Vapor phase aldol condensation of acetaldehyde on metal oxide catalysts

Vapor phase aldol condensation of acetaldehyde on silica supported alkali metal containing solid bases, silica supported tungsten and molybdenum oxides and other unsupported oxides were studied. It was found that silica supported alkali metal containing solid base catalysts can effectively catalyze the aldol condensation of acetaldehyde to produce crotonaldehyde as well as crotyl alcohol with a good selectivity (ca. 90%) and stability and moderate conversion. MoO₃/SiO₂ or MoO_x/SiO₂ (reduced state) was found to be almost inactive for this reaction. Contrary to MoO₃(MoO_x)SiO₂, WO₃(WO_x)/SiO₂ catalysts are active and more selective. Most of other unsupported oxides tested in this study are not attractive for the low activity and selectivity or poor stability. ZrO₂ or pre-treated ZrO₂-SO^2-₄ was initially quite active, but deactivated quickly. Co-feeding hydrogen can enhance the activity of ZrO₂ or inhibit deactivation of pre-treated ZrO₂-SO^2-₄. The super acidic site associated with SO^2-₄ species has a great contribution to coking but little to aldol condensation.