Gas-phase hydration of ethene over tungstena–zirconia

Applied Catalysis A: General 259 (2004) 199–205

Gas-phase hydration of ethene over tungstena–zirconia Wenling Chu a,1 , Tsuneo Echizen a , Yuichi Kamiya b , Toshio Okuhara a,∗ a

Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan b Japan Science and Technology Corporation, 4-1-8 Honcho, Kawaguchi 332-0012, Japan

Received 14 July 2003; received in revised form 13 September 2003; accepted 13 September 2003

Abstract Hydration of ethene over a variety of tungstena–zirconia (WO3 /ZrO2 ) catalysts of different W/Zr ratios using different calcination temperatures is studied at 473 K using a continuous fixed-bed reactor at atmospheric pressure. The maximum activity for ethene hydration is obtained by calcination at 1073 K with a W/Zr ratio of 0.4, while skeletal isomerization of n-butane is found to exhibit a maximum rate at W/Zr = 0.1. The most active WO3 /ZrO2 catalysts is demonstrated to be superior in activity to other typical solid acids such as H4 SiW12 O40 /SiO2 , zeolites (H-ZSM-5 and H-␤), various mixed oxides, and polymer resins. Lifetime tests reveal that constant activity of WO3 /ZrO2 obtained at 72 h is about 70% of initial activity and that the activity is largely recovered by calcination at 773 K in air. The specific activity (per surface area) of WO3 /ZrO2 is correlated with the hydrophobicity as estimated from the adsorption of water rather than with that estimated from the density of acid sites measured by NH3 -TPD. Therefore, the pronounced activity of WO3 /ZrO2 is mainly brought about by the high hydrophobicity of the surface along with the inherent acidity. © 2003 Elsevier B.V. All rights reserved. Keywords: Tungstena–zirconia; Hydration; Ethene; Ethanol; Hydrophobicity

1. Introduction Ethanol is one of the largest-volume organic chemicals and consumer products. The processes used in the last five decades for the synthesis of ethanol on an industrial scale involve the gas-phase hydration of ethene using phosphoric acid supported on silica [1] and liquid-phase synthesis using H2 SO4 . The solid catalyst-process is desirable from the viewpoint of environmental protection. However, the present commercial catalyst, supported-phosphoric acid, still exhibits the problem of vaporization of phosphoric acid from the silica during operation, which causes a decline in catalytic activity and corrosion of equipment. Thus, it is necessary to develop a new solid acid from which the active component dose not vaporize; it must be recoverable as well as active and selective for this reaction. Until now, there have been many reports on the hydration of ethene to ethanol over solid acids such as tungstosilicic acid on silica gel [2], ZnO–TiO2 [3], Nb2 O5 ·nH2 O [4], W–P ∗

Corresponding author. Tel./fax: +81-11-706-4513. E-mail address: [email protected] (T. Okuhara). 1 On leave from State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, PR China. 0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.09.041

mixed oxides [5], metal phosphates [6], and zeolites [7,8]. However, these catalysts are not sufficient for commercial use because of deactivation, difficulties in reproducibility, and low catalytic activities. Recently, several solid acids active for aqueous reactions have been reported [9]. High-silica H-ZSM-5, Cs2.5 H0.5 PW12 O40 , Nb2 O5 ·nH2 O, MoO3 /ZrO2 , NbOPO4 , and polymer resins are examples. Only H-ZSM-5 zeolite has been commercially used as a water-tolerant solid acid for liquid-phase hydration of cyclohexene by Asahi Chemical [10,11]. Hydrophobicity of catalyst surface as well as an acidic property is considered to dominate catalytic performances for acid-catalyzed reactions in the presence of excess water [9,10,12]. Oxides and mixed oxides as solid acids are usually contaminated by water and deactivate in water [9]. Furthermore, solid acids such as SiO2 –Al2 O3 are also deactivated in H2 O atmosphere for the gas-phase reaction [13]. Tungsten oxide-based catalysts have attracted much attention because of the unique catalysis exhibited in acidcatalyzed reactions [14–17]. It is worthy of note that the catalysts are extremely stable at high temperatures in H2 , O2 or H2 O atmospheres [18] since it is calcined at above 1073 K. Wakamatsu et al. [19] have described in a patent that WO3 supported on ZrO2 (WO3 /ZrO2 ) is active in hydration of

