Impact of Hydrotalcite’s Basic Sites on the Catalyst Stability and the Branching Selectivity in α‑Hydroxyketone Aldolization

• Published in: ACS catalysis 2023-12, Vol.13 (24), p.15811-15823
• Main Authors: Rammal, Fatima, Matthijssen, Joost, Khalil, Ibrahim, Calderon-Ardila, Sergio, Makshina, Ekaterina, Sels, Bert F.
• Format: Article
• Language: English
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.3c03975

Reconstructed hydrotalcites serve as powerful catalysts for the aldolization of α-hydroxyketones, exemplified here by the glycerol-derived dihydroxyacetone (DHA), toward the formation of branched hexoses (dendroketoses). Due to the multichemical functionality of these hydroxyketones, various competitive reactions such as keto-aldehyde isomerization (e.g., dihydroxyacetone (DHA)/glyceraldehyde (GLA) equilibrium) are possible, reducing the branching selectivity of the aldolization reaction. This study reveals that the nature of the basic sites (as determined via CDCl3 probe FT-IR and CO2-TPD experiments) strongly affects the branching selectivity of the condensation reaction as well as the stability of the hydrotalcite catalyst. For instance, strong basic sites not only allow the undesired keto-aldehyde isomerization but also promote the Cannizzaro reactions toward the formation of organic carboxylic acids leading to the catalyst leaching and waste generation. Thus, subsequent chemical transformations of branched sugars cannot occur without prior purification. For instance, the hydrogenation toward biobased branched polyol, which is the industrial target in casu base condensation of DHA. Tuning the basic properties of the reconstructed hydrotalcite based on this knowledge ultimately led to an active, selective, and stable catalyst with improved regeneration possibility.