Photocatalytic oxidative of Keggin-type polyoxometalate ionic liquid for enhanced extractive desulfurization in binary deep eutectic solvents

Linlan Wu,Zhengxin Jiao,Suhang Xun,Minqiang He,,Lei Fan,Chao Wang,Wenshu Yang,Wenshuai Zhu,,Huaming Li

1 School of Chemistry and Chemical Engineering,Institute for Energy Research,Jiangsu University,Zhenjiang 212013,China

2 School of Environment and Safety Engineering,Institute of Environmental Health and Ecological Security,Jiangsu University,Zhenjiang 212013,China

3 School of Chemistry and Chemical Engineering,Yangzhou University,Yangzhou 225002,China

Keywords:Deep eutectic solvents Extractive desulfurization Ionic liquid Photocatalytic oxidative Aerobic Fuel

ABSTRACT A series of novel binary deep eutectic solvents(DESs)composed of choline chloride(ChCl)and formic acid(HCOOH)with different molar ratios have been successfully synthesized and applied in extractive desulfurization (EDS).Keggin-type polyoxometallate ionic liquid [TTPh]3PW12O40 was prepared and used as catalyst to enhance the EDS capacity by means of photocatalytic oxidative process.Both of the DESs and[TTPh]3PW12O40 ionic liquid catalyst were characterized in detail by Fourier transform infrared spectroscopy spectra (FT-IR),elemental analysis,and X-ray photoelectron spectroscopy (XPS).It was found that the molar ratios of ChCl:HCOOH had a major impact on desulfurization performance,and the optimal desulfurization capacity 96.5% was obtained by ChCl/5HCOOH.Besides dibenzothiophene (DBT),the desulfurization efficiencies of 4-methylbenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene(4,6-DMDBT),two kinds of DBT derivatives,were also investigated under the same experimental conditions.Moreover,the free radical scavenging experiments manifested that superoxide radical (·O2-) and hole (h+) played important roles in the desulfurization system.After further analysis of the oxidation products by gas chromatography-mass spectrometry (GC–MS),the possible reaction mechanism was proposed.Thus,photocatalytic oxidative has been proved to be one of the efficient approaches for enhancing the extractive desulfurization performance in DES.

1.Introduction

For many years,fossil fuels such as coal,natural gas and oil have remained the main energy sources of human beings,accounting for over forty percent of the total energy [1].It is worth noting that there are many kinds of sulfur compounds impurities in fuel,which will release SOxafter combustion,causing acid rain,haze and other serious environmental pollution problems[2].Therefore,strict environmental regulations on sulfur content of fuel have been established in many countries and regions.Currently implemented standards for diesel and gasoline in our country require sulfur content to be limited less than 10 mg?kg-1,and ultra-low content or even sulfur free fuel standard might be put forward in the future[3,4].In order to meet the requirements of environmental protection regulations,desulfurization technology has become one of the hot topics in petrochemical industry [5].

Traditionally hydrodesulfurization (HDS) technology can efficiently remove mercaptan,thioether and disulfide [6],but for dibenzothiophene (DBT) and its derivatives,HDS faces a setback due to its low efficiency and the increasing operation cost [7].As a result,various of non-HDS technologies have attracted more and more attentions [8,9],such as adsorptive desulfurization(ADS) [10],extractive desulfurization (EDS) [11],biological desulfurization (BDS) [12],oxidative desulfurization (ODS) [13],etc.Among them,EDS is considered as one of the ideal desulfurization methods because of its mild conditions,does not change the quality of fuel,and no hydrogen consumption [14].However,EDS is usually difficult to remove the sulfur content to less than 10 mg?kg-1in oil,therefore,many assistive technologies have been used to achieve the purpose of deep desulfurization of fuel [15].Photocatalytic oxidative desulfurization (PODS) is essentially a kind of advanced oxidative technique composed of high efficiency catalyst and light source(ultraviolet or visible light),which can be carried out at ambient temperature and atmospheric pressure,as well as low operation cost and eco-friendly,is regarded as a promising auxiliary method [16].

The main oxidants used in ODS process are hydrogen peroxide(H2O2)[17,18],ozone(O3)[19],oxygen(O2)[20],tert-butyl hydrogen peroxide (TBHP) [21] and so on,among which O2is undoubtedly the most economical and green one [22].In order to activate O2and improve the photocatalytic oxidative desulfurization efficiency,multifarious semiconductor photocatalysts have been reported,such as CeO2/TiO2[23],CeO2/attapulgite/g-C3N4[24]and Pt-MTcPc/SnO2[25].In addition,the selection of extractants has an important influence on the desulfurization efficiency.Commonly used extractants include methanol,acetonitrile,ionic liquids (ILs),and so on [11].But,there are still some problems,such as excessive use of extractants,prolonged illumination,toxic and volatile extractants,etc.Therefore,the development of green solvents can eliminate the environmental impact of toxic organic solvents and achieve greener production processes [26].

