(235f) Acetone Condensation over Ion-Exchanged Hydroxyapatite Catalysts

Biomass can be used to produce platform molecules such as ethanol, acetone, and hydroxymethylfurfural, but further processing is needed to convert renewable feedstocks into useful products. One promising solution is bimolecular coupling of alcohols or aldehydes to form higher-chained commodity chemicals and transportation fuels. Recent reports in literature have shown that hydroxyapatite (HAP; Ca₅(PO₄)₃OH) is efficient at catalyzing a variety of coupling reactions, such as aldol condensation, Knoevenagel condensation, and Guerbet coupling; however, the mechanism and site requirements for these reactions remain a point of debate. [1,2]

In this work, we investigate the gas-phase condensation of acetone to mesityl oxide over HAP as a model reaction. Using a post-synthesis ion-exchange method, a series of divalent cation-substituted HAP catalysts were prepared to understand how different elements influence reaction rates. Initial characterization by XRD and BET suggest that the surfaces of exchanged-HAP catalysts are enriched with the introduced dopant while retaining an apatitic structure. To overcome equilibrium limiations, catalysts were tested in a fixed bed reactor using a physical mixture of HAP and Pd/SiO₂ to hydrogenate unsaturated products with H₂. Our results show that the nature of the cation in HAP plays a key role in governing catalytic activity. Isotopic labeling and KIE experiments reveal that enolate formation and C-C coupling are fast and reversible under the range of reaction conditions tested. The rate-limiting step occurs after the formation of diacetone alcohol, which is an intermediate in the reaction. A relationship between key descriptors of HAP structure and microkinetic parameters of acetone condensation will be discussed.

[1] Hanspal, S.; Young, Z. D.; Shou, H.; Davis, R. J. ACS Catal. 2015, 5, 1737â??1746.

[2] Ho, C. R.; Shylesh, S.; Bell, A. T. ACS Catal. 2016, 6, 939â??948.