Effect of acidic promoters on the titania nanotubes supported V2O5 catalysts for the selective oxidation of methanol to dimethoxymethane

Chinese Journal of Catalysis 34 (213) 211 2117 催化学报 213 年第 34 卷第 11 期 www.chxb.cn available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/chnjc Article (Dedicated to Professor Yi Chen on the occasion of his 8th birthday) Effect of acidic promoters on the titania nanotubes supported V2O5 catalysts for the selective oxidation of methanol to dimethoxymethane Jingxuan Cai, Yuchuan Fu #, Qing Sun, Minhui Jia, Jianyi Shen * Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 2193, Jiangsu, China A R T I C L E I N F O Article history: Received 7 July 213 Accepted 16 August 213 Published 2 November 213 Keywords: Selective oxidation of methanol Dimethoxymethane Acidic additive Surface acidity Surface redox property A B S T R A C T The effect of acidic promoters on the titania nanotubes (TNT) supported V2O5 catalysts () was investigated. The structure of TNT was quite stable after the treatment with sulfuric, phosphoric, and phosphotungstic acids, respectively. The acid modified catalysts were tested for the selective oxidation of methanol to dimethoxymethane (DMM). It was found that only the modified with sulfuric acid followed by calcination at 673 K exhibited the significantly enhanced selectivity to DMM with high methanol conversions. The calcination created some sulfate groups strongly interacted with vanadium species, which enhanced the strengths of surface acidity without weakening the redox ability of vanadium sites. The addition of phosphoric and phosphotungstic acids might enhance the surface acidity of V2O5/TiO2, but weakened its redox ability, and therefore had the negative effect for the target reaction. 213, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. 1. Introduction Titania nanotubes (TNT) possess high surface areas, thermal stability, and photoactivity, and are used in many applications such as environmental protection [1], chemical synthesis [2], catalyst supports [3] and dye sensitized solar cell fabrications [4,5]. Among these applications, TNT was intensively studied as catalyst supports. For example, TNT has been used to support noble metals for some reactions such as double bond migration [6,7], water gas shift reaction [8], and CO2 adsorption and hydrogenation [1]. Recently, we found that TNT supported V2O5 catalysts (shorted as ) exhibited high conversion of methanol (64%) with high selectivity to dimethoxymethane (DMM) (9%) at a relatively low reaction temperature (43 K) for the selective oxidation of methanol [3]. This exellent performance of at low reaction temperature was ascribed to the high surface area of TNT, which was stable during the catalyst preparation and catalytic reaction. The prepared must be properly modified by an acidic component in order to enhance the surface acidity to achieve a high selectivity to DMM at high conversion of methanol. As reported previously [3,9 11], sulfates (SO4 2 ) have been used as effective acidic promoters for. However, the sulfates decomposed during the calcination of catalysts, leading to the loss of some sulfates, so that it was difficult to control accurately the amount of remaining sulfates in the catalysts. In the present work, we tried other acidic components to modify the to see their effects on * Corresponding author. Tel: +86 25 8359435; Fax: +86 25 83317761; E mail: jyshen@nju.edu.cn # Corresponding author. Tel: +86 25 8359435; Fax: +86 25 83317761; E mail: ycfu@nju.edu.cn This work was supported by the National Natural Science Foundation of China (217388), the Specialized Research Fund for the Doctoral Program of Higher Education (28284138), the National Basic Research Program of China (25CB2214), and the Fundamental Research Funds for the Central Universities. DOI: 1.116/S1872 267(12)669 1 http://www.sciencedirect.com/science/journal/1872267 Chin. J. Catal., Vol. 34, No. 11, November 213

