Conversion of Biomass-Derived 2-Hexanol to Liquid Transportation Fuels: Study of the Reaction Mechanism on Cu–Mg–Al Mixed Oxides

The reaction mechanism of 2-hexanol conversion to high molecular weight compounds to be used as liquid transportation fuels was studied on MgO, Cu/SiO₂ and a bifunctional Cu–Mg–Al mixed oxide with 8 wt% Cu (catalyst 8.0CuMgAl). Catalysts were characterized by several physical and spectroscopic techniques. The evolution of 2-hexanol conversion and yields in inert (N₂) and reducing (H₂) reaction atmospheres at different contact times (W/F⁰) was investigated, which allowed distinguishing between primary and secondary products. In H₂, at W/F⁰ = 500 g h/mol, the bifunctional 8.0CuMgAl catalyst yielded more than 90 % of branched C9–C24 oxygenates and hydrocarbons that were obtained via sequential steps comprising dehydrogenation, C–C coupling, dehydration and hydrogenation reactions. The metal-base bifunctional nature of this reaction network on 8.0CuMgAl was elucidated: nano-sized Cu⁰ particles promote dehydrogenation and hydrogenation steps whereas Mg–O pairs participate mainly in C–C coupling reactions. The product distribution depended on the reaction atmosphere. In H₂, the reaction pathways leading to formation of even carbon atom number products (C12, C18 and C24) were favored and hydrocarbons were the main products at high conversion levels. In N₂, significant amounts of odd carbon atom number products (C9, C15 and C21) were formed with a higher contribution of oxygenates to the product pool.