Selective synthesis of α-olefins by dehydration of fatty alcohols over alumina–thoria mixed catalysts

Abstract

The selective and high-yield production of α-olefins by alcohol dehydration is challenging because the isomerization and polymerization olefin products are more thermodynamically stable. In this study, we prepared a physical mixture of nano-sized alumina (Al₂O₃ⁿ) and thoria (ThO₂) with high efficiency and selectivity towards the production of α-olefins from stearic alcohol (100% conversion, 92% yield of α-olefin, rate: 2.925 mmol g⁻¹ h⁻¹, solvent: dodecane, 300 °C). To the best of our knowledge, this is the most efficient system developed for fatty α-olefin production from fatty alcohols, as compared to literature. Dehydration experiments of cis- and trans-2-methyl cyclohexanol over Al₂O₃ⁿ revealed the reaction to proceed by anti-elimination, which was favored by the abundance of Lewis acidic sites (LAS, 0.457 mmol g⁻¹) and Lewis basic sites (LBS, 0.567 mmol g⁻¹) on the porous surface of Al₂O₃ⁿ. LAS adsorbed and eliminated the hydroxyl groups (–OH) of stearic alcohol, generating a carbonium ion (carbocation) intermediate, which was subsequently, converted to the alkene as final products over the LBS. However, the selectivity and yield of α-olefin were comparatively low over pure Al₂O₃ⁿ due to the formation of isomeric olefin products. Interestingly, the addition of ThO₂ increased yield of α-olefin up to 92% owing to the strong adsorption of this compound on the catalytic surface at high temperatures (300 °C), as evidenced by in situ Fourier transform infrared spectroscopy (FTIR)-ethylene adsorption analyses. Consequently, ThO₂ hindered the undesired isomerization and oligomerization of α-olefin taking place over LAS, thereby enhancing the yield of α-olefin products.