Mohammad Ali Rezvani *,Sahar KhandanNegin SabahiHamid Saeidian

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

2 Department of Chemistry,Payam Noor University,Zanjan,Iran

Keywords:Oxidative desulfurization Sandwich-type polyoxometalate β-Cyclodextrin Heterogeneous catalyst

ABSTRACT In this work,in order to obtain deep clean gas oil,a novel organic-inorganic hybrid(n-C4H9)4N)7H5Si2W18Cd4O68@β-cyclodextrin(abbreviated as TBA-SiWCd@β-CD)composite was synthesized by supporting quaternary ammonium salt of sandwich-type polysilicotungstate on β-cyclodextrin(TBA-SiWCd@β-CD)as an efficient catalyst for oxidative desulfurization(ODS)of gas oil.The successful composition of the materials explained by the formation of host-guest inclusion complex,which confirmed through FTIR,UV-vis,XRD,SEM,and EDX characterization analyses.Experimental results revealed that the levels of sulfur content and mercaptan compounds of gas oil lowered with 97%removal efficiency.Compared with the ODS treatment of gas oil,the TBA-SiWCd@β-CD composite showed an outstanding catalytic performance for the oxidation of dibenzothiophene(DBT)in the prepared model fuel.The main factors that influence the desulfurization efficiency and the kinetic study of the ODS process were investigated.The prepared heterogeneous catalyst was found to give remarkable reusability for five runs without a discernible decrease in its activity.This study suggested the potential application of the TBA-SiWCd@β-CD catalyst for removal of hazardous sulfur compounds from gas oil fuel.

1.Introduction

Fossil fuels,including coal,oil and to a lesser extent gas,naturally contain sulfur in both organic and inorganic form.Over the last years,production of transportation fuel with substantially lowered sulfur content has been an area of major concern in the refinery industry.The combustion of hydrocarbon fuels containing sulfur emits sulfur oxides into the atmosphere.All of these toxic gases do not only affect air pollution but also endanger human health,plant and animal life due to acid rain and global warming[1-3].In addition,sulfur compounds in fossil-derived fuels poison the catalysts in the emission control system and limit their activity in various aspects[4].As a consequence,the many topics of researches deal with desulfurization of liquid fuels recently[5-7].The conventional hydrodesulphurization(HDS)method shows high efficiency in elimination of aliphatic and acyclic sulfur compounds(e.g.thiols,sulfides,and disulfides)in the petroleum refinery.However,it is less effective in treating refractory alkylated organic sulfur compounds like dibenzothiophene,and its derivatives caused by the steric hindrance of these compounds[5,6].Mentioned the HDS is carried out over Co-Mo/Al2O3or Ni-Mo/Al2O3catalyst at elevated operating temperatures(300-340 °C)and high hydrogen pressures(2-10 MPa)in middle-distillate fuels[8,9].Also,the polyaromatic sulfur molecules such as benzothiophene(BT),dibenzothiophene(DBT)and their alkyl-substituted derivatives have low reactivity in hydrogenation treatments[10].In light of the sustainable chemistry,oxidative desulfurization(ODS)is considered the promising alternative strategy to gain the desired low levels of sulfur in fuels under mild conditions.As an interesting fact,ODS is capable to easily remove of refractory sulfur compounds[11,12].High electron density on the sulfur atom renders aromatic sulfides more susceptible to the electrophilic attack of the oxidizing agent and the oxidation to their corresponding sulfones[13,14].

Polyoxometalates(POMs)are polynuclear metal-oxygen clusters based on early,high valent transition metals(typically molybdenum,tungsten or vanadium)[15].POMs have attracted the considerable attention of researchers in catalysis due to their fascinating structures and excellent physic-chemical characteristics[16].Among their various structures,sandwich-type transition metals POMs have been developed as catalysts for the oxidation of alcohols,sulfides,and for the epoxidation of hydrocarbons[17].The clusters of POMs are soluble in most of the polar solvents,causing difficulties in the separation and recovery,which is an important restriction for catalytic systems[18].To confront this limitation,many approaches were proposed to architect the POMbased heterogeneous catalysts.A feasible strategy is loading metal-oxo anions on the surface of the appropriate supports.Thus,it is the substantial factor to choose the type of supports[19].