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propene. Li et al. [12] reported that WO3 /ZrO2 calcined at 1073 K exhibited high activity during hydrolysis of ester in water and esterification. However, the hydration of ethene over WO3 /ZrO2 has not been reported, to the best of our knowledge. The present study attempts to elucidate the catalytic property of WO3 /ZrO2 in the gas-phase hydration of ethene in comparison with a variety of solid acids. The effects of W/Zr atomic ratio and calcination temperature on the hydration activity of WO3 /ZrO2 are systematically examined. The remarkable impacts of W/Zr ratio and calcination temperature on catalytic activity are discussed with respect to characterization studies into acidic properties as measured by NH3 -TPD and hydrophobicity as estimated from the adsorption of water. The stability of WO3 /ZrO2 during the reaction and the recoverability by calcination are investigated in comparison with typical solid acids such as H4 SiW12 O40 /SiO2 , and polymer resins.

2. Experimental 2.1. Preparation of catalysts Tungstena–zirconia, WO3 /ZrO2 , catalysts of different W/Zr ratios were prepared by an impregnation method. Powder of ammonium para-tungstate, (NH4 )10 W12 O41 ·5H2 O, was first added to hot water (not dissolved instantly) and then the suspension was stirred under ultrasonic radiation for 5 min to form the solution. Zr(OH)4 (10 g, Daiichi Kigenso Co.) that had been dried at 373 K overnight was impregnated with an aqueous solution (0.016 mol dm−3 ) of (NH4 )10 W12 O41 ·5H2 O at room temperature. The solid was dried overnight at 373 K, and calcined at a certain temperature in air for 3 h. These catalysts are denoted by WO3 /ZrO2 (W/Zr = 0.1), where the figure in parentheses is the atomic ratio of W to Zr. As references, Cs2.5 H0.5 PW12 O40 and silica-supported H4 SiW12 O40 were prepared according to the literature [20,21] and calcined at 523 K in air for 4 h. The loading amount of H4 SiW12 O40 was adjusted to 40 wt.%. H-ZSM5 (JRC-Z5-70H, Si/Al = 40, 400 m2 g−1 ), H-mordenite (JRC-Z-HM-20(3), Si/Al = 9 359 m2 g−1 ) and H-␤ (Tosoh, Si/Al=25, 730 m2 g−1 ) were used after calcination at 773 K in air for 3 h. Nb2 O5 ·nH2 O (CBMM, HY-340, AD/2322, 131 m2 g−1 ) calcined at 573 K in air for 3 h was also used. Sulfated zirconia (denoted by SO4 2− /ZrO2 , 90 m2 g−1 ) was prepared according to the literature [22] and calcined at 893 K in air for 3 h. WO3 /TiO2 (W/Ti = 0.1) was prepared by impregnating TiO2 (Aerosil P25, 46 m2 g−1 ) with an aqueous solution of (NH4 )10 W12 O41 ·5H2 O and calcined at 1073 K in air for 3 h (27 m2 g−1 ). As polymer resins, Amberlyst 15 (Organo Co., 50 m2 g−1 ), Nafion-SiO2 composite (Du Pont, SAC-13, Nafion 13 wt.%, 344 m2 g−1 ) and Aciplex-SiO2 composite [23] (Asahi Chemical, 1.3 m2 g−1 ) were also used as received.

