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Superior deeply desulfurization of real diesel over MoO3-silica gel as an efficient catalyst for oxidation of refractory compounds Bita Mokhtari, Azam Akbari, and Mohammadreza Omidkhah Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01646 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 28, 2019

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Energy & Fuels

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Superior deeply desulfurization of real diesel over MoO3-silica gel as an

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efficient catalyst for oxidation of refractory compounds

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Bita Mokhtaria, Azam Akbari b,, Mohammadreza Omidkhahc

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a

Mazandaran University of Science and Technology, Mazandaran, Iran

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b

Chemistry and Chemical Engineering Research Center of Iran, P.O. Box 14335-186, Tehran, Iran

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c

Faculty of Chemical Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran

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Abstract

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In this work, deep extractive-catalytic oxidative desulfurization (ODS) of hydrotreated real diesel

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fuel was attained using a synthesized catalyst of supported molybdenum oxide on mesoporous

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silica gel (MoO3/SG) with a high surface area. The characterization of the catalyst was assessed

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by FT-IR, XRD, BET, BJH, N2 adsorption-desorption, SEM and EDS analysis. The effect of

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molybdenum loading, Mcatalyst/Voil, H2O2/sulfur molar ratio, temperature, and reaction duration

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were examined on the ODS of diesel aiming to set at the optimum values. Different extraction

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solvents were evaluated, where acetonitrile processed with the best performance among all. About

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99.9% of sulfur in real diesel was selectively removed using MoO3(5wt.%)/SG catalyst under the

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optimal mild reaction conditions of 45°C, O/S molar ratio of 8, Mcatalyst/Voil of 0.1 g/mL, and

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reaction time of 90 min employing only one step extraction of ODS products by acetonitrile.

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Comparing to previous researches on ODS of real diesel, an obviously superior performance was

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exhibited by the introduced catalyst in this study because of the sufficiently large pore diameter,

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considerably higher surface area (346.5 m2/g) of the catalyst, well distribution of active phase on

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the SG surface and ODS operating at the optimal conditions. Interestingly, no significant loss in

Corresponding

author, Email: [email protected], Tel.: +98-21-44787720

1 ACS Paragon Plus Environment

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catalyst activity was revealed after 7 recycling in ODS of DBT model; however, the catalyst

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reusability was relatively limited by the interactions of various hydrocarbons in the real diesel. The

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studies on overall kinetic and reaction pathway were conducted as well.

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Keywords: Oxidative desulfurization; Real diesel; Heterogeneous catalyst; Thiophenic

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compound; MoO3; Silica gel;

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1. Introduction

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The organosulfur compounds in untreated diesel fuels mainly involved benzothiophenes (BTs),

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dibenzothiophenes (DBTs) and its alkylated derivatives [1]. During diesel combustion, these sulfur

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compounds are converted to SOx which causes the acid rain and environmental problems.

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Accordingly, stringent environmental regulations have been imposed worldwide to limit the sulfur

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content of diesel fuels to a very low level (< 10 ppmw) [2-4]. Therefore, ultra-deep desulfurization

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of fuels has attracted a lot of attention [5,6]. The commercial Hydrodesulfurization (HDS) is the

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most commonly used method for the removal of sulfur compounds from petroleum fuels in

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industry. This technology is usually applied employing Ni, Co and/or Mo supported on Al2O3

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catalysts under temperatures range of 290-455 °C and H2 pressures of 150-3000 psi [7]. Unlike the

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high efficiency of this technology in the elimination of aliphatic and acyclic sulfur compounds, it

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bears some operational drawbacks with the removal of DBT and its alkylated derivatives specially

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4,6-dimethyl dibenzothiophene (4,6-DMDBT) because of the high steric hindrance of them. These

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large size and refractory organosulfur compounds usually remain in diesel fuel after HDS making

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about 500 ppmw sulfur concentration [8]. Deeply removal of these compounds by HDS requires

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the use of more severe temperatures and pressures, more active catalysts and longer residence time, 2 ACS Paragon Plus Environment

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which impose additional investments and higher economic costs [6,9]. To overcome these

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problems, several alternative approaches have been developed, such as biodesulfurization [10,11],

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selective adsorption [12], extraction [13,14] and oxidative desulfurization (ODS) [15-18]. Among

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these processes, ODS has been known as a more cost-effective method for replacing or completing

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the HDS process, because it can deeply remove the refractory sulfur compounds of fuel under mild

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reaction temperature and pressure without the need of expensive hydrogen [19,20]. This process

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takes place in two steps; first, sulfur compounds in the fuel oxidize to sulfoxides and their sulfones.

