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

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

Journal of Colloid and Interface Science 335 (2009) 216–221 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.e...

406KB Sizes 0 Downloads 8 Views

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

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Preparation and characterization of mesoporous VOx–TiO2 complex oxides for 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 2009 Accepted 10 March 2009 Available online 2 April 2009 Keywords: Mesoporous VOx–TiO2 Dispersion of vanadia Surface acidic and redox properties Selective oxidation of methanol Synthesis of dimethoxymethane

a b s t r a c t Mesoporous VOx–TiO2 with high surface areas were prepared using the procedure of evaporationinduced self-assembly combined with ammonia posttreatment. The samples were characterized by Xray 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 oxidation of methanol to dimethoxymethane (DMM). It was found that the VOx–TiO2 materials exhibited high surface areas with pore diameters of 4 nm. The vanadia species were highly dispersed in the VOx–TiO2 within 30 wt% VOx content, evidenced by the results of XRD and LRS. The VOx–TiO2 samples exhibited both surface acidic and redox properties. The surface acidity was further enhanced on the addition of SO4 2 . The catalyst SO4 2 =30VOx —TiO2 exhibited good performance for the selective oxidation of methanol (57% conversion) to DMM (83% selectivity) at 423 K. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction Vanadium-based catalysts are extensively used for a large number of oxidation reactions such as catalytic oxidation of methanol to formaldehyde [1] or methyl formate [2], ammoxidation of aromatic hydrocarbons [3], selective catalytic reduction (SCR) of NOx [4], and oxidative dehydrogenations [5]. The interaction of the supports or other components with vanadium species can generally influence both the dispersion and the structure of vanadium species, resulting in different chemical reactivities. Specifically, TiO2 is one of the most commonly used supports because of the remarkable activity of V2O5/TiO2 catalysts. Titania exists in three ordinary structure polymorphs (anatase, rutile, and brookite), in which rutile is the most thermodynamically stable and anatase is the most effective titania used in catalysis. However, the specific surface areas as well as their thermal stability of these materials are usually not satisfactory. Thus, efforts have been devoted to develop TiO2 and VOx–TiO2 with high surface areas, particularly in the field of porous materials [6–12]. Ten years ago, Brinker et al. reported a synthetic approach called evaporation-induced self-assembly (EISA) [13]. This method allows the tuning of the inorganic condensation with the formation of meso-organized liquid template. With the evaporation of solvent from the original diluted solution, a liquid crystalline mesophase is gradually formed. Yang et al. first introduced this method to prepare thermally stable mesoporous TiO2 with Pluronic triblock copolymers (P123) as structure-direct* Corresponding author. Fax: +86 25 83594305. E-mail address: [email protected] (J. Shen). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.03.027

ing agents [6]. Other poly(ethylene oxide)-based surfactants [8] and quaternary ammonium bromide [14] were also successfully applied as templates for TiO2 synthesis. For the EISA method, a postsynthesis treatment with gaseous NH3 was found to be of great importance in increasing the thermal stability of mesoporous titania [9]. Cassiers et al. treated the as-prepared titania hybrid with an aqueous NH4OH solution and consequently prepared a thermally stable mesoporous titania calcined at 873 K [11,12]. Similarly, Segura and co-workers prepared a mesoporous TiO2 to support vanadia for the selective catalytic reduction of NOx [15]. Among the aforementioned catalytic reactions, the oxidation of methanol has been widely used as a probe reaction to characterize the activity of oxide catalysts and to correlate the structures and surface acidic and redox properties [16]. It has been reported that methanol could be converted to dimethyl ether (DME) on acidic surfaces, to formaldehyde (FA) and methyl formate (MF) on oxidative surfaces, and to dimethoxymethane (DMM) on acidic and oxidative bifunctional surfaces [16]. In a previous work, the V2O5/TiO2 modified with Ti(SO4)2 catalysts were proved to be effective for the selective oxidation of methanol to dimethoxymethane [17]. In the present work, mesoporous VOx–TiO2 oxides with high surface areas were synthesized using the EISA method combined with an ammonia solution posttreatment. The mesoporous VOx–TiO2 with vanadia content as high as 30 wt% retained high surface areas (>230 m2 g1) even after calcination at high temperatures (e.g., at 673 K). These samples were characterized by XRD, laser Raman spectroscopy, N2 adsorption, transmission electron microscopy (TEM), temperature-programmed reduction (H2-TPR), microcalorimetric adsorption of NH3, and isopropanol probe reaction. The

