Vapor-phase intramolecular aldol condensation of 2,5-hexanedione over yttrium zirconate catalyst

Abstract

Vapor-phase intramolecular aldol condensation of 2,5-hexanedione (2,5-HD) to 3-methyl-2-cyclopentenone (MCP) was performed over several rare-earth zirconate catalysts, with yttrium zirconate (YZrO) emerged as the most efficient catalyst. Several parameters, such as Y content, calcination temperature, reaction temperature, and contact time, highly influenced the catalytic performance of the YZrO catalyst: YZrO calcined at 700 °C exhibited the highest catalytic activity but still suffered from catalyst deactivation due to coke formation. Modifying the YZrO with Ag and employing the modified catalyst in an H₂ flow effectively reduced the coke accumulation, thus improving the catalytic stability during the aldol condensation of 2,5-HD at 350 °C. The poisoning experiment with the co-fed of acidic CO₂ gas and basic molecules of 2,6-, and 3,5-dimethylpyridine suggested that the aldol condensation proceeded via an acid-base concerted mechanism.