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Thereafter, the more polar oxidation products are easily removed by an appropriate process such

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as solvent extraction, adsorption or distillation [21]. Previous studies on ODS process have

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employed different oxidizing agent such as H2O2, ozone, t-butyl hydroperoxide (TBHP), t-butyl

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hypochlorite (t-BuOCl), cyclohexanone peroxide (CYHPO) and etc. [22-25]. Among them,

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hydrogen peroxide is widely used due to its low cost, high oxidation capacity and non-toxic by-

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products [26]. To enhance the oxidation activity of the oxidants, an active catalyst is also required

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from a homogenous or heterogeneous type such as polyoxometalates [27,28], organic acids [29],

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ionic liquids (ILs) [2,30-32] and solid catalysts [33- 35]. Due to limitations for separation and

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reusing of the homogeneous catalysts, the heterogeneous types are more important and applicable

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[36]. Heterogeneous transition metal oxide catalysts on porous silica or alumina supports, such as

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MoO3/V2O5/MCM-41 [37], Mo/V/W/Al2O3 [38], Mo-V on alumina pellets [39], mesoporous V-

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Ti-CMK-3 [40], MoOx-VOx/Al2O3 [41], are an interest group of active catalysts which can be

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easily recycled but often increase the ODS reaction time relatively. Among them, it seems that

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molybdenum oxides on a support material have been preferred in previous ODS studies [20,42].

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For example, Wang et al. [43] and Gutierrez et al. [44] focused on ODS of model compounds by

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Mo/Al2O3 catalyst and t-butyl hydroperoxide. Chang et al. [42] evaluated ODS of DBT model


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utilizing cumene hydroperoxide and modified MoO3/SiO2 catalyst with alkaline earth metals. The

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cost-effective catalysts of molybdenum supported on Ti-pillared interlayer clay (Ti-PILC) [23]

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and 4A molecular sieve as a microporous material [24], as well as cobalt supported on a cubic

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mesoporous silica named KIT-6 [25], have been prepared and investigated in ODS of BT and DBT

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models using cyclohexanone peroxide (CYHPO) as an oil-soluble oxidant leading to interesting

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results. Moreover, in previous researches of our group [20,45] the ODS of model diesel using

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MoO3/Al2O3-H2O2 was facilitated by ultrasonic irradiation. It was also explored that the reactivity

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of refractory model sulfur compounds decreased in the order of DBT> DMDBT>> BT in

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MoO3/Al2O3-H2O2 system [45]. Such supported heterogeneous catalysts with important features

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of a low cost and easy preparation, have been commercially significant as were used for many

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industrial chemical processes [20,37]. Nevertheless, it should be point out that catalytic ODS of

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real diesels was less focused in literature, while this is crucial for a practical application. In brief,

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ODS of Mexican diesel fuel with 320 ppmw sulfur by Mo/Al2O3 and H2O2 [15], ODS of diesel oil

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employing MoO3/Al2O3 catalysts with V, Ti, and W, as dopants and co-dopants [38], Fe/MoO3–

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PO4/Al2O3 catalyst investigation in ODS of Malaysian diesel with TBHP-DMF system [46], ODS

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kinetics of middle distillates in cumene hydroperoxide-MoO3/Al2O3 system [8], and ODS

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reactivity of sulfur compounds in light gas oil containing 39 ppmw sulfur by Mo/Al2O3 catalyst

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and t-BuOOH oxidant [47], have been the researches which more focused on commercial

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feedstocks. Except these researches, usually the model oils were used for various ODS studies,

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which may face to some limitations for application of industrial feedstocks. Moreover, the small

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pore volume and low surface area of the investigated catalysts may limit the activity in ODS of

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different large size refractory sulfur compounds in real fuels. Considering these operational

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challenges, the efficient and selective deep ODS of hydrotreated real diesel (containing 590 ppmw


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sulfur) having refractory sulfur compounds was aimed in this work. The suggested and used

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catalyst was Mo oxide loaded on mesoporous silica gel (SG) having profitable textural properties

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involved high surface area and appropriate pore size for efficient oxidation of large refractory

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sulfur compounds having alkyl substituents. This new suggested catalyst was synthesized and then

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characterized by different methods. The effect of process variables was examined during ODS of

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the real diesel. The catalyst reusability in real diesel treatment and overall kinetic at the optimal

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conditions, and the reaction pathway were also studied. This cost-effective and active catalyst

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having a simple preparation method, could be valuable for a promising industrial application.

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2. Experimental

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2.1 Materials

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Ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24.4H2O), silica gel (SG), acetonitrile,

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methanol (99%), H2O2 (30 wt.% aqueous solution), n-Octane and dibenzothiophene (DBT) were

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purchased from Merck company, German. A hydrotreated real diesel containing 590 ppmw of

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sulfur were supplied from Tehran Oil Refinery.

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2.2 Catalyst preparation

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Molybdenum oxide supported on silica gel (MoO3/SG) with Mo loading ranging from 5 up to 30

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wt.%, was prepared via incipient wetness impregnation (IWI) method. According to the weight

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percent requirement of MoO3, a desirable amount of ammonium heptamolybdate was dissolved in

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a specific volume of distilled water and subsequently slowly added to the support at ambient

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temperature. The wet mixture was dried in a vacuum rotary and then calcined at 500°C for 8 h in

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air.


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2.3 Catalysts characterization techniques

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Fourier transform infrared (FT-IR) spectroscopy were recorded using PerkinElmer FT-IR

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spectrometer. X-ray diffraction (XRD) patterns of the synthesized catalysts were obtained using

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X’Pert MPD model of Philips diffractometer utilizing high-intensity Cu/kα radiation (40kV;

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40mA; 1.78897Å) and the step scan technique at 2θ angles range between 3 and 90°. The values

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of specific surface area and pore volume of the catalysts were calculated according to BET method

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by means of Belsorp-mini II and the pore-size distribution was determined using desorption branch

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of isotherm with the Barrett-Joyner-Halenda (BJH) method. Surface morphology of the catalyst

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was determined using scanning electron microscopy (SEM) by Tescan-VEGA, Czech. The

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elemental composition of synthesized catalyst was investigated by Energy Dispersive X-ray

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spectroscopy (EDS) analysis. TEM measurements were performed on a Philips CM 120

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microscope operated at 100 kV and by using 200 mesh Au grids.