2.2. Characterization NH3 -TPD was carried out with a Multitask TPD system (BEL Japan Inc.) equipped with a mass spectrometer. After the sample (200 mg) was pretreated in a flow of He at 773 K for 30 min, it was exposed to NH3 (100 Torr) for 30 min at 373 K. The chamber was subsequently purged with an He flow at 373 K for 30 min to remove physisorbed ammonia. The temperature of the sample was raised at a rate of 10 K min−1 to 1073 K, and the desorption rate of NH3 was monitored using m/e = 16 and 17. The adsorption isotherm of water was measured at 298 K with an automatic adsorption apparatus (Belsorp 18, BEL Japan Inc). Before measurements, the samples were preevacuated at 573 K for 2 h. Water (purified with a Milli-Q system (Millipore)) was degassed by freeze-thaw cycles prior to use in adsorption measurements. The surface area of the catalysts was determined by the BET method using N2 with an automatic gas adsorption apparatus (Belsorp 28SA, BEL Japan Inc.) after the sample was evacuated at 573 K for 2 h. XRD patterns were recorded at room temperature using an X-ray diffractometer (Rigaku Geigerflex 2027). 2.3. Catalytic reactions Gas-phase hydration of ethene was performed in a continuous flow reactor (Pyrex tube, 10 mm of inside diameter) at atmospheric pressure. The catalyst powder was pelletized, followed by crushing and sieving to collect the 40–60 mesh fraction. Prior to use in the reaction, the catalyst was pretreated in a He flow (30 cm3 min−1 ) at 573 K for 1 h. Reactions were carried out at 473 K using 0.5 g of the catalyst with a mixture of ethene (23 vol.%) and water (23 vol.%) diluted with He (balance) at a total flow rate of 57.9 cm3 min−1 . At the outlet of the reactor, ethanol and water were collected in a trap cooled at about 200 K for 2 h. Both the amount of product ethanol and gas-phase at the outlet were determined using an FID-GC (Shimadzu 8A) equipped with a dioctyl phthalate (DOP) 30% Uniport R column (3 mm × 2 m). Skeletal isomerization of n-butane was carried out in a flow reactor. Prior to the reaction, the catalyst (1.0 g, 40–60 mesh) was pretreated in a flow of He (50 cm3 min−1 ) for 1 h at 573 K. Then n-butane (10.2 vol.%) in He (balance) was fed at a flow rate of 5 cm3 min−1 . Reaction products were analyzed with an FID-GC (Shimadzu GC-14B) equipped with a VZ-10 column (3 mm × 3 m).

3. Results 3.1. Catalytic reactions The conversions of ethene in the hydration and of n-butane in the skeletal isomerization for WO3 /ZrO2 cal-

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Fig. 1. Conversion of (䊊) ethene in hydration and (䊉) n-butane in skeletal isomerization as a function of WO3 -surface density on WO3 /ZrO2 calcined at 1073 K. Hydration of ethene: 473 K with a mixture of ethene 23%, water 23% and He (balance) at atmospheric pressure, catalyst 0.5 g, and total flow rate 57.9 cm3 min−1 . The skeletal isomerization of n-butane: 573 K with a mixture of n-butane 10.2% and He (balance) at atmospheric pressure, catalyst 1.0 g, and total flow rate 5.0 cm3 min−1 .

cined at 1073 K are plotted in Fig. 1 as a function of WO3 -surface density of WO3 /ZrO2 . The surface density (SD; W-atom nm−2 ) was calculated from Eq. (1) using WO3 loading amount (WL; wt.%), formula weight of WO3 (FWWO3 ; 231.8), and surface area (SA; m2 g−1 ) [24]. SD =

WL × (6.02 × 1023 ) 100 × FWWO3 × (SA × 1018 )

(1)

Pure ZrO2 did not exhibit any activity in either reaction under these reaction conditions. In the case of the hydration of ethene, the conversion increased as WO3 -surface density increased to 24.8 W-atom nm−2 (corresponds to W/Zr = 0.4), while the selectivities to ethanol were above 99% independently of the WO3 -surface density. In contrast, the maximum activity of the skeletal isomerization of n-butane was achieved at 5.7 W-atom nm−2 of WO3 -surface density (W/Zr = 0.1). The influence of calcination temperature on the catalytic activity of WO3 /ZrO2 (W/Zr = 0.1) for the hydration of ethene is shown in Fig. 2. The equilibrium conversion of ethene is 0.4% under the reaction conditions. The catalytic activity of WO3 /ZrO2 was found to depend strongly on calcination temperature. While WO3 /ZrO2 calcined at 773 K was much less active (only 0.001% of conversion), the increase in calcination temperature from 773 to 1073 K resulted in a great increase in catalytic activity. Further increasing the temperature caused a decrease in activity. As a result, maximum activity was obtained by calcination of WO3 /ZrO2 at 1073 K. Surface areas and acid amounts of the WO3 /ZrO2 (W/Zr = 0.1) catalysts calcined at different temperatures are also shown in Fig. 2. As the calcination temperature increased, a gradual decrease in both surface area and total acid amount measured by NH3 -TPD became obvious. The

Fig. 2. Effects of calcination temperature on (䊉) conversion for hydration of ethene, (䊏) surface area and (䊐) total acid amount of WO3 /ZrO2 (W/Zr = 0.1). Reaction conditions: 473 K with a mixture of ethene 23%, water 23% and He (balance) at atmospheric pressure, catalyst 0.5 g, and total flow rate 57.9 cm3 min−1 .

