Journal of Colloid and Interface Science 335 (2009) 216–221

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Preparation and characterization of mesoporous VOx–TiO2 complex oxidesfor the selective oxidation of methanol to dimethoxymethane

Jingwei Liu, Qing Sun, Yuchuan Fu, Jianyi Shen *

Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China

a r t i c l e i n f o

Article history:Received 13 January 2009Accepted 10 March 2009Available online 2 April 2009

Keywords:Mesoporous VOx–TiO2

Dispersion of vanadiaSurface acidic and redox propertiesSelective oxidation of methanolSynthesis of dimethoxymethane

0021-9797/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.jcis.2009.03.027

* Corresponding author. Fax: +86 25 83594305.E-mail address: jyshen@nju.edu.cn (J. Shen).

a b s t r a c t

Mesoporous VOx–TiO2 with high surface areas were prepared using the procedure of evaporation-induced self-assembly combined with ammonia posttreatment. The samples were characterized by X-ray diffraction (XRD), laser Raman spectroscopy (LRS), transmission electron microscopy (TEM), N2

adsorption, temperature-programmed reduction (H2-TPR), microcalorimetry for the adsorption of NH3,and isopropanol probe reaction. Their catalytic activities were evaluated for the reaction of selective oxi-dation of methanol to dimethoxymethane (DMM). It was found that the VOx–TiO2 materials exhibitedhigh surface areas with pore diameters of 4 nm. The vanadia species were highly dispersed in theVOx–TiO2 within 30 wt% VOx content, evidenced by the results of XRD and LRS. The VOx–TiO2 samplesexhibited both surface acidic and redox properties. The surface acidity was further enhanced on the addi-tion of SO4

2�. The catalyst SO42�=30VOx—TiO2 exhibited good performance for the selective oxidation of

methanol (57% conversion) to DMM (83% selectivity) at 423 K.� 2009 Elsevier Inc. All rights reserved.

1. Introduction

Vanadium-based catalysts are extensively used for a large num-ber of oxidation reactions such as catalytic oxidation of methanolto formaldehyde [1] or methyl formate [2], ammoxidation of aro-matic hydrocarbons [3], selective catalytic reduction (SCR) of NOx

[4], and oxidative dehydrogenations [5]. The interaction of the sup-ports or other components with vanadium species can generallyinfluence both the dispersion and the structure of vanadium spe-cies, resulting in different chemical reactivities. Specifically, TiO2

is one of the most commonly used supports because of the remark-able activity of V2O5/TiO2 catalysts. Titania exists in three ordinarystructure polymorphs (anatase, rutile, and brookite), in which ru-tile is the most thermodynamically stable and anatase is the mosteffective titania used in catalysis. However, the specific surfaceareas as well as their thermal stability of these materials are usu-ally not satisfactory. Thus, efforts have been devoted to developTiO2 and VOx–TiO2 with high surface areas, particularly in the fieldof porous materials [6–12]. Ten years ago, Brinker et al. reported asynthetic approach called evaporation-induced self-assembly(EISA) [13]. This method allows the tuning of the inorganic conden-sation with the formation of meso-organized liquid template. Withthe evaporation of solvent from the original diluted solution, a li-quid crystalline mesophase is gradually formed. Yang et al. firstintroduced this method to prepare thermally stable mesoporousTiO2 with Pluronic triblock copolymers (P123) as structure-direct-

ll rights reserved.

ing agents [6]. Other poly(ethylene oxide)-based surfactants [8]and quaternary ammonium bromide [14] were also successfullyapplied as templates for TiO2 synthesis. For the EISA method, apostsynthesis treatment with gaseous NH3 was found to be of greatimportance in increasing the thermal stability of mesoporous tita-nia [9]. Cassiers et al. treated the as-prepared titania hybrid withan aqueous NH4OH solution and consequently prepared a ther-mally stable mesoporous titania calcined at 873 K [11,12]. Simi-larly, Segura and co-workers prepared a mesoporous TiO2 tosupport vanadia for the selective catalytic reduction of NOx [15].

Among the aforementioned catalytic reactions, the oxidation ofmethanol has been widely used as a probe reaction to characterizethe activity of oxide catalysts and to correlate the structures andsurface acidic and redox properties [16]. It has been reported thatmethanol could be converted to dimethyl ether (DME) on acidicsurfaces, to formaldehyde (FA) and methyl formate (MF) on oxida-tive surfaces, and to dimethoxymethane (DMM) on acidic and oxi-dative bifunctional surfaces [16]. In a previous work, the V2O5/TiO2

modified with Ti(SO4)2 catalysts were proved to be effective for theselective oxidation of methanol to dimethoxymethane [17].

