Dehydration of 1,3-butanediol over rare earth oxides

Dehydration of 1,3-butanediol over rare earth oxides

Applied Catalysis A: General 377 (2010) 92–98 Contents lists available at ScienceDirect Applied Catalysis A: General journal homepage: www.elsevier...

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Applied Catalysis A: General 377 (2010) 92–98

Contents lists available at ScienceDirect

Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Dehydration of 1,3-butanediol over rare earth oxides Hiroshi Gotoh, Yasuhiro Yamada, Satoshi Sato * Graduate School of Engineering, Chiba University, Yayoi, Inage, Chiba 263-8522, Japan

A R T I C L E I N F O

A B S T R A C T

Article history: Received 4 November 2009 Received in revised form 28 December 2009 Accepted 13 January 2010 Available online 20 January 2010

Vapor-phase catalytic dehydration of 1,3-butanediol was investigated over rare earth oxides (REOs) calcined at different temperatures. In the dehydration of 1,3-butanediol over REOs such as Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3, Lu2O3, and Y2O3, 3-buten-2-ol and 2-buten-1-ol were preferentially produced. REOs exhibited different catalytic activities in the dehydration of 1,3-butanediol depending on their crystal structures. CeO2 showed the highest formation rate with the highest selectivity to the unsaturated alcohols among the REOs. Cubic REOs also selectively produced the unsaturated alcohols: cubic Er2O3, Yb2O3, and Lu2O3 showed high formation rate of the unsaturated alcohols. Since the formation rates of the unsaturated alcohols over Er2O3 and CeO2 were suppressed in CO2 and NH3 carrier gas flows more than in H2 flow, it is probable that the acid–base sites play a major role of the formation of the unsaturated alcohols. ß 2010 Elsevier B.V. All rights reserved.

Keywords: Unsaturated alcohols 3-Buten-2-ol 1,3-Butanediol Dehydration Rare earth oxide Acid–base property

1. Introduction We have recently reported the vapor-phase catalytic dehydration of 1,4-butanediol over pure rare earth oxides (REOs) calcined at different temperatures [1–4]. In the dehydration of 1,4butanediol, 3-buten-1-ol was selectively produced over REOs. Weakly basic heavy REOs, such as Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3, Lu2O3, and Y2O3, showed high selectivity to 3-buten-1-ol. Heavy REOs exhibited different catalytic activities in the dehydration of 1,4-butanediol depending on their crystal structures [4]: cubic REOs with bixbyite structure produced 3-buten-1-ol with selectivity higher than 80 mol%; in particular, cubic Er2O3, Yb2O3, and Lu2O3 showed the highest formation rate of 3-buten-1-ol. In addition, supported REOs, such as Yb2O3 supported on monoclinic ZrO2, are also effective for the formation of 3-buten-1-ol from 1,4butanediol [5]. In the reaction of 1,4-butanediol, the catalytic function of REOs is related to the basic properties of REO that originated in lanthanide contraction [6]. The formation rate of 3-buten-1-ol over Er2O3 was suppressed in either acidic CO2 or basic NH3 carrier gas flow. The active centers for the formation of 3-buten-1-ol are composed of both basic and acidic sites [4]. The acid–base property of ZrO2 also plays a vital role in the formation of 3-buten-1-ol from 1,4-butanediol [7]. Cubic In2O3 with bixbyite structure shows excellent and stable catalytic activities for the formation of 3-

* Corresponding author. Tel.: +81 43 290 3377; fax: +81 43 290 3401. E-mail address: [email protected] (S. Sato). 0926-860X/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.01.025

buten-1-ol from 1,4-butanediol [8]. In2O3 and Yb2O3 catalyze the effective formation of 4-penten-1-ol from 1,5-pentanediol [8,9]. We have also reported that CeO2 catalyzes the dehydration of 1,3-diols into unsaturated alcohols [3,10–13]: 3-buten-2-ol and trans-2-buten-1-ol are produced from 1,3-buanediol at 325 8C with selectivities of 57 and 36 mol%, respectively. In2O3 also shows the catalytic activity in the dehydration of 1,3-butanediol to produce 3-buten-2-ol and trans-2-buten-1-ol [8], whereas it is deactivated in a short period of time. In contrast, in the dehydration of 1,3butanediol over other REOs, 3-buten-2-ol and trans-2-buten-1-ol are produced with the maximum of the total selectivity as high as 40 mol% [6], while side reactions include the decomposition to ethanol, ethanal, methanol, propanone, and 2-propanol and the dehydrogenation–hydrogenation to produce 3-buten-2-one, butanone, 2-butanol, and 1-butanol.