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

catalytic behavior of the samples was tested for the selective oxidation 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 concentrated hydrochloride solution (36–38% HCl) and refluxed at 378 K to obtain 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], mesoporous VOx–TiO2 was synthesized as follows. The solution containing vanadium was added into an ethanolic solution of cetyltrimethylammonium bromide (CTAB) to form a mixture solution, which was then added dropwise into the solution containing titanium under vigorous stirring. The resulting transparent solution was subsequently transferred into an open petri dish and maintained at 333 K for 7 days to evaporate the solvent, yielding a hybrid. The molar ratio used was V:Ti:CTAB:HCl:H2O:EtOH = x:1:0.16:1.4:17:20, where x was a variable according to the desired ratio of V/Ti. The dried hybrids were posttreated with ammonia solution. One gram of hybrid was refluxed with 50 ml of ammonia solution (6 wt%) at pH 9–10 for 2 days. Afterward, the hybrid was filtered, dried, and calcined under flowing air (60 ml min1) at 673 K for 5 h. In this way, the samples TiO2, 10VOx–TiO2, 20VOx–TiO2 and 30VOx–TiO2, were prepared, in which the amounts of V2O5 (weight percentage) were 0%, 10%, 20%, and 30%, respectively. 2.1.2. Preparation of SO4 2 =VOx —TiO2 The SO4 2 =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 solution containing 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 a Philips X’Pert Pro diffractometer using Ni-filtered CuKa radiation (k = 0.15418 nm), operated at 40 kV and 40 mA at a scanning rate of 0.417° per second. The amount of anatase in the sample was estimated by using the Spurr equation:

 F A ¼ 100 

 1 100 1 þ 0:8IA ð1 0 1Þ=IR ð1 1 0Þ

where FA, IA, and IR are represented as the amount of anatase, the intensities of anatase (1 0 1), and rutile (1 1 0) reflections, respectively. Nitrogen adsorption–desorption isotherms were derived at the liquid nitrogen temperature using a Micromeritics ASAP 2020 analyzer. Prior to a measurement, the sample was degassed to 103 Torr at 573 K. Pore size distribution and pore volume were determined by the Barrett–Joyner–Halenda (BJH) method according to the desorption branch of an isotherm. Elemental analysis was performed on an ARL-9800 X-ray fluorescence spectrometer. TEM images were obtained from a JEOL JEM 2100 transmission electronic microscope with an accelerating voltage of 200 kV. The samples were dispersed in ethanol under ultrasonic conditions and deposited onto copper grids coated with ultrathin carbon films. Laser Raman spectra were acquired on a Renishaw in Via Raman microscope with the 514.5 nm line of an Ar ion laser as the excitation source of about 2 mW. Spectra were recorded with 1 cm1 resolution and 3 scans.

