V oxide catalysts

Chemical Engineering Journal 244 (2014) 168–177

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Gas phase oxidehydration of glycerol to acrylic acid over Mo/V and W/V oxide catalysts Lingqin Shen, Hengbo Yin ⇑, Aili Wang, Xiufeng Lu, Changhua Zhang Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Glycerol oxidehydration was

Gas phase glycerol oxidehydration was effectively catalyzed by Mo/V or W/V oxide catalysts. Mo/V and W/V oxide catalysts had acidic and oxidative activities for glycerol oxidehydration reaction. Acid sites on Mo/V and W/V oxide catalysts catalyzed glycerol dehydration to acrolein and acetaldehyde. Metallic cations with low valences in Mo/V and W/V oxide catalysts gave high oxidation activity for the formation of acrylic acid, acetic acid, CO, and CO2.

effectively catalyzed by Mo/V or W/V oxide catalysts.  Mo/V or W/V catalyst had acidic and oxidative activities for oxidehydration reaction.  Acid sites on catalysts catalyzed glycerol dehydration to acrolein and acetaldehyde. n+ (n+1)+  M /M pairs gave high oxidation activity for the formation of acrylic acid.

a r t i c l e

i n f o

Article history: Received 31 October 2013 Received in revised form 17 January 2014 Accepted 20 January 2014 Available online 28 January 2014 Keywords: Glycerol Acrylic acid Oxidehydration Mo/V oxide catalysts W/V oxide catalysts

a b s t r a c t Gas phase oxidehydration of glycerol over Mo/V and W/V oxide catalysts was investigated in a fixed bed reactor. The Mo/V and W/V oxide catalysts were facilely prepared by calcination method and characterized by XRD, XPS, Raman, and NH3-TPD techniques. In addition to MoO3 or V2O5, Mo6V9O40 was formed in Mo/V oxide catalysts. The presence of V in W/V oxide catalysts caused the formation of hexagonal or orthorhombic WO3 while monoclinic WO3 was formed in the absence of V component. For Mo/V oxide catalysts, Mo1V0.25 oxide catalyst gave the maximum acrylic acid yield of 20.1% and the maximum acetic acid yield of 20.8% at the reaction temperature of 300 °C. Mo1V4 oxide catalyst gave the maximum acrolein yield of 28.8% at 300 °C. For W/V oxide catalysts, W1V0.25 oxide catalyst gave the maximum acrylic acid yield of 25.7% and the maximum acetic acid yield of 21.2% at 300 °C. However, pure WO3 catalyst gave the highest acrolein yield of 50.3% at 300 °C. The yields of CO and CO2 increased with increasing V content in both bimetal oxide catalysts. More Mo5+, W5+, and V4+ cations present in the bimetal oxide catalysts gave high catalytic oxidation activity. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction

⇑ Corresponding author. Tel.: +86 (0)511 88787591; fax: +86 511 88791800. E-mail address: [email protected] (H. Yin). http://dx.doi.org/10.1016/j.cej.2014.01.051 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.

With the rapid growth of biodiesel production, glycerol, a byproduct of ca. 10% biodiesel, is considered as one of important biomaterials for producing valuable chemicals [1–11]. Conversion

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L. Shen et al. / Chemical Engineering Journal 244 (2014) 168–177

of surplus glycerol to valuable chemicals, such as acrylic acid [12– 18], acrolein [12–29], propanediol [30], and lactic acid [31–34], is attracting researchers’ attention. As compared to the present commercial acrylic acid production process via two continuous oxidation steps starting from petroleum-derived propylene over Bi/Mo and Mo/W/V oxide catalysts in series, direct conversion of glycerol to acrylic acid is a promising eco-friendly alternative [35]. Oxidehydration of glycerol to acrylic acid can be categorized into one- and two-step methods. The two-step method includes glycerol dehydration to acrolein and subsequent acrolein oxidation to acrylic acid in two separate catalyst beds. Sooknoi et al. [14] used zeolites as catalysts for glycerol dehydration to acrolein and silica-supported Mo/V oxides as catalysts for acrolein oxidation to acrylic acid. They claimed that the two bed system gave acrylic acid yield of 40% while the mixed catalyst bed gave acrylic acid yield of 30% as well as considerable amounts of acetic acid and acetaldehyde. Andersson et al. [18] configured zirconia-supported W/Nb and W/V/Mo catalysts as consecutive beds in one reactor, the yields of acrylic acid and acrolein were 44.1% and 7.8%, respectively. However, when they mixed W/Nb and W/V/Mo catalysts in one bed, almost no acrylic acid was formed. For the one-step method, the catalysts with both acidic and oxidative functions have been investigated. Wang et al. [13] investigated iron oxide (FeOx)-embedded iron orthovanadate (FeVO4) bifunctional catalysts for glycerol oxidehydration, the highest acrylic acid yield of 14% was obtained. Ueda et al. [12] prepared Mo/V (Nb, Te) and W/V oxide catalysts, such as Mo3VO, MoVNbTeO, and W3VO, by hydrothermal method. They reported that the Mo3VO, MoVNbTeO, and W3VO catalysts gave acrylic acid yields of 26.3%, 28.4%, and 23.7%, respectively. Nieto et al. [15] investigated the effect of V content on the catalytic activity of hydrothermally synthesized W/V oxide catalysts in glycerol oxidehydration reaction. It was found that there was an optimal atomic ratio range of V to W + V of 0.12–0.21 for glycerol oxidehydration to acrylic acid, giving the maximum yield of 25%. Subsequently, they improved the catalyst by the addition of Nb in the W/V oxide catalyst, the acrylic acid yield increased to 34% [16]. Recently, Mota et al. [17] reported the glycerol oxidehydration over zeolitesupported V oxide catalysts. The highest acrylic acid yield of 25% was obtained at 275 °C. From the previous work on catalytic oxidehydration of glycerol, it is found that Mo, W, and V oxides are effective components for glycerol oxidehydration to acrylic acid because Mo, W, and V oxides not only have acid sites but also have oxidation ability [36–40]. Furthermore, glycerol oxidehydration to acrylic acid over bifunctional catalyst is a potentially economic route in the viewpoint of low production cost. However, the yield of acrylic acid is still unsatisfactory in the viewpoint of atomic economy. The effect of surface structures and acidities of the oxide catalysts on their catalytic activities in the glycerol oxidehydration reaction was not systematically investigated. Catalytic oxidehydration of glycerol to acrylic acid is still worth of investigation. Herein, we report the glycerol oxidehydration in air to acrylic acid over Mo/V and W/V oxide catalysts by one-step method. The Mo/V and W/V oxide catalysts were facilely prepared by directly drying the aqueous solutions of their precursors (ammonium salts) and subsequently followed by calcination at high temperature. By this method, the catalyst components could well react or mix with each other. The effect of catalyst components on the oxidehydration reaction was investigated in detail. The oxide catalysts were characterized by XRD, XPS, Raman, NH3-TPD, and BET techniques. It was found that the addition of vanadium in the oxide catalysts favored glycerol oxidehydration to acrylic acid. The element valence state and acid strength of the oxide catalysts played important roles in the oxidehydration reaction. Possible reaction routes were also briefly discussed.

