J . Phys. Chem. 1990, 94, 8939-8945
8939
Fourier Transform Infrared Spectroscopic Studies of the Reactivity of Vanadia-Titania Catalysts toward Olefins. 1. Propylene Vicente Sanchez Escribano? Guido Busca,* and Vincenzo Lorenzelli Istituto di Chimica, Facoltd di Ingegneria, Universitri di Genova, P.1e Kennedy, I-16129 Genova, Italy (Received: April 12, 1990; In Final Form: June 20, 1990)
The adsorption and transformation of propylene and of several 0-containing C3organic molecules (namely isopropyl alcohol, acetone, allyl alcohol, acrolein, and acrylic acid) on vanadia-titania in the temperature range 150-673 K have been investigated by 1R spectroscopy. At low temperatures ( 1 50-200 K), propylene adsorbs as such through *-bonding and reacts later to give an adsorbed species identified tentatively as an allyl radical. Alternatively, it undergoes addition by a surface OH group to give an isopropoxy species. At least two main surface pathways for propylene transformation are active, leading to surface species functionalized at C2(isopropoxy species and acetone) and at C, (acrolein and acrylates) as well as products arising from coupling of the allyl radical. The results are discussed in relation to literature data concerning the heterogeneously catalyzed selective oxidation of propylene carried out on vanadia-titania as well as on bismuth-molybdate-based catalysts.
Introduction Vanadium-titanium complex oxides represent the basic form of catalysts used industrially for the production of phthalic anhydride and of aromatic nitriles via selective oxidation and ammoxidation of alkyl Materials belonging to this system have also been tested as catalysts for other partial oxidation reactions."' In particular, propylene oxidation on vanadia-titania catalysts has been in~estigated:~-"selective oxidation products have been obtained with nonnegligible selectivities only at low conversions. When conversions rise to high values, carbon oxides are the main reaction products. So although vanadia-titania is not an excellent catalyst for the selective oxidation of propylene, it can be used as a model catalyst for mechanistic studies. Vanadia-titania can be prepared having a medium surface area and very small particle size. Consequently, this material is very useful for IR studies of adsorbed species. In previous papers, we have reported the results of our investigations of the mechanisms of the selective oxidation of toluene,i2benzene,13 and butadieneI4 on vanadia-titania. In all three cases, surface intermediates have been observed spectroscopically, and a surface reaction path consistent with the kinetic data has been proposed. This research concerns the adsorption and reaction of olefins and of corresponding oxygenated molecules on vanadia-titania. The aim is to identify the surface pathways for olefin transformation on vanadia-titania and to obtain information on the nature of the sites for both selective and unselective oxidation on oxide catalyst surfaces. In the first paper of this series, we report on the adsorption and oxidation of propylene, while in the second part of this series, the behavior of the less reactive olefin ethylene is discussed. Experimental Section The vanadia-titania catalyst (9.6% V205by weight) has been prepared by impregnation of Ti02 (P25 from Degussa, Hanau, West Germany) with a NH4V03-boiling water solution, followed by calcination at 723 K for 3 h. The surface area is 50 m2/g. The catalyst has been pressed into self-supporting disks and activated by heating in air at 673 K for 30 min and then evacuated at 673 K for 1 h in the IR cell. Adsorption has been carried out at room temperature or lower temperatures using a liquid nitrogen cooled cell. Successive heat treatments have been carried out under evacuation. Propylene was taken from commercial cylinders from SI0 (Milano, Italy). Acetone, isopropyl alcohol, acrylic acid, acrolein, and allyl alcohol were hyperpure products from Carlo Erba (Milano, Italy). They have been purified by multiple freeze-
* To whom correspondence should be addressed. 'On leave from Departamento de Quimica Inorganica, Facultad de Ciencias Quimicas, Universidad de Salamanca, Plaza de 10s Caidos, E-37008 Salamanca, Spain.