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2.4 Catalytic ODS method and analysis

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Catalytic ODS experiments were carried out in a 50 mL glass batch reactor with a magnetic stirrer,

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cooling reflux and a water bath for temperature controlling. In a typical run, the catalyst and diesel

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sample, were introduced to the reactor at a constant temperature of 45°C under the stirring speed

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of 1000 rpm. For ODS runs of real diesel, simultaneously, a same volume of acetonitrile was added

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as an extraction solvent. To initiate the oxidation reaction, an excess amount of hydrogen peroxide

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was added to mixture when the temperature was stabilized at a desired value. After a specified

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reaction time, the stirring was stop and the oil sample was extracted after phase separation. The

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sulfur compounds of real diesel were detected by gas chromatograph with Pulsed Flame


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Photometric detector (GC-PFPD) (Varian CP-3800 with a capillary column of VF-1 with 60 m ×

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0.25 mm id inner diameter). Total sulfur of the real diesel was determined by Multi EA 5000

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(Analytik Jena, Germany), according to the international standard of ASTM D5453. Flame

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ionization detector (FID) was utilized to measure ODS yield of model fuel. The desulfurization

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yields were reported by an approximate ±5 % error estimated from duplication of the experiments.

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The following equation was utilized for calculating the desulfurization yield:

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yield (%) = [1 - (Ct/C0)] ×100

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Where C0 and Ct (ppmw) respectively represented the initial and final sulfur concentration of diesel

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fuel. GC–MS (Varian, Saturn 2200, GC/MSD; VF-5 MS column, 30 m × 250 m id × 0.25 m)

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was employed to characterize desulfurization products. Hexadecane and DBT were used as the

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internal standard in GC-FID and GC-PFPD analysis respectively, which was added to the samples

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before injecting to GC.

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3. Result and discussion

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3.1 Catalyst characterization

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3.1.1 FTIR

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Fig. 1 illustrated the FTIR spectra of MoO3/SG catalysts having different contents of molybdenum

155

oxide (0, 5, 10, 15, 20 and 30 wt.%). The obvious bands around 3500 cm-1 and 1630 cm-1 in all

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spectra, were corresponded to the symmetrical stretching vibration of O-H and deformation of the

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adsorbed H2O on SG, respectively [48]. FTIR spectra of SG without Mo loading displayed two

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peaks at 464.8 and 825.4 cm-1 associated to the symmetric stretching of Si-O-Si, and one peak at

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1110 cm-1 assigned to the asymmetric stretching of Si-O-Si [49]. At 5 wt.% Mo loading, the

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formation of Mo bound on the surface of SG was appeared at 460.7, 809.2 and 1098.7 cm-1. When


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Mo loading increased to 10 wt.%, the stretching vibration of Mo=O bond on the surface of SG was

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appeared at 816 cm-1. By more increasing Mo loading up to 15 wt.%, two distinct bands were

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appeared at 602 and 830 cm-1, which were respectively ascribed to the formation of bridged Mo-

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O-Mo and weak microcrystallites of MoO3 [37]. Therefore, polymolybdate was began to be

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formed at 15 wt.% of Mo loading. The bands of MoO3 crystals were emerged clearly at 526 and

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850 cm-1 when Mo loading was increased up to 20 wt.%.

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168 169

Fig. 1. FTIR spectra of SG (a) and supported catalysts with Mo loading of 5 wt.% (b), 10 wt.% (c), 15 wt.% (d) and

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20 wt.% (e).

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3.1.2 XRD 8 ACS Paragon Plus Environment

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XRD patterns of the synthesized catalyst samples having different contents of molybdenum oxide

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were presented in Fig. 2. From the results, the absence of the characteristic peaks associated to

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MoO3 crystals at 5 wt.% Mo loading revealed a high dispersion of Mo active phases on the SG

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support. However, the increasing of molybdenum content from 10 wt.% up to 20 wt.% led to

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apparent the peaks corresponded to MoO3 crystals with broad reflection centering at 2θ of 12.81,

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23.40, 25.69, 27.38 and 39.60° [50]. These results were in a good agreement with FTIR analysis

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which proved the formation of polymolybdate structures and MoO3 crystals by increasing Mo

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loading on the SG support.

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Fig. 2. XRD patterns of SG (a) and the supported MoO3/SG catalyst with 5 wt. % (b), 10 wt.% (c), 15 wt.% (d) and

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20 wt.% (e) Mo loading

183 184

3.1.3 BET, BJH and N2 adsorption-desorption isotherms


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The textural properties of the SG before and after supporting Mo oxide with 10 wt.% loading were

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assessed by BET and BJH method. The results were summarized in Table 1. The reduction of BET

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surface area from 490.6 to 346.49 m2/g confirmed well dispersion of Mo oxide on the surface of

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mesoporous SG support without polymerization or agglomeration in a good agreement with the

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XRD results. The Mo oxide species may also be located partially inside the pores resulted in some

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pore blockage and then reduction of average pore volume from 0.69 to 0.58 cm3/g (Table 1).

