A novel synthesis of MnO2 , nanoflowers as an efficient heterogeneous catalyst for oxidative desulfurization of thiophenes

Nano-Structures & Nano-Objects 20 (2019) 100392

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A novel synthesis of MnO2 , nanoflowers as an efficient heterogeneous catalyst for oxidative desulfurization of thiophenes Mustafa A. Alheety a , Subhi A. Al-Jibori b , Ahmet Karadağ c,d , Hüseyin Akbaş c , ∗ Mukhtar H. Ahmed e , a

Department of Biology, Midle East University College, Baghadad, Iraq Department of Chemistry, College of Science, Tikrit University, Tikrit, Iraq c Department of Chemistry, College of Arts, Science, Gaziosmanpasa University, Tokat, Turkey d Biotechnology Department, College of Science, Bartın University, Bartın, Turkey e SISAF-Drug Delivery Nanotechnology, Ulster University, Belfast, UK b

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Article history: Received 19 March 2019 Received in revised form 2 September 2019 Accepted 19 September 2019 Keywords: [Mn2 (bit)4 (H2 O)2 ] complex MnO2 nanoflowers Oxidative desulfurization Tertiary-butylperoxide

a b s t r a c t [Mn2 (bit)4 (H2 O)2 ] complex was used in this study as a novel precursor for the synthesis of MnO2 nanoflowers. The inorganic precursor was synthesized from the reaction between chloride salt of Mn(II) and benzisothiazolinate. The chemical composition and the nanostructure of the prepared MnO2 catalyst was characterized using several identification techniques (XRD, Thermal gravimetry, FTIR, N2 physisorption, and SEM-EDX). The results showed that the new precursor is a suitable Mn source for the synthesis of highly crystalline MnO2 nanoflowers of a small particle size with a large surface area. Manganese dioxide nanoflowers were examined for its catalytic ability in the oxidative desulfurization process of a model and real diesel fuels. Oxidative desulfurization conditions have been found to influence the removal efficiency of thiophene, 4,6-dimethyldibenzothiophene and dibenzothiophene. The optimum desulfurization parameters were examined, and the results show that the amount of catalyst is 0.5 g, extraction solvent/ oil ratio is 6, reaction time is 35 min, reaction temperature is 65◦ C and oxidant: sulfur (O/S) mole ratio is 4. Under these optimum conditions, the MnO2 catalyst is capable of removing more than 95% of thiophenes from commercial and crude diesel, in the presence of tert-butyl hydroperoxide (TBHP) as an oxidant. The newly prepared catalyst is recyclable for seven sequential oxidative desulfurization runs without the requirement for reactivation steps. This catalytic reaction including TBHP as an effective oxidant was presented through a suggested mechanistic pathway. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Benzisothiazolinone (Hbit) is a polyfunctional heterocyclic ligand, containing sulfur, nitrogen as well as oxygen atoms that can coordinate to metal centers in various coordination modes (monodentate, bridging and chelating bidentate) [1,2] as shown in Fig. 1. There is only little-reported information regarding benzisothiazolinone complexes [1–3] with no information of Mn-bit complexes. Manganese dioxide is observed to be a good potential candidate for catalytic oxidative desulfurization owing to its low cost, high surface area and eco-friendly nature [4]. Various synthetic methods for this oxide have been reported, for instance hydrothermal [5,6], thermal degradation [7], sol–gel [8], electro-deposition [9] and microwave-assisted synthesis [10]. ∗ Corresponding author. E-mail address: [email protected] (M.H. Ahmed). https://doi.org/10.1016/j.nanoso.2019.100392 2352-507X/© 2019 Elsevier B.V. All rights reserved.

Fig. 1. Bonding modes of benzisothiazolinone anion to a metal cation.

Different morphologies concluded nanowires [11], nanorods [12], nanoflakes [13] and nanoflowers [14] have been synthesized. Despite the synthesis methods or morphologies, MnSO4 , KMnO4 or

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Fig. 2. FTIR spectrum of[Mn2 (bit)4 (H2 O)2 ].

Fig. 3. FTIR spectrum of MnO2 nanoflowers.

