Thermochemistry of the selective dehydrogenation of cyclohexane to benzene on Pt surfaces

We use a quasiempirical valence bond (QVB) scheme [E.A. Carter, Chem. Phys. Lett. 169 (1990) 218] 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 cyclohexane to benzene over Pt surfaces. We estimate heats of adsorption and formation for adsorbed cyclohexyl (c-C₆H₁₁), a cycloallylic intermediate (c-C₆H₉), cyclohexadiene (c-C₆H₈), cyclohexadienyl (c-C₆H₇), phenyl (c-C₆H₅), and benzyne (c-C₆H₄) on Pt surfaces. Estimates of these needed formation energies are then combined with the experimentally measured adsorption energies of cyclohexane (c-C₆H₁₂), cyclohexene (c-C₆H₁₀), and benzene (C₆H₆), 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 species (c-C₆H₉) is shown to be an important stable intermediate in cyclohexane dehydrogenation 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 Pt(111) can now give a fairly complete description of the energetics of this prototypical hydrocarbon conversion reaction on Pt(111) surfaces.