pump-thaw cycles and evaporated under vacuum. IR spectra have been recorded at room temperature or lower temperatures by a Nicolet M X l Fourier transform instrument, equipped with a conventional evacuation/gas-manipulationramp ( Torr) and IR cells (NaCI windows) built by Glass Emery (Genova, Italy). The spectra of the adsorbed species are presented after subtraction of the spectrum of the activated catalyst disk from that recorded after adsorption. Results and Discussion For reading purposes, the experiments concerning the adsorption of oxygenated compounds will be discussed first. The structure of the adsorbed species and the surface reactions for which we have direct or indirect evidence are summarized in Scheme I. ( a ) Acetone Adsorption. The spectra relative to the adsorption and reaction of acetone on the surface of vanadia-titania are reported in Figure I . The spectra show evidence that acetone adsorbs a t room temperature without reaction (Figure 1, a). In fact, all bands detected can be assigned to acetone adsorbed as such. The C = O stretching frequency is observed, sharp and very intense, at 1682 cm-', Le., at a frequency significantly lower than that of the gas-phase molecule (1734 cm-I Is). This suggests that it interacts with surface Lewis sites through coordination of one oxygen lone pair. This is also confirmed by the relevant upward shift of the asymmetric C-C-C stretching mode detected by us at 1248 c d , near the value measured for the adduct of acetone with the Lewis acid TiCI4 (v(C=O) at 1665 cm-I, v,,(CCC) at 1245 cm-' 16), with respect to the gas-phase value of 1215 cm-'.I5
( I ) Wainwright, M. S.; Forster, N. R. Caral. Reu. Sci. Eng. 1979, 19, 21 I . (2) Gellings, P. J. Catalysis; The Royal Society of Chemistry: London, 1985; Vol. 7, p 105. (3) Cavani, F.; TriWr6, F. Chim. Ind. (Milan) 1988, 70,58. (4)Miyamoto, A.; Mori, K.;Inomata, M.; Murakami, Y. Proc. 8th ICC;
Berlin. 1984: Vol. IV. D 285. (5)'Busca; G.; Marchetti, L.; Centi, G.;Trifid, F. J. Chem. Soc.,Faraday Trans. I 1985,81, 1003. (6)Slinkard, W. E.; Degroot, P. B. J . Catal. 1981, 68, 423. (7) Busca, G.; Centi, G.; Trifir6. F. ADDI.Catal. 1986. 25, 265. (8) Garcia, Fierro, J. L.; Arrua, L. A.;'iopez Nieto, J. M.; Kremenic, G. Appl. Catal. 1988, 37, 323. (9)Doulov, A.; Forissier, M.; Noguerol Perez, M.; Vergnon, P. Bull. Soc. Chim. Fr., Part I , 1979, 129. (IO) Ono,T.;Nagakawa, Y.; Miyata. H.; Kubokawa, Y. Chem. Chem. SOC.Jpn. 1984, 57, 1205. ( I I ) Martin, C.; Rives, V. J . Mol. Catal. 1988, 48, 381. (12) Busca, G . ; Cavani, F.; Trifid, F. J. Catal. 1987, 106. 471. (13) Busca, G.; Ramis, G.; Lorenzelli, V. J. Mol. Catal. 1989, 55, 1. (14)B u m , G.; Ramis, G.; Lorenzelli, V. In New deuelopments in selecfiue oxidation; Trifir6, F., Centi, G . , Eds.; Elsevier: Amsterdam, 1990; p 825. (I 5) Delle Piane, G.; Overend, J. Spectrochim. Acta 1966, 22, 593. (16) Cassimatis, D.;Sisz, B. P. Helu. Chim. Acta 1960, 43, 852.
0022-3654/90/2094-8939%02.50/0 0 1990 American Chemical Society
8940
rhe Journal of Physical Chemistry, Vol. 94, No. 26, 1990 I
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Figure 1. FT-IR spectra of the adsorbed species arising from contact of activated vanadia-titania with acetone vapor (P < I Torr) at room temperature (a) and following evacuation for IO min at 330 K (b), 373 K ( c ) , 423 K (d), and 473 ( e ) .
The other bands (2970, 2925 cm-I, CH stretchings; 1421, 1368 cm-I, CH3 bendings; 1090 cm-I, CH3 waggingIs are poorly perturbed with respect to those measured for the pure compound. This species is stable even under evacuation at 330 K, as deduced by the persistence of the bands under these conditions (Figure I , b). Instead, new bands grow at the expense of those of coordinated acetone (that accordingly decrease slowly) by heating under evacuation at 373 and 423 K (Figure 1, c and d). These new bands are observed as a broad absorption having a sharp maximum at 1560 cm-l and a broad shoulder at 1535 cm-l, together with two sharp bands at 1444 and 1380 cm-I. Further heating at 473 K under evacuation (Figure I , e) causes the complete disappearance of the bands of adsorbed acetone, as well as of the new features at 1565 cm-l (sharp) and at 1380 cm-'. After this treatment, the spectrum shows three absorption bands near 1540 (rather broad), 1448 (slightly sharper and more intense), and 1355 cm-' (much weaker). This spectrum is exactly that observed after adsorption of acetic acid and evacuation (Figure 2, a). We can consequently interpret these spectra considering that acetone transforms to give a labile species characterized by the rather sharp bands at 1565 and 1380 cm-I, as well as acetate ions that are very stable on the surface. Decomposition of acetone, giving acetates and gas-phase methane, has already been observed on other oxide surfaces such as alumina" and tin oxideIs at even lower temperatures. To attempt the identification of the surface species responsible for the bands at 1565 and 1380 cm-I, we can use previous results relative to acetone adsorption on the surface of Fe203.19 In this case, it was concluded that acetone enolate anion is formed by proton extraction, responsible for a band at 1540 cm-l, and can react with gas-phase acetone, giving crotonic condensation, as also found on TiOz (rutile20). Enolate anions have been found to be (17) Knozinger, H.; Krietenbrink. H.: Muller, H. D.: Schulz, W. Proc. 6th ICC, London 1976, 183. (18) Thornton, E. W.; Harrison, P.G. J . Chem. Soc., Faraday Trans. I 1915, 71, 2468. (19) Busca, G.: Lorenzelli, V. J . Chem. Sor.. Faraday Trans. I 1982, 78,
2911.