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Typical pore diameter distribution of the samples evaluated by BJH method, was illustrated in Fig.

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3. The results indicated an increase in the average pore diameter from 5.63 to 6.79 nm after Mo

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immobilization on SG, as observed in some previous developed catalysts [20,39,51], possibly

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coming from the filling of some smaller pores by the Mo oxide species. It may also be caused by

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the thin wall thickness and low thermal stability resulted in partial pore collapse of SG support

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during preparation and, specially, calcination of the catalyst at high temperature of 500 °C and 8

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h duration [17, 52]. However, the mesoporous structure of SG support was retained after being

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impregnated by Mo. Because of the large molecular size of the refractory thiophenic reactants

199

having methyl groups, the higher pore diameter of the catalyst was further appropriate for

200

reduction of diffusion limitations. Fig. 3 showed N2 adsorption-desorption isotherms of the support

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and catalyst samples. The isotherms exhibited a typical type IV isotherm revealing a mesoporous

202

structure [53]. Furthermore, the observed H2 hysteresis loops represented the bottle ink pores for

203

both samples.

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Table 1. Textural properties of the SG support and the synthesized catalyst with 10 wt.% Mo loading. Sample

Surface area (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

SG

490.67

0.69

5.63

MoO3/SG

346.49

0.58

6.79

205 10 ACS Paragon Plus Environment

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Fig. 3. N2 adsorption-desorption isotherms and BJH pore size distribution of SG and MoO3(10 wt.%)/SG catalyst.

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3.1.4 SEM and EDS

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The SEM images of the SG before and after Mo oxide immobilization were shown in Fig. 4. There

211

was no change in the size and morphology of SG after Mo immobilization which exhibited that

212

the support particles were not damaged during impregnation. A relatively uniform dispersion of

213

Mo oxide on SG was also observed which was in agreement with XRD and BET results. Typical

214

EDS spectra of the support and catalyst samples were also illustrated in Fig. 5. The successfully

215

immobilization of molybdenum oxide on the surface of SG was established by EDS spectra of

216

MoO3(10 wt.%)/SG catalyst sample in Fig. 5 (b). The EDS analysis result approximated an average

217

value of 8.9 wt.% for molybdenum loading on the catalyst surface. As can be seen in Figs. 6 & 7,

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EDX elemental maps of Mo, Si and O, and TEM images of MoO3/SG catalyst, represented the


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well uniformly distribution of Mo, in agreement with the above-mentioned XRD, FTIR and BET

220

results.

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Fig. 4. SEM images of SG (a) and MoO3(10 wt.%)/SG catalyst (b)

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224 225

Fig. 5. EDS images of SG (a) and MoO3 (10 wt.%)/SG catalyst (b)


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Fig. 6. SEM image of MoO3/SG catalyst (a) and the corresponding EDS elemental maps for Mo (b), Si (c) and O (d)

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Fig. 7. TEM images of MoO3/SG catalyst

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3.2 Influence of molybdenum loading and extraction solvent

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To find the optimum loading of molybdenum oxide on SG support, different catalyst samples

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having 0, 5, 10, 12, 15, 20 and 30 wt.% Mo content were prepared and assessed in ODS of the real

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diesel. The results were listed in Table 2. At the mentioned experimental condition, only 6.5 % of

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sulfur was removed by mesoporous SG without Mo loading in the absence of acetonitrile. In the 13 ACS Paragon Plus Environment

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presence of acetonitrile 43% of sulfur has been eliminated, while achieving 91.3 % efficiency by

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5 wt.% Mo loading on SG. This indicated highly catalytic ability of the prepared Mo oxide on the

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mesoporous SG. Subsequently, the catalyst activity gradually decreased by increasing Mo loading

239

from 5 up to 30 wt.%. Such effect may be attributed to the accumulation of molybdenum oxides

240

on the catalyst surface at Mo loading above 5 wt% which was confirmed by FTIR and XRD

241

analysis. Therefore, 5 wt.% Mo loading (approximately actual loading amount of 4.4% by EDS

242

analysis) was selected for the examination of MoO3/SG catalyst in this work.

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Table 2. Influence of Mo loading on the catalyst performance in ODS of real diesel (Reaction conditions: T=45°C,

244

t=90 min, O/S=5, Mcatalyst/Voil=0.1 g/mL and Vacetonitrile/Voil=1/1) Mo loading on the support (wt.%)

Sulfur removal (%)

0 (SG in the absence of acetonitrile)

6.5

0 (SG + acetonitrile)

43

5

91.3

10

91.0

12

89.6

15

86.9

20

78.1

30

67.0

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An appropriate solvent for efficiently extraction of ODS products was needed in this work.

