Catalysis of Organic..

68Synthesis of MIBKwe examined the aldol condensation step to MO. In this study we have addedpalladium or nickel to facilitate the hydrogenation of MO to MIBK. Othergroups have also used copper (7) as well as palladium (3, 8) and nickel (9).The possible reactions that can take place are outlined in Figure 1 (6).Typical by-products of the aldol reaction are phorone and isophorone, howeveras can be seen in the figure other by-products can be formed, especially in thepresence of a hydrogenating functionality.OOHO-H 2 OO+H 2OOH+H 2MIBCMOMIBK+(CH 3 ) 2 CO+(CH 3 ) 2 CO+(CH 3 ) 2 COOOOPHORONEO+H 24,4'-DIMETHYL HEPTA-2,6-DIONE3,3,5-METHYL-CYCLOHEXANONE-H 2 O1,6-ALDOL2,4-DIMETHYLHEPT-2,4-DIEN-6-ONE1,6 MICHAELO2,6-DIMETHYLHEPT-2-EN-4-ONE+H 2+H 2OISOPHORONEOOMESITYLENE2,6-DIMETHYL HEPTAN-4-ONEFigure 1. Mechanistic scheme for main reaction and by-products.Results and DiscussionThe Pd-KOH/silica, Pd-CsOH/silica, Ni-KOH/silica and Ni-CsOH/silicacatalysts were tested at various temperatures. The selectivity to MIBK, MIBC,and isophorone as a function of temperature after 24 h on-line is shown inFigure 2. The conversion at the three temperatures is shown in Figure 3.All the catalysts showed high selectivity until 673 K where there was adramatic breakdown in selectivity over the nickel catalysts. Note that this lossin selectivity was not associated with a dramatic increase in conversion. Indeed