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 6 7 0 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.
8945
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
The Journal of Physical Chemistry, Vol. 94, No. 26, 1990
8946
Sanchez Escribano et al.
SCHEME 1
CH3COOH
T
.1 CH,= CH,
.L
.OH d
CH,
I
7
CH3 l CH
-
CH3 1 C
ox +
II 0
(e
/I”
ox
*\
0’ -‘O
1.L
1
co2
(e)
CH2
pI
5
-H2
-H+
CH,
II CH
I
0
+
1
ox
y z 4% \
-OH
0
0
1
1 .L
2
HCHo
1 ox
-H,
ox
H
*
1
6.b
1
found are constituted by supported or unsupported vanadia doped with Pd.M Titania-supported Pd/V205 is reported to be the most performant.6 Also pure vanadia has been found to act as a partial oxidation catalyst for ethylene, giving nonnegligible yields in acetaldehyde and acetic acid5 at temperatures of the order of 500 K, although its performance is greatly enhanced by doping with Pd. Titania-supported vanadia was found by Mori et al.’ to be significantly more active than pure V205in ethylene oxidation, although the very high temperature used by these authors (773 K) prevented the detection of any partial oxidation product. The present paper summarizes the results of our IR studies of ethylene adsorption and oxidation on vanadia-titania: the aim of this work is to know if active sites for ethylene selective oxidation are present on the vanadia-titania surface and what are the reasons of its poor efficiency as a selective oxidation catalyst for ethylene.
(9)
0 -
0
1
1
-
I
I
I
1
I
I
1800
1700
1600
1500
1400
1300
co (PI -OH
Experimental Section Preparation and characteristics of the vanadia-titania catalyst and experimental methods have been reported in the first paper of this seriesS8 Ethylene was taken from commercial cylinders from SI0 (Milano, Italy). Acetic acid, acetaldehyde, and ethanol were hyperpure products from Carlo Erba (Milano, Italy).
Results and Discussion In order to have an easier description of the spectra obtained after oxidation of C2 molecules, we will first describe the results obtained by adsorption of the more oxidized molecules. In Scheme I, the proposed structure for the surface species detected and a proposed reaction pathway are summarized. ( a ) Adsorption ofAcetic Acid. The IR spectra of the adsorbed species arising from acetic acid adsorption at room temperature on vanadia-titania are reported in Figure I . Two bands, at 17 18 and 1295 cm-I. are only observed in contact with acetic acid vapor (Figure I , a) but disappear by evacuation at room temperature. They can be assigned to v(C-0). and to a coupled C--0 stretching/COH deformation mode of weakly adsorbed acetic acid (I770 and 1264 cm-I, respectively, in gaseous acetic acidg). All P.; Montarsal, R.J. Cural. 1980,63, 182, 191. (6) Van der Heide, E.; de Wind, M.; Gerritsen, A. W.; Scholten, J. J. F. froc. SICC, Calgary 1988, 1648. (7) Mori, K.;Miyamoto, A.; Murakami, Y.J. fhys. Chem. 1984,88, 2741. (8) Sanchez Escribano, V.; Busca, G.;Lorenzelli, V. J. Phys. Chem., preceding paper in this issue. ( 5 ) Sloane, J . L.; Boutry,
1900
wavenumbers
1200
cm-‘
Figure 1. FT-lR spectra of the adsorbed species arising from acetic acid adsorption on vanadia-titania at room temperature (a) and evacuation at room temperature (b), 330 K (c), 373 K (d), 423 K (e), 473 K (0, 573 K (g), and 623 K (h).
other bands are only weakly affected by evacuation at room temperature. Evacuation at increasing temperatures in the 300-473 K range (Figure 1, b-e) causes the progressive decrease down to the disappearance of another strong band near 1660 cm-’ as well as of a broad absorption centered near 3200 cm-l. These bands can be assigned to u(C=O) and v ( 0 H ) of another more strongly adsorbed form of intact acetic acid. Instead, evacuation upon heating in this temperature range only slightly perturbs the (9) Haurie, M.; Novak, A. J. Chim. fhys. 1965, 62, 146.
