Study of the catalyst deactivation in the base-catalyzed oligomerization of acetone

The deactivation of unpromoted MgO and alkali-promoted MgO catalysts in the vapor-phase self-condensation of acetone was studied. The reaction was catalyzed by basic sites and major products were mesityl oxide, isomesityl oxide and isophorone. Catalysts deactivated because of coke formation. Both the initial catalyst deactivation (d(0), h(-1)) and the product distribution depended on contact time (W/F-0): d(0), and the selectivity to mesityl oxide increased when W/F-0 was increased. It is proposed that non-cyclic trimers, such as phorone, which are produced by aldol condensation of mesityl oxide with acetone, are the key intermediate species for coke formation. These non-cyclic trimers are highly 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 increased the d(0) value measured on unpromoted MgO following the sequence Li < Na < K < Cs: the stronger the promoter oxide basicity, the higher the catalyst deactivation. Enhancement of the MgO basicity by alkali addition strengthens the interaction between the solid surface and non-cyclic trimers, and improves the catalyst ability for abstracting the alpha-proton from the acetone molecule. As a consequence, the aldol condensation synthesis of tetramers and heavier polymers is favored which results in an increasing coke formation. (C) 1998 Elsevier Science B.V.