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In this regard, the choice of a suitable solvent with a high ability is very important. The

248

typical solvents of acetonitrile, acetone and methanol were investigated as an extraction

249

solvent in ODS of real diesel and the results were compared in Fig. 8. In the absence of

250

any extraction solvent, 53% of sulfur content was reduced. This concluded that mesoporous

251

SG could also play a selective adsorption role for some ODS products. When an extraction


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solvent was employed, the ODS efficiency increased. The highest efficiency was reached

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by acetonitrile as an aprotic solvent that has also been stated by previous researches

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[35,37]. Furthermore, in order to more explain the pure extraction property of acetonitrile,

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a comparative extraction experiment in the absence of catalyst and H2O2 was done that

256

resulted 38% yield of total sulfur removal. Therefore, not only the oxidized sulfur

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compounds products could well remove from diesel by acetonitrile, some non-oxidized

258

sulfur hydrocarbons were also extracted. The consideration of the possible diesel loss in

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the extraction step by acetonitrile was also important for practical application. Therefore,

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the solubility of the diesel fuel in acetonitrile was measured using the gravimetric method

261

as stated by Gao et al. [54] and replicated 2 times. An average amount of 2.1 wt.% was

262

attained for diesel solubility in the solvent.

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Fig. 8. Comparison of different extraction solvents in ODS of real diesel by MoO3/SG catalyst at T=45°C, t=90 min,

265

O/S=5, Mcatalyst/Voil=0.1 g/mL

266 267

3.3 Influence of main process variables

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The effect of reaction time on the ODS of the real diesel over MoO3/SG catalyst was demonstrated

269

in Fig. 9. The results illustrated the increase of ODS efficiency along the time and the maximum

270

amount of 99.9 % was reached after 90 min under the mentioned experimental conditions. While 15 ACS Paragon Plus Environment

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increasing the reaction time more than 90 min had no obvious effect on ODS efficiency. As a

272

result, the minimum duration of 90 min was offered for further ODS experiments in this work.

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274 275

Fig. 9. Influence of reaction time on the sulfur elimination of real diesel at T=45°C, O/S=8 mol/mol and

276

Mcatalyst/Voil= 0.1 g/mL

277 278

The catalyst/oil (Mcatalyst/Voil) is a crucial parameter for the scale-up and industrialization of

279

the catalysts. Fig. 10 showed the influence of Mcatalyst/Voil ratio on ODS of real diesel by the

280

introduced MoO3/SG catalyst at the mentioned experimental conditions. The reaction efficiency

281

was enhanced by increasing Mcatalyst/Voil ratio from 0.01 up to 0.1 g/mL. This could be arisen from

282

more exposed active sites resulted in an improved molybdenum peroxide formation from catalytic

283

reaction of H2O2. Nevertheless, the sulfur removal slightly decreased by further increase in

284

Mcatalyst/Voil ratio from 0.1 to 0.12 g/mL. This effect could come from the increase of SG support

285

having hydrophilic surface with OH functional groups in the reaction media, which possibly made

286

physical adsorption of H2O2 on the non-active surface. This issue has also been previously verified

287

by our group for hydrophilic MoO3/γ-Al2O3 catalyst [20]. Hence, at extra catalyst amount of 0.12

288

g/mL, a large portion of excess H2O2 may rapidly be trapped by highly accessible porous SG,

289

instead of staying near the active sites and reaction with Mo oxides. So the observed decline in 16 ACS Paragon Plus Environment

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290

ODS potential of the catalyst was resulted. Therefore, the optimal Mcatalyst/Voil ratio of 0.1 g/mL

291

was preferred in ODS of real diesel in this work.

292

293 294

Fig. 10. Influence of Mcatalyst/Voil rario on ODS of real diesel at T=45°C, t=90 min and O/S= 8 mol/mol)

295 296

The influence of reaction temperature on sulfur elimination was also examined and the results were

297

presented in Fig. 11. The increase of reaction temperature from 25 up to 45°C, improved ODS

298

efficiency of the catalyst via accelerating oxidation reactions. Afterwards, the more elevating of

299

temperature from 45 to 60 °C resulted in decrease of sulfur elimination from the maximum amount

300

of 99.9% to 91.6%. According to the results obtained in this work and previous studies [2,45,55],

301

it could be concluded that ODS efficiency relatively decreased at reaction temperatures above a

302

certain value (45 °C in this work) due to H2O2 loss by thermal decomposition of it to H2O and O2.

303

This optimal temperature has also been directly influenced by the nature of the catalyst and

304

refractory sulfur compounds in fuel. For instance, G.Gutierrez et al. [15] reported a higher optimal

305

temperature of 80 °C for ODS of a commercial Mexican diesel fuel with 320 ppmw sulfur over

306

Mo/γ-Al2O3 catalyst.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

307 308

Fig. 11. Influence of reaction temperature on ODS efficiency of real diesel at Mcatalyst/Voil=0.1 g/mL, O/S= 8

309

mol/mol and t=90 min

310

The determination of an appropriate H2O2/sulfur (O/S) molar ratio was important because of its

311

direct effect on ODS performance of the catalyst and the hazardous issues of H2O2 [56]. According

312

to the stoichiometry of ODS reactions, 2 moles of H2O2 is necessary to convert one mole of sulfur

313

compound to its corresponding sulfone, but practically an excess H2O2 amount is used [57]. The

314

effect of excess O/S molar ratio on ODS efficiency of the real diesel over the introduced MoO3/SG

315

catalyst was demonstrated in Fig. 12. By increasing O/S molar ratio from 2 up to 8, the removal

316

of sulfur compounds was enhanced to its highest level. This was attained from a more accessibility

317

of oxidant which improved catalytic oxidation reaction of sulfur compounds. Subsequently, ODS

