SO2 and NH3 adsorption calorimetry in the study of the surface acid/base properties of oxide catalysts for acrolein production

Acrolein, the simplest unsaturated aldehyde, is widely used in chemical industry as intermediate for the synthesis of various widely used chemical products as biocides, acrylic acid, methionine and other numerous chemicals [1,2]. With the aim to find renewable materials easy to transform into acrolein, minimizing the investment on the existing plants, the oxidation of biosourced alcohols has been recently proposed. In this process, the oxidation of methanol and ethanol to formaldehyde and acetaldehyde and the aldol-condensation and dehydration of the two aldehydes to acrolein, are carried out simultaneously, contrarily to most of research works published up to now. The acid-base properties of the solid catalysts play a key-role in the aldol condensation reactions. Basic sites are essential to increase the activity and selectivity of aldolisation reactions, but their strength plays also an important role in side reactions. The quantification of the acid/base sites concentration and strength on the surface of oxide catalysts is even more crucial when the reaction is carried out in presence of oxygen, thus leading to over-oxidation reactions. In the present study, a series of oxide catalysts obtained by deposition of basic oxides (Na2O, K2O, MgO, CaO) on a silica support has been considered in the cross-condensation of aldehyde and formaldehyde to produce acrolein. The addition of different guest oxides of the first and second group of the periodic table, allowed to tune the acidity and basicity of the catalyst surface. The basic and acidic properties of the catalysts have been investigated by adsorption calorimetry (see Figure) in order to obtain the concentration, strength and strength distribution of the active sites. Finally, the catalytic reaction has been performed in presence of oxygen with the aim to verify how in oxidizing conditions the acid/base properties can impact the activity and selectivity towards acrolein.