BASE CATALYSIS FOR THE SYNTHESIS OF ALPHA,BETA-UNSATURATED KETONES FROM THE VAPOR-PHASE ALDOL CONDENSATION OF ACETONE

The vapor-phase aldol condensation of acetone was studied over MgO pro moted with 0.7-1.0 wt.-% of alkali (Li, Na, K and Cs) or alkaline eart h (Ca, Sr and Ba) metal ions. The basic properties of the samples were characterized by chemisorption of carbon dioxide. The basicity of MgO increased on addition of the promoter following the basicity order of the promoter oxide: the stronger the electron donor properties of the promoter, the greater the generation of surface basic sites. Major re action products were mesityl oxide (MO), isomesityl oxide (IMO) and is ophorone (IF). The selectivity to (MO + IMO + IF) over unpromoted MgO was practically 100%, thereby showing that magnesium oxide is suitable for selectively obtaining alpha,beta-unsaturated ketones. The reactio n was totally inhibited by co-feeding acetic acid along with acetone w hereas the co-injection of pyridine did not affect the acetone convers ion. This indicated that the self-condensation of acetone over MgO-bas ed catalysts is catalyzed by basic sites. The promoter addition increa sed the activity of the MgO catalyst and a good correlation was obtain ed between catalyst activity and the concentration of basic sites. Suc h a proportionality between activity and surface basicity was an addit ional evidence that the rate-determining step in the aldol condensatio n mechanism is controlled by the surface base property. All the cataly sts exhibited similar IP/(IMO + MO) selectivity ratio, except the Li/M gO sample which produced substantially larger amounts of isophorone. B ecause the tricondensation of acetone to give isophorone requires stro ng basic sites, the higher selectivity toward isophorone was indicativ e of the presence of stronger surface basic sites in the Li/MgO sample . Results from carbon dioxide chemisorption confirmed that Li/MgO exhi bited the strongest basic properties. The generation of high-strength basic sites was explained by assuming that the addition of lithium cau ses a structural promotion of the MgO sample by replacing the Mg2+ ion s by Li+ in the MgO lattice. The replacement would result in strained Mg-O bonds and formation of [Li+O-] species, which causes the generati on of stronger basic sites.