Deep eutectic solvents (DESs) are a kind of eutectic mixture,which is regarded as ILs analogues because of the similar characteristics,and they were first proposed by Abbottet al.in 2003[27].As shown in Fig.S1 (see Supplementary Material ),DESs are consisted of two non-toxic substances,one of which is the hydrogen bond receptor HBA (quaternary ammonium salts,tetraalkylammonium or phosphonium salts) and the other is the hydrogen bond donor HBD (alkyd amine or carbohydrate) [28].As a kind of ideal green solvents,DESs have many advantages[28],for instance,low cost,simple to prepare,biodegradable,low melting point and vapor pressure,low volatility and designability.More importantly,DESs in desulfurization system are beneficial to enhance the contact reaction between catalyst and sulfur compounds,so as to effectively improve the desulfurization efficiency[29].At present,DESs have played an increasingly important role in desulfurization field [30].

In this work,a photocatalytic oxidative enhanced extractive desulfurization (POEDS) system is constructed.A series of binary DESs were successfully prepared and used as extractants by mixing different molar ratios of choline chloride (ChCl) and formic acid(HCOOH).A kind of Keggin-type polyoxometallate ionic liquid[TTPh]3PW12O40was synthesized and employed as photocatalyst,since polyoxometallates have been widely used as environmentally friendly catalysts in the fields of esterification[31],alkylation[32] and photocatalysis [33],due to the unique acid catalytic and redox catalytic properties.Desulfurization performances of different kinds and dosages of DESs were tested in detail,and the enhancement effect of photocatalytic oxidative on desulfurization efficiency was studied.Moreover,possible mechanism of the POEDS system was proposed based on the free radical scavenging experiments and determination of oxidation products.

2.Experimental

Materials and reagents used in this work and the characterization methods were revealed in Supplementary Material .All chemicals were used directly after purchased.

2.1.Preparation of DESs and [TTPh]3PW12O40

In a typical process,ChCl and HCOOH were mixed with a certain molar ratio in oil bath at 80 °C,and then stirred vigorously with a magnetic stirrer(800 r?min-1)for 4 h until two ingredients became homogeneous liquid phase.The molar ratios of HBA:HBD were 1:1,1:2,1:3,1:4 and 1:5,respectively.After reaction,the homogeneous liquid was cooled to room temperature naturally,marked as ChCl/xHCOOH,x=1,2,3,4 and 5.The solution remained in a homogeneous liquid state after cooling and prolonged placement that indicating the formation of DESs[27,34].Conversely,the crystallized or precipitated solvent was a heterogeneous mixture.And the results were listed in Table S1 in the Supplementary Material .

Table 1 Elemental analysis of C,H and N in DESs

[TTPh]3PW12O40ionic liquid catalyst was synthesized by facile ion exchange method.Trihexyl tetradecyl phosphine chloride([TTPh]Cl) and phosphotungstic acid (H3PW12O40) were dissolved in 50 ml anhydrous ethanol at the molar ratio of 3:1,respectively.Then,the two ethanol solutions were mixed under stirring at room temperature and white precipitate was generated.After stirring continuously for 4 h,the precipitate was gathered by filtering and washed by anhydrous ethanol till no chloride ions were detected by silver nitrate.Finally,the precipitate was dried at 100 °C overnight and ionic liquid catalyst was obtained,marked as [TTPh]3PW12O40.

2.2.Desulfurization experiment

Desulfurization experiments were carried out under the following steps.First,0.05 g[TTPh]3PW12O40catalyst,3 ml DES and 20 ml model oil were added into a home-made flask reactor in turn.A circulator bath instrument was used to control the reaction temperature at 30 °C and the solution in the flask reactor was stirred for 30 min in dark to establish extraction equilibrium.Next,200 μl iso-butyraldehyde (IBA) was added and air was blowed into the reaction system,and the photocatalytic oxidation enhancement process was started under the UV irradiation (a 250 W high pressure light source).Then,the upper oil phase was collected,centrifuged and tested by gas chromatography (Agilent 7890 A)every 15 min.

