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

Abstract

A mechanistic understanding of catalytic reactions at solid‐liquid interface is limited both experimentally and theoretically but attracts much interest. Using density functional theory calculations (DFT) and ab initio molecular dynamics (AIMD) simulations, we investigated the effect of liquid water on α‐H abstraction, C−C coupling, and dehydration steps of aldol condensation of formaldehyde and acetone on an anatase TiO2(101) surface. The existence of the aqueous phase lowered the Gibbs energy of activation of dehydration step pronouncedly from 187 to 74 kJ/mol through proton transfer mechanism, making the hydrogenation pathway more favorable in the aqueous phase. In contrast, a mixed route prevails in the vapor phase. This work provides insights into the effect of the bulk water through a proton transfer mechanism on the dehydrogenation, C−C coupling, and dehydration steps.

Elucidation of Active Sites in Aldol Condensation of Acetone over Single-Facet Dominant Anatase TiO2 (101) and (001) Catalysts

“… 1 − 3 This reaction is a key step in many biomass conversion processes, including the Guerbet reaction of ethanol 4 − 8 and saccharides to biofuel. 9 Because of their high efficiency and low toxicity and cost, metal oxides including TiO 2 , 10 − 20 ZrO 2 , 18 , 19 CeO 2 , 13 , 18 , 21 and MgO 12 , 22 , 23 and their combinations 24 , 25 are widely used as solid acid–base catalysts in the aldol condensation reaction. Understanding the elementary steps and the active site requirements for this reaction is crucial to improving the design of the catalyst and the atomic efficiency of the oxygenate transformation processes.…”

“…TiO 2 is one of the most studied and commercialized catalytic materials. 26,27 The mechanism of aldol condensation has been extensively investigated on both anatase [10][11][12]14,20 and rutile TiO 2 . 10 It is generally accepted that aldol condensation on TiO 2 is catalyzed by Lewis acid−base site pairs (Ti−O pairs) and proceeds via enolation of the aldehyde or ketone to form an enolate intermediate, followed by C−C coupling between the enolate and an adjacent aldehyde or ketone.…”

“…10 It is generally accepted that aldol condensation on TiO 2 is catalyzed by Lewis acid−base site pairs (Ti−O pairs) and proceeds via enolation of the aldehyde or ketone to form an enolate intermediate, followed by C−C coupling between the enolate and an adjacent aldehyde or ketone. [10][11][12]14,20,28 A more detailed proposed reaction pathway is as follows (Scheme 1): (1) first, the aldehyde or ketone molecule adsorbs on a Lewis acid site (i.e., a Ti δ+ cation) ( The unstable aldol products then dehydrate to unsaturated aldehydes or ketones (Step 5). Although these proposed elementary steps are widely accepted, there are still debates about which step(s) are kinetically relevant.…”

“…The catalytic aldol condensation of carbonyl compounds such as aldehydes and ketones forms C–C bonds and eliminates O; therefore, it has great potential for the successful upgrading of biomass-derived feedstocks, which contain smaller C chains and excess O compared to the desired fuel and chemical products. − This reaction is a key step in many biomass conversion processes, including the Guerbet reaction of ethanol − and saccharides to biofuel . Because of their high efficiency and low toxicity and cost, metal oxides including TiO 2 , − ZrO 2 , , CeO 2 , ,, and MgO ,, and their combinations , are widely used as solid acid–base catalysts in the aldol condensation reaction. Understanding the elementary steps and the active site requirements for this reaction is crucial to improving the design of the catalyst and the atomic efficiency of the oxygenate transformation processes.…”

“…TiO 2 is one of the most studied and commercialized catalytic materials. , The mechanism of aldol condensation has been extensively investigated on both anatase − ,, and rutile TiO 2 . It is generally accepted that aldol condensation on TiO 2 is catalyzed by Lewis acid–base site pairs (Ti–O pairs) and proceeds via enolation of the aldehyde or ketone to form an enolate intermediate, followed by C–C coupling between the enolate and an adjacent aldehyde or ketone. − ,,, A more detailed proposed reaction pathway is as follows (Scheme ): (1) first, the aldehyde or ketone molecule adsorbs on a Lewis acid site (i.e., a Ti δ+ cation) (Step 1); (2) the adjacent basic site (an O δ− anion) then extracts the α-H of the adsorbed aldehyde or ketone to generate an enolate (Step 2); (3) the enolate CC bond nucleophilically attacks the carbonyl carbon of a vicinal aldehyde or ketone molecule to create an intermolecular CC bond (Step 3); and (4) the reprotonation of the C–C bond in the coupled intermediate generates an aldol and completes the catalytic cycle (Step 4). The unstable aldol products then dehydrate to unsaturated aldehydes or ketones (Step 5).…”