Microporous and Mesoporous Materials 116 (2008) 614–621

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Microporous and Mesoporous Materials

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TiO2 nanotubes supported V2O5 for the selective oxidation of methanol todimethoxymethane

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

Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, 22 Hankou Road, Nanjing 210093, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 January 2008Received in revised form 24 April 2008Accepted 22 May 2008Available online 7 July 2008

Keywords:Mesoporous TiO2 nanotubesDispersion of V2O5

Surface acidic and redox propertiesSelective oxidation of methanolSynthesis of dimethoxymethane

1387-1811/$ - see front matter � 2008 Elsevier Inc. Adoi:10.1016/j.micromeso.2008.05.032

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

Chlorine free mesoporous TiO2 nanotubes (TNT) with high surface areas were prepared and used to sup-port V2O5. The addition of V2O5 increased the surface acidity of TNT, which was further enhanced by theaddition of Ti(SO4)2. The materials were characterized by X-ray diffraction (XRD), laser Raman spectros-copy (LRS), transmission electron microscopy (TEM), N2 and O2 adsorption, X-ray photoelectron spectros-copy (XPS), temperature-programmed reduction (H2-TPR), and microcalorimetry and infraredspectroscopy (FTIR) for the adsorption of NH3. The catalytic behavior for the selective oxidation of meth-anol to dimethoxymethane (DMM) was evaluated. It was found that V2O5 was highly dispersed on TNTwith the V2O5 loading lower than 20 wt%. The 6%SO2�

4 /20%V2O5/TNT displayed the strong surface acidicand redox characters and exhibited excellent performance for the selective oxidation of methanol toDMM. The methanol conversion reached 64% with 90% selectivity to DMM over the SO2�

4 /20%V2O5/TNTat 403 K.

� 2008 Elsevier Inc. All rights reserved.

1. Introduction

Since the discovery of carbon nanotubes, such one dimensionalnano-sized materials have received extensive attention from aca-demic and technologic fields [1]. Inorganic metal oxide nanotubeshave also been successively developed [2–5], among which, TiO2

nano-sized tubes (TNT) might have potential advantages in manyfields such as photocatalysis [6,7], catalyst supports [8–11], sup-ercapacitors [12], hydrogen storage [13,14], sensors [15], magneticnanodevices [16], solar cells [17,18] and lithium cells [19,20]. Gen-erally, the fabrication of TiO2 nanotube consists of three chemicalmethods [21], namely, template assisted, anodic oxidation andalkaline hydrothermal, in which hydrothermal treatment in con-centrated alkali solution was considered to be a cost-saving, effi-cient and facile way [22].

The high surface areas and open mesoporous morphology ofTNT facilitate the transportation of reagents to active sites duringcatalytic reactions. The semiconducting property of TNT might fa-vor the electronic interactions between the support and supportedactive phases, leading to the improved catalytic redox reactions[21]. TNT has been used to support noble metals for some reactionssuch as double-bond migration [23], water–gas shift reaction[24,25] and photocatalytic reactions [26]. CuOx and WOx have alsobeen supported on TNT for the reactions of selective catalyticreduction [8] and oxidation of dibenzothiophene [27].

ll rights reserved.

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V2O5/TiO2 catalysts are extensively studied in various reactionssuch as selective oxidation of methanol to formaldehyde [28] andmethyl formate [29], selective oxidation of aromatic compoundsto aromatic aldehydes [30], selective oxidation of olefins to phtha-lic anhydride [31], ammoxidation of aromatic hydrocarbons [32]and selective catalytic reduction of NOx [33]. In particular, the oxi-dation of methanol has been widely used as a probe reaction tocharacterize the surface acidic and redox properties of metal oxidecatalysts [34]. It has been reported that methanol could be con-verted into dimethyl ether (DME), formaldehyde (FA), methyl for-mate (MF), and dimethoxymethane (DMM) on acidic, oxidativeand bifunctional (acidic and oxidative) surfaces. The V2O5/TiO2 cat-alysts modified by Ti(SO4)2 might exhibit excellent performancefor the selective oxidation of methanol to DMM [35]. Since TNTmight have higher surface areas than the conventional TiO2 (suchas P25), we synthesized TNT and used it to support V2O5 andTi(SO4)2 for the selective oxidation of methanol to DMM. In thisway, higher conversion of methanol was achieved for the produc-tion of DMM.

