J. Phys. Chem. 1991,95, 5541-5545
5541
FT-IR Studles of the Reactlvlty of Vanadla-Titanla Catalysts toward Olefins. 3. n-Butenes and Isobutene Vicente Sanchez Escribano: Guido Busca,* and Vicenzo Lorenzelli Istituto di Chimica, Facoltb di Ingegneria. Universitb di Genoua, P. le Kennedy, I-I6129 Genooa, Italy (Received: June 21. 1990: In Final Form: January 28, 1991) The interaction of vanadia-titania catalyst with 1-butene, cis-2-butene. trans-2-butene,and isobutene has been investigated by FT-IR spactrosoopy in the temperature range 150-673 K. The three normal butenes give at room temperature 2-butoxide species that can undergo at higher temperatures oxidative dehydrogenation to methyl ethyl ketone and later oxidative C-C bond cleavage to acetate species. Methyl-allyl species are also formed and can be isolated by low-temperature experiments in the case of 1-butene. These speces follow a different oxidative path with formation of butadiene, furan, and finally maleic anhydride. Starting from near 200 K,isobutene oligomerizes giving its dimer 2,4,4-trimethylpent-l-eneand higher oligomers. At room temperature, hydration to t-butoxide species is also observed. Further heating causes oxidative decomposition to acetone, carboxylate species, and, from the oligomers, cyclic anhydrides. Only traces of allylic oxidation products of isobutene can be found. All these reactions m u r in the absence of gas-phase oxygen.
Iatrodllction Vanadia-titania mixed or supported oxides constitute wellknown catalysts for the selective oxidation of alkyl They have also been tested as catalysts for the oxidation of C4 unsaturated hydrocarbons: significant selectivities to maleic anhydride have been reported in the oxidation of butadiene,'.' while the main selective product in the oxidation of 1-butene is acetic acid;6 in the oxidation of isobutene, methacrolein, acetone, and acetic acid are produced at small conversions.' Supported vanadia catalysts are believed to work by Mars-Van Krevelen mechanisms! involving reaction of the oxidized catalyst surface with hydrocarbons giving selectiveoxidation products and following reoxidation of the catalyst by gaseous oxygen. Previous m-IR studies concemin the reactivity of vanadia-titania with ethylene9 and propylene' supported this view. In fact, reactive adsorption of these olefins produces adsorbed selective oxdiation products in the absence of gas-phase oxygen. The reason for the relatively poor selectivity in olefin oxidation on vanadia-titania has been attributed mainly to overoxidation of the selective products giving finally carbon oxides.9J0 The present paper reports data relative to the oxidative adsorption of the three n-butenes and of isobutene on model vanadia-titania catalysts. The aim is to complete the picture concerning light olefin oxidation pathways on this catalytic system and to compare the data with those arising from butadiene oxidation experiments."
!
Experimental Seetion The vanadia-titania catalyst (9.6% VzO, by weight) has been prepared by impregnation of TiOZ(P25 from Degussa, Hanau, West Germany) with NH4V03boiling water solution, followed by calcination at 723 K for 3 h. The surface area is 50 mz/g. The catalyst has been pressed into self-supporting disks and activated by heating in air at 673 K for 30 min and then placed at 673 K for 1 h in an evacuated IR cell. Adsorption has been camed out at room or lower temperatures by using a liquid-nitrogencooled cell. Successively heat treatments have been carried out under vacuum. I-butene, cis-2-butene, tram-2-butcne, and isobutene were taken from commercial cylinders from SI0 (Milano, Italy). 2-butanol and ?-butyl alcohol were pure products from Carlo Erba (Milano, Italy) and have been purified by multiple freeztplmpthaw cycles and evaporated under vacuum. Ir spectra have been recorded at room or lower temperatures by a Nicolet MXI Fourier transform instrument, quipped with To whom correspondence should be a d d r d .
'On leave from Departamento de Quimica Inorganice, Facultad de
Cicnciu Quimicar, Univmidad de Salamanca, Plaza de loa Caidos, E-37008 Salamanca, Spin.
