A FT-IR Study of the Reactivity of Tungsta-Supported Catalysts toward

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A FT-IR Study of the Reactivity of Tungsta-Supported Catalysts toward Butan-2-ol S. R. G. Carraza´n, C. Martı´n, G. Solana, and V. Rives* Departamento de Quı´mica Inorga´ nica, Universidad de Salamanca, 37008-Salamanca, Spain Received March 22, 2001. In Final Form: June 5, 2001 The surface acid properties of WOx supported on different oxides (alumina, silica, magnesia, titania, zirconia, and niobia), as well as the decomposition of butan-2-ol on these systems, have been studied by FT-IR spectroscopy. Incorporation of supported tungsten-containing species significantly increases the surface acid properties, and new surface acid sites develop, except for in the magnesia-supported system. The increase in surface acidity is responsible for the high selectivity to butene formation on these systems, except for in the magnesia-supported system, where a larger selectivity to methyl ethyl ketone is observed.

Introduction The catalytic decomposition of alcohols on oxidic catalysts, via dehydration and/or dehydrogenation reactions, is considered a straightforward means of producing materials of prime industrial importance. Consequently, this subject has been widely studied in the past decades by many authors.1-11 Different reaction mechanisms (e.g., E1, E2, or E1CB) have been proposed depending on the type of surface sites participating in these reactions, that is, surface acid or basic sites in dehydration leading to alkenes, or basic or redox sites in dehydrogenation leading to carbonyl compounds. So, the decomposition of alcohols has become a well-established probe reaction to study the surface acid-base and redox properties of heterogeneous oxide catalysts.8-10 Vanadium and molybdenum oxides, whether unsupported or supported on other oxides, are currently used as catalysts for the selective oxidation of organic compounds. It is noticeable that tungsten oxide, despite exhibiting structure and surface acidic properties similar to those of the former oxides, has been only scarcely used in these reactions. However, in the past decade, it has been shown that tungsten oxide is one of the most selective catalysts for benzene oxidation to maleic anhydride,12 toluene oxidation to benzaldehyde, alkene isomerization, and also in several hydrotreating processes.13-15 In ad* To whom correspondence should be addressed. Tel: +34923294400 (switchboard 1545). Fax: +34923294574. E-mail: [email protected]. (1) Wender, I. Catal. Rev. 1984, 26, 304. (2) Figueras Roca, F.; de Mourgues, L.; Tranbouze, Y. J. Catal. 1969, 14, 107. (3) Kno¨zinger, H.; Scheglila, A. J. Catal. 1970, 17, 252. (4) Kno¨zinger, H.; Kohne, R. J. Catal. 1966, 5, 264. (5) Krylov, O. V. Catalysis by Non-Metals; Academic Press: New York, 1970; p 115. (6) Deo, A. V.; Chuang, T. T.; Dalla-Lana, I. G. J. Phys. Chem. 1971, 75, 234. (7) Zaki, M. I.; Sheppard, N. J. Catal. 1983, 80, 114. (8) Grybowska-Swierkosz, B. Mater. Chem. Phys. 1987, 17, 121. (9) Nondek, L.; Sedlacek, J. J. Catal. 1975, 40, 34. (10) Ouqour, A.; Coudurier, G.; Vedrine, J. C. J. Chem. Soc., Faraday Trans. 1993, 89, 3151. (11) Ramis, G.; Busca, G.; Lorenzelli, V. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1591. (12) Haber, J.; Janas, J.; Schiavello, M.; Tilley, J. D. J. Catal. 1993, 82, 395. (13) Harrison, W. T.; Choudry, U.; Machiels, C. J.; Sleigh, A. W.; Cheetham, A. K. J. Solid State Chem. 1985, 82, 101. (14) van Roosmalen, A. J.; Mol, J. C. J. Catal. 1982, 78, 17. (15) Mol, J. C.; Moulijn, J. A. In Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1987; p 8.

