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Mechanism of Dehydrocyclization of 1-Hexene to Benzene on Cu3Pt(111): Identification of 1,3,5-Hexatriene as Reaction Intermediate Andrew V. Teplyakov,* Alejandra B. Gurevich, Eva R. Garland, and Brian E. Bent Chemistry Department, Columbia University, New York, New York 10027
Jingguang G. Chen Exxon Research and Engineering Company, Annandale, New Jersey 08801 Received July 3, 1997. In Final Form: October 2, 1997 We report here ultrahigh vacuum studies of the dehydrocyclization reaction of submonolayer coverages of 1-hexene to benzene on a Cu3Pt(111) single crystal surface, using reflection-absorption infrared spectroscopy (RAIRS), near edge X-ray absorption fine structure (NEXAFS) studies, and temperatureprogrammed reaction/desorption (TPR/D) spectrometry. As discussed in a previous TPR/D paper, at surface coverages up to 13% of monolayer saturation, 1-hexene forms benzene on a Cu3Pt(111) surface. Selectivity to benzene formation is 70 ( 10%, with the remaining 30 ( 10% of the adsorbed 1-hexene dehydrogenating irreversibly to surface carbon and H2. For higher coverages, molecular desorption commences. Spectroscopic identification of the intermediates of the reaction of 1-hexene and other model compounds, such as a 1,3,5-hexatriene, with a Cu3Pt(111) surface suggests that 1-hexene and 1,3,5-hexatriene have a common intermediate, and this intermediate has been identified as a rehybridized hexatriene species. Other model compounds, such as trans-3-hexene, have also been used to provide further understanding of the mechanism of the aromatization reaction.
1. Introduction Despite the practical importance of the catalytic transformation of aliphatic hydrocarbons into aromatic molecules, the mechanistic aspects of this process have not been studied thoroughly. It can be largely explained by the absence of experimental techniques and conditions amenable to such studies. The majority of the industrial catalysts for the aromatization reaction to date are primarily based on supported platinum.1-4 Hence, the major experimental technique for characterization of such systems was chromatography. Other techniques, such as infrared spectroscopy5 and temperature-programmed reaction,6 were rarely used because of the experimental difficulties. On the other hand, the variety of the available ultrahigh vacuum (UHV) techniques, capable of studying mechanisms and intermediates of different reactions, often require single crystal surfaces as model catalysts. Platinum in the form of a single crystal has been extensively studied as a catalyst for the dehydrocyclization reaction.7-12 * Author to whom the correspondence should be addressed: New York University, Chemistry Department, 100 Washington Square East, Room 1018, New York, NY 10003; tel, (212) 998-8441; fax, (212) 260-7905; e-mail,
[email protected]. (1) Joshi, P. N.; Bandyopadhyay, R.; Awate, S. V.; Shiralkar, V. P.; Rao, B. S. React. Kinet. Catal. Lett. 1994, 53, 231-236. (2) Dai, L.-X.; Sakashita, H.; Tatsumi, T. J. Catal. 1994, 147, 311321. (3) Kharson, M. S.; Dzigvashvili, T. R.; Dolidze, A. V.; Kiperman, S. L. Kinet. Catal. 1991, 32, 344-348. (4) Zheng, J.; Dong, J.-L.; Yan, A.-Z. Appl. Catal. A 1995, 126, 141. (5) Dimitrov, C.; Bezouhanova, T. P.; Kovacheva, P. H.; Dineva, R. K. Dokl. Bolg. Akad. Nauk 1979, 32, 1231-1234. (6) Zimmer, H.; Rozanov, V. V.; Sklyarov, A. V.; Paa´l, Z. Appl. Catal. 1982, 2, 51. (7) Davis, S. M.; Zaera, F.; Somorjai, G. A. J. Catal. 1984, 85, 206223.