0022-3654/91/2095-5541$02.50/0
a conventional evacuation/gas-manipulationramp (1VSTom) 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 The interaction of C4 olefins with the vanadia-titania surface has been investigated with two different procedures. In a series of experiments, adsorption has been carried out at room temperature, and temperature was later increased progressively under evacuation. In another series of experiments, butenes are adsorbed at 150 K and the spectra of the adsorbed species were recorded upon evacuation at increasing temperatures from 150 up to 673 K. Adsorption of o-Butenes. The spectra of the adsorbed species resulting from adsorption of 1-butene at room temperature on vanadia-titania and their evolution by heating under vacuum are reported in Figure 1. The spectrum recorded at room temperature before heating can be compared with those relative to adsorption of cis- and trans-2-butene, reported in Figure 2. Several features are present in all three cases and correspond to CH3 asymmetric deformations/CH2 scissorings (complex band in the 1480142O-cm-' region, main maximum at 1462 cm-'), CH, symmetric deformations (complex band in the 1400-136O-cm-' region), C-H bending (1332 cm-') and C-O/C-C stretchings (three maxima at 1115,1108, and 1096 cm-I) of secondary butoxide species. They correspond in fact to bands arising from 2-butanol adsorbed in the same conditions. The behavior of linear C4 olefins, consequently, is parallel to that of the C3olefin propylene, giving easily an addition of an VOH group following the Markovnikov rule, resulting in 2-butoxide species. Weak broad bands formed in all cases, already at room temperature in the region 170-1500 cm-l correspond to oxidation compounds. Also, the spectra in the vcH region are almost identical in the three cases (Fig. 3a for 1-butene). (1) Wainwright, M.S.; Foster, N. R. Catal. Rev.Sei. Eng. 1979,19,211. (2) Gellings, P. J. In CatalysiG The Royal Society of Chemistry: London, 1985, Vol. 7, p 105. (3) Bond, G. C.; Flamerz, S.; Sului, R. Discuss. Faraday Soc. 1989, No. 87, 65. (4) Bond, G.C.; Sarkany, A. J.; Parfitt, G.D. J. Catal. 1979, 57,476. (5) Busca, G.; Marchetti, L.; Centi, G.;and Trifirb, F. J . Chem. Soc., Faraday Trans. I 1985,81, 1003. (6) Slinkard, W. E.;and Degroot, P. B. J. Catal. 1981, 68, 423. (7) Garcia Fierro, J. L.; Arrua, L. A.; Lopez Nieto, J. M.;Kremenic, 0. Appl. Calal. 1988,37, 323. (8) Srivastava, R. D.Heterogeneous Catalytic Science; CRC Press: Boca Raton, FL, 1988. (9) Sanchez Escribano, V.;Busca, 0.;Lorenzelli, V. J . Phys. Chem. 1990, 94, 8945. (Part 2 of this series.) (10) Sanchez Epcrbano, V.; Busat, 0.;Lorenzclli,V. J. Phys. Chem. 1990, 94, 8939. (Part 1 of this series.) (11) B u m , G.; Ramis, G.;Lorenzelli, V. J . Mol. Catal. 1989, 55, 1.
Q 1991 American Chemical Society
5542 The Journal of Physical Chemistry, Vol. 95, No.14, 1991
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Figwe 1. FT-IR spectra of adsorbed species arising from adsorption of I-butene on vanadia-titania: (a) at 300 K (b) with succespive evacuation at room temperature: (c) following successive heating under evacuation at 423 K; (d) at 473 K (e) at 523 K.
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Figure 2. FT-IR spectra of adsorbed species arising from adsorption of (a) cis-2-butene and (b) rrans-2-buteneat 300 K,followed by evacuation at 300 K.