Cyclodextrins(CDs)are a family of cyclic oligosaccharides composed of-(1,4)linked glucopyranose units[20].The shapes of CDs molecules are toroidal or cone-like with a hydrophobic central cavity and a hydrophilic outer surface.The most unique property of CDs is the ability to admit a variety of guest molecules into their cavity such as alcohols,acids,small inorganic anions,aromatic and aliphatic hydrocarbons[21,22].β-type of CDs are the attractive class of supramolecular compounds because of the high availability,biocompatibility,low cost and catalytic features[23,24].Owing to their favorable properties,it seems that β-CDs are suitable carriers for loading of the POMs and enhancement of their catalytic activity.

In consideration for application of the organic-inorganic hybrid composites,in this work,the TBA-SiWCd@β-CD inclusion complex was synthesized and found to be a high-performance catalyst for the ODS process.The oxidation reactions were performed on the model fuel and typically gas oil.The sulfur-containing molecules were oxidized using in situ production of peroxyacetic acid(HOOAc)in the presence of TBA-SiWCd@β-CD catalyst and then extracted by acetonitrile(MeCN)solvent.The effect of the nature of substrates,types of catalyst,types of oxidation system,dosage of catalyst,reaction temperature and time on the ODS efficiency evaluated.Also,the kinetics and mechanism of the oxidation reactions and the recyclability of the prepared catalyst in the designed ODS system(TBA-SiWCd@β-CD/H2O2/HOAc/MeCN)further elucidated.

2.Experimental

2.1.Materials and methods

All reagents and solvents used in this work are available commercially and were used as received unless otherwise indicated.Benzothiophene(BT),dibenzothiophene(DBT),4-methyldibenzothiophene(4-MDBT),4,6-dimethyldibenzothiophene(4,6-DMDBT),n-heptane,hydrogen peroxide(H2O2,30 vol%),acetic acid(HOAc),acetonitrile(MeCN),sodium tungstate dihydrate(Na2WO4?2H2O),sodium metasilicate(Na2SiO3),beta-cyclodextrin(β-CD),cadmium nitrate tetrahydrate(Cd(NO3)2?4H2O),and tetrabutylammonium bromide(TBAB)were purchased from Sigma-Aldrich without purification.The compound of A-β-Na9[β-SiW9O34]?23H2O(abbreviated as A-β-SiW9)was prepared according to the method reported in the literature[25].Typical gas oil was used with the following specification:density of 0.8355 g·ml-1at 15°C and total sulfur content of 0.9856 wt%.

The Fourier transform infrared(FTIR)spectra of the samples were recorded on a Thermo-Nicolet-iS10 spectrometer,using KBr disks in the scanning wavelength range of 400-4000 cm-1.Ultraviolet-visible(UV-vis)spectra were measured with a double beam Thermo-Heylos spectrometer in the range of 200-500 nm.The crystal structure of the materials was determined by X-ray diffractometry(XRD)using a Bruker D8 Advance powder X-ray diffractometer with CuKα(λ=0.15406 nm)radiation in the 2θ range from 5°to 60°.The surface morphologies were examined by scanning electron microscope(SEM)by LEO 1455 VP equipped with an energy dispersive X-ray(EDX)spectroscopy apparatus.The content of total sulfur of gas oil and model fuel was determined using X-ray fluorescence(XRF)with a TANAKA X-ray fluorescence spectrometer RX-360 SH.

2.2.Preparation of catalyst

2.2.1.Preparation of K12[Si2W18Cd4(H2O)2O68]

A-β-SiW9(0.50 g)was added to a solution which contained Cd(NO3)2?4H2O(0.15 g)in 25 ml of distilled water with vigorous stirring.The solution was heated at 50°C for 3 h and then cooled to room temperature.An excess amount of potassium chloride(0.75 g)was added to the solution,and the mixture was stirred for 15 min and filtered.The filtrate K12[Si2W18Cd4(H2O)2O68](abbreviated as KSiWCd)was recrystallized at 60°C and dried under vacuum.