We expect that the catalytic activities of heavy REOs for the dehydration of 1,3-butanediol vary with their crystal structure, as we have found in the dehydration of 1,4-butanediol [4]. In this paper, we report the dehydration of 1,3-butanediol over REOs having different crystal structures at different calcination temperatures. We also present the results of poisoning experiments for CeO2 and Er2O3 using H2, CO2 and NH3 gasses to clarify the features of active sites. Then, we discuss the catalytic properties of REOs in relation to their basic and redox properties as well as in comparison with the performance in the dehydration of 1,4butanediol.

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Fig. 1. Changes in conversion and selectivity in the dehydration of 1,3-butanediol over REO samples, (A) CeO2, (B) Tb4O7, and (C) Sc2O3. (a) Circles, conversion of 1,3butanediol; (b) triangles, selectivity to sum of the unsaturated alcohols, such as 3-buten-2-ol, 3-buten-1-ol, cis- and trans-2-buten-1-ol; (c) squares, selectivity to sum of the oxidized products, such as butanone, 3-buten-2-one, and propanone. Letters in the figure indicate crystal structures of sample: M, monoclinic; C, cubic; CF, cubic fluorite. The number in parenthesis is the calcination temperature of the sample (8C). Conversion and selectivity were averaged in the initial 5 h. W/F = 0.15 g h cm 3, reaction temperature = 325 8C, N2 carrier gas flow rate = 20 cm3 min 1.

2. Experimental 2.1. Preparation of samples 1,3-Butanediol was purchased from Wako Pure Chemical Industries, Japan, and was used for the catalytic reaction without further purification. REO samples were purchased from Kanto Kagaku Co. Ltd., Japan. It has been reported that commercial REO samples are prepared by decomposing corresponding chlorides at temperatures >2000 8C for ca. 2 s in the vapor phase. The commercial REO samples were calcined at temperatures between 500 and 1000 8C. 2.2. Characterization The specific surface areas (SAs) and crystal structures of the REOs used in this study are listed in Table 1 of Ref. [14]. The SA values can be seen in Figs. 1 and 2, which also display the crystal structure of samples calcined at different temperatures. The numbers and densities of basic sites of several REOs are summarized in Table 2 of Ref. [14].

the reactor top at a liquid-feed rate of 2.0 cm3 h 1 together with N2 flow of 20 cm3 min 1. The effluent collected every hour was analyzed by gas chromatography (GC-8A, Shimadzu, Japan) with a capillary column (TC-WAX, 30 m, GL Science Inc., Japan) and a flame ionization detector. Since the catalytic activity is stable, as previous papers have reported [2,6], both the conversion of 1,3-butanediol and the selectivity to each product were averaged in the initial 5 h to evaluate the catalytic activity. The conversion of 1,3-butanediol is defined as the amount of 1,3-butanediol consumed in the reaction. The selectivity to each product represents molar selectivity. In order to elucidate the catalytic functions of REOs, we carried out a poisoning experiment: dehydration of 1,3-butanediol was performed in acidic, basic, and reductive gasses. In the poisoning experiment, either CO2 or H2 was used as a carrier gas at 30 cm3 min 1 instead of N2. An equimolar mixture of NH3 and N2 at the total flow rate of 30 cm3 min 1 was also used as carrier gas. 3. Results 3.1. Dehydration of 1,3-butanediol over REOs calcined at different temperatures

2.3. Catalytic reaction The dehydration of 1,3-butanediol was carried out in a fixedbed flow reactor. Prior to the reaction, a sample (0.30 g) was preheated in the flow reactor in N2 flow at 500 8C for 1 h. After the catalyst bed was cooled to 325 8C, 1,3-butanediol was fed through

Sesquioxides of trivalent rare earth metals have A-, B-, and Ctype crystal structures at temperatures lower than 2000 8C [15– 17]. The A-, B-, and C-type crystal structures are hexagonal, monoclinic, and cubic, respectively. We have previously reported the structures of REOs calcined at different temperatures [14]:

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Fig. 2. Changes in conversion and selectivity in the dehydration of 1,3-butanediol over REO samples, (A) Dy2O3, (B) Ho2O3, (C) Y2O3, (D) Er2O3, (E) Tm2O3, (F) Yb2O3, and (G) Lu2O3. (a) Circles, conversion of 1,3-butanediol; (b) triangles, selectivity to sum of the unsaturated alcohols, such as 3-buten-2-ol, 3-buten-1-ol, cis- and trans-2-buten-1-ol. Captions are the same as those in Fig. 1.