In the present work, mesoporous VOx–TiO2 oxides with high sur-face areas were synthesized using the EISA method combined withan ammonia solution posttreatment. The mesoporous VOx–TiO2

with vanadia content as high as 30 wt% retained high surface areas(>230 m2 g�1) even after calcination at high temperatures (e.g., at673 K). These samples were characterized by XRD, laser Ramanspectroscopy, N2 adsorption, transmission electron microscopy(TEM), temperature-programmed reduction (H2-TPR), microcalori-metric adsorption of NH3, and isopropanol probe reaction. The

J. Liu et al. / Journal of Colloid and Interface Science 335 (2009) 216–221 217

catalytic behavior of the samples was tested for the selective oxida-tion of methanol to dimethoxymethane (DMM).

2. Experimental

2.1. Materials

2.1.1. Preparation of mesoporous VOx–TiO2

The amount of 5.0 g V2O5 was added into 33.3 ml concentratedhydrochloride solution (36–38% HCl) and refluxed at 378 K to ob-tain a solution. Titanium isopropoxide (Ti(OiPr)4, Aldrich, 97%)was dissolved in ethanolic HCl solution.

In reference to the work of Cassiers et al. [11,12], mesoporousVOx–TiO2 was synthesized as follows. The solution containing vana-dium was added into an ethanolic solution of cetyltrimethylammo-nium bromide (CTAB) to form a mixture solution, which was thenadded dropwise into the solution containing titanium under vigor-ous stirring. The resulting transparent solution was subsequentlytransferred into an open petri dish and maintained at 333 K for7 days to evaporate the solvent, yielding a hybrid. The molar ratioused was V:Ti:CTAB:HCl:H2O:EtOH = x:1:0.16:1.4:17:20, where xwas a variable according to the desired ratio of V/Ti. The dried hy-brids were posttreated with ammonia solution. One gram of hybridwas refluxed with 50 ml of ammonia solution (6 wt%) at pH 9–10for 2 days. Afterward, the hybrid was filtered, dried, and calcinedunder flowing air (60 ml min�1) at 673 K for 5 h. In this way, thesamples TiO2, 10VOx–TiO2, 20VOx–TiO2 and 30VOx–TiO2, were pre-pared, in which the amounts of V2O5 (weight percentage) were 0%,10%, 20%, and 30%, respectively.

2.1.2. Preparation of SO42�=VOx—TiO2

The SO42�=VOx—TiO2 catalysts were prepared by the incipient

wetness impregnation method. Specifically, for each preparation,a known amount of VOx–TiO2 was added into the aqueous solutioncontaining the desired amount of Ti(SO4)2 solution with 10 wt%SO4

2� prior to calcination. After being dried at room temperature,the SO4

2�=VOx—TiO2 catalysts were calcined at 673 K for 5 h.

2.2. Characterization

Powder X-ray diffraction (XRD) patterns were collected on aPhilips X’Pert Pro diffractometer using Ni-filtered CuKa radiation(k = 0.15418 nm), operated at 40 kV and 40 mA at a scanning rateof 0.417� per second. The amount of anatase in the sample wasestimated by using the Spurr equation:

FA ¼ 100� 11þ 0:8IAð101Þ=IRð110Þ

� �100

where FA, IA, and IR are represented as the amount of anatase,the intensities of anatase (101), and rutile (110) reflections,respectively.

Nitrogen adsorption–desorption isotherms were derived at theliquid nitrogen temperature using a Micromeritics ASAP 2020 ana-lyzer. Prior to a measurement, the sample was degassed to10�3 Torr at 573 K. Pore size distribution and pore volume weredetermined by the Barrett–Joyner–Halenda (BJH) method accord-ing to the desorption branch of an isotherm. Elemental analysiswas performed on an ARL-9800 X-ray fluorescence spectrometer.TEM images were obtained from a JEOL JEM 2100 transmissionelectronic microscope with an accelerating voltage of 200 kV. Thesamples were dispersed in ethanol under ultrasonic conditionsand deposited onto copper grids coated with ultrathin carbonfilms. Laser Raman spectra were acquired on a Renishaw in Via Ra-man microscope with the 514.5 nm line of an Ar ion laser as theexcitation source of about 2 mW. Spectra were recorded with1 cm�1 resolution and 3 scans.

