Ring Opening of Biomass-Derived Cyclic Ethers to Dienes\nover Silica/Alumina

Abstract

We\nshow that cyclic ethers, such 2-methyltetrahydrofuran (2-MTHF),\ncan undergo dehydration to produce pentadienes over SiO₂/Al₂O₃. The catalyst exhibited reversible deactivation\ndue to coke deposition, with the yield to pentadienes decreasing from\n68% to 52% at 623 K over 58 h time on stream. A reaction network for\n2-MTHF dehydration was proposed on the basis of the results of space\ntime studies. Pentadienes can be produced directly by a concerted\nhydride shift and dehydration of carbenium intermediates or indirectly\nthrough dehydration of pentanal and pentenol. Reaction kinetics studies\nwere performed at temperatures ranging from 573 to 653 K and 2-MTHF\npartial pressures from 0.21 to 2.51 kPa. The apparent activation energy\nbarrier for 2-MTHF conversion to pentadienes and the reaction rate\norder for ring opening were determined to be 74 kJ mol^–1 and 0.24, respectively, indicating strong interaction between 2-MTHF\nand the SiO₂/Al₂O₃ surface. Other\nsolid acids such as γ-Al₂O₃, H-ZSM-5,\nand Al-Sn-Beta were found to be active for 2-MTHF dehydration to pentadienes.\nThe rate of ring opening decreased in the order 2,5-dimethyltetrahydrofuran\n> 2-MTHF > tetrahydropyran > tetrahydrofuran. Over SiO₂/Al₂O₃, the dehydration of 2,5-dimethyltetrahydrofuran\nresulted in 75% yield to hexadiene isomers.