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Table 1 Dehydration of 1,3-butanediol over REOs calcined at 1000 8Ca. REO

Conversion (%)

La2O3 CeO2 Pr6O11 Nd2O3 Sm2O3 Eu2O3 Gd2O3 Tb4O7 Dy2O3 Ho2O3 Y2O3 Er2O3 Tm2O3 Yb2O3 Lu2O3 Sc2O3

2.4 72.6 4.4 1.7 3.1 2.7 2.8 6.9 7.7 10.9 33.5 26.3 19.9 33.4 29.0 25.9

Selectivity (mol%)b 3-Buten-2-ol

2-Buten-1-ol

3-Buten-1-ol

Oxidized products

7.1 58.0 9.0 9.0 11.5 17.9 16.1 49.2 47.1 53.4 54.4 54.4 50.9 51.2 52.2 38.4

1.9 35.7 2.6 4.4 4.0 9.0 8.3 30.1 32.0 33.9 31.1 33.9 37.8 35.2 40.7 45.4

0.4 0.4 0.5 1.3 1.1 4.9 8.8 0.5 0.5 0.6 0.4 0.8 0.7 0.9 1.0 7.6

25.3 2.0 40.3 5.3 33.9 14.8 18.3 4.2 7.9 3.0 8.3 3.4 4.8 4.2 3.3 4.1

a Conversion and selectivity were averaged in the initial 5 h. W/F = 0.15 g h cm 3 where W and F are catalyst weight and flow rate of reactant fed, respectively. Reaction temperature = 325 8C, N2 carrier flow rate = 20 cm3 min 1. b 2-Buten-1-ol is a mixture of cis- and trans-isomers. Oxidized products are butanone, 3-buten-2-one, and propanone.

heavy REOs, such as Dy2O3, Ho2O3, Y2O3, Tm2O3, and Lu2O3, exhibited transformations of crystal structure from monoclinic to cubic with increasing calcination temperature. At intermediate temperatures, the heavy REOs consisted of both monoclinic and cubic structures. Figs. 1 and 2 also display the transformation of crystal structure. CeO2 and Tb4O7 have the cubic fluorite structure, and Sc2O3 has the cubic bixbyite structure [14]. Table 1 summarizes conversion and selectivity in the dehydration of 1,3-butanediol at 325 8C over REOs calcined at 1000 8C. REO samples are listed in the order of ionic radius. Except CeO2, light REOs, such as La2O3, Pr6O11, Nd2O3, Sm2O3, Eu2O3, and Gd2O3, mainly catalyzed the oxidation to produce butanone, 3-buten-2one, and propanone. In contrast, CeO2 and heavy REO catalysts, such as Tb4O7, Dy2O3, Ho2O3, Y2O3, Er2O3, Tm2O3, Yb2O3, Lu2O3, and Sc2O3, showed high selectivity to unsaturated alcohols with a small amount of oxidized products, such as butanone, 3-buten-2one and propanone. The sum of the selectivities to the unsaturated alcohols exceeded 94 mol% over CeO2. Table 2 shows typical conversion and selectivity data in the dehydration of 1,3-butanediol at 325 8C over Er2O3 calcined at temperatures between 500 and 1000 8C. The selectivities to unsaturated alcohols such as 3-buten-2-ol and 2-buten-1-ol increased with increasing calcination temperature, whereas the selectivities to 3-buten-1-ol and to the oxidized products such as butanone, 3-buten-2-one and propanone, decreased. This difference comes from the variation of the crystal structure of Er2O3 with the calcination temperature: Er2O3 shows monoclinic crystal