Reactivity of Vanadia-Titania toward Ethylene
3100
3000 wavenumbers
2900 cm
The Journal of Physical Chemistry, Vol. 94, No. 26, 1990 8947
2800 .i
Figure 2. FT-IR spectra (vCH) region) of the adsorbed species arising from adsorption of acetic acid (a), acetaldehyde (b), and ethanol (c) all at room temperature and followed by evacuation at room temperature; ethanol evacuated at 423 K (d), 473 K (e), and 523 K (f).
complex absorptions in the 1600-1380-~m-~ region. This complex absorption is composed of a couple of very strong bands. The higher frequency one after evacuation at rwm temperature shows the main maximum at 1530 cm-I with a sharp shoulder near 1500 cm-l, but the relative intensities of them are inverted after evacuation at temperatures higher than 330 K (Figure 1, c). The lower frequency component of this couple is the more intense and always shows the main maximum at 1442 cm-I and a lower frequency shoulder near 1418 cm-l. Two sharp and weak bands at 2940 (Figure 2, a) and 1350 cm-l also behave in parallel with the bands of this couple: all of them can be assigned to adsorbed acetate ions (2940 cm-l, symmetric CH3 stretching; 1530 and 1500 c d , asymmetric COz stretching; 1442 and 1418 cm-I, symmetric C 0 2 stretching with superimposed CH, asymmetric deformation; 1350 cm-l, CH, symmetric deformation9) having bidentate coordination.10 The intensity of the bands of the acetate species is substantially stable up to 473 K but drops at higher temperatures (Figure 1, g) to disappear at 620 K (Figure 1, h), without leaving any other surface species. At intermediate temperatures, the asymmetric C 0 2 stretching shifts again to 1535 cm-I, probably due to some kind of rearrangement of the adsorbed species. The dissociative adsorption of acetic acid on vanadia-titania parallels that already observed on other ionic oxide catalysts."-I3 The products of such a decomposition, bidentate acetates, occur in the absence of oxygen strongly bonded on the surface. Their decomposition is in fact observed only above 473 K. (b) Adsorprion of Acetic Aldehyde. The spectra relative to the adsorption of acetic aldehyde are reported in Figure 3. After adsorption and evacuation at room temperature, a complex pattern of bands is evident in the range below 1800 cm-'. Some of these bands can be assigned to acetaldehyde adsorbed as such. They are at 1682 c d , sharp and very strong (u(C=O)); 1355 cm-', sharp and medium (symmetric CH3 deformation); and 1 132 cm-I, coupled CC stretching and CH3 and CCO deformations.14 The corresponding absorptions on gas-phase acetaldehyde have been observed at 1746, 1353, and 1 1 14 cm-l, r e ~ p e c t i v e 1 y . l ~A~ ~ ~ complex weak absorption is also detected in the u(CH) region (Figure 2, b). The relevant shift down of the C O stretching suggests that this species is coordinated on Lewis acid sites through one of the oxygen lone pairs. Another weak, sharp component (IO) Deacon, G. B.; Phillips, R. J.
Coord. Chem. Rev. 1980, 33, 227. (1 I ) Griffiths, D. M.; Rochester, C. H. J . Chem. Soc., Faraday Trans. 1977, 74, 1988. (12) Lorenzelli, V.; Busca, G.; Sheppard, N . J . Carol. 1980, 66, 28. ( I 3) Sclafani, A.; Palmisano, L.; Schiavello, M.; Augugliaro, V.; Coluccia, S.;Marchese, L. New J . Chem. 1988, 12, 129. (14) Hollenstein, H.; Gunthard, H. H. Specfrochim. Acta 1971, 27A. 2027. (IS) Wiberg, K. B.;Walters, V.; Colson, S.D. J . Phys. Chem. 1984, 88, 4723.
1800
1700
1600
1500
1400
1300
wavenumbers
1200
cm-
1100
1
Figure 3. FT-IR spectra of the adsorbed species arising from acetaldehyde adsorption on vanadia-titania at room temperature and evacuation at room temperature (a), 373 K (b), 423 K (c), 473 K (d), and 623 K (e).