surface area of pure ZrO2 decreased monotonously from 100 to 6 m2 g−1 with an increase in calcination temperature from 773 to 1173 K, as reported previously [12]. Similarly, the surface area of WO3 /ZrO2 decreased as calcination temperature increased, but the surface area was still 35 m2 g−1 after calcination at 1173 K (Fig. 2B). This indicates that the presence of WO3 suppresses the aggregation of primary particles of ZrO2 . Catalytic results for the hydration of ethene over various solid acids are summarized in Table 1. At the initial stage of the reaction (2 h), the following activity order was obtained: WO3 /ZrO2 (W/Zr = 0.4) ≈ 40 wt.%H4 SiW12 O40 /SiO2 > Aciplex-SiO2 > Nafion-SiO2 > H-␤ > Cs2.5 H0.5 PW12 O40 ≈ Amberlyst-15 > WO3 /TiO2 (W/Ti = 0.1) ≈ SO4 2− /ZrO2 > H-ZSM-5 (Si/Al = 40). The WO3 /ZrO2 gave high selectivities to ethanol (99%). Acetaldehyde is by-product in this case. The selectivity for 40 wt.%H4 SiW12 O40 /SiO2 was relatively low due to the formation of butenes, while this catalyst showed nearly the same activity as WO3 /ZrO2 (W/Zr = 0.4). The polymer resin-SiO2 composites such as Nafion-SiO2 and Aciplex-SiO2 were highly active and selective, but these catalysts seriously deactivated under the reaction conditions, as will be described below. Changes in conversions in the hydration of ethene over WO3 /ZrO2 during the reaction are shown in Fig. 3, along with results for 40 wt.%H4 SiW12 O40 /SiO2 and two polymer resin-SiO2 composites (Aciplex-SiO2 and

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Table 1 Catalytic activities of various solid acids for hydration of ethene Catalyst ZrO2 WO3 /ZrO2 (W/Zr = 0.1) WO3 /ZrO2 (W/Zr = 0.2) WO3 /ZrO2 (W/Zr = 0.3) WO3 /ZrO2 (W/Zr = 0.4) WO3 /ZrO2 (W/Zr = 0.5) 40 wt.%H4 SiW12 O40 /SiO2 Cs2.5 H0.5 PW12 O40 H-mordenite H-ZSM-5 (Si/Al = 40) H-␤ (Si/Al = 25) Nb2 O5 ·nH2 O SO4 2− /ZrO2 WO3 /TiO2 (W/Ti = 0.1) Amberlyst-15 Nafion-SiO2 Aciplex-SiO2

WO3 -surface densitya (W-atm nm−2 ) 5.7 11.2 15.4 24.8 32.1

Calcination temperature (K)

Surface area (m2 g−1 )

Acid amountb (mmol g−1 )

Conversion of ethenec (%)

Selectivity to ethanol (%)

1073 1073 1073 1073 1073 1073 523 523 773 773 773 573 893 1073 As received As received As received

11 72 63 61 45 39 145 130 359 400 730 131 90 27 50 344 1

0.01 (0.01) 0.29 (0.20) 0.23 (0.17) 0.24 (0.17) 0.18 (0.13) 0.15 (0.11) 0.56 0.15 0.75 0.40 1.80d 0.25 0.17 0.12 4.70e 0.80f 0.46

0.00 (0.00) 0.12 (1.43) 0.12 (1.42) 0.16 (1.93) 0.17 (2.05) 0.13 (1.64) 0.17 (2.02) 0.09 (1.15) 0.05 (0.51) 0.06 (0.73) 0.12 (1.44) 0.001 (0.02) 0.07 (0.82) 0.08 (0.92) 0.09 (1.08) 0.13 (1.64) 0.16 (1.93)

– 99 99 99 99 99 91 93 96 95 93 100 99 98 87 99 99

a

WO3 -surface density was calculated from Eq. (1) (see text) [24]. Acid amount was measured by NH3 -TPD. The figures in parentheses are the acid amount of strong acid sites (see text). c The hydration of ethene was performed at 473 K with a mixture of ethene 23%, water 23% and He (balance) at atmospheric pressure. Catalyst 0.5 g and total flow rate 57.9 cm3 min−1 . The data were collected at 2 h. The figures in parentheses are the rate of ethanol formation (␮mol min−1 g−1 ). d Estimated from the Si/Al ratio. e From a technical description of Organo Co. The acid amount corresponds to the ion-exchange capacity. f From a technical description of Du Pont. The acid amount corresponds to the ion-exchange capacity. b

