyst has been applied to remove OSCs from model fuels and real gasoline through ODS method. The experimental results indicated that the CuSPOM@PbO@PVA composite exhibited obvious catalytic activity toward the reduction of sulfur concentration in fuel. Moreover, the synthesized hybrid composite can significantly

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increase the ODS efficiency up to 96% at 35 °C after 1 h. Emphases were addressed on the efficient catalytic performance of Cu-SPOM@PbO@PVA nanohybrid catalyst and its easier recovery from the reaction media. 2. Experimental 2.1. Materials The following chemicals and reagents were used as received without further purification: tungstate

dihydrate

(Na2WO4·2H2O),

copper

(II)

acetate

monohydrate

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sodium

(Cu(CH3COO)2·H2O), hydrogen peroxide 30 vol.% (H2O2), glacial acetic acid (CH3COOH), thiophene (Th), benzothiophene (BT), dibenzothiophene (DBT), n-heptane, acetonitrile

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(CH3CN), and polyvinyl alcohol (PVA) were obtained from Sigma-Aldrich. Lead nitrate (Pb(NO3)2) and citric acid monohydrate (C6H8O7·H2O) were purchased from Merck. It should

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be pointed out that the PbO NPs was prepared as previously described [20].

The

Cu-substituted

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2.2. Synthesis of Cu-SPOM

polyoxometalate,

Na13[(CuW9O34)2H3Cu4(H2O)2]·39H2O

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(abbreviated as Cu-SPOM), was prepared according to the literature [40]. A solution of Na2WO4·2H2O (5.00 g) in 15 mL of distilled water (DW) was heated to 75 °C and mixed with

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a solution of Cu(CH3COO)2·H2O (7.86 g) in 10 mL of DW. The pH was adjusted to 3-4 using CH3COOH and the mixture was stirred at 75 °C for 1 h. After completing the reaction, the

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mixture was cooled to room temperature and a green solid product was separated by filtration. Afterward, the filtrate was dried at 70 °C for 3 h to obtain the Cu-SPOM compound. 2.3. Synthesis of Cu-SPOM@PbO In a typical procedure, Pb(NO3)2 (3.90 g) and C6H8O7·H2O (1.90 g) were dissolved in 50 mL of DW, separately. The Pb(NO3)2 solution was dripped in the C6H8O7·H2O solution. The

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mixed solution was heated to 80 °C with stirring for 30 min. Then, the synthesized Cu-SPOM (0.10 g) was dissolved in 5 mL boiling DW and added into the reaction vessel. After heating the above solution at 80 °C for 1 h, a porous gel was formed. To prepare Cu-SPOM@PbO powder, the resultant gel was aged at 70 °C for 2 h and calcined at 400 °C for 4 h. 2.4. Synthesis of Cu-SPOM@PbO@PVA nanohybrid catalyst The inorganic-organic Cu-SPOM@PbO@PVA composite has been synthesized according to the following procedure: a solution of Cu-SPOM@PbO (0.10 g) in 10 mL of DW

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was dispersed using an ultrasonic water bath for 30 min to reach a uniform suspension. In the other container, PVA (0.10 g) was dissolved in 40 mL of DW and stirred at 80 °C for 30 min. Then, the suspension of Cu-SPOM@PbO was added dropwise to the PVA solution and

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clean glass petri dish and dried at 80 °C for 2 h.

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vigorously stirred for 1 h to form a viscose gel. At the end, the obtained gel was placed in a

2.5. Characterization methods

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The Fourier transform infrared (FT-IR) spectra of the materials in KBr pellets were obtained using a Thermo-Nicolet-iS10 spectrometer. The powder X-ray diffraction (XRD) was

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analyzed by a Bruker D8 Advance X-ray diffractometer with monochromatic Cu (Kα) radiation, the voltage 40 kV and current 30 mA. The morphological properties of the samples

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were studied by scanning electron microscopy (SEM) on LEO 1455 VP coupled with an energy

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dispersive X-ray (EDX) spectroscopy apparatus. The ultrasonic water bath was used by a Bandelin Sonorex Digitec frequency 35 kHz mains connection 230 V. The total sulfur content in real gasoline and thiophenic model fuels were determined by X-ray fluorescence (XRF) with a TANAKA X-ray fluorescence spectrometer RX-360 SH. 2.6. ODS of thiophenic model fuels