dition, synergic effects have been put into evidence when tungsten oxide is added to the commercial V2O5/TiO2 catalysts in the selective reduction of nitrogen oxide with ammonia (SCR).16 In the present paper, we report the effect of the incorporation of tungsta on several oxides (Al2O3, SiO2, MgO, TiO2, ZrO2, Nb2O5), with different acid-base properties, on the catalytic decomposition of butan-2-ol. The surface acid properties of the solids, as well as the analysis of the species formed in this test reaction, have been studied by the FT-IR spectroscopy monitoring of the adsorption of several probe molecules (pyridine, butan2-ol, and methyl ethyl ketone). Experimental Section Materials and Sample Preparation. The supports were obtained from Degussa (γ-Al2O3 Aluminium Oxid C, SiO2 Aerosil200, and TiO2 P-25), Janssen Chemica (ZrO2 r.a., ref 55420), Panreac (MgO r.a., ref 151276), or Niobium Products Co. (Nb2O5 to H8Nb6O19‚H2O, ref HY340); pyridine, methyl ethyl ketone (MEK), and butan-2-ol were obtained from Fluka or Janssen Chimica (r.a.). The catalysts were prepared following the conventional impregnation method. Prior to impregnation, the support was calcined for 12 h at 773 K in an open oven to remove adsorbed hydrocarbon impurities; niobia and zirconia were calcined, however, at 723 K to maintain a high specific surface area and crystallinity and to avoid undesired phase changes. Fifteen grams of the support was impregnated at room temperature with 150 mL of an aqueous solution containing ammonium paratungstate (NH4)10H2(W2O7)6 (from Fluka, r.a.). The amount of the tungsten salt was chosen in each case to obtain a monolayer of tungsten oxide, WO3, as calculated from the specific surface area of the support (determined by the BET method) and the area covered by a “molecule” of WO3 (17 × 104 pm2).17 Once the mixture had been homogenized by magnetic stirring, the solvent was slowly evaporated in a water bath, and the solid was dried in an air-open oven a 373 K for 18 h, handground in an agate mortar, and then calcined in air at 723 K for 3 h. The catalysts are named as WX, where X stands for the first letter of the chemical symbol of the metal forming the oxide support. Techniques. X-ray diffraction profiles were recorded in a Siemens D-500 instrument, using Ni-filtered Cu KR1 radiation (λ ) 1.54 Å) interfaced to a DACO-MP data acquisition microprocessor equipped with Diffract/AT software. The Raman spectra in the 200-1200 cm-1 range were recorded on a computercontrolled Jobin Yvon spectrometer, model U-100, using the 514(16) Chen, J. P.; Yang, R. T. Appl. Catal. 1992, 80, 135. (17) Vermaire, D. C.; van Berge, P. C. J. Catal. 1989, 116, 309.

10.1021/la010428r CCC: $20.00 © 2001 American Chemical Society Published on Web 10/09/2001

FT-IR Study of Tungsta-Supported Catalysts 515 nm line from a Spectra Physics model 165 Ar+ laser as the exciting source (ca. 400 mW on the sample, 5 cm-1 spectra shiftwidth). X-ray absorption (XAS) data at the W L3 edge were collected at 77 K on wiggler station 9.2 at the Daresbury Synchrotron Radiation Source (UK) with an electron ring running at 2 GeV and 210-230 mA. Temperature-programmed reduction analysis was carried out in a Micromeritics TPR/TPD 2900 instrument at a heating rate of 10°/min and using ca. 15 mg of sample and a H2/Ar (5% vol) mixture as the reducing agent (60 mL/min). Surface acidity characterization was carried out through the FT-IR spectroscopy monitoring of pyridine adsorption in a Perkin-Elmer 16PC spectrometer coupled to a high-vacuum Pyrex system. Self-supported disks were used, degassed in situ in a special cell (built in Pyrex, but with CaF2 windows, transparent to IR radiation in the required wavenumbers range) at 673 K for 2 h prior to pyridine adsorption. The spectrometer was coupled to a PC, and commercial software from PerkinElmer was used to process the spectra. One hundred scans were taken to improve the signal-to-noise ratio at a nominal resolution of 2 cm-1. The gas was admitted to the IR cell at room temperature. After 15 min of equilibration, the gas phase was removed by outgassing at different temperatures (293-673 K), and the spectrum was recorded. This same technique and apparatus were used to monitor the adsorption of butan-2-ol and MEK (an expected reaction product). For butan-2-ol, the spectra were recorded with and without previous outgassing of the gas phase. Pyridine, butan-2-ol, and MEK were submitted to several freeze-pump-thaw cycles before being adsorbed on the catalysts.

