Effect Of The Acid-Base Properties Of Mg-Al Mixed Oxides On The Catalyst Deactivation During Aldol Condensation Reactions

Abstract

The effect of chemical composition of Mg-Al mixed oxides on both the acid-base properties and the deactivation process during the gas phase self-condensation of acetone was studied. The activity and selectivity for acetone oligomerization depended on the catalyst acid-base properties. Mgrich catalysts selectively yielded mesityl oxides whereas Al-rich MgyAlOx oxides produced mainly isophorone. The initial deactivation rate, increased linearly with the density of surface basic sites, thereby suggesting that although MgyAlOx oxides promote the self-condensation of acetone by both acid- and base-catalyzed mechanisms, the deactivation rate would be closely related to the surface basic properties. The MgyAlOx activity declines in the acetone oligomerization reaction due to a blockage of both base and acid active sites by a carbonaceous residue formed by secondary reactions. The amount and the nature of the carbon deposits were characterized by temperatureprogrammed oxidation technique. MgyAlOx and Al2O3 formed more and heavier coke than pure MgO but the latter deactivates faster. The deactivation rate and coke composition are defined by the nature of the active site involved in the coke-forming reactions at different catalyst compositions rather than by the carbon amount or polymerization degree.