Mn(CH3 COO)2 have been used in all the previous works as manganese precursors from the viable chemical compound source. Upon our best information, the production of MnO2 as nanoflowers from its complex source has not been described yet. Sulfurcontaining compounds such as (H2 S, organic sulfides, mercaptans, disulfides, thiophine, and dibenzothiophene) are considered as undesirable oil pollutants, and a number of organo-sulfur compounds will be conveyed to diesel oil through the refining process [15–17]. Environmental regulations have been applied in numerous nation-states to decrease sulfur-containing compound levels in diesel and different fuels because it converted into SO2 as toxic emissions [18]. organosulfur elimination method still the main operational and economic challenges for the petroleum refining process [19]. Several solid catalysts have been used either alone or supported for the process of ODS such as heteropoly acid [20–22] or metal oxides, such as the oxides of; zinc, molybdenum, cobalt, manganese, iron [23–25]. The most common catalysts are used in removing organo-sulfur compounds especially that has low molecular weight (thiols, mercaptans, sulfides, disulfides) are Ni-Mo/Al2 O3 and Co-Mo/Al2 O3 via hydrodesulfurization process [26–28]. These catalysts worked at elevated temperature (over 300 ◦ C) and pressure (20–100 atm H2 ) while our catalyst was found to be effective in mild conditions. However, multiring aromatic compounds and alkyl-substituted derivatives of dibenzothiophene such as 4,6-dimethyldibenzothiophene are relatively inactive compared to hydrodesulfurization due to their highly steric effect, making deep desulfurization by the hydrodesulfurization process exceedingly difficult [29,30]. Therefore, removal of sulfur by catalytic oxidative desulfurization (oxidation of sulfur followed by solvent extraction) has gained a lot of attention and documentation in recent articles [31,32] and with our catalyst, the results show that the ODS is very promising for

releasing thiophene derivatives. Nowadays, the catalytic desulfurization is an excellent method used to remove all thiophene derivatives because it is easily oxidized to their corresponding sulfones in the presence of an oxidant at low pressure and temperature conditions. The catalysts that have molybdenumbased have been extensively used in previous works and found to be appropriate in Catalytic-ODS methods in the presence of hydrogen peroxide or organic-hydroperoxide. Mo/Al2 O3 and H2 O2 oxidant used to remove 97.8% of sulfur [33]. While the newly prepared catalyst removes over the 95%. In this study, we report the use of an inorganic compound as a precursor for nanomaterial (MnO2 nanoflowers). We also report the results of the deep elimination of organo-sulfur compounds (Th, DBT, 4,6-DMDBT) from model diesel oil via catalytic oxidative desulfurization under mild reaction conditions. The potential MnO2 catalyst with nanoflowers structures was additional explored using commercial and crude diesel. However, regarding unsupported manganese dioxide nanoflowers catalyst, there was no report done before. 2. Experimental section 2.1. Chemicals used All chemical compounds obtained from Sigma-Aldrich. MnCl2 .4H2 O [CAS Number: 13446-34-9], benzisothiazolinone (Hbit) [CAS Number 2634-33-5] were used for preparation of [Mn2 (bit)4 (H2 O)2 ]. KMnO4 [CAS Number 7722-64-7] and H2 SO4 [CAS Number 7664-93-9] were used for the synthesis of MnO2 nanoflowers. The oxidizing agent used in Cat-ODS was tert-butyl hydroperoxide (TBHP) (70 wt.% in H2 O) [CAS Number 75-912] and dimethyl-formamide (DMF) [CAS Number: 68-12-2] was

