Synthesis of methacrylic acid by aldol condensation of propionic acid with formaldehyde over acid–base bifunctional catalysts

A variety of catalysts consisting of MgO, SiO2, Al2O3 and ZrO2, with and without added cesium, were evaluated in the aldol condensation of propionic acid with formaldehyde to produce methacrylic acid. Reactions were performed in a fixed-bed reactor operating at atmospheric pressure and 598 K. Stepwise TPD of pre-adsorbed NH3 and CO2 was used to estimate the number and strength of surface acid and base sites, respectively. Both a strong solid base such as MgO and a non-basic solid such as SiO2 were ineffective at catalyzing the aldol condensation reaction. However, Cs-loaded silica and alumina exhibited moderate activity and high selectivity for the reaction. Results from reactivity measurements and surface characterization suggest that both acid and base sites are needed to catalyze the reaction, and the observed rates over the bifunctional ZrO2 catalyst support this hypothesis. A reversible inhibition of the rate by added water was observed over Cs–SiO2 and is attributed to migration and agglomeration of the cesium on the support in the presence of water.