Nafion-SiO2 ). Conversions over WO3 /ZrO2 (W/Zr = 0.4) and 40 wt.%H4 SiW12 O40 /SiO2 were found to be almost constant for approximately the first 20 h of stream. In contrast, the polymer resin-SiO2 composites deactivated rapidly within 10 h. Near constant conversions over WO3 /ZrO2 and H4 SiW12 O40 /SiO2 were obtained after about 60 h, with WO3 /ZrO2 giving the greater rate. The reaction was suspended once at 72 h, during which the spent catalysts were calcined in air again. As shown in Fig. 3, the WO3 /ZrO2 re-calcined at 773 K in air for 5 h exhibited activity close

Fig. 3. Time courses for hydration of ethene over (䊊) WO3 /ZrO2 (W/Zr = 0.4), (䊉) 40 wt.%H4 SiW12 O40 /SiO2 , (䊐) Aciplex-SiO2 and (䉱) Nafion-SiO2 . Reaction conditions: 473 K with a mixture of ethene 23%, water 23% and He (balance) at atmospheric pressure, catalyst 0.5 g, and total flow rate 57.9 cm3 min−1 . At 72 h (broken line), the spent catalysts were calcined at 773 K in air for 5 h and at 573 K for WO3 /ZrO2 (W/Zr = 0.4) and 40 wt.%H4 SiW12 O40 /SiO2 , respectively.

to the initial activity. For 40 wt.%H4 SiW12 O40 /SiO2 , however, the initial activity was not recovered by re-calcination (573 K, 5 h). 3.2. Characterization of catalyst Fig. 4 shows XRD patterns of WO3 /ZrO2 samples with different WO3 -surface densities calcined at 1073 K for 3 h. ZrO2 calcined at 1073 K exhibited characteristic peaks

Fig. 4. The XRD patterns of WO3 /ZrO2 with different W/Zr atomic ratios. All samples were calcined at 1073 K for 3 h. The marks (䊉) and (䊐) show tetragonal and monoclinic ZrO2 , respectively. (a) ZrO2 , (b) W/Zr = 0.1, (c) W/Zr = 0.2, (d) W/Zr = 0.3, (e) W/Zr = 0.4, and (f) W/Zr = 0.5.

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Fig. 5. TPD spectra of NH3 from WO3 /ZrO2 with different W/Zr ratios. (a) ZrO2 , (b) W/Zr = 0.1, (c) W/Zr = 0.2, (d) W/Zr = 0.3, (e) W/Zr = 0.4, (f) W/Zr = 0.5 and (g) WO3 . All samples were calcined at 1073 K for 3 h. The broken lines represent the deconvoluted profiles for spectrum (f).

corresponding to the monoclinic phase of ZrO2 , with the main peaks appearing at 2θ = 28.2◦ and 31.5◦ [25]. As the WO3 -surface density increased, the fraction of monoclinic zirconia decreased. When the W/Zr ratio exceeded 0.1 (corresponding to a surface density of 5.7 W-atom nm−2 ), only the tetragonal phase of ZrO2 was detected. In addition, the bulk WO3 peaks around 25◦ were observed for WO3 -surface densities higher than 11.2 W-atom nm−2 (W/Zr = 0.2), with the intensity of these peaks increasing with WO3 -surface density. TPD spectra of NH3 from the WO3 /ZrO2 catalysts of different WO3 -surface densities (W/Zr ratio) are presented in Fig. 5, along with those obtained for pure WO3 and ZrO2 . The TPD spectra for all of the WO3 /ZrO2 samples exhibited similar shapes. Deconvoluted spectra are shown by the broken lines, revealing that each spectrum actually consists of two peaks, centered around 473 and 573 K. Peak temper-

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atures did not depend greatly on the WO3 -surface density, indicating similar acid strengths. However, pure ZrO2 and WO3 exhibited desorption peaks at much lower temperatures. Fig. 5 further revealed that as the WO3 -surface densities increased, the areas of the TPD spectra increased up to a maximum at a WO3 -surface density of 5.7 W-atom nm−2 (W/Zr = 0.1) before decreasing. In Fig. 6, adsorption isotherms of water at 298 K for WO3 /ZrO2 with various W/Zr ratios calcined at 1073 K are shown. All isotherms obtained are Type III according to IUPAC classification [26]; these are often observed when weak interaction between the adsorbate and adsorbent occurs. The W/Zr ratio was observed to greatly influence the amount of water adsorbed, increasing with W/Zr ratio to a peak at W/Zr = 0.1, then decreasing.