2. Experimental 2.1. Materials Glycerol, ammonium metavanadate (NH4VO3), ammonium tungstate ((NH4)10W12O415H2O), ammonium heptamolybdate ((NH4)6Mo7O244H2O), acetic acid, and acrylic acid were of reagent grade and were purchased from Sinopharm Chemical Reagent Co., Ltd. Acrolein was of reagent grade and was purchased from Xiya Chemical Reagent Co., Ltd. All chemicals were used as received without further purification. Distilled water was used in all of the experiments. 2.2. Catalyst preparation Mo/V and W/V oxide catalysts were prepared by the calcination method. Given amounts of (NH4)6Mo7O244H2O, NH4VO3, and (NH4)10W12O415H2O were first dissolved in distilled water. Then the solution was evaporated in an electric oven under stirring to obtain a dried salt mixture. The dried sample was further dried in an oven at 120 °C for 12 h and then calcined at 550 °C for 2 h to obtain Mo/V and W/V oxide catalysts. The as-prepared catalysts were pressed at 20 MPa to form pellets and the pellets were crushed to small-sized particles with particle sizes ranging from 0.18 to 0.425 mm. Hereafter, the as-prepared oxide catalysts were denoted as MoaVb and WaVb, where, a and b denote the atomic ratio of Mo to V or W to V in the oxide catalysts. For comparison, pure MoO3, WO3, and V2O5 oxides were also prepared starting from (NH4)6Mo7O244H2O, (NH4)10W12O415H2O, and NH4VO3, respectively, by the calcination method. The preparation procedures were the same as those mentioned above. 2.3. Catalyst characterization Powder X-ray diffraction (XRD) patterns of the fresh and spent oxide catalysts were obtained on a Bruker D8 Advance X-ray diffractometer using Ni-filtered Cu Ka (k = 0.15406 nm) radiation operated at 40 kV and 20 mA. The XRD patterns were collected in the 2h range of 10–60° (step size 0.02°). The crystallite sizes of catalyst components were calculated by Scherrer’s equation: D = Kk/(Bcos h), where K was taken as 0.89 and B was the full width of the diffraction line at half of the maximum intensity. The crystallite sizes are listed in Table 1. X-ray photoelectron spectra (XPS) of the fresh and spent oxide catalysts were acquired on a PHI 5000 Versa Probe with a Al Ka X-ray source operated at 150 W (15 kV) at a pressure of 109 mbar. All binding energies were calibrated by using contaminant carbon (C1s 284.6 eV) as a reference. Curve fitting was performed with the software XPS PEAK 4.1. Raman measurements were performed on a Renishaw RM1000 with a confocal microscope Raman system using an excitation wavelength of 514 nm supplied by a Renishaw HPNIR laser Table 1 Crystallite sizes of the Mo/V and W/V oxide catalysts. Catalysts

Crystallite sizes (nm) Mo6V9O40 (0 2 2)

Mo1V4 Mo1V1 Mo1V0.25 MoO3 W1V4 W1V1 W1V0.25 WO3 V2O5

65.6 63.0 50.9

MoO3 (0 2 1)

WO3 (2 0 0)

V2O5 (0 0 1) 48.4

/ 54.3 38.6 38.6 97.6 88.0 30.9

57.4 / / 83.1

L. Shen et al. / Chemical Engineering Journal 244 (2014) 168–177

(10 mW). The powder samples were pressed to form disks. The Raman spectra were collected at room temperature in air and in the region of 100–1200 cm1. The total acidities and acid strengths of the oxide catalysts were estimated by temperature-programmed desorption of ammonia (NH3-TPD) in a fixed-bed continuous flow micro-reactor at atmospheric pressure. The samples (0.1 g) were dried at 200 °C for 1 h to remove physically absorbed water and then ammonia-saturated in a NH3 stream at 60 °C for 0.5 h. After purging with helium (30 mL/min) at 60 °C for 0.5 h to remove the physically adsorbed NH3, the samples were heated at a linear heating rate of 10 °C/ min up to 600 °C. In order to determine the acidity from the NH3 desorption profiles, the areas under the curves were integrated by Gaussian deconvolution of the peaks. The acidity was expressed micromoles per packing volume of catalyst. Surface areas of the oxide catalysts were determined by multipoint N2 adsorption at 196 °C in a Micromeritics ASAP 2000 physical adsorption apparatus. The data were treated in accordance with the BET method. Before measurement of surface area, the samples were degassed under vacuum at 300 °C for 1 h. 2.4. Catalytic test The catalytic test was performed in a stainless steel tubular fixed-bed micro-reactor with diameter and length of 8 mm and 200 mm, respectively, packed with 5.0 ± 0.1 mL of catalyst with particle sizes ranging from 0.18 to 0.425 mm, operating at 250, 300 and 350 °C, respectively. A stream of glycerol aqueous solution (20%) was fed into the reactor at a flow rate of 6 mL/h. Air flow rate was 50 mL/min. The reaction products were condensed in a cold trap kept in liquid nitrogen and collected at different reaction temperatures after reaction for 1 h. The compositions of liquid reaction products were analyzed on a gas chromatograph, equipped with a flame ionization detector (temperature program: 70 °C, 2 min; 25 °C/min; 240 °C, 4 min) and a PEG packed capillary column (0.25 mm  30 m). The quantification of product was carried out using sec-butyl alcohol as an internal standard. For the analysis of gas phase products, such as CO and CO2, a wide-bore column (TDX-01) was used at an oven temperature of 80 °C using thermal conductivity detector. The glycerol conversion and the product yields were the parameters used to evaluate the catalyst performance. They are calculated according to the following equations.