217

H2-TPR measurements were carried out in a continuous mode using a U-type quartz microreactor (3.5 mm in diameter) equipped with a thermal conductivity detector (TCD). A sample of about 50 mg was contacted with a H2:N2 mixture (5.13% volume of H2 in N2) at a flow rate of 40 ml min1. The sample was heated at a rate of 10 K min1 from room temperature to 1250 K. Microcalorimetric adsorption of ammonia was performed at 423 K by using a Tian-Calvet type heat flux Setaram C80 calorimeter. The calorimeter was connected to a volumetric system equipped with a Baratron capacitance manometer for the pressure measurement and gas handling. About 0.1 g sample was pretreated in 500 Torr O2 at 573 K for 1 h, followed by evacuation at the same temperature for 1 h. The probe molecule ammonia was purified with the successive freeze–pump–thaw cycles. 2.3. Catalytic reactions The probe reaction of isopropanol conversion was carried out in a fixed-bed glass tube reactor. About 100 mg of a sample was loaded for each reaction. Isopropanol was introduced onto the catalyst by bubbling air through a glass saturator filled with isopropanol maintained at 295 K. Isopropanol and reaction products were analyzed by an online gas chromatograph, using a PEG 20M packed column connected to a flame ionization detector (FID). Each catalyst was pretreated by heating in air at 673 K for 1 h and then cooled in the same flow to the reaction temperature (403 K). The reaction of selective oxidation of methanol was carried out at atmospheric pressure in a fixed-bed microreactor (glass) with an inner diameter of 6 mm. Methanol was introduced into the reaction zone by bubbling O2/N2 (1/5) through a glass saturator filled with methanol (99.9%) maintained at 278 K. In each test, 0.2 g of a catalyst was loaded, and the gas hourly space velocity (GHSV) was maintained at 11,400 ml g1 h1. The feed composition was methanol:O2:N2 = 1:3:15 (v/v). Methanol, dimethoxymethane, formaldehyde, and other organic compounds were analyzed by using a GC equipped with FID and TCD detectors connected to Porapak N columns. CO and CO2 were detected by using another GC with a TCD connected to a TDX-01 column. The gas lines were kept at 373 K to prevent condensation of reactants and products. Selectivity was reported on a carbon basis as the percentage of the 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 with different contents of V2O5. For the TiO2 without VOx, typical diffraction peaks of anatase and rutile were observed. It was estimated to have about 69% of anatase and 31% of rutile (by molar fraction) according to the Spurr equation. With the incorporation of VOx, intensities of peaks attributed to anatase increased, while those corresponding to rutile were weakened (see Table 1), suggesting the promotion effect of VOx for the formation of anatase phase. 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 in the VOx–TiO2 samples, indicating that the VOx species were highly dispersed with probably microcrystals less than 4 nm. Laser Raman spectra of the mesoporous VOx–TiO2 materials are depicted in Fig. 2. The bands at 152, 199, 399, 510, and 637 cm1 belonging to anatase were observed for all the samples [20], while the bands due to rutile (250, 437, and 610 cm1) were not apparent [20]. With the addition of vanadia, two features appearing at 960 and 817 cm1 were characteristic of polymeric V–O–V [21] and

218

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

Fig. 1. XRD patterns of the TiO2 and VOx–TiO2 samples with different contents of V2O5.

octahedral decavanadate V10O28 species (or isolated VO4 3 tetrahedral structure) [3,22]. The band at 1020 cm1 due to the symmetric vibration of vanadyl species was not observed. The intensity of this peak was usually affected by moisture in air [23]. The N2 adsorption–desorption isotherms and pore size distributions of the VOx–TiO2 samples with different contents of V2O5 are shown in Fig. 3. The BET surface area, pore size, and pore volume of the samples are listed in Table 1. All the samples exhibited the type IV nitrogen isotherm with a H2 hysteresis loop, characteristic of mesoporous materials. A clear hysteresis loop was shown at a relative pressure between 0.4 and 0.7, related to the capillary condensation associated with mesoporous channels. However, at higher relative pressure between 0.8 and 1.0, the shape of the hysteresis loops was of the type H3 associated with aggregates of platelike particles that formed slitlike pores. Similar adsorption behavior was also observed for titania tubules [24]. The monomodal pore size distribution curves derived from desorption branches (Fig. 3, inlet) implied that the materials had relatively regular pore channels in the mesoporous region. The BET surface area, pore size, and pore volume of the samples are listed in Table 1. The parameters are, respectively, equal to 326 m2 g1, 3.7 nm, and 0.34 ml g1. With the incorporation of 10 wt% V2O5, the surface area of the 10VOx–TiO2 decreased, while the average pore size increased and pore volume remained constant. Further increase in vanadia content led to the gradual decrease in surface area, while the variations of pore size and pore volume were not striking. The morphology and pore structure of the samples were observed by transition electron microscopy (TEM) as shown in Fig. 4. The TiO2 exhibited uniform wormlike pore arrangement with pore size of about 4 nm, in accordance with the pore size determined by N2 adsorption. These wormlike pores seemed to remain but slightly disordered with the addition of V2O5. The mesoporosity was probably due to the intra- and interparticle voids. The results from N2 adsorption–desorption and TEM clearly demonstrate that the VOx–TiO2 prepared were mesoporous materials. Table 1 Anatase content, surface area, pore size, and pore volume of the mesoporous TiO2 and VOx–TiO2 with different contents of V2O5. Sample

