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 CeO2(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 CH3CHOHCHCHO adsorbed on an oxygen vacancy. It constitutes a deep energy minimum, rendering oxygen vacancies an inactive site for CrH formation under given conditions.
Highlights
Energy and chemical production from renewable sources including biomass has been a major driver for both academic and industrial research for the past two decades.[1−4] Lignocelluloses can be deconstructed via physiochemical or biological methods, such as pyrolysis or hydrolysis, to create mixtures composed of small organic oxygenates that need to be upgraded to fuel or higher value products
The most intense peak in 1300−1800 cm−1 is located at 1700 cm−1, which we assign to the νC O mode in AcH.[32]
By monitoring acetaldehyde (AcH) flown over ceria nanooctahedra (o-ceria nanoparticles (CeNPs)) with in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and by comparison with density functional theory (DFT)-derived simulated IR spectra of relevant reaction intermediates on CeO2(111), we conclusively show the formation of crotonaldehyde (CrH) as the primary aldol condensation product of AcH at ambient temperature
Summary
Energy and chemical production from renewable sources including biomass has been a major driver for both academic and industrial research for the past two decades.[1−4] Lignocelluloses can be deconstructed via physiochemical or biological methods, such as pyrolysis or hydrolysis, to create mixtures composed of small organic oxygenates that need to be upgraded to fuel or higher value products. There is a consensus that the mechanism of aldol condensation generally involves the formation of an enol or enolate through dehydrogenation of the α carbon in a carbonyl compound forming a nucleophilic C center, which attacks the electrophilic C in the C O of another carbonyl compound, followed by the elimination of a H2O molecule. This can occur between two molecules of the same kind (selfcondensation) or different kinds (cross condensation). To understand the aldol condensation of aldehydes and ketones, as well as related reactions such as the Guerbet reaction, many studies have been carried out on a variety of solid compounds, including CeO2, ZrO2, TiO2, Al2O3, MgO, mixed oxides, layered double hydroxides, and amorphous aluminophosphates.[10−20] Much effort has been made to improve catalytic performance, through modifying the availability or strength of acid and base sites, geometry and coordination of metal ions, or extent of reduction or through mixing different oxides.[12,18,21−25] The use of different oxides and different reaction conditions, and possibly co-catalysts (e.g., Cu, Pd)[15,26] or co-reactants (e.g., H2, alcohols),[17,26,27] complicates a comprehensive understanding of the mechanism
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