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used for the extraction of sulfones, n-heptane [CAS Number 14282-5] was used as a solvent for thiophenes (Th) [CAS Number 142-82-5], dibenzothiophene (DBT) [CAS Number: 132-65-0] and 4,6-dimethyldibenzothiophene (4,6-DMDBT) [CAS Number 120712-1]. Model diesel was made with 600 ppm of sulfur; 200 ppm for each of the following thiophenes Th, DBT and 4,6-DMDBT. 2.2. Instrumentations FT-IR spectrum of the complex was measured by SHIMADZU 8400S. All other FT-IR spectra were measured by (JASCO FT/IR430) spectrophotometer within the range of 400–4000 cm−1 . Melting points were measured using SMP40 melting point apparatus supplied by Stuart Company, UK. The elemental analysis (CHN) of the complexes was measured by EURO/EA 3000. Electronic study (UV–VIS) spectrum of the complex was recorded by Perkin Elmer Spectrophotometer at 25 ◦ C, over a spectral range between 200 nm and 1000 nm. Scanning electron microscopy (SEM) with Energy dispersive X-ray analysis (EDX) of MnO2 nanoflowers was measured using Oxford instruments SEM Tech. The Mn ions were determined using Inductively Coupled Plasma Optical-Emission Spectrometry (ICP-OES), Thermo ScientificTM iCAPTM 7000 Series ICP-OES, Cambridge, UK, operation condition: RF generator power, 1.150 W; coolant gas flow rate, 12 l min−1 ; auxiliary gas, 0.5 l min−1 ; pump rate, 50 rpm; nebulizer gas flow, 0.5 l min−1 ; nebulizer gas pressure, 230 kPa; wash time, 20 s; plasma view, Axial; number of measurements, 3. The XRD pattern of MnO2 nanoflowers was recorded with a Shimadzu X6000 instrument with Nickel–Cooper filter for the X-ray radiation (Cu Kα , λ = 1.5406 Å). Thermogravimetric analysis (TG) was achieved using the PerkinElmer, Inc., Shelton, CT, USA at 351000 ◦ C with dry N2 inert gas; pan, pt; Celsius per minute, 10. Gas Chromatography (Agilent 6890N GC) equipped with Flame Photometric Detector (GC-FPD) was used to determine the sulfur contents. At 250 ◦ C, helium gas (He) was used as a carrier and hydrogen (H2 ) as a fuel for FPD. 2.3. Synthesis of the Na[bit] A solution of Hbit (0.5 g, 3.306 mmol) in Ethanol (10 ml) was added drop wise under vigorous stirring to a solution of sodium hydroxide (0.1323 g, 3.306 mmol) in hot EtOH (15 ml) at 55– 60 ◦ C, A precipitate was observed immediately with each added drop. Thereafter, the mixture was stirred for further hour at 35 ◦ C. The resulting clear solution was evaporated on a steam bath to give a white solid. The white solid was collected and dried under vacuum (yield: 0.507 g, 96.3%), mp: 310–314 ◦ C [2]. 2.4. Synthesis of [Mn2 (bit)4 (H2 O)2 ] A solution of MnCl2 .4H2 O (0.330 g, 1.67 mmol) in distilled water (5 ml) was added to a colorless solution of Na[bit] (0.578 g, 3.34 mmol) in distilled water (10 ml). Off-white precipitate was formed directly, the mixture was stirred for 10 min at room temperature and refluxed on a steam bath for further 1 h to give an off-white precipitate. The mixture was filtered, washed with hot distilled water and dried in a vacuum oven (yield: 0.575 g, 92%). Anal. Calc. for C28 H20 Mn2 N4 O6 S4 : C, 45.04; H, 2.70; N, 7.50; S, 17.18%; Found: C, 45.20; H, 2.50; N, 7.33; S, 17.40%. Molar conductivity in DMSO (5.5 Ω−1 cm−1 mol−1 ), electronic spectrum (UV–Vis) 74,813 cm−1 Decompose at 340 ◦ C.

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2.5. Preparation of MnO2 nanoflowers catalyst Solid [Mn2 (bit)4 (H2 O)2 ] was grounded well in a mortar and then sieved using a 100-micrometer mesh. The sieved powder was immersed in deionized water (solid to liquid ratio 1:100) to reduce aggregation. The suspension solution (2.000 g, 2.678 mmol) was subsequently ultrasonicated in order to break particle agglomerates followed by the addition of (0.8464 g, 5.355 mmol) of KMnO4 in 300 ml deionized water at room temperature followed by 5 ml of H2 SO4 solution (1: 1 ratio of H2 SO4 to deionized water). The resulting mixture was stirred at 60 ◦ C for 6 h to form a brown gel-like mixture, which was filtered, washed 10 times with hot distilled water and directly transferred to a burning furnace and it was calcinated at different temperatures for different times at; 180 ◦ C for 60 min, 350 ◦ C for 30 min and 500 ◦ C for 30 min respectively to form a brown–black precipitate of MnO2 nanoflowers. 2.6. Catalytic experiments at optimal conditions The oil bath was first heated and stabilized at 60–62 ◦ C during a typical ODS run. To a twenty ml of simulated diesel or real diesel (commercial or crude ) respectively, TBHP (TBHP/S = 4) and catalyst dosage (0.500 g) were added and refluxed for 35 min under strong stirring (900 rpm). Thereafter the DMF to diesel ratio of 1 to 6 was used to extract the resulting sulfones from oxidized diesel fuel. The extraction mixture was stirred for 35 min and the diesel was separated from DMF in a separating funnel [34]. The sulfur contents after the reaction were monitored using Gas Chromatography [35]. A 50 m × 0.25 mm i.d ×0.25 µm film thickness HP-1 capillary column was used for separation. Without any dilution, 1.0 ml of the sample volume was injected in GC-FPD to determine the sulfur concentration in the studied samples. The concentration of sulfur contents that were removed by Cat-ODS was calculated using the following equation: (Co − Ct)