at 1705 cm-I can be associated to u(C=O) of a more weakly perturbed form of adsorbed acetaldehyde. These bands are almost unaffected by heating at 330 K but disappear progressively by heating up to 473 K (Figure 3, b-d), when they are no more observed. Other bands instead are already present after contact at room temperature and increase in intensity by heating up to 373 K (Figure 3, b), but later their intensity drops rapidly. They are all sharp at 1635, 1615, and 1186 cm-l. Finally, other bands grow by heating adsorbed acetldehyde up to 473 K and disappear progressively only in the temperature range 473-623 K (Figure 3, d and e). They are concident to the bands arising also from adsorption of acetic acid and are already assigned above to the bidentate acetate species. The formation of acetate species from acetaldehyde corresponds to oxidation by the surface, also already found, for example, on Fe2O3I2 and for the interaction of formaldehyde with vanadiatitania.I6 No evidence is found for a disproportionation of the aldehyde. The bands at 1635,1615, and 1186 cm-', which behave in parallel, are due to the product of a different reaction of acetaldehyde; this species is slightly more stable on the surface than acetaldehyde itself but is only detected together with acetaldehyde. A tentative identification of this species can be done in parallel to the behavior of adsorbed acetone on the same surface. We have found evidence previouslyI6that acetone dissociates in its enolate anion on vanadia-titania. Indeed, the bands observed can be assigned to an enolic form CH2=CHO-: it has in fact been shown that in vinyl ethers the v(C=C) band is split due to a Fermi resonance with the first overtone of the CH2 wagging 1node."9~* The band at 1 186 cm-I could be due to the u(C-0) of this species. An alternative possible assignment for these bands is to the crotonic condensation product c r ~ t o n a l d e h y d e . ~ ~ We then conclude that acetaldehyde is stable as such on the surface of vanadia-titania only below 320 K but can be easily (16) Busca, G.; Elmi, A. S.;Forzatti, P. J . fhys. Chem. 1987, 91, 5263. (17) Owen, N. L.; Sheppard, N . Trans. Faraday Soc. 1964, 60, 634. ( I 8) Sullivan, J. F.; Dickson, T. J.; Durig, J. R. Specrrochim. Acta 1986, 42A. 113. (19) Bowles, A. J.; George, W. 0.;Maddams, W. F. J . Chem. Soc. B 1%9, 810.
8948 The Journal of Physical Chemistry, Vol. 94, No. 26, I990
Sanchez Escribano et al. I
I
I
1900
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1700
I
I
1600
1500
I
I 1EOO
1700
1600
1500
1400
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1200
navenumbers
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cm
i
Figure 4. FT-IR spectra of the adsorbed species arising from ethanol adsorption on vanadia-titania at room temperature and evacuation at room temperature (a), 373 K (b), 423 K (c), 473 K (d), 523 K (e), and 573 K
(0.
oxidized to acetate ions that are strongly retained on the surface. Enolization, with possibly subsequent crotonic condensation, can also occur. (c) Adsorption of Ethanol. Adsorption of ethanol at room temperature (Figure 4, a) results in the formation of strong, sharp bands at 1470, 1448, 1387, and 1358 cm-', with weaker bands at 1270 cm-I, all assigned to different C H deformation modes of the ethyl group," and at 1095 cm-', due to C - 0 stretching. All these bands, as well as the very strong C H stretchings at 2982, 2942, and 2878 cm-I (Figure 2, c and d) can be assigned to ethoxy groups produced by ethanol dissociation, while the features of undissociated adsorbed ethanol2' are practically undetected. These bands are also not affected by heating under evacuation at 330 K. By heating at 373 and 420 K (Figure 4, b and c), all the above bands progressively decrease in intensity while a sharp band progressively grows at 1682 cm-I. This band coincides with that due to adsorbed acetic aldehyde (v(C=O), see above). According to this assignment and to the behavior of adsorbed acetaldehyde, described above, this band decreases in intensity to disappear in the temperature range 423-523 K (Figure 4, c-e) while bands 1635 and 1615 cm-I growth at 473 K but have already disappeared at 523 K. Their detection even under these conditions, where the concentration of adsorbed acetaldehyde is certainly small, so preventing condensation, can be taken as a confirmation that these bands are due to the "monomeric" enol species more than to the "dimeric" crotonaldehyde. At temperatures higher than 523 K (Figure 4, 9 only bands at 1535, 1440, and 1380 cm-' are still present. They can be assigned to adsorbed acetates, as discussed above, also according to their stability. However, we can note that the intensity ratio of the two C 0 2 stretchings is apparently inverted in this case with respect to the spectrum of acetates produced by acetic acid adsorption. Moreover, two bands are also observed near 1380 and 1370 cm-I. It seems likely that the spectrum observed after ethanol oxidation on vanadia-titania is due to the superimposition of the spectrum of adsorbed acetates with that of adsorbed formates. These species, when formed by formic acid adsorption on vanadia-titania, are responsible for an intense band at 1560 cm-l (asymmetric COO stretching) and for a weaker doublet at 1378 and 1370 cm-' (CH deformation and symmetric COO ~tretching'~).It seems relevant to note that formate species can be formed by oxidation at the C=C double bond of the enolic form of acetaldehyde. This species can be formed starting from ethanol by two different mechanisms: (i) oxidation to formaldehyde and subsequent abstraction of a proton; (ii) dehydrogenation (loss of H,)of ethoxy species. So the enolate/aldehyde relative concentration could be higher when starting from ethanol than from the aldehyde itself. If formates are mainly produced by oxidation of the enolate, it is reasonable to find a (20) Perchard, J . P.; Josien, M. L. J . Chim. Phys. 1968, 65. 1834. G.;Lorenzelli, V . J . Cbem. Soc., Furuday Trans.