3.Results and Discussion

3.1.Structure characterizations

FT-IR analysis was taken to investigate the structure and compare the changes of ChCl and HCOOH in different kinds of DESs[35].As shown in Fig.1,DESs with different molar ratios of HBA:HBD provide similar peaks.In Fig.1(a),the peaks around 954 and 1479 cm-1are belonged to C-C bonds and the bending vibration in plane of C-H of methylene group,respectively.The peaks ranging from 1230 to 1030 cm-1belongs to the vibration of the C-N bond while the absorption peak around 1083 cm-1represents C-O stretching vibration.In Fig.1(b),the absorption peak around 1362 cm-1is assigned to the bending vibration of O-H in plane of formic acid.It can be found that the peak is disappeared or redshift(from 1362 cm-1to 1350 cm-1) in Fig.1(c)–(g),the as-prepared DESs,indicating the formation of hydrogen bond [36,37].In addition,the strong peak around 1717 cm-1in Fig.1(b) is attributed to the C=O bond in formic acid[38].However,the strong hydrogen bond between carboxylic acid transformed into the weak hydrogen bond of DESs,resulting in the blueshift of C=O(from 1717 cm-1to 1723 cm-1) [36].The main peaks of ChCl and HCOOH can be observed in the prepared DESs,indicating that the macromolecular structure of ChCl and HCOOH are maintained.

Fig.1.FT-IR spectra of the samples:(a)ChCl,(b)HCOOH,(c)ChCl/HCOOH,(d)ChCl/2HCOOH,(e) ChCl/3HCOOH,(f) ChCl/4HCOOH,(g) ChCl/5HCOOH.

Fig.2.FT-IR spectra of:(a) [TTPh]3PW12O40,(b) H3PW12O40.

Then,elemental analysis was employed to investigate the elemental composition of ChCl/xHCOOH and the results are listed in Table 1.The experiment values of C,H and N in DESs are not very difference from the theoretical values,indicating that no chemical reaction between ChCl and HCOOH and hydrogen bond is formed.After that,the elemental composition of [TTPh]3PW12O40catalyst was also characterized by elemental analysis in Table S2.The theoretical molar ratio of cation to anion is 3:1,and the experiment values of C and H are close to the theoretical ones.It is proved that[TTPh]3PW12O40catalyst contained three carbon chain cations,which is consistent with the A3B structure.

Fig.3.XPS spectra of (a) survey spectra and high-resolution XPS spectra of (b) W 4f,(c) O 1s.

Fig.4.Desulfurization performances of different DESs.Conditions: T=30 °C, m(catalyst)=0.05 g, VDES(ChCl/xHCOOH)=3 ml, t=90 min,UV, VIBA=200 μl,v(air)=300 ml?min-1, V(model oil)=20 ml.

The structural information of the prepared catalyst [TTPh]3-PW12O40was further explored by FT-IR analysis revealed in Fig.2.A series of peaks at 1080 cm-1,983 cm-1,891 cm-1,799 cm-1in Fig.2(b) are assigned to P-Oa,W=Od,W-Ob-W,W-Oc-W,respectively,the asymmetric stretching vibration bands of Keggin structure in H3PW12O40[39].All of the characteristic absorption peaks can be found in the FT-IR spectrum of [TTPh]3-PW12O40in Fig.2(a),indicating that the Keggin structure is well maintained.Additionally,the peaks around 2927 cm-1,2854 cm-1and 1409 cm-1observed in Fig.2(a) are ascribed to the quaternary phosphonium salt cation of the catalyst.It is thus confirmed that the successful preparation of the ionic liquid catalyst.

To further acquire the chemical composition and valence states of elements of [TTPh]3PW12O40,XPS analysis was taken.The wide survey spectrum in Fig.3(a) shows that there are obvious peaks of O 1s,C 1s,P 2p and W 4f in the as-prepared catalyst,meaning that the sample contains O,C,P and W elements,further indicating that H3PW12O40has reacted with [TTPh]Cl.In Fig.3(b),the highresolution XPS spectrum of W 4f,four peaks are observed.The main peaks of W 4f7/2and W 4f5/2are located at the binding energies of 36.3 and 38.4 eV,respectively,which are assigned to the W-O-W bonds[40].Meantime,the binding energies of W 4f indicate that the oxidation state of W is +6,which are belonged to[PW12O40]3-Keggin anions [41].In addition,the low binding energy peaks at 34.9 and 37.0 eV are corresponding to the oxidation state of W5+,this may be because of a small amount of electronic transfer from [TTPh]Cl to H3PW12O40[41,42].Furthermore,the peak of O 1s in Fig.3(c) can be deconvolved into two peaks,with the binding energies of 531.2 and 532.5 eV,which are separately ascribed to W-O-W and W-O-P,respectively [43,44].The results of XPS spectra well support the conclusion of FT-IR and elemental analysis characterizations.