Jingxuan Cai et al. / Chinese Journal of Catalysis 34 (213) 211 2117 2111 the selective oxidation of methanol to DMM as well as on the structure and surface areas of. Specifically, phosphoric acid (H3PO4) and phosphotungstic acid (H3PW12O4, PTA) were added respectively into in order to compare with sulfuric acid (H2SO4) for the modification of. The acidity of phosphoric acid is weaker than sulfuric acid while that of PTA is stronger than sulfuric acid [12]. Moreover, PTA is of strong redox property besides the strong acidity. The modification with PTA might enhance both the surface acidity and redox ability of catalysts. 2. Experimental 2.1. Preparation of samples TNT were prepared via a hydrothermal method from a commercial TiO2 (Degussa P25, SBET = 52 m 2 g 1 ) and concentrated NaOH according to the previous work [3]. A catalyst with 2 wt% of V2O5 was prepared by the incipient wetness impregnation method by using an aqueous solution of vanadium oxalate as the vanadium precursor [13]. The obtained sample was divided into four shares. One was impregnated with diluted H2SO4 solution and dried at room temperature and then at 393 K for 12 h (denoted as S 393). Then, part of S 393 was calcined at 673 K for 1 h, and a sample S 673 was obtained. In a similar way, samples P 393, P 673, W 393, and W 673 were prepared by using the aqueous solutions containing H3PO4 (for P) and PTA (for W), respectively. The amount of an acid added was 5% by weight of. 2.2. Characterization Powder X ray diffraction (XRD) patterns were collected on a Philips X Pert Pro diffractometer using Ni filtered Cu Kα radiation (λ =.15418 nm), operated at 4 kv and 4 ma at a scanning rate of.417 per second. Transmission electron microscopy (TEM) was performed on a JEOL JEM 21 microscope with an accelerating voltage of 2 kv. The samples were dispersed in ethanol under ultrasonic conditions and deposited onto copper grids coated with ultrathin carbon films. Nitrogen adsorption desorption isotherms were measured at the liquid nitrogen temperature using a Micromeritics ASAP 22 analyzer. Prior to a measurement, the sample was degassed to 1 1 3 Torr at 473 K. Pore size distribution and pore volume were determined by the Barrett Joyner Halenda (BJH) method according to the desorption branch of an isotherm. The Fourier transform infrared spectroscopy (FTIR) spectra were recorded with a Bruker Vector 22 Fourier transform spectrometer in the range of 4 4 cm 1, with a resolution of 2 cm 1 and 2 acquisition scans. Laser Raman spectra (LRS) were acquired on a Renishaw invia Raman microscope with 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 1 scans. Elemental analysis was performed with an X ray fluorescence (XRF) spectrometer (ARL98XP+, ARL Co., Switzerland) operated at 5 kv and 5 ma. Hydrogen temperature programmed reduction (TPR) measurements were carried out in a continuous mode using a quartz microreactor (3.5 mm in diameter). A sample of about 5 mg was contacted with a 5.13% H2/N2 mixture at a total flow rate of 4 ml min 1. The sample was heated at a rate of 1 K min 1 from room temperature to 117 K. Hydrogen consumption was monitored by using a thermal conductivity detector (TCD). The reducing gas first passed through the reference arm of the TCD before entering the reactor. The flow out of the reactor was directed through a trap filled with Mg(ClO4)2 (to remove water from the reduction) and then to the second arm of the TCD. Microcalorimetric adsorption of ammonia was performed at 423 K by using a Tian Calvet type heat flux Setaram C8 calorimeter. The calorimeter was connected to a volumetric system equipped with a Baratron capacitance manometer for the pressure measurement and gas handling. About.1 g sample (, S 673, P 673, or W 673) was pretreated in 5 Torr O2 at 573 K for 1 h, followed by evacuation at the same temperature for 1 h. Similarly, the S 393, P 393 and W 393 were pretreated at 473 K. Ammonia as a probe molecule was purified with the successive freeze pump thaw cycles. 