Few additional features are detected in the case of 1-butene adsorption at room temperature (Figure la): a sharp band at 1638 cm-' is due to adsorbed unreacted 1-butene ( v M l 2 ) and disap pears by evacuation, while a band is also evident at 1160 cm-I. This absorption is almost nonexistent in the case of adsorption of 2-butenes at room temperature, but it disappears quickly also in the case of 1-butene upon heating. Heating under vacuum causes very similar transformations of the adsorbed species arising from adsorption of the three linear butenes. The spectra relative to 1-butene adsorbed species are shown in Figure 1, and are representative of what happens also for 2-butenes. A sharp band grows at 1678 cm-I simultaneously with the decrease of the bands, cited above, characterizing the sec-butoxide species: in particular the CH deformation at 1332 cm-' and the uco/vcc complex near 1100 cm-' progressively disappear. After heating at 423 K,the band at 1678 cm-I, due to C - 0 stretching of methyl ethyl ketone, raises its maximum intensity when the bands of sec-butoxides have disappeared. Simultaneously, relatively sharp bands at 1615 and 1560 cm-' raise also their maximum. Further heating (Figure 5d,e) causes the sudden disappearance of the above bands, and the further (12) B u n , G.; Ramis, G.; Lorenzelli, G.; Janin, A,; Lavalley, J. C. Spectrochlm. Acta 1987, 43A, 489. ( 13) Bsllamy, L. J. The In/rord Spectra of Complex Molecules, 2nd ed.; Chapmann and Hall: London, 1980; Vol. 11.
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. Figure 3. FT-IR spectra (ucHregion) of adsorbed species arising from adsorption of I-butene: (a) adsorption at 300 K followed by evacuation at 300 K; (b) adsorption at 150 K followed by evacuation at 150 K (c) absorption at 150 K followed by evacuation at 250 K. wevcnumbars
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growth of strong bands at 1535 cm-' (broad, having a shoulder near 1600 cm-I) and in the region 1480-1380 cm-* (showing a sharp strong component at 1448 cm-I, a broad component near 1430 cm-I, and a very weak multiple component at 1375, 1368, 1355 cm-I). These complex absorptions are certainly due to different carboxylate species (asymmetric and symmetric COO stretchings near 1540 and 1440 cm-I), to which acetate species, characterized by CH3 deformation vibrations at 1448 and 1355 cm-' certainly participate. Heating at temperatures higher than 573 K causes the decomposition of carboxylate species with the progressive decrease of all bands detected. The above results indicate that the more evident reaction pathway for C4linear olefins on vanadia-titania is parallel to that already established for propylene and ethylene?JO The same alkoxide species are formed by addition of a VOH species to the three isomeric linear butenes. These alkoxides easily undergo oxidative dehydrogenation at the expense of oxidized V centers (probably W O H species) to the corresponding ketones (in this case methyl ethyl ketone) that can also give enolic compounds. According to previous results, these enolic compounds are likely responsible for the bands at 1615 and 1560 cm-I. Decomposition of these ketones (possibly uia the enolate species) consists of oxidative breaking of C-C bonds, producing carboxylate species. Breaking at C2-C3gives acetates that can either desorb as acetic acid, which is the main selective product in flow-reactor catalytic oxidation? or further decompose to C02. As remark4 before, some additional absorption detected after 1-butene adsorption at room temperature indicates that only in this case is some other compound formed, although it decomposes readily. The only band evident of this species is at 1160 cm-I. As will be shown below (Figure 8,d), this band coincides with the most intense band of adsorbed t-butoxide species, arising from isobutene impurities in 1-butene gas or produced by 1-butene skeletal isomerization. Alternatively, this band could be assigned to a skeletal vibration