phase at 500 and 600 8C and cubic bixbyite phase at 900 and 1000 8C [14]. Fig. 1 shows changes in conversion and selectivity with specific surface area in the dehydration of 1,3-butanediol over CeO2 (Fig. 1A), Tb4O7 (Fig. 1B) and Sc2O3 (Fig. 1C). The selectivity to the unsaturated alcohols, such as 3-buten-2-ol, 2-buten-1-ol and 3buten-1-ol, increased with increasing calcination temperature over CeO2. However, the selectivity to unsaturated alcohols over cubic fluorite Tb4O7 showed a maximum at a calcination temperature of 800 8C. Over cubic bixbyite Sc2O3, the selectivity to unsaturated alcohols exceeded 90 mol% at calcination temperatures higher than 700 8C. Fig. 2 shows changes in conversion and selectivity with specific surface area in the dehydration of 1,3-butanediol over REO samples, such as Dy2O3, Ho2O3, Y2O3, Er2O3, Tm2O3, Yb2O3, and Lu2O3. Dy2O3 (Fig. 2A) and Ho2O3 (Fig. 2B) had reasonable changes in conversion with specific surface area: the conversion of 1,3butanediol decreases with decreasing specific surface area. In contrast, Y2O3, Er2O3, Tm2O3, Yb2O3, and Lu2O3 (Fig. 2C–G) showed notable changes in the conversion. For example, Yb2O3 showed little change in the conversion with specific surface area. Y2O3 showed an opposite change to those of Dy2O3 and Ho2O3. The selectivity to the unsaturated alcohols generally increased with decreasing the specific surface area over these REOs. In the dehydration of 1,3-butanediol, REOs with cubic bixbyite structure had higher selectivity to unsaturated alcohols than REOs with monoclinic structure did.

Table 2 Dehydration of 1,3-butanediol over Er2O3 calcined at different temperaturesa. Calcination (8C)

SAb (m2 g

500 600 700 800 900 1000

33.5d 32.3d 23.4e 21.5e 16.4f 13.6f

a b c d e f

1

)

Conversion (%)

23.8 21.4 21.3 24.9 26.5 26.3

Selectivity (mol%)c 3-Buten-2-ol

2-Buten-1-ol

3-Buten-1-ol

Oxidized products

29.4 33.6 39.1 46.6 54.3 54.4

14.7 18.0 23.2 29.0 34.4 33.9

3.5 3.8 2.7 1.2 0.5 0.8

36.3 30.3 20.2 13.2 5.2 3.4

Conversion and selectivity at 325 8C were averaged in the initial 5 h. W/F = 0.15 g h cm 3. N2 carrier flow rate = 20 cm3 min Specific surface area cited from Table 2 of Ref. [4]. 2-Buten-1-ol is a mixture of cis- and trans-isomers. Oxidized products are butanone, 3-buten-2-one, and propanone. Crystal phase: monoclinic structure. Crystal phase: mixture of monoclinic structure and cubic bixbyite. Crystal phase: cubic bixbyite.

1

.

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Table 3 Dehydration of 1,3-butanediol over CeO2 and Er2O3 under different atmospheresa. Catalyst Carrier gas

Conversion (%)

Selectivity (mol%)b 3-Buten-2-ol

2-Buten-1-ol

3-Buten-1-ol

71.5 65.5 62.6 71.0

57.1 56.8 55.4 54.0

35.3 38.7 36.8 36.9

0.4 0.4 0.3 0.4

1.5 0.6 3.6 3.9

N2d NH3 + N2c,d CO2d H2d

28.7 20.9 17.9 24.0

53.0 53.9 43.8 46.8

32.6 34.5 31.4 30.5

0.5 0.6 0.8 0.5

1.4 2.7 3.7 10.5

Er2O3 N2e NH3 + N2c,e CO2e H2e

89.7 78.9 41.9 82.6

56.2 53.2 38.0 50.3

36.2 37.0 23.5 35.1

0.7 0.9 0.6 0.7

3.2 3.5 20.8 9.4

N2 NH3 + N2c CO2 H2

27.1 22.9 18.6 26.7

57.5 52.1 25.4 49.0

34.9 35.4 13.1 32.4

0.8 2.2 0.5 1.0

1.8 2.1 34.7 9.2

CeO2 N2 NH3 + N2c CO2 H2

a Conversion and selectivity at 325 8C were averaged in the initial 5 h. W/F = 0.15 g h cm 3. Total flow rate of carrier gas = 30 cm3 min calcined at 1000 8C. b 2-Buten-1-ol is a mixture of cis- and trans-isomers. Oxidized products are butanone, 3-buten-2-one, and propanone. c An equimolar mixture of NH3 and N2 was used as carrier gas. d Reaction temperature = 275 8C, W/F = 0.30 g h cm 3. e Reaction temperature = 325 8C, W/F = 0.45 g h cm 3.