2. Experimental

2.1. Preparation of samples

TNT were prepared via a hydrothermal synthesis from a com-mercial TiO2 (Degussa P25, SBET = 52 m2 g�1) and concentratedNaOH, according to the previous work [36,37]. In a typical proce-dure, 0.9 g P25 was dispersed in 30 ml of 10 M NaOH solution

J. Liu et al. / Microporous and Mesoporous Materials 116 (2008) 614–621 615

and stirred. After 30 min, the mixture was transferred into a Tef-lon-lined stainless steel autoclave and heated at 403 K for 24 h.The as-synthesized paste was washed first by 0.1 M HNO3 and then0.05 M HNO3. Finally it was further washed by distilled water at318 K until pH of the rinsing solution reached 7. The white pasteobtained was dried at 313 K overnight and then calcined at 673 Kfor 1 h.

V2O5/TNT catalysts were prepared by the incipient wetnessimpregnation method. Specifically, for each preparation, a knownamount of TNT was added into the aqueous solution containingthe desired amount of vanadium oxalate ((NH4)2[VO(C2O4)2H2O]-H2O) and stirred. After being dried at room temperature, the sam-ple was impregnated into a Ti(SO4)2 solution, with 8 wt% of SO2�

4

prior to calcination. After being dried again at room temperature,the V2O5/TNT and SO2�

4 /V2O5/TNT samples were calcined at 673 Kfor 1 h.

2.2. Characterization

Powder X-ray diffraction (XRD) patterns were collected on Phi-lips X’Pert Pro diffractometer using Ni-filtered Cu Ka radiation(k = 0.15418 nm), operated at 40 kV and 40 mA at a scanning rateof 0.417� per second. Thermal analyses were carried out on a NET-ZSCH STA409C thermal analysis system at a heating rate of10 K min�1 over a temperature range of 313–1073 K in a flowingN2 of 40 ml min�1.

Nitrogen adsorption–desorption isotherms were measured atthe liquid nitrogen temperature using a Micromeritics ASAP 2020analyzer. Prior to a measurement, the sample was degassed to 10�3

Torr at 473 K. Pore size distribution and pore volume were deter-mined by the Barrett–Joyner–Halenda (BJH) method according tothe desorption branch of an isotherm. Transmission electronmicroscopy (TEM) was performed on a JEOL JEM 2100 microscopewith an accelerating voltage of 200 kV. The samples were dis-persed in ethanol under ultrasonic conditions and deposited ontocopper grids coated with ultrathin carbon films. Laser Raman spec-tra (LRS) were acquired on a Renishaw inVia Raman microscopewith the 514.5 nm line of an Ar ion laser as the excitation source(2 mW). Spectra were recorded with 1 cm�1 resolution and 5 scans.

The dispersion of vanadium species was measured by using thehigh temperature oxygen chemisorption method (HTOC) [38].About 0.1–0.2 g sample was reduced in flowing H2 (40 ml min�1)at 640 K for 2 h, and evacuated at the same temperature for0.5 h. Oxygen uptake was measured at 640 K in a home-made con-ventional volumetric apparatus equipped with an accurate pres-sure gauge.

X-ray photoelectron spectra (XPS) measurement was performedon a SSI 301 instrument equipped with a hemispherical electronanalyzer and an Al anode (Al Ka = 1486.6 eV). Spectra were ac-quired using the pass energy of 40 eV with an X-ray powered at150 W. The residual pressure in the spectrometer chamber duringdata acquisition was 5 � 10�8 Pa. A value of 284.6 eV for the bind-ing energy of the main C1s component was used as an internal cal-ibration standard.

H2-TPR measurements were carried out in a continuous modeusing a U-type quartz microreactor (3.5 mm in diameter). A sampleof about 50 mg was contacted with a H2:N2 mixture (5.13% volume ofH2 in N2) at a total flow rate of 40 ml min�1. The sample was heatedat a rate of 10 K min�1 from room temperature to 1250 K. The hydro-gen consumption was monitored using a thermal conductivitydetector (TCD). The reducing gas was first passed through the refer-ence arm of the TCD before entering the reactor. The flow out of thereactor was directed through a trap filled with Mg(ClO4)2 (to removewater from the product) and then to the second arm of the TCD.