2.3. Catalytic tests 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,.2 g of a catalyst was loaded, and the gas hourly space velocity (GHSV) was 114 ml g 1 h 1. The feed composition was maintained as methanol:o2:n2 = 1:3:15 (v/v). Methanol, DMM, 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 1 column. The gas lines were kept at 373 K to prevent condensation of reactants and products. The, S 673, P 673, and W 673 were pretreated by heating in air at 673 K for 1 h and then cooled in the same flow to the reaction temperature. The S 393, P 393, and W 393 were pretreated at 473 K. 3. Results and discussion 3.1. Structural characterizations Table 1 shows the composition, BET surface area, average pore diameter, and pore volume of the catalysts modified by different acids. It is seen that the possessed a quite high surface area (192 m 2 g 1 ) and pore volume (.8 cm 3 g 1 ), which were decreased upon the addition of acids. The surface area and pore volume were further decreased after calcination

2112 Jingxuan Cai et al. / Chinese Journal of Catalysis 34 (213) 211 2117 Table 1 Composition, BET surface area, average pore diameter, and pore volume of the catalysts modified by different acids. Catalyst Composition a (wt%) SBET Pore size Pore volume V2O5 SO3 P2O5 WO3 (m 2 g 1 ) (nm) (cm 3 g 1 ) 19.7 192 13.5.8 S 393 18.9 4.5 166 13.4.71 S 673 19.4 1.5 136 16.4.73 P 393 19.2 3.5 177 13.2.73 P 673 18.4 3.5 163 14.1.75 W 393 18.6.2 4.9 176 13..72 W 673 18.6.2 4.8 162 14.4.74 a Measured by XRF. at 673 K. This effect was more pronounced for the sulfuric acid modified catalyst. However, the pore sizes were slightly increased for the catalysts after the calcination, probably owing to the increase of interspaces among catalyst particles rather than the increase of pore sizes of the TNT support. After calcination at 673 K, the sulfur content (based on the amount of SO3 remained) in the sulfuric acid modified catalyst decreased from 4.5 to 1.5 wt%, indicating that only small amount of SO4 2 could be strongly bonded on the catalyst surface. On the other hand, the contents of other two acids added were not decreased upon the calcination at 673 K. Figure 1 shows the TEM images of TNT and TNT supported V2O5 catalysts. It is seen that the TNT had the homogeneous diameter of about 5 nm (Fig. 1 ). After loading V2O5 by impregnation, the morphology of TNT did not seem to be changed significantly (only a few TNT were ruptured, Fig. 1 (b)). More TNT seemed to be ruptured upon the addition of sulfuric acid (Fig. 1 and (d)), but the tubular morphology of TNT was mostly remained. Similar changes were observed for the catalysts upon the addition of phosphoric acid and PTA (Fig. 1 (e h)). Figure 2 shows the nitrogen adsorption desorption isotherms and corresponding BJH pore size distributions for the and H2SO4 modified samples before and after the calcination. These isotherms belong to the type IV with an H3 hysteresis loop, characteristic of slit shaped mesoporous structure. Thus, the mesoporous structure of TNT remained after the addition of vanadia and sulfuric acid followed by calcination at 673 K. The catalysts modified by phosphoric acid and PTA displayed the similar adsorption desorption isotherms and corresponding BJH pore size distributions (not shown). Figure 3 shows the XRD patterns for the unmodified and modified with sulfuric acid, phosphoric acid, and PTA. No crystalline V2O5 was observed in the XRD profiles, indicating (b) (d) (e) (f) (g) (h) Fig. 1. TEM images of TNT and TNT supported V2O5 catalysts. TNT; (b) ; S 393; (d) S 673; (e) P 393; (f) P 673; (g) W 393; (h) W 673. Volume adsorbed (cm 3 g 1, STP) 6 5 4 3 2 1 Pore Volume ( cm 3 g -1 nm -1 ) 1.8 1.6 1.4 1.2 1..8.6.4.2. 1 2 3 4 5 Pore width (nm)..2.4.6.8 1. Relative pressure (p/p ) Volume adsorbed (cm 3 g 1, STP) 6 5 4 3 2 1 Pore Volume ( cm 3 g -1 nm -1 ) 1.8 1.6 1.4 1.2 1..8.6.4.2. 1 2 3 4 5 Pore width (nm) (b)..2.4.6.8 1. Relative pressure (p/p) Volume adsorbed (cm 3 g 1, STP) 6 5 4 3 2 1 Pore Volume ( cm 3 g -1 nm -1 ) 1.8 1.6 1.4 1.2 1..8.6.4.2. 1 2 3 4 5 Pore width (nm)..2.4.6.8 1. Relative pressure (p/p) Fig. 2. Nitrogen adsorption desorption isotherms and corresponding BJH pore size distributions for, S 393 (b), and S 673.