Anatase content in TiO2 (%)

SBET (m2 g1)

Pore size (nm)

Pore volume (cm3 g1)

TiO2 10VOx–TiO2 20VOx–TiO2 30VOx–TiO2

69 74 76 83

326 300 260 236

3.7 4.3 4.6 4.4

0.34 0.35 0.33 0.28

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).

3.2. Redox and acidic properties TPR is frequently used to study the redox properties of metal oxide catalysts. The TPR profiles for the VOx–TiO2 samples are shown in Fig. 5. It is seen that no hydrogen consumption peak was observed until 1100 K for the TiO2, indicating that the sample was not reduced up to this temperature. A single broad reduction peak in the 740–765 K range occurred for each of the VOx–TiO2 samples, corresponding to the reduction of highly dispersed vanadia species from V5+ to V3+ [25,26]. With the increase of vanadium content, the area of the reduction peak increased and the peak maximum (TM) shifted to higher temperatures. The single reduction peak seemed to suggest the highly dispersed vanadia species in the VOx–TiO2 samples, consistent with the results of XRD and LRS. Microcalorimetric adsorption of ammonia has been used to determine the number, strength, and strength distribution of surface acidities [27]. In Fig. 6, differential heats versus coverage for NH3 adsorption on the TiO2, 30VOx–TiO2, and SO4 2 =30VOx —TiO2 samples are depicted. The TiO2 exhibited an initial heat of 181 kJ mol1 and ammonia coverage of 649 lmol g1, indicating its strong surface acidity. The 30VOx–TiO2 displayed the initial heat of 147 kJ mol1 and ammonia coverage of 860 lmol g1. It seemed that the incorporation of vanadia produced more acidic sites in the VOx–TiO2 samples. Addition of SO4 2 onto the 30VOx–TiO2 further decreased the initial heat to 116 kJ mol1, but increased the coverage for the adsorption of ammonia to 1000 lmol g1. The decreased initial heat on the addition of sulfate was due to the endothermic interaction of ammonia with polymerized sulfates in the sample. A similar phenomenon was reported for the sulfated titania materials by Desmartin-Chomel et al., and the existence of polymerized sulfate species was confirmed by FTIR [28]. Thus, the microcalorimetric adsorption of ammonia did not seem to titrate the acidic strength of sulfated samples. However, the SO4 2 =30VOx —TiO2 did exhibit more surface acid sites (1001 lmol g1) than the 30VOx–TiO2 (860 lmol g1). The conversion of isopropanol (IPA) has been frequently used as a probe reaction for the characterization of acid–base and redox properties of oxide catalysts [29–31]. It is commonly accepted that isopropanol is dehydrated to produce propylene (PPE) and

219

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

240

240

2.0

2.0

(b)

1.6

1.6 -1

Pore volume (cm g nm )

0.8

0.4

0.0

3 -1

3 -1

160

200

1.2

Volume adsorbed (cm g )

3 -1

Volume adsorbed (cm g )

200

3 -1

-1

Pore volume (cm g nm )

(a)

1

120

10 Pore width (nm)

100

80 40

160

1.2

0.8

0.4

0.0 1

120

240

80

0.2 0.4 0.6 0.8 Relative pressure (P/P 0)

1.0

0.0

0.2 0.4 0.6 0.8 Relative pressure (P/P0)

2.0

0.4

10

100

Pore width (nm)

80

160

(d)

1.6

-1 3 -1

3 -1

0.8

1

1.2

0.8

0.4

0.0

120

1

10

100

Pore width (nm)