generally rather intense in the spectrum of a,&unsaturated carbonyl compounds. The origin of this band has not been investigated further. The spectrum resulting from contact of 1-butene with the surface at 150 K corresponds to that of the intact weakly adsorbed or liquid-like molecule (Figures 3b and 4a).I2 By warming up to 250 K,the olefin desorbs, leaving a different species whose spectrum is reported in Figures 3c and 4b. This spectrum shows a complex CH, asymmetric deformation band having a shoulder at 1478 cm-' and the maximum at 1472 cm-' but also a broader component near 1440 cm-'. At lower frequencies, a band constituted by a sharp strong feature at 1370 cm-' (probably a
The Journal of Physical Chemistry, Vol. 95, NO.14, 1991 5543
Reactivity of Vanadia-Titania Catalysts toward Olefins
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Figure 4. FT-IR spectra of adsorbed species arising from adsorption of I-butene at 150 K on vanadia-titania and successive evacuation: (a) at 150 K (b) at 250 K (c) at 330 K; (d) at 373 K. symmetric methyl deformation) and a weaker absorption at 1390-1380 cm-', is observed together with two bands at 1240 and 1 150 an-'. The spectrum of this species in the wcHregion (Figure 3c) is dominated by bands typical of methyl groups (2958 and 2870 cm-l, asymmetric and symmetric stretchings) and does not contain the feature typical of saturated methylene groups (asymmetric stretching near 2925 cm-'). Other weaker absorp tions (3025 an-',very weak, 2995 an-',shoulder) are also present, probably due to unsaturated =CH2 and/or =CH- groups. No bands are detected in the region near 1100 cm-', evidencing the absence of C-O bonds. We must conclude that this species corresponds to an hydrocarbon entity. Nevertheless, the spectrum does not scem to be compatible with that of 1-butene po1~mers.I~ Correspondingly, heating of this species does not cause the formation of carbonyl compounds (no bands near 1680 cm-l). On the contrary, bands typical of carboxylate species grow by heating under vacuum (broad features at 1550 and 1400 cm-l, Figure 4c,d), but features of acetates are absent or very weak. At temperatures higher than 473 K additional bands at 1865 and 1790 cm-' also appear. These bands are typical of cyclic anhydrides (symmetric and asymmetric C 4 stretching of the 0-cO - C 4 system1)and closely correspond to those of adsorbed maleic anhydride." A broad component near 1620 cm-l is also detected near 423 K. We previously found" that adsorbed furan species, either formed by oxidation of butadiene or arising from adsorption of furan itself, are responsible for a broad band at this frequency. The spectra detected in these conditions closely resemble those observed after butadiene adsorption on the same surface." We conclude that a second pathway is detected under these conditions, different from what we have discussed previously, resulting from the first run of experiments. The key intermediate in this pathway would be the hydrocarbon entity responsible for the spectrum of Figure 4b. This compound certainly contains a methyl group responsible for the bands at 2958,2870,1472,1370 cm-'. The other low-frequency bands of these species resemble those that have been detected after propylene adsorption in the same conditions1° and suggest an assignment to an anionic or radical-like methyl-allyl surface species, CH3--CH=CHrrCH2. According to literature data, tentative although reasonable assignments can be given to the bands at 1440 and 1150 cm-l to the asymmetric and symmetric C=C=C stretching, respectively. In fact, these modes have been detected at 1458 and 1192 cm-I for *-bonded allyl species, such as in the case of C3H,PdCI2ISand (14) Goldbach, G.; Peitscher, G. J . Polym. Sci. 1968, E6,783.
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Figure 5. FT-IR spectra of adsorbed species arising from.adsorptionof isobutene on vanadia-titania: (a) at 150 K (b) with succaive warming under evacuation at 210 K, (c) at 330 K.
