Synthesis of 1,6-hexanediol from HMF over double-layered catalysts of Pd/SiO₂ + Ir-ReO_x/SiO₂ in a fixed-bed reactor

1,6-Hexanediol (1,6-HDO) was effectively prepared from 5-hydroxymethylfurfural (HMF) over double-layered catalysts of Pd/SiO₂ + Ir-ReO_x/SiO₂ in a fixed-bed reactor. Under optimal reaction conditions (373 K, 7.0 MPa H₂, in solvent mixtures of 40% water and 60% tetrahydrofuran (THF)), 57.8% yield of 1,6-HDO was obtained. The double-layered catalysts loaded in double-layered beds showed much superior performance compared to that of a single catalyst of Pd-Ir-ReO_x/SiO₂, even when the same amount of active components were used in the catalysts. The reaction solvent significantly affected product distributions, giving a volcano-shape plot for the 1,6-HDO yield as a function of the ratio of water to THF. Brønsted acidic sites were generated on the catalyst in the presence of water which played determining roles in 1,6-HDO formation. A high pressure of H₂ contributed to 1,6-HDO formation by depressing the over-hydrogenolysis of reaction intermediates and products to form hexane and hexanol. The reaction route was proposed for HMF conversion to 1,6-HDO on the basis of conditional experiments.