Acrolein direct synthesis from methanol and ethanol on hybrid FeMoOx - Na,K,Ca,Mg/SiO2 oxide catalysts

n the third decade of the past century, acrolein, a key intermediate in the synthesis of two important commodity chemicals, namely acrylic acid and methionine, was industrially produced by aldolisation of acetaldehyde and formaldehyde. Later, another synthetic way based on propylene oxidation was applied and largely studied. More recently, due to the raw materials diversification and the need to decrease the greenhouse gas emissions, new processes starting from renewable feedstock have been considered [1,2]. With the aim of minimizing the investment on the existing plants, the oxidation of biosourced alcohols is a promising way to produce acrolein when compared to glycerol dehydration. In this work, the oxidation of a mixture of methanol and ethanol to formaldehyde and acetaldehyde on FeMoOx and simultaneous aldol-condensation and dehydration to acrolein on Na,K,Ca,Mg/SiO2, was carried out; while most of research works published previously, were considering each reaction individually. 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 oxidative coupling of alcohols 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 respectively by sulphur dioxide and ammonia adsorption calorimetry in order to obtain the number, strength and strength distribution of the active sites. Finally, the catalytic reaction has been performed with the aim to verify how in oxidizing conditions the acid/base properties can impact the activity and selectivity towards acrolein. Good performances were obtained for Na/SiO2 catalyst with 25% acrolein yield and 10% CO+CO2 yield. K/SiO2 and Ca/SiO2 showed the lower acrolein production; they promoted side oxidation reactions that overoxidized acrolein (or reactants) into CO and CO2, due to the presence of oxygen. At 5000 h-1 GHSV and a temperature of 280 °C for K/SiO2 and of 320 °C for Ca/SiO2, the CO+CO2 yields were already higher than 10 mol%, thus limiting the possibility to further increase the reaction temperature to enhance acetaldehyde conversion and acrolein selectivity. Mg/SiO2 presented the best results (at 320 °C and 5000 h-1 GHSV) with 29% and 38% of acrolein yield and selectivity, respectively, and a CO+CO2 yield of 7% (see Figure). The presence of both acid and basic sites seems to increase the selectivity towards acrolein. References 1. JL Dubois, C. Duquenne, W. Holderich, J Kervennal, Patent US7655818 BB, Arkema France,2006 2. D. Stosic, S. Bennici, J.-L. Couturier, J.-L. Dubois, A. Auroux, Catal. Commun. 17 (2012) 23-28. Acknowledgements: the present research has received support from ADEME BIOMA+.