Aldol condensation among acetaldehyde and ethanol reactants on TiO2: Experimental evidence for the kinetically relevant nucleophilic attack of enolates

Recent advances in selective C-C bond coupling for ethanol upgrading over balanced Lewis acid-base catalysts

“…If the aldehydes adsorbed on the surface of the metal oxide catalyst were abundant, the formation rate of C-C bond would not change much by only increasing the partial pressure of the aldehydes. Therefore, it brought up another reaction mechanism of C-C bond formation named Guerbet reaction [14] with ketones or aldehydes as the key reaction intermediates.…”

“…During C-C bond coupling through aldol condensation (step (3)), a proton could be extracted from the β position of an aldehyde to form an surface active species called the enolate (•CH 2 CHO*A in Scheme 2), which would attack the carbonyl group of the adjacent adsorbed acetaldehyde to generate a C-C bond and obtain one molecule of adsorbed aldol. These two steps were proposed to be the kinetically relevant step separately following the Guerbet reaction by several groups working on the mechanisms of the C-C bond formation from ethanol [14]. The reaction can be accomplished both in gas and liquid phase, which would result in different product distributions (some-times different reaction mechanisms) due to the difference in the transferring/returning of proton between gas and liquid phase during molecular activation and/or elementary steps of products generation that is related to the concentrations of the proton on the surface of the catalyst.…”

“…Compared to the prime homogeneous O In order to optimize the reaction performance of C-C bond coupling over Mg-AlO x catalyst, various metal/ metal oxides were attempted to be loaded onto the surface of Mg-AlO x to fine tune the physical properties of the catalyst. For example, Marcu et al [22] reported that Cu [14,15,47,55,[60][61][62][63][64]. Among them, the anatase TiO 2 has been considerably reported that it possesses high activity and selectivity for aldol condensation reaction [60][61][62].…”

“…Furthermore, we recently reported the study about the kinetically relevant nucleophilic attack of enolates to the aldehyde in the aldol condensation by feeding ethanol and acetaldehyde mixtures over anatase TiO 2 [14]. Product distribution results show 25% C 4 selectivity (2-bu-tenal, 2-buten-1-ol and 1-butanol), 11% C 6 -C 8 , 25% C 8 aromatics, 58% esters (ethyl acetate and δ-hexalactone) and <5% unknown products at 10% acetaldehyde conversion over TiO 2 (TiO 2 annealed at~673 K).…”

“…Extensive studies suggest that the formation of C-C bond requires to control the strength of Lewis acid and base sites. Most of the studies on C-C bond coupling following aldol condensation reaction were over metal oxide catalysts with modest Lewis acid sites or Brønsted base sites [13], such as TiO 2 [14], ZrO 2 [15] and MgO (Lewis base) [16]. However, the uncontrollable Lewis acid (or base) density/strength would lead to the formation of side-product rather than selective C-C bond coupling, which arouses a lot of studies about mixed metal oxide catalysts with balanced Lewis acid-base pairs [10,13,17,18].…”