Co-precipitated La-modified boehmite-derivatives as solid base catalysts for the intramolecular aldol condensation of 2,5-hexanedione

Abstract

This study focuses on the design and evaluation of lanthanum (La)-modified boehmite (AlOOH)-derived catalysts synthesized by a urea homogeneous precipitation (hp-) method (non-calcined), with performance compared to catalysts prepared by a conventional impregnation (imp-) method (calcined). These catalysts were applied in the intramolecular aldol condensation of 2,5-hexanedione (HD) to produce 3-methyl-2-cyclopentenone (MCP). A range of La loadings and calcination temperatures were investigated. Structural characterization by XRD and N₂ physisorption was conducted to correlate physical properties with catalytic performance. The hp-La–Al–(OH) catalyst exhibited high activity even without calcination with 54% yield and 82% selectivity, which was comparable to the imp-La/AlOOH catalyst requiring thermal treatment to become active with 60% yield and 71% selectivity, in 3.0 mmol scale of the reaction. These differences were attributed to distinct surface characteristics and active site distributions arising from the preparation method. Poisoning experiments using benzoic acid, 2,6-dimethylpyridine and 3,5-dimethylpyridine reagent indicated that the reaction proceeds via a base mechanism over the non-calcined hp-La–Al–(OH) catalyst, and via an acid–base cooperative mechanism over the calcined imp-La/AlOOH catalyst.