Glycerol conversion ¼ ðM GLY;in  M GLY;remained Þ=MGLY;in  100%

ð1Þ

Product yield ¼ M i =M GLY;in  100%

ð2Þ

residual peaks appearing at 2h = 12.75°, 23.33°, 25.68°, 27.32°, 33.75°, 38.96°, 45.76°, 46.31°, 49.29°, 55.22°, and 58.81° are attributed to those of MoO3 (JCPDS: 35-0609). All the XRD peaks of Mo1V1 catalyst are consistent with those of Mo6V9O40 (JCPDS: 34-0527). However, the mole ratio of Mo to V in starting materials was larger than that in Mo6V9O40, indicating that well dispersed MoO3 could be formed in Mo1V1 catalyst. The XRD spectra of Mo1V4 catalyst show the peaks appearing at 2h = 18.31°, 21.58°, 24.99°, 27.63°, 33.20°, 33.87°, 46.90°, 50.33° and 51.03°, which are in agreement with those of Mo6V9O40 (JCPDS: 34-0527). The residual peaks appearing at 2h = 15.32°, 20.46°, 26.05°, 30.93°, and 32.44° could be attributed to those of orthorhombic V2O5 (JCPDS: 41-1426). The results revealed that Mo6V9O40 was formed when Mo/V oxide catalysts were prepared by using ammonium metavanadate and ammonium heptamolybdate as starting materials. At the same time, MoO3 or V2O5 was formed when excessive ammonium heptamolybdate or ammonium metavanadate was supplied. It was also found that the crystallite sizes (0 2 2) of Mo6V9O40 increased from 50.9 to 65.6 nm with increasing the molar ratio of V to Mo from 0.25:1 to 4:1 (Table 1). However, the crystallite size of MoO3 present in Mo/V oxide catalyst was higher than that of pure MoO3 catalyst, indicating that the presence of V enhanced the crystal growth of residual MoO3. In contrast, the presence of Mo inhibited the crystal growth of residual V2O5 in Mo/V oxide catalyst. For the W1V0.25 and W1V1 catalysts, the XRD peaks appearing at 2h = 13.96, 22.72°, 24.33°, 26.85°, 28.17°, 36.57°, 49.96°, 55.33°, and 55.51° are in agreement with those of hexagonal WO3 (JCPDS:

A

g f e d c b

a

10

3.1.1. XRD analysis Fig. 1 shows the XRD patterns of fresh and spent Mo/V and W/V oxide catalysts. When ammonium heptamolybdate, ammonium metavanadate, and ammonium tungstate were calcined at 550 °C for 2 h, orthorhombic MoO3 (JCPDS 35-0609), orthorhombic V2O5 (JCPDS 41-1426), and monoclinic WO3 (JCPDS 43-1035) were synthesized, respectively. For the fresh bimetal oxide catalysts, the XRD spectra of Mo1V0.25 catalyst show the peaks appearing at 2h = 18.31°, 21.58°, 24.99°, 27.63°, 33.20° and 50.31°, which are in agreement with those of Mo6V9O40 (JCPDS: 34-0527). And the

30

40

50

B

60

i* h* g* f*

Intensity (a. u.)

3.1. Characterization of catalysts

20

2θ (º)

where MGLY,in is the initial mole number of glycerol. MGLY,remained is the mole number of unreacted glycerol. Mi represents the mole number of product i. 3. Results and discussion

i h

Intensity (a. u.)

170

e* d* c* b*

a*

10

20

30

40

50

60

2θ (º) Fig. 1. XRD patterns of fresh (A) and spent (B) Mo/V and W/V oxide catalysts. a, MoO3; b, Mo1V0.25; c, Mo1V1; d, Mo1V4; e, WO3; f, W1V0.25; g, W1V1; h, W1V4; i, V2O5. * , spent catalysts.

171

L. Shen et al. / Chemical Engineering Journal 244 (2014) 168–177

Mo 3d3/2

Mo 3d5/2

d* d

Intensity (a. u.)

c* c b* b a*

a

240

238

236

234

232

230

228

Binding Energy (eV)

B

V 2p3/2

h* h g* g

Intensity (a. u.)

