Open AccessElucidating the Mechanism of Ambient-Temperature Aldol Condensation of Acetaldehyde on Ceria
Author(s) -
Suman Bhasker-Ranganath,
Md. Saeedur Rahman,
Chuanlin Zhao,
Florencia Calaza,
Zili Wu,
Ye Xu
Publication year - 2021
Publication title -
acs catalysis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 4.898
H-Index - 198
ISSN - 2155-5435
DOI - 10.1021/acscatal.1c01216
Subject(s) - chemistry , crotonaldehyde , aldol condensation , acetaldehyde , desorption , photochemistry , density functional theory , diffuse reflectance infrared fourier transform , reactivity (psychology) , adduct , associative substitution , aldol reaction , catalysis , adsorption , computational chemistry , organic chemistry , photocatalysis , medicine , alternative medicine , pathology , ethanol
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.