Thermochemistry of the Selective Dehydrogenation of Cyclohexane to Benzene on Pt Surfaces
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Journal of Molecular Catalysis A: Chemical 131Ž. 1998 39±53 Thermochemistry of the selective dehydrogenation of cyclohexane to benzene on Pt surfaces Bruce E. Koel a,), David A. Blank b, Emily A. Carter b a Department of Chemistry, UniÕersity of Southern California, Los Angeles, CA 90089-0482, USA b Department of Chemistry and Biochemistry UniÕersity of California, Los Angeles, CA 90095-1569, USA Received 14 August 1997; revised 18 October 1997; accepted 24 October 1997 Abstract We use a quasiempirical valence bondŽ. QVB schemew E.A. Carter, Chem. Phys. Lett. 169Ž. 1990 218x for calculating the heats of formation of adsorbed species on surfaces to provide reliable estimates of the relative stabilities of several of the surface intermediates and adsorbate-surface bond strengths that are important in the selective dehydrogenation of cyclohex- ane to benzene over Pt surfaces. We estimate heats of adsorption and formation for adsorbed cyclohexylŽ. c-C611 H , a cycloallylic intermediateŽ. c-C69 H , cyclohexadiene Ž. c-C 68 H , cyclohexadienyl Ž. c-C 67 H , phenyl Ž. c-C 65 H , and benzyne Ž.c-C64 H on Pt surfaces. Estimates of these needed formation energies are then combined with the experimentally measured adsorption energies of cyclohexaneŽ. c-C612 H , cyclohexene Ž. c-C 610 H , and benzene Ž. C 66 H , to provide heats of reaction and an equilibrium thermodynamic description of a selective dehydrogenation mechanism that involves the step-wise, sequential removal of one H atom at a time from cyclohexane to form benzene. In addition, several further decomposition products of benzene are considered as precursors to undesirable carbon-forming reactions. In agreement with experimental observations, a cycloallylic speciesŽ. c-C69 H is shown to be an important stable intermediate in cyclohexane dehydrogena- tion that could also be involved in the catalytic rate-limiting step. Carbon±carbon bond cleavage and other possible surface reaction pathways are not considered herein. Addition of measured or estimated values for the activation barriers involved in the reaction on PtŽ. 111 can now give a fairly complete description of the energetics of this prototypical hydrocarbon conversion reaction on PtŽ. 111 surfaces. q 1998 Elsevier Science B.V. Keywords: Cyclohexane; Benzene; Pt 1. Introduction ates, i.e., cyclohexene and a C69 H species, have been identified and the rate constants for some The dehydrogenation of cyclohexane to ben- of the sequential reaction steps have been mea- zene is a prototypical hydrocarbon reforming suredwx 3±5 . Dehydrogenation of cyclohexane to reactionwx 1,2 that has been studied extensively benzene proceeds efficiently on PtŽ. 111 , but at the molecular level in ultrahigh vacuum very little benzene is desorbed into the gas Ž.UHV . On Pt Ž. 111 , which is the substrate most phase since the Pt surface readily dehydro- thoroughly investigated, some of the intermedi- genates the benzene product. Studies of the cyclohexene intermediate have been carried out. ) Corresponding author. Also, studies are available for cyclohexyl and 1381-1169r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S1381-1169Ž. 97 00255-0 40 B.E. Koel et al.rJournal of Molecular Catalysis A: Chemical 131() 1998 39±53 cyclohexadiene, which are likely intermediates lution electron energy loss spectroscopy in the reaction. In related work, the influence of Ž.HREELSwx 13,23 and reflection-absorption IR Biwx 6 and Cs wx 7 adatoms on this reaction, as spectroscopyŽ. RAIRSwx 20 have shown that well as many aspects of this reaction on ordered cyclohexane is adsorbed with the molecular wx Pt±Sn alloys 8±10 have also been investi- plane parallel to the surface with C3v symmetry, gated. probably in a chair configuration with three Even with all of the experimental work, sev- axial H atoms projecting toward the surface, eral of the likely intermediates associated with forming hydrogen bonds. Adsorption leads to a this reaction are still unidentified. The primary broad, low frequency C±H stretching band due challenge is to observe and identify metastable to extensive `softening' of the axial C±H bonds. intermediates by surface spectroscopic methods The surface coverage in the monolayer is be- and this is an elusive goal. Another key goal is tween 0.13wx 8 to 0.21 wx 6 monolayerŽ. ML . to determine the adsorption bond strengths and Upon heating, some cyclohexane desorbs, but surface bond energies for the reactant, products, most of itŽ 68%wx 6. converts to adsorbed ben- and intermediates in the reaction in order to zene which decomposes to hydrogen and ad- provide a thermochemical description of the sorbed carbon. Under UHV conditions, no sig- reaction. Temperature programmed desorption nificant gas-phase benzene is produced follow- Ž.TPD and laser induced thermal desorption