Results Characterization of the Catalysts. The nature of the phases existing in these samples has been studied following different experimental techniques, namely, X-ray diffraction, FT-IR, Raman, and XAS spectroscopies, and temperature-programmed reduction (TPR); the results have been reported elsewhere,18-21 and here they are summarized. For catalyst WS, a crystalline phase is formed similar to ammonium tungstosilicate, (NH4)2H2SiW12O40, where tungsten ions are octahedrally coordinated by oxide ions.20 However, for catalysts WA, WT, WZ, and WN, a dispersed amorphous phase is formed where tungsten is in different coordinations: [WO4], [WO5], and [WO6].18,21 Finally, an amorphous MgWO4 phase with tetrahedral [WO4] units has been detected in catalyst WM.19 Calcination of the samples after tungsta loading leads to a decrease in the specific surface area,18 except in the case of the magnesia-supported sample (WM). The partial sintering observed in all other cases (Table 1) has been widely reported and is favored by the presence of the supported phase (tungsta, molybdena, vanadia, etc.). As a consequence, the actual loading of tungsta exceeds the value corresponding to the monolayer, with the excess depending on the sintering degree. The behavior observed for the magnesia system is, however, unique. In this case, a remarkable increase in the specific surface area is observed, so the actual final loading is much lower than the monolayer value. This is due to the reversible chemical/structural changes from MgO (periclase, rock-salt structure) to Mg(OH)2 (brucite, CdI2-like structure) on wetting MgO in a paratungstate aqueous solution; the acidic nature of this solution leads (18) Solana, G. Ph.D. Thesis, University of Salamanca, Spain, 1996. (19) Martı´n, C.; Malet, P.; Rives, V.; Solana, G. J. Catal. 1997, 169, 516. (20) Martı´n, C.; Malet, P.; Solana, G.; Rives, V. J. Phys. Chem. B 1998, 102, 2759. (21) Malet, P.; Martı´n, C.; Solana, G.; Rives, V. Aspects of the Surface Chemistry of Oxidic Systems; Edizioni Librerı´a Cortina: Torino, Italy, 1999; p 39.

Langmuir, Vol. 17, No. 22, 2001 6969 Table 1. General Characteristics of the Catalysts Studied catalyst

%w (weight)

SBET (m2 g-1)a

% change SBETb

WA WS WT WZ WN WM WO3

14.7 24.8 8.1 8.1 9.5 9.0 80.0

88 120 42 44 51 245

-12 -40 -21 -17 -19 +290

R(%)c 32 89 64 50 57 5 80

a Specific surface area (BET method) of the WX catalyst. Percentage change in SBET from the pure X support to catalyst WX. c Reduction percentage up to 1200 K with respect to W6+ f W0.

b

Table 2. Positions (cm-1) of the ν(CCN) Bands Recorded after Adsorption of Pyridine at Room Temperature and Outgassing at the Same Temperature on the Supports and on the Catalystsa Bpy sample

8a

19a

Lpy 19b

Al2O3 WA

1636

1490

1542

SiO2 WS

1638

1487

1538

1639

1490

1537

1639 1635 1635

1488 1488 1487

1538 1540 1535

TiO2 WT ZrO2 WZ Nb2O5 WN MgO WM WO3

8a

8b

19a

19b

1622 1614 1596 1614 1596b 1596b 1613 1596b 1604 1610 1602 1608 1605 1608 1598b 1599c 1608