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Figure 2 FT-IR spectra of adsorbed acetates (a) and acrylates (b) arising from contact of vanadia-titania with acetic acid and acrylic acid vapor, respectively (less than 1 Torr) and evacuation at 523 K (a) and 423 K ( b ) .
responsible for bands in these regions: for example, the most intense absorptions of the sodium salt of malonaldehyde in DMSO solution are quoted at 1582 and 1374 cm-' (asymmetric and symmetric stretching of a -O-C==C/O=C-C- anionic system.*' Supplementary acetone adsorption experiments have been carried out also on the pure TiOz support. Indeed, we have found that the formation of the enolate anion occurs much faster at room temperature on the pure TiOz support and is consequently inhibited by the presence of the vanadia monolayer. This effect agrees with the base-catalyzed nature of crotonic condensation on the surface of metal oxides and with the lower basicity of vanadia-titania with respect to pure titania. In conclusion, acetone chemisorbs strongly as such on the surface of vanadia-titania at room temperature and, by increasing the temperature, dissociates to its enolate anion, which is relatively labile, and decomposes at 473 K to the strongly bonded acetate ions. (b) Isopropyl Alcohol Adsorption. Isopropyl alcohol adsorption at room temperature on vanadia-titania (Figure 3, a) results in the appearance of a number of sharp bands, all assignable to an adsorbed isopropoxide species, in agreement with the discussion reported in ref 22, relative to isopropyl alcohol adsorption on the TiOz support. These bands are a t 2975 (strong), 2935 (strong), 2920,2900, and 2870 cm-' (CH stretchings), 1468 and 1455 cm-' (asymmetric CH3 bendings), 1385 and 1370 c d (symmetric CH3 bendings), 1330 cm-I (CH out-of-plane bending), and 1168, 1140, and 1125 cm-' (C-C and C - 0 stretchings and CH, rocking). Evidence is found for the presence of very small amounts of undissociatively adsorbed isopropyl alcohol, characterized by a typical OH deformation band near 1290 cm-', that is present in much higher amounts after adsorption on the pure titania support.22 These bands of isopropoxide species are almost unaffected by treatment at 330 K under evacuation (Figure 3, b) but begin to decrease in intensity by treatment at higher temperatures and almost disappear at 423 K (Figure 3, c). At this temperature, (20) Griffiths, D. M.; Rochester, C. M.; J . Chem. Soc.. Faraday Trans. I 1978, 74. 403.
(21) George, W. 0.;Mansell, V . G. Spectrochim. Acra 1968, 24, 145. (22) Rossi, P. F.; Busca, G.; Lorenzelli, V.; Saur, 0.; Lavalley, J. C. Langmuir 1987, 3, 5 2 .
Reactivity of Vanadia-Titania toward Propylene
The Journal of Physical Chemistry, Vol. 94, No. 26, 1990 8941
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1 CH2=CH C H O,,, other bands grow, due to transformation products. They are observed at 1680 (very strong and sharp), 1420, and 1250 cm-I. These are the most evident bands due to adsorbed acetone, as discussed above. These bands grow by heating to 423 K and later decrease and disappear at 523 K (Figure 3, d). Intermediately at 473 K bands are also detected at 1565 (sharp), 1540 (broad shoulder), and 1440 cm-l, assigned above to the acetone enolate anion and to acetate species. Starting from 523 K, only the bands of surface acetate species are observed, which decrease progressively by further heating (Figure 3, e) and then disappear. It is then concluded that isopropyl alcohol is oxidized at temperatures of the order of 373 K by the surface of vanadia-titania to acetone, which later behaves as already reported by its direct adsorption. (c) Acrylic Acid, Acrolein, and Allylic Alcohol Adsorption. To test the existence of spectral features arising from the oxidation of propylene at the methyl group, the IR spectra of adsorbed acrylic acid and aldehyde and of allyl alcohol on vanadia-titania have been also studied. The spectra of the adsorbed species arising from acrylic acid adsorption at room temperature followed by evacuation at 373 K or higher temperatures (Figure 2, b) are all and only those we expected for adsorbed acrylate species, according to the parallelism of the spectrum we observed with that of sodium acrylate.23 The bands observed are assigned to C=C stretching (1635 cm-I), asymmetric C 0 2 stretching (I495 cm-', with a tail at higher frequencies), CH2 scissoring superimposed on the symmetric C 0 2stretching band (1440 cm-I); CH bending (1 375 cm-I), (23) Feairheller, W. R.: Katon, J . E. Specrrochim. Acta 1967, 23A, 2225.