Although these preliminary studies provided some insight into reactivity of platinum toward the C-C and C-H bond activation, results related to the aromatization reaction were complicated due to existence of other competing reactions (isomerization, hydrogenolysis, etc.) on a platinum surface. One way to avoid these competing reactions was suggested in a previous TPR/D study of 1-hexene on a Cu3Pt(111) surface.13 As pointed out by de Jongste, reactivity and selectivity of platinum surface toward the dehydrocyclization reaction could be varied by alloying it with copper,14-16 and the Cu3Pt(111) surface was suggested to possess the properties necessary for this reaction to occur on a single crystal surface under ultrahigh vacuum conditions. Our previous temperature-programmed reaction/desorption (TPR/D) studies of 1-hexene reaction with this surface13 showed that 1-hexene chemically reacts with a Cu3Pt(111) surface at low coverages, while upon increasing coverage molecular desorption of 1-hexene commences. In the course of this reaction 70 ( 10% of reacted 1-hexene produces benzene and hydrogen, with the other 30 ( 10% decomposing to leave carbon deposited on the surface as confirmed by Auger electron spectroscopy (8) Joyner, R. W.; Lang, B.; Somorjai, G. A. J. Catal. 1972, 27, 405415. (9) Gillespie, W. D.; Herz, R. K.; Petersen, E. E.; Somorjai, G. A. J. Catal. 1981, 70, 147-159. (10) Dauscher, A.; Garin, F.; Maire, G. J. Catal. 1987, 105, 233-244. (11) Garin, F.; Aeiyach, S.; Legaire, P.; Maire, M. J. Catal. 1982, 77, 323-337. (12) Garin, F.; Maire, G. Acc. Chem. Res. 1989, 22, 100-106. (13) Teplyakov, A. V.; Bent, B. E. J. Phys. Chem. B 1997, 101, 90529059. (14) de Jongste, H. C.; Kuijers, F. J.; Ponec, V. Proc. Int. Cong. Catal., 6th 1977, 2, 915-926. (15) de Jongste, H. C.; Ponec, V. J. Catal. 1980, 63, 389-394. (16) de Jongste, H. C.; Ponec, V.; Gault, F. G. J. Catal. 1980, 63, 395-403.
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(AES). Comparison of the rates of benzene and hydrogen evolution from the aromatization of 1-hexene on a Cu3Pt(111) surface with those of reaction of cyclohexene, 1,3cyclohexadiene, 1,4-cyclohexadiene, 1,3-hexadiene, and 1,3,5-hexatriene suggests that the rate-determining step in the dehydrocyclization of 1-hexene is not the dehydrogenation step. Instead, the cyclization accompanied by the loss of two hydrogen atoms is identified as the rate-limiting step.13 However, the previous TPR/D studies13 did not answer the following two questions: (1) does the dehydrogenation of 1-hexene lead to the formation of 1,3,5-hexatriene or do these two compounds have a common intermediate; (2) what role does the cis-trans isomerization with respect to the internal double bond (for example, in 1,3,5hexatriene) play in the dehydrocyclization reaction. The main objective of the present paper is to provide mechanistic insights regarding these two questions. Studies presented in this paper have been performed in UHV conditions and made use of near edge X-ray absorption fine structure (NEXAFS), and reflectionabsorption infrared spectroscopy (RAIRS) to identify surface intermediates and products of the reaction of 1-hexene on a Cu3Pt(111) single crystal surface. Because this aromatization process is a multiple step reaction, various relevant model compounds were used to confirm the reaction mechanism. The aromatization of 1-hexene was compared with reactions of 1,3,5-hexatriene and trans3-hexene, as well as with the adsorption of benzene on a Cu3Pt(111) surface. Our results suggest that 1-hexene rehybridizes on a Cu3Pt(111) surface at temperatures as low as 95 K to form most likely a di-σ-bonded species. With an increase of the surface temperature, this intermediate loses four hydrogen atoms at temperatures below 350 K and forms a new intermediate that has been identified as hexa-σ-bonded triene by comparison with the intermediate formed by 1,3,5-hexatriene at the same temperature. Upon further heating this intermediate undergoes cyclization, promptly releasing the product benzene to the gas phase. It should be pointed out that, unlike 1-hexene, 1,3,5-hexatriene keeps its π-system intact at 95 K, which can probably be explained by a high degree of conjugation in this compound. After being heated to 200 K it rehybridizes to form an intermediate which can be best described as a hexa-σ-bonded species. The intermediates formed on a Cu3Pt(111) surface by both 1-hexene and 1,3,5-hexatriene are practically identical from the RAIRS and NEXAFS results, and they are very different from benzene physisorbed on the same surface. As supported by NEXAFS studies, benzene stays intact on a Cu3Pt(111) surface up to the temperature of the molecular desorption. Even though comparison with literature studies of benzene on other surfaces suggests that its π-system is slightly perturbed by the interaction with a Cu3Pt(111) surface,17 our results show that the π-orbitals of benzene do not undergo significant rehybridization, resulting in a reversible adsorption of benzene on the surface without decomposition. 2. Experimental Section The RAIRS and temperature-programmed reaction/desorption (TPR/D) results presented here were obtained in a UHV chamber with background pressure of ∼5 × 10-10 Torr equipped with an Auger electron spectrometer (AES), a high-resolution electron energy loss spectrometer, a differentially-pumped quadrupole (17) Sto¨hr, J. NEXAFS Spectroscopy; Springer-Verlag: New York, 1992; Vol. 25.