318

yield has been dropped by more raising O/S molar ratio from 8 to 10. The decline in ODS

319

efficiency was attributed to the highly adsorption of more H2O content from the initial aqueous

320

oxidant solution (30 wt.% H2O2) to the catalyst surface. H2O adsorption could occupy the Mo

321

oxide active centers and so decline effective surface area [20]. In conclusion, the optimal O/S

322

molar ratio of 8 was used for the next experiments. Fig. 13 displayed the GC-PFPD results of the

323

refractory sulfur compounds in hydrotreated real diesel before and after ODS by the synthesized

324

catalyst at the above-mentioned optimal mild conditions. The corresponded ODS efficiency was

325

99.9 %. BT (used as the internal standard for GC analysis), 2-MBT, DBT and 4,6-DMDBT were 18 ACS Paragon Plus Environment

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Energy & Fuels

326

identified by the model standard samples. The other sulfur compounds groups in GC-PFPD were

327

characterized by comparing the identified peaks in GC-MS, based on the known sulfur compounds

328

in diesel fuels which are usually the alky derivatives of BTs and DBTs [58]. The color of diesel

329

fuel was slightly changed from light brown to yellow after ODS. This may be mainly ascribed to

330

the separation of sulfur hydrocarbons which first were selectively oxidized to high polar sulfones

331

by the catalyst and oxidant, and then extracted from diesel by acetonitrile. Additionally, the partial

332

extraction of some non-sulfur hydrocarbons from diesel has been possible in low concentrations,

333

which were further confirmed by GC-MS of acetonitrile after ODS treatment of diesel.

334 335

Fig. 12. Influence of H2O2/sulfur molar ratio on sulfur removal of real diesel at T=45°C, Mcatalyst/Voil =0.1 g/mL and

336

t=90 min

337


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

338 339

Fig. 13. GC-PFPD analysis of the real diesel before and after ODS under the optimal reaction conditions of T=45°C,

340

O/S= 8 mol/mol, Mcatalyst/Voil =0.1 g/mL and t=90 min

341 342

3.4 Catalyst recycling

343

The catalyst recycling and reusing potential in ODS of real diesel was practically highly important.

344

Fig. 14 revealed the recyclability of the catalyst tested for up to 4 cycles. For this examination,

345

after each experimental run and phase separation, the used catalyst was filtered from diesel, washed

346

with fresh methanol under magnetic stirrer at 60°C for 1hr and finally dried at 80°C overnight.

347

Thereafter, the catalyst was used in the subsequent ODS run under the determined optimal

348

conditions in this work. The results were presented in Fig. 14. The ODS yield of real diesel dropped

349

from 99.9 % in the first run up to 91% in the second reaction cycle and subsequently reduced to

350

82 and 68% in the next ODS cycles. Based on the obtained results in this work and previous

351

researches [20,44], the observed decline in ODS efficiency was due to the adsorption of water and

352

sulfones products on the catalyst surface, as well as possible leaching of Mo oxides. However, in

353

this work, the presence of various non-sulfur hydrocarbons in real diesel may also cause further 20 ACS Paragon Plus Environment

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Energy & Fuels

354

catalyst deactivation. To more clarify this effect, the catalyst reusability was evaluated in ODS of

355

DBT in n-octane as a model fuel at a same experimental conditions of real diesel treatment. The

356

results shown in Fig. 15, explained no significant reduction in catalyst activity after 7 recycling.

357

Therefore, the main factor affecting the catalyst reusability in ODS of real diesel was the

358

interactions of various non-sulfur hydrocarbons. These compounds possibly caused to make such

359

stronger adsorption of ODS products and other impurities on the surface of SG, which could not

360

be exactly peeled off from the catalyst surface by washing with the solvent. Therefore, some more

361

effective treatments such as ultrasonication, thermal procedures or stronger solvents, could be

362

offered for more efficiently cleaning and regeneration of the used catalyst.

363

364 365

Fig. 14. Recycling potential of the catalyst in ODS of real diesel (T=45°C, t=90 min, O/S=8 mol/mol,

366

Mcatalyst/Voil=0.1 g/mL)

367


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

368 369

Fig. 15. Recycling potential of the catalyst in ODS of DBT at reaction conditions mentioned in Fig. 14

370 371

3.5 Comparison of ODS ability of MoO3/SG catalyst with previous researches

372

In order to more find out the beneficial behavior of MoO3/SG catalyst in this work, a comparison

373

with some Mo containing ODS catalysts referred in previous studies on the commercial feedstocks,

374

was carried out as summarized in Table 3. This comparison explained the excellent performance

375

of MoO3/SG catalyst while, obviously, a relatively more moderate experimental conditions were

376

desirable as well. According to the obtained results and discussion in this work, high surface area

377

and desirable pore structure of SG comparing to some other supports, and well dispersion of Mo

378

oxide on the catalyst surface, as well as the optimization of operating conditions during the real

379

diesel treatment might allow to achieve the obvious higher efficiency. For example, a better

380

effectiveness than the catalysts suggested later by Safa et al. [8] using cumene hydroperoxide

381

(CHP), and Ishihara et al. [47] using TBHP was provided in the present work due to the substantial

382

higher surface area and pore volume of SG comparing to γ-Al2O3. Nevertheless, the type of sulfur

383

and non-sulfur hydrocarbons in real feedstocks were also important on the catalyst effectiveness.

