acrolein production from a mixture of alcohols in gas phase on silica supported basic oxides catalysts: on which sites?

We studied the production of acrolein from a mixture of alcohols in a two stages gas-phase process to understand and decouple the reaction mechanism. Oxidation of methanol and ethanol on FeMo catalyst takes place in the first stage reactor [1]; then the formaldehyde and the acetaldehyde produced feed the second stage reactor where acrolein is produced by aldol condensation (in the presence of O2 and other 1st stage reaction gases) on silica-supported CaO, MgO, Na2O and K2O catalysts [2]. We characterized the catalysts for their acid and base surface properties, by adsorption calorimetry, using NH3 and SO2 respectively as probe molecules (Fig.1). The other physico-chemical properties were also determined by complementary techniques as XRD, XPS, N2-adsorption at 77K. We tested the catalysts couples at temperatures chosen in order to maintain the CO+CO2 yield below 10 mol %. K-Si and Ca-Si catalysts showed the highest oxidation features and could not be tested above 270 and 300 °C, respectively, despite the acrolein selectivity that might improve at higher temperatures. In each case, the catalysts produced mainly acrolein, acetaldehyde, formaldehyde, CO and CO2. MgO-containing catalyst (Mg-Si) is the most promising and shows the highest acrolein selectivity and the highest amount of both acid and base strong sites (>100 kJ/mol) (Fig.2). Strong basic and acidic sites, as well as a good dispersion of the active oxides on the silica surface, drive the catalytic performances. The fine tuning of the surface properties is crucial to improve the acrolein selectivity and to minimize the over oxidation behavior.