Experimental and Theoretical Study for Vapor Phase Aldol Condensation of Methyl Acetate and Formaldehyde over Alkali Metal Oxides Supported on SBA-15

Alkali metal oxides supported on SBA-15, M/SBA-15 (M = Li, Na, K, Rb, and Cs), prepared by the impregnation method were characterized and tested for the aldol condensation of methyl acetate and formaldehyde to methyl acrylate (MA). M/SBA-15 catalysts are active for the formation of MA while the support is inert. The supported alkali metals react with hydroxyl groups on SBA-15 by replacing the H atoms and generate weak and medium basic sites. The latter plays an important role in promoting the reaction. Coke formation results in deactivation of Cs/SBA-15, and the deactivated catalyst can be regenerated by burning off the coke. Two possible reaction pathways for the formation of MA were explored using quantum calculations. The predicated activity for M/SBA-15 is consistent with the experimental observations. A pathway with adsorbed enol molecule as an intermediate was suggested to be predominant for the formation of MA.