384

As a result, the developed profitable MoO3/SG catalyst could be considered as a well promising

385

choice for practical applications in ODS process.

386 22 ACS Paragon Plus Environment

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Energy & Fuels

387

Table 3. Comparison of MoO3/SG catalyst performance in this work with the previous ODS studies on commercial

388

feedstocks. Catalyst

Oxidant

Real fuel type

S0 (ppmw)

Sremoval (%)

Reaction condition

Ref.

MoO3/SG

H2O2

Commercial diesel

590

99.9

T=45°C t=90 min O/S=8

This work

Mo/Al2O3

H2O2

Commercial diesel

320

97.8

T=60°C t=60 min O/S=11

[15]

Fe/MoO3PO4/Al2O3

TBHP

Commercial diesel

440

96

T=45°C t=30 min O/S=3

[46]

V/W/MoO3/Al2O3

TBHP

Commercial and Crude diesel

550

>90

T=60°C t=30 min O/S=3

[38]

MoO3/Al2O3

CHP

Hydrotreated middle distillate

445

>80

T=90°C t=2h O/S=20

[8]

Mo@COMOC-4

TBHP

Commercial diesel

639

74

T=70°C t=50 h O/S=12

[35]

MoO3NiO/Al2O3

TBHP

Commercial diesel

387

96

T=140°C t=2h O/S=22

[59]

MoO3/Al2O3

TBHP

Light gas oil

39

89.5

T=100°C t=3h O/S=15

[47]

389 390

3.6 ODS kinetic and mechanism study

391

During ODS process, the organosulfur compounds (R-S) catalytically react with H2O2 oxidant to

392

form the unstable sulfoxides (R-SO) which then rapidly oxidize to the respected sulfones (R-SO2)

393

through a same oxidation way [15]. The oxidation reactions were mentioned below:

394

R ― S + H2O2

395

R ― SO + H2O2

k

R ― SO + H2O k′

R ― SO2 + H2O


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 45

396

Since the conversion of sulfoxides to sulfones was very fast, the first reaction has been the

397

controlling step. When this reaction was assumed to fit pseudo-first-order kinetic and excess H2O2

398

concentration was supposed to be approximately constant, the reaction rate can be expressed as:

399

rS =

400

ln

401

Where rS is the reaction rate, CS and CS0 represent the sulfur concentration before reaction and

402

after a certain time of t (min), respectively; k is the reaction rate constant which can be calculated

403

based on the slope of linear plot of ln CS versus reaction time (t) at a constant temperature. When

dCS

CS C S0

dt

= ―kCS

= ―kt

CS

0

404

CS

an average rate for sulfur removal from real diesel was assumed, ln CS versus reaction time (t) was 0

405

plotted as shown in the Fig. 16. The attained correlative coefficient, R2, was 0.974 which validated

406

a pseudo-first-order kinetic for total sulfur elimination. Accordingly, the average ODS

407

desulfurization rate constant was calculated as 0.074 min-1.

408 409

Fig. 16. The pseudo-first-order kinetic for ODS of real diesel at T=45°C using MoO3/SG catalyst


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Energy & Fuels

410

.

411

Since the exact characterization of each individual sulfur hydrocarbon of the real diesel was

412

difficult, the ODS products of DBT model was identified by GC-MS analysis. Accordingly, ODS

413

reaction of DBT in n-octane over MoO3/SG catalyst was run at 45°C, O/S molar ratio of 8,

414

Mcatalyst/Voil of 0.1, and during 90 min. After the reaction, the catalyst was filtered off and washed

415

with methanol solvent under stirring for 60 min. Afterwards, the oil and methanol samples were

416

analyzed by GC-MS using internal standard. The results shown in Fig. 17, proved DBT (m/z=184)

417

and DBTO2 (m/z= 216) product in methanol phase which was same as some previous reports

418

[2,57,60]. No sulfur compound was found in the diesel phase because of the complete elimination

419

of DBT and DBTO2 by MoO3/SG catalyst from model fuel at the used conditions for

420

experimentation. Furthermore, due to the fast conversion of sulfoxide to sulfone, no DBTO as the

421

oxidation product was observed by GC-MS analysis. Additionally, the MoO3/SG catalyst was able

422

to partially extract some of DBT without oxidation leading to the appearance of DBT in the catalyst

423

phase in Fig. 17. However, the oxidation of sulfur hydrocarbons and then extraction by acetonitrile

424

was more profitable because of an easier separation of polar sulfones from acetonitrile comparing

425

to non-oxidized compounds.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

426 427

Fig. 17. GC-MS of the oil and catalyst phase after ODS of DBT using MoO3(5wt.%)/SG

428 429

According to above results and the previous reports [45,44,51,15], a plausible mechanism for ODS

430

in MoO3/SG-H2O2 system could be proposed as shown in Scheme 1. This pathway involved first

431

production of Mo peroxide intermediate from reaction of H2O2 with Mo oxide as the catalyst active

432

sites. The generated highly active Mo peroxide oxidized DBT to DBTO and then to DBTO2 in a

433

same manner by nucleophilic reaction of sulfur with Mo peroxide under the used mild reaction

434

conditions. Based on the results, the pore size of the catalyst was perfect for promoting the

435

adsorption of thiophenic reactants on the surface. Finally, the produced sulfones and water, and

436

the remained unreacted H2O2 were extracted from fuel by acetonitrile solvent.