other cirboxy!z:es ---)
d
1 CH,=CHCOOH,,) and C-C stretching (1275 cm-I). After evacuation at room temperature, broad absorptions are also detected at 1680 and near 1600 cm-I, which disappear by evacuation at higher temperatures. These bands can be due to weakly adsorbed forms of acrylic acid in a liquid-like form.23,24 Acrylate species are stable upon evacuation at temperatures up to 473 K, while their absorption bands decrease progressively in intensity by treatment in the temperature range 473-623 K, leaving only small bands due probably to further oxidation products at 1515 and 1485 cm-I. Acrolein is adsorbed as such at room temperature on vanadia-titania: in fact, the observed bands (Figure 4, a) are all and only those we expected for the aldehyde perturbed by coordination on Lewis acid sites. The most intense band is observed to shift from 1653 to 1647 cm-I by increasing the evacuation temperature and is assigned to the C=C stretching made (1724 cm-I in the gas2S)strongly shifted down by coordination. Two shoulders are also observed at 1625 cm-I and, much weaker, at 1610 cm-I, probably due to u(C==C) (1 625 cm-I in the gas25)for two different conformers, an s-trans one and an s-cis one, respectively.26 Other bands are observed at 1430, 1372, 1282, and 1 188 cm-I and are assigned to CH2 scissoring, two C H deformation, and C-C stretching modes r e ~ p e c t i v e l y . ~ ~ ~ ~ ~ By heating under outgassing, acrolein starts to transform at 373 K and is present no more on the surface at 473 K. However, (24) Krause, P. F.; Katon, J. E.; Smith, K. K. Spectrochim. Acta 1976, 32A, 957.
(25) Harris, R. K. Spectrochim. Acra 1964, 20, 1129. (26) Bowles, A. J.; George, W. 0.;Maddams, W. F. J . Chem. Soc. B 1%9,
8 IO.
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8942 The Journal of Physical Chemistry, Vol. 94, No. 26, 1990
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Figure 3. FT-IRspectra of the adsorbed species arising from contact of activated vanadia-titania with isopropyl alcohol vapor (P < 1 Torr) at room temperature and following evacuation for 10 min at room temperature (a), 330 K (b), 423 K (c), 523 K (d), and 573 K (e).
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Figure 4. FT-IRspectra of the adsorbed species arising from contact of activated vanadia-titania with acrolein vapor ( P < 1 Torr) at room temperature and following evacuation for IO min at room temperature
(a), 423 K (b), and 523 K ( c ) .
among its transformation products (Figure 4, b and c). acrylates are present together with several other species, possibly arising from dimerization, polymerization, and/or other oxidation reactions; their identification will be not attempted here.
wavenumbers
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Figure 5. FT-IR spectra of the adsorbed species arising from contact of activated vanadia-titania with allyl alcohol vapor (P< 1 Torr) at room temperature (a), and following evacuation for IO min at room temperature (b) and 523 K (c).
Allyl alcohol adsorbed at rmm temperature (Figure 5 , a) gives rise to a spectrum that can be interpretd on the basis of that of allyl alcohol, as reported in the literat~re.~'A medium intensity band at 1645 cm-I is assigned to v(C=C) stretching; a much stronger and very sharp band at 1422 cm-I cm due to the scissoring of the vinylic = C H 2 group, while broader bands at 1445 and 1345 cm-' with a shoulder at 1360 and at 1 1 IO cm-' are assigned to the scissoring of the -CH,-0 group, CH2 wagging, and C-0 stretchings. The absence of the absorption bands related to the OH group (OH stretching and in-plane deformation) and the values of the v(C=C) and v ( C - 0 ) frequencies, shifted down and up, respectively, with respect to the gas-phase allyl alcohol (v( C 4 )at 1654 cm-l, v(C-0) at 1038 cm-l 27) indicate that these bands, together with weaker ones at 1288, 1235, 1145 cm-l, are due to the allyl alcoholate group, also according to the behavior of other adsorbed alcohols, including isopropyl alcohol (see above). The progressive growth of bands such as that at 1185 cm-' indicates that acrolein, not present at all immediately after allyl alcohol adsorption, is formed progressively by dehydrogenation upon evacuation at room temperature (Figure 5, b) and increasing temperatures. In fact, heating under vacuum causes the progressive decrease of the intensities of the bands assigned above to allyl alcoholate and the progressive growth of all bands discussed in relation to acrolein adsorption. At 423 K, allyl alcoholates and acrolein are present together on the surface. Above this temperature (Figure 5, c) both species disappear, while the spectrum appears to be dominated by two strong absorptions near 1550 cm-I (more intense) and near 1400-1450 cm-I (less intense), together with several weaker maxima. This spectrum is typical of carboxylate species, although several forms are certainly present together. The spectrum does not correspond to that of acrylate species only (Figure 2, b). It is then concluded that allyl alcohol is very easily oxidized to acrolein. However, acrolein, either adsorbed as such or produced by oxidation of allyl alcohol, reacts at moderate temperatures (starting from 373 K ) not only at the aldehyde group but (27) Silvi, B.; Perchard, J . P. Specrrochim. Acto 1976, 32, 1 1
