Aqueous-Phase Acetic Acid Ketonization over Monoclinic Zirconia

Heterogeneous catalysis in the aqueous phase is paramount to the catalytic conversion of renewable biomass resources to transportation fuels and useful chemicals. To gain fundamental insights into how the aqueous phase affects catalytic reactions over solid catalysts, vapor- and aqueous-phase acetic acid ketonization over a monoclinic zirconia (m-ZrO2) catalyst had been comparatively investigated using ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations. The monoclinic zirconia was modeled by the most stable ZrO2((1) over bar 11) surface structure. The aqueous phase consisted of 111 explicit water molecules with a density of 0.93 g/cm(3). The AIMD simulation results reveal that the aqueous phase/ZrO2((1) over bar 11) interface is highly dynamic. At the typical reaction temperature of 550 K, similar to 67% 6-fold-coordinated Zr-6c Lewis acidic sites are occupied by either water molecules or hydroxyls, while all 2-fold-coordinated O-2c sites are protonated as hydroxyls. As a result, it is expected that there are limited active sites on the ZrO2((1) over bar 11) surface for acetic acid adsorption in the aqueous phase. Acetic acid ketonization on the ZrO2((1) over bar 11) surface in both vapor and aqueous phases is assumed to be proceeded via the beta-keto acid intermediate. In the vapor phase, an alternative Langmuir-Hinshelwood mechanism in which the neighboring coadsorbed acetic acid and dianion can directly combine together and form the CH3COOHCH2COO* intermediate is identified as the more feasible pathway than the traditional C-C coupling step via the combination of acyl and dianion. In the aqueous phase, our DFT results demonstrate that water molecules actively participate in the deprotonation and protonation steps via the Grotthuss proton transfer mechanism. Furthermore, our results suggest that an Eley-Rideal mechanism pathway for the formation of the beta-keto acid intermediate is feasible in the aqueous phase on the basis of the observed energetic analysis. However, the low availability of dianion is also a key factor that inhibits the ketonization reaction in the aqueous phase. The effects of dynamic aqueous phase on the key surface reaction steps are further confirmed by sampling different reaction configurations from AIMD trajectories.