1579

1493

1339

1577

1490

1449

1577

1487

1447b 1451

1575 1575 1573 1573 1575 1576

1490 1490 1487 1487 1488 1487

1576 1579

1493 1490

1445 1445 1443 1443 1444 1444 1442b 1442c 1445

a Bpy represents pyridine bonded to surface Bro ¨ nsted acid sites; Lpy represents pyridine coordinated to surface acid sites. b Phyc sisorbed or hydrogen-bonded pyridine. Weakly coordinated pyridine.

to partial dissolution of the starting MgO and its transformation to brucite, with periclase forming afterward upon calcination at 723 K. Reducibility of the tungsten species is also affected by the support used (Table 1); temperature-programmed reduction of unloaded tungsta up to 1200 K leads to 80% reduction (assuming 100% for total reduction from W6+ to W0). The reduction percentage is lower for all the supported samples, except for sample WS, and reached a minimum value for sample WM.18 Surface Acidity. The spectra of pyridine adsorbed on supports and supported tungsten oxide were recorded between 1700 and 1400 cm-1, and the bands observed were assigned according to Parry22 and Boehm and Kno¨zinger.23 The results are summarized in Table 2. Most of the supports exhibit surface Lewis acidity; silica and magnesia, however, show only a weak band at low wavenumbers (1598 and 1596 cm-1, respectively), which is easily removed upon outgassing at 373 K, suggesting it corresponds to weakly adsorbed (physisorbed) or hydrogen-bonded pyridine. Only niobia shows, in addition to bands due to coordinated pyridine, other bands at 1635 and 1540 cm-1 due to the pyridinium species, thus indicating the existence of surface Bro¨nsted acid sites on (22) Parry, E. P. J. Catal. 1963, 2, 371. (23) Boehm, H. P.; Kno¨zinger, H. In Catalysis; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1983; Vol. 4, p 40.

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Carraza´ n et al.

Figure 1. FT-IR spectra recorded after adsorption of pyridine at room temperature and outgassing at (a) room temperature, (b) 373, (c) 473, (d) 573, and (e) 673 K on catalysts WA (left) and WZ (right).

Figure 2. FT-IR spectra recorded after adsorption of methyl ethyl ketone at room temperature and outgassing at (a) room temperature, (b) 373, and (c) 473 K on catalysts WA (left) and WZ (right).

this support. For Al2O3, the band due to the 8a mode of ν(CCN) vibrations splits into three components at 1622, 1614, and 1596 cm-1, indicating the existence of different types of surface Al3+ cations. This splitting has been previously ascribed24 to the adsorption of pyridine on surface Al3+ cations in octahedral (1596 cm-1) or tetrahedral (1622 and 1614 cm-1) coordination, although Connell and Dumesic25 ascribe the band at 1614 cm-1 to octahedrally coordinated Al3+ ions and that at 1622 cm-1 to tetrahedral sites. Except for silica and magnesia, surface Lewis acid sites on the supports are rather strong. The bands due to adsorption on Lewis surface acid sites persisted in the spectra even after outgassing at 573 K. As already reported by other authors,26 only surface Lewis sites are observed for bulk tungsta, with bands at 1608, 1579, 1490, and 1445 cm-1, although the spectra are poorly defined, probably because of the extremely low specific surface area of the oxide (ca. 5 m2/g). As it can be concluded from data in Table 2, surface acidity is enhanced in all cases after the incorporation of tungsten oxide. All spectra recorded after the adsorption of pyridine at room temperature and outgassing at the same temperature (some representative spectra for catalysts WA and WZ are shown in Figure 1) show bands due to coordinated pyridine (Lewis acid sites) sometimes slightly shifted with respect to their positions in the spectra of the original supports. In addition, bands due to the pyridinium vibration modes develop, except for sample WM, thus indicating that the incorporation of tungsten has led to the development of surface Bro¨nsted acid sites. The bands recorded after the adsorption of pyridine on sample WM can be ascribed only to the adsorption on weak surface Lewis sites, as the bands are removed easily after outgassing at 473 K. On the contrary, outgassing at increasing temperatures up to 673 K does not have any effect on the bands recorded for all other catalysts, both those corresponding to adsorption on surface Lewis and surface Bro¨nsted acid sites, thus indicating the adsorption sites are rather strong. Such a change in the surface acidity of oxides when mixed with other oxides has been previously reported in the literature, and several models have been suggested to account for this change. Tanabe and Takeshita27 ascribed this effect to the presence of charge-unbalanced, localized M-O-X bonds (X being the new, in our case W,

