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Mesoporous titania–silica–polyoxometalate

Cite this:Catal.Sci.Technol.,2013,3,1985

nanocomposite materials for catalytic oxidation desulfurization of fuel oil

Xue-Min Yan,*a Ping Mei,a Lin Xiong,a Lin Gao,a Qifeng Yang a and Lunjun Gong*b

A series of mesoporous titania–silica–polyoxometalate (HPW–TiO 2–SiO 2)nanocomposite materials with di?erent silica–titania ratios were prepared in the presence of non-ionic surfactant by evaporation induced self-assembly method.XRD,TEM,nitrogen adsorption–desorption isotherm and FTIR measurements indicated these materials possess mesoporous structure with relatively uniform channel-like pores and large internal BET surface areas.The incorporated polyoxometalate clusters preserve intact their Keggin structure into the mesoporous frameworks.The nanocomposites were used as catalysts for oxidative desulfurization of model fuel,which was composed of dibenzothiophene (DBT)and hydrocarbon,while H 2O 2is used as oxidant.The catalytic oxidation results show that the catalysts are very active in refractory bulky molecule organosulfur compounds in fuel oil.The mesoporous

HPW–TiO 2–SiO 2also shows high selectivity for DBT oxidation in the presence of benzene and achieved the goal of desulfurization.The surface acid strength and acidic sites were characterized by NH 3–TPD and Py-FT-IR.It revealed that Lewis acidic sites play an important role in the removal of organic sulfur compounds from the DBT–petroleum ether–benzene system.After regeneration of used catalysts three times,the activity of catalysts has not obviously decreased.

1.Introduction

Deep desulfurization of transportation fuels has become an important research subject due to the increasingly stringent regulations and fuel specifications (S content o 10ppm)in many countries for environmental protection purposes.1The current method of desulfurization of fuel oil is hydrodesulfurization (HDS).Although the HDS process is highly e?ective in removing thiols,sulfides and disulfides,it is di?cult to remove alkyl-substituted dibenzothiophenes like 4,6-dimethyldibenzothiophene,which are resistant to HDS due to steric hindrance.2–4In order to achieve low sulfur goals with current HDS technology,higher temperature,higher pressure,larger reactor volume,and more active catalysts are essential.In addition,the severe conditions lead to negative e?ects,such as the decrease of catalyst life,higher hydrogen consumption,and higher yield losses resulting in higher costs.5Owing to these defects of HDS technology,alternative

technologies have been investigated widely,among which oxidative desulfurization (ODS)is considered to be one of the promising new methods for super deep desulfurization of fuel oil.6,7In the ODS process,the refractory dibenzothiophene (DBT)and 4,6-dimethydibenzothiophene (4,6-DMDBT)are oxidized to their corresponding sulfones under mild conditions,which are subsequently removed by extraction,adsorption,distillation,or decomposition.8–12

Various oxidants were used in ODS such as nitric acid,13hydrogen peroxide,14,15molecular oxygen,16air,1tert -butyl-hydroperoxide,17K 2FeO 418and solid oxidizing agents.19Among these oxidants,H 2O 2was the most chosen oxidant as it only produces water as a byproduct.Together with H 2O 2,various catalysts have been used,such as acetic acid,20formic acid,21heteropolyacids,22,23ionic liquids 24and some solid catalysts including Ti molecular sieves,25,26WOx/ZrO 227and Mo/Al 2O 3.28Among these catalysts,heteropolyacids (HPAs)are of special interest because they have tremendous merits such as pseudo-liquid phase behaviour and tunable catalytic property.29How-ever,the disadvantages of bulk HPA catalysts are their low surface area (o 10m 2g à1)and high solubility in polar media,which lead to poor accessibility to the catalytic activity sites and make them di?cult to separate after reaction.To overcome

College of Chemistry and Environmental Engineering,Yangtze University,Jingzhou 434023,PR China.E-mail:yanzhangmm2002@https://www.wendangku.net/doc/6d18162537.html,;Fax:+867168060650;Tel:+867168060984b