H2-TPR measurements were carried out in a continuous modeusing a U-type quartz microreactor (3.5 mm in diameter) equippedwith a thermal conductivity detector (TCD). A sample of about50 mg was contacted with a H2:N2 mixture (5.13% volume of H2

in N2) at a flow rate of 40 ml min�1. The sample was heated at arate of 10 K min�1 from room temperature to 1250 K.

Microcalorimetric adsorption of ammonia was performed at423 K by using a Tian-Calvet type heat flux Setaram C80 calorime-ter. The calorimeter was connected to a volumetric systemequipped with a Baratron capacitance manometer for the pressuremeasurement and gas handling. About 0.1 g sample was pretreatedin 500 Torr O2 at 573 K for 1 h, followed by evacuation at the sametemperature for 1 h. The probe molecule ammonia was purifiedwith the successive freeze–pump–thaw cycles.

2.3. Catalytic reactions

The probe reaction of isopropanol conversion was carried out ina fixed-bed glass tube reactor. About 100 mg of a sample wasloaded for each reaction. Isopropanol was introduced onto the cat-alyst by bubbling air through a glass saturator filled with isopropa-nol maintained at 295 K. Isopropanol and reaction products wereanalyzed by an online gas chromatograph, using a PEG 20M packedcolumn connected to a flame ionization detector (FID). Each cata-lyst was pretreated by heating in air at 673 K for 1 h and thencooled in the same flow to the reaction temperature (403 K).

The reaction of selective oxidation of methanol was carried outat atmospheric pressure in a fixed-bed microreactor (glass) with aninner diameter of 6 mm. Methanol was introduced into the reac-tion zone by bubbling O2/N2 (1/5) through a glass saturator filledwith methanol (99.9%) maintained at 278 K. In each test, 0.2 g ofa catalyst was loaded, and the gas hourly space velocity (GHSV)was maintained at 11,400 ml g�1 h�1. The feed composition wasmethanol:O2:N2 = 1:3:15 (v/v). Methanol, dimethoxymethane,formaldehyde, and other organic compounds were analyzed byusing a GC equipped with FID and TCD detectors connected toPorapak N columns. CO and CO2 were detected by using anotherGC with a TCD connected to a TDX-01 column. The gas lines werekept at 373 K to prevent condensation of reactants and products.Selectivity was reported on a carbon basis as the percentage ofthe converted CH3OH appearing as a given product.

3. Results and discussion

3.1. Structure and texture characterization

Fig. 1 shows the XRD patterns of mesoporous VOx–TiO2 withdifferent contents of V2O5. For the TiO2 without VOx, typical dif-fraction peaks of anatase and rutile were observed. It was esti-mated to have about 69% of anatase and 31% of rutile (by molarfraction) according to the Spurr equation. With the incorporationof VOx, intensities of peaks attributed to anatase increased, whilethose corresponding to rutile were weakened (see Table 1), sug-gesting the promotion effect of VOx for the formation of anatasephase. This observation did not agree with the previous studies,which might be due to the different preparation methods [18,19].No diffraction peaks due to crystalline vanadia were detected inthe VOx–TiO2 samples, indicating that the VOx species were highlydispersed with probably microcrystals less than 4 nm.

Laser Raman spectra of the mesoporous VOx–TiO2 materials aredepicted in Fig. 2. The bands at 152, 199, 399, 510, and 637 cm�1

belonging to anatase were observed for all the samples [20], whilethe bands due to rutile (250, 437, and 610 cm�1) were not apparent[20]. With the addition of vanadia, two features appearing at 960and 817 cm�1 were characteristic of polymeric V–O–V [21] and

Fig. 2. Raman spectra of the TiO2 (a) and VOx–TiO2 with different contents of V2O5:10VOx–TiO2 (b), 20VOx–TiO2 (c), and 30VOx–TiO2 (d).

Fig. 1. XRD patterns of the TiO2 and VOx–TiO2 samples with different contents ofV2O5.

218 J. Liu et al. / Journal of Colloid and Interface Science 335 (2009) 216–221

octahedral decavanadate V10O28 species (or isolated VO43� tetrahe-

dral structure) [3,22]. The band at 1020 cm�1 due to the symmetricvibration of vanadyl species was not observed. The intensity of thispeak was usually affected by moisture in air [23].