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.

The NH3 adsorption FTIR spectra were recorded with a BrukerVector 22 Fourier transform spectrophotometer in the range of4000–400 cm�1, with a resolution of 2 cm�1 and 40 acquisitionscans. About 15 mg Sample was pressed into a self-sustaining waferwith 13 mm in diameter and mounted in a quartz IR cell with CaF2

windows. Prior to each test, the sample was pretreated in the samemanner as that for microcalorimetric studies. Then the spectrum ofthe fresh wafer was recorded as a background. Subsequently, thesample was exposed to NH3 (�30 Torr) at room temperature for30 min. After evacuation for 30 min at room temperature, the IRspectrum for the adsorbed ammonia was collected.

2.3. Catalytic test

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 ofcatalyst was loaded, and the gas hourly space velocity (GHSV)was 11,400 ml g�1 h�1. The feed composition was maintained asmethanol: O2:N2 = 1:3:15 (v/v). Methanol, DMM, formaldehydeand other organic compounds were analyzed by using a GCequipped with FID and TCD detectors connected to Porapak N col-umns. CO and CO2 were detected by using another GC with a TCDconnected to a TDX-01 column. The gas lines were kept at 373 K toprevent condensation of reactants and products.

3. Results and discussion

3.1. Structural characterizations

Fig. 1 shows the XRD patterns of the samples. The TNT beforecalcination exhibited the diffraction pattern of H2Ti2O4(OH)2 [39].The thermalgravimetric analysis (Fig. 2) showed that its weightloss reached 18 wt% at 700 K, corresponding to the loss of 2 H2Omolecules in a H2Ti2O4(OH)2. After calcination, anatase was foundto be the main phase in the TNT with trace of TiO2–B. Addition of20% V2O5 or less did not seem to change the XRD patterns, and onlythe phase of anatase was observed, indicating the high dispersionof V2O5 on the TNT. The crystalline phase of V2O5 was observedwhen the loading increased to 30 wt%.

Fig. 3 shows the Raman spectra of the TNT and V2O5/TNT sam-ples. Before calcination, the TNT exhibited the weak and broadbands around 191, 268, 450 and 675 cm�1, agreeing with the datareported previously and might be assigned to the vibrations inH2Ti2O4(OH)2 [40]. After calcination at 673 K, strong bands at148, 196, 397, 513 and 638 cm�1 due to anatase phase were clearlyseen. Weak bands around 247, 285 and 465 cm�1 might be relatedto the trace of TiO2–B in the TNT [19,20]. No vibrations due to Na–O–Ti (280 cm�1) or short Ti–O (involving nonbridging oxygen coor-dinated with Na+ at 920 cm�1) were observed [41], indicating thatall the Na+ were washed out from the TNT.

No Raman vibrations due to V@O or V–O–V species were ob-served for the V2O5/TNT samples with 20% of loading or less,implying the highly dispersion of V2O5 on TNT. However, the typ-ical Raman feature of crystalline V2O5 (997 cm�1) was clearly seenfor the 30%V2O5/TNT, suggesting the agglomeration of loaded V2O5

on the surface of TNT, consistent with the result of XRD.

Fig. 1. XRD patterns of TNT (a) before calcination as well as TNT (b), 5%V2O5/TNT(c), 10%V2O5/TNT (d), 20%V2O5/TNT (e) and 30%V2O5/TNT (f), after calcination at673 K.

Fig. 2. Thermalgravimetric (TG) analysis of TNT before calcination.

Fig. 3. Raman spectra of TNT (a) before calcination as well as TNT (b), 5%V2O5/TNT(c), 10%V2O5/TNT (d), 20%V2O5/TNT (e) and 30%V2O5/TNT (f), after calcination at673 K.

616 J. Liu et al. / Microporous and Mesoporous Materials 116 (2008) 614–621

The TEM images of TNT, 20%V2O5/TNT and SO2�4 /20%V2O5/TNT

were presented in Fig. 4. Before calcination, the nano-tubular mor-phology of TNT was clearly seen with the length of 100–200 nmand diameter of 10–20 nm. After calcination at 673 K for 1 h, thetubular structure of TNT was remained. Addition of 20 wt% V2O5

did not seem to change the tubular morphology of TNT. However,the nano-tubular morphology of TNT seemed to be changed by thefurther loading of Ti(SO4)2. Nanoparticles and nanorods were

formed upon the addition of Ti(SO4)2. The similar phenomenonwas reported for the WOx/TNT catalysts [27].