Institute of Modern Physics,Faculty of Materials,Optoelectronics and Physics,Xiangtan University,Xiangtan 411105,China.E-mail:ljgong@https://www.wendangku.net/doc/6d18162537.html,

Received 30th October 2012,Accepted 29th March 2013DOI:10.1039/c3cy20732c

https://www.wendangku.net/doc/6d18162537.html,/catalysis

Catalysis

Science &Technology

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these drawbacks,HPA can be made into insoluble solid catalysts with high surface area by fixing them onto suitable supports,such as mesoporous SiO 2,30TiO 2,31ZrO 2,32Al2O3,33Ta 2O 5,34and activated carbon.35

In our previous study,30,31mesoporous HPW–SiO 2and HPW–TiO 2nanocomposites with various HPW contents were synthesized by evaporation-induced self-assembly method.It has been demonstrated that these mesoporous composites catalysts are e?ective for oxidation of the refractory sulfur compounds in fuel oil when using H 2O 2as the oxidant.However,the selectivity of these catalysts for sulfides in fuels is not high,and some unsaturated hydrocarbon components in the fuel are also oxidized,which impede the desulfurization e?ciency of catalysts.Therefore,the HPA-based catalysts with high selectivity are highly desirable in heterogeneous ODS system.In the present work,a series of mesoporous titania–silica–polyoxometalate nanocomposite materials with di?erent silica–titania ratios were prepared in the presence of non-ionic surfactant by evaporation induced self-assembly method.The nanocomposites were characterized by various analytical and spectroscopic techniques such as XRD,nitrogen adsorption–desorption isotherm,and FTIR.The surface acidity of composites was analyzed by NH 3–TPD and FTIR measurement of adsorbed pyridine;the catalytic activity and selectivity of the catalysts were system-atically evaluated in the desulfurization of model fuel.

2.Experimental

2.1.

Synthesis of mesoporous HPW–TiO 2–SiO 2

The block copolymer EO 20PO 70EO 20(Pluronic P123)was received from Aldrich Corporation.Tetraethoxysilane (TEOS),tetrabutyl titanate (Ti(C 4H 9O)4),12-phosphotungstic acid (H 3PW 12O 40án H 2O),hydrogen peroxide,acetonitrile,petroleum ether (90–1201C),benzene and ethanol were purchased from Shanghai Chemicals Corporation.Dibenzothiophene (DBT,99%)was received from Acros Organics.The mesoporous HPW–SiO 2–TiO 2were prepared by using P123as a structure-directing agent,TEOS as the silica precursor and tetrabutyl titanate Ti(C 4H 9O)4as the titania precursor.In a typical synthesis,an amount of Ti(C 4H 9O)4was dissolved in the mixture of 2.40g acetic acid and 20mL ethanol with stirring.The mixture was adjusted to pH 1–2by adding hydrochloric acid (HCl,36%),and then a certain amount of TEOS ethanol solution was added into the above mixture.Finally,an amount of 12-phosphotungstic acid was added slowly to form sol A.Appropriate P123was dissolved in ethanol to prepare solution B.After the sol A was magnetically stirred for about 2h at room temperature,the solution B was filled slowly and magnetically stirred for another 2h.The resulting sol solution with molar composition x Ti(C 4H 9O)4:y TEOS :(0.02x +0.016y )P123:4x HOAc :(60x +20y )C 2H 5OH :(20x +15y )HPW was poured into a Petri dish and maintained for 2days at 401C and then was transferred into a 601C oven and aged for another 2days.As-synthesized mesostructured composites were calcined at 4001C in air for 10h (ramp rate 11C min à1)to obtain mesoporous HPW–TiO 2–SiO 2with 20wt%HPW concentration.The four samples with x :y value of 2:1,1:1,1:2and 1:3were obtained and labeled as

HPW–TiO 2–SiO 2(2:1),HPW–TiO 2–SiO 2(1:1),HPW–TiO 2–SiO 2(1:2)and HPW–TiO 2–SiO 2(1:3),respectively.2.2.

