9650
J. Phys. Chem. B 2006, 110, 9650-9659
Thermal Chemistry of 1-Methyl-1-Cyclopentene and Methylene Cyclopentane on Pt(111) Surfaces: Evidence for Double-Bond Isomerization Ricardo Morales and Francisco Zaera* Department of Chemistry, UniVersity of California, RiVerside, RiVerside, California 92521 ReceiVed: February 16, 2006; In Final Form: March 27, 2006
The thermal chemistry of 1-methyl-1-cyclopentene (1MCpd) and methylene cyclopentane (MeCp) was investigated on clean and hydrogen- and deuterium-predosed Pt(111) single-crystal surfaces by temperatureprogrammed desorption and reflection-absorption infrared spectroscopy. It was found that MeCp isomerizes easily to 1MCpd but that the reverse reaction does not occur, at least under our experimental conditions. The MeCp to 1MCpd isomerization is aided by the presence of coadsorbed hydrogen, and occurs through the formation of a common 1-methyl-1-cyclopentyl (1MCp-Pt) surface intermediate; that intermediate then undergoes β-hydride elimination selectively at the ring position to form the 1MCpd product. In addition to this isomerization, both 1MCpd and MeCp also dehydrogenate on the Pt(111) surface to form a methylcyclopentadiene species, at 325 and 350 K, respectively. A small amount of benzene desorption is detected above 500 K with both reactants, indicative of a ring enlargement reaction. No evidence for the activation of any of the allylic hydrogens was obtained in either molecule.
Introduction
SCHEME 1: Horiuti-Polanyi Mechanism
The catalytic conversion of olefins under reducing conditions is one of the simplest and most studied catalyzed reactions.1-3 When promoted by solid transition-metal catalysts, the ensuing hydrogenation, dehydrogenation, H-D exchange, and double bond isomerization reactions are accounted for by the so-called Horiuti-Polanyi mechanism (Scheme 1).4,5 This deceivingly simple scheme, which is based on a series of stepwise hydrogenation-dehydrogenation steps, can be used to explain many experimental observations, but also hides a number of complications associated with the regio- and stereospecificity of the β-hydride elimination step from the intermediate surface alkyl species. For instance, it has recently been pointed out by us that the exchange of the hydrogen atoms around a CdC double bond by deuterium is accompanied by a cis-trans interconversion.6 Also, many alkyl surface groups, branched alkyls in particular, may have several β-hydrogens in different environments, and elimination of each of those may lead to different products. Some trends have been identified by past surface-science studies in connection with this issue,7-10 namely: (1) β-hydride elimination takes place preferentially from the more substituted β-carbons; (2) the desorption temperature of the resulting alkenes shifts to lower temperature with either decreasing chain length or increasing number of substitutions; and (3) the alkene yield is higher (relative to alkane production by the competing reductive elimination reaction) for branched alkyls compared to linear alkyls. Finally, there is the potential of a competing mechanism being operative in olefin conversion involving an initial dehydrogenation at an allylic position.1-3,8 Indeed, some allyl intermediates are particularly stable, especially those based on cyclic species,11-14 and are therefore likely to be involved in either hydrogenation15 or selective oxidation16 catalytic processes. In the present study, the surface chemistry of 1-methyl-1cyclopentene (1MCpd) and methylene cyclopentane (MeCp) * Corresponding author. E-mail:
[email protected].