80

40

40 0.0

Pore volume (cm g nm )

200

1.2

Volume adsorbed (cm g )

3 -1

Volume adsorbed (cm g )

3 -1

-1

Pore volume (cm g nm )

1.6

0.0

120

1.0

240

2.0

(c)

160

100

40

0.0

200

10 Pore width (nm)

0.2

0.4 0.6 0.8 Relative pressure (P/P0)

1.0

0.0

0.2 0.4 0.6 0.8 Relative pressure (P/P0)

1.0

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), and 30VOx–TiO2 (d).

diisopropyl ether (DIPE) on acidic sites, while it undergoes a dehydrogenation reaction to acetone (ACE) on basic sites in the absence of oxygen. On the other hand, IPA can also be oxidatively dehydrogenated to ACE in an oxidative atmosphere. Thus, the surface acidic and redox properties could be described by the isopropanol probe reaction in the presence of oxygen. Table 2 presents the results for isopropanol probe reaction over the mesoporous TiO2 and VOx–TiO2 at 403 K. The TiO2 exhibited low conversion of isopropanol (0.3%) with 100% selectivity to propylene, suggesting its acidic character without any redox property, consistent with the results from ammonia adsorption microcalorimetry and TPR. The introduction of vanadium species greatly enhanced the conversion of isopropanol. It seemed that the conversion of isopropanol increased with the increase of vanadia content. The conversions of isopropanol on 10VOx–TiO2, 20VOx–TiO2, and 30VOx–TiO2 were 10%, 14%, and 17%, respectively. Meanwhile, the dehydration products propylene and diisopropyl ether were greatly decreased and acetone became the main product, indicating that the VOx–TiO2 exhibited mainly the redox property. The addition of sulfate ions onto VOx–TiO2 further increased the conversion of isopropanol, but the selectivity to acetone was decreased. This was due to the significantly enhanced surface acidity which catalyzed the dehydration of isopropanol to propylene and DIPE. 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 derivatives are widely used in large amounts as, for example, synthetic dyestuffs, resins, drugs, and perfumes. In addition, the oxidation of methanol can be used as a probe reaction to characterize the surface acidic and redox properties of catalysts according to the different products (formaldehyde, dimethyl ether, dimethoxymethane, methyl formate, carbon oxides, and hydrocarbons) [16]. It has been reported that methanol could be converted into dimethyl ether (DME), formaldehyde (FA), methyl formate (MF), dimethoxymethane (DMM) on acidic, oxidative, and acidic and oxidative bifunctional surfaces. Therefore, the surface acidic and redox properties of mesoporous TiO2, VOx–TiO2, and SO4 2 =VOx —TiO2 were characterized via the oxidation of methanol in this work. The results are presented in Table 3. According to the above characterization results, the TiO2 exhibited some surface acidity without any redox ability. Thus, the conversion of methanol was low (0.2%) over the TiO2 at 403 K, and DME was the only product. The incorporation of 10 wt% vanadia significantly increased the conversion of methanol (8%) and the

220

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

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 of V2O5.

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

Fig. 6. Differential heat versus coverage for NH3 adsorption at 423 K over the TiO2, 30VOx–TiO2 and SO4 2 =30VOx —TiO2 samples.

Table 2 Conversion of isopropanol over the TiO2, VOx–TiO2, and SO4 2 =VOx —TiO2 samples in air at 403 K. Catalyst

TiO2 10VOx–TiO2 20VOx–TiO2 30VOx–TiO2 SO4 2 =10VOx —TiO2 SO4 2 =20VOx —TiO2 SO4 2 =30VOx —TiO2

Isopropanol conversion (%)

Selectivity (%) Propylene

Diisopropyl ether

Acetone

0.3 10 14 17 20 21 24

100 10 11 16 43 44 32

0 6 8 9 24 25 17

0 84 81 75 33 31 51

J. Liu et al. / Journal of Colloid and Interface Science 335 (2009) 216–221 Table 3 Oxidation of methanol over the TiO2, VOx–TiO2, and (SO4)2/VOx–TiO2 samples. Catalyst