∗ 100 Co Where; initial overall sulfur concentration in the diesel (Co), the total concentration of sulfur in the processed diesel after t (time) minutes reaction (Ct). Following the catalytic reaction on the diesel the FTIR spectrum has also been obtained. Y =

3. Results and discussion 3.1. Characterization of [Mn2 (bit)4 (H2 O)2 ] Addition of one equivalent of an aqueous solution of manganese chloride tetrahydrate to an aqueous solution of sodium benzisothiaolate resulted in the formation of [Mn2 (bit)4 (H2 O)2 ]. The elemental analysis supports the replacement of chloride ions from MnCl2 . The low molar conductivity confirmed the non– ionic character of the [Mn2 (bit)4 (H2 O)2 ]. All attempts to get crystals of the [Mn2 (bit)4 (H2 O)2 ] suitable for the X-ray diffraction study were unsuccessful. The FTIR spectrum in Fig. 2, of the [Mn2 (bit)4 (H2 O)2 ] shows absorption peaks at 3601, 3055 and 1614 cm−1 , which corresponds to the ν (OH) of H2 O, ν (==C–H) and ν (C==C), respectively [36]. The intense band at 1614 cm−1 corresponds to ν (C==O) [37] shifted to a lower frequency than that in the bit anion (1642 cm−1 ) by 28 cm−1 . The other shifted peak was at 1490 cm−1 attributes to ν (C–N) shifted to a lower frequency than that in the bit anion (1506 cm−1 ). The lower shifts of ν (C==O) and ν (C–N) indicate coordination of bit through the exocyclic oxygen atom of C==O and the endocyclic nitrogen atom of C–N groups [2,38]. The spectrum is similar to that reported of the corresponding Cu(II) complex [38], which has been

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Fig. 4. XRD diffractogram of MnO2 nanoflowers.

Fig. 6. EDX of MnO2 nanoflowers.

Fig. 5. SEM images of (a) 100 nm and (b) its magnification of MnO2 nanoflowers. Fig. 7. N2 adsorption–desorption isotherm of MnO2 nanoflowers.

considered by an X-ray diffraction study to be binuclear with four 1,2-Benzisothiazolin-3-one (bit) ligands bridging the two copper cations. Therefore, we suggest that the structure of the present Mn-bit complex to be similar to the Cu complex and has the following structure [Mn2 (bit)4 (H2 O)2 ]. Another improvement for this structure is tightly comes from the elemental analysis of this complex because we observed a correlation between practical and theoretical values and therefore the proposed structure exactly matches the actual structure. See Section 2.3. 3.2. Characterization of MnO2 nanoflowers catalyst The resulting MnO2 nanoflower was characterized by the following techniques: 3.2.1. FTIR study Fig. 3 displays the FTIR spectrum of MnO2 . The intense band at 535 cm−1 may well be attributed to the ν (Mn–O), demonstrating the existence of manganese-oxygen bond within manganese dioxide configuration. Whilst the mid intense absorption bands at 1640 cm−1 and 1100 cm−1 resemble to the bending vibrations of the O–H group (in absorbed water) joined with manganese atoms [39]. In addition, the more intense broad band at 3408 cm−1 attributes to the ν (O–H) of the hydroxyl group of the absorbed water molecules [40–42]. The fact of the existence of the water molecules within the MnO2 compound due to the high surface area of the nano-sized manganese dioxide. 3.2.2. XRD study To prove the type of manganese oxide formed, X-ray was used because it gives more characteristic signals than in IR. XRD pattern of MnO2 nanoflowers is shown in Fig. 4 are the major phases in this powder. The values 2θ of ∼11.4◦ , 24.0◦ , 37.2◦ for (0 0 1), (0 0 2), (1 0 0) and (1 1 0) characteristic planes are assigned to the layered structure of δ -MnO2 whereas the (1 0 0) plane indicates the crystallized water and the formation of water MnO2 interlayer [42]. The measurement proves the absence of

Fig. 8. TG diagram of MnO2 nanoflowers.

additional peaks which are associated with the presence of impurities. According to Debye–Scherrer equation [43], the particle size of the newly prepared MnO2 nanoflowers was determined to be 40 nm.