was studied on Pt(111) in order to test the potential contribution of allylic intermediates to the conversion of those olefins. These molecules were chosen because of the lability of their allylic hydrogens and the steric constraints for their hydrogenation imposed by their cyclic nature. It was hypothesized that these two factors may hinder the Horiuti-Polanyi mechanism at the expense of the formation of an allylic intermediate, but no evidence for this was found. Two more issues were addressed in this study: (1) the selectivity of the β-H elimination step from the common 1-methyl-1-cyclopentyl (1MCp-Pt) intermediate that controls the isomerization between MeCp and 1MCpd and (2) the possible rearrangements of the carbon skeleton to produce other hydrocarbons, in particular a ring enlargement to yield benzene. Temperature-programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS) studies were carried out on the thermal decomposition, hydrogenation, and H-D exchange of MeCp and 1MCpd on Pt(111) under ultrahigh vacuum (UHV) conditions. Our results show no evidence of an allyl intermediate, but the production of a small amount of benzene. It was also found that both molecules easily undergo alkene-alkyl interconversion reactions, and that MeCp isomerizes preferentially to 1MCpd, in particular in the presence of coadsorbed hydrogen. Experimental Methods All the TPD and RAIRS experiments were performed in a two-level UHV chamber cryopumped to a base pressure below 3 × 10-10 Torr described in detailed previously.15,17 The main stage of this chamber is equipped with a UTI 100C quadrupole
10.1021/jp061004e CCC: $33.50 © 2006 American Chemical Society Published on Web 04/27/2006
Thermal Chemistry on Pt Surfaces mass spectrometer retrofitted with a retractable nose cone. That cone ends in a 5 mm diameter aperture, which can be placed within 1 mm of the front face of the single crystal used as the solid sample for the selective detection of molecules desorbing from its surface. The mass spectrometer is interfaced to a personal computer capable of monitoring the time evolution of up to 15 different masses in a single TPD experiment. A constant heating rate of 10 K/s was used in all TPD runs with the aid of homemade electronics, and a bias of -100 V was applied to the crystal in order to avoid any chemistry induced by stray electrons from the ionizer of the ion gauge or the mass spectrometer.18 The mass spectrometer signals in the TPD spectra are reported in arbitrary units, but scales are provided in each figure to allow for relative comparisons. Desorption yields, calculated by integration of the TPD curves after appropriate calibration, are also reported. The second level of our surface-characterization chamber, which can be reached with a long-travel manipulator, is used for the RAIRS experiments. The IR beam from a Bruker Equinox 55 FT-IR spectrometer is passed through a polarizer and focused through a NaCl window onto the sample at grazing incidence (∼85°). The reflected beam is then passed through a second NaCl window and focused onto a mercury-cadmiumtelluride (MCT) detector. The entire beam path is enclosed in a sealed box purged with dry air, purified using a scrubber for CO2 and water removal. All spectra presented in this paper correspond to averages of over 2000 scans taken with 4 cm-1 resolution and ratioed against similarly obtained spectra for the clean surface prior to gas dosing. The Pt(111) single crystal, a disk 8 mm in diameter and 2 mm in thickness, was mounted on the sample holder via spotwelding to a pair of tantalum wires attached to corresponding copper electrical feedthroughs. This arrangement allowed for the Pt crystal to be cooled to approximately 80 K by using a continuous flow of liquid nitrogen, and to be heated resistively to up to 1100 K. The temperature was measured with a chromealumel thermocouple spot-welded to the side of the crystal. The sample was routinely cleaned by cycles of oxidation in 2 × 10-6 Torr oxygen at 700 K and annealing in a vacuum at 1100 K, and occasionally by Ar+ sputtering, although the use of the ion bombardment was minimized to avoid the creation of surface defects. The 1-methyl-1-cyclopentene (1MCpd, 98% purity) and methylene cyclopentane (MeCp, 97% purity) liquid samples were obtained from Aldrich and treated via a series of freezepump-thaw cycles before use. Their purity was checked periodically by mass spectrometry. Gas exposures were performed by backfilling of the vacuum chamber, and are reported in langmuirs (1 langmuir ≡ 10-6 Torr‚s), not corrected for differences in ion gauge sensitivities. All dosings were done at temperatures below 100 K unless otherwise indicated. Results a. TPD. Figure 1 shows the molecular and hydrogen TPD spectra for 1MCpd on the Pt(111) surface as a function of initial dose. The signals for 67 and 2 amu are reported in this figure, but the identity of the products was corroborated by following several other masses. According to these results, a hightemperature hydrocarbon desorption peak is seen at 260 K starting at 1.5 langmuir exposures, while monolayer and multilayer desorptions start above 2.0 and 2.5 langmuirs and occur at 165 and 150 K, respectively. Assuming a preexponential factor of 1015 s-1 and using a Redhead analysis,19 desorption energies of 11.1 and 13.9 kcal/mol are estimated for the monolayer and high-temperature desorption, respectively. The
J. Phys. Chem. B, Vol. 110, No. 19, 2006 9651
Figure 1. Temperature-programmed desorption (TPD) data for 1-methyl1-cyclopentene (1MCpd) adsorbed on Pt(111) showing the profiles for molecular desorption (left) and hydrogen production (right) at different exposures.