Teplyakov et al. mass spectrometer, an ion gun for surface cleaning, a low-energy electron diffraction apparatus (LEED), and a setup for reflectionabsorption infrared spectroscopy. A detailed description of this vacuum system is published elsewhere.18 The Cu3Pt(111) single crystal was obtained from MaterialTechnologie & Kristalle GmbH (Ju¨lich, Germany) as a circular disk (1 cm diameter and 2 mm thickness), polished to a mirror finish on one of the (111) surfaces. The crystal was mounted on a resistive heating element attached to a manipulator. The temperature of the crystal was measured by a chromel-alumel thermocouple whose junction was wedged into the hole on the side of the crystal. Crystal temperatures as low as 110 K could be achieved by cooling with liquid nitrogen. Heating was provided by a dc power supply (Hewlett-Packard, 6291A), controlled by a temperature programmer (Eurotherm 818 P). A temperature ramp of 3 K/s was used in the temperature-programmed desorption (TPD) studies. The crystal was cleaned as described in ref 19 by Ar+ sputtering at 550 K for 15 min followed by annealing in UHV at 840 K for 20 min to free the surface of carbon, sulfur, and oxygen as confirmed by AES. A (1×1) diffraction pattern was observed by LEED.20 All hydrocarbons used here were purchased from Aldrich and had a purity not less than 99%. They were purified by several freeze-pump-thaw cycles before introduction into the chamber, and the purity of the dosing gas was confirmed in situ by mass spectrometry. All exposures are reported in langmuirs, where 1 langmuir is 10-6 Torr‚s. For the TPD experiments, the adsorbate-covered surface was positioned line-of-sight to the mass spectrometer, about 2 mm from a 2 mm diameter sampling aperture, which was used to detect selectively the molecules desorbing from the center of the single crystal surface. For the RAIRS studies, the infrared light from a Fourier transform infrared (FT-IR) spectrometer (Perkin-Elmer 1800) is focused through a differentially pumped barium fluoride window onto the sample at an incidence angle of 82.5° from the surface normal. The reflected light in the specular direction is collected by a narrow band mercury cadmium telluride (MCT) detector (EG&G Judson, Model No. J15-D14-M216-S-01M-YS-WE, Ge window, D* > 4 × 1010). The optical path is purged with CO2and H2O-free air generated by an FT-IR purge gas generator (Balston 75-62). Each spectrum presented here is taken with an 8 cm-1 resolution and represents an average of several thousand scans (∼4 min/1000 scans). The number of scans corresponding to each spectrum is indicated in the figure caption. The near-edge X-ray absorption fine structure (NEXAFS) measurements were conducted on the U1 beamline of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. A detailed description of the experimental end station apparatus has been given elsewhere.21 The twostage UHV chamber is equipped with an ion sputtering gun, a quadrupole mass spectrometer, and an Auger electron spectrometer. All spectra were recorded with a partial electron yield (18) Jenks, C. J.; Bent, B. E.; Bernstein, N.; Zaera, F. J. Am. Chem. Soc. 1993, 115, 308-314. (19) Castro, G. R.; Schneider, U.; Busse, H.; Janssens, T.; Wandelt, K. Surf. Sci. 1992, 269/270, 321-325. (20) The surface structure and composition of Cu3Pt(111) has recently been studied by Wandelt and co-workers, and their early papers reported a (2×2) LEED pattern for this surface.20a-c This is the diffraction pattern that one would expect for ideal termination of the bulk lattice, and it corresponds to a lattice of isolated Pt atoms, each surrounded by six nearest neighboring Cu atoms in the plane. More recent papers from Wandelt et al. report a (1×1) pattern for Cu3Pt(111)20d,e which is the same as that observed here. The similarities between the bonding and reactions of CO and H2 on the surfaces with these two different LEED patterns suggest that in both cases the surface Pt atoms are surrounded by Cu even though long range order of the Pt atoms is absent on the (1×1) surface. Low-energy ion scattering studies indicate that the surface is enriched in copper by only 5%. (a) Schneider, U.; Busse, H.; Linke, R.; Castro, G. R.; Wandelt, K. J. Vac. Sci. Technol., A 1994, 12, 2069. (b) Linke, R.; Schneider, U.; Busse, H.; Becker, C.; Shro¨der, U.; Castro, G. R.; Wandelt, K. Surf. Sci. 1994, 307, 407. (c) Becker, C.; Shro¨der, U.; Castro, G. R.; Schneider, U.; Busse, H.; Linke, R.; Wandelt, K. Surf. Sci. 1994, 307, 412. (d) Shen, Y. G.; O’Connor, D. J.; Wandelt K.; MacDonald, R. J. Surf. Sci. 1995, 328, 21. (e) Shen, Y. G.; O’Connor, D. J.; Wandelt K.; MacDonald, R. J. Surf. Sci. 1995, 331-333, 746. (21) Fisher, D. A.; Colbert, J.; Gland, J. L. Rev. Sci. Instrum. 1989, 60, 1596.
