Catalytic performance of the dodecatungstophosphoric acid on different supports

Applied Catalysis A: General 256 (2003) 141–152

Catalytic performance of the dodecatungstophosphoric acid on different supports J. Haber∗ , K. Pamin, L. Matachowski, D. Mucha Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Krakow, Poland Received 9 October 2002; received in revised form 23 December 2002; accepted 27 December 2002

Abstract Varying amounts of tungstophosphoric acid (HPW) have been supported on Y-type zeolite and silica using the incipient wetness method. The catalytic activity of the catalysts was studied in the vapor-phase dehydration of ethanol. The reaction was carried out in a conventional flow-type reactor under atmospheric pressure at temperatures changing from 398 to 773 K. Unexpectedly, deposition of heteropolyacid on zeolite HY with the loadings decreasing from 46.8 to 0.00016 wt.% results in catalysts characterized by the same catalytic activity indicating that the catalytic behaviour is determined by few very active sites, composed of Keggin anions and lacunary Keggin anions located in the supercages opened at the surface, which has a honeycomb structure. Contrary to the series of HPW/Y catalysts, HPW supported on SiO2 shows decreasing catalytic activity when its concentration lowers, the monolayer being catalytically inactive in acid–base reactions. © 2003 Elsevier B.V. All rights reserved. Keywords: Tungstophosphoric acid; Zeolite HY; Silica; Ethanol; Dehydration

1. Introduction Heteropolyacids (HPAs) with Keggin structure are widely used as acid catalysts, due to their very strong Brønsted acidity and their structural properties [1–3]. Heteropolyacids containing tungsten as addenda atoms exhibit a strong acidity, high thermal stability and low oxidation potential [1], which allow them to be used as catalysts in various reactions at moderate temperatures [4]. The acidity of heteropolyacid can be controlled by selecting the suitable HPA building components, the partial neutralization of the acid or the dispersion on different supports [5]. ∗ Corresponding author. Tel.: +48-12-6395101; fax: +48-12-4251923. E-mail address: [email protected] (J. Haber).

Supported heteropolyacids are important for many applications, because bulk HPAs have low specific surface area (1–10 m2 /g). In the case of unsupported heteropolyacids, when the reactants have a polar character, the catalytic reactions occur not only at the surface but also in the bulk of the solid heteropolyacids [6]. This is the reason why despite their low surface areas they demonstrate quite high catalytic activity. When non-polar reactants are used, it is important to increase the surface area or even better to increase the number of accessible acid sites of the HPA. This can be achieved by dispersing the heteropolyacids on solid supports with high surface areas [7–9]. It is known, that HPAs strongly interact with supports at low loading levels, while the bulk properties of heteropolyacids prevail at higher loadings [7]. Acidic or neutral substances like SiO2 , active carbons

0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00395-8

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or aluminosilicates are suitable as supports, but silica is the most often used. When tungstophosphoric acid (HPW) is supported on silica two distinct forms of heteropolyacid are deposited on the surface of SiO2 : the bulk crystalline phase and “the interacting form” [10]. Due to the interaction of HPA and the OH surface groups protons are transferred to the silica surface which results in a decrease of acidity and the appearance of redox properties of HPAs. Interacting with the surface occurs very readily for samples with loadings lower than 30 wt.% [7]. In this paper, we present the studies of the catalytic behaviour of tungstophosphoric acid deposited on Y-type zeolite and silica. Another important issue discussed here is the state of the heteropolyacid structure after deposition on the zeolite HY as it is known that the structure and properties of heteropolyacid dispersed at the silica surface (at low coverages) influence its behavior in catalytic reactions [11]. Tungstophosphoric acid was deposited on zeolite HY in the wide range of concentrations. For comparison catalysts with heteropolyacid supported on silica were prepared. Heteropolyacid after impregnation was or was not washed with water. The catalysts were tested in the vapour-phase dehydration of ethanol, the reaction which is considered to be a “test reaction” for acidic catalysts. 2. Experimental