4. Discussion 4.1. Bulk structure of WO3 /ZrO2 One of the critical roles of loaded WO3 was found to be suppression of the thermal sintering of ZrO2 , as shown in Table 1. The loaded tungsten may form a thermally stable layer to protect the oxide particles from sintering. At the same time, the phase-transformation of ZrO2 from tetragonal to monoclinic was inhibited (Fig. 4). These changes in the phase-transformation with the addition of WO3 are in agreement with the literature [27,28]. Aside from WO3 , other oxides including SO4 2− , Cr2 O3 and MoO3 were also reported to suppress the sintering of ZrO2 crystallite, thus maintaining the surface area [12,29,30]. With the tungsten loading exceeding 11.2 W-atom nm−2 (W/Zr = 0.2), the formation of free WO3 crystal was detected by XRD (Fig. 4). The formation of the WO3 phase was also detected by Raman spectroscopy [31,32]. This surface density (11.2 W-atom nm−2 ) corresponds to a nearly doubly accumulated layer of WO3 , since a monolayer of WO3 is estimated to be 5–7 W-atom nm−2 from its surface area [33–35]. 4.2. Acidic properties of WO3 /ZrO2

Fig. 6. Adsorption isotherms of water over WO3 /ZrO2 with different W/Zr ratios. (䊏) W/Zr = 0.1, (䊉) W/Zr = 0.2, (䉱) W/Zr = 0.3, (䊊) W/Zr = 0.4, (䊐) W/Zr = 0.5 and ( ) )ZrO2 . All samples were calcined at 1073 K for 3 h.

As shown in Fig. 5, all of the NH3 -TPD profiles of WO3 /ZrO2 exhibited a low-temperature desorption peak centered at around 473 K, and a shoulder peak at around 573 K, both of which are nearly independent of the surface density of WO3 . Typical strong solid acids produced NH3 desorption peaks at about 650 K (H-ZSM-5) [20], 700 K (H-mordenite) [36], and 830 K (Cs2.5 H0.5 PW12 O40 ) [20]. The results shown in Fig. 5 indicate that the acid strengths of these WO3 /ZrO2 catalysts are far lower than those of the above typical solid acids. Combining NH3 -TPD and IR spectra of NH3 adsorbed on WO3 /ZrO2 , Iglesia and coworkers [18] found that changes in WO3 -surface coverage neither influenced the NH3 desorption profiles nor

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the position of the ν(N–H) infrared band. Thus, they concluded that no significant changes in acid strength among WO3 /ZrO2 catalysts with different WO3 -surface densities were evident. In addition, they observed that densities of the acid sites (per nm2 of surface area) increased with increasing WO3 -surface density up to the monotungstate saturation coverage (∼4 W nm−2 , from Raman and UV-visible spectra) [37]. Furthermore, the densities of the acid sites were found to reach constant values as the WO3 -surface density increased, indicating that two-dimensional polytungstate structures and three-dimensional WO3 domains possess similar densities of the acid sites. The present data indicate that the densities of the acid sites are almost independent of WO3 -surface density over 5.7 W nm−2 , which is consistent with previous reports [18,37,38]. Barton et al. [37] proposed a model of acid sites for WO3 /ZrO2 in which WO3 clusters of intermediate size delocalize a net negative charge caused by a slight reduction in W6+ centers in reactant environments containing H2 or hydrocarbons. This temporary charge imbalance leads to the formation of Brønsted acid (WO3 )m {W6−n O3 }{n-H+ }. 4.3. Hydrophobicity and catalytic activity of WO3 /ZrO2 Table 1 clearly demonstrates that WO3 /ZrO2 is far superior to typical solid acids such as zeolites (H-ZSM-5, H-␤ and H-mordenite) and other oxide catalysts, while WO3 /ZrO2 possesses a smaller number of acid sites than these solid acids, and has the lowest acid strength. Therefore, the activity order of these solid acids suggests that some factors besides acidity determines the catalytic activity for the hydration of ethene. This idea is supported by Fig. 7, which shows that the specific activity of WO3 /ZrO2 is not correlated to the density of acid sites. The unique change in specific activity as a function of WO3 -surface density (W/Zr ratio) cannot be explained in terms of acid strength or the density of acid sites. However, it is clear that the specific activity of WO3 /ZrO2 correlates well with surface hydrophobicity (Fig. 7). Some studies on water-tolerant catalysis by solid acids have demonstrated that hydrophobicity is a very important factor influencing catalytic activity [10,12]. The importance of the hydrophobic nature of the surface of solid catalysts is typically demonstrated in the hydration of cyclohexene over H-ZSM-5 zeolites [14]. In addition, good correlation between specific activity and hydrophobicity was revealed by water-participating reactions over MoO3 –ZrO2 [12]. Eguchi et al. [8] have demonstrated that the hydrophobic character of catalysts is advantageous in olefin hydration. In the present study, to estimate hydrophobicity semi-quantitatively, the reciprocal of the density of adsorbed water (1/D) is used as a measure of the hydrophobicity of the surface [12]. As the WO3 -surface density increases, similar trends in variations in specific activity (Fig. 7A) and hydrophobicity (Fig. 7B) were found. These results suggest that the hydrophobicity of this catalyst plays a cru-