Reactivity of Vanadia-Titania toward Propylene
The Journal of Physical Chemistry, Vol. 94, No. 26, 1990 8943
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up to now indirect evidence, we have also studied the adsorption of propylene at lower temperatures. The spectra relative to the adsorbed species arising from propylene adsorption at 150-1 9 0 K (Figure 7, a and b) are similar to those of propylene adsorbed as such on pure titania." The most evident perturbation is related to the downward shift and the splitting of the v ( C = C ) stretching (two components are observed at 1643 and 1632 cm-I, with respect to the gas-phase value of 1653 cm-' 28), due to the interaction with two different adsorbing centers. However, by warming under evacuation, this spectrum progressively disappears and new features appear. At 210 K, weak bands are apparent at 1468, 1373, 1165, and 1126 cm-' (shown by arrows in Figure 7, c). Further warming at 250 K (Figure 7, d) causes the appearance of bands that correspond to those already assigned to isopropoxy species while acetone is probably present starting from 330 K and is very evident at 420 K (Figure 7, f). Bands due to carboxylate species (again different from those of acetates) grow progressively starting from 330 K and become very intense. However, after heating at 473 K, two new bands also appear at 1870 and 1790 cm-' that grow again up to 573 K (Figure 7, g). The unusual position and the relative intensity of these bands, not observed after adsorption of propylene at room temperature followed by heating, strongly suggest their assignment to a cyclic anhydride (symmetric and asymmetric C=O stretchings in the system O=C-0-C= such as maleic anhydride. A study of the adsorption of this compound of vanadia-titania has been published recentlyi3 and confirms this assignment. In summary, the above low-temperature experiment produced some new information, as follows: (i) The reaction of propylene with surface OH's, producing isopropoxy species, takes place at a temperature as low as 210 K. (ii) C-C bond formation certainly occurs at least at low temperatures, giving rise finally to the production of the C4oxidized compound maleic anhydride. (iii) At 210 K, an intermediate species is detected that is responsible for weak bands at 1468, 1373, 1165, and 1126 cm-'. The absence of strong bands in the u(C=O) or v ( V - 0 ) regions allows us to propose that this species does not contain oxygen, so it is an hydrocarbon. To attempt an assignment, on tentative bases, we can compare these bands with the most intense ones of *-bonded allyl species in the complex (C3H5PdC1)z(1458, 1383, 1228,and 1192 cm-' 31). Also the allyl radical, isolated in cryogenic matrices,32 is responsible for bands not far from those of this intermediate species. We can consequently propose that *-bonded propylene can lose an hydrogen atom to give a a-bonded allyl species that can later undergo dimerization, possibly by radical coupling, to a C6compound and be finally oxidized to give the C4 compound maleic anhydride. ( e ) Surface Pathways for Propylene Transformation. The above results show evidence that more than one pathway is active on the surface of vanadia-titania for propylene transformation, as summarized in Scheme I. The formation of isopropoxide species and acetone, as well as of its enolate ion, from propylene represents the first pathway leading to C3compounds functionalized at Cz. In terms of a mechanism, propylene undergoes an electrophilic attack by active proton of surface OH's to give isoproxide species. Previous characterization studies have in fact shown that vanadia-titania is a weak Bronsted acidic ~urface.'~According to previous studies, propylene interaction on TiOz, which is not a Bronsted acidic surface, only produces weak adsorption without chemical reaction.z8 Instead, on strongly Bronsted acidic surfaces, such as tungsta-titania and titania-containing sulfates, propylene 0 2 9 9 3 0 ) ,
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Figure 6. FT-IR spectra of the adsorbed species arising from contact of activated vanadia-titania with propylene gas ( P = 50 Torr) at room temperature and following evacuation for IO min at room temperature (a), 393 K (b), 473 K (c), and 573 K (d).
