Base Catalysis for the Synthesis of Alpha,Beta-Unsaturated Ketones from the Vapor-Phase Aldol Condensation of Acetone

DICOSIMO, JI; DIEZ, V.K.; APESTEGUIA, CR

APPLIED CATALYSIS A-GENERAL

Elsevier

Año: 1996 vol. 137 p. 149 - 166

0926-860X

The vapor-phase aldol condensation of acetone was studied over MgO promoted with 0.7-1.0 wt.-% of alkaline (Li, Na, K and Cs) or alkaline earth (Ca, Sr and Ba) metal ions. The basic properties of the samples were characterized by chemisorption of CO2. The basicity of MgO increased on addition of the promoter following the basicity order of the promoter oxide: the higher the electron-donor properties of the promoter, the higher the generation of surface basic sites. Major reaction products were mesityl oxide (MO), isomesityl oxide (IMO) and isophorone (IP). The selectivity to (MO+IMO+IP) over unpromoted MgO was practically 100%, thereby showing that MgO is an effective solid base for selectively obtaining a,b-unsaturated ketones. The reaction was totally inhibited by the adding of acetic acid to the reactants whereas the coinjection of pyridine did not affect the acetone conversion. This indicated that the self-condensation of acetone over MgO-based catalysts is catalyzed by basic sites. The promoter addition increased the activity of the MgO catalyst and a good correlation was obtained between catalyst activity and the concentration of basic sites. It is suggested that the improvement in catalytic activity is related to the enhancement in the ability by the oxide catalyst for abstracting the proton in a-position to the carbonyl group in the acetone molecule. All the catalysts exhibited similar IP/(IMO+MO) selectivity ratio, excepting the Li/MgO sample which produced substantially larger amounts of isophorone. Because the tricondensation of acetone to give isophorone requires strong basic sites, the higher selectivity toward isophorone exhibited by the Li/MgO sample was indicative of the presence of stronger basic sites in this sample. Results from CO2 chemisorption confirmed that Li/MgO exhibited the strongest basic properties. The generation of high-strength basic sites was explained by assuming that the addition of Li causes a structural promotion of the MgO sample by replacing the Mg2+ ions by Li+ in the MgO lattice. The replacement would result in strained Mg-O bonds and formation of [Li+O-] species which causes the generation of stronger basic sites.