3.2. Dehydration of 1,3-butanediol over REOs in different carrier gasses Table 3 summarizes the conversion and the selectivity in the dehydration of 1,3-butanediol with different carrier gases. CO2, NH3, and H2 were used as carrier gas instead of N2 over CeO2 and Er2O3 samples calcined at 1000 8C. At high conversions over CeO2 and Er2O3, the conversion of 1,3-butanediol as well as the selectivity to unsaturated alcohols, such as 3-buten-2-ol, 2buten-1-ol, and 3-buten-1-ol, showed a little change with CO2 and NH3 carrier gases compared to N2 carrier gas. H2 carrier gas affected the conversion and the selectivity over CeO2 and Er2O3 very slightly. At low conversions over Er2O3, the conversion of 1,3-butanediol and the selectivity to 3-buten-2-ol, 2-buten-1-ol, and 3-buten-1-ol decreased in CO2 and NH3 carrier gases while the selectivity to butanone, 3-buten-2-one and propanone increased. CO2 was more efficient for poisoning than NH3. In particular, Er2O3 showed significant decrease in the conversion and the selectivity to unsaturated alcohols in CO2 carrier gas. Thus, the selectivity to oxidized products increased in CO2 carrier gas over Er2O3. At 275 8C, CeO2 was poisoned by CO2 and NH3 in a similar way to Er2O3 at 325 8C. H2 carrier gas slightly affected the conversion and the selectivity over CeO2 and Er2O3. The poisoning effect of the gases on the catalytic activity increased in the order of H2 < NH3 < CO2.

Oxidized products

1

. CeO2 and Er2O3 samples were

ol, and 3-buten-1-ol, generally increased with increasing calcination temperature. The increase in the intrinsic catalytic activity of REO is caused by the transformation of monoclinic into cubic structure at high calcination temperature. In fact, cubic bixbyite Lu2O3, Yb2O3, Er2O3, Tm2O3, and Y2O3 showed high catalytic activity. Particularly, cubic fluorite CeO2 showed the highest formation rate of unsaturated alcohols at all calcination temperatures. We have previously reported the conversion of

3.3. Intrinsic catalytic activity of REOs for formation of the unsaturated alcohols from 1,3-butanediol Fig. 3 shows the formation rate of the sum of the unsaturated alcohols based on unit surface area calculated from the conversion data at 325 8C, which represents the intrinsic catalytic activity of REO for the formation of the unsaturated alcohols. The formation rates of the sum of the unsaturated alcohols over REOs calcined at 1000 8C are summarized in Table 4. Over the REOs, the formation rate of the unsaturated alcohols, such as 3-buten-2-ol, 2-buten-1-

Fig. 3. Changes in the formation rate of sum of the unsaturated alcohols, such as 3buten-2-ol, 3-buten-1-ol, cis- and trans-2-buten-1-ol, in the dehydration of 1,3butanediol over REOs with calcination temperature. The formation rate based on surface area was calculated under the conditions specified in Figs. 1 and 2. Values of Ce (a) are from Fig. 1A, and those of Ce (b) are calculated from the data cited from Table 2 in Ref. [13].

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Table 4 Formation rate of unsaturated alcohols over REOs calcined at 1000 8C and In2O3. REO

Ria (nm)

Formation rate of unsaturated alcohols (mmol h b

La2O3 CeO2 Pr6O11 Nd2O3 Sm2O3 Eu2O3 Gd2O3 Tb4O7 Dy2O3 Ho2O3 Y2O3 Er2O3 Tm2O3 Yb2O3 Lu2O3 Sc2O3 In2O3 a b c d

0.1032 0.0970 0.0990 0.0983 0.0958 0.0947 0.0938 0.0923 0.0912 0.0901 0.0900 0.0890 0.0880 0.0868 0.0861 0.0745 0.0800

1

m

2

)

From 1,3-butanediol at 325 8C

From 1,4-butanediol at 375 8Cc

0.020 3.853 0.090 0.049 0.059 0.039 0.093 0.439 0.410 0.637 0.985 1.282 0.804 1.382 1.442 0.571 3.523d

0.032 1.644 0.062 0.426 0.167 0.200 0.370 2.495 1.486 1.819 3.097 4.527 2.718 5.010 5.215 0.866 4.923d

Ionic radius of trivalent rare earth cation with coordination number 6 except Ce4+ with coordination number 8 [18]. The formation rate based on surface area was calculated from the conversion data in Table 1. Data cited from Fig. 6 of Ref. [4]. Data calculated from the conversion data in Tables 2 and 3 of Ref. [8].

