Depolymerization of Lignin over a Ni–Pd Bimetallic Catalyst Using Isopropanol as an in Situ Hydrogen Source

Performance of Ni–Pd bimetallic catalysts on depolymerization of lignin into monomeric phenols was investigated using isoproponal as an in situ hydrogen source. The performance of catalysts and the effects of temperature and starting lignins were extensively examined. Ni50Pd50/SBA-15 exhibited the best performance in the depolymerization of cellulolytic enzyme corn stalk lignin, affording the highest total monophenols yield of 8.14 wt % which was 1.96 times that of Ni/SBA-15 and 1.44 times that of Pd/SBA-15. For the depolymerization of acid-extracted birch lignin, 18.52 wt % monophenols were obtained over Ni50Pd50/SBA-15 at 245 °C for 8 h, and 4-propylsyringol had 8.89 wt % yield with 47.97% selectivity. The physicochemical characteristics of the catalysts were elucidated with ascanning electron microscopy, high-resolution transmission electron microscopy, nitrogen adsorption/desorption, X-ray diffraction, and X-ray photoelectron spectroscopy. The bimetallic catalyst had the characteristics of narrow distribution of metal particles with a small size, well-retained hexagonal pore structure of SBA-15, higher dispersion of Pd, and more Pd0 and Ni0 as compared with monometallic catalysts. The properties of the bimetallic catalyst provide more contact opportunities between the abundant active centers and lignin fragments, and therefore significantly promote the hydrogenolysis process to produce monomeric phenols.