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Figure 2. Comparison of RAIRS spectra for 1-hexene dehydrocyclization to benzene on a Cu3Pt(111) surface with TPR/D results for this reaction. The multilayer 1-hexene and 1,3,5hexatriene IR spectra (1000 scans each) are compared with a submonolayer 1,3,5-hexatriene IR spectrum and with the IR spectra for 0.5 langmuir exposure of 1-hexene, which were taken after briefly annealing the surface at the temperatures indicated in the TPR spectrum. Each of these submonolayer IR spectra represents the average over 2000 scans.
Figure 1. TPR/D studies of the dehydrocyclization reaction in 1-hexene (top) and 1,3,5-hexatriene (middle) compared to the molecular desorption of benzene (bottom) from a Cu3Pt(111) single crystal surface. detector with a retarding voltage of -200 eV. The resolution of the synchrotron monochromator was set at 0.4 eV near the carbon K-edge region. All NEXAFS spectra reported have been divided by the signals from a reference grid, which measures the incident beam intensity simultaneously with the NEXAFS spectra, and then by the corresponding ratio of the spectra of a clean surface taken at the same incidence angle. The validity of such a treatment has been discussed in detail previously.22
3. Results and Interpretation This section will be subdivided into four parts. Section 3.1 reviews the results of TPR/D analysis of 1-hexene, 1,3,5-hexatriene, and benzene interaction with a Cu3Pt(111) surface. Section 3.2 presents the RAIRS studies of these reactions and proposes the structure of the intermediate for the dehydrocyclization reaction. These results are substantiated by NEXAFS studies presented in section 3.3. Finally, the analysis of possible isomerization with respect to the double bond is given in section 3.4. 3.1. TPR/D Studies of 1-Hexene, 1,3,5-Hexatriene, and Benzene Interaction with a Cu3Pt(111) Surface. The detailed TPR/D studies of the interaction of 1-hexene, 1,3,5-hexatriene, and benzene with a Cu3Pt(111) surface are presented in ref 13 and are briefly summarized in Figure 1. 1-Hexene chemically reacts with a Cu3Pt(111) surface at low coverages (up to 0.5 langmuir or 13% of the 1-hexene monolayer saturation coverage), while upon increasing coverage molecular desorption of 1-hexene commences. In the course of this reaction 70 ( 10% of reacted 1-hexene produces benzene (evolved at 405 K) and hydrogen; the other 30 ( 10% decompose to leave carbon deposited on the surface as confirmed by AES. (22) Outka, D. A.; Sto¨hr, J. J. Chem. Phys. 1988, 86, 3539.