3.1.2. XPS analysis We performed the XPS analysis to determine the valence states of elements on the surfaces of fresh and spent catalysts. The XPS profiles of V 2p3/2, Mo 3d, and W 4f of the fresh and spent catalysts are shown in Fig. 2. The profiles of V 2p3/2, Mo 3d, and W 4f lines were slightly asymmetric, suggesting the co-presence of V5+/V4+, Mo6+/Mo5+, and W6+/W5+ pairs on the surfaces of catalysts, respectively. Deconvolution of XPS spectra based on a Gaussian signal was carried out [15,41–45]. The XPS deconvolution of Mo 3d, V 2p3/2, W 4f of the representative catalysts, Mo1V0.25 and W1V4, is shown in Fig. 3. The detailed deconvolution of XPS spectra is supplied in supporting information (Fig. 1s). The results are listed in Table 2. For the fresh MoO3, V2O5, and WO3 catalysts, the portions of Mo5+, Mo6+, V4+, V5+, W5+, and W6+ were 73.6%, 26.4%, 17.0%, 83.0%, 88.0%, and 12.0%, respectively, indicating that the metallic cations with different valences existed on the catalyst surfaces. After taking part in the glycerol oxidehydration reaction, the portions of the metallic cations with more oxidized states in the spent oxide catalysts were higher than those in the fresh oxide catalysts, indicating that the crystal oxygen of the oxide catalysts probably took part in the glycerol oxidehydration reaction. For the fresh Mo1V0.25, Mo1V1, and Mo1V4 oxide catalysts, the portions of Mo5+ and Mo6+ ranged from 92.3% to 97.2% and 2.8% to 7.7%, respectively. When V was present in the Mo/V oxide catalysts, the portions of Mo6+ decreased by ca. 20% as compared to that in pure MoO3 catalyst. The presence of V component in the Mo/V oxide catalysts changed the portions of Mo cations with different valences. However, the portions of V4+ and V5+ in the Mo/V oxide catalysts were similar to those in pure V2O5 oxide catalyst. For the spent Mo/V oxide catalysts, the portions of Mo6+ and V5+ cations slightly increased as compared to those in the fresh Mo/V oxide catalysts. For the fresh W1V0.25, W1V1, and W1V4 oxide catalysts, the portions of W5+ and W6+ ranged from 93.1% to 97.1% and 2.9% to 6.9%, respectively. When V was present in the W/V oxide catalysts, the portions of W6+ decreased by ca. 59% as compared to that in pure WO3 catalyst. Additionally, the portions of V5+ in the W/V oxide catalysts decreased by ca. 729% as compared to that in pure V2O5 oxide catalyst. The oxidized states of surface cations were significantly affected by the catalyst composition. Interestingly, the portions of W6+ in the spent W/V oxide catalysts increased to 79.095.1%. And the portions of V5+ also obviously increased as compared to that in the fresh W/V oxide catalysts. As compared to the fresh oxide catalysts, the changes in valence states of surface cations in the spent oxide catalysts indicated that the crystal oxygen of the bimetal oxide catalysts probably took part in the glycerol oxidehydration reaction.

A

f* f e* e d* d c* c b* b 528

526

524

522

520

518

516

514

Binding Energy (eV)

C

W 4f5/2

W 4f7/2

g* g f* f

Intensity (a. u.)

33-1387). No XRD peaks related to vanadium compounds were detected, indicating that V2O5 probably well dispersed in the W/V oxide catalysts. The XRD spectra of W1V4 catalyst show the peaks appearing at 2h = 15.35°, 20.26°, 21.71°, 26.13°, 31.00°, 32.36°, 34.28°, and 41.26°, which are in agreement with those of V2O5 (JCPDS: 41-1426). The residual peaks appearing at 2h = 23.22° and 23.99° could be attributed to those of orthorhombic WO3 (JCPDS: 20-1324). With increasing V content in W/V oxide catalysts, WO3 phase changed from monoclinic to hexagonal and orthorhombic. Meanwhile, the crystallite sizes (2 0 0) of WO3 in W/V oxide catalysts also obviously changed with changing V content (Table 1). For the spent catalysts, their XRD spectra are similar to those of the fresh oxide catalysts, respectively, meaning that the crystal structures of the oxide catalysts did not obviously change after taking part in glycerol oxidehydration reaction.

e*

e i*

i

44

42

40

38

36

34

32

Binding Energy (eV) Fig. 2. XPS patterns of Mo/V and W/V oxide catalysts. a, MoO3; b, Mo1V0.25; c, Mo1V1; d, Mo1V4; e, W1V0.25; f, W1V1; g, W1V4; h, V2O5; i, WO3. *, spent catalysts.

3.1.3. Raman spectra Fig. 4 shows the Raman spectra of fresh and spent Mo/V and W/ V oxide catalysts in the region of 100–1200 cm1. The Raman spectra of fresh MoO3 catalyst show the peaks appearing at 114, 129, 158, 281, 335, 665, 819, and 997 cm1, corresponding to those of orthorhombic MoO3 [46]. The spectra of fresh V2O5 catalyst show the characteristic peaks of orthorhombic V2O5, appearing at 139, 282, 405, and 990 cm1, respectively [47,48]. For the fresh Mo1V0.25 catalyst, the Raman spectra show the peaks similar to those of orthorhombic MoO3 except a new peak at 871 cm1. For the fresh Mo1V1 and Mo1V4 catalysts, their Raman spectra show

172

L. Shen et al. / Chemical Engineering Journal 244 (2014) 168–177 Mo 3d5/2

Mo1V0.25

238

236

Intensity (a. u.)

Intensity (a. u.)

Mo 3d3/2

240

234

232

230

Mo1V0.25*

Mo 3d5/2

228

Mo 3d3/2

240

Binding Energy (eV)

238

236

234

232

230

228

Binding Energy (eV)

V 2p3/2

V 2p3/2

Mo1V0.25*

Intensity (a. u.)

Intensity (a. u.)

Mo1V0.25

520

518

516

514

512

520

518

Binding Energy (eV)

516

512

Binding Energy (eV) W1 V 4

W 4f7/2

514

W1 V 4 *

W 4f7/2

Intensity (a. u.)

W 4f5/2

Intensity (a. u.)

W 4f5/2

40

38

36

34

32

40

Binding Energy (eV) V 2p3/2

38

36

34

32

Binding Energy (eV) V 2p3/2

W1 V 4 *

Intensity (a. u.)

Intensity (a. u.)

