Selective dehydra-decyclization of cyclic ethers to conjugated dienes over zirconia

ZrO₂ provides high selectivity (>90%) to conjugated pentadienes through the dehydra-decyclization of C-5 cyclic ethers, even at high conversions. 1,3-Pentadiene was the major product in both reaction of 2-methyltetrahydrofuran and tetrahydropyran over ZrO₂. The reaction of 3-methyltetrahydrofuran produced nearly stoichiometric amounts of isoprene. Other catalysts, including TiO₂, γ-Al₂O₃, and H-ZSM-5, were generally much less selective and produced a mixture of diene isomers. A combination of TPD and steady-state measurements revealed that both piperylenes are exclusively produced through primary catalytic pathways from 2-methyltetrahydrofuran, avoiding any isomerization once formed. First-principle calculations on ZrO₂ imply the presence of an energetically favored, surface isomerization of ring-opened intermediates to conjugated alkenolates that selectively dehydrate to conjugated dienes, providing high selectivity to the desired products. The stabilization of the conjugated alkenolate is key for understanding the ability of ZrO₂ to selectively produce conjugated dienes from cyclic ethers, without the need for the thermo-limited diene isomerization.