The silica having a BET total surface area of ca. 306 m2 /g, a pore diameter of 90 Å, and a pore volume of 0.90 ml/g was used as other support of the catalysts. Supports before impregnation were sieved into grains of 0.2–0.4 mm. Dodecatungstophosphoric acid (Merck) was supported on zeolite HY and silica by means of incipient wetness impregnation. The known amount of tungstophosphoric acid was dissolved in water and the hot support was added to the solution. When the whole solution was absorbed by the solid, the obtained slurry was shaken for 15 min at room temperature and dried in the oven at 363 K for a few hours. Following described above procedure so called “unwashed” catalysts, denoted HPW/Y and HPW/SiO2 were prepared. Washed samples were obtained when the heteropolyacid after deposition on support was washed with water to dissolve that part of the heteropolyacid which was present in the form of small crystallites and was not bound to the support. They are designated HPW/Y* and HPW/SiO2 *. Unwashed catalysts were prepared by deposition of HPW on the supports with coverages changing in the range from 46.8 to 0.00016 wt.% for zeolite HY and of 24.2, 14.5 and 0.4 wt.% for silica. Washed samples were synthesized with loadings of 39 wt.% HPW in the case of zeolite HY and of 14.5 wt.% HPW for silica. Assuming that the Keggin anions are supported mainly in mesopores which have the surface area of 251.1 m2 /g, the theoretical surface concentration of the HPW can be estimated to be about 82% of monolayer.

2.1. Materials 2.2. Techniques Zeolite NaY, used as the parent material, was dealuminated chemically by means of ethylenediaminetetra-acetic acid (H4 EDTA). Dealumination was carried out under stirring at 363 K and the process was continued for 4 h. Resulting Si/Al ratio was determined to be 4.53. The dealuminated sample was washed with hot distilled water and dried. Zeolite was four times ion-exchanged with 0.5 M NH4 NO3 solution at room temperature, washed with water until no nitrate ions were present in the solution, dried and calcined in the oven at 723 K for 2 h. It had a BET total surface area of ca. 690 m2 /g, a pore diameter of 9 Å (micropores) and 35 Å (mesopores), and a pore volume of 0.45 ml/g. It was used as support for the catalysts in the dehydration of ethanol.

2.2.1. Nitrogen physisorption measurements BET surface areas, pore areas and pore volumes distributions of the catalysts were calculated from nitrogen adsorption isotherms at 77 K in an Autosorb-1, Quantachrome equipment. 2.2.2. Infrared spectroscopy FT-IR spectra were recorded on a Nicolet 800 spectrometer at room temperature in KBr pellets over the range of 400–1400 cm−1 under the atmospheric conditions. 2.2.3. X-ray diffraction Powder X-ray diffraction patterns were recorded on a Siemens D5005 automatic diffractometer using Cu

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K␣ radiation (55 kV, 30 mA) selected by a graphite monochromator in the diffracted beam. 2.2.4. Catalytic tests Vapour-phase dehydration of ethanol (99.8%, POCh) was carried out in a conventional flow-type reactor under atmospheric pressure, with a 0.3 ml of catalyst packed in a quartz reactor. Detail reaction conditions are described elsewhere [12]. The reactants feed was a mixture of helium and ethanol (94.3/5.7 mol%). The products were analyzed by the Perkin-Elmer 900 gas chromatograph equipped with a FID detector and Porapak S column, when steady-state reaction conditions were reached. In order to trace the coke formation the coking test for sample of heteropolyacid on zeolite Y and silica was carried out for 3 days. As the result only slight decrease of catalytic activity was observed. 3. Results 3.1. Pore size distribution and surface area 3.1.1. HY zeolite Zeolites are characterized by high porosity, varying between a few hundred and one thousand m2 /g. In an

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unmodified zeolite Y structure having pore openings of ca. 7.4 Å, dimensions of Keggin anion with diameter of 12 Å prevent the molecule from entering the pores of zeolite through the small window openings and the Keggin anion can be adsorbed only on the external surface. Dealumination of the faujasite develops the secondary pore structure. Fig. 1 shows the pore size distribution as a function of the pore diameter for micro- (Fig. 1a) and mesopores (Fig. 1b). The typical diameter of mesopores in the secondary pore system is 35 Å so that, in the dealuminated sample, Keggin anion can locate itself in micropores larger than 12 Å, which constitute about 30% of all micropores, in mesopores and on the external surface of zeolite. The nature of possible interactions between the heteropolyacid and the zeolitic matrix was studied earlier by NMR [13]. The presence of two forms of heteropolyacid: unperturbated Keggin units weakly interacting with the support and the Keggin anions interacting with OH groups of the zeolite was discovered even for the catalysts with the low (2 wt.% of HPW) concentration of heteropolyacid on faujasite. But while for low loadings HPW is distributed evenly between non-interacting and interacting species, for catalysts with 16 wt.% of HPW in sample or more the non-interacting Keggin units become predominant.