3.2.3. SEM study Mostly, the morphology cannot be determined by X-ray, therefore, this technique was used to improve the nanostructure because the nano shape is an important factor in such reactions. Typical SEM images of dried MnO2 nanoflowers synthesized from [Mn2 (bit)4 (H2 O)2 ] are presented in Fig. 5a,b. From the SEM micrograph, the estimated size of these flowers is determined to be 240 nm. The petals of the MnO2 nanoflower appear as a small sheet-like structure (thickness 5–7 nm, length 40–45 nm) [13,42, 44]. The sheet-like morphology would significantly increase the surface area, hence improving the catalytic effect and adsorption capacity of MnO2 .

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Fig. 9. Reaction time effect on total sulfur removal efficiency. Conditions for reaction: n-heptane containing (Th, DBT, 4,6-DMDBT), 0.45 g, O/S 4 mol ratio, 35 min, about 900 rpm at 60 ◦ C.

Fig. 10. Temperature (◦ C) effect on the overall sulfur removal efficiency. Conditions for reaction: n-Heptane (Th, DBT, 4,6-DMDBT) containing at different temperatures (30, 40, 50, 60, 70 and 80 ◦ C), catalyst amount 0, 43 g, O/S 4 mol ratio, 35 min and 900 rpm.

3.2.4. Energy dispersive X-ray analysis (EDX) This measurement (Fig. 6) was conducted on the MnO2 sample to determine the presence and the weight% of the Mn and O in it. The analysis showed the manganese peaks at 5.9 and 0.5 KeV with 62.1% of weight ratio whilst the oxygen peak at 0.51 KeV with 36.4% of weight ratio. It is obvious that the peaks position of the Mn and O are in excellent agreement with the result that published previously [44]. The wt.% of the manganese and oxygen contents in the sample are in agreement with the theoretical wt.% (Mn 63.19 and O 36.81). The carbon peak at 0.25 KeV could be attributed to the traces of carbon remnants after oxidation of benzisothiazolinone. 3.2.5. N2 adsorption–desorption isotherm and texture properties The Nitrogen adsorption–desorption isotherm for MnO2 sample is shown in Fig. 7. The Type IV is detected in this isotherm as in the IUPAC classification. The Type IV isotherm is typical of mesopores solids; involving of greater mesopores [45,46]. The textural properties of the manganese dioxide catalyst obtained by nitrogen adsorption–desorption isotherm. The BET surface area of catalyst was found to be 172 m2 /g and the pore volume 0.21 cm3 /g.

Fig. 11. Optimal O/S mol ratio study. Conditions for reaction: n-Heptane (Th, DBT, 4,6DMDBT), 0, 47 g, 60 ◦ C, 35 min, 900 rpm and O/S mol ratio of (2–6).

Fig. 12. Catalyst dosage effect on total sulfur removal efficiency. Conditions for reaction: n-Heptane (Th, DBT, 4,6DMDBT), O/S 4 mol ratio, 60 ◦ C, 35 min, 900 rpm and (0.1–0.6) g of catalyst.

Fig. 13. Study on catalyst reusability. Conditions for reaction: n-Heptane (Th, DBT, 4,6DMDBT), O/S 4 mol ratio, 0.5 g catalyst, 35 min and 900 rpm at 60 ◦ C.

nano MnO2 surface. This weight loss indicating that the H2 O content on the manganese dioxide nanoflowers was about 3.5%. The second main weight loss, of approximately 9% between 375 ◦ C and 1200 ◦ C was attributed to the evolution of oxygen through decomposition of MnO2 with reduction of manganese from MnO2 , Mn(IV) to Mn2 O3 Mn(III) form as shown in the following equation [41].

