Langmuir 1991, 7, 3034-3040
3034
0-H and C-H Bond Activation in Cyclohexanol by Atomic Oxygen on Ag( 110): Alkoxide Formation and Selective Dehydrogenation to Cyclohexanone Philip B. Merrill and Robert J. Madix* Departments of Chemistry and Chemical Engineering, Stanford University, Stanford, California 94305 Received February 13,1991. I n Final Form: September 19, 1991
The adsorption and reaction of C6HllOH on clean and oxygen-covered Ag(ll0) was studied by temperature-programmed reaction spectroscopy. Similarly, the adsorption of C6HloO on clean Ag(ll0) was investigated. On the clean Ag( 110) surface C6HllOH desorbs molecularly from the saturated monolayer at 250 K with an activation energy, E,, of 64 kJ/mol assuming a preexponential factor, u, of 1013s-l, and from a multilayer state at 195 K with an activation energy of 65 kJ/mol. C6HloO desorbed molecularly from the monolayer at 230 K with E, = 60.0 f 3.9 kJ/mol and v = 1013.3*0.8 s-l, and from a multilayer state at 165 K with an activation energy of 58 kJ/mol. On the partially oxidized Ag(ll0) surface, in addition to forming molecular multilayer and monolayer states, CeHllOH reacts with atomic oxygen selectively to form adsorbed cyclohexoxy (C6H110(,,)and HzO. Selectivity was maximized with oxygen coverages below 0.25 monolayer and saturation C6HllOH exposures. The surface alkoxide is stable up to a temperature of 297 K, where decomposition via C-H bond breaking and re-formation occurred, leading to the evolution of cyclohexanone, hydrogen, water, and cyclohexanol. The activation energy for the decomposition is 76 f 2 kJ/mol, assuming a preexponential factor of 1013s-l. 1. Introduction
Reactions of alcohols with adsorbed atomic oxygen on Ag and Cu surfaces have been studied extensive1y.l-l5The accepted mechanism involves 0-H bond activation by the surface oxygen, resulting in the formation of an adsorbed alkoxy intermediate and water.
In the case of primary and secondary alcohols a @-hydrogen ( p to the surface) is activated and available for C-H bond activation. Activated C-H bonds are defined16 as C-H bonds that are CY to a heteroatom or unsaturated center. The activated C-H bonds are more easily cleaved because the carbon-centered radicals resulting from homolytic cleavage will be stabilized by the adjacent heteroatom or unsaturated center. Further reaction thus results in the formation of an aldehyde or ketone along with the parent alcohol, hydrogen, and water from recombination reactions.
* To whom correspondence should be addreseed.
(1) Wachs, I. E.; Madix, R. J. Surf. Sci. 1978, 76, 531. (2) Wachs, I. E.; Madix, R. J. J. Cotal. 1978, 53, 208. (3) Wachs, I. E.; Madix, R. J. Appl. Surf. Sci. 1978, 1, 303. (4) Sexton, B. Surf. Sci. 1979, 88, 299. (5) Sexton, B. J. Vac. Sci. Technol. 1979, 16, 1033. (6)Ryberg, R. Chem. Phys. Lett. 1981,83, 423. (7) Bowker, M.; Madix, R. J. Surf. Sci. 1982, 116, 549. (8) Sexton, B. Surf. Sci. 1985, 155, 366. (9) Outka, D. A.; Madix, R. J.; Stohr, J. Surf. Sci. 1985, 164, 235. (IO) Prabhakaran,K.; Sen, P.;Rao, C. N. R. Surf. Sci. 1986,169, L301. (11) Capote, A. J.; Madix, R. J. J.Am. Chem. SOC.1989, 1 1 1 , 3570. (12) Capote, A. J.; Madix, R. J. Surf. Sci. 1989, 214, 276. (13) Brainard, R. L.; Madix, R. J. J.Am. Chem. SOC.1989,111,3826. (14) Brainard, R. L.; Peterson, C. G.; Madix, R. J. J. Am. Chem. SOC. 1989,111,4553. (15) Brainard, R. L.; Madix, R. J. J.Am. Chem. SOC.1989,109,8082. (16) Janowicz, A. H.; Bergman, R. G. J. Am. Chem. SOC.1983, 105, 3929.