at 1477 and 1242 cm-l for the free allyl radical.I6 We assigned bands at 1468 and 1165 cm-' to the same modes of an allyl species in our previous paper concerning propylene adsorption and oxidation.1° The bands at 3015,2995, 1390, and 1240 cm-' can be assigned to stretching and deformation modes of unsaturated =CH2 and/or =CH- groups. From the chemical point of view, the identification of this intermediate as a methyl-allyl species agrees with its behavior. In fact, loss of a second hydrogen atom would give butadiene. Previous studies evidenced that butadiene reacts easily at room temperature with surface vanadyl species, giving furan and, later, maleic anhydride. The spectra we observe after low-temperature adsorption of 1-butene followed by evacuation and further heating agree with those observed after oxidative adsorption of butadiene" and give unequivocal evidence of the formation of maleic anhydride with an indication of the intermediate formation of furan. Our experiments consequently provided evidence for two different and independent pathways for 1-butene transformation. In the first series of experiments, where the surface species resulting from room-temperature adsorption are progressively heated, compounds activated at C2 (sec-butoxides, methyl ethyl ketone, acetates probably desorbing in part as acetic acid) are predominant. In the second series of experiments, where adsorbed species produced at 150 K are progressively warmed and heated, a second path is instead evident, giving compounds activated at CI-C4 (methylallyl, butadiene, furan, maleic anhydride). It is probable that the first step for C2 functionalization (Le., addition of a VOH group to C - C ) is slower than that giving Cl-C4 functionalization (hydrogen abstraction). Consequently, by carrying out adsorption at 150 K followed by evacuation, the hydrogen abstraction product can be isolated, and its evolution evidenced. If adsorption is carried out at room temperature, the spectroscopic features of the products functionalized at C2 predominante and mask the intermediates of the alternative pathway. The spectra recorded after low-temperature adsorption of 2butenes appear to be a mixture of those recorded starting from 1-buteneadsorption at room and lower temperatures. Compounds functionalized both at C2 and at CI-C4are detectable together. This probably reflects the higher reactivity of the C = C bond of internal olefins with respect to the terminal olefin 1-butene toward electrophilic attack by a VOH group. Alternatively, it can also reflect the lower reactivity of allylic methyl with respect to allylic methylene groups to hydrogen abstraction. Adoorption of Isobutene. The spectrum of the adsorbcd species arising from contact at 150 K of activated vanadia-titania with (151 Sourisscau. C.: Pasauier. B. Can. J. Smctrosc. 1973. 18. 91. (16) Maier, G.;'Reisena;er, H. P.; Rohde,'B.; Dehnicke, K. Chrm. Ber. 1983, 116,132.
5544 The Journal of Physical Chemistry, Vol. 95, No. 14, 1991
Sanchez Escribano et al.
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Figure 6. FT-IR spectra (pCH region) of adsorbed species arising from adsorption of isobutene on vanadia-titania: (a) at 150 K, (b) with successive warming under evacuation at 210 K.
isobutene gas (Fi ures Sa and 6a) corresponds closely to that of liquid isobutene,A with very small frequency shifts. Warming under evacuation up to 210 K (Figures 5b and 6b) results in a complete change of the spectrum. The spectroscopic features of isobutene as such are no more present at all. Nevertheless, the spectrum is still certainly due to a hydrocarbon species. The absorptions in the methyl symmetric deformation region (where two components are evident at 1392 and 1368 cm-I, the lower frequency one being the more intense) and in the C.C stretching region (with an intense band at 1238 cm-' and a weaker one at 1203 cm-') are typical of the t-butyl group.13 Also the complex feature in the asymmetric methyl deformation region (four components at 1478, 1468, 1450, and 1440 cm-') as well as in the CH stretching regions (Figure 6b; main maxima at 2958,2900, and 2875 cm-I) indicates the presence of several methyl groups. Moreover, the sharp band at 1642 cm-' indicates the presence of according to the dea terminal C==C bond (vc- ~tretching)'~ tection of a weak band at 3075 cm-'that is evidence of an olefinic CHF group (asymmetric CH2stretching). The overall spectrum agress with that of the isobutene dimer 2,4,4-trimethylpent-lene.'*J9 This compound is typically formed by proton-catalyzed cationic dimerization of isobutene, but can also be the result of Lewis-acid-catalyzed dimerization. This reaction, occurring at so low a temperature as 200 K evidences the stronger reactivity of the branched olefin isobutene with respect to the linear ones. After evacuation at room or slightly higher temperature, these bands decrease in intensity and are also slightly modiied, showing that a different species is present, whose spectral features were previously masked by those of the isobutene dimer. In particular, the methyl asymmetric deformation band, formerly complex, is now broad and almost componentless at 1475 cm-', while in the vcC region, a single band at 1230 cm-'is now detected. Morever, the bands associated with the olefin bond are no more evident. The spectrum is consistent with that of isobutene polymers.20.2' It is then reasonable to propose that warming from 150 to 210 K results in the oligomerization of isobutene forming not only the dimer but also higher oligomers. Progressive heating of the sample where these oligomers are adsorbed causes progressively their oxidation (Figure 7) with the growth of bands typical of carboxylates, having broad maxima at 1550 and 1430 cm-I. However, weaker sharp bands are also evident after heating at 570 K at 1870 (weaker) and 1790 cm-' (stronger; Figure 3b). typical of cyclic anhydride^.'^ These compounds can form when hydrocarbons having chains of four ~~~~~
~
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Luttke, W.;Braun, S . Ber. Bunsen-Grs. Phys. Chem. 1%7, 71,34. (IS) Pouchcrt, C. J. The Aldrich Library of m-IR spectra. 1985. (19) Tohra. N.;Matsuda, M.;Shirai, 1. Bull. Chem. Soc. Jpn. 1%2,38. 371. (20) Kozyreva, M.S . Opt. Speclrosc. 1959, 6, 303. (17)
(21) An Infrared Spectroscopy Atlas for the Cwtfng Industrv; Federation of Societica for Coating Technology: Philadelphia, Pa, 1980 p 321.