2.2.2.Preparation of[(n-C4H9)4N)]7H5[Si2W18Cd4(H2O)2O68]

The KSiWCd(0.20 g)was dissolved in 55 ml of distilled water at 50°C under stirring condition.A solution of TBAB(0.10 g)in 5 ml of distilled water was added drop-wise to the solution of KSiWCd.The mixture was magnetically stirred at 60 °C for 3 h to form a white precipitate.The obtained precipitate[(n-C4H9)4N)]7H5[Si2W18Cd4(H2O)2O68](abbreviated as TBA-SiWCd)was filtered,then washed thoroughly with acetonitrile and ether,and air dried.

2.2.3.Preparation of organic-inorganic[(n-C4H9)4N)]7H5[Si2W18Cd4(H2O)2O68]@β-cyclodextrin composite

In a typical procedure,the solid TBA-SiWCd(0.05 g)was dissolved in 20 ml of distilled water at 60°C.This solution was added drop-wise to the solution of β-CD(0.10 g)in 30 ml of distilled water under magnetic stirring.Subsequently,the resulting mixture was maintained at 60°C under stirring for 45 min to form viscose solution.The prepared solution was dried at 80°C for 4 h.The remained white solid[(n-C4H9)4N)]7H5[Si2W18Cd4(H2O)2O68]@β-cyclodextrin(abbreviated as TBA-SiWCd@β-CD)was washed several times with distilled water to remove excess β-CDs.

2.3.Catalytic activity tests

2.3.1.Oxidative desulfurization of model fuel

Appropriate amounts of the thiophenic compounds(TCs)such as BT,DBT,4-MDBT,and 4,6-DMDBT were dissolved in n-heptane to prepare the different model fuels.The initial concentration of each TC in the model fuels was 500 ppmw(parts per million by weight).The ODS process was carried out in a 100-ml round-bottom flask coupled in a temperature-controlled water bath.In a typical procedure,the temperature of the water bath was fixed at 60°C.The prepared model fuel(50 ml)in the closed flask was mixed with H2O2/HOAc(6 ml)in a volume ratio of 1/1 and TBA-SiWCd@β-CD catalyst(0.10 g).The reaction proceeded under vigorous stirring(500 r·min-1)at 60°C for 2 h.Afterwards,the mixture was cooled to room temperature,followed by addition of MeCN(10 ml)to extract the oxidation products.A formed immiscible mixture of n-heptane and water was separated by a separation funnel.The upper liquid phase(oil phase)was analyzed using the X-ray fluorescence spectrometer according to D-4294 and D-3227 ASTM.The TCs removal efficiency(%)was calculated using Eq.(1),in which Tiis the initial concentration,and Tfis the final concentration of TCs after oxidation treatment.

2.3.2.Oxidative desulfurization of real gas oil

Desulfurization of gas oil was processed in the same manner as described above.After heating the water bath,real gas oil(50 ml)was added to the round-bottom flask,and during the experiment,the temperature was maintained constant at 60 °C.The solution of H2O2/HOAc(6 ml)and TBA-SiWCd@β-CD catalyst(0.10 g)were added to the flask under stirring condition(500 r·min-1)for 2 h.After the completion of the reaction,the mixture was cooled to room temperature.Now,MeCN(10 ml)was added to extract the oxidized sulfur compounds from the oil phase.The total sulfur and mercaptan content of gas oil before and after the ODS treatment were determined by using X-ray fluorescence.The ODS efficiency(%)was expressed by the following Eq.(2),where Siand Sfare the concentrations of total sulfur content in gas oil before and after treatment,respectively.

