Elucidating the Mechanism of Ambient-Temperature Aldol Condensation of Acetaldehyde on Ceria

Abstract

Using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations, we conclusively demonstrate that acetaldehyde (AcH) undergoes aldol condensation when flown over ceria octahedral nanoparticles, and the reaction is desorption-limited at ambient temperature. trans -Crotonaldehyde (CrH) is the predominant product whose coverage builds up on the catalyst with time on stream. The proposed mechanism on CeO 2 (111) proceeds via AcH enolization (i.e., α C–H bond scission), C–C coupling, and further enolization and dehydroxylation of the aldol adduct, 3-hydroxybutanal, to yield trans -CrH. The mechanism with its DFT-calculated parameters is consistent with reactivity at ambient temperature and with the kinetic behavior of the aldol condensation of AcH reported on other oxides. The slightly less stable cis -CrH can be produced by the same mechanism depending on how the enolate and AcH are positioned with respect to each other in C–C coupling. All vibrational modes in DRIFTS are identified with AcH or trans -CrH, except for a feature at 1620 cm –1 that is more intense relative to the other bands on the partially reduced ceria sample than on the oxidized sample. It is identified to be the C=C stretch mode of CH 3 CHOHCHCHO adsorbed on an oxygen vacancy. It constitutes a deep energy minimum, rendering oxygen vacancies an inactive site for CrH formation under given conditions.

Hydrolysis of Acetamide on Low-Index CeO2 Surfaces: Ceria as a Deamidation and General De-esterification Catalyst

“…The minimum-energy reaction energy profiles based on the intermediate states and elementary steps proposed in Table 2 are mapped out in Figure 5 . Since free-energy contributions at the gas/solid interface are expected to be minor for reactions of small organic compounds on ceria under ambient conditions, 84 they are omitted from Figure 5 .…”

“…Instead, we make the assumption that the vdW and solvation effects on a TS are the average of these effects on the IS and FS of an elementary step. This approach would yield errors of 0–0.15 eV for the stability of TS based on the results of our previous study on aldol condensation of acetaldehyde on CeO 2 (111) . Under this assumption, conversion from the η 1 state to the TI is seen to be slightly outcompeted by acetamide desorption on all three facets, of which the (110) facet is the most efficient at directing a given molecular flux toward acetamide deamidation versus acetamide desorption, followed by (100), and then (111).…”

“…50 A U eff value of 2 eV was used in this study based on better agreements between GGA+U predictions obtained at U eff = 2−3 eV and available experimental measurements, including the bulk reduction energy of CeO 2 , 80,81 chemisorption energy of CO on CeO 2 (110), 82 and peak desorption temperatures of AcH. 83 The calculated equilibrium lattice constants of 5.476 Å (GGA-PW91) and 5.452 Å (optB86b-vdW) 84 are in good agreement with the experimental value of 5.41 Å. 85,86 The effects of solvation by water were modeled implicitly by treating the solvent as a dielectric continuum.…”

“…Our prior work 52,84 suggests that vdW interactions contribute to the stability of organic molecules as small as acetaldehyde when they are adsorbed on ceria. Thus, we have recalculated the minimum-energy reaction energy profiles on the three lowindex facets using the optB86b-vdW functional (red dashed traces, Figures 5a−c).…”

“…This approach would yield errors of 0−0.15 eV for the stability of TS based on the results of our previous study on aldol condensation of acetaldehyde on CeO 2 (111). 84 Under this assumption, conversion from the η 1 state to the TI is seen to be slightly outcompeted by acetamide desorption on all three facets, of which the (110) facet is the most efficient at directing a given molecular flux toward acetamide deamidation versus acetamide desorption, followed by (100), and then (111). On the other hand, the adsorption energy of acetic acid (ΔE ads SCCS , Table 3) remains at −1 eV on (111) and lower on ( 110) and ( 100) (ΔE ads SCCS , Table 3), suggesting that product desorption continues to be limiting, particularly on the two more open facets, even when assisted by an aqueous phase.…”