(21) Ramis, G.;Busca, I 1987. 83. 1591.
2000
1400
1300
-1
wavenumbers cm
Figure 5. FT-IR spectra of the adsorbed species arising from ethylene adsorption on vanadia-titania at 150 K and evacuation at 150 K (a) and 170 K (b).
higher formate concentration when ethanol instead of acetaldehyde is oxidized. In conclusion, ethanol can be oxidized by the surface of vanadia-titania to acetaldehyde at temperatures as low as 330 K and can produce by further oxidation acetate species. Moreover, ethoxy groups can be directly dehydrogenated to enolate species whose C=C bond can be broken to give formate species. (6)Adsorption of Ethylene. When vanadia-titania is put into contact with ethylene gas at room temperature, no adsorbed species is detected. Instead if contact is carried out at 150 K (Figure 5, a) the spectral features of poorly perturbed adsorbed ethylene are observed.,, In particular, we detect strong bands at 3092 and 2978 cm-I, due to the two IR active CH2 stretching modes, with a weaker combination mode at 3065 cm-I. In the lower frequency region, the two IR active bands at I898 (first overtone of a out-of-plane CH2deformation mode) and at 1442 cm-' (CH2 asymmetric scissoring) are observed to be strong, while two weaker bands at 1620 and 1342 cm-' are also seen, corresponding to two IR inactive (Raman active) bands (C=C stretching and CH2 symmetric scissoring). Evacuation at 170 K (figure 5, b) causes the strong decrease of the intensity of these bands, although the lower frequency bands are still detectable at 161 2, 1442, and 1340 cm-'. Under these conditions, the relative intensity of the Raman active bands (1 6 12 and 1340 cm-l) is much stronger than at higher coverages, suggesting that they are due to truly adsorbed molecular ethylene, interacting through its *-type orbital on cationic sites.22 The strong IR bands observed at higher coverage (150 K) are instead assigned to condensed liquid ethylene. Evacuation at temperatures higher than 190 K leads to the complete desorption of ethylene. Reactive adsorption of ethylene is clearly found if contact of vanadia-titania with the gas is carried out at 373 K or higher temperatures. After contact at 373 K (Figure 6, a) several weak bands appear in the region 1800-1000 cm-I. In particular bands are detected at 1675, 161 5, and 1560 cm-' with a broader shoulder at 1540, 1440, 1400 (broad), 1385 (very weak), and 1355 cm-I. ~
~~
~
~~
-
(22) Busca, G.;Ramis, G.;Lorenzelli, V ; Janin, A : Lavalley, J C Specrrochrm Acta 1987, 43A, 489
Reactivity of Vanadia-Titania toward Ethylene -
1
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The Journal of Physical Chemistry, Vol. 94, No. 26, I990 8949
0)
U
m n 0 Y)
n
1800
1700
1600
1200
wavenumbers cm-'
Figure 6. FT-IR spectra of the adsorbed species arising from ethylene adsorption on vanadia-titania at 373 K (a) and at 423 K (b) and following evacuation at 423 K (c) and 473 K (d).