1,3-butanediol over CeO2 [3,12,13], which is prepared by calcining cerium hydroxide. The previous data of CeO2 are fitted with the present formation rate for CeO2 (Fig. 3). 4. Discussion 4.1. Correlation between ionic radius of rare earth cation and catalytic activity of REOs We have discussed the correlation between the radius of the rare earth cation and the catalytic activity of REOs in the reaction of 1,4-butanediol [4]. REOs ranging from Lu (ionic radius 0.0861 nm) to Tb (0.0923 nm), namely Lu2O3, Yb2O3, Er2O3, Y2O3, Tm2O3, and Tb4O7, show high formation rate of 3-buten-1-ol with high selectivity in Fig. 5 of Ref. [4], from which the data cited are also listed in Table 4. We have concluded that the surface acid–base properties regulated by the surface geometry of heavy REOs affect

Fig. 4. Changes in the formation rate of sum of the unsaturated alcohols, such as 3buten-2-ol, 3-buten-1-ol, cis- and trans-2-buten-1-ol, in the dehydration of 1,3butanediol over REOs at 325 8C with ionic radius of rare earth metals. (a) Open circles, REO samples calcined at 500 8C; (b) closed circles, REOs calcined at 1000 8C; (c) triangle, In2O3, the datum cited from Table 4.

the selectivity to unsaturated alcohols. In the dehydration of 1,4butanediol, however, CeO2 is less active and less selective than those of heavy REOs. Fig. 4 depicts the relationship between the ionic radius of the rare earth cation [18] and the formation rate of unsaturated achohols in the dehydration of 1,3-butanediol over REOs calcined at 500 and 1000 8C. In the dehydration of 1,3-butanediol over heavy REOs calcined at 1000 8C, the formation rate of unsaturated alcohols is increased with decreasing ionic radius (Fig. 4). The results in the dehydration of 1,3-butanediol have the same tendency as the dehydration of 1,4-butanediol over heavy REOs such as Lu2O3, Yb2O3, Er2O3, Y2O3, and Tm2O3 [4]. It is important that heavy REOs with ionic radii from 0.086 to 0.089 nm are effective in the dehydration of diols with the carbon number of 4. In the dehydration of 1,4-butanediol over heavy REOs with cubic bixbyite structure, it is reported that the formation rate of 3buten-1-ol increases with increasing calcination temperature [4]. In particular, cubic Lu2O3, Yb2O3, and Er2O3 show high formation rates of 3-buten-1-ol, whereas cubic fluorite CeO2 shows much lower formation rate than heavy REOs. In contrast to the dehydration of 1,4-butanediol, in the dehydration of 1,3-butanediol, cubic fluorite CeO2 shows a much higher formation rate of unsaturated alcohols than heavy REOs (Figs. 3 and 4). Table 4 compares the catalytic activities of REOs in the dehydration between 1,3- and 1,4-butanediols. The formation rates over heavy REOs in the dehydration of 1,4-butanediol at 375 8C are three or four times larger than those of 1,3-butanediol at 325 8C (Table 4). If one considered the difference in the reaction temperature of 50 8C, the reactivity of the diols could be similar to each other over heavy REOs. In contrast, CeO2 is extremely active for the dehydration of 1,3-butanediol, since it has the highest formation rate of unsaturated alcohols at 3.85 mmol h 1 m 2 even at 325 8C. CeO2 is exceptionally active for the dehydration of 1,3butanediol, since 1,3-butanediol is dehydrated at lower reaction temperatures than 1,4-butanediol. In contrast to cubic fluorite CeO2, however, cubic fluorite Tb4O7 shows the formation rate of 0.439 mmol h 1 m 2, which is less active than CeO2. There is a difference in the radius values of the rare earth cation: Ce4+ (0.0970 nm) and Tb4+ (0.0923 nm). The small difference in the cation size would affect the activity of the oxide in the dehydration of 1,3-butanediol. This indicates that a specific interaction between CeO2 surface and 1,3-butanediol would contribute to the dehydration.