3.Results and Discussion

3.1.Characterization of the materials

FT-IR spectroscopic analysis was carried out to demonstrate the successful preparation of the TBA-SiWCd@β-CD catalyst.Fig.1 displays the spectra of(a)TBA-SiWCd,(b)β-CD,and(c)TBA-SiWCd@β-CD composite.In the spectrum of sandwich-type TBA-SiWCd,the characteristic peaks at around 700-1100 cm-1are attributed to the absorption of the metal-oxygen stretching modes(Fig.1(a)).The vibrational frequencies at 716,902,949,and 1084 cm-1are revealed the stretching modes of W--Oc--W(edge-sharing octahedral),W--Ob--W(corner-sharing octahedral),Si--Oa,and W--Od(terminal),respectively.The Keggin structure of H4SiW12O40(denoted as SiW12)displays characteristic bonds at 785,880,930,and 1020 cm-1[26].From the comparison,it can be concluded that the stretching vibration frequency of Si--Oain both the sandwich-and Keggin-type polyanions is almost the same.This similarity indicates that the tetrahedral symmetry of the SiO4group is retained in the structure of[Si2W18Cd4(H2O)2O68]12-anions[27].Further,the stretching vibrations of TBA-SiWCd have slightly shifted in comparison with SiW12.This phenomenon can be explained in terms of the change of the polyanions symmetry caused by the substitution of cadmium atoms,which also results in the high negative charge density of anions[28].Also,it has been reported that the peaks around 500 and 1400 cm-1are the characteristic bands of Cd--O[29].The bands at 1616 and 3453 cm-1are assigned to the H--O--H bending and O--H stretching vibration,respectively,which are related to a large amount of water molecules in the TBA-SiWCd structure.The absorption bands at 2810 and 2997 cm-1are ascribed to the symmetric and asymmetric stretching modes of--CH2group in the hydrocarbon chains[30].The spectrum of bulk β-CD is shown in Fig.1(b).The peaks at 577 and 608 cm-1corresponded to the vibration modes of alkyl groups in long alkyl chains and the skeleton vibration of CD,respectively.The stretching vibrations of the C--O--C band at 1026,1155,and 1424 cm-1are observed due to glucose linkages in β-CD structure.The absorption band at 1648 cm-1is associated with the bending vibrations of H--O--H,while the peak at 3391 cm-1can identify the stretching vibrational modes of O--H belonging to different hydroxyl groups(primary,secondary,and H--O--H).The peak at 2928 cm-1is also evident the C--H stretching band[31,32].Fig.1(c)illustrates the absorption bands of TBA-SiWCd@β-CD catalyst.It is clear that the characteristic peak of TBA-SiWCd cannot be detected in the spectra of the catalyst and the FT-IR signature of β-CD is still present.These results indicating that TBA-SiWCd is covalently attached to the β-CD via the reaction of the oxygen functional groups of polyoxoanions with the hydroxyl groups of β-CD[33].Moreover,the blue shift in the peak position of C--O--C at 1157 cm-1proves the successful encapsulation of TBA-SiWCd and the formation of the host-guest inclusion complex[34,35].

Fig.1.FT-IR spectra of(a)TBA-SiWCd,(b)β-CD,and(c)TBA-SiWCd@β-CD.

The UV-vis spectra of the materials were recorded in aqueous solution.As shown in Fig.2(a),the two unique characteristics peaks of TBA-SiWCd at 220 and 235 nm are mainly accounted for the charge transfer(CT)bands of terminal oxygen(Od→W)and bridge oxygen to tungsten(W--Ob/c→W),respectively[26].The shoulder peaks at about 340-360 nm are considered as CT transition of oxygen to cadmium(O→Cd)[36].The identifiable absorption band for β-CD is observed around 270-290 nm in Fig.2(b)[37].The absorption peaks of TBASiWCd@β-CD catalyst are shown at 250 and 365 nm,which indicated that the structure of the polyoxoanion is retained after incorporation with β-CD molecules(Fig.2(c and d)).Also,the maximum absorption wavelength of the inclusion complex is shifted,which confirmed the interactions between materials and the results of FT-IR studies.

Fig.2.UV-vis spectra of(a)TBA-SiWCd,(b)β-CD,and(c,d)TBA-SiWCd@β-CD.

The materials were characterized by XRD technique in the scanning range 5°≤2θ≤60°.As shown in Fig.3(a),the XRD pattern of TBA-SiWCd exhibited the unique sharp and narrow diffraction peaks at 2θ values of 5.5°,9°-11.7°,15.8°-20°,and 24°-29.7°.The crystalline nature of β-CD presented characteristic peaks at 6.2°,8.9°,12.4°,and 27.9°,which revealed that the CDs represented a cage-type structure(Fig.3(b))[32].The X-ray diffraction profiles of TBA-SiWCd@β-CD indicated the appearance of new peaks,disappearance of some peaks,and also reduction in intensity of some more peaks corresponding to the individual constituents(Fig.3(c)).The results suggest the formation of TBA-SiWCd@β-CD inclusion complexes as well as reported by Gao et al.[38].Besides,the peaks at 12.5° and 18.8° are evidence of the transformation of CDs from the cage-to channel-type structure[39,40].