Fig. 7. Effects of WO3 -surface density on (䊊) specific activities for hydration of ethene and (䊉) skeletal isomerization of n-butane, ( ) the hydrophobicity and density of (䊐) total and (䊏) strong acid sites over WO3 /ZrO2 calcined at 1073 K.

cial role in the catalytic activity. The hydrophobic nature of the catalyst could reasonably either increase the access of ethene to active sites or suppress the poisoning effect of water. The recent kinetic model proposed by Kochkar and Figueras [39] may explain the influence of hydrophobicity on activity. They presumed that the hydrophobic catalyst preferentially attacked the C=C bond in the olefin. The reason for the enhancement of hydrophobicity with an increase in the WO3 -surface density has not yet been elucidated. By covering the ZrO2 surface with a WO3 network, which is probably hydrophobic, the number of OH groups on the surface could be reduced. 4.4. Deactivation and regeneration of WO3 /ZrO2 The stability and recoverability of a catalyst are very important in practical use. As can be seen in Fig. 3, WO3 /ZrO2 with a W/Zr ratio of 0.4 exhibited excellent stability and recoverability compared to H4 SiW12 O40 /SiO2 . The formation of coke and/or the irreversible adsorption of by-products could be reasons for the deactivation. Since deactivated WO3 /ZrO2 was regenerated by calcination in air, the coke formed could be removed by treatment. In other words,

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regeneration of the WO3 /ZrO2 was successfully achieved by controlled burning of the deposited coke in air at 773 K. In the case of H4 SiW12 O40 /SiO2 , the calcination temperature was adjusted to 573 K because of the relatively low thermal stability of H4 SiW12 O40 [40]. In addition, protons of heteropolyacid such as H3 PW12 O40 begin to desorb in water at above 623 K, with the Keggin structure decomposing at above 723 K [40]. In fact, calcination of H4 SiW12 O40 /SiO2 at 573 K did not recover the initial activity (Fig. 3). The polymer resin-SiO2 composites deactivated rapidly (Fig. 3), although Nafion-H itself was claimed to be stable up to 483 K [41]. The desulfonation and the drop in activity of Nafion-SiO2 were observed during the operation at 423 K [23].

5. Conclusion The W/Zr atomic ratio and calcination temperature of WO3 /ZrO2 influenced sensitively the structure, acid properties, hydrophobicity, and catalytic activity. The maximum conversion of ethene was obtained over WO3 /ZrO2 (W/Zr = 0.4), which had the highest hydrophobicity. Increased hydrophobicity with W/Zr ratio and calcination temperature was found to be responsible for higher activity in ethene hydration. Spent WO3 /ZrO2 (W/Zr = 0.4) was found to be readily regenerated by calcination at 773 K in air. WO3 /ZrO2 (W/Zr = 0.4) was demonstrated to be far superior in activity to other typical solid acids.

Acknowledgements One of the authors, Dr. Wenling Chu, is supported by a grant from the Japan Science Promotion Society (JSPS). This work is also supported by Core Research for Evolution Science and Technology (CREST) of Japan Science and Technology Corporation (JST).

References [1] Y. Ogino, Sekiyu Gakkaishi 17 (1974) 166. [2] J. Muller, H.I. Waterman, Brennstoffchem. 38 (1957) 321. [3] K. Tanabe, C. Ishiya, I. Matsuzaki, I. Ichikawa, H. Hattori, Bull. Chem. Soc. Jpn. 45 (1972) 47. [4] K. Ogasawara, T. Iizuka, K. Tanabe, Chem. Lett. (1984) 645.