By increasing the contact temperature up to 420 K (Figure 6, b), all bands grow, although that at 1400 cm-' becomes sharper near 1415 cm-I. Further evacuation at 420 K (Figure 6, c) causes the complete disappearance of the band at 1615 cm-I, a slight decrease of the band at 1675 cm-I, and further growth of the bands at 1560 (very sharp), 1535 (rather broad), 1440, and 1355 cm-I. Further heating up to 473 K (Figure 6, d) causes the complete desappearance of the band at 1675 cm-I, the decreases of that at 1560 cm-l (completely disappeared at 523 K), and the further growth of those at 1535, 1440, and 1355 cm-', while a new, very weak band appears at 1370 cm-l together with a broad one near 1250 The sharp band at 1675 cm-' is certainly due to a u(C=O) stretching mode of a carbonylic compound and can be assigned, together with the weaker band near 1390 cm-I, to adsorbed acetadehyde (see above). Formation of acetaldehyde from ethylene is likely obtained by previous reaction of ethylene with surface hydroxy groups, giving intermediate ethoxy species that are later oxidized. The lack of detection of bands associated with ethoxy species in these conditions agrees with the previous result that showed that at 373 K ethoxy groups produced by ethanol adsorption are already largely oxidized to acetaldehyde. This mechanism is also based on the following data: (i) parallel reactions are observed at lower temperature starting from more electron-rich olefins (Le., propylene* and isobuteneZ3),according to a electrophilic attack by active H+; (ii) this parallel reaction observed with higher olefins at lower temperatures gives alkoxy groups that, by heating, can produce carbonyl compounds.s The bands at 1535, 1440, and 1380 cm-I are assigned to carboxylate groups, mainly acetates, according to the data arising from acetic acid adsorption. However, we may note that the spectra of the species arising from ethylene oxidation show the band at 1535 cm-' being more intense than that at 1440 cm-I, while in the case of surface acetates the inverse is observed. Moreover, sharp bands at 1565 and 1370 cm-' are also observed, relative to species having a lower stability than acetates. These bands can be assigned to a relatively labile form of surface for(23) Sanchez Escribano, V.; Busca, G.; Lorenzelli, V. To be published.
mates, due to their coincidence with bands observed after formic acid adsorption.16 Another less labile form of formate species on this surface is characterized by bands at 1540 (more intense) and 1358 cm-'.I6 These absorptions are probably superimposed to those of acetates in the spectra of Figure 6, d. The band detected near 1615 cm-' is relative to another rather labile adsorbed species, whose identification is difficult. Assuming that this band, relatively sharp, is due to u(C==C), we can propose on a very tentative basis that this species is something like a -OCH=CHOspecies. ( e ) Surface Pathways for Ethylene Transformation. Our data indicate that ethylene reacts with the surface of vanadia-titania catalysts starting from near 373 K. The lower reactivity of ethylene with respect to more electron-rich olefins, such as propylene and isobutene, that have been found to react at much lower temperature^^*^^ suggests that electrophilic attacks undergone by olefins are mainly involved. At least two types of reaction pathways are envisaged between ethylene and the vanadia-titania surface. Adsorbed acetaldehyde is clearly identified as a reaction product of ethylene, together with its overoxidation products acetate ions. According to the parallelism of the behavior of other olefins in analogous condition^,^.^^ it seems clear that ethylene undergoes an electrophilic attack by protons arising from surface hydroxy groups, giving ethoxy species that at the temperature where this reaction starts are immediately dehydrogenated to acetaldehyde. This species is detectable on the surface only up to 423 K: at higher temperatures, it is desorbed or, mostly, oxidized to acetates. Acetates are very strongly bonded to the surface, disappearing only by heating at temperatures higher than 573 K, probably due to their slow decomposition. On vanadiasilica catalysts, acetaldehyde is produced by oxidation of ethanol with total selectivity at temperatures below 500 K, while acetic acid is produced in the range 500-600 K.24 At higher temperatures carbon oxides are formed together with the dehydration product ethylene as well as with acetaldehyde. Adsorbed acetates are probably the surface precursors for both acetic acid and carbon oxides. As discussed above, at least another pathway is also present, leading to formate species. Evidence is found for a side reaction of ethoxy groups, leading to, by dehydrogenation, enol-like species CH2=CHO- that undergo a breaking reaction at the C=C double bond, produced following the C I surface formate species. Intermediate in this way could be the -OCH=CHOspecies that we have identified on very tentative bases above as that responsible for a band at 1615 cm-I. To justify the formation of such a species, we can invoke a dehydrogenation of a -OCH2CH20- intermediate that could alternatively decompose, giving formaldehyde that later oxidizes to formates. Indeed, formaldehyde has been detected as a partial oxidation product of ethylene on vanadia-based catalysts.2s C=C breaking df ethylene to give formate has already been observed by ethylene interaction with a typical nonselective oxidation catalyst such as iron oxide.26 v) Heterogeneously Catalyzed Oxidation of Ethylene. Working at 773 K, Mori et ale7investigated the behavior of vanadia-titania in the unselective oxidation of ethylene. They found that C 0 2 was the main reaction product, but also CO was produced by a parallel mechanism. Different authors reported the formation of partial oxidation products from ethylene, such as acetaldehyde and acetic acids and also formaldehydeZSon different vanadia-based catalysts. The surface pathways for ethylene transformation on vanadia-titania, discussed above, provide information on the mechanisms of both selective and unselective ethylene oxidation, as proposed in Scheme I. Previous studies concerning formaldehyde and formic acid oxidation on vanadia-titaniaZ7gave evidence that CO is the main product at (24) Oyama, S . T.; Lewis, K. B.; Carr, A. M.; Somorjai, G. A. Proc. 9KC, Calgary 1988, 1489. (25) Margolis, L. Y . Adu. Catal. 1963, 14, 429. (26) Busca, G.; Zerlia, T.; Lorenzelli, V.; Girelli, A. J . Cataf. 1984, 88. 12s.