Unlike 1-hexene, 1,3,5-hexatriene only needs to undergo a cyclization step to evolve benzene, and no additional dehydrogenation is necessary. Tracing of m/e+ ) 80 shows no 1,3,5-hexatriene molecular desorption for the exposures up to 3.0 langmuirs, which corresponds to 75% of a monolayer saturation coverage. However, such a high reactivity is coupled with extremely low selectivity toward benzene formation, since the corresponding AES studies and TPR/D peak area analysis show that only 30 ( 10% of the hexatriene molecules reacted to actually form benzene at 406 K, with the other 70 ( 10% decomposing to leave carbon on a surface. Figure 1 shows that the interaction of benzene with a Cu3Pt(111) surface was reversible. At low exposures the desorption temperature of benzene was found to be 310 K. This temperature shifts down to 200 K upon increasing benzene exposure to 1 langmuir. Further increase of exposure results in a second desorption feature at 162 K (not shown) which is saturated at 3 langmuirs. Given that the desorption temperature of benzene from the second layer is ∼15 K lower (the results not presented here), this feature can be assigned to the desorption from a different site on the alloy surface. The major conclusion from results presented in Figure 1 and from other accompanying13 TPR/D studies is that dehydrocyclization of 1-hexene to benzene on a Cu3Pt(111) surface involves two steps: dehydrogenation below 300 K and cyclization at ∼400 K. 3.2. RAIRS Studies of 1-Hexene and 1,3,5-Hexatriene Interaction with a Cu3Pt(111) Surface. The next point to confirm is whether 1-hexene simply forms 1,3,5-hexatriene, which in turn undergoes cyclization at higher temperatures, or if 1-hexene and 1,3,5-hexatriene have a common intermediate in the dehydrocyclization reaction. Temperature dependence of RAIRS spectra, in the C-H stretch region, of 1-hexene has been analyzed and compared to the spectra of 1,3,5-hexatriene. Shown in Figure 2 is the spectrum taken after exposing the surface to 0.5 langmuir of 1-hexene at 150 K. This exposure corresponds to ∼13% of a monolayer coverage of 1-hexene, at which all 1-hexene molecules undergo chemical transformation at higher temperatures. When the surface is heated to 250 K, which is the temperature right before the onset of the low-temperature feature of hydrogen
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Figure 3. NEXAFS spectra of the multilayer coverages of benzene, 1,3,5-hexatriene, and 1-hexene taken at normal incidence.
evolution in TPR/D spectra, no changes are observed. After the surface is heated to 350 K, the temperature at which dehydrogenation step has been shown to be completed based on the H2 desorption peak,13 all C-H stretching features of the RAIRS spectra disappear. At the same time TPR/D studies show that at 350 K no hydrocarbon fragments have been desorbed from the surface.13 According to the surface dipole selection rule, this means that any hydrocarbon fragments present on a surface, after its exposure to 0.5 langmuir of 1-hexene and heating to 350 K, should have their C-H bonds approximately parallel to the surface. The RAIRS and TPR/D results in Figure 2 suggest that at 350 K 1-hexene converts to a dehydrogenated intermediate. The RAIRS spectra of 1,3,5-hexatriene at submonolayer and multilayer coverages are also presented in Figure 2. These spectra show that at a submonolayer exposure, 1,3,5-hexatriene does not exhibit any features in the C-H stretching region, whereas at multilayer coverages some features corresponding to the C-H stretches of this compound do show up in the RAIRS spectrum. Once again, this means that all hydrocarbon species, after exposing Cu3Pt(111) to submonolayer coverages of 1,3,5-hexatriene, have their C-H bonds approximately parallel to the surface. The submonolayer RAIRS spectra of 1,3,5-hexatriene did not show any change after the surface was heated up to 350 K (spectrum not shown). 3.3. NEXAFS Studies of 1-Hexene. 1,3,5-Hexatriene, and Benzene Interaction with a Cu3Pt(111) Surface. NEXAFS is a very useful technique in the determination of molecular orientation of unsaturated hydrocarbons on a surface, as well as for the degree of rehybridization of the π-orbitals of these compounds on
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Figure 4. NEXAFS spectra of the submonolayer coverage (1 langmuir) of benzene taken at normal incidence at 95 K and at glancing incidence at 95 and 180 K.