0743-7463/91/2407-3034$02.50/0
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The absence of a @-hydrogenin tertiary alcohols results in a different decomposition pathway at significantly higher temperatures.13J5 In this work reactions of cyclohexanol, C G H ~ ~ Oand H, cyclohexanone, C6Hlo0, were studied upon Ag(ll0) using temperature-programmed reaction spectroscopy (TPRS). The overall purpose of the work was to ascertain whether the ring hydrogens introduced competing pathways into the oxidation scheme above. The reaction of cyclohexanol with the Ag(ll0) surface partly covered with adsorbed oxygen was compared to desorption of the alcohol and cyclohexanone on the clean surface. The surface intermediate resulting from oxidation of cyclohexanolwas more stable than either the alcohol or ketone bound to the clean surface. Decomposition of the intermediate on the clean surface liberated the ketone to the gas phase. Additionally, molecular hydrogen, water, and the alcohol itself were liberated by a reaction limited in rate by the decomposition. Decomposition of the intermediate in the presence of excess, coadsorbed atomic oxygen has more complex chemistry and will be the subject of another study. The choice of cyclohexanol for the work discussed in this paper has the following motivation. As a cyclic compound it is unique from the alcohols previously studied, and reactions on the ring skeleton may alter the mechanism and/or yield useful products. As an example 0 1991 American Chemical Society
0-H and C-H Bond Actiuation in Cyclohexanol consider the reactions of cyclohexene, CGH10, and phenol, CGH~OH, on the oxidative Ag(ll0) surface. Due to the presence of acidic C-H bonds on the rings, cyclohexene oxydehydrogenates below 200 K to benzene, which desorbs at 275 K.17 Phenol, which lacks a @-hydrogen,as is the case with the tertiary alcohols, appears to react with this surface to form a phenoxide,ls which is thermally stable up to (and possibly beyond) 375 K. Phenoxide formation from cyclohexanol is possible if ring C-H bonds are activated, since dehydrogenation of the partially unsaturated ring, as with cyclohexene, may yield the stable phenoxide before @-elimination could occur to allow cyclohexenone desorption. The relative energetics of these pathways determine product selectivity. Since the ring hydrogens are not very acidic, we expect highly selective oxidation to cyclohexanone, and in this study this hypothesis has been tested and found to be correct. 2. Experimental Section Temperature-programmed reaction spectroscopyexperiments were conducted in an ultrahigh vacuum chamber described previ~usly.~*~ The base pressure was typically 2 X 10-loTorr. The Ag(ll0) crystal, mounted on a rotable manipulator with three degrees of translational freedom, was heated resistively through its tantalum support with a linear heating profile. Heating rates, 6, ranged from 0.3 to 5 K s-l; fl was 4 K s-l in the results reported here unlessotherwise specified. Coolingto about 110 K was accomplished by conduction from a liquid nitrogen reservoir. Desorbing species were detected with a UTI Model lOOC quadrupole mass spectrometer. The ionization cage was enclosed by a glass cap which limited analysis to products desorbing in line of sight from the area of the crystal positioned directly in front of the cap opening. The quadrupole mass spectrometer was computer-multiplexed using software previously described,lgwhich enabled up to 100masses to be monitored simultaneously, thus allowing the masses of significant cracking fragments to be determined. Scan times over this entire range of masses limited the temperature resolution of the TPRS to about 10 K. Higher temperature resolution was achieved by monitoring fewer masses. With scan times of 25 ms for each mass to be monitored, a complete scan of 10masses with a heating rate of 4 K s-l could be accomplished with a resolution of 1K. Clean Ag(ll0)surfaces were prepared by Ar ion bombardment followed by annealing to 800 K. Residual carbon and/or hydrocarbon species were removed by dosing oxygen onto the crystal held at 500 K. A t this temperature 02 dissociativelyadsorbs on the Ag(ll0) surface, and previous studies verify the rapid desorption of oxidation products COz and HZO.