cation) formed in the mixed oxide. Kung28 has suggested an unlocalized model. Both models are applied to diluted oxide mixtures, where cation substitution has taken place. An alternative model has been proposed by Connell and Dumesic25,29 for mixed oxides where such a cation substitution does not take place, but only incorporation of the “guest” onto the surface of the oxide support (i.e., Fe3+/ SiO2, Fe3+/TiO2, Al3+/SiO2, etc.). These authors assume that the formal charge on the supported cation is balanced by surface coordination of lattice oxide anions, leading, in some cases, to coordinatively unsaturated cations able to behave as surface Lewis acid sites. The strength of these sites depends on the electronegativity of the guest metal cation. Adsorption of Methyl Ethyl Ketone (MEK). This is one of the expected reaction products of butan-2-ol; its adsorption on the catalysts has been studied following the same procedure as for the adsorption of pyridine, and spectra are reported in the 1800-1000 cm-1 range, where the most representative bands are expected. All spectra recorded after the adsorption of MEK are rather similar, and some are shown in Figure 2. Bands recorded at room temperature, and after outgassing at the same temperature, can be ascribed to molecularly adsorbed MEK (unreactive adsorption), although some small differences in the position of the band due to the υ(CdO) mode are observed, probably due to slightly different interactions between the oxygen atom and the catalyst’s surface. In the spectra corresponding to the samples outgassed at room temperature, this band is observed at 1690, 1711, 1703, 1680, 1680, and 1669 cm-1, respectively, for catalysts WA, WS, WM, WT, WZ, and WN. Other bands recorded at 1460 and 1390 cm-1 are due to deformation modes of the ethyl and methyl groups. Bands recorded at 15701540 and 1480-1440 cm-1 in all samples, except for WS, can be ascribed to the υas and υs modes, respectively, of carboxylate groups. The intensities of the bands due to molecularly chemisorbed MEK decrease, and the bands shift toward lower wavenumbers as the outgassing temperature is increased. Simultaneously, bands due to carboxylate species become stronger; nevertheless, the MEK bands are still observed for most of the samples, even after outgassing at 573 K. In sample WM, the MEK bands disappear on outgassing at 473-573 K and in WS at 373 K. Adsorption of Butan-2-ol. Adsorption of this alcohol has been studied in the absence of oxygen, with or without outgassing after adsorption. When the alcohol (ca. 20 Torr)

(24) Morterra, C.; Chiorino, A.; Ghiotti, G.; Garone, E. J. Chem. Soc., Faraday Trans. 1 1979, 75, 271. (25) Connell, G.; Dumesic, J. A. J. Catal. 1986, 102, 216. (26) Meijers, S.; Gielgens, L. H.; Ponec, V. J. Catal. 1995, 156, 147. (27) Tanabe, K.; Takhesita, T. Adv. Catal. 1967, 17, 315.

(28) Kung, H. J. Solid State Chem. 1984, 52, 191. (29) Connell, G.; Dumesic, J. A. J. Catal. 1986, 102, 103.

