(15a) Condensation Reactions of Propanal Over Ceria-Zirconia Mixed Oxide Catalysts

Vapor phase aldol condensation and ketonization reactions of propanal were investigated over Ce_xZr_1-xO₂ mixed oxides as a model reaction to produce gasoline range molecules from short aldehydes found in bio-oil mixtures. Several operating parameters were investigated. They include the type of carrier gas used (H₂ or He) and the incorporation of acids and water in the feed. Propanal is converted to higher carbon chain oxygenates on Ce_xZr_1-xO₂ by two pathways, aldol condensation and ketonization. The major products of these condensation reactions include 3-pentanone, 2-methyl-2-pentenal, 2-methylpentanal, 3-heptanone and 4-methyl-3-heptanone. It is proposed that the primary intermediate for the ketonization path is a surface carboxylate. The presence of acids in the feed inhibits the aldol condensation pathway by competitive adsorption that reduces the aldehyde conversion. Water also promotes ketonization and inhibits aldol condensation by increasing the concentration of surface hydroxyl groups that enhance the formation of surface carboxylates with the aldehyde. Hydrogen enhances cracking and production of light oxygenates and hydrocarbons. The light oxygenates may in turn be reincorporated into the reaction path, giving secondary products. However, the hydrocarbons do not react further. The analysis of the fresh and spent catalysts by XPS showed varying degrees of reduction of the oxide under different operating conditions and was in agreement with the reaction results. Changing the proportion of the parent oxides showed that increased Zr favored formation of aldol products while increased Ce favored ketonization. This occurred by shifting the balance of the acid-base properties of the active sites.