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4.2. Differences among REOs in the 1,3-butanediol dehydration The transformation of crystal structure from monoclinic to cubic bixbyite structure during calcination affects the selectivity to unsaturated alcohols of the resulting REOs. In the dehydration of 1,3- and 1,4-butanediols, the cubic bixbyite structures of heavy REOs, such as Lu2O3, Yb2O3, Er2O3, Y2O3, and Tm2O3, have advantages for the formation of unsaturated alcohols. In the formation of unsaturated alcohols such as 3-buten-2-ol and 2buten-1-ol (Table 1), cubic fluorite CeO2 and bixbyite, such as Dy2O3, Ho2O3, Y2O3, Er2O3, Tm2O3, Yb2O3 and Lu2O3, are more advantageous than hexagonal REOs such as La2O3 and Nd2O3 and monoclinic REOs such as Sm2O3, Eu2O3 and Gd2O3 and cubic fluorite Pr6O11 and Tb4O7. C-type REOs with cubic bixbyite constitutionally favor the unsaturated alcohols, while A- and B-type REOs do not essentially favor the formation of unsaturated alcohols. We have recently reported that cubic bixbyite surface structure of In2O3 (2 2 2) plane (Fig. 5D in Ref. [8]), and discussed the catalytically active sites on the plane. Four-oxygen defects regularly exist over In2O3 (2 2 2) plane, and many In3+ ions are exposed on the center. The three In3+ ions exposed on the defects would be the active sites: a b-hydrogen and OH groups of butanediol adsorb on the three In3+ ions, and are abstracted as a water via the redox scheme over In2O3 (Fig. 8B in Ref. [8]). The original idea comes from the catalytic reaction over cubic fluorite CeO2 [11], and the most stable (1 1 1) plane of CeO2 has an oxygen defect [19–24]. In a similar manner to In2O3, cubic REOs such as Dy2O3 Ho2O3, Y2O3, Er2O3, Tm2O3, Yb2O3 and Lu2O3 should have four-oxygen defects over the (2 2 2) plane. 4.3. Proposed mechanism of 1,3-butanediol dehydration In the TPD profiles of CO2 adsorbed on heavy REOs calcined at 1000 8C [14], desorption volume and temperature decrease with decreasing ionic radius. The heavy REOs have few acid sites, as judged from the TPD results of adsorbed NH3 [14]. The heavy REOs can be stated to be weak basic and non-acidic. However, not only acidic CO2 but also basic NH3 carrier gases suppress the formation rate of unsaturated alcohols over Er2O3 (Table 3). Thus, the acid– base property of REOs would affect the formation. In H2 carrier gas, on the other hand, there is no effect of poisoning in the dehydration of 1,3-butanediol over Er2O3. Therefore, the reaction of 1,3butanediol and the formation of unsaturated alcohols depend on basic and acidic sites over cubic Er2O3. We have speculated that the redox sites, such as Ce4+ cations exposed on the oxygen defect site, would be the active sites [11,25,26]. However, the speculation was not supported by experimental evidence in the past studies. In H2 carrier gas over CeO2 at 275 8C, the dehydration of 1,3-butanediol is slightly poisoned by H2. This may be the evidence for the past speculation. But CeO2 was seriously poisoned by CO2 and NH3 rather than by H2 at 275 8C (Table 3). The poisoning results are clear evidence for the active sites on CeO2. It is probable that very weak basic sites and acidic sites act as abstraction sites and adsorption sites, respectively. CeO2 has the lowest basicity among the REOs [14]. The poisoning experiment in this work indicates that acid–base sites could be the active sites. In2O3, which has a cubic bixbyite crystal structure that is the same as those of heavy REOs, has redox property rather than acid– base nature [8]. Activity data for In2O3, which are calculated from the conversion data of Ref. [8], are added to Table 4 and Fig. 4. The catalytic activity of In2O3 was similar to that of cubic fluorite CeO2, rather than those of heavy REOs with acid–base property. We still have a question about how the redox sites of CeO2 produce the active sites, since there is no evidence that denies the speculation that the redox sites are the active sites. Indeed, H2 slightly poisons