also on its skeletal chain, most probably at the C=C bond, producing by oxidation carboxylate species other than acrylates. ( d ) Propylene Adsorption. The spectrum arising from propylene adsorption at room temperature followed by evacuation at the same temperature is shown in Figure 6, a. Very evident and sharp bands are present at 1670, 1468 and 1455 (doublet), 1388 and 1372 (doublet), 1328,1250,1170,1132, and 1095 cm-l, the last being very strong. All these bands are the same as those observed and assigned above after isopropyl alcohol adsorption and correspond to a mixture of isopropoxy species (strongest band, v ( C - 0 ) at 1095 cm-I) and coordinated acetone (strongest band, v(C=O) near 1670 cm-l). Other bands are also detected, broad, near 1635 cm-' and in the region 1620-1500 cm-', having a maxium at 1565 cm-'. By increasing the evacuation temperature (Figure 6, b), the bands relative to adsorbed acetone grow progressively in intensity together those in the region 1650-1500 cm-', while the adsorptions associated with the isopropoxy species progressively decrease in intensity and then disappear. At even higher temperatures (Figure 6, c and d) up to 523 K, acetone also disappears while the spectrum becomes dominated by a very strong band near 1540 cm-' (rather broad) and another absorption in the region 1450-1 350 cm-' where a sharp component at 1448 cm-' eventually predominates. The overall appearance of the spectrum suggests the presence of carboxylate species. However, the spectrum does not correspond either to that of acetate species only (Figure 2, a) or to that of acrylate species only (Figure 2, b). The detection of a very weak sharp band at 1275 cm-' suggests that acrylate species are indeed present together with other carboxylates, altogether responsible for the nearly superimposed COz asymmetric and symmetric stretching absorptions in the 16001500- and 1450-1 400-cm-' regions. The detection at temperatures below 473 K of a distinct absorption band (relatively sharp) in the region 1640-1 620 cm-', not present after isopropyl alcohol and acetone adsorption, could be associated to the presence of allyl alcoholate, acrolein, and/or acrylate species (v(C=C), see above). The detection of acetone and isopropyl alcohol proves that proplylene reacts with the surface at its Cz position. To obtain further information on other reaction pathways, for which we have
(28) Busca, G.; Ramis, G.; Lorenzelli, V.; Janin, A.; Lavalley, J . C. Spectrochim. Acra 1987, 43A, 489. (29) Bellamy, L. J.; Connelly, B. R.; Philpotts, A. R.; Williams, R. L. Z. Electrochem. 1960, 64. 563. (30) Mirone. P.; Chiorboli, P. Specrrochim. Acra 1962, 18, 1425. (3 I ) Sourisseau, C.; Guillermet, J.; Pasquier, B. Chem. Phys. Leu. 1974, 26, 564. (32) Maier, G.; Reisenauer, H . P.; Rohde, B.; Dehnicke, K. Chrm. Ber. 19a3.116.132. ( 3 3 ) Busca, G . Langmuir 1986, 2, 511.
8944 The Journal of Physical Chemistry, Vol. 94, No. 26, I990
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wavenumbers cm Figure 7. FT-IR spectra of the adsorbed species arising from contact of activated vanadia-titania with propylene gas (P = 5 0 Torr) at 173 K and following evacuation for IO min at 173 K (a), 193 K (b), 213 K (c), 253 K (d), 333 K (e), 523 K (f), and 573 K (g).
These results compare well with previous literature data, such reacts with acive protons to give an isopropyl carbocation that initiates a surface cationic-type oligomerization chain r e a ~ t i o n . ~ ~ . ~as~ the following: The case of vanadia-titania is an intermediate one, and the iso(i) Benzene is frequently found as a byproduct of propylene oxidation, as well as 1,5-hexadiene,thought to be its precursor.374 propyl group formed by proton attack on propylene remains (ii) Benzene4and c& hydrocarbons can be oxidized to maleic strongly associated to the oxide ion, giving an isopropoxide species. Variable-temperature experiments have shown that the reaction anhydride on vanadia-titania as well as on other vanadiumcontaining catalysts.41 between propylene and surface OH'S takes place already near 230 K. Isopropoxide species are subsequently dehydrogenated to (iii) Toluene reacts easily at the methyl group on the surface acetone starting at temperatures near 300-373 K. This result can of vanadia-titania, giving an intermediate thought to be a benzyl species having a radical nature.I2 be compared with those relative to the catalytic decomposition of isopropyl alcohol, giving mostly acetone at temperatures near Consequently, we can propose the following mechanism for the second reaction pathway at the methyl group, leading to C, 473 K for vanadia-titania "monolayer" catalyst^.