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

Abstract

The reaction mechanism of 2-hexanol conversion to high molecular weight compounds to be used as liquid transportation fuels was studied on MgO, Cu/SiO 2 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 (N 2 ) and reducing (H 2 ) reaction atmospheres at different contact times (W/F 0 ) was investigated, which allowed distinguishing between primary and secondary products. In H 2 , at W/F 0 = 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 Cu 0 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 H 2 , 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 N 2 , significant amounts of odd carbon atom number products (C9, C15 and C21) were formed with a higher contribution of oxygenates to the product pool.

The Role of Copper in the Upgrading of Bioalcohols

“…Cu and Ni are the metals most often used for this purpose owing to their activity in alcohol dehydrogenation.D iC osimo et al obtained am ixture of C9-C24 oxygenates and hydrocarbons with yields up to 91 %t hrough gas-phase upgrading of 2-hexanol at 300 8Co ver aC u-Mg-Al oxide containing 8% of Cu. [16] The authors compared these results with those obtained in the presence of aC u/SiO 2 catalyst with the same loading of Cu but in this case no CÀCb onds were formed, thus suggesting that Cu 0 particlesa re responsible only for the dehydrogenation-hydrogenation steps. In contrast, coupling reactions are promoted by Mg 2 + ÀO 2À couples but the authors found an optimal amount of Al, surroundingt hese couples and giving account of the very high selectivity observed.…”

“…Di Cosimo et al. obtained a mixture of C9–C24 oxygenates and hydrocarbons with yields up to 91 % through gas‐phase upgrading of 2‐hexanol at 300 °C over a Cu‐Mg‐Al oxide containing 8 % of Cu . The authors compared these results with those obtained in the presence of a Cu/SiO 2 catalyst with the same loading of Cu but in this case no C−C bonds were formed, thus suggesting that Cu 0 particles are responsible only for the dehydrogenation–hydrogenation steps.…”

“…Also in the case of Cu‐Mg‐Al systems, a structure in which Cu is surrounded by Al was proposed . It is worth noting that this structure favors also consecutive steps leading to C6 products in the case of ethanol over Cu‐Ceria/C and C24 ones in the case of hexanol over Cu‐Mg‐Al systems.…”

“…The strength of the support basic sites was found to correlate with activity and selectivity following the order TiO 2 < ZrO 2 < Al 2 O 3 .A ne xtensive investigation on the activity of Cu, Ni and Co supported on alumina conducted at 240 8Ca nd pressures up to 100 bar,s howed that 1-butanol was selectively formed (selectivity = 60-65 %a t2 5% conver-sion )o ver 19 %N i/Al 2 O 3 whereas 16 %C o/Al 2 O 3 gave ethyl acetate with up to 85 %s electivity. [29] The behavior of 1.8 %C u and 4.5 %C uc atalysts was quite different, with the first one giving 65 %b utanol and 22 %e thyl acetate, whereas the one with the higher loading gave 55 %b utanol and3 7% ethyl acetate (Table 1, entries [14][15][16][17].…”

Ethanol dehydrogenative reactions catalyzed by copper supported on porous Al–Mg mixed oxides

“…The use of aluminum and magnesium mixed oxides derived from hydrotalcites as support for impregnation of copper species and forward application in the dehydrogenation of alcohols is motivating. Aer heat treatment, these mixed oxides have various unique properties as providing an ideal balance of acid and base sites [13][14][15][16][17] and unique textural properties, which have impact on metals dispersion and therefore on ethanol conversion to acetaldehyde and ethyl acetate. 3,5 In a previous study, [18][19][20] an emulsion-mediated method was used to produce Al-Mg mixed oxides from hierarchically ordered hydrotalcite precursors that gave a high specic surface area.…”