Jingxuan Cai et al. / Chinese Journal of Catalysis 34 (213) 211 2117 2113 Intensity (a. u.) Anatase TiO 2 -B ( d') 1 2 3 4 5 6 7 8 2 (Degree) the high dispersion of V2O5 in the catalysts even after the calcination at 673 K. Figure 4 shows the skeletal FTIR spectra of the catalysts. Two IR bands around 98 and 82 cm 1 were observed for (Fig. 4 ), which could be assigned to monovanadate and polyvanadate species bonded to the surface of TiO2, respectively [14]. Addition of sulfuric acid did not have any effect on the two bands, indicating that no strong interaction occurred between vanadium species and sulfuric acid in the S 393 dried at 393 K. The intensities of the two bands increased for the sample (S 673) after calcination at 673 K, indicating the increased interaction between the surface vanadium species and sulfates. For comparison, the IR spectra for TNT supported sulfuric acid were obtained. Three bands around 125, 113, and 155 cm 1 were observed for the sample prepared upon the addition of sulfuric acid onto TNT (STNT) followed by drying at 393 K (STNT 393). These bands were characteristic features of sulfate groups on the surface of TiO2 [15]. After calcination at 673 K, the band around 125 cm 1 decreased its intensity, while the two bands around 113 (d) ( c') ( b') Fig. 3. XRD patterns of, S 393 (b), S 673 (b ), P 393, P 673 (c ), W 393 (d), and W 673 (d ). (b) and 155 cm 1 merged into a broad band around 112 cm 1. These results indicated that most sulfate groups were decomposed while the remaining ones strengthened their interactions with the surface of TNT upon the calcination at 673 K. The FTIR bands for sulfate groups in the S 393 were around 12, 112, and 155 cm 1 with some blue shifts as compared to those of STNT (125 and 113 cm 1 ), indicating the interactions of sulfate groups with vanadium species in the catalyst. After calcination at 673 K, the two bands around 12 and 155 cm 1 disappeared due to the decomposition of sulfate groups while the one around 112 cm 1 remained. Thus the band around 112 cm 1 might belong to the sulfate groups interacted strongly with surface vanadium species. Addition of phosphoric acid onto TNT resulted in an IR band around 14 cm 1 (for PTNT 393) as shown in Fig. 4(b). This band remained (for PTNT 673) after the calcination at 673 K. It is typical of phosphate groups and was significantly broadened due to the reduced symmetry upon the dispersion on the support [16]. This band shifted to 11 cm 1 when phosphate groups were added into, indicating the interaction of phosphate groups with surface vanadium species [17]. After calcination at 673 K, the band around 11 cm 1 remained, while the bands around 98 and 82 cm 1 for V=O and V O V vibrations were weakened, which might be due to the interaction of vanadium species with phosphate groups or the coverage of some vanadium species by phosphate groups [17], leading to the decrease of oxidation ability of the catalyst. Addition of PTA onto TNT resulted in two IR bands around 98 and 82 cm 1 (for WTNT 393), as shown in Fig. 4, which could be assigned to phosphotungstate anions [18]. After the calcination at 673 K, these bands remained (for WTNT 673) with decreased intensities, indicating the decomposition of some Keggin structures of phosphotungstate anions. Since these bands overlapped with those of surface vanadium species, the addition of PTA into resulted in the increased intensities of the two bands. According to the IR result of WTNT 673, it was expected that some Keggin structures of phosphotungstate anions remained in the W 673 even after it was calcined at 673 K. Figure 5 shows the Raman spectra of the catalysts from 9 to 11 cm 1. The band around 13 cm 1 might be assigned to the terminal vibrations of V=O in, while the band around S-673 (b) P-673 113 S-393 P-393 155 11 12 113 82 82 98 TNT 98 TNT PTNT-393 STNT-393 125 113 155 STNT-673 PTNT-673 112 14 14 12 1 8 6 14 12 1 8 6 Wavenumber (cm -1 ) Wavenumber (cm -1 ) Transmittance (a.u.) Transmittance (a.u.) Transmittance (a.u.) W-673 W-393 82 98 TNT WTNT-393 WTNT-673 98 82 14 12 1 8 6 Wavenumber (cm -1 ) Fig. 4. Skeletal FT IR spectra of catalysts modified by H2SO4, H3PO4 (b), and H3PW12O4.