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The ODS process was studied by employing some thiophenic OSCs which are commonly found in real fuel. The model fuels were prepared by dissolving Th, BT, and DBT compounds in n‐heptane solvent, separately. The initial concentration of each sulfur compound was 500 ppmw (ppm by weight). In a typical run, 50 mL of the prepared model fuel was introduced into a two‐necked flask (100 mL) equipped with a stirrer and thermometer. The flask was placed in a water bath at a constant temperature (25, 30, 35, and 40 °C) and then the CuSPOM@PbO@PVA nanohybrid catalyst was added to the fuel. The reaction mixture was

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continuously stirred (700 rpm) and heated to the set temperature under atmospheric pressure. Afterward, 3 mL of oxidant (the mixture of H2O2/CH3COOH in v/v ratio of 2:1) was added into the flask, and this was recorded as the initial time of the ODS process. Then, the solution

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was cooled to room temperature, followed by addition of CH3CN as an extraction solvent. The v/v ratio of CH3CN /fuel was 1:5. The formed biphasic mixture was decanted by a separation

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funnel to separate the oil phase from the water phase. The treated oil phase was analyzed using

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the XRF spectrometer for the determination of the sulfur amount. The removal efficiency (RE, %) of OSCs was calculated according to the following equation: (1)

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 C   100 RE  1   C0 

where C0 and C represent the concentration of OSCs (ppmw) before and after ODS process,

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respectively.

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2.7. ODS of real gasoline

The ODS experimental procedure of real gasoline was conducted in the same manner as

in the way described above. Briefly, 50 mL of gasoline (containing 4996 ppmw of sulfur) was added to the flask and placed in the water bath at 35 °C. The Cu-SPOM@PbO@PVA nanohybrid catalyst (0.10 g) and oxidant (3 mL) were added into the reactor, respectively. The resulting mixture was stirred for 1 h to complete the oxidation reaction. After that, the solution 7

was cooled to ambient temperature and 10 mL of CH3CN extraction solvent was utilized. When the ODS run was finished, the mixture was kept still for 15 min. In the next step, the gasoline phase was separated from the water phase and then analyzed by XRF based on D-4294 and D3227 ASTM standard tests. The removal efficiency of sulfur content and mercaptan compounds in the real gasoline were calculated using Eq. 1. 3. Results and discussion

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3.1. Characterization of materials The interaction between inorganic metal oxides and the organic polymer was confirmed by FT-IR studies. Fig. 1 shows the FT-IR spectra of PVA, PbO, Cu-SPOM, and Cu-

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SPOM@PbO@PVA nanohybrid catalyst in the wave number ranging from 400-4000 cm-1. The absorption bands for the pure PVA at 884 and 1435 cm-1 are assigned to the stretching and

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bending vibrations of H–C–H, respectively (Fig. 1(a)). The peaks at 1096 and 1733 cm-1

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corresponds to the stretching vibration modes of C–O–C and C=O in acetyl and vinyl acetate groups of PVA, respectively [23,24]. Also, a strong broad band appeared at 3512 cm-1 has been attributed to the stretching vibrations of the O–H groups. Fig. 1(b) represents the FT-IR

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spectrum of PbO NPs. The peaks observed at 470 and 683 cm-1 are related to the Pb–O and Pb–O–Pb bands [20]. Concerning Cu-SPOM, the characteristic vibrations bands at 584, 700,

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807, and 934 cm-1 can be assigned to the vibration bands of the metal-oxide skeleton, involving,

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Cu–Oa, W–Oc–W, W–Ob–W, and W=Od, respectively [40,41]. The band at 1483 cm-1 is associated with the stretching vibration of the C=O bond due to physically surface-adsorbed CO2 [42]. Also, the absorption peaks located at 1617 and 3419 cm-1 indicate the bending and stretching vibrations of crystal H2O molecules, respectively. In the spectrum of CuSPOM@PbO@PVA, the characteristic peaks of PbO and Cu-SPOM between 700 and 1100 cm-1 are identified. By comparing the spectra of pure PVA and Cu-SPOM@PbO@PVA

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nanohybrid, the peak at 3512 cm-1 is shifted towards lower wavenumber of 3441 cm-1 after the composition of the materials. It is attributed to the formation of hydrogen bond between the hydroxyl

functional

groups

of

PVA

matrix

and

the

oxygen

groups

of

[(CuW9O34)2H3Cu4(H2O)2]n- anion and PbO [43]. (Fig. 1) Further evidence of the successful preparation of Cu-SPOM@PbO@PVA nanohybrid compound can be obtained from XRD analysis. Fig. 2 depicts the X-ray diffraction patterns of