Fig. 1. Micropore (a) and mesopore (b) size distribution of dealuminated HY zeolite as a function of pore diameter.

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Fig. 2. X-ray diffraction patterns of (a) parent zeolite NaY, (b) dealuminated material, (c) 46.8 wt.% HPW/Y and (d) H3 PW12 O40 .

In Fig. 2 the X-ray diffraction patterns of parent zeolite NaY (a), dealuminated material (b), dealuminated material with supported heteropolyacid (c) and tungstophosphoric acid (d) are compared. The diffraction pattern of 46.8 wt.% HPW/Y sample indicates the existence of two crystalline phases: zeolite matrix and tungstophosphoric acid. No evidence of a collapse of the faujasite structure of the support was observed despite its exposure to strong acidic medium of HPW. After treating the 46.8 wt.% HPW/Y sample with water the HPW loading decreases to 39 wt.% as the result of dissolution of crystalline HPW deposited at the external surface of the zeolite. The HPW remaining in the sample is spread over the surface of mesopores and external surface of zeolite particles. Taking into account the specific surface area of mesopores amounting to about 251.1 m2 /g the theoretical surface coverage with Keggin anions can be estimated to amount to about 82% of monolayer. The Keggin anions lining out the mesopores are probably less accessible for the reactants of the catalytic reaction due to the diffusional limitations. Moreover, it was discussed earlier [13] that the Keggin units adsorbed in the mesopores can interact strongly with the sur-

rounding pores. The species adsorbed on the external surface are not sensitive to such interactions. Table 1 compares total surface area, surface area of micro- and mesopores and pore volume measured for zeolite HY and illustrates the changes of those parameters after heteropolyacid deposition. The BET measurements were also normalised to 1 g of pure zeolite in a sample. These data confirm that the structure of zeolite HY does not collapse upon supporting the tungstophosphoric acid. Taking into account normalized data, sorption properties of the samples compared in Table 1 demonstrate no differences between 46.8 wt.% HPW/Y catalyst and zeolite HY. In the case of 39.0 wt.% HPW/Y* catalyst a slight increase in sorption values is probably caused by the water treatment of the as-synthesized material. However, normalized sorption data fluctuate around the data measured for zeolite matrix. Remembering the fact that only small amount of heteropolyacid was washed away, the BET results confirm the conclusion that heteropolyacid exists on the external surface and surface of mesopores of zeolite mainly in monomolecular dispersion. Similar opinion was presented by Marme et al. [14] suggesting that isolated

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Table 1 Changes of the zeolite HY porosity after heteropolyacid deposition Sample

Total surface area (m2 /g)

Mesopore area (m2 /g)

Micropore area (m2 /g)

Total pore volume (cm3 /g)

Mesopore volume (cm3 /g)

Micropore volume (cm3 /g)

Y

689.6

251.1

438.5

0.45

0.23

0.22

46.8 wt.% HPW/Y

354.2 665.8

124.4 233.8

229.8 423.0

0.24 0.45

0.12 0.23

0.12 0.22

39.0 wt.% HPW/Y*

422.1 728.1

165.4 285.3

256.7 442.8

0.30 0.52

0.17 0.30

0.13 0.22

Values given in italic were recalculated taking into account 1 g of pure zeolite in a sample.