(27) Elmi, A. S.;Tronconi, E.; Cristiani, C.; Gomez Martin, J. P.; Forzatti, G.Ind. Eng. Chem. Res. 1989, 28, 387.
P.: Busca,
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The Journal of Physical C h e m i s t r y , Vol. 94, No. 26, I990
temperatures higher than 473 K and derives from the decomposition of surface formate species as follows: HCO, = CO OH-. On the contrary, decomposition of acetate species could produce
+
c02. These results indicate that vanadia-titania catalysts possess the active sites necessary for the selective oxidation of ethylene to useful products such as acetaldehyde and acetic acid. However, due to the kinetic inertness of ethylene, this reaction occurs at temperatures at which the desired product acetaldehyde is already further oxidized. Acetate species are produced in these conditions that are strongly bonded and can either desorb as acetic acid or decompose to C02. So poor performance of this catalyst is due to the successive oxidation of the partial oxidation products. The performances could be improved by using low surface area catalysts and by working with small contact times. As for the role of the different components in the Pd/V20s/ Ti02 and Pd/V2OS/AI2O3heterogenized Wacker catalysts, working at a temperature of 373 K in the presence of water, previous data suggested that supported vanadia acts essentially as the support for Pd and vanadium centers are only involved in the reoxidation of the Pd active center.6 The mechanism of the heterogeneous reaction should be the same of the homogeneous one, with copper substituted by vanadium centers. A role of the support has been mentioned in the formation of byproducts in the oxidation of 1-butene on Pd/V20s/Ti0,.28 However, our data indicate that at the same temperature Pd-free vanadia-titania is able to oxidize at the surface ethylene to acetaldehyde and, even more, higher olefins to carbonyl compounds.8 It has also been reported that the presence of Pd influences the reducibility of vanadiaS and should consequently enhance the activity of vanadia-titania. It is so possible to propose two alternative paths, one at vanadium, certainly active on Pd-free catalysts, and the other at palladium, as in the homogeneous case. It is possible that both paths are operating on Pd/V20S/Ti02and dominate each other in different conditions. (g) Actiue S i t e s for Olefin Oxidation on Oxide C a t a l y s t s . At least three types of olefin partial oxidation reactions can be distinguished formally. Their products arise from (i) allylic oxidation (e.g., acrolein from propylene), (ii) hydration/dehydrogenation (e.g., acetone from propylene and acetaldehyde from ethylene), and (iii) oxidative cleavage of the C=C bond (acetaldehyde or acetic acid from propylene or 2-butene, formaldehyde from ethylene). From the above data, products arising from all three reactions can be detected on the surface of vanadia-titania while on other catalysts (such as, for example, Fe2O3) only reaction iii is observed to occur at the surface.26 From the above discussion, on vanadia-titania the oxidative cleavage of the olefinic C=C bond (reaction iii) should represent a side reaction of path ii that also involves an attack at the C=C double bond. Consequently, the number of active sites for olefin activation toward oxidation reduces to two. The hydration/dehydrogenation reactions are very well detected as surface reactions on vanadia-titania and can be related to the medium strength of the Bronsted acid sites of this material, coupled with the medium redox properties of vanadate centers. As discussed previously,a alkoxy species are in fact formed from olefins (28) Van der Heide, E.; Ammerlaan, J . A. M.; Gerritsen, A. W.; Scholten, J . J . F. J . Mol Cafal. 1989, 55, 320.