FT-IR Study of Tungsta-Supported Catalysts

is adsorbed at room temperature on catalyst WZ and outgassed (10-4 Torr) at the same temperature, bands are recorded at 2978, 2939, and 2887 cm-1 for the ν(CH) mode, 1470 and 1389 cm-1 for the δ(CH) mode, and 1166, 1130, 1113, and 1090 cm-1 for the ν(C-C)/ν(C-O)/r(CH3) mode of the sec-butoxy species. These positions are similar to those previosuly reported by Busca et al.30 after the adsorption of butan-2-ol on MgFe2O4. An additional band is observed at 1338 cm-1, almost identical to that corresponding to the deformation mode, δ(OH), of molecular butan-2-ol (1330 cm-1). That is, the alcohol is both dissociatively and undissociatively adsorbed. Outgassing at increasing temperatures gives rise to a general decrease in the intensities of all bands, which are completely removed after outgassing at 473 K, without detection of reactive adsorption. Similar spectra, with only minor changes in the positions of the bands, are recorded for catalysts WA, WT, WN, and WM; however, for this last sample, a weak band is recorded at 1690 cm-1 upon outgassing at 473 K which can be ascribed to the ν(CdO) mode of MEK. Adsorption of the alcohol and outgassing at room temperature on catalyst WS give rise to IR bands at 2978, 2934, 2870, 1470, 1459, and 1382 cm-1. No band can be clearly recorded below ca. 1350 cm-1 due to the strong absorption bands of silica in this wavenumber range; however, the bands recorded are conclusive enough to be ascribed to the formation of the sec-butoxy species and molecularly adsorbed butanol. The interaction with the surface is, however, rather weak, as the bands are easily removed when the sample is outgassed at 373 K. It can be also observed that the weak bands (slightly stronger for WS and WM) between 3775 and 3650 cm-1 due to the ν(OH) mode of surface hydroxyl groups are removed after adsorption of the alcohol, with the simultaneous development of a new, broad band between 3400 and 3500 cm-1. Upon outgassing, this band is removed, and the original band is restored. This easy process suggests that this broad band is due to coordinated molecular butanol or hydrogen bonded to the surface; it may be alternatively due to the formation of new, surface hydroxyl groups. To ascertain the nature of the gas phase weakly bonded to the surface of the catalyst, some experiments were also carried out by recording the spectra on nonoutgassed systems after adsorption of the alcohol at room temperature. The spectra recorded after the adsorption of butan2-ol on catalyst WZ (spectra were similar for catalysts WT, WN, and WA) are shown in Figure 3. In addition to the bands described above and due to molecular butanol and sec-butoxy species, other bands are recorded at 1630 and 3040 cm-1 after heating at 473 K. These bands can be ascribed to the ν(HCdCH) and ν(HCd) modes of cis2-butene, as the bands are recorded for this compound in the gas phase at 1660 and 3040 cm-1.31 Shifts toward lower wavenumbers are typical of alkene species interacting with the CdC double bond flat on the surface through an interaction of the π-orbital with surface electron-withdrawing centers. The high preference of the cis product on dehydration of secondary butanol has been explained by Pines and Haag.32 The presence of butene-1 can be discarded, as bands at 1856-1800 and 3097-3075 cm-1 due to the overtone of the out-of-plane δ(CH2) mode and ν(CH) mode of the vinyl group are not recorded. When (30) Busca, G.; Finocchio, E.; Lorenzelli, V.; Trombetta, M.; Rossine, S. A. J. Chem. Soc., Faraday Trans. 1996, 92, 4687. (31) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman and Hall: London, 1975. (32) Pines, H.; Haag, W. O. J. Am. Chem. Soc. 1961, 83, 2847.

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Figure 3. FT-IR spectra recorded after adsorption of butan2-ol at (a) room temperature, (b) 473, and (c) 573 K on catalyst WZ.