^^ The higher functionalized products as well as to C-C bond formation products. temperature with respect to that at which we find dehydrogenation Propylene can undergo hydrogen abstraction from the methyl is probably due to the need for energy to allow desorption of group, as already proposed for toluene,I2 leading to an allyl inacetone. Acetone can decompose further to acetates (and probably termediate, possibly of the radical type. This intermediate can methane, in the absence of oxygen as in our conditions) in the be "frozen in" in relevant amounts only by low-temperature adtemperature range 373-473 K. sorption under our conditions and can consequently dimerize to The difference of the spectra of carboxylates arising from give 1,Shexadiene. By successive heating, this c6 compound can acetone decomposition (that are due to acetate species) and from propylene oxidation shows that this pathway is not the only one be oxidized to maleic anhydride. If the adsorption experiment for propylene transformation, finally giving other carboxylate is carried out at higher temperatures, the concentration of the allyl species. From our data, evidence is found for a reaction at the species is much lower, and they most probably react preferentially methyl group, as follows: with surface oxide species to give allyl alcoholates that are rapidly oxidized to acrolein. (i) After propylene adsorption at room temperature, other species are found together with the previously cited ones reThe main features of the mechanism of propylene oxidation sponsible for a relatively sharp band near 1630 cm-'; this band to acrolein on bismuth-molybdate-based catalysts have been could correspond to the v(C=C) of adsorbed allyl alcoholate established on the basis of reactor experiments using isotopically and/or acrylate species, observed almost at the same frequency. labeled propylene^.^^-^^ This mechanism involves (i) a slow first hydrogen abstraction from the methyl group, giving a symmetrical (ii) By further heating, in the complex pattern due to a mixture n-bonded allyl species; (ii) a fast reversible formation of a u C-0 of adsorbed carboxylates, a few features probably relative to adsorbed acrylate species are also detectable. bond at the allylic position, giving an allyl alcoholate; and (iii) (iii) After propylene adsorption at low temperatures, an ina fast second hydrogen abstraction, giving acrolein. termediate hydrocarbon species is detected, whose IR absorptions The same mechanism very probably apply also to vanadiacan support its tentative identification as an allyl species. titania catalysts. However, from our experiments, acrolein reacts (iv) By heating the sample after low-temperature propylene easily at the C==C bond on the surface of vanadia-titania (starting adsorption, bands due to the C4 oxidized compound maleic anfrom 373 K), giving by oxidation a mixture of carboxylate species hydride are found that are certainly due to oxidation of species (possibly formates, acetates, acrylates, oxalates, etc.). arising from previous C-C bond formation. ~~
~~~
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(34) AI-Mashta, F.;Sheppard, N . ; Davanzo, C. U . Marer. Chem. Phys. 1985, 13, 315. (35) Ramis, G.: Busca, G.; Lorenzelli, V . In Struclure and Reactioity of Surfaces; Morterra, C., et al., Eds.; Elsevier: Amsterdam, 1989; p 777. (36) Grzybowska-Swierkosz. B. Muter. Chem. Phys. 1987. 17, 121.
(37) Grzybowska, B.; Haber, J.; Janas, J . J . Coral. 1977, 49, 150. (38) Grasselli, R. K.;Burrington, J . D. Adu. Caral. 1982, 30. 133. (39) Snyder, T.P.;Hill, C. G. C a r d . Reo.-Sci. Eng. 1989, 31, 43. (40) Howe, R. F.; Minming, H. Proc. 9fCC. Calgary 1988, 1585. (41) Centi, G.; Trifir6, F.; Busca, G.; Ebner, J. R.; Gleaves, J. T. froc. 91CC. Calgary 1988, 1538.
J. Phys. Chem. 1990,94, 8945-8950 (r) Mechanism of the Catalytic Oxidation of Propylene. The catalytic oxidation of propylene on vanadia-titania has been studied by several authorse1’ at temperatures ranging between 550 and 670 K. Acrolein and acetone together with carbon oxides are mentioned as the main products, although smaller amounts of propionaldehyde, acetaldehyde, and acrylic and acetic acids are also cited. Oxidation reactions on vanadia-based catalysts have been generally described as being of the redox type, Le., being due to oxidation of the reactant by the catalyst surface with subsequent surface reoxidation by gas-phase oxygen (Mars-Van Krevelen m e ~ h a n i s m ~ ~ .This ~ ~ ) .has also been concluded for propylene oxidation on vanadia-titania on the basis of reactor experiments carried out with isotopic oxygen.I0 In this sense, our experiments detecting the oxidation of propylene and of several C3 potential intermediates by the surface can be taken as relevant with respect to the catalytic reaction mechanism. Indeed, the above-cited reaction pathways justify the main selective oxidation products detected, acetone and acrolein, as well as acrylic and acetic acids and agree with the mechanisms proposed in the literature for propylene o ~ i d a t i o n . ~ ’The - ~ ~ formation of propionaldehyde can be interpreted as evidence of surface hydrogenation/dehydrogenation between isopropyl alcohol and acrolein, resulting in acetone and propionaldehyde. A similar reaction could involve acetic acid to give acetaldehyde. It seems relevant to show that the surface reactions we detected occur at lower temperatures than those used in the catalytic reaction experiments.”’ However, we should mention that the desired selective oxidation products that we have indeed found to be formed at the surface are all strongly adsorbed and need energy to be desorbed. However, at higher temperatures, their transformation is observed, leading to overoxidation products that are also very strongly adsorbed (carboxylates) and eventually give (42) Mars, P.; Van Krevelen, D. W. Chem. Eng. Sci. Suppl. 1954, 3, 41. (43) Srivastava, R. D. Heterogeneous Catalytic Science; CRC Press: Boca Raton, FL, 1988.