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Energy & Fuels

437 438

Scheme 1. The overall mechanism of ODS system.

439 440

4. Conclusion

441

The new porous MoO3/SG catalyst with a high surface area of 346.5 m2/g was synthesized by a

442

simple incipient wetness impregnation (IWI) method and for the first time was tested in deep ODS

443

of real diesel. A high dispersion of Mo oxides on SG surface without Mo crystallization or

444

polymolybdate formation was demonstrated by FTIR and XRD results at 5 wt.% loading of Mo

445

which provided a catalyst with the best ODS performance. This new supported catalyst was

446

capable to efficiently produce active Mo peroxide from H2O2 and then oxidize sulfur compound

447

to stable sulfones at a mild reaction condition. Despite adsorption of some ODS products by SG,

448

acetonitrile was also employed as an extraction solvent for completely removal of sulfones and

449

water products from real diesel. The influence of main process variables as temperature, O/S molar

450

ratio, Mcatalyst/Voil ratio and duration of reaction were investigated. The superior ODS potential of

451

99.9% was achieved for total sulfur removal of real diesel at the determined optimal conditions.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

452

The high ODS ability of the catalyst could be due to the suitable textural properties such as

453

sufficiently large pore diameter and the higher surface area of mesoporous SG and highly

454

dispersion of Mo oxides. These factors caused to more activity of the catalyst for removal of main

455

refractory sulfur compounds remained in diesel after HDS. The examination revealed that the

456

catalyst reusability was limited by the various hydrocarbons present in the real diesel while it could

457

be reused 7 times in ODS of DBT in model oil without any significant loss of activity.

458

Consequently, some more effective treatments such as ultrasonication and thermal procedures

459

have been suggested for more peeling off impurities from the catalyst after each ODS run of real

460

diesel. A pseudo-first-order macro kinetic could be assumed for sulfur elimination from real diesel.

461

In conclusion, the developed MoO3/SG catalyst could be considered as a profitable promising

462

choice for industrial applications.

463 464

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465

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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figures and Schemes:

635

636 637

Fig. 1. FTIR spectra of SG (a) and supported catalysts with Mo loading of 5 wt.% (b), 10 wt.% (c), 15 wt.% (d) and

638

20 wt.% (e).


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639 640

Fig. 2. XRD patterns of SG (a) and the supported MoO3/SG catalyst with 5 wt. % (b), 10 wt.% (c), 15 wt.% (d) and

641

20 wt.% (e) Mo loading

642 643

Fig. 3. N2 adsorption-desorption isotherms and BJH pore size distribution of SG and MoO3(10 wt.%)/SG catalyst.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

644

645 646

Fig. 4. SEM images of SG (a) and MoO3(10 wt.%)/SG catalyst (b)

647 648

649 650

Fig. 5. EDS images of SG (a) and MoO3 (10 wt.%)/SG catalyst (b)


Page 38 of 45

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651 652

Fig. 6. SEM image of MoO3/SG catalyst (a) and the corresponding EDS elemental maps for Mo (b), Si (c) and O (d)

653

654 655

Fig. 7. TEM images of MoO3/SG catalyst

656


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

657 658

Fig. 8. Comparison of different extraction solvents in ODS of real diesel by MoO3/SG catalyst at T=45°C, t=90 min,

659

O/S=5, Mcatalyst/Voil=0.1 g/mL

660

661 662

Fig. 9. Influence of reaction time on the sulfur elimination of real diesel at T=45°C, O/S=8 mol/mol and

663

Mcatalyst/Voil= 0.1 g/mL

664 665

Fig. 10. Influence of Mcatalyst/Voil rario on ODS of real diesel at T=45°C, t=90 min and O/S= 8 mol/mol)


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666 667

Fig. 11. Influence of reaction temperature on ODS efficiency of real diesel at Mcatalyst/Voil=0.1 g/mL, O/S= 8

668

mol/mol and t=90 min

669 670

Fig. 12. Influence of H2O2/sulfur molar ratio on sulfur removal of real diesel at T=45°C, Mcatalyst/Voil =0.1 g/mL and

671

t=90 min


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

672 673

Fig. 13. GC-PFPD analysis of the real diesel before and after ODS under the optimal reaction conditions of T=45°C,

674

O/S= 8 mol/mol, Mcatalyst/Voil =0.1 g/mL and t=90 min

675 676 677


Page 42 of 45

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678 679

Fig. 14. Recycling potential of the catalyst in ODS of real diesel (T=45°C, t=90 min, O/S=8 mol/mol,

680

Mcatalyst/Voil=0.1 g/mL)

681 682

683 684

Fig. 15. Recycling potential of the catalyst in ODS of DBT at reaction conditions mentioned in Fig. 12


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

685 686

Fig. 16. The pseudo-first-order kinetic for ODS of real diesel at T=45°C using MoO3/SG catalyst

687

688 689

Fig. 17. GC-MS of the oil and catalyst phase after ODS of DBT using MoO3(5wt.%)/SG


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690 691

Scheme 1. The overall mechanism of ODS system.

692


Silica Gel as

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