W1 V 4

520

518

516

514

512

Binding Energy (eV)

520

518

516

514

512

Binding Energy (eV)

Fig. 3. XPS deconvolution of Mo 3d, V 2p3/2, W 4f of the representative Mo1V0.25 and W1V4 catalysts. *, spent catalysts.

peaks appearing at 143, 280, 401, 872, and 979; 140, 281, 404, 876, and 987 cm1, respectively, which are similar to those of orthorhombic V2O5 except the peaks at 872 and 876 cm1. The peaks appearing at 871876 cm1 are ascribed to the vibration of MoOV, indicating the formation of MoOV oxide in Mo/V oxide catalysts [49]. As certified by XRD analysis, the MoOV

oxide compound had the formula of Mo6V9O40. However, for the fresh Mo1V1 catalyst, the characteristic peak of MoO3 at 819 cm1 disappeared, indicating that in addition to the Mo6V9O40 phase, amorphous or disordered MoO3 was probably formed. For the spent MoO3 and V2O5 catalysts, their Raman spectra show the peaks appearing at 115, 130, 158, 282, 336, 667, 820,

L. Shen et al. / Chemical Engineering Journal 244 (2014) 168–177 Table 2 XPS results of Mo/V and W/V oxide catalysts. Catalysts

Portions of metallic ions

Mo1V4 Mo1V4a Mo1V1 Mo1V1a Mo1V0.25 Mo1V0.25a W1V4 W1V4a W1V1 W1V1a W1V0.25 W1V0.25a V2O5 V2O5a MoO3 MoO3a WO3 WO3a a

Mo5+

Mo6+

V4+

V5+

92.9 91.1 97.2 91.2 92.3 89.6

7.1 8.9 2.8 8.8 7.7 10.4

13.5 11.0 19.5 17.0 15.7 10.3 24.2 23.9 46.2 21.6 39.2 19.9 17.0 4.7

86.5 89.0 80.5 83.0 84.3 89.7 75.8 76.1 53.8 78.4 60.8 80.1 83.0 95.3

73.6 57.9

W5+

W6+

97.1 4.9 93.1 8.8 97.0 21.0

2.9 95.1 6.9 91.2 3.0 79.0

88.0 16.8

12.0 83.2

26.4 42.1

The spent catalysts.

i

A

h

Intensity (a.u.)

g f e d c b a

200

400

600

800

1000

1200

173

143, 282, 404, 874, and 989; 140, 281, 406, 880, and 1000 cm1, respectively. The Raman spectra of the spent catalysts are similar to those of the fresh ones, indicating that the bulk chemical structures of the spent catalysts are similar to those of the fresh ones. However, the Raman peaks of the spent Mo/V oxide catalysts slightly shifted to greater wavenumbers than those of the fresh ones, indicating that the spent catalysts were probably in more oxidized states. The Raman spectra of fresh WO3 catalyst show the characteristic peaks of monoclinic WO3 at 133, 270, 327, 715, and 807 cm1, respectively [50]. The Raman spectra of the fresh W1V0.25 and W1V1 catalysts show the peaks appearing at 185, 263, 455, 689, and 779 cm1, which are attributed to those of hexagonal WO3 [50]. Furthermore, a peak at 968 cm1 was also observed, which could be explained by the presence of W@O bonds generated by a structure defect due to vanadium incorporation and V@O bonds associated to polymeric VAOAW chains [15]. The Raman spectra of fresh W1V4 catalyst with the highest vanadium content show the peaks appearing at 139, 281, 405, and 989 cm1, attributed to those of orthorhombic V2O5. The peak appearing at 799 cm1 is ascribed to the characteristic peak of orthorhombic WO3 [51]. The Raman analysis certified that there was an interaction between WO3 and V2O5 in W/V oxide catalysts. For the spent WO3 catalyst, the Raman spectra show the peaks appearing at 135, 270, 331, 716, and 808 cm1, indicating that the spent WO3 catalyst was present in monoclinic WO3 phase. For the spent W1V0.25, W1V1, and W1V4 catalysts, their Raman spectra show the peaks appearing at 185, 263, 299, 327, 456, 690, 780 and 966; 195, 263, 299, 327, 456, 696, 784, and 974; 139, 190, 282, 405, 805, and 992 cm1, indicating that the bulk chemical structures of the spent catalysts are similar to those of the fresh ones. However, the Raman peaks of the spent W/V oxide catalysts slightly shifted to greater wavenumbers than those of the fresh ones, indicating that the spent catalysts were in more oxidized states [50]. It was found that the results obtained from Raman analysis for both fresh and spent catalysts were consistent with those by XRD and XPS analyses.

Raman shift (cm-1)

B i* h*

Intensity (a.u.)

g* f* e* d* c* b* a*

200

400

600

800

1000

1200

Raman shift (cm-1) Fig. 4. Raman spectra of Mo/V and W/V oxide catalysts. a, MoO3; b, Mo1V0.25; c, Mo1V1; d, Mo1V4; e, WO3; f, W1V0.25; g, W1V1; h, W1V4; i, V2O5. *, spent catalysts.

and 997; 139, 282, 405, and 990 cm1, respectively, indicating that the spent MoO3 catalyst was in orthorhombic MoO3 phase and the spent V2O5 catalyst in orthorhombic V2O5 phase. For the spent Mo1V0.25, Mo1V1, and Mo1V4 catalysts, their Raman spectra show the peaks appearing at 111, 122, 281, 336, 818, 873, and 992;

