摘要
The complete conversion of castor oil with 96.0% selectivity toward the hydrodeoxygenation (HDO) product octadecane in 3 h at 250 °C, 40 bars H2 and 20 bars CO2 pressure with a high castor oil to catalyst ratio (w/w) of 18.4 was demonstrated using a SiO2-TiO2-encapsulated one-pot Fe-Au-Pd@SiO2-TiO2 catalyst. This octadecane selectivity was 72.6% higher than what was obtained using a Fe-Ni-Pd@SiO2-TiO2 catalyst under the same reaction conditions, but with only half the castor oil/catalyst ratio as 9.2. At three times the castor oil/catalyst ratio of 27.6, the selectivity toward C18 using Fe-Au-Pd@SiO2-TiO2 was still 66.8% higher. The one-pot catalysts were also superior to a mesocellular foam-supported Fe-Pd-Ni catalyst synthesized using the conventional two-step wet impregnation method. Pressurized CO2 in hexane facilitated the HDO pathway obeying Le Chatelier's principle and by lowering the viscosity of the reactant solution. Based on the observed product spectrum, a reaction pathway for the HDO of castor oil over Fe-Au-Pd@SiO2-TiO2 was proposed. Catalyst characterization revealed different types of active sites in Fe-Au-Pd@SiO2-TiO2, acting synergistically to catalyze the HDO of castor oil. Single-crystal X-ray diffraction showed that the Fe-Au-Pd ensemble had a tetragonal dipyramidal crystal system with an Au centric nanostructure. Fe-Au-Pd@SiO2-TiO2 possessed impressive H2 spillover capability driven by the Fe-Au-Pd nanoparticles and a large number of isolated electron-deficient Pdδ+ surface species arising out of Pd-Au interaction via charge transfer. The Fe component of Fe-Au-Pd@SiO2-TiO2 existed as individual Fe, bimetallic Fe-Au/Fe-Pd, or trimetallic Fe-Au-Pd nanoparticles. The Fe surface of these nanoparticles in its Fe state was the primary HDO site. Due to one-pot synthesis, the catalyst could contain high practical metal loading (96.7% for Fe), resulting in abundant active sites for HDO. Fe-Au-Pd@SiO2-TiO2 was also tested to be stable over at least five cycles.