Characterization of the catalysts

The obtained mesoporous HPW–TiO 2–SiO 2samples were first ground into fine powder for characterization.X-ray di?raction (XRD)patterns were recorded on a Rigaku D/MAX-RB di?racto-meter with a Cu Ka radiation operating at 40kV,50mA.Transmission electron microscopy (TEM)images and elemental microprobe analyses were taken with a transmission electron microscope (TEM,JEOL 2100)equipped with an Oxford instrument energy dispersive X-ray spectroscopy (EDS)system at the accelerating voltage of 200kV.Nitrogen adsorption–desorption data were measured with a Quantachrome Autosorb-1analyzer at 77K.The samples were outgassed at 2001C for 6h prior to the measurements.The surface area was calculated by the Brunauer–Emmett–Teller (BET)method.The pore-size distribu-tion was derived from the adsorption branches of the isotherms by using the Barrett–Joyner–Halenda (BJH)method.FTIR spectra (1400–600cm à1)were recorded on a Digilab-FTS60spectrometer.The samples were pressed with KBr in the ratio of 1:150.The HPW content in solid samples was determined by the results of inductively coupled plasma analysis (ICP,Perkin-Elmer 3300DV).

NH 3–TPD experiments were conducted on Autochem 2910chemical adsorption instrument (Micromeritics).The samples were heated to 673K in flowing N 2(30mL min à1)for 1h and then cooled to room temperature.Adsorption of ammonia was carried out at 323K to saturation.And then ammonia was replaced with N 2at 373K for 1.5h to remove the physical adsorption ammonia,followed by the desorption of ammonia with increasing temperature from 323to 973K at a rate of 10K min à1.The acidity of catalysts was analyzed by FTIR measurement of adsorbed pyridine using the same IR spectrometer with a 4cm à1resolution.The samples were pressed into a self-supporting wafer,followed by evacuation at 623K for 0.5h.After cooled to 333K,pyridine was adsorbed on the samples until saturation.Subse-quently,the samples were outgassed for 0.5h at di?erent temperatures,and the spectra were recorded.2.3.

Desulfurization of model fuel

The experiments of oxidative desulfurization were performed in a 150mL flask equipped with a stirrer and a condenser.Two kinds of model fuels with sulfur content of 1000m g g à1were obtained by dissolving DBT in petroleum ether and petroleum ether +benzene (with volume ratio of 3:1),respectively.In the typical run,the water bath was firstly heated and then stabilized to a certain temperature.0.2g of catalyst was added to a mixture of 20mL of model fuel and 20mL of acetonitrile.Then a certain amount of 30%aqueous H 2O 2was added to start the reaction.The resulting mixture was stirred for 2h at the reaction temperature.The catalyst was centrifuged o?,the oil phase was analyzed by WK-2C total sulfur analyzer and a gas chromatograph equipped with a pulsed flame photometric detector (PFPD,agilent-6890,capillary column HP-5).Infrared spectroscopy analyses of DBT solution before and after oxidation

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were carried out by using FT/IR spectrometer (Nicolet 6700).The catalyst was washed with toluene several times,heated at 573K for 4h and reused in the next run.

3.Results and discussion

3.1

Characterization of the catalysts

Fig.1shows small-angle and wide-angle powder XRD patterns of mesoporous HPW–TiO 2–SiO 2samples with di?erent Ti–Si molar ratios.As is observed in Fig.1a,the samples present a well-resolved di?raction peak at 2y =1–31,typical for mesos-tructured materials.Only one XRD peak in the low angle range indicates that no long-range order pore arrangement exists in the samples.With the increase of the value of the Ti–Si molar ratio,the intensity of the di?raction peak becomes weak.When the Ti–Si molar ratio reaches 2:1,significant shrinkage of the di?raction peak is observed,which suggests the destruction of