Fig.3.XRD patterns of(a)TBA-SiWCd,(b)β-CD,and(c)TBA-SiWCd@β-CD.

SEM images were acquired to elucidate the microstructure and morphologies of the β-CD and TBA-SiWCd@β-CD composite.The image of bulk β-CD gives crystalline irregular block structures with a size of several micrometers(Fig.4(a)).In case of TBA-SiWCd@β-CD,the non-uniform surface morphology and the agglomerated particles are observed(Fig.4(b)).Comparing Fig.4(c)and(d),the mapping micrographs detected by EDX analysis identified an accumulation of silicon(Si)and tungsten(W)elements in the TBA-SiWCd@β-CD composite.The SEM-EDX results confirmed successful incorporation of the polyoxoanion clusters and β-CD molecules,which are in good agreement with the results of XRD studies.

3.2.Oxidative desulfurization results of gas oil and model fuel

The catalytic activity of TBA-SiWCd@β-CD composite was assessed by the removal of sulfur compounds from gas oil using H2O2/HOAc as the oxidizing agent.The attained results after oxidation treatments were reported in Table 1.The total sulfur content of was reduced drastically from 0.9856 wt%to 0.0306 wt%at 60°C after 2 h.Also,the concentration of the mercaptan compounds was lowered from 287 to 9 ppm.It was worth noting that the other specifications of gas oil remained unchanged.After the first ODS run,the heterogeneous catalyst was recovered and reused in the next oxidation reaction under the same reaction conditions.The removal efficiency of total sulfur content was still remained 97%,revealing that the use of TBA-SiWCd@β-CD/H2O2/HOAc/MeCN system can be a promising strategy for implementing and sustaining efficient ODS treatments.

Fig.4.SEM image of(a)β-CD and(b)TBA-SiWCd@β-CD.EDX results of(c)β-CD and(d)TBA-SiWCd@β-CD.

Table 1 ODS of gas oil by TBA-SiWCd@β-CD catalyst

3.3.Effect of the nature of the substrates

Desulfurization of model fuel was investigated under the same reaction condition in order to examine the performance of TBASiWCd@β-CD on the removal of each TCs.As shown in Fig.5,the model sulfur-containing compounds consisting of BT,DBT,4-MDBT,and 4,6-DMDBT were removed from n-heptane with 95,98,96,and 97%efficiency,respectively.The oxidation reactivity of the refractory TCs decreased was in the order of DBT＞4,6-DMDBT＞4-MDBT＞BT.Besides,the partial electron charge on the sulfur atom of these compounds was reported as DBT(5.758),4,6-DMDBT(5.760),4-MDBT(5.759),and BT(5.739)[41].The lowest reactivity of BT in the ODS treatments resulted from its low electron density.While the reactivity of DBT and 4,6-DMDBT were governed by the steric hindrance of the attached CH3groups due to the insignificance difference in their electron densities[42].Moreover,it was interesting to note that the removal of 4-MDBT was higher than that of BT substrate.The reason could be ascribed to the electronic donation effect of the alkyl group,which enhanced the electron density of sulfur atom in 4-MDBT[43].The results indicated that the sulfur removal efficiency of TCs was affected by both their electron density and steric hindrance.The results attended in this work correspond to that reported for a POM/H2O2/carboxylic acid system[42,44].DBT was selected as a refractory thiophenic sulfur compound representative for the following oxidation experiments.

Fig.5.Effect of the nature of the substrates on sulfur removal efficiency using 0.10 g of TBA-SiWCd@β-CD catalyst.