2114 Jingxuan Cai et al. / Chinese Journal of Catalysis 34 (213) 211 2117 Intensity (a. u.) 945 13 9 95 1 15 11 Ramanshift (cm -1 ) 945 cm 1 for V O V vibrations could not be clearly observed [19]. Upon the addition of sulfuric acid, the intensity of band around 13 cm 1 increased [2], indicating more V=O sites appeared on the surface of S. In contrast, the intensity of band around 13 cm 1 was weakened upon the addition of phosphoric acid, indicating the decreased number of surface V=O groups, probably due to the interaction of vanadium species with phosphate groups or the coverage of some vanadium species by phosphate groups [17], in consistence with the IR results presented above. The band around 13 cm 1 was significantly broadened for the W 673, suggesting the interactions of V=O with added phosphotungstate anions. 3.2. Surface redox and acidic properties ( d') Figure 6 shows the TPR profiles of catalysts modified by sulfuric acid, phosphoric acid, and PTA. For comparison, the TPR profiles of different acids supported on TNT were also presented. TNT itself did not have a peak [5]. After loading vanadia, the exhibited an intensive TPR peak around 79 K (Fig. 6 ), corresponding to the reduction of highly dispersed vanadia from V 5+ to V 3+ [21]. There were two peaks higher than 79 K for sulfuric acid supported on TNT. However, these two (d) ( c') ( b') Fig. 5. Raman spectra of, S 393 (b), S 673 (b ), P 393, P 673 (c ), W 393 (d), and W 673 (d ). (b) peaks disappeared when sulfuric acid was supported on, indicating the interaction of sulfate groups with vanadium species. In fact, the TPR peak was broadened for S 393, as compared to that of, indicating the reduction of sulfate groups with vanadium species around 78 K. The TPR peak of S 673 was similar to that of, owing to the fact that most sulfate groups had been decomposed upon the calcination at 673 K. Although TPR peaks higher than 79 K were observed for phosphoric acid supported on TNT (Fig. 6 (b)), addition of phosphoric acid into did not seem to have any effect on the reduction of vanadium species in the P catalysts because similar TPR peaks were observed for P and. The TPR peaks were hardly seen for PTA supported on TNT (Fig. 6 ). However, addition of PTA into decreased the temperatures of TPR peaks for W catalysts, suggesting the enhanced redox ability of promoted by PTA. The surface acidities of modified by different acids were probed by the microcalorimetric adsorption of ammonia. Figure 7 presents the results. Figure 7 shows that the exhibited the high initial heat (2 kj mol 1 ) and coverage (4.9 μmol m 2 ) for the adsorption of ammonia. Addition of sulfuric acid followed by drying at 393 K decreased the initial heat to 174 kj mol 1 but increased the ammonia coverage to 6.97 μmol m 2. After calcination at 673 K, the initial heat increased to 23 kj mol 1, but the coverage decreased to 6 μmol m 2 for the S 673. Apparently, the calcination at 673 K decomposed most sulfate groups in the catalyst but created some sulfate groups that were strongly interacted with titania and vanadia species and were thus highly acidic [22,23]. Addition of phosphoric acid into greatly decreased the initial heat (11 kj mol 1 ) for the adsorption of ammonia on P 393 (Fig. 7 (b)). This might be due to the formation of some V P O compounds that possessed relatively low surface acidity [24]. However, the coverage of ammonia was significantly increased (6.63 μmol m 2 ), indicating the increased density of surface acid sites. The calcination at 673 K further decreased the initial heat, but the density of acid sites was not changed, as seen in Fig. 7 (b) for the adsorption of ammonia on P 673. Addition of PTA into decreased the initial heat a little (from 2 to 186 kj mol 1 ) as seen in Fig. 7 for the adsorption of ammonia on W 393. However, the calcination at 673 K significantly decreased the initial heat (to 133 kj mol 1 ), H2 consumption (a. u.) 78 S-673 78 S-393 STNT-673 STNT-393 79 4 6 8 1 Temperature (K) H2 consumption (a. u.) 788 (b) P-673 79 P-393 PTNT-673 PTNT-393 79 4 6 8 1 Temperature (K) H2 consumption (a. u.) 77 W-673 776 W-393 WTNT-673 WTNT-393 2 79 4 6 8 1 Temperature (K) Fig. 6. H2 TPR profiles of catalysts modified by H2SO4, H3PO4 (b), and H3PW12O4.