ing cyclohexane adsorption. From the fraction Ž.LITD have been used for estimating the ad- of cyclohexane desorbed in TPD at 228 K, an sorption energies of cyclohexane, cyclohexene, adsorption energy of 13.9 kcalrmol can be and benzene. Unfortunately, these methods can- estimatedwx 8 . Dehydrogenation begins at 180± not be used for several of the adsorbed hydro- 195 K, corresponding to an activation energy, r carbon intermediates, since the barriers to sub- Ea, of 9.5±13.5 kcal mol over the 5±30% ML sequent surface reactions are less than the corre- coverage rangewx 5 . Benzene is not formed until sponding desorption activation energies and thus 300 Kwx 3±5,23 , in contrast to early reports of decomposition reactions occur rather than des- benzene formation at 200 Kwx 6,18 . Unfortu- orption into the gas phase. Furthermore, ad- nately, cyclohexeneŽ. c-C610 H remains as the sorbed molecules of this size are beyond the only species from cyclohexane dehydrogenation scope of current first principles theoretical that has been identified on PtŽ. 111wx 3 , although methods. Yet, it is essential to obtain equilib- an intermediate with C69 H stoichiometry has rium thermodynamic data on the relative stabili- been inferred to exist over the temperature range ties of surface species in order to better under- 180±270 K using LITD combined with Fourier stand the mechanism of this reaction. In this transform mass spectroscopyŽ. FTMSwx 4 and paper, we utilize a scheme that can be used to bismuth-postdosing thermal desorption spec- estimate the stabilities of surface species in troscopyŽ. BPTDSwx 22 , with the form of this order to provide an energetic foundation for the species favored to be a cycloallylicŽ. c-C69 H mechanism of selective dehydrogenation of cy- structure, rather than a simple loss of three axial clohexane to benzene. Below, we briefly sum- hydrogens, as deduced from HREELS datawx 23 . marize the experimental observations for this No direct evidence exists for cyclohexylŽ c- reaction and various mechanisms that have been C611 H.Ž. , cyclohexadiene c-C 68 H , or cyclo- postulated to explain the experimental data. hexadienylŽ. c-C67 H formation from cyclohex- The adsorption and decomposition of cyclo- ane. hexane on the PtŽ. 111 surface have been studied Some information has recently become avail- using a variety of surface science methodsw 3± able for the reactivity of the chemisorbed cyclo- 8,11±24x . Cyclohexane adsorbs molecularly at hexyl species on PtŽ. 111 . Using electron in- 100 K. Vibrational studies with both high reso- duced dissociationŽ. EID , low energy electron B.E. Koel et al.rJournal of Molecular Catalysis A: Chemical 131() 1998 39±53 41 bombardment of physisorbed layers of cyclo- remaining benzene to decompose to hydrogen hexane was used to cleanly prepare monolayer and adsorbed carbon. coverages of adsorbed cyclohexyl specieswx 8,25 . The adsorption and reaction of both 1,3- In TPD experiments, H2 was the dominant reac- cyclohexadiene and 1,4-cyclohexadiene on tion product desorbed, with no cyclohexene and PtŽ. 111 have been investigatedw 4,10,11,15, only small amounts of benzene desorption ob- 24,26x . Cyclohexadiene chemisorption is com- served. The H2 evolution in the TPD spectra pletely irreversible and upon heating it dehydro- was H2 desorption rate-limited, and so no infor- genates to form benzene. Further heating causes mation is available on the activation energy for this adsorbed benzene product to either desorb subsequent C±H bond breaking in adsorbed cy- from the surface or dehydrogenate completely clohexyl. However, on two ordered SnrPtŽ. 111 to form surface carbon. HREELSwx 26 and SFG surface alloys which are less reactive than wx24 indicate that the nature of the surface bond- PtŽ. 111 , cyclohexene desorption is observed in ing is a quadra-s-type with four covalent Pt±C two peaks at 208 and 345 K. The peak at 208 K bonds with sp3 hybridization at those carbons. is cyclohexene desorption rate-limited with a In the SFG studies, a red-shift of the C±H desorption energy of 12.6 kcalrmol, and so we stretching frequency to 2770 cmy1 was ob- can place an upper limit on the activation bar- served and interpreted as a strong interaction wx rier for the reaction of cyclohexyl on PtŽ. 111 to between the surface and the CH2 groups 24 . be 12.6 kcalrmol. Saturation coverage for chemisorbed cyclohexa- Cyclohexene adsorption on PtŽ. 111 has been diene is about 0.18 MLwx 10,26 . LITDrFTMS investigated by several groupsw 3,4,6,9,11,15, experiments show that the activation energy for 22,24x . Generally, it is thought that cyclohexene conversion of cyclohexadienes to benzene is adsorbs molecularly on the PtŽ. 111 surface at very lowŽ at low coverages conversion occurs at ( r wx 100 K. TPD and HREELS show two different 115 K, Ž..Ea 7 kcal mol 4 , but studies using states of molecularly adsorbed cyclohexene in BPTDS indicate that dehydrogenation of both the first adsorbed layer.