205

[5] T. Ushikubo, M. Kurashige, T. Koyanagi, H. Ito, Y. Watanabe, Catal. Lett. 69 (2000) 83. [6] A. Isobe, Y. Yabuuchi, N. Iwasa, N. Takezawa, Appl. Catal. A 194/195 (2000) 395. [7] M. Iwamoto, M. Tajima, S. Kagawa, J. Chem. Soc., Chem. Commun. (1985) 228. [8] K. Eguchi, T. Tokiai, H. Arai, Appl. Catal. 34 (1987) 275. [9] T. Okuhara, Chem. Rev. 102 (2002) 3641. [10] H. Ishida, Catal. Surv. Jpn. 1 (1997) 241. [11] K. Tanabe, W.F. Hoelderich, Appl. Catal. A 181 (1999) 399. [12] L. Li, Y. Yoshinaga, T. Okuhara, Phys. Chem. Chem. Phys. 1 (1999) 4913. [13] L. Li, Y. Yoshinaga, T. Okuhara, Phys. Chem. Chem. Phys. 4 (2002) 6129. [14] M. Hino, K. Arata, Appl. Catal. A 169 (1998) 151. [15] M.G. Falco, S.A. Canavese, R.A. Comelli, N.S. Figoli, Appl. Catal. A 201 (2000) 37. [16] R.A. Comelli, S.A. Canavese, N.S. Figoli, Catal. Lett. 55 (1998) 177. [17] R.D. Wilson, D.G. Barton, C.D. Baertsch, E. Iglesia, J. Catal. 194 (2000) 175. [18] C.D. Baertsch, S.L. Soled, E. Iglesia, J. Phys. Chem. B 105 (2001) 1320. [19] S. Wakamatsu, M. Shimura, S. Asaoka, Y. Shirato, Preprints of the Annual Meeting of the Japan Petroleum Institute, Tokyo, 1995, B34. [20] T. Okuhara, T. Nishimura, M. Misono, J. Mol. Catal. 74 (1992) 247. [21] A. Miyaji, T. Echizen, K. Nagata, Y. Yoshinaga, T. Okuhara, J. Mol. Catal. A 201 (2003) 145. [22] M. Hino, K. Arata, Catal. Lett. 30 (1995) 25. [23] K. Okuyama, X. Chen, K. Takata, D. Odawara, T. Suzuki, S. Nakata, T. Okuhara, Appl. Catal. A 190 (2000) 253. [24] N. Naito, N. Katada, M. Niwa, J. Phys. Chem. B 103 (1999) 7206. [25] Selected Powder Diffraction Data for Metals and Alloys, Data Book, 1st ed., vol. II, Card No. 13-307 and No. 24-1164, International Center For Diffraction Data (JCPDS), Swarthmore, USA. [26] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982, pp. 248–282. [27] M. Hino, K. Arata, J. Chem. Soc., Chem. Commun. (1987) 1259. [28] D.S. Kim, M. Ostromecki, I.E. Wachs, J. Mol. Catal. A 106 (1996) 93. [29] T. Yamaguchi, K. Tanabe, Mater. Chem. Phys. 16 (1986) 67. [30] M. Valigi, A. Cimono, D. Cordischi, S. DeRossi, C. Ferrari, G. Ferraris, D. Gazzoli, V. Indovina, M. Occhiuzzi, Solid State Ionics 63/65 (1993) 136. [31] D.-S. Kim, M. Ostromecki, I.E. Wachs, J. Mol. Catal. A 106 (1996) 93. [32] J.-R. Sohn, M.-Y. Park, Langmuir 14 (1998) 6140. [33] K.C. Pratt, J.V. Sanders, V. Christov, J. Catal. 124 (1990) 416. [34] Y.-C. Xie, Y.-Q. Tang, Adv. Catal. 37 (1997) 1. [35] M.A. Vuurman, I.E. Wachs, J. Phys. Chem. 96 (1992) 5008. [36] M. Niwa, N. Katada, Catal. Surv. Jpn. 1 (1997) 215. [37] D.G. Barton, M. Shtein, R.D. Wilson, S.L. Soled, E. Iglesia, J. Phys. Chem. B 103 (1999) 630. [38] I.E. Wachs, Catal. Today 27 (1996) 437. [39] H. Kochkar, F. Figueras, J. Catal. 171 (1997) 420. [40] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 221. [41] M.A. Harmer, E. William, Q. Sun, J. Am. Chem. Soc. 118 (1996) 7708.