Fig. 5 shows the nitrogen adsorption-desorption isotherms andcorresponding BJH pore size distributions for the TNT and V2O5/TNT samples. According to IUPAC classification [42], these iso-therms belong to the type IV with a H3 hysteresis loop, character-istic of slit-shaped mesoporous structure. The calcined TNT had asurface area of 305 m2 g-1 with a pore diameter centered at13 nm (see Table 1). Upon the addition of V2O5, the surface areaand pore volume of V2O5/TNT decreased continuously with the in-crease of loading, whereas the pore size was almost unaffected.Since BJH method accounts for the pores not only inside the tubesbut also the secondary pores formed between the tubes, the aver-age pore size also might mainly depend on the sizes of the second-ary pores which were not significantly affected by loading of V2O5.In addition, filling TiO2 nanotubes with small particles might notchange the average diameter of pores inside the tubes due to thecylindrical geometry of the pores. Additional loading of SO2�

4 on20%V2O5/TNT brought about the further decrease of surface areaand pore volume, probably resulted from the partial destructionof the nano-tubular morphology of TNT as seen in the TEM images(Fig. 4e and f).

3.2. Surface redox and acidic properties

O2 chemisorption was sometimes used to estimate the disper-sion of supported V2O5. Nag et al. devised a low temperature oxy-gen adsorption method (LTOC) [43] while Oyama et al. proposed ahigh temperature oxygen adsorption method (HTOC) [38]. It wassuggested that the HTOC might exert less possibility of bulk reduc-tion and sintering [44], and we used this technique for the rough

Fig. 4. TEM images of TNT (a) before calcination as well as TNT (b), 20%V2O5/TNT (c, d) and SO2�4 /20%V2O5/TNT (e, f), after calcination at 673 K.

J. Liu et al. / Microporous and Mesoporous Materials 116 (2008) 614–621 617

estimation of the dispersion of vanadium species. In this technique,the samples were first reduced in H2 at 640 K, at which only sur-face vanadium species were supposedly reduced [38]. O2 adsorp-tion was then performed at 640 K to titrate the number ofreduced vanadium cations on the surface, supposing the ratio ofO/V = 1.

Table 1 gives the O2 uptakes and corresponding V densities forthe TNT and V2O5/TNT samples with different V2O5 loadings. Thetotal number of vanadium sites loaded in terms of unit surface areawas also given for comparison. It was found that little O2

(0.0065 mmol g�1) was chemisorbed on TNT, in agreement withthe result reported [44]. Thus, O2 was mainly adsorbed on vana-dium species in the V2O5/TNT samples. The uptake of O2 and thesurface density of vanadium calculated accordingly were increasedwith the vanadia loading. Wachs et al. reported that the theoreticaldensity for the monolayer dispersion of vanadia on TiO2 was

7.9 V nm�2 [45]. The data in Table 1 shows that except for the30%V2O5/TNT, all other samples had the loading less than mono-layer dispersion. Thus, high dispersion of vanadia was titrated forthese samples. If we define the dispersion as the ratio of theamount of surface vanadium sites titrated by O2 adsorption andthat of total vanadium loaded, the dispersion was found to be100%, 89%, 90% and 96% for the samples 5%V2O5/TNT, 10%V2O5/TNT, 20%V2O5/TNT and SO2�

4 /20%V2O5/TNT, respectively. The30%V2O5/TNT had the loading (18.4 V nm�2) that was much higherthan the monolayer dispersion capability. The surface densityaccording to O2 adsorption was calculated to be 14.3 V nm�2 forthis sample. This was much higher than the theoretical value, indi-cating that the HTOC technique did titrate some bulk V2O5. Addi-tion of sulfate seemed to increase the surface vanadium density.

XPS was applied to detect the contents and chemical states ofelements in the subsurface regions. Table 2 gives the data about

Fig. 5. N2 adsorption–desorption isotherms and pore size distributions derivedfrom the desorption branches for TNT (a), 20%V2O5/TNT (b) and SO2�

4 /20%V2O5/TNT(c), after calcination at 673 K.