Figure 4. FT-IR spectra recorded after adsorption of butan2-ol at (a) room temperature, (b) 473, and (c) 573 K on catalysts WS (left) and WM (right).

the temperature is raised to 573 K, the bands due to molecular butan-2-ol and alkoxide species almost completely vanish, and, together with the bands described above and due to butene-2, a new band at 1690 cm-1 corresponding to the ν(CdO) mode of carbonyl species develops; this band is similar to that recorded after the adsorption of MEK on this same sample. The spectra recorded prior to outgassing after adsorption at room temperature on catalyst WS, Figure 4, show the same bands due to molecularly adsorbed alcohol and to alkoxide species, as previously described. When the temperature is raised to 473 K, new bands are observed at 3040, 1711, and 1630 cm-1; these bands become stronger (and those due to molecular butan-2-ol and butoxy species become weaker) when the temperature is further raised. The new bands recorded can be ascribed to the simultaneous presence of cis-2-butene and MEK, products formed upon dehydration and dehydrogenation of the alcohol, respectively. Bands which originated from molecularly adsorbed butan-2-ol and alkoxide species are again recorded when the alcohol is adsorbed at room temperature or 373 K on catalyst WM, Figure 4. At 473 K, a new band at 1700 cm-1 develops which is due to the ν(CdO) mode of MEK, as it is similar to that recorded after the adsorption of MEK on this catalyst. The new band’s intensity is further enhanced upon heating at 573 K. Weak bands are recorded at 1540 and 1440 cm-1, together with the intense band at 1700 cm-1, when the sample is heated at 673 K (these weak bands are only recorded in the transmittance mode, as the simultaneous presence of a strong absorption due to MgO in this wavenumber region makes their detection in the difference mode extremely difficult). These two bands can be ascribed to the antisymmetric and symmetric

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stretching modes of the carboxylate species, formed upon oxidation of the carbonyl group. Similar bands had been detected when adsorbed MEK was heated at high temperatures. Simultaneously, a weak band due to the molecular water mode, δ(H2O), is recorded at ca. 1620 cm-1 in all spectra.

Carraza´ n et al. Scheme 3

Discussion Catalytic decomposition of alcohols on metal oxides generally undergoes the following cases:2-4,9 (i) dehydration to the alkene which is a rather common acid-base reaction involving surface Bro¨nsted acid sites (E1 mechanism), acid-base pairs (E2 mechanism), or strong basic sites (E1CB mechanism); (ii) simple dehydrogenation, for which the presence of gaseous O2 is not essential, which forms the carbonyl compound and H2 on basic catalysts; (iii) oxidative dehydrogenation which forms the carbonyl and water (Mars and van Krevelen mechanism33,34), where oxygen from the gas phase is required to reoxidize the active sites; and (iv) total oxidation to CO (or CO2) and water. In the studies described here, we have found dehydrogenation and dehydration processes. The FT-IR results indicate that adsorption of butan-2-ol takes place in all cases with the formation of alkoxide species (sec-butoxy) and molecularly adsorbed butan-2-ol. As the surface of the solids is essentially dehydroxylated, formation of the alkoxide has probably taken place through Scheme 1, although the simultaneous Scheme 2 cannot be fully discarded, especially with hydroxylated surfaces. This second mechanism gives rise to the formation of molecular water. Although the band due to the δ(H2O) mode is only clearly detected for sample WM, it could be overlapped by other bands (probably those due to the ν(CdC) mode of butene) in the other cases. Scheme 1

Scheme 4

However, several authors have proposed6,37,38 alternative mechanisms where the intermediate is not the alkoxide, but where the carbanions (isomerization of longchain alcohols) or the adsorbed alcohol is the alkoxide. Dehydrogenation of butan-2-ol to MEK, observed at 473 K on catalyst WS and at 573 K on catalysts WZ, WN, WA, and WT, probably proceeds via an oxidative mechanism (Scheme 5), as all these catalysts show surface redox sites (W6+) which are readily reducible, especially in the case of catalyst WS, as the TPR results summarized in Table 1 show. On the contrary, formation of MEK on catalyst WM most likely takes place through a simple dehydrogenation, as in this catalyst the redox sites are only slightly reducible, Scheme 6. Scheme 5