Temperature (K)

Conversion (%)

Selectivity (%) DMM

FA

MF

DME

TiO2

403 413 423 433 403 413 423 433 403 413 423 433 403 413 423 433 403 413 423 433 403 413 423 433 403 413 423 433

0.2 0.6 0.9 1.4 8 11 15 19 15 20 26 33 23 32 38 47 11 16 19 28 18 23 29 40 27 41 57 75

0 0 0 0 93 82 61 39 90 75 45 17 86 62 33 12 94 83 74 36 90 74 51 27 94 90 83 68

0 0 0 0 4 11 21 30 6 16 36 46 6 15 27 31 0 9 13 39 5 15 29 42 0 0 1 6

0 0 0 0 3 7 17 31 4 8 19 36 8 22 39 57 1 2 4 12 3 7 15 25 5 9 14 24

100 100 100 100 0.0 0.2 0.2 0.3 0.1 0.1 0.2 0.2 0.1 0.2 0.3 0.3 5 7 9 12 2 3 5 6 1 1 1 2

10VOx–TiO2

20VOx–TiO2

30VOx–TiO2

SO4 2 =10VOx —TiO2 a

SO4 2 =20VOx —TiO2 a

SO4 2 =30VOx —TiO2 a

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

seemed that the surface acidity of the VOx–TiO2 catalysts were not strong enough for the condensation reaction to form DMM. The addition of SO4 2 onto the VOx–TiO2 resulted in the significant increase of methanol conversion and DMM selectivity. For example, at 423 K, the conversion of methanol and the selectivity to DMM on the 30VOx–TiO2 were 38% and 33%, respectively, while they were 57% and 83%, respectively, on the SO4 2 =30VOx —TiO2 . The considerable increase in the methanol conversion and DMM selectivity was surely due to the enhanced surface acidity on the addition of SO4 2 . The increased surface acidity promoted the condensation of formaldehyde with methanol, and thus enhanced the selectivity to DMM. In our previous work, we prepared an optimized catalyst 10%V2O5/TiO2–Ti(SO4)2, which exhibited 38% conversion of methanol and 94% selectivity to DMM at 423 K [17]. Apparently, the present catalyst SO4 2 =30VOx —TiO2 showed the better catalytic reactivity for the selective oxidation of methanol to DMM under the same reaction conditions. 4. Conclusions Mesoporous VOx–TiO2 with high surface areas were successfully synthesized via the evaporation-induced self-assembly method combined with the ammonia solution posttreatment. The meso-

