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

Journal Article · · JACS Au
 [1];  [1];  [1];  [2];  [3];  [4];  [1];  [4]
  1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
  2. Tiangong University, Tianjin (China)
  3. Pacific Northwest National Lab. (PNNL), Richland, WA (United States); Tiangong University, Tianjin (China)
  4. Pacific Northwest National Lab. (PNNL), Richland, WA (United States); Washington State Univ., Pullman, WA (United States)

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 struc-tural heterogeneity of catalysts such as commercial TiO2 powders. Here, we provide a deep understanding of the reaction mechanisms, kinetics, and structure–function relationships for vapor phase acetone aldol condensation through the con-trolled 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-OsH) and acetone bonded to Lewis acid sites (acetone-Ti5c) on surface of both {101} and {001} facet dom-inant TiO2. Intermolecular C–C coupling of enolate intermediate from acetone-Ti5c and a vicinal acetone-OsH is a kinetically relevant step, which is consistent with kinetic and isotopic studies and DFT calculations. The {001} facet showed a lower apparent activation energy (or higher reactivity) 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 intermo-lecular C–C bond formation than the {101} facet.

Research Organization:
Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES)
Grant/Contract Number:
AC05-76RL01830
OSTI ID:
1783813
Report Number(s):
PNNL-SA--157855
Journal Information:
JACS Au, Journal Name: JACS Au Journal Issue: 1 Vol. 1; ISSN 2691-3704
Publisher:
ACS PublicationsCopyright Statement
Country of Publication:
United States
Language:
English

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