Jingxuan Cai et al. / Chinese Journal of Catalysis 34 (213) 211 2117 2115 Differential heat (kj mol -1 ) 2 16 12 8 4 S-393 S-673 1 2 3 4 5 6 7 8 Coverage ( mol m -2 ) Differential heat (kj mol -1 ) 2 16 12 8 4 (b) P-393 P-673 1 2 3 4 5 6 7 Coverage ( mol m -2 ) Differential heat (kj mol -1 ) 2 16 12 8 4 W-393 W-673 1 2 3 4 5 6 Coverage ( mol m -2 ) Fig. 7. Microcalorimetric adsorption of ammonia on the and modified by different acids at 423 K. probably due to the decomposition of crystalline PTA. Addition of PTA did not seem to change the density of acid sites in the W catalysts. 3.3. Catalytic reactions Table 2 presents the results for the selective oxidation of methanol to DMM over catalysts modified with different acids. The possessed redox and acidic properties. The activity and selectivity of this reaction depend on the surface properties of catalysts as well as the reaction conditions (especially the reaction temperature). Generally, methanol was oxidized on redox sites to form adsorbed formaldehyde, which was then condensed with methanol to form DMM on acidic sites [25]. The intermediate product, formaldehyde, could also be further oxidized into methyl formate and COx [26]. The data in Table 2 shows that the exhibited high selectivity to DMM (93%) at 383 K. However, with the increase of reaction temperature, the selectivity to DMM on the decreased rapidly, indicating that the surface acidity of was not strong enough so that the high selectivity to DMM could not be maintained at the high methanol conversions. The modification of surface acidity was a way to enhance the yield of DMM [3]. Addition of sulfuric acid into greatly increased the surface acidity of S 393 but inhibited its redox property. Accordingly, the selectivity to DMM at the higher reaction temperatures was increased while the conversion of methanol was decreased on the S 393. As discussed above, the calcination at 673 K decomposed most surface sulfate groups, and only some of them were remained. Such sulfate groups were Table 2 Selective oxidation of methanol to dimethoxymethane over acid modified catalysts at 43 K. Catalyst Methanol Selectivity (%) conversion (%) DMM FA MF DME COx 51 51 24 25 S 393 34 82 13 3 2 S 673 58 92 2 6 P 393 37 76 18 6 P 673 31 85 14 1 W 393 54 37 33 3 1 W 673 44 55 26 19 Feed conditions: methanol/o2/n2 = 1/3/15, GHSV = 114 ml g 1 h 1. DMM: dimethoxymethane; FA: formaldehyde; MF: methyl formate; DME: dimethyl ether; COx: CO2 and/or CO. those interacted strongly with vanadia and enhanced the surface acidity of S 673 without weakening its redox ability. Thus, the selectivity to DMM could be remained high (92%) with the high conversion of methanol (58%) over the S 673 at 43 K. Although the addition of phosphoric acid into increased the surface acidity of P, its redox ability was weakened, as revealed by the results of microcalorimetric adsorption of ammonia and FTIR. Thus, the selectivity to DMM was increased on P with decreased conversion of methanol. PTA is known to be a strong oxidative agent and a strong acid simultaneously. The TPR results in Fig. 6 show that the redox ability of was somewhat enhanced upon the addition of PTA. On the other hand, the addition of PTA into increased the acid catalyzed products on W 393. These results indicated that the addition of PTA enhanced the surface acidity more than redox ability for the probe reaction. However, most PTA might be decomposed, and the resulted compounds might cover the redox sites of vanadia species. Thus, the effect of addition of PTA was similar to that of phosphoric acid for the selective oxidation of methanol to DMM. Figure 8 shows the dependence of selectivity to DMM on the conversion of methanol for the modified with different acids. Apparently, only the S 673 exhibited the positive effect, i.e., the high selectivity to DMM could only be obtained on the S 673 with high conversion of methanol. Addition DMM selectivity (%) 1 8 6 4 2 S-393 S-673 P-393 P-673 W-393 W-673 2 4 6 8 1 Methanol conversion (%) Fig. 8. Selectivity to DMM vs conversion of methanol over the and modified by different acids catalysts. Feed conditions: methanol/o2/n2 = 1/3/15 (v/v), GHSV = 114 ml g 1 h 1. Data were collected at the temperatures 383 423 K.