The N2 adsorption–desorption isotherms and pore size distribu-tions of the VOx–TiO2 samples with different contents of V2O5 areshown in Fig. 3. The BET surface area, pore size, and pore volumeof the samples are listed in Table 1. All the samples exhibited thetype IV nitrogen isotherm with a H2 hysteresis loop, characteristicof mesoporous materials. A clear hysteresis loop was shown at arelative pressure between 0.4 and 0.7, related to the capillary con-densation associated with mesoporous channels. However, at high-er relative pressure between 0.8 and 1.0, the shape of thehysteresis loops was of the type H3 associated with aggregates ofplatelike particles that formed slitlike pores. Similar adsorptionbehavior was also observed for titania tubules [24]. The monomo-dal pore size distribution curves derived from desorption branches(Fig. 3, inlet) implied that the materials had relatively regular porechannels in the mesoporous region. The BET surface area, pore size,and pore volume of the samples are listed in Table 1. The parame-ters are, respectively, equal to 326 m2 g�1, 3.7 nm, and 0.34 ml g�1.With the incorporation of 10 wt% V2O5, the surface area of the10VOx–TiO2 decreased, while the average pore size increased andpore volume remained constant. Further increase in vanadia con-tent led to the gradual decrease in surface area, while the varia-tions of pore size and pore volume were not striking.

The morphology and pore structure of the samples were ob-served by transition electron microscopy (TEM) as shown inFig. 4. The TiO2 exhibited uniform wormlike pore arrangementwith pore size of about 4 nm, in accordance with the pore sizedetermined by N2 adsorption. These wormlike pores seemed to re-main but slightly disordered with the addition of V2O5. The meso-porosity was probably due to the intra- and interparticle voids. Theresults from N2 adsorption–desorption and TEM clearly demon-strate that the VOx–TiO2 prepared were mesoporous materials.

Table 1Anatase content, surface area, pore size, and pore volume of the mesoporous TiO2 andVOx–TiO2 with different contents of V2O5.

Sample Anatase contentin TiO2 (%)

SBET

(m2 g�1)Pore size(nm)

Pore volume(cm3 g�1)

TiO2 69 326 3.7 0.3410VOx–TiO2 74 300 4.3 0.3520VOx–TiO2 76 260 4.6 0.3330VOx–TiO2 83 236 4.4 0.28

3.2. Redox and acidic properties

TPR is frequently used to study the redox properties of metaloxide catalysts. The TPR profiles for the VOx–TiO2 samples areshown in Fig. 5. It is seen that no hydrogen consumption peakwas observed until 1100 K for the TiO2, indicating that the samplewas not reduced up to this temperature. A single broad reductionpeak in the 740–765 K range occurred for each of the VOx–TiO2

samples, corresponding to the reduction of highly dispersed van-adia species from V5+ to V3+ [25,26]. With the increase of vanadiumcontent, the area of the reduction peak increased and the peakmaximum (TM) shifted to higher temperatures. The single reduc-tion peak seemed to suggest the highly dispersed vanadia speciesin the VOx–TiO2 samples, consistent with the results of XRD andLRS.

Microcalorimetric adsorption of ammonia has been used todetermine the number, strength, and strength distribution of sur-face acidities [27]. In Fig. 6, differential heats versus coverage forNH3 adsorption on the TiO2, 30VOx–TiO2, and SO4

2�=30VOx—TiO2

samples are depicted. The TiO2 exhibited an initial heat of181 kJ mol�1 and ammonia coverage of 649 lmol g�1, indicatingits strong surface acidity. The 30VOx–TiO2 displayed the initial heatof 147 kJ mol�1 and ammonia coverage of 860 lmol g�1. It seemedthat the incorporation of vanadia produced more acidic sites in theVOx–TiO2 samples. Addition of SO4

2� onto the 30VOx–TiO2 furtherdecreased the initial heat to 116 kJ mol�1, but increased the cover-age for the adsorption of ammonia to 1000 lmol g�1. The decreasedinitial heat on the addition of sulfate was due to the endothermicinteraction of ammonia with polymerized sulfates in the sample.A similar phenomenon was reported for the sulfated titania materi-als by Desmartin-Chomel et al., and the existence of polymerizedsulfate species was confirmed by FTIR [28]. Thus, the microcalori-metric adsorption of ammonia did not seem to titrate the acidicstrength of sulfated samples. However, the SO4

2�=30VOx—TiO2

did exhibit more surface acid sites (1001 lmol g�1) than the30VOx–TiO2 (860 lmol g�1).