reactive surfaces. We will first show NEXAFS spectra of multilayer coverages of 1-hexene, 1,3,5-hexatriene, and benzene. These data are presented in Figure 3. In agreement with the literature data,17 benzene exhibits split π* and σ* transitions, whose energies are the same as those given in the literature.17 The C K-edge features of 1,3,5-hexatriene are quite similar to those of benzene. This is expected because hexatriene has a conjugated π-system that is very similar to that of benzene. On the other hand, the NEXAFS spectrum of 1-hexene is typical for a monounsaturated compound. The π* transition in 1-hexene, with respect to other C K-edge features, is not as intense as that in benzene or hexatriene. However, the σ*C-H transition in 1-hexene is much more intense, which is expected, because 1-hexene has more C-H bonds than the other two unsaturated compounds. The other σ* transitions can be assigned to C-C single bonds and CdC double bonds, whereas for benzene and 1,3,5hexatriene both these transitions were associated with the π-system (bonding and antibonding orbitals).17 Figure 4 presents the NEXAFS studies of the interaction of a benzene monolayer with a Cu3Pt(111) surface. Consistent with previous studies of benzene on unreactive metal surfaces, the spectra taken at the glancing incidence show an intense π* transition at ∼286 eV. This transition is very weak when the incident photon beam is normal to the surface, indicating that benzene molecules at submonolayer coverages (here ∼30% of a monolayer) are adsorbed on a Cu3Pt(111) surface with their π-orbitals perpendicular to the surface; i.e., the plane of the molecule is oriented parallel to the surface. The reverse angular dependence of intensity for the σ*1 and σ*2 transitions further supports this hypothesis. Comparison of the π*
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Figure 5. Temperature dependence of the NEXAFS spectra of the submonolayer coverages (2 langmuirs) of 1,3,5-hexatriene taken at glancing and normal incidence.
transition intensity in benzene on Cu3Pt(111) with the same transition in benzene on a Ag(110) surface (which is completely inert toward benzene decomposition23,24) and on a Mo(110) surface (where benzene decomposition takes place23,25) suggests that the π-system of benzene is only slightly perturbed by the Cu3Pt(111) surface. This conclusion is made based on a slightly lower intensity of a π* transition with respect to the height of the edge jump in benzene on Cu3Pt(111) than on Ag(110),23,24 while the shape and the peak position for this transition are practically the same on these two surfaces. On the other hand, on the reactive Mo(110) surface23,25 π* transition in benzene is shifted to higher resonance energies and the width of the peak is significantly greater than that observed in our experiments. Figure 4 also shows that the NEXAFS spectra of benzene taken at different temperatures do not show significant differences up until the temperature where molecular desorption of benzene commences, as indicated by the top spectrum which was recorded after heating the adsorbed layer to 180 K. NEXAFS studies of 1,3,5-hexatriene on a Cu3Pt(111) surface are shown in Figure 5. The π* transition evidenced at 95 K alters its intensity, shape, and peak energy upon heating the surface to 120 K. Analogous changes in the π* transition for different unsaturated compounds on reactive metal surfaces have been reported previously17,23,25 and were typically associated with the rehy(23) Liu, A. C.; Sto¨hr, J.; Friend, C. M.; Madix, R. J. Surf. Sci. 1990, 107. (24) Solomon, J.; Madix, R. J.; Sto¨hr, J. Surf. Sci. 1991, 255, 12. (25) Liu, A. C.; Friend, C. M. J. Chem. Phys. 1988, 89, 4396.
bridization of the π-system of these hydrocarbons. On the basis of the temperature dependence of NEXAFS spectra taken at both glancing and normal incidences for the surface temperatures varied from 120 to 350 K, we can conclude that the rehybridization process is complete at temperatures below 250 K and that the reaction intermediate existing on a surface at this temperature interval can be assigned to the hexa-σ-bonded triene structure. Figure 6 presents the NEXAFS studies of 1-hexene on a Cu3Pt(111) surface. 1-hexene rehybridizes upon adsorption at 95 K, as suggested by the observation that the π* transition in this compound has a wider peak shape, lower intensity, and higher peak energy than those in the multilayer, analogously to the previous studies on the reactive metal surfaces.17,23,25 Consistent with IR data presented in section 3.2, no changes occur on a surface precovered with 0.13 monolayer coverage of 1-hexene upon heating to 250 K. However, after heating to 350 K, certain changes are evidenced in the spectra taken both at glancing and normal incidences. These changes are associated with more intense σC-H transitions at normal incidence, which is consistent with IR studies, suggesting that at 350 K C-H bonds in the surface intermediate are approximately parallel to the surface. Finally, Figure 7 presents the comparison of the NEXAFS spectra for 1-hexene and 1,3,5-hexatriene taken both at normal and at glancing incidences after the surface is annealed to 350 K. The two sets of spectra show remarkable similarity, suggesting a common intermediate for the dehydrocyclization reaction for these two com-
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Figure 6. Temperature dependence of the NEXAFS spectra of the submonolayer coverages (0.5 langmuir) of 1-hexene taken at glancing and normal incidence.
