Reversibly interconverted Cu+/Cu+-H species as active sites for selective hydrogenation of fatty acid methyl esters to fatty alcohol over layered double hydroxide derived CuMgAlOx catalysts

Abstract

High fatty alcohol production (>90 % selectivity) can be achieved though the selective hydrogenation of fatty acid methyl esters over layered double hydroxide derived CuMgAlO_x catalysts in a fixed-bed reactor at 250 °C under atmospheric H₂. ∼17 wt.% Cu loading CuLDHs with different Mg²⁺/Al³⁺ ratios (CuMg₆₀Al₄₀O_x, CuMg₇₀Al₃₀O_x, CuMg₇₅Al₂₅O_x, and CuMg₈₀Al₂₀O_x) were prepared by co-precipitation-hydrothermal method. The Cu dispersion and species were determined by H₂-TPR, consecutive H₂-TPR, N₂O-dissociative reaction, and in situ TR-XANES. Highly dispersed Cu metal along with cationic Cu(I) species were obtained for all CuMgAlO_x. The cationic Cu(I) species (Cu⁺/Cu⁺-H) content increased with Mg²⁺ content. In the presence of H₂, the cationic Cu(I) species undergo reversible interconversion between Cu⁺ and Cu⁺-H species, facilitating hydrogen dissociation/evolution. The hydrogenation activity was governed by the balance of the metallic Cu surface and the cationic Cu(I) species. The Cu⁺ species allow preferential adsorption of CO ester for selective hydrogenation of FAMEs to fatty alcohol (>90 % selectivity). With Cu_surface/Cu(I) ratio at 0.25 (CuMg₇₅Al₂₅O_x), the fatty alcohol production rate of 49.3 h⁻¹ was obtained with high stability due to the reversible interconversion of Cu⁺/Cu⁺-H that prevented product re-adsorption and side reactions.