2116 Jingxuan Cai et al. / Chinese Journal of Catalysis 34 (213) 211 2117 Chin. J. Catal., 213, 34: 211 2117 Graphical Abstract doi: 1.116/S1872 267(12)669 1 Effect of acidic promoters on the titania nanotubes supported V2O5 catalysts for the selective oxidation of methanol to dimethoxymethane Jingxuan Cai, Yuchuan Fu *, Qing Sun, Minhui Jia, Jianyi Shen * Nanjing University Addition of sulfate groups followed by calcination at 673 K strengthened the surface acidity of V2O5/TiO2 and thus promoted the selectivity to dimethoxymethane at the high methanol conversions, while the addition of phosphoric and phosphotungstic acids in any case or the addition of sulfate groups without a high temperature calcination had the negative effect because they decreased either the surface acidity or redox ability of V2O5/TiO2. DMM selectivity (%) 1 8 6 4 2 2 S2-393 S2-673 P2-393 P2-673 W2-393 W2-673 2 4 6 8 1 Methanol conversion (%) of phosphoric acid and PTA had only the negative effect. Without calcination at the high temperature, the addition of sulfuric acid also had the negative effect for the selective oxidation of methanol to DMM. Thus, the catalyst for the selective oxidation of methanol to DMM should be preferably modified by the small amount of sulfate groups interacted strongly with the redox sites of surface vanadium species formed during the calcination at 673 K. 4. Conclusions In order to enhance the surface acidity of titania nanotubes supported vanadia catalyst (2%V2O5/TNT) to increase the selectivity to dimethoxymethane (DMM) from the oxidation of methanol, the V2O5/TNT was modified with sulfuric, phosphoric, and phosphotungstic acids, respectively. Although the addition of these acids led to some ruptures of the TNT structure, the tubular morphology of TNT was mostly remained. The addition of these acids without calcination enhanced the surface acidity but weakened the redox ability of V2O5/TNT, leading to the decreased activity for the selective oxidation of methanol to DMM. After the calcination at 673 K, the strengths of surface acidity of the catalysts modified by phosphoric and phosphotungstic acids were decreased while that modified by sulfuric acid increased. In any case, the modification with phosphoric or phosphotungstic acids had the negative effect for the oxidation of methanol to DMM because they might cover some redox sites of vanadia species. In contrast, the sulfate groups strongly interacted with redox sites of vanadia species remained after the calcination enhanced the surface acidity without weakening the redox ability of vanadia species, and thus promoted the selectivity to DMM at the high methanol conversions. References [1] Yu K P, Yu W Y, Ku M C, Liou Y C, Chien S H. Appl Catal B, 28, 84: 112 [2] Vijayan