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Figure 7. FT-IR spectra of adsorbed species arising from adsorption of isobutene at 150 K on vanadia-titania and successive warming to mom temperature and heating under evacuation to (a) 373 K and (b) 623 K.
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Figure 8. FT-IR spectra of adsorbed species arising from adsorption of isobutene on vanadia-titania: (a) at room temperature; with successive heating under evacuation to (b) 373 K and (c) 438 K. Spectrum d is for adsorbed species arising from t-butyl alcohol adsorption on vanadia-titania at room temperature.
or more C atoms are oxidized on the vanadia-titania surface, as observed for butadiene." So their detection implies that oxidation of isobutene oligomers. The spectrum of the adsorbed species arising from the contact of vanadia-titania with isobutene at room temperature (first experimental procedure, Figure 8a) is partly different from that recorded after adsorption at 150 K and successive slow warming to room temperature. In fact, together with the bands assigned to oligomeric species, a strong band is also observed centered at 1160 cm-' but having also shoulders at 1175 and 1190 cm-'. This band is the most intense one when t-butyl alcohol is adsorbed on vanadia-titania (Figure 8d) and is due to the V ~ / V C Ccoupled stretchings of t-butoxy The other bands of this species are nearly superimposed on those of isobutene oligomers. (22) Ramia, G.; B u m , G.; Lorenzelli, V.J. Chem. Soc., Faraday Tram. 1 1987, 83, 1591.
J. Phys. Chem. 1991,95, 5545-5551 Moreover, other absorption bands are evident at 1580 cm-' (probably the most intense asymmetric COz- stretching of carboxylates) and near 1620 c d . The latter band is not detected after adsorption and oxidation of t-butyl alcohol. This would support a tentative assignment of this band to products of allylic oxidation. Further heating at 373-423 K (Figure 8b,c) causes also the growth of a sharp shoulder at 1675 cm-I, appearing with close correlation with the disappearance of the absorptions cited above due to t-butoxide species. The band at 1675cm-'falls at the same frequency as the vco band of acetone adsorbed on this surface.'O t-butoxide species probably decompose with loss of methane, giving acetone. Accordingly, acetone and acetic acid, which is formed by its further are detected as products of isobutene catalytic oxidation on similar catalysts.' At higher temperatures, the predominant features are those of adsorbed carboxylates, while, by use of this experimental procedure, cyclic anhydrides are not observed.