Scheme 2

Molecular adsorption of the alcohol may take place through hydrogen bonding to the surface (Scheme 3a and b) or through coordination to surface Lewis sites, according to Scheme 3c. All these mechanisms would account for the broad band recorded at 3500-3400 cm-1 after the adsorption of butan-2-ol. The bands which originated from undissociated butan2-ol and the alkoxy species disappear from the spectra when the formation of butene-2 or MEK is observed, thus indicating that the former species are intermediates in the formation of the latter through dehydration or dehydrogenation of the alcohol. Dehydration to butene-2, observed on all samples except on catalyst WM, may be assumed to take place through decomposition of the alkoxy species to the alkene in the gas phase and re-formation of surface hydroxyl groups,35,36 Scheme 4. (33) Mars, P.; Van Krevelen, D. W. Chem. Eng. Sci. 1954, 3, 41. (34) Doornkamp, C.; Ponec, V. J. Mol. Catal. A: Chem. 2000, 162, 19.

Scheme 6

Further oxidation of MEK to the carboxylate species has only been detected on catalyst WM at 673 K and, even in this case, only in a rather low percentage. (35) Ipatieff, V. N. Catalysis Reactions at High Pressures and Temperatures; Macmillan: New York, 1936; p 60. (36) Nakajima, T.; Miyata, H.; Kubokawa, Y. Bull. Chem. Soc. Jpn. 1982, 55, 609.

FT-IR Study of Tungsta-Supported Catalysts

The different selectivities observed for these catalysts (at least, from a qualitative point of view) to butene-2 or to MEK seem to be related to the surface acid-base and redox properties of these catalysts. Catalysts WZ, WN, WA, and WT, for which the surface species are rather similar (amorphous WOx species) and with a rather high surface concentration of strong acid (both Lewis and Bro¨nsted) sites, are especially selective to butene-2 formation; moreover, the presence of redox sites (W6+) favors the formation of MEK at rather high temperatures. Sample WS, for which crystalline supported phases, (NH4)2H2SiW12O40, have been detected, is more easily reducible than the amorphous WOx species in the other catalysts; this catalyst is strongly acidic, and, in addition to its selectivity to butene-2, it is selective to the formation of MEK at lower reaction temperatures. Finally, catalyst WM contains amorphous magnesium tungstates which are hardly reducible. The surface Lewis acid sites are rather weak and contain a high surface concentration of basic sites (even the pure support is the most basic one among the supports tested); therefore, this catalyst is more selective to MEK formation through a simple dehydrogenation route. We should also not forget that the surface area development in this catalyst has probably led to the exposure of basic sites associated to the support itself. (37) Pines, H.; Manassen, J. Adv. Catal. 1966, 16, 49. (38) Witmore, F. C. J. Am. Chem. Soc. 1932, 54, 3274.

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Conclusions This study further confirms the applicability of alcohol conversion to alkenes or carbonyl compounds in the absence of oxygen to characterize the surface acid-base and redox properties of metal oxide catalysts. All systems tested show strong surface Lewis and Bro¨nsted acid sites, except WM which only shows weak surface Lewis acid sites. Incorporation of tungsten on the surface of the supports, in addition to the development of new surface acid sites (mainly Bro¨nsted type), also produces surface redox sites (W6+ species), which are responsible for the oxidative dehydrogenation to MEK, detected on catalysts WA, WN, WZ, WT, and WS. Because of their high surface acidity, these samples are also rather selective to the formation of butene-2, even at rather low reaction temperatures (473 K). On the other hand, the lower surface acidity and reducibility of sample WM make it more selective to MEK formation via simple dehydrogenation. Acknowledgment. The authors thank CICYT (Spain, Grant PB93-0633) and Junta de Castilla y Leo´n (Consejerı´a de Educacio´n y Cultura, Grant 71/99) for financial support. G.S. acknowledges a leave from Universidad de Guanajuato (Mexico) and CONACYT (Mexico, Grant 85003). LA010428R