3.1.4. NH3-TPD analysis The temperature-programmed desorption of NH3 was used to estimate the acidities of catalysts. Although this method does not give exact values for acid strength, it allows a ready comparison of the acidities of different catalysts. The NH3-TPD profiles of MoO3, V2O5, WO3, Mo/V, and W/V oxide catalysts are shown in Fig. 5 and the total acidities are listed in Table 3. According to NH3 desorption temperature, the acid sites are usually classified into three types, weak- (150300 °C), medium(300500 °C), and strong- (500650 °C) strength [27]. Broad NH3-TPD peaks at ca. 210 °C were detected for all the Mo/V oxide catalysts. The NH3 desorption temperatures ranged from ca. 100 to 500 °C, revealing that the Mo/V oxide catalysts had both weak- and medium-strength acid sites. According to the shapes of NH3-TPD peaks, it was found that weak-strength acid sites were dominant in Mo/V oxide catalysts. The NH3-TPD peaks of MoO3 and V2O5 appeared at ca. 210 °C. Weak-strength acid sites were dominant in both pure oxide catalysts. For the Mo/V oxide catalysts, their acidities in unit of lmolNH3/mLcat were in an order of Mo1V4  Mo1V1  Mo1V0.25 > MoO3 > V2O5. The Mo/V oxide catalysts had higher acidities than pure MoO3 and V2O5 catalysts. For the W/V oxide catalysts, main NH3-TPD peaks appeared at ca. 220 °C for W1V4, W1V1, and W1V0.25 catalysts, respectively. The NH3-TPD results showed that W/V oxide catalysts had both weak- and medium-strength acid sites. And the weak-strength acid sites were dominant. The acidities in unit of lmolNH3/mLcat

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were in an order of W1V0.25 > W1V1 > W1V4 > WO3 > V2O5. The acidities of W/V oxide catalysts were higher than those of WO3 and V2O5 catalysts. However, at low V content, W/V oxide catalyst had high acidity. It could be explained as that the interaction between WO3 and V2O5 gave high acidity. At high V content, the presence of large portion of separated V2O5 decreased the catalyst acidity. 3.1.5. Specific surface areas The surface areas of Mo/V and W/W oxide catalysts were in a range of 10.412.3 m2/g (Table 3). The surface areas of MoO3, WO3, and V2O5 catalysts were in a range of 23.5 m2/g. The Mo/ V and W/V oxide catalysts had higher surface areas than pure MoO3, WO3, and V2O5 catalysts. 3.2. Catalysis results Fig. 6 shows the glycerol conversions and product yields in the glycerol oxidehydration over MoO3, V2O5, and Mo/V oxide catalysts at different reaction temperatures. For pure MoO3 and V2O5 catalysts, glycerol was completely converted when the reaction temperature was 300 °C. When MoO3 was used as the catalyst at 300 °C, the maximum yields of acrolein (9.2%) and acetic acid (4.1%) were obtained. When V2O5 was used as the catalyst at 300 °C, the maximum yields of acrolein (5.9%) and acetic acid (12.2%) were obtained. The yields of acetaldehyde for MoO3 and V2O5 catalysts ranged from 5.5% to 6.9% and 7.2% to 7.9%, respectively, at reaction temperatures of 250350 °C. High acetaldehyde yield was obtained at 250 °C. However, no acrylic acid was

Intensity (a. u.)

A

a b c d e

100

200

300

400

500

400

500

Temperature (ºC)

Intensity (a. u.)

B

f g h i e

100

200

300

Temperature (ºC) Fig. 5. NH3-TPD profiles of fresh Mo/V (A) and W/V (B) oxide catalysts. a, Mo1V4; b, Mo1V1; c, Mo1V0.25; d, MoO3; e, V2O5; f, W1V4; g, W1V1; h, W1V0.25; i, WO3.

detected over both pure MoO3 and V2O5 catalysts. With increasing reaction temperature, the yields of CO and CO2 increased. At the reaction temperature of 350 °C, the yields of CO and CO2 for V2O5 catalyst were 35.3% and 32.4%, respectively. The yields of CO and CO2 for MoO3 catalyst were 20.2% and 10.5%. The results indicated that V2O5 catalyst had higher oxidation activity than MoO3 catalyst. From the carbon balance (Table 4), it was found that MoO3 catalyst caused more coking than V2O5 catalyst. For the Mo/V oxide catalysts, glycerol conversion increased with increasing reaction temperature. When the reaction temperature was 350 °C, the conversion of glycerol reached 100%. The yields of acrolein increased with increasing V content in Mo/V oxide catalysts. The maximum acrolein yield of 28.8% was obtained at 300 °C when Mo1V4 oxide was used as the catalyst. In contrast to the acrolein yield, the yields of acrylic acid and acetic acid increased with decreasing V content. The maximum yields of acrylic acid (20.1%) and acetic acid (20.8%) were obtained at 300 °C when Mo1V0.25 oxide was used as the catalyst. At the reaction temperatures of 250350 °C, the yields of acetaldehyde were less than 4.3% for all the Mo/V oxide catalysts. To obtain high yields of acrolein, acrylic acid, and acetic acid, the optimal reaction temperature was 300 °C. The yields of CO and CO2 increased with increasing reaction temperature and V content. The carbon balance increased with lowering reaction temperature and increasing V content (Table 4), indicating that low reaction temperature and high V content inhibited coking in the glycerol oxidehydration reaction over Mo/V oxide catalysts. As compared to pure MoO3 catalyst, Mo1V0.25 catalyst gave high yields of acrylic acid and acetic acid. It can be explained as that with the addition of V in the catalyst, the total acidity and acid strength were increased, giving high catalytic activity for the dehydration of glycerol to acrolein and acetaldehyde. The formation of Mo6V9O40 in Mo1V0.25 catalyst gave high catalytic oxidation activity for the oxidation of acrolein to acrylic acid and acetaldehyde to acetic acid. Meanwhile, the yields of CO and CO2 over Mo1V0.25 catalyst were higher than those over pure MoO3 catalyst, certifying that Mo1V0.25 catalyst had higher catalytic oxidation activity than pure MoO3 catalyst. It was concluded that the formation of Mo6V9O40 phase not only increased total acidity and acid strength but also increased the catalytic oxidation activity. Furthermore, as shown in XPS analysis, the presence of V in Mo/V oxide catalysts caused the formation of more Mo5+ cations as compared to pure MoO3 catalyst. The presence of more Mo5+ cations probably caused the formation of more active oxygen in the reaction process, giving high catalytic oxidation activity. As compared to the fresh Mo/V oxide catalysts, the change in the portions of Mo5+, Mo6+, V4+, and V5+ in the spent Mo/V oxide catalysts gave clues that crystal oxygen probably took part in the oxidation reaction. For the Mo1V1 catalyst, with the formation of more Mo6V9O40 phase, the total acidity and acid strength increased as compared to pure V2O5 and MoO3 catalysts. Although the high total acidity and acid strength favored the dehydration of glycerol to acrolein, the formation of more Mo6V9O40 phase also increased the catalytic oxidation activity for the formation of CO and CO2. With further increasing V content, in addition to the formation of Mo6V9O40, V2O5 was also formed in Mo1V4 catalyst. The high total acidity and acid strength favored the glycerol dehydration to acrolein. At the same time, the presence of V2O5 with high catalytic oxidation activity caused the formation of more CO and CO2. Fig. 7 shows the glycerol conversions and product yields in the glycerol oxidehydration over WO3 and W/V oxide catalysts at different reaction temperatures. For pure WO3 catalyst, glycerol was completely converted when the reaction temperature were 300 °C. The maximum acrolein yield of 50.3% was obtained at 300 °C. At the reaction temperatures of 250350 °C, the yields of acetic acid and acetaldehyde ranged from 2.1% to 3.9% and 5.1%