Keggin anions are immobilized in the channels of hexagonal mesoporous silica by weak interaction with walls by hydrogen bonds. The Keggin units retain their structure and are well dispersed as long as their content is lower than the monolayer coverage. 3.1.2. Silica It has been shown in the literature [15] that, the coverage of the support surface by vanado-molybdophosphoric acid is not complete, even at high acid content. The heteropolyacid deposits on silica in form of blocks of different height covering maximally 20% of the SiO2 surface, even when the acid content is equal to one theoretical monolayer. Tungstophosphoric acid similarly forms crystallites of heteropolyacid, sticking to the surface of silica, and only the first layer at the interphase with silica is modified by the interaction with the support. Being soluble in water the acid crystallites are washed away by water treatment, leaving at the surface of silica monomolecular islands [12]. Thus, the amount of HPW taken for preparation of the catalyst was calcu-

lated under assumption that 30% of silica should be covered with the active phase and the excess present as crystalline phase was removed by washing with water, leaving islands of monomolecular layer of Keggin anions strongly bound to the surface of the support. In the case of silica (Table 2) both surface area and volume of mesopores decrease due to the deposition of heteropolyacid. Simultaneously, micropores area can practically be neglected and its increase observed after deposition of HPW on silica can be explained by the presence of micropores in HPW formed after its adsorption on silica. Similar behaviour of tungstophosphoric acid deposited on high surface area silica resulting in development of micropores was described in [16]. Part of tungstophosphoric acid is deposited on silica in the form of crystallites soluble in water and as such can be easily washed away. Water treatment decreases initial surface concentration of HPW from 24.2 to 14.5 wt.% resulting in the growth of the surface area which approaches the value (332.9 m2 /g) close to the area of the silica support. The slight increase in total surface area is probably induced by the

Table 2 Changes of the silica porosity after heteropolyacid deposition Sample

Total surface area (m2 /g)

Mesopore area (m2 /g)

SiO2

306.0

301.4

24.2 wt.% HPW/SiO2

211.8 290.0 249.0a

14.5 wt.% HPW/SiO2 *

287.0 350.8 332.9a

Micropore area (m2 /g)

Total pore volume (cm3 /g)

Mesopore volume (cm3 /g)

Micropore volume (cm3 /g)

4.6

0.89

0.88

0.01

178.5 244.4

33.3 45.6

0.56 0.77

0.54 0.74

0.02 0.03

267.8 327.3

19.2 22.5

0.80 0.98

0.79 0.97

0.01 0.01

Values given in italic were recalculated taking into account 1 g of pure zeolite in a sample. a This value was calculated after the subtraction of the micropores area developed due to the deposition of HPW.

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water treatment of the sample. Decrease of micropores area observed after removal of the excess crystalline phase confirms that micropores were formed mainly in the deposited heteropolyacid. The differences in quantity of HPW washed away from zeolite HY and silica may be related to the difference in its accessibility due to the variation in size of mesopores which amounts to 35 and 90 Å, respectively. Moreover, in larger pores of silica HPW may be deposited in form of larger crystallites, less perturbed by the support and therefore showing better solubility. 3.2. FT-IR spectroscopy The spectrum of tungstophosphoric acid consists of five characteristic bands in the region 1100–400 cm−1 : 1080, 982, 893, 812 and 595 cm−1 (Fig. 3). Positions of these bands can be assigned, in turn, to the stretching vibrations of P–O, W=O and W–O–W, and the bending vibration of P–O [3]. When heteropolyacid is deposited on silica, lowering of the intensities of the characteristic bands is observed, while their positions do not change. The removal of the bulk heteropolyacid by washing with water causes further decrease of intensities of the characteristic bands, while the stretching vibration of the W–O–W band at 893 cm−1 vanishes. In the conditions prevailing at the silica surface hydrolysis of the W–O–W bonds occurs leading to the opening of the structure of Keggin anion followed by its transformation into the constituent triads [12]. Similar mechanism was postulated for the low-loaded molybdophosphoric acid on silica, in which the Mo–O–Mo bonds undergo hydrolysis [11]. Moreover, it could be expected that in such circumstances the acidity of the supported heteropolyacids decreases or disappears exposing the redox properties of the catalytic system. Thus, when Keggin anions are present at the silica surface in molecular dispersion, they undergo hydrolysis and practically lose their acidic properties and hence become inactive as acid–base catalysts. Fig. 4 presents FT-IR spectra of tungstophosphoric acid deposited on zeolite HY. Similarly as for silica impregnation of HPW on the surface of zeolite HY also results in lowering of the characteristic IR vibrations while no shift of IR bands was observed. The removal of the excess of heteropolyacid from the 46.8 wt.% HPW/Y catalyst was followed by further