~OThe cleanliness of the sample was verified by monitoring the temperatures of desorption and relative sizes of COz and 02 desorption peaks in a temperature-programmed desorption (TPD) experiment following an 02 dose at 300 K (02titration). A clean surface was defined as one which possessed an uncorrected COz/ 02 peak area ratio less than 0.05. Reagent grade C6HllOH (Baker analyzed) was stirred under a NZpurge over KzC03 drying agent for about 24 h, vacuum transferred to a gas handling tube containing additional drying agent,and degassed by freezepumpthaw cycles. CsHloO (Baker analyzed) was similarly prepared without drying. Oxygen (Matheson, extra dry) was used as received. The purity of the vapor over the liquids was routinelychecked by leaking the sample into the chamber and measuring its cracking pattern with the mass spectrometer. Full mass spectra over the range of m/q = 2-105 were recorded. The spectra obtained were consistent with published cracking patterns.21 Exact cracking fractions for these compounds for this mass spectrometer under current operating parameters were determined by measuring the cracking patterns (17) Roberts, J. T.; Madix, R. J. Surf. Sci. Lett. 1990,226, L71. (18) Roberta, J. T.; Madix, R. J. Unpublished results. (19) Liu, A. C.; Friend, C. M. Reu. Sci. Instrum. 1986, 57 (a), 1519. (20) Bowker, M.; Barteau, M. A.; Madix, R. J. Surf. Sci. 1980,92,528. (21) The Mass Spectrometry Data Centre. The Eight Peak Index of Muss Spectru;The Royal Societyof Chemistry: Nottingham, U.K., 1983.
Langmuir, Vol. 7,No. 12,1991 3035 Table I. Relative Yields of Ions Used for Determination of the Cvclohexanone Product pure 300 K residual pure 300 K cyclohexanol after cyclohexanol cyclohexanone mi9 product samplea deconvolution sample 2 18 32 42 44 55 57 67 69 70 82
280.8 432.8 0.3 329.7 60.8 182.9 100.0 27.9 18.9
13.0d 11.8
80.0 125.0
0.3
110.0 56.0 62.5 100.0 25.0 5.75 8.0d 10.5
91.4* 14O.lc 0.0 100.0 2.2 54.8 0.0 1.3 6.0 2.0 0.6
19.4 (18.9) 20.1 (18.2) 0.0 (10.0) 100.0 1.0 (10.5) 83.0 (115.) 1.0 (10.2) 1.5 (10.7) 13.4 (13.8) 11.0 (15.5) 0.3 (10.2)
Cyclohexanol data from multilayer peak of excess cyclohexanol dose on oxygen predosed Ag(ll0) surface (Figure3). The results are within error bars of pure cyclohexanol sample and are internally consistent with the data of deconvolution. * Value represents ion yield from both cyclohexanoneand excess Hz. e Value represents ion yield from both cyclohexanoneand excess HzO. These values for mlq = 70 used softwarelgwhich tracked 100 masses simultaneously. The software which collected the remaining data was limited to 10 masses but gave more accurate results. of a desorbing multilayer. All exposures were calculated with uncorrected ion gauge readings and are expressed in langmuirs (1langmuir = 10" Torr-s). Direct dosing through array dosers provided an estimated enhancement factor of lo3 for the condensables. Stated exposures include a correction for this enhancement. Identification of desorbing species utilized mass spectral fragmentation/cracking patterns. Multilayer desorption peaks from our experiments on the clean surface provided the cracking patterns for pure cyclohexanol and cyclohexanone. These patterns accounted for all desorption features on the clean surface; i.e., only molecular desorption of the species adsorbed was observed. Identification of the products of cyclohexanol oxidation at 300 K utilized our experimental cracking patterns for cyclohexan01 and cyclohexanone as well as literature valuesz1for cyclohexenol, 2-cyclohexen-1-01 and 3-cyclohexen-1-01. Although cleavage of a nonactivated C-H bond was not expected, the possibility of evolution of a cyclohexenol specieswas not ignored. Simultaneous evolution of cyclohexanol with the product made subtraction of the alcohol contribution necessary. Elimination of the contribution from cyclohexanol allowed discrimination of the possible products. The parent fragment of cyclohexanolwas not observed, and thus the cracking pattern of cyclohexanol and cyclohexanone are so similar that no unique masses exist which independently track and differentiate the evolution of the products. Table I showsthe data which best differentiate between