desorption energy of the multilayer, according to a leadingedge analysis,20 is calculated to be 8.6 kcal/mol, in reasonably good agreement with the reported value for the heat of condensation, ∆H°cond) -7.8 ( 0.1 kcal/mol.21 The dehydrogenation of 1MCpd, reflected by the H2 TPD data in the right panel of Figure 1, occurs in a stepwise manner: two main desorption peaks are seen at 280 and 520 K, together with a third broad desorption region around 600800 K. This behavior is similar to that reported for other cycloalkenes,13,22-24 and even for other alkenes,18,25-27 on Pt(111). The relative areas of the H2 TPD peaks follow a relation of approximately 3:3:4, suggesting the formation of at least two partially dehydrogenated surface intermediates with C6H7 and C6H4 average stoichiometries before total dehydrogenation to carbonaceous species is reached above 800 K. The desorption peak at 280 K may be desorption-limited, suggesting that the initial dehydrogenation steps may occur at rather low temperatures. The higher temperature peaks, on the other hand, do reflect the kinetics of the corresponding C-H bond-scission steps. Analogous molecular and hydrogen TPD spectra for MeCp on Pt(111) are shown in Figure 2. MeCp molecular desorption is seen at 160, 180, and 260 K, starting at 3.0, 2.5, and 1.5 langmuirs, respectively. This corresponds to desorption activation energies of 7.4, 12.2, and 13.9 kcal/mol. The hydrogen peaks are seen at 300, 540, and 600-800 K, at slightly higher temperatures than with 1MCpd, and an additional small desorption feature at 440 K is also evident at exposures above 2.0 langmuirs. The relative peak areas in this case follow ratios of 3:1:2:4, indicating intermediates with C6H7, C6H6, and C6H4 average stoichiometries on this surface at ∼ 350, 450, and 600 K. The yields obtained from the TPD uptakes in Figures 1 and 2 are summarized in Figure 3. These data were calculated by integration of the corresponding desorption traces after scaling by the appropriate sensitivity factors for each species. The coverages are reported relative to monolayer saturation on the clean surface. The data for 1MCpd and MeCp share several trends, among them: (1) only total dehydrogenation is seen at low coverages; (2) hydrocarbon desorption from the monolayer starts around 2.0 langmuirs; (3) desorption from the monolayer increases with increasing dose, even after the onset of molecular
9652 J. Phys. Chem. B, Vol. 110, No. 19, 2006
Morales and Zaera
Figure 2. Temperature-programmed desorption (TPD) data for methylene cyclopentane (MeCp) adsorbed on Pt(111) showing the profiles for molecular desorption (left) and hydrogen production (right) at different exposures. Figure 4. Reflection-absorption infrared spectra (RAIRS) for 1MCpd adsorbed on Pt(111) at 100 K as a function of exposure.
Figure 3. Yields from the TPD experiments with 1MCpd (left) and MeCp (right) adsorbed on Pt(111) reported in Figures 1 and 2.
desorption from the multilayer (at ∼ 3.0 langmuirs); and (4) the sticking coefficient of the hydrocarbon is approximately constant throughout the saturation of the monolayer but increases significantly afterward as additional 1MCpd or MeCP condenses in multilayers. Also note that dehydrogenation and H2 desorption account for approximately 60% of the saturated monolayer in both cases. b. RAIRS. Figures 4 and 5 show the RAIR spectra for the uptake of 1MCpd and MeCp on Pt(111) at 100 K, respectively. The corresponding mode assignments, made by comparison with reported liquid- and gas-phase data,28,29 with our data for methylcyclopentane,30 and with reported spectra for cyclopentene in the gas phase31 and on Pt(111),23 are provided in Tables 1 and 2. The good matches between the spectra for the adsorbed species and those for the solid, liquid, and gas phases indicate that at this temperature the adsorption is molecular. Also, for both molecules and after all exposures, the most intense vibrational modes are those due to the CH stretchings (28003000 cm-1 region), followed by the CH2 scissoring and CH2 wagging at ∼1440 and ∼1000 cm-1, respectively. In the case of 1MCpd, the low intensities of the CdC and dC-H stretching modes at submonolayer (0.5 langmuir
vapor/solid phasesb,c
817 (s) 873 (vw) 899 (vw) 925 (vw) 997 (s), 1062 (w) 1010 (sh), 1024 (w) 1156 (w) 1133 (w) 1200 (m) 1260 (w), 1296 (w), 1330 (w) 1371 (w) 1423 (m), 1444 (m) 1475 (w) 1661 (w) 2847 (s), 2876 (sh), 2893 (vs) 2932 (s) 2956 (vs) 3047 (w)
817 (s) 886 (w) 907 (m) 923 (s) 1000 (vs), 1064 (m) 1010 (s), 1032 (m) 1148 (w) 1135 (m) 1210 (m) 1261 (w), 1293 (m), 1328 (m) 1375 (s) 1436 (s), 1448 (vs) 1460 (w) 1655 (m) 2858 (vvs), 2876 (sh), 2894 (vs) 2936 (vvs) 2959 (vvs) 3046 (vs)
a Nomenclature: F, rocking; δ, deformation; τ, twisting; ω, wagging; γ, scissoring; ν, stretching. Subindices: a, asymmetric; s, symmetric; oop, out-of-plane. b Relative peak intensities provided in parentheses: w, weak; m, medium; s, strong; v, very strong; sh, shoulder; br, broad. c From ref 29.
