“Brønsted solid acids for ionone synthesis”

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

Ancient Harvor, Genova, (Italy).

Congreso; 6th Congress on Catalysis by Acids and Bases; 2009

Ionones are valuable chemicals. Whereas b-ionone is used in vitamin A synthesis, a- and g-ionone are employed in fragrances. Ionones are produced from citral via a two-step process (Figure 1) that involves liquid catalysts. The second step, the cyclization of pseudoionone (PS) promoted by sulfuric acid gives ionone yields of 70-90 % but works using solid acids report much lower yields. In this work, we postulate different solid acid catalysts to convert PS into ionones in high yields. We investigate the effect of the acid species (Brønsted or Lewis) and reaction conditions on the ionone yield and isomer distribution. Four silica-supported tungstophosphoric acid catalysts (HPAS-x; x: wt.% of HPA) and Cs-HPA were prepared. Commercial HBEA, SiO2-Al2O3 and HPA were also used. Total acid site densities (na) were measured by TPD of NH3. The presence of Lewis and Brønsted surface acid species was determined by FTIR of pyridine (Py). PS cyclization was performed at 343-383 K and 250 kPa in a batch reactor, with typically a Toluene/PS = 71 molar ratio and a catalyst/PS = 56 wt. % ratio. HBEA and SiO2-Al2O3 presented Lewis and Brønsted surface acid sites whereas the HPA-based catalysts were essentially Brønsted acids. Table 1 shows that na, PS conversion (XPS) and ionone yield (hIONONE) increased with the HPA loading for the HPAS catalysts. Furthermore, a correlation was found between the initial ionone formation rate and the Brønsted acid site band area, thereby confirming that the reaction is promoted by this kind of surface sites. HBEA and unsupported HPA contained sites accessible for NH3 and Py adsorption but not available for the reaction due to diffusional and spatial constraints. The best hIONONE at 353 K was obtained on HPAS-59 which presented the highest density of available Brønsted sites. On the latter the hIONONE reached 79 % in just 1.5 h at 383 K, Figure 3. Also, as shown in Figure 3, at 383 K the contribution of the a isomer was enhanced for reaction times longer than 1.5 h (time for 100 % PS conversion), reaching ~ 80 % of the total ionones at the end of the run, whereas b ionone remained constant. An isomerization process took place in which the least stable g isomer converted to a-ionone. Ionone synthesis is efficiently promoted on the Brønsted acid sites of silica-supported heteropolyacids. A 79 % ionone yield was obtained at 383 K in 1.5 h, a value comparable to that of the sulfuric acid-catalyzed ionone synthesis.  The main isomer (a) is favored at high temperatures and reaction times.