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L. Shen et al. / Chemical Engineering Journal 244 (2014) 168–177 Table 3 Packing densities, surface areas, and acidities of Mo/V and W/V oxide catalysts. Packing density (g/mL)

Surface areas (m2/g)

NH3 desorption peak positions (°C)

Acidities (lmolNH3/mLcat.)

Mo1V4 Mo1V1 Mo1V0.25 W1V4 W1V1 W1V0.25 MoO3 WO3 V2O5

0.80 0.85 1.01 1.01 1.44 2.37 1.11 2.75 0.65

12.3 11.6 10.9 11.5 10.7 10.4 3.5 2.2 2.0

213 210 210 220 218 217 210 215 208

43.6 42.3 45.3 69.1 87.6 131.5 23.3 59.4 10.9

100 80 60 40 20 100 80 60 40 20 100 80 60 40 20 100 80 60 40 20 100 80 60 40 20 0

250ºC

300ºC

350ºC

V2O5

Mo1V4

Mo1V1

Mo1V0.25

MoO3

GLY

ACR

AA

ACA

ACE

CO

CO2

Fig. 6. Glycerol conversions and product yields in the glycerol oxidehydration catalyzed by Mo/V oxide catalysts at different reaction temperatures. ACR, acrolein; AA, acrylic acid; ACA, acetic acid; ACE, acetaldehyde; CO, carbon monoxide; CO2, carbon dioxide.

Table 4 Carbon balances over Mo/V and W/V oxide catalysts at different reaction temperatures. Catalysts

Reaction temperatures (°C)

Carbon balancesa (mol%)

Mo1V4

250 300 350

89.6 72.7 42.5

Mo1V1

250 300 350

76.4 70.1 40.3

Mo1V0.25

250 300 350

63.3 61.5 41.4

W1V4

250 300 350

77.1 69.2 45.8

W1V1

250 300 350

71.3 71.8 44.2

W1V0.25

250 300 350

63.6 77.4 49.5

MoO3

250 300 350

19.2 24.1 23.5

WO3

250 300 350

59.2 67.9 44.3

250 300 350

27.6 32.5 38.6

V2O5

P Carbon balance = (Yi  ni)/(3XGLY). Yi, yield of product i; ni, carbon number in molecule of product i; XGLY, glycerol conversion. a

to 7.5%, respectively. However, no acrylic acid was obtained over pure WO3 catalyst. With increasing reaction temperature, the yields of CO and CO2 increased. At the reaction temperature of 350 °C, the yields of CO and CO2 reached 37.8% and 18.4%, respectively. As compared to V2O5 catalyst, WO3 catalyst had high acidity and weak oxidation activity in the glycerol oxidehydration reaction, giving high acrolein yield. For the W/V oxide catalysts, glycerol conversion increased with increasing reaction temperature. When the reaction temperature was 350 °C, the conversions of glycerol over the W/V oxide catalysts reached 100%. The yields of acrolein increased with increasing the V content in W/V oxide catalysts. The maximum acrolein yield of 29.2% was obtained when W1V4 oxide was used as the catalyst at the reaction temperature of 300 °C. In contrast to the acrolein yield, the yields of acrylic acid and acetic acid increased with decreasing V content. The maximum yields of acrylic acid (25.7%) and acetic acid (21.2%) were obtained over W1V0.25 catalyst at 300 °C. At the reaction temperatures of 250350 °C, the yields of acetaldehyde were less than 4.8% for all the W/V oxide catalysts. On the other hand, the yields of CO and CO2 increased with increasing reaction temperature and V content in the W/V oxide catalysts. With low V content, W1V0.25 catalyst gave high yield of acrylic acid while pure WO3 and V2O5 oxides had no catalytic activity for the formation of acrylic acid. WO3 and V2O5 in W/V oxide catalysts synergistically catalyzed the oxidation of acrolein to acrylic acid. With further increasing the V content in W/V oxide catalysts, the yields of acrylic acid decreased while the yields of CO and CO2 increased. It can be explained as that more V2O5 with high oxidation activity was formed at high V content, giving high catalytic oxidation activity for the oxidation of acrylic acid to CO and CO2.

Conversion & Yield (mol%)

Conversion & Yield (mol%)

Catalysts

100 80 60 40 20 100 80 60 40 20 100 80 60 40 20 100 80 60 40 20 100 80 60 40 20 0

250 ºC

300 ºC

350 ºC

V2O5

W1V4

W1V1

W1V0.25

WO3

GLY

ACR

AA

CA

ACE

CO

CO2

Fig. 7. Glycerol conversions and product yields in the glycerol oxidehydration catalyzed by W/V oxide catalysts at different reaction temperatures. ACR, acrolein; AA, acrylic acid; ACA, acetic acid; ACE, acetaldehyde; CO, carbon monoxide; CO2, carbon dioxide.

