(520f) Role of Enol-Keto Tautomerization in m-Cresol Selective Hydrogenation on Ni(111) and Nife(111) Surface

Using density functional theory, the reaction mechanism for selective hydrogenation of m-cresol on Ni (111) and NiFe (111) has been systematically investigated. Our results show that on Ni (111), the initial hydrogenation occurs at the nearest adjacent site of phenyl-OH forming 3-methyl-1,3-cyclohexadienol followed by the formation of unsaturated intermediate 3-methyl-cyclohexadienol, whereas on NiFe(111), the first step is the enol-keto tautomerization forming 3-methyl-3,5-cyclohexadienone which is more favorable than hydrogenation of the phenyl ring. After that, two products, i.e., 3-methylcyclohexanone and 3-methylcyclohexanol, can be formed on Ni(111), where the former is through enol-keto tautomerization and the latter is through further hydrogenation. In comparison, 3-methyl-3,5-cyclohexadienone intermediate adsorbed on NiFe(111) can proceed via partial hydrogenation forming 3-methylcyclohexanone or via complete hydrogenation forming 3-methylcyclohexanol. The reaction pathway via phenoxy intermediate is less likely on either Ni (111) or NiFe(111) due to strongly-bound phenoxy intermediate which can only serve as a spectator. The calculated reaction energy scales well with activation energy for representative elementary steps, demonstrating good Brønstedâ??Evansâ??Polanyi relationship. Moreover, the calculated reaction energies agree well with the experimental observations that the conversion from 3-methylcyclohexanone to 3-methylcyclohexanol or vice versa on Ni(111) is more favorable than that on NiFe(111). Overall, selective/complete hydrogenation of m-cresol on NiFe (111) towards final products of ketone/alcohol is kinetically more favorable than that on Ni(111). Our calculations confirms that alloying Fe into Ni can promote the alternative reaction pathway, i.e., via enol-keto tautomerization, rather than the conventional hydrogenation of phenyl ring of m-cresol on Ni(111).