The conversion of isopropanol (IPA) has been frequently used asa probe reaction for the characterization of acid–base and redoxproperties of oxide catalysts [29–31]. It is commonly accepted thatisopropanol is dehydrated to produce propylene (PPE) and

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Fig. 3. N2 adsorption–desorption isotherms and pore size distributions for the TiO2 (a) and VOx–TiO2 with different contents of V2O5: 10VOx–TiO2 (b), 20VOx–TiO2 (c), and30VOx–TiO2 (d).

J. Liu et al. / Journal of Colloid and Interface Science 335 (2009) 216–221 219

diisopropyl ether (DIPE) on acidic sites, while it undergoes a dehy-drogenation reaction to acetone (ACE) on basic sites in the absenceof oxygen. On the other hand, IPA can also be oxidatively dehydro-genated to ACE in an oxidative atmosphere. Thus, the surface acidicand redox properties could be described by the isopropanol probereaction in the presence of oxygen.

Table 2 presents the results for isopropanol probe reaction overthe mesoporous TiO2 and VOx–TiO2 at 403 K. The TiO2 exhibitedlow conversion of isopropanol (0.3%) with 100% selectivity to pro-pylene, suggesting its acidic character without any redox property,consistent with the results from ammonia adsorption microcalori-metry and TPR. The introduction of vanadium species greatlyenhanced the conversion of isopropanol. It seemed that the conver-sion of isopropanol increased with the increase of vanadia content.The conversions of isopropanol on 10VOx–TiO2, 20VOx–TiO2, and30VOx–TiO2 were 10%, 14%, and 17%, respectively. Meanwhile,the dehydration products propylene and diisopropyl ether weregreatly decreased and acetone became the main product, indicat-ing that the VOx–TiO2 exhibited mainly the redox property. Theaddition of sulfate ions onto VOx–TiO2 further increased theconversion of isopropanol, but the selectivity to acetone was de-creased. This was due to the significantly enhanced surface aciditywhich catalyzed the dehydration of isopropanol to propylene andDIPE. The enhancement of surface acidity on the addition of sulfate

in vanadia–titania catalysts was also observed by Baraket et al.[32].

3.3. Selective oxidation of methanol

Methanol is an important chemical and intermediate. Its deriv-atives are widely used in large amounts as, for example, syntheticdyestuffs, resins, drugs, and perfumes. In addition, the oxidation ofmethanol can be used as a probe reaction to characterize the sur-face acidic and redox properties of catalysts according to the differ-ent products (formaldehyde, dimethyl ether, dimethoxymethane,methyl formate, carbon oxides, and hydrocarbons) [16]. It has beenreported that methanol could be converted into dimethyl ether(DME), formaldehyde (FA), methyl formate (MF), dimethoxyme-thane (DMM) on acidic, oxidative, and acidic and oxidative bifunc-tional surfaces. Therefore, the surface acidic and redox propertiesof mesoporous TiO2, VOx–TiO2, and SO4

2�=VOx—TiO2 were charac-terized via the oxidation of methanol in this work. The results arepresented in Table 3.

According to the above characterization results, the TiO2 exhib-ited some surface acidity without any redox ability. Thus, the con-version of methanol was low (0.2%) over the TiO2 at 403 K, andDME was the only product. The incorporation of 10 wt% vanadiasignificantly increased the conversion of methanol (8%) and the

Fig. 4. TEM images of the TiO2 (a and b) and 30VOx–TiO2 (c and d) samples.

Fig. 5. H2-TPR profiles of the TiO2 and VOx–TiO2 samples with different contents ofV2O5.

Fig. 6. Differential heat versus coverage for NH3 adsorption at 423 K over the TiO2,30VOx–TiO2 and SO4

2�=30VOx—TiO2 samples.

Table 2Conversion of isopropanol over the TiO2, VOx–TiO2, and SO4

2�=VOx—TiO2 samples inair at 403 K.