Fig. 3. FT-IR spectra of (a) 14.5 wt.% HPW/SiO2 *, (b) 24.2 wt.% HPW/SiO2 and (c) H3 PW12 O40 .

rather insignificant lowering of the characteristic HPW bands but—at variance with the case of silica—the band associated with the presence of W–O–W bonds at 893 cm−1 did not vanish. Even for the 0.016 wt.% HPW/Y sample, characteristic band at 893 cm−1 is still observed (marked by the arrow on curve c, Fig. 4) which suggests that despite very low concentration of HPW Keggin anions are present in the faujasite structure. However, it can be argued whether in such diluted catalytic systems structure of Keggin anion is retained. In aqueous solutions Keggin anion has a limited stability range. At pH 1.5–2 it is reversibly and quickly transformed into the lacunary anion

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Fig. 5. The mode of deposition of Keggin anion on zeolite HY.

Fig. 4. FT-IR spectra of (a) 39 wt.% HPW/Y*, (b) 46.8 wt.% HPW/Y, (c) 0.016 wt.% HPW/Y, (d) zeolite HY and (e) HPW.

[PW11 O39 ]7− : [PW12 O40 ]3− + 5OH− ⇔ [PW11 O39 ]7− + [HWO4 ]− + 2H2 O Aqueous solution of lacunary anions is stable at pH between 2 and 6. Thus, in the conditions prevailing during the synthesis of the low loaded catalysts the equilibrium between Keggin and lacunary anions is shifted to the right. Therefore, both types of anions become deposited on the HY zeolite. The presence of lacunary species can be observed in infrared spectrum where the P–O stretching vibration at 1080 cm−1 assigned to [PW12 O40 ]3− splits into two bands 1085 and 1040 cm−1 when [PW11 O39 ]7− is formed [17]. Un-

fortunately, after deposition of heteropolyacid on the faujasite the P–O vibrations are not visible, screened by a strong absorption band of zeolite present exactly in this region. The anion of HPW with Keggin structure is a sphere with diameter of 12 Å. The supercages of zeolite Y have a diameter of 13 Å and are ideal place for the location of HPW molecules. During the zeolite surface pretreatment and preparation for deposition of the heteropolyacid the faujasite crystals disintegrate and supercages break forming “bowls”, the exposed surface resembling a honeycomb. Similar behaviour was described by Corma et al. [18] where the MWW structure undergoes delamination leading to the formation of half opened cavities on the external surface, which are called “cups”. Keggin anions fit very well into these “bowls”. Their location in the “bowls” prevents the hydrolysis of the Keggin anion structure and formation of triads like it was postulated in the case of silica. Such deposition of HPW takes place at the external surface of the zeolite and to some extent at the surface of mesopores, the Keggin anions becoming completely isolated from each other and showing unusually high catalytic activity. The scheme of such a behaviour of Keggin anion deposited on the zeolite HY is presented in Fig. 5. For low loaded catalysts, when Keggin anions are in equilibrium with lacunary anions, [PW11 O39 ]7− anion can also be immobilized in the “bowls”. 3.3. Catalytic activity in dehydration of ethanol Two series of catalysts presented in Table 3, were prepared and their catalytic activities were tested in the dehydration of ethanol.

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Table 3 The list of the studied catalysts Catalysts not treated with water (wt.%)

Catalysts treated with water (wt.%)

HPW/Y 46.8, 1.56, 0.78, 0.42, 0.16, 0.078, 0.016, 0.0078, 0.0016, 0.00016

HPW/Y* 39.0

HPW/SiO2 24.2, 14.5, 0.4

HPW/SiO2 * 14.5

Silica is very suitable as support, frequently used for depositing different heteropolyacids because it is relatively inert towards them in the reaction conditions. On the other hand those catalysts quite often work in the reaction conditions in which water is involved, i.e. hydration or dehydration [12,19]. For this reason they should be resistant to water. In order to prepare such catalysts, the samples were dipped in water and thoroughly washed until all the crystallites, not bound to the support were dissolved and removed. The surface coverage of the samples after water treatment decreases from 24.2 to 14.5 wt.% leaving at the surface a monolayer of heteropolyacid interacting with the support and not soluble in water. The choice of zeolite HY as the support for tungstophosphoric acid was based on its large surface area and the dimension of supercage. Catalytic tests were performed for the whole series of the HPW/Y catalysts with the loading of heteropolyacid chang-