B, Dimitrijevic N M, Rajh T, Gray K. J Phys Chem C, 21, 114: 12994 [3] Liu J W, Fu Y C, Sun Q, Shen J Y. Microporous Mesoporous Mater, 28, 116: 614 [4] Jiang G D, Lin Z F, Zhu L H, Ding Y B, Tang H Q. Carbon, 21, 48: 3369 [5] Cummings F R, Le Roux L J, Mathe M K, Knoesen D. Mater Chem Phys, 21, 124: 234 [6] Torrente Murciano L, Lapkin A A, Bavykin D V, Walsh F C, Wilson K. J Catal, 27, 245: 272 [7] Sato Y, Koizumi M, Miyao T, Naito S. Catal Today, 26, 111: 164 [8] Idakiev V, Yuan Z Y, Tabakova T, Su B L. Appl Catal A, 25, 281: 149 [9] Fu Y C, Shen J Y. Chem Commun, 27: 2172 [1] Zhao H Y, Bennici S, Shen J Y, Auroux A. J Catal, 21, 272: 176 [11] Guo H Q, Li D B, Jiang D, Li W H, Sun Y H. Catal Commun, 21, 11: 396 [12] Mizuno N, Misono M. Chem Rev, 1998, 98: 199 [13] Bond G C. Appl Catal A, 1997, 157: 91 [14] Sun Q, Fu Y C, Liu J W, Auroux A, Shen J Y. Appl Catal A, 28, 334: 26 [15] Chen J P, Yang R T. J Catal, 1993, 139: 277 [16] Samantaray S K, Parida K. J Mol Catal A, 21, 176: 151 [17] Blanco J, Avila P, Barthelemy C, Bahamonde A, Odriozola J A, De LaBanda J F G, Heinemann H. Appl Catal, 1989, 55: 151 [18] Stanger U L, Orel B, Regis A, Colomban P. J Sol Gel Sci Technol, 1997, 8: 965 [19] Deo G, Wachs I E, Haber J. Crit Rev Surf Chem, 1994, 4: 141 [2] Dunn J P, Jehng J M, Kim D S, Briand L E, Stenger H G, Wachs I E. J Phys Chem B, 1998, 12: 6212 [21] Besselmann S, Freitag C, Hinrichsen O, Muhler M. Phys Chem Chem Phys, 21, 3: 4633 [22] Lin C H, Chien S H, Chao J H, Sheu C Y, Cheng Y C, Huang Y J, Tsai C H. Catal Lett, 22, 8: 153 [23] Bavykin D V, Friedrich J M, Walsh F C. Adv Mater, 26, 18: 287 [24] Avila P, Blanco J, Bahamonde A, Palacios J M, Barthelemy C. J Mater Sci, 1993, 28: 4113 [25] Liu H C, Bayat N, Iglesia E. Angew Chem Int Ed, 23, 42: 572 [26] Tatibouet J M. Appl Catal A, 1997, 148: 213

Jingxuan Cai et al. / Chinese Journal of Catalysis 34 (213) 211 2117 2117 酸性助剂对 V 2 O 5 /TiO 2 催化剂甲醇选择氧化为甲缩醛的影响 蔡景轩, 傅玉川 # *, 孙清, 贾敏慧, 沈俭一南京大学化学化工学院介观化学教育部重点实验室, 江苏南京 2193 摘要 : 研究了酸性助剂对 TiO 2 纳米管 (TNT) 负载的 V 2 O 5 催化剂 (V 2 O 5 /TNT) 性能的影响, 发现经硫酸 磷酸或磷钨酸处理后, TNT 的结构稳定, 但表面酸性和氧化 - 还原性发生了变化, 从而改变了甲醇选择氧化为甲缩醛的催化性能. 实验结果表明, V 2 O 5 /TNT 催化剂经硫酸修饰和 673 K 焙烧, 其甲缩醛选择性显著提高, 且维持了较高的甲醇转化率. 催化剂表征表明, 高温焙烧促进了硫酸根与钒物种之间的强相互作用, 从而提高了催化剂的表面酸性而没有降低钒的氧化 - 还原性. 磷酸和磷钨酸修饰虽然也提高了 V 2 O 5 /TNT 催化剂的表面酸性, 但降低了其中钒氧化物的氧化 - 还原能力, 反而降低了催化剂的活性. 关键词 : 甲醇选择氧化 ; 甲缩醛 ; 酸性助剂 ; 表面酸性 ; 表面氧化 - 还原性 收稿日期 : 213-7-7. 接受日期 : 213-8-16. 出版日期 : 213-11-2. * 通讯联系人. 电话 : (25)8359435; 传真 : (25)83317761; 电子信箱 : jyshen@nju.edu.cn # 通讯联系人. 电话 : (25)8359435; 传真 : (25)83317761; 电子信箱 : ycfu@nju.edu.cn 基金来源 : 国家自然科学基金 (217388); 高等学校博士学科点专项科研基金 (28284138); 国家重点基础研究发展计划 (973 计划, 25CB2214); 中央高校基本科研业务费专项基金. 本文的英文电子版由 Elsevier 出版社在 ScienceDirect 上出版 (http://www.sciencedirect.com/science/journal/1872267).