Conclusions The results concerning the adsorption of n-butenes evidence the existence of two oxidative pathways. In the case of 1-butene these two pathways can be separated by working under different conditions. One gives, via hydration-oxidation, compounds oxidized at Cf, as already shown from ethylene and propylene oxidation. However, ketones can also undergo oxidative cleavage of the C-C bond, possibly through previous enolization, giving finally acetate species, which can be obtained by oxidation of both C3 and C4 olefins. A second pathway is detected, involving allylic hydrogen abstraction. We have an indication here suggesting that this reaction occurs faster on allylic methylene groups (1 -butene) than on allylic methyl groups (2-butenes). This reaction in the case of propylene
5545
finally produces products functionalized at Cl, acrolein, and/or acrylic acid. In the case of C4 compounds instead, allylic hydrogen abstraction represents the first step for oxidative dehydrogenation, giving butadiene, furan, and finally, maleic anhydride. The branched olefm isobutene shows as expected a much higher reactivity toward electrophilic attack from a proton, resulting in polymerization at low temperature as a competitive reaction with respect to alcoholate formation. The ternary alcoholate t-butoxide at higher temperature decomposes giving acetone and finally acetate species. Allylic hydrogen abstraction is detected almost indirectly in this case. Only traces of products functionalyzed at CI (such as methacrolein or their overoxidation compounds) can be found. The infrared investigation presented here allowed us, as in the previous cases concerning C2and C3 olefins, to detect and justify on mechanistic bases the formation of the main selective oxidation products of C4 olefins. The data obtained with Cz, C3,and C4 olefins constitutes a self-consistent set of data that also accords with the well-known chemistry of simple olefins. It is concluded that active sites for olefin selective oxidation are present on vanadia-titania surfaces, although the excessive reactivity of the surface toward the selective oxidation products reduces the performance of the catalyst. The selective oxidation products are formed on the clean surface without gas-phase oxygen, so confirming that Mars-Van Krevelen-type mechanisms occur. 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). R w t r y No. V,O,, 1314-62-1; Ti02, 13463-67-7; I-butene, 106-98-9; cis-2-butene, 590-18-1; rranr-2-butene, 624-64-6; isobutene, 115-11-7.
Dynamlc Phase Behavlor of Base MetaVNoble Metal Catalyst Partlcles. 3. Graphite-Supported I ron/Irldlum John W.Cobes, 111, and Jonathan Phillips* Department of Chemical Engineering, The Pennsylvania State University, 133 Fenske Laboratory. University Park, Pennsylvania 16802 (Received: August 1, 1990)
As pert of a program designed to study the structure of bimetallic particle phase behavior and surface chemistry, iron-iridium particles supported on graphite were studied by true differential calorimetry, Mossbauer spectroscopy, X-ray diffraction, and transmission electron microscopy. The phase behavior of the supported particles was determined to be a complex function of the sequence of oxidation and reduction treatments. One point stands out as being of particular significance to the catalysis community: at least two chemically different reduced surfaces were produced. One reduced surface was iron-like, and the other the surface of a true iron-iridium alloy. The type of dynamic phase behavior observed is similar to the behavior of iron-rhodium particles extensively studied in earlier work.
Introduction Although bi- and trimetallic catalysts are important industrially, particularly in petroleum the phase behavior of the systems is poorly understood. A better understanding could lead to new applications of these catalysts. Indeed, only recently it was shown that bimetallic catalysts, specifically iron-rhodium, can have two distinct reduced surfaces." Previously, it was thought that base metallnoble metal bimetallic catalysts had only a single reduced surface structure.'* Each of these surfaces can have distinct activity and selectivity. For example, studies of 1-butene hydrogenation over the two surfaces of iron-rhodium To whom correspondence should be addressed.
0022-3654191/2095-5545S02.50/0
showed one surface state to be highly selective for isomerization and the other to be an excellent hydrogenation ~ a t a l y s t . ' ~This (1) Sinfelt, J. H.Bimerallic Caralysrs: Discoucries, Concepts utut A p plications; Wiley: New York, 1983. (2) Sinfelt, J. H. US. Patent 3,753,368, 1976. (3) Sinfelt, J. H.Platinum Mer. Chem. 1976, 20, 114. (4) Kluksdahl, H.E. U.S. Patent 3,415,737, 1968. (5) Pullitzer, E. L.Plarinum Mer. Rco. 1972, 16, 72. (6) Burch, R. Plarinum Met. Reu. 1978, 22, 57. (7) Rasscr, J. C. PIarinumlridiwn Re/orming Caralysrs; Delft University Press: Delft, The Netherlands, 1977. (8) Taylor, K. C.; Schlatter, J. C. J. Carol. 1980,63, 53. (9) Schlatter, J. C.; Taylor, K. C. J . Coral. 1977, 19, 42. (10) Schuit, 0 . C. A.; Gatca, B. C. AIChE 1. 1973, 19,417.
0 1991 American Chemical Society