TABLE 2: Vibrational Mode Assignment for Methylene Cyclopentane (MeCp) on Pt(111), 100 Kb vibrational modea
0.0-0.5 langmuir
>0.5 langmuir
vapor/liquid phasesb,c
ω(dCH2) δs(ring) F(dCH2) τ(dCH2) ω(CH2) τ(CH2) δ(ring) γ(dCH2) δ(CH2) γ(CH2) δoop(CH2) ν(CdC) νs(R-CH2)oph νs(β-CH2)oph νs(CH2)iph νs(β-CH2)iph νa(β-CH2)iph νa(CH2)oph νa(dCH2)
876 (s) 901 (vw)
875 (vs) 899 (m) 926 (m) 978 (vw) 1015 (w) 1150 (w) 1215 (m) 1403 (w) 1424 (m) 1447 (m) 1473 (w) 1653 (s) 2833 (m) 2890 (m) 2860 (s) 2902 (m) 2960 (vs) 2946 (vs) 3070 (m)
877 (s) 896 (sh, m) 923 (m) 977 (vvw) 1015 (m) 1150 (w) 1218 (m) 1405 (sh, m) 1430 (s) 1459 (vs) 1477 (m) 1655 (s) 2842 (m) 2883 (vs) 2850 (sh, m) 2908 (m) 2972 (vvs) 2950 (sh, m) 3084 (vs)
1015 (vw) 1150 (vw) 1215 (vw) 1422 (vw) 1450 (vw) 1651 (w) 2833 (w) 2890 (w) 2860 (w) 2903 (w) 2960 (m) 2945 (m)
a Nomenclature: ω, wagging; F, rocking; τ, twisting; δ, deformation; γ, scissoring; ν, stretching. Subindices: a, asymmetric; s, symmetric; iph, in-phase; oph, out-of-phase. R and β refer to the carbon position from the methylene group. b Relative peak intensities provided in parentheses: w, weak; m, medium; s, strong; v, very strong; sh, shoulder. c From ref 30.
for MeCp show clear evidence of the ν(CdC) and νs(dCH2) stretching modes at all exposures, indicating that the carboncarbon double bond is tilted away from the surface. Although the IR spectra of the condensed phase show that these two vibrational modes are more intense on MeCp than on 1MCpd,28,29
Figure 6. RAIRS spectra for 1MCpd (left) and MeCp (right) as a function of adsorption temperature. Exposures of 6.0 langmuirs were used in all cases. Several surface species can be identified here, including methyl cyclopentadiene at 325 and 350 K for 1MCpd and MeCp, respectively.
their clear detection with MeCp even at very low (e0.1 langmuir) exposures suggests that the adsorption geometry of the two molecules is significantly different. This could explain the differences observed in the TPD results, especially in connection with the hydrogenation, isomerization, and H-D exchange reactions shown later. To identify the surface reactions of 1MCpd and MeCp on the Pt(111) surface, we turn to the results from our RAIRS characterization of these molecules at different adsorption temperatures (Figure 6). These spectra were obtained by starting with a multilayer (a 6.0 langmuir exposure) of the appropriate chemical and then annealing at the indicated temperatures. In general, the features in the 900-1700 cm-1 region of the spectra are quite weak and not very informative; the most intense peaks are seen in the C-H stretching (2700-3000 cm-1) region. With 1MCpd (Figure 6, left panel), the spectra at 175 and 200 K are similar but different to the one obtained at 100 K after low exposures (