the dehydration of 1,3-butanediol over CeO2 in H2 flow at a low temperature of 275 8C. Thus, we still need further investigation for discussing the reaction mechanism. It is obvious that cubic REOs are active for the dehydration of 1,3-butanediol: the catalytic activities depend on their crystal structures but not strongly correlated to surface area. The experimental results strongly support the correlation between catalytic activities and crystal structures. Either fluorite or bixbyite crystal structure of cubic REOs including In2O3 would be inevitable for the activation of 1,3-butanediol. We also need further investigation, i.e. quantum chemical calculations, for discussing the inevitabilities of specific crystal phase for the activation of 1,3butanediol. 5. Conclusions Vapor-phase catalytic dehydration of 1,3-butanediol over rare earth oxides (REOs) calcined at different temperatures was investigated. 3-Buten-2-ol and 2-buten-1-ol were mainly produced, together with dehydrogenated products such as butanone, 3-buten-2-one, and propanone. The catalytic activities depend on their crystal structures but not strongly correlated to surface area. Heavy REOs exhibited different catalytic activities in the dehydration of 1,3-butanediol depending on their crystal structures. Cubic bixbyite REOs selectively produced the unsaturated alcohols such as 3-buten-2-ol and 2-buten-1-ol, while monoclinic REOs were less active and less selective than cubic REOs. In the REOs, cubic fluorite CeO2 showed the highest formation rate of the unsaturated alcohols. Both basic and acidic sites of CeO2 and Er2O3 serve as active centers for the formation of the unsaturated alcohols, since the formation rate of the unsaturated alcohols is suppressed under either acidic CO2 or basic NH3 flow. References [1] S. Sato, R. Takahashi, T. Sodesawa, N. Yamamoto, Catal. Commun. 5 (2004) 397–400. [2] A. Igarashi, S. Sato, R. Takahashi, T. Sodesawa, M. Kobune, Catal. Commun. 8 (2007) 807–810. [3] M. Kobune, S. Sato, R. Takahashi, J. Mol. Catal. A: Chem. 279 (2008) 10–19. [4] S. Sato, R. Takahashi, M. Kobune, H. Inoue, Y. Izawa, H. Ohno, K. Takahashi, Appl. Catal. A: Gen. 355 (2009) 64–71. [5] H. Inoue, S. Sato, R. Takahashi, Y. Izawa, H. Ohno, K. Takahashi, Appl. Catal. A: Gen. 352 (2009) 66–73. [6] S. Sato, R. Takahashi, T. Sodesawa, A. Igarashi, H. Inoue, Appl. Catal. A: Gen. 328 (2007) 109–116. [7] N. Yamamoto, S. Sato, R. Takahashi, K. Inui, J. Mol. Catal. A: Chem. 243 (2006) 52–59. [8] M. Segawa, S. Sato, M. Kobune, T. Sodesawa, T. Kojima, S. Nishiyama, N. Ishizawa, J. Mol. Catal. A: Chem. 310 (2009) 166–173. [9] S. Sato, R. Takahashi, N. Yamamoto, E. Kaneko, H. Inoue, Appl. Catal. A: Gen. 334 (2008) 84–91. [10] S. Sato, R. Takahashi, T. Sodesawa, N. Honda, H. Shimizu, Catal. Commun. 4 (2003) 77–81. [11] S. Sato, R. Takahashi, T. Sodesawa, N. Honda, J. Mol. Catal. A: Chem. 221 (2004) 177–183. [12] A. Igarashi, N. Ichikawa, S. Sato, R. Takahashi, T. Sodesawa, Appl. Catal. A: Gen. 300 (2006) 50–57. [13] A. Igarashi, N. Ichikawa, S. Sato, R. Takahashi, T. Sodesawa, Appl. Catal. A: Gen. 314 (2006) 134. [14] S. Sato, R. Takahashi, M. Kobune, H. Gotoh, Appl. Catal. A: Gen. 355 (2009) 57–63. [15] M.P. Rosynek, Catal. Rev.-Sci. Eng. 16 (1977) 111–154. [16] L. Eyring, in: G. Meyer, L.R. Morss (Eds.), Synthesis of Lanthanide and Actinide Compounds, Kluwer Academic Publisher, Netherlands, 1991, pp. 187–224. [17] G. Adachi, N. Imanaka, Chem. Rev. 98 (1998) 1479–1514. [18] R.D. Shanon, Acta Crystallogr. A 32 (1976) 751–767. [19] T.X.T. Sayle, S.C. Parker, C.R.A. Catlow, Surf. Sci. 316 (1994) 329–336. [20] J.C. Conesa, Surf. Sci. 339 (1995) 337–352. [21] H. No¨renberg, G.A.D. Briggs, Surf. Sci. 402–404 (1998) 734–737. [22] H. No¨renberg, G.A.D. Briggs, Surf. Sci. 424 (1999) L352–L355. [23] K. Fukui, Y. Namai, Y. Iwasawa, Appl. Surf. Sci. 188 (2002) 252–256. [24] Y. Namai, K. Fukui, Y. Iwasawa, Catal. Today 85 (2003) 79–91. [25] N. Ichikawa, S. Sato, R. Takahashi, T. Sodesawa, J. Mol. Catal. A: Chem. 231 (2005) 181–189. [26] N. Ichikawa, S. Sato, R. Takahashi, T. Sodesawa, H. Fujita, T. Atoguchi, A. Shiga, J. Catal. 239 (2006) 13–22.