3.2.6. Thermal gravimetric analysis The thermal gravimetric curve (air atmosphere) for MnO2 nanoflowers (Fig. 8) showed two main losses in weight between 35–1000 ◦ C. The first minor wt.% loss of approximately 3.5% below 200 ◦ C ascribed to the loss of absorbed water on the

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Fig. 14. FTIR spectra of (black line) fresh (red line) spent catalyst. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 15. ODS series on real (commercial and crude) diesel with O/S 4 mol ratio, 0.5 g catalyst, 35 min and 900 rpm at 60 ◦ C (DMF volume ratio to diesel: 1.0 per batch).

3.3. Optimal condition studies for desulfurization in model diesel 3.3.1. Reaction time As shown in Fig. 9, 30–90 min reaction times were examined to the catalytic Th, DBT and 4,6-DMDBT removal efficiency. The data show that with an increase in reaction time the removal of sulfur-containing compounds increases. Besides, unimportant removal of thiophenes after 35 min of reaction time was observed. It is expected that the percentage of desulfurization will have little influence at prolonged reaction times of more than 50 min due to the degradation of most TBHP oxidants.

that the efficiency of sulfur removal increases when the O/S ratio increases. We started this experiment using the O/S ratio of 2, recording a 60% reduction in sulfur content and therefore could be considered a feasible method but to obtain a higher removal efficiency, the O/S ratio of 3 was used which caused an increase of approximately 20% when compared to the O/S ratio of 2. When O/S is equivalent to 4 molar ratios which is essentially greater than reaction stoichiometry, sulfur elimination was 89%. When the O/S molar ratio has been increased above 4, the oxidant content reduces the efficiency of sulfur removal and thereafter becomes constant. This is due to unproductive decomposition of TBHP to O2 and alcohol and this reduces the oxidation efficiency.

3.3.2. Reaction temperature The reaction temperature was studied in the series of 35–80 ◦ C and shown in Fig. 10. The increasing of reaction temperature from 30 to 60 ◦ C increased the removal efficiency of sulfur till it reached 89% at 60 ± 5 ◦ C. At 70 ± 2 ◦ C removal starts to decrease. The optimum temperature was 65 ◦ C as high temperatures initiate the decomposition of TBHP and then destroy the oxidation capability [47,48] and the evaporation of n-heptane at higher temperatures. We were able to control the loss of nheptane by proper condensation of vapors. The results revealed that increasing the temperature increases the interior energy between the reacting compounds and increases the rapid molecular movement that increases the efficiency of sulfur removal.

3.3.4. Effect of catalyst mass Different masses of catalyst of 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6 g were examined to conclude the ideal catalyst quantity required for the catalytic reaction as shown in Fig. 12. The efficiency of sulfur removal is significantly increased by increasing the amount of the catalyst and the maximum sulfur removal of 94% was noticed when the catalyst weight is 0.5 g. The addition of extra catalyst tended to reduce the efficiency of sulfur removal to around 83% at a weight of 0.6 g. The decrease in catalyst efficiency would be attributed directly to the accumulation and aggregation effects that reduce the number of active sites on the catalytic surface, thus reducing the effective surface area [39].

3.3.3. O/S ratio The effect of the used oxidizing agent (TBHP) on the desulfurization efficiency is shown in Fig. 11. The results demonstrate

3.3.5. Catalyst reusability and stability studies The catalyst reuse study under optimal reaction conditions was examined and the results are illustrated in Fig. 13. After

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Scheme 1. Suggest reaction mechanism for thiophene removal using TBHP as oxidant and MnO2 nano flowers as catalyst.

the optimal conditions (0.5 g, O/S 4 mol, 60 ◦ C, 900 rpm) were installed, the process of Cat-ODS was applied followed by a decantation process to recover the MnO2 nanoflowers catalyst. The decanted catalyst was then used for further desulfurization run after adding the oxidizing agent (TBHP) and the solution of thiophenes under the same optimal conditions. The present work has shown that the examined catalyst is able to reuse up to 7 cycles with an activity decrease of only 9%. The MnO2 catalyst stability in oxidative desulfurization process is improved by ICP-OES analysis of a liquescent specimen after each run which was then digested by concentrated HNO3 to convert manganese oxide into manganese ion. The ICP-OES results were used to determine the quantity of metal leakage and compared with the real quantity of Mn presented in the catalyst before the Cat-ODS reaction. After 7 cycles of reaction, only 0.05% of Mn was lost. FTIR analysis (Fig. 14) also confirmed that after Cat-ODS reaction, the characteristic peaks of the MnO2 catalyst remain unaffected. Specifically, in Fig. 13, the continued presence of the absorption bands at 998 and 1100 cm−1 becomes more intense due to organics coating of MnO2 [48,49].