cyclohexanol and cyclohexanone components in the 300 K peak. One of the strongest signals for cyclohexanol, mlq = 57, was assigned an intensity value of 100,and other cracking fragments were given values relative to this. For a pure sample of cyclohexanone, the mlq = 57 signal is 1%of the intensity of the strongest signal, m/q = 42. The cracking fraction for m/q = 57 was not reported in the literature for any of the cyclohexenol species;presumably it is not large. For this reason, we assumed all detected signal at mlq = 57 represented only cyclohexanol. Using the intensity of this signal and the fragmentation pattern for cyclohexanol, the contribution from cyclohexanol to every other m / q signal was subtracted. The resulting cracking pattern represented the products left after deconvoluting cyclohexanol from the product. Comparison of this pattern to that expected for cyclohexanoneshowedexcellentagreement with the exception of an excess of mlq = 2 and mJq= 18which represent hydrogen and water, respectively, which can be products evolved at this temperature. The data rules out the possibility of evolution of the cyclohexenol species. For all of these compounds, mlq = 70 represents the most abundant cracking fragment,but for the 300 K product, this signal represented only 13% of the mlq = 57 signal before deconvolution. After subtraction of the contribution due to cy-
Merrill and Madix
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3036 Langmuir, Vol. 7, No. 12, 1991 Cyclohexanol / Ag( 1 10)
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100
200
300
400
Temperature (K)
Figure 1. Temperature-programmed reaction spectra for various exposures of cyclohexanol to the clean Ag(ll0) surface at 110 K. Shown is-the CsHllOH (mlq = 57) signal.
clohexanol, mlq = 70 represented only 3 % of the mlq = 42 signal, the most abundant ion for cyclohexanone, which is expected from the cyclohexanone alone. In addition to considering the most abundant ion for the cyclohexenol compounds, all published cracking fragments of the cyclohexenol compounds were investigated and showed no apparent correlation to the desorbing product at 300 K. 3. Results and Discussion 3.1. Cyclohexanol Adsorption on Clean Ag( 110). 3.1.1. Temperature-Programmed Desorption (TPD) of Cyclohexanol. On the clean Ag(ll0) surface adsorption of CsHllOH is reversible. The cracking fractions of the species evolved a t 250 K correspond to those determined for cyclohexanol during desorption of the multilayer a t 200 K. Additionally, carbon-containing products are not obtained in significant amounts from 0 2 titrations following adsorption and desorption of the alcohol, indicating the absence of adsorbed residues. Temperatureprogrammed desorption profiles for a range of exposures of 110 K are shown in Figure 1. For low exposures, t I 2 langmuirs, a single desorption state is observed at 248 K. With increasing exposure, the peak temperature, Tp, shifts t o slightly higher temperatures. For higher exposures, t > 2 langmuirs, the peak temperature remains nearly constant and a second desorption state appears a t 192 K. The first state saturates a t high exposure, but the state a t 192 K does not, indicative of desorption from the multilayer a t 190-200 K. The small desorption features of 220 and 285 K are due t o unavoidable minor 0 2 and H20 impurities in the CsHllOH sample. As will be discussed in section 3.3, the oxygen impurity contributes to product formation and its subsequent decomposition. The magnitude of these features depended upon the extent of the impurities. Cracking fractions of the species desorbing a t these temperatures indicated that cyclohexanol desorbs at 220 K and a combination of cyclohexanone and cyclohexanol evolves a t 285 K. 3.1.2. Kinetics and Energetics of Cyclohexanol Desorption. The desorption features of 192-200 K are attributed to multilayer formation, since the peak cannot be saturated with a reasonable exposure. An analysis for zero-order kinetics was performed.22 From a plot of In (22) Yates, J. T. The Thermal Desorption of Adsorbed Species. In Methods of Experimental Physics, Volume 22; Solid State Physics: Surfaces; Park, R. L., Lagally, M. G., Eds.; Academic Press: New York, 1985; p 425.
