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 CuMgAlOx catalysts in a fixed-bed reactor at 250 °C under atmospheric H2. ∼17 wt.% Cu loading CuLDHs with different Mg2+/Al3+ ratios (CuMg60Al40Ox, CuMg70Al30Ox, CuMg75Al25Ox, and CuMg80Al20Ox) 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 CuMgAlOx. 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 C[dbnd]O ester for selective hydrogenation of FAMEs to fatty alcohol (>90 % selectivity). With Cusurface/Cu(I) ratio at 0.25 (CuMg75Al25Ox), 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.