3.4.Effect of different catalysts

In order to investigate the effect of different catalysts on the removal of TCs and total sulfur content of gas oil,a series of oxidation reactions were taken,and the comparative results were listed in Table 2.The results clearly showed that in the absence of the catalyst(blank experiment)the removal efficiency of BT,DBT,4-MDBT,and 4,6-DMDBT was found 18%,20%,18%,and 19%,respectively.Additionally,17%of the sulfur content of gas oil was removed.The pure β-CD did not show significant catalytic performance compared with and TBASiWCd.It is indicated that the polyoxoanions play an essential role in the enhancement of desulfurization efficiency because of the formation of peroxometalate species.Further,the high sulfur removal yield of the TBA-SiWCd@β-CD inclusion complex exhibited a higher activity than unsupported TBA-SiWCd clusters.It is also interesting to compare the catalytic activity of the different type heteropolyoxoanions for the oxidation of various organic sulfur substrates.In this work,their catalytic activity was followed in order of[Si2W18Cd4(H2O)2O68]12-＞[SiW12O40]4-＞[P5W30O110]14-＞[P2W18O62]6-.It was reflected that the sandwich-type polyoxometalate could significantly promote the desulfurization rate in the ODS system[41-44].

Table 2 Effect of different catalysts on the ODS of different sulfur compounds①

3.5.Effect of catalyst dosage

The optimum dosage of the TBA-SiWCd@β-CD catalyst was also determined for the oxidation of sulfur content of gas oil and the oxidation of TCs present in model fuel containing 500 ppmw of DBT(Fig.6).It could be deduced that the desulfurization efficiency increased with increasing catalyst dosage,although no significant change was observed when the catalyst dosage was increased from 0.10 to 0.12 g.No increase in the ODS efficiency at high dosage might be attributable to the accumulation of the accelerated peroxometalate species that hinders effective sulfide adsorption and oxidation.The optimum catalyst dosage(0.10 g)was selected as a basis for the forthcoming experiments.

Fig.6.Effect of TBA-SiWCd@β-CD catalyst dosage on removal efficiency of DBT and the sulfur content of gas oil.

3.6.Effect of reaction temperature and time

As shown in Fig.7,the catalytic activity of the TBA-SiWCd@β-CD in the oxidation of sulfur compounds at different temperatures 30,40,50,and 60°C were compared.The results showed that changing the temperature and time affect the rate of the ODS reactions.Despite the experimental data,when the reaction was carried out at 30°C,the desulfurization percent was approximately 60%.While,at the temperature of 60°C,the highest sulfur removal efficiency was obtained.The reaction time was the most important design parameter that affects the performance of ODS process.The removal efficiency of sulfur compounds at 60°C rapidly increased from 25%in the 10 min of reaction to 97%as the stirring was increased to 2 h.In presence of the catalyst,the removal efficiency of DBT and sulfur content of gas oil was achieved 98%and 97%,respectively,at 60°C after 2 h.

Fig.7.Effect of reaction temperature and time on removal efficiency of(a)DBT and(b)the sulfur content of gas oil.

3.7.Effect of oxidation system

Effect of the oxidation system on the removal of TCs and sulfur content of gas oil was investigated.In this regard,different organic and inorganic acids such as acetic acid,oxalic acid,benzoic acid,sulfuric acid,and carbonic acid were used to acidify the system.The volume ratio of H2O2/acid was 1/1 in each oxidation reaction.As reported in Table 3,a poor sulfur removal percentages were attributed to carbonic acid,while,the superior catalytic performance was achieved by acetic acid.It was concluded that the oxidation reactivity of organic acids was higher than inorganic acids due to the in situ production of per-acids as very powerful oxidizing agents and efficiently oxidation of organic sulfur compounds to sulfones without forming a substantial amount of residual product[45,46].

Table 3 Effect of different oxidation systems on the ODS of different sulfur compounds①

3.8.The kinetic studies

The kinetics of DBT and sulfur oxidation at various temperatures were examined using pseudo-first-order model.The reaction rate constant k was calculated by plotting the lnC/C0or C/C0against t as follows:

In the formula,C0and C are the initial and final sulfur concentrations,respectively.According to Fig.8 and Eq.(5),a linear relationship was observed between C/C0and t parameters and the correlation coefficients(R2)obtained close to unity(Table 4).It was indicated that the kinetic results fitted the pseudo-first-order kinetic model.Further,the affiliation of k on the reaction temperature could be express by the well-known Arrhenius equation in which,A,Ea,R,and T are the preexponential factor,the apparent activation energy,the universal gas constant,and the reaction temperature,respectively Eq.(6).The Eavalue was calculated using the plot of lnk versus 1/T(Fig.9).The Eavalues were 24.66 and 22.62 kJ·mol-1for oxidation of DBT and total sulfur content of gas oil,respectively.The calculated Eavalues are in good agreement with the reported literature for ODS of gas oil and DBT compounds[13].