the ordered mesostructure.The wide-angle XRD patterns of the mesoporous HPW–TiO 2–SiO 2samples are presented in Fig.1b.The wider di?raction peak at 251usually belongs to the amorphous SiO 2characteristic peak.However,with the increase of the value of the Ti–Si molar ratio,the intensity of the di?raction peak has been strengthened.The possible reason is that the broadened (101)peak of anatase appeared in the same position,which suggests the existence of nano-crystalline anatase in mesostructure framework due to the increasing Ti content.No evident peak for HPW crystalline phases is found on wide-angle XRD patterns of these samples,which indicates that HPW clusters are successfully dispersed in the mesoporous framework rather than existing as free solid acid.The HPW contents of mesoporous HPW–TiO 2–SiO 2samples in the final products detected by ICP are listed in Table 1.The determined loadings of HPW are close to the expected values,indicating that the losses of HPW are insignificant in our prepared process.

Fig.2a displays a typical TEM image obtained from meso-porous HPW–TiO 2–SiO 2(1:3)sample.A typical mesostructure with a wormhole framework can be observed in the HPW–TiO 2–SiO 2(1:3)sample,which is consistent with the low-angle XRD result.To ascertain the presence of HPW within the inorganic frameworks,we also determined the elemental composition of the local structure of HPW–TiO 2–SiO 2(1:3)sample by TEM-EDS spectra analysis.The TEM-EDS spectra (Fig.2b)acquired across a thin area of the mesoporous structures indicate the atomic percents of Si,Ti and W are 30.12,10.2and 2.60,respectively.These ratios correspond to a weight percentage of HPW loading at 19.2for HPW–TiO 2–SiO 2(1:3)sample,which is consistent very well with the ICP result.This suggests that the HPW clusters are mainly located within the frame-works and not aggregated as a separated phase.

FT-IR spectra of HPW and mesoporous HPW–TiO 2–SiO 2samples are shown in Fig.3,It has been widely reported that HPW with Keggin structures shows several strong,typical IR bands at ca.1079cm à1(stretching frequency of P–O in the central PO 4tetrahedron),983cm à1(terminal bands for W Q O in the exterior WO 6octahedron),889cm à1and 805cm à1(bands for the W–O b –W and W–O c –W bridge,respectively).36,37The peaks of mesoporous HPW–TiO 2–SiO 2samples appearing in the range from 1100to 700cm à1are ascribed to the stretch vibrations of P–O,W Q O,and W–O–W bonds of the Keggin units,indicating that the primary Keggin structures of these polyoxotungstates remain intact after the formation of the composites.It also can be found that the W–O b –W mode

shifts

Fig.1Small-angle (A)and wide-angle (B)XRD patterns of mesoporous HPW–TiO 2–SiO 2samples with various Ti–Si molar ratios:(a)2:1,(b)1:1,(c)1:2and (d)1:3.

Table 1

The HPW content and structural parameters of di?erent catalysts

Samples

HPW content

Pore size (nm)S BET (m 2g à1)Pore volume (cm 3g à1)Gel (wt%)Product (wt%)HPW–TiO 2–SiO 2(1:3)2019.1 3.73700.32HPW–TiO 2–SiO 2(1:2)2019.2 3.73340.26HPW–TiO 2–SiO 2(1:1)2018.9 3.72850.25HPW–TiO 2–SiO 2

(2:1)

19.1

3.7

254

0.26

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from 889cm à1

to 895cm à1

,and the W–O c –W mode shifts from 805cm à1to 817cm à1for HPW–TiO 2–SiO 2samples.This can be explained as an e?ect of chemical interactions between the HPW anion and the mesostructure framework.38

Fig.4shows the N 2adsorption isotherms of the mesoporous HPW–TiO 2–SiO 2samples with various Ti–Si molar ratios.All samples display type IV isotherms with H2hysteresis loops,which are related to the mesoporous structure.H2hysteresis loops are observed for materials with relatively uniform channel-like pores,when the desorption branch happens to be located at relative pressure around P /P 0=0.4.BJH pore size distribution plots are shown in Fig. 5.The samples exhibit uniform pore size distributions with the mean pore size of about 3.7nm.The BET surface area and pore volume of the mesoporous HPW–TiO 2–SiO 2samples are summarized in Table 1.The BET surface area and pore volume of HPW–TiO 2–SiO 2(1:3)sample are 370m 2g à1and 0.32cm 3g à1;the corresponding values of HPW–TiO 2–SiO 2(1:2),