pounds. Further explanation of these NEXAFS spectra is given in the discussion section. 3.4. TPR/D Studies of trans-3-Hexene Interaction with a Cu3Pt(111) Surface. Another question which has not been answered by previous studies is how the isomerization with respect to the internal double bond affects the rate of the dehydrocyclization reaction. This question did not arise during the studies of 1-hexene, because this compound has only one external double bond and all the transformations, including dehydrogenation, depend on its conformation and geometrical allocation on a surface. On the other hand, 1,3,5-hexatriene used in the studies presented here was purchased from Aldrich and predominantly consisted of trans-isomer. Due to the fact that the exact ratio of cis/trans isomers in that mixture was not known and that ∼70% of the reacted hexatriene at the submonolayer coverage decomposed on a Cu3Pt(111) surface, a conclusion about the effect of cis vs trans internal double bond on the rate and the mechanism of dehydrocyclization could not be made. In order to understand the mechanism of the dehydrocyclization reaction in a hydrocarbon where the geometry is given by a trans-isomer with respect to the internal double bond, the TPR/D studies of trans-3-hexene interaction with a Cu3Pt(111) surface have been performed. As shown in the scheme below, this compound has to isomerize with respect to its double bond in order to form benzene by a dehydrocyclization pathway. As shown in Figure 8, no molecular desorption of trans3-hexene is observed from a Cu3Pt(111) surface for exposures up to 0.25 langmuir, which corresponds to approximately 8% of a monolayer. When the exposures
Scheme 1
exceed 0.25 langmuir, molecular desorption of trans-3hexene commences, which is evidenced by a TPD peak at ∼270 K. The temperature of this peak decreases with increasing exposure of trans-3-hexene and reaches 215 K at a monolayer coverage. A multilayer TPD peak starts to grow at exposures above 3 langmuirs and has a temperature of 134 K, as shown in Figure 8. At the same time, for trans-3-hexene exposures less than 0.25 langmuir, the only hydrocarbon product found to evolve from the surface was benzene as proven by monitoring m/e+ ) 78 and m/e+ ) 77 at an electron impact ionization energy of 70 eV and m/e+ ) 78 at electron impact ionization energy of 9 eV.26,27 On the basis of TPR/D and AES studies, it was suggested that out of 8% of a monolayer coverage of trans-3-hexene chemically reacted with a Cu3Pt(111) surface, 70 ( 20% form benzene, with the remaining 30 ( 20% decomposing on the surface. The observation that the formation of benzene by trans3-hexene was detected on a Cu3Pt(111) surface indicates that isomerization, with respect to an internal double bond, occurs on this surface, according to Scheme 1. The rate and activation parameters of this isomerization process should be comparable to those of molecular desorption of trans-3-hexene, because otherwise no dehydrocyclization (26) Xi, M.; Bent, B. J. Vac. Soc. Technol., B 1992, 10, 2440-2446. (27) Lin, J.-L.; Bent, B. E. J. Am. Chem. Soc. 1993, 115, 2849.
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Figure 7. Comparison of the NEXAFS spectra of the submonolayer coverages of 1,3,5-hexatriene and 1-hexene taken at glancing and normal incidence after annealing the surface to 350 K.