221

porous materials exhibited acidic and redox bifunctional characters that catalyzed the selective oxidation of methanol to dimethoxymethane (DMM). The surface acidity of these materials was further enhanced by the addition of sulfate anions, and the sulfated (SO2 4 Þ/30VOx ATiO2 was found to catalyze the selective oxidation of methanol to DMM with high conversion (57%) and selectivity to DMM (83%) at 423 K. Acknowledgments We acknowledge financial support from the NSFC (20673055) and the MSTC (2005CB221400 and 2004DFB02900). References [1] F. Roozeboom, P.D. Cordingley, P.J. Gellings, J. Catal. 68 (1981) 464. [2] A.S. Elmi, E. Tronconi, C. Cristiani, J.P. Gomez Martin, P. Forzatti, G. Busca, Ind. Eng. Chem. Res. 28 (1989) 387. [3] M. Sanati, A. Andersson, J. Mol. Catal. 59 (1990) 233. [4] P.H. Mutin, A.F. Popa, A. Vioux, G. Delahay, B. Coq, Appl. Catal. B 69 (2006) 49. [5] R. Grabowski, S. Pietrzyk, J. Sloczynski, F. Genser, K. Wcislo, B. GrzybowskaSwierkosz, Appl. Catal. A 232 (2002) 277. [6] P.D. Yang, D.Y. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Nature 396 (1998) 152. [7] B. Smarsly, D. Grosso, T. Brezesinski, N. Pinna, C. Boissiere, M. Antonietti, C. Sanchez, Chem. Mater. 16 (2004) 2948. [8] D. Grosso, G. Soler-Illia, E.L. Crepaldi, F. Cagnol, C. Sinturel, A. Bourgeois, A. Brunet-Bruneau, H. Amenitsch, P.A. Albouy, C. Sanchez, Chem. Mater. 15 (2003) 4562. [9] D. Grosso, G. Soler-Illia, F. Babonneau, C. Sanchez, P.A. Albouy, A. BrunetBruneau, A.R. Balkenende, Adv. Mater. 13 (2001) 1085. [10] B.Z. Tian, X.Y. Liu, B. Tu, C.Z. Yu, J. Fan, L.M. Wang, S.H. Xie, G.D. Stucky, D.Y. Zhao, Nat. Mater. 2 (2003) 159. [11] K. Cassiers, T. Linssen, M. Mathieu, Y.Q. Bai, H.Y. Zhu, P. Cool, E.F. Vansant, J. Phys. Chem. B 108 (2004) 3713. [12] K. Cassiers, T. Linssen, V. Meynen, P. Van der Voort, P. Cool, E.F. Vansant, Chem. Commun. (2003) 1178. [13] C.J. Brinker, Y.F. Lu, A. Sellinger, H.Y. Fan, Adv. Mater. 11 (1999) 579. [14] G. Soler-Illia, A. Louis, C. Sanchez, Chem. Mater. 14 (2002) 750. [15] Y. Segura, L. Chmielarz, P. Kustrowski, P. Cool, R. Dziembaj, E.F. Vansant, J. Phys. Chem. B 110 (2006) 948. [16] J.M. Tatibouet, Appl. Catal. A 148 (1997) 213. [17] Y. Fu, J. Shen, Chem. Commun. (2007) 2172. [18] M.A. Banares, L.J. Alemany, M.C. Jimenez, M.A. Larrubia, F. Delgado, M.L. Granados, A. Martinez-Arias, J.M. Blasco, J.L.G. Fierro, J. Solid State Chem. 124 (1996) 69. [19] W.E. Slink, P.B. DeGroot, J. Catal. 68 (1981) 423. [20] B.M. Reddy, A. Khan, Y. Yamada, T. Kobayashi, S. Loridant, J.C. Volta, J. Phys. Chem. B 107 (2003) 5162. [21] C.L. Zhao, I.E. Wachs, Catal. Today 118 (2006) 332. [22] G.T. Went, S.T. Oyama, A.T. Bell, J. Phys. Chem. 94 (1990) 4240. [23] C. Cristiani, P. Forzatti, G. Busca, J. Catal. 116 (1989) 586. [24] T.Y. Peng, A. Hasegawa, J.R. Qiu, K. Hirao, Chem. Mater. 15 (2003) 2011–2016. [25] S. Besselmann, C. Freitag, O. Hinrichsen, M. Muhler, Phys. Chem. Chem. Phys. 3 (2001) 4633. [26] G.C. Bond, J.P. Zurita, S. Flamerz, P.J. Gellings, H. Bosch, J.G. Van Ommen, B.J. Kip, Appl. Catal. 22 (1986) 361. [27] A. Auroux, Top. Catal. 4 (1997) 71. [28] A. Desmartin-Chomel, J.L. Flores, A. Bourane, J.M. Clacens, F. Figueras, G. Delahay, A.G. Fendler, C. Lehaut-Burnouf, J. Phys. Chem. B 110 (2006) 858. [29] D. Kulkarni, I.E. Wachs, Appl. Catal. A 237 (2002) 121. [30] X. Gu, J. Ge, H. Zhang, A. Auroux, J. Shen, Thermochim. Acta 451 (2006) 84. [31] F.M. Bautista, J.M. Campelo, D. Luna, J. Luque, J.M. Marinas, Catal. Today 128 (2007) 183. [32] L. Baraket, A. Ghorbel, P. Grange, Appl. Catal. B 72 (2007) 37. [33] Q. Sun, Y. Fu, J. Liu, A. Auroux, J. Shen, Appl. Catal. A 334 (2008) 26.