HPW–TiO 2–SiO 2(1:1)and HPW–TiO 2–SiO 2(2:1)samples are 334m 2g à1and 0.26cm 3g à1,285m 2g à1and 0.25cm 3g à1,and 254m 2g à1and 0.26cm 3g à1,respectively.With the increase of Ti content in samples,the BET surface area and pore volume have slightly decreased,resulting from the oligomerization of Ti species or the presence of very small TiO 2nanostructure.

Ammonia TPD characterization is a well-known method for determination of surface acid strength of solid heterogeneous catalysts.In the NH 3–TPD curves,peaks are generally distributed into two regions :below and above 4001C,referred to as low-temperature (LT)and high-temperature (HT)regions,respectively.39The peaks in the HT region can be attributed

to the desorption of NH 3from strong Bro

¨nsted and Lewis type acid sites,and the peak in the LT region is assigned as the desorption of NH 3from some relatively weak acid sites.40NH 3–TPD profiles of HPW–TiO 2–SiO 2catalysts with di?erent Ti–Si molar ratios are shown in Fig.6.Catalysts exhibited a broad and asymmetric ammonia desorption peak in the range of 180–5001

Fig.2Typical TEM images (A)and TEM-EDS spectra (B)of mesoporous HPW–TiO 2–SiO 2(1:3)sample.The copper peaks result from the TEM Cu

grid.

Fig.3IR spectra of various samples:(a)HPW,(b)HPW–TiO 2–SiO 2(2:1),(c)HPW–TiO 2–SiO 2(1:1),(d)HPW–TiO 2–SiO 2(1:2),and (e)HPW–TiO 2–SiO 2(1:

3).

Fig.4The N 2adsorption–desorption isotherms of di?erent samples.

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corresponding to the occurrence of di?erent acid sites.How-ever,the amount of NH 3desorbed on HPW–TiO 2–SiO 2samples with di?erent Ti–Si molar ratios are di?erent.The amount of NH 3desorbed on the samples has increased with the decrease of Ti content.It indicates that more acidic sites occurred in the catalyst with lower Ti content and the weak acid sites are predominant.

FT-IR measure of adsorbed pyridine has been performed as further determination for the nature of acidity on the surface of HPW–TiO 2–SiO 2samples.Py-FTIR spectra of samples were measured in the region of 1700–1400cm à1and are shown in Fig.7.According to the literature,41the bands at 1540and 1639cm à1correspond to the characteristic of the protonated

pyridine for Bro ¨nsted acid sites.The bands at 1447,1575and 1610cm à1

connected to pyridine molecules are coordinated with Lewis acid sites.Further,pyridine co-adsorbed on both Bro

¨nsted acid sites and Lewis acid sites give rise to a band at 1490cm à1.Fig.7clearly shows that all the catalysts exhibit a

larger number of Lewis acid sites than the Bro

¨nsted acid sites.The number of Lewis acid sites in these samples should be proportional to the increase of the number of exposed atoms,since silica shows only weak Lewis acidity.Consequently,the high intensity of the band due to pyridine adsorbed on Lewis acid sites in the samples results from coordinately unsaturated Ti 4+.The Py-FTIR results show that more Lewis acid sites exist in the sample with Ti–Si molar ratio of 1:3compared to other samples.With the increase of Ti content,the coordinately unsaturated Ti 4+tends to decrease due to the oligomerization of Ti species to form very small TiO 2nanostructures,which has

been observed from the results of XRD.The surface Bro

¨nsted acid sites in these samples are associated with tungsten species

(W 6+–OH).The intensity of the bands representing Bro

¨nsted acidity has increased with the decrease of Ti content in the range of Ti–Si molar ratio from 1:1to 1:3.It means that the higher Ti content in the samples led to a decrease in the acidity of the supported HPW.This suggests a change in the HPW structure,brought about by the dispersion and/or the stronger interaction of HPW with the supports.423.2.

