Balanced anchoring sites and volatile matter in biochar render Ni/biochar with higher metal dispersion and superior activity in hydrogenation of vanillin

Abstract

Metal/carbon catalysts could be prepared via simple calcination of biochar impregnated with salt of metal precursors. Abundance of oxygen-containing functionalities on biochar affects dispersion of nickel via influencing availability of anchoring sites for nickel ions in impregnation step. Biochar with high content of volatile matter tended to have more oxygen-containing species, but the volatiles released in calcination of Ni/biochar could experience secondary condensation, forming carbonaceous deposit and covering nickel sites. Therefore, the balanced distribution of oxygen-containing functionalities and volatile matter in biochar on dispersion of nickel were investigated via preparation of Ni/biochar catalysts with biochar support obtained at varied pyrolysis temperature (200, 300, 400 and 500 °C). The results indicated that the calcination of Ni/biochar-200 with biochar produced via pyrolysis at 200 °C encapsulated nickel from the deposit of secondary condensation of the volatiles, while the lack of abundance of O-containing species of biochar-400 and biochar-500 led to the growth of nickel species large crystals in their supported catalysts. In comparison, Ni/biochar-300 showed the highest dispersion of nickel and thus the highest activity for hydrogenation of vanillin to produce 2-methoxy-4-methylphenol (yield: 89.4%). The in-situ IR characterization of functionalities of biochar versus temperature and residence time indicated that the pyrolysis at 300 °C enhanced formation of –OH groups, the main anchoring sites of nickel ions. Nonetheless, abundant C = O at 400 °C and developed aromatic ring structures at 500 °C were not effective for dispersion of nickel species.