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

Abstract

Aldol condensations of carbonyl compounds for C–C bond formation are a very important class of reactions in organic synthesis and upgrading of biomass-derived feedstocks. However, the atomic level understanding of reaction mechanisms and structure–activity correlation on widely used transition metal oxide catalysts are limited due to the high degree of structural heterogeneity of catalysts such as commercial TiO 2 powders. Here, we provide a deep understanding of the reaction mechanisms, kinetics, and structure–function relationships for vapor phase acetone aldol condensation through the controlled synthesis of two catalysts with high surface areas and clean, dominant facets, coupled with detailed characterization and kinetic studies that are further assisted by density functional theory (DFT) calculations. Temperature-dependent diffuse reflectance infrared Fourier transform spectroscopy showed the existence of abundant acetone bonded to surface hydroxyl groups (acetone-O s H) and acetone bonded to Lewis acid sites (acetone-Ti 5c ) on the surface of both {101} and {001} facet dominant TiO 2 . Intermolecular C–C coupling of theenolate intermediate from acetone-Ti 5c and a vicinal acetone-O s H is a kinetically relevant step, which is consistent with kinetic and isotopic studies as well as DFT calculations. The {001} facet showed a lower apparent activation energy (or higher activity) than the {101} facet. This is likely caused by the weaker Lewis acid and Brønsted base strengths of the {001} facet which favors the reprotonation–desorption of the coupled intermediate, making the C–C coupling step more exothermic on the {001} facet and resulting in an earlier transition state with a lower activation barrier. It is also possible that the {001} facet has a smoother surface configuration and less steric hindrance during intermolecular C–C bond formation than the {101} facet.

Probing Acid–Base Properties of Anatase TiO2 Nanoparticles with Dominant {001} and {101} Facets Using Methanol Chemisorption and Surface Reactions

“…It was also discovered that 2-propanol dehydration only occurs on Lewis acid−base sites and Brønsted acidity plays no role for this catalytic reaction. In our more recent acetone aldol condensation studies over the same catalysts, 39 it was further demonstrated that the TiO 2 (001) catalyst is more active for this reaction, likely because of the following reasons: first, the kinetically relevant C−C coupling step requires proton transfer from reactive surface hydroxyls to the aldol precursor and then partial desorption to form the adsorbed aldol. The weaker Brønsted base and Lewis acid strengths of the {001} facet favor the proton transfer and desorption, making the C−C coupling step more exothermic on the {001} facets; second, the {001} facet has a smoother surface configuration than the {101} facet (Scheme 1), making the C−C coupled transition state better stabilized on the {001} facet.…”

“…We note that the anatase materials used in the present study were from different batches of synthesis, as opposed to the catalysts used in our previous studies. 38,39 Even though they display similar physicochemical properties, expected from similar synthesis procedures used, maintaining identical particle size distribution and dominant facet exposure from batch to batch, unfortunately, was found challenging. In particular, the TiO 2 (001) sample used in this study has a larger average particle size and less exposed {001} facets than TiO 2 (001) samples used in our previous studies.…”

“…In particular, the TiO 2 (001) sample used in this study has a larger average particle size and less exposed {001} facets than TiO 2 (001) samples used in our previous studies. 38,39 3.2. Surface Acidity of the TiO 2 (101) and TiO 2 (001) Nanoparticles.…”

“…This notion appears to be consistent with our prior finding that Lewis acid sites on TiO 2 (001) are weaker than those on TiO 2 (101), which explains why acetone C−C coupling occurs more readily on TiO 2 (001). 39 However, Figure 5 shows that some methoxides do not couple to DME but rather thermally decompose (R5). This portion of methoxides, as suggested earlier, is attributed to surface defects.…”

Elucidating the Cooperative Roles of Water and Lewis Acid–Base Pairs in Cascade C–C Coupling and Self-Deoxygenation Reactions

“…Over Lewis acid–base pairs, although acetone aldolization has been well studied and understood, 14 − 16 reports on ATIB (i.e., C–C coupling followed by self-deoxygenation) are still scarce. 7 , 8 , 12 A spectroscopic study by Zaki et al showed that the formation of isobutene arises from C–C bond cleavage of acetone oligomers, i.e., MSO.…”