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PVA, PbO, Cu-SPOM, and Cu-SPOM@PbO@PVA in the range of 5° ≤ 2θ ≤ 60°. In Fig. 2(a), the pure PVA shows some broad peaks around the 2θ values of 19.20, 22.94, and 41.12°, which assigned to its crystalline form (JCPDS No 53-1487) [25]. According to Fig. 2(b), the

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reflections at 28.55, 29.03, 31.73, 35.61, 48.52, 56.67, and 59.61° are corresponded to (111), (002), (200), (210), (022), (222), and (311) crystal planes of the PbO NPs (JCPDS No 00-005-

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0570), respectively [20].The XRD pattern of Cu-SPOM indicates characteristic peaks at 2θ

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values of 9.04, 15.26, 22.64, 25.52, 33.02, 35.18, 41.12, 43.04, 48.14, and 49.64° as shown in Fig. 2(c). These diffraction peaks can be indexed to monoclinic WO3 (JCPDS 43-1035) [44]. The XRD profile of Cu-SPOM@PbO@PVA nanohybrid reveals the presence of PVA, PbO,

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and Cu-SPOM diffraction peaks, confirming their successful incorporation. The crystallite size of the Cu-SPOM@PbO@PVA nanohybrid was determined from the XRD pattern using

k  cos 

(2)

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d

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Scherrer’s equation [16]. The value provided by the equation was 33.30 nm.

where, λ is the wavelength of the X-ray used (λ = 1.541 Å), k is the shape factor (for spherical particles k = 0.9), β is the full peak width at half max (FWHM) in radians, and θ is the diffraction angle in degree. (Fig. 2)

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The surface morphology of the samples has been analyzed by SEM method. Fig. 3(a) shows that the surface of pure PVA is rather smooth and flat. From the SEM image of PbO NPs, it can be found that their particles are agglomerated and spherical in shape (Fig. 3(b)). Furthermore, the morphology of the Cu-SPOM sample implies irregular particles with different sizes (Fig. 3(c)). According to Fig. 3(d), the SEM photograph of Cu-SPOM@PbO@PVA nanohybrid catalyst presents the nano-sized spherical particles which are homogeneously dispersed on the surface of PVA polymer. The corresponding particle size distribution of the

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Cu-SPOM@PbO@PVA in Fig. 4(a) clearly indicates that the particles are in the range of 3040 nm. Also, the EDX spectrum confirms an accumulation of C, O, Cu, W, and Pb elements in the synthesized nanohybrid composite with an approximated percentage (wt.%) of 28.70,

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44.86, 4.66, 6.88, and 14.90%, respectively (Fig. 4(b)). Although, the intensity of peaks corresponded to Cu and W is low due to very small amounts of them in Cu-

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on the surface of the PVA polymer.

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SPOM@PbO@PVA. But, the results show the successful immobilization of Cu-SPOM@PbO

(Fig. 3)

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(Fig. 4)

3.2. ODS results of thiophenic model fuels and real gasoline

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In order to accurately test the applicability of Cu-SPOM@PbO@PVA as the nanohybrid catalyst, the ODS treatments were applied to the thiophenic model fuels and real gasoline. In

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this study, the mixture of H2O2/CH3COOH in v/v ratio of 2:1 was carefully chosen as the oxidant given the previous high performance obtained [24, 25, 27, 39]. Moreover, the CH3CN solvent was utilized based on its high adsorption capacity for the extraction of ODS products [45]. Table 1 reports the desulfurization results of real gasoline using the CuSPOM@PbO@PVA catalyst at 35 °C after 1 h. As can be concluded, 97% (0.4847 wt.%) of the sulfur content of gasoline was removed during the ODS process. Fascinatingly, the Cu10

SPOM@PbO@PVA nanohybrid demonstrated considerable catalytic performance for removal of mercaptan compounds. As shown in Table 1, the concentration of mercaptan compounds was lowered from 98 to 4 ppm. In case of model fuels, the ODS treatments of thiophenic substrates were done under the condition of Cu-SPOM@PbO@PVA catalyst = 0.10 g, model fuel = 50 mL, oxidant = 3 mL, extraction solvent = 10 mL, temperature = 35 °C, and time = 1 h. Under the above conditions, the oxidation removal of Th, BT, and DABT compounds from n-heptane was performed with the efficiency of 97, 98, and 98%, respectively.