618 J. Liu et al. / Microporous and Mesoporous Materials 116 (2008) 614–621

the surface composition and binding energy for the TNT, 20%V2O5/TNT and SO2�

4 /20%V2O5/TNT. No signal of sodium ions was de-tected, indicating again that all the Na+ was fully removed uponwashing. The atomic ratio of O/Ti for TNT is about 2.3, slightlyhigher than the stoichiometric composition of TiO2. The bindingenergy of Ti 2p3/2 was 458.5 eV for TNT, respectively, reflecting a+4 oxidation state of Ti. The binding energy of V 2p3/2 in the20%V2O5/TNT was found to be 517.2 eV, indicating that +5 oxida-tion state of vanadium, in agreement with the result reported[46]. Addition of sulfate did not seem to change the binding

energies of Ti and V. The added S had the binding energy of168.3 eV, indicating the form of SO2�

4 on the surface [46]. The glo-bal and surface amounts of SO2�

4 were found to be 6.2% and 4.5%,respectively, by the elemental analysis (with an ARL-9800 X-rayfluorescence spectrometer) and XPS for the SO2�

4 /20%V2O5/TNT.Temperature programmed reduction (TPR) is a technique that

can be used to probe the reducibility of metal oxides. Fig. 6 showsthe TPR profiles of the TNT and V2O5/TNT samples after calcinationat 673 K. No TPR peaks were detected for the TNT, indicating thenon-redox feature of the support, consistent with the result of O2

adsorption. Two H2 consumption peaks were observed around718 K and 779 K for the 5%V2O5/TNT, which might be ascribed tothe reduction of monomeric and polymeric surface VOx speciesfrom V5+ to V3+, respectively [47–49]. These two peaks shifted tohigher temperatures with the increase of vanadia loading. Com-pared to the bulk V2O5, the reduction temperatures were signifi-cantly lowered for the V2O5/TNT samples, suggesting thesignificantly enhanced redox ability of the V2O5/TNT samples. Fur-ther addition of sulfate had little effect on the TPR profile.

Microcalorimetric adsorption of ammonia has been used todetermine the number, strength and strength distribution of sur-face acidities [50]. Differential heats versus coverage for NH3

adsorption on the TNT and V2O5/TNT samples are depicted in Fig.7. The TNT exhibited an initial heat of 144 kJ mol�1 with the NH3

saturation coverage of 2.3 lmol m�2. Addition of vanadia on theTNT enhanced the surface acidity since both the initial heat and sat-uration coverage of NH3 increased. Specifically, the initial heat andammonia coverage increased to 150 kJ mol-1 and 2.8 lmol m�2,respectively, for the 20%V2O5/TNT. Addition of SO2�

4 into the20%V2O5/TNT did not seem to affect the initial heat, but the satura-tion coverage of NH3 (4.7 lmol m�2) was increased significantly.Thus, the SO2�

4 /20%V2O5/TNT might possess much higher surfaceacidity than the 20%V2O5/TNT.

Fig. 8 shows the FTIR spectra for the adsorption of NH3 on theTNT, 20%V2O5/TNT and SO2�

4 /20%V2O5/TNT calcined at 673 K. It isknown that the IR peaks around 1446, and 1649 cm�1 are due tothe deformation vibrations of NHþ4 produced by NH3 adsorptionon Brønsted acid sites, while the peaks around 1600 and1168 cm�1 are due to coordinatively adsorbed NH3 on Lewis acidsites [51]. Fig. 8 shows that the TNT possessed weak surface aciditybecause only weak bands were detected for the adsorbed ammo-nia. Addition of V2O5 on the TNT increased the Brønsted aciditysince the intensity of the band around 1434 cm�1 increased signif-icantly. Further addition of SO2�

4 seemed to increase the surface Le-wis acidity, since the intensity ratio for the bands around 1220(Lewis) and 1420 cm�1 (Brønsted) was increased. Zhao et al. re-ported that on sulfated zirconia, the strong Lewis sites were dueto the chemisorbed SO3, which could be partially converted intosurface sulfated species acting as strong Brønsted sites [52].