3.9.Proposed mechanism pathway

On the basis the experimental results and discussion above,the elimination process of sulfur-containing compounds catalyzed by TBASiWCd@β-CD was proposed as follows:the H2O2reacts with CH3COOH to produce the in situ production of peroxyacetic acid(CH3COOOH)as an oxygen supplier.

Fig.8.Plots of C/C0against time for the ODS of(a)DBT and(b)the sulfur content of gas oil.

The interaction of CH3COOOH with(n-C4H9)4N)7H5Si2W18Cd4(H2O)2O68generates the peroxo-polyoxometalate intermediates species.The hydrophobic alkyl chain of(n-C4H9)4N)countercation helps to attract the weakly sulfide molecules closer to the intermediate phase via hydrophobic-hydrophobic interactions[47].At this point,the electrophilic active oxygen in peroxometalate species attacks the sulfur atom at a high electron density for oxidation of substrates[48].The reaction products(sulfoxides and sulfones)accumulate in the water phase due to their nature of polarity,which is of benefit to separate by polar extraction solvent such as MeCN(Fig.10).

3.10.Recycling performance of heterogeneous catalyst

The need to implement green chemistry principles is a driving force towards the design and development of recyclable catalysts.The recycling process was carried out under optimized conditions for the desulfurization of DBT as the substrate by the heterogeneous TBASiWCd@β-CD catalyst.At the end of the oxidation reaction,the catalyst was separated by simple filtration as a low cost and simple recoverymethod,washed with dichloromethane,and then dried at 90 °C for 1 h.The recovered catalyst was processed for the subsequent reaction using the fresh oxidant(H2O2/HOAc)and extraction solvent(MeCN).The results were noted in Table 5.The desulfurization efficiency of DBT is only slightly lower(about 3.1%)than the fresh catalyst for the five times of reuse.The decrease in the catalytic activity of TBASiWCd@β-CD maybe attributed to the adsorption of reaction products(DBTO and DBTO2molecules)on the surface of the catalyst,which inhibited the reaction rate of DBT.It means that the designed ODS system(TBA-SiWCd@β-CD/H2O2/HOAc/MeCN)had an excellent recyclability and prospect of industrial application.

Table 4 Pseudo-first-order rate constants and correlation factors of the ODS of sulfur content of gas oil and DBT

Fig.9.Arrhenius plots for the ODS of(a)DBT and(b)the sulfur content of gas oil.

Table 5 Recycling performance of TBA-SiWCd@β-CD catalyst for ODS of DBT①

Fig.10.Schematic illustration of the proposed ODS process of DBT as a sulfur-containing compound catalyzed by TBA-SiWCd@β-CD heterogeneous nanocatalyst.

4.Conclusions

In summary,the TBA-SiWCd@β-CD inclusion complex was designed as a catalyst for ODS of sulfur-containing model fuel and gas oil.The characterization techniques were confirmed that the successful incorporation of the synthesized materials.The oxidation experimental results demonstrated that the refractory aromatic sulfur-containing compounds could be efficiently removed at 60°C after 2 h.The catalyst showed excellent catalytic activity in the oxidation of DBT molecules with 98%efficiency.Oxidation reactivity of TCs decreased according to the following order:DBT＞4,6-DMDBT＞4-MDBT＞BT using H2O2/HOAc oxidizing system.The results indicated that the kinetics of sulfur oxidation fitted the pseudo-first-order kinetic model.The evaluation of reusability of the heterogeneous catalyst showed that the ODS system could be reused up to five runs conveniently.This work was introduced as a facile strategy for providing the ODS treatments to promote the quality of liquid fuels.