"Synthesis of ionone precursosrs by aldol condensation on alkali-promoted MgO catalysts"

VERÓNICA K. DÍEZ; CARLOS R. APESTEGUÍA; J. ISABEL DI COSIMO

Seattle, Estados Unidos

Congreso; Annual Meeting of the Pacific Coast Catalysis Society; 2006

Pacific Coast Catalysis Society

Pseudoionones are valuable acyclic precursors for the synthesis of a- and b-ionones, which are extensively used as pharmaceuticals and fragrances. The b-ionone isomer is the preferred reactant for different synthesis processes leading to vitamin A whereas a-ionone is in high demand in the fragrance industry because of its sweet-floral, reminiscent of violet scent.  Currently, pseudoionones are commercially produced via the aldol condensation of citral with acetone in a liquid-phase process involving the use of diluted bases, such as NaOH, Ba(OH)2 or LiOH, which entails concerns derived from toxicity, corrosion, and disposal of the spent base. The consecutive cyclization of pseudoionones to yield a- and b-ionones is catalyzed by strong liquid acids. The liquid-phase synthesis of pseudoionones by cross-aldol condensation of citral with acetone was studied on alkali-promoted MgO catalysts. Alkaline metals (A) such as Li, Na, K and Cs were added to a high-surface area MgO in A/Mg molar ratios of up to 0.01. Promoters of bigger ionic radius than that of Li blocked the catalyst pores of MgO causing a decrease of both surface area and catalyst activity. In contrast, Li addition enhanced the pseudoionone yield of parent MgO.  This beneficial effect of Li was further investigated by preparing, characterizing and testing several Li/MgO catalysts with different Li loadings.  Li loadings of up to 0.5 wt. % increased the total base site density of parent MgO by mainly increasing the density of very active strong base sites and thereby promoted pseudoionone formation. A pseudoionone yield of 93 % was obtained at 353 K with a catalyst/citral weight ratio of 0.2 for the 0.5 wt. % Li/MgO catalyst at the end of the 6-hour catalytic run. Increasing the Li concentration further caused particle agglomeration and formation of unreactive carbonates that block the active sites. The citral/acetone aldol condensation mechanism on Li-MgO catalysts was also investigated and a Langmuir-Hinshelwood-Hougen-Watson kinetic expression was developed to account for the initial pseudoionone formation rate and to interpret experimental data. It was found that the rate-determining step is the abstraction of the a-proton from the acetone molecule that takes place on strong Brönsted base sites.