3.3. Catalytic reaction

Methanol and its derivatives have been widely studied due totheir industrial importance [34]. In addition, the catalytic oxidationof methanol is a convenient structure-sensitive reaction, which hasbeen used to characterize the surfaces of metal oxides for theiracid/base and redox properties [34,53]. Methanol is converted toformaldehyde (FA) and methyl formate (MF) on redox sites, to di-methyl ether (DME) on acidic sites, and to dimethoxymethane(DMM) on redox and acidic bifunctional sites. Hence, the conver-sion of methanol was used to evaluate the surface properties ofthe V2O5/TNT samples in this work. The results are given in Table 3.

The conversion of methanol was very low (<1.5%) over the TNTat all the temperatures employed in this work, and the only prod-uct was DME. This is due to that the TNT was lack of redox prop-erty, as evidenced by the results of O2 adsorption and TPR. The

Table 1Surface areas, pore parameters and O2 uptakes for TNT and TNT supported V2O5

Sample SBET (m2 g�1) Pore size (nm) Pore vol. (cm3 g�1) Total loading (V nm�2) O2 uptake (mmol g�1) VOx density (V nm�2) Dispersion (%)b

TNT after calcination 305 13.1 1.30 –a 0.0065 –a –a

5%V2O5/TNT 273 11.2 0.94 1.2 0.272 1.2 10010%V2O5/TNT 235 11.7 0.82 2.8 0.480 2.5 8920%V2O5/TNT 213 11.2 0.75 6.2 0.993 5.6 9030%V2O5/TNT 108 14.9 0.49 18.4 1.286 14.3 78SO2�

4 /20%V2O5/TNT 177 9.2 0.46 7.5 1.055 7.2 96

a Not available.b Dispersion is defined as the V sites titrated by O2 adsorption divided by the total V sites added.

Table 2XPS analysis for TNT, 20%V2O5/TNT and SO2�

4 /20%V2O5/TNT

Sample Binding energy (eV) Composition (at%)

Ti V S O V Ti S

TNT (673 K calcined) 458.5 – – 70.21 – 29.79 –20%V2O5/TNT 458.7 517.2 – 68.13 7.47 24.40 –SO2�

4 /20%V2O5/TNT 458.5 517.1 168.3 69.25 6.70 22.86 1.19

Fig. 6. H2-TPR profiles for TNT and V2O5/TNT samples after calcination. A bulk V2O5

was also included for comparison.

Fig. 7. Differential heat versus coverage for NH3 adsorption at 423 K over the TNT,20%V2O5/TNT and SO2�

4 /20%V2O5/TNT calcined at 673 K.

Fig. 8. FTIR of NH3 adsorption on TNT (a), 20%V2O5/TNT (b) and SO2�4 /20%V2O5/TNT

(c) after calcination at 673 K.

J. Liu et al. / Microporous and Mesoporous Materials 116 (2008) 614–621 619

addition of V2O5 greatly enhanced the catalytic activity. For exam-ple, the conversion of methanol was 23% with DMM as the mainproduct (77% selectivity) on the 10%V2O5/TNT at 393 K. Theincreased activity and selectivity to DMM were due to the en-hanced surface acidic and redox properties of the 10%V2O5/TNT.The 20%V2O5/TNT exhibited the highest catalytic activity amongthe V2O5/TNT samples studied in this work. The conversion ofmethanol reached 38% with a DMM selectivity of 69% over the20%V2O5/TNT at 393 K. With the increase of reaction temperature,

Table 3Selective oxidation of methanol to dimethoxymethane

Catalyst Temp.(K)

Conversion(%)

Selectivity (%)

FAa MFa DMMa DMEa CO

TNT 393 0 0 0 0 0 0403 0.8 0 0 0 100 0413 1.1 0 0 0 100 0423 1.3 0 0 0 100 0

10%V2O5/TNT 393 23 12 11 77 0 0403 30 21 16 63 0 0413 38 37 30 33 0 0423 55 42 54 4 0 0

20%V2O5/TNT 393 38 16 15 69 0 0403 45 32 32 36 0 0413 71 26 71 3 0 0423 96 33 36 0 0 31

30%V2O5/TNT 393 27 6 5 89 0 0403 34 21 12 67 0 0413 41 38 28 34 0 0423 56 35 58 7 0 0

SO2�4 /20%V2O5/TNT

393 42 0 5 95 0 0

403 64 1 9 90 0 0413 83 5 24 70 1 0423 91 10 78 8 1 3

a FA = formaldehyde; MF = methyl formate; DMM = dimethoxymethane;DME = dimethyl ether.

