STUDY OF THE CATALYST DEACTIVATION IN THE BASE-CATALYZED OLIGOMERIZATION OF ACETONE

The deactivation of unpromoted MgO and alkali-promoted MgO catalysts i n the vapor-phase self-condensation of acetone was studied. The reacti on was catalyzed by basic sites and major products were mesityl oxide, isomesityl oxide and isophorone. Catalysts deactivated because of cok e formation. Both the initial catalyst deactivation (d(0), h(-1)) and the product distribution depended on contact time (W/F-0): d(0), and t he selectivity to mesityl oxide increased when W/F-0 was increased. It is proposed that non-cyclic trimers, such as phorone, which are produ ced by aldol condensation of mesityl oxide with acetone, are the key i ntermediate species for coke formation. These non-cyclic trimers are h ighly unsaturated compounds that remain strongly bound to the catalyst surface yielding higher non-volatile oligomeric compounds which block basic active sites. Promotion of MgO with alkaline metal ions increas ed the d(0) value measured on unpromoted MgO following the sequence Li < Na < K < Cs: the stronger the promoter oxide basicity, the higher t he catalyst deactivation. Enhancement of the MgO basicity by alkali ad dition strengthens the interaction between the solid surface and non-c yclic trimers, and improves the catalyst ability for abstracting the a lpha-proton from the acetone molecule. As a consequence, the aldol con densation synthesis of tetramers and heavier polymers is favored which results in an increasing coke formation. (C) 1998 Elsevier Science B. V.