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Scheme 1. Reaction routes in glycerol oxidehydration over Mo/V and W/V oxide catalysts.

Although the interaction between WO3 and V2O5 endowed W/V oxide catalyst with higher acidity than pure WO3 catalyst, the acrolein yields over W/V oxide catalysts were less than those over pure WO3 catalyst. The reason can be explained as that V2O5 present in W/V oxide catalyst had high catalytic oxidation activity for deep oxidation of resultant acrolein to CO and CO2, giving low acrolein yield. On the other hand, as shown in XPS analysis, W/V oxide catalysts had more W5+ and V4+ cations than pure WO3 and V2O5 catalysts. The presence of more W5+ and V4+ cations probably caused the formation of more active oxygen in the reaction process, giving high catalytic oxidation activity. Generally, the carbon balance increased with lowering reaction temperature (Table 4), indicating that low reaction temperature inhibited coking in the glycerol oxidehydration reaction over W/V oxide catalysts. 3.3. Reaction routes of glycerol oxidehydration When Mo/V and W/V oxides were used as bifunctional catalysts in the glycerol oxidehydration, the product distributions were similar. The detected products included acrolein, acrylic acid, acetic acid, acetaldehyde, CO, and CO2. At the same time, polymer-like residue was also formed according to carbon balance calculation, indicating that the catalysts also catalyzed the intermolecular reaction among glycerol molecules and resultant product molecules. The reaction routes in the glycerol oxidehydration catalyzed by Mo/V and W/V oxide catalysts were suggested as Scheme 1. Mo/V and W/V oxide catalysts had acid sites on their surfaces, taking as acid catalysts for glycerol dehydration reaction. Protonation of glycerol could occur either on the primary or on the secondary hydroxyl group. The former caused the formation of acetaldehyde and the latter induced the formation of acrolein via elimination of two H2O molecules [19–29]. The intermediate 3-hydroxypropionaldehyde could undergo dehydration and crack reactions to form acrolein and acetaldehyde, respectively [19–29]. Under the present experimental conditions, 3-hydroxypropionaldehyde was not detected, indicating that 3-hydroxypropanal was rapidly dehydrated or cracked. The resultant acrolein and acetaldehyde were oxidized to acrylic acid and acetic acid, respectively, on the oxidation active sites of the catalysts, such as Mo5+, W5+, and V4+ cations. Furthermore, acrylic acid or acetic acid could be further oxidized to form CO and CO2, successively. 4. Conclusions When Mo/V oxide catalysts were prepared starting from ammonium heptamolybdate and ammonium metavanadate by the calcination method, Mo6V9O40 phase was formed. The presence of V in

W/V oxide catalysts caused the formation of hexagonal or orthorhombic WO3 while monoclinic WO3 was formed in the absence of V component. XPS analysis shows that the presence of V component in Mo/V and W/V oxide catalysts caused the formation of more Mo5+, W5+, and V4+ ions as compared to pure MoO3, WO3, and V2O5 oxides. The presence of V in Mo/V and W/V oxide catalysts increased their acidities as compared to pure MoO3, WO3, and V2O5 catalysts, respectively. With low V content, Mo1V0.25 and W1V0.25 catalysts gave high yields of acrylic acid while pure MoO3, WO3, and V2O5 oxides had no catalytic activity for the formation of acrylic acid. The formation of Mo6V9O40 in Mo/V oxide catalysts favored the oxidation of acrolein to acrylic acid. WO3 and V2O5 in W/V oxide catalysts synergistically catalyzed the oxidation of acrolein to acrylic acid. With high V content, Mo/V and W/V oxide catalysts gave low yields of acrylic acid but high yields of CO and CO2. The formation of V2O5 phase at high V content gave high catalytic oxidation activity for the oxidation of resultant acrylic acid to CO and CO2. Acknowledgments This work was financially supported by research funds from Jiangsu Provincial Department of Education (CXZZ12-0683, 11KJB53002, 1102120C), Sichuan Province Nonmetallic Composites and Functional Materials Key Laboratory Project (10zxfk35), and Ministry of Education of the People’s Republic of China (2011M500866). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2014.01.051. References [1] C. Wang, B.L. Dou, H.S. Chen, Y.C. Song, Y.J. Xu, X. Du, T.T. Luo, C.Q. Tan, Hydrogen production from steam reforming of glycerol by Ni–Mg–Al based catalysts in a fixed-bed reactor, Chem. Eng. J. 220 (2013) 133–142. [2] R.R. Pawar, S.V. Jadhav, H.C. Bajaj, Microwave-assisted rapid valorization of glycerol towards acetals and ketals, Chem. Eng. J. 235 (2014) 61–66. [3] J.L. Wang, M. Zhang, Z. Zheng, F.W. Yu, J.B. Ji, The indirect conversion of glycerol into 1,3-dihydroxyacetone over magnetic polystyrene nanosphere immobilized TEMPO catalyst, Chem. Eng. J. 229 (2013) 234–238. [4] F.J.G. Ortiz, P. Ollero, A. Serrera, S. Galera, Optimization of power and hydrogen production from glycerol by supercritical water reforming, Chem. Eng. J. 218 (2013) 309–318. [5] R.P.V. Faria, C.S.M. Pereira, V.M.T.M. Silva, J.M. Loureiro, A.E. Rodrigues, Glycerol valorisation as biofuels: Selection of a suitable solvent for an innovative process for the synthesis of GEA, Chem. Eng. J. 233 (2013) 159–167. [6] M. Ayoub, A.Z. Abdullah, Diglycerol synthesis via solvent-free selective glycerol etherification process over lithium-modified clay catalyst, Chem. Eng. J. 225 (2013) 784–789.

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