Density Functional Theory Study of Carboxylic Acids Adsorption and Enolization on Monoclinic Zirconia Surfaces

Computational modeling is a valuable tool for understanding atomic-level catalytic processes on metal oxides. Carboxylic acid adsorption and enolization with varied degrees of alpha branching on monoclinic zirconia's most important surfaces, ((1) over bar 11), (111), and ((1) over bar 01), have been studied by the density functional theory (DFT). Carboxylates on zirconia ((1) over bar 11) and (111) surfaces are preferentially stabilized in the bidentate bridging mode, with O-H bond dissociation and hydrogen bonding to a 2-fold coordinated (2-fc) lattice oxygen. Carboxylic alpha hydrogen abstraction by another 2-fc lattice oxygen results in enolization of adsorbed carboxylates most readily on the ((1) over bar 11) surface of monoclinic zirconia with activation energy similar to 25 kcal/mol, which is not sensitive to acid branching. Enolization on the (111) surface requires higher activation energy, 29-33 kcal/mol depending on acid branching. This study demonstrates the origin of an important intermediate in the carboxylic acid ketonization mechanism-often named surface ketene.