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(Table 1) It was of interest to explore the catalytic activity of pure PVA, PbO, and Cu-SPOM in the ODS of the OSCs. As listed in Table 2, the efficiency of ODS processes using synthesized

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materials decreases in the order of Cu-SPOM@PbO@PVA > Cu-SPOM > PbO > PVA. The results evidently indicated that the immobilization of Cu-POM compound on PbO within a

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polymer matrix offers an attractive route for the preparation of the high-performance inorganic-

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organic nanohybrid catalyst. Also, the reaction without catalyst as a reference experiment was tested in order to confirm that the catalytic activity is due to Cu-SPOM@PbO@PVA catalyst. The results clearly showed that the removal efficiencies of Th, BT, and DBT were examined

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in the absences of the catalyst 13, 15, and 16%, respectively (Fig. 5). (Table 2)

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The effect of Cu-SPOM@PbO@PVA catalyst dosage (g) on the ODS process of

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thiophenic sulfur compounds was studied and the results are shown in Fig. 5. Various amounts of nanohybrid catalyst (0-0.12 g) were added to 50 mL of model fuels and stirred at 35 °C for 1 h. It was found that the significant ODS rate was observed when the applied catalyst dosage was ≥ 0.10 g. The removal efficiency of Th, BT, and DBT reached 97, 98, and 98% using only 0.10 g of nanohybrid catalyst, respectively. It can be attributed to the high catalytic activity of Cu-SPOM@PbO@PVA nanohybrid catalyst. When the dosage increased to 0.12 g, the yield

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of the ODS process was similar to that of using 0.10 g. The optimum Cu-SPOM@PbO@PVA dosage (0.10 g) was selected as a basis for the forthcoming oxidation experiments. (Fig. 5) The influence of reaction temperature was investigated by conducting ODS experiment at 25, 30, 35, and 40 °C. As illustrated in Fig. 6, the removal efficiency of sulfur compounds greatly increased with raising the temperature and reached to maximum amounts at 35 °C. The favorable temperature obtained for the removal of OSCs was lower than the other ODS systems

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[46-48]. (Fig. 6)

Besides the effect of temperature, the ODS process was also influenced by the reaction

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time. The effect of process time was investigated from 0 to 1 h under otherwise identical conditions as mentioned above. According to Fig. 6, it was established that the removal rate of

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OSCs was rapid in the beginning but it decreased with time until it reached equilibrium. The

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maximum desulfurization efficiencies can be achieved from less than 30% at the time of 5 min to nearly 98% after 1 h. Consequently, the time of 1 h was optimized for ODS treatments of

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thiophenic model fuels. 3.3. Kinetics of ODS process

is used.

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For determination of the order of ODS reaction, equation of pseudo-first-order reaction

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 dC     kC  dt 

(3)

If C0 and Ct are respectively assumed as concentration of OSCs at time = 0 and time = t (min), the rate constant (k) can be found as follows:

 C0     e kt  Ct 

(4)

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The variation of Th, BT, and DBT concentration (C0/Ct) with time (t) are shown in Fig. 7. It is obvious that the plot of (C0/Ct) or ln (C0/Ct) versus t gives a straight line, which indicates that the oxidation rates of Th, BT, and DBT over Cu-SPOM@PbO@PVA catalyst can be appropriately described using the pseudo-first-order model. Table 3 shows the values of rate constant (k) and correlation coefficient (R2) of thiophenic compounds at 25, 30, 35, and 40 °C. It can be concluded that the R2 values are found close to unity (~ 1). It is also noted that the k parameter increases with increasing the reaction temperature. The apparent activation energies

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can be calculated by plotting a graph of ln k versus 1/T and obtaining their slopes (Fig. 8). The assessed Ea values for the ODS of Th, BT, and DBT were 52.68, 48.34, and 48.30 kJ/mol, respectively.

 Ea 1 ( ) R T

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ln k 

(5)

reaction temperature (Kelvin), respectively.

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(Fig. 7)

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where Ea is the apparent activation energy; R is the gas constant (8.314 J/mol.K ); and T is the

(Fig. 8)

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(Table 3)

3.4. Proposed pathway of ODS process

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The mechanism of sulfur oxidation in the presence of Cu-SPOM@PbO@PVA