Desulfurization over mesoporous HPW–TiO 2–SiO 2

The activity of the catalysts was evaluated in the ODS of model fuel of DBT–petroleum ether system containing 1000m g g à1S at the selected conditions of oxidation temperature:601C,oxidation time:120min and O/S molar:2:1.In this paper,acetonitrile has been added in each run.The experiment results are shown in Table 2.When the catalysts and hydrogen peroxide are all absent,a small reduction (20%)of sulfur content was observed since the organic sulfur compounds in model fuel were extracted with acetonitrile.In the presence of catalyst and acetonitrile but without hydrogen peroxide,the sulfur content reduced by above 55.1%,which could be attributed to the adsorption capacity of catalysts.The adsorption performance of catalysts increased with the decrease of Ti content in mesoporous HPW–TiO 2–SiO 2samples.There are two reasons for the reduced

adsorption

Fig.5The BJH pore size distribution curves of di?erent

samples.

Fig.6NH 3–TPD profiles of di?erent catalysts:(a)HPW–TiO 2–SiO 2(2:1),(b)HPW–TiO 2–SiO 2(1:1),(c)HPW–TiO 2–SiO 2(1:2),and (d)HPW–TiO 2–SiO 2(1:

3).

Fig.7FTIR spectra of pyridine adsorbed on di?erent catalysts:(a)HPW–TiO 2–SiO 2(1:3),(b)HPW–TiO 2–SiO 2(1:2),and (c)HPW–TiO 2–SiO 2(1:1).

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capacity of catalysts with higher Ti content.On the one hand,the higher Ti content resulted in the destroyed mesostructure of the sample,and then led to the lower BET surface area.On the other hand,according to the surface acidity results of di?erent sam-ples,the higher Ti content in the samples led to a decrease in the surface acidity of catalysts.It is well known that the DBT series compounds can display some Lewis basic character due to the available free lone electron pairs of these compounds.Therefore,more adsorption active sites could be provided by the catalyst with higher acidity.The prepared catalysts exhibit remarkably high activities for the removal of sulfur compounds by mild oxidation in the present of hydrogen peroxide.The content of sulfur decreases from 1000to 15m g g à1on using HPW–TiO 2–SiO 2(2:1)catalyst after 120min of reaction,while the sulfur compound can be eliminated completely by using HPW–TiO 2–SiO 2(1:2)and HPW–TiO 2–SiO 2(1:3)catalysts in the same conditions.The higher desulfurization e?ciency of HPW–TiO 2–SiO 2catalysts could be attributed to the mesopore size required for the bulky sulfur compounds,and high surface area of catalysts which allows more exposed active and adsorptive centers for reactants.In addition,the coordinately unsaturated Ti 4+existing in the catalysts are also considered responsible for the peroxo species activation.

In order to confirm the oxidation products of DBT,the DBT solutions before and after oxidation were firstly extracted by acetonitrile,and then the acetonitrile phases were analyzed by FTIR measurement after the solvents have volatilized.Fig.8shows the IR spectra of DBT before and after oxidation.The specific infrared absorption of sulfone (1165cm à1,1289cm à1)43was observed in the IR spectrum of DBT after oxidation (Fig.8b),which indicated that sulfone was formed by oxidation.Therefore,in the ODS process,the refractory DBT are oxidized to their corres-ponding sulfones under mild conditions,which are subsequently removed by extraction due to its higher polarity.

Although the mesoporous HPW–TiO 2–SiO 2has shown e?cient catalytic activity in the DBT–petroleum ether system,

the selectivity of the mesoporous HPW–TiO 2–SiO 2for sulfides should be investigated for the reason that some components of the fuel,such as alkenes and aromatic hydrocarbons,can also be oxidized,which hinders the activation of sulfur compounds.The selective desulfurization capacity of mesoporous HPW–TiO 2was carried out in the DBT–petroleum ether–benzene system.The adsorption capacity of catalysts were detected in the absence of hydrogen peroxide.As is shown in Table 2,the adsorption capacity of catalysts towards DBT increases with the decrease of Ti content in mesoporous HPW–TiO 2–SiO 2sam-ples.The di?erence of adsorption capacity of di?erent catalysts could also be attributed to the di?erent BET surface area and surface acidity in samples.Among these catalysts,the HPW–TiO 2–SiO 2(1:3)catalyst shows the highest desulfurization e?ciency in the present of hydrogen peroxide.The removal of sulfur has reached 96%with HPW–TiO 2–SiO 2(1:3)catalyst in the DBT–petroleum ether–benzene system.In our previous