620 J. Liu et al. / Microporous and Mesoporous Materials 116 (2008) 614–621

the conversion of methanol increased. However, the selectivity toDMM decreased while that to formaldehyde and methyl formateincreased. At 423 K, great amount of CO were produced over the20%V2O5/TNT. The formation of large amount of formaldehydeand methyl formate indicated that the V2O5/TNT exhibited mainlythe strong redox properties at the reaction temperatures. Additionof 6% SO2�

4 into the 20%V2O5/TNT enhanced the activity for theselective oxidation of methanol to DMM. At 403 K, the conversionof methanol increased from 45% to 64% and the selectivity to DMMfrom 36 to 90%, over the 20%V2O5/TNT, upon the addition of SO2�

4 .Mechanistically, DMM was produced first by the oxidation ofmethanol to formaldehyde on redox sites followed by the conden-sation of formaldehyde with additional methanol to DMM onacidic sites [35,54]. Apparently, the addition of SO2�

4 enhancedthe surface acidity without lowering the surface redox ability ofthe 20%V2O5/TNT. In addition, the other products were about 1%formaldehyde and 9% methyl formate. No CO or CO2 was foundin this case since the reaction temperature was relatively low(403 K). Since methyl formate was also a useful product, the totalselectivity to DMM and methyl formate reached 99%.

In this work, we also prepared a V2O5/TiO2 by using a commer-cial TiO2 (ISK MC-150, SBET = 300 m2 g�1). Previously, we prepareda V2O5/TiO2 by using a Degussa P25 TiO2 (SBET = 52 m2 g�1). Thesetwo catalysts were modified with Ti(SO4)2. The loading used was5% and 20% V2O5 on the P25 and MC-150, respectively, accordingto their surface areas. Their catalytic behavior for the oxidationof methanol to DMM was tested and compared with that of TNTsupported ones. At 403 K, the SO2�

4 /5%V2O5/P25 exhibited highselectivity to DMM (99%), but the conversion of methanol waslow (8%) due to the low surface area of P25. Although the MC-150 possessed high surface area before the calcination, the surfacearea decreased to 98 m2 g�1 upon the support of V2O5 and SO2�

4 fol-lowed by calcination at 673 K. The conversion of methanol was 27%with 95% selectivity to DMM at 403 K over this catalyst. On theother hand, the SO2�

4 /20%V2O5/TNT possessed the surface area of177 m2 g�1 after calcination at 673 K. The conversion of methanolwas 64% with 90% selectivity to DMM over the SO2�

4 /20%V2O5/TNT at 403 K. Apparently, the TNT supported V2O5 catalyst exhib-ited more stable surface area and thus much better reactivity forthe selective oxidation of methanol to DMM than its counterparts.

The results appear promising for the industrial application.Thus, we demonstrated in this work that the TNT could act asthe appropriate support for V2O5 and SO2�

4 to achieve the excellentperformance for the selective oxidation of methanol to DMM.

4. Conclusions

TiO2 nanotubes (TNT) were successfully synthesized from acommercial TiO2 (P25) in concentrated NaOH via a hydrothermalmethod. The TNT possessed high surface area (305 m2 g�1) andlarge pore diameter centered at 13 nm. The TNT could be used tosupport V2O5 with high loadings (e.g., 20%) and high dispersions(near to the monolayer dispersion). The V2O5/TNT displayed thestrong redox and acidic characters. The surface acidity could befurther enhanced by the addition of SO2�

4 , without lowering thesurface redox ability. The sulfated V2O5/TNT was found to exhibitthe excellent performance for the selective oxidation of methanolto dimethoxymethane (DMM). Specifically, the conversion ofmethanol reached 64% with 90% selectivity to DMM over the6%SO2�

4 /20%V2O5/TNT at the mild reaction temperature (403 K).

Acknowledgments

We acknowledge the financial support from the National Sci-ence Foundation of China (20673055), the Ministry of Scienceand Technology of China (2005CB221400 and 2004DFB02900).

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