Aqueous-Phase Acetic Acid Ketonization over Monoclinic Zirconia

Abstract

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(1̅11) surface structure. The aqueous phase consisted of 111 explicit water molecules with a density of 0.93 g/cm3. The AIMD simulation results reveal that the aqueous phase/ZrO2(1̅11) interface is highly dynamic. At the typical reaction temperature of 550 K, ∼67% 6-fold-coordinated Zr6c Lewis acidic sites are occupied by either water molecules or hydroxyls, while all 2-fold-coordinated O2c sites are protonated as hydroxyls. As a result, it is expected that there are limited active sites on the ZrO2(1̅11) surface for acetic acid adsorption in the aqueous phase. Acetic acid ketonization on the ZrO2(1̅11) surface in both vapor and aqueous phases is assumed to be proceeded via the β-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 β-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.

Effect of Zirconia Polymorph on Vapor-Phase Ketonization of Propionic Acid

“…The total amount of adsorbed propionic acid (peak area of 1080 cm −1 ) (A), the amount of monodentate propionate (peak area of 1590-1597 cm −1 ) and bidentate propionate (peak area of 1560 and 1510 cm −1 ) (B) on ZrO2 normalized by surface area. Scheme 2 displays the possible surface reaction of propionic acid ketonization over ZrO2 based on the in situ DRIFTS results and the previous reports [19,25,26]. Propionic acid dissociative adsorbs on the acid-base site as monodentate and bidentate adsorption modes, then forms enolate through α-H abstraction by the vicinal base site.…”

“…Furthermore, the *C 2 H 5 COO* decreases slowly and there is less contrast with C 2 H 5 COO* after the removal of propionic acid, owing to the stronger binding with catalysts surface. The *C 2 H 5 COO* may even be converted to C 2 H 5 COO*, directly proceeding ketonization, or be desorbed [19,25,26,32].…”

“…The amphoteric metal oxides show higher catalytic activity, such as MnO 2 , TiO 2 , ZrO 2 , CeO 2 , etc. [12,[17][18][19][20][21][22]. The reaction activity of carboxylic acid ketonization is strongly influenced by the surface structure of metal oxides [23][24][25][26][27].…”

Aqueous Phase Aldol Condensation of Formaldehyde and Acetone on Anatase TiO2(101) Surface: A Theoretical Investigation

“…The effects of explicit water molecules in the aqueous phase on catalytic reaction have been investigated on both metal and metal oxide surfaces . Desai et al.…”

“…Cai et al. comparatively studied vapor‐ and aqueous‐phase acetic acid ketonization over a monoclinic zirconia surface and concluded that the Eley‐Rideal mechanism, instead of Langmuir‐Hinshelwood mechanism, is the major C−C coupling pathway for the formation of β‐keto acid in aqueous phase . The hydrophilic acetic acid molecule/acetate, which was stabilized by surrounding water molecules in aqueous phase, can directly react with the di‐anion species, resulting in β‐keto acid.…”