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nanohybrid catalyst can be understood based on the proposed pathway. According to the top photograph of Scheme 1, it is observed that the various organic thiophenic compounds are found in the oil (fuel) phase. In order for activation of the catalyst, hydrogen peroxide (H2O2) reacts with acetic acid (CH3COOH) to form peracetic acid (CH3COOOH) and water (Step 1, Scheme 1). At the same time, the oxygen can be transferred from CH3COOOH to terminal metal-oxygen groups (M=Od) of Cu-SPOM and active metal-peroxo intermediates (M OO ) are 13

generated [37,49]. In this catalytic ODS system, the oxidation of DBT as a sulfur-containing compound occurs via the electrophilic attack of active oxygen in M OO on the electron pairs of sulfur atoms. As a result, DBT sulfoxide (DBTO) and/or sulfone (DBTO2) are produced, which they are highly polar in comparison with their initial form before the oxidation (Step 2 and 3, Scheme 1). Hence, the polar extraction solvent can be used to easily remove the polar products and obtain the clean fuel [49]. (Scheme 1)

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3.5. Regeneration of the Cu-SPOM@PbO@PVA nanohybrid catalyst

The regeneration and eco-friendliness are the important properties of catalysts. In this regard, a set of five successive ODS experiments were carried out for the evaluation of Cu-

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SPOM@PbO@PVA reusability. At the end of each test, the hybrid catalyst was separated from

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the reaction media by simple filtration, washed with the dichloromethane solvent, and then dried at 75 °C for 2 h. The separated catalyst was used in the consequent reaction under the

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same conditions using the fresh model fuel, oxidant, and CH3CN solvent. After ODS, the model fuel phase was analyzed to determine the DBT concentration. As reported in Table 4, the

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catalytic oxidation ability of nanohybrid catalyst in the oxidation reaction of DBT decreased from 98 to 95% after the catalyst was regenerated for five times. It was reflected that the

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designed Cu-SPOM@PbO@PVA nanohybrid is a promising catalyst in terms of reusability. However, a little decrease in the removal efficiency of DBT compound was due to the leaching

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of the active species. Also, Yang et al. reported that increasing the recycle times leads to increasing the adsorption of oxidation products (DBTO and DBTO2) on the surface of the catalyst and decreasing the ODS efficiency [50]. The Cu-SPOM@PbO@PVA recovered aftere the reaction was characterized, in order to check the IPOM–PVA stability. Fig. 9 illustrates IR spectrum of Cu-SPOM@PbO@PVA after five catalytic cycles, respectively. Even after five

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runs of the reaction, the catalytic activity of Cu-SPOM@PbO@PVA was almost the same as that of freshly used catalyst. (Table 4) (Fig. 9) 4. Conclusions In summary, the fabrication of inorganic-organic Cu-SPOM@PbO@PVA nanohybrid

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and its application as a new catalyst for ODS treatment were reported. Pure Cu-SPOM compound was first synthesized and coated on PbO NPs. Then, the obtained material (CuSPOM@PbO) was immobilized on PVA to prepare the Cu-SPOM@PbO@PVA catalyst. The

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catalytic activity of all prepared samples was evaluated in the OSCs removal reactions. It was found that the nanohybrid material exhibited an outstanding performance under a relative mild

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condition with less reaction time. Experimental results evidenced that the concentration of Th, BT, and DBT molecules was reduced in model fuels with the efficiency of 97, 98, and 98%,

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respectively. In a word, the combination of inorganic metal oxides with organic polymer could provide ODS efficiency. Also, the desulfurization process of OSCs was significantly

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influenced by key operational factors, including, the dosage of catalyst, reaction temperature, and reaction time. According to the results of catalyst regeneration, it was noted that the

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recycled use of Cu-SPOM@PbO@PVA nanohybrid for five times did not conspicuously affect

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its activity. We believe the synthesis of Cu-SPOM@PbO@PVA composite can provide valuable knowledge for the development of highly efficient nanohybrid catalysts for the production of sulfur-free fuels.

Conflict of interests

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I would be grateful if you could kindly make the necessary arrangements for assessment of this manuscript by referees. Your early response will be greatly appreciated. I look forward to hearing from you in due course.

The manuscript has not been previously published, is not currently submitted for other journal, and will not be submitted. All authors listed, agree to the content and publication of the submitted manuscript. I hereby certify that this paper consists of original, unpublished work which is not under

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consideration for publication elsewhere. All authors have seen and approved the final version of the manuscript being submitted. They warrant that the article is the authors' original work, hasn't received prior publication and isn't under

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consideration for publication elsewhere.

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Figures captions

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Fig. 1. FT-IR spectra of (a) PVA, (b) PbO, (c) Cu-SPOM, and (d) Cu-SPOM@PbO@PVA.

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Fig. 2. XRD patterns of (a) PVA, (b) PbO, (c) Cu-SPOM, and (d) Cu-SPOM@PbO@PVA.