Catalyst Isopropanolconversion (%)

Selectivity (%)

Propylene Diisopropylether

Acetone

TiO2 0.3 100 0 010VOx–TiO2 10 10 6 8420VOx–TiO2 14 11 8 8130VOx–TiO2 17 16 9 75SO4

2�=10VOx—TiO2 20 43 24 33SO4

2�=20VOx—TiO2 21 44 25 31SO4

2�=30VOx—TiO2 24 32 17 51

220 J. Liu et al. / Journal of Colloid and Interface Science 335 (2009) 216–221

selectivity to DMM (93%). Apparently, the increase in activity andselectivity to DMM was due to the bifunctional properties (acidicand redox) of VOx–TiO2 samples. The conversion of methanol in-creased with the increase of vanadium content. At 403 K, the con-version of methanol was 8%, 15%, and 23% over the 10VOx–TiO2,20VOx–TiO2, and 30VOx–TiO2, respectively. With the increase ofreaction temperature, the conversion of methanol increased, whilethe selectivity to DMM decreased rapidly with the increased selec-tivity to the oxidation products formaldehyde and methyl formate.It has been reported that the oxidation of methanol to DMM mightinvolve two steps: (1) oxidation of methanol to formaldehyde onredox sites and (2) condensation of formaldehyde produced withadditional methanol to DMM on acidic sites [17,33]. Thus, it

Table 3Oxidation of methanol over the TiO2, VOx–TiO2, and (SO4)2/VOx–TiO2 samples.

Catalyst Temperature(K)

Conversion(%)

Selectivity (%)

DMM FA MF DME

TiO2 403 0.2 0 0 0 100413 0.6 0 0 0 100423 0.9 0 0 0 100433 1.4 0 0 0 100

10VOx–TiO2 403 8 93 4 3 0.0413 11 82 11 7 0.2423 15 61 21 17 0.2433 19 39 30 31 0.3

20VOx–TiO2 403 15 90 6 4 0.1413 20 75 16 8 0.1423 26 45 36 19 0.2433 33 17 46 36 0.2

30VOx–TiO2 403 23 86 6 8 0.1413 32 62 15 22 0.2423 38 33 27 39 0.3433 47 12 31 57 0.3

SO42�=10VOx—TiO2

a 403 11 94 0 1 5413 16 83 9 2 7423 19 74 13 4 9433 28 36 39 12 12

SO42�=20VOx—TiO2

a 403 18 90 5 3 2413 23 74 15 7 3423 29 51 29 15 5433 40 27 42 25 6

SO42�=30VOx—TiO2

a 403 27 94 0 5 1413 41 90 0 9 1423 57 83 1 14 1433 75 68 6 24 2

a The content of sulfate was 8.2%, 6.1%, and 4.5%, respectively, as determined byXRF.

J. Liu et al. / Journal of Colloid and Interface Science 335 (2009) 216–221 221

seemed that the surface acidity of the VOx–TiO2 catalysts were notstrong enough for the condensation reaction to form DMM.

The addition of SO42� onto the VOx–TiO2 resulted in the signif-

icant increase of methanol conversion and DMM selectivity. Forexample, at 423 K, the conversion of methanol and the selectivityto DMM on the 30VOx–TiO2 were 38% and 33%, respectively, whilethey were 57% and 83%, respectively, on the SO4

2�=30VOx—TiO2.The considerable increase in the methanol conversion and DMMselectivity was surely due to the enhanced surface acidity on theaddition of SO4

2�. The increased surface acidity promoted the con-densation of formaldehyde with methanol, and thus enhanced theselectivity to DMM. In our previous work, we prepared an opti-mized catalyst 10%V2O5/TiO2–Ti(SO4)2, which exhibited 38% con-version of methanol and 94% selectivity to DMM at 423 K [17].Apparently, the present catalyst SO4

2�=30VOx—TiO2 showed thebetter catalytic reactivity for the selective oxidation of methanolto DMM under the same reaction conditions.

4. Conclusions

Mesoporous VOx–TiO2 with high surface areas were successfullysynthesized via the evaporation-induced self-assembly methodcombined with the ammonia solution posttreatment. The meso-

porous materials exhibited acidic and redox bifunctional charac-ters that catalyzed the selective oxidation of methanol todimethoxymethane (DMM). The surface acidity of these materialswas further enhanced by the addition of sulfate anions, and thesulfated (SO2�

4 Þ/30VOx ATiO2 was found to catalyze the selectiveoxidation of methanol to DMM with high conversion (57%) andselectivity to DMM (83%) at 423 K.

Acknowledgments

We acknowledge financial support from the NSFC (20673055)and the MSTC (2005CB221400 and 2004DFB02900).

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