Sulphated zirconia catalysts for the gas-phase dehydration of glycerol into acrolein: the control of selectivity and a study on phenomena contributing to catalyst deactivation

Glycerol, the co-product in biodiesel synthesis, will play a crucial role in future biorefineries, because its derivatives find use in various sectors, such as fuels, chemicals, pharmaceutical, detergent, and building industries. In this context, in recent years catalytic dehydration of glycerol to acrolein has attracted great interest; acrolein is mostly used as an intermediate for the synthesis of acrylic acid. Catalysts offering the best performance include WO3/ZrO2, supported Keggin polyoxometalates, V/P/O, rare earth pyrophosphates and H-ZSM5. In this communication, we report about a study on the synthesis of acrolein by means of glycerol dehydration in the gas phase, catalysed by sulphated zirconia. The reaction parameters affecting glycerol conversion and selectivity to acrolein, the contribution of homogeneous reactions, and the role of diffusion constraints were examined, in relation to the complex reaction network. The best selectivity to acrolein, 42% obtained at 49% glycerol conversion, was achieved by combining conditions that limit the occurrence of consecutive reactions from acrolein, leading to acetaldehyde and to heavy compounds, whilst minimizing mass and heat transfer inter-particle limitations. Catalyst deactivation did not only occur because of the formation of heavy compounds, which accumulated on catalysts in the form of coke, but also by other, someway unexpected, events that contributed to catalyst deactivation especially when the reaction was carried out under anaerobic conditions. The catalytic behaviour was affected by the sulphate content in catalysts, not only because of the different number of acid sites, but also because of the self-reduction of sulphuric into sulphurous groups, a phenomenon that however occurred only in the sample having the greater S content, and the leaching of S from catalysts, due to the hydrolysis of sulphate groups and the formation of volatile organic esters.