Table 4 Turnover frequencies (TOF) calculated for unsupported heteropolyacid and series of heteropolyacid deposited on zeolite HY Catalyst

TOF [mol EtOH/(mol Keggin anion s)]

HPW + quartz HPW/Y (46.8 wt.%) HPW/Y (1.56 wt.%) HPW/Y (0.78 wt.%) HPW/Y (0.42 wt.%) HPW/Y (0.16 wt.%) HPW/Y (0.078 wt.%) HPW/Y (0.016 wt.%) HPW/Y (0.0078 wt.%) HPW/Y (0.0016 wt.%) HPW/Y (0.00016 wt.%)

0.068 0.024 1.48 1.56 2.76 6.47 15.19 73.90 169.30 781.25 7917.20

ing from 46.8 to 0.00016 wt.% (Fig. 6). The samples were not treated with water. Surprisingly, it turned out that catalytic activities of all catalysts of this series are similar indicating that they contain at the surface a very small concentration of extremely active sites responsible practically for their catalytic behaviour. Two products of ethanol dehydration were observed: diethyl ether at low temperatures and ethylene at higher temperatures. As an example, the selectivities to diethyl ether and ethylene, as function of temperature, observed in the case of 0.00016 wt.% HPW/Y are shown in Fig. 7. Table 4 presents turnover frequencies (TOF) calculated for the series of HPW/Y catalysts at 448 K. Total

Fig. 6. Catalytic activity of H3 PW12 O40 deposited on zeolite HY with different concentrations: (a) 0.0078 wt.% HPW/Y, (b) 0.00016 wt.% HPW/Y, (c) 0.016 wt.% HPW/Y, (d) 46.8 wt.% HPW/Y and (e) zeolite HY. Catalysts were not treated with water.

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Fig. 7. Selectivities to ethylene (a) and diethyl ether (b) as the function of temperature for 0.00016 wt.% sample.

amount of active sites in catalyst samples was taken for calculations. For comparison TOF value for the physical mixture of heteropolyacid and quartz (amount of HPW in this mixture is equivalent to the contents of heteropolyacid in 46.8 wt.% HPW/Y catalyst) was determined. This value is comparable with TOF calculated for the 46.8 wt.% HPW/Y catalyst. As the overall catalytic activity of all catalysts of this series is similar, the catalytic activity per Keggin anion increases as the amount of heteropolyacid on the surface of zeolite HY decreases. Keggin and lacunary anions placed in “bowls” formed by large cavities opened at the surface are encapsulated there and due to this are resistant to further hydrolysis by water molecules, being at the same time isolated active centers. Taking into account that lacunary Keggin anions are accompanied by seven protons, these centers are very strong and to a large extent responsible for catalytic activity. Keggin anions deposited in mesopores are less accessible for reactants and in this way do not strongly influence the catalytic activity. Lowering the loading of HPA deposited on zeolite HY surface in the range 46.8–0.00016 wt.% implies that at the lowest loadings only isolated centers are present. Changes of the TOF values describing the behaviour of heteropolyacid supported on zeolite HY illustrates that isolated acidic centers appear for the

catalyst holding 0.016 wt.%. It is interesting to point out that catalytic activity of the 39.0 wt.% HPW/Y* is very similar to the activity of parent catalyst 46.8 wt.% HPW/Y. This fact confirms that water treatment of 46.8 wt.% HPA/Y results in the decrease of the HPA concentration on zeolite HY to 39.0 wt.% due to washing away heteropolyacid from mesopores and without removal of isolated active centers. For comparison, the series of samples synthesized by depositing on silica the amount of heteropolyacid corresponding to 24.2, 14.5 and 0.4 wt.% of monolayer were prepared for catalytic tests in dehydration of ethanol. Additionally, 14.5 wt.% HPW/SiO2 * catalyst was obtained by water treatment of the parent sample 24.2 wt.% HPW/SiO2 , after which only monolayer of Keggin anions remains at the surface. Fig. 8 shows differences in catalytic activity between catalysts having different HPW loadings on silica. Lowering the concentration of the deposited heteropolyacid results in the decrease of the catalytic activity. In this case the reaction proceeds in so-called pseudo-liquid phase [6]. Due to the ability of solid heteropolyacid to absorb a large amount of polar molecules like alcohols or ethers in the catalyst bulk and the extremely high proton mobility, the catalytic reaction may occur not only on the surface but also in the crystalline HPA. Catalytic activities of the two samples, treated and