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Fig. 3. SEM images of (a) PVA, (b) PbO, (c) Cu-SPOM, and (d) Cu-SPOM@PbO@PVA.

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Fig. 4. (a) The particle size distribution histogram and (b) EDX analysis of the Cu-

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SPOM@PbO@PVA nanohybrid catalyst.

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Fig. 5. Effect of Cu-SPOM@PbO@PVA catalyst dosage on the ODS process of (a) Th, (b) BT, (c) DBT, and (d) sulfur compounds in gasoline. Condition of ODS reaction: 50 mL of real gasoline or model fuel (500 ppmw of Th, BT, and DBT in n-heptane), 0.10 g of catalyst, 3 mL of oxidant, 10 mL of acetonitrile, reaction temperature = 35 °C, and reaction time = 1 h.

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Fig. 6. Effect of reaction temperature and time on the ODS of (a) Th, (b) BT, and (c) DBT by

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Cu-SPOM@PbO@PVA nanohybrid catalyst.

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Fig. 7. Plots of C0/Ct against time for the ODS of (a) Th, (b) BT, and (c) DBT.

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Fig. 8. Plots of ln k against 1/T for the ODS of (a) Th, (b) BT, and (c) DBT.

Fig. 9. Comparison of IR spectra of (a) free Cu-SPOM@PbO@PVA (b) after first run reuse (c) after 3 rune reuse and (d) after 5 rune reuse. 28

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Scheme 1. Schematic illustration of the proposed pathway of the ODS process by Cu-

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SPOM@PbO@PVA nanohybrid catalyst.

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Tables captions Table 1. ODS results of real gasoline by Cu-SPOM@PbO@PVA nanohybrid catalyst. Table 1. ODS results of real gasoline by Cu-SPOM@PbO@PVA nanohybrid catalyst. Before After ODS ODS a 1 Total sulfur content by XRF wt.% ASTM D 4294 0.4996 0.0149 2 Mercaptan compounds ppm ASTM D 3227 98 4 3 Density by hydrometer @ 15 °C g/mL ASTM D 1298 0.7995 0.7994 4 Salt ptb ASTM D 3230 17 17 5 Water content by distillation vol.% ASTM D 4006 Nil. Nil. IBP 48.7 48.5 °C FBP 209.6 209.4 10 68.7 68.6 6 Distillation 50 ASTM D 86 119.4 119.3 vol.% 90 187.4 187.2 95 207.7 207.6 a Condition of ODS reaction: 50 mL of gasoline, 0.10 g of catalyst, 3 mL of oxidant, 10 mL of acetonitrile, reaction temperature = 35 °C, and reaction time = 1 h. Properties of gasoline

Unit

Method

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Table 2. Catalytic activity of synthesized materials in the ODS of thiophenic model fuels and real gasoline.

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Table 2. Catalytic activity of synthesized materials in the ODS of thiophenic model fuels and real gasoline.a ODS efficiency (%) Th Cu-SPOM@PbO@PVA 97 Cu-SPOM 65 PbO 50 PVA 30 Without catalyst 13 a Condition of ODS reaction: 50 mL of real gasoline or model fuel (500 ppmw of Th, BT, and DBT in n-heptane), 0.10 g of catalyst, 3 mL of oxidant, 10 mL of acetonitrile, reaction temperature = 35 °C, and reaction time = 1 h.

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Table 3. Pseudo-first-order rate constants and correlation factors of thiophenic sulfur compounds at different temperatures. Table 3. Pseudo-first-order rate constants and correlation factors of thiophenic sulfur compounds at different temperatures.

Rate constant (k) Th BT DBT 0.019 0.021 0.022 0.024 0.025 0.030 0.047 0.045 0.048 0.047 0.050 0.053

Correlation factor (R2) Th BT DBT 0.9125 0.9187 0.9445 0.8777 0.8892 0.9513 0.9278 0.9174 0.8862 0.9278 0.9477 0.9515

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Temperature (°C) 25 30 35 40

Table 4. Regeneration of the Cu-SPOM@PbO@PVA nanohybrid catalyst for the ODS of DBT

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Table 4. Regeneration of the Cu-SPOM@PbO@PVA nanohybrid catalyst for the ODS of DBT compound. a

Run

ODS efficiency (%)

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Condition of ODS reaction: 50 mL of model fuel (500 ppmw of DBT in nheptane), 0.10 g of catalyst, 3 mL of oxidant, 10 mL of acetonitrile, reaction temperature = 35 °C, and reaction time = 1 h.

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