3.3.6. Effect of DMF/diesel ratio DMF was selected for the extraction experiment. Toteva et al. [50] confirmed that DMF is the best low boiling solvent for diesel treatment. The effect of the diesel ratio to DMF on the desulfurization efficiency was examined and the results show that the 6 diesel:1DMF is the most economical and appropriate ratio. 3.4. The Cat-ODS treatment series on real diesel Fig. 15 displays the series of Cat-ODS on real diesel using MnO2 nanoflowers catalyst. The real crude and commercial diesel fuel oils contain 8000 ppm and about 550 ppm of sulfur respectively. After a runs of Catalyst-ODS reaction at optimum conditions (0.5 g, O/S 4 mol, 60 ◦ C, 900 rpm), the sulfur content levels of crude diesel and commercial diesel were achieved to the Euro III Standard and (approximately 10 ppm) respectively. These levels are in accordance with the International Standard Diesel Fuel (ISDF). In commercial diesel, approximately 81.9% of sulfur was removed following the first Catalyst-ODS process. A further second extraction was carried out to remove the remaining sulfur-containing

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compounds with another fresh DMF from the same commercial diesel sample. The second extraction reduced another 14.9% of sulfur, that indicated Catalyst-ODS was an effective production method for green diesel. Although the second Cat-ODS/5 extraction process was applied to commercial diesel sample produced by one oxidation process and two extraction processes, we did not notice a significant change in sulfur content (only 0.9%). In the case of crude diesel, up to five extraction cycles were treated with a double Cat-ODS process to reach the Euro III standard (< 350 ppm). This assignment, due to the presence of very high sulfur content in crude diesel with different sulfur types, was difficult to remove most of the sulfur compounds in the crude diesel sample. 3.5. Suggested Cat-ODS mechanism The nanoflower structured MnO2 is found to be effective as well as can be reused to remove three types of thiophenic compounds. Therefore, Scheme 1 proposes the catalytic mechanism that is feasible. The reaction is supposed to be initiated by a nucleophilic attack of TBHP on the Mn==O sites of nano-MnO2 catalyst surface, which produced a manganese monoperoxides (active intermediate). Nucleophilic attack of the exocyclic unpaired electron of S atom (in thiophenes) on the active intermediate is then carried out to form sulfoxide and regenerate manganese dioxide. The sulfoxide species is subsequently further oxidized by the use of manganese peroxide which converts it into the analogous sulfone and regenerates O==Mn==O for the close reaction cycle. The higher quantity of Mn, smaller particle size and larger surface area confirm that additional O==Mn==O sites are available. As a one site mechanism was illustrated in Scheme 1. 4. Conclusion In this research, we established that the [Mn2 (bit)4 (H2 O)2 ] complex is a promising material for the synthesis of MnO2 nanoflowers. This nanomaterial is excellent for removing of sulfur -containing compounds in model diesel and real diesel fuel samples. Further improvement on optimal sulfur removal conditions indicated that the oxidative desulfurization (ODS) of the diesel model was achieved by more than 95% by 0.5 g, 35 min, 60– 62 ◦ C, (O/S) 4 and 900 rpm. In contrast, the catalyst is found to be very stable and could be reused seven more times with little variation in activity. For the real samples under optimum conditions, the organo sulfur content in crude diesel achieved the Euro III Standard level while it reached 10 ppm in commercial diesel. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to acknowledge Bartin UniversityTurkey, Gaziosmanpasa University-Turkey and Tikrit University for the financial support. References [1] D.M. Griffith, A. Haughey, S. Chahal, H. Müller-Bunz, C.J. Marmion, Novel palladium(II) and platinum(II) complexes of biocidal benzisothiazolinone (Bit); X-ray crystal structures of co-crystallised Bit/BitO and cis-Pd(en)(Bit-1H)2 .H2 O, Inorg. Chim. Acta 363 (2010) 2333–2337.

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