Sanchez Escribano et al. because active protons are present, but they are not so strong to induce polymerizati~n~~ and consecutive transformations such as dehydrocyclization, coking, and cracking,30 as detected on true Bronsted acid solids. This type of reaction is not observed on solids that do not show Bronsted acidity such as Ti0222and Fe203.26 Alkoxy groups are easily oxidized near 373 K because of the oxidizing ability of V* centers ( n most probably = 5 ) . This agrees with the medium catalyst acidity found to be necessary to carry out this reaction on heterogeneous catalysts" and with the favoring effect of feeding water together with the olefin and oxygen.31 We can then propose as the active site for this reaction a M"+-OH group where M"+is likely V5+on vanadia-titania and Mo6+on molybdena-alumina32 and on molybdenum zeolites33 that have been found to be active for oxidation reactions, giving Wacker-type products, e.g., acetone from propylene. The allylic oxidation on oxide catalysts has been investigated deeply34,3sand could be expected to involve sites similar to those involved in benzylic oxidation (synthesis of benzaldehyde from toluene or phthalic anhydride from o-xylene). We have previously concludeda that on vanadia-titania this reaction occurs with a mechanism similar to that proposed on industrial multicomponent propylene oxidation catalysts (e.g., bismuth molybdate^^^.^^). The key active sites for this reaction would be those allowing the proton abstraction from the methyl group; there are several indications that this abstraction occurs both in propylene and toluene oxidation, by homolytic cleavage to give H' and allyl or benzyl r a d i ~ a l . ~ In ~ - the ~ ~ case of vanadia-titania, we may suppose, according to previous indications, that the H' abstracting sites are coordinatively unsaturated paramagnetic V 0 2 + centers, for whose existence evidence has been f o ~ n d . ~It~seems, ~ ~ * in fact, reasonable to propose that the homolytic cleavage of the C-H bond is favored by sites having available d-type electrons and are able to perturb C-H bonds due to their Lewis a ~ i d i t y . ~Accordingly, ~,~ Doulov et al. reported evidence that V4+ is involved in the active site for propylene selective oxidation on vanadia-titania.41 Due to the lower reactivity of vinylic hydrogens with respect to the allylic and benzylic ones, the hydrogen abstraction mechanism is probably unactive in the case of ethylene adsorption and oxidation. 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 investigador). (29) Ramis, G.; Busca, G.; Lorenzelli, V. In Sfrucrure and Reacriuiiy of Surfaces; Morterra, C., et al. Eds.; Elsevier: Amsterdam, 1989; p 777. (30) Ramis, G.; Rossi, P. F.; Busca. G.; Lorenzelli, V.; La Ginestra, A,; Patrono, P. Langmuir 1989, 5, 917. (31) Moro-oka, Y.; Takita, Y.; Ozaki, A . J . Caial. 1972, 27, 185. (32) Giordano, N.; Vaghi, A,; Bart, J. C . J.; Castellan, A . J . Cafa1.1972, 38, 1 I . (33) Howe, R. F.; M i m i n g , H. Proc. 91CC, Calgary 1988, 1585. (34) Grasselli, R. K.; Burrington, J . D. Ado. Caial. 1981, 30, 133. (35) Snyder, T. P.; Hill, C . G . Cafal. Rev.-Sci. Eng. 1989, 31, 43. (36) Busca, G.; Cavani, F.; Trifir6, F. J . Cafal. 1987, 106, 171. (37) Busca, G.;Centi, G.; Marchetti, L.; Trifir6, F. Langmuir 1986, 2, 568. (38) Centi, G. Discuss. Faraday Soc. 1989,87, 93. (39) AI-Mashta, F.; Davanzo, C . U.; Sheppard, N . J . Chem. Soc., Chem. Commun. 1983, 1258. (40) Busca, G.; Centi, G.; Trifir6, F. J . Am. Chem. SOC.1985, 107,7757. (41) Doulov, A,: Forissier, M.: Noguerol Perez, M.; Vergnon. P. Bull. Soc. Chim. Fr. 1979, 1-129.