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mainly by oxidation and decomposition carbon oxides. Consequently, we can conclude that the vanadia-titania surface contains very active sites for propylene selective oxidation. However, its excessive activity and/or acidity results in a too strong retention of the selective oxidation products and to their overoxidation. This agrees with the observation of Martin and Rives” that doping vanadia-titania with sodium, leading to a less acidic surface, enhances the selectivity in acrolein production from propylene. It must also be mentioned that the cited references concerning propylene oxidation on vanadia-titania refer to a catalyst having a relatively high surface area (1 5-50 m2/g),9-11favoring consecutive reactions. Indeed vanadia-titania catalysts are useful for selective oxidations whose products are chemically relatively inert and resist overoxidation, such as phthalic anhydride and aromatic nitriles. The comparison of the behavior of vanadia-titania with that of pure titania toward propylene adsorption shows evidence of the strickingly different reactivity of the surface sites of these materials. While the pathway leading to acetone seems to be related to the weak Bronsted acidity of vanadia-titania, the pathway leading to C I functionalized products would involve sites able to abstract hydrogen. In a previous publication concerning toluene oxidation,I2 vanadium centers in the form of V02+ have been proposed as the active site. Both the data presented here, showing that this abstraction would occur well below room temperature, and those reported previously (showing a benzyl radical formed at room temperature from toluene) show evidence for the very high reactivity of these centers for the activation of hydrocarbons. Acknowledgment. This work has been supported by CNR, Progetto finalizzato Chimica Fine 11. V.S.E. acknowledges the Ministerio de Educacion y Ciencia, Spanish Government, for a research grant (beca de formacion del profesorado y personal investigator). Registry No. Vanadia, 1314-62-1;titania, 13463-67-7; propylene, 1 15-07-1; acetone, 67-64-1; isopropyl alcohol, 67-63-0; acrylic acid, 7910-7; acrolein, 107-02-8;allylic alcohol, 107-18-6.
Fourier Transform Infrared Spectroscopic Studies of the Reactivity of Vanadia-Titania Catalysts toward Olefins. 2. Ethylene Vicente Sanchez Escribano,+Guido Busca,* and Vincenzo Lorenzelli Istituto di Chimica, Facoltd di Ingegneria, Universitd di Genova, P.le Kennedy, I - 1 6 / 2 9 Genova, Italy (Received: April 12. 1990: In Final Form: June 20, 1990)
The adsorption and transformation of ethylene and of some 0-containing C2 molecules (namely ethanol, acetaldehyde and acetic acid) on vanadia-titania in the temperature range 150-673 K have been investigated by IR spectroscopy. Ethylene only adsorbs as such, without reaction, at low temperature. Reactive adsorption is found to start from 373 K. Chemisorbed acetaldehyde and, by further heating, acetate and formate ions are produced. Other species, identified tentatively as an enol-like species CH2=CH-0as well as -0-CH=CH-0--, are responsible for typical absorptions. The surface reaction pathways are discussed in relation to literature data concerning ethylene oxidation on heterogenous oxide catalysts. The active sites for olefin selective oxidation on vanadia-titania are proposed to be V5+OHgroups (for hydration/dehydrogenation reactions) and coordinatively unsaturated V4+=0 groups for allylic oxidation of propylene.
Introduction The heterogenousiy catalyzed ethylene oxidation is carried Out industrially on silver catalysts to produce ethylene oxide.’ Acetaldehyde is produced by ethylene oxidation in solution using Pd/Cu homogeneous catalysts (Wacker process2). The production
* To whom correspondence should be addressed.
‘On leave from Departamento de Quimica Inorganica, Facultad de
Ciencias Quimicas. Universidad de Salamanca. Plaza de 10s Caidos, E-37008 Salamanca, Spain.
0022-3654/90/2094-8945$02.50/0
of acetaldehyde and/or acetic acid from ethylene by an heterogenously catalyzed process should be very desirable. However, up to now, attempts to heterogenize efficiently the Wacker process have been unsu~cessful.~”The better heterogeneous catalysts ( I ) Berty, J . M. In AppLied Industrial Catalysis; Leach, B. E., Ed.; Academic Press: New York, 1983; Vol. I , p 207. (2) Smidt, J.; Hafner, W.; Jira, R.; Sedlmeier, R.; Siebner, R.; Ruttinger, R.; Kojer, H . Angew. Chem. 1959, 71, 176. ( 3 ) Evnin, A. B.; Rabo, J . A,; Kasai, P. H. J . Carol. 1973, 30, 109. (4) Forni, L.; Terzoni, G. Ind. Eng. Chem. Process Des. Den 1977, 16, 288.
0 1990 American Chemical Society