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Fig. 8. The catalytic activity of the heteropolyacid supported on silica: (a) 24.2 wt.% HPW/SiO2 , (b) 14.5 wt.% HPW/SiO2 , (c) 14.5 wt.% HPW/SiO2 * and (d) 0.4 wt.% HPW/SiO2 .

not treated with water, with the same concentration of 14.5 wt.% HPW on silica were different. In the case of silica covered with the crystalline heteropolyacid all protons are easily available for the reagents except those directly bound to the surface. When monolayer of Keggin anions is formed on the silica surface, pro-

tons become employed in protonation of the surface Si–OH groups and do not participate in the catalytic reaction. Catalytic properties of tungstophosphoric acid supported on silica and zeolite HY show differences in the dehydration of ethanol (Fig. 9). Comparison of

Fig. 9. Comparison of catalytic activity of pure silica and zeolite HY with catalysts containing the same quantity of tungstophosphoric acid: (a) 0.4 wt.% HPW/Y, (b) zeolite HY, (c) 0.4% HPW/SiO2 and (d) silica.

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two catalysts containing the same quantity of heteropolyacid deposited on different supports (0.4 wt.% HPW/SiO2 and 0.42 wt.% HPW/Y) shows that 0.4 wt.% HPW/SiO2 catalyst has the same activity like pure silica up to 673 K. On the other hand, 0.42 wt.% HPW/Y sample is much more active than the parent zeolite HY. Analysis of those results indicates that the state of the monolayer of heteropolyacid on silica and zeolite HY should be different. Heteropolyacid exists on zeolite HY in form of isolated, Keggin or lacunary anions which are characterized by a very high catalytic activity. As the lacunary anion can be coordinated maximally by seven protons, it exhibits very strong acidic properties stronger than in the case of Keggin anion. This may be the reason why the catalysts with low concentration of HPW show unusually high catalytic activity in the dehydration of ethanol. High catalytic activity of lacunary anion-based catalysts was also observed in oxidation reactions [20–22]. Conversely, concentration of heteropoly molecules deposited on SiO2 is sufficient to react with existing hydroxyl groups undergoing hydrolysis, consuming protons from silica surface [10] and as a consequence, lowering to the large extent catalytic activity.

4. Conclusions Differences in catalytic performance of tungstophosphoric acid supported on zeolite HY and silica in dehydration of ethanol were observed. Dimensions of Keggin anion prevent the molecule from entering the pores of zeolite through the window openings and heteropolyacid is located on external surface and in mesopores. Lowering the concentration of HPW on zeolite HY causes that on the surface both Keggin or lacunary anions deposit as isolated centers. They are located in “bowls” formed by supercages opened at the surface, which resemble a honeycomb. The Keggin anions of 12 Å diameter fit perfectly to the 13 Å “bowls” of supercages, which prevents their hydrolysis. Isolated lacunary anions coordinating seven protons behave as sites of very high catalytic activity. As the result, turnover frequency (TOF) reaches the highest value for the lowest loading. In the case of HPW supported on silica, the catalytic activity drops with the decreasing concentration be-

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cause the quantity of so-called “pseudo-liquid phase” decreases. The removal of bulk heteropolyacid from the silica surface leaves only monolayer of Keggin anions bound strongly to the surface due to hydrolysis into separate triads. Such monolayer is catalytically less active.

Acknowledgements The authors wish to thank Miss Z. Czuła for performing BET measurements and Joanna Krysciak, M.Sc., for FT-IR spectra. The financial support of the Polish Committee for Scientific Research within grant no. 3 T09A 107 16 is also gratefully acknowledged.

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