Table 2

The adsorption performance and catalytic activity of di?erent catalysts in model fuels a

System

Catalyst

Oxidant S content (m g g à1)

Total desulfurization rate (%)Feed Oil phase DBT–petroleum ether

HPW–TiO 2–SiO 2(1:3)—100032767.3H 2O 210000100HPW–TiO 2–SiO 2(1:2)—100034165.9H 2O 210000100HPW–TiO 2–SiO 2(1:1)—100039560.5H 2O 21000499.6HPW–TiO 2–SiO 2(2:1)

—100044955.1H 2O 210001598.5DBT–petroleum ether–benzene

HPW–TiO 2–SiO 2(1:3)—100047452.6H 2O 210004096HPW–TiO 2–SiO 2(1:2)—100051548.5H 2O 210005694.4HPW–TiO 2–SiO 2(1:1)—100056843.2H 2O 210007292.8HPW–TiO 2–SiO 2(2:1)

—100059440.6H 2O 2

1000

90.6

Operating parameters—reaction time:2h;reaction temperature:343K;O/S :12;catalysts amount:0.2

Fig.8IR spectra of DBT before (a)and after (b)oxidation.

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study,44the DBT removal was only 85%after 120min of desulfurization of the DBT–petroleum ether–benzene system when using mesoporous HPW–SiO 2catalyst.The higher desul-furization rate of HPW–TiO 2–SiO 2(1:3)catalyst could be attributed to the abundant surface Lewis acid sites.DBT,as Lewis basic character compound,is more basic than benzene.Hence,the sulfur compounds can be preferentially adsorbed on catalyst surface according to a Lewis-type acid–base interaction.The adsorption of DBT on the catalyst surface can increase the collision probability of DBT molecule and the catalytic active sites,which leads to the selective oxidation of DBT.The observed trend for the removal of sulfur with di?erent catalysts is HPW–TiO 2–SiO 2(1:3)4HPW–TiO 2–SiO 2(1:2)4HPW–TiO 2–SiO 2(1:1)4HPW–TiO 2–SiO 2(2:1),which is in agree-ment with the sequence of the amount of Lewis acidity sites.Therefore,Lewis acidity sites are responsible for the selective oxidation of the DBT.

The recycling of the catalyst in the oxidation of DBT was also investigated in DBT–petroleum ether–benzene system with mesoporous HPW–TiO 2–SiO 2(1:3)as catalyst and the results are shown in Fig.9.After each run,the catalyst was separated by filtration,washed with toluene,heated at 573K for 4h,and then placed into a fresh reagent mixture.The removal of sulfur for the three runs was 96,94.6and 93.5%,respectively,indicating that the catalyst was stable under operating conditions.

4.Conclusions

In conclusion,mesoporous HPW–TiO 2–SiO 2composites were successfully prepared by evaporation induced self-assembly method.The HPW clusters are well dispersed in the mesoporous framework and the Keggin structure of HPW is preserved in the formed composite.These composites show excellent catalytic activity and selectivity towards DBT in oxidation of DBT in model fuel.The high surface area,su?cient pore size and surface Lewis

acidity sites are the reasons for its high selectivity and catalytic activity.In addition,the mesoporous HPW–TiO 2–SiO 2catalyst shows excellent reusing ability,which makes it a promising catalyst in oxidative desulfurization process.

Acknowledgements

The authors are grateful for the financial support provided by Chinese National Science Foundation (21106008)and Nature Science Foundation of Hubei Province (2011CDB007).

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