reaction would be detected. However, the mechanism of this isomerization with respect to an internal double bond cannot be determined based on the results presented here. 4. Discussion As shown by Teplyakov et al.,13 the dehydrocyclization process can be successfully mimicked at the UHV conditions for the conversion of 1-hexene to benzene on a Cu3Pt(111) surface. This reaction has been studied by TPR/D and has been shown to consist of two steps: dehydrogenation and cyclization. However, TPR/D studies could not provide any information about the structure of the reaction intermediate and only indirectly suggested the overall formula of this intermediate leading to the formation of benzene. Furthermore, the issue of whether the isomerization with respect to an internal double bond occurs before or after the formation of such an intermediate was not discussed. The results presented in this paper answer both questions and thus, combined with the results of ref 13, cover most of the subject regarding the reaction mechanism of the dehydrocyclization reaction of 1-hexene on the Cu3Pt(111) surface. First, the RAIRS studies presented here support the findings of the previous studies that the significant change on the surface of copperplatinum alloy occurs between 250 and 350 K, the temperature where the dehydrogenation step of the reaction was postulated to take place. The obvious changes in the RAIRS spectra of the C-H stretching region of 1-hexene upon increasing the surface temperature from
250 to 350 K suggest the existence of a planar intermediate on this surface prior to the cyclization step. The RAIRS studies alone, however, did not allow for the identification of the surface intermediate. The NEXAFS technique served as an important complementary tool for the identification of the surface intermediates and their molecular orientation with increasing surface temperatures. The results obtained by NEXAFS showed that 1-hexene and 1,3,5-hexatriene have a common intermediate in the reaction of dehydrocyclization, and this intermediate was suggested to have a structure of hexa-σ-bonded triene. The spectroscopic signature of this intermediate is distinctly different from that of benzene and from multilayer coverages of 1-hexene or 1,3,5hexatriene. It should also be noted that 1-hexene rehybridizes upon adsorption at temperatures as low as 95 K and remains unchanged until the temperature of the surface is raised to above 250 K, which is in complete agreement with TPR/D studies of ref 13 and with RAIRS studies presented here. Strong conjugation of the π-system in 1,3,5-hexatriene allows it to keep the π-system intact upon adsorption at 95 K, but heating to ∼150 K leads to a complete rehybridization of 1,3,5-hexatriene, leading to the formation of the hexa-σ-bonded hexatriene intermediate that subsequently converts to benzene at 405 K. It should be noted that 1,3,5-hexatriene has been suggested previously as a possible intermediate in the dehydrocyclization reaction of linear chain hydrocarbons on supported metal catalyst.28,29 However, the existence
1344 Langmuir, Vol. 14, No. 6, 1998
Teplyakov et al.
Figure 8. TPR/D studies of trans-3-hexene dehydrocyclization on a Cu3Pt(111) surface tracing m/e+ ) 84 (trans-3-hexene molecular desorption) and m/e+ ) 78 (benzene formation). The inset shows the balance of peak areas evidenced in the TPR/D plots of m/e+ ) 84 and m/e+ ) 78 corrected for the mass spectrometry sensitivity plotted versus trans-3-hexene exposure.
of this hypothetically feasible intermediate and its bonding to the surface have never been supported spectroscopically. The final issue addressed in the studies presented here is a possibility of isomerization with respect to the internal double bond in unsaturated linear chain hydrocarbons. Previous TPR/D studies13 did not address this question, although the reactivity and selectivity of a mixture of cisand trans-isomers of 1,3,5-hexatriene (30 ( 10% of reacted 1,3,5-hexatriene formed benzene and 70 ( 10% decomposed to leave carbon on a Cu3Pt(111) surface) are consistent with ∼30/7029 ratio of cis- and trans isomer in a thermodynamic mixture. The issue of isomerization with respect to the internal double bond has been addressed here by studying the dehydrocyclization in trans-3-hexene. The fact that dehydrocyclization leading to the benzene formation has been observed in this system suggests that isomerization with respect to the internal double bond successfully competes with desorption. This observation also suggests that the activation energy and kinetic parameters of this reaction are comparable with those of desorption of trans-3-hexene. 5. Conclusions In agreement with previous TPR/D studies of the dehydrocyclization reaction on a Cu3Pt(111) surface, the (28) Paa´l, Z.; Te´te´nyi, P. Acta Chim. Acad. Sci. Hung. 1968, 55, 273286. (29) Paa´l, Z.; Te´te´nyi, P. Acta Chim. Acad. Sci. Hung. 1968, 58, 105108.
results presented here suggest that there are two steps involved in this transformation: dehydrogenation below 300 K and cyclization at ∼400 K. Spectroscopic identification of the intermediates of dehydrocyclization reaction in 1-hexene and 1,3,5-hexatriene indicates a common intermediate existing on a Cu3Pt(111) surface prior to the cyclization step. Furthermore, this intermediate has been identified as a hexa-σ-bonded triene, that is, bonded with the C-H bonds nearly parallel to the surface. These spectroscopic results also resolved the origin of the common intermediate in the dehydrocyclization reaction for these two compounds as opposed to the formation of hexatriene from 1-hexene followed by rehybridization. Another key result of the present study concerns the competition between the cyclization step and trans-cis isomerization with respect to the internal double bond in unsaturated compounds. The TPR/D results of dehydrocyclization in trans-3-hexene indicate that these two processes have comparable activation energies and kinetic parameters, since they coexist on a Cu3Pt(111) surface. Acknowledgment. Financial support from the National Science Foundation (Grant No. CHE-93-18625), The Dow Chemical Company, and Union Carbide as part of their Innovation Recognition Program is gratefully acknowledged. LA970731L