rate vs 1/T, where rate represents the rate of desorption and T is the temperature, the energy for desorption from the multilayer is found to be 66 kJimo1. This value may be compared to the heat of vaporization, AHH,,equal to 49.9 kJ/mo1.23 The heat of fusion, equal to 1.8 kJ/mol, is only a small contribution to this difference. The fact that the peak temperature increases with coverage in the monolayer indicates that the desorption kinetics are not simple. Clustering of alcohol molecules on the clean Ag(ll0) surface is suggested by the fact that the low-temperature state, clearly due to molecular multilayer formation a t high exposures, forms well before saturation of the monolayer occurs. These lateral interactions are due to a combination of hydrogen or van der Waals bonding between coadsorbed molecules. Similarly, the increase in the peak temperature of the state near 250 K with coverage is consistent with attractive lateral interactions in the monolayer.24 On the basis of simple first-order desorption kinetics with coverage-independent rate parameters, using a peak temperature of 254 K, a normal first-order preexponential factor of 1013s-l, and an experimental heating rate of 4.2 K s-l, the desorption energy for this state was estimated to be 64 k J / m 0 1 . ~The ~ peak symmetry for each desorption trace a t a given exposure is not that expected for a first-order desorption, however. The ratio of the coverage a t the peak to the total initial coverage, up/utotal,was 0.44, where 0.37 is expected for perfect first-order behavior, 0.5 for second-order, and 0 for zero-order. First-order, leadingedge analysis of the desorption traces over the coverage range of 6 = 5-15% of the initial coverage yielded an activation energy of 90.3 f 3.9 kJ/mo1.22 Isotherm-isostere analysis was performed on data from both low- and high-coverage regimes.26 For all temperatures chosen the observed isothermal reaction order was substantially less than 1(0.7 f 0.1). For given coverages, values of rate and temperature may be obtained from these isotherms in order to determine isosteric values of the desorption energyF6 For the coverage range investigated (0.05-1 monolayer (ML), where saturation of the 250 K state represents 1 ML), the E, was 76.3 f 6.5 kJ/mol and the preexponential factor was 1015.5*1.3s-l. Unfortunately, due to the complexities of the kinetics, the desorption energy is not accurately known; it appears to lie between 70 and 90 kJ/ mol. 3.2. Cyclohexanone Adsorption on Clean Ag( 110). 3.2.1. Temperature-ProgrammedDesorption (TPD) of Cyclohexanone. The desorption of CGH100 from the clean Ag(ll0) surface is qualitatively similar to that of CGH~IOH;the desorption is also reversible, and no decomposition occurs. Figure 2 shows the temperatureprogrammed desorption spectra for a range of coverages following adsorption at 110 K. The first desorption state of cyclohexanone appears a t 230 K. There is a slight shift in peak temperature to lower values for increasing coverage, but after an exposure of 2.5 langmuirs the peak temperature remains constant. Again, a low-temperature state is observed a t 165 K with increasing peak temperature. The initial state can be saturated with a 4-langmuir exposure, whereas the state at 165K cannot, and the latter is attributed to a multilayer. 3.2.2. Kinetics and Energetics of Cyclohexanone Desorption. On the basis of a zero-order leading-edge (23) C.R.C.Handbook of Chemistry and Physics, 66th ed.; CRC Press: Boca Raton, FL, 1985, p C-672. (24) Niemantaverdriet, J. W.; Markert, K.; Wandelt, K. Appl. Surf. Sci. 1988, 31, 211. (25) Readhead, P. A. Vacuum, 1962,12, 203. (26) Falconer, J. L.; Madix, R. J. J. Catal. 1977, 48, 262.
0-H and C-H Bond Actiuation in Cyclohexanol
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