3.2.Desulfurization performances of different DESs

The desulfurization performances of different DESs were evaluated in Fig.4.Under the same conditions,the desulfurization efficiency is only 33.4%when HBA:HBD=1:1.Then,it can be seen that with the increasing proportion of HBD,the removal of DBT enhanced gradually,meaning that the kind of DES is an important factor in the desulfurization system.The corresponding desulfurization efficiencies are 48.9%,70.8% and 85.9% when the molar ratios of HBA:HBD are set as 1:2,1:3 and 1:4,respectively.When the molar ratio of HBA:HBD=1:5,a maximum desulfurization efficiency of 96.5%is obtained.Herein,the subsequent experiments all use the DES prepared with this molar ratio.

Fig.5.Effect of DES dosage on desulfurization efficiency.Conditions: T=30 °C, m(catalyst)=0.05 g, t=90 min,UV, VIBA=200 μl, v(air)=300 ml?min-1, V(model oil)=20 ml.

3.3.Effect of DES dosage

Fig.5 exhibits the desulfurization performance with different dosages of DES(ChCl/5HCOOH)of the system under the same conditions.It is found that underV(DES)=1 ml and 2 ml for a reaction of 90 min,the desulfurization efficiencies increase continuously and reach 81.1% and 91.6%,respectively.By further increasing the dosage of DES to 3 ml,it exhibits the uppermost extractive capacity and the desulfurization rate reaches 96.5% within 90 min.The excellent desulfurization performance is attributed to the enhancement of DES dosage,manifesting that the amount of EDS plays an important role in sulfur removal.Therefore,3 ml DES is selected as the optimal value in the following investigations.

3.4.Enhanced performances of different reaction systems

Fig.6.Enhanced performances of different reaction systems.Conditions:T=30°C,m(catalyst)=0.05 g, VDES(ChCl/5HCOOH)=3 ml, t=90 min,UV, VIBA=200 μl,v(air)=300 ml?min-1, V(model oil)=20 ml.

Fig.7.Removal ability for DBT derivatives.Conditions:T=30°C,m(catalyst)=0.05 g,VDES(ChCl/5HCOOH)=3 ml, t=90 min,UV, VIBA=200 μl, v(air)=300 ml?min-1,V(model oil)=20 ml.

Fig.8.Effect of other composition on sulfur removal.Condition: T=30 °C, m(catalyst)=0.05 g, VDES(ChCl/5HCOOH)=3 ml, t=90 min,UV, VIBA=200 μl,v(air)=300 ml?min-1, V(model oil)=20 ml.

To investigate the enhanced performances of different reaction systems,a series of experiments were carried out in Fig.6.It can be found that the desulfurization efficiency is only 5.4% without the extractive of DES.With the help of photochemical oxidation and IBA in DES,the removal of DBT increased to 39.1%.Then,[TTPh]3-PW12O40is employed as catalyst,and a photocatalytic oxidation enhanced extractive desulfurization system is established.The desulfurization efficiency increased sharply to 96.5%and the residual content of DBT is only 0.7 mg?kg-1.Moreover,there is almost no desulfurization efficiency in the absence of IBA or UV irradiation,and the removals of DBT are only 2.6%and 5.2%,respectively.The above experimental results demonstrate that constructing a photocatalytic oxidation system is an efficient way to enhance the extractive desulfurization capacity in DES.

3.5.Desulfurization performances for different sulfur compounds

Fig.9.Radical scavengers under UV irradiation.Condition: T=30 °C, m(catalyst)=0.05 g, VDES(ChCl/5HCOOH)=3 mL, t=90 min,UV, VIBA=200 μL,v(air)=300 mL?min-1, V(model oil)=20 mL.

It is well known that there are many kinds of DBT derivatives in real oil,which require harsh reaction conditions to remove by HDS process [37].Here,4-Methylbenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) were selected as typical derivatives to explore the adaptability of the desulfurization system.As reported by Otsuki and Li [45,46],the electron densities of the sulfur atoms in 4,6-DMDBT,4-MDBT,and DBT were 5.760,5.759,and 5.758,respectively.The difference among them is so small that it has little effect on the efficiency of desulfurization.However,due to the steric hindrance effect of methyl groups,the removal of 4-MDBT and 4,6-DMDBT are more difficult than DBT[46,47],and the desulfurization rates decrease in the following order:DBT >4-MDBT >4,6-DMDBT.As shown in Fig.7,the removals of the three sulfur compounds in 90 min are 96.5%,91.0% and 78.8%,respectively.After prolonging the reaction to 180 min,the removal of 4,6-DMDBT can reach 90.7% in Fig.S2.

3.6.Effect of oil composition

In addition to DBT and its derivatives,there are also a certain amount of olefins and aromatic compounds in real oil.Hence,it is essential to further investigate the effect of oil composition on desulfurization efficiency of the system,and cyclohexene andpxylene were chose as representative compounds to examine their effects on DBT removal.As depicted in Fig.8,the desulfurization rate decreases from 96.5%to 69.9%with the addition of 1%(mass)p-xylene after a reaction of 90 min.What’s more,when 1% (mass)cyclohexene is added,the desulfurization rate decreased significantly to 17%,indicating that cyclohexene can suppress the removal of DBT more strongly thanp-xylene.This may be due to the fact that olefins are more susceptible to be oxidized in strong oxidization condition.Moreover,the C=C bond is easier to absorb UV light,which hinders the absorption of UV light of DBT [36].

Fig.10.GC–MS analysis of the DES phase after reaction.

Fig.11.Schematic depiction of the desulfurization process under UV irradiation.

3.7.Possible mechanism study

In order to reveal the mechanism of photocatalytic oxidative enhanced extractive desulfurization performance,a series of free radical scavenging experiments were performed.Generally,there are three kinds of active species including superoxide radicalhydroxyl radical (·OH) and hole (h+) that exist in the photocatalytic process [48].In this work,benzoquinone (BQ),isopropanol (IPA) and disodium ethylene diamine-tetraacetate(EDTA-2Na) were employed as the corresponding scavengers for the above active species,respectively [49].As displayed in Fig.9,compared with the experiment without any free radical scavenger,BQ and EDTA-2Na can significantly suppressed the photocatalytic reaction,and the desulfurization rates decrease to 15.7% and 28.3%,respectively.In addition,the addition of IPA also reduces the desulfurization efficiency from 96.5% to 62.1%.Therefore,it can be illustrated thatand h+are the main active species,while·OH also participated in the photocatalytic oxidative of DBT.

The determination of products is also of great help to understand the reaction mechanism,so that gas chromatography-mass spectrometry (GC–MS) analysis of the DES phase was carried and the result was shown in Fig.10.After the reaction,the DES phase was extracted by the tetrachloromethane prior to analysis.It can be seen that two main peaks be detected in the DES phase,which are belonged to DBTO(m/z=200)and DBTO2(m/z=216)[50].The results of GC–MS illustrate that DBT is oxidized into the corresponding sulfoxide (DBTO) and sulfone (DBTO2) after photocatalytic oxidation process.

Based on the above experimental results,a possible mechanism for the photocatalytic oxidation of[TTPh]3PW12O40ionic liquid for enhanced extractive desulfurization in DES is schematically illustrated in Fig.11.At the beginning of the reaction,DBT in oil phase is extracted into the DES (ChCl/5HCOOH) phase before reaching extraction equilibrium.Then,DBT is oxidized into DBTO and DBTO2byh+and ·OH in DES.Due to the enhanced polarity,the resulting oxidation products are retained in DES phase,which promoted the continued extraction of DBT from the oil phase into the DES phase until ultra-low sulfur content oil is obtained.That is to say,with the help of photocatalytic oxidation,DBT in oil phase can be continuously extracted into DES phase,and the extractive desulfurization capacity is enhanced significantly.The proposed mechanism has been widely accepted by previous studies [36,41].

4.Conclusions

In summary,a new class binary DES as extractant was investigated for extractive desulfurization.[TTPh]3PW12O40was prepared and used as catalyst for photocatalytic oxidation of DBT to enhance the extractive desulfurization capacity in DES.The ChCl/5HCOOH DES was proved to have the best extractive performance through adjusting the molar ratios of HBA and HBD.As a result,a photocatalytic oxidation for enhanced extractive desulfurization in DES system was constructed,and the desulfurization efficiency could reach 96.5%(residual 0.7 mg?kg-1DBT)in 90 min.The synthesized[TTPh]3PW12O40catalyst was analyzed by FT-IR,elemental analysis and XPS.The kinds of free radicals formed during the photocatalytic oxidation process and the products after oxidation were studied,and then,a possible reaction mechanism was proposed.The above study provides a new pathway for enhancing the extractive desulfurization capacity with green extractant.

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.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No.21808091),Natural Science Foundation of Jiangsu Province (Nos.BK20200896,BK20190243),Key Laboratory of Tropical Medicinal Plant Chemistry of Ministry of Education,Hainan Normal University (20150376),China Postdoctoral Foundation(No.2020M671365),and the Student Innovation and Entrepreneurship Training Program (202010299457X).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.04.031.