One-step butadiene synthesis via gas-phase prins condensation of propylene with formaldehyde over heteropolyacid catalysts

Gas-phase continuous synthesis of butadiene from propene and formaldehyde has been studied over heteropolyacids (HPA) supported on silica. Silicotungstic acid (SiW) is found to be the most active and selective among other HPA. At low SiW loadings, catalysts have low selectivity due to the formation of a butanal by-product, while at high loadings (≥40 wt %), selective butadiene formation and stable performance are achieved. Strong Brønsted acid sites associated with intrinsic SiW protons are shown to be more active in butadiene synthesis as compared with Si−OH₂⁺ sites formed by the interaction of HPA protons with a silica support. The kinetic study and product cofeeding experiments suggest that propene activation or C−C bond formation is the rate-determining step and that both butadiene and butanal are formed over strong acid sites starting from a common intermediate. Over weaker acid sites, condensation of butanal with formaldehyde leads to 2-ethylacrolein. Besides that, the reaction is complicated by butadiene interaction with formaldehyde leading to 2,3-dihydropyran, propene oligomerization and cracking, formaldehyde decomposition toward CO and H₂, and extensive coke formation leading to catalyst deactivation. However, the optimization of reaction temperature, SiW loadings